versión impresa ISSN 0366-1644
Bol. Soc. Chil. Quím. v.45 n.2 Concepción jun. 2000
METAL ION INTERACTIONS WITH POLY(2-ACRYLAMIDE-2-
METHYL-1-PROPANESULFONIC ACID-co-METHACRYLIC ACID).
a) Departamento de Polímeros, Facultad de Ciencias Químicas,
Universidad de Concepción, Casilla 160-C, Concepción, Chile;
FAX: 56-41-245974; e-mail: firstname.lastname@example.org
b) Instituto de Química, Universidad Austral de Chile, Casilla 567, Valdivia, Chile;
FAX: 56-63-221597; e-mail:mailto: email@example.com
(Received: January 6, 2000 - Accepted: January 27, 2000)
In memoriam of Doctor Guido S. Canessa C.
Three copolymers of 2-acrylamide-2-methyl-1-propanesulfonic acid and methacrylic acid have been synthesized. The interactions of these copolymers and the corresponding homopolymers with Cu2+, Cd2+, Co2+, Cr3+, Zn2+, Ni2+, and Ag+ have been analyzed as a function of the pH by the use of the ultrafiltration technique. A typical polyelectrolyte behavior has been observed.
KEY WORDS: Ultrafiltration, polymer-metal ion interactions, water-soluble polymers, polyelectrolyte.
Se han sintetizado tres copolímeros a partir de los monómeros ácido 2-acrilamida-2-metil-1-propansulfónico y ácido metacrílico. Se han analizado las interacciones en función del pH de estos copolímeros y de los correspondientes homopolímeros con los iones metálicos Cu2+, Cd2+, Co2+, Cr3+, Zn2+, Ni2+ y Ag+, utilizando la técnica de ultrafiltración. Los polímeros muestran un comportamiento típico de polielectrólitos.
PALABRAS CLAVES: Ultrafiltración, interacciones polímero-ion metálico, polímeros solubles en agua, polielectrólitos.
Interactions between polymers and different metal ions are an interesting field of study due to potential analytical and technological applicability. Water-insoluble resins have been extensively used to selectively concentrate and separate metal ions from water solutions1,2). Water-soluble polymers may also be applied for these purposes and different techniques such as dialysis3,4) or ultrafiltration5,6) have been developed to study the interactions of metal ions with these polymers. Upon the use of the ultrafiltration technique in a batch-like method, a profile for the retention of the different metal ions by the polymer during filtration can be obtained. In order to extract the maximum information from the retention profiles, a model has been presented that, under some assumptions, allows to calculate the amounts of metal ions bound to the polymer in every instant, as well as the amounts of metal ions free in the solution, with the aid of some semiempirical equations7). This information may be useful to face the analyses of the interactions of water-soluble polymers with metal ions.
In this paper, the interactions between Cu2+, Cd2+, Co2+, Cr3+, Zn2+, Ni2+, and Ag+ with three copolymers of 2-acrylamide-2-methyl-1-propanesulfonic acid and methacrylic acid are analyzed as a function of the pH by the use of the ultrafiltration technique, and the results are compared with those of the corresponding homopolymers.
The synthesis of poly(2-acrylamide-2-methyl-1-propanesulfonic acid) (PAMPS), poly(methacrylic acid) (PMAA), and three corresponding copolymers (AMPS-MAA-01, AMPS-MAA-02, AMPS-MAA-03) was carried out through radical polymerization. The initiator was ammonium persulfate (1 mol %) 8). The compositions of the copolymers are listed in Table 1. Metal nitrates were used to prepare metal solutions.
|Table I: List of the amounts of polymer used and their composition.|
|PAMPS||41||1 / 0||0.2|
|PMAA||17||0 / 1||0|
|MAA-01||100||0.58 / 0.42||0.37|
|MAA-02||59||0.57 / 0.43||0.22|
|MAA-03||76||0.39 / 0.61||0.21|
|a) Obtained by 1H-NMR|
The unit used for retention studies consisted of a filtration cell with a magnetic stirrer, a membrane with an exclusion rating of 5,000 or 100,000 g mol-1 (Filtron), a reservoir, a selector, and a pressure source. Metal ion concentrations were measured by atomic absorption on a Perkin Elmer 3100 spectrometer. The pH was controlled on a H. Jürgens & Co. pH meter.
Polymeric molecular weight fractions over 100,000 g mol-1 were dissolved in bidistilled water, and metal nitrates were added (5 mmol of each metal ion). The exact polymer amounts used are listed in Table 1. The solutions were brought to 20 ml of total volume and the pH was adjusted. The pH of the water contained in the reservoir was adjusted to the same value. The filtration runs were carried out over a membrane with an exclusion rating of 5,000 g mol-1 under a total pressure of 3 bar, keeping constant the total volume in the cell by creating a continuous flux of liquid through the cell solution from the reservoir. Filtration fractions were collected and the metal concentrations analyzed.
Retention profiles at pH 1.
The retention profiles as a function of the pH of the two homopolymers and the three copolymers are shown in Figure 1. These polymers are polyelectrolytes. PAMPS is a strong polyelectrolyte that is dissociated in water. On the other hand, PMAA is a weak polyelectrolyte, which is partially dissociated from pH over 3. The carboxylate groups have the ability to form coordination bonds with the metal ions studied while the sulfonate groups do not have this ability. The interactions of the copolymers with the metal ions should be due to both long distance electrostatic interactions and short distance coordinating interactions. At pH 1 the carboxylate groups are protonated, and their ability to coordinate metal ions is decreased. On the other hand, it has been shown how the ionic strength influences the rate of interaction of polyelectrolytes with sulfonate groups with the metal ions7). At pH 1, the high ionic strength decreases the ability of the metal ions studied to bind electrostatically the polymer, and they are consequently eluted out of the cell. Note that all the divalent metal ions show similar retention profiles due to their similar charges, while the elution of Ag+ is faster, as this metal ion has lower charge, and the elution of Cr3+ is slower as it has higher charge.
|Fig.1. Retention profiles of Cu2+, Cd2+, Co2+, Cr3+, Zn2+, Ni2+, and Ag+ at pH 1. PMMA (), PAMPS (¨), AMPS-MMA-01 (ª ), AMPS-MMA-02 (*), AMPS-MMA-03 (x).|
This fact may be quantified7). The retention profiles may be adjusted to the plot of the so-called retention functions, which are functions of the type RZ = uZ + vZ exp (-kZ · F), where Z is the charge of the metal ions and uZ, vZ, and kZ are experimental parameters. From these retention functions the dissociation constant of the equilibrium established between the metal ions and the polymer can be calculated. The retention profiles obtained for the divalent metal ions with the copolymers at pH 1 may be adjusted to functions of the type R = exp (-kZ · F), where kZ takes values around 0.8. This shows a value for the dissociation constant of 4. The dissociation constants found at this pH for the interactions of the copolymers with the trivalent metal ion Cr3+ are in the range of 0.89 to 1.38. The dissociation constants found for Ag+ tend to infinite, indicating that the monovalent metal ion do not interact with the polymer at this pH. The values of the dissociation constants for the homopolymer PAMPS are lower comparing with the copolymers. For the divalent metal ions they take values around 1. This fact may be explained considering that the homopolymer has a higher linear charge density than the copolymers, due to the absence of the MAA units. The differences found between the retention profiles corresponding to the three copolymers may be due to their different composition, the differences in the values of the linear charge density, the differences on the overall sulfonate concentration, and conformational effects.
Retention profiles at pH 3.
The retention profiles corresponding to the interactions between the divalent metal ions with the three copolymers and the homopolymers at pH 3 are shown in Figure 2. At this pH the carboxylic groups are not dissociated, and so, the methacrylic units do not show the ability to bind the metal ions. On the contrary, the ionic strength at this pH is much lower than at pH 1. So, the metal ions interact electrostatically with the polymers, and they are retained inside the cell.
Fig. 2. Retention profiles of Cu2+, Cd2+, Co2+, Zn2+, and Ni2+ at pH 3. PMMA (), PAMPS (¨), AMPS-MMA-01 (ª ), AMPS-MMA-02 (*), AMPS-MMA-03 (x).
Two effects may be observed analyzing these retention profiles. The retention profiles of AMPS-MAA-03 reach the lowest value of the retention at F = 10. The amount of sulfonate groups in solution for this polymer is equivalent to the amount of sulfonate groups in AMPS-MAA-02 and PAMPS. The differences in the retention profiles are due to the different linear charge density of the polymers. From Manning´s Counterion Condensation Theory, it can be obtained an expression that relates the linear charge density with the retention parameter u2:
Here, x is the linear charge density, cp is the concentration of polymeric charged groups, and c2init is the initial metal ion amount. The last two variables are constant in the three experiments. Then, comparing two of these three experiments, namely a and b, it can be obtained the expression:
The theoretical value of the linear charge density for PAMPS is 2.8, assuming the polymer as a straight linear arrange of charges. The values of the theoretical linear charge density obtained for the copolymers are shown in Table 2. The linear charge density is proportional to the fraction of AMPS units in the copolymers. The theoretical values obtained are not in agreement with this fact, and they appear not to be proportional to the fraction of AMPS units found in the copolymers. It is normal to use the linear charge density as an adjustable parameter4,9). The empirical values of x (xeff) that agree with equation (2) and are proportional to the fraction of AMPS units in the copolymers are shown in Table 2. The effective charge density is very high. For PAMPS it corresponds to an effective distance between the charges of 7.14 · 10-9, instead of the theoretical 2.55 · 10-8. This fact has been interpreted by the influence of the polymer conformation. The high charge density is associated with a coiled conformation of the polymer chain10). Even if the polymer is highly charged, hydrophobicity in the backbone of the chains affects its conformation11).
|Table II: Calculation of the theoretical values of x and its comparison with the empirical values xeff.|
Another effect that can be observed is the influence of the polymer concentration. AMPS-MAA-01 and AMPS-MAA-02 have very similar compositions. But the ultrafiltration experiment performed with the former was done with a higher polymer concentration. As there are more polymeric charged groups in the solution, the metal ions are strongly retained inside the cell.
Retention profiles at pH 5 and 7.
The retention profiles at these two pH have been obtained. At pH 5 MAA units are partially dissociated, and at pH 7 they are totally dissociated. This fact is reflected on the retention values corresponding to PMMA at F = 10 (see Figure 3). At these pHs the ionic strength is very low. The high charge of the polymers and the low ionic strength make that all the metal ions studied are completely retained, with the exception of the monovalent Ag+. Cr3+ was not used at these pHs, and Cu2+ was not used at pH 7 to avoid precipitation problems. The values of the retention at F = 10 for Ag+ are shown in Table 3. The differences found between the values of the retention may be explained by the differences on the amounts of charged groups in solution, the different linear charge densities of the polymers, and the complexation ability of MAA units.
Fig. 3. Retention values at F = 10 for the interactions of PMMA with Cu2+ (t), Cd2+ (n), Co2+ ( ), Zn2+ (+), Ni2+ (¨), and Ag+ (l) as a function of the pH.
The interactions of three copolymers of 2-acrylamide-2-methyl-1-propanesulfonic acid and methacrylic acid with Cu2+, Cd2+, Co2+, Cr3+, Zn2+, Ni2+, and Ag+ have been analyzed as a function of the pH by the use of the ultrafiltration technique. The results were compared with those of the corresponding homopolymers. A typical polyelectrolyte behavior has been observed. The retention profiles found may be explained considering the relative strength of the polyelectrolytes as a function of the pH, the concentration of polymeric charged groups and the ionic strength.
The authors thank the Fondecyt (Grant No 8990011) and the Dirección de Investigación of the University Austral de Chile (Grant No S-199906) for financial support.
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