Cr(VI) removal using different reducing agents combined with fly ash leachate: A comparative study of their efficiency and potential mechanisms

Remediation of high concentrations of Cr(VI) in wastewater involves its chemical reduction to Cr(III), a product with low toxicity that can be easily removed. To date, NaBH₄ has rarely been used to reduce Cr(VI). This article reports a comparative study of Cr(VI) removal by NaBH₄ and five sulfur-based reducing agents (FeSO₄, Na₂S₂O₅, NaHSO₃, Na₂S₂O₃, and Na₂SO₃). The potential mechanisms of Cr(VI) removal by these six reducing agents with and without fly ash leachate (FAL) are also discussed. The results revealed that the reduction and subsequent removal of Cr(VI) are influenced by the hydrolysis and ionization of the reducing agents in solution. Thus, the reduction reaction was significantly enhanced when Na₂S₂O₅ and NaHSO₃ were added in excess of 600 mg L⁻¹. Combined with FAL, smaller amounts of NaBH₄ were required to reduce Cr(VI) to Cr(III) at pH 3.0 compared to those with the other reducing agents. NaBH₄ combined with FAL at a dose of 100 mg L⁻¹ afforded a total Cr (Cr_T) removal of 96.32% within 20 min, a value much higher than that obtained with the other reducing agents. The catalytic mechanism of NaBH₄ for such a FAL-catalyzed Cr(VI) reduction system is similar to that of acid catalysis via the hydrolysis of the Fe(III) and Al(III) species in FAL. Improvement of the Cr_T removal was also observed via Cr(VI) entrapment in the structure of Fe(III) and Al(III) metal hydroxides. These results indicate that relatively low loadings of NaBH₄ combined with FAL show great promise for Cr(VI) pollution remediation.