Magnetic vinylphenyl boronic acid microparticles for Cr(VI) adsorption: Kinetic, isotherm and thermodynamic studies

Accepted Manuscript Title: Magnetic vinylphenyl boronic acid microparticles for Cr(VI) adsorption: Kinetic, isotherm and thermodynamic studies Author: Ali Kara Emel Demirbel Nalan Tekin Bilgen Osman Necati Bes¸irli PII: DOI: Reference:

S0304-3894(14)00994-7 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.12.011 HAZMAT 16445

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

12-2-2014 19-11-2014 8-12-2014

Please cite this article as: Ali Kara, Emel Demirbel, Nalan Tekin, Bilgen Osman, Necati Bes¸irli, Magnetic vinylphenyl boronic acid microparticles for Cr(VI) adsorption: Kinetic, isotherm and thermodynamic studies, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2014.12.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Magnetic vinylphenyl boronic acid microparticles for Cr(VI) adsorption: Kinetic, isotherm and thermodynamic studies

Ali Kara1,*, Emel Demirbel1, Nalan Tekin2, Bilgen Osman1, Necati Beşirli1 1

Kocaeli University, Faculty of Arts and Science, Department of Chemistry, 41380 Kocaeli, Turkey

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Uludag University, Faculty of Arts and Science, Department of Chemistry, 16059 Bursa, Turkey

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* Correspondence Address

Ali Kara, Ph.D.

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Görükle Campus, Nilüfer

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16059, Bursa, Turkey

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Tel and Fax: (90) 224 2941733

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e-mail: [email protected]

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Magnetic Vinylphenyl Boronic Acid Microparticles for Cr(VI) Adsorption: Kinetic, İsotherm and Thermodynamic Studies

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Highlights

• Cr(VI) can oxidize biological molecules and be one of the most harmful substance. • Magnetic seperation techniques are used on different applications in many fields. • Magnetic processor.

systems can be used for rapid and selective removal as a magnetic

• We investigate

properties of both new material and other magnetic adsorbents

reported in the literatures on the adsorption of Cr(VI) ions. • No researchments were reported on adsorption of Cr(VI) with magnetic vinylphenyl boronic acid microparticles. Word Account: 78 Abstract Magnetic vinylphenyl boronic acid microparticles, poly(ethylene glycol dimethacrylate(EG)boronic

acid(VPBA))

[m-poly(EG-VPBA)],

produced

by

suspension

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vinylphenyl

polymerization and characterized, was found to be efficient solid polymer for Cr(VI)

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adsorption. The m-poly(EG-VPBA) microparticles were prepared by copolymerizing of

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ethylene glycol dimethylacrylate (EG) with 4-vinyl phenyl boronic acid (VPBA). The -

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poly(EG-VPBA) microparticles were characterized by N2 adsorption/desorption isotherms,

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ESR, elemental analysis, scanning electron microscope (SEM) and swelling studies. The m-

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poly(EG-VPBA) microparticles were used at adsorbent/Cr(VI) ion ratios. The influence of

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pH, Cr(VI) initial concentration, temperature of the removal process was investigated. The

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maximum removal of Cr(VI) was observed at pH 2. Langmuir isotherm and Dubinin-

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Radushkvich isotherm was found to better fit the experiment data rather than Fruendlich isotherm. The kinetics of the adsorption process of Cr(VI) on

the m-poly(EG-

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VPBA)microparticles were investigated using the pseudo first-order, pseudo-second-order,

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Ritch-second-order and intraparticle diffusion models, results showed that the pseudo-second order equation model provided the best correlation with the experimental results. The

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thermodynamic parameters (free energy change, ∆G0 enthalpy change, ∆H0; and entropy change, ∆S0) for the adsorption have been evaluated.

Keywords: magnetic polymers; adsorption isotherm; adsorption kinetic; adsorption thermodynamic; Cr(VI) ions

Notation concentration of Cr(VI) ions at equilibrium (mg.L-1)

C0

initial concentration of Cr(VI) ions in solution of Cr(VI) ions (mg. L-1)

Ea

activation energy of adsorption (kJ mol-1)

Efe

free energy of adsorption (kJ mol-1)

∆G°

Gibbs free energy of adsorption (J.mol-1)

∆H°

isosteric enthalpy of adsorption (J.mol-1)

∆S°

entropy change of the adsorption process (J.mol-1.K-1)

qe

the amount of Cr(VI) ions adsorbed on the adsorbent at equilibrium (mg.g-1)

qt

the amount of Cr(VI) ions adsorbed on the adsorbent at any time (mg.g-1)

qm

the maximum amount of Cr(VI) ions adsorbed per unit mass adsorbent (mg.g-1)

QL

the maximum amount of Cr(VI) ions adsorbed per unit mass adsorbent (mg g-1)

KL

the Langmuir constant related to the affinity of binding sites (mL mg-1)

n

the heterogenity factor

KF

the Freundlich constant

QD-R

the maximum amount of Cr(VI) ions adsorbed per unit mass adsorbent (mg.g-1)

KD-R

the Dubinin-Radushkevich constant (mol2 J-2)

ε

the polanyi potential (J.mol-1)

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k2

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k1

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RL

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Ce

the dimensionless separation factor the rate constant of pseudo first-order adsorption (min-1) the rate constant of pseudo second-order adsorption ((g/mg).min-1)

kR

the rate constant for the modified Ritchie’s-second-order model (min-1)

ki

the intraparticle diffusion rate constant (mg/g min0.5)

R2

linear regression coefficient

t

time (min)

temperature (K)

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1. Introduction

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Environmental pollution by heavy metals has become an ecotoxicological hazard of

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prime interest and increasing significance [1]. Heavy metal ions like as chromium are often

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detected in industrial wastewaters, which originate from metal plating, mining activities,

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smelting, battery manufacture, tanneries, petroleum refining, paint manufacture, pesticides,

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pigment manufacture, printing and photographic industries, etc [2]. Chromium is the second

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most abundant inorganic groundwater contaminant at hazardous waste sites. It was observed

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that Cr(III) readily oxidizes to Cr(VI) when typical oxidation conditions are present. The toxicity of Cr depends primarily on its chemical form. Trivalent chromium compounds are

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much less toxic than those of hexavalent Cr, since hexavalent chromium is known to be very

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mobile and hazardous to human health through inhalation, skin contact, and ingestion, being highly toxic, carcinogenic and mutagenic to living organisms, even when present in very low concentrations in water [3]. Exposure to Cr(VI) causes problems of skin dermatitis, liver and kidney damage, respiratory sensitization (asthma), carcinogenicity, etc [4]. Because of its negative impact on the aquatic ecosystem, the hexavalent state is more concerned by the

research. It is highly toxic because of its oxidizing properties and tends to accumulate in living organisms causing serious damages for bacteria, plants and animals (i.e. reduction of fish production). Cr(VI) is classified by the International Agency for Research on Cancer (IARC) at the top priority list of toxic pollutants. The maximum authorized in water is restricted at 5 mg L− 1 and the threshold concentration is a challenge for the water quality [5].

Several treatment technologies have been developed to remove chromium from water

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and wastewater. Common methods include chemical precipitation, ion exchange, membrane

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separation, ultrafiltration, flotation, electrocoagulation, solvent extraction, sedimentation, precipitation, electrochemical precipitation, soil flushing/washing, electrokinetic extraction,

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phytoremediation, reduction, reverse osmosis, dialysis/electrodialysis, adsorption/filtration,

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evaporation, cementation, dilution, air stripping, steam stripping, flocculation, and chelation

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[6]. Conventional techniques for removing metal ions from wastewater include chemical

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precipitation, membrane separation, reverse osmosis, evaporation and electrochemical

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treatment, and solvent extraction [7]. Adsorption process has been extensively examined for

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the elimination of the organic substances or heavy metal ions from water and waste water [8].

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In recent years much attention has been paid to adsorption techniques and the design and

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synthesis of new sorbents for toxic metal ions. Boronic acid carrying agarose or acrylamidebased polymeric beads have been used in chromatographic studies involving separation.

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Recently Shimomura et al. [9] have synthesized a solid magnetic support by modifying

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magnetite particles with graft polymerization of acrylic acid. They have immobilized dihydroxyphenylboronic acid (DHPB) on modified magnetite surface through amide linkages and exploited the binding nature of various oligosaccharides with these boronicacid functionalized particles. However, these polymer or magnetic polymer based supports suffer from severe drawbacks owing to its less surface area which ultimately results in less binding

efficiency. Boronic acid functionalized adsorption was based on the complex formation between boronic acid groups of support materials. 4-vinylphenylboronic acid functional group was used for isolation of RNA, nucleodites, glycoenzymes [10], adsorption of sugar [9].

The applications of magnetic polymer microspheres became a new topic of research. Magnetic polymer microsphere indeed has its strength because it can be benefit from both components: magnetic particles and polymer. The magnetic particles make the rapid and

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facile separation possible in an external magnetic field [11]. Magnetic separation is relatively

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rapid and easy, cost effective and highly efficient. To make full use of the magnetic separation technology, magnetic carriers with good properties are necessary and indispensable. There are

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two main ways to prepare the magnetic polymer microbeads. One is coating or encapsulating of

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magnetic particles with performed polymer. The other is using monomer polymerization, which

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is used more widely. Several kinds of polymerization can be used, such as emulsion

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polymerization, dispersion polymerization, suspension polymerization and microemulsion

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polymerization, etc [12]. To make full use of the advantages of magnetic separation, magnetic

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supports with good physiochemical properties are necessary and desirable. The ideal magnetic

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supports are expected to be of high magnetic properties, small size and narrow size

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distribution, high content of surface functional groups and nontoxicity [13]. Magnetic separation technique has some advantages, such as high efficiency, cost-effectiveness.

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Magnetic carriers are usually composed of the magnetic cores to ensure a strong magnetic

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response and a polymeric shell to provide favorable functional groups and features for various applications. In addition, magnetite (Fe3O4) has been widely used as magnetic material due to their excellent magnetic properties, chemical stability and biocompatibility [14].

In this paper, boronic acid carrying m-poly(EG-VPBA) microparticles were prepared by the suspension polymerization and characterized. We show that the m-poly(EG-VPBA) microparticles can be used directly for adsorption of Cr(VI) ions from aqueous solutions. In addition, we have conducted kinetic, isothermal and thermodynamic analysis. The results show that such boronic acid modified superparamagnetic particles are efficient support for the adsorption and repeated use.

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2. Experimental

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2.1. Materials

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Ethylene glycol dimethacrylate (EGDMA) was obtained from Merck (Darmstadt,

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Germany). N-Vinyl imidazole (VIM, Aldrich, Steinheim, Germany) was distilled under vacuum

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(74–76οC, 10 mm Hg). 2,2’-Azobisisobutyronitrile (AIBN) was obtained from Fluka A.G.

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(Buchs, Switzerland). Poly(vinyl alcohol) (PVAL; Mw: 100.000, 98% hydrolyzed) was supplied

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from Aldrich Chem. Co. (USA). Magnetite nanopowder (Fe3O4; diameter 20–30 nm) was

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obtained from Aldrich (USA). All other reagents were of analytical grade and were used without

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further purification. All water used in the binding experiments was purified using a Barnstead (Dubuque, IA) ROpure LPw reverse osmosis unit with a high flow cellulose acetate membrane

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(Barnstead D2731) followed by a Barnstead D3804 NANOpurew organic/colloid removal and

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ion exchange packed-bed system.

2.2. Synthesis of the m-poly(EG-VPBA) microparticles

EG and VPBA were copolymerized in suspension by using AIBN and poly(vinyl alcohol)

as the initiator and the stabilizer, respectively. Toluene was included in the polymerization recipe as the diluent (as a pore former). A typical preparation procedure was exemplified below. Continuous medium was prepared by dissolving poly(vinyl alcohol) (200 mg) in the purified water (50 ml). For the preparation of dispersion phase, EG (21,2 mmol) magnetite Fe3O4 nanopowder (1.0 g) and toluene (10 ml) were stirred for 10 min at room temperature. Then, VPBA (3,38 mmol) and AIBN (200 mg) were dissolved in the homogeneous organic phase. The organic phase was dispersed in the aqueous medium by stirring the mixture magnetically (375

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rpm), in a sealed-cylindrical Pyrex polymerization reactor. The reactor content was heated to

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polymerization temperature (i.e., 70oC) within 4 h and the polymerization was conducted for 1 h with a 375 rpm stirring rate at 80oC. The final microparticles were extensively washed with

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ethanol and water to remove any unreacted monomer or diluents and then dried at 50oC in a

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vacuum oven. Table 1 shows recipe and polymerization conditions for preparation of the m-

Table 1

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poly(EG-VPBA) microparticles.

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2.3. Characterization of the m-poly(EG-VPBA) microparticles

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The porosity of the microparticles was measured by a N2 gas adsorption/desorption

isotherm technique (Quantachrome Corporation, Poremaster 60, USA). The specific surface

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area of beads in a dry state was determined by a multipoint Brunauer-Emmett-Teller (BET) apparatus (Quantachrome Corporation, Autosorb-6, USA). The porosity and the specific surface area of the microparticles are investigated by the experiments which were given in data terms (Table 2). Pore volumes and average pore diameter for the beads were determined

by the BJH (Barrett, Joyner, Halenda) model. The average size and size distribution of the beads were determined by screen analysis performed using standard sieves (Model AS200, Retsch Gmb & Co., KG, Haan, Germany). Micro-structures of the beads were observed using a scanning electron microscopy (SEM, CARL ZEISS EVO 40, UK). In order to evaluate the degree of VIM incorporation, the synthesized m-poly(EGDMA-VPBA) microparticles were subjected to elemental analysis using a Leco Elemental Analyzer (Model CHNS-932, USA). The magnetization curve of the bead sample was measured by a vibrating sample

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magnetometer (VSM, Princeton Applied Research, Model 150A, USA). The presence of

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magnetite nano-powders in the bead samples was investigated with an electron spin resonance

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Table 2

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(ESR) spectrophotometer (EL 9, Varian, USA).

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2.4. Adsorption experiments

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Batch experiments were conducted in a temperature-controlled shaker (Clifton, England).

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The speed of the shaker was fixed at 300 rpm for all experiments. In order to optimize Cr(VI)

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removal conditions, the effect of initial pH, initial Cr(VI) concentration and temperature were

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investigated. For batch adsorption experiments, 20 mL of Cr(VI) solution was mixed with 20 mg of the m-poly(EG-VPBA) microparticles at different initial concentrations (200-10000

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mg/L) and pH values (2–5) in 50 mL erlenmeyer flasks. The pH values were adjusted by

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using 0.1 M HCl or 0.1 M NaOH solution. After equilibration, the suspensions were separated and analyzed for metal ions by an UV-vis spectrophotometer (Shimadzu-2100 UV-vis, Japan). The equilibrium adsorption capacity was calculated using the following equation:

q e = (C 0 − C e )

V m

(1)

where, qe (mg g-1) is the equilibrium adsorption capacity, C0 and Ce are the initial and equilibrium concentration (mg L-1) of Cr(VI), V (L) is the volume of the Cr(VI) solution and m (g) is the weight of the dried adsorbent. 2.5. Desorption and repeated use experiments

Reusability of the adsorbents is one of the most important considerations in the adsorption and removal studies of the heavy metals. For batch desorption studies, the Cr (VI)-

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loaded m-poly(EGD-VPBA) microparticles in 1.0 M HNO3 and 1.0 M NaOH solutions was

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desorbed in a shaking water bath at 300 rpm for 24 h at room temperature. The desorption ratio was calculated from the amount of Cr(VI) ions adsorbed on the m-poly(EG-VPBA)

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microparticles and the final Cr(VI) concentration in the desorption medium, by using the

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following expression.

Amount of Cr(VI) ions desorbed to the desorption medium

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Desorption ratio =

x 100

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Amount of Cr(VI) ions adsorbed on the microparticles

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(2)

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Ten consecutive adsorption–desorption cycles were conducted to test the reusability of the

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magnetic microparticles by using the same magnetic microparticles.

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3. Results and Discussion

3.1. Characterization of the m-poly(EG-VPBA) microparticles

The suspension polymerization procedure provided cross-linked m-poly(EG-VPBA) micro-dimensional particles. The N2 adsorption/desorption isotherm and corresponding pore size distribution curve for the m-poly(EG-VPBA) microparticles are shown in Fig. 1. The surface area (SBET), pore volume (VP) and pore size are given in Table 2. As shown in Fig. 1, the m-poly(EG-VPBA) microparticles are found to be type IV (based on IUPAC classification) isotherms for the typical hysteresis loop in the mesopore range. We can see that the adsorption at relative pressure below 0.2, which is ascribed to micropore adsorption.

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Along with the increase of relative pressure, increases in adsorption capacity are caused by

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the monolayer/multilayer adsorptions of nitrogen molecules on the mesopores [13-14]. Changing of pore sizes in the range of 1,8 nm (micro)-3.7 nm (meso) indicates that the

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magnetic particles contain both micropores and mesopores.

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Fig. 1

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Fig. 1 N2 sorption isotherms and pore size distribution (inset) of m-poly(EG-VPBA)

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microparticles.

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The surface morphology and bulk structures of the m-poly(EG-VPBA) microparticles

are visualized by SEM (see Fig. 2). All the micro-dimensional particles have rough surface. It can be seen from Fig.2 that a large quantity of well-distributed pores could be observed and they have netlike structure. The m-poly(EG-VPBA) microparticles have this characteristic which increases the specific surface area, the binding capacity of micro-dimensional particles,

as well as the mass transfer rate of binding. The equilibrium swelling ratio for m-poly(EGVPBA) microparticles are 73% in water.

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Fig. 2

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Fig. 2 The SEM images of the m-poly(EG-VPBA) microparticles.

Magnetic characteristics of catalyst are related to their type generally, while those of

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magnetic materials are usually related to the content of magnetic component inside. So, Fe3O4 content is very important to the magnetic responsibility of magnetic materials. In general, the higher Fe3O4 content shows the stronger magnetic responsibility [9]. For this reason, the average Fe3O4 content of the m-poly(EG-VPBA) microparticles were determined by density analysis. The hydrated density of the m-poly(EG-VPBA) microparticles measured at 25oC

was 1.71 g mL-1. By the same procedure, the density of Fe3O4 particles was found to be 4.94 g mL-1. The density of non-magnetic poly(EG-VPBA) measured at 25oC was 1.08 g mL-1. The magnetic particles volume fraction in the m-poly(EG-VPBA) can be calculated from the following equation derived from the mass balance:

φ=

ρ A , ρ C and ρ M

(3)

are the densities of non-magnetic poly(EG-VPBA), Fe3O4

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where,

ρC − ρ M ρC − ρ A

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nanopowder, and the m-poly(EG-VPBA), respectively. Thus the m-poly(EG-VPBA) gel volume fraction in the magnetic micro-dimensional particles was estimated to be 83.7%.

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Therefore, the average Fe3O4 content of the resulting m-poly(EG-VPBA) was 16.3%. The

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presence of magnetite nanopowder in the polymer structure was also confirmed by the ESR

Fig. 3

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spectrum as shown in Fig. 3.

Fig.

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ESR

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spectrum of the m-poly(EG-VPBA).

A peak of magnetite was detected in the ESR spectrum. It should be noted that the non-magnetic beads cannot be magnetized under this condition. It reflects response ability of magnetic materials to the change of external magnetic field firstly and it characterizes the ability of magnetic materials to keep magnetic field strength when the external magnetic field is removed. In order to show the magnetic stability, the m-poly(EG-VPBA) was kept in

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distilled water and ambient air for 3 months, and the same ESR spectrum was obtained. With the goal of testing the mechanical stability of the m-poly(EG-VPBA), a bead sample was

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treated in a ball mill for 12 h. SEM photographs show that a zero percentage of the sample

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was broken. The g factor given in Fig. 3 can be considered as quantity characteristic of the

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molecules in which the unpaired electrons are located, and it is calculated from Eq. (4). The

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measurement of the g factor for an unknown signal can be a valuable aid in the identification

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of a signal. In the literature, the g factor for Fe+3 is determined within the range of 1.4–3.1 for

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low spin and 2.0–9.7 for high spin complexes [9]. The g factor was found to be 2.5 for the m-

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poly(EG-VPBA).

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g= hϑ / β H r

(4)

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Here, h is the Planck constant (6.626x10-27 erg s-1); β is Universal constant (9.274x10-21 erg

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G-1); ϑ is frequency (9.707x109 Hz) and H r is resonance of magnetic field (G).

3.2. Adsorption of Cr(VI) ions from aqueous solutions

3.2.1. Effect of pH on adsorption

The pH of the Cr (VI) solution is an important factor which controls the adsorption process particularly the adsorption capacity. This parameter causes the change of surface charge of the sorbent, conversion of the chromium species and other ions present in the solution, and extent of dissociation of functional groups on the active sites of the adsorbent. Therefore, in this study, the adsorption of Cr(VI) ions onto the m-poly(EG-VPBA)

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microparticles studied as a function of pH. The initial pH values of Cr(VI) solutions were kept

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between 2.0 and 5.0. The results are depicted in Fig. 4, showing that the maximum adsorption on m-poly(EG-VPBA) microparticles are 85.08 mg g-1 occurred at pH 2, it dropped to 1.20

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mg g-1 at pH of 5.0, respectively. The Cr(VI) exists in various forms in the aqueous solution,

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including H 2 CrO4 , HCrO4− , CrO42− and Cr2 O72− [15]. H 2 CrO4 is predominant when the

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pH<1.0, HCrO4− and Cr2 O72− are predominant at pH values ranging from 2.0 to 6.0 and

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CrO42− is predominant at pH>6.0 [16]. But the relative abundance of them are dependent on

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solution pH and Cr(VI) concentration [17].

The adsorption of Cr(VI) ions depends on the protonation or unprotonation of

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functional groups on surface of the microbeads. At acidic pH, the imidazole groups of the m-

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poly(EG-VPBA) microparticles are positively charged [18]. These features were favorable for Cr(VI) adsorption onto the adsorbent by electrostatic interaction and ion exchange. At pH

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values ranging from 2.0 to 6.0 which HCrO4− and Cr2 O72− exist in aqueous medium, adsorption of Cr(VI) on m-poly(EG-VPBA) microparticles take place through ion exchange mechanism involving positively charged imidazole group and negatively charged hydrogen chromate or dichromate ion depending on the condition. While increasing pH would increase

the anions such as OH- in the solution, leading to a competitive adsorption with negative Cr(VI) forms. Therefore, adsorption capacity of Cr(VI) decreases with the increase of pH.

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Fig. 4

Fig. 4 Effect of pH on adsorption of Cr(VI) ions of the m-poly(EG-VPBA) microparticles.

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(initial solution concentration: 500 mg.dm-3; temperature: 25 °C; contact time: 510 min)

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3.2.2. Effect of temperature on adsorption

The uptake capacity of Cr(VI) (mg.g-1) decreases with increasing the temperature in

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Fig. 5. This may be due to that, the stability of the interaction between the metal ions and the active groups of the m-poly(EG-VPBA) microparticles decreases at higher temperatures. The magnitude of the heat of adsorption can provide useful information concerning the nature of the surface and the adsorbed phase [19].

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Fig. 5

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Fig. 5 Effect of temperature on adsorption of Cr(VI) ions of the m-poly(EG-VPBA)

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microparticles (initial solution concentration: 500 mg.dm-3; pH: 2.0)

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3.2.3. Effect of initial concentration of Cr(VI) ions on adsorption

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The influence of the initial Cr(VI) concentration on the adsorption capacity of m-

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poly(EG-VPBA) microparticles are investigated at various concentrations. As shown in Fig. 6, when the initial Cr(VI) concentration increases from (200–10000) mg L-1, the adsorption

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capacity of the microspheres increases from (30.14 to 711.2) mg g-1. This result showed that the initial Cr(VI) concentration provided a driving force to overcome the mass transfer resistances between the adsorption medium and adsorbent [20]. With more Cr(VI) existed in solution, more active sites of the m-poly(EG-VPBA) microparticles were involved in the adsorption process. Higher ions concentration enhanced the mass transfer driving force, and

increased the metal ions uptake capacity at equilibrium [21]. In addition, with the increasing of the metal ions concentration, the number of collisions between metal ions and adsorbent increased, which enhanced the adsorption process [22]. The m-poly(EG-VPBA) microparticles provide various types of functional groups, such as imidazole group. The binding Cr(VI) can be due to ion-exchange, adsorption, microprecipitation, complexation on the m-poly(EG-VPBA) microparticles.

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Fig. 6

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Fig. 6 Effect of initial concentration of Cr(VI) ions onto the m-poly(EG-VPBA)

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microparticles microparticles at various temperatures. (pH 2.0)

3.3 Adsorption Kinetics

To investigate the mechanism of adsorption and its potential rate-controlling steps that include mass transport and chemical reaction processes, kinetic models have been exploited to analyze the experimental data. In addition, information on the kinetics of metal uptake is required to select the optimum condition for full scale batch metal removal processes [23]. Several kinetic models have been applied to find out the adsorption mechanism. The equation of the four kinetic models is expressed as follows:

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The pseudo-first-order kinetic model of Lagergren is given as follows [24];

(5)

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log(q e − q t ) = log q e − k1t / 2.303

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where qe and qt (mg/g) are the amounts of the Cr(VI) ions adsorbed at equilibrium and at

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time (min), respectively. k1 (1/min) is the rate constant of pseudo-first-order adsorption and

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qe is the adsorption capacity at equilibrium.

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The pseudo-second-order kinetic model can be expressed as [25];

(6)

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t 1 1 = + t 2 qt k 2 qe q e

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where qe and qt (mg/g) have the same definitions as in Eq. (6), k 2 is the pseudo-secondorder rate constant at the equilibrium ((g/mg)/min). The initial adsorbent rate h ((mg/g)/min) can be determined from k 2 and qe values using the following equation:

h = k 2 q e2

(7)

The modified Ritchie’s-second-order kinetic model [26];

1 1 1 = + qt k R q e t q e

Where qt

(8)

and q e (mg/g) have the same definitions as in Eq. (8), k R is the rate constant

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(1/min) of the modified Ritchie’s-second-order kinetic model.

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The intraparticle diffusion model can be described as [27];

(9)

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qt = k i t 1 2

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Where qt (mg/g) has the same definitions as in Eq. (9), k i is the intraparticle diffusion rate

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constant ((mg/g)/min1/2).

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The uptake time data obtained was treated in the form of four kinetic models including

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represented in Fig. 7a-d. Additionally, the regression coefficients and predicted q e values of

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these four kinetic models for all temperatures studied are all listed in Table 3. As shown in

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Table 3, the values of R2 for pseudo-second-order are extremely high, all greater than 0.99 for all temperatures studied. In other words, the pseudo-second-order model better described the

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sorption of Cr(VI) ions onto the m-poly(EG-VPBA) microparticles. Besides, the values of equilibrium adsorptive capacities of Cr(VI) predicted by pseudo-second-order model are closer to the experimental results (Table 3). The pseudo-second-order model is more likely to predict the adsorption behavior over the whole range of adsorption than other kinetic models,

suggesting that pseudo-second-order model represents the present adsorption system, which implies that the adsorption is an ion exchange process [17].

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Fig. 7

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Fig. 7 (a)

Fig. 7 (b)

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Fig. 7 (c)

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Fig. 7 (d) Fig. 7 Adsorption kinetics of adsorption of Cr(VI) ions by the m-poly(EG-VPBA) microparticles at different temperatures:(a) pseudo-first-order (b) pseudo-second-order (c)

A

Ritchie’s-second-order (d) intraparticle diffusion.

Table 3

PT

The values of the pseudo-second order rate constant, k2, were found to increase from

RI

5.075x10-5 to 9.432x10-5 g/mg min, for an increase in the solution temperature of 277 to 338 K. There is a linear relationship between the k2 and temperature (Fig. 8). The adsorption rate

SC

constant is usually expressed as a function of solution temperature by the following Arrhenius

A

Ea RT

(10)

D

M

ln k 2 = ln k 0 −

N

U

type relationship [28]:

TE

where k2 is the rate constant of pseudo-second-order of adsorption (g/(mg min)), ko is the independent temperature factor (g/(mgmin)), R is the gas constant (8.314 J mol-1 K-1), and T

A

CC

EP

is the solution temperature (K).

Fig. 8

PT RI

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Fig. 8 Arrhenius plot.

U

Therefore, the relationship between k2 and T can be represented in an Arrhenius form as:

(11)

M

A

N

 905.4  ln k 2 = 4.323 −  −  T  

D

From this equation, the rate constant for adsorption, k0, is 75.42 g/mg min and the activation

TE

energy for adsorption, Ea, is 7.528 kJ/mol. The magnitude of activation energy explains the

EP

type of sorption. Two main types of adsorption can occur, physical or chemical. In physical adsorption, the equilibrium is usually attained rapidly and easily reversible, because the

CC

energy requirements are small. Chemical adsorption is specific and involves forces much

A

stronger than physical adsorption. The range of 5–40 kJ/mol of activation energy correspond a physisorption mechanism or the range of 40–800 kJ/mol suggests a chemisorption mechanism [29]. The result obtained in this study is 7.528 kJ/mol which indicating that the adsorption is a physisorption.

3.4

Adsorption isotherm

Adsorption isotherm is critical in optimizing the use of adsorbent, since it can be used to not only assess the adsorption capacity of adsorbent, but also describe how the adsorbate interacts with adsorbent [30].The relationship between the amount of Cr(VI) ions adsorbed onto the adsorbent surface and the remaining Cr(VI) ions concentration in the aqueous phase at equilibrium can be observed by the adsorption equilibrium isotherm analysis as shown in

PT

investigate the effect of initial concentration of Cr(VI) ions. This relationship showed that the

RI

adsorption capacity increased with the equilibrium concentration of the Cr(VI) ions in solution, progressively reaching saturation of the adsorbent. The Langmuir, Freundlich and

SC

Dubinin–Radushkevich (D–R) isotherm models were applied to simulate the experimental

N

U

data.

A

The Langmuir model assumes that a monomolecular layer is formed when adsorption

M

takes place without any interaction between the adsorbed molecules. The Langmuir model can

TE

D

be represented as [31]:

(12)

CC

EP

Ce C 1 = + e q e QL K L QL

where QL (mg/g) is the maximum amount of Cr(VI) per unit weight of the m-poly(EGD-

A

VPBA) microparticles to form complete monolayer coverage on the surface bound at high equilibrium Cr(VI) concentration C e , and KL is Langmuir constant related to the affinity of binding sites (L/mg). QL represents a particle limiting adsorption capacity when the surface is fully covered with Cr(VI) and assists in the comparison of adsorption performance. QL and KL

are calculated from the slopes and intercepts of the straight lines of plot of

Ce versus C e (not qe

shown).

The Freundlich model involves equilibrium on a heterogeneous surface, where the sorption energy is not homogeneous for all sorption sites. This isotherm model is defined by

1/ n

(13)

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qe = K f Ce

PT

the equation [32] below:

U

1 ln C e n

(14)

A

N

ln q e = ln K f +

SC

The linear form of the Freundlich adsorption isotherm model is expressed as follows,

M

where q e is the amount of metal ions adsorbed at equilibrium time (mg g-1), C e is the

D

equilibrium concentration of the metal ions in solution (mg L-1). K f (mg g1)(L mg-1)1/n and

TE

n are isotherm constants which indicate capacity and intensity of the adsorption, respectively.

EP

The values of K f and n were calculated from the slope and intercept of the plot ln q e versus

CC

ln C e (not shown).

A

The calculated values of the Langmuir and Freundlich equation’s parameters are given

in Table 4. It can be seen that the linear coefficients of determination (R2) for the Freundlich isotherm model are lower than for Langmuir isotherm, which indicate that this model does not describe very well the adsorption processes of m-poly(EG-VPBA) microparticles for Cr(VI). It also can be found from Table 4 that R2 values for the Langmuir isotherm model are higher

than >0.99. Obviously, the Langmuir model is much better to describe the adsorption of Cr(VI) onto the m-poly(EG-VPBA) microparticles than the Freundlich model. Thus, the sorption behavior of Cr(VI) onto m-poly(EG-VPBA) microparticles is considered to be representative of sorption onto a monolayer [33]. The experimental Cr(VI) uptake is lower than the theoretical maximum adsorption capacity. It may be attributed to the incomplete contact of Cr(VI) and the adsorbent. Additionally, both QL and KL decreased with increasing temperature, indicating the bonding between heavy metals and active sites of the adsorbent

A

CC

EP

TE

D

M

A

N

U

SC

RI

PT

weakened at higher temperature and the adsorption process is exothermic.

PT

The essential features of a Langmuir isotherm can be expressed in terms of a

RI

dimensionless constant separation factor or equilibrium parameter, RL which is defined by

U

1 1 + KC e

(15)

A

N

RL =

SC

following equation [34]:

M

where, K is the Langmuir constant (dm3 mg-1) and C e is the initial metal ion concentration

TE

D

(mg dm-3).

EP

Parameter RL indicates the shape of isotherm as follows:

CC

Value of RL Type of isotherm Unfavorable

RL = 1

Linear

0< RL <1

Favorable

RL = 0

Irreversible

A

RL > 1

RL value between 0 and 1 indicates a favorable adsorption. Here in, the values of RL between 0 and 1 indicate a favorable adsorption. The calculated values of the dimensionless factor RL are included in Table 4. All RL values obtained are in the ranges 0.1687-0.9549 indicates the favorable adsorption of Cr(VI) by m-poly(EG-VPBA) microparticles under consideration. This means that the equilibrium isotherms can be well described by the Langmuir model, and the adsorption process is monolayer adsorption onto a surface with finite number of identical

PT

sites, which are homogeneously distributed over the adsorbent surface.

RI

Although the Langmuir and Freundlich isotherm models are widely used, they do not

SC

give information on the adsorption mechanism. To this aim, the equilibrium data were tested with the Dubinin-Radushkevich isotherm model (D-R isotherm). The Dubinin-Radushkevich

U

model is essential, because with this model makes it possible to determine the order process

N

adsorption, chemical or physical and it is used to calculate the mean free energy of adsorption.

(16)

TE

D

Qe = Q D − R exp(− K D − R ε 2 )

M

A

The non-linear D-R isotherm is expressed as [18]:

EP

and the linearized form of the equation is given as:

(17)

A

CC

ln Qe = ln Q D − R − K D − R ε 2

where Qe is the amount of solute adsorbed per mass of adsorbent (mg g-1), QD-R is the maximum adsorption capacity (mg g-1), KD-R is the D-R constant (mol2/J2) and ε is the Polanyi potential (J mol-1), which can be calculated as:

ε = RT (ln 1 + 1 / C e )

(18)

where R is the gas constant (J/molK), T the absolute temperature (K) and Ce the equilibrium concentration of the adsorbate in aqueous solution (mg/L) [35-37]. The values of QD-R and KDR

were calculated and are shown in Table 4.

The mean free energy of adsorption (Efe) was calculated from the KD-R values using the

1

(19)

SC

− 2K D−R

U

E fe =

RI

PT

equation:

N

The E fe value gives information about adsorption mechanism, physical or chemical. If it lies

M

A

between 8 and 16 kJ mol-1, the adsorption process takes place chemically, while, E fe < 8 kJ mol-1, the adsorption process proceeds physically [38]. All the parameters calculated were

D

presented in Table 4. In this study, D–R isotherm model fits with the experimental data

TE

correlation coefficient ranges 0.9947–0.9973, and the E fe values obtained using the D–R

EP

constant, in the non-linear form, were 5.482 kJ/mol for 277 K, 5.475 kJ/mol for 298 K, and

CC

6.066 kJ/mol for 338 K. The E fe values obtained for all temperatures studied in this research were lower than 8 kJ mol-1, where can say that the adsorption of Cr(VI) onto m-poly(EG-

A

VPBA) microparticles is obtained by physisorption.

3.5 Adsorption thermodynamics

Thermodynamic considerations of an adsorption process are necessary to conclude whether the process is spontaneous or not. The experimental data obtained at different temperatures were used in calculating the thermodynamic parameters such as Gibbs free energy change ( ∆G 0 ), enthalpy change ( ∆H 0 ) and entropy change ( ∆S 0 ). These parameters were calculated by the following van’t Hoff equation [18] (Eq. 20) and Eq. 21, ∆S 0 ∆H 0 1 − ( ) R R T

(20)

PT

ln K L =

(21)

RI

∆G 0 = ∆H 0 − T ∆S 0

SC

where KL is Langmuir constant, R is gas constant (8.314 J mol-1 K-1), and T is temperature

U

(K). The values of ∆H 0 and ∆S 0 can be calculated from the slope and intercept of the linear

A

CC

EP

TE

D

M

A

N

plot of ln K L versus 1/T, respectively.

Fig. 9 The plot of lnKL versus 1/T for the determination of thermodynamic parameters for adsorption of Cr(VI) ions on the m-poly(EG-VPBA) microparticles.

The thermodynamic parameters of the adsorption were reported in Table 5. The Gibbs free energy change values were -7.470, -6.630 and –7.049 kJ/mol at 277, 298 and 338K, respectively. The negative values of ∆G 0 confirm the feasibility of the process and the spontaneous nature of adsorption with a high preference for Cr(VI) m-poly(EG-VPBA) microparticles. The change in free energy for physisorption is usually between −20 and 0 kJ mol−1, whereas that for chemisorption is often in the range of −80 to −400 kJmol−1 [39]. The ∆G0 value obtained in this research indicated that the adsorption is physisorption. The

PT

standard enthalpy and entropy changes of adsorption were −8.875 kJ mol−1 and -6.001 J mol−1

RI

K−1, respectively (Table 5). The value of ∆H 0 is negative, indicating that the adsorption

SC

reaction is exothermic; hence, the amount adsorbed at equilibrium must decrease with increasing temperature (Fig. 5). Additionally, the reason why adsorption capacity increased

U

with increasing temperature can be explained by the value of ∆H 0 . The negative of ∆S 0

N

indicated that the randomness was decreased at the solid/liquid interface during the adsorption

M

A

of Cr(VI) ions onto m-poly(EG-VPBA) microparticles. The low value of ∆S 0 indicated that

TE

D

no remarkable changed on entrophy occurs [18].

CC

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3.6 Desorption and reusability study

The use of an adsorbent in the adsorption process depends not only on the adsorptive

A

capacity, but also on how well the adsorbent can be regenerated and used again. In addition, reusability is also one of the most important considerations in the applicability of adsorbents [16]. For repeated use of an adsorbent, adsorbed metal ions should be easily desorbed under suitable conditions. Desorption of the adsorbed Cr(VI) ions from the m-poly(EG-VPBA) microparticles were also studied in a batch experimental system. Desorption experiments put

into evidence that after 24 hours contact NaOH solutions (1.0 mol/L, desorption percentage 96%) were more efficient than HNO3 solutions (1.0 mol/L, desorption percentage 29%) to desorbs Cr(VI) ions for the adsorbent. The repeated use for NaOH solutions of m-poly(EGVPBA) microparticles show that adsorption–desorption process is reversible process. Five cycles of adsorption–desorption experiments were conducted to examine the capability of the m-poly(EG-VPBA) microparticles to retain Cr(VI) ions removal capacity. The adsorption capacity of the m-poly(EG-VPBA) microparticles was decreased only 6% during a 5

PT

adsorption–desorption cycle, which indicated that m-poly(EG-VPBA) microparticles had a

U

SC

3.7 Comparison of some adsorbents on Cr(VI) adsorption

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good reusability in the practical application.

N

A comparison between the adsorption capacities of m-poly(EG-VPBA) microparticles

A

and other magnetic adsorbents under similar conditions is presented in Table 6. From Table 6,

M

it can be seen that the maximum adsorption capacity of the m-poly(EG-VPBA) microparticles

D

were evaluated to be 816.8 mg/g, which was much higher than those of other magnetic

EP

TE

adsorbents reported in the literatures.

CC

4. Conclusions

A

In this study, the m-poly(EG-VPBA) microparticles were prepared suspension

polymerization and used for the removal of Cr(VI) in batch experiments. The effects of the pH, initial Cr(VI) concentration, temperature, and reusability were investigated. The optimum pH value for Cr (VI) adsorption is found at pH 2. Equilibrium isotherm data were fitted using Langmuir, Freundlich, and Dubinin-Raduskhevich (D-R) models. Among these models,

Langmuir and Dubinin-Raduskhevich (D-R) models are in good agreement with the experimental data with high R2. The kinetic parameters were evaluated utilizing the pseudofirst-order, pseudo-second-order, Ritchie’s-second-order, and the intraparticle diffusion kinetic models. The adsorption kinetics followed the mechanism of the pseudo-second-order equation for all systems studied. The adsorption of Cr(VI) dependence on temperature was investigated and the thermodynamic parameters ∆G0, ∆H0 and ∆S0 were calculated. The results show a feasible, spontaneous and exothermic adsorption process. The mechanism of

PT

adsorption includes mainly electrostatic interactions (physical interactions) between Cr(VI)

RI

ions and m-poly(EG-VPBA) microparticles. So the adsorption is a physisorption process. The adsorption–desorption cycle results demonstrated that the regeneration and subsequent use m-

SC

poly(EG-VPBA) microparticles would enhance the economics of practical applications for the

N

U

removal of Cr(VI) from water and wastewater.

A

Acknowledgement

M

This work was supported by the Research Fund of The University of Uludag Project Number:

A

CC

EP

TE

D

KUOP-2013/29 and Project Number: OUAP(F)-2012/28.

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Table Captions

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D

M

A

N

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ions, J. Mater. Sci. 45 (2010) 5291–5301.

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Table 1 Recipe and polymerization conditions for preparation of the m-poly(EG-VPBA)

A

microparticles. Table 2 BET surface area, pore volume and pore size of m-poly(EG-VPBA) microparticles. Table 3 Kinetic parameters for the adsorption of Cr(VI)ions onto the m-poly(EG-VPBA) microparticles.

Table 4 Parameters of Langmuir, Freundlich and Dubinin-Raduskhevich isotherm models, for the adsorption of Cr(VI) ions onto the m-poly(EG-VPBA) microparticles. Table 5 The thermodynamic values of the adsorption for the different solution temperatures used for Cr(VI) ions removal with the m-poly(EG-VPBA) microparticles. Table 6 Adsorption properties comparison of various magnetic adsorbents for removal of

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Cr(VI).

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Figure Captions

N

Figure 1 N2 sorption isotherms and pore size distribution (inset) of m-poly(EG-VPBA)

A

microparticles.

M

Figure 2 The SEM images of the m-poly(EG-VPBA) microparticles.

D

Figure 3 ESR spectrum of the m-poly(EG-VPBA).

TE

Fig. 4 Effect of pH on adsorption of Cr(VI) ions of the m-poly(EG-VPBA) microparticles.

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(initial solution concentration: 500 mg.dm-3; temperature: 25 °C; contact time: 510 min) Fig. 5 Effect of temperature on adsorption of Cr(VI) ions of the m-poly(EG-VPBA)

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microparticles (initial solution concentration: 500 mg.dm-3; pH: 2.0).

A

Figure 6 Effect of initial concentration of Cr(VI) ions onto the m-poly(EG-VPBA) microparticles at various temperatures. (pH: 2.0). Figure 7 Adsorption kinetics of adsorption of Cr(VI) ions by the m-poly(EG-VPBA) microparticles at different temperatures:(a) pseudo-first-order (b) pseudo-second-order (c) Ritchie’s-second-order (d) intraparticle diffusion.

Figure 8 Arrhenius plot. Figure 9 The plot of lnKL versus 1/T for the determination of thermodynamic parameters for

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PT

adsorption of Cr(VI) ions on the m-poly(EG-VPBA) microparticles.

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Table 1 Recipe and polymerization conditions for preparation of the m-poly(EG-VPBA)

N

U

microparticles.

M

Reactor volume: 100 mL Stirring rate: 375 rpm o

o

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TE

D

Temperature and time: 70 C (4h), 80 C (1h)

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Aqueous Dispersion Phase

Organic Phase

Distilled water: 50 mL

VPBA: 0.5 g

PVAL: 200 mg

AIBN: 200 mg

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Polymerization Conditions

Toluene: 10 mL EGDMA: 4.0 mL Fe3O4: 1.0 g