Microwave assisted preparation of glycidyl methacrylate grafted cellulose adsorbent for the effective adsorption of mercury from a coal fly ash sample

Journal of Environmental Chemical Engineering 1 (2013) 1359–1367

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Microwave assisted preparation of glycidyl methacrylate grafted cellulose adsorbent for the effective adsorption of mercury from a coal fly ash sample A. Santhana Krishna Kumar, M. Barathi, Swetha Puvvada, N. Rajesh * Department of Chemistry, Birla Institute of Technology and Science, Pilani-Hyderabad Campus, Jawahar Nagar, Shameerpet Mandal, R.R. Dist, 500 078 AP, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 8 June 2013 Received in revised form 4 October 2013 Accepted 8 October 2013

Biopolymer materials are known for their excellent attributes in diverse applications. In this work, we present a novel microwave assisted preparation of glycidyl methacrylate (GlyMA) grafted cellulose adsorbent for the effective adsorption of mercury. The graft polymerization of glycidyl methacrylate onto cellulose in polar aprotic solvent dimethylformamide (DMF) medium was investigated comprehensively for the removal of mercury. The grafting was confirmed through various characterization techniques such as FT-IR, powder XRD, SEM and EDS analysis. The adsorption was effective at pH 5.0 in 1.0 M NaCl medium with 25 mL of 50 mg L1 Hg(II) solution and a Langmuir adsorption capacity of 37.03 mg g1 could be attained. The electrostatic interaction between the tetrachloromercurate(II) anion and the protonated oxygen atoms in the monomer GlyMA is the plausible mechanism. The adsorption process is spontaneous and endothermic with a positive entropy change and the pseudo second order kinetics favours the interaction. Effective desorption of mercury is achieved using KI thereby regenerating the adsorbent material. The utility of the adsorbent is recognized in its application towards the adsorption of mercury from a coal fly ash sample. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: Cellulose Glycidyl methacrylate Mercury Microwave Coal fly ash

Introduction Heavy metal toxicity is a global concern and mercury is not an exception. The bioaccumulation of mercury [1], neurotoxicity, cell damage effects [2] and the stringent limit of 2.0 mg L1 [3] as specified by EPA in drinking water warrants effective methods for its detoxification. Contamination of mercury could arise from chlor alkali industries, battery, coal fly ash and electronic manufacturing plants [4]. The permitted discharge limit of wastewater for total mercury is 10.0 mg L1 [5]. A variety of methods such as ion exchange [6], chelating [7,8] and adsorption techniques [9] are known for the removal of mercury. Synthetic triethylenetetramine modified polystyrene resin shows good potential towards the removal of mercury [10]. The limitations of some of the above methods are in the removal at low concentration levels [11]. Considering the eco friendly and cost-effective methods, biopolymers prove to be a viable and affordable alternative for the adsorption of mercury. As a natural ubiquitous biopolymer, cellulose is a chief polysaccharide of plant cell walls [12] known for its ability to adsorb metal ions [13–15]. In view of the lower adsorption capacity of unmodified cellulose, functionalization or immobilization techniques are necessary to enhance the selectivity towards a particular metal ion [16]. Functionalization of

* Corresponding author. Tel.: +91 40 66303503; fax: +91 40 66303998. E-mail address: [email protected] (N. Rajesh). 2213-3437/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.10.004

cellulose in its hydroxyl group is used for the adsorption of heavy metals and these yield desirable properties such as higher adsorption capacity, selectivity and fast sorption kinetics. Different cellulosic adsorbents can be synthesized by functionalization as well as immobilization on its surface. Specifically, two main approaches are utilized for adsorbing heavy metal ions from aqueous solution: (i) direct modification, involving the cellulose backbone by introducing chelating functionalities, (ii) grafting of specific monomers to the cellulose backbone and subsequent functionalization of these grafted polymer chains. Densification, grafting, amination and ethylation are some of the commonly used methods for modifications. Lei et al. [17] have reported the preparation of an anion exchanger based on TiO2 densified cellulose for expanded bed adsorption. Surface functionalization of cotton cellulose with glycidyl methacrylate has been utilized for the adsorption of aromatic pollutants from wastewaters [18]. Functional polymers that contain donor atoms such as N, S, O, and P can form coordinate bonds with many heavy metals [19]. Aniurudhan et al. [20,21] have reported the application of amino functionalized poly(glycidyl methacrylate)-grafted cellulose in the removal and recovery of vanadium(V) and arsenic from aqueous solutions. Shibi and Anirudhan [22] grafted acrylamide onto banana stalk using the ferrous ammonium nitrate/H2O2 redox initiator for the adsorption of mercury. The material was then aminated by reacting it with ethylenediamine and then refluxed with succinic anhydride to functionalize it with carboxylate groups. Bicak et al. [23] grafted polyacrylamide onto cotton

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cellulose using ceric ammonium sulphate as initiator for the removal of Hg(II) from aqueous solution and achieved a uptake level of 12.5 mg g1. The adsorption of Hg(II) onto microwave induced poly(acrylic acid) modification of Cassia javanica seed gum grafted copolymer was studied by Singh et al. [24]. Some of the distinct functionalization reagents for cellulose include o-benzenedithiol [25], polyethyleneimine [26], 6-deoxy-6-mercaptocellulose [27] and polyacrylamide grafted coconut coir pith [28]. Microwave irradiation [29–34] is one of the very effective methods for activating chemical reactions in a homogeneous and selective manner. Orlando et al. [35] have studied the application of microwave radiation for the adsorption of copper and mercury from the reaction of urea with the active sites present in sugarcane bagasse. The innovations in the methodologies have evolved with a view to overcome some of the drawbacks of the existing methods when applied to a specific problem. Since, each methodology differs with respect to selectivity and sensitivity, their field application depends on the nature of the samples to be treated. A cautious introspection reveals that there is an undeniable need to look for more effective remediation strategies. Taking into cognizance these aspects and other factors such as availability, biodegradability and a greener approach efforts were focused towards the development of a novel microwave assisted approach in the preparation of glycidyl methacrylate grafted cellulose adsorbent for mercury detoxification from a coal fly ash sample.

where V is the volume of the aqueous solution (L) and W is the weight of the GlyMA-cellulose adsorbent (g). The CVAAS calibration data were acquired by reduction using SnCl2 wherein, the elemental mercury that is released absorbs the radiation at 253.7 nm and this is correlated to the mercury content in the aqueous solution. Instrumentation Mercury analysis was performed using a MA 5840, ECIL India model analyser by the cold vapor atomic absorption technique in which stannous chloride in acidic medium is utilized to reduce the Hg2+ to mercury vapor. The pH optimization was done using an LI127 pH metre supplied by Elico, India. Orbital Incubator shaker was procured from Biotechnics, India. A Jasco-4200 FT-IR spectrometer was used to characterize the functional groups in the GlyMA grafted cellulose adsorbent in the range 400–4000 cm1. Powder XRD (Philips PANalytical X’PERT PRO diffractometer operating at 40 kV and 30 mA, step size of 0.0178) was utilized to record the characteristic changes in the diffraction pattern of the GlyMA grafted cellulose adsorbent. The scanning electron microscopy (SEM) micrographs and Energy dispersive spectral analysis (EDS) of the adsorbent prior and consequent to the adsorption of mercury were recorded with a JEOL JSM-6390 analyzer. Results and discussion

Experimental Characterization of the glycidyl methacrylate-cellulose adsorbent Chemicals Analytical grade reagents were used and Milli Q water (Elix-3) was used to prepare aqueous solutions of Hg2+ of different concentrations. The cellulose biopolymer was obtained from Himedia, India. Mercury(II)chloride was procured from Merck, India. The monomer glycidyl methacrylate and dimethylformamide (DMF) were obtained from Sisco Research Laboratories, India. A working solution of 50 mg L1 Hg2+ was prepared by appropriate dilution from 1000 mg L1 stock Hg2+. All other necessary reagents required for optimization of various parameters were procured from Merck, India. Adsorbent preparation and batch adsorption studies

The FT-IR spectra of the glycidyl methacrylate grafted cellulose and Hg(II)-treated adsorbent were examined to understand the mechanism of interaction of Hg(II) with the adsorbent (Fig. 1). The glycidyl methacrylate grafted cellulose adsorbent shows characteristic peaks of cellulose O–H, C–O, glycosidic linkages [38] and a distinct peak at 1714 cm1 due to C5 5O group in the glycidylmethacrylate monomer [39] as well as three bands at 1321, 816, and 699 cm1 attributed to epoxy ring [39]. This confirms the grafting of GlyMA onto the cellulose biopolymer surface. The O–H, C–H stretching peaks appear at 3343 cm1, 2895 cm1 and after the adsorption of mercury the changes are reflected in the shift in the O–H, C5 5O peaks to 3356 cm1 and 1720 cm1, respectively. This shows that the tetrachloromercurate anion interacts with

[(Fig._1)TD$IG]

Cellulose (6.0 g), in 15 mL of dimethylformamide (DMF) and glycidylmethacrylate (0.023 mole) were taken in a 100 mL R.B flask and irradiated in a Godrej (Model No. GMS 17M07 WHGX, 1200W) domestic microwave oven for 3 min. The percentage of grafting is obtained using the expression [36] %Grafting ð%GÞ ¼

W1  W0  100 W0

(1)

where W1, W0 denote the weight of the grafted cellulose and the weight of original cellulose. The percentage grafting was found to be 91.6. The batch experiments were performed by equilibrating 0.5 g of GlyMA grafted cellulose adsorbent material with a known volume (V) (25 mL of 50 mg L1 Hg(II) solution in 1.0 mol L1 NaCl) adjusted to pH 5 in an orbital incubator shaker for different time intervals and the concentration of Hg(II) in the solution phase was found out by Cold Vapour-Atomic Absorption Spectrophotometric (CV-AAS) technique [37]. The amount of mercury adsorbed (qe) in mg g1 was calculated from the difference between the initial Hg(II) concentration Co and equilibrium concentration, Ce (mg L1) using the expression: qe ¼

ðC o  C e ÞV W

(2)

Fig. 1. FT-IR spectrum showing the involvement of different functional groups in (A) glycidyl methacrylate grafted cellulose adsorbent. (B) After the adsorption of Hg2+.

[(Fig._2)TD$IG]

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Fig. 2. Powder XRD pattern of (A) glycidyl methacrylate grafted cellulose adsorbent. (B) After the adsorption of Hg2+.

protonated hydroxyl and carbonyl oxygen atoms through electrostatic interaction. The interaction of glycidyl methacrylate with cellulose results in the appearance of peaks in the XRD pattern (Fig. 2) at 2u values 15.948, 18.108, 22.18, 25.268, 25.748 and 39.238, respectively [40]. The sharpness of the peak reflects the crystalline nature of the grafted adsorbent. The secondary interaction of mercury with the cellulose-grafted with glycidyl methacrylate adsorbent gives characteristic new peaks at 16.08 and 48.28. These results indicate that the mercury ion could adsorb onto the adsorbent through the OH, C5 5O, CH–OH functional groups through electrostatic interaction. The SEM images obtained prior and after the adsorption of mercury show (Fig. 3) some apparent changes in the surface morphology of the GlyMA-cellulose adsorbent with regard to the porosity and surface homogeneity. Cellulose is characterized by the distinct intermolecular hydrogen bonds. However, the porous SEM image of grafted and grafted cellulose (Fig. 3) indicated that the addition of monomeric units onto cellulose impede the formation of the intramolecular hydrogen bonds [39] resulting in a loose porous structure favourable for the metal ion adsorption on its surface. Indeed, after the adsorption of mercury, the active adsorption sites are covered with the metal ion as evident from the shiny white surface that exists in the SEM image after adsorption. The adsorption of mercury on the surface of GlyMA-cellulose adsorbent was also confirmed through the energy dispersive X ray (EDS) spectral analysis (Fig. 4). The characteristic peak in the range 1–3 keV confirms the adsorption of mercury on the surface of the GlyMA-cellulose surface [41].

Interaction of mercury(II) with GlyMA-cellulose adsorbent The hydroxyl groups play an important role during microwave irradiation. The localized rotations on an almost immobile OH group arising as a result of the dielectric heating involve energy transfer to surrounding solvent molecules [42]. The cleavage of the OH bond results in the ensuing interaction with GlyMA (Fig. 5) with the lowering of Gibb’s energy of activation [43]. The adsorption of mercury(II) was found to be quantitative (99.8%) in the pH range 4–6 with 1.0 mol L1 NaCl [44]. This was ascertained from the filtrate that was analysed for the mercury content as the difference in the concentration (;Co–Ce) remaining in solution. The tetrachloromercurate(II) anion by virtue of its high stability constant [44] is prone to interact with the protonated OH groups in the cellulose surface through electrostatic interaction. Furthermore, after grafting the ester carbonyl groups also could be protonated in weakly acidic medium and this further reinforces the electrostatic interaction with tetrachloromercurate(II) anion. At higher pH (greater than 6), Hg(II) could hydrolyse to give the species HgOHCl; HgOHþ and HgðOHÞ2 [45] which leads to the decrease in the percentage adsorption of mercury(II). These hydroxyl species could also compete for the active adsorption sites in the grafted polymer surface thereby resulting in a decrease in the percentage adsorption of mercury. Adsorption kinetics and isotherm models The amount of Hg(II) adsorbed increased with time (Fig. 6) and reached saturation within 45 min of equilibration. The

[(Fig._3)TD$IG]

Fig. 3. SEM images showing the surface morphology of (A) glycidyl methacrylate grafted cellulose adsorbent. (B) After the adsorption of Hg2+.

[(Fig._4)TD$IG]

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Fig. 4. EDS spectral analysis showing the presence of adsorbed mercury along with other elements.

[(Fig._5)TD$IG]

Fig. 5. Scheme depicting the interaction of cellulose, glycidyl methacrylate and tetrachloromercurate(II) anion.

[(Fig._6)TD$IG]

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2.45 2.40 2.35

-1

qt /(mg g )

2.30 2.25 2.20 2.15 2.10 2.05 2.00

0

10

20

30

40

50

Time /(min) Fig. 6. Plot of amount of mercury adsorbed qt against time.

experimental data obtained from the adsorption of mercury onto the GlyMA grafted cellulose adsorbent were fitted using the Lagergen [46] and Ho [47] first and second order kinetic models. The expressions relating these are logðqe  qt Þ ¼ log qe 

k1 t 2:303

t 1 t ¼ þ qt k2 q2e qe

(3)

(4)

The kinetic plot of log(qe  qt) against t (Fig. 7A) and t/qt against t (Fig. 7B) gives the respective rate constants. The higher regression coefficient value obtained from the pseudo second order model is an appropriate fit to the adsorption data. The qe calculated and qe exp values were found to be 2.52 and 2.43 mg g1 and this further confirms the applicability of this model in describing the adsorption kinetics. The kinetic constants k1 and k2 were found to be 0.093 min1 and 0.202 g mg1 min1 with regression coefficients 0.86 and 0.99, respectively. In general, pore, surface and intraparticle diffusion could be the probable modes of adsorption of mercury onto the GlyMA grafted cellulose adsorbent surface. Intraparticle diffusion can be best described using the Weber–Morris [48] diffusion model which relates the amount of mercury adsorbed at time t to the intraparticle rate constant (kint) pffiffi (5) qt ¼ kint t þ C A plot of qt against the square root of time gives a non-zero intercept (Fig. 7C) and hence, boundary layer effect [49] could also influence the adsorption kinetics of mercury onto the GlyMA grafted cellulose adsorbent. The intraparticle rate constant kint was found to be 0.099 mg g1 min(1/2). The amount of mercury(II) adsorbed qt increases with the time of adsorption t, and this indicates that in the initial stages the external surface diffusion could control the adsorption kinetics followed by intraparticle diffusion process in defining the rate of adsorption of mercury onto the adsorbent surface. The interaction between the GlyMA grafted cellulose sorbent and Hg(II) from aqueous solution in dynamic equilibrium can be represented as

GlyMA-cellulose þ

2 HgCl4

2 , HgCl4

Essentially, the Langmuir isotherm assumes monolayer coverage, with equivalency in all sites and also negligible interaction between the adsorbed molecules. The site equivalency and the ability of a molecule to adsorb at a particular site independent of the occupation of the neighbouring sites also implies that the enthalpy of adsorption is identical for all sites and independent of the surface coverage. The well-established Langmuir, Freundlich and several other familiar isotherm models (Table 1) were used to fit the experimental adsorption data [50–55]. The isotherm constants (Table 1) are obtained from the corresponding isotherm plots (Fig. 7D–H). The maximum adsorption capacity (qo) and the adsorption energy b in the Langmuir isotherm are obtained from the plot of Ce/qe against Ce (Fig. 7D). A good Langmuir adsorption capacity of 37.03 mg g1 signifies the ability of GlyMA grafted cellulose to adsorb mercury (II) from solution. The dimensionless separation [56] factor obtained (RL = 1/1 + bCo) from the Langmuir model which is less than 1 further augments the interaction between the graft copolymer and the metal ion. The applicability of Langmuir isotherm model could also be corroborated from value of the exponent g obtained from the Redlich–Peterson (R–P) isotherm model (Fig. 7E). A value close to unity in R–P isotherm model indicates the extent to which the Langmuir isotherm model could be utilized to fit the experimental adsorption data. In the present adsorption system, the value of the exponent (0.68) and low regression coefficients in both these models show that there could be other isotherms that would give a better fit to the adsorption data. The Freundlich isotherm plot of log qe versus log Ce (Fig. 7F) gives the index of adsorption n and the adsorption capacity KF. The adsorption intensity n and the high regression coefficient of 0.92 reflect the effective uptake of mercury by the GlyMA grafted cellulose adsorbent. The mean free energy of adsorption, EDR obtained from the Dubinin–Raduksveich (D–R) model (Fig. 7G) indicates physical adsorption between mercury and the GlyMA grafted cellulose adsorbent. The low binding energy b obtained in the present study involving the Temkin isotherm (Fig. 7H) illustrates the electrostatic interaction between the tetracholoromercurate(II) anion and the GlyMA grafted cellulose adsorbent. The Elovich isotherm are obtained from the plot relating ln(qe/Ce) against qe as given in (Fig. 7I) as given in Table 1.

Table 1 Various isotherm parameters studied for the adsorption of mercury. Isotherm

Linearized expression

Parameters

Values

Langmuir

Ce qe

qo (mg g1) b (L mg1) RL r2

37.03 0.0126 0.613 0.68

Freundlich

log qe ¼ log K F þ 1n log C e

KF (mg11/n g1 L1/n) n r2

1.6307 1.967 0.92

Redlich–Peterson

lnðAC e =qe  1Þ ¼ g lnðC e Þ þ lnðBÞ

g B (L mg1) A (L g1) r2

0.686 0.0619 0.466 0.88

Dubinin– Radushkevich

ln qe ¼ ln qm  be2

qm (mg g1) b (mol2 kJ2) E (kJ mol1) r2

9.69 0.5596 0.9452 0.32

Elovich

ln

Temkin

ln A þ RT ln C e qe ¼ RT b b

¼ q1b þ Cq e o

  qe Ce

o

¼ lnðK E qm Þ  qqe

 GlyMA-cellulose ðsurfaceÞ

The free Hg(II) and the adsorbed mercury on the GlyMA cellulose surface in such a dynamic equilibrium can be described by simple isotherms such as Langmuir and Freundlich models.

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m

qm (mg g1) KE (L mg1) r2 B A (L mg1) r2

19.84 0.031 0.33 4.243 0.497 0.67

[(Fig._7)TD$IG]

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Fig. 7. Kinetics of adsorption of mercury. (A) Pseudo first order kinetics, (B) pseudo second order kinetics, (C) intraparticle Weber–Morris plot of qt against t1/2, (D) Langmuir isotherm plot of (E) Redlich–Peterson isotherm, (F) Freundlich isotherm, (G) Dubinin–Radushkevich isotherm, (H) Temkin isotherm, (I) Elovich isotherm and (J) Van’t Hoff plot showing the variation of ln K with temperature.

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Thermodynamics of adsorption The adsorption thermodynamics gives more insight into the mechanistic aspects of the interaction between the metal ion and the GlyMA grafted cellulose. This is obtained through the Gibb’s free energy (DG8), enthalpy (DH8) and entropy (DS8) changes in the overall adsorption process and the Van’t Hoff equations [57] are used to calculate these parameters:

DG ¼ RT ln K ln K ¼

DH DS þ RT R

(6) (7)

The equilibrium constant K is obtained from the ratio of concentration of Hg(II) adsorbed on the GlyMA grafted cellulose adsorbent to that in the solution. The DH8 values determine the endothermic or exothermic nature of the interaction. The entropy and enthalpy parameters are acquired from the slope and intercept of ln K against 1/T plot (Fig. 7J). The negative free energy (DG8) values and the magnitude of DH8 gives information about the adsorption mechanism and for physical adsorption, DH8 is by and large lower than 80 kJ mol1. The equilibrium constant increases with temperature and hence higher temperatures favour the endothermic adsorbent–adsorbate interaction. The positive entropy change associated with the electrostatic interaction also reflects the increased randomness at the GlyMA grafted cellulose adsorbent-solution interface. The DG8 values at four different temperatures (293, 303, 313 and 323 K) were found to be 1.40, 3.04, 4.72 and 6.94 kJ mol1, respectively. The entropy and enthalpy changes were found to be 180 J mol1 K1 and 51 kJ mol1, respectively. When the monomer, GlyMA is dispersed in DMF medium, the molecules tend to be disordered in order to facilitate the interaction with cellulose resulting in a gain in the translational entropy arising from the GlyMA grafted biopolymer. As a result there is an increase in the entropy of adsorption. The free energy and entropy changes associated with the interaction of the monomer with cellulose plays a significant role in the ensuing interaction with mercury(II). The hydroxyl groups in the monomer GlyMA orient in such a manner so as to facilitate the effective electrostatic interaction with tetrachloromercurate(II) anion. The change in the free energy can be expressed as the difference in the enthalpy and the entropy changes of the respective components as

DGGlyMA ¼ DHðGlyMAÞ  T DSðGlyMAÞ

(8)

DGadsorption ¼ DHadsorption  TðDScellulose þ DSGlyMA þ DSHgðIIÞ Þ

(9)

The overall entropy contribution arises from the individual entropy changes associated with the cellulose, monomer and the mercury(II) ions in aqueous solution. Hence, the sum of these individual entropy changes is positive and this is reflected in the increased randomness at the GlyMA grafted biopolymer-solution interface. The large entropy and the positive enthalpy contribution (DH > 0) could also be ascribed to the desolvation with the corresponding decrease in the degree of hydration [58]. The increase in entropy that stems from the decrease in the structural constraint on the solvent leads to clustering of the hydrophobic groups in the biopolymer. The positive entropy change in this adsorption process augments the total degrees of freedom and this could be attributed to the effective interaction between the mercury and the GlyMA grafted cellulose sorbent. Column study A glass column (25 cm length) was filled with 3 g of the GlyMA grafted cellulose adsorbent up to a height of 3.0 cm. A 200 mL volume of 10 mg L1 Hg(II) was delivered onto the column at a

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flow rate of 10 mL min1 .The retention of mercury as tetrachloromercurate(II) anion with the protonated hydroxyl group in the GlyMA grafted cellulose adsorbent column was quantitative up to 250 mL sample volume. Above 250 mL, there is a decrease in the amount of mercury adsorbed and this could be attributed to the column expansion [45] thereby disturbing the close packing of the GlyMA-cellulose adsorbent in the column. An optimum flow rate of 10 mL min1 ensures effective contact between mercury(II) and the polymeric adsorbent. The performance of an adsorbent could also be assessed based on the adsorbent exhaustion rate. This parameter is expressed as the ratio of the mass of the GlyMA-cellulose adsorbent to the maximum sample volume in litres [59]. With a short laboratory column containing 3.0 g of the adsorbent, the exhaustion rate of the adsorbent was found to be 12 g L1. A lower value denotes the efficacy of the adsorbent towards tolerating a particular sample volume. With an increase in the amount of the adsorbent, the sample volume tolerated would also enhance thereby leading to a higher efficiency in adsorption. Adsorbent regeneration As the mercury(II) ion approaches the GlyMA grafted cellulose surface, the potential energy decreases as it becomes physisorbed effectively. The regeneration of the adsorbent and the subsequent desorption of mercury from the GlyMA grafted cellulose surface is an activated process since the adsorbed metal ion needs to effectively lifted from the base of the potential energy well. The choice of a reagent for desorption depends on its ability to interact with mercury(II) in relieving the adsorbent–adsorbate interaction. In this regard, reagents such as KI, EDTA, KSCN and thiourea [45,60,61] were explored and we observed that KI was the most efficient in desorbing mercury as its tetraiodomercurate(II) anion into the aqueous phase. The soft–soft interaction (Hg–I) favours the facile desorption in transfer of the mercury(II) ion from the GlyMA grafted cellulose adsorbent surface. Application to coal fly ash sample Fly ash emanating from thermal power plants constitutes a very prominent source of mercury pollution [62]. A known weight of the sample collected from a thermal power plant was digested using HF–HNO3–H2SO4 mixture [45,63] filtered and diluted to a known volume. Hg(II) was adsorbed on the column as tetrachloromercurate(II) anion with the GlyMA grafted cellulose adsorbent material. The concentration of mercury in the aqueous phase was measured using cold vapor atomic absorption technique. The concentration of mercury in the fly ash sample was found to be 0.30  0.05 mg g1 with three replicate measurements. The GlyMA grafted cellulose adsorbent was effective in the adsorption of mercury and furthermore, the adsorbed mercury could also be desorbed quantitatively using KI.

Table 2 Comparison of adsorption capacity against other sorbent materials. Adsorbent material

Adsorption capacity (mg g1)

References

Grafted polyacrylamide onto cotton cellulose o-benzenedithiol-modified cellulose Naphthalimide-functionalizedFe3O4@SiO2 core/shell nanoparticles MCM-41 modified with 2-mercaptopyridine Dihydroxy azacrown ether crosslinked chitosan GlyMA grafted with cellulose (present study)

12.5 23 30

[23] [25] [64]

20 22.1 37.03

[65] [66] Present study

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Comparison against other adsorbents The efficacy of GlyMA grafted cellulose adsorbent was compared against other absorbents [23,25,64–66]. The comparison given in Table 2 shows that the GlyMA grafted cellulose adsorbent shows good adsorption capacity for effective adsorption of mercury. Conclusions This work has demonstrated the ability of a novel microwave assisted method in the preparation of GlyMA grafted cellulose adsorbent for the effective adsorption of mercury. The adsorbent could be prepared in 3 min with a high grafting yield. A Langmuir adsorption capacity of 37.03 mg g1, pseudo second order kinetics and the spontaneous endothermic adsorption process is ably supported by the electrostatic interaction between HgCl42 and the grafted polymer. The entropy of adsorption, DSads is positive and this reflects the increased randomness at the GlyMA-cellulose adsorbent-solution interface. The adsorption of mercury is also well characterized through various analytical techniques and the regeneration of the GlyMA grafted cellulose sorbent is easily accomplished using potassium iodide. Overall, this grafted polymer shows good stability and has proved to be very useful in the adsorption of mercury from a coal fly ash sample as a practical application. Acknowledgements We thank Alagappa University, Karaikudi, India, and Karunya University, Coimbatore, India for their assistance in characterization of the adsorbent material. References [1] Y. Li, C.Y. Wu, Role of moisture in adsorption, photocatalytic oxidation, and reemission of elemental mercury on a SiO2–TiO2 nanocomposite, Environ. Sci. Technol. 40 (2006) 6444–6448. [2] H.Y. Jeong, B. Klaue, J.D. Blum, K.F. Hayes, Sorption of mercuric ion by synthetic nanocrystalline mackinawite (FeS), Environ. Sci. Technol. 41 (2007) 7699–7705. [3] E.P.A., National Primary Drinking Water Regulations, US Environmental Protection Agency (EPA), Washington, DC, 2002 http://www.access.gpo.gov/nara/cfr/ waisidx 02/40cfr141 02.html. [4] R. Say, E. Birlik, Z. Erdemgil, A. Denizli, A. Erso¨z, Removal of mercury species with dithiocarbamate-anchored polymer/organosmectite composites, J. Hazard. Mater. 150 (2008) 560–564. [5] J.A. Ritter, J.P. Bibler, Removal of mercury from wastewater: large-scale performance of an ion-exchange process, Water Sci. Technol. 25 (1992) 165–172. [6] S. Chiarle, M. Ratto, M. Rovatti, Mercury removal from water by ion exchange resins adsorption, Water Res. 34 (2000) 2971–2978. [7] A.A. Atia, A.M. Donia, K.Z. Elwakeel, Selective separation of mercury(II) using a synthetic resin containing amine and mercaptan as chelating groups, React. Funct. Polym. 65 (2005) 267–275. [8] J. Chwastowska, E. Kosiarska, Synthesis and analytical characterization of a chelating resin loaded with dithizone, Talanta 35 (1988) 439–442. [9] J.U.K. Oubagaranadin, N. Sathyamurthy, Z.V.P. Murthy, Evaluation of Fuller’s earth for the adsorption of mercury from aqueous solutions: a comparative study with activated carbon, J. Hazard. Mater. 142 (2007) 165–174. [10] C. Xiong, C. Yao, Synthesis, characterization and application of triethylenetetramine modified polystyrene resin in removal of mercury, cadmium and lead from aqueous solutions, Chem. Eng. J. 155 (2009) 844–850. [11] J. Choong, K.H. Park, Adsorption and desorption characteristics of mercury(II) ions using aminated chitosan bead, Water Res. 39 (2005) 3938–3944. [12] https://www.novapublishers.com/catalog/product_info.php?products_id=11476. [13] G. Crini, Recent developments in polysaccharide-based materials used as adsorbents in wastewater treatment, Prog. Polym. Sci. 30 (2005) 38–70. [14] D. William, C. O’Connell, Birkinshaw, T.F. O’Dwyer, Heavy metal adsorbents prepared from the modification of cellulose: a review, Bioresour. Technol. 99 (2008) 6709–6724. [15] E. Guibal, Interactions of metal ions with chitosan-based sorbents: a review, Sep. Purif. Technol. 38 (2004) 43–74. [16] S. Kamel, E.M. Hassan, M. El-Sakhawy, Preparation and application of acrylonitrile-grafted cyanoethyl cellulose for the removal of copper(II) ions, J. Appl. Polym. Sci. 100 (2006) 329–334.

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