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Microwave-assisted green synthesis of xanthan gum grafted diethylamino ethyl methacrylate: An efficient adsorption of hexavalent chromium
Carbohydrate Polymers 222 (2019) 114989
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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Microwave-assisted green synthesis of xanthan gum grafted diethylamino ethyl methacrylate: An efficient adsorption of hexavalent chromium
T
⁎
Edwin Makhadoa,c, Sadanand Pandeya,b, , James Ramontjaa a
Department of Applied Chemistry, Centre for Nanomaterials Science Research, University of Johannesburg, P.O. Box 17011, Doornfontein 2028, Johannesburg, South Africa b Department of Chemistry, College of Natural Sciences, Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk, 38541, Republic of Korea c Department of Chemistry, School of Physical and Mineral Sciences, University of Limpopo (Turfloop) Polokwane, South Africa
A R T I C LE I N FO
A B S T R A C T
Chemical compounds studied in this article: Xanthan gum (PubChem CID: 7107) N, N-diethylamino ethyl methacrylate (PubChem CID: 61012) Acetone (PubChem CID: 180) Potassium dichromate (PubChem CID: 24502) Ammonium persulfate (PubChem CID: 62648) Sodium hydroxide (PubChem CID: 14798) Hydrochloric acid (PubChem CID: 313)
We report the development of a novel graft copolymer, diethylamino ethyl methacrylate grafted xanthan gum (mwXG-g-DEAEMA), by microwave heating. The synthesized graft copolymer was used for potential application of Cr(VI) adsorption. The structure, thermal stability and morphologies of XG and mwXG-g-DEAEMA were characterized to verify the adsorbent formed under optimized reaction conditions. FTIR, XRD, TGA and SEM techniques were used for characterization of XG and mwXG-g-DEAEMA. Furthermore, 1H NMR spectroscopic analyses predict the probable structure of copolymer. Based on the NMR data, a plausible mechanism for copolymer formation has been proposed. The effects of adsorbent loading, pH, contact time and equilibrium concentration of the Cr(VI) adsorption were investigated batch wise. The Cr(VI) adsorption process followed the pseudo-second-order rate model and equilibrium data were best described by Freundlich isotherm model. This work will encourage researchers to focus on this facile green technique for the synthesis of adsorbent with enhanced adsorption capacity.
Keywords: Biopolymer Graft co-polymerisation Microwave-assisted synthesis Adsorption Cr(VI) removal
1. Introduction Water pollution as a result of toxic metal ions, such as chromium (Cr) is of great concern in wastewater treatment. Most of the Cr compounds exist as trivalent chromium Cr(III) or hexavalent chromium Cr (VI). The latter is more toxic as a result of its high water solubility and mobility (Benoit, 1976). The utilization of this toxic metal in various industrial processes like electroplating, leather tanning, and production of dyes, batteries, photographs, and cement, etc. has led to the need to treat wastewater effluent prior to release into the environment. Swallowing of Cr(VI) can cause diseases such as cancer abdominal pain, diarrhea, heart failure damages to the gut, liver, and kidneys (Li et al., 2008; Pandey, 2017; Mehta, Mazumdar, & Singh, 2015). The US Environmental Protection Agency (EPA) has capped the release of Cr(VI) into wastewater at 0.5 mg/L (Barai & Engelken, 2002). In addition, a maximum contamination level for Cr in drinking water is capped at 0.1 mg/L (De Gisi, Lofrano, Grassi, & Notarnicola, 2016). For this
reasons, there is a need to remove Cr(VI) from Cr-containing industrial effluents before it is discharged into the soil and water. Treatment of industrial effluents has steered researchers in the direction of engineering efficient and reliable techniques for the removal of metal ions from contaminated water. Adsorption displays many advantages in removing toxic metal ions due to its operational simplicity, low cost and simplicity of design when compared to conventional techniques (De Gisi et al., 2016). Adsorption is the most promising and widely used technique for the removal of metal ions and synthetic dyes from wastewater. Hybridization of natural polymers with synthetic polymers or copolymers led to the formation of new materials with improved properties such as better processability and biodegradability. In this regard, naturally derived carbohydrate based polymers have gain significant attention globally as a result of their properties which include biocompatibility, biodegradability, hydrophilicity, non-toxicity, cost-effectiveness, and renewable and recyclable nature. Xanthan gum (XG) an efficient polyelectrolyte due to the presence of tunable
⁎ Corresponding author at: Department of Chemistry, College of Natural Sciences, Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk, 38541, Republic of Korea. Tel.: (+82) 10-6286-7573 E-mail addresses: [email protected], [email protected], [email protected], [email protected] (S. Pandey).
https://doi.org/10.1016/j.carbpol.2019.114989 Received 20 February 2019; Received in revised form 27 May 2019; Accepted 10 June 2019 Available online 12 June 2019 0144-8617/ © 2019 Elsevier Ltd. All rights reserved.
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were analytical grade. For all the experiments, deionized water (DI) was used. The stock solutions of Cr(VI) (1000 mg/L) was prepared by dissolving an appropriate amount of K2Cr2O7 in 1000 mL of DI, and the stock solution was further diluted for batch experiments.
hydroxyl groups. Graft copolymers based on XG have been effectively used for the adsorption of the heavy metals and organic dyes from aqueous solutions (Ghorai et al., 2014; Singh, Tiwari, Pandey, & Preeti Sanghi, 2015). Efforts have been made on the exploitation of graft copolymers for various applications including chemical sensor (Pandey & Nanda, 2016; Pandey & Ramontja, 2016; Pandey, 2016), drug delivery systems (Sen, Mishraa, Jhaa, & Pal, 2010), and wastewater treatment (Sarkar, Aniruddha Pal, Ghorai, Mandre, & Pal, 2014; Fosso-Kankeu, Van den Berg, Waanders, & Pandey, 2018, b, Ghorai, Sarkar, Panda, & Pal, 2013; Pal et al., 2015; Pandey & Mishra, 2012). The use of graft copolymers for wastewater treatment has been significantly focused on in the recent reports. Hajeetha, Sudhab, Vijayalakshmib, and Gomathi (2014) reported the synthesis of cellulose grafted polyacrylonitrile for removal of Cr(VI) from aqueous solution via the conventional method using ceric ammonium nitrate as initiator. In recent years, the exploitation of microwave irradiation has emerged as an effective process for preparation of the graft copolymers. This process is considered due to its advantages of high reproducibility, fast reaction process and ease of operation (Makhado, Pandey, Nomngongo, & Ramontja, 2017; Makhado, Pandey, Nomngongo, & Ramontja, 2017). Hence, recently several researchers have prepared graft copolymers via microwave irradiation techniques. For example, Pal, Majumder, and Bandyopadhyay (2016) grafted poly (acrylic acid) onto xanthan gum by employing microwave-assisted technique and the as-synthesized graft copolymer was used as a biosorbent for the removal of Hg(II) from water. Sharma and Mishra (2010) reported the synthesis of chitosangrafted polystyrene via microwave initiated method and they applied this material for removal of Cr(VI). This adsorbent was found to be more effective for Cr(VI) removal. In another study by Kumar, Kumar, Rajesh, and Rajesh (2014) n-butylacrylate grafted chitosan was prepared by microwave-assisted method and they employed this material as adsorbent for the removal of Cr(VI) from aqueous solution. In their study the adsorption kinetic was well described by pseudo-second order and the maximum adsorption capacity was found to be 17.14 mg/g (Kumar et al., 2014). Recently, Li et al. (2018), reported the preparation of chitosan grafted with a hyperbranched polymer for adsorption of Cr (VI) from aqueous solution. The adsorption kinetics and isotherm models were well fitted with pseudo-second-order and Freundlich isotherm model, respectively. They obtained a maximum adsorption capacity of 194.55 mg/g. It is well documented the presence poly(DEAEMA) imparts pH and temperature responsive character to a hydrogel. (Karthika & Vishalakshi, 2015; Liu et al., 2002; Schmalz, Hanisch, Schmalz, & Müller, 2010). To the best of our knowledge, there is no literature about the hybridization of XG with DEAEMA. Therefore, the objectives of this study were (i) to synthesize mwXG-g-DEAEMA by employing microwave assisted strategy, using APS as the free radical initiator and characterize the structure of the synthesized mwXG-g-DEAEMA; (ii) to investigate the interactive effects of microwave-assisted process parameters including monomer, initiator, biopolymer, microwave exposure time and microwave power; (iii) to investigate the possibility of utilizing mwXG-g-DEAEMA as adsorbent for the removal of Cr(VI) from aqueous solution.
2.2. Microwave-assisted synthesis of mwXG-g-DEAEMA In order to synthesis of mwXG-g-DEAEMA, 0.1 g XG was dissolved in required amount of distilled water at 25 °C. To this solution, calculated amount of DEAEMA and MBA were added to the reaction flask and allowed to react for 5 min. Then catalytic amount of (NH4)2S2O8 was added dropwise to the mixture to initiate polymerization and allowed to react for another 2 min at 25 °C. The flask was exposed under fixed microwave power for a definite time period in a domestic microwave oven with a frequency of 2450 MHz. A power output from 0 to 1000 W with continuous adjustment was used for all the experiments. After each exposure, the temperature of the reaction mixture was determined with thermometer. After desired time period, the mwXG-gDEAEMA was precipitated by pouring the reaction mixture into large quantity of acetone and washed well to remove adhered homopolymer if any is present along with graft copolymers. The precipitated copolymer was filtered and the copolymer samples obtained were finally dried under vacuum at 60 ⁰C to a constant weight and the pulverized in a ball mill. The percentage grafting (%G) and percentage grafting efficiency (%GE) of mwXG-g-DEAEMA were calculated using the following Eqs. (1) and (2) (Fanta & Doane, 1986).
%G=
W1 − W 2 × 100 W2
%GE=
W1 − W 2 × 100 W3
(1)
(2)
where, W1, W2 and W3 stand for the weight of XG, mwXG-g-DEAEMA and DEAEMA, respectively. 2.3. Adsorption studies of mwXG-g-DEAEMA to Cr(VI) All the adsorption experiments were performed in a batch system. Typically, 20.0 mg of mwXG-g-DEAEMA adsorbent was added in 20.0 mL of Cr(IV) solution of known concentration and the mixture was stirred at 160 rpm and room temperature for adsorption to proceed. In order to determine the optimal pH of solution for metal ion adsorption, the Cr(IV) removal by mwXG-g-DEAEMA was evaluated in the pH range from 1.0 to 7.0. The pH of Cr(IV) solutions were adjusted using 0.1 M HCl or NaOH solution. After the final stage, the adsorbent was filtered through 0.45 μm PVDF syringe filters from the Cr(IV) solution and the residual Cr(IV) concentration was measured using the inductively coupled plasma optical emission spectrometry (ICP-OES) at maximum wavelength of metal ion. It should be mentioned that all experiments were performed in triplicates. The percentage adsorption was calculated using Eq. (3):
Adsorption % =
2. Materials and methods
Co − Ce × 100 Co
(3)
and the equilibrium uptake was calculated using Eq. (4):
2.1. Materials
qe = (Co − Ce ) × Xanthan gum (XG) from Xanthomonas campestris (average molecular weight of pure xanthan gum is 4,500,000) and N, N-diethylamino ethyl methacrylate (DEAEMA, 99%) monomer were purchased from SigmaAldrich (South Africa). Acetone and hydrochloric acid (HCl) were procured from Merck (South Africa). Initiator ammonium persulfate (APS) (≥98%), Potassium dichromate (K2Cr2O7) and sodium hydroxide (NaOH) were procured from Sigma-Aldrich, South Africa. All solvents
V W
(4)
where Co denotes the initial and the Ce denotes the equilibrium concentrations (mgL−1) of the Cr(VI) solution, respectively. Furthermore, qe is the equilibrium capacity of Cr(VI) on the adsorbent (mg/g), V is the volume of the Cr(VI) solution used (L) and W is the weight of adsorbent (g) used. Experiments were conducted with different mwXG-gDEAEMA grades, pH, adsorption time and adsorbent concentration. 2
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3. Characterization of mwXG-g-DEAEMA adsorbent
copolymerization. The decrease in the %G and %GE after the optimum concentration might be due to the increase in the rate of termination reactions. Similar, phenomena were observed concerning the effect of initial concentration on the grafting parameters (Mostafa, Naguib, Sabaa, & Mokhtar, 2005).
FT-IR spectra of all samples were recorded in the range of 4000–400 cm−1 on Spectrum-100 Perkin Elmer, USA spectrometer using KBr pellet. X-ray diffractometer was employed for acquiring XRD pattern with a (Rigaku Ultima IV, X-ray diffractometer, Japan) by employing CuKα radiation of the wavelength of 1.5406 Å with visible slights at 45 kV/40 mA. XRD spectra were collected in the 2θ range between 5° and 90° with a step size of 0.01°, and a scan speed of 1°/min. The morphologies of the samples were carried out by using scanning electron microscopy (SEM), (TESCAN, VEGA SEM, Czech Republic) at a 20 kV electron acceleration voltage. The surfaces of the samples were coated with carbon to avoid charging. Thermogravimetric analyses (TGA) of samples were carried out by a Q500 TA Instruments, USA from room temperature to 900 °C at a heating rate of 10 °C min−1 under a nitrogen atmosphere. 1H NMR spectrometry was performed with 500 MHz Bruker Avance III HD spectrometer (Germany) using deuterium oxide for XG and d-chloroform for mwXG-g-DEAEMA as the lock solvent. Samsung (Model No. ME9114W1; 1500 W, Made: Malaysia) domestic microwave oven having 2450 MHz microwave frequency and a power output from 0 to 1000 W was used for synthesis of mwXG-gDEAEMA. The pH measurements were made with HI 9811-5/HI 1285-5 (Romania). A Femto spectrophotometer, model 700 Plus using a 1.0 cm glass cell, adjusted in the wavelength of 540 nm, was used for the quantification of Cr(VI) in order to compare with the ICP-OES results.
4.1.3. Effect of XG concentration The effect of increasing XG concentration on grafting parameters is shown in (Fig. 1(c)). According to (Fig. 1(c)), it was observed that %G decreased continuously with increasing XG concentration from 0.1 to 0.5 g. in contrary, the %GE with increase in the concentration of the XG. The decrease in the %G beyond optimum value could be attributed to the increase in the viscosity of the reaction medium as the concentration of the polymer increases, which hinders the movement of APS molecules, monomer molecules and active chains, thus the grafting decrease. Additionally, a high XG concentration could generate higher number of XG macro-radicals, which could combine with one another to terminate the reaction, as a result the grafting parameters values decrease. Similar observations have also been reported in relation to the effect of the polymer concentration on the grafting parameters (Trivedi, Kalta, Patel, & Trivedi, 2005; Yigitoglu, Isiklan, & Ozmen, 2007). 4.1.4. Effect of time of exposure The effect of microwave exposure time on grafting reaction was investigated by changing time interval from 20 to 100 s keeping the concentration of XG (0.1 g), DEAEMA (15 × 10−5 M), APS (300 × 10-4 M), microwave power (80%) and the total reaction volume of 25 mL fixed. The results are displayed in (Fig. 1(d)). The %G and %GE were found to be increasing from (237.1 and 33.7) to (292.7 and 41.7), respectively, with increased exposure time from 20 to 40 s. This increment in the graft parameters was then followed by gradual decrease in grating with prolonged exposure to microwave.
4. Results and discussion 4.1. Effect of mwXG-g-DEAEMA of reaction mixture Effect of reaction parameters such as monomer, initiator, biopolymer, microwave exposure time and microwave power were investigated in order to get optimum grafting conditions (Eqs. (1) and (2)). Fig. 1(a–e) present the effect of various reaction parameters on grafting and grafting efficiency. Mechanism for the synthesis of mwXGg-DEAEMA is displayed in (Fig. S1).
4.1.5. Effect of the microwave power The effect of microwave power on grafting reaction was studied by altering microwave power from 50 to 100%, at the fixed concentration of XG (0.1 g), APS (60 × 10−4 M), DEAEMA (15 × 10-5 M), exposure time (40 s) and the total reaction volume of 25 mL. The results are shown in (Fig. 1(e)). The %G and %GE were increased almost linearly with microwave power up to 70% thereafter the %G (292.7) and %GE (41.7) was decreased. The reason of this behaviour is attributed to the formation of more monomer and macro-radicals due to fast energy transfer among the molecules (Wan et al., 2011). The higher microwave power results in the formation of homopolymer and decomposition of the grafting copolymers (Biswal et al., 2007; Huacai, Wan, & Dengke, 2006).
4.1.1. Effect of DEAEMA concentration The percentage grafting in mwXG-g-DEAEMA was examined by varying the monomer concentration from 6 × 10−5 M to 18 × 10−5 M (Fig. 1(a)). The %G was found to be linearly proportional to monomer concentration from 3 × 10−5 M up to 15 × 10−5 M. Beyond 15 × 10−5 M the %G decreased with increasing the DEAEMA concentration. Similar trend was observed for the %GE at the fixed concentration of XG (0.1 g), APS (60 × 10-4 M), exposure time (60 s), microwave power (80%) and the total reaction volume of 25 mL. Beyond optimum concentration, the decrease in the %G and %GE values could be associated with the depletion of the active sites on the biopolymer as a result of the predominance of homopolymerization with further increase in DEAEMA concentration in the polymerization medium. Furthermore, the increase in homopolymer increases the viscosity of the medium making monomers to diffusion into the XG backbone difficult. Similar behaviours have been reported in other system (Makhlouf, Marais, & Roudesli, 2007).
4.2. Characterization of XG and mXG-g-DEAEMA 4.2.1. X-ray diffraction (XRD) characterization Characterization techniques used in this study are given in the supporting file. The XRD analysis was carried out to investigate the structural parameters. Fig. 2(a) displays the X-ray diffractogram of XG and mwXG-g-DEAEMA. The diffractogram of XG exhibits two prominent diffractions, one at low diffraction angle (2θ = 23.6°) values, which is indicative of the mostly amorphous structure of the XG and another one at 2θ = 37.5°, showing some crystalline content of the XG (Kumar, Singh, & Ahuja, 2009; Pandey & Mishra, 2011) On account of mwXG-g-DEAEMA, the XRD pattern showed that the characteristic peak of XG shifted to 16.9°. Furthermore, additional peaks appeared at 20.7° and 29.7° may be due to grafting of DEAEMA onto XG.
4.1.2. Effect of APS concentration The percentage grafting in mwXG-g-DEAEMA was examined by varying the initiator concentration from 60 × 10−4 M to 360 × 10−4 M at fixed concentration of XG (0.1 g), DEAEMA (15 × 10-5 M), exposure time (60 s), microwave power (80%) and the total reaction volume of 25 mL, the results are presented in (Fig. 1(b)). It was found that percentage grafting and grafting efficiency increased to 270.8 to 23.3 up to concentration 0.03 M of APS. Further raise in APS concentration decreased the percentage grafting and grafting efficiency. The increase in APS concentration enhances the number of radical species XG-O∙ radicals which in the presence of monomer radicals increases graft
4.2.2. FTIR spectroscopy To confirm the structure of products, the reactions were monitored by FTIR, as shown in Fig. 2(b). The FT-IR spectrum of spectrum of pure XG showed peaks around 3246, 2932 and 1404 cm−1, demonstrating 3
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Fig. 1. Optimization of mwXG-g-DEAEMA variables, %G and %GE by varying (a) monomer (DEAEMA); (b) Initiator (APS); (c) Biopolymer (XG); (d) MW exposure time (s); (e) MW power (%).
characteristic stretching vibration of eOH, aliphatic eCH stretching of alkyl group, and eCH bending of methyl groups, respectively. In addition, the absorption bands appeared at 1169 and 1053 cm−1 correspond to eCO bending (Makhado, Pandey, Nomngongo, & Ramontja, 2018). The FT-IR spectrum of XG-g-DEAEMA, shows a broad and intense band at 3345 cm−1, corresponding to the stretching of eOH bonds. The absorption bands at 2844 and 2950 cm−1 were attributed to the stretching vibration of eCH2 aromatic and eCH3 methyl groups of DEAEMA. The band at 1734 cm−1 is ascribed to eC]O stretching of carboxylic ester (Geng et al., 2016). The transmission intensity of mwXG-g-DEAEMA was higher as a result of additional concentration of carboxylic ester. The absorption bands at 1123 and 1473 cm−1 were attributed to the bending of ester eOeCH2 bond and CeN stretching
vibrations in DEAEMA, respectively. The new intense peaks appeared at 623 cm−1. This result confirms that DEAEMA was chemically bonded to the XG molecule. In the spectrum of mwXG-g-DEAEMA after adsorption of Cr(VI) ions showed that the intensity of characteristic bands at 1734, 1473, 1123 and 623 cm−1 were reduced. The reduction in the intensity of characteristic bands after adsorption of Cr(VI) ions confirms that the interaction occurred between Cr(VI) ions on to mwXG-g-DEAEMA.
4.2.3. Thermal analysis Thermogravimetric analysis (TGA) is a simple method for studying the decomposition pattern and the thermal stability of the compounds. TGA curves of XG and mwXG-g-DEAEMA are depicted in Fig. 2(c,d). There are remarkable differences of the thermal stability for XG and 4
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Fig. 2. Characterization of XG and mwXG-g-DEAEMA (a) XRD; (b) FTIR; (c) TGA; and (d) DTGA. 1 H NMR spectra of neat DEAEMA, XG and mwXG-g-DEAEMA are showed in Fig. 3(a–c), respectively. 1H NMR spectrum of pure XG was performed in deuterated water (D2O), while mwXG-g-DEAEMA was dissolved in d-chloroform (CDCl3). The characteristic neat XG peaks were observed at δ 3.51–3.64 ppm, which can be attributed to the hydrogen associated to proton peaks of glycosidic CH2 and alcoholic CH2; The peak at δ 4.88 ppm, ascribed attributed to the hydrogen linked to the C1, δ 4.00 ppm, from the hydrogen atoms associated to the C2 and C3 carbons δ 3.91-3.84 ppm attributed to the hydrogen associated to the C4 carbons and δ 3.72 ppm attributed to the hydrogen associated to the C5 carbons. The peak of D2O used for dissolving neat XG was observed at δ 4.30 ppm (Makhado et al., 2017a, 2017b). In the 1H NMR spectra of mwXG-g-DEAEMA, additional signals appeared at δ 2.72 and 0.86 ppm are attributed to proton peaks of CH2(CH3) and CH2(CH3) (denoted by A and B) for DEAEMA, respectively. δ 2.93 ppm is assigned to proton peaks of CH2N (denoted by C). δ 4.28 ppm ascribed to the hydrogen associated to the CH2 (denoted by D) carbons. δ 1.91 ppm is assigned to proton peaks of CH3 (denoted by E). The peaks in mwXG-g-DEAEMA at δ 4.01, 3.65, and 2.15 ppm were assigned to proton peaks of the XG backbone. The peak of CDCl3 solvent used for dissolving mwXG-g-DEAEMA was observed at δ 7.24 ppm. In the 1H NMR spectra of mwXG-g-DEAEMA, there was slight shift in the proton peaks at their corresponding range. This result is due to the fact that chemical shifts are slightly different in different solvents, depending on electronic solvation effects. The resolution of the peaks appeared in mwXG-g-DEAEMA was poor as compared to pure XG; this may be due to poor solubility of mwXG-g-DEAEMA in solvent. These observations suggest that the copolymer DEAEMA had been effectively grafted onto the XG backbone as intended in Fig. S1. Furthermore, the obtained results are in agreement with the 1H-NMR spectra recorded by other researchers who worked with pure and modified XG (Makhado et al., 2017a, 2017b; Kumar, Deepak Sharma, Srivastava, & Kumar, 2017; Pal et al., 2016).
mwXG-g-DEAEMA. TGA of XG demonstrated three-step characteristic thermogram; the first degradation occurred over the range 32–120 °C associated with the endothermic peak might be attributed dehydration (Makhado, Pandey, & Ramontja, 2018). The rate of weight reduction increases with an increase in the temperature. Furthermore, TGA curve of XG demonstrated the second degradation process in the range 270–350 °C accompanied by a weight loss exceeding 22% is assigned to the decomposition of XG backbone. Finally, total of 69% weight loss occurs around 600 °C (Makhado, Pandey, Nomngongo et al., 2018). The thermogram of mwXG-g-DEAEMA showed four degradation stages. The initial decomposition stage was observed in the range 265–292 °C and weight reduction of about 7% was observed, shows lower H2O content of the grafted sample. The second decomposition stage weight loss of approximately 53% was observed in the temperature range 230–322 °C. This may be due to the degradation of xanthan gum backbone as well as decomposition of functional group of grafted DEAEMA chain. The vast majority of the weight loss occurred when the temperature was above 230 °C. The third gradual degradation step accounting for about 77% weight loss occurred in the temperature range 415–450 °C. Finally, total 98% weight loss occurred around 610 °C, which is for the decomposition of the grafted polymer backbone. The DTGA curves (Fig. 2(d)) provide clear information of TGA data and also confirms the lower thermal stability of mwXG-g-DEAEMA when compared to XG. This result may be attributed to the low thermal stability of DEAEMA. In a study conducted by (Madill, Garcia-Valdez, Champagne, & Cunningham, 2017) DEAEMA was found to exhibit two decomposition peaks at 320–370 °C and 390–450 °C. Furthermore, DEAEMA showed lower thermal stability than pure chitosan and graft copolymer sample (Madill et al., 2017). 4.2.4. NMR analyses In order to confirm synthesized graft copolymerization (mwXG-gDEAEMA) 1H NMR spectroscopic analysis was investigated. The proton, 5
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Fig. 3. 1H NMR spectra of (a) DEAEMA, (b) XG and (c) mwXG-g-DEAEMA.
morphology of mwXG-g-DEAEMA exhibits rough irregular surface morphology shown in Fig. 4(c and d). The mwXG-g-DEAEMA after Cr (VI) ions adsorption appears to have irregular morphology with heterogeneous surface (Fig. 4(e and f)). Fig. 4(g and h) shows the EDS spectrum of mwXG-g-DEAEMA before adsorption of Cr(VI) ions and mwXG-g-DEAEMA after removal of Cr(VI) ions, respectively. The presence of Cr(VI) ions in the surface of graft copolymer (mwXG-gDEAEMA) was further confirmed by EDS analysis. EDS results were in
4.2.5. Scanning electron microscopy characterization The morphology of XG, mwXG-g-DEAEMA and mwXG-g-DEAEMA Cr(VI) ions loaded were studied by SEM. The SEM micrograph of XG Fig. 4(a and b) showed the granular morphology, due to the amorphous nature of the biopolymer (Makhado et al., 2017a, 2017b). In comparison to the pure XG (Fig. 4(a and b)) (Makhado et al., 2017a, 2017b), the grafting of DMAEMA onto XG shows considerable changes in the shape and size of the granular morphology of XG particles. The surface 6
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Fig. 4. SEM images of XG at different magnification (a) 2kx, (b) 4kx; mwXG-g-DEAEMA at different magnifications (c) 2kx, (d) 4kx; mwXG-g-DEAEMA Cr(VI) ions loaded at different magnification (e) 2kx, (f) 4kx, (g) EDS analysis of mwXG-g-DEAEMA, and (h) EDS analysis of mwXG-g-DEAEMA Cr(VI) ions loaded.
4.3. Adsorption of hexavalent chromium
agreement with the FTIR spectra of reported in Fig. 2(b).
4.3.1. Effect of pH on the adsorption of Cr(VI) The solution pH is significant parameters in the sorption process as a result of its remarkable effect on the adsorption sites on the adsorbent (mwXG-g-DEAEMA) surface and it also the speciation of Cr(VI) in the solution. The initial pH of the solution was investigated in the range of 1–6 and the obtained results are shown in Fig. 5(a). The conditions employed to study the effect of pH on the adsorption of Cr(VI) are stated in Fig. 5(a). Percentage adsorption for Cr(VI) ions on mwXG-gDEAEMA was attained at pH 2. Similar trend are also reported in literatures (Garg, Kaur, Garg, & Sud, 2007). Thereafter sharply decrease in the adsorption was noticed with increasing in the initial pH of the solutions. A possible reason for these observations is that the adsorbent was positively charged with the protonated tertiary amino group while
4.2.6. Elemental analysis The elemental analysis of XG and the graft copolymer was performed by using PerkinElmer® 2400 Series II CHNS/O Elemental Analyzer (2400 Series II) at a high temperature combustion method at 960 °C for complete combustion. Samples were analyzed in synthetic air using oxygen as the combustion gas and helium as the carrier gas. Analyses were performed in triplicate. The estimated analysis of three different elements, carbon (C), hydrogen (H) and nitrogen (N), was undertaken. Elemental analysis were conducted to confirm the graft procedure was effective and findings are displayed in Table S1. The results obtained show that grafted sample contain nitrogen. Since XG does not have no or negligible amount of nitrogen in its composition, identification of this element further confirms the grafting. 7
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Fig. 5. Effect of initial (a) pH; (b) absorbent dose; and (c) time for Cr(VI) sorption by mwXG-g-DEAEMA.
by the internal active sites. Finally, all active sites of the adsorbent are occupied and the amounts of adsorbed Cr(VI) reach constant values. Kinetic model provides a relationship between the rate of sorption and the type of reaction mechanism involved. Two kinetic models, namely pseudo-first-order and pseudo-second-order model have been adopted to describe kinetic data (Eq. (S1-2)). The fitting effects of experimental data with pseudo-first-order and pseudo-second-order kinetic model are shown in Fig. 6(a) and (b), respectively. Kinetic model parameters and correlation coefficient (R2) are summarized in Table 1. The best fitting of the adsorption kinetics models was determined on the basis of correlation coefficients (R2), pseudo-second-order approached the experimental points better than pseudo-first-order. It was concluded that pseudo-second-order was more precise in evaluating the adsorption kinetics. Since pseudosecond-order model is based on the assumption of chemisorption, these results indicate that the limiting rate in the adsorption of Cr(VI) is a chemisorption mechanism through electrostatic attraction between active sites of adsorbent and metal ion (Fig. 6).
on the other hand the sorbate was negatively charged, since Cr(VI) at lower pH exist manly as HCrO4−and Cr2O7. 4.3.2. Effect of adsorbent dose Fig. 5(b) presents the effect of adsorbent dosage on the adsorption of Cr(VI) from aqueous solution. The adsorption of Cr(VI) initially increased as the adsorbent dose was increased, maximum percentage adsorption was found to be 45.2% at 30 mg loading. This result may be due to the aggregation of adsorption sites and enhanced diffusion path length, which lead to a decrease of total available surface area. The percentage adsorption results were helpful in determining the optimum amount of adsorbent. 4.4. Effect of contact time and adsorption kinetics of mwXG-g-DEAEMA for Cr(VI) The effect of contact time on the removal of Cr(VI) by mwXG-gDEAEMA was evaluated with the initial dye concentration of 40 mg L−1, at 25 °C temperature and pH 2; because the adsorption rate is a crucial parameter in the practical applications of adsorbents. The effect of contact time on the adsorption of Cr(VI) by the mwXG-gDEAEMA is shown in Fig. 5(c). It is clear that about 80% adsorptions occur during the first 2 h and the amounts of adsorbed Cr(VI) increase with prolonging contact time and achieve equilibrium at about 4 h for Cr(VI) and no more significant Cr(VI) removal was observed beyond 4 h. As surface adsorption sites become occupied, the Cr(VI) gradually penetrate into the inner of mwXG-g-DEAEMA and are slowly absorbed
4.5. Adsorption isotherm studies Adsorption isotherms analysis was also performed to describe the adsorption mechanism and also indicate how adsorbate molecules distribute between aqueous and solid phases. The experimental adsorption data of the mwXG-g-DEAEMA were interpreted using three common isotherm models, Langmuir, Freundlich models and Temkin which are widely used to describe the adsorption behavior. 8
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Fig. 6. Sorption kinetic plots of Cr(VI) ions onto mwXG-g-DEAEMA (a) pseudo-first-order and (b) pseudo-second-order sorption kinetics.
DEAEMA, we compared the obtained results in this study with some previously reported adsorbents that have been used for Cr(VI) removal. Maximum adsorption capacity used for adsorption experiment are listed in Table 3 (Karthik & Meenakshi, 2015; Kumar et al., 2014; Lu et al., 2017; Nithya et al., 2016; Sharma et al., 2017; Wu et al., 2017). The maximum adsorption capacity of mwXG-g-DEAEMA was much higher than some of the adsorbents found in the recent literature. The exceptional high adsorption capacity of mwXG-g-DEAEMA arises from the protonated tertiary amino group, which facilitate the adsorption of Cr(VI) from aqueous solution. On the basis of these results it may be concluded mwXG-g-DEAEMA can be considered as a good adsorbent for large scale wastewater purification due to high adsorption capacity, and facile preparation process from low cost and nontoxic materials such as XG.
Table 1 Kinetic model parameters for Cr (VI) ions adsorption onto the mwXG-gDEAEMA at 40 mg/L concentration, pH 2 and temperature 25 ⁰C. Kinetic Model
Kinetic Parameters
mwXG-g-DEAEMA
Pseudo first-order
K1 qe R2 K2 qe R2
0.550 0.787 0.707 0.027 47.35 0.989
Pseudo-second-order
Table 2 Adsorption Isotherm study of Cr(VI) ions onto the mwXG-g-DEAEMA at three different temperatures. Isotherm Model
Langmuir
Freundlich Temkin
Isotherm Constants
qm(mg/g) b RL R2 n R2 AT (L/g) bT β (J. mol−1) R2
mwXG-g-DEAEMA
5. Conclusion
298.15
308.15
318.15
58.8 0.014 0.879–0.592 0.588 1.323 0.978 0.244 292.7 0.118 0.880
41.7 0.026 0.792–0.432 0.632 1.604 0.944 0.332 326.4 0.003 0.824
34.5 0.054 0.651–0.271 0.925 1.932 0.992 0.536 355.4 0.003 0.929
In this work, graft polymerization was employed to synthesis adsorbent using DEAEMA and XG for hexavalent chromium removal from aqueous solution. Optimum reaction conditions for maximum %G and %GE values were found to be 292.7 and 41.7, respectively. The structure and morphology properties of the synthesized mwXG-g-DEAEMA was supported by FTIR, 1NMR, XRD and SEM. The effects of pH, dose, contact time, and equilibrium concentration on Cr(VI) adsorption were investigated batch wise. The adsorption of Cr(VI) was found to be highly pH dependent, and the maximum adsorption was observed under acidic condition of pH 2. Adsorption kinetic data fitted well with pseudo-second order model, showing the chemical adsorption of mwXG-g-DEAEMA to Cr(VI). The maximum adsorption capacity of Cr (VI) onto the mwXG-g-DEAEMA was found to be 58.8 mg/g. The experimental data were explained by the Freundlich isotherm model, which suggests the adsorption occurred by the formation of Cr(VI) multilayer at the adsorbent surface. In conclusion, the mwXG-gDEAEMA can be considered as a good adsorbent for large scale wastewater purification due to high adsorption capacity, and facile preparation process from low cost and nontoxic materials such as XG.
The equilibrium adsorption isotherm models were studied in details since they provide insight into the adsorption mode between the adsorbate and adsorbent surface (Eq. (S3-10)) and the results are summarized in Table 2. The best fitting of the adsorption isotherms models was determined on the basis of correlation coefficients (R2). The fitting degree follows the following sequence: Freundlich (best fit) > Temkin > Langmuir. Fig. 7 The Freundlich isotherm model showed good correlation with the sorption process compared with other investigated isotherm models. Fig. 7 Therefore, adsorption process is multilayer coverage and the adsorption site on adsorbent surface is heterogeneous. It is well known that Freundlich constant (n) gives an idea about the favorability of the adsorption process. The value of n should be between 1 and 10 for favorable adsorption. In the present study the value of n ranges between 1.323 and 1.932 indicating that the adsorption of Cr(VI) onto mwXG-g-DEAEMA was favourable process. According to the Langmuir isotherm model the maximum adsorption capacity of Cr(VI) on mwXG-g-DEAEMA was found to be 58.8 mg/g.
Acknowledgements The authors would like to thank the National Research Foundation (NRF), South Africa (Grant No:116679) and other affiliated organizations for financial support. Contributions Conceived and designed the experiments: S.P. Execution of experiments: S.P. and E.M. Data analysis: S.P. and E.M. Data interpretation: S.P. Author S.P and E.M. wrote the main manuscript text. All authors (E.M., S.P. and J.R.) reviewed the manuscript thoroughly before submission.
4.6. Comparison of mwXG-g-DEAEMA with other adsorbents for Cr(VI) removal For a better understanding of the adsorption ability of the mwXG-g9
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Fig. 7. Langmuir, Freundlich and Temkin isotherm for the adsorption of Cr(VI) ions onto mwXG-g-DEAEMA at three different temperatures (a) 298.15 K, (b) 308.15 K and (c) 318.15 K. Table 3 Comparison of mwXG-g-DEAEMA with other adsorbents employed for removal of Cr(VI). Adsorbents
Maximum adsorption capacity Qmax (mg/g)
Reference
n-butylacrylate grafted chitosan Bamboo charcoal grafted by Cu2+ N-aminopropyIsilane complexes Magneticnanoparticle-multiwalled carbon nanotube composites Chitosan-cl-poly(alginiacid)nano hydrogel Glutaraldehyde crosslinked silica gel/chitosn-g-poly(butylacrylate) Sodium alginate-polyaniline (SAP) nanofibers mwXG-g-DEAEMA
17.14 17.93 19.65 26.49 55.71 73.34 58.8
(Kumar et al., 2014) (Wu et al., 2017) (Lu et al., 2017) (Sharma et al., 2017) (Nithya et al., 2016) (Karthik & Meenakshi, 2015) Present work
Appendix A. Supplementary data
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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Microwave-assisted green synthesis of xanthan gum grafted diethylamino ethyl methacrylate: An efficient adsorption of hexavalent chromium
T
⁎
Edwin Makhadoa,c, Sadanand Pandeya,b, , James Ramontjaa a
Department of Applied Chemistry, Centre for Nanomaterials Science Research, University of Johannesburg, P.O. Box 17011, Doornfontein 2028, Johannesburg, South Africa b Department of Chemistry, College of Natural Sciences, Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk, 38541, Republic of Korea c Department of Chemistry, School of Physical and Mineral Sciences, University of Limpopo (Turfloop) Polokwane, South Africa
A R T I C LE I N FO
A B S T R A C T
Chemical compounds studied in this article: Xanthan gum (PubChem CID: 7107) N, N-diethylamino ethyl methacrylate (PubChem CID: 61012) Acetone (PubChem CID: 180) Potassium dichromate (PubChem CID: 24502) Ammonium persulfate (PubChem CID: 62648) Sodium hydroxide (PubChem CID: 14798) Hydrochloric acid (PubChem CID: 313)
We report the development of a novel graft copolymer, diethylamino ethyl methacrylate grafted xanthan gum (mwXG-g-DEAEMA), by microwave heating. The synthesized graft copolymer was used for potential application of Cr(VI) adsorption. The structure, thermal stability and morphologies of XG and mwXG-g-DEAEMA were characterized to verify the adsorbent formed under optimized reaction conditions. FTIR, XRD, TGA and SEM techniques were used for characterization of XG and mwXG-g-DEAEMA. Furthermore, 1H NMR spectroscopic analyses predict the probable structure of copolymer. Based on the NMR data, a plausible mechanism for copolymer formation has been proposed. The effects of adsorbent loading, pH, contact time and equilibrium concentration of the Cr(VI) adsorption were investigated batch wise. The Cr(VI) adsorption process followed the pseudo-second-order rate model and equilibrium data were best described by Freundlich isotherm model. This work will encourage researchers to focus on this facile green technique for the synthesis of adsorbent with enhanced adsorption capacity.
Keywords: Biopolymer Graft co-polymerisation Microwave-assisted synthesis Adsorption Cr(VI) removal
1. Introduction Water pollution as a result of toxic metal ions, such as chromium (Cr) is of great concern in wastewater treatment. Most of the Cr compounds exist as trivalent chromium Cr(III) or hexavalent chromium Cr (VI). The latter is more toxic as a result of its high water solubility and mobility (Benoit, 1976). The utilization of this toxic metal in various industrial processes like electroplating, leather tanning, and production of dyes, batteries, photographs, and cement, etc. has led to the need to treat wastewater effluent prior to release into the environment. Swallowing of Cr(VI) can cause diseases such as cancer abdominal pain, diarrhea, heart failure damages to the gut, liver, and kidneys (Li et al., 2008; Pandey, 2017; Mehta, Mazumdar, & Singh, 2015). The US Environmental Protection Agency (EPA) has capped the release of Cr(VI) into wastewater at 0.5 mg/L (Barai & Engelken, 2002). In addition, a maximum contamination level for Cr in drinking water is capped at 0.1 mg/L (De Gisi, Lofrano, Grassi, & Notarnicola, 2016). For this
reasons, there is a need to remove Cr(VI) from Cr-containing industrial effluents before it is discharged into the soil and water. Treatment of industrial effluents has steered researchers in the direction of engineering efficient and reliable techniques for the removal of metal ions from contaminated water. Adsorption displays many advantages in removing toxic metal ions due to its operational simplicity, low cost and simplicity of design when compared to conventional techniques (De Gisi et al., 2016). Adsorption is the most promising and widely used technique for the removal of metal ions and synthetic dyes from wastewater. Hybridization of natural polymers with synthetic polymers or copolymers led to the formation of new materials with improved properties such as better processability and biodegradability. In this regard, naturally derived carbohydrate based polymers have gain significant attention globally as a result of their properties which include biocompatibility, biodegradability, hydrophilicity, non-toxicity, cost-effectiveness, and renewable and recyclable nature. Xanthan gum (XG) an efficient polyelectrolyte due to the presence of tunable
⁎ Corresponding author at: Department of Chemistry, College of Natural Sciences, Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk, 38541, Republic of Korea. Tel.: (+82) 10-6286-7573 E-mail addresses: [email protected], [email protected], [email protected], [email protected] (S. Pandey).
https://doi.org/10.1016/j.carbpol.2019.114989 Received 20 February 2019; Received in revised form 27 May 2019; Accepted 10 June 2019 Available online 12 June 2019 0144-8617/ © 2019 Elsevier Ltd. All rights reserved.
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were analytical grade. For all the experiments, deionized water (DI) was used. The stock solutions of Cr(VI) (1000 mg/L) was prepared by dissolving an appropriate amount of K2Cr2O7 in 1000 mL of DI, and the stock solution was further diluted for batch experiments.
hydroxyl groups. Graft copolymers based on XG have been effectively used for the adsorption of the heavy metals and organic dyes from aqueous solutions (Ghorai et al., 2014; Singh, Tiwari, Pandey, & Preeti Sanghi, 2015). Efforts have been made on the exploitation of graft copolymers for various applications including chemical sensor (Pandey & Nanda, 2016; Pandey & Ramontja, 2016; Pandey, 2016), drug delivery systems (Sen, Mishraa, Jhaa, & Pal, 2010), and wastewater treatment (Sarkar, Aniruddha Pal, Ghorai, Mandre, & Pal, 2014; Fosso-Kankeu, Van den Berg, Waanders, & Pandey, 2018, b, Ghorai, Sarkar, Panda, & Pal, 2013; Pal et al., 2015; Pandey & Mishra, 2012). The use of graft copolymers for wastewater treatment has been significantly focused on in the recent reports. Hajeetha, Sudhab, Vijayalakshmib, and Gomathi (2014) reported the synthesis of cellulose grafted polyacrylonitrile for removal of Cr(VI) from aqueous solution via the conventional method using ceric ammonium nitrate as initiator. In recent years, the exploitation of microwave irradiation has emerged as an effective process for preparation of the graft copolymers. This process is considered due to its advantages of high reproducibility, fast reaction process and ease of operation (Makhado, Pandey, Nomngongo, & Ramontja, 2017; Makhado, Pandey, Nomngongo, & Ramontja, 2017). Hence, recently several researchers have prepared graft copolymers via microwave irradiation techniques. For example, Pal, Majumder, and Bandyopadhyay (2016) grafted poly (acrylic acid) onto xanthan gum by employing microwave-assisted technique and the as-synthesized graft copolymer was used as a biosorbent for the removal of Hg(II) from water. Sharma and Mishra (2010) reported the synthesis of chitosangrafted polystyrene via microwave initiated method and they applied this material for removal of Cr(VI). This adsorbent was found to be more effective for Cr(VI) removal. In another study by Kumar, Kumar, Rajesh, and Rajesh (2014) n-butylacrylate grafted chitosan was prepared by microwave-assisted method and they employed this material as adsorbent for the removal of Cr(VI) from aqueous solution. In their study the adsorption kinetic was well described by pseudo-second order and the maximum adsorption capacity was found to be 17.14 mg/g (Kumar et al., 2014). Recently, Li et al. (2018), reported the preparation of chitosan grafted with a hyperbranched polymer for adsorption of Cr (VI) from aqueous solution. The adsorption kinetics and isotherm models were well fitted with pseudo-second-order and Freundlich isotherm model, respectively. They obtained a maximum adsorption capacity of 194.55 mg/g. It is well documented the presence poly(DEAEMA) imparts pH and temperature responsive character to a hydrogel. (Karthika & Vishalakshi, 2015; Liu et al., 2002; Schmalz, Hanisch, Schmalz, & Müller, 2010). To the best of our knowledge, there is no literature about the hybridization of XG with DEAEMA. Therefore, the objectives of this study were (i) to synthesize mwXG-g-DEAEMA by employing microwave assisted strategy, using APS as the free radical initiator and characterize the structure of the synthesized mwXG-g-DEAEMA; (ii) to investigate the interactive effects of microwave-assisted process parameters including monomer, initiator, biopolymer, microwave exposure time and microwave power; (iii) to investigate the possibility of utilizing mwXG-g-DEAEMA as adsorbent for the removal of Cr(VI) from aqueous solution.
2.2. Microwave-assisted synthesis of mwXG-g-DEAEMA In order to synthesis of mwXG-g-DEAEMA, 0.1 g XG was dissolved in required amount of distilled water at 25 °C. To this solution, calculated amount of DEAEMA and MBA were added to the reaction flask and allowed to react for 5 min. Then catalytic amount of (NH4)2S2O8 was added dropwise to the mixture to initiate polymerization and allowed to react for another 2 min at 25 °C. The flask was exposed under fixed microwave power for a definite time period in a domestic microwave oven with a frequency of 2450 MHz. A power output from 0 to 1000 W with continuous adjustment was used for all the experiments. After each exposure, the temperature of the reaction mixture was determined with thermometer. After desired time period, the mwXG-gDEAEMA was precipitated by pouring the reaction mixture into large quantity of acetone and washed well to remove adhered homopolymer if any is present along with graft copolymers. The precipitated copolymer was filtered and the copolymer samples obtained were finally dried under vacuum at 60 ⁰C to a constant weight and the pulverized in a ball mill. The percentage grafting (%G) and percentage grafting efficiency (%GE) of mwXG-g-DEAEMA were calculated using the following Eqs. (1) and (2) (Fanta & Doane, 1986).
%G=
W1 − W 2 × 100 W2
%GE=
W1 − W 2 × 100 W3
(1)
(2)
where, W1, W2 and W3 stand for the weight of XG, mwXG-g-DEAEMA and DEAEMA, respectively. 2.3. Adsorption studies of mwXG-g-DEAEMA to Cr(VI) All the adsorption experiments were performed in a batch system. Typically, 20.0 mg of mwXG-g-DEAEMA adsorbent was added in 20.0 mL of Cr(IV) solution of known concentration and the mixture was stirred at 160 rpm and room temperature for adsorption to proceed. In order to determine the optimal pH of solution for metal ion adsorption, the Cr(IV) removal by mwXG-g-DEAEMA was evaluated in the pH range from 1.0 to 7.0. The pH of Cr(IV) solutions were adjusted using 0.1 M HCl or NaOH solution. After the final stage, the adsorbent was filtered through 0.45 μm PVDF syringe filters from the Cr(IV) solution and the residual Cr(IV) concentration was measured using the inductively coupled plasma optical emission spectrometry (ICP-OES) at maximum wavelength of metal ion. It should be mentioned that all experiments were performed in triplicates. The percentage adsorption was calculated using Eq. (3):
Adsorption % =
2. Materials and methods
Co − Ce × 100 Co
(3)
and the equilibrium uptake was calculated using Eq. (4):
2.1. Materials
qe = (Co − Ce ) × Xanthan gum (XG) from Xanthomonas campestris (average molecular weight of pure xanthan gum is 4,500,000) and N, N-diethylamino ethyl methacrylate (DEAEMA, 99%) monomer were purchased from SigmaAldrich (South Africa). Acetone and hydrochloric acid (HCl) were procured from Merck (South Africa). Initiator ammonium persulfate (APS) (≥98%), Potassium dichromate (K2Cr2O7) and sodium hydroxide (NaOH) were procured from Sigma-Aldrich, South Africa. All solvents
V W
(4)
where Co denotes the initial and the Ce denotes the equilibrium concentrations (mgL−1) of the Cr(VI) solution, respectively. Furthermore, qe is the equilibrium capacity of Cr(VI) on the adsorbent (mg/g), V is the volume of the Cr(VI) solution used (L) and W is the weight of adsorbent (g) used. Experiments were conducted with different mwXG-gDEAEMA grades, pH, adsorption time and adsorbent concentration. 2
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3. Characterization of mwXG-g-DEAEMA adsorbent
copolymerization. The decrease in the %G and %GE after the optimum concentration might be due to the increase in the rate of termination reactions. Similar, phenomena were observed concerning the effect of initial concentration on the grafting parameters (Mostafa, Naguib, Sabaa, & Mokhtar, 2005).
FT-IR spectra of all samples were recorded in the range of 4000–400 cm−1 on Spectrum-100 Perkin Elmer, USA spectrometer using KBr pellet. X-ray diffractometer was employed for acquiring XRD pattern with a (Rigaku Ultima IV, X-ray diffractometer, Japan) by employing CuKα radiation of the wavelength of 1.5406 Å with visible slights at 45 kV/40 mA. XRD spectra were collected in the 2θ range between 5° and 90° with a step size of 0.01°, and a scan speed of 1°/min. The morphologies of the samples were carried out by using scanning electron microscopy (SEM), (TESCAN, VEGA SEM, Czech Republic) at a 20 kV electron acceleration voltage. The surfaces of the samples were coated with carbon to avoid charging. Thermogravimetric analyses (TGA) of samples were carried out by a Q500 TA Instruments, USA from room temperature to 900 °C at a heating rate of 10 °C min−1 under a nitrogen atmosphere. 1H NMR spectrometry was performed with 500 MHz Bruker Avance III HD spectrometer (Germany) using deuterium oxide for XG and d-chloroform for mwXG-g-DEAEMA as the lock solvent. Samsung (Model No. ME9114W1; 1500 W, Made: Malaysia) domestic microwave oven having 2450 MHz microwave frequency and a power output from 0 to 1000 W was used for synthesis of mwXG-gDEAEMA. The pH measurements were made with HI 9811-5/HI 1285-5 (Romania). A Femto spectrophotometer, model 700 Plus using a 1.0 cm glass cell, adjusted in the wavelength of 540 nm, was used for the quantification of Cr(VI) in order to compare with the ICP-OES results.
4.1.3. Effect of XG concentration The effect of increasing XG concentration on grafting parameters is shown in (Fig. 1(c)). According to (Fig. 1(c)), it was observed that %G decreased continuously with increasing XG concentration from 0.1 to 0.5 g. in contrary, the %GE with increase in the concentration of the XG. The decrease in the %G beyond optimum value could be attributed to the increase in the viscosity of the reaction medium as the concentration of the polymer increases, which hinders the movement of APS molecules, monomer molecules and active chains, thus the grafting decrease. Additionally, a high XG concentration could generate higher number of XG macro-radicals, which could combine with one another to terminate the reaction, as a result the grafting parameters values decrease. Similar observations have also been reported in relation to the effect of the polymer concentration on the grafting parameters (Trivedi, Kalta, Patel, & Trivedi, 2005; Yigitoglu, Isiklan, & Ozmen, 2007). 4.1.4. Effect of time of exposure The effect of microwave exposure time on grafting reaction was investigated by changing time interval from 20 to 100 s keeping the concentration of XG (0.1 g), DEAEMA (15 × 10−5 M), APS (300 × 10-4 M), microwave power (80%) and the total reaction volume of 25 mL fixed. The results are displayed in (Fig. 1(d)). The %G and %GE were found to be increasing from (237.1 and 33.7) to (292.7 and 41.7), respectively, with increased exposure time from 20 to 40 s. This increment in the graft parameters was then followed by gradual decrease in grating with prolonged exposure to microwave.
4. Results and discussion 4.1. Effect of mwXG-g-DEAEMA of reaction mixture Effect of reaction parameters such as monomer, initiator, biopolymer, microwave exposure time and microwave power were investigated in order to get optimum grafting conditions (Eqs. (1) and (2)). Fig. 1(a–e) present the effect of various reaction parameters on grafting and grafting efficiency. Mechanism for the synthesis of mwXGg-DEAEMA is displayed in (Fig. S1).
4.1.5. Effect of the microwave power The effect of microwave power on grafting reaction was studied by altering microwave power from 50 to 100%, at the fixed concentration of XG (0.1 g), APS (60 × 10−4 M), DEAEMA (15 × 10-5 M), exposure time (40 s) and the total reaction volume of 25 mL. The results are shown in (Fig. 1(e)). The %G and %GE were increased almost linearly with microwave power up to 70% thereafter the %G (292.7) and %GE (41.7) was decreased. The reason of this behaviour is attributed to the formation of more monomer and macro-radicals due to fast energy transfer among the molecules (Wan et al., 2011). The higher microwave power results in the formation of homopolymer and decomposition of the grafting copolymers (Biswal et al., 2007; Huacai, Wan, & Dengke, 2006).
4.1.1. Effect of DEAEMA concentration The percentage grafting in mwXG-g-DEAEMA was examined by varying the monomer concentration from 6 × 10−5 M to 18 × 10−5 M (Fig. 1(a)). The %G was found to be linearly proportional to monomer concentration from 3 × 10−5 M up to 15 × 10−5 M. Beyond 15 × 10−5 M the %G decreased with increasing the DEAEMA concentration. Similar trend was observed for the %GE at the fixed concentration of XG (0.1 g), APS (60 × 10-4 M), exposure time (60 s), microwave power (80%) and the total reaction volume of 25 mL. Beyond optimum concentration, the decrease in the %G and %GE values could be associated with the depletion of the active sites on the biopolymer as a result of the predominance of homopolymerization with further increase in DEAEMA concentration in the polymerization medium. Furthermore, the increase in homopolymer increases the viscosity of the medium making monomers to diffusion into the XG backbone difficult. Similar behaviours have been reported in other system (Makhlouf, Marais, & Roudesli, 2007).
4.2. Characterization of XG and mXG-g-DEAEMA 4.2.1. X-ray diffraction (XRD) characterization Characterization techniques used in this study are given in the supporting file. The XRD analysis was carried out to investigate the structural parameters. Fig. 2(a) displays the X-ray diffractogram of XG and mwXG-g-DEAEMA. The diffractogram of XG exhibits two prominent diffractions, one at low diffraction angle (2θ = 23.6°) values, which is indicative of the mostly amorphous structure of the XG and another one at 2θ = 37.5°, showing some crystalline content of the XG (Kumar, Singh, & Ahuja, 2009; Pandey & Mishra, 2011) On account of mwXG-g-DEAEMA, the XRD pattern showed that the characteristic peak of XG shifted to 16.9°. Furthermore, additional peaks appeared at 20.7° and 29.7° may be due to grafting of DEAEMA onto XG.
4.1.2. Effect of APS concentration The percentage grafting in mwXG-g-DEAEMA was examined by varying the initiator concentration from 60 × 10−4 M to 360 × 10−4 M at fixed concentration of XG (0.1 g), DEAEMA (15 × 10-5 M), exposure time (60 s), microwave power (80%) and the total reaction volume of 25 mL, the results are presented in (Fig. 1(b)). It was found that percentage grafting and grafting efficiency increased to 270.8 to 23.3 up to concentration 0.03 M of APS. Further raise in APS concentration decreased the percentage grafting and grafting efficiency. The increase in APS concentration enhances the number of radical species XG-O∙ radicals which in the presence of monomer radicals increases graft
4.2.2. FTIR spectroscopy To confirm the structure of products, the reactions were monitored by FTIR, as shown in Fig. 2(b). The FT-IR spectrum of spectrum of pure XG showed peaks around 3246, 2932 and 1404 cm−1, demonstrating 3
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Fig. 1. Optimization of mwXG-g-DEAEMA variables, %G and %GE by varying (a) monomer (DEAEMA); (b) Initiator (APS); (c) Biopolymer (XG); (d) MW exposure time (s); (e) MW power (%).
characteristic stretching vibration of eOH, aliphatic eCH stretching of alkyl group, and eCH bending of methyl groups, respectively. In addition, the absorption bands appeared at 1169 and 1053 cm−1 correspond to eCO bending (Makhado, Pandey, Nomngongo, & Ramontja, 2018). The FT-IR spectrum of XG-g-DEAEMA, shows a broad and intense band at 3345 cm−1, corresponding to the stretching of eOH bonds. The absorption bands at 2844 and 2950 cm−1 were attributed to the stretching vibration of eCH2 aromatic and eCH3 methyl groups of DEAEMA. The band at 1734 cm−1 is ascribed to eC]O stretching of carboxylic ester (Geng et al., 2016). The transmission intensity of mwXG-g-DEAEMA was higher as a result of additional concentration of carboxylic ester. The absorption bands at 1123 and 1473 cm−1 were attributed to the bending of ester eOeCH2 bond and CeN stretching
vibrations in DEAEMA, respectively. The new intense peaks appeared at 623 cm−1. This result confirms that DEAEMA was chemically bonded to the XG molecule. In the spectrum of mwXG-g-DEAEMA after adsorption of Cr(VI) ions showed that the intensity of characteristic bands at 1734, 1473, 1123 and 623 cm−1 were reduced. The reduction in the intensity of characteristic bands after adsorption of Cr(VI) ions confirms that the interaction occurred between Cr(VI) ions on to mwXG-g-DEAEMA.
4.2.3. Thermal analysis Thermogravimetric analysis (TGA) is a simple method for studying the decomposition pattern and the thermal stability of the compounds. TGA curves of XG and mwXG-g-DEAEMA are depicted in Fig. 2(c,d). There are remarkable differences of the thermal stability for XG and 4
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Fig. 2. Characterization of XG and mwXG-g-DEAEMA (a) XRD; (b) FTIR; (c) TGA; and (d) DTGA. 1 H NMR spectra of neat DEAEMA, XG and mwXG-g-DEAEMA are showed in Fig. 3(a–c), respectively. 1H NMR spectrum of pure XG was performed in deuterated water (D2O), while mwXG-g-DEAEMA was dissolved in d-chloroform (CDCl3). The characteristic neat XG peaks were observed at δ 3.51–3.64 ppm, which can be attributed to the hydrogen associated to proton peaks of glycosidic CH2 and alcoholic CH2; The peak at δ 4.88 ppm, ascribed attributed to the hydrogen linked to the C1, δ 4.00 ppm, from the hydrogen atoms associated to the C2 and C3 carbons δ 3.91-3.84 ppm attributed to the hydrogen associated to the C4 carbons and δ 3.72 ppm attributed to the hydrogen associated to the C5 carbons. The peak of D2O used for dissolving neat XG was observed at δ 4.30 ppm (Makhado et al., 2017a, 2017b). In the 1H NMR spectra of mwXG-g-DEAEMA, additional signals appeared at δ 2.72 and 0.86 ppm are attributed to proton peaks of CH2(CH3) and CH2(CH3) (denoted by A and B) for DEAEMA, respectively. δ 2.93 ppm is assigned to proton peaks of CH2N (denoted by C). δ 4.28 ppm ascribed to the hydrogen associated to the CH2 (denoted by D) carbons. δ 1.91 ppm is assigned to proton peaks of CH3 (denoted by E). The peaks in mwXG-g-DEAEMA at δ 4.01, 3.65, and 2.15 ppm were assigned to proton peaks of the XG backbone. The peak of CDCl3 solvent used for dissolving mwXG-g-DEAEMA was observed at δ 7.24 ppm. In the 1H NMR spectra of mwXG-g-DEAEMA, there was slight shift in the proton peaks at their corresponding range. This result is due to the fact that chemical shifts are slightly different in different solvents, depending on electronic solvation effects. The resolution of the peaks appeared in mwXG-g-DEAEMA was poor as compared to pure XG; this may be due to poor solubility of mwXG-g-DEAEMA in solvent. These observations suggest that the copolymer DEAEMA had been effectively grafted onto the XG backbone as intended in Fig. S1. Furthermore, the obtained results are in agreement with the 1H-NMR spectra recorded by other researchers who worked with pure and modified XG (Makhado et al., 2017a, 2017b; Kumar, Deepak Sharma, Srivastava, & Kumar, 2017; Pal et al., 2016).
mwXG-g-DEAEMA. TGA of XG demonstrated three-step characteristic thermogram; the first degradation occurred over the range 32–120 °C associated with the endothermic peak might be attributed dehydration (Makhado, Pandey, & Ramontja, 2018). The rate of weight reduction increases with an increase in the temperature. Furthermore, TGA curve of XG demonstrated the second degradation process in the range 270–350 °C accompanied by a weight loss exceeding 22% is assigned to the decomposition of XG backbone. Finally, total of 69% weight loss occurs around 600 °C (Makhado, Pandey, Nomngongo et al., 2018). The thermogram of mwXG-g-DEAEMA showed four degradation stages. The initial decomposition stage was observed in the range 265–292 °C and weight reduction of about 7% was observed, shows lower H2O content of the grafted sample. The second decomposition stage weight loss of approximately 53% was observed in the temperature range 230–322 °C. This may be due to the degradation of xanthan gum backbone as well as decomposition of functional group of grafted DEAEMA chain. The vast majority of the weight loss occurred when the temperature was above 230 °C. The third gradual degradation step accounting for about 77% weight loss occurred in the temperature range 415–450 °C. Finally, total 98% weight loss occurred around 610 °C, which is for the decomposition of the grafted polymer backbone. The DTGA curves (Fig. 2(d)) provide clear information of TGA data and also confirms the lower thermal stability of mwXG-g-DEAEMA when compared to XG. This result may be attributed to the low thermal stability of DEAEMA. In a study conducted by (Madill, Garcia-Valdez, Champagne, & Cunningham, 2017) DEAEMA was found to exhibit two decomposition peaks at 320–370 °C and 390–450 °C. Furthermore, DEAEMA showed lower thermal stability than pure chitosan and graft copolymer sample (Madill et al., 2017). 4.2.4. NMR analyses In order to confirm synthesized graft copolymerization (mwXG-gDEAEMA) 1H NMR spectroscopic analysis was investigated. The proton, 5
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Fig. 3. 1H NMR spectra of (a) DEAEMA, (b) XG and (c) mwXG-g-DEAEMA.
morphology of mwXG-g-DEAEMA exhibits rough irregular surface morphology shown in Fig. 4(c and d). The mwXG-g-DEAEMA after Cr (VI) ions adsorption appears to have irregular morphology with heterogeneous surface (Fig. 4(e and f)). Fig. 4(g and h) shows the EDS spectrum of mwXG-g-DEAEMA before adsorption of Cr(VI) ions and mwXG-g-DEAEMA after removal of Cr(VI) ions, respectively. The presence of Cr(VI) ions in the surface of graft copolymer (mwXG-gDEAEMA) was further confirmed by EDS analysis. EDS results were in
4.2.5. Scanning electron microscopy characterization The morphology of XG, mwXG-g-DEAEMA and mwXG-g-DEAEMA Cr(VI) ions loaded were studied by SEM. The SEM micrograph of XG Fig. 4(a and b) showed the granular morphology, due to the amorphous nature of the biopolymer (Makhado et al., 2017a, 2017b). In comparison to the pure XG (Fig. 4(a and b)) (Makhado et al., 2017a, 2017b), the grafting of DMAEMA onto XG shows considerable changes in the shape and size of the granular morphology of XG particles. The surface 6
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Fig. 4. SEM images of XG at different magnification (a) 2kx, (b) 4kx; mwXG-g-DEAEMA at different magnifications (c) 2kx, (d) 4kx; mwXG-g-DEAEMA Cr(VI) ions loaded at different magnification (e) 2kx, (f) 4kx, (g) EDS analysis of mwXG-g-DEAEMA, and (h) EDS analysis of mwXG-g-DEAEMA Cr(VI) ions loaded.
4.3. Adsorption of hexavalent chromium
agreement with the FTIR spectra of reported in Fig. 2(b).
4.3.1. Effect of pH on the adsorption of Cr(VI) The solution pH is significant parameters in the sorption process as a result of its remarkable effect on the adsorption sites on the adsorbent (mwXG-g-DEAEMA) surface and it also the speciation of Cr(VI) in the solution. The initial pH of the solution was investigated in the range of 1–6 and the obtained results are shown in Fig. 5(a). The conditions employed to study the effect of pH on the adsorption of Cr(VI) are stated in Fig. 5(a). Percentage adsorption for Cr(VI) ions on mwXG-gDEAEMA was attained at pH 2. Similar trend are also reported in literatures (Garg, Kaur, Garg, & Sud, 2007). Thereafter sharply decrease in the adsorption was noticed with increasing in the initial pH of the solutions. A possible reason for these observations is that the adsorbent was positively charged with the protonated tertiary amino group while
4.2.6. Elemental analysis The elemental analysis of XG and the graft copolymer was performed by using PerkinElmer® 2400 Series II CHNS/O Elemental Analyzer (2400 Series II) at a high temperature combustion method at 960 °C for complete combustion. Samples were analyzed in synthetic air using oxygen as the combustion gas and helium as the carrier gas. Analyses were performed in triplicate. The estimated analysis of three different elements, carbon (C), hydrogen (H) and nitrogen (N), was undertaken. Elemental analysis were conducted to confirm the graft procedure was effective and findings are displayed in Table S1. The results obtained show that grafted sample contain nitrogen. Since XG does not have no or negligible amount of nitrogen in its composition, identification of this element further confirms the grafting. 7
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Fig. 5. Effect of initial (a) pH; (b) absorbent dose; and (c) time for Cr(VI) sorption by mwXG-g-DEAEMA.
by the internal active sites. Finally, all active sites of the adsorbent are occupied and the amounts of adsorbed Cr(VI) reach constant values. Kinetic model provides a relationship between the rate of sorption and the type of reaction mechanism involved. Two kinetic models, namely pseudo-first-order and pseudo-second-order model have been adopted to describe kinetic data (Eq. (S1-2)). The fitting effects of experimental data with pseudo-first-order and pseudo-second-order kinetic model are shown in Fig. 6(a) and (b), respectively. Kinetic model parameters and correlation coefficient (R2) are summarized in Table 1. The best fitting of the adsorption kinetics models was determined on the basis of correlation coefficients (R2), pseudo-second-order approached the experimental points better than pseudo-first-order. It was concluded that pseudo-second-order was more precise in evaluating the adsorption kinetics. Since pseudosecond-order model is based on the assumption of chemisorption, these results indicate that the limiting rate in the adsorption of Cr(VI) is a chemisorption mechanism through electrostatic attraction between active sites of adsorbent and metal ion (Fig. 6).
on the other hand the sorbate was negatively charged, since Cr(VI) at lower pH exist manly as HCrO4−and Cr2O7. 4.3.2. Effect of adsorbent dose Fig. 5(b) presents the effect of adsorbent dosage on the adsorption of Cr(VI) from aqueous solution. The adsorption of Cr(VI) initially increased as the adsorbent dose was increased, maximum percentage adsorption was found to be 45.2% at 30 mg loading. This result may be due to the aggregation of adsorption sites and enhanced diffusion path length, which lead to a decrease of total available surface area. The percentage adsorption results were helpful in determining the optimum amount of adsorbent. 4.4. Effect of contact time and adsorption kinetics of mwXG-g-DEAEMA for Cr(VI) The effect of contact time on the removal of Cr(VI) by mwXG-gDEAEMA was evaluated with the initial dye concentration of 40 mg L−1, at 25 °C temperature and pH 2; because the adsorption rate is a crucial parameter in the practical applications of adsorbents. The effect of contact time on the adsorption of Cr(VI) by the mwXG-gDEAEMA is shown in Fig. 5(c). It is clear that about 80% adsorptions occur during the first 2 h and the amounts of adsorbed Cr(VI) increase with prolonging contact time and achieve equilibrium at about 4 h for Cr(VI) and no more significant Cr(VI) removal was observed beyond 4 h. As surface adsorption sites become occupied, the Cr(VI) gradually penetrate into the inner of mwXG-g-DEAEMA and are slowly absorbed
4.5. Adsorption isotherm studies Adsorption isotherms analysis was also performed to describe the adsorption mechanism and also indicate how adsorbate molecules distribute between aqueous and solid phases. The experimental adsorption data of the mwXG-g-DEAEMA were interpreted using three common isotherm models, Langmuir, Freundlich models and Temkin which are widely used to describe the adsorption behavior. 8
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Fig. 6. Sorption kinetic plots of Cr(VI) ions onto mwXG-g-DEAEMA (a) pseudo-first-order and (b) pseudo-second-order sorption kinetics.
DEAEMA, we compared the obtained results in this study with some previously reported adsorbents that have been used for Cr(VI) removal. Maximum adsorption capacity used for adsorption experiment are listed in Table 3 (Karthik & Meenakshi, 2015; Kumar et al., 2014; Lu et al., 2017; Nithya et al., 2016; Sharma et al., 2017; Wu et al., 2017). The maximum adsorption capacity of mwXG-g-DEAEMA was much higher than some of the adsorbents found in the recent literature. The exceptional high adsorption capacity of mwXG-g-DEAEMA arises from the protonated tertiary amino group, which facilitate the adsorption of Cr(VI) from aqueous solution. On the basis of these results it may be concluded mwXG-g-DEAEMA can be considered as a good adsorbent for large scale wastewater purification due to high adsorption capacity, and facile preparation process from low cost and nontoxic materials such as XG.
Table 1 Kinetic model parameters for Cr (VI) ions adsorption onto the mwXG-gDEAEMA at 40 mg/L concentration, pH 2 and temperature 25 ⁰C. Kinetic Model
Kinetic Parameters
mwXG-g-DEAEMA
Pseudo first-order
K1 qe R2 K2 qe R2
0.550 0.787 0.707 0.027 47.35 0.989
Pseudo-second-order
Table 2 Adsorption Isotherm study of Cr(VI) ions onto the mwXG-g-DEAEMA at three different temperatures. Isotherm Model
Langmuir
Freundlich Temkin
Isotherm Constants
qm(mg/g) b RL R2 n R2 AT (L/g) bT β (J. mol−1) R2
mwXG-g-DEAEMA
5. Conclusion
298.15
308.15
318.15
58.8 0.014 0.879–0.592 0.588 1.323 0.978 0.244 292.7 0.118 0.880
41.7 0.026 0.792–0.432 0.632 1.604 0.944 0.332 326.4 0.003 0.824
34.5 0.054 0.651–0.271 0.925 1.932 0.992 0.536 355.4 0.003 0.929
In this work, graft polymerization was employed to synthesis adsorbent using DEAEMA and XG for hexavalent chromium removal from aqueous solution. Optimum reaction conditions for maximum %G and %GE values were found to be 292.7 and 41.7, respectively. The structure and morphology properties of the synthesized mwXG-g-DEAEMA was supported by FTIR, 1NMR, XRD and SEM. The effects of pH, dose, contact time, and equilibrium concentration on Cr(VI) adsorption were investigated batch wise. The adsorption of Cr(VI) was found to be highly pH dependent, and the maximum adsorption was observed under acidic condition of pH 2. Adsorption kinetic data fitted well with pseudo-second order model, showing the chemical adsorption of mwXG-g-DEAEMA to Cr(VI). The maximum adsorption capacity of Cr (VI) onto the mwXG-g-DEAEMA was found to be 58.8 mg/g. The experimental data were explained by the Freundlich isotherm model, which suggests the adsorption occurred by the formation of Cr(VI) multilayer at the adsorbent surface. In conclusion, the mwXG-gDEAEMA can be considered as a good adsorbent for large scale wastewater purification due to high adsorption capacity, and facile preparation process from low cost and nontoxic materials such as XG.
The equilibrium adsorption isotherm models were studied in details since they provide insight into the adsorption mode between the adsorbate and adsorbent surface (Eq. (S3-10)) and the results are summarized in Table 2. The best fitting of the adsorption isotherms models was determined on the basis of correlation coefficients (R2). The fitting degree follows the following sequence: Freundlich (best fit) > Temkin > Langmuir. Fig. 7 The Freundlich isotherm model showed good correlation with the sorption process compared with other investigated isotherm models. Fig. 7 Therefore, adsorption process is multilayer coverage and the adsorption site on adsorbent surface is heterogeneous. It is well known that Freundlich constant (n) gives an idea about the favorability of the adsorption process. The value of n should be between 1 and 10 for favorable adsorption. In the present study the value of n ranges between 1.323 and 1.932 indicating that the adsorption of Cr(VI) onto mwXG-g-DEAEMA was favourable process. According to the Langmuir isotherm model the maximum adsorption capacity of Cr(VI) on mwXG-g-DEAEMA was found to be 58.8 mg/g.
Acknowledgements The authors would like to thank the National Research Foundation (NRF), South Africa (Grant No:116679) and other affiliated organizations for financial support. Contributions Conceived and designed the experiments: S.P. Execution of experiments: S.P. and E.M. Data analysis: S.P. and E.M. Data interpretation: S.P. Author S.P and E.M. wrote the main manuscript text. All authors (E.M., S.P. and J.R.) reviewed the manuscript thoroughly before submission.
4.6. Comparison of mwXG-g-DEAEMA with other adsorbents for Cr(VI) removal For a better understanding of the adsorption ability of the mwXG-g9
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Fig. 7. Langmuir, Freundlich and Temkin isotherm for the adsorption of Cr(VI) ions onto mwXG-g-DEAEMA at three different temperatures (a) 298.15 K, (b) 308.15 K and (c) 318.15 K. Table 3 Comparison of mwXG-g-DEAEMA with other adsorbents employed for removal of Cr(VI). Adsorbents
Maximum adsorption capacity Qmax (mg/g)
Reference
n-butylacrylate grafted chitosan Bamboo charcoal grafted by Cu2+ N-aminopropyIsilane complexes Magneticnanoparticle-multiwalled carbon nanotube composites Chitosan-cl-poly(alginiacid)nano hydrogel Glutaraldehyde crosslinked silica gel/chitosn-g-poly(butylacrylate) Sodium alginate-polyaniline (SAP) nanofibers mwXG-g-DEAEMA
17.14 17.93 19.65 26.49 55.71 73.34 58.8
(Kumar et al., 2014) (Wu et al., 2017) (Lu et al., 2017) (Sharma et al., 2017) (Nithya et al., 2016) (Karthik & Meenakshi, 2015) Present work
Appendix A. Supplementary data
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