Adsorption of Metal Ions on Novel 3n-Propyl ( Methylpyridinium ) Silsesquioxane Chloride Polymers Surface . Study of Heterogeneous Equilibrium at the Solid-Solution Interface

Os polímeros Si3PyCl (A) e Si4PyCl (B), cloretos de 3-n-propil(3-metilpiridínio) silsesquioxano e 3-n-propil(4-metilpiridínio)silsesquioxano respectivamente, foram preparados pelo método de síntese sol-gel. Os valores de capacidade de troca iônica (mmol g) foram iguais a 0,90 e 0,81 para (A) e (B), respectivamente. Considerando a reação nSiPyCl (s) + M (sln) + 2Cl (sln) (SiPy) n MCl 2+n n(s) , as constantes de equilíbrio calculadas para cada etapa foram: (A): Cu, logK (2) = 4,14(0,03), logK (2) = 2,91(0,04); Zn, logK (2) = 3,3(0,3), logK (2) = 4,9(0,3); (B): Cu, logK (2) = 3,82(0,03), logK (2) = 3,00(0,04); Zn, logK (2) = 3,93(0,03), logK (2) = 3,55(0,08), onde K (2) 1 e K (2) 2


Introduction
In recent years the use of solids, modified or not with functional groups, presenting affinities towards several metal ions in solution phase has been reported.A large variety of materials such as zeolite, alumina and silica have been commonly used in metal adsorption processes.9][20] However, as these methods are time consuming procedures, in the last decade a new class of organofunctionalized materials has been obtained by using the sol-gel processing method.2][23][24][25] Prepared hybrid xerogels, in almost all solids obtained, present homogeneous distribution of the organic groups throughout the matrix volume.][28][29] Despite the several works on the use of these modified solids reported in the literature, a better understanding of the adsorption mechanism on the surface of these solids is necessary.Normally, the model used has been the ideal process where interactions between the adsorbed species on the surfaces are neglected. 30,313][34] Methods based on this model of non ideal character have enabled the determination of stoichiometric composition and affinity constants of the species in equilibrium at the solid-solution interface. 35he present work describes two new hybrid materials obtained by sol-gel synthesis, denominated as 3-n-propyl-(3-methylpyridinium) silsesquioxane chloride (Si3Py + Cl -) and 3-n-propyl-(4-methylpyridinium) silsesquioxane chloride (Si4Py + Cl -), aiming at their use as adsorbent materials.They were applied in the adsorption processes of Cu II and Zn II from ethanol solution.The mechanism of interaction between the adsorbed species on the solid surface is discussed in detail.

Experimental
Preparation of Si3Py + Cl -and Si4Py + Cl - The materials were prepared according to a previously described method. 36The idealized structure of the ion exchanger polymers Si3Py + Cl -and Si4Py + Cl -is shown in Figure 1.
Briefly, tetraethylorthosilicate (TEOS), ethanol and aqueous HCl solution were mixed in a round bottomed flask, and the resulting solution was stirred for 2.5 h at room temperature (298 K, step (a) in Figure 1).To this solution 3-n-chloropropyltrimethoxysilane, CPTS, was added, and the solution stirred for 2 h at room temperature (step (b)).The temperature of the solution was raised to 328 K and the mixture was allowed to stand for 60 h open to the ambient atmosphere until the gelation process occurred.The resulting gel was powdered, washed with ethanol and then dried under vacuum (133 x 10 -3 Pa) at room temperature.The dry gel was immersed in a round bottomed flask containing a solution prepared by mixing pure 3-methylpyridine or 4-methylpyridine and dry toluene (step (c)).Each mixture was heated at the reflux temperature of the solvent for approximately 3 h.The solids were filtered, washed with ethanol and dried for 2 h under vacuum (133 x 10 -3 Pa) at room temperature.

Chemical analyses
The elemental analyses were carried out on a Perkin-Elmer model 2400 CHN analyzer.Triplicate measurements were made for each sample.
To determine the concentration of exchangeable chloride ions, approximately 0.050 g of material was immersed into 30 mL of 0.1 mol L -1 HNO 3 aqueous solution.The chloride ions released to the solution phase were titrated with standard 0.01 mol L -1 AgNO 3 solution, using the potentiometric titration method.

Physical measurements
The CP-MAS 13 C NMR spectra of the samples were obtained on a Bruker AC 300P spectrometer.A pulse sequence with contact time of 1ms, interval between pulses of 2 s and acquisition time of 133 ms was used.
The thermogravimetric analyses were made on a Dupont TGA 2050 apparatus.About 10 mg of each material was heated with a heating rate of 10 K min -1 in the temperature range between 298 and 1243 K, under nitrogen atmosphere.

MCl 2 adsorption isotherms
The adsorption of CuCl 2 and ZnCl 2 from ethanol solutions by Si3Py + Cl -and Si4Py + Cl -was studied at 298 K by the batch technique.Solutions (50 mL) containing variable concentrations of the metal and about 50 mg of the adsorbent solid were shaken for 3 h at 298 K.The amount of metal ion adsorbed by the solid phase was calculated by applying the equation: where N i is the initial mole number of metal added, N s is the mole number of the metal in the solution phase in equilibrium with the solid phase, and m is the mass of the solid phase.N s was determined by complexometric titration using 0.01 mol L -1 standard edta solution.

Characteristics of the material
The results of elemental analyses and determination of exchangeable chloride ions in SiPy + Cl -are summarized in Table 1.
The results for N and Cl -contents indicate that the materials were prepared with good yield (practically 100%), i.e., in step (c) of Figure 1, practically all metylpyridine reacted with the chloropropyl pendant groups.The molar ratios N/Cl -obtained from experimental data are 1.07 and 0.99 for Si3Py + Cl -and Si4Py + Cl -, respectively.
The 13 C NMR spectral data for Si3Py + Cl -(a) and for Si4Py + Cl -(b) are summarized on Table 2.][39][40][41][42] Infrared spectra of both samples showed two medium intensity absorption bands at 1635 and 1511 cm -1 (Si3Py + Cl -) and at 1636 and 1447 cm -1 (Si4Py + Cl -), assigned respectively to the ring coupled stretching vibration (νCC + νCN) and the deformation mode of the methyl group (δCH 3 ) of the pyridinium group attached to the silsesquioxane framework (IR spectra are not shown). 43he results of thermogravimetric analyses (Figure 2) show that the materials are stable up to 522 K, decomposing above this temperature with loss of methyl pyridine molecules.
The adsorption process occurs according to the equilibrium reaction described by equation (1).The metal  ion diffuses into the solid-solution interface followed by the counterion Cl -, forming the MCl 2+n n-complex on the surface, in order to keep the electroneutrality: (1)   where SiPy + Cl -represents the 3-n-propyl(3-methylpyridinium) or 3-n-propyl(4-methylpyridinium) chloride pendant functional groups attached to the silica framework, n is number of the pendant groups in the reaction site and (s) and (sln) are the solid and solution phases, respectively.If n > 1, the pendant functional groups act as a set of spatially oriented polydentate cationic ligands.The dissociation and autocomplexation of MCl 2 in solution phase can be neglected, since they do not distort the main conclusions about the adsorption processes.To describe quantitatively the adsorption equilibrium, one has to estimate the adsorption capacity of the material, t Q (in mol g -1 ), in respect to MCl 2 and constants of adsorption equilibria.The concentration of active binding groups ("effective capacity") may differ significantly from the experimental capacity found by elemental analyses of N f or from the ion exchange capacities determined by the ionized chloride.It depends on the stoichiometry of interactions of MCl 2 with the active surface groups SiPy + Cl -, accessibility to sorption centers, their affinity by metal chlorides, nature of solvent and other factors.Therefore, this quantity should be determined together with the composition of surface complexes and their thermodynamic stability.All these tasks are solved by means of application of meaningful models for describing experimental adsorption isotherms.
In the simplest case, adsorption of metal chlorides may be described as a reaction between a sorbate entity S and an active sorption center: (2)   where S is MCl 2(sln) , -Q is the fixed active center (SiPy + Cl - ) is the surface complex, and β is the heterogeneous global or overall stability constant.When complexes -SQ of only one type are formed, all sorption centers -Q are energetically homogeneous and lateral interactions are absent, the sorption is of ideal character and can be described by the Langmuir equation in the linear form, represented by equation 3: (3) where [ -SQ] is the specific concentration of adsorbed species S (in mol g -1 ), [S] is the equilibrium concentration (mol L -1 ), D -1 is the reciprocal of the distribution coefficient (in g L -1 ).
If the plot of D -1 versus [S] is linear, the model of ideal sorption is considered to be valid. 45The values of unknown parameters of the model, t Q and β, are calculated from the coefficients of a linear D -1 versus [S] dependence.
Unfortunately, this simple model did not fit properly the experimental adsorption isotherms of MCl 2 .At the lower CuCl 2 and ZnCl 2 concentrations, deviation of the experimental D -1 values from the straight lines is observed (Figure 4), pointing to the non-ideal character of the adsorption process.
This non-ideal character of adsorption was interpreted in terms of the cooperativity effect, which will be discussed in detail at the end of this section.The affinity of the MCl 2+n nfor the cationic pendant groups may be enhanced (positive cooperativity) or depressed (negative cooperativity) by a previous occupation of the site by a metal complex. 46The description of the non-ideality in terms of cooperativity effects was performed with the aid of the model of polidentate binding. 47According to it, the surface of a sorbent is considered as an assemblage of independent centers containing several SiPy + cations and the respective number of Cl -ions as counterions.The simplest version, the model of bidentate binding, represents the adsorption process by the following equilibrium reactions: (4)     where M = Cu II or Zn II , β 1 (2) and β 2 (2) are the global or overall stability constants: The relation between , where K (2)  i are the stepwise stability constants.
2 is defined as: In the equations, the subscript corresponds to the number of functional groups −Py + Cl -included into one sorption center (in the present case, two sorption sites in center) and the superscript is the order of constant, i.e., the number of MCl 2 entities attached to a sorption center.In equation (1) we observe that M 2+ and Cl - dissociated species are adsorbed as neutral species MCl 2 , since in the diffusion process of M II into the solid-solution interface it is followed by the counterions.
The occupation of all sorption centers with MCl 2 was not achieved in any case even at the highest MCl 2 concentration studied.Because of this, the maximum reached N f values gave only rough estimations of the adsorption capacity, t Q .Therefore, it was necessary to estimate simultaneously all fitting parameters of the model, i.e., t Q , β (2) 1 and β (2) 2 .The possible values of sorption capacities t Q were scanned, and for each tested t Q value, the values of fitting parameters β (2) 1 and β (2) 2 were calculated through minimization of the criterion: (9)   where k is the k th experimental points, ∆ = N f calc -N f exp , and w k is the statistical weight assigned as (10)   with σ r = 0.05 (5%) as the relative random error in the N f determination.The global criterion χ 2 was applied to test the statistical adequacy of the models.The models were assumed to be adequate if the following inequality was fulfilled: where χ 2 (5%) is the 5% point of the chi-squared distribution with f degrees of freedom (f = N-Z, where Z is the number of unknown equilibrium constants and N is the total number of experimental points).The t Q value (and the corresponding values of equilibria constants), which resulted in the minimum value of χ 2 exp , was accepted as the most trustworthy estimations that provided the best fit of the measured sorption isotherm.All calculations for the model of bidentate binding were performed with the aid of the CLINP 2.1 software program. 48,49The results of simulations are summarized in Table 3.The calculated isotherms (dotted lines) are shown in Figure 3.
In order to have a better understanding of the adsorption mechanism, and taking into account that all stepwise constants were calculated, the distribution coefficients, α i , were determined for S 1 , S 2 and S 3 species by applying the equation: (12)   where (13)   The dependencies of the distribution coefficients, α i, , of the surface complex species, MCl 4 2-(S 2 ) and MCl 3 -(S 3 ), and the corresponding concentration of the free pendant center SiPy + Cl -(S 1 ) are shown in Figure 5.
The calculated values of α i for CuCl 2 adsorption on Si3Py + Cl -and Si4Py + Cl -(Figure 4a and 4c) and for ZnCl 2 adsorption on Si4Py + Cl -(Figure 4d) indicate that both species MCl 4 2-and MCl 3 -are adsorbed on the adsorbents surface over the whole concentration range of MCl 2 in solution.The correlation between stepwise equilibrium constants for these three cases (Table 3 , showing that the negative cooperativity effect is operating.In this case, the formation of MCl 4 2-species (S 2 ) on the adsorbents surface reduces the affinity of the surface by MCl 2 , and consequently the formation of MCl 3 -species (S 3 ), according to the reaction described in equation ( 4),  is depressed.For adsorption of ZnCl 2 on Si3Py + Cl -(Figure 4b), Table 3 shows that K 2 (2) > K 1 (2) , indicating a positive cooperativity effect, i.e., adsorption and formation of MCl 3 is enhanced in the presence of MCl 4 2-in the adsorption site.ZnCl 3 -species is formed predominantly over the whole concentration range of ZnCl 2 in the solution phase.
Adsorption of ZnCl 2 and CuCl 2 from ethanol solutions on the sorbent materials occurred considering that each center of adsorption was formed by two neighboring SiPy + Cl -functional groups spatially orientated as a bidentate ligand.Considering the ability of copper and zinc chlorides in forming MCl 2+n n-complexes, 46 the values of t Q and the two stepwise equilibrium constants were calculated for MCl 2 adsorption on both Si3Py + Cl -and Si4Py + Cl -solid surfaces.The simulations demonstrated that, for systems where the negative cooperativity effects are observed, both MCl 3 -and MCl 4 2-species can coexist on the surface, except for ZnCl 2 adsorption on Si3Py + Cl -, where positive cooperativity is observed.
Taking into account the high affinity shown by both materials, Si3Py + Cl -and Si4Py + Cl -, to Cu II and Zn II chlorides, the novel silsesquioxane polymers modified with 3-and 4-methylpyridinium chloride are potentially useful in adsorption processes of these metals and extensible for other similar metal halides.

Figure 1 .
Figure 1.Fluxogram of the preparation of SiPy + Cl -by the sol-gel method.

Table 1 .
Results of elemental analyses (C,H,N) and determination of ionized chloride (Cl -)

Table 3 .
Calculated values of the adsorption capacity, t Q , global stability constants, β