Oxidation of hydrogen sulfide by hydrous Fe(III) oxides in seawater

Abstract

The rate of oxidation of hydrogen sulfide by hydrous Fe(III) oxides has been determined as a function of pH (4.0–8.5), temperature (5–45 °C) and ionic strength (0–4 m) in seawater. The effect of the major ions (Ca2+, Mg2+, SO42−, B(OH)4− and HCO4−), minor solutes (PO43−, Si(OH)4, NH4+ and Mn2+) and some organic ligands (EDTA, Tris, oxalate, humic acid and fulvic acid) on the rates of oxidation was examined. The overall rate constant, k, is first order with respect to both hydrogen sulfide and hydrous Fe(III) oxides: −d[H2S]Tdt = k[H2S]T [Fe(OH)3(s)] The rate constant, k, for seawater (S = 36, pH = 8.0) at 25 °C is 1.48 × 102 M−1 min−1 or 0.0138 m−2 1 min−1 when [Fe(OH)3(s)] is expressed in surface area (m21). The apparent activation energy was found to be 4.5 ± 0.2 kJ mol−1. The oxidation rate is strongly dependent on pH with a maximum near 6.5. The pH dependence of the oxidation rate can be adequately described by the reaction pathways including the formation of surface complexes between sulfide species and surface sites of Fe(OH)3(s), followed by electron transfer. The major divalent ions (SO42−, Mg2+ and Ca2+) decrease the reaction rate with the effect of SO42− being greater than Mg2+ and Ca2+. Phosphate and silicate inhibit the reaction significantly due to the adsorption on the particle surface. The reactivity of Fe(OH)3(s) is highly dependent on the condition of its formation. Some organic ligands such as EDTA and Tris can enhance the Fe(II)–Fe(III) redox cycle. The dominant product from the oxidation of H2S by Fe(OH)3(s) is elemental sulfur (S °).