Porosity refinement and intensification of triarylmethane dyes adsorption on bauxite residue <i>via</i> chemical activation: parametric optimization and theoretical insights

The widespread commercial application and low-cost synthesis of triarylmethane dyes, particularly in aquaculture as fungicides, have led to their excessive release into aquatic environments, raising serious environmental concerns. This study presents an effective strategy for removing triarylmethane dyes using chemically activated bauxite residue (BR), a waste product of the alumina industry. Chemical activation with ZnCl₂ significantly enhanced the surface area and porosity of BR, as confirmed by Brunauer-Emmett-Teller (BET) analysis and electron microscopy, leading to improved dye adsorption capacity. Response surface methodology (RSM) was employed for process optimization, achieving a removal efficiency of over 96% under optimal conditions: adsorbent dose of 40 g/L, dye concentration of 40 mg/L, pH 7, temperature 30°C, and stirring speed 150 rpm. The adsorption capacity at equilibrium (qₘ) reached 64.93 mg/g and 41.84 mg/g for malachite green (MG) and brilliant green (BG), respectively. Model validation revealed high reliability, with correlation coefficients (R2) of 0.989 (MG) and 0.977 (BG), and adjusted/predicted R2 values of 0.955/0.906 (MG) and 0.962/0.916 (BG), respectively. Among the kinetic models tested, the Eovich model provided the best fit (R2 ≈ 0.99) for MG, suggesting a chemisorption-dominated mechanism. Additionally, the treated effluent exhibited no antibacterial effect against Escherichia coli, indicating its suitability for reuse. These findings highlight the potential of ZnCl₂-activated BR as a sustainable and efficient adsorbent for removing triarylmethane dyes, with promising prospects for further scale-up and environmental deployment.