多孔介質(zhì)中微塑料的環(huán)境行為研究進(jìn)展

李宵慧,徐紅霞,孫媛媛,吳吉春*

多孔介質(zhì)中微塑料的環(huán)境行為研究進(jìn)展

李宵慧1,2,徐紅霞1*,孫媛媛1,吳吉春1**

(1.南京大學(xué)地球科學(xué)與工程學(xué)院,南京大學(xué)表生地球化學(xué)教育部重點(diǎn)實(shí)驗(yàn)室,江蘇 南京 210023；2.南京師范大學(xué)海洋科學(xué)與工程學(xué)院,江蘇 南京 210023)

微塑料的環(huán)境污染問(wèn)題已成為當(dāng)前國(guó)內(nèi)外研究的前沿?zé)狳c(diǎn)之一,近年來(lái)土壤和地下水系統(tǒng)中微塑料的環(huán)境行為研究得到越來(lái)越多的關(guān)注.在論述多孔介質(zhì)中微塑料來(lái)源和危害的基礎(chǔ)上,聚焦國(guó)內(nèi)外學(xué)者在微塑料環(huán)境行為方面的研究,綜合評(píng)述了微塑料穩(wěn)定性及其在多孔介質(zhì)中運(yùn)移行為的最新進(jìn)展,重點(diǎn)介紹了物理、化學(xué)和生物三方面因素的影響及作用機(jī)制.從微塑料對(duì)其他污染物之間的吸附/解吸作用以及共運(yùn)移行為兩個(gè)方面匯總分析了微塑料和其他污染物間相互作用的最新研究進(jìn)展.今后需要在不同類型微塑料、多因素耦合效應(yīng)以及微塑料與微生物之間的相互作用等方面進(jìn)一步深入研究,以期系統(tǒng)掌握多孔介質(zhì)中微塑料的運(yùn)移行為及作用機(jī)制.

微塑料；多孔介質(zhì)；環(huán)境行為；復(fù)合污染

塑料制品由于成本低廉、延展性強(qiáng)和性質(zhì)穩(wěn)定等特點(diǎn)被廣泛應(yīng)用于人類生產(chǎn)和生活[1-2].塑料因回收率低及難以分解等特點(diǎn)導(dǎo)致了嚴(yán)重的環(huán)境污染問(wèn)題.其中,微塑料[3]作為一種新型污染物引起了國(guó)內(nèi)外的高度關(guān)注.微塑料難以降解,對(duì)污染物有較強(qiáng)的荷載作用并可被動(dòng)植物攝取,嚴(yán)重威脅生態(tài)環(huán)境和人類健康[4-5].

微塑料按化學(xué)組成可分為聚乙烯(PE)、聚氯乙烯(PVC)、聚丙烯(PP)、聚苯乙烯(PS)和聚酰胺(PA)等,按來(lái)源則可分為初生微塑料和次生微塑料[3].初生微塑料是指在生產(chǎn)中被制成的微米級(jí)塑料顆粒,常用于工業(yè)制造、個(gè)人護(hù)理產(chǎn)品生產(chǎn)等[6-8];次生微塑料一般由大塊塑料垃圾經(jīng)物理、化學(xué)和生物過(guò)程而造成分裂和體積減小而成,來(lái)源于洗衣機(jī)廢水中的合成纖維、農(nóng)用地膜、塑料制品等[9-11].目前, 微塑料已在世界范圍內(nèi)的海洋環(huán)境[12-15]、地表水系統(tǒng)(湖泊、河流等)[16-17]、陸地系統(tǒng)(土壤、污泥等)[18-21]和空氣粉塵中[22-23]被廣泛檢出.

隨著研究的不斷深入,科學(xué)家們逐漸認(rèn)識(shí)到除海洋環(huán)境和地表水環(huán)境外,包括土壤和地下水在內(nèi)的地下環(huán)境亦是微塑料的重要儲(chǔ)存庫(kù)[9,24],甚至有研究指出陸地中的微塑料豐度可能是海洋的2～23倍[25].德國(guó)科學(xué)家Rillig等[24]最早關(guān)注并強(qiáng)調(diào)了土壤微塑料污染這一科學(xué)問(wèn)題.多領(lǐng)域科研工作者相繼開(kāi)展了相關(guān)研究,并呼吁提高對(duì)地下環(huán)境中微塑料污染問(wèn)題的重視程度[11,20,26-27].近年來(lái),地下環(huán)境中微塑料污染的來(lái)源與分布、分離與檢測(cè)方法、生態(tài)毒性和環(huán)境行為等方面的研究不斷推進(jìn),并取得了一定進(jìn)展[28-29].其中,微塑料在多孔介質(zhì)中的運(yùn)移等環(huán)境行為受到越來(lái)越多的關(guān)注,已成為相關(guān)研究領(lǐng)域的熱點(diǎn)問(wèn)題.掌握微塑料在多孔介質(zhì)中的運(yùn)移規(guī)律和影響機(jī)制,對(duì)于準(zhǔn)確評(píng)價(jià)其環(huán)境風(fēng)險(xiǎn)和制定有效防治措施至關(guān)重要[9,24].迄今為止,關(guān)于多孔介質(zhì)中微塑料運(yùn)移行為的研究雖明顯增加,但總體上仍處于起步階段,面臨諸多挑戰(zhàn)[5,9,30].現(xiàn)有關(guān)于地下環(huán)境中微塑料污染的綜述文章主要側(cè)重于總結(jié)其來(lái)源與分布、分離與檢測(cè)方法、生態(tài)毒性等方面的內(nèi)容[4,11,26,31-33],對(duì)多孔介質(zhì)中微塑料的運(yùn)移行為研究則有待進(jìn)一步梳理和歸納.

本文在簡(jiǎn)要論述地下環(huán)境中微塑料來(lái)源與危害的基礎(chǔ)上,聚焦國(guó)內(nèi)外微塑料環(huán)境行為研究方面的最新進(jìn)展,總結(jié)并評(píng)述了影響微塑料穩(wěn)定性及其在多孔介質(zhì)中運(yùn)移行為的物理、化學(xué)和生物因素,以及微塑料和其他污染物之間的相互作用及共遷移行為,并對(duì)今后該領(lǐng)域需要進(jìn)一步深入的研究方向進(jìn)行了展望.

1 多孔介質(zhì)中微塑料的來(lái)源與危害

1.1 多孔介質(zhì)中微塑料的來(lái)源

地下環(huán)境中的微塑料污染主要由人類活動(dòng)產(chǎn)生,按來(lái)源可分為面源污染和點(diǎn)源污染.點(diǎn)源污染主要包括污泥的使用、有機(jī)肥的施用、廢水灌溉、污水排放等,面源污染則主要來(lái)自農(nóng)業(yè)塑料薄膜的使用、垃圾焚燒、垃圾填埋及大氣沉降等[24,34-35](圖1).來(lái)自各污染途徑的微塑料數(shù)量巨大,有報(bào)道稱2016年約有39t微塑料通過(guò)個(gè)人護(hù)理產(chǎn)品進(jìn)入自然環(huán)境中[36],澳大利亞每年污泥施用所產(chǎn)生的微塑料總量高達(dá)2.8×103～1.9×104t[37].據(jù)估算,通過(guò)各種途徑進(jìn)入到地下環(huán)境中的微塑料可能超過(guò)40萬(wàn)t[25].另外,當(dāng)前全球新冠(COVID-19)大流行,造成口罩和手套等個(gè)人防護(hù)用品用量激增,很大程度上加劇了微塑料污染的環(huán)境風(fēng)險(xiǎn).微塑料進(jìn)入到地下環(huán)境之后,可通過(guò)不同途徑在土壤-地下水系統(tǒng)中進(jìn)行遷移與輸送,亦可通過(guò)植物吸收、食物鏈傳遞等方式進(jìn)入地上環(huán)境[32,38].

1.2 多孔介質(zhì)中微塑料的危害

地下環(huán)境中的微塑料污染對(duì)生態(tài)環(huán)境和人體健康造成巨大威脅[38].首先,微塑料自身即對(duì)地下環(huán)境健康造成一定風(fēng)險(xiǎn),已有研究表明微塑料顆粒會(huì)影響土壤的結(jié)構(gòu)、功能性及生物多樣性[21,39],但具體的影響程度及作用機(jī)制、對(duì)土壤物質(zhì)循環(huán)是促進(jìn)還是抑制尚不明確[2,39].最新研究表明微塑料的存在會(huì)通過(guò)影響菌根真菌的形成及多樣性進(jìn)而影響植物-土壤系統(tǒng)[40-41].研究表明微塑料顆?？杀煌寥牢⑸?、土壤動(dòng)物及植物等攝取,繼而可在細(xì)胞、組織、個(gè)體到生態(tài)環(huán)境等多個(gè)維度產(chǎn)生危害[39,42].隨著研究的深入,微塑料對(duì)人類健康的潛在風(fēng)險(xiǎn)被進(jìn)一步揭示.最新研究發(fā)現(xiàn)亞微米級(jí)甚至微米級(jí)的微塑料均可被小麥和生菜吸收并進(jìn)入相應(yīng)可食用部位[43],而在此之前科學(xué)家們認(rèn)為微米級(jí)的塑料在日常食用的蔬菜和農(nóng)作物中是不可能存在的.微塑料因性質(zhì)穩(wěn)定會(huì)在環(huán)境中長(zhǎng)期存在,在其生產(chǎn)過(guò)程中添加的增塑劑、阻燃劑等有害物質(zhì)會(huì)不斷被釋放并通過(guò)食物鏈累積[39,44-45].除微塑料自身毒害效應(yīng)外,微塑料與其他污染物形成的復(fù)合污染也引起了國(guó)內(nèi)外學(xué)者的重點(diǎn)關(guān)注.微塑料具有比表面積大、疏水性強(qiáng)等特點(diǎn),對(duì)有機(jī)物、重金屬、致病菌等污染物有吸附富集作用,且可作為載體攜帶污染物遷移[19,46-48],繼而可能對(duì)生物和環(huán)境造成復(fù)合生態(tài)毒性效應(yīng)[44,49-54].Liu等[55]研究發(fā)現(xiàn)PS微球能夠顯著促進(jìn)有機(jī)污染物芘和4-壬基酚在飽和壤砂介質(zhì)中的遷移能力.據(jù)此可推測(cè),地下環(huán)境中有機(jī)物、重金屬等污染區(qū)域中微塑料的存在可能會(huì)進(jìn)一步加劇污染物的環(huán)境風(fēng)險(xiǎn),甚至導(dǎo)致污染物伴隨微塑料向深層地下水遷移.此外,全球范圍應(yīng)對(duì)新冠疫情的口罩和手套等個(gè)人防護(hù)用品在使用后可能攜帶細(xì)菌、病毒等病原體,這類塑料制品如果未經(jīng)處理進(jìn)入環(huán)境,將會(huì)對(duì)環(huán)境和人類造成雙重威脅,加劇微塑料污染的同時(shí)還可能增加病原體的傳播風(fēng)險(xiǎn).

2 微塑料的穩(wěn)定性

掌握微塑料的穩(wěn)定性及影響機(jī)制是闡明其在多孔介質(zhì)中運(yùn)移行為的基礎(chǔ).有關(guān)微塑料的穩(wěn)定性研究一般采用團(tuán)聚動(dòng)力學(xué)實(shí)驗(yàn)法,即通過(guò)動(dòng)態(tài)光散射等技術(shù)對(duì)目標(biāo)條件下微塑料的水合粒徑隨時(shí)間的變化速率進(jìn)行測(cè)定和表征,依據(jù)獲得的團(tuán)聚動(dòng)力學(xué)曲線計(jì)算附著系數(shù)()和臨界聚沉濃度(CCC),進(jìn)而定量和定性分析微塑料在不同條件下的穩(wěn)定性[56-58].此外,多種現(xiàn)代分析技術(shù)如粒徑分析儀、Zeta電位儀、掃描電鏡-能譜(SEM-EDS)、原子力顯微鏡(AFM)、傅里葉變換紅外光譜(FTIR)、X射線光電子能譜(XPS)等被廣泛應(yīng)用于微塑料的檢測(cè)和相關(guān)形態(tài)、官能團(tuán)等的表征分析[6,59-60].鑒于檢測(cè)分析在微塑料污染研究中的重要性,已有多篇中文文獻(xiàn)專門綜述了微塑料檢測(cè)與分析方法的最新進(jìn)展[10,61-62].總體上,目前關(guān)于微塑料在水環(huán)境中穩(wěn)定性的研究以實(shí)驗(yàn)為主,相關(guān)的模擬計(jì)算和理論研究還非常缺乏.

圖1 地下環(huán)境中微塑料污染來(lái)源及其在多孔介質(zhì)中運(yùn)移行為示意

微塑料的穩(wěn)定性與其自身物理化學(xué)性質(zhì)以及所處環(huán)境體系的水化學(xué)條件密切相關(guān)[56,59-60,63-65].此外,微塑料的穩(wěn)定性很大程度上也決定了其生態(tài)毒理效應(yīng)、對(duì)污染物的吸附能力和載體作用等.

2.1 微塑料理化性質(zhì)對(duì)微塑料穩(wěn)定性的影響

微塑料自身的大小、形狀、表面形貌、密度等物理特性直接決定其穩(wěn)定性.不同粒徑的微塑料可能會(huì)呈現(xiàn)出不同的表面電荷密度以及擴(kuò)散動(dòng)力學(xué)過(guò)程,進(jìn)而表現(xiàn)出穩(wěn)定性上的差異[63,66-67],但已有研究尚難以明確微塑料穩(wěn)定性與其粒徑大小之間的規(guī)律.自然環(huán)境中微塑料形狀多樣,包括球狀、纖維狀、線狀等[9],但是已有研究多采用商業(yè)生產(chǎn)的圓球狀微塑料,關(guān)于不同形狀微塑料的穩(wěn)定性研究還有待進(jìn)一步拓展[68].除了形態(tài)和大小等物理性質(zhì)上的差異,微塑料表面官能團(tuán)種類的不同在造成表面電荷差異的同時(shí)[69],還影響其對(duì)有機(jī)物和金屬陽(yáng)離子的吸附能力,是影響微塑料穩(wěn)定性的重要化學(xué)因素[15,69].例如,Song等[70]發(fā)現(xiàn)PS微球表面的氨基利于溶解態(tài)有機(jī)物的吸附,卻抑制了Ca2+和懸浮有機(jī)質(zhì)的吸附,而羧基則對(duì)以上吸附反應(yīng)均起促進(jìn)作用.此外,PS微球的臨界聚沉濃度隨顆粒表面羧酸根離子濃度的增加或羧基中和程度的增加而顯著增大,這意味著微塑料表面官能團(tuán)的數(shù)量對(duì)其穩(wěn)定性的影響同樣不可忽視.最新研究表明,微塑料進(jìn)入自然環(huán)境后所經(jīng)歷的機(jī)械摩擦、化學(xué)氧化等老化過(guò)程可能導(dǎo)致其表面粗糙度、電負(fù)性及官能團(tuán)組成等理化性質(zhì)發(fā)生變化,進(jìn)而影響其穩(wěn)定性[59,63].Liu等[64]利用紫外光照射來(lái)模擬研究老化對(duì)納米聚苯乙烯微球(PSNPs)穩(wěn)定性的影響,發(fā)現(xiàn)老化使得PSNPs的表面電負(fù)性增強(qiáng),在NaCl溶液中的穩(wěn)定性增加,而紫外照射老化后PSNPs表面所形成的羧基則通過(guò)增強(qiáng)其與Ca2+之間的相互作用而加劇了微塑料在CaCl2溶液中的團(tuán)聚現(xiàn)象.Mao等[60]發(fā)現(xiàn)經(jīng)過(guò)UV-H2O2老化處理之后,納米PS表面形成的羰基增強(qiáng)了塑料微粒的極性并降低了疏水性,因而增強(qiáng)了納米PS 的穩(wěn)定性.有學(xué)者注意到微塑料表面生物膜對(duì)其穩(wěn)定性的關(guān)鍵作用,Michels等[71]發(fā)現(xiàn)PS微球表面形成生物膜之后粘度增強(qiáng),導(dǎo)致穩(wěn)定性降低.Liu等[72]研究了生物膜的重要組成成分-細(xì)胞胞外聚合物對(duì)微塑料穩(wěn)定性的影響及作用機(jī)制,研究發(fā)現(xiàn)細(xì)胞胞外聚合物通過(guò)空間位阻效應(yīng)抑制了PSNPs在NaCl溶液中的凝聚現(xiàn)象, 而在CaCl2溶液中則通過(guò)雙電層壓縮和分子橋連作用促進(jìn)了PSNPs的團(tuán)聚,使其穩(wěn)定性降低.截至目前,關(guān)于老化過(guò)程及生物膜等因素對(duì)微塑料穩(wěn)定性影響的研究還比較缺乏,有待進(jìn)一步加強(qiáng)和深入以全面揭示相關(guān)影響機(jī)理.

2.2 水化學(xué)條件對(duì)微塑料穩(wěn)定性的影響

除自身理化性質(zhì)外,微塑料的穩(wěn)定性還取決于所處環(huán)境體系的水化學(xué)條件,受到水溶液中的離子類型、離子強(qiáng)度和離子的水化能力等不同因素的綜合控制.目前這方面的研究多考慮金屬陽(yáng)離子對(duì)微塑料穩(wěn)定性的影響.在達(dá)到CCC之前,體系中金屬離子濃度增大一般會(huì)導(dǎo)致微塑料顆粒表面電負(fù)性的降低而使其團(tuán)聚增強(qiáng)、穩(wěn)定性下降;超過(guò)CCC進(jìn)入擴(kuò)散控制區(qū)之后,金屬離子濃度的繼續(xù)增大則不會(huì)對(duì)其穩(wěn)定性造成進(jìn)一步的改變[60,73-74].離子價(jià)態(tài)對(duì)微塑料穩(wěn)定性的影響機(jī)制符合Schulze-Hardy定律,即離子價(jià)態(tài)越高,對(duì)微塑料穩(wěn)定性的抑制作用越顯著,這主要是因?yàn)楦邇r(jià)態(tài)離子具有更強(qiáng)的電荷中和能力,且可以通過(guò)連接顆粒間的官能團(tuán)起到架橋作用[59-60,66,73].根據(jù)Hofmeister序列,對(duì)于同種價(jià)態(tài)的離子,其水化半徑越小則越利于微塑料顆粒發(fā)生聚沉.然而相關(guān)研究表明該因素的影響并不是決定性的,具體的影響趨勢(shì)和作用機(jī)制由微塑料自身性質(zhì)及其所處體系其他水化學(xué)條件等多因素共同控制[59-60,67].作為水化學(xué)條件的關(guān)鍵因子,pH值直接影響微塑料的表面電荷、官能團(tuán)的質(zhì)子化和去質(zhì)子化過(guò)程[74-75],因而pH值的影響程度與微塑料本身的性質(zhì)密切相關(guān).例如,Mao等[60]指出隨著水溶液的pH值從3.5增至9.5,粒徑為100nm的PS微球表面電負(fù)性相應(yīng)增大,進(jìn)而使得穩(wěn)定性增強(qiáng);而Lu等[59]研究發(fā)現(xiàn)由于顆粒間靜電斥力較強(qiáng),平均粒徑為300～ 400nm PS微球的穩(wěn)定性在pH值為3.0～8.0的范圍內(nèi)均未發(fā)生明顯變化.水環(huán)境中的可溶性有機(jī)物(腐殖酸、富里酸、蛋白質(zhì)、多糖等)廣泛存在,也被證明是影響膠體穩(wěn)定性的重要因素[46,53-54,56,60,76],且具體的影響程度及作用機(jī)制與可溶性有機(jī)物的種類及存在狀態(tài)、膠體的自身性質(zhì)、體系的離子組成等條件密切相關(guān)[58,63,77].目前關(guān)于可溶性有機(jī)物對(duì)微塑料穩(wěn)定性的報(bào)道中考慮的條件較為單一,有待進(jìn)一步加強(qiáng)研究深度和廣度,以闡明可溶性有機(jī)物對(duì)微塑料穩(wěn)定性的影響及作用機(jī)制.

值得指出的是,已有關(guān)于微塑料穩(wěn)定性的研究絕大多數(shù)都針對(duì)微塑料顆粒之間的均質(zhì)團(tuán)聚,然而自然環(huán)境中廣泛存在的金屬氧化物、高嶺土、蒙脫土等自然膠體與微塑料顆粒形成異質(zhì)團(tuán)聚的可能性更大[5,78].PS微球與Fe2O3、二氧化鈦等納米顆粒形成異質(zhì)團(tuán)聚后穩(wěn)定性下降的現(xiàn)象已經(jīng)見(jiàn)諸報(bào)道[79-80].但由于檢測(cè)方法等技術(shù)限制,目前關(guān)于微塑料與其他膠體之間異質(zhì)團(tuán)聚行為的研究嚴(yán)重缺乏[5],使得準(zhǔn)確預(yù)測(cè)微塑料的環(huán)境行為面臨困難.

總體而言,關(guān)于微塑料穩(wěn)定性的研究還很不充分,存在研究對(duì)象和考慮因素單一等局限性,相關(guān)影響因素和作用機(jī)制尚不完全明確,為更好的闡明微塑料在地下環(huán)境中的運(yùn)移等環(huán)境行為,有必要進(jìn)一步深入開(kāi)展不同種類微塑料在自然條件下的穩(wěn)定性研究.

3 微塑料在多孔介質(zhì)中的運(yùn)移行為

微塑料在多孔介質(zhì)中的運(yùn)移行為是其環(huán)境歸趨中的重要環(huán)節(jié)(圖1).開(kāi)展實(shí)驗(yàn)室內(nèi)一維運(yùn)移模擬實(shí)驗(yàn)是研究微塑料在多孔介質(zhì)中運(yùn)移行為的常用方法,一般采用玻璃珠、石英砂、土壤等填充砂柱作為多孔介質(zhì),通過(guò)運(yùn)移模擬實(shí)驗(yàn)獲取不同情形下微塑料的穿透曲線以及在砂柱中的滯留分布來(lái)分析其在多孔介質(zhì)中的運(yùn)移過(guò)程和影響機(jī)制[5,9,81].針對(duì)不同種類及性質(zhì)的微塑料,可采用可見(jiàn)-紫外分光光度計(jì)、熒光分光光度計(jì)、流式細(xì)胞儀等儀器進(jìn)行濃度測(cè)定[55,81-82].

研究表明微塑料的運(yùn)移行為受微塑料自身性質(zhì)、介質(zhì)性質(zhì)、水流等多方面因素的影響.在檢索國(guó)內(nèi)外相關(guān)研究進(jìn)展的基礎(chǔ)上,本文對(duì)微塑料在多孔介質(zhì)中遷移行為的主要研究進(jìn)展進(jìn)行了總結(jié)和梳理,從物理、化學(xué)和生物三方面因素予以分述.

3.1 物理因素的影響

3.1.1 微塑料物理性質(zhì) 微塑料顆粒的自身物理特性首先決定其在多孔介質(zhì)中的運(yùn)移能力[35,83].真實(shí)自然環(huán)境中存在的微塑料在大小、形狀、表面形態(tài)(粗糙度、開(kāi)裂、凹凸)等方面差異顯著,然而已有研究選擇對(duì)象非常局限.微塑料粒徑大小是決定其遷移范圍的基本要素,已有研究中選用微塑料顆粒大小為納米到微米尺寸,不同粒徑微塑料可通過(guò)影響瀝濾作用、堵塞作用等機(jī)制控制而表現(xiàn)出不同的遷移能力.一般地,粒徑小的微塑料更容易向下遷移,從表層土壤進(jìn)入到地下水中.例如,Dong等[84]發(fā)現(xiàn)隨著PS微塑料粒徑由2.0mm 減至0.8mm,其在飽和石英砂介質(zhì)中的穿透率由13.6%增至41.3%,這主要是因?yàn)闉r濾作用隨微塑料粒徑的減小而逐漸減弱.從形狀上看,已有研究對(duì)象多局限于表面平滑的規(guī)則球狀微塑料顆粒,對(duì)纖維狀、碎片狀等微塑料遷移行為的報(bào)道甚少.微塑料的形狀已被證實(shí)會(huì)影響土壤團(tuán)聚體形成和土壤系統(tǒng)中有機(jī)物分解反應(yīng)[85],而對(duì)其運(yùn)移行為的影響及作用機(jī)制尚不明確.另外,除微塑料原有性質(zhì)外,自然環(huán)境中化學(xué)氧化、機(jī)械摩擦、微生物作用等通過(guò)改變微塑料的理化性質(zhì)對(duì)其運(yùn)移行為的影響不容忽視,有研究發(fā)現(xiàn)UV和O3老化處理可使PS微球表面電負(fù)性增強(qiáng)、疏水性降低,從而顯著提高微塑料在飽和壤砂土介質(zhì)中的運(yùn)移能力[55],而其他老化因素對(duì)微塑料運(yùn)移行為影響亟需進(jìn)一步探究[86].從材料類型角度來(lái)看,已有研究多針對(duì)聚苯乙烯類微塑料,雖然不同種類的微塑料具有相似的特性,但在塑性、密度等方面的差異也會(huì)影響其在地下環(huán)境中的垂向遷移[4].目前對(duì)其他種類微塑料在多孔介質(zhì)中的運(yùn)移行為的研究還非常有限,因此要全面掌握微塑料的運(yùn)移等環(huán)境行為,必須對(duì)其他種類和來(lái)源的微塑料展開(kāi)進(jìn)一步研究[5,87].

3.1.2 介質(zhì)物理因素 多孔介質(zhì)的介質(zhì)粒徑、非均質(zhì)性、飽和度等物理性質(zhì)是決定微塑料在其中運(yùn)移行為的重要因素.一般認(rèn)為,介質(zhì)粒徑越大越利于膠體在其中的運(yùn)移,粒徑越小的多孔介質(zhì)具有更小的孔喉和滲透率,且可提供更多的吸附位點(diǎn)[88-89].地下環(huán)境中廣泛存在的介質(zhì)非均質(zhì)性被認(rèn)為是導(dǎo)致膠體運(yùn)移行為理論預(yù)測(cè)與實(shí)際實(shí)驗(yàn)結(jié)果之間不相符的重要原因.宏觀的非均質(zhì)性會(huì)導(dǎo)致優(yōu)勢(shì)流,Rillig等[35]研究發(fā)現(xiàn)介質(zhì)的大孔隙(孔徑>0.08mm)中液體流動(dòng)性較強(qiáng),進(jìn)而微塑料的運(yùn)移能力隨之提高. Majdalani等[90]發(fā)現(xiàn)由于土壤干燥出現(xiàn)的裂縫有助于微塑料向更深處運(yùn)移.介質(zhì)表面粗糙度等微觀非均質(zhì)性被證明會(huì)通過(guò)影響介質(zhì)表面與膠體之間的作用力、削弱水動(dòng)力剪切力等機(jī)制控制膠體的運(yùn)移行為[91],然而目前關(guān)于微塑料運(yùn)移行為的已有研究多選用形狀規(guī)則、表面光滑的石英砂或玻璃珠等作為砂柱填充介質(zhì)[5],介質(zhì)粗糙度對(duì)微塑料運(yùn)移行為的影響程度及作用機(jī)制研究有待加強(qiáng).尤其當(dāng)微塑料顆粒與多孔介質(zhì)同時(shí)具有較強(qiáng)的非均質(zhì)性時(shí),目前已有結(jié)論和規(guī)律是否適用還需進(jìn)一步探索和驗(yàn)證.相較于飽和多孔介質(zhì),非飽和介質(zhì)中膠體運(yùn)移行為及作用機(jī)制更加復(fù)雜,Sirivithayapakorn等[92]研究發(fā)現(xiàn)水-汽界面的存在利于促進(jìn)塑料微球在多孔介質(zhì)中的沉積,當(dāng)水汽界面消失,該部分微塑料便會(huì)釋放到水相中.20世紀(jì)90年代即有學(xué)者開(kāi)展了塑料微球在非飽和多孔介質(zhì)中的運(yùn)移行為[93],但是相關(guān)作用機(jī)制至今尚不十分明確[94-95],微塑料粒徑大小、表面電荷、親疏水性等自身特性與飽和度的耦合作用亟需進(jìn)一步探究.另外,多孔介質(zhì)中孔隙水流速也是影響微塑料在介質(zhì)表面的沉積與釋放的重要因素[96].一般情況下,水流剪切力隨流速增大而增大,不利于膠體在介質(zhì)中的滯留[88].

3.2 化學(xué)因素的影響

3.2.1 微塑料化學(xué)因素 除微塑料自身物理性質(zhì)外,其化學(xué)成分、表面官能團(tuán)、親疏水性等化學(xué)性質(zhì)同樣決定其運(yùn)移行為[63].微塑料種類繁多,但現(xiàn)有研究大多以PS微塑料為研究對(duì)象,對(duì)其他種類微塑料運(yùn)移行為的掌握還很有限.對(duì)于同種微塑料,表面官能團(tuán)組成上的差異可能會(huì)使其呈現(xiàn)出不同的運(yùn)移能力.例如,Dong等[97]發(fā)現(xiàn)表面被羧基(NPC)、磺酸基(NPS)、低密度氨基(NPA)、高密度氨基(NPA)官能團(tuán)修飾過(guò)的PS微球在海水飽和石英砂介質(zhì)中的回收率分別為19.69%、16.37%、13.33%、9.78%,這主要是因?yàn)槲⑺芰项w粒表面電荷的差異導(dǎo)致的.微塑料進(jìn)入環(huán)境之后,自身化學(xué)性質(zhì)經(jīng)歷過(guò)生物、化學(xué)等老化過(guò)程會(huì)發(fā)生變化,運(yùn)移行為也會(huì)隨之改變.Liu等[55]發(fā)現(xiàn)PS微球在UV和O3下暴露氧化之后顆粒表面電負(fù)性和親水性增強(qiáng),在壤土介質(zhì)中的運(yùn)移能力顯著增強(qiáng).目前老化作用對(duì)微塑料運(yùn)移行為的影響研究還很缺乏,要準(zhǔn)確預(yù)測(cè)真實(shí)環(huán)境中微塑料的環(huán)境行為,必須加強(qiáng)這一方面的研究.

3.2.2 介質(zhì)化學(xué)性質(zhì) 介質(zhì)的礦物組成、表面電荷、化學(xué)非均質(zhì)性等性質(zhì)均是影響膠體運(yùn)移的重要因素,對(duì)微塑料來(lái)講同樣如此.自然環(huán)境介質(zhì)復(fù)雜多樣,然而目前對(duì)微塑料運(yùn)移行為的柱實(shí)驗(yàn)研究多選用性質(zhì)比較單一的石英砂和玻璃珠作填充多孔介質(zhì)[11,98-100].研究表明微塑料在自然介質(zhì)中的滯留量一般高于其在純凈石英砂介質(zhì)中的滯留量. Bouchard等[101]認(rèn)為粗糙介質(zhì)表面可為微塑料提供更多的沉積位點(diǎn),介質(zhì)表面的氫氧化鋁可提供正電荷,與帶負(fù)電的微塑料顆粒之間形成靜電引力. Quevedo等[89]則將壤土中較高的微塑料滯留量歸因于壤土與沙土粒徑分布的差別以及壤土表面的非均質(zhì)性.Wu等[98]研究發(fā)現(xiàn)PS微塑料在不同類型土壤(沙土、黑土、紅土)中的運(yùn)移行為與土壤的鐵鋁化合物含量及pH值密切相關(guān),具體表現(xiàn)為滯留量與土壤的鐵鋁化合物含量成正比,與土壤pH值成反比,這主要取決于土壤介質(zhì)與微塑料顆粒之間的靜電作用.要進(jìn)一步掌握微塑料在真實(shí)環(huán)境中的遷移行為,則需要進(jìn)一步探索和完善不同種類自然介質(zhì)中微塑料的運(yùn)移與分布情況.

3.2.3 水化學(xué)因素 相較于其他因素,關(guān)于多孔介質(zhì)體系中水化學(xué)因素對(duì)微塑料運(yùn)移行為的影響研究數(shù)量較多,但總體來(lái)看考慮的因素單一.水流的電解質(zhì)組成對(duì)微塑料運(yùn)移行為的影響研究得到了較多關(guān)注.研究發(fā)現(xiàn)離子強(qiáng)度增大一般會(huì)壓縮介質(zhì)和微塑料顆粒雙電層,減小其表面電負(fù)性,從而降低兩者之間的靜電斥力,抑制微塑料的運(yùn)移能力[77,99].相同離子強(qiáng)度條件下,高價(jià)態(tài)離子對(duì)微塑料運(yùn)移能力的抑制程度更強(qiáng),這是因?yàn)楦邇r(jià)離子具有更強(qiáng)的電荷中和能力,且可以在介質(zhì)與微塑料顆粒之間起架橋作用[98].另外,離子強(qiáng)度的影響與微塑料粒徑相關(guān),Dong等[84]發(fā)現(xiàn)海水鹽度的變化(35～3.5PSU)并沒(méi)有顯著影響較大粒徑微塑料顆粒(2.0～0.8mm)的遷移,而當(dāng)海水鹽度從35PSU下降到17.5或3.5PSU時(shí),小粒徑微塑料(0.6～0.1mm)的聚集被顯著或完全抑制.作為水化學(xué)條件的關(guān)鍵因子,pH值對(duì)微塑料的運(yùn)移行為的影響研究還很缺乏.Cai等[79]研究了PS微球與TiO2在不同pH值條件下在飽和石英砂介質(zhì)中的共遷移行為,發(fā)現(xiàn)不同pH值條件下涉及的作用機(jī)制不同.在pH=5的情況下,PS-TiO2異質(zhì)團(tuán)聚體的形成以及介質(zhì)表面預(yù)先沉積的TiO2提供額外吸附位點(diǎn)是導(dǎo)致微塑料沉積增多的主要機(jī)制;pH=7時(shí),除上述原因外,由于TiO2在石英砂表面沉積造成石英砂表面粗糙度的增加也被認(rèn)為是促進(jìn)微塑料滯留的重要機(jī)制之一.腐殖酸等可溶性有機(jī)物被證明可通過(guò)改變微塑料及多孔介質(zhì)的表面電荷、形成空間位阻效應(yīng)等作用機(jī)制改變微塑料在多孔介質(zhì)中的運(yùn)移行為,不同種類可溶性有機(jī)物對(duì)不同種類微塑料的影響程度及作用機(jī)制研究亟需進(jìn)一步開(kāi)展[102].

3.3 生物因素的影響

多孔介質(zhì)中的蚯蚓、跳蟲(chóng)、螨蟲(chóng)等土壤生物被證明是影響微塑料遷移與分布行為的重要生物因素.一方面,蚯蚓等生物在地下活動(dòng)會(huì)形成土壤空隙、毛細(xì)管、洞穴等,助于微塑料向土壤深層遷移[26,103].另一方面,土壤生物可通過(guò)攝食、排泄、表面附著、抓、推等生物活動(dòng)成為微塑料的重要運(yùn)輸媒介,加速微塑料在地下環(huán)境中的擴(kuò)散和遷移,亦可能進(jìn)一步污染地下水資源[2,24].

作為陸地生態(tài)系統(tǒng)的重要組成,植物在地下環(huán)境污染物的遷移轉(zhuǎn)化中承擔(dān)重要角色[21,104].植物會(huì)通過(guò)吸收將微塑料傳輸至地上環(huán)境中,有學(xué)者揭示了作物吸收微塑料的通道與機(jī)制,發(fā)現(xiàn)塑料顆?？梢源┩感←満蜕烁颠M(jìn)入植物體,并能在蒸騰拉力的作用下通過(guò)導(dǎo)管系統(tǒng)隨水流和營(yíng)養(yǎng)流進(jìn)入作物可食用部位[43,105].植物對(duì)微塑料的吸收富集可以改變微塑料的環(huán)境行為,同時(shí)也意味著微塑料可能通過(guò)食物鏈傳遞而威脅人類健康及農(nóng)業(yè)環(huán)境的可持續(xù)發(fā)展.此外,植物分泌的有機(jī)物質(zhì)也可能會(huì)影響微塑料在土壤中的穩(wěn)定性和運(yùn)移行為,目前此類研究幾乎為空白.

圖2 多孔介質(zhì)中微塑料運(yùn)移行為的影響因素

要準(zhǔn)確預(yù)測(cè)微塑料的遷移行為,自然環(huán)境中廣泛存在的生物膜的影響不可忽略.生物膜可以改變介質(zhì)的比表面積、表面電荷、粗糙度、親疏水性等性質(zhì),可通過(guò)影響靜電作用、空間位阻效應(yīng)等對(duì)不同性質(zhì)膠體的運(yùn)移能力產(chǎn)生抑制或促進(jìn)作用[106-108],目前關(guān)于生物膜對(duì)微塑料運(yùn)移行為及作用機(jī)制的影響研究還較為薄弱[37,109].Mitzel等[109]發(fā)現(xiàn)生物膜的存在促進(jìn)了PS微球在石英砂介質(zhì)中的滯留,且具體的影響效應(yīng)與生物膜親疏水性相關(guān),PS在疏水性更強(qiáng)的PAO1生物膜附著的石英砂介質(zhì)中更加穩(wěn)定.由于微生物自身種類繁多、以及附著介質(zhì)性質(zhì)豐富多樣,導(dǎo)致生物膜本身具有很強(qiáng)的物理化學(xué)非均質(zhì)性[110],因此要真正揭示自然環(huán)境中生物膜對(duì)微塑料運(yùn)移行為的影響及作用機(jī)制,目前的研究還遠(yuǎn)遠(yuǎn)不足[37].

4 微塑料與污染物之間相互作用

微塑料與污染物之間相互作用形成的復(fù)合污染是導(dǎo)致微塑料污染問(wèn)題備受關(guān)注的重要原因之一[5,26,111].研究表明由于微塑料比表面積大、疏水性強(qiáng)等特性,對(duì)多種污染物表現(xiàn)出吸附和富集能力[9,45,48,50,112],然而這方面的研究多側(cè)重于探究微塑料對(duì)污染物的吸附能力及影響因素,對(duì)微塑料吸附污染物前后發(fā)生的性質(zhì)、生態(tài)毒性、以及環(huán)境行為等方面變化的關(guān)注和掌握還遠(yuǎn)遠(yuǎn)不足,因此目前國(guó)內(nèi)外學(xué)者對(duì)微塑料與污染物復(fù)合效應(yīng)的觀點(diǎn)存在一定爭(zhēng)議.近年來(lái),針對(duì)微塑料與污染物在多孔介質(zhì)中的共遷移行為研究逐漸增多,為微塑料與其他污染物的復(fù)合風(fēng)險(xiǎn)評(píng)估提供了一定參考價(jià)值[5,9].本文在梳理微塑料對(duì)污染物吸附、解吸行為研究進(jìn)展的基礎(chǔ)上,總結(jié)了微塑料與其他類型污染物在多孔介質(zhì)中的共遷移行為(表1).

4.1 微塑料對(duì)污染物的吸附作用

微塑料對(duì)重金屬、有機(jī)物、微生物等污染物的吸附作用受微塑料自身理化性質(zhì)、污染物種類及濃度、所處反應(yīng)體系的水化學(xué)條件等多種因素的共同影響[50,113-114].

微塑料可通過(guò)直接吸附作用、疏水分配作用、陰陽(yáng)離子之間的化學(xué)鍵作用等對(duì)重金屬進(jìn)行吸附[115].付東東等[116]研究了不同粒徑PS微球?qū)u2+的吸附情況,發(fā)現(xiàn)粒徑為0.5 μm的PS微球因比表面積更大、微孔隙更多,比粒徑為0.6μm的PS微球吸附能力更大.另外,吸附量隨Cu2+濃度增加及微塑料濃度減小而增大,而吸附速率卻隨Cu2+濃度增加及微塑料濃度降低而降低.微塑料老化之后表面物理結(jié)構(gòu)及化學(xué)性質(zhì)的變化是導(dǎo)致其吸附能力差異的重要因素[117].Turner等[118]發(fā)現(xiàn)聚乙烯顆粒經(jīng)過(guò)老化之后表面電負(fù)性增強(qiáng),對(duì)銅、銀等重金屬吸附速率加快.Lang等[119]使用H2O2老化和芬頓老化兩種方式對(duì)PS微球進(jìn)行處理,發(fā)現(xiàn)老化之后塑料微球表面發(fā)生氧化并形成微裂紋,對(duì)Cd2+的吸附能力顯著提高,而且芬頓老化處理使微球表面吸附位點(diǎn)增多,進(jìn)而表現(xiàn)出更大的吸附能力提升.張瑞昌等[120]發(fā)現(xiàn)酸處理、堿處理、氧化處理(30% H2O2)和高溫-凍融處理四種老化方式使 PE 微塑料表面出現(xiàn)大量的粗糙褶皺結(jié)構(gòu),導(dǎo)致PE對(duì)Zn的吸附量增加.

表面吸附、靜電作用、分子間作用力、疏水性作用等是影響微塑料吸附有機(jī)污染物的重要機(jī)制[45].楊杰等[121]研究了PE、PS、PVC在不同條件下對(duì)四環(huán)素的吸附情況,發(fā)現(xiàn)不同種類微塑料因親疏水性、表面電荷、極性等性質(zhì)不同對(duì)四環(huán)素的吸附存在顯著差異,吸附能力強(qiáng)弱順序?yàn)镻E>PVC> PS,且pH值、有機(jī)物、陽(yáng)離子組成等土壤環(huán)境條件明顯影響吸附效果.Velzeboer等[112]研究了微米/納米尺度PE和PS微球?qū)Χ嗦嚷?lián)苯的吸附作用,結(jié)果表明納米PS的吸附能力比微米尺度PE強(qiáng)1～2個(gè)數(shù)量級(jí),這是由于納米PS具有更高的芳香度和比表面積.Bakir等[113]研究了微塑料顆粒對(duì)不同種類有機(jī)污染物的競(jìng)爭(zhēng)吸附,發(fā)現(xiàn)微塑料對(duì)有機(jī)物的吸附取決于微塑料、有機(jī)物質(zhì)的疏水作用,以及微塑料孔隙的密集程度.另外,老化作用同樣影響微塑料對(duì)有機(jī)污染物的吸附能力.徐鵬程等[122]研究發(fā)現(xiàn)PS經(jīng)過(guò)紫外光照射發(fā)生老化之后,表面結(jié)晶度和官能團(tuán)數(shù)量增加,導(dǎo)致其對(duì)多溴聯(lián)苯醚的吸附量降低42.38%,而老化前后PE對(duì)污染物的吸附量沒(méi)有發(fā)生變化.

相比于微塑料對(duì)重金屬和有機(jī)污染物的吸附研究,針對(duì)微塑料對(duì)致病菌等微生物吸附的研究還較為缺乏.塑料碎片疏水表面可為各種微生物群落提供穩(wěn)定的棲息場(chǎng)所,利于微生物的聚集,并加速生物膜的形成[123].Harrison等[53]研究也發(fā)現(xiàn)海洋沉積物中的細(xì)菌可以迅速定植于微塑料中.值得注意的是,微塑料表面生物膜的形成可能會(huì)改變其對(duì)其他污染物的吸附能力,Wang 等[124]對(duì)比了PE微球加載生物膜前后對(duì)重金屬Cu2+和四環(huán)素的吸附性能及吸附機(jī)制,發(fā)現(xiàn)生物膜的存在顯著增強(qiáng)了對(duì)污染物的吸附能力以及污染物在微塑料表面的穩(wěn)定性,原始微塑料和加載生物膜的微塑料對(duì)污染物的主導(dǎo)吸附機(jī)制分別為顆粒內(nèi)擴(kuò)散和膜擴(kuò)散,另外,表面加載生物膜的PE微球還受絡(luò)合作用和競(jìng)爭(zhēng)作用的影響.

4.2 微塑料與污染物之間的解吸作用

微塑料對(duì)污染物的解吸作用是微塑料與污染物之間相互作用的重要組成部分.污染物從微塑料解吸的過(guò)程中,微塑料便成為了污染物的“源”,因此闡明微塑料與污染物之間的解吸作用及其機(jī)理對(duì)于準(zhǔn)確評(píng)估微塑料的環(huán)境風(fēng)險(xiǎn)同樣重要[125].相比于微塑料與污染物之間吸附作用的研究,聚焦微塑料與污染物之間解吸作用的報(bào)道還較少.已有研究表明,該解吸過(guò)程同樣受微塑料自身性質(zhì)、污染物性質(zhì)、反應(yīng)體系的水化學(xué)條件等多因素共同作用[125-127].作為微塑料的重要組成部分,熒光物質(zhì)、顏料和雙酚A等各類添加劑均可從微塑料釋放進(jìn)入環(huán)境中,對(duì)環(huán)境和生物產(chǎn)生二次污染,具體釋放程度與速率取決于與微塑料本身性質(zhì)及其所處的環(huán)境條件[128-131].

除微塑料添加劑外,微塑料表面吸附的其他環(huán)境污染物也可能通過(guò)解吸作用重新釋放到環(huán)境或生物體內(nèi).Zhou等[126]研究了不同條件下微塑料表面Cd的解吸過(guò)程,發(fā)現(xiàn)人工蚯蚓腸環(huán)境相比于沉積環(huán)境更加利于Cd的解吸,且腐殖酸的存在對(duì)解吸過(guò)程呈促進(jìn)作用.Zuo等[127]研究了可生物降解塑料聚己二酸丁二醇酯(PBAT)對(duì)有機(jī)物菲的吸附和解吸行為,發(fā)現(xiàn)解吸過(guò)程主要取決于PBAT中橡膠亞組分的豐度.與吸附作用相同,老化作用同樣是影響微塑料對(duì)污染物解吸過(guò)程的重要因素.劉學(xué)敏[132]研究發(fā)現(xiàn)UV/H2O2/Chlorine等老化過(guò)程顯著影響不同種類微塑料內(nèi)部雙酚A的釋放,低密度聚乙烯經(jīng)老化后不再釋放雙酚A,而老化聚碳酸酯微塑料上的單體雙酚A則會(huì)源源不斷的泄露.

4.3 微塑料與污染物在多孔介質(zhì)中的共遷移

表1 微塑料與污染物之間的相互作用

微塑料與其他污染物在多孔介質(zhì)中的共遷移行為,是微塑料的重要環(huán)境行為之一,也是直接決定微塑料-污染物復(fù)合效應(yīng)的重要過(guò)程.Liu 等[55]研究發(fā)現(xiàn)PS微球可作為載體促進(jìn)芘在飽和壤土中的遷移能力,而且微塑料經(jīng)過(guò)UV和O3老化處理之后,對(duì)芘和4-壬基酚的吸附能力顯著增強(qiáng),進(jìn)而顯著提高對(duì)兩者遷移能力的促進(jìn)程度.Zhao等[81]研究了PS微球和四環(huán)素在飽和多孔介質(zhì)中的共遷移行為,發(fā)現(xiàn)兩者共存時(shí)運(yùn)移行為變得更加復(fù)雜,PS微球的存在一定程度上抑制了四環(huán)素的遷移能力,具體的遷移行為與體系的離子強(qiáng)度和陽(yáng)離子組成有關(guān).Li等[114]的研究指出,不同粒徑的微塑料顆粒對(duì)鐵氧化物在石英砂中的遷移和沉積行為具有不同的影響,這主要是微塑料和鐵氧化物形成的共聚物以及微塑料提供吸附點(diǎn)差異所導(dǎo)致的.微塑料與污染物濃度比值同樣影響共遷移過(guò)程,Dong等[100]研究了富勒烯(C60)與納米塑料(NPs)在海砂多孔介質(zhì)中的協(xié)同遷移,發(fā)現(xiàn)隨NPs/C60濃度比不斷下降(1～1/3),由于較大次生團(tuán)聚體的形成,NPs的運(yùn)移能力逐漸受到抑制;當(dāng)比值繼續(xù)下降到1/10時(shí),兩者的運(yùn)移過(guò)程主要由C60決定.值得注意的是,目前關(guān)于微塑料與污染物之間協(xié)同運(yùn)移研究多側(cè)重關(guān)注微塑料對(duì)污染物運(yùn)移的影響,污染物對(duì)微塑料在多孔介質(zhì)中運(yùn)移與分布行為的影響及作用機(jī)制有待進(jìn)一步擴(kuò)展和深入.微生物作為一種生物膠體,與微塑料之間可能發(fā)生共遷移現(xiàn)象.He等[82]重點(diǎn)關(guān)注了微塑料對(duì)細(xì)菌BL21在飽和石英砂介質(zhì)中遷移行為的影響,發(fā)現(xiàn)在較高離子強(qiáng)度條件下(50mM NaCl,5mM CaCl2),不同粒徑(0.02～2μm)的塑料顆粒通過(guò)影響塑料-細(xì)菌-石英砂之間的吸附情況促進(jìn)了細(xì)菌的運(yùn)移.微塑料表面生物膜的存在可通過(guò)其表面官能團(tuán)組成、親疏水性、粘性等進(jìn)而影響其運(yùn)移等環(huán)境行為[129].附著在微塑料上的微生物會(huì)伴隨著微塑料一起遷移,導(dǎo)致微生物進(jìn)入到其他生態(tài)系統(tǒng)中,可能會(huì)對(duì)原有生態(tài)系統(tǒng)的群落和功能產(chǎn)生影響.另外,致病菌隨微塑料遷移的過(guò)程中產(chǎn)生的環(huán)境風(fēng)險(xiǎn)可能會(huì)被放大,此外,新冠(COVID-19)爆發(fā)和流行背景下,闡明微塑料與細(xì)菌、病毒等病原體在地下環(huán)境中的共遷移行為及作用機(jī)制對(duì)于保護(hù)生態(tài)安全和人類健康的重要意義更加凸顯.

5 未來(lái)研究展望

綜上所述,目前對(duì)于微塑料在多孔介質(zhì)中的遷移行為研究尚存不足,需要進(jìn)行更加系統(tǒng)和深入的研究,闡明不同因素的影響程度及作用機(jī)制,揭示微塑料與其他污染物之前的共遷移行為及其復(fù)合效應(yīng),進(jìn)而為準(zhǔn)確預(yù)測(cè)微塑料在地下環(huán)境中的歸趨提供理論指導(dǎo)和科學(xué)依據(jù).進(jìn)一步研究應(yīng)重點(diǎn)關(guān)注以下幾點(diǎn):

5.1 自然環(huán)境中微塑料種類繁多、理化性質(zhì)迥異,而已有研究多以商業(yè)生產(chǎn)的形狀規(guī)則、表面平滑的PS微球?yàn)檠芯繉?duì)象,故而研究結(jié)論存在明顯的局限性.要想對(duì)微塑料的環(huán)境行為進(jìn)行全面準(zhǔn)確評(píng)估,今后應(yīng)加強(qiáng)對(duì)不同來(lái)源、不同種類的初生和次生微塑料的研究.

5.2 自然環(huán)境中膠體廣泛存在,且對(duì)污染物環(huán)境歸趨的影響不可忽視.厘清膠體與微塑料之間的異質(zhì)團(tuán)聚行為以及兩者在多孔介質(zhì)中的共遷移機(jī)制,對(duì)于準(zhǔn)確預(yù)測(cè)微塑料的環(huán)境行為及環(huán)境風(fēng)險(xiǎn)至關(guān)重要.

5.3 微塑料對(duì)污染物的吸附富集和運(yùn)移載體功能是評(píng)價(jià)其環(huán)境風(fēng)險(xiǎn)的重要內(nèi)容,關(guān)于微塑料與其他污染物協(xié)同運(yùn)移行為及作用機(jī)制的研究亟需進(jìn)一步完善.此外,新冠(COVID-19)疫情爆發(fā)和流行背景下,針對(duì)微塑料與細(xì)菌、病毒等病原體在地下環(huán)境中的共遷移行為研究面臨新的挑戰(zhàn).

5.4 多孔介質(zhì)是集多種因素為一體的復(fù)雜系統(tǒng),而目前已有研究大多考慮各因素的單一影響,且對(duì)諸如介質(zhì)非均質(zhì)性、飽和度、生物膜等重要因素的影響研究嚴(yán)重缺乏.今后在豐富這方面研究的同時(shí),可深入研究多因素耦合作用下微塑料在多孔介質(zhì)中的運(yùn)移行為,為微塑料污染的有效防控及精準(zhǔn)治理提供理論依據(jù).

5.5 重視多孔介質(zhì)中的微塑料污染對(duì)生物多樣性、群落結(jié)構(gòu)等方面的影響,并加強(qiáng)塑料生物降解方面的研究.

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Review on the environmental behaviors of microplastics in porous media.

LI Xiao-hui1,2, XU Hong-xia1*, SUN Yuan-yuan1, WU Ji-chun1**

(1.Key Laboratory of Surficial Geochemistry of Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China；2.College of Marine Science and Engineering, Nanjing Normal University, Nanjing 210023, China)., 2021,41(6)：2798～2811

Microplastics (MPs) pollution is among the global environmental concerns and has become one of the frontier research. Increasing attention has been paid to the environmental behaviors of MPs in soil and groundwater system in recent years. Based on a brief summary of the sources and hazard of MPs, this paper mainly focused on the research progress in MPs environmental behaviors, and the latest progress in the stability and transport behaviors of MPs in porous media was reviewed, with emphasis on the influence and mechanism of physical, chemical and biological factors. Besides, adsorption/desorption of pollutants on/from MPs and their cotransport were also summarized to discuss the interaction between MPs and other pollutants. To systematically understand the transport behaviors and mechanisms of MPs in porous media, further research should address different MPs types, multi-factor coupling effect, and the interaction between MPs and microorganisms, etc.

microplastics；porous media；environmental behaviors；combined pollution

X502

A

1000-6923(2021)06-2798-14

2020-11-06

國(guó)家自然科學(xué)基金資助項(xiàng)目(41730856, 41877182);中央高?；究蒲袠I(yè)務(wù)費(fèi)專項(xiàng)資金資助項(xiàng)目(020614380106)

* 責(zé)任作者, 副教授, hxxu@nju.edu.cn;**教授, jcwu@nju.edu.cn

李宵慧(1991-),女,山東濟(jì)寧人,講師,博士,主要從事污染物遷移、轉(zhuǎn)化及修復(fù)研究.發(fā)表論文8篇.