Arsenic Partitioning during Schwertmannite Dissolution and Recrystallization in the Presence of Fe(II) and Oxalic Acid

Fe(II)-driven transformation of schwertmannite can strongly impact the transport and redistribution of coprecipitated As in reducing acid sulfate soil (ASS)/acid mine drainage (AMD)-contaminated environments. Oxalic acid is prevalent in the natural environment, but its influence on Fe(II)-induced schwertmannite conversion and the repartitioning behavior of coprecipitated As is poorly characterized. In the present study, we examined the impact of oxalic acid on Fe(II)-induced As(V)-coprecipitated schwertmannite transformation and As redistribution between dissolved, adsorbed, and incorporated phases during mineral transformation at a pH of 5.5 and in 1 mM Fe(II). According to Mössbauer analysis, in 1 mM Fe(II) without oxalic acid treatment, schwertmannite was converted into lepidocrocite (65%) and goethite (35%). In mixed Fe(II)–oxalic acid systems, oxalic acid played a synergistic role in the dissolution and transformation of Fe(II)-induced schwertmannite. In 1 mM Fe(II) with 0.1/0.5/1 mM oxalic acid reaction systems after 14 day incubation, the fractions of goethite were 68% (lepidocrocite occupied 32%), 100%, and 100%, respectively. In all treatments, the PO43–-extractable fraction of As increased during mineral transformation, indicating that partially coprecipitated As of the original mineral transferred to the surface of the resulting products. Compared with mixed Fe(II)–oxalic acid systems, more adsorbed As formed in Fe(II) treatment without oxalic acid, which might correspond to the speciation of the resulting minerals. Although Fe(II) was a potential reductant for As(V), no As(III) was detected in any reaction system by the end of incubation. These findings have important implications for accurately predicting the geochemical behavior of As in AMD-contaminated areas.