It has long been recognized that the formation of soluble arsenic sulfur complexes plays a key role for the mobility and toxicity of arsenic in sulfate-reducing environments. Knowledge of the exact arsenic species is essential to understand the behavior of arsenic in sulfidic aquifers and to develop remediation strategies. In the past, monomeric and trimeric thioarsenites were assumed to be the existing species in sulfidic systems. In this study, thioarsenates were identified by IC-ICP/MS in arsenite- and sulfide-containing solutions as well as in a reduced groundwater from a contaminated site. The unexpected finding of an oxidation of As(lll) to As(V) in thioarsenates in strongly reducing systems can be explained by the high affinity between As(Ill) and sulfur. In sulfide-containing solutions without oxidant, As(lll) therefore undergoes disproportionation to thioarsenates (As(V)) and elemental arsenic. It has previously been supposed that mobility as well as toxicity of arsenic increases if the redox state decreases. For sulfidic waters, the opposite is probably the case. Thus, the formation of thioarsenates could be used in connection with remediation strategies. Thioarsenates are highly sensitive to oxygen and pH. This is important for analytical procedures. A loss of soluble arsenic as well as a conversion to arsenite and arsenate may occur if water samples containing thioarsenates are analyzed with conventional methods.
Groundwater from a depth of 100-200 m is the main source of public water supply in most municipalities in the Pannonian basin in central and southeastern Europe. Even though its quality does not always meet EU standards for drinking water-including those regarding arsenic-in many villages and even in some major cities no treatment except chlorination takes place. Of the several alternatives to improve the water supply situation in the Autonomous Province of Vojvodina in the northern part of the Republic of Serbia, re-orientation towards more centralized systems combined with river bank filtration as an additional and sustainable raw water resource was evaluated as the best. A hydrogeological and hydrochemical survey of the Tisa (or Tisza) River alluvium in the Padej test field confirmed the aptness of this approach. A good connection between the Tisa River bed and the alluvial aquifer consisting of fine-grained sand was found (average hydraulic conductivity of 5×10 −5 m/s). With appropriately designed and managed wells, 80-100 l/s bank filtrate per km of river bank can be produced for water supply. Comprehensive analysis of the river water and river bank filtrate as well as a pilot treatment of the bank filtrate suggest that aeration-oxidation-flocculation-filtration-disinfection is a suitable technology for the Tisa River bank filtrate.
Due to the geological and hydrochemical characteristics, the majority of the ground waters in the Vojvodina possess an unusual chemistry. Mainly this concerns their high alkalinity, low hardness and high contents in natural organic matter. Moreover the concentrations of arsenic, boron, ammonia and methane are elevated in many waters. Arsenic is present in the inorganic species arsenate(III) and arsenate(V), which are toxic to humans. Complex and expensive technology is required for these types of waters to produce drinking water for the public supply. Amongst others, the treatment process would have to include flocculation with high dosages of chemicals and for some waters also reverse osmosis, resulting in waste water amounts of up to 30%. The latter is critical since the exploited aquifer recharges very slowly. The Tisza River is an alternative resource, especially in the case that bank filtrate extraction could be implemented.
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