Sulfonation of crosslinked styrene/divinyl benzene
copolymer beads formed from porous foam and ion
adsorption of copper by them: column adsorption
modeling
Necla Barlik and Bülent Keskinler
ABSTRACT
The porous foam is made by the polymerisation of a high internal phase emulsion and it is a highly porous, low density, open cellular material. Surface properties of the foam were chemically modified via a sulfonation process. Sulfonation addedSO
3Hþgroups to the polymer matrix. The ion adsorption behavior of copper ions on sulfonated polymer beads, depending on inlet concentration (10–60 mg/L), pH of inlet solution (2.00–5.20) and flow velocity (1.7–11.4 m/h) was studied. It was shown that the amount of copper adsorbed was not affected with increasing concentration of feed solutions andflow velocity. Also the process was highly pH dependent. The maximum removal was 117.96 mg Cu/g dry adsorbent atflow velocity 11.4 m/h. Column experimental tests were conducted to provide data for theoretical modeling and to verify the system performance of the process. A theoretical column model adopted in this work was found to describe well the ion adsorption breakthrough characteristics.
Necla Barlik (corresponding author) Bülent Keskinler
Engineering Faculty,
Department of Environmental Eng., Ardahan University, Ardahan, Turkey E-mail: neclabarlik@ardahan.edu.tr and Engineering Faculty,
Department of Environmental Eng., Gebze Institute of Technology, Gebze,
Kocaeli, Turkey
Key words|Bohart-Adams, column adsorption, copper removal, porous beads, sulfonation
INTRODUCTION
Ion exchange is defined as a process where an insoluble sub-stance removes ions of positive or negative charge from an electrolytic solution and releases other ions of like charge into solution in a chemically equivalent amount. These inso-luble solid materials are called an ion exchanger. Many different natural and synthetic products show ion exchange properties. The most important of these are ion exchange resins, ion exchange coals, mineral ion exchangers, and syn-thetic inorganic ion exchangers. Ion exchange resins are known as the most important class of ion exchangers. Their framework, the so-called matrix, consists of an
irregu-lar, macromolecular, three dimensional network of
hydrocarbon chains (Helfferich).
Porous styrene/divinylbenzene copolymer beads are produced by oil-in-water (O/W) suspension polymerization. The monomers are diluted with an inert organic liquid called porogen that introduces pores during the copolymer-ization. The porosity is controlled by the amount and the type of porogen and crosslinkage, i.e., divinylbenzene. Styrene/divinylbenzene monomers produce hydrophobic
polymer surface. These beads are converted in order to be used as adsorbents, support for catalysts, anion exchangers, cation exchangers, etc. (Ahmed et al.).
Microcellular open-celled polymeric foams have found applications in a wide range of areas as ion exchange resin, controlled release systems and biotechnological applications (Wakeman et al.;Hentze & Antonietti;Park et al. ). Different methods exist for their preparation. However, those based on the polymerization of the continuous phase of a highly concentrated emulsion (Deleuze et al.) seem the most attractive by their simplicity andflexibility. One of the newest methods for producing porous materials with a more regular structure has been developed based on high-internal-phase emulsion. These foams are highly porous, low density, open cellular materials. The characteristics and syntheses of such materials have already been reviewed (Burke et al. ). In the majority of the applications, the polymerizable phase is a mixture of styrene and divinylbenzene as cross-linking agent however the few attempts to prepare non-styrenic porous foam (Busby et al.).
A cation exchanger can be prepared by sulfonation of a crosslinked polystyrene bead and this topic has been studied by a number of researchers (Alexandratos). Sulfonation alters the surface of polymer beads from hydrophobic to hydrophilic.
Sulfonation is defined as a substitution reaction used to attach theSO3H group on a molecule of an organic
com-pound via chemical bond to carbon or, less frequently, to a nitrogen atom of the organic compound. Compounds such as H2SO4, SO3, acyl and alkyl sulfates and chlorosulfonic
acid are commonly used as sulfonating agents (Kučera &
Jančář ).
Sulfonated polymeric beads are generally supplied in the
form of polymer beads of diameters between 0.3–1.0 mm
(Hart et al.), and therefore have relatively small fluid/ surface contact areas. To increase the surface area, beads of smaller diameter would have to be used which would cause a decrease in permeability of the adsorbent bed. The structure of beads formed from porous foam can provide larger contact surface areas with the benefit of higher per-meability. Both the outside and the inside surface area of the beads can be used as an adsorbent, so they can ensure larger adsorption surface in comparison with conventional suspension beads.
Anthropogenic sources of copper in the aquatic environ-ment include mining, metal plating, and domestic and industrial wastes. Copper is used throughout the electrical industry for wire, armature windings, water pipes, cooking utensils, stills, roofing materials, pigments, and chemical and pharmaceutical equipment. Copper is extremely toxic to aquatic biota. Algae are especially sensitive to copper, with both marine and freshwater species being adversely impacted at concentrations as low as 1–5 mg/L (Evangelou ). According to European Community (EU) legislation the maximum acceptable concentration for copper in coastal and estuarine waters has been tentatively established at environmental quality standard of 5μg L1 (Dangerous Substance Directive 76/464/EEC) (Lu & Gibb).
The removal of copper and its complexes from aqueous solutions can be achieved by several processes, such as chemical precipitation, adsorption, solvent extraction, reverse osmosis, ultrafiltration or ion exchange (Kim et al. ; Cséfalvay et al.; Hamdaoui ; Zhang et al. ).
In this work, porous foam beads were produced and their surface modification was carried out by a sulfonation process. Dynamic ion adsorption tests conducted removal of copper from aqueous solution using the sulfonated beads.
METHOD
MaterialsStyrene (STY), divinyl benzene (DVB), potassium persul-phate, sorbitan monoleate (Span 80), and isopropanol
were purchased from Sigma Aldrich. H2SO4 and
CuSO4.5H2O were reagent-grade Merck products.
Preparation of porous foam
The preparation of high internal phase emulsions and their use in the production of porous foam materials have been
described by several research groups (Wakeman et al.
;Mercier et al.;Barlık ). In this work the con-tinuous oil phase of the emulsion was made from a mixture of styrene crosslinked with divinyl benzene and a water-in-oil emulsifier, Span 80. The oil phase composition was styrene 59%, divinyl benzene 26%, and Span 80 15% (by volume). The dispersed aqueous phase (internal phase) comprises of a solution of polymerization initiator such as potassium per-sulphate (0.4% by mass of the internal phase), in distilled water. Enough amount of aqueous phase was dosed into the stirred oil phase until an aqueous to oil phase ratio of 90:10 (by volume) was achieved. The amount of internal phase was typically 54 ml.
The mixing was carried out using twoflat impellers perpen-dicular to each other so that thefinal level of the emulsion is about 1 cm above the top impeller. The lower impeller on the stirrer shaft was as close to the bottom surface of the vessel as possible. The processing conditions were dosing rate of the aqu-eous phase, RD¼ 3 cm3min1, impeller speed,Ω ¼ 300 rpm,
and total mixing time (including the dosing time), 60 min. As the emulsionflows quite readily, it is easily
polymer-ized in moulds of any shape. After emulsification, the
emulsion was transferred to containers and the emulsion was polymerized at 60W
C for 4 h. Then, the solid polymer was removed from the container and dried of the aqueous phase by standing in an oven at 60W
C for overnight. Result-ing foam had 90% of void volume and 14% of crosslinkResult-ing degree. Washing was performed in isopropanol and then with distilled water. Mould polymer foam was broken manu-ally into pieces of about 1–1.5 cm.
Sulfonation of porous beads
Sulfonation in concentrated sulfuric acid (98%) was carried out by a mechanical stirrer 3 g of the foam beads in 100 ml
of the acid at 100W
C for period of 2 h. After sulfonation the foam beads were separated from the acid, washed with dilute sulfuric acid solutions of increasing water content. Finally, they were washed with deionized water until neu-tralized, and allowed to dry. Then, sulfonated beads were sieved.
Ion-exchange capacity (IEC) and degree of sulfonation (DS) determination
The capacity of an ion exchanger is done by evaluating the
number of ionogenic groups contained in the ‘specific
amount’ of the material. The specific amount is defined as the amount which weighs one gram when the material is completely converted to the Hþor Clform and is free of sorbed solutes and solvents. So rigorous a definition is necessary because the weight of a given amount of the ion exchanger depends on the experimental conditions, for example, on its ionic form. The characteristic constant obtained in this way is usually called ‘ion-exchange (or scientific weight) capacity’ and is expressed in milliequiva-lents per gram of dry resin in Hþ or Clform (Helfferich ).
The DS is defined as the number of —SO3H groups per
repeating unit. Under normal conditions only monosulfona-tion occurs so that only one —SO3H group is present on
each of the benzene rings. One, two or three —SO3H
groups may be attached to one carbon atom of the aliphatic chain, whereas only one—SO3H group may be attached to
the carbon atom of an aromatic ring (Kučera & Jančář ;Wakeman et al.).
A simple back titration technique was used to determine the number of Hþions attached to the copolymer chain. The titration reactions can be summarized as follows:
R SO3Hþ NaOH ! R SO3Naþ H2O (1)
NaOH(excess)þ HCl ! NACl þ H2O (2)
250 mg of washed and dried sulfonated beads was added to 150 ml of 0.1 N NaOH aqueous solution. After a 24 h period of shaking the resultant solution was back titrated to a phenolphthalein endpoint using 0.1 N HCl.
The IEC of the beads was calculated from the following equation (Shin et al.):
IEC(meq=g dry resin) ¼ [(VNaOH× NNaOH)
(VHCl× NHCl)]=Wdry (3)
where NNaOHand NHClare the concentrations of the NaOH
and HCl solutions, respectively, and VHCland VNaOHare the
volumes of HCl and NaOH solutions, respectively. Wdryis
mass of dry sulfonated beads.
Value of IEC calculated from Equation (3) was then used to obtain the DS of the sulfonated beads using the relationship shown in Equation (4) (Idibie et al.):
DS¼ IEC × Mð CÞ= 1 (IEC × M½ SO3H) (4)
where IEC is the IEC (mol/g), MCis the molecular weight of
the non-sulphonated copolymer (g/mol) and MSO3H is the molecular weight of SO3H (g/mol).
Ion adsorption of copper on sulfonated porous beads Traditionally, adsorption isotherms have been used for testing the overall performance of the adsorbent. But, in practice, the final technical systems normally use column-type oper-ation. Moreover, simple isotherms cannot give accurate scale up data in afixed bed system (Al-Degs et al.).
Fixed bed experiments were conducted using a vertical glass column of 10 cm height and 1.16 cm diameter at room temperature. The dried mass of sulfonated beads (0.5–0.6 mesh size) of 0.4 g filled the column generating the bed height of 6 cm. A scheme of the system is shown inFigure 1.
Aqueous solutions of copper were prepared by
dissol-ving analytical grade CuSO4.5H2O. The feed was
introduced using peristaltic pumps. Inlet concentration of 10, 25, 40 and 60 mg Cu/L, flow velocities of 1.7, 2.8, 5.7 and 11.4 m/h, and pH of 3.00, 4.00 and 5.20 were chosen. pH of the inlet solutions adjusted with 1 N HCl. The
experimental tests were performed in upflow mode. In all tests the outlet samples (C ) were regularly collected until the saturation of the bed (C/C0¼ 1) occurred. The
concen-tration of copper ions was measured using an UV-160 Shimadzu spectrophotometer.
RESULTS AND DISCUSSION
Sulfonated porous beadsReaction of foam with sulfuric acid produced a sulfonated bead surface characteristics is very different from the orig-inal materials. Before sulfonation the beads exhibited hydrophobic properties but after sulfonation the materials became hydrophilic.
Equation (5) shows the chemical structure of sulfonated crosslinked polystyrene assuming that all the available ben-zene rings become sulfonated (100% sulfonation). The actual mechanisms of sulfonation of low molar mass hydro-carbon compounds are expected to be the same as in the case of high molecular mass polymer substrates. Sulfonated
ionomers have been defined as macromolecular compounds
containing sulfonic (—SO3) groups (Kučera & Jančář ).
(5) The IEC of beads was determined by titrimetric method as 4.56 meq/g or 144.78 mg Cu/g. The DS was calculated using the exchange capacity, as 78%.
Dynamic ion adsorption of copper on the sulfonated porous beads
Ion adsorption is a process whose performance is affected by a number of operational parameters, including the equi-librium resin adsorption capacity and the mass transfer rate. Prediction of column performance usually involves the resolution of a set of non-linear partial differential equations, although as an alternative, a macroscopic ver-sion was developed from the original microscopic model
by Bohart and Adams (Bohart & Adams ) and
Thomas (Thomas) and for describing the behavior of a column absorber. The main advantages of this model are its simplicity and reasonable accuracy in predicting the breakthrough curves under various operating con-ditions. In most instances the equation which represented the macroscopic model is written as:
ln C0 C C ¼ ln exp k1Qem f 1 k1C0t (6)
where C is the solute concentration in the outlet solution at time t (mg/l), C0is the inlet solute concentration (mg/l), k1
the rate constant of adsorption (l/(mg h)), Qe the
equili-brium solid-phase (i.e. resin) concentration of sorbed solute (gram of solute per gram of adsorbent), m the mass of adsorbent (g), f the flow rate (l/h) and t the time (h). Due to relatively larger value of exp k½ð 1QemÞ=f than 1 in
most instances, Equation (6) can be rewritten as:
ln C C0 C ¼ k1C0t k1Qem f ¼ k1C0 t Qem C0f (7)
According to Equation (7), the left-hand quantity, ln C½ = Cð 0 CÞ, is a linear function of time for a given set
of system and operating conditions. Hence, a linear plot of ln C½ = Cð 0 CÞ versus t yields a slope and intercept for
estimation of the model parameters. It is noted that Qem= Cð 0fÞ in Equation (7) represents the time when
ln C= C½ ð 0 CÞ and k1 is determined from the slope. The
Figure 2|Plot of ln[C/(C0–C)] vs. ion exchange time (t) for copper adsorption by
sulfo-nated porous beads for 25 mg/L feed solution concentration and 5.20 pH at 25W
two parameters in Equation (7) can be readily determined by plotting ln C½ = Cð 0 CÞ against t using the observed data. Figure 2 shows the plot ln C= C½ ð 0 CÞ versus t for flow
rates.
The parameters determined for the sulfonated beads of copper for different inlet concentrations, pH of feed sol-utions and flow velocities are listed in Table 1. Using the estimated parameters, the breakthrough curves for different inlet concentration, pH of feed solutions andflow velocity conditions could be reconstructed for copper adsorption.
Figures 3–5 compare the theoretical and observed break-through curves for different inlet concentrations, pH of feed solutions and different flow velocities, respectively. It is apparent that the model predictions compare reasonably well with the observed data. Hence the simplified version
of the logistic equation, as represented by Equation (7), can be used for convenient representation of the column ion adsorption process under the present experimental conditions.
CONCLUSIONS
In this study,firstly porous foam beads were produced and their surface modification was carried out by a sulfonation process. Then, the removal of copper ions from aqueous sol-utions onto the beads were investigated under different experimental conditions such as the feed concentration,
pH andflow rate.
Table 1|Parameters of theoretical and experimental breakthrough
Inlet conc. (Co) (mg/L) pH of inlet solutions Flow velocity (qo) (m/h) k1*103(L*mg1*min1) Qtheo(mg Cu/g) Qexp(mg Cu/g) R2
10 5.20 5.7 2.69 104.10 103.17 0.845 25 5.20 5.7 2.94 104.62 106.00 0.962 40 5.20 5.7 1.13 104.91 101.15 0.909 60 5.20 5.7 1.58 105.21 100.35 0.933 25 2.00 5.7 3.47 88.71 88.70 0.885 25 3.00 5.7 2.08 110.71 108.77 0.901 25 4.00 5.7 2.62 122.10 118.78 0.855 25 5.20 1.7 0.65 98.59 97.77 0.867 25 5.20 2.8 1.68 108.44 109.02 0.949 25 5.20 11.4 6.14 119.81 117.96 0.972
Figure 3|Comparison of predicted (solid lines) and observed (circles, solid circles, triangles and stars) copper adsorption breakthrough curves of sulfonated porous beads for different inlet copper concentrations (5.7 m/h offlow vel-ocity and 5.20 pH at 25W
C).
Figure 4|Comparison of predicted (solid lines) and observed (circles, solid circles, triangles and stars) copper adsorption breakthrough curves of sulfonated porous beads for pH of feed solutions (5.7 m/h offlow velocity and 25 mg/L inlet concentration at 25W
Titrimetric analysis determines the number of ionic groups for a given amount of resin; when in the hydrogen form this can be interpreted as the total or ‘scientific’ capacity. As the DS is a measure of the number of —SO3
groups attached to the polymer structure, this result was expected as more ionic groups provide more positions for the adsorption (or exchange) of ions from solution. Under some operating conditions not all the available counter ions may take part in the exchange process, giving rise to the capacity under specific operating conditions; this is the so-called apparent or effective capacity. The apparent capacity (Qexp) was estimated from the effluent profiles by
calculating the area between the adsorption curve and a line representing the concentration of the feed solution as shown inTable 1.
The effect of pH on the dynamic adsorption of copper ions on the beads was studied within the pH range of 2.00–5.20. It shown that pH of solution has played an impor-tant role in the adsorption process. At pH between 1 and 6 there are three species (Cuþ2, Cu(OH)þ, Cu(OH)2) present
in solution, and Cu(OH)2concentration increases in the
sol-ution when the pH is above 5.00 (Elliott & Huang). The highest adsorption amount of copper ions was obtained at pH 4.00. The pH has a considerable effect on mobility, so, the mobility of copper ions increases with decreasing pH.
Furthermore, at lower pH value, the Hþ ions compete
with Cuþ2for the exchange sites in the system, thereby par-tially releasing the latter.
A theoretical model was adopted for representing the column breakthrough. Under dynamic conditions, the
increase in the copper concentration and the flow
vel-ocity cause a decrease of the breakthrough
performance. A theoretical model was adopted for repre-senting the column performance. The model with its
model parameters properly identified was observed to
predict reasonably well the experimental breakthrough curves.
Hydrodynamics of the beads structure ensured contact
between the adsorbent and fluid. This led to sulfonated
beads being capable of treating large volume of liquid before breakthrough of ions at the downstream end of the column.
The potential of the beads formed from porous foam structure as an effective adsorbent is demonstrated, and would be enhanced if full sulfonation were achieved.
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