versión impresa ISSN 0366-1644
Bol. Soc. Chil. Quím. v.47 n.3 Concepción sep. 2002
Bol. Soc. Chil. Quím., 47, 253-258 (2002) ISSN 0366-1644
MAXIMUM RETENTION CAPACITY OF A STRONG
POLYELECTROLYTE FOR DI-AND TRIVALENT CATIONS BY
LIQUID-PHASE POLYMER-BASED RETENTION (LPR) TECHNIQUE
BERNABÉ L. RIVAS* , S. AMALIA POOLEY,
EDUARDO PEREIRA, PAOLA GALLEGOS
Department of Polymers, Faculty of Chemistry, University of Concepción,
Casilla 160-C, Concepción, Chile. email@example.com
(Received: March 6, 2002 - Accepted: May 28, 2002)
The maximum retention capacity (MRC), at different pH and polymer-metal ion ratio, of a water-soluble polymer containing a strong acid group was determined. The metal ions investigated were: Cu(II), Co(II), Ni(II), and Cr(III). There was an important effect of the pH on the MRC. The MRC values of the divalent metal ions were very similar and higher than that for the trivalent cation, would indicate that the polymer-metal ion interaction was basically through electrostatic type.
Key words: Water-soluble polymer, metal ions, maximum retention capacity.
Se determinó la capacidad máxima de retención (CMR) de un polímero soluble en agua que contiene un grupo ácido fuerte a diferentes pH y distintas razones polímero-ion metálico. Los iones metálicos investigados son: Cu(II), Co(II), Ni(II) y Cr(III). Hay un importante efecto del pH sobre la CMR. Los valores de CMR de los iones metálicos divalentes son muy similares y mayores que los del cation trivalente, lo que indicaría que la interacción polímero-metal fue básicamente de tipo electrostático.
Palabras claves: Polímero soluble en agua, iones metálicos, capacidad máxima de retención.
The removal and separation of metal ions are a technological challenge with respect to industrial and environmental applications. Conventional methods include the use of water-insoluble polymers, that allow the quantitative and partially selective enrichment of ions [1-5]. However, these heterogenous methods require additional steps, for example, back extraction, elution, etc. and show less favorable kinetics.
On the contrary, the use of inert membranes enables separation to be achieved that cannot be succesfully conducted by other means. The possibility of preconcentration and separation of different species without separating agents is another advantage of some membrane techniques. Membrane separation is most selective, if soluble reagents are added. Hydrophilic polymers with complexing groups, termed polychelatogens, have been tested to show the applicability of the method to the separation of various metal cations and anionic species for analytical and technological purposes. The ligands and their metal-complex can be retained by an ultrafilter, whereas the free metal ions pass through the filtration membrane. This method is called liquid-phase polymer based retention [6-9]. A series of polymers have been designed and investigated with respect to the analytical determination of metal ions [10-15]. The advantages of the method are the high selectivity of the separation, owing to the use of a selective binding, and the low energy requirements involved in ultrafiltration.
One of the most important properties of one chelating polymer is their maximum retention capacity (MRC) for a metal ion. It depends strongly on the pH. Therefore, the aim of this paper is to suggest a polymer-metal ion interaction mechanism through the MRC values. Accordingly, the MRC was determined for a strong polyelectrolyte, the polychelatogen poly(2-acrylamido-2-methyl-1-propanesulfonic acid) respect to Cu(II), Co(II), Ni(II), and Cr(III) at different pH and polymer-metal ion ratio.
The polymer was synthesised by radical polymerization, purified, and characterized previously .
The metal nitrates (Merck, analytical grade) were used as received.
The maximum retention capacity (MRC) by the enrichment method according to the liquid-phase polymer based retention (LPR) technique was determined. In this method, a metal ion solution of known concentration is passed through of the polymer solution P(APS) (20 mL). The volume of the polymer solution is kept constant and the metal ion content is determined in the different filtrate volume. With these data, the plots of metal ion concentration in the filtrate as function of filtrate volume are obtained. For this study 165.8 mg (0.8 mmole of repeat units) of P(APS) dissolved in 20.0 mL of bidistillated water adjusted at the respective pH. According to the solubility properties of the metal ions three pH were studied: 3, 5, and 7. Two metal ion concentrations were investigated: 1.0 mM and 4.0 mM.
The retention profiles at Z = 10 were carried out considering a metal ion concentration of 0.005 mmol in 20 mL. Polymer concentration is 0.2 mmol of repeat unit in 20 mL.
The pH was determined with a Jenco Electronics 1671 pH-meter. For the LPR technique, a membrane filtration system was employed to test the coordinating properties of the polychelatogens. A Unican Solaar M5 Atomic Absorption Spectrometer was used for the determination of the metal ion concentrations in the filtrate.
RESULTS AND DISCUSSION
The polyelectrolyte poly(2-acrylamido-2-methyl-1-propane sulfonic acid) P(APS) obtained by radical polymerization  was studied as polychelatogen to determine the maximum retention capacity (MRC) by liquid-phase polymer based retention (LPR) technique through the enrichment method. The metal ion retention profiles were also determined previously .
The metal-ion retention at filtration factor, Z =10 depended on the pH. The retention at pH 1 is very low for Cr(III) and there is not retention for all the other metal ions. It is due to the competition between the hydrogen and metal ion by the ligand or exchange group. The highest values are achieved at pH = 7 for Co(II) and Ni(II) and pH =3 and pH = 5 for Cr(III) and Cu(II) respectively (see table 1). With this information on metal ion retention, the maximum retention capacity under different experimental conditions was studied.
Table I. Metal ion retention values (mmol metal ion/mmol repeat unit) at different pH and Z=10.
By comparison the curves of the metal ion concentration in the filtrate versus filtrate volume for the polymer with the corresponding control, it is possible obtain the necessary filtrate volume to achieve the MRC for each metal ion (see figure 1).
Figure 1. Retention of Co(II) at pH = 3 (a); pH = 5 (b); and pH = 7 (c)
With this volume and by the relationship:
MRC = (M x V)/Pm
MRC : mg of metal ion retained per g of polymer
M: metal ion concentration (mg/L)
V: filtrate volume through the membrane free of metal ion (L)
Pm: mass of polymer (g)
the MRC was calculated and the results for the metal ions at different pH and concentrations are summarized in table 2.
Table II. MRC values for di- and tri-valent metal ions under different experimental conditions.
For the three metal ions investigated, the values are higher that those obtained from the retention profiles at Z=10 (see table 1). These values would indicate that at this metal ion concentration the active sites are not saturated.
There is a clear difference between the MRC values for the divalent and trivalent metal ions. It is lower for Cr(III) and the MRC values for the divalent cations are very similar. It would demonstrate that the polymer-metal ion interaction is the electrostatic type (see figure 2).
Figure 2. Electrostatic polymer-metal ion interactions
It would explain why the MRC for a trivalent cation like Cr(III) is lower than that a divalent cation like Cu(II), Co(II), and Ni(II). It is due to theoretically, one Cr3+ ion will interact with three negative charges coming from repeat units P(APS), whereas a divalent cation as Co2+ will do only with two. Hence, to neutralize completely the charges in P(APS), partially or completely dissociated, it will be necessary a higher number of divalent ions than that trivalent cations, and consequently the MRC will be higher for the formers. Theoretically, for completely dissociated P(APS), the MRC value for a trivalent cation is 0.33 (mole of M3+/mol of repeat unit), whereas for a divalent cation is 0.5 (mole of M2+/ mol of repeat unit).
Moreover, the MRC values increases as increases the pH, achieving the highest value at pH = 5. This behavior is similar for all the metal ions (see figure 3). It can be attributed to the interactions between the metal ions and P(APS) through electrostatic interaction, which depend on the dissociation degree of the sulfonic acid. The dissociation increases as increases the pH. At pH = 5, the dissociation is practically complete because there are not important differences between the MRC values at pH = 5 and pH = 7. Besides, the MRC values at pH = 7, for Co(II) : 0.43 (mol/mol repeat unit) and Ni(II) : 0.43 (mol/mol repeat unit) are close to theoretical value, 0.5 (mol/mol of repeat unit). The difference is due to carry out the calculations it was assumed that all the sulfonate groups are available to interact with the metal ions. The availability of these groups is determined by the conformation of the polymer chain in solution. This conformation is very influenced by the ionic strength of the solution, due to that it is possible assume the the prefered conformation will be a coil. Accordingly, not all the sulfonate groups will be available to interact with the metal ions, giving MRC values lower than those theoretical values.
Figure 3. Maximum retention capacity, MRC, for Co(II), Ni(II), Cu(II), and Cr(III) ions at different pH.
On the other hand, the metal ion retention behavior of P(APS) can be treated according to the condensation theory [17-19]. In this sense, it is suggested that there is a number of condensated counterions to the polyelectrolyte in a determinated volume. These condensated ions are in equilibrium with a determinated number of counterions out the condensation volume. The amount of free and condensated ions depend basically on the lineal charge density parameter x. It assumes that never the theoretical value for the metal ion retention with a polyelectrolyte will be achieved.
The LPR technique allows the determination of the maximum retention capacity (MRC) for metal ions with different charge density by using a polyelectrolyte containing sulfonic acid groups. The MRC values are higher for the divalent cations respect to the trivalent cations. The MRC depended strongly of the pH. Thus, the highest values for Co(II) and Ni(II) ions (0.43 mol of metal ion/mol of repeat unit) were obtained at pH = 7. It demonstrated that the electrostic interaction forces are very important in this polymer-metal ion interactions.
The authors would like to thank FONDECYT (Grant Líneas Complementarias No 8990011).
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