海洋多孔介質(zhì)中微塑料和富勒烯的共遷移

張 文,董志強(qiáng),黃 睿,景昕宇,呂祥偉,邱宇平*

海洋多孔介質(zhì)中微塑料和富勒烯的共遷移

張 文1,2,董志強(qiáng)1,2,黃 睿1,景昕宇1,呂祥偉1,邱宇平1,2*

(1.同濟(jì)大學(xué)環(huán)境科學(xué)與工程學(xué)院,污染控制與資源化研究國(guó)家重點(diǎn)實(shí)驗(yàn)室,上海 200092；2.上海污染控制與生態(tài)安全研究院,上海 200092)

采用海水和天然海砂,模擬構(gòu)建了一維柱狀的海洋多孔介質(zhì)體系,研究了粒徑1μm的聚苯乙烯微塑料(PS)與富勒烯(C60)在海水飽和砂柱中的共遷移行為.結(jié)果發(fā)現(xiàn):單體系下,15mg/L PS的穿透率(eff)和最大穿透濃度(MEC)可分別達(dá)到36.8%和0.42;而15mg/L的C60團(tuán)聚明顯,其eff和MEC值僅分別為16.8%和0.22.當(dāng)15mg/L PS與15mg/L C601:1共存時(shí),PS能與部分C60形成穩(wěn)定共團(tuán)聚體,促進(jìn)C60遷移;但反之C60卻抑制了PS遷移.如果將雙體系下PS的濃度由15mg/L增至45mg/L,PS對(duì)C60的遷移促進(jìn)則轉(zhuǎn)變?yōu)檫w移抑制,這主要源于PS-C60共團(tuán)聚體體積的增大和數(shù)量的增加.

微塑料；富勒烯；海洋體系；共遷移

近岸海洋環(huán)境是陸源顆粒態(tài)污染物蓄積的重要場(chǎng)所[1].微塑料(粒徑小于5mm)和人工納米工程材料(如富勒烯)是典型的顆粒污染物[2-3],前者源于顆粒塑料產(chǎn)品的人為排放和大體積塑料垃圾的環(huán)境分解;而后者在催化和生物醫(yī)藥領(lǐng)域有著廣泛的應(yīng)用,會(huì)隨廢水的排放進(jìn)入海洋環(huán)境[4-5].研究指出:微塑料在海洋多孔介質(zhì)中的團(tuán)聚狀態(tài)和遷移能力取決于塑料顆粒的大小[6];對(duì)比而言,富勒烯在高鹽度水體環(huán)境中(>120mmol/L NaCl)基本以顆粒團(tuán)聚和低遷移特征為主[7-8].微塑料與富勒烯兩類(lèi)膠體共存于海洋環(huán)境中是否會(huì)發(fā)生相互作用,進(jìn)而影響各自的團(tuán)聚和遷移是本文關(guān)注的焦點(diǎn).

本研究將探索聚苯乙烯(PS)微塑料小球和富勒烯(C60)在海水飽和一維砂柱中的共遷移行為,考察海水中二元膠體的團(tuán)聚特征,微觀觀測(cè)海砂表面膠體顆粒的保留狀況并輔以DLVO相互作用能計(jì)算,揭示PS和C60在海洋多孔介質(zhì)中的共遷移機(jī)制.

1 材料與方法

1.1 材料

天然海砂收集于福建平潭海峽,干燥后過(guò)濾除去雜質(zhì)(如海洋生物殘留物等),進(jìn)一步篩分得到75~1000μm的砂粒,最后用去離子水反復(fù)沖洗,于105°C烘干后使用.激光粒度儀(LA-960, HORIBA, Ltd., Japan)測(cè)得海砂平均粒徑為(0.44±0.05)mm.孔隙率為44.6%.

熒光聚苯乙烯微球(PS)購(gòu)自上海輝質(zhì)生物科技有限公司,粒徑約1μm,表面含有少量的羧基(~100mmol/g).富勒烯(C60)購(gòu)自上海阿拉丁試劑公司,純度99.9%.采用甲苯萃取法配制C60膠體溶液[9-10].使用總有機(jī)碳分析儀(日本島津股份有限公司)測(cè)得C60懸浮液中的總有機(jī)碳濃度約為33mg/L,避光4 °C保存?zhèn)溆?人工海水根據(jù)Kester法配置[11].

1.2 PS和C60在海水中的團(tuán)聚與沉降

通過(guò)馬爾文激光粒度儀(Marlvern Instruments, Ltd., Worcestershire, UK)測(cè)定35‰ 鹽度海水溶液中,PS和C60的水合粒徑(DLS)和電位.其中DLS值每隔2min 記錄一次數(shù)據(jù),連續(xù)記錄40min,得到團(tuán)聚動(dòng)力學(xué)曲線.將PS和C60膠體溶液置于石英比色皿中,采用紫外分光光度計(jì)(UV-2550, Shimadzu Scientific Instruments, Columbia, MD)測(cè)定上清液40min內(nèi)吸光度的變化,得到沉降曲線.

1.3 PS和C60飽和填充柱遷移實(shí)驗(yàn)

采用一維柱動(dòng)態(tài)遷移實(shí)驗(yàn)探究PS和C60在海洋多孔介質(zhì)中的遷移行為.有機(jī)玻璃柱長(zhǎng)10cm,內(nèi)徑為1cm,柱中裝填約(11.50±0.5)g天然海砂,砂柱平均長(zhǎng)度約為(9.00±0.20)cm.利用膠管連接進(jìn)水口和蠕動(dòng)泵,以此控制溶液注入速度.柱子的頂端和底端存在50μm不銹鋼篩網(wǎng),以防止天然海砂流失.

實(shí)驗(yàn)前,以(2.20±0.11)mL/min的流速,自下而上持續(xù)通入100mL海水使填充柱達(dá)到飽和.同時(shí),用橡膠球敲打填充柱以排除砂子間的氣泡,并起到穩(wěn)定柱子孔隙率的作用.將配置好的PS和C60溶液先用超聲波清洗機(jī)(SK3200HP, KUDOS, 中國(guó))超聲10min,然后用于柱遷移實(shí)驗(yàn).膠體溶液自下而上從500mL玻璃燒杯中連續(xù)泵入填充柱40min后,改用背景海水溶液注入柱內(nèi)10min.每隔2min(約1.47孔隙體積數(shù),PV)收集出流液,以測(cè)定遷移過(guò)程中PS和C60的濃度.在單體系中,PS和C60的濃度,采用紫外分光光度計(jì)(UV-2550, Shimadzu Scientific Instruments, Columbia, MD)測(cè)定,波長(zhǎng)為350nm(標(biāo)準(zhǔn)曲線見(jiàn)圖1a).在雙體系中,采用熒光分光光度計(jì)(FluoroMax-4, HORIBA, America)直接測(cè)定PS的濃度,其中激發(fā)/發(fā)射波長(zhǎng)分別為488nm/525nm,激發(fā)和發(fā)射狹縫寬度均為5nm,C60的存在不影響PS熒光強(qiáng)度(標(biāo)準(zhǔn)曲線見(jiàn)圖1b).C60的濃度可通過(guò)差減法計(jì)算獲得:根據(jù)PS的濃度反算出PS在350nm紫外吸收率,通過(guò)混合體系總吸光度減去PS吸光度,進(jìn)一步得到C60在350nm的吸光度,最后根據(jù)紫外吸光標(biāo)準(zhǔn)曲線得到C60的濃度[8,12].每組實(shí)驗(yàn)進(jìn)行3次以上平行實(shí)驗(yàn),實(shí)驗(yàn)數(shù)據(jù)取平均值.

(a)紫外分光光度計(jì), (b)熒光分光光度計(jì)

此外,為了直觀地描述PS和C60在砂柱中的保留情況,通過(guò)掃描電子顯微鏡(S4800, Hitachi, Japan)觀察填充柱入口端砂樣表面顆粒附著狀態(tài).

1.4 相互作用能的計(jì)算

本研究采用DLVO理論計(jì)算PS-PS,C60-C60和 PS-C60的相互作用能(TOT),即范德華力(VDW)和靜電力(EDL)之和:

范德華作用能[13-14]:

式中:1和2是膠體粒子的半徑;為膠體離子間相互作用距離;是有效Hamaker常數(shù)(PS為4.04× 10?21J,C60為6.70×10?21J,PS與C60為5.20×6.70× 10?21J)[15-16].

雙電子層相互作用[17]:

式中:B為Boltzmann常數(shù)(1.38×10?23J/K);為開(kāi)爾文溫度(298K);為不同的離子價(jià)態(tài)(本文中=1,因?yàn)楹Ｋ拇蟛糠殖煞譃镹aCl);e是元電荷(1.6×10?19C);為德拜長(zhǎng)度(500mmol/L NaCl溶液中值約2.3× 109m?1);為表面電勢(shì)能,無(wú)量綱參數(shù):

式中:是膠體表面電位[18].

2 結(jié)果與討論

2.1 單體系中PS和C60的團(tuán)聚

單獨(dú)的PS和C60在海水體系中的團(tuán)聚曲線和沉降曲線如圖2所示.PS(15和45mg/L)水合粒徑(DLS)保持在1μm左右(圖2a),接近初始尺寸(~1μm),說(shuō)明PS在海水體系中未發(fā)生團(tuán)聚.沉降曲線表明,40min時(shí)上清液的/0(相對(duì)濃度)高達(dá)0.94,顯示PS具有較好的穩(wěn)定性(圖2b).相比而言,C60(15mg/L) 在海水中具有較低的穩(wěn)定性:在40min內(nèi),其水合粒徑從(0.274±0.067)μm迅速增加到(0.727±0.090)μm (圖2a);相對(duì)濃度(/0)在40min內(nèi)下降為0.79 (圖2b).透射電鏡結(jié)果進(jìn)一步表明:PS和C60在海水中分別呈出單分散(圖3a)和團(tuán)聚狀態(tài)(圖3b).

△ 15mg/L PS ▲ PS/C60=1:1 □ 45mg/L PS ■ PS/C60=3:1 ○ 15mg/L C60

圖3 單體系和雙體系下海水中PS和C60的透射電鏡圖

表1 不同實(shí)驗(yàn)條件下PS和 C60的Zeta電位與遷移參數(shù)

注:MEC為最大穿透濃度;eff為穿透率.

圖4 單體系下PS-PS (15mg/L)和C60-C60及雙體系下PS- C60 (1:1)之間的DLVO能量相互作用分布

膠體表面電荷是決定膠體穩(wěn)定性的主要因素[19-20].表面電荷越多,膠體穩(wěn)定性越高[11].海水體系中PS和C60的Zeta 電位存在較大的差異:其中,PS具有較高的負(fù)電位值(-27.5±1.5)mV(15mg/L)和(-29.0±1.3)mV (45mg/L);而15mg/L C60的Zeta電位則僅為(-11.3±0.6)mV (表1). DLVO能量計(jì)算(圖4)進(jìn)一步顯示:海水中PS(15mg/L)顆粒間存在較高能壘,約284.3BT,這說(shuō)明顆粒間靜電斥力占主導(dǎo)作用,能維持PS相對(duì)穩(wěn)定;而C60顆粒間不存在能壘,表明C60顆粒間范德華引力作用高于靜電排斥作用,導(dǎo)致C60易發(fā)生團(tuán)聚[17-21].

2.2 PS和C60的共團(tuán)聚

PS和C60混合體系(質(zhì)量比1:1和3:1)中,膠體的水合粒徑逐漸增加,并最終超過(guò)單體系下PS和C60任一膠體的粒徑(圖2a),這說(shuō)明雙體系中生成了更大團(tuán)聚體.40min時(shí)雙體系的/0為0.82,略高于單體系15mg/L C60的相對(duì)懸浮濃度(0.79),明顯低于15mg/L PS的/0(0.91~0.94) (圖2b),這暗示C60有可能主導(dǎo)雙體系的穩(wěn)定性.

值得注意的是,PS/C60=3:1雙體系在40min內(nèi)的團(tuán)聚粒徑(1.544μm)明顯高于其1:1混合時(shí)的團(tuán)聚粒徑(1.203μm)(圖2a).這表明增加PS的質(zhì)量可抑制體系的穩(wěn)定性.而之前的研究表明,0.2μm PS和C60雙體系的穩(wěn)定性卻隨著PS質(zhì)量的增加而提升[13].造成現(xiàn)象反差的原因在于:相同質(zhì)量濃度下,1μm PS的顆粒數(shù)量遠(yuǎn)小于0.2μm PS的數(shù)量(約為后者的1/125),因此PS更容易被大量C60所覆蓋,從而形成大顆粒團(tuán)聚體.當(dāng)1μm PS濃度增加時(shí),這些被C60覆蓋的PS,碰撞幾率得到提升,團(tuán)聚體粒徑會(huì)進(jìn)一步增加.透射電鏡證實(shí),與PS/C60=1:1的初級(jí)團(tuán)聚(圖3c)相比,當(dāng)PS/C60=3:1時(shí),C60能通過(guò)架橋作用將PS緊密地連在一起,從而形成更大的團(tuán)聚體.

PS-C60共團(tuán)聚體的形成可能與膠體溶液的Zeta電位有關(guān),雙體系下膠體的電位(-16.7±1.6)mV要高于單體系PS的電位(-27.5±1.5)mV(表1).根據(jù)DLVO理論計(jì)算可知,PS和C60在海水中的作用能壘不存在(圖4),說(shuō)明PS和C60之間的范德華引力占主導(dǎo),從而形成了共團(tuán)聚體[20,22].此外,PS和C60表面所帶的苯環(huán),可通過(guò)形成π-π作用促進(jìn)共團(tuán)聚體的形成[16,22-23].

2.3 單組份PS和 C60的遷移

15mg/L PS最大穿透濃度(MEC,相對(duì)濃度)為 0.42,溶質(zhì)流出率(eff)為36.8%(表1).PS的穿透曲線呈現(xiàn)出緩慢上升的趨勢(shì)(圖5a),表明海水中PS在多孔介質(zhì)中的遷移行為主要受到阻隔作用的影響[24].原因在于:沙粒表面可供PS附著的位點(diǎn)有限,隨著PS的不斷注入,位點(diǎn)數(shù)逐漸減少,進(jìn)而導(dǎo)致隨后的PS不能有效附著,因此顆粒遷移能力逐漸增加.掃描電鏡圖顯示,附著在沙粒表面的PS主要以單獨(dú)個(gè)體存在(圖6a).這從側(cè)面證實(shí)PS占據(jù)了附著位點(diǎn)后,會(huì)排斥其他PS顆粒[8,24].45mg/L PS的穿透曲線與15mg/L PS 基本重合(圖5a),說(shuō)明本研究條件下濃度變化并不影響PS的遷移行為.

圖5b顯示,C60的穿透曲線呈現(xiàn)下降趨勢(shì),即隨著C60的不斷注入,其在多孔介質(zhì)的保留能力不斷增大,遷移能力逐漸減弱,這種遷移行為被稱(chēng)為成熟現(xiàn)象[6,25].這一方面是因?yàn)镃60的粒徑隨著時(shí)間不斷增加,導(dǎo)致后面注入的C60粒徑較大,更易于被多孔介質(zhì)截留[26].另一方面是由于C60顆粒間吸引作用較強(qiáng),已附著在介質(zhì)表面的C60,為隨后注入的C60提供了更多的附著位點(diǎn),使得多孔介質(zhì)表面的納米顆粒沉積層由單層轉(zhuǎn)變?yōu)槎鄬覽27].觀察圖6b可知,沙粒表面確實(shí)存在大量的C60團(tuán)聚體或多層附著的C60.

2.4 PS和C60的共遷移

與單體系的15mg/L PS相比,雙體系(1:1)下PS的穿透能力明顯降低,其MEC和eff分別降為0.18和14.8%(表1).這表明多孔介質(zhì)中共存C60膠體顯著抑制了PS的遷移.通常而言,共存膠體可以通過(guò)降低原膠體表面電荷和形成共團(tuán)聚體等作用,來(lái)抑制原膠體遷移[6,28].表1顯示,雙體系膠體溶液的表面負(fù)電荷含量(-16.7±1.6)mV,低于單體系 PS的表面負(fù)電荷含量(-27.5±1.5)mV.根據(jù)DLVO理論,納米顆粒表面負(fù)電的減少會(huì)導(dǎo)致其與表面負(fù)電多孔介質(zhì)間的靜電排斥作用減弱,從而增加顆粒在介質(zhì)表面的附著,降低顆粒遷移能力[29].透射電鏡結(jié)果揭示,大量的C60附著在PS的表面,增加了膠體的粒徑(圖3c).這種粒徑的增加,將顯著提升遷移過(guò)程中的截留作用,從而導(dǎo)致其遷移能力的下降[30].此外,C60的附著還會(huì)使得PS的表面更加粗糙,這也會(huì)增強(qiáng)PS在沙粒表面的保留能力[31].

圖6 PS和C60在柱入口端沙粒表面的掃描電鏡圖

C60共存使得PS穿透曲線呈現(xiàn)下降趨勢(shì)(圖5a).這說(shuō)明C60影響下PS的遷移從阻隔作用主導(dǎo)變?yōu)槌墒熳饔弥鲗?dǎo).圖6c顯示,大顆粒的團(tuán)聚體(PS-C60-PS)出現(xiàn)在沙粒的表面,證實(shí)了PS間阻隔作用的消失,成熟現(xiàn)象的發(fā)生.主要原因可能是:在多孔介質(zhì)的孔道中,由于空間狹小,顆粒間的碰撞幾率更大[24,32].

同樣,PS也影響著C60的遷移.1:1雙體系下,C60的穿透曲線要高于單體系(圖5b);同時(shí)C60的MEC從0.22升至0.27,eff從16.8%提升為23.2%(表1),這說(shuō)明PS的存在提升了C60的遷移能力.這可能與雙體系下膠體的表面負(fù)電荷(-16.7±1.6)mV高于單體系C60表面負(fù)電荷(-11.3±0.6)mV有關(guān).另一方面,在海水及其飽和沙柱中,PS的表面附著大量C60,使得PS充當(dāng)載體攜帶C60遷移(圖6c).

之前的研究表明,0.2μm PS 與C601:1 混合,C60的eff高達(dá)29.3%[8],而本研究中1μm PS存在時(shí),C60的eff僅為23.2%(表1).這是因?yàn)橄嗤|(zhì)量濃度下,1μm PS的顆粒數(shù)量較少,不能夠提供足夠多的載體攜帶C60,因此C60團(tuán)聚體依然存在于共遷移過(guò)程中(圖6c).

如將共存1μm PS的濃度提升至45mg/L (即PS/C60=3:1),PS與C60各自的遷移能力均會(huì)低于二者單體系下的遷移能力(圖5).其中C60最大穿透濃度從0.22降低至0.20,PS 的最大穿透濃度從0.44降為0.13(表1).質(zhì)量濃度比3:1混合的PS/C60(-17.3± 1.1)mV比1:1混合的PS/C60(-16.7±1.6)mV具有更負(fù)的表面電荷(表1),這本應(yīng)導(dǎo)致前者具有更高的遷移能力,但結(jié)果并非如此[29].考察團(tuán)聚體粒徑變化發(fā)現(xiàn), 3:1的PS/C60能形成更大且復(fù)雜的團(tuán)聚體(PS- C60-PS)(圖3d),進(jìn)而導(dǎo)致PS/C60在多孔介質(zhì)中的遷移能力顯著下降[30].SEM(圖6d)也輔證,遷移后沙粒表面存在著大量的PS-C60-PS復(fù)雜團(tuán)聚體.因此,提升1μm PS的質(zhì)量濃度并不能起到進(jìn)一步促進(jìn)C60遷移的效果,反而會(huì)抑制其遷移.

3 結(jié)論

3.1 單體系下,由于靜電斥力和勢(shì)壘的存在,1μm的PS在海水體系中穩(wěn)定分散,在海砂中遷移能力較強(qiáng);而C60因其表面電荷少,在海水中團(tuán)聚與沉降明顯,遷移能力較弱.

3.2 雙體系下,PS和C60容易形成共團(tuán)聚.當(dāng)PS/ C60=1:1時(shí),部分C60附著在PS表面,形成PS/C60初級(jí)團(tuán)聚體,抑制PS的遷移;同時(shí)PS可作為載體,在一定程度提升C60的遷移能力.

3.3 當(dāng)PS/C60=3:1時(shí),初級(jí)團(tuán)聚體數(shù)量增加,并能通過(guò)C60的架橋作用互相連接,形成大塊的次級(jí)團(tuán)聚體,使得PS和C60發(fā)生相互抑制遷移現(xiàn)象.

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Cotransport of microplastics and fullerene in marine porous media.

ZHANG Wen1,2, DONG Zhi-qiang1,2, HUANG Rui1, JING Xin-yu1, Lü Xiang-wei1, QIU Yu-ping1,2*

(1.State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China；2.Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China)., 2019,39(12)：5063~5068

A simulated one-dimensional column system was established with seawater and natural sea sand to investigate the cotransport of 1μm polystyrene microplastics (PS) and fullerene (C60) in seawater-saturated marine media. The results showed that, in single suspension, the mass percentage recovered from the effluent (eff) and maximum effluent concentration (MEC) of the well-dispersed PS (15mg/L) were 36.8%, and 0.42, respectively, while the aggregation (agglomeration) of C60at 15mg/L was obviously presented with 16.8% ofeffand 0.22 of MEC, respectively. When 15mg/L PS and 15mg/L C60were coexisted, PS might form stable co-aggregates with part of C60thereby promote the transport of C60. Conversely, C60inhibited the transport of PS. If the concentration of PS in binary suspension increased from 15mg/L to 45mg/L, the enhancing effect of PS on transport of C60was eventually transformed into the inhibition of C60transport, which was mainly due to the increase in the volume and number of PS-C60co-aggregates.

microplastics；fullerene (C60)；marine environment；cotransport

X145

A

1000-6923(2019)12-5063-06

張 文(1994-),女,江蘇蘇州人,同濟(jì)大學(xué)碩士研究生,主要研究方向?yàn)槲⑺芰檄h(huán)境過(guò)程與歸趨.發(fā)表論文4篇.

2019-06-03

中央高?；究蒲袠I(yè)務(wù)費(fèi)(22120180244)

* 責(zé)任作者, 教授, ypqiu@#edu.cn