Magnetically recoverable activated carbon/CMC–β-cyclodextrin composite sponge for high-performance Cr(VI) adsorption, reduction, and sustainable wastewater treatment

Abstract

Hexavalent chromium (Cr(VI)) is a dangerous and mobile contaminant with high toxicity that can threaten aquatic environments and human health. A magnetically recoverable activated carbon composite sponge (MACC) was synthesized by the in-situ growth of Fe₃O₄ NPs into activated carbon followed by encapsulation in a crosslinked carboxymethyl cellulose (CMC)/β-cyclodextrin (β-CD) biopolymer matrix. The structural characterization and physicochemical properties of the synthesized composite were carried out using XRD, BET surface area analysis, FTIR, SEM-EDS, XPS, and vibrating sample magnetometry. Batch adsorption tests indicated that the removal of Cr(VI) was significantly influenced by solution pH, adsorbent dosage, and contact time. The adsorption kinetics were best described by the pseudo-second-order model, suggesting that the mechanism was controlled by chemisorption. Equilibrium data fitted well to the Langmuir isotherm model, indicating monolayer adsorption onto homogeneously distributed active sites with a maximum adsorption capacity (qₘ) of 528.75 mg.g⁻¹. Process optimization was performed using response surface methodology based on Box–Behnken Design (BBD), where a quadratic polynomial regression model was developed to evaluate individual and interactive effects of operational variables; numerical optimization via desirability function revealed that optimal adsorption conditions were obtained at pH 4, adsorbent dosage 0.02 g, and contact time 100 min with a predicted adsorption capacity about 530 mg.g⁻¹ very close to experimental results. In addition, the MACC composite showed good regeneration ability and structural stability after repeated adsorption–desorption cycles without any detectable framework deterioration; these results confirmed that the MACC composite sponge is an efficient magnetically separable sustainable adsorbent for Cr(VI) remediation from aqueous environments.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-46358-4.

Introduction

The removal of chromium, especially hexavalent chromium (Cr(VI)), from wastewater is very important because of its high toxicity, persistence, and large amounts released by industries. Cr(VI) is described as a very soluble, mobile, and bioavailable contaminant that can easily cross biological membranes and cause carcinogenic, mutagenic, and teratogenic effects even at low concentrations¹. Its presence is associated with some of the most severe issues, such as kidney and liver damage, lung disease, skin ulceration, and an increased risk of cancer. From an ecological viewpoint, chromium pollution has negative effects on aquatic environments by reducing microbial activity and leading to its accumulation in sediments and food webs; thus posing long-term ecological hazards. Since conventional wastewater treatment methods are usually not effective in removing trace amounts of Cr(VI), regulatory bodies worldwide have established strict discharge regulations to prevent contamination of soil and groundwater. Therefore, there is a need for the development of efficient, cost-effective, selective, and regenerable adsorbents for chromium removal in order to protect public health as well as aquatic life and support sustainable industrial operations while complying with environmental legislation².

Chromium, particularly in its hexavalent state, poses serious risks to human health when found in wastewater due to its high toxicity, strong oxidative abilities, and cancer-causing potential³. Prolonged exposure to Cr(VI)-contaminated water can lead to a wide range of serious health issues, including lung cancer, cancers of the nasal cavity and sinus, and gastrointestinal malignancies due to its ability to easily penetrate cell membranes and cause DNA damage. Additionally, it has been associated with renal and hepatic failure because it bioaccumulates in these organs as they try to detoxify chromium. Skin contact with chromium-contaminated water may result in dermatitis, allergic reactions, skin ulcers, and chronic skin diseases known as “chrome ulcers”. Drinking this contaminated water might cause gastrointestinal problems such as abdominal pain, nausea, and bleeding from the digestive tract⁴. Breathing in aerosolized forms of contaminated wastewater can trigger respiratory diseases like asthma, bronchitis, perforation of the nasal septum, and reduced lung function. Also, exposure to Cr(VI) has been linked with reproductive toxicity developmental disorders impaired immune responses and genetic mutations making it especially dangerous for at-risk groups such as pregnant women and children. Given the seriousness and extent of these health effects regulating chromium in wastewater is crucial for protecting public health and maintaining environmental quality⁵.

Among the many methods used for removing heavy metals from wastewater such as chemical precipitation, coagulation and flocculation, ion exchange, membrane filtration, electrochemical treatments and biological processes adsorption has become very popular because it is one of the most effective and flexible techniques due to its specific operational as well as environmental advantages⁶. Though seemingly simple, conventional methods such as precipitation and coagulation often lead to high sludge production and are not effective at low metal concentrations. On the other hand, advanced methods like ion exchange, membrane separation, and electrochemical techniques achieve high removal efficiencies but are limited by cost issues, fouling problems, and significant energy or chemical needs⁷. On the other hand, adsorption achieves a good balance between effectiveness, economy, and sustainability. This method can reach high removal efficiencies even at low metal concentrations because of the strong attraction between adsorbent surfaces and metal ions; this is more pronounced when functional groups like –OH, –COOH, and –NH₂, are present. It also produces little secondary waste͏, has easy design and operation, and allows use of many different materials both natural and synthetic as well as those coming from waste such as activated carbon͏, biochar͏, zeolites͏, clays͏, chitosan͏; new adsorbents like metal-organic frameworks8. Furthermore, several adsorbents can be easily regenerated and reused by mild chemical treatments, increasing the economic viability of the process for continuous applications. Taken together, these properties high selectivity and efficiency at low concentrations with little sludge production in addition to being regenerative and environmentally friendly make adsorption a preferred technique as well as a common solution for heavy metal removal in modern wastewater treatment systems9. Activated carbon is a very good material for taking heavy metals out of waste water because it has a very large surface area, a clear pore structure, and many functional groups on its surface that help in forming strong bonds with metal ions10. The activated carbon has a micro- and mesoporous structure that provides a large number of active sites for adsorption, so it can achieve high uptake capacities even when the concentrations of metal ions are low. In addition, activated carbon contains many oxygen-containing functional groups, including hydroxyl, carbonyl, carboxyl, and phenolic groups. These increase the binding of metals through different mechanisms like electrostatic attraction, ion exchange, complexation, and surface precipitation11. Another great advantage is its chemical and thermal stability, which lets activated carbon keep working well over a wide range of pH values, temperatures, and salt levels. Also, activated carbon can be changed physically or chemically, or mixed with special agents to greatly improve its attraction for certain heavy metals. This increases both selectivity and adsorption capacity. In addition, this material creates very little secondary waste, can be reused many times through thermal or chemical methods, and is easily available at a low price, especially when it comes from agricultural by-products (like coconut shells, sawdust, and rice husks). All these features high adsorption capacity, tunable surface chemistry, operational stability, reusability, and economic feasibility make activated carbon one of the most useful materials for taking out heavy metals in environmental clean-up projects12.

Magnetic activated carbon embedded in a biopolymer matrix of β-CD and CMC offers an effective, multifunctional, and recoverable system as an adsorbent for the removal of heavy metals and chromium in its hexavalent form (Cr(VI)) from wastewater¹³. The activated carbon has a large surface area with many micro/mesopores and different types of oxygen-containing functional groups. These properties help strong adsorption processes like electrostatic attraction, ion exchange, complexation, and surface redox reactions^14,15. These mechanisms are very important for reducing Cr(VI) to the less dangerous Cr(III)). The encapsulation by β-CD brings in hydrophobic cavities and more hydroxyl groups, which help improve host–guest interactions and increase the binding strength for both organic–inorganic metal complexes and oxyanion species like HCrO₄⁻ and Cr₂O₇²⁻. Also, CMC brings carboxylate and hydroxyl groups that greatly enhance metal chelation, increase surface hydrophilicity, and give structural stability to the composite. The biopolymer structure also helps reduce the clumping of activated carbon particles, keeping high dispersion and ensuring active adsorption sites are available. The addition of magnetic nanoparticles gives superparamagnetic properties to this system so that it can be easily separated from an external magnetic field, thus reducing sludge production and allowing easy regeneration and reuse. To sum up, a hybrid adsorbent with high surface area activated carbon, multifunctional β-CD and CMC as well as magnetic recoverability results in enhanced capacity, selectivity, stability, and operational convenience for removal of various heavy metals including Cr(VI) under real wastewater treatment conditions¹⁶.

Using the Box-Behnken Design (BBD) to optimize Cr(VI) adsorption has many methodological and statistical advantages, showing that it is suitable for adsorption studies¹⁷. Compared to full factorial or central composite designs, BBD requires a relatively small number of experimental trials; hence it saves time, cost, and material consumption while maintaining a strong and reliable optimization process. This design does not involve extreme combinations of factors in its experimental set-up and therefore reduces the chance of very adverse conditions that may lead to the degradation or instability of the adsorbent during Cr(VI) treatment. Moreover, BBD effectively evaluates both the individual and interactive impacts of parameters like pH, adsorbent dosage, contact time, temperature, and initial Cr(VI) concentration for correct modeling on their respective contributions toward removal efficiency. The resulting quadratic polynomial model permits accurate response predictions as well as finding optimal operating conditions supported by statistical validation through ANOVA and response surface methodologies plus desirability functions. Also, BBD helps understand complex adsorption dynamics better by revealing synergistic or antagonistic interactions among variables that are often overlooked in single-factor studies. In short, the Box–Behnken Design is a powerful, low-cost method with scientific credibility for improving Cr(VI) adsorption processes and optimizing advanced adsorbent materials¹⁸.

Modified activated carbon, polymeric hydrogels, and magnetic nanocomposites have been intensively studied as functionalized adsorbents for the removal of Cr(VI). Most of these systems suffer from poor regeneration efficiency, difficult post-treatment separation, and adsorption mechanisms dominated by weak physical interactions. The present MACC composite combines a porous activated carbon framework with magnetically responsive Fe₃O₄ nanoparticles and a dual-functional CMC/β-CD biopolymer matrix to allow enhanced adsorption through synergistic electrostatic attraction, surface complexation, and redox-driven reduction of Cr(VI) to Cr(III). The integration of response surface methodology based on the Box–Behnken Design allows systematic optimization of operational parameters which improves adsorption performance and provides a statistically supported approach for practical wastewater treatment applications.

The originality of this manuscript lies in developing a multifunctional magnetic activated carbon composite that is separately encapsulated by two biopolymers β-CD and CMC. This composite is designed for highly efficient magnetically retrievable removal of Cr(VI) and other heavy metals from aqueous systems. Unlike regular activated carbon adsorbents, the addition of β-CD enables host–guest inclusion interactions as well as provides extra binding sites. At the same time, CMC brings many carboxylate functionalities that enhance chelation, increase surface hydrophilicity, and improve structural stability. The use of magnetic nanoparticles permits quick separation with external magnets so there is no need for filtration which greatly improves operational practicality and regeneration. It also allows synergistic adsorption mechanisms such as electrostatic interactions, complexation, reduction of Cr(VI) to Cr(III), and host–guest entrapment that cannot be achieved with single-component materials. Further, the adsorption method is optimized using Box–Behnken Design which provides a statistically validated and energy-efficient way to find the best parameters for Cr(VI) removal. In short, results describe a new sustainable magnetically recoverable adsorbent with better performance selectivity and reusability marking an important step forward from current materials used for Cr(VI) adsorption.

Experimental

Materials and instruments

The supplementary materials, specifically Tables S1 and S2, offer comprehensive descriptions of the resources utilized for each component and apparatus, serving as visual aids for reference purposes.

Synthesis of adsorbent

Synthesis of activated carbon

The first step in preparing activated carbon is to wash the pomegranate peel with hot water and then dry it in a furnace at 100 °C. The next step involves soaking the dried pomegranate peel with a 25% by weight solution of citric acid at a ratio of biomass to citric acid solution of 1:2 for 24 h at room temperature. After pre-treatment, hydrothermal treatment was carried out at 80 °C on the material. Rather than using centrifugation at 10,000 rpm, the product was obtained by filtration. The process of carbonization is then accelerated by microwave irradiation at a power of 800 W for 15 min under an inert nitrogen atmosphere with a constant flow rate of 100 mL/min. The sample that has been carbonized was refluxed in water for three hours and washed with boiling distilled water until the pH of the filtered water became neutral. After activation, carbon was dried for 24 h at 110 °C in an oven and stored properly for further use¹⁹.

Synthesis of Fe₃O₄

In preparing the Fe₃O₄ nanoparticles, ferrous chloride (FeCl₂·4H₂O) and ferric chloride (FeCl₃·6H₂O) were mixed with deoxygenated deionized water in a 1:2 molar ratio. The reaction took place at 80 °C under continuous stirring and was sustained in a nitrogen environment. Then, the pH of this solution was raised slowly to about 10 by adding a 25% aqueous ammonia solution dropwise. This change brought about instant precipitation of Fe₃O₄ as indicated by the formation of a black solid. The solution was kept stirring for another half hour to ensure full nucleation and crystallization, plus an extra ten minutes of sonication. The next step involved retrieving the Fe₃O₄ nanoparticles using magnetic separation followed by washing with ethanol and distilled water to get rid of any leftover contaminants. Finally, samples were dried at 65 °C²⁰.

Synthesis of magnetic activated carbon composite sponge (MACC)

Magnetic activated carbon (MAC) was synthesized by dispersing 5.0 g of synthesized activated carbon in 100 mL deionized water and subjecting the mixture to ultrasonication for 15 min to ensure a good dispersion. In another container, 2.1 g of Fe₃O₄ nanoparticles (which is equivalent to 30 wt% of the final composite) were dispersed in 50 mL deionized water under sonication for proper dispersion²¹. The suspension of Fe₃O₄ nanoparticles was then added dropwise into the activated carbon dispersion under stirring, and the mixture kept at 70 °C for 1 h for even deposition of magnetic nanoparticles on the surface of carbon via electrostatic and physical interactions. The resulting magnetic activated carbon (MAC) was magnetically separated, washed several times with deionized water until neutral pH achieved, and dried overnight at 60 °C²². The MAC was then encapsulated in a crosslinked biopolymer matrix by dissolving 1.0 g of carboxymethyl cellulose (CMC) and 1.0 g of β-cyclodextrin (β-CD) in 100 mL deionized water to obtain a homogeneous solution. The MAC dried was added to the polymeric solution and ultrasonicated for 10 min for dispersion. The pH was set at nearly 11 with 0.1 M NaOH, after which epichlorohydrin was added dropwise, totaling 0.28 mL as crosslinking agent; this corresponds to an epichlorohydrin-to-(CMC/β-CD) mass ratio of about 0.14:1 (w/w)²³. The reaction mixture was kept at 40 °C with stirring for six hours so that ether linkages could form between the hydroxyl groups of CMC and β-CD, creating eventually a stable three-dimensional polymeric network that would physically trap MAC particles inside it. After the crosslinking reaction was over, neutralizing the mixture with dilute HCl gave the final MACC composite sponge which was magnetically separated from the solution, washed several times with deionized water and ethanol to purge unreacted reagents, and dried at a temperature between 50 °C and 60 °C until further adsorption study experiments were carried out (Fig. 1)²².

Fig. 1.

A schematic representation illustrating the synthesis process of the MACC composite sponge.

Removal of Cr(VI) and batch studies of the MACC composite sponge

An extensive study was undertaken to assess the ability of the MACC composite sponge to adsorb under varying conditions of pH, dosage, concentration, time, and temperature. Using a batch system with a total volume of 25 mL, the adsorption behavior towards Cr(VI) was studied within the concentration range of 40 to 620 mg/L²⁴. The pH values were adjusted sequentially from 2 to 8. This pH range was based on the pH-dependent speciation of Cr(VI) in aqueous media and on the conditions that can be found in real wastewater. Under such conditions, chromium is generally present as HCrO₄⁻ and Cr₂O₇²⁻ at acidic to near neutral pH levels (pH 2–6), but it will start to change into CrO₄²⁻ at higher pH values. This range also includes the pH_pzc of the MACC composite sponge (3.52), which makes it possible to systematically assess adsorption behavior through the transition from positively to negatively charged surface states²⁵. The contact time used ranged between 5 and 100 min. The amount of adsorbents was varied between 0.02 and 0.50 g, and the temperature was varied from 20 to 45 °C during the study. To adjust the pH, 0.1 N solutions of HCl and/or NaOH were used. The experimental data were fitted in isotherm studies, kinetic studies, and temperature effect on thermodynamic parameters related to the adsorption process. The samples of research were analyzed using a flame atomic absorption spectrometer. The sorption capacity for each sample was obtained as an average of three independent experiments. Aqueous samples after each adsorption experiment were first separated using an external magnet then filtered through a 0.45 μm membrane filter to remove any suspended adsorbent particles before analysis. The filtrates were acidified with dilute nitric acid in order to stabilize the dissolved chromium species and prevent precipitation during measurement. If necessary, deionized water was used for proper dilution so that Cr(VI) concentration would fall within the linear calibration range of flame atomic absorption spectroscopy. Calibration curves were constructed from standard chromium solutions prepared under similar acidic conditions from a certified stock solution as described above. The analytical method was validated for limit of detection (LOD), precision, and recovery. Detection limit was determined based on three times standard deviation of blank measurements; precision was checked by triplicate analysis of selected samples giving relative standard deviation (RSD) values below 5%¹¹. The amount of Cr(VI) that was taken up, known as q_e, was calculated together with the percentage of removal (% RE) using Eqs. (1) and (2) for their respective calculations.

1

2

Experimental design

Response Surface Methodology (RSM) is a statistical approach that constructs empirical models when there are several possible responses. The primary aim of RSM is to describe the relationship between the response variable and the factors affecting it, while also using these models to optimize the process under study. The sorption capacities for each sample were determined as an average of three separate experimental runs. The removal percentage (% RE) and the amount of Cr(VI) adsorbed (q_e) were calculated using Eqs. (1) and (2), respectively. RSM basically conducts a series of planned experiments to alter some key features of a process in order to find the best answer within the whole context of the method. One of the most common methods utilized for optimizing process parameters is Central Composite Design (CCD). Equation 1 describes the quantity of Cr(VI) adsorbed per gram of adsorbent at different times t considering MACC composite sponge. Additionally, effectivity assessment regarding removal process can be obtained by using Eq. 2²⁶. The three main factors that significantly affect the process are “adsorbent mass”, “contact time”, and “solution pH”. These factors were chosen based on their negative effects on adsorption capacity, as shown in Table S3 that gives the extreme values for each parameter studied, taken from a broad study carried out with Design Expert Software. This table is meant to show different combinations of parameter limits and their resulting effects. It mainly focuses on center runs (P), axial runs (2×m), and factorial runs (2^m), along with other experimental designs and mixtures used in this study. Such diversity of methods reflects how complex the testing strategies were. Equation (3) is the mathematical equation used to calculate the total number of experiments needed, which depends on how many input variables are included in the analysis.

3

P denotes the total number of significant experiments performed, while N is the number of process variables that influence the responses. In this study, the value of the defined parameter “m” is 3. The three main steps of the central composite design are model coefficients estimation, experimental design, and model prediction. A careful and detailed examination of the results obtained in each step is necessary. After completing the previous tasks, a systematic empirical model has been established to evaluate how well the function performs for different combinations of input variables. Thus, a quadratic regression model has been developed as shown in Eq. (4):

4

In this analysis, the resistance constant is denoted by “i”, and the velocity coefficient is represented by “j”. The parameters β₀, β_i, β_ii, and β_ij are constants related to resistance, interaction, velocity, and the constant term, respectively. The goodness-of-fit of the proposed polynomial perfection equation was evaluated using the metrics R², R²_Adj, and R²_Pred (see Table 1). A high R² value indicates good predictive accuracy for the model and reflects a strong relationship between the model and the experimental data observed. The Box-Behnken Design numerical optimization based on the desirability function determined that the maximum Cr(VI) adsorption onto MACC composite sponge occurs at a solution pH of 4, an adsorbent dosage of 0.02 g, and a contact time of 100 min. Under these optimized conditions, the quadratic model predicted a maximum adsorption capacity in the range of about 520–530 mg.g⁻¹ which was experimentally obtained as 528.75 mg.g⁻¹; this finding validated both the adequacy and reliability of the developed RSM model to predict adsorption performance accurately.

Table 1.

Study of the adsorption of Cr(VI) and its related interactions on the surface in relation to the central composite design is important.

Run	Real variables			Adsorption capacity (mg/g)
	pH	Time (min)	Dose (g)	Investigational	Predicted	Residue
1	5	100	0.02	529.87	501.95	27.92
2	5	5	0.02	69.03	89.18	-20.15
3	5	52.5	0.26	352.28	352.28	0.0000
4	5	52.5	0.26	352.28	352.28	0.0000
5	8	52.5	0.02	366.37	370.21	-3.84
6	5	52.5	0.26	352.28	352.28	0.0000
7	5	5	0.5	43.15	71.07	-27.92
8	8	52.5	0.5	241.04	237.10	3.93
9	2	100	0.26	274.55	298.53	-23.99
10	2	5	0.26	35.77	11.69	24.08
11	5	52.5	0.26	352.28	352.28	0.0000
12	8	5	0.26	47.61	23.62	23.99
13	8	100	0.26	365.43	389.51	-24.08
14	2	52.5	0.5	218.08	214.24	3.84
15	5	52.5	0.26	352.28	352.28	0.0000
16	5	100	0.5	331.17	311.02	20.15
17	2	52.5	0.02	286.23	290.16	-3.93

Results and discussion

Characterization of MACC composite sponge

Patterns of X-ray diffraction (XRD)

The XRD patterns shown in Fig. S1(a, b) verify the successful synthesis of Fe₃O₄ nanoparticles and their incorporation into activated carbon matrix for MACC composite sponge. As illustrated in Fig. S1(a), the diffraction peaks at about 2θ values of 30.2°, 35.5°, 43.3°, 53.6°, 57.2° and 62.8° are well matched with characteristic crystal planes of magnetite Fe₃O₄ i.e., (220), (311), (400), (422), (511) and (440). This is consistent with the standard JCPDS card No: 00-900-8318 proving that highly crystalline Fe₃O₄ nanoparticles have been formed here. The XRD pattern for ACC given in Fig S1(b) shows a broad diffraction band around the angular range of approximately 22–26 degrees which indicates an amorphous carbon structure typically found in activated carbon materials having disordered graphitic layers²⁷. After magnetic modification, the MACC composite exhibits both broad peaks due to amorphous carbon and sharp peaks corresponding to Fe₃O₄. This indicates that magnetite nanoparticles were successfully deposited onto the surface of activated carbon without changing its original porous structure. The absence of any additional impurity peaks confirms high phase purity as well as effective integration of Fe₃O₄ within the composite matrix; Further, a slight reduction in peak intensity observed in the MACC pattern can be attributed to uniform dispersion of Fe₃O₄ nanoparticles and partial encapsulation by CMC/β-CD biopolymer coating that reduces crystallite visibility. These results together prove that a magnetically functionalized activated carbon composite has been made with preserved structural integrity which is necessary for better adsorption performance and easier magnetic recoverability during Cr(VI) removal. The pattern of X-ray diffraction shown in Fig. 2(a) describes the crystal structure of the MACC made. The peaks of diffraction reveal a broad peak placed within the range of 2θ = 25–45°, which is a sign of activated carbon’s amorphous nature. This pattern matches with the (002) plane related to graphitic carbon layers that have low crystallinity. An absence of sharp and distinct peaks indicates that most of the carbon domains are disordered; thus, it suggests high surface area for materials usually known as activated carbon²⁸. The broadness of the observed peak indicates partial graphitization with turbostratic stacking of the carbon sheets. The general diffraction pattern shows that the addition of Fe₃O₄ nanoparticles and their subsequent encapsulation with CMC and β-CD has not significantly changed the amorphous carbon structure. A very weak shoulder and even weaker diffraction features that appear near 2θ ≈ 35–43° can be related to the introduction of magnetite (Fe₃O₄) nanocrystals into the carbon matrix, which proves successful magnetic modification. The lack of any sharp crystalline peaks for Fe₃O₄ is due to the even distribution of nanosized particles and also due to a biopolymer coating that reduces X-ray scattering intensity. These XRD results confirm that a magnetically responsive, amorphous carbon-based composite has been successfully synthesized wherein Fe₃O₄ is homogeneously integrated within the CMC/β-CD polymeric matrix while at the same time keeping intact the porous and amorphous nature of the carbon structure which is essential for adsorption applications²⁹.

Fig. 2.

(a) XRD pattern of MACC composite sponge, (b) N₂ adsorption/desorption isotherm of MACC and Cr@MACC, (c) Pore radius distribution, (d) Pore size distribution, (e) FT-IR of MACC and Cr@MACC composite sponge, and (f) Magnetization of Fe₃O₄ and MACC composite sponge.

N₂ adsorption/desorption isotherm

The nitrogen adsorption and desorption isotherm for the MACC composite sponge, as shown in Fig. 2(b), reveals a Type IV isotherm with an H3-type hysteresis loop, according to IUPAC classification. This indicates that there is some mesoporous structure made up of slit-shaped pores created by the aggregation of plate-like particles³⁰. The gradual increase in the adsorption process followed by a hysteresis loop at relative pressure P/P₀ = 0.4–1.0 supports this fact since it falls within the range indicative of capillary condensation occurring inside the mesopores, as discussed in Table S4. Specific surface area for MACC composite sponge measured by BET method was found to be 84.8 m²/g, which indicates a highly developed porous network suitable for adsorption applications^31,32. When Cr(VI) ions were adsorbed, the surface area decreased to 72.6 m²/g due to some mesopores and micropores being blocked by the chromium species that had attached themselves. This change also resulted from surface complexes forming between Cr(VI) ions and functional groups on the composite’s surface, as indicated in Table S5. The pore size distribution in Fig. 2(c) also proves this point. The MACC composite sponge had an average pore size of 2.40 nm, meaning it is mesoporous. After Cr(VI) was adsorbed, the pore size reduced to 2.12 nm³³. This shows that the chromium ions got inside the pores since the effective pore size became smaller. In addition, analysis of pore size distribution and volume shown in Fig. 2(d) indicates that total pore volume dropped from 1.12 cm³/g for pristine MACC to 0.88 cm³/g after Cr(VI) adsorption. This significant decrease in both surface area and pore volume confirms that Cr(VI) ions adsorbed not only on the outer surface but also inside the internal pore structure, indicating an effective use of the porous framework. Together, nitrogen adsorption-desorption measurements confirm mesoporous property of MACC composite sponge and demonstrate that adsorption of Cr(VI) ions leads to filling up pores and covering surfaces which then reduces textural parameters for material as shown in Fig. 2(d).

FT-IR

The FT-IR spectrum of activated carbon (ACC), shown in Fig. S1(c), reveals the surface functional groups that are mainly responsible for adsorption interactions. There is a broad absorption band at 3400–3450 cm⁻¹ due to stretching vibrations of hydroxyl (–OH) groups from surface phenolic functionalities and physically adsorbed moisture. Weak peaks at 2920–2850 cm⁻¹ are due to asymmetric and symmetric stretching vibrations of aliphatic C–H bonds in the carbonaceous framework³⁴. A sharp band around 1620–1650 cm⁻¹ belongs to stretching vibrations for carbonyl (C = O) groups or aromatic C = C bonds present within the graphitic structure of activated carbon. Also, the strong band observed in the region between 1100 and 1030 cm⁻¹ can be assigned to C–O stretching vibrations from ether, alcohol, or carboxylic acid groups which means that some oxygen-containing surface functionalities exist³⁵. The peak which is below 800 cm⁻¹ is normally associated with out-of-plane bending vibrations of aromatic C–H groups. These functional groups, especially hydroxyl, carbonyl, and ether moieties, are very important in the process of electrostatic attraction, surface complexation as well as hydrogen bonding with Cr(VI) ions so that they increase the adsorption efficiency of ACC by acting as active binding sites for further modifications during MACC composite formation. The FT-IR spectra for MACC and Cr@MACC composite sponges are shown in Fig. 2(e). They help to explain the functional groups that take part in the adsorption of Cr(VI) and also the various interactions that occur on composite surfaces. In the FT-IR spectrum of as-received MACC, a broad absorption band was seen at around 3420 cm⁻¹. This usually relates to stretching vibrations from hydroxyl groups (–OH) coming from CMC, β-CD, and physically adsorbed water molecules. The absorption peaks found at about 2920 cm⁻¹ and 2850 cm⁻¹ belong to both asymmetric and symmetric C–H stretching vibrations of aliphatic –CH₂ groups in the organic framework. An important band was found at 1630 cm⁻¹ which corresponds to stretching vibrations for carbonyl groups (C = O) in carboxylate functionalities; this overlaps with bending mode related to adsorbed water. The shoulder around 1420 cm⁻¹ is attributed to the bending vibrations of –CH₂ groups and deformation of hydroxyl bonds in CMC and β-CD. A strong peak between 1080 and 1030 cm⁻¹ is due to C–O–C and C–O stretching vibrations from polysaccharide units, confirming effective crosslinking between β-CD and CMC through epichlorohydrin^36,37. The weak absorption band at about 570 cm⁻¹ is due to Fe–O stretching vibrations, indicating that Fe₃O₄ nanoparticles have been successfully incorporated into the composite matrix. After Cr(VI) was adsorbed, significant shifts and changes in intensity in the FT-IR spectrum of Cr@MACC confirmed chemical interactions between active functional groups and chromium species. In particular, broadening of the –OH stretching band with a small shift to lower wavenumbers indicated hydrogen bonding plus complex formation with Cr(VI) ions³⁸. Also, an intensity decreases for the C = O stretching band near 1630 cm⁻¹ suggested coordination between carboxyl groups and chromium ions. The characteristic vibrations C–O–C within 1080–1030 cm⁻¹ showed reduced transmittance as well; this further supports that ether and hydroxyl groups are involved in the adsorption mechanism. The observation of the Fe–O band at about 570 cm⁻¹ in the spectrum of Cr@MACC means that magnetic Fe₃O₄ core is still there since it was not removed during adsorption. All these changes in spectra together show that Cr(VI) adsorbs onto MACC composite sponge by different ways like attracting oppositely charged particles, forming new compounds with some functional groups, and maybe even changing oxidation states through its –OH, –COOH, and ether groups. This proves chromium can be effectively held inside the structure of the composite³⁹.

Magnetization

Vibrating sample magnetometry was employed to assess the magnetic properties of synthesized MACC composite sponge, specifically its magnetization characteristics in order to confirm the successful incorporation of Fe₃O₄. This assessment plays an important role in verifying that the composite can be efficiently extracted from aqueous environments using an external magnetic field, which is a basic condition for its use in water treatment processes. The effectiveness as well as the speed with which adsorbent can be recovered are crucial factors determining both reusability and overall economic feasibility of the method applied. The magnetic hysteresis loop shown in Fig. 2(f) reveals that MACC composite sponge has a saturation magnetization (Ms) value equal to 50.16 emu/g; however, pure Fe₃O₄ presents higher saturation magnetization at 104.7 emu/g. Even though there is some reduction in total magnetic strength caused by the introduction of Fe₃O₄ into biopolymer-coated MACC composite sponge, the residual magnetization remains sufficiently high for effective magnetic separation. This also means that this composite still maintains its superparamagnetic nature even after surface modification and structural encapsulation took place. The fact that these properties are kept makes it possible to have an easy solid-liquid separation process without needing extra filtration measures. As a result, the findings from this magnetization test back up the theoretical plan for creating an adsorbent that can be reused and retrieved with magnets, which fits very well with the needs for effective and large-scale removal of pollutants²⁰.

EDX analysis

The Energy Dispersive X-ray (EDX) spectrum of the MACC composite sponge shown in Fig. 3(a) gives clear signals for oxygen, iron, carbon, and a much smaller signal for nitrogen. The atomic weight% (wt%) values given in the inset are: carbon (C) 16.08%, nitrogen (N) 14.00%, oxygen (O) 53.54%, and iron (Fe) 16.37%⁴⁰. The strong signal for oxygen shows that there are many groups with oxygen in the CMC/β-CD coating; these groups include hydroxyl (–OH), carboxylate (–COO–), and ether (C–O–C), plus some water that has been adsorbed. The presence of an iron peak confirms that Fe₃O₄ nanoparticles have been successfully integrated to give magnetic property to the composite material⁴¹. The relatively lower amount of carbon could come from the carbon support, which is partly hidden by the dense polysaccharide shell because of the limited sampling depth of the EDX technique. The nitrogen content that can be measured, around 14%, might come from very small amounts of nitrogen-containing residues or adsorbed materials; this includes possible contamination and should be looked into further if it is important to the study. In general, what was found by EDX backs up a view in which Fe₃O₄ is evenly mixed with an oxygen-rich polymeric material sitting on a carbon base. It would be good to add elemental mapping results for better knowledge about surface chemistry and oxidation states like that shown in Fig. 3(a).

Fig. 3.

EDX examination: (a) MACC composite sponge, and (b) Cr@MACC composite sponge.

The energy-dispersive X-ray (EDX) spectrum for the Cr@MACC composite sponge, shown in Fig. 3(b), confirms that chromium species have been effectively adsorbed and stabilized on the surface of the magnetic activated carbon composite. The elemental composition analysis yields these proportions: C = 12.74%, Cr = 25.04%, N = 6.20%, O = 32.80%, and Fe = 23.22%. Strong chromium peaks are evident in this spectrum, especially near 0.6 keV and 5.4 keV; such findings provide compelling proof about the presence of chromium on the surface of the composite material and thus validate that Cr(VI) ions have been adsorbed⁴². Compared to the clean MACC composite in Fig. 3(a), there is a significant increase in iron and oxygen content, while carbon content decreases slightly. This suggests that the surface may be covered by chromium oxide or hydroxide species. The high level of oxygen indicates that Cr(VI) adsorption involves surface redox reactions and complexation with existing functional groups like –OH, –COOH, and C–O–C originating from CMC and β-CD coating. The iron peaks confirm the stability and exposure of the magnetic Fe₃O₄ nanoparticles after adsorption, while the mild nitrogen signal indicates that the functionalities of the polymeric coating are preserved. The high chromium content (25.04%) proves that the MACC composite sponge has a high adsorption capacity for Cr(VI), which is probably due to mechanisms like electrostatic attraction, ion exchange, and complex formation between chromium species and functional groups on the surface of the composite. In conclusion, EDX data confirm that Cr@MACC composite sponge captured and held chromium ions on its surface without damaging the structural integrity of its magnetic framework⁴³. This makes it a potential candidate for heavy metal removal from aqueous environments as shown in Fig. 3(b).

SEM

Figure 4(a) presents the SEM micrograph of the MACC composite sponge, which shows a solid but very porous and interconnected three-dimensional structure with aggregated carbonaceous domains within the crosslinked CMC/β-CD matrix. The surface has a wrinkled and folded texture with many cavities, voids, and gaps between particles all over the matrix. These irregular concave areas and open channels create an uninterrupted porous pathway that allows fluids to enter easily and reach internal adsorption sites more quickly⁴⁴. The micro voids seen here plus layered projections mean there is a hierarchical pore system developed by polymeric crosslinking plus magnetic nanoparticle infusion confirming that spongy structure has been made which will enhance mass transfer as well as ion diffusion inside this composite matrix. Three-dimensional grayscale reconstruction of the MACC composite sponge from SEM data reveals significant surface morphology variation. The elevation profile features steep peaks, ridge-like projections, and interlinked valleys that form a porous structure. Such height variations correspond to hierarchical pore channels and internal voids characteristic of sponge morphology, indicative of an open network for facilitating mass transfer and diffusion of adsorbate species during adsorption (Fig. 4(b)). The 3D thermal color-scale view of MACC composite sponge illustrates surface elevation differences. The intensity colors from low to high indicate depth variations that reveal cavities, protrusions, and connected pores⁴⁵. These gradients are a representation of a non-uniform porous structure with shallow depressions and deeper holes, which is an indicator of a hierarchical pore network. Such irregularities increase the internal accessibility and thus the adsorption efficiency (Fig. 4(c)). The MACC composite sponge elevation profile in blue tone appears in Fig. 4(d). It shows the spatial continuity and depth distribution of surface features. The differences in blue-green intensity reflect variations in surface height and reveal the presence of interrelated pore channels, microcavities, and layered protrusions within the topology. These characteristics indicate that there is a hierarchical porous network throughout the composite matrix. The continuity of elevated and depressed regions also indicates a sponge-like architecture which is very important for fluid permeability as well as for holding adsorbate species during adsorption⁴⁶.

Fig. 4.

(a) SEM of MACC composite sponge, (b) Grayscale 3D map, (c) Thermal color scale, (d) Blue tones.

The SEM micrograph of the Cr@MACC composite sponge in Fig. 5(a) depicts substantial alterations in surface morphology subsequent to Cr(VI) ion adsorption. The surface appears more compact and densely aggregated, with pore openings partially blocked by chromium species. Flake-like clusters and layered formations are indicative of chromium complex accumulation within the porous framework. These changes are a direct result of effective Cr(VI) ion interaction with functional groups present in the composite matrix, which leads to surface coverage and partial pore filling. Reduced void spaces as well as compact domain formation serve as evidence for chromium immobilization while keeping structural integrity necessary for adsorption. The surface reconstruction of Cr@MACC composite sponge in 3D grayscale displays the topographical variations due to chromium adsorption. The elevation profile has densely packed protrusions with fewer surface depressions compared to pristine MACC, which indicates that chromium has occupied some pores. The interconnected ridges are an indication that surface-bound chromium complexes have modified the original porous network. Such modifications are a clear representation of effective deposition and immobilization of Cr(VI) ions within this sponge-like structure; hence, creating a more consolidated surface while still maintaining the structural integrity of the matrix for adsorption purposes (Fig. 5(b)). The thermal color-scale 3D topographic view of the Cr@MACC composite sponge in Fig. 5(c) depicts variations in surface elevation after the adsorption of Cr(VI). The gradient between the low- and high-intensity areas indicates differences in surface depth and height due to newly formed aggregated domains. An increased intensity in elevated regions suggests accumulation of chromium species within the porous framework, partially filling cavities and changing the original pore architecture. These changes further reveal that surface-bound chromium complexes have formed within the interconnected network, confirming that effective adsorption and immobilization of Cr(VI) ions has occurred. The Cr@MACC composite sponge in blue-tone three-dimensional elevation profile shown in Fig. 5(d) indicates surface changes after chromium adsorption⁴⁷. The blue–cyan intensity variations indicate surface height and depth differences, thus highlighting the areas of compaction and reduced pore accessibility. Interconnected elevated regions and less pronounced depressions are indicative of partial filling of pore channels by adsorbed chromium species, resulting in a denser but still open framework. These features confirm that Cr(VI) ions have been successfully immobilized within the porous network while maintaining the integrity of interconnected adsorption pathways⁴⁸.

Fig. 5.

(a) SEM of Cr@MACC composite sponge, (b) Grayscale 3D map, (c) Thermal color scale, (d) Blue tones.

SEM mapping

Elemental mapping via SEM-EDS for the MACC composite sponge reveals an even spread of major elements: carbon (C), nitrogen (N), oxygen (O), and iron (Fe) throughout the structure. SEM images display surface porosity that matches known microstructural characteristics of MACC material. Maps show well-distributed carbon (11%) and nitrogen (8%) from biopolymeric coating CMC and β-CD. A large oxygen signal (34%) indicates many oxygen-rich functional groups –OH, –COOH, and C–O–C which are important in binding processes with metal ions. The strong, uniform iron signal (47%) proves that Fe₃O₄ nanoparticles have been successfully integrated into the activated carbon matrix with good interfacial adhesion between them⁴⁹. No agglomerations appear in the Fe map; this means that the nanoparticles are integrated instead of being clustered together, which promotes magnetic homogeneity and stability. These results confirm the synthesis of a magnetic activated carbon composite with a uniform distribution of Fe₃O₄ nanoparticles firmly bound to the CMC/β-CD-coated carbon substrate for enhanced magnetic responsiveness and adsorptive capability toward heavy metals (Fig. 6(a)).

Fig. 6.

SEM mapping of: (a) MACC composite sponge, and (b) Cr@MACC composite sponge.

SEM-EDS mapping of the Cr@MACC composite sponge indicates successful chromium uptake and its uniform distribution over the surface of MACC. The SEM image presents a rough but compact morphology with partially filled pores by chromium ions, proving strong interaction with Cr(VI). Elemental maps show distributions for carbon (C), nitrogen (N), oxygen (O), iron (Fe), and chromium (Cr). The signals from carbon (9%) and nitrogen (4%) represent the CMC/β-CD polymer coating that has retained its structure after adsorption. Oxygen at 28% corresponds to various functional groups such as –OH, –COOH, and C–O–C which are likely involved in Cr(VI) adsorption as well as reduction. The signal for iron at 24% proves even dispersion confirming stability for Fe₃O₄ nanoparticles post chromium uptake. Chromium mapping shows a consistent presence indicating effective surface coverage as well as homogeneous immobilization⁵⁰. The adsorptions might take place due to the electrostatic attraction, ion exchange, complexation and possible reduction of Cr(VI) to Cr(III) at active sites on the magnetic core and polymer coating. The insignificant aggregation here proves that stability is maintained by the composite after adsorption. SEM-EDS results confirm that Cr@MACC composite sponge has uniform deposition of chromium, structural homogeneity and strong retention of properties which implies its competence in remediation of heavy metals from aqueous systems (Fig. 6(b)).

XPS

The high-resolution X-ray photoelectron spectroscopy (XPS) data for the C1s region of MACC and Cr@MACC show major shifts in surface chemistry pre- and post-chromium adsorption. The unmodified MACC has a C1s spectrum with four main peaks: one at 284.76 eV (34.61%) is assigned to C–C/C = C, which corresponds to the graphitic carbon backbone; another peak at 286.23 eV (52.23%) is assigned to C–O/C–OH, associated with hydroxyl, ether, and glycosidic functionalities from CMC and β-CD; an additional peak at 288.79 eV (13.16%) is due to carbonyl/ester C = O -COOH and O–C–O groups that may be involved in complexation with metals⁵¹. The C1s spectrum of Cr@MACC after adsorption of Cr(VI) showed significant energy shifts and intensity changes, which is an indication of the interactions between chromium and surface functional groups (Fig. 7). The peak C − C/C = C shifts to 284.53 eV (23.67%), while that for C − O/C − OH moves to 285.64 eV (50.81%), both shifts being indicative of electron withdrawal. The C = O peak shifted to 288.25 eV with a significant reduction to 25.51%, which reflects the consumption of carboxyl and carbonyl groups during binding with chromium. Furthermore, broadening in the region from 289 to 291 eV can be attributed to formation of new surface complexes Cr − O−C. These spectral changes are clear evidence that Cr(VI) is adsorbed onto MACC through complexation, electrostatic interaction, and partial reduction into Cr(III). This emphasizes effective chemical immobilization of chromium in the composite matrix.

Fig. 7.

XPS of MACC and Cr@MACC composite sponge.

The XPS spectra of the N1s region for both MACC and Cr@MACC (Fig. 7) offers substantial evidence concerning the chemical states of nitrogen in the composite material and their transformation after Cr(VI) adsorption. The spectrum of pristine MACC exhibits two major peaks, one at 400.93 eV (61.34%) which is attributed to nitrogen in environments, while the second peak at 399.54 eV (38.66%) indicates amine-type nitrogen that denotes incorporation of nitrogen in composite. The spectrum shows marked changes in the N1s envelope for Cr@MACC confirming that these functionalities are indeed playing a role in binding chromium as it exposes itself to Cr(VI). Three distinct peaks appear: one high-energy peak at 406.7 eV (46.02%) is attributed to oxidized nitrogen species due to effects caused by coordination with Cr(VI); another smaller secondary peak at 404.0 eV (16.66%) relates to protonated nitrogen species indicating surface oxidation and the third one at 399.51 eV (37.32%) relates to pyridinic/amine nitrogen which though indicates stability is most probably involved via electrostatic interaction with chromium; presence of these new N–O and N⁺ species plus shifts among various states strongly suggests that these nitrogen’s are mainly responsible for immobilization of Cr(VI) through coordination as well as redox transformation thus confirming their importance within adsorption mechanisms related to this MACC composite⁵².

The high-resolution O1s XPS spectra for MACC and Cr@MACC were used to understand better the different oxygen-containing functional groups that take part in the adsorption of chromium. The unmodified MACC composite shows an O1s region with two main peaks: one at 533.18 eV (51.4%) for oxygen in C–O, C–OH, and C–O–C environments from the CMC/β-CD polymeric coating and another at 532.32 eV (48.6%) associated with carbonyl and carboxyl groups in the crosslinked biopolymer matrix (Fig. 7). Changes can be seen in the O1s profile of Cr@MACC after Cr(VI) adsorption; this proves that functional groups with oxygen are involved in binding chromium. The peak at 533.1 eV (30.87%) is lowered, which indicates partial coordination with chromium because it was expected for unmodified MACC composite material; hence it proves that these functional groups are active toward binding metals such as chromium ions rather than other species present in solution during testing⁵³. The major oxygen component shifts slightly to 532.08 eV (51.35%). This shift indicates a change in electron density on carbonyl/carboxyl oxygen due to complexation. A new peak appears at about 531.55 eV with a relative intensity of 17.78%, which can be attributed to the formation of metal-oxygen bonds (Cr-O/Cr-O-C). This observation is an indirect proof of the surface complexation of chromium species on the polymer support matrix material surface through XPS analysis results interpretation. The intensity reduction of C–O/C–OH oxygen, the peak shift, and the emergence of a Cr–O peak underscore that hydroxyl and carboxyl groups are crucial for chromium immobilization through complexation and coordination, demonstrating the MACC composite’s strong affinity for Cr(VI) species⁵².

The high-resolution Fe 2p XPS spectra for MACC and Cr@MACC composites were analyzed to show the chemical states and stability of Fe₃O₄ nanoparticles before and after chromium adsorption. In the spectrum from unmodified MACC, the Fe 2p region clearly exhibits a spin-orbit doublet that is indicative of magnetite (Fig. 7). This has an Fe 2p₃/₂ component at about 711 eV and an Fe 2p₁/₂ component between 724 and 725 eV, suggesting both oxidation states (Fe²⁺ and Fe³⁺) as typically found in Fe₃O₄. The high energy peak around 711 eV relates to octahedral coordination of Fe³⁺–O, while the shoulder at lower energy corresponding to Fe²⁺–O confirms its inverse spinel structure. After chromium (VI) adsorption, Cr@MACC’s Fe 2p spectrum maintains its doublet; this means that no oxidation or structural degradation took place during adsorption proving that the core is chemically stable. Some peak shifts and intensity changes are due to interactions between chromium species and oxygen bonded to iron in Fe₃O₄, alongside electron redistribution at the Fe-O-Cr interface. Both oxidation states being present in Cr@MACC composite imply that the magnetic phase is preserved as confirmed by strong recoverability magnetization studies after adsorption. The very slight broadening of satellite peaks indicates a formation of surface Cr-O-Fe interactions; however, it does not affect the core electronic structure of Fe₃O₄. This finding demonstrates the structural integrity, redox stability, and interfacial compatibility of MACC-embedded Fe₃O₄ nanoparticles toward sustained effectiveness in magnetic separation and adsorption applications after heavy metal uptake⁵⁴.

The Cr 2p high-resolution XPS spectrum of the Cr@MACC composite provides strong evidence for chromium adsorption and its different oxidation states on the surface (Fig. 7). In the Cr 2p₃/₂ region, three main components are noticed: one peak at 579.02 eV (42.35%) is due to Cr(VI) in chromate/dichromate forms; another at 576.36 eV (17.89%) shows that this has Cr(III) species such as Cr₂O₃ or Cr(OH)₃; an intermediate at 586.31 eV (22.62%) indicates partially reduced chromium species. The region for Cr 2p₁/₂ has a peak at 589.23 eV (17.14%), which further confirms the presence of Cr(VI). The fact that both forms are present highlights the complexity of the adsorption process involving physical binding and electrostatic interactions along with substantial redox transformations taking place in it. Some of the less toxic transformations from Cr(VI) to Cr(III) can be seen through interaction with electron-rich functional groups as demonstrated in O1s and N1s spectra. The large intensity of peaks for Cr(VI) indicates very strong surface complexation, while validation for Cr(III) supports that it has been reduced effectively by the MACC matrix. This proves that Cr@MACC immobilizes chromium via mechanisms like electrostatic attraction, ligand exchange, surface complexation, and redox reduction to achieve stable retention of both oxidation states⁵⁵.

XPS survey spectra of MACC and Cr@MACC confirm the adsorption of chromium on the surface of the composite (Fig. 7). The unmodified MACC spectrum shows peaks for C 1s, O 1s, N 1s, and Fe 2p that are consistent with activated carbon, a carboxymethyl cellulose/β-cyclodextrin polymer, and Fe₃O₄ nanoparticles. In Cr@MACC after Cr(VI) adsorption, major elemental signatures are retained with a slight decrease in intensities of C 1s and O 1s peaks because they are partially covered by adsorbed chromium species; however, the signal for Fe 2p remains intact indicating that no degradation has occurred to the Fe₃O₄ core. The appearance of a new signal for Cr 2p in the spectrum of Cr@MACC provides proof for immobilization of chromium⁵⁵. Variations in the intensities of N 1s and O 1s signals are seen as an evidence of strong interaction between Cr(VI)/Cr(III) and the MACC functional groups. This supports a reduction-complexation adsorption mechanism. To summarize, iron integration, high availability of functional groups, and efficient chromium uptake by MACC were all presented in this analysis to emphasize its potential for Cr(VI) removal (Table S6).

Point zero of charge

The point zero of charge (pH_pzc) of the MACC composite sponge was obtained from the plot of ΔpH against initial pH by taking the value where the curve crosses zero⁵⁶. It is found to be at 3.52 and plays a significant role in determining surface charge properties and hence adsorption capacity for Cr(VI). At pH values less than 3.52, functional surface groups in the MACC composite mainly –OH, –COOH, and C–O functional groups coming from CMC/β-CD matrix. After this point, as pH increases above pH_pzc these functional groups start losing protons resulting into generation of many negatively charged sites (–O⁻ and –COO⁻), which will dominate surface interaction⁵⁷. Results presented in this paper show that removal of Cr(VI) is favored at pH values above pH_pzc due to adsorption-reduction synergy rather than simple electrostatic attraction. Although Cr(VI) predominantly exists as negative charge carrying oxyanions (HCrO₄⁻, CrO₄²⁻, Cr₂O₇²⁻), under deprotonated MACC conditions it favors strong inner sphere complexation, ligand exchange and redox reactions where electron-rich functional groups plus Fe²⁺ sites within Fe₃O₄ become electron donors leading to partial reduction of Cr(VI) into Cr(III). The produced Cr(III) will then be immobilized by complexing with oxygenated groups or precipitating as Cr(OH)₃ within the pores matrix supported by shifts observed in FT-IR spectra, XPS deconvolution, decreased BET pore volume and SEM–EDS mapping of chromium⁵⁸. Hence, the negative surface charge observed above the pH_pzc greatly intensifies the chemical interaction and redox-driven attraction between the adsorbent and chromium species, establishing pH levels above 3.52 as optimal for enhancing Cr(VI) uptake by the MACC composite sponge, as shown in Fig. S2(a).

Batch experiments

Effect of pH

The pH effect on Cr(VI) adsorption by the MACC composite sponge was closely related to the surface charge properties of the adsorbent and chromium speciation in solution. Adsorption capacity gradually increased from 264 mg/g at pH 2 to a maximum of 452 mg/g at pH 4 (see Fig. S2(b)). This best performance at pH 4 is very near the pH_pzc value of 3.52 determined from the ΔpH vs. initial pH plot⁵⁸. This shows that the MACC surface changes from a positive charge to a negative one near this pH. Below the pH_pzc, the surface has protonated functional groups which attract Cr(VI) oxyanions (HCrO₄⁻, Cr₂O₇²⁻). But too much protonation at very low pH (2 to 3) limits access to active complexation sites and blocks redox interactions so that adsorption is lower. When the pH goes just over the pH_pzc, especially around pH 4, the surface starts to lose protons and shows electron-rich –O⁻ and –COO⁻ groups. This situation also improves electrostatic attraction, ligand exchange, inner-sphere complexation, and the reduction of Cr(VI) into Cr(III). The good balance between protonation and deprotonation at pH 4 creates a suitable chemical condition for strong binding on surfaces and redox immobilization of chromium species which explains why there is such an increase in adsorption capacity observed⁵⁹. On the other hand, at high pH values (> 5), the decrease in q_e from 426 to 286 mg/g happens because there is less attraction due to charges plus more competition from OH⁻ ions making it harder for chromate to get onto the surface. So, maximum adsorption seen at pH 4 indicates how surface charge dynamics, functional-group reactivity, and Cr(VI) speciation all interact intricately under control by the pH_pzc of the MACC composite sponge.

Effect of dose

The relationship between MACC sponge dosage and Cr(VI) adsorption shows an inverse correlation between adsorption capacity (q_e) and adsorbent quantity, while removal efficiency consistently increases within the tested dosage range. An increase from 0.02 to 0.50 g adsorbent raised removal efficiency from 56.69 to 97.60%⁵⁸. Higher MACC dosage means more active sites, better surface area exposure, and more functional groups to interact with Cr(VI). The q_e values decreased from 266.25 to 16.25 mg/g with increasing dosage; this happens because at a fixed Cr(VI) concentration, the mass of adsorbent has increased so that now there are fewer Cr(VI) ions adsorbed per gram and maybe even some overlapping or aggregation of active sites at higher dosages. Lower doses (0.02 to 0.10 g) have higher q_e values which indicate quickly saturated available sites due to strong affinity between MACC and Cr(VI) through means such as electrostatic attraction and ligand exchange. After this point, increasing dosages lead to an increase in available active sites beyond the availability of Cr(VI), thus causing a decline in q_e even though near-complete removal efficiency is maintained at doses of 0.40 g or more. It would be fair to conclude from the results that lower dosages increase adsorption capacity while higher dosages increase removal effectiveness for practical applications depending on what is wanted out there in the wastewater treatment scenario (Fig. S2(c)).

Effect of Cr(VI) concentration

The effect of initial Cr(VI) concentration on the adsorption performance of MACC composite sponge was studied, results showed that with the increase of Cr(VI) concentration, adsorption capacity increased and removal efficiency decreased after a certain extent. The initial Cr(VI) concentration increasing from 40 to 620 mg/L changed the adsorption capacity from 50.82 to 528.75 mg/g⁶⁰. This rise shows that at higher concentrations, a greater mass-transfer driving force results in more rapid diffusion of chromate ions to the active sites on the adsorbent. At lower concentrations (40 to 150 mg/L), removal efficiency stays high (96–98%) because there are enough functional groups (–OH, –COOH, C–O–C) and Fe₃O₄ sites on MACC to capture almost all Cr(VI). Beyond 250–300 mg/L, as removal efficiency falls from 96 to 68%, it indicates that adsorption sites have become saturated and that the composite has a limited capability for removing Cr(VI) under conditions of high ionic load⁶¹. The further increase in adsorption capacity at even higher concentrations (400–620 mg/L) proves the good utilization of the porous structure together with multilayer interactions such as electrostatic attraction and ligand exchange plus partial reduction of Cr(VI) to Cr(III). It is an example where with increasing occupation of active sites, competition for adsorption increases and thus removal efficiency decreases even though q_e values are increasing; this proves that while increasing concentration of Cr(VI) enhances adsorption capacity too much concentration will exceed available binding sites hence degrading removal efficiency illustrating the delicate balance between mass-transfer forces and site saturation in MACC composite sponge (Fig. S2(d)).

Effect of contact time

The Cr(VI) adsorption kinetics on the MACC composite sponge showed fast uptake at first then slowly reach equilibrium as seen in Fig. S2(e). In the initial 20 to 30 min, it rose steeply to about 60–65% of its max capacity. This fast uptake is due to many active sites being available on the outer surface of the MACC composite, which has hydroxyl, carboxyl, and ether functional groups plus surface oxygen linked with Fe₃O₄. The strong difference in concentration between solution and adsorbent allows quick movement and strong interactions like electrostatic attractions, ligand exchanges, and early complexation⁶². The adsorption kinetics of Cr(VI) on the MACC composite sponge is presented in Fig. S2(e). The uptake was fast at the beginning and then gradually reached equilibrium. It increased sharply in the first 20 to 30 min to about 60–65% of its maximum capacity. This fast uptake can be attributed to a large number of active sites available on the outer surface of the MACC composite, which contains hydroxyl, carboxyl, and ether functional groups as well as surface oxygen linked with Fe₃O₄. There is a strong driving force due to a high concentration difference between the solution and adsorbent that enhances rapid transport and intensifies interactions such as electrostatic attractions, ligand exchanges, and initial complexation.

Effect of temperature

The temperature effect on the MACC composite sponge’s Cr(VI) removal efficiency shows that adsorption improves significantly at higher temperatures. This indicates that the adsorption process is endothermic. As seen in Fig. S2(f), raising the temperature from 20 to 45 °C results in a large increase in removal efficiency from 48.4 to 98.1%. This observation highlights very strong thermally activated features of interaction between Cr(VI) species and functional groups in the composite material. At lower temperatures (20–25 °C), limited molecular movement reduces the diffusion rate of Cr(VI) ions toward active sites⁶³. The energy needed to overcome activation barriers associated with surface complexation and redox reactions is not enough, leading to low removal efficiencies. When temperature increases, the kinetic energy of chromate ions helps in boundary-layer diffusion, improves intraparticle transport, and allows better interaction with electron-donating groups like –OH, and –COOH. This thermal activation helps not only in binding through electrostatics and ligand exchange but also increases the reduction of Cr(VI) into Cr(III). This conversion becomes energetically favorable at higher temperatures. Higher temperatures can also improve pore accessibility by expanding the structure of the polymeric network so that internal adsorption sites become more accessible. The fact that there is an increase in removal efficiency up to 45 °C supports the idea that the adsorption of Cr(VI) onto MACC composite sponge happens with endothermicity, diffusion enhancement, and strong support from thermally driven redox and complexation processes.

Adsorption isotherm

The utilization of various adsorption isotherm models, namely Langmuir⁶⁴, Freundlich⁶⁵, Dubinin–Radushkevich (D–R)⁶⁶, Temkin⁶⁷, Jossens⁶⁸, and Toth, enables an in-depth analysis of the adsorption properties of Cr(VI) on the MACC composite sponge. It effectively captures the details of surface roughness, interaction energies, and site availability for adsorption. The Langmuir model is very useful since it describes the capacity for monolayer adsorption and evaluates whether adsorption can take place on a uniform surface with few binding sites; this is an important factor in determining how much Cr(VI) can be maximally adsorbed by the MACC composite. On the other hand, the Freundlich model accepts heterogeneity and multilayer adsorption complexities, thus allowing an assessment of surface irregularities, different functional groups present, and different strengths of adsorptions related to the CMC/β-CD-coated activated carbon matrix. The D–R isotherm gives insight into what kind of mechanism underlies the adsorption by calculating mean free adsorption energy (E), which helps identify if it is physical adsorption, chemisorption, or ion-exchange processes that are very important for knowing more about partial reduction from Cr(VI) to Cr(III). The Temkin model, which considers the interactions between adsorbent and adsorbate, helps identify the nature of heat distribution and whether the adsorption process becomes more favorable with an increase in temperature. This agrees with the endothermic nature of Cr(VI) binding observed in MACC. The Jossen isotherm improves fitting for systems that show sigmoidal or cooperative adsorption behavior, thus helping to understand possible adsorbate–adsorbate interactions that could take place at high concentrations of Cr(VI). Also, the Toth isotherm overcomes some limitations of both Langmuir and Freundlich models by providing great flexibility for heterogeneous surfaces without assuming a homogeneous energy distribution. Such feature makes it very suitable for the multifunctional and mesoporous structure of the MACC composite sponge as shown in Tables S7 and S8. In conclusion, the full use of this set of isotherm models ensures a detailed evaluation of Cr(VI) adsorption effectiveness, explains the main removal processes involved, and improves how well predictions apply to real-world wastewater treatment situations, as shown in Table S9.

The adsorption of Cr(VI) on MACC composite sponge fits well with the Langmuir isotherm model because the experimental value of maximum adsorption capacity (qₘ, exp = 528.75 mg/g) is very close to the value predicted by Langmuir for monolayer coverage (qₘ = 530.62 mg/g)⁶⁴. This high concordance suggests that Cr(VI) adsorption follows a Langmuir isotherm, indicating monolayer coverage on a surface with homogeneous and well-defined energetic active sites. The Langmuir constant K_L = 0.076 L/mg represents the strong affinity of Cr(VI) oxyanions to the functional groups in MACC composite, implying very strong binding through mechanisms such as electrostatic attraction, ligand exchange, and inner-sphere complexation. The separation factor R_L = 0.057 is much less than one, meaning it lies between 0 < R_L < 1 which is an indication of a very favorable adsorption process; hence, the low R_L value implies that MACC composite still has considerable Cr(VI) adsorption capacity at high concentrations due to its large surface area and abundant oxygen-containing functional groups together with reduction ability from Fe₃O₄. Therefore, the Langmuir fitting results confirm that the MACC composite sponge has excellent monolayer adsorption property for Cr(VI), controlled by strong and selective interaction between chromate species and tailored functional domains of the composite as illustrated in Fig. S3(a). the adsorption of Cr(VI) onto MACC composite sponge was liner fitted to Langmuir as represented at Fig. S3(b).

The Freundlich isotherm model has shown how Cr(VI) adsorption on the MACC composite sponge takes place by explaining the surface heterogeneity and multilayer adsorption tendency of this material. The Freundlich exponent n equals 3.46, which means that a very favorable adsorption process is occurring with a strong affinity between Cr(VI) oxyanions and heterogeneous functional groups on the MACC surface⁶⁵. This number means that the mix has many different binding spots with different energy levels, which matches well with the added effects of activated carbon microporosity, CMC/β-CD hydroxyl and carboxyl groups, and the surface oxygen functionalities of Fe₃O₄. Also, the Freundlich constant (K_F = 127.65 mg/g (L/mg)¹⁄ⁿ) supports the great adsorption ability of the MACC composite. This shows clear surface interactions like electrostatic attraction, hydrogen bonding, ligand exchange, and surface complexation. The strong fit of the Freundlich model proves that Cr(VI) uptake is not limited to perfect single-layer formation; instead, it probably includes multi-layer adsorption and slow pore filling which goes along with observed increases in adsorption ability at higher amounts of pollutants. In short, these Freundlich parameters back up the claim that this MACC composite sponge has a highly uneven and energetically good surface that can easily grab Cr(VI) ions from very high to very low amounts, as shown in Fig. S3(b).

The Dubinin–Radushkevich isotherm provides important information about the mechanism behind the adsorption of Cr(VI) on the MACC composite sponge. It explains the energy features and type of sorption process⁶⁶. The D–R model gave a theoretical maximum adsorption capacity (Qᴅʀ = 471.27 mg/g) that matched both the high experimental results and the capacities derived from Langmuir, confirming that MACC composite has an efficiently microporous-mesoporous structure allowing large uptake of Cr(VI). Using Kᴅʀ (D–R constant = 1.103 × 10⁻⁵ mol².kJ⁻²), the mean free energy of adsorption was calculated as Eₐ = 29.42 kJ/mol which is much greater than the value of 8 kJ/mol that differentiates physisorption from chemisorption. This energy value clearly indicates that the process for removing Cr(VI) by MACC is mainly dominated by chemisorption processes involving stronger interactions such as ligand exchange, inner sphere complexation, and redox-driven binding mechanisms. The specific Eₐ value here supports the role of electron donating surface functional groups (–OH, –COOH, C–O–C) and Fe²⁺ sites in Fe₃O₄ for partial reduction of Cr(VI) into Cr(III), which matches findings from FT-IR and XPS analyses. Therefore, according to D-R model the adsorption mechanism can be described as being controlled chemically with high energy intensity rather than purely physical or diffusional to further verify very strong affinity and good structural compatibility of MACC composite sponge towards remediation of Cr(VI).

The Temkin isotherm model describes better the adsorption of Cr(VI) on MACC composite sponge by taking into account the adsorbate-adsorbent interactions and the variation of adsorption energy with surface coverage. The Temkin binding constant (Kₜ = 1.18 L/mol) indicates a strong affinity between Cr(VI) oxyanions and functional groups of MACC composite, thus confirming that significant interactions like electrostatic attraction, hydrogen bonding, and ligand exchange exist. The heat of adsorption parameter (bₜ = 24.068 J/mol) suggests that moderate positive adsorption energy means that the process is endothermic and becomes more favorable with increasing temperature, which is what was experimentally observed in terms of removal efficiency at higher thermal conditions⁶⁷. This parameter also means that the energy required for Cr(VI) binding decreases gradually with increasing surface coverage, confirming that the MACC surface has a heterogeneous distribution of active sites that interact with Cr(VI) in a non-uniform manner. Together, the parameters obtained from the Temkin isotherm support the idea that Cr(VI) adsorption onto the MACC composite sponge is controlled by a thermally activated chemisorption process influenced by surface interactions, thus complementing results from Langmuir, Freundlich, and D–R isotherm models.

The Jossens isotherm is a model that explains the distribution of adsorption sites and interactions affecting Cr(VI) uptake by the MACC composite sponge. The model constant (K = 45.17 L/mg) reflects a strong interaction between Cr(VI) oxyanions and active sites on the adsorbent, which means there is considerable affinity of the MACC surface for chromate ions due to electrostatic attraction, ligand exchange, and redox-driven complexation. The exponent (n = 0.09) well below one indicates the system’s energetic heterogeneity leading to cooperative uptake behavior at higher concentrations. A low n value implies that even with increasing Cr(VI) levels, binding strength remains strong as more sites become available due to structural changes or surface complexation. This indicates that Cr(VI) adsorption on MACC does not obey ideal monolayer behavior but rather involves strong site saturation at lower concentrations combined with cooperative enhancement at higher ones. Therefore, Jossens isotherm is an appropriate model for describing complex characteristics of Cr(VI) elimination by MACC sponge while enriching insight from other models such as Langmuir, Freundlich, and Dubinin-Radushkevich⁶⁸.

The Toth isotherm model was applied as an appropriate model to describe Cr(VI) adsorption on MACC composite sponge, which is a hybrid of Langmuir type monolayer and Freundlich surface heterogeneity. The maximum adsorption capacity obtained from the Toth model (qₘ = 578.13 mg/g) is greater than that experimentally determined, indicating that MACC composite possesses abundant active sites for Cr(VI) binding over a wide concentration range. The Toth affinity constant (Kₜ = 9.705 L/mg), a relatively high value, reveals strong binding interactions between Cr(VI) oxyanions and various functional groups existing in the composite such as –OH, –COOH, and C–O–C. Also, the Toth heterogeneity parameter t = 0.912 is close to unity; this means that although some energetic heterogeneity exists on MACC surface it behaves mostly like a quasi-homogeneous surface with well-defined active sites. This finding agrees with contributions from structural features of activated carbon micropores, crosslinked CMC/β-CD matrix and incorporation of Fe₃O₄ nanoparticles which support uniform adsorption and reduction processes. The strong fit of the Toth model to the experimental data confirmed that Cr(VI) adsorption involves monolayer chemisorption followed by sequential pore filling and strong surface complexation; thus underscoring high efficacy as well as flexibility of MACC composite sponge toward remediation of chromium-laden wastewater⁶⁸.

The adsorption isotherm parameters discussed in Table S9 the enhanced adsorption performance of the MACC composite sponge towards Cr(VI) ions compared to pristine MACC. As per the Langmuir model, maximum adsorption capacity (qₘ, exp) experimentally increased significantly from 473.47 mg.g⁻¹ for MACC to 528.75 mg.g⁻¹ for the MACC composite sponge with a notable increase in Langmuir affinity constant (Kₗ) from 0.0239 to 0.076 L.mg⁻¹ confirming stronger adsorbent–adsorbate interactions after polymer encapsulation. The Freundlich parameters further support this improvement where adsorption intensity (n) increased from 2.92 to 3.46 and Freundlich constant (K_F) increased from 80.77 to 127.65 mg.g⁻¹ (L.mg⁻¹)¹⁄ⁿ indicating a more heterogeneous surface with energetically favorable multilayer adsorption sites in the composite sponge. Also, Dubinin­Radushkevich mean adsorption energy Eₐ increased from 25.4 to 29.42 kJ.mol⁻¹ suggesting that Cr(VI) removal onto the MACC composite sponge is mainly controlled by chemisorption mechanisms such as surface complexation and redox-driven reduction rather than weak physical adsorption; this enhancement can be seen in the Temkin binding constant which increased significantly from 0.298 to 1.18 L.mol⁻¹ reflecting stronger electrostatic attraction and chemical affinity between Cr(VI) oxyanions and functional groups of the composite matrix. Likewise, the Jossens constant K increased from 18.52 to 45.17 confirming improved cooperative adsorption behavior within sponge-like porous frameworks while Toth heterogeneity parameter t increased from 0.72 to 0.912 indicating a broader distribution of active binding sites. These findings confirm that incorporation of CMC/β-CD biopolymer network and Fe₃O₄ nanoparticles into MACC structure improves surface functionality, adsorption energy, pore accessibility thereby enhancing adsorption capacity, binding strength as well as overall removal efficiency of Cr(VI) ions making MACC composite sponge a more effective sustainable adsorbent for wastewater treatment applications (Fig. 8(a)). Adsorption isotherm of Cr(VI) onto MACC was fitted to Langmuir (Fig. 8(b)).

Fig. 8.

(a) Adsorption isotherm models, (b) Langmuir linear fit, (c) Adsorption kinetic models, (d) Pseudo-second-order linear fit for adsorption of Cr(VI) and MACC.

Adsorption kinetics

The utilization of various adsorption kinetic models including the Pseudo-First-Order⁶⁹, Pseudo-Second-Order⁷⁰, Intraparticle Diffusion⁷¹, and Elovich models provides an in-depth understanding of the mechanistic processes involved in the removal of Cr(VI) using the MACC composite sponge⁷². The Pseudo-First-Order model describes the influence of initial physical interactions and surface diffusion, thus serving to assess the kinetics of how fast Cr(VI) molecules approach free external active sites. The Pseudo-Second-Order model, however, is known as one of the most reliable models for chemisorption systems because it investigates electron-sharing or electron-exchange interactions between Cr(VI) species and functional groups on MACC composite like hydroxyl, and carboxyl; this elucidates whether the rate-limiting step is controlled by chemical bonding or redox reaction. The Intraparticle Diffusion model helps evaluate how much pore diffusion and internal mass transfer contribute to the total kinetic rate a key factor for a sponge-like mesoporous adsorbent wherein Cr(VI) species move into deeper pore channels after fast surface adsorption (refer to Tables S7 and S8). The Elovich model is best suited for describing chemisorption on very heterogeneous surfaces making it highly relevant to the multilayered and functionally diverse MACC composite where adsorption rates decline over time due to increasing site saturation and changes in surface energy. In totality, these kinetic models allow for a multilevel approach toward understanding the adsorption mechanism that talks about physical versus chemical control, surface versus intraparticle diffusion, homogeneous versus heterogeneous site behavior—bringing one closer to an accurate as well as mechanistically detailed picture of how Cr(VI) gets trapped and immobilized by the MACC composite sponge (Table S10).

The Pseudo-First-Order kinetic model is applied to evaluate the early adsorption kinetics of Cr(VI) on the composite sponge MACC⁶⁹. The resulting rate constant (K₁ = 0.28977 × 10⁻² min⁻¹) suggests a fast uptake in the beginning stages of the adsorption process. This moderate value of the PFO rate means that physical interactions like boundary-layer diffusion, surface migration, and early electrostatic attraction between protonated functional groups and chromate ions are important factors influencing the process during the first few minutes. This aligns with what was observed regarding the quick increase in q_t from experimental data. It should be noted, however, that the PFO model usually underestimates equilibrium adsorption capacity when chemical bonding or redox processes are dominant. This indicates that although physical adsorption is very important at first, it does not represent the rate-limiting step for Cr(VI) removal through MACC composite. The behavior described by the PFO model rather highlights how accessible Cr(VI) molecules are to plentiful external adsorption sites before they move into deeper mesoporous areas or participate in chemisorption. Thus, this K₁ value confirms that diffusion facilitates and surface interactions control the first stage of adsorption while an overall mechanism would be better related to chemisorption-based models like Pseudo-Second-Order model.

The Pseudo-Second-Order (PSO) kinetic model showed an excellent fit with the experimental data for Cr(VI) adsorption onto the MACC composite sponge, indicating that the process is mainly controlled by chemisorption and valence electron interaction between adsorbent surface and Cr(VI) species. The equilibrium adsorption capacity obtained from the PSO model (qₑ = 532.4 mg/g) matches well with the experimental one, confirming the applicability of this model as well as its high predictive ability⁷⁰. The PSO rate constant (K₂ = 3.59 × 10⁻⁵ g.mg⁻¹min⁻¹) represents a slower yet more controlled phase of chemical interaction that follows an initial rapid physical diffusion stage. The relatively low value of K₂ indicates that the rate-limiting step involves mechanisms such as electron sharing, ligand exchange, and partial reduction of Cr(VI) to Cr(III), especially through interaction between chromate ions with hydroxyl, carboxyl, ether, and Fe²⁺/Fe³⁺ functional sites in the MACC composite. The good fit of the PSO model indicates that the adsorption process is mainly controlled by chemical affinity rather than mass transfer limitations which supports multi-step mechanisms in Cr(VI) removal involving surface complexation, pore diffusion, and redox immobilization. In summary, parameters from the PSO kinetic analysis further verify that adsorption of Cr(VI) onto MACC composite sponge is mainly controlled by chemisorption giving better description of kinetics than Pseudo-First-Order model as shown in Fig. S3(c). Fitting Cr(VI) adsorption to PSO was done on linear model as presented at Fig. S3(d).

The Intraparticle Diffusion model was used to evaluate the effect of internal mass-transfer resistance on the overall rate of adsorption of Cr(VI) onto the MACC composite sponge. The intraparticle diffusion constant (K_i = 56.94 mg.g⁻¹min⁻^1/2) indicates a high initial diffusion rate, showing that Cr(VI) oxyanions rapidly move from bulk solution into macroporous and mesoporous structures within the composite during early stages of adsorption⁷¹. This high K_i value is consistent with the sponge-like structure, rough surface, and interconnected pore structure seen in SEM images that favor fast external and internal diffusion processes. The non-zero boundary-layer thickness parameter (X = 32.64 mg/g) confirms that intraparticle diffusion is not the only rate-controlling step. It shows a multi-stage kinetic mechanism involving: (i) fast surface adsorption by electrostatic attraction and ligand exchange; (ii) slow intraparticle diffusion into deeper pores; and (iii) chemisorption and partial reduction of Cr(VI) at internal active sites. In addition, the presence of a boundary-layer effect means that surface resistance plays a role in the kinetics before intraparticle diffusion becomes dominant. In other words, parameters from the IPD model describe how Cr(VI) adsorbs onto the MACC composite sponge through a complex combination of external mass transfer and pore diffusion followed by strong chemical interactions that finally control adsorption equilibrium (Fig. S3(c)).

The Elovich kinetic model describes the heterogeneous chemisorption kinetics of Cr(VI) adsorption onto MACC composite sponge. The value of β was found to be 170.17 g/mg, which has an inverse relation with surface coverage and activation energy. This means that in the adsorption process, active sites become less available gradually due to surface heterogeneity and an increase in the energy barrier for adsorption. It represents a typical phenomenon that matches the MACC composite’s complex structural properties where functional groups like –OH, –COOH, and C–O–C along with Fe²⁺/Fe³⁺ sites participate through multistep binding as well as partial reduction mechanisms. The initial rate of adsorption α = 0.00141 mg.g⁻¹min⁻¹ is relatively low, which means that this composite adsorbs Cr(VI) mainly by electrostatic attraction and surface diffusion; however, further steps such as chemisorption and redox reaction are slower due to high energetic diversity plus low surface site accessibility⁷². This observation agrees with the experimental kinetic profile that shows an immediate uptake followed by a slower phase controlled by diffusion and reaction kinetics. The Elovich model is applicable, which means that the adsorption of Cr(VI) on the MACC composite mainly takes place via heterogeneous chemisorption affected by different activation energy values. This is especially true for site saturation, changes in structure, and immobilization of chromium through redox reactions, as shown in Fig. S3(d).

The kinetic parameters of adsorption for Cr(VI) on MACC and the pristine MACC composite sponge, discussed in the Table S10, indicate that after composite formation there is a large increase in the rate of adsorption and interaction mechanism. The pseudo-first-order rate constant (K₁) changed from 0.167 × 10⁻² min⁻¹ for MACC to 0.28977 × 10⁻² min⁻¹ for the MACC composite sponge which means that now it is faster due to better surface accessibility and more functional groups inside CMC/β-CD matrix. More importantly, with the pseudo-second-order model showing an equilibrium adsorption capacity (qₑ) increased significantly from 471.6 mg.g⁻¹ for MACC up to 532.4 mg.g⁻¹ for this new composite sponge confirmed that chemisorption is indeed proving dominance as the rate-controlling mechanism since such a structure would provide more active binding sites towards Cr(VI) immobilization. The small drop in the K₂ value for the composite sponge can be explained by stronger chemical interactions that make it take longer before reaching stable surface complexation. Also, the intraparticle diffusion constant (K_i) went up from 52.82 to 56.94 mg.g⁻¹min⁻¹/², with a rise in boundary layer thickness parameter (X) going from 28.96 to 32.64 mg.g⁻¹; this means better internal pore diffusion and mass transfer in the porous network of sponge-like material. Elovich kinetic parameters also showed a major increase in β constant from 0.022 to 170.17 g.mg⁻¹ which means adsorbent-adsorbate interactions are stronger and surface heterogeneity is high for MACC composite compared to pristine MACC. All these kinetic results together suggest that adding Fe₃O₄ nanoparticles and crosslinked CMC/β-CD biopolymer network not only speeds up adsorption but also enhances diffusion pathways and chemisorptive interactions to improve overall efficiency and stability of MACC composite sponge for removing Cr(VI) from water systems (Fig. 8(c)). Adsorption kinetic of Cr(VI) onto MACC was fitted to Pseudo-second-order (Fig. 8(d)).

Diffusion mechanism

The intraparticle diffusion study of Cr(VI) adsorption onto the MACC composite sponge, as shown in Fig. S3(e), reveals a clear three-stage diffusion mechanism. This means that the whole adsorption process is affected by both mass transfer and reaction events, not just one single rate-limiting step. In the first part, indicated by a steep linear segment, external film diffusion is mainly seen, where Cr(VI) oxyanions quickly move from the bulk solution to the surface of the MACC sponge. This fast uptake happens due to a large concentration gradient and an immediate presence of many active functional groups like –OH, –COOH, and Fe²⁺/Fe³⁺ sites on the surface. The next part characterized by an intermediate linear section of the curve shows intraparticle diffusion; during this phase, Cr(VI) species slowly penetrate into mesoporous channels within complex carbon architectures and polymer-coated magnetic areas of composites as described in Fig S3(e). This stage is slower since there is more resistance to diffusion and steric hindrance inside the pores but still important because it has an interconnected porous network that allows deeper transport. In stage III which has a plateau-like region diffusion sharply declines when adsorbent approaches its saturation point Internal site occupation shifts adsorption process toward an equilibrium controlled by chemisorption involving mechanisms such as ligand exchange formation of inner sphere complexes partial reduction from Cr(VI) to Cr(III)⁷¹. The observation of a non-linear relationship and non-zero intercepts indicates that intraparticle diffusion does not solely dictate the rate; rather, the process entails a synergistic sequence that includes film diffusion, pore diffusion, and surface reaction, thereby highlighting the intricate multi-mechanistic nature of Cr(VI) removal facilitated by the MACC composite sponge, as illustrated in Fig. S3(f).

Adsorption thermodynamics

The adsorption of Cr(VI) onto the MACC composite sponge was studied in detail with respect to its thermodynamic properties using the Van’t Hoff and Arrhenius models. This provided an in-depth perspective on the energetic dynamics and inherent spontaneity associated with the adsorption phenomenon⁷³. The Van’t Hoff plot of ln Kc vs. 1/T shown in Fig. S4(a) yielded a positive enthalpy change (ΔH° = 85.62 kJ/mol) and a significantly positive entropy value (ΔS° = 298.32 J/mol.K). This indicated that the adsorption of Cr(VI) onto MACC is mainly endothermic, becoming more favorable at higher temperatures. The high value for ΔH° suggests that the adsorption mechanism is mainly due to chemisorption involving strong interactions as in ligand exchange, inner-sphere complex formation, and partial reduction of Cr(VI) to Cr(III). The positive value for ΔS° indicates an increase in disorder at the solid–solution interface which could be due to structural rearrangement of water molecules, enhanced pore accessibility, and release of hydration shells from chromate ions when they interact with functional groups of MACC. Furthermore, the negative values of Gibbs free energy (ΔG° from − 1.78 to − 9.23 kJ/mol) imply that this process is spontaneous with increasing spontaneity at higher temperatures confirming the endothermic character of this system⁷⁴. The Arrhenius plot of ln K₂ vs. 1/T (Fig. S4(b)) gave an activation energy of 46 kJ/mol, which clearly places the adsorption in the region of chemical adsorption since E_a is more than 40 kJ/mol. This activation energy supports the fact that the rate-determining step involves strong covalent-like interactions and electron transfer between Cr(VI) and oxygen-rich functional groups as well as Fe²⁺/Fe³⁺ sites in the MACC composite material. The analyses conducted using both Van’t Hoff and Arrhenius methods clearly show that Cr(VI) adsorption on MACC is endothermic, spontaneous, thermally favorable, and chemisorption controlled by energetic and structural factors enhanced at higher temperatures (Table S11).

Mechanism of interaction

The adsorption of Cr(VI) onto the MACC composite sponge presents a complex and synergistic multi-step mechanism, involving functional groups from the CMC/β-CD coating, magnetic Fe₃O₄ nanoparticles, and the porous activated carbon matrix. Under acidic medium conditions, MACC surface protonation develops a large positive charge that electrostatically attracts Cr(VI) oxyanions predominantly existing as HCrO₄⁻ and Cr₂O₇²⁻ for immediate surface binding⁷⁵. An ion-exchange mechanism follows this, where Cr(VI) displaces surface hydroxyl groups and oxygenated biopolymer sites, thus forming C–O–Cr linkages; these were detected in FT-IR and XPS spectra by changes. Strong inner-sphere complexation takes place next through the coordination of Cr(VI) with carbonyl, carboxyl, and hydroxyl groups in the polymeric coating. Simultaneously, conjugated carbon structures as well as hydroxyl/carboxyl groups partially reduce Cr(VI) to Cr(III); this is proved by seeing both Cr(VI) and Cr(III) in an XPS Cr 2p spectrum. The process happens when newly formed Cr(III) gets immobilized either by precipitation as Cr(OH)₃ or by forming stable complexes with –OH or –COO⁻ groups. BET shows that pore volume and surface area decreased which means intraparticle diffusion and pore filling took place while SEM/EDX mapping indicated even distribution of chromium on the composite surface. In summary, processes such as electrostatic attraction, ion exchange, inner-sphere complexation between MACC composite sponge material and chromium species in solution redox reactions between species happening inside the solution as well as outside it over specific functional groups plus pore entrapment help make possible an efficient yet stable removal of Cr(VI).

Effect of interfering ions

Based on the ion selectivity results presented in the figure, the adsorption performance of the MACC composite sponge toward Cr(VI) ions was significantly affected by the presence of competing background ions that are commonly found in real contaminated water matrices. In the absence of interfering ions, the adsorption capacity was 488.4 mg.g⁻¹; however, with monovalent cations such as Na⁺ and K⁺ introduced, it slightly reduced to 452.7 and 478.3 mg.g⁻¹, respectively indicating their relatively weak competition for active adsorption sites (Fig. S5). On divalent cations having a more pronounced inhibitory effect where Ca²⁺ and Mg²⁺ reduced adsorption capacities down to 427.7 and 386.8 mg.g⁻¹ respectively due to stronger electrostatic interactions and competition with Cr(VI) for negatively charged functional groups on the surface of the hydrogel Similarity in presence anions that included NO₃⁻, Cl⁻, SO₄²⁻, and PO₄³⁻ which resulted in varying degrees of reductions in Cr(VI) uptake, i.e., 441.74, 472, 447.8 and 411.22 mg.g⁻¹, respectively can be attributed to ionic atmospheres or metal–anion complexes hindering diffusion and accessibility of Cr(VI) ions toward adsorption sites However relatively high adsorption capacities retained in all cases confirm strong selectivity and practical applicability of MACC composite sponge for efficient Cr(VI) removal from multi-ionic wastewater systems⁷⁶. The small decrease in how well Cr(VI) is removed with competing ions shows that the MACC composite has a stronger attraction to Cr(VI) species. This proves that it can be used effectively in treating complex wastewater systems that contain many dissolved metal ions.

Reusability

In terms of large-scale applications, the ability to recycle the adsorbent after recovering the pollutant would be a significant advantage. To see how many times the MACC composite sponge could be reused, a continuous adsorption/desorption cycle experiment was performed using a 0.2 M NaOH solution as eluent⁷⁷. As shown in Fig. S6, it was found that the adsorption capacity of MACC composite sponge performed better during first three cycles of the experiment with no noticeable decrease in adsorption capacity observed, which means that there is an indication that it can be reused several times without degrading its performance. On the contrary, during the last three cycles, the efficiency of Cr(VI) adsorption by adsorbent decreased probably due to incomplete desorption of Cr(VI) from MACC composite sponge material. Also, conversion of Cr(VI) into Cr(III) might have prevented desorption chromium from desorption solution. Stability of membrane was checked by studying XRD patterns shown in Fig. S7 after regeneration process; results confirm that structural integrity of adsorbent held even after six cycles use⁷⁸. The XRD pattern of the regenerated MACC composite showed no meaningful structural change after several adsorption–desorption cycles. This confirmed that both the amorphous carbon structure and the Fe₃O₄ phase within it were kept intact, suggesting strong resistance to leaching of nanoparticles during reuse. The magnetic carbon-based nanocomposite material demonstrates excellent stability and reusability in repeated adsorption–desorption cycles, along with easy magnetic separation. This property suggests that the MACC composite can be reused effectively without a significant loss in performance, thus lowering material consumption and operational costs in real-world applications for treating wastewater⁷⁹.

Comparison with other adsorbents

Table S12 shows a detailed and qualified study of the performance properties of the synthesized adsorbent concerning its efficiency in adsorbing different substances intended for the elimination of Cr(VI), as documented in the relevant academic literature. The results indicate significant differences in the maximum adsorption capacities of the materials assessed. Results indicate that the maximum adsorption capabilities of materials varied significantly. Among these, it is worth noting that the synthesized adsorbent had an exceptionally high experimental maximum capacity with q_max=533.8 mg/g which makes this material potentially useful and effective as an interim solution for removing Cr(VI)^80,81.

Statistical analysis

ANOVA

The results of ANOVA for the adsorption of Cr(VI) onto the MACC composite sponge show very clearly that the fitted quadratic model is statistically significant (F = 52.28, p < 0.0001). This means that all selected variables together have a strong effect on the adsorption response⁸². Among the linear terms, contact time (B) was most significant (p < 0.0001), indicating that it has a dominant role in controlling Cr(VI) uptake. The adsorbent dose (C, p = 0.0008) and pH (A, p = 0.0267) followed in significance order. The significant BC interaction (p = 0.0128) reveals that the effect of dose is strongly dependent on the contact time, while other interactions AB and AC were insignificant. Curvature in the response was confirmed through significant quadratic terms for pH A² at p = 0.0010 and time B² with p < 0.0001, whereas C² remained insignificant, indicating primarily linear behavior of dose within the studied range⁸³. The high R² value of 0.9853 adjusted R² value of 0.9665 together with an adequate precision value gives excellent agreement between model and experimental data; however, predicted R² is low at 0.7655 which indicates moderate predictability for the model and also implies that further validation experiments can increase robustness for this model prediction. Though lack-of-fit sum of squares has been reported here but due to no replicated runs pure error is zero hence a reliable statistical test for lack-of-fit cannot be conducted. In summary, ANOVA results confirm that the developed model can adequately describe Cr(VI) adsorption onto MACC and that time, pH and dose particularly with non-linear contributions from pH and time are primary factors controlling the adsorption process (Table 2)⁸⁴.

Table 2.

A statistical analysis of variance has been performed on the models that have been used.

Source	Sum of squares	df	Mean squares	F-value	P-value
Model	3.187E + 05	9	35416.41	52.28	< 0.0001	significant
A-pH	5295.53	1	5295.53	7.82	0.0267
B-Time	2.130E + 05	1	2.130E + 05	314.49	< 0.0001
C-Dose	21848.15	1	21848.15	32.25	0.0008
AB	1561.95	1	1561.95	2.31	0.1727
AC	817.61	1	817.61	1.21	0.3083
BC	7467.03	1	7467.03	11.02	0.0128
A²	19705.12	1	19705.12	29.09	0.0010
B²	44698.86	1	44698.86	65.99	< 0.0001
C²	148.71	1	148.71	0.2195	0.6536
Residual	4741.70	7	677.39
Lack of Fit	4741.70	3	1580.57
Pure Error	0.0000	4	0.0000
Cor Total	3.235E + 05	16
Std. Dev.	26.03
Mean	268.81
C.V. %	9.68
R²	0.9853
Adjusted R²	0.9665
Predicted R²	0.7655
Adeq Precision	24.5605
PRESS	75867.25
-2 log Likelihood	143.97
BIC	172.30
AICc	200.64

The analysis presented in Table S13 shows that the quadratic model is the best fit for describing the process of Cr(VI) adsorption onto the MACC composite sponge in terms of statistical significance and consistency. Even though the linear and two-factor interaction (2FI) models have relatively high sums of squares, 83,317.16 and 73,470.58, respectively, their sequential p-values, 0.0004 for the linear model and 0.7250 for the 2FI model, indicate that only the linear model significantly contributes to explaining variations in the response variable; hence, the 2FI model does not improve predictive ability. Additionally, both models produce much lower adjusted and predicted R² values—0.6830/0.5647 for the linear model and 0.6366/0.2649 for the 2FI model—which reflects poor explanatory power and low reliability in predictions. On the other hand, a quadratic model has an extremely significant sequential p-value (0.0002), meaning that its inclusion of quadratic terms greatly improves modeling performance by capturing more curvature present in experimental data; this is further supported by high values of adjusted R² (0.9665) and predicted R² (0.7655) indicating good agreement between actual and predicted results thereby validating exceptional predictive capability of this model The cubic model is considered aliased and therefore not fit for interpretation since it brings in terms that cannot be separately evaluated because of design limitations. Based on this assessment, it can be concluded that the quadratic model is indeed the best-fitting model providing an optimal compromise among statistical significance explanatory power as well as predictive performance concerning accurate description Cr(VI) adsorption behavior^85,86.

The polynomial equation that has been encoded serves as the statistical model used to define the relationship between the adsorption capacity (q_e) of the MACC composite sponge for Cr(VI) and three operational parameters (A, B, and C) expressed in coded form. These coded parameters represent actual experimental conditions, including solution pH, adsorbent dosage, and contact duration normalized to a dimensionless scale (e.g., − 1, 0, + 1), thus enabling rigorous regression analysis^87,88. The model contains linear, interaction, and quadratic terms to represent not only the direct effect of each parameter on the uptake of Cr(VI) but also those combined interactions that affect the overall removal efficiency. Positive coefficients for certain terms indicate enhancement in adsorption performance with an increase in these parameters; negative coefficients, on the other hand, would indicate inhibitory effects or possible saturation phenomena at high levels. The interaction terms (AB, AC, BC) show how the efficiency of one factor depends on another in this case showing that synergistic improvements can be achieved by optimizing both pH and adsorbent dosage together while some adverse combinations may reduce efficiency. Also, the presence of negative quadratic terms means there is curvature in the response surface which implies that beyond optimal conditions adsorption will fall off possibly due to active site saturation or competition among Cr(VI) species. All these factors combined make this coded equation a good mathematical tool for predicting Cr(VI) adsorption behavior, finding best operating parameters, and explaining complicated interactions of MACC composite sponge physicochemical properties when used to clean pollutants as described in Eq. (5).

5

Equation (6) is the coded quadratic model that expresses the adsorption capacity (q_e) of the MACC composite sponge for Cr(VI) as a function of three operational variables: solution pH, contact time, and adsorbent dose. The positive coefficients for pH, time, and dose (82.47, 8.52239, and 134.165) suggest that optimizing each of these parameters independently will increase Cr(VI) uptake by enhancing active site availability and mass transfer. Interaction terms are used to understand how combined variable effects influence adsorption. The positive coefficient for pH × Time (0.138672) means there is a synergistic effect when both parameters are optimized together; however, negative coefficients for pH × Dose (− 19.8568) and Time × Dose (− 3.79) indicate antagonistic interactions where too much adsorbent dose or long contact time may reduce efficiency due to either site saturation or particle agglomeration^87,88. The negative quadratic coefficients for pH², Time², and Dose² (− 7.60114, − 0.045666, and − 103.178) indicate curvature in the response surface, showing that increasing these factors past certain limits decreases adsorption capacity. This suggests a transition from chemisorption dominance to equilibrium. Overall, the model accurately predicts Cr(VI) adsorption behavior and clarifies the interactions of pH, time, and dose, aiding in identifying optimal conditions for enhancing MACC composite sponge efficiency as outlined in Eq.(6).

6

Response surface analysis and modelling of experimental designs

Figure S8(a) shows the normal probability plot of the externally studentized residuals. The residuals closely follow the reference line, indicating that the experimental data are normally distributed and that there is no indication of systematic deviation in the model^89,90. The points are scattered pretty evenly around the line without much bending or grouping, which supports the idea of homoscedasticity. This means that the variance of the errors stays consistent across the experimental range. Also, since there are no extreme outliers, it strengthens the claim that the quadratic regression model developed fits well with the adsorption system and that data transformation is not needed. This strong linearity proves that ANOVA’s basic assumptions are met, confirming that the model used to predict Cr(VI) adsorption onto the MACC composite sponge is appropriate and reliable, as shown in Figure S8(a).

Figure S8(b) compares predicted and actual values, showing excellent agreement between experimental adsorption capacities and those from the proposed quadratic model. Most data points fall on the diagonal line, meaning that the model describes well the adsorption process of Cr(VI) onto the MACC composite sponge with no significant errors or biases. Most points are close to this line; therefore, prediction errors are small, supporting the reliability of this model in estimating responses across all experimental conditions. Also, since there is no random dispersion or systematic patterns, it further confirms that selected variables and their interactions have been properly integrated into the model framework. This strong linear relationship supports both the robustness of this model and its ability to predict accurately, justifying its use for optimization as well as for predicting accurately Cr(VI) adsorption behavior under different operational scenarios^89,90.

The Box-Cox plot in Fig. S8(c) assesses the need for a power transformation of the response variable meant to stabilize variance and enhance the normality of residuals in the Cr(VI) adsorption model. Its minimum occurs close to λ = 0, within the region defined by a 95% confidence interval, which means that no substantial transformation is required. The vertical green line at λ = 0 further substantiates this claim; it implies that either the natural logarithmic transformation or even the original data would be sufficient to meet al.l assumptions required by the regression model. Since λ is now very close to unity and falls within an acceptable range, this indicates that variance is stable in this model and that residual distribution fits well—hence no more transformations are necessary. Results like these validate that original experimental data can be optimized and indicate how well a statistical model captures Cr(VI) adsorption dynamics concerning MACC composite sponge^89,90.

In Fig. S8(d), the externally studentized residuals are plotted against the predicted adsorption values. This plot is used to evaluate both the adequacy and randomness of the proposed quadratic model. The residuals show a random pattern around the zero-reference line with no discernible trends, curvature, or clustering; such behavior indicates that error terms are independent and homoscedastic. Additionally, all data points fall within upper and lower control limits (± 4.81963), thus further validating that there are no significant outliers and that no single observation has an undue influence on the model. This random distribution also supports the assumptions of constant variance and independence of residuals. Therefore, it can be concluded that this model describes adsorption phenomena without any systematic error, indicating that MACC composite sponge model accurately predicts Cr(VI) removal over tested conditions^91–93.

The perturbation plot in Fig. S8(e) shows how sensitive the adsorption capacity (qₑ) of the MACC composite sponge is for Cr(VI) to changes in each independent variable from a reference point, while keeping all other factors constant^91–93. The pronounced curvature observed for variable B (adsorbent dose) indicates that this parameter has the most substantial effect on Cr(VI) uptake, suggesting that even minor adjustments in dosage lead to significant changes in adsorption capacity. Conversely, variable A (solution pH) displays a moderate positive slope, affirming that adsorption efficiency is enhanced as pH approaches its optimal range, likely attributable to increased electrostatic interactions and greater availability of active functional groups. On the other hand, variable C (contact time) shows a nearly horizontal and slightly negative inclination, indicating that this variable contributes the least to the adsorption response once the surface-active sites are nearing saturation. The distinct slopes and orientations of the curves distinctly delineate the relative importance of the experimental parameters affirming that dose is the predominant factor succeeded by pH while time effect remains minimal under conditions examined. This analysis corroborates significance of variables as indicated by model and reinforces optimization strategy implemented to improve Cr(VI) removal.

The cube plot in Fig. S8(f) displays the effect of pH (A), contact time (B), and adsorbent dosage (C) on the adsorption capacity (qₑ) of MACC composite sponge for Cr(VI) removal. The adsorption response values at the cube vertices indicate how different combinations of these parameters affect system efficiency. The minimum adsorption capacity of 10.9894 mg/g is obtained at low pH, contact time, and dosage, which shows that under inadequate conditions, active sites are not available to interact with Cr(VI) ions and the sponge. Conversely, the maximum capacity of 493.329 mg/g occurs when both pH and contact time are at their highest values with medium dosed levels; this means that under such conditions more Cr(VI) can be taken up by the material. Dosage has a very pronounced effect on adsorption capacity whereby too high an amount can reduce capacity to as low as − 5.66948 mg/g because of overpopulation and competition between adsorbates for active sites. Thus, from the cube plot, it could be inferred that pH and contact time interact synergistically in enhancing adsorption while dosage effects are rather complex and non-linear; hence optimal performance requires strategic balancing of these parameters towards achieving greater Cr(VI) removal efficiency^91–93.

Model adequacy checking

The combined impacts of adsorbent dose and contact time on the adsorption capacity (qₑ) of MACC composite sponge are shown in Fig. 9(a) through three-dimensional interaction, contour, and desirability plots. From the three-dimensional response surface plot, it is clear that qₑ increases significantly with increasing contact time at medium levels of adsorbent dose. This means that increasing the interaction time facilitates the diffusion of Cr(VI) ions toward active binding sites within the matrix of the sponge. However, beyond a certain optimal adsorbent dosage, there is a marked reduction in adsorption capacity which could be due to excessive aggregation of sponge particles possibly masking active functional groups or changes in speciation for Cr(VI) at higher dosages. The contour plot confirms this as it shows smooth curvature with well-defined gradients delineating the operational range for maximum adsorption; high values of qₑ are seen at longer contact times with an appropriate dose indicating synergistic enhancement in adsorption ability^94–96. The desirability plot further confirms this optimization with values close to unity (0.97) at the same factor levels, indicating that the selected combination of process parameters satisfies the multiple criteria required for optimal Cr(VI) removal. Altogether, these graphical representations emphasize the need for careful tuning of adsorbent dosage and sufficient contact time to maximize the adsorption performance of the MACC composite sponge.

Fig. 9.

Contour, 3D interaction, and desirability (a) Dose & time, (b) pH & dose, (c) pH & time.

Figure 9(b) shows the effect of solution pH and adsorbent dosage on adsorption capacity of Cr(VI) (qₑ) as three-dimensional response surfaces, contour, and desirability plots. The interaction surface indicates a nonlinear relationship, where qₑ increases with an increase in pH from acidic to neutral, reaching its maximum at about pH 5. This is due to less competition from protons at moderate pHs which enhances electrostatic attraction of Cr(VI) oxyanions towards functional groups on MACC sponge. At very high pHs, however, qₑ decreases because of repulsive forces between negatively charged sites on the sponge and negatively charged species of Cr(VI), such as CrO₄²⁻ or Cr₂O₇²⁻. The contour plot further validates this optimal range since extensive yellow areas indicating high qₑ are found around moderate pH and dosage levels. Excessive adsorbent dosage brings about a plateau or even a decrease in qₑ probably due to saturation of sites and limitations in mass transfer. The desirability plot indicates values close to 1.0 for the optimal condition that maximizes adsorption while minimizing material consumption. From this figure, it can be concluded that pH has a primary effect on the removal efficiency of Cr(VI), with dosage having a secondary but significant effect; thus, optimal pH should be maintained for the MACC composite sponge^94–96. The interactive effects of solution pH and contact time on the adsorption capacity (qₑ) of the MACC composite sponge for Cr(VI) are illustrated in Fig. 9(c). The three-dimensional plot exhibits that qₑ increases greatly with increasing contact time and pH, reaching a maximum at certain optimal values before decreasing gradually. It means that sufficient duration is needed to allow Cr(VI) ions to diffuse into the material and interact with its functional groups. Longer contact times stabilize the adsorption equilibrium without additional increases. The contour plot further confirms that maximum adsorption occurs at an intermediate pH value (about 5) under prolonged contact, which implies favorable electrostatic interaction as well as better accessibility to active sites. At low pH, competition from protons restricts Cr(VI) uptake; at high pH, adsorption is hindered by repulsion from negatively charged surfaces of the sponge. The desirability plot suggests conditions for maximum qₑ close to 1.0, which concurs with the objective of the optimization model. In general, this points out that there are very important combined effects of pH and contact time on adsorption efficiency and emphasizes accurate pH control and adequate exposure time to achieve effective Cr(VI) remediation using MACC composite sponge^94–96.

Validation of models and the desirability approach

Figure S9(a) shows the individual desirability and factor effect plots for pH, contact time, and adsorbent dose in the optimization of Cr(VI) adsorption capacity (qₑ) on the MACC composite sponge. The desirability profiles indicate optimal values close to 1.0, which is an indication of ideal operational conditions for enhancement of Cr(VI) removal. The pH desirability curve has a parabolic shape with its apex at about pH 4–5, where functional groups on the surface of the sponge are protonated to their maximum level; thus, electrostatic attraction toward Cr(VI) oxyanions is at its maximum for improved adsorption. Contact time desirability clearly increases until longer times reach peaks that favor ion diffusion and attachment until equilibrium is achieved. Conversely, adsorbent dose desirability shows first an increase and later a decrease at higher levels indicative of an optimal dose above which particle crowding and overlap of active sites render sorptive effectiveness less efficient. The lower panel qₑ plots validate these trends by quantitatively linking variations in adsorption capacity to changes in parameters. Maximum predicted qₑ values coincide with highest desirability points, thus confirming the chemical significance of this optimization process. Plots here basically show that pH is the most important variable influencing uptake followed by contact time then adsorbent dose; hence stressing balancing these factors for better performance. Desirability and response outcomes prove MACC composite sponge effective under conditions identified for Cr(VI) removal maximization⁹⁷.

Figure S9(b) shows a desirability bar chart of the optimized conditions for Cr(VI) adsorption onto MACC composite sponge. The individual desirability values of the three independent variables, namely pH, contact time, and adsorbent dose, are very close to 1.0, with specific values being 0.999998, 0.999997, and 0.999999, respectively. This indicates that each parameter is at its optimum to maximize the adsorption response. Therefore, these results imply that the selected operating conditions fulfill all required constraints for achieving optimal Cr(VI) uptake simultaneously. Furthermore, the response desirability for qₑ is 0.887777 which represents a high efficiency of the adsorption process though it is slightly less than that of input variables due to limitations from adsorption equilibrium and saturation of active sites. The overall model performance indicated by the collective desirability value is very good with a value of 0.970678 suggesting that this particular set of factors has a very strong effect on achieving both high Cr(VI) removal and economic use of MACC composite sponge. Thus, this chart can be taken as proof concerning how well an optimization algorithm works in practice and also validates these recommended operating conditions as being most suitable for actual use⁹⁷.

Figure S9(c) shows the desirability ramp function used to find the best operating conditions for Cr(VI) adsorption on the MACC composite sponge. The ramps describe how each independent variable: pH, contact time, and adsorbent dosage affects the overall desirability index. Model optimal conditions are at a pH value of 4.0, an approximate contact time of 100 min, and very low adsorbent dosage (0.02 g) with predicted adsorption capacity (qₑ) equal to 474.424 mg/g. Each parameter reaches its maximum desirability at these specific values as shown by red markers located at the top of each ramp. The cumulative desirability value is 0.971; this means that highly effective optimization results indicate that this particular combination of factors meets all criteria for maximized adsorption efficiency simultaneously. It further emphasizes the validation trustworthiness results based on response surface methodology (RSM) optimization strategy highlighting how MACC composite sponge performs best in removing Cr(VI) under these defined optimized conditions⁹⁷.

Conclusion

This study has presented a novel magnetically recoverable activated carbon composite sponge (MACC) loaded with Fe₃O₄ nanoparticles into the porous activated carbon structure and encapsulated within a crosslinked CMC/β-CD biopolymer matrix to create an adsorption platform that works through electrostatic attraction, surface complexation, and partial reduction of Cr(VI) into less toxic Cr(III) species. The batch adsorption tests showed that removal efficiency can be greatly affected by solution pH, adsorbent dosage, and contact time where adsorption kinetics follows a pseudo-second-order model while equilibrium data fits well with the Langmuir isotherm which confirms chemisorption-controlled monolayer adsorption mechanism having maximum adsorption capacity (qₘ) equal to 528.75 mg.g⁻¹. Response surface methodology optimization based on Box–Behnken Design indicated that the best adsorption condition was at pH 4, adsorbent dosage of 0.02 g, and contact time of 100 min with a predicted adsorption capacity close to 530 mg.g⁻¹. The MACC composite has shown competitive adsorption performance among magnetic or biopolymer-functionalized adsorbents previously reported for Cr(VI) removal, along with better operational features such as fast magnetic separation, structural stability over many cycles of regeneration and reusability without significant deterioration of the framework. All these findings qualified the MACC composite sponge as a promising environmentally friendly adsorbent for effective remediation of Cr(VI) from water environments plus providing a statistically optimized approach toward practical applications in wastewater treatment. The Box–Behnken optimization indicated that the maximum Cr(VI) uptake can be obtained at pH 4 by using 0.02 g of MACC in a contact time of 100 min with a predicted adsorption capacity of about 530 mg.g⁻¹. Future efforts will be directed toward assessing the selective adsorption capabilities of MACC within multi-component aqueous systems that include competing inorganic ions. This approach aims to more accurately replicate conditions found in actual wastewater environments.

Supplementary Information

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Author contributions

All authors participated directly in the current research work.

Data availability

All relevant data are within the manuscript and available from the corresponding author upon request.

Declarations

Competing interests

The authors declare no competing interests.

Consent to Participate

All authors participated directly in the current research work.