Stabilisation of thioarsenates in iron-rich waters – the effect of nitrogen, ethanol, NaCl and EDTA
Judith Forberg (11/2010-08/2011)
Support: Britta Planer-Friedrich, Elke Süß
Thioarsenates contribute significantly to arsenic speciation in sulfidic, iron-containing waters and differ considerably in their biogeochemical behavior from arsenate and arsenite. Howev-er, especially in the presence of iron stabilization of thioarsenates is challenging due to spec-iation changes by oxidation and arsenic losses by co-precipitation with iron-hydroxides. To develop suitable stabilization methods, thioarsenate behavior in Fe-As-S-systems was studied. Monothioarsenate was found to be stable in the presence of iron(II) under anoxic conditions, but converted to arsenate via arsenite in the presence of oxygen and iron(III). Stabilization experiments were performed with synthetic thioarsenate- and arsenite-sulfide-solutions in the absence and presence of iron(II). Cool storage of samples in nitrogen-filled bottles turned out to be most effective for stabilization over one week and more efficient than flash-freezing. The addition of sodium chloride (0,1 mol/L) to increase matrix strength or ethanol (1 %) with slightly reducing effect did not increase thioarsenate stability, but reduced arsenite oxidation in arsenite-sulfide-mixes. Addition of 25 mM EDTA also increased thioarsenate stability in the presence of iron by complexation, especially in combination with flash-freezing. However, arsenite oxidation and thioarsenate formation were induced. Following the laboratory experiments, the most effective stabilization methods were applied for sampling of natural waters from Czech mineral springs near Franzensbad enriched in iron (0,04 – 1,1 mmol/L), sulfide (0,9 – 24 µmol/L), and arsenic (0,8 – 2,3 µmol/L). Thioarsenates accounted for up to 50 % of the arsenic species and could be stabilized for six days by cool storage in nitrogen-filled bottles, in some cases with addition of ethanol.