Low molecular weight organic acids strengthen the electron transfer of natural FeS2/biochar composite for Cr(VI) reduction: Experimental observations and governing mechanisms

Abstract

Low molecular weight organic acids with low cost and environmentally friendly characteristics can enhance the electron transfer between materials and contaminants. In this study, oxalic acid (OA), citric acid (CA), and ethylenediaminetetraacetic acid (EDTA) were selected to strengthen ball milled natural FeS2/biochar composite (FeS2/biochar) for aqueous Cr(VI) removal. 100% of Cr(VI) was removed when 5 mM of OA and 0.25 g·L−1 FeS2/biochar were used together. In contrast, the removal rate of Cr(VI) by the individual FeS2/biochar and OA was 57.9% and 6.3%, respectively. The experimental and characterization results suggested that OA was adsorbed on the surface of FeS2/biochar through the conjugated structure of pine biochar, thereby enhancing the dissolution rate of Fe(II) and S(-II) in the FeS2/biochar composite, and facilitating the conversion of Fe(II), S(-II), and Cr(VI) to Fe(III), SO42-, and Cr(III), respectively. In addition, the complexation of OA with Fe(III) and Cr(III) reduced the formation of Fe(OH)3 and Cr(OH)3, exposing more surface active site of FeS2/biochar for the reduction of Cr(VI). As an electron donor, the addition of OA enhanced the electron transfer process of FeS2/biochar to Cr(VI) on the one hand, and on the other hand promoted the redox cycle of Fe(II) through the electron transfer between OA and Fe ions. FeS2/biochar showed better stability even after 5 cycles (in 5 mM OA system). The present study demonstrated the promise of low molecular weight organic acids in strengthening materials for environmental remediation.