The effect of iron reduction on the long-term stability of scorodite in the presence of enolic hydroxyl groups and mineral transformation

Scorodite waste during its short-term storage in an iron environment has proved to pose an arsenic pollution risk. However, the long-term stability of scorodite is not completely understood. This work investigated the effect of iron reduction by enolic hydroxyl groups as reductive natural organic matter on long-term stability of scorodite. The results showed that 38.3% and 31.3% of As was released into the aqueous phase after 99.4% and 82.6% of Fe(III) was reduced to Fe(II) at pH 6 and 7 for 134 days, respectively. Compared with the observations regarding 3-day reactions in our previous work, arsenic was further dissolved and the degree of Fe(III) reduction increased over time. In contrast, the release of Fe(II) at pH 6 and 7 exhibited a descending trend from the 240th h, with the percentage of aqueous Fe(II) falling to 9.1% and 5.6%, respectively. Most of Fe(II) and As(V) were immobilized by solids. Crystalline parasymplesite as the only secondary solid was responsible for the fixation of arsenic. Chemical analysis together with solid characterization indicated that the transformation of the secondary solid involved the conversion of FeHAsO4 (Fe/As molar ratio of 1) to Fe3(AsO4)2·8H2O (Fe/As molar ratio of 1.5), causing more As release. Scorodite, therefore, was unstable during its long-term storage in an Fe-reducing environment at (slightly) circum-neutral pH. •Arsenic is further released by long-term reaction of scorodite with ascorbic acid.•FeHAsO4 as the intermediate product is converted to Fe3(AsO4)2·8H2O.•Parasymplesite is the only secondary solid for Fe(II) and As(V) immobilization.•The decomposition of ascorbic acid for the formation of oxalate is not observed.•Scorodite is not long-term stable in Fe(III)-reducing environment at pH 6 and 7.