Bachelor Thesis

Formation of thioarsenates upon sorption of arsenic to mackinawite

Moritz Zeising (11/2014-12/2015)

Support: Britta Planer-Friedrich, Thomas Borch, Regina Lohmayer

Knowledge of the sorption behavior of arsenic (As) is important because of its natural occurrence and toxicity. Previous studies described higher sorption onto iron sulfide minerals for arsenite, As(III), and arsenate, As(V), than for thioarsenates, thiolated pentavalent arsenic species. This study investigated the formation of thioarsenates in a system containing sulfur and iron. Two mineral formation processes of mackinawite (FeS) were compared, one in the absence (preformed mineral) and the other in the presence (concurrent formation) of As during synthesis. Preformed mineral experiments resulted in high removal of As from suspension. Speciation was dominated by As(V). Higher mobility of As was observed in concurrent formation experiments. Speciation was dominated by As(III) and thioarsenates. The reduced species As(III) was likely to react with elemental sulfur to thiolated As species. Formation of thioarsenates may be the reason for higher mobility of As in the aqueous phase as shown by other studies. Confirmation of thioarsenates in Fe-rich suspensions is difficult because of precipitation of iron during storage, As co-precipitation, and As speciation changes. In this study, recovery problems were found using the anoxic, ethanolic and cool storage method. Flash-frozen storage with EDTA addition probably led to artificial thioarsenate formation. In a natural environment, concurrent formation of mackinawite in the presence of As is important, because of reductive dissolution of iron oxides with sorbed As. Transformation to iron sulfides may lead to mobilization of As due to the observed thioarsenate formation.