Effects Of DOM And Light Radiation On Mercury Transformation In Water And Their Kinetic Study

Mercury, a toxic metal element, is widely distributed in the environment and is considered to be a global pollutant, since it can deposit to remote area after a long distance transfer for its significant volatility property.About two thousand tons of mercury was discharged in to the environment artificially every year. Since methylmercury was found to be responsible for the Minamata disease in Japan in the middle of last century, much more concern was focused on the transfer and transformation of the mercury.Water body plays an important role in the geochemical cycle of mercury, and it is both a source and sink to mercury. Elemental mercury releases from water body after reduction from Hg2+ to Hg0 and deposites again to the earth after oxidation or adsorption to particles. Many factors can influence mercury chemical behavior in water, such as dissolved organic matter (DOM) and ultraviolet (UV). Mehthylmercury is much more toxic than inorganic mercury, and it is the main speciation to pose a health risk to people. Hence, methylation and demethylation of mercury in water is also a hot topic in the research of mercury environmental chemistry.In this study, simulated experiment was employed to detect the mercury reduction in water with the treatments of DOM from different source and various light conditions. The influence of DOM and light radiation was on mercury reduction in water was studied. The mechanism of photo-induced redox of mercury in the presence of NO3" was analyzed. In addition, kinetic study on the methylation and demethylation of mercury in water under various light conditions was conducted.The results showed that all of the DOM extracted from humic soil (DOMH), rice straw (DOMR) and compost (DOMS) inhibited mercury reduction at a higher total organic carbon (TOC) concentrtion in water, and the strongest inhibition was observed with DOMH. It seemed that the strongest inhibition of DOMH was due to its lower average molecular weight and higher saturation degree. DOM at a lower TOC concentration stimulated mercury reduction in water. It suggested that DOM act as a ligand at higher TOC concentration and as a transmitter of electron at lower TOC concentration. To estimate influence of complex reaction on mercury reduction, mercury flux from Hg(NO3)2 solution containing EDTA was measured. The result showed that mercury flux decreased with increasing EDTA concentration. It suggested that complex reaction inhibited mercury reduction in water.Light radiation enhanced HgCl2 reduction but hindered Hg(NO3)2 reduction. Most probably, the·OH, which was produced via photolysis of NO3- reoxidized the dissolved gas mercury (DGM). The influence of light intensity and wavelength on Hg(NO3)2 reduction was studied, and the result showed that Hg(NO3)2 reduction increased with light intensity and UVB induced the fastest reduction rate.Through the interpretation of DGM formation and emission rate, the lag time between the DGM accumulation and volatilization was pose to explain the weak relationship between mercury flux and light intensity in some field studies.The kinetic study of mercury reduction under natural light was conducted through both concentration gradient method and trial method. The pseudofirst rate constant was estimated to be 7×10-4 min-1.The experiments of mercury reduction treated by KNO3 and CH3OH, which was used as a NO3-provider and a·OH scavenger respectively, indicated that·OH which was produced via direct photolysis of NO3- in water should be responsible for the unexpected inhibition of UV to Hg(NO3)2 reduction. The result suggested that light radiation did not always mediate mercury reduction as that had been firmly believed and the importance of coexisting negative radicals should be not ignored.There was no methylmercury produced was observed under dark in the reaction solution of Hg(NO3)2 and acetic acid, while in the UVB, UVA and natural light treatments, methylmercury concentration increased significantly in 17 d. The result suggested that light radiation can stimulate methylation of Hg(NO3)2 in water. Methylation rate of each treatment was in the order of UVA> UVB>natural light>dark.The demethylation rate differed from each other with the treatment of UVB, UVA, natural light and dark, and it was in the order UVB>UVA>natural light>dark. Although the rate of each treatment was different from each other, all the reaction solution under different light radiation obtained the same final balance, and the percent of methylmercury was 10% to total mercury. Since this study was conducted in lab, and there was no other factor except light radition influencing the reaction system, then the proportion of the methylmercury can be considered as aThe kinetic study proved that demethylation of methylmercury under light radiation was a second-order reaction, but the reaction order in dark could not be concluded with the information from this study. The product, HCHO was observed in the reactions in light radiation but not in the dark. Based on the product and pH variation of the solution, it is concluded that different demethylation mechanism was involved in the reactions under dark and light radiation. The half-life time of methylmercury under UVB, UVA and natural light was 0.62 d,1.3 d和1.08 d respectively.