何 強(qiáng),顏 正,智 悅,錢深華,王小銘,陳 一,劉彩虹,程 呈,胡學(xué)斌

植物對(duì)PFAS的富集機(jī)制研究進(jìn)展

何 強(qiáng),顏 正,智 悅*,錢深華,王小銘,陳 一,劉彩虹,程 呈,胡學(xué)斌

(重慶大學(xué),三峽庫區(qū)生態(tài)環(huán)境教育部重點(diǎn)實(shí)驗(yàn)室,重慶 400045)

本文通過文獻(xiàn)梳理,分析了全(多)氟烷基化合物(PFAS)對(duì)植物的暴露途徑;統(tǒng)計(jì)了已發(fā)表文獻(xiàn)中36種植物對(duì)19種PFAS的轉(zhuǎn)運(yùn)、富集特征;系統(tǒng)地闡釋了PFAS從環(huán)境介質(zhì)到植物組織內(nèi)的遷移、積累機(jī)制;討論了PFAS分子結(jié)構(gòu)(如全氟碳鏈長度、頭部官能團(tuán))、植物生理特性、環(huán)境因素對(duì)該富集過程的影響,并提出了未來有關(guān)植物富集PFAS可關(guān)注的重點(diǎn)和方向,以期能深入認(rèn)識(shí)PFAS在環(huán)境介質(zhì)-植物根際-植物組織內(nèi)的賦存與遷移轉(zhuǎn)化特征,更好地管控評(píng)估PFAS污染場地并制定植物修復(fù)方案,為開展生態(tài)與健康風(fēng)險(xiǎn)評(píng)價(jià)提供參考.

全(多)氟類化合物；PFAS；植物；轉(zhuǎn)運(yùn)；富集

全(多)氟烷基化合物(PFAS)是一類高度氟化的有機(jī)物,具有優(yōu)良的表面活性、疏水疏油性能、化學(xué)惰性和耐高溫能力[1-2],已被廣泛應(yīng)用于工業(yè)生產(chǎn)和日常生活中[3-6].近10a來,PFAS在全球范圍內(nèi)各種環(huán)境介質(zhì)中被廣泛檢出,包括大氣、水、土壤、沉積物、植物、動(dòng)物(如南極企鵝,海鷗等)和極地冰蓋,發(fā)達(dá)國家PFAS污染比發(fā)展中國家更為嚴(yán)重[7-13].目前人體血液和尿液中已經(jīng)多種PFAS[14-15],其中全氟辛烷磺酸(PFOS)和全氟辛烷羧酸(PFOA)在人體內(nèi)半衰期平均值分別為3.4a和2.7a[15].毒理學(xué)研究表明PFAS會(huì)對(duì)人體產(chǎn)生多種毒性作用,包括腎臟、免疫系統(tǒng)、神經(jīng)、發(fā)育和內(nèi)分泌干擾毒性[16-19].

由于PFAS特殊的理化性質(zhì),其在進(jìn)入環(huán)境中后可遠(yuǎn)距離輸送[20-22].PFAS可以通過大氣干濕沉降、地表徑流和固廢(如工業(yè)固廢、生活垃圾、污泥)處置等多種途徑進(jìn)入土壤造成污染[23-25],因此土壤是PFAS重要的“匯”.賦存于土壤中的PFAS可從土壤顆粒脫附、遷移、轉(zhuǎn)運(yùn)、富集于植物組織,并通過陸生食物鏈傳遞和生物放大,危害生態(tài)系統(tǒng)和人類健康[26].通常污染場地中的PFAS污染面積廣、濃度痕量、環(huán)境持久(難以被光解、水解、生物降解)[27-29],故PFAS污染場地修復(fù)技術(shù)已成為亟待解決的挑戰(zhàn).植物修復(fù)是一種低成本、低擾動(dòng)、低二次污染的原位場地修復(fù)技術(shù),已被證明是可用于痕量、微量PFAS污染治理的處理技術(shù)之一[30-31].然而目前對(duì)于植物吸收PFAS的研究集中于農(nóng)作物等可食用植物,研究的重點(diǎn)圍繞在食物鏈富集和放大以及生態(tài)環(huán)境風(fēng)險(xiǎn).研究的污染物對(duì)象集中在全氟烷基羧酸和磺酸(PFCA和PFSA)兩類,尤其是PFOA和PFOS,而對(duì)于其他復(fù)雜分子結(jié)構(gòu)PFAS的植物富集行為還鮮有報(bào)道.植物特性、環(huán)境因素對(duì)植物轉(zhuǎn)運(yùn)、富集PFAS的影響尚不明確.本文對(duì)已發(fā)表的文獻(xiàn)數(shù)據(jù)進(jìn)行了梳理,統(tǒng)計(jì)了植物對(duì)包括PFCA、PFSA、全氟聚醚(PFEA)在內(nèi)的多種PFAS的生物富集行為;闡釋了PFAS在植物體內(nèi)的積累富集機(jī)制;討論了PFAS分子結(jié)構(gòu)、植物生理特性、環(huán)境因素對(duì)生物富集過程的影響,并提出了未來有關(guān)植物富集PFAS研究可以關(guān)注的重點(diǎn)和方向.以期為全面認(rèn)識(shí)PFAS在土-植物體系中的賦存狀況與遷移轉(zhuǎn)化機(jī)制、更好地管控評(píng)估PFAS污染場地制定植物修復(fù)方案以及為生態(tài)與健康風(fēng)險(xiǎn)評(píng)價(jià)提供參考.

1 PFAS對(duì)植物的暴露途徑

PFAS對(duì)植物的暴露途徑包括由植物根系暴露在PFAS污染地表徑流或污染土壤,或者植物葉片暴露于大氣中攜帶PFAS的顆粒和水滴[23,32-34].土壤中PFAS的來源包括大氣沉降、地表徑流和生活垃圾或污水廠活性污泥的堆放、垃圾場滲濾液(特別是食品包裝材料、消防防火服等垃圾)等多種污染途徑[35-37].具體來說,由于PFAS難以被傳統(tǒng)污水處理工藝降解和去除[22,36,38-39],通過污水管網(wǎng)進(jìn)入到污水廠的生產(chǎn)廢水及醫(yī)療廢水中的大量PFAS無法被降解和去除,其大部分會(huì)沉積、吸附在活性污泥相.部分污泥會(huì)作為土壤改良劑被回用于農(nóng)田[39-41],土壤改良劑投入農(nóng)田后其中的PFAS首先解吸、脫附,再重新吸附于表層土壤顆粒,并隨灌溉水向土壤深處滲流.垃圾滲濾液也是PFAS進(jìn)入土壤和水環(huán)境的一種重要載體,其可以通過直接(滲濾液泄露或穿透防滲層)或間接(滲濾液經(jīng)污水處理后排放)的方式被排入環(huán)境[42],進(jìn)而暴露于植物根系.

空氣中以蒸汽相和顆粒相呈現(xiàn)的PFAS及其前體物可以吸附到植物葉片和樹皮等部位并被植物吸收,即從周圍空氣中直接吸收氣相化學(xué)物質(zhì)或通過質(zhì)量交換的方式吸收[43-44].也可通過大氣沉降以雨雪水的方式被帶回陸地表面,進(jìn)入土壤水體污染植物根系[45-46].有報(bào)道稱垃圾填埋場、氟化物加工廠中的揮發(fā)性和半揮發(fā)性PFAS前體物可能會(huì)逸入到大氣中.在此過程中,離子型PFAS前體如氟調(diào)聚物醇(FTOH)、全氟辛烷磺酰胺(FOSA)和全氟辛烷磺酰胺乙醇(FOSE),先通過空中運(yùn)輸,后被植物葉片吸收,再在植物體內(nèi)被進(jìn)一步代謝為PFCA和PFSA[47]. Tian等[24]分析了天津垃圾填埋場附近的植物中PFAS的濃度水平和分布情況,證實(shí)植物葉片能有效吸收空氣中多種PFAS,包括近來受到廣泛關(guān)注的PFAS前體物多氟烷基磷酸二酯(diPAPS),植物葉片被發(fā)現(xiàn)能有效地反映PFAS環(huán)境污染情況,因此可作為一種被動(dòng)采樣方法,用于監(jiān)測植物點(diǎn)源附近的空氣質(zhì)量.

2 植物對(duì)PFAS的生物富集

2.1 植物對(duì)PFAS的生物富集濃度

目前研究最廣泛的PFAS種類是PFCA和PFSA 2個(gè)系列,分別被研究了108次和84次,前期研究集中于PFOA(C7,-COOH)和PFOS(C8,-SO3H)兩種物質(zhì).36種實(shí)驗(yàn)植物包括草本植物及木本植物.表1列出了植物對(duì)PFAS的富集濃度水平.針對(duì)已報(bào)道的36種植物經(jīng)由暴露培養(yǎng)后對(duì)19種PFAS均有生物富集(表1).

總結(jié)前期研究結(jié)果發(fā)現(xiàn),對(duì)于同種植物,PFAS的全氟碳鏈鏈長和頭部官能團(tuán)均會(huì)影響其在植物體內(nèi)的轉(zhuǎn)運(yùn)和富集.如木賊()對(duì)全氟己烷羧酸(PFHxA,C5,-COOH)的富集濃度高達(dá)2.4×104ng/g,但對(duì)全氟己烷磺酸(PFHxS,C6,-SO3H)的富集濃度只有2.8×102ng/g;黑柳()對(duì)全氟戊烷羧酸(PFPeA,C4,-COOH)的富集濃度為3.2×104ng/g,而對(duì)PFOA(C7,-COOH)的富集濃度僅為3.7×103ng/g.除了PFCA和PFSA外,Zhang等[48]研究了包含全氟-3-甲氧基丙酸(PFMoPrA)、全氟-4-甲氧基丁酸(PFMoBA)、全氟(2-甲基-3-氧代己酸)銨(GenX)、4,8-二氧-3氫-全氟壬酸銨(ADONA)、氯代多氟烷基醚磺酸鹽(F-53B)在內(nèi)的PFEA,相比于PFCA和PFSA,長毛苔草()作為研究對(duì)象對(duì)5種PFEA的富集濃度范圍僅為8.0～1.3×102ng/g,而同為莎草科的紙莎草()對(duì)PFCA和PFSA則顯示出更高的富集能力,對(duì)PFOA和PFOS的富集濃度分別達(dá)到3.4×103ng/g和1.2×104ng/g,然而,目前對(duì)PFEA的研究數(shù)據(jù)還相對(duì)較少,且暴露時(shí)間、環(huán)境因素(溫度、光照、濕度等)、培養(yǎng)介質(zhì)中的初始濃度等均對(duì)富集效果有顯著影響.相比于典型PFAS,新型PFAS醚基類替代品是否具有相似的生物富集行為亟需更多數(shù)據(jù)支持.

圖1 植物對(duì)PFAS的生物富集數(shù)據(jù)匯總

已報(bào)道數(shù)據(jù)截至2021年4月,色帶的厚度表示所對(duì)應(yīng)的PFAS或植物的研究類型多少,植物或者PFAS前、后的數(shù)字表示其被研究的頻率

比較植物對(duì)PFAS生物富集水平可篩選出具有植物修復(fù)潛力的PFAS高耐性物種.已發(fā)表文獻(xiàn)中,植物對(duì)PFAS生物富集濃度范圍為1.4～8.8×105ng/g, 不同植物科屬對(duì)PFAS的富集能力差異較大(表1).其中燈芯草(燈芯草科)顯示出最高的富集能力(8.8×105ng/g,干重),其次是大安水蓑衣(爵床科)4.6×104ng/g (干重),皆遠(yuǎn)超其他植物種類.相較而言,富集能力最低的物種為萬壽菊(菊科)和線葉水馬齒(水馬齒科),其對(duì)PFAS的富集能力僅為1.40～95.00ng/g(干重)和6.56～ 14.16ng/g(干重).由表1可知,針對(duì)禾本科類植物研究報(bào)道最多,且其具有較高的PFAS生物富集能力,6種禾本科類植物(蘆葦、狗牙根、紫陽茅、水茅、雙雄雀麥和蘆竹)對(duì)7種PFAS(全氟丁烷羧酸PFBA、PFPeA、PFHxA、PFOA、全氟葵烷羧酸PFDA、全氟丁烷磺酸PFBS、PFHxS和PFOS)的富集濃度在2.2×102～3.4×104ng/g之間.包含玉米、水稻、燕麥在內(nèi)的禾本科植物已被證明對(duì)重金屬[49]、有機(jī)物(如多環(huán)芳烴[50])及石油污染土壤[51]有高生物量耐性,其可能是因?yàn)楹瘫究浦参锏母稻哂袃?yōu)良的根系形態(tài),可以通過根系固定、根系吸收和傳輸、植物揮發(fā)、植物降解及根系微生物降解過程去除PFAS.

相比于草本植物,目前對(duì)木本植物的研究較少(圖1),木本植物由于其多年生,根莖粗壯,木質(zhì)部發(fā)達(dá)的特點(diǎn),也具有PFAS污染場地修復(fù)的潛力. Gobelius和Huff調(diào)研了瑞典斯德哥爾摩機(jī)場附近消防訓(xùn)練場[含PFAS的水性滅火泡沫(AFFF)污染場地]植物群落的不同植物物種即白樺()、挪威云杉()、鳥櫻桃()、山灰()、地接骨木()、長山毛櫸()和野草莓()對(duì)PFAS的富集行為.其發(fā)現(xiàn)白樺葉和云杉針葉中PFAS的濃度分別高達(dá)97ng/g和94ng/g(濕重),單棵樹可以分別富集高達(dá)11mg和1.8mg的總PFAS[52].該研究表明木本植物每年可以從重度PFAS污染場地去除 1.4g總PFAS/hm2,是一種可行的場地修復(fù)手段[34,52].

表1 植物對(duì)PFAS的富集濃度水平

注:所列濃度為整株植物對(duì)該種PFAS富集的濃度值(ng/g); ND為濃度數(shù)據(jù)缺失.

2.2 根系富集系數(shù)(RCF)

水培或土培過程中,植物根系中PFAS的濃度與培養(yǎng)基質(zhì)中PFAS的濃度比為根系富集系數(shù)(RCF,公式1),該參數(shù)表示植物根系對(duì)PFAS的富集潛力,是描述PFAS在植物體內(nèi)累積趨勢的重要指標(biāo).

由圖2a所示,36種植物對(duì)19種PFAS(包括PFCA、PFSA、PFEA系列)的RCF值介于0.04～1030,跨越6個(gè)數(shù)量級(jí);PFAS鏈長和頭部官能團(tuán)均會(huì)影響RCF值,對(duì)于PFCA系列,從PFBA(C3,-COOH)到PFHxA(C5,-COOH),RCF的數(shù)值變化不顯著(1.295～ 1.69),而從PFHxA(C5,-COOH)到全氟十二烷羧酸(PFDoDA,C12,-COOH),RCF的數(shù)值隨全氟碳鏈鏈長顯著增加(1.69～701.33);PFSA系列(C4,C6,C8)也顯示出類似趨勢,RCF也隨著全氟碳鏈長度的增加顯著升高(2.42～81.84),該趨勢說明隨著碳鏈長度的增加,根系對(duì)PFAS的富集能力呈現(xiàn)遞增的現(xiàn)象.前期研究顯示長鏈PFAS(如PFOS、PFOA)顯示出較高的RCF,易在根部富集,且長鏈PFAS的富集程度與根部蛋白質(zhì)、脂質(zhì)含量存在顯著正相關(guān)關(guān)系[63-65].

2.3 轉(zhuǎn)運(yùn)系數(shù)(TF)

PFAS從根部向上的轉(zhuǎn)移潛力通常用轉(zhuǎn)運(yùn)系數(shù)(TF)來表示,其等于植物地上部分PFAS含量與地下部分PFAS含量的比值.

對(duì)于11種PFCA、3種PFSA和5種PFEA,其TF的范圍為0.01～18.54.隨著鏈長的增加,PFCA和PFSA系列的TF值均顯示出下降趨勢(圖2b).TF隨鏈長降低可能是由于長鏈PFSA疏水性更強(qiáng),易于吸附在植物根系,并受卡氏帶的限制,從而向上轉(zhuǎn)移的潛力較弱,而溶解度更高、疏水性較弱、碳鏈更短的PFAS可以穿越卡氏帶屏障,通過蒸騰流被轉(zhuǎn)運(yùn)莖部[54,58].對(duì)于新型氧雜型PFEA,TF的范圍為0.1～ 14.2,分子質(zhì)量最高的6:2Cl-PFESA(F-53B的主要成分)明顯低于其他4種PFEA.PFEA相比于相似分子質(zhì)量的PFCA、PFSA,其TF值略高,說明植物對(duì)其向上轉(zhuǎn)運(yùn)的能力更強(qiáng).由于目前PFEA的數(shù)據(jù)樣本較少,上述現(xiàn)象還亟需更多的實(shí)驗(yàn)證實(shí).

比較圖2a和圖2b,TF(0.01～18.54)的值要遠(yuǎn)小于RCF(0.04～1030)的值,其可能是因?yàn)楦S管組織的吸附能力遠(yuǎn)大于莖維管組織的吸附能力或根表面與卡氏帶之間的植物組織的吸附能力[66],這也是植物吸收污染物的重要機(jī)制之一.

3 PFAS在“土-水-根-莖-葉”多介質(zhì)體系的傳質(zhì)過程與機(jī)制分析

3.1 PFAS在“土-水-根”體系的傳質(zhì)過程

前期研究表明,土壤是PFAS在生態(tài)系統(tǒng)不同環(huán)境介質(zhì)中交換的重要周轉(zhuǎn)站,土壤-水是PFAS進(jìn)入植物體內(nèi),并且進(jìn)一步進(jìn)入食物鏈的重要途徑[67-68]. PFAS首先從其污染源直接或間接釋放進(jìn)入土壤環(huán)境,由于PFAS難以被降解,吸附和淋溶是影響PFAS在土壤-水體系中遷移和生物有效性的兩個(gè)重要過程[69-70].PFAS與土壤顆粒之間的吸附-淋溶機(jī)制包括疏水作用、靜電作用、氫鍵、離子交換和范德華力等,以上幾種作用力可能同時(shí)存在[70].分子結(jié)構(gòu)上, PFAS在土壤顆粒-孔隙水體系中的吸附和淋溶行為與C-F鏈長、特征官能團(tuán)有關(guān);前期針對(duì)PFAS在沉積物中的吸附行為研究發(fā)現(xiàn):碳氟鏈上每增加一個(gè)CF2基團(tuán),logOC(有機(jī)碳吸附常數(shù))值即增加0.3～ 0.6;碳鏈長度相同時(shí),磺酸類比羧酸類在沉積物上吸附高約0.2個(gè)對(duì)數(shù)單位[71].土壤理化性質(zhì)方面,土壤對(duì)PFAS的吸附、淋溶和吸附滯后性與土壤有機(jī)質(zhì)含量、pH值、陰/陽離子交換容量、共存陽離子、離子強(qiáng)度、比表面積、礦物質(zhì)含量等有關(guān)[70,72-73],而且主流研究認(rèn)為土壤中有機(jī)質(zhì)含量是最主要的因素[71,74],此結(jié)論不僅僅針對(duì)典型傳統(tǒng)PFAS,最新的研究表明,新型醚類替代物F-53B的最大吸附量也與土壤有機(jī)質(zhì)含量呈正相關(guān)[75].

3.2 植物根系對(duì)PFAS的吸收以及PFAS的跨膜機(jī)制

PFAS吸附在植物根系表面之后,先被運(yùn)輸?shù)礁砥ぜ?xì)胞,再被徑向運(yùn)輸?shù)狡?附著在根表皮的PFAS可通過共質(zhì)體運(yùn)輸和質(zhì)外體運(yùn)輸兩種途徑被根吸收[55,76-77].共質(zhì)體運(yùn)輸是指植物細(xì)胞間通過胞間連絲進(jìn)行PFAS轉(zhuǎn)運(yùn),而質(zhì)外體運(yùn)輸是指植物細(xì)胞間通過原生質(zhì)體以外的自由空間進(jìn)行PFAS轉(zhuǎn)運(yùn).前期研究指出,PFAS可通過質(zhì)外體和共質(zhì)體途徑被轉(zhuǎn)運(yùn).在質(zhì)外體運(yùn)輸中,PFAS會(huì)被內(nèi)皮層中垂直周壁分布的卡氏帶(Casparian strip)阻斷,由于卡氏帶由疏水的木栓質(zhì)和木質(zhì)素組成,故對(duì)長鏈、大分子PFAS在植物體內(nèi)的轉(zhuǎn)運(yùn)有阻礙作用,而小分子PFAS可通過卡氏帶被徑向轉(zhuǎn)運(yùn)[78].

植物根系對(duì)PFAS的吸收包括伴隨蒸騰流,通過擴(kuò)散作用進(jìn)入植物體內(nèi)的被動(dòng)吸收和通過植物細(xì)胞膜對(duì)污染物選擇性運(yùn)輸?shù)闹鲃?dòng)吸收(圖3).主動(dòng)和被動(dòng)過程對(duì)PFAS吸收的作用取決于植物生理、PFAS的親脂性、分子結(jié)構(gòu)以及培養(yǎng)方式[79-80].有研究者推測PFAS(特別是小分子短鏈PFAS)可以通過被動(dòng)擴(kuò)散的方式被植物細(xì)胞吸收.有研究者基于PFAS在磷脂雙分子層膜中的平衡分配發(fā)現(xiàn),PFAS與磷脂膜的融合程度與氟化碳原子的個(gè)數(shù)(n)成正相關(guān),且全氟磺酸(-SO3H)的融合程度強(qiáng)于全氟羧酸(-COOH)[53].

圖3 PFAS在植物細(xì)胞中的潛在主動(dòng)運(yùn)輸通道

相比之下,PFAS的主動(dòng)運(yùn)輸可能需要依賴膜上的載體蛋白、陰離子通道和水通道蛋白[33,53,55,81].當(dāng)前關(guān)于PFAS跨膜機(jī)制的研究,主要是通過添加代謝抑制劑(停止能量依賴過程)、水和陰離子通道蛋白(水轉(zhuǎn)運(yùn)蛋白)抑制劑來推測其吸收機(jī)制.為揭示植物吸收PFAS的路徑,Wen等[77]通過使用2,4-二硝基苯酚(2,4-DNP)、NaN3和Na3VO4作為代謝抑制劑,發(fā)現(xiàn)PFOA被玉米()根部吸收的過程是需要消耗能量的主動(dòng)運(yùn)輸,而PFOS則是在載體幫助下的被動(dòng)擴(kuò)散過程;Wen等[76]發(fā)現(xiàn)小麥()對(duì)PFBA、PFHxA、PFOA和PFOS的吸收也是需要消耗能量的主動(dòng)過程;有研究者分別通過添加甘油和硝酸銀競爭、抑制水通道蛋白,Wen等[77]發(fā)現(xiàn)水通道蛋白被抑制后,小麥對(duì)PFOA的吸收無影響,而PFOS的吸收過程被壓制,甘油組和硝酸銀組PFOA/PFOS植物吸收分別降低25%和31%[77];而Wang等[53]發(fā)現(xiàn)加入甘油和硝酸銀后,東方澤瀉()對(duì)PFOA和PFOS的吸收均受到了不同程度的抑制(PFOA降低25.3%～30.9%、PFOS降低22.3%～33.9%),表明水通道蛋白是東方澤瀉根部吸收PFOA和PFOS的重要通道.另外,通過加入9-蒽甲酸(9-AC)、4,4￠-二異硫氰酰-2,2￠-基二磺酸二鈉鹽(DID)和5-硝基-2-(3-苯丙胺)苯甲酸(NPPB)作為陰離子通道阻斷劑,Wen等[77]發(fā)現(xiàn)玉米對(duì)PFOA的富集僅受9-AC影響,而對(duì)PFOS的富集卻主要受DID和NPPB影響,且PFOS和PFOA共存時(shí),植物對(duì)他們的吸收不具有競爭關(guān)系,添加PFOS前后PFOA的生物富集濃度無顯著性變化,說明植物根部對(duì)不同PFAS物質(zhì)具有不同的轉(zhuǎn)運(yùn)路徑.相比之下,Wang等[53]卻發(fā)現(xiàn)PFOS和PFOA在東方澤瀉根部的吸收過程同樣依賴陰離子通道的參與,但PFOA和PFOS卻顯示出競爭關(guān)系,添加PFOS前后PFOA生物富集濃度變化顯著(<0.01),在PFOS和PFOA同時(shí)存在時(shí),PFOS和PFOA的富集濃度分別降低14.7%和31%.另一方面,Zhang等[82]研究了小麥對(duì)PFBA、PFHxA、PFOA和PFOS的生物富集,發(fā)現(xiàn)甘油、硝酸銀、9-AC、DID和NPPB對(duì)小麥吸收上述PFAS均無影響,推測其轉(zhuǎn)移過程是借由小麥根部特異性載體蛋白的主動(dòng)運(yùn)輸.上述結(jié)果的不一致可部分歸因?yàn)橹参锓N間的巨大差異.綜上,目前學(xué)術(shù)界對(duì)PFAS的跨膜機(jī)制尚未統(tǒng)一,植物對(duì)PFAS生物轉(zhuǎn)運(yùn)富集機(jī)制尚不明確,特別是植物對(duì)分子結(jié)構(gòu)不同PFAS的選擇性生物富集亟待明確.

3.3 PFAS從根系向地上部分的轉(zhuǎn)運(yùn)機(jī)制

有機(jī)化合物被植物根系吸收后,穿過表皮、皮層、內(nèi)皮層和維管束鞘到達(dá)維管束,然后通過木質(zhì)部從根部向地上部分傳輸并積累[55,61,83-84].在此過程中,位于木質(zhì)部中的卡氏帶會(huì)限制PFAS從根向中央維管系統(tǒng)的遷移,卡氏帶通過封閉細(xì)胞和細(xì)胞膜(質(zhì)外體)之間的非活性空間,防止化合物通過內(nèi)皮層的被動(dòng)運(yùn)輸[85].Wang等[55]利用DESI-MS(吸附解吸電噴霧電離質(zhì)譜成像)和TEM-EDS(透射電子顯微鏡結(jié)合X射線能譜)觀測了PFOA和PFOS在8種濕地植物中向上轉(zhuǎn)運(yùn)運(yùn)輸?shù)穆窂?其中再力花、蘆葦僅通過共質(zhì)體運(yùn)輸向上轉(zhuǎn)移,而美人蕉、風(fēng)車草則是通過共質(zhì)體運(yùn)輸和質(zhì)外體運(yùn)輸兩種過程同時(shí)向上轉(zhuǎn)移,推測其可能是與植物特異性向上轉(zhuǎn)移模式和植物不同的莖截面生理結(jié)構(gòu)差異性有關(guān).

PFAS從植物根系向地上部分傳輸?shù)尿?qū)動(dòng)力為植物蒸騰作用產(chǎn)生的水勢梯度.蒸騰作用的強(qiáng)度可通過測定植物蒸騰速率()和氣孔導(dǎo)度(Gs)來量化,其受植物生理結(jié)構(gòu)(如氣孔的數(shù)量和大小)和環(huán)境因素(如光照、溫度、濕度、風(fēng)速)影響[81].蒸騰流富集因子(TSCF)是木質(zhì)部汁液中污染物濃度與根區(qū)水培液或土壤溶液中污染物濃度的比值,被廣泛用于描述化學(xué)物質(zhì)在植物中的傳輸能力.TSCF可以根據(jù)植物組織中PFAS的質(zhì)量和累積蒸騰水分的體積估算:

根據(jù)化合物的TSCF數(shù)值大小可將化合物分為3類:植物對(duì)化合物的吸收速度高于對(duì)水分的吸收速度,TSCF值>l;與水分吸收具有相同的速度的化學(xué)物質(zhì),TSCF=1;吸收速度低于水分的化合物,TSCF值

4 植物吸收PFAS的影響因素

PFAS在植物中的吸收、富集和轉(zhuǎn)運(yùn)特征與PFAS分子結(jié)構(gòu)(如logow值、全氟碳鏈長度、官能團(tuán)類型、分子尺寸)[66,86,89–92]、植物根和莖的組成(如蛋白質(zhì)含量、脂質(zhì)含量、蛋白質(zhì)結(jié)構(gòu))[77,81,93]、植物根和莖的生理特征(如水分蒸騰量、氣孔導(dǎo)度、生物量、根生理形態(tài))[66,89,94-95]、環(huán)境因素(培養(yǎng)基質(zhì)性質(zhì)、土壤有機(jī)質(zhì)含量、鹽度、濕度和溫度)[87,96]、暴露濃度[86]、時(shí)間(如污染物濃度、污泥施用頻率)[83,90,97]有關(guān).

4.1 PFAS分子結(jié)構(gòu)

PFAS在植物中的吸收、富集和轉(zhuǎn)運(yùn)特征與PFAS分子結(jié)構(gòu)有關(guān).Felizeter等[66]發(fā)現(xiàn)PFCA系列(C6-C11)的RCF和logow呈現(xiàn)正相關(guān)關(guān)系,但是在PFDoDA(C12)之后,RCF不會(huì)進(jìn)一步增加,其推測原因?yàn)樘兼溸^長的PFAS物質(zhì)親脂性過高,分子尺寸過大,其在植物根部表皮的吸附達(dá)到飽和.Wang等[55]基于可視化技術(shù)DESI-MS和TEM-EDS觀察到PFOS和PFOA在植物表皮細(xì)胞、皮層和維管束中均有分布,但其在維管束中的濃度遠(yuǎn)低于維管束周圍相鄰皮層中濃度,表明長鏈物質(zhì)如PFOA和PFOS主要是吸附于植物根系.綜上所述,根對(duì)PFAS的吸收很可能受到其在植物根類脂質(zhì)固體上吸附作用的支配,而從根到葉的遷移受到PFAS疏水性(ow)、分子量的限制;隨碳鏈長度的增加,根轉(zhuǎn)運(yùn)系數(shù)RCF呈現(xiàn)遞減的現(xiàn)象.由此,短鏈PFAS(如PFBA、PFHxA)更易向上運(yùn)輸,并更多的富集在莖葉、果實(shí)中;而長鏈PFAS(如PFOS、PFOA)則顯示出較高的根富集因子(RCF)和較低的根-莖葉遷移因子(TF),更易在根部富集[63-64].

PFAS頭部官能團(tuán)也會(huì)影響其生物富集行為.對(duì)比同種植物(燈芯草,菊苣)對(duì)相同鏈長(C3、C5、C8)的PFCA和PFSA的RCF值發(fā)現(xiàn),其富集PFCA和PFSA的RCF值分別在0.12～13和0.23～50.97之間;所綜述植物的根系對(duì)PFOS的RCF整體介于2.3～1030,其數(shù)值顯著高于PFOA(RCF 介于1.4～72),因此可以推斷官能團(tuán)對(duì)根系的生物富集行為有顯著性影響[60,62,83,98].其可能是由于磺酸基比帶羧基的PFCA更容易吸附到土壤中,即與同鏈長的PFCA相比,PFSA具有更低的根際土壤遷移能力和生物有效性,故其更不容易被富集到植物中.

4.2 植物的生理特征

植物體內(nèi)的蛋白質(zhì)、脂質(zhì)及其物理特征等都會(huì)影響其吸收PFAS.植物細(xì)胞內(nèi)的蛋白質(zhì)是影響植物吸收PFAS的重要因素.不同的蛋白質(zhì)可能在載體(或通道)相關(guān)和蒸騰驅(qū)動(dòng)的吸收PFAS的過程中發(fā)揮不同的作用,且蛋白質(zhì)的類型和功能在植物品種中也具有一定的特異性.之前的文獻(xiàn)發(fā)現(xiàn),PFAS與某些動(dòng)物組織中的特定蛋白具有較高的親和力[81],且檢測出PFAS在富含蛋白質(zhì)的組織中分布更多[93]. Wen等[93]和Yu等[81]指出植物體內(nèi)蛋白質(zhì)越高,植物體內(nèi)富集的PFAS可能越高,即PFAS含量與植物地上部份蛋白質(zhì)和植物根系蛋白質(zhì)含量的比值呈正相關(guān),即蛋白質(zhì)含量越高的植物,顯示出更高的RCF和TF值.但如前文所述,目前學(xué)術(shù)界關(guān)于植物蛋白對(duì)PFAS富集過程的影響尚不明確.

植物組織的脂質(zhì)含量也會(huì)影響PFAS的富集水平.植物細(xì)胞中存在的脂質(zhì)有助于形成細(xì)胞膜并調(diào)節(jié)細(xì)胞極性,從而影響PFAS在植物細(xì)胞的進(jìn)出.Wen等[77]報(bào)道植物根系脂質(zhì)對(duì)根系中PFAS的積累具有抑制作用,并且對(duì)PFSA的抑制作用強(qiáng)于PFCA. PFAS在根系的富集量與脂質(zhì)的含量呈負(fù)相關(guān)[93].脂質(zhì)可能與PFAS競爭根上的蛋白質(zhì)結(jié)合位點(diǎn),故對(duì)PFAS吸收的抑制能力取決于特定化合物與蛋白質(zhì)的結(jié)合強(qiáng)度.然而Wang等[55]的研究顯示脂質(zhì)含量與PFAS的RCF值呈正相關(guān),這與Wen等[77]的研究結(jié)論相悖.由于目前的研究數(shù)量有限,未來亟需后續(xù)的研究補(bǔ)充數(shù)據(jù),進(jìn)一步揭示脂質(zhì)對(duì)PFAS富集過程的作用機(jī)制.

除此之外,植物的生理形態(tài)特性包括根系形態(tài)[99]、根系比表面積[100]、氣孔導(dǎo)度[81]等也會(huì)影響植物富集PFAS.由于土壤相中PFAS的流動(dòng)性低于水相,由此植物根系的長度和比表面積對(duì)傳質(zhì)過程至關(guān)重要.氣孔導(dǎo)度則主要影響蒸騰作用,由于植物富集PFAS的驅(qū)動(dòng)力為蒸騰拉力,所以氣孔導(dǎo)度(氣孔的數(shù)量和大小)會(huì)影響蒸騰作用的強(qiáng)度,進(jìn)一步影響PFAS在植物內(nèi)的轉(zhuǎn)運(yùn)效率.

4.3 環(huán)境因素和暴露方式

環(huán)境介質(zhì)中的pH值、鹽度、土壤有機(jī)碳含量、溫度均會(huì)影響植物吸收PFAS.首先,Krippner等[92]研究了pH值(水培實(shí)驗(yàn),培養(yǎng)液pH值分別為5、6、7)對(duì)玉米富集PFAS的影響,發(fā)現(xiàn)只有PFDA的富集速率隨pH值變化有顯著的區(qū)別(pH=5時(shí)2.51μg/(g·d),干重,pH=7時(shí)1.52μg/(g·d),干重),而其他PFAS對(duì)pH值的依賴性表現(xiàn)并不一致,如PFBA和PFHxA的富集速率在pH=7時(shí)較pH=5時(shí)有明顯的提高,這與PFDA的表現(xiàn)恰好相反,而其他類型的PFAS(PFPeA,全氟庚烷羧酸PFHpA, PFOA,全氟壬烷羧酸PFNA, PFBS, PFHxS和PFOS)隨pH值變化無顯著差異.pH值變化對(duì)PFAS富集的潛在機(jī)制仍然未知,亟需后續(xù)研究進(jìn)一步解釋.

培養(yǎng)基質(zhì)鹽度的增加有利于水培系統(tǒng)植物對(duì)PFAS的吸收,而在土培系統(tǒng)中,隨著鹽度的增高, PFAS吸附在土壤顆粒上的比例反而增高[101].鹽度效應(yīng)主要影響PFAS的根吸收和轉(zhuǎn)運(yùn),高鹽度可能會(huì)通過鹽析機(jī)制促使PFAS更多地進(jìn)入根系的有機(jī)成分中,PFAS富集的增強(qiáng)可歸因?yàn)橹参镌谌芤弘娊赓|(zhì)濃度升高的情況下增加水分吸收以維持適當(dāng)?shù)臐B透壓,隨著鹽度的增加,溶液的離子強(qiáng)度增加,這可能導(dǎo)致植物根系對(duì)PFAS的吸收更高[102],在Zhao等[101]的研究中發(fā)現(xiàn),相比對(duì)照組,小麥在高鹽度水培介質(zhì)中對(duì)所研究的4種PFCA(PFBA、PFHpA、PFOA和PFDoDA)的TF值高出3.1～4.9倍,而在土壤介質(zhì)中高鹽度則是增強(qiáng)了土壤的吸附性,通過靜電相互作用使得PFAS更多的附著在土壤表面,但目前并沒有鹽度對(duì)植物在土培條件下吸收PFAS的影響研究,還需要后續(xù)實(shí)驗(yàn)證實(shí).

土壤有機(jī)碳含量(oc,%)也對(duì)植物吸收PFAS有重要的影響[89,103].PFAS在土壤有機(jī)質(zhì)中的吸附會(huì)導(dǎo)致有機(jī)質(zhì)微孔的重排、構(gòu)象變化或改變,進(jìn)而增加PFAS在土壤相的賦存和滯留,降低PFAS的生物可利用性[104-105].Higgins等[106]證實(shí)PFAS在土壤中的吸附與土壤有機(jī)碳含量成正相關(guān),風(fēng)化的有機(jī)質(zhì)具有更強(qiáng)的吸附能力[107-108],影響土壤中PFAS的生物有效性,并抑制其向植物體內(nèi)積累.

溫度是影響植物生長的另一個(gè)環(huán)境因素,可以通過影響水和營養(yǎng)物質(zhì)的吸收間接影響植物對(duì)污染物的吸收,也可以促進(jìn)植物的生理活動(dòng),如蒸騰作用.一般情況下,當(dāng)植物生長在適宜溫度范圍內(nèi)時(shí),生長溫度的升高可以提高養(yǎng)分的擴(kuò)散速率,降低水分的粘度,為光合作用提供更多的能量,從而增加對(duì)污染物的吸收.Zhao等[102]認(rèn)為溫度的升高也有利于植物對(duì)PFAS的吸收,隨著溫度升高,所研究的4種PFCA(PFBA、PFHpA、PFOA和PFDoDA)的TF值顯著升高,且植物吸收長鏈PFCA相較短鏈PFCA對(duì)溫度的變化更明顯.

除此之外,植物組織中PFAS的富集濃度與培養(yǎng)介質(zhì)的暴露濃度相關(guān).Blaine等[89]發(fā)現(xiàn)在萵苣葉中所有PFAS的濃度均顯示出與灌溉水中PFAS濃度(50～105ng/L)存在正相關(guān)關(guān)系.在此范圍內(nèi),植物組織中檢測到的 PFAS 對(duì)數(shù)濃度隨灌溉水中PFAS 線性增加而未達(dá)到飽和.Zhang等[62]在高濃度(640～ 4300μg/L)下對(duì)燈芯草富集8種PFAS(PFBA、PFPeA、PFHxA、PFHpA、PFOA、PFBS、PFHxS、PFOS)進(jìn)行了研究,發(fā)現(xiàn)在高濃度下植物對(duì)PFOA、PFHxS和PFOS之外的PFAS都沒有生物富集效果,得出結(jié)論燈芯草對(duì)PFAS的吸收能力可能隨溶液初始濃度增加而降低.

5 結(jié)論與展望

本文綜述了目前已發(fā)表文獻(xiàn)中36種植物對(duì)19種PFAS的暴露途徑、生物富集水平以及在植物內(nèi)的轉(zhuǎn)運(yùn)、積蓄、跨膜機(jī)制,并且系統(tǒng)地分析了PFAS分子結(jié)構(gòu)、植物生理、環(huán)境因素和暴露方式對(duì)該生物富集行為的影響.本文解析了PFAS分子結(jié)構(gòu)(全氟鏈長、特征頭部官能團(tuán)等)與多環(huán)境多介質(zhì)的行為(RCF、TF、TSCF)的相關(guān)關(guān)系,這對(duì)于正確評(píng)估PFAS環(huán)境行為和潛在的生態(tài)風(fēng)險(xiǎn)效應(yīng)具有指導(dǎo)意義.另外,部分植物顯示出對(duì)PFAS脅迫的高耐受性,可用于植物場地修復(fù)技術(shù),作為成本低廉、環(huán)境友好、低影響的手段去除污染場地中各類有機(jī)氟化污染物.

綜上,目前植物富集PFAS的研究還較少,亟需后續(xù)的研究進(jìn)行擴(kuò)充論證:①研究的植物的種類集中于農(nóng)作物,需要補(bǔ)充大量的數(shù)據(jù),遴選出具有高生長速率和對(duì)PFAS高耐受性物種;②由于PFAS普遍分布于水環(huán)境中,并可能沿水生食物鏈生物放大,故對(duì)藻類和水生植物的研究十分必要;③目前的研究集中在PFSA和PFCA,對(duì)于新型替代品、前體物質(zhì)鮮有涉及,而PFAS的前體物質(zhì)可能作為PFAS生產(chǎn)過程中的副產(chǎn)品進(jìn)入環(huán)境,也可能在生產(chǎn)過程中逃逸到大氣中,故還需要進(jìn)一步探索前體物質(zhì)的遷移、植物轉(zhuǎn)運(yùn)富集過程,明確前體物的生態(tài)環(huán)境風(fēng)險(xiǎn);④大部分對(duì)植物中PFAS的實(shí)驗(yàn)集中于室內(nèi)水培,忽略了根際效應(yīng),即根際土、根際微生物對(duì)PFAS的作用,且實(shí)驗(yàn)室內(nèi)的植物生長條件與野外實(shí)際修復(fù)環(huán)境差異較大,實(shí)驗(yàn)室條件下得出的結(jié)論還需要進(jìn)一步田間實(shí)驗(yàn)驗(yàn)證.

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Research progress on bioaccumulation of per-and polyfluoroalkyl compounds by plants.

HE Qiang, YAN Zheng, ZHI Yue*, QIAN Shen-hua, WANG Xiao-ming, CHEN Yi, LIU Cai-hong, CHENG Cheng, HU Xue-bin

(Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environment, Ministry of Education, Chongqing University, Chongqing 400045, China)., 2022,42(11)：5395～5407

This review summarized the literature data, including bioaccumulation of 19 kinds of per- and polyfluoroalkyl compounds (PFAS) by 36 plants. Exposure, transport, and bioaccumulation feature data are summarized and interpreted; the mechanism of migration and accumulation of PFAS from environmental media to plant tissues was systematically elucidated; effects of PFAS molecular structure (such as perfluorocarbon chain length and head functional group), plant physiological characteristics, and environmental factors on the bioaccumulation process were discussed. In addition, future studies of phytoremediation and combined remediation of PFAS contaminated sites were prospected. The information can be used to manage and evaluate PFAS-contaminated sites, formulate phytoremediation plans, and assess ecological and health risk of PFAS contaminated sites.

poly- and perfluoroalkyl substances；PFAS；plants；translocation；bioaccumulation

X173

A

1000-6923(2022)11-5395-13

何 強(qiáng)(1965-),男,江蘇江陰人,教授,博士,研究方向?yàn)樾〕擎?zhèn)污水處理、城市排水管網(wǎng)、城市水環(huán)境綜合整治.發(fā)表論文200余篇.

2022-04-15

國家自然科學(xué)基金資助項(xiàng)目(U20A20326,52200225);中國博士后科學(xué)基金資助項(xiàng)目(265648)

* 責(zé)任作者, 助理研究員, yuezhi6170@163.com