Removal of metal ions from aqueous solution by chelating polymeric microspheres bearing phytic acid derivatives

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EUROPEAN POLYMER JOURNAL

European Polymer Journal 44 (2008) 1183–1190

www.elsevier.com/locate/europolj

Removal of metal ions from aqueous solution by chelating polymeric microspheres bearing phytic acid derivatives Francesca Iemma *, Giuseppe Cirillo, Umile Gianfranco Spizzirri, Francesco Puoci, Ortensia Ilaria Parisi, Nevio Picci Dipartimento di Scienze Farmaceutiche, Universita` della Calabria, 87036 Rende (CS), Italy Received 13 July 2007; received in revised form 21 December 2007; accepted 15 January 2008 Available online 20 January 2008

Abstract The aim of this study was to investigate the possibility to synthesize new chelating polymeric microspheres owing immobilized biocompatible agent as chelating functional groups and to evaluate their performance in metal ions removal from aqueous solution. The microparticles were synthesized via precipitation polymerization of 4-O-(4-vinylbenzyl)-myo-inositol 1,3,5-orthoformate with ethylene glycol dimethacrylate (EGDMA) and subsequent exhaustive phosphorylation of myo-inositol groups using phosphoric acid. Spherical geometry with monodisperse nature of the polymeric microparticles was confirmed by scanning electron micrographs (SEM) and dimensional analysis. A large surface area of the microspheres provided a maximum interaction of metal ions and the chelating functional groups on the surface. Absorption capacity of the beads for the selected metal ions, i.e., Cu(II), Ni(II), Fe(III), was investigated in detail in aqueous solution at pH 5.0 utilizing UV/Vis spectroscopy. This study showed that the macromolecular systems are very effective in chelating these metal ions and the affinity order of the microbeads toward metal ions is: Fe(III) > Ni(II) > Cu(II). The chelating beads can be easily regenerated by 1.0 M HNO3 with high effectiveness. These features make the synthesized beads a potential candidate for metal ions removal at high capacity. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Metal removal; Phytic acid derivatives; Chelating polymer; Spherical microparticles; Precipitation polymerization

1. Introduction Environmental contamination by heavy metal ions is a serious problem owing to their tendency to accumulate in living organisms and toxicities in * Corresponding author. Tel.: +39 0984 493011; fax: +39 0984 493298. E-mail address: [email protected] (F. Iemma).

relatively low concentration [1–3]. Different technologies and processes are currently used to reduce heavy metal concentration in the waste water. Biological treatments [4], membrane processes [5], advanced oxidation processes [6], chemical and electrochemical techniques [7,8], and absorption procedures [9,10] are the most widely used techniques for metals removing from industrial effluents. Among these technologies, many researches concentrated

0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.01.024

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on metal ions recovery using chelating polymers because they are reusable, easy handling and have higher absorption capacities, efficiencies as well as high selectivity to some metal ions [11,12]. Hence, numerous chelating resins have been prepared through polymerization of conventional chelating monomers, such as acrylic acid [13], allylthiourea [14], vinyl pyrrolidone [15], and vinyl imidazole [16]. Additionally, modification of a synthetic [17– 19] or natural polymeric matrix [20,21] by functionalization reactions has also been used to form chelating polymers. In this study our attention was focussed on the preparation of new chelating resins by immobilizing chelating biomolecule on a polymeric support. Phytic acid was chosen as starting compound to confer high chelating properties to a synthetic polymeric resins. Phytic acid and its analogues, are known to play a crucial role in reducing the concentration of free metal ions, such as calcium, magnesium, iron, zinc, copper, and nickel [22]. Recently, the possibility to bind phytic acid derivatives to polymeric chains without losing antioxidant properties was shown and antioxidant activity of water-soluble and insoluble copolymers based on phytic acid derivatives in rat liver microsomal membranes was successfully determined [23,24]. The aim of this work was to evaluate the possibility to employ polymeric microspheres bearing phytic acid derivatives in heavy metals removal from aqueous solutions. Our approach involved the preparation of chelating matrices owing a spherical shape to maximize the interactions between metal ions and chelating functional groups on the microspheres surface. An interesting way to prepare mono-dispersed insoluble microspheres is the precipitation polymerization. The technique was applied using as functional monomer a precursor of the chelating group such as 4-vinylbenzyl myo-inositol orthoformate and ethylene glycol dimethacrylate (EGDMA) as crosslinking agent. The so obtained intermediate copolymer was subsequently phosphorylated using phosphoric acid to carry out the final chelating microspheres. Both unphosphorylated and phosphorylated polymers by SEM photography and dimensional analysis were characterized. Binding experiments were performed to check the chelating properties of synthesized materials. First of all, iron(III) was chosen as model metal to prove the preserved chelating properties in the functionalized crosslinked system. This choice was dictated by

the fact that iron(III) is a metal with a very high affinity for phytic acid. Subsequently, the possibility to employ these resins in heavy metals removal was tested by using Nickel(II) and Copper(II) as divalent model ions. The sequestering ability of polymeric microspheres toward iron(III), copper(II), and nickel(II) cations was evaluated by using, for each metal, appropriated UV–Vis colorimetric methods. Finally, repeated use of the ion-chelating beads is also discussed. The synthesized macromolecular system could be very useful from an environmental and biological point of view, to reduce the availability of the pollutant in aqueous systems such as natural, industrial waters and biological fluids. 2. Experimental 2.1. Materials Myo-inositol, triethyl orthoformate, p-toluensulfonic acid, sodium hydride (80% dispersion), 4vinylbenzylchloride, ethylene glycol dimethacrylate (EGDMA), 2,20 -azobisisobutyronitrile (AIBN), ammonium molybdate, ammonium vanadate, phosphoric acid (85% wt), nitric acid (63% v/v), hydrochloride acid (37% v/v), trifluoroacetic acid (98% wt), ammonia solution (30% w/w), phosphoric anhydride, thiobarbituric acid, dimethylglyoxime, iodine, citric acid, ferric nitrate, cupric nitrate, nickel nitrate, potassium iodide were obtained from Sigma (Sigma Chemical Co., St. Louis, MO) and Carlo Erba Reagents (Milan, Italy). N,N-dimethylformamide (DMF), acetonitrile, chloroform, ethyl acetate, hexane, 2-propanol, methanol, acetone, ethanol were obtained from Fluka Chemika-Biochemika (Buchs, Switzerland) and Carlo Erba Reagents (Milan, Italy) and were reagent grade. DMF was dried before use. Acetonitrile was HPLC grade. 2.2. Instrumentation UV spectra were made by spectrometer UV/Vis Jasco V-530 at 25 °C. Scanning electron microscopy (SEM) photographs were obtained with a Jeol JSMT 300 A; the surface of the samples was made conductive by deposition of a gold layer on the samples in a vacuum chamber. Particles size distribution was carried out using an image processing and analysis system, a Leica DMRB equipped with a LEICA Wild 3D stereomicroscope. The shaker and centrifu-

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gation systems consisted of a wrist action shaker (Burrell Scientific) and an ALC micro-centrifugette 4214, respectively.

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added to 100 ml. The solutions were analyzed by UV–Vis spectrophotometer (k = 420 nm). 2.6. Spectrophotometric determination of Fe(III)

2.3. Microspheres preparation 4-Vinylbenzyl myo-inositol orthoformate (VBMO) was synthesized as described in the literature [23]. For microspheres preparation, 1.96 mmol of VBMO and 5.88 mmol of EGDMA were dissolved in 55 ml of acetonitrile in a 100 ml round bottom flask and then 100 mg of AIBN were added. The polymerization mixture were degassed in a sonicating water bath, purged with nitrogen for 10 min cooling with an ice-bath. The flask was then gently agitated (40 rpm) in an oil bath. The temperature was increased from room temperature to 60 °C within 2 h, and then kept at 60 °C for 24 h. At the end of the reaction, the particles were filtered, washed with 100 ml of 2-propanol, 100 ml of methanol, and 100 ml of acetone. Particles were successively dried under vacuum overnight at 40 °C. 2.4. Phosphorylation of myo-inositol residues Five hundred milligram of polymeric microparticles were added at 50 ml phosphoric acid (85 wt%) and heated at 120 °C for 12 h. After cooling, the particles were filtered, washed with distilled water to pH 7.0 and then with 100 ml of 2-propanol, 100 ml of methanol and 100 ml of acetone. Particles were successively dried under vacuum overnight at 40 °C. 2.5. Analysis of phosphate groups Ten gram of ammonium molybdate and 0.50 g of ammonium vanadate were dissolved in HNO3 (70 ml, 63% v/v) and distilled water (430 ml). In 100-ml volumetric flasks, five solutions were prepared introducing 25 ml of the vanade–molybdate reagent, 13, 15, 27, 19, 21 ml of P2O5 solution (200 mg/l), respectively, and distilled water was added to 100 ml. These solutions were analyzed by UV–Vis spectrophotometer (k = 420 nm) in order to obtain a calibration curve. Polymeric particles (100 mg) was added to a solution of 5 ml distilled water, 5 ml HCl (12 mol l1) and 5 ml HNO3 (14.5 mol l1) and heated at 100 °C for 15 h. After cooling, the particles were filtered and the solutions were added in a 100-ml volumetric flask to 25 ml of vanade–molybdate solution and distilled water was

To record the calibration curve, in 100-ml volumetric flasks, five solutions were prepared introducing 20 ml of TBA solution (5.10 g in 500 ml of NaOH solution 0.05 M), 7.5, 15, 22.5, 30, and 72.2 ml of Fe(NO3)3 solution (0.67 g in 500 ml of water, pH 5.0) and distilled water to 100 ml, to obtain final concentrations of 4.16  104; 8.31  104; 1.25  103; 1.66  103; 4.0  103, respectively. These solutions were analyzed by UV–Vis spectrophotometer (k = 532 nm) to plot a calibration curve and the correlation coefficient (R2) of the regression equation obtained by the method of least square was calculated. 2.7. Spectrophotometric determination of Cu(II) In 100 ml volumetric flasks, 0.15; 0.20; 0.25; 0.30; 0.40 g of Cu(NO3)2 were dissolved in water (pH 5.0) to raise the final concentrations of 8.00  103; 1.06  102; 1.33  102; 1.59  102; 2.12  102 mol l1, respectively. In order to obtain the calibration curve, 40 ml of each solution were introduced in 100 ml volumetric flask and, after neutralization with concentrated ammonia solution (30% wt) to pH 7.0, ammonia solution (3.00 mol l1) was added to 100 ml. The final concentrations were 3.20  103; 4.24  103; 5.32  103; 6.36  103; 8.48  103, respectively. The resulting solutions were analyzed by UV–Vis spectrophotometer (k = 610 nm) to obtain a calibration curve and the correlation coefficient (R2) of the regression equation obtained by the method of least square was calculated. 2.8. Spectrophotometric determination of Ni(II) To obtain the calibration curve, in 100 ml volumetric flasks, five solutions were prepared introducing, respectively, 7.50; 15; 30; 40; 50 ml of Ni(NO3)2 solution (1.29  104 mol l1, pH 5), 0.30 ml of nitric acid (15% wt), 10 ml of citric acid buffer (pH 4.7), 5 ml of iodine solution (8.3 g of KI and 6.3 g of I2 in 500 ml of water), 20 ml of dimethylglyoxime solution (0.1 g in 50 ml of ammonia solution (30% wt) and ethanol to 100 ml) and distilled water to 100 ml, to obtain the final concentrations of 9.76  106; 1.94  105; 3.87  105; 5.16  105;

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6.45  105 mol l1. These solutions were analyzed by UV–Vis spectrophotometer (k = 530 nm) to obtain a calibration curve and the correlation coefficient (R2) of the regression equation obtained by the method of least square was calculated. 2.9. Metal ions binding experiments Evaluation of the capacity of the polymeric matrix to recognize and bind metal ions in water solution (pH 5.0) was performed. Briefly, 40 ml of an aqueous solution (2.0  102 mol l1) of each metal ion (Fe(NO3)3; Cu(NO3)2; and Ni(NO3)2) were mixed with 0, 25, 50, 75, 100 mg of polymeric particles in 50 ml conical centrifugation tube and sealed. The suspensions were mixed with the help of an ultrasonic bath and the tubes were oscillated by a wrist action shaker in a water bath for 3 h. The mixtures were brought to the desired pH by adding sodium hydroxide (NaOH) and nitric acid (HNO3) and maintained in a range of ±0.1 U. The suspensions were then centrifuged for 10 min (7000 rpm) and, after dilution, the metal ions concentration in the liquid phase was measured by UV–Vis spectrophotometer as before reported. The same experiments were performed using unphosphorylated polymeric microparticles. 2.10. Desorption and repeated use Desorption of metal ions from the beads was carried out using 1.0 M HNO3 solution. The chelating microbeads (100 mg) were placed in the elution medium and stirred continuously at room tempera-

ture. The desorption ratio was calculated from the amount of metal ions adsorbed on microspheres and the final metal ions concentration in the desorption medium. In order to test the reusability of the chelating beads, metal ion absorption–desorption cycle was repeated five times using the same beads. 3. Results and discussion 3.1. Synthesis and characterization of polymers To prepare chelating polymeric microspheres, myo-inositol orthoformate monomer, obtainable from commercial myo-inositol [23], and ethylene glycol dimethacrylate (EGDMA) as cross-linker were used. To synthesize microspheres the precipitation polymerization was chosen as synthetic procedure [25,26]. The proposed mechanism is characterized by two parts: nucleation and growth of microspheres. The reaction begins as an usual solution polymerization. The monomers and initiator are dissolved in the organic solvent and during the polymerization oligomers are formed. After a certain period of time, the concentration of oligomers becomes sufficiently high to allow radical polymerization of oligomers to form a microgel (Nucleation). Each seed (microgel) then grows by continuous capture of oligomers. This last effect prevents the occurrence of any further nucleation and hence uniformly-sized particles are produced. During polymerization the growing polymer chains are separated from the continuous medium by enthalpic precipitation in cases of non-favorable

Fig. 1. Photomicrography of spherical unphosphorylated copolymer (a) and spherical phosphorylated copolymer (b).

F. Iemma et al. / European Polymer Journal 44 (2008) 1183–1190

polymer–solvent interactions or entropic precipitation in cases where crosslinking prevents the polymer and the solvent from freely mixing [27]. The spherical geometry and the practically monodispersion of prepared sample were confirmed by scanning electron micrographs (SEM) (Fig. 1a) and dimensional analysis (Fig. 2). The mean particle sizes and the polydispersity of the microspheres were 4.69 and 1.01 lm, respectively. Cleavage of the orthoformate and phosphorylation of all the hydroxyl groups in phosphoric acid at 120 °C carried out to the final microparticles (Fig. 3). This simple and quickly synthetic strategy allows to obtain the complete phosphorylation of myo-inositol residues in one-pot reaction without further purification processes. Microparticles SEM (Fig. 1b) clearly show that the morphologic characteristics of prepared samples did not change after exhaustive phosphorylation. Dimensional analysis showed that the phosphoryla-

Fig. 2. Copolymers dimensional analysis.

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tion did not interfere significantly with particles dimensions and polydispersity values (4.76 and 1.02 lm, respectively) (Fig. 2). Microparticles were also characterized by quantitative determination of phosphate amount in order to check the functionalization degree. This procedure involves the hydrolysis of the phosphate groups from the polymeric backbone and the subsequent reaction of freed phosphates with a vanade– molybdate reactive. The phosphate content was 7.20  107 ± 0.26 mol per mg of polymer. 3.2. Metal removal from aqueous solutions The prepared functionalized microbeads should be suitable for complexation of a wide range of transition metal ions in a solution-like behaviour. To investigate the complexation properties, binding experiments, in which the microbeads were treated with different metal ions solutions, were performed. In particular, the chelating properties of microparticles towards Fe(III), Cu(II), and Ni(II) ions were tested. For this purpose, nitrate salts were used as metal ions source, the beads were suspended in solutions containing an excess of these salts, and mixed with the help of an ultrasonic bath to ensure complete mixing. After incubation time, the beads were filtered off and the amount of bounded metal was determined using an adequately chelating agents, selected for each metal, to form a complex UV– Vis detectable. In our experiments, nitrate salts (8.0  104 mol) were mixed with different amount of polymeric microspheres from 0 to 100 mg in 40 ml of distilled

Fig. 3. Synthesis of copolymer. Reagents and condition: (i): AIBN, CH3CN, 60 °C, 24 h; (ii): H3PO4, 120 °C, 12 h; P, phosphate group.

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Fig. 4. Bounded Iron(III) (D), Copper(II) (), Nickel(II) (h) (%) expressed as a function of the amount of chelating polymer. mb, Bounded metal mass; m0, initial metal mass.

phorylated polymeric microparticles were treated with trifluoroacetic acid solution to make the myo-inositol residues freed of orthoformate groups. The metal ions absorption on the unphosphorylated microspheres is very low, confirming that chelating properties of synthesized materials could be ascribe to specific interactions between solved ions and phosphoric groups on the surface of the microbeads, while free hydroxyl groups of myo-inositol are ineffective.

water for 3 h and, after microbeads removal, unbounded metal concentration was determined by UV–Vis measurement as follows: iron–thiobarbituric acid adduct at 532 nm; copper–ammonia adduct at 610 nm, and nickel–dimethylglyoxime adduct at 530 nm. The polymer was found to be stronger chelating agent towards metal ions and the relationship between amount of polymeric microspheres and bound metals is shown in Fig. 4. Increasing the amount of polymer in the binding solution, an increase of chelated metal was raised. The absorption capacities of beads referred to 100 mg of polymeric support are 30.1 mg for Fe(III), 29.0 mg for Ni(II), and 22.8 mg for Cu(II). Metal ions affinity order is as follows: Fe(III) > Ni(II) > Cu(II) (Table 1). The non-specific absorption of metal ions onto the blank beads (without phosphoric groups on the surface) was also evaluated under the same experimental conditions. For this purpose, unphos-

3.3. Desorption and repeated use The repeated use, i.e., regenerability, of the polymer beads is likely to be a key factor in improving process economics. Desorption of the adsorbed metal ions from the microbeads was also studied in a batch experimental set-up. The microspheres beads loading the maximum amounts of the respective metal ions were placed within the desorption medium containing 1.0 M HNO3 and the amount

Table 1 Bounded metal ions (%) Polymer (mg)

Bounded metal Fe(III)

25 50 75 100

Cu(II)

Ni(II)

mb (mg)

mb/m0 (%)

mb (mg)

mb/m0 (%)

mb (mg)

mb/m0 (%)

15.0 ± 1.6 24.3 ± 1.8 28.5 ± 1.3 30.1 ± 0.4

33.3 ± 3.7 54.1 ± 4.1 63.4 ± 2.0 67.0 ± 1.1

7.6 ± 1.1 13.5 ± 1.0 18.2 ± 1.4 21.4 ± 1.2

14.9 ± 2.1 26.4 ± 1.9 35.7 ± 2.8 41.9 ± 2.4

8.3 ± 2.1 17.2 ± 2.6 24.1 ± 4.4 29.0 ± 2.0

17.6 ± 4.4 36.7 ± 5.7 51.2 ± 3.1 61.8 ± 4.4

mb, bounded metal mass; m0, initial metal mass.

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of metal ions desorbed in 24 h was measured. For all metal ions, recorded desorption ratios were up to 92% ± 4.8. In order to obtain the reusability of the synthesized materials, the absorption–desorption cycle was repeated five times by using the same adsorbent. The absorption capacity of the adsorbent for all metal ions did not significantly change during the repeated absorption–desorption operations.

[3]

[4]

[5] [6]

4. Conclusions Heavy metal ions are known to be toxic and especially cadmium, arsenic, mercury, chromium, copper, lead, nickel, selenium, silver, and zinc are released into the environment in quantities that pose a risk to living systems. Absorption technology enables the use of polymeric beads for rapid, costeffective and selective heavy metal removal. In this study, the possibility to immobilize a biocompatible chelating agent on polymeric support were evaluated. Chelating microspheres, via a two steps procedure, were successfully synthesized and applied to remove copper(II), iron(III), and nickel(II) ions from aqueous solutions. The first step is the precipitation polymerization of myo-inositol derivatives in presence of a cross-linker agent, the second one is the exhaustive phosphorylation of the myo-inositol derivatives. Particles morphology shows a spherical shape and a micrometric and monodisperse size distribution. The affinity order of metal ions is as follows: Fe(III) > Ni(II) > Cu(II). Repeated absorption and desorption cycles showed the feasibility of these newly synthesized beads for metal removal. These proposed macromolecular systems could be very helpful in environmental and biological fields, to reduce heavy metal pollution in natural, industrial waters or biological fluids.

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Acknowledgments This work was financially supported by MIUR (Programma di ricerca di rilevante interesse nazionale 2005) and University funds.

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