Synthesis and Characterization of Poly (1,4-Benzenedimethylene Phthalate) and the Study

Poly (1,4-benzenedimethylene phthalate) was synthesized by condensation of phthaloyl chloride with 1,4-benzenedimethanol in the presence of pyridine in dry THF at 30 °C . The resulting polymer white powder was characterized by viscosity measurement, FT-IR, 1H and 13C NMR, elemental analysis, thermal (TGA-DSC) methods and scanning electron microscopy. The uptake properties of Pb(II), Zn(II) and Cd(II) metals by the polymer from aqueous solutions were studied by the batch and column techniques as a function of pH, temperature, concentration and contact time. The uptake increased with increasing pH reaching a maximum at pH = 6.00 for all ions. The polymer showed high uptake capacities toward Pb(II) and Zn(II) ions, but medium uptake capacity toward Cd (II). The linearized forms of Langmuir, Freundlich and Dubinin-Radushkevich adsorption isotherms indicate studied ions Pb(II), Zn(II), and Cd(II). The adsorption capacity follow the order: Pb(II) > Zn(II) > Cd(II). The thermodynamic functions, ΔG°, ΔH° and ΔS° were determined for Pb(II), Zn(II) and Cd(II); the values of ΔG° indicated that the adsorption process of these metal ions on the polymer is favorable. In this work, the values of ΔGǂ, ΔHǂ and ΔSǂ were determined by using Eyring-Polanyi equation and Arrhenius equation. The results indicated that ΔGǂ=72.8, 78.6, 87.8kJ/mol for Pb, Zn, Cd ions, respectively. The column experiments for metal ion uptake were conducted at pH= 6.0, 25.0 °C, and initial concentration of 150.0mg/L. The loaded concentrations were 76.42, 49,05 and 28.47 ppm for Pb, Zn, Cd ions, respectively. The efficiency for recovery of metal ions after adsorption by treatment of the loaded polymer with 0.1M HNO3, 0.1M EDTA, gave good percent recovery for 0.1M HNO3

There are two types of polyesters: Aliphatic polyesters and aromatic polyesters. The synthesis of aliphatic polyesters has been well established for several years. However, these polyesters possess low thermal stability due to their low melting points and glass transition temperatures owing to their low molecular weight. These properties resulted in limited usage and few applications of aliphatic polyesters, yet they showed potential as biodegradable polymers. On the other hand, aromatic polyesters display an excellent pattern of physical properties. They are strongly resistant to hydrolysis, bacterial and fungal attack, they also remain unaltered in the environment [11,12], Combining aromatic and aliphatic units in the same polyester chain has been envisaged as an attractive approach to obtain novel products encompassing biodegradability and high performance properties [11].

Preparation of The Polymer
Poly(1,4-benzenedimethylene phthalate) was synthesized by polycondensation using single phase organic solvent polymerization. 1,4-benzenedimethanol ( 4.97 g, 0.036 mol), pyridine (8.54 g, 0.108 mol) and a catalytic amount of 4-DMAP were dissolved in THF (60 mL). To this solution, a solution of phthaloyl chloride (7.307 g, 0.036 mol) in THF (30 mL) was added drop wise with stirring. The reaction mixture was stirred for 1 h at (30)(31)(32)(33)(34)(35) o C and then for 3 days at room temperature. The polymer precipitated as a white solid from the THF solution. The solvent was evaporated and the solid was dissolved in chloroform (150mL) and washed with water (2x500 mL), (6% v/v) HCl solution (1 x 150 mL), and finally with distilled water (3 x 500mL). The chloroform solution was dried over anhydrous sodium sulfate, and was then concentrated to about 100 mL of solution. The polymer was precipitated by drop wise addition of chloroform solution to 500.0 mL of methanol. The precipitated polymer was then filtered and dried at 55.0 oC under vacuum to give a white powder 66.0g, (68 % yield)..

Preparation of Stock Solutions
Stock solutions (1000.0 mg/L) of the three metal ions were prepared by dissolving specific amounts of the salts of Pb(II), Zn(II), and Cd(II), in 0.1 M NaClO 4 which was adjusted to the desired pH. The stock solutions were used to prepare solution with different concentrations (20.0, 40.0, 50.0, 60.0, 80.0, 100 and 150.0mg/L). The dilution is achieved by using 0.10 M NaClO 4 and adjusted by 0.10 M HClO 4 to achieve the desired pH= 4.00, 5.00 and 6.00.

Study of Metal Uptake Characteristics of The Polymers By Batch Technique
The metal uptake characteristics for each metal ion were studied using batch equilibrium technique. An aqueous solution of known metal ion concentration (25.0mL) was added to polymer powder (0.10 g), the mixed solutions were mechanically shaken, after a certain period of time at 25.0 o C, 35.0 o C, 45.0 o C, the mixture was filtered and the amount of the metal ion remaining in the filtrate solution was determined by atomic absorption spectrometry after constructing up an analytical calibration curve for each element (Pb(II), Zn(II), and Cd(II)).

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shaken. The contact time was varied from 5 minutes to 48 hours at 25.0 o C, 35 o C, 45 o C. The mixture was filtered and the amount of the metal ion remaining was determined with atomic absorption spectrometry. The amount of metal ion uptake by the polymer (qe), may be obtained from the following relation:

( )
i eq e C C V q m − = q e : Metal ion uptake by the polymer (in mg M(II)/ g polymer).
C i : Initial metal ion concentration (mg/L). And the percentage of metal ion loading by the polymer expressed as % uptake was [13]:

Effect of pH on The Metal-Ion Uptake
Similar experiments were also carried out, under different pH values of 4.00, 5.00 and 6.00 for fixed contact time of 24 hours to determine the effect of pH on the metal ion uptake by the polymer.

Adsorption Isotherms Studies
The adsorption of Pb(II), Zn(II), and Cd(II) was carried out by taking a known mass of 100.0g ± 0.1mg of the polymer swelled with 25.0mL of solutions of concentration variation ranging from (20.

Metal Ion-Uptake By The Polymer Using Column Experiment
Glass column of 30.0 cm length and 1.5cm inner diameter was used in this experiment. The column was packed with 1.00g ± 0.1mg dried polymer. A sample volume of 150.0mL containing Pb(II) of 1000mg/L was passed through the column at a flow rate of 1.0mL/4min. The eluate was collected in a 100.0mL volumetric flask, and concentration of the metal ion was then determined by AAS. The same experimental conditions were used for the determination of Zn(II), and Cd(II) ions uptake, where the sample which passed through the column was 150mg/L of these metal ions.

Desorption studies
The desorption of the Pb(II), Zn(II), and Cd(II), ions was carried under column condition, where the polymer was loaded with each metal ion as described before, using 50.0mL of two eluting agents, 0.10M HNO 3 and 0.10M EDTA were used for polymer recovery from adsorbed metal ion, keeping the flow rate of elution at (1mL/4min). The concentration of metal ion in the eluate was collected in five 10.0mL portions, and was then determined by AAS.

Water regain ( α ):
Water regain is defined as the amount of water absorbed by 1000.0mg ± 0.1 mg of polymer [14]. A sample of dry polymer was suspended in water, and was left for 2 and 24 hours. The polymer was filtered and weighed, dried at 60.0 o C and then re-weighed. Water regain (α) was calculated from the mass difference (eq).
Mass of polymer bound water (g) á Mass of dry polymer (g) =

Polymer synthesis
The synthesis of poly (1,4-benzenedimethylene phthalate), from equimolar amounts of 1,4-benzenedimethanol and phthaloyl chloride was performed by solution polycondensation in THF at 30.0 °C in the presence of excess pyridine as acid scavenger and 4-DMAP as the catalyst. The reaction proceeded by pyridinecatalyzed nucleophilic displacement of the chloride of the phthaloyl chloride with the alcoholic group of 1,4-benzenedimethanol. The relatively high yield of the polymer may have been due to the high reactivity of phthaloyl chloride group. The resulted polymer, which was obtained as a powder was found to be insoluble in many common organic solvents such as tetrahydrofuran (THF), diethyl ether, acetone and methanol but soluble in chloroform.

Solution viscosity
The inherent viscosity of poly(1,4-benzenedimethylene phthalate) solution was calculated from viscosity measurements of dilute polymer solutions (0.5g/dL) in chloroform at 25 °C. The polymer had an inherent viscosity of 0.22dL/g. This value indicates that the polymer had an intermediate inherent viscosity which implies that it had moderate molecular masses. This value is higher than the those obtained at 0-5° for poly(bisphenol-Aphthalate), poly(bisphenol-Asuccinate), and poly(cyclohexanedimethylene phthalate) which had the values of 0.11dL/g , 0.13dL/g, and 0.11dL/g, respectively [15].

Infrared spectroscopy
The polymer was analyzed by FT-IR spectroscopy. The FT-IR spectrum exhibits characteristic absorption bands for the major bonds involved in the polymer. The FTIR spectrum ( Figure 1) showed two strong absorption bands for the stretching vibration of the carbonyl group (C=O) of the phthalate ester group at 1726cm -1 and for C-O-C in the range from 1124 to 1279cm -1 . Another strong IR band was observed at 2951cm -1 assigned to the C-H stretching in the 1,4-benzendimethanol moiety. These wave numbers, which are typical for the ester group are conformed to the reported literature [15], and thus confirm the formation of the postulated polymer Scheme 1 and Figure 1.   The Nuclear magnetic resonance (nmr) spectra for polyesters  H-NMR spectrum: The polymer was analyzed by NMR spectroscopy in order to elucidate its chemical structure and support its formation. In the 1 H-NMR spectrum of poly(1,4benzendimethylene phthalate), the signal of the aromatic protons of the phenylene ring of benzendimethylene unit was observed as singlet at δ = 7.36ppm, whereas that of the aliphatic methylene protons attached to the oxygen of the ester group was observed as singlet at δ = 5.22ppm. The aromatic methylene protons of the phthalate unit in ortho and meta positions to the ester were observed at δ = 7.71ppm and 7.50ppm, respectively. The 1H-NMR spectrum is shown in Figure 2. In the 13 C-NMR spectrum of poly(1,4benzendimethylene phthalate), the signal of the aromatic carbon atoms of the phenylene ring of benzendimethylene unit appeared at δ = 129ppm. The signal of the quaternary aromatic carbon atoms appeared at δ = 136ppm. The signal of the aliphatic methylene carbon atoms attached to the oxygen of the ester group appeared at δ = 67ppm. The signal of the quaternary aromatic carbon atoms of the phthalate unit to which the ester group is attached appeared Arc Org Inorg Chem Sci

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at δ = 132ppm. The signals of the aromatic carbon atoms in ortho and meta positions to the ester group appeared at δ = 129ppm and 133ppm, respectively. The signal of the carbonyl carbon atom of the ester group appeared at δ = 167 ppm. The 13C-NMR spectrum of the polymer is shown in Figure (2), and the1H-NMR and 13 C-NMR spectral data for the polymer assigned to the various proton and carbon atoms are presented in Table 1

Thermal Properties
The thermal properties of polyesters synthesized were also investigated by differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) under dry N 2 atmosphere.

Thermal Transition
The thermal properties of polymer were investigated with DSC and TGA. The T g value of the polymer was 52°C. This value is considered lower than expected for such an aromatic polymer such as poly(ether carbonate) containing aromatic -aromatic ether showed T g values from room temperature up to 47°C [16] and polyquinoxalines and other aromatic polymers were studied T g from 215.5 to 394.5°C [17]. This T g value may have been due to the imparted flexibility effects of the aliphatic methylene groups of the 1,4 -benzenedimethanol. The presence of the aliphatic moieties in the polymer backbone imparted flexibility to polymer segments to move under the effect of temperature. This ease of motion is reflected in the slightly low T g value of the polymer [16]. The DSC thermogram of the polymer is shown in Figure 4.

Thermal stability
The thermal stability of the polymer was investigated by TGA under dry nitrogen. Table 1 summarizes the initial thermal decomposition (onset) temperature T d i , T d 5% ,T d 10% , and T d 50% decomposition temperatures, which correspond to the temperatures at which 1, 5, 10, and 50% loss of mass of polymer occurred, respectively. The table 2 also shows the residual mass percent remaining after heating the polymer to 673.1 °C which was found to be 8.74%. The thermogram of the polymer Figure 5 Arc Org Inorg Chem Sci 6 displayed a typical one-stage characteristic with a relatively fast mass loss occurring at temperatures between 400 and 450 °C. The fast mass losses may have been due to decomposition of the polymer backbone. These values are higher than the corresponding values obtained in the case of poly (1,4-cyclohexandimethlenephthalate), which occurred between 350-450°C and the residual mass equal 0.44% [15].

Morphological Characterization
The surface of the polymer was also characterized by SEM before and after metal sorption. The SEM micrographs for the polymer synthesized are shown in Figure 6. The pores are distributed on the rough surface of the polymer, the small pores which are similar to the forms of flowers are located on the surface of the rods with an average size <5µm. The existence of these pores provides convenient diffusion channels for metal ions into the interior of the polymer when it is used for adsorption of metal ions from aqueous solution. The interior structure of the polymer showed randomly distributed large gaps and air pockets created during polymerization. The spongy structure of the inner rods maximize the contact surface between the polymer and the solution which led to increased metal ion uptake, this is shown in Figure 6a for the polymer before metal sorption. However, after adsorption of metal ion by the polymer, a slight loss occurred to the composition of surface features and of the channel and a small part of its surface became smooth. This is represented for the polymer surface loaded with Cd(II) ions and is shown in Figure 6b & c. It has been found by SEM investigation that loading the polymer with zinc ions changed its surface topography, increased the proportion of smooth surface and resulted in narrow channels and pores Figure 6d & 6e. The highest uptake of metal ions by polymer was for Pb(II), the polymer loaded with lead ions was studied by SEM to observe the changes in surface topography that took place, the entire surface became smooth and the pores disappeared due to full metal ion coverage. SEM for our polymer was similar to that of poly(cychlohexandimethelen succinate) (Al-Dweri, 2010). The images are presented in Figure 6f  8

Water Regain or Water Content ( α )
Water regain experiment was performed to determine the water regain ratio (α) for the polymer. Water regain is usually correlated with the hydrophilic character of the polymer, the higher the water regains, and the more hydrophilic the polymer is. The water regain value for the polymer was found to be 0.019 g/g after 2h of stirring and 0.029g/g after 24h stirring, this indicates that the polymer has a low hydrophilic character. These values are higher than the corresponding values obtained by poly(1,3-cyclohexylene oxalate) polymer which was found to have values of 0.011g/g and 0.014g/g, respectively. Based on these values the polymer is considered to be low hydrophilic in nature [18] and these values are smaller than the corresponding values obtained for poly (1,4-cyclohexanedimethylene oxalate) polymer which was found to have values of 0.064g/g and 0.087g/g [19]. Principally, the water molecules are polar and would interact with the polar groups of the polymer (the ester group), this interaction explains the water regain properties of the polymer.

The rate of metal ion uptake by the Polymer
The adsorption kinetics of metal ions on the surface of the polymer was investigated as shown in Figure 7 for example. The adsorption of metal ions increase with time until complete saturation.  Table 3 and shown in Figure 8 & (Table 2) (20-23).  In the Langmuir model, the values of correlation regression coefficient (R 2 ) are greater than 0.90 and had excellent linearity. This indicates that homogenous sites of interaction are better to describe the process and the maximum sorption capacities (qm) deduced from these results indicated that the polymers has the highest capacity towards Pb(II) ions, but it shows a lower capacity towards Cd(II). It is observed that the adsorption capacity  The trend in q m values in poly(1,4-cyclohexanedimethylen phthalate) at pH= 6.00 and 25.0 o C is Pb(II) > Cd(II), in (mg/g) were 53.5>17.7 respectively [15]. The trend in qm values for poly(1,4cyclohexanedimethylene oxalate) at pH= 4.00 and 25.0 oC is Pb(II) > Cd(II) > Zn(II), in (mg/g) were 31.2 > 29.8 > 15.9, respectively [19]. The trend for Pb(II) and Cd(II) ions in poly(hydroquinone oxalate), and in poly(neopentyl oxalate) polymer, were as follows: Pb(II)>Cd(II) [24][25][26][27] and thus the results similar in order the metal ion with our results in the literature [23] that is q m values 207.7 >30.7 >19.1 for Pb(II) >Zn(II) >Cd(II) in poly(bisphenol A oxalate).
The results indicate that our polymer has reasonable q m values. In the Freundlich model, both K F and nare Freundlich constants, being indicative of the adsorption capacity and the adsorption intensity respectively. High value of n between (1.4-3.6) indicates that adsorption is good over the entire range of concentration studied, while small values of n means that the adsorption is good at high concentrations but much less at lower concentrations and the values of n were greater than one indicating that the adsorption was favorable. A greater value of KF indicates a higher capacity for the adsorption than smaller values [28].   As illustrated in Tables 3-5, the values of E for Pb (II) are (0.110-0.600 kJ/mol), for Cd(II) are (0.080 -0.200 kJ/mol) and for Zn(II) are (0.180-0.310kJ/mol). All values are less than 8.00kJ/mol, this indicates that physical forces affect adsorption. The result of the concentration variation isotherms of polymer (Table 5) and the plots of Dubinin-Radushkevich at pH= 4, 5 and 6 at 25 °C, 35 °C, 45 °C are shown in Figure 9.

The Effect of Temperature on The Uptake
The effect of varying temperature on the % uptake of metal ions was also investigated. The results are obtained by plotting % uptake at different pH against temperature and are presented in Figure 10.
The results obtained showed that the adsorption process of Pb(II) , Zn(II) and Cd(II) onto the surface of the polymer is an endothermic process since the % uptake increases as the temperature increases nearly at all tested pH values.    Arrhenius equation can be used to determine the activation energy and pre-exponential factor for a reaction were calculated from Figures 11 and shown in Table 6. The results A and Ea values that obtained in Figure 11 and Table 6 are calculated from Arrhenius equation: Table 7) Where k is the rate constant, Ea activation energy, R gas constant and A pre-exponential factor. The values of A and Ea can be calculated from intercept and slope of a straight line of a plot of Ln k against 1/ T. The pre-exponential factor A is the constant of proportionality between the concentration of the reactants and the rate at which they collide. The activation energy Ea is the minimum kinetic energy required for a collision to result in reaction, through the more favorable molecular orientations. The factor exp (−Ea / RT) represents the fraction of molecular collisions that have an energy value equal to or greater than the activation energy Ea. At higher temperatures a larger portion of reactant molecules will have the required Ea to react. Thus, the reaction rates depend on Ea, the reactant orientations (relative positions) during collisions and the temperature. Both A and Ea are approximately constant over a moderate range of temperature (50K) [29]. The order of Ea values for Pb, Zn and Cd is (19.2, 32.1, 83.9kJ/mol) respectively. This low value of Ea indicates a reaction rate slightly sensitive with temperature [30]. So, we cannot explain the mechanism of the process from activation energy only but needed to use Eyring equation [31].

The Eyring equation (activated complex theory)
Determination of activation energy, entropy and enthalpy of activation by this equation  Table 8.    The Eyring-Polanyi equation has been applied to rate processes and the calculation of values of enthalpies and entropies of activation without pointing out the significance of the obtained values and the difference between the activation energy values found by using the Arrhenius equation.
Ln k /T = -∆H ǂ / R T + Ln KB /h + ∆S ǂ / R……… (6) A plot of (Ln k/T) versus 1/T gives a straight line with a slope of -∆H ǂ /R from which the enthalpy of activation can be derived and with intercept of ln (kB/h) +∆S ǂ /R from which the entropy of activation is derived. From the values of the free energy of activation the real energy requirements are known, thus, suggesting the use of the Eyring-Polanyi equation mainly as a tool for gaining a deeper understanding of the actual processes at work and not only as a tool for predicting reaction rates based on measured rate constants. This relation is usually used for the suggestion of a mechanism for a certain reaction in the following way: the certain reaction is performed at various temperatures where the reaction rate constant is measured. The pre-exponential factor A of Arrhenius equation has been related to ΔS ǂ of Eyring equation.
Low values of lnA correspond to negative values of ΔS ǂ , the activated complex in the transition state has a more organized, more ordered and more rigid structure than the reactants. This happens when bonds are formed or substances are absorbed, and high values of lnA correspond to positive (or less negative) values of ΔS ǂ , a positive value for the entropy of activation indicates that the transition state is disordered (less organized), compared to the state of the reactants. This happens when bonds are broken or substances are desorbed. The calculated value of the entropy of activation is used for the suggestion of a mechanism i.e. in replacement reactions: Associative (ΔS ǂ < 0), Dissociative (ΔS ǂ > 0), Interchange (ΔS ǂ = 0). [32] In our results in Table 8, the values of ΔS ǂ for Pb(II), Zn(II) and Cd(II) are -188.7, -164.7, -21.3 J/mol.K, respectively all the values of ΔS ǂ< 0 indicated that the process has Associative mechanism. The relation between Ea of the Arrhenius and ΔΗ ǂ of the Eyring equation (activated complex theory) is: There was no comparison between Ea values and the ΔG ǂ values that lead to the conclusion that Ea does not represent the full energetics of a process. Thus ∆G ǂ is the critical factor and not Ea. Thus, near room temperature (the thermodynamic temperature, 25 o C), Ea is roughly 2.5 kJ mol -1 larger than ∆H ǂ. [32] Through Table  (4.68) can be calculated divide between ∆H ǂ ǂ and Ea for Pb(II), Zn(II) and Cd(II) that equal (2.6, 2.6, 2.5 kJ mol -1 ) respectively, there results indicated of the temperature independence on reaction rate and the temperature influences is room temperature 25 o C [32]. For, so, we needed calculate the free energies of activation ΔG ǂ . It has been found that ΔG ǂ gives a more realistic/true value of the "activation the processes need in order to take place and not Ea or ΔS ǂ alone. The free energy of activation ΔG ǂ includes not only the ΔH ǂ component (= Ea−RT) but also the ΔS ǂ component that may be important. The term −TΔS ǂ that has to be added to ΔH ǂ in order to give ΔG ǂ which may be critical, Determines the spontaneity of the reaction ΔG ǂ ΔG ǂ greater than zero reaction is spontaneous ΔG ǂ 0 = system at equilibrium, no net change occurs ΔG ǂ less than zero reaction is not spontaneous [33]. The results of ΔG ǂ in Table 8 were explaining that all the values of ΔG ǂ > 0. So, this process is spontaneous (physical process).

Distribution Coefficient (Kd)
The distribution coefficient is defined as the final concentration of metal ion in the sorbed form on polymer divided by its final concentration in solution. It is regarded a standard parameter in the assessment of the physicochemical behavior of metal ions between solid and liquid phases. It is calculated by the following equation. Kd = qe / Ce = KL qm-KL qe …………….... (9)  14 Where K d is the distribution coefficient (L/g). Thus, a plot of (qe / Ce) against (qe) should be a straight line with slope= -K and an intercept= q m K if Langmuir equation is applicable The distribution coefficients (K d ) is calculated for the polymer at different pH values (4.0, 5.0, and 6.0) and temperatures (25°C, 35°C, and 45°C) are given in Table 9.

Thermodynamics of Adsorption on the Polymer
In order to understand the possible adsorption mechanism involved in the removal process, thermodynamic functions for the system, including changes in Gibbs free energy (ΔG°), change in enthalpy of adsorption (ΔH°) and changes in entropy of adsorption (ΔS°), were calculated using the following equation Using the following equation: Where K d is the equilibrium constant, R is the gas constant and T is the temperature in Kelvin.    The results of the studies on the influence of temperature on metal ions adsorption are presented in Table 10 above. The positive values of enthalpy indicate that the adsorption removal increased with increase of the solution temperature. This shows that the adsorption process is an endothermic one. ΔG°, ΔH°, and ΔS° are the thermodynamic functions related to the experiment conditions. Spontaneity and favorability of the adsorption process is established by decrease in Gibbs free energy values, ΔG°. The value of ΔG° between (0.16 -1.04) indicates the degree of favorability of the adsorption process, so the values of ΔG presented in Table 10 indicate that the adsorption of Pb(II), Zn(II) and Cd(II) is a favorable process [28]. All the values of ΔG° are very small and positive which suggests that the adsorption of metal ions onto polymer require some small amount of energy to convert reactants into products [34]. This is agreeing with values of Table 10 which represent to the degree of favorability of adsorption, the decrease of ΔG° values of Pb(II) >Zn(II) >Cd(II). As shown in Table 10 all ΔH values are positive this suggests the endothermic nature of metal adsorptions. One possible explanation of this is the well-known fact that heavy metal ions used are well solvated in water. In order for these ions to be adsorbed, they are denuded to some extent of the hydration sheath. This dehydration process of ions requires energy for removal of water from ions is essentially an endothermic process [35]. We Arc Org Inorg Chem Sci 15 assume that the energy of dehydration exceeds the exothermicity of the ions attaching to the surface [36]. The implicit assumption here is that after adsorption the environment of the metal ions is less aqueous than it was in the solution state.
The endothermic interactions between polymer surface and metal ions were accompanied by small positive values of entropy, which was the driving force for adsorption. The positive values of ΔS signify an increased state of randomness at the solidsolution interface following adsorption. Also the positive entropy of adsorption reflects the affinity of adsorbent for metal ions used. The adsorbed water molecules, which are displaced by the adsorbate species, gain more translational energy than is lost by the adsorbate ions, thus allowing the prevalence of randomness in the system [28]. The entropy changes were most likely to be due to structural changes and adjustments in the adsorbate as well as the adsorbent. The structural changes arise from the release of ions like H+ from the polymer surface into the solution and also from partial solvation of the metal ions in water. The adsorptions of Pb(II), Cd(II) and Zn(II) on polymer were associated with entropy decrease in conformity with the general situation of ions existing in a more chaotic random distribution in aqueous solution compared to their adsorbed and immobilized states [37].

Metal ion uptake by the polymer
The metal ion uptake by the polymer using column experiment for Pb(II), Zn(II) and Cd(II) was determined at pH 6.0 and 25.0 o C, initial concentration of 150.0 mg/L and a flow rate 1 mL/4min. The uptake for metal ions is represented in Table 11. It can be seen that the uptake capacities of the polymer with the metal ions fall in the order; This result is similar to the order of the metal ions in the batch experiment. However, the values of percent uptake for the metal ions in column experiment are lower than those obtained in batch experiments, because in order to achieve the complete saturation a much greater time is needed. On the other hand, there is no mechanical shaking associated with the column experiments, which result a decrease in percent of metal uptake.

Desorption Studies
Two eluting agents, 0.10 M HNO 3 and 0.10 M EDTA were used for removal of metal ions, keeping the flow rate of elution 1 mL/4 min. The fluent was collected in five portions, 10.0mL for each portion; the results are expressed as percent recovery and represented in Table 12. The eluting agents react in two different ways: HNO 3 act as proton-exchange agent and the second a complex-forming agent as EDTA. Depending on the values of the % accumulative recovery, in Table 12, the following trend was observed for the eluting agents of metal ions from the polymer: 0.1 M HNO 3 > 0.1 M EDTA The experiments confirmed that maximum metal desorption can be achieved with mineral acids in concentrations of 0.1M solutions. This could be attributed to cation exchange between the proton and metal sorbed. However, this method is more complex than protonation [8,38].

Conclusion
In this study, we prepared a polymer containing phthalate function group and capable of adsorbing the metal ion by solution polycondensation and characterization of poly(1,4benzenedimethelene Phthalate). The structure and properties Arc Org Inorg Chem Sci 16 of polymer was confirmed by FT-IR, 1 H NMR, 13 C NMR, elemental analysis, SEM and thermal analysis. The sorption properties of the synthesized polymer toward Pb(II), Zn(II), and Cd(II) in aqueous solutions were examined under various experimental conditions using both batch and column experiments. The effective desorption for the metal ions was studied, and the coefficient of recovery of sorption ability was also investigated. The polymer has high sorption rate for Pb(II) observed during the first 24h with high percentage of uptakes toward Pb(II), Zn(II) and low percentage of uptakes toward Cd(II) ions. The influence of different pH on metals uptake showed that the metal-ion uptake by the polymer increased with increasing pH and reached a maximum at pH=6 for Pb(II), Zn(II), and Cd(II). The best conditions for adsorption of metal ions and maximum adsorption capacity (q m ) on polymer surface are pH=6, T= 45°C and initial metal concentration of 150 ppm.
The obtained adsorption data showed fitting for Langmuir, Freundlich and Dubinin-Radushkevich adsorption isotherm models. The application of the Eyring equation to literature data i.e. the calculation of ΔH ǂ , ΔS ǂ and ΔG ǂ , has pointed out that in geochemical transformations it is necessary to calculate the entropy of activation along with the enthalpy of activation in order to fully characterize a process energetically. A column packed with the polymer has good metal uptake properties toward all metal ions, and followed the order: Pb(II) > Zn(II)> Cd(II) at pH 4.0 and 25 o C and flow rate 1 mL/4min. The efficiency of recovery of metal ions after adsorption can be carried out by treatment of the loaded polymer with 0.1M HNO 3 and 0.1M EDTA with good percent recovery.