張永祥,杜 偉,李雅君,趙崇輝

納米零價(jià)鐵在水處理中的應(yīng)用研究綜述

張永祥*,杜 偉,李雅君,趙崇輝

(北京工業(yè)大學(xué)城市建設(shè)學(xué)部,北京 100124)

納米零價(jià)鐵(nZVI)作為最常用的納米顆粒之一,在去除環(huán)境水體中的污染物方面開(kāi)展了大量的研究.本篇綜述通過(guò)系統(tǒng)全面地總結(jié)nZVI的相關(guān)進(jìn)展,介紹nZVI的各個(gè)方面進(jìn)而指導(dǎo)其發(fā)展方向.其中,研究?jī)?nèi)容主要包括制備方法、改性方式、去除不同水中污染物的作用機(jī)理和催化機(jī)理、在場(chǎng)地研究中的應(yīng)用以及毒性作用機(jī)制.本文發(fā)現(xiàn),納米零價(jià)鐵仍存在缺少綜合評(píng)價(jià)方法、應(yīng)用受限、各項(xiàng)研究不同步等問(wèn)題.nZVI未來(lái)的發(fā)展,應(yīng)具備考慮反應(yīng)性、穩(wěn)定性、遷移性、毒性的評(píng)價(jià)方法,避免同一改性材料在不同研究方向的時(shí)間差異性,讓nZVI的應(yīng)用更好地適用實(shí)地研究.

納米零價(jià)鐵；材料制備；材料改性；作用機(jī)理；致毒機(jī)理

近年來(lái),納米零價(jià)鐵(nZVI)研究廣泛.人工制備的納米零價(jià)鐵由于易被氧化,會(huì)在鐵殼外形成氧化層,構(gòu)成核殼結(jié)構(gòu).核殼結(jié)構(gòu)成分復(fù)雜包括鐵礦(FeOOH)、方鐵礦(FeO)、磁鐵礦(Fe3O4)、磁赤鐵礦(gFe2O3)和赤鐵礦(a-Fe2O3)[1].氧化物外殼的存在不僅避免了nZVI鐵核的腐蝕,也會(huì)轉(zhuǎn)移鐵殼電子到活性位點(diǎn)上的污染物,實(shí)現(xiàn)還原作用.但nZVI的單獨(dú)使用難以滿足復(fù)雜環(huán)境修復(fù)的需要,通過(guò)改性方式等可發(fā)展基于nZVI的多功能修復(fù)材料.現(xiàn)有的改性方式研究頗多,不同改性方式也兼具不同優(yōu)勢(shì).從工程角度來(lái)看,直接在實(shí)際應(yīng)用中使用納米級(jí)的材料會(huì)存在操作問(wèn)題,且循環(huán)使用性能差,可能對(duì)接收的生態(tài)系統(tǒng)構(gòu)成潛在風(fēng)險(xiǎn)[2],這就需要對(duì)nZVI可能產(chǎn)生的環(huán)境問(wèn)題進(jìn)行整理匯總,指導(dǎo)nZVI的實(shí)際應(yīng)用.已有研究[3]總結(jié)了實(shí)地應(yīng)用中nZVI對(duì)微生物群落的影響,展示出考察nZVI遷移能力對(duì)于研究其毒性的重要性.盡管存在一定應(yīng)用風(fēng)險(xiǎn),nZVI因其優(yōu)點(diǎn),如今已經(jīng)廣泛應(yīng)用于污染物去除,例如:重金屬、抗生素、鹵代污染物;nZVI懸浮液也被注入地下水中,開(kāi)展場(chǎng)地治理,成為環(huán)境科學(xué)近年來(lái)的熱點(diǎn)之一.根據(jù)研究分析,現(xiàn)有nZVI綜述整理歸納各類(lèi)反應(yīng)機(jī)理并不全面,缺少對(duì)不同污染物的論述和反應(yīng)式[4-5].綜上,本文旨在回顧nZVI制備和改性方法,并輔以反應(yīng)方程式;評(píng)估改性nZVI的性能,整理并清晰闡述nZVI對(duì)于不同污染物的去除機(jī)理以及nZVI在高級(jí)氧化中的催化作用.此外,概述nZVI的實(shí)地應(yīng)用中的運(yùn)作模式和影響因素,描述nZVI對(duì)細(xì)胞的毒性機(jī)理,探究不同因素的影響,整體介紹nZVI的發(fā)展現(xiàn)狀.

1 納米零價(jià)鐵的制備方法

納米零價(jià)鐵的制備方法根據(jù)nZVI的形態(tài)變化可分為分解法和合成法兩類(lèi)[6].分解法是將大顆粒鐵轉(zhuǎn)為納米級(jí)別的顆粒.合成法則相反,將分子級(jí)別甚至更小的鐵,轉(zhuǎn)化為納米級(jí).

常見(jiàn)的制備方法見(jiàn)表1,除了表1介紹方法外,nZVI制備方法還有很多,比如易于調(diào)節(jié)顆粒形態(tài)的氣相沉積法、增強(qiáng)顆粒穩(wěn)定性的蒸汽冷凝法等[5].不同方法不僅影響了nZVI的粒徑和表面積,并且對(duì)nZVI最終的形態(tài)也有影響.球磨法因?yàn)檠心スに嚩a(chǎn)生了薄片狀的納米顆粒,這明顯區(qū)別于其它方法.綠色合成法制備的nZVI生態(tài)安全性更加可靠,且制備材料可再生,經(jīng)濟(jì)性良好.激光熱解法易得到更小粒徑的納米顆粒.

表1 nZVI制備方法匯總

1.1 液相還原法

液相還原法是目前實(shí)驗(yàn)室規(guī)模研究制備nZVI最普遍的方法.但成本高、存在環(huán)境風(fēng)險(xiǎn)制約著大規(guī)模的生產(chǎn)應(yīng)用.并且在還原過(guò)程中,nZVI顆粒易摻入雜質(zhì).液相還原法反應(yīng)原理是通過(guò)BH4-還原水中的Fe2+或Fe3+,在水中生成懸濁的nZVI,或直接還原在負(fù)載材料的表面,實(shí)現(xiàn)負(fù)載改性,見(jiàn)式(1)～(2)[15].值得注意的是BH4-與水同樣可以反應(yīng),見(jiàn)式(3)～(4)[16],這會(huì)影響nZVI的產(chǎn)量.此外,Fan等[17]將FeSO4·7H2O和KBH4混合研磨,將固體混合物加入淀粉溶液中,通過(guò)流變相法制備出了分散性和穩(wěn)定性良好的材料.在制備過(guò)程中也可加入乙醇,構(gòu)建醇水體系,可降低溶液的電離常數(shù),提高nZVI的負(fù)載率.乙醇不僅可以防止nZVI氧化,而且隨著乙醇含量在去離子水和乙醇共溶劑溶液中分別增加,nZVI的表面積顯著增加[18].但實(shí)驗(yàn)室通過(guò)液相還原法制備的nZVI,多采用磁鐵進(jìn)行分離,這可能會(huì)影響nZVI的飽和磁化強(qiáng)度[19].

溶劑熱法是液相還原法的一種分支方法,在封閉系統(tǒng)改變反應(yīng)過(guò)程的溫度與壓力,更利于nZVI的生成.相對(duì)于NaBH4,溶劑熱制備nZVI時(shí),水合肼(N2H4·H2O)的化學(xué)性質(zhì)更穩(wěn)定,成本更低,并產(chǎn)生N2,見(jiàn)式(5)[20].

盡管液相還原法存在一定的成本問(wèn)題,但估計(jì)未來(lái)的一段時(shí)間液相還原法仍是制備nZVI的主要方法.一方面是因?yàn)閷?shí)驗(yàn)室規(guī)模的研究仍占據(jù)大多數(shù);另一方面,液相還原法是與改性方式最有效的結(jié)合手段,比如負(fù)載改性、雙金屬改性、硫化改性以及表面改性都存在與液相還原法相結(jié)合的方式.

1.2 機(jī)械球磨法

機(jī)械球磨法是在球磨機(jī)中,通過(guò)研磨球和原料之間不斷的摩擦、碰撞,從而得到更小平均粒徑顆粒的方法.與液相還原法相比,機(jī)械球磨法具有更好的經(jīng)濟(jì)性,更差的環(huán)境性,這是因?yàn)檠心シ椒〞?huì)因生產(chǎn)過(guò)程中的高能耗而導(dǎo)致較大的環(huán)境影響[21].機(jī)械球磨法具有巨大的應(yīng)用潛力,主要體現(xiàn)在便于nZVI的大規(guī)模生產(chǎn),但隨著nZVI正向著多功能材料方向發(fā)展,機(jī)械球磨法難以與改性方式相結(jié)合是限制其應(yīng)用的關(guān)鍵一步.機(jī)械球磨法包含兩種方法.一種是干磨,球磨過(guò)程中會(huì)與惰性氣體介質(zhì)接觸,干磨比濕磨可以更有效地減小顆粒[22],但干磨過(guò)程中,磨球和原料容易粘附在一起[23],此時(shí)可以及時(shí)分離,避免過(guò)磨.另一種是濕磨,濕磨可以在水中或非水介質(zhì)中(如單乙二醇).其中使用水溶液時(shí),鐵顆粒和水會(huì)發(fā)生氧化反應(yīng),見(jiàn)式(6)[24],可能導(dǎo)致nZVI的產(chǎn)量大幅下降.為了減少氧化過(guò)程,需要更加可控的反應(yīng)介質(zhì)[25].而用乙二醇研磨的過(guò)程中,顆粒球磨時(shí)無(wú)法與空氣接觸以防止氧化,不會(huì)發(fā)生反應(yīng)和脆化,顆粒在研磨介質(zhì)的作用下形成薄片,形狀不規(guī)則[26].在研磨過(guò)程中,研磨環(huán)境對(duì)控制最終顆粒形態(tài)、尺寸和相演變有很大影響.Ribas等[26]發(fā)現(xiàn)在非水介質(zhì)中,球磨時(shí)加入超細(xì)氧化鋁能改善球磨過(guò)程,可以有效減少研磨過(guò)程中的殘余薄片,增多了活性位點(diǎn)和比表面積.這種研磨產(chǎn)生的顆粒沒(méi)有連續(xù)且厚的氧化層,其穩(wěn)定性和流動(dòng)性遠(yuǎn)高于商業(yè)顆粒[9].

Fe0+2H2O?Fe2++H2+2OH-(6)

1.3 微乳液法

微乳液法是利用金屬鹽和一定的沉淀劑形成微乳液,通過(guò)控制膠粒成核生長(zhǎng),熱處理后得到納米微粒的方法[27].微乳液具有高的包封無(wú)機(jī)粒子的能力,消除了有害的有機(jī)溶劑,并形成高度均勻的微結(jié)構(gòu),所獲得的納米粒子尺寸接近于作為納米反應(yīng)器的原始單體液滴的尺寸[28].但是該方法制備工藝較為復(fù)雜,對(duì)操作有一定要求.

1.4 電沉積法

電沉積法是通過(guò)電解含有Fe3+/Fe2+的電解質(zhì),使得nZVI在惰性陰極沉積的生產(chǎn)方法.該過(guò)程在電弧的作用下,電極表面金屬汽化,遇水冷卻在電極上,形成nZVI.但產(chǎn)生的納米級(jí)粒子傾向于聚集在一起,在電極上納米粒子可以聚結(jié)以增加其尺寸并降低其反應(yīng)性[11].因此在過(guò)程中可以添加分散劑、表面活性劑、通過(guò)超聲波震蕩輔助來(lái)避免團(tuán)聚.Long等[12]通過(guò)電沉積法將Fe/Pb負(fù)載在活性炭電極上,顆粒分布均勻,電極切面干凈.區(qū)別于液相還原法的分步制備法,電沉積法可以將Fe和Pb同時(shí)負(fù)載在電極材料上,但電沉積法的負(fù)載過(guò)程能耗更高,并且對(duì)要做陰極的負(fù)載材料有更高的要求.此外,在實(shí)地研究中通常需要幾十千克的nZVI,通過(guò)電沉積法制備不僅能耗大,而且改性時(shí)需要消耗大量的陰極材料.因此,近幾年通過(guò)電沉積法制備nZVI的研究很少.Huang等[29]開(kāi)發(fā)了一種全新的電沉積方法,首先將納米顆粒沉積到連續(xù)旋轉(zhuǎn)的金屬箔,然后通過(guò)機(jī)械分層收集,進(jìn)而合成了銅、金、鎳等納米顆粒.這種方法中的電解液可以重復(fù)利用,降低了生產(chǎn)成本,例如納米銀的工業(yè)生產(chǎn)成本降低了90%.但最終是否能應(yīng)用于易團(tuán)聚的nZVI,有待考察和驗(yàn)證.

1.5 綠色合成方法

綠色合成方法多是采用植物中提取的天然化合物還原原材料,使Fe2+或Fe3+離子與提取物中的酚(-OH)基團(tuán)、蛋白質(zhì)或有機(jī)酸反應(yīng),制備nZVI的方法.采用這些物質(zhì)制備的材料具有一定的抗氧化性,同時(shí)也為合成nZVI提供分散劑和掩蔽劑,防止nZVI在合成過(guò)程中聚集和氧化[30].除此之外,細(xì)菌、病毒、真菌和藻類(lèi)等微生物也可用于綠色合成nZVI[31],但通過(guò)培養(yǎng)微生物生產(chǎn)時(shí)效性差并且nZVI的產(chǎn)量也很難控制.總體上,綠色合成法其顯著特點(diǎn)是價(jià)格低廉、環(huán)境友好、可再生,并且避免了高能耗的高壓高溫條件,為nZVI的大規(guī)模生產(chǎn)提供了一種思路,目前具有極大的應(yīng)用潛力.Martins等[32]進(jìn)行生命周期評(píng)估研究,其中經(jīng)濟(jì)評(píng)價(jià)表明傳統(tǒng)的合成方法比綠色合成方法成本約高出8倍,且綠色合成方法相比傳統(tǒng)方法對(duì)環(huán)境的影響更低,大約降低50%.但有研究表明從不同植物提取物中合成的納米顆粒在形狀、大小和特性上都不同,這改變了其從水溶液中去除污染物的能力[13].盡管針對(duì)從植物提取物中綠色合成納米鐵顆粒進(jìn)行了許多研究,但這些過(guò)程中涉及的活性成分在很大程度上仍然未知[31].這將很大程度上影響nZVI作用機(jī)理的探究,所選材料中許多未知基團(tuán)可能會(huì)被合成在nZVI表面進(jìn)而影響nZVI反應(yīng)性和復(fù)雜性.

1.6 碳熱還原法

碳熱還原法是在高溫下(溫度>600℃),以碳為還原劑,將含鐵化合物還原成nZVI的方法[33].這種方法的特點(diǎn)之一是碳既是nZVI的還原劑,又可做nZVI的負(fù)載材料.而碳材料具有高表面積、豐富的官能團(tuán)和良好的化學(xué)穩(wěn)定性,比其它材料更加優(yōu)異[34].制備過(guò)程中,影響復(fù)合材料制備的重要因素包括含鐵化合物的類(lèi)型、合適的鐵碳比以及制備溫度.常用的鐵化合物包括Fe3O4、Fe2(SO4)3、Fe(NO3)3·9H2O[35].以Fe3O4為例,可表述為式(7)[36].Meng等[37]發(fā)現(xiàn)鐵碳比太高太低都不利于nZVI的生成,而合適的鐵碳比有利于nZVI在碳骨架內(nèi)部或外部具有合理排列方式,這可能對(duì)鐵基炭復(fù)合材料的反應(yīng)性起重要作用.此外,制備過(guò)程中的高熱反應(yīng)溫度會(huì)造成孔隙坍塌,降低復(fù)合材料的比表面積[38],但低熱反應(yīng)溫度可能降低nZVI的產(chǎn)量.此外,有機(jī)物在一定程度上可能對(duì)nZVI的產(chǎn)量起負(fù)面作用,因?yàn)樗赡鼙Ｗo(hù)磁鐵礦核心或誘導(dǎo)鐵物種的氧化反應(yīng)[33].與常規(guī)液相還原法相比,它具有以下優(yōu)點(diǎn):碳能夠承載大量的nZVI,并且nZVI在空氣中的穩(wěn)定性得到提高[39].

1.7 輔助方法

超聲輔助是最常用的輔助方法.通過(guò)液相還原制備時(shí),不斷提高超聲功率,nZVI顆粒結(jié)構(gòu)由球形變?yōu)榘鍫?再變?yōu)獒槧頪8].超聲波的空化作用會(huì)讓水熱解產(chǎn)生·H自由基,進(jìn)而可以與含硫有機(jī)物生成H2S.這一過(guò)程可以被用于改性硫化納米零價(jià)鐵的制備.另外,微波輔助也可用于機(jī)械球磨過(guò)程,通過(guò)微波和球磨的協(xié)同作用,在碰撞和剝落過(guò)程中不斷產(chǎn)生大量高活性的nZVI;在微波和研磨的耦合效應(yīng)中產(chǎn)生的·OH自由基和H+也提高了反應(yīng)速率[10].除上述方法外,Wang[40]使用帶有不銹鋼絲網(wǎng)填料(孔隙率為0.85,表面積為860m/m3)的旋轉(zhuǎn)填充床反應(yīng)器連續(xù)制備平均尺寸為14nm的nZVI顆粒.采用輔助方法的優(yōu)點(diǎn)是可以得到性能更加優(yōu)良的改性nZVI材料,但在研究中并未發(fā)現(xiàn)場(chǎng)地制備的nZVI采用輔助方法.這可能是因?yàn)檩o助方法對(duì)性能提升并不符合經(jīng)濟(jì)效益.

2 納米零價(jià)鐵的常見(jiàn)改性方法

圖1 nZVI常見(jiàn)改性方式示意

盡管納米零價(jià)鐵被認(rèn)為是有效的污染物去除材料,但nZVI顆粒由于其磁性而迅速聚集,這導(dǎo)致它在水處理中的有效性降低.為了克服nZVI的缺點(diǎn),對(duì)nZVI進(jìn)行改性被視為一種可行的方法.而改性效果的評(píng)估也多是在理想條件下,借助去除率展示,不符合應(yīng)用實(shí)際的.因?yàn)槿コ什粌H受材料本身影響,也受材料投加量等影響.這必然造成去除污染物成本提高,而實(shí)際應(yīng)用nZVI時(shí)有必要考慮經(jīng)濟(jì)效益. Liu等[41]提出了基于經(jīng)濟(jì)效益的電子效率,將其定義為特定時(shí)間內(nèi)還原過(guò)程利用的電子相對(duì)于nZVI提供電子的百分比.顯然,這種評(píng)估方式的優(yōu)點(diǎn)是優(yōu)化成本,充分利用電子;缺點(diǎn)是不能反應(yīng)污染物去除速率.Zhou等[42]綜述了提高nZVI電子選擇性的方式,論證了這種經(jīng)濟(jì)效益的評(píng)估重要性,但表示仍然需要更加經(jīng)濟(jì)和快速的評(píng)估方式.而本文發(fā)現(xiàn),在大多數(shù)關(guān)于納米零價(jià)鐵的研究中,針對(duì)材料改性的評(píng)價(jià)大多只是基于不同物理化學(xué)條件下反應(yīng)性、穩(wěn)定性或毒性的探究.而這種分離式的研究方式可能會(huì)造成改性材料評(píng)價(jià)的分歧.例如,Duan等[43]發(fā)現(xiàn)腐殖酸會(huì)通過(guò)空間位阻效應(yīng)阻礙表面改性nZVI與污染物接觸而降低反應(yīng)性;Hou等[44]的研究表明腐殖酸的存在提高了表面改性nZVI的穩(wěn)定性.此外,單一影響因素的反應(yīng)性探究仍是大多數(shù)研究的主要內(nèi)容,而單因素的探究代表性差,難以接合nZVI的實(shí)際應(yīng)用.如Jung等[45]的研究表明,在腐殖酸存在時(shí),Na+、Ca2+對(duì)表面改性的nZVI的穩(wěn)定性沒(méi)有顯著影響.而Wu等[46]發(fā)現(xiàn)Na+、Mg2+、Ca2+的存在對(duì)nZVI的穩(wěn)定性無(wú)明顯影響,但其與腐殖酸形成的絡(luò)合物會(huì)增強(qiáng)nZVI的團(tuán)聚.因此,新的評(píng)估方式還應(yīng)對(duì)改性納米零價(jià)鐵進(jìn)行多方面(反應(yīng)性、穩(wěn)定性、遷移性、毒性)和多因素的評(píng)價(jià).

2.1 負(fù)載改性

負(fù)載改性是將nZVI均勻分散地負(fù)載到材料表面,以減少nZVI的團(tuán)聚,如圖1(a)所示.負(fù)載改性最主要的作用,一是抑制納米顆粒的團(tuán)聚現(xiàn)象,二是提高了改性材料的比表面積.具有吸附性能的載體材料通過(guò)吸附污染物,加快污染物跟nZVI的接觸過(guò)程,促進(jìn)了污染物的降解.負(fù)載改性的nZVI多采用液相還原法進(jìn)行制備.前文提到碳熱還原法也可制備負(fù)載型的nZVI材料,不同碳材料通過(guò)碳熱還原制備的復(fù)合材料見(jiàn)表2.通過(guò)不同碳材料制備的負(fù)載nZVI粒徑尺寸皆比較優(yōu)良.負(fù)載改性選擇的材料一般具有高表面積、多孔結(jié)構(gòu)的特點(diǎn),如粘土類(lèi)、高表面積的碳材料等.粘土中的2:1型由于晶格取代帶負(fù)電,因此容易懸浮在水中并吸附陽(yáng)離子物種,同時(shí)具有較高的比表面積、良好的熱穩(wěn)定性和可接受的機(jī)械強(qiáng)度的特點(diǎn)[47].但純粘土礦物對(duì)水相微污染物的修復(fù)效率不高.脫吸附也會(huì)產(chǎn)生二次污染,吸附后粘土難以從溶液中分離和回收,降低了其在工業(yè)廢水處理中的使用量[48].改性常見(jiàn)的粘土材料有高嶺石、蒙脫石、膨潤(rùn)土等.碳類(lèi)材料的顯著優(yōu)點(diǎn)是比表面積大,孔隙率高,具有良好的吸附性能.為了進(jìn)一步抑制材料聚集,表面活性劑和聚合物可用于生成nZVI空間穩(wěn)定的懸浮液,提升負(fù)載改性材料的分散性能[49].例如Zhang等[50]用羧甲基纖維素穩(wěn)定新型生物碳負(fù)載的nZVI去除水中Cr(IV),在最優(yōu)條件下的吸附量為132.8mg/g,相比同類(lèi)材料,吸附量有了顯著提升.然而,商用活性炭?jī)r(jià)格昂貴,相比之下,利用農(nóng)業(yè)廢棄物如秸稈、谷殼、水果等生產(chǎn)活性炭是一個(gè)不錯(cuò)的選擇[51].負(fù)載改性因負(fù)載材料提供較大的表面積幾乎可以和其它任何改性方式結(jié)合,是發(fā)展多功能納米零價(jià)鐵材料的重要手段.

表2 碳熱還原法制備的nZVI

2.2 雙金屬改性

雙金屬改性是將nZVI和另一種金屬結(jié)合,增強(qiáng)反應(yīng)活性的改性方式,因此常用于需要脫氫的污染物,但第二種金屬可能產(chǎn)生的環(huán)境問(wèn)題不可忽略,如圖1(b).其原理是第二種金屬與nZVI結(jié)合,形成原電池,可以增強(qiáng)反應(yīng)性,加速電子轉(zhuǎn)移[12].金屬催化作用是將分子氫分解成原子氫,成為一種強(qiáng)大的還原劑,見(jiàn)式(8)～(9)[53].常用于雙金屬改性的金屬有鈀(Pd)、鉑(Pt)、鎳(Ni)、銅(Cu)、銀(Ag).其中鈀表現(xiàn)出極高的脫氯效率,幾乎所有的研究都表明鈀是提高nZVI反應(yīng)活性最有效的催化劑[54],但由于鈀成本較高,應(yīng)用受到限制[55].研究表明,鈀鐵系統(tǒng)中鐵和脫氯副產(chǎn)物(如氯乙烯、二氯乙烯)的濃度比鐵系統(tǒng)低一個(gè)數(shù)量級(jí)[56].He等[57]發(fā)現(xiàn)Fe/Pd、硫化改性nZVI、Fe/Ni、Fe/Cu和Fe/Ag對(duì)三氯乙烯的表面積歸一化還原率分別為nZVI的約6800、190、130、20和8倍.雙金屬nZVI/Pd和nZVI/Ni對(duì)二氯乙烯、三氯乙烯和四氯乙烯的降解效果優(yōu)于加入Ag或Cu的nZVI.但nZVI/Pd納米粒子表面能高,導(dǎo)致嚴(yán)重的聚集,可能導(dǎo)致反應(yīng)性降低,導(dǎo)致耐久性和機(jī)械強(qiáng)度降低[58].因此可以采用其它改性方式,開(kāi)發(fā)多功能的復(fù)合材料.例如,可以利用多孔材料作為機(jī)械支撐進(jìn)行負(fù)載改性,提高雙金屬在水中的穩(wěn)定性.Venkateshaiah等[59]發(fā)現(xiàn),在合成地下水和真實(shí)地下水條件下,相比于鈀改性,鎳參雜的nZVI同樣可以完全降解二氯乙烷、三氯乙烯和四氯乙烯(99.9%).然而,鎳鐵雙金屬納米顆粒仍然容易受到氧化失效和聚集等缺陷的影響[60].此外,鎳和銀具有一定的毒性,而鉑作為一種摻雜合成貴金屬,同樣應(yīng)用受限,相比之下,銅的成本相對(duì)較低,毒性也較低[61].

2.3 硫化改性

硫化納米零價(jià)鐵(S-nZVI)比nZVI具有更大的比表面積和更強(qiáng)的還原能力,其在污染物修復(fù)中的潛在作用引起了廣泛關(guān)注,且生成的硫化鐵可作為電子轉(zhuǎn)移介質(zhì),可有效地將電子從nZVI核心轉(zhuǎn)移到其表面的目標(biāo)污染物[62],如圖1(c).需要補(bǔ)充說(shuō)明的是S-nZVI的外殼并不是單一的硫化鐵,而是由復(fù)雜的硫化物構(gòu)成,正是這些硫化物改善了nZVI的疏水性能、電子轉(zhuǎn)移性能.此外,S-nZVI在一定程度上抑制了鐵的析氫速率,提高礦化程度和去除效率,表現(xiàn)出良好的耐鹽性[63].Su等[64]發(fā)現(xiàn)老化對(duì)S-nZVI的去除能力沒(méi)有負(fù)面影響,氧破壞了S-nZVI的結(jié)構(gòu),將鎘的去除能力從85mg/g提高到約120mg/g.但硫化改性會(huì)使nZVI對(duì)pH值的變化更加敏感.S-nZVI因其獨(dú)特優(yōu)勢(shì),已經(jīng)成為nZVI改性的熱點(diǎn)方向,在制備、改性、場(chǎng)地研究、反應(yīng)性、電子選擇性、毒性方面都有較多研究;而其它改性方式一般只集中在其中的幾個(gè)方面.到目前為止,已經(jīng)開(kāi)發(fā)了兩種用于nZVI硫化的方法.分別是一步法:NaBH4和Na2S2O4加入到鐵溶液中,見(jiàn)式(10)～(13).二步法:先通過(guò)NaBH4合成nZVI,然后加入Na2S,見(jiàn)式(14)～(18)[65].一步法制備S-nZVI的實(shí)際硫含量和疏水性均顯著高于兩步法S-nZVI,具有更明顯的選擇性.而兩步法,不論是提高硫投加量還是延長(zhǎng)硫化時(shí)間,只能十分有限地提高實(shí)際硫含量[66].研究表明,采用不同的鐵和硫試劑,可以顯著影響所合成的S-NZVI材料的理化性質(zhì)、反應(yīng)活性和選擇性[67].除此之外,Zhan等[68]通過(guò)生物沉淀方法,使硫酸鹽還原菌在nZVI表面原位生成二硫化亞鐵,制備出了沉降性能良好的硫化納米顆粒,避免了有害化學(xué)硫試劑的使用.硫化納米鐵的壽命在很大程度上取決于硫化鐵的結(jié)構(gòu)和均勻性.外殼有效地將電子從核心轉(zhuǎn)移到其表面以減少污染物,同時(shí)保護(hù)核心免受缺氧腐蝕[69].重金屬離子與nZVI的反應(yīng)是還原作用或表面絡(luò)合反應(yīng),取決于它們的電極電位,而重金屬與S-nZVI的反應(yīng)機(jī)理是硫化物形成或表面絡(luò)合,硫在反應(yīng)過(guò)程中也被氧化為S0和SO42-[70].

2.4 表面改性

表面改性是在水介質(zhì)中分散和遷移nZVI的有效方法,這是由于表面改性試劑提供的涂層分子通過(guò)空間位阻效應(yīng)降低了納米顆粒間的作用力,提高了nZVI的穩(wěn)定性,如圖1(d)所示.表面活性劑溶液存在臨界膠束濃度,此時(shí)膠體質(zhì)點(diǎn)和離子存在平衡,超過(guò)此濃度,溶液的表面張力和界面張力等性質(zhì)顯著變化.因此制備表面改性nZVI時(shí),表面活性劑的濃度是重要影響因素.nZVI表面的電荷在涂層改性后發(fā)生變化,會(huì)減少靜電吸引和聚集[71].在較低表面活性劑濃度下,表面涂層一方面通過(guò)提供更多的可用表面位置來(lái)降低污染物濃度,另一方面卻占據(jù)了反應(yīng)位點(diǎn),并通過(guò)靜電或空間斥力抑制了污染物從水到顆粒表面的傳質(zhì)[72].表面改性的缺點(diǎn)不僅表現(xiàn)在堵塞活性位點(diǎn),也包括減少擴(kuò)散通道、抑制nZVI向污染物的電子轉(zhuǎn)移、清除活性自由基以及污染物向表面活性劑的分配[15].Wan等[73]對(duì)SiO2包覆nZVI去除二四二氯酚的中間產(chǎn)物進(jìn)行研究,表明了SiO2包覆nZVI和未包覆nZVI對(duì)二四二氯酚的去除表現(xiàn)出相同的降解路徑,但傳質(zhì)模式不同.親水二氧化硅涂層強(qiáng)烈吸附Fe2+和Fe3+,減緩其從內(nèi)表面到外表面的遷移,延遲氧化鐵的形成速度,提高納米顆粒鈍化.在腐殖酸存在時(shí),表面穩(wěn)定劑和腐殖酸之間的相互作用對(duì)nZVI的穩(wěn)定性有很大影響.Dong等[74]研究發(fā)現(xiàn)腐殖酸通過(guò)與表面穩(wěn)定劑(吐溫-20或淀粉)發(fā)生作用作為顆粒之間的連接,導(dǎo)致nZVI顯著的聚集和沉降.

3 納米零價(jià)鐵的作用機(jī)理

nZVI目前的應(yīng)用大多基于nZVI強(qiáng)還原性和較大的表面積的特性.強(qiáng)還原性是因其電極電位為-0.44V,可以還原電極電位更高的物質(zhì).nZVI已被證實(shí)能夠有效地轉(zhuǎn)化多種環(huán)境污染物,包括含氯有機(jī)物、偶氮染料、磷酸鹽、硝酸鹽、重金屬[75],反應(yīng)過(guò)程中產(chǎn)生的氫氧化物,毒性更低,也是良好的絮凝劑[76].本節(jié)將根據(jù)污染物類(lèi)別,具體闡述nZVI的作用機(jī)理,同時(shí)說(shuō)明高級(jí)氧化中nZVI催化產(chǎn)生自由基的機(jī)理.

3.1 nZVI去除污染物機(jī)理

重金屬難以生物降解,且多以陽(yáng)離子形式存在,而六價(jià)鉻以陰離子形態(tài)存在水體中.nZVI由于核殼結(jié)構(gòu)的存在表面帶正電,更容易吸附陰離子. nZVI對(duì)金屬元素的去除機(jī)理包括還原、氧化、吸附、沉淀以及共沉淀,見(jiàn)式(19)～(23)[5].去除過(guò)程不是單一的反應(yīng),而是以上不同機(jī)理的組合,并且不具有同時(shí)性.表3展示了nZVI對(duì)常見(jiàn)金屬元素機(jī)理作用的不同點(diǎn).

Me++me-?Me(n-m)+/Me0(19)

Me+-me-?Me(n-m)(20)

Me++氧化鐵及氫氧化物?Me+@氧化鐵及氫氧化物(21)

Me++OH-?Me(OH)(22)

Me++氧化鐵及氫氧化物?Me-Fe-OOH (23)

表3 常見(jiàn)金屬元素的去除機(jī)理[79]

nZVI最主要的作用機(jī)理是還原性,其中nZVI的氧化物外殼作為電子通道,將電子轉(zhuǎn)移到鐵核,實(shí)現(xiàn)電子交換.核殼結(jié)構(gòu)還提供了活性位點(diǎn),通過(guò)靜電吸附或絡(luò)合吸附污染物.但由于nZVI強(qiáng)還原性,總是不可避免地發(fā)生副反應(yīng),如式(6),這會(huì)競(jìng)爭(zhēng)nZVI的活性位點(diǎn).除核殼結(jié)構(gòu)提供的活性位點(diǎn),S-nZVI的硫鐵化物外殼,提供了帶活性位點(diǎn)的羥基,不同改性材料也存在如羥基、羧基、磷酸基等官能團(tuán)提供更多活性位點(diǎn)[77].而且沉淀、共沉淀作用形成的沉淀物可以一定程度上起到吸附的作用,并且鐵的氧化物具有鐵磁性,與鐵同樣易于處理后的分離,降低環(huán)境風(fēng)險(xiǎn).當(dāng)下已經(jīng)開(kāi)展了大量關(guān)于nZVI機(jī)理的定性研究,但缺乏對(duì)不同作用機(jī)理的定量研究.對(duì)于某種污染物各個(gè)機(jī)理的重要性往往需要實(shí)驗(yàn)與表征相結(jié)合才能得出結(jié)論.例如nZVI在去除六價(jià)鉻時(shí),共沉淀發(fā)揮的作用強(qiáng)于吸附作用,這可能是因?yàn)殍F鉻形成的羥基殼足夠穩(wěn)定,能負(fù)擔(dān)多物種同時(shí)共沉淀[78].

nZVI對(duì)于鹵代有機(jī)物的去除主要依靠提供電子,置換鹵族元素,見(jiàn)式(24)～(26)[80].具體可分為3步:首先有機(jī)化合物和nZVI結(jié)合形成復(fù)合物;其次電子從nZVI轉(zhuǎn)移到鹵代有機(jī)物;最終反應(yīng)并釋放還原產(chǎn)物[81].反應(yīng)或腐蝕產(chǎn)生的Fe2+,被氧化成Fe(OH)3,隨后經(jīng)鱗石英、針鐵礦和磁鐵礦等形態(tài)形成了鐵氫化物[82].

水中的硝酸鹽、磷酸鹽、溴酸鹽等水中陰離子也可被nZVI去除,但機(jī)理并不相同,區(qū)別見(jiàn)表4.nZVI的活性位點(diǎn)首先吸附氫離子,表現(xiàn)出正電荷,其次活性位點(diǎn)與水中的陰離子結(jié)合[83].發(fā)生還原作用后,溴酸鹽和高氯酸鹽被nZVI還原成Br-和Cl-,而nZVI對(duì)硝酸鹽的還原作用,能將硝酸鹽還原為NH4+.雖然水中的陰離子一般不是水中的主要污染物,但它們的存在會(huì)阻礙nZVI還原其它污染物,降低nZVI的效率.需要補(bǔ)充的是,當(dāng)今機(jī)理的論證,更多的根據(jù)反應(yīng)產(chǎn)物推導(dǎo)反應(yīng)途徑,而缺少對(duì)反應(yīng)過(guò)程的正向論證.這種情況就使得反應(yīng)過(guò)程中多種途徑不能得到量化,難以確定主要降解途徑.為了更好地理解反應(yīng)機(jī)理,許多研究通常采用反應(yīng)動(dòng)力學(xué)和吸附等溫線的方式描述反應(yīng)過(guò)程,但這些模型比較簡(jiǎn)單不能表明各自機(jī)理發(fā)揮的作用.值得注意的是,在一些論文中根據(jù)Eley-Rideal機(jī)理建立了模型,模擬污染物去除過(guò)程和去除率,并且可以量化不同反應(yīng)機(jī)理的占比[83-84].這顯然提供了一種nZVI機(jī)理探究思路,可通過(guò)建模的方式提供反應(yīng)方程式,根據(jù)實(shí)驗(yàn)結(jié)果確定方程式的參數(shù),量化吸附和降解過(guò)程,并最終通過(guò)nZVI固體等不參與模型中變化計(jì)算的數(shù)據(jù)論證機(jī)理.

表4 常見(jiàn)陰離子去除機(jī)理[85-88]

3.2 nZVI催化機(jī)理

高級(jí)氧化技術(shù)可產(chǎn)生強(qiáng)氧化能力的自由基,能有效去除醇、酚等多種有機(jī)物,可以分為光催化、聲催化、化學(xué)氧化等多種方法.nZVI與微尺度的其它鐵材料相比,具有更高的反應(yīng)速率,因此可作為非均相環(huán)境修復(fù)的有效催化劑.例如,nZVI可用于催化H2O2,產(chǎn)生羥基自由基,見(jiàn)式(27).

反應(yīng)產(chǎn)生的Fe3+再與nZVI反應(yīng)或與超氧化氫反應(yīng),生成Fe2+,見(jiàn)式(28)～(29).但反應(yīng)(29)過(guò)程緩慢,若不使用nZVI,整個(gè)催化體系會(huì)積累Fe3+,進(jìn)而使得反應(yīng)系統(tǒng)的pH敏感性提升.綜上,nZVI在產(chǎn)生強(qiáng)氧化自由基的過(guò)程中主要起到兩個(gè)作用,一是與水反應(yīng)提供參與反應(yīng)的Fe2+,二是快速還原反應(yīng)后的Fe3+為Fe2+.并且反應(yīng)結(jié)束后,nZVI可利用磁性從水中分離回收,二次利用,這是均相催化體系所不具備的.

需要注意的是,nZVI活化溶解氧和鈍化層中Fe2+活化分子氧都可以產(chǎn)生H2O2,使單獨(dú)的nZVI材料也具有弱氧化性,見(jiàn)式(30)-(32)[89].

一硫酸鹽(PMS)和過(guò)二硫酸鹽(PDS)在常溫下結(jié)構(gòu)穩(wěn)定,難以發(fā)生分解反應(yīng).而nZVI可催化這兩種過(guò)硫酸鹽產(chǎn)生硫酸根自由基,見(jiàn)式(33)～(35)[90-91].通過(guò)這些反應(yīng)式可以看出,nZVI的作用同樣是產(chǎn)生Fe2+參與反應(yīng),生成具有強(qiáng)氧化性的自由基,達(dá)到去除污染物的目的.

此外,過(guò)多的nZVI產(chǎn)生的Fe2+會(huì)與硫酸鹽自由基反應(yīng)而抑制對(duì)污染物的處理效果,如下式(36)[92],因此有必要選擇合適的nZVI用量,提高污染物去除效率.

本文在梳理催化機(jī)理相關(guān)資料時(shí),發(fā)現(xiàn)與nZVI的去除機(jī)理相比,采用模型分析催化效果的研究較少.一方面可能是因?yàn)榇呋瘷C(jī)理中nZVI的催化作用存在副反應(yīng)較多(如式(6)和(30)),另一方面自由基難以在模型中表述.

4 納米零價(jià)鐵的實(shí)地研究

納米零價(jià)鐵已應(yīng)用于可滲透反應(yīng)墻,如Eijamal等[93]將雙金屬改性的nZVI作為填充材料,除磷性能比nZVI提高了2.2倍.nZVI顆粒尺寸小,比含水層孔隙小得多,可以分散在含水泥漿中,也能直接注入底土中[94],而不是作為可滲透反應(yīng)墻的填充介質(zhì).如今大多數(shù)的nZVI研究都是基于實(shí)驗(yàn)室內(nèi)短時(shí)間的單因素實(shí)驗(yàn),改性材料去除有機(jī)污染物也存在成本高昂等問(wèn)題[95].這種研究系統(tǒng)遠(yuǎn)離實(shí)際的環(huán)境水域,會(huì)導(dǎo)致對(duì)nZVI去除性能的高估[96].而闡述不同影響因素之間的作用也具有重要意義.其中響應(yīng)面法(RSM)是一種科學(xué)、實(shí)用的實(shí)驗(yàn)設(shè)計(jì)方法,可以避免傳統(tǒng)方法的不足[97];另一方面,開(kāi)展場(chǎng)地研究是長(zhǎng)時(shí)間評(píng)估nZVI能力的最有效手段.

nZVI也已經(jīng)開(kāi)展了多項(xiàng)實(shí)地研究.主要可分為污染物去除和nZVI在地下水的遷移兩類(lèi).表5展示了不同nZVI的實(shí)地應(yīng)用研究.由表5可知,不同改性已應(yīng)用于實(shí)地研究,但仍有差別.臺(tái)灣地區(qū)的采用的鈀金屬改性,15d左右對(duì)氯乙烯的去除率就達(dá)到很高的水平(約90%);而采用生物炭負(fù)載的nZVI和聚丙烯酸表面改性的nZVI,在約40d實(shí)現(xiàn)了同樣比例的鹵代污染物的去除.可見(jiàn),無(wú)論是實(shí)驗(yàn)室內(nèi)研究和實(shí)地研究,雙金屬改性是快速去除污染物的方式.需要注意的是,去除效率的研究不能排除場(chǎng)地因素的影響,這也是實(shí)地研究與實(shí)驗(yàn)室內(nèi)研究的區(qū)別.在實(shí)驗(yàn)室中常用的負(fù)載改性,實(shí)地應(yīng)用中占比很少,這很可能是因?yàn)槭芟抻谪?fù)載材料的流動(dòng)性.而針對(duì)nZVI遷移性和流動(dòng)性的研究,集中在硫化改性和表面改性,這要?dú)w因于改性的納米零價(jià)鐵具有良好的分散性,并能在地下水介質(zhì)中遷移.所以場(chǎng)地的改性方式的選擇要根據(jù)場(chǎng)地實(shí)際情況而定.

如圖2所示,nZVI懸浮液通過(guò)重力注入或者壓力注入等方式進(jìn)入注入井.注入井中可采用隔斷器控制懸浮液的注入范圍.nZVI在地下水中經(jīng)過(guò)一斷時(shí)間的遷移可以在監(jiān)測(cè)井中檢測(cè)到.因此,想要測(cè)定nZVI遷移能力在地下水中的各向異性,有必要在注入井的周?chē)贾帽O(jiān)測(cè)井.根據(jù)本文研究總結(jié),監(jiān)測(cè)井的布置距離一般不超過(guò)5m.正如表5總結(jié),幾項(xiàng)nZVI的遷移研究,其在地下水中的移動(dòng)距離都被控制在3m內(nèi).除了測(cè)定nZVI以外,在注入井添加熒光素后,取樣器通過(guò)泵在監(jiān)測(cè)井自動(dòng)取樣,完成示蹤試驗(yàn),可以獲取本區(qū)域的含水層的水動(dòng)力特性.在一些研究中,也會(huì)在注入井和監(jiān)測(cè)井之間布置觀察井,主要目的是為了提取巖心數(shù)據(jù),分析含水層地質(zhì)條件[98].

圖2 nZVI實(shí)地應(yīng)用的現(xiàn)場(chǎng)布置示意

場(chǎng)地研究需要考慮更多因素,如nZVI注入井和指標(biāo)監(jiān)測(cè)井的布置,注入井和監(jiān)測(cè)井的距離等,都影響著污染物濃度的測(cè)定和nZVI在地下水中遷移距離的監(jiān)測(cè).合適的布置不僅能精準(zhǔn)評(píng)價(jià)nZVI的遷移性和穩(wěn)定性,也能詳細(xì)描述出nZVI長(zhǎng)時(shí)間的濃度變化和形態(tài)轉(zhuǎn)變.nZVI的使用也要考慮地質(zhì)條件,在密集的地質(zhì)地層中使用nZVI進(jìn)行修復(fù)的效率較低[99].此外,nZVI的注射方式也是影響nZVI遷移的重要因素.例如在沙箱實(shí)驗(yàn)中,首次注入改性的nZVI可以幫助后續(xù)注入的nZVI輸運(yùn),提高了nZVI總的遷移性,但羽流擴(kuò)散較小[100].對(duì)此, Asad等[101]通過(guò)COMSOL模擬nZVI在多孔介質(zhì)中的輸運(yùn),他們認(rèn)為注入速率可以改變井場(chǎng)周?chē)牡叵滤俣冗M(jìn)而影響nZVI的運(yùn)動(dòng),而地下水流速和水力梯度在統(tǒng)計(jì)上沒(méi)有顯著改變,不會(huì)對(duì)nZVI的遷移產(chǎn)生較大影響.由此可見(jiàn),對(duì)于場(chǎng)地研究采用試驗(yàn)和模型相結(jié)合的手段,可以更好地探究不同影響因素對(duì)nZVI的運(yùn)作模式.但據(jù)研究,采用二維數(shù)值模型分析納米顆粒遷移的不多.這可能是因?yàn)殚_(kāi)展場(chǎng)地研究難度大,運(yùn)行周期長(zhǎng),并且在nZVI遷移過(guò)程中影響因素眾多,難以準(zhǔn)確提供長(zhǎng)期穩(wěn)定的運(yùn)行參數(shù).

表5 nZVI的實(shí)地研究匯總

續(xù)表5

實(shí)際場(chǎng)地中,由于nZVI表面能高、磁性相互作用的缺點(diǎn),其在反應(yīng)過(guò)程中容易快速聚集,導(dǎo)致原位修復(fù)效率低,制約原位修復(fù)效果[78].因此避免nZVI在場(chǎng)地團(tuán)聚是實(shí)地研究的重點(diǎn).此外容易氧化和鈍化的問(wèn)題也縮短nZVI壽命,抑制了nZVI的現(xiàn)場(chǎng)應(yīng)用[37].這也證實(shí)了改性在實(shí)際應(yīng)用中存在必要性.除了上述問(wèn)題,在修復(fù)地下水污染過(guò)程中形成的nZVI雜化物幾乎不可能去除,改性材料同樣會(huì)留在水中長(zhǎng)期影響地下水環(huán)境.而nZVI原位修復(fù)的長(zhǎng)期成功往往依賴(lài)于短期非生物脫氯后的生物轉(zhuǎn)化,必須注意不能抑制健康微生物群落的生長(zhǎng)[100].因此,對(duì)于任何基于nZVI的地下水修復(fù)項(xiàng)目,根據(jù)場(chǎng)地情況針對(duì)性的改性nZVI是場(chǎng)地污染物去除的關(guān)鍵,而調(diào)查nZVI的毒性和轉(zhuǎn)化途徑是降低nZVI使用風(fēng)險(xiǎn)的關(guān)鍵.

5 納米零價(jià)鐵的致毒機(jī)制

納米顆?？梢酝ㄟ^(guò)多種方式進(jìn)入人體,如空氣傳播或水體傳播等.現(xiàn)在普遍認(rèn)為氧化應(yīng)激是nZVI的主要致毒機(jī)制,主要包含兩方面,見(jiàn)圖3;一方面來(lái)自胞外nZVI產(chǎn)生的活性氧類(lèi)(ROS)進(jìn)入胞內(nèi)[110],而ROS的存在會(huì)造成蛋白質(zhì)、核酸的損失;另一方面,nZVI可以通過(guò)吸附性能物理?yè)p傷改變細(xì)胞膜的通透性,導(dǎo)致胞外環(huán)境中的Fe2+大量跨膜擴(kuò)散進(jìn)入胞內(nèi)[111].并且,這種情況下在無(wú)氧條件下更加嚴(yán)重,這是因?yàn)闊o(wú)氧條件下nZVI不易腐蝕,加速了對(duì)細(xì)胞膜的損傷[112].一旦Fe2+進(jìn)入細(xì)胞后,可以與線粒體發(fā)生反應(yīng),再次生成活性氧物種.除此之外,Lin等[113]還發(fā)現(xiàn),高劑量的nZVI可以附著在細(xì)菌表面,直接干擾各項(xiàng)生理活動(dòng).針對(duì)這種因nZVI劑量而產(chǎn)生的不同效果,Jiang等[114]證明了nZVI濃度效應(yīng)的重要性.通過(guò)研究好氧條件下不同劑量nZVI對(duì)副球菌菌株還原硝酸鹽的影響,發(fā)現(xiàn)生物硝酸鹽處理中,使用低濃度nZVI(50～500mg/L)作為電子源可能會(huì)加速硝酸鹽還原;而高濃度nZVI(1000mg/L)抑制了硝酸鹽的生物還原作用.這是因?yàn)檫^(guò)量攝入Fe2+,會(huì)對(duì)細(xì)胞造成氧化性的損傷.綜上可以推斷出,納米顆粒的劑量使用和團(tuán)聚情況是影響其毒性的最關(guān)鍵因素,這不僅是因?yàn)榧{米顆粒尺寸的增大會(huì)導(dǎo)致其生物利用度降低,也因?yàn)榧{米顆粒的獨(dú)特性質(zhì).此外,陸賢等[115]證明了nZVI的作用時(shí)間也是其毒性影響的關(guān)鍵因素.nZVI濃度為5000mg/L時(shí),12h后對(duì)耐四環(huán)素菌的抑制率為80%;而反應(yīng)時(shí)間為48h,nZVI對(duì)對(duì)耐四環(huán)素菌具有促進(jìn)作用.針對(duì)可能產(chǎn)生的毒性,本文整理了nZVI對(duì)不同生物的毒性研究情況,見(jiàn)表6.從中可以看出,nZVI會(huì)導(dǎo)致人的淋巴細(xì)胞中線粒體功能障礙,這與上面Fe2+與線粒體反應(yīng)的情況相吻合.與對(duì)細(xì)胞影響不同的是,nZVI對(duì)于生物體的影響過(guò)程復(fù)雜,只能通過(guò)存活率、發(fā)芽率等指標(biāo)反應(yīng),并且實(shí)驗(yàn)周期并不長(zhǎng),最高為48h,這難以反映nZVI長(zhǎng)期毒性.而開(kāi)展場(chǎng)地研究中的nZVI處理地下水時(shí)間可達(dá)半年以上,這就大大提高了應(yīng)用風(fēng)險(xiǎn).針對(duì)這種情況,應(yīng)該開(kāi)展更長(zhǎng)時(shí)間的毒性實(shí)驗(yàn)觀察.此外,生物群包括了不同生物以及不同發(fā)育階段的生物體,而毒理研究往往集中于某個(gè)階段的生物,探究nZVI對(duì)同一生物不同發(fā)育階段的影響,也是nZVI未來(lái)大規(guī)模應(yīng)用要面臨的重要課題,我們希望以后的毒理作用研究可以從此著手,例如在nZVI存在時(shí),開(kāi)展對(duì)水蚤卵孵化成功率以及孵化后成活率研究.

對(duì)于研究nZVI生態(tài)安全性時(shí),改性也是消除其潛在生物毒性的重要研究?jī)?nèi)容.研究發(fā)現(xiàn)可以通過(guò)不同方式,降低nZVI的毒性,進(jìn)一步增加nZVI的環(huán)境友好性.如表面改性通過(guò)減少沉降來(lái)降低nZVI的毒性,從而限制顆粒接觸細(xì)胞[116].相比之下,雙金屬改性會(huì)促進(jìn)nZVI產(chǎn)生活性氧類(lèi),從而增加nZVI對(duì)細(xì)胞的毒性.硫化改性由于Fe0含量較低,以及硫酸鹽和氧化鐵沉積在顆粒表面,產(chǎn)生的毒性低于nZVI,在實(shí)際環(huán)境中的應(yīng)用風(fēng)險(xiǎn)很小,并且隨著時(shí)間的推移,毒性潛力會(huì)降低[117].影響nZVI的改性方式也多是實(shí)地研究中采用的方式,固集中考察某種改性方式的實(shí)地情況和毒理性質(zhì)是必要的.比如,上文提到硫化改性具有良好的遷移性并且毒性低,且其本身選擇性強(qiáng),抗腐蝕性好,那么硫化納米零價(jià)鐵很可能繼續(xù)在未來(lái)很長(zhǎng)一段時(shí)間成為研究熱點(diǎn).

圖3 nZVI對(duì)細(xì)胞的致毒機(jī)制

除了改性對(duì)毒性影響外,水中的胞外聚合物,既促進(jìn)了nZVI形成毒性較小的鐵(氫)氧化物物種,又抑制nZVI氧化產(chǎn)生活性氧類(lèi),也能實(shí)現(xiàn)nZVI的毒性減弱[118].而水中的鈣離子具有雙重性,一方面可以促進(jìn)nZVI的團(tuán)聚和沉積,降低其毒性[119];另一方面,鈣離子的存在會(huì)促進(jìn)nZVI附著在細(xì)菌表面,對(duì)細(xì)胞產(chǎn)生毒性[120].對(duì)此,Cheng等[117]通過(guò)添加不同離子監(jiān)測(cè)硫化納米零價(jià)鐵對(duì)大腸桿菌的滅活作用,發(fā)現(xiàn)典型地下水成分(Ca2+、SO42-、HCO3-、腐殖酸)的存在可以不同程度地降低納米顆粒的毒性作用,并具有濃度依賴(lài)性.此外,存在于生物組織內(nèi)的許多不同蛋白質(zhì)可在nZVI周?chē)纬傻鞍坠?阻止其向細(xì)胞內(nèi)轉(zhuǎn)移[121].綜上,nZVI的毒性,與其本身的濃度和所處的環(huán)境密切相關(guān),同時(shí)也與生物本身相關(guān),這就提高了nZVI毒性機(jī)理研究的難度.而且毒理研究?jī)?nèi)容多基于實(shí)驗(yàn)室條件,實(shí)際場(chǎng)地的環(huán)境背景值更加復(fù)雜,實(shí)驗(yàn)室內(nèi)的研究可能不適用于實(shí)地研究.因此,未來(lái)毒理研究應(yīng)積極依托真實(shí)的地下水環(huán)境,但為了避免材料未知的風(fēng)險(xiǎn),可以在實(shí)驗(yàn)室模擬真實(shí)地下水條件,考察復(fù)雜環(huán)境因素下nZVI的致毒作用.

表6 nZVI對(duì)不同生物體的毒性

6 結(jié)論與展望

近年來(lái)對(duì)nZVI進(jìn)行了大量研究,集中在單因素的實(shí)驗(yàn)水平,目的是創(chuàng)新nZVI改性方式,克服nZVI存在的缺點(diǎn).但改性材料的轉(zhuǎn)化途徑和遷移性有待考察,這是限制著nZVI場(chǎng)地研究的關(guān)鍵一項(xiàng).綜上,提出以下幾點(diǎn)展望:

6.1 目前基于實(shí)際環(huán)境條件,除了考量多種因素的影響,也應(yīng)注重材料的實(shí)用性和經(jīng)濟(jì)效益.比如響應(yīng)面法是考慮多因素的設(shè)計(jì)實(shí)驗(yàn),但在去除率最大的情形下,并不能反應(yīng)材料的成本是否合理.雖然已經(jīng)有了基于電子轉(zhuǎn)移效率的評(píng)價(jià),但也需要更加綜合全面和便捷的方法,評(píng)價(jià)反應(yīng)性、穩(wěn)定性、毒性等.并且在使用nZVI復(fù)合材料時(shí),多是與裸nZVI比較,這并不能證明該材料相比于其它改性更加優(yōu)異.希望有更加全面、量化的方法評(píng)選nZVI 的改性方式,在不造成二次污染的情況下,達(dá)到以低成本完成高效去除的目的.另外本文也發(fā)現(xiàn)單一的改性方式優(yōu)缺點(diǎn)很明顯,很多研究通過(guò)采用多種改性方法結(jié)合的方式,提升了改性材料的綜合性能.因此,未來(lái)改性方式的發(fā)展方向可能是通過(guò)低成本開(kāi)發(fā)多功能的納米零價(jià)鐵復(fù)合材料.

6.2 明確作用機(jī)理是增強(qiáng)nZVI應(yīng)用性的必然要求,實(shí)際上闡明nZVI去除污染物的機(jī)理十分困難.對(duì)于nZVI的作用機(jī)理的探究需要左右論證,而常用的材料表征手段如X射線光電子能譜(XPS)只能定量地反應(yīng)元素的占比情況,難以詳細(xì)描述污染物或者nZVI的轉(zhuǎn)化,而X射線吸收精細(xì)結(jié)構(gòu)譜(XAFS)雖然可以測(cè)定固相中Fe的不同形態(tài),但XAFS在國(guó)內(nèi)設(shè)備平臺(tái)資源有限.因此,研究可以提出更全面的理論,綜合考慮不同機(jī)理的占比和互相影響,亦或者有針對(duì)性的大量總結(jié)整理特定污染物的去除過(guò)程,細(xì)化反應(yīng)過(guò)程.其次,開(kāi)展試驗(yàn)可以結(jié)合污染物形態(tài)選擇表征方法并盡可能依托更更精細(xì)的探測(cè)手段.

6.3 nZVI的場(chǎng)地研究是nZVI走向?qū)嶋H應(yīng)用的重中之重,尤其當(dāng)下研究中不僅包括自然環(huán)境污染物的去除,也涵蓋了工業(yè)場(chǎng)地以及獨(dú)立反應(yīng)器.這不僅需要改性材料功能和結(jié)構(gòu)適應(yīng)場(chǎng)地或反應(yīng)器,更需要確定nZVI去除污染物的非環(huán)境影響因素,推動(dòng)nZVI走向大規(guī)模應(yīng)用.而同樣重要的,是研究nZVI在場(chǎng)地中是否有潛在危害.不同改性nZVI遷移性不同,在水體中轉(zhuǎn)化途徑復(fù)雜,存在多種中間體,且nZVI與其它污染物之間也會(huì)形成的絡(luò)合物,這增大了毒理研究難度.除此之外,上文提到,多功能的復(fù)合材料將是nZVI未來(lái)的發(fā)展趨勢(shì),這種復(fù)雜的反應(yīng)性也進(jìn)一步增加了研究難度.因此,應(yīng)將材料的毒性研究作為一種必要的研究?jī)?nèi)容,或者提供一種能夠普遍地、有效地評(píng)價(jià)材料毒性的方式,避免nZVI的潛在毒性限制nZVI的應(yīng)用.另外,毒理研究一定程度上需要與實(shí)地研究具有同樣的時(shí)間跨度,充分評(píng)估nZVI材料安全性、可靠性.

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A review of nano zero valent iron in water treatment.

ZHANG Yong-xiang*, DU Wei, LI Ya-jun, ZHAO Chong-hui

(Faculty of Urban Construction, Beijing University of Technology, Beijing 100124, China)., 2022,42(11)：5163～5178

As one of the most commonly used nanoparticles, nano zero valent iron (nZVI) had been extensively studied in the removal of pollutants from environmental water. This review systematically and comprehensively summarized the related progress of nZVI, introduced various aspects of nZVI and then guided its development direction. Among them, the research contents mainly included preparation method, modification method, mechanism of action and catalytic mechanism for removing different pollutants in water, application in site research, and mechanism of toxicity. In this paper, it was found that there were still problems such as lack of comprehensive evaluation methods, limited application, and asynchronous research on nano zero valent iron. The future development of nZVI should have evaluation methods that consider reactivity, stability, mobility, and toxicity, to avoid the time difference of the same modified material in different research directions, so that the application of nZVI can be better applied to field research, and promote the application of nZVI in the field.

nZVI；material preparation；material modification；mechanism of action；pathogenic mechanism

X703.5

A

1000-6923(2022)11-5163-16

張永祥(1962-),男,黑龍江省綏化市人,教授,博士,主要從事水資源和污染控制模擬技術(shù)研究.發(fā)表論文150余篇.

2022-04-06

國(guó)家重點(diǎn)研發(fā)計(jì)劃(2016YFC0401404)

* 責(zé)任作者, 教授, yxzhang@bjut.edu.cn