Ind. Eng. Chem. Res. 2004, 43, 6095-6101
6095
Methacryloylamidoglutamic Acid Incorporated Porous Poly(methyl methacrylate) Beads for Heavy-Metal Removal Adil Denizli,*,† Nurullah Sanli,‡ Bora Garipcan,† Su 1 leyman Patir,§ and ‡ Gu 1 leren Alsancak Departments of Chemistry and of Science Education, Hacettepe University, Ankara, Turkey, and Department of Chemistry, Su¨ leyman Demirel University, Isparta, Turkey
Poly(methyl methacrylate-methacryloylamidoglutamic acid) beads (average diameter 150-200 µm) were prepared by copolymerizing methyl methacrylate (MMA) with methacryloylamidoglutamic acid (MAGA). Poly(MMA-MAGA) beads were characterized by swelling and surface area studies, scanning electron microscopy, and elemental analysis. These porous beads with a swelling ratio of 36.4% and containing 696 µmol of MAGA/g were used for heavy-metal removal involving cadmium, mercury, and lead. Poly(MMA-MAGA) beads have a specific surface area of about 67.8 m2/g. Metal adsorption results were found to be a function of solution properties (i.e., medium pH and metal concentration) and the types of metals to be adsorbed. We have obtained adsorption capacities as 29.9 mg/g (149 µmol/g) for Hg(II), 28.2 mg/g (250 µmol/g) for Cd(II), and 65.2 mg/g (314 µmol/g) for Pb(II). The adsorption capacities on an observed molar basis were in the order of Pb(II) > Cd(II) > Hg(II). Adsorption of heavy-metal ions from synthetic wastewater was also studied. The adsorption capacities are 22.4 mg/g for Hg(II), 24.2 mg/g for Cd(II), and 52.6 mg/g for Pb(II) at 0.5 mmol/L initial metal concentration. Of course, depending on the desired goals, the beads containing metal could be regenerated for appropriate disposal. Our results suggest that poly(MMA-MAGA) beads can be good metal adsorbers and have great potential applications in environmental protection. Introduction Industrial wastewater is one of the major sources of environmental pollution. Among the environmental pollutants, heavy metals have gained relatively more significance in view of their persistence and toxicity.1 They can cause mental retardation, cancer, and nervous system damage.2 Heavy metals are nonbiodegradable and, therefore, must be removed from water.3 Heavy metals in wastewater coming from battery manufacturing, painting, printing, coal combustion, sewage wastewaters, automobile emissions, mining activities, tanneries, alloy industries, and the utilization of fossil fuels are just a few examples.4 Various methods of heavymetal removal from wastewaters have been reported in the literature; among these methods are precipitation, membrane filtration, neutralization, ion exchange, and adsorption. Among these techniques, adsorption is generally preferred for the removal of heavy metal ions because of its high efficiency, ease of handling, and availability of different adsorbents.5 The search for costeffective adsorbents has also become the focus of attention of many scientists.6 Heavy-metal removal by using the chelating polymers would be of great importance in environmental applications.7-16 Several criteria are important in the design of chelating polymers with substantial stability for the selective removal of metal ions: specific and fast complexation of the metal ions as well as the reusability of the chelating polymers. An expensive and critical step in this preparation process * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemistry, Hacettepe University. ‡ Su¨leyman Demirel University. § Department of Science Education, Hacettepe University.
is coupling of a chelating ligand to the adsorption matrix. The major issue is that of slow release of this covalently bonded chelator off the matrix. Release is a general problem encountered in any ligand adsorption technique that caused a decrease in the adsorption capacity.17-20 The time-consuming, cost of chelating procedure has inspired a search for suitable low-cost and reusable adsorbents. For these reasons, we have focused our attention on the development of chelating compounds for the assembly of a new class of novel heavy-metal adsorbents. In this work we show that poly(methyl methacrylatemethacryloylamidoglutamic acid) [poly(MMA-MAGA)] beads can be used directly for heavy-metal removal. This novel approach for the preparation of a metal-chelating matrix has many advantages over conventional preparation techniques. In a conventional approach, it is necessary to activate the matrix for metal-complexing ligand immobilization. In this procedure, comonomer MAGA acted as the metal-complexing ligand, and there is no need to activate the matrix for the chelating ligand immobilization. Therefore, the metal-complexing ligand immobilization step was also eliminated. MAGA was polymerized with MMA, and no leakage of the metalcomplexing ligand was also necessary. This paper describes the preparation and characterization of MAGA and poly(MMA-MAGA) beads for heavy-metal removal. The results of adsorption/elution studies with Cd(II), Hg(II), and Pb(II) ions are reported here. Experimental Section Materials. L-Glutamic acid hydrochloride and methacryloyl chloride were supplied by Sigma (St. Louis, MO). MMA and ethylene glycol dimethacrylate (EGD-
10.1021/ie030204z CCC: $27.50 © 2004 American Chemical Society Published on Web 07/28/2004
6096 Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004
MA) were obtained from Fluka AG (Buchs, Switzerland), distilled under reduced pressure in the presence of a hydroquinone inhibitor, and stored at 4 °C until use. Benzoyl peroxide (BPO) was obtained from Fluka (Buchs, Switzerland). Poly(vinyl alcohol) (PVAL; MW 100 000, 98% hydrolyzed) was supplied from Aldrich Chemical Co. (Milwaukee, WI). All other chemicals were of reagent grade and were purchased from Merck AG (Darmstadt, Germany). All water used in the adsorption experiments was purified using a Barnstead (Dubuque, IA) ROpure LP reverse osmosis unit with a high-flow cellulose acetate membrane (Barnstead D2731) followed by a Barnstead D3804 NANOpure organic/colloid removal and ion-exchange packed-bed system. The resulting purified water (deionized water) has a specific conductivity of 0.6 µS/cm. (i) Synthesis of MAGA. For the synthesis of MAGA, the following experimental procedure was applied: 5.0 g of L-glutamic acid hydrochloride and 0.2 g of hydroquinone were dissolved in 100 mL of dichloromethane. This solution was cooled to 0 °C. Then, 13.0 g of triethylamine was added to the solution, and 4.0 mL of methacryloyl chloride was poured slowly into this solution under a nitrogen atmosphere. This solution was stirred magnetically at room temperature for 2 h. At the end of the chemical reaction period, unreacted methacryloyl chloride was extracted with a 10% NaOH solution. The aqueous phase was evaporated in a rotary evaporator, and the residue (i.e., MAGA) was dissolved in ethanol. (ii) Preparation of poly(MMA-MAGA) Beads. Poly(MMA-MAGA) beads were prepared by suspension polymerization. A typical procedure may be summarized as follows: The stabilizer, PVAL, was dissolved in 50 mL of deionized water for the preparation of the continuous phase. The dispersion phase was prepared by mixing MMA (4.0 mL), EGDMA (8.0 mL), MAGA (1.0 g), and toluene (12.0 mL) in a test tube. The initiator, BPO (100 mg), was dissolved in this homogeneous solution. The dispersion phase was added to the continuous medium in a glass-sealed polymerization reactor (100 mL) placed in a water bath equipped with a temperature-controlled system. The polymerization reactor was heated to 65 °C within about 30 min by stirring the polymerization medium at 600 rpm. The polymerizaton was conducted at 65 °C for 4 h and at 90 °C for 2 h. After completion of polymerization, the reactor content was cooled to room temperature. A washing procedure was applied after polymerization to remove the diluent and any possible unreacted monomers from the beads. The polymer beads were filtered and resuspended in ethyl alcohol. The bead suspension was stirred for about 1 h at room temperature and the beads were separated by filtration. The beads were washed twice with ethyl alcohol and then four times with deionized water using the same procedure. When not in use, the beads were kept under refrigeration in a 0.02% sodium azide solution for prevention of microbial contamination. Characterization Studies. FTIR spectra of MAGA and poly(MMA-MAGA) beads were obtained by using a FTIR spectrophotometer (FTIR 8000 series; Shimadzu, Japan). The dry beads (about 0.1 g) was thoroughly mixed with KBr (0.1 g, IR grade; Merck, Germany), the resulting mixture was pressed into a pellet, and the FTIR spectrum was then recorded.
The 1H NMR spectrum of the MAGA monomer was taken in CDCl3 on a JEOL GX-400 300 MHz instrument. The residual nondeuterated solvent (CHCl3) served as an internal reference. Chemical shifts are reported in ppm (δ) downfield relative to CHCl3. The surface area of the poly(MMA-MAGA) beads was measured by the Brunauer-Teller-Emmett method with an ASAP2000 instrument (Micromeritics, Norcross, GA). The average size and size distribution of the beads were determined by screen analysis performed by using Tyler standard sieves. The water uptake ratio of the poly(MMA-MAGA) beads was determined in distilled water. The experiment was conducted as follows: initially dry beads were carefully weighed before being placed in a 50 mL vial containing distilled water. The vial was put into an isothermal water bath with a fixed temperature (25 °C) for 2 h. The bead sample was taken out from the water, wiped using a filter paper, and weighed. The weight ratio of dry and wet samples was recorded. The water content of the poly(MMA-MAGA) beads was calculated by using the following expression:
water uptake ratio % ) [(ms - mo)/mo] × 100 (1) where mo and ms are the weights of beads before and after uptake of water, respectively. The surface morphology of the beads was examined using scanning electron microscopy (SEM). The beads were initially dried in air at 25 °C for 7 days before being analyzed. A fragment of the dried bead was mounted on a SEM sample mount and was sputter-coated for 2 min. The sample was then mounted in a scanning electron microscope (JEOL, JEM 1200 EX, Tokyo, Japan). The surface of the sample was then scanned at the desired magnification to study the morphology of the poly(MMA-MAGA) beads. To evaluate the degree of MAGA incorporation, the produced poly(MMA-MAGA) beads were subjected to elemental analysis using a Leco elemental analyzer (model CHNS-932; St. Joseph, MI). Adsorption Studies. Adsorption of heavy-metal ions from aqueous solutions was investigated in batch experiments. Effects of the initial heavy-metal ion concentration and the pH of the medium on the equilibrium adsorption time and adsorption capacity were studied. Aliquots (25 mL) of aqueous solutions containing different amounts of heavy-metal ions (in the range of 10700 mg/L) were treated with the chelating beads. Nitrate salts were used for the metal ion source. Adsorption flasks were stirred magnetically at 600 rpm. The suspensions were brought to the desired pH by adding sodium hydroxide and nitric acid. Investigations were made for pH values in the range of 2.0-6.0. After a sufficient amount of time for reaching equilibrium (i.e., 120 min), the solution was centrifuged, and the supernatant was removed and analyzed for remaining metal ions. In all experiments, the polymer amount was kept constant at 100 mg for a 25 mL solution. Blank trials without bead addition were performed for each tested metal concentration. The concentration of the sample was analyzed by a graphite furnace atomic absorption spectrophotometer (AAS AA800, Perkin-Elmer, Bodenseewerk, Germany) at appropriate intervals. The concentration of Hg(II) in the supernatant was measured by using a graphite furnace AAS connecting a cold vapor unit. A Photron mercury hallow cathode lamp was used. The working current/wavelength values and the
Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6097
optimized experimental conditions for mercury measurements were as follows: working current/wavelength, 6 mA/253.6 nm; concentration of SnCl2, 1% (w/ v); concentration of KMnO4, 0.5% (w/v); concentration of H2SO4, 5% (w/v). The working current/wavelength values for cadmium and lead determinations were 8 mA/ 228.8 nm and 10 mA/283.3 nm, respectively. The instrument response was periodically checked with known metal solution standards. The experiments were performed in replicates of three, and the samples were analyzed in replicates of three as well. For each set of data present, standard statistical methods were used to determine the mean values and standard deviations. Confidence intervals of 95% were calculated for each set of samples in order to determine the margin of error. Adsorption experiments were carried out at 20 °C. The adsorption capacity q (mg/g of polymer) was obtained as follows: q ) [(C0 - Ce)V]/m, where C0 and Ce are the initial and equilibrium concentrations (mg/L), respectively, V is the volume of the aqueous phase (L), and m is the mass of the beads used (g). Heavy-Metal Adsorption from Synthetic Wastewater. Adsorption of heavy-metal ions from synthetic wastewater was carried out in a batch system. A wastewater sample (25 mL) containing 0.5 mmol/L from each metal ion [i.e., Cd(II), Pb(II), and Hg(II)] was incubated with the poly(MMA-MAGA) beads at a pH of 7.0 at room temperature, in the flasks stirred magnetically at 600 rpm. In all experiments, the polymer amount was kept constant at 100 mg for a 25 mL solution. Synthetic wastewater also contains Ni(II), Zn(II), Fe(II), Co(II), Sn(II), and Ag(I). The concentration of each metal ion in synthetic wastewater is 0.1 mmol/ L. To adjust the salinity, 700 mg/L NaCl was added into the synthetic wastewater. A synthetic wastewater solution was prepared according to the European Union Directive 91/271/EEC. After adsorption, the concentration of the metal ions in the remaining solution was determined by AAS as described above. Elution/Reusability Studies. The elution efficiency from the poly(MMA-MAGA) beads was measured for all of the metals. Solutions of the same concentrations as the adsorption experiments were used. The initial and final concentrations were measured for the adsorption solutions to estimate the amount of metal ion removed. The elution efficiency was calculated by comparing this value with the amount of metal ion eluted and measured. Elution of the metals from the polymer beads was carried out in 25 mL of a 0.1 M HNO3 solution for 30 min. The chelating beads (100 mg) were placed in the elution medium and stirred with a magnetic stirrer at 600 rpm at room temperature. The released metal ion amount was determined using a graphite furnace AAS according to the guidelines of the manufacturers. To determine the reusability of the chelating beads, consecutive adsorption-elution cycles were repeated five times by using the same chelating beads. Results and Discussion Properties of Polymer Beads. Poly(MMA-MAGA)] beads were in the spherical form in the size range of 150-200 µm. The specific surface area of the beads was found to be 67.8 m2/g. The beads are cross-linked matrixes. The equilibrium swelling ratio of the poly(MMA-MAGA) beads is 36.4%. Compared with poly(MMA) (5.6%), the water uptake ratio of the poly(MMA-
Figure 1. Molecular formula of poly(MMA-MAGA) beads.
Figure 2. FTIR spectrum of the MAGA monomer.
MAGA) beads increased. Several explanations can be offered. First, incorporating MAGA actually introduces more hydrophilic functional groups into the polymer chain, which can interact with more water molecules within the polymer matrixes. Second, reacting MAGA with MMA effectively decreased the molecular weight. Therefore, water molecules penetrate into the polymer network more easily, resulting in an improvement of the water uptake in aqueous solutions. These beads are also quite rigid, strong enough, and suitable for column applications. MAGA was selected as the metal-complexing ligand. In the first step, MAGA was synthesized from Lglutamic acid hydrochloride and methacryloyl chloride. Then, MAGA was incorporated into the bulk structure of the poly(MMA) beads by suspension polymerization. The molecular formula of the poly(MMA-MAGA) beads is given in Figure 1. The FTIR spectrum of MAGA has the characteristic stretching vibration carboxyl-carbonyl and amide-carbonyl absorption bands at 1732 and 1655 cm-1, respectively, as shown in Figure 2. The N-H bending peak appearing at 1540 cm-1 is associated with the amide vibration of MAGA. 1H NMR was used to determine the synthesis of the MAGA structure. Figure 3 shows the 1H NMR spectrum of MAGA. The 1H NMR spectrum is shown to indicate the characteristic peaks from the groups in the MAGA monomer. These characteristic peaks are as follows: 1H NMR (DMSO) δ 1.19-1.23 (m; 2H, CH2), 1.50-1.55 (m; 2H, CH2), 1.90 (s; 3H, CH3), 4.49-4.59 (m; 1H, CNH), 5.33 (s; 1H, H2CdC), 5.73 (s; 1H, H2CdC), 6.87 (δ; 1H, NH), 9.58 (δ; 6s, 2H, OH). The FTIR spectrum was undertaken to determine the structure of the copolymer beads (Figure 4). The FTIR spectrum of poly(MMA-MAGA) has characteristic peaks appearing at 3450 cm-1 (characteristic hydroxyl, OH stretching vibration), 2920 cm-1 (CH3 stretching vibra-
6098 Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004
Figure 3.
1H
NMR spectrum of the MAGA monomer.
Figure 4. FTIR spectrum of poly(MMA-MAGA) beads.
tion), 2870 cm-1 (CH2 stretching vibration), and 1740 cm-1 (carboxyl-carbonyl stretching vibration). The carbonyl peak appearing at 1670 cm-1 is associated with the amide and carbonyl vibrations of MAGA. These data confirmed that the poly(MMA-MAGA) copolymer beads were formed with functional group MAGA. The surface morphology and bulk structure of poly(MMA-MAGA) beads are exemplified by the electron micrographs in Figure 5. As is clearly seen here (Figure 5A), the beads have a spherical form and rough surface. The micrograph in Figure 5B was taken with broken beads to observe the internal structure of the polymer. The presence of pores within the bead interior is clearly seen in this photograph. It can be concluded that the beads have a microporous interior surrounded by a rough surface. The roughness of the bead surface should be considered as a factor, providing an increase in the surface area. In addition, these micropores reduce the diffusional resistance and facilitate mass transfer because of their high internal surface area. To determine the degree of MAGA incorporation, elemental analysis of the synthesized poly(MMAMAGA) was performed. The incorporation of MAGA was found to be 696 µmol of MAGA/g of polymer from nitrogen stoichiometry. Effect of pH on Metal Binding. pH is a critical parameter in adsorption because it influences the equilibrium by affecting the speciation of the heavy-metal ions in solution, the concentration of competing hydrogen ions, and the chemistry of active binding sites on the adsorbent. At acidic pH values, metals exist as free cations and are thus available for adsorption. On the other hand, a decrease in pH results in an increase in the hydrogen ion concentration, and hence possible competition with the metal ions for the available adsorption sites. The results presented in Figure 6 show that poly(MMA-MAGA) chelating beads exhibited a low affinity for heavy-metal ions in acidic conditions (pH < 4.0) and a somewhat higher affinity between pH 5.0 and
Figure 5. SEM micrographs of poly(MMA-MAGA) beads: (A) surface; (B) cross section.
Figure 6. Effect of pH on the adsorption of metal ions: MAGA loading, 696 µmol/g; initial concentration of metal ions, 500 mg/ L; T, 20 °C.
6.0. The difference in the adsorption behavior of heavymetal ions can be explained by the different affinities of heavy-metal ions for the donor atoms in the metalcomplexing amino acid ligand/comonomer MAGA. A difference in the metal coordination behavior is most probably also the case for the incorporated MAGA ligand, resulting in a relatively high adsorption of metal ions at high pH under noncompetitive adsorption conditions.
Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 6099 Table 1. Isotherm Model Constants and Correlation Coefficients for Adsorption of Heavy-Metal Ions from Aqueous Solutions Langmuir
Freundlich
metal ion
Qm (mg/g)
a (mL/mg)
R2
a
n
R2
Cd(II) Hg(II) Pb(II)
29.8 31.4 68.9
0.043 0.030 0.007
0.996 0.997 0.996
2.67 3.23 6.97
0.390 0.373 0.371
0.92 0.91 0.90
Table 2. Competitive Metal Ion Adsorption Capacity of Poly(MMA-MAGA) Beads: Concentration of Each Metal Ion, 0.5 mmol/L; pH, 6.0; T, 20 °C adsorption capacity metal ion
Figure 7. Adsorption isotherms of heavy-metal ions onto poly(MMA-MAGA) beads: MAGA loading, 696 µmol/g; pH, 6.0; T, 20 °C.
Equilibrium Adsorption Isotherms. Figure 7 shows the Cd(II), Hg(II), and Pb(II) adsorption isotherms of the metal-chelating beads. The amount of metal ions adsorbed per unit mass of the beads increased first with the initial concentration of the metal ions and then reached a plateau value, which represents saturation of the active adsorption sites (which are available and accessible for metal ions) on the beads. Adsorption of Hg(II) reached a saturation level at a lower bulk concentrations, i.e., at about 100 mg/L, but adsorption of Cd(II) and Pb(II) ions reached saturation at higher concentration, i.e., about 500 mg/L. Because of the possibility of precipitation of the metal ions, we did not increase the initial concentration over 700 mg/L. The binding capacities of the chelating beads are 29.9 mg/g (149 µmol/g) for Hg(II), 28.2 mg/g (250 µmol/g) for Cd(II), and 65.2 mg/g (314 µmol/g) for Pb(II). This shows that the affinity of the poly(MMA-MAGA) beads toward the adsorption of Pb(II) was stronger than that toward Cd(II) and Hg(II). The molar basis of calculation is the only accurate way of investigating the competition in multicomponent metal mixtures. Molar basis units are measured as micromoles per gram of dry adsorbent. It is evident from the results that the order of capacity of the metal-chelating beads on a molar basis for the single-component metals is Pb(II) > Cd(II) > Hg(II). To optimize the design of an adsorption system to remove heavy-metal ions, it is important to establish the most appropriate correlation for the adsorption isotherms. Two isotherm equations have been tested in the present study, namely, Langmuir and Freundlich. The most widely used adsorption isotherm for modeling equilibrium data is the Langmuir equation, which for dilute solutions may be represented as
Qe ) QmaCe/(1 + aCe)
(2)
Qm is the maximum adsorption capacity, and a is the characteristic of the Langmuir equation and can be determined from a linearized form of the above equation:
Ce/Qe ) 1/Qm + (a/Qm)Ce
(3)
Therefore, a plot of Ce/Qe vs Ce gives a straight line of slope a/Qm and intercept 1/Qm. The constant Kd (Qma) is the Langmuir equilibrium constant, and the ratio a/Kd
Cd(II) Hg(II) Pb(II) Ni(II) Zn(II)
mg/g
µmol/g
24.2 ( 1.1 215 ( 9.7 22.4 ( 1.4 111 ( 6.9 52.6 ( 1.8 253 ( 8.7 1.2 ( 0.2 20 ( 1.5 1.7 ( 0.2 26 ( 1.1
adsorption capacity metal ion
mg/g
µmol/g
Fe(II) Co(II) Sn(II) Ag(I)
1.5 ( 0.1 1.2 ( 0.1 1.4 ( 0.1 1.2 ( 0.1
26 ( 1.0 20 ( 1.4 11 ( 0.8 11 ( 0.6
gives the theoretical monolayer saturation capacity. The Langmuir equation is applicable to homogeneous adsorption, where each metal ion-polymer bead adsorption process is characterized by an equal adsorption energy distribution. The Langmuir adsorption activation energy obeys Henry’s law at low concentrations. The Freundlich expression is an empirical equation based on adsorption on a heterogeneous surface. The Freundlich equation is commonly presented as
Qe ) aCen
(4)
and the equation may be linearized by taking logarithms
ln Qe ) n ln Ce + ln a
(5)
Therefore, a plot of ln Qe vs ln Ce enables the constant a and exponent n to be determined. The Langmuir and Freundlich constants, along with the correlation coefficients (R2), have been calculated from the corresponding plots for adsorption of heavymetal ions on the adsorbents, and the results are presented in Table 1. The correlation regression coefficients show that the adsorption process can be welldefined by the Langmuir equation. The Langmuir fit is considered to be evidence that adsorption stops at one monolayer, consistent with specific and strong adsorption onto specific binding sites. Because the exchange reaction between surface binding sites and previously adsorbed metal ions is only a monolayer or less, there is an accumulation of matter at the solid-solution interface, without the creation of a three-dimensional structure. Adsorption from Synthetic Wastewater. Adsorption capacities of the poly(MMA-MAGA) beads from synthetic wastewater for Cd(II), Pb(II), and Hg(II) are shown in Table 2. It is worth noting that the adsorption capacities of the poly(MMA-MAGA) beads from synthetic wastewater for all metal ions are lower than those of the single solutions. The adsorption capacities are 24.2 mg/g for Cd(II), 22.4 mg/g for Hg(II), and 52.6 mg/g for Pb(II). The chelating beads exhibit the following metal ion affinity sequence on a molar basis: Pb(II) > Cd(II) > Hg(II). In this case, chelating beads adsorbed other metal ions also [i.e., Ni(II), Zn(II), Fe(II), Co(II), Sn(II), and Ag(I)]. It is obvious that the presence of other metal ions in the synthetic wastewater decreases the
6100 Ind. Eng. Chem. Res., Vol. 43, No. 19, 2004 Table 3. Heavy-Metal Ion Adsorption Capacity of Chelating Beads after Repeated Adsorption-Elution Cycles: Initial Concentrations of Metal Ions, 50 mg/L; pH, 6.0; T, 20 °C Cd(II)
Hg(II)
Pb(II)
cycle no.
adsorption (mg/g)
elution (%)
adsorption (mg/g)
elution (%)
adsorption (mg/g)
elution (%)
1 2 3 4 5
138.0 ( 3.2 137.2 ( 4.1 137.0 ( 3.8 136.2 ( 3.4 135.5 ( 3.8
97.2 ( 1.1 96.4 ( 1.2 96.5 ( 1.2 97.0 ( 1.0 97.4 ( 1.0
86.0 ( 2.5 85.2 ( 2.8 84.7 ( 3.2 84.0 ( 3.5 83.1 ( 3.0
96.9 ( 1.6 98.0 ( 1.7 97.5 ( 1.7 97.1 ( 1.2 97.9 ( 1.8
160.0 ( 1.2 158.2 ( 1.6 157.0 ( 1.9 155.9 ( 1.0 154.0 ( 1.4
97.2 ( 3.2 98.0 ( 3.3 98.0 ( 3.4 99.0 ( 3.2 97.6 ( 3.1
adsorption capacities of chelating beads for Cd(II), Pb(II), and Hg(II) ions. Elution and Repeated Use. Table 3 indicates that an aqueous solution of nitric acid (0.1 M) is capable of eluting the heavy-metal ions [i.e., Cd(II), Hg(II), and Pb(II)] from the poly(MMA-MAGA) material; up to 99% metal ion recovery was achieved. One of the important criteria for any metal adsorbent is its capability for reuse because this factor improves both the materials cost and the economics of related metal separation technology. Multiple-cycle adsorption/elution equilibrium data (Table 3) have confirmed the feasibility of reusing poly(MMA-MAGA) material. No decay of the heavy-metal adsorption capacity was observed after five cycles of repeated use. Conclusion The wastewaters discharged from chemical industries that may contain heavy-metal ions have a toxic effect on all living organisms. Because of this, thier disposal to the environment is a major threat to both human health and the ecosystem. So, the development of new technologies is required to treat wastewaters as an alternative to traditional physicochemical processes.21-23 Adsorption represents a potentially cost-effective way of eliminating toxic heavy metals from industrial wastewaters. Reusable polymer-based adsorbents have been recognized as a promising class of low-cost adsorbents for the removal of heavy-metal ions from aqueous waste streams. In this study, metal-chelating beads were prepared and applied directly without any modification to the removal of lead, mercury, and cadmium ions from aqueous solutions including synthetic wastewater. This novel approach for the preparation of a metal-chelating matrix has many advantages over the conventional adsorbents: those need the activation of the polymeric matrix for metal-complexing ligand immobilization. In this procedure, MAGA acts as the metal-complexing group, and there is no need to activate the matrix for metal-complexing ligand immobilization. MAGA is polymerized with MMA, and no leakage of the ligand is necessary. Some results were given as follows: The adsorption capacities of the chelating beads are 28.2 mg/g for Cd(II), 29.9 mg/g for Hg(II), and 65.2 mg/g for Pb(II). The affinity order of metal ions on a molar basis is as follows: Pb(II) > Cd(II) > Hg(II). pH significantly affected the adsorption capacity of the adsorbent. Adsorption of heavy-metal ions from synthetic wastewater was also studied. The adsorption capacities are 22.4 mg/g for Hg(II), 24.2 mg/g for Cd(II), and 52.6 mg/g for Pb(II) at a 0.5 mmol/L metal concentration. Repeated adsorption-elution operations showed the feasibility of these chelating beads for heavy-metal adsorption. These results suggest that poly(MMA-MAGA) beads can be good metal adsorbers and have great potential applications in environmental protection.
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Received for review March 3, 2003 Revised manuscript received December 17, 2003 Accepted December 17, 2003 IE030204Z
