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

宿彥彬(Yen-Bin Su)

指導教授 ： 林居慶

國立中央大學/工學院/環境工程研究所/碩士(2015年)

摘要

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。

關鍵字

汞循環 ； 環境生地化 ； 甲基汞生成 ； 水稻田 ； 火力發電廠

並列摘要

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.

參考文獻

75. Stein, E. D., Y. Cohen, and A. M. Winer, “Environmental distribution and transformation of mercury compounds”, Critical Reviews in Environmental Science and Technology, vol. 26, pp. 1-43,(1996).

59. Parks, J. M., A. Johs, M. Podar, R. Bridou, R. A. Hurt, Jr., S. D. Smith, S. J. Tomanicek, Y. Qian, S. D. Brown, C. C. Brandt, A. V. Palumbo, J. C. Smith, J. D. Wall, D. A. Elias, and L. Liang, “The genetic basis for bacterial mercury methylation”, Science, vol. 339, pp. 1332-1335,(2013).

1. Aiken, G. R., C. C. Gilmour, D. P. Krabbenhoft, and W. Orem, “Dissolved organic matter in the Florida Everglades: implications for ecosystem restoration”, Critical Reviews in Environmental Science and Technology, vol. 41, pp. 217-248,(2011).

3. Barkay, T., M. Gillman, and R. R. Turner, “Effects of dissolved organic carbon and salinity on bioavailability of mercury”, Appl Environ Microbiol, vol. 63, pp. 4267-4271,(1997).

5. Barringer, J. L., and Z. Szabo, “Overview of investigations into mercury in ground water, soils, and septage, new jersey coastal plain”, Water, Air, and Soil Pollution, vol. 175, pp. 193-221,(2006).