Capability of cationic water-soluble polymers in conjunction with ultrafiltration membranes to remove arsenate ions.
Arsenic contamination of drinking water is considered a principal environmental health threat throughout the world. Chronic intake is associated with an increased risk to cancer, diabetes, and cardiovascular disease, and recent studies suggest increased health risks at levels as low as 5-10 ppb [1, 2].
The II Region in northern Chile is the most important copper mining area in the world, and it shows the highest lung cancer mortality rate in the country . The population in Antofagasta, the principal city in the II Region, was exposed between 1958 and 1970 to high arsenic concentrations in drinking water, which has been currently decreased to about 20 times.
In the search for a water treatment solution to protect the environment, the development of polymers as anionic exchangers has emerged as an important area of study. Polymers with chelating groups have been employed for the removal of metal ions from diluted solutions, and they imply potential technical applications [4-8]. However, an efficient, selective separation of inorganic ions has also been achieved using water-soluble polymeric reagents [6-13], which present several advantages in comparison to heterogeneous phase polymer reagents.
The presence of arsenic in water in amounts above the permitted level (0.05 ppm, according to the World Health Organization, WHO) can be detrimental to live organisms [4-15]. Toxic arsenic in water is presented mainly as As(V) and As(III), as well as in its inorganic hydrolysis species. Several methods have been developed to remove arsenic from aqueous solutions, including precipitation-coagulation with inorganic oxides, adsorption on impregnated and metal-loaded chelating resins, and membrane processes like reverse osmosis .
Additionally, there are different natural and synthetic products that present ion-exchange properties, where the organic resins are by far the most important ion exchangers. Owing to their exceptional ion exchange properties, numerous commercially available or easy-to-make water-soluble polymers can be advantageously used to remove ions from aqueous medium. Polyelectrolytes may be distinguished from chelating polymers (polychelatogens), where polyelectrolytes are characterized by ionizable groups in aqueous solution, while polychelatogens contain groups that have the ability to form coordination bonds with targeted species. Quaternary ammonium groups have been especially investigated and extensively used as ion-exchanged cationic groups in polymeric electrolytes [7-18].
As an example, it has been demonstrated that quarternized poly(methyleneimine) is an efficient polymer reagent for As(V) species removal, using the liquid-phase polymer-based retention (LPR) technique .
This paper reports on the ability of different cationic polymers containing an exchanger quaternary ammonium group, to remove As(V) species from aqueous media. These polymers have the following advantages: to be soluble in water with a high content of functional groups, to exchange anions with arsenic species and, therefore, a higher efficiency, and the possibility of polymer reuse can be expected. Essays using the washing method at constant ionic strength and at pH 4, 6, and 8 are also detailed in this article.
The monomers [2-(acryloyloxy)ethyl]trimethylammonium chloride (ClAETA) and [2-(acryloyloxy)ethyl]trimethylammonium methyl sulfate (SAETA) were used as received (Aldrich). Ammonium persulfate (AP, Aldrich) was used as a polymerization initiator. A solution of 1000 ppm of [Na.sub.2]HAs[O.sub.4] x 7[H.sub.2]O (Merck) was used.
Two homopolymers were synthesized by solution radical polymerization. Specifically, 5 g of each monomer and 1 mol% of AP were dissolved in 40 mL of water, and the reaction mixture was stirred at 70[degrees]C over [N.sub.2] for 24 h. The products are water-soluble, and subsequently, the solutions were freeze-dried. The polymers obtained were dissolved in water, purified using an ultrafiltration membrane, and then fractionated with ultrafiltration membranes with different exclusion molecular weight limits (10,000; 30,000; 50,000; and 100,000 g [mol.sup.-1]). The different fractions were characterized by Fourier transformed infrared spectra (FT-IR) and Proton nuclear magnetic resonance ([.sup.1.H] NMR) spectroscopy, and TG-DSC. The [.sup.1.H] NMR spectra of monomers and homopolymers were comparatively analyzed. Polymerization occurred, as indicated, by the absence of the signal at 5.44 and 5.68 ppm corresponding to the ethylene and vinyl protons in the monomers. The polymerization yield was 97.2% for the 100,000 g [mol.sup.-1] fraction and ~95.0% for the filtration fraction of poly[2-(acryloyloxy)ethyl]trimethylammonium chloride (P(ClAETA)) and poly[2-acryloyloxy)ethyl]trimethylammonium methyl sulfate (P(SAETA)), respectively. The polymers showed to be highly hydroscopic.
Using the washing method, 0.2 mmol of the polymer in the range above 100,000 g [mol.sup.-1] are dissolved in twice-distilled water, and then the solution containing 0.01 mmol of arsenic is added to the cell solution. Based on our experience with cation ions, the ratio used between the polymer functional groups and the arsenic was selected to ensure an excess of ligand groups and increase in retention. The solutions are brought to 20 mL of total volume, and the pH is adjusted by adding 0.1 M NaOH or 0.1 M HN[O.sub.3]. The reservoir's washing water is at the cell's pH value. Filtration runs were performed under a total pressure of 3.5 bar, using an ultrafiltration membrane with an exclusion rating of 10,000 g [mol.sup.-1]. The total volume (20 mL) in the cell was kept constant. Fractions of 20 mL were collected by filtration, and the metal ion concentration was analyzed. A blank experiment on a polymer-free solution was also performed. The enrichment method by maximum capacity measurements was also applied. The experimental conditions used were a 4 mM solution of [Na.sub.2]HAs[O.sub.4] and 0.8 mmol of P(CIMPTA) in 20 mL of cell solution. One blank experiment, without polymer, was also performed.
To study the effect of anions on the LPR essays, anion exchange was performed using a 2 M excess of sodium sulfate and chloride salts. The experimental setup was similar to that used for the LPR technique with the washing method, although without arsenic. Mixtures of sodium salt and P(SAETA) at different pH were stirred for 24 h. Then the excess of anions that did not exchange with methyl sulfate was removed by washing with water of adjusted pH. These conditions were selected according to the pre-experiment results. Although the temperature and time can also be studied, at this point in the research, only these effects were considered. In this study, the time and the temperature were fixed at determined values. To ensure equilibrium between the polymer functional groups and the arsenic species, time was set at 24 h.
Arsenic concentration was measured in the filtrate by atomic absorption spectrometry, using a Perkin Elmer 3100 spectrometer; the quantity of arsenic species retained was determined by the difference with the initial concentration. pH was measured with a pH meter (H. Jurgen and Co).
The FT-IR were recorded with Magna Nicolet 550 and Nexus Nicolet spectrometers. For quantitative analysis, 1 mg of the sample per 100 mg of KBr was employed. The NMR spectra were recorded in [D.sub.2]O at room temperature with a multinuclear Bruker AC 250 spectrometer 250 MHz. The thermal behavior was studied under [N.sub.2] using a thermogravimetric analyzer (TGA), with a TGA 625 from Polymer Laboratories. The heating rate was 10[degrees]C [min.sup.-1], the temperature range was 25-500[degrees]C, and the sample weight was 0.5-3 mg.
The polymers were characterized by FT-IR and [.sup.1.H] NMR spectroscopy. The FT-IR studies were performed in the range of 400-4000 [cm.sup.-1] for all cases. The spectra showed the following main characteristic absorption bands (in [cm.sup.-1]): 1735 (C=O) of ester bond, 1482 bending band of the quaternary ammonium groups, (-[N.sup.+](C[H.sub.3])), 1175 (C-O) of ester. P(SAETA) shows the distinctive asymmetric stretching (1253-1219 [cm.sup.-1]) and symmetric stretching (1061-1018 [cm.sup.-1]) (S=O) bands of the tetrahedral S[O.sub.4.sup.2-] group.
[.sup.1.H] NMR spectra of monomers and homopolymers were recorded in deuterated water. The polymerization occurred, demonstrated by the absence of the signals at 5.44 and 5.68 ppm, which correspond to the hydrogen atoms of the carbon-carbon double bond of the monomer, with the observation of two signals attributed to the C[H.sub.2] and CH (single C-C bond) groups at 1.7 and 2.5 ppm, respectively, corresponding to the polymer chain. The signal at 3.14 ppm was attributed to nine protons of the three methyl substitutes on the quaternary ammonium group. A characteristic strong signal observed at 3.89 ppm corresponds to the proton of the OS[O.sub.3]CH group coupled with one from a methylene group. Another methylene group of the C[H.sub.2]C[H.sub.2] side chain shows a signal at 4.6 ppm. The signal of the solvent appears at 4.8 ppm.
TG-DSC essays indicated a weight loss in two steps at different decomposition temperature maximum. For the two homopolymers, the weight loss was nearly 85%, which occurred at 267 and 332[degrees]C for P(ClAETA) and P(SAETA), respectively. The melting temperature range was narrower for P(ClAETA) than for P(SAETA), with a maximum at 267 and 332[degrees]C, respectively. This difference suggests a slightly better crystallinity in P(ClAETA). The second peak at 420[degrees]C was attributed to exothermic reactions corresponding to ammonium salt decomposition (Figs. 1 and 2).
RESULTS AND DISCUSSION
The cationic polymers were synthesized by solution radical polymerization. The structure of both synthesized polymers P(CIAETA) and P(SAETA) is shown below:
Arsenic Retention Properties of the Water-Soluble Polymers
Figures 3-5 present the As(V) retention profiles as a function of pH for the different cationic polymers. These retention profiles show the arsenic retention, R versus Z, where Z is the filtration factor defined as the ratio of the filtrate volume ([V.sub.f]) to the cell volume ([V.sub.o]). In general, As(V) species are more efficiently retained at higher pH (6 and 8) than at lower pH. At pH 4, the ability to bind metal ions is lower or the number of effective active sites for homopolymers is low.
The predominating species in solution are monovalent ([H.sub.2]As[O.sub.4.sup.-]) anions in equilibrium with the nondissociated salt. It is assumed that the functional groups' polarity is one of the parameters controlling the exchange selectivity. At pH 6, there is equilibrium between monovalent ([H.sub.2]As[O.sub.4.sup.-]) and divalent (HAs[O.sub.4.sup.2-]) anions in solution, which suggests that the anionic exchanger prefers the arsenic divalent anion, with respect to the monovalent anions under the same (acid pH) conditions. Therefore, this explanation is corroborated by the polymer's higher retention ability at basic pH, where divalent species are the predominating species. The polymer binding capacity is attributed to the anionic exchange between the anions associated to the polymer's cationic groups and the arsenate anions, and the binding of the arsenate anions with the ammonium quaternary cationic groups. The results presented in Figs. 3 and 4 show that the polyelectrolytes with exchanger chloride anions, such as the poly[2-acryloyloxy)ethyl]trimethylammonium chloride P(ClAETA), present a higher selectivity than P(SAETA), containing methylsulfate groups as exchanger anions. Polymers with chloride exchanger groups presented the highest retention (100%) at basic and neutral pH. This result can be attributed to an easier release of the [Cl.sup.-] anions, in comparison to the methyl sulfate anions, associated with the quaternary ammonium groups. Monovalent ions are retained strongly to quaternary ammonium hydrophobic sites due to differences in ionic size, solvability, and polarity. A larger, more polarizable ion produces an easier disruption of the local water structure, enabling an easier association with a given quaternary ammonium ion. A well-hydrated anion, such as chloride, will be more easily released into the solution than poorly hydrated anions, such as bromide or iodide anions. The more hydrophobic character of the monovalent anion (OS[O.sub.3]C[H.sub.3])[.sup.-], which contains a methyl group, could explain its efficient retention in the polyammonium receptor.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Figure 4 shows the results obtained with the P(SAETA), which contains methyl sulfate anions instead of chloride anions as exchanger groups. Consequently, the difficulty to exchange the hydrophobic voluminous OS[O.sub.3]C[H.sub.3]--of P(SAETA) in comparison with the more polarizable [Cl.sup.-] anion in P(CIAETA) resulted in a 40-60% reduction in arsenate retention.
The effect of the exchange of methyl sulfate anions with S[O.sub.4.sup.2-] anions was also studied. A P(SAETA) solution containing an equimolar amount of sulfate anions was stirred at room temperature for 24 h, and the resulting mixture's retention properties for arsenic species were studied using the LPR technique. The results are presented in Fig. 5.
The retention decreased up to 40% at pH 8. It is probable that the decrease in the retention ability of the cationic polymer exchanger is due to an increase in the ionic strength of the solution following the addition of [Na.sub.2]S[O.sub.4], which induced a change in polarization. Moreover, the exchange in the polymeric structure of the methyl sulfate species by the sulfate anions turned out to be difficult. Divalent sulfate ions have a great affinity for water, because they are well-hydrated anions. These ions presumably have less affinity for the more hydrophobic alkylammonium sites in the exchanger polymer. The high affinity of a divalent anion to a resin site is attributed to electroselectivity . On the other hand, it is known  that adsorption diminishes as the media's ionic strength increases in certain cases of nonspecific adsorption (outer-sphere surface complex).
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
Another essay was performed to study the exchange properties of a polymer solution that was contacted with [Cl.sup.-], before the washing method. A P(SAETA) solution containing a twofold molar excess of chloride anions was shaken for 24 h at room temperature, and the retention properties of the resulting solution were studied using the LPR technique. Results are shown in Fig. 6. It is probable that some (OS[O.sub.3]C[H.sub.3])[.sup.-] groups were exchanged for [Cl.sup.-] anions. In these experimental conditions, the essays showed different retention profiles in comparison to those obtained without chloride anions (Fig. 5). The R value significantly increased at pH 8 and 6, although no profound modification was observed at pH 4 (Fig. 7).
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
The results of the essays performed using the enrichment method with P(ClAETA) and P(SAETA), to determine the maximum retention capacity (MRC) for arsenic anions in aqueous solutions at pH 8, are shown in Figs. 7 and 8, respectively.
The MRC is defined as
MRC = MV/[P.sub.m]
where [P.sub.m] is the amount of polymer, (g); M is the initial concentration of As(V) (mg [L.sup.-1]); and V is the filtrate volume, (defined volume) through of membrane free of As(V), (mL).
[FIGURE 8 OMITTED]
MRC was calculated as 142 mg/g for P(ClAETA) and 75 mg/g for P(SAETA), corresponding to the total filtrate volume of 300 mL.
Assuming a quantitative As(V) retention, the enrichment factor (E = 3.2 for P(ClAETA) and E = 2.5 for P(SAETA)) is determined according to the following relationship:
E = (P x MRC)/M
where P is the concentration of polymer (g [L.sup.-1]); MRC is the maximum retention capacity of the polymer (mg/g); and M is the initial concentration of the metal salt (mg/L).
The binding capacity of the homopolymer P(ClAETA) with E = 3.2 was better than P(SAETA) with E = 2.5. The type of anionic exchanger group was found to be an important factor in arsenate retention.
Two cationic polymers synthesized by radical polymerization and containing different exchangeable anions have presented different arsenate ion retention properties. Quaternary ammonium cationic soluble polymer with exchangeable chloride counteranions effectively removes arsenate ions at high pH (pH 8).
The polymer P(SAETA), which contains the voluminous groups (OS[O.sub.3]C[H.sub.3])[.sup.-] that are less polarizable and more hydrophobic than [Cl.sup.-], exhibits a lower (~50%) arsenate ion retention ability. The maximum binding capacity values for P(SAETA) is two times less than for P(ClAETA). However, the amount of anions that each group was able to bind (E factor) is not very different. Thus, the nature of the anionic exchanger group appears to be an important factor in arsenate retention by water-soluble, cationic polymers.
Additionally with P(SAETA), the modification of the medium's ionic strength following [Na.sub.2]S[O.sub.4] addition induced a change in polarization, without exchange of the methyl sulfate species for divalent sulfate anions, decreasing the cationic exchange ability for arsenate ion retention.
LPR Liquid-phase polymer-based retention P(CIAETA) Poly[2-(acryloyloxy)ethyl]trimethylammonium chloride P(SAETA) Poly[2-(acryloyloxy)ethyl]trimethylammonium methyl sulfate FT-IR Fourier transformed infrared [.sup.1.H] NMR Proton nuclear magnetic resonance TGA Thermogravimetry analysis DSC Differential scanning calorimetry R Retention Z Filtration factor MRC Maximum retention capacity E Enrichment factor
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Bernabe L. Rivas, (1) Maria del Carmen Aguirre, (1) Eduardo Pereira, (1) Jean-Claude Moutet, (2) Eric S. Aman (2)
(1) Faculty of Chemistry, University of Concepcion, Concepcion, Chile
(2) Laboratoire d'Electrochimie Organique et de Photochimie Redox, UMR CNRS 5630, ICMG-FR CNRS 2607, Universite Joseph Fourier Grenoble 1, 38041 Grenoble Cedex 9, France
Correspondence to: Bernabe L. Rivas; e-mail: email@example.com
Contract grant sponsor: FONDECYT; contract grant number: 3050057; contract grant sponsor: CNRS-CONICYT; contract grant number: 18641.
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|Author:||Rivas, Bernabe L.; del Carmen Aguirre, Maria; Pereira, Eduardo; Moutet, Jean-Claude; Aman, Eric S.|
|Publication:||Polymer Engineering and Science|
|Article Type:||Technical report|
|Date:||Aug 1, 2007|
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