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

Abstract

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 (MBCN700) and hypoxic (MBCA700) conditions. X-ray diffraction analysis revealed that α-Fe0 and γ-Fe0 were the dominant Fe species in MBCN700 and MBCA700, respectively. The effects of different process parameters, including composite dosage, initial Cr(VI) concentration, pH, dissolved O2 (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 MBCA700 was greater than that of MBCN700, 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 MBCN700 occurred via adsorption, FeC micro-electrolysis, and co-precipitation. In contrast, the removal mechanism for MBCA700 included adsorption, reduction, and electrostatic attraction. Thus, ZVI biochar composites can serve as potential remediators of Cr(VI)-contaminated wastewater.