Effect of iron reduction by enolic hydroxyl groups on the stability of scorodite in hydrometallurgical industries and arsenic mobilization.

Scorodite (FeAsO4·2H2O) is an important arsenic-bearing solid waste in hydrometallurgical industries, but its stability in reducing environments is not well understood. This study investigated the effect of Fe(III) reduction by enolic hydroxyl groups on the stability of scorodite and arsenic mobilization at various pH values and ascorbic acid/scorodite molar ratios (AH2/Sc). The results showed that 47-89% Fe(III) reduction by ascorbic acid caused approximately 10-69% (~ 37-260 mg L-1) arsenic release and 4.5-63% (~ 13-176 mg L-1) Fe(II) release at pH 5-8. The releases of arsenic and Fe(II) increased with increasing AH2/Sc, whereas they decreased as pH increased. The results of the solid characterization and chemical analysis indicated that the mixture of poorly crystalline parasymplesite and probably amorphous FeHAsO4⋅xH2O was the new arsenic sink. The high solubility of this ferrous arsenate with the Fe(II)/As(V) molar ratio > 1 was deemed to be a major contributor to the relatively high arsenic release. This work differed from our previous finding that almost all arsenic was retained in the solid phase after similar Fe(III) reduction in scorodite with hydroquinone. Phenolic hydroxyl groups complexed with aqueous Fe(II), unlike enolic hydroxyl groups, was possibly the dominant reason for the formation of different secondary minerals, which strongly influenced arsenic redistribution between aqueous and solid phases.