曹惜霜,信 欣,2*,楊 豪,鄂 荻

CTS@Fe3O4-COOH對(duì)低pH值采收性能及機(jī)理

曹惜霜1,信 欣1,2*,楊 豪1,鄂 荻1

(1.成都信息工程大學(xué)資源環(huán)境學(xué)院,四川 成都 610225；2.中-塞環(huán)境與能源“一帶一路”聯(lián)合實(shí)驗(yàn)室,四川 成都 610225)

采用共沉淀法制備得到磁性材料殼聚糖@檸檬酸改性Fe3O4(CTS@Fe3O4-COOH),通過單因素與正交試驗(yàn)考察了不同條件下其對(duì)小球藻()的采收效率.結(jié)合XRD、FT-IR和VSM等材料的結(jié)構(gòu)性質(zhì)表征、表面Zeta電位及Derjaguin-Landau-Verwey-Overbeek(DLVO)理論分析,探討CTS@Fe3O4-COOH對(duì)小球藻的絮凝采收機(jī)理.結(jié)果表明,CTS@Fe3O4-COOH對(duì)小球藻具有高效采收效率,與未改性相比采收效率提高約30%.單因素試驗(yàn)表明材料投加量與pH值對(duì)小球藻采收效率的影響較大;正交試驗(yàn)表明當(dāng)CTS@Fe3O4-COOH投加量為4.5g/L時(shí),在pH 4的條件下,經(jīng)500r/min 快攪3min后再70r/min慢攪5min,對(duì)小球藻的采收效率高達(dá)98.35%.DLVO等理論分析表明,CTS@Fe3O4-COOH對(duì)小球藻的采收機(jī)理為電荷中和、靜電修補(bǔ)、吸附架橋與整體絮凝聯(lián)合作用.本文結(jié)果為CTS@Fe3O4-COOH采收固定煙氣能源微藻的實(shí)際應(yīng)用提供數(shù)據(jù)支持.

微藻采收；CTS@Fe3O4-COOH；小球藻；磁絮凝

當(dāng)前為應(yīng)對(duì)氣候變化,實(shí)現(xiàn)“雙碳”目標(biāo),CO2的減排顯得尤為重要.目前常用的減排技術(shù)為化學(xué)吸收或物理吸附CO2、CO2封存等,但均存在能耗過高的問題[1].近幾年,微藻工藝被廣泛應(yīng)用于環(huán)境處理[2],微藻通過光合作用將CO2轉(zhuǎn)化為生物能,實(shí)現(xiàn)了CO2固定與資源化利用[3],伴有可再生生物燃料的產(chǎn)生[4],同時(shí)緩解了我國能源緊缺的問題.但由于微藻一般懸浮于培養(yǎng)液中,導(dǎo)致其采收難度增加[5],采收環(huán)節(jié)成本約占整個(gè)微藻生產(chǎn)成本的20%～ 30%[6],因此,尋找一種低成本、高效率的微藻采收技術(shù)有利于推動(dòng)微藻行業(yè)的發(fā)展.

目前微藻采收技術(shù)主要包括離心、膜過濾、浮選和絮凝[7],其中磁絮凝技術(shù),不僅具有顯著的微藻收獲能力,在一定輔助條件下在促進(jìn)微藻細(xì)胞裂解的同時(shí)還有利于油脂的釋放,從而簡化了微藻的下游工藝[8].另外絮凝后的材料經(jīng)過物理去磁后還能重復(fù)利用[9].由于四氧化三鐵(Fe3O4)這種磁性材料具有較大的比表面積,磁性也較強(qiáng),用于收獲微藻的磁性材料和磁性絮凝劑通常圍繞這種物質(zhì)展開[10-11].但表面裸露的Fe3O4在有氧條件下化學(xué)性質(zhì)不穩(wěn)定[12],且在水溶液中其表面為負(fù)電荷,難以采收微藻.為了提高Fe3O4對(duì)微藻的采收率,廣泛采用將陽離子基團(tuán)或聚合物來涂覆改性.部分研究[13-14]將Fe3O4功能化制得Fe3O4-PEI材料,對(duì)等采收效率均可達(dá)98%以上.Li等[15]通過合成鐵納米顆粒對(duì)的采收效率在分散1min可達(dá)98.3±1.8%,與預(yù)合成的Fe3O4納米顆粒比較采收效率明顯提高.劉詩楠[16]功能化合成Fe3O4/ PAC材料在3min內(nèi)對(duì)微藻采收效率可達(dá)95%以上.由此可見,對(duì)磁性材料表面改性能夠顯著提高微藻的采收效率,但大部分改性材料存在價(jià)格昂貴、改性過程復(fù)雜的缺點(diǎn),難以大規(guī)模使用[17].殼聚糖是一種天然高分子聚合物,其分子鏈上含有豐富的-NH2和-OH,可以與其他材料共價(jià)結(jié)合賦予復(fù)合材料更優(yōu)異的性能[18-19],價(jià)格便宜且無害,使其成為了微藻采收的研究熱點(diǎn)之一[20].相關(guān)研究[21-22]使用殼聚糖對(duì)Fe3O4納米顆粒改性后用于采收,其對(duì)小球藻均具有高效采收效果,可達(dá)95%.但殼聚糖直接包覆于Fe3O4上容易出現(xiàn)脫附的情況,故需先對(duì)Fe3O4表面進(jìn)行改性,而檸檬酸為常用的小分子表面改性劑,表面具有3個(gè)羧基官能團(tuán),其中一個(gè)或者兩個(gè)羧基被吸附于材料表面,但至少有一個(gè)羧基是自由的,這有助于避免具有超順磁性的含鐵粒子團(tuán)聚,利于材料分散[23-24].

基于此,本研究利用檸檬酸為改性劑,殼聚糖為包覆劑對(duì)磁性Fe3O4表面進(jìn)行改性,制備得到了殼聚糖@檸檬酸改性磁性Fe3O4(CTS@Fe3O4-COOH),采用磁絮凝法對(duì)小球藻進(jìn)行采收實(shí)驗(yàn),探討其在不同環(huán)境條件下對(duì)小球藻的絮凝采收效率的影響;結(jié)合磁性材料的表征以及Derjaguin-Landau-Verwey- Overbeek(DLVO)理論等分析,深入探究其對(duì)小球藻絮凝采收機(jī)理,并得到高效經(jīng)濟(jì)采收小球藻的磁絮凝材料的采收最優(yōu)條件.結(jié)果可為該磁性材料經(jīng)濟(jì)高效采收固碳能源微藻提供理論依據(jù).

1 實(shí)驗(yàn)與方法

1.1 實(shí)驗(yàn)材料

1.1.1 主要試劑 FeCl2· 4H2O,FeCl3· 6H2O,氫氧化鈉,無水乙醇,一水檸檬酸,戊二醛,醋酸,本文中所有試劑均為分析純.

1.1.2 材料的制備 本文共制備了3種磁性材料,分別為磁性納米Fe3O4、檸檬酸改性Fe3O4(Fe3O4- COOH)與殼聚糖@檸檬酸改性Fe3O4(CTS@Fe3O4- COOH),具體步驟如下:

(1)磁性Fe3O4和Fe3O4-COOH的制備

采用共沉淀法[25]制備磁性Fe3O4.稱取摩爾比為1/2的Fe2+/Fe3+溶于100mL去離子水中,反應(yīng)(40℃, 1h)后調(diào)節(jié)pH=9,接著攪拌(40℃,1h)后用永磁體分離,超純水洗滌3次,最后用無水乙醇洗滌后真空干燥(60℃,12h)得到磁性納米Fe3O4.然后稱取2g制備好的磁性納米Fe3O4均勻分散于無水乙醇(20%)中,并倒入溶解有0.33g一水檸檬酸的30mL去離子水與之混合,接著攪拌反應(yīng)(40℃, 2600r/min,1h),待溶液冷卻到室溫用永磁體分離,分離后的產(chǎn)物用去離子水和無水乙醇交替洗滌至pH=7,最終將產(chǎn)物于真空干燥箱內(nèi)干燥,干燥后研磨封存得到檸檬酸改性Fe3O4-COOH[26],反應(yīng)均在氮?dú)饬鞅Ｗo(hù)下進(jìn)行.

(2)Fe3O4-COOH表面殼聚糖修飾

稱取1.5g Fe3O4-COOH于100mL 3%殼聚糖的醋酸溶液(2%)中,水浴條件下超聲分散(40℃,20min);然后加入1.35mL 25%的戊二醛溶液,水浴條件下機(jī)械攪拌(40℃,4h);最后將得到的黑色凝膠塊干燥(60℃,12h),并依次使用2%醋酸、熱水和冷水交替洗滌數(shù)次,再將洗滌干凈的黑色固體真空干燥(50℃, 12h);最后研磨過200目篩得到CTS@Fe3O4-COOH,并置于干燥器內(nèi)儲(chǔ)存待用[27].

1.1.3 藻類培養(yǎng) 固定煙氣CO2的能源微藻來自本課題組,為.采用BG-11培養(yǎng)基培養(yǎng),并通入15%CO2與85%N2的混合氣體,在通氣速率0.075vvm、溫度28.5℃、光照強(qiáng)度4950Lux的條件下培養(yǎng)至穩(wěn)定期末[28]后取出進(jìn)行采收實(shí)驗(yàn).

1.2 實(shí)驗(yàn)方法

1.2.1 采收方法 實(shí)驗(yàn)前取適量培養(yǎng)到生長末期的藻液(ρ=1.94g/L,OD680=1.805),在OD680測定藻液初始吸光度,記錄數(shù)據(jù).然后將適量磁性材料超聲分散于5mL去離子水中,再將其與待采收的藻液相混合,在混凝攪拌機(jī)下于300r/min快速攪拌1min后在50r/min慢速攪拌1min進(jìn)行采收.攪拌結(jié)束放置于永磁體上靜置沉降,分別取1,2,5,10,20,30min時(shí)上清液測定OD680的值,直到OD680的值基本不變時(shí),表明采收完成,通過初始吸光度與采后吸光度的值計(jì)算采收效率,計(jì)算公式如下:

式中:為微藻采收效率;OD680i為收獲前小球藻藻液液吸光度;OD680f為收獲后上清液小球藻吸光度.

1.2.2 單因素與正交試驗(yàn) 單因素試驗(yàn)時(shí),按照1.2.1中采收方法首先在1～10g/L范圍內(nèi)對(duì)CTS@Fe3O4-COOH、Fe3O4-COOH、Fe3O4、殼聚糖4種材料及用量進(jìn)行篩選,然后在此基礎(chǔ)上依然按照1.2.1中的采收方法,采用控制變量法探究出最佳pH值、攪拌速率、攪拌時(shí)間.探究pH值對(duì)采收效果的影響時(shí),使用0.1mol/L HCl與0.1mol/L NaOH將pH調(diào)至4～9;探究材料在最佳pH值條件下攪拌速率對(duì)采收效率的影響時(shí),分別設(shè)置快攪速率300, 400,500,600,700,800r/min與慢攪速率30,50,70,100, 120,150r/min;探究攪拌時(shí)間對(duì)采收效率影響時(shí),設(shè)置采收時(shí)間分別為1,3,5,7,9,11min;最后選取對(duì)采收效率影響最大的3個(gè)單因素,基于單因素實(shí)驗(yàn)結(jié)果設(shè)置3因素3水平的正交試驗(yàn)得出采收效率最佳的采收條件.每組試驗(yàn)設(shè)置2組平行試驗(yàn).

1.3 試驗(yàn)材料性能評(píng)價(jià)

1.3.1 材料表征 采用X射線衍射(XRD, D8Advance,德國布魯克公司)、傅里葉變換紅外光譜(FT-IR,Thermo IS 50型,美國Thermo Scientific公司)測試制得材料的晶體結(jié)構(gòu)與分子結(jié)構(gòu);采用磁滯回線(VSM,MPMS(SQUID)XL,美國Quantum Design公司)測定化合物磁學(xué)性質(zhì);采用激光粒度分析儀、Zeta電位(Zetasizer Nano,英國馬爾文公司生產(chǎn))、光學(xué)顯微鏡(CX31RBSFA,奧林巴斯株式會(huì)社生產(chǎn))測試制備出的材料粒徑、表面Zeta電位與形貌.

1.3.2 DLVO理論模型 在DLVO理論中,顆粒間的相互作用力是指顆粒間產(chǎn)生的范德華力(VDW)與壓縮雙電層產(chǎn)生的靜電力(EDL)之和[29].藻細(xì)胞與CTS@Fe3O4-COOH之間發(fā)生絮凝過程所產(chǎn)生的作用力可用DLVO理論進(jìn)行深入分析.范德華力與靜電力均可用球盤模型描述,其計(jì)算公式分別如下[30-31]:

式中:p為材料粒徑大小;為材料與藻細(xì)胞間的距離;為反應(yīng)特征波長,一般為100nm;132為哈馬克常數(shù),可近似估算為1.76x10-21J;

式中:0為真空介電常數(shù),為8.854′10-12C/(V×m);r為水的相對(duì)介電常數(shù),為78.5(無量綱);p和c為顆粒和藻細(xì)胞的表面電位,由實(shí)驗(yàn)測得;Z為離子價(jià)態(tài),用HCl或NaOH調(diào)節(jié)pH值時(shí)溶液中的離子價(jià)態(tài)為±1;為電子的帶電量,為1.602′10-19C;n0為離子濃度, mol/L,根據(jù)溶液pH值計(jì)算得到H+和OH-的濃度;為玻爾茲曼常數(shù),為1.38′10-23J/K;為絕對(duì)溫度,為298K.

2 結(jié)果與討論

2.1 表征與分析

2.1.1 XRD表征 圖1(a)為實(shí)驗(yàn)所用4種材料的X射線衍射圖.由圖1(a)可知,磁性Fe3O4的主要衍射峰位于18.28°、30.12°、35.48°、43.16°、53.48°、57.08°、62.69°處,與Fe3O4的標(biāo)準(zhǔn)卡片(PDF No. 88-035)進(jìn)行比對(duì),發(fā)現(xiàn)主要特征峰均吻合,如(110)、(220)、(311)、(400)、(422)、(511)和(440),且無其它雜峰出現(xiàn),說明所制備材料為Fe3O4,且雜質(zhì)含量少.由其他2種改良材料的XRD圖可以看到衍射峰位置未變,只是強(qiáng)度有所差異,說明在用檸檬酸鈉和殼聚糖對(duì)Fe3O4進(jìn)行包埋的過程對(duì)Fe3O4的結(jié)構(gòu)未造成改變,得到的改性材料也為尖晶石結(jié)構(gòu).同時(shí)由圖可以看到CTS@Fe3O4-COOH的衍射峰窄而尖銳,說明此時(shí)衍射峰強(qiáng)度最強(qiáng),Fe3O4的結(jié)晶性最好.

2.1.2 FT-IR表征 如圖1(b)所示,對(duì)于磁性Fe3O4粒子,在3440cm-1附近出現(xiàn)的峰是由O-H 鍵伸縮振動(dòng)產(chǎn)生,其彎曲振動(dòng)所對(duì)應(yīng)的譜帶在1120cm-1附近.在580cm-1附近出現(xiàn)的吸收峰是磁性Fe3O4的特征峰,由Fe-O鍵伸縮振動(dòng)產(chǎn)生[32];Fe3O4-COOH的紅外圖譜不僅具有580cm-1處Fe3O4特征吸收峰與3440cm-1處由O-H鍵形成的振動(dòng)峰,還多了1620, 1380cm-1附近-COOH基團(tuán)的不對(duì)稱伸縮振動(dòng)峰和對(duì)稱伸縮振動(dòng)峰,說明檸檬酸根與Fe3O4粒子表面的羧基成功發(fā)生了脫水反應(yīng),磁性Fe3O4粒子成功被羧基化;對(duì)于殼聚糖,在1156,1380,1660,2880,3430cm-1附近均出現(xiàn)了吸收峰,分別是-C-O-C-的不對(duì)稱伸縮振動(dòng)峰[33]、C-H鍵的彎曲振動(dòng)峰、C=O鍵的伸縮振動(dòng)峰、C-H鍵的伸縮振動(dòng)峰與O-H鍵和N-H鍵重疊產(chǎn)生的特征吸收峰[34];CTS@Fe3O4-COOH與殼聚糖相比出現(xiàn)了在1560cm-1附近的-NH-CO-結(jié)構(gòu)特征的峰,是由殼聚糖分子鏈上的-NH2與Fe3O4-COOH粒子上的-COOH共價(jià)結(jié)合形成的,由此可知?dú)ぞ厶浅晒Π裼贔e3O4-COOH上.

2.1.3 VSM表征 由圖1(c)可以看到, CTS@ Fe3O4-COOH的磁滯回線是一條過原點(diǎn)的平滑曲線,不存在磁滯現(xiàn)象,雖然CTS@Fe3O4-COOH的飽和磁化強(qiáng)度與Fe3O4相比有所降低,約為31.72emu/g,這是因?yàn)闄幟仕徕c濃度會(huì)影響改性后粒子的形貌和結(jié)晶性,從而影響其磁學(xué)性能[35].尤雯[36]制備出的磁性材料MC-g-PAM磁飽和強(qiáng)度為9.63emu/g,低于CTS@Fe3O4-COOH的飽和磁化強(qiáng)度,但對(duì)外加磁場仍具有較高的響應(yīng)力,因此,本試驗(yàn)中制備的CTS@Fe3O4-COOH仍能夠有效的進(jìn)行磁分離.

2.2 單因素實(shí)驗(yàn)結(jié)果分析

2.2.1 不同材料投加量對(duì)采收效率的影響 磁性材料的用量已被確定為影響磁絮凝過程中微藻收獲效率和運(yùn)行成本最重要因素之一[37].由圖2(a)可知,4種材料均能有效采收小球藻,隨著投加量的增加,采收效率均明顯提高,當(dāng)投加量為4.5g/L時(shí),大部分小球藻被高效的絮凝收獲,采收效率開始趨于穩(wěn)定.絮凝劑采收效率達(dá)到穩(wěn)定時(shí)的最小劑量通常用于評(píng)估其絮凝能力,4種材料絮凝采收效率達(dá)到穩(wěn)定時(shí)的最小劑量為5g/L,此時(shí)CTS@Fe3O4-COOH的采收效率明顯高于其他3種材料,為92.3%,其余3種材料Fe3O4-COOH、殼聚糖、Fe3O4采收效率依次為79.9%、77.3%、63.5%.同時(shí)可以看到,對(duì)小球藻的采收效率由高到低依次為CTS@Fe3O4- COOH>Fe3O4- COOH>殼聚糖>Fe3O4,改性材料效果明顯優(yōu)于未改性材料,這是因?yàn)榻?jīng)過表面修飾后材料表面可以提供足夠的官能團(tuán)和正活性位點(diǎn)進(jìn)行吸附[38],增強(qiáng)了對(duì)小球藻的采收效率.雖然當(dāng)材料投加量大于4.5g/L時(shí)對(duì)小球藻的采收效率仍在增加,但其增長速度過慢,因此考慮經(jīng)濟(jì)性選取投加量為4.5g/L的CTS@Fe3O4- COOH為采收材料進(jìn)行后續(xù)單因素實(shí)驗(yàn).

此外,其他研究中的采收材料采收效果,如Huang等[39]采用0.075g/L NiO能夠100%采收小球藻(藻液密度約為1g/L),相比CTS@Fe3O4-COOH用量低,但Ni容易對(duì)藻細(xì)胞造成損害.在本文中, CTS@Fe3O4-COOH在最佳采收條件下,當(dāng)投加量為4.5g/L,能夠采收98.35%的小球藻(藻液密度約為1.946g/L),而仍有其他采收小球藻的材料,如Fraga- García等[40]采用裸Fe3O4采收95%的小球藻(藻液密度0.6g/L),每克小球藻需Fe3O410g,而本文材料僅需4.5g.Lee等[41]采用AC-nZVI采收藻密度約為1.5g/L小球藻時(shí)(低于本研究采收的密度),當(dāng)AC- nZVI投加量為20g/L時(shí)采收效率約為100%,Lee的采收效率與本研究相當(dāng),但其用量相當(dāng)于本研究的4倍.又如Kim等[42]采用鋅鎂鐵氧體采收99%藻密度為本文1/4的微藻,每克微藻需投加材料2.5g,高于本文.同時(shí)相關(guān)研究表明在采用絮凝劑采收微藻時(shí),其投加量通常會(huì)隨著藻細(xì)胞密度的增加而成倍增加[43],而微藻采收效率往往隨著絮凝劑投加量的增加而增加.因此本試驗(yàn)材料仍具有良好的經(jīng)濟(jì)性與高效采收效率.

2.2.2 pH值對(duì)采收效率的影響 由圖2(b)可知,未加入絮凝劑時(shí),在任何pH值條件下對(duì)小球藻的采收效率都很低(4.2%～13.8%),且隨著pH值的升高而降低.在加入CTS@Fe3O4-COOH的實(shí)驗(yàn)組,可以看到與空白組相似的變化規(guī)律.隨著pH值的逐漸升高,CTS@Fe3O4-COOH對(duì)小球藻的采收效率逐步降低.這是由于經(jīng)過殼聚糖功能化后的材料,表面的-NH2在低pH值的條件下容易質(zhì)子化生成-NH3+,從而與帶負(fù)電的小球藻細(xì)胞相互吸引,且殼聚糖本身所帶陽離子繼續(xù)吸引小球藻形成更大的絮凝體從而達(dá)到高效采收.當(dāng)pH>7時(shí),溶液中OH-比H+占優(yōu)勢,-NH2難以質(zhì)子化,故采收效率降低.在所有pH值范圍內(nèi)采收效率均穩(wěn)定于80%以上,當(dāng)pH=4時(shí)對(duì)小球藻的采收效率最高為94.2%,同時(shí)經(jīng)15%煙氣培養(yǎng)后用于收獲的小球藻自身pH值就處于4～5,故選擇pH=4為最佳采收pH值.

2.2.3 攪拌時(shí)間、速率對(duì)采收效率的影響 由圖2(c) 可知,攪拌速率對(duì)小球藻的采收影響不大,幾乎無差別,所有攪拌速率情況下對(duì)小球藻的采收效率均高于91%,這可能是由于小球藻體積較大,容易與CTS@Fe3O4-COOH結(jié)合,對(duì)外界攪拌要求低.圖2(d)為攪拌時(shí)間對(duì)小球藻采收效率的影響,不論快攪或慢攪,其對(duì)小球藻的采收效率均隨著時(shí)間的增加先增加而后慢慢降低.當(dāng)體系處于慢攪時(shí),隨著攪拌時(shí)間增加,攪拌速率低,剪切力小,增加了CTS@Fe3O4- COOH與小球藻碰撞的機(jī)會(huì),且已形成的絮凝體繼續(xù)吸引其他未被采收的小球藻形成更大的絮凝體采收沉降;但當(dāng)體系處于快攪時(shí),CTS@Fe3O4- COOH與小球藻間碰撞的幾率增大,因此CTS@Fe3O4-COOH可以迅速捕集小球藻沉降,但攪拌時(shí)間越長,剪切力過大容易將已形成的絮凝體打散且難以重新組合,從而降低采收效率[18].當(dāng)慢攪時(shí)間為5min、快攪時(shí)間為3min時(shí),采收效率分別最高為94.35%、94.90%.

2.3 正交試驗(yàn)優(yōu)化

由單因素試驗(yàn)結(jié)果可知,材料投加量、pH值與攪拌時(shí)間對(duì)CTS@Fe3O4-COOH采收小球藻的影響較大,因此選擇以上3個(gè)因素進(jìn)行3因素3水平正交試驗(yàn)優(yōu)化采收效率,結(jié)果如表1.

其中,均值表示任一列上因素水平結(jié)果的算術(shù)平均值.一般來說,極差越大,表明因素的水平對(duì)結(jié)果的影響越大;均值越大,對(duì)應(yīng)的因素設(shè)計(jì)水平越優(yōu).如表1所示,計(jì)算得到(pH值)>(投加量)>(攪拌時(shí)間).比較各因素水平均值,并結(jié)合單因素實(shí)驗(yàn)結(jié)果,得到投加量、pH值、快速攪拌時(shí)間與慢速攪拌時(shí)間最優(yōu)分別為4.5g/L、pH=4、3min與5min時(shí)對(duì)小球藻的采收效率可達(dá)98.35%.

表1 正交試驗(yàn)方案與結(jié)果

2.4 采收絮凝機(jī)理探討

2.4.1 DLVO作用力分析 (1)材料及微藻的Zeta電位表面電位直接決定采收材料能否與微藻細(xì)胞結(jié)合.CTS@Fe3O4-COOH材料表面帶正電荷,與未經(jīng)包覆的Fe3O4相比變化顯著,能夠與小球藻細(xì)胞有效結(jié)合采收.當(dāng)裸露的 Fe3O4顆粒溶解在藻液中,Fe3O4表面捕獲的H+比OH?多,導(dǎo)致 Fe3O4表面富含羥基[44],無法吸引小球藻細(xì)胞并有效采收.經(jīng)檸檬酸殼聚糖包覆后表面形成-CO-NH-,表面從負(fù)電荷到正電荷,同時(shí)增加了Fe3O4表面所帶等位點(diǎn)與表面積,所帶的正電荷電位較高為16.4mV,微藻細(xì)胞的表面電荷為-15.8mV,為后續(xù)材料對(duì)微藻的高效采收奠定了基礎(chǔ).

圖3 CTS@Fe3O4-COOH粒徑分析

(2)材料與微藻DLVO作用力分析由圖3可以看到,88.88%的材料粒徑分布在0～20μm之間,且超過70%的材料粒徑分布在10～20μm,故取平均粒徑為15μm.由圖4可知,當(dāng)材料與小球藻直接相互作用距離大于10nm時(shí),范德華力與靜電斥力趨于0,DLVO作用力也為0.當(dāng)相互作用距離低于10nm時(shí),雖然各作用力均為負(fù)值,但其絕對(duì)值足夠小,膠體間作用力也較小.小球藻細(xì)胞表面存在著顯著的勢壘,能夠使得小球藻細(xì)胞足夠的懸浮穩(wěn)定,從而無法與磁性材料相接觸而被采收[45].由圖4可知,投加了CTS@Fe3O4- COOH的藻液DLVO作用力為負(fù)值,不存在勢壘,這是由于在材料與小球藻絮凝過程中,靜電力與范德華力均為負(fù)值,且占主導(dǎo)地位的為靜電力,靜電力表現(xiàn)為靜電引力的形式,范德華力的作用非常小,此時(shí)DLVO作用力表現(xiàn)為引力.因此,CTS@Fe3O4- COOH的投加能夠越過小球藻細(xì)胞間的勢壘,使小球藻細(xì)胞脫穩(wěn)而與材料發(fā)生絮凝作用得到采收.

2.4.2 采收機(jī)理討論 (1)采收前后顯微鏡分析由圖5可知,在水溶液中CTS@Fe3O4-COOH呈黑色顆粒狀,具有良好的分散性;小球藻細(xì)胞之間高度分散,互相獨(dú)立不接觸;圖5(c)為采收后絮凝體,可以看到,雖然溶液中仍有部分小球藻未得到采收,CTS@Fe3O4- COOH表面吸附有大量小球藻,且緊密結(jié)合.

(2)采收機(jī)理分析基于以上試驗(yàn)結(jié)果與理論分析,本研究探索并闡明CTS@Fe3O4-COOH與小球藻間絮凝采收機(jī)制,即采收體系外部條件與CTS@Fe3O4-COOH材料自身性質(zhì)的聯(lián)合作用.磁性Fe3O4溶于水后表面形成的鐵羥基與檸檬酸根中部分羧酸基團(tuán)發(fā)生反應(yīng)形成Fe3O4-COOH,然后殼聚糖經(jīng)戊二醛交聯(lián)反應(yīng)包裹于Fe3O4-COOH上,形成一個(gè)網(wǎng)格結(jié)構(gòu),使其具有良好的穩(wěn)定性和抗剪性,不易破損.因此,CTS@Fe3O4-COOH與表面裸露的Fe3O4相比,其表面分散性、穩(wěn)定性與表面積得到了大大提高.同時(shí),由材料采收投加量的對(duì)比可知, CTS@Fe3O4-COOH材料3g投加量就可達(dá)到60%采收效率,同等采收率條件下Fe3O4與Fe3O4-COOH各需投加5 和3.5g,當(dāng)CTS@Fe3O4-COOH投加量為4.5g時(shí)對(duì)小球藻采收效率可達(dá)90%以上,改性材料絮凝采收性能明顯增強(qiáng). CTS@Fe3O4-COOH對(duì)小球藻的采收效率受體系pH值影響較大,攪拌速率與時(shí)間影響較小.隨著采收體系pH值的提高,采收效率逐漸降低,當(dāng)pH=4時(shí),對(duì)小球藻的采收效率最高為94.2%,酸性條件下材料表面所帶-NH2質(zhì)子化生成-NH3+,與帶負(fù)電的小球藻相吸引得到采收;經(jīng)500r/min 快攪3min后在70r/min慢攪5min的條件下,提高了CTS@Fe3O4-COOH與小球藻間的碰撞幾率,增強(qiáng)了采收效率,對(duì)小球藻的采收效率高達(dá)98.35%.

本研究中CTS@Fe3O4-COOH對(duì)小球藻的采收具體可以分為4個(gè)方面:(1)帶電物質(zhì)的電荷中和.小球藻細(xì)胞表面呈負(fù)電荷,功能化后的CTS@Fe3O4-COOH表面為正電荷,同時(shí)殼聚糖由氨基(-NH2)和羥基(-OH)等活性吸附位點(diǎn)組成,陽離子密度高[46],從而將帶負(fù)電荷的小球藻細(xì)胞強(qiáng)烈吸附于表面達(dá)到電荷中和采收.(2)靜電修補(bǔ)機(jī)制[47]. CTS@Fe3O4- COOH與具有相反電荷的小球藻細(xì)胞結(jié)合,從而在小球藻細(xì)胞表面形成電荷修補(bǔ),小球藻細(xì)胞之間通過相反的修補(bǔ)相互連接電荷而導(dǎo)致絮凝收獲,這與DLVO作用受靜電引力主導(dǎo)所一致.(3)吸附架橋作用.殼聚糖具有吸附架橋作用弱的特點(diǎn),但其表面存在大量-NH2與-OH,-NH2易于藻液中H+質(zhì)子化形成-NH3+與大量帶負(fù)電的小球藻細(xì)胞吸附絮凝收獲;同時(shí)-NH2基團(tuán)可以很容易地與小球藻細(xì)胞表面的-COOH和-OH基團(tuán)發(fā)生反應(yīng)[48],形成具有富胺結(jié)構(gòu)的功能性磁性顆粒,有助于小球藻的采收;-OH與小球藻之間能通過氫鍵作用形成絮凝體,加上殼聚糖具有長鏈構(gòu)象,可以同時(shí)結(jié)合在2個(gè)小球藻細(xì)胞表面并在表面產(chǎn)生1個(gè)“橋梁”,通過架橋作用吸附小球藻.(4)整體絮凝. CTS@Fe3O4-COOH能與小球藻間形成1個(gè)三維網(wǎng)狀帶正電荷的絮凝體[46],從而增強(qiáng)對(duì)其他分散小球藻的捕集作用,形成一個(gè)更大的絮凝體進(jìn)而整體沉降得到高效采收.CTS@Fe3O4- COOH材料的制備和其與小球藻絮凝機(jī)理示意圖如圖6所示.因此,CTS@Fe3O4-COOH具有良好的分散性與穩(wěn)定性,且表面帶有較高的正電荷,通過電荷中和、靜電修補(bǔ)、架橋作用與整體絮凝作用同小球藻細(xì)胞高效絮凝采收.

圖5 采收前后光學(xué)顯微鏡圖

圖6 CTS@Fe3O4-COOH與小球藻絮凝機(jī)理

3 結(jié)論

3.1 CTS@Fe3O4-COOH具有超順磁性、優(yōu)良分散性、Zeta電位與裸露Fe3O4相比從負(fù)到正,具備對(duì)小球藻高效采收的基本條件,磁性能分析表明其磁飽和強(qiáng)度為31.72emu/g,對(duì)外加磁場具有良好的響應(yīng),能夠得到有效分離回收.

3.2 CTS@Fe3O4對(duì)小球藻采收時(shí)的最佳材料投加量、pH值、攪拌時(shí)間分別為4.5g/L、4、慢攪5min與快攪3min,攪拌速率對(duì)采收效率無明顯影響;當(dāng)CTS@Fe3O4-COOH投加量為4.5g/L,在pH=4的條件下,經(jīng)500r/min 快攪3min后再在70r/min慢攪5min后,對(duì)小球藻的采收效率最高為98.35%.

3.3 CTS@Fe3O4-COOH通過電荷中和、靜電修補(bǔ)機(jī)制、吸附架橋作用與整體絮凝四者的聯(lián)合機(jī)制,與小球藻之間形成一個(gè)三維網(wǎng)狀絮凝體從而對(duì)小球藻有高效采收效率.

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The harvesting performance and mechanism of CTS@Fe3O4-COOH on low pH.

CAO Xi-shuang1, XIN Xin1,2*, YANG Hao1, E Di1

(1.College of Resources and Environment, Chengdu University of Information Technology, Chengdu 610225, China；2.China-Serbia “the Belt and Road” Joint Laboratory on Environment and Energy, Chengdu 610225, China)., 2022,42(5)：2169～2178

The magnetic material chitosan@citric acid modified Fe3O4(CTS@Fe3O4-COOH) was synthesized by co-sedimentation method. The harvesting efficiency ofunder different conditions was investigated by single-factor and orthogonal tests. The combination with the structural property characterization of XRD, FT-IR and VSM, surface Zeta potential and the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory analysis to investigate the flocculation harvesting mechanism of CTS@Fe3O4-COOH on. The results implied that CTS@Fe3O4-COOH had high harvesting efficiency for, with an increase in harvesting efficiency of about 30% compared to unmodified. The single-factor test showed that the magnetic material dosage and pH had a greater effect on the harvesting efficiency of. The orthogonal test showed the best flocculation conditions of investigated variables were: 4.5g/L of CTS@Fe3O4-COOH, pH=4, rapid stirring at 500r/min in 3min, slow stirring 70r/min in 5min and the harvesting efficiency can reach 98.35%. Theoretical analyses such as DLVO suggested the harvesting mechanisms of CTS@Fe3O4-COOH onwere charge neutralization, electrostatic path, adsorption bridging and sweeping flocculation. The results provide data support for the practical application of CTS@Fe3O4-COOH harvesting of fixed flue gases from energy microalgae.

microalgae harvesting；CTS@Fe3O4-COOH；；magnetic flocculation

X703.5

A

1000-6923(2022)05-2169-10

曹惜霜(1997-),女,四川瀘州人,碩士研究生,主要從事水污染控制理論與技術(shù)研究.發(fā)表論文3篇.

2021-09-29

四川省科技廳國際合作項(xiàng)目(2019YFH0133)

* 責(zé)任作者, 教授, 178920302@qq.com