Kinetics of sulfur isotope exchange between aqueous sulfide and thiosulfate involving intra- and intermolecular reactions at hydrothermal conditions

Sulfur isotope exchange between sulfide (H2S) and thiosulfate (HSSO3H) can be described by the general rate law for a two-compound system (X and AB) with three exchangeable atoms (X, A, and B) proposed by [X. Chu, H. Ohmoto, Kinetics of isotope exchange reactions involving intra- and intermolecular reactions: I. Rate law for a system with two chemical compounds and three exchangeable atoms. Geochim. Cosmochim. Acta 55 1991 1953–1961]. According to the rate law, the isotope exchange reaction is comprised of one overall intramolecular exchange between sulfane (–SH or SH) and sulfonate (–SO3H or SO3H) sulfurs of thiosulfate (i.e., SH⇔SO3H in thiosulfate) and two overall intermolecular exchanges between sulfide and sulfane sulfur of thiosulfate (i.e., H2S⇔SH of thiosulfate) and between sulfide and sulfonate sulfur of thiosulfate (i.e., H2S⇔SO3H of thiosulfate). The rate constants for the three overall exchange reactions and the equilibrium isotopic fractionation factors among sulfide, sulfane, and sulfonate of thiosulfate were estimated by fitting [F. Uyama, H. Chiba, M. Kusakabe, H. Sakai, Sulfur isotope exchange reaction in the aqueous system: thiosulfate–sulfide–sulfate at hydrothermal temperature. Geochem. J. 19 1985 301–315] experimental data on sulfur isotope exchange between aqueous H2S and sodium thiosulfate by the least squares method. At temperatures of 100–170 °C, the equilibrium fractionation factors (in per mil) can be expressed as: 1000 ⁢ ln α H 2 S – SH = − 0.327 ± 0.055 ( 10 12 / T 4 ) + 2.676 ± 0.341 ( 10 6 / T 2 ) 1000 ⁢ ln α SO 3 H – SH = − 0.352 ± 0.009 ( 10 12 / T 4 ) + 7.523 ± 0.054 ( 10 6 / T 2 ) and 1000 ⁢ ln α SO 3 H – H 2 S = − 0.0293 ± 0.058 ( 10 12 / T 4 ) − 4.871 ± 0.357 ( 10 6 / T 2 ) (T in K). At near-neutral pH, the overall rate (m−1 s−1) for the sulfur isotope exchange between H2S and –SO3H of thiosulfate is described by log k SO 3 H ⇔ H 2 S = − 5.14 ( 10 3 / T ) + 10.35 (T in K) with an activation energy of 98.3 kJ/mol at 100–170 °C. arison of the rates of sulfur exchanges among H2S, –SH, and –SO3H of thiosulfate with the rates of polysulfide–thiosulfate formation and disproportion reactions determined by [W.F. Giggenbach, Kinetics of the polysulfide–thiosulfate disproportionation up to 240 °C. Inorg. Chem. 13 1974b 1730–1733] suggests that the sulfur isotope exchanges between aqueous sulfide and thiosulfate may proceed via the formation and disproportionation of polysulfides (e.g., S3S2−, S4S2−, etc.):10H2S+3S2O32−=4S3S2−+2H++9H2OandSnS2−+SSO32−=Sn+1S2−+SO32−. The disproportionation reaction of polysulfides appears to control the exchange rate between S2− and S6+ atoms in thiosulfate and is considered the rate-determining step in the sulfate–sulfide exchange reaction rather than the intramolecular exchange of thiosulfate proposed by [H. Ohmoto, A.C. Lasaga, Kinetics of reactions between aqueous sulfates and sulfides in hydrothermal systems. Geochim. Cosmochim. Acta 46 1982 1727–1745]. Therefore, polysulfides may play an important role in the chemical and isotopic reactions between aqueous sulfide and sulfate under hydrothermal conditions.