versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.53 n.1 Concepción mar. 2008
J. Chil. Chem. Soc, 53, N° 1 (2008)
BINDING OF URANYL IONS BY WATER-INSOLUBLE POLYMERS CONTAINING MULTILIGAND GROUPS
BERNABÉ L. RIVAS,1* IVÁN M. PERIC 2 SANDRA VILLEGAS,1 BEATRIZ RUF,1
1Polymer Department, Faculty of Chemical Sciences, University of Concepción, Concepción, Chile,e-mail: email@example.com
2analytical and Inorganic Chemistry Department, Faculty of Chemical Sciences, University of Concepción, Concepción, Chile
The crosslinked resins poly(N-(3-dimethylamino)propylmethacrylamide) P(NDAPA), poly((3-dimethylamino)propylacrylate)P(DAPA), poly(4-acryloylmorpholine-co-acrylic acid) P(AMo-AA), poly(N-(3-dimethylamino)propylmethacrylamide-co-4-vinyl pyridine) P(NDAPA-VPy) and poly((3-dimethyl amino)propylacrylate-co-acrylic acid) P(DAPA-AA), poly(N-(3-dimethylamino)propylmethacrylamide-co-acrylic acid) P(NDAPA-AA) were obtained by radical polymerization. These resins were completely insoluble in water. The uptake metal ion properties were studied by batch equilibrium procedure for copper(II) and uranyl ions.
Keywords: resin; radical polymerization; metal ions; polymer-metal complexes
Environmental conservation is of increasing social and economic importance. A particular pollution problem is the contamination of water by metal ions. The recovery of uranium from contaminated water of flooded mines (01-15 mg u/m3) also presents a very important problem to be solved. Hence, the design of effective host molecules for uranyl ions is related with the economic importance of selective extraction of uranium from water.13
The process using adsorbents is thought to be the most effective method for recovery of uranium because of the high selectivity for uranium, the case of handling, the safety to the environment, etc. Examples of well-studied polymer ligands of uranyl ions are: carboxylate such as EDTA analogues, phenol, p-ketone, ammonium, sulfonic acid, and amidoxime. 4"24
Various separation/pre-concentration techniques have been used for uranyl ion such as liquid extraction and solid phase extraction using cation exchangers, chemically modified or impregnated silica, active carbon, and Amberlite XAD resins.25"32
Other technique for the preparation of solid phase sorbent is the imprinting technique that could be used for selective separation and pre-concentration of trace metal ions as uranium.33"34
Synthetic polymers can modify the properties of functional groups attached to the backbone and improve the effectiveness of various molecular functions. In addition, modification of the microenvironments of synthetic polymers often improves the effectiveness of the functional groups. Thus, the chemical modification of the macromolecular backbones can improve the uranyl-binding properties.
The aim of the current manuscript is the synthesis of resins containing several functional groups to remove efficiently copper and uranyl ions at different pH.
Acrylic acid (AA, Merck), 4-Acryloylmorpholine (AMo, 97% Aldrich), 4-vinyl pyridine (VPy, 95% Aldrich), 1,4-divinylbenzene (DVB, Fluka), N-(3-dimethylamino)propylmethacrylamide (NDAPA, Aldrich), 3-dimeth ylamino)propylacrylate (DAPA, Aldrich) were purified by distillation. 2,2'-Azoisobutyronitrile (AIBN, Merck) was recrystallized from methanol, N,N'-methylene-bis-acrylamide (MBA, 99% Aldrich) and ammonium persulfate (AP, 95% Aldrich) were used without further purification.
The analytical grade, copper(II) nitrate, uranyl acetate, nitric acid, perchloric acid, sodium hydroxide and sodium carbonate were purchased from Merck.
Synthesis of the resins P(NDAPA), P(DAPA), P(AMo-AA), P(NDAPA-AA), P(NDAPA-VPy), and P(DAPA-AA)
The resins P(NDAPA), P(DAPA), P(AMo-AA), P(NDAPA-AA), P(ND APA-VPy ),andP(D APA-AA) were synthesizedby radical polymerization by mixing the corresponding reagents in a polymerization flask as follows:
P(NDAPA): NDAPA (0.0552 mole, 10 mL), MBA (0.00228 mole, 0.3552 g) and AP (0.000299 mole, 0.0684 g) in 10 mL of water. Yield = 91.0 %.
P(DAPA): DAPA (0.0552 mole, 10 mL), MBA (0.00232 mole, 0.3621 g) and AP (0.000285 mole, 0.0651 g). Yield = 82.0%.
P(AMo-AA): AMo (0.04 mole, 5 mL), AA (0.043 mole, 3 mL), DVB (0.0035 mole, 0.5 mL) and AIBN (0.00039 mole, 0.0651 g) (Merck) in 8 mL of dry toluene. Yield = 95.0 %.
P(NDAPA-AA): NDAPA (0.0276 mole, 5 mL), AA (0.029 mole, 2 mL), MBA (0.0021 mole, 0.3461 g) and AP (0.00027 mole, 0.0605 g) in 7 mL of water. Yield = 97.0 %.
P(NDAPA-VPy): NDAPA (0.0276 mole, 5 mL), VPy (0.0278 mole, 3 mL), MBA (0.0022 mole, 0.3450 g) and AP (0.00031 mole, 0.0697 g) in 8 mL of water. Yield =87.0%.
P(DAPA-AA): DAPA (0.0295 mols, 5 mL), AA (0.0292 mole, 2 mL), MBA (0.0023 mole, 0.3659 g) and AP (0.00027 mole, 0.0618 g) in 7 mL of water. Yield = 99.0%.
The polymerization mixtures were kept under nitrogen at 70°C for 8 h (see Scheme 1). Then methanol was added and the resin was removed from the flask, filtered and washed with abundant water. The resin was dried under vacuum at 40°C and then milled and sized by screening. The fraction with mesh size in the range of 250-500 mm was chosen.
Resin metal ion uptake
The effect of the pH on the metal ion retention properties was studied by batch equilibrium procedure. These batch metal uptake experiments were carried out using the standard metal salts Cu(N03)2 and U02(CH3COO)2 at pH range 1-5. All experiments were carried out in flasks placed on a shaker at 20°C. The retention ability for Cu(II) and U(VI), under noncompetitive and competitive conditions was determined as a function of the pH. Batches of 50.0 mg of resin were used, together with a mixture of 5.00 mL of metal ion(s) solution. Under noncompetitive conditions the resin/metal ion ratio (in mol) is 20:1.0. Under competitive conditions, solutions were prepared as before but considering the sum total mmole of the metal ions involved to keep constant the resin/metal ion ratio. After a shaking time of 1 h at a desired temperature, each sample was filtered and washed with deionized water at the corresponding pH. Filtrate was collected into calibrated 100-mL volumetric flask and made up to volume. In all cases, copper(II) was determined by atomic absorption spectrometry (AAS) and uranium(VI) colorimetrically.24
Temperature effect on Cu(II) and U(VI) ion retention was performed with all the resins at 20, 25, 30, 35 and 40°C under shaking for 1 h.
To obtain the maximum adsorption capacity for the metal ions, batches of 50.0 mg resin were used together with a mixture of 25.00 niL of 1.0 g/L metal ion solution. After a shaking time of 1 h at 20°C, each sample was filtered. Then, the batches were reconstituted by adding 25.00 niL of fresh metal ion solution. The process was repeated three times. The last filtration step was followed by a washing step with water at the corresponding pH. Filtrate was collected into calibrated 100 mL volumetric flask and made up to volume.
RESULTS AND DISCUSSION
Synthesis and characterization of the resins
Six water-insoluble resins were synthesized by radical polymerization. Resins were characterized by FT-IR spectroscopy. Table I shows the most characteristic absorption bands which confirm the polymerization process. Resin compositions were determined from their FT-IR spectra by comparison of the areas corresponding to each monomer.
The thermal study under dynamic conditions demonstrates that resins P(NDAPA), P(NDAPA-VPy), and P(DAPA-AA) were very stable up to 200°C, whereas P(DAPA), P(AMo-AA) and P(NDAPA-AA) were more unstable showing a thermal loss up to 14.5 % at the same temperature. At a higher temperature, the weight loss increases remarkably and comparatively for the resin P(AMo-AA) at 300°C and 400°C. These results are summarized in Table II.
Metal ion adsorption studies Effect of the pH
This study was carried out basically to compare the affinity and selectivity of the resins toward copper(II) and uranium(VI). pH influences the polymer surface chemistry and the solution chemistry of the metal ions under study. It is known that variation of the pH can affect the surface charge of the adsorbent, and the degree of ionization and speciation of adsórbate.36 Hence, the adsorption dependence on resins at different pH for the metal ions was determined (see Figures 1-3).
The contact time used was 1 h insofar as all the metal ions achieve the equilibrium close to 30 min which is considered too fast as the reactions occur in heterogeneous media.
short adsorption equilibrium time take place probably due to a high complexation rate (i.e. high affinity), between Cu(II) and U(VI) ions and the functional group anchored in the resin structure.
The influence of pH on the adsorption of Cu(II) ions can be explained on the basis of the well known electrostatic interaction model. 37 As the pH decreases, the surface of the resin enhances its positive character. At very low pH values copper(II) ion is positive charged and its adsorption is not favored at all. Competition between H+ and metal cations minimizes the extent of the metal ion adsorption indeed. As the pH increases, the adsorbent surface enhances its negative character and therefore, the adsorption of copper(II) becomes favorable. These studies were carried out only up to pH 5 because above this value the precipitation of Cu(OH)2 starts to occur.
The high retention of U(VI) observed at low pH can be explained by considering that the nitrogen atom of the pyridine moiety is protonated at low pH, particularly at the pH range 1-2, forming -NH+A" - groups. These protonated groups have a nitrogen atom without a free-electron pair capable of forming coordination bonds with the U022+ ions. Therefore, it could be possible to suggest that an ion-exchange process could be involved in the retention of U022+ ions. In this case, it would exchange the anion by [U02(N03)J.38, M Particularly, P(AMo-AA) shows a good selectivity for U(VI) at pH 3 and pH 5 toward Cu(II). On the other hand, P(DAPA) shows a higher selectivity for Cu(II) toward U(VI) atpH 3.
Maximum retention capacity (MRC)
In order to explore the applications of the adsorbents, it is very important to know the adsorption capacity of resins for the metal ions studied. Results are summarized in Figure 4. Examination of these results shows that resins P(NDAPA) and P(DAPA) have a higher MRC for Cu(II) toward U(VI) whereas P(AMo-AA), P(NDAPA-AA), P(NDAPA-VPy), and P(DAPA-AA) have the greatest MRC values for U(VI) toward Cu(II). Consequently, when pyridine or carboxylic moieties are present the retention of Cu(II) decreases. In this context the resin P(AMo-AA) shows a remarkable selectivity for U(VI) toward Cu(II), at optimum pH within experimental error.
Effect of the temperature solution
In consideration to the results of the effect of pH on the retention, the effect of the temperature solution was studied for the retention of Cu(II) and U(VI) atpH 5.
A general behavior was found, retention of both cations decrease as the temperature increases, from 20°C to 40 °C (data not shown). Hence, knowing that the adsorption reaction is exothermic, higher temperatures could induce an increasing decomposition of the chelate or the ion exchanger group, which can explain the drop of the adsorption rate. The optimal solution temperature was 20 °C under the range of temperatures assayed. An example of this behavior is shown for P(NDAPA) in Figure 5.
Regeneration of resins is likely to be an important factor when improving process economics. In metal remediation processes, metal ions should be easily eluted from loaded resins under suitable conditions. Therefore, elution was evaluated by performing it in a batch experimental set-up. Various factors are mostly involved in the metal ion elution, such as the extent of hydration of the metal ions and adsorbent microstructure. Nevertheless, a key factor appears to be the binding strength. When HC104 or HN03 are used as the elution media, the coordination spheres of the chelated metal ions are disrupted and, subsequently, these ions are released from the resin. In this study the elution time was 1 h. These elution studies suggest the possibility to elute selectively Cu(II) from U(VI) when resin P(NDAPA) is used (see Figures 6 and 7). The ability to reuse the resins was carried for the resins P(DAPA) as well as P(AMo-AA) (see Figures 8 and 9). For the resin P(DAPA) as well as P(AMo-AA) the adsorption behavior of U(VI) is almost stable for four cycles of use. The adsorption capacity of the recycled adsorbent can be maintained at a greater than 80% level at the fourth cycle.
The crosslinked P(NDAPA), P(DAPA), P(AMo-AA), P(NDAPA-AA), P(NDAPA-VPy), and P(DAPA-AA) resins were synthesized by solution radical polymerization. Among these resins those which contain pyridine and carboxylic acid groups showed a higher affinity in the removal of U(VI) ions from an aqueous solutions below pH 3.
The authors thank FONDECYT (Grant No 1070542), CIPA, and the Dirección de Investigación, Universidad de Concepción the partial financial support
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(Received: 26 September 2007 -Accepted: 20 December 2007)