Langmuir 1994,10,4688-4692
4688
Ligand-Modified Polyelectrolyte-EnhancedUltrafiltration with Electrostatic Attachment of Ligands. 1. Removal of Cu(I1) and Pb(I1) with Expulsion of Ca(I1) Melda Tunqay,t2$ Sherril D. Christian,*>$?§ Edwin E. Tucker,$>$ Richard W. Taylor,$>$ and John F. Scamehorn$vIl Department of Chemistry, Faculty of Engineering, University of Istanbul, Istanbul, Turkey, Institute for Applied Surfactant Research, University of Oklahoma, Norman, Oklahoma 73019, Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019,and Department of Chemical Engineering, University of Oklahoma, Norman, Oklahoma 73019 Received April 4, 1994. I n Final Form: August 19, 1994@ A water-soluble mixture of a cationic polyelectrolyte, poly(diallyldimethylammonium chloride) or PDADMAC, and an anionic ligand, 4,5-dihydrox 1,3-benzenedisulfonicacid, or Tiron, is used as an agent for binding divalent cations, such as Cu2+and Pbl;. In equilibrium dialysis experiments, the PDADMACTiron mixture is shown to be effective in binding Cu2+and Pb2+in the retentate, while enhancing the concentrationofthe noncomplexed cation, Ca2+,in the permeate. In the pH range employed, the uncomplexed and complexed forms of Tiron exist primarily as divalent anions. As a consequence they are bound electrostatically to the cationic polyelectrolyte,and little of the ligand is lost through the membrane. The effects of pH, cation concentration, and presence of a competing cation (Ca2+),and added NaCl on the retention of Cu2+and Pb2+are discussed.
Introduction During the past decade, we have developed a number of colloid-enhanced ultrafiltration separation methods for removing molecular or ionic impurities from contaminated aqueous streams.1,2 In these processes, a water-soluble colloid is added to the polluted feed stream, binding or solubilizingthe target species. When the combined stream is passed through an ultrafiltration (UF) membrane having suitable pore sizes, the colloid and pollutants largely remain in the retentate stream and purified water passes through the membrane a s apermeate stream. Both aqueous surfactant micelles and polyelectrolytes have been utilized as the dissolved colloid in separation processes termed micellar-enhanced u l t r a f i l t r a t i ~ n ~ and -~ polyelectrolyte-enhanced ult~-afiltration.~-~' In numerous studies of colloid-enhanced ultrafiltration separations, we
* To whom correspondence should be addressed. Department of Chemistry, University of Istanbul. t Institute for Applied Surfactant Research, University of Oklahoma. 8 Department of Chemistry a n d Biochemistry, University of Oklahoma. II Department of Chemical Engineering, University of Oklahoma. Abstract published in Advance A C S Abstracts, November 1, 1994. (1)Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. A m . Environ. Lab. 1990,2(l), 13. (2)Christian, S.D.; Scamehorn, J. F.; Tucker, E. E.; O'Rear, E. A.; Hanvell, J. H. InHandbook on Removal ofHeavy Metalsfrom Industrial Wastewater; Peters, R. W., Ed.; CRC Press: Boca Raton, FL, in press. (3)Christian, S. D.; Scamehorn, J. F. InSurfactant-BasedSeparation Processes; Scamehorn, J . F., Hanvell, J. H., Eds.; Marcel Dekker: New York, 1989;Chapter 1. (4)Scamehorn, J.F.;Christian, S. D.; Ellington, R. T. In SurfactantBased Separation Processes; Scamehorn, J . F., Hanvell, J. H., Eds.; Marcel Dekker: New York, 1989;Chapter 2. (5)Dunn, R.0.;Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol. 1986,20,257. (6)Dunn, R. 0.; Scamehorn, J. F.; Christian, S. D. Colloids Surf. 1989,35, 49. (7)Scamehorn, J. F.;Ellington, R. T.; Christian, S. D.; Penney, B. W.; Dunn, R. 0.; Bhat, S. N. In Recent Advances in Separation Techniques-IIl; Li, N., Ed.; AICHE Symp. Ser. No. 250; American Institute of Chemical Engineers: New York, 1986;Vol. 82,p 48. (8) Christian, S. D.; Bhat, S. N.; Tucker, E. E.; Scamehorn, J. F.; El-Sayed, D. A. M C h E J . 1988,34,189.
have been able to show that in the direct methods (micellarenhanced ultrafiltration or polyelectrolyte-enhanced ultrafiltration), the quality of the product water (permeate stream) can be predicted from results of small-scale equilibrium binding experiments, including equilibrium dialysis and related physicochemical techniques. Therefore, because these direct colloid-enhanced ultrafiltration separations are equilibrium-controlled, rather than kinetically-controlled, many of the problems involved in scale-up for industrial and environmental applications are simplified, and thermodynamic relationships can be used to predict separation e f f i c i e n c i e ~ . ~ v ~ J ~ - ~ ~ Several recent studies from our laboratory have shown the advantage of using ligands solubilized by surfactant micelles in equilibrium dialysis and UF separations to remove particular metal ions in the presence of other ions of the same charge.12J6J7 For example, ligand-modified micellar-enhanced ultrafiltration (LM-MEUF)performed with N-alkyliminodiacetic acid orN-alkyltriamine ligands has been shown to be effective in removing cations such as Cu2+almost quantitatively from a feed stream.12J6J7 The metal ions are complexed by ligands attached to the colloidal (micellar) species and remain in the retentate, while uncomplexed ions like Ca2+pass through the UF
@
(9)Sasaki, K. J.; Burnett, S. L.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1989,5, 363. (10)Tucker, E. E.; Christian, S. D.; Scamehorn, J. F.; Uchiyama, H.; Guo, W. Transport and Remediation ofsubsurface Contaminants;ACS Symposium Series No. 491;American Chemical Society: Washington, DC. 1992:D 86. (11)Volchek, K.;Krenstel, E.; Zhilin, Y.; Shtereva, G.; Dytnersky, Y~. . J.Membr. Sci. 1993. 79.253. (12jKlepacjJ.; Simmons, D. L.; Taylor, R. W.; Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol. 1991,26,165. (13)Mahmoud, F.Z.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F . J . Phys. Chem. 1989,93,5903. (14)Smith, G. A,; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. J . Solution Chem. 1986,15, 519. (15)Christian, S.D.; Smith, G. A,; Tucker, E. E.; Scamehorn, J. F. Langmuir 1985,I , 564. (16)Dharmawardana, U. R.; Christian, S. D.; Taylor, R. W.; Scamehorn, J. F. Langmuir 1992,8 , 414. (17)Simmons, D. L.;Schovanec, A. L.; Scamehorn, J. F.; Christian, S. D.; Taylor, R. W. Environmental Remediation: Removing Organic and Metal Ion Pollutants; ACS Symposium Series No. 509,American Chemical Society: Washington, DC, 1992;p 180. - - - - I
- I - - -
0743-7463/94/241Q-4688$04.50/00 1994 American Chemical Society
Enhanced Ultrafiltration
Langmuir, Vol. 10,No. 12, 1994 4689
determined using a total organic carbon analyzer (Dohrman membrane. In LM-MEUF, the synthesized ligands conModel DC-180). The ligand, 4,5-dihydroxy-1,3-benzenedisulfonic tain long-chain alkyl or alkyVary1 moieties which cause acid, disodium salt monohydrate ("iron), was obtained from them to be almost quantitatively solubilized in surfactant Aldrich Chemical Co. and used without further purification. micelles without greatly decreasing their afinity for the All of the reagents and chemical compounds used in the target metal ion(s). An analogous approach using soluble experiments, 4-(2-pyridylazo)resorcinol(PAR)monosodium salt polymers as the colloidal species involves covalent atmonohydrate, Cu(NO&, CaC12, anhydrous Pb(N03)2,CH3C02tachment of chelating moieties to the polymer.18 When Na, CHSCO~H, NaOH, and HC1, were of analytical reagent grade. the ligand is bound to a positively charged (cationic) Test solutions were prepared by weight from the polyelectrolyte, surfactant micelle, a n electrostatic effect involving the ligand, and metal ion stock solutions in double-distilleddeionized water. pH adjustments ofthe solutions were monitored with an charged colloid and like-charged co-ions causes the Orion EA 940 pH meter, using an Orion Research Sure-Flow concentrations of cations that are not complexed by the Combination Electrode. ligand to become enhanced in the permeate ~ t r e a m . ~ Ross ~ ~ ~ ~ Equilibrium dialysis (ED) experiments were performed as This is referred to as an ion expulsion effect, although in described p r e v i o ~ s l y using ~ ~ - ~equilibrium ~ dialysis cells from UF separations under reasonable operating conditions, Fisher Scientific having volumes of approximately 5 mL in each the concentration of co-ions expelled is generally much compartment and employing 6000 molecular weight cutoff less than the equilibrium value, because ion expulsion is regenerated cellulosemembranes. The test solutions were placed controlled by kinetic factors.20 in one compartment of the dialysis cell and pure water was placed An inherent disadvantage of LM-MEUF and ligandin the other. Cells were thermostatted a t 25 "C for 24 h, by which time equilibrium was attained.15 The initial concentrations modified polyelectrolyte-enhanced ultrafiltration (LMof PDADMAC and Tiron in the retentate were 20 and 1.0 mM, PEUF) is the necessity of developing synthetic methods respectively, in all experiments. for covalently attaching the ligand subunits to hydrophobic Metal ion concentrations in the samples ofthe initial solution, groups or directly to soluble polymers. An alternative retentate, and permeate were determined using an atomic method for binding ligands to the water-soluble polyelecabsorption (AA)analyzer, Model Varian SpectrAA-20. For AA trolytes or micelles in colloid-enhanced UF separations is analysis ofthe retentate samples, standard solutions containing to utilize ligands having sufficient charge to bind nearly the target metal ions were prepared in the presence of 20 mM quantitatively to oppositely-charged colloids. Our previPDADMAC and samples were diluted to the same concentration ous research on the binding of organic anions to micelles of PDADMAC. "iron concentrations were determined by direct or polyelectrolytes has indicated that a net ionic charge spectrophotometric measurements. Calibration equations, resulting from standards prepared in the presence and absence of of -2 should be sufficient to cause these species to bind PDADMAC,were used for calculating the retentate and permeate almost quantitatively, by electrostatic attraction to opconcentrations. Spectrophotometric analyses were performed positely-charged colloids, even in the presence of moderate with an HP8452A diode-array spectrophotometer, using a l-cm concentrations of added salt^.^^,^^ Numerous polyanionic cell with 5-s signal averaging. For the analysis of copper, the ligands have found use as analytical reagents for particular PAR method was also used, as described e l ~ e w h e r e . ~ ~ . ~ ~ metals and in other applications where selective complexation of cations is required. It should be feasible, Results and Discussion therefore, to utilize readily available ligands having a Equilibrium dialysis experiments, utilizing 20 mM negative charge, bound to a n excess of a positively charged PDADMAC a t pH = 5.5,were carried out in the absence colloid, a s additives to contaminated feed streams, and to of any added metal cations, anions, or NaC1. The rejection obtain separations similar to those obtained in LM-MEUF coefficient, Rx(%),of a species is calculated using the of LM-PEUF processes. expression In the present study, the cationic polyelectrolyte poly(diallyldimethylammonium chloride), PDADMAC, is used together with a n anionic ligand, 4,5-dihydroxy-l,&benzenedisulfonic acid, Tiron. This reagent is known to be effective in binding metal cations like Cu2+and Pb2+.22 where [X]p and [X]R are the measured concentrations of When a mixture comprising a t least a 20:l mole ratio of the species of interest in the permeate (filtrate) and PDADMAC to Tiron is employed, the Tiron is almost retentate, respectively. The values of [XI, and [XIR, in quantitatively bound and retains its ability to complex terms of the monomer subunit, were determined by total Cu2+and Pb2+,while unbound ions like Ca2+are expelled carbon analysis after 24 h and the rejection of PDADMAC into the permeate. We describe here results of equilibrium was calculated to be 98.9%. The small amount of polymer dialysis experiments performed to determine the potential lost most likely comprised low molecular weight fragments utility of LM-PEUF with electrostatic attachment of not completelyremoved during the initial cleanup process. ligands in treating waste water or industrial process Equilibrium dialysis experiments were performed with streams. aqueous solutions of Tiron and PDADMAC in order to determine the effectiveness of the polyelectrolyte in Experimental Section retaining (rejecting) the anionic forms of the complexing agent. ED results obtained using 1.0 mMTiron as a single PDADMAC was obtained from Calgon Corp. (Merck)as a 40% solute indicated that Tiron reaches equilibrium across solution; it was purified by removing low molecular weight the dialysis membrane within 24 h. When 20 mM compounds by ultrafiltration using a 10 000 molecular weight cut-off membrane. The concentration of the stock solution was PDADMAC (concentration expressed in terms of monomeric subunit) was included along with 1.0 mM Tiron, the rejection of Tiron after 24 h was determined to be (18)Geckeler, IC;Lange, G.; Eberhardt, H.; Bayer, E. Pure Appl. Chem. 1980,52, 1883. 99.0%. In the presence of 50 mM NaCl and 20 mM (19)Christian, S.D.; Tucker, E. E.; Scamehorn, J. F.; Sasaki, K. J.; PDADMAC, the rejection of Tiron was 98.7%. No meaLee, B.-H.Langmuir 1989, 5, 876. surable difference in the permeate Tiron concentration in (20)Krehbiel, D. K.;Scamehorn, J. F.; Ritter, R.; Christian, S. D.; Tucker, E. E. Sep. Sci. Technol. 1992,27, 1775. the permeate was discernible for ED experiments extended (21)Dunaway, C.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. to 48 and 72 h. Manuscript in preparation. (22)Smith, R.M.; Martell, A. E. In Critical Stability Constants; Smith, R. M., Martell, A. E., Eds.; Plenum Press: New York, 1977,Vol. 3,p 205; 1982,Vol. 5, p 342;1989,Vol. 6,p 381.
(23)Anderson, R. G.;Nickless, G. Analyst 1969,92, 207 (24)Iwamoto, T. Bull. Chem. SOC.Jpn. 1969, 34, 605.
Tuncay et al.
4690 Langmuir, Vol. 10, No. 12, 1994 Table 1. Equilibrium Dialysis Results for Cu2+and Pb2+ with Tiron (1.0 mM)/PDADMAC (20 mM) Mixtures
0.6 ,
(A) DH= 5.5
0.5 1
[Cu2+Y[Tironl Rcu (Red %" 1.03 0.68 0.51 0.31
[Cu2+1/[Tironl RcU(Rcdc),%a
69.6 (49.7) 81.8 (79.2) 94.3 99.7
0.20 0.11 0.08
99.9 299.9 299.9
(B)uH = 6.5 ~~
1.09 0.69
[Cu2+Y[Tiron]
72.5 (45.7) 87.3 (78.2)
0.30 0.15
(C) pH = 2.0 [Cu2+]p,%b [Cu2+Y[Tironl
1.02 0.82
64.6 65.1
99.7 299.9
57.4 (49.1) 86.0 (80.3)
[Cu2+]p,%*
0.49
69.2
0.30 0.15
99.7 299.9
84.4 85.9 89.9
0.38 0.22 0.19
90.5 90.5 98.2
RM(%) given by eq 1;Redc(%) given by eq 4. [Cu2+]p(%) given by eq 2.
Equilibrium calculationsz5using the protonation constants for TironZ2predict the following species distribution of ligand (H,Tn-4) in aqueous solution in the pH range 2.0-5.5;HzTZ-(>99.2%),HT3- (-0.8%). At pH = 6.5, the distribution would be HzT2- (92.8%)and HT3- (7.2%).Thus for the pH range utilized the ligand exists predominantly as the divalent anion with appreciable amounts of the trivalent anion present at pH = 6.5. The distribution of polymer-associated Tiron may be shifted to increase the fraction in the trivalent form, owing to increased electrostatic interaction between the polymer and the more highly charged form of the ligand. Preliminary ED studies of 1,5-naphthalenedisulfonatewith PDADMAC under similar conditions to those used here give rejection values between 95 and 99% for this divalent anion.21 The results for Tiron and naphthalenedisulfonate indicate that ligands with a formal charge of -2 will be retained by PDADMAC to a degree sufficient for use in metal ion separations. The behavior of the PDADMAC (20 mM)-Tiron (1.0 mM) system with individual divalent cations (Cu2+,Pb2+) was studied using ED to determine the effects of metal ion to Tiron ratio, pH, and excess electrolyte (NaCl). The initial [Cu2+l:[Tironlratios and the corresponding percent rejection values are listed in Table 1. Results plotted in Figure 1 show the effect of varying the retentate Cu2+concentration on the permeate Cu2+ concentration at various pH values, for constant initial Tiron and PDADMAC concentrations. Most of the experiments were carried out a t pH = 5.5,both to ensure a high degree of complexation of the metal ion and to avoid precipitation of hydroxides. Lowering the pH to 2.0 reduces the effective metalligand complexation constant because of the increased competition for the ligand by protons, decreasing the binding of Cu2+ and hence its rejection. Equilibrium studiesz2 indicate that three types of copper-Tiron (25) Perrin, D. D.;
Sayce, I. G. Tuluntu 1967, 14, 833.
pH 6.5 o pH 5.5 A
A
pH2.0
0.4 -
-+ (u
a
A
o_
0.3
4
0.2
W I-
5w
1
OM
0.1 -
a
0
m
0
00.
0
(E)p H = 5.5 1.06 0.79 0.41
E
Y
0
(D) pH = 5.5, 50 mM NaCl in Retentate [Cu2+Y[Tironl Rcu (Redc), [Cu2+Y[!I'ironl RcU(Redc), 1.04 0.67
A
0 D
0.2
0.4
0.6
0.8
RETENTATE [Cu2+](mM) Figure 1. Effect of retentate [Cu2+1on permeate [Cu2+1as a function of pH in the presence of 1.0 mM Tiron and 20 mM PDADMAC.
Table 2. Equilibrium Dialysis Results for Equimolar Cu2+and Ca2+with TironlPDADMAC MixturesQ [M2+y[Tiron] RcU(Realc),%b [Ca2+]p,%c 1.10 0.73 0.41 0.17
68.8 (45.1) 80.3 (74.2) 97.2 97.4
75.8 75.3 69.5 68.2
a pH = 5.5, [Tiron] = 1.0 mM, [PDADMAC] = 20 mM; RM(%) given by eq 1. Rcdc(%) given by 4. [Cu2+1p(%) given by eq 2.
complexes are formed in aqueous solution: Cu(Tiron)H-, Cu(Tiron)2-, and Cu(Tiron)&. Calculations for the CuTiron system in water at pH = 2.0 indicate that 299.5% of the total copper exists as uncomplexed Cu2+,while the major form of Cu-Tiron complex is Cu(Tiron)H- (-0.4%). The plot in Figure 1, corresponding to the data at pH = 2.0, indicates that Cu2+becomes concentrated in the permeate relative to the retentate. Separate measurements showed that the presence of Cu2+does not increase the concentration of Tiron in the permeate compared to runs in which the divalent metal ion is not present. Therefore, one can conclude that most of the Cu2+is in the uncomplexed form for the ED runs at pH = 2.0. For these experiments, where there is no rejection of metal ion, we report the percent metal ion in the permeate [MI,
= 100[MIp'Wlp
+ [MI,)
(2)
where [Mlp and [MIRare the metal ion concentrations in the permeate and retentate, respectively. The results listed in Table 1C show that ion expulsion effects are important and that -65-69% of the Cu2+is concentrated in the permeate. Similar distributions of the uncomplexed metal ion were observed in ED experiments where Ca2+ was added to the Cu2+-Tiron-PDADMAC system (see Table 2). For the ED studies at pH values of 5.5 and 6.5 the rejection of copper is 299.7% for [Cu2+l:[Tironlratios less than 0.5. As this ratio increases from 0.5 to -1, the copper rejection decreases, falling t o -70% a t the highest Cu2+ concentrations. Examination ofthe values in parts A and B of Table 1shows a slight increase in the copper rejection for corresponding [Cu2+l:[Tironlratios as the pH increases from 5.5 to 6.5. The plots of the ED data at pH 5.5 and 6.5 shown in Figure 1and the values ofRculisted in parts A and B of Table 1 suggest that Cu(Tiron)z6- is the predominant complex species, particularly for solutions where [Cu2+l:[TironlI0.5. For aqueous solutions with [Tironlbt,l = 2[Culbtd = 1.0 mM, calculation^^^ using known equilibrium contants22
Langmuir, Vol.10,No. 12, 1994 4691
Enhanced Ultrafiltration predict the following percentages of Cu(Tiron)2- and Cu(Tiron)z6-: pH = 5.5(92.4%/5.3%);pH = 6.5 (34.2%/65.6%). Analogous calculations for solutions where [Tironlbtd = [Cu2+lbd = 1.0 mM give [Cu(Tiron)2~l:[Cu(Tiron)~6~l distributions as follows: pH = 5.5(90.0%/0.8%);pH = 6.5 (94.5%/2.6%). The protonated complex, Cu(Tiron)H-, accounts for only 0.1% to 0.9% of the total copper under the cond'tions described above. These calculations reveal that the ormation of 1:2 Cu-Tiron complexes is highly dependent on the pH and the ratio of metal to ligand concentrations for conditions similar to those employed in the ED experiments. In discussing the behavior of the ED runs a t pH = 5.5 to 6.5, it is useful to consider the disproportionation reaction for the Cu(Tiron)2- complex:
b
2Cu(Tirod2- == Cu(Tiron):-
+ Cu2+
(3)
By use of known stability constants,22a value of -0.01 is calculated for the equilibrium constant of this reaction in aqueous solutions. For systems containing the cationic polymer PDADMAC, electrostatic interactions should cause the equilibrium in eq 3 to shift to the right, favoring increased formation of the more highly charged complex, Cu(Tiran)z6-. This appears to be the case when the [CUI: [Tiron] ratio is less than 0.5, because rejection values of 99.7% or greater are obtained for ED runs under these conditions. For ED runs where [Cul:[Tironl > 0.5 the distinct decrease in copper rejection suggests several possibilities. At one extreme, one can assume that the added Cu2+reacts quantitatively with Cu(Tiron)z6- but that the resulting Cu(Tiron)2- complexes are not strongly bound to the polymer. However, since the HzTiron2- species has a rejection value of 98.9%, it is reasonable to assume that any Cu(TironlZ- complexes formed would be retained by the polymer to a similar degree; i.e., Rc,, > 98% for 0.5 < ([Cul:[Tironl) < 1.0. At the other extreme, one can assume that the added Cu2+ does not react appreciably with Cu(Tiron)zs- and that some fraction of the uncomplexed Cu2+is expelled into the permeate. If only Cu(Tiron)Z6-complexes form, and assuming that these complexes are retained almost completely (299.7%)by the polymer, a theoretical value for the copper rejection for this situation may be calculated from
+
Rcalc(%) = 1 0 o u - WCUIkee)/((l - f)[CU1kee
[Cu(Tiron)~-l)}(4) where [Cu(Tiron)z6-1= 0.5 mM, [Culfiee = [Culctd - 0.5 mM, and f is the fraction of uncomplexed Cu2+found in the permeate. On the basis of results a t pH = 2.0 and for experiments with added Ca2+,the value off is -0.650.70. Examination of Table lA,B,D and 2 shows that the R d cvalues based on the assumption of complete formation of Cu(Tiron)z6- are always lower than the experimental values. Therefore, the disproportionation is not complete and mixtures of CuZf and the 1:l and 1:2 Cu-Tiron complexes exist in this concentration region. The experimental data presented in Figure 2 and the results listed in Table 1D show that the presence of 50 mM NaCl influences the rejection of Cu2+. Addition of salt decreases the rejection of Cu2+from 69.6 to 57.4% for the highest [Cu2+y[Tironlratios; however, at mole ratios less than 0.5, the added salt has relatively little effect. The addition of excess electrolyte decreases the ion expulsion e f f e ~ ton ~ the ~ , uncomplexed ~~ Cu2+for [Cu2+1: Tiron > 0.5, thus reducing the copper rejection. At [Cu2+l: [Tironl ratios less than 0.5, there is very little free Cu2+
sE
1::
7 With 50 mM NaCl
A
v
-+
0.4 1
N
S
2 W I-
< W
0.3 1 0.2
A 0
:
5 W
n 0
0
001 0
,
0
0.2 0.4 0.6 0.8 RETENTATE [Cu2+](mM) Figure 2. Effect of retentate [Cu2+]on permeate [Cu2+]in the absence and presence of 50 mM NaCl at pH = 5.5 with 1.0 m M 0
Tiron and 20 mM PDADMAC. 0.25
0
W
I-
4
5
t
0.1
0 0
0.05
W
n
0
0-
I
0
e
oe
o
o '
0.2
.
I
0.4
,
'
,
,
0.6
,
'
'
0.e
RETENTATE [M2+](mM) Figure 3. Effect of retentate [M2+] on permeate [M2+]for of Cu2+and Pb2+,individually,at pH = 5.5 in the presence of 1.0 mM Tiron and 20 mM PDADMAC.
and the added electrolyte would not be expected to affect the interaction of Cu(Tiron)z6- with the polymer sufficiently to cause a measurable decrease in binding. The results presented up to this point pertain to the removal of divalent copper ion. ED experiments analogous to those with Cu2+a t pH = 5.5 were performed with Pb2+ as the target metal ion. The results are listed in Table 1E and a comparison with Cuz+is shown in Figure 3. In contrast with the Cu2+-Tiron systems, the behavior observed in the ED experiments with lead suggests that Pb-Tiron complexes have predominately 1:1stoichiometry for the conditions employed in this study. If one assumes that the Pb(Tironl2- complexes present are retained by the polymer to the same degree as the ligand species HzTironz-, then it is also apparent that Pb2+is not complexed as strongly as Cu2+by Tiron in the PDADMAC systems. Calculationsz5 for aqueous solutions where [PbZ+lbtd= [TironIwhl= 1.0 mM using known equilibrium constantsz2 predict that the distribution of Pb2+ and Pb(Tiron)2-is 29% and 71%, respectively a t pH = 5.5 and 3.6% and 96.4% a t pH = 6.5. Pb(Tiron)z6- accounts for less than 0.09% under these conditions. This behavior is similar to that found for solutions containing the polymer. The metal ion selectivity of Tiron-PDADMAC was also tested by performing ED experiments in which equal concentrations of Cu2+and Caz+were introduced into the retentate compartment. When copper and calcium ions are both present in the feed solution, the copper ions are almost completely complexed with Tiron, leaving most of
4692 Langmuir, VoE. 10, No. 12, 1994 0.7
1
Tuncay et al.
Conclusions
I
0
cu
v Ca
W
I-
0.3
3
0.2 1
4 W
a
7
0 I
0.1 1
0
0 ,
0
,
, 0,
0
0.2
0.4
0.6
RETENTATE [M2+](mM) Figure 4. Effect of retentate [M2+1on permeate [M2+l for equimolar mixtures of Cu2+and Ca2+a t pH = 5.5 in the presence of 1.0 mM Tiron and 20 mM PDADMAC.
the calcium ions free. The results are listed in Table 2, and Figure 4 shows the individual permeate metal concentrations for the Cu2+/Ca2+system as a function of retentate concentrations. As can be seen from Figure 4, it is possible to concentrate particular multivalent cations such as Ca2+in the permeate while concentrating other ions of the same charge, e.g., Cu2+,that bind selectively to the ligand in the retentate. Comparison of the copper rejection for systems where copper ion is present alone with those containing a n equal concentration of calcium ion showed no obvious difference. That is, the Cu2+ concentration in the permeate for experiments done with both Cu2+ and Ca2+ present is approximately equal to the permeate Cu2+concentration for experiments done in the presence of only Cu2+under the same conditions. Because of the great difference in the tendency of Cu2+and Ca2+to bind to Tiron, there is no deleterious effect of the metals being present as a mixture at the concentrations employed in the ED experiments. Thus, the excellent selectivity in removal of Cu2+in the presence of Ca2+that was observed in LMMEUF experiments using iminodiacetic acid ligands12J6 is also maintained in the ED experiments with electrostatic attachment of Tiron.
A water-soluble mixture of the cationic polyelectrolyte PDADMAC and the anionic ligand Tiron has been shown to bind transition-metal and heavy-metal cations, such as Cu2+and Pb2+,while expelling noncomplexed alkalineearth ions such as Ca2+. The metal ion may be removed from the polymer-associated ligand by lowering the pH of the retentate. The PDADMAC-Tironmixture should find utility in UF separations analogous to LM-MEUF and LM-PEUF, which can be designed to remove certain ions selectively, while expelling others of the same charge type. 12~16~17~19,20 The fact that small molecular ligands can be attached electrostatically to a n oppositely-charged polyelectrolyte, rather than covalently bonded, should make the design of soluble colloids for use in LM-MEUF and LM-PEUF much simpler. It should be noted that negatively-charged anions, such as sulfate, selenate, and arsenate will be rejected by the UF membrane along with the complexed divalent cations that bind to the PDADMAC-Tiron mixture. Therefore, employing the mixed-colloid PDADMAC-Tiron in UF separations is quite similar to the use of mixed-bed ion exchange, in that both anions and cations can bind to the polymer and be removed from the processed aqueous stream. Provided regeneration of the colloid and ligands can be accomplished, the colloid-enhanced UF processes may have the advantage over ion exchange of operating in a continuous or steady-state mode, using only homogeneous aqueous solutions.
Acknowledgment. Financial support for this work was provided by the National Science Foundation Grant No. CTS-9123388;the Center for Waste Reduction Technologies of the American Institute of Chemical Engineers, Agreement No. N12-Nl0; Department of Energy Office of Basic Energy Sciences Grant No. DE-FG05-84ER13678; E. I. Du Pont de Nemours & Co.; IC1 Chemicals; and KerrMcGee Corporation. We appreciate the contributions of Mr. James Roach in performing atomic absorbance and total organic carbon measurements and of Ms. Connie Dunaway and Mr. Roach in helping in the preparation of this manuscript.
