Novel Magnetically Doped Epoxide Functional Cross-linked

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Novel Magnetically Doped Epoxide Functional Cross-linked Hydrophobic Poly(lauryl methacrylate) Composite Polymer Particles for Removal of As(III) from Aqueous Solution Rukhsana Shabnam, Muhammad A. Rahman, Muhammad A. J. Miah, Mostafa K. Sharafat, Hasan M. T. Islam, Muhammad A. Gafur, and Hasan Ahmad Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01741 • Publication Date (Web): 14 Jun 2017 Downloaded from http://pubs.acs.org on June 15, 2017

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Industrial & Engineering Chemistry Research

Novel Magnetically Doped Epoxide Functional Cross-linked Hydrophobic Poly(lauryl methacrylate) Composite Polymer Particles for Removal of As(III) from Aqueous Solution

Rukhsana Shabnam†, Muhammad A. Rahman†, Muhammad A. J. Miah†, Mostafa K. Sharafat†, Hasan M. T. Islam‡, Muhammad A. Gafur§, Hasan Ahmad*† †

Department of Chemistry, Rajshahi University, Rajshahi 6205, Bangladesh

‡

Department of Chemistry, Begum Rokeya University Rangpur, Rangpur 5400, Bangladesh

§

Pilot Plant and Process Development Centre, BCSIR, Dhaka 1205, Bangladesh

*Phone +88-0721-711107; E-mail: [email protected]; [email protected]

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ABSTRACT: Superparamagnetic iron oxide nanoparticles have been found suitable as adsorbent materials for the removal of As(III) from aqueous solution. In this investigation the usefulness of magnetically doped epoxide functional cross-linked poly(lauryl methacrylate) (PLMA) composite polymer particles as an adsorbent bed for the removal of As(III) ions has been evaluated. The epoxide functional composite polymer particles are prepared by seeded polymerization of glycidyl methacrylate (GMA) in presence of crosslinked poly(LMAdivinylbenzene), P(LMA-DVB), seed particles. The surface of prepared composite polymer particles is finally doped with Fe3O4 nanoparticles. The epoxide functional magnetic composite polymer particles have been named as P(LMA-DVB)/PGMA/Fe3O4. A pH and contact time dependent adsorption behavior of As(III) is observed on P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles. The equilibrium (qe) reached after 180 min and a highest removal efficiency of 57.98% is attained at pH 5.0. The adsorption isotherm strictly followed Langmuir model with maximum theoretical adsorption capacity (qm) reached 66.23 mg/g of particles at 323K. Batch kinetic sorption experiments showed that a pseudo-second-order rate kinetic model is more applicable. The study of thermodynamic equilibrium parameters suggested that adsorption of As(III) is endothermic and spontaneous. The adsorbent could be regenerated partially by treatment with 0.01 M NaOH, retaining about 40% of the adsorption capacity after first time adsorption and then only slightly decreased.


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1. INTRODUCTION The water that we use in many ways is highly contaminated by toxic metal ions which is a worldwide environmental problem including Bangladesh. Due to undegradable nature, toxic metal ions accumulate in humans and other living organisms which in turn enter the food chains.1 Among the different toxic metals existed in water effluents, arsenic (As) is the most hazardous element. It is a metalloid, possessing both metallic and non-metallic properties. Short and long term intake of As contaminated water can possess risks for human health related problems such as spontaneous pregnancy loss, respiratory complications, immunological system disorders, kidney cancer as well as changes in pigmentation, skin thickening (hyperkeratosis), neurological disorders, muscular weakness, loss of appetite, nausea and black foot disease.2-7 Acute As poisoning causes vomiting, oesophageal and abdominal pain, and bloody “rice water” diarrhea.8-10 According to US Environmental Protection Agency, EPA, the maximum contaminant level is 50 µgL-1 which has recently been refixed to 10 µgL-1.11,12 As is mobilized in surface and ground water by a variety of ways such as natural weathering, chemical and biological reactions, volcanic eruptions and other anthropogenic activities.13 In addition mining activities, combustion of fossil fuels, uses of arsenic containing pesticides, herbicides, bactericides, wood preservatives, paints, drugs, dyes as well as arsenic additives to livestock feed add extra impact to the problem.14,15 The oxidation state of As plays an important role since it determines the toxicity, the sorption behavior and the mobility in the aquatic environment. As exists in four different oxidation states namely −3, 0, +3 and +5.16 Two inorganic forms of As are common in natural waters: arsenite (AsO33−) and arsenate (AsO43−), referred to as As(III) and As(V). Pentavalent (+5) or arsenate species are AsO43−, HAsO42−, H2AsO4−, while trivalent (+3) arsenites include 3 ACS Paragon Plus Environment


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As(OH)3, As(OH)4−, AsO2OH2− and AsO33−. Pentavalent species predominates and are stable in oxygen rich aerobic environments while trivalent arsenites predominate in moderately reducing anaerobic environments such as surface and groundwater.17 As(V) is believed to be less toxic than As(III) and is the main species in natural waters.10 Numerous technologies such as oxidation,18 precipitation or coprecipitation,19,20 coagulation,21,22 sorption,23,24 ion-exchange,25,26 reverse osmosis,27 and electrokinetic methods28 have been studied for the removal of As from water. But the use of these methods is limited due to high operation and waste treatment costs, high consumption of reagents and large volume of sludge formation.29 But generation of high quality effluent adsorption technique is recognized as a most promising versatile approach for the treatment of As contaminated water because of simplicity, low cost, high concentration efficiency, flexibility in design, operation and environment friendliness. In addition, adsorption processes are mostly reversible in nature. The adsorbents can be regenerated by suitable desorption processes for multiple use.30 Desorption processes has low maintenance cost, high efficiency, and ease of operation.31 Fe(III)-bearing materials has attracted much interest in As adsorption because of their high selectivity and affinity compared to other conventional adsorbent materials like activated carbon, soil, resin and alumina.32-35 In a research Yean et al. reported that magnetite iron oxide adsorbed As at pH below 9 but desorbed while the pH is adjusted to more than 10.36 Chowdhury et al. studied the adsorption of As and Cr by mixed magnetite and maghemite nanoparticles from aqueous solution and obtained maximum adsorption at pH 2.37 Morillo and his group studied the removal of As(III) and As(V) from acidic solutions with novel forager sponge-loaded superparamagnetic iron oxide nanoparticles.38 In acidic pH range, most of the As species in aqueous solution are negatively charged. Thus electrostatic attraction among magnetite4 ACS Paragon Plus Environment

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maghemite nanoparticles and metal species favored the removal of As compounds from water solution. However, the naked iron oxide magnetic nanoparticles are often insufficient for their poor

colloidal

stability,

hydrophilicity,

contamination

and

difficulty

for

further

functionalization.39 Moreover, particles in the nano-range despite being having high surface activity are not suitable for designing chromatographic adsorption bed because of the drainage probability with the effluent and hence could produce iron oxide contaminated effluent. In this regard for efficient removal of As, the use of micrometer-sized composite materials with magnetite (Fe3O4) shell layer could limit the washing out effect of adsorbent with discharged effluent. In this investigation preparation and finally application of micron-sized epoxide functional

magnetically

doped

hydrophobic

poly(lauryl

methacrylate-

divinylbenzene)/poly(glycidyl methacrylate)/Fe3O4 abbreviated as P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles will be discussed by measuring the adsorption behavior of As(III) from aqueous solution. LMA is a well-known hydrophobic long alkyl chain monomer and its polymer/copolymer offers extensive application potential as resins for chromatographic separation, water purification, oil absorbency agents, viscosity modifiers and oil-soluble drag reducers.40-42 The modification of crosslinked hydrophobic P(LMA-DVB)/PGMA composite particles by Fe3O4 nanoparticles is expected to promote easy separation as well as recovery from the dispersion medium following applications. Moreover the presence of epoxide functionality on the surface would help to bind the iron oxide nanoparticles with magnetic composite polymer particles via complexation and therefore would prevent their leaching into the effluent during water treatment.43 One more advantage of this hydrophobic micron-sized epoxide functional P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles is that they are useful for the removal



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of hydrophobic contaminants like textile dyes, organic pollutants, pesticides etc. from wastewater.44

2. Experimental Section 2.1. Chemicals and Instruments. LMA from Fluka Chemika (Switzerland) was washed with 10% NaOH aqueous solution to remove any inhibitor and finally passed through activated basic alumina by column chromatography. Crosslinking agent DVB from Sigma-Aldrich, Chemie (USA) (80% grade) was purified with aqueous 10% NaOH solution and subsequently dehydrated by stirring with anhydrous CaCl2. Benzoyl peroxide (BPO) from BDH Chemicals Ltd. (UK) was recrystallized from methanol and preserved in the refrigerator before use. Cationic azo-initiator 2,2′-azobis(2-amidinopropane)hydrochloride (V-50) from LOBA Chem., India, was recrystallized from water before use. GMA Fluka Chemika (Switzerland) and PVA from Thomas Baker (Chemicals) Limited (India) of molecular weight 1.4 x 104 gmol-1 were used without purification. As2O3 from May & Baker (UK), ferric chloride hexahydrate (FeCl3.6H2O), ferrous sulfate (FeSO4), NH4OH, oleic acid, KI, SnCl2, CHCl3, sodium diethyldithio-carbamate (NaS.CS.N(C2H5)2.3H2O), Zn-granules and other chemicals were of analytical grade. Deionized water was distilled using a glass (Pyrex) distillation apparatus. Scanning electron microscopy, SEM was performed to see the particle size distribution with a SU8000 microscope (Hitachi, Japan) operating at a voltage of 20 kV. Vibrating sample magnetometer, VSM (MicroSense, EV9, USA) was used for studying the magnetic property of composite polymer particles. FTIR (Perkin Elmer, FTIR-100, USA) was used to see the structural composition of the particles surface before and after As(III) adsorption. Adsorption


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study was performed by SP-300 (OPTIMA, Japan) single and UV-1650 pc (Shimadzu, Japan) double beam UV-visible spectrophotometers. 2.2. Preparation of Crosslinked P(LMA-DVB)/PGMA/Fe3O4 Composite Polymer Particles. P(LMA-DVB) polymer latex particles were prepared by suspension polymerization of LMA (3 g) and DVB (3 g) in presence of polymeric stabilizer PVA (1.2 g) in 200 mL distilled water using BPO (0.12 g) as oil soluble initiator. The polymerization was carried out in a threenecked round bottomed flask, mechanically stirred at 100 rpm and maintained at 75°C for 24 h under a nitrogen atmosphere. The P(LMA-DVB) copolymer particles were washed repeatedly with double distilled water. P(LMA-DVB)/PGMA composite polymer particles were prepared by seeded polymerization of GMA (1.5 g) in presence of P(LMA-DVB) latex particles (3 g) as seed particles dispersed in 150 g of distilled water utilizing V-50 (0.03 g) as initiator. The polymerization was continued for 12 h in a three-necked round bottomed flask at 70°C under a nitrogen atmosphere. P(LMA-DVB)/PGMA composite particles were washed by replacing the continuous phase with double distilled water following repeated centrifugation prior to the characterization. Magnetically doped P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles were prepared by co-precipitation of Fe2+ (0.556 g) and Fe3+ (0.6255 g) from their alkali aqueous solution (molar ratio 1: 2) containing 20 g of 25% NH4OH and 2.5 g of P(LMA-DVB)/PGMA composite polymer particles in 150 g of water. The magnetic composite polymer particles were washed repeatedly by magnetic separation and decantation. 2.3. pH- and Contact Time-Dependent Adsorption Study. The extent of As(III) adsorption was studied using a batch mode of experiment which was carried out by mixing 0.05



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g of P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles with 30 mL of 10 mg L−1 As(III) solution in a 100 mL stopper glass bottle. For pH dependent study the experiments were conducted at variable initial pH (3 to 6). The arsenic-composite mixture was magnetically stirred for 180 min at 303K. The composite polymer particles were finally separated by applying external magnetic field followed by centrifugation at 12,000 rpm. Centrifugation technique was employed to avoid the presence of any nonmagnetic dust particles and to improve the accuracy of the measurement. The supernatant was transferred to As generator and 5 mL conc. HCl, 2 mL KI solution and 0.5 mL SnCl2 were added in it. The content of the flask was swirled and then allowed to stand for about 15 min to ensure complete reduction of As to +3 state. The absorber tube was charged with 4 mL of the AgS.CS.N(C2H5)2. Granular Zn (5 g) was added to the solution in the flask and then the hydrogen sulfide scrubber was immediately inserted. The evolution of arsine (AsH3) was 99% completed within 40 min and the mixture was heated for another 5 min in water-bath at 75°C to complete the reaction. The absorber solution was transferred and its absorbance was measured at the wavelength of 535 nm by a UV-visible spectrophotometer. The amount of As adsorbed was calculated by subtracting the concentration in the medium from that of initial concentration. Calibration graph was used for this purpose. Likewise contact time-dependent adsorption measurements with variable contact time (5300 min) were carried out at the pH value of maximum adsorption (optimized from the above experiment) using the same procedure. For comparison, reference P(LMA-DVB)/PGMA composite polymer particles were also used to check the adsorption of As(III) under identical conditions at pH 5.0, optimized from pH dependent adsorption measurement on magnetic composite polymer particles.


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2.4. Adsorption Isotherm of As(III) onto the Composite Polymer Particles and Thermodynamic Parameters. Adsorption isotherm experiments were performed at different temperatures (293, 303, 313 and 323K) with different concentrations (5 to 15 mg L−1). The adsorption experiment was carried out for 180 min at pH 5.0 (optimum as found from the previous experiment). For each measurement, a mixture of 30 mL of respective As(III) solution was mixed with 0.05g composite particles. The absorption behavior of As on the composite polymer particles was measured using UV-Vis spectrophotometer as discussed above. The Langmuir model was used to explain adsorption process of homogeneous monolayer surfaces with the basic assumption that adsorption occurred at specific homogeneous sites of the adsorbent. Another model frequently used was the Freundlich to describe heterogeneous systems. Temkin adsorption isotherm that explains the chemisorptions mechanism of adsorption phenomenon was also used. The variation of the equilibrium association constant (Ka = KL) with temperature was analyzed in terms of van’t Hoff plots ln = −

∆

. +

∆

(1)

from which the thermodynamic parameters: free energy change (∆G◦); enthalpy change (∆H◦), and entropy change (∆S◦) were extracted. From the slope of the plot lnKa vs. 1/T of equation (1), the value of enthalpy change was determined. From the relationships: ∆ = − ln

(2)

and ∆ = ∆ − ∆

(3)

the values of ∆G◦ and ∆S◦ were estimated. 9 ACS Paragon Plus Environment


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2.5. Kinetics of As(III) Adsorption. The time dependent rate of As(III) adsorption was determined by batch experiments. For this experiment, 0.3 g of P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles were thoroughly mixed with 180 g of 10 mg L−1 As(III) solution. The pH value of the mixture was adjusted to 5.0 and the mixture was magnetically stirred at 303K for a period of 5–300 min (varying). Aliquots of residual As were analyzed at the different time intervals by a UV-visible spectrophotometer as described above. The adsorption kinetic data were fitted with a pseudo-first (Eq. 4) and pseudo-second-order models (Eq. 5): − = −

!"

=!

#

+$

(4)

(5)

% % !#

Where qe and qt are the amounts of adsorbed As (mg g−1) at equilibrium and at any time t (min), respectively. k1 (min−1) and k2 (g mg−1 min−1) are the equilibrium rate constants for pseudo-first and -second-order adsorptions, respectively. 2.6. Desorption of As(III). In order to evaluate the reuse of P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles adsorption-desorption cycles were carried out. At first, 30 mL of 10 mg L-1 As(III) solution was mixed with 0.05 g of P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles. The pH value was adjusted to 5.0 and the mixture was allowed to stand under stirring at 303K for 180 min. The composite particles were magnetically separated. The amount of adsorption was measured. Then for desorption, As(III)-adsorbed P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles were dried at 40°C for 24 h and then weighed up. Subsequently, the adsorbed As(III) on P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles was extracted by treating with 30 mL of 0.01~0.03 M NaOH as eluent for 24 h at room temperature. Then, the composite polymer particles were magnetically separated followed by centrifugation at 12000 rpm, and the concentration of desorbed As(III) in the supernatant was determined by UV-Vis


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spectrophotometer as discussed above. The recovered composite polymer particles were reused as adsorbent for As(III) adsorption. 2.7. Analytical Methods. At time t, the amount of adsorption, qt (mg g-1) was calculated using the following formula: =

'( )'" *

(6)

+

where Ct (mg L-1) is the liquid phase concentrations of As(III) at any time, C0 (mg L- 1) is the initial concentration of the dye in solution. V is the volume of the solution (L) and W is the mass of dry adsorbent (g). The amount of equilibrium adsorption, qe (mg g-1), was calculated using the formula =

'( )'# *

(7)

+

where C0 and Ce (mg L-1) are the liquid-phase concentrations of As(III) present initially and at equilibrium. The As(III) removal percentage,η was calculated as follows:

η=

'( )'# '(

× 100

(8)

where C0 and Ce (mg L-1) are the initial and equilibrium concentrations of the As(III) in solution. The difference between experimental and calculated values was measured by the root mean square (rms) parameter and used to determine how well models represent the experimental data. Chi-square statistic (χ2) was determined as follows: /0 = ∑

2!#34 )!5(6 7

%

(9)

!5(6

where qmod is the modeled amount of As(III) adsorbed (mg g−1) and qexp is the experimental amount of As(III) adsorbed (mg g−1). If data from the model are similar to the experimental data, χ2 will be small. Therefore, using the non-linear Chi-square test, it is necessary to analyze the data set to confirm the best-fit isotherm.28 11 ACS Paragon Plus Environment


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3. RESULTS AND DISCUSSION SEM image of P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles shows in Figure 1 implies that particles are bit polydispersed. This is the common feature normally observed for polymer particles prepared by suspension polymerization.45,46 The heterogeneous protruding surface structure of composite polymer particles suggests the deposition of Fe3O4 nanoparticles. The heterogeneous non-smooth surface structure confirms the modification of particle surface by Fe3O4 nanoparticles. Some free Fe3O4 nanoparticles may also have by-produced during in situ precipitation of Fe2+ and Fe3+ from alkaline solution. The average size and coefficient of variation of composite polymer particles are around 11.3 µm and 46% respectively. The EDX spectrum of P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles (not shown) indicates the presence of signal for Fe (2.54 atom%) and the amount of O (atom%) increases to 23.49% from 21.9% after magnetization of P(LMA-DVB)/PGMA composite particles.

Figure 1. SEM image of P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles.


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The magnetization curve of separated P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles was recorded using VSM at room temperature (Figure S1). The nearly zero coercivity and the reversible nature of the magnetization curve confirm the superparamagnetic character of composite particles at room temperature. The value of saturation magnetization is ~3.6 emu/g. This result suggests that the prepared P(LMA-DVB)/PGMA/Fe3O4 composite particles possess strong magnetic property. It was also possible to separate the magnetic composite polymer particles from the treatment solution using external magnetic field without employing time consuming separation process like centrifugation and sedimentation.

3.1. Adsorption Behavior of As(III). The application potential of the prepared P(LMADVB)/PGMA/Fe3O4 composite polymer particles was evaluated by studying adsorption behavior of As(III) from aqueous solution. The parameters measured are detailed below.

3.1.1. Effect of pH. The adsorption of adsorbate is often controlled by the pH of the solution as it will change both the activity of active sites of adsorbent and the state of adsorbate. Therefore it is necessary to optimize the pH value of the solution for understanding the adsorption behavior of As(III) on P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles. Figure 2 shows that the adsorption of As(III) increases from 1.95 mg g-1 at pH 3.0 to 3.48 mg g-1 at pH 5.0 and then decreases with further increase in pH value. Consequently, the removal or uptake efficiency of As(III) by P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles also shows similar behavior and reaches maximum (57.98 %) at pH 5.0 (see inset Figure). In general variation of pH affects the surface chemistry of the iron oxides. It is noteworthy to mention that above the pH value of 3.0, the hydroxyl groups at the surface of the iron oxide are doubly



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protonated (-FeOH2+) and the surface charge of iron oxide is thus positive.47 While at a certain pH, the hydroxyl group is protonated with single proton (-FeOH) having no (net) surface charge on the iron oxide. The value of this pH, called the point of zero charge, ranges between 5.5 and 9.0 for iron oxides.47 At pH value higher than point of zero charge the hydroxyl group is deprotonated (-FeO-), and thus the iron oxide surface bears a negative charge. In the present investigation the maximum adsorption of As(III) at pH 5.0 is therefore due to the electrostatic attraction between the positive charge of the iron oxide surface and the anionic form of arsenite (AsO33-). The iron oxide is negatively charged at pH values above the point of zero charge which causes electrostatic repulsive forces with negatively charged arsenite and hence decreases the amount of As(III) adsorption. Comparatively the adsorption magnitude of As(III) at pH 5.0 on the reference P(LMA-DVB)/PGMA composite polymer particles was almost zero (data not shown). This result indicates that the adsorption of As(III) by P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles is resulted from the interaction with iron oxide nanoparticles rather than with functional epoxide groups of the polymer matrix. Few literatures are also available which supports the interaction of As with iron oxide nanoparticles.37,38


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3.8 3.4 3.0 60

Removal of Arsenic (%)

qe (mg g-1)

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2.6 2.2

50

40

30 2.5

3.5

4.5

5.5

6.5

pH

1.8 2.5

3.5

4.5 pH

5.5

6.5

Figure 2. Effect of pH on adsorption of As(III) on P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles at ambient temperature. Inset Figure shows the removal percentage of As(III). Conditions: As(III), 10 mg/L; Polymer solid, 0.05 g; contact time, 180 min; total volume, 30 mL; temperature, 303K. 3.1.2. Effect of Contact Time. Contact time is crucial for the determination of rate of adsorption process. The effect of contact time on the adsorption amount (mg g-1) of As(III) was measured up to 300 min at pH 5.0 where the initial arsenic concentration was 10 mg L-1 for 0.05 g of P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles. Figure 3 shows that initially the adsorption of As(III) is slow and gradually increases until attaining a steady value after reaching the equilibrium at about 180 min. This contact time was selected as the equilibrium time for subsequent experiments. The initial fast adsorption rate is due to higher initial concentration of As and large number of available vacant active sites on the composite particle surface. The adsorption did not increase beyond the equilibrium time owing to the significant decrease in the number of active sites on the adsorbent. It is worthwhile to note that a higher amount of



composite polymer particles (~0.1 g) is necessary to effectively reduce the As(III) concentration from 10 mg L-1 to below the US EPA recommended level of 10 µg L-1. 3.5

3.2 qe (mg g-1)

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2.9

2.6

2.3 0

100

200

300

Contact time (min)

Figure 3. Effect of contact time on the adsorption of As(III) on P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles. Conditions: As(III), 10 mg/L; Polymer solid, 0.3 g; total volume, 180 mL; pH, 5.0; temperature, 303K. 3.1.3. Effect of Initial Concentration and Temperature. The effects of initial As concentrations (C0) in the range of 5 to 15 mg L-1 on the equilibrium amount of adsorption (qe) (investigated under the optimized conditions; pH: 5.0 and contact time: 180 min) at different temperatures are shown in Figure 4. This measurement was carried out to see the effect of initial concentration and temperature on the amount of adsorption rather than to find the equilibrium initial concentration. In the experimental range the amount of adsorption at different temperatures increases with the increase of initial concentration. The increase of initial concentration enhances the driving force for transferring As(III) ions from the aqueous phase to the composite particle surface and hence increases the interaction between As(III) and magnetic


Page 17 of 39

nanoparticles present at the particle surface. This effect is more pronounced at higher temperature. Therefore, a slightly higher temperature is found to be favorable for the adsorption of As(III) onto P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles. 6

5 293 K

qe (mg g-1)

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4

303 K 313 K 323 K

3

2

1 0.004

0.008

0.012

0.016

Initial Concentration, C0 (mg mL-1)

Figure 4. Initial As(III) concentration and temperature dependent adsorption behavior on P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles. Conditions: Polymer solid, 0.05 g; total volume, 30 mL; contact time, 180 min; pH, 5.0. 3.1.4. Equilibrium Study. The factors such as heterogeneity/homogeneity of adsorbents, the type of coverage and possibility of interaction between the adsorbate species determine the distribution of the adsorbate species among liquid and adsorbent which can be described by the mathematical models of adsorption isotherms like Langmuir, Freundlich and Temkin. These isotherms relate equilibrium amount adsorbed per unit mass of adsorbent, qe, to the equilibrium concentration of adsorbate in the bulk fluid phase Ce.



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The main point of assumptions of the Langmuir model48,49 are as the maximum adsorption occurs when a saturated monolayer of solute molecules is present on the adsorbent surface, there is no migration of adsorbate molecules in the surface plane and the energy of adsorption is constant. The Langmuir isotherm is given by: =

!5 89 '# :89 '#

(10)

By plotting (1/qe) versus (1/Ce), the constants in the Langmuir isotherm can be determined making use of above equation rewritten as:

!#

= ! + ! 5

5 89

'

(11)

#

where the Langmuir constants, qm represents the maximum adsorption capacity for the solid phase loading and KL represents the energy constant related to the heat of adsorption. The plots of 1/qe against 1/Ce for the experimental data (Figure 5) exhibit high correlation coefficient (R2>0.99) of the linearized Langmuir equation (Table S1). Comparative results reveal that the adsorption data fits the Langmuir adsorption isotherm best at 323K as correlation coefficient is maximum at this temperature. Overall, Langmuir model can explain the adsorption of As(III) on P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles. The calculated values of qm and KL obtained from the intercept and slope of the respective straight line are presented in Table S1. The maximum adsorption capacity (qm) of P(LMA-DVB)/PGMA/Fe3O4 for As(III) is 21.51 mg g-1 at 293K and increases with increasing temperature. Thus corresponding to maximum adsorption capacity it is favorable to carry out adsorption at higher temperature. It can be mentioned that the surface of P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles is homogeneous and the adsorption of As(III) formed a monolayer on its outer surface.50


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0.90

0.70

1/qe

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0.50

293 K 303 K 313 K

0.30

323 K

0.10 0.10

0.17

0.24

0.31

0.38

0.45

1/Ce

Figure 5. Langmuir isotherms of As(III) adsorption on P(LMA-DVB)/PGMA/Fe3O4 composite polymer particles at different temperatures. Conditions: Polymer solid, 0.05 g; total volume, 30 mL; pH 5.0; contact time, 180 min. The dimensionless constant called the separation factor (RL) is an essential feature of the Langmuir isotherm model. This shows the nature of the adsorption process as unfavorable (RL>1), favorable (0

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