SPION nanocomposite hydrogel for adsorption of m-cresol and o-chlorophenol

Sustainable Chemistry and Pharmacy 6 (2017) 96–106

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Pectin-crosslinked-guar gum/SPION nanocomposite hydrogel for adsorption of m-cresol and o-chlorophenol

MARK

⁎

Gaurav Sharmaa, , Amit Kumara, Chetali Chauhana, Andrew Okrama, Shweta Sharmaa, Deepak Pathaniab, Susheel Kaliac a b c

School of Chemistry, Shoolini University, Solan 173212, Himachal Pradesh, India Department of Environmental Sciences, Central University of Jammu, Rahya Suchani, Samba/Bagla- 181143, Jammu and Kashmir, India Department of Chemistry, Army Cadet College Wing, Indian Military Academy, Dehradun 248007, India

A R T I C L E I N F O

A B S T R A C T

Keywords: Pectin Guar gum Nanocomposite hydrogel Adsorption

Pectin-crosslinked-guar gum/superparamagnetic iron oxide (Pc-cl-GG/SPION) nanocomposite hydrogel have been fabricated through co-precipitation/polymerization method. During this process, methylenebisacrylamide was used as a sole cross linker and it plays a vital role for the enhancement of mechanical stability of the nanocomposite hydrogel. This magnetic nanocomposite hydrogel was characterized using various techniques such as Fourier-Transform Infrared (FTIR) Spectroscopy, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) Transmission Electron Microscopy (TEM) and Vibrating Sample Magnatometery (VSM). The prepared nanocomposite hydrogel was subjected to adsorption of organic pollutants from aqueous system. Experimental results indicated that Langmuir isotherm fitted best to the adsorption of MC and OCP onto Pc-cl-GG/SPION nanocomposite hydrogel. Maximum adsorption capacity of Pc-cl-GG/SPION nanocomposite hydrogel has been found to be 176.1 and 75.6 mg/g for m-cresol and o-chlorophenol, respectively.

1. Introduction Rapid pace of industrialization and large scale rise in aqueous pollution has led to global concern over availability of clean water. With increasing water requirements, technology driven efforts have been made by scientific and industrial community for removal of noxious pollutants and toxins from water resources. The organic pollutants, such as dyes, phenols, antibiotics, persistent organic pollutants (POPs) and pesticides, etc are harmful for aquatic animals as well as humans. They contribute significantly to various diseases, such as hypertension, respiratory disorders and organ damage, etc. in living beings. POPs have adverse impact on human health and environment. These are organic compounds that are resistant to environmental degradation through biological, chemical and photolytic processes. Some of them arise from nature, for example volcanoes and biosynthetic pathways, but most of them are man-made via total synthesis. The pulp mills, textile and dyestuff industries discharge highly colored waste water to the streams which has provoked serious aquatic environmental threats. The water becomes highly unsuitable for drinking, irrigation and other useful purposes (Sanghavi et al., 2013). Various methods which can be used for the removal of harmful organic pollutants and heavy metals from water bodies include ion

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exchange (Alam et al., 2014), adsorption (Albadarin et al., 2012; Hameed and Daud, 2008; Hena, 2010), photolysis (Franco et al., 2009; Wang et al., 2010) and membrane filtration (Schwarze et al., 2015) etc. But now-a-days biopolymers fabricated nanohydrogels have gained mounting attention in the field of waste water treatment. Biopolymers are biodegradable, non-toxic and renewable materials which are gaining importance due to their perspective applications in different fields. They are extremely useful in performing functions like storage of energy, preservation and transmittance of genetic information and cellular construction. Biopolymers based on synthetic methods are used in manufacturing of substrate mats, as packaging materials and in surgical implants. It has been observed that these materials were used efficiently for waste water treatment (Gupta et al., 2014; Sharma et al., 2014a, 2014b; Thakur et al., 2017). Various biopolymer based nanocomposite hydrogels have been synthesized in the recent years such as chitosan-crosslinked-poly(alginic acid) nanohydrogel (G. Sharma et al., 2017c), chitosan-g-poly(acrylamide)/Zn nanocomposite hydrogel (Pathania et al., 2016a) and starch/poly(alginic acid-cl-acrylamide) nanohydrogel (G. Sharma et al., 2017d) etc. Moreover, these biopolymer-based materials possess better mechanical properties. The composite materials comprises of organic polymers as supporting materials and inorganic precipitates as ion exchanger. Therefore,

Corresponding author. E-mail address: [email protected] (G. Sharma).

http://dx.doi.org/10.1016/j.scp.2017.10.003 Received 15 August 2017; Received in revised form 9 October 2017; Accepted 15 October 2017 2352-5541/ © 2017 Elsevier B.V. All rights reserved.

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HO

CH2 OH O OH OH

CH2

O OH

OH

O

HO

OH

O

CO2H O

O

CH2 OH O OH OH

O

OH

CO2CH3 O

+

HO

+ n O

OH

m

O H2 C

O

CH2OH O OH OH

O

O OH

O NH

CH2

NH

0

Ammonium Persulphate

65 C

CO2H O O

O CH2

Fe3O 4

O

CH3 O

OH

O

O OH

OH

m HO

O

O

NH NH

O

O

OH

CO2 H

O

O CO2CH3

O

CO2CH3 O

O O

CH2 O

n

OH

O

Fe3O 4

OH

OH

OH

HO

OH

CO2CH3

O

n

OH

Fe3O 4

OH

O OH

CO2CH3

O

OH

O

O Fe3O 4

CO2CH3 O

HO

Pectin

Guar Gum

Fe3O 4

O

OH

O m

O CH2OH

O

O CH2 OH

O Scheme 1. Synthesis of Pc-cl-GG/SPION nanocomposite hydrogel.

Fig. 1. FTIR spectrum of Pc-cl-GG/SPION nanocomposite hydrogel.

biopolymers have flexible organic backbone with fixed inorganic groups (Naushad, 2014; Rezvanain et al., 2017). Nanocomposite hydrogels are effective in pollution controlling due to their selectivity, specificity and higher range of usability (G. Sharma

et al., 2017b)(Gadalla et al., 2016; Pathania et al., 2016b). Many advanced technologies have been employed for improving mechanical, thermal and chemical stabilities of nanocomposite hydrogel for remediation of heavy metals and organic pollutants (Islam and Patel, 97

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substances from aqueous solution. In the present work, the combination of pectin-crosslinked-guar gum hydrogel with embedded SPION particles has been reported. Synthesized nanocomposite hydrogel provides enormous surface area which makes it an effective material for potential application in removal of pollutants from the water. 2. Experimental 2.1. Material Guar gum, pectin, the initiator ammonium per sulfate and crosslinker N, N′-methylenebis-acrylamide of analytical grade were purchased from Hi Media, India. NH4OH, FeCl2·4H2O and FeCl3.6H2O (Sigma Aldrich) was used for SPION particle synthesis. All solutions were prepared in double distilled water.

Fig. 2. XRD pattern of Pc-cl-GG/SPION nanocomposite hydrogel.

2.2. Preparation of biopolymer gel

2008; G. Sharma et al., 2017d). The nanocomposite hydrogels have received more and more attention due their stability, high efficiency and target identification. They are easy to manipulate from synthesis to disposal (Ahmed, 2015; Anjum et al., 2016). Moreover, nanocomposite hydrogels can effectively resist changes in solvent, temperature, electric field and so on. In modern world, the nanosized materials have various applications in many fields such as separation, tissue engineering (Pal et al., 2008) and drug delivery etc (Benamer et al., 2006; Kumar et al., 2008; Rosiak et al., 1995; Sengupta et al., 2007). They have modified surface functionalities which have been employed for the removal of toxic

In a typical procedure 0.6 g of guar gum and pectin were dissolved separately in 10 mL distilled water. The solutions were then mixed and stirred continuously for 1hr at 35 °C to obtain biopolymer gel. 2.3. Preparation of SPION nanoparticles sol SPION nanoparticles were prepared by co-precipitation method using solutions of ferric chloride and ferrous chloride (Liao & Chan, 2001). For this, equimolar solutions (0.3 mol/L) of ferric chloride and ferrous chloride were prepared in double distilled water and mixed

Fig. 3. SEM images showing the morphologies at different magnifications for Pc-cl-GG/ SPION nanocomposite hydrogel.

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Fig. 6. pHpzc of Pc-cl-GG/SPION nanocomposite hydrogel.

stirred for 4 h at 65̊ °C vigorously. The obtained nanocomposite hydrogel of Pc-cl-GG/SPION was washed with double distilled water and dried at 50 °C for 12 h. The resultant yield of the nanohydrogel was calculated by using the following formula (Mahdieh et al., 2016)

%yield =

WR − WP X 100 WR

(1)

Where WR is weight of reactant and WP is weight of the product. 2.5. Characterization Fourier transform infrared spectrum of Pc-cl-GG/SPION nanocomposite hydrogel was taken by KBr method. The material was thoroughly mixed with KBr, powdered and a disc was made by applying pressure. The FTIR spectrum was recorded in the range of400–4000 cm−1. X-ray diffraction analysis of the Pc-cl-GG/SPION was recorded by X-ray diffractometer (Brucker D8 Advance). It uses Cu Kα radiation (K=1.5418 A°) and a Ni- filter. Pc-cl-GG/SPION was finely powdered before subjecting to X- ray radiation. The angle of scattering of diffracted beam was being measured with respect to the incident Xray beams. Scanning electron microscopy of Pc-cl-GG/SPION was performed at different magnifications. The particle size of Pc-cl-GG/SPION was found by placing a drop of suspension of nanocomposite hydrogel prepared in ethanol onto a carbon copper grid. The grid was analyzed under transmission electron spectroscopy.

Fig. 4. TEM images showing morphologies at different magnifications for Pc-cl-GG/ SPION nanocomposite hydrogel.

2.6. Adsorption studies of m-cresol and o-chlorophenol onto Pc-cl-GG/ SPION nanocomposite hydrogel Adsorption experiments onto Pc-cl-GG/SPION were performed using standard batch method. 0.5 g of Pc-cl-GG/SPION was placed in set of 250 mL flasks with 100 mL of organic pollutants i.e., meta-cresol (MC) and ortho-chlorophenol (OCP) solutions each of different concentrations. The pH of each solution was adjusted using 0.1 N NaOH solutions. The mixtures were agitated in thermoshaker at constant temperature and speed for a particular time. The equilibrium concentrations of OCP and MC in the solutions were measured at 340 nm and 272 nm respectively using UV–visible spectrophotometer (Kennedy et al., 2007; Shah et al., 2017). The experimental conditions were optimized for meta-cresol and ortho-chlorophenol concentration (50–250 mgL−1), temperature (35°−75 °C), pH (2−13), adsorbent amount (0.050–0.4 g) and contact time (1–6 h).

Fig. 5. M-H curve of Pc-cl-GG/SPION nanocomposite hydrogel.

together. To the mixture, ammonium hydroxide solution (17.02 mL NH4OH in 6.25 mL water) was added drop wise with constant stirring and the pH was adjusted to nearly 10. The obtained precipitates were kept with mother liquor for complete digestion. 2.4. Synthesis of Pc-cl-GG/SPION nanocomposite hydrogel Magnetic SPION nanoparticles prepared in above sol was added to the biopolymeric gel of pectin and guar gum. To the above reaction mixture, ammonium per sulfate (8%) as initiator and methylenebisacrylamide (4%) as across linker was added drop wise with constant stirring using magnetic stirrer. The resultant reaction mixture was

2.7. Adsorption isotherms The adsorption isotherms provide information about the distribution of organic pollutant molecules onto adsorbent Pc-cl-GG/SPION at equilibrium. The relationship between the concentration of the 99

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Fig. 7. Optimization of various parameters for the adsorption of meta- cresol onto Pc-cl-GG/SPION (a) concentration of meta- cresol (b) amount of Pc-cl-GG/SPION (c) pH (d) temperature (e) time.

(Mohammadi et al., 2011). The values of slope and intercept corre1 spond to 1 and q K obtained from the linear plots of Ce versus Ce/qe.

pollutant adsorbed and left out in the solution was determined by the Freundlich and Langmuir isotherms (Kadam et al., 2016). Isotherm study helps in predicting the type of adsorption taking place (Alhogbi, 2017).

qm

1 C 1 = e + qe qm KL qm

2.7.1. Langmuir isotherm Langmuir model assumes that monolayer adsorption takes place on energetically homogeneous surface (Mittal et al., 2016). In addition, it is also assumed that adsorption energy is independent of adsorbed quantity. The Langmuir model is explained by the equation:

qe =

qm KL Ce 1 + KL Ce

m L

Value of qm and KL can be calculated from the Langmuir linear equation:

(3)

2.7.2. Freundlich isotherm Freundlich isotherm is based on the assumption that a reversible adsorption occurs on the energetically heterogeneous surface (Sharma et al., 2017). It favors the multilayer adsorption of adsorbate molecules onto the adsorbent. Freundlich model can be well explained by the equation: -

(2)

Where qe is the amount of organic pollutant adsorbed at equilibrium (mg g−1) and Ce is the concentration of pollutant in solution at equilibrium (mg L−1). Here qm and KL are the Langmuir constants related to the adsorption capacity and adsorption energy respectively

log qe = log Kf + 100

1 Ce n

(4)

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Fig. 8. Optimization of various parameters for the adsorption of ortho-chlorophenol onto Pc-cl-GG/SPION (a) concentration of meta- cresol (b) amount of Pc-cl-GG/SPION (c) pH (d) temperature (e) time.

process. This model is generally given by the equation (Febrianto et al., 2009):

Where Kf and n are Freundlich constants and determined from the linear plot of log Ce versus logqe. The parameters Kf and 1/n are related to adsorption capacity and adsorption intensity of the system. Value of n helps in generalizing the adsorption intensity (Sharma et al., 2017a). Value of n = 1 generalizes that the partition between the two phases are independent of the concentration. If 1/n is less than 1, it shows that normal adsorption is taking place; however, 1/n > 1indicates the cooperative adsorption (Ho, 2006; Naushad et al., 2015).

ln (qe − qt ) = ln qe − k1 t

(5)

Where qt is the quantity of MC and OCP adsorbed at time t (mg/g); qe is the quantity of MC and OCP adsorbed at equilibrium (mg/g) and k1 is the rate constant. Pseudo- second order kinetic model generalizes the chemical interactions between the adsorbate molecules and the adsorbent. This model can be well explained by the equation:

2.8. Adsorption kinetics

1 1 t = + t qt k2 qe 2 qe

Pc-cl-GG/SPION was dispersed in MC and OCP solutions for generalizing the adsorption ability at various time intervals. pH of the solution was adjusted using 0.1 N NaOH solution. The solution was then placed in an incubator shaker at 150 rpm. Absorbance was noted after pre-determined time intervals. Two models were used to evaluate the adsorption kinetics namely, pseudo- first order and pseudo- second order kinetic models. Pseudofirst order kinetics generalizes the physical nature of the adsorption

(6)

Where qt is the quantity of MC and OCP adsorbed at time t (mg/g); qe is the quantity of MC and OCP adsorbed at equilibrium (mg/g) and k2 is the rate constant. Thermodynamic studies have also been carried out in order to determine the enthalpy, entropy and Gibbs free energy values. Enthalpy value evaluates the endothermic or exothermic nature of the adsorption 101

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Fig. 9. Adsorption isotherms; Langmuir isotherm for (a) meta-cresol (b) orthochlorophenol and Freundlich isotherm for (c) meta-cresol (d) ortho-chlorophenol.

3.2. Characterization

Table 1 Values of various Langmuir and Freundlich isotherm constants. Pollutant

Meta- cresol Ortho- chlorophenol

Langmuir

Fig. 1(a) shows the characteristic peaks of Pc-cl-GG/SPION nanocomposite hydrogel. Peaks at 1743.7 cm−1and 3408.2 cm−1are assigned to C˭O and -OH stretching respectively for the biopolymers (Kadam et al., 2016). The N-H diffraction band overlaps with O-H stretching band in the range between 3282 and 3364 cm−1. Peak at 815 and 832.10 cm−1 are ascribed to the C-H bending vibrations. The C-H stretching vibrations have been confirmed by the occurrence of peaks at 2934.2, 1401.7 and 2085.11 cm−1 (Chetouani et al., 2017). The peak at 1642 cm−1 corresponds to C˭O stretching of amides. Asymmetric bending vibrations of –CH3 has been attributed to the peaks at 1533.8 and 935.70 cm−1 (Inamuddin et al., 2007). Peaks at 1052.5 and 1034.4 cm−1 are due to the C-O stretching of the guar gum unit (Maity and Ray, 2016). Strong absorption peak at 535.9 cm−1 is ascribed to the Fe-O vibrations generalizing the formation of SPION particles (Asgari et al., 2014). C-O-C stretching has been confirmed by the peak at 1106.4 cm−1 (Likhitha et al., 2014). Fig. 2 shows the XRD pattern of Pc-cl-GG/SPION nanocomposite hydrogel. The X-ray diffraction pattern of the material was recorded in powdered form (Fig. 2) which suggests the semi-crystalline nature of nanocomposite hydrogel. The peaks at 30.7°, 35.8°, 54.9°and 63° correspond to the (220), (311), (422) and (440) diffraction planes of the spinel cubic structure of SPION (López et al., 2012; Safee et al., 2010; Vaidyanathan et al., 2007). The pure guar gum shows characteristic peaks at 20.6° which correspond to the peak of GG (Mudgil et al., 2012). And the diffraction peak at 21.36° corresponds to presence of pectin (Dafe et al., 2017). Scanning electron micrographs of Pc-cl-GG/SPION nanocomposite hydrogel were studied using scanning electron microscope. Fig. 3(a-d) shows the SEM images of Pc-cl-GG/SPION at different magnifications. These images indicate the porous surface of the Pc-cl-GG/SPION nanocomposite hydrogel. Fig. 3(b) shows the imprinting of SPION particles onto the porous surface of Pc-cl-GG backbone confirming the fabrication of Pc-cl-GG/

Freundlich

qm

KL

R2

Kf

n

R2

176.1 75.6

−0.036 −0.017

0.971 0.791

1.872 2.432

−2.805 −0.729

0.891 0.640

process and entropy determines the randomness of the process (Zhu et al., 2007).

3. Results and discussion 3.1. Synthesis of Pc-cl-GG/SPION nanocomposite hydrogel Pc-cl-GG/SPION nanocomposite hydrogel has been synthesized by co-precipitation/polymerization method. Initiator, APS helped in the initiation of reaction so to form long polymeric networks. The two polymeric units; pectin and guar gum has been cross- linked with the help of a cross- linker; methylenebisacrylamide. Their cross-linking resulted increase in the mechanical strength and also increased resistant ability towards heat and attack by solvents. After cross-linking, magnetically active SPOIN particles were imprinted onto the polymeric unit. Due to the presence of magnetically active SPOIN particles, the synthesized Pc-cl-GG/SPION nanocomposite hydrogel was easily separable from the solution with the help of magnetic bar. The reported yield for synthesis of Pc-cl-GG/SPION is 89%. The Scheme 1 depicts the synthesis of Pc-cl-GG/SPION nanocomposite hydrogel.

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Fig. 10. Adsorption kinetics: Pseudo first order kinetics for (a) Meta-cresol (b) Orthochlorophenol; Pseudo second kinetics for (c)Meta-cresol (d) Orthochlorophenol.

Table 2 Values of various rate constants and correlation coefficient (R2) for pseudo-first order and pseudo-second order kinetics. Kinetic model

Pseudo- first order Pseudo- second order

Meta- cresol

Table 3 Values of ΔH°, ΔS° and ΔG° for the adsorption of meta- cresol and orthochlorophenol onto Pc-cl-GG/SPION. Pollutant

Ortho- chlorophenol

k1

qe

R2

k1

0.010 k2 3.32

−3.37 qe 14.71

0.906 R2 0.943

0.117 k2 0.717

R2

qe −11.63 qe 6.71

0.896 R2 0.979

MC OCP

ΔH0 (kJ/ mol)

6.91 7.24

ΔS0 (kJ/ mol/K)

0.03 0.04

ΔG0 (kJ/mol) Helmholtz free energy at various temperatures 308 K

318 K

328 K

338 K

−18.43 −19.68

−18.79 −20.09

−19.17 −20.49

−19.55 −20.87

(Alqadami et al., 2017). The magnetization-hysteresis (M-H) curve for Pc-cl- GG/SPION is given in Fig. 5. It is clear that the nanocomposite hydrogel possesses superparamagnetic character with a saturation magnetization of 21 emu/g. The nearly zero coercivity supports the superparamagnetism of hydrogel. The magnetization is high enough for magnetic separation of the sample. The magnetic character of the composite hydrogel is due to super fine particles of Fe3O4 which possess superparamagnetic character. Fig. 6 shows the pHpzc of the synthesized Pc-cl-GG/SPION nanocomposite hydrogel. It generalizes that the pHpzc

SPION nanocomposite hydrogel. TEM images of Pc-cl- GG/ SPION nanocomposite hydrogel are shown in Fig. 4(a,b) at different magnifications. Fig. 4(a) shows the porous surface of the synthesized nanocomposite hydrogel with average particle size ranges between 30–50 nm. Fig. 4(b) shows the HRTEM image of the fabricated Pc-clGG/SPOIN nanocomposite hydrogel. The lattice fringe spacing of the SPION in Pc-cl-GG/SPION has been well elaborated and d- spacing value has been found to be 0.245 nm which corresponds to the (311) plane. It supported the formation of a cubic structure of SPIONs

Fig. 11. Thermodynamic studies for (e) Meta-cresol (f) Orthochlorophenol.

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temperature can result because of two factors operating complementary to each other i.e., firstly rise in temperature opens up the matrix of Pccl-GG/SPION nanocomposite hydrogel and also the diffusion rate of adsorbate molecule across the external and internal pores of the adsorbent particle increases (Naushad et al., 2016). But beyond 65 °C temperature, the rate of adsorption decreases. This may be slight deformation of adsorbent structure at higher temperature and also desorption is higher than adsorption beyond optimized temperature.

of the adsorbent of 3 representing that the surface of Pc-cl-GG/SPION nanocomposite hydrogel is neutral at pH 3. Above this pH, surface is negatively charged and below this pH, it is positively charged. 3.3. Application of Pc-cl-GG/SPION nanocomposite hydrogel for removal of organic pollutants The optimization of different reaction parameters such as initial organic pollutant concentration, reaction temperature, dosage, pH and contact time for adsorption of MC and OCP onto Pc-cl-GG/SPION nanocomposite hydrogel has been explored.

3.3.5. Effect of contact time Fig. 7(e) and Fig. 8(e) shows the effect of contact time on the removal of MC and OCP by Pc-cl-GG/SPION nanocomposite hydrogel. It has been revealed that the adsorption efficiency of adsorbent reached its maximum value after 2 h. It was studied in the range of 1–6 h at optimized temperature. Extent of removal of the organic pollutant by Pc-cl-GG/SPION nanocomposite hydrogel found to decrease (after 2 h) with the increase in the contact time. After 2 h, there was gradual decrease in the rate of adsorption as with the passage of time the remaining vacant sites were unable for the pollutant molecules due to the repulsive forces between the solute molecules onto the adsorbent and in liquid phase (G. Sharma et al., 2017c).

3.3.1. Effect of initial organic pollutant concentration The initial concentration of adsorbate i.e., MC and OCP is one of the most important factors affecting their adsorption on to the adsorbent i.e., Pc-cl-GG/SPION nanocomposite hydrogel. It has been found that at lower initial MC and OCP concentrations, their adsorption percentage was higher at constant dosage of Pc-cl-GG/SPION nanocomposite hydrogel. The amount of pollutants adsorbed onto the Pc-cl-GG/SPION nanocomposite hydrogel decreases if amount of adsorbent is kept unchanged and the concentration of pollutants is increased. As here the amount of Pc-cl-GG/SPION nanocomposite hydrogel is fixed, the number of active sites for adsorption is approximately same hence initial concentration of MC and OCP i.e., 50 ppm, completely saturates the adsorption sites (Savić and Vasić, 2006). The effect of initial organic pollutant concentration was carried out in range 50–250 ppm as shown in Figs. 7(a) and 8(a). The percent adsorption decreases from 58% to 5% for MC and 44–1% for OCP with increase in organic pollutants concentration. It was observed that% adsorption is higher for MC then to OCP which can be due to greater interactions between the MC and Pc-cl-GG/SPION as compared to OCP and Pc-cl-GG/SPION.

3.4. Isotherm study Adsorption isotherms describe the mutual behavior between concentration of organic pollutant in solution and amount of pollutant adsorbed on an adsorbent (Alqadami et al., 2016; Maryam Ahmadzadeh Tofighy, 2011). Herein, Langmuir and Freundlich models were used to analyze the experimental data. Langmuir and Freundlich isotherm models have been presented in Fig. 9 and various fitting parameters have been summarized in Table 1. For the adsorption of MC and OCP, Langmuir model presented a better fit with high correlation coefficient (R2) values, 0.971 for MC and 0.791 for OCP as compared to the R2 values of Freundlich isotherm model, 0.891 for MC and 0.640 for OCP. Maximum adsorption capacities of MC and OCP as determined from the Langmuir isotherm are 176.1 mg/g and 75.6 mg/g, respectively which is due to the difference in their nucleophilicity values. As the adsorption process fitted better to the Langmuir isotherm, it follows that monolayer adsorption of MC and OPC is taking place onto Pc-cl-GG/SPION nanocomposite hydrogel.

3.3.2. Effect of adsorbent dosage A varying concentration effect of adsorbent dosage i.e., Pc-cl-GG/ SPION nanocomposite hydrogel was studied for adsorption of MC and OCP. The experiments were performed in range of 50 − 400 mg. The adsorption of OCP pollutant increase upto 150 mg (Fig. 7b) and then decreases whereas for MC at 50 mg amount of Pc-cl-GG/SPION maximum % adsorption was observed (Fig. 8b). The maximum adsorption of MC was 44% at 50 mg and for OCP it was 72% at 100 mg concentration of Pc-cl-GG/SPION. As the amount of Pc-cl-GG/SPION is increased but the concentration of pollutants MC and OCP was constant, the Pc-cl-GG/SPION reaches equilibrium adsorption for both the pollutants at different concentrations. And the value of qe keeps on decreasing throughout the process.

3.5. Adsorption kinetics and thermodynamic studies Adsorption kinetics helps in generalizing the nature of bonding between the adsorbate and adsorbent molecules (Dada et al., 2012; Yagub et al., 2014). For the adsorption of meta-cresol and orthochlorophenol on the Pc-cl-GG/SPION, all the optimized parameters were used as the test set condition. Equilibrium point was attained within 120 min for both the pollutants (Fig. 10a-d). In order to investigate the complete adsorption kinetics, two models were mainly studied: Pseudo- first order and pseudo- second order kinetic models. Various constants and correlation coefficient (R2) values of both kinetic models have been presented in Table 2. Table 2 shows that the kinetics involved in the adsorption of metacresol and orthochlorophenol indicates the cooperative adsorption. It can be well explained by pseudo- second order kinetics generalizing the chemical interactions between the adsorbent and adsorbate molecules. The k2 value 3.32 for meta- cresol is much higher than 0.717 for orthochlorophenol, indicating the strong affinity of Pc-cl-GG/SPION nanocomposite hydrogel for meta- cresol as compared to orthochlorophenol. The kinetic data for both the pollutants fitted well to the pseudo- second order kinetics with high correlation coefficient value of 0.943 and 0.979 for meta- cresol and orthochlorophenol respectively (Oyelude et al., 2017). Fig. 11(a,b) shows the thermodynamic study graph for meta- cresol and orthochlorophenol. Values of ΔH°, ΔS° and ΔG° for both the

3.3.3. Effect of pH It was observed that at low pH the presence of hydrogen ions facilitate the adsorption process onto Pc-cl-GG/SPION as seen in Fig. 7(c) and Fig. 8(c) for m-cresol and o-chlorophenol. pHpzc of Pc-cl-GG/ SPION is 3 denoting the neutral surface of the nanocomposite hydrogel at pH 3. Below this pH, the surface of the hydrogel is positively charged and above this, it is negatively charged. Maximum adsorption of both the pollutants occurred at pH 1. MC and OCP both are good nucleophiles and thus showed best interactions with the positively charged surface at low pH leading to maximum adsorption. However, at pH higher than 3, the surface of the nanocomposite hydrogel became negatively charged and showed repulsion forces to the negatively charged MC and OCP molecules, leading to their decreased adsorption. 3.3.4. Effect of temperature The effect of temperature on adsorption of MC and OCP onto Pc-clGG/SPION nanocomposite hydrogel was shown in Fig. 7(d) and Fig. 8(d). It was revealed that the rate of adsorption increases with the rise temperature from 30 °C to 65 °C, indicating that the process is endothermic in nature. The increase in adsorption with rise in 104

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pollutants have been presented in Table 3. Negative values of ΔG° for meta- cresol and orthochlorophenol suggest the spontaneous nature of their adsorption on the Pc-cl-GG/SPION. Intensification of the negative value of ΔG° with increasing temperature shows that the adsorption was more feasible at higher temperatures. The positive value of ΔH° determines the endothermic nature of the adsorption of both the pollutants. `

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4. Conclusion In the present paper, an efficient technique has been developed for multi-pollutant removal by synthesizing nanocomposite hydrogel of SPION and pectin crosslinked guar gum hydrogel by a facile co-precipitation/polymerization method. The adsorption studies revealed that Pc-cl-GG/SPION nanocomposite hydrogel was used as an efficient adsorbent for removal of organic pollutants meta-cresol (MC) and orthochlorophenol (OCP) from water system. It has been observed that the adsorption of MC was more favorable as compared to OCP. MC act as a better nucleophile than OCP, so better adsorption results were observed for MC. The difference in the nucleophilicities of the two pollutants was due to the presence of two different groups (methyl of MC and chlorine of OCP) which were responsible for the difference in their nucleophilicities and ultimate interactions with the Pc-cl-GG/SPION nanocomposite hydrogel. The Langmuir model has been found to fit the data perfectly as compared to Freundlich model. Adsorption of MC was found to be more as compared to OCP as is relevant from its high qm value, 176.1 mg/g for MC and 75.6 mg/g for OCP. Synthesized nanocomposite hydrogel shows promising applications as adsorbent for organic pollutants. References Ahmed, E.M., 2015. Hydrogel: preparation, characterization, and applications: a review. J. Adv. Res. 6, 105–121. Alam, M.M., ALOthman, Z.A., Naushad, M., Aouak, T., 2014. Evaluation of heavy metal kinetics through pyridine based Th(IV) phosphate composite cation exchanger using particle diffusion controlled ion exchange phenomenon. J. Ind. Eng. Chem. 20, 705–709. Albadarin, A.B., Mangwandi, C., Al-Muhtaseb, A.H., Walker, G.M., Allen, S.J., Ahmad, M.N.M., 2012. Kinetic and thermodynamics of chromium ions adsorption onto lowcost dolomite adsorbent. Chem. Eng. J. 179, 193–202. Alhogbi, B.G., 2017. Potential of coffee husk biomass waste for the adsorption of Pb(II) ion from aqueous solutions. Sustain. Chem. Pharm. 6, 21–25. Alqadami, A.A., Naushad, M., Abdalla, M.A., Ahamad, T., Abdullah Alothman, Z., Alshehri, S.M., 2016. Synthesis and characterization of Fe3O4@TSC nanocomposite: highly efficient removal of toxic metal ions from aqueous medium. RSC Adv. 6, 22679–22689. Alqadami, A.A., Naushad, M., Abdalla, M.A., Ahamad, T., Abdullah ALOthman, Z., Alshehri, S.M., Ghfar, A.A., 2017. Efficient removal of toxic metal ions from wastewater using a recyclable nanocomposite: a study of adsorption parameters and interaction mechanism. Journal of Cleaner Production. Elsevier Ltd. Anjum, S., Gupta, A., Sharma, D., Gautam, D., Bhan, S., Sharma, A., 2016. Development of novel wound care systems based on nanosilver nanohydrogels of polymethacrylic acid with Aloe vera and curcumin. Mater. Sci. Eng. C. 64, 157–166. Asgari, S., Fakhari, Z., Berijani, S., 2014. Synthesis and characterization of Fe3O4 magnetic nanoparticles coated with carboxymethyl chitosan grafted sodium methacrylate. J. Nanostruct. 4, 55–63. Benamer, S., Mahlous, M., Boukrif, A., Mansouri, B., Youcef, S.L., 2006. Synthesis and characterisation of hydrogels based on poly(vinyl pyrrolidone). Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. At. 248, 284–290. Chetouani, A., Follain, N., Marais, S., Rihouey, C., Elkolli, M., Bounekhel, M., Benachour, D., Le Cerf, D., 2017. Physicochemical properties and biological activities of novel blend films using oxidized pectin/chitosan. Int. J. Biol. Macromol. 97, 348–356. Dada, A.O., Olalekan, A.P., OLatunya, A.M., Dada, O., 2012. Langmuir, Freundlich, Temkin and Dubinin – Radushkevich Isotherms Studies of Equilibrium Sorption of Zn2+. Unto Phosphoric Acid. Modif. Rice Husk. J. Appl. Chem. 3, 38–45. Dafe, A., Etemadi, H., Dilmaghani, A., Mahdavinia, G.R., 2017. Investigation of pectin/ starch hydrogel as a carrier for oral delivery of probiotic bacteria. Int. J. Biol. Macromol. 97, 536–543. Febrianto, J., Kosasih, A.N., Sunarso, J., Ju, Y.-H., Indraswati, N., Ismadji, S., 2009. Equilibrium and kinetic studies in adsorption of heavy metals using biosorbent: a summary of recent studies. J. Hazard. Mater. 162, 616–645. Franco, A., Neves, M.C., Carrott, M.M.L.R., Mendonça, M.H., Pereira, M.I., Monteiro, O.C., 2009. Photocatalytic decolorization of methylene blue in the presence of TiO2/ ZnS nanocomposites. J. Hazard. Mater. 161, 545–550. Gadalla, H.H., El-Gibaly, I., Soliman, G.M., Mohamed, F.A., El-Sayed, A.M., 2016.

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