鄭懷禮,鐘 政,鄒 宏,白 瑩,趙 瑞,丁 魏,蔣君怡

磁性陽離子型殼聚糖絮凝劑去除Cr(Ⅵ)

鄭懷禮1*,鐘 政1,鄒 宏2,白 瑩3,趙 瑞1,丁 魏1,蔣君怡1

(1.重慶大學(xué),三峽庫區(qū)生態(tài)環(huán)境教育部重點實驗室,重慶 400045；2.重慶藍潔廣順凈水材料有限公司,重慶 4024652；3.中海油天津化工研究設(shè)計院有限公司,天津 300131)

以二氧化硅和硅烷偶聯(lián)劑(KH-570)包裹的磁性Fe3O4作為磁芯,殼聚糖(CS)、丙烯酰胺(AM)和丙烯酰氧乙基三甲基氯化銨(DAC)作為接枝單體,通過低壓紫外光引發(fā)合成新型磁性殼聚糖絮凝劑 (FSCAD),研究該材料對Cr(Ⅵ)的去除性能.采用傅立葉變換紅外、熱重分析、X射線衍射圖譜和振動樣品磁強計對絮凝劑進行表征,顯示材料成功制備并具有良好的磁響應(yīng)性.系統(tǒng)探究了pH值、絮凝劑投加量、反應(yīng)時間、干擾離子對絮凝性能的影響并擬合絮凝劑絮動力學(xué)模型.結(jié)果表明,絮凝劑絮凝動力學(xué)符合擬二級動力學(xué)方程,在投加量為900mg/L、pH值為3、反應(yīng)時間為60min時FSCAD對低濃度Cr(Ⅵ)廢水的去除效果可達到90.48%.

磁絮凝劑；低壓紫外光；殼聚糖；接枝改性；六價鉻

重金屬污染物中,Cr(VI)具有氧化電位高、易于穿透生物膜的性質(zhì),是一種毒性極強的重金屬元素[1].Cr(VI)主要來源于鉻酸鹽制造[2]、電鍍、金屬拋光[3]、紡織印染[4]、合金和鋼鐵制造等工業(yè)過程[5].含Cr(VI)廢水的主要處理方法包括化學(xué)還原法[2]、光催化還原法[6-7]、離子交換法[8]、膜過濾法[9]以及絮凝法[10]等.其中絮凝法是水處理中常用的工藝之一,具有工藝簡單、適用性廣的優(yōu)點[11].然而傳統(tǒng)的絮凝劑對Cr(VI)廢水的去除效率低、對pH值的依賴性強.因此需要開發(fā)高效的絮凝劑來強化絮凝性能.

磁絮凝技術(shù)是一種常用的強化混凝技術(shù),在原混凝體系中加入高比表面積的磁性納米顆粒MNPs(如納米氧化鐵)作為載體,加強磁性物質(zhì)、絮凝劑與水體污染物之間的碰撞,形成的絮體密實,混凝去除效率提高[12-13].然而,這類常規(guī)磁絮凝也存在磁粉與絮體的結(jié)合程度較低、處理效果不穩(wěn)定等局限性.磁性復(fù)合絮凝劑由磁粉與傳統(tǒng)絮凝劑復(fù)合制成,絮凝后形成的絮體具有磁性,易于分離對水體中重金屬離子的去除有良好的效果[14].本文磁性復(fù)合絮凝劑中的磁芯采用磁性Fe3O4納米顆粒,為屏蔽Fe3O4納米顆粒的團聚作用同時提高其抗氧化性和抗酸性,對其進行無機物包覆.二氧化硅(SiO2)具有良好的化學(xué)穩(wěn)定性、生物相容性和親水性,同時SiO2能阻止Fe3O4納米顆粒的聚集[15],為MNPs表面的進一步修飾提供活性位點,本文采用二氧化硅對磁芯進行包裹. C=C通過硅烷偶聯(lián)劑接枝到磁芯表面,為聚合物提供足夠的接口.殼聚糖具有成本低、無毒、可生物降解等優(yōu)勢,已有研究表明殼聚糖對Cr(VI)有良好的去除效果[16-17],因此本文采用殼聚糖作為接枝絮凝材料.

在絮凝劑制備領(lǐng)域中,最廣泛應(yīng)用的光引發(fā)方式是以高壓紫外燈管作為光源的高壓紫外光引發(fā)聚合法[18].高壓紫外光引發(fā)法反應(yīng)溫度高,需加入冷凝管回流裝置進行降溫,增加了操作的復(fù)雜性.近年來研究的低壓紫外光引發(fā)法具有反應(yīng)溫度低、操作簡單、聚合物不易交聯(lián)、更加的節(jié)能環(huán)保的優(yōu)勢[19].本研究采用低壓紫外光引發(fā)接枝聚合反應(yīng),以SiO2和硅烷偶聯(lián)劑(KH-570)包裹的磁性Fe3O4作為磁芯,引入氨基、羥基、季銨鹽基團,制備出了Fe3O4@SiO2@C=C@殼聚糖-丙烯酰胺-丙烯酰氧乙基三甲基氯化銨磁性絮凝劑(FSCAD).通過一系列表征,研究磁性絮凝劑的合成與反應(yīng)機理,將FSCAD應(yīng)用于Cr(VI)模擬廢水的處理,研究絮凝劑對Cr(VI)的最佳去除率.

1 材料和方法

1.1 實驗材料

殼聚糖(CS,乙?；?95%)、戊二醛(50% wt)、丙烯酰氧乙基三甲基氯化銨(DAC,80% wt)、Fe3O4納米顆粒均購自成都麥卡西化工有限公司.其他化學(xué)品購自成都科龍化學(xué)試劑有限公司.超純水由LZ-PJ-10A凈化系統(tǒng)(中國重慶利振科技有限公司)制成.所有試劑均為分析級,使用時無需進一步純化.

1.2 磁絮凝劑的制備

1.2.1 Fe3O4@SiO2的制備 采用溶膠凝膠法制備Fe3O4@SiO2殼核納米顆粒.將磁性納米顆粒Fe3O4分散于酒精和去離子水體積比為2:1的混合溶液中,超聲10min.注入質(zhì)量分數(shù)為25%的氨水溶液.在120r/min攪拌下,緩慢連續(xù)加入TEOS(正硅酸四乙酯),TEOS與氨水的體積比為3:1,室溫攪拌下反應(yīng)8h.反應(yīng)完成后先后用酒精和去離子水洗滌Fe3O4@SiO2,去除未反應(yīng)的TEOS.最后在60℃下真空干燥至恒重.

1.2.2 Fe3O4@SiO2@C=C的制備 采用含有乙烯基的硅烷偶聯(lián)劑3-(甲基丙烯酰氧基)丙基三甲基氧基硅烷(KH-570)對Fe3O4@SiO2表面進行修飾,引入C=C,提供充分的接枝位點.具體操作方法:將Fe3O4@SiO2納米顆粒分散于裝有酒精的三頸圓底燒瓶中超聲處理10min.吹氮氣10min,然后加熱到78℃.在加熱至62℃時,緩慢加入KH-570醇溶液,然后用氮氣鼓泡10min.將該氣密混合物回流并在78℃下以180r/min攪拌12h.反應(yīng)完成后,用外部磁體分離表面上被C=C鍵修飾的Fe3O4@SiO2殼核納米顆粒,用乙醇純化6～8次,并在60℃真空干燥至恒重,制得Fe3O4@SiO2@C=C.

1.2.3 FSCAD的制備 FSCAD通過低壓紫外光引發(fā)的接枝共聚制備.將殼聚糖溶解在100mL 1%乙酸溶液中,加入2g Fe3O4@ SiO2@C=C納米顆粒和丙烯酰胺(AM),充分振搖并超聲處理10min.吹氮氣10min,加入1.5mL過硫酸鉀溶液(0.1g/mL),繼續(xù)吹氮氣,加入9mL丙烯酰氧乙基三甲基氯化銨(DAC)溶液.搖晃均勻,在低壓紫外線燈下照射3h.制備的磁性絮凝劑用乙醇純化6～8次,然后真空60℃干燥至恒重.

1.3 分析方法

采用傅里葉變換紅外光譜(FTIR;Nicolet iS10,美國)表征具有KBr顆粒的FSCAD官能團.利用具有CuKα輻射(=1.54056?)的X射線衍射圖(XRD; DMAX/2C,日本Rigaku),以表征FSCAD的相結(jié)構(gòu),表面化學(xué)結(jié)構(gòu)利用Axis Ultra X射線光電子能譜儀(XPS;Empyrean,PANalytical B.V.,荷蘭)檢測.

1.4 實驗方法

采用模擬Cr(VI)廢水評估FACAD的絮凝性能.將重鉻酸鉀溶于超純水中制備濃度為500mg/L的Cr(VI)標(biāo)準(zhǔn)水溶液,再用超純水稀釋至預(yù)定濃度.分別用0.01mol/L的HCl/NaOH調(diào)節(jié)pH值.首先將混合溶液在300r/min的高速下攪拌5min,在50r/min的低速下攪拌60min.隨后,在磁鐵的幫助下分離絮狀物.抽吸液體表面以下2cm的上清液以測量金屬離子濃度.重金屬離子的濃度采用中國標(biāo)準(zhǔn)(GB/ T9723-2007)方法[20]測定.用紫外可見分光光度計(T6Pgeneral Co.,Ltd,中國)在540nm波長下測量上清液的透光率.所有樣品濃度結(jié)果采用3個平行測量值的平均值表示,并通過計算3個值的標(biāo)準(zhǔn)偏差獲得誤差限.污染物去除效率計算公式如下:

式中:C和C分別表示的Cr(VI)初始濃度和最終濃度, mg/L.

2 結(jié)果與討論

2.1 磁性混凝劑的特性

2.1.1 FTIR光譜分析 如圖1所示,對于Fe3O4@SiO2,559.4cm?1處的吸附峰對應(yīng)典型的Fe- O拉伸振動[19].在794.1和1064.5cm?1處的吸附峰分別與Si-O-Si的不對稱/對稱振動和彎曲振動有關(guān).945.7cm?1處的峰值是由Si-O-H振動[21]引起的.這些峰證實了Fe3O4@SiO2中Fe3O4和SiO2的存在.Fe3O4@SiO2@C=C中1661.7cm?1與C=C的振動有關(guān),這表明KH-570的成功接枝.1030.8,1156.6, 1447.8,1614.2和3340.4cm?1分別與C-OH、C-O-C、-CH2、N-H和-OH/-NH2交聯(lián)CS的振動有關(guān)[22-23]. 1730.1cm?1的峰歸屬于DAC中O-C=O的伸縮振動,955cm?1處的吸附帶歸屬于季銨基[24],表明DAC單體成功接枝.而1536.9和1655.8cm?1的峰分別屬于AM中的酰胺II和酰胺I譜帶[25].上述特征峰表明,利用AM和DAC對CS進行了成功的改性.

圖1 FSCAD的紅外光譜

2.1.2 XRD圖譜分析 如圖2所示,Fe3O4@SiO2的特征峰位于2θ=18.5°,30.3°,35.6°,43.3°,53.6°,57.2°和62.8°,分別歸結(jié)為(1 1 1),(2 2 0),(3 1 1),(4 0 0),(4 2 2),(5 1 1)和(4 4 0),這表明制備的Fe3O4具有良好的結(jié)晶度.對于FSCAD,這些沒有變化的峰表明,無論是SiO2還是CS-AM-DAC外殼都不會對Fe3O4磁芯[26]的尺寸和晶相造成任何損害.此外,SiO2和有機共聚物均未見峰,說明其結(jié)構(gòu)為無定形[27].

圖2 FSCAD X射線衍射表征結(jié)果

圖3 FSCAD的磁化混線

2.1.3 磁性能分析 圖3為±20000Oe磁場下,用VSM分析Fe3O4@SiO2、Fe3O4@SiO2@C=C、FSCAD的磁性能圖.3條曲線均呈對稱分布,未觀察到明顯的磁滯現(xiàn)象,說明所有樣品均具有超順磁性特征[28-29].Fe3O4@SiO2的飽和磁化強度為57.9emu/g;接枝C=C后,Fe3O4@SiO2@C=C的飽和磁化強度分別為55.9emu/g,接枝AM與DAC后FSCAD的飽和磁化強度為12.7emu/g.這一現(xiàn)象是因為磁芯引入了非磁性材料MPS和有機共聚物.雖然FSCAD的飽和磁化值遠低于Fe3O4@SiO2的飽和磁化值,但在外加磁鐵的幫助下,仍能快速有效地從水溶液中分離出來.

2.1.4 熱重分析 如圖4所示,對于Fe3O4@SiO2來說只有微弱的失重,這部分失重由SiO2造成,證明了SiO2成功包裹在了Fe3O4的表面且Fe3O4@SiO2具有優(yōu)越的熱穩(wěn)定性;Fe3O4@SiO2@C=C與Fe3O4@SiO2相比也有很小一部分的失重量,這是因為接枝KH-570引入了C=C的原因,證明了C=C雙鍵的成功引入,也證明了Fe3O4@SiO2@C=C良好的熱穩(wěn)定性.KH-570的沸點為255℃,若Fe3O4@SiO2與KH-570為物理吸附,則在255℃前就會發(fā)生較大失重,因此可以推斷出改性后Fe3O4@SiO2與KH- 570通過共價鍵相連[30];而對于FSCAD來說有4個明顯的失重階段:其中第1個失重階段(T1-T2)是親水性基團吸附水分子蒸發(fā)所致[31],失重率為9.3%.第2個失重階段(T2～T3)與亞胺反應(yīng)和酰胺基團熱分解有關(guān)[32-33],失重率為25.6%.第3個失重階段(T3-T4)與DAC的O=C-O分解有關(guān),失重率為23.4%.第4個失重階段(T4-800 °C)由共聚物主鏈的熱分解造成[34],失重率為11.2%.

圖4 FSCAD熱重表征結(jié)果

2.2 混凝特性分析

圖5 FSCAD投加量對去除Cr(VI)的影響

2.2.1 投加量的影響 如圖6所示,絮凝劑的離子去除率(RE(CV))隨著投加量的增加而升高,并在投加量為0.9mg/mL后增長速率減緩.從RE(CV)來看, FSCAD對低濃度Cr(VI)的去除效率更高.在絮凝的過程中,各種絮凝劑都有一個最佳的投加量.在低用量的情況下,材料鏈上沒有足夠的活性基團與重金屬反應(yīng),導(dǎo)致Cr(VI)的去除量較低.隨著投加量的增加,活性基團數(shù)量增加,殼聚糖中極其活潑且相鄰的羥基與氨基對Cr(VI)的絡(luò)合吸附能力增加,陽離子單體DAC引入的季銨鹽基團對水溶液中鉻酸鹽的靜電吸附能力不斷加強,Cr(VI)濃度不斷降低.同時由于絮凝劑的吸附架橋作用,絮體和團聚體迅速形成.當(dāng)投加量進一步增加時,大分子鏈中存在的過多的正電荷會增加反應(yīng)的靜電斥力,從而影響絮凝劑對Cr(VI)的進一步去除.本實驗結(jié)果顯示,絮凝劑投加量在1000mg/L以內(nèi)時去除率有減緩趨勢但無明顯下降趨勢.從經(jīng)濟性的角度出發(fā),選取900mg/L作為后續(xù)實驗的絮凝劑投加量.

2.2.2 溶液pH值的影響 如圖6所示,在酸性條件下,由于質(zhì)子化氨基的形成和季銨鹽基團的存在使得FSCAD的Zeta電位為正值,隨著pH值的增大,氨基的去質(zhì)子化作用加強Zeta電位減小,但由于季銨鹽基團的存在FSCAD在整個pH值范圍內(nèi)表面都帶正電荷.因此,改性后的絮凝劑具有更大去除水體陰離子污染物的潛力.

圖6 FSCAD的Zeta電位與pH值的關(guān)系

Cr(VI)在水中的存在形式與水中氫離子濃度有關(guān).如圖7(a)所示,在Cr(VI)濃度為1mmol/L的情況下,當(dāng)pH<1時,主要以H2CrO4的形式存在;pH= 2～6時,主要為HCrO4-與CrO42-的共同存在;pH=6～ 10時,主要以CrO42-的形式存在, pH>10時幾乎只有CrO42-存在.因此本文選取pH值范圍為1～10范圍內(nèi)進行研究.如圖7(b)所示, pH=1～3時,絮凝劑對Cr(VI)的去除率逐漸增加,并在pH=3時分別對5, 20與50mg/L Cr(VI)獲得最高去除率90.48%、84.91%和55.39%.pH=3～10時,去除率逐漸下降. FSCAD中的羥基、氨基、季銨鹽基團通過氫鍵、靜電吸附、表面絡(luò)合與Cr(VI)結(jié)合[35-36],從而有效地去除了Cr(VI).pH<3時去除率減小的原因可以從Cr(VI)在該pH值下的存在形式和電性結(jié)合起來分析. FSCAD中的氨基在低pH值(pH=3)下Cr(VI)主要以HCrO4-的形式存在,氨基與氫離子結(jié)合形成質(zhì)子化氨基-NH3+,表面帶正電(+23.52mV),質(zhì)子化氨基與季銨鹽基團可以與溶液中不同形式的鉻酸鹽發(fā)生靜電相互作用,從而形成最大化的絡(luò)合延伸[16],當(dāng)pH<3時,水體中存在一定比例的H2CrO4,間接降低了溶液中鉻酸鹽的含量,H2CrO4存在比例越大,靜電相互作用越弱.pH值在3-5時,溶液中Cr(VI)以HCrO4-與CrO42-共同存在,但此時質(zhì)子化氨基的數(shù)量隨著pH值增加而減少,正電性減弱,靜電相互作用減小.當(dāng)pH值增加至堿性時,CrO42-為主要存在形式,質(zhì)子化氨基的數(shù)量進一步減少.由于絮凝劑正電性下降、CrO42-所占據(jù)活性位點多于HCrO4-[36]、OH-與CrO42-競爭活性位點的現(xiàn)象越來越強烈的原因,導(dǎo)致靜電作用不斷下降、去除率不斷降低.

2.2.3 共存離子影響 研究了在水環(huán)境中廣泛存在的陽離子(Na+、K+、Ca2+)和陰離子(Cl?、NO3?、SO42?)對Cr(VI)去除率的影響[37-38].采用50mmol/L的Na+、K+、Ca2+、Cl?、NO3?、SO42?離子模擬水體中的共存離子, Cr(VI)濃度為20mg/L, pH值調(diào)節(jié)為7,絮凝劑投加量為900mg/L.如圖8(a)所示,共存陰離子對Cr(VI)離子的絮凝去除均表現(xiàn)出不利影響,其影響程度依次為:Cl?

2.2.4 絮凝時間的影響 絮凝速率是評估特定絮凝劑應(yīng)用潛力的重要因素,通常對于目標(biāo)污染物的去除速率越快絮凝劑越理想.分別配制5,20mg/L和50mg/L的Cr(VI)離子溶液,在中性條件下進行實驗,絮凝劑投加量為900mg/L.如圖8(b)所示,絮凝劑對重金屬Cr(VI)離子的吸收較快,在反應(yīng)開始30min后FSCAD對5和20mg/L Cr(VI)離子的去除率超過70%;對50mg/L的Cr(VI)離子的去除率超過40%.隨后去除率緩慢增加并達到平衡值,FSCAD對Cr(VI)離子的相對最優(yōu)去除率分別為76.58%、76.59%、44.76%.最初的去除率快速上升現(xiàn)象是由目標(biāo)污染物與氨基、羥基和季銨鹽基團的相互作用引起,初期活性位點充足,能迅速進行有效反應(yīng).但隨著反應(yīng)時間的延長,重金屬與FSCAD之間形成絮體,活性位點被占據(jù),線性分子鏈發(fā)生卷曲,造成重金屬與活性位點之間的碰撞效率降低.絮凝劑FSCAD的快速初始吸附-絮凝速率表明其在應(yīng)急水污染凈化中具有良好的應(yīng)用潛力.根據(jù)上述動力學(xué)數(shù)據(jù), 40min后絮凝劑的反應(yīng)趨近平衡,本文慢速攪拌時間選擇60min可以確保磁性絮凝劑去除Cr(VI)達到平衡.

圖9 FSCAD在Cr(VI)不同初始濃度條件下的動力學(xué)擬合

表1 FSCAD的模型擬合結(jié)果

2.2.5 絮凝劑去除Cr(VI)的絮凝動力學(xué) 如圖9和表1所示,FSCAD的絮凝反應(yīng)過程與準(zhǔn)二級動力學(xué)擬合最好,在準(zhǔn)二次動力學(xué)反應(yīng)過程中,反應(yīng)物之間的離子交換是限制反應(yīng)速率的關(guān)鍵[42-45].因此,離子交換在FSCAD的絮凝反應(yīng)中起重要作用.

3 結(jié)論

3.1 表征結(jié)果表明,絮凝劑成功制備,具有良好的熱穩(wěn)定性與磁響應(yīng)性.

3.2 絮凝實驗證明,在pH=3、絮凝劑投加量為900mg/L、絮凝程序按設(shè)計條件操作的條件下, FACAD對5mg/L Cr(VI)的最佳去除率約為90.48%;對20mg/L Cr(VI)的最佳去除率約為84.91%;對50mg/L Cr(VI)的最佳去除率約為55.39%.絮凝劑分子鏈上的氨基、羥基和季銨鹽基團可以快速去除水中的Cr(VI).

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Synthesis of magnetic cationic chitosan flocculant by low-pressure UV light for the removal of Cr(Ⅵ).

ZHENG Huai-li1*, ZHONG Zheng1, ZOU Hong2, BAI Ying3, ZHAO Rui1, DING Wei1, JIANG Jun-yi1

(1.Key Laboratory of the Three Gorges Reservoir Region's Eco-Environment, StateMinistry of Education, Chongqing University, Chongqing 400045, China；2.Chongqing Lanjie Guangshun Water Treatment Materials Co.Ltd, Chongqing 402465, China；3.CenerTech Tianjin Chemical Research and Design Institute Co. Ltd, Tianjin 300131, China)., 2022,42(2)：745～752

A new magnetic chitosan flocculant FSCAD was synthesized byusing magnetic Fe3O4coated with silica and silane coupling agent (KH-570) as magnetic core, chitosan, acrylamide and acryloyloxyethyl trimethylammonium chloride as graft monomers and low-pressure ultraviolet light as initiator,the Cr(Ⅵ) removal properties of the material were studied. The flocculants were characterized by fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray diffractometer and vibration sample magnetometer, showing the materials were successfully prepared and have good magnetic response. The effects of pH, dosage, reaction time and interference ions on flocculation properties were explored and fitted to the flocculation dynamics model. The flocculation kinetics accorded with the pseudo second order kinetic equation and the removal effect of FSCAD on low concentration Cr(Ⅵ) wastewater reached 90.48% at the added amount of 900mg/L, pH of 3 and the reaction time of 60min.

magnetic flocculant；low-pressure UV；chitosan；grafting modification；Cr(Ⅵ)

X703.1

A

1000-6923(2022)02-0745-08

鄭懷禮(1957-),男,重慶人,教授,博士,主要從事水處理及水處理劑研究.發(fā)表論文400多篇.

2021-07-10

國家自然科學(xué)基金資助項目(21677020);重慶市技術(shù)創(chuàng)新與應(yīng)用示范專項重點研發(fā)項目(cstc2018jszx-zdyfxmX0002)

* 責(zé)任作者, 教授, zhl@cqu.edu.com