Enhanced adsorption of Fe(II) from synthetic wastewater using modified bentonite: Isotherms, kinetics, thermodynamics, and adsorption mechanisms

Abstract

The introduction of Fe²⁺ ions into water systems poses significant environmental risks globally. This study investigates the adsorption efficiency of bentonite from the Kongyrtog deposit in Uzbekistan and its modified forms for Fe²⁺ removal from contaminated water. Batch adsorption experiments were conducted with Fe²⁺ concentrations of 2.0–20 mg/L, at pH 7.0, an adsorbent dose of 1 g/L, and temperatures between 288 and 308 K. The bentonites were named MED (modified with ethylenediammoniumdihydrochloride) and MGD (modified with hexamethylenediammonium dihydrochloride), while NKB refers to natural bentonite. Multiple linear regression (MLR) modeling validated the adsorption process, with high R² correlation coefficients supporting the Langmuir model. Optimal adsorption conditions included a dose of 1 g/L, pH 5.2, a contact time of 40 min, and a temperature of 308 K, with the adsorption kinetics fitting a pseudo-second-order model. Using the Langmuir isotherm model, the maximum adsorption capacities (q_max) were determined as 8.47 mg/g for NKB, 8.51 mg/g for MGD, and 10.84 mg/g for MED. Among the modified bentonites, Fe²⁺ adsorption followed the activity sequence MED > MGD > NKB, with intraparticle diffusion modeling suggesting a two-stage adsorption process. The negative Gibbs free energy values (ΔG°) confirmed the process was spontaneous and endothermic. These modified bentonites offer a cost-effective alternative to activated carbon with the ability to regenerate up to four times. Initial regeneration efficiencies were 78 % for NKB, 83 % for MGD, and 86 % for MED. This study demonstrates the potential of modified bentonites as sustainable adsorbents for Fe²⁺ removal, contributing to advancements in eco-friendly water treatment technologies.