Application of green adsorbent in the removal of malachite green from wastewater: Isotherm, kinetics and toxicity studies

Malachite green (MG), a cationic dye widely used in the textile industry and poultry farming as an antiseptic, is persistent in the environment and poses significant toxicological risks to both flora and fauna. This study investigated the removal of MG from aqueous solutions using a low-cost Moringa oleifera seed husk-derived adsorbent (RMO) in single and binary systems, with Methylene blue (MB) as co-contaminant. Batch adsorption experiments were conducted to evaluate the effects of adsorbent dosage, pH, initial dye concentration, contact time, and temperature on removal efficiency. Optimal MG removal of 95 % and 97 % was achieved at pH 9 for single and binary systems, respectively, attributed to favorable electrostatic interactions above the point of zero charge (pH pzc = 5.7). Increasing adsorbent dosage enhanced removal efficiency to 99.89 %, while equilibrium was attained at 240 and 180 min for single and binary systems, respectively. Kinetic data conformed best to the pseudo-second order model, indicative of chemisorption-controlled adsorption, while equilibrium data were best described by the Dubinin-Radushkevich and Langmuir isotherm models for single and binary systems, respectively. Thermodynamic analysis revealed endothermic adsorption (ΔH = +6.92 and + 21.59 kJ/mol for single and binary systems, respectively), with contrasting spontaneity between the two systems. Post-treatment toxicity evaluation using the Allium cepa bioassay demonstrated a significant reduction in toxicity, with the EC 40 increasing from 12 to 23 mg/L following adsorption, confirming the environmental safety of the treated effluent. These findings establish RMO as an effective, sustainable, and eco-friendly adsorbent for the treatment of dye-contaminated industrial wastewater.