Activation Chemistry-Driven Enhancement of Calotropis procera-Derived Fe₃O₄/Activated Carbon Nanocomposites for Efficient Treatment of Paint Industry Wastewater

Abstract

The wastewater generated from the paint industry consists of very complicated organic contaminants that result in higher biochemical oxygen demand (BOD) and chemical oxygen demand (COD). In order to solve such wastewater problems, there is a need to develop efficient techniques. This paper focuses on the preparation of Fe₃O₄/activated carbon (AC) nanocomposites via activation by H₃PO₄ and KOH and comparative analysis of efficiency in treating paint industry wastewater. For characterization purposes, the obtained nanocomposites were analyzed using UV–vis spectroscopy, dynamic light scattering, X-ray diffraction, Brunauer–Emmett–Teller (BET) surface analysis, vibrating sample magnetometry, thermogravimetric analysis, scanning electron microscopy, and Fourier-transform infrared spectroscopy. It was found from BET analysis that H₃PO₄-AC/Fe₃O₄ had a higher surface area of 393.58 m² g^–1 compared to KOH-AC/Fe₃O₄ with a surface area of 368.92 m² g^–1. Both exhibited mesoporous structures with pore diameters of 2.98 and 2.87 nm, respectively. The highest efficiency (95.05% BOD and 97.34% COD) was recorded under optimal conditions (30 min; 2 mg of adsorbent; pH 4 for H₃PO₄-AC/Fe₃O₄ and pH 6 for KOH-AC/Fe₃O₄). Reusability studies demonstrated sustained adsorption performance over eight cycles, with H₃PO₄-AC/Fe₃O₄ exhibiting superior regeneration stability and pollutant removal efficiency compared to KOH-AC/Fe₃O₄. According to the Langmuir isotherm analysis, the highest adsorption capacities (165.93 mg g^–1 of BOD and 213.94 mg g^–1 of COD) were achieved using H₃PO₄-AC/Fe₃O₄, whereas the pseudo-first-order kinetics modeling revealed the best fit (R ² = 0.99936–0.99992). The process of intraparticle diffusion contributed to the adsorption but did not control the process. Thermodynamic parameters proved the spontaneous nature of the adsorption process as being endothermic and driven by entropy. The proposed mechanism of adsorption included the following processes: film diffusion, pore filling, intraparticle diffusion, hydrogen bonding, π–π interaction, van der Waals interactions, and interactions with oxygen-containing surface groups. These findings demonstrate that phosphoric acid activation significantly enhances the physicochemical properties and adsorption performance of biomass-derived Fe₃O₄/AC nanocomposites for efficient treatment of complex industrial wastewater.

1. Introduction

Industrial effluents continue to be a significant source of water pollution worldwide, especially in developing areas, where treatment systems are often inadequate. The paint industry, in particular, produces complex effluents rich in organic compounds, pigments, and refractory chemicals that pose risks to aquatic ecosystems and human health if they are not properly treated. , These pollutants lead to high chemical oxygen demand (COD), coloration, and toxicity in water bodies, resulting in reduced dissolved oxygen levels and harm to aquatic life. , Traditional wastewater treatment techniques, such as biological oxidation, coagulation, and advanced oxidation, frequently struggle to effectively lower persistent organic loads, particularly when industrial discharges vary. This situation calls for the creation of efficient, cost-effective, and sustainable adsorbents capable of targeting a broad spectrum of organic pollutants with a minimal operational complexity.

Activated carbon (AC) is well known for its effectiveness in water purification due to its large surface area, extensive porosity, and strong affinity for organic molecules, making it particularly adept at removing dyes, phenols, and other organic pollutants through adsorption. , Biomass-derived AC has gained popularity as a renewable and low-cost alternative to commercial AC, transforming agricultural waste into valuable environmental remediation products. − Research has successfully produced AC from various agricultural residues, such as shells, husks, and leaves, demonstrating promising adsorption performance in water systems. Calotropis procera is prevalent in tropical regions and generates significant biomass waste in the form of shells, leaves, and wood. Previous studies have shown that C. procera leaves and other biomass components can serve as effective low-cost biosorbents for dye removal, indicating significant surface functional groups and porosity that facilitate adsorption. In this light, C. procera leaves have demonstrated substantial methylene blue dye uptake from aqueous solutions, underscoring their potential as an adsorbent material. However, most studies have focused on untreated biomass or simple AC forms, with limited exploration of advanced material modifications to enhance performance further.

Over the past two decades, adsorption has shown great potential as one of the most versatile and cost-effective processes for the treatment of wastewater in industry because of its simple operation, high removal capacity, low energy consumption, and applicability to a variety of pollutants. The conventional adsorbents such as AC, zeolites, clay minerals, silica-based materials, polymeric resins, biochar, carbon nanotubes, graphene derivatives, metal oxides, metal–organic frameworks, and magnetic nanocomposites have been thoroughly examined for the adsorptive removal of dyes, heavy metals, pharmaceuticals, pesticides, phenols, and other persistent organic pollutants from textile, paint, petrochemical, pharmaceutical, mining, leather, food processing, and electroplating wastewater streams. Of all the above adsorbents, biomass-derived ACs have attracted much more interest due to their high abundance, inexpensive preparation, environmental friendliness, and good porous properties, whereas magnetic functionalization of them using Fe₃O₄ nanoparticles would help realize fast magnetic separation, avoid secondary sludge formation, and improve the recyclability of the materials. Some recent reviews have pointed out that future adsorbents should be prepared from renewable biomass and involve nanotechnology to reach high adsorption capacity, easy regeneration, and a scalable process for the treatment of industrial wastewater instead of a laboratory model system.

However, substantial knowledge deficiencies continue to exist regarding the use of less-studied lignocellulosic biomass, effects of activation chemistry on the structure and adsorption characteristics of magnetically ACs, and validation via application of complex industrial effluents containing several types of organic contaminants. It can be seen that the vast majority of the existing literature on the topic relates to synthetic dye solutions or one-pollutant systems, but the literature concerning the practical usage of such materials for treating real paint industry wastewater and simultaneous removal of biochemical oxygen demand (BOD) and COD is rather scarce. Although numerous research works have been performed concerning biomass-based Fe₃O₄/AC nanocomposites, the scientific novelty of this research lies in the synthesis and comparative analysis of H₃PO₄- and KOH-activated C. procera carbon as a support for Fe₃O₄ nanoparticles for the treatment of real paint industry wastewater. − The difference from other similar research lies in the fact that this research reveals the connection between the activation chemistry, physicochemical parameters (specific surface area, pore size distribution, point of zero charge, crystallite size, and surface functional groups), and simultaneous removal of complex organic pollutants represented by BOD and COD. Taking into account this fact, the aim of the current study can be stated as the creation of AC/Fe₃O₄ nanocomposites based on the renewable biomass C. procera via H₃PO₄ and KOH activations and study of their physicochemical properties and adsorption behavior in terms of paint industry wastewater treatment.

Additionally, even though the process involves activation and thermal processing stages that may result in energy costs and high production costs, the employment of cheap biomass as raw material with conventional processing methods makes it promising for industrialization purposes. However, it is necessary to carry out technoeconomic analysis, energy optimization, and pilot testing in order to evaluate the commercial viability of the adsorbents developed in this study. It is thus anticipated that the results obtained in this study will aid in the development of environmentally friendly, cost-effective, recyclable, and scalable adsorbents for wastewater treatment.

While great developments have been made in the area of magnetic carbon-based adsorbents for wastewater treatment, there have not been many studies conducted on the use of C. procera biomass as a raw material for developing Fe₃O₄-functionalized AC nanocomposites for paint industry wastewater treatment. Earlier investigations were concentrated largely on traditional forms of AC obtained from agricultural waste materials or general Fe₃O₄/carbon-based nanocomposites designed for removing individual contaminants instead of complicated organic compounds that cause BOD and COD in actual industrial wastewater. What makes this work unique is that it proposes an environmentally friendly Fe₃O₄-functionalized AC nanocomposite fabricated from C. procera biomass through two kinds of chemical activation processes (H₃PO₄ and KOH) and an investigation into the effect of the chemistry of the activation process on the properties of the developed nanocomposites.

2. Materials and Method

2.1. Materials

Analytical grade ferric chloride hexahydrate (FeCl₃·6H₂O, Sigma-Aldrich), analytical grade ferrous chloride tetrahydrate (FeCl₂·4H₂O, Sigma-Aldrich), analytical grade sodium hydroxide (NaOH, Sigma-Aldrich), analytical grade phosphoric acid (H₃PO₄, Sigma-Aldrich), analytical grade potassium hydroxide (KOH, Sigma-Aldrich), analytical grade hydrochloric acid (HCl, Sigma-Aldrich), and deionized water were employed in all the experiments. All reagents and chemicals were of analytical grade and did not undergo further purification.

2.2. Methods

2.2.1. Sample Collection and Pretreatment

2.2.1.1. Plant Collection and Pretreatment

The fresh leaves of the Sodom apple (C. procera) were collected from various locations within the Minna metropolis, Niger State, Nigeria. These leaves were washed with distilled water, air-dried in the laboratory, and subsequently pulverized before the phytochemical screening.

2.2.1.2. Wastewater Sample Collection

Paint industry wastewater samples were gathered from Minna, Niger State, Nigeria, and transferred into a 100 L container initially treated with. These samples were then transported to the laboratory and stored in a refrigerator at 4 °C until analysis to minimize microbial activity and prevent physicochemical changes in the wastewater composition. The physicochemical analysis included measurements of turbidity, pH, conductivity, phosphate, carbonate, dissolved oxygen, COD, and BOD, using standard procedures.

2.2.1.3. Screening the Plant Leaves for Phytochemicals

Before the synthesis of the nanoparticles, the selected C. procera leaves were classified by a taxonomist at the Department of Plant Biology, Federal University of Technology, Minna, Niger State, Nigeria. After the classification of the leaves, phytochemical screening was performed to select a suitable leaf sample for the formation of the nanoparticles. Qualitative analysis of the leaves was performed using quantitative analysis based on the total tannins present in the leaves and the total phenolic contents.

2.2.1.4. Leaf Extract Preparation

A known mass (20 g) of dried leaves of C. procera was pulverized using a mortar and pestle, followed by grinding using a mechanical blender; then, 200 mL of deionized water was added. After stirring, the mixture was heated to 75 °C for 10 min and subsequently cooled to room temperature. It was then filtered using a muslin cloth, followed by using Whatman no. 1 filter paper, and the resulting filtrate (aqueous leaf extract) was stored in a refrigerator.

2.2.1.5. Synthesis of Fe₃O₄ Nanoparticles Using C. procera Leaf Extract

Magnetite nanoparticles (Fe₃O₄-NPs) were synthesized using a green chemistry approach, whereby C. procera leaf extract served as a reducing and stabilizing agent. The 2:1 molar mixture of ferric chloride (Fe³⁺) and ferrous chloride (Fe²⁺) was used as a precursor salt. 40 mL of this iron salt solution was mixed with 100 mL of aqueous leaf extract, and its pH was adjusted to 10. The mixture was stirred continuously for 60 min, and 1.0 M NaOH was added gradually to maintain a pH of 10. The mixture gradually turned black, confirming the formation of the magnetite nanoparticles.

The colloidal solution was aged for 24 h to complete the nucleation process. The nanoparticles were precipitated, and washing was carried out several times with deionized water until a neutral pH of 7 was reached. The precipitate was dried at 80 °C for 12 h, and the magnetite nanoparticles were calcined at 450 °C for 2 h to improve crystallinity and burn off organic residues. The final product, a dark gray magnetite powder, was stored in airtight containers for further use and characterization.

2.2.1.6. Synthesis of Activated Carbon

AC was derived from the stems of C. procera via the chemical activation method. The stems were washed thoroughly using deionized water, dried under the sun, and then cut into small pieces. The biomass was first carbonized in a muffle furnace at 400 °C in a nitrogen environment for 2 h.

For the activation, the char obtained from the carbonization process was treated separately with phosphoric acid (H₃PO₄) or potassium hydroxide (KOH) in a 1:1 ratio. Each mixture was then soaked in the respective acid/base solution for 12 h to ensure the penetration of the chemicals into the char. The treated char was then placed in a muffle furnace and heated to 500–600 °C in a nitrogen environment for 1 h.

The AC derived from the individual treatment of the char with H₃PO₄ or KOH has different pore structures that are beneficial in the removal of organic contaminants from wastewater treatment plants.

2.2.1.7. Fabrication of Activated Carbon Decorated with Fe₃O₄ Nanocomposites

Magnetite (Fe₃O₄) nanocomposite AC was prepared by the wet impregnation process. The same amount of weight of previously prepared Fe₃O₄ nanoparticles and chemically AC (H₃PO₄ activation and KOH activation) was used to provide a 1:1 weight ratio. Both materials were mixed in deionized water to give a homogeneous suspension. The suspension was stirred continuously with a magnetic stirrer for 2 h at room temperature to enable the uniform impregnation of Fe₃O₄ nanoparticles on the AC surface. The formed slurry was dried at 80 °C in a hot air oven for 12 h to make sure the removal of the solvent.

The dried composite was calcined at 500 °C for 2 h in a muffle furnace in ambient air to optimize the interface bonding between AC and Fe₃O₄ nanoparticles and increase the crystallinity of the magnetic phase and decomposition of any organics remaining in the composite. Once calcination was completed, the nanocomposites were left to cool to ambient temperature in air. They were then ground to a fine powder and kept in sealed containers for further analysis and investigation.

The nanocomposites prepared from H₃PO₄-AC and Fe₃O₄ and KOH-AC and Fe₃O₄ in a 1:1 mass ratio were designated as H₃PO₄-AC/Fe₃O₄ and KOH-AC/Fe₃O₄, respectively.

2.3. Characterization of Materials

The initial monitoring of magnetite nanoparticle synthesis was conducted using UV–visible spectroscopy (Model Cary 100, Agilent Technologies, USA) to verify the formation of nanoparticles. Following this, both the magnetite nanoparticles and the hybrid nanocomposites were characterized to assess their structural and surface attributes. The microstructural characteristics of the synthesized materials were examined using high-resolution transmission electron microscopy (Zeiss Auriga), while the surface morphology of the materials was analyzed through high-resolution scanning electron microscopy, also with a Zeiss Auriga instrument. The specific surface area of the nanocomposite was measured using the Brunauer–Emmett–Teller (BET) method with a NOVA4200e surface area analyzer (Quantachrome, UK). The crystalline phase composition and structural characteristics were evaluated using X-ray diffraction (XRD) analysis with a Shimadzu XRD-6000 diffractometer. To assess colloidal stability, the zeta potential of the nanomaterials was determined using a ZetaView PMX100 microelectrophoresis analyzer (Particle Metrix GmbH, Meerbusch, Germany).

2.4. Batch Adsorption Experiment

To ensure that the results of the adsorption tests were reliable and repeatable, all adsorption tests were repeated three times under the identical conditions. The values presented in the paper are an average of three separate measurements, and there was no significant variation among them.

2.4.1. Zeta Potential Measurements

The point of zero charge of the synthesized nanocomposites was determined from ζ-potential measurements by adjusting the suspension pH over an appropriate range and identifying the pH at which the measured ζ-potential crossed zero.

2.4.2. Effect of pH

The adsorption of BOD and COD from paint industry wastewater using the AC/Fe₃O₄ nanoparticles and the corresponding nanocomposites as an adsorbent was studied by measuring 100 cm³ of the paint wastewater into separate conical flasks. The conical flasks were corked, and the solution was stirred continuously at 200 rpm at various pH levels (1–8) at 30 °C and with an adsorbent dosage of 2 mg. The liquid phases were filtered out of the solution using Whatman no 1 filter paper.

2.4.3. Effect of Contact Time

The adsorption of BOD and COD from paint industry wastewater using the AC/Fe₃O₄ nanoparticles and the corresponding nanocomposites as an adsorbent was studied by measuring 100 cm³ of the paint wastewater into separate conical flasks. The conical flasks were corked, and the solution was stirred continuously at 200 rpm at various contact times (5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, and 60 min) at 30 °C and with an adsorbent dosage (2 mg) and pH of 4 for H₃PO₄-AC/Fe₃O₄ and KOH-AC/Fe₃O₄ for 6. The liquid phases were filtered out of the solution using Whatman no. 1 filter paper.

2.4.4. Effect of Adsorbent Dosage

The adsorption capacity of BOD and COD from paint wastewater at various adsorbent dosages of 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 mg per 100 cm³ wastewater was studied at adsorption conditions of temperature 30 °C and pH of 4 for H₃PO₄-AC/Fe₃O₄ and KOH-AC/Fe₃O₄ for 6. Every aliquot was quantitatively transferred to a 250 mL Erlenmeyer conical flask, tightly stopped, and continuously agitated using a magnetic stirrer at 250 rpm to avoid inhomogeneous mixing and enhance mass transfer between the solution and adsorbent. The residual concentrations of BOD and COD were determined by referring to the standard method.

2.4.5. Effect of Reaction Temperature

Adsorption characteristics of BOD and COD in paint industry wastewater were examined at various temperatures of 30, 35, 40, 45, 50, 55, and 60 °C at 30 °C and with an adsorbent dosage (2 mg) and pH of 4 for H₃PO₄-AC/Fe₃O₄ and KOH-AC/Fe₃O₄ for 6, controlled using a thermostatically stirred shaker. A constant volume of 50 cm³ of paint wastewater was precisely measured into a series of single 250 mL Erlenmeyer conical flasks, each tightly corked to prevent volatilization and contamination. The solutions were maintained under constant magnetic stirring at 250 rpm to ensure an equal interaction between the solution and the adsorbent.

Upon completion of each run of adsorption, suspensions were filtered through a Whatman no. 1 filter paper, and the filtrates were stored for subsequent physicochemical analyses.

2.4.6. Desorption/Regeneration Study

The spent adsorbent used in each adsorption test was separated from the sewage wastewater using the centrifugation technique and rinsed with distilled water to remove impurities and any remaining solution. After that, 0.1 g of the recovered adsorbent was treated with 50 mL of 0.1 M nitric acid (HNO₃) as the desorbing agent for 60 min under constant stirring to enable desorption of the adsorbed heavy metal ions. The regenerated adsorbent was then repeatedly washed with distilled water to neutralize it and dried in a hot-air oven at 105 °C for 2 h before the next adsorption step.

The regeneration method was chosen to check the reusability and operability of the adsorbents. Only distilled water is not effective for the desorption of tightly adsorbed heavy metals because their adsorption occurs mainly due to electrostatic attraction, surface complexation, and ion exchange. An acidic medium protonates oxygen-containing functional groups on the adsorbent surface and facilitates the desorption of the metal ions by weakening interactions between the metal ions and the adsorbent surface. Despite the good results of regeneration with nitric acid in this study, the chosen regeneration strategy can be considered to be preliminary.

2.4.7. Data Analysis

The data collected in this analysis were used to fit different kinetic models and adsorption isotherms to determine the adsorption efficiency and adsorption mechanism for the adsorption between BOD and COD. Eqs and were used to calculate the equilibrium adsorption capacity and percent (%) removal for the desired impurities

$$q_{\mathrm{e}}=\frac{(C_{\mathrm{i}}-C_{\mathrm{f}})\times V}{m}$$ 1

$$\%\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=\frac{(C_{\mathrm{i}}-C_{\mathrm{f}})}{C_{\mathrm{i}}}\times 100\%$$ 2

where q _e represents the amount of heavy metal adsorbed at equilibrium (mg/L), C _i and C _f represent the initial and final BOD and COD concentrations in the paint refinery wastewater, respectively, V represents the volume of solution (L), and m represents the mass of the adsorbent (g).

2.4.8. Pseudo-First-Order Model

The kinetic data were analyzed according to the pseudo-first-order kinetic model, which is based on the assumption that the rate of adsorption is proportional to the number of vacant sites and that adsorption occurs only on isolated sites, and ions do not interact with each other. The equation for the pseudo-first-order kinetic model is given by eq

$$\ln (q_{\mathrm{e}}-q_{\mathrm{t}})=\ln q_{\mathrm{e}}-k_1\mathrm{t}$$ 3

where q _e (mg/g) is the mass of metal adsorbed at any time (t) and k ₁ (min^–1) is the pseudo-first-order adsorption equilibrium rate constant. The values of k ₁ and qe are determined by the slope and intercept of the plot of log­(q _e – q _t) vs t, respectively.

2.4.9. Pseudo-Second-Order Model

The pseudo-second-order model for the kinetic process in the adsorption of metal ions onto the adsorbent material has been discussed by Revellame et al. The pseudo-second-order model is used for the prediction of the whole adsorption range by considering chemical sorption or chemisorption as the rate-controlling step in the adsorption process. In this model, the rate of adsorption was controlled by the adsorption capacity instead of the concentration of the adsorbent. The pseudo-second-order model can be defined by eq

$$\frac{t}{q_{\mathrm{t}}}=\frac{1}{k_2{q}_{\mathrm{e}}^2}+\frac{1}{q_{\mathrm{e}}}\mathrm{t}$$ 4

2.4.10. Intraparticle Diffusion Model

Weber and Morris developed and proposed the intraparticle diffusion model, which also accounts for the pore diffusivities. Intraparticle diffusivities are considered the slowest step in the adsorption process, and the rate-controlling step occurs. Instantaneous adsorption in the intraparticle diffusion model was proposed by Pholosi et al. The adsorption equation is presented in eq

$$q_{\mathrm{t}}=k_{\mathrm{i}}{t}^{0.5}+C$$ 5

The pseudo-second-order rate constant (g/mg/min) is denoted as k ₂. The slope of the plot of t/q _t vs t determines the value of the q _e.

2.4.11. Elovich Model

The Elovich model is used for the prediction of the nature of adsorption in wastewater treatment processes. It has been suggested that the rate of adsorption decreases exponentially with an increase in the amount of adsorbed solute. Eq is the Elovich equation

$$q_{\mathrm{t}}=\frac{1}{\beta }\ln \alpha \beta -\frac{1}{\beta }\ln (t)$$ 6

where α is the initial adsorption rate (mg g^–1 min^–1), β is the desorption constant (g mg^–1). The value of the reciprocal of β reflects the number of sites available for adsorption, whereas the value of the adsorbed quantity when ln t is equal to zero is given by 1/βln αβ.

2.4.12. Adsorption Isotherms

Adsorption isotherms provide basic physicochemical information necessary for assessing the suitability of the adsorption process as a unit operation. Freundlich adsorption is concerned with heterogeneous adsorption, whereas Langmuir adsorption is concerned with monolayer adsorption. The adsorption isotherm model proposed by Temkin predicts that the heat of adsorption of the molecules present in the adsorbed phase decreases linearly instead of logarithmically with the coverage of the molecules. Another adsorption isotherm model is the Dubinin–Radushkevich model, which assumes adsorption to be both homogeneous and heterogeneous. The Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich adsorption isotherm equations are presented in Table .

1. Equation of Adsorption for Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R) Isotherms.

isotherm	equation number
Langmuir	$\frac{C_{\mathrm{e}}}{q_{\mathrm{e}}}=\frac{1}{\mathrm{b}{q}_{\mathrm{m}}}+\frac{C_{\mathrm{e}}}{q_{\mathrm{m}}}$ (7)
Freundlich isotherm	$\ln q_{\mathrm{e}}=\ln K_{\mathrm{f}}+\frac{1}{n}\ln C_{\mathrm{e}}$    (8)
Temkin	q e = Bln A T + Bln C e    (9)
D–R	ln q e = ln q m – K D–Rs 2 (10)

where q _e (mg/g) is the equilibrium adsorption capacity, q _max (mg/g) is the maximum adsorption capacity, K _L (L/mg) is the Langmuir constant describing adsorption affinity for the adsorbent, K _F, is the Freundlich constant, 1/n is the Freundlich constant related to the multilayer adsorption capacity, and n is the heterogeneity factor, which represents the extent to which the adsorption depends on the equilibrium concentration. b (g J mol^–2) is the Temkin constant, R is the universal gas constant (8.314 J K^–1), T is the temperature in K, C _e (mg/L) is the equilibrium adsorbate concentration C _o (mg/L) is the initial concentration, k _D–R is a constant related to the adsorption energy, ε is the Polanyi potential, which is related to the equilibrium concentration, and RL is the separation factor. The essential feature of the Langmuir equation can be expressed in terms of a separation factor, RL (dimensionless), which is defined by the following equation.

2.4.13. Thermodynamic Evaluation of the Adsorption Process

The thermodynamic aspects of BOD and COD removal from laundry graywater adsorption by the synthesized nanomaterials were investigated to understand the adsorption mechanism and thermodynamic energies involved in the adsorption process. The calculation of the standard Gibbs free energy change (ΔG°) (eq ), standard enthalpy change (ΔH°) (eq ), and standard entropy change (ΔS°) (eq ) helped understand the thermodynamic aspects of the adsorption phenomenon. The distribution equilibrium coefficient (K _d) (eq ) was calculated at different temperatures, and the linear plot of ln K _d vs 1/T was used to determine ΔH° and ΔS° from the slope and intercept of the van’t Hoff plot, respectively. These thermodynamic parameters helped understand the thermodynamic aspects of the adsorption phenomenon, i.e., whether the adsorption of BOD and COD was spontaneous (ΔG° < 0), endothermic or exothermic (ΔH° > or <0), and increased or decreased randomness at the solid–liquid interface (ΔS° > or <0)

$$\mathrm{\Delta }G=-RT\ln K_{\mathrm{d}}$$ 11

$$\mathrm{\Delta }G=\mathrm{\Delta }H-T\mathrm{\Delta }S$$ 12

$$K_{\mathrm{d}}=\frac{q_{\mathrm{e}}}{C_{\mathrm{e}}}$$ 13

where ΔG° is the standard Gibbs’ free energy change for the adsorption (J/mol), R is the universal gas constant, T is the temperature (K), K _d is the distribution coefficient of the adsorbate, R = 8.3149 × 10³ (ΔH°) is the standard enthalpy change, and (ΔS°) is the standard entropy change.

3. Results and Discussion

3.1. Phytochemical Screening of C. procera Leaves

The phytochemical data presented in Table for the leaves of the C. procera plant show the presence of bioactive components that can reduce and stabilize the precursor to produce nanoparticles. The data report on the presence of phenols, flavonoids, tannins, saponins, and alkaloids in the plant (19.94 mg/100 g). The presence of these phytochemicals in plants has been confirmed in various studies. The studies reported that flavonoids and phenolic compounds are the major phytochemicals responsible for the antioxidant and reducing activities that are important in the synthesis of nanoparticles. The presence of higher levels of saponins in the plant is supported by the literature that reports the presence of saponin glycosides in the plant extracts. Saponins were reported to be responsible for stabilization of the surface of the nanoparticles. The moderate levels of phenols and flavonoids in the plant are supported by the literature. The studies reported the presence of quantified phenolics (14.73 mg/100 g) and flavonoids (6.88 mg/100 g) in the plant. The presence of these compounds was reported to be responsible for reducing and capping activities. The literature supports the low levels of alkaloids and tannins in the plant. Studies reported that these compounds were responsible for metal ion complexation. Table is scientifically valid and comparable to the literature, confirming that the synergistic presence of these phytochemicals underpins the effectiveness of C. procera leaves in green synthesis of Fe₃O₄ nanoparticles.

2. Phytochemical Contents of C. procera Leaves .

parameter	value
phenols (mg/100 g)	14.73c ± 0.46
flavonoids (mg/100 g)	6.88c ± 0.23
tannins (mg/100 g)	4.76c ± 0.17
saponins (mg/100 g)	19.94a ± 0.89
alkaloids (mg/100 g)	3.90b ± 0.35

^aData represent triplicates as mean ± standard error. Values with different superscripts within a column are significantly different (p ≤ 0.05).

3.2. UV–Visible Analysis of Magnetite Nanoparticles

The UV–vis spectral pattern of the green-synthesized Fe₃O₄ nanoparticles is comparable to the previously reported optical properties of magnetite nanostructures. The band shown in Figure S1 at 278.73 nm is generally assigned to ligand-to-metal charge transfer transitions between O^2– and Fe³⁺ ions in the spinel structure, which is generally true for all iron oxide nanoparticles. Compared to the UV–vis spectrum of noble metal nanoparticles, which display well-defined surface plasmon resonance bands, the UV–vis spectrum of Fe₃O₄ nanoparticles displays a broad band in the UV region due to the semiconductor character of the material.

The high absorbance in the region between 200 and 300 nm and the gradual decrease in absorbance toward the visible region are band gap-related transitions. The absence of peak maxima in the visible region indicates minimal aggregation and good colloidal stability, which is commonly achieved in green synthesis through the capping action of phytochemicals such as polyphenols/flavonoids. Apart from colloidal stability, phytochemicals are also likely to contribute to absorbance at longer wavelengths through additional electronic transitions.

Moreover, the tailing of the spectrum at longer wavelengths is indicative of polydispersity in the nanoscale and effective bioreduction of Fe²⁺/Fe³⁺ precursors to magnetite nanoparticles. This has been consistently reported in the case of plant-based synthesis of Fe₃O₄ nanoparticles.

3.3. Dynamic Light Scattering Analysis

Dynamic light scattering (DLS) measurements are well accepted for assessing the quality of nanoparticle dispersion in relation to their hydrodynamic diameter. It is an appropriate tool for iron oxide nanoparticle-based nanocomposites. The Z-average diameter, which was 77.47 nm, and polydispersity index (PDI) of 0.248 for the Fe₃O₄ nanoparticles are within the expected range for moderately monodisperse magnetic nanoparticles, with PDI <0.3 for well-controlled particle size and stability (see Figure S2a–c).

The decrease in the hydrodynamic diameter of the H₃PO₄- and KOH-AC/Fe₃O₄ nanocomposites is consistent with the literature, which states that the surface modification of iron oxide with porous materials such as carbon prevents the aggregation of iron oxide particles through dipole–dipole interactions.

The decreasing Z-average values and PDI values, especially in the KOH-activated materials (57.14 nm and PDI = 0.235), indicate better monodispersity and stabilization. AC exhibits a high surface area and functional groups that facilitate better particle separation and interaction with the solvent. This helps prevent agglomeration. The functional groups in the AC assist in the prevention of agglomeration. The results obtained in the DLS analysis prove that carbon activation enhances the stability of the Fe₃O₄-based catalysts.

3.4. Scanning Electron Microscope Analysis

The distinct effect of activation chemistry and nanoparticle addition is evident in the scanning electron microscopy (SEM) images. In Figure a, H₃PO₄-AC is seen to have a highly porous structure with a “honeycomb-like” morphology. This is characteristic of phosphoric acid activation, which involves dehydration and cross-linking reactions to improve carbon surface area. On the other hand, Figure b indicates that KOH-AC has a relatively rough surface with larger pores. This is characteristic of KOH activation, which involves a redox reaction to etch the carbon surface and improve its microporosity. ,

1.

(a) Phosphoric acid (H₃PO₄) AC, (b) potassium hydroxide (KOH) activated, (c) phosphoric acid (H₃PO₄) AC decorated with Fe₃O₄ nanoparticles, and (d) potassium hydroxide (KOH) AC decorated with Fe₃O₄ nanoparticles.

After Fe₃O₄ addition, Figure c indicates that H₃PO₄-AC/Fe₃O₄ has uniformly distributed spherical Fe₃O₄ particles. This suggests a strong interaction between Fe₃O₄ and oxygen-functional groups in the carbon surface, which improves nanoparticle dispersion.

On the contrary, the morphology of KOH-AC/Fe₃O₄ in Figure d presents a grape-like structure of Fe₃O₄ nanoparticles, indicating localized growth and magnetic dipole–dipole interactions, resulting in partial agglomeration, a common observation for magnetite-based nanomaterials. From the SEM results, it is confirmed that the decoration of Fe₃O₄ nanoparticles on both supports was successful, and the morphology of the particles was influenced by the activation pathways. The morphology of the particles is significant in the adsorption and catalytic activity of carbon–magnetite nanocomposites.

3.5. Brunauer–Emmett–Teller Analysis

Furthermore, this BET analysis result reinforces the mesoporous nature of both nanocomposites based on type IV isotherms and hysteresis loops occurring in the high relative pressure (>0.8) regime, as demonstrated in Figure A­(a,b). It is found that the H₃PO₄-AC/Fe₃O₄ nanocomposite has a higher specific surface area (393.58 m²/g) compared to KOH-AC/Fe₃O₄ (368.92 m²/g), which is in line with the previous reports that phosphoric acid activation increases the development of porous structures and oxygen-containing surface functional groups. The pore size distribution plot indicates that the two nanocomposites indeed have mesoporous structures with average pore sizes of 2.98 nm for H₃PO₄-AC/Fe₃O₄ and 2.87 nm for KOH-AC/Fe₃O₄, which lie in the mesoporous range (2–50 nm) and thus are advantageous for the removal of organic contaminants in wastewater. While KOH-AC/Fe₃O₄ possesses slightly larger pore volume (0.0163 cm³/g) than H₃PO₄-AC/Fe₃O₄ (0.0158 cm³/g), the difference is not significant. It is thus reasonable to conclude that the better adsorption capability of H₃PO₄-AC/Fe₃O₄ is mainly due to its large specific surface area and surface modification.

2.

(A) (a,b) N₂ adsorption–desorption isotherms of (a) KOH-AC/Fe₃O₄ and (b) H₃PO₄-AC/Fe₃O₄; (B) vibrating sample magnetometry (VSM) plots for (a) KOH-AC/Fe₃O₄ and (b) H₃PO₄-AC/Fe₃O₄; (C) XRD pattern of (a) KOH-AC; (b) H₃PO₄-AC; (c) Fe₃O₄; (d) KOH-AC/Fe₃O₄; and (e) H₃PO₄-AC/Fe₃O₄; (D) (a) Fe₃O₄, (b) KOH-AC/Fe₃O₄, and (c) H₃PO₄-AC/Fe₃O₄; (E) Fourier-transform infrared (FTIR) spectra for (a) KOH-AC/Fe₃O₄ and (b) H₃PO₄-AC/Fe₃O₄; and (F) EDX spectra for (a) KOH-AC/Fe₃O₄ and (b) H₃PO₄-AC/Fe₃O₄.

3.6. Vibrating Sample Magnetometry Analysis

The VSM magnetization curves of both nanocomposites display superparamagnetic properties, which are indicated by the S-shaped hysteresis loops with zero coercivity and remanence, as shown in Figure B. H₃PO₄-AC/Fe₃O₄ displays a higher saturation magnetization of 21 emu/g, whereas KOH-AC/Fe₃O₄ displays a slightly lower saturation magnetization of 20 emu/g.

The slightly lower saturation magnetization of both composites compared with that of Fe₃O₄ is due to the diamagnetic nature of the AC matrix, which dilutes the ferromagnetic properties of Fe₃O₄. However, both composites display adequate saturation magnetization, which indicates easy separability of these composites from aqueous solutions with the help of an external magnet.

3.7. X-ray Diffraction Analysis

XRD patterns further validated that the AC, magnetite nanoparticles, and their respective nanocomposites have been successfully synthesized (Figure C­(a–e)). AC samples showed a wide diffraction peak centered at 2θ = 20–25° for the (002) reflection plane of turbostratic carbon, signifying the amorphous structure of the carbon matrix with a low level of graphitization. This wide peak is unique to biomass-derived AC and results from the formation of disordered aromatic carbon layers.

XRD patterns of magnetic nanoparticles (Figure C­(c)) demonstrated clear peaks at around 30.2°, 35.5°, 43.2°, 53.6°, 57.1°, and 62.8°, attributed to the (220), (311), (400), (422), (511), and (440) planes of cubic spinel Fe₃O₄, respectively. These peaks match perfectly with those in the standard XRD pattern of magnetite (JCPDS/PDF no. 19-0629 or equivalent spinel Fe₃O₄ pattern), implying successful synthesis of the crystalline magnetic nanoparticles without any impurity phase.

The XRD spectra of the H₃PO₄-AC/Fe₃O₄ and KOH-AC/Fe₃O₄ nanocomposites displayed the typical diffraction peaks of the Fe₃O₄ spinel crystal phase, which include diffraction peaks at 2θ ≈ 30.20° (220), 35.50° (311), 43.20° (400), 53.60° (422), 57.10° (511), and 62.80° (440) along with the wide diffraction band of AC with a maximum diffraction peak at 2θ ≈ 23.00°, corresponding to the (002) plane. The preservation of these characteristic reflections confirms the successful incorporation of Fe₃O₄ nanoparticles into the AC matrix without altering the crystalline spinel structure of the magnetic phase. Thus, magnetite nanoparticles have been successfully embedded in the amorphous carbon network without modification of the spinel crystalline structure of Fe₃O₄ nanoparticles. The lower intensity and broadening in the diffraction peaks of nanocomposites can be attributed to the coating effect of the AC and also to the small crystallite size of the Fe₃O₄ nanoparticles. Crystallite sizes determined by using the Debye–Scherrer formula were found to be 25.84 nm for H₃PO₄-AC/Fe₃O₄ and 32.98 nm for KOH-AC/Fe₃O₄. Thus, the smaller crystallite size in the case of phosphoric acid-activated nanocomposite indicates the effective inhibition of crystal growth due to phosphoric acid activation, which was reflected in the high BET surface area and better adsorption performance.

3.8. Thermogravimetric Analysis

The thermogravimetric analysis (TGA) of the AC/Fe₃O₄ nanocomposites indicates that the materials have unique thermal decomposition characteristics that depend on the materials’ composition and surface chemistry. The TGA curve of the pure Fe₃O₄ nanoparticles, Figure D­(c), indicates that the sample has minimal weight loss over the range of 25–600 °C. The weight loss is around 3–4%, which is consistent with the high thermal stability of inorganic Fe₃O₄ materials. The absence of organic materials in the Fe₃O₄ nanoparticles also means that the weight loss is minimal. The TGA curve of the H₃PO₄-AC/Fe₃O₄ nanocomposites, Figure D­(a), indicates that the sample has a higher weight loss of around 22–23% over the range of 25–600 °C. The weight loss occurs in multiple stages: the initial weight loss at low temperatures (<100 °C) followed by degradation of oxygenated and phosphate-containing functional groups (150–300 °C), and gradual carbon matrix decomposition at higher temperatures. This behavior is typical of phosphoric acid-ACs, which possess abundant thermally labile surface functionalities.

KOH-AC/Fe₃O₄ (Figure D­(b)) exhibits relatively lower weight loss, around 18–19%, which indicates that it is thermally stable due to the presence of more thermally stable carbon structures as a result of KOH activation. The stepwise weight loss patterns in both composites are consistent with TGA patterns for carbon–metal oxide composites. However, it is observed that all samples are thermally stable at 400 °C and beyond.

3.9. Fourier-Transform Infrared Analysis

The FTIR spectra of KOH-AC/Fe₃O₄ (Figure E­(a)) and H₃PO₄-AC/Fe₃O₄ (Figure E­(b)) show successful grafting of oxygen-containing functional groups and Fe₃O₄ nanoparticles into the AC matrix. The broad absorption peak at 3513.52 cm^–1 is assigned to O–H stretching vibrations of surface hydroxyl groups and water molecules adsorbed on the sample. The absorption peak at 3058.82 cm^–1 is assigned to C–H stretching vibrations of aromatic hydrocarbons, showing that there are some graphitic carbons in the material. The absorption peaks at 2573.95 cm^–1 and 2462.93 cm^–1 correspond to C–H stretching vibrations of carbonaceous groups present in the material as a result of the chemical activation process. The absorption peak at 1638.23 cm^–1 is assigned to CC stretching vibrations of aromatic carbons and bending vibration of adsorbed O–H groups. The absorption peak at 1462.84 cm^–1 corresponds to CH₂ bending vibrations, while the absorption peak at 742.92 cm^–1 corresponds to the C–H out-of-plane bending of aromatic groups. Most importantly, the absorption peak at 625.73 cm^–1 is assigned to the Fe–O stretching vibration. When compared to KOH-AC/Fe₃O₄, H₃PO₄-AC/Fe₃O₄ showed a difference in the intensities of the O–H, CC, and Fe–O peaks due to an increased level of surface functionalization achieved through phosphoric acid activation. This is in accordance with the findings from the BET analysis, which demonstrated that H₃PO₄-AC/Fe₃O₄ has a larger specific surface area and better pore development.

3.10. EDX Analysis

The EDX spectra of (a) KOH-AC/Fe₃O₄ and (b) H₃PO₄-AC/Fe₃O₄ nanocomposites are shown in Figure F. The presence of the major elements in these nanocomposites is confirmed by these spectra. Both spectra are comprised mainly of peaks for carbon (C), oxygen (O), and iron (Fe), thus implying that there is the formation of Fe₃O₄ nanocomposites with AC. For the KOH-AC/Fe₃O₄ nanocomposite, it has 58.67 wt % C, 34.69 wt % Fe, and 6.64 wt % O. This implies that there is a lot of carbon on the surface of the material with a relatively high amount of Fe₃O₄. In the case of the H₃PO₄-AC/Fe₃O₄ nanocomposite, there is 52.77 wt % C, 31.98 wt % Fe, and 15.25 wt % O. The noticeably higher oxygen content in the H₃PO₄-activated sample suggests the introduction of additional oxygen-containing surface functionalities during phosphoric acid activation, which is consistent with enhanced surface oxidation. The presence of oxygen-containing functional groups will help to increase surface polarity, hydrophilicity, and the availability of active adsorption sites. In addition, the lower content of iron in H₃PO₄-AC/Fe₃O₄ could mean a larger amount of oxygenated carbon species compared to the magnetic component. , Thus, EDX analysis confirms the successful preparation of both magnetic nanocomposites and proves the great influence of the activation procedure on the elements’ content and surface chemistry.

4. Batch Adsorption Study

The point of zero charge (pH_pzc) is the pH at which the net surface charge of an adsorbent is zero and is a key parameter governing adsorbent–adsorbate interactions. The pH_pzc was determined using the zeta potential method by measuring the surface charge of the adsorbents over a range of pH values and identifying the pH at which the zeta potential became zero. The pH_pzc values were found to be 4.0 for H₃PO₄-AC/Fe₃O₄ and 6.0 for KOH-AC/Fe₃O₄ (Figure A,B). In the case of a solution pH less than pH_pzc, the surface of the adsorbent will be positively charged due to the protonation of the surface groups. In such an acidic medium, there will be a competition between the high concentration of H⁺ ions and organic compounds present in the wastewater for the available adsorption sites; therefore, the adsorption capacity will decrease. In contrast, when the pH value is greater than the pH_pzc, the surface of the adsorbent will be negatively charged due to deprotonation of the surface groups. As BOD and COD reflect the total oxygen consumption required by a mixture of various biodegradable and oxidizable organic substances and not by specific ionized entities, the adsorption process here can be explained by the cumulative effect of surface charge, porous structure, and surface functional groups and not just by electrostatic attraction. It can be inferred that the reduced value of pH_pzc for H₃PO₄-AC/Fe₃O₄ suggests that its surface has a higher acidity due to a higher presence of oxygen-based functional groups.

3.

(A,B) Point of zero charge plot for KOH-AC/Fe₃O₄ and H₃PO₄-AC/Fe₃O_4; (C,D) effect of solution pH on the removal of BOD and COD; (E,F) effect of contact time on the removal of BOD and COD; (G,H) effect of adsorbent dosage on the removal of BOD and COD; (I,J) effect of temperature on the removal of BOD and COD; and (K,L) reusability plots for KOH-AC/Fe₃O₄ and H₃PO₄-AC/Fe₃O₄.

4.1. Effect of Solution pH on Biochemical Oxygen Demand and Chemical Oxygen Demand Removal

The solution pH significantly influences the removal efficiency of BOD and COD due to its effect on surface charge and pollutant speciation (see Figure C,D). For H₃PO₄-AC/Fe₃O₄, the removal efficiency shows an extremely sharp increase from pH 2 (14.57% BOD, 16.93% COD) to pH 4 (96.95% BOD, 98.64% COD) and remains high (97–99%) up to pH 8. The sharp increase in removal efficiency is consistent with its lower pH_pzc (4). At pH < 4, the surface is positively charged, and the electrostatic repulsion and competition between the positively charged surface and H^– ions are significant. At pH ≥ 4, the surface is negatively charged, and the electrostatic attraction, surface complexation, and pore diffusion of organic pollutants are significant.

For KOH-AC/Fe₃O₄, the removal efficiency shows a gradual increase from pH 2 (15.46% BOD, 26.62% COD) to pH 6 (88.84% BOD, 90.57% COD) and a slight increase from pH 7 to 8 (89% BOD, 94% COD). The gradual increase is consistent with its higher pH_pzc (∼6). The optimal adsorption occurs at a pH close to neutral and slightly alkaline. At low pH, the protonation is less favorable, while the increased removal efficiency at high pH is consistent with the improved electrostatic attraction and reduced competition from H^– ions.

Thus, maximum removal occurs near or slightly above pH_pzc, confirming that electrostatic interactions and surface functional groups govern adsorption under the given conditions (2 mg dosage, 30 min, 30 °C).

4.2. Effect of Contact Time on Biochemical Oxygen Demand and Chemical Oxygen Demand Removal

The influence of contact time on BOD and COD removal by KOH-AC/Fe₃O₄ and H₃PO₄-AC/Fe₃O₄ nanocomposites in Figure E,F showed the typical adsorption behavior with an initial fast adsorption phase and later equilibrium phase as the adsorption sites were progressively saturated. In the case of the KOH-AC/Fe₃O₄ nanocomposite, the removal efficiencies of BOD and COD increased from 23.76% and 32.58%, respectively, at 5 min to 86.11% and 82.95% at 30 min, showing that maximum adsorption occurs within the first 30 min. After this duration, the rate of adsorption was lowered due to the saturation of adsorption sites. Equilibrium was achieved after 35 and 60 min of contact time. The maximum BOD and COD removal efficiencies were achieved at 60 min, being 95.94% and 97.96%, respectively.

Likewise, the H₃PO₄-AC/Fe₃O₄ nanocomposite showed relatively faster kinetics of adsorption with higher removal efficiencies as compared to those of the KOH-AC/Fe₃O₄ nanocomposite. Removal efficiencies increased very quickly from 44.82% (BOD) and 33.94% (COD) at 10 min to 95.05% and 97.34%, respectively, after 30 min. Beyond this period, only marginal increases in adsorption efficiency were observed, with equilibrium likewise established between 35 and 60 min The maximum BOD removal efficiency was found to be about 97.99%, whereas the COD removal efficiency was recorded at 99.27% in the equilibrium region. The improved adsorption capacity of the H₃PO₄-AC/Fe₃O₄ nanocomposite is due to its enhanced specific surface area, well-defined porous structure, and more oxygen-containing functional groups, formed as a result of phosphoric acid activation, which provide a higher number of available adsorption sites for organic pollutants.

Both of the nanocomposites showed considerable BOD and COD removal efficiencies in just 30–35 min of their exposure to water samples; afterward, the adsorption rate became much slower owing to the increased saturation of adsorption sites and the decreasing amount of free pores. Taking into account that there is not much difference in the efficiency of the process after 30 min, this time period can be considered the optimal time for carrying out further adsorption experiments.

4.3. Effect of Adsorbent Dosage on Biochemical Oxygen Demand and Chemical Oxygen Demand Removal

The effect of adsorbent dosage on BOD and COD removal was investigated under controlled conditions, including a 40 min contact time, 30 °C, and optimal pH values (pH 4 for H₃PO₄-AC/Fe₃O₄ and pH 6 for KOH-AC/Fe₃O₄). The results revealed that the removal efficiencies are a function of adsorbent dosage for both adsorbents.

Moreover, the removal efficiencies for BOD and COD increased continuously for the range of 2–14 mg for the adsorbent material, increasing to 94.85% and 96.47%, respectively. The results indicate that increasing the adsorbent material dosage is beneficial for increasing the available adsorption sites. , For the range 14–20 mg, the results plateaued to 95–97%, indicating that the available adsorption sites are saturated, and the adsorption sites may overlap, resulting in no significant improvement (see Figure G,H).

In the same way, H₃PO₄-AC/Fe₃O₄ shows an increased trend from 2 mg (BOD 33.85%, COD 35.49%) to almost complete removal at 16–20 mg (BOD ∼96.49–96.99%, COD ∼97.89–98.46%). However, it is important to note that the composite shows better efficiency over most dosages, as expected from its improved surface area and functional groups.

The efficiency of the adsorbent is improved with increased dosages, and the optimal amount is found to be 14–16 mg, beyond which the improvement.

4.4. Effect of Temperature on Biochemical Oxygen Demand and Chemical Oxygen Demand Removal

The effect of temperature on BOD and COD was also investigated at a fixed adsorbent dosage (14 mg), contact time (40 min), and pH values (pH 4 for H₃PO₄-AC/Fe₃O₄ and pH 6 for KOH-AC/Fe₃O₄). The results showed that increasing the temperature enhances the adsorption efficiency for both adsorbents.

For the adsorbent material, the results showed that the removal efficiencies increased from 30 °C (BOD = 32.55%, COD = 43.82%) to 55–60 °C (BOD = 88.93–88.99%, COD = 91.58–92.84%). The results showed a significant improvement in adsorption efficiency from 30 to 50 °C, indicating that the adsorbent material is improving the diffusion rate of pollutants and increasing the interaction with active sites, as shown in Figure I,J. The results showed that after 55 °C, the system approaches equilibrium, with minimal further improvement. −

4.5. Reusability Study on Biochemical Oxygen Demand and Chemical Oxygen Demand Removal

These findings on reusability indicate that both KOH-AC/Fe₃O₄ and H₃PO₄-AC/Fe₃O₄ nanocomposites showed relatively high efficiencies for the removal process during 8 cycles, although there was a progressive decrease in their performance (see Figure K,L). The BOD removal efficiency of the nanocomposite KOH-AC/Fe₃O₄ was 98.56% in the first cycle and 69.64% in the eighth cycle, while the COD removal efficiency was 97.95% in the first cycle and 75.65% in the eighth cycle. In turn, H₃PO₄-AC/Fe₃O₄ nanocomposites had a BOD removal efficiency of 97.89% in the first cycle and 74.32% in the eighth cycle, and COD removal efficiency was 99.86% and 78.97% in the first and the eighth cycles, correspondingly. Therefore, the residual efficiency of the phosphoric acid-activated composite suggests its stability during regeneration and resistance to performance degradation. , This improved recyclability is attributed to the enhanced surface oxygen-containing functional groups generated during phosphoric acid activation, which promote stronger interactions with organic contaminants and help preserve active adsorption sites during repeated use. , The decreasing effectiveness of the two adsorbents can be attributed to incomplete desorption of the pollutants, clogging of the pore system, and partial use or modification of the active surface area after multiple regeneration operations. , However, the two nanocomposites still had more than 69% of BOD removal and 75% of COD removal after eight cycles of adsorption–regeneration. This illustrates the stability of the adsorbents’ structure and shows their ability for reuse.

4.6. Adsorption Isotherm Studies

Equilibrium data for the adsorption of BOD and COD on the KOH-AC/Fe₃O₄ and H₃PO₄-AC/Fe₃O₄ nanocomposites have been analyzed using the Langmuir, Freundlich, and Temkin isotherms (Table a,b). All these models describe the adsorption process well, since their correlation coefficient is rather high (R ² > 0.99). All three models can be used to describe the adsorption process because all of them provide high values of R ² (0.9929–0.9983) in addition to low SSE and χ² values; that is, the deviation between the experimental and calculated values is minimal and suggests favorable monolayer adsorption from energetically uniform sites. − Moreover, the value of the Langmuir separation factor (R _l < 1) proves the favorability of adsorption at the studied conditions, and maximum adsorption capacity shows high performance of H₃PO₄-AC/Fe₃O₄ (165.93 and 213.94 mg/g for BOD and COD, respectively) in comparison with KOH-AC/Fe₃O₄ (143.82 and 198.84 mg/g, respectively). The Freundlich model provides even slightly higher R ². Nevertheless, the fact that the Temkin isotope has higher values for SSE and χ² means that the error involved in predicting the trend is higher as compared to the Langmuir isotherm. In essence, the adsorption can be explained as involving mainly monolayer adsorption that takes place on a heterogeneous surface with a distribution of adsorption sites. −

3. Adsorption Isotherm Parameters for the Removal of BOD and COD onto KOH-AC/Fe₃O₄ Nanoadsorbent; (b) Adsorption Isotherm Parameters for the Removal of BOD and COD onto H₃PO₄-AC/Fe₃O₄ Nanoadsorbent.

isotherm model	parameters	BOD	COD
(a)
Langmuir	q max (mg/g)	143.82	198.84
	K L (L/mg)	0.396	0.0513
	R L	0.0783	0.00261
	R 2	0.9929	0.9937
	SSE	0.0812	0.0506
	χ2	0.00472	0.00351
Freundlich	K f (mg/g)	7.74	11.85
	1/n	0.482	0.599
	R 2	0.9968	0.9998
	SSE	8.041	6.58
	χ2	0.876	0.711
Temkin	B T (kJ/mol)	0.479	0.731
	K T (L/g)	0.395	2.26
	R 2	0.9957	0.9991
	SSE	3.952	3.157
	χ2	0.558	0.493
(b)
Langmuir	q max (mg/g)	165.93	213.94
	K L (L/min)	0.513	0.0637
	R L	0.09641	0.0242
	R 2	0.9956	0.99834
	SSE	0.0533	0.02813
	χ2	0.00852	0.00692
Freundlich	K f (mg/g)	8.93	16.064
	1/n	0.627	0.741
	R 2	0.9988	0.9999
	SSE	6.825	4.93
	χ2	0.858	0.579
Temkin	B T (kJ/mol)	0.531	0.892
	K T (L/g)	0.723	2.88
	R 2	0.9972	0.9994
	SSE	2.16	2.09
	χ2	0.643	0.862

It is worth noting that BOD and COD are bulk wastewater quality indicators reflecting the oxygen demand of complex combinations of organic substances that can be biologically degraded and chemically oxidized and not an individual adsorbate molecule. Thus, the adsorption capacities (q _max) reported here should be treated as apparent adsorption capacities, indicating the removal of organic substances responsible for BOD and COD according to the performed experiments. Future research with total organic carbon (TOC) determination, representative organic substance identification, and proper blank control will give a much better estimation of the adsorption capacity and mechanism.

4.7. Adsorption Kinetic Studies

Adsorption kinetic analysis of BOD and COD using KOH-AC/Fe₃O₄ and H₃PO₄-AC/Fe₃O₄ was done using PFO, PSO, and IPD models (Table a,b). All models gave excellent fitting with experimental data (R ² > 0.996), meaning that the adsorption process has more than one mass-transfer rate process. In comparison, the PFO model showed the highest correlation coefficients (R ² = 0.9991–0.9999) in combination with the lowest SSE and χ² for both adsorbents and, hence, provided the best fit to the experimental data. These results mean that the initial phase of the adsorption process is controlled by rapid occupation of available adsorption sites on the surface. ,, Even though the PSO model had somewhat lower correlation coefficients and SSE, χ² values, and relatively low q _e1 values, the relatively high equilibrium adsorption capacities (q _e2 = 11.83–15.15 mg/g) can be explained by the contribution of the chemisorption process to the overall adsorption process at later adsorption stages. Moreover, the IPD model had a high R ² value (0.9982–0.9999), and since the intercept values were not equal to zero (C ≠ 0), it can be said that intraparticle diffusion occurred but was not the only factor controlling the process. The increased kinetic parameters (K ₁ and K _id) and q _e2 values for H₃PO₄-AC/Fe₃O₄ confirm a faster adsorption rate, improved diffusion, and better adsorption capacity than KOH-AC/Fe₃O₄. ,

4. (a) Adsorption Kinetic Model Parameters for the Removal of BOD and COD onto KOH-AC/Fe₃O₄ Nanoadsorbent; (b) Adsorption Kinetic Model Parameters for the Removal of BOD and COD onto H₃PO₄-AC/Fe₃O₄ Nanoadsorbent.

kinetic model	parameter	BOD	COD
(a)
pseudo-first order	R 2	0.99912	0.9998
	K 1 (1/min)	0.653	0.7153
	q e1 (mg/g)	5.821	7.058
	SSE	0.563	0.471
	Χ2	0.0145	0.0113
pseudo-second order	R 2	0.99683	0.99891
	K 2 (g/mg min)	0.369	0.402
	q e2 (mg/g)	11.83	13.64
	SSE	5.84	4.856
	Χ2	2.642	1.914
intraparticle diffusion	R 2	0.99821	0.99984
	K id	1.942	2.742
	C	0.592	0.689
	SSE	0.692	0.493
	Χ2	0.283	0.245
(b)
pseudo-first order	R 2	0.99936	0.99992
	K 1 (1/min)	0.747	0.853
	q e1 (mg/g)	5.991	7.732
	SSE	0.483	0.393
	Χ2	0.0136	0.0125
pseudo-second order	R 2	0.99791	0.99901
	K 2 (g/mg min)	0.383	0.492
	q e2 (mg/g)	13.23	15.15
	SSE	4.93	3.842
	Χ2	2.245	1.743
intraparticle diffusion	R 2	0.99882	0.99991
	K id	2.821	3.127
	C	0.624	0.783
	SSE	0.597	0.455
	Χ2	0.263	0.213

4.8. Adsorption Thermodynamic Studies

The thermodynamic parameters for the adsorption process of BOD and COD onto KOH-AC/Fe₃O₄ and H₃PO₄-AC/Fe₃O₄ show important information on spontaneity, heat, and disorder of the process. The positive values of ΔH obtained for the two materials, i.e., 39.86–53.14 kJ/mol for KOH-AC and 35.96–48.13 kJ/mol for H₃PO₄-AC, confirm the fact that the process is endothermic and indicates that the higher temperature facilitates the removal of organic pollutants (see Table S1 in Supporting Information).

The positive values of ΔS obtained, i.e., 0.268–0.295 J/mol K for KOH-AC and 0.248–0.462 J/mol K for H₃PO₄-AC, indicate an increase in the level of disorder in the process, which is characteristic of a process in which the structure of water around the adsorbate molecules becomes disordered upon adsorption of these compounds onto the solid surfaces.

The negative values of ΔG obtained for all temperatures confirm that the process is spontaneous, and ΔG values obtained decrease with an increase in temperature. For example, in the case of BOD adsorption onto H₃PO₄/AC/Fe₃O₄, ΔG values obtained decrease from −104.10 J/mol at 30 °C to −113.34 J/mol at 50 °C, indicating that the higher temperatures facilitate the spontaneity of the process.

Comparing both materials, it can be noted that H₃PO₄-AC/Fe₃O₄ has more negative values of ΔG than KOH-AC/Fe₃O₄, indicating that it has a greater driving force and better adsorption efficiency. The adsorption process is endothermic, spontaneous, and entropy-driven, with the H₃PO₄-activated magnetite exhibiting a greater affinity and adsorption capacity for BOD and COD removal.

The choice of the adsorption method has a profound influence on adsorption kinetics and thermodynamics, implying that H₃PO₄-AC/Fe₃O₄ is a more efficient adsorbent than KOH-AC/Fe₃O₄ under different temperatures.

4.9. Scanning Electron Microscopy Analysis of Spent Analysis

SEM analysis of spent AC/MNPs nanocomposites after treatment of wastewater from paint industry. From the SEM images of the spent adsorbents, it is seen that significant changes in surface morphology have occurred after BOD and COD removal from the paint industry wastewater.

The spent KOH-AC/Fe₃O₄ shown in Figure a has undergone significant changes in surface morphology compared to fresh KOH-AC/Fe₃O₄. The individual spherical nanoparticles of Fe₃O₄ that were clearly visible on the surface of fresh KOH-AC/Fe₃O₄ appear to be somewhat obscured and embedded in a heterogeneous mixture of surface-adsorbed organic pollutants. The surface is rougher and exhibits signs of fouling, aggregation, and pore clogging, indicating significant adsorption of paint wastewater pollutants including organic dyes and solids.

4.

(a) Potassium hydroxide (KOH) AC decorated with Fe₃O₄ nanoparticles, (b) phosphoric acid (H₃PO₄) AC decorated with Fe₃O₄ nanoparticles.

The spent H₃PO₄-AC/Fe₃O₄ shown in Figure b exhibits significant changes in surface morphology compared to fresh H₃PO₄-AC/Fe₃O₄. The grape-like clusters of Fe₃O₄ nanoparticles appear to be heavily coated with surface-adsorbed organic pollutants and appear to be smoother and more aggregated. The surface-adsorbed organic pollutants appear to have coated the surfaces of nanoparticles rather thickly. The changes in the surface characteristics of the spent adsorbents, such as the filling of pores, coating of the surface, and agglomeration of particles, indicate the involvement of the adsorption sites in the treatment of the wastewater. The extent of surface coverage in the H₃PO₄-AC/Fe₃O₄ nanocomposites was greater because of the higher pore volume (0.0163 cm³/g), which led to the retention of more impurities. The changes in the surface characteristics of the spent adsorbents validate the efficacy of the nanocomposites in the removal of complex organic impurities responsible for BOD and COD in paint industry wastewater. ,

4.10. Comparative Adsorption Study

The adsorption capacities for Fe₃O₄/AC composite adsorbents, as presented, indicate that both adsorbents, namely, KOH-AC/Fe₃O₄ and H₃PO₄-AC/Fe₃O₄, showed high adsorption efficiency for BOD and COD removal from paint industry wastewater, with q _max ranging from 143.83 to 213.94 mg/g, as presented in Table (see Table ). The adsorption capacities are competitive compared with other Fe₃O₄/AC composite adsorbents for the adsorption of organic pollutants. For instance, AC-Fe₃O₄ composite adsorbents for the adsorption of methylene blue dye showed higher adsorption capacities, reaching 321 mg/g, attributed to the high affinity of AC for specific dye molecules, while regenerated AC-Fe₃O₄ composite adsorbents showed adsorption capacities ranging from 265 to 366 mg/g for dyes, such as methylene blue and malachite green.

5. Reported Adsorption Capacities (q _max, mg/g) of Fe₃O₄/AC-Based Adsorbents.

adsorbent	pollutant	q max (mg/g)	notes/reference
KOH-AC/Fe 3 O 4	BOD equilibrium adsorption	165.93	this study
KOH-AC/Fe 3 O 4	COD equilibrium adsorption	198.84	this study
H 3 PO 4 -AC/Fe 3 O 4	BOD equilibrium adsorption	143.83	this study
H 3 PO 4 -AC/Fe 3 O 4	COD equilibrium adsorption	213.94	this study
magnetic AC-Fe3O4 composite	methylene blue dye	321	high-capacity AC Fe3O4 for dye removal
regenerated AC + Fe3O4	methylene blue/malachite green dyes	265–366	regenerated AC used for dye adsorption
Fe3O4/biochar hybrid	PFOS/PFOA	21.8–51.9	organic micropollutant study (not AC)
heterophase iron-oxide/carbon composite	crystal violet/Congo red dyes	35–45	magnetic iron-oxide/carbon composite
Fe3O4-AC for dye removal (general)	various dyes	85–107	Fe3O4/ACcomposite adsorbing dyes in literature

In contrast, Fe₃O₄/biochar hybrids and heterophase iron oxide/carbon composites have shown relatively lower adsorption capacities (21.8–51.9 mg/g and 35–45 mg/g, respectively) for micropollutants and cationic/anionic dyes, indicating the significant role of the surface area, pore structure, and chemical groups in the adsorption process. Overall, the results have shown that the H₃PO₄-AC/Fe₃O₄ nanocomposites have slightly better adsorption performance compared to the KOH-AC/Fe₃O₄ nanocomposites for BOD and COD, although the adsorption capacities are found to be relatively lower compared to the specific adsorbents designed only for the removal of dyes. However, the adsorption capacity is found to be very high compared to other wastewater treatment processes.

4.11. Adsorption Mechanism

Figure S3 presents the proposed mechanism of adsorption for the elimination of organic components causing BOD and COD in the wastewater of paint industries through the use of nanoadsorbents based on KOH-AC/Fe₃O₄ and H₃PO₄-AC/Fe₃O₄. First, the organic molecules are diffused from the bulk liquid phase to the external surface of the adsorbent by film diffusion; then, intraparticle diffusion within the micro- and mesoporous structures takes place. From the BET analysis, both of the nanocomposites have a large specific surface area and developed mesoporosity that makes enough adsorption sites available. Then, adsorption is facilitated by pore filling and surface interaction due to oxygen-containing functional groups identified by FTIR, such as hydroxyl (−OH), carbonyl (CO), and Fe–O. Such functional groups facilitate hydrogen-bonding, dipole–dipole interaction, van der Waals interaction, π–π interaction between aromatic carbon moieties and the organic molecules, and electrostatic interaction in which the wastewater components are ionizable at such pH values. Adsorption therefore involves various mechanisms as opposed to just one kind of adsorption mechanism, as supported by the kinetics and isotherm study that shows fast surface adsorption, followed by diffusion control on heterogeneous adsorption sites. Superior adsorptive capacity of H₃PO₄-AC/Fe₃O₄ is due to high specific surface area, small crystallite size, low pH_pzc, and presence of more oxygen functional groups, providing better active sites for adsorption of the variety of organic materials that cause the observed BOD and COD.

5. Conclusion

In this work, H₃PO₄- and KOH-activated nanocomposites comprising Fe₃O₄ particles and AC from the plant C. procera have been successfully synthesized via chemical activation, green synthesis of Fe₃O₄ nanoparticles, and fabrication of the composite, which can be used for the purification of real industrial wastewaters. The comprehensive characterization of the material by UV–vis, DLS, XRD, BET, VSM, TGA, SEM, and FTIR spectroscopies revealed that the mesoporous magnetic nanocomposites containing the highest amount of oxygen-containing functional groups have been successfully prepared. H₃PO₄-AC/Fe₃O₄ showed better physicochemical properties in terms of the highest specific surface area (393.58 m² g^–1), average pore diameter (2.98 nm), and smaller crystallite size (25.84 nm) compared to KOH-AC/Fe₃O₄. Optimum adsorption conditions were obtained with 30 min of contact time and 2 mg of the adsorbent, with pH of 4 for H₃PO₄-AC/Fe₃O₄ and 6 for KOH-AC/Fe₃O₄. Adsorption was observed to increase with temperature. Therefore, an endothermic process takes place. The equilibrium data fit well into the Langmuir, Freundlich, and Temkin isotherms, with the Langmuir equation having the least error and maximum adsorption capacity of 165.93 mg g^–1 (BOD) and 213.94 mg g^–1 (COD) for H₃PO₄-AC/Fe₃O₄. Reusability assessment confirmed stable adsorption over eight cycles, with H₃PO₄-AC/Fe₃O₄ demonstrating superior regeneration efficiency and long-term operational durability compared to KOH-AC/Fe₃O₄. In kinetic studies, it was observed that the pseudo-first-order kinetic equation provided the best fit to the experimentally obtained data, with pseudo-second-order and intraparticle diffusion kinetics showing the involvement of chemisorption and pore diffusion processes in adsorption, respectively. It was seen from thermodynamic analysis that adsorption was an exothermic, entropy-driven, and spontaneous process. The proposed adsorption mechanism included film diffusion, pore filling, intraparticle diffusion, hydrogen-bonding, π–π interaction, van der Waals forces, and interactions between oxygen-containing surface functional groups and organic components responsible for the bulk parameters BOD and COD.

Despite the developed nanocomposites possessing high potential in the treatment of wastewater from the paint industry, more research work is needed. Future research must take into account the effects of various competing inorganic ions and dissolved organic matter on adsorption processes, the effects of regeneration, stability, magnetic recovery efficiency, and Fe leaching due to repeated adsorption/desorption cycles, and also the relevant control experiments, which should include blank wastewater, filter only, AC only, Fe₃O₄ only, and AC without Fe₃O₄, to distinguish adsorption from other processes such as filtration, precipitation, coagulation, and pH effects. In addition to that, the full carbon mass balance analysis using the TOC method and organic pollutant characterization along with BOD and COD measurements is strongly recommended to make sure about the actual performance of the adsorption processes.

Detailed analytical results and interpretations are provided in the main manuscript.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c03864.

• Additional experimental details; characterization results; adsorption thermodynamic parameters; Supporting Information tables (Table S1) and Figures (S1–S3); and other Supporting Information related to the materials, methods, and findings of the study (PDF)

I.A.B. (conceptualization, data curation, formal analysis, methodology, investigation, software, resources, project administration, writingoriginal draft, writingreview and edit; validation); A.J.O. (conceptualization, data curation, funding, project administration, formal analysis, methodology, writingoriginal draft, writingreview and edit; visualization); A.N. (data curation, formal analysis, investigation, visualization, resources); Z.A.A.A.Z. (formal analysis, funding, project administration, writingreview and edit). All authors contributed substantially to the conception, design, execution, or analysis of this research. The final manuscript has been reviewed and approved by all authors, who hold joint responsibility for the accuracy and integrity of the reported data.

This work was supported by the Deanship of Scientific Research, Vice Presidency for Graduate Studies and Scientific Research, King Faisal University, Saudi Arabia [Grant No. KFU264334].

The authors declare no competing financial interest.