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

Abstract

Sulfur isotope exchange between sulfide (H 2S) and thiosulfate (HSSO 3H) 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 (–SO 3H or SO 3H) sulfurs of thiosulfate (i.e., SH⇔SO 3H in thiosulfate) and two overall intermolecular exchanges between sulfide and sulfane sulfur of thiosulfate (i.e., H 2S⇔SH of thiosulfate) and between sulfide and sulfonate sulfur of thiosulfate (i.e., H 2S⇔SO 3H 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 H 2S 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 H 2S and –SO 3H 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. A comparison of the rates of sulfur exchanges among H 2S, –SH, and –SO 3H 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., S 3S 2−, S 4S 2−, etc.): 10H 2S+3S 2O 3 2−=4S 3S 2−+2H ++9H 2O and S n S 2−+SSO 3 2−=S n+1 S 2−+SO 3 2− . The disproportionation reaction of polysulfides appears to control the exchange rate between S 2− and S 6+ 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.