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Faculty for Biology, Chemistry, and Earth Sciences

Environmental Geochemistry Group - Prof. Dr. Britta Planer-Friedrich

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Bachelor Thesis

Sorption of arsenic-sulfur species on goethite

Julia Arndt (03/2010-09/2010)

Support: Britta Planer-Friedrich, Elke Süß

Arsenic is an ubiquitous element. Its abundance in groundwater and the related world-wide problems of toxic effects due to chronic exposure from drinking water are regulated by arsenic mobilization from surrounding sediments. Iron hydroxides are one of the most important sorbents for arsenic in natural systems and during rehabilitation measures. Hence, sorption behavior of the inorganic species, arsenite and arsenate, has been widely studied. In suboxic sulfidic aquatic environments, however, these are only minor species, while arsenic-sulfur species, so-called thioarsenates, predominate. To date, nothing has been published about their sorption behavior and mobility.
In the present bachelor thesis, mono- and tetrathioarsenate were selected as representative members of the homologue series of thioarsenates. Batch experiments were conducted to determine their sorption kinetics and maximum sorption capacities on goethite (α‑FeOOH) compared to arsenite and arsenate. Maximum sorption capacity was found to decrease in the order arsenite > arsenate > monothioarsenate > tetrathioarsenate with 27, 22, 11, and 4 µmol arsenic sorbed per gram FeOOH. The pH was not adjusted or buffered over the course of the experiments and was determined to be 10.4, 9.2, 11.5 and 12.2, respectively. Sorption equilibrium was reached after 30 minutes for arsenite, 48 hours for arsenate, 7 days for monothioarsenate, and not at all within 14 days for tetrathioarsenate.
Problems encountered when trying to fit the tetrathioarsenate sorption isotherms are most likely related to significant speciation changes observed during the experiment. At low concentrations, tetrathioarsenate decomposed at pH 11 as expected to trithioarsenate and arsenite. At higher concentrations (5.3 mM), however, unexpectedly substantial formation of arsenate (50.3 % of total arsenic) was found. At the high pH of the original tetrathioarsenate solution, arsenate sorption was significantly less than what was observed in the arsenate experiment at pH 9.2 (only 16.5 %). Total sorption capacity could be increased for tetrathioarsenate to 615 µmol As /g FeOOH by increasing the experimental As:Fe ratio from 0.004 to 0.48, however the maximum sorption capacity of goethite (due to further linear increase) was still not reached.
Overall, it can be concluded that monothioarsenate and especially tetrathioarsenate are much more mobile than arsenite and arsenate. Predictions based on sorption constants from arsenite and arsenate will thus significantly underestimate the arsenic release in sulfidic environments.

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