水凝膠負(fù)載BiOI活化過一硫酸鹽降解尼泊金甲酯

胡優(yōu)優(yōu),李正魁

水凝膠負(fù)載BiOI活化過一硫酸鹽降解尼泊金甲酯

胡優(yōu)優(yōu),李正魁*

(南京大學(xué)環(huán)境學(xué)院,污染控制與資源化研究國家重點(diǎn)實(shí)驗(yàn)室,江蘇 南京 210023)

以輻射聚合法合成的水凝膠為載體,采用化學(xué)沉淀法制備水凝膠基碘氧化鉍復(fù)合光催化劑(p(HEA-APTMACl)-BiOI).應(yīng)用掃描電子顯微鏡(SEM),X射線能譜儀(EDS)和X射線粉末衍射儀(XRD)對材料進(jìn)行表征,結(jié)果顯示,BiOI成功負(fù)載于水凝膠上.p(HEA-APTMACl)-BiOI活化過一硫酸鹽(PMS)于可見光下可有效降解尼泊金甲酯(MP).探究氧化劑投加量、BiOI負(fù)載量、MP初始濃度等因素對MP降解效率的影響,結(jié)果表明,p(HEA-APTMACl)、BiOI與PMS存在協(xié)同作用,且在MP初始濃度為0.328mmol/L,PMS濃度為1.5mmol/L,催化劑投加量為0.1g條件下,2h內(nèi)MP去除率高達(dá)99%.Cl-對MP降解速率有促進(jìn)作用,SO42-則相反,且HCO3-會(huì)抑制MP降解效率,NO3-和H2PO4-對MP降解影響較小.

水凝膠；碘氧化鉍；過一硫酸鹽；尼泊金甲酯

尼泊金甲酯,又名對羥基苯甲酸甲酯(MP),其重點(diǎn)污染源主要是污水處理廠.由于MP難以降解和傳統(tǒng)污水處理技術(shù)的局限性,污水中MP仍以ng/L的水平存在[1-2].此外它具有弱雌激素活性以及潛在的內(nèi)分泌干擾效應(yīng),人體長期接觸可能會(huì)引發(fā)乳腺癌[3].

與傳統(tǒng)水處理方法相比,基于硫酸根自由基(·SO4-)的高級氧化技術(shù)擁有比羥基自由基(·OH)更高的氧化還原電位,較長的半衰期,較寬的pH值耐受范圍等優(yōu)點(diǎn)[4],有利于處理難降解有機(jī)物.過硫酸鹽(PS)和過一硫酸鹽(PMS)是常見的產(chǎn)生·SO4-的氧化劑,可通過紫外光、熱、過渡金屬離子等方法活化[5].目前, PMS的活化多采用負(fù)載型光催化劑[6-7],一方面催化性能高效,另一方面有利于光催化劑的回收利用.研究發(fā)現(xiàn)負(fù)載型Co3O4活化PMS可以高效降解雙氯芬酸,其降解效率達(dá)100%[8].

碘氧化鉍(BiOI)是一種新型可見光催化劑,具有特殊的層狀結(jié)構(gòu)和良好的光催化性能[9].有研究表明BiOI/Fe3O4可活化PS降解污染物[10].水凝膠因三維大孔結(jié)構(gòu)而成為BiOI理想的載體.本文通過化學(xué)沉淀法將BiOI負(fù)載于水凝膠上,制備得到水凝膠基碘氧化鉍復(fù)合催化劑p(HEA-APTMACl)-BiOI,有效地將p(HEA-APTMACl)-BiOI作為PMS活化劑引入高級氧化體系中,考察了其活化PMS降解MP效果,同時(shí)對主要影響因素進(jìn)行探討.

1 材料與方法

1.1 實(shí)驗(yàn)材料

對羥基苯甲酸甲酯,購自麥克林有限公司(中國上海);2-丙烯酸羥乙酯(HEA)和五水合硝酸鉍(Bi(NO3)3·5H2O),購自Aladdin公司(中國上海);(3-丙烯酰胺丙基)三甲基氯化銨(APTMACl),購自日本東京化成工業(yè)株式會(huì)社;碘化鉀(KI),購自國藥集團(tuán)化學(xué)試劑有限公司;過硫酸氫鉀Oxone(PMS)購自羅恩公司(中國上海),以上試劑均為分析純;溶劑甲醇為色譜純,購自默克公司(中國上海).

1.2 催化劑制備與表征

首先應(yīng)用輻射聚合法合成復(fù)合水凝膠:將57mL去離子水、12.7mLAPTMACl和30.4mLHEA分別加入250mL棕色瓶中,超聲30min使其均勻混合;隨后向棕色瓶中通入氮?dú)?保證混合溶液處于無氧狀態(tài);再將棕色瓶置于含適量乙醇和干冰的保溫桶內(nèi),在-78℃溫度下采用60Co-γ射線輻照24h,輻照劑量設(shè)定為20kGy;最后,所得水凝膠在常溫下解凍,并切割成方塊,多次清洗,獲得復(fù)合水凝膠p(HEA- APTMACl).

采用化學(xué)沉淀法制備復(fù)合光催化劑,具體操作如下:稱取一定質(zhì)量的復(fù)合水凝膠p(HEA- APTMACl),分散于0.5mol/LHNO3中,加入1mmolBi(NO3)3·5H2O,混合均勻后置于恒溫振蕩器中反應(yīng)24h;濾出上述水凝膠,清洗,將上述水凝膠移至50mL0.02mol/L的KI溶液中,振蕩24h,水凝膠顏色最終呈現(xiàn)橘黃色,表明BiOI成功負(fù)載于復(fù)合水凝膠上;最后將反應(yīng)后的水凝膠過濾,多次洗滌后于60℃烘箱內(nèi)干燥,即得水凝膠基碘氧化鉍復(fù)合催化劑p(HEA-APTMACl)-BiOI.

BiOI的制備除了不加復(fù)合水凝膠外,其余步驟與上述相同.

材料的表面形貌表征是采用掃描電子顯微鏡(SEM, FEI Quanta 250FEG)結(jié)合X射線能譜儀(EDS),并使用X射線粉末衍射儀(XRD, X' TRA)測定材料的晶體結(jié)構(gòu).

1.3 MP降解實(shí)驗(yàn)

MP的降解實(shí)驗(yàn)在光催化反應(yīng)裝置中進(jìn)行,以500W氙燈為可見光光源,在(20±2)℃條件下,配置并移取50mL0.32mmol/L的MP溶液,再向石英管中加入0.1g催化劑,1.5mmol/LPMS,放置于光催化反應(yīng)儀中,可見光照射反應(yīng)2h,分別在一定間隔時(shí)間內(nèi)取樣,用0.22μm濾膜過濾,利用高效液相色譜法(HPLC)測定殘余的MP濃度.每組樣品設(shè)置2個(gè)平行樣,取其平均值進(jìn)行分析.

MP濃度測定的具體條件為:C18反相色譜柱(Agilent Technologies,5μm,4.6×150mm),柱溫為25℃,流動(dòng)相為甲醇與0.1%乙酸溶液(體積比為65:35),流速為1.0mL/min,進(jìn)樣量為10μL,采用二極管陣列檢測器(DAD),檢測波長為254nm. MP去除率按照式(1)計(jì)算:

MP降解率=(0-)/0×100% (1)

2 結(jié)果與討論

2.1 水凝膠負(fù)載BiOI催化劑表征分析

通過SEM分析材料表面的形貌,如圖1所示,可以看出復(fù)合水凝膠表面光滑,有豐富的孔隙結(jié)構(gòu);而p(HEA-APTMACl)-BiOI表面堆疊了二維納米片,這可能是BiOI以微粒的形式負(fù)載于復(fù)合水凝膠的表面或孔隙中.而EDS表征結(jié)果證明了p(HEA- APTMACl)-BiOI存在Bi、O、I等元素.

從圖2可以看出,其衍射峰與BiOI標(biāo)準(zhǔn)圖譜(JCPDDS 73-2062)匹配度較高.此外,復(fù)合催化劑具有29.7°,31.7°,45.4°和55.3°的衍射峰,分別對應(yīng)(012),(110),(014),(114),(122)晶面的衍射峰,說明BiOI成功負(fù)載于復(fù)合水凝膠上.

圖2 p(HEA-APTMACl)-BiOI與BiOI標(biāo)準(zhǔn)卡片的XRD圖譜對比圖

2.2 MP降解性能對比分析

在MP初始濃度為0.328mmol/L,PMS濃度為1.5mmol/L,催化劑投加量為0.1g條件下,考察了不同材料活化PMS催化降解MP的效果.由圖3(a)可知,單獨(dú)的PMS和BiOI/PMS體系中,2h內(nèi)MP的降解效率分別為5%和20%.當(dāng)p(HEA- APTMACl)-BiOI單獨(dú)光降解MP時(shí),污染物去除率為10%左右;而向該體系加入PMS,MP的降解效率為99%,遠(yuǎn)高于單獨(dú)的BiOI/PMS體系.此外, p(HEA-APTMACl)/ PMS體系對MP的降解效率影響不大.劉曼等[11]研究摻銅介孔碳(Cu-OMC)活化PMS降解雙酚A(BPA),結(jié)果發(fā)現(xiàn)在活化PMS降解BPA的過程中,起作用的是Cu與OMC間協(xié)同作用.上述結(jié)果同樣證明p(HEA-APTMACl)、BiOI與PMS間存在協(xié)同作用,有利于提高污染物的去除效率.

圖3(b)為p(HEA-APTMACl)-BiOI光催化活化PMS降解MP過程中TOC變化情況.從圖中可以看出,TOC去除率在0,30,60,90,120min處分別為0%、32.8%、65.1%、69.1%和78.3%,隨著反應(yīng)時(shí)間的延長,污染物的礦化率逐漸增加,表明p(HEA- APTMACl)-BiOI/PMS/Vis體系對MP不僅有著良好的去除效果,而且其礦化效果比較優(yōu)異.

(a) 不同體系中MP的降解效率;(b) p(HEA-APTMACl)-BiOI/PMS/Vis體系TOC去除率

2.3 氧化劑投加量的影響

為了研究氧化劑投加量對MP降解效率的影響,分別投加0,0.32,0.75,1.5,3.0mmol/L的PMS,控制其他條件不變,MP去除效果如圖4所示.從圖4可知,當(dāng)體系中未投加PMS時(shí),MP在2h內(nèi)去除率不到10%;隨著PMS投加量的增加,MP降解明顯加快.當(dāng)PMS濃度由0.32mmol/L增加至1.5mmol/L時(shí),2h內(nèi)MP降解率可由42%提升到99%,這是因?yàn)镻MS濃度越高,產(chǎn)生的活性自由基數(shù)量越多[12];但進(jìn)一步增強(qiáng)PMS濃度,MP降解率提高不顯著,說明反應(yīng)活性位點(diǎn)有限,且催化過程中產(chǎn)生的·SO4-自由基本身會(huì)發(fā)生重組(式2),·SO4-會(huì)與多余的過硫酸氫鹽HSO5-反應(yīng)(式3),生成氧化能力較弱的·SO5-[12];此外,過量的PMS導(dǎo)致SO42-的增多,而《地表水環(huán)境質(zhì)量標(biāo)準(zhǔn)(GB-2002)》[13]和《生活飲用水衛(wèi)生標(biāo)準(zhǔn)(GB5749- 2006)》[14]規(guī)定的SO42-濃度限值為250mg/L (2.6mmol/L)[15],因此反應(yīng)體系中氧化劑的投加量應(yīng)從經(jīng)濟(jì)成本和環(huán)境保護(hù)綜合考慮,選擇1.5mmol/L的過硫酸鹽投加量.

×SO4-+×SO4-?S2O82-(2)

HSO5-+×SO4-?×SO5-+SO42-+H+(3)

圖4 PMS投加量對MP降解的影響

2.4 BiOI負(fù)載量的影響

在相同負(fù)載條件下,將不同濃度(1,5,10, 20mmol/L)的BiOI負(fù)載到復(fù)合水凝膠上,并維持MP初始濃度為0.328mmol/L,PMS濃度為1.5mmol/L,催化劑投加量為0.1g等條件不變,考察不同負(fù)載量的水凝膠基碘氧化鉍復(fù)合催化劑在體系中對MP催化性能的影響.如圖5所示,隨著BiOI負(fù)載量的增加,MP去除率和降解速率均呈現(xiàn)上升的趨勢.當(dāng)BiOI負(fù)載量為1mmol/L時(shí),2h內(nèi)MP去除率和降解速率分別可達(dá)87%和0.0248min-1;當(dāng)BiOI負(fù)載量由5mmol/L增加到20mmol/L時(shí),MP去除率在2h內(nèi)未發(fā)生明顯變化,仍保持99.9%的MP去除率,但其降解速率從0.0336min-1增長至0.0667min-1,與TiO2太陽光誘導(dǎo)均相光降解MP的降解趨勢一致[16];該現(xiàn)象主要是因?yàn)樵谙嗤姆磻?yīng)條件下,高負(fù)載量的催化劑為PMS提供更多的反應(yīng)位點(diǎn)[17],產(chǎn)生更多的強(qiáng)氧化性自由基,有效提高體系中MP的降解效能.

2.5 MP初始濃度的影響

圖6 不同初始濃度對MP降解的影響

如圖6所示,當(dāng)MP初始濃度為0.066mmol/L時(shí),在50min內(nèi)MP降解率可達(dá)最優(yōu);當(dāng)MP初始濃度為0.132mmol/L時(shí),MP降解率仍有99.7%,其反應(yīng)速率略高于0.066mmol/L條件下的反應(yīng)速率.這是由于體系中污染物初始濃度較低時(shí),污染物濃度的增加使得污染物和催化劑充分接觸,污染物被催化劑表面的活性自由基氧化降解[18];當(dāng)污染物濃度繼續(xù)升高時(shí),一方面,催化劑表面的部分活性位點(diǎn)被污染物所占據(jù),不利于活化PMS[19];另一方面,MP降解過程中的中間產(chǎn)物也與目標(biāo)污染物MP間存在活性自由基的競爭現(xiàn)象;而MP的降解取決于體系中產(chǎn)生的活性自由基.所以,要達(dá)到相同的去除效果所需的反應(yīng)時(shí)間延長,MP降解速率有所下降.

2.6 水中陰離子的影響

自然水體中廣泛存在各種陰離子,例如Cl-、HCO3-、H2PO4-、NO3-以及SO42-,這些離子的存在或多或少會(huì)影響污染物的去除效率.

在體系中,控制MP初始濃度為0.328mmol/ L,PMS濃度為1.5mmol/L,催化劑投加量為0.1g等條件,分別研究10mmol/LNO3-、SO42-、H2PO4-、Cl-、HCO3-等無機(jī)陰離子對MP去除效率的影響.如圖7所示,體系中NO3-、H2PO4-對MP降解影響比較微弱,Cl-的存在明顯加速M(fèi)P的降解速率,而SO42-的介入使MP的降解速率發(fā)生遲滯現(xiàn)象,同時(shí)HCO3-的引入導(dǎo)致MP的降解率下降了近10%.

圖7 無機(jī)陰離子對MP降解的影響

Cl-增強(qiáng)MP降解速率的主要原因在于Cl-與PMS反應(yīng)生成具有強(qiáng)氧化性的HClO,從而電離出ClO-[20-21][式(4)~(8)],進(jìn)一步提高M(jìn)P的降解速率;此外,Cl-與·SO4-和·OH反應(yīng)所產(chǎn)生的·Cl和·ClOH-(式8~9)的氧化還原電位與·SO4-和·OH的氧化還原電位差異不大[22],也能夠降解MP,一定程度上抵消·SO4-和·OH的消耗.總之,MP降解速率的提高是由Cl-的存在引起的.

HCO3-對MP降解效率產(chǎn)生抑制作用,主要是因?yàn)镠CO3-扮演·SO4-和·OH淬滅劑的角色[23],一方面HCO3-與·SO4-和·OH發(fā)生淬滅反應(yīng)[式(10)~(11)],結(jié)果·SO4-和·OH自由基數(shù)量減少,另一方面淬滅反應(yīng)所產(chǎn)生的·HCO3、·CO3-的還原電位明顯低于·SO4-和·OH[24],最終導(dǎo)致MP降解效率的降低.同樣地,SO42-也會(huì)消耗·SO4-和·OH[式(12)~(13)],伴隨著S2O82-和OH-的生成,一定程度上抑制了MP的降解速率,但最終仍能達(dá)到先前的MP降解效率.

HSO5-+Cl-→HOCl+SO42-(4)

HSO5-+2Cl-+H+→H2O+Cl2+SO42-(5)

Cl2+H2O→HOCl+H++Cl-(6)

HOCl→H++ClO-(7)

×SO4-+Cl-→×Cl+SO42-(8)

×OH+Cl-→×ClOH-(9)

HCO3-+×SO4-→×HCO3+SO42-(10)

HCO3-+×OH→×CO3-+H2O (11)

SO42-+×SO4-→S2O82-+e-(12)

SO42-+×OH→×SO4-+OH-(13)

3 結(jié)論

3.1 采用輻射聚合法成功制備了一種新型復(fù)合水凝膠 p(HEA-APTMACl);利用化學(xué)沉淀法負(fù)載碘氧化鉍,合成水凝膠基碘氧化鉍復(fù)合催化劑p(HEA- APTMACl)-BiOI.

3.2 相對于單一的BiOI/PMS體系,p(HEA- APTMACl)-BiOI能高效活化PMS降解MP,取得良好的去除效果,并具有一定的礦化能力.

3.3 當(dāng)MP初始濃度為0.328mmol/L,PMS濃度為1.5mmol/L,催化劑投加量為0.1g時(shí),2h內(nèi)MP去除率高達(dá)99%; NO3-、H2PO4-對MP降解影響不明顯,Cl-促進(jìn)MP降解,而SO42-和HCO3-抑制MP降解.

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Activation of peroxymonosulfate by hydrogel supported BiOI for methylparaben degradation.

HU You-you, LI Zheng-kui*

(State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, China)., 2019,39(8)：3249~3254

P(HEA-APTMACl)-BiOI was prepared by chemical precipitation method and the hydrogel synthesized by radiation polymerization was used as catalyst carriers. The catalyst was characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). These results showed that BiOI was successfully loaded on the hydrogel. The p(HEA-APTMACl)-BiOI/peroxymonosulfate (PMS) presented high methylparaben (MP) degradation efficiency under visible light, and the effects of several operating parameters (PMS dosage, BiOI loadings and initial MP concentration) on the MP degradation efficiency were also explored. The results demonstrated that there was a synergistic effect between p(HEA-APTMACl), BiOI and PMS. In addition, the MP (0.328mmol/L) degradation efficiency reached 99% within 2h when PMS concentration was 1.5mmol/L and the catalyst dosage was 0.1g. The addition of Cl-accelerated the degradation rate of MP, while the SO42-lead to hysteresis in MP degradation. Moreover, the MP degradation efficiency was reduced by the presence of HCO3-, but it was little influenced by NO3-and H2PO4-.

hydrogel；BiOI；peroxymonosulfate；methylparaben

X703

A

1000-6923(2019)08-3249-06

胡優(yōu)優(yōu)(1994-),女,安徽安慶人,南京大學(xué)碩士研究生,研究方向?yàn)樗w污染物去除技術(shù).發(fā)表論文1篇.

2018-12-29

國家水體污染控制與治理科技重大專項(xiàng)(2017ZX07204-004, 2018ZX07208-001)

* 責(zé)任作者, 教授, zhkuili@nju.edu.cn