Abiotic Redox Transformation of Hg under Dissimilatory Iron-Reducing Conditions and its Impact on Groundwater Quality

• Main Authors: Ping-Chieh Liao, 廖炳傑
• Other Authors: Chu-Ching Lin
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/83199628278669653008

碩士 === 國立中央大學 === 環境工程研究所 === 102 === Groundwater contamination with mercury (Hg) is an increasing problem worldwide, as Hg is one of the most toxic elements on Earth. Hg present in groundwater may not only render this water resource unsuitable for domestic use, it may also be transported to surface waters, where it would be converted by microbes to methylmercury, a potent neurotoxin, thus posing a greater threat to public and ecosystem health. However, the mechanisms that lead to transformation and movement of Hg in the groundwater environment are still largely unknown. Nonetheless, it has been suggested that Hg-contaminated groundwater may result from stimulation of microbial activities in subsurface environments. Relevantly, recent studies have shown that certain strains of dissimilatory iron-reducing bacteria (DFeRB) are able to reduce Hg(II) via an unknown pathway that is independent of mer activities, particularly when iron minerals are supplemented as terminal electron acceptors for DFeRB to respire. All these observations suggest that biogenic ferrous species produced by DFeRB may be a key player in the redox transformation of Hg in the subsurface anoxic zone. As a result, in this study laboratory microcosm experiments were conducted to explore the interaction between Hg and ferrous-iron under environmentally relevant iron-reducing conditions to test the following hypotheses: (i) in anoxic groundwater environments Hg(II) reduction under iron-reducing conditions is predominantly carried out by biogenic surface-bound and crystalline ferrous species, instead of dissolved ferrous; (ii) the degree to which the redox transformation of Hg by biogenic Fe(II) proceeds is primarily modulated by surface chemistry of iron minerals. Results from these microcosm tests showed that under neutral pH conditions when ferrous species were bound to goethite and hematite, two of the most commonly found ferric (oxyhydr)oxides in the environment, enhanced reduction of Hg(II) to Hg(0) was observed. In addition, siderite, one of secondary iron minerals frequently formed during the growth of iron-reducers in carbonate-buffering sedimentary environments, showed the capacity to reduce Hg(II) as well. Surprisingly, Hg(II) reduction by dissolved ferrous species was also observed when complexation of Hg(II) with amino-functionalized organic matter in the solution. Taken together, these results imply that conventional in situ bio-remedial actions relying on stimulation of indigenous microbial activities to enhance bio-degradation and reductive transformation of pollutants in contaminated subsurface sediments should be more cautious, as they might inadvertently mobilize Hg in the aquifer.