Adsorption behavior and mechanism of malachite green dye onto natural bentonite from Sa Bot district, Lopburi province, Thailand

Dye-laden wastewater poses a persistent environmental challenge due to its toxicity and resistance to degradation. This study reports the novel application of natural bentonite sourced from Sa Bot district, Lopburi province, Thailand, for the adsorption of cationic malachite green (MG) dye. The distinct geological origin of this bentonite, which has not been previously investigated for MG dye removal, underscores its unique physicochemical properties and local relevance. Comprehensive characterization was performed using BET surface area analysis, FTIR spectroscopy, SEM-EDXS, and determination of the point of zero charge (pHpzc). Batch adsorption experiments systematically examined the effects of pH, adsorbent dose, contact time, initial dye concentration, and temperature. Optimal adsorption occurred at pH 9, with equilibrium being reached within 180 minutes, yielding a maximum adsorption capacity of 35.97mg/g at 30°C. Equilibrium data were well fitted to both Langmuir and Freundlich models, while the Redlich–Peterson model indicated surface heterogeneity. Kinetic analysis confirmed pseudo-second-order behavior, with intraparticle diffusion contributing alongside surface adsorption. Thermodynamic evaluation revealed that the process was endothermic and spontaneous across 20-40°C, as evidenced by negative ∆G values throughout this temperature range. Mechanistic insights highlighted electrostatic interactions, hydrogen bonding, n–π interactions, pore filling, and ion exchange. Furthermore, the bentonite demonstrated excellent reusability, maintaining stable performance over five regeneration cycles. These findings establish locally sourced bentonite as a cost-effective, sustainable adsorbent with practical significance for industrial wastewater treatment.