*WINNER* A study on the Spectrophotometric Analysis of Hg(II) using Dithizone under Conditions Pertinent to Hg(II) Reduction in Aquatic Systems

Abstract

The reduction of mercuric mercury (Hg(II)) in aquatic systems contributes to the transformation, transportation, and fate of mercury in the environment. Solar radiation has been identified as the driving force of Hg(II) reduction to volatile dissolved gaseous mercury (DGM) in aquatic bodies and is hypothesized to be linked to processes mediated by dissolved organic carbon (DOC). Superoxide has also been hypothesized to mediate Hg(II) photoreduction in aquatic systems but the proposed mechanism has recently been rejected in literature. We investigated the applicability of the dithizone method for spectrophotometric analysis of Hg(II) under different environmental conditions pertinent to the study on aquatic Hg(II) reduction, particularly superoxide-mediated Hg(II) reduction. We studied the effects of organic acids (e.g., citrate, cysteine), pH, and reagents used to generate superoxide (e.g., xanthine, xanthine oxidase). Our study showed that some organic acids lowered the sensitivity of the method but the calibration curves in all scenarios retained good linearity. We concluded that the mercury-dithizone method was valid for the study of superoxide mediated Hg(II) reduction. Our preliminary study of this reduction was performed in the absence of light and under controlled conditions (e.g., pH 7.2, 25°C, and 5 µM initial Hg(II) concentration). We observed a decrease of the absorbance at 496 nm (i.e. reduction of Hg(II)) with various rates of superoxide production and various concentrations of organic acids. This preliminary research suggests that superoxide could be an important intermediate for Hg(II) reduction even in the absence of sunlight.