| 摘要: | 汞是毒性最高的元素之一，當地下水源受汞污染時對公眾與生態是一大威脅，尤其當汞藉由地下水傳輸進入表面水體並進一步形成甲基汞後更為嚴重，但目前對於汞在地下水環境的轉換與流佈機制仍不清楚。即使如此，現有的調查資料顯示地下水汞污染可能跟厭氧微生物的代謝活動與其所引起的地化條件有關，且近期文獻亦指出部份鐵還原菌可不需透過生物性抗汞機制即可將汞還原，尤其當以固態三價鐵礦作為電子接受者時，此效果更加顯著。這些線索說明鐵還原菌生長時所形成的生源性亞鐵在汞氧化態的轉換作用上可能扮演著關鍵性的角色。為此，本研究藉由模擬地下水的環境條件，設計非生物性的汞還原縮模試驗，以測試「造成汞還原的物種乃是生源性的吸附型與晶格型亞鐵，且其還原程度由表面化學所主導」的假說。實驗結果證實當溶解態亞鐵於pH中性條件下與環境中常見的三價鐵礦goethite與hematite形成吸附型亞鐵時，有顯著的汞還原效應產生；此外，鐵還原菌利用鐵礦生長後所形成的二次礦物siderite，同樣對汞的還原效用也相當顯著。然而，在測試溶解態亞鐵與Hg(II)反應時，藉由PHREEQC對反應系統內的化學組成模擬時意外的發現，當Hg(II)與帶有胺基的有機物錯合時，將可能會提高汞本身被溶解態亞鐵離子還原的可能性，使得近80%的 Hg(II)被還原成Hg(0)。本研究結果意味著目前常用的現地生物整治工程需更為謹慎，因同樣依賴受污染地下含水層中的厭氧菌，使其刺激後進行污染物降解，過程中可能會無預警地改變汞或其他非整治標地污染物於地下水中的流佈行為，導致二次污染的情形發生。; 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. |
