Chlorine isotope fractionation between chloride (Cl-) and dichlorine (Cl2)

The use of chlorine stable isotopes (Cl-35 and Cl-37) can help to constrain natural processes that involve chlorine species with different oxidation states. Theoretical studies based on thermodynamic and quantum mechanical approaches predict large isotope fractionation during redox reactions but to date, these reactions have not been studied experimentally. Here, we explore the chlorine isotope fractionation during the oxidation of hydrated Cl- (redox state of -I) to Cl-2 (redox state of 0) at 25 degrees C and at 0 degrees C. Our apparatus consists of a sealed glass reactor where liquid HCl is mixed with liquid H2O2, a strong oxidant. Following complex reaction pathways, this mixture ultimately leads to the oxidation of Cl- and to the formation of Cl-2 gas. As long as it is degassing, the Cl-2 gas is flushed out of solution using N-2 as a vector-gas from the glassreactor to a potassium hydroxide (KOH) solution (pH 14) where it disproportionates into soluble species: Cl- and ClO-. After each experiment, the chlorine isotopic composition was measured in the recovered KOH-trap solution, as well as in the residual HCl solution. Consistent with theoretical predictions, the produced Cl-2 gas is always enriched in the heavier Cl-37 as compared to the initial Cl-reservoir. The following isotope fractionation factors are obtained: At 0 degrees C the isotopic fractionation 1000lna alpha(Cl2-Cl-) is 4.51 (+ 1.65/-0.49)% At 25 degrees C the isotopic fractionation 1000ln alpha(Cl2-Cl-) is 3.94 (+ 0.69/-0.18)%. From the obtained data it is suggested that the production of Cl-2 gas in our experiments is best described by a closedsystem distillation. Our results are in agreement with published theoretical ab-initio calculations. (C) 2017 Elsevier Ltd. All rights reserved.