利用一種新的合成物殼聚糖生物吸附劑去除Cr中英文翻譯資料

1、利用一種新的合成物殼聚糖生物吸附劑去除Cr()利用一種葡(萄)糖胺生物聚合物殼聚糖涂膜在氧化鋁陶瓷上制備新型殼聚糖復(fù)合生物吸附劑，由高溫?zé)崃呀?、孔性?jì)、電鏡掃描、X射線光電子能譜法測(cè)定其特征. 在25進(jìn)行間歇等溫吸附平衡和連續(xù)塔吸附試驗(yàn)以檢驗(yàn)它從鍍鉻設(shè)施廢水中去除六價(jià)鉻,并研究 pH值,硫酸鹽、氯離子對(duì)吸附的影響。鉻()飽和的生物吸附劑可以在0.1M 氫氧化鈉溶液再生。 對(duì)比目前的調(diào)查結(jié)果與文獻(xiàn)報(bào)道表明氧化鋁表面的殼聚糖具有更大的鉻(VI)吸附能力。 另外,實(shí)驗(yàn)平衡數(shù)據(jù)擬合Langmuir和Freundlich等溫吸附式和得出的等溫參數(shù)值, Langmuir模型所得最大的容量是153.85毫克

2、/克殼聚糖。概述 從采礦、電鍍?cè)O(shè)備、發(fā)電設(shè)備、電子器件制造單位和皮革廠排放的廢水中金屬離子濃度往往高于當(dāng)?shù)嘏欧艠?biāo)準(zhǔn),這些廢水中含有有毒重金屬如鉻、鎘、鉛、汞、鎳、銅等。在采礦、電鍍、工業(yè)加工、核燃料合成、軍事基地分局周圍的地下水含有害成分. 根據(jù)環(huán)保法規(guī),污水或水中含有重金屬在排放之前一律進(jìn)行處理?；瘜W(xué)沉淀,氧化/還原, 機(jī)械過(guò)濾、離子交換、膜分離、 碳吸附等各種處理方法廣泛應(yīng)用于去除廢水中的有毒重金屬。 近年來(lái)生物吸附被公認(rèn)為減少地表水和工業(yè)廢水金屬污染的一種有效方法。 生物吸附是指利用生物材料從溶液中去除金屬或非金屬單質(zhì),化合物和離子.Olin和Bailey等開(kāi)展了廣泛的資料研究,以找出潛

3、在的低成本吸附劑處理重金屬污染的水和重金屬?gòu)U水. 他們鑒別了12種潛在吸附劑去除鉛、鎘、銅、鋅、汞,其中殼聚糖具有最高的金屬離子的吸附容量。 殼聚糖是從蝦、螃蟹、某些真菌、甲殼類生物等萃取得到的甲殼素通過(guò)脫乙酰作用獲得. 殼聚糖在自然界中不僅豐富廉價(jià),同時(shí)它又是一個(gè)良好的重金屬吸附劑,殼聚糖可以螯合超過(guò)甲殼素5-6倍量的金屬，這是因?yàn)樵跉ぞ厶抢镆蛎撘阴Ｗ饔么嬖谧杂砂被?。研究人員先后多次企圖改進(jìn)殼聚糖以使其更容易傳質(zhì)和釋放活性官能團(tuán)來(lái)增強(qiáng)吸附能力。嫁接功能團(tuán)到原殼聚糖主鏈來(lái)進(jìn)一步提高其吸附性能。Kawamura, Hsein 和 Rorrer等研究了多孔殼聚糖和化學(xué)交聯(lián)殼聚糖粒對(duì)重金屬的吸附,與

4、天然殼聚糖相比，多孔殼聚糖顆粒、化學(xué)交聯(lián)殼聚糖粒子、 冠醚殼聚糖、浸微殼聚糖, 金屬離子絡(luò)合殼聚糖樹脂明顯提高了吸附能力。 volesky,Holan ,Waseand 和Forster 討論了幾種生物吸附劑對(duì)金屬包括放射性的物質(zhì)鈾、釷等的吸附能力,認(rèn)為這些生物吸附劑應(yīng)對(duì)材料進(jìn)一步改進(jìn)和向商品化發(fā)展。它們的天然形式是軟性的和其水溶液有一種結(jié)塊或形成凝膠的傾向. 此外，自然形成的活性官能團(tuán)不易速效吸附，而在工藝流程設(shè)計(jì)過(guò)程中該基團(tuán)對(duì)傳遞金屬污染物起著十分重要的作用，它還提供應(yīng)用處理中所需的物質(zhì)支持和增大金屬結(jié)合基團(tuán)接納金屬的可達(dá)性。 因此,本次研究試圖制備一種將殼聚糖涂在氧化鋁表面的生物吸附劑。

5、 這種由氧化鋁為載體的生物吸附劑由高溫?zé)崃呀?、孔性?jì)、電子顯微鏡掃描、X射線光電子能譜測(cè)定其特征 。在Brunauer-Emmett-Teller (BET)吸附等溫線基礎(chǔ)上，它的表面積、孔徑 、孔徑分布由氮孔率決定。 這項(xiàng)研究的目的是制備一種殼聚糖合成物，了解其吸附特征，檢驗(yàn)合成物和天然樣本的去除鉻()的能力以及在間歇和連續(xù)模型中的等溫吸附平衡時(shí)的吸附容量。 另外,還應(yīng)獲得與Langmuir和Freundlich等溫吸附式擬合的實(shí)驗(yàn)平衡數(shù)據(jù)和等溫參數(shù)值，并用同樣試劑進(jìn)行塔吸附試驗(yàn)，以及pH值對(duì)鉻()吸附的影響程度，也將研究殼聚糖生物吸附劑在0.1M氫氧化鈉溶液中的再生能力。實(shí)驗(yàn)內(nèi)容化學(xué)樣品

6、來(lái)自Aldrich化工股份有限公司(Milwaukee, WI)的重鉻酸鉀、活性氧化鋁、殼聚糖、1,5二苯卡巴，其中活性氧化鋁是標(biāo)準(zhǔn)級(jí)150目brockman I。由Fisher 化工(Fair Lawn, NJ)生產(chǎn)的氯化鉀、氫氧化鈉。來(lái)自EM 科學(xué)(Gibbstown, NJ)的硫酸鉀。所有的鹽類都是ACS(美國(guó)化學(xué)學(xué)會(huì))認(rèn)證等級(jí)或更好。 所有溶液由ASTM(美國(guó)材料試驗(yàn)學(xué)會(huì))的去離子水制備(18 M-H2O grade Barnstead Nanopure)。生物吸附劑準(zhǔn)備 由殼聚糖凝膠覆蓋陶瓷的生物吸附劑的制備過(guò)程如下:將150目氧化鋁陶瓷在110烘箱干燥4小時(shí)后在室溫下用草酸攪拌混合

7、4小時(shí)進(jìn)行表面涂層, 然后從酸中過(guò)濾出的氧化鋁用去離子水洗兩次，再在70真空烘箱中干燥24小時(shí)，將約50克中等分子量殼聚糖徐徐加入1000毫升質(zhì)量分?jǐn)?shù)為10%草酸溶液并攪拌。 加熱至40-50使其容易混合形成酸和殼聚糖的粘性混合物(凝膠)。取大約500毫升的殼聚糖凝膠用水稀釋2倍并加熱至40-50，將約500克的酸處理后的氧化鋁緩慢加入稀釋凝膠并攪拌約36小時(shí)之后靜置澄清。再用Whatman41濾紙?jiān)谡婵諚l件下過(guò)濾出上清液，將得到的合成物用去離子水洗兩次,然后在55真空烘箱中干燥24小時(shí), 最后在涂過(guò)一層生物吸附劑的氧化鋁上進(jìn)行重復(fù)涂層處理以增加殼聚糖的負(fù)載量，大約用時(shí)24小時(shí)。合成過(guò)程中過(guò)量

8、的草酸用氫氧化鈉溶液中和處理. 再將兩次涂膜的混合物用Whatman41濾紙過(guò)濾,并用2500毫升的去離子水洗，及過(guò)濾之后在55真空烘箱干燥48小時(shí)左右，轉(zhuǎn)移到玻璃瓶后存放在干燥器內(nèi)。 生物吸附劑的特征 生物吸附劑特性包括:(1)熱解,(2)孔徑分析,(3)電子顯微鏡掃描,(4)XPS分析。(1)熱裂解技術(shù)測(cè)定氧化鋁負(fù)載的殼聚糖。 測(cè)量生物吸附劑在裂解中減少的重量得到在氧化鋁上負(fù)載的殼聚糖的量。將準(zhǔn)確稱量后的干燥生物吸附劑放入瓷瓶?jī)?nèi)放入一個(gè)750 馬弗爐內(nèi)6小時(shí)，然后在干燥空氣中冷卻,稱量得到生物吸附劑減輕的重量。用空瓷瓶、純氧化鋁、酸處理氧化鋁、純殼聚糖和生物吸附劑做各進(jìn)行三次的對(duì)照實(shí)驗(yàn)。(

9、2)由孔性計(jì)確定的表面積和孔徑。 使用一個(gè)微型的BET 測(cè)定儀在零下196下超純度的氮?dú)鈼l件下測(cè)定生物吸附劑的表面積、孔容和孔徑，它們的平均值分別是125.24 sq.m/g、0.1775cm3/g、71.125Å。 (3)電子顯微鏡掃描， 以電子顯微鏡研究表面形態(tài)。殼聚糖生物吸附劑的電子顯微鏡掃描 (SEMs)由環(huán)境掃描電子顯微鏡(XL30-ESEM-FEG, FEI 公司, Hillsboro, OR,U.S.A.)獲得，見(jiàn)圖1(a)、(b)。圖1 放大100倍（a）和800倍（b）的殼聚糖生物吸附劑的掃描電子顯微鏡像 (4)X射線光電子能譜 殼聚糖生物吸附劑XPS譜在PHI模型

10、5400AXIS Ultra Kratos 分析儀(Manchester,U.K.)得出，列于圖2。 圖3是在鉻液反應(yīng)后吸附劑的XPS譜,顯示了吸附鉻的2個(gè)高峰。圖2圖3等溫吸附平衡. 研究適量重鉻酸鉀溶解在去離子水得到的Cr()溶液中的間歇等溫吸附平衡。用原子吸收光譜法和紫外光譜儀測(cè)定溶液的金屬濃度, 在25±0.5進(jìn)行大量的100至500毫克殼聚糖生物吸附劑的等溫平衡研究, pH4.0的鉻溶液(50毫升)與殼聚糖生物吸附劑在200RPM的攪拌水浴24小時(shí)后達(dá)到平衡 ,之后從溶液中用Whatman41濾紙過(guò)濾出該生物吸附劑，分析濾液中的金屬含量。 每單位生物吸附劑的金屬吸附量qe(

11、毫克) ,由下式式得出 : 其中Ci和Ce分別是初始和平衡時(shí)的濃度（毫克/升）, M是生物吸附劑的干重（克）,V是溶液體積（升）.分別在不同的pH值下得出pH在平衡吸附試驗(yàn)中對(duì)吸附過(guò)程的影響，以及檢驗(yàn)負(fù)離子即硫酸鹽、氯化物鉻()吸附的影響. 實(shí)驗(yàn)中硫酸鹽、氯化物濃度控制在1毫摩爾水平。 塔吸附試驗(yàn) 在內(nèi)徑約1厘米長(zhǎng)30厘米床容為30cm3的玻璃柱內(nèi)進(jìn)行動(dòng)態(tài)吸附試驗(yàn)，實(shí)驗(yàn)中柱完全泡在用一個(gè)恒溫neslab 和masterflex泵的25±0.5循環(huán)水浴中，塔底采用孔隙100微米聚乙烯濾盤托住吸附劑。當(dāng)柱充滿干燥吸附劑時(shí)震蕩以使空隙和空氣量減到最少，將塔溶液作適當(dāng)稀釋后用分光光度計(jì)測(cè)定不

12、同時(shí)期的濃度，當(dāng)塔中鉻飽和后泵入空氣清洗剩余水溶液以使其在0.1M氫氧化鈉溶液再生，并在解吸過(guò)程的第5、10、20、30分鐘進(jìn)行采樣分析。再生后，用去離子水清洗塔以備以后吸附時(shí)使用。分析過(guò)程 在酸性介質(zhì)中測(cè)量鉻(六)與1,5-二苯卡巴反應(yīng)形成紅紫色化合物可以測(cè)定六價(jià)鉻測(cè)定。通過(guò)紫外可見(jiàn)分光光度計(jì)測(cè)量得到該化合物的最大吸光度在540納米，用重鉻酸鉀標(biāo)準(zhǔn)溶液標(biāo)定六價(jià)鉻，為了顯色將等溫吸附樣品用0.2摩爾硫酸調(diào)整pH到1.0±0.3，則樣品濃度根據(jù)鉻(六)標(biāo)準(zhǔn)溶液的吸光度與濃度關(guān)系曲線得到。精確研究顯示，分析程序的再生性優(yōu)于1毫克/升。結(jié)論涂膜過(guò)程制備了一種穩(wěn)定的、wheatish彩色的顆

13、粒生物吸附劑合成物。 方案一生物吸附劑的性質(zhì) 前面講述了通過(guò)高溫?zé)崃呀獾姆椒y(cè)定覆蓋在150目氧化鋁上的草酸和殼聚糖的平均量。結(jié)果表明，純氧化鋁損失約2.1%，草酸處理過(guò)的凈重減少4.5%，單一殼聚糖膜氧化鋁凈損失7.8%，而二次殼聚糖膜的生物吸附劑凈重減少21.1%，純殼聚糖在750裂解后殘留重量0.7%。殼聚糖從螃蟹殼中由酸堿提取甲殼素通過(guò)作用獲得的，所以該殘留物可能是少量的碳酸鈣混入甲殼素而產(chǎn)生的，但殘留量很少，沒(méi)必要為此在殼聚糖凈重上進(jìn)行修正。 草酸這個(gè)羧酸在氧化鋁與殼聚糖之間起橋梁作用. 正如方案1中，一個(gè)羧酸基與氧化鋁形成較強(qiáng)的螯合連環(huán)酯,而另外一個(gè)與殼聚糖的-NH3+形成離子(或

14、電子)鍵。 生物吸附劑中的草酸還可與-OH、 -CH2OH或者-NH2形成的氫鍵。圖1（a）（b）的殼聚糖氧化鋁的掃描電子顯微鏡像(SEMS)表明顆粒的平均粒徑為100-150微米,且合成物顆粒一般為球形。 一些顆粒由單個(gè)粒子聚集成團(tuán)的，生物吸附劑的細(xì)孔面積僅為3.3m2/g，而總表面積達(dá)到了105.2 m2/g，這表明吸附劑相對(duì)來(lái)說(shuō)是無(wú)孔的。在圖2的XPS光譜中表明在結(jié)合能為289 eV (C 1s)、535 eV (O 1s)、402 eV (N 1s), 和78 eV (Al 2p)時(shí)碳、氧、氮、和鋁為表面觀測(cè)到的主要元素。在此結(jié)合能的基礎(chǔ)上，殼聚糖的一半表面可以用-CH2OH、 -CO

15、和-NH2鑒別。在圖3中展示了在鉻溶液中暴光后的鉻的光譜，它表明鉻已部分地（約67%）轉(zhuǎn)化為鉻(III)，此結(jié)果與Dambies等觀測(cè)結(jié)果一致。平衡等溫線 在25°C和pH為4條件下鉻(六)的等溫吸附平衡的結(jié)果如圖4。圖4等溫先表明增大吸附質(zhì)的平衡濃度可以促進(jìn)吸附。生物吸附劑的吸附容量為153.8毫克鉻(六)/克殼聚糖。有資料報(bào)道，每單位質(zhì)量的天然殼聚糖吸附劑吸附鉻(六)的最大值為27毫克，Ni2+印記殼聚糖樹脂為51毫克，而化學(xué)交聯(lián)和非交聯(lián)殼聚糖分別為78毫克和50毫克，這里報(bào)道的兩次涂膜的生物吸附劑大大好于其他物質(zhì)可以看出：生物吸附劑殼聚糖合成物比天然殼聚糖有更大的吸附能力，即涂

16、膜過(guò)程促進(jìn)了殼聚糖的吸附能力，這可能是因?yàn)樵龃罅吮砻娣e和促進(jìn)了鉻離子向殼聚糖結(jié)合部位的傳遞。圖5表明pH值對(duì)生物吸附劑吸附鉻(六)的影響。圖5從圖中看出，低pH有利于吸附而增大pH值時(shí)吸附作用降低，dantas和schmuhl等也得到了類似的結(jié)果。鉻(六)以幾種穩(wěn)定的形式存在。如Cr2O72-、HCr2O7-、.和Cr2O72-,HCr2O7-, HCrO4-,和CrO42-,它的存在形式主要取決于鉻離子的濃度和溶液的pH值.在較低的pH值時(shí),吸附劑由于氨基而帶正電,而吸附質(zhì)中的鉻酸鹽離子以陰離子形式存在從而形成吸附劑與吸附質(zhì)的靜電吸引。因此,在較低的pH值時(shí)可增大吸附.若增大溶液的pH值則吸

17、附劑會(huì)受到非質(zhì)子化和吸附容量的降低.當(dāng)超過(guò)某一pH值,僅僅吸附過(guò)程回影響從水介質(zhì)中去除鉻()。因此所有數(shù)據(jù)都是在pH值為4.0時(shí)得到的。在圖6中表示氯化物和硫酸鹽以及兩者的交互作用對(duì)鉻()吸附作用的影響。圖6陰離子也會(huì)對(duì)生物吸附劑吸附Cr2O72-有輕微的抑制,可以預(yù)計(jì),單價(jià)氯離子對(duì)吸附的抑制會(huì)小于二價(jià)的硫酸根離子,但氯化物與硫酸鹽兩者的抑制作用并未出現(xiàn)疊加.在別的文獻(xiàn)中也有報(bào)道,一般與陰離子競(jìng)爭(zhēng)表面結(jié)合部位的吸附都有類似的抑制。Gao等人提出殼聚糖像氧離子或氯離子在樣品溶液中餓離子交換機(jī)制一樣定量地吸附某些金屬,這意味著交互作用發(fā)生在殼聚糖的氨基功能團(tuán)與Cr2O7-之間,而且這種交互作用主要

18、是靜電引力.Fu等證實(shí)在紅外線與紫外線光譜之間存在靜電引力.XPS研究提供了涉及物質(zhì)吸附部位的認(rèn)別,并發(fā)現(xiàn)鉻(六)吸附發(fā)生在高分子物質(zhì)的胺官能團(tuán)上,如方案2所示:雖然離子(或電子)吸引是吸附劑與吸附質(zhì)的主要因素,但其他因素在低和高pH值的條件下會(huì)對(duì)吸附產(chǎn)生重要影響.例如在低和高pH值時(shí)就可能出現(xiàn)金屬吸附質(zhì)或與殼聚糖羥基和羧基的羥基化后的吸附質(zhì)進(jìn)行氫鍵結(jié)合.在低和高的pH值時(shí)鉻存在其他不同形式,而且這些形式歸因于不同pH值時(shí)的吸附曲線,所以殼聚糖生物吸附劑對(duì)鉻的總吸收量取決于:1 離子吸引力,2 氫鍵結(jié)合,3 較弱的范德華力。 Langmuir和Freundlich模型 Langmuir和Fre

19、undlich模型是描述液相與固相之間的吸附組成的最簡(jiǎn)單和最常用的等溫線. Langmuir模型假定單層吸附,而Freundlich模式是經(jīng)驗(yàn)公式.對(duì)數(shù)據(jù)進(jìn)行分析得到Langmuir和Freundlich參數(shù), Langmuir模型的數(shù)學(xué)公式是: 式中Ce是溶液中吸附劑的平衡濃度(mg/l),qe是平衡時(shí)吸附劑的吸附量(mg/l),Q是飽和吸附量(mg/l),b是吸附系數(shù),有Langmuir模型對(duì)平衡濃度和吸附量作直線得出參數(shù)Q和b.在坐標(biāo)紙上的Ce/qe對(duì)Ce的直線表明:殼聚糖生物吸附劑對(duì)鉻()的吸附符合Langmuir模型,從實(shí)驗(yàn)中推斷出的Q和b的值分別為153.85mg/L和0.023L

20、/mg,相關(guān)的R2的值為0.9896. Freundlich模型表示為:式中K和1/n是Freundlich等溫常數(shù),從模型中得到它們分別為0.9565和1.4047,其相關(guān)的R2值為0.9972。塔吸附研究 圖7表示的是在pH值為4.0和25條件下生物吸附劑頭兩個(gè)周期從綜合廢水中吸附鉻(六)的實(shí)驗(yàn)曲線。圖7:從圖中明顯可以看出當(dāng)開(kāi)始濃度(C1)約100mg/L是床容數(shù)在40以下都無(wú)鉻流出,當(dāng)床容數(shù)大于40時(shí),塔出水濃度逐漸上升,直到約200床容是達(dá)到進(jìn)水濃度.相對(duì)于進(jìn)水濃度增長(zhǎng)減慢的出水濃度表明吸附的動(dòng)力減少了,當(dāng)生物吸附劑鉻飽和時(shí),泵入空氣排出塔中溶液.吸附塔在流量2.6ml/min,0.

21、1M的NaOH溶液中可以再生,其解吸曲線如圖8所示。圖8:在NaOH溶液中,解吸的最大值發(fā)生在第5床容,而且在底20床容時(shí)完成再生,再生床用于之后的鉻的吸附,它的第二周期的曲線如圖7。對(duì)周期一和周期二比較發(fā)現(xiàn),生物吸附劑對(duì)廢水中的鉻(六)的吸附容量并未減少.在pH>10時(shí),殼聚糖的鉻(六)解吸主要是由于殼聚糖帶正電的胺基團(tuán).圖9表示的鍍鉻設(shè)施廢水的鉻(六)吸附曲線。圖9此廢水中包含有鐵(13mg/l),鎘(0.0065mg/l),鉛(48mg/l),硫酸(69mg/l),硝酸鹽(11mg/l),氰化物(0.32mg/l),以及磷酸鹽(17mg/l)。比較綜合出水的結(jié)果,廢水開(kāi)始稀釋至鉻約

22、100ppm并調(diào)整pH為4.0,則用周期一和周期而分別代表純生物吸附劑和再生生物吸附劑的吸附曲線如圖10。圖10相應(yīng)的未稀釋水樣(C1=1253ppm,pH=2.0)的吸收和解吸曲線見(jiàn)圖11和圖12。圖11圖12可以看出塔出水(C0)直到第15床容都無(wú)鉻的出現(xiàn),然后隨初始濃度緩慢增大濃度直到第45床容,它的最大解吸量發(fā)生在第3床容.在高初始濃度和低pH值時(shí)生物吸附劑顯示了更大的吸附能力。綜上所述,涂膜過(guò)程改進(jìn)了殼聚糖對(duì)六價(jià)鉻的吸附能力,所以應(yīng)更多地活化生物吸附劑合成物的活性部位,而塔的吸附解吸研究表明新研究合成的殼聚糖生物吸附劑可用于去除工業(yè)廢水中的鉻()。感謝作者感謝來(lái)自Dr.Richard

23、 Haas和Grant DEFG02-91-ER45439美國(guó)能源部部分資助的Illionois大學(xué)材料精細(xì)分析中心提供的X-射線光電子能譜分析,同時(shí)感謝Scott J.Robinson,成像技術(shù)集團(tuán),Backman尖端科技研究所,以及Illionois大學(xué)掃描電子顯微鏡像的幫助。參考文獻(xiàn)(略) 原文Removal of Hexavalent Chromium from Wastewater Using a New Composite Chitosan BiosorbentU.S. Army Construction Engineering Research Laboratories,Cham

24、paign, Illinois 61826-9005, and Illinois Waste Management and Research Center, Illinois Department of Natural Resources, University of Illinois at Urbanas Champaign, Champaign, Illinois 61820A new composite chitosan biosorbent was prepared by coating chitosan, a lucosamine biopolymer, onto ceramic a

25、lumina. The composite bioadsorbent was characterized by high-temperature pyrolysis, porosimetry, scanning electron microscopy, and X-ray photoelectron spectroscopy.Batch isothermal equilibrium and continuous column dsorption experiments were conducted at 25 °C to evaluate the biosorbent for the

26、 removal of hexavalent chromium from synthetic as well as field samples obtained from chrome plating facilities. The effect of pH, sulfate, and chloride ion on adsorption was also investigated. The biosorbent loaded with Cr(VI) was regenerated using 0.1 M sodium hydroxide solution. A comparison of t

27、he results of the present investigation with those reported in the literature showed that chitosan coated on alumina exhibits greater adsorption capacity for chromium(VI). Further, experimental equilibrium data were fitted to Langmuir and Freundlich adsorption isotherms, and values of the parameters

28、 of the isotherms are reported. The ultimate capacity obtained from the Langmuir model is 153.85 mg/g chitosan.IntroductionProcess waste streams from mining operations, metal-plating facilities, power generation facilities, electronic device manufacturing units, and tanneries often contain metal ion

29、s at concentrations above local discharge limits. These waste streams contain toxic heavy metals such as chromium,cadmium, lead, mercury, nickel, and copper. Groundwater around many mining, plating, and processing industries,nuclear fuel complexes, and military bases often gets contaminated with haz

30、ardous components. To meet environmental regulations, effluents or water contaminated with heavy metals must be treated before discharge. Chemical precipitation, oxidation/reduction, mechanical filtration, ion exchange, membrane separation, and carbon adsorption are among the variety of treatment pr

31、ocesses widely used for the removal of toxic heavy metals from the waste streams.In recent years biosorption has been recognized as an effective method of reduction of metal contamination in surface water and in industrial effluents (1).Biosorption is defined as the removal of metal or metalloid spe

32、cies, compounds, and particulates from solution by biological material (2). Olin et al. (3) and Bailey et al. (4) conducted an extensive literature search to identify low cost sorbents with potential for treatment of heavy metal contaminated water and waste streams. They identified 12 potential sorb

33、ents for lead, cadmium, copper, zinc, and mercury. Among the sorbents identified, chitosan has the highest sorption capacity for metal ions (5).Chitosan is obtained by deacetylation of chitin, which is extracted from shrimp, crab, some fungi, and other crustaceans.Chitosan is not only inexpensive an

34、d abundant in nature, but it also is a good adsorbent for heavy metals.Chitosan chelates five to six times greater amounts of metals than chitin. This is attributed to the free amino groups exposed in chitosan because of deacetylation of chitin (6).Several investigators have attempted to modify chit

35、osan to facilitate mass transfer and to expose the active binding sites to enhance the adsorption capacity. Grafting specific functional groups onto native chitosan backbone allows its sorption properties to be enhanced (7). Kawamura et al. (8),Rorrer et al. (9), and Hsein and Rorrer (10) evaluated

36、the sorption of heavy metals on the porous chitosan beads and chemically cross-linked beads of chitosan. Chitosan azacrown ethers (11, 12), chitosan impregnated with microemulsions (13), and chitosan resins imprinted with metal ions (14) showed remarkable increase in adsorption capacity compared to

37、an untreated sample.Volesky and Holan (1) andWaseand Forster (15) discussed several biosorbents and their metal binding capacity including that for radioactive species such as uranium and thorium.It has also been recognized that these biosorbents need further modificationanddevelopment for commercia

38、lization.Biosorbents, in their natural form, are soft and have a tendency in aqueous solutions to agglomerate or to form a gel. In addition, the active binding sites are not readily available for sorption in their natural form. Transport of the metal contaminants to the binding sites plays a very im

39、portant role in process design. It was also necessary to provide physical support and increase the accessibility of the metal binding sites for process applications. Hence, an attempt was made in the present investigation to prepare a biosorbent by coating chitosan on alumina.An alumina supported bi

40、osorbent is characterized in this paper by high-temperature pyrolysis, scanning electron microscopy, and X-ray photoelectron spectroscopy. The surface area, pore diameter, and pore diameter distribution are determined with the nitrogen porosimeter on the basis of Brunauer-Emmett-Teller (BET) adsorpt

41、ion isotherm.The objectives of this study were to prepare a composite chitosan biosorbent, to characterize the sorbent, and to evaluate the removal of hexavalent chromium from synthetic as well as field samples. The adsorption capacity of the biosorbent was evaluated by studying the equilibrium adso

42、rption isotherms of Cr(VI) in batch and flow modes.Further, the equilibrium data were fitted to Langmuir and Freundlich adsorption isotherms, and the values of parameters of the isotherms were obtained. Column adsorption experiments are also performed with a field sample. In addition, the effect of

43、pH on the extent of adsorption of Cr(VI) on the biosorbent was examined. Regenerability of the composite biosorbent using 0.1 M sodium hydroxide also was examined.Experimental Section Chemicals. Potassium dichromate, activated alumina, chitosan,and 1,5-dipheny- lcarbazide were procured from Aldrich

44、Chemical Co. (Milwaukee, WI). The activated alumina was Brockman I, standard grade, 150 mesh. Potassium chloride and sodium hydroxide were obtained from Fisher Chemicals (Fair Lawn, NJ). Potassium sulfate was obtained from EM Science (Gibbstown, NJ). All salts were ACS certified grade or better. All

45、 solutions were prepared with ASTM type I deionized water (18 M¿-H2O grade Barnstead Nanopure).Preparation of Biosorbent. Composite chitosan biosorbent was prepared by coating the ceramic substrate with chitosan gel as follows. Ceramic alumina 150 mesh was dried in oven for 4 h at 110 °C.

46、The dried alumina was stirred with oxalic acid for 4 h at room temperature to coat the surface.The alumina was filtered from the acid, washed twice with DI water, and dried in an oven at 70 °C under vacuum for 24 h. About 50 g of medium molecular weight chitosan was slowly added to 1000 mL of 1

47、0 wt%oxalic acid solution with stirring. The acid and chitosan form a viscous mixture (gel),which must be heated to 40-50 °C to facilitate mixing. Approximately 500 mL of the chitosan gel was diluted 2-fold with water and heated to 40-50 °C. About 500 g of the acidtreated aluminawasslowly

48、added to the diluted gelandstirred for about 36 h. The contents were allowed to settle, and the clear liquid was filtered out under vacuum with Whatman 41 filter paper. The composite biosorbent was washed twicewith DI water and dried in the oven at 55 °C under vacuum for 24 h. The coating proce

49、ss was then repeated on the oncecoated biosorbent to increase loading of chitosan on the alumina. Twenty-four h were used in the second coating process. Excess oxalic acid in the composite biosorbent was neutralized by treatment with aqueous NaOH. The mixture was then filtered with Whatman 41 filter

50、 paper, washed with _2500 mL of DI water, and filtered. The twice-coated biosorbent was then dried in the oven under vacuum at 55 °C for about 48 h and transferred to a glass bottle for storage in a desiccator.Characterization of the Biosorbent. Characterization of the composite biosorbent incl

51、uded the following: (a) pyrolysis,(b) porosimetry, (c) scanning electron microscopy,and (d) XPS analysis.(a) Determination of Chitosan Loading on Alumina by Pyrolysis Technique. The amount of chitosan coated on the alumina was obtained by measuring the weight loss of biosorbent from pyrolysis. Dried

52、 biosorbent was accurately weighed into a ceramic boat and placed in a muffle furnace.The biosorbent was muffled for 6 h at 750 °C. Afterward the oven was cooled in dry air, and weight loss of the biosorbent was obtained. Control experiments with empty boat, pure alumina, acid-treated alumina,

53、pure chitosan, and biosorbent were also carried out. All the experiments were conducted in triplicate.(b) Determination of Surface Area and Pore Diameter by Porosimetry. Surface area, pore volume, and pore diameter of the composite biosorbent were determined witha Micromeritics BET instrument by mea

54、ns of adsorption of ultra purity nitrogen at -196 °C. Average values of these properties are 125.24 sq.m/g, 0.1775 cm3/g, and 71.125 Å respectively.(c) Scanning Electron Microscopy. Surface morphology was studied with an electron microscope. The scanning electron micrographs (SEMs) of comp

55、osite chitosan biosorbent,obtained with an Environmental Scanning Electron Microscope (XL30-ESEM-FEG, FEI Company, Hillsboro, OR,U.S.A.), are presented in Figure 1(a),(b).(d) X-ray Photoelectron Spectroscopy An XPS spectrum of the composite chitosan biosorbent, obtained on a PHI model 5400AXIS Ultra

56、 Kratos Analytical instrument (Manchester,U.K.), is shown in Figure 2. Figure 3 is an XPS spectrum of the sorbent after exposure to chromium solution. Figure 3 shows the chromium 2p peaks.Equilibrium Adsorption Isotherms. Batch equilibrium adsorption isotherm studies were conducted with aqueous solu

57、tions of Cr(VI) prepared by dissolving appropriate amounts of potassium dichromate in deionized water. The concentrations of the prepared metal solutions were verified using atomic absorption spectroscopy and a UV-Vis spectrometer.Equilibrium isotherm studies were conducted at 25 ( 0.5 °C with

58、the mass of composite biosorbent varied from 100 to 500 mg. Chromium solutions (50 mL) at pH 4.0 were allowed to equilibrate with the composite biosorbent for 24 h in an oscillating water bath agitated at 200 rpm. After equilibration, the biosorbent was filtered from the solution (Whatman 41 filter

59、paper), and the filtrate was analyzed for metals.The amount of the metal adsorbed (mg) per unit mass of biosorbent, qe, was obtained by using the equationwhere Ci and Ce are initial and equilibrium concentrations in mg/L, M is the dry mass of biosorbent in grams, and V is volume of solution in liters. Equilibrium adsorption experiments were conducted at various pHs to evaluate the pH profile of the