Adsorption performance and mechanism of Fe-loaded biochar derived from waste zanthoxylum branch for removing Cr(VI) from aqueous solution

Zanthoxylum branch became agricultural waste without proper utilization, and the pristine biochar (PB) had the limited removal capacity of Cr(VI) (21.75 mg g −1 ). In this study, zanthoxylum branch was fabricated to modified biochar (MZB) through impregnating with 0.4, 0.6, 0.8, and 1.0 mol L −1 Fe (NO 3 ) 3 solution and then pyrolyzing at 500 and 900 ℃. After sieving by batch adsorption experiment of Cr(VI), the optimal MZB was prepared with impregnating solution concentration of 0.4 mg L −1 and pyrolysis temperature of 900 ℃ (900MZB4), and displayed the outstanding Cr(VI) removal capacity (110.43 mg g −1 ). Besides, the effect of initial solution pH was evaluated, and removal process of Cr(VI) was a reaction of consuming protons. The fitting results of Langmuir model and pseudo-second-order kinetic model confirmed that the Cr(VI) removal reaction was a homogeneous chemisorption. Thermodynamic parameters demonstrated that the adsorption of Cr(VI) was an endothermic spontaneous process. The results of SEM and XRD suggested that zero-valent iron (Fe 0 ) particles were located on 900MZB4 surface and participated in the removal reaction of Cr(VI). The XPS analysis indicated 78.37% of Cr(VI) was reduced to Cr(III) by the cooperation of oxygen-containing functional groups (OFG) and Fe 0 . More OFG and higher surface area (130.36 m 2  g −1 ) revealed by FTIR and BET also contributed to the better characteristic of 900MZB4. In addition, 900MZB4 maintained removal efficiency at 81.22% after sixth regeneration cycles. Therefore, it is feasible to prepare high-efficient adsorbent from zanthoxylum branch for wastewater treatment. Graphical abstract