Exploring Clay-Based Catalysts for Wastewater Remediation: Catalytic Degradation of Congo Red Using Montmorillonite and Response Surface Methodology

Introduction

Freshwater is Earth’s most vital resource, essential for supporting human life and ecosystems. However, modern agricultural and industrial activities are threatening this finite supply through pollution. The increasing demand for food and housing is exacerbating the problem, leading to higher volumes of industrial waste and sewage being dumped into rivers and nearby land—a trend that shows no signs of slowing down ¹. According to the NORMAN Network’s findings, water bodies are contaminated with numerous hazardous substances, such as agricultural chemicals, heavy metals, petroleum byproducts, and artificial dyes. Such irresponsible practices will inevitably impose a heavy burden on present and future generations. Experts predict that worldwide pollution levels will keep increasing, resulting in severe consequences ². According to the United Nations WWAP, global water demand is expected to exceed 55% by the end of the next twenty years ³.Recently, several researchers, particularly those associated with ecological science, have emphasized the importance of synthetic dyes as harmful pollutants. The WHO has classified synthetic dyes as one of the ten most prevalent chemical contaminants ⁴.

Congo red (CR) is a synthetic dye often used in many consumer items, such as fabrics, hair dyes, leather, and ceramics, mostly to augment their aesthetic appeal ^5,6. This dye may be categorized according to its application technique, chemical composition, solubility, and constituent elements ⁷. CR has two main parts: the auxochrome, which helps it dissolve better and stick to things, and the chromophore, which gives the dye its color⁸. Despite its extensive use, an increasing amount of research has shown negative health effects associated with exposure to this dye. Exposure to this dye can lead to health issues, such as an elevated heart rate ⁹, malfunction of the reproductive system ¹⁰, and respiratory difficulties¹¹. The toxicological effects are mostly ascribed to the aromatic rings and azo functional groups inside the chromophore units of the dye molecule ^12,13. Due to the possible threats to human health and the environment, there is an urgent need to devise and execute sustainable and effective remediation measures to alleviate the effects of this harmful synthetic dye².

Synthetic dyes are highly persistent and difficult to fully break down through natural processes like photolysis, biodegradation, or chemical decomposition. To minimize their release into the environment, several treatment methods can be employed, including ion exchange, chemical precipitation, electrochemical treatment, membrane filtration, and adsorption techniques ^14–16. However, these conventional treatment methods have significant drawbacks, such as incomplete dye mineralization, concentrated waste disposal challenges, high energy demands, costly operations, and excessive sludge production, along with other limitations ^17–19. Consequently, there is a pressing need for highly efficient and advanced technology for the thorough elimination of pollutants from aquatic environments ^20,21. One effective way to treat wastewater is through advanced oxidation processes (AOPs), which can change harmful substances into safe minerals. While direct oxidation can effectively degrade pollutants in wastewater, the need for extreme conditions—such as high temperature and pressure—to decompose certain compounds significantly raises the overall treatment costs ^22–24. Catalytic oxidation stands out as a highly efficient advanced oxidation process, leveraging powerful radicals generated through catalytic reactions under mild conditions. This method harnesses the formation of potent oxidizing radicals capable of degrading nearly all wastewater pollutants. Its core objective is converting synthetic dyes into harmless, non-toxic compounds ²⁵. In catalytic processes, reactions primarily occur on the catalyst’s surface, making key characteristics such as porosity, surface area, selectivity for radical generation, structural stability, particle uniformity, and cost critical determinants of overall catalytic efficiency^22–24. Moreover, Catalytic degradation is an effective technique for decomposing organic contaminants in wastewater using a combination of agents capable of oxidizing, reducing, and accepting electrons.  Hydrogen peroxide (H₂O₂) acts as an oxidizing agent, generating hydroxyl radicals that break down complex pollutants into smaller components.²⁶. Sodium borohydride (NaBH₄) functions as a reducing agent, facilitating the elimination of chlorine or organic contaminants such as azo dyes²⁷. Potassium bromate (KBrO₃) accelerates the process by absorbing electrons, thereby enhancing the stability of intermediate stages and facilitating electron transfer²⁸  . When combined, these agents work together to improve the effectiveness of catalytic degradation, making it a practical option for cleaning wastewater and restoring the environment²⁹.

Bentonite clay (BC), distinguished by its widespread availability, affordability, and exceptional adsorption capability, has become a very promising material for color removal applications ^30,31. Previous studies have confirmed bentonite clay’s efficacy in adsorption and separation processes, underscoring its exceptional cation-exchange capacity, structural stability, and high adsorption affinity for various contaminants ^32,33. To fully harness bentonite clay’s adsorption potential, chemical treatments are typically required to expand its surface area and increase active site availability, thereby dramatically improving its dye removal efficiency³⁴. Bentonite consists of layers including two tetrahedral sheets of silica interspersed with one octahedral sheet of alumina. Furthermore, Bentonite has a lasting negative charge because some aluminum ions (Al³⁺) in the tetrahedral layer are replaced by silicon ions (Si⁴⁺). In the octahedral layer, magnesium ions (Mg²⁺) substitute aluminum ions (Al³⁺). Within the lattice framework, exchangeable cations (Ca²⁺, Na⁺, etc.) neutralize the negative charge. This equilibrium facilitates the adherence of cationic pollutants to the structure ^35–37. Bentonite may be readily organo-modified to adsorb nonionic contaminants by the partition method, in addition to cation adsorption ^38,39 .

The Box-Behnken design is a prevalent and efficient experimental design technique in Response Surface Methodology (RSM) that enhances processes and systems by elucidating the link between many independent variables (factors) and a dependent variable (response)⁴⁰. The design functions at three tiers: low (-1), middle (0), and high (+1). Each combination of factor values happens with equal frequency ⁴¹. Unlike full factorial designs, Box-Behnken does not assess all possible factor combinations, leading to a lower number of experiments. It thoroughly analyzes the interrelations among components, which is crucial for a complete understanding of the process. This architecture effectively handles several variables, often between three and seven⁴³.

This work introduces an innovative method using chemically purified nanostructured Iraqi bentonite clay (NIBC) as an economical and eco-friendly catalyst for the breakdown of Congo Red dye. This study uses a three-part system (H₂O₂, NaBH₄, KBrO₃) for breaking down Congo Red dye, which was improved using Box-Behnken Design (BBD) and evaluated with Response Surface Methodology (RSM), unlike earlier studies that only focused on adsorption or photocatalysis. The addition of scavenger trapping studies helps us understand the main reactive oxygen species at work, showing that superoxide and hydroxyl radicals are the key agents that break down the dye. This detailed study expands the use of natural clays in advanced oxidation processes (AOPs) and develops a scientifically backed model for treating wastewater with locally sourced, modified bentonite, marking a significant improvement in environmental chemistry and sustainable engineering.

Figure 1: The diagram indicates the purification of NIBC.Click here to View Figure

Experimental Part

Instrumentation

A thorough material evaluation was conducted to assess the physicochemical characteristics of the nanostructured Iraqi bentonite clay (NIBC). Field-Emission Scanning Electron Microscopy (FE-SEM, Quanta 4500 FEI, USA) was used to examine the surface morphology and microstructural characteristics of the purified clay. The component composition was determined by energy-dispersive X-ray spectroscopy (EDX), yielding a semi-quantitative assessment of constituent distribution. The crystallinity and phase purity of the clay were evaluated using X-ray Diffraction (XRD) analysis (model to be provided). UV-visible spectrophotometric examination was performed using a PerkinElmer Lambda 25 UV-Vis spectrophotometer (USA) to observe absorbance variations in Congo red (CR) dye at a maximum wavelength (λ_max) of 498 nm, assessing adsorption and catalytic degradation over time.

Chemicals

All reagents used in this work were of analytical quality and utilized as supplied without additional purification. Raw bentonite clay was sourced from the Al-Anbar/Akashat area of the Western Desert in Iraq. Congo red dye (C.I. 22120) was procured from ROTH GmbH (Germany). Oxidizing and reducing agents—hydrogen peroxide (H₂O₂, 30% w/v), sodium borohydride (NaBH₄, 98%), and potassium bromate (KBrO₃)—were obtained from Carl Roth GmbH. Glacial acetic acid (CH₃COOH, 70%), sodium hydroxide (NaOH, 99%), and hydrochloric acid (HCl, 37%) were procured from Merck. Isopropyl alcohol (IPA) was provided by Fluka, while ascorbic acid (AA) was supplied by Scharlau Chemicals. Deionized (DI) water was used in all steps for solution preparation, clay washing, and dilution.

Chemical Purification of Bentonite Clay

The raw bentonite was chemically purified to remove inorganic carbonates that might hinder catalytic surface reactions, hence enhancing its catalytic activity. Sixty grams of the clay sample were suspended in 600 mL of 10% (v/v) glacial acetic acid, continuously stirred, and kept at 80–90 °C for four hours. The progression of CO₂ gas, signifying the elimination of carbonate, was seen till termination. The successful decarbonation was verified using a turbidity test with lime water. The suspension was left to cool at room temperature for 24 hours, thereafter undergoing filtering by vacuum suction. The retained material was repeatedly rinsed with distilled water until the filtrate displayed a neutral pH (~7), so validating the elimination of residual acetic acid. The purified sample was subjected to drying in a hot air oven at 120–130 °C for 5 hours and thereafter stored in a desiccator. The resulting dry mass of 56.8 g signifies the effective extraction of about 3.2 g of carbonate material. This technique guaranteed the preservation and activation of montmorillonite as the predominant mineral phase in the clay.

Catalytic Degradation Experiments

The catalytic efficacy of NIBC in the degradation of Congo red dye was examined at different reagent concentrations using a statistically determined methodology. A constant quantity of NIBC (50 mg) was introduced to 50 mL of CR solution (starting concentration: 10 ppm) and underwent catalytic treatment with varying concentrations of H₂O₂, NaBH₄, and KBrO₃, spanning from 0 to 20 mM. A three-level, three-factor Box-Behnken Design (BBD) inside the Response Surface Methodology (RSM) framework was used to investigate the impacts and interactions of these chemicals on degrading efficiency, improving the process with a limited number of experimental runs (n=16). Before catalytic treatment, the CR solution containing NIBC was subjected to magnetic stirring in darkness for 50 minutes to attain adsorption-desorption equilibrium. The catalytic process commenced with the introduction of oxidizing and reducing agents, and the system was sustained at ambient conditions (pH = 7, 25 ± 2 °C) for 60 minutes. At specified times, 3 mL aliquots were extracted, centrifuged at 10,000 rpm for 1 minute to eliminate solid residues, and the supernatant was evaluated using UV-Vis spectroscopy at 498 nm.

To identify dominant ROS, scavengers were introduced:

Isopropanol (IPA) for hydroxyl radicals (•OH)

p-Benzoquinone (p-BQ) for superoxide anions (O₂⁻•)

Sodium oxalate (Na₂C₂O₄) for holes (h⁺)

Potassium bromate (KBrO₃) for electrons (e⁻) ^44–48.

Table 1: Parameters’ levels examined in the BBD statistical experiment.

Results and Discussion

Surface morphology and elemental composition

The surface charge was ascertained using point of zero charge (pHpzc) analysis. The isoelectric point (zero charge point, pHpzc) was found to be about pH 8.4, where montmorillonite has no charge; therefore, all experiments were done at neutral pH levels to ensure a steady pH effect ^49–54 .  At the point where the pH differential equals zero, one can determine the pHZC ⁵⁵ .

The SEM image of NIBC, shown in Figures 2A and B, shows a different and very porous structure made of uneven, flat particles. The 15.00 KX zoom shows the clumping of flat and layered particles, which is typical for montmorillonite minerals. The particles have a coarse surface roughness, indicating the existence of many micro- and mesopores, advantageous for adsorption applications. The interconnected porous structure and the existence of gaps within the particles increase the material’s surface area, making it appropriate for wastewater treatment and other adsorption-based applications. The observed shape and structure indicate the presence of montmorillonite, the primary constituent of the material.

Figure 2: SEM images of Bentonite clay at (A) 1μm and (B) 2 μm magnification.Click here to View Figure

Energy-dispersive X-ray (EDX) analysis compared the elemental composition of Iraqi bentonite clay before and after montmorillonite synthesis (Figure 3), revealing dominant peaks for oxygen (63.6%), silicon (18.89%), and aluminum (14.47%) characteristic of aluminosilicates, along with trace amounts of calcium, iron, titanium, sulfur, and chlorine. While these impurities were present in negligible quantities, they may occupy active surface sites and reduce the material’s available surface area, potentially diminishing its adsorption capacity ⁵⁶ .

Figure 3: EDX spectrum and comprehensive elemental ratio map of NIBC.Click here to View Figure

Catalytic degradation

The study employed natural Iraqi bentonite clay-derived NIBC in a catalytic degradation process, systematically evaluating the influence of H₂O₂, NaBH₄, and KBrO₃ through carefully designed experiments and statistical analysis. Key operational parameters and their optimized values are comprehensively outlined in Tables 1 and 2, providing a detailed framework for the reaction conditions.

Response surface methodology

Response Surface Methodology (RSM) is considered as a statistical modeling technique that analyzes experimental data to construct predictive regression equations, enabling systematic optimization of process variables and their interactive effects on degradation efficiency. By integrating designed experiments with empirical results, RSM enhances the understanding of how operational parameters influence reaction dynamics ^57,58.

Box-Behnken design (BBD)

The research used the widely used BBD. A notable advantage of the BBD technique is its ability to reduce the overall number of experiments required to determine the yield of catalytic degradation ^58–61. We executed the BBD, concentrating on three specific variables: H₂O₂, NaBH₄, and KBrO₃. The assessed response variable was the decreasing fraction of CR. Table 2 delineates each independent variable accompanied by three separate levels. Sixteen experimental studies were selected and further examined.

Table 2: Findings of the BBD including fitted and observed values.

 Note:It is important to note that the pH was maintained at 7, and during the one-hour catalytic degradation, all samples were equilibrated for adsorption and desorption in the dark for one hour.

Table 3 shows a clear summary of how well different models explain the breakdown of Congo Red (CR) using montmorillonite as the catalyst. The assessment carefully compares different regression models—linear, two-factor interaction (2FI), quadratic, and cubic—based on important statistics like the sum of squares, degrees of freedom (df), mean square, F-value, and p-value. The quadratic model is the most statistically robust, shown by a high F-value of 32.26 and a low p-value of 0.0004. These measurements demonstrate that the quadratic model substantially improves the fit relative to the 2FI model; hence, it accounts for more variance in the response variable. The model is designated as “suggested,” underscoring its statistical validity and appropriateness for characterizing the observed experimental behavior.The linear model has no statistical significance relative to the mean, as shown by an F-value of 0.1112 and a p-value of 0.9519. This indicates that the linear model helps insignificantly elucidate the variation in the data. The 2FI model, with an F-value of 0.0138 and a p-value of 0.9976, inadequately captures the interacting effects and is considered insufficient. The cubic model has a substantial F-value (18.58) and a minimal p-value (0.0193); nevertheless, it is classified as “aliased.” This indicates confusion among variables owing to an inadequate number of experimental trials, making the cubic model statistically inaccurate and impracticable for analysis. The leftover error, which is calculated as 6.81 over 3 degrees of freedom, is quite low, supporting that the quadratic model fits well. The overall sum of squares is 4841.23 across 16 degrees of freedom, indicating that the model accounts for a considerable extent of the variability. In general, these results highlight that the quadratic model is the best and most important way to represent how Congo Red breaks down using montmorillonite. This model strikes a good balance between being complicated and being able to make accurate predictions, without issues of overfitting or aliasing, making it a great choice for further improvement and study.

Table 3: Model fit summary for catalytic degradation of CR over  montmorillonite.

Table 4 shows the analysis of variance (ANOVA) for breaking down Congo Red (CR), revealing that several factors greatly influence the breakdown process. The main effects of the three chemicals—hydrogen peroxide (H₂O₂), sodium borohydride (NaBH₄), and potassium bromate (KBrO₃)—are important, with p-values of 0.0373, 0.0279, and 0.0217, showing that each one helps improve the degradation process on its own. The squared terms of the three variables (A², B², and C²) are very important, with p-values under 0.0005, showing a strong nonlinear relationship between the amounts of these chemicals and how well degradation happens. Additionally, some interaction terms like A²C and AB² are important, with p-values of 0.0169 and 0.0109, showing that the way some reagents work together, especially in complex ways, is important for the degradation process. Nonetheless, the interaction terms AB, AC, BC, A²B, and the three-way interaction ABC are not statistically significant, suggesting little interactive impact among these components. The ANOVA model is important, showing an F-value of 86.33 and a p-value of 0.0018, which confirms that it effectively explains the differences in the percent degradation of Congo Red. These results underscore the significance of both the individual and interaction impacts of the reagents, particularly their nonlinear contributions, in optimizing catalytic degradation for efficient wastewater treatment.

Table 4: ANOVA for catalytic degradation of CR dye.

The effectiveness of breaking down Congo Red (CR) dye was predicted using regression analysis based on three factors: potassium bromate (KBrO₃, called C), sodium borohydride (NaBH₄, called B), and hydrogen peroxide (H₂O₂, called A). To represent the intricate behavior of the system, the second-order polynomial regression model included linear, interaction, and quadratic factors. The intercept value, which represents the expected deterioration efficiency at the middle values of all variables, was 32.49, as seen in Table 5. The positive number (+2.70) for H₂O₂ in the linear factors shows that it helps improve CR breakdown, probably by increasing the formation of hydroxyl radicals (•OH). NaBH₄ and KBrO₃ had negative coefficients (−3.02 and −3.32, respectively), meaning that high amounts of these substances can lower degradation efficiency due to competing reactions or scavenging of radicals.Significant impacts were also seen in the interactions between the factors. The positive coefficients of the AC and BC terms (0.4047 and 1.56, respectively) showed that H₂O₂ works well with KBrO₃ and NaBH₄ works well with KBrO₃ together. Interestingly, the BC interaction had the most favorable impact, suggesting that KBrO₃ and NaBH₄ work together to strongly affect the degradation process. The curvature of the response surface was reflected in the negative values of the three quadratic components (A2, B, and C) (−13.51, −12.18, and −14.37, respectively), which also indicated that the degrading efficiency was decreased by excessively high or low reagent concentrations. The cubic interaction terms also showed that the system was much more intricate. The negative value for A²C (−6.03) suggests that having a lot of H₂O₂ with KBrO₃ has a negative effect, while the positive value for AB² (5.14) shows that combining H₂O₂ with a higher amount of NaBH₄ greatly improves degradation. In summary, the statistical model showed how important the reagents’ individual and combined impacts were on the degrading process. The results, shown in Table 5, suggest that a second-order polynomial model, such as the one from the Box–Behnken Design, is effective for improving the degradation conditions and accurately predicting how well CR can be removed.

Table 5: Regression equation coefficients for CR dye degradation.

Figure 4 shows a detailed evaluation of how well Congo Red breaks down using a nanocomposite catalyst made with NaBH₄ (NIBC). Figure 4A presents a contour plot showing the interaction effects of H₂O₂ and NaBH₄ concentrations on degradation efficiency. The figure exhibits a symmetrical arrangement of circular contour lines, with the middle red area signifying the peak deterioration efficiency, recorded in experiment 14. The color gradient transitions from blue, denoting the minimal degradation efficiency of 0.49%, to red, signifying the peak efficiency of 34.57%, demonstrating the vital importance of maintaining an appropriate ratio between the oxidant (H₂O₂) and the electron donor (NaBH₄). This nonlinear connection indicates that both excessive and inadequate quantities of either reagent diminish degradation performance, underscoring the need to attain an exact equilibrium for efficient catalysis.Figure 4B illustrates a parity map that contrasts the projected and actual degradation efficiencies of Congo Red. The proximity of data points to the diagonal reference line indicates a robust connection between the experimental outcomes and the model’s forecasts. Even though there are small differences at lower degradation levels, the clustering of high efficiency values near the line shows that response surface methodology (RSM) is effective in modeling and predicting how the system degrades. The results from Figures 4A and 4B together show that the NIBC catalyst is effective in breaking down Congo Red and suggest that the statistical model provides accurate predictions for improving degradation efficiency.

Figure 4: (A) Contour plot for maximum degradation experiment 14, (B) predicted vs. actual degradation of Congo red over NIBC.Click here to View Figure

Scavenger trapping experiments

Table 6 shows how well Congo Red dye breaks down when different trapping agents are used, pointing out the role of various reactive species in the breakdown process. Without a scavenger, the breakdown efficiency reaches its highest at 36.9%, serving as the control where all reactive species can act freely. Adding Na₂C₂O₄, which captures holes, slightly lowers the breakdown to 32.56%, suggesting that holes (h⁺) play a significant role in the process. In the absence of a scavenger, the degradation efficiency peaks at 36.9%, functioning as a control in which all reactive species are unrestricted. The addition of Na₂C₂O₄, a hole scavenger, results in a small decrease in degradation to 32.56%, indicating a considerable involvement of holes (h⁺) in the process. The addition of KBrO₃ (an electron scavenger) further reduces the degradation to 25.33%, indicating the involvement of electrons (e⁻). A notable reduction is seen with IPA (isopropanol), a hydroxyl radical (•OH) scavenger, where the degradation decreases to 16.67%, indicating the vital role of •OH radicals. The greatest significant reduction is seen with ascorbic acid (A.A.), a superoxide radical (•O₂⁻) scavenger, leading to just 9.46% breakdown. This big drop shows that superoxide radicals are the most active particles in breaking down Congo Red, followed by hydroxyl radicals, electrons, and then holes.

Table 6: Scavengers trapping experiments versus percent of degradation for CR

Conclusion

The study demonstrates that nanostructured Iraqi bentonite clay (NIBC) is a cost-effective and environmentally friendly catalyst for breaking down Congo Red (CR), a harmful synthetic dye. Glacial acetic acid significantly improved the surface activity and porosity of NIBC during chemical purification. The process of breaking down Congo Red (CR) was improved by using a method called Box-Behnken Design, which looked at how hydrogen peroxide, sodium borohydride, and potassium bromate affected how well CR was removed. The optimal degradation conditions achieved a maximum CR elimination effectiveness of 36.9% at neutral pH and ambient temperature. Scavenger trapping tests showed that superoxide and hydroxyl radicals are the main reactive oxygen species (ROS), while electrons and holes play supporting roles. The modified natural clay offers local resources for eco-friendly water treatment and allows for the breakdown of pollutants using radicals in gentle conditions without expensive catalysts or harsh working environments. Future studies should explore NIBC’s reuse, stability, effectiveness against other organic pollutants, and its potential for larger or industrial use. Combining this system with other advanced oxidation processes or mixed treatment methods could improve efficiency and support stronger, more eco-friendly water cleaning systems.

Acknowledgment

The authors would like to express their sincere gratitude to the Ministry of Higher Education and Scientific Research in Kurdistan, as well as the University of Zakho, for their invaluable support and contributions to this work

Conflict of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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