| Abstract
| - The role of dissolved oxygen (DO2) on the oxidation ofhydrosulfide ions (HS-;= 50−150 μmol/L) intopolysulfides (; n = 2−9), colloidal sulfur, and oxysulfurspecies with iron(III) trans-1,2-diaminocyclohexanetetraacetate (iron(III)-cdta;= 50−300 μmol/L) complexesin alkaline solutions (pH 9−10.2) was investigated at 25± 1 °C. At higher pH, oxygen was seen to slow down thehydrosulfide conversion rate. For instance, the HS- half-life was 24.8 min in a DO2-saturated iron(III)-cdta solutioncompared to 11.3 min in the corresponding anoxicsolution (pH 10.2,= 80 μmol/L,= 200 μmol/L).In anoxia, HS- oligomerizes into chain-like polysulfideswhich behave as autocatalysts on the HS- conversion rates.The presence of DO2 disrupts the HS- oligomerizationprocess by generating thiosulfate precursors from polysulfides,a pathway that impedes the HS- uptake. At loweralkaline pH where the hydroxide-free Fe3+cdta4- is theprevailing iron(III)-cdta species, the “iron(II)-cdta + DO2”oxidative reaction becomes crucial. Oxidative regenerationof iron(III) as Fe3+cdta4- (being more reactive thanFe3+OH-cdta4-) offsets to some extent the restrictive roleof oxygen on the accumulation of polysulfides. Thiosulfateand sulfate were the main end-products for the currentexperimental conditions to the detriment of colloidal sulfur,which did not form in DO2-saturated solutions.
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