Aqueous photoreduction of oxidized mercury species in presence of selected alkanethiols

Mercury is a global environmental contaminant with severe toxicity impact. The chemical processes resulting in the transformation of oxidized mercury species (Hg²⁺) to elemental mercury (Hg⁰), greatly affects the fate and transport of mercury in the natural environment. We hereby provide the first study on the photochemistry of Hg²⁺ with selected alkanethiols (R-SH) as model compounds to represent thiols and thiol-type binding sites on humic substances in natural waters because of the common sulfhydryl functional group (-SH). Kinetic studies were performed using cold vapor atomic fluorescence spectroscopy (CVAFS), the formation of Hg²⁺-thiol complexes (Hg(SR)₂) were confirmed by UV-visible spectrometry and Atmospheric Pressure Chemical Ionization-Mass Spectrometry (APCI-MS), and the reaction products were analyzed using Electron Impact-Mass Spectrometry (EI-MS) and Solid Phase Microextraction coupled with Gas Chromatography-Mass Spectrometry (SPME/GC-MS). Our results indicated that the photoreduction of Hg²⁺ by selected alkanethiols may be mediated by Hg²⁺-thiol complexes to produce Hg⁰. Under our experimental conditions, the apparent first order rate constants obtained for 1-propanethiol, 1-butanethiol, and 1-pentanethiol were (2.0 ± 0.2) × 10⁻⁷ s⁻¹, (1.4 ± 0.1) × 10⁻⁷ s⁻¹, (8.3 ± 0.5) × 10⁻⁸ s⁻¹, respectively. The effects of ionic strength, dissolved oxygen or chloride ion on reaction rates were found to be minimal under our experimental conditions. The identified products of the reaction between oxidized mercury species with selected alkanethiols (C₃-C₅) were Hg⁰ and disulfides (RS-SR). The potential environmental implications are herein discussed.