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SEPARATIONS Sorption of Heavy Metals onto Selective Ion-Exchange Resins with Aminophosphonate Functional Groups Randolf Kiefer† and Wolfgang H. Ho1 ll*,‡ Krupp-Uhde GmbH, EL-PR, Friedrich-Uhde-Strasse 15, 44141 Dortmund, Germany, and Forschungszentrum Karlsruhe, Institute of Technical Chemistry, Section WGT, P.O. Box 3640, D-76021 Karlsruhe, Germany
The ion-exchange equilibria for the exchange of heavy metal ions (Cu2+, Ni2+, Cd2+, Zn2+, and Co2+) and of Ca2+, Na+, and NH4+ ions by two commercially available chelating ion-exchange resins with aminophosphonate functional groups (Purolite S 940 and S 950) have been studied. From simple binary systems of the exchange of each of the species for H+ ions, the series of selectivity for these species were determined. Evaluation of the binary data by means of the surface complexation theory led to sets of binary equilibrium parameters that remain unchanged in multicomponent systems. By means of these parameter values, ternary and quaternary equilibria and equilibria with complexing-agent-bearing wastewaters from metal surface treatments were predicted and compared with experimental results. In all cases, very satisfactory agreement was found. Introduction The removal of undesirable ionic impurities from wastewaters and process streams has led to the development of several types of selective ion exchangers. The most common of these resins have imino diacetic acid functional groups and are widely applied for final polishing of industrial wastewaters from metal finishing processes and other applications. Thiol and thiouronium functional groups provide excellent uptake of mercury species. Organic chemistry offers a wide range of functional groups or molecules that are selective for certain target species and that can be introduced into the matrix of ion exchangers. Additional selective resins have been synthesized, however, without being introduced into commercial applications, partly because of poor chemical and physical stability and partly because of the small amount required by commercial applications. A comprehensive summary can be found in the work by Sahni and Reedijk.1 Resins with aminoalkylphosphonate functional groups were first synthesized by Kennedy and Ficken2 in 1956 and by Manecke and Heller in 1960.3 Exchange resins with aminomethylphosphonic groups similar to modern resins were first prepared in 1960. The syntheses and applications of actual resins of this type have been comprehensively described by Klipper4 and Millar5 and co-workers. These exchangers have attracted increasing technical interest since about 1980. Their structure is schematically shown in Figure 1. One of their great advantages is the preferred uptake of alkaline earth ions from concentrated brine solutions down to concentrations of less than 0.02 mg/L. As a * Author to whom correspondence should be addressed. Phone: +49 (0)7247 82 3775. Fax: +49 (0)7247 82 3478. † Krupp-Uhde GmbH. ‡ Forschungszentrum Karlsruhe.
Figure 1. Structural elements of aminophosphonate resins.
consequence, pretreatment of such solutions prior to electrolysis is one of their most important fields of application. Klipper and co-workers4 presented separation factors for the exchange of calcium for sodium in the range of R(Ca/Na) ≈ 21 500. Only a few authors, however, have investigated resins with aminophosphate groups with respect to the sorption of heavy metals. Melling and West6 compared one commercially available resin with other chelating exchangers. Their investigations exhibited a fairly good uptake of copper mainly at pH values >3. Jacquin and co-workers7 studied the recovery of trivalent gallium from acid leach liquors of zinc ores and found a strong preference over zinc mainly at low pH values of 0.5-1.5. Leinonen et al. applied aminophosphonate resins to the elimination of zinc and nickel from wastewaters of the plating industry,8 whereas Lehto et al. used such resins for the elimination of zinc from wastewaters.9 Investigation of the applicability for heavy metal uptake was one of the main objectives of a Brite Euram EU research project carried out between 1996 and 1999.10 The present paper summarizes the results of systematic fundamental studies on equilibria in the uptake of heavy metals by some commercially available resins with aminophosphonic functional groups.
10.1021/ie010182l CCC: $20.00 © 2001 American Chemical Society Published on Web 09/15/2001
Ind. Eng. Chem. Res., Vol. 40, No. 21, 2001 4571 Table 1. Manufacturer’s Data on the Exchange Resins Purolite S 940
Purolite S 950
matrix
polystyrene/DVB, polystyrene/DVB, macroporous macroporous particle size, mm 0.425-0.85 0.3-1.2 bed density, g/L 710-745 710-745 particle density, g/cm3 1.11 1.11 moisture content, % 61 61 pH range 2-11 2-11 total capacity, mequiv/g (wet) H+ form 1.70 2.40 Na+ form 1.00 1.20 2+ Ca uptake 2.28 2.50 Table 2. Total Exchange Capacities of Centrifuged Wet Resin Material in the Respective Form qmax(Cu) qmax(Ni) qmax(Zn) qmax(Cd) qmax(Co) qmax(Ca) qmax(Na)
Purolite S 940
Purolite S 950
1.77 1.82 1.64 1.46 1.65 1.70 1.36
2.37 2.24 2.03 2.00 2.16 2.20 1.87
Experimental Section Exchange Resins. For experimental investigations, two commercially available exchangers with aminophosphonate functional groups and two additional resin samples provided by Purolite International were used. The resins and their properties are listed in Table 1.11 Ion-Exchange Capacities. To obtain reliable reproducibility of experimental data, the resin material as received from the manufacturers was treated three times with large excess volumes of 1 M HCl and 1 M NaCl to elute impurities and monomers. Resin samples in the sodium form were eventually converted to the metal forms by means of 0.5 M metal chloride solutions. Resin samples in the metal forms were weighed after 20 min of centrifugation to exclude water adhering to the outer surface of the resin material. Equilibrium Measurements. For measurements of resin capacities (maximum metal uptake), two samples of 1.5 g were contacted with 200 mL of 0.1 M or 1 M HCl for 7 days. The resin capacities are summarized in Table 2. For equilibrium experiments, usually four series of six to eight samples in the metal form (0.5-12 g) were contacted with 200 mL of solution. The contact time usually amounted to 7 days at a constant temperature of 25 °C. For studies of binary equilibria, the solutions contained either 10 mmol/L HCl or 10 mmol/L HCl plus 5, 10, or 25 mmol/L metal chloride of the metal originally on the exchange resin. In the case of ternary and quaternary equilibria, the solution contained 10 mmol/L HCl and metal chlorides of one or two metals originally not on the exchanger. Furthermore, simulated industrial wastewaters were equilibrated with resin material in the calcium form. The composition of these solutions is listed in Table 3. From the equilibrated solutions, samples were taken to determine the metal concentrations by means of ICP or atomic absorption spectroscopy. pH values were also measured. Mathematical Modeling of Equilibria General. The surface complexation theory as presented in several earlier publications 12-17 was applied
Table 3. Composition of Simulated Wastewaters model solution
composition
Watt’s nickel bath, diluted 1:100 sulfamate/nickel bath, diluted 1:100 weakly acidic galvanic bath, diluted 1:34 bath solution for cadmium removal, diluted 1:13 acid copper bath, diluted 1:80
6 mmo/L NiSO4 x 7 H2O, 4 mmol/L NiCl2 x 6 H2O, 5 mmol/L H3BO3, 10 mmol/L HCl 10 mmol/L Ni(SO3NH2)2 x 4 H2O, 5 mmol/L H3BO3, 10 mmol/L HCl 10 mmol/L ZnCl2, 50 mmol/L NH4Cl, 10 mmol/L HCl 10 mmol/L Cd(NO3)2, 10 mmol/L NH4NO3, 10 mmol/L HNO3 10 mmol/L CuSO4 x 5 H2O, 5 mmol/L H2SO4
for evaluation of experimental results and prediction of equilibria. In the surface complexation theory, it is assumed that the ion exchanger can be considered as a plane surface across which the functional groups are uniformly distributed. Surface charges are generated by the dissociation or protonation of surface groups. As a consequence, it can be assumed that protons are adsorbed directly on the surface. The majority of the other ions are located in Stern layers parallel to the surface and with individual distances from the surface. The remaining counterions and also coions are distributed across the diffuse layer, which normally can be neglected, as shown in the previous publications.12-17 Because of the electrical charges of each layer, the entire sequence can be considered as an electric capacitor. It is furthermore assumed that electrostatic interactions as well as swelling phenomena can be neglected. An exchanger valency zR of negative sign is defined that corresponds to the smallest common multiple of the counterion valencies. Activity coefficients in the resin phase are assumed to be 1. Equilibrium Relationships for a Binary System. The sorption of counterions is considered as a local equilibrium reaction. For a metal ion, this reaction can be written as
R-PO32- + A2+ w R-PO3A
(1)
Application of the mass action law to this reaction yields the constant of formation of this surface complex
KMe )
c(R-PO3A)
(2)
c(R-PO32-) c(A2+)St,A
The uptake of a competing ion B can be considered in a similar way. Taking A2+ as the reference ion, the ratio of formation constants
KAB )
c(R-PO3A) c(B2+)St,B c(R-PO3B) c(A2+)St,A
(3)
expresses the preference, or lack thereof, for the uptake of one of the species. The preference for A leads to numerical values KAB > 1; that for B, to values 1 are found. With respect to the meaning of this parameter, it can therefore be concluded that hydrogen ions are generally preferred by both resins. Nevertheless, copper, zinc, and cadmium are still sorbed relatively strongly. With the exception of cobalt and calcium, resin S 940, with its smaller total capacity, exhibits a stronger uptake of metal ions than does resin S 950. This is in agreement with theoretical considerations.19 From these results, the following series of selectivity can be deduced17
S 940: H+ > Cu2+ > Cd2+ > Zn2+ > Ni2+ > Co2+ > Ca2+ > Na+ S 950: H+ > Cu2+ > Zn2+ > Cd2+ > Ca2+ > Co2+ > Ni2+ > Na+ Comparison with results of equilibrium studies of the exchange of different metals against hydrogen ions on iminodiacetate resins reveals that the aminophosphonate resins studied have a smaller preference for copper and nickel but a much stronger preference for calcium
Ind. Eng. Chem. Res., Vol. 40, No. 21, 2001 4573
Figure 2. Development of generalized separation factor for the exchange of different metals for hydrogen ions. Resins: Purolite S 940 (left) and Purolite S 950 (right). Table 5. Equilibrium Parameters m(H,i) of the Exchange for Protons m(H,Cu) m(H,Ni) m(H,Zn) m(H,Cd) m(H,Co) m(H,Ca) m(H,Na) m(H,NH4) Figure 3. Development of generalized separation factor for the exchange of ammonium for hydrogen ions. Resin: Purolite S 950. Table 4. Equilibrium Parameters log KH i of the Exchange for Protons
log log log log log log log log
KH Cu KH Ni KH Zn KH Cd KH Co KH Ca KH Na H KNH 4
Purolite S 940
Purolite S 950
1.44 2.63 1.96 1.65 3.11 3.37 1.34 --
1.66 3.03 2.30 2.44 2.96 2.81 1.95 1.29
and sodium.13 The preference for all metal ions is considerably stronger than that of normal weakly acidic resins.12 Ternary Systems. The results of the investigation of ternary systems with protons and two divalent metal cations are plotted in Figure 4. The left-hand side shows the results for copper and nickel; those for zinc and cadmium are on the right-hand side. The equilibria for the commercially available exchangers S 940 and S 950
Purolite S 940
Purolite S 950
2.15 4.22 2.02 3.28 2.22 5.62 3.71 -
1.56 3.31 1.18 1.66 2.11 4.81 3.71 5.11
are represented as dimensionless metal loadings as a function of pH, together with the theoretical loadings predicted from the binary equilibrium constants. As can be seen, the theoretical approach provides excellent agreement with the experimental data. The increase of the relative loading with the metal ion originally on the resin at higher pH values is due to the increasing quantity of resin in these samples. Equilibria for protons and calcium with nickel or cobalt are plotted in Figure 5. Again, very satisfactory agreement between experimental and predicted sorption behavior is found. From these results, the pH values of maximum resin loading for each metal can be deduced. The respective values are summarized in Table 6.17 Quaternary Equilibria. The results of the investigation of different quaternary systems with hydrogen and three heavy metal ions are shown in Figure 6. Taking into account the difficulties and inaccuracies of the analytical determination of three metals in one sample, the agreement between the experimental data and the theoretical predictions is still quite satisfactory.
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Figure 4. Comparison of experimental and predicted resin loadings for the ternary systems Cu2+/Ni2+/H+ (left) and Cd2+/Zn2+/H+ (right). Resins: Purolite S 940 (top) and Purolite S 950 (bottom). Table 6. pH Value of Maximum Metal Loading Cu2+ Zn2+ Cd2+ Co2+ Ni2+
Purolite S 940
Purolite S 950
3.0-3.1 3.2-3.3 3.4-3.5 3.6 4.0-4.2
2.8-2.9 3.0 3.2-3.4 3.6 3.9-4.0
Similar agreement is found in systems with protons, calcium, and two heavy metal ions (Figure 6, right-hand side). Both the ternary and quaternary equilibria reveal the strong preference of copper over all other metals and the rather poor uptake of nickel.17 Equilibria with Simulated Wastewaters. In a final series of experiments, equilibria with simulated industrial wastewaters were investigated. The composition of these solutions is shown in Table 3. Because of its stronger selectivity and higher capacity, only Purolite S 950 in the calcium form was used for these experiments. Results for the four simulated solutions are shown in Figure 7. The diagrams again demonstrate very satisfactory predictions of the sorption behavior by means of the theoretical approach.17 Conclusions
Figure 5. Comparison of experimental and predicted resin loadings for the ternary systems Ca2+/Ni2+/H+ for Purolite S 940 (top) and Ca2+/Co2+/H+ for Purolite S 950 (bottom).
The comprehensive investigation of exchange equilibria of aminophosphonate resins with respect to various metals has demonstrated the following: The preference for heavy metals copper and nickel is smaller than that with iminodiacetate resins, whereas calcium and also sodium are much more strongly preferred. For most metal ions, a stronger preference is found for the resin with the smaller total capacity. The preference of calcium is stronger for the resin
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Figure 6. Comparison of experimental and predicted resin loadings for the quaternary systems Ni2+, Cu2+/Zn2+/H+ (left); Ca2+, Ni2+/ Cd2+/H+; and Ca2+, Cu2+/Ni2+/H+ (right). Resins: Purolite S 940 (top) and Purolite S 950 (bottom).
Figure 7. Comparison of experimental and predicted resin loadings for systems with simulated wastewaters containing NiCl2/NiSO4/ HCl/H3BO3 (top left), Ni(SO3NH2)2/HCl/H3BO3 (bottom left), ZnCl2/NH4Cl/HCl (top right), and Cd NO3/HNO3/NH4NO3 (bottom right). Resin: Purolite S 950 in Ca2+ form.
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modification with increased total capacity and even exceeds that for nickel and cobalt. Binary equilibria of the exchange for protons can be described by means of a set of two equilibrium parameters that remain constant for a wide range of ionic strengths. These constants need not be adapted or modified for multicomponent equilbria. In fact, a comparison of ternary and quaternary equilibria calculated by means of binary equibrium parameters shows excellent agreement with experimental results. The theoretical approach also allows for a satisfactory prediction of equilibria that include complexing agents in the liquid phase. The description of multispecies and reactioncoupled equilibria, therefore, allows for the prediction of both sorption kinetics and column dynamics,17,20,21 which might be useful in the industrial application of such exchange resins. Acknowledgment The authors are indebted to the European Union for financial support through the Brite Euram project BRPR-CT96-0196. Furthermore, they thank all partners in the project for the fruitful discussions and assistance, especially Purolite International for the supply of exchange resins. Notation Ao ) specific surface area, m2/g c(i) ) concentration of species i, mol/L c(i)St ) concentration of species i in Stern layer, mol/L c(i)x ) concentration of species i at position x in an electrical field, mol/L C(i,j) ) electric capacitance of capacitor formed by the layers of species i and j, F/m2 F ) Faraday constant, A s Kij ) constant of surface reaction, equilibrium constant m(i,j) ) abbreviation, defined by eq 8 q(i) ) resin loading with species i, equiv/g qmax ) maximum resin loading, equiv/g Qij ) generalized separation factor R ) gas constant, J/(mol K) T ) temperature, K y(i) ) dimensionless loading with species i z(i) ) valency of species i zR ) valency of resin with respect to surface complexes Ψx ) electrical potential at position x, V Counterions adsorbed at the surface are designated with chemical symbols without charge sign
Literature Cited (1) Sahni, S. K.; Reedijk, J. Coordination chemistry of chelating resins and ion exchangers. Coord. Chem. Rev. 1984, 59, 1. (2) Kennedy, J.; Ficken, G. E. Synthesis of metal-complexing polymers and R-aminophosphonate polymers. J. Appl. Chem. 1958, 8, 465. (3) Manecke, G.; Heller, H. Amphotere Ionenaustauscherharze vom Aminophosphonsa¨ure- und Aminocarbonsa¨ure-Typ. Angew. Chem. 1960, 72, 523. (4) Klipper, R. M.; Hoffmann, H.; Mitschker, A.; Wagner R. The influence of morphology and degree of substitution on the selectiv-
ity of chelating resins. In Ion Exchange for Industry; Streat, M., Ed.; Ellis Horwood Ltd,: Chichester, U.K., 1988; p 243. (5) Millar, J. E.; Petruzzelli, D.; Tiravanti, G. Some problems in the use of chelating resins for environmental protection from heavy metals. In Recent Developments in Ion Exchange; Williams, P., Hudson, M., Eds.; Elsevier Applied Science: London, 1990; p 337. (6) Melling, J.; West, D. W. A comparative study of some chelating ion-exchange resins for application in hydrometallurgy. In Ion Exchange for Industry; Streat, M., Ed.; Ellis Horwood Ltd.: Chichester, U.K., 1988; p 724. (7) Jacquin, O.; Faux-Mallet, S.; Cote, G.; Baur, D. The recovery of gallium(III) from acid leach liquors of zinc ores using selective ion-exchange resins. In Recent Developments in Ion Exchange; Williams, P., Hudson, M., Eds.; Elsevier Applied Science: London, 1990; p 213. (8) Leinonen, H.; Lehto, J.; Ka¨kela¨, A. Purification of nickel and zinc from wastewaters of metal plating plants by ion exchange. React.Funct. Polym. 1994, 23, 221. (9) Lehto, J.; Vaaramaa K.; Leinonen, H. Ion Exchange of Zinc by an Aminophosphonate Resin. React.Funct. Polym. 1997, 33, 13. (10) Lehto, J.; Streat, M.; Ho¨ll, W. H.; Dale, J.; Greig, J.; YliPentti, A. Development of advanced ion exchange materials and methods for the removal of toxic metals from metallurgical waste effluents. In Proceedings of the TRAWMAR Annual Workshop, Berlin, Germany, Oct 4-7, 2000; MIRO: Lichfield, Staffordshire, U.K., 2000. (11) Purolite International Ltd., Middlesex, U.K. Purolite Ion Exchange Resins, Product information, 1997. (12) Horst, J.; Ho¨ll W. H.; Eberle, S. H. Application of the surface complex formation model to exchange equilibria on ionexchange resins. Part I: Weak acid resins. React. Polym. 1990, 13, 209. (13) Ho¨ll, W. H.; Horst J.; Wernet, M. Application of the surface complex formation model to exchange equilibria on ion-exchange resins. Part II: Chelating resins. React. Polym. 1991, 14, 251. (14) Ho¨ll, W. H.; Horst J.; Franzreb M. Application of the surface complex formation model to exchange equilibria on ionexchange resins. Part III: Anion exchangers. React. Polym. 1993, 19, 123. (15) Ho¨ll, W. H.; Horst J.; Franzreb, M.; Eberle, S. H. Description of ion-exchange equilibria by means of the surface complexation theory. In Ion Exchange and Solvent Extraction, A Series of Advances; Marinsky, J., Marcus, Y., Eds., Marcel Dekker: New York, 1993; Vol. 11, p 151. (16) Horst J.; Ho¨ll W. H. Application of the surface complex formation model to exchange equilibria on ion-exchange resins. Part IV: Amphoteric sorption onto γ-aluminium oxide. J. Colloid Interface Sci. 1997, 195, 250. (17) Kiefer, R. Untersuchungen zur Elimination und Trennung von Schwermetallen aus Abwa¨ssern mit Kationenaustauschverfahren. Ph.D. Dissertation, University and Forschungszentrum Karlsruhe, Karlsruhe, Germany, 1999. (18) Bronstein, I. N. Semendjajev, K. A. Taschenbuch der Mathematik; Harri Deutsch: Frankfurt, Germany, 1989. (19) Helfferich, F. Ion Exchange; McGraw-Hill: New York, 1962; p 157. (20) Franzreb, M.; Ho¨ll, W. H.; Eberle, S. H. Liquid-phase mass transfer in multicomponent ion exchange. II. Systems with irreversible chemical reactions in the film. Ind. Eng. Chem. Res. 1995, 34, 2670. (21) Kalinitchev, A. I.; Ho¨ll, W. H. Computerized description of the concentration waves in nonlinear ion exchange systems on the basis of the surface complexation model. In Ion Exchange at the Millenium; Greig, J., Ed.; Imperial College Press: London, 2000; p 201.
Received for review February 22, 2001 Revised manuscript received July 12, 2001 Accepted July 22, 2001 IE010182L
