Equilibrium Treatment for Highly Selective Sulfonated Microcapsules

Article pubs.acs.org/IECR

Equilibrium Treatment for Highly Selective Sulfonated Microcapsules Containing Di(2-ethylhexyl)phosphoric Acid Á ngela Alcázar, Cristina Gutiérrez, Antonio de Lucas, Manuel Carmona, and Juan F. Rodríguez* Department of Chemical Engineering, Institute of Chemical and Environmental Technology, University of CastillaLa Mancha, Avenida de Camilo José Cela s/n, 13071 Ciudad Real, Spain ABSTRACT: Microcapsules containing di(2-ethylhexyl)phosphoric acid (DEHPA) within a sulfonated poly(styrene-codivinylbenzene) shell (SMC-DEHPA), were characterized by different analytical techniques including the stepwise isothermal analysis and FT-IR, which indicated the presence of DEHPA, sulfonic groups, and sulfone cross-linking in the material. The separation of copper from aqueous solutions was investigated by equilibrium studies. According to the ideal mass action law model, the useful capacity of this material was 1.782 ± 0.013 equiv kg−1, where DEHPA represents half of this value (in agreement with its content). On the other hand, nonsulfonated microcapsules or those without DEHPA presented lower useful capacities. Furthermore, sulfonated material exhibited an enhancement of the selectivity for copper, as confirmed by the equilibrium constants (Ksulfonated > 104Knonsulfonated). Thus, the incorporation of sulfonic groups into the polymeric shell favors the mobility of the counterions in the microcapsule, making available the active centers of the extractant.

1. INTRODUCTION Industrial activity constitutes the main responsibility for the discharge of dangerous substances into the aquatic environment such as heavy metals, which are bioaccumulated in living organisms and can even affect human health.1 Thus, the search for new environmentally friendly technologies capable of removing these pollutants from water is a matter of great concern. Complexing microcapsules, which consist of the encapsulation of an extractant agent within a polymeric shell, are shown as a promising technology to achieve the removal of metals. This kind of material is proposed as an excellent alternative to the traditional major-industrial-scale techniques for separation and recovery of metals, solvent extraction and ion exchange,2−5 as it combines the advantages of both methods. Complexing microcapsules are highly selective, present a large interfacial area, can be easily separated from water media, and minimize the use of organic solvents.6−9 Consequently, they overcome the limitations of not only traditional methods but also those shown by other separation systems, such as solvent impregnated resins or supported liquid membranes. The encapsulation of extractant agents of different natures has been carried out, fundamentally within poly(styrene) [P(St)], poly(divinylbenzene) [P(DVB)], poly(styrene-codivinylbenzene) [P(St−DVB)] and poly(styrene-co-ethylene glycol dimethacrylate) [P(St−EGDMA)] shells.3−6,10,11 For microcapsules containing cationic extractants such as the acid organophosphorus compounds di(2-ethylhexyl)phosphoric acid (DEHPA) and 2-ethylhexylphosphonic acid mono-2ethylhexyl ester (EHPNA), high selectivities for the metallic ions similar to those presented by the pure extractant agents were obtained. Nevertheless, compared with conventional organic ion exchangers, these materials show relatively slow kinetics (especially at low metal concentrations) due to the low permeability of the hydrophobic membrane to ions. This restriction could be solved by the incorporation of active sulfonic sites in the polymeric shell. The negative charge of the © 2016 American Chemical Society

sulfonate groups will improve the mass transfer of the metal cations from the external liquid phase to the core (the selective part of the microcapsule), since they transform the hydrophobic cover into one with hydrophilic character. In this way, our research group reported in a previous work12 the synthesis of DEHPA microcapsules with a functionalized P(St−DVB) shell, containing 15.02 wt % DEHPA and a particle size of ∼500 μm. It was found that sulfonation by means of H2SO4 during 20 min was enough to provide them the common characteristics of an ion exchanger bead. The quantification of the encapsulated extractant agent was carried out by conventional ramp analysis. Nevertheless, due to the unsuccessful results obtained for microcapsules having sulfonated shells,12 a different analytical technique for the proper characterization should be developed. The synthesis and behavior of polysulfone microcapsules which immobilized DEHPA and Cyanex 923 has been described in the literature.13,14 These materials attained equilibrium in less than 30 min, while nonsulfonated ones reached this condition after 200 min. Thus, the aim of this work is to apply different analytical techniques for the characterization of this kind of microcapsule, including a stepwise isothermal technique, not previously described for the quantification of encapsulated materials. Additionally, the equilibrium behavior of DEHPA sulfonated microcapsules for the binary system H+/Cu2+ was studied by means of the ideal mass action law model (IMAL), which has not been previously reported in the literature for complexing microcapsules. Received: Revised: Accepted: Published: 1033

October 15, 2015 January 5, 2016 January 9, 2016 January 10, 2016 DOI: 10.1021/acs.iecr.5b03871 Ind. Eng. Chem. Res. 2016, 55, 1033−1042

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Industrial & Engineering Chemistry Research

Figure 1. (a) Schematic diagram of the experimental procedure for equilibrium studies. (b) Profiles of pH and conductivity for the first five equilibrium steps using the blank solution.

2. EXPERIMENTAL SECTION 2.1. Materials. Copper(II) nitrate trihydrate (99 wt %, Panreac Chemical Co.) was used to prepare the mother solution (0.55 N Cu2+, pH 3.6). Water, with a conductivity value of 1 μS cm−1, was obtained by distillation and subsequent deionization using ion exchange resins. In this study three different materials were used. Microcapsules from P(St−DVB) containing 34.86 wt % DEHPA (97 wt %, Aldrich Chemical Co.), denoted as MC-DEHPA, were synthesized following the procedure described by Alcázar et al.12 This material presented an average particle size in number of 475.17 μm. Sulfonated microcapsules (SMC-DEHPA) were obtained by sulfonation of MC-DEHPA using H2SO4 (96 wt %, Sigma-Aldrich Chemical Co.) according to the method described elsewhere.12 SMC-DEHPA showed a swelling degree of 25.81 wt % and sulfur content of 39.42 g kg−1. Finally, sulfonated microcapsules without DEHPA (SMCNonDEHPA) were obtained after extraction from SMCDEHPA using acetone as solvent. 2.2. Microcapsule Characterization. 2.2.1. Environmental Scan Electron Microscopy (ESEM). The morphology and the surface features of sulfonated and nonsulfonated microcapsules were observed by using a Quanta 250 (FEI Co.) with a tungsten filament operating at a working potential of 12.5 kV. A gaseous secondary electron detector (GSED) was employed. The electron beam resolution at this vacuum mode is 2.0−3.0 nm at 30 kV. 2.2.2. Thermogravimetric Analysis (TGA). The thermal stability and DEHPA content of the synthesized microcapsules were obtained by means of a TA Instruments SDT Q600 simultaneous differential scanning calorimetry (DSC)−TGA. A

stepwise isothermal technique was used from room temperature to 973.15 K at a heating rate of 283.15 K min−1 under a nitrogen atmosphere. In the stepwise isothermal technique, the material is heated at a constant rate until a specified rate of weight change is detected and then held isothermally until the weight change is completed. This heating−isothermal process is repeated up to the final temperature. This equipment presents a calorimetric accuracy/precision of ±2% (based on metal standards) and a balance sensitivity of 0.1 μg. 2.2.3. Particle Size and Particle Size Distribution. The average particle size of the microcapsules in number (dpn0.5) and volume (dpv0.5) were determined by low angle laser light scattering (LALLS) using a Mastersizer Hydro 2000 equipped with a Scirocco 2000 unit that works with air as dispersing agent. Each sample was analyzed at least three times to ensure reliable measurements with a precision of 1%. 2.2.4. Functional Group Determination. Fourier transform infrared (FT-IR) spectroscopy was utilized in order to identify the characteristic functional groups of sulfonated (SMCDEHPA) and nonsulfonated microcapsules (MC-DEHPA). Spectra were obtained on a Varian 640-IR FT-IR spectrometer equipped with an attenuated total reflectance (ATR) accessory (with a wavelength accuracy and precision of >0.01 cm and >0.005 cm, respectively). This accessory contains a pressure mechanism for good sample-to-crystal contact. All infrared spectra were collected using 16 scans and 8 cm−1 resolution from 650 to 2000 cm−1. 2.2.5. Transmission Electron Microscopy. Transmission electron microscopy (TEM) analyses employed a JEOL JEM4000EX unit with an accelerating voltage of 400 kV. 2.3. Application: Equilibrium Studies. Equilibrium experiments were generated for the binary system H+/Cu2+ at 1034

DOI: 10.1021/acs.iecr.5b03871 Ind. Eng. Chem. Res. 2016, 55, 1033−1042

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process. The copper was analyzed by using a Varian 710-ES inductively coupled plasma atomic emission spectrometer.

298.15 K in a Metrohm automated system specifically designed for that purpose and not previously described in the literature, following the procedure shown schematically in Figure 1a. The reactor was initially charged with 0.15 dm3 of water and 1.0 g of microcapsules. The suspension formed by the solid material and the solution was vigorously agitated (600 rpm) with a stirring propeller with three wings and digital speed control (802 Stirrer). This glass reactor has a lateral outlet located at the bottom with a fritted glass disk which permits the liquid discharge from the vessel retaining the solid microcapsules. On the other hand, a 0.55 N copper solution was placed in the storage reagent unit for its further addition. This system is equipped with the Tiamo 2.1 program, which controls the stirrer, the addition of the mother solution, and extraction of the equilibrium solution from the reactor. In addition, the Tiamo 2.1 program registers and records the pH and conductivity data measured by a pH meter (Ecotrode Plus, 809 Titrando) and conductimeter (five-ring conductivity measuring cell c = 1.0 cm−1 with Pt1000, 856 Conductivity Module). After a period of 600 s (time necessary to achieve the homogenization of the system), different sequential volumes of 1, 2, 5, 10, and 20 cm3 of the equilibrium solution were discharged from the reactor every 3000 s (sufficient time to reach the equilibrium). Once the time required for each equilibrium step was elapsed, the same volume of mother solution was poured into the reactor to maintain a constant volume. The copper content in each step was analyzed in triplicate by a flame atomic absorption spectrometer (FAAS; SpectrAA 220 Fast Sequential, Varian), with a detection limit of 1 μg L−1 and standard deviation of ±0.1%. This extraction−addition method of the solutions was repeated five times for each of the volumes studied, bringing the number of experiments to 25. Note that each of these experiments corresponds to a point of the equilibrium curve. Figure 1b shows the profiles of pH and conductivity for the first five steps, when the extraction−addition volume was 1 mL. The solid phase concentration in equilibrium with the liquid phase was obtained by the following mass balance equation: qR − i = zi([i]0 − [i])V /W

3. RESULTS AND DISCUSSION 3.1. Microcapsule Characterization. As aforementioned, conventional TGA does not allow the accurate determination of DEHPA content in sulfonated microcapsules since both desulfonation and extractant evaporation occur simultaneously from 473.15 to 623.15 K.12,16,17 In this work, an isothermal stepwise technique was used in order to quantify accurately the amount of encapsulated DEHPA, whose evaporation is distinguished from sulfonic group degradation since they occur at different temperatures. The thermogravimetric (TG) and differential thermogravimetric (DTG) curves for the extractant, P(St−DVB) polymer, and the studied materials defined in section 2 are shown in Figure 2. The TG plot for pure DEHPA (Figure 2a) indicates that 75 wt % of this compound evaporates at 483.55 K leading to a 8.48 wt % of residue after 973.15 K, due to the phosphorus content of the extractant molecule that forms an insulating char layer providing resistance to heat and mass transfer.18,19 On the other hand, neither P(St−DVB) (Figure 2b) nor SMCNonDEHPA (Figure 2e) showed any weight loss at this temperature. In MC-DEHPA (Figure 2c), the weight loss at 495.75 K can be related to the amount of DEHPA encapsulated, containing 34.43 wt % extractant agent. Moreover, the remaining material after the analysis (8.20 wt %) confirmed the increase in the thermal stability of the polymeric matrix due to the presence of phosphorus from DEHPA. These results were in good agreement with those obtained by the conventional TGA ramp method (34.86 wt % for DEHPA and 9.97 wt % for the residue),12 which means that the stepwise isothermal method is a suitable and accurate technique for thermal characterization. Likewise, SMC-DEHPA (Figure 2d) showed a weight loss at 452.86 K which corresponds to 17.69 wt % of DEHPA. Nonetheless, this value was higher than that determined gravimetrically by extracting DEHPA with acetone.12 This variation could indicate that the presence of sulfonic groups into the microcapsule avoids the easy elution of the extractant agent, which remains entrapped in the polymeric matrix. Additionally, these microcapsules presented two other degradation steps: the first step before 373.15 K (2.15 wt %) corresponds to the water physically absorbed by the sulfonated materials20 since sulfonic groups give a certain hydrophilic character to the hydrophobic polymeric shell; the second step at 588.5 K (5.21 wt %) is due to the desulfonation process. Moreover, the residue was increased up to 10.19 wt % due to the contribution of sulfonic groups in addition to the phosphorus compounds from DEHPA. Finally, sulfonated microcapsules after the extraction of DEHPA with acetone (Figure 2e) also showed a weight loss at 588.2 K (3.03 wt %), which was slightly less than the value exhibited by SMC-DEHPA. This decrease could be attributed to the DEHPA retained in sulfonic groups that is removed during desulfonation, and it was only observed in SMCDEHPA. Since SMC-DEHPA and SMC-NonDEHPA only differ in DEHPA content, part of the weight loss at 588.2 K should be included for DEHPA quantification considering that 75% of the extractant agent evaporates before 600 K. The following expression was proposed to determine the total amount of DEHPA:

(1) +

2+

where i represents the cations H and Cu ; qR−i corresponds to the equilibrium concentration of each cation in the microcapsule (equiv kgdry microcapsules−1), zi is electrochemical valence of these ions (zH+ = 1, zCu2+ = 2); [i]0 and [i] are the initial and equilibrium concentrations of each cation in the liquid phase (mol dm−3), respectively; V represents the initial volume (dm3) and W is the weight of dry solid (kg). 2.4. Microcapsule Regeneration. Hydrochloric acid at 6 wt % was chosen as the regenerant agent according to our previous results.15 It was selected because it demonstrated a high efficiency in the regeneration of strong acid cationic exchanger (Amberlite IR-120) loaded with Cu2+. A 0.1 g sample of SMC-DEHPA was saturated with Cu2+ by using 0.1 dm−3 of a 0.1 N CuCl2 solution, and further it was in contact with 0.1 dm−3 of the regenerant agent. The regeneration was used with the aim to turn back the microcapsules to the primitive state, whatever the DEHPA and sulfonic groups in H+ form. Experiments were carried out at 298.15 K and vigorous stirring. The capacity of regenerated SMC-DEHPA was obtained from the analysis of the total copper eluted from the microcapsules by the HCl solution after the regeneration 1035


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Industrial & Engineering Chemistry Research DEHPA Total (wt %)

= (DEHPA (wt %)483K + x·DEHPA (wt %)588K )/0.75 (2)

where x represents the difference in weight loss between SMCDEHPA and SMC-NonDEHPA at 588 K. From eq 2, the total DEHPA in SMC-DEHPA was 26.49 wt %. This value did not show a satisfactory agreement with the DEHPA content previously reported using the conventional TG analysis,12 concluding that the stepwise isothermal technique is preferable for the quantification of extractant agents contained in the sulfonated polymeric shell. Furthermore, this value was lower than the amount of extractant present in the microcapsules prior to their sulfonation (34.86 wt %), indicating that sulfuric acid extracts part of the DEHPA during the functionalization process. In accordance with the residue value (11.46 wt %), the nondegraded sulfonic and sulfone cross-linking groups are more thermally stable, being part of the residue, which exhibited a lower value than in SMC-DEHPA owing to the reduction of phosphorus compounds. Figure 3 shows the ESEM photographs of MC-DEHPA and SMC-DEHPA. According to Figure 3, nonsulfonated microcapsules had a spherical shape with a regular and soft surface and a particle size appropriate to be used as an ion exchanger. On the other hand, sulfonated microcapsules showed some fissures and cracks, probably due to the tension generated in the polymer matrix by the incorporation of −SO3H groups, although they were not broken.21 A slight particle size decrease owing to the sulfonation process was confirmed by LALLS analysis: from 340.83 to 316.21 μm for dpn0.5 and from 561.49 to 517.81 μm for dpv0.5. This fact may be due to the existence of undesirable sulfone bridging, which deactivates the functional sulfonic groups and decreases the average size of the network structure.22 The FT-IR spectra of sulfonated and nonsulfonated microcapsules were obtained in order to confirm the presence of phosphorus and sulfonic groups inside the beads. Figure 4 shows the infrared spectra of MC-DEHPA and SMC-DEHPA, where four additional bands appear in sulfonated microcapsules. The stretching vibrations associated with sulfonation are indicated by arrows in Figure 4. The in-plane bending vibrations of the aromatic rings para-substituted with the sulfonate group and the sulfonate anion attached to the aromatic ring are represented at 831 and 1004 cm−1 and 1124

Figure 2. TG and DTG curves of (a) pure DEHPA, (b) P(St−DVB), (c) nonsulfonated, (d) sulfonated, and (e) extracted sulfonated without DEHPA (SMC-NonDEHPA) microcapsules.

Figure 3. ESEM micrographs of (a) MC-DEHPA and (b) SMC-DEHPA. 1036


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Figure 5. pH profiles for blank and treated solutions for each of the materials at 298.15 K.

Figure 4. FT-IR spectra of (a) SMC-DEHPA and (b) MC-DEHPA.

cm−1, respectively. The band at 1031 cm−1 represents the symmetric stretching vibrations of the sulfonate group.23,24 On the other hand, MC-DEHPA and SMC-DEHPA showed characteristics peaks of DEHPA.25 Absorption bands at 1600, 1451, and 903 cm−1 (indicated by asterisks in Figure 4) are associated with P(O) (OH), P−CH2, and P−O−C stretching vibrations, respectively. In addition, MC-DEHPA presented a stretching band at 1268 cm−1 related to the PO bond. Thus, sulfonation by using concentrated sulfuric acid could be used to introduce sulfonic groups into the polymer network although a non-negligible amount of the DEHPA is eluted by this agent. Once SMC-DEHPA were characterized, their ability as extractant material for copper removal in the binary system H+/ Cu2+ was studied. 3.2. Determination of Equilibrium Parameters. 3.2.1. Initial Considerations. Typically, the ion-exchange mechanism of conventional resins has been defined as a reversible process according to reaction 3: KAB

β A r α + + α Bs β + ←→ β A s α + + α Br β +

indicating the release of protons during the load of copper ions. Thus, the IMAL model can be utilized for the equilibrium characterization of this type of materials. The diminution of the pH value by adding Cu(NO3)2 in the blank can be explained by the copper dissociation as a function of this parameter. According to Faur-Brasquet et al.28 and Santana-Casiano et al.,29 copper will be either in its free form (Cu2+) or in the hydrolyzed forms, CuOH+ and Cu(OH)2, following the reactions K1

Cu 2 + + H 2O ↔ CuOH+ + H+

(4)

K2

Cu 2 + + 2H 2O ↔ Cu(OH)2 + 2H+

(5) 27,28

The reported constant values from the literature (K1 = 5.012 × 10−7 and K2 = 3.981 × 10−14) and the maximum working concentration in this research were considered for determining the distribution of copper species in pure water as a function of pH. As can be seen in Figure 6, Cu2+ ion is the major species in water up to pH 8.0; at pH 4.6−8.0, aqueous CuOH+ is prevalent and, at pH 5.8−14.0, the aqueous Cu(OH)2 predominates. Since the working pH value for the experiments was lower than 4.7, Cu(OH)2 can be neglected. Nonetheless, CuOH+ could be present in solution and, hence, the value of K1 was checked experimentally. Both mass and charge balances were utilized giving rise to eq 6:

(3)

where “r” represents the ion exchanger in its initial A form and “s” is the solution in which the counterion B is present; α and β denote the valences of the ionic species A and B, respectively. When the ion exchanger is brought into contact with the electrolyte solution, A ions are replaced for a stoichiometrically equivalent amount of B ions of the same sign, preserving the electroneutrality. At the equilibrium, the solid and liquid phases contain both counterion species A and B, though not necessarily in the same concentration ratio since the distribution of the ions in both phases is given by the selectivity of the ion exchanger. Complexing microcapsules have been treated as traditional adsorbents and therefore, typical Langmuir, Freundlich, and Redlich−Peterson isotherm models have been applied to define their behavior.7,11,14,26,27 Nevertheless, in accordance with the above explanation, complexing microcapsules should behave as an ion exchanger realizing, as is the case of DEHPA, protons into the aqueous phase as the removal of the heavy metal cations proceeds. In this way, previous polluted solutions would become more acidic. Figure 5 shows the profiles of pH over the course of the experiments for each of the materials as well as that for the blank solution. As can be seen, the pH value of the aqueous solutions decreased for all the materials studied,

[H+]2 + (K1 − [H+]0 )[H+] − ([Cu 2 +]0 + [H+]0 )K1 = 0 (6) 2+

where [Cu ]0 is the initial concentration of copper in mol dm−3 and [H+]0 and [H+] are the initial and equilibrium proton concentrations in solution in mol dm−3, respectively. The unique unknown parameter, K1, was determined by a fitting tool solving the nonlinear equation for [H+] based on the Marquardt algorithm minimizing the error between experimental and theoretical values of [H+]. The K1 value and its confidence interval were 4.290 × 10−8 ± 6.582 × 10−9 (confidence level of 95% [α = 0.05]). This value was 1 order of magnitude lower than the theoretical one.28 In accordance with the K1 value, a relationship [CuOH+]/[ Cu2+] ≪ 10−3 was obtained regardless the material, finding that the CuOH+ species was almost negligible under the experimental 1037


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Industrial & Engineering Chemistry Research Q = z H+qR − H+ + z Cu2+qR − Cu2+

(10)

where Q is the maximum capacity dependent on the type of material (MC-DEHPA, SMC-DEHPA, SMC-NonDEHPA). Clearing qR−Cu2+ from eq 9 and replacing it in eq 10, Q could be expressed by Q − z H+qR − H+ − z Cu2+K H+Cu2+qR − H+ 2[Cu 2 +]/[H+]2 = 0 (11)

where qR − H+ = (Q − z Cu2+([Cu 2 +]0 − [Cu 2 +])V /W )/z H+ (12)

From the charge balance in the liquid phase, eq 13 was obtained: [H+]0 + 2[Cu 2 +]0 − [H+] − 2[Cu 2 +] = 0

where [H+]0 and [Cu2+]0 correspond to the initial and nonexchanged negative ions in the liquid phase. Considering the charge balance (eq 13) and mass balance in the solid phase (eqs 8 and 11), a set of two nonlinear equations with two unknown variables [H+] and [Cu+] and two fitting parameters KH+Cu2+ and Q were obtained. The concentration of the ions in the liquid phase was obtained calculating the roots of the above-mentioned equations for each step by the Newton− Raphson method. A fitting tool based on the Marquardt algorithm (confidence level of 95% [α = 0.05]) was developed for predicting KH+Cu2+ and Q by minimizing the error between experimental and theoretical values for [H+] and [Cu+]. The equilibrium parameters for each type of microcapsules and their confidence interval as well as R2 are summarized in Table 1.

Figure 6. Theoretical speciation of copper in pure water for a maximum total concentration of 0.202 mol dm−3 using K1 and K2 from Faur-Brasquet et al.28 and Santana-Casiano et al.,29 respectively.

conditions. Bearing in mind this result, the ion exchange equilibrium in the solid phase would be represented by the following reaction scheme: K H+Cu2 +

Cu 2 + + 2RH+ ←⎯⎯⎯⎯→ RCu 2 + + 2H+

(7) 2+

Thus, the equilibrium concentration of Cu in the solid phase determined from eq 1 could be expressed as qR − Cu2+ = z Cu2+([Cu 2 +]0 − [Cu 2 +])V /W

(8)

3.2.2. Equilibrium Studies. So as to define the equilibrium behavior of SMC-DEHPA and the influence of the presence of sulfonic groups in their shells on the copper uptake, it is necessary to know how MC-DEHPA and SMC-NonDEHPA work. On the one hand, the latter materials possess unique active centers, those from DEHPA and −SO3H, respectively. On the other hand, SMC-DEHPA are constituted by both functional groups. In spite of the fact that these microcapsules present a heterogeneous structure, it was considered as a homogeneous solid for the modeling treatment. If ideal behavior for both phases is assumed, the ideal mass action law can be utilized to represent the solid−liquid equilibrium from eq 7. This way, the equilibrium constant for Cu2+ will be given by eq 9: K H+Cu2+ = qR − Cu2+[H+]2 /(qR − H+ 2[Cu 2 +])

(13)

Table 1. Fitting Parameters for the Equilibrium of H+/Cu2+ for the Materials Studied material

KIE (kg dm−3)

Q (equiv kg−1)

MC-DEHPA SMC-NonDEHPA SMC-DEHPA

(1.650 ± 1.081) × 10−5 0.272 ± 0.050 0.135 ± 0.022

0.185 ± 0.009 0.963 ± 0.005 1.782 ± 0.013

These values were used to plot the theoretical isotherms together with the experimental equilibrium data for Cu2+, depicted in Figure 7. According to Figure 7, experimental data correlated very well with the proposed model. Moreover, sulfonated materials almost reached their total capacity even at low copper concentrations, whereas MC-DEHPA achieved it at higher concentrations. Taking into account the total capacity of the pure extractant (3.10 equiv kg−1) (Alcázar et al.30) and the total amount of DEHPA encapsulated in MC-DEHPA, the total capacity of these microcapsules should have been 1.081 equiv kg−1. Nonetheless, it is 1 order of magnitude lower. This fact indicates that the shell would act as a barrier which makes difficult the diffusion of copper ions into the capsule due to its hydrophobic nature. On the other hand, when the shell was sulfonated, the capacity was 13 times higher indicating that the presence of sulfonic groups increases the mobility of metal ions from the aqueous phase up to the extractant agent, since they turn the hydrophobic nature of the shell into one with hydrophilic character. In addition, these groups would increase the porosity of the polymer. Attending to Figure 8, which shows the effect of the sulfonation process on the surface of P(St−DVB) beads

(9)

−1

where qR−Cu2+ is expressed in mol kg , qR−H+ represents the active sites of the microcapsules which correspond with the nonsubstituted sulfonic groups and DEHPA encapsulated (mol kg−1), [H+] and [Cu2+] are the equilibrium concentrations of both ions in the liquid (mol dm−3), and KH+Cu2+ (kg dm−3) denotes the ideal ion exchange equilibrium constant for the binary system H+/Cu2+. When heterovalent ions are exchanged, the maximum capacity of an ion exchanger is variable in molar units. Nevertheless, this design parameter is constant when it is expressed in equivalents. For this reason, the total capacity of the materials studied was determined employing equiv kg−1 units. Assuming that no electric current develops and that there are no vacant ionic sites inside the materials, the following general electroneutrality equation is verified: 1038


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Figure 7. Equilibrium isotherms based on IMAL model for copper at 298.15 K with the synthesized materials. Experimental data and theoretical predictions are represented by symbols and lines for MCDEHPA (■, ---), SMC-NonDEHPA (○, ), and SMC-DEHPA (▲, ···), respectively.

under extreme conditions of temperature, macroporous structures can be distinguished in the ESEM photograph (Figure 8a). Moreover, the growth of pores in microcapsules during the sulfonation process is observed in TEM images. The size of the pores in MC-DEHPA (Figure 8b) was about 0.2 μm, and the presence of DEHPA was confirmed in the darker areas. The increase in the macropore size in SMC-DEHPA was confirmed in the TEM image shown in Figure 8c, which is in good agreement with the results shown by the ESEM photograph. As shown in TEM images, in MC-DEHPA and SMC-DEHPA the extractant agent is encapsulated inside the polymer matrix. The capacity of SMC-NonDEHPA was almost twice as low as that of SMC-DEHPA. This fact shows that copper exchange is not only due to sulfonic groups but also is a consequence of the active groups from DEHPA, whose active centers were available once the microcapsules had been sulfonated. As demonstrated, the total capacity of sulfonated microcapsules containing DEHPA (QSMC‑DEHPA) is given by both total DEHPA encapsulated (QDEHPA) and total sulfonic groups from the polymeric matrix (QSMC‑NonDEHPA): Q SMC‐DEHPA = Q DEHPA + Q SMC‐NonDEHPA

Figure 8. (a) ESEM photograph of SMC-DEHPA under extreme conditions of temperature; (b) TEM image of MC-DEHPA; (c) TEM image of SMC-DEHPA.

(14)

The capacity of the sulfonated shell or the capacity of SMCNonDEHPA was quantified by means of equilibrium experiments, resulting in 0.963 ± 0.005 equiv kg−1. From TGA it was found 3.03 wt % sulfonic groups which would correspond with a capacity of 0.374 equiv kg−1 indicating that this method underpredicts the amount of sulfonic groups, since they not only decompose in this temperature range. Furthermore, QSMC‑NonDEHPA was 1.3 times lower than that obtained from the sulfur content (39.42 g kg−1). This decrease in the capacity demonstrates once more the presence of sulfone cross-linking groups which do not participate in ion exchange. If SMC-DEHPA are considered as a heterogeneous solid, the following equilibrium parameters for DEHPA could be obtained based on eqs 15−18: K DEHPA + 2+

Cu 2 + + 2RDEHPA ←⎯⎯⎯⎯→ RCu 2 + + 2H+ H Cu

K HDEHPA = qRDEHPA − Cu2+[H+]2 /(qRDEHPA 2[Cu 2 +]) + Cu 2 +

(16)

Q DEHPA = z DEHPAqRDEHPA + z Cu2+qRDEHPA − Cu2+

(17)

z Cu2+qR − Cu2+ + z Cu2+qRDEHPA − Cu2+ − z Cu2+ ([Cu 2 +]0 − [Cu 2 +])V /W = 0

(18)

−3 where KDEHPA H+Cu2+ (kg dm ) is the ideal ion exchange equilibrium constant for the binary system H+/Cu2+ in DEHPA; qRDEHPA represents the active sites of the microcapsules which correspond with the nonsubstituted DEHPA (mol kg−1); qRDEHPA−Cu2+ is the equilibrium concentration of Cu2+ in the extractant encapsulated (mol kg−1); zDEHPA = 1, the electrochemical valence of DEHPA.

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as its respective curve lay above the diagonal. On the contrary, copper uptake by MC-DEHPA was only effective for highly concentrated solutions, showing more preference for the H+ of its extractant agent than for the metallic cation. These findings indicate that the presence of sulfonic groups in the shell increases the mobility of metal ions from the aqueous phase up to the extractant agent. 3.4. Regeneration of the Microcapsules. One of the most important properties for handling of any material in ion exchange processes is its reusability. Nevertheless, there is an important lack of information concerning the proper regenerant and its concentration. In a previous work, the minimizing of the environmental impact of the regeneration process using an ion exchange bed of Amberlite IR-120 charged with copper, zinc, and cadmium was studied.15 Different concentrations of NaCl were studied, ranging from 4 to 24 wt %, and it was found that 6 wt % seemed to be the optimal concentration. The results using HCl at the same concentration were closer but even faster. With the aim to ensure the full regeneration of DEHPA, 6 wt % HCl solution was selected as the regenerant agent since the extractant agent prefers proton ions to sodium ones. Figure 10 shows the TG analyses for SMC-DEHPA and those charged with copper (Figure 10a) and regenerated with HCl (Figure 10b) for two different operating cycles. The incorporation of copper to the SMC-DEHPA (Figure 10a) provided higher thermal stability, and a shift in degradation peaks was observed. Nevertheless, the weight losses at different steps were very close to the original ones; any significant difference was observed in both first charged and

Solving the nonlinear equations (eqs 13 and 18), the parameters KDEHPA H+Cu2+ and QDEHPA could be determined following the fitting procedure described above. The equilibrium constant determined for DEHPA was KIEDEHPA = 0.309 ± 0.062 kg dm−3. As expected, it was greater than that for sulfonic groups (SMCNonDEHPA) shown in Table 1. This fact confirms the high selectivity of the extractant for copper under the working experimental conditions. The QDEHPA value of 0.815 ± 0.033 equiv kg−1 coincides with the result obtained from the percentage of DEHPA given by the TG analysis (∼25 wt %). This fact confirms the reliability of the stepwise isothermal technique for measuring the content of extractant in a sulfonated material. Furthermore, the regressed value of the DEHPA capacity also agrees with that determined from eq 14, which led to 0.819 equiv kg−1. The final capacity was smaller than those of commercial materials (4.5 equiv kg−1),31 but higher than those found by Nishihama et al.,3 who synthesized nonsulfonated P(St−DVB) microcapsules containing DEHPA. This research group obtained a loading capacity of 0.5 equiv kg−1 working at pH 5 using Zn2+ as cation. 3.3. Determination of the Selectivity of the Materials. According to the ion exchange behavior of the microcapsules, the ionic concentration for a binary system can be expressed as ionic fraction according to expressions 19 and 20:

xi = Ci*/N

(19)

yi = qi*/Q

(20)

where xi and yi represent the ionic fraction of the ion i in the liquid and solid phases, respectively; N is the total concentration of the ions in the solution phase (equiv dm−3); Q is the useful capacity of the material (equiv kgdry microcapsules−1). Figure 9 shows the experimental and theoretical comparisons of MC-DEHPA and SMC-DEHPA for the system studied in their dimensionless forms. According to Figure 9, theoretical results for the ionic fraction of the metal ion in the microcapsules are in good agreement with experimental values. As expected, equilibrium was very favorable for SMC-DEHPA

Figure 9. Dimensionless isotherms for the binary system H+/Cu2+ at 298.15 K. Experimental data and theoretical predictions are represented by symbols and lines for MC-DEHPA (■, ---) and SMC-NonDEHPA (○, ), respectively.

Figure 10. TG curves for SMC-DEHPA: (a) charged with copper and (b) regenerated with HCl. 1040


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phosphoric acid extractant. Ind. Eng. Chem. Res. 2004, 43 (3), 751− 757. (4) Yang, W. W.; Luo, G. S.; Gong, X. C. Extraction and separation of metal ions by a column packed with polystyrene microcapsules containing Aliquat 336. Sep. Purif. Technol. 2005, 43 (2), 175−182. (5) Kamio, E.; Fujiwara, Y.; Matsumoto, M.; Valenzuela, F.; Kondo, K. Investigation on extraction rate of lanthanides with extractantimpregnated microcapsule. Chem. Eng. J. 2008, 139 (1), 93−105. (6) Araneda, C.; Fonseca, C.; Sapag, J.; Basualto, C.; YazdaniPedram, M.; Kondo, K.; Kamio, E.; Valenzuela, F. Removal of metal ions from aqueous solutions by sorption onto microcapsules prepared by copolymerization of ethylene glycol dimethacrylate with styrene. Sep. Purif. Technol. 2008, 63 (3), 517−523. (7) Barassi, G.; Valdés, A.; Araneda, C.; Basualto, C.; Sapag, J.; Tapia, C.; Valenzuela, F. Cr(VI) sorption behavior from aqueous solutions onto polymeric microcapsules containing a long-chain quaternary ammonium salt: kinetics and thermodynamics analysis. J. Hazard. Mater. 2009, 172 (1), 262−268. (8) Trochimczuk, A. W.; Kabay, N.; Arda, M.; Streat, M. Stabilization of solvent impregnated resins (SIRs) by coating with water soluble polymers and chemical crosslinking. React. Funct. Polym. 2004, 59 (1), 1−7. (9) Cox, M.; Liquid−Liquid Extraction and Liquid Membranes in the Perspective of the Twenty-First Century. In Solvent Extraction and Liquid Membranes: Fundamentals and Applications in New Materials; Aguilar, M., Cortina, J. L., Eds.; CRC Press: 2008; pp 1−18. (10) Kamio, E.; Matsumoto, M.; Kondo, K. Extraction Mechanism of Rare Metals with Microcapsules Containing Organophosphorus Compounds. J. Chem. Eng. Jpn. 2002, 35 (2), 178−185. (11) Kamio, E.; Matsumoto, M.; Valenzuela, F.; Kondo, K. Sorption behavior of Ga(III) and In(III) into a microcapsule containing longchain alkylphosphonic acid monoester. Ind. Eng. Chem. Res. 2005, 44 (7), 2266−2272. (12) Alcázar, A.; Pérez, A.; de Lucas, A.; Carmona, M.; Rodríguez, J. F. Sulfonation of microcapsules containing selective extractant agents. J. Chem. Sci. Technol. 2013, 2 (2), 25−38. (13) Yang, W. W.; Luo, G. S.; Wu, F. Y.; Chen, F.; Gong, X. C. Di-2ethylhexyl phosphoric acid immobilization with polysulfone microcapsules. React. Funct. Polym. 2004, 61 (1), 91−99. (14) Ozcan, S.; Tor, A.; Aydin, M. E. Removal of Cr(VI) from aqueous solution by polysulfone microcapsules containing Cyanex 923 as extraction reagent. Desalination 2010, 259 (1−3), 179−186. (15) Valverde, J. L.; De Lucas, A.; Carmona, M.; Pérez, J. P.; González, M.; Rodríguez, J. F. Minimizing the environmental impact of the regeneration process of an ion exchange bed charged with transition metals. Sep. Purif. Technol. 2006, 49, 167. (16) Rikukawa, M.; Sanui, K. Proton-conducting polymer electrolyte membranes based on hydrocarbon polymers. Prog. Polym. Sci. 2000, 25 (10), 1463−1502. (17) Swier, S.; Ramani, V.; Fenton, J. M.; Kunz, H. R.; Shaw, M. T.; Weiss, R. A. Polymer blends based on sulfonated poly(ether ketone ketone) and poly(ether sulfone) as proton exchange membranes for fuel cells. J. Membr. Sci. 2005, 256 (1−2), 122−133. (18) Chattopadhyay, D. K.; Webster, D. C. Thermal stability and flame retardancy of polyurethanes. Prog. Polym. Sci. 2009, 34 (10), 1068−1133. (19) Sherazi, A.; Ahmad, S.; Kashmiri, M. A.; Kim, D. S.; Guiver, M. D. Radiation-induced grafting of styrene onto ultra-high molecular weight polyethylene powder for polymer electrolyte fuel cell application. II. Sulfonation and characterization. J. Membr. Sci. 2009, 333 (1−2), 59−67. (20) Velencoso, M. M.; Ramos, M. J.; Klein, R.; de Lucas, A.; Rodriguez, J. F. Thermal degradation and fire behaviour of novel polyurethanes based on phosphate polyols. Polym. Degrad. Stab. 2014, 101, 40−51. (21) Coutinho, F. M. B.; Souza, R. R.; Gomes, A. S. Synthesis, characterization and evaluation of sulfonic resins as catalysts. Eur. Polym. J. 2004, 40 (7), 1525−1532.

reused microcapsules. The main change with respect to the original SMC-DEHPA is observed in the residue, which increased up to 19.83 wt % probably due to the flame retardant properties provided by the metallic ion. Figure 10b showed that the regeneration process of SMCDEHPA by using HCl is feasible. A different behavior with respect to the copper effect was observed for the regenerated microcapsules, since all curves are close to the original SMCDEHPA. Any significant difference between the weight losses for DEHPA and sulfonic groups was observed, revealing that the capacity of the microcapsules was fully recovered. Besides, the capacity of the regenerated SMC-DEHPA obtained by elemental analyses was very similar in both cycles to the value predicted by the IMAL model: 1.72 equiv kg−1 in the first cycle and 1.77 equiv kg−1 in the second one. Hence, it was verified that the regeneration process of SMC-DEHPA by using HCl was feasible since the capacity of the microcapsules was fully recovered.

4. CONCLUSIONS A new suitable technique, stepwise isothermal thermogravimetric analysis, can be employed in order to quantify the content of DEHPA encapsulated within a sulfonated polymeric shell, thereby avoiding the problems resulting from the conventional ramp method. The sulfonation process not only gave rise to sulfonic groups but also promoted the existence of nonexchangeable sulfone cross-linking groups, which reduced the useful capacity of the materials. SMC-DEHPA presented two different active groups, those from the extractant and the sulfonic groups from the shell, exhibiting a maximum capacity of 1.782 ± 0.013 equiv kg−1. This value was 13 times higher than that of nonsulfonated microcapsules and almost twice that for the sulfonated shell without DEHPA inside. Thus, the presence of the sulfonic groups in the polymeric shell enhances the diffusion of ions from the aqueous phase to the interior of the microcapsule, since they make available the active centers of the extractant.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +34 926 295300, ext. 3416. Fax: +34 926 295242. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of Ministerio de Economiá y Competitividad through the project Ref. CTQ2008-03474/PPQ.

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REFERENCES

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Equilibrium Treatment for Highly Selective Sulfonated Microcapsules

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