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

Abstract 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 (AH 2) 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 AH 2 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 AH 2 upon Fe(III) reduction was lower than that in oxic environment. Compared with produced Fe(II,III) (hydr)oxides in the presence of hydroquinone (QH 2), humboldtine was formed during the long-term reactions of Fe(III)-As(V) co-precipitate with AH 2. 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. Highlights • Fe(III)-As(V) co-precipitate in ascorbic acid solutions is unstable at pH 5−9. • More As release occurs upon Fe reduction at higher pH or lower Fe/As molar ratio. • Oxalate is the final product of ascorbic acid oxidation under Fe-reducing condition. • Oxalate competes with arsenate for Fe(II) inhibiting parasymplesite accumulation. • Parasymplesite and humboldtine are main secondary solids for As and Fe fixation. [ABSTRACT FROM AUTHOR]