Low Molecular Weight Organic Acids On Mercury Redox And Transformation

There has been a global concern of mercury (Hg) contamination, due to its special physicochemical property and extreme toxicity. Hg biogeochemistry cycle are of great importance for understanding its toxicity and cycling in the environment, and these reactions are controlled by both biotic and abiotic factors, especially involving plants. As the first producer of terrestrial ecosystems, not only plant leaves exchange Hg with atmosphere, but also plant root exudates can activate soil heavy metals.Reservoir is an important and typical eco-sensitive areas due to its special "Hg activation effect". Three Gorges Reservoir is the most distinguished extra-large reservoir, which has vertical height of30m, an area of over400km2flooding areas called water-level fluctuation zones resulted in long-term wet-dry alternation after completion of impoundment. Water-level fluctuation zones is in dry period every year showing an special local characteristics that plentiful vegetations grow in these large areas due to sufficient rainfall and appropriate temperature, especially some parts of these areas are usually used by local people for cultivation. The vegetation in the growth process releases large amounts of root exudates to fluctuating zone soil, during the flood period root exudates will follow into the water. Thus, the process could make important impacts on the form transformation and redox behavior of Hg in these areas. Thus, based on above consideration between Hg and fluctuating zone vegetation root exduates, a serie experiments were conducted in this study, effects of low molecular weight organic acid (LMWOA), the most important components in vegetation root exduates, on Hg transformation and redox behavior were studied, which may provide an additional way to interpret environmental effect of fluctuating zone vegetation root exduates on the fate of reservoir Hg.Simulated experiments were employed to dectect the influence of LMWOA, citric, tartaric, malic, succinic acid, on reduction of Hg2+, oxidation of Hg0, methylation of Hg2+and demethylation of MeHg. In all, based on above study works, following main results obtained:(1) All four organic acids had significantly effects on the Hg reduction reaction. Both tartaric and malic acid can reduce Hg2+. The Hg reduction rate by tartaric acid treatment was second-order with respect to Hg2+concentration. And, the reduction capacity of tartaric acid was stronger than malic acid. Citric acid inhibited the Hg reduction reaction. Succinic acid improved the Hg reduction reaction at low concentration, and inhibited the reaction at high concentration. Tartaric and malic acids were final electron donors for Hg2+reduction reaction, while succinic in low concentration was only act as electron transfer. In addition, mixed acid (citric, tartaric, malic, succinic acid concentration ratio was1:1:1:1) can reduce Hg2+, and reduction capacity was stronger than single organic acid treatment.(2) All four organic acids showed a capacity for oxidating Hg0in the early stage, but the oxidized Hg0was subsequently reduced. The oxidation rate of Hg0was induced by citric acid, followed by tartaric and malic acid, and succinic acid.(3) All four organic acids unable to provide suitable mehyl donor for Hg2+, thus unable to realize inorganic mercry methylation.(4) Methylmercury degradation under light treatment was siginificantly higher than dark condition. There had different mechanisms, which the former including photodegradation and microbal degradation, the latter just microbal degradation.All four organic acids had significantly effects on MeHg demethylation.Under light and dark condition, both citric and succinic acid had significantly inhibitory effects on MeHg demethylation, which succinic was stronger than citric. Tartaric and malic acid inhibited under light condition, while improved under dark treatments.(5) The reactions of all four organic acids with Hg2+or Hg0were dominated by redox, while with MeHg were dominated by complexation.(6) Redox properties of all four organic acids were closely related to their structure and functional groups. The electron donating group a—OH is the functional group for reduction properties of tartaric and malic acid. Citric acid comprises three internal strong electron withdrawing group—carboxyl group, and therefore it had relatively stronger oxidizability. Succinic acid only has two carboxyl groups, both reactive functional groups and molecular weight are the smallest of four, therefore it has the least interactive active sites for interaction with Hg.