ABSTRACT

Sulfur isotope exchange reactions in the aqueous system H₂S-S₂O₃^2--SO₄^2- were studied experimentally in the temperature range from 50 to 170°C and at near-neutral pH using sealed Pyrex glass tubes as reaction vessels. The exchange reaction between thiosulfate and sulfide readily took place above 100°C. The sulfur isotope exchange reaction between inner (sulfonate) and outer (sulfane) sulfurs of thiosulfate was accelerated by coexisting sulfide ions. In other words, the sulfide ions played a catalytic role in the intramolecular sulfur isotope exchange reaction through the formation of polysulfide (e.g., S₃S^2-, S₄S^2-) from thiosulfate and sulfide. On the other hand, the sulfur isotope exchange reaction between thiosulfate and sulfate and that between sulfate and sulfide hardly proceeded in about 500 hours even at temperatures as high as 150°C, whereas the sulfane sulfur of thiosulfate exchanged sulfur isotopes with sulfide even at 50°C. The isotope fractionation factor between the sulfonate and sulfane sulfurs of thiosulfate was determined at 120, 130, 150 and 170°C. The results are approximated by the following equation with ³⁴S being favored in sulfonate sulfur. 1000 ln α = −4.94 × 10⁶/T² + 4.13 × 10⁴/T − 39.03 The half-time of the intramolecular exchange reaction of thiosulfate under presence of sulfide ions was about 15 hours at 120°C and 490 hours at 100°C, respectively. The present results support the view that thiosulfate is an intermediate in the istope exchange between sulfate and sulfide. However further study is required to assess if the intramolecular exchange is the rate-determinant step in the sulfate-sulfide exchange reaction as was suggested by Ohmoto and Lasaga (1982). Variations of the sulfur isotope ratios in natural systems, which possibly involve thiosulfate, were discussed in view of the present results.