背景離子類(lèi)型和濃度對(duì)聚苯乙烯微塑料/鉛在飽和石英砂中共運(yùn)移的影響

常博焜,陳怡汀,曹 鋼,胡 良,呂家瓏,杜 偉**,胡斐南,2*

背景離子類(lèi)型和濃度對(duì)聚苯乙烯微塑料/鉛在飽和石英砂中共運(yùn)移的影響

常博焜1,陳怡汀1,曹 鋼1,胡 良1,呂家瓏1,杜 偉1**,胡斐南1,2*

(1.西北農(nóng)林科技大學(xué)資源與環(huán)境學(xué)院,陜西 楊凌 712100；2.中國(guó)科學(xué)院水利部水土保持研究所,陜西 楊凌 712100)

為闡明水化學(xué)條件對(duì)微塑料和重金屬運(yùn)移的影響,初步明確兩種環(huán)境污染物共運(yùn)移過(guò)程中的耦合效應(yīng)及其對(duì)環(huán)境條件的響應(yīng)機(jī)制,研究了背景電解質(zhì)離子不同價(jià)態(tài)和濃度組成條件下1μm 聚苯乙烯微塑料(PS-MPs)和Pb2+在飽和一維砂柱中的單獨(dú)及共遷移行為.結(jié)果表明:背景離子濃度的增加或價(jià)態(tài)的升高均會(huì)抑制PS-MPs的單獨(dú)運(yùn)移能力,當(dāng)Na+濃度從1mmol/L增加到100mmol/L,PS-MPs和石英砂之間的排斥勢(shì)壘下降了1348kT;當(dāng)Ca2+濃度從1mmol/L增加到100mmol/L,PS-MPs和石英砂之間的排斥勢(shì)壘下降了956kT. PS-MPs/Pb2+二元體系中Pb2+能降低PS-MPs的遷移能力,背景離子濃度和價(jià)態(tài)的提升可削弱Pb2+對(duì)PS-MPs運(yùn)移能力的抑制性.當(dāng)Na+濃度從1mmol/L增加到100mmol/L,PS-MPs和石英砂之間的排斥勢(shì)壘下降了1100kT;當(dāng)Ca2+濃度從1mmol/L增加到100mmol/L,PS-MPs和石英砂之間的排斥勢(shì)壘下降了543kT.背景離子濃度的增加或價(jià)態(tài)的升高能促進(jìn)Pb2+的單獨(dú)運(yùn)移能力.PS-MPs/Pb2+二元體系中,PS-MPs能夠促進(jìn)Pb2+的運(yùn)移,背景離子濃度較低時(shí),PS-MPs負(fù)載Pb2+的遷移率較高,反之亦然.對(duì)于PS-MPs和Pb2+單運(yùn)移體系,背景陽(yáng)離子濃度和價(jià)態(tài)的提升可進(jìn)一步屏蔽PS-MPs及石英砂表面負(fù)電荷,競(jìng)爭(zhēng)吸附石英砂表面結(jié)合位點(diǎn),抑制PS-MPs運(yùn)移,促進(jìn)Pb2+運(yùn)移;對(duì)于PS-MPs和Pb2+共運(yùn)移體系,背景離子濃度和價(jià)態(tài)的提升可通過(guò)調(diào)節(jié)Pb2+與PS-MPs及石英砂表面的相互作用,削弱Pb2+對(duì)PS-MPs遷移能力的抑制作用.背景離子對(duì)PS-MPs表面位點(diǎn)的競(jìng)爭(zhēng)吸附及對(duì)電荷的屏蔽效應(yīng)影響PS-MPs對(duì)Pb2+的負(fù)載遷移能力.

微塑料；聚苯乙烯；重金屬；鉛離子；多孔介質(zhì)；共遷移

大量塑料制品暴露于水域和陸地生態(tài)系統(tǒng),并通過(guò)物理、化學(xué)、生物等過(guò)程降解為微塑料,由此引起的環(huán)境污染問(wèn)題已經(jīng)逐漸受到世界各地的關(guān)注[1-2].陸地生態(tài)系統(tǒng)中微塑料釋放量是海洋系統(tǒng)釋放量的4～23倍,是微塑料的重要源-匯[3-4].因此開(kāi)展陸地生態(tài)系統(tǒng)中微塑料環(huán)境行為的研究十分必要.

多孔介質(zhì)中的微塑料運(yùn)移過(guò)程是其在陸地生態(tài)系統(tǒng)中普遍存在的環(huán)境行為.通常,微塑料以膠體懸浮態(tài)或者顆粒凝聚形式賦存于環(huán)境中.已有研究表明,微塑料的尺寸、形狀、類(lèi)型、聚集狀態(tài)以及不同的溶液化學(xué)性質(zhì)、多孔介質(zhì)類(lèi)型、溫度等運(yùn)移環(huán)境,共同影響并決定了多孔介質(zhì)中微塑料復(fù)雜的運(yùn)移過(guò)程[5-6].微塑料運(yùn)移可對(duì)環(huán)境生態(tài)安全產(chǎn)生多重威脅.一方面,微塑料表面能夠負(fù)載重金屬等污染物,擴(kuò)大污染元素的空間傳輸范圍[7-8];另一方面,重金屬離子也可借助離子—微塑料表面絡(luò)合作用,改變微塑料膠體粒徑和其表面電荷,影響微塑料在運(yùn)移過(guò)程中的穩(wěn)定性和流動(dòng)性[9-10].

聚苯乙烯(PS)是生產(chǎn)生活中廣泛使用的熱塑性材料,其微顆粒具有粒徑小、比表面積大的特點(diǎn),在自然環(huán)境中極易吸附和攜帶重金屬離子[11-12].PS吸附的重金屬濃度比一般材料高出10～100倍[13].PS制品在經(jīng)過(guò)物理、化學(xué)和生物處理時(shí)產(chǎn)生的PS-MPs進(jìn)入土壤后必然對(duì)環(huán)境質(zhì)量提升和人體健康保障產(chǎn)生威脅.鉛具有急性毒性、長(zhǎng)期積累和持久性的特點(diǎn),是陸地環(huán)境中最常見(jiàn)的重金屬污染物.可在農(nóng)作物不同部位富集,經(jīng)由食物鏈進(jìn)入人體,極大地?fù)p害身體健康[14].已有研究表明,MPs對(duì)Pb2+有較高的吸附能力,土壤中的MPs可積累大量的Pb2+,其吸附量隨著環(huán)境中Pb2+濃度的增加而增大[15-18].Pb2+可通過(guò)特異性吸附及顆粒內(nèi)擴(kuò)散作用與MPs結(jié)合[19].因此,自然環(huán)境中PS-MPs與Pb2+可相互作用并通過(guò)共運(yùn)移形成協(xié)同污染,對(duì)生態(tài)環(huán)境構(gòu)成潛在威脅.PS- MPs表面富含-CH官能團(tuán),極易與環(huán)境中的Pb2+結(jié)合,并隨著環(huán)境化學(xué)條件變化將吸附態(tài)Pb2+再次釋放[20].研究表明,pH和鹽度可通過(guò)改變PS-MPs和Pb2+的電荷狀態(tài)影響兩者間相互作用[21],但溶劑化學(xué)條件對(duì)二者在環(huán)境中的遷移行為的影響仍需進(jìn)一步探究.因此,本研究將探索不同離子價(jià)態(tài)和離子濃度下PS-MPs和Pb2+在飽和一維砂柱中的單獨(dú)運(yùn)移和共遷移行為,并基于DLVO理論和ADE模型,揭示PS-MPs和Pb2+在飽和多孔介質(zhì)中的運(yùn)移機(jī)制,為微塑料和重金屬協(xié)同污染的防控提供一定理論基礎(chǔ).

1 材料與方法

1.1 供試材料的制備

1.1.1 PS-MPs懸浮液 10g/L的單分散熒光聚苯乙烯懸浮液(粒徑約1μm,密度為1.05g/cm3,最大激發(fā)波長(zhǎng)470nm,最大發(fā)射波長(zhǎng)526nm,激發(fā)和發(fā)射狹縫寬度均為5nm)購(gòu)自天津倍思樂(lè)色譜技術(shù)開(kāi)發(fā)中心.先前針對(duì)微塑料運(yùn)移和沉積行為的研究中使用的微塑料顆粒濃度通常在1～30mg/L,考慮到自然水生環(huán)境中Na+、Ca2+濃度普遍低于1000mmol/L[22-23].因此選擇用1mmol/L,10mmol/L, 100mmol/L NaCl和CaCl2作為背景電解質(zhì)溶液將PS-MPs懸浮液稀釋為10mg/L.使用100mmol/L NaOH和HCl將懸浮液的初始pH值調(diào)整為6.0±0.3.

1.1.2 鉛溶液 去離子水中溶解適量的Pb(NO3)2,制備Pb2+儲(chǔ)備溶液(1000mg/L).自然水體中Pb2+的濃度范圍一般在ng/L至mg/L[24].然而探索Pb2+和PS- MPs的共運(yùn)移行為,需要PS-MPs能夠吸附一定量的Pb2+.因此,本研究將Pb2+稀釋至實(shí)驗(yàn)條件所需的10mg/L(高于自然水體中的濃度).使用0.1mol/L NaOH和HCl將溶液的初始pH值調(diào)整為6.0±0.3,以避免Pb2+的水解和沉淀[25].

1.1.3 石英砂前處理 選取平均粒徑為0.75mm的石英砂(SiO2)(周至縣石英砂有限公司,中國(guó)陜西)作為柱遷移實(shí)驗(yàn)中的多孔介質(zhì).用自來(lái)水、0.1mol/L HCl和0.1mol/L NaCl依次清洗石英砂以去除其中的鐵、鋁等氧化物,最后用去離子水多次沖洗,在105℃條件下烘干后備用[26].

1.2 飽和石英砂柱中PS-MPs/Pb2+遷移實(shí)驗(yàn)

采用柱遷移實(shí)驗(yàn)研究PS-MPs和Pb2+在飽和多孔介質(zhì)中的垂向遷移行為.采用濕填法,將洗凈烘干的石英砂填滿長(zhǎng)15cm,內(nèi)徑3cm的有機(jī)玻璃柱,填充柱的孔隙體積(PV)約45mL,平均孔隙率(砂柱的孔隙體積與總體積之比)約為0.42.為支撐多孔介質(zhì),在柱頂及柱底各放一張孔徑為50μm的尼龍網(wǎng)[27].使用硝酸鉀溶液進(jìn)行非反應(yīng)性示蹤實(shí)驗(yàn),利用Hydrus- 1D擬合求得對(duì)流彌散系數(shù),以確定溶質(zhì)流動(dòng)性和柱性能[26].柱遷移實(shí)驗(yàn)包括在不同水化學(xué)條件下(背景離子分別為Na+/Ca2+,離子濃度均為1,10,100mmol/L, pH為6.0)PS-MPs和Pb2+在砂柱中的單獨(dú)運(yùn)移和共同運(yùn)移.實(shí)驗(yàn)開(kāi)始前利用蠕動(dòng)泵(HL-2B,上海瀘西分析儀器廠有限公司,中國(guó))自下而上注入至少10PV去離子水,以排除柱中石英砂之間的氣泡.接著通入至少3PV背景電解液,以穩(wěn)定溶液化學(xué)條件.實(shí)驗(yàn)中先通入3PV相同背景溶液的PS-MPs/Pb2+/混合物懸浮液(考慮到懸浮液的穩(wěn)定性,在進(jìn)樣前進(jìn)行超聲處理10min),再通入3PV背景電解液,控制流速為1ml/min,使用四通道部分自動(dòng)收集器(EBS-20,上海瀘西分析儀器廠有限公司,中國(guó))按10min時(shí)間間隔連續(xù)收集流出液.遵循以上操作路徑,開(kāi)展PS-MPs和Pb2+的單獨(dú)運(yùn)移和共運(yùn)移試驗(yàn),采用熒光分光光度法[28](LS55,珀金埃爾默,美國(guó))和火焰原子吸收光譜法(AA530,珀金埃爾默,美國(guó))分別測(cè)定流出液中PS-MPs與Pb2+的濃度,以確定PS-MPs與Pb2+在遷移過(guò)程中對(duì)應(yīng)的濃度變化,并獲得穿透曲線,各處理重復(fù)兩次,利用Hydrus-1D擬合穿透曲線.

1.3 性質(zhì)表征及模型預(yù)測(cè)

1.3.1 性質(zhì)表征 使用動(dòng)態(tài)光散射儀(Nanobook Omni,布魯克海文公司,美國(guó)),設(shè)置入射波長(zhǎng)為635nm,散射角90°工作條件下測(cè)定PS-MPs懸浮液的水動(dòng)力學(xué)直徑.將10g石英砂與40mL背景電解質(zhì)溶液置于100mL三角瓶中,用細(xì)胞破碎儀(XO-900D,南京先歐儀器制造有限公司,中國(guó))在50%功率(450W)條件下超聲分散10min,采用高靈敏度zeta電位分析儀(ZETA PALS,布魯克海文公司,美國(guó))測(cè)定不同水化學(xué)條件下PS-MPs和石英砂的zeta電位,并基于測(cè)定結(jié)果進(jìn)行DLVO能量計(jì)算.使用傅里葉變換紅外光譜(Nicolet iS 50,Thermo Scientific, America)分析PS-MPs吸附Pb2+前后官能團(tuán)的變化情況.通過(guò)掃描電子顯微鏡-能量色散光譜儀(SEM-EDS)(S-4800,日立,日本)觀察了PS-MPs的表面形態(tài)和初級(jí)尺寸以及砂柱口石英砂表面PS-MPs的沉積狀況和表面元素(圖1).

圖1 吸附Pb2+前后PS-MPs的FTIR光譜圖(a), PS-MPs粒度分布圖(b), PS-MPs運(yùn)移后砂柱入口處石英砂表面SEM-EDS圖像(c)及PS-MPs與Pb2+共同運(yùn)移后砂柱入口處石英砂表面SEM-EDS圖像(d)

1.3.2 膠體運(yùn)移行為的評(píng)估 對(duì)流-彌散模型是描述溶質(zhì)運(yùn)移的常用數(shù)學(xué)模型.利用一維飽和流條件下的對(duì)流彌散方程分析飽和多孔介質(zhì)中NO3-的遷移規(guī)律,使用Hydrus-1D計(jì)算對(duì)流彌散系數(shù)并擬合穿透曲線.數(shù)學(xué)模型如下[26]:

式中:w是水流中溶質(zhì)的濃度,mg/L;是阻滯因子,表示介質(zhì)中平衡反應(yīng)的級(jí)數(shù);是彌散系數(shù),cm2/min;水流速度,cm/min;是膠體聚沉速度系數(shù),min?1.

1.3.3 PS-MPs與介質(zhì)表面間相互作用能的表征 應(yīng)用DLVO理論計(jì)算PS-MPs顆粒間和PS- MPs與石英砂表面間的相互作用能.DLVO能量(DLVO)由范德華力相互作用能(LW)和靜電相互作用能(EL)組成.利用公式(3)和(4)計(jì)算PS-MPs與石英砂之間的LW和EL,利用公式(5)和(6)計(jì)算PS-MPs顆粒之間的LW和EL[29-31]:

式中:132是PS-MPs-水-石英砂的Hamaker常數(shù)(9.8×10-21J),是PS-MPs-水的Hamaker常數(shù)(4.6× 10-21J)其中PS-MPs、石英砂、水的Hamaker分別為6.78×10-20J、8.8×10-20J、3.7×10-20J[32];p1p2是PS-MPs的顆粒半徑;是相互作用表面之間的距離;0是真空介電常數(shù),常溫下為8.854×10-12C/V/m;是水的相對(duì)介電常數(shù)(78.4);是德拜長(zhǎng)度的倒數(shù),常溫下取0.104nm;是玻爾茲曼常數(shù)(1.38×10-23J /K);是絕對(duì)溫度;是某離子價(jià)態(tài);e是電子電荷量(1.6×10-19C);0是膠體與固相顆粒表面間的最小平衡接觸距離(0.157nm);是極性作用力在水溶液中的特征衰減長(zhǎng)度(0.6nm);ps分別是PS-MPs和石英砂的zeta電位.

2 結(jié)果與討論

2.1 不同離子濃度/價(jià)態(tài)條件下PS-MPs的單獨(dú)遷移特征及其影響機(jī)制

由圖2可知,隨著背景離子濃度的增大,NaCl和CaCl2體系中PS-MPs的最大出流濃度比分別從0.78和0.45下降到0.25和0.14,穿透率分別從78.8%和42.8%下降到24.8%和13.6%.說(shuō)明在所有試驗(yàn)條件下,PS-MPs的穿透能力均隨著背景離子濃度的增大而減弱,這可能由于高濃度背景離子使PS-MPs大量沉積在石英砂表面,導(dǎo)致能夠穿透多孔介質(zhì)的PS-MPs逐漸變少[33].溶膠是熱力學(xué)不穩(wěn)定系統(tǒng),因此引用經(jīng)典DLVO理論描述PS-MPs顆粒之間以及PS-MPs與石英砂之間的相互作用.根據(jù)雙電層理論可知,膠體穩(wěn)定性與膠體表面電荷性質(zhì)有關(guān),相鄰PS-MPs顆粒表面同種電荷數(shù)量越多,它們之間的靜電排斥力越強(qiáng),PS-MPs顆粒間的穩(wěn)定性越好[34-37].此時(shí),PS-MPs不易凝聚沉積,具有較好的遷移性,反之亦然.本研究中,Na+/Ca2+陽(yáng)離子可通過(guò)靜電作用吸附在帶負(fù)電的PS-MPs顆粒表面并削弱其表面電負(fù)性(表1),使PS-MPs顆粒間(圖4)以及PS-MPs顆粒與石英砂之間(圖3)的靜電排斥力減弱, PS-MPs膠體穩(wěn)定性及遷移性相繼降低.隨著背景陽(yáng)離子濃度的提升,PS-MPs顆粒表面電負(fù)性進(jìn)一步削弱.如表1所示,當(dāng)Na+和Ca2+離子濃度分別從1mmol/L增至100mmol/L時(shí),PS-MPs的zeta電位(絕對(duì)值)分別降低了55.09%和65.13%, PS-MPs電負(fù)性的迅速下降必將導(dǎo)致其膠體穩(wěn)定性驟降,加劇其在石英砂顆粒表面的絮凝沉積.由圖3可知,當(dāng)Na+和Ca2+濃度為1mmol/L時(shí),兩體系中PS-MPs與石英砂之間的DLVO合力表現(xiàn)為斥力,說(shuō)明在低濃度背景電解液中,石英砂與PS-MPs之間的相互作用由靜電排斥力主導(dǎo),石英砂對(duì)PS-MPs的吸附量相對(duì)較小.且砂柱中PS-MPs的沉積量較少,不會(huì)因其對(duì)砂柱孔隙的大量堵塞而阻礙PS-MPs的運(yùn)移.當(dāng)Na+和Ca2+濃度分別從1mmol/L增加到100mmol/L 時(shí),兩體系排斥勢(shì)壘分別下降了1348kT和955kT.說(shuō)明隨著背景離子濃度的升高,二者表面的電負(fù)性因陽(yáng)離子對(duì)表面負(fù)電荷逐漸增強(qiáng)的屏蔽效應(yīng)而減弱,導(dǎo)致PS-MPs與石英砂之間的斥力勢(shì)能減少,勢(shì)壘降低,總勢(shì)能逐漸由范德華力構(gòu)成的引力勢(shì)能主導(dǎo).此時(shí)PS-MPs因與石英砂相互吸引而嚴(yán)重抑制其自身運(yùn)移,且穿透曲線出現(xiàn)明顯的熟化現(xiàn)象[28],表明已經(jīng)沉積在石英砂表面的PS-MPs可能會(huì)繼續(xù)通過(guò)靜電吸引的方式吸附更多的PS-MPs,使介質(zhì)表面PS-MPs沉積逐層增多,大量沉積態(tài)PS-MPs導(dǎo)致飽和多孔石英砂介質(zhì)孔隙堵塞.

比較兩種背景電解液的穿透曲線(圖2)可知, Ca2+對(duì)PS-MPs運(yùn)移的抑制作用強(qiáng)于Na+.例如,Ca2+濃度為1mmol/L時(shí),PS-MPs的最大出流濃度比為0.45,穿透率為42.8%,低于1mmol/L NaCl時(shí)PS-MPs的最大出流濃度比(0.78)和穿透率(78.8%); Ca2+濃度為10mmol/L時(shí),PS-MPs的最大出流濃度比為0.25,穿透率為24.1%,遠(yuǎn)低于10mmol/L NaCl時(shí)PS-MPs的最大出流濃度比(0.45)和穿透率(43.9%).不同價(jià)態(tài)離子對(duì)膠體凝聚作用的影響符合Schulze- Hardy原則,即與低價(jià)離子組成體系相比,高價(jià)離子組成體系中PS-MPs膠體更易凝聚并沉積在石英砂柱中,這主要是因?yàn)镃a2+比Na+具有更強(qiáng)的屏蔽表面負(fù)電荷的能力,繼而導(dǎo)致更低的PS-MPs表面電位[38].由表1可知,在相同離子濃度下,PS-MPs在CaCl2溶液中的zeta電位明顯低于NaCl溶液,表列結(jié)果驗(yàn)證了推論的正確性.另外,通過(guò)圖3,圖4可知,相同離子濃度下CaCl2溶液中PS-MPs與石英砂間以及PS-MPs顆粒間具有較NaCl溶液中更低的排斥勢(shì)壘,說(shuō)明高價(jià)態(tài)陽(yáng)離子對(duì)PS-MPs的表面電位、膠體穩(wěn)定性及其在多孔介質(zhì)表面的沉積效應(yīng)的確產(chǎn)生了更顯著的影響[39].此外,Ca2+可通過(guò)陽(yáng)離子橋接作用促進(jìn)PS-MPs之間的團(tuán)聚,增強(qiáng)膠體的瀝濾作用[40-41],亦可通過(guò)橋接作用連接PS-MPs和石英砂顆粒,使后者表面粗糙度增加,形態(tài)更不規(guī)則, PS-MPs更易滯留在砂柱中并可能堵塞多孔介質(zhì)之間的孔隙[26].上述結(jié)果及分析清晰地表明,不同價(jià)態(tài)金屬陽(yáng)離子濃度的升高均會(huì)抑制PS-MPs的運(yùn)移,且高價(jià)態(tài)陽(yáng)離子對(duì)PS-MPs運(yùn)移的抑制作用要比低價(jià)態(tài)陽(yáng)離子更加明顯.

表1 實(shí)驗(yàn)條件和理化信息

續(xù)表1

注:MEC為流出液中溶質(zhì)的最大濃度百分比eff為溶質(zhì)的穿透率DLS diameter PS-MPs為通入石英砂柱的懸濁液中PS-MPs的水合粒徑.

2.2 不同離子濃度/價(jià)態(tài)條件下Pb2+的單獨(dú)遷移特征及其影響機(jī)制

石英砂表面帶負(fù)電荷,帶正電荷的Pb2+易通過(guò)靜電引力吸附在石英砂表面.在不同背景離子及其濃度條件下,Pb2+的遷移滯留能力有較大差異.如圖5所示,隨著背景電解液中Na+和Ca2+的濃度從0.1mmol/L增至100mmol/L,Pb2+的最大出流率分別從0.60和0.68上升到0.87和0.93,穿透率分別從50.1%和71.6%上升到92.6%和99.1%,這表明介質(zhì)對(duì)Pb2+的吸附能力不斷減弱,更多的Pb2+能夠穿透石英砂柱.背景離子濃度與Pb2+的遷移速率呈正相關(guān),這主要是因?yàn)殡S著背景離子濃度的增大,石英砂表面負(fù)電位降低,對(duì)Pb2+的靜電引力變?nèi)?同時(shí), Na+/Ca2+與Pb2+競(jìng)爭(zhēng)吸附有限的石英砂結(jié)合位點(diǎn), Na+/Ca2+離子濃度的增加會(huì)使其位點(diǎn)競(jìng)爭(zhēng)能力增強(qiáng),且石英砂表面的吸附態(tài)Pb2+也可能因高濃度的Na+和Ca2+而解吸附,重新回到孔隙水并以溶解態(tài)Pb2+的形式流出[42].

由圖5可知,Pb2+的運(yùn)移穿透曲線表現(xiàn)出明顯的不對(duì)稱(chēng)性,并伴隨拖尾現(xiàn)象.不對(duì)稱(chēng)性有可能是因?yàn)槭⑸皩?duì)Pb2+的吸附能力有限,隨著Pb2+的不斷注入,石英砂表面對(duì)Pb2+的吸附能力逐漸減弱,進(jìn)而導(dǎo)致其遷移能力逐漸增強(qiáng).拖尾現(xiàn)象也說(shuō)明,部分吸附在石英砂表面的Pb2+受到水流的沖刷作用易造成脫附現(xiàn)象[14].拖尾現(xiàn)象隨著背景離子溶液濃度的增加或離子價(jià)態(tài)的升高愈發(fā)顯著,這說(shuō)明此時(shí)石英砂對(duì)Pb2+的吸附能力較弱,導(dǎo)致在水流沖刷作用下更多的Pb2+能夠脫離石英砂表面重新回到孔隙水中,并以溶解態(tài)鉛的形式穿透砂柱[19,43];隨著離子價(jià)態(tài)及其濃度的升高,穿透曲線對(duì)稱(chēng)性逐漸顯現(xiàn),Pb2+穿透能力較強(qiáng),在石英砂中不會(huì)占據(jù)過(guò)多的結(jié)合位點(diǎn),并對(duì)隨后通過(guò)石英砂表面的Pb2+的運(yùn)移能力造成較大的影響.

如圖5所示,相同濃度的不同價(jià)態(tài)離子對(duì)Pb2+在飽和多孔介質(zhì)中的運(yùn)移影響迥異.Ca2+濃度為1mmol/L時(shí),Pb2+的最大出流濃度比為0.68,穿透率為71.6%,高于1mmol/L Na+時(shí)Pb2+的最大出流濃度比(0.60)和穿透率(50.1%);當(dāng)Ca2+濃度為10mmol/L時(shí),Pb2+的最大出流濃度比為0.84,穿透率為86.4%,運(yùn)移能力遠(yuǎn)強(qiáng)于同濃度Na+體系中Pb2+的運(yùn)移能力,與100mmol/L Na+體系中Pb2+的最大出流濃度比(0.87)和穿透率(92.6%)接近,這是因?yàn)镃a2+的電荷中和效應(yīng)強(qiáng)于Na+,能夠更有效的屏蔽石英砂表面的負(fù)電荷,從而通過(guò)減弱石英砂對(duì)Pb2+的靜電吸引促進(jìn)Pb2+的運(yùn)移[38].

2.3 不同離子濃度/價(jià)態(tài)條件下PS-MPs和Pb2+的共遷移特征及其影響機(jī)制

PS-MPs和Pb2+在飽和石英砂介質(zhì)中的單獨(dú)運(yùn)移實(shí)驗(yàn)結(jié)果表明,背景離子濃度/價(jià)態(tài)的確對(duì)兩種環(huán)境污染物的傳輸產(chǎn)生了重要影響.為探究二者共運(yùn)移過(guò)程中污染物遷移特征以及離子濃度/價(jià)態(tài)對(duì)共遷移過(guò)程產(chǎn)生的影響開(kāi)展了不同背景離子及其濃度條件下PS-MPs和Pb2+的共運(yùn)移實(shí)驗(yàn).

2.3.1 Pb2+對(duì)PS-MPs運(yùn)移的影響 通過(guò)比較PS-MPs在單獨(dú)運(yùn)移(圖2)和共遷移(圖6a,b)體系中的穿透曲線可知,其在共運(yùn)移體系中的最大出流濃度比和穿透率均低于相同條件下單一體系中對(duì)應(yīng)的最大流出濃度比和穿透率,說(shuō)明Pb2+共存條件下PS-MPs的穿透能力明顯下降.最近的研究證明,微塑料對(duì)重金屬的吸附是符合Freundlich模型的物理吸附過(guò)程[19-20].通過(guò)FTIR分析PS-MPs吸附Pb2+前后官能團(tuán)的變化情況.有趣的是,PS-MPs在吸附Pb2+后,沒(méi)有出現(xiàn)或消失新的特征峰.結(jié)果表明,吸附過(guò)程中可能以物理吸附為主.一方面, PS-MPs的穿透能力明顯下降可能是由于吸附在PS-MPs表面帶正電荷的Pb2+能夠橋接表面帶負(fù)電荷的PS-MPs和石英砂,促使更多的PS-MPs沉積在石英砂表面[43].另一方面,由表1可知,共運(yùn)移體系中PS-MPs的zeta電位均低于單體系PS-MPs的zeta電位,意味著Pb2+的存在使得PS-MPs的zeta電位更接近正值,這將降低PS-MPs顆粒間和PS-MPs與石英砂之間由靜電力支配的排斥勢(shì)壘,導(dǎo)致更多的PS-MPs沉積在石英砂表面,進(jìn)一步抑制其運(yùn)移能力[44-45].通過(guò)對(duì)比單獨(dú)運(yùn)移和共運(yùn)移體系中PS-MPs和石英砂間的DLVO相互作用(圖3和圖7)可知,在相同的水化學(xué)條件下,共運(yùn)移體系中PS-MPs與石英砂之間的排斥勢(shì)壘均低于單一體系中二者之間的排斥能壘.當(dāng)Na+濃度為1,10,100mmol/L時(shí),與單獨(dú)運(yùn)移體系相比,共運(yùn)移體系中PS-MPs與石英砂之間的排斥勢(shì)壘分別下降了704kT,662kT,456kT;當(dāng)Ca2+濃度為1, 10mmol/L時(shí),排斥勢(shì)壘分別下降了413kT,192kT (單獨(dú)運(yùn)移和共運(yùn)移體系,Ca2+濃度為100mmol/L時(shí)不存在排斥能壘),計(jì)算結(jié)果佐證了以上推論.

圖7 不同電解質(zhì)濃度條件下PS-MPs和Pb2+在NaCl(a)和CaCl2(b)體系中共運(yùn)移的DLVO能量分布

2.3.2 PS-MPs對(duì)Pb2+運(yùn)移的影響 Pb2+能夠抑制PS-MPs在多孔介質(zhì)中的遷移.同時(shí),PS-MPs對(duì)Pb2+的吸附也提高了后者在環(huán)境介質(zhì)中的運(yùn)移能力[46].如圖(6c,d)所示,相同條件下共遷移體系中Pb2+的最大出流濃度比和穿透率均高于單一體系(圖5a,b)中對(duì)應(yīng)的最大出流濃度比和穿透率.這說(shuō)明:第一, PS-MPs的存在能夠增加多孔介質(zhì)中Pb2+的遷移能力,擴(kuò)大其污染范圍;第二,Pb2+在PS-MPs表面的負(fù)載遷移是其穿透石英砂柱的重要途徑.顯然,PS- MPs和Pb2+的相互作用為共運(yùn)移過(guò)程中后者的大量遷移提供了可能.現(xiàn)有理論認(rèn)為,重金屬可通過(guò)靜電力、表面絡(luò)合和沉淀吸附在微塑料表面,亦可被微塑料表面形成的水膜吸附,這些直接/間接相互作用對(duì)環(huán)境中微塑料和重金屬的共遷移產(chǎn)生了至關(guān)重要的影響[47-48].

2.3.3 離子濃度/價(jià)態(tài)對(duì)PS-MPs和Pb2+共運(yùn)移的影響 背景電解液中的Na+和Ca2+可通過(guò)干預(yù)PS- MPs膠體和石英砂顆粒的表面電荷性能影響Pb2+的吸附能力.如表1所示,當(dāng)背景離子濃度從1mmol/L增至100mmol/L時(shí),PS-MPs(Pb2+)在單運(yùn)移和共運(yùn)移體系中的出流濃度比之差逐漸減小,說(shuō)明隨著背景離子濃度的增加,Pb2+對(duì)PS-MPs運(yùn)移的抑制作用和PS-MPs對(duì)Pb2+運(yùn)移的促進(jìn)作用均逐漸減弱.一方面,較高背景離子濃度導(dǎo)致PS-MPs穩(wěn)定性下降,易團(tuán)聚沉積并可能占據(jù)石英砂表面的吸附位點(diǎn),致使微塑料攜帶Pb2+共同運(yùn)移的能力降低,即PS-MPs對(duì)Pb2+運(yùn)移的促進(jìn)作用減弱;另一方面,背景離子濃度的增加使其競(jìng)爭(zhēng)吸附表面位點(diǎn)的能力增強(qiáng)[49-50],可能導(dǎo)致Pb2+在PS-MPs和石英砂之間的橋聯(lián)作用及Pb2+對(duì)PS-MPs運(yùn)移的抑制作用相繼減弱.離子價(jià)態(tài)對(duì)共遷移過(guò)程中PS-MPs和Pb2+運(yùn)移的影響與單獨(dú)運(yùn)移的影響趨勢(shì)相同,在共運(yùn)移體系中,高價(jià)態(tài)背景離子會(huì)抑制PS-MPs的運(yùn)移并促進(jìn)Pb2+的運(yùn)移.與PS-MPs單獨(dú)運(yùn)移試驗(yàn)的穿透曲線相比,共運(yùn)移試驗(yàn)中Pb2+對(duì)PS-MPs運(yùn)移的抑制作用在Ca2+體系中更弱.因?yàn)橄啾扔贜a+,Ca2+具有更強(qiáng)的表面吸附位點(diǎn)競(jìng)爭(zhēng)和膠體雙電層壓縮能力,因此Ca2+體系中Pb2+對(duì)PS-MPs運(yùn)移能力的抑制作用將顯著減弱.然而,不同價(jià)態(tài)的背景離子對(duì)共運(yùn)移中Pb2+的運(yùn)移影響并不顯著,這可能是因?yàn)橛坞x態(tài)Pb2+是單體系中Pb2+遷移的唯一形式,而共遷移體系中Pb2+可以通過(guò)PS-MPs負(fù)載和游離態(tài)兩種形式穿透石英砂柱,致其最大出流濃度比接近1,Ca2+和Na+對(duì)共運(yùn)移試驗(yàn)中Pb2+的運(yùn)移沒(méi)有產(chǎn)生明顯影響.

3 結(jié)論

3.1 PS-MPs單獨(dú)運(yùn)移體系中,背景陽(yáng)離子濃度的增大能夠增強(qiáng)其對(duì)PS-MPs與石英砂表面負(fù)電荷的屏蔽效應(yīng),致使二者之間的排斥勢(shì)壘及PS-MPs膠體的穩(wěn)定性下降,最終導(dǎo)致PS-MPs在石英砂表面沉積,運(yùn)移能力減弱.Ca2+較Na+有更強(qiáng)的表面電荷屏蔽能力,且其能在PS-MPs和石英砂之間扮演橋聯(lián)作用,導(dǎo)致Ca2+體系中PS-MPs遷移能力更弱.

3.2 Pb2+單獨(dú)運(yùn)移體系中,背景陽(yáng)離子濃度的增大使其對(duì)石英砂表面負(fù)電荷的屏蔽效應(yīng)及競(jìng)爭(zhēng)石英砂表面吸附位點(diǎn)的能力增強(qiáng),繼而使Pb2+不易附著在石英砂表面,運(yùn)移能力逐漸增強(qiáng).Ca2+較Na+有更強(qiáng)的表面電荷屏蔽能力及吸附位點(diǎn)競(jìng)爭(zhēng)能力,因此相同濃度條件下Ca2+體系中Pb2+的運(yùn)移能力更強(qiáng).

3.3 PS-MPs與Pb2+共運(yùn)移受到背景離子濃度、價(jià)態(tài)以及遷移物特性的共同影響.Pb2+能夠通過(guò)物理吸附行為吸附在PS-MPs表面,并橋聯(lián)、屏蔽石英砂與PS-MPs表面的負(fù)電荷降低二者間的排斥勢(shì)壘,減弱PS-MPs顆粒間的穩(wěn)定性,繼而抑制PS-MPs的遷移能力,該過(guò)程中背景離子濃度和價(jià)態(tài)的提升削弱了Pb2+抑制PS-MPs運(yùn)移的能力;PS-MPs能夠促進(jìn)Pb2+的運(yùn)移,Pb2+在PS-MPs表面的負(fù)載遷移是其穿透石英砂柱的重要途徑.背景離子的濃度較低時(shí),PS-MPs負(fù)載Pb2+的遷移率較高,反之亦然.該過(guò)程中背景離子對(duì)PS-MPs表面位點(diǎn)的競(jìng)爭(zhēng)吸附及對(duì)電荷的屏蔽效應(yīng)發(fā)揮了重要作用.

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Effects of background ion types and concentrations on the co-transport of polystyrene microplastics / lead in saturated quartz sand.

CHANG Bo-kun1, CHEN Yi-ting1, CAO Gang1, HU Liang1, Lü Jia-long1, DU Wei1**, HU Fei-nan1,2*

(1.College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi 712100, China；2.Institute of Soil and Water Conservation, CAS & Ministry of Water Resources, Yangling, Shaanxi 712100, China)., 2022,42(7)：3193～3203

In order to elucidate the effect of hydrochemical conditions on the transport of microplastics and heavy metals, and to clarify the coupling effect in the co-transport process of the two environmental pollutants and their response mechanism to environmental conditions preliminarily, the effects of background electrolyte ions with different valence and concentrations on the individual and co-transport behaviors of 1μm polystyrene microplastics (PS-MPs) and Pb2+in a saturated one-dimensional sand column were studied. The experimental results showed that the increase in the background ion concentration or the valence would inhibit the individual transport ability of PS-MPs. When the Na+concentration increased from 1mmol/L to 100mmol/L, the repulsive barrier between PS-MPs and quartz sand decreased by 1348kT; when the Ca2+concentration increased from 1mmol/L to 100mmol/L, the repulsive barrier between PS-MPs and quartz sand decreased by 956kT. Pb2+in PS-MPs/Pb2+binary system could reduce the transport ability of PS-MPs, and the increase of the background ion concentration and the valence can weaken the inhibition of Pb2+on the transport ability of PS-MPs. When the Na+concentration increased from 1mmol/L to 100mmol/L, the repulsive barrier between PS-MPs and quartz sand decreased by 1100kT; when the Ca2+concentration increased from 1mmol/L to 100mmol/L, the repulsive barrier between PS-MPs and quartz sand decreased by 543kT. The increase in the background ion concentration or the valence can promote the individual transport ability of Pb2+. PS-MPs can promote the transport of Pb2+in the binary system of PS-MPs/Pb2+. When the background ion concentration was low, the transport of Pb2+loaded by PS-MPs was higher, and vice versa. For PS-MPs and Pb2+individual transport systems, the increase in background cation concentration and valence can further shield the negative charges on the surface of PS-MPs and quartz sand, competitive adsorption the surface binding sites of quartz sand, inhibits the transport of PS-MPs while promoting Pb2+transport. For the co-transport system of PS-MPs and Pb2+, the increase in background ion concentration and valence can weaken the inhibitory effect of Pb2+on the transport ability of PS-MPs by adjusting the interaction between Pb2+and PS-MPs and the surface of quartz sand. The competitive adsorption of background ions to PS-MPs’ surface sites and the shielding effect on charges affect the transport ability of Pb2+loaded by PS-MPs.

microplastic；polystyrene；heavy metals；lead ion；porous media；co-transport

X145

A

1000-6923(2022)07-3193-11

常博焜(1998-),男,黑龍江穆棱人,西北農(nóng)林科技大學(xué)碩士研究生,主要從事微塑料在多孔介質(zhì)中的運(yùn)移方面研究.

2021-12-30

陜西省自然科學(xué)基礎(chǔ)研究計(jì)劃(2021JQ-170)

*責(zé)任作者, 副研究員, hufeinan-629@163.com, **, 講師, weidu0932@126.com