朱崢嶸,王明新,張金永,葉 倩,周 杰

Fe(VI)/CaO2雙氧化體系降解水環(huán)境中的DMP

朱崢嶸,王明新*,張金永,葉 倩,周 杰

(常州大學(xué)環(huán)境與安全工程學(xué)院,常州 213164)

研究了高鐵酸鹽(Fe(VI))/過氧化鈣(CaO2)聯(lián)合處理對(duì)鄰苯二甲酸二甲酯(DMP)的降解性能及機(jī)理,分析了共存離子的影響.基于響應(yīng)面方法中的Box-Behnken實(shí)驗(yàn)設(shè)計(jì)進(jìn)行了多因素實(shí)驗(yàn),采用二次多項(xiàng)式和逐步回歸法擬合了DMP去除率與Fe(VI)投加量、CaO2投加量和反應(yīng)時(shí)間之間的關(guān)系,對(duì)實(shí)驗(yàn)條件進(jìn)行了優(yōu)化分析.結(jié)果表明,在初始pH值為中性條件下,Fe(VI)/CaO2可以有效去除DMP,降解過程符合準(zhǔn)一級(jí)動(dòng)力學(xué)模型.當(dāng)Fe(VI)/CaO2/DMP物質(zhì)的量比為1.1/4.4/1時(shí),模型預(yù)測(cè)DMP最大降解率為95.57%,和驗(yàn)證實(shí)驗(yàn)得出的降解率90.14%相近,表明該響應(yīng)面模型具有較好的模擬和預(yù)測(cè)能力.HCO3-和Mg2+對(duì)DMP的降解有一定抑制作用.自由基清除實(shí)驗(yàn)結(jié)果顯示,羥基自由基(HO·)和超氧自由基(O2-·)對(duì)DMP的降解具有重要貢獻(xiàn),表明Fe(VI)和CaO2可形成類芬頓試劑協(xié)同降解污染物.DMP的主要降解途徑包括酯基水解、側(cè)鏈氧化和苯環(huán)的羥基化;主要降解產(chǎn)物為鄰苯二甲酸單甲酯,2,5-二羥基苯甲酸,間苯二甲酸,草酸等.

高鐵酸鹽；過氧化鈣；鄰苯二甲酸二甲酯；響應(yīng)面法；降解機(jī)制

鄰苯二甲酸酯(PAEs)是一種廣泛使用的增塑劑,主要用于提高塑料的可塑性和柔韌性[1-3].PAEs具有潛在的內(nèi)分泌干擾效應(yīng),對(duì)內(nèi)分泌系統(tǒng)的激素活性有一定的抑制作用[4-5].鄰苯二甲酸二甲酯(DMP)是最常見的一種PAEs,水環(huán)境中的半衰期約為3.2a[6].據(jù)報(bào)道,在各類水體、土壤和大氣中均檢測(cè)到DMP[7-8].DMP易在環(huán)境中累積和富集,對(duì)人類健康和生態(tài)系統(tǒng)造成了潛在的威脅[9],已引起越來越廣泛的關(guān)注.

生物降解法可以有效地去除水環(huán)境中的DMP[10],但降解速率較慢,且在實(shí)際應(yīng)用中受到諸多環(huán)境因素,如溫度、溶解氧和pH值的限制.化學(xué)氧化技術(shù)具有反應(yīng)速率高、能夠有效地去除大多數(shù)有機(jī)污染物等特點(diǎn),大量應(yīng)用于污染場(chǎng)地的修復(fù)[11-12].其中,芬頓和類芬頓反應(yīng)的高氧化性能受到廣泛關(guān)注.芬頓反應(yīng)通過Fe2+催化過氧化氫(H2O2)產(chǎn)生羥基自由基(HO·)氧化污染物,HO·具有極強(qiáng)的氧化性,可以氧化多種有機(jī)和無機(jī)污染物[13-14].然而,在地下環(huán)境中液體H2O2的不穩(wěn)定性使得Fenton反應(yīng)在污染場(chǎng)地修復(fù)中的應(yīng)用受到限制[15].研究表明,過氧化鈣(CaO2)可以作為H2O2的固體來源,1g CaO2最多可以生成 0.47g H2O2,具有較強(qiáng)的氧化能力[16]. CaO2是一種環(huán)境友好型氧化劑,在污染物降解過程中不會(huì)對(duì)環(huán)境造成污染,而且還能刺激微生物生長(zhǎng),從而提高生物修復(fù)效率.Qian等[17]在研究石油烴類污染物時(shí)發(fā)現(xiàn),pH6時(shí),CaO2可以在3d內(nèi)有效去除甲苯及其中間體.Qian等[18]研究表明,CaO2在5d內(nèi)可完全降解2,4-二氯苯酚,主要的降解產(chǎn)物為苯酚,2-氯酚和4-氯酚.Arienzo[19]在研究CaO2降解水和土壤中的2,4,6-三硝基甲苯時(shí)發(fā)現(xiàn),CaO2對(duì)TNT污染土壤的原位修復(fù)具有良好的效果.

在過去10a中,基于CaO2的類芬頓技術(shù)廣泛應(yīng)用于地下水修復(fù)[20],但也存在一些問題,如單獨(dú)使用CaO2去除難降解有機(jī)污染物的速度相對(duì)較慢、Fe(Ⅱ)和Fe(Ⅲ)在循環(huán)轉(zhuǎn)化過程中容易累積形成氫氧化物,導(dǎo)致降解速率下降等.因此許多學(xué)者在類芬頓反應(yīng)體系中利用零價(jià)鐵、針鐵礦、赤鐵礦等作為替代鐵源[21-25],或者通過添加檸檬酸、草酸、谷氨酸等螯合劑螯合Fe(Ⅱ)和Fe(Ⅲ),避免其在中性或堿性條件下沉淀[26-28].高鐵酸鉀(Fe(VI))是一種高效的綠色氧化劑,集絮凝、氧化、消毒殺菌作用為一體,在氧化過程中生成的氫氧化鐵通常為納米級(jí)顆粒,具有較好的絮凝作用,可同時(shí)吸附多種有機(jī)污染物,廣泛應(yīng)用于有機(jī)污染物的去除[29].Yang等[30]研究發(fā)現(xiàn),Fe(VI)在弱堿性條件下可以有效去除羥基化的二苯甲酮衍生物二苯酮-3,主要的降解產(chǎn)物為4-甲氧基二苯甲酮和4-甲氧基苯甲酰陽離子.Chen等[31]研究表明,pH為8.0時(shí),Fe(VI)可在240秒內(nèi)快速去除五氯二苯基硫化物.

Fe(VI)具有較高的反應(yīng)速率,CaO2則是緩釋氧化劑,兩者在水溶液中均呈堿性,可以較好共存.構(gòu)建Fe(VI)/CaO2雙氧化體系,在發(fā)揮各自氧化效能的同時(shí),可充分利用Fe(VI)分解產(chǎn)生的Fe(Ⅱ)或Fe(Ⅲ)催化CaO2分解產(chǎn)生的H2O2,形成類芬頓試劑,進(jìn)一步提高其降解能力.因此,本研究以DMP為目標(biāo)污染物,研究Fe(VI)和CaO2單獨(dú)和聯(lián)合處理對(duì)DMP的降解性能及機(jī)理,通過響應(yīng)面法優(yōu)化實(shí)驗(yàn)設(shè)計(jì),確定最佳反應(yīng)條件;擬合降解動(dòng)力學(xué)模型,鑒別主要活性物質(zhì);確定降解產(chǎn)物,推斷降解路徑,旨在為DMP污染水環(huán)境的修復(fù)提供參考.

1 材料和方法

1.1 實(shí)驗(yàn)試劑

分析純DMP,硝酸鈉,硫酸鈉,碳酸氫鈉,氯化鎂,氫氧化鈉,氯化鉀,亞硫酸鈉,異丙醇,硫酸,鹽酸,乙酸,乙酸鈉,鄰菲羅啉,鹽酸羥胺和硫酸亞鐵銨購自Sinopharm Chemical Reagents公司(中國上海).Fe(VI)購自MACKLIN Reagent公司(中國上海).甲醇和乙腈(HPLC級(jí))購自Merck(Darmstadt,Germany)公司.CaO2、甲基叔丁基醚(GC級(jí)),GC衍生化試劑六甲基二硅氮烷和三甲基氯硅烷購自Aladdin Reagent公司(中國上海).實(shí)驗(yàn)用水由Milli-Q水純化系統(tǒng)制取超純水.

1.2 實(shí)驗(yàn)設(shè)計(jì)

Fe(VI)與CaO2聯(lián)合處理實(shí)驗(yàn)在恒溫振蕩器中進(jìn)行.在100mL反應(yīng)溶液中加入不同劑量的Fe(VI)和CaO2,反應(yīng)開始后動(dòng)態(tài)取樣檢測(cè).取1.00mL DMP樣品到含有200μL亞硫酸鈉(0.10mol/L)的色譜小瓶中,用于HPLC檢測(cè).在Fe(VI)和CaO2不同投加量下分別測(cè)量水溶液中二價(jià)和三價(jià)鐵離子,pH值和h的變化.二價(jià)和三價(jià)鐵離子采用鄰二氮菲分光光度法測(cè)量.

自由基清除實(shí)驗(yàn):在100mL反應(yīng)溶液中加入Fe(VI)/CaO2/DMP物質(zhì)的量比為2/5/1的Fe(VI)和CaO2作為對(duì)照,另外2組實(shí)驗(yàn)中分別再加入一定劑量的異丙醇和苯醌,反應(yīng)開始后動(dòng)態(tài)取樣檢測(cè)DMP.

所有樣品使用0.45μm濾膜過濾,取2次平行樣平均值作為當(dāng)前DMP濃度,以計(jì)算反應(yīng)速率.

1.3 響應(yīng)面法實(shí)驗(yàn)設(shè)計(jì)

表1 實(shí)驗(yàn)自變量因素的編碼水平

采用Box-Behnken模型設(shè)計(jì)正交實(shí)驗(yàn)[32],選取Fe(VI)投加量(1)、CaO2投加量(2)和反應(yīng)時(shí)間(3)為3個(gè)主要因素,并以+1,0,-1分別代表自變量的高、中、低3因素水平,響應(yīng)值為DMP的降解率.實(shí)驗(yàn)設(shè)計(jì)結(jié)果如表1所示.

1.4 鑒定降解產(chǎn)物

Fe(VI)和CaO2聯(lián)合處理時(shí)投加量物質(zhì)的量比2/2、2/5、5/2、0/5,反應(yīng)12h.取1.0mL樣品通過真空冷凍干燥濃縮,殘余物溶于4mL甲基叔丁基醚(MTBE)中,然后在70℃下用0.2mL六甲基二硅氮烷和0.1mL三甲基氯硅烷衍生化30min,經(jīng)0.45μm濾膜過濾后進(jìn)樣檢測(cè)[33].GC/MS用于分析中間產(chǎn)物.

1.5 分析方法

DMP采用高效液相色譜儀(LC-20AT,日本島津公司)分析.色譜柱:C18反相柱(SinChrom ODS-BP (5μm,4.6mm×250mm)).流動(dòng)相:水/乙腈=25/75(體積比),流速:1.0mL/min.進(jìn)樣體積:20μL,檢測(cè)波長(zhǎng): 225nm.

降解產(chǎn)物用Thermo Trace 1300GC-MS檢測(cè),色譜柱:TG-5MS.采用GC-MS系統(tǒng)(TSQ Quantum XLS Ultra TM,Thermo Scientific,USA)分析數(shù)據(jù)[21].載氣:氮?dú)?流速:1mL/min.進(jìn)樣口溫度:280℃.升溫程序:在80℃下保持3min,以30℃/min升溫至120℃,再以18℃/min升溫至140℃,再以30℃/min升溫至210℃,再以30℃/min升溫至250℃,最后在250℃保持1min.

2 結(jié)果與討論

2.1 Fe(VI)和CaO2聯(lián)合降解DMP的影響因素

圖1 單獨(dú)Fe(VI)對(duì)DMP的去除

2.1.1 Fe(VI)和CaO2單獨(dú)處理對(duì)DMP的降解效果 圖1顯示了DMP降解效率隨反應(yīng)時(shí)間和Fe(VI)投加量的變化,反應(yīng)結(jié)束時(shí)間為3h.當(dāng)Fe(VI)的投加量(Fe(VI):DMP的物質(zhì)的量比)從2:1增加到20:1時(shí),反應(yīng)開始10min到90min時(shí),DMP的去除效率從7.1%增加到74.3%.單獨(dú)CaO2對(duì)DMP的降解效果如圖2所示,反應(yīng)結(jié)束時(shí)間為12h.隨著CaO2投加量(CaO2:DMP的物質(zhì)的量比)從2:1增加到5:1時(shí),反應(yīng)結(jié)束后DMP的去除效率從49.2%增加到83.7%.隨著CaO2投加量的進(jìn)一步增加,DMP得到進(jìn)一步去除.表明單一Fe(VI)體系或者單一CaO2體系可以有效地去除DMP,但是需要消耗大量的氧化劑以及反應(yīng)時(shí)間.

圖2 單獨(dú)CaO2對(duì)DMP的去除

2.1.2 Fe(VI)和CaO2聯(lián)合對(duì)DMP的降解效果 不同投加量的Fe(VI)和CaO2對(duì)DMP的降解效果如圖3所示, 反應(yīng)結(jié)束時(shí)間為12h.就單獨(dú)Fe(VI)投加量(Fe(VI):DMP的物質(zhì)的量比)為2:1而言,反應(yīng)結(jié)束后DMP的去除率為7.3%;單獨(dú)CaO2投加量(CaO2: DMP的物質(zhì)的量比)分別為2:1和5:1時(shí),反應(yīng)結(jié)束后DMP的去除率分別為49.2%和83.7%;而Fe(VI)和CaO2聯(lián)合降解DMP(Fe(VI)/CaO2/DMP的物質(zhì)的量比為2/5/1)時(shí),DMP的去除率為99.5%.表明單獨(dú)Fe(VI)體系中,低劑量的Fe(VI)無法有效去除DMP;單獨(dú)CaO2體系中,CaO2通過分解產(chǎn)生H2O2破壞DMP分子的化學(xué)鍵,而H2O2的氧化還原電位為1.77V較低,導(dǎo)致無法完全去除污染物及其中間體;此外,CaO2產(chǎn)生的超氧自由基存活時(shí)間短和CaO2本身的緩釋性進(jìn)一步限制了對(duì)DMP的降解效率. Fe(VI)和CaO2在降解DMP過程中存在協(xié)同效應(yīng),CaO2分解釋放的H2O2與Fe(VI)的還原產(chǎn)物Fe(II)和Fe(III)迅速反應(yīng),生成氧化還原電位較高的羥基自由基,可以提高污染物的去除率.DMP的快速降解主要發(fā)生時(shí)前6h,在接下來的6h內(nèi)反應(yīng)速率較慢.推測(cè)當(dāng)Fe(VI)和CaO2投加到反應(yīng)溶液中,首先發(fā)生劇烈的氧化反應(yīng),在這一過程中,Fe(VI)被還原成Fe(II)和Fe(III),同時(shí)CaO2分解產(chǎn)生H2O2形成芬頓反應(yīng),進(jìn)一步降解污染物.但是,隨著反應(yīng)的繼續(xù)進(jìn)行,溶液中Ca(OH)2逐漸累積,Fe(II)和Fe(III)形成氫氧化物沉淀,Fe(II)和Fe(III)再循環(huán)受到抑制,從而減緩了反應(yīng)速率.

圖3 Fe(VI)和CaO2聯(lián)合處理對(duì)DMP的降解效果

在不同的Fe(VI)和CaO2投加量條件下,擬合了DMP的降解動(dòng)力學(xué)曲線.DMP的降解遵循準(zhǔn)一級(jí)動(dòng)力學(xué)模型,實(shí)驗(yàn)數(shù)據(jù)具有良好線性擬合(2=0.99),準(zhǔn)一級(jí)速率常數(shù)(app)計(jì)算方法如下:

式中:C是時(shí)刻(min)的DMP濃度,0是DMP初始濃度,app是準(zhǔn)一級(jí)動(dòng)力學(xué)常數(shù),是反應(yīng)時(shí)間.

如圖4所示,當(dāng)Fe(VI)投加量(Fe(VI):DMP的物質(zhì)的量比)為5:1,CaO2投加量(CaO2:DMP的物質(zhì)的量比)從0.5:1逐漸增加到5:1時(shí),app從0.05369h-1增加到0.21063h-1.隨著CaO2投加量的增加,DMP的去除率和反應(yīng)速率顯著增加.提高CaO2的用量可以產(chǎn)生更多的H2O2,從而提高芬頓反應(yīng)的效率,加快污染物的降解.當(dāng)CaO2投加量(CaO2:DMP的物質(zhì)的量比)為2:1,Fe(VI)投加量(Fe(VI):DMP的物質(zhì)的量比)從2:1逐漸增加到5:1時(shí),app從0.06821h-1增加到0.12267h-1;當(dāng)CaO2投加量(CaO2:DMP的物質(zhì)的量比)為5:1,Fe(VI)投加量(Fe(VI):DMP的物質(zhì)的量比)從2:1逐漸增加到5:1時(shí),app從0.19432h-1增加到0.21063h-1.CaO2投加量較低時(shí),隨著Fe(VI)投加量的增加,DMP的去除率和反應(yīng)速率大幅上升,提高Fe(VI)的用量可以加快催化H2O2從而反應(yīng)速率; CaO2投加量較高時(shí),繼續(xù)增加Fe(VI)的用量對(duì)反應(yīng)速率的影響不大,可能是因?yàn)镃aO2濃度高時(shí),反應(yīng)體系的pH值較高,Fe(VI)易產(chǎn)生Fe (OH)3,從而影響Fe(II)和Fe(III)的循環(huán)轉(zhuǎn)化,導(dǎo)致反應(yīng)速率變化不大.

圖4 Fe(VI)和CaO2聯(lián)合降解DMP動(dòng)力學(xué)

2.2 響應(yīng)面分析

2.2.1 模型擬合 采用3因素3水平實(shí)驗(yàn)設(shè)計(jì), Box-Behnken響應(yīng)面優(yōu)化實(shí)驗(yàn)設(shè)計(jì)實(shí)驗(yàn)結(jié)果和預(yù)測(cè)值見表2.

表2 RSM實(shí)驗(yàn)設(shè)計(jì)與結(jié)果

采用軟件Design-Experts8.0.5對(duì)擬合結(jié)果進(jìn)行方差分析,利用值對(duì)每個(gè)因素進(jìn)行顯著性檢測(cè),值<0.05說明回歸模型顯著[34-35].擬合得到如式(2)所示的二次多項(xiàng)式模型:

=-0.254+0.1101+0.2622+0.1493-0.01612-

6.450×10-313+2.353×10-323+1.083×10-312-

0.026×10-322-9.496×10-332(2)

式中:為響應(yīng)值,代表DMP的降解率,1、2、3分代表Fe(VI)投加量、CaO2投加量和反應(yīng)時(shí)間.式中正項(xiàng)系數(shù)表示增加此因素值會(huì)提高響應(yīng)值;反之,負(fù)項(xiàng)系數(shù)表示增加此因素值會(huì)降低響應(yīng)值[32].模型決定系數(shù)2為0.9679,證明獨(dú)立變量之間的相關(guān)性較好,該模型可用于實(shí)際預(yù)測(cè).3個(gè)因素的值均小于0.05,表明這3個(gè)因素對(duì)響應(yīng)值影響極為顯著; AB、AC、BC的值均小于0.05,表明其兩兩因素的交互作用顯著.

為獲得DMP去除率最大值的優(yōu)化條件,設(shè)定優(yōu)化目標(biāo)為去除率最高,約束條件為Fe(VI)投加量、CaO2投加量和反應(yīng)時(shí)間在本實(shí)驗(yàn)設(shè)定的取值范圍內(nèi),得到的最優(yōu)反應(yīng)條件為:Fe(VI)/CaO2/DMP(物質(zhì)的量比)為1.1/4.4/1時(shí),模型預(yù)測(cè)的污染物降解率為95.57%,和實(shí)驗(yàn)得出的降解率94.14%數(shù)值接近,表明擬合方程具有較高的精確度.

2.2.2 響應(yīng)曲面分析 圖5顯示了Fe(VI)投加量和CaO2投加量?jī)蓚€(gè)因素的交互影響.對(duì)應(yīng)的等高線圖呈現(xiàn)橢圓形趨勢(shì),說明兩個(gè)因素存在交互效應(yīng)[34].響應(yīng)值隨著Fe(VI)和CaO2投加量的增加而增加.在投加量較小時(shí),等高線斜率較高、響應(yīng)值變化較大.當(dāng)繼續(xù)增加投加量時(shí),等高線逐漸變平.這說明在反映初始階段,污染物濃度較高,氧化劑與污染物接觸效率高,降解速率較快.Fe(VI)在氧化目標(biāo)污染物時(shí)自身分解產(chǎn)生Fe(II)和Fe(III),進(jìn)一步活化CaO2產(chǎn)生的H2O2生成HO·,而且HO·可以進(jìn)一步與H2O2反應(yīng)產(chǎn)生O2-·,從而提高了降解效率.

HO·+H2O2→HO2·+ H2O (3)

HO2·?O2-·+H+(4)

圖6顯示了Fe(VI)投加量和反應(yīng)時(shí)間2個(gè)因素的交互影響.對(duì)應(yīng)的等高線圖呈現(xiàn)橢圓形,說明兩個(gè)因素交互效應(yīng)明顯.當(dāng)Fe(VI)投加量一定時(shí),隨著反應(yīng)時(shí)間的增加,響應(yīng)值先增高后趨于平緩并有降低趨勢(shì).Fe(VI)在降解污染物過程中,Fe(III)易形成氫氧化物沉淀,Fe(II)和Fe(III)的再生循環(huán)受到限制.同時(shí),Fe(II)也會(huì)捕捉反應(yīng)溶液中的HO·使活性物質(zhì)降低(方程(5)),從而抑制降解速率[34].

HO·+ Fe2+→Fe3++ HO-(5)

圖5 Fe(VI)和CaO2投加量對(duì)DMP去除率的影響

圖6 Fe(VI)投加量和反應(yīng)時(shí)間對(duì)DMP去除率的影響

圖7 CaO2投加量和反應(yīng)時(shí)間對(duì)DMP去除率的影響

圖7顯示了CaO2投加量和反應(yīng)時(shí)間的交互作用.對(duì)應(yīng)的等高線圖呈現(xiàn)橢圓形,說明兩個(gè)因素交互效應(yīng)明顯.響應(yīng)值隨著CaO2投加量的增加而增加,并隨著時(shí)間的延長(zhǎng)趨于平緩.CaO2在釋放H2O2過程中生成Ca(OH)2,在堿性體系中,H2O2的釋放速率降低而O2的釋放速率上升,導(dǎo)致整個(gè)反應(yīng)體系的降解速率下降[36].此外,隨著反應(yīng)時(shí)間的推進(jìn),溶液中H2O2也會(huì)捕捉HO·并與之反應(yīng)[37].因此,過量投加CaO2和延長(zhǎng)反應(yīng)時(shí)間對(duì)于進(jìn)一步提高DMP降解速率的作用較小.

2.3 自由基對(duì)DMP降解的影響

如前所述,Fe(VI)/CaO2雙氧化體系中可能產(chǎn)生的自由基主要是HO·和O2-,實(shí)驗(yàn)采用異丙醇和苯醌作為探針化合物特異性結(jié)合HO·和O2-.Tang等[36]研究Fe(II)/CaO2體系降解四氯化碳的結(jié)果表明,O2-·在污染物降解過程中占主導(dǎo)作用.Lewis等[20]在利用螯合劑改性芬頓試劑降解三氯乙烯(TCE)過程中,發(fā)現(xiàn)HO·是主要的功能自由基.Zhang等[38]在研究異丙醇對(duì)HO·的掩蔽作用時(shí)發(fā)現(xiàn),向反應(yīng)溶液添加30mmol/L的異丙醇時(shí),TCE的降解受到大幅度抑制.如圖8所示,向反應(yīng)溶液中添加異丙醇和苯醌后,DMP的降解速率受到抑制,且在Fe(VI)/CaO2雙氧化體系降解DMP的過程中,HO·的貢獻(xiàn)大于O2-·.

圖8 自由基對(duì)DMP降解的影響

2.4 共存離子對(duì)DMP降解的影響

在天然地下水中,DMP的去除效率會(huì)受到一些金屬離子和無機(jī)陰離子的影響.K+,Mg2+,HCO3-, SO42-和 NO3-離子對(duì)DMP去除效果的影響如圖9所示.其中,Mg2+和HCO3-離子的抑制作用較為明顯;K+,SO42-和 NO3-離子表現(xiàn)出輕微的促進(jìn)作用.

圖9 共存離子對(duì)DMP降解的影響

HCO3-離子是地下最常見的一種離子,不同地區(qū)水環(huán)境中的HCO3-離子含量從幾十到幾百′10-6,它對(duì)HO×具有一定的猝滅作用(方程(6~8))[39].Mg2+在反應(yīng)溶液中易形成氫氧化物附著在Fe(VI)表面,從而抑制降解速率(方程(9)).添加K+可以加速DMP的去質(zhì)子化,從而促進(jìn)DMP的降解.水環(huán)境中的NO3-離子含量在10-5~10-3mg/L,在有機(jī)物降解過程中可以充當(dāng)光敏劑促進(jìn)反應(yīng)速率[40].Qian等[18]在研究納米級(jí)CaO2降解2,4-二氯苯酚時(shí)也發(fā)現(xiàn)向反應(yīng)溶液中添加適量的SO42-可以促進(jìn)自由基對(duì)目標(biāo)污染物的降解,這與本實(shí)驗(yàn)結(jié)論相一致.

HO· + HCO3-→CO3·-+ H2O (6)

O2-· + O2-·→HO2-+ CO3·-(7)

CO3·-+ O2-·→CO32-+ O2(8)

Mg2++ 2HO·→2OH-+ Mg2+→Mg(OH)2(9)

2.5 DMP降解過程中鐵離子、pH值及Eh變化

Fe(VI)/CaO2/DMP物質(zhì)的量比為2/5/1條件下,DMP降解過程中鐵離子濃度的變化如圖10所示;pH值及Eh變化如圖11所示.

隨著反應(yīng)時(shí)間的增加,Fe2+的含量先上升后下降,Fe3+的含量緩慢增加后趨于穩(wěn)定.隨著反應(yīng)不斷進(jìn)行,溶液中H2O2含量增加,Fe(VI)被還原成Fe2+和Fe3+.Fe2+不斷催化H2O2產(chǎn)生HO?,自身被氧化為Fe3+,由于溶液呈堿性,Fe3+容易沉淀形成納米級(jí)絮狀Fe(OH)3.

圖10 DMP降解過程中鐵離子濃度變化

圖11 DMP降解過程中pH值及Eh變化

圖11顯示,溶液pH值隨著反應(yīng)時(shí)間的增加逐漸降低,這歸因于DMP的幾種中間產(chǎn)物,包括鄰苯二甲酸、間苯二甲酸和草酸等.h則隨著反應(yīng)的推進(jìn)呈上升趨勢(shì),但由于受pH值的影響,相對(duì)較低[41].

2.6 中間產(chǎn)物和降解途徑

通過GC/MS分析,檢測(cè)到的主要中間產(chǎn)物包括鄰苯二甲酸單甲酯,2,5-二羥基苯甲酸,間苯二甲酸和草酸等.圖12為不同氧化劑投加量下DMP的主要中間產(chǎn)物的演變.隨著氧化劑投加量的增加,DMP逐漸被降解為毒性較低的小分子,最終被氧化成二氧化碳和水.

圖12 不同氧化劑投加量下DMP的降解產(chǎn)物變化

圖13 Fe(VI)/CaO2體系中DMP降解路徑推測(cè)

基于產(chǎn)物鑒定和自由基清掃實(shí)驗(yàn),提出了DMP可能的降解路徑,包括酯基水解、Fe(VI)的親電氧化效應(yīng)及單電子耦合機(jī)制、自由基(HO·和O2-·)攻擊苯環(huán)和碳鏈.如圖13所示,首先,DMP在堿性溶液中先水解生成鄰苯二甲酸單甲酯(MMP),進(jìn)一步水解生成鄰苯二甲酸并異構(gòu)化成形式更穩(wěn)定的間苯二甲酸(IPA);然后,Fe(VI)通過締合反應(yīng)與活化的間苯二甲酸形成氫鍵,經(jīng)歷電子轉(zhuǎn)移過程破壞碳氧鍵生成中間體苯甲酸;進(jìn)一步,反應(yīng)溶液中的HO·和O2-·繼續(xù)攻擊羧基鄰位和間位的碳原子發(fā)生加成反應(yīng),生成中間體2,5-二羥基苯甲酸(2,5-DHBA);Fe(VI)和2種自由基按照上述方式繼續(xù)攻擊苯環(huán)和碳鏈,進(jìn)一步降解成小分子有機(jī)物,最終礦化成CO2和H2O.

3 結(jié)論

3.1 Fe(VI)/CaO2體系在氧化污染物的同時(shí)形成類芬頓試劑聯(lián)合降解污染物,并且聯(lián)合降解效果優(yōu)于單獨(dú)降解效果.

3.2 DMP的去除效果取決于Fe(VI)投加量、CaO2投加量、反應(yīng)時(shí)間和共存離子等因素.DMP的降解符合準(zhǔn)一級(jí)動(dòng)力學(xué)模型.通過響應(yīng)面法優(yōu)化降解參數(shù),最優(yōu)投加量Fe(VI)/CaO2/DMP物質(zhì)的量比為1.1/ 4.4/1、溶液初始pH為中性條件下,DMP去除率達(dá)到90%以上.

3.3 基于自由基清掃實(shí)驗(yàn)和中間產(chǎn)物鑒定,提出可能的降解機(jī)制包括酯基水解、Fe(VI)的親電氧化效應(yīng)及單電子耦合機(jī)制、自由基(HO·和O2-·)攻擊苯環(huán)和碳鏈.在中性pH值和室溫下,Fe(VI)/CaO2體系對(duì)DMP污染水環(huán)境具有很大的修復(fù)潛力.

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Degradation performance of dimethyl phthalate from aqueous environment by dual oxidant of ferrate/calcium peroxide.

ZHU Zheng-rong, WANG Ming-xin*, ZHANG Jin-yong, YE Qian, ZHOU Jie

(School of Environmental & Safety Engineering, Changzhou University, Changzhou 213164, China)., 2019,39(7)：2838~2846

The degradation efficiency and mechanism of ferrite (Fe(VI))/calcium oxide (CaO2) combined treatment on dimethyl phthalate (DMP) were studied. Multi-factor experiments were carried out based on the Box-Behnken experimental design in Response Surface Method. Relationships between the removal rate of DMP and Fe(VI)dosage, CaO2dosage and reaction time were fitted by quadratic polynomial and stepwise regression method, and the experimental conditions were optimized. DMP could be effectively removed by Fe(VI)/CaO2under neutral pH conditions, and the degradation process conformed to the pseudo-second-order kinetic model. The maximum degradation rate of DMP predicted by the model reached 95.57% when the molar ratio of Fe(VI)/CaO2/DMP was 1.1/4.4/1, which was close to 90.14% obtained by verification experiments, indicating that the model possessed a good simulation and predictive performance. HCO3-and Mg(Ⅱ) inhibited the degradation of DMP to a certain extent. The results of free radical scavenging experiments revealed that hydroxyl radical (HO·) and superoxide radical (O2-·) made significant contributions to the degradation of DMP, demonstrating that Fe(VI)and CaO2could form Fenton-like reagents to synergistically degrade pollutants. The main degradation pathways of DMP included hydrolysis of ester group, oxidation of side chain and hydroxylation of benzene ring. The main degradation products were monomethyl phthalate, 2,5-dihydroxybenzoic acid, isophthalic acid, oxalic acid, etc.

ferrate；calcium peroxide；dimethyl phthalate；response surface method；degradation mechanisms

X703

A

1000-6923(2019)07-2838-09

朱崢嶸(1994-),女,江蘇南通人,常州大學(xué)環(huán)境與安全工程學(xué)院碩士研究生,主要研究水環(huán)境中鄰苯二甲酸酯污染物的去除.發(fā)表論文2篇.

2018-11-05

國家自然科學(xué)基金資助項(xiàng)目(41772240,41641032)

* 責(zé)任作者, 教授, wmxcau@163.com