J . Chem. Tech. Biotechnol. 1990,49, 395404

Metal Recovery Using Chitosan” Edvar Onsrayen & Oyvind Skaugrud Protan A/S, PO Box 420, 3002 Drammen, Norway (Received 29 September 1989; accepted 18 January 1990)

ABSTRACT Chitosan is a natural polycationic polymer which possesses valuable properties as a metal recovering and water purifying agent. Applications are -waste water treatment for heavy metal and radio isotope removal and valuable metal recovery, -potable water puriJication for reduction of unwanted metals, -agriculture-controlled release of trace metals essential to plant growth, -food-complex binding of iron in precooked food to reduce ‘warmed-over flavour’. The interactions of metals with chitosan are complex, probably simultaneously dominated by adsorption, ion-exchange and chelation. To study this it is of utmost importance to work with well characterized chitosans. This has been a problem as available characterizing methodology is limited. Degree of polymerization and deacetylation and the distribution of acetyl groups along the polymer chain is of crucial importance for chitosan metal interacting characteristics. Making chemical derivatives is a way to alter the metal interacting characteristics of chitosan. Chitosan possesses general coagulantlflocculant characteristics towards bio-molecules and surfaces. Key words: chitosan, metal recovery, complex binding of metal, chelation of metal, removal of toxic metal, waste water treatment, water purification.

* Paper presented at the meeting ‘Recovery/Removalof Metals by Biosorption- A Commercial Reality or a Scientist’sDream?, organised by the Solvent Extraction and Ion Exchange Group of the Society of Chemical Industry and held in London on 18 May 1989. 395 J . Chem. Tech. Biotechnol. 0268-2575/90/$03.50 0 1990 SCI. Printed in Great Britain

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1 CHITOSAN PRODUCTION Industrial production of chitosan is based on chitin derived from shells of crabs and shrimps. The amount of chitin available from the fisheries of crustaceans is estimated to more than 40 000 tons per year. The key steps in the extraction of chitin from shells are the removal of proteins and minerals, such as calcium carbonate and calcium phosphate, by treatment with alkali and acid respectively. Before chemical treatment, the shells are ground to improve the efficiency of treatment, and after final rinsing the chitin is dried and can be stored as a stable intermediate. This chitin production process is summarized in Fig. 1. The deacetylation of chitin through treatment with strong NaOH is carried out at elevated temperature over a controlled period of time. To achieve the highest possible yield the material is kept in the solid state. Chitosan manufactured in this way may consequently contain foreign material not dissolved and removed during manufacture. To prepare a filterable grade product, an additional step is added, including solubilization in an appropriate organic acid. The acid solution is then filtered through specified mesh size filter before spray drying. In contrast to the regular grade chitosan, this latter salt form is water soluble. Water-soluble industrial-grade chitosan is made by dry blending regular chitosan with an appropriate acid. These chitosan production processes are summarized in Fig. 2.

2 PHYSICAL AND CHEMICAL PROPERTIES OF CHITOSAN 2.1 Molecular structure

Similar to cellulose, chitin and chitosan are long linear polymeric molecules of j(1+4)-linked glycans. The repeating unit in chitin is 2-acetamido-2deoxy-~glucose-(N-acetylglucosamine), while chitosan comprises a non-homogeneous mixture with the deacetylated form (glucosamine)(Fig. 3). The molecular weight of Crab/Shrimp shells

1 Grinding

1 Removal of proteins by NaOH treatment

1 Rinse

1 Removal of minerals by HCL treatment .L

Rinse

1 Dry

1 Chitin Fig. 1. Chitin production process.

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Chitin

1 Deacetylation

1 Drying

1 Chitosan /

Sea CureTMflakeL

Milling

Dissolving (acid)

1

Sea CureTMpowder

I

Filtering

1

Blending

Spray drying Sea CureTM+

Sea Cure SDTMpowder

Fig. 2. Manufacture of alternative commercial forms of chitosan.

fH3

co

~

o

\

H

~

H

HO H H

co \

CH, OH

H H

Chitin

l H

co

\

H CH,OH

HO H

H

H

H

Chitosan

Fig. 3. Chemical structure of chitin and chitosan

chitosan for commercial products depends on the processing conditions, and grades within the range 1 0 0 W 1 OOOOOO daltons are available. The mole fraction of deacetylated units (glucosamine), defined as the degree of deacetylation, is usually 7&90%. 2.2 Solubility

Standard grades of chitosan require addition of acid to enable solubilization in water. Acetic acid is most commonly used in quality control and standard procedure measurements. Other organic as well as mineral acids are used successfully in various applications. The required amount of acid to solubilize

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chitosan depends on the characteristics of the acid used. Viscosity values for 1 "/, (w/w) chitosan in various organic acids at different acid concentrations are given in Table 1. Chitosan is partially soluble in mineral acids such as hydrochloric, nitric and perchloric acid, but practically insoluble in sulphuric and phosphoric acids. Compared with the more common organic acids the solubility in inorganic acids seems more limited with regards to the concentration ratio chitosan/acid. Figure 4 describes the viscosity of 1 % (w/w) and 5 % (w/w) chitosan solutions at increasing concentrations of hydrochloric acid. Chitosan samples of different molecular weight have been chosen to achieve viscosity values of the same magnitude for the two chitosan concentrations. Chitosan is a weak base, and a minimum quantity of acid is required to convert the glucosamine units to the soluble form, R-NH:. Due to this more acid is required TABLE 1 Brookfield Viscosity and pH of 1 % (w/w)Chitosan in Various Organic Acids 1 % ( w / w ) Acid

Acid

Acetic acid" Propionic acid" Formic acid" Lactic acid" Pyruvic acid Malonic acid Adipic acid Malic acid Succinic acid Tartaric acid Citric acid Oxalic acid

5 % (w/w)Acid

10% ( w / w ) Acid

Viscosity (mPa s )

pH

Viscosity (mPa s )

pH

Viscosity (mPa s )

pH

260 260 240 235 225 195 190 180 180 52 35 12

4.1 4.3 2.6 3.3 2.1 2.5 4.1 3.3 3.8 2.8 3.0 1.8

260

3.3

260

2.9

185 235

2.0 2.7

185 270

1.7 2.1

205

2.3

220

2.1

135 195 100

2.0 2.3

160 215 100

1.7 2.0 0.8

1.1

'Weight of acid as 100%.

-

u!

6 5 4

3% 2 1

0 0

0.5

1.0

1.5

HCI

Fig. 4 Viscosity (-)

and pH (----)

of 1

2.0

23

3.0

Yo(W/W)

(w/w) (A)and 5 % (w/w)(0) chitosan in HCI.

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to solubilize 5 % (w/w) than 1 % (w/w) chitosan, as illustrated by the shift in maximum for the two viscosity curves. The viscosity profiles show a maximum, followed by an approximately constant level before dropping off. This can be explained as a change in conformation of chitosan as a polyelectrolyte due to increase in ionic strength.

2.3 Salt effects The repelling effect of each positively charged deacetylated unit on neighbouring glucosamine units will result in an extended conformation of the polymer in solution. The addition of electrolytes will reduce this effect, resulting in a more random coil like conformation of the molecule. At higher concentrations of electrolytes a salting-out effect will occur, precipitating the chitosan from the solution. The viscosity pattern in Fig. 5 , where measurements are carried out with three different salts at increasing concentrations, confirms the effects described above. The viscosity pattern in Fig. 4 can be explained in a similar way. 2.4 Compatibility Studies of the solubilities of chitosan in acids have shown that workable solutions can be achieved with acid concentrations as high as 50% (acetic, lactic and propionic acid). With formic acid it is even possible to dissolve chitosan in the pure acid. Figure 6 shows viscosities of aqueous chitosan solutions in presence of acetone, ethanol and methanol. At organic solvent concentrations as high as 50 %, chitosan still functions as a viscosifier without any precipitation. The compatibility of chitosan with organic compounds like cationic and nonionic polymers is good, while multivalent anions easily crosslink with chitosan to form gels and precipitates.'-3

2.5 Rheological properties 2.5.1 Viscosifier

Due to the high molecular weight and the linear unbranched structure of the 300r

Salt concentration (mold r i 3 )

Fig. 5. Salt effect. 0, NaCI; A,KCI; V, KBr.

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h I

0

I

I

20 40 60 Concentration of organic solvent % ( w l w )

I 80

Fig. 6. Compatibility with organic solvents. 0, Acetone; A, ethanol; V, methanol.

14r

Chitosan concentration % ( w l w )

Fig. 7. Viscosity as a function of chitosan concentration.

molecule, chitosan is an excellent viscosifier in acid environments. The viscosity as a function of chitosan concentration is shown in Fig. 7. 2.5.2 Gelling agent Reacting chitosan with a controlled amount of a multivalent anion will result in crosslinking between the chitosan m~lecules."~The network formed will have the ability to retain large amounts of water, in some systems as much as 95 % or more. The crosslinking can be done in acid, neutral or basic environments, depending on the method applied. Several gelling counterions are available of which some are listed in Table 2.

2.5.3 Film forming agent In principle a film can be considered as a thin gel, normally containing less water than in the case of ordinary gels. In practice, virtually the same methods are applied in making films as used for gel formation. In addition films can be made by careful evaporation of water from a pure chitosan solution. Membranes and fibres can be

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Counterions

401

for

TABLE 2 Ionotropic Chitosan

Gelation

of

Low molecular weight counterions Pyrophosphate Tripolyphosphate Tetrapolyphosphate Octapolyphosphate Hexametaphosphate (Fe(CN),(4-/(Fe(CN),)3 High molecular weight counterions Alginate K-Carrageenan Poly-1-hydroxy-1-sulphonate-propene-2 Hydrophobic counterions Octylsulphate Laurylsulphate Hexadecylsulphate Cetylsterarylsulphate

considered as films with extremely low water contents, made by precipitation of chitosan with a counterion.

3 COMPLEX BINDING OF METALS WITH CHITOSAN A large number of studies of the complex binding of metals to chitosan have been reported, where the influence of the raw material (chitin), the degree of deacetylation, effects of salt form, as well as physical behaviour of the chitosan have been investigated.’-I5

3.1 Influence of the raw material of chitin Chitin is known to exist in several forms depending on the species of origin of the raw material. The effect on the complexing ability is probably of minor importance and more a result of the chitin/chitosan process than the raw material itself.

3.2 Effect of degree of deacetylation Initial studies on the effect of the degree of deacetylation on complexing ability shows no significant differences even though some metals behaved differently. Subsequently it was apparent that the complexability of chitosan compares favourably with that of chitin. 3.3 Chitosan salts and derivatives

To avoid solubilization of chitosan when used in a resin form, various ways of stabilizing the solid state have been investigated. By crosslinking with phosphates

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and sulphates salt forms are achieved that with respect to most practical purposes are insoluble. In some cases improved affinity to the metal re~ults.'~.'' More recently interest in derivatives of chitin and chitosan has increased due to the fact that not only insoluble chitosan forms, but also substantial increases in complexing capacity' are achieved. Derivatives are, however, often more expensive to use and from a cost/benefit viewpoint less interesting for bulk applications. Another aspect of derivatization is the fear of residual amounts of unreacted compounds in case these could create undesired effects.

3.4 Chitosan as a coating material Based on positive results with complex binding of metals onto membranes a newly developed technique for coating of expanded silica with a large surface area with chitosan has been examined. Preliminary results indicate very high complex binding capacities when simulating removal of metals from drinking water by filtration purification. For example a reduction from 10 ppm down to 0.5 ppm in quantities of water adequate for single house operations has been found (unpublished).

4 MECHANISMS IN METAL UPTAKE ON CHITOSAN21-24 The mechanism of complex formation of metals with chitosan is manifold and probably dominated by different processes such as adsorption, ion-exchange and chelation under different conditions. Most studies of the mechanism so far are restricted to the Cuxhitosan complex, and variations in mechanism are to be expected with different metals. Adsorption studies indicate that both boundary layer resistance as well as intraparticle diffusion could be the ratecontrolling step. In such studies it is of outmost importance to work with wellcharacterized chitosan, knowing for instance not only degree of deacetylation, but also something about the distribution of acetyl groups along the molecule and throughout the chitosan particle. Spectroscopic studies have shown that different complexes exist depending on pH. The dominant complex in the case of Cu is proposed to have two O H groups and one NH, group as ligands and the fourth site either occupied by a water molecule or by the OH group on the C3 carbon atom. At higher pH values the CUNO, structure is proposed to change to a CuN,02 structure.

5 COMMERCIAL APPLICATIONS OF CHITOSAN-COMPLEX BINDING OF METALS The only field of applications where complex binding of metals with chitosan currently is of commercial importance is in waste-water treatment. More applications are expected to develop over the next years where substantial

Metal recovery using chitosan

403

quantities of chitosan will be required. (1) Waste-water treatment Higher demand in reduction of harmful metals from waste water to protect the environment and increased scepticism towards the use of synthetic flocculents make chitosan interesting in removal of heavy metals and radioisotopes, and in the recovery of valuable metals.’-’

(2) Water purification Drinking water supply for single and small numbers of users seldom includes sophisticated purification processes and reduction of pollutants like toxic metals is done by installation of filters with easily replaceable cartridges. Recycling of larger amounts of ‘process water’ often requires removal of accumulated metals. Chitosan products exist for removal of iron and manganese from swimming pools and spas. (3) Agriculture Chitosan has applications in controlled release of trace metals essential to plant growth.

(4) Food Storage of precooked meat results in development of bad flavours and odours caused by catalytic oxidation of unsaturated fatty acids. The catalytic oxidation depends on the available iron in the meat, and can be reduced considerably by complex binding of the iron.25.26 ( 5 ) Biomedical

Chitosan has shown excellent properties as a hypocholesterolemic agent. Coprecipitation in the intestine of trace metals (Fe in particular) has to be limited by careful selection of appropriate chitosan q ~ a l i t y . ~ ’ - ~ ’

REFERENCES I . Rippon, J . A., Improving the dye coverage of immature cotton fibers by treatment with chitosan. J . Soc. Dyers Colourists, 100 (1984) 298. 2. Knorr, D., Dye binding properties of chitin and chitosan. J . Food Sci., 48 (1983) 36. 3. Aiba, S., Fujiwara, Y., Hideshima, T., Hwang, C., Kakizaki, M., Monoura, N., Rha,C. K., Shoij, T., Sinskey, A. J. & Tsutsumi, A., Filmogenic properties ofchitin/chitosan. In Chitin in Nature and Technology, ed. R. A. A. Muzzarelli, C. Jeuniaux & G. W. Gooday. Plenum Press, New York and London, 1986, p. 389. 4. Yamaguchi, R., Hirano, S . , Arai, Y. & Ito, T., Chitosan salt gels thermally reversible gelation of chitosan. Agric. Biol. Chem., 42(10) (1981-1982) 1978. 5. Knorr, D. & Daly, M., Mechanics and diffusional changes observed in multi-layer chitosan/alginate coacervate capsules. Process Biochem., 23 (1988) 48. 6. Knorr, D. & Daly, M., Chitosan-alginate complex coacervate capsules: Effects of calcium chloride, plasticizers, and polyelectrolytes on mechanical stability. Biotechnol. Progress, 4(2) (1988) 76. 7. Lepri, L., Desideri, P.G. & Tanturli, G., Chromatographic behaviour of inorganic ions on chitosan thin layers and columns. J . Chromatography, 147 (1978) 375. 8. Muzzarelli, R. A. A. & Rocchetti, R., Enhanced capacity ofchitosan for transition-metal ions in sulphuric acid solutions. Tulata, 21 (1974) 1137.

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9. Yang, T. C. & Zall, R. R., Absorption of metals by natural polymers generated from seafood processing wastes. lnd. Eng. Chem. Prod. Res. Deu., (1984) 168. 10. Hauer, H., The chelating properties of Kytex H chitosan. In Proceedings of The First International Conference on ChitinlChitosan, ed. R. Muzzarelli & E. Pariser. MIT Sea Grant Report, 1978. 1 1 . Madhavan, P. & Ramachandran Nair, K. G., Metal-binding property of chitosan from prawn waste. In Proceedings of The First International Conference on ChitinlChitosan, ed. R. Muzzarelli & E. Pariser. MIT Sea Grant Report, 1978. 12. Yaku, F. & Koshijima, T., Chitosan-metal complexes and their function. In Proceedings of The First International Conference on ChitinlChitosan,ed. R. Muzzarelli & E. Pariser. MIT Sea Grant Report, 1978. 13. Muzzarelli, R. A. A., The chelating ability of amino-acid glucons and sugar acid chitosans. In Chitin in Nature and Technology, ed.R. A. A. Muzzarelli, C. Jeuniaux & G. W. Gooday. Plenum Press, New York and London, 1986, p. 321. 14. Muzzarelli, R. A. A., Natural Chelating Polymers. Pergamon Press, Oxford, 1973, p. 96. 15. Ogawa, K., Oka, K., Miyanishi, T. & Hirano, S., X-Ray diffraction study on chitosanmetal complexes. In Chitin, Chitosan and Related Enzymes, ed. J. P. Zikakis. Academic Press, New York, 1984, p. 327. 16. Sakaguchi, T., Horikoshi, T. & Nakajima, A., Adsorption of uranium by chitin phosphate and chitosan phosphate. Agric. Biol. Chem., 45 (10) (1981) 2191. 17. Masri, M. S. & Randall, V. G., Chitin and chitosan derivatives for removal of toxic metallic ions from manufacturing-plant waste streams. In Proceedings of The First International Conference on ChitinlChitosan, ed. R. Muzzarelli & E. Pariser. MIT Sea Grant Report, 1978. 18. Moore, G. K. &Roberts, G. A. F., Reactions ofchitosan: 4. Preparation oforgansoluble derivatives of chitosan. l n t . J . Biol. Macromol., 4 (1982) 246. 19. Muzzarelli, R. A. A., Chitin and its derivatives: New trends of applied research. Carbohydrate Polymers, 3 (1983) 53. 20. Muzzarelli, R. A. A., Chemically modified chitosans. In Chitin in Nature and Technology, ed. R. A. A. Muzzarelli, C. Jeuniaux & G. W. Gooday. Plenum Press, New York and London, 1986, p. 295. 21. Rinando, M. & Domard, A., Solution properties of chitosan. In Chitin and Chitosp: Sources, Chemistry, Biochemistry, Physical Properties and Applications, ed. G. SkjakBrrek, T. Anthonsen & P. Sandford. Elsevier Applied Science, London and New York, 1989, p. 71. 22. Domard, A., pH and c.d. measurements on a fully deacetylated chitosan: application to Cu"-polymer interactions. l n t . J . Macromol., 9 (1987) 98. 23. Park, J . W., Park, M. & Park, K. K., Mechanism of metal ion binding for chitosan in solution. Cooperative inter- and intramolecular chelations. Bull. Kor. Chem. SOC.,5 (1984) 108. 24. Ogawa, K., Miganiski, T. & Hirano, S., X-Ray diffraction study on chitosan-metal complexes. In Advances in Chitin, Chitosan and Related Enzymes, Proceedings of the Joint US-Japan Seminar. University of Delaware, USA. Academic Press, Orlando, FL, 1984, p. 327. 25. Coyner, M., Enright, N. & Powers, V., Does your hamburger lose its flavor on the refrigerator shelf overnight? Add some crab shells. In News Service, The American Chemical Society, 1&15 September 1989. 26. Angelo, A. J. St. & Bailey, M. E., Warmed-Ouer Flavor of Meat. Academic Press, New York, 1987. 27. Gordon, D. T. & Besch-Williford, C., Action of amino polymers on iron status, gut morphology, and cholesterol levels in the rat. In Chitin, Chitosan and Related Enzymes, ed. J. P. Zikakis. Academic Press, New York, 1984, pp. 97-1 17. 28. Nauss, J. L., Thompson, J. L. & Nagyvary, J., The binding of micellar lipids to chitosan. Lipids, 18 (10) (1983) 714. 29. Furda, I., Aminopolysaccharides- Their Potential as Dietary Fiber. American Chemical Society, 1983, p. 105.