Bachelor Thesis

Efficiency of SONO filters for removal of monothioarsenate

Fabian Wismeth (12/2014-11/2015)

Support: Britta Planer-Friedrich, Stephan Hug, Jörg Schaller

Due to the extensive arsenic problems within the drinking water supply system in Bangladesh, easy arsenic removal systems and filter processes such as SONO Filter become the center of interest. In this thesis these filters were recreated at laboratory scale. Their efficiency in regard to remove monothioarsenate (AsVSO33-) was examined in comparison to arsenite (AsIIIO33-) and arsenate (AsVO43-). Batch experiments were conducted previously to analyze the sorption characteristics of arsenite, arsenate and monothioarsenate. The resulting samples were examined via ICP-MS for determining the total concentrations of various elements and additionally via IC-ICP-MS for analyzing the concentration of the different arsenic species. The concentration-dependent batch experiments showed differences between the arsenic species regarding their sorption characteristics. Under pH-neutral conditions arsenate was removed best and arsenite worst. The sorption of monothioarsenate was located between these two species. Arsenic adsorption onto iron(III)(hydr)oxide was lowest for experiments containing phosphate due to the inhibiting effect of phosphate. The experiments conducted in an UV-Box revealed that monothioarsenate is stabile to UV-Light and the sorption was not affected. Using silicate-free synthetic groundwater monothioarsenate was removed at first but followed by a remobilization of monothioarsenate leading to an increase of the monothioarsenate concentration by a factor of two. Due to experiments from the past with arsenate it is known that in absence of silicate the formation of crystalline iron phases increases. Hence, iron tends to form the energy-efficient compact structure replace adsorbed arsenic out of the structure back into the solution. The results of the columns experiments confirmed the results of the batch experiments concerning the order of the sorption characteristics of arsenite, arsenate and monothioarsenate. Moreover, the inhibiting effect of phosphate on the sorption of monothioarsenate became evident. During the column experiments filtered and unfiltered samples were compared to distinguish between the soluble and colloidal arsenic in solution. It seems as if the amount of arsenic in solution decreases faster than the amount of colloidal arsenic increases with rising numbers of column flows whereby the total amount of arsenic decreased. Distinct species transformations were solely observed in the batch experiments where monothioarsenate disproportionation to elemental sulfur and arsenite, which could be further oxidized to arsenate. On the other hand, the resulting arsenite may also react to dithioarsenate via ligand exchange and addition of S0.