title: Hydrosulfide oxidation pathways in oxic solutions containing iron(III) chelates.
abstract: The role of dissolved oxygen (DO2) on the oxidation of hydrosulfide ions (HS-; C(HS-)0 = 50-150 micromol/L) into polysulfides (S(n)2-; n = 2-9), colloidal sulfur, and oxysulfur species with iron(III) trans-1,2-diaminocyclohexanetetraacetate (iron(III)-cdta; C(Fe(III)0 = 50-300 micromol/L) complexes in alkaline solutions (pH 9-10.2) was investigated at 25 +/- 1 degree 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-)0 = 80 micromol/L, C(Fe(III))0 = 200 micromol/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 Fe(3+)cdta(4-) is the prevailing iron(III)-cdta species, the "iron(II)-cdta + DO2" oxidative reaction becomes crucial. Oxidative regeneration of iron(III) as Fe(3+)cdta(4-) (being more reactive than Fe(3+)OH(-)cdta(4-)) 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.
authors.affiliation: Department of Chemical Engineering, Laval University, Québec, Canada.
authors.forename: Simon
authors.initials: S
authors.lastname: Piché
authors.forename: Faïçal
authors.initials: F
authors.lastname: Larachi
mesh_terms: D004492:Edetic Acid / Q000031:analogs & derivatives* / Q000737:chemistry; D007502:Iron Chelating Agents / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D012996:Solutions; D013440:Sulfides / Q000737:chemistry*; D013885:Thiosulfates / Q000737:chemistry
chemical_list: D007502:Iron Chelating Agents; D012996:Solutions; D013440:Sulfides; D013885:Thiosulfates; C005390:CDTA; D004492:Edetic Acid; D010100:Oxygen