Removal of Hexavalent Chromium by Using Sustainable Green Materials as Low-Cost Adsorbents

The possibility of using three types of NaOH-treated tea residues (green tea, pu-erh, and tieguanyin) as low-cost adsorbents for Cr(VI) treatment was investigated. The surface charge, composition, morphology, structure, and functional groups in the obtained biosorbents were characterized by pH zpc , cellulose content, SEM, BET, XRD, and FTIR spectroscopy. The nonlinear pseudo-first-order, pseudo-second-order, and Elovich models were used to investigate adsorption kinetics at various initial concentrations. The adsorption processes were more consistent with the pseudo-second-order kinetic model in the range of 5-50 mg L -1 . The adsorption isotherm at 298 K was described using the nonlinear Langmuir, Freundlich, Temkin-Pyzhev, and Dubinin-Radushkevich models, indicating that the process was favorable and complex with maximum adsorption amounts of 6.15, 19.50, and 12.31 mg g -1 for green tea, pu-erh, and tieguanyin residues, respectively. Thermodynamic analysis revealed that the adsorption was a spontaneous, endothermic process. The results demonstrated that all materials had the potential to successfully remove Cr(VI) from the aqueous solution.