Reductive dissolution of mercury-bearing iron(III) (oxyhydr)oxides by dissimilatory iron-reducing bacteria and the potential to mobilize mercury in its elemental form

• Main Authors: Shih-Yun Wang, 王詩芸
• Other Authors: 林居慶
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/59339272316789556488

碩士 === 國立中央大學 === 環境工程研究所 === 104 === The groundwater contamination with mercury (Hg) is an increasing problem worldwide, and since Hg is highly toxic, contamination may render this water resource unsuitable for intented use. In many cases, the external pollution sources are not well defined, and samples taken from monitoring wells are oftentimes characterized with (i) elevated levels of organic carbon and (ii) a positive correlation between concentrations of total Hg or elemental Hg and dissolved iron (Fe). On the basis of these observations, possible mechanisms of the biogeochemical processes that underlie the contamination scenarios have been proposed, including the most acceptable one that postulates “growth and associated metabolisms of indigenous iron-reducers may have been the primary cause for the alternation of Hg speciation and mobility in the aquifer”. However, such hypothesis is completely derived from results of the studies that only investigated redox transformations of Hg in the aqueous phase of a heterogeneous system, which may not be representative of the real situation encountered in the aquifer, as Hg is considered originating from saturated Hg-bearing sediment (i.e., the solid phase). To validate this hypothesis, laboratory microcosm experiments were conducted to simulate processes of microbially-induced reductive dissolution of Fe(III) minerals by equilibrating Hg(II) adsorption onto synthetic hematite and goethite (two of the most commonly found iron oxyhydroxides in the environment used as model minerals) prior to the addition and growth of the model bacterium, Shewanella oneidensis MR-1 (an iron reducer). Hg(0), dissolved Hg(II) and total Fe(II) were measured periodically over the course of the experiment to monitor the trends of phase distribution, redox transformation and thus mobility of Hg resulted from microbial growth coupled with transit of secondary iron mineral formation. It was observed that equilibrium of Hg(II) adsorption onto Fe(III) minerals was reached rapidly presumably due to strong electrostatic interactions, and was not significantly disturbed after the spike of bacterial cells. In addition, formation of Hg(0) corresponding to the growth of cells, as well as little detection of dissolved Hg(II) were observed. These results support the original hypothesis and indicate that Hg(II) deposited in the sedimentary zone would be reduced and released as Hg(0) on account of biogenic Fe(II) produced by iron-reducing bacteria and therefore the entire process potentially promotes the movement of Hg in the aquifer environment.