Reduction of mercury(II) by tropical river humic substances (Rio Negro) - A possible process of the mercury cycle in Brazil

The evolution of elemental Hg from its environmental compounds has already been supposed to be an important process within the global mercury cycle. T he present study characterizes the abiotic reduction of Hg(II) ions by typi cal river humic substances (HS) conventionally pre-isolated by the adsorben t XAD 8 from the "Rio Negro" near Manaus, Brazil. For the investigation of this reduction process a special reaction and Hg(0) trapping unit combined with cold-vapor atomic absorption spectrometry (CVAAS) was developed. Preco ncentration of traces of mercury(II), if required, was obtained by a home-m ade FIA system using microcolumns filled with the Hg(II)-selective collecto r CheliteS(R) (Serva Company). The effect of environmentally relevant param eters such as the pH value, the Hg(II)/HS ratio and the HS concentration on the I-IE;(II) reduction process was studied as a function of the time. The Hg(0) production was highest at pH 8.0 and in the case of decreasing HS am ounts (0.5 mg) when about 65% of initially 1.0 mug Hg(H) was reduced within 50 h. Moreover, the reduction efficiency of HS towards HE;(II) strongly de pended on the HS concentration but hardly on the Hg(II)/HS ratio. The reduc tion kinetics followed a relatively slow two-step first-order mechanism wit h formal rate constants of about 0.1 and 0.02 h(-1), respectively. Based on these findings the possible relevance of the abiotic evolution of mercury in humic-rich aquatic environments is considered. (C) 2000 Elsevier Science B.V. All rights reserved.