Gamma Radiation-Polymerized

Gamma Radiation-Polymerized Zn(II) Methacrylate as a Sorbent for Removal of Pb(II) Ions from Wastewater F. Uren˜a-Nun˜ez,*,† C. Barrera-Dı´az,‡ and Bryan Bilyeu§ Instituto Nacional de InVestigaciones Nucleares, A.P. 18-1027, Col. Escando´n, Delegacio´n Miguel Hidalgo, C.P. 11801, Me´xico, D.F., Me´xico, UniVersidad Facultad de Quı´mica, Auto´noma del Estado de Me´xico, Paseo Colo´n interseccio´n Paseo Tollocan S/N, C.P. 50120, Toluca, Estado de Me´xico, Me´xico, and Department of Materials Science and Engineering, UniVersity of North Texas, P.O. Box 305310, Denton, Texas 76203-5310 In this work, Zn(II) polymethacrylate (Zn(II)PMA) is synthesized by ç irradiation of the corresponding monomer and evaluated for use as a lead ion absorbent. The polymer powder was mixed with Pb(II) aqueous solutions to determine its capacity at removing the heavy metal ion from water. The Pb(II) removal occurs via a Langmuir-type adsorption mechanism; the Pb(II) removal was found to be a function of the contact time between the polymer and the solution. A decrease of 94% of Pb(II) concentration in the liquid phase was achieved when an initial Pb(II) concentration was 25 mg L-1. The polymer was characterized using scanning electron microscopy, energy dispersion analysis, electron paramagnetic resonance, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The use of Zn(II)PMA for the treatment of wastewater containing heavy metals is an innovative method that constitutes a simple, effective, and economical mean for wastewater treatment. Introduction Most heavy metal ions are toxic or carcinogenic and hence present a threat to human health and the environment when they exist in or are discharged into various water resources.1,2 Heavy metal pollution exists in wastewater discharge of many industries among which the plating facilities, tanneries, and mining operations are easily distinguishable due to their severe envi- ronmental impacts and ever present risks associated with mismanagement.3 One of the most promising alternative methods for heavy metal removal is the sorption of pollutant ions on waste materials, both organic and inorganic, which usually are abundant and inexpensive.4 Moreover, after these inexpensive sorbents have been expended, they can be easily disposed or regenerated.5 This feature is also common in various macro- molecules such as polymeric materials. Adsorption is one of the methods commonly used to remove heavy metal ions from low-concentration aqueous solutions. The efficiency of adsorption relies on the capability of the adsorbent to concentrate or adsorb metal ions from the solutions onto its surfaces, removing the ions from the solution. There are many types of adsorbents, including activated carbon, oxide minerals, resins, polymer fibers, and biosorbents, which have been used to adsorb metal ions or to enrich trace amounts of metals from aqueous solutions. Due to the relatively large external specific surface areas, various fibers have attracted certain attention as an adsorbent to remove heavy metal ions from water or wastewater in recent years.6,7 In the adsorption process, metal ions in the aqueous solutions may be transported through diffusion or convection to the surface of the adsorbent and then become attached to the surfaces due to various physical or chemical interactions between the metal ions and the surface functional groups of the adsorbent.8 The use of polymeric materials is based on inexpensive substitutes for wastewater treatment. A way to enhance the adsorption capacity is by grafting of synthetic polymers followed by functionalization.9 Surface functional groups effective for metal ion adsorption are grafted onto the fibers through chemical reactions.10 Therefore, the chemical components of the fibers play important roles in the introduction of the functional groups on the surface of the fibers. Polyacrylonitrile fibers have been studied to introduce carboxyl, hydrazine, or imidazoline groups on the surfaces for adsorbing some metal ions from aqueous solutions.9 New material research is needed to enhance metal uptake. Polymer production using irradiation techniques presents the following advantages over traditional methods: the synthesis is carried out in the absence of catalysts and initiators; moreover, polymerization and cross-linking may occur simultaneously.11 Furthermore, there is no need to add solvents to perform the polymerization. Thus, this technique could be considered as a clean way to obtain polymeric materials. In this study, the polymerization of Zn(II) methacrylate was induced by ç radiation. The Zn(II) polymethacrylate (Zn(II)- PMA) was used as a sorbent in a series of batch experiments to investigate its capacity in removing lead ions from aqueous solutions. In order to characterize the material composition and the mechanisms involved, the following techniques were used: scanning electron microscopy (SEM), energy dispersion analysis (EDX), electron paramagnetic resonance (EPR), X-ray diffrac- tion, Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). Materials and Methods Synthesis of Zn(II) Methacrylate (Zn(II)MA). Zn(II)MA was synthesized in the following steps: an aqueous solution of NaHCO3 was treated with methacrylic acid and the mixture was stirred for 30 min (eq 1); then ZnCl2 was added and stirred again for 1 h at 40 °C (eq 2). Once the reaction took place, the * To whom correspondence should be addressed. Tel.: + (52)- 53297200. Fax:+ (52)-53297301. E-mail address fun@nuclear.inin.mx. † Instituto Nacional de Investigaciones Nucleares. ‡ Auto´noma del Estado de Me´xico. § University of North Texas. 3382 Ind. Eng. Chem. Res. 2007, 46, 3382-3389 10.1021/ie0612111 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/14/2007 insoluble Zn(II) methacrylate precipitate was filtered out, washed with distilled water, and dried under vacuum. Polymerization of Zn(II)MA. The ç-ray induced polymer- ization of the monomer was carried out in a ç irradiation unit ALC Gammacell-220, supplied with a 60Co source. A 20 kGy dose was applied at a 0.5 kGy h-1 rate. It has been shown elsewhere by our group12 that these conditions induce complete polymerization of the monomer with the greatest crystallinity index. The polymerization reaction is shown in eq 3. Zn(II)MA and Zn(II)PMA Characterization. SEM and EDX. The SEM characterization was carried out on samples of both monomer and polymer, using a JEOL JSM-5900 LV microscope to obtain information on the composition and general features of the structures. Scanning electron microscopy provides secondary electron images of the surface with resolution in the micrometer range, while energy dispersive X-ray spectroscopy offers in situ chemical analysis of the bulk. Images were observed at 20 kV. The chemical composition of the polymer was determined by a DX-4 analyzer coupled to the SEM, before and after contact with the aqueous solution. Electron Paramagnetic Resonance. The polymer was analyzed using EPR to confirm the presence of free radicals during the polymerization. This study was done with a Varian E-15 spectrometer operating at X-band. The EPR spectrometer settings were as follows: 1.0 mW microwave power; the field modulation at 100 kHz was 0.4 mT; the magnetic field was set around 330 mT; the scan range was 40 mT linear sweep in 8 min. X-ray Diffraction. The crystallinity of the Zn(II)PMA was analyzed with an X-ray diffractometer scanning in the 2ı range 0-60. Copper radiation was used with a diffracted beam monochromator tuned to KR radiation Fourier Transform Infrared Spectroscopy. The monomer and polymer were analyzed with a Nicolet Magna-IR 550 to observe the changes in the chemical bonds and structure and to ensure that polymerization had taken place. Thermogravimetric Analysis (TGA). This analysis was performed on a TA Instruments TGA 51 thermogravimetric analyzer, which was operated in a nitrogen atmosphere at a heating rate of 10 °C min-1 from 25 to 800 °C. X-ray Photoelectron Spectroscopy. XPS analyses of the Zn- (II)PMA after the lead adsorption was carried out on an AXIS HIS spectrometer (Kratos Analytical Ltd.) with an Al ka X-ray source to determine the C, O, Zn, and Pb atoms present on the surface of the polymer. Surface Area Measurements. The polymer surface area was determined by standard multipoint techniques using a Mi- Figure 1. Secondary electron image of the Zn(II)MA monomer. The magnification marker is 50 ím. Figure 2. Secondary electron image of the Zn(II)PMA polymer. The magnification marker is 1 ím. Ind. Eng. Chem. Res., Vol. 46, No. 10, 2007 3383 cromeritics Gemini 2360 instrument. Prior to analysis, the samples were dehydrated at 80 °C for 2 h. Adsorption of Pb(II) on Polymer. In order to evaluate the Pb(II) removal capacity of the polymer, batch equilibrium tests were conducted at constant temperature (18 ( 0.5 °C). The powdered Zn(II) polymethacrylate samples were put in contact with the aqueous Pb(II) solutions. All solutions were prepared with analytical grade reagents, using deionized water (18 M¿ cm resistivity). The mixtures were stirred, and then the phases were separated by filtration and the Pb(II) in solution was evaluated. The selected parameters mass/volume ratio, initial metal concentration, and contact time were studied. Duplicate experiments permitted us to average the results. Quantification of Pb(II)and Zn Concentration in Solution. The concentration of Pb(II) and Zn ions in solution, before and after the sorption process, was determined using a Perkin-Elmer 2380 atomic absorption spectrophotometer. All calibrations and procedures were carried out in accordance with AWWA standards.13 The effect of the pH on lead sorption was measured by changing the pH of the aqueous solution in a range of 1-6 pH units. Thermodynamic Study. The existence of Pb(II) complexes in aqueous solution has been reported.17,18 Using this informa- tion, the distribution diagrams of chemical species were calculated using the MEDUSA program.19 Results and Discussion SEM Analysis of Zn(II)MA and Zn(II)PMA. Figure 1 shows the secondary electron images of the monomer of zinc methacrylate recorded at 500�. It can be seen that the Zn(II)- MA is formed by small aggregates containing small laminar fiberlike structures (see Figure 1). Figure 2 shows the SEM image of the Zn(II)PMA recorded at 20000�. It is observed that after the ç polymerization a number of flake structures are formed. The size of these particles is within 1-ím width range. The elemental analysis of the polymer indicates the presence of C, O, and Zn, as shown in Table 1 Electron Paramagnetic Resonance. The EPR spectrum of PMZn was measured with a Varian E-15 spectrometer operating at the X-band of the microwaves. The EPR spectrum was recorded as the first derivative of the absorption spectrum. All measurements were performed at room temperature, and the instrument settings were as follows: magnetic field 330 mT, scan range 40 mT, scan time 8 min, magnetic field modulation amplitude 0.1 mT, modulation frequency 100 kHz, microwave power 2.0 mW (nominally 1.0 mW per half of the dual cavity); receiver gain and time constant were always adjusted according to the signal intensity. The EPR spectra of the zinc polymethacrylate obtained at 20 kGy dose of radiation is shown in Figure 3. The signal intensities are complex; however, it has been shown that the propagative free radical is of the type This spectrum is associated with free radicals at 328 mT. X-ray Diffraction. The X-ray spectrum of Zn(II)MA is shown in Figure 4. It is important to note that the peak signals are well defined, with the largest peak at 7.5 units, followed in Table 1. Elemental Composition of Zinc Polymethacrylate Obtained by X-ray Microanalysis elemental composition/ % atomic compound C O Zn Zn(II)PMAa 41.04 27.91 31.05 theoretical calculation 40.85 27.23 27.66 a The total addition is 100%, but the equipment cannot detect the Hydrogen contribution, this is the reason on the difference. Figure 3. EPR spectra of Zn(II)PMA. Signal at 330 indicates the presence of free radicals. Figure 4. X-ray diffractogram of (a) Zn(II)MA and (b) Zn(II)PMA. Note that in both cases the peaks are clearly shaped, indicating a crystalline array. R â - CH2 R - Cœ - (CH3)COOH 3384 Ind. Eng. Chem. Res., Vol. 46, No. 10, 2007 a lesser extend by signals at 15 and 22.5 units. A similar spectrum was recorded in the case of Zn(II)PMA. Note that the overall shape of the peaks indicates that this material has a crystalline structure. FT-IR Analysis. FT-IR spectra of Zn(II)MA, Zn(II)PMA and Zn(II)PMA after the contact with the lead solution are shown in Figure 5. This technique was used to identify important functional groups. Figure 5a shows that the FT-IR spectra of the monomer displayed a small band at 3080 cm-1 indicating a stretching of the alkene group, at 1860 is the characteristic overtone of the double bond, while at 1640 there is the confirmation of the carbon-carbon double bond. The five characteristics bands of a carboxylic acid are replaced by two bands at 1560 and 1430 cm-1, which correspond to the conversion of the inorganic salt. In 2970 and 2930 cm-1, there are signals corresponding to the symmetric and asymmetric movements of the C-CH3 bond. Finally, the methyl signal at 1375 cm-1 is observed. On the other hand, it is observed in Figure 5b that there is no signal at 3080 cm-1, indicating the polymerization has occurred. Thermogravimetric Analysis. The weight percent loss thermogram of Zn(II)MA and Zn(II)PMA shown in Figure 6 indicates that degradation of the monomer and polymer begins at 212 and 387 °C, respectively. This is an important result since once the polymer adsorbs Pb(II), it could be disposed off in a furnace where the polymer will degrade. The monomer starts to degrade at a lower temperature than the polymer due to the increased thermal stability of cross- linking. However, after 500 °C, both materials have a similar degradation trend. Surface Area Measurements. The result of the BET analysis of the surface area was 1.65 m2 g-1. This value is relatively smaller when compared with mineral carbon. However, sorption results indicate that good adsorption occurs onto this material. pH Effect on Lead Sorption. Lead concentration and pH define the different Pb chemical species present in aqueous solution. The distribution of the Pb species depends on the pH of the aqueous solution. In Figure 7, the distribution of the chemical species in a 150 mg L-1 lead aqueous solution as a function of pH is presented. Note that there are two species, namely, Pb2+ and Pb(OH)2. Lead will be present as a free ion up to pH of 6, when the fraction of lead hydroxide present in aqueous solution is the equal fraction amount. The most important information that this diagram provides is that it indicates that precipitation of lead will occur when the aqueous solution is above a pH of 6. Therefore, all sorption experiments were carried at a pH of 5.5. The effect of solution pH values on the adsorption of lead ion on the polymer is shown in Figure 8. It can be observed that by increasing the pH of the aqueous solution the lead absorption is increased. At a pH below 2, lead adsorption is not detected; however, the adsorption amount of lead ions onto polymer increased consistently for pH from 3 to 6. This behavior has been observed previously when using aminated polyacry- lamide fibers, and the explanation is based on the electrical repulsion forces present in acidic pH.9 On the other hand, on increasing pH values, the electrical repulsion force became weaker and the lead ions interactions with the polymer become stronger. Figure 5. FT-IR spectra of (a) Zn(II)MA and (b) the Zn(II)PMA. Figure 6. Weight loss on heating in a nitrogen atmosphere of the Zn(II)MA (0) and Zn(II)PMA ([). The first weight loss corresponds to water loss from the material, while the second one indicates the onset of material degradation. Ind. Eng. Chem. Res., Vol. 46, No. 10, 2007 3385 Adsorption Results. Two initial sorbent amounts were tested: 50 and 100 mg of the polymer and two initial Pb(II) aqueous solution concentrations, 25 and 150 mg L-1, at a pH of 5.5. Figure 9 shows the experimental plots obtained for Pb(II) removal from aqueous solution as a function of contact time with the polymer. Note that, at a concentration of 150 mg/L of Pb, the Zn(II)PMA presents at 70 min a maximum of 67% removal; beyond that removal the system remains stable. Similar behavior is observed at a concentration of 25 mg L-1, but a 94% removal is achieved. Pb(II) Adsorption Isotherms. Lead adsorption isotherms were fitted to Langmuir and Freundlich equations in order to calculate the maximum adsorption capacity of the polymers. Conditions used to obtain experimental sorption data are shown in Table 2. The Langmuir equation is based on the assumption of a structurally homogeneous adsorbent where all sorption sites are identical and energetically equivalent. It is assumed that, once a metal ion occupies a site, no further adsorption takes place in this site. Langmuir constants q0 (sorption capacity of the material, mg g-1) anb b (energy of adsorption) can be graphically obtained when plotting Ce/q0 versus Ce, which has a slope of 1/q0 and a intercept of 1/qob. Ce is the equilibrium concentration of lead ion (presented in Table 2). The linear equation is shown in eq 4.11 The Freundlich model assumes that the adsorbent consists of a heterogeneous surface composed of different adsorption sites. Freundlich parameters Kf (related with sorption capacity) and 1/n (intensity of the adsorption) can be obtained from the linearized plots of log qe versus log Ce. Equation 5 shows the Freundlich isotherm model.12 Figure 10 shows the linearized isotherm of Zn(II)PMA after contact with an aqueous solution of Pb for the (a) Langmuir model and (b) Freundlich model. SEM and EDS of the Zn(II)PMA after the Lead Contact. The morphology and topography of the Zn(II)PMA after contact with the aqueous lead solution is shown in Figure 11. The flakelike octagonal structures of the Zn(II)PMA seen in the Figure 7. Predominant lead species in aqueous solution. [Pb] ) 150 mg L-1. Pb2+ (]) and Pb(OH)2 (b). Figure 8. pH effect on lead adsorption. Figure 9. Pb(II) concentration in aqueous solution as a function of contact time. Initial concentration of 150 mg L-1 (0) and initial concentration of 25 mg L-1 (b). Table 2. Experimental Data Used To Fit the Adsorption Data Pb(II) initial concn (Co/mg L-1) Pb(II) equilibrium concn (Ce/mg L-1) 25 1.5 50 8.69 75 16.94 100 32.4 150 65 Ce/q ) (1/q0)b + (1/q0)Ce (4) Log qe ) log Kf + 1/n log Ce (5) 3386 Ind. Eng. Chem. Res., Vol. 46, No. 10, 2007 unexposed polymer are still present; however, they are more compacted and brighter. The brightness is likely due to the lead that is on the surface of the polymer after the sorption process. Figure 12 shows the energy dispersion analysis of the Zn- (II)PMA before and after the sorption experiments. Before the sorption, C, O, and Zn are the principle constituents of the polymer (Figure 12a, Table 1); however, after contact with the solution, Pb is incorporated on the polymer surface as seen in Figure 12b. X-ray Photoelectron Spectroscopy. XPS is used to identify the interaction of a metal ion with surface chemical groups on an adsorbent during adsorption. Interactions between a metal ion and an atom on the surface of the adsorbent change the distribution of the electrons around the corresponding atoms: electron-donating ligands can lower the binding energy (/BE) of the core electrons, while electron-withdrawing ligands can increase it. The XPS spectra of the Zn(II)PMA after contact with lead ions in aqueous solution is shown in Figure