The stability of Fe(III)-As(V) co-precipitate in the presence of ascorbic acid: Effect of pH and Fe/As molar ratio

The potential hazards of Fe(III)-As(V) co-precipitate under reducing conditions are incompletely known. This work investigated the effect of Fe(III) reduction by ascorbic acid (AH₂) on the stability of Fe(III)-As(V) co-precipitate at different pHs and Fe/As molar ratios. The results showed that As (14-98.9%) and Fe (27.9-99.3%) were significantly released into solution by 79.9-97.5% Fe(III) reduction of the co-precipitate (Fe/As molar ratios of 3 and 5) at pH 5-9. More As release was observed with the increase of pH (6-9) or decrease in Fe/As molar ratio (from 5 to 3). This could be attributed by oxalate, the final product of AH₂ decomposition, which strongly competed with As(V) for Fe(II) at higher pH or lower Fe/As molar ratio, inhibiting parasymplesite accumulation and then causing more As mobilization. The stability of Fe(III)-As(V) co-precipitate with AH₂ upon Fe(III) reduction was lower than that in oxic environment. Compared with produced Fe(II,III) (hydr)oxides in the presence of hydroquinone (QH₂), humboldtine was formed during the long-term reactions of Fe(III)-As(V) co-precipitate with AH₂. The findings of this study implied that parasymplesite and humboldtine as secondary solid products were environmental relevant and mainly responsible for As(V) and Fe(II) immobilization.