Effective elimination of Congo red dye from tannery wastewater using Fe-pillared bentonite: Synthesis, adsorption isotherm, kinetics, and optimization via Box-Behnken design

Abstract

Untreated tannery wastewater with a high concentration of Congo Red (CR) dye can harm the ecosystem and public health. Therefore, Iron-pillared bentonite (Fe-PB) was synthesized and evaluated for its ability to remove CR dye from tannery wastewater. The effect of OH⁻/Fe³⁺ molar ratio within the range of 1.2–2.4 on the physico-chemical properties of Fe-PBs was examined. The solids obtained were characterized by XRF, XRD, BET, and SEM analyses, which confirmed that the pillaring process was successful. The adsorption process was optimized using response surface methodology combined with the Box-Behnken design (RSM-BBD). For both raw bentonite (RB) and Fe-PB_2.0 clays, the optimized conditions were found at 0.03 g adsorbent mass, pH solution 2, 45 °C temperature, and 115 min contact time. The maximum adsorption capacity of CR reached 95.72 mg/g for RB and 175.75 mg/g for Fe-PB_2.0, due to an increase in specific surface area, from 78 m²/g for RB to 190 m²/g for Fe-PB_2.0. The pseudo-second-order kinetic and Freundlich models indicated a better fit with the adsorption data. Moreover, five adsorption-desorption cycles were used in regeneration studies, and the removal rate of Fe-PB_2.0 varied from 98 to 86 %, indicating the material's great potential for reuse. Before and after the adsorption experiments, several physicochemical characteristics of the tannery wastewater were examined, including pH, conductivity, turbidity, BOD₅, COD, and CR content. The study highlights the potential of the pillared bentonite clay as a cost-effective adsorbent for treating tannery wastewater and indicates the effectiveness of the RSM-BBD approach for modeling and optimization.