PNNL

Abstract

Previous studies have demonstrated that gas phase H2S can immobilize certain redox-sensitive contaminants (Cr, U, Tc, etc.) in vadose zone environments. A key issue for effective and efficient delivery of H2S in these environments is its reactivity with indigenous iron oxides. To elucidate the factors that control the transport of H2S in the vadose zone, laboratory experiments were conducted to determine reaction mechanisms and rates of H2S oxidation by iron oxide coated sands using several carrier gas compositions and variable flow rates. Most experiments were conducted using ferrihydrite-coated sand. Additional studies were conducted with goethite and hematite coated sand and a natural sediment. Column experiments were conducted with 200 ppmv H2S in a carrier gas at three flow rates. Three carrier gases were used; N2, Air and O2. Selective extractions were conducted at the end of each column experiment to determine the mass balance of the reaction products for support of the postulated reaction pathways. XPS was used to confirm the presence of the reaction products. Results from the column experiments containing ferrihydrite indicate that when N2 is used as the carrier-gas, the major H2S oxidation products are FeS and elemental sulfur (S0). The ratios of FeS/S0 at the end of these experiments were consistent with the stoichiometry of the postulated reactions. When air or O2 were used as the carrier gas, S0 become the dominant reaction product along with FeS2 and smaller amounts of FeS, sulfate and thiosulfate.