鄭瑩瑩,張可佳,岑 程,毛如寅,張土喬

供水管網(wǎng)中微/納米塑料的賦存及污染特性

鄭瑩瑩,張可佳*,岑 程,毛如寅,張土喬

(浙江大學(xué)建筑工程學(xué)院,浙江 杭州 310058)

微/納米塑料具有分布廣泛、粒徑小、難降解、可吸附有毒物質(zhì)等特性,其去除效果及對(duì)環(huán)境污染的現(xiàn)狀尚不明晰,是當(dāng)前研究熱點(diǎn)和難點(diǎn).本文從供水管網(wǎng)中微/納米塑料的來(lái)源、分布規(guī)律及水質(zhì)安全危害等角度,概述了全球范圍內(nèi)供水管網(wǎng)中微/納米塑料的研究現(xiàn)狀.結(jié)果表明,給水處理廠出廠水?dāng)y帶殘留的微/納米塑料顆粒進(jìn)入供水管網(wǎng),而塑料材質(zhì)管道在水力、水質(zhì)作用下(流速、機(jī)械磨損、消毒劑等)同樣存在微/納米塑料釋放可能.微/納米塑料自身的密度、電荷等固有特性影響了它在供水管網(wǎng)中的空間分布規(guī)律,且在飲用水輸配過(guò)程中,微/納米塑料可同有機(jī)物、微生物等物質(zhì)反應(yīng),從而影響供水管網(wǎng)的水質(zhì)安全.本文旨在提出未來(lái)的研究重點(diǎn)和方向,從而為進(jìn)一步了解微/納米塑料對(duì)飲用水安全的影響及如何控制其污染提供理論基礎(chǔ).

微/納米塑料；供水管網(wǎng)；分布規(guī)律；水質(zhì)安全

塑料污染已成為一個(gè)世界性問(wèn)題,一旦釋放到環(huán)境中,其在環(huán)境中物理化學(xué)和生物因素的作用下會(huì)逐步分解成微塑料(尺寸<5mm,MPs)、甚至納米塑料(尺寸<1μm,NPs)[1].研究表明,近10%的塑料最終將以微/納米塑料顆粒(MPs/NPs)的形式進(jìn)入水環(huán)境[2].因粒徑小、難降解、可吸附有毒物質(zhì)等特性,大量的MPs將對(duì)生物體產(chǎn)生毒性作用,造成嚴(yán)重的生態(tài)風(fēng)險(xiǎn).而NPs由于粒徑更小,比表面更大,細(xì)胞親和力更高,更易于穿透細(xì)胞[3],使生物原有機(jī)能遭到損傷,如誘導(dǎo)氧化應(yīng)激導(dǎo)致炎癥[4]、DNA損傷、細(xì)胞凋亡、脂質(zhì)/能量代謝的改變等[5-6],因此MPs/NPs的危害都不容忽視.在對(duì)人體糞便進(jìn)行MPs/NPs檢測(cè)時(shí),結(jié)果表明95.8%的樣本呈陽(yáng)性[7],且在母嬰的胎盤(pán)中同樣發(fā)現(xiàn)了MPs/NPs[8],這說(shuō)明MPs/NPs不僅可經(jīng)消化道排泄還可通過(guò)血液或者臍帶等方式在體內(nèi)傳播甚至停留,從而威脅人類健康[9].人體攝入MPs/NPs的途徑有皮膚接觸[10]、大氣呼吸[11]、食物鏈富集[12]等.其中由于MPs/NPs廣泛分布在水生環(huán)境中,飲用水的使用在日常生活中更是不可避免,因此有必要加深對(duì)供水管網(wǎng)中MPs/NPs危害風(fēng)險(xiǎn)的認(rèn)識(shí).

當(dāng)前凈水處理工藝對(duì)MPs/NPs的去除效果有限以及輸配過(guò)程中外源侵入[13-14],導(dǎo)致在飲用水中檢測(cè)到MPs/NPs的存在.對(duì)全球14個(gè)國(guó)家的159個(gè)飲用水樣本檢測(cè)發(fā)現(xiàn)81%的樣本中含有MPs/NPs.其中,美國(guó)有94%的龍頭水樣品中檢出了MPs/NPs,英國(guó)、德國(guó)和法國(guó)等歐洲國(guó)家的飲用水中MPs/NPs檢出率達(dá)72%[15].在對(duì)中國(guó)長(zhǎng)沙供水流程中MPs/ NPs的豐度研究發(fā)現(xiàn)水源、出廠水和龍頭水中的豐度分別為2173～3998(平均=2753)、338～400(平均= 351.9)和267～404(平均=343.5)顆粒/L[16],且大多數(shù)聚合物由聚乙烯(PE)、聚丙烯(PP)和聚對(duì)苯二甲酸乙二醇酯(PET)組成[17].在塑料供水管道大量使用的今天,我國(guó)塑料管道占比已超過(guò)55%[18],英國(guó)已達(dá)到63%,聚氯乙烯(PVC)在美國(guó)供水市場(chǎng)中占66%[19].因此,輸配水管道對(duì)龍頭水存在MPs/NPs也可能起到重要作用.一旦在供水管網(wǎng)中出現(xiàn)MPs/NPs釋放,且與管道內(nèi)物質(zhì)發(fā)生反應(yīng)產(chǎn)生有害物質(zhì),將會(huì)嚴(yán)重危害龍頭水水質(zhì)安全.但迄今為止,僅已確定MPs/NPs會(huì)存在于水源地、遷移于凈水工藝、殘留于飲用水中,但大部分都只停留在檢測(cè)和表征層面,對(duì)其在供水管網(wǎng)中的分布規(guī)律及與水中其他物質(zhì)的復(fù)合作用機(jī)理研究較少,由此造成的飲用水污染及對(duì)人體健康影響尚不明晰.基于此,本文重點(diǎn)分析了供水管網(wǎng)中MPs/NPs的來(lái)源,討論了MPs/NPs在供水管網(wǎng)中的分布規(guī)律及賦存特征,總結(jié)了MPs/NPs自身及同管網(wǎng)中其他物質(zhì)復(fù)合作用對(duì)管網(wǎng)水質(zhì)安全的影響及危害,并基于MPs/NPs的管網(wǎng)水質(zhì)安全問(wèn)題,提出未來(lái)的研究工作方向及有效控制的措施.

1 供水管網(wǎng)中MPs/NPs的來(lái)源

1.1 前驅(qū)來(lái)源

供水管網(wǎng)中MPs/NPs的前驅(qū)來(lái)源主要來(lái)自兩方面,一方面是由于自來(lái)水廠原水受到MPs/NPs污染,但其自身去除效果有限;另一方面是部分處理工藝導(dǎo)致塑料碎片化,增加MPs/NPs豐度.水體中的MPs/NPs主要來(lái)自陸地運(yùn)輸,漁業(yè)活動(dòng)和船舶排放物等,它們通過(guò)地表徑流、土壤滲透和大氣運(yùn)輸?shù)冗M(jìn)入水環(huán)境.隨著水系統(tǒng)的自然循環(huán),MPs/NPs同樣污染著淡水環(huán)境甚至飲用水水源,其中可作為飲用水水源取水點(diǎn)的太湖[20]西北部表層水中的MPs/NPs含量最高可達(dá)25800顆粒/m3,浮游生物樣品中的MPs含量比美國(guó)大湖地區(qū)高2個(gè)數(shù)量級(jí).而丹江口水庫(kù)盡管主區(qū)的污染水平較低[21],但丹江口大壩和人口稠密地區(qū)的采樣點(diǎn)仍然存在污染,調(diào)查結(jié)果分別為(6087±6981)和(15017±454)顆粒/m3,這導(dǎo)致丹江口凈水廠的原水中含有一定量的MPs/NPs.

自來(lái)水廠對(duì)MPs/NPs的去除作用受自身粒徑的影響[22],大于20μm的MPs/NPs去除率可為99%左右,而小于20μm的MPs/NPs去除率則受處理工藝和原水等不同的影響.Mintenig等[23]對(duì)德國(guó)西北部以地下水為水源的自來(lái)水廠的廢水MPs/NPs檢測(cè)結(jié)果就表明,粒徑大于20μm的豐度僅為0～7顆粒/m3.Pivokonsky等[24]也證實(shí)了這一結(jié)果,WTP1、WTP2和WTP3(捷克共和國(guó)境內(nèi))自來(lái)水廠的粒徑為0.2～100μm的MPs/NPs去除率分別為70%,81%和83%,其中殘留MPs/NPs中粒徑范圍占比分別如下,大于50μm幾乎為0%、5～10μm為30%～50%、1～5μm為25%～60%.Wang等[14]對(duì)以長(zhǎng)江下游為水源的自來(lái)水廠進(jìn)行研究發(fā)現(xiàn)混凝可以很好地去除MPs/NPs,混凝裝置的總體去除效率約為40.5%～54.5%.其中MPs(>10μm)幾乎被全部去除,而MPs(5～10μm)的去除率僅為44.9%～75.0%.這主要是由于較大尺寸MPs/NPs良好的沉降特性以及纖維狀MPs/NPs更易附著于絮狀物上的結(jié)構(gòu)特性.即使Ma等[25]表明添加陰離子聚丙烯酰胺(PAM)可以大大增加鋁鹽去除PE顆粒的百分比,但最終去除率也僅在60%左右.而其他的水處理工藝如砂濾去除MPs的效率僅為29.0%～44.4%.其中纖維、球體和碎片的去除率分別為30.9%～49.3%、23.5%～50.9%和18.9%～27.5%.在臭氧化和活性炭(GAC)過(guò)濾過(guò)程中,纖維、球體和碎片的去除率分別為38%～52.1%、76.8%～86.3%和60.3%～69.1%[14].由此可見(jiàn),臭氧化和GAC過(guò)濾的去除效果明顯高于常規(guī)處理,但仍然存在一定量的殘留物.

而在水處理過(guò)程中有數(shù)據(jù)證明MPs可能會(huì)破碎成NPs,從而增加了水中MPs/NPs的豐度[26].Wang等[14]發(fā)現(xiàn)臭氧化使1～5μmMPs的量增加了2.8%～16.0%,這可能是由于剪切流將MPs分解成更小的顆粒和纖維,這與在廢水處理廠中觀察到的情況一致[27].Wu等[22]研究表明生物處理和臭氧化出水中5～20μm顆粒狀MPs/NPs的豐度增加.生物處理中小于20μm的MPs/NPs的豐度平均增加了149.58%,臭氧化中10～20μm的MPs/NPs從1204.7顆粒/L增加到1700.0顆粒/L;5～10μm的MPs/NPs從3233.8顆粒/L增加到3352.8顆粒/L.這可以解釋為MPs/ NPs首先被氧化削弱,然后由于水流的剪切力和空氣曝氣等外力而分解,從而產(chǎn)生較小的MPs/NPs.因此總結(jié)上文,常規(guī)的自來(lái)水廠并未針對(duì)去除MPs/ NPs設(shè)計(jì)優(yōu)化專門(mén)的處理工藝,如PP、聚苯乙烯(PS)等MPs/NPs就難以被截留處理而隨出廠水進(jìn)入到供水管網(wǎng)中[28].其中小于20μm的MPs/NPs占主導(dǎo)地位可達(dá)到99.94%,且小于5μm的甚至占到80%以上[14].

表1 各飲用水處理廠對(duì)MPs/NPs的去除效果匯總表

1.2 管網(wǎng)中潛在來(lái)源

龍頭水中的MPs/NPs與供水管網(wǎng)中管材有關(guān),且供水管網(wǎng)存在釋放MPs/NPs的可能.Gomiero等[32]對(duì)挪威中型城市地區(qū)飲用水供應(yīng)管網(wǎng)不同收集點(diǎn)進(jìn)行MPs/NPs測(cè)定時(shí)發(fā)現(xiàn),PE在水平分布線上顯示出顯著的增加,可能是由于PE管道被機(jī)械磨損造成的.同樣Weber等[33]也報(bào)道了供水管道磨損是德國(guó)北部一個(gè)由地下水供應(yīng)的飲用水供應(yīng)系統(tǒng)中MPs/ NPs分布的主要驅(qū)動(dòng)因素.最近我國(guó)長(zhǎng)沙[16]、泰國(guó)[34-35]、瑞典[36]等地龍頭水中MPs/NPs的調(diào)研也均證實(shí)了這一推論.

同時(shí),管道環(huán)境下不同因素會(huì)影響MPs/NPs的釋放.在不同管道液體流動(dòng)條件下獲得的剪切應(yīng)力、溫度等作用力有助于MPs產(chǎn)生NPs[26].Ekvall等[37]模擬了自然機(jī)械力下的微塑料分解過(guò)程,僅通過(guò)5min就產(chǎn)生了NPs.同樣高溫下聚合物與水接觸水解切斷化學(xué)官能團(tuán)使其分子量降低,導(dǎo)致聚合物變得更易碎.夏季高溫暴曬條件下的塑料管道則存在著一定的管道老化、碎化風(fēng)險(xiǎn).而持續(xù)的老化則會(huì)使表面越來(lái)越粗糙,形成裂紋和缺陷,最終導(dǎo)致大塊塑料碎裂成更小的顆粒[38].如Hernandez等[39]發(fā)現(xiàn)在泡制溫度(95℃)下浸泡一個(gè)塑料袋泡茶會(huì)釋放約116億MPs和31億NPs.Chamas等[40]估計(jì)高密度聚乙烯(HDPE)在紫外線照射條件下其比表面降解率可達(dá)到9.5μm/yr.

此外,供水管道管壁附著的生物膜以及殘留的消毒劑在一定程度上也會(huì)對(duì)塑料進(jìn)行生物分解,從而產(chǎn)生更小的塑料碎片.Shi等[41]則發(fā)現(xiàn)雨水管道中,塑料可在生物作用下連續(xù)分解成較小的塑料碎片.同時(shí)飲用水消毒中最常見(jiàn)的消毒劑氯對(duì)塑料管道也有一定的影響.Mitroka等[42]將HDPE管道浸沒(méi)在氯濃度分別為50,250和500mg/L氯化水溶液中160d的研究發(fā)現(xiàn)次氯酸(HOCl)會(huì)加速管道的老化,此外,在消毒劑的作用下,還可能導(dǎo)致MPs/NPs的碎片化.此外,水解是通過(guò)聚合物在其表面的斷鏈作用降解聚合物的過(guò)程.一項(xiàng)研究報(bào)告表明[43],當(dāng)羧基在酸性條件下自催化水解時(shí),含有雜原子(PU和PET)的聚合物會(huì)被水解降解.當(dāng)微塑料顆粒長(zhǎng)時(shí)間暴露在低pH值條件和光氧化作用下時(shí), MPs的水解可能會(huì)加速提高NPs的豐度.

因此在供水管網(wǎng)中,泵開(kāi)啟產(chǎn)生的機(jī)械磨損、水流下的剪切力、溫度、微生物以及消毒劑等因素都會(huì)在一定程度上影響塑料管道中MPs/NPs的釋放,從而增加龍頭水中MPs/NPs的豐度.

表2 管網(wǎng)釋放MPs/NPs的影響因素及作用機(jī)理

2 供水管網(wǎng)中MPs/NPs的分布規(guī)律

在長(zhǎng)時(shí)間長(zhǎng)距離的配送過(guò)程中,密度較小的MPs/NPs會(huì)相互聚集成粒徑較大的團(tuán)聚體,隨后在重力等作用下沉積至管壁.同時(shí)管壁上的生物膜可能對(duì)沉積在管壁附近的MPs/NPs有包裹作用. MPs/NPs聚集體的形成在很大程度上受不同的pH值、有機(jī)物以及其他因素的影響.密度相對(duì)較小、懸浮在水中的MPs/NPs在水基質(zhì)中存在有機(jī)物或抗衡離子的情況下會(huì)形成團(tuán)聚體.MPs/NPs間及與其他物質(zhì)的附著力主要受其表面電荷的影響.由于靜電引力,帶正電的MPs/NPs與細(xì)胞緊密結(jié)合,而其他帶負(fù)電的MPs/NPs通過(guò)范德華力、酸堿相互作用以及靜電力與細(xì)胞松散地結(jié)合[48].而較大密度的MPs/NPs會(huì)在水流的作用下沉積至管底管壁,其中老化的聚苯乙烯微塑料(PS-MPs)比原始PS-MPs更分散,沉降更慢[49].Vahidi等[50]在檢測(cè)管道沉積物中微塑料顆粒特性時(shí),發(fā)現(xiàn)相對(duì)密度較大的PVC占比較高.且彎曲的管段處相較于直管段能收集到更多的MPs/NPs沉積物,Wu等[15]的研究證實(shí)了這一論斷.此外,MPs/NPs對(duì)生物膜的附著力主要受其表面電荷的影響,生物膜中重要成分細(xì)胞外聚合物(EPS)始終帶負(fù)電,其可以通過(guò)靜電相互作用與帶正電的污染物聚合體結(jié)合[52].

部分沉積、附著甚至被包裹的MPs/NPs在水流剪切力的作用下,會(huì)隨著水流的沖刷,重新釋放到管網(wǎng)水中.Madejski等[53]對(duì)沿水運(yùn)輸路線收集的一些樣品測(cè)量了MPs/NPs,結(jié)果顯示MPs/NPs主要集中在入口、入口閥等管道內(nèi)流速短時(shí)間內(nèi)變化的地方,因此這一作用在流速突變的情況下尤其明顯.管網(wǎng)水中殘留的消毒劑氯等物質(zhì)同樣對(duì)MPs/NPs表面包裹的生物膜具有消解作用,Zhong等[54]研究表明隨著投氯量升高, PE和不銹鋼管內(nèi)生物量均逐漸降低.在此作用下生物膜-MPs/NPs聚積物隨其密度減少又重新懸浮到水中,直接進(jìn)入龍頭水中.總之,供水管網(wǎng)中的MPs/NPs經(jīng)過(guò)一系列的反應(yīng)作用,一部分同管網(wǎng)中的有機(jī)物、微生物等物質(zhì)作用從而影響管網(wǎng)水質(zhì)的安全,一部分則隨著水流直接進(jìn)入龍頭水被用戶所使用.

3 微/納米塑料對(duì)水質(zhì)安全的危害

3.1 微/納米塑料直接影響水質(zhì)安全

MPs/NPs是一種新的微生物生態(tài)位,又可作為一些潛在病原體的載體,從而影響管網(wǎng)中微生物的數(shù)量.Zhang等[55]對(duì)中國(guó)廣東11個(gè)地點(diǎn)的沿海沉積物中細(xì)菌群落和MPs/NPs豐度進(jìn)行檢測(cè)分析,發(fā)現(xiàn)MPs/NPs豐度與假單胞菌、芽孢桿菌等呈正相關(guān),且<0.5mm的尺寸與芽孢桿菌的正相關(guān)性比其他尺寸的更大.MPs/NPs具有巨大的比表面積,這增加了它們與微生物細(xì)胞的潛在接觸面積.在有氧或厭氧條件下,自由基和分子雙氧的催化反應(yīng)可以在它們的表面發(fā)生產(chǎn)生活性氧自由基(ROS)從而誘導(dǎo)細(xì)胞毒性[56].Liu等[57]研究結(jié)果表明,NPs能夠誘導(dǎo)Coelicolor M 145菌株細(xì)胞產(chǎn)生過(guò)量的ROS.低聚苯乙烯濃度會(huì)增加ROS的產(chǎn)生,而高ROS水平會(huì)誘導(dǎo)抗氧化相關(guān)基因的表達(dá),抗氧化相關(guān)基因表達(dá)與抗氧化酶活性相關(guān)[58].

MPs/NPs還可改變細(xì)胞的生物學(xué)特質(zhì),易導(dǎo)致微生物產(chǎn)生應(yīng)激反應(yīng).其中NPs可以和蛋白質(zhì)結(jié)合成“蛋白質(zhì)電暈”的形式使其更易穿透細(xì)胞膜并與細(xì)胞結(jié)構(gòu)相互作用[59].隨后通過(guò)吞噬作用、胞飲作用、巨胞飲作用或被動(dòng)轉(zhuǎn)運(yùn)和吸收進(jìn)入細(xì)胞膜和各種生物結(jié)構(gòu),改變均質(zhì)膜的性質(zhì)(如機(jī)械軟化等)或誘導(dǎo)溶酶體破壞進(jìn)而誘導(dǎo)細(xì)胞死亡[60-61].NPs進(jìn)入生物體后還可通過(guò)阻斷或促進(jìn)氧化應(yīng)激和免疫系統(tǒng)有關(guān)的基因表達(dá)來(lái)誘導(dǎo)毒性.Cui等[62]研究結(jié)果表明,高濃度的PS-NPs由于破壞了淡水水蚤的抗氧化系統(tǒng),從而降低了淡水水蚤的抗氧化酶基因表達(dá)能力. Zhang等[63]研究發(fā)現(xiàn)水蚤暴露于NPs(75nm)21d將會(huì)抑制谷胱甘肽轉(zhuǎn)移酶(GST)的表達(dá),導(dǎo)致氧化應(yīng)激從而降低聚合物對(duì)生物體本身的毒性.

一項(xiàng)針對(duì)不同管材上(球墨鐵管、聚乙烯管、不銹鋼)微生物的研究表明,盡管PE管壁生物膜上的微生物群落數(shù)較低,但單位群落數(shù)的產(chǎn)嗅能力是最強(qiáng)的[64-65].而國(guó)內(nèi)外在靠近用戶端的“最后一公里”普遍采用PE、PVC管作為供水管道,由此造成的異嗅異味問(wèn)題占到總飲用水嗅味事件的73%[66-67],且通過(guò)微生物甲基化作用產(chǎn)生致嗅物是龍頭水異嗅異味問(wèn)題的重要途徑之一[68].供水管網(wǎng)中的MPs/ NPs或許能夠在一定程度上干擾管網(wǎng)生物膜中的微生物細(xì)胞,影響抗氧化酶活性,誘導(dǎo)抗氧化相關(guān)基因的表達(dá),激發(fā)微生物產(chǎn)生更多的ROS[69],從而促進(jìn)氧化應(yīng)激.在此研究基礎(chǔ)上,本文得到以下推論: MPs/NPs豐度與假單胞菌等產(chǎn)嗅優(yōu)勢(shì)菌呈正相關(guān),產(chǎn)嗅優(yōu)勢(shì)菌在上述作用下氧化應(yīng)激將產(chǎn)生更多的嗅味物質(zhì)影響龍頭水的口味,從而影響供水的水質(zhì)安全.

3.2 MPs/NPs與其他污染物的復(fù)合危害

MPs/NPs具有粒徑小、難降解、可吸附有毒物質(zhì)等特性,而NPs由于高細(xì)胞親和力和高表面積,更易于與其他污染物一起進(jìn)入生物體,然后在細(xì)胞和分子水平上共同誘導(dǎo)生物毒性.影響其毒性的主要因素有濃度、粒徑、暴露時(shí)間和所攜帶污染物等[70].此外,生物膜可以改變MPs/NPs的表面性質(zhì),使表面減少疏水性,從而影響MPs/NPs本身的吸附毒性[71].研究表明MPs/NPs可吸附Cu、Cd、Zn、Ni和其他重金屬[72].Sun等[73]發(fā)現(xiàn)PS-MPs在共暴露的最初24h內(nèi)減輕Ag+對(duì)大腸桿菌的細(xì)胞毒性.一方面,部分Ag+吸附在粒徑為0.1和1.0μm的PS-MPs表面降低了Ag+的生物利用度,從而較少的游離Ag+可以與細(xì)胞膜相互作用并破壞細(xì)胞膜.另一方面, PS-MPs可以直接粘附在細(xì)胞表面,保護(hù)大腸桿菌細(xì)胞膜不與Ag+相互作用.同時(shí)有研究表明吸附的抗生素會(huì)改變共存的微生物的群落結(jié)構(gòu).如MPs/NPs吸附抗生素后,改變了跳蟲(chóng)腸道微生物組的組成和結(jié)構(gòu),并降低了腸道細(xì)菌的多樣性[74].同時(shí)有研究表明潛在病原體Legionella菌的存在與塑料材質(zhì)密切相關(guān),其在塑料管壁生物膜上以及管網(wǎng)水中的數(shù)量同鑄鐵管相比要高出好幾倍.其中在HDPE中增加了5倍,而在未增塑聚氯乙烯(PVC-U)的情況下則增加了約30倍[75].

圖1 供水管網(wǎng)內(nèi)MPs/NPs的分布規(guī)律及對(duì)水質(zhì)安全的危害

吸附作用在老化的MPs/NPs表面更為明顯.研究表明老化的MPs/NPs因其表面結(jié)構(gòu)的變化,對(duì)有害物質(zhì)的吸附親和力更高[76].Kelkar等[77]在實(shí)驗(yàn)室條件下將原始塑料暴露于飲用水消毒劑量下(CT值50～150mg min/L),HDPE和PS不能完全抵抗氯化引起的氧化侵蝕,且HDPE新形成了碳-氯(C-Cl)鍵.碳-氯鍵的形成增加了毒性,從而導(dǎo)致聚合物更加疏水,更利于有害物質(zhì)的吸附和積累.Wang等[78]查閱總結(jié)了MPs/NPs對(duì)抗生素類、農(nóng)藥類(滴滴涕)、內(nèi)分泌干擾物(雙酚A)、多氯聯(lián)苯、多環(huán)芳烴等有機(jī)污染物的吸附作用.主要機(jī)制包括疏水相互作用、表面吸附和氫鍵等.而多氯聯(lián)苯類藥物具有致癌性、致突變性和致畸性,滴滴涕則會(huì)導(dǎo)致不良的神經(jīng)系統(tǒng)影響和免疫缺陷[79].

綜上所述,MPs/NPs對(duì)水質(zhì)安全的危害主要有以下兩方面:(1)MPs/NPs將影響到生物膜上微生物菌群,例如產(chǎn)嗅菌的數(shù)量和種類,從而誘導(dǎo)產(chǎn)生種類更多、濃度更大的致嗅物,還更可能會(huì)促進(jìn)潛在致病菌類群的積累[75],從而嚴(yán)重影響龍頭水的品質(zhì)及適飲性;(2)管網(wǎng)水中的有機(jī)污染物、重金屬及殘留的消毒劑等都將與MPs/NPs產(chǎn)生復(fù)合效應(yīng),主要表現(xiàn)在強(qiáng)大的比表面積將加速對(duì)污染物、病原體的吸附.而最初在微塑料表面吸附的污染物,當(dāng)MPs/NPs被攝入并停留在人體內(nèi),由于pH值較低、溫度較高、存在消化液體等環(huán)境,生物體中吸附污染物的解吸附率比海水中快得多[80].因此MPs/NPs對(duì)水質(zhì)安全及人體的危害可能遠(yuǎn)大于現(xiàn)有研究的結(jié)論.

4 結(jié)論與展望

4.1 結(jié)論

供水管網(wǎng)中MPs/NPs的來(lái)源主要有兩大類:第一類是由于未針對(duì)MPs/NPs去除進(jìn)行專門(mén)的工藝優(yōu)化,自來(lái)水廠現(xiàn)有常規(guī)或深度處理對(duì)MPs/NPs的去除率僅為70%～80%左右,且部分水處理工藝還會(huì)使塑料碎片化,導(dǎo)致出廠水仍殘留大量的MPs/NPs;第二類是供水管網(wǎng)管道本身存在釋放可能,溫度、泵開(kāi)啟產(chǎn)生的機(jī)械磨損、水流作用下的剪切力、微生物以及消毒劑等因素都會(huì)在一定程度上影響塑料管道中MPs/NPs的釋放.

MPs/NPs因自身密度、材質(zhì)、電荷等原因,在管道內(nèi)可能會(huì)發(fā)生沉積、聚集等現(xiàn)象.同時(shí)會(huì)同管網(wǎng)中有機(jī)物質(zhì)、微生物、消毒劑等物質(zhì)發(fā)生作用,出現(xiàn)MPs/NPs破裂碎化、吸附有害物質(zhì)復(fù)合作用以及改變微生物群落,產(chǎn)生應(yīng)激反應(yīng)等現(xiàn)象,從而影響供水管網(wǎng)水質(zhì)安全問(wèn)題.

4.2 展望

①模擬真實(shí)環(huán)境下的飲用水管道系統(tǒng),考察不同環(huán)境下,如流速、水溫、pH值、不同塑料材質(zhì)的管材、管齡等,塑料管道中原位釋放的MPs/NPs的賦存特征(含量及粒徑分布);②明確MPs/NPs對(duì)真實(shí)環(huán)境下供水管道系統(tǒng)水質(zhì)安全問(wèn)題的危害,從而采取措施保障龍頭水的安全及適飲性問(wèn)題;③加強(qiáng)如何提高或優(yōu)化現(xiàn)有工藝針對(duì)MPs/NPs的去除效率的研究,從源頭上對(duì)MPs/NPs進(jìn)行控制;④研制新材料作為供水管材或進(jìn)一步提高塑料管材的穩(wěn)定性,從而防止供水管網(wǎng)MPs/NPs釋放的可能.

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The abundance and contamination characteristics of micro/nano-plastics in drinking water distribution systems.

ZHENG Ying-ying, ZHANG Ke-jia*, CEN Cheng, MAO Ru-yin, ZHANG Tu-qiao

(College of Civil Engineering and Architecture, Zhejiang University, Hangzhou 310058, China)., 2022,42(4)：1610～1617

Micro/nano-plastics are becoming a research hotspot due to the characteristics of wide distribution, small particle size, difficulty in degradation, and can adsorb toxic substances, etc. However, the information on the status of environmental contamination and removal is rare. In this review, the research status of micro/nano-plastics in drinking water distribution systems (DWDS) was discussed, which was from the perspectives of the sources, migration characteristics and safety hazards of water quality. The results showed that residual micro/nano-plastics can be carried into DWDS by the effluent of water treatment plants. Besides, pipe material, flow velocity, mechanical wear and residual chlorine may affect the release of micro/nano-plastics. The spatial and temporal variation of micro/nano-plastics was affected by the inherent characteristics (e.g., density and charge) in DWDS. In the process of drinking water transmission and distribution, the water quality and safety of the DWDS has been considered that is relevant to the reaction between micro/nano-plastics with organics, microorganisms and other substances. Therefore, the review makes a prospect for the future research direction, in order to provide theory basis for further understanding of the impact of micro/nano-plastics on the drinking water safety, in addition to the control of micro/nano-plastics.

micro/nanoplastics；drinking water distribution system；abundance characteristics；water quality safety

X131.2

A

1000-6923(2022)04-1610-08

鄭瑩瑩(1998-),女,浙江溫州人,浙江大學(xué)碩士研究生,主要研究方向?yàn)楣┧芫W(wǎng)水質(zhì)安全保障.發(fā)表文章4篇.

2021-09-30

國(guó)家自然科學(xué)基金資助項(xiàng)目(51978602,51778561)

*責(zé)任作者, 副教授, zhangkj@zju.edu.cn