徐海玉,張明青,陳翌昱

有機(jī)凹凸棒石負(fù)載納米零價(jià)鐵去除水中六價(jià)鉻

徐海玉,張明青*,陳翌昱

(中國礦業(yè)大學(xué)環(huán)境與測(cè)繪學(xué)院,江蘇 徐州 221116)

以黏土為載體負(fù)載納米零價(jià)鐵(nZVI,nanoscale zero-valent iron)可改善零價(jià)鐵顆粒的團(tuán)聚行為,增強(qiáng)其反應(yīng)性.在負(fù)載過程中鐵和黏土用量比例不同,對(duì)nZVI顆粒團(tuán)聚和反應(yīng)性的影響也不同.以有機(jī)凹凸棒石(CTMAB/A)為載體,采用液相還原法在1:3和1:5兩種鐵土質(zhì)量比條件下制備負(fù)載型納米零價(jià)鐵復(fù)合材料CTMAB/A-nZVI-3和CTMAB/A-nZVI-5.利用X射線衍射儀(XRD)和掃描電子顯微鏡(SEM)表征了兩種負(fù)載樣品和不添加黏土載體時(shí)nZVI的分散差異,并通過X射線光電子能譜儀(XPS)分析樣品與Cr(VI)反應(yīng)后的產(chǎn)物分布.結(jié)果表明:降低負(fù)載反應(yīng)時(shí)的鐵土比能明顯改善nZVI粒子的團(tuán)聚現(xiàn)象、減小nZVI平均粒徑、提高小粒徑顆粒占比、增強(qiáng)其反應(yīng)活性.小于10nm的nZVI顆粒在CTMAB/A-nZVI-3和CTMAB/A-nZVI-5負(fù)載nZVI總數(shù)中占比分別為3.60%和7.60%,在不負(fù)載的nZVI樣品中占比為0.鐵的投加量同為0.75g/L時(shí),CTMAB/A-nZVI-5中鐵對(duì)Cr(VI)去除率為86.20%,CTMAB/A-nZVI-3中為78.00%.

凹凸棒石；納米零價(jià)鐵；鐵土比；六價(jià)鉻

近年來,nZVI作為還原劑被越來越多的應(yīng)用于治理環(huán)境中具有氧化性的有毒有害物質(zhì)如As(V)、Cr(VI)、Pb(Ⅱ)等[1].歐美一些國家和地區(qū)將nZVI用于污染地下水中六價(jià)鉻Cr(VI)的原位治理,取得了良好的效果[2].已有研究證實(shí)nZVI顆粒具有核-殼結(jié)構(gòu),外層殼多為鐵的氧化物,核多為Fe0.水中污染物首先被表層氧化殼吸附,然后才被核中Fe0還原[3]. nZVI顆粒巨大的表面積和存在于表面上的大量活性位點(diǎn)加速了反應(yīng)過程的進(jìn)行.但是,巨大的比表面積和超高的表面能加之鐵的磁性,也使nZVI顆粒很容易團(tuán)聚.團(tuán)聚影響了顆粒在含水層中的流動(dòng)性和反應(yīng)活性,因此限制了nZVI在地下水處理中的應(yīng)用[4].為了克服nZVI易團(tuán)聚的缺陷,用活性炭、各類黏土、浮石等作負(fù)載材料制備負(fù)載型nZVI已成為目前的研究熱點(diǎn)[5-8].

凹凸棒石(Attapulgite)是一種具有納米棒狀晶體形貌(長約0.5~5μm,直徑約20~70nm)和規(guī)整納米孔道(0.37nm×0.64nm)的層鏈狀黏土礦物.較大的比表面積和陽離子交換能力使凹凸棒石在環(huán)境污染控制和修復(fù)領(lǐng)域得到廣泛應(yīng)用[9].但凹凸棒石結(jié)構(gòu)中帶有負(fù)電荷,不能直接去除環(huán)境中帶負(fù)電的重鉻酸根,所以一些學(xué)者研究有機(jī)改性后的凹凸棒石或凹凸棒石直接負(fù)載nZVI對(duì)Cr(VI)的去除效果,取得了一定效果[10-13].劉少?zèng)_[10]研究發(fā)現(xiàn)經(jīng)聚乙烯亞胺改性后的凹凸棒石對(duì)Cr(VI)的最大吸附量為51.81mg/g. Quan等[11]研究表明,4g/L的凹凸棒石負(fù)載nZVI復(fù)合材料在中性條件下處理含20mg/L Cr(VI)的水溶液,其對(duì)鉻的去除率可達(dá)98.73%.

本文以陽離子表面活性劑十六烷基三甲基溴化銨(CTMAB)改性后的有機(jī)凹凸棒石為載體,在1:3和1:5兩種鐵土質(zhì)量比條件,以及不添加附載體條件下制備負(fù)載型納米材料(記為CTMAB/A-nZVI-3、CTMAB/A-nZVI-5和nZVI),并表征材料的微觀形貌和nZVI粒子的分散狀態(tài),考察負(fù)載材料對(duì)水中Cr(VI)的去除規(guī)律,探討不同鐵土比對(duì)nZVI顆粒粒徑、分散性和反應(yīng)性的影響.

1 材料與方法

1.1 試劑、材料和儀器

試劑與材料:硼氫化鈉(NaBH4)、七水合硫酸亞鐵(FeSO4×7H2O)、無水乙醇、磷酸、鹽酸、重鉻酸鉀(K2Cr2O7)、1,5-二苯基碳酰二肼,以上藥劑均為分析純.CTMAB改性后的凹凸棒石,由江蘇盱眙啟睿礦業(yè)有限公司提供,粒度小于200目.

儀器:高速旋轉(zhuǎn)離心機(jī)(上海盧湘儀離心機(jī)儀器, TG1650-WS);可見分光光度計(jì)(上海舜宇恒平科學(xué)儀器,722型);掃描電子顯微鏡(德國Zeiss,SUPRA 40);X射線衍射儀(德國 Bruker,D8ADVANCE);X射線光電子能譜儀(美國Thermo Fisher,ESCALAB 250Xi).

1.2 樣品制備與表征

以有機(jī)凹凸棒石為負(fù)載材料,采用傳統(tǒng)的液相還原法,利用NaBH4還原硫酸亞鐵制備nZVI.反應(yīng)方程式如下所示:

于1000mL的三口燒瓶中加入300mL的醇水混合液(乙醇/水=1:4),定量投加FeSO4×7H2O,室溫下攪拌15min至FeSO4×7H2O完全溶解,之后定量投加有機(jī)凹凸棒石.攪拌使有機(jī)凹凸棒石充分分散后,將現(xiàn)配的濃度為2mol/L的NaBH4溶液以每秒1滴的速度滴入三口燒瓶中,NaBH4的滴加量以硼鐵物質(zhì)的量比為2:1計(jì).滴加結(jié)束后,再用磁力攪拌器攪拌1h使反應(yīng)完全,之后靜置20~30min.傾去上清液,用磁鐵分離出樣品,用脫氧去離子水反復(fù)洗滌3遍.以上操作均在高純N2保護(hù)下進(jìn)行.樣品在50℃條件下真空干燥6h,研磨過200目篩,得到CTMAB/A-nZVI樣品.制備鐵土比分別為1:3和1:5兩種負(fù)載材料,對(duì)應(yīng)FeSO4×7H2O添加量分別為7.45,4.96g,有機(jī)凹凸棒石添加量分別為4.50,5.00g.制備nZVI時(shí),不添加有機(jī)凹凸棒石,其他條件均相同.

利用SEM和XRD對(duì)所制樣品形貌進(jìn)行表征.根據(jù)SEM表征結(jié)果,利用Nano Measurer 1.2粒徑分析軟件統(tǒng)計(jì)計(jì)算nZVI顆粒粒徑分布規(guī)律及平均粒徑,統(tǒng)計(jì)樣本不少于100[14].

1.3 Cr(VI)去除試驗(yàn)方法及表征

在多個(gè)250mL錐形瓶中加入由K2Cr2O7配制初始濃度0為20mg/L的Cr(VI)溶液100mL,用鹽酸調(diào)節(jié)pH值至5,然后定量投加負(fù)載型納米材料,在25℃環(huán)境溫度中進(jìn)行還原試驗(yàn).每隔一定時(shí)間取出1個(gè)樣品瓶,用高速離心機(jī)在5000r/min條件下離心10min,采用二苯碳酰二肼分光光度法于波長540nm處測(cè)定上清液中Cr(VI)濃度e[15],計(jì)算Cr(VI)去除率.

計(jì)算公式如下:

吸附平衡后,用高速離心機(jī)在5000r/min條件下離心10min分離出負(fù)載型納米材料,60℃過夜真空干燥,之后利用XPS對(duì)樣品表面鉻進(jìn)行分析.

2 試驗(yàn)結(jié)果與分析

2.1 材料表征

由圖1可知,8.4°、19.8°和35.3°是凹凸棒石的特征峰,20.9°和26.7°是石英的特征峰.44.6°是零價(jià)鐵的特征峰,CTMAB/A-nZVI-3和CTMAB/A- nZVI-5兩種負(fù)載樣品在此處中都出峰明顯,說明nZVI成功負(fù)載在CTMAB/A表面.此外,35°~37°是鐵氧化物的峰值范圍,說明樣品中部分零價(jià)鐵被氧化.

未加負(fù)載材料的nZVI和兩種負(fù)載樣品的SEM形貌分析結(jié)果如圖2所示.從圖2(a)可以看出,nZVI呈球形或橢球形,由小球體團(tuán)聚成的簇團(tuán)大量存在.nZVI粒子發(fā)生團(tuán)聚主要是源于巨大的比表面積,此外,nZVI表面的Fe3O4磁性氧化物層也有助于粒子間的磁性相吸[16].負(fù)載樣品的XRD分析表明樣品中含有鐵的氧化物,其應(yīng)該是Fe3O4磁性氧化物.圖2(b)和圖2(c)顯示,nZVI簇團(tuán)明顯減少,顆粒較為均勻的分散在呈針棒狀或纖維狀凹凸棒石表面,說明黏土負(fù)載有效改善了nZVI顆粒容易團(tuán)聚的缺陷.與CTMAB/A-nZVI-3相比,CTMAB/A-nZVI-5中nZVI粒子更為分散,有念珠狀分布現(xiàn)象存在,幾乎未見明顯簇團(tuán),說明鐵土比的降低更能夠阻止nZVI顆粒團(tuán)聚,提高其分散性.

由圖3可知,未負(fù)載的nZVI顆粒平均粒徑為69.70nm,CTMAB/A-nZVI-3和CTMAB/A-nZVI -5平均粒徑分別為71.50, 62.90nm.雖然CTMAB/A- nZVI-3表面nZVI的平均粒徑比負(fù)載樣品略有增大,但其中大于100nm的顆粒含量為0,而未負(fù)載的nZVI中大于100nm的顆粒含量為8.80%.小于10nm的顆粒在CTMAB/A-nZVI-3和CTMAB/A-nZVI-5樣品中所占比例分別為3.60%和7.60%,在未加負(fù)載材料的nZVI中占比為0.可見,凹凸棒石的加入可降低大粒徑顆粒份額;負(fù)載反應(yīng)時(shí)的鐵土比越小,小粒徑顆粒所占份額越大.

圖1 CTMAB/A、CTMAB/A-nZVI-3和CTMAB/A-nZVI-5的XRD圖

A-凹凸棒石;Q-石英;F-鐵氧化物

圖2 nZVI和兩種凹凸棒負(fù)載樣品的SEM圖

a: nZVI b: CTMAB/A-nZVI-3 c: CTMAB/A-nZVI-5

圖3 nZVI和兩種凹凸棒負(fù)載樣品粒度分布

2.2 去除水中Cr(VI)的規(guī)律

圖4是未加負(fù)載材料的nZVI、CTMAB/A和兩種負(fù)載樣品分別在投加量為4g/L、pH值為5、Cr(VI)初始濃度為20mg/L條件下反應(yīng)時(shí)間對(duì)Cr(VI)去除規(guī)律的影響.可以看出,隨著反應(yīng)時(shí)間的延長,Cr(VI)的去除率先快速增加,30min后逐漸趨于穩(wěn)定.這一趨勢(shì)與黃園英等[17]的研究結(jié)果相同.后續(xù)試驗(yàn)的反應(yīng)時(shí)間均設(shè)定為30min.反應(yīng)平衡后CTMAB/A- nZVI-3和CTMAB/A-nZVI-5兩種負(fù)載樣品對(duì)Cr(VI)的去除率分別為98.23%和91.85%,而未加負(fù)載材料的nZVI僅為72.78%.可見在投加量相同的條件下,雖然負(fù)載樣品的鐵含量比未負(fù)載的nZVI樣品少,但去除率卻明顯提高.反應(yīng)平衡后CTMAB/A對(duì)Cr(VI)的去除率為10.40%,說明負(fù)載樣品中CTMAB/A對(duì)Cr(VI)的去除率占比較小.所以,負(fù)載樣品與未加負(fù)載材料的nZVI相比,其較高的去除率是由于所負(fù)載零價(jià)鐵顆粒的粒徑和聚集狀態(tài)發(fā)生改變所導(dǎo)致的.程浪等[18]認(rèn)為,凹凸棒石經(jīng)CTMAB改性后,其帶電性由負(fù)轉(zhuǎn)正,因此對(duì)Cr(VI)的去除主要是靜電吸附作用.

圖4 反應(yīng)時(shí)間對(duì)Cr(VI)去除率的影響

pH值:5,Cr(VI)初始濃度:20mg/L,投加量:4g/L

從圖5中Cr(VI)去除率隨3種樣品投加量的變化規(guī)律可以看出,Cr(VI)的去除率隨投加量增加而增加.投加量為6g/L時(shí),兩種負(fù)載樣品的Cr(VI)去除率幾乎達(dá)100%,未加負(fù)載材料的nZVI去除率為99.06%.當(dāng)樣品投加量小于6g/L時(shí),CTMAB/ A-nZVI-3的去除率始終高于CTMAB/A-nZVI-5.這是由于樣品投加量相同時(shí)CTMAB/A-nZVI-3中鐵的含量高于CTMAB/A-nZVI-5所致.

圖5 樣品投加量對(duì)Cr(VI)去除率的影響

pH:5,Cr(VI)初始濃度20mg/L,反應(yīng)時(shí)間30min

圖6 3種樣品中鐵含量與Cr(VI)去除率的關(guān)系

pH:5, Cr(VI)初始濃度20mg/L,反應(yīng)時(shí)間30min

為了明確零價(jià)鐵顆粒粒徑變化對(duì)其反應(yīng)性的影響,本文計(jì)算了負(fù)載樣品中鐵的含量及由nZVI所貢獻(xiàn)的Cr(VI)去除率,如當(dāng)CTMAB/A-nZVI-3樣品投加量為2g/L時(shí),樣品中鐵的量為0.5g/L,CTMAB/ A為1.5g/L.此負(fù)載樣品對(duì)Cr(VI)的總?cè)コ蕿?3.90%,其中包括被CTMAB/A樣品吸附的Cr(VI)和被所負(fù)載nZVI吸附還原的Cr(VI)兩部分.在此條件下Cr(VI)在CTMAB/A表面的最大吸附量為0.52mg/g,由此算得2g/L的負(fù)載樣品中由黏土CTMAB/A吸附去除的Cr(VI)為3.90%,由nZVI吸附還原去除的Cr(VI)為50.00%.計(jì)算結(jié)果如圖6所示,可以看出,鐵投加量為0.25,1.00g/L時(shí),兩種負(fù)載樣品中由nZVI貢獻(xiàn)的Cr(VI)去除率差異不大,分別約為28%和90%,而未加負(fù)載材料的nZVI去除率僅為6.25%和23.45%,說明黏土負(fù)載顯著提高了nZVI的反應(yīng)性.當(dāng)鐵投加量介于0.25g/L和1.00g/L之間時(shí),CTMAB/A-nZVI-5中nZVI對(duì)Cr(VI)的去除明顯高于CTMAB/A-nZVI-3,如鐵含量為0.75g/L,二者分別為86.20%和78.00%.這一趨勢(shì)和樣品中nZVI顆粒的粒徑分布一致.Yan等[19]研究認(rèn)為零價(jià)鐵與污染物之間的反應(yīng)首先在顆粒的表面進(jìn)行,所以顆粒的比表面積大小對(duì)反應(yīng)效率具有較大的貢獻(xiàn).在圖3中可以看出,CTMAB/A-nZVI-5樣品中nZVI顆粒不僅平均粒徑比CTMAB/A-nZVI-3小,而且小粒徑顆粒所占比重明顯比CTMAB/A-nZVI-3多,鐵含量相同時(shí),nZVI粒徑越小,比表面積越大,在負(fù)載樣品中對(duì)Cr(VI)的去除率貢獻(xiàn)也就越大.這一發(fā)現(xiàn)與王雅楠等[20]的研究結(jié)果相同.

2.3 反應(yīng)產(chǎn)物分析

圖7分別為CTMAB/A、CTMAB/A-nZVI-3和CTMAB/A-nZVI-5處理水中Cr(VI)后的XPS分析譜圖.圖7(a)出現(xiàn)兩個(gè)峰,其電子結(jié)合能分別位于585.2,575eV附近,575eV處的峰是衛(wèi)星峰, 585.2eV處為Cr(VI)特征峰[21-22],表明水中Cr(VI)直接吸附在CTMAB/A樣品表面,沒有發(fā)生價(jià)態(tài)變化.圖7(b)和7(c)中兩個(gè)峰的電子結(jié)合能分別位于585eV附近和577eV附近,577eV為Cr(III)的特征峰[21].說明在負(fù)載樣品表面同時(shí)存在Cr(VI)和Cr(III).由此可以推斷,部分Cr(VI)在負(fù)載樣品表面與nZVI顆粒發(fā)生了氧化還原反應(yīng),生成Cr(III).圖7(c)中577eV處的鋒面積明顯大于585eV處的峰面積,可以推斷樣品表面鉻主要以Cr(III)形式存在,說明絕大部分Cr(VI)已經(jīng)被nZVI還原成Cr(III),僅存在少部分Cr(VI).

圖7(b)和圖7(c)是在相同條件下, CTMAB/A- nZVI-3和CTMAB/A-nZVI-5投加量分別為3,4.5g時(shí)的XPS譜圖,兩種樣品中鐵含量相同,均為0.75g/L,圖7(b)中585eV處峰面積明顯大于圖7(c)中該處峰面面積,說明CTMAB/A-nZVI-3表面所含的Cr(VI)量超過CTMAB/A-nZVI-5,這進(jìn)一步印證了CTMAB/A-nZVI-5表面nZVI顆粒能將更多的Cr(VI)氧化為Cr(III),說明降低鐵土比有利于nZVI反應(yīng)活性的提高,這也與nZVI顆粒粒徑降低有直接關(guān)系.

3 結(jié)論

3.1 在制備有機(jī)凹凸棒石負(fù)載納米零價(jià)鐵的復(fù)合材料時(shí),nZVI顆粒的粒徑分布和分散效果受鐵土比的影響明顯.鐵土比越小,負(fù)載樣品中小粒徑的nZVI顆粒越多,顆粒分散性也越好.小于10nm的nZVI顆粒在CTMAB/A-nZVI-3所負(fù)載nZVI顆?？倲?shù)中占3.60%,在CTMAB/A-nZVI-5中占7.60%,在為負(fù)載的樣品中的占比為0.

3.2 負(fù)載樣品中小粒徑nZVI所占比重越多,樣品反應(yīng)活性越高.鐵的投加量同為0.75g/L時(shí),鐵土比為1:5的CTMAB/A-nZVI-5樣品中鐵對(duì)Cr(VI)去除的貢獻(xiàn)為86.20%,鐵土比為1:3的CTMAB/A-nZVI-3為78.00%.

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Removal of Cr(VI) from aqueous solution using organically modified attapulgite-supported nanoscale zero-valent iron.

XU Hai-yu, ZHANG Ming-qing*, CHEN Yi-yu

(School of Environmental Science and Spatial Informatics, China University of Mining and Technology, Xuzhou 221116, China)., 2019,39(12)：5079~5084

Loading on clay mineral can improve the agglomeration behavior of zero-valent iron particles and enhance their reactivity. Particle agglomeration and reactivity are related to the mass ratio of iron to clay. In this study, CTMAB/A-nZVI-3 and CTMAB/A-nZVI-5 were prepared using organic attapulgite (CTMAB/A) as carrier by liquid phase reduction method when the mass ratio of iron to clay were set to 1:3 and 1:5, respectively. The loaded sample and the unloaded nZVI sample were characterized by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The product of the sample after reaction with Cr(VI) was analyzed by X-ray photoelectron spectroscopy (XPS). The results showed that with the ratio of iron to clay decreasing, the agglomeration of nZVI particles were improved and the particle size distributions of nZVI were changed. The nZVI particles with particle size less than 10nm accounted for 3.60% in CTMAB/A-nZVI-3, 7.60% in CTMAB/A-nZVI-5, and zero in unloaded nZVI samples. When the amount of iron in supported samples was 0.75g/L, the Cr(VI) removal rateresulting from iron was 86.20% in CTMAB/A-nZVI-5samples and 78.00% in CTMAB/A-nZVI-3samples.

attapulgite；nanoscale zero-valent iron；mass ratio of ion to clay；Cr(VI)

X703

A

1000-6923(2019)12-5079-06

徐海玉(1994-),女,山東威海人,中國礦業(yè)大學(xué)碩士研究生,主要從事水中重金屬污染的治理研究.

2019-05-20

國家自然科學(xué)基金資助項(xiàng)目(51874304);徐州市科技計(jì)劃項(xiàng)目(KC16SG262)

* 責(zé)任作者, 副教授, zmqcumt@163.com