Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

W. H. Patrick, Jr


Studies dealing with the speciation, species transformations, and solubility of selenium and arsenic as affected by soil or sediment redox potential and pH were initiated because of a lack on information and the need for a better understanding of selenium and arsenic chemistry in wetland soils and sediments. Analytical techniques were developed that permitted the determination of selenium and arsenic species commonly encountered in soils and sediments. The speciation and redox chemistry of selenium and arsenic was studied in selected soils and sediments. Under reduced conditions ($-$200 mV), selenium solubility in sediments from Kesterson Reservoir (CA) and Hyco Reservoir (NC) was low and (elemental selenium + selenides) comprised 80-100% of the total soluble selenium. Experimental data and equilibrium thermodynamic calculations suggest that insoluble metal selenides, particularly FeSe, controlled selenium solubility under reduced conditions. Upon oxidation form $-$200 to 500 mV selenium solubility increased approximately 20 times in both the Kesterson and Hyco Reservoir sediments. Under moderately reduced conditions (0-200 mV), selenite was the dominant (45 to 100%) soluble selenium species. At redox levels above 200 mV, selenite was further oxidized to selenate and selenium solubility reached a maximum. An alkaline pH resulted in greater dissolved selenium concentrations. The experimental data also illustrated the importance of biomethylation in selenium chemistry. Under oxidized conditions, dimethyl selenide constituted 15 to 36% of the total soluble selenium. Redox potential and pH were also shown to exhibit a major impact on arsenic speciation, and solubility in Hyco Reservoir (NC) sediments and in an arsenic contaminated soil from Kolin (La). In contrast to selenium, arsenic solubility increased with decreasing redox. In both studies, arsenate was the major ($>$80%) arsenic species present under oxidized conditions. Upon reduction, arsenite became the major dissolved arsenic species, and arsenic solubility increased. Upon reduction from 500 to $-$200 mV, total arsenic in solution increased 25 and 13 times in the Hyco Reservoir and Kolin soil, respectively. The importance of adsorption-desorption and precipitation-dissolution reactions in controlling arsenic chemistry was illustrated in the Kolin soil.