腐植酸對氧化鋅吸附Cu(Ⅱ)的影響

謝發(fā)之,謝志勇,李國蓮,李海斌,汪雪春,岳先名,李振宇

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腐植酸對氧化鋅吸附Cu(Ⅱ)的影響

謝發(fā)之1,2*,謝志勇1,3,李國蓮1,2,李海斌1,2,汪雪春1,3,岳先名1,3,李振宇1,3

(1.水污染控制與廢水資源化安徽重點實驗室,安徽合肥 230022；2.安徽建筑大學(xué)環(huán)境與能源工程學(xué)院,安徽合肥 230022；3.安徽建筑大學(xué)材料與化學(xué)工程學(xué)院,安徽合肥 230022)

為研究復(fù)雜三元體系中HA(Humic acid,腐殖酸)對氧化鋅吸附Cu(Ⅱ)的影響,探究HA對重金屬Cu(Ⅱ)在環(huán)境中遷移轉(zhuǎn)化的作用機理,通過改變HA的添加順序及添加量,系統(tǒng)研究了HA在不同環(huán)境條件下對氧化鋅吸附Cu(Ⅱ)的影響作用.結(jié)果表明,添加HA能顯著降低氧化鋅對Cu(Ⅱ)的吸附量,在pH為5.05時添加50mg/L的 HA, Cu(Ⅱ)的最大吸附量降低了61.74%;離子強度增大會促進(jìn)氧化鋅對Cu(Ⅱ)的吸附,在NaNO3濃度為1.0mol/L時最大吸附量為21.79mg/g;pH值為5.05時吸附量達(dá)到最大值,升溫有利于氧化鋅對Cu(Ⅱ)吸附,氧化鋅對Cu(Ⅱ)的吸附符合Freundlich模型.HA不同添加順序?qū)u(Ⅱ)的吸附量的影響表現(xiàn)為對照>后添加HA>同時添加HA>先添加HA.紅外光譜特征表明,HA和Cu(Ⅱ)在氧化鋅表面吸附位點形成競爭吸附,并且氧化鋅表面的羥基在吸附HA和Cu(Ⅱ)的過程中產(chǎn)生重要作用.

腐植酸；氧化鋅；Cu(Ⅱ)；吸附

腐殖酸(HA)是由動植物殘體在微生物以及地球化學(xué)作用下分解和合成的一類有機大分子聚合物,其廣泛存在于土壤和水體等環(huán)境介質(zhì)中[1].由于HA含有豐富的羧基和酚基等官能團,HA在水體或水體沉積物中能與多價陽離子發(fā)生相互作用,從而對多價陽離子在環(huán)境中的遷移和吸附產(chǎn)生重要影響[[2-3].由于氧化鋅在催化及電子工業(yè)等領(lǐng)域的廣泛使用,氧化鋅被不斷釋放到水環(huán)境系統(tǒng)中,嚴(yán)重危害水生生物及飲水安全[4].水環(huán)境系統(tǒng)中存在氧化鋅不僅影響氮磷的遷移轉(zhuǎn)化,而且影響金屬陽離子的遷移轉(zhuǎn)化[5-6].因此,研究HA與氧化鋅之間的相互作用以及HA存在下氧化鋅對重金屬陽離子吸附行為的作用機制,對全面了解環(huán)境中氧化鋅對重金屬離子遷移轉(zhuǎn)化行為的影響具有重要意義.

重金屬Cu(Ⅱ)作為典型的水體污染物,其過量存在具有毒性和致癌性.沉積物中金屬氧化物的吸附作用和水體中廣泛存在的溶解有機質(zhì)對Cu(Ⅱ)的絡(luò)合作用,會對Cu(Ⅱ)在水體中的毒性及其遷移轉(zhuǎn)化行為產(chǎn)生重要影響[7-8],溶解有機質(zhì)存在下Cu(Ⅱ)與金屬氧化物之間的作用機制尚不明確[9-10].已開展的研究主要集中在二元體系,如Bagheri等[11]研究了氧化鋅薄膜吸附水溶液中的Cu(Ⅱ),氧化鋅通過表面的羥基去質(zhì)化絡(luò)合Cu(Ⅱ),吸附等溫線符合Azizian-Volkov模型.Yang等[12]從滇池沉積物中逐步分離獲得4種HA,分別對Cu(Ⅱ)進(jìn)行吸附實驗,研究表明前期提取的HA因含有較多的羧基對Cu(Ⅱ)有較高的吸附容量.Bian等[13]研究了HA存在條件下,HA可以吸附在納米氧化鋅表面,從而納米氧化鋅在高離子強度的水溶液中的穩(wěn)定性得到提高.基于HA-ZnO、HA-Cu(Ⅱ)發(fā)生相互作用二元體系,進(jìn)一步研究揭示三元體系HA-ZnO-Cu(Ⅱ)相互作用規(guī)律有其必要性.

該研究通過改變HA和氧化鋅添加至Cu(Ⅱ)溶液中的順序,模擬不同環(huán)境條件下氧化鋅對Cu(Ⅱ)的吸附,系統(tǒng)研究了離子強度、溶液初始pH、不同(HA)、吸附溫度、初始〔Cu(Ⅱ)〕對氧化鋅吸附Cu(Ⅱ)的影響,以期為重金屬Cu(Ⅱ)在水體中的遷移轉(zhuǎn)化及其污染防治技術(shù)提供科學(xué)依據(jù).

1 材料與方法

1.1 實驗材料

商品氧化鋅由徐州試劑二廠生產(chǎn),分子量為81.38.HA購自SIGMA-ALDRICH公司.鹽酸、硝酸鈉均購自國藥集團上?；瘜W(xué)試劑公司,分析純.實驗用水為超純水.

1.2 實驗方法

將不同0〔Cu(Ⅱ)〕溶液25mL加入50mL離心管中,背景溶液[以(NaNO3)計]為0.1mol/L,調(diào)節(jié)pH后,先后加入氧化鋅0.02g和不同濃度的HA定容至50mL,(25±0.5) ℃置于振蕩器中振蕩吸附48h(根據(jù)吸附動力學(xué)試驗表明,48h可達(dá)吸附平衡),離心后測定上清液中ρ[Cu(Ⅱ)].ρ[Cu(Ⅱ)]通過WFX-1E2型原子吸收光光度計(北京瑞利分析儀器公司)使用標(biāo)準(zhǔn)曲線法測定,儀器條件:波長為324.7nm,燈電流為2.0mA,光譜通帶為0.4nm,空氣流量為6.5L/min,乙炔流量為2.5L/min,燃燒高度為8mm.

HA與氧化鋅添加順序為:先添加HA(先加HA后添加氧化鋅)、同時添加HA(HA與氧化鋅同時添加)和后添加HA(先添加氧化鋅至吸附平衡后添加HA),然后以不添加HA組(對照組)實驗作為對比.氧化鋅對Cu(Ⅱ)吸附量用(1)計算,并利用Langmuir模型(2)和Freundlich(3)模型擬合吸附過程[14-15].

式(1)中:e為平衡吸附量,mg/g;0、e為吸附前后〔Cu(Ⅱ)〕,mg/L;為溶液總體積,mL;為針鐵礦的用量,mg.(2)中e為吸附平衡濃度mg/L;m為單分子層的最大吸附量,g/g;K為Langmuir吸附常數(shù)(L/mg).(3)中e為吸附平衡濃度(mg/L);F(mg1-1/nL1/ng-1)為符合Freundlich常數(shù),為符合Freundlich的指數(shù).

2 結(jié)果與討論

2.1 氧化鋅的表征

該研究中所用商品氧化鋅的X射線衍射(XRD)圖譜如圖1所示,通過尖銳衍射峰及其他主要峰的2位置分析,其與標(biāo)準(zhǔn)圖譜(PDF#36- 1451)能較好吻合,確定該氧化鋅為六方紅鋅礦結(jié)構(gòu),晶格常數(shù)分別為a=3.249?,c= 5.206?.圖2為該氧化鋅的SEM圖,該氧化鋅團聚較為嚴(yán)重,顆粒大小不均勻,部分細(xì)小顆粒在60nm左右,其比表面積為24.6m2/g,表面Zeta電位為-19.8± 0.6mV.

2.2(HA)的影響

如圖3所示,在離子強度為0.1mol/L,0〔Cu(Ⅱ)〕為25mg/L,pH為5.05,溫度為(25±0.5)℃時,Cu(Ⅱ)的吸附量隨(HA)在0～75mg/L范圍內(nèi)增加而降低.HA含有豐富的羥基和羧基等官能團,大量的官能團通過靜電作用、離子交換和絡(luò)合作用增強對陽離子的吸附能力[16-17].氧化鋅吸附Cu(Ⅱ)時,由于HA與Cu(Ⅱ)既能對氧化鋅產(chǎn)生競爭吸附,同時由于HA能吸附Cu(Ⅱ),使得Cu(Ⅱ)在水溶液中部分被固定,因而被氧化鋅吸附Cu(Ⅱ)的量會降低,因此隨著HA濃度的增加,氧化鋅對Cu(Ⅱ)的吸附量降低[18].由于HA和Cu(Ⅱ)均能被氧化鋅吸附,HA與Cu(Ⅱ)競爭氧化鋅表面的吸附位點,不添加HA時不存在競爭吸附,因而吸附量最大[11,19].添加HA時,后添加的HA將競爭氧化鋅表面的吸附位點,已被吸附的Cu(Ⅱ)將會被部分替換下來,因而略比不添加HA時的吸附量低.而先添加HA時,HA先與Cu(Ⅱ)發(fā)生內(nèi)層絡(luò)合釋放氫離子,抑制氧化鋅表面的羥基去質(zhì)子化(≡ZnOH+Cu2+?≡ZnOCu+H+),氧化鋅吸附產(chǎn)生的絡(luò)合物及Cu(Ⅱ)被抑制,導(dǎo)致氧化鋅對Cu(Ⅱ)的吸附量最小[11,20].同時添加HA,相當(dāng)于腐植酸改性氧化鋅吸附Cu(Ⅱ),Cu(Ⅱ)競爭氧化鋅表面的HA,吸附量大小處在先添加HA與后添加HA的吸附量之間.因此,在該研究條件下HA的添加順序?qū)u(Ⅱ)吸附量的影響表現(xiàn)為對照(不添加HA)>后添加HA>同時添加HA>先添加HA.

2.3 離子強度的影響

如圖4所示,在pH為5.05,(HA)為50mg/L,0〔Cu(Ⅱ)〕為25mg/L,溫度為(25±0.5)℃,離子強度從0.1增至1.0mol/L時,對照組Cu(Ⅱ)的吸附量由14.12mg/g增至21.79mg/g.添加HA后,Cu(Ⅱ)的吸附量較對照組顯著降低,同時Cu(Ⅱ)的吸附量也隨離子強度增大而增大,在離子強度為1.0mol/L時達(dá)到相同吸附量21.79mg/g.未添加HA時,由于氧化鋅在水溶液中與Cu(Ⅱ)形成內(nèi)層絡(luò)合,因而Cu(Ⅱ)的吸附量隨離子強度增大而增大[21-22].由于氧化鋅吸附HA的量隨離子強度的增加而減小,則氧化鋅吸附HA以外層絡(luò)合為主[23].因外層絡(luò)合比內(nèi)層絡(luò)合更易受到離子強度的影響,添加HA時,Na+依靠靜電引力中和溶液中的負(fù)電荷,使HA因為失去部分水合分子而降低其穩(wěn)定性,同時由于HA的負(fù)電性減弱易發(fā)生團聚,導(dǎo)致HA分子結(jié)構(gòu)空間易阻礙與Cu(Ⅱ)絡(luò)合,因而離子強度增大抑制HA吸附Cu(Ⅱ)的能力.當(dāng)Na+大量存在時(即離子強度為1.0mol/L時),因為Na+能與HA中所能絡(luò)合Cu(Ⅱ)的官能團COOH-和OH-反應(yīng),離子強度增加會使HA絡(luò)合Cu(Ⅱ)的能力降低,在離子強度為1.0mol/L時HA絡(luò)合Cu(Ⅱ)的能力降低至極限,因此在此離子強度下,不同添加順序HA的吸附量與對照組相同[24-25].圖5為氧化鋅吸附Cu(Ⅱ)后在40℃條件下干燥獲得的樣品,根據(jù)圖5可知3383cm-1處為羥基吸收峰,HA在該處存在酚羥基和醇羥基吸收峰,而在1113cm-1附近有明顯的伸縮振動變化,即為氧化鋅吸附Cu(Ⅱ)后形成絡(luò)合物的峰,添加HA后該區(qū)域為脂肪族與羧酸等官能團中的C=O、C-N等鍵的伸縮振動峰,峰減弱表明了氧化鋅吸附HA.HA不同添加順序的紅外光譜很相似,表明HA在氧化鋅的表面以外層絡(luò)合為主[26-27].

2.4 pH的影響

如圖6所示,在離子強度為0.1mol/L(HA)為50mg/L,0〔Cu(Ⅱ)〕為25mg/L,溫度為(25±0.5)℃,對照組吸附量在pH值3.98~8.02范圍內(nèi)呈先增加后降低趨勢,在pH值為5.05時達(dá)到最大吸附量14.12mg/g.添加HA的吸附量顯著降低,其變化趨勢與對照組基本相同,最大吸附量均出現(xiàn)在pH值5.05時,此pH值下對照組的吸附量比添加HA時在pH值的最低吸附量高80.08%.由于在水溶液中Cu(Ⅱ)存在不同形態(tài),如圖7所示.在pH為5.05時,Cu(Ⅱ)主要以Cu2+和Cu(OH)+形式存在,而氧化鋅表面Zeta電位為-19.8±0.6mV,對帶正電的陽離子吸附能力強,因此在pH為5.05時吸附量最大.隨著pH值增加,Cu(Ⅱ)主要以不帶電及帶負(fù)電的離子存在,氧化鋅靜電吸引能力減弱導(dǎo)致吸附量降低[28].添加HA,隨pH增加帶負(fù)電荷的HA排斥帶負(fù)電荷的水合Cu(Ⅱ)離子,HA及Cu(Ⅱ)均難以被氧化鋅吸附,因此吸附量存在明顯降低.同時,氧化鋅在pH為5.05時,氧化鋅表面主要發(fā)生(ZnO(s)+2H+(aq)?Zn2+(aq)+ H2O)反應(yīng),如圖8所示.此時有利于對Cu2+和Cu(OH)+形式的吸附,而再降低或升高pH時,因Cu(Ⅱ)離子形態(tài)及氧化表面形成帶負(fù)電的離子均有對吸附量降低的影響[13,30].

2.5[Cu(Ⅱ)]的影響

如圖9所示,在離子強度為0.1mol/L,HA)為50mg/L,pH為5.05,溫度為(25±0.5)℃,0[Cu(Ⅱ)]在10~75mg/L范圍內(nèi),對照組Cu(Ⅱ)的吸附量隨0[Cu(Ⅱ)]的增加而增加,達(dá)到平衡時的吸附量為26.85mg/g.添加HA時,吸附量的變化趨勢與對照組變化趨勢相同,但吸附量較對照組顯著降低,降低最顯著的后添加HA組的平衡吸附量比對照組低61.74%.0[Cu(Ⅱ)]增加,氧化鋅的吸附位點逐漸達(dá)到飽和,吸附量呈現(xiàn)先增加后保持不變的趨勢.通過Langmuir和Freundlich方程擬合,結(jié)果如圖10、11所示.pH為5.05的條件下,對照組氧化鋅對Cu(Ⅱ)的吸附符合Langmuir模型,屬于單層吸附;而添加HA, 氧化鋅對Cu(Ⅱ)的吸附符合Freundlich模型,屬于表面非均勻的多層吸附,可見HA影響氧化鋅均勻性吸附Cu(Ⅱ)[30-31].同時添加HA最為符合Freundlich模型,相當(dāng)于HA改性氧化鋅導(dǎo)致氧化鋅表面不均勻性最強[32].而后添加HA,即HA在干擾氧化鋅吸附Cu(Ⅱ),影響氧化鋅表面均勻性比同時添加HA弱.先添加HA,HA吸附了Cu(Ⅱ)后,氧化鋅再對水溶液中Cu(Ⅱ)吸附,此時水溶液中Cu(Ⅱ)的組成比其他情況更為復(fù)雜,氧化鋅線性相關(guān)系數(shù)擬合程度更為傾向于Freundlich模型.因此,通過Langmuir和Freundlich方程擬合能揭示HA影響氧化鋅吸附Cu(Ⅱ)程度的強弱.

Fig.9 Effects of initial Cu(Ⅱ)concentration on the binding of Cu(Ⅱ)to Zinc Oxide

2.6 溫度的影響

如圖12所示,在離子強度為0.1mol/L,0[Cu(Ⅱ)]為25mg/L,pH為5.05,(HA)為50mg/L時,溫度從25℃升至45℃,對照組的Cu(Ⅱ)吸附量增大了24.46%,后添加HA、先添加HA和同時添加HA分別增加了72.78%、69.89%和41.73%.HA由于靜電庫侖力作用在氧化鋅表面緩慢形成有機薄膜,升溫使溶液中的Cu(Ⅱ)克服氧化鋅表面的液膜阻力增強,有利于Cu(Ⅱ)由微孔向內(nèi)部擴散,因而氧化鋅對Cu(Ⅱ)的吸附位點增多,有利于氧化鋅對Cu(Ⅱ)的吸附[33].而升溫會使得HA容易解離,減小HA對Cu(Ⅱ)的競爭吸附作用,因而升溫有利于氧化鋅對Cu(Ⅱ)吸附[34].對照組氧化鋅對Cu(Ⅱ)的吸附符合Langmuir模型,意味著升溫有利于吸附.添加HA,雖然吸附模型較為符合Freundlich模型,但升溫依然有利于氧化鋅對Cu(Ⅱ)吸附.添加HA,其吸附機理可能有氫鍵、偶極作用力、范德華力和化學(xué)鍵合等作用,吸附機理圖如圖13所示.

3 結(jié)論

3.10[Cu(Ⅱ)]在10~75mg/L范圍內(nèi),氧化鋅對Cu(Ⅱ)吸附隨0[Cu(Ⅱ)]增加而增大,對照組最大平衡吸附量為26.85mg/g,添加HA顯著降低了氧化鋅對Cu(Ⅱ)的吸附量.對照組及不同HA添加順序下,離子強度增大均促進(jìn)氧化鋅對Cu(Ⅱ)的吸附,在1.0mol/L時達(dá)到相同吸附量21.79mg/g.

3.2 pH值影響Cu(Ⅱ)吸附量變化,添加HA吸附量變化趨勢與對照組基本相同,最大吸附量均在5.05時,此pH值時對照組吸附量比添加HA時在pH值的最低吸附量高80.08%,偏酸性條件有利于氧化鋅對Cu(Ⅱ)吸附.HA與Cu(Ⅱ)競爭氧化鋅表面的吸附位點,增加(HA)會降低氧化鋅對Cu(Ⅱ)的吸附量,升溫均有利于對照組和添加了HA體系中氧化鋅對Cu(Ⅱ)的吸附.

3.3 HA的添加順序?qū)u(Ⅱ)吸附量的影響表現(xiàn)為對照組(不添加HA)>后添加HA>同時添加HA>先添加HA,水體中存在HA將降低了氧化鋅吸附Cu(Ⅱ)的能力,因此不利于氧化鋅在水體中固定Cu(Ⅱ)污染.

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Effects of humic acid on the adsorption of copper(Ⅱ) onto zinc oxide.

XIE Fa-zhi1,2*, XIE Zhi-yong1,3, LI Guo-lian1,2, LI Hai-bin1,2, WANG Xue-chun1,3, YUE Xian-ming1,3, LI Zhen-yu1,3

(1.Anhui Key Laboratory of Water Pollution Control and Wastewater Resource, Hefei 230601, China；2.School of Environment and Energy Engineering, Anhui Jianzhu University, Hefei 230601, China；3.School of Materials and Chemical Engineering, Anhui Jianzhu University, Hefei 230601, China)., 2017,37(8)：2970~2977

To study the influence of humic acid on the adsorption of Cu (II) by zinc oxide, the adsorption of Cu (II) on zinc oxide in different environment conditions were investigated, by changing the order of addition and dosage of HA. The mechanism of fate and transport of Cu (II) in the water environment was also proposed. The results showed that the adsorption Cu(II) on zinc oxide could be significantly reduced by adding HA, and that the Cu (II) maximum adsorption decreased 61.74% by adding 50mg/L HA in the pH of 5.05. The Cu (II) adsorption increased by increasing the ionic strength, the maximum adsorption was 21.79mg/g when the concentration of NaNO3was 1.0mol/L. When the pH value was 5.05 the adsorption reached the maximum; and temperature was in favor of Cu (II) adsorption on zinc oxide. Cu (II) adsorption on zinc oxide fitted the Freundlich model well. The adding orders of HA influence the Cu (II) adsorption as the following: non-adding > adding after > adding simultaneously > adding ahead. The characteristics of infrared spectra showed that HA and Cu (II) form competitive adsorption on the surface adsorption and surface hydroxyl of zinc oxide play an important role in adsorption of HA and Cu (II).

humic acid；zinc oxide；copper (Ⅱ)；adsorption

X703.1

A

1000-6923(2017)08-2970-08

謝發(fā)之(1976-),男,安徽定遠(yuǎn)人,教授,博士,主要從事環(huán)境污染控制方面的研究.發(fā)表論文50余篇.

2017-02-02

安徽省自然科學(xué)基金項目(1608085MB43);國家自然基金項目(21107001);安徽省教育廳自然科學(xué)研究重點項目(KJ2016A154);安徽省2014年高校優(yōu)秀青年人才支持計劃項目

* 責(zé)任作者, 教授, fzxie@ahjzu.edu.cn