A new method for the investigation of mercury redox chemistry in natural waters utilizing deflatable Teflon® bags and additions of isotopically labeled mercury

The toxicity and behavior of mercury (Hg) varies greatly between its chemical species, yet the mechanisms which control the redox chemistry of Hg in natural waters are still poorly understood. Previous studies have identified these processes and compared the Hg redox chemistry between water types but have been hampered by errors associated primarily with the type of reaction vessels utilized, and the inability of the methods to simultaneously measure oxidation and reduction. Presented here are the results of experiments which demonstrate the validity of a new method that addresses both these issues through the design and use of a new reaction vessel, a 5 L PFA Teflon® incubation bag, and by the addition of isotopically labeled inorganic Hg species (both HgII and Hg0). The method development showed that mm thick FEP Teflon® is permeable to Hg0, and therefore unsuitable. Application of this method showed that both oxidation and reduction occurred simultaneously in natural waters exposed to ambient sunlight and that the rate of these transformations were of similar order (10−3 to 10−4 s−1). Given such reaction rates, the characteristic time to equilibrium is rapid, on the order of hours. The method is applicable for tracer studies, and the method was able to quantify rate constants of greater than 10−5 s−1. Overall, this study suggests that the rates of reaction are faster than previously predicted primarily because previous studies did not account for the fact that both reactions are occurring simultaneously in natural waters.