Enhanced hexavalent chromium (Cr(VI)) removal from aqueous solution by Fe–Mn oxide-modified cattail biochar: adsorption characteristics and mechanism

In the study, a novel Fe–Mn oxide-modified cattail biochar was synthesised and Cr(VI) adsorption on Fe–Mn oxide-modified biochar in water was investigated. XRF, XRD and FTIR results indicated that Fe and Mn were successfully loaded on the surface of Fe–Mn oxide-modified biochar. Batched experiments showed that Cr(VI) adsorption on Fe–Mn oxide-modified biochar fitted well with the pseudo-second-order kinetics model. The Freundlich isotherm adsorption model could better describe the isothermal adsorption process of Cr(VI) onto Fe–Mn oxide-modified biochar. The maximum saturated adsorption capacity data Qm of Fe–Mn oxide-modified biochar at 20°C, 30°C, and 40°C were respectively 46.43, 52.12, and 59.81 mg·g−1. The lower pH contributed to the adsorption of Cr(VI) onto Fe–Mn oxide-modified biochar. The effects of coexisting anions on the Cr(VI) removal efficiency were insignificant. Leaching experiments showed that the leakage risk of Mn2+ during the adsorption process was negligible. The adsorption mechanisms of Cr(VI) onto Fe–Mn oxide-modified biochar mainly involve electrostatic attraction, ligand exchange and complexation. All the results suggested that Fe–Mn oxide-modified biochars could serve as promising adsorbents for Cr(VI) pollution treatment.