PARA-MAGNETICPOLYVINYLBUTYRALPARTICLESCONTAININGACTIVATEDCARBONASANEWADSORBENT

ZAFERDEMIRCANEVREN TEKOLDENIZ TANYOLAC�AHMETR. O« ZDURAL

Chemical Engineering Department,Hacettepe University,Beytepe, Ankara, Turkey

In this study, a new type of magnetic particle was prepared in a uniform and

spherical form for the range of 750mm<Dp<1000mm from commercially

available polyvinylbutyral (Mowital B30H, Hoechst), magnetite, and acti-

vated carbon using solvent evaporation technique. Magnetite (Dp<5 mm) was

successfully embedded in the adsorbent particles. The surface and cross-sec-

tional views of p-magnetic particles were investigated with a Scanning Elec-

tron Microscope (SEM). The intensity of magnetization was measured with a

vibrating-sample magnetometer. The adsorption studies of phenol and

chromium (Cr(VI)) onto the adsorbent particles were carried out batch-wise.

The effects of activated carbon and magnetite percentages on the adsorption

capacity were investigated, and different initial adsorbate concentration and

pH values were examined. Desorption experiments were carried out with

different concentrations of NaOH, and it was determined that approximately

95% of adsorbed ions were desorbed. It was concluded that the para-magnetic

polyvinylbutyral particles containing activated carbon and magnetite could

effectively be used in the adsorption of organic and inorganic pollutants.

Keywords: Magnetic particles; Solvent evaporation; Activated carbon;

Phenol; Chromium; Adsorption

Received 15 June 2001; in final form 14 March 2002.

Address correspondence to Ahmet R. Ozdural, Chemical Engineering Department,

Hacettepe University, Beytepe, Ankara, 06532 Turkey. E-mail: ozdural@hacettepe.edu.tr

Chem. Eng. Comm.,190: 831�842, 2003Copyright# 2003 Taylor & Francis

0098-6445/03 $12.00+ .00

DOI: 10.1080/00986440390207440

831

INTRODUCTION

Numerous wastes that contain heavy metals and organic pollutants are ofmajor concern due to their toxicity to many life forms. Heavy metal ionsand phenol can be removed by adsorption and for the purpose activatedcarbon, metal oxides, silica gel, and ion-exchange resins have been usedto remove and concentrate trace solutes from waste and drinking waters(Reed and Nonavinakere, 1992). Activated carbon first came into pro-minence through its use as an adsorbent in gas masks in World War I.However the knowledge that carbon produced by the decomposition ofwood can remove coloring matter from solutions dates back to the fif-teenth century. The first commercial application of this property was notmade, however, until 1794, when charcoal filters were used in Britishsugar refinery industries (Shreve, 1967). Water treatment is one of themost important uses of activated carbons because of their extendedsurface area, high adsorption capacity, microporous structure, and spe-cial surface reactivity. Besides these examples, the adsorption capacity ofa given carbon is strongly influenced by the chemical nature of the sur-face. The adsorption by activated carbon has become the water industry’sstandard for the reclamation of municipal and industrial waste water to apotable water quality (Slejko, 1985). Several workers have used activatedcarbons to extract organic contaminants and heavy metal ions frompotable water (Lee et al., 1998; Chatzopoulos and Varma, 1995; Hand etal., 1997; Wang et al., 1997).

The kinetics of the adsorption processes are strongly influenced bythe diameter of the adsorbent particles such that mass transfer increaseswith decreasing particle diameter. The reasons for this are the higherspecific outer surface of smaller particles and the shorter diffusion pathlength in the adsorbent particle. In the case of mass transfer limitedexchange processes, the aim is therefore to apply particles of minimumsize. However, these efforts are counteracted by the fact that the pressuredrop in fixed beds strongly increases with reduction in mean particle size.Consequently, other handling techniques, such as fluidized beds, areused. Besides the reduced pressure drop, fluidized beds are additionallycompatible with crude unclarified feed solutions; this means that a front-end filtration step is often not required.

Magnetically stabilized fluidized beds (MSFB) exhibit a uniquecombination of packed-bed and fluidized-bed properties. The MSFB is atraditional fluidized bed with the addition of a magnetic field. One of theconsequences of the application of the magnetic field is the enhancementof the mass transfer between the liquid and magnetically susceptiblecomposite beads while aiding in the beads’ retention within the bed.Enhanced mass transfer and conversion rates, similar to those obtained inpacked beds, are routinely obtained while the characteristics of fluidized

832 Z. DEMIRCAN ET AL.

beds—low pressure drop and ability to process solids—are maintained.The use of magnetic adsorbent materials in a fixed-bed mode surroundedby an electromagnet was shown to be very efficient for the removal ofplutonium and americium from wastewater (Ebner et al., 1999).

This paper presents experimental results obtained for the preparationof para-magnetic adsorbent particles to be used in magnetic field-enhanced process. In this study we attempted to eliminate the dis-advantages of the use of powdered activated carbon, such as separationdifficulties from adsorption media, by incorporating para-magneticproperties to the adsorbent particles. A commercially available resin,polyvinylbutyral (Mowital B30H, Hoechts), and magnetite were used asthe carrier polymer matrices and magnetic material, respectively, toproduce para-magnetic polyvinylbutyral particles containing activatedcarbon (PVBAC) beads. The work elaborated the method of solventevaporation for spherical particle preparation (Tanyolac� and Ozdural,2000a, 2000b).

MATERIALS

Activated carbon washed phosphoric acid and sulfuric acid as a genuineadsorbent was obtained from Sigma (St. Louis, MI, USA). Poly-vinylbutyral (Mowital# B30H) from Hoechst (Frankfurt, Germany) isthe polymeric component for the preparation of magnetic supports.Chloroform (BDH, Dorset, UK) was used as solvent to its high dissol-ving capacity of Mowital and quick evaporation rate at room tempera-ture. Magnetite with spherical particle shape having a diameter of <5 mm,(Fe3O4) was obtained from Aldrich (Taufkirchen, Germany) as magneticmaterial. The average PVA (Polyvinyl Alcohol) and SDS (SodiumDodecyl Sulphate) was obtained from Sigma (St. Louis, MI, USA) asstabilizer and emulsifier, respectively.

For adsorption experiments phenol (Aldrich, Taufkirchen, Germany)and potassium chromate, K2CrO4 (Sigma, St. Louis, MI, USA), wereused to prepare sorbate solutions. Phosphate, acetate, and tetraboratebuffer solutions for phenol experiments were purchased from Merck,BDH, and Sigma, respectively.

Preparation ofMagnetic Polyvinylbutyral (PVB) BeadsContaining Activated Carbon (PVBAC)

Magnetic microbeads were prepared by a modified method of solventevaporation in a well-stirred 1L glass reactor. The prescribed amount ofPVB was dissolved in chloroform, and the required amounts of magnetiteand activated carbon were transferred into this solution and thoroughlymixed. Then the mixture was quickly poured into a dispersion medium

PARA-MAGNETIC PARTICLES 833

and continued to be mixed. The dispersion medium contains stabilizerPVA and emulsifier SDS in a continuously agitated water phase. Thecomponents and experimental conditions for the preparation of para-magnetic PVBAC are given in Table I. The dispersion medium wasstirred mechanically at 700 rev/min at 20�C. After 4 h the chloroform wasevaporated, leaving magnetite and activated carbon completely encap-sulated in PVB microbeads. The produced microbeads were washed withdistilled and deionized water to remove impurities physically adsorbedonto the surface of the microbeads.

Analysis of the Structure andMagnetism

A vibrating-sample magnetometer (Princeton Applied Research Cor-poration, Oakridge, USA) was used to measure the degree of magnetismfor magnetic microbeads (PVBAC) in a magnetic field. To observe thesurface topography of the PVBAC microbeads, scanning electronmicrographs (SEM) of gold-coated samples were taken with an SEMdevice (Model: Raster Electronen Microscopy, Leitz-AMR-1000,Germany).

Adsorption Experiments

Both phenol and chromium (VI) adsorption experiments were conductedbatch-wise for 8 h at 25�C in the temperature-controlled shaker. In atypical adsorption experiment, an appropriate amount of phenol andchromium (VI) was diluted from main solution at 1.06 10�2 M and 1.9210�2 M, respectively. One gram of microbeads was added to start theadsorption reaction. For equilibrium data, microbeads were removed

Table I The Components and Experimental Conditions for the Preparation of Para-

magnetic Polyvinylbutyral Particles Containing Activated Carbon

Component/condition Quantity

Polyvinylbutyral (Mowital) 1 g

Magnetite 0.55 g

Chloroform 10mL

PVA 4 g

SDS 2 g

Activated carbon 4 g

Distilled water 400mL

Temperature 20�C

Processing time 4 h

Stirring rate 700 rpm

834 Z. DEMIRCAN ET AL.

from the medium by filtration after equilibrium was reached. The amountof adsorbed phenol and chromium (VI) onto the microbeads (adsorptioncapacity) was determined by measuring the initial and final concentrationof phenol and chromium (VI) in the medium spectrophotometrically at465 nm and 540 nm, respectively, using a calibration curve preparedpreviously.

Adsorption Isotherms

Langmuir and Freundlich isotherms have been widely used for describingthe equilibrium nature of uptake of many pollutants. The Langmuir andFreundlich isotherms are reported by the following expressions:

Langmuir Isotherm: q�¼ðQ0C�Þ=ðKþ C�Þ; ð1Þ

Freundlich Isotherm: q�¼KFðC�Þ1=n; ð2Þ

where C* and q* are the equilibrium values of solution concentration(mmol/mL) and amount of adsorbate adsorbed (mmol/g), respectively.Q0 and K are the Langmuir isotherm and KF and (1/n) are Freudlichisotherm constants. Q0 indicates maximum amount of adsorbed by unitweight of adsorbent particles. The amount of adsorption per unit mass ofthe adsorbent particles was evaluated by using the following expression:

q¼ððC0 � CÞ=mÞV; ð3Þ

where q is the amount of adsorbed onto unit mass of adsorbent particles(mmol/g), C0 and C are concentrations of the initial and final solutions ofthe liquid phase, respectively (mmol/mL), V is the volume of liquid phase,and m is the amount of adsorbent particles (g) placed into adsorptionmedia.

RESULTSANDDISCUSSION

In this study, well-shaped spherical and magnetic microbeads containingactivated carbon in the diameter range of 750 mm<Dp<1000 mm with anaverage diameter of 850 mm were produced. The mechanical strength ofthe particles change with the preparation conditions. Carbon fines weredetected in the sorbate solution, when the activated carbon content ofparticles exceeds 80% by weight. However for 80% activated carbonparticles, spectrophotometric measurements in the visible range showedthat, under the experimental conditions, carbon fines release was negli-gible. The behavior of magnetic beads in a magnetic field using a

PARA-MAGNETIC PARTICLES 835

vibrating magnetometer is shown in Figure 1 in terms of voltage vs.magnetic field intensity for 80%, 50%, and 25% w/w of activated carbon.The voltage measured was proportional to amount of magnetite used. Inthe graph, 4KG magnetic field was found sufficient to excite all of thedipole moments of 1 g microbeads for all activated carbon ratios. Thevoltage measured was decreased when the amount of activated carbonwas increased. Low magnetic beads containing more activated carbonrequired lower magnetic field intensity to reach the plateau voltage value.This plateau value will be an important design parameter for a magne-tically fluidized bed or for magnetic filtration using these particles. In theliterature, this value changes from 8KG to 20KG for various applica-tions, thus our magnetic beads will need less magnetic intensity in amagnetically fluidized bed or for a magnetic filter (Svoboda, 1994;Williams et al., 1994).

Figure 2 shows the SEM micrographs of PVBAC beads. The parti-cles in Figure 2a have a spherical shape. Figure 2b presents the cross-sectional view of the microbeads that are containing 80% activatedcarbon. Figure 2b indicates that the particles are in porous nature and theconstituents are distributed relatively evenly in the structure.

Adsorption and Desorption Studies

Prior to the kinetic studies, the effect of the amount of activated carbonand magnetite inside the particles and medium pH on the adsorptioncapacity was investigated to reach the optimum adsorption conditions.

Figure 1. The behavior of magnetic beads in magnetic field using a vibrating magnetometer.

836 Z. DEMIRCAN ET AL.

Figure 2. Scanning electron microscope (SEM) micrographs of the magnetic 80% PVBAC

beads, (a) surface viewing of PVBAC and (b) cross-sectional viewing of PVBAC.

PARA-MAGNETIC PARTICLES 837

Figure 3 presents the change of the amount of adsorbed phenol andCr(VI) with respect to pH values. Maximum adsorption of phenol andCr(VI) was obtained around pH 8 and pH 1 at initial phenol and Cr(VI)concentration of 2.66 10�4 M and 4.81 10�4 M at 25�C, respectively.

Figure 4 shows the amount of adsorbed phenol and Cr(VI) withrespect to activated carbon content of the particles. As expected, both forphenol and Cr(VI) adsorption, the adsorption capacity of the PVBACbeads increased with the increase of the percentage of activated carbon.However, due to mechanical reasons, the maximum value of activatedcarbon w/w remain at 80. It may be seen that the adsorption capacity ofadsorbent containing 80% w/w activated carbon is about three timesmore than adsorbent noncontaining activated carbon. Figure 5 showsthat the amount of magnetite inside the particle has minor effects on theadsorption capacity of the microbeads.

Langmuir and Freundlich adsorption isotherms were tested, andadsorption constants were evaluated as shown in Figure 6 and Figure 7,respectively. It was concluded that (by comparison of R2 values) thefittings of the experimental data to the Freundlich model were poor ascompared with the Langmuir model where R2 values were above 0.999.These figures and the corresponding correlation coefficients illustrate thatbehavior of phenol and Cr(VI) adsorption for containing 80% ACmicrobeads is typically represented with Langmuir-type monolayerkinetics. The maximum adsorption capacity Q0 for phenol and Cr(VI)was calculated as 0.4318 (mmol/g) and 0.4524 (mmol/g), and the con-stant, K, was calculated as 0.000777M and 0.00149M, respectively.

Figure 3. The effect of pH on adsorption capacity.

838 Z. DEMIRCAN ET AL.

Activated carbons from different sources exhibit different surface areasand different Q0 values. In the literature (Nevskaia et al., 1999; Moreno-Castilla et al., 1995) for phenol adsorption, Q0 values were cited as 1.03and 2.316 (mmol/g), whereas surface area values (SBET) were given as 490and 1114m2/g, respectively. The surface area of (SBET) para-magneticpolyvinylbutyral particles used in this study was measured as 123.31m2/g, and the Q0 value for phenol was determined as 0.4318 (mmol/g). The

Figure 4. The effect of activated carbon percentage on adsorption capacity.

Figure 5. The effect of magnetite percentage on adsorption capacity.

PARA-MAGNETIC PARTICLES 839

results indicate that the maximum adsorption capacity of original acti-vated charcoal is decreased in favor of incorporating para-magneticproperties to the adsorbent.

For assuring the release of adsorbed phenol and Cr(VI) on PVBACmicrobeads, desorption experiments were carried out batch-wise withvarious concentrations of NaOH solutions. Figure 8 shows that after 6 happroximately 95% of the adsorbed phenol and Cr(VI) was recovered in

Figure 7. Freundlich adsorption isotherms of phenol and Cr(VI).

Figure 6. Langmuir adsorption isotherms of phenol and Cr(VI).

840 Z. DEMIRCAN ET AL.

the 1 M NaOH and 2 M NaOH solutions from PVBAC microbeads,respectively.

CONCLUSION

The para-magnetic polyvinyl butyral particles containing activated car-bon and magnetite can effectively be used in adsorption of organic andinorganic pollutants. The PVBAC particles eliminate disadvantages ofpowdered activated carbon such as separation difficulties from adsorp-tion media. The magnetic separations of such matrices from adsorbatesolution are simple to perform. When the magnetic field pulls the para-magnetic adsorbent particles to the side of the vessel, other ingredientsremain in the solution or fall to the bottom. Furthermore, these magneticbeads might help in overcoming film diffusion limitation and create apossible alternative to conventional mixing system, due to the planetarymotion of individual beads when an external rotating magnet is appliedfrom the outside of the vessel. The PVBAC particles enable the use ofmagnetic processing techniques for large scale, rapid, and continuousselective separations. The equilibrium data shows that the Langmuirisotherm model satisfactorily represents the equilibrium conditions forboth phenol and Cr(VI) ions. The Langmuir constants for phenol andCr(VI) ions were determined as Q0¼ 0.4318 (mmol/g), K¼ 0.000777Mand Q0¼ 0.4524 (mmol/g), K¼ 0.00149M, respectively. The desorptionexperiments show that approximately 95% of adsorbed ions are desorbedwith different concentrations of NaOH.

Figure 8. PVBAC particles: adsorption and desorption of phenol and Cr(VI).

PARA-MAGNETIC PARTICLES 841

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