全氟辛烷磺酰胺在小麥和蚯蚓中的富集與轉化

吳思寒,吳雨濛,王雙杰,陳 萌,劉 青,祝凌燕

全氟辛烷磺酰胺在小麥和蚯蚓中的富集與轉化

吳思寒,吳雨濛,王雙杰,陳 萌,劉 青,祝凌燕*

(南開大學環境科學與工程學院,環境污染過程與基準教育部重點實驗室,天津城市生態環境修復與污染防治重點實驗室,天津 300350)

研究了不同培養介質和培養方式下全氟辛烷磺酰胺(PFOSA)在小麥、蚯蚓體內的生物富集和轉化.結果表明:小麥根系可以從培養介質中吸收PFOSA并向上轉運至莖葉.土壤中PFOSA生物有效性受總有機碳(TOC)的影響顯著,高TOC含量土壤中PFOSA的生物有效性降低,導致其在小麥和蚯蚓中的生物富集因子分別由(61.24±8.42)和(21347.91±208.86)降至(5.61±0.23)和(1404.92±108.21).PFOSA在小麥的根和莖葉以及蚯蚓中都可以轉化為PFOS,但在蚯蚓中的轉化率((3.87±1.71)%)顯著低于小麥((26.39±3.02)%).小麥根中PFOS的支鏈異構體(-PFOS)比例在低、高TOC含量時分別為(14.8±2.0)%、(66.1±26.2)%,低于莖葉(分別為(63.0±21.3)%、(85.2±2.4)%)),可能是由于根部轉化生成的-PFOS更容易向莖葉轉運.小麥特別是小麥莖葉中的-PFOS比例((85.2±2.4)%)顯著高于蚯蚓((16.5±4.0)%).小麥的存在可以提高土壤中PFOSA的生物有效性,從而促進蚯蚓對PFOSA的富集,但對其轉化影響不大.本文為小麥和蚯蚓中PFOSA的富集和轉化提供了證據,有助于探索環境中PFOS的間接來源.

全氟辛烷磺酰胺(PFOSA)；小麥；蚯蚓；生物富集；生物轉化

全氟和多氟化合物(PFASs)因其優異性質已在日常生活中被廣泛使用,如食品包裝材料、消防泡沫、表面活性劑、紡織品等[1].PFASs結構中的C-F鍵是最強的共價單鍵,因此PFASs極為穩定,難以發生生物和非生物降解(如水解、光解),可在環境中長期存在,并發生生物富集、生物放大效應[2-3].全氟辛烷磺酸(PFOS)是研究最多、環境中最常見的一種PFASs[4],已在環境介質和人體中[5-10]廣泛檢出,并對生殖、免疫、內分泌和神經系統以及肝臟均具有毒害作用[11-12],因此于2009年列入斯德哥爾摩公約,被逐步禁用[13].除直接排放外,環境中PFOS的另一主要來源是環境中前體物(PreFOS)的轉化[14].據報道,1970～2002年間,PreFOS的最大歷史排放量(6800 ～45250t)要遠高于PFOS(450～2700t)[15].全氟辛烷磺酰胺(PFOSA)是一種在環境中廣泛檢出的典型PreFOS[16-19].它是許多其他大分子PreFOS轉化為PFOS的中間物質,且是限速步驟[20-26].工業生產的PFOSA中,支鏈異構體(-PFOSA)比例為24.7%[27]. PFOSA的轉化存在異構體選擇性,其異構體轉化特征會對環境中PFOS的異構體分布特征產生影響,有助于追溯環境中PFOS的來源[28].但目前關于PFOSA在陸生動植物中的特異性轉化研究較少,因此對其進行研究十分必要.

土壤是PFASs在環境中的重要歸屬地之一[6], PFASs能通過點源污染、大氣沉降、地面徑流等進入土壤系統[29].例如,在美國空軍設施附近的土壤中PFOSA最高濃度甚至已達到20000ng/g[6].土壤中PFASs可以通過植物或動物進入食物鏈,經食物鏈的富集產生生物放大效應,對人體健康產生危害.小麥(L.)是一種世界各地都廣泛種植的禾本科農作物,是人類食物的重要來源之一[12].蚯蚓()是土壤中生物量最大的無脊椎動物,其生命活動會直接或間接地對有機污染物在土壤中的遷移和轉化產生影響[30].土壤的性質、生物種類等都會影響PFOSA的生物富集和生物轉化,因此本文以小麥、蚯蚓為受試生物,PFOSA為目標污染物,研究不同培養介質和培養方式下PFOSA在小麥、蚯蚓體內的生物富集和轉化,為探索環境中PFOS的間接來源、評價PFOSA的人類健康和生態風險以及環境修復提供理論參考.

1 材料與方法

1.1 實驗材料

實驗所用試劑如下:全氟辛烷磺酰胺工業品(PFOSA,>90%,北京百靈威科技有限公司),全氟辛烷磺酸標準品及其內標(PFOS和13C-PFOS,>99%)、全氟辛烷磺酰胺標準品及其內標(PFOSA和13C- PFOSA, >99%)購自加拿大惠靈頓實驗室.甲醇(色譜純)、過氧化氫30%(H2O2,質量分數,分析純)均購自天津市化學試劑供銷公司,甲基叔丁基醚(分析純,天津市康科德科技有限公司),氫氧化鈉(NaOH,優級純)、四丁基硫酸氫銨(TBAHS,分析純)、無水碳酸鈉(Na2CO3,優級純)均購自天津市津科精細化工研究所.甲酸(色譜純,上海安譜實驗科技股份有限公司)、氨水(色譜純,阿拉丁)、甲醇(HPLC級)和乙腈(HPLC級)購自美國Fisher公司.

實驗所用小麥種子(L.)購自天津農科院,所用赤子愛勝蚓()購于天津當地養殖場.

1.2 暴露實驗

1.2.1 小麥在土壤和石英砂介質中的暴露實驗 首先將小麥種子于3% H2O2(質量分數)中浸泡30min消毒,用Milli-Q水沖洗5次后,用Milli-Q水浸泡一夜,之后在鋪有濕濾紙的底部未全封閉的透氣托盤架上均勻鋪開,使之于22～27℃下發芽,托盤架上部鋪一層鋁箔以保證含水率,防止水分蒸發過快.為了研究土壤中TOC對PFOSA生物有效性的影響,實驗采用2種培養介質種植小麥,分別為土壤和石英砂,設立4組實驗:有毒有植物、無毒有植物、有毒無植物和無毒無植物,染毒濃度為200ng/g,每組設置3個平行.將發芽3d、大小均一的小麥幼苗移植到1L塑料花盆中,每盆裝有650g培養介質,種植7株小麥.將花盆放置于采光較好的天臺處培養,每天隨機更換盆的位置以保證小麥受陽光照射條件良好,每天澆水使土壤含水率保持在30%左右,14d后收獲小麥.收獲時用蒸餾水沖洗小麥根部,之后用濾紙擦干,將小麥根和莖葉分離,棄去根莖連接處,于-20℃冰箱中儲存.

1.2.2 蚯蚓在不同TOC含量土壤中的暴露實驗 將蚯蚓于未染毒土壤中避光馴養14d.實驗采用土壤和土壤/石英砂(8:2,質量比)兩種培養介質,染毒濃度為0, 200ng/g,設置3個平行.將馴養好的蚯蚓放入250mL燒杯,每杯裝有100g培養介質,放入10條蚯蚓.將燒杯用鋁箔包裹,避光培養,每天加水保持土壤濕度在30%左右,分別培養7和14d后收獲蚯蚓.收獲時從土壤中取出蚯蚓并用水清洗,在裝有潮濕脫脂棉的玻璃杯中放置24h清腸后用水沖洗,再用濾紙吸去水分,于-20℃冰箱中儲存.

1.2.3 蚯蚓-小麥聯合暴露實驗 小麥種子的培育見1.2.1,蚯蚓馴養見1.2.2.將赤子愛勝蚓與小麥聯合培養,采用土壤和土壤/石英砂(8:2)兩種培養介質,染毒濃度為0, 200ng/g,設置3個平行.將發芽3d的小麥和馴養完成的蚯蚓放入1L塑料花盆中,每盆裝有650g培養介質,加入7株小麥和10條蚯蚓.用鋁箔包裹花盆邊緣并高出邊緣3cm,以防蚯蚓在實驗期間爬出.每天加水保持土壤濕度在30%左右,14d后收獲小麥和蚯蚓.小麥和蚯蚓的取樣方式同1.2.1和1.2.2.

1.3 樣品前處理

生物樣前處理采用Chen等[31]的方法并稍作改進.將生物樣置于冷凍干燥機中-50℃下干燥48h,剪碎.稱取一定質量干重樣品(根取0.01g,莖葉取0.05g,蚯蚓取0.1g)于10mL PP 離心管中,加入2ng內標(包括M4-PFOS和M8-PFOSA),渦旋振蕩,使其充分混勻,平衡至少2h.依次加入1mL 0.5mol/L的TBAHS 溶液(pH值用氫氧化鈉溶液調至10)和2mL 0.25mol/L的Na2CO3緩沖液,充分混勻后,加入4mL甲基叔丁基醚,混合液于250r/min下震蕩20min,然后在4℃、6000r/min下離心10min使有機相與水相分離,將上層有機相轉移至新10mL離心管中.萃取過程重復2次,萃取液氮吹至干.用2mL甲醇復溶,加入50mg Carb填料以去除色素等雜質,混勻后在4℃,11000r/min下離心15min.取上清液至新離心管中氮吹干.用1mL甲醇復溶,過0.22μm尼龍濾膜后轉移到進樣小瓶中,放在凍存盒中于-20℃冰箱中保存,以待分析.

培養介質前處理采用Zhao等[30]的方法并稍作改進.將培養介質置于冷凍干燥機中-50℃下干燥48h,將1g干燥的介質置于10mL PP離心管中,加入2ng內標(包括M4-PFOS和M8-PFOSA),渦旋振蕩,使其充分混勻,平衡至少2h.加入5mL甲醇,然后于250r/min下震蕩10min.將離心管于40℃超聲10min.將該混合物在3000r/min下離心10min.重復提取3遍,并將上清液合并到另一個新的離心管中.萃取液氮吹至2mL.加入50mg Carb填料以去除色素等雜質,混勻后在4℃,11000r/min下離心15min.取上清液至新離心管中氮吹干.用1mL甲醇復溶,過0.22μm尼龍濾膜后轉移到進樣小瓶中,放在凍存盒中于-20℃冰箱中保存,以待分析.

1.4 儀器分析

在負電噴霧電離模式下,使用Waters超高效液相色譜-質譜聯用儀(ACQUITY-UPLC/XEVO-TQS)分析樣品中各物質含量.色譜柱采用FluoroSep-RP Octyl柱(150mm′2.1mm,3μm粒徑; ES Industries, West Berlin,NJ).柱溫為38℃,進樣體積為10μL,流動相流速為0.15mL/min.對于PFOSA異構體,流動相A、B分別為3mmol/L甲酸的水溶液(pH值用氨水調至4.15)和乙腈溶液.流動相初始比例為60%A、40%B,保持1min,在3min變為74%B,在35min變為80%B,在35.1min變為100%B并保持至40min,在45min回到初始比例.對于PFOS異構體,流動相A、B分別為3mmol/L甲酸的水溶液(pH值用氨水調至4.15)和甲醇溶液.流動相初始比例為60%A、40%B,保持0.3min,在1.9min變為64%B,在5.9min變為66%B,在7.9min變為70%B,在26min變為74%B,在30min變為100%B并保持至33min,在35min回到初始比例并保持至40min.質譜條件為:毛細管電壓2700V;離子源溫度150℃;去溶劑溫度350℃;錐孔氣流速150L/Hr;去溶劑氣流速800L/Hr;霧化氣流速7bar.目標化合物的定量離子、錐孔電壓及碰撞能參數如表1所示.

表1 目標化合物的定量離子、錐孔電壓、碰撞能

注:a3-和5-PFOS異構體無法進行基線分離,因此被合并為3+5-PFOS.

1.5 質量保證與質量控制

回收率實驗通過向空白基質中加入標準品進行.在樣品前處理過程中添加過程空白以校正背景污染.根據樣品峰面積與內標峰面積之比對物質濃度進行定量.方法檢出限(MDL)定義為信噪比為3:1時物質濃度.各物質的回收率和檢出限如表2所示.本實驗中,各基質中PFOSA和PFOS的回收率均在80%～100%范圍內,因此不使用回收率對測得的各物質濃度進行校正.

在暴露實驗開始前,小麥中就檢測到一定濃度的PFOSA(根(11.71±0.93) ng/g,莖葉(2.64±0.44) ng/g)和PFOS(根(16.89±4.25) ng/g,莖葉(4.20±2.56) ng/g).在暴露14d后,空白小麥中檢測到相似水平的PFOSA(根(9.53±1.74)～(10.38±1.60) ng/g,莖葉(2.56±0.59)～(3.34±1.64) ng/g )和PFOS(根(15.31±1.31)～(16.24±1.96) ng/g,莖葉(1.64±0.54)～(1.71±0.47) ng/g).空白蚯蚓體內PFOSA和PFOS的濃度遠低于實驗組蚯蚓(相差103～105個數量級).由此判斷,空白組小麥和蚯蚓中PFOSA和PFOS來源于背景污染.本文所用暴露組濃度均扣除了空白組濃度.

表2 目標化合物的回收率和方法檢出限(MDL)

1.6 數據分析

對小麥的根富集因子(RCF)、蚯蚓的生物富集因子(BSAF)和轉化率(TR)進行計算,公式如下:

式中:m為培養介質中PFOSA的濃度, ng/g;root為小麥根中PFOSA濃度, ng/g dw;e為蚯蚓中PFOSA濃度, ng/g dw;PFOS為小麥或蚯蚓中PFOS濃度, ng/g dw;PFOSA為小麥或蚯蚓中PFOSA濃度, ng/g dw.

使用IBM SPSS Statistics 22進行數據分析.使用配對檢驗評估各組間RCF、TR和-PFOS比例的差異,當< 0.05時認為存在顯著性差異,當< 0.01時認為差異極顯著.

2 結果與討論

2.1 小麥和蚯蚓的生理狀況

在整個實驗周期內,小麥和蚯蚓生理狀況良好,沒有不良反應、死亡等現象,空白對照組和實驗組小麥和蚯蚓生物量無顯著性差異(> 0.05).

2.2 小麥對PFOSA的富集和轉化

通過公式(1)計算小麥的RCF,發現PFOSA在石英砂培養小麥中的RCF((61.24±8.42))遠大于土壤中培養的小麥((5.61±0.23)),存在顯著性差異(<0.01),說明培養基質中的總有機碳含量(TOC)會顯著影響其生物有效性.Liu等[32]研究表明土壤TOC含量與所吸附的PFOS量之間存在顯著的線性關系(< 0.01).土壤中TOC含量高于石英砂,對PFOSA具有較強的吸附能力,使得土壤中可生物利用的PFOSA減少,導致在土壤中生長的小麥的RCF較低.

在小麥的根和莖葉中均測得轉化產物PFOS.通過公式(3)計算了小麥中PFOSA的轉化率,發現莖葉中轉化率遠高于根中(<0.01,圖1).有實驗表明,當辛醇-水分配系數(logow)>4時,疏水性有機化合物會強烈吸附于根的上表皮,難以轉運到莖葉[33].Zhao等[34]研究也表明當有機化合物logow>4時,該物質從根到莖葉轉運因子(TF)隨logow的增加而指數遞減.PFOSA的logow(5.62)小于PFOS(logow= 6.43)[35],表明PFOS較PFOSA更難從根轉運至莖葉,因此莖葉中較高的轉化率表明葉中PFOS不僅來源于根部轉運,還來源于莖葉的生物轉化.此外還發現PFOSA在土壤培養的小麥根(<0.05)和莖葉(< 0.01)的轉化率均大于石英砂中小麥(圖1).這可能是由于PFOSA被土壤中微生物轉化為PFOS[36],生成的PFOS被小麥富集.土壤中有機碳可以為微生物的生長提供充足的碳源,從而提高微生物數量和活性[37].Bizkarguenaga等[38]也觀察到在高TOC的土壤中生菜對PFOSA降解更強.

圖1 生長在不同培養基質中的小麥對PFOSA的轉化率

**、*分別表示差異極顯著(<0.01)和顯著(<0.05)

除石英砂培養的小麥根外((14.8±2.0)%),小麥中-PFOS比例均顯著高于24.7%(土壤中小麥根:(66.1±26.2)%,土壤中小麥葉:(85.2±2.4)%,石英砂中小麥葉:(63.0±21.3)%,圖2),說明小麥中-PFOSA比-PFOSA優先發生轉化.小麥中-PFOS比例呈現以下趨勢:根中-PFOS比例小于莖葉,石英砂培養的小麥中-PFOS比例小于土壤.小麥根中- PFOS比例要小于莖葉,這是因為直鏈異構體比相應的支鏈異構體具有更高的疏水性[18],導致-PFOS比-PFOS更易從根轉運至莖葉.石英砂介質培養的小麥根中-PFOS比例低于24.7%,可能也是由于根中生成的-PFOS部分轉運至莖葉.已有研究表明,PFOSA是N-EtFOSE生物轉化過程的中間產物[21,39].Liu等[25]在好氧土壤中觀察到N-EtFOSE的異構體特異性生物轉化并檢測到-PFOS的生成.Chen等[31]表明,-PFOSA在生物體內會優先轉化為-PFOS.因此可以推測,土壤中微生物對-PFOSA進行優先轉化,生成更多的-PFOS并被小麥吸收,從而導致土壤中培養的小麥中-PFOS比例高于石英砂中培養的小麥.

圖2 小麥根和莖葉中br-PFOS比例

2.3 蚯蚓對PFOSA的富集和轉化

通過計算蚯蚓對PFOSA的生物富集因子(BSAF),發現隨培養時間增加,PFOSA在蚯蚓體內的BSAF增加(圖3).培養14d后,土壤/石英砂(8:2)介質中PFOSA在蚯蚓體內的BSAF值(21347.91±208.86)遠高于土壤中(1404.92±108.21),是土壤中的15.20倍(圖3),該趨勢與PFOSA在小麥體內富集的結果相同,進一步說明土壤中TOC對PFOSA具有較強的吸附能力,降低了土壤中PFOSA的生物有效性.

圖3 不同培養時間和培養介質下蚯蚓中PFOSA的BSAF

在蚯蚓中同樣檢測到PFOSA轉化產物PFOS的生成,但其TR相對較低.在土壤中培養了14d的蚯蚓中PFOSA的TR((3.87±1.71)%)顯著低于小麥中((26.39±3.02)%),表明不同生物對PFOSA的轉化能力存在很大差異[40].

圖4 不同培養時間和培養介質下蚯蚓中br-PFOS比例

不同培養介質對蚯蚓中-PFOS比例會產生影響,土壤/石英砂(8:2)介質中,-PFOS比例更高(圖4,<0.05).這可能是由于土壤/石英砂(8:2)介質加入了石英砂,土壤黏性降低,孔隙度高,通氣透水性強,更適宜蚯蚓生存,其中蚯蚓的活性更高.在生物體內-PFOSA異構體會優先代謝[31,41],生成-PFOS.土壤/石英砂(8:2)介質僅加入20%的石英砂,TOC含量與土壤差距不大,所以推測其微生物數量和活性差異較小,對PFOSA轉化的影響也差別不大.因此土壤/石英砂(8:2)介質中蚯蚓體內-PFOS比例高.與小麥特別是莖葉中較高的-PFOS比例不同,在土壤中培養了14d的蚯蚓中-PFOS比例為(16.5±4.0)%,小于24.7%.這可能是由于蚯蚓通過大量排出糞便向外排出污染,而-PFOS比-PFOS更易從體內排出[31].

2.4 小麥共同生長對蚯蚓中PFOSA的富集和轉化的影響

蚯蚓和小麥聯合培養時,土壤/石英砂(8:2)介質中PFOSA在蚯蚓體內的BSAF(32176.65±844.49)高于土壤中(26743.14±11311.21)(圖5),進一步說明土壤TOC會顯著影響其中PFOSA的生物有效性.由圖5可知,聯合培養時PFOSA在蚯蚓體內的BSAF顯著高于蚯蚓單獨培養時的結果(<0.05).Kelsey等[42]、Zhao等[34]也發現南瓜和小麥會促進蚯蚓對p,p'-DDE和PFASs的富集.這主要是由于植物根系分泌物會提高PFOSA從土壤中的解吸,提高其生物有效性.小麥存在時,蚯蚓中PFOSA轉化率與單獨培養時無顯著性差異(>0.05),表明小麥的存在對蚯蚓中PFOSA轉化率無影響.

圖5 不同培養方式和培養介質下蚯蚓中PFOSA的BSAF

E:蚯蚓單獨培養; E+W:蚯蚓和小麥聯合培養

3 結論

3.1 小麥根能從土壤溶液中吸收PFOSA并轉運至莖葉.土壤TOC含量顯著影響其中PFOSA的生物有效性,TOC含量高,其生物有效性降低.PFOSA在小麥的根和莖葉中都可以轉化為PFOS,且土壤培養的小麥中PFOSA的轉化率更高.小麥中-PFOS比例關系為根<莖葉,石英砂<土壤.

3.2 蚯蚓能有效從土壤中富集PFOSA并將其轉化為PFOS.土壤/石英砂(8:2)介質培養的蚯蚓-PFOS比例更高.蚯蚓中PFOSA轉化率和-PFOS比例均低于小麥.

3.3 小麥提高了蚯蚓對PFOSA的富集,但對蚯蚓中PFOSA轉化率影響不大.

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Bioaccumulation and biotransformation of perfluorooctane sulfonamide in wheat and earthworms.

WU Si-han, WU Yu-meng, WANG Shuang-jie, CHEN Meng, LIU Qing, ZHU Ling-yan*

(Key Laboratory of Pollution Processes and Environmental Criteria of Ministry of Education, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China)., 2021,41(5)：2434～2440

The bioaccumulation and biotransformation of perfluorooctane sulfonamide (PFOSA) in wheat and earthworms was investigated in different culture media by different culture methods. The results indicated that PFOSA was effectively absorbed by wheat roots from the culture media and translocated from roots to shoots. The bioavailability of PFOSA in soil was significantly affected by the soil total organic carbon (TOC) content. The bioavailability of PFOSA in the soil with higher TOC content was reduced, resulting in the bioaccumulation factors in wheat and earthworms decreased from (61.24±8.42) and (21347.91±208.86) to (5.61±0.23) and (1404.92±108.21), respectively. PFOSA could be transformed into PFOS in the earthworms as well as in the roots and shoots of wheat, but the transformation rate of PFOSA in the earthworms ((3.87±1.71)%) was significantly lower than that in the wheat ((26.39±3.02)%). The ratio of branched PFOS isomers (-PFOS) in the wheat roots was (14.8±2.0)% and (66.1±26.2)% at low and high TOC content, respectively, lower than those in the shoots ((63.0±21.3)% and (85.2±2.4)%), respectively), which might be because it was easier to translocate-PFOS formed in roots to shoots. The ratio of-PFOS in wheat, especially in wheat shoots ((85.2±2.4)%), was significantly higher than that in earthworms ((16.5±4.0)%). The presence of wheat enhanced the bioavailability of PFOSA in the soil, thereby promoted the accumulation of PFOSA in earthworms, but had little effect on the transformation of PFOSA. The results provided evidence for the bioaccumulation and biotransformation of PFOSA in wheat and earthworms, and were helpful to explore the indirect sources of PFOS in the environment.

PFOSA；wheat；earthworm；bioaccumulation；biotransformation

X131

A

1000-6923(2021)05-2434-07

吳思寒(1998-),女,河南安陽人,南開大學碩士研究生,主要從事全氟和多氟化合物環境行為研究.

2020-10-08

國家重點研發計劃(2018YFC1801003,2019YFC1804203),國家自然科學基金資助項目(41991313,21737003)

* 責任作者, 教授, zhuly@nankai.edu.cn