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1. Introduction

The removal and separation of metal ions in aqueous solution play an important role for the analysis of wastewaters, industrial and geological samples as well as for environmental remediation^{1, 2}. Chelating resins have been widely utilized for the removal of the undesired metal ions from these aqueous solutions^{3, 4}. The development of high performance chelating resins for removing heavy metal ions from aqueous solution is considered not only as research priority in the environmental field but also as an area of interest in inorganic catalyst and recovery of valuable trace metal ions^5-8. These resins show greater selectivity compared to the conventional types of ion exchangers^9-11. Besides, they show good physical and chemical properties such as porosity, high surface area, durability and purity^12-14. Many chelating resins with different functionalities like quaternary amine^{11, 12}, sulfonamides¹⁵, Schiff base^{7, 8,16-20}, sulfonic acid²¹, hydroxamic acid²² and amidoxime^{23, 24} have been emphasized for interaction with metal ions.

Schiff bases having multidentate coordination sites are known to form complexes with transition metal ions readily. Present in a polymer matrix, they are expected to show affinity and selectivity towards the metal ions at an appropriate pH^{7, 8, 16-20}. This led us to synthesize chelating resins, which will show affinity for the metal ions at appropriate pH.

In the present work Schiff base chelating resins have been prepared. The adsorption behavior of the resin obtained towards heavy metal ions (II); Cu, Cd, Co, Zn, Hg, and Pb have been studied. Both kinetic thermodynamic parameters of the adsorption process were also calculated. It also reported the thermal behavior, degradation kinetic, thermodynamic parameters and the thermal stability of the Ricaa91 and Ricaa73 resin at a single heating rate (10 °C min^-1).

2. Experimental

2.1 Chemicals

All starting materials were reagent grade: solvents, indicators and metal ions (Cu2+, Cd²⁺, Co²⁺, Zn²⁺, Hg²⁺, and Pb²⁺) were purchased from BDH chemical company - England, anthranilic acid, and absolute ethanol from Fluka chemical company-USA, 1,4-phenylenediamine and benzoyl peroxide from Aldrich chemical company-Germany, and cinnamaldehyde and 1,2 azobisisobutyronitrile (AIBN) were from HiMedia laboratories- India.

Anthranilic acid was purified and recrystallized before use from distilled water. 1,4-Phenylenediamine was recrystallized from benzene. Cinnamaldehyde and 1,2 azobisisobutyronitrile were used as received.

2.2 Techniques

Melting points were determined on Stuart Scientific Electro-thermal melting point apparatus. FTIR spectra were recorded using the KBr disc technique on a JASCO 410 FTIR Spectrophotometer. Elemental analyses (CHN) were performed on an elemental analyses system, GmbH VARIOEL V_2.31998 CHNS Mode. ¹H-NMR spectra were recorded in d₆ DMSO on VARIAN 300 MHz FT-NMR Spectrometer (δ in ppm) using TMS as an internal reference. The TGA/DrTGA, DTA/DrDTA thermo-analytical curves were measured in a heating range (25-800 °C) and at a rate of 10 °C min^-1 under nitrogen using Shimadzu TGA-50H and Shimadzu DTA-50H thermal analyzers, respectively.

2.3 Synthesis of Schiff bases

2.3.1 Synthesis of cinnamaldehyde anthranilic acid (ciaa)

The Schiff base ciaa was prepared by adding cinnamaldehyde (0.05 mol, 6.38 mL) to solution of anthranilic acid (0.05 mol, 6.86 gm) in 20 mL absolute ethanol with stirring and gentle heating. The formed precipitate was filtered, washed with ethanol and then dried in air (Scheme 1). Color yellow. Mwt. 251. Yield 85%. M.p. 140 °C.

Elemental analysis (% CHN): Calcd. (Found) C: 76.47 (76.23), H: 5.21 (5.07), N: 5.57 (5.32). ¹HNMR data (DMSO-d₆): (ppm), 47 - 8.00 (m, CH vinyl, CH arm), 9.65 (d, CH=N). IR (KBr) ν/cm^-1: 3100 - 2634 (broad band OH_str), 3063, 3029 (=CH vinyl, arm), 1704 (C=O), 1616 (CH=N), 1603, 1586, 1509 (C=C vinyl, arm), 747, 687 (=CHoop).

2.3.2 Synthesis of cinnamaldehyde-1,4-phenylenediamine (cipn)

To a solution of 1,4-phenylenediamine (0.05 mol, 5.41gm) in 20 mL absolute ethanol, a cinnamaldehyde (0.1 mol, 12.7 mL) was added with stirring and gentle heating. The precipitate obtained was filtered, washed thoroughly with ethanol and then dried in air (Scheme 2). Color, pale yellow. Mwt. 336. Yield 80%. M.p. 227 °C.

Elemental analysis (% CHN): Calcd. (Found) C: 85.68 (85.06), H: 5.99 (5.86), N: 8.32 (7.90). 1HNMR data (DMSO-d6): δ(ppm), 7.1 - 7.6 (m, CH vinyl, CH arm), 8.4 (d, CH=N). IR (KBr) ν/cm-1: 3077, 3039, 3029 (=CHstr), 1626 (CH=N), 1603, 1599, 1586, 1572 (C=C vinyl, arm), 838, 745, 693(=CHoop).

2.4 Synthesis of (ciaa – cipn) copolymers (chelating resins)

A general heterogeneous copolymerization method was used in the preparation of the chelating resins in this study²⁵. Each of the comonomers, ciaa and cipn, were mixed individually with AIBN (1 mol/100 mol of common mixture) as a polymerization initiator in 5 mL DMF. The radical copolymerization was maintained by mixing together the two comonomer mixture solutions, then heating to 75 °C under reflux for 24 h. After cooling the chelating resin was precipitated in methanol, filtered off, washed several times with methanol and finally, dried in air (Scheme 3). Since the Schiff base cipn possess two vinyl (- CH₂=CH₂-) moieties on either sides of its chemical structure, it was deliberately used as the cross-linking agent in the preparation of the chelating resins.

The resin of (ciaa-cipn) copolymer was prepared as crosslinked chelating resins in two different ratios of ciaa and cipn mixtures²⁶; a mixture of ciaa (9 gm, 35.9×10^-3 mol) and cipn (1 gm, 3×10^-3 mol) to form the resin Rciaa91, and that of ciaa (7 gm, 27.9×10^-3 mol) and cipn (3 gm, 8.9×10^-3 mol) to form the resin Rciaa73.

2.5 Metal ion adsorption measurements using a batch method.

Stock solutions of the metal ions were prepared in distilled water. A stock solution of EDTA (5.0 x 10^-3 mol L^-1) was prepared and standardized against a solution of MgSO₄.7H₂O using Eriochrome Black-T (EBT) as an indicator²⁷. Buffers of acetic acid/sodium acetate and ammonium hydroxide/ammonium chloride were used for the experiments carried out under a controlled pH.

The metal ion adsorption experiments using the batch method were carried out at a controlled pH by placing 0.05 g chelating resin with 50 mL metal ion at initial concentration 5.0 x 10^-3 mol L^-1. The contents of the flask were equilibrated on a Gallenkamp flask shaker at room temperature for 60 min at 150 rpm. The pH of the solution was adjusted using a suitable buffer. Then, 5 mL of the solution (free of suspended solid) were taken at the end of the experiment at different intervals. The residual concentration of the metal ion solution was determined via titration against 5.0 x 10^-3 mol L^-1 EDTA using EBT indicator^{27, 28}.

2.6 Effect of pH on the uptake of metal ions

The effect of pH of the metal ion test solution is an important parameter for adsorption of metal ions because it affects the solubility of the metal ions, concentration of the counter ions, on the functional groups of the adsorbent and the degree of ionization of the adsorbent²⁹. Adsorption measurements under pH control were carried out following the above procedures of uptake experiments. The pH was controlled using NH₄OH/NH₄Cl buffer solution to study the uptake behavior at the alkaline mediums (pH 6–12) and acetic acid/sodium acetate (AcOH/NaOAc) buffer was used to study the uptake in the acidic medium (pH 4).

3. Results and discussion

3.1 Synthesis and Characterization

The Schiff bases, ciaa and cipn were prepared as described in the experimental part, by the reaction of cinnamaldehyde (ci) with anthranilic acid (aa), and 1,4-phenylenediamine (pn), respectively. The Schemes 1 and 2 display the structures of the synthesized Schiff bases ciaa, cipn, respectively. Two chelating resins, Rciaa91 and Rciaa73 were then synthesized using free radical polymerization reaction of the synthesized Schiff bases with different compositional weight ratios (ciaa:cipn), 9:1 and 7:3, respectively, (Scheme 3).

The synthesized Schiff bases were subjected to elemental analysis (CHN) and their found values were in good agreement with those of the calculated values for the suggested formulas of the prepared samples. The melting points were sharp, indicating the purity of prepared Schiff bases.

3.2 Spectral analysis FTIR spectra

The chemical structures of the synthesized Schiff bases were confirmed by the IR and ¹HNMR spectra. The FTIR spectra of the synthesized Schiff bases, ciaa and cipn, showed sharp and strong characteristic absorption peaks. Figure 1 displays the FTIR spectra of the synthesized Schiff bases, and their corresponding copolymer Rciaa91. The disappearance of the aldehydic carbonyl ν(C=O) band and the amino ν(NH₂) bands of the starting materials, and the appearance of the azomethine ν(C=N) band confirmed the formation of the Schiff bases. The ν(C=N) stretching frequencies of the Schiff bases, ciaa and cipn appeared at 1616, 1626, respectively. Upon copolymerization of the Schiff bases ciaa and cipn, the IR spectra of the chelating resins showed broader and less intense peaks compared to their corresponding comonomers. The medium broad peak in the range of 3400 – 3200 cm^-1 could be due to the adsorbed water molecules on the Schiff base chelating resin because of its hygroscopic nature¹⁸.

Figure 2 shows the ¹HNMR spectra of the Schiff base, ciaa and cipn. The ¹HNMR spectra of the Schiff bases, ciaa and cipn showed doublet signals for the δ (CH=N) protons at 9.65 and 8.45, respectively. The multiple peaks in the range δ 6.47 - 8.00 ppm were attributed to the vinyl and aromatic protons of the conjugated structure of the Schiff bases. The singlet peak at 2.51 ppm in the spectra of is due to the DMSO solvent used in the analysis. The appearance of a broad signal at δ 3.2 ppm in ¹HNMR spectrum of the Schiff base cipn could be attributed of the presence of traces of unreacted NH₂ in its chemical structure³⁰.

3.3 Uptake of metal ions (Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Hg²⁺, Pb²⁺) by the chelating resins

3.3.1 Optimum pH of the metal ion uptake

The preliminary uptake experiments of the metal ions; Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Hg²⁺ and Pb²⁺ by the chelating resin Rciaa91 and Rciaa73 at different pH values between 4 and 10 were examined by batch technique for 60 minutes as a reaction time²⁶. The results are shown in Table 1. In general, the uptake of the metal ions increased with increasing pH, until reaching a maximum value and then followed by a decrease in the uptake values at higher pH values, due to the precipitation of the metal hydroxides in the basic solution³¹.

The optimum pH values obtained for the metal ions under study; Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Hg²⁺, Pb²⁺ are 8, 9, 7, 8, 5, 7, respectively.

Table 1 showed that the chelating resins Rciaa91 and Rciaa73 absorbed the metal ions in the following order Hg²⁺ > Cu²⁺ > Zn²⁺ > Pb²⁺ > Co²⁺ > Cd²⁺ each metal at its optimum pH and at the same reaction time and ion concentration. A special feature seen in this study is that Hg²⁺ ion showed the highest uptake affinity at low pH compared to the rest of the metal ions under study. This allows separation of Hg²⁺ ions, selectively, in the presence of other metal ions.

3.3.2 Effect of contact time

The interaction between the resin and metal ion continues until most of effective sites of the resin are occupied by the metal ion. At this point the resin is said to have reached the equilibrium state with the metal ions³². The time required to reach the equilibrium state for the uptake processes of each metal ion with the chelating resins Rciaa91 and Rciaa73 was evaluated at their optimum pH values. Table 2 and Figures 3 and 4 show the relationship between the uptake processes of each metal ion and the equilibrium reaction time, for the chelating resins Rciaa91 and Rciaa73, respectively. In the case of the chelating resins Rciaa91, the equilibrium state was attained within 30 min for all the metal ions and the time required for 50% uptake was about 10 min. On the other hand, for the chelating resin Rciaa73, the equilibrium state was attained within 60 min for the metal ions under investigation and the time required for 50% uptake was about 30 min.

From the data obtained it could be concluded that the uptake values depend on both the metal ion and the pH values. The uptake capacity of the chelating resins, Rciaa91 and Rciaa73 for different metal ions showed a maximum value of 2.9 mmol g^-1 resin and 2.6 mmol g^-1 resin for Hg²⁺ ions and a minimum value of 0.98 mmol g^-1 resin and 0.88 mmol g^-1 resin for Cd²⁺ ions, respectively. The uptake capacities of the rest of the metal ions were in between for both the chelating resins Rciaa91 and Rciaa73 as seen in Figures 3 and 4, respectively.

The differences in the uptake capacities may be attributed to i) the differences in the stability constants of the formed complexes between the different metal ions and the resin^{33, 34}. For this reason, the differences in the stability constants may explain the high uptake capacities of Hg²⁺ for both the chelating resins, in spite of the large ionic radii of the mercury (II) ion compared to the other ions under investigation, ii) the differences in ionic radii of the metal ions; the smaller the ion size the easier it can penetrate through the network of the resin³³. This explains why, the smallest ion Cu²⁺, (ionic radii 0.72Å) showed high uptake capacities, 1.84 mmol g^-1 and 1.40 mmol g^-1, whereas the largest ion Cd²⁺, (ionic radii 0.97 Å) showed low uptake capacities, 0.90 mmol g^-1 and 0.88 mmol g^-1, for Rciaa91 and Rciaa73, respectively.

3.4 The effect of compositional ratio of the chelating groups and degree of cross-linking between Rciaa91 and Rciaa73 on the uptake processes.

Table 3 and the Figure 5 show the effect of chelating group and degree of cross-linking ratio of Rciaa91 and Rciaa73 on the uptake process of the metal ions at the same pH medium, initial metal concentration and reaction time.

The results obtained indicated that the resin Rciaa91 has higher uptake efficiency than Rciaa73 at the same conditions. This is due to the difference in the compositional ratio of chelating groups and degree of cross-linking of the chelating resins Rciaa91 and Rciaa73. The resin Rciaa91 with ratio (9:1) by weight, has about 20% of coordinating groups RCH=N-C₆H₄-COOH more than the resin Rciaa73 with compositional ratio (7:3) by weight. Therefore, the low metal ion uptake capacity of the chelating resin Rciaa73 could be attributed to its higher degree of cross linking. This presumably hinders the approach of the metal ions to the polymeric matrix and, hence, limits the chelating reaction³⁵.

The uptake of the resin Rciaa91 was about 20% more than that of the resin Rciaa73 for the small metal ions under investigation (Cu²⁺, Co²⁺, Zn²⁺) whose ionic radii was nearly 0.7 Å. This ratio is nearly close to the theoretical ratio of chelating groups between the two resins. On the other hand, the uptake of the large ions (Cd²⁺, Hg²⁺and Pb²⁺) with ionic radii of about 1 Å, by the resin Rciaa91 was about 11% more than that of the resin Rciaa73. This shortage in uptake capacity maybe due to the larger ionic volume of metal ions (Cd²⁺, Hg²⁺ and Pb²⁺) and the increasing of degree of cross linking in the resin Rciaa73^{26, 33}.

3.5 The effect of the degree of cross linking on the optimum reaction time for the uptake processes.

The data in the Table 2 and Figures 3 and 4 showed that the resin Rciaa91 with a lower degree of cross linking reached the optimum reaction time for the uptake process at half time period compared to the resin Rciaa73. For example, the equilibrium state for the uptake of Cu²⁺ ions by Rciaa91 at the optimum pH 8 was attained after 30 min, whereas the equilibrium state for the uptake of Cu²⁺ ions by Rciaa73 was reached after 60 min. Also, it was calculated from the data obtained that the time required to achieve 50% of the metal ion uptake was within 5 min for Rciaa91, whereas, it was within 20 min for Rciaa73.

The comparison between the optimum reaction time for the resins, Rciaa91 and Rciaa73, which have the same chemical structure, but different degree of cross-linking proved that, the higher the degree of cross-linking the higher the optimum reaction time, at the same pH medium.

The resin Rciaa73 has about 200% more cross linking through RCH=NC₆H₄N=CHR than the resin Rciaa91. A high degree of cross-linking makes the polymer chains more rigid and more hindering for the metal ions to penetrate the network of the resin in order to act with the active sites. This in turn, will lead to a decrease in the rate of uptake process and increase in the time required to reach the equilibrium state during the uptake^{26, 33}.

3.6 Thermal Studies

The TGA and DTA curves of the prepared Rciaa91 and Rcaa73 resins are given in Figures 6.1 and 6.2. These curves characterize and compare the thermal degradation of these two resins at 10 °C min^-1 heating rate, under nitrogen and over the range 20-800 °C. For the evaluation of the thermal degradation kinetics parameters at a single heating rate (10 °C min^-1), the activation energy (E_a) and pre-exponential factor (Z) are determined by using the Coats-Redfen method³⁶ for the reaction order n ≠ 1. When the Coats-Redfern method is linearized for a correctly-chosen order of reaction (n) yields the activation energy (E_a) from the slope of the equation:

where: α = fraction of weight loss, T = temperature (K), Z = pre-exponential factor, R = molar gas constant, q = heating rate and n = reaction order; estimated by Horovitz-Metzger method³⁷.

The thermodynamic parameters of the thermal degradation step; enthalpy (ΔH*), entropy (ΔS*), and Gibbs energy (ΔG*) of activation are calculated using the following standard equations:

The characteristics of the thermal degradation of these two resins recorded on the TG/DTG/DTA curves, their kinetics and thermodynamics parameters extracted from these curves are given in Tables 4 and 5.

The thermal degradation TG curves of Rciaa91 and Rciaa73 resin (Figures 6.1 and 6.2) showed a very slow and continuous mass bleeding (i.e. no plateau from the start up to about 450 °C) of about 37.7% and 27.8%, respectively. Consequently, very small and weak DTG peaks are observed on the DTG curves of these resins and this is indicative to the very slow degradation process in both resins at the about 35-450 °C range. Therefore, the prominent steps of these two resins that occur at about 470-641 °C range with large and strong DTG peak at 580 °C (Rciaa91) and 584 °C (Rciaa73) are considered as characteristic steps for which the thermal degradation, kinetics and thermodynamics parameters are determined and hence compared (Tables 4 and 5).

The Rciaa91 resin showed a mass loss (61.3%) at the prominent step (470-634 °C) with strong ^TDTG at 584 °C and exothermic TDTA peak at 580 °C. The calculated activation energy E_a of the Rciaa91 resin for this prominent step is 184 kJ mol^-1, while -167 J K^-1 mol^-1, 83 kJ mol^-1 and 226 kJ mol^-1 are the values of entropy (ΔS*), enthalpy (ΔH*), and Gibbs energy (ΔG*) changes. The prominent step (480-641 °C) of the Rciaa73 resin showed also exothermic mass loss (64.7%) with strong ^TDTG (580 °C) and ^TDTA (592 °C). The activation energy E_a, entropy (ΔS*), enthalpy (ΔH*), and Gibbs energy (ΔG*) changes are 180 kJ mol^-1, -211 J K^-1 mol^-1, 49 kJ mol^-1 and 226 kJ mol^-1, respectively.

If the initial molecular structure destruction temperature (T_i) of the Ricaa91 and Ricaa73 resin is taken as a measure of the thermal stability, these two resins are of similar thermal stability for their similar T_i (35 °C). It can be concluded that although the two resins Ricaa91 and Rciaa73 are of similar chemical structure but different degree of cross-linking, their TGA curves and the data extracted from them (Tables 4 and 5) indicate almost similar thermal degradation behavior, kinetics and thermodynamics parameters as well as similar thermal stability regardless the difference in the degree of the cross-linking 1:3 (Rciaa91:Rciaa73). The resin Rciaa91 losses 37.7% of its mass slowly and 61.3% rapidly with 1% residue, while Rciaa73 losses 28% slowly and 64.7% rapidly with 7.5% residue. This difference in mass loss degradation process may be due to the cross-linking variation.

4. Conclusions

The effect of the compositional ratio of the chelating groups and the degree of the cross-linking of the Schiff base chelating resins, Rciaa91 and Rciaa73 on the uptake behavior of the heavy metal ions (Cu²⁺, Cd²⁺, Hg2+, Co²⁺, Pb²⁺ and Zn²⁺) was the main aim of this research. A special feature observed in this study was that Hg²⁺ ions showed the highest uptake affinity at low pH compared to the rest of the metal ions under study for both resins, Rciaa91 and Rciaa73. The difference in the uptake affinity of the chelating resins maybe due to i) the ionic radii of the metal ions; the smaller the ion size the easier it can penetrate through the network of the resin, ii) the stability constants which explains the high uptake capacities of Hg²⁺ for both the chelating resins, in spite of the large ionic radii of the mercury (II) ion compared to the other ions under investigation.

When comparing between the uptake affinity and the optimum reaction time for the chelating resins Rciaa91 and Rciaa73, which have the same chemical structure, but different degree of cross linking, one concludes that the higher the degree of cross-linking the higher the optimum reaction time and the lower the uptake affinity towards the heavy metal ions.

The chelating resins showed good thermal stability. The thermal degradation behavior and the kinetic parameters of the resin Rciaa91 and Rciaa73 are almost similar, indicating that the differences in the degree of cross-linking are almost of no significant effects.

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Author notes

f_alyusufy@yahoo.com

IR