利用一种新的合成物壳聚糖生物吸附剂去除Cr中英文翻译资料.doc

利用一种新的合成物壳聚糖生物吸附剂去除Cr()利用一种葡(萄)糖胺生物聚合物壳聚糖涂膜在氧化铝陶瓷上制备新型壳聚糖复合生物吸附剂，由高温热裂解、孔性计、电镜扫描、X射线光电子能谱法测定其特征. 在25进行间歇等温吸附平衡和连续塔吸附试验以检验它从镀铬设施废水中去除六价铬,并研究 pH值,硫酸盐、氯离子对吸附的影响。铬()饱和的生物吸附剂可以在0.1M 氢氧化钠溶液再生。 对比目前的调查结果与文献报道表明氧化铝表面的壳聚糖具有更大的铬(VI)吸附能力。 另外,实验平衡数据拟合Langmuir和Freundlich等温吸附式和得出的等温参数值, Langmuir模型所得最大的容量是153.85毫克/克壳聚糖。概述 从采矿、电镀设备、发电设备、电子器件制造单位和皮革厂排放的废水中金属离子浓度往往高于当地排放标准,这些废水中含有有毒重金属如铬、镉、铅、汞、镍、铜等。在采矿、电镀、工业加工、核燃料合成、军事基地分局周围的地下水含有害成分. 根据环保法规,污水或水中含有重金属在排放之前一律进行处理。化学沉淀,氧化/还原, 机械过滤、离子交换、膜分离、 碳吸附等各种处理方法广泛应用于去除废水中的有毒重金属。 近年来生物吸附被公认为减少地表水和工业废水金属污染的一种有效方法。 生物吸附是指利用生物材料从溶液中去除金属或非金属单质,化合物和离子.Olin和Bailey等开展了广泛的资料研究,以找出潜在的低成本吸附剂处理重金属污染的水和重金属废水. 他们鉴别了12种潜在吸附剂去除铅、镉、铜、锌、汞,其中壳聚糖具有最高的金属离子的吸附容量。 壳聚糖是从虾、螃蟹、某些真菌、甲壳类生物等萃取得到的甲壳素通过脱乙酰作用获得. 壳聚糖在自然界中不仅丰富廉价,同时它又是一个良好的重金属吸附剂,壳聚糖可以螯合超过甲壳素5-6倍量的金属，这是因为在壳聚糖里因脱乙酰作用存在自由氨基。研究人员先后多次企图改进壳聚糖以使其更容易传质和释放活性官能团来增强吸附能力。嫁接功能团到原壳聚糖主链来进一步提高其吸附性能。Kawamura, Hsein 和 Rorrer等研究了多孔壳聚糖和化学交联壳聚糖粒对重金属的吸附,与天然壳聚糖相比，多孔壳聚糖颗粒、化学交联壳聚糖粒子、 冠醚壳聚糖、浸微壳聚糖, 金属离子络合壳聚糖树脂明显提高了吸附能力。 volesky,Holan ,Waseand 和Forster 讨论了几种生物吸附剂对金属包括放射性的物质铀、钍等的吸附能力,认为这些生物吸附剂应对材料进一步改进和向商品化发展。它们的天然形式是软性的和其水溶液有一种结块或形成凝胶的倾向. 此外，自然形成的活性官能团不易速效吸附，而在工艺流程设计过程中该基团对传递金属污染物起着十分重要的作用，它还提供应用处理中所需的物质支持和增大金属结合基团接纳金属的可达性。 因此,本次研究试图制备一种将壳聚糖涂在氧化铝表面的生物吸附剂。 这种由氧化铝为载体的生物吸附剂由高温热裂解、孔性计、电子显微镜扫描、X射线光电子能谱测定其特征 。在Brunauer-Emmett-Teller (BET)吸附等温线基础上，它的表面积、孔径 、孔径分布由氮孔率决定。 这项研究的目的是制备一种壳聚糖合成物，了解其吸附特征，检验合成物和天然样本的去除铬()的能力以及在间歇和连续模型中的等温吸附平衡时的吸附容量。 另外,还应获得与Langmuir和Freundlich等温吸附式拟合的实验平衡数据和等温参数值，并用同样试剂进行塔吸附试验，以及pH值对铬()吸附的影响程度，也将研究壳聚糖生物吸附剂在0.1M氢氧化钠溶液中的再生能力。实验内容化学样品 来自Aldrich化工股份有限公司(Milwaukee, WI)的重铬酸钾、活性氧化铝、壳聚糖、1,5二苯卡巴，其中活性氧化铝是标准级150目brockman I。由Fisher 化工(Fair Lawn, NJ)生产的氯化钾、氢氧化钠。来自EM 科学(Gibbstown, NJ)的硫酸钾。所有的盐类都是ACS(美国化学学会)认证等级或更好。 所有溶液由ASTM(美国材料试验学会)的去离子水制备(18 M-H2O grade Barnstead Nanopure)。生物吸附剂准备 由壳聚糖凝胶覆盖陶瓷的生物吸附剂的制备过程如下:将150目氧化铝陶瓷在110烘箱干燥4小时后在室温下用草酸搅拌混合4小时进行表面涂层, 然后从酸中过滤出的氧化铝用去离子水洗两次，再在70真空烘箱中干燥24小时，将约50克中等分子量壳聚糖徐徐加入1000毫升质量分数为10%草酸溶液并搅拌。 加热至40-50使其容易混合形成酸和壳聚糖的粘性混合物(凝胶)。取大约500毫升的壳聚糖凝胶用水稀释2倍并加热至40-50，将约500克的酸处理后的氧化铝缓慢加入稀释凝胶并搅拌约36小时之后静置澄清。再用Whatman41滤纸在真空条件下过滤出上清液，将得到的合成物用去离子水洗两次,然后在55真空烘箱中干燥24小时, 最后在涂过一层生物吸附剂的氧化铝上进行重复涂层处理以增加壳聚糖的负载量，大约用时24小时。合成过程中过量的草酸用氢氧化钠溶液中和处理. 再将两次涂膜的混合物用Whatman41滤纸过滤,并用2500毫升的去离子水洗，及过滤之后在55真空烘箱干燥48小时左右，转移到玻璃瓶后存放在干燥器内。 生物吸附剂的特征 生物吸附剂特性包括:(1)热解,(2)孔径分析,(3)电子显微镜扫描,(4)XPS分析。(1)热裂解技术测定氧化铝负载的壳聚糖。 测量生物吸附剂在裂解中减少的重量得到在氧化铝上负载的壳聚糖的量。将准确称量后的干燥生物吸附剂放入瓷瓶内放入一个750 马弗炉内6小时，然后在干燥空气中冷却,称量得到生物吸附剂减轻的重量。用空瓷瓶、纯氧化铝、酸处理氧化铝、纯壳聚糖和生物吸附剂做各进行三次的对照实验。(2)由孔性计确定的表面积和孔径。 使用一个微型的BET 测定仪在零下196下超纯度的氮气条件下测定生物吸附剂的表面积、孔容和孔径，它们的平均值分别是125.24 sq.m/g、0.1775cm3/g、71.125。 (3)电子显微镜扫描， 以电子显微镜研究表面形态。壳聚糖生物吸附剂的电子显微镜扫描 (SEMs)由环境扫描电子显微镜(XL30-ESEM-FEG, FEI 公司, Hillsboro, OR,U.S.A.)获得，见图1(a)、(b)。图1 放大100倍（a）和800倍（b）的壳聚糖生物吸附剂的扫描电子显微镜像 (4)X射线光电子能谱 壳聚糖生物吸附剂XPS谱在PHI模型5400AXIS Ultra Kratos 分析仪(Manchester,U.K.)得出，列于图2。 图3是在铬液反应后吸附剂的XPS谱,显示了吸附铬的2个高峰。图2图3等温吸附平衡. 研究适量重铬酸钾溶解在去离子水得到的Cr()溶液中的间歇等温吸附平衡。用原子吸收光谱法和紫外光谱仪测定溶液的金属浓度, 在250.5进行大量的100至500毫克壳聚糖生物吸附剂的等温平衡研究, pH4.0的铬溶液(50毫升)与壳聚糖生物吸附剂在200RPM的搅拌水浴24小时后达到平衡 ,之后从溶液中用Whatman41滤纸过滤出该生物吸附剂，分析滤液中的金属含量。 每单位生物吸附剂的金属吸附量qe(毫克) ,由下式式得出 : 其中Ci和Ce分别是初始和平衡时的浓度（毫克/升）, M是生物吸附剂的干重（克）,V是溶液体积（升）.分别在不同的pH值下得出pH在平衡吸附试验中对吸附过程的影响，以及检验负离子即硫酸盐、氯化物铬()吸附的影响. 实验中硫酸盐、氯化物浓度控制在1毫摩尔水平。 塔吸附试验 在内径约1厘米长30厘米床容为30cm3的玻璃柱内进行动态吸附试验，实验中柱完全泡在用一个恒温neslab 和masterflex泵的250.5循环水浴中，塔底采用孔隙100微米聚乙烯滤盘托住吸附剂。当柱充满干燥吸附剂时震荡以使空隙和空气量减到最少，将塔溶液作适当稀释后用分光光度计测定不同时期的浓度，当塔中铬饱和后泵入空气清洗剩余水溶液以使其在0.1M氢氧化钠溶液再生，并在解吸过程的第5、10、20、30分钟进行采样分析。再生后，用去离子水清洗塔以备以后吸附时使用。分析过程 在酸性介质中测量铬(六)与1,5-二苯卡巴反应形成红紫色化合物可以测定六价铬测定。通过紫外可见分光光度计测量得到该化合物的最大吸光度在540纳米，用重铬酸钾标准溶液标定六价铬，为了显色将等温吸附样品用0.2摩尔硫酸调整pH到1.00.3，则样品浓度根据铬(六)标准溶液的吸光度与浓度关系曲线得到。精确研究显示，分析程序的再生性优于1毫克/升。结论涂膜过程制备了一种稳定的、wheatish彩色的颗粒生物吸附剂合成物。 方案一生物吸附剂的性质 前面讲述了通过高温热裂解的方法测定覆盖在150目氧化铝上的草酸和壳聚糖的平均量。结果表明，纯氧化铝损失约2.1%，草酸处理过的净重减少4.5%，单一壳聚糖膜氧化铝净损失7.8%，而二次壳聚糖膜的生物吸附剂净重减少21.1%，纯壳聚糖在750裂解后残留重量0.7%。壳聚糖从螃蟹壳中由酸碱提取甲壳素通过作用获得的，所以该残留物可能是少量的碳酸钙混入甲壳素而产生的，但残留量很少，没必要为此在壳聚糖净重上进行修正。 草酸这个羧酸在氧化铝与壳聚糖之间起桥梁作用. 正如方案1中，一个羧酸基与氧化铝形成较强的螯合连环酯,而另外一个与壳聚糖的-NH3+形成离子(或电子)键。 生物吸附剂中的草酸还可与-OH、 -CH2OH或者-NH2形成的氢键。图1（a）（b）的壳聚糖氧化铝的扫描电子显微镜像(SEMS)表明颗粒的平均粒径为100-150微米,且合成物颗粒一般为球形。 一些颗粒由单个粒子聚集成团的，生物吸附剂的细孔面积仅为3.3m2/g，而总表面积达到了105.2 m2/g，这表明吸附剂相对来说是无孔的。在图2的XPS光谱中表明在结合能为289 eV (C 1s)、535 eV (O 1s)、402 eV (N 1s), 和78 eV (Al 2p)时碳、氧、氮、和铝为表面观测到的主要元素。在此结合能的基础上，壳聚糖的一半表面可以用-CH2OH、 -CO和-NH2鉴别。在图3中展示了在铬溶液中暴光后的铬的光谱，它表明铬已部分地（约67%）转化为铬(III)，此结果与Dambies等观测结果一致。平衡等温线 在25C和pH为4条件下铬(六)的等温吸附平衡的结果如图4。图4等温先表明增大吸附质的平衡浓度可以促进吸附。生物吸附剂的吸附容量为153.8毫克铬(六)/克壳聚糖。有资料报道，每单位质量的天然壳聚糖吸附剂吸附铬(六)的最大值为27毫克，Ni2+印记壳聚糖树脂为51毫克，而化学交联和非交联壳聚糖分别为78毫克和50毫克，这里报道的两次涂膜的生物吸附剂大大好于其他物质可以看出：生物吸附剂壳聚糖合成物比天然壳聚糖有更大的吸附能力，即涂膜过程促进了壳聚糖的吸附能力，这可能是因为增大了表面积和促进了铬离子向壳聚糖结合部位的传递。图5表明pH值对生物吸附剂吸附铬(六)的影响。图5从图中看出，低pH有利于吸附而增大pH值时吸附作用降低，dantas和schmuhl等也得到了类似的结果。铬(六)以几种稳定的形式存在。如Cr2O72-、HCr2O7-、.和Cr2O72-,HCr2O7-, HCrO4-,和CrO42-,它的存在形式主要取决于铬离子的浓度和溶液的pH值.在较低的pH值时,吸附剂由于氨基而带正电,而吸附质中的铬酸盐离子以阴离子形式存在从而形成吸附剂与吸附质的静电吸引。因此,在较低的pH值时可增大吸附.若增大溶液的pH值则吸附剂会受到非质子化和吸附容量的降低.当超过某一pH值,仅仅吸附过程回影响从水介质中去除铬()。因此所有数据都是在pH值为4.0时得到的。在图6中表示氯化物和硫酸盐以及两者的交互作用对铬()吸附作用的影响。图6阴离子也会对生物吸附剂吸附Cr2O72-有轻微的抑制,可以预计,单价氯离子对吸附的抑制会小于二价的硫酸根离子,但氯化物与硫酸盐两者的抑制作用并未出现叠加.在别的文献中也有报道,一般与阴离子竞争表面结合部位的吸附都有类似的抑制。Gao等人提出壳聚糖像氧离子或氯离子在样品溶液中饿离子交换机制一样定量地吸附某些金属,这意味着交互作用发生在壳聚糖的氨基功能团与Cr2O7-之间,而且这种交互作用主要是静电引力.Fu等证实在红外线与紫外线光谱之间存在静电引力.XPS研究提供了涉及物质吸附部位的认别,并发现铬(六)吸附发生在高分子物质的胺官能团上,如方案2所示:虽然离子(或电子)吸引是吸附剂与吸附质的主要因素,但其他因素在低和高pH值的条件下会对吸附产生重要影响.例如在低和高pH值时就可能出现金属吸附质或与壳聚糖羟基和羧基的羟基化后的吸附质进行氢键结合.在低和高的pH值时铬存在其他不同形式,而且这些形式归因于不同pH值时的吸附曲线,所以壳聚糖生物吸附剂对铬的总吸收量取决于:1 离子吸引力,2 氢键结合,3 较弱的范德华力。 Langmuir和Freundlich模型 Langmuir和Freundlich模型是描述液相与固相之间的吸附组成的最简单和最常用的等温线. Langmuir模型假定单层吸附,而Freundlich模式是经验公式.对数据进行分析得到Langmuir和Freundlich参数, Langmuir模型的数学公式是: 式中Ce是溶液中吸附剂的平衡浓度(mg/l),qe是平衡时吸附剂的吸附量(mg/l),Q是饱和吸附量(mg/l),b是吸附系数,有Langmuir模型对平衡浓度和吸附量作直线得出参数Q和b.在坐标纸上的Ce/qe对Ce的直线表明:壳聚糖生物吸附剂对铬()的吸附符合Langmuir模型,从实验中推断出的Q和b的值分别为153.85mg/L和0.023L/mg,相关的R2的值为0.9896. Freundlich模型表示为:式中K和1/n是Freundlich等温常数,从模型中得到它们分别为0.9565和1.4047,其相关的R2值为0.9972。塔吸附研究 图7表示的是在pH值为4.0和25条件下生物吸附剂头两个周期从综合废水中吸附铬(六)的实验曲线。图7:从图中明显可以看出当开始浓度(C1)约100mg/L是床容数在40以下都无铬流出,当床容数大于40时,塔出水浓度逐渐上升,直到约200床容是达到进水浓度.相对于进水浓度增长减慢的出水浓度表明吸附的动力减少了,当生物吸附剂铬饱和时,泵入空气排出塔中溶液.吸附塔在流量2.6ml/min,0.1M的NaOH溶液中可以再生,其解吸曲线如图8所示。图8:在NaOH溶液中,解吸的最大值发生在第5床容,而且在底20床容时完成再生,再生床用于之后的铬的吸附,它的第二周期的曲线如图7。对周期一和周期二比较发现,生物吸附剂对废水中的铬(六)的吸附容量并未减少.在pH10时,壳聚糖的铬(六)解吸主要是由于壳聚糖带正电的胺基团.图9表示的镀铬设施废水的铬(六)吸附曲线。图9此废水中包含有铁(13mg/l),镉(0.0065mg/l),铅(48mg/l),硫酸(69mg/l),硝酸盐(11mg/l),氰化物(0.32mg/l),以及磷酸盐(17mg/l)。比较综合出水的结果,废水开始稀释至铬约100ppm并调整pH为4.0,则用周期一和周期而分别代表纯生物吸附剂和再生生物吸附剂的吸附曲线如图10。图10相应的未稀释水样(C1=1253ppm,pH=2.0)的吸收和解吸曲线见图11和图12。图11图12可以看出塔出水(C0)直到第15床容都无铬的出现,然后随初始浓度缓慢增大浓度直到第45床容,它的最大解吸量发生在第3床容.在高初始浓度和低pH值时生物吸附剂显示了更大的吸附能力。综上所述,涂膜过程改进了壳聚糖对六价铬的吸附能力,所以应更多地活化生物吸附剂合成物的活性部位,而塔的吸附解吸研究表明新研究合成的壳聚糖生物吸附剂可用于去除工业废水中的铬()。感谢作者感谢来自Dr.Richard Haas和Grant DEFG02-91-ER45439美国能源部部分资助的Illionois大学材料精细分析中心提供的X-射线光电子能谱分析,同时感谢Scott J.Robinson,成像技术集团,Backman尖端科技研究所,以及Illionois大学扫描电子显微镜像的帮助。参考文献(略) 原文Removal of Hexavalent Chromium from Wastewater Using a New Composite Chitosan BiosorbentU.S. Army Construction Engineering Research Laboratories,Champaign, 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 alumina. 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 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 the 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 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 ions 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 hazardous 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 processes 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 species, 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 sorbents 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 and 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 chitosan 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 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 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 commercialization.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 important 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 biosorbent 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) adsorption 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 adsorption 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 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 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 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. 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 10 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 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 process 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 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 included 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 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, 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 means 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 respectively.(c) Scanning Electron Microscopy. Surface morphology was studied with an electron microscope. The scanning electron micrographs (SEMs) of composite 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 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 solutions 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 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 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