張文靜,秦運(yùn)琦,劉 丹,馬添翼,厲曉飛

微生物在多孔介質(zhì)中環(huán)境行為研究

張文靜1,2*,秦運(yùn)琦1,2,劉 丹1,2,馬添翼1,2,厲曉飛1,2

(1.吉林大學(xué)地下水資源與環(huán)境教育部重點(diǎn)實(shí)驗(yàn)室,吉林 長(zhǎng)春 130021；2.吉林大學(xué)新能源與環(huán)境學(xué)院,吉林 長(zhǎng)春 130021)

多孔介質(zhì)中的致病細(xì)菌及病毒等病原微生物遷移研究對(duì)保護(hù)飲用水資源具有重要意義.在歸納總結(jié)國(guó)內(nèi)外已有研究資料的基礎(chǔ)上,對(duì)微生物在多孔介質(zhì)中遷移行為進(jìn)行了分析.研究表明病原微生物作為生物膠體的一種,在遷移過(guò)程中受多種遷移機(jī)制共同作用.除常見水動(dòng)力條件和水化學(xué)條件外,微生物在遷移過(guò)程中還受自身性質(zhì),介質(zhì)顆粒及環(huán)境因素的影響.在此基礎(chǔ)上,分別討論了微生物在飽和多孔介質(zhì)與非飽和多孔介質(zhì)中的遷移行為,并對(duì)近年來(lái)研究熱點(diǎn)及進(jìn)展進(jìn)行了簡(jiǎn)要概括.目前微生物在地下水中遷移研究日益豐富,但由于地下環(huán)境復(fù)雜性,微生物的易變異性,對(duì)于地下水系統(tǒng)中微生物造成的污染去除修復(fù)還需進(jìn)一步探討.

微生物；多孔介質(zhì)；遷移機(jī)制；生物膠體；運(yùn)移和沉積

研究發(fā)現(xiàn),由于污水處理不當(dāng)使得含水層,地下水和飲用水中都存在大量致病微生物,對(duì)土壤及地下水造成嚴(yán)重污染[1-3].膠體由于其具有較大的比表面積及大量吸附位點(diǎn)可作為一種吸附載體.在地下水資源保護(hù)方面,膠體可實(shí)現(xiàn)對(duì)地下水系統(tǒng)中其他微量元素及有機(jī)組份的協(xié)同遷移,同時(shí)對(duì)去除污染物也有重要作用[4].病原微生物如病毒, 細(xì)菌等屬于生物膠體范疇,在地下水系統(tǒng)中不但可以自身遷移,也可作為載體實(shí)現(xiàn)與其他污染物的協(xié)同遷移.因此探討微生物在多孔介質(zhì)中的遷移不僅對(duì)地下水資源保護(hù),地下水中病原體風(fēng)險(xiǎn)評(píng)估具有重大意義,也對(duì)生物修復(fù)治理,供水系統(tǒng)設(shè)計(jì)具有重大意義.

病毒及細(xì)菌常被用作生物指示劑,反映生物膠體在多孔介質(zhì)中的遷移情況.為探究微生物在多孔介質(zhì)中的運(yùn)移過(guò)程,國(guó)內(nèi)外學(xué)者開展了大量室內(nèi)柱實(shí)驗(yàn)及場(chǎng)地試驗(yàn).孔隙尺度可視化技術(shù)的改進(jìn)為微生物在飽和及非飽和介質(zhì)中的遷移研究提供了便利[1].隨著研究的不斷深入,國(guó)內(nèi)外專家在實(shí)驗(yàn)設(shè)計(jì)、理論研究、測(cè)試方法、探究?jī)?nèi)容等方面都有較新進(jìn)展.我國(guó)學(xué)者畢永強(qiáng)等[2]發(fā)現(xiàn)采油微生物在多孔介質(zhì)中運(yùn)移受到平衡吸附和架橋篩分的共同作用.同時(shí)唐朱睿等[3]指出胞外呼吸菌在與污染物協(xié)同遷移中可以降解污染物,對(duì)污染物原位修復(fù)上具有重大意義.本課題組主要討論了不同水動(dòng)力、水化學(xué)條件下,大腸桿菌噬菌體在飽和多孔介質(zhì)及非飽和多孔介質(zhì)中的遷移轉(zhuǎn)化,同時(shí)對(duì)噬菌體與其他膠體的協(xié)同遷移展開探討[4-5].在指示微生物選擇方面,相比于常見的陸地微生物, Ghanem等[6]選取了海洋噬菌體探究其在多孔介質(zhì)中的遷移規(guī)律.同時(shí)微生物與其他膠體或污染物的協(xié)同作用也成為當(dāng)先研究熱點(diǎn).Walshe等[7]研究發(fā)現(xiàn)高嶺石膠體的存在可以促進(jìn)病毒礫石含水層中的遷移.這些研究為保護(hù)地下水資源、飲用水資源和評(píng)估病原體污染的風(fēng)險(xiǎn)提供了諸多參考資料.

本文在檢索國(guó)內(nèi)外相關(guān)研究的基礎(chǔ)上,主要總結(jié)了微生物在多孔介質(zhì)中遷移的研究現(xiàn)狀與研究進(jìn)展,對(duì)其在多孔介質(zhì)中的沉積與遷移機(jī)制進(jìn)行了總結(jié)與討論.同時(shí)對(duì)微生物在飽和多孔介質(zhì)與非飽和多孔介質(zhì)中的遷移行為進(jìn)行分析,并總結(jié)了微生物在多孔介質(zhì)中的研究熱點(diǎn),為今后微生物在多孔介質(zhì)中的遷移研究提供依據(jù)與參考.

1 微生物在多孔介質(zhì)中的遷移機(jī)理

微生物在多孔介質(zhì)中的遷移過(guò)程實(shí)際上是膠體隨水流在孔隙中的運(yùn)移過(guò)程[8].在運(yùn)移中,膠體會(huì)發(fā)生運(yùn)移、釋放、沉積.遷移過(guò)程受多種機(jī)制共同作用,同時(shí)也受到膠體粒徑、介質(zhì)顆粒大小、介質(zhì)形狀、表面粗糙程度、孔隙尺寸、孔隙水化學(xué)性質(zhì)等因素影響[9].遷移機(jī)理的中文表述目前國(guó)內(nèi)還未統(tǒng)一,這里給出較常用的表述方法.微生物在地下多孔介質(zhì)運(yùn)移過(guò)程圖參照Z(yǔ)hong等[10]對(duì)生物表面活性劑增細(xì)菌在多孔介質(zhì)中的遷移研究見圖1.

圖1 微生物在地下多孔介質(zhì)中運(yùn)移過(guò)程示意

1.1 對(duì)流-彌散

微生物作為生物膠體,在多孔介質(zhì)中的遷移受地下水水力驅(qū)動(dòng),與溶質(zhì)運(yùn)移基本相似.微生物在隨孔隙水運(yùn)動(dòng)時(shí)會(huì)發(fā)生對(duì)流,受水力梯度,孔隙度和滲透率等影響[11].由于流場(chǎng)的不均一性以及膠體在通過(guò)多孔介質(zhì)時(shí)發(fā)生扭轉(zhuǎn),對(duì)流過(guò)程中也會(huì)產(chǎn)生彌散.由于布朗運(yùn)動(dòng)引起的擴(kuò)散與分子或膠體顆粒的質(zhì)量成反比,因此溶質(zhì)遷移程度比生物膠體高.流場(chǎng)具有復(fù)雜的孔隙網(wǎng)絡(luò),粒徑較小的微生物更易沿流線運(yùn)動(dòng)至滯留區(qū),在滯留區(qū)發(fā)生沉積.對(duì)于較大的微生物,相比于擴(kuò)散阻塞占主導(dǎo)地位.

1.2 孔隙阻塞

孔隙阻塞同樣被認(rèn)為是自然環(huán)境中影響微生物遷移的重要機(jī)制[12-13].孔隙阻塞是指多孔介質(zhì)的部分孔隙由于過(guò)小而不能使生物膠體通過(guò)的過(guò)程[14-15].孔隙阻塞速率系數(shù)與膠體和介質(zhì)直徑的比值呈線性正相關(guān)[15-16].介質(zhì)顆粒的形狀與大小具有不規(guī)則性,其表面粗糙程度也被認(rèn)為是影響孔隙阻塞發(fā)生的重要因素[17-20].自然條件下,由于顆粒組成復(fù)雜,多孔介質(zhì)的孔隙尺寸范圍較大[21].同時(shí)研究指出物理、化學(xué)因素以及流動(dòng)流體力學(xué)因素共同影響多孔介質(zhì)中的膠體孔隙阻塞[22].

1.3 吸附

在多孔介質(zhì)中孔隙的尺寸遠(yuǎn)大于微生物粒徑,因此微生物在多孔介質(zhì)中的主要的沉積行為為吸附[23].靜電作用力、范德華力、疏水作用力等共同影響微生物沉積于多孔介質(zhì)表面[24].基于DLVO 理論,微生物顆粒的吸附過(guò)程可以分為可逆吸附和不可逆吸附.可逆吸附是一種微生物與介質(zhì)之間相對(duì)較弱的相互作用,作用強(qiáng)度取決于溶液中離子強(qiáng)度及化學(xué)成分.不可逆吸附是一種涉及大量能量的永久相互作用.隨著離子強(qiáng)度的增加勢(shì)壘逐漸消失,微生物在介質(zhì)表面發(fā)生不可逆沉積[8].疏水相互作用是影響細(xì)菌粘附的一個(gè)重要因素[25].

1.4 沉積

沉積作用同樣被認(rèn)為是影響微生物在多孔介質(zhì)中沉積的重要影響因素[26].這里介紹兩種沉積機(jī)制(阻塞型沉積和熟化型沉積).阻塞型沉積是指當(dāng)微生物沉積于介質(zhì)表面時(shí)會(huì)減少其他細(xì)胞附著的附著面積[27].國(guó)外學(xué)者早期研究發(fā)現(xiàn),阻塞型沉積的發(fā)生會(huì)降低膠體對(duì)土壤的整體沉積,并且沉積發(fā)生時(shí)對(duì)介質(zhì)的覆蓋與膠體的類型有關(guān)[28].熟化型沉積是指已經(jīng)沉積于介質(zhì)的微生物作為額外的收集器為在多孔介質(zhì)中移動(dòng)的生物膠體提供沉積位點(diǎn).當(dāng)微生物沉積于介質(zhì)表面時(shí)會(huì)增加沉積點(diǎn)位進(jìn)而促進(jìn)其他膠體粒子的沉積.

2 微生物環(huán)境行為研究理論及檢測(cè)方法

描述微生物在多孔介質(zhì)中的行為特征與膠體相似,大多采用依據(jù)穿透曲線計(jì)算膠體質(zhì)量回收率,對(duì)膠體沉積與運(yùn)移進(jìn)行判斷[29-30].利用DLVO理論,判斷膠體在水溶液中的穩(wěn)定情況,進(jìn)而判斷膠體的沉積與運(yùn)移為微生物在多孔介質(zhì)中的運(yùn)移行為影響提供理論依據(jù)[31-32].除此之外隨著研究的不斷發(fā)展,在基本理論基礎(chǔ)之上,新的研究理論不斷豐富來(lái)支撐研究結(jié)果.在這里給出幾種經(jīng)典理論及最新理論研究.

2.1 Re及質(zhì)量回收率

R是生物膠體穿透曲線質(zhì)心和示蹤劑穿透曲線質(zhì)心的比值,通常反映了在整個(gè)實(shí)驗(yàn)過(guò)程(包括注入和沖洗階段)流速對(duì)微生物遷移的影響[33].微生物(或示蹤劑)的質(zhì)量回收率則表示細(xì)菌及病毒在實(shí)驗(yàn)過(guò)程中的流出端檢測(cè)到的遷移量占注入膠體總量的百分比,反映微生物在多孔介質(zhì)遷移過(guò)程中沉積量與遷移量的關(guān)系.本課題組前期在探究病毒在飽和多孔介質(zhì)的實(shí)驗(yàn)中,選用碘化鉀作為示蹤劑,并計(jì)算R值.結(jié)果表明隨著離子強(qiáng)度的增加R值隨之增加,說(shuō)明膠體病毒的遷移率逐漸降低[4].

除測(cè)量出流處質(zhì)量回收率外,近期研究中還對(duì)沉積在介質(zhì)上的膠體計(jì)算質(zhì)量回收率. Dong等[34]在遷移實(shí)驗(yàn)后,將石英砂分為10份放入帶有福爾馬林及NaOH的水溶液中震蕩30min,使吸附于介質(zhì)表面的大腸桿菌解吸,計(jì)算其質(zhì)量回收率.這種方法使得沉積量的計(jì)算更為精確,但也不排除微生物自身失活等帶來(lái)的誤差.

2.2 膠體過(guò)濾理論

膠體過(guò)濾理論基于一維對(duì)流擴(kuò)散方程來(lái)研究膠體在多孔介質(zhì)中的遷移規(guī)律.經(jīng)典膠體過(guò)濾理論定量比較了生物膠體在介質(zhì)上的附著情況,用膠體沉積速率常數(shù)(att)反映膠體沉積速率的大小[35]. Syngouna等[36]在探究人腺病毒與黏土膠體和二氧化鈦納米顆粒協(xié)同遷移的實(shí)驗(yàn)中,利用膠體過(guò)濾理論判斷病毒在多孔介質(zhì)表面的沉積情況.計(jì)算結(jié)果表明,在協(xié)同條件下碰撞效率隨流速增加而增大.說(shuō)明在較大流速下,病毒在被介質(zhì)吸附的概率更大.

然而研究表明經(jīng)典膠體過(guò)濾理論也有一定的局限性.Sasidharan等[37]認(rèn)為依據(jù)膠體過(guò)濾理論研究細(xì)菌在多孔介質(zhì)中的遷移和滯留時(shí)通常假設(shè)影響因素與流速無(wú)關(guān).然而隨著研究發(fā)現(xiàn),流速明顯影響膠體過(guò)濾理論中的眾多參數(shù).

2.3 DLVO理論及XDLVO理論

DLVO理論常被用來(lái)描述生物膠體在多孔介質(zhì)中的穩(wěn)定性[38].膠體顆粒之間或顆粒與介質(zhì)之間存在相互吸引的范德華力和相互排斥的靜電斥力.兩者之間的合力反映膠體顆粒是發(fā)生吸附聚沉還是保持分散穩(wěn)定的狀態(tài)[33].Syngouna等[39]指出當(dāng)總的勢(shì)能為正,表明膠體間為斥力,膠體在懸浮液中較為穩(wěn)定,不易發(fā)生沉積.勢(shì)壘的變化與介質(zhì)表面粗糙度密切相關(guān).介質(zhì)表面越粗糙,微生物與介質(zhì)之間的勢(shì)壘越低[40].

研究發(fā)現(xiàn)由于存在范德華力及靜電力以外的作用力,傳統(tǒng)的DLVO理論并不能很好地解釋實(shí)驗(yàn)結(jié)果.隨著探究的不斷深入,國(guó)內(nèi)外專家學(xué)者提出了擴(kuò)展DLVO(XDLVO)理論. Attinti等[41]用DLVO及XDLVO理論討論了人類腸道病毒在有針鐵礦包覆的砂表面沉積情況.Park等[31]利用DLVO理論與XDLVO理論表征大腸桿菌噬菌體在氧化鐵包覆的砂上沉積情況,其計(jì)算結(jié)果沒(méi)有明顯不同.事實(shí)上XDLVO理論中除考慮靜電力及范德華力外還考慮疏水作用力,對(duì)比可知在沉積過(guò)程中疏水作用力不起主要作用.Bai等[42]在討論在非飽和條件下細(xì)菌多孔介質(zhì)中的沉積發(fā)現(xiàn)初級(jí)勢(shì)阱不存在,表明細(xì)菌在固-水及氣-水界面上不會(huì)發(fā)生沉積.

2.4 檢測(cè)方法

微生物在多孔介質(zhì)中遷移研究除用到上述的理論基礎(chǔ)外,還有許多檢測(cè)手法來(lái)表征膠體在多孔介質(zhì)中的遷移行為.電鏡掃描(SEM)是常見測(cè)試手法,主要用來(lái)觀察微觀條件下介質(zhì)顆粒表面特征,微生物自身形狀,及膠體在多孔介質(zhì)中的沉積情況[4].馬爾文檢測(cè)在微生物遷移研究中用來(lái)判斷不同條件下微生物表面電勢(shì)及粒徑大小[42].這里總結(jié)了微生物研究中常見的幾種檢測(cè)方法見表1.

在基礎(chǔ)的檢測(cè)方法上,一些新的檢測(cè)手段也被廣泛用于微生物遷移實(shí)驗(yàn)中.磁共振成像被用來(lái)研究微生物在微觀孔隙中的行為[43].原子力顯微鏡(AFM)具有高精度以及高靈敏度,被用來(lái)探究微生物的立體形態(tài)和結(jié)構(gòu)等[44].Wang等[45]用SEM及AFM檢測(cè)膠體的粒徑高度及形態(tài)等.在其他物質(zhì)的行為判斷中,一些學(xué)者利用三維熒光光譜技術(shù)探究如苯乙烯等有機(jī)物,以探究污水處理情況[46-47].三維熒光光譜技術(shù)在微生物方面的應(yīng)用有待驗(yàn)證.

表1 地下水中常見微生物檢測(cè)方法

3 微生物在多孔介質(zhì)中遷移影響因素

微生物在多孔介質(zhì)中遷移受諸多因素影響.除膠體自身性質(zhì)和介質(zhì)外,水動(dòng)力、水化學(xué)因素,及環(huán)境因素都會(huì)對(duì)微生物遷移產(chǎn)生影響[38].近年來(lái),國(guó)內(nèi)外專家除對(duì)單一因素開展更為細(xì)致深入的討論外,對(duì)多種因素協(xié)同作用進(jìn)行了分析研究,同時(shí)對(duì)微生物與其他膠體及污染物的協(xié)同作用也開展了大量探究.盡管如此微生物在多孔介質(zhì)中的遷移仍是極為復(fù)雜的過(guò)程,需要不斷深入思考.

3.1 膠體自身因素

關(guān)于微生物自身性質(zhì)對(duì)其在多孔介質(zhì)中遷移的影響,隨著探究的深入及拓展得到不斷發(fā)展.一些研究表明微生物自身形態(tài)及粒徑大小主要影響生物膠體在多孔介質(zhì)中的阻塞和吸附過(guò)程[53-54]. Pelley等[55]指出膠體粒徑越大與多孔介質(zhì)發(fā)生碰撞效率的機(jī)會(huì)越大.關(guān)于細(xì)胞自身結(jié)構(gòu)探究,早期Gannon等[53]認(rèn)為細(xì)胞在土壤中的遷移與其大小密切相關(guān),與鞭毛的存在無(wú)關(guān).近來(lái)一些學(xué)者通過(guò)研究發(fā)現(xiàn),鞭毛可使細(xì)胞自身具有運(yùn)動(dòng)能力,增加細(xì)胞與介質(zhì)表面的碰撞效率,從而增強(qiáng)細(xì)菌的吸附能力[56].其他類型生物膠體則主要靠水流或布朗運(yùn)動(dòng)向介質(zhì)表面移動(dòng)[57].

在病毒研究方面,除常見的大腸桿菌噬菌體(MS2,T-3,ΦX174)外,人腺病毒(hAdVs)也用作探究對(duì)象來(lái)研究病毒微生物在多孔介質(zhì)中的遷移. Kokkinos等[58]指出相同孔隙水流速實(shí)驗(yàn)條件下,人腺病毒的質(zhì)量回收率都小于大腸桿菌噬菌體.近些年海洋噬菌體作為新的指示劑來(lái)探究微生物在多孔介質(zhì)中的遷移.Ghanem等[6]通過(guò)5種海洋噬菌體及兩種常見大腸桿菌噬菌體探究噬菌體大小,形態(tài)及表面性質(zhì)對(duì)其在多孔介質(zhì)中遷移的影響.

3.2 水動(dòng)力及水化學(xué)因素

微生物在多孔介質(zhì)中遷移受水動(dòng)力條件影響,遷移行為隨擾動(dòng)強(qiáng)度的變化而變化.此外, Vasilladou等[59]研究發(fā)現(xiàn)較高的孔隙水速度會(huì)導(dǎo)致細(xì)菌在介質(zhì)中的停留時(shí)間縮短,從而降低細(xì)菌吸附的可能性.Masciopinto等[60]指出裂隙流的擾動(dòng)會(huì)導(dǎo)致病毒的再懸浮,土壤的不均一性也會(huì)使在裂隙水流中產(chǎn)生較強(qiáng)的瞬時(shí)解吸.

近年來(lái),對(duì)水動(dòng)力因素研究更為細(xì)致,研究發(fā)現(xiàn),流速影響眾多參數(shù)變化.Sasidharan等[61]從介質(zhì)表面的相互作用能、細(xì)胞停留時(shí)間和轉(zhuǎn)矩平衡等方面討論孔隙水流速影響大腸桿菌在多孔介質(zhì)中遷移行為.研究發(fā)現(xiàn)隨著滯留時(shí)間的增加及流速的減小,細(xì)胞與介質(zhì)表面的粘滯力增加.

微觀角度及生物化學(xué)角度應(yīng)用于解釋水化學(xué)條件對(duì)微生物遷移的影響.pH值的增加會(huì)導(dǎo)致細(xì)胞壁中羧基和氨基基團(tuán)的離解,從而使細(xì)胞表面帶負(fù)電荷.使微生物在多孔介質(zhì)中的沉積量降低[62]. Zhang等[63]認(rèn)為離子強(qiáng)度的增加會(huì)促進(jìn)細(xì)菌及病毒的吸附,主要原因是由于離子強(qiáng)度的增加會(huì)在生物沉積過(guò)程中使勢(shì)壘衰減.多價(jià)陽(yáng)離子具有增強(qiáng)細(xì)胞吸附和減少細(xì)胞遷移的能力[64-65].與單價(jià)陽(yáng)離子相比,二價(jià)離子(如Mg2+和Ca2+)的存在明顯減弱了細(xì)菌的轉(zhuǎn)運(yùn)[66].

由于地下水環(huán)境的復(fù)雜性,對(duì)其他鹽溶液也進(jìn)行了討論研究.在考慮離子強(qiáng)度對(duì)細(xì)菌遷移影響的同時(shí),探討多種因素的協(xié)同作用機(jī)制.Dong等[34]研究表明懸浮液中的硅酸鹽存在使細(xì)菌在無(wú)鐵包覆的石英砂中遷移量減少.相比之下,在有鐵礦物包覆砂中遷移量增加.原因推測(cè)為硅酸鹽離子會(huì)與細(xì)菌爭(zhēng)奪有鐵礦物包裹砂上的吸附點(diǎn)位.

3.3 介質(zhì)顆粒因素

介質(zhì)顆粒大小及介質(zhì)表面性質(zhì)同樣影響微生物在多孔介質(zhì)中的遷移.Chrysikopoulos等[67]認(rèn)為MS2和ΦX174在石英砂上的沉積主要取決于石英砂的粒徑大小,噬菌體的沉積與介質(zhì)顆粒的大小呈負(fù)相關(guān).

多孔介質(zhì)表面結(jié)構(gòu)復(fù)雜多樣.一些多孔介質(zhì)表面含有金屬氧化物等礦物改變介質(zhì)表面電荷,對(duì)微生物的吸附和遷移具有影響[31,62].Yang等[68]研究發(fā)現(xiàn)附著在多孔介質(zhì)表面的有機(jī)物會(huì)增加陽(yáng)離子交換能力,從而影響細(xì)菌的遷移.同時(shí)Dong等[69]指出在硅酸鹽會(huì)與大腸桿菌爭(zhēng)奪鐵礦包覆砂上的吸附點(diǎn)位,促進(jìn)細(xì)菌的遷移.Bozorg等[70]研究發(fā)現(xiàn),相比于單一的玻璃珠,有生物膜包裹的玻璃珠對(duì)微生物的吸附量更大.

天然環(huán)境下,由于粘土顆粒的比表面積較高,使微生物與土壤顆粒間的作用力較強(qiáng)[59,71-72].Balkhair等[73]指出,相比于農(nóng)業(yè)土壤,糞便細(xì)菌在城市土壤的沉積量更多,推測(cè)可能由于城市土壤大多為粒徑較小的砂,介質(zhì)面積更大有利于細(xì)菌的沉積.同時(shí)相比于農(nóng)業(yè)環(huán)境,城市中水流速較低同樣有利于細(xì)菌沉積.

3.4 其他因素

微生物濃度同樣影響生物膠體在多孔介質(zhì)中的遷移和沉積[74].Zhang等[75]認(rèn)為,病毒在介質(zhì)表面沉積量是濃度的函數(shù),增加注入病毒的濃度可以增加膠體在介質(zhì)表面的沉積.溫度是決定吸附動(dòng)力學(xué)和熱力學(xué)的關(guān)鍵因素.溫度影響細(xì)胞表面疏水性、細(xì)胞外聚合物在細(xì)菌表面的構(gòu)造[23,76-77].同時(shí)溫度對(duì)微生物的活性也具有影響[8].Chrysikopoulos等[67]指出,在靜態(tài)條件下,溫度對(duì)石英砂吸附病毒有顯著影響.

表面活性劑具有修飾細(xì)菌細(xì)胞和多孔介質(zhì)表面性質(zhì)的能力成為研究熱點(diǎn).Grna等[78]和Zhang等[79]指出表面活性劑降低了細(xì)菌的表面疏水性,增強(qiáng)了細(xì)菌在多孔介質(zhì)中的遷移能力.由于細(xì)胞表面存在羧基和磷酸鹽等陰離子表面基團(tuán),細(xì)菌細(xì)胞表面帶負(fù)電荷.離子表面活性劑可以通過(guò)附著在細(xì)胞表面改變細(xì)菌細(xì)胞表面電荷,來(lái)中和或增強(qiáng)細(xì)胞表面的原始表面電荷[80].

4 微生物在多孔介質(zhì)中的遷移

地下水環(huán)境結(jié)構(gòu)復(fù)雜,微生物作為生物膠體,既可以獨(dú)自在多孔介質(zhì)中遷移,也可與污染物結(jié)合通過(guò)多孔介質(zhì)到達(dá)地下含水層[81].研究微生物在多孔介質(zhì)中的沉積遷移情況,對(duì)保護(hù)地下水資源,實(shí)現(xiàn)地下水修復(fù)具有重要意義.圖2為微生物在對(duì)飽和多孔介質(zhì)及非飽和多孔介質(zhì)中遷移示意.

圖2 微生物在對(duì)飽和多孔介質(zhì)和非飽和多孔介質(zhì)中遷移示意

4.1 微生物在飽和多孔介質(zhì)中遷移

飽和多孔介質(zhì)主要指地下水系統(tǒng)中的飽水帶,又稱飽和帶.飽水帶巖石孔隙全部為液態(tài)水充滿,且水體是連續(xù)分布的,在具有水頭差的作用下可發(fā)生連續(xù)運(yùn)動(dòng).在飽和多孔介質(zhì)條件下,不同水動(dòng)力條件及不同水化學(xué)條件成為影響膠體遷移的主要影響因素[12,61,82-83].微生物在飽和多孔介質(zhì)中的沉積和運(yùn)移行為一直受到眾多學(xué)者的關(guān)注與研究.

從理論上講,微生物隨流體在介質(zhì)中運(yùn)移時(shí),受水動(dòng)力的影響會(huì)沿流體的流線方向運(yùn)動(dòng),當(dāng)流線與介質(zhì)顆粒表面之間的距離小于微生物的半徑時(shí),細(xì)菌及病毒被攔截于介質(zhì)表面.微生物的布朗運(yùn)動(dòng)也會(huì)使自身接近于介質(zhì)表面時(shí),發(fā)生吸附.微生物還受到重力的影響沉積于介質(zhì)表面,即重力沉降[29].對(duì)微生物在飽和多孔介質(zhì)中的遷移規(guī)律研究大多在實(shí)驗(yàn)室利用柱子實(shí)驗(yàn)進(jìn)行.理論研究及實(shí)驗(yàn)研究都探討了孔隙水溶液化學(xué),流體速度,介質(zhì)結(jié)構(gòu),溫度等諸多條件的作用[61,67,84].微生物的遷移能力與其表面電荷密切相關(guān)[85].研究表明, pH值在2~8的范圍內(nèi),微生物普遍帶負(fù)電[86].微生物表面負(fù)電荷減少,降低微生物與介質(zhì)間的斥力,抑制了細(xì)胞的遷移. Huysman等[87]研究發(fā)現(xiàn)表面疏水的細(xì)菌及病毒能更有效的吸附于介質(zhì)表面.微生物的穩(wěn)定性也受表面疏水性影響[88].若微生物為疏水性膠體則較不穩(wěn)定,易聚集成大顆粒,影響膠體在飽和多孔介質(zhì)中的遷移[89].

Syngouna等[36]指出在飽和多孔介質(zhì)條件下人腺病毒的遷移受黏土及納米顆粒影響,協(xié)同遷移的過(guò)程中,隨著流速增加,膠體顆粒的碰撞效率也隨之增加.Zhang等[90]對(duì)飽和條件下病毒與針鐵礦間的相互作用開展了室內(nèi)實(shí)驗(yàn).研究表明,隨著溶液pH值的降低,噬菌體在針鐵礦上的附著量增加.王芳等[91]指出水體中膠體對(duì)銅綠微囊藻生長(zhǎng)具有促進(jìn)作用.朱維晃等[92]探討了微生物異化還原針鐵礦膠體的動(dòng)力學(xué)特征及影響因素為有機(jī)物及重金屬造成的水環(huán)境污染提供參考.

4.2 微生物在非飽和多孔介質(zhì)中遷移

微生物通常由土壤表面或隨污水廢水,化糞池系統(tǒng)等進(jìn)入地下環(huán)境.相比于飽和地下水系統(tǒng),微生物在非飽和系統(tǒng)下的沉積及遷移行為更加復(fù)雜[32].除了在飽和多孔介質(zhì)中發(fā)生的沉積機(jī)制外,由于氣相的存在為膠體沉積提供了新的吸附位點(diǎn)會(huì)使病毒的遷移與沉積發(fā)生改變[93-94].微生物在非飽和多孔介質(zhì)中的行為本質(zhì)上是與空氣、水和固體顆粒相互接觸的行為.此外,在非飽和多孔介質(zhì)中,微生物可以在包裹于介質(zhì)顆粒的薄膜中保留下來(lái)[95-96].

微生物在非飽和多孔介質(zhì)中的遷移同樣受到水動(dòng)力和水化學(xué)因素的影響.當(dāng)非飽和多孔介質(zhì)存在時(shí),瞬態(tài)效應(yīng)更加復(fù)雜.非飽和介質(zhì)中存在優(yōu)先流,同時(shí)非均勻性會(huì)引起流速的較大變化增加擴(kuò)散.Mitropoulou等[32]研究了不同飽和條件下親水性乳膠粒的遷移和沉積行為,發(fā)現(xiàn)隨著含水量的降低,膠體沉積速率增加.地下水中瞬變流的條件,及變飽水度對(duì)病毒沉積于固-水界面及氣-水界面產(chǎn)生連續(xù)再分配.微生物顆粒之間的相互作用可以聚集成團(tuán)運(yùn)移或吸附于介質(zhì)表面,也可以與多孔介質(zhì)體系中的氣泡一同運(yùn)移.人們生產(chǎn)生活中的農(nóng)業(yè)灌溉,人工回灌及自然界中的降雨入滲,也會(huì)對(duì)微生物形成再分配進(jìn)而影響微生物的遷移過(guò)程.

國(guó)外學(xué)者開展了病毒在飽水帶和包氣帶遷移行為研究的室內(nèi)柱實(shí)驗(yàn),實(shí)驗(yàn)表明由于氣體的存在,病毒在非飽和介質(zhì)中的沉積量明顯高于飽和介質(zhì)中的沉積量.并且介質(zhì)含水量越低,沉積的病毒量越多[30].Torkzaban等[97]和Kim等[98]針對(duì)氣-水界面在非飽和介質(zhì)中對(duì)微生物沉積的影響展開了實(shí)驗(yàn)研究,結(jié)果表明靜電作用對(duì)微生物在氣-水界面上沉積的影響比微生物自身的親疏水性要強(qiáng).其他探究指出,溶液的離子強(qiáng)度會(huì)顯著影響生物膠體的沉積和遷移[99].隨著離子強(qiáng)度的降低,細(xì)菌對(duì)帶負(fù)電荷石英砂的粘附效率和粘附率降低[100].這是由于細(xì)菌單收集器表面的雙電層厚度增加.

5 微生物在多孔介質(zhì)中遷移進(jìn)展討論與展望

隨著研究的不斷發(fā)展,對(duì)微生物在多孔介質(zhì)中遷移行為的研究更為深入,研究角度更為開闊.由于地下環(huán)境中物質(zhì)的復(fù)雜多樣性,除常見的影響因素外,一些專家學(xué)者考慮到多學(xué)科交叉影響,對(duì)生物因素也進(jìn)行了討論.Zhong等[101]討論了低濃度的鼠李糖脂對(duì)銅綠假單胞菌在理想多孔介質(zhì)種的遷移影響.實(shí)驗(yàn)過(guò)程中選取表面親水及表面疏水的玻璃珠用作填充介質(zhì).結(jié)果表明表面疏水介質(zhì)對(duì)細(xì)菌在多孔介質(zhì)表面沉積具有重要作用,低濃度的鼠李糖脂促進(jìn)細(xì)菌在介質(zhì)中的遷移.Dan等[102]還探究了雙酚A(Bisphenol A)對(duì)細(xì)菌在石英沙中遷移的影響,結(jié)果表明分子疏水性使雙酚A與細(xì)菌競(jìng)爭(zhēng)介質(zhì)表面的吸附位點(diǎn)增加細(xì)菌在多孔介質(zhì)中的遷移量.袁瑞強(qiáng)等[103]認(rèn)為在不利的吸附條件下,通過(guò)提高環(huán)境的pH值可以使吸附在初級(jí)勢(shì)阱中的膠體解吸下來(lái),從而降低膠體的吸附能力.

依據(jù)已有的室內(nèi)實(shí)驗(yàn)研究,微生物在多孔介質(zhì)中的行為在實(shí)際場(chǎng)地進(jìn)行開展.Kvitsand等[52]認(rèn)為在流速較快的場(chǎng)地條件,寒冷的氣候中和氧化物的存在阻礙了病毒的遷移,同時(shí)降低了病毒的活性.同時(shí)場(chǎng)地環(huán)境中存在優(yōu)先流以及污染物的可逆吸附會(huì)增加污染物運(yùn)移的風(fēng)險(xiǎn).在低氧環(huán)境下微生物的去除是有限的.Hornstra等[104]研究表明,在低氧條件下,遷移時(shí)間、垂向遷移距離以及孔隙水流速均對(duì)病毒的遷移具有影響.同時(shí)微模型及流通池被用來(lái)研究細(xì)菌及病毒的沉積與解吸[105-107].

對(duì)于今后微生物在多孔介質(zhì)中的遷移研究,應(yīng)在已有的研究理論基礎(chǔ)上繼續(xù)豐富研究方法和測(cè)試手段,為微生物在多孔介質(zhì)中遷移提供可靠依據(jù).同時(shí)多種因素的共同作用對(duì)微生物的遷移影響仍需進(jìn)一步探究.相比于飽和多孔介質(zhì),微生物在非飽和多孔介質(zhì)中的遷移更為復(fù)雜.由于氣-水界面的存在,對(duì)微生物在多孔介質(zhì)中的遷移有膠體影響.探究不同水動(dòng)力水化學(xué)因素及環(huán)境因素對(duì)生物膠體在非飽和多孔介質(zhì)中的遷移影響對(duì)地下水污染修復(fù)具有重大意義.微生物與其他膠體及污染物的協(xié)同遷移同樣對(duì)地下水污染治理具有重要意義.在修復(fù)已污染的地下水過(guò)程中,地下水的保護(hù)及污染物的源頭治理更為重要,應(yīng)引起廣泛關(guān)注.

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致謝：對(duì)課題組所有科研工作人員,及從事污染水文地質(zhì)的科研人員表示感謝.

Transport behavior of microorganism in the porous media.

ZHANG Wen-jing1,2*,QIN Yun-qi1,2, LIU-Dan1,2, MA Tian-yi1,2, LI Xiao-fei1,2

(1.Key Laboratory of Groundwater Resources and Environment, Ministry of Education, Jilin University, Changchun 130021, China；2.College of New Energy and Environment, Jilin University, Changchun 130021, China)., 2018,38(10)：3975~3984

Understanding the fate and transport of pathogenic microorganisms in porous media is of great significance to protect water resources. On the basis of the previous research at home and abroad, the migration behaviour of microorganisms in porous media has been summarized. Several studies have shown that, pathogenic microorganisms as a kind of biocolloid, its transport will be controlled by a series of complex mechanism. In addition to the common hydrodynamic and hydrochemical conditions, microorganisms are also affected by itself characteristics, medium particles and environmental factors during the migration process. On this basis, the migration behaviours of microorganisms in saturated porous media and unsaturated porous media have been discussed, and the research focus in recent years have also been briefly summarized. At present, a great number of related researches about this topic have been published. However, due to the complexity of underground environment and the variability of microorganisms, it is necessary to further discuss the remediation of microorganism pollution in groundwater system.

microorganism；porous medium；migration mechanism；biocolloid；transport and deposition

X172

A

1000-6923(2018)10-3975-10

張文靜(1980-),女,山東臨沂人,教授,博士,主要從事污染水文地質(zhì)方面研究.發(fā)表論文20篇.

2018-03-24

國(guó)家自然科學(xué)基金資助項(xiàng)目(41472215,41877175);高等學(xué)校學(xué)科創(chuàng)新引智計(jì)劃資助項(xiàng)目(B16020)

* 責(zé)任作者, 教授, zhangwenjing80@ hotmail.com