Dramatically enhanced aerobic Cr(VI) reduction with scrap zero-valent aluminum induced by oxalate

In the present study, the flexibility of utilizing scrap zero-valent aluminum (SZVAl) for Cr(VI) reduction mediated by oxalate was evaluated. Results show that the produced H2O2 and the reactive electrons releasing from metal aluminum were the primary electron donors for the preferable Cr(VI) reduction in anaerobic SZVAl/Cr(VI)/oxalate system. The reduction rate of Cr(VI) decreased with the increase of both initial pH value of 1.5–4.5 and Cr(VI) initial concentration of 50–300 μM, increased with oxalate concentration from 0 to 4.0 mM, and markedly retarded under anaerobic condition due to the completely inhibited H2O2 production. Notably, the accelerated Cr(VI) reduction induced by oxalate can be ascribed to the fact that the affinity of oxalate to Cr(VI)/Cr(V) species resulted in the expansion of Cr(VI)/Cr(V) coordination from tetrahedron to hexahedron, which was preferred for hexahedral Cr(III) species. Consequently, the presence of oxalate can significantly accelerate Cr(VI) reduction rate, and acted as an alternative electron donor simultaneously for Cr(VI) reduction through intramolecular electron transfer reaction. As compared with zero-valent aluminum (ZVAl), SZVAl exhibited superior reducing capacity for Cr(VI) reduction due to the presence of numerous galvanic cells in SZVAl corrosion process. The appreciable treatment efficiency of real chrome electroplating effluent indicates SZVAl/oxalate system presents as a preferable case of “waste control by waste” strategy. Moreover, the present study provided a new perspective on the role of oxalate in Cr(VI) reduction, which was significant in the detoxification of Cr(VI) process.