Improving the adsorption efficiency of a low-cost natural adsorbent for the removal of an organic pollutant: optimization and mechanism study†

Abstract

Water scarcity and contamination due to organic pollutants (dyes, antibiotics, pesticides, etc.) have constituted a major problem for humanity in recent decades. In this context, the present work explores the adsorption capacity of modified clay as an adsorbent for the removal of crystal violet (CV) dye from aqueous solutions. The modification combines the basic activation and thermal treatment processes at various temperatures (350 °C to 750 °C). The adsorption performance of the chosen adsorbent was systematically evaluated through batch adsorption experiments using the RSM-Doehlert methodology. The effect of different factors, namely, the adsorbent dose (AD), contact time (CT), and initial CV concentration (IC), at natural pH (pH = 5.29) and room temperature (T = 23 ± 2 °C) on the adsorption capacity of CV was evaluated. The significance of the model was assessed using analysis of variance (ANOVA). Results indicate that the optimum conditions for maximum removal are as follows: AD = 0.5 g L⁻¹, CT = 95 min, and IC = 118.8 mg L⁻¹. Kinetics and isotherms suggest that the experimental data are better described by the nonlinear pseudo-second-order (PSO) model and nonlinear Langmuir isotherm. The adsorption capacity obtained based on the Langmuir isotherm was 1199.93 mg g⁻¹. Moreover, the adsorption process is spontaneous and exothermic, as determined through the thermodynamic study. The proposed adsorption mechanism involves hydrogen bonding, n–π interactions, and cationic substitutions in the interfoliar space of the AC-750 °C adsorbent. This approach offers a promising pathway for incorporating sorbents derived from natural resources, such as clays, into advanced water treatment engineering practices, achieving high adsorption capacity for various organic pollutants, including cefazolin and tetracycline.