劉 建,何 亮,駱成杰,宋 凱,王 飛,白 雪,李清毅

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生活垃圾焚燒飛灰固化體重金屬動(dòng)態(tài)浸出規(guī)律

劉 建*,何 亮,駱成杰,宋 凱,王 飛,白 雪,李清毅

(西南交通大學(xué)地球科學(xué)與環(huán)境工程學(xué)院,四川 成都 610031)

為研究實(shí)際生活垃圾衛(wèi)生填埋場中滲濾液長期作用于飛灰固化體的浸出行為,分別采用pH值為5.5的醋酸-醋酸鈉溶液和pH值為7.0的去離子水作為初始浸提液,建立飛灰固化體的動(dòng)態(tài)浸出試驗(yàn).初始浸提液pH值的差異會(huì)影響飛灰固化體的結(jié)構(gòu)完整性和固化體表面晶體的生成情況,當(dāng)初始浸提液為醋酸-醋酸鈉溶液時(shí),實(shí)驗(yàn)過程中飛灰固化體會(huì)破碎;當(dāng)初始浸提液為去離子水時(shí),飛灰固化體表面生成的CaCO3晶體可能會(huì)抑制重金屬浸出.當(dāng)實(shí)驗(yàn)進(jìn)行至約120d時(shí),2組實(shí)驗(yàn)浸出液的pH值和重金屬濃度基本穩(wěn)定,且pH值相差不大;但采用去離子水浸提時(shí),Pb、Cr的累計(jì)浸出量明顯更高,且Pb的浸出濃度(0.29mg/L)甚至一度超過填埋場入場限值,其潛在風(fēng)險(xiǎn)應(yīng)受到關(guān)注.

飛灰；固化；重金屬；動(dòng)態(tài)浸出；填埋場

截至2016年,我國生活垃圾焚燒處理量已達(dá)7.38′107t/a[1],但焚燒后會(huì)產(chǎn)生30%~35%的固體殘?jiān)?其中飛灰約占5%[2].生活垃圾焚燒產(chǎn)生的飛灰中富集了多種重金屬和二噁英等污染物[3-5],我國《生活垃圾填埋場污染控制標(biāo)準(zhǔn)》(GB16889-2008)[6]規(guī)定,生活垃圾焚燒產(chǎn)生的飛灰需經(jīng)過預(yù)處理達(dá)到有關(guān)標(biāo)準(zhǔn)后才可進(jìn)入生活垃圾填埋場進(jìn)行填埋處置.

目前,生活垃圾焚燒飛灰的預(yù)處理技術(shù)主要有介質(zhì)固化技術(shù)、高溫處理技術(shù)和化學(xué)藥劑穩(wěn)定化技術(shù)[7].其中,水泥固化法因工藝成熟、操作簡單、成本低成為常用處理技術(shù)[8].為降低飛灰水泥固化體對環(huán)境的危害,國內(nèi)外學(xué)者對固化體的物理特性[8-11]、重金屬固化效果[12-13]開展了大量研究.用于飛灰固化體重金屬浸出測試的方法有很多,包括國標(biāo)法HJ/T 300[14]和EPA推薦的TCLP法[15]均是(18±2)h的短期測試,不能充分模擬長期浸出條件下重金屬的浸出過程[16].因此,水槽法和柱浸出法在實(shí)際研究工作中得以應(yīng)用.陳林等[17]、Hassan等[18]采用水槽法分別研究了飛灰和底灰污染物在瀝青路面中的溶出特性;何品晶等[19]、Hyks等[20]采用柱浸出法考察了飛灰及其穩(wěn)定化產(chǎn)物的長期浸出行為,認(rèn)為短期測試可能低估穩(wěn)定化處理飛灰在填埋初期的重金屬浸出風(fēng)險(xiǎn).上述嘗試較好地彌補(bǔ)了HJ/T 300 2007[14]、TCLP[15]和GB5086.1-1997[21]等短期、靜態(tài)測試方法的局限性[22],但仍不能很好地刻畫并模擬飛灰固化體進(jìn)入實(shí)際填埋場后的動(dòng)態(tài)浸出行為.

本實(shí)驗(yàn)用醋酸、醋酸鈉和去離子水配置pH值為5.5、7.0的浸提液,對飛灰固化體進(jìn)行動(dòng)態(tài)浸出.根據(jù)浸出液中pH值、重金屬濃度和浸出前后飛灰固化體的SEM和XRD圖譜,探討飛灰固化體中重金屬的浸出機(jī)理,為飛灰與生活垃圾共處置的長期環(huán)境影響評估和風(fēng)險(xiǎn)管控提供理論依據(jù).

1 材料與方法

1.1 實(shí)驗(yàn)材料與設(shè)備

實(shí)驗(yàn)材料:飛灰(取自成都某大型生活垃圾焚燒發(fā)電廠),普通硅酸鹽水泥,醋酸,醋酸鈉,去離子水.

實(shí)驗(yàn)設(shè)備:100目篩,電子天平(XB622,上海精科天美貿(mào)易有限公司),pH計(jì)(pH-220,杭州齊威儀器有限公司),電感耦合等離子體發(fā)射光譜儀(ICP-7400,賽默飛世爾科技),X射線衍射儀(D8ADVANCE,德國BRUKER-AXS有限公司),掃描電子顯微鏡(JSM-6510,日本電子公司).

1.2 飛灰固化體的制備

將飛灰過100目篩以去除其中大塊徑的石灰和活性炭,用電子天平稱取飛灰和水泥,按照20%的水泥摻入比,將二者混合并攪拌均勻;按固液比S:L=0.5加入去離子水,攪拌均勻后置入PVC模具(100mm×100mm×40mm)中,用振動(dòng)板壓實(shí),待硬化成型后去除模具,放置于陰涼處養(yǎng)護(hù)28d備用.

1.3 實(shí)驗(yàn)設(shè)計(jì)與分析方法

實(shí)驗(yàn)裝置為矩形有機(jī)玻璃槽體,見圖1.槽內(nèi)放置飛灰固化體,并確保四周均有10mm間隙以使浸提液可與飛灰固化體充分接觸.配置好的浸提液置于儲(chǔ)液槽內(nèi),并快速添加至浸出區(qū)直到液面與溢流口相平,然后用蠕動(dòng)泵按照設(shè)定的補(bǔ)給流量向浸出區(qū)連續(xù)補(bǔ)充浸提液,經(jīng)浸出后的溶液自流進(jìn)入出液槽.泵開始補(bǔ)充浸提液時(shí),浸出區(qū)的溢流口便會(huì)有溶液流出,此時(shí)開始計(jì)時(shí).

圖1 動(dòng)態(tài)浸出實(shí)驗(yàn)裝置 Fig.1 Setting of dynamic leaching experiment

采用《固體廢物浸出毒性浸出方法－醋酸緩沖溶液法》(HJ/T 300-2007)[14]對原灰進(jìn)行浸出.由表1浸出結(jié)果可知,原灰中重金屬Pb、Zn、Cr(總Cr,下同)和Cd均有不同程度浸出,且部分指標(biāo)超出了填埋場的入場限值,故須對原灰進(jìn)行預(yù)處理至其浸出液中有害物質(zhì)濃度低于《生活垃圾填埋場污染控制標(biāo)準(zhǔn)》(GB 16889-2008)[6]中表1規(guī)定限值后方可進(jìn)入填埋場進(jìn)行處置.

生活垃圾填埋場初期(5a以下)滲濾液呈弱酸性,中期(5a以上)呈中性[23].本實(shí)驗(yàn)采用醋酸-醋酸鈉配置成pH值為5.5的弱酸溶液模擬垃圾填埋場初期滲濾液的弱酸環(huán)境,采用去離子水模擬填埋中期的中性環(huán)境.補(bǔ)給徑流量根據(jù)當(dāng)?shù)啬杲涤炅亢徒涤耆霛B系數(shù)進(jìn)行估算,設(shè)置為700mL/d,實(shí)驗(yàn)周期為120d,取樣時(shí)間參考《放射性廢物固化體長期浸出實(shí)驗(yàn)》(GB 7023-1986)[24]并進(jìn)行適當(dāng)調(diào)整.從浸出區(qū)出液口取樣測試,浸出液的pH值用pH計(jì)測定,Pb、Zn、Cr、Cd的濃度用ICP-OES測定,固化體中的晶體結(jié)構(gòu)用XRD分析,固化體的形貌用SEM觀測.

設(shè)置pH值為5.5(編號(hào)1#)、7.0(編號(hào)2#)的2組浸提液用于分別模擬填埋場初期和中期滲濾液的酸堿環(huán)境,按照設(shè)計(jì)的時(shí)間間隔(0.17d、0.33d、1d、3d、5d、7d、11d、14d、21d、28d、42d、63d、91d、120d)取飛灰固化體的浸出液,測定其pH值和Pb、Zn、Cr、Cd濃度.

表1 原灰浸出結(jié)果(mg/L) Table 1 Leaching results of raw ash(mg/L)

2 結(jié)果與討論

2.1 浸出液中pH值的變化規(guī)律

如圖2所示,1#浸出液前期pH值呈弱酸性,隨后快速升高并于21d左右至強(qiáng)堿性,63d達(dá)到峰值;2#浸出液的pH值在初期時(shí)迅速上升至強(qiáng)堿性,隨后緩慢升高并于77d左右達(dá)到峰值.

2組實(shí)驗(yàn)浸出液中pH值變化規(guī)律存在顯著差異,其原因主要是:1#浸提液為弱酸性緩沖體系,能在實(shí)驗(yàn)前期維持浸出液pH值的弱酸性,反應(yīng)機(jī)理見式1.1#固化體在浸出21d出現(xiàn)開裂,63d破碎,不同時(shí)間的形態(tài)見圖3.固化體的開裂增加了浸提液和固化體的接觸面積,促進(jìn)固化體內(nèi)部Ca(OH)2、CaCO3等物質(zhì)不斷地消耗H+(式2),致使浸出液的pH值進(jìn)一步上升.2#實(shí)驗(yàn)未發(fā)現(xiàn)固化體開裂,其浸出液pH值達(dá)到峰值的時(shí)間較1#實(shí)驗(yàn)更長.當(dāng)實(shí)驗(yàn)進(jìn)行至后期時(shí),固化體中的物質(zhì)溶出達(dá)到動(dòng)態(tài)平衡,pH值也趨于穩(wěn)定,但歸因于1#浸提液的pH值更低,因此1#浸出液的pH值在實(shí)驗(yàn)期內(nèi)也略低.值得注意的是,無論是初始浸提液為pH=5.5的醋酸-醋酸鈉溶液還是pH=7.0的去離子水,飛灰固化體浸出至120d時(shí)其浸出液仍有較強(qiáng)的堿性.

緩沖平衡體系:CH3COOH ?CH3COO-+ H+,

CH3COONa ?CH3COO-+ Na+(1)

酸化過程反應(yīng):Ca(OH)2+ 2H+= Ca2++ 2H2O,

CaCO3+ 2H+= Ca2++ CO2↑+ H2O (2)

圖2 pH隨浸出時(shí)間的變化規(guī)律

圖3 1#固化體隨時(shí)間開裂情況Fig.3 Cracking condition of 1# solidified body with time

2.2 重金屬的浸出特性

2.2.1 浸出濃度隨時(shí)間變化規(guī)律 重金屬以酸提取態(tài)、可還原態(tài)、可氧化態(tài)、殘?jiān)鼞B(tài)的形態(tài)存在于固化體中,其中酸提取態(tài)最易浸出,而殘?jiān)鼞B(tài)能較為穩(wěn)定存在于環(huán)境中[25].浸出液中Pb、Cr、Zn和Cd濃度變化見圖4(2#實(shí)驗(yàn)中無Cd檢出).由圖4可知,1#實(shí)驗(yàn)浸出液中各重金屬離子濃度變化趨勢不盡相同,而2#實(shí)驗(yàn)浸出液中Pb、Cr和Zn濃度均呈先上升后下降,然后趨于穩(wěn)定的變化趨勢.總體上,當(dāng)實(shí)驗(yàn)進(jìn)行至120d時(shí),2組實(shí)驗(yàn)浸出液中重金屬濃度均趨于穩(wěn)定.

由表2可知,Cd的氫氧化物可溶于酸,Pb、Cr、Zn的氫氧化物可溶于堿(式3),所以1#實(shí)驗(yàn)先后經(jīng)歷“浸出-沉淀-溶解”的過程.1#實(shí)驗(yàn)初期各重金屬的浸出濃度較高,但隨著浸出液pH值升高達(dá)到各自沉淀范圍先后開始生成氫氧化物沉淀,所以浸出液中濃度較2#低;當(dāng)浸出液至較強(qiáng)堿性環(huán)境時(shí)沉淀又開始溶解[26],此時(shí)重金屬的浸出過程同2#實(shí)驗(yàn).

表2 20℃下重金屬氫氧化物沉淀范圍及溶解性統(tǒng)計(jì) Table 2 Precipitation range and solubility of heavy metal hydroxide under 20℃

在實(shí)驗(yàn)中后期,1#實(shí)驗(yàn)的固化體出現(xiàn)了開裂甚至破碎,增加了固體與液體的接觸面積,但由于此時(shí)浸出液的堿性會(huì)被弱酸性浸提液部分中和(式4),所以對重金屬氫氧化物沉淀的溶解能力沒有2#浸出液強(qiáng),2#浸出液中重金屬濃度更高也就不難理解.值得注意的是,2#浸出液中Pb的最高濃度曾一度達(dá)到0.29mg/L,超過了填埋場入場限值,其潛在風(fēng)險(xiǎn)應(yīng)受到關(guān)注.可通過適當(dāng)增加水泥摻入量[16]、加入添加劑[27]或固化前酸洗[28]等方法減少或降低Pb的浸出量.

M(OH)+OH-= [M(OH)2n]-(M為Pb、Cr、Zn) (3)

OH-+ CH3COOH ?CH3COO-+ H2O (4)

2.2.2 單位累計(jì)浸出量隨時(shí)間變化規(guī)律 為方便比較分析兩組實(shí)驗(yàn)的重金屬浸出量,采用式(5)計(jì)算單位質(zhì)量固化體的累計(jì)浸出量,該式中同時(shí)考慮了浸出區(qū)內(nèi)儲(chǔ)留液體和流出浸出區(qū)液體中重金屬的浸出量.

由圖5可知,飛灰固化體中重金屬累積浸出量隨時(shí)間增加而增加,但因隨著實(shí)驗(yàn)的進(jìn)行固化體中重金屬含量逐漸減少且浸出難度增大,使得浸出速率隨時(shí)間在降低,因此曲線越到后期越漸平緩.

實(shí)驗(yàn)開始的很短一段時(shí)間內(nèi),1#實(shí)驗(yàn)浸出區(qū)為弱酸環(huán)境,固化體中重金屬開始快速溶出,但隨著飛灰和水泥中堿性物質(zhì)的溶出,pH值快速升高,使得固化體中浸出的重金屬量越漸減少;2#實(shí)驗(yàn)初期浸出區(qū)為中性環(huán)境,但隨著固化體內(nèi)飛灰和水泥中堿性物質(zhì)的溶出,浸出區(qū)內(nèi)pH值很快便升高至堿性環(huán)境并維持較高的pH值.Pb、Zn、Cr為兩性重金屬,在強(qiáng)酸、強(qiáng)堿性環(huán)境下均有較好的溶解性,因此,隨著浸出區(qū)pH值的升高,重金屬的可溶態(tài)絡(luò)合物[29]便會(huì)存在于浸出液中.盡管酸性環(huán)境可以促進(jìn)重金屬浸出,但由于維持的時(shí)間太短且中后期還存在酸堿中和導(dǎo)致1#浸出區(qū)的pH值比2#更低,因此總體上2#實(shí)驗(yàn)Pb、Cr累積浸出量比1#實(shí)驗(yàn)大,而Zn則由于主要受前期酸性溶出影響,因此1#實(shí)驗(yàn)的累計(jì)浸出量比2#實(shí)驗(yàn)大.綜上,浸出酸堿環(huán)境對重金屬的溶出起著關(guān)鍵作用[30],除應(yīng)單獨(dú)分區(qū)填埋飛灰固化體,盡量避免酸性滲濾液與其接觸以加速重金屬的溶出[31]外,還應(yīng)做好覆蓋和防滲措施,盡量避免雨水或地下水進(jìn)入填埋場與之接觸.

2.3 飛灰固化體SEM和XRD分析

2.3.1 SEM形貌分析 從圖6可以看出,隨著浸出過程的進(jìn)行,1#實(shí)驗(yàn)飛灰固化體的表面由光滑變得粗糙并出現(xiàn)孔隙,在后期有不規(guī)則顆粒晶體物質(zhì)堆積在固化體表面.從2#實(shí)驗(yàn)SEM掃描結(jié)果看,飛灰固化體表面逐漸出現(xiàn)不規(guī)則顆粒晶體物質(zhì),在實(shí)驗(yàn)中后期隨時(shí)間增加堆積愈多.

1#實(shí)驗(yàn)過程中固化體表面的孔隙結(jié)構(gòu)有利于溶液與固化體內(nèi)部接觸,若在合適的pH值范圍,可以促進(jìn)固化體中重金屬的浸出;1#、2#實(shí)驗(yàn)后期在固化體表面均沉積了不規(guī)則顆粒晶體物質(zhì),且2#晶體尺寸比1#大,該晶體有可能阻礙浸出液與固化體的接觸,抑制重金屬的浸出.

圖6 動(dòng)態(tài)浸出過程固化體SEM變化Fig.6 SEM change of solidified body in dynamic leaching(a)上圖為1#固化體SEM; (b)下圖為2#固化體SEM

2.3.2 XRD晶體結(jié)構(gòu)分析 從圖7可以看出,飛灰固化體中的堿性物質(zhì)Ca(OH)2被溶出(式6),是造成浸出液呈堿性的主要原因.1#固化體中盡管CaCO3遇酸會(huì)溶解,但因其溶解度很小且浸出液中含有的大量Ca2+會(huì)促使其再生成(式2、7),所以浸出后1#固化體仍含有大量的CaCO3,另外還有SiO2、Ca8Si5O18晶體.在2#實(shí)驗(yàn)中浸出的Ca2+生成難溶CaCO3晶體[32]附在固化體表面,但由于CaCO3晶體的生成需空氣中CO2溶解于水中再與Ca2+反應(yīng)因而進(jìn)行得較緩慢,所以生成的CaCO3晶體可能對實(shí)驗(yàn)中前期飛灰固化體內(nèi)物質(zhì)的浸出行為影響不大.

Ca(OH)2= Ca2++ 2OH-(6)

Ca2++ CO2+ H2O = CaCO3↓+ 2H+(7)

3 結(jié)論

3.1 無論初始浸提液為pH=5.5的醋酸-醋酸鈉溶液還是pH=7.0的去離子水,實(shí)驗(yàn)至120d左右浸出液中pH值和重金屬濃度均基本達(dá)到穩(wěn)定,但這對于pH值環(huán)境常在變化的實(shí)際生活垃圾填埋場來講,穩(wěn)定的過程和時(shí)間可能需要更長.

3.2 浸出環(huán)境的pH值、飛灰固化體的結(jié)構(gòu)及表面晶體生成情況是影響飛灰固化體中重金屬浸出的關(guān)鍵因素.浸提液為醋酸-醋酸鈉溶液時(shí)固化體重金屬的浸出主要集中在實(shí)驗(yàn)初期,而浸提液為去離子水時(shí)則主要集中在初-中期;實(shí)驗(yàn)后期沉積在固化體表面的CaCO3晶體有可能會(huì)抑制重金屬的浸出.

3.3 本實(shí)驗(yàn)條件下,2組實(shí)驗(yàn)后期浸出液的pH值均較高且2#浸出液中Pb的濃度甚至一度超過了填埋場入場標(biāo)準(zhǔn).因此,在實(shí)際過程中,應(yīng)通過優(yōu)化固化工藝、加入添加劑或固化前酸洗等方法降低飛灰中重金屬的可浸出量,再將飛灰單獨(dú)分區(qū)填埋并做好防雨、防滲措施,盡量避免滲濾液、雨水和地下水接觸飛灰固化體從而導(dǎo)致其中重金屬的溶出.同時(shí),今后研究工作中還應(yīng)關(guān)注飛灰固化體在生活垃圾填埋場中更為長時(shí)間和pH值環(huán)境更為復(fù)雜時(shí)的浸出風(fēng)險(xiǎn).

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Dynamic leaching rule of heavy metals in solidified body of fly ash from MSW incineration.

LIU Jian*, HE Liang, LUO Cheng-jie, SONG Kai, WANG Fei, BAI Xue, LI Qing-yi

(Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu 610031, China)., 2019,39(3)：1087~1093

A dynamic leaching experiment was established using CH3COOH-CH3COONa solution with a pH of 5.5 and deionized water with a pH of 7.0 as initial leach liquor, aiming at studying the long-term leaching behavior of fly ash solidified body in the domestic waste sanitary landfill site. The difference in pH of the initial leach liquor would affect the structural integrity and the formation of crystal on the surface of fly ash solidified body. When CH3COOH-CH3COONa solution was used, the fly ash solidified body would be broken during the experiment. While deionized water was used, CaCO3crystals formed on the surface of fly ash solidified body would inhibit the leaching of heavy metals. Both pH value and heavy metal concentration of the two groups’ leachate tended to be stable until the experiment lasted about 120 days, and the final pH value of them were close to each other. However, in the group of deionized water, the amount of cumulative leaching of Pb and Cr were significantly higher than that of CH3COOH-CH3COONa solution, and the leaching concentration of Pb (0.29mg/L) even once exceeded the landfill entry limit, which should be concerned for the potential risk.

fly ash；solidification；heavy metal；dynamic leaching；landfill

X705

A

1000-6923(2019)03-1087-07

劉 建(1982-),男,四川威遠(yuǎn)人,副研究員,博士,研究方向?yàn)楣腆w廢物處理與處置、工程水環(huán)境效應(yīng)及其控制.發(fā)表論文20余篇.

2018-08-09

國家自然科學(xué)基金資助項(xiàng)目(41602241,U1734205)

* 責(zé)任作者, 副研究員, liukai-102@163.com