Fractionation of stable chlorine isotopes during transport through semipermeable membranes

Abstract

The stable isotope ratio of chlorine is a potential hydrogeochemical tool. A recently-developed technique allows detection of differences in the isotope ratio of water samples. Measured variations cover a range of 3.0 ± 0.12 per mil. In deep aquifer brines, enrichment in 37C1 seems to increase with increasing chloride concentration. Two types of experiments were performed, short-term (9-20 days) and long-term (five months), in an effort to produce fractionation in the laboratory. The short-term experiments involved isothermal diffusion of chloride across six synthetic, semipermeable, cation-exchange membranes. Variables in the experiments were cation-exchange capacity, pore size, concentration, and stirring. The long-term experiment involved the forced flow of a sodium chloride solution through a bentonite membrane. Both types of experiments produced over 1.2 ± 0.12 per mil fractionation, with isotopically heavier effluent solutions. Fractionation appears to increase with increasing cation-exchange capacity. Conceptual and mathematical models are presented.