2674
Ind. Eng. Chem. Res. 1996,34, 2574-2583
Poly(ethy1ene imine)-BasedGranular Sorbents by a New Process of Templated Gel-Filling. High Capacity and Selectivity of Copper Sorption in Acidic and Alkaline Media Manas Chandat and Garry L. Rempel* Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
A new process has been developed for making granular gel-type sorbents from chelating resins using metal ion as template. Named as templated gel-filling, the process uses the chosen metal as templating host ion on high-surface-area silica to build a templated gel layer from a solution of the chelating resin in a suitable solvent in which the resin is soluble but its metal complex is insoluble. After cross-linking the templated gel layer, the silica support is removed by alkali to produce a hollow shell of the templated gel. The shells are then soaked in a concentrated aqueous solution of the same metal ion and suspended in the same resin solution to afford gel-filling. The shells thus filled with metal-templated gel are treated with cross-linking agent, followed by acid to remove the template ion and activate the resin for metal sorption. Poly(ethy1ene imine) and its partially ethylated derivative have been used to produce granular gel-type sorbents by this process, with Cu(I1) as the template ion. These sorbents are found to offer high capacity and selectivity for copper over nickel, cobalt, and zinc in both acidic and alkaline media. Containing a relatively high fraction of imbibed water, the sorbents exhibit markedly enhanced rate behavior, in both sorption and stripping.
Introduction The three important functional properties of ionexchange resins that influence their applications are resin capacity, selectivity, and rate behavior for loading and elution. Researches in resin development aim at enhancing these properties. The equilibrium sorption capacity of a commercial resin is oRen found to be much less than the theoretical capacity calculated from resin composition. This is especially true for resins synthesized from functional group bearing monomers. The nonattainment of theoretical capacity in such cases is attributed to the inaccessibility of many sorption sites buried inside the resin matrix. For example, commercial poly(4-vinylpyridine) has a measured proton capacity of 5.7 mequivlg of dry resin (Chanda et al., 1983) as compared to the theoretical proton capacity of ca. 8 mequivlg of dry resin. Similarly commercial polybenzimidazole has a measured proton capacity of 4.5 mequivlg of dry resin (Chanda et al., 1985) as compared to the theoretical capacity of 6.5 mequiv/g of dry resin. The discrepancy between measured and theoretical capacities is often found to be much higher for other larger sorbate species (Chanda et al., 1983). The chemical reaction at the sorption sites in the resin being usually too fast to affect the overall sorption rate, the rate-determining step in most cases of ion-exchange sorption has been established to be either intraparticle diffusion or external film diffusion (Helfferich, 1991). While the film diffusional resistance can be eliminated or minimized by effective agitation of the external liquid, intraparticle diffusional resistance is largely dependent on the physical characteristics of the resin matrix. With increasing conversion of the resin bead, the diffusion path becomes longer and more tortuous depending on the nature of the matrix. The rate of sorption therefore decreases significantly with progressive resin conversion, necessitating prolonged contact with the sorbate to reach equilibrium. + O n leave from Indian Institute of Science, Bangalore, India.
Since both the problems of nonattainment of theoretical capacity a t equilibrium and slow rate of attainment of equilibrium sorption are related to degrees of inaccessibility of sorption sites in the interior of resin beads, a significant improvement in both these respects results from preparing the sorbent as a thin layer on high surface area substrate. This consideration prompted us (Chanda and Rempel, 1993, 1994) to develop a process of gel coating a weak-base resin poly(4-vinylpyridine) and high quaternized poly(4-vinylpyridine) on highsurface-area silica. These sorbents, as expected, exhibited significantly faster kinetics and greater attainment of the theoretical capacity of the coated resin. However, since the coating constitutes only about 5% (w/w)of the composite sorbent, the capacity per unit volume of the sorbent amounts t o only a small fraction of that of the coated resin. As a result, the application potential of surface-coated sorbents remains essentially limited to relatively small scale separations as in chromatography. Their application on a large scale for metal recovery and purification becomes uneconomic because of the requirement of large volume to overcome the reduced overall capacity of the sorbent. This deficiency of the coated sorbent prompted us to undertake development of stable gel-type porous sorbents with relatively large fraction of imbibed water which would afford higher capacity (due t o greater accessibility of sorption sites) coupled with faster kinetics (due t o imbibed water), as compared to conventional bead-type, rigid sorbents. The effect of imbibed water is explained by the model of Mackie and Meares (1955) which relaies diffusion coefficient of a species in ion exchanger (D) to the diffusion coefficient in solution (D)by D = D[q1(2 - q)I2, where 7 is the fractional intraparticle void volume (satisfactorily approximated by weight fraction of imbibed solvent); a larger amount of water in the resin thus contributes to greater diffusivity. Commerical cation-exchange resins containing sulfonic or carboxylic acid functional groups have limited potential for removal and recovery of heavy metals from process solutions and waste streams because of their
0888-588519512634-2574$09.00/0 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995 2575 low selectivity. A number of chelating ion-exchange resins have been produced by resin manufacturers to overcome this problem. Iminodiacetic acid resins, polyamine (24aminomethyl)pyridine) resins, and aminophosphonic acid resins are some of the more widely used resins in this category (Wolff, 1982; Green and Hancock, 1982; Jones and Grinstead, 1977; Grinstead, 1979). Cross-linking of polymer-ligand chain maintained at the optimal conformation for the coordination sphere of a metal ion to enhnace the sorption selectivity of the same metal ion was first attempted by Nishide et al. (1976, 1977). They made a soluble complex between a polymer ligand, such as poly(4-vinylpyridine) (PVP),and a chosen metal ion, and then cross-linked the free pyridine sites by adding a cross-linking agent to the solution of the polymer complex. The template ion was then removed by treatment with an acid to activate the resin for metal sorption. If the conformation of the polymer-ligand chain is maintained a t best for the metal ion used as template, it is expected that the resin will preferentially form a complex with the same metal ion when the resin is added to a solution containing various metal ions. Nishide et al. (1977)used a low-molecular-weightPVP having an average degree of polymerization 122 as the polymer ligand, copper(II), cobalt(II), zinc(II), and cadmium(I1) as template ions, and 1,Cdibromobutane as the cross-linking agent. They studied the sorption of these metal ions on the template-quaternized resins after removal of the template ions and found the maximum effect for copper(I1) used as template ion, while for other metal ions the template effect was not significant. Copper was also used as the templating host ion in the earlier gel-coating process developed by us (Chanda and Rempel, 1993)for coating PVP on high-surface-area silica. Extending this work further, the same template ion has now been used to fill, by reacting with a chelating resin, the empty space created by removing the silica support with alkali. Subsequent removal of the template metal ions after cross-linking the gel with a suitable reagent then yields a porous granular sorbent with high capacity and fast kinetics. Using this new process of templated gel-filling (TGF),we have prepared granular sorbents from poly(ethy1ene imine) (PEI) and partially ethylated poly(ethy1eneimine) (EPEI), employing in each case copper(I1) as the template ion. The process is, however, general and can be used with other chelating resins and metal ions, the work on which is under way. In addition to producing granular sorbents based on chelating resins, the process offers the possibility of introducing memory effect and hence greater selectivity for the metal ion used as template. However, the extent to which the selectivity is enhanced will be influenced by several factors, such as relative sizes of the ions to be separated, inherent sorption preference of the chelating resin, and the extent of retention of ligand orientation even after the removal of the template ion. A high selectivity may be achieved when the net effect of all these factors is favorable for the chosen metal ion. In the present paper the TGF process has been described in detail and PEI- and EPEI-based sorbents made by this process have been used for studying sorption characteristics and selectivity for copper over nickel, cobalt, and zinc in both acidic and alkaline media. The results are presented below.
l
1
1
1
1
1
1
1
1
1
55
60
l
1
1
1
1
1
50 PPm
1
1
1
1
45
1
1
1
40
lW ETHYLENE PROTONS
ETHYL PROTONS
-
1
1
1
5.0
1
1
1
l
1
4.0
1
1
1
1
1
3.0
1
1
1
1
1
2.0
1
1
1
1
1
1
1
1 .o
PPm Figure 1. (a) 13C-NMR of poly(ethy1ene imine) in water. (b) Proton NMR of partially ethylated poly(ethy1ene imine) in DzO.
Experimental Section Preparation of Sorbents. A commercial branched poly(ethy1ene imine) (PEI) (Aldrich Catalog No. 18,1978)of average molecular weight 50 000-60 000 was used for making a granular cross-linked sorbent by the TGF process. The polymer was analyzed by 13C-NMR(Bruker -400, 100.6 MHz) in water solution. The direction and magnitude of the shift in the spectrum (Figure la) are determined by the number and proximity of exchangeable NHs and are described by a simple formula (Pierre and Geckle, 1985). The calculated results from Figure l a indicate primary, secondary, and tertiary amine distributions as 38%, 34%, and 28%, which are in good agreement with the respective values 38%, 36%, and 26% reported (Pierre and Geckle, 1985) for a branched poly(ethy1ene imine), PEI-18 (Dow Chemical). The above PEI from Aldrich was also converted partially alkylated by reacting with ethyl bromide in methanol solution. Ethyl bromide (0.75 mol) was added to 65 g of 50% aqueous solution of PEI (0.75 mol) dissolved in 1L of methanol. The solution was refluxed with mixing for 48 h, after which an aqueous solution of NaHC03 (500 mL, 10% w/v) was added. Methanol and water were removed by distillation under vacuum. From the residue containing ethylated PEI and sodium salts, the former was extracted with methanol and recovered by further removing the methanol by distillation. The semisolid product was analyzed to determine the extent of ethylation. A proton NMR spectrum of the ethylated PEI in DzO is shown in Figure lb. From the relative areas of ethyl protons and ethylene protons (the imine protons do not show up due to rapid exchange with deuterium of DzO), the extent of ethylation was calculated to be about 22%.
2576 Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995
-
P E I in n-butonol
Glutoroldehyde in
_____)
n-butonol
LTemplote
P E I loyer (Cu complex)
hort ion (C3+)
PEI loyer (Cu complex)
of croralinked P E I
cu so4
roln. (04
-
-
Glutoroldehyde in
6)- 0 4
7
n- butanol
(ii) NoOH
TGF-PEI granular sorbent
PEIin n-butonol
dr
o r linked P E I gel (Cu complex)
’
PE I gel (CU complex)
‘Crorrlinked PEI(Cu complex) outer shell
Crosslink
Figure 2. Schematic of templated gel-filling process for making gel-type granular sorbent from poly(ethy1eneimine) using Cu2+as template.
Partial and not full alkylation is essential for the TGF process used in the present work as glutaraldehyde cross-linking cannot be performed on tertiary polyamine. The TGF process developed for making granular sorbents is illustrated schematically in Figure 2 for poly(ethylene imine). The process essentially consists of two steps. In the first step, a cross-linked layer of the chelating polymer is formed on a high-surface-area silica by using the desired metal as the templating host ion, followed by leaching successively of the metal ion and silica to obtain a porous hollow shell of the cross-linked polymer. In the next step, the shells are filled with concentrated aqueous solution of the same metal ion which now acts as the host ion to fill up the shell by accepting the chelating polymer from a suitable nonaqueous medium in which the polymer is soluble but the polymer-metal complex is insoluble. The polymer is then cross-linked with a suitable cross-linking agent and the chelating host ion removed by acid leaching to obtain a microporous gel-type granular sorbent. Both poly(ethy1ene imine) (PEI) and partially ethylated poly(ethy1ene imine) (EPEI) were used to produce granular sorbents by the aforesaid process and were designated TGF-PEI and TGF-EPEI, respectively. In a typical procedure of making TGF-PEI with Cu(I1) as the template ion, 100 g of silica gel (Aldrich Catalog No. 23,608-2,35-60 mesh) was soaked in 250 mL of 2% (w/ v) CuSOs5HzO solution and evaporated to dryness on a water bath with continuous mixing (I). PEI (20 g) available as 50% aqueous solution was dissolved in 400 mL n-butanol(I1). I was added t o I1 and shaken on a gyratory shaker for 6 h. The solution was decanted off, and the gel was washed several times with n-butanol. The gel-coated solid was then dispersed in 400 mL of 10% (w/v)glutaraldehyde solution in n-butanol and the mixture was vigorously agitated a t room temperature for 12 h. The PEI-Cu(I1) complex gel-coated on silica and cross-linked by reaction with glutaraldehyde in this way was then treated successively with 2 N HzS04 and 2 N NaOH to leach out Cu(I1) and SiOz, respectively, leaving behind porous, hollow shells of cross-linked PEI. The shells were then impregnated with CuSO4 solution by soaking in 30% (w/v)CuSO4 solution for a few hours and then shaken on a gyratory shaker at room temperature in a solution of PEI (80 g, 50% aqueous solution)
Table 1. Elemental Compositions of Sorbents Used for Comparison of Copper Sorption and Selectivity elements (wt %) sorbent C H N 0 TGF-PEI 57.5 10.1 13.4 16.6 TGF-EPEI 58.5 10.3 12.5 15.4 SiOZ. .[PEI14 4.0 0.7 0.9 0.8 The balance being silica.
in n-butanol(800 mL) for 12 h. This resulted in filling of the hollow shells with a gel of PEI-Cu(I1) complex which was then cross-linked by treating with 500 mL 10% (w/v) glutaraldehyde solution in n-butanol for 12 h at room temperature. The cross-linkedand templated PEI granular gel was washed successively with 2 N Hzso4 and 0.1 N NaOH, and then thoroughly washed with water till free of alkali. The resulting granular sorbent designated TGF-PEI was stored in wet condition. A granular gel-type sorbent designated TGF-EPEI was also made using the ethylated PEI prepared from the same PEI and the above process of templated gelfilling with Cu(I1) as the templating host ion. The sorbents were used for studying their sorption behavior and copper selectivity relative to nickel, cobalt and zinc. For comparison, poly(ethy1ene imine) coated on silica gel, a commercial product from Aldrich (Catalog No. 24,674-31,was also used in the sorption studies. This product, containing about 94% silica and 6% (w/w) crosslinked PEI, is designated in this study as SiOz**[PEIl, in accordance with the system of notation given by Warshawsky and Upson (1989). The elemental compositions of the sorbents are given in Table 1, and the properties of the sorbents are given in Table 2. The pore size distribution of the dried sorbent TGFPEI is shown in Figure 3 which indicates that all the pores in the dry sorbent particle are less than 100 A with most of the pores being on the order of 20 A. The TGF process thus yields a microporous gel-type sorbent. However, since the dry sorbent imbibes a considerable amount of water, the pore sizes in the water-swollen gel would be expected to be significantly larger than in the dry gel. Sorption Experiments. For measurement of equilibrium sorption of metal ions, small-scale dynamic contacts between the sorbent and aqueous solutions of
Ind. Eng. Chem. Res., Vol. 34,No. 8, 1995 2577 Table 2. Properties of Sorbents Used for Comparison of Copper Sorption and Selectivity sorbent properties water content (g/g of wet sorbent) swelling (%) particle diameter, (mm) wet dry BET surface areaa (m2/gof dry) pore volumeb (cm3/gof dry) proton capacity (mequivlg of dry)
TGF-PEI
TGF-EPEI
0.79
0.80
SiOz. *[PEIl
65
70
28
0.26-0.85 0.22-0.73 1.6
0.32-0.90 0.27-0.75 3.2
0.34-1.16 0.31-1.05 107
3.7
3.8
0.52
9.0
8.5
0.6
Results and Discussion
Dried sorbent. Estimated from water imbibed by dry sorbent. 0 r)
e,
b
LL
1.60
0 0.001 LL
' 20
'
Analysis. Metal ions in aqueous solution were estimated with an ARL (Applied Research Laboratories) Fissions Spectraspan 7 (SS-7) DCP emission spectrometer. Chemical methods and W/visible spectrophotometry (Perkin-Elmer Lambda 3B) based on complexes with specific reagents were used for comparative and supplementary analysis. CoppedII) was estimated from the absorbance at 612 nm after addition of NfiOH with the help of calibration chart. Volumetric EDTA methods were used for other metal ions (Vogel, 1961). BET surface areas of the three sorbents and pore size distribution of TGF-PEI were measured on a Quantachrome Autosorb Automated Gas Sorption System.
??s.&t 50
! '
100 200
:
'
V I
500 1000
RADIUS, A Figure 3. Pore size distribution of TGF-PEI gel-type resin (dry).
the metal ion of specified concentration were effected in tightly stoppered flasks for 12 h on a gyratory shaker. The extent of sorption was calculated from the residual concentration of the sorbate in the equilibrated solution. For determination of sorption kinetics wet-sieved sorbents of narrow particle size range were used. Dynamic contacts between sorbent and solution were effected in small batches, separately for each time period. For relatively long contact periods ( Z 1min), the suspension of sorbent in solution was shaken on a gyratory shaker and the system was frozen after a specified period by draining the solution quickly through a screen with openings small enough to retain the resin particles. The ratio of drainage time to the time of shaking was much less than 0.1 for dynamic contact periods greater than 1 min. For experiments involving relatively short dynamic contacts ( Ni(I1) > Co(I1)= Zn(I1). This order of sorption capacities is more clearly revealed by sorption isotherms. Sorption Isotherm. The equilibrium sorptions of Cu(II), Ni(II), Co(II),and Zn(I1)were measured on TGFPEI, TGF-EPEI, and SiOz**[PEI]both in acidic media at pH 4.5 and in ammoniacal media a t pH 11.0. These sorption data are plotted against equilibrium concentration in Figures 6-9. The equilibrium sorption of Cu(11)is seen to be much less dependent on the sorbate concentration than those of Ni(II), Co(II), and Zn(II), implying that the former is associated with stronger binding than the latter. The equilibrium sorption data in Figures 6-9 fitted well to the Langmuir isotherm, yielding correlation coefficients in the range 0.974-0.999. The parameters A, and Kb,representing, respectively, the saturation sorption capacity (mmol metayg dry resin) and sorption binding constant (Umol), the Langmuir isotherm is written as
where x* is the equilibrium sorption (mmol of metayg of dry resin) and C*is the equilibrium sorbate concentration (mmoYL). The values Of Kb and A, determined by least squares fit of the sorption data in Figures 6-9 are presented in Table 3. It is seen from the data in Table 3 that, among the four metal ions, only Cu(I1)has the combination of high capacity and high binding constant on both TGF-PEI and TGF-EPEI. For both Cu(I1) and Zn(I1) the binding constant on TGF-PEI and TGF-EPEI is an order of magnitude greater in acidic media than in alkaline media, while for Ni(I1) and Co(I1)the binding constants
2578 Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995
0.8 0.6 0.4
0.2 2
3
0.0
4 5 PH PH Figure 4. Effect of pH on the sorption of (a) Cu(I1) and (b) Ni(I1) on sorbents. Initial concentration 10 mmol/L; loading 4.0-8.0 g (wet)/ L; temperature 25 "C.
3
2
0.8 0.6
1
0.4
0.2 0.0
0
0.0
PH PH Figure 5. Effect of pH on the sorption of (a) Co(I1) and (b) Zn(I1) on sorbents. Initial concentration 10.0 mmol/L; temperature 25 "C.
3
(b) Ni (II). pH 4.5
1
0.4
0.3 0.2 0.1 0
2 4 6 8 1 EQUILIBRIUM CONC., mmol C u (II)/L
w
0.0
0
EQUILIBRIUM CONC., mmol N i (II)/L
Figure 6. Sorption isotherms of (a) Cu(I1) and (b) Ni(I1) on sorbents in mildly acidic media. Loading 4.0-8.0 g (wet)& temperature 25 "C.
are of similar order of magnitude. In comparison, the sorbent SiOy {PEII exhibits both higher sorption capacity and higher binding constant in alkaline media than in acidic media for all four metal ions. This is due to the large contribution of Si02 which constitutes about 94% (w/w) of the sorbent. For metal ions with no significant size difference, the sorption selectivity is largely influenced by the sorption affinity represented by the Kb values. A positive template effect would therefore lead to higher Kb values. This is clearly supported by a comparison of the Kb values for Cu(I1) sorption on TGF-PEI and SiOy {PEI] at pH 4.5. The template effect on the sorption capacity
is, however, not significant as is revealed by a comparison of the A, value for Cu(I1)sorption at pH 4.5 on TGFPEI with that on SiOz.*[PEIl which, it may be noted, contains only about 6% (w/w) PEI. Selectivity for Cu(I1). The selectivity of the sorbents for Cu(I1) over Ni(II), Co(II), and Zn(I1) in both acidic and alkaline media was determined by calculating the separation factor Scm, defined by X*CUC*M
SCurM
where x*cU and
X*M
= x*
MC*Cu
represent equilibrium sorptions
Ind. Eng. Chem. Res., Vol. 34,No. 8, 1995 2679
3 (a) Co (ll). pH 4.5
- 0.3 - 0.2
EQUILIBRIUM CONC., EQUILIBRIUM CONC., mmol Zn(n)/L mmol Co (II)/L Figure 7. Sorption isotherms of (a) Co(I1) and (b) Zn(I1) on sorbents in mildly acidic media. Loading 4.0-8.0 g (wet)&; temperature 25 "C.
"1
EQUILIBRIUM CONC., mmol Cu (II)/L
(b) Ni (ll)/NHQH
pH 11.0
EQUILIBRIUM CONC., mmol Ni(II)/L
Figure 8. Sorption isotherms of (a) Cu(I1) and (b) Ni(I1) on sorbents in ammoniacal media. Loading 4.0-8.0 g (wet)%; temperature 25 "C.
EQUILIBRIUM CONC., mmoi C o (II)/L
EQUILIBRIUM CONC., mmol Zn(n)/L
Figure 9. Sorption isotherms of (a) Co(1I) and (b) Zn(I1) on sorbents in ammoniacal media. Loading 4.0-8.0 g (wet)/L; temperature 25 "C.
(mmol/g of dry resin) of Cu(I1) and metal M (other than CUIin a mixture of Cu and M in aqueous solution; C*cu and C*Mare the equilibrium concentrations (mmoVL) of Cu(I1)and M in solution in the presnce of the sorbent. The equilibrium sorptions of Cu(II), Ni(II), Co(II),and Zn(I1) on TGF-PEI, TGF-EPEI, and SiOz*.[PEI] were measured employing binary mixtures of Cu(I1) and one
of the other three metals, each having an initial concentration of 10 mmoVL. The separation factor values calculated from eq 3 are presented in Table 4. The results in Table 4 show that while both the Cu(11)templated gel-filled sorbents derived from PET and EPEI exhibit high selectivity for Cu(I1) over Ni(II), Co(11),and Zn(I1)in both acidic and alkaline (ammoniacal)
2580 Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995 Table 3. Langmuir Isotherm Parameters A. and Kb for Sorption of Cu(II), "I), Sorbents at pH 4.5 and 11.0
3.62 4.10 0.19 4.43 4.07 0.21 2.56 2.76 0.21 2.73 2.78 0.19
Cu(II)/TGF-PEI Cu(II)/TGF-EPEI Cu(II)/SiO2..[PEIl Ni(II)/TGF-PEI Ni(II)/TGF-EPEI Ni(II)/Si02**[PEIl CO(1I)PTGF-PEI Co(II)/TGF-EPEI Co(II)/SiO2.-[PEIl Zn(II)/TGF-PEI Zn(II)/TGF-EPEI Zn(II)/Siz.-[PEIl
4080 3470 370 156 202 159 470 484 192 1385 1930 302
0.994 0.998 0.997 0.995 0.996 0.974 0.993 0.992 0.995 0.996 0.997 0.999
Table 4. Separation Factor for Selectivity of Cu(I1) over Ni(II), Co(II),and Zn(I1) in Acidic and Alkaline Media Scu/Ni
sorbent TGF-PEI TGF-EPEI SiOz..[PEI]
scuico
SCuiZn
PH4.5 PH 11.0 PH4.5 pH 11.0 pH 4.5 8.5 11.6 1.5
15.0 18.4 8.0
12.6 17.5 4.8
18.4 25.8 2.2
20.5 32.6 11.9
pH 11.0
98 124 0.1
media, the selectivity is significantly higher in alkaline media than in acidic media. Moreover, the sorbent derived from EPEI exhibits higher selectivity than the one derived from PEI. The separation factor values for Si020 .CPEIl which are dominated by the contribution of the Si02 used as support show that while Cu(I1) has higher selectivity over Ni(I1) and Co(I1) in both acidic and ammoniacal media, Zn(I1) has higher selectivity over Cu(I1)in ammoniacal media as would be expected in view of the significantly higher binding constant for Zn(I1) on SiO2**[PEI]a t pH 11.0 (Table 3). It may be mentioned that while the CuUIbtemplated TGF sorbent exhibits, as noted above, high selectivity for Cu(I1) over each of the other metals Ni(II), Co(II), and Zn(II), no enhancement of selectivity by the template effect was observed for Ni(II), Co(II), and Zn(I1) in the presence of Cu(I1). This may be attributed to the inherently high selectivity and strong binding of Cu(I1) on PEI, as there are no significant size differences among these metal ions. Experiments were conducted to determine the effect of high background concentrations of sodium salts such as NaCl and Na2S04. It was found that neither of these salts, even in 100-fold higher concentrations than Cu(111, has any significant effect on the Cu(I1) sorption capacity of both TGF-PEI and TGF-EPEI. Kinetic Considerations. Since TGF-EPEI has the highest selectivity for Cu(I1) among the three sorbents, kinetic experiments were performed only with this sorbent. The sorption of copper on TGF-EPEI from copper sulfate solutions of different initial concentrations both in acidic medium at pH 4.5 and in alkaline medium in NH40H a t pH 11.0 was measured as a function of time under conditions of vigorous agitation at ambient temperature. Such data are plotted as fractional attainment of equilibrium sorption U t )as a function of time t in Figure 10. It is seen from Figure 10 that the concentration of the sorbate has a strong effect on the rate of sorption of Cu(I1) in both acidic and alkaline media, though the effect is relatively more pronounced in acidic media. Interestingly, the rate of sorption is remarkably faster in alkaline media than in acidic media, the t l l z values for 50%attainment of equilibrium sorption being 8 min
Co(II), and Zn(I1) on Different
4.17 4.56 0.67 2.43 2.15 0.52 1.47 1.51 0.77 0.80 0.82 0.62
465 522 742 214 260 414 263 208 557 238 185 1194
0.995 0.999 0.988 0.997 0.995 0.979 0.999 0.947 0.988 0.999 0.994 0.986
and 27 s at pH 4.5 and 11.0, respectively, for an initial sorbate concentration of 10.0 mmol of Cu(II)/L. The enhancement of sorption rate in ammoniacal media may be attributed a t least partly t o the observed fact that the gel-type sorbent undergoes greater swelling in ammoniacal media. The sorption of Cu(I1)by the gel-type polyamine resin involves mass transfer accompanied by chemical reactions. Since relatively few sorption data are available, simple models were used t o describe the rate of mass transfer. The following two one-parameter models were used: (1)film diffusion in the fluid and (2) heterogeneous diffusion in the resin (progressive shell mechanism). A film-diffision-controlledbatch ion-exchange experiment can be described with the following equation given by Helfferich (1962)
where U t )is the fractional attainment of equilibrium sorption a t time t , D is the diffusion coefficient in the film, 6 is the film thickness, ro is the radius of the resin particle, P i s the resin volume, V is the solution volume, C is the counterion concentration in solution, and is the counterion concentration in the resin. For film diffusion control, a plot of -ln[l - U(t)l uersus t should be linear. Equation 3 was tested with sorption data for Cu(I1) on TGF-EPEI resin. In all cases, the plot was curved, indicating that the sorption was not under film diffusion control. The description of the rate of sorption according t o a shell progressive mechanism for particle diffusion control might be valid for species with a high affinity for the resin resulting in a quasi-irreversible sorption. Shell diffusion involves a moving boundary transport, and several investigators presented solutions to this type of problem. An analytical solution is available for shell progressive mechanism of sorption kinetics with constant bulk concentration (Helfferich, 1965; Schumckler and Goldstein, 19741, which is referred to as infinite solution volume condition. The model, however, does not apply to batch experiments with limited solution volume or changing bulk concentration, which are more convenient to perform for comparative evaluation of sorbents. An analytical solution has been derived (Chanda and Rempel, 1994) for a shell progressive model which assumes quasi-stationary diffusion but allows change of bulk
Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995 2581
Figure 10. Rate a t (a) pH 4.5 and (b) pH 11.0 (ammoniacal). Resin loading 4.0-15.0 g (wet&; temperature 25 "C.
0
8
16 24 32 (t-to), min
40
..
0
1
2 3 4 (t-to), min
5
Figure 11. Test of eq 4 for sorption of Cu(I1) on TGF-EPEI resin of bead size (diameter) of 0.60-0.82 mm at (a) pH 4.5 and (b) pH 11.0 (ammoniacal). Temperature 25 "C; vigorous agitation.
solution concentration with progressive conversion of resin bead. The model is described by the equation
Table 5. Values of nf, for Sorption of Cu(I1) on TGF-EPEI in Acidic and Alkaline Media at Different Concentrations of External Solution A i j (cm2/s)from eq 4 concn of Cu(I1) in soln (mmoVL) DH 4.5 DH 11.0 2.5 10-5 4.0 3.2 x 2.7 10-5 6.0 3.6 x 10.0 4.4 x 10-6 2.1 x 10-5
c, is the
where
sorption capacity per unit volume of the unreacted resin bead, V is the volume of the solution, and COis the initial concentration of the sorbate. The term 1in eq 4 is the molar distribution coefficient. For any given run 1 is assumed to be constant. The right hand side of eq 4 can be plotted against (t - to), and can be evaluated from slope of the linear plot. The sorption data for Cu(I1) given in Figure 10 are thus plotted in Figure 11. Linear-plots show the validity of the model. The values of AD obtained from the slopes are recorded in Table 5. Since the gel-filled sorbent used for rate measurements was a mixture of different shapes, though belonging to a narrow size range, the radius of an equivalent sphere ( A r i s , 1957) having volume equal to the average volume of pgrticles in the size fraction was used for calculation of AD. The values of AD are seen to be essentially independent of concentration, which justifies the assumption of constancy of 1 in deriving the above model equation. The
a
p =(
3 1 1 3
The term t o in eq 4 is the initial time at which a measurable conversion UOof the sorbent is obtained. In eq 7, derived from mass balance for a shell-core system, n is the number of sorbent particles in each test,
2582 Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995 100
80
K
$ -
60
a
a
E
40
II)
20 v
0
10
20
30
40
50
TIME, s Figure 12. Stripping of TGF-EPEI with excess of acid under vigorous agitation at 25 "C. Before stripping the sorbent was loaded with Cu(I1) by equilibration with 10 mmoVL CuSO4 solution.
resin phase diffusivity in alkali media is several times higher than that in acidic media in agreement with the significantly higher rate of sorption observed in alkaline media. Stripping Behavior. Since the metal sorption capacity of the polyamine sorbents is nearly zero a t pH levels below 2 and above 12, both acid and alkali stripping can be used to recover the sorbed metal. However, since the sorption capacity is more sensitive t o pH in the acidic range, acid stripping will be preferred. The stripping characteristics of copperloaded TGF-EPEI resin with HC1 and HzS04 are shown in Figure 12. Very rapid stripping is obtained with acids, the stripping with HC1 being relatively faster than with H2S04. With 1 N HCl the stripping of the sorbed copper is nearly instantaneous with more than 90% stripping being attained in 10 s, compared to about 83% stripping with 1 N HzS04 in the same period under similar conditions. The stripping rate is, however, dependent on the strength of the acid. For example, 0.1 N HC1 produces only about 70% stripping in 10 s under similar conditions. The markedly rapid stripping behavior of the sorbent would be attributed to the high accessibility of the sorption sites in the gel-filled sorbent coupled with the high diffusivity of acids in the sorbent phase.
Conclusions
A new process of templated gel-filling (TGF) has been developed t o produce gel-type granular sorbents using chelated resins and metal ions as the template. The process depends on the use of a suitable solvent, referred to as a "gel-filling solvent", in which the chelating resin is soluble but its metal complex is insoluble. In the first step, hollow shells of the resin are made by building a templated gel layer of the resin on high-surface-area silica preloaded with the templating metal as the host ion, followed by cross-linking the resin with a suitable reagent and leaching out the silica with alkali. In the second step of the process, the hollow shells are filled with concentrated aqueous solution of a salt of the chosen metal and suspended with agitation in a solution of the resin in a gel-filling solvent in order to fill the shells with templated resin. After performing crosslinking reaction on the templated resin, metal ions are leached out with a suitable reagent to obtain a porous,
granular sorbent of the cross-linked chelating resin with possible memory for the template metal in favorable cases. Using this process, granular sorbents designated TGF-PEI and TGF-EPEI, have been prepared from poly(ethylene imine) and partially ethylated poly(ethy1ene imine). The sorption characteristics of Cu(II),Ni(II), CO(II), and Zn(I1)in both acidic and alkaline (ammoniacal) media on these sorbents have been determined and compared with those on a commerical product (Aldrich) of poly(ethy1ene imine) coated on silica, designated SiOy {PEII. In acidic media, the gel-filled sorbents have significantly higher capacity than the coated sorbent due largely to a low content (-6% by weight) of the resin coat on silica support. This demonstrates the utility of the TGF process of making granular sorbents. Both sorbents, TGF-PEI and TGF-EPEI, have high selectivity for Cu(I1) over Ni(II), Co(II), and Zn(I1) in both acidic and alkaline media, the selectivity in alkaline media being greater than that in acidic media. The selectivity of Si020 .[PEI] is very low, in comparison, due to sorption by silica which constitutes about 94% by weight of the sorbent. The TGF sorbents exhibit a high rate of sorption depending on pH and the concentration of the sorbate in the substrate. In a 10 mmol/L CuSO4 solution, for example, the tliz for 50% attainment of equilibrium sorption on TGF-EPEI is 8 min in an acidic medium at pH 4.5 and 27 s in ammoniacal medium at pH 11.0. The kinetics is even faster in the stripping process. With 1 N HC1, for example, the stripping of Cu(I1) loaded on TGF-EPEI is nearly instantaneous, with more than 90% stripping being attained in 10 s. The rate data for Cu(I1) sorption on TGF-EPEI, in both acidic and alkaline media, fit a finite bath shell progressive model well, yielding AD values of 3.7 x and 2.4 x cm2/s for sorption a t pH 4.5 and 11.0, respectively. The significantly faster rate of sorption in alkaline media is attributed, at least in part, to greater swelling of the sorbent in alkaline medium. The significantly higher sorption capacity of the gelfilled sorbents, as compared t o the resin-coated silica sorbent, while retaining in a considerable measure the fast kinetic attributes of the latter, is an important advantage of the templated gel-filling process for making gel-type granular sorbents from chelating resins. The selectivity enhancement, however, is dependent on several factors. In situations where the metal ions to be separated differ greatly in size but have no significant difference in affinities to a chelating resin, a TGF sorbent made with this resin and the smaller size metal ion as the template would be expected to provide greater selectivity for this metal.
Acknowledgment The financial support of research from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
Nomenclature A, = saturation sorption capacity of resin, mmol (g of dry
resin)-' C = sorbate concentration in solution, mmol L-l = sorbate concentration in resin, mmol L-l C* = equilibrium sorbate concentration in solution, mmol
c
L-1
CO= initial sorbate concentration in solution, mmol L-1
Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995 2583
cr= sorption capacity per unit volume of unreacted resin
bead at equilibrium, mmol L-1 D = diffision coefficient in the film, cm2 s-l D = effective diffisivity in reacted layer, cm2s-l Kb = binding constant, L mol-' ro = initial radius of sorbent particle, cm R* = (1 - LJ)1/3 R*o = (1 - Uo)ll3 t = time, s t o = initial time at which a measurable conversion of resin is obtained, s x* = equilibrium sorption on resin, mmol (g of dry resin)-l U = fractional attainment of equilibrium sorption of beadform resin Greek Symbols a = parameter, defined by eq 7 p = [(l - a)/aI1l3 A = molar distribution coefficient (dimensionless)
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Received for review September 6 , 1994 Revised manuscript received January 10, 1995 Accepted January 19, 1995" IE940526Q
Abstract published in Advance A C S Abstracts, J u n e 15, 1995.
