Preparation of low-cost biochar from discarded tea waste for aqueous phase removal of hexavalent chromium [Cr(VI)]: kinetics, isotherm, and recyclability analysis

The paper reports the application of iron–Jatropha leaf extract functionalized biochar derived from discarded tea waste to remove hexavalent chromium from aqueous matrices. The surface of the FeJa@TB was prepared from discarded tea waste and was functionalized by FeSO4, FeCl3, and Jatropha leaf extract. The batch mode adsorption experiments with various experimental conditions such as pH, contact time, initial adsorbate concentration, and temperature, were conducted to evaluate the ability of the adsorbent to remove Cr(VI). Kinetics of the sorption were analyzed by fitting the sorption data on various models namely, pseudo-first-order, pseudo-second-order, and intra-particle diffusion models. The kinetic study concluded that chemisorption was the chief mechanism for the adsorptive removal of Cr(VI) as the sorption followed a pseudo-second-order model. Sorption data fitted better to the Freundlich isotherm model, suggesting multilayer Cr(VI) coverage onto the biochar surface. Thermodynamic parameters ΔG°, ΔH°, and ΔS° were computed to observe the influence of temperature on the process of sorption. The study concluded that the adsorptive removal of Cr(VI) onto biochar was non-spontaneous and exothermic with fewer randomness at the solid–liquid interface. The prepared biochar was recyclable up to six times with insignificant removal efficiency loss.