Biochar-anchored low-cost natural iron-based composites for durable hexavalent chromium removal

Current studies have confirmed that sulfidation is an effective method to enhance the reduction capacity of zero-valent iron (ZVI), however, the cost of sulfidation and the durability of the material are still the challenges of material reducibility. In this work, ternary composites of natural pyrite (FeS2), zero-valent iron powder, and pine wood biochar (BC) were synthesized by simple ball milling technology (BM) and applied for the removal of hexavalent chromium (Cr(VI)) in aqueous solution. Batch sorption experiments of each component and composites at various mass rates conformed that the adsorption capacity of BMFeS2/ZVI/BC at 1:1:1 (81.5 mg·g−1) exceeded two to three-fold of Cr (VI) than their individuals. Comparative experiments on two groups with various raw material ratios indicated the balance between biochar-related adsorption capacity and reduction-related S/Fe was imperative. The analysis and calculation of the products yielded 70.07 % of Cr(VI) removal due to reduction/precipitation and 29.93 % was ascribed to surface sorption at the equilibrium Cr(VI) concentration of 33.76 mg·L−1. The electron selectivity (ES) and utilization rate (UR) of 1:1:1 toward Cr(VI) were determined to be 14.20 % and 48.08 %, respectively. A negligible metal ion leakage was observed upon pH > 5.5. Shielding experiments of Fe2+, TEM, and XPS analysis proved the reduction of Cr(VI) occurred mainly on solid surfaces. The role of biochar in the reaction system was identified as conductor, adsorber, and disperser. The multiple interfaces caused by ball milling provide different reduction pathways. Nevertheless, reduction, adsorption, and surface complexation were the dominant mechanisms for Cr(VI) removal. This work demonstrates the potential of natural pyrite for the preparation of BMFeS2/ZVI/BC composite to remove Cr(VI) from water and wastewater.