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Effect of abiotic factors on the mercury reduction process by humic acids in aqueous systems

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Mercury (Hg) in the environment can have serious toxic effects on a variety of living organisms, and is a pollutant of concern worldwide. The reduction of mercury from the toxic Hg2+ form to Hg0 is especially important. One pathway for this reduction to occur is through an abiotic process with humic acids (HA), which is controlled by a number of factors, including initial mercury and HA concentrations, pH, temperature and light, all of which were investigated in this study. Comparison of the reduction capacity of HA involving the initial addition status (aqueous or solid HA) and different Hg sources were also examined. Results indicated that HA were able to reduce mercury abiotically, and the reduction process can be fitted by a pseudo-first order equation. Higher initial mercury and HA concentrations inhibited the reduction process, and low (3.6) or high (8.1) solution pH values also decreased the HA reduction capacity. In addition, Hg0 production rate increased with increasing temperature, and the same trend was observed with light exposure. Humic acids added as an aqueous solution resulted in significantly greater Hg0 production than addition as a bulk solid. Finally, the mercury reduction rate and capacity varied significantly (P<0.05) with HA from different sources due to the different characteristics of the HA compounds. These included phenolic hydroxyl, carboxyl, quinone group and E4/E6, all of which showed that the HA extracted from a forest humus soil had the greatest reduction capacity, compared to two other types of commercially produced HA. These research findings shed light on understanding the role that HA play in the mercury abiotic reduction process, and underlying mechanisms in aqueous environments. Meanwhile, the results also evidentially demonstrated the existence of a possible pathway of Hg2+ reduction, which indicated that humic substances in natural environments, especially in water bodies, should be considered not only as strong sinks for Hg, but also as an Hg volatilization source.
Tao Jiang , Shi-Qiang Wei , D. C. Flanagan , Meng-Jie Li , Xue-Mei Li , Qiang Wang , Chang Luo
USDA Scientist Submission
Pedosphere 2014 v.24 no.1
Journal Articles, USDA Authors, Peer-Reviewed
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