Experimental study of mass-independence of Hg isotope fractionation during photodecomposition of dissolved methylmercury

Abstract

Experiments modelling photolytic decomposition of methylmercury chloride in aqueous solutions of different chemical composition have been performed. Ion-exchange chromatographic separation using Chelex^® 100 resin was used in order to separate methylmercury from inorganic mercury prior to the isotope ratio measurements by solution nebulization multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The performance of the chromatographic separation has been evaluated in terms of recovery of both methylmercury and inorganic Hg using synthetic solutions. Both mass-dependent and mass-independent fractionation of Hg isotopes concomitant with the decomposition process have been observed. Mass-independent Hg isotope fractionation (MIF) resulted in selective enrichment of ¹⁹⁹Hg and ²⁰¹Hg relative to the other isotopes in the methylmercury molecules and has been attributed to the magnetic isotope effect. The highest extent of MIF of Hg isotopes, expressed as Δ¹⁹⁹Hg and Δ²⁰¹Hg values, has been observed in acidified solution with low concentration of total dissolved solids (TDS). Progressive decrease in Δ¹⁹⁹Hg and Δ²⁰¹Hg values in acidified solution with higher concentration of TDS, alkaline solutions of both low and high concentration of TDS, and in a solution of ascorbic acid has been attributed to suppression of the radical pair reaction mechanism, responsible for the occurrence of the magnetic isotope effect, by substances acting as radical scavengers, such as OH⁻ or ascorbic acid. The data obtained in this study demonstrate the significance of spin chemistry effects in the isotope fractionation of mercury.