Ind. Eng. Chern. Res. 1993,32,2170-2178
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Synthesis of Porous-Magnetic Chitosan Beads for Removal of Cadmium Ions from Waste Water Gregory L. Rorrer' and Tzu-YangHsien Department of Chemical Engineering, Oregon State University, Coruallis, Oregon 97331
J. Douglas Way* Department of Chemical Engineering and Petroleum Refining, Colorado School of Mines, Golden, Colorado 80401
Chitosan is a glucosamine bipolymer capable of adsorbing transition-metalions from aqueous solution. Highly porous chitosan beads were prepared by dropwise addition of an acidic chitosan solution into a sodium hydroxide solution precipitation bath. The gelled chitosan beads were cross-linked with glutaraldehyde and then freeze dried. Beads of 1-and 3-mm diameter were prepared. Beads of 1-mm diameter possessed surface areas exceeding 150 m2/gand mean pore sizes of 560 A and were insoluble in acid media a t p H 2. Well-mixed batch adsorption experiments revealed that both metal and hydronium ions compete for available adsorption sites by a chelation mechanism. Adsorption isotherms at 25 OC and pH 6.5 over the concentration range 1-1690 mg Cd2+/Lpossessed a stepped shape and maximum adsorption capacities for the 1-and 3-mm beads were 518 and 188 mg of Cd/g of bead, respectively. The stepped shape of the isotherm was explained by a pore-blockage mechanism. (a) Chitosan
Introduction Process waste streams from mining operations, metal plating facilities, and electronic device manufacturing operations often contain very dilute concentrations of heavy-metal ions. Furthermore, the ground water surrounding many manufacturing sites, nuclear fuel processing facilities, and military bases is contaminated with low levels of heavy metal ions such as chromium and cadmium. Since heavy-metal ions are often toxic at low concentrations and are not biodegradable, they must be physically removed from the contaminated water in order to meet increasingly stringent environmental quality standards. This will require the development of new separations technologies utilizing novel solid adsorbents that are capable of recovering low-concentration heavy metal ions from waste water (King, 1987). Polysaccharide biopolymers isolated from marine organisms are a new class of potentially inexpensive and environmentally benign solid adsorbents that exhibit a high specificity toward metal ions. Of particular interest is the amine biopolymer chitosan, which selectively binds to virtually all group I11 transition-metal ions at low concentrations but does not bind to groups I and I1 alkali and alkaline-earth metal ions (Muzzarelli, 1973,1977).As shown in Figure 1, chitosan is a linear glucosamine biopolymer possessing an average molecular weight of 120 000. Chitosan is derived from the deacetylation of chitin, a linear N-acetylglucosamine biopolymer. Chitin is the principal component of the shells of crustacean organisms and the second most abundant biopolymer in nature next tocellulose. The amine groups on the chitosan chain serves as a chelation site for transition-metal ions, and the @1,4 glycosidic linkages joining glucosamine units resist both chemical and biological degradation. The observation that chitosan has a high affinity for virtually all non-alkali, group I11transition-metal cations at 20-40 ppm concentrations is well documented (Muzzarelli, 1973, 1977; Masri et al., 1974), but the effects of key process variables and adsorbent properties on ad-
* Corresponding authors. 0888-5885/93/2632-2170$04.00f 0
amlne group
(b) Crosslinking of Chitosan
H900 -chilcsan chain
NCHO
"2
H& CHO
glularaklehyde
I ICH21 I
CHO
I I
[CH2)
2
Figure 1. Chitosan: (a) structure; (b) cross-linking with glutaral-
dehyde. sorption behavior are still not well characterized. Recently however, McKay et al. (1989) measured adsorption isotherms and heats of adsorption for Cu2+,Hgz+, Ni2+, and Zn2+on chitosan powder as function of particle size and temperature (25-60 "C) at near-neutral pH. The isotherm data were fitted to the Langmuir model over the 5-300 ppm metal ion concentration range, and the adsorption capacity decreased with increasing temperature. The maximum adsorption capacities for Hg2+, Cu2+, Ni2+,and Zn2+were 815,222,164, and 75 mg of metal/g of chitosan, respectively, for 1-mm particles at 25 "C. Additional isotherm data for Cu, Zn, Cd, Cr(III), and P b at similar conditions are provided by Yang and Zall(1984). Jha et al. (1988) measured adsorption isotherms and adsorption kinetics for Cd2+ on chitosan powder over a narrow concentration range of 1-10 ppm at 25 "C and pH 6.5 at various particle sizes (0.037-0.328 mm), fitted the data to 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 9,1993 2171
a Freundlich isotherm model, and found that decreasing the particle size increased the adsorption capacity. The increase in adsorption capacity with decreasing particle size implied that the metal preferentially adsorbed on the outer surface and did not fully penetrate the particle, an observation also made by Maruca et al. (1982) for adsorption isotherms of Cr(II1) on chitosan flake of 0.44-mm nominal particle size. Jhaet al. (1988)further showed that (1)competing alkali ions (as Ca2+) a t 100 ppm did not significantly inhibit Cd2+removal rates and adsorption capacity and (2) the adsorption capacity decreased with decreasing pH, particularly below pH 4. Maruca et al. (1982) also observed that adsorption capacity decreased with decreasing pH for the adsorption of Cr(II1) ionson chitosan. These results suggest that hydronium ions compete with metal ions for available amine sites on chitosan. Finally, Jha et al. (1988) demonstrated that 90% of the Cd adsorbed on chitosan powder could be released back to the liquid phase within 8 h using 0.01 HC1 a t pH 2, and Coughlin et al. (1990) showed that Ni, Cu, and Cr(II1) adsorbed on chitosan flake could be desorbed at low pH, even under repeated adsorptionfdesorption cycles. Thus, the metal 'ion adsorption process on chitosan is reversible, making adsorbent regeneration and metal recovery at low pH feasible. Udaybhaskar et al. (1990) considered the effect of pH, ionic strength, and particle size on Cr (VI) adsorption kinetics at 23 "C, and fitted liquid-phase chromium concentration vs time data to a pseudo-first-order rate equation. A first-order rate constant of 1.92 h-' at 23 OC and pH 4 was obtained for a 1-L Cr(V1) solution of initial concentration 5 ppm contacted with 0.5 g of chitosan powder (0.37 mm). At pH 4, Udaybhaskar et al. noted that Cr(V1) is speciated primarily as the anion HCr04- at concentrations less than 0.1 mM, whereas amine sites on chitosan are fully protonated. This implies that chitosan is also capable of an alternative anion-exchangemechanism for adsorption of oxymetal anions at low pH. Although chitosan has an intrinsically high affinity and selectivity for transition-metal ions, the adsorbent raw material is not suitable for processing aqueous waste streams or treatment of ground water. Chitosan is usually obtained in a flaked or powdered form that is both nonporous and soluble in acidic media. The low internal surface area of the nonporous material limits access to interior adsorption sites and hence lowers metal ion adsorption capacities and adsorption rates. The solubility of chitosan in acidic media prevents its use in recovery of metal ions from waste water a t low pH (Masri et al., 1978). Furthermore, the flaked or powdered form of chitosan swells and crumbles easily and thus does not behave ideally in packed-column configurations common to pump-andtreat adsorption processes. The reprocessing of flaked or powdered chitosan into a highly porous, chemically cross-linked bead with a magnetic component can overcome many process limitations. The initial development of non-cross-linked, porous beads of chitosan for biomedical applications was first reported by Bodmeier et al. (19891, who claimed that freeze-dryingof gelled chitosan particles produced a highly porous matrix that easily dissolved in acid solution. For metal ion adsorption applications, the high internal surface area of the porous beads could boost the metal binding capacity and also increase the transport rate of metal ions into the particle. Chemical cross-linking of the linear chitosan chains with the bifunctional reagent glutaraldehyde (see Figure lb) can render chitosan insoluble in acidic media and improve resistance to chemical and
biological degradation. Cross-linking could also improve the mechanical strength and abrasion resistance of the bead so that the adsorbent is suitable for use in a packed column. Finally, by incorporating a magnetic component to the bead, many novel on-site cleanup processes using magnetic chitosan beads are possible, including the use of wet-drum magnetic separators to seledivelyretrieve metaladsorbed magnetic chitosan beads directly from a contaminated surface water site or the use of magnetically stabilized fluidization technologies to remove toxic metal ions in silt-laden ground water (Rorrer and Way, 1991). The purpose of this work was to prepare highly porous, magnetic beads of chemically cross-linked chitosan, and then investigate how these beads adsorb dilute concentrations of cadmium ions from aqueous solution at nearneutral pH. Specifically, chitosan was cast into porous beads by a phase-inversion technique. The chitosan beads were cross-linked with glutaraldehyde and then freezedried to remove water without collapsing the porous internal structure. Magnetite was added to the beads during the casting process to incorporate the magnetic component. The adsorption of cadmium ions (Cd2+)onto the chitosan beads was followed in a well-mixed batch contactor. Adsorption isotherms and kinetic data for beads of 1-and 3-mm nominal diameter were obtained over a very broad range of equilibrium cadmium ion concentrations ranging from
