Chromium(VI) removal from synthetic solution using novel zero-valent iron biochar composites derived from iron-rich sludge via one-pot synthesis

In this study, Fe-rich sludge obtained from coagulation–flocculation of swine wastewater was processed into zero-valent iron (ZVI) biochar composites through one-pot pyrolysis at 700 °C under anoxic (MBCN 700 ) and hypoxic (MBCA 700 ) conditions. X-ray diffraction analysis revealed that α-Fe 0 and γ-Fe 0 were the dominant Fe species in MBCN 700 and MBCA 700 , respectively. The effects of different process parameters, including composite dosage, initial Cr(VI) concentration, pH, dissolved O 2 (DO), and contact time on Cr(VI) removal from synthetic solutions were evaluated. The results showed that both the acidic initial pH and DO contributed to Cr(VI) removal; the highest removal rate was observed at pH 3. Based on the Langmuir isotherm model, the predicted maximum removal capacity of MBCA 700 was greater than that of MBCN 700 , primarily because of its core-shell structure, less corrosive ZVI, and higher degree of graphitization. X-ray photoelectron spectroscopy analysis coupled with fitting of the kinetic (pseudo-second-order) and isothermal (Freundlich) data suggested that Cr(VI) removal by MBCN 700 occurred via adsorption, Fe C micro-electrolysis, and co-precipitation. In contrast, the removal mechanism for MBCA 700 included adsorption, reduction, and electrostatic attraction. Thus, ZVI biochar composites can serve as potential remediators of Cr(VI)-contaminated wastewater. • Fe 0 biochar composites were obtained by pyrolysis of coagulation flocculation sludge. • Dissolved oxygen and low pH favored the aqueous Cr(VI) removal by the composites. • Pseudo-second-order kinetic model better described the Cr(VI) removal. • Graphitic carbon structures facilitated the electron transfer from Fe 0 to Cr(VI).