農(nóng)田土壤重金屬污染化學(xué)鈍化修復(fù)研究進(jìn)展

胡紅青，黃益宗，黃巧云，劉永紅，胡超

（1 華中農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院，武漢 430070；2 農(nóng)業(yè)部環(huán)境保護(hù)科研監(jiān)測(cè)所，天津 300191）

農(nóng)田土壤重金屬污染化學(xué)鈍化修復(fù)研究進(jìn)展

胡紅青1，黃益宗2，黃巧云1，劉永紅1，胡超1

（1 華中農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院，武漢 430070；2 農(nóng)業(yè)部環(huán)境保護(hù)科研監(jiān)測(cè)所，天津 300191）

土壤重金屬化學(xué)鈍化修復(fù)是指向污染土壤中添加鈍化劑，使重金屬由活性向穩(wěn)定化形態(tài)轉(zhuǎn)化，以降低重金屬的遷移和生物可利用性，從而修復(fù)重金屬污染土壤的方法。本文綜述了近些年國(guó)內(nèi)外各類鈍化材料修復(fù)重金屬污染土壤的作用效果和機(jī)理、實(shí)例等方面的研究進(jìn)展，并討論了原位修復(fù)土壤重金屬中亟待解決的問(wèn)題，旨在為農(nóng)田土壤重金屬污染的化學(xué)鈍化劑篩選與應(yīng)用提供參考依據(jù)。

重金屬；土壤污染；化學(xué)鈍化；鈍化材料

隨著城鎮(zhèn)化、工業(yè)化的發(fā)展和城市污泥、廢棄物進(jìn)入農(nóng)業(yè)生態(tài)系統(tǒng)，土壤重金屬污染態(tài)勢(shì)日趨嚴(yán)峻。據(jù)國(guó)家環(huán)保部、國(guó)土資源部等的調(diào)查[1]，我國(guó)土壤各種污染物超標(biāo)點(diǎn)位占調(diào)查總點(diǎn)位的16.1%；而耕地土壤點(diǎn)位超標(biāo)率高達(dá)19.4%，污染情形不容樂(lè)觀。

由于我國(guó)人口壓力大，優(yōu)質(zhì)耕地資源短缺與糧食生產(chǎn)需求的矛盾異常突出，不可能將污染土壤進(jìn)行大規(guī)模休閑、種植非糧食作物或開(kāi)展植物修復(fù)；工程措施則代價(jià)高昂難以實(shí)施，且污染土壤填埋并不去除重金屬類污染物，所以對(duì)農(nóng)田重金屬污染土壤而言，切實(shí)可行且能保證作物安全生產(chǎn)的修復(fù)措施應(yīng)是化學(xué)鈍化，尤其是對(duì)中輕度污染的農(nóng)田土壤。

化學(xué)鈍化修復(fù)是向污染土壤中施入各種鈍化劑，利用吸附、沉淀、氧化還原、絡(luò)合等機(jī)制，改變污染物的形態(tài)與活性，使其轉(zhuǎn)化成非活性、植物難吸收的組分，從而實(shí)現(xiàn)修復(fù)利用的技術(shù)。目前采用的鈍化劑主要包括各類含磷物質(zhì)、粘土礦物、生物炭、氧化物、有機(jī)物等，它們對(duì)不同污染物以及土壤類型、污染程度的修復(fù)效果有一定差異，相關(guān)綜述論文也常見(jiàn)報(bào)道。本文就一些主要的化學(xué)鈍化材料修復(fù)重金屬污染研究進(jìn)展作一概述，為進(jìn)一步推動(dòng)農(nóng)田重金屬污染土壤修復(fù)研究與應(yīng)用提供參考。

1 含磷物質(zhì)對(duì)污染土壤中重金屬的鈍化

含磷物質(zhì)除提供植物磷營(yíng)養(yǎng)外，對(duì)重金屬的鈍化修復(fù)是當(dāng)前土壤重金屬污染修復(fù)研究的熱點(diǎn)領(lǐng)域之一，也是一種廉價(jià)、環(huán)境友好的修復(fù)材料[2]，其可以通過(guò)釋放磷來(lái)有效地固定土壤中的重金屬。在實(shí)際應(yīng)用中，常見(jiàn)的含磷材料有磷酸及可溶性磷酸鹽，磷酸鈣、磷灰石、磷礦粉、骨粉等難溶含磷材料[3–5]，以及活化磷礦粉、溶磷菌–磷礦粉、動(dòng)物糞便–磷礦粉堆肥等復(fù)合含磷材料[6–9]。含磷材料修復(fù)的對(duì)象主要包括 Pb、Cd、Cu、Zn、Ni、Hg、Cr、Co 以及 As 等[2–9]。

磷酸鹽可以直接參與土壤重金屬的鈍化，也常與其他礦物材料混合使用。采用磷酸酸化磷礦粉處理Pb污染的土壤，可將土壤中非殘?jiān)鼞B(tài)Pb轉(zhuǎn)化為殘?jiān)鼞B(tài)，降低土壤中Pb的淋溶毒性[10]。Pb與磷形成了極穩(wěn)定的磷氯鉛礦[Pb5(PO4)3Cl]，明顯降低了植物對(duì)Pb的吸收[11]。另外，用磷酸化生物炭處理鉛污染土壤，發(fā)現(xiàn)其有很好的修復(fù)效果[12]。與磷酸的鈍化作用相比較，可溶性磷酸鹽 (如磷酸銨、磷酸氫鉀) 等也可直接參與重金屬的鈍化作用。用磷酸氫二銨處理土壤60天后，Cd的溶出量從306 mg/kg降低到34 mg/kg，磷含量增加會(huì)相應(yīng)提高Cd的穩(wěn)定效果[13]。雷鳴等[14]研究了磷酸氫二鈉對(duì)污染土壤中重金屬(Pb、Cd、Zn) 向水稻遷移的影響，發(fā)現(xiàn)其顯著提高了土壤pH值，降低了土壤中交換態(tài)Pb、Cd、Zn含量，同時(shí)明顯降低了水稻各器官中Pb、Cd的含量。

過(guò)磷酸鈣和重過(guò)磷酸鈣等也被用于修復(fù)重金屬污染的土壤。用過(guò)磷酸鈣修復(fù)Pb、Cu污染的土壤，一段時(shí)間培養(yǎng)后，Pb和Cu大幅度轉(zhuǎn)化為殘?jiān)鼞B(tài)[15]。重過(guò)磷酸鈣用于鈍化修復(fù)Pb、Cu和Zn復(fù)合污染土壤，4周后發(fā)現(xiàn)可有效地降低提取態(tài)Pb和Cu，但對(duì)土壤中Zn的穩(wěn)定化影響較?。涣滋幚砜梢种芇b和Cu在土壤剖面中的徑向遷移[16]。在Pb、Cd、Cu和Ni污染的土壤中施加重過(guò)磷酸鈣處理后，Pb和Cd向殘?jiān)鼞B(tài)轉(zhuǎn)化[17]，降低大白菜對(duì)重金屬的吸收[18]。林笠等[19]采用盆栽試驗(yàn)研究了重金屬Cd、Pb復(fù)合污染土壤中添加磷對(duì)草莓累積重金屬的影響，結(jié)果表明，添磷后不僅能顯著降低Cd、Pb對(duì)草莓產(chǎn)量和品質(zhì)的影響，還能降低Cd、Pb在各組織中的累積。

含磷材料還包括磷酸鈣、天然磷灰石、磷礦粉、骨粉等難溶磷酸鹽礦物，它們是堿性礦物，有效磷遠(yuǎn)低于可溶性磷酸鹽及磷肥。用磷礦粉處理重金屬污染的土壤能增加植物對(duì)As的吸收，降低蕨類植物體內(nèi)Pb、Cd含量[20]。羥基磷灰石可顯著降低土壤中Pb、Zn、Cd、Co和Ni的生物有效性，增強(qiáng)它們的地球化學(xué)穩(wěn)定性[21–22]。納米磷材料的性質(zhì)有別于普通含磷礦物，用納米Ca3(PO4)2處理射擊場(chǎng)的重金屬Pb、Cu、Zn污染后，土壤中可提取態(tài)重金屬大幅度降低，部分Cu和Pb結(jié)合在納米磷酸鈣表面[23]；而用負(fù)載納米羥基磷灰石的生物炭原位修復(fù)Pb污染土壤，Pb的固定率達(dá)到74.8%，殘?jiān)鼞B(tài)增加到66.6%，土壤中生物有效性Pb顯著減少[24]。

難溶磷礦物的磷有效性低，為提高有效磷的釋放，溶磷菌–磷礦粉、有機(jī)酸活化磷礦粉、動(dòng)物糞便–磷礦粉堆肥等也被用于處理不同污染程度的土壤。磷礦粉經(jīng)處理后，有效磷含量提高，對(duì)重金屬的鈍化效率也高于原磷礦粉。Park等[6]利用溶磷菌處理磷礦粉后，固定污染土壤中的Pb效果更強(qiáng)。與溶磷菌相比，草酸處理磷礦粉后，能更好地鈍化土壤中重金屬Pb、Cu、Cd，毒性淋溶分析顯示Pb含量低于美國(guó)EPA標(biāo)準(zhǔn)[9]；磚紅壤中施加磷礦粉和草酸活化磷礦粉后，交換態(tài)鉛含量下降，穩(wěn)定態(tài)Pb、Cu含量增加，且活化磷礦粉的效果更佳[7]。許學(xué)慧等[8]在Cd、Cu污染的礦區(qū)土壤中添加磷礦粉和活化磷礦粉，可降低土壤中交換態(tài)重金屬的含量，減少萵苣對(duì)重金屬Cd和Cu的吸收；施加活化磷礦粉后萵苣根和地上部重金屬含量比對(duì)照最高可降低55%和59%。

含磷材料在土壤重金屬原位修復(fù)中具有重要的實(shí)際意義。該方法對(duì)土壤環(huán)境的擾動(dòng)少，除了提供磷素外，大部分磷材料可提高土壤的pH，影響重金屬在土壤中的形態(tài)，加快重金屬由可溶性向難溶性的轉(zhuǎn)化，減少植物對(duì)重金屬的吸收。現(xiàn)有研究表明，含磷材料主要對(duì)重金屬Pb、Cd、Cu等有較好的鈍化效果，其機(jī)理表現(xiàn)在以下方面[3, 24–25]：提高土壤pH，使重金屬離子生成氫氧化物沉淀；利用釋放的磷酸根與重金屬離子作用，生成溶解度更小的磷酸鹽礦物 (磷氯鉛礦等)；土壤重金屬離子與含磷礦物晶格中的陽(yáng)離子發(fā)生同晶置換而被固定；金屬陽(yáng)離子在礦物表面發(fā)生靜電吸附和共沉淀作用被固定(圖1)，實(shí)際環(huán)境中這幾種作用機(jī)理可能是共存的。

2 無(wú)機(jī)礦物對(duì)重金屬的鈍化

無(wú)機(jī)礦物也常用于土壤重金屬的化學(xué)鈍化，主要包括膨潤(rùn)土、凹凸棒石、海泡石、沸石等無(wú)機(jī)礦物，赤泥、飛灰、磷石膏和白云石殘?jiān)裙I(yè)副產(chǎn)物。此外，還有一些化學(xué)制品，如硫酸亞鐵等。這些通常不提供植物營(yíng)養(yǎng)成分，而且可以改良土壤性質(zhì)。

2.1 粘土礦物鈍化劑

用于土壤污染物鈍化的粘土礦物主要包括海泡石、凹凸棒石、膨潤(rùn)土 (蒙脫石) 等，它們較大的比表面積決定了其良好的吸附性能，可通過(guò)吸附、離子交換、配位反應(yīng)和共沉淀等反應(yīng)鈍化重金屬[26]。

圖1 含磷材料對(duì)土壤中重金屬的鈍化作用機(jī)理Fig. 1 Immobilization of heavy metals in soil by phosphorous-containing materials

凹凸棒石也稱坡縷石，對(duì)Cd、Pb和Cu污染土壤具有良好的修復(fù)效果[27]。其對(duì)Zn的鈍化以吸附和表面絡(luò)合為主[28]，對(duì)Cd以碳酸鹽、氫氧化物或表面絡(luò)合的形式固定[29]。謝晶晶等[30]認(rèn)為，Zn2+在凹凸棒石表面先發(fā)生快速吸附，其后為慢速沉淀，表面快速水化時(shí)可提高懸浮液的pH值，誘導(dǎo)了Zn2+水解沉淀。

Zhang等[31]實(shí)驗(yàn)證明凹凸棒石添加量為紅壤的1%～4% (質(zhì)量比) 時(shí)，土壤中可提取態(tài)重金屬的濃度都有明顯降低。殷飛等[28]發(fā)現(xiàn)添加20%凹凸棒石降低可提取態(tài) Pb、Cd、Cu、As的比例最高達(dá)35%～54%，植物易吸收的可交換態(tài)Pb顯著減少，殘?jiān)鼞B(tài)Pb顯著增加。Liang等[29]也表明，凹凸棒石能降低水稻土中Cd的可交換態(tài)，增加碳酸鹽結(jié)合態(tài)和殘?jiān)鼞B(tài)，并降低糙米中23%～56%的Cd。

凹凸棒石對(duì)重金屬的吸附能力可通過(guò)改性得到加強(qiáng)。將凹凸棒石改性成微納米網(wǎng)加入污染土壤，能明顯降低土壤 Cr (Ⅵ) 的淋洗量，并能將 Cr (Ⅵ) 還原成Cr (Ⅲ)[32]。添加10%富鈣凹凸棒石可以分別降低土壤酸溶態(tài) Cd 56% 和 Pb 82%[33]。凹凸棒石–磁鐵復(fù)合物在去除U (Ⅵ) 方面比單一組分更優(yōu)越[34]。

蒙脫石摻入沉積物后可固定Zn，但不能提高Cu的穩(wěn)定性[35]。0.5%膨潤(rùn)土可明顯降低Pb、Zn和Cd的水溶性[36]。Zhang等[31]發(fā)現(xiàn)蒙脫石對(duì)Cu吸附量可達(dá)3741 mg/kg，按2%施入土壤可降低對(duì)蚯蚓60%的重金屬毒性。

相比較單一蒙脫石，其改性產(chǎn)物的環(huán)境應(yīng)用正引起更多關(guān)注。蒙脫石–OR–SH (鈣基蒙脫石酸活化后，在乙醇–水–巰基硅烷溶液中分散) 飽和吸附的Cd無(wú)毒性，在連續(xù)盆栽4季作物后，對(duì)土壤Cd仍保持顯著的鈍化效果[37]。施加巰基化改性膨潤(rùn)土能有效固定土壤Cd和Pb，顯著降低土壤中重金屬的活性態(tài)含量，并將其轉(zhuǎn)化為穩(wěn)定的鐵錳結(jié)合態(tài)，有較好的鈍化長(zhǎng)效性。另外，蒙脫石與有機(jī)聚合物的復(fù)合研究也有大量報(bào)道[38–39]。將殼聚糖加載到蒙脫石后，該復(fù)合物對(duì)Pb2+、Cu2+和Cd2+的最大吸附量分別為 49.3、28.2 和 20.6 mg/g[40–41]。

海泡石有較好的重金屬吸附能力[42]，能降低水稻土中可交換態(tài)Cd并增加碳酸鹽結(jié)合態(tài)和殘?jiān)鼞B(tài)，使Cd以碳酸鹽、氫氧化物或者表面絡(luò)合的形式被固定[29]。添加0.5%～5%的海泡石可降低菠菜對(duì)Cd吸收量的28.0%～72.1%，當(dāng)5%海泡石加入土壤，酶活性和微生物量也得以恢復(fù)[43]。海泡石的添加可使TCLP-Cd降低0.6%～49.6%，而植物吸收降低14.4%～84.1%[44]。將1%～5%海泡石加入土壤后，Cd、Zn和Pb的淋洗量降低60%～70%，而苜蓿莖稈中Zn的濃度最高降低45%。當(dāng)添加量為5%時(shí)，土壤呼吸活性、脫氫酶和堿性磷酸酶活性分別增加了25%、138% 和42%[45]。Li等[46]的實(shí)驗(yàn)則表明，可交換態(tài)Cd降低14.3%～49.0%，而糙米中的Cd含量降低34.5%～44.4%。海泡石改性后有更好的鈍化效果，如經(jīng)過(guò)氧化氫改性后可極大地促進(jìn)其對(duì)Pb的吸附，比天然海泡石提高43.5%[47]。

2.2 工業(yè)廢棄物的應(yīng)用

粉煤灰顆粒呈多孔型蜂窩狀結(jié)構(gòu)，比表面積大，堿性，具有較高的吸附重金屬能力[48]?？墒┤胛廴就寥酪怨潭ㄖ亟饘賉49]。實(shí)驗(yàn)表明，經(jīng)粉煤灰改良后，土壤中Hg、Cd和Pb有效態(tài)含量平均降低 24.4%～31.8%，鈍化作用明顯[50]。

自然沸石或改性沸石均可用于穩(wěn)定土壤中重金屬污染物[51]。其作用機(jī)理是通過(guò)增加堿度而促進(jìn)表面對(duì)重金屬的吸附；或重金屬離子與沸石內(nèi)陽(yáng)離子的交換。通過(guò)在沸石的孔口附近交換陽(yáng)離子來(lái)改變其孔道的尺寸，可賦予沸石新的吸附性能[52]。研究表明，2%沸石在土壤中培養(yǎng)一個(gè)月可導(dǎo)致Zn、Pb的生物有效性降低 15.9%和6.1%[43]。污染土壤中添加沸石可增加淋出液pH并降低Pb的生物有效性[53]。硝酸鉀、氫氧化鈉改性沸石比天然沸石能更顯著地降低土壤酸提取態(tài)Zn的含量[52]。

赤泥是鋁土礦經(jīng)強(qiáng)堿浸出氧化鋁后產(chǎn)生的殘?jiān)?。在含Pb 913 mg/kg的土壤中加入1%赤泥，可以使NH4NO3提取Pb降低90%[54]。添加5%赤泥可使土壤交換態(tài)Pb和Zn降低99%以上[55]。2個(gè)月赤泥處理使生物有效性Cu含量比對(duì)照降低 13.2%[56]。但也有研究表明，5%的赤泥添加使Cd、Ni、Pb和Zn的不穩(wěn)定態(tài)降低22%～80%，而As和Cu的不穩(wěn)定態(tài)卻分別增加了24%和47%，當(dāng)赤泥添加量為5%或更高時(shí)，Cd、Ni、Pb和Zn流動(dòng)性的降低更甚于 As、Cu、Cr和 V[54]。

2.3 其他材料的應(yīng)用

一些鐵基材料也用于土壤重金屬的鈍化。如鋼渣具有較高的pH值，導(dǎo)致重金屬形成化學(xué)沉淀。據(jù)殷飛等[28]報(bào)道，添加20%鋼渣能顯著降低土壤中可交換態(tài)Cd以及可交換態(tài)和碳酸鹽結(jié)合態(tài)Zn含量，并顯著增加殘?jiān)鼞B(tài)Cu含量。據(jù)報(bào)道，硫酸亞鐵加入土壤1個(gè)月后，土壤酸提取態(tài)As含量比對(duì)照處理降低86.6%，2個(gè)月后，土壤As的生物有效性含量比對(duì)照降低90.8%，優(yōu)于骨炭、磷酸二氫鈣和堆肥[57]。隨硫酸鐵用量增加，對(duì)有效態(tài)As的固定效果明顯增加；當(dāng)Fe3+/ PO43–摩爾比為 7.2 時(shí)，7 d 后土壤有效態(tài)Pb、Cd、As去除率分別為99%、41%、69%。

Rinklebe等[58]比較了活性炭、膨潤(rùn)土、生物炭、殼聚糖、粉煤灰、有機(jī)粘土、沸石等對(duì)Cu污染土壤的修復(fù)能力。除有機(jī)粘土和沸石外，其他改良劑均明顯增加土壤pH。Tica等[59]比較了磷灰石和Slovakite (白云巖、膨潤(rùn)土、沸石等的混合物) 的鈍化效果，兩者均能有效降低重金屬Pb、Zn、Cu和Cd的毒性，而Slovakite效果更佳。

大量天然及廢棄物材料因廉價(jià)易得吸引了許多研究者的關(guān)注。目前對(duì)這些材料的應(yīng)用特性和效能已有許多試驗(yàn)，但以下方面尚需進(jìn)一步加強(qiáng)研究：1) 單一礦物對(duì)重金屬的微觀穩(wěn)定機(jī)制；2) 鈍化劑加入后重金屬的長(zhǎng)期穩(wěn)定性；3) 粘土礦物的改性及產(chǎn)物的效能。

3 生物炭對(duì)重金屬的鈍化

3.1 生物炭鈍化土壤重金屬的效果及機(jī)理

生物炭是土壤重金屬修復(fù)研究中的一種重要材料。田間試驗(yàn)證明，小麥秸稈生物炭可有效固定土壤中的Cd和Pb[60]。將稻稈和稻殼生物炭施入土壤，短期內(nèi)可以有效鈍化重金屬[61–62]。生物炭對(duì)重金屬生物有效性的影響源于改變土壤pH，增加土壤有機(jī)質(zhì)含量，改變土壤氧化還原狀況及微生物群落組成等多種機(jī)制的協(xié)同作用，而生物炭對(duì)重金屬的吸附機(jī)理主要有靜電作用、離子交換、陽(yáng)離子π鍵、沉淀反應(yīng)等[63]。

生物炭對(duì)重金屬的鈍化效果受到多因素的影響，如生物炭的來(lái)源、制備條件 (溫度、炭化時(shí)間等)、土壤性質(zhì)、重金屬種類及污染程度等。生物炭的表觀性質(zhì)在一定程度上決定了其對(duì)重金屬的固定能力。不同原材料和熱解溫度會(huì)得到性質(zhì)不同的生物炭，對(duì)土壤重金屬的修復(fù)效果和機(jī)制也有差別。硬木在600℃時(shí)制得的生物炭對(duì)Cu和Zn的吸附量高于棉花秸稈450℃時(shí)制得的生物炭[64]。將竹炭和水稻秸稈生物炭按不同比例施加到Cu、Pb、Zn、Cd污染土壤中，發(fā)現(xiàn)后者鈍化效果更好[65]。

3.2 生物炭復(fù)合材料的研究

生物炭因其在高溫裂解過(guò)程中部分基團(tuán)損失、吸附后分離難等不足，已有學(xué)者開(kāi)始研究將生物炭與其他材料復(fù)合或者進(jìn)行化學(xué)改性，加強(qiáng)其吸附能力。主要有以下方法：1) 用 KOH、H2O2、O3、H2SO4/HNO3等改性生物炭，提高生物炭的比表面積，增加其表面官能團(tuán) (如羧基)，提高對(duì)污染物的固定能力[66–67]； 2) 與磁性吸附劑 (如納米氧化鐵、零價(jià)鐵等)復(fù)合，可以賦予生物炭磁性[68]，利于回收；3) 結(jié)合納米技術(shù)制備新型復(fù)合材料，提高生物炭的封存和處理能力；4) 用化學(xué)修飾法將錳或鎂氧化物、過(guò)磷酸鈣等與生物炭復(fù)合，在生物炭表面添加一些能與污染物相互作用的基團(tuán)，提高吸附效果[69–70]。

Inyang等[67]對(duì)比了甘蔗渣生物炭與經(jīng)厭氧消化的甘蔗渣制備的生物炭對(duì)水中Pb2+的去除效果，發(fā)現(xiàn)后者對(duì)Pb2+的最大吸附量是前者的20倍。Agrafioti等[71]分別將CaO溶液、FeO粉末、FeCl3溶液與稻殼、有機(jī)固體廢棄物混合，用于As (V) 的去除，發(fā)現(xiàn)其對(duì)As的去除率顯著高于原始生物炭。Zhao等[70]研究表明用生物炭與磷肥共熱解后增加生物炭對(duì)重金屬的固定率。

4 石灰對(duì)重金屬的鈍化

4.1 石灰對(duì)土壤重金屬的鈍化效果與機(jī)制

鈣可與鎘發(fā)生同晶替代作用。試驗(yàn)表明，施用生石灰處理在2年中可使糙米中鎘含量降低至國(guó)家食品衛(wèi)生標(biāo)準(zhǔn)限值 (0.2 mg/kg) 以下[72]。Pandit等[73]研究發(fā)現(xiàn)施石灰能降低菠菜中鎘的濃度。Tan等[74]研究石灰鈍化土壤后5種蔬菜 (萵苣、大白菜、花椰菜等) 體內(nèi)含鎘量的變化，發(fā)現(xiàn)其降低40%～50%。

施用石灰可降低土壤中有效態(tài)銅含量[75]。鉛污染土壤經(jīng)石灰處理后，玉米對(duì)鉛的吸收明顯下降，其籽粒含鉛量可達(dá)到國(guó)家食品衛(wèi)生標(biāo)準(zhǔn)[76]。吳烈善等[77]在人工污染的黃色黏土中添加石灰處理，土壤Pb、Cu、Cd、Zn的穩(wěn)定率可達(dá)98.5%～99.8%。石灰對(duì)鉻 (Cr6+) 和汞 (Hg2+) 的吸附很穩(wěn)定[78]，施用 6% 石灰后，土壤能固定69%的Cr6+和63%的Hg2+。

石灰通過(guò)降低土壤中H+濃度，增加土壤顆粒表面負(fù)電荷，促進(jìn)對(duì)重金屬離子的吸附，降低重金屬的遷移性。另外，石灰可改變重金屬形態(tài)，促進(jìn)金屬碳酸鹽形成，減少活性重金屬的比例[79]。

4.2 石灰與其他材料配施的效果

2% 石灰–燒石膏–木炭 (質(zhì)量比 3∶1∶2) 施用在湖南衡陽(yáng)一土壤中，鎘固定率達(dá)58.9%[80]。2%天然腐熟牛糞 +2%石灰組合施用，Pb、Cu、Cd、Zn穩(wěn)定效率達(dá)95.9%～99.4%[77]。石灰和有機(jī)肥復(fù)合施用使土壤中交換態(tài)Cd含量降低54.7%，遠(yuǎn)高于單獨(dú)施用石灰的[81]。Wang等[82]在草甸土進(jìn)行Cd的鈍化實(shí)驗(yàn)，0.2%石灰 +5%蛇紋石復(fù)配的效果最好，處理60 d后有效態(tài)Cd含量降低29.1%。He等[83]研究施用石灰、礦渣和甘蔗渣在第四紀(jì)紅黏土的鈍化效果，發(fā)現(xiàn)復(fù)合施用效果最佳，鎘含量降低58.3%～70.9%，結(jié)合種植低Cd積累的水稻品種，可使糙米中的Cd含量降至污染物限度。

5 其他鈍化劑對(duì)重金屬的鈍化

5.1 有機(jī)鈍化劑

有機(jī)物料不僅提供植物養(yǎng)分，改良土壤，同時(shí)也是有效的土壤重金屬吸附、絡(luò)合劑，被廣泛應(yīng)用于土壤重金屬污染修復(fù)中。有機(jī)物通過(guò)提升土壤pH、增加土壤陽(yáng)離子交換量、形成難溶性金屬–有機(jī)絡(luò)合物等方式來(lái)降低土壤重金屬的生物可利用性[84–86]。目前常用的有機(jī)鈍化劑主要包括植物秸稈[87–88]、畜禽糞便[89–90]、城市污泥和有機(jī)堆肥等[84, 91]。

紫云英施入農(nóng)田中，土壤有效銅和鎘的含量降低，同時(shí)降低了稻草和谷粒中銅和鎘的含量[87]。水稻秸稈和磷肥混施可降低土壤中重金屬的植物有效性[92]。水稻秸稈堆肥施用增加了農(nóng)田土壤中重金屬Zn、Cd和Pb的碳酸鹽結(jié)合態(tài)、鐵錳氧化物結(jié)合態(tài)、有機(jī)質(zhì)結(jié)合態(tài)和殘?jiān)鼞B(tài)重金屬的比例，也降低了農(nóng)田土壤中重金屬的生物有效性[88]。

家禽糞便、生物固體等可增加土壤中溶解性有機(jī)質(zhì)含量，并與重金屬形成較穩(wěn)定的金屬–有機(jī)絡(luò)合物，降低重金屬的生物可利用性，特別是腐熟度較高的有機(jī)質(zhì)可通過(guò)形成粘土–金屬–有機(jī)質(zhì)三元復(fù)合物增加重金屬吸附量[93]。施用豬糞后，稻麥兩季表層土壤重金屬Cu、Zn含量略有升高，靜態(tài)環(huán)境容量均降低[94]。家禽糞便、生物固體等使用后，可強(qiáng)烈地與Hg結(jié)合而固定之[95]。在農(nóng)田土壤中添加豬糞，可使土壤有效銅、鎘顯著降低，同時(shí)也極大降低稻草和谷粒中銅、鎘的含量[87]。

Hashimoto等[89]研究了畜禽糞便對(duì)Pb淋溶的影響，發(fā)現(xiàn)畜禽糞便能顯著降低土壤水溶態(tài)及可交換態(tài)Pb含量，促使其向殘留態(tài)轉(zhuǎn)化，降低其遷移和生物可利用性。張亞麗等[90]向Cd污染土壤施加豬糞等有機(jī)物，也得到類似結(jié)果。施用15 g/kg的糞肥和壓濾泥漿均降低了土壤外源Ni的植物有效性[86]。

腐熟堆肥施入土壤后可減少重金屬的生物有效性[96]，不但可以顯著降低污染土壤中As、Cd、Pb、Zn等的生物有效態(tài)含量，還可顯著降低植物對(duì)重金屬的吸收[96]。添加生物堆肥到銅污染土壤中，顯著降低了CaCl2提取的銅含量，增加了土壤的pH值[97]。

腐殖酸能與重金屬結(jié)合，也是土壤重金屬的鈍化劑。用腐植酸與膨潤(rùn)土 (或過(guò)磷酸鈣) 處理Pb污染土壤，發(fā)現(xiàn)分別投加20%腐植酸與20%膨潤(rùn)土、10%腐植酸與6%過(guò)磷酸鈣，固定40 d后土壤中有效態(tài)鉛含量均大幅降低[98]。添加主要成分為腐殖酸的褐煤到銅污染土壤中，顯著降低了土壤中CaCl2提取的銅含量[97]。

5.2 鐵粉

納米鐵或含鐵納米材料在土壤重金屬治理過(guò)程中也發(fā)揮著重要的作用。有研究者利用零價(jià)納米鐵降低污染土壤中Cd、Cr和Zn的有效性，發(fā)現(xiàn)其能明顯提高金屬的穩(wěn)定性，對(duì)Cr的修復(fù)效果和穩(wěn)定性很好[98]。研究證實(shí)，有機(jī)堆肥配合鐵砂等在鈍化重金屬污染物時(shí)表現(xiàn)出加和作用，可顯著降低重金屬的生物有效性，并可能超過(guò)無(wú)機(jī)鈍化劑的單獨(dú)作用[91]。

納米零價(jià)鐵粉施于砷污染土壤中，能使砷由水溶態(tài)和吸附態(tài)向非晶質(zhì)鐵鋁氧化物結(jié)合態(tài)和晶質(zhì)鐵鋁氧化物態(tài)轉(zhuǎn)化，其中水溶態(tài)和吸附態(tài)砷可減少70%和18%，而非晶質(zhì)鐵鋁氧化物結(jié)合態(tài)和晶質(zhì)鐵鋁氧化物態(tài)砷分別最大增加42%和51%，并顯著降低三七中的砷含量[99]。磷酸鐵納米材料可以顯著降低土壤中水溶態(tài)、可交換態(tài)和碳酸鹽結(jié)合態(tài)Cu含量，促使Cu向殘?jiān)鼞B(tài)轉(zhuǎn)化；鐵納米材料可顯著降低土壤淋洗液中Cr含量[100]。

納米零價(jià)鐵配合低分子量有機(jī)酸施用可增加農(nóng)田土壤中鉛的去除，0.2 mol/L檸檬酸配合2.0 g/L零價(jià)鐵對(duì)農(nóng)田土壤鉛的去除效率能增加83%[101]。生物炭負(fù)載納米零價(jià)鐵能有效固定土壤中鉻，當(dāng)施用8 g/kg生物炭負(fù)載納米零價(jià)鐵于土壤中15 d后，土壤中六價(jià)鉻不可檢出，進(jìn)而降低鉻在土壤–植物系統(tǒng)的轉(zhuǎn)移[102]。

6 結(jié)語(yǔ)

隨著我國(guó)農(nóng)田土壤重金屬污染面積的增加，尋找切實(shí)可行的處置方法刻不容緩。從國(guó)內(nèi)外的研究與實(shí)踐來(lái)看，土壤重金屬的化學(xué)鈍化措施可以較好地固定重金屬，降低重金屬的活性和環(huán)境風(fēng)險(xiǎn)，但是該技術(shù)在實(shí)際應(yīng)用中尚有一些亟待深入研究的問(wèn)題。

1) 鈍化與其他技術(shù)聯(lián)用 鈍化能使重金屬的形態(tài)暫時(shí)改變，但并未從土壤中徹底根除。當(dāng)外界條件改變時(shí)，固定的重金屬還可能重新釋放，導(dǎo)致二次污染。微生物修復(fù)技術(shù)利用微生物產(chǎn)生的硫化物等來(lái)固定土壤中重金屬，具有持久性作用。此外，利用作物輪作–磷修復(fù)措施也可以較好地修復(fù)農(nóng)田重金屬污染。

2) 方案優(yōu)選及鈍化劑改性 污染土壤常是多種重金屬共存的體系，同時(shí)地域、氣候等環(huán)境因素對(duì)鈍化劑的要求不完全相同。因此，必須結(jié)合每種重金屬的性質(zhì)來(lái)選擇不同的鈍化劑和修復(fù)措施。鈍化劑改性可以根據(jù)不同重金屬特性增強(qiáng)其鈍化功能，形成廣譜性多功能鈍化材料。

3) 新型高效環(huán)保鈍化劑研發(fā) 鈍化劑包括人工合成的材料和天然材料，有些天然材料中含有重金屬以及放射性物質(zhì)，遺留在土壤環(huán)境中也會(huì)對(duì)環(huán)境造成一定的副作用，當(dāng)它們累積到一定量時(shí)，這些材料的環(huán)境負(fù)效應(yīng)就需要考慮了。因此在選用不同材料修復(fù)被重金屬污染的土壤時(shí)，必須環(huán)境友好，同時(shí)要提高其修復(fù)效率。

4) 鈍化機(jī)理與產(chǎn)物穩(wěn)定性 鈍化劑的性質(zhì)是決定鈍化重金屬機(jī)理的主要因素。當(dāng)前，宜對(duì)不同材料鈍化重金屬機(jī)制開(kāi)展深入研究，為進(jìn)一步的實(shí)踐奠定理論基礎(chǔ)。在所形成的重金屬難溶物中，氫氧化物和碳酸鹽的溶解度要大于磷酸鹽沉淀物的溶解度，所以，利用重金屬的溶解性選用不同的鈍化劑和措施可以有效地降低重金屬的生物活性，更多地將重金屬離子轉(zhuǎn)化為活性更低的難溶礦物，以達(dá)到更強(qiáng)的鈍化效果。

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Research progress of heavy metals chemical immobilization in farm land

HU Hong-qing1, HUANG Yi-zong2, HUANG Qiao-yun1, LIU Yong-hong1, HU Chao1
( 1 College of Resource and Environment, Huazhong Agricultural University, Wuhan 430070, China;2 Agro-Environmental Protection Institute, Ministry of Agriculture, Tianjin 300191, China )

Soil heavy metal pollution remediation by chemical immobilization is a method that applies immobilization reagent to reduce the mobilization and availability of heavy metals. This paper summarized the research progress of efficiency and mechanism of various immobilization materials to agricultural land, and discussed the main problems for in situ remediation. The purpose of the paper is to provide base for choice and application of immobilization materials.

heavy metal; soil pollution; chemical immobilization; immobilization materials

2017–07–31 接受日期：2017–09–29

國(guó)家科技支撐計(jì)劃課題（2015BAD05B02）資助。

胡紅青（1966—），男，湖北荊門市人，博士，教授，主要從事土壤重金屬污染修復(fù)領(lǐng)域研究。E-mail：hqhu@mail.hzau.edu.cn