The role of dissolved oxygen (DO2) on the oxidation of hydrosulfide ions (HS-;C_(HS-)~O = 50-150 mu mol/L) into polysulfides (S_n~(2-);n = 2-9),colloidal sulfur,and oxysulfur species with iron(III) trans-1,2-diaminocyclohexanetet-raacetate (iron(III)-cdta;C_(Fe)~O = 50-300 mu mol/L) complexes in alkaline solutions (pH 9-10.2) was investigated at 25 +- 1 °C.At higher pH,oxygen was seen to slow down the hydrosulfide conversion rate.For instance,the HS~ half-life was 24.8 min in a DO2-saturated iron(III)-cdta solution compared to 11.3 min in the corresponding anoxic solution (pH 10.2,C_(HS-)~O = 80 mu mol/L,C_(Fe(III))~O = 200 mu mol/L).In anoxia,HS~ oligomerizes into chain-like polysulfides which behave as autocatalysts on the HS~ conversion rates.The presence of DO2 disrupts the HS~ oligomerization process by generating thiosulfate precursors from polysulfides,a pathway that impedes the HS~ uptake.At lower alkaline pH where the hydroxide-free Fe3+cdta~4- is the prevailing iron(III)-cdta species,the "iron(II)-cdta + DO2" oxidative reaction becomes crucial.Oxidative regeneration of iron(III) as Fe3+cdta4- (being more reactive than Fe3+OH-cdta4-) offsets to some extent the restrictive role of oxygen on the accumulation of polysulfides.Thiosulfate and sulfate were the main end-products for the current experimental conditions to the detriment of colloidal sulfur,which did not form in DO2-saturated solutions.
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