Title

Oxidation of hydrogen sulfide by manganese(IV) and iron(III) (hydr)oxides in seawater

Date of Award

1995

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Marine and Atmospheric Chemistry

First Committee Member

Frank J. Millero, Committee Chair

Abstract

The oxidation rates of hydrogen sulfide by Mn(IV) and Fe(III) (hydr)oxides were determined as a function of pH, temperature and ionic strength in seawater. The effects of major ions (Ca$\sp{2+}$, Mg$\sp{2+}$, SO$\sb4\sp{2-}$, B(OH)$\sb4\sp-$ and HCO$\sb3\sp-$), minor components (PO$\sb4\sp{3-}$, Si(OH)$\sb4$, NH$\sb4\sp+$, Fe$\sp{2+}$ and Mn$\sp{2+}$) and some organic ligands (EDTA, TRIS, humic acid and fulvic acid) on the oxidation rates were examined. The mineral phases used included $\delta$MnO$\sb2, \alpha$FeOOH, $\beta$FeOOH, $\alpha$Fe$\sb2$O$\sb3$ and freshly precipitated Fe(OH)$\sb3$(s). The redox reactions were found to be surface-controlled processes and first order with respect to the concentrations of both sulfide and metal oxides. Ca$\sp{2+}$, Mg$\sp{2+}$, SO$\sb4\sp{2-}$ and PO$\sb4\sp{3-}$ were found to decrease the oxidation rates due to their interactions with the oxide surfaces. EDTA and TRIS enhanced the redox cycling of Fe by keeping Fe(III) in solution or through the forming of more reactive Fe(III) colloids. The strong pH dependence of the rate of sulfide oxidation is probably attributed to the formation of surface complexes. The major product from H$\sb2$S oxidation by Mn(IV) and Fe(III) (hydr)oxides is elemental sulfur (S$\sp\circ$), except at high MnO$\sb2$/H$\sb2$S ratios when S$\sb2$O$\sb3\sp{2-}$ and SO$\sb4\sp{2-}$ become important.Adsorption kinetics and equilibrium of phosphate on $\delta$MnO$\sb2$ were determined in seawater as a function of pH, temperature and salinity. The effects of Ca$\sp{2+}$, Mg$\sp{2+}$, SO$\sb4\sp{2-}$ and humic acid on the adsorption were investigated. The data have been modeled using a triple layer surface complexation model. The results suggest that manganese oxides can act as important adsorbents of phosphate in natural waters as well as in surface sediments.The Pitzer specific interaction model and the ion pairing model have been combined to examine the speciation of trace divalent and trivalent metals in natural waters, including anoxic waters. This model has also been used to examine the solubility of Fe in seawater.A number of chemical parameters (H$\sb2$S, O$\sb2$, pH, TA, TCO$\sb2$, NH$\sb4\sp+$, PO$\sb4\sp{3-}$, Si(OH)$\sb4$, Mn$\sp{2+}$ and Fe$\sp{2+}$) were measured in the Framvaren Fjord, Norway. In the anoxic waters of the fjord, high C:N and C:P ratios were found, while the ratio of N:P was similar to the Redfield ratio. Based on the C:N:P ratio of 155:16:1 in organic matter, about 30% of sulfide produced from sulfate reduction was estimated to be removed by processes such as oxidation, formation of FeS$\sb2$, degassing and incorporation into organic matter. The oxidation rates of H$\sb2$S by Mn(IV) and Fe(III) (hydr)oxides in the waters near the O$\sb2$/H$\sb2$S interface were slightly faster than the observed values in the laboratory, possibly due to the presence of bacteria.

Keywords

Physical Oceanography; Biogeochemistry; Geochemistry

Link to Full Text

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