Geochemcial Controls on Arsenic and Phosphorus in Natural and Engineered Systems

This thesis elucidates fundamental reactions that can control concentrations of arsenic and phosphate in water sources. High levels of arsenic or phosphorus have significant implications for the environment-- arsenic is extremely toxic to humans while phosphorus can cause eutrophication.

Initial work focused on arsenic solids that might exert geochemical control on soluble arsenic. Formation of proposed iron, barium, copper and zinc-arsenic solids were systematically examined under realistic environmental conditions. Thermodynamically favored copper, ferrous and barium solids did not form under circumstances of significance to drinking water sources. However, sorption of arsenic to iron, zinc and copper solids was discovered to be very significant, depending on the pH and solids age.

Given the established importance of sorption in arsenic and phosphate chemistry, two key constituents (silica and sulfide) implicated in mobilization of sorbed arsenic or phosphate were examined in detail. The addition of silica, which competes with arsenate or phosphate for sorption sites on Al(OH)3 and Fe(OH)3 hydroxides, caused release of 0-30% sorbed As and P at pHs between 7.0 and 8.5. Reaction of sulfide with Fe(OH)3 led to instantaneous release of 50-95% of sorbed As and P through a reductive dissolution mechanism. This instantaneous release was slowly reversed as orpiment (As2S3) and vivianite [Fe3(PO4)2] slowly precipitated, but under other circumstances, these solids would not be expected to form. Modeling results suggest that arsenic and phosphate concentrations could either increase or decrease in response to reaction between Fe(OH)3 and sulfides, thereby reconciling literature reports that seemed to contradict one another.