Adsorption of Trace Heavy Metals: Application of Surface

Ind. Eng. Chem. Res. 2005, 44, 8671-8681

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Adsorption of Trace Heavy Metals: Application of Surface Complexation Theory to a Macroporous Polymer and a Weakly Acidic Ion-Exchange Resin B. Saha* and M. Streat Advanced Separation Technologies Group, Department of Chemical Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, United Kingdom

A Hypersol-Macronet polymer, MN-600, and a weakly acidic ion exchanger, C-104E containing mainly carboxylic functionality, have been characterized to evaluate their performance for trace heavy metal removal. Characterization of these polymers in the form of scanning electron micrographs, Brunauer-Emmett-Teller (BET) and Langmuir surface area measurements, Fourier transform infrared spectroscopy analysis, X-ray photoelectron spectroscopy analysis, atomic composition measurement, elemental analysis, sodium capacity determination, and zeta potential measurements has been conducted. Density functional theory has been used to analyze the pore size distribution data of MN-600. The BET and Langmuir surface areas of MN-600 are 951 and 1118 m2 g-1, respectively. The zeta potentials of polymers at different pH values indicate zero crossover points at a pH of 1.2 and 1.8 for MN-600 and C-104E, respectively. The sodium capacity values of C-104E and MN-600 are 9.65 and 0.45 mequiv g-1, respectively. The oxygen content of C-104E is 42.12%, whereas MN-600 contains 15.37% oxygen in the polymer matrix. The higher sorption capacity of C-104E can be attributed to the presence of a greater number of oxygen-containing groups in the polymer (particularly carboxylic functionality). C-104E contains about 1.4% nitrogen, whereas MN-600 contains only 0.47% nitrogen. The applicability of surface complexation theory for the sorption of heavy metals onto these sorbents has been explored. The sorption of copper, nickel, and zinc ions from aqueous solution on these sorbents has been studied in batch equilibrium experiments for binary, ternary, and quaternary systems to determine the equilibrium parameters for modeling of ion-exchange equilibria. From the experimentally determined equilibrium parameters, ion-exchange equilibria have been calculated for a wide range of initial conditions. The experimental results for the Macronet polymer as well as the weakly acidic ion exchanger have been compared with the results obtained using the surface complexation model for the prediction of binary, ternary, and quaternary ion-exchange equilibria. Introduction Environmental pollution by toxic metals occurs globally through waste disposal from agricultural processes, metallurgical industries, and electroplating industries and contaminated groundwater from hazardous waste sites. Ion-exchange finds application in analytical chemistry, chemical processing, hydrometallurgy, and environmental-pollution control as a technique for the recovery and refinement of soluble metallic species. However, ion-exchange sorption is a complex phenomenon and it is difficult to predict quantitative sorption behavior in multicomponent systems. Synthetic ion-exchange resins based on crosslinked polystyrene-divinylbenzene polymers are well established for the decontamination of effluent streams. However, many polymers offer relatively low surface area at high cost. Recently, a new series of polymeric networks1 originally invented and developed by Davankov and Tsyurupa2 have been commercially developed. These resins are based on a spherical styrenedivinylbenzene copolymer that is crosslinked when the * To whom correspondence should be addressed. Tel.: (+) 44-(0)1509-222505; Fax: (+) 44-(0)1509-223923. E-mail: [email protected].

polymer is in the swollen state to yield various pore structures and surface functionalities.3 Their unique morphology results from expanding the polymeric chains in a good (swelling) solvent and fixing them to methylene crosslinked bridges created by Friedel-Crafts reactions of chloromethyl groups.4 The formation of an expanded and rigid polymeric network in the presence of large amounts of a solvating media leads to two fundamentally important criteria. First, a crosslinked material is formed, with a markedly low density of chain packing due to the large number of bridges, revealing new types of porosity with high surface areas (1000-1900 m2/g). Second, polymeric adsorbents with strong inner stresses are created. The latter stems from the natural tendency of the polymers to shrink and become denser during the removal of the solvent. Both criteria are the reason that this group of polymers exhibit exceptionally high adsorption capacity. In the 1980s it became apparent that these polymers were extremely versatile. The geometric structure of the pores of the polymers was easily adjusted, and polymers with microporous, mesoporous, and macroporous structures were produced. Moreover, the polymer surface chemistry could be changed by using comonomers with the desired functional groups at the stage of copolymerization or by chemical transformation of the polymer

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formed using appropriate modifying agents.5 The adsorption capacity of the hypercrosslinked polymers increases with the number of active functional groups provided that they are accessible to adsorbate molecules. They have become widely employed for the removal of organic substances from water,6-8 treatment of wastewaters,3 and chromatographic analysis (column packing for HPLC, gel-permeation chromatography, and gas chromatography).9,10 Recently Tsyurupa and Davankov11 published a review discussing the basic principle of formation of hypercrosslinked polymers demonstrating different ways of obtaining these polymers, i.e., via polymerization and polycondensation processes as well as by crosslinking linear macromolecules of different nature. It was found that the synthesis of polymers in polymer chains offers the flexibility for the sorbate to penetrate into the polymer mass. It was concluded that it is necessary to take into account adsorption by accumulation of sorbate on the border between polymer and fluid phase and adsorption of sorbate into the polymer matrix when assessing the adsorption mechanism.12 Kunin was undoubtedly the pioneer in the characterization of ion-exchange polymers made from methacrylic or acrylic acid crosslinked with various divinyl monomers.13 Divinylbenzene is most commonly encountered in this role in commercially available polymers. The possibility of using carboxylic cation-exchange resins in nuclear water purification systems has received considerable interest in recent years.14 Carboxylic polymers are attractive for nuclear applications because they do not contribute sulfate to the treated water and because of easier incineration of acrylic polymers than styrene polymers.14 The sorption of copper, cadmium, nickel, and zinc ions on thiol-based chelating polymeric resins was investigated by Saha et al.15 Pesavento et al.16 reported the sorption properties of a weak acid cation-exchange resin containing carboxylic groups on the basis of the Donnan model. In another publication, Pesavento and Biesuz17 used the GibbsDonnan model to describe and predict the sorption equilibria of metal ions on chelating resins. Horst et al.18 and Ho¨ll et al.19 described the ion-exchange equilibria of weak acid resins by means of surface complexation theory. Sto¨hr et al.20 applied the surface complexation model to predict equilibria for the uptake of heavy metal salts by a weakly basic anion exchanger. Kiefer and Ho¨ll21 studied the sorption of heavy metals onto selective ion-exchange resins with aminophosphonate functional groups, and they also predicted the ion-exchange equilibria using the surface complexation model. Very recently, Jeon and Ho¨ll22 applied the surface complexation model for heavy metal sorption equilibria onto aminated chitosan. However, the application of surface complexation theory toward heavy metal removal by Macronet polymers and weakly acidic polymeric resins has not been published to date. In the present work, a Hypersol-Macronet polymer, MN-600, and a weakly acidic ion exchanger, C-104E containing mainly carboxylic functionality, have been characterized to evaluate their performance for trace heavy metal removal. To predict the performance of these ion exchangers in actual industrial processes and to understand the mechanism of metal uptake by polymeric resins, it was thought desirable to model the ion-exchange equilibria for these resins. In this work,

the applicability of surface complexation theory for the sorption of heavy metals onto these sorbents has been explored. Experimental Section Polymeric Resins. Samples of Macronet polymer, MN-600, and acrylic resin, C-104E, used in this work were obtained from Purolite International Ltd., U.K. Macronet polymer is based on a spherical styrenedivinylbenzene copolymer that is crosslinked when the polymer is in the swollen state.3 Tsyurupa and Davankov11 described that hypercrosslinked polystyrene was obtained on the basis of preformed polystyrene chains dissolved in or swollen with a good organic solvent by means of converting two phenyl rings of the initial polystyrene chains into one (or more) crosslinking bridges of a very restricted conformational mobility. C-104E is a gel type acrylic polymer manufactured also by Purolite International Ltd., U.K. As-received samples of polymeric resins were converted to hydrogen form. This was done by passing 2 L of 5% (w/w) hydrochloric acid through a resin bed containing 20 g of the sorbent. The resin bed was washed with deionized water, and the conductivity of the washed liquid was measured using a conductivity meter (Kent EIL 5007 model). The washing was continued until the conductivity of the washed liquid reached that of deionized water. Chemicals. The acids used were laboratory grade hydrochloric acid and nitric acid, and the alkali used was sodium hydroxide. All these materials were obtained from Fisher, U.K. Analytical grade CuCl2‚2H2O, NiCl2‚6H2O, and ZnCl2 and spectroscopic grade potassium bromide were obtained from Fisher, U.K. Standard metal ion solutions used for analysis in the atomic absorption spectrophotometer were obtained from Fisher, U.K. Analysis. Solutions were analyzed for metal ions using a Varian SpectrAA 200 spectrophotometer in flame absorption mode. Copper, nickel, and zinc concentrations were analyzed at wavelengths of 216.5, 232.0 or 352.5, and 213.9 nm, respectively. The solution pH was measured using a Mettler Toledo 340 pH meter. The conductivity was measured using an ABB KentTaylor EIL 5007 conductivity meter. Characterization Procedure. (a) Scanning Electron Micrography (SEM). Scanning electron microscopy was used to observe the surface morphology of both polymeric materials. The surface of an MN-600 bead and a C-104E particle cut in half were scanned by a Cambridge Instruments 360 scanning electron microscope. The preparation of the samples involved 24 h of drying in an oven at 378 K and subsequent storage in a desiccating jar over silica gel prior to analysis. The samples were attached onto aluminum platforms using PVA glue and sputter-coated with gold. (b) Surface Area and Pore Size Distribution Measurements. Surface area and porosity measurements were carried out by the nitrogen adsorption and desorption method using a Micromeritics ASAP 2000 automatic analyzer fitted with an optional high stability 133.3 N m-2 pressure transducer.3,23 Weighed samples of polymers were prepared by outgassing for a minimum period of 24 h at 373 K on the degas ports of the analyzer. Adsorption isotherms were generated by dosing nitrogen (>99.99% purity) onto the adsorbent contained within a bath of liquid nitrogen at approximately

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77 K. Surface area was measured for the linear relative pressure range between 0.1 and 0.2. (c) Fourier Transform Infrared (FTIR) Spectroscopy Analysis. FTIR spectra were recorded on a Pye Unicam (Mattson 300) Cambridge, U.K., infrared spectrophotometer. Samples of polymeric material in the particle size range 8-30 µm were mixed with finely divided spectroscopic grade potassium bromide in the ratio 1:50 to enable absorbance values of less than 1 to be recorded. Background spectra of potassium bromide and water vapor were recorded every 100 min. Spectra were recorded at a resolution of 4 cm-1 using a minimum of 400 scans. (d) X-ray Photoelectron Spectroscopy (XPS) Analysis. XPS spectra were recorded at a takeoff angle of 90° (with respect to the sample surface) on a VG ESCALAB MK I spectrometer using a nonmonochromatic Al KR X-ray source (1486.6 eV) at a pressure of ∼10-7 mbar. Quantification was achieved by measurement of the peak area following the subtraction of a Shirley type background. Correction was made for photoelectron cross section, inelastic mean free path, transmission of the energy analyzer, and angular asymmetry in photoemission (where appropriate).15 The calibration of the spectrometer energy scale was conducted according to the peak of carbon contamination with the binding energy (Eb) 285.0 eV. Survey spectra were obtained at a pass energy of 100 eV using 0-1100 eV scan (0.3 eV step size) and a dwell time of 50 ms. Analysis of the C 1s peak was conducted, over the range 280-295 eV, at an analyzer pass energy of 35 eV and a step size of 0.025 eV. All spectra were obtained using an anode power of 200 W (10 kV, 20 mA). (e) Zeta Potential Measurement. The zeta potential of the polymers in the size range 0-4 µm was measured using a Malvern Zetamaster instrument. The measurements were based on a laser Doppler electrophoresis technique. The technique operates by measuring the interference fringes of two laser beams at the point where the beams cross. Particles that cross the beams will cause the interference fringes to shift, and this can be related back to the particle’s velocity and hence to the electrophoretic mobility.24 This technique offers several advantages over traditional microscopic methods. It averages the measurement over thousands of readings, generating an intensity distribution, greatly reducing statistical errors. Very low or zero zeta potential can also be measured accurately by virtue of an optical modulator which causes a Doppler shift in one of the beams. About 0.02 g samples of polymers were equilibrated with different amounts of 0.1 M HCl and 0.01-0.1 M NaOH in 0.5 M NaCl solution. Deionized water was added so that the chloride ion concentration remained the same in all solutions. The samples were kept under gentle stirring conditions, and the solution pH and zeta potential were measured after 24 h. (f) Elemental Analysis. Samples were analyzed on a Perkin-Elmer series II 2400 elemental microanalyzer in the Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, U.K. Each sample was accurately weighed (1-2 mg) and wrapped in tin foil prior to analysis by the instrument. The analysis required the combustion of the sample in the combustion tube at 2073 K in a controlled environment in the presence of pure oxygen (purging of the chamber with He before admitting oxygen ensures no presence of any

other gases during combustion). The gaseous combustion products were further decomposed in the presence of a series of catalysts. A thermal conductivity detector quantified the percentage of carbon, hydrogen, and nitrogen. The oxygen content was subsequently determined by difference. (g) Ion-Exchange Equilibrium Experiments. The performance of Macronet MN-600 and the weakly acidic cation exchanger (C-104E) was investigated for the uptake of metals from aqueous solutions. This is to compare the results of the sorbents under study with an ion exchanger with a known quantity of a particular functional group. Sorption experiments were performed to determine the capacity of these polymeric materials for the uptake of copper, zinc, and nickel from water. As-received samples of ion-exchange materials were converted to hydrogen form. This was done by passing 2 L of 5% (w/w) HCl through a resin bed containing 20 g of sorbent. The resin bed was washed with deionized water, and the conductivity of the washed liquid was measured. The washing was continued until the conductivity of the washed liquid reached that of deionized water. The samples were dried in an oven at 378 K for 24 h after the washing cycles. They were kept in a desiccator overnight and then weighed. This procedure was repeated until no significant change in weight could be measured. The converted H+ resins were rinsed with deionized water to achieve neutral pH. The resin samples were then loaded with the desired metal ion from 0.5 M metal chloride solution. Interstitial metal ions were removed by rinsing the metal-loaded resin material with deionized water. Resin samples in the metal forms were weighed after 20 min of centrifugation to exclude water adhered to the outer surface of the resins. For the equilibrium experiments, a series of samples of resin in the respective metal form were equilibrated with 100 mL of 0.01 M hydrochloric acid solution for 7 days at 298 K. The equilibrated solutions were analyzed by an atomic absorption spectrophotometer (AAS) for metal ion concentration, and the corresponding solution pH values were also noted. The binary equilibrium parameters were determined from the equilibrium metal ion concentrations and pH values. For determination of ternary (e.g., H+/Zn2+/Ni2+ and H+/Cu2+/Ni2+) and quaternary (e.g., H+/Cu2+/Zn2+/Ni2+) ion-exchange equilibria, the resins were loaded with nickel by contacting them with 0.5 M nickel chloride solution for 7 days. The interstitial metal ions were removed by rinsing the metal-loaded resins with deionized water. A series of samples of resin in nickel form were centrifuged for 20 min to convert them into a reference state for the equilibrium experiments. Different amounts of the centrifuged resins were equilibrated with 100 mL of 0.01 M hydrochloric acid solution containing 5-10 mM metal chloride for 7 days at 298 K. The equilibrium parameters were determined from the equilibrium metal ion concentrations and pH values. Results and Discussion Scanning Electron Micrography. Figure 1 illustrates the scanning electron micrographs of C-104E and MN-600 indicating the presence of micropores and macropores for both sorbents. However, the different surface morphology of hypercrosslinked polymer MN600 and acrylic polymer (gel type) C-104E is clearly visualized in the scanning electron micrographs.

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Figure 1. Scanning electron micrographs of sliced (a) C-104E and (b) MN-600 beads at 40 K magnification.

Surface Area and Pore Size Distribution. Surface area and pore size distribution analysis for both samples were carried out by a N2 adsorption/desorption method at 77 K. The porosity distribution of MN-600 was calculated using the density functional theory (DFT) model based on nitrogen adsorption assuming slit pore geometry. The adsorption isotherm is known to convey a great deal of information about the energetic heterogeneity and geometric topology of the sample under study. The data of physical adsorption have been used for many years as the basis for methods to characterize the surface area and porosity of adsorbents. However, real solid surfaces rarely approach ideal uniformity of structure. It is generally accepted that nonporous materials present surface area of greater or lesser attraction for adsorbed molecules. This energetic heterogeneity greatly affects the shape of the adsorption isotherm with the result that simple theories such as Langmuir and Brunauer-EmmettTeller (BET) formulas can, at best, give only approximate estimates of surface area. Porous solids are almost never characterized by a single pore dimension but exhibit a more or less wide distribution of sizes. The observed adsorption isotherm for a typical material is therefore the convolution of an adsorption process with the distribution of one or more properties that affect the

process. This was first stated mathematically by Ross and Olivier25 for the case of surface energy distribution and has become known as the integral equation of adsorption. During the past two decades, great advances have been made in understanding the structure and thermodynamics of inhomogeneous systems of simple molecules, including the surface tension and density profile of free solid surfaces and fluids confined by parallel or cylindrical walls. DFT has been especially useful in these investigations, together with computer simulations that can serve as a definitive reference.26 It seems clear that any improved method for extracting micropore and mesopore size information from adsorption data must be as much as possible in accord with these data. The DFT model is now recognized as a powerful tool for studying inhomogeneous classical fluids.15,27 The modeled system consists of a single pore represented by two parallel walls separated by a specific distance. The pore is considered to be open and immersed in a single component fluid at a fixed temperature and pressure. The fluid responds to the walls and reaches equilibrium under such conditions. In this condition, by definition, the chemical potential at every point equals the chemical potential of the bulk fluid. The bulk fluid is a homogeneous system of constant density, and its chemical potential is determined by the pressure of the system using standard equations. The fluid near the walls is not of constant density, and its chemical potential is composed of several positiondependent contributions that must total at every point to the same value as the chemical potential of the bulk fluid. At equilibrium, the entire system has a minimum Helmholtz free energy, thermodynamically known as the grand potential energy. DFT describes the thermodynamic grand potential as a function of the single particle density distribution and, therefore, calculates the density profile that minimizes the grand potential energy and yields the equilibrium density profile. The calculation method requires the solution of a system of complex integral equations that are implicit functions of the density vector. Since analytical solutions are not possible, the problem has been solved using iterative numerical methods. An in-depth discussion of density functional theory and mathematical formulations was reported by Olivier27 and Balbuena and Gubbins.28 Inversion of the integral equation of adsorption to determine micropore size distribution from experimental isotherms using the DFT model usually produces results showing double minima, regardless of the simulation method used. This is assumed to be a model-induced artifact. The inclusion of surface heterogeneity in the model, while more realistic, does not change this observation significantly. The strong packing effects exhibited by a rigid parallel wall model seem likely to be the dominant feature causing the double minima in the derived pore size distributions. C-104E is a gel type acrylic polymer, and hence the resin structure collapsed during the DFT analysis. As a result, the surface area and pore size distribution of C-104E polymeric resin could not be measured. Using the DFT model, pore size distributions were obtained by deconvolution from experimental data for MN-600. Figure 2 shows a large volume of macropores are present within the polymer matrix of MN-600. The BET and Langmuir surface areas of MN-600 are 951 and 1118 m2 g-1, respectively.


Figure 2. DFT pore size distribution of MN-600.

Figure 4. C 1s spectra of (a) C-104E and (b) MN-600. Figure 3. FTIR spectra of MN-600.

FTIR Spectroscopy Analysis. FTIR spectra of MN600 can be seen in Figure 3. MN-600 shows characteristic bands attributable to the vibrations of >CH-, -CH2-, benzene rings, and oxygen functionality. The aromatic out-of-plane C-H vibrations and ring out-ofplane vibrations in the region 900-700 cm-1 signify this type of aromatic substitution. The strong adsorption bands at 819 and 702 cm-1 suggest the presence of p-substituted benzene rings. The adsorption band at 702 cm-1 gives evidence of monosubstituted benzene rings, e.g., uncrosslinked polystyrene. The adsorption bands at 1680-1700 cm-1 suggest the presence of carbonyl (>CdO) functional groups, and the band at 1604 cm-1 may be due to the O-H deformation vibration of carboxylic groups. Evidence of aryl ketones is shown by the presence of an adsorption band at 1200 cm-1, the phenyl carbon stretch, and several medium intensity bands at 1295 and 1384 cm-1, due to C-C-C bending and C-CO-C. The adsorption band at 1295 cm-1 and another band at 1018 cm-1 suggest the possibility of an alkyl aryl ether group. The spectrum of C-104E shows a strong band at ∼1700 cm-1 which suggests the presence of carbonyl (>CdO) functional groups. Because of CdO stretching vibrations and O-H in-plane and out-of-plane deformation vibrations, a peak has been found at ∼1200-1100 cm-1. This suggests the possibility of carboxylic groups. The aromatic out-of-plane C-H vibrations and ring outof-plane vibrations in the region 900-650 cm-1 signify aromatic substitution.

XPS Analysis. XPS was conducted to identify the surface concentration and the chemical state of oxygen in the resin matrix, identified by the FTIR analysis. XPS spectra of the C 1s orbital of C-104E and MN-600 resins are shown in Figure 4. The shake-up lines in the spectra indicate the presence of the polystyrene aromatic backbone of the resins. Actually, these shake-up lines are due to π-π* transitions involving the two highest filled orbitals and the lowest unfilled orbital. The spectra also confirmed the presence of oxygen in the polymers. The concentration of oxygen is greater at the external surface of the polymer (depth < 10 nm) compared to the bulk material, which suggests that there is a concentration gradient in the polymer matrix. The analysis of the polymer beads indicated higher surface oxygen concentrations that may be attributed to heterogeneity of the polymer or the chemisorption of molecular oxygen at the resin surface. All this information shows that the oxygen-containing groups are not just physically adsorbed to the polymer but are chemisorbed to the resin surface. Figure 4a indicates that the chemical shift is caused by oxygen. The C 1s peak at 288.3 eV suggests the presence of carboxylic groups on C-104E. Zeta Potential and pH Titration Measurements. The zeta potential of polymers at different pH values is shown in Figure 5, indicating zero crossover points at a pH of 1.2 and 1.8 for MN-600 and C-104E, respectively. In this work, C-104E showed considerably higher sorption capacity than MN-600. The point where the pH titration curve crosses the pH axis is called point of zero charge (PZC). This has been defined as the pH value at which the surface charge is zero at a particular ambient temperature, applied pressure, and aqueous solution

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Figure 5. Zeta potential of polymers at various pH values. Table 1. Elemental Analysis of Polymeric Resins atomic composition sorbent

carbon (%)

hydrogen (%)

nitrogen (%)

oxygen (%)

C-104E MN-600

51.28 78.42

5.48 5.74

1.12 0.47

42.12 15.37

composition. PZC will change depending on the type and amount of functional groups present on the ion exchanger. It is shifted to a lower value with an increase in acidic functional groups, e.g., carboxyl, phenolic, carbonyl, and lactonic groups. The PZC of MN-600 was found to be higher than that of C-104E because of the presence of higher acidic functional groups in C-104E compared to MN-600. The surface charge of C-104E is more negative because of added functionality in the polymeric resin. The sodium capacities of C-104E and MN-600 are 9.65 and 0.45 mequiv g-1, respectively. Elemental Analysis. The elemental analysis results of these polymers are presented in Table 1. The results show that there was a significant amount of oxygen present in C-104E compared to MN-600. The oxygen content of C-104E is 42.12%, whereas MN-600 contains 15.37% oxygen in the polymer matrix. The higher sorption capacity of C-104E can be attributed to the presence of a greater number of oxygen-containing groups in the polymer (particularly carboxylic functionality), which is consistent with other characterization results. MN-600 contains mostly carboxylic groups (∼0.37 mequiv g-1), and very few carbonylic (∼0.11 mequiv g-1) and phenolic groups (∼0.04 mequiv g-1) are present in the polymeric resin. C-104E bead contains about 1.4% nitrogen, whereas MN-600 contains only 0.47% nitrogen. Mathematical Modeling of Ion-Exchange Equilibria. The application of surface complexation theory for the prediction of ion-exchange equilibria has been published in several earlier publications.18-22,29,30 However, the application of surface complexation theory toward heavy metal sorption by Macronet polymer and weakly acidic acrylic polymeric resin has not been published yet. The surface complexation theory considers the sorption of ions as a local equilibrium reaction that is caused by the amphoteric behavior of the surface. It considers the resin as a plane sheet with a fictitious surface area. It is assumed that functional sites are uniformly distributed across this surface and counterions are arranged in Stern layers parallel to the surface of the sorbent. Fixed sites and counterions form so-called surface-complexes, and these can be characterized by complexation constants. Due to the presence of coun-

Figure 6. Schematic diagram of surface, counterions, layers, and surface potential for the surface complexation model.

terions in individual layers, any two adjacent layers form an electric capacitor. Hence, the resin surface can be considered as a series of capacitors in a multicomponent system. There are no electrostatic interactions between adjacent surface groups and between counterions in the same or in different layers. It is furthermore assumed that electrostatic interactions as well as swelling phenomena can be neglected. An exchanger valency zR of negative sign is defined that corresponds to the smallest common multiple of the counterion valencies. Activity coefficients in the resin phase are assumed to be unity. The complete details of this theory and mathematical derivation have been described by Horst et al.18 and Ho¨ll et al.19 In this section, we briefly describe the surface complexation model development for binary and multicomponent ion-exchange equilibria. Calculation of Binary Ion-Exchange Equilibria. In a binary exchange system H+-A2+, there is only one Stern layer in which A2+ ions are located, whereas protons are sorbed directly on the surface since they are forming undissociated carboxylic acid groups (see Figure 6). The surface complexation model holds for carboxylic exchangers. The sorption of metal counterions A2+ in a layer parallel to the surface is considered as the formation of complexes (a local equilibrium reaction), which can be formulated as

R(COO-)2 + A2+ S R(COO)2A

(1)

Application of the mass action law to this reaction yields the constant of formation of this surface complex

KA )

c(R(COO)2A) c(R(COO)22-)c(A2+)ST,A

(2)

Unlike associated protons, A2+ counterions are assumed to be located in a layer with a characteristic distance from the surface where the electric potential is ΨST,A. The uptake of a competing ion B2+ can be considered in a similar manner. Considering A2+ as the reference ion, the ratio of formation constants can be expressed as follows:


KAB )

c(R(COO)2A)c(B2+)ST,B c(R(COO)2B)c(A2+)ST,A

(3)

This represents preference or lack thereof for the uptake of one of the species. If A2+ is preferred over B2+, the numerical value of KAB should be greater than unity. In eq 3, the concentrations of A2+ or B2+ in the Stern layers cannot be measured and hence the following PoissonBoltzmann relationship needs to be applied to measure their concentrations in the free solution.

[

c(i)x ) c(i) exp -

]

z(i) Ψ RT x

(4)

where c(i) ) concentration of species i, mol/L; c(i)x ) concentration of species i at position x in an electrical field, mol/L; z(i) ) valency of species i; R ) gas constant, J mol-1 K-1; T ) temperature, K; Ψx ) electrical potential at position x, V. Consideration of the relationships for multilayer capacitors eventually leads to the ratio of formation constants as follows:

KAB

qmax F2 c(A) c(B2+) zR ) c(B) c(A2+) ln 10 A0CST(A,B)RT

(5)

where A0 ) specific surface area, m2 g-1; CST(A,B) ) electric capacitance of the capacitor formed by the layers of species A and B, F m-2; F ) Faraday constant, A s; qmax ) maximum resin loading, equiv g-1. The first term on the right-hand side of eq 5 can be written as

QAB )

c(A) c(B2+) c(B) c(A2+)

(6)

From the model of surface complexation for binary systems, the term QAB is defined as the generalized separation factor.19 Replacing the ratio of resin phase concentrations by the ratio of the loadings, solving eq 5 for QAB, and taking the logarithm and after mathematical manipulation, we get eq 7.

log QAB ) log KAB + m(A,B)y(B)

(7)

qmax F2 zR ln 10 A0CST(A,B)RT

(8)

where

m(A,B) )

and y(B) ) dimensionless loading of species B. All the quantities on the right-hand side of eq 6 can be evaluated from experiments. It is evident from eq 7 that the ratio of the formation constants of surface complexes for species A and B, log KAB, and the slope m(A,B) can be determined from the plot of log QAB vs y(B) and by linear regression.19 Hence, evaluation of binary ion-exchange equilibria leads to two equilibrium parameters. The preference for A leads to positive values of log KAB. The slope m(A,B) contains the electric capacitance of the capacitor formed by the layers of A and B ions. Both of these parameters should be constant for the sorption of a metal ion by a specific sorbent and can be used for the prediction of multicomponent

Table 2. Binary Equilibrium Parameters of Polymeric Resins resin

exchange

log K(H,Me)

m(H,Me)

C-104E

Cu2+/H+ Zn2+/H+ Ni2+/H+ Cu2+/H+ Zn2+/H+ Ni2+/H+

0.58 1.39 1.85 3.02 3.42 3.69

0.94 0.71 2.15 3.15 2.96 3.94

MN-600

equilibria. The lower the value of log KAB, the more preferred is the metal species (Me) by the sorbent. The greater the value of m(H,Me), the greater the distance of the Stern layers of the corresponding metal ion from the surface of the sorbent. From the plot of log QAB vs y(B), two characteristic equilibrium parameters for each of the examined binary system are given in Table 2. From Table 2, the following sequence of selectivity of the metal ions can be derived for both polymeric resins: Cu2+ . Zn2+ . Ni2+. And the sequence of the distance of Stern layers of the metal ions is as follows: Zn2+ . Cu2+ . Ni2+. For simple cation exchange, the equilibrium can be expressed as + + + R-SO3- HResin + M+ aq S R-SO3 MResin + Haq (9) + R-CO2 HResin + M+ aq S R-CO2 MResin + Haq (10)

The equilibrium is largely determined by (i) the pKa of the functional group, i.e., pH of the solution, and (ii) the affinity of the functional group for the metal ion that is largely a function of the charge-to-radius ratio of the cation. Hence, the affinity of cation-exchange resins for metal ions is in the same order as the ionic charge: M4+ > M3+ > M2+ > M+ ≈ H+. For weakly acidic cationexchange resin, this order is slightly altered. Certain transition metals have a greater affinity for such resins due to the covalent character of the metal-resin bond. For example, the strength of sorption of a species such as Cu2+ on a weakly acidic cation-exchange resin is substantially greater than the divalent and trivalent nontransition metals. Desorption of metals from the resin is accomplished by shifting the equilibrium [eqs 9 and 10] to the left through mass action by increasing the concentration of hydrogen ion or of other competing ionic species. The pKa of the functional group plays an important role in desorption. Thus, weakly acidic ionexchange resins can be regenerated with essentially a stoichiometric amount of acid, whereas strongly acidic cation-exchange resins require substantially larger amounts and higher concentrations. For weakly acidic cation-exchange resins, sorption of metals is generally limited to a pH of 2 or greater. Owing to the small pKa of the functional group, even the very strongly sorbed transition metals cannot compete with the hydrogen ion at high acidities. Where the carboxylic acid functional group is substantially ionized, the sorption behavior of weak acid resin parallels that of strong acid resin except for the preference of those resins for certain transition metals. Calculation of Multicomponent Ion-Exchange Equilibria. The advantage of this method is the fact that both the parameters obtained from binary ionexchange equilibria should be constant for the sorption of a metal ion by a specific sorbent and can be used for the prediction of multicomponent equilibria. Hence,

8678


multicomponent equilibria can be considered as the superposition of two or more binary equilibria. For a system of hydrogen ion and three different metal ions (e.g., A, B, and C), the system of equations can be written as follows: H log QH A ) log KA + m(H,A)y(A + B + C)

(11)

log QAB ) log KAB + m(A,B)y(B + C)

(12)

log QBC ) log KBC + m(B,C)y(C)

(13)

If the equilibrium parameters of the exchange of divalent metal ions A, B, and C for the reference H+ ion are known, the unknown parameters log KAB, log KBC, and the slopes m(A,B) and m(B,C) result from eqs 14 and 15.

log KH C zH RC

)

log KH A zH RA

+

log KAB zARB

(14)

where zR ) valency of resin with respect to surface complexes.

m(H,C) ) m(H,A) + m(A,B)

(15)

Moreover, the conditions of electroneutrality for the liquid and resin phases need to be maintained. After taking into account the mass balances for each metal ion, there are eight equations that need to be solved for the determination of four unknown equilibrium resin loadings and liquid phase concentrations each. Finally, the formation of water from hydrogen ions and hydroxyl ions was needed for the calculations. The resulting set of nonlinear relationships were solved numerically using a suitable mathematical method, e.g., the NewtonKantorovitch method.18 Modeling of Binary Ion-Exchange Equilibria. Binary equilibria are described by a logarithmic equilibrium parameter or generalized separation factor, which is a linear function of the composition in the resin phase (see eqs 11-13). Multicomponent equilibria are considered as a superposition of several binary equilibria. This approach considers the pH-dependent dissociation of functional groups. In the exchange system comprising metal cations and hydrogen ions, the latter dominates the exchange process and is preferred by resins rather than metal ions. Hence, the equilibrium isotherms can be well represented by plotting the dimensionless resin loading of the metal ions against the equilibrium pH. The sorption process can be mathematically described by the surface complexation theory after determining the equilibrium parameters, log KH Me and m(H,Me) for the binary system (H+/Me2+), where Me is any metal ion. Graphical representations of some representative generalized separation factors for the exchange of nickel and zinc ions (separately) for hydrogen ion of C-104E and MN-600 are shown in Figures 7 and 8, respectively. It can be seen that the numerical values of the separation factors follow a straight line, from which the equilibrium parameters were derived. For resin valencies zR ) -2 for the exchange of protons for divalent counterions and zR ) -1 for monovalent ions, the respective numerical values of log KH Me and m(H,Me) are summarized in Table 2. The log KH Me values signify that Cu2+ is the most preferred for both

Figure 7. (a) Development of the generalized separation factor for the exchange of nickel ion for hydrogen ion of C-104E. (b) Development of the generalized separation factor for the exchange of zinc ion for hydrogen ion of C-104E.

Figure 8. (a) Development of the generalized separation factor for the exchange of nickel ion for hydrogen ion of MN-600. (b) Development of the generalized separation factor for the exchange of zinc ion for hydrogen ion of MN-600.


Figure 9. Ternary ion exchange equilibria for (a) C-104E (H+/ Zn2+/Ni2+) and (b) MN-600 (H+/Cu2+/Ni2+). Initial concentration: C(H+) ) 10 mM; C(Cu2+)/C(Zn2+) ) 5 mM.

polymers. These results also reveal that C-104E exhibits stronger uptake of metal ions than MN-600 and the distance of the Stern layers of the corresponding metal ion from the surface of MN-600 is higher than that of C-104E. Modeling of Ternary and Quaternary Ion-Exchange Equilibra. For modeling of ternary and quaternary systems, two more characteristic parameters based on surface complexation theory have to be added to the set of equations for binary systems. After setting the order of Stern layers according to the m values of the corresponding binary systems, another two equilibrium parameters are to be calculated. Their values were calculated using the binary constants with H+ as reference ions. For the modeling of ternary systems, eqs 14 and 15 have to be extended for three components, whereas for quaternary systems those are to be determined for four components. For ternary systems, the resulting set of 9 nonlinear equations contains the following 9 unknown quantities: x(H+), x(Me1), x(Me2), y(H+), y(Me1), y(Me2), c(OH-), Q(H,Me1), and Q(Me1,Me2), where Me1 and Me2 are two different metal ions. In addition to these 9 unknown quantities, a quaternary system contains another 3 parameters, e.g., x(Me3), y(Me3), and Q(Me1,Me2), which results in a set of 12 parameters. The other equations were obtained by mass balance of each species (using dimensionless concentration) in the solution as well as resin phase and also from the ionic product of water. These sets of equations were solved using the same numerical method used for the

Figure 10. Quaternary ion-exchange (H+/Cu2+/Zn2+/Ni2+) equilibria for (a) C-104E and (b) MN-600. Initial concentration: C(H+) ) 10 mM; C(Cu2+) ) 5 mM; C(Zn2+) ) 5 mM. Table 3. Calculated Equilibrium Parameters of Polymeric Resins resin

log K(Zn,Cu)

m(Zn,Cu)

log K(Cu,Ni)

m(Cu,Ni)

C-104E MN-600

-0.81 -0.40

0.23 0.19

1.27 0.67

1.21 0.79

determination of binary ion-exchange equilibria. The detailed mathematical derivations for application of the surface complexation model to exchange equilibria on ion-exchange resin was published by Horst et al.18 The values of the equilibrium parameters used for the determination of ternary and quaternary systems are given in Table 3. Examples of the calculated equilibrium isotherms for ternary (e.g., H+/Zn2+/Ni2+ and H+/Cu2+/Ni2+) and quaternary (e.g., H+/Cu2+/Zn2+/Ni2+) systems and the respective experimental data are plotted in Figures 9 and 10. Ternary equilibria (H+/Cu2+/Ni2+ and H+/Zn2+/ Ni2+) of C-104E and MN-600 are shown in Figure 9. The results shows that at very low resin dosage, there is hardly any exchange of Ni2+ by H+/Cu2+ or Zn2+ ions. However, with an increase in resin dosage, Ni2+ ions are more likely to be replaced by H+/Cu2+ or Zn2+ ions and hence the solution pH increases. The model predicts that at very high resin dosage most of the Ni2+ ions will be replaced by more preferred H+/Cu2+ or H+/Zn2+ ions. The shape of the ternary equilibrium curve for MN600 is slightly different than that of C-104E (see Figure 9). The Ni2+ capacity of MN-600 is much lower than that

8680


of C-104E. As a consequence, at a very low resin dosage, the amount of Ni2+ ions loaded with resin is very low and hence the exchange of Ni2+ by H+/Cu2+ or Zn2+ is negligible. However, with an increase in resin dosage, Ni2+ ions are more rapidly exchanged by H+/Cu2+ or Zn2+ ions and finally at very high resin dosage most of the Ni2+ ions are replaced by H+/Cu2+ or Zn2+ ions and the solution pH increases as high as 5.8. For quaternary (e.g., H+/Cu2+/Zn2+/Ni2+) ion-exchange equilibria (Figure 10), at very low resin dosage (in Ni2+ form), the solution pH was very low and Ni2+ ions were exchanged more rapidly by Zn2+ ions than Cu2+ ions. The sequence of the distance of Stern layers of metal ions is Zn2+ . Cu2+ . Ni2+, and hence Zn2+ ions form a Stern layer adjacent to the resin surface compared to Cu2+ and Ni2+ ions. The exchange of Ni2+ ions by Cu2+ ions increases with an increase in resin dosage, and finally equilibrium is established between Ni2+/Cu2+ and Ni2+/Zn2+ exchange. This is because of the higher selectivity of Cu2+ compared to Zn2+ for both polymers. However, at very high resin dosage most of the loaded Ni2+ ions were replaced by more preferred Cu2+ and Zn2+ ions. From Figures 9 and 10, it is quite evident that the surface complexation model showed good agreement with the experimental results for the weakly acidic ion exchanger (C-104E) as well as the Macronet polymer (MN-600). Conclusions Pore size distribution measurements indicate that MN-600 contains macropores in the polymer matrix. Infrared spectroscopic analysis indicated the presence of mainly carboxylic functional groups on the sorbent surface. XPS analysis, elemental analysis, and atomic composition measurements showed that the percentage of oxygen groups in C-104E is markedly higher than that in MN-600. C-104E showed much better performance than MN-600 for the sorption of copper, nickel, and zinc because of the presence of a relatively greater number of carboxyl groups in its matrix. The metal sorption capacity of the sorbents follows the order Cu2+ > Zn2+ > Ni2+. The binary ion-exchange equilibria for hydrogen ion were described by a set of two equilibrium parameters that remained constant for a wide range of ionic strengths. These constants were modified for the prediction of multicomponent equilibria. The surface complexation model showed good agreement with the experimental results for Macronet polymer as well as weakly acidic ion exchanger for binary, ternary, and quaternary ion-exchange equilibria. Acknowledgment The financial support by the EC (Contract No. BRPRCT96-0158) is gratefully acknowledged. We are thankful to Professor Wolfgang Ho¨ll and his research group for valuable discussions and helpful comments for the development of the surface complexation model. We thank Purolite International Ltd., U.K., for kindly supplying the polymeric resins. Nomenclature A0 ) specific surface area, m2 g-1 c(i) ) concentration of the species i in solution, mol/L c(i)ST ) concentration of species i in the Stern layer, mol/L

c(i)x ) concentration of species i at position x in an electrical field, mol/L CST(A,B) ) electric capacitance of the capacitor formed by the layers of species A and B, F m-2 F ) Faraday constant, A s log KAB ) the ratio of the formation constants of surface complexes for species A and B m(A,B) ) characteristic equilibrium parameter for binary exchange of species A and B Mei ) metal ion i qi ) resin loading for species i, equiv g-1 qmax ) maximum resin loading, equiv g-1 QAB ) generalized separation factor R ) gas constant, J mol-1 K-1 T ) temperature, K y(i) ) dimensionless metal loading of the resin for a specific metal species i z(i) ) valency of species i zR ) valency of resin with respect to surface complexes Ψx ) electrical potential at position x, V

Literature Cited (1) Hypersol-MacronetTM Sorbent Resins, Purolite Technical Bulletin; The Purolite Co.: Pontyclun, Wales, U.K., 1995. (2) Davankov, V. A.; Tsyurupa, M. P. Structure and properties of porous hypercrosslinked polystyrene sorbents ‘Styrosorb’. Pure Appl. Chem. 1989, 61, 1881. (3) Tai, M. H.; Saha, B.; Streat, M. Characterisation and Sorption Performance of a Hypersol-Macronet Polymer and an Activated Carbon. React. Funct. Polym. 1999, 41, 149. (4) Veverka, P.; Jerˇa´bek, K. Mechanism of hypercrosslinking of chloromethylated styrene-divinylbenzene copolymers. React. Funct. Polym. 1999, 41, 21. (5) Belyakova, L. D.; Schevchenko, T. I.; Davankov, V. A.; Tsyurupa, M. P. Sorption of vapours of various substances by hypercrosslinked “STYROSORB” polystyrenes. Adv. Colloid Interface Sci. 1986, 25, 249. (6) Streat, M.; Sweetland, L. A.; Horner, D. J. Removal of Pesticides from Water using Hypercrosslinked Polymer Phases: Part 3. Mini-column Studies and the Effect of Fulvic and Humic Substances. Trans. Inst. Chem. Eng. (Part B) 1998, 76, 135. (7) Streat, M.; Horner, D. J. Adsorption of highly soluble herbicides from water using activated carbon and hypercrosslinked polymers. Trans. Inst. Chem. Eng. (Part B) 2000, 78, 363. (8) Streat, M.; Malik, D. J.; Saha, B. Adsorption and ionexchange properties of engineered activated carbons and carbonaceous materials. In Ion Exchange and Solvent Extraction; Sen Gupta, A. K., Marcus, Y., Eds.; Marcel Dekker: New York, 2004; pp 1-84. (9) Tsyurupa, M. P.; Malsova, L. A.; Andreeva, A. I.; Mrachkovskaya, T. A.; Davankov, V. A. Sorption of organic compounds from aqueous media by hypercrosslinked polystyrene sorbents ‘Styrosorb’. React. Polym. 1995, 25, 69. (10) Beth, M.; Unger, K. K.; Tsyurupa, M. P.; Davankov, V. A. Microporous hypercrosslinked polystyrene Styrosorb as a restricted access packing in sample cleanup for high-performance Liquid-Chromatography. 1. Evaluation of restricted access properties. Chromatographia 1993, 36, 351. (11) Tsyurupa, M. P.; Davankov, V. A. Hypercrosslinked polymers: Basic principle of preparing the new class of polymeric materials. React. Funct. Polym. 2002, 53, 193. (12) Veverka, P.; Jerˇa´bek, K. Influence of hypercrosslinking on adsorption and absorption on or in styrenic polymers. React. Funct. Polym. 2004, 59, 71. (13) Kunin, R.; Barry, R. Ind. Eng. Chem. 1949, 41, 1269. (14) Saha, B.; Streat, M. Sorption of Trace Heavy Metals by Hypersol-Macronet Polymer and Weakly Acidic Ion Exchanger. Proceedings of IEX 2000 Ion Exchange at the Millennium, Cambridge; Greig, J. A., Ed.; Imperial College Press: London, 2000; p 183. (15) Saha, B.; Iglesias, M.; Cumming, I. W.; Streat, M. Sorption of Trace Heavy Metals by Thiol Containing Chelating Resins. Solvent Extr. Ion Exch. 2000, 18, 133. (16) Pesavento, M.; Biesuz, R.; Cortina, J. L. Anal. Chim. Acta 1994, 298, 225.

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Received for review November 29, 2004 Revised manuscript received January 18, 2005 Accepted January 20, 2005 IE048848+

Adsorption of Trace Heavy Metals: Application of Surface

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