徐 雷,代惠萍,魏樹和

淋洗劑在重金屬污染土壤修復(fù)中的研究進(jìn)展

徐 雷1,3,代惠萍2*,魏樹和1**

(1.中國(guó)科學(xué)院沈陽(yáng)應(yīng)用生態(tài)研究所污染生態(tài)與環(huán)境工程重點(diǎn)實(shí)驗(yàn)室,遼寧 沈陽(yáng) 110016；2.陜西理工大學(xué)生物科學(xué)與工程學(xué)院,陜西 漢中 7230013；中國(guó)科學(xué)院大學(xué)資源與環(huán)境學(xué)院,北京 100039)

針對(duì)鎘/鉛等一類重金屬污染土壤、砷污染土壤,以及鎘-鉛-砷等復(fù)合污染土壤(三類不同重金屬污染土壤),就淋洗劑在污染土壤修復(fù)中的選用等問(wèn)題進(jìn)行綜述.總的來(lái)看,對(duì)于鎘/鉛等一類重金屬污染土壤,螯合劑的去除率較高.如GLDA(谷氨酸N,N-二乙酸)和檸檬酸等,不僅去除率高,而且環(huán)境較友好對(duì)于砷污染土壤,復(fù)合淋洗劑的去除效果比較顯著,如NaOH-EDTA對(duì)As的去除率較高對(duì)于鎘-鉛-砷等復(fù)合污染土壤,復(fù)合淋洗劑則更能發(fā)揮其所含各類淋洗劑的優(yōu)勢(shì).如NaOH-H3PO4與單一淋洗劑相比,對(duì)土壤中多種重金屬的去除率均較高. 筆者認(rèn)為,同時(shí)對(duì)土壤中多種重金屬均有較高去除率且二次污染較小的復(fù)合淋洗劑將是未來(lái)的研究重點(diǎn).

土壤淋洗技術(shù)；淋洗劑；重金屬；砷

2014年由原環(huán)境保護(hù)部和國(guó)土資源部聯(lián)合發(fā)布的《全國(guó)土壤污染狀況調(diào)查公報(bào)》表明[1],我國(guó)土壤總的超標(biāo)率為16.1%,污染類型以無(wú)機(jī)型為主,無(wú)機(jī)污染物超標(biāo)點(diǎn)位數(shù)占全部超標(biāo)點(diǎn)位的82.8%,無(wú)機(jī)污染物以重金屬為主.可見(jiàn),土壤重金屬污染問(wèn)題亟待解決.目前常用的修復(fù)方法主要有電動(dòng)修復(fù)、固化/穩(wěn)定化、土壤淋洗以及生物修復(fù)等.在這些技術(shù)中,土壤淋洗技術(shù)因其操作簡(jiǎn)單、高效徹底的優(yōu)勢(shì),在場(chǎng)地污染土壤修復(fù)中應(yīng)用比較廣泛.土壤淋洗主要是指利用淋洗劑或化學(xué)助劑與土壤中的重金屬結(jié)合,通過(guò)其解吸、螯合、溶解和固定等化學(xué)作用將重金屬?gòu)耐寥乐辛芟闯鰜?lái),進(jìn)而達(dá)到修復(fù)污染土壤的目的[2].

通常認(rèn)為,土壤重金屬污染是指場(chǎng)地土壤中As、Cd、Cr(六價(jià))、Cu、Pb、Hg和Ni 7種重金屬中的某一種或兩種以上含量超過(guò)我國(guó)《土壤環(huán)境質(zhì)量建設(shè)用地土壤污染風(fēng)險(xiǎn)管控標(biāo)準(zhǔn)(試行)》GB36600-2018的風(fēng)險(xiǎn)管控值,或農(nóng)田土壤中As、Cd、Cr(六價(jià))、Cu、Pb、Hg、Ni和Zn 8種重金屬中的某一種或兩種以上含量超過(guò)我國(guó)《土壤環(huán)境質(zhì)量農(nóng)用地土壤污染風(fēng)險(xiǎn)管控標(biāo)準(zhǔn)(試行)》GB15618-2018的風(fēng)險(xiǎn)管控值(超標(biāo)).砷雖然不是重金屬,但因其毒性等性質(zhì)與鎘/鉛等相類似而人為地劃入重金屬一類.在絕大多數(shù)的重金屬淋洗試驗(yàn)中,也是將砷與其它重金屬混為一談的.但這種研究方法,往往忽視了砷與其它重金屬淋洗特性的差異.因?yàn)楫?dāng)土壤的pH值升高或下降時(shí),砷與鎘或鉛等其它重金屬的有效性往往呈相反的變化趨勢(shì),進(jìn)而淋洗效果也是相反的.砷在高pH值的條件下遷移能力增強(qiáng),淋洗去除率較高[3]鎘或鉛等重金屬則在較低pH條件下容易被淋出[4].因此,如果將砷與鎘或鉛等重金屬不加區(qū)分地混在一起進(jìn)行淋洗研究,就可能忽視了砷的淋洗劑的研究或可以同時(shí)高效淋洗砷、鎘、鉛等混合重金屬淋洗劑的開發(fā).例如,尹雪等[5]研究表明,在EDTA和檸檬酸復(fù)合淋洗條件下,土壤中Cd、Pb、Cu、As的最高去除率可分別達(dá)43.39%、27.17%、24.36%和11.72%[5]. As雖然也可以被同時(shí)淋洗出來(lái),但去除率顯然不高.因此,在采用淋洗技術(shù)對(duì)重金屬污染土壤進(jìn)行修復(fù)時(shí),關(guān)于淋洗劑的選用,需要區(qū)別進(jìn)行.可以考慮分為以下三種不同的污染土壤類型.第一類是鎘/鉛等一類重金屬污染土壤,主要由Cd、Cr(六價(jià))、Cu、Pb、Hg、Ni和Zn這7種重金屬中的某一種或二種以上污染的土壤第2類是砷污染土壤,也即土壤中只有砷含量超標(biāo)而其它重金屬不超標(biāo)的土壤以及第3類,鎘-鉛-砷等復(fù)合污染土壤,也就是As與Cd、Cr(六價(jià))、Cu、Pb、Hg、Ni和Zn這7種重金屬中的某一種或二種以上復(fù)合污染的土壤.其中第一類重金屬污染土壤的淋洗劑的研究最為普遍.本文針對(duì)這三類污染土壤,分別對(duì)淋洗劑的淋洗效果和研究現(xiàn)狀進(jìn)行綜述,對(duì)該領(lǐng)域的研究前景進(jìn)行展望,以期為重金屬污染土壤淋洗劑的選用提供一定參考.

1 鎘/鉛等一類重金屬污染土壤淋洗劑的研究

1.1 無(wú)機(jī)淋洗劑

常見(jiàn)的無(wú)機(jī)淋洗劑多為酸溶液和含氯離子的鹽溶液,如HCl、FeCl3、CaCl2等.酸類淋洗劑主要作用是提供酸性環(huán)境,有利于重金屬離子的解離和轉(zhuǎn)化[6].Wang等[6]對(duì)比了H3PO4、HNO3等 6種酸溶液對(duì)Cd和Pb污染土壤的淋洗效果,發(fā)現(xiàn)HNO3對(duì)土壤中的Cd和Pb去除率最高,分別為75.7%和60.6%.原因在于HNO3溶液可以解離出大量的H+,從而促進(jìn)土壤中重金屬的解吸[7].Moon等[8]研究發(fā)現(xiàn)HCl、HNO3、H2SO4對(duì)Zn的淋洗去除率也較高,主要原因也在于降低土壤pH值增強(qiáng)了重金屬的解吸溶出.顯然,這些強(qiáng)酸型的淋洗劑對(duì)土壤環(huán)境的破壞力極強(qiáng),在導(dǎo)致土壤酸化的同時(shí)也會(huì)破壞土壤結(jié)構(gòu)[9].相對(duì)而言, FeCl3對(duì)土壤環(huán)境的影響比上述的強(qiáng)酸要小得多,淋洗效果也很好.陳春樂(lè)等[10]研究表明,FeCl3對(duì)黏壤土中Cd的淋洗效果明顯優(yōu)于NaCl和CaCl2兩種鹽溶液.在某些優(yōu)化的淋洗條件下,FeCl3對(duì)重金屬的去除率甚至高于檸檬酸,如Alaboudi等[11]研究發(fā)現(xiàn)0.5M的FeCl3對(duì)沙質(zhì)土淋洗1h,對(duì)土壤中Pb、Cd和Cr的去除率最高可分別達(dá)到93.79%、97.4%和81.75%.原因可能在于FeCl3水解過(guò)程中產(chǎn)生的酸環(huán)境使Pb等的溶解性提高[4].陳欣園和仵彥卿的研究就表明,經(jīng)FeCl3淋洗修復(fù)后,水稻土對(duì)pH由6.44變?yōu)?.11[12].

FeCl3雖然較上述強(qiáng)酸對(duì)土壤環(huán)境的破壞力相對(duì)的弱一些,但其過(guò)量的無(wú)機(jī)離子可能導(dǎo)致土壤鹽漬化.因此,在兼顧污染土壤的生產(chǎn)等用途時(shí),這一類淋洗劑也應(yīng)慎用.

1.2 螯合劑

螯合劑通過(guò)螯合作用可以與許多種重金屬離子形成穩(wěn)定的水溶性絡(luò)合物,使重金屬由不溶態(tài)轉(zhuǎn)為可溶態(tài)而從土壤表面解吸出來(lái).

通常認(rèn)為,EDTA是最常用的、對(duì)土壤中多種重金屬都具有很強(qiáng)螯合去除能力的化學(xué)淋洗劑之一,但因其很難生物降解,在應(yīng)用方面受到較多限制.近年來(lái)研究發(fā)現(xiàn),谷氨酸N,N-二乙酸(GLDA)的淋洗能力也很強(qiáng)且很容易生物降解. Gluhar等[13]對(duì)比了EDTA和GLDA、亞氨基二琥珀酸四鈉(IDS)、乙二胺二琥珀酸(EDDS) 三種可生物降解螯合劑對(duì)污染土壤中Pb、Zn、Cd的去除效果.結(jié)果發(fā)現(xiàn);在后三種螯合劑中GLDA的去除效果最好,對(duì)Cd和 Zn 去除率分別為76%和33%,大體上與EDTA(去除率分別為71%和29%)相當(dāng),但對(duì)Pb的去除率(58%)低于EDTA(75%),這與Wang等[14]的研究結(jié)論一致.Kaurin等[15]研究結(jié)果也表明,EDTA的淋洗去除效率比GLDA、EDDS和IDS稍高一些.對(duì)于其它的淋洗效果較弱的螯合劑,Feng等[16]研究了乙二胺四亞甲基膦酸(EDTMP)和聚丙烯酸(PAA)對(duì)土壤Cd、Pb、Zn的淋洗效果,EDTMP的去除率較高. EDTMP的修復(fù)可能以螯合作用為主,適合于強(qiáng)酸和強(qiáng)堿性的土壤環(huán)境[16],而PAA的淋洗過(guò)程可能以靜電吸附為主,適用于弱酸和中性條件下的污染土壤的修復(fù).

EDTA在光化學(xué)降解和生物降解方面的性能較差,容易在土壤環(huán)境中長(zhǎng)期存在[15,17],影響土壤有機(jī)質(zhì)含量,降低總氮以及有效態(tài)鉀、鈣、鈉、鎂的含量,進(jìn)而降低土壤肥力[18],對(duì)植物和微生物的生長(zhǎng)造成嚴(yán)重影響[19].與EDTA相比, GLDA最大的特點(diǎn)在于具有良好的生物可降解性,在土壤中28d可降解80%以上,且降解產(chǎn)物對(duì)土壤環(huán)境無(wú)不良影響[20-21],被認(rèn)為是一種較新型的高效的化學(xué)螯合劑,優(yōu)于之前報(bào)道的另一種環(huán)保螯合劑EDDS.此外,亞氨基二琥珀酸四鈉(IDS)也被認(rèn)為是一種新型的螯合劑,其生物降解速度更快,7d可降解80%,毒性較低,環(huán)境友好[15].

1.3 小分子有機(jī)酸

這一類物質(zhì)能使吸附在黏土、腐殖質(zhì)等顆粒表面上的重金屬解吸或溶解,使之?dāng)U散到土壤溶液中,或與有機(jī)酸根離子形成可溶性的絡(luò)合物,從而增加金屬離子的活動(dòng)性而從土壤中淋洗出來(lái)[22].

常見(jiàn)的小分子有機(jī)酸有檸檬酸和酒石酸等[23].黎詩(shī)宏等[24]研究發(fā)現(xiàn);檸檬酸和酒石酸的淋洗效果較好,對(duì)Cd的去除率分別達(dá)到73%和62%,而乙酸和草酸去除能力較弱.檸檬酸和酒石酸具有較高的淋洗去除率與以下兩個(gè)方面原因有關(guān);一是酸溶作用,檸檬酸和酒石酸的初始pH值分別為1.6和1.3,有利于對(duì)重金屬的去除二是螯合作用,它們一般含有多個(gè)配位體,有利于螯合重金屬.檸檬酸的淋洗效果優(yōu)于酒石酸的主要原因在于檸檬酸去除了可交換態(tài)和大部分的可還原態(tài)重金屬,而酒石酸則只去除了可交換態(tài)的重金屬[25].其它的有機(jī)酸,如草酸對(duì)Pb、Zn、Cu、Cd的去除率比較低,這可能是由于草酸與重金屬離子形成的難溶的草酸鹽沉淀,不利于離子洗脫原因造成的[26].

小分子有機(jī)酸在土壤中均易降解,如檸檬酸20d便可降解70%,對(duì)土壤和作物的影響較小[27].因此,雖然其淋洗效果不如螯合劑如EDTA高,但在考慮不造成土壤環(huán)境不良影響的條件下,也得到較廣泛的應(yīng)用.

1.4 表面活性劑

表面活性劑主要通過(guò)改變土壤表面性質(zhì),提高配體的溶解性,或是通過(guò)離子交換等過(guò)程,促進(jìn)金屬陽(yáng)離子或其配合物從固相轉(zhuǎn)移到液相從而使重金屬?gòu)耐寥乐辛芟闯鰜?lái).常見(jiàn)的化學(xué)表面活性劑主要有十二烷基苯磺酸鈉(SDBS)、聚山梨脂(Tween-80)和曲拉通(Triton)等. 生物表面活性劑主要有鼠李糖脂、皂素和烷基多苷等.

鼠李糖脂是目前研究較多的糖脂類生物表面活性劑[28],由親水部分中的一個(gè)或兩個(gè)鼠李糖基團(tuán)和疏水部分中的最多三個(gè)羥基脂肪酸(C8–C14)組成.鼠李糖脂具有較低的臨界膠束濃度,能夠顯著降低溶液表面張力.分子中的羥基和羧基基團(tuán)還可與土壤中的重金屬離子絡(luò)合,使重金屬?gòu)耐寥乐薪j(luò)合出來(lái)[29].Cd的淋洗率隨鼠李糖脂濃度的升高而增大[29],這可能是因?yàn)楫?dāng)鼠李糖脂濃度較低時(shí),其膠束結(jié)構(gòu)不足以形成囊泡狀,不易將Cd 從土壤中解吸出來(lái)隨著濃度的升高,鼠李糖脂膠束結(jié)合形成更大的膠團(tuán),溶出更多的有機(jī)質(zhì),進(jìn)而降低土壤-溶液界面張力,削弱土壤對(duì)Cd 的粘附性,促進(jìn)土壤中Cd的釋放.

皂素是近年來(lái)研究較多的另一種生物表面活性劑[30-31].皂素淋洗去除土壤中重金屬主要是通過(guò)絡(luò)合和增溶作用實(shí)現(xiàn)的.皂素分子外部的羧基、羥基等基團(tuán)可以與重金屬離子形成絡(luò)合物.Ko等[32]研究結(jié)果表明,皂素比相同濃度下的EDTA和EDDS對(duì)Cd有著更高的淋洗去除率,且去除率與淋洗液濃度成正比,這與Zhang等[30]的研究結(jié)果相類似.但也有研究指出,皂素對(duì)污染土壤中Cd的淋洗去除率較低,僅為17.71%[33].造成研究結(jié)果差異較大的原因是多方面的,土壤類型、有機(jī)質(zhì)含量等都會(huì)顯著影響土壤中重金屬的去除率.Gusiatin等[34]的研究表明;皂素對(duì)壤質(zhì)沙土中Cu的去除率為82%,壤土為67%,粉質(zhì)黏土則最低為20%.顯然,土壤中有機(jī)質(zhì)含量和粘性土質(zhì)占比越大,Cu的淋洗去除越困難.但通常認(rèn)為,螯合劑如EDTA的淋洗效果要好于表面活性劑,鼠李糖脂的淋洗效率好于皂素.相較于化學(xué)表面活性劑,生物表面活性因其較好的生物可降解性研究的更多一些.

1.5 植物淋洗劑

主要是通過(guò)植物提取液中的羧基、羥基、酰胺等物質(zhì)與土壤中的重金屬發(fā)生絡(luò)合或離子交換等作用,將重金屬溶解到土壤液體中,最終淋洗出土體[35-36].

Feng等[35]研究了和兩種植物材料浸提液作為淋洗劑對(duì)土壤中Pb、Zn和Cd的淋洗去除效果.結(jié)果表明前者對(duì)Pb、Zn和Cd的去除率顯著高于后者,但去除率相對(duì)于螯合劑而言并不高,分別為5.98%～6.83%、21.82%～27.94%和39.90%～40.74%.徐小遜等[37]研究發(fā)現(xiàn),駁骨丹()浸提液對(duì)土壤中的Cd和Pb淋洗去除率分別為59.81%～72.45%和13.27%～17.27%.淋洗機(jī)理可能與植物材料中含有的官能團(tuán)與土壤中重金屬進(jìn)行離子交換和絡(luò)合反應(yīng)有關(guān)[38].陳月[36]研究發(fā)現(xiàn);透莖冷水花的()、野蕎麥()和異藥花()的水浸提液對(duì)土壤中Pb、Zn和Cd的淋洗去除率分別為38.99%～57.79%、6.93%～6.97%和23.93%～33.98%.淋洗率的高低可能與浸提液中磷酸羥基、氨基、羧基和羰基等含量有關(guān).

植物淋洗劑最大的優(yōu)點(diǎn)是其優(yōu)良的生物降解性,對(duì)土壤環(huán)境干擾較小,淋洗后土壤的有機(jī)碳、全氮、速效氮和全鉀的含量甚至還有所增加.但目前并沒(méi)有廣泛應(yīng)用,影響的因素主要有兩個(gè).一是淋洗劑的原料較為短缺,因?yàn)橥瑢僦参镏g,甚至同一植物不同部分之間對(duì)重金屬的去除效果都可能顯著不同.二是其淋洗效率較螯合劑等較低.多數(shù)研究表明,植物淋洗劑可有效去除土壤中易遷移轉(zhuǎn)化的可交換態(tài)和碳酸鹽結(jié)合態(tài)重金屬,但對(duì)不易遷移轉(zhuǎn)化的鐵錳氧化物結(jié)合態(tài)、活性較差的有機(jī)結(jié)合態(tài)及不能被生物利用的殘?jiān)鼞B(tài)的淋洗去除能力就有限[35,37].

1.6 復(fù)合淋洗劑

上述不同種類的單一淋洗劑由于其組成成分以及淋洗平衡時(shí)間等作用方式的不同,往往有著不同的應(yīng)用范圍和最佳的淋洗條件.單一淋洗劑往往也很難對(duì)多種重金屬同時(shí)具有較好的去除效果.根據(jù)淋洗劑與土壤重金屬的淋洗機(jī)理,將不同類型的單一淋洗劑進(jìn)行優(yōu)化復(fù)配,可能達(dá)到更好的淋洗修復(fù)效果、擴(kuò)大應(yīng)用范圍或減少淋洗劑的用量[29].

Zhang等[30]研究發(fā)現(xiàn),相較于單獨(dú)的檸檬酸淋洗液,加入少量的皂素會(huì)顯著提高Pb的去除率.對(duì)于礦區(qū)Pb污染土壤,對(duì)Pb的去除率最高可達(dá)56.2%. Wang等[9]用黃腐酸鉀和檸檬酸以4:1的體積比制備的復(fù)合淋洗劑,在最佳淋洗條件下,對(duì)Cu和Pb的去除率分別為42.92%和50.46%,均優(yōu)于其單一條件下的淋洗率,解決了單一黃腐酸鉀淋洗劑對(duì)重金屬去除能力較差和單一檸檬酸淋洗劑易導(dǎo)致土壤酸化的問(wèn)題.陳春樂(lè)等[10]比較了3種鹽溶液(NaCl、CaCl2、FeCl3)與HCl復(fù)合的淋洗劑對(duì)Cd污染土壤的淋洗效果,HCl-FeCl3的復(fù)合對(duì)Cd去除率最高,達(dá)78.9%.陶虎春等[39]對(duì)Pb、Zn污染土壤進(jìn)行了淋洗試驗(yàn),淋洗劑對(duì)土壤中Pb、Zn的去除率由高到低依次是草酸-FeCl3復(fù)合淋洗劑>FeCl3>草酸,這也表明復(fù)合的淋洗劑確實(shí)具有協(xié)同作用.

將去除重金屬效果較好的螯合劑如EDTA等,與其它種類淋洗劑如表面活性劑、生物螯合劑等復(fù)配,可以提高對(duì)多種重金屬的淋洗去除效果.EDTA- 2Na和鼠李糖脂復(fù)合淋洗劑 (體積比為1.5:1)就顯著提高了土壤中Cd的去除率[29],淋洗后的土壤中交換態(tài)、碳酸鹽結(jié)合態(tài)和有機(jī)結(jié)合態(tài)Cd含量比淋洗前均下降98%以上,而鐵錳氧化物結(jié)合態(tài)和殘?jiān)鼞B(tài)含量也分別下降了43.66%和18.71%.可能的作用機(jī)制是EDTA的螯合作用和鼠李糖脂的膠束增溶作用產(chǎn)生的協(xié)同效應(yīng)更有利于重金屬的去除.

表1 關(guān)于鎘/鉛等一類重金屬污染土壤的淋洗劑

筆者針對(duì)鎘/鉛等一類重金屬污染土壤的淋洗劑的優(yōu)點(diǎn)和缺點(diǎn)進(jìn)行了簡(jiǎn)要的總結(jié),并綜合其淋洗效果及對(duì)土壤環(huán)境的影響程度,給出了一些推薦意見(jiàn),如表1所示.

2 砷污染土壤淋洗劑的研究

砷是一種變價(jià)元素,包括-3、0、+3、+5四種價(jià)態(tài),在土壤環(huán)境中,砷多以+3或+5價(jià)帶負(fù)電荷的砷含氧陰離子存在,如HAsO42-、H2AsO4-等,砷在環(huán)境中的轉(zhuǎn)化、遷移以及毒性等與其形態(tài)密切相關(guān)[41].與其它重金屬污染土壤的淋洗修復(fù)不同,單一淋洗劑如EDTA等已被證明效果有限,并易對(duì)土壤性質(zhì)產(chǎn)生不利影響[42].目前對(duì)砷污染土壤的淋洗研究相對(duì)較少,報(bào)道主要針對(duì)砷的淋洗劑研究如表2所示.

唐敏等[41]的研究表明;隨著檸檬酸濃度、液土比、淋洗時(shí)間的增加,砷的淋洗率逐漸增大.在最佳淋洗條件下,檸檬酸對(duì)砷的去除率最高可達(dá)70.58%.但陳尋峰等[3]研究卻顯示,檸檬酸在所研究的幾種淋洗劑中對(duì)As的去除能力最低.這些研究結(jié)果說(shuō)明檸檬酸淋洗效果可能受多種因素的影響較大,應(yīng)用性有待商榷.

較多的研究結(jié)果表明堿類淋洗劑如NaOH對(duì)As去除率較高[3,43],其原因主要是NaOH是強(qiáng)堿,會(huì)使土壤中的As在較高的pH條件下遷移能力提高,并能溶解土壤中的礦物成分,有利于離子交換作用過(guò)程,解吸出土壤中的As.盡管NaoH對(duì)砷淋洗效果較好,但其對(duì)土壤環(huán)境的破壞程度也較大.

表2 關(guān)于砷污染土壤的淋洗劑

相比于單一淋洗劑,復(fù)合淋洗劑對(duì)砷的淋洗去除效果更好[3].陳尋峰等[3]先用0.5mol/LNaOH淋洗4h然后再0.1mol/LEDTA淋洗4h進(jìn)行復(fù)合二步淋洗時(shí),土壤As的去除率從66.73%提高到91.83%.陳燦等[43]先用0.5mol/L NaOH 淋洗4h,然后再用0.5mol/L KH2PO4淋洗4h進(jìn)行復(fù)合二步淋洗時(shí),可使土壤中砷的去除率提高到82.60%.主要原因除了較高pH條件下As的遷移能力增強(qiáng)有利于其淋洗去除外,磷酸鹽和砷酸鹽在結(jié)構(gòu)上的相同晶型也利于磷酸根和砷酸根發(fā)生同晶置換,從而將As置換出來(lái)[3,43].此外,Lee等[42]的研究結(jié)果表明,草酸-抗壞血酸復(fù)合淋洗劑對(duì)As的去除量是草酸和抗壞血酸單獨(dú)去除As量總和的3.5倍(pH=2) 和13倍(pH=7).草酸主要是通過(guò)配體促進(jìn)反應(yīng)從土壤中提取As,抗壞血酸鹽主要通過(guò)還原反應(yīng)提取As,二者的協(xié)同作用共同提高了砷的去除率.

總的來(lái)看,關(guān)于砷復(fù)合淋洗劑的研究還較少,且缺少對(duì)高效去除機(jī)理的探究.

3 鎘-鉛-砷等復(fù)合污染土壤淋洗劑的研究

目前對(duì)于鎘-鉛-砷等復(fù)合污染土壤的研究也很少,涉及到的淋洗劑如表3所示.Torres等[44]研究了表面活性劑對(duì)鎘-鉛-砷等復(fù)合污染土壤中重金屬的去除率.結(jié)果表明,Tween-80對(duì)重金屬的總?cè)コ首罡?為67.1%.對(duì)于As,Polafix CAPB的去除率最高(49.7%),其次是Surfacpol 14104 (43.1%)和Tween-80 (42.6%).李尤等[45]研究結(jié)果表明,鼠李糖脂對(duì)Cd、Zn和Cu的去除率較好,分別為53.8%、44.7%、63.2%,但對(duì)Cd和As的去除效果較差,分別為23.1%和34.4%. Bi等[46]從風(fēng)化褐煤中提取一種腐殖物質(zhì),研究其作為一種淋洗劑對(duì)Cd、As污染的三種土壤(紅壤、黑土、潮土)中的污染物去除能力,最優(yōu)淋洗條件下,Cd的去除率為68.2%～88.1%對(duì)As的去除率為44.2%～57.4%,總體去除效率一般.

單一淋洗劑在同時(shí)去除砷和其他重金屬時(shí)具有局限性[47],如EDTA-2Na在去除鎘或鉛等效果較好,但去除砷效果較差[48].相對(duì)的,多種淋洗劑的復(fù)配較單一淋洗劑對(duì)鎘-鉛-砷等復(fù)合污染土壤的淋洗效果更好一些.Wei 等[48]研究表明,淋洗劑EDTA-2Na與磷酸復(fù)合比其各自單獨(dú)使用時(shí)對(duì)污染土壤中As、Cd、Cu和Pb的去除率都高,分別為42.0%、52.0%、31.1%和55.6%.尹雪等[5]研究發(fā)現(xiàn),EDTA-檸檬酸復(fù)合淋洗劑(摩爾比為1:1)在最佳淋洗條件下,對(duì)As、Cd、Cu和Pb的去除率較單獨(dú)的EDTA和檸檬酸均有提高.然而,復(fù)配的淋洗劑有時(shí)也不一定會(huì)同時(shí)提高對(duì)所有重金屬的淋洗去除率.郭軍康等[49]用草酸和EDTA對(duì)Cd和As污染土壤進(jìn)行淋洗研究,草酸單獨(dú)使用時(shí)對(duì)As的去除率為90%EDTA單獨(dú)使用時(shí)對(duì)Cd的去除率為70%.但EDTA和草酸進(jìn)行復(fù)合淋洗時(shí),對(duì)As和Cd的去除率則分別降為80%和50%,說(shuō)明在這方面的研究還有待提高.

表3 關(guān)于鎘-鉛-砷等復(fù)合污染土壤的淋洗劑

4 結(jié)語(yǔ)

一般來(lái)說(shuō),在不考慮污染土壤用途的情況下,對(duì)于重金屬污染土壤來(lái)說(shuō),淋洗技術(shù)修復(fù)效果比較好.然而,從長(zhǎng)遠(yuǎn)的發(fā)展的角度來(lái)看,對(duì)于農(nóng)田重金屬污染土壤或相當(dāng)多的場(chǎng)地污染土壤來(lái)說(shuō),兼顧修復(fù)技術(shù)對(duì)污染土壤環(huán)境的影響,特別是不造成二次污染是大勢(shì)所趨.據(jù)此,筆者在進(jìn)行了簡(jiǎn)要總結(jié)的基礎(chǔ)上,給出了相應(yīng)的選用意見(jiàn).雖然如此,目前關(guān)于淋洗劑的研究仍然存在一些不足,如對(duì)于淋洗過(guò)程中輔助技術(shù)的應(yīng)用關(guān)注較少;更為迫切的是對(duì)于砷以及砷與鎘/鉛等其它重金屬?gòu)?fù)合污染土壤研究較少,可選擇的高效淋洗劑較少,今后的研究重點(diǎn)可能在以下幾個(gè)方面;

1)砷污染土壤或砷與鎘/鉛等復(fù)合污染土壤也是較突出的問(wèn)題,加強(qiáng)這方面淋洗劑的研究,特別是復(fù)合淋洗劑的研究可能更有實(shí)踐意義.

2)復(fù)合淋洗劑對(duì)污染土壤中多種重金屬的去除效率較高,二次污染相對(duì)較小,是未來(lái)研究的重點(diǎn).當(dāng)前研究在淋洗效率上和應(yīng)用方面上均有欠缺,未來(lái)應(yīng)加強(qiáng)對(duì)復(fù)合淋洗劑的研究.

3)目前研究多集中于對(duì)淋洗劑的篩選,對(duì)去除機(jī)理的強(qiáng)化研究可能更有利于高效淋洗劑的應(yīng)用.同時(shí),類似于超聲波、微波或電動(dòng)等輔助技術(shù)的聯(lián)合應(yīng)用也應(yīng)關(guān)注.

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Advances of washing agents in remediation of heavy metal contaminated soil.

XU Lei1,3, DAI Hui-ping2*, WEI Shu-he1**

(1.Key Laboratory of Pollution Ecology and Environment Engineering, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China；2.College of Biological Science & Engineering, Shaan Xi University of Technology, Hanzhong 723001, China；3.College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100039, China)., 2021,41(11)：5237～5244

The research progresses of leaching agents in the remediation of heavy metal contaminated soils were reviewed with focused on three different pollution types, including the soils mainly contaminated by cadmium (Cd) and lead (Pb) of the same category, by arsenic (As), and by the combination of the first two types of heavy metals. In general, the chelating agent for the first type of polluted soil showed higher removal efficiency, especially for GLDA (Glutamic acid, N, N-Diacetic acid), citric acid and so on, which were environment friendly either. For the second type of soil mainly contaminated by As, the removal rate of composite washing agent (CWA) was relatively high like NaOH-EDTA (NaOH-Ethylenediamine tetraacetic acid). In particular, the CWAs, just like NaOH-H3PO4, was with very high removal rates compared to the single washing agents for the third type polluted soil. It was suggested that the CWAs with higher removal efficiency for several heavy metals in soil together and less secondary pollution will be the key research points in the future.

soil washing；washing agents；heavy metal；arsenic

X53

A

1000-6923(2021)11-5237-08

徐 雷(1993-),男,山東青島人,中國(guó)科學(xué)院大學(xué)博士研究生,主要從事重金屬污染土壤修復(fù)研究.發(fā)表論文8篇.

2021-04-25

國(guó)家重點(diǎn)研發(fā)計(jì)劃項(xiàng)目(2018YFC1800501);秦巴生物資源與生態(tài)環(huán)境國(guó)培重點(diǎn)實(shí)驗(yàn)室開放課題(SLGPT2019KF04-02);陜西省市校共建項(xiàng)目(SXJ-2101);國(guó)家高端外國(guó)專家項(xiàng)目(G20200241015)

* 責(zé)任作者, 教授, daihp72@snut.edu.cn; ** 研究員, shuhewei@iae.ac.cn