Adsorptive Removal of Cr(VI) from Synthetic Wastewater Using PVA-Capped CuS/Bentonite Nanocomposites: Kinetic and Isotherm Studies

This study investigated the adsorptive removal of Cr (VI) from synthetic wastewater using polymer-capped Copper sulfide incorperated with bentonite nanocomposites . The adsorbent was prepared by homogeneous precipitation method of Copper sulfide nanoparticles using a thiourea-based pyrrolidone ligand, followed by polymer capping agent with PVA (polyvinyl alchol) immobilisation on bentonite clay. Batch adsorption experiments were conducted to evaluate the effects of solution pH, initial Cr(VI) concentration, adsorbent dosage, contact time and temperature. The nanocomposite showed strong pH-dependent adsorption, with optimum Cr(VI) removal at pH 2.0, where HCrO₄⁻ species were favourably attracted to protonated surface sites. Under simulated optimum conditions of 50 mg/L Cr(VI), 0.5 g/L adsorbent dosage, 90 min contact time and 298 K, the composite achieved 96.4% removal and an adsorption capacity of 132.8 mg/g. Equilibrium was reached within 90 min, with rapid uptake during the first 30 min due to abundant vacant active sites. Kinetic modelling showed better agreement with the pseudo-second-order model (R² = 0.997) than the pseudo-first-order model, suggesting that chemisorption contributed significantly to Cr (VI) uptake. The Langmuir isotherm provided the best fit (R² = 0.994), indicating monolayer adsorption on homogeneous active sites, with a maximum capacity of 148.6 mg/g. Regeneration studies showed adsorption or 86.7% removal efficiency after five cycles, confirming good reusability .