A comparative study of Schiff base chelating resins: synthesis, uptake of heavy metal ions, and thermal studies

Two new chelating resins (Rciaa91 and Rciaa73) with different compositional chelating groups and degree of cross-linking were prepared by free radical copolymerization of Schiff bases obtained from condensation reaction of cinnamaldehyde (ci) with anthranilic acid (aa) and 1,4-phenylenediamine (pn) monomers. The synthesized materials were characterized using CHN analyses, FTIR, 1H-NMR, and thermal analyses (TGA, DTA). Batch technique was applied, and the contact time, pH and initial concentration of the metal ions were investigated as factors affecting the uptake behavior. The results obtained indicated that the chelating resin with larger compositional ratio of chelating moieties and lower degree of cross-linking showed lower optimum reaction time and higher uptake affinity towards the metal ions Cu(II), Cd(II), Co(II), Zn(II), Hg(II), and Pb(II), under the same conditions. Both the chelating resins showed uptake behavior of 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. The thermal degradation behavior and stability of the resins were investigated by using non-isothermal thermogravimetric analysis (TGA/DTG/DTA), at 10 °C min-1 heating rate and under nitrogen. The Coats-Redfern method was used to evaluate the kinetic and thermodynamic parameters (ΔG*, ΔH* and ΔS*) for the prominent degradation steps in the TGA curves at 450-660 °C range.


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][6][7][8] . These resins show greater selectivity compared to the conventional types of ion exchangers [9][10][11] . Besides, they show good physical and chemical properties such as porosity, high surface area, durability and purity [12][13][14] . Many chelating resins with different functionalities like quaternary amine 11,12 , sulfonamides 15 , Schiff base 7,8,[16][17][18][19][20] , sulfonic acid 21 , hydroxamic acid 22 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 ABSTRACT: Two new chelating resins (Rciaa91 and Rciaa73) with different compositional chelating groups and degree of crosslinking were prepared by free radical copolymerization of Schiff bases obtained from condensation reaction of cinnamaldehyde (ci) with anthranilic acid (aa) and 1,4-phenylenediamine (pn) monomers. The synthesized materials were characterized using CHN analyses, FTIR, 1 H-NMR, and thermal analyses (TGA, DTA). Batch technique was applied, and the contact time, pH and initial concentration of the metal ions were investigated as factors affecting the uptake behavior. The results obtained indicated that the chelating resin with larger compositional ratio of chelating moieties and lower degree of cross-linking showed lower optimum reaction time and higher uptake affinity towards the metal ions Cu(II), Cd(II), Co(II), Zn(II), Hg(II), and Pb(II), under the same conditions. Both the chelating resins showed uptake behavior of the metal ions in the following order Hg 2+ > Cu 2+ > Zn 2+ >Pb 2+ >Co 2+ > Cd 2+ each metal at its optimum pH and at the same reaction time and ion concentration. The thermal degradation behavior and stability of the resins were investigated by using non-isothermal thermogravimetric analysis (TGA/DTG/DTA), at 10 °C min -1 heating rate and under nitrogen. The Coats-Redfern method was used to evaluate the kinetic and thermodynamic parameters (ΔG * , ΔH * and ΔS * ) for the prominent degradation steps in the TGA curves at 450-660 °C range. towards the metal ions at an appropriate pH 7,8,[16][17][18][19][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 ).
Anthranilic acid was purified and recrystallized before use from distilled water. 1,4-Phenylenediamine was recrystallized from benzene.

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 V2.31998 CHNS Mode. 1 H-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.

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

Synthesis of (ciaacipn) copolymers (chelating resins)
A general heterogeneous copolymerization method was used in the preparation of the chelating resins in this study 25 . 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 mixtures 26 ; 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.

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 indicator 27 . 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 .

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 adsorbent 29 . 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). 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).

Synthesis and Characterization
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.

Spectral analysis FTIR spectra
The chemical structures of the synthesized Schiff bases were confirmed by the IR and 1 HNMR 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 (C=O) band and the amino (NH2) bands of the starting materials, and the appearance of the azomethine  (C=N) band confirmed the formation of the Schiff bases. The (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 nature 18 .

Uptake of metal ions (Cu
The optimum pH values obtained for the metal ions under study; Cu 2+ , Cd 2+ , Co 2+ , Zn 2+ , Hg 2+ , Pb 2+ 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 Hg 2+ > Cu 2+ > Zn 2+ > Pb 2+ > Co 2+ > Cd 2+ each metal at its optimum pH and at the same reaction time and ion concentration. A special feature seen in this study is that Hg 2+ ion showed the highest uptake affinity at low pH compared to the rest of the metal ions under study. This allows separation of Hg 2+ ions, selectively, in the presence of other metal ions.

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 ions 32 . 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 Hg 2+ ions and a minimum value of 0.98 mmol g -1 resin and 0.88 mmol g -1 resin for Cd 2+ 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 Hg 2+ 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 resin 33 . This explains why, the smallest ion Cu 2+ , (ionic radii 0.72Å) showed high uptake capacities, 1.84 mmol g -1 and 1.40 mmol g -1 , whereas the largest ion Cd 2+ , (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.

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 reaction 35 .
The uptake of the resin Rciaa91 was about 20% more than that of the resin Rciaa73 for the small metal ions under investigation (Cu 2+ , Co 2+ , Zn 2+) 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 (Cd 2+ , Hg 2+ and Pb 2+ ) 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 (Cd 2+ , Hg 2+ and Pb 2+ ) and the increasing of degree of cross linking in the resin Rciaa73 26, 33 .

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 Cu 2+ ions by Rciaa91 at the optimum pH 8 was attained after 30 min, whereas the equilibrium state for the uptake of Cu 2+ 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 .

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 (Ea) and pre-exponential factor (Z) are determined by using the Coats-Redfen method 36 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 (Ea) from the slope of the equation:    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: max ln kT 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 Ea 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 Ea, 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 (Ti) 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 Ti (35 °C). It can be concluded that although the two resins Ricaa91 and Rciaa73 are of similar chemical structure but different degree of crosslinking, 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.

Conclusions
The effect of the compositional ratio of the chelating groups and the degree of the crosslinking of the Schiff base chelating resins, Rciaa91 and Rciaa73 on the uptake behavior of the heavy metal ions (Cu 2+ , Cd 2+ , Hg 2+ , Co 2+ , Pb 2+ and Zn 2+ ) was the main aim of this research. A special feature observed in this study was that Hg 2+ 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 Hg 2+ 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.