Application of chemical kinetics to mercury cycling in the aquatic environment: Photoreduction of mercury(II) and binding of mercury(II) and methylmercury(+) to natural ligands

The thesis comprises four main chapters on chemical reactions and kinetics of some of the processes involved in the global mercury cycle. In the first chapter, the UVA irradiation of aqueous acidic mercuric chloride in the presence of large excess of Fe(III) organic diacid complexes results in partial reduction of the mercuric ion to elemental mercury. The pseudo-first-order rate constant (k) for photoreduction reaction is pH-dependent. Similar results were obtained using visible irradiation although the rates were ca. 10 times slower. The mechanism of photoreduction is inferred to involve reaction of Hg(II) with a secondary photoproduct, the strongly reducing radical anion CO 2-•. No other previous reports have suggested the involvement of this radical in mercury reduction. In the presence of dissolved oxygen, competition for CO2-• between Hg(II) and O2 reduces the rate and efficiency of mercuric ion reduction. The O2-•/HO2 products do not reduce Hg(II). On the contrary, their disproportionation leads to the formation of H2O2 which causes a slow reoxidation of Hg(0). In the second chapter, the reaction rate of UVA photoreduction of Hg(II) ions by fulvic and humic acids was found to have higher values in the pH range of 5 to 6 which is relevant to most aquatic environments, within this pH range, speciation calculations show that most of Hg(II) will bind to DOC. The effects of environmentally relevant parameters such as Hg(II)/HS ratio, and chloride concentration were investigated and the likely mechanism identified. The interaction of DOC with Hg species is not only limited to photoreactions but also the complexation reaction affects the bioavailability and speciation of Hg. This was the topic of the last two chapters. The kinetic stability of Hg-HS and McHg-HS complexes was characterized by different combined techniques; the competitive ligand exchange method (CLEM) combined with inductively coupled plasma-mass spectrometer (CLEM-ICP-MS) and the tangential flow ultrafiltration (TFUF) combined to CLEM-ICP-MS. The Hg-HS complexes can be described by at least two kinetically distinguished components; the more inert (slow) with a dissociation rate constant in the order of 10-5 s -1, and a labile component with a dissociation rate constant in the order of 10-3 s-1, protons (H+) was found to compete with Hg2+ for the strong binding sites, Hg/HS concentration ratio also was an important parameter, when [Hg 2+] was high then the strong binding sites was saturated and the excess Hg bound to the weak sites. (Abstract shortened by UMI.