An eco-friendly synthesis, characterization, morphology and ion exchange properties of terpolymer resin derived from p-hydroxybenzaldehyde

2.3.1 Physicochemical and Elemental Analysis

The terpolymer resin was subject to micro analysis for C, H and N on an Elementar Vario EL III Carlo Erba 1108 elemental analyzer. The number average molecular weight $\bar{M}n$ was determined by conductometric titration in DMSO medium using ethanolic KOH as the titrant by using 25 mg of sample. A plot of the specific conductance against the milliequivalants of potassium hydroxide required for neutralization of 100 g of terpolymer was made. Inspection of such a plot revealed that there were many breaks in the plot. From this plot the first break and the last break were noted. The calculation of $\bar{M}n$ by this method is based on the following consideration (Rahangdale et al., 2009): (1) the first break corresponds to the stage at which the first acidic phenolic hydroxyl group is neutralized, and (2) the last break observed beyond the first break represents the stage at which phenolic hydroxyl group of all the repeating units are neutralized. On the basis of the average degree of polymerization, ( $\overline{\mathit{DP}}$ ) the average molecular weight has to be determined by the following Eq. (1)…

2.3.2 Spectral and surface analysis

Electronic (UV–visible) absorption spectra of the terpolymer in DMSO was recorded with a double beam spectrophotometer fitted with an automatic pen chart recorder on themosensitive paper in the range of 200–850 nm at L.I.T. RTM, Nagpur University Nagpur. Infrared spectra of p-HBBF terpolymer resin were recorded in najol mull with a Perkin-Elmer-Spectrum RX-I, FT-IR spectrophotometer in KBr pallets in the range of 4000–500 cm⁻¹ at Sophisticated Analytical Instrumentation Facility, Punjab University, Chandigarh. Proton NMR and ¹³C NMR spectra were recorded with Bruker Adanve-II 400 NMR spectrophotometer using DMSO-d₆ as a solvent, at Sophisticated Analytical Instrumentation Facility, Punjab University, Chandigarh. The surface analysis was performed using scanning electron microscope at different magnifications. SEM has been scanned by a JEOL JSM-6380A Analytical Scanning Electron Microscope at VNIT, Nagpur.

The results of elemental analysis and spectral analysis determine that the p-HBBF terpolymer is composed by p-hydroxybenzaldehyde, biuret and formaldehyde.

2.4.1 Determination of metal uptake in the presence of electrolytes of different concentrations

The terpolymer sample (25 mg) was suspended in an electrolyte solution (25 ml) of known concentration. The pH of the suspension was adjusted to the required value by using either 0.1 N HCl or 0.1 N NaOH. The suspension was stirred for a period of 24 h at 25 °C. To this suspension was added 2 ml of a 0.1 M solution of metal ion and the pH was adjusted to the required value. The mixture was again stirred at 25 °C for 24 h and filtered (Zalloum and Mubarak, 2008; Patle Deepti et al., 2011). The solid was washed and the filtrate and washings were combined and the metal ion content was determined by titration against standard EDTA. The amount of metal ion uptake of the polymer was calculated from the difference between a blank experiment without polymer and the reading in the actual experiments (Patle Deepti et al., 2010). The experiment was repeated in the presence of other three electrolytes such as NaCl, NaClO₄ and Na₂SO₄.

2.4.2 Evaluation of the rate of metal ion uptake

In order to estimate the time required to reach the state of equilibrium under the given experimental conditions, a series of experiments of the type described above were carried out, in which the metal ion taken up by the chelating resin was determined from time to time at 25 °C (in the presence of 25 ml of 1 M NaNO₃ solution). It was assumed that under the given conditions, the state of equilibrium was established within 24 h. The rate of metal uptake is expressed as percentage amount of metal ions taken up after certain time related to that at the state of equilibrium.

2.4.3 Evaluation of the distribution of metal ions at different pH

The distribution of each one of the six metal ions i.e. Fe(III), Cu(II), Cd(II), Zn(II), Ni(II) and Pb(II) between the polymer phase and the aqueous phase was determined at 25 °C and in the presence of a 1 M NaNO₃ solution. The experiment was carried out as described earlier at different pH values. The distribution ratio “D” is defined by the following relationship $$\mathbf{D}=\frac{\{\text{Wt}.(\text{in mg})\text{of metal ions taken up by}1\text{gm of terpolymer}\}}{\{\text{Wt}.(\text{in mg})\text{of metal ions present in 1 ml of terpolymer}\}}$$

3.3.1 Effect of electrolytes and their concentration on the metal ion uptake capacity

We examined the influence of NO₃⁻, Cl⁻, ${\mathrm{SO}}_4^{2-}$ and ClO₄⁻ at various concentrations on the equilibrium of metal-resin interaction of constant pH. Different metal ions have different pH in solution, which have been mentioned in Figs. 6–9, which show that the amount of metal ions taken up by a given amount of terpolymer depends on the nature of concentration of the electrolyte present in the solution.

Generally as concentration increases of the electrolyte, the ionization decreases, the number of ligands decreases in the solution which form the complex with less metal ions and more ions are available for adsorption. The variable metal ions uptake capacity of p-HBBF terpolymer may be due to the strong and weak complex formation between electrolyte ligand and metal ion. When the concentration of electrolyte is zero then there is no negative ion (ligand) in metal ion solutions, no complex formation. All metal ions may be available to the adsorption on the polymer. Hence show maximum uptake of ion by p-HBBF terpolymer. As there is no complex formation, there is no problem of strong and weak nature, hence all metal ions show comparable uptake on the polymer at zero concentration. Zero concentration of electrolyte may not affect the metal uptake capacity of polymer. Hence on increasing concentration there should be an increase in uptake of metal ion. But trend disturbed due to formation of more stable complex with more number of ligands which decrease the number of metal ions available for adsorption, hence uptake decreases.

If electrolyte ligand–metal ion complex is weaker than polymer metal ion chelates, the more metal ion can form complex with polymer hence uptake of metal ion is more. But if this complex is strong than polymer – metal ion chelates, more metal ions form strong complex with electrolyte ligand which make metal uptake capacity lower by polymer.

After adsorption when the solution is filtered and the polymer is shaking with dilute HCl, there is an exchange of metal ion and H⁺ ions, metal ions enter in the acid solution, this phenomena is known as desorption, or regeneration of polymer. The regenerated polymer can again be used for re-adsorption by shaking it with metal ion solution. Adsorption and re-adsorption depend on concentration of metal ions but desorption does not.

In the presence of nitrates, perchlorate and chloride ions, the uptake of Fe(III), Cu(II) and Ni(II) ions increases with increasing concentration of electrolytes, whereas in the presence of sulfate ions, the amount of above mentioned ions taken up by the terpolymer resin decreases with increasing concentration of the electrolyte (Vogel, 1978; Agrawal et al., 2011). Moreover, the uptake of Zn(II), Cd(II) and Pb(II) ions increases with decreasing concentration of the nitrates, perchlorate, chloride and sulfate ions. Above NO₃⁻, Cl⁻ and ClO₄⁻ ions form weak complex with the above metal ions, while ${\mathrm{SO}}_4^{-2}$ form stronger complex thus the equilibrium is affected. This may be explained on the basis of the stability constants of the complexes with those metal ions and nature of ligands. The stability of the complexes depends on the charge of metal ions and nature of ligands. Among all above four ligands ${\mathrm{SO}}_4^{2-}$ is a strong ligand, due to having more number of electrons for donation to the meal ion during complex formation, forms strong and stable complex with all the six metal ions under study, therefore overall metal uptake is less in sodium sulfate electrolyte and on increasing concentration of ${\mathrm{SO}}_4^{2-}$ ions in solution, more and more metal ions can form complex with ${\mathrm{SO}}_4^{2-}$ ligands, less number of ions remain left available for uptake of polymer, decreasing metal uptake capacity in ${\mathrm{SO}}_4^{2-}$ electrolyte, while the ligands NO₃⁻, Cl⁻ and ClO₄⁻ may form weak complex with Fe³⁺, Cu²⁺and Ni²⁺metal ions as pH may be lower, therefore may increase the metal uptake capacity of the polymer. While the ligands ${\mathrm{NO}}_3^{-}$ , Cl⁻ and $\mathrm{C}1{\mathrm{O}}_4^{-}$ may form strong complex with Cd²⁺, Zn²⁺ and Pb²⁺ at some higher pH, therefore if the concentration of these ligands increases the more complex formation might be possible, which decreases the metal uptake capacity of the terpolymer. This type of trend has also been observed by other investigators in this field (Rahangdale et al., 2010; Singru et al., 2010). The ratio of physical core structure of the resin is significant in the uptake of different metal ions by the resin polymer. The amount of metal ion uptake by the p-HBBF terpolymer resin is found to be higher when comparing to the other polymeric resins.

3.3.2 Rate of metal ion uptake as a function of time

The rate of metal adsorption was determined to find out the shortest period of time for which equilibrium could be carried out while operating as close to equilibrium condition as possible. During rate of metal ion determination, the concentration of metal ion and electrolyte solution and pH of the solution remain constant and pH of each metal ion is different which is given in Fig. 10. As shaking time increases the polymer gets more time for adsorption, hence uptake of metal ions increases. Fig. 10 shows the results of rate of uptake of metal ion on p-HBBF terpolymer resin. The rate refers to the change in the concentration of the metal ions in the aqueous solution which is in contact with the given terpolymer. The Fig. 10 shows that the time taken for the uptake of the different metal ions at a given stage depends on the nature of metal ions under given conditions. It is found that Fe(III) ions required about 3 h for the establishment of the equilibrium, whereas Cu(II), Ni(II), Zn(II) and Pb(II) ions required about 5 or 6 h. Thus the rate of metal ions uptake follows the order Fe(III) > Cu(II) > Ni(II) > Zn(II) > Cd(II) > Pb(II) for the terpolymer (Samir et al., 2004; Rahangdale et al., 2010).

The rate of metal uptake may depend upon hydrated radii of metal ions. The rate of uptake for the post transition metal ions exhibit other trend for Cd(II), the rate of uptake is comparable with that of Pb(II) because of difference in ‘d’ orbital. In this evaluation we find out the shortest period of time is close as the equilibrium condition is acquired. At the equilibrium condition metal has the highest percentage rate of uptake, which is acquired due to 24 h staking. The observation obtained indicates that, time required for the rate of metal ion uptake depends on the nature of the metal ions and may be depend on the ionic size. Thus the rate of metal ion uptake follows the order:

Fe³⁺ > Cu²⁺ ≈ Ni²⁺ > Zn²⁺ > Cd²⁺ ≈ Pb²⁺

Ionic size 0.55 0.57 0.69 0.90 1.10 1.19

The sequences of rate of metal ion uptake indicate that the rate may depend on size of the ion. The rate is directly proportional to the size of the metal ion. For example Fe³⁺ has more charge and small size, therefore equilibrium is attained within three hours, while other first transition ions Cu²⁺, Ni²⁺, and Zn²⁺ have nearly equal cationic size, charges are some: therefore required 5 h to attain equilibrium, while Cd²⁺ and Pb²⁺ have large atomic size, therefore requiring 6 h to attain equilibrium. The trend is in good agreement with earlier co-workers (Rahangdale et al., 2008).

3.3.3 Distribution ratios of metal ions at different pH

The distribution of metal ion depends upon pH of the solution. By increasing pH, the H⁺ ion concentration in the solution decreases and only metal ion in the solution available for adsorption increases uptake of metal ions.

The effect of pH on the amount of metal ions distributed between two phases can be explained by the results given in Fig. 11. The data on the distribution ratio as a function of pH indicate that the relative amount of metal ions taken up by the terpolymer increase with increasing pH of the medium (Patel and Manavalan, 1984; Rahangdale et al., 2010; Singru et al., 2010; Riswan Ahamed et al., 2010; Burkanudeen et al., 2010). The magnitude of increase, however, is different for different metal cations.

The study was carried from 2.5 up to pH 6.5 to prevent hydrolysis of metal ions at higher pH. For Fe³⁺ ion the highest working pH is 2.5/3, has lower distribution ratio since Fe³⁺ forms octahedral complex with ligand of electrolyte, which shows crowding effect. This steric hindrance may be lower the distribution ratio of Fe³⁺ ion. Cu²⁺ and Ni²⁺ have higher distribution ratio over pH range of 2.5–6.5 which may be due to the less steric hindrance. Thus the value of distribution ratio for given pH depends upon the nature and stability of chelates formation for particular metal ion. The data of distribution ratio show a random trend in certain cases. This may be due to the amphoteric nature of the p-HBBF. From the result given in Fig 11, it reveals that with decrease in atomic number, the ion uptake capacity is increasing at that particular pH. In case of Cd(II) and Pb(II), purely electrostatic factors are responsible. The ion uptake capacity of Cd(II) is lower owing to the large size of its hydrated ion than that of Cu(II). The steric influence of the methyl group and hydroxyl group in p-HBBF resin is probably responsible for their observed low binding capacities for various metal ions. Thus the interaction of this resin material with various metal ions in an aqueous environment may largely limit the binding sites, which are suitably, disposed in a conformational favorable manner on the surface layer. The higher value of distribution ratio for Cu(II) and Ni(II) at pH 2.6–6.0 may be due to the formation of most stable complex with chelating ligands.

Therefore the polymer under study has more selectivity of Cu²⁺ and Ni²⁺ ions at pH 4.0–6.0 then other ions which from rather weak complex. While at pH 3 the terpolymer has more selectivity of Fe³⁺ ions. The p-HBBF terpolymer resin take up Fe(III) ion more selectively than any other metal ions under study. The order of distribution ratio of metal ions measured in pH range 2.5–6.5 is found to be Fe(III) > Cu(II) > Ni(II) > Zn(II) > Cd(II) ≈ Pb(II). Thus the results of such type of study are helpful in selecting the optimum pH for a selective uptake of a particular metal cation from a mixture of different metal ions. For example, the result suggests the optimum pH 6.0, for the separation of Cd(II) and Ni(II) with distribution ratio ‘D’ are 273.4 and 778.2, respectively using the p-HBBF terpolymer resin as ion-exchanger.

Similarly, for the separation of Cu (II) and Fe (III), the optimum pH is 3, at which the distribution ratio ‘D’ for Cu(II) is 32.6 and that for Fe(III) is 326.2. The lowering in the distribution of Fe(III) was found to be small and, hence, efficient separation could be achieved. In order to assess the potential for separation of metal ions Fe³⁺ from other metal ions, the following combinations of metal solutions were prepared: (1) Fe³⁺ and Cu²⁺ (2) Fe³⁺ and Ni²⁺ (3) Fe³⁺ and Zn²⁺ (4) Fe³⁺ and Cd²⁺ (5) Fe³⁺ and Pb²⁺. The solution for separations were prepared by mixing 1 ml of 0.1 M solutions of Fe³⁺ with 1 ml of 0.1 M solution of Cu²⁺, Ni²⁺, Zn²⁺ Cd²⁺ and Pb²⁺. Selective uptake of the metal ions was studied by adjusting the optimum pH of 3. Distribution ratios of Fe³⁺ at pH = 3 in the mixture with metal ions Cu²⁺, Ni²⁺, Zn²⁺ Cd²⁺ and Pb²⁺ were found to be 293.6, 302.9, 310.9, 309.6, 314.9 and 307.6, respectively, i.e. slight lower than 326.2 found when Fe³⁺ alone was studied. The lowering in the distribution ratios of Fe³⁺ was found to be small and hence efficient separation could be achieved. The structure of polychelate of terpolymer metal ion chelate complex has been given in Fig. 12.

Close

Figure 12

Polychelate formation of p-HBBF terpolymer resin.

Export to PPT