Engineering novel ceramic metal oxides with residual carbon for efficient sequestration of Auramine O from wastewater

Abstract

Auramine O is a hazardous cationic dye that threatens aquatic ecosystems by suppressing photosynthesis, and it poses health risks to humans through toxicity and bioaccumulation. Its chemical persistence allows it to remain in water bodies and enter food chains, which amplifies environmental and medical concerns. Here, we report a facile Pechini sol–gel route to synthesize CuO/MgCr₂O₄/C and CuO/MgCr₂O₄/MgO/C at 600 and 800 °C nanohybrids labeled MCC600 and MCC800, respectively, for efficient Auramine O removal. XRD confirms monoclinic CuO and cubic MgCr₂O₄ in MCC600 and reveals additional cubic MgO in MCC800 with average crystallite sizes of 60.97 nm and 75.35 nm, respectively, which reflects thermally driven grain growth. EDX verifies C, O, Mg, Cr, and Cu, with carbon decreasing and chromium increasing at higher temperature due to enhanced organic burnout and phase development. FE-SEM shows cauliflower-like agglomerates with fine nanospheres for MCC600 and a more uniform granular texture for MCC800, while HR-TEM reveals ultrafine quasi-spherical clusters for MCC600 and larger faceted polyhedral grains for MCC800. The capacities reach 442.48 mg/g for MCC600 and 298.51 mg/g for MCC800. Kinetic and thermodynamic analyses demonstrate pseudo-first-order behavior with exothermic spontaneous and physical adsorption that follows the Langmuir isotherm. Nearly complete acid-driven regeneration and stable multicycle reuse are achieved, and high performance is retained in real wastewater, which confirms suitability for practical treatment.

Introduction

Water contamination by organic dyes arises mainly from rapid growth of textile printing, paper, leather, cosmetic, pharmaceutical, and laboratory activities that release colored effluents into surface and ground waters^1–4. These dyes are designed to be chemically stable and highly visible, which means that even small discharges can cause intense coloration and long-term persistence in aquatic systems. Poor wastewater treatment infrastructure and uncontrolled disposal further accelerate the spread of dye pollutants in natural water bodies. Organic dyes present significant risks to ecosystems as well as human health because they block light penetration, inhibit photosynthesis, and disturb oxygen balance in water^5–7. Several dyes along with their degradation byproducts display toxic, mutagenic, or carcinogenic behavior, which makes long-term exposure dangerous for living organisms. Contaminated water can also affect food chains when dyes accumulate in plants, fish, and sediments^8–10. Auramine O is a synthetic cationic dye widely utilized for coloring and as a biological stain and it is known to be particularly harmful. It can cause irritation to skin and eyes and prolonged exposure is associated with liver and kidney damage. In aquatic environments Auramine O is toxic to microorganisms and aquatic life and it resists natural degradation which leads to persistent pollution¹¹. Several technologies have been developed to remove organic dyes from water, including membrane filtration¹², adsorption¹³, coagulation/flocculation¹⁴, electrodialysis¹⁵, bioremediation¹⁶, and photocatalytic degradation^17,18. Each method has its own advantages, but many suffer from high operational cost, generation of secondary waste, low selectivity, or slow treatment rates. Photocatalysis and membrane processes often require complex equipment^19,20, while biological treatments are limited by the toxicity of dyes to microorganisms²¹. Conventional biological treatment approaches such as anaerobic digestion have been widely employed for organic wastewater remediation; however, their effectiveness toward synthetic dyes such as Auramine O remains limited. Many dyes exhibit strong chemical stability and resistance to biodegradation, which reduces microbial conversion efficiency and prolongs treatment time. Anaerobic treatments typically demand tight operational control and long retention times, and they can be hindered by dye toxicity, which suppresses microbial activity and reduces process efficiency^22,23. Catalytic oxidation/reductive degradation routes have also been explored for persistent dyes; for example, noble-metal-modified semiconductors (e.g., Sn₃O₄@Au) can accelerate dye degradation by enhancing interfacial charge transfer²⁴. However, such catalytic approaches often require added reagents and/or tight process control, which can increase operational complexity and cost. Among these methods, adsorption has gained wide attention because it is simple, efficient, and adaptable to different water matrices. It can remove dyes without producing harmful byproducts, and it allows recovery and reuse of the adsorbent. Adsorption also works effectively at low dye concentrations where many other methods become inefficient²⁵. Nanotechnology provides materials with unique surface and structural features that enhance interaction with pollutants^26–28. Metal oxide nanoparticles and their composites are especially attractive for dye adsorption because they combine chemical stability, high surface area, and tunable surface chemistry^29–34. Composites formed with polymers or other metal oxides further improve adsorption by introducing multiple functional sites and by creating synergistic interactions that strengthen dye binding^35–39. A wide range of synthesis strategies has been developed for preparing nanostructured metal oxides and ceramic-based materials, including hydrothermal and solvothermal methods^40,41, co-precipitation⁴², solid-state reactions⁴³, carbothermal reduction⁴⁴, high-energy ball milling⁴⁵, spark plasma sintering⁴⁶, combustion synthesis⁴⁷, and microwave-assisted heating⁴⁸. Although these techniques can successfully produce crystalline nanomaterials, many of them suffer from drawbacks such as poor compositional homogeneity, broad particle-size distribution, high energy consumption, prolonged processing time, limited control over phase purity, or difficulties in scaling up. Microwave-assisted synthesis, for example, has been reported to accelerate phase formation through rapid electromagnetic heating⁴⁹. However, challenges related to non-uniform heat distribution, hot-spot formation, and restricted control over defect chemistry may arise, particularly in complex multicomponent systems. In contrast, the Pechini sol–gel method provides molecular-level mixing of metal precursors via chelation and polymeric network formation, which enables precise stoichiometric control, homogeneous elemental distribution, reduced diffusion distances during calcination, and improved phase integration in multiphase nanohybrids. These features make the Pechini approach especially suitable for constructing well-dispersed oxide/carbon nanostructures with controlled composition and enhanced functional performance⁵⁰. The Pechini sol–gel technique offers a potent route for producing uniform metal oxide nanostructures with controlled composition and high purity. It allows intimate mixing of metal ions at the molecular level, which leads to well-dispersed phases after calcination. This method also produces materials with high surface area and homogeneous distribution of active sites, which are essential for efficient adsorption^51–53. Various adsorbents have been reported for Auramine O removal, including non-imprinted polymer⁵⁴, activated carbon⁵⁵, ZnS/Cu/activated carbon composite⁵⁶, hydroxyapatite⁵⁷, Fe₃O₄⁵⁸, crosslinked chitosan-kaolin composite⁵⁹, and cellulose⁶⁰. Although some of these materials show moderate capacities, such as non-imprinted polymers (203.99 mg/g) and hydroxyapatite (132.57 mg/g), many others, such as activated carbon, Fe₃O₄, and cellulose, display very low uptake. These limitations arise from low surface reactivity, limited porosity, weak dye affinity, and poor stability, which highlights the need for new advanced adsorbents with higher efficiency and durability. This work introduces novel CuO/MgCr₂O₄/C and CuO/MgCr₂O₄/MgO/C nanohybrids synthesized by the Pechini-assisted sol gel route with annealing at 600 and 800 °C for effective Auramine O removal. The combination of multiple oxide phases in asingle structure produces a synergistic interaction that enhances the number as well as diversity of active binding centers. This design leads to stronger dye interaction and higher adsorption performance compared with conventional single-phase materials. The selection of each component in the present nanohybrids was strategically designed to maximize adsorption efficiency through complementary physicochemical properties. CuO was selected due to its strong surface reactivity, abundance of surface hydroxyl groups, and well-documented affinity toward cationic organic pollutants through electrostatic interaction and surface complexation. MgCr₂O₄ spinel was incorporated because of its high structural stability, chemical durability, and resistance to dissolution under alkaline conditions, which ensures long-term adsorption performance and preserves active sites during repeated operation. In the material calcined at 800 °C, MgO was intentionally generated as an additional basic oxide phase capable of enhancing surface alkalinity and strengthening electrostatic attraction toward the cationic Auramine O molecules at pH values above pH_PZC. Carbon, originating from controlled partial organic burnout during the Pechini process, was retained within the structure to increase electrical conductivity, provide π–π interaction domains for aromatic dye binding, and contribute to improved surface area and porosity. The integration of these phases into a single multiphase nanohybrid was therefore not arbitrary but was guided by the objective of combining oxide-based electrostatic adsorption, spinel structural robustness, basic surface enhancement, and carbon-assisted interfacial interactions within one synergistic framework. Beyond dye adsorption, multiphase oxide/carbon nanohybrids of this type are relevant to a broad spectrum of environmental and functional applications. Mixed metal oxides such as CuO and Mg-based spinels have been widely investigated for heavy metal ion sequestration, photocatalytic degradation of organic contaminants, antibacterial activity, and catalytic oxidation reactions due to their redox activity and surface hydroxyl functionality. The presence of carbon domains further extends potential applications to electrochemical energy storage, electrocatalysis, and sensing platforms by improving electrical conductivity and interfacial charge transfer. In addition, spinel-type oxides are known for their thermal stability and mechanical robustness, making them suitable for high-temperature catalytic systems and multifunctional ceramic coatings. Therefore, although the present study focuses on Auramine O removal, the designed CuO/MgCr₂O₄/C and CuO/MgCr₂O₄/MgO/C nanohybrids possess structural and physicochemical features that could enable their utilization in wastewater treatment, pollutant immobilization, heterogeneous catalysis, and advanced environmental remediation technologies. Future work will investigate additional applications of the prepared nanohybrids, particularly their catalytic activity and energy-related functionalities such as electrocatalysis and energy storage behavior, and the results will be presented in subsequent publications.

Materials and methods

Materials

Tartaric acid (C₄H₆O₆), chromium nitrate nonahydrate (Cr(NO₃)₃.9H₂O), sodium hydroxide (NaOH), polyethylene glycol 400 (H(OCH₂CH₂)_nOH), hydrochloric acid (HCl), magnesium nitrate hexahydrate (Mg(NO₃)₂.6H₂O), potassium chloride (KCl), copper acetate monohydrate (Cu(CH₃COO)₂.H₂O), and Auramine O dye (C₁₇H₂₁ClN₃) were obtained from Sigma-Aldrich and employed directly without additional purification.

Synthesis of CuO/MgCr₂O₄/C and CuO/MgCr₂O₄/MgO/C nanohybrids

The synthesis route depicted in Fig. 1 was performed through a Pechini sol–gel strategy based on organic complexation and polymeric network formation. A chelating solution was first prepared through dissolving 35 g of tartaric acid in about 100 mL of deionized water. Separately, 20 g of Mg(NO₃)₂.6H₂O, 20 g of Cr(NO₃)₃.9H₂O, as well as 20 g of Cu(CH₃COO)₂.H₂O were solubilized in 200 mL of deionized water to obtain a uniform mixed precursor solution. After that, the tartaric acid solution was then combined with the metal precursor solution with constant agitation for a duration of 30 min to ensure effective coordination between the metal ions and the organic ligand. After this step, 15 mL of polyethylene glycol 400 was introduced as a polycondensation facilitator, and the blend was heated at 200 °C under constant stirring until the solvents were fully removed and a consolidated gel-like mass was formed. The resulting dried material was ground into a powder and calcined for 3 h at 600 °C or 800 °C to generate the targeted nanohybrids. The product obtained at 600 °C was labeled MCC600 and corresponds to the CuO/MgCr₂O₄/C nanohybrid, while the material produced at 800 °C was labeled MCC800 and corresponds to the CuO/MgCr₂O₄/MgO/C nanohybrid.

Fig. 1.

Schematic diagram of the Pechini sol gel procedure utilized to produce CuO/MgCr₂O₄/C and CuO/MgCr₂O₄/MgO/C nanohybrids.

Instrumentation

Crystal phase identification for all prepared samples was carried out applying an X-ray diffraction diffractometer (XRD, D8 Discover, Bruker, USA). Microstructural features as well as elemental signals were obtained applying a field emission scanning electron microscope equipped with energy dispersive X-ray spectroscopy (FE-SEM/EDX, Quanta 250 FEG, Thermo Fisher Scientific, USA). High-resolution imaging was performed with a high-resolution transmission electron microscope (HR-TEM, JEM 2100Plus, JEOL Ltd, Japan) to assess the fine nanostructure and particle characteristics. Fourier transform infrared (FTIR) spectra were recorded using a Nicolet IS10 spectrometer (Thermo Fisher Scientific, USA) in the range of 400–4000 cm⁻¹ to identify surface functional groups and to monitor changes before and after dye adsorption. Quantification of Auramine O in the treated solutions was conducted applying a UV–Vis spectrophotometer (Cintra 3030, GBC, Australia).

Removal of Auramine O dye pollutant from aqueous solutions

Batch adsorption tests were performed under the operating windows listed in Table 1 using a magnetic stirrer to evaluate the influence of pH (2–10), interaction time (10–100 min), temperature (298–328 K), as well as preliminary Auramine O concentration (100–400 mg/L) on the removal efficiency of the synthesized nanohybrids. Upon completion of each adsorption test, the solid phase was isolated through centrifugal separation, and the remaining Auramine O concentration in the clear filtrate was determined using a UV–Vis spectrophotometer at 432 nm.

Table 1.

Experimental parameters affecting the removal of Auramine O using synthesized nanohybrids.

Influence	Studied conditions
	V (mL)	Co (mg/L)	T (K)	W (g)	t (min)	pH
pH	150	300	298	0.1	300	2–10
Contact time	150	300	298	0.1	10–100	10
Solution temperature	150	300	298–328	0.1	60 (MCC600) 80 (MCC800)	10
Concentration of Auramine O dye	150	100–600	298	0.1	60 (MCC600) 80 (MCC800)	10

To ensure that all subsequent adsorption experiments were conducted under equilibrium conditions, the contact time was optimized by monitoring Auramine O uptake over 10–100 min. The adsorption profiles reveal that MCC600 reaches a steady state after 60 min, whereas MCC800 requires 80 min to attain equilibrium, as no further improvement in removal efficiency was detected beyond these durations. Accordingly, contact times of 60 min for MCC600 and 80 min for MCC800 were fixed in all later parametric, isotherm, regeneration, and interference studies to guarantee complete surface saturation and consistent comparison among the investigated operating variables.

The removal percentage (% R) was calculated using Eq. 1^61,62. The adsorption capacity (Q, mg/g) was calculated using Eq. 2^9,63.

1

2

In these expressions, C_o and C_e are the initial and equilibrium dye concentrations. V is the solution volume, and W is the adsorbent mass.

Regeneration of the Auramine O-loaded adsorbents was performed by contacting the spent solids with 100 mL of HCl at 1 M, 1.5 M, or 2 M under magnetic stirring for 60 min, then collecting the eluate for dye analysis. The desorption efficiency was calculated using Eq. 3⁶⁴.

3

In this expression, C_d denotes the dye concentration in the desorption medium, and V_d is the eluent volume.

Reusability experiments were conducted for five consecutive cycles by treating 0.1 g of the adsorbent with 150 mL of Auramine O solution at a concentration of 300 mg/L at pH 10 and 298 K while applying contact times of 60 min for MCC600 and 80 min for MCC800. After each adsorption step, the spent material was regenerated using 100 mL of 2 M HCl and then reused in the next cycle under the same operating conditions to evaluate the stability and durability of the nanohybrids.

Competitive adsorption experiments were performed to examine how coexisting ions affect Auramine O uptake by MCC600 and MCC800. The operating conditions were 298 K and pH 10 with Auramine O adjusted to 300 mg/L and the dye to competitor ratio maintained at 1:1 on a molar basis. Coexisting species were introduced as Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl⁻, NO₃⁻, and SO₄²⁻ together with crystal violet and acid blue 25 to simulate common ionic and dye components in aqueous systems. Batch tests were carried out by contacting 0.1 g of adsorbent with 150 mL of Auramine O solution containing the selected competitor. The suspensions were magnetically stirred for 60 min with MCC600 and for 80 min with MCC800 to reach equilibrium. Residual Auramine O in the supernatant was quantified by UV Vis spectrophotometry.

Measurement of the pH_PZC of the developed adsorbent substances

The pH_PZC of MCC600 and MCC800 nanohybrids was assessed via batch experiments in 50 mL of 0.01 M KCl serving as the background electrolyte, where the initial pH (pH_I) of each suspension was controlled, then 0.15 g of the nanohybrid was incorporated, and the mixtures were stirred for 24 h before taking the final pH reading (pH_F). The pH_PZC was obtained from the △pH versus pH_I plot after calculating △pH by Eq. 4 and identifying the pH_I value at which △pH equals zero ^65–67.

4

Results and discussion

Characterization results

XRD

As shown in Fig. 2A, the XRD pattern of the MCC600 nanohybrid confirms the formation of monoclinic CuO that matches card No. JCPDS-00-005-0661 and cubic MgCr₂O₄ that matches card No. JCPDS-00-010-0351. The characteristic CuO reflections appear at 2θ = 32.56, 38.91, 46.35, 48.84, 58.23, 61.68, 66.37, 68.16, 72.45, and 75.20 and they are indexed to (110), (200), (− 1 1 2), (− 2 0 2), (202), (− 1 1 3), (− 3 1 1), (220), (311), and (004), respectively. The MgCr₂O₄ reflections occur at 2θ = 18.48, 30.34, 35.60, 37.11, 43.31, 53.68, 57.39, and 63.06 and they are indexed to (111), (220), (311), (222), (400), (422), (511), and (440), respectively. In Fig. 2B, the MCC800 nanohybrid exhibits the same CuO phase with monoclinic symmetry and card No. JCPDS-00-005-0661 together with the MgCr₂O₄ spinel phase with cubic symmetry and card No. JCPDS-00-010-0351 and it additionally reveals cubic MgO corresponding to card No. JCPDS-01-078-0430. The MgO peaks are observed at 2θ = 62.23 and 78.65 and they are indexed to (220) and (222), respectively. The crystallite size was calculated using the Debye–Scherrer equation⁶⁸ employing all clearly resolved reflections belonging to the detected crystalline phases in each sample, and the final reported value represents the arithmetic average obtained from all selected peaks. Besides, the average crystallite size increases from 60.97 nm for MCC600 to 75.35 nm for MCC800 which is consistent with thermally driven crystallite growth at the higher treatment condition where enhanced atomic diffusion promotes coalescence and grain growth while reducing lattice defects and microstrain.

Fig. 2.

XRD diffractograms of (A) MCC600 and (B) MCC800 nanohybrids.

EDX

As shown in Fig. 3, the EDS spectra of MCC600 (Fig. 3A) and MCC800 (Fig. 3B) confirm the presence of C, O, Mg, Cr, and Cu, which supports the successful formation of the intended oxide-based nanohybrids. Table 2 indicates that the surface elemental distribution changes between the two samples, which is expected because increasing the thermal treatment promotes further decomposition of the organic matrix produced by the Pechini sol–gel approach. The carbon signal is mainly associated with residual species originating from tartaric acid that chelates the metal ions and from polyethylene glycol 400 that builds the polymeric network, and at the higher temperature these organics are more completely oxidized, so the measured carbon contribution decreases. The remaining differences in elemental percentages arise from temperature-driven changes in phase development and surface exposure, where crystallization and grain growth can alter which oxide domains dominate the particle surface, leading to an apparent redistribution of Cu and Cr, while Mg and O vary more moderately because they are common constituents of the major oxide phases.

Fig. 3.

EDS spectral profiles of (A) MCC600 and (B) MCC800 nanohybrids.

Table 2.

Elemental composition of MCC600 and MCC800 nanohybrids.

Nanohybrid	Atomic percentages
	% C	% O	% Mg	% Cr	% Cu
MCC600	10.4	45.7	18.4	6.2	19.3
MCC800	7.8	44.8	19.1	11.5	16.8

Although EDX confirms the presence of elemental carbon, it does not define its bonding state; therefore, we do not claim carbon incorporation (doping) into the CuO or MgCr₂O₄ lattices. Instead, the carbon signal is attributed to a residual carbonaceous phase originating from incomplete decomposition of the Pechini polymeric matrix at 600 °C, while the lower carbon fraction at 800 °C reflects more effective organic burnout at higher temperature. This interpretation is consistent with Pechini-derived oxide/carbon composites reported in the literature⁶⁹, where carbon-rich products form at 600 °C due to incomplete thermal decomposition of organic precursors, whereas 800 °C promotes more extensive removal of organic residues and a carbon-reduced composite.

Figure 4 presents the elemental mapping images of MCC600 (Fig. 4A) and MCC800 (Fig. 4B) nanohybrids, which provide clear evidence of the spatial distribution of the constituent elements within the synthesized structures. In both samples, the mapping images confirm the homogeneous dispersion of C, O, Mg, Cr, and Cu across the examined regions without noticeable elemental segregation or clustering. The carbon mapping further demonstrates that the carbon phase derived from the Pechini sol–gel process is not localized but rather well-dispersed throughout the oxide framework, which is essential for providing π–π interaction domains and enhancing interfacial contact with dye molecules. Comparing MCC600 and MCC800, the elemental distribution remains homogeneous in both cases, although MCC800 exhibits relatively stronger and more defined oxide signals, consistent with enhanced crystallization and organic burnout at higher calcination temperature.

Fig. 4.

Mapping images of (A) MCC600 and (B) MCC800 nanohybrids.

FE-SEM

As shown in Fig. 5A, the MCC600 nanohybrid is composed of strongly agglomerated particles that appear as irregular compact clusters with a cauliflower-like texture, and the surface is decorated with fine quasi-spherical nanoparticles. Figure 5B shows that MCC800 exhibits a more homogeneous granular morphology that is dominated by densely packed quasi-spherical nanoparticles with clearer interparticle necks and small intergranular voids. The difference between the two structures is mainly attributed to the higher thermal treatment for MCC800, which enhances the decomposition of the residual organic matrix from the Pechini sol–gel route and promotes improved crystallization and particle rearrangement, which produces a more uniform nanoparticle assembly, while MCC600 retains a less matured framework where incomplete burnout and limited diffusion favor the formation of coarser, irregular aggregates and stronger agglomeration.

Fig. 5.

FE-SEM morphological features of (A) MCC600 and (B) MCC800 nanohybrids.

HR-TEM

Figure 6A shows MCC600 as an assembly of ultrafine quasi-spherical nanocrystallites that form highly agglomerated porous clusters. The gradual contrast and the blurred particle boundaries indicate a high fraction of very small particles, which is consistent with restrained crystal growth at the lower treatment condition. Figure 6B shows MCC800 with larger faceted polyhedral particles together with plate-like and ellipsoidal grains that are joined into dense aggregates. The clearer faceting and the larger particle size are attributed to enhanced atomic diffusion at the higher temperature, which promotes coalescence and sintering of neighboring nanocrystallites and converts the initially fine nuclei into more mature grains. The observed difference therefore reflects temperature-driven structural evolution of the Pechini-derived network, where more organic removal and improved crystallinity at the higher condition yield coarser, well-defined particles compared with the finer nanosphere-rich texture in MCC600.

Fig. 6.

HR-TEM morphological features of (A) MCC600 and (B) MCC800 nanohybrids.

Figure 7 shows that MCC600 exhibits a noticeably higher N₂ uptake than MCC800 over the whole relative pressure range, confirming a more developed porous structure and larger accessible surface. This trend is consistent with the quantitative textural parameters in Table 3, where MCC600 presents a BET surface area of 14.15 m²/g compared with 5.72 m²/g for MCC800, along with a higher total pore volume of 3.03 × 10⁻² cc/g for MCC600 versus 1.26 × 10⁻² cc/g for MCC800. Despite this difference in surface area and pore volume, both materials exhibit comparable average pore diameters within the mesoporous range, namely 4.32 nm for MCC600 and 4.43 nm for MCC800, indicating that the main change with higher calcination temperature is the loss of accessible porosity rather than a shift in pore size regime. The lower adsorption uptake of MCC800 in Fig. 7 can therefore be attributed to thermally induced particle coalescence and framework densification at 800 °C, which reduces the number of accessible pores and the exposed surface area. Consequently, the higher N₂ uptake and superior textural properties of MCC600 provide a clear structural basis for its enhanced Auramine O adsorption performance by offering more available active sites and facilitating mass transfer within the porous network.

Fig. 7.

N₂ adsorption/desorption isotherms of MCC600 and MCC800 nanohybrids.

Table 3.

Surface textures of MCC600 and MCC800 nanohybrids.

Surface textures	Sample
	MCC600	MCC800
BET surface area (m2/g)	14.15	5.72
Total pore volume (cc/g)	3.03 × 10−2	1.26 × 10−2
Average pore diameter (nm)	4.32	4.43

Optimization

Effect of solution pH on removal efficiency

Figure 8 shows that Auramine O removal is very low at pH 2, with MCC600 achieving 2.35% and MCC800 achieving 1.68%, while it becomes much higher at pH 10, where MCC600 reaches 96.77% and MCC800 reaches 64.58%.

Fig. 8.

Influence of pH on the uptake efficacy of Auramine O utilizing MCC600 and MCC800 nanohybrids.

Mechanism of adsorption Figure 9 indicates pH_PZC values of 7.32 for MCC600 and 8.55 for MCC800, which means that at pH of 2 the dye solution is below pH_PZC and the surfaces are mainly positively charged. Under these conditions, electrostatic repulsion between the positively charged surface as well as the cationic dye suppresses adsorption. At pH 10, the solution is above pH_PZC, and the surfaces become negatively charged, which promotes electrostatic attraction and a sharp increase in removal efficiency. Figure 10 summarizes these two regimes, and the higher efficiency of MCC600 at pH 10 suggests a stronger affinity and greater availability of effective adsorption sites under alkaline conditions.

Fig. 9.

pH_PZC of MCC600 and MCC800 nanohybrids.

Fig. 10.

Removal mechanism of Auramine O using MCC600 and MCC800 nanohybrids.

FTIR analysis was performed before (Fig. 11A) and after (Fig. 11B) Auramine O adsorption on MCC600 as a representative example to elucidate the underlying adsorption mechanism. Figure 11 provides direct molecular-level insight into the adsorption mechanism of Auramine O onto MCC600 through simultaneous preservation of oxide lattice vibrations and emergence of dye-specific bands. The bands at 405, 474, 633, and 757 cm⁻¹, retained before and after adsorption, correspond respectively to M–O lattice bending, Cu–O stretching of monoclinic CuO, and Cr–O and Mg–O vibrations of the MgCr₂O₄ spinel framework. The stability of these vibrations confirms that the crystalline oxide matrix remains structurally intact, indicating that dye removal proceeds via surface interaction rather than structural transformation or phase change. The slight shift from 888 to 881 cm⁻¹, assigned to surface oxygenated/carbonate-type species, reflects modification of surface electronic density after adsorption, suggesting electrostatic interaction between negatively charged surface sites and the cationic Auramine O molecules. Similarly, the displacement of the C–O stretching band from 1116 to 1136 cm⁻¹ indicates direct involvement of surface oxygen-containing functional groups in binding, most likely through hydrogen bonding or electrostatic attraction. The shift from 1440 to 1482 cm⁻¹ is tentatively assigned to surface carbonate-related vibrations; however, this region can also overlap with contributions from other surface species such as CH₃ bending modes and/or carboxylate (COO⁻) vibrations from oxygenated surface residues or adsorbed dye fragments. Therefore, the band displacement is interpreted more generally as evidence of surface functional-group perturbation after adsorption, consistent with electrostatic interaction under alkaline conditions (pH > pH_PZC). More importantly, the appearance of new bands at 819 and 943 cm⁻¹, assigned to aromatic C–H bending vibrations, and the strong aromatic skeletal vibration at 1592 cm⁻¹, confirm immobilization of the dye’s conjugated chromophore onto the adsorbent surface. This indicates π–π stacking interactions between the aromatic rings of Auramine O and the carbon domains within MCC600. Additionally, the emergence of 1040 and 1323 cm⁻¹ bands, corresponding to C–N stretching vibrations of the dye’s amine groups, together with the 1392 cm⁻¹ CH₃ deformation band, confirms that the amine-containing moieties participate in surface binding. These interactions likely occur via electrostatic attraction and possible hydrogen bonding with surface hydroxyl groups^64,66. Collectively, the preserved oxide lattice bands, the adsorption-induced shifts of surface functional-group vibrations, and the emergence of aromatic and amine-related dye bands in Fig. 11 demonstrate that Auramine O removal occurs through a synergistic mechanism dominated by electrostatic attraction between negatively charged oxide surfaces and the cationic dye, reinforced by π–π stacking interactions with carbon domains and hydrogen bonding with surface oxygen functionalities, without altering the crystalline structure of MCC600.

Fig. 11.

FTIR spectra of MCC600 (A) before and (B) after adsorption of Auramine O dye.

Role of contact time in Auramine O uptake

Figure 12 indicates that Auramine O uptake increases quickly at the beginning and then slows as equilibrium is approached, which is consistent with rapid binding on readily available surface sites followed by a diffusion-limited stage as the number of vacant sites decreases. After 10 min, MCC600 achieves 47.84% removal, whereas MCC800 shows 25.84%, which demonstrates faster early-stage adsorption for MCC600 under identical conditions. At the equilibrium time, MCC600 reaches 96.71% at 60 min, while MCC800 reaches 64.42% at 80 min, which confirms that MCC600 provides both higher efficiency and a shorter time to reach steady state. The removal does not improve at times beyond 60 min for MCC600 or beyond 80 min for MCC800 because the adsorbent surfaces become saturated.

Fig. 12.

Time-dependent uptake of Auramine O by MCC600 and MCC800 nanohybrids.

The adsorption kinetics were evaluated using the pseudo-first-order model expressed by Eq. 5 and the pseudo-second-order model expressed by Eq. 6^70–72.

5

6

In these equations, Q_e is the adsorption capacity at equilibrium and Q_t is the adsorption capacity at time t, while K₁ is the pseudo first-order rate constant, and K₂ is the pseudo second-order rate constant.

Figure 13A–B shows that the pseudo-first-order plots provide an appropriate linear fit for both MCC600 and MCC800, and Table 4 confirms this behavior because the calculated Q_e values from the pseudo-first-order model are very close to the experimental capacities (Q_Exp) with R² values of 0.9999 for both samples. By contrast, the pseudo-second-order model gives lower R² values and Q_e values that deviate more strongly from Q_Exp, which indicates that Auramine O adsorption on these nanohybrids is better described by the pseudo-first-order kinetics.

Fig. 13.

(A) Pseudo-first-order and (B) pseudo-second-order representation for Auramine O elimination utilizing MCC600 and MCC800 nanohybrids.

Table 4.

Kinetic constants for Auramine O elimination using MCC600 and MCC800 nanohybrids.

Nanohybrid	QExp (mg/g)	Pseudo-1st-order			Pseudo-2nd-order
		K1 (1/min)	R2	Qe (mg/g)	K2 (g/mg.min)	R2	Qe (mg/g)
MCC600	435.21	0.06734	0.9999	433.76	0.0001227	0.9964	552.49
MCC800	289.91	0.05048	0.9999	287.92	0.0001376	0.9959	367.65

Role of temperature in Auramine O uptake

Figure 14 illustrates a clear temperature effect on Auramine O removal for both materials between 298 and 328 K. At 298 K, MCC600 delivers 96.71% removal, whereas MCC800 provides 64.42%, which highlights the superior performance of MCC600 under identical conditions. When the solution temperature rises to 328 K, the elimination efficacy decreases to 90.81% for MCC600 and to 54.43% for MCC800. The reduction over this temperature interval equals 5.90% for MCC600 and 9.99% for MCC800, which indicates that MCC800 experiences a larger performance drop as temperature increases. Despite this decline, MCC600 remains consistently more effective than MCC800 at both temperatures, and the same overall decreasing tendency is observed for the two samples, as shown in Fig. 14.

Fig. 14.

Influence of solution temperature on the uptake percentage of Auramine O by MCC600 and MCC800 nanohybrids.

Figure 15 shows the linear Van’t Hoff relationships for Auramine O adsorption on MCC600 and MCC800, and Table 5 compiles the associated thermodynamic parameters that describe the adsorption driving forces. The thermodynamic parameters were obtained from Eqs. 7, 8, and 9^73,74.

7

8

9

Fig. 15.

Van’t Hoff analysis based on the ln K_d–1/T plots for Auramine O elimination utilizing MCC600 and MCC800 nanohybrids.

Table 5.

Thermodynamic parameters for Auramine O adsorption on MCC600 and MCC800 nanohybrids.

Nanohybrid	△So (kJ/mol.K)	△Ho (kJ/mol)	△Go (kJ/mol)
			298	308	318	328
MCC600	0.06807	− 29.76	− 50.05	− 50.73	− 51.41	− 52.09
MCC800	0.02838	− 11.01	− 19.47	− 19.75	− 20.03	− 20.32

In these equations, K_d is the distribution coefficient, T is the absolute temperature, R is the universal gas constant, while ΔH°, ΔS°, and ΔG° are the standard enthalpy, entropy, and Gibbs free energy changes, respectively.

As reported in Table 5, ΔH° is negative for both MCC600 and MCC800, which verifies that adsorption proceeds exothermically, and the magnitude of ΔH° remains below 40 kJ/mol, which supports a physical adsorption mechanism. The ΔG° values are negative at 298, 308, 318, and 328 K for each sample, which indicates that adsorption is spontaneous throughout the studied temperature range. The positive ΔS° values for MCC600 and MCC800 further indicate feasibility because the overall disorder increases at the adsorbent solution interface during uptake of the dye. The larger negative ΔH° and more negative ΔG° values obtained for MCC600 compared with MCC800 indicate a stronger thermodynamic preference for Auramine O adsorption, which is consistent with the trend evident in Fig. 15 and Table 5.

Although ΔG° values more negative than − 20 kJ/mol are often associated with chemisorption, adsorption classification should not rely solely on the magnitude of ΔG° and must be assessed together with ΔH° and the overall interaction picture. In the present study, the moderate ΔH° values below 40 kJ/mol support that the adsorption is mainly governed by physical interactions such as electrostatic attraction and hydrogen bonding, while a limited contribution from surface complexation or coordination with metal–oxide hydroxyl sites cannot be excluded⁷⁵. Therefore, Auramine O uptake is best described as predominantly physisorption-controlled with minor chemisorptive contributions at specific high-energy surface sites, which explains the strong negative ΔG° values without contradicting the thermodynamic enthalpy criterion.

Role of starting concentration in Auramine O uptake

Figure 16 establishes that the elimination efficiency declines as the starting concentration of Auramine O increases for both adsorbents, and the decrease is substantially larger for MCC800. At 100 mg/L, MCC600 attains 98.94% removal, whereas MCC800 achieves 92.71%, which shows that MCC600 provides stronger dye capture under dilute conditions. At 350 mg/L, the removal drops to 83.07% for MCC600 and to 55.52% for MCC800, which indicates that the higher solute loading overwhelms the available binding sites and weakens the overall removal performance. The consistently higher values for MCC600 across the concentration range confirm that it sustains better removal performance than MCC800, and this behavior is clearly reflected in Fig. 16.

Fig. 16.

Influence of preliminary Auramine O concentration on elimination of dye by MCC600 and MCC800 nanohybrids.

Figure 17 together with Table 6, summarizes the isotherm analysis for Auramine O uptake on MCC600 and MCC800 using the Langmuir (Fig. 17A) and Freundlich (Fig. 17B) approaches. The Langmuir model is written according to Eq. 10, whereas the Freundlich model is written according to Eq. 11^76–78.

10

11

Fig. 17.

(A) Langmuir and (B) Freundlich adsorption isotherms for Auramine O on MCC600 and MCC800 nanohybrids.

Table 6.

Langmuir and Freundlich isotherm parameters for Auramine O adsorption on MCC600 and MCC800 nanohybrids.

Nanohybrid	Langmuir			Freundlich
	Qmax (mg/g)	R2	K3 (L/mg)	K4 (mg/g)(L/mg)1/n	1/n	R2
MCC600	442.48	0.9994	0.6125	210.98	0.1581	0.6354
MCC800	298.51	0.9997	0.2018	128.08	0.1587	0.6955

In these equations, Q_e is the equilibrium adsorption capacity expressed as the amount of dye retained per unit mass of adsorbent. Q_max denotes the theoretical monolayer capacity whereas K₃ represents the Langmuir affinity constant. K₄ represents the Freundlich capacity constant whereas n reflects adsorption intensity through the term 1/n.

The coefficients of determination reported in Table 6 indicate that the Langmuir model describes the equilibrium data more accurately for both samples because the Langmuir R² values are close to unity compared with the Freundlich R² values. This outcome supports a predominantly monolayer adsorption pattern and aligns with the trends depicted in Fig. 17A, B. The parameters in Table 6 further show that MCC600 has a larger Q_max than MCC800, which confirms its higher capacity for Auramine O under the examined conditions.

Table 7 provides a consolidated comparison of the highest adsorption capacities for Auramine O reported for different adsorbent materials, and it clearly demonstrates that MCC600 and MCC800 exhibit much higher Q_max values than the other systems listed. The values obtained for conventional adsorbents such as activated carbon, Fe₃O₄, cellulose, and several polymer-based and mineral-based composites are relatively low, which indicates limited availability of effective binding sites and weaker interactions with the dye molecules. In contrast, the remarkably high Q_max values achieved by MCC600 and MCC800 reflect their enhanced ability to accommodate a larger amount of Auramine O per unit mass. This superior performance originates from the synergetic influence resulting from the presence of multiple phases within a single nanohybrid structure, which generates a high density of diverse and energetically favorable active sites. Such multiphase integration promotes stronger and more numerous interactions with Auramine O and facilitates more efficient monolayer coverage and pore filling. As a result, MCC600 and MCC800 stand out in Table 7 as highly effective materials for Auramine O adsorption, and they surpass the previously reported adsorbents by a wide margin.

Table 7.

Literature comparison of Auramine O maximum adsorption capacities Q_max and those achieved in this work.

Adsorbent	Qmax (mg/g)	Ref.
Non-imprinted polymer	203.99	54
Activated carbon	12.55	55
ZnS/Cu/activated carbon composite	38.10	56
Hydroxyapatite	132.57	57
Fe3O4	6.11	58
Crosslinked chitosan–kaolin composite	35.85	59
Cellulose	20.00	60
MCC600	442.48	This study
MCC800	298.51	This study

The superior adsorption performance of MCC600 can be partially attributed to its smaller crystallite size. Smaller crystallites generally provide a higher specific surface area than larger crystallites, which increases the number of exposed surface atoms and the density of accessible adsorption sites available for dye binding. This higher accessible surface area promotes faster diffusion of Auramine O toward active centers and enhances the adsorption capacity. In contrast, the larger crystallites observed for MCC800 due to thermal grain growth typically exhibit a lower specific surface area and fewer accessible surface sites, which reduces the availability of adsorption centers and contributes to its comparatively weaker adsorption performance.

Desorption and reuse performance

Figure 18 confirms that stronger acidic conditions promote more effective release of Auramine O from both MCC600 and MCC800. Using 1 M HCl, the desorption reaches 87.19% for MCC600 and 82.24% for MCC800, which designates efficient dye stripping even at the lowest acid concentration examined. Increasing the acid concentration to 2 M raises the desorption to 99.89% for MCC600 and 99.68% for MCC800, which reflects an almost complete recovery of the adsorbed dye and a strong regeneration potential for both nanohybrids. The high desorption efficiency of HCl can be attributed to intensive protonation at the solid–liquid interface, which weakens the dye-adsorbent interactions by neutralizing or reversing the surface charge and by suppressing specific binding contributions, thereby favoring the transfer of Auramine O back into the aqueous phase.

Fig. 18.

Desorption efficiency of Auramine O using HCl eluent for MCC600 and MCC800 nanohybrids.

Figure 19 indicates that the adsorption performance remains largely stable during successive reuse for both MCC600 and MCC800 with only a modest decrease after repeated cycles. At cycle 0, MCC600 achieves 96.71% removal, while MCC800 reaches 64.42%, which establishes the baseline efficiency of each material. By cycle 5, the removal becomes 92.52% for MCC600 and 57.01% for MCC800, which shows that the materials retain most of their initial activity despite multiple regeneration steps. The progressive but limited decline suggests that only a small fraction of active sites becomes less accessible over cycling while the overall adsorption framework remains effective. Across all cycles, MCC600 preserves higher removal than MCC800, and the same gradual downward trend for both samples is evident in Fig. 19.

Fig. 19.

Reusability of MCC600 and MCC800 nanohybrids over five adsorption–desorption cycles.

To evaluate the structural stability of the nanohybrids after the adsorption–regeneration cycles, XRD patterns were collected for the regenerated MCC600 and MCC800 samples and compared with those of the fresh materials. The post-use diffractograms retained the characteristic reflections of the original phases without the appearance of new peaks or any meaningful peak shift, indicating that no significant phase transformation or structural degradation occurred during operation (Figures omitted for brevity). In addition, the filtrates collected after the adsorption and regeneration steps were analyzed by inductively coupled plasma (ICP), and no detectable Mg, Cr, or Cu ions were found, which confirms negligible metal leaching and supports the chemical stability of the prepared nanohybrids under the investigated conditions.

Mechanistically, the acidic eluent protonates surface oxygenated adsorption sites (M–O⁻/M–OH), converting them into neutral or positively charged groups (M–OH and M–OH₂⁺). This protonation competes directly with the cationic Auramine O molecules for the same binding domains, weakens electrostatic attraction, and promotes desorption through charge neutralization or charge reversal when pH becomes far below pH_PZC. Therefore, acid-driven regeneration is primarily governed by competitive proton adsorption and disruption of electrostatic and hydrogen-bonding interactions rather than chemical degradation of the dye. To support this interpretation, FTIR spectra were collected after the desorption step (figure omitted for brevity); the post-desorption spectra retain the characteristic M–O lattice vibrations, confirming preservation of the oxide framework and indicating that adsorption–desorption proceeds via largely reversible surface interactions.

Effect of interference

Table 8 shows that common inorganic ions only moderately affect Auramine O uptake, while dye co-solutes exert the strongest competition for both adsorbents. Relative to the control condition, divalent cations cause the largest decreases among the ions, with Ca²⁺ and Mg²⁺ reducing Q more than Na⁺ and K⁺, whereas the anions Cl⁻, NO₃⁻, and SO₄²⁻ produce only minor reductions. Crystal violet leads to the greatest suppression of adsorption for MCC600 and MCC800, while Acid Blue 25 has a limited influence compared with the cationic dye. Overall, MCC600 maintains higher Q values than MCC800 under all interference conditions, as summarized in Table 8.

Table 8.

Influence of interfering ions on uptake of Auramine O dye employing MCC600 and MCC800 nanohybrids.

Interfering ions	Q of MCC600 (mg/g)	Q of MCC800 (mg/g)	Reduction in Q of MCC600 (mg/g)	Reduction in Q of MCC800 (mg/g)
None (Control)	442.48	298.51	–	–
Na+	432.23	290.53	10.25	7.98
K+	430.33	289.23	12.15	9.28
Mg2+	407.93	271.03	34.55	27.48
Ca2+	401.63	267.03	40.85	31.48
Cl−	441.73	296.49	0.75	2.02
NO3−	441.83	297.49	0.65	1.02
SO42−	438.63	297.03	3.85	1.48
Crystal violet dye	356.43	233.03	86.05	65.48
Acid blue 25 dye	439.93	297.83	2.55	0.68

Evaluation of Auramine O elimination from wastewater

Practical applicability was examined by treating actual laboratory wastewater collected from the students’ chemistry facilities at the College of Science, Imam Mohammad Ibn Saud Islamic University in Riyadh, Saudi Arabia. The sample was fortified with Auramine O to reach an initial concentration of 300 mg/L to mirror the single solute tests while keeping the native ionic background of the real environment unchanged. The characterized wastewater contained K⁺ (12.7 mg/L), Na⁺ (58.4 mg/L), Ca²⁺ (24.3 mg/L), Mg²⁺ (7.9 mg/L), NO₃⁻ (21.6 mg/L), Cl⁻ (71.5 mg/L), HCO₃⁻ (118.0 mg/L), SO₄²⁻ (46.2 mg/L), and PO₄³⁻ (4.6 mg/L), along with trace Cu (0.06 mg/L), Fe (0.23 mg/L), and Zn (0.11 mg/L), and no other detectable dyes besides the added Auramine O. Under the optimized conditions, MCC600 achieved an uptake capacity of 418.12 mg/g, and MCC800 reached 274.13 mg/g, which are marginally below the capacities measured in deionized water at the same starting concentration, 442.48 mg/g and 298.51 mg/g, respectively. The modest reduction is consistent with competition from dissolved ions and partial occupation of surface binding domains by matrix constituents, yet both nanohybrids remained highly effective, and MCC600 consistently delivered higher uptake, which supports their use for polishing dye-contaminated student laboratory effluents.

Conclusions

Despite the large number of adsorbents previously reported for Auramine O removal, most conventional materials suffer from limited adsorption capacity, insufficient structural stability, or single-phase compositions that provide a restricted variety of binding sites. Moreover, many studies focus primarily on adsorption performance without integrating controlled multiphase engineering to enhance synergistic surface interactions. Therefore, a clear research gap exists in the rational design of structurally integrated oxide/carbon nanohybrids capable of delivering both high adsorption capacity and long-term operational stability. The present work addresses this gap by constructing a multiphase CuO/MgCr₂O₄/C (MCC600) and CuO/MgCr₂O₄/MgO/C (MCC800) framework through a controlled Pechini sol–gel route, enabling synergistic electrostatic, hydrogen bonding, π–π, and surface coordination interactions within a single architecture. The structural analyses confirmed that thermal treatment controls phase development and particle growth, which in turn governs surface accessibility and adsorption efficiency. The superior performance of MCC600 arises from its finer nanostructure and higher density of exposed active sites compared with the more coarsened MCC800. The optimum adsorption conditions were achieved under an alkaline medium at pH 10 using 0.1 g nanohybrid in 150 mL dye solution with a preliminary Auramine O concentration of 300 mg/L at 298 K and equilibrium times of 60 min for MCC600 and 80 min for MCC800. Adsorption proceeds through a physical mechanism driven by favorable surface interactions and is well described by monolayer coverage and pseudo-first-order uptake behavior. The materials also show strong tolerance to competing ions and retain most of their capacity after repeated regeneration, which highlights their durability. Successful operation in real laboratory wastewater demonstrates that these nanohybrids are not limited to ideal conditions but can function in complex aqueous matrices. These outcomes indicate that the designed multiphase oxide/carbon framework offers a reliable and scalable option for treating dye-contaminated effluents and provides a foundation for further development of advanced nanostructured adsorbents. From a broader perspective, the findings of this study open new opportunities for tailoring multiphase ceramic–carbon nanostructures for advanced environmental remediation. Future investigations may focus on scaling-up synthesis, continuous-flow system evaluation, selective removal of mixed organic pollutants, and exploring catalytic or energy-related functionalities of these materials. Such developments could further expand the practical impact of the designed nanohybrids beyond batch adsorption systems.

Author contributions

Reem M. Alghanmi (Conceptualization, Visualization, Writing—review and editing); Ehab A. Abdelrahman (Methodology, Conceptualization, Writing—Review & Editing).

Data availability

All data generated or analyzed in this study are included in the article.

Decalarations

Competing interests

The authors declare no competing interests.