Towards Universal Wavelength-Specific Photodegradation Rate Constants for Methyl Mercury in Humic Waters, Exemplified by a Boreal Lake-Wetland Gradient

We report experimentally determined first-order rate constants of MeHg photolysis in three waters along a Boreal lake-wetland gradient covering a range of pH (3.8–6.6), concentrations of total organic carbon (TOC 17.5–81 mg L^–1), total Fe (0.8–2.1 mg L^–1), specific UV_{254 nm} absorption (3.3–4.2 L mg^–1 m^–1) and TOC/TON ratios (24–67 g g^–1). Rate constants determined as a function of incident sunlight (measured as cumulative photon flux of photosynthetically active radiation, PAR) decreased in the order dystrophic lake > dystrophic lake/wetland > riparian wetland. After correction for light attenuation by dissolved natural organic matter (DOM), wavelength-specific (PAR: 400–700 nm, UVA: 320–400 nm and UVB: 280–320 nm) first-order photodegradation rate constants (<i>k</i>_pd) determined at the three sites were indistinguishable, with average values (±SE) of 0.0023 ± 0.0002, 0.10 ± 0.024 and 7.2 ± 1.3 m² E^–1 for <i>k</i>_pdPAR, <i>k</i>_pdUVA, and <i>k</i>_pdUVB, respectively. The relative ratio of <i>k</i>_pdPAR, <i>k</i>_pdUVA, and <i>k</i>_pdUVB was 1:43:3100. Experiments conducted at varying MeHg/TOC ratios confirm previous suggestions that complex formation with organic thiol groups enhances the rate of MeHg photodegradation, as compared to when O and N functional groups are involved in the speciation of MeHg. We suggest that if the photon fluxes of PAR, UVA, and UVB radiation are separately determined and the wavelength-specific light attenuation is corrected for, the first-order rate constants <i>k</i>_pdPAR, <i>k</i>_pdUVA, and <i>k</i>_pdUVB will be universal to waters in which DOM (possibly in concert with Fe) controls the formation of ROS, and the chemical speciation of MeHg is controlled by the complexation with DOM associated thiols