Submitted to: Pedosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/5/2012
Publication Date: 2/1/2014
Publication URL: http://handle.nal.usda.gov/10113/59351
Citation: Jiang, T., Wei, S., Flanagan, D.C., Li, M., Li, X., Wang, Q., Luo, C. 2014. Effect of abiotic factors on the mercury reduction process by humic acids in aqueous systems. Pedosphere. 24(1):125-136. Interpretive Summary: Mercury is a toxic element that can seriously affect human health and other organisms. It is normally present in the soil, water, and atmosphere at relatively low levels, but in certain circumstances or situations the amount of mercury can increase to more dangerous levels. This study conducted basic research to better understand some of the chemical transformations of mercury between ionic forms in water solutions to the stable elemental mercury that typically exists as either a liquid or a vapor. In particular, we examined how humic acids, which are a component of soil organic matter, can act to reduce mercury from its Hg2+ ionic form to its Hg0 elemental form in water solutions. Different factors affecting this reaction were also studied, including the amounts of mercury and humic acids present, the type of humic acids used, solution acidity, temperature and the presence of light. We found that the humic acids that were obtained from a forest humus soil had the greatest ability to reduce the mercury. Humic acids added as a solid and mixed with the water had less of an effect than those added that were already dissolved and in solution. Increasing temperatures as well as light also increased the chemical reaction, which tells us that in the natural environment this process likely occurs more rapidly in the summertime. Organic materials are known to be a sink (collector) of mercury ions, but this work shows that they can also be a pathway that the ionic forms of mercury can be reduced to the elemental form, that can then leave the system as a gas. This research impacts other scientists as well as environmental protection agency personnel seeking to better understand the transport and fate of mercury in natural systems. Better knowledge of the chemical reactions associated with mercury may allow development of better monitoring, control, and/or remediation methods.
Technical Abstract: 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.