水 /氣界面間汞交換通量的研究進展＊

閆海魚馮新斌

(中國科學院地球化學研究所,環(huán)境地球化學國家重點實驗室,貴陽,550002)

水 /氣界面間汞交換通量的研究進展＊

閆海魚＊＊馮新斌

(中國科學院地球化學研究所,環(huán)境地球化學國家重點實驗室,貴陽,550002)

本文綜述了近年來國內(nèi)外大量文獻,對有關水體和大氣中汞的存在形態(tài)、特性及水/氣間汞交換通量的影響因素進行了總結,描述了目前國內(nèi)外相關領域的研究現(xiàn)狀,并對該領域下一步的研究方向進行了探討.

水 /氣界面,汞,通量.

汞是惟一可以在常溫下以液態(tài)存在并具有揮發(fā)性的金屬,它以多種化學形態(tài)存在于環(huán)境中,并在水體、大氣、土壤和生物體間不斷遷移轉(zhuǎn)化.其中,水體與大氣互為汞的“源”或“匯”,一方面,汞從水體向大氣的釋放減輕了水中汞的負荷,另一方面,也增加了大氣中汞的含量,反之亦然.水體和大氣界面間汞的交換通量決定著汞傳輸方式——進入大氣向周邊環(huán)境擴散,還是留在水中進一步轉(zhuǎn)化為甲基汞并隨水生食物鏈逐級富集到大型食肉性魚類體內(nèi),最終對人類健康構成威脅.因此,水體和大氣界面間汞交換通量的研究受到國內(nèi)外研究者的廣泛關注.

1 水體和大氣中汞的存在形態(tài)與特性

1.1 汞在大氣中的存在形態(tài)與性質(zhì)

單質(zhì)汞在常溫下具有很高的飽和蒸汽壓,且多數(shù)汞化合物也具有較強的揮發(fā)性.大氣中,95%以上的汞是氣態(tài)單質(zhì)汞 (gaseous elemental mercury,GEM即 Hg0)[1-3],其余不足 1%—5%是活性氣態(tài)汞(reactive gaseousmercury,RG M)和顆粒態(tài)汞 (particulate gaseousmercury,PG M)[4-5].GEM揮發(fā)性高和水溶性低且在大氣中相對惰性[6-10],可以長時間 (約 0.5—2年)滯留在大氣中[11-12],并隨全球大氣循環(huán)發(fā)生大范圍長距離的遷移.RG M主要是以 HgCl2為主的汞的鹵化物[5,13-14],具有較高的表面活性和水溶性[15],很容易通過降雨再次進入陸地生態(tài)系統(tǒng),對局地或區(qū)域性環(huán)境具有重要影響[5-16].PEM根據(jù)其顆粒粒度的大小不同,主要沉降在污染源附近,并隨污染源距離的增加含量逐漸減少.

1.2 汞在水環(huán)境中的存在形態(tài)和性質(zhì)

水環(huán)境中,汞的主要存在形態(tài)為溶解氣態(tài)汞 (dissolved gaseous mercury,DG M,主要是 Hg0)、二價無機汞 (Hg2+)、單甲基汞 (methylmercury,MeHg)和少量二甲基汞 (dimethylmercury,DMeHg).DG M在全球海水中占總汞的 10%—30%[17-18].DG M主要存在于淺表水層,Lindberg等[19]曾搜集了 11組水體表層 1cm、100cm和沉積物上部 5cm的水樣進行研究,結果發(fā)現(xiàn)有 9組 DG M含量是隨著水深增加而減少的,這支持了 DG M源于表層水的觀點.水中DG M含量的高低受幾個競爭性因素控制:可利用溶解態(tài)活性 HgⅡ(DRM),DRM光致還原率或其它產(chǎn)生過程,新生 DG M的水平對流,包括氧化和釋放在內(nèi)的DG M的損失[19].

由于 Hg0在水中的溶解度很低,通常它在天然水體,特別是表層水體中處于超飽和狀態(tài),導致大量H會從水體向大氣釋放[5].研究表明,水體是大氣汞的重要自然釋放源之一[4,20-21],而水體向大氣的排汞過程成為汞從水體移除的一個主要途徑[22].目前對水中 DG M形成的機理還不清楚,但現(xiàn)有的研究顯示水中DG M形成的機理可能很多,其中最重要的是水中微生物將二價汞還原為單質(zhì)汞[23],或者是非生物在腐殖質(zhì)存在的情況下將二價汞還原為單質(zhì)汞[24],或者有機汞化合物的降解產(chǎn)生單質(zhì)汞[18,25].最近研究也顯示,二價汞的光致還原是 DG M產(chǎn)生的另一個重要的機制[26-27].水中 DG M的損失主要是向大氣的釋放和被氧化成二價汞 (如在氯離子存在的情況下)[28].

2 水/氣界面間的汞交換及其影響因素

水-氣間汞交換過程是汞在大氣、陸地和水體之間生物地球化學循環(huán)的一個非常重要的環(huán)節(jié)[21].一方面,水體向大氣釋放單質(zhì)汞,減輕了水體汞的負荷,減少了水中汞的甲基化幾率,從而減少了魚體內(nèi)甲基汞富集的可能.另一方面,大氣中單質(zhì)氣態(tài)汞可參與全球大氣循環(huán)傳播和擴散,隨大氣干、濕沉降進入更廣闊的環(huán)境系統(tǒng),因此水體又是大氣汞的重要來源.

大量的研究顯示,水體每年向大氣釋放的汞約占大氣汞天然來源的 32%—77%[4,20,29-30].王定勇[31]對重慶土壤和水體汞釋放通量的研究結果顯示,水體的汞釋放通量明顯高于土壤.由此可見,水體對大氣汞的貢獻是非?？捎^的.水-氣間汞交換通量方面的研究,一直受到研究者的廣泛關注.目前,這方面的研究主要集中在水-氣間 Hg0的交換通量上[31-35],也有少量研究對水-氣間甲基汞的釋放通量進行了初步探討[36].

水-氣間汞交換通量的研究顯示,汞交換通量存在如下三個規(guī)律:晝夜變化規(guī)律、季節(jié)變化規(guī)律和晴雨天變化規(guī)律.(1)晝夜變化規(guī)律.水體向大氣汞的釋放通量極大地取決于光照強度,并與光照強度具有顯著的相關性,這種現(xiàn)象主要表現(xiàn)為水體向大氣汞的釋放通量在白天和光照較強的時間段出現(xiàn)峰值,夜晚和光照較弱的時間段出現(xiàn)谷值[37-41].G?rdfeldt[42]等的研究認為這主要是因為白天比夜晚有更強的紫外光照射,特別是 UVA,促進了汞的光致還原作用,從而使表層水中產(chǎn)生更多的 DG M,他的研究結果也證明了這點,即 DG M的釋放通量與光照呈現(xiàn)顯著相關性,相關系數(shù)R2=0.99.在此基礎上,部分環(huán)境參數(shù)的改變?nèi)缢?、DOC含量,將改變水體垂直方向上的受光照程度,從而產(chǎn)生一般水體的 DG M隨水深減少的趨勢,以及水氣界面汞通量晴天大于陰天,白天大于夜晚的現(xiàn)象.另外兩個點出現(xiàn)底部水中DG M含量也較高,這意味著除了光照之外,還有別的條件會影響到DG M的產(chǎn)生和逸出.(2)季節(jié)變化規(guī)律.春、夏季節(jié) (或暖季節(jié))釋放通量高于秋、冬季節(jié) (或冷季節(jié))[31,35,40-41].如 Zhang和 Dill[43]對一個水庫進行為期一年的研究顯示,水中DG M含量在夏季的六到八月最高,之后逐漸降低,12月達到最低值,春夏比秋冬季節(jié)有更高的DG M平均產(chǎn)率,且季節(jié)變化趨勢也顯示出似乎接近日光輻射變化規(guī)律.其次,光照對溫度的影響會影響到水溫,溫度的變化會影響到 DG M在水中的飽和度,暖季節(jié)水溫上升,DG M在水中的溶解度下降,趨向于向大氣的釋放,因此導致水體向大氣汞釋放通量增加.(3)晴雨天的變化規(guī)律.晴天表現(xiàn)為水體向大氣汞的釋放,陰天和降雨期間表現(xiàn)為大氣向水體的汞沉降[35,44].對雨天汞出現(xiàn)負通量的原因和機理還不清楚,目前認為可能的原因是 Hg被通量箱壁或水表本身吸收,一旦太陽出來,蒸發(fā)開始,通量再次變成正值[34].

就目前的認識來看,水體中的DG M主要來自于表層水中二價汞的還原,水中DG M的產(chǎn)生量與驅(qū)動DG M釋放的因素是直接影響水體向大氣汞釋放通量的主要因素.對于 DG M從水中向大氣擴散的驅(qū)動力方面,則普遍認為主要是受到水氣界面間DG M濃度梯度及熱力學方面的影響.另外,許多物理化學參數(shù)可以加速這種轉(zhuǎn)化增加水體向大氣汞的釋放量,包括光照強度、水溫、pH、和 DOC濃度[26,28,45-47]等.這些參數(shù)中,光照強度是影響DG M產(chǎn)生的主要因素,光照的增加會促進DG M的產(chǎn)生,同時,光照增加使水體溫度增加,降低了DG M在水中的溶解度,從而表現(xiàn)為水體向大氣釋放汞的通量增加[39,48-49],而其它參數(shù)對天然水體中DG M的產(chǎn)生所起的作用仍然存在爭議.比如:對貴州省境內(nèi)的紅楓湖水體與大氣間交換通量的最新研究顯示,在天氣光照不足 140W·m-2的陰雨天氣,水體與大氣間汞交換通量并沒有明顯的晝夜變化規(guī)律,且與光照沒有顯著正相關關系[33,50].此外,水體汞釋放還受鹽度、風速、浪花等影響[44],其相互的關系也不是簡單的正負相關關系,而是比較復雜的函數(shù)關系.而紫外光的類型 (UV-A或UV-B)對 DG M也有影響,春夏比秋冬季節(jié)有更高的DG M平均產(chǎn)率,且季節(jié)變化趨勢也顯示出似乎接近日光輻射變化規(guī)律.

為了查明各項參數(shù) (特別是光照強度及其與之共同作用的其它參數(shù))對水體與大氣間 DG M交換量的主要影響機制,除了野外的實測,也有人進行了部分室內(nèi)模擬研究.研究發(fā)現(xiàn),日光輻射通過一系列的途徑產(chǎn)生活性氧化形態(tài),且常常只有不到 1s的環(huán)境半衰期.水中 DG M被羥基[7,42]O、O3[6,51],有機過氧功能團[52],和氯化物及有機化合物所氧化[26,28],導致含量降低.然而,也有研究發(fā)現(xiàn)DG M在黑暗中也會減少[22,26,49,53].這意味著水中存在受長效光介質(zhì)控制的氧化作用或者是存在非光致氧化作用.法國的一項研究顯示[54],顆粒相 (微生物、氧化物等)與還原過程有關.該研究對過濾和不過濾的天然水用 300—450nm的光在有氧和無氧狀況下照射 4d,發(fā)現(xiàn) DG M在黑暗期間也能觀察到,且在過濾的水中 DG M似乎形成的量很低,而在未過濾水樣中觀察到明顯的還原過程,且只有在通氮氣時該過程變的較為顯著.

3 問題與展望

據(jù) Park等[55]對韓國南部 Juam水庫的研究和近年來美國部分湖泊、中國貴州省的百花湖夏季 DG M濃度的比較結果顯示,中國的淡水湖水體 DG M遠高于韓國,而韓國又高于美國.他認為韓國被研究區(qū)域水體沒有直接的人為汞污染源,其DG M偏高的原因除了源于本地少數(shù)上風向的工廠之外,很有可能是來自中國這樣的高汞釋放源區(qū).本文作者對貴州省百花湖水體向大氣釋汞通量的現(xiàn)場測定也顯示較高的結果:其中平均值為 4.0—9.7 ng·m-2·h-1,最大值高達 51 ng·m-2·h-1[35],顯著高于意大利的西西里海峽 (0.1—0.3ng·m-2·h-1[40])、美國北明尼蘇達湖 (0.04—0.05 ng·m-2·h-1[56]) 、美國阿拉斯加湖(均值為 1.2 ±0.4,范圍為 0.5—1.7 ng·m-2·h-1[57])、美國威斯康辛湖 (0.4—2.3 ng·m-2·h-1[58])、加拿大安大略湖 (0.04—1.3 ng·m-2·h-1[59];中值為 0.8—1.9 ng·m-2·h-1[33])和密西根湖 (1.0±0.6 ng·m-2·h-1[60]).由于我國是目前公認的大氣汞排放國家,以上兩個事例暗示我國較豐富的湖泊 /水庫系統(tǒng)可能是我國及周邊地區(qū)大氣汞的一個重要的天然來源.由于我國相關研究十分有限,因此對不同類型水域的深入研究十分必要.第二,盡管向大氣汞的貢獻主要來自于海洋,但對于淡水系統(tǒng)向大氣釋放汞的研究可作為海洋相關研究的重要補充.第三,根據(jù)目前國內(nèi)外研究現(xiàn)狀可知,水/氣界面間甲基汞通量測定方面,無論是實驗方法還是實驗結果都很少.第四,盡管環(huán)境樣品中甲基汞的含量極低,但因其毒性很強,仍是應當嘗試研究的領域.在DG M的形成機制和 DG M向水體釋放的驅(qū)動機制方面,需要更詳細的室內(nèi)模擬實驗和野外測試結果來驗證.

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ABSTRACT

Recentwork reported in a large number of references on the water/air exchange flux ofmercury from both China and abroad is reviewed in this paper.Mercury species and characteristics in the water and air environment as well as the main factors affecting the mercury exchange flux be tween water and air are summarized.Based on the summary,the current research status and future research direction are discussed.

Keywords:water/air interface,mercury,exchange flux.

RESEARCH DEVELOPM ENT ONWATER/A IR EXCHANGE FLUX OF M ERCURY

YAN HaiyuFENG X inbin
(State KeyLaboratory of Environmental Geochemistry,Institute of Geochemistry,Guiyang,550002,China)

2010年 3月 12日收稿.

＊國家自然科學基金資助項目(No.40803036,No.40973083).

＊＊通訊聯(lián)系人,E-mail:yanhaiyu@vip.skleg.cn