陳愛(ài)俠,田 錚,衛(wèi) 瀟,胡蕊蕊,張奕軒,關(guān)娟娟

一步法制備磁性多孔碳及高效吸附卡馬西平

陳愛(ài)俠*,田 錚,衛(wèi) 瀟,胡蕊蕊,張奕軒,關(guān)娟娟

(長(zhǎng)安大學(xué)水利與環(huán)境學(xué)院,長(zhǎng)安大學(xué)旱區(qū)地下水文與生態(tài)效應(yīng)教育部重點(diǎn)實(shí)驗(yàn)室,陜西 西安 710054)

為了資源化利用廢棄農(nóng)林生物質(zhì)并有效去除水體中卡馬西平(CBZ).受“發(fā)酵”策略的啟發(fā),以KHCO3為活化劑,FeCl3·6H2O為磁性前驅(qū)體,通過(guò)簡(jiǎn)便的一步法將楊木屑轉(zhuǎn)化為具有發(fā)達(dá)孔結(jié)構(gòu)的磁性多孔碳(MPC).通過(guò)調(diào)整原料配比得到了最佳產(chǎn)物MPC-2-0.3,其比表面積為1002.48m2/g,對(duì)50mg/L的CBZ吸附容量為202.70mg/g.SEM和XRD等表征均證明鐵納米粒子成功摻雜到多孔碳中,使多孔碳具有優(yōu)異的磁分離能力.吸附過(guò)程的動(dòng)力學(xué)和等溫線更符合擬二級(jí)動(dòng)力學(xué)和Langmuir模型,闡明其主要吸附機(jī)理為吸附位點(diǎn)、π-π相互作用和氫鍵作用.MPC-2-0.3在室溫和較寬的pH值范圍內(nèi)具有良好的吸附性能,并易于分離和再生.

鐵摻雜；多孔碳；一步法；卡馬西平

卡馬西平(CBZ)被廣泛用于治療癲癇和神經(jīng)痛,在水體中常被檢出[1-5].先前的研究已證實(shí)CBZ可能導(dǎo)致血小板、粒細(xì)胞和白細(xì)胞的數(shù)量減少,此外,CBZ的長(zhǎng)期攝入會(huì)導(dǎo)致肝腎功能衰竭[6-7].而傳統(tǒng)的水處理工藝對(duì)此類污染物去除效果較差.為了解決CBZ帶來(lái)的一系列環(huán)境問(wèn)題,吸附[8]、催化[9]、膜分離[10]和高級(jí)氧化工藝[11]等被廣泛應(yīng)用.其中吸附法簡(jiǎn)便、環(huán)保,無(wú)疑是去除水體中CBZ更經(jīng)濟(jì)有效的方法.用于水處理的理想吸附劑應(yīng)當(dāng)對(duì)環(huán)境無(wú)害,并具有良好的吸附能力.因此,由綠色、清潔可再生的生物質(zhì)為前體碳化合成的吸附劑引起了廣泛關(guān)注[12-15].

然而,將生物質(zhì)直接碳化孔結(jié)構(gòu)有限,其對(duì)污染物的吸附性能和選擇性具有一定局限性.研究發(fā)現(xiàn),經(jīng)活化的多孔結(jié)構(gòu)碳材料在吸附領(lǐng)域表現(xiàn)出巨大潛力,實(shí)現(xiàn)了對(duì)有機(jī)污染物的高效吸附[16-17]通常,激活生物碳的方法有物理法和化學(xué)法.相較于利用二氧化碳、蒸汽或氧氣在高溫下活化的物理方法,化學(xué)活化法因效率高且產(chǎn)量大而更具前景[18].該方法主要是將活化劑(通常為強(qiáng)酸或強(qiáng)堿)與原料混合在惰性氣體(N2、Ar)保護(hù)下高溫煅燒.在眾多活化劑中,KOH因效果突出且價(jià)格低廉而備受研究人員青睞[19].Guo等[20]的研究表明碳源與KOH的添加比例是影響材料比表面積的重要因素.盡管KOH可以有效地構(gòu)建多孔結(jié)構(gòu)并提高比表面積,但它的強(qiáng)腐蝕性和環(huán)境毒性限制了大規(guī)模生產(chǎn)和多孔碳的實(shí)際應(yīng)用.Wang[21]和Lu[22]等人提出了一種類似“發(fā)酵”的造孔過(guò)程.利用KHCO3熱解過(guò)程中產(chǎn)生的大量氣體形成豐富孔結(jié)構(gòu)的分層多孔碳.此外,研究證實(shí)材料的吸附能力與形成的微孔結(jié)構(gòu)密切相關(guān),隨著污染物濃度增加,中孔也起到一定的吸附作用.因此, KHCO3作為活化劑除了具有強(qiáng)力的造孔能力外,對(duì)環(huán)境也相對(duì)友好.

如上所述,活化后的多孔碳具有良好吸附能力,但在廢水處理過(guò)程中,如何分離粉末狀的多孔碳并回收利用是個(gè)亟待解決的問(wèn)題.如今,將Fe[23]、Co[24-25]、Ni[26]、Cu[27]等金屬離子摻雜至多孔碳中,可通過(guò)磁分離進(jìn)行快速分離.而來(lái)源廣泛、價(jià)格低廉的Fe是一種優(yōu)秀的磁前驅(qū)體[28].相關(guān)研究表明[29-30],含有Fe納米顆粒的磁性多孔碳不僅能提高吸附能力而且表現(xiàn)出優(yōu)異的磁分離和再生性能,被認(rèn)為是廢水處理中極具潛力的材料.然而磁性多孔碳的制備方法往往是多步過(guò)程,先合成多孔碳,然后再進(jìn)行過(guò)渡金屬元素?fù)诫s.因此,探索制備高性能磁性多孔碳的簡(jiǎn)便合成方法具有重要意義.

本文以楊木屑為碳源,FeCl3·6H2O為磁前驅(qū)體,KHCO3為活化劑.通過(guò)簡(jiǎn)便的一步熱解法實(shí)現(xiàn)同步磁化和活化,基于KHCO3“發(fā)酵式”的造孔過(guò)程,將廢物生物質(zhì)轉(zhuǎn)化為具有豐富孔結(jié)構(gòu)的磁性多孔碳.通過(guò)調(diào)節(jié)原料配比,探究最佳制備條件,并以在水體中難去除的CBZ為目標(biāo)污染物,建立制備條件與吸附性能之間的構(gòu)效關(guān)系,揭示去除機(jī)理及適用性,為CBZ的高效去除提供新途徑.

1 材料與方法

1.1 主要試劑

楊木屑來(lái)自西安當(dāng)?shù)氐囊患夷静膹S.為了去除雜質(zhì),首先將木屑與0.5mol鹽酸反應(yīng)24h,用去離子水洗滌多次后在烘箱中60?C下干燥備用. CBZ (C15H12N2O, 98%),KHCO3和FeCl3·6H2O購(gòu)自阿拉丁化學(xué)試劑有限公司(上海),所有的試劑均為分析純.

1.2 主要儀器

通過(guò)掃描電子顯微鏡(SEM,Sigma500, ZEISS,德國(guó))表征了BC(生物碳)、PC(多孔碳)和MPC- 2-0.3(磁性多孔碳)的形貌特征;X射線衍射(XRD, smartlab9K, Rigaku,日本)和傅立葉變換紅外光譜(FTIR, Thermo Scientific Nicolet iS10,美國(guó))分別測(cè)試樣品的晶型結(jié)構(gòu)和官能團(tuán)組成;氮?dú)馕?解吸等溫線由比表面積分析儀(ASAP2020PLUS, Micromeritics,美國(guó))在77K下測(cè)定;采用X射線光電子能譜儀(XPS,K-Alpha,Thermo Scientific,美國(guó))對(duì)樣品的表面元素組成及化學(xué)態(tài)定性分析.磁滯回線由振動(dòng)樣品磁強(qiáng)計(jì)(VSM, LakeShore7404,美國(guó))測(cè)定;CBZ剩余濃度由紫外-可見(jiàn)光分光光度計(jì)(UV-Vis,美譜達(dá),上海)測(cè)定;管式爐(SK-G06123K,天津中環(huán)實(shí)驗(yàn)設(shè)備有限公司,中國(guó))實(shí)現(xiàn)熱解過(guò)程的可控升溫.

1.3 材料制備

將1.0g木屑與不同質(zhì)量KHCO3和FeCl3·6H2O混合于含有60mL去離子水的燒杯中.上述溶液在80?C下磁性攪拌至溶液蒸發(fā).將形成的粘稠狀混合物轉(zhuǎn)移至烘箱,40?C下干燥過(guò)夜后研磨并置于管式爐中.在氮?dú)夥諊?由室溫5?C/min升至750?C并保留2h.將得到產(chǎn)物用0.5mol鹽酸和去離子水各洗滌3次,烘干后得到MPC.根據(jù)原料添加比例,產(chǎn)物命名為MPC--,其中,分別代表KHCO3和FeCl3·6H2O與木屑的質(zhì)量比.未添加鐵鹽得到的PC和木屑直接煅燒得到的BC均與上述制備方法相同.

1.4 吸附實(shí)驗(yàn)

吸附實(shí)驗(yàn)在50mL離心管中進(jìn)行,將4.0mg MPC--與20mL不同濃度的CBZ溶液混合并吸附12h,取出溶液用紫外-可見(jiàn)光分光光度計(jì)(波長(zhǎng)285nm)分析,計(jì)算上清液中CBZ的剩余濃度.為確定最佳制備條件,按照以上步驟,探究了原料摻比,pH值(2～12)和共存離子濃度對(duì)吸附CBZ的影響.為了數(shù)據(jù)的準(zhǔn)確性,所有實(shí)驗(yàn)均重復(fù)3次,并給出平均值及標(biāo)準(zhǔn)差.CBZ的平衡吸附容量通過(guò)下式計(jì)算:

式中:0和e分別代表CBZ溶液的初始濃度和吸附平衡濃度,mg/L.是溶液體積,mL.是吸附劑的質(zhì)量,g.此外, 分別在298、303和308K進(jìn)一步研究了吸附等溫線模型,分析了MPC-2-0.3對(duì)CBZ的吸附行為.同時(shí),根據(jù)CBZ的平衡吸附容量(式(1))分析了吸附熱力學(xué)行為.

在吸附動(dòng)力學(xué)研究中,分別稱取4.0mg的MPC-2-0.3樣品在303K下吸附20mL初始濃度為10,30和50mg/L的CBZ溶液,測(cè)定不同接觸時(shí)間(5～720min)CBZ的剩余濃度C(mg/L).接觸時(shí)間為時(shí)刻的吸附容量Q(mg/g)如式(2)所示

1.5 再生實(shí)驗(yàn)

使用甲醇作為脫附劑,將吸附CBZ至平衡后的MPC-2-0.3加入含有30mL甲醇溶液的燒杯中,在水浴振蕩器中常溫(298K)震蕩12h后用去離子水洗滌至中性后烘干.將4.0mg再生的MPC-2-0.3加入50mg/L的CBZ溶液中,實(shí)驗(yàn)步驟皆與吸附實(shí)驗(yàn)相同.按照以上實(shí)驗(yàn)步驟,將吸附-脫附重復(fù)4次,并測(cè)試每個(gè)周期的吸附容量.

2 結(jié)果與討論

2.1 材料表征分析

2.1.1 SEM分析 如圖1所示,未添加活化劑的BC(圖1(a1),(a2))呈現(xiàn)出條狀結(jié)構(gòu),基本以2.0nm左右的孔隙為主且數(shù)量較少.而木屑經(jīng)活化后得到的PC(圖1(b1),(b2))和MPC(圖1(c1),(c2))形成了大量的微孔孔隙,這是由于KHCO3反應(yīng)時(shí)產(chǎn)生大量氣體造成的.此外,高溫活化過(guò)程生成了具有腐蝕性K+與碳基體結(jié)合促進(jìn)了微孔和介孔結(jié)構(gòu)的產(chǎn)生,為吸附提供反應(yīng)位點(diǎn),凸顯出活化劑是影響制孔的關(guān)鍵因素.該反應(yīng)過(guò)程可由以下化學(xué)式描述:

2KHCO3→K2CO3+H2O+CO2(3)

K2CO3→K2O+CO2(4)

CO2+C→2CO (5)

K2O+C→2K+(6)

與PC相比,MPC-2-0.3也表現(xiàn)出豐富的孔結(jié)構(gòu),因表面附著大量游離的Fe納米顆粒顯得更為粗糙,并使材料的孔隙和比表面積增加,有利于高效吸附目標(biāo)污染物.

圖1 BC、PC和MPC-2-0.3的掃描電鏡圖(SEM)

2.1.2 BET分析 在圖2(a)中,PC和MPC的N2吸附-脫附行為表現(xiàn)為I型和IV型組合等溫線,表明存在微孔和介孔.由于微孔吸附占主導(dǎo)作用,導(dǎo)致等溫線在低壓區(qū)(/0=0～0.1)N2吸附量增長(zhǎng)迅速并趨于飽和.此外,在/0=0.4處出現(xiàn)明顯的H4型滯回環(huán),該特征說(shuō)明MPC-2-0.3具有結(jié)構(gòu)均勻的介孔.但未添加KHCO3活化的BC其N2吸附-脫附等溫線主要表現(xiàn)為以微孔為主導(dǎo)的I型等溫線.圖2(b)展示了三種多孔碳的孔徑分布證明了該結(jié)果,MPC-2-0.3中小于2.0nm的微孔大幅度增多,且2.0nm～4.0nm的介孔結(jié)構(gòu)可以有效增加接觸面積,加快傳質(zhì)過(guò)程,因此MPC-2-0.3的孔結(jié)構(gòu)相較PC和BC更有優(yōu)勢(shì).由表1可知,KHCO3作為活化劑在制備分層結(jié)構(gòu)的多孔碳上表現(xiàn)出巨大潛力,被KHCO3活化后的PC表面積是BC的3.31倍.磁前驅(qū)體鍛燒產(chǎn)生的納米Fe顆粒附著在多孔碳表面進(jìn)一步增大了比表面積,使MPC-2-0.3比表面積高達(dá)1002.48m2/g.此外,MPC- 2-0.3比表面積升高、微孔體積占比下降且平均孔徑增大,約2.37nm,推測(cè)同步活化磁化的過(guò)程也促進(jìn)了介孔和大孔的生成.

圖2 三種吸附劑的N2吸附-脫附等溫線和孔徑分布

表1 BC、PC和MPC-2-0.3的孔隙特征

2.1.3 XRD分析的XRD衍射圖譜如圖3(a)所示, PC和MPC-2-0.3在2=26°和29°左右都存在明顯的特征峰,分別對(duì)應(yīng)的是石墨碳和無(wú)定形碳的衍射峰[31].在MPC-2-0.3中,2=26°的衍射峰增強(qiáng),說(shuō)明石墨化程度更高.并且在2=44o和65o出現(xiàn)了Fe的衍射峰,與Fe的(110),(200)晶面相對(duì)應(yīng)(PDF#06- 0696).XRD結(jié)果說(shuō)明,木屑與鐵鹽混合煅燒成功將Fe摻雜于多孔碳中.

2.1.4 FTIR分析 如圖3(b)所示,PC和MPC-2-0.3的FTIR光譜圖均在3483和1600/cm處出現(xiàn)-OH與C=O伸縮振動(dòng)峰[32].與鐵源混合煅燒得到的MPC-2-0.3其光譜圖在2340和583/cm處出現(xiàn)了新的特征峰,分別對(duì)應(yīng)O=C-O累積雙鍵、Fe-O伸縮振動(dòng),證實(shí)了Fe納米顆粒被成功負(fù)載[33].

2.1.5 XPS分析 通常多孔碳的吸附性能與其表面化學(xué)性質(zhì)密切相關(guān).如圖4(a)所示MPC-2-0.3的元素成分主要有C、O、N、Fe,其含量分別為75.64%、16.57%、5.96%和1.12%.其中N元素含量相較原木屑有所增加.在圖4(b)中,C 1s圖譜可分為三個(gè)峰,分別位于284.8、285.6和289.4eV,代表三種不同類型的碳功能基團(tuán)(C-C、C-O和O=C-O),這表明存在含氧功能基團(tuán).在圖4(c)中,O 1s圖譜在531.8、532.9和534.3eV處的峰分別屬于C=O、C-O和O=C-O.在吸附過(guò)程中,這些含氧功能基團(tuán)和污染物分子之間可以形成氫鍵,這有利于提高吸附性能.圖4(d)為木屑和MPC-2-0.3的N 1s高分辨圖譜,均以吡啶-N、吡咯-N和石墨-N形式存在.由于氮?dú)庠诟邷叵聟⑴c了活化過(guò)程,并轉(zhuǎn)化為吡咯-N,而吡啶-N在高溫下轉(zhuǎn)變?yōu)橄鄬?duì)穩(wěn)定的石墨-N結(jié)構(gòu)[34].相比之下,MPC-2-0.3中吡咯-N和石墨-N峰強(qiáng)度上升而吡啶-N下降.此外,這些基團(tuán)可以充當(dāng)多孔碳表面上的活性位點(diǎn),有助于增強(qiáng)π-πEDA作用和吸附劑與CBZ之間的疏水效應(yīng),從而提高吸附速率[35]. Fe 2P高分辨圖譜如圖5(e)所示,分別在728.1、724.2、714.7、710.7和709.5eV處出現(xiàn)獨(dú)立的峰,表明Fe離子具有Fe0、Fe2+和Fe3+三種存在形式[36].推測(cè)鐵物種在高溫活化過(guò)程中的轉(zhuǎn)變途徑如下:Fe3+首先在低溫下水解為非晶Fe物種(例如Fe(OH)3和FeO(OH))[37].非晶Fe物種繼續(xù)轉(zhuǎn)化為Fe3O4,然后被高溫?zé)峤膺^(guò)程中生成的H2、CO和C還原為FeO,并進(jìn)一步還原為Fe0[38].

圖3 PC和MPC-2-0.3的XRD和FTIR圖譜

圖4 木屑和MPC-2-0.3的XPS掃描全譜圖及高分辨率譜圖

圖5 MPC-2-0.3的磁滯回線和磁選效應(yīng)

2.1.6 VSM分析 圖5可以觀察到MPC-2-0.3的磁滯回線呈現(xiàn)S型且不過(guò)原點(diǎn),閉合曲線包圍面積小、無(wú)磁滯現(xiàn)象,證明該材料為易于磁化和退磁的軟磁材料[39].結(jié)果還表明,在外部磁場(chǎng)的作用下MPC-2-0.3從CBZ溶液中被輕易分離,從而為吸附劑回收和再利用提供途徑.

2.2 材料制備參數(shù)對(duì)吸附容量的影響

圖6(a)可以看出,活化劑的添加比例對(duì)CBZ的吸附效果影響較大.當(dāng)KHCO3的加入使吸附容量明顯增加,隨著摻比例增至1:2時(shí),吸附容量由68.0mg/g驟增至202.7mg/g.但KHCO3的過(guò)量加入反而使吸附容量降低.導(dǎo)致該現(xiàn)象的主要原因是CBZ的吸附主要依賴于微孔和介孔,但這些孔結(jié)構(gòu)持續(xù)被過(guò)多的KHCO3活化后擴(kuò)大,從而導(dǎo)致吸附容量下降.圖6(b)表明,FeCl3·6H2O的摻雜不僅使吸附容量上升53.25mg/g,還賦予了材料優(yōu)秀的磁分離能力.隨著FeCl3·6H2O比例增加,吸附量逐漸下降,可能是由于附著在多孔碳表面的Fe納米顆粒越來(lái)越多,造成孔隙堵塞.

初始濃度:50mg L?1;pH值:7.0;溫度:303K

此外,活化溫度也與孔結(jié)構(gòu)存在密切關(guān)系,從而影響吸附容量.如圖6(c)所示,在較低的活化溫度下,由于KHCO3與碳之間的反應(yīng)不充分,孔隙的形成受到限制,因此650℃時(shí)吸附容量較低.隨著活化溫度的升高,加快KHCO3分解,導(dǎo)致孔徑擴(kuò)大.吸附容量在750℃時(shí)最高,表明此時(shí)能較好地實(shí)現(xiàn)自活化反應(yīng).然而溫度進(jìn)一步提高至850℃時(shí)吸附容量大幅度下降,歸因于活化溫度過(guò)高導(dǎo)致孔結(jié)構(gòu)被破壞.綜上所述,因750℃下獲得的MPC-2-0.3具有較好的吸附能力和磁分離能力,因此后續(xù)試驗(yàn)主要以該磁性多孔碳為研究對(duì)象.

2.3 溶液pH值對(duì)吸附容量的影響

圖7 溶液pH對(duì)MPC-2-0.3吸附CBZ的影響

初始濃度:50mg/L;溫度:303K

溶液pH值是影響吸附行為的重要參數(shù).由Zeta電位測(cè)定結(jié)果可知(圖7(b)),MPC-2-0.3在pH值2～12范圍內(nèi)均呈負(fù)電性.而當(dāng)pH=12時(shí),材料的Zeta電位出現(xiàn)了顯著上升,這是由于調(diào)節(jié)溶液pH值時(shí)引入的Na+、K+等陽(yáng)離子與反離子電荷相同,并產(chǎn)生排斥作用使MPC-2-0.3電勢(shì)升高,電負(fù)性減少.因此,MPC-2-0.3的吸附能力主要取決于CBZ分子的離子形式.在圖7(a)中,在pH 2～12的范圍內(nèi)MPC- 2-0.3的吸附容量整體呈現(xiàn)輕微地先上升后下降的趨勢(shì),但基本保持穩(wěn)定,在pH=6時(shí)吸附容量最大為224.722mg/g,與Ncibi等[40]報(bào)道相一致.這意味著pH值不是影響吸附CBZ效果的關(guān)鍵參數(shù),吸附劑在寬泛的pH值范圍都有著對(duì)CBZ良好的吸附效果.可以推測(cè),在所研究的pH值范圍內(nèi)(2～12),CBZ分子保持電負(fù)性,其與MPC-2-0.3的結(jié)合不受靜電相互作用的約束,因此CBZ 的吸附可能主要由疏水性,π-π相互作用和氫鍵控制.

2.4 離子強(qiáng)度和腐植酸對(duì)吸附容量的影響

為了探究MPC-2-0.3的抗干擾能力,選取了4種水體中常見(jiàn)的金屬離子(Na+、K+、Ca2+、Mg2+)作為干擾離子.結(jié)果表明(圖8(a)),在這4種金屬離子存在的情況下,吸附劑的吸附容量波動(dòng)甚微,同時(shí)也應(yīng)證了靜電作用對(duì)CBZ的吸附影響并不顯著.

同時(shí)考察了腐殖酸對(duì)吸附能力的影響,如圖8(b)所示,隨著腐植酸濃度逐漸上升,MPC-2-0.3對(duì)CBZ的吸附容量逐漸下降,由218.8mg/L降至169.5mg/L.總體上來(lái)說(shuō),MPC-2-0.3的吸附容量與腐植酸濃度呈負(fù)相關(guān),且線性關(guān)系良好(2=0.9184).推測(cè)導(dǎo)致該現(xiàn)象的原因是腐植酸分子與MPC-2-0.3通過(guò)范德華力和π-π EDA相互作用占據(jù)了一部分表面結(jié)合位點(diǎn),引起腐植酸分子與CBZ分子競(jìng)爭(zhēng)吸附位點(diǎn),從而導(dǎo)致吸附量有明顯下降[41].

圖8 離子強(qiáng)度和腐植酸對(duì)MPC-2-0.3吸附CBZ的影響

初始濃度:50mg/L;pH值:7.0;溫度:303K;共存離子濃度:100mmol/L

2.5 吸附行為研究

2.5.1 吸附動(dòng)力學(xué) 為估算MPC-2-0.3的吸附速率,選定不同濃度的CBZ溶液(10,30,50mg/L)進(jìn)行吸附動(dòng)力學(xué)研究.圖9(a)～(c)顯示了吸附容量隨時(shí)間變化的數(shù)據(jù)及使用擬一級(jí)動(dòng)力學(xué)和擬二級(jí)動(dòng)力學(xué)模型對(duì)相關(guān)數(shù)據(jù)擬合后的曲線.可以看出,MPC-2-0.3對(duì)CBZ的吸附容量在最初60min內(nèi)急速上升,表明在該時(shí)間段內(nèi)吸附迅速.隨著時(shí)間推移,吸附速率減慢,在300min左右接近平衡.這說(shuō)明該吸附劑對(duì)CBZ的吸附過(guò)程分為兩步,首先CBZ分子在多孔碳表面被快速吸附,當(dāng)表面的吸附位點(diǎn)被占據(jù)時(shí),CBZ分子向多孔碳內(nèi)部擴(kuò)散的過(guò)程中由于阻力增加,導(dǎo)致吸附速率減慢.此外,MPC-2-0.3的吸附容量伴隨著CBZ初始濃度的升高而增加,這是由于溶液濃度與液相到固相的傳質(zhì)推動(dòng)力成正比,濃度升高促使更多的CBZ分子擴(kuò)散至多孔碳中.由圖9(a)～(c)還可以看出,擬二級(jí)動(dòng)力學(xué)模型相較于擬一級(jí)動(dòng)力學(xué)模型具有更好的線性相關(guān)性.在表2中,擬二級(jí)動(dòng)力學(xué)擬合曲線的相關(guān)系數(shù)(2)都大于0.98,也表明擬二級(jí)動(dòng)力學(xué)模型能很好地描述MPC-2-0.3對(duì)CBZ的吸附行為,是通過(guò)電子交換或共價(jià)鍵完成的化學(xué)吸附過(guò)程[21].

如圖9(d),為顆粒內(nèi)擴(kuò)散模型的擬合圖形,其中所有線段都可擬合為3段,表明MPC-2-0.3對(duì)CBZ的吸附過(guò)程主要分為3階段,首先CBZ分子從溶液快速擴(kuò)散到吸附劑表面,然后CBZ通過(guò)多孔碳的孔隙被吸附于MPC-2-0.3的內(nèi)表面上,最后一個(gè)階段則對(duì)應(yīng)的是吸附平衡[42].且3個(gè)線段均不是直線,意味著除粒子擴(kuò)散外,還有控制吸附的其他過(guò)程[33].表3列出了顆粒內(nèi)擴(kuò)散的各參數(shù),進(jìn)一步證明了圖9(b)的結(jié)果.

表2 MPC-2-0.3吸附CBZ的動(dòng)力學(xué)參數(shù)

表3 MPC-2-0.3吸附CBZ的顆粒內(nèi)擴(kuò)散模型參數(shù)

圖10 MPC-2-0.3的吸附等溫線

2.5.2 吸附等溫線 由圖10可以看出,與Freundlich模型相比較,Langmuir非線性擬合效果更佳,并且MPC-2-0.3的吸附容量隨著CBZ濃度和溫度上升而增加.這意味濃度和溫度的升高增加了傳質(zhì)推動(dòng)力,使更多的CBZ分子與吸附劑表面接觸.表4顯示MPC-2-0.3在298,303和308K溫度下, Langmuir模型擬合的相關(guān)系數(shù)(2)均大于0.99,由該模型計(jì)算出的m分別為 292.5、302.8和307.3mg/g.表明吸附過(guò)程更傾向于均勻吸附.另外,1/值落在0.3左右的范圍內(nèi),也證明升溫可促進(jìn)吸附.表5列出了文獻(xiàn)中報(bào)道的其他吸附劑的m、比表面積、吸附劑添加量及吸附條件,結(jié)果發(fā)現(xiàn),MPC-2-0.3對(duì)CBZ的吸附能力相比于其他吸附材料具有明顯優(yōu)勢(shì).

表4 Langmuir和Freundlich等溫線模型參數(shù)

2.5.3 吸附熱力學(xué) 為探究不同接觸溫度對(duì)吸附性能的影響,對(duì)MPC-2-0.3在不同溫度下對(duì)CBZ的吸附數(shù)據(jù)進(jìn)行熱力學(xué)擬合.從表中可知,當(dāng)溫度從293K升至308K時(shí),DG的值逐漸減少(由-5.054變化為-6.029kJ/mol)且均呈現(xiàn)負(fù)值,這表明較高的溫度有利于吸附CBZ并存在自發(fā)的吸附過(guò)程.ΔH大于0,證實(shí)了吸附過(guò)程是吸熱反應(yīng),與吸附等溫線分析結(jié)果一致,因此通過(guò)適當(dāng)提高溫度可以促進(jìn)吸附.除此之外DS為正值,表明CBZ分子在溶液與MPC-2-0.3界面的吸附過(guò)程是隨機(jī)的,因?yàn)镃BZ分子被吸附在吸附劑表面是一個(gè)去溶劑化過(guò)程[48].

表5 MPC-2-0.3和其他吸附劑對(duì)CBZ的吸附能力比較

圖11 MPC-2-0.3的熱力學(xué)分析

表6 MPC-2-0.3吸附CBZ的熱力學(xué)參數(shù)

2.6 吸附機(jī)理分析

實(shí)驗(yàn)結(jié)果表明,MPC-2-0.3對(duì)CBZ的吸附可能包括物理吸附和化學(xué)吸附,但物理吸附主導(dǎo)CBZ的吸附過(guò)程.由于MPC-2-0.3具有高比表面積和豐富的孔結(jié)構(gòu),為CBZ分子提供了吸附位點(diǎn)和傳質(zhì)通道,從而達(dá)到高效吸附的目的,其中范德華力也起到一定作用.從pH值和共存離子對(duì)吸附過(guò)程影響的研究中也可以看出靜電作用對(duì)吸附影響并不顯著.MPC- 2-0.3富含給電子基團(tuán)可以作為π電子供體,具有酰胺基團(tuán)的CBZ可作為π電子受體,所以吸附劑CBZ分子之間存在π-π相互作用以及氫鍵作用(圖12)[49-50].此外,存在于MPC-2-0.3與CBZ之間的疏水作用也是控制吸附過(guò)程不可或缺的因素[32].

圖12 MPC-2-0.3對(duì)CBZ的吸附機(jī)理

2.7 再生性和穩(wěn)定性

從實(shí)際應(yīng)用的角度出發(fā),吸附劑的再利用具有重大意義.如圖13(a)所示,吸附劑即使在經(jīng)歷4個(gè)循環(huán)后對(duì)CBZ的吸附容量?jī)H下降了25.63%,仍表現(xiàn)出良好的吸附能力.在吸附過(guò)程中,鐵離子溶出的濃度很低,如圖13(b)所示,隨著吸附進(jìn)行至8h,溶液中的鐵離子濃僅為0.89mg/L.以上結(jié)果表明MPC-2-0.3再生性和穩(wěn)定性較好,并具有應(yīng)用于實(shí)際廢水處理的潛力.

圖13 MPC-2-0.3的再生性及穩(wěn)定性

3 結(jié)論

3.1 基于KHCO3“發(fā)酵”策略,通過(guò)同步磁化和活化成功制備出一系列楊木屑基磁性多孔碳.在最佳條件下獲得的MPC-2-0.3具有高表面積(1002.48m2/g)和優(yōu)秀的吸附性能(308K時(shí)飽和吸附量為307.3mg/g).

3.2 CBZ的吸附過(guò)程由物理吸附主導(dǎo),同時(shí)符合擬二級(jí)動(dòng)力學(xué)和Langmuir模型.整個(gè)吸附過(guò)程涉及、π-πEDA、氫鍵和疏水作用.

3.3 該吸附劑也具有良好的抗酸堿鹽溶液能力,此外,成功的引入磁性Fe納米顆粒,使吸附劑通過(guò)磁分離可以輕松回收,并體現(xiàn)出良好的再生性能.

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One step preparation of biomass based magnetic porous carbon and efficient adsorption of CBZ.

CHEN Ai-xia*, Tian Zheng, WEI Xiao, Hu Rui-rui, Zhang Yi-xuan, Guan Juan-juan

(Key Laboratory of Subsurface Hydrology and Ecological Effects in Arid Region, Ministry of Education School of Water and Environment,, Chang￠an University, Xi’an 710054, China)., 2022,42(4)：1714～1725

In order to utilize waste agricultural and forestry biomass as resources and effectively remove carbamazepine (CBZ) from water. Inspired by the “l(fā)eavening” strategy, poplar sawdust was transformed into magnetic porous carbon (MPC) with developed pore structure by using KHCO3as activator and FeCl3·6H2O as magnetic precursor. The optimum product MPC-2-0.3 was prepared by adjusting the ratio of raw materials. Its specific surface area was 1002.48m2/g and the CBZ adsorption capacity of 50mg/L was 202.70mg/g. SEM and XRD showed that Fe nanoparticles were successfully doped into porous carbon, giving porous carbon excellent magnetic separation ability. The kinetics and isotherms of the adsorption process were more in line with the pseudo second-order kinetics and Langmuir model. It was clarified that the main adsorption mechanisms were adsorption sites, π-π interaction and hydrogen bonding. MPC-2-0.3 had excellent adsorption performance at room temperature and a wide pH range, and it was easy to separate and regenerate.

iron doping；porous carbon；one step method；carbamazepine

X703.5

A

1000-6923(2022)04-1714-12

陳愛(ài)俠(1967-),女,陜西富平人,副教授,博士,主要從事水污染控制和生態(tài)修復(fù)研究.發(fā)表論文50余篇.

2021-09-30

陜西省自然科學(xué)基礎(chǔ)研究計(jì)劃項(xiàng)目(2021JM-153);長(zhǎng)安大學(xué)研究生科研創(chuàng)新實(shí)踐項(xiàng)目(30010374013)

*責(zé)任作者, 副教授, aixiach@chd.edu.cn