Effect of hydroquinone-induced iron reduction on the stability of Fe(III)-As(V) Co-precipitate and arsenic mobilization

Long-term storage of Fe(III)-As(V) co-precipitate wastes derived from the industrial As removal process poses the risk of secondary arsenic pollution to the local environment. However, its stability in only iron-reducing environment remains unclear. The effect of iron reduction by hydroquinone (QH2) on the stability of co-precipitate at Fe/As molar ratio of 5 was investigated in this study. The results showed that 16.5-93% of Fe(III) reduction in NaOH-neutralized co-precipitate caused 0.1-11% (0.1-8.5 mg L-1) As release and 0.9-46% (2.6-130 mg L-1) Fe(II) release at pH 4 and 6. Most of the As(V) and more than half of the Fe(II) generated by the reductive decomposition of Fe(III)-As(V) co-precipitate were retained in solid phases. Solid characterization and thermodynamic calculation indicated that amorphous Fe3(AsO4)2 was probably the major secondary mineral for As immobilization. At pH 8, 11-39% As and less than 4% Fe(II) were released into solution after 3.6-87% Fe(III) was reduced to Fe(II). Both amorphous Fe3(AsO4)2 and Fe(II,III) (hydr)oxides were responsible for As retention. The solubility of NaOH-neutralized co-precipitates in terms of dissolved As concentration upon Fe(III) reduction at pH 4, 6 and 8 with QH2/Fe(III) mol of 1 was lower than that in oxic environment. It was also lower than the solubility of Ca(OH)2-neutralized co-precipitate after Fe(III) reduction at pH 4 and 6.