Mercury is an environmental contaminant of global concern. The reduction of oxidized mercury species (Hg(II)) by organic acids to elemental mercury (Hg0) is significant for understanding the cycling of mercury between the atmosphere and aqueous systems. This study focused on the reduction of Hg(II) by small, semivolatile dicarboxylic acids (C2-C4). The reaction kinetics was studied using cold vapor atomic fluorescence spectroscopy (CVAFS), and the products of the reaction were analyzed using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and nuclear magnetic resonance (NMR) spectrometry. The effects of light, dissolved oxygen and chloride ion on reaction rates were also investigated. The highest reaction rates were observed in systems free of both oxygen and chloride ion with the second-order apparent rate constants of 1.2 x 10(4), 4.9 x 10(3), and 2.8 x 10(3) (L x mol(-1) x s(-1)) for oxalic, malonic, and succinic acids at pH 3.0 and T = 296 +/- 2 K, respectively. The photoreduction of Hg(II) was mediated by the complexes formed between Hg" and dicarboxylic acids, and the identified products were Hg0, hydroxycarboxylic acids and monocarboxylic acids. Our results also indicated that the presence of chloride ion significantly reduced the reduction rate by competing with the complexation of Hg" with dicarboxylic acids, while dissolved oxygen retarded the production of Hg0 by involving in the reoxidation of reduced Hg species to Hg(II). Based on our experimental results, a tentative mechanism is proposed and the potential environmental implications are discussed.

Mercury is one of the most toxic metals and has global importance due to the biomagnification and bioaccumulation of organomercury via the aquatic food web. The physical and chemical transformations of various mercury species in the atmosphere strongly influence their composition, phase, transport characteristics and deposition rate to the ground. Modeling efforts to evaluate the mercury cycling in the environment require an accurate understanding of atmospheric mercury chemistry. We focus this article on recent studies (since 2015) on improving our understanding of the atmospheric chemistry of mercury. We discuss recent advances in (i) determining the dominant atmospheric oxidant of elemental mercury (Hg 0 ); (ii) understanding the oxidation reactions of Hg 0 by halogen atoms and by nitrate radical (NO 3 ); (iii) the aqueous reduction of oxidized mercury compounds (Hg II ); and (iv) the heterogeneous reactions of Hg on atmospherically-relevant surfaces. The need for future research to improve understanding of the fate and transformation of mercury in the atmosphere is also discussed.

Mercury (Hg) is a key toxic global pollutant. Studies in aquatic environment have suggested that thiols could be important for mercury speciation. Thioglycolic acid has been detected in various natural water systems and used as a model compound to study the complicated interaction between mercury and polyfunctional dissolved organic matter (DOM). We herein presented the first evidence for mercury particle formation during kinetic and product studies on the photochemistry of divalent mercury (Hg(2+)) with thioglycolic acid at near environmental conditions. Mercuric sulfide (HgS) particles formed upon photolysis were identified by high-resolution transmission electron microscopy coupled with energy dispersive spectrometry and select area electron diffraction. Kinetic data were obtained using UV-visible spectrophotometry and cold vapour atomic fluorescent spectrometry. The apparent first-order reaction rate constant under our experimental conditions was calculated to be (2.3±0.4)×10(-5) s(-1) at T=296±2 K and pH 4.0. It was found that (89±3)% of the reactants undergo photoreduction to generate elemental mercury (Hg(0)). The effects of ionic strengths, pH and potassium ion were also investigated. The formation of HgS particles pointed to the possible involvement of heterogeneous processes. Our kinetic results indicated the importance of weak binding sites on DOM to Hg in photoreduction of Hg(2+) to Hg(0). The potential implications of our data on environmental mercury transformation were discussed.

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.

Mercury (Hg) pollution in the environment is a global issue and the toxicity of mercury depends on its speciation. Chemical redox reactions of mercury in an aquatic environment greatly impact on Hg evasion to the atmosphere and the methylation of mercury in natural waters. Identifying the abiotic redox pathways of mercury relevant to natural waters is important for predicting the transport and fate of Hg in the environment. The objective of this review is to summarize the current state of knowledge on specific redox reactions of mercury relevant to natural waters at a molecular level. The rate constants and factors affecting them, as well as the mechanistic information of these redox pathways, are discussed in detail. Increasing experimental evidence also implied that the structure of natural organic matter (NOM) play an important role in dark Hg(II) reduction, dark Hg(0) oxidation and Hg(II) photoreduction in the aquatic environment. Significant photooxidation pathways of Hg(0) identified are Hg(0) photooxidation by hydroxyl radical (OH•) and by carbonate radical (CO3−•). Future research needs on improving the understanding of Hg redox cycling in natural waters are also proposed.