Acetate improves catalytic performance for rapid removal of Cr(VI) by sodium borohydride in aqueous environments

An efficient method of remediating Cr(VI) involves the conversion of Cr(VI) to Cr(III) using sodium borohydride (NaBH₄). However, information regarding the role of organic substances in Cr(VI) remediation is limited. As such, in this study, we aimed to evaluate the role of common low-molecular-weight acetate in the rapid chemical reduction of Cr(VI) by NaBH₄ and subsequent total chromium (TCr) removal. The results showed that acetate promoted the transformation of high-concentration Cr(VI) to Cr(III) by NaBH₄. The presence of 0–4 mM acetate did not affect the subsequent precipitation of Cr(III). In the presence of 4 mM acetate, Cr(VI) conversion was 99.03%, and subsequent TCr removal was 95.87% within 15 min without adding precipitant. The addition of acetate reduced the requirement for a low initial pH to achieve high Cr(VI) reduction efficiency. In particular, in the presence of 10 mM acetate, the conversion of 100 mg/L Cr(VI) by NaBH₄ at an initial pH of 4 was 99.26% compared with 36.89% in the absence of acetate. Regarding the promotion mechanism of acetate for the reduction of Cr(VI) by NaBH₄, we did not observe direct electron transfer from acetate to Cr(VI) at the origin of the reduction, but the reduction of Cr(VI) was promoted by inhibiting the production of OH^– through buffering. Cr in HCrO₄^- was the favorable site for nucleophilic reaction, which could be attacked by H^- from BH₄^-. According to the results of qualitative and semiquantitative XPS, Fourier transform infrared spectroscopy, and elemental analyses of sediments, a small amount of Cr(VI) and approximately 3% of acetate was removed through precipitation adsorption by the precipitated Cr(III). This novel acetate–Cr(VI)–NaBH₄ remediation technology, which is simple and easy to use, improves the removal of high-concentration Cr(VI) from wastewater.