Oxidation of atomic mercury by hydroxyl radicals and photoinduced decomposition of methylmercury in the aqueous phase

The rate constant for Hg-o + OH, k(Hgo)+(.OH) = (2.4 +/- 0.3)x 10(9) M-1 s( -1), in the aqueous phase was determined using a relative rate technique wi th methyl mercury as reference compound. The OH initiated mercury reaction proceeds via the molecular Hg(I) radical which is oxidised to Hg(II) by dis solved O-2. The reaction can be of importance under certain atmospheric cir cumstances, such as when the aqueous phase capacity of forming OH radicals is significant and the gas phase concentration of ozone drops. The same end product, i.e. Hg(II) was observed from the photodegradation of methylmercu ry hydroxide. In this case, molecular Hg(I) radicals are again likely to be formed after photodegradation of the Hg-C bond with subsequent oxidation. A lifetime of 230 h of methylmercury at outdoor conditions was estimated du e to this reaction. The action of OW on methylmercury species also involves breaking of organometallic bonds and formation of Hg(II), Speciation of th ese reaction products from methylmercury is important for the estimation of biogeochemical cycling of mercury. (C) 2001 Elsevier Science Ltd. All righ ts reserved.