吸附汞之三價鐵礦於生物還原溶解過程中元素汞的生成與移動潛勢;Reductive dissolution of mercury-bearing iron(III) (oxyhydr)oxides by dissimilatory iron-reducing bacteria and the potential to mobilize mercury in its elemental form

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Title:	吸附汞之三價鐵礦於生物還原溶解過程中元素汞的生成與移動潛勢;Reductive dissolution of mercury-bearing iron(III) (oxyhydr)oxides by dissimilatory iron-reducing bacteria and the potential to mobilize mercury in its elemental form
Authors:	王詩芸;Wang,Shih-Yun
Contributors:	環境工程研究所
Keywords:	異化性鐵還原;地下含水層;元素汞生成;汞移動潛勢;dissimilatory iron reducing bacteria;aquifers;formation of elemental mercury;mercury mobility
Date:	2016-08-29
Issue Date:	2016-10-13 15:26:30 (UTC+8)
Publisher:	國立中央大學
Abstract:	汞是毒性最強的元素之一，當地下水遭受汞污染時將導致此水資源無法被適當利用。在許多案例中，污染場址並未發現到有明顯外來點源的存在，且水質監測的結果常顯示(1)受污染的地下水含有較高濃度的有機碳，以及(2)水中的總汞、甚至元素汞濃度的增加與溶解鐵濃度的增加具有正相關等特性。雖然幾種可能的生地化機制已被提出用以解釋及推估污染事件背後的成因，但這些機制目前皆未被證實，包括「當含水層處於以鐵還原為主要的最終電子接受的狀態時，汞化性與移動性的轉變極有可能與系統中的異化性鐵還原菌之生長及代謝有關」這樣的論點，其推論主要是基於過往的研究已證實鐵還原菌本身以及因這類微生物生長代謝而衍生的二次礦物可將汞直接還原。然而，文獻上所得的這些汞還原作用其實驗設計皆是針對水溶液中的汞進行探討，無法貼近於地下水污染案例中汞物種真實的分布情況。有鑑於此，本研究藉由模擬的方式以可進行胞外呼吸作用的鐵還原菌 Shewanella oneidensis MR-1 為模式生物，環境中最常見的三價鐵礦hematite (赤鐵礦)與 goethite (黃鐵礦)為模式礦物進行厭氧縮模試驗，待 Hg(II)與hematite 或 goethite 於試驗培養液中達吸附平衡後加入 MR-1，並隨時間分析系統中的溶解汞、元素汞與總亞鐵的含量變化，以觀測地下含水層中當異化性鐵還原菌受刺激生長後二次鐵礦的形成及汞的氧化還原轉換與移動潛勢。試驗與模擬的結果顯示，Hg(II)與 Fe(III)鐵礦可能因靜電吸引作用的關係而迅速地達到吸附平衡，且此平衡在添加 MR-1 菌株後並未受到劇烈破壞。此外，汞的還原程度雖然隨著 MR-1 對不同鐵礦的使用而有所差異，但不論是在含有 hematite 或 goethite的系統，元素汞的生成皆可對應細胞的生長代謝，且過程中所檢測到的溶解汞濃度的變化皆不甚明顯。這些結果支持原本的假說，即「地下飽和含水層中原吸附/鑲嵌於鐵礦的 Hg(II)在受到鐵還原菌生長代謝的影響下，將因生源性 Fe(II)的生成而易被還原成 Hg(0)，使得整體 Hg 於地下水環境中的移動性提高，進而促成汞的污染擴散」。;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.
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