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

Hiroshi Ohmoto  David R.Cole  
【摘要】:正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℃,the equilibrium fractionation factors(in per mil) can be expressed as: 10001nα_(H_2S-SH) = -0.327±0.055(10~(12)/T~4) + 2.676±0.341(l0~6/T~2) 10001nα_(SO_3H-SH)= -0.352±0.009(10~(l2)/T~4) + 7.523±0.054(10~6/T~2) and 10001nα_(SO_3H-H_2S)= -0.0293±0.058(10~(l2)/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 logk_(SO_3H)(?)H_2S=-5.14(10~3/T) + 10.35 (T in K) with an activation energy of 98.3 kJ/mol at 100-170℃. 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℃.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_nS~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.

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