掩埋場滲出水環境之汞甲基化潛勢探討;Investigation of mercury methylation potential in the landfill leachate environment

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Title:	掩埋場滲出水環境之汞甲基化潛勢探討;Investigation of mercury methylation potential in the landfill leachate environment
Authors:	徐志昆;Hsu,Chih-Kun
Contributors:	環境工程研究所
Keywords:	掩埋場;甲基汞;生物可利用性;生物地質化學;生態衝擊;landfills;methylmercury;bioavailability;biogeochemistry;ecological impact
Date:	2016-08-25
Issue Date:	2016-10-13 15:26:05 (UTC+8)
Publisher:	國立中央大學
Abstract:	汞雖是具有神經毒性的微量元素，但因其獨有的物化特性，至今仍常用於某些工業、家用、醫療品項，以及消費性的電子產品。這些含汞的物件在廢棄後，最終將會不可避免地流入垃圾掩埋場中，特別是在垃圾分類及資源回收較不積極實施或推廣的地區。掩埋場中的汞由於可能因滲出水浸出以及揮發作用恐存在著衝擊周圍生態系統的污染潛勢問題，但過往的研究多僅著重於氣相汞的逸散調查，對於滲出水的汞流出以及隨後的環境污染問題則甚少著墨。然而，相較於其他地下環境，掩埋場所具有的高鹽度、高有機質濃度、顯著氧化還原狀態的改變 (即好氧、脫硝、硫酸鹽還原、鐵還原以及甲烷化彼此間的轉換) 等特性都將可能廣泛且深遠地影響汞在此系統內的化學組成、轉化作用以及移動潛力，尤其當操控生物性汞甲基化的基因組在經半世紀的努力後終於近期被鑑定出來，並藉此進一步地指出可現地生成甲基汞的厭氧微生物也包括常見的發酵菌與甲烷生成菌後，更加凸顯這類過往被忽略的地下場址需重新檢視其內汞物種的生地化轉換與流佈機制。有鑑於此，本研究先選定台灣北部三座鄰近的垃圾掩埋場作為研究場址，採樣分析其滲出水中與汞甲基化作用相關之生地化參數，藉以釐清掩埋場中汞的傳輸、宿命與污染潛勢。初步的調查結果如同預期，所選定之掩埋場因已停埋多年 (掩埋齡逾20年)，總汞的濃度僅約69 – 246 pM，且COD與TOC分別低於1,000 mg/L與200 mg/L，暗示著這些場址已達第三期末或第四期之掩埋成熟度。然而，所測得的DO與ORP則分別低於0.2 mg/L與介於20 – 200 mV，代表掩埋場滲出水仍為厭氧狀態 (貼近鐵還原狀態)，因此本研究進一步以滲出水為介質，並額外添加無機汞與甲基汞，以及汞甲基化純菌進行縮模實驗，用以模擬滲出水受汞污染時，汞的生物可利用性、(生物性與非生物性的) 氧化/還原與甲基化/去甲基化的程度，其結果指出滲出水將汞還原的能力甚低，進一步推估可能存在中性汞錯合物並透過被動擴散攝取至細胞體內生成甲基汞，然而現地環境中汞的生物可利用性仍不高，並於生物性去甲基化迅速的作用下抑制甲基汞累積，滲出水因而不具顯著的汞甲基化潛勢，代表著台灣掩埋場無須過度擔心甲基汞對環境造成的危害風險。然而，不同時期與型態的掩埋場於汞甲基化潛勢皆有所差異，期望本研究之結果可做為往後掩埋場評估之依據，同時對法規、管理層面以及風險評估上的科學佐證皆有所助益。;While mercury (Hg) is a well-known neurotoxic element, it is still being used for or incorporated into certain industrial processes and commercial products as a result of its unique physicochemical properties, which leads to a situation that some of these Hg-bearing solid wastes may eventually be disposed in conventional landfills, especially in places where systematic recycling practices are difficult to implement. The presence of Hg in landfills is a concern due to the potential for it to volatilize or leach from the landfill site and impact local ecosystems. However, earlier studies were merely focused on documenting the emissions of Hg as landfill gas. Little has been undertaken to investigate the possible release of Hg in leachate, as well as the biogeochemical processes that control Hg transformations in landfills. Given that the genomic information derived from a recently identified gene cluster (i.e., hgcAB) encoding for proteins essential for microbial Hg methylation has indicated that the anaerobic conditions in landfills can foster the growth of some fermentative bacteria, syntrophs and methanogens that were previously unrecognized but now are acknowledged as Hg-methylators, it is important to re-examine the speciation and in particular methylation potential of Hg within the landfill environment. Accordingly, in this study three landfills designated as A, B and C in northern Taiwan were chosen as the research sites to obtain leachate samples. Specific emphases were placed on characterization of geochemical conditions and determination of Hg bioavailability and transformations including reduction, methylation and demethylation in these settings. Preliminary results show that, as expected, concentrations of total Hg measured in the leachate samples (69-246 pM) lie within the range reported for the background environmental Hg level, presumably due to both the thorough and consistent implementation of recycle/reuse policy in Taiwan and a prolonged period of landfill termination. Levels of analyzed geochemical parameters such as 1000 mg/L COD, 200 mg/L TOC, 0.2 mg/L DO, as well as 20 – 200 mV ORP also support the observation that the landfill maturation of the study sties has reached at or beyond the stage of Phase IV. Further, microcosm experiments conducted with spike of inorganic Hg(II), methylmercury and Geobacter sulfurreducens PGA (i.e., a model Hg-methylating bacterium) into leachates in the presence or absence of bacterial growth inhibitors reveal that at this stage, the potential for in situ Hg methylation seems to be limited owing to poor bioavailability of inorganic Hg and vigorous biotic demethylation of organomercury. Together, not only can these results be used as the baseline data for future comparisons, but they also indicate that the impact of Hg contamination from the three studied landfill sites on the surrounding environment is likely trivial. Nonetheless, considering the fact that the chemical and microbiological conditions vary in different phases of the landfill maturation process, future study on the relative importance of these conditions in governing mercury transformations in prior phases, particularly Phases II & III, is warranted.
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