title: Stabilization of thioarsenates in iron-rich waters.
abstract: In recent years, thioarsenates have been shown to be important arsenic species in sulfidic, low-iron waters. Here, we show for the first time that thioarsenates also occur in iron-rich ground waters, and that all methods previously used to preserve arsenic speciation (acidification, flash-freezing, or EDTA addition) fail to preserve thioarsenates in such matrices. Laboratory studies were conducted to identify the best approach for stabilizing thioarsenates by combination and modification of the previously-applied methods. Since acidification was shown to induce conversions between thioarsenates and precipitation of arsenic-sulfide minerals, we first conducted a detailed study of thioarsenate preservation by flash-freezing. In pure water, thioarsenates were stable for 21d when the samples were flash-frozen and cryo-stored with a minimal and anoxic headspace. Increasing headspace volume and oxygen presence in the headspace were detrimental to thioarsenate stability during cryo-storage. Addition of NaOH (0.1M) or EtOH (1% V/V) counteracted these effects and stabilized thioarsenates during cryo-storage. Addition of Fe(II) to thioarsenate solutions caused immediate changes in arsenic speciation and a loss of total arsenic from solution during cryo-storage. Both effects were largely eliminated by addition of a neutral EDTA-solution, and thioarsenates were significantly stabilized during cryo-storage by this procedure. Neutralization of EDTA was required to prevent alteration of thioarsenate speciation through pH change. With the modified method (anoxic cryo-preservation by flash-freezing with minimal headspace after addition of neutralized EDTA-solution), the fractions of mono- and dithioarsenate, the two thioarsenates observed in the iron-rich ground waters, remained stable over a cryo-storage period of 11d. Further modifications are needed for the higher SH-substituted thioarsenates (tri- and tetrathioarsenate), which were not encountered in the studied iron-rich ground waters.
authors.affiliation: University of Bayreuth, Environmental Geochemistry, Universitaetsstrasse 30, Bayreuth, Germany. elke.suess@uni-bayreuth.de
authors.forename: Elke
authors.initials: E
authors.lastname: Suess
authors.forename: Dirk
authors.initials: D
authors.lastname: Wallschläger
authors.forename: Britta
authors.initials: B
authors.lastname: Planer-Friedrich
mesh_terms: D001149:Arsenates / Q000737:chemistry*; D004492:Edetic Acid / Q000737:chemistry; D005615:Freezing; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000737:chemistry*; D013457:Sulfur Compounds / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000737:chemistry*
chemical_list: D001149:Arsenates; D013457:Sulfur Compounds; D014874:Water Pollutants, Chemical; D004492:Edetic Acid; D007501:Iron