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-以北投垃圾焚化爐為例

Probing the biogeochemical processes of methylmercury formation and accumulation in the paddy system in the vicinity of a municipal solid waste incinerator

張惟竣(Wei-Chun Chang)

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

自工業革命開始，環境中汞的排放源已知主要與人為活動有關，特別是焚化廠及燃煤發電廠的運作。汞經排出後，部分(當中絕大多數為氧化態的無機二價汞)會因沉降作用降至附近地表，並有機會被現地異營性的厭氧微生物轉化成毒性更強的甲基汞。早期的研究認為，甲基汞的曝露與中毒都是透過魚類海鮮的攝取而造成，但近期文獻發現，高濃度的甲基汞可從生長在離汞排放源相近的稻米中檢測出來，暗示著陸域生態系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，因此對於水稻田為何易成為甲基汞的生成環境，生地化循環如何涉入其過程，以及排放源造成甲基汞在稻米的累積效應為何是值得深入探討的課題。
有鑑於此，本研究針對北投垃圾焚化廠周圍水稻田的表土、表水、根系土壤與其孔隙水中的總汞與甲基汞，以及可能影響無機汞生地化循環(特別是與甲基汞生成相關)的參數進行樣品的採樣分析，並同時分析當地稻作的總汞和甲基汞含量，以了解此農地場址的汞物種背景值與生物有效性程度；除此之外，也藉由培養現地土壤的縮模試驗及分生技術調查場址內可能將汞甲基化的主要厭氧微生物族群；最後，利用水耕植栽試驗探討培養液中的化學組成對稻作吸收與累積甲基汞的影響。
調查結果指出，所選場址的土壤、孔隙水，以及栽種在此區的稻米，其總汞與甲基汞含量皆無超過法規的農地與食用米標準，且與文獻中所提的背景值相近，顯示排放源的空污防治設備可有效管控汞的排放，未造成此區稻田顯著的汞累積。而根據現地地質化學與微生物族群的分析可知：(1)孔隙水中的甲基汞與無機汞濃度呈顯著正相關；(2)場址內的硫與鐵條件適合汞甲基化菌群的生長；(3)硫酸鹽還原菌可能是場址內主要的汞甲基化菌群。綜合這些因素，暗示著若研究場址的總汞濃度增加，其根系環境將具有促進甲基汞生成與累積的潛勢，故定期監測此敏感生態系統的汞濃度變化有其必要。此外，藉由添加不同形式的配位基而改變稻作培養液的甲基汞化學組成後發現，配位化學可造成稻作不同程度的甲基汞攝取及累積，然此部分的機制原理仍需進一步的探究。

關鍵字

甲基汞生成 ； 水稻田 ； 現地環境生物地質化學 ； 焚化爐

並列摘要

Since the industrial revolution, the release of mercury (Hg) from emission sources to the environment has been predominantly resulted from human activities, with burning of fossil fuels and waste being the leading contributors. Once released, a partial amount of Hg (mostly in its divalent inorganic forms) would return to the Earth’s surface by wet or dry deposition and then be converted in situ by certain heterogeneous anaerobic bacteria to methylmercury (MeHg), the most toxic form of Hg and known for its great bioaccumulation tendency . While consumption of predatory fish and seafood has been considered the primary route for human exposed to MeHg, recent studies have reported high levels of MeHg in rice grown in the vicinity of anthropogenic Hg releasing sources, suggesting that ingestion of crops from the terrestrial food chain may be another critical route of human exposure to MeHg. Given that (i) rice is a staple food in Taiwan and throughout Asia and (ii) the potential for maternal MeHg 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 route 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, four paddy sites within the agricultural area of the Beitou municipal solid waste (MSW) incinerator were chosen to sample surface water, topsoil and root soil. Total Hg, MeHg, as well as ancillary geochemical/microbiological parameters in soil, porewater, and rice grains were analyzed. In addition, microcosm and hydroponic experiments were carried out to probe (i) the primary Hg methylators in the root soil of the study sites and (ii) the influence of coordination chemistry on the uptake of MeHg by roots of rice plants. Results showed that the levels of total Hg and MeHg in paddy soil and rice grains did not exceed the current standards set for farmland soil and edible rice, suggesting that our study sites are not contaminated with Hg and the air control devices employed in the Beitou MSW incinerator may have been efficient for the control of Hg emission. However, it is observed that both the bioavailability of inorganic Hg and the activity of Hg-methylating microbes increased during the early and mid rice growing season, presumably due to the anoxia created under flooded conditions. This suggested that the paddy ecosystem has a great potential for enhanced Hg-methylation if elevated inputs of Hg occurred, and hence there is a need for constant monitoring of the Hg level in this area. Results of microcosm experiments revealed that sulfate-reducing bacteria may be the principal Hg-methylators in the rhizospheric zones of the study sites. Molecular identification of the hgcA gene in the root soil samples further confirmed the existence of Hg-methylating microbes. Lastly, using different forms of ligands to alter MeHg speciation in the growing medium, preliminary results from the hydroponic culturing of rice implied that both passive diffusion and active transport mechanisms may all take place in the root uptake of MeHg in rice.

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