Applications of chelating resin for heavy metal removal from wastewater

2.1 Materials

All materials mentioned in the synthetic process such as iminodiacetic acid, NaOH, glycidyl methacrylate, N,N′-Methylene bisacrylamide (MBA) and 4,4′-azobis (4-cyarovaleric acid) (V501) were all reagent grade and used as-received from the supplier (Chemically pure, Shanghai Chemical Reagent Co. Ltd., Shanghai, China) without further treatments. The heavy metal ions solutions are prepared from deionized water and the following metal salts: CuSO₄·5H₂O, Pb(NO₃)₂, CdCl₂·2.5H₂O, ZnSO₄·7H₂O, CoCl₂·6H₂O, NiCl₂·6H₂O, CrCl₃·6H₂O and Mn(Ac)₂·4H₂O.

2.2 Measurement

Fourier transform infrared (FT-IR) measurement was performed with KBr pellets on a Perkin-Elmer 577 FT-IR spectrometer in the range 400–4000 cm^-1. A JENWAY 3010 model pH meter was used to measure an expected pH value. Here, the initial and final concentrations of metal ions were determined by using a UNICAM 930 model flame atomic adsorption spectrometer equipped with deuterium lamp background correction, Hallow Cathode Lamp and air acetylene burner. Thermal gravimetric analysis (TGA) was conducted on a Universal V3.7A TA instrument under flowing pure N₂ gas (20 ml/min) at a heating rate of 10°C min^-1 from room temperature to 800°C. Elemental analyses for C, H and N were done on an EA1110-CHNS elemental analyzer. Scanning electron microscope (SEM) images were taken on a Hitachi S-4700 SEM.

2.3 Synthesis of the resin PIDA

The resin of poly(iminodiacetic acid) (PIDA) was prepared by a cross-linking polymerization as shown in Scheme 1. The monomer was obtained from the reaction of glycidyl methacrylete and sodium 2,2′-azanediyldiacetate under 65°C as reported (11). The polymerization procedure was as follows: the mixture of monomer, MBA and V501, with a molar ratio of 100:10:1 (31.9 g:4.63 g:0.84 g), was stirred at 70°C for 3 h. The obtained polymer was washed with distilled water to remove excess monomers for three times and dried before any other chemical treatment. This experiment can also amplify in kilogram reaction. The ratio of the reactants is similar with the above (3.2 kg:5 kg:1 kg), but the reaction temperature will be 80°C, which is higher to ensure complete polymerization.

Scheme 1:

The polymerization process of chelating resin PIDA.

2.4 Adsorption experiments

In most cases, metal ion solutions were prepared from the reagents with 3000 mg/l and diluted to appropriate concentration with pH adjustment. All adsorption experiments were carried out batchwise, and the adsorption equilibrium was obtained by stirring the mixture. When the adsorption process was completed, the solution was filtered and the metal ions remaining in solution were measured after dilution. Every experiment was repeated at least three times to ensure the accuracy of data. The amount of metal ion adsorption was calculated from the ratio of value difference between the initial and final concentrations in aqueous solution. The amount of metal ion absorbed at time t, q_t was calculated from the mass balance equation

where Q is the amount of metal ions adsorbed onto the amount of the resin (mmol g^-1), C₀ and C_t the concentration of metal ions in the initial and equilibrium concentrations of the metal ions in aqueous phase (mmol l^-1), V is the volume of the aqueous phase (L), and M is the dry weight of the resin (g).

3.1 Characterization of the resin

The obtained resin was opaque, and the morphology of the resin was observed by SEM scanning, as shown in Figure 1A. The clearly observed porous surface was one of the main reasons for the large chelating capacity and fast adsorption speed. Moreover, we measured the contact angle of the resin. According to Figure 1B, the small contact angle showed that the resin was ultra-hydrophilic and the water could wet the solid easily.

Figure 1:

(A) The SEM photograph of bifunctional chelating resin and (B) contact angle of bifunctional chelating resin PIDA with an 8 μl water droplet.

The thermal analysis TGA and differential scanning calorimetry of the cross-linking polymer PIDA was measured in the range of 20–600°C. As shown in Figure S1, The PIDA showed a good thermal stability with decomposition temperature close to 200°C. Its glass transition temperature is about 72°C, similar to those analogues such as methyl methacrylate polymer (12).

3.2 Effect of pH Value

It is well known that the pH value of solutions plays an important role in coordination reactions and electrostatic interactions in physical adsorption processes. The hydrophilic groups such as hydroxyl, carboxyl and carbonyl groups in PIDA affect the pH value of its solution. The effect of pH value on metal ions adsorption was studied by changing the initial pH value of the solutions with the addition of dilute hydrochloric acid and potassium hydroxide solution (13). Results are shown in Figure 2; it should be noticed that the adsorption of Pb²⁺ and Co²⁺ was improved slightly with increased pH value. According to the results of other metal ions, all adsorption capacities of different metal ions were particularly low at low pH values. It might be attributed to the competitive adsorption between H₃O⁺ ions and metal ions for the limited active adsorption sites in the PIDA resin. However, with the increased pH value, by decreasing the proton concentration, the adsorption capacities increased significantly. When the pH values increased in the appropriate range, the interactions between the resin and metal ions increased, which enhanced the adsorption capacity. The optimum pH values at which the resin showed the maximum adsorption capacity were 5.0 for Cu²⁺and Cd²⁺, and 6.0 for Mn²⁺, Co²⁺, Ni²⁺, Cr³⁺, Pb²⁺and Zn²⁺. Compared to the previous reports (14), the optimum pH value of the resin adsorption of Cu²⁺, Cd²⁺ and Pb²⁺ was about 5.0, which was consistent with our result except for the minor difference with the Pb²⁺. It might be due to the different structures of the two kinds of resin.

Figure 2:

The effect of pH on the adsorption of Cd(II), Co(II), Cu(II), Ni(II), Pb(II), Mn(II), Zn(II) and Cr(III) metal ions for 1 h.

3.3 Effect of reaction time

The adsorption capacity for different metal ions was measured as a function of time to determine an optimum time for the adsorption (15). Figure 3 showed the effect of reaction time on the removal of Cd(II), Co(II), Cu(II), Ni(II), Pb(II), Mn(II), Zn(II) and Cr(III) from solutions by PIDA. The removed metal ions increased and achieved equilibrium after about 30 min. As shown in Figure 3, the adsorption rate was fast in the first 6 min, which was because of the exchange ability of chelating groups (-COO^-Na⁺) in PIDA resin. However, the amount of adsorbed metal ions reached a maximum value and was kept constant in the following period, which meant adsorption equilibrium. It was obvious that the largest capacity was shown on the adsorption of Cd (II). The maximum adsorption capacity of resin for each metal ion is shown in Table 1. From the values listed in Table 1, it was easy to conclude that the resin had much higher affinity to Cd (II) than any other ions. And the adsorption capacity of Cd (II) was higher than the previous report, such as 0.92 mmol/g for S930 (16), 0.004 mmol/g for sporopollenin (17) and 0.035 mmol/g for PAA-PVC (18).

Figure 3:

Effect of contact time on the different metal ions (Cd(II), Co(II), Cu(II), Ni(II), Pb(II), Mn(II), Zn(II) and Cr(III)) using PIDA (initial concentration of metal ions: 3000 ppm (100 ml), amount of resin: 1 g, temperature: 25±1°C, stirring time: 1 h, initial pH: the optimum pH values).

FT-IR spectra for the PIDA resin with and without metal ions adsorbed in were measured to study the adsorption mechanism. Carboxylic acid groups of the resin showed two bands: an asymmetrical stretching band near 1650 cm^-1 and a weaker symmetrical stretching band near 1400 cm^-1. According to previous reports (19), the shift of adsorption bands for the free carbonyl bond (C=O, 1650 cm^-1) could illustrate whether the bonding between the ligand and each metal ion in solid phase was covalent or ionic. The larger the red shift is, the more covalent bonds it has. The covalent bonds are formed by the N and O atoms and metal ions. And the infrared data are shown in Table 2, which are in line with the actual situation. The adsorption band of carbonyl bond (C=O, 1650 cm^-1) red-shifted due to the interaction with different ions. According to the results, Cd(II) ion showed the strongest affinity to carboxylic acid via covalent bond, and the affinity of Pd (II) to carboxylic acid was the weakest. The infrared data coincided with the descending orders of adsorption of 1 g resin, Cd²⁺>Cu²⁺>Mn²⁺>Ni²⁺>Zn²⁺>Co²⁺>Cr³⁺>Pb²⁺, from former experiments. As a result, the properties of covalent bond between carboxylic acid group in resin and metal ion would be used to explain the difference of adsorption capacity between each metal ion. However, the adsorption mechanism of Cd(II) is different from the S930 resin. In that system, the adsorption band of carbonyl bond (C=O, 1650 cm^-1) show small changes after adsorbing Cd(II), which reveal that the N and O atoms in the resin and Cd (II) do not show significantly coordinated behavior. It is because the S930 is a medium with more rigid LEWIS alkali which is unlike PIDA, so the Cd (II) (LEWIS acid) will not be coordinated with N and O atoms in LEWIS alkali. And in our system, from the results of FT-IR spectra, Cd(II) ion showed the strongest affinity to carboxylic acid via covalent bond which was embodied by the larger red shift of adsorption band of carbonyl bond (C=O, 1650 cm^-1) after adsorption. Moreover, we have also studied on the reuse of the resin. The PIDA resin could be reused for adsorbing metal ions after adding a dilute aqueous hydrochloric acid solution (1%). The Cd ions were used as an example to do the experiment, and we found that the resin could be used repeatedly for more than eight times in which it had the same amount of adsorption.

3.4 Effect of initial metal concentration and adsorption isotherms

In fact, the adsorption of metal ions highly depended on the initial metal concentration and the amount of resin. Therefore, we changed the initial concentration of metal ions and kept the amount of resin consistent, to examine the conditions of adsorption equilibrium. The initial metal ion concentrations were 500, 1000, 1500, 2000, 2500 and 3000 ppm, respectively (10 ml), and the weight of the resin was 100 mg in all solutions. The adsorption experiments were carried out in the conditions of optimal pH value, and the reaction time was 1 h. As shown in Figure 4, the metal adsorption capacities significantly increased with the increasing initial metal concentration until it reached the saturated adsorption of adsorbent. The attendant might be due to the high driving force for mass transfer. In other words, for the same amount of adsorbent, the higher the initial concentration is, the faster the adsorption equilibrium reaches.

Figure 4:

Effect of initial concentration on adsorption of heavy metal by resin PIDA (amount of resin: 100 mg, temperature: 25±1°C, stirring time: 1 h, initial pH: the optimum pH values).

The adsorption isotherm is defined as the relation between the concentration of adsorbent in the bulk and the amount of adsorbed metal ions on the interface. Therefore, we need to analyze sorption isotherms to form an equation that can accurately represent the results and could be used for the purpose of design. Herein, two popular adsorption models, Langmuir and Freundlich models, were applied to evaluate the experimental data.

3.5 Chelating isotherm

The non-competitive Langmuir has been a useful tool for the description and comparison of heavy metal sorption by different adsorbents (11, 20). Those isotherms elaborate the relation between the amount of metal taken up per unit weight of adsorbent q_e and the equilibrium adsorbate concentration in the bulk fluid phase C_e.

The Langmuir isotherm is given as

1qe=1Q0+1bQ0Ce

where q_e is the amount of solute adsorbed on the surface of the adsorbent (mmol g^-1), C_e is the equilibrium ion concentration in the solution (mmol l^-1), Q⁰ is the maximum surface density at monolayer coverage and b is the Langmuir adsorption constant (l mmol^-1). The plots of 1/q_e vs. 1/C_e gave a straight line, and the values of Q⁰ and b can be calculated from the intercept and slope of the plots, respectively. Experimental data obtained from the effect of initial concentrations on the adsorption of heavy metals on PIDA were also evaluated by applying this equation, and the corresponding constants are given in Table 3.

3.6 Effect of resin amount on ion exchange

As mentioned above, the adsorption of metal ions highly depended on the amount of resin. Figure 5 shows the removal of heavy metal ions from solution as a function of PIDA resin amount at their optimum pH values. The amount of resin varied from 0.0 to 1.2 g, and the reaction time was 1 h constantly. From the results, it was clear that the removal of metal increased along with the increased amount of resin in the same initial concentration, 3000 ppm. Among these ions, Pb(II)showed the maximum adsorption capacity (75%) by using the smallest amount of resin. This phenomenon may be due to the property of Pb as a heavy metal, the nucleus of which has weaker binding capacity to atomic electrons, so the ion exchange between Pb and resin is faster (21, 22). Similarly, the ion adsorption capacity of Cd reached about 48%, which might be because of the formation of stable bonds between Cd and the resin, leading to adsorption equilibrium (23, 24).

Figure 5:

Effect of the quantity of resin on the adsorption equilibrium in PIDA (initial concentration: 3000 ppm, temperature: 25+1°C, stirring time: 1 h, initial pH: the optimum pH values).

Other metal ions such as Zn²⁺, Mn²⁺ and Co²⁺ exhibited similar adsorption rate. However, the metal removal rate slowed down when the amount of resin continued to increase. Since the portion of metal ions removed from the aqueous phase increased as the sorbent dosage increased in the batch vessel with a fixed initial concentration, the curves in Figure 5 approach asymptotic values from 0 to 1.2 g resin. Less time was required to reach the equilibrium adsorption when the amount of sorbent increased. Therefore, we concluded that the adsorbent dosage would increase the amount and rate of removed metal ions but would reduce the efficiency of exchange. This was because the ion exchange capacity of PIDA was not saturated. A small amount of adsorbent which attributes to a lower chelating density would increase the efficiency of the adsorbent (8). However, the speed of adsorption might be reduced. As expected, for an identical initial concentration of metal ions, the equilibrated concentration decreased when the dosage of adsorbent increased. The reason lies on the fact that increased adsorbent provides a greater surface area or chelating sites for a fixed concentration (21, 25, 26).

3.7 Thermodynamics of the adsorption

The effect of temperature on the adsorption of heavy metal ions into the resin PIDA is shown in Figure 6. In a typical experiment, 10 ml of aqueous solution containing metal ions with the initial concentration of 3000 ppm and 100 mg of resin was kept at the optimum pH value. The experimental temperatures are 288, 303, 318 and 333 K, respectively. The effect of temperature on the equilibrium constant of ion exchange between metal and resin was investigated in this stage. From Figure 6, it was observed that the metal ions’ adsorbent capacity increased slightly along with the increasing temperature and showed the endothermic nature of the adsorption. This is due to the exothermic chelating reactions of the metal ions with the resin PIDA. The equilibrium constants in the experiments increased with the increase of temperature. The carboxylic groups in the structure of PIDA are partially protonated at all temperature levels, resulting in only a slight increase in metal adsorption capacities at high temperature. Here, only thermodynamics of ion exchange is involved; the temperature effect on dynamics of adsorption is relatively small. The chelating process is the major mechanism for the removal of metal ions from solution, and the basic process is essentially competitive ionic attraction for the ionic site.

Figure 6:

Effect of temperature on heavy metal adsorption using resin PIDA (initial concentration: 3000 ppm (10 ml), amount of resin: 100 mg, stirring time: 1 h, initial pH: the optimum pH values).