Removal of Hexavalent Chromium from Wastewater Using a New

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Environ. Sci. Technol. 2003, 37, 4449-4456

Removal of Hexavalent Chromium from Wastewater Using a New Composite Chitosan Biosorbent V E E R A M . B O D D U , * ,†,| K R I S H N A I A H A B B U R I , ‡,§ JONATHAN L. TALBOTT,‡ AND E D G A R D . S M I T H †,‡,| U.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 UrbanasChampaign, Champaign, Illinois 61820

A new composite chitosan biosorbent was prepared by coating chitosan, a glucosamine 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 adsorption 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.

Introduction Process 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. * Corresponding author phone: (217)398-5511; fax: (217)373-3430; e-mail: [email protected]. † U.S. Army Construction Engineering Research Laboratories. ‡ University of Illinois at UrbanasChampaign. § Present address: Department of Chemistry, S. V. University, Tirupati 517502, India. | Present address: Environmental Processes Branch, U.S. Army Construction Engineering Research Laboratories, Champaign, IL 61826-9005. 10.1021/es021013a CCC: $25.00 Published on Web 09/05/2003

 2003 American Chemical Society

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) and Wase and 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 modification and development 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. VOL. 37, NO. 19, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Scanning electron micrographs of the composite chitosan biosorbent at (a) 100× and (b) 800× magnification.

Experimental Section Chemicals. Potassium dichromate, activated alumina, chitosan, and 1,5-diphenylcarbazide 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 MΩ-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 alumina was slowly added to the diluted gel and stirred 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 twice with 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. 4450

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(b) Determination of Surface Area and Pore Diameter by Porosimetry. Surface area, pore volume, and pore diameter of the composite biosorbent were determined with a 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 Å 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 equation

qe )

(Ci - Ce)V M

(1)

where 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 adsorption process. The effect of competing anions, namely sulfate and chloride, on the adsorption of Cr(VI) was also evaluated. Sulfate and chloride concentrations were maintained at 1 millimolar levels in the experiments. Column Adsorption Experiments. Dynamic flow adsorption experiments were conducted in a glass column of dimensions about 1 cm internal diameter by 30 cm length. The bed volume of the column was 30 cm3. The column was fully jacketed allowing experiments to be carried out at constant temperature of 25 ( 0.5 °C using a circulating water

FIGURE 2. Composite chitosan biosorbent-survey.

FIGURE 3. Chromium loaded composite chitosan biosorbent. bath, a Neslab Thermostat, and a Masterflex Pump. Column ends were fitted with polyethylene filter disks of 100 µm pore size to retain the composite material. Columns were shaken while being packed with dried adsorbent to minimize void volumes and air gaps. Concentrations of column effluents obtained at various intervals were monitored spectrophotometrically after making appropriate dilutions. After the column was saturated with chromium, it was drained of remaining aqueous solution by pumping air prior to regeneration with 0.1 M sodium hydroxide solution. Samples at 5, 10, 20, and 30 min intervals from the start of the desorption process were collected for analysis. After regeneration, the column was washed with DI water before use in subsequent adsorption runs. Analytical Procedure. Hexavalent chromium was determined colorimetrically (16) by measurement of the intense red-violet complex formed by reaction of chromium(VI) with

1,5-diphenylcarbazide in an acidic medium. A Cary 3E UVvisible spectrophotometer was used to obtain measurements of the chromophore complex at its absorbance maximum of 540 nm. Standard stock solutions prepared from potassium dichromate were used to calibrate the instrument for hexavalent chromium. The pH of adsorption isotherm samples was adjusted to 1.0 ( 0.3 with 0.2 N sulfuric acid for color development. Isotherm sample concentrations were determined from an absorbance versus concentration calibration curve constructed from standard Cr(VI) solutions. A precision study revealed that the analytical procedure is reproducible to better than 1 mg/L.

Results and Discussion The coating process yielded a stable, granular composite biosorbent that was wheatish in color. VOL. 37, NO. 19, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Equilibrium adsorption of chromium(VI) on composite chitosan biosorbent.

FIGURE 5. Effect of pH on the adsorption of chromium(VI) on composite chitosan biosorbent at an initial concentration of 100 mg/L of Cr(VI).

SCHEME 1

Characterization of Biosorbent. The average amounts of oxalic acid and of chitosan coated on to 150-mesh alumina were obtained by the high-temperature pyrolysis method described earlier. The results show pure alumina lost about 2.1 wt %. Alumina treated with oxalic acid lost a net weight of 4.5% and a single coating of chitosan on alumina lost 7.8 net wt %. The net amount of chitosan on the twice-coated biosorbent was 21.1 wt %. Pure chitosan leaves a residue of about 0.7 wt % after pyrolysis at 750 °C. Chitin is obtained from crab shells by acid-base extraction. And chitosan is obtained from chitin by a deacetylation process. The residue may be due to a small amount of calcium carbonate that remains bound with the chitin. Since the residue is such a small amount, no attempt was made to correct the chitosan net weight for this. Oxalic acid is a dicarboxylate used to form a bridge between alumina and chitosan. As illustrated in in Scheme 1, one carboxylate group forms a relatively strong surface chelate via ester linkage with the alumina (17) while the other one forms ionic (or electrostatic) bonds with -NH3+ groups present in chitosan. The oxalic acid could also form hydrogen bonds with -OH, -CH2OH, or -NH2 groups on the biopolymer.Scanning electron micrographs (SEMs) of chitosan coated alumina in Figure 1a,b illustrates that the average size of particles is 100-150 microns and that the shape of the composite particle can be described as spherical. Some particles are agglomerated clusters of the individual particles. The micropore area of the biosorbent is only 3.3 m2/g compared to the total surface area (105.2 m2/g). This indicates that the sorbent is relatively nonporous. The XPS spectrum provided in Figure 2 reveals carbon, oxygen, nitrogen, and aluminum are the predominant elements observed on the surface from binding energies at 289 eV (C 1s), 535 eV (O 1s), 402 eV (N 1s), and 78 eV (Al 2p). Based upon binding energies, surface moieties of the composite biosorbent are identified as -CH2OH, -CO, and -NH2. Chromium is observed in the spectra after exposure to chromium solution (Figure 3). Figure 3 reveals that the chromium is partially reduced (∼67%) to chromium(III). These results are consistent with the observations of Dambies et al. (35). 4452

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FIGURE 6. Effect of anions on the adsorption of chromium(VI) on composite chitosan biosorbent. Equilibrium Isotherms. Results of the equilibrium adsorption isotherm for Cr(VI) at 25 °C and pH 4 are shown in Figure 4. The isotherm indicates that adsorption increases with an increasing equilibrium concentration of the sorbate. The sorption capacity of the composite biosorbent was found to be 153.8 mg Cr(VI)/g of chitosan. Maximum values of Cr(VI) adsorbed per unit mass of adsorbent reported in the literature are 27 mg for chitosan (5), 51 mg for Ni2+ imprinted chitosan resin (14), and 78 mg and 50 mg for cross-linked and non-cross-linked chitosan (18). The value reported here on the twice-coated biosorbent is considerably greater than that reported elsewhere (5) indicating that the chitosan in the composite biosorbent has greater adsorption capacity than unsupported chitosan and the coating process improved adsorption capacity of chitosan. This improvement may be attributed to increased surface area and facilitation of transport of chromium ions to the binding sites on chitosan.The effect of pH on the adsorption of Cr(VI) on the biosorbent is shown in Figure 5. Adsorption is greater at low pH and decreases with increasing pH. Similar behavior was observed by de Dantas et al. (13) and Schmuhl et al. (18). Cr(VI) can exist in several stable forms such as Cr2O72-, HCr2O7-, HCrO4-, and CrO42-, and the relative abundance of a particular complex depends on the concentration of the chromium ion and the pH of the solution (28). At lower pH the sorbent is positively charged due to the protonation of amino groups, while the sorbate, dichromate ion, exists mostly as an anion leading to the electrostatic attraction between sorbent and sorbate. This results in increased adsorption at low pH. As pH of the solution increases the sorbent undergoes deprotonation and the adsorption capac-

TABLE 1. Adsorption Capacities of Different Adsorbents for Chromium(VI) adsorbent

maximum adsorption capacity (mg/g)

pH

activated carbon (Filtrosorb 400) sawdust coconut shell activated carbon activated carbon (Filtrosorb 400) sphagnum moss peat compost leaf mould sawdust sugar beat pulp maize cob sugar cane bagasse coconut husk fibers palm pressed-fibers pinus sylvestris bark activated by 0.05 N NaCl leather based activated carbon chitosan chitosan cross-linked chitosan metal ion imprinted chitosan chitosan cross-linked with epichlorohydrin metal ion imprinted chitosan cross-linked with epichlorohydrin chitosan cross-linked with ethylene glycol diglycidyl ether chitosan cross-linked with epichlorohydrin composite chitosan biosorbent

125.5 3.3 20.0 145.0 119.0 101.0 43.0 39.7 17.2 13.8 13.4 29.0 15.0 19.5 241.0 78.0 27.2 50.0 51.0 52.3 51.0 56.8 11.3 153.8a

6.0 6.0 2.5 2.5 1.5 1.5 2.0 2.0 2.0 1.5 2.0 2.1 2.0 4.5 3.0 5.0 ? 5.0 5.5 5.5 5.5 5.5 3.0 4.0

maximum initial concn (mg/L) 260.0 50.0 1000.0 1000.0 1000.0 1000.0 1000.0 500.0 300.0 500.0 20.0 1000.0 1000.0 ? 1000.0 1000.0 1000.0 1000.0 1000.0 100.0 5000.0

ref 19 20 21 22 22 22 22 23 23 23 23 24 24 25 26 26 5 18 14 14 14 14 27 present study

a Based on the amount of chitosan (21.1 wt %) on the composite biosorbent. This corresponds to a maximum capacity of 35.4 mg/g of composite biosorbent.

SCHEME 2

ity decreases. Above this pH, only the adsorption process influences the removal of Cr(VI) from aqueous medium (22). In view of this, all experimental data were collected at pH 4.0. The effects of sulfate, chloride, and both sulfate and chloride on the biosorption of the Cr(VI) are shown in Figure 6. Anions slightly inhibited the adsorption of Cr2O72- on the composite biosorbent. The inhibition on adsorption by monovalent Cl- is less than that of divalent SO42-, as can be expected. Inhibition effects of both chloride and sulfate do not appear to be additive. In general, similar reductions in adsorptions due to anion competition for surface binding

sites have been reported elsewhere in the literature (29-32). Gao et al. (33) reported that chitosan adsorbs some metals quantitatively as oxyanions or anionic chloro complexes in sample solutions by an ion-exchange mechanism. This implies that the interaction occurs between NH3+ functional groups in chitosan and Cr2O72- and that the interaction is chiefly electrostatic attraction in nature. Fu et al. (34) confirmed an electrostatic attraction by IR and UV spectral studies. XPS studies (35), which provide identification of the sorption sites involved as well as the forms of species sorbed, found that sorption of Cr(VI) does indeed occur on amine functional groups of the biopolymer as shown in Scheme 2 VOL. 37, NO. 19, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. Experimental breakthrough curves for adsorption of chromium(VI) from aqueous solution on composite chitosan biosorbent. Bed volume ) 30 mL, pH ) 4.0, flow rate ) 2.4 mL/min.

FIGURE 8. Regeneration of composite chitosan biosorbent loaded with chromium(VI) using 0.1 M NaOH. Bed volume ) 30 mL, flow rate ) 2.6 mL/min.

FIGURE 10. Regeneration of composite chitosan biosorbent loaded with chromium(VI) from field sample using 0.1 M NaOH. Bed volume ) 30 mL, flow rate ) 2.5 mL/min.

FIGURE 9. Experimental breakthrough curves for adsorption of chromium(VI) from field sample on composite chitosan biosorbent. Bed volume ) 30 mL, pH ) 4.0, flow rate ) 2 mL/min.

FIGURE 11. Experimental breakthrough curve for adsorption of chromium(VI) from field samples on composite chitosan biosorbent. Bed volume ) 30 mL, pH ) 1.99, flow rate ) 2.0 mL/min.

(34). Though the ionic (or electrostatic) attraction between the sorbent and sorbate is the dominate mechanism, other mechanisms at low and high pH could be responsible for adsorption. For example at low and high pH, hydrogen bonding of the metal sorbate (M) or hydroxylated sorbate to hydroxyl and carboxyl groups of chitosan could occur. Other forms of chromium exist at both low and high pH and these

forms should contribute to the overall pH adsorption profile. Thus the total uptake of Cr(VI) by the composite chitosan biosorbent is due to (i) ionic attraction, (ii) hydrogen bonding, and (iii) and weak van der Waals forces. Langmuir and Freundlich Models. Langmuir and Freundlich models are the simplest and most commonly used isotherms to represent adsorption of components from a

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FIGURE 12. Regeneration of composite chitosan biosorbent loaded with chromium(VI) from field sample (Co ) 1253 ppm) using 0.1 M NaOH. Bed volume ) 30 mL, flow rate ) 2 mL/min. liquid phase on to a solid phase (36). The Langmuir model assumes monolayer adsorption while the Freundlich model is empirical in nature. The data are analyzed to obtain Freundlich and Langmuir parameters. The mathematical expression of the Langmuir model is

qe )

Q0bCe (1 + bCe)

(2)

where Ce is the equilibrium concentration of sorbent in solution (mg/L), qe is the equilibrium loading of sorbate on sorbant (mg/g), Q0 is the ultimate adsorption capacity (mg/ g), and b is the relative energy of adsorption (L/mg). The Langmuir model can be linearized to obtain the parameters Q0 and b from experimental data on equilibrium concentrations and adsorbent loading. A linear plot of Ce/qe verses Ce was observed and illustrates that adsorption of Cr(VI) on the composite chitosan biosorbent follows the Langmuir model. The values of Q0 and b, obtained from experimental data, are 153.85 mg/g and 0.0023 L/mg, respectively, with an R2 value of 0.9896. The Freundlich model is expressed as

qe ) kCe1/n

(3)

where k and 1/n are Freundlich isotherm constants. The constants obtained from the model and are 0.9565 and 1.4047, respectively, with an R2 value of 0.9972. Column Adsorption Studies. Experimental breakthrough curves for Cr(VI) adsorption from synthetic effluents on the biosorbent (first two cycles, I and II) at pH 4.0 and 25 °C are shown in Figure 7. As evident from the figure, no leakage of Cr(VI) occurs up to 40 bed volumes with an initial influent concentration (Ci) of approximately 100 mg/L. After 40 bed volumes, the column effluent concentration (Co) increases gradually and attains the influent concentration at around 200 bed volumes. The relatively slow increase of effluent to influent concentration with bed volumes indicates slower kinetics of adsorption. When the biosorbent is fully loaded with chromium, pumping air drains the solution in the column. The column is regenerated with 0.1 M sodium hydroxide flowing at 2.6 mL/min. The desorption curve is shown as cycle I in Figure 8. Maximum desorption occurs within 5 bed volumes and complete regeneration occurs within 20 bed volumes with 0.1 M sodium hydroxide. The regenerated bed is used for subsequent adsorption of chromium, and the breakthrough curve is shown in Figure 7 as cycle II. A comparison of the breakthrough curves of cycle I and cycle II indicates that there is no apparent reduction in the adsorption capacity of the biosorbent for Cr(VI) from

synthetic effluents. Desorption of Cr(VI) from chitosan is believed to be mainly due to deprotonation of chitosan amine groups above pH 10. The adsorption breakthrough curves for Cr(VI) from wastewater of a chromium plating facility are illustrated in Figure 9. The wastewater from the chrome plating facility also contained iron (13 mg/L), cadmium (0.0065 mg/L), lead (48 mg/L), sulfate (69 mg/L), nitrate (11 mg/L), fluoride (0.32 mg/L), and phosphate (17 mg/L). To compare with the results of the synthetic effluents, the wastewater was first diluted to about 100 ppm of Cr(VI) and adjusted to pH 4.0. As above, cycles I and II represent the adsorption on virgin biosorbent and regenerated biosorbent. Desorption curves for diluted field samples are shown in Figure 10. In the case of field samples the adsorption capacity of regenerated adsorbent is reduced to some extent in the second cycle. Breakthrough and desorption curves for an undiluted field sample (Ci ) 1253 ppm) at a pH 2.0 are provided in Figures11 and 12, respectively. No chromium is found in the column effluent (Co) up to 15 bed volumes, and then the concentration increases very slowly to the initial influent concentration at around 45 bed volumes. Maximum desorption occurs within 3 bed volumes. The biosorbent exhibits greater adsorption capacity due to higher initial concentration and lower pH. To summarize, the results show that the coating process has improved adsorption capacity of chitosan for hexavalent chromium and suggests that more active sites are exposed on the composite biosorbent. Column adsorption-desorption studies indicate that the newly developed composite chitosan biosorbent could be used to remove chromium(VI) from industrial effluents.

Acknowledgments The authors would like to acknowledge analytical support (X-ray Photoelectron Spectroscopy) received from Dr. Richard Haas and the Center for Microanalysis of Materials, University of Illinois, which is partially supported by the U.S. Department of Energy under Grant DEFG02-91-ER45439. The authors also thank Mr. Scott J. Robinson, Imaging Technology Group, Beckman Institute for Advanced Science and Technology, University of Illinois for help with the scanning electron micrographs.

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Received for review November 4, 2002. Revised manuscript received July 21, 2003. Accepted July 21, 2003. ES021013A