長(zhǎng)期凍融循環(huán)下固化鉛鋅鎘復(fù)合重金屬污染土抗剪強(qiáng)度及浸出特征研究

王 瑤,楊忠平,周 楊,常嘉卓,李登華,羅 俊

長(zhǎng)期凍融循環(huán)下固化鉛鋅鎘復(fù)合重金屬污染土抗剪強(qiáng)度及浸出特征研究

王 瑤1,2,楊忠平2*,周 楊1,常嘉卓2,李登華2,羅 俊1

(1.中機(jī)中聯(lián)工程有限公司,重慶 400039；2.重慶大學(xué)土木工程學(xué)院,重慶 400044)

為了評(píng)估長(zhǎng)期凍融循環(huán)后(最長(zhǎng)90d(次))固化復(fù)合重金屬污染土的的抗剪強(qiáng)度及浸出特征, 采用水泥、生石灰和粉煤灰按比例混合的復(fù)合固化劑固化/穩(wěn)定化鉛鋅鎘復(fù)合重金屬污染土進(jìn)行三軸壓縮試驗(yàn)及毒性特征浸出程序試驗(yàn).結(jié)果表明,固化污染土體的內(nèi)摩擦角僅在凍融循環(huán)3次內(nèi)有明顯增加,增加率高達(dá)96.3%;粘聚力在凍融循環(huán)30次內(nèi)總體趨勢(shì)不斷下降,之后無(wú)顯著變化,最終下降率達(dá)到54.23%;Pb2+、Zn2+、Cd2+浸出濃度與凍融循環(huán)次數(shù)呈正比;EC值與凍融循環(huán)次數(shù)在總體上正相關(guān);長(zhǎng)期凍融循環(huán)作用后浸出液的pH值降低.并通過(guò)掃描電子顯微鏡,進(jìn)一步探究長(zhǎng)期凍融循環(huán)下固化污染土抗剪強(qiáng)度及浸出特征的劣化機(jī)理,結(jié)果顯示在凍融循環(huán)后期固化土體內(nèi)生成了大量的延遲鈣礬石,這些延遲鈣礬石在形成過(guò)程中的膨脹作用是引起抗剪強(qiáng)度損失、重金屬浸出濃度升高的主要原因.

長(zhǎng)期凍融循環(huán)；復(fù)合重金屬污染土；抗剪強(qiáng)度；浸出特征；劣化機(jī)理

自20世紀(jì)90年代起,根據(jù)我國(guó)城市總體規(guī)劃及發(fā)展的需要,許多低技術(shù)高污染的城市企業(yè)被關(guān)閉或搬遷,遺留下了許多廢棄的工業(yè)場(chǎng)地[1-2].這些遺留下來(lái)的工礦廢棄荒地不僅侵占了大量土地資源,還帶來(lái)了高水平的土壤污染[3].而且,工業(yè)廢棄場(chǎng)地的重金屬污染土壤一般是受某一重金屬元素污染為主,同時(shí)伴隨著其他重金屬元素污染的復(fù)合重金屬污染土壤[4].

固化/穩(wěn)定化(Solidification/Stabilization,簡(jiǎn)寫(xiě)為S/S)技術(shù)是目前金屬及類(lèi)金屬污染場(chǎng)地修復(fù)工程的首選技術(shù)方案[5-11].S/S添加固化劑到重金屬污染土壤,通過(guò)固化劑與重金屬的物理或化學(xué)反應(yīng)來(lái)固定重金屬,防止它們遷移和擴(kuò)散到環(huán)境中,以此來(lái)達(dá)到降低重金屬毒性和遷移率的目的[6,12-15].但是S/S技術(shù)并沒(méi)有去除污染土中的重金屬,而且S/S材料的壽命也是一個(gè)不確定性的因素,所以固化重金屬污染土的長(zhǎng)期穩(wěn)定性問(wèn)題亟待解決[16].已有研究表明,極端的環(huán)境變化,例如凍融交替變換[17-18]、酸雨的滲入[13,19-21]、干濕交替循環(huán)[22-23]、硫酸鹽侵蝕[24]等,均可能導(dǎo)致固化/穩(wěn)定化的重金屬污染土壤性質(zhì)發(fā)生變化,影響固化/穩(wěn)定化重金屬污染土壤的強(qiáng)度和浸出性[25-26],甚至導(dǎo)致穩(wěn)定的污染物再次活化而造成二次污染.Yang等[27]研究發(fā)現(xiàn),凍融循環(huán)作用會(huì)降低固化/穩(wěn)定化鉛污染土的無(wú)側(cè)限抗壓強(qiáng)度.Liu等[28]研究表明,經(jīng)歷凍融循環(huán)后,固化/穩(wěn)定化鋅污染土壤的應(yīng)力-應(yīng)變曲線逐漸趨于硬化模型,抗壓強(qiáng)度明顯下降.夏兆君[29]研究發(fā)現(xiàn)中國(guó)凍土分布范圍較廣,季節(jié)性凍土區(qū)域所占比例高達(dá)53.5%.事實(shí)上,不僅是中國(guó),季節(jié)性凍土遍布世界各地,如俄羅斯的歐洲部分[30]和西西伯利亞的南部[31].

目前已有大量文獻(xiàn)報(bào)道了凍融循環(huán)對(duì)固化/穩(wěn)定化污染土壤工程特性的影響.Yang等[32]研究了凍融循環(huán)作用對(duì)固化鉛污染土無(wú)側(cè)限抗壓強(qiáng)度和滲透系數(shù)的影響;Liu等[33]通過(guò)半動(dòng)態(tài)淋濾試驗(yàn),研究了凍融循環(huán)作用下固化鋅污染土的長(zhǎng)期穩(wěn)定性.但是現(xiàn)有研究多是針對(duì)單一重金屬污染的土壤,而對(duì)于污染場(chǎng)地而言,多是同時(shí)存在多種重金屬的復(fù)合重金屬污染土壤.并且大多數(shù)學(xué)者只研究了短期凍融循環(huán)(小于15d)對(duì)固化/穩(wěn)定化重金屬污染土壤的影響,而沒(méi)有考慮長(zhǎng)期凍融循環(huán)[34],而季節(jié)性凍土區(qū)固化/穩(wěn)定化的重金屬污染土壤在季節(jié)交替過(guò)程中會(huì)長(zhǎng)期處于凍融交替狀態(tài).

因此,基于以上原因,本文通過(guò)三軸壓縮試驗(yàn)及毒性特征浸出試驗(yàn),得到長(zhǎng)期(最長(zhǎng)時(shí)間為90d(次))凍融循環(huán)作用后固化復(fù)合重金屬污染土的抗剪強(qiáng)度指標(biāo)及毒性浸出特征,以此評(píng)價(jià)長(zhǎng)期凍融作用下固化復(fù)合重金屬污染土的抗剪強(qiáng)度及浸出特征.并通過(guò)掃描電子顯微鏡(SEM),從微觀角度進(jìn)一步探究長(zhǎng)期凍融循環(huán)作用下固化復(fù)合重金屬污染土抗剪強(qiáng)度及浸出特征的劣化機(jī)理.

1 材料與方法

1.1 試驗(yàn)材料

1.1.1 試驗(yàn)用土 試驗(yàn)用土取自重慶市高廟村某工地,為紅褐色的黏性土.采用電熱恒溫鼓風(fēng)干燥箱(100 ℃,24h)對(duì)其進(jìn)行烘干處理,并通過(guò)1mm篩.按照《土工試驗(yàn)方法標(biāo)準(zhǔn)》(GB/T 50123-2019),對(duì)篩分過(guò)的土壤進(jìn)行常規(guī)土工試驗(yàn),得到試驗(yàn)用土的基本物理性質(zhì)參數(shù)如表1所示.并通過(guò)X射線熒光(X-Ray Fluorescence,即XRF)分析技術(shù)檢測(cè)了土壤的化學(xué)成分如表2所示.

表1 土壤的基本物理性質(zhì)指標(biāo)

表2 土壤的化學(xué)成分

注:①:表示未檢測(cè)到.

1.1.2 重金屬污染物 選擇Pb2+、Zn2+和Cd2+作為污染離子,并且鑒于硝酸根離子的高溶解度(高陽(yáng)離子活動(dòng)性)及其對(duì)固化/穩(wěn)定化過(guò)程的小干擾性[35-36],以硝酸鉛(Pb(NO3)2)、硝酸鋅(Zn(NO3)2)和硝酸鎘(Cd(NO3)2)作為重金屬污染物添加到采集的土壤中.其中,污染物Pb(NO3)2(分析純)、Cd(NO3)2·4H2O(高純?cè)噭?和Zn(NO3)2·6H2O(分析純)均采購(gòu)自重慶躍翔化工有限公司.

1.1.3 固化劑 李登華[37]及Yang等[17]研究表明,當(dāng)水泥、生石灰及粉煤灰以2:1:1的比例混合,共同修復(fù)重金屬污染土?xí)r效果可以達(dá)到較好的修復(fù)效果,故將水泥、生石灰、粉煤灰分別以干土質(zhì)量的5%、2.5%、2.5%比例混合,制成復(fù)合固化劑,以此復(fù)合修復(fù)鉛鋅鎘復(fù)合污染土.水泥為普通硅酸鹽水泥(OPC325),生石灰主要成分為CaO(分析純),粉煤灰取自重慶某電廠(F類(lèi)).三種固化劑均過(guò)0.075mm的篩子,以保持其均勻性,并且通過(guò)X射線熒光(XRF)分析技術(shù)檢測(cè)了水泥、生石灰、粉煤灰的化學(xué)成分,結(jié)果如下表3所示.由表3可知,粉煤灰中SiO2,Al2O3和Fe2O3的總量超過(guò)了70%,因此根據(jù)標(biāo)準(zhǔn)《用于水泥和混凝土的粉煤灰》(GB/T 1596-2017),該粉煤灰可被歸為F類(lèi)粉煤灰[34].

表3 三種固化劑的化學(xué)成分

注:①:表示未檢測(cè)到.

1.1.4 去離子水 本研究涉及重金屬離子濃度,而自來(lái)水中含有Ca2+、Mg2+、Zn2+等金屬陽(yáng)離子,故選擇去離子水作為試驗(yàn)用水.

1.2 試驗(yàn)方法

1.2.1 試樣配比設(shè)計(jì) 試樣設(shè)計(jì)配合比如表4所示.去離子水的含量取最優(yōu)含水率(11.8%)的120%.將《土壤環(huán)境質(zhì)量標(biāo)準(zhǔn)》規(guī)定的Pb、Zn、Cd三級(jí)標(biāo)準(zhǔn)限制分別放大16、10、400倍,得到試樣重金屬Pb、Zn、Cd的濃度分別為8000、5000、400mg/kg;水泥摻量為5%,粉煤灰及生石灰摻量均為2.5%.上文所提到的固化劑的摻量、重金屬污染物的濃度均以制備試樣所需干土質(zhì)量為基準(zhǔn)計(jì)算.

表4 試樣配比設(shè)計(jì)

1.2.2 試樣制備 根據(jù)表4用分析天平稱取一定量的去離子水和硝酸鉛、硝酸鎘、硝酸鋅,將二者混合均勻后加入到一定質(zhì)量的干土中,攪拌均勻后裝進(jìn)保鮮袋,密封后放入恒溫保濕箱(溫度:22℃;相對(duì)濕度:95%)養(yǎng)護(hù)30d,得到鉛鋅鎘復(fù)合重金屬污染土.根據(jù)設(shè)計(jì)的固化劑摻量,稱取一定質(zhì)量的水泥、生石灰和粉煤灰加入養(yǎng)護(hù)完成的鉛鋅鎘復(fù)合重金屬污染土,攪拌均勻后放入保鮮袋,密封后放入恒溫保濕箱(溫度:22℃;相對(duì)濕度:95%)悶養(yǎng)24h,得到固化復(fù)合鉛鋅鎘重金屬污染土.采用擾動(dòng)土干堆法,分5次將土樣裝填到尺寸為F39.1′80mm的圓柱狀模具中.每次裝填土樣質(zhì)量相同,且每次裝填后用擊實(shí)器對(duì)裝填土壤進(jìn)行擊實(shí),使其達(dá)到固定高度,并對(duì)擊實(shí)表面進(jìn)行刮毛,最終得到三軸壓縮試驗(yàn)的樣品.最后將制備好的試樣以及制備試樣后剩余的固化復(fù)合鉛鋅鎘重金屬污染土樣放入標(biāo)準(zhǔn)養(yǎng)護(hù)室(溫度:22 ℃;相對(duì)濕度:95%)中養(yǎng)護(hù)56d.在養(yǎng)護(hù)期間每隔7d翻轉(zhuǎn)1次試樣,使試樣水分分布均勻.

1.2.3 凍融循環(huán)交替試驗(yàn) 養(yǎng)護(hù)完成后,將六組試樣放入高低溫交替試驗(yàn)箱(生產(chǎn)廠家:重慶泰斯特試驗(yàn)儀器有限公司TC401)進(jìn)行凍融試驗(yàn).凍融循環(huán)分為0、3、7、14、30、90次,一共6個(gè)階段,每組試樣對(duì)應(yīng)一個(gè)凍融循環(huán)階段.每次凍融均在24h內(nèi)完成,試驗(yàn)設(shè)置的凍結(jié)溫度為-10℃[38].一次(天)凍融周期設(shè)置如圖1所示.

圖1 單次凍融循環(huán)溫度控制時(shí)間線

1.2.4 三軸壓縮試驗(yàn) 三軸試驗(yàn)方法為固結(jié)不排水(CU)試驗(yàn),試驗(yàn)儀器為環(huán)境巖土全自動(dòng)滲透儀(生產(chǎn)廠家:英國(guó)GDS儀器).選取100kPa、200kPa以及300kPa作為試驗(yàn)圍壓.按照《土工試驗(yàn)方法標(biāo)準(zhǔn)》[38](GB/T 50123-2019)3.2.4條的抽氣飽和法,對(duì)取出的凍融完成后的試樣進(jìn)行真空飽和.將完成飽和的試樣取出裝到壓力室中,然后按照飽和(反壓力飽和)—固結(jié)—剪切的順序?qū)υ囼?yàn)程序進(jìn)行設(shè)置.壓力架下降速率為0.2mm/min,剪切應(yīng)變速率設(shè)置為每分鐘0.08%,當(dāng)軸向應(yīng)變達(dá)到20%時(shí)停止剪切.

1.2.5 毒性特征浸出試驗(yàn) 本次試驗(yàn)按照《ASTM Method 1311》規(guī)范,采用固體廢物的毒性浸出試驗(yàn)方法,對(duì)不同凍融循環(huán)次數(shù)(0、3、7、14、30及90次)下固化/穩(wěn)定化鉛鋅鎘復(fù)合污染土中的重金屬進(jìn)行提取.其中,酸性浸取液的類(lèi)型取決于土壤樣品自身的pH值大小,具體確定步驟見(jiàn)下圖2,TCLP毒性浸出試驗(yàn)的主要參數(shù)見(jiàn)表5.

圖2 TCLP規(guī)范浸取液的確定方法

表5 TCLP毒性浸出試驗(yàn)的主要參數(shù)

使用Optima 8000電感耦合等離子體發(fā)射光譜儀(ICP-OES)測(cè)定浸出液重金屬離子濃度;使用雷磁PHS-3E型pH計(jì)測(cè)定浸出液pH值;使用雷磁DDSJ-308A型電導(dǎo)率儀測(cè)浸出液電導(dǎo)率值.

1.2.6 掃描電子顯微鏡(SEM) 本次試驗(yàn)在重慶大學(xué)分析測(cè)試中心完成,使用TM4000Plus Ⅱ臺(tái)式掃描顯微鏡.對(duì)于不導(dǎo)電的試樣,為了使其表面具有良好的導(dǎo)電性能,需要將試樣抽真空,在其表面進(jìn)行噴金處理,噴金所用儀器為MSP-mini離子濺射儀裝置.將噴金處理后試樣放進(jìn)臺(tái)式掃描電鏡儀中(電壓:20kV)中,對(duì)試樣進(jìn)行1000倍的圖像拍攝.

2 結(jié)果與分析

2.1 長(zhǎng)期凍融循環(huán)對(duì)固化復(fù)合重金屬污染土抗剪強(qiáng)度的影響

試驗(yàn)得到內(nèi)摩擦角和粘聚力見(jiàn)下表6.

表6 不同凍融循環(huán)作用下固化/穩(wěn)定化鉛鋅鎘復(fù)合污染土壤的抗剪強(qiáng)度指標(biāo)

為評(píng)價(jià)長(zhǎng)期凍融循環(huán)對(duì)固化/穩(wěn)定化鉛鋅鎘復(fù)合重金屬污染土體抗剪強(qiáng)度指標(biāo)的影響,分別采用式(1)、(2)計(jì)算土體、值在不同凍融循環(huán)次數(shù)時(shí)的變化率:

RC=(|0-C|)/0×100% (1)

RC=(|0-|)/0×100% (2)

式中:RC、RC分別為、值變化率;0、0分別為凍融循環(huán)次數(shù)為0次時(shí)的、值;C、φ分別為凍融循環(huán)(3,7,14,30,90)次后的、值.RC、RC與凍融循環(huán)次數(shù)關(guān)系曲線見(jiàn)圖3.

由圖3可知,固化/穩(wěn)定化鉛鋅鎘復(fù)合重金屬污染土壤樣品的粘聚力變化率總體上隨著凍融循環(huán)次數(shù)的增加而增加.這主要是因?yàn)榉磸?fù)的凍融作用會(huì)引起固化/穩(wěn)定化鉛鋅鎘復(fù)合重金屬污染土壤樣品內(nèi)部連接的破壞,導(dǎo)致粘聚力顯著降低,且這種破壞是不斷累積的.粘聚力的變化率在本次實(shí)驗(yàn)的最終達(dá)到54.23%,基本穩(wěn)定在60%左右.

圖3 長(zhǎng)期凍融循環(huán)下固化/穩(wěn)定化鉛鋅鎘復(fù)合污染土的粘聚力及內(nèi)摩擦角變化率

由圖3可觀察到,在凍融循環(huán)次數(shù)小于3次時(shí),固化/穩(wěn)定化鉛鋅鎘復(fù)合重金屬污染土壤樣品的內(nèi)摩擦角的變化率與凍融循環(huán)次數(shù)正相關(guān),這主要是因?yàn)榉磸?fù)的凍融交替作用后土壤顆?？赡懿粫?huì)完全回到原來(lái)的位置,土壤顆粒會(huì)發(fā)生不規(guī)則重排,大小孔隙面積占總面積的比例會(huì)變化,造成土壤顆粒移動(dòng)的難度和顆粒間附著面積的增加,最終導(dǎo)致內(nèi)摩擦角的大小不斷增加[39-40].但是,當(dāng)凍融循環(huán)3次后,內(nèi)摩擦角的變化率達(dá)到96.30%,之后再增加凍融循環(huán)次數(shù),變化率幾乎保持不變.故凍融循環(huán)作用前期內(nèi)摩擦角劇烈增加,增加率高達(dá)96.30%;凍融循環(huán)后期內(nèi)摩擦角保持穩(wěn)定,無(wú)明顯變化.

2.2 長(zhǎng)期凍融循環(huán)對(duì)固化復(fù)合重金屬污染土毒性浸出特征的影響

2.2.1 重金屬離子浸出濃度 由圖4(a)可知,Pb2+浸出濃度整體上與凍融循環(huán)作用次數(shù)呈正相關(guān).而且,在凍融循環(huán)前Pb2+浸出濃度低于國(guó)家環(huán)境保護(hù)總局在《危險(xiǎn)廢物鑒別標(biāo)準(zhǔn)浸出毒性鑒別》(GB 5085.3-2007)中所規(guī)定的浸出液中Pb2+濃度限值(5mg/L);當(dāng)凍融循環(huán)達(dá)到3次時(shí),Pb2+浸出濃度略微高于其濃度限值,隨后隨著凍融循環(huán)次數(shù)的增加, Pb2+浸出濃度越來(lái)越高,到凍融次數(shù)達(dá)到90次時(shí), Pb2+浸出濃度遠(yuǎn)遠(yuǎn)高于其濃度限值,約為其限值的40倍.

由圖4(b)可知,Zn2+浸出濃度整體上與凍融循環(huán)作用次數(shù)呈正相關(guān).在凍融循環(huán)次數(shù)達(dá)到7次時(shí), Zn2+浸出濃度超過(guò)國(guó)家環(huán)境保護(hù)總局在《危險(xiǎn)廢物鑒別標(biāo)準(zhǔn)浸出毒性鑒別》(GB 5085.3-2007)中所規(guī)定的浸出液中Zn2+濃度限值(100mg/L);隨后隨著凍融循環(huán)次數(shù)的增加,Zn2+浸出濃度上升速率逐漸變緩.直到凍融循環(huán)次數(shù)達(dá)到90次時(shí),Zn2+的浸出濃度約為其限值的1.7倍.

由圖4(c)可知,Cd2+浸出濃度與凍融循環(huán)作用次數(shù)呈正相關(guān).在凍融循環(huán)次數(shù)達(dá)到7次時(shí),Cd2+濃度超過(guò)國(guó)家環(huán)境保護(hù)總局在《危險(xiǎn)廢物鑒別標(biāo)準(zhǔn)浸出毒性鑒別》(GB 5085.3-2007)中所規(guī)定的浸出液中Cd2+限值(1mg/L);當(dāng)凍融次數(shù)超過(guò)14次后,Cd2+浸出濃度升高的速度變慢,直到凍融循環(huán)次數(shù)達(dá)到90次時(shí),Cd2+的浸出濃度約為其限值的5倍.

綜上所述,凍融循環(huán)作用會(huì)增加固化/穩(wěn)定化鉛鋅鎘復(fù)合重金屬污染土Pb2+、Zn2+、Cd2+浸出濃度.這主要是因?yàn)?凍融交替過(guò)程中存在著放熱和吸熱,會(huì)對(duì)溫度造成影響,進(jìn)而影響到土體對(duì)重金屬陽(yáng)離子的吸附作用(包括非專(zhuān)性吸附與專(zhuān)性吸附),加速固化劑水化產(chǎn)物、土壤有機(jī)質(zhì)表面吸附的金屬陽(yáng)離子(如Pb2+、Cd2+、Cu2+等)解吸[41],最終影響溶液中重金屬的濃度和毒性[42].

而且,雖然Pb2+、Zn2+、Cd2+的浸出濃度都與凍融循環(huán)次數(shù)正相關(guān),但是浸出液中Pb2+的濃度的增加速度一直較快,而Zn2+、Cd2+的濃度在凍融循環(huán)后期增長(zhǎng)逐漸變緩.這是因?yàn)槲廴就寥乐械腜b2+主要是通過(guò)吸附到水合凝膠上而固化的[43],而Zn2+、Cd2+可能在水泥/粉煤灰產(chǎn)生的堿性環(huán)境中沉淀,并在水合凝膠中被包封或吸附到水合凝膠上[44],所以相對(duì)而言,Pb2+的浸出濃度直接受溫度的影響更大,它會(huì)一直受凍融循環(huán)作用的影響.

2.2.2 浸出液EC值 在液體中常以電阻的倒數(shù)—電導(dǎo)率(Electrical Conductance,簡(jiǎn)寫(xiě)為EC)作為評(píng)估溶液導(dǎo)電能力大小的指標(biāo).通常水溶液導(dǎo)電的程度與電解質(zhì)的濃度有關(guān),所以水溶液的電導(dǎo)率值可反映出水中含有電解質(zhì)的程度.因此,一般可認(rèn)為電導(dǎo)率值與溶液中的離子濃度有很好的關(guān)聯(lián)性.在TCLP試驗(yàn)的基礎(chǔ)上,測(cè)試了浸出液的EC值,得到的結(jié)果如圖5所示,該圖顯示了固化/穩(wěn)定化鉛鋅鎘復(fù)合重金屬污染土TCLP浸出液電導(dǎo)率EC值與凍融循環(huán)次數(shù)的關(guān)系.

由圖5可知,浸出液EC值先是隨著凍融次數(shù)的增加快速增加,增加到最高點(diǎn)后又驟然降低,隨后緩緩增加.但是與凍融循環(huán)前相比,凍融循環(huán)90次后,固化鉛鋅鎘復(fù)合重金屬污染土浸出液的EC值增加了10.96%,這與重金屬浸出濃度與凍融循環(huán)次數(shù)正相關(guān)的結(jié)果相符.在凍融循環(huán)次數(shù)為7～14次時(shí),EC值的降低主要是由于在凍融循環(huán)作用進(jìn)行的同時(shí),水化反應(yīng)和火山灰反應(yīng)仍然在進(jìn)行,不斷生成CSH膠體及Ca(OH)2結(jié)晶體,溶液中的Ca2+和氫基離子(OH-)減少,而同時(shí)生成的CSH水合凝膠會(huì)包封或吸附溶液中的Zn2+、Cd2+等金屬陽(yáng)離子,使浸出液中的金屬陽(yáng)離子減少,促使浸取液的電導(dǎo)率降低.但是在凍融循環(huán)作用后期,隨著凍融循環(huán)次數(shù)的增加,水化反應(yīng)消耗的Ca2+,以及生成的水化產(chǎn)物固化的Pb2+、Zn2+、Cd2+的數(shù)量遠(yuǎn)遠(yuǎn)小于由于長(zhǎng)期反復(fù)的凍融交替作用而浸出的Pb2+、Zn2+、Cd2+的數(shù)量,所以可以觀察到EC值很明顯的上升.

圖5 長(zhǎng)期凍融循環(huán)下固化/穩(wěn)定化鉛鋅鎘復(fù)合重金屬污染土的TCLP浸出液電導(dǎo)率

圖6 長(zhǎng)期凍融循環(huán)下固化/穩(wěn)定化鉛鋅鎘復(fù)合重金屬污染土的TCLP浸出液pH值

2.2.3 浸出液pH值 從圖6可以看出,雖然在凍融循環(huán)次數(shù)為0～7次時(shí),TCLP浸出液pH值的大小與凍融循環(huán)次數(shù)正相關(guān);在凍融循環(huán)次數(shù)為超過(guò)7次后,TCLP浸出液pH值的大小與凍融循環(huán)次數(shù)負(fù)相關(guān).所以,與凍融循環(huán)作用前相比,長(zhǎng)期凍融循環(huán)作用會(huì)降低浸出液的pH值,即降低浸出后固化/穩(wěn)定化重金屬污染土壤的堿性和緩沖能力.

2.3 長(zhǎng)期凍融循環(huán)對(duì)固化復(fù)合重金屬污染土微觀結(jié)構(gòu)的影響

從圖7中可以看到,凍融循環(huán)作用下的固化/穩(wěn)定化鉛鋅鎘復(fù)合重金屬土樣中,因?yàn)橥翗?固化劑之間的各種物理化學(xué)反應(yīng),生成了大量板狀氫氧鈣石、凝膠狀水化產(chǎn)物(如CAH、CSH等)等,它們會(huì)物理吸附或包裹Zn2+、Cd2+等重金屬離子,填充土體孔隙,降低土體孔隙比,進(jìn)而促使土體結(jié)構(gòu)致密.凍融循環(huán)次數(shù)為0、3、7次時(shí),固化/穩(wěn)定化鉛鋅鎘復(fù)合重金屬污染土的SEM圖像中存在許多還未水化的粉煤灰顆粒,當(dāng)凍融循環(huán)次數(shù)達(dá)到14次后,SEM圖像中幾乎看不到完整的粉煤灰顆粒.該現(xiàn)象印證了重金屬鉛、鋅、鎘的存在能夠延緩固化/穩(wěn)定化過(guò)程中的水化反應(yīng).這也是為什么在凍融循環(huán)前期固化/穩(wěn)定化鉛鋅鎘復(fù)合污染土強(qiáng)度仍偶有增加.

(a)凍融循環(huán)0次;(b)凍融循環(huán)3次;(c)凍融循環(huán)7次;(d)凍融循環(huán)14次;(e)凍融循環(huán)30次;(f)凍融循環(huán)90次

從圖7中還可以看出,從(a)～(f)孔隙和裂隙特征越來(lái)越明顯,細(xì)小顆粒增多,這說(shuō)明凍融循環(huán)作用會(huì)破壞土體結(jié)構(gòu),使得原有膠結(jié)程度較好的顆粒因水分物相的改變而破碎成小顆粒.當(dāng)凍融循環(huán)次數(shù)增加到90次時(shí),能夠看到大量的針狀的水泥芽孢桿菌(通常稱為鈣礬石,見(jiàn)圖7(f)),因此可知固化體在凍融循環(huán)后期生成了大量的延遲鈣礬石,由于鈣礬石含有的結(jié)晶水較多,所以這些延遲鈣礬石在形成過(guò)程中會(huì)發(fā)生膨脹,損壞土體的內(nèi)部結(jié)構(gòu),最終造成了強(qiáng)度的明顯損失[45-47].不僅如此,鈣礬石的膨脹作用還會(huì)增加將被固定的有毒重金屬成分釋放到相鄰水體中的可能性,使得重金屬離子浸出濃度升高.

3 結(jié)論

3.1 三軸壓縮試驗(yàn)結(jié)果表明,凍融循環(huán)作用在短期內(nèi)會(huì)增加固化土體的內(nèi)摩擦角,當(dāng)凍融循環(huán)3次后,的增加率高達(dá)96.35%,之后再無(wú)顯著變化;凍融循環(huán)作用會(huì)顯著降低固化土體的粘聚力,當(dāng)凍融循環(huán)次數(shù)達(dá)到90次時(shí),值下降率高達(dá)54.23%,并基本保持穩(wěn)定.

3.2 毒性特征浸出試驗(yàn)(TCLP)結(jié)果表明,長(zhǎng)期凍融循環(huán)作用會(huì)增加固化/穩(wěn)定化鉛鋅鎘復(fù)合重金屬污染土Pb2+、Zn2+、Cd2+的浸出濃度、浸取液EC值,降低浸取液pH值.

3.3 SEM結(jié)果顯示,固化/穩(wěn)定化鉛鋅鎘復(fù)合重金屬污染土壤在凍融循環(huán)后期生成了大量的延遲鈣礬石,這些延遲鈣礬石在形成過(guò)程中的膨脹作用損壞了土體的內(nèi)部結(jié)構(gòu),最終導(dǎo)致強(qiáng)度出現(xiàn)明顯損失,且重金屬離子浸出濃度升高.

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致謝：在此特別感謝重慶大學(xué)楊忠平教授提供優(yōu)良的試驗(yàn)條件,同時(shí)感謝常嘉卓、李登華在試驗(yàn)過(guò)程中所給予的幫助.

Shear strength and leaching characteristics of solidified lead-zinc-cadmium composite heavy metal contaminated soil under long-term freeze-thaw cycles.

WANG yao1,2, YANG Zhong-ping2*, ZHOU Yang1, CHANG Jia-zhuo2, LI Deng-hua2, LUO Jun1

(1.China Machinery China United Engineering Co., Ltd., Chongqing 400039, China；2.School of Civil Engineering, Chongqing University, Chongqing 400044, China)., 2022,42(7)：3276～3284

In order to evaluate the shear strength and leaching characteristics of the solidified composite heavy metal contaminated soil after long-term freeze-thaw cycles (up to 90days (times)), triaxial compression test and toxicity characteristic leaching procedure test were carried out on the lead-zinc-cadmium composite heavy metal contaminated soil after solidification/stabilization with composite curing agent of cement, quicklime and fly ash. The results show that the angle of internal friction only increased significantly to 3 freeze-thaw cycles, and the increase rated was as high as 96.3%; while the overall trend of cohesion continued to decline by 30 freeze-thaw cycles, and there was no significant change afterwards, and the final decline rate reached 54.23%; the leaching concentration of Pb2+, Zn2+, Cd2+, and the EC value of the extract is proportional to the number of freeze-thaw cycles; the pH value of the extract decreases after long-term freeze-thaw cycles. And through scanning electron microscopy, we further explored the degradation mechanism of the shear strength and leaching characteristics of solidified contaminated soil under long-term freeze-thaw cycles. The results show that a large amount of delayed ettringite is formed in the solidified soil at the end of the freeze-thaw cycle, and its expansion during the formation process is the main reason for the loss of shear strength and the increase of heavy metal leaching concentration.

long-term freeze-thaw cycle；composite heavy metal contaminated soil；shear strength；leaching characteristics；degradation mechanism

X53

A

1000-6923(2022)07-3276-09

王 瑤(1996-),女,四川自貢人,碩士,主要從事重金屬污染土研究.發(fā)表論文2篇.

2021-12-20

國(guó)家自然科學(xué)基金資助項(xiàng)目(42177125,41772306)

* 責(zé)任作者, 教授, yang-zhp@163.com