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研究生： 宿彥彬
Yen-Bin Su
論文名稱： 鄰近汞排放源之水稻田受現地地質化學與微生物影響之甲基汞生成與累積作用 -以台中火力發電廠為例
Probing the biogeochemical processes of methylmercury formation and accumulation in the paddy system in the vicinity of a coal-fired power plant station
指導教授： 林居慶
口試委員:
學位類別： 碩士
Master
系所名稱： 工學院 - 環境工程研究所
Graduate Institute of Environmental Engineering
論文出版年： 2015
畢業學年度： 103
語文別： 中文
論文頁數： 107
中文關鍵詞： 汞循環 、環境生地化 、甲基汞生成 、水稻田 、火力發電廠
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    • 汞由於其獨特的化性，已被認定為是全球性污染物。目前已知釋放至環境中的汞，主要是人為活動所產生，尤以燃煤發電廠及焚化廠的貢獻量最大。氧化態的無機汞因其大氣停留時間相對短暫，被排放後將因乾濕沉降作用回到地表，並有機會被環境中某些特定的厭氧菌群轉化成為毒性更強的甲基汞，之後再經食物鏈的累積放大效應而對生態與人類造成健康威脅。以往的觀念中，甲基汞所帶來的毒害問題幾乎都是經由對魚類海鮮的攝食所造成；然而，近期的文獻顯示，生長在離汞排放源相近田地上的稻米已被檢測出含高濃度的甲基汞，暗示著除了一般所認知的水域生態系統外，陸域生態系統中的食物也可能成為甲基汞的攝食途徑之一。由於稻米是台灣，也是許多亞洲地區人民的主食，雖然藏於米粒內的汞濃度或許不高，但若以長期攝取的總量觀點來看，其對健康所帶來的影響值得關注。為此，對於水稻田為何易成為甲基汞生成的環境，環境生地化的作用與循環機制如何涉入其過程，以及特定(潛在)排放源對於鄰近地區的水稻田系統的甲基汞累積效應為何，有待進一步的研究與探討。
    本研究以台中火力發電廠周圍的水稻田為研究場址，對其表水、表土、根際土與其孔隙水、以及場址內收成之稻米進行總汞、甲基汞及可能影響汞甲基化反應之地化參數進行分析，盼藉此明瞭汞於現地場址的生物有效性程度，並同時將現地根際土壤當做植種源進行縮模試驗，搭配汞甲基化基因作為生物標記，進一步分析現地根際圈內可能的主要汞甲基化菌群。除此之外，也藉由水耕植栽試驗，在調控稻作培養液內不同甲基汞的配位化學條件下，初步探究孔隙水的化學組成對於稻作吸收與累積甲基汞的效應為何。調查結果指出，由現地不論是根際土、根際土壤之孔隙水、表面土以及稻米的總汞與甲基汞濃度來看，相較於過去文獻與法規值，本研究所挑選鄰近台中火電廠的兩水稻田場址均屬於未受汞污染之地區，推測一直以來台中火力發電廠對於廠內所設置的與汞排放相關之空氣污染防治措施應相當完善，使得排出的廢氣並未對鄰近的水稻田農地造成汞污染與累積。而根據地化參數分析、縮模試驗及分生試驗的結果得知，水稻田在覆有表面灌溉水的生長期間，根際土內具有最高的微生物活性以及汞的生物可利用性，且硫酸鹽還原菌群可能為現地主導汞甲基化的主要菌群。總結上述調查結果，暗示著覆水的水稻田為具有高汞甲基化潛勢的場址，因此一旦場址在水稻生長期間受到外來汞污染，其根際土環境很有可能會將無機汞進一步轉化成甲基汞，進而造成場址內甲基汞的生成及後續稻米內的累積問題。最後，水耕植栽試驗的結果指出不同型態的甲基汞確實會對稻作的吸收造成影響，且由結果初步推測稻作吸收甲基汞的背後可能同時隱含著被動擴散與主動運輸等機制，但實際為何仍有待後續的研究進一步確認。

    Mercury is a highly toxic trace element that has been recognized internationally as a global priority pollutant. Current inventories of mercury emissions indicate that anthropogenic activities are the major sources of mercury inputs to the environment, with coal combustion and solid waste incineration accounting for more than half of the total emissions. Once released, inorganic oxidized forms of mercury with relatively short atmospheric residence time would be deposited locally, then be converted by specific groups of anaerobic bacteria to methylmercury, a potent neurotoxin that can readily accumulate and magnify in biota, particularly in the aquatic food web. However, in terrestrial food chains, because lowland rice paddies display ecological functions similarly to wetlands that have been known as important sites for methylmercury formation, the paddy system can be potentially considered “hotspots” of mercury methylation. Indeed, recent studies have reported that aside from consumption of fish and seafood, high levels of methylmercury are detected in rice grown in the vicinity of anthropogenic mercury emission sources, suggesting that ingestion of rice may be another important human exposure route to methylmercury. Given that rice is a staple food in Taiwan and throughout Asia and the potential for maternal methylemrcury exposure (even at low-level) through ingestion of rice that may subsequently impact health of the offspring, it is important to conduct thorough investigation of this exposure pathway by examining why rice paddies are conductive for Hg methylation, which biogeochemical reactions may have been involved in this process, and also how additional inputs resulted from anthropogenic perturbations may eventually lead to the potential accumulation of Hg and MeHg in rice plants.
    In this study, surface water, surface soil and rhizospheric soil and porewater in two rice fields near the Taichung Coal-Fired Power Plant Station were sampled. Analyses included total mercury, methylmercury and the geochemical parameters which may influence the mercury methylation cycle. In addition, microcosm, gene-probing and hydroponic experiments were carried out to investigate the primary microbes and processes that might have controlled the production of methylmercury in our study sites. Our results suggest that levels of total Hg and MeHg in paddy soil and rice grains did not exceed the current control standards set for farm land and edible rice, suggesting that the study sites are not contaminated with Hg and the air control devices employed in the coal-fired power plant may have been efficient for the control of Hg emission. However, it is observed that both bioavailability of inorganic Hg and the activity of Hg-methylating microbes were increased during the early and mid rice growing season. Results of soil incubation experiments and molecular probing revealed that sulfate-reducing bacteria may be the principal Hg-methylators in the rhizospheric zones of the study sites, suggesting that the paddy ecosystem has a great potential for enhanced Hg-methylation if elevated inputs of Hg occurred. Finally, results of hydroponic experiments implied that both passive diffusion and active transport may take place in the root uptake of MeHg in rice plants.

    目錄 第一章 前言 1 1.1研究背景 1 1.2 研究目的 3 第二章 文獻回顧 4 2.1. 環境中汞的排放源與甲基汞的危害 4 2.2. 濕地為汞甲基化的熱點場址 6 2.3. 環境因子對汞甲基化反應的影響 7 2.3.1 微生物 7 2.3.2 硫 10 2.3.3 鐵 12 2.3.4 鹽度 14 2.3.5 有機質 15 第三章 實驗材料、方法與設備 21 3.1 實驗架構 21 3.3 現地採樣實驗規劃與樣品前處理 24 3.4 藥品與試劑 26 3.5 地質參數分析 26 3.5.1 總汞分析 26 3.5.2 甲基汞分析 28 3.5.3 有機質分析 29 3.5.4 鐵分析 29 3.5.5 孔隙水溶解性硫化物分析 31 3.5.6 酸揮發性硫化物(Acid-volatile sulfide ,AVS)分析 31 3.5.7 硫酸鹽濃度分析 33 3.6 微生物縮模實驗 33 3.7 植栽試驗 38 3.8 分生試驗 41 3.8.1 土壤DNA萃取 42 3.8.2 樣品PCR放大 42 3.8.3 TA cloning 42 3.8.4 質體抽取 44 第四章 結果與討論 46 4.1 現地環境地質參數分析 46 4.2 稻田土壤及孔隙水中總汞與甲基汞濃度討論 54 4.3 現地根際土所主導的汞基化菌群探討 66 4.4 分生試驗結果探討 73 4.5 不同型態甲基汞對於稻作攝取的影響探討 77 4.6 本研究場址與北投場址比較 81 第五章 結論與建議 85 5.1 結論 85 5.2 建議 86 參考文獻 89   圖目錄 圖2.1 無機汞自人為設施排放後的傳輸途徑(Carpi, 1997) 5 圖3.1 實驗架構圖 22 圖3.2 台中火力發電廠周圍地圖 23 圖3.3 採樣點與汞排放源的相對位置 24 圖3.4 採樣管所採集的相對位置。洞口處為採樣後所留下的痕跡，所採集的土樣主要以臨近植物根部的根際土壤為主。 25 圖3.5 分光光度計分析Fe(II)之檢量線 30 圖3.6分光光度計分析S(-II)之檢量線 31 圖3.7 AVS之purge and trap實驗圖 32 圖3.8 微生物縮模實驗設立完成圖 37 圖3.9 稻作植栽實驗設立完成圖。上圖為植栽試驗初步設立完成圖，而下圖則為稻作的生長階段。 40 圖3.10 分生試驗流程示意圖 41 圖4.1 汞與甲基汞於稻田中的傳輸、轉換示意圖。黃色實心圈圈表示地化參數可能對於汞甲基化反應所帶來的影響。一旦無機汞進入至稻田場址，現地土壤的地化參數會改變無機汞的型態而影響了微生物對於無機汞的生物可利用性。此外，現地所存在的地化條件亦會決定現地所主導的微生物菌群。當無機汞被微生物甲基化成甲基汞後，甲基汞亦會因現地地化作用形成不同型態的甲基汞，而何種型態的甲基汞容易進入稻作中，其背後可能的吸收機制將在本章節進行探討。 46 圖4.2 台中各稻田在不同生長期間內根際土壤之總鐵(上圖)與亞鐵(下圖)濃度變化 49 圖4.3台中各稻田在不同生長期間內孔隙水的總鐵(上圖)與亞鐵(下圖)變化 50 圖4.4 台中各稻田在不同生長期間孔隙水硫酸鹽濃度變化 52 圖4.5 台中各稻田在不同生長期間根際土之AVS濃度變化 53 圖4.6 根際土中亞鐵與AVS的相關性 53 圖4.7 兩稻田根際土中的總汞濃度變化 56 圖4.8 兩稻田孔隙水中的總汞濃度變化 56 圖4.9 兩稻田根際土中的甲基汞濃度變化 59 圖4.10 兩稻田孔隙水中的甲基汞濃度變化 60 圖4.11 孔隙水甲基汞濃度與根際土AVS濃度的相關性 60 圖4.12 孔隙水甲基汞濃度與根際土亞鐵濃度的相關性 61 圖4.13 孔隙水甲基汞濃度與孔隙水總鐵濃度的相關性 61 圖4.14 孔隙水甲基汞濃度與孔隙水亞鐵濃度的相關性 62 圖4.15 孔隙水中甲基汞濃度與孔隙水總汞的相關性 62 圖4.16 縮模試驗經培養數週後硫酸鹽還原菌組別之液體硫化物濃度 68 圖4.17 縮模試驗經培養數週後鐵還原菌組之液體亞鐵濃度 68 圖4.18 縮模試驗經培養數週後甲烷菌組別之氣體甲烷濃度 69 圖4. 19兩場址之根際土壤分別添加不同刺激劑/抑制劑與無機汞，培養一段時間後其系統內的甲基汞生成濃度(ng / L)。其中control組別(包含SRB、IRB及Methanogen)為利用已滅過菌土壤當做植種源的控制組，In situ表示未添加任何刺激劑與抑制劑的組別，sti(包含SRB、IRB及Methanogen)為添加針對三種菌群之刺激劑組別，in(包含SRB及Methanogen)則表示為添加抑制劑的組別。 71 圖4.20 利用針對hgcA的引子對現地樣本做跑膠分析。 74 圖4.21 經Transformation後所呈現的藍白菌落。 75 圖4.22不同濃度與不同配位基下，甲基汞累積於稻作植物中的濃度 81 圖4.23 北投垃圾焚化爐周圍圖與採樣農地彼此以及與焚化爐煙囪的相對位置 82 表目錄 表3.1微生物縮模試驗培養液組成 35 表3.2針對現地場址本身汞甲基化組別的培養液組成 36 表3.3 植作試驗培養液成份 39 表3.4 TA cloning反應之ligation配置劑量 44 表4.1 鄰近台中火力發電廠兩稻田場址內的表面土、根際土、稻米總汞與甲基汞濃度與場址內稻米的生物累積因子平均值整理 66 表4.2 以PHREEQC模擬縮模試驗鐵還原菌群組別之主要物種分佈 72  

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