116 - Role of Volatile Organic Compounds Derived from Organic Matter on the Photodemethylation of Monomethylmercury

"The decomposition of monomethylmercury (MMHg) can be abiotically induced by photodegradation in the water column or biotically by certain microorganisms. Our aim in this work was to elucidate the abiotic photodemethylation pathway in a model freshwater (Suwannee River Fulvic acid with DOC =6ppm, and [MMHg] = 50 ppb at pH 6.5). We investigated simultaneous photodemethylation to Hg(II) and photoreduction to Hg(0) with a comparison of oxic and anoxic conditions. We, therefore, irradiated MMHg freshwater solution through exposure to three ranges of solar wavelength under full light (280-800 nm), without short UVB (305-800 nm), and visible light (420-800 nm). We performed kinetic experiments following dissolved and gaseous Hg species concentrations (MMHg, iHg(II), Hg(0)). The organic matter (OM) in the remained solution was characterized by measuring Dissolved Organic Carbon (DOC), and Specific UV Absorbance (SUVA) calculation. Reactive oxygen species (ROS) production was determined by chemical quenching methods. Production of Volatile organic compounds (VOCs) was measured using Selected Ion Flow Tube Mass Spectrometry (SIFT-MS).

A comparison between two methods of post-irradiation and continuous Hg(0) trapping confirmed MMHg photodecomposition to Hg(0) is mainly induced by a first photodemethylation step to iHg(II) followed by a photoreduction step to Hg(0). Photodemethylation under anoxic conditions showed a higher kinetic rate at 1.42±0.18 h-1 compared to that under oxic conditions at 0.27±0.01 h-1. Moreover, photoreduction increased up to three-fold under anoxic conditions. Both results using ROS quenching methods and VOCs measurements revealed the occurrence and production of low molecular weight organic compounds (molecular mass/ charge < 300) as photoreactive intermediates responsible for MMHg photodemethylation as a dominant pathway. This study also supports the role of dissolved oxygen as an inhibitor for the photodemethylation pathways driven by low molecular weight organic matter photosensitizers."