Journal of the Chilean Chemical Society
versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.52 n.2 Concepción jun. 2007
J. Chil. Chem. Soc, 52, Nº 2 (2007) págs.: 1164-1168
REMOVAL OF METAL IONS WITH IMPACT ON THE ENVIRONMENT BY WATER-INSOLUBLE FUNCTIONAL COPOLYMERS: SYNTHESIS AND METAL ION UPTAKE PROPERTIES.
BERNABÉ L. RIVAS*1, SANDRA VILLEGAS, 1 BEATRIZ RUF, 1 IVÁN M. PERIC,2
1Polymer Department, Faculty of Chemistry, University of Concepción, Casilla 160-C, Concepción Chile 2Analytical and Inorganic Chemistry Department, Faculty of Chemistry, University of Concepción, Casilla 160-C, Concepción Chile
The crosslinked resins poly(4-acryloylmorpholine-co-4-vinyl pyridine) P(AMo- VPy), poly(N-(3-dimethylamino)propylmethacrylamide-co-4-vinyl pyridine) P(NDAPA-VPy), and poly((3-dimethylamino)propylacrylate-co-4-vinyl pyridine) P(DAPA-VPy) were obtained by radical polymerization. The resins were completely insoluble in water. The uptake metal ion properties are studied by batch equilibrium procedure for the following metal ions: silver(I), copper(II), cadmium(II), zinc(II), lead(II), mercury(II), chromium(III), and aluminum(III). P(AMo-VPy) resin shows a lower metal ion affinity than P(DAPA-VPy), except for Hg(II) which is retained in almost 100 % at pH 2. At pH 5, the resin shows the higher affinity for Ag(I) (80%) and Cu(II) (60%) but it is very low for Zn(II) and Cr(III). The polymer ligand metal ion equilibrium is achieved during the first 20 min. By changing the pH is possible to remove between 50 % and 70 % of Cd(II) and Cu(II) ions by using (1 M, 4 M) HClO4 and (1 M, 4 M) HNO3.
Keywords: resin, radical polymerization, metal ions, polymer-metal complexes.
Heavy metal ions dissolved in water are one of the most pollutants of the environment as it accumulates in living tissues, causing many harmful effects. Many industries are responsible for pollutating the environment with the heavy metal ions contained in their waste-waters. The effluents generated by industries as petroleum refineries, non-ferrous metal works, aircrafts plating, etc., generally, have a complex composition which includes metals, (ions or complexes) suspended solids, and other components.  As example, copper is known to be one of the most toxic heavy metals to living organisms and it is one of the most widespread heavy metal contaminants on the environment.  It is due to that the copper has become one of the most widely used metals. Thus, it is used or produced in the manufacture of printed circuit boards, tannery operations, mining drainage, chemical manufacturing, metal finishing industries, etc. Therefore, significant copper-containing waste streams are produced from these industries.  Mercury also causes a severe pollution arising mainly from metal smelting and caustic-chlorine production in mercury cells, metal processing plating, and metal finishing industries. Mercury causes significant economic and public health problems by its presence in aquatic ecosystems.  Mercury is converted in more toxic form, i.e., methyl mercury chloride by aquatic living-organisms, and accumulated in the tissue of fishes and birds. The illness, which come to be known as Minamata disease, was caused by mercury poisoning as a result of eating contaminated fish. 
The toxicity of heavy metal ions may be caused by the following mechanisms: blocking the essential functional groups of biomolecules such as enzymes; displacing essential metal from biomolecules; modifying the active conformation of biomolecules, especially enzymes and disrupting the integrity of the biomembranes; and modifying some other biologically active agents. These toxicities mechanism are all based on the strong binding abilities of these metallic ions. Thus, mercury has very high tendency to bind proteins and it mainly affects the renal and nervous systems. [6-7]
Several techniques have been proposed for the removal of metals, such as flotation, precipitation, solvent extraction, ion exchange, adsorption onto different adsorbents, such as activated carbons, clays, zeolites, bioadsorbents, manganese nodules, materials containing carbon, etc., cementation on iron, membrane processing and electrolytic methods. [8-11]
One of the most investigated supports to remove metal ions corresponds to polymers. The main uses and applications for insoluble polymeric matrix are related with the metal recovery from dilute solutions. [12-27] Reactive polymeric materials for wastewaters treatment and metal recovery have received much attention. Metal ions are non-biodegradable in nature; their intake at a certain level is toxic. Attempts have therefore been made to incorporate a vast number of chelating groups into a polymer network.
There are different natural and synthetic products that show ion-exchange properties. The organic resins are by far the most important ion exchangers. The main advantages are a high chemical and mechanical stability, high ion-exchange capacity, and ion-exchange rate. Another advantage is the possibility of selecting the fixed ligand groups and the degree of cross-linking. However, due to a wide range of materials and methods of synthesis, it is not surprising that the chelating exchanger physical form may vary from a rock-hard material to soft gel.
The selective separation of metals from different fluids and from contaminated areas is a challenge of great interest. Materials to perform this task should be highly selective, non-hydrophobic, and easy to regenerate. The idea of hard and soft acid base (HSAB) concept  and stability of the metal complexes are the valuable guidelines for the active site selection. Anchoring the active site to a solid support in a polymer matrix provides an immobilized active surface capable of selective and quantitative separation of cations from an aqueous solution.
The aim of this paper is to study the ability of three resins, containing amide, amine, pyridine, and carboxylate groups, to recover by batch equilibrium procedure, metal ions with environmental impact.
4-Acryloylmorpholine (AMo, 97% Aldrich), 4-vinyl pyridine (VPy, 95% Aldrich), divinylbenzene (DVB, Fluka), N-(3-dimethylamino)propylmethacry lamide (NDAPA, Aldrich), 3-dimethylamino)propylacrylate (DAPA, Aldrich) were purified by distillation. 2,2-Azoisobutyronitrile (AIBN, Merck) was recrystallized from methanol, N,N-methylene-bisacrylamide (MBA, 99% Aldrich) and ammonium persulfate (AP, 95% Aldrich) were used without further purification.
The nitrate salts of silver(I), copper(II), cadmium(II), zinc(II), lead(II), mercury(II), chromium(III), and aluminum(III) (all from Merck) were used. The analytical grade, nitric acid, perchloric acid, and sodium hydroxide were purchased from Merck.
Synthesis of the resins P(AMo-VPy), P(NDAPA-VPy), and P(DAPA-VPy)
The resins P(AMo-VPy), P(NDAPA-VPy), and P(DAPA-VPy) were synthesized by radical polymerization by mixing the corresponding reagents in a polymerization flask as follows:
P(AMo-VPy): AMo (0.04 mole, 5 mL), VPy (0.046 mole, 5 mL), DVB (0.0035 mole, 0.5 mL) and AIBN (0.00040 mole, 0.0653 g) (Merck) in 9.5 mL of dry toluene. Yield = 91.0 %.
P(NDAPA-VPy): NDAPA (0.0276 mole, 5 mL), VPy (0.0278 mole, 3 mL), MBA (0.0022 moles, 0.3450 g) and PSA (0.00031 mole, 0.0697 g) in 8 mL of water. Yield = 87.0 %.
P(DAPA-VPy): DAPA (0.0295 mole, 5 mL), VPy (0.0297 mole, 3.2 mL), MBA (0.0024 mole, 0.3646 g), and AP (0.00028 mole, 0.0629 g). Yield = 90.0%.
The polymerization mixtures were kept under nitrogen at 70ºC for 8 h (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 40ºC 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 standard metal salts AgNO3, Cu(NO3)2, Cd(NO3)2, Zn(NO3)2, Pb(NO3)2, Cr(NO3)3, Al(NO3)3, and Hg(NO3)2 at pH range 1 5 depending on the metal ion. All experiments were performed in flasks placed on a shaker at 20ºC. The retention ability for Ag(I), Cu(II), Cd(II), Zn(II), Pb(II), Cr(III), Al(III) and Hg(II), under noncompetitive and competitive conditions, was determined as a function of the pH. Batches of 50.0 mg 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 mmol of the metal ions involved for 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. Filtrate was collected into calibrated 100-mL volumetric flask and diluted to volume. The metal ion concentrations were determined by atomic absorption spectrometry (AAS).
Time effect on Ag(I) and Hg(II) ion retention was determined with the resin P(AMo-VPy) at 5, 15, 30 and 120 minutes under shaking at 20ºC.
Temperature effect on Ag(I), Cu(II), Cd(II), Zn(II), Pb(II), Cr(III), Al(III), and Hg(II) ion retention was performed with the resins P(AMo-VPy) and P(NDAPA-VPy) at 20, 25, 30, 35 and 40ºC 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 mL of 1.0 g/L metal ion solution. After a shaking time of 1 h at 20ºC, each sample was filtered. Then, the batches were reconstituted by adding 25.00 mL of fresh metal ion solution. The process was repeated three times. The last filtration step was followed by a washing step with water at a desired pH. Filtrate was collected into calibrated 100 mL volumetric flask and diluted to volume with water at the corresponding pH. The metal ions were determined by AAS.
To study the recovery of the resin, 0.1 g of metal ion loaded resin were contacted with 10 mL of the elution medium at 20ºC. The solution was filtered and the metal ions were determined by AAS.
To carry out the charge-discharge cycles, 10 mL of a metal ion solution with 0.1 g of dry resin were stirred by 1 h at 20ºC and then filtered and received in 100 mL completing the volume with water at the same pH. Then, the resin was placed with 10 mL of elution medium for 1 h at 20ºC. It was filtered and diluted to 100 mL with water at the corresponding pH. This procedure was repeated three times. The metal ion was determined by AAS.
The elution media assayed were (1M, 4 M) HClO4 and (1M, 4 M) HNO3.
A Julabo air-batch shaker was used for shaking the solution at a desired temperature. The pH was measured with a digital H. Jürgens & Co. pH meter. A Unicam Solaar 5M series atomic absorption spectrometer (UK) was used for the determination of single and mixed metal ions. The FTIR spectra of the sample were recorded with a Magna Nicolet 550 spectrophotometer. The thermograms of the loaded and unloaded resins were recorded on a STA-625 thermoanalyzer (Polymer Laboratories). Approximately 5 mg of the dry sample were heated at 10°C/min heating rate under dynamic nitrogen atmosphere.
In a similar way, the effect of the temperature on the uptake metal ion behaviour was studied at 20 ºC, 25 ºC, 30 ºC, and 40 ºC respectively.
RESULTS AND DISCUSSION
Synthesis and characterization of the resins
Three water-insoluble copolymers were synthesized by radical copolymerization. These resins were characterized by FT-IR spectroscopy. The spectra show the most characteristic absorption bands (see table I), which confirm the copolymerization reaction. The copolymer composition was determined from their FT-IR spectra by comparison of the areas corresponding to each monomer.
The thermal study under dynamic conditions demonstrates that the three resins are very stable up to 200ºC. They lose only water occluded into the polymer matrix. Up to 300ºC the thermal loss is lower than 6 %, and from this temperature, the weight loss increases particularly for the resin P(NDAPA-VPy) which lose a 46.7 % and 93.9 % at 400 ºC and 500 ºC respectively (see table II).
Metal ion adsorption studies
Effect of the pH
The removal of pollutants from wastewaters by adsorption is highly dependent on sorbate solution pH. pH is the most critical parameter as it influences both the polymer surface chemistry as well as the solution chemistry of soluble metal ions. Variation of the pH can affect the surface charge of the adsorbent, and the degree of ionization and speciation of adsorbate. Therefore, the adsorption dependence on the pH for the metal ions was examined (see figs. 1-3).
As the equilibrium condition is reached close to 30 min, which is considered too fast for reactions occurring in heterogenous media, the contact time used was 1 h in all cases. This fast adsorption equilibrium time is probably due to high complexation rate (i.e. high affinity), especially between Ag(I) and Hg(II) ions and the functional group anchored to the chains in the resin structure.
The influence of pH on the removal of metal ions, except for Hg(II) can be explained on the basis of the electrostatic interaction model.  As the pH decreases, the surface of the resin becomes more positively charged, therefore the adsorption of cationic species result unfavorable. Effectively, competition between H+ and metal cations minimizes the extent of the metal ion adsorption. On the contrary, as the pH increases, the adsorbent surface becomes more negatively charged and therefore the adsorption of positively metal ion species is more favorable. These studies were carried out only up to pH 5 because as the pH increase above this value the precipitation starts to occur. Thus, above pH 5 occurs the precipitation of Cu(OH)2 and not the adsorption capability of the resins.
Respect to the high retention of Hg(II) at low pH, a possible explanation consider that the nitrogen atom of the pyridine moiety was probable protonated at low pH, particularly at pH range 1-2, forming -NH+A- - groups. In these protonated groups, the nitrogen atom has not a free-electron pair capable of forming coordination bonds with the metal ions. Therefore, it could be possible to suggest that an ion-exchange process could be involved in the retention of Hg(II) ions. In this case, it would exchange the anion by the anion [Hg(NO3)4]2-.  Particularly, P(NDAPA-VPy) shows a good selectivity for Hg(II) or Ag(I) at pH 1 toward the other ions considered. On the other hand, P(AMo-VPy) and P(DAPA-VPy) show a good selectivity only for Hg(II) among the cations studied at pH 1.
Effect of the temperature solution
According to the results of the effect of pH, the effect of the temperature
solution was investigated for the retention of Ag(I) and Hg(II) at pH 5 and 2 respectively. The results for P(AMo-VPy) are summarized in fig. 4.
As can be observed, in all cases the retention of both cations decreases as the temperature increases from 20 ºC to 40 ºC. Therefore, by considering that the adsorption reaction is exothermic, this observed effect corresponds to the desorption of the metal ion by increase of the temperature. The optimal solution temperature was 20 ºC.
Maximum retention capacity (MRC)
To explore the applications of the adsorbents, it is relevant to obtain knowledge on the adsorption capacity of the resin for the metal ions studied (see Table III). Examination of these results indicate that resin P(DAPA-VPy) shows the greatest MRC for Ag(I) and Hg(II) at the corresponding optimum pH within experimental error. Moreover, the resin P(NDAPA-VPy) shows a remarkable MRC for Al(III).
The regeneration of the resin is likely to be a key factor in lowering the process costs. To be useful in metal remediation processes, metal ions should be easily eluted under suitable conditions. Elution of the metal ions from the loaded resins was performed in a batch experimental set-up. Various factors are probably involved in the metal ion elution, such as the extent of hydration of the metal ions and adsorbent microstructure. However, among others an important factor appears to be binding strength. When HClO4 or HNO3 are used as the elution media, the coordination spheres of the chelated metal ions, but especially Ag(I) and Hg(II) are disrupted and, subsequently, these ions are released from the polymer surface into the desorption medium. In this study the elution time is 1 h. These elution studies suggest the possibility to elute selectively Zn(II), Pb(II), Cr(III), and/or Al(III) from Ag(I), Cu(II), Cd(II) and/or Hg(II) when resin P(AMo-VPy) is used (data not shown). On the other hand, when resin P(DAPA-VPy) is used most of the cations elute less than 20 %, except Ag(I) which elutes almost 100 % when HNO3 is employed as the elution medium (see fig. 5). The ability to reuse the resins is shown in the figures 6 and 7. For the resin P(AMo-VPy) the adsorption behavior of Ag(I) is stable for four cycles of use. The adsorption capacity of the recycled adsorbent can be maintained at 98 % level at the fourth cycle. On the other hand, the resin P(DAPA-VPy) as well as P(NDAPA-VPy) (data not shown) shows a slightly decrement in the Ag(I) as well as Hg(II) adsorption behavior from one cycle to the other.
The metal ion retention under competitive conditions is shown in fig. 8. Resin P(AMo-VPy) was studied by considering a Ag(I)-Hg(II) mixture and in these conditions it shows a high selectivity for Ag(I). Furthermore, resin P(DAPA-VPy) was tested by considering a Cd(II), Zn(II), Pb(II), and Hg(II) mixture and in these conditions it shows not a well defined selectivity (data not shown).
The effect of the contact time on retention of Hg(II) and Ag(I) was carried out with the resin P(AMo-VPy) (see fig. 9) where Hg(II) shows a faster kinetic than that Ag(I) under the experimental conditions.
The crosslinked P(AMo-VPy), P(DAPA-VPy), and P(NDAPA-VPy) resins, that were synthesized by solution radical polymerization, showed a great affinity in the removal of mercury as well as silver ions from an aqueous solutions at pH 2 and pH 5 respectively. P(DAPA-VPy) shows an elution of Ag(I) over 90% and Hg(II) less than 45% were obtained using 1-4M HClO4 as elution medium. Consecutive adsorption and desorption showed the feasibility of this resin for Hg(II) adsorption.
The authors thank to Dirección de Investigación, Universidad de Concepción the partial financial support.
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