Reductive Sequestration of Chromate with Pyrite-Loaded nZVI@biochar Composites

Various green materials like biochar and Fe⁰ (nano-scale zerovalent iron, nZVI) have been applied to remediate aqueous Cr(VI) contamination, but few studies have tried to further improve the performance of nZVI and/or biochar composites with different sulfidation methods. Here, we modified a hybrid material of nZVI@biochar with Na₂S and pyrite (FeS₂), applied it to remove aqueous Cr(VI) under different experimental conditions, and revealed key factors influencing Cr(VI) removal performance. The results show that pyrite loading is an effective sulfidation method to increase the Fe and S contents in composites. FeS_x-nZVI@BC (1:1) had a Cr(VI) removal efficiency of ~95% with 5 mg/L Cr(VI) loaded, which was much higher than other hybrid composites. The Cr(VI) removal efficiency of FeSx-nZVI@BC showed a decreasing trend under pH conditions that increased from pH 3 to pH 9. The presence of dissolved oxygen and aqueous Cu²⁺ and Cd²⁺ could significantly suppress the removal of aqueous Cr(VI), while humic acids at different concentrations did not suppress Cr(VI) removal. After the reaction, it was observed with an energy-dispersive spectrometer (SEM-EDS) that most Cr in the solid phase was closely associated with pyrite minerals. X-ray photoelectron spectroscopy (XPS) spectra, together with the Fe²⁺-quenching method, confirmed that Fe (Fe²⁺ or Fe⁰) acted as the main electron donor, contributing to ~90% of the Cr(VI) reduction. Our study indicates that pyrite loading could further improve the performance of remediation materials and that the pyrite-loaded nZVI@BC composite is a green material with strong potential to be applied in the remediation of water contaminated by Cr(VI).