Adsorptive removal of hexavalent chromium [Cr(VI)] from water using green-synthesized carbon quantum dot–zinc ferrite nanocomposites

Hexavalent chromium [Cr(VI)] is a harmful, water-soluble pollutant. Groundnut shell-derived carbon quantum dots (CQDs) were functionalized using hydrothermal treatment and coupled with zinc ferrite (ZnFe₂O₄) through co-precipitation to create a GSCQD/ZnFe₂O₄ nanocomposite. Successful conjugation was established by characterization, which produced a magnetically recoverable adsorbent. Batch studies recorded a maximum of 95% Cr(VI) removal at pH 2, 0.2 g/L adsorbent dosage, 20 mg/L initial concentration, and 50 min contact time. Kinetics was pseudo-second-order (R² = 0.9927), indicating chemisorption, while isotherms conformed to Langmuir (qmax = 79.5 mg/g, R² = 0.9993), reflecting monolayer adsorption. Thermodynamic parameters indicated a spontaneous (ΔG < 0), endothermic (ΔH = 15.9 kJ/mol) process. The mechanism of adsorption included electrostatic attraction, surface complexation, pore filling, and Cr(VI) reduction to Cr(III). The results of reusability tests indicated efficiencies of 80.4%, 80.1% and 73.4% in three cycles. Therefore, GSCQD/ZnFe₂O₄ is a suitable, green, and scalable adsorbent for water treatment.