Fabrication of high performance biodegradable Holarrhena antidysenterica fiber based adsorption devices

Jitender Dhiman; B.S. Kaith

doi:10.1016/j.arabjc.2020.10.004

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Original article

13 (

); 8734-8749

doi:

10.1016/j.arabjc.2020.10.004

Fabrication of high performance biodegradable Holarrhena antidysenterica fiber based adsorption devices

Jitender Dhiman^{^a,^b,⁎}, B.S. Kaith^{^b}

a Environmental Management Division, Center for Pulp and Paper Research Institute, Uttar Pradesh, India

b Chemistry Department, Dr B R Ambedkar NIT, Jalandhar, Punjab, India

⁎Corresponding author. jitenderdhiman81@gmail.com (Jitender Dhiman)

Received: 2020-8-5, Accepted: 2020-10-5, Published: 2020-12-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The current investigation highlights the synthesis of adsorption device MHa-g-poly(AN)-AE by graft copolymerization of acrylonitrile (AN) onto Holarrhena antidysenterica fiber in the presence of air along with Ferrous ammonium sulfate (FAS) and Potassium persulfate (KPS) initiators followed by quaternization process. Synthesized samples and backbone were studied using different techniques such as Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction spectroscopic (XRD) and TGA/DTA/DTG studies. High efficiency of dye adsorption (99% of malachite green dye) was achieved using an initial dye concentration of 10.0 mg L⁻¹ with an adsorbent dose of 500 mg 50 ml⁻¹ within the time duration of 165 min at neutral pH and 25 °C. Adsorption data best fit with Langmuir Isotherm, pseudo second-order kinetics model and follow both macro & micro-pore intra-particle diffusion.

Keywords

Anion exchanger

Quaternization

Adsorption rate and biomass

Show Related Articles from PubMed

MHa: Chemically modified Holarrhena antidycentrica fiber
MHa-g-poly(AN): Chemically modified Holarrhena antidycentrica graft poly(AN)
MHa-g-poly(AN)-AE: Chemically modified Holarrhena antidycentrica graft poly(AN) ion exchanger
MG: Malachite Green

1 Introduction

The current scenario of water contamination is the worst in human history due to the discharge of untreated effluent in all kinds of water bodies. Chief contaminants present in this discharged water are heavy metals, organic load and synthetic dyes, released from miscellaneous industrial sectors of the world which includes textile, printing, paper, fertilizers, leather, pesticide and pharmaceutical (Grassi et al., 2012; Idris et al., 2012). More than 100,000 dyes both organic and synthetic are used all over the world. Water pollution caused as a result of the enormous release of industrial effluents containing synthetic dyes produce a large number of environmental issues. Even minute traces of these synthetic dyes have carcinogenic, mutagenic and allergic impacts on aquatic ecosystem along with human beings (Badruddoza et al., 2013; Zhu et al., 2012; Camci-Unal and Pohl, 2010; Güçlü et al., 2003; Kiefer and Höll, 2001; Liu et al., 2010; Murshed et al., 2016 ). Miscellaneous techniques such as chemical oxidation, activated carbon adsorption, biological treatment membrane technology, photo-catalytic degradation and ion exchange have been used widely (De Gisi et al., 2016). Ion exchange is examined to be a cost-effective, environment-friendly and reliable process as it can expel dye molecules without any alteration in its molecular structure (AL-Othman et al., 2011; AlOthman et al., 2013; Cesur and Balkaya, 2007; Gupta and Suhas, 2009; MacHida et al., 2012; Zhang et al., 2011 ).

In the earlier decades, different materials such as activated charcoal, zeolites, agro waste, clays, biomass and natural polymers were reported to be effective in dyes and toxic metal ions removal. These materials have limited scope as they are less efficient. The expanding consideration leads to inspiration for the development of high-end adsorbents which are highly efficient in dye removal from wastewater. Fibers are macromolecules having a long chain-like structure with a huge number of active sites where stubborn functional groups can be attached using different chemical and radiation assisted methods (Cheng et al., 2011; Crini and Badot, 2008; Crini and Morcellet, 2002 ).

The selection of material for the synthesis of ion exchangers is a crucial factor. Synthetic fibers can be synthesized easily but the major problem with them is their non-biodegradability which poses a threat to our environment whereas natural fibers can be utilized to serve the same purpose without any environmental issues. In addition to this, natural fiber possesses various other qualities over synthetic fibers such as high selectivity, good physicochemical stability, more thermal stability and enhanced porosity. Natural backbones are obtained from renewable sources and therefore did not cause any harm to the environment. Organic polymers such as pectin, chitin, chitosan, cyclodextrin, polystyrene, starch, cellulose and polyacrylamide are widely utilized for the ion-exchangers synthesis. Natural polymers can be easily modified which enables the researchers to add the desired functional group on them and convert them in ion exchangers. Over time a large number of methods were developed by researchers for serving the purpose of polymer modification. Methods like radiation-induced modification, plasma treatment; graft copolymerization, gas-phase oxidation, ultrasonic method and wet chemical or electrochemical oxidation are commonly used for the same purpose. Graft copolymerization is a method that widely used for polymer modification which increases the density of active sites on the polymer that helps in adsorption of target metal ions and dyes (Bismarck et al., 1999; Cao et al., 2005; Denison et al., 1988; Huang et al., 2002; Kaith et al., 2009b; Kaith and Kalia, 2007; Fu et al., 1998; Montes-Morán et al., 2001; Ramanathan et al., 2001; Zhang et al., 2004 ).

Redox copolymerization has been used frequently for polymer grafting. In this mechanism, free radicals are generated by the oxidation of the substrate, which initiates the polymerization process. Oxidants such as peroxides, persulphates and permanganates form potential redox system in the presence of various reducing agents like alcohols, amines, amides, aldehydes, ketones and acids which helps in aqueous polymerization of vinyl monomers on the backbone (Cakmak et al., 1991; Yildiz et al., 2012; Arslan et al., 2001; Savaskan et al., 1996; Yıldız and Hazer, 2000 ).

Researchers revealed that cationic dyes possessing positive charge had great affection toward negatively charged sites of anion exchangers and thus are highly effective in the removal of cationic dyes such as malachite green from wastewater. This feature of anion exchanger is encouraged to use it as cationic dye removal from wastewater (Abdel-Halim, 2013; Oladipo et al., 2014).

H. antidysenterica fiber can be utilized for the synthesis of ion exchangers. It is a bio-polymer obtained from H. antidysenterica tree (Fig. 1). This tree is a small to medium-size and grows in the wild mountains of India. It is commonly known as Bitter Oleander, Connessi Bark, Kurchi Bark, Dysentery Rose Bay and Tellicherry Bark in India.

The main focus of this study is to synthesize an anion exchanger using H. antidysenterica fiber (MHa) and assess its dyes adsorption behavior in the aqueous medium. The goal has been achieved by graft copolymerization of AN (acrylonitrile) onto MHa and followed by quaternization using methyl iodide. Synthesized anion exchanger was finally evaluated for its dye adsorption capacity against malachite green as a model cationic dye.

2 Experimental

2.1

2.1 Material and methods

In Present research work Holarrhena antidysenterica (Ha) fiber was used as backbone which was collected from the hilly areas of district Kangra, Himachal Pradesh, India. Collected bio-polymer was purified using soxhlet extraction for 72 h in acetone. Acrylonitrile (AN, SD Fine Chemicals), ferrous ammonium sulfate (FAS, SD Fine Chemicals), dimethyl formamide (DMF, E-Merk Chemicals) and potassium persulfate (KPS, SD Fine Chemicals) of analytical grade were used as received without any further purification. Diethyl ether, lithium aluminum hydride, dimethyl sulfate, methyliodide and dioxane were procured from SD Fine Chemicals. Malachite green dyes were purchased from E-Merk Chemicals.

2.2

2.2 Chemical modification of Holarrhena antidysenterica fiber (MHa)

Two stage modification of H. antidysenterica fiber was carried-out in which first stage involves defatting process and second stage involves delignifcation process. Defatting process involves soxhlet extraction of H. antidysenterica fiber for 72 h in the presence of acetone solvent system. Defatting process eliminate the wax present on sample which cause interference in graft copolymerization (Mohanty et al., 2000). Delignifcation process was carried-out by treating defatted fiber with 40% sodium chlorite solution at 65 °C reaction temperature for 4 h. The pH of reaction mixture was maintained at 4.0 throughout the reaction by slow addition of acetic acid. Reaction mixture cooled down to room temperature which was followed by repeated distilled water washings. Washed fiber was treated with 2% sodium bisulfite solution and finally acid traces were removed from the fiber by 10–15 distilled water washings. Resulted fiber was dried at 45 °C in hot air oven to ensure the moisture content upto 5–10% (Kaith et al., 2015). Delignifcation of sample increases the number of active reaction sites by exposing the surface of cellulose fiber (Ilyas et al., 2017).

2.3

2.3 Graft copolymerization of acrylonitrile onto MHa

MHa fiber (0.5 g) was dipped in 100 ml of distilled water overnight which activates its reactive sites. Further the fiber was graft copolymerized using acrylonitrile alongwith optimized molar ratio of FAS-KPS which serve as initiator in reaction at definite temperature for specific period of time. Homo-polymer present on graft copolymer was separated using Soxhlet extraction in DMF solvent system. Resulted graft copolymer was dried at 50 °C in hot air oven until constant weight was obtained. Percentage graft yield (%GY) was calculated as per the Eq. (1) (Kaith et al., 2009b):

(1)

Percentage graft yield (% GY) = \frac{W_{f} {- W}_{i}}{W_{i}} \times 100

where W_i = initial wt. of sample; W_f = final wt. of sample (after removal of homopolymer).

2.4

2.4 Synthesis of anion exchangers

The nitrile group (—C≡N) of graft copolymer was reduced to primary amine (—CNH₂) group by using LiAlH₄. 3.8 g LiAlH₄ was added in 200 ml of ice cooled anhydrous ethyl ether. Resulted solution was mixed with 12.5 g of powdered graft copolymer MHa-g-poly(AN) taken in 20 ml of anhydrous ethyl ether. To the reaction mixture 4.0 ml of water and 3.0 ml of 20% NaOH was added followed by addition of 14.0 ml of water. The reduced graft copolymer obtained after filtration was converted to anion exchanger by quaternization process. 0.5 g of reduced graft copolymer was treated with 1.0 ml of methyl iodide and 1.0 g of KHCO₃ taken in 20 ml of methanol at room temperature for 24 h (Amundsen and Nelson, 1951; Chen and Benoiton, 2006). The sample was dried in hot air oven at 40 °C till the constant weight obtained.

2.5

2.5 Anion exchange capacity

Anion exchange capacity (IEC) of an anion exchanger is the number of exchangeable anionic groups (equivalents) per gram weight of dry exchanger. Anion exchange capacity of MHa-g-poly(AN)-AE was determined by back titration method. Cl⁻ ions were found to replace I⁻ ions present on the surface of anion exchanger on immersing in 0.01 mol L⁻¹ HCl solution for 24 h. The decrease in concentration of Cl⁻ ions from the HCl solution was equal to number of I⁻ ions released by anion exchanger and was equal to anion exchange capacity of MHa-g-poly(AN)-AE (Eq. (2)) (Vengatesan et al., 2015).

(2)

Anion exchange capacity (AEC) = \frac{{(V}_{HCl} \times C_{HCl} {) - (V}_{NaOH} \times C_{NaOH})}{W_{dry}}

where V_HCl = volume of HCl added, V_NaOH = volume of of NaOH consumed, C_NaOH = concentrations of NaOH, C_HCl = concentrations of HCl and W_dry = weight of dried exchanger.

2.6

2.6 Characterization

Fourier transform infrared (FTIR) spectra of backbone, graft copolymer and anion exchanger was recorded using Agilent Carry 630 FTIR spectrometer using KBr pellets. SEM-EDX analysis was carried out using FEI Quanta 200 microscope for morphological and elemental analysis of the samples. Thermal properties of synthesized samples were evaluated in temperature range of 50–700 °C at a heating rate of 10 °C min⁻¹ in air using TG/DTA 6300, SII EXSTAR 6000. X-ray diffraction studies of the samples were performed on instrument named Bruker D8, USA, under ambient conditions using Cu-Kα (1.5418 Å). % crystallinity was calculated using Eq. (3) (Kaith et al., 2009a):

(3)

% Cr = \frac{I_{22}}{I_{18} {+ I}_{22}} \times 100

where I₁₈ and I₂₂ are the crystalline and amorphous intensities at 2θ-scale close to 18° and 22°, respectively.

2.7

2.7 Dye removal studies

Dye adsorption studies of anion exchanger were carried-out using malachite green dye. Each experiment was carried-out using 50 ml of dye solution to study the effect of contact time, initial concentration and dose of anion exchanger. The initial concentration of dye was varied from 5 to 25 mg L⁻¹ using 250–1250 mg anion exchanger at pH 7.0 and 25 °C till the equilibrium was attained. Three replications of each experiment were performed. The concentration of dye at various stages was measured using UV spectrophotometer operated at λ_max 365 nm. The calibration curve was plotted in order to calibrate the instrument to find-out the concentration of unknown samples. Percentage of dye removal was calculated using the Eq. (4) (Fan et al., 2012):

(4)

% Dye removal = \frac{C_{o} {- C}_{eq}}{C_{o}} \times 100

where C_o and C_eq are the initial and equilibrium concentration of dye in mg L⁻¹.

2.8

2.8 Adsorption isotherm modeling

Different adsorption isotherms models such as Langmuir, Freundlich and Temkin were applied on experimental data to predict the adsorption of malachite green dye on the surface of adsorbent (anion exchanger). Experimental data at equilibrium position i.e, amount of dye absorbed (q_e) and adsorbent concentration (C_e) at constant pH and temperature were used to find optimum isotherm which best fit on given situation. Research data was described using linear forms of Langmuir, Freundlich and Temkin equations (Table 1) (Elmorsi, 2011; Ho et al., 2005).

Table 1 Different isotherm models.

Isotherm	Linear form	Plot
Langmuir	$\frac{1}{q_{e}} = [\frac{1}{K_{L} q_{m}}] \frac{1}{C_{e}} + [\frac{1}{q_{m}}]$	$\frac{1}{q_{e}} VS \frac{1}{C_{e}}$
Freundlich	$log q_{e} = log K_{F} + \frac{1}{n} log C_{e}$	$log q_{e} VS log C_{e}$
Temkin	$q_{e} = \frac{RT}{b_{T}} log K_{F} + \frac{RT}{b_{T}} log C_{e}$	$q_{e} VS ln C_{e}$

2.8.1

2.8.1 Langmuir isotherm

The Langmuir isotherm model based on the assumption that adsorbed material (in liquid phase, malachite green) adsorbed over the surface of adsorbent (solid phase, anion exchanger) in the form of a monolayer at a constant temperature. It also assumes that equilibrium control the distribution of the compound between the two phases. Therefore, at the equilibrium state adsorption and desorption rates are equal. The value of K_L and q_m is estimated using Langmuir equation (Eq. (5)) (Annadurai et al., 2002; Elmorsi, 2011; Zhao et al., 1989 ):

(5)

\frac{1}{q_{e}} = [\frac{1}{K_{L} q_{m}}] \frac{1}{C_{e}} + [\frac{1}{q_{m}}]

where q_m = maximum capacity of adsorption (mg g⁻¹), K_L = affinity constant (L mg⁻¹), q_e = dye adsorbed at equilibrium and C_e = dye concentration at equilibrium.

2.8.2

2.8.2 Freundlich isotherm

Basis of Freundlich isotherm is the assumption that adsorption of dye on adsorbent took place on a heterogeneous surface. The linear form of Freundlich is given as (Eq. (6)) (Annadurai et al., 2002; Zhao et al., 1989):

(6)

log q_{e} = log K_{F} + \frac{1}{n} log C_{e}

where K_F = adsorption capacity (L mg⁻¹) and

\frac{1}{n}

 = adsorption intensity.

2.8.3

2.8.3 Temkin isotherm

Adsorption potential of malachite green onto anion exchanger was also studied using temkin isotherm model. Temkin model took into account the effects of indirect adsorbate interactions on adsorption phenomena. The linear form of Temkin is given by following equation (Eq. (7)) (Elmorsi, 2011):

(7)

q_{e} = \frac{RT}{b_{T}} log K_{F} + \frac{RT}{b_{T}} log C_{e}

where R = 0.008314 kJ mol⁻¹K⁻¹, T = absolute temperature (K), 1/b_T = absorption potential of the adsorbent (KJ mol⁻¹) and K_T = adsorption capacity (L g⁻¹).

3 Result and discussion

3.1

3.1 Graft copolymerization

Fig. 2 depicted the mechanism for chemical modification of H. antidysenterica fiber (MHa), which was carried-out to remove impurities like hemi-cellulose and lignin from the surface of fiber. Further Fig. 2 represent the incorporation of vinyl monomer acrylonitrile onto MHa in the presence of ferrous ammonium sulfate (FAS) and potassium persulfate (KPS) initiator system was discussed in same (Das and Saikia, 2000; Kaur et al., 2010; Kumar et al., 2013; Lv et al., 2009; Thakur et al., 2013; Zahran and Hebeish, 1993 ).

Schematic mechanism of in-air graft copolymerization of AN onto MHa, further conversion of graft copolymer into anion exchanger.

Reaction parameters which effect the percentage grafting were optimized to obtain maximum graft yield. Parameters such as reaction time, reaction temperature, initiator ratio, monomer concentration, pH and amount of solvent were optimized (Al-Hoqbani et al., 2014; Behari et al., 2001; Liu et al., 2006; Singha and Rana, 2012; Tripathy et al., 2009; Witono et al., 2012 ). Maximum graft copolymerization (500.7%) was achieved with reaction time, 180 min; reaction temperature, 60 °C; initiator ratio, 0.5: 1.0; solvent volume, 40 ml; pH of medium, 7.0 and AN, 0.92 × 10⁻⁴ mol L⁻¹ (Fig. 3a-f).

Effect of different reaction parameters on percentage graft yield in case of MHa-g-poly(AN) (a) Reaction Time; (b) Reaction Temperature; (c) Amount of Solvent; (d) Initiator Ratio; (e) pH of Reaction Medium; (f) Monomer Concentration.

In the beginning, graft yield increases with increase in reaction time till its maximum value and thereafter a decrease in graft yield was observed with further increase in time interval. This observed increase in graft yield might be due to the increased interaction of primary free radicals with the monomer and backbone MHa. However, beyond the optimum reaction time the percentage graft yield started decreasing, this could be due to predominance of homopolymerization, thereby suppressing the graft copolymerization. Similarly, graft yield increases with increase in reaction temperature and become constant, after optimum value it starts decreasing. An increase in graft yield with the rise of temperature was due to the generation of OH* and SO₄²⁻* which led to generation of active sites on the backbone. Live poly(AN) chains approaches the active sites of backbone which results in graft copolymerization. Beyond optimum reaction temperature homopolymerization take over the graft copolymerization and thus graft yield decreased. Variation in pH of reaction medium from acidic to basic range results in premature termination of graft copolymerization and thereby decreased graft yield was observed. Change in concentration of initiators lead to increase in graft yield till optimum molar ratio of initiator beyond which a decrease was observed in percentage grafting. This was due to the reduction of Fe³⁺ ions to Fe²⁺, thereby resulting in termination of graft copolymerization, hence decreased graft yield (Kaith et al., 2013; Roy et al., 2009).

Graft yield was found to increase with increase in solvent volume and then attained maximum value, further addition of solvent in reaction mixture lowers the graft yield. The decrease in graft yield with increase in solvent volume was due to decline in number of free radicals per unit volume leading to decreased interaction between the active sites of backbone and live chains. Similar trend of grafting was observed with increase in concentration of monomer in reaction medium. The reason behind decrease in graft percentage beyond optimum monomer concentration was due to chain transfer reactions which resulted in premature termination of live chains (Chauhan et al., 2005; Kaur et al., 2013; Roy et al., 2009; Sharma et al., 2003; Shen et al., 2014; Singha et al., 2014; Wan et al., 2011 ).

3.2

3.2 Ion-exchange capacity

The maximum anion exchange capacity of synthesized ion exchanger was found to be 1.43 mmol g⁻¹.

3.3

3.3 Characterization

3.3.1

3.3.1 FTIR

FTIR spectrum of backbone showed a broad peak at 3319.4 cm⁻¹ due to the presence of free —OH groups, at 2928.9 cm⁻¹ and 1024.5 cm⁻¹ due to C—H and C—O stretchings, respectively (Fig. 4a). An additional peak at 2241.4 cm⁻¹ was observed in FTIR spectrum of MHa-g-poly(AN) showed due to the presence of C≡N stretching and thus confirmed the graft copolymerization of acrylonitrile onto MHa backbone (Fig. 4b). Also, intensity of peak of —OH functional group decreased due to involvement of these active sites with poly(AN) live chains in graft copolymerization process (Choi and Nho, 2000). In the FTIR spectrum of MHa-g-poly(AN)-AE another additional peaks appeared at 1486.1 cm⁻¹ and 1386.5 cm⁻¹ which support the quaternization of amino group present on reduced graft copolymer (Fig. 4c). Peak present at 2241.4 cm⁻¹ due to C≡N stretching disappeared from spectrum which resulted in conclusion that amino groups of reduced graft copolymer were converted into quaternized functional groups (Liu et al., 2015).

FTIR spectra of (a) MHa; (b) MHa-g-poly(AN); (c) MHa-g-poly(AN)-AE.

3.3.2

3.3.2 SEM and energy dispersive X-ray studies

Careful observation of SEM images recorded for MHa revealed that fiber possess continuous morphology with homogeneous surface (Fig. 5a). SEM images of MHa-g-poly(AN) showed that surface of grafted fiber became heterogeneous and irregular which confirm the grafting of poly(AN) onto MHa (Fig. 5b). Increase in roughness and morphology was further observed when took in account the SEM images of MHa-g-poly(AN)-AE which evidently indicated the quaternization of reduced graft copolymer (Fig. 5c).

SEM images of (a) MHa; (b) MHa-g-poly(AN); (c) MHa-g-poly(AN)-AE.

In case of EDX studies data showed that MHa possessed 61.54% (54.57%) and 38.46% (45.43%) of carbon and oxygen in atomic % (weight %), respectively which confirmed the ligno-cellulosic nature of the backbone (Fig. 6a). Also, EDX image of MHa-g-poly(AN) (Fig. 6b) showed the presence of nitrogen in atomic % (weight %) of 11.02% (11.32%) alongwith carbon with atomic % (weight %) of 53.82% (47.42%) and oxygen with atomic % (weight %) of 35.15% (41.26%). Thus, the presence of nitrogen and decrease in carbon content clearly indicated the incorporation of poly(AN) chains onto active sites of MHa backbone (Kaur et al., 2013). EDX image of anion exchanger confirmed the presence of iodine (0.16%) alongwith nitrogen (3.10%), oxygen (62.97%) and carbon (33.78%). The weight % for iodine, nitrogen, oxygen and carbon was 1.34, 2.94, 68.24 and 27.48, respectively. Results clearly showed the presence of iodine in the sample alongwith change in the atomic % of nitrogen, oxygen and carbon in MHa-g-poly(AN)-AE and therefore confirmed the quaternization of reduced MHa-g-poly(AN) (Fig. 6c).

EDX images of (a) MHa; (b) MHa-g-poly(AN); (c) MHa-g-poly(AN)-AE.

3.3.3

3.3.3 X-ray diffraction

XRD pattern of MHa backbone showed sharp peak at 22° on 2θ scale (Fig. 7a). Percentage crystallinity of MHa has been found to be 50.1%. MHa-g-poly(AN) XRD pattern showed two peaks at 17° and 22.5° on 2θ-scale. The percentage crystallinity was found to be 35.5% (Fig. 7b). Decrease in crystallinity has been found on the graft copolymerization with acrylonitrile. This decrease in crystallinity was due to poor order of orientation of crystallites with respect to central axis, after incorporation of poly(AN) chains onto backbone (Kaith et al., 2009a). The XRD studies of anion exchanger MHa-g-poly(AN)-AE showed five peaks. Three major peaks were found at 17.4°, 23.3° and 29.7°, whereas, two minor peaks were observed at 36.8° and 63.6° (Fig. 7c). An increase in % crystallinity from 35.5% to 45.1% was found on quaternization of reduced graft copolymer. This could be explained on the basis of formation of some large-size crystallites in the amorphous region of the reduced graft copolymer due to electrostatic interaction between different polymeric chains, resulting in alignment with the axis of the polymer and attributing to increase in overall percentage crystallinity of the anion-exchanger (Rashid et al., 2014).

XRD of (a) MHa (b) MHa-g-poly(AN); (c) MHa-g-poly(AN)-AE.

3.3.4

3.3.4 Thermal studies

Thermogravimetric analysis (TGA) of MHa (Fig. 8a) was carried-out as a function of percentage weight loss with respect to temperature. Dehydration, glycogen formation and depolymerization are the processes which take place during the degradation.

TGA/DTA/DTG of (a) MHa; (b) MHa-g-poly(AN); (c) MHa-g-poly(AN)-AE.

MHa was found to exhibits two stage decomposition. First stage was observed in the temperature range between 227.9 and 339.7 °C with 58.2% weight loss and the second stage from 339.7 − 495.9 °C with 30.6% weight loss. Initial weight loss was due to dehydration, loss of volatile matters and depolymerization processes. MHa was found to show 0.287 mg min⁻¹ and 0.100 mg min⁻¹ rate of weight loss at 300.4 °C and 445.6 °C, respectively. DTA analysis showed exothermic reactions which took place during thermal degradation at 310.4 °C and 445.6 °C with the loss of 25.8 μV and 20.6 μV energy, respectively.

TGA analysis of MHa-g-poly(AN) showed one stage degradation in temperature range of 297.6–609.8 °C with 82.9% weight loss (Fig. 8b). The IDT and FDT were found to be 297.6 °C and 609.8 °C which were higher than that of MHa backbone. Thus, the graft copolymerization of acrylonitrile onto MHa fiber could increase the thermal stability. DTG studies showed that in case of MHa-g-poly(AN) the rate of weight loss was 0.153 mg min⁻¹ and 0.654 mg min⁻¹ at 344.1 °C and 534.1 °C, respectively. On the other hand, DTA showed two exothermic peaks at 258.1 °C and 535.6 °C with 2.4 µV and 7.9 µV energy loss, respectively during thermal degradation process. Thus, the graft copolymers MHa-g-poly(AN) was found to possess higher final decomposition temperatures and DTA studies also showed decomposition peaks at higher temperature than that of MHa. Therefore, graft copolymers MHa-g-poly(AN) was found more stable than MHa backbone.

Quaternized anion exchanger MHa-g-poly(AN)-AE showed two stage thermal degradation (Fig. 8c). First stage degradation was observed in the temperature range of 159.0–487.2 °C with 34.1% weight loss. While second stage degradation included 16.6% weight loss in the range of 487.2–715.8 °C with 40.2% residue left. Final decomposition temperature (FDT) of MHa-g-poly(AN)-AE was higher than that of graft copolymer and backbone. DTG studies revealed that rate of weight loss of MHa-g-poly(AN)-AE was 0.583 mg min⁻¹, 0.274 mg min⁻¹ and 0.239 mg min⁻¹ at 174.5 °C, 270.1 °C and 530.2 °C, respectively. It was observed that at lower temperature the rate of weight loss was higher. In DTA studies both endothermic and exothermic processes were observed at 175.9 °C and 494.3 °C, with −12.3 µV and 49.2 µV values, respectively. Thus, TGA, DTG and DTA studies showed that the quaternization of reduced graft copolymer enhanced the thermal stability (Khan and Baig, 2014).

3.4

3.4 Dye removal studies

Malachite green (MG) dye was used as model dye to study the adsorption behavior of MHa-g-poly(AN)-AE. Anion exchanger was observed to be effective in the removal of 99% malachite green from aqueous system. Reason behind removal of dye could be the electrostatic attraction between –N⁺(CH₃)₃I⁻ group of MHa-g-poly(AN)-N⁺(CH₃)₃I⁻ and = N⁺(CH₃)₂ groups of malachite green dye (Fig. 9a). The adsorption of MG could also be due to physical adsorption on the surface of anion exchanger and electrostatic attraction between negatively charged iodine atom present on quaternized anion exchanger and positively charged nitrogen atom of MG (Fig. 9b). Mechanism depicting dye adsorption by anion exchanger is shown in Fig. 9 (Naushad et al., 2016; Reddy and Lee, 2013).

(a) Electrostatic interaction and (b) dye adsorption reaction between quaternized anion exchanger and malachite green.

3.4.1

3.4.1 Effects of different parameters on dye removal

3.4.1.1

3.4.1.1 Effect of initial concentration of dye

Percentage dye adsorption by MHa-g-poly(AN)-AE was studied as a function of initial dye concentration by keeping all other parameters like contact time and dose concentration constant. The impact of initial dye concentration was studied using dye solution of 5.0 mg L⁻¹, 10.0 mg L⁻¹, 15.0 mg L⁻¹, 20.0 mg L⁻¹ and 25.0 mg L⁻¹ with fixed amount of anion-exchanger (500 mg) and contact time 210 min. Initially with increase in concentration from 5.0 mg L⁻¹ to 10.0 mg L⁻¹ of malachite green there was an increase in percentage dye adsorption from 98% to 99%. Further increase in dye concentration from 10.0 mg L⁻¹ to 25.0 mg L⁻¹ resulted in decreased dye adsorption (Fig. 10a). Initially with increase in concentration of malachite green the rate of dye adsorption was found to increase due to increase in interaction between dye and adsorbent. However, further increase in dye concentration resulted in decreased available active sites of adsorbent for dye molecules and became the limiting factor. Therefore, at low concentration molecules of dye have more chances to interact with adsorbent active sites which enhanced the initial rate of MG dye adsorption (Greluk and Hubicki, 2013a; Miranda et al., 2014).

Effect of (a) contact time and initial concentration of dye; (b) dose concentration of MHa-g-poly(AN)-AE on % dye removal.

3.4.1.2

3.4.1.2 Effect of contact time in dye removal

Effectiveness of MHa-g-poly(AN)-AE in dye adsorption was studied as a function of contact time. Dye adsorption was maximum at initial stage and rate of adsorption increases with time till optimum value was reached. Dye adsorption took place in three phases (Fig. 10a). In First phase, sharp slope was observed in adsorption curve from 0 to 75 min. Second phase included a slow increase in percentage dye removal and became approximately constant in third phase. During initial stage the percentage dye uptake was largely due to the presence of large number of active sites on the surface of adsorbent. In second phase of adsorption a decreased adsorption rate was due to reduced number of active sites for adsorption of dye molecules. This phase of slow adsorption can also be attributed to intra-particle diffusion which is a slow process. Whereas, constant slope of dye adsorption curve in final phase indicated that no further dye adsorption was taking place which was due to the saturation of active sites present on the surface of anion exchanger. Equilibrium attained with different concentration of dye 5.0 mg L⁻¹, 10.0 mg L⁻¹, 15.0 mg L⁻¹, 20.0 mg L⁻¹ and 25.0 mg L⁻¹ was at 135 min, 165 min, 180 min, 195 min and 195 min, respectively. Maximum percentage of dye removal was at the time interval of 165 min with initial dye concentration of 10 mg L⁻¹ (Greluk and Hubicki, 2013b, 2011a).

3.4.1.3

3.4.1.3 Effect of adsorbent dose

The impact of adsorbent dose on percentage dye removal was studied by varying the amount of adsorbent from 250 mg 50 ml⁻¹ to 1250 mg 50 ml⁻¹ while keeping other parameters like initial dye concentration and contact time constant (Fig. 10b). Dye uptake capacity of anion exchanger was observed to increase with the increase in adsorbent dose from 250 mg 50 ml⁻¹ to 500 mg 50 ml⁻¹. Further increase in amount of adsorbent beyond optimal value (500 mg 50 ml⁻¹) resulted in decreased rate of dye removal. In the initial stage the rate of dye adsorption increased from 98% to 99% with increase in adsorbent dose from 250 mg 50 ml⁻¹ to 500 mg 50 ml⁻¹ which was due to the availability of active sites. Afterward decreased rate of adsorption is attributed to aggregation of adsorption sites which decreased the surface area of adsorbent available to dye adsorption and increased the diffusion path length. As a result an increase in anion exchanger dose beyond optimal value gave rise to slow dye uptake. Moreover, time required to reach equilibrium also increased with increase in adsorbent dose (Greluk and Hubicki, 2011b; Shan et al., 2014).

3.5

3.5 Isotherms for the dye adsorption

3.5.1

3.5.1 Langmuir isotherms

Fig. 11a showed linear fit of Langmuir isotherm for adsorption of malachite green onto MHa-g-poly(AN)-AE. Table 2 showed the calculated values of q_m, K_L and R² using Langmuir isotherm. Calculated values using Langmuir isotherm showed that minimal deviation from the fitted equation i.e. R² was 0.983 (Table 2) which is quite high. For the value of R² greater than 0.89, it was proposed that adsorption data followed the Langmuir isotherm. Also, the value of q_m which is the measurement of adsorption upto maximum capacity was found to be 0.71 (Journal et al., 2009; Miah et al., 2010).

Isotherm model (a) Langmuir; (b) Freudich; (c) Temkin.

Table 2 Langmuir; Freundlich and Temkin constants for the adsorption process.

Langmuir Constants			Freundlich Constants			Temkin Constants
q_m (mg g⁻¹)	K_L (L mg⁻¹)	R²	K_F (L mg⁻¹)	n	R²	b_T (KJ mol⁻¹)	K_T (L mg⁻¹)	R²
0.71	0.024	0.983	2.062	0.68	0.964	4.701	0.477	0.864

3.5.1.1

3.5.1.1 Separation factor

Separation factor (R_L) is a dimensionless constant and is a characteristic of Langmuir isotherm. It can be expressed using Eq. (8):

(8)

R_{L} = \frac{1}{1 + K_{L} C_{O}}

where C_o = highest initial concentration of malachite green (mg L⁻¹) and K_L = Langmuir constant. The significance of R_L is that it represents the shape of isotherm which may be linear (R_L = 1), favorable (0 < R_L < 1), unfavorable (R_L greater than 1), or irreversible (R_L = 0). The calculated value of R_L for present study lies in between the range 0 < R_L < 1(Table 3) which means the adsorption of malachite green on anion exchanger is a favorable process and present data fits Langmuir isotherm model. Therefore, it was concluded that MHa-g-poly(AN)-AE is a good absorber of MG dye.

Table 3 Separation factor (R_L) value at different dye concentration.

Conc of dye	5	10	15	20	25
R_L	0.89	0.81	0.74	0.67	0.63

3.5.2

3.5.2 Freundlich isotherm

Adsorption of MG onto MHa-g-poly(AN)-AE also studied using Freundlich Isotherm model (Fig. 11b). Using this model value of K_L and value of 1/n was calculated alongwith the value of R² (Table 2).The value of R² (0.9647) in this case is lower than the value of R² (0.983) of Langmuir isotherm. Thus, Freundlich Isotherm model is less appropriate to describe the dye adsorption process onto MHa-g-poly(AN)-AE as compare to Langmuir isotherm (Annadurai et al., 1999; Zhao et al., 1989).

3.5.3

3.5.3 Temkin isotherm

The linear plot of Temkin isotherm is shown in Fig. 11c. Using the linear plot value of K_T and b_T was calculated for adsorption of MG onto MHa-g-poly(AN)-AE with the help of Temkin equation. Lower value of adsorption capacity K_T (-0.477 L/g) and R² (0.864) (Table 2) indicated that Temkin isotherm poorly describe the adsorption of MG on anion exchanger.

Since the R² value was highest in case of Langmuir Isotherm, thus adsorption data best fit for Langmuir Isotherm followed by Freundlich Isotherm and least fit with Temkin isotherm.

3.6

3.6 Adsorption kinetics

Kinetic studies of malachite green dye adsorption onto anion exchanger was studied using different kinetic models such as pseudo-first-order (Annadurai et al., 2002), the pseudo-second order (Hameed et al., 2007; Ho and Mckay, 1999) and an intra-particle diffusion (Allen et al., 1989). The experimental data was fitted into different models of kinetics to interpret the results.

3.6.1

3.6.1 Pseudo first-order equation

The rate constant of malachite green adsorption was determined using pseudo first order equation, which could be expressed as Eq. (9):

(9)

Log (q_{e} - q_{t}) = L o g q_{e} - \frac{K_{1} t}{2.303}

where q_e = amount of MG adsorbed at equilibrium (mg L⁻¹), q_t = amount of MG adsorbed at time t, k₁ = Adsorption rate constant (min⁻¹)

Values of k₁ and q_e were calculated from the graph plotted between log(q_e-q_t) and t. k₁ evaluated from slope whereas q_e calculated using intercept of the graph (Fig. 12a). Results calculated using Eq. (9) showed that value of R² was low and the experimental value of q_e did not well agreed with calculated value (Table 4). This indicated that adsorption of MG onto anion exchanger poorly agree with first order kinetics.

(a) Pseudo-first order kinetics; (b) Pseudo-second order kinetics; (c) intra- particlediffusion plot.

Table 4 kinetic parameters for Malachite green adsorption onto MHa-g-poly(AN)-AE.

First order kinetic model						Second order kinetic model
C_O	q_e (exp)	q_e (cal)	K₁	R²	SSE	q_e (cal)	K₂	R²	SSE
5 mg L⁻¹	0.49	0.31	0.028	0.98	0.020	0.33	0.0343	0.96	0.0014
10 mg L⁻¹	0.99	0.74	0.030	0.95	0.008	0.91	0.0080	0.93	0.0005
15 mg L⁻¹	1.48	1.30	0.028	0.62	0.032	1.12	0.0031	0.99	0.0023
20 mg L⁻¹	1.97	1.98	0.029	0.72	0.008	1.87	0.0015	0.99	0.0006
25 mg L⁻¹	2.46	4.00	0.029	0.07	0.013	2.30	0.0003	0.99	0.0009

3.6.2

3.6.2 Pseudo second-order equation

Pseudo second-order equation can be expressed as following (Eq. (10)):

(10)

\frac{t}{q^{t}} = \frac{1}{K_{2} q_{e}^{2}} + [\frac{t}{q_{e}}]

where k₂ = second order adsorption rate constant q_e and K₂ can be calculated from the slope and intercept of the graph between t/q_t and t respectively. Close analysis of Fig. 12b indicated that linear plots for all different initial concentartion studies of MG adsorption accuires high values of R² (Table 4). There was found a very good agreement between claculated and experimental values of q_e. Thus, the adsorption process of MG onto synthesized anion exchanger could be represented by pseudo second order kinetics (Allen et al., 1989).

Validity of different kinetic models i.e. pseudo-first order and pseudo-second order models for the absorption of malachite green dye onto MHa-g-poly(AN)-AE were verified at different initial concentration dye using the sum of squared error (SSE, percentage). SSE can be evaluated using Eq. (12):

(12)

SSE = \frac{\sqrt{{(q_{e, exp} - q_{e, c a l})}^{2}}}{N}

where N = number of data points used in plot for each model

Model validity was found out by comparison between values of SSE%, lower the value better the plot fit. Values of SSE for two models were shown in Table 4. Analysis of table revealed that pseudo first order kinetics yielded high SSE value (0.0020–0.0013) as compare to pseudo second order kinetics (0.0014–0.0009). Since, pseudo first order kinetics possessed high value of SSE, this indicated that MG adsorption onto MHa-g-poly(AN)-AE did not followed pseudo first order kinetics which further support earlier prediction of adsorption model based on R² and q_e,cal values (Hameed et al., 2007).

3.6.3

3.6.3 Intra-particle diffusion study

Intraparticle diffusion based mechanism was used to study the adsorption process of maclachite green onto anion exchanger. Adosrption process of malachite green dye was studied using Eq. (11).

(11)

q_{t} = K_{id} t^{\frac{1}{2}}

where q_t = amount of dye adsorbed at time t, K_id = the intra-particle diffusion rate constant (mg g⁻¹min^−3/2). The rate parameter K_id of stage i is obtained from the slope of the straight line of graph between q_t and t^1/2.

If the adsorption followed Intra-particle diffusion mechanism then the graph between q_t and t^1/2 should be straight line passing through origin. On the other hand if graph is non-linear then some another mechanism was also involved during dye adsorption process alongwith above mentioned mechanism. Fig. 12c. showed that graph between q_t and t^1/2 was non-linear which pointed that more than one mechanism was followed during MG adsorption. Further, it was observed that intra-particle diffusion of MG occurred in two phases. First portion of graph is gradually increasing which indicated the macro-pore diffusion, whereas, second portion of graph is linear which attributed to micro-pore diffusion (Fig. 12c) (Jadhav and Vanjara, 2004). Values of Intra-particle diffusion constant are shown in Table 5. These results supported the fact that dye adsorption followed more than one process and intra-particle transport is not rate-limiting step. Further the rate of MG diffusion was slow which increased as a result of increase in the concentration of dye. This means MG dye molecules are bulky and diffuse slowly on adsorbent (Jadhav and Vanjara, 2004).

Table 5 Weber-Morris parameter.

C_o (mg L⁻¹)	K_id (mg g^−1/2 min^−3/2)	R²
5 mg L⁻¹	0.0328	0.88
10 mg L⁻¹	0.0764	0.89
15 mg L⁻¹	0.1133	0.83
20 mg L⁻¹	0.1709	0.89
25 mg L⁻¹	0.2097	0.88

4 Conclusions

Anion exchanger MHa-g-poly(AN)-AE was successfully synthesized, which was confirmed by various characterization techniques such as FTIR, SEM-EDX and XRD. TGA/DTA/DTG studies indicated that thermal stability of anion exchanger was enhanced after quaternization of reduced copolymer. Dye removal studies showed that MHa-g-poly(AN)-AE is capable of removing 99% of malachite green dye from waste water. Adsorption data best fit with Langmuir Isotherm, since, the R² value was highest in its case, followed by Freundlich Isotherm and least fit with Temkin isotherm. Further, adsorption data follow pseudo second-order kinetics as plot for second-order kinetics give highest R² value and minimum difference between estimated and calculated values of maximum adsorption capacity (q_m). Intra-particle diffusion studies showed that adsorption process followed more than one mechanism. Studies indicated that adsorption process followed both macro-pore diffusion and micro-pore diffusion. In last the validity of kinetic models was studied which revealed that experimental data properly fit with pseudo second order kinetics model as it acquired minimum SSE value.

Acknowledgements

Authors are highly grateful to DST-FIST New Delhi for providing financial assistance in procuring the FTIR and UV-Vis spectrophotometer which were used for the analysis of the samples. Authors also thankful to TEQUIP-II for providing financial assistance for present research work.

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Fabrication of high performance biodegradable Holarrhena antidysenterica fiber based adsorption devices

Abstract

Keywords

Anion exchanger

Quaternization

Adsorption rate and biomass

Abbreviations

1 Introduction

2 Experimental

2.1 Material and methods

2.2 Chemical modification of Holarrhena antidysenterica fiber (MHa)

2.3 Graft copolymerization of acrylonitrile onto MHa

2.4 Synthesis of anion exchangers

2.5 Anion exchange capacity

2.6 Characterization

2.7 Dye removal studies

2.8 Adsorption isotherm modeling

2.8.1 Langmuir isotherm

2.8.2 Freundlich isotherm

2.8.3 Temkin isotherm

3 Result and discussion

3.1 Graft copolymerization

3.2 Ion-exchange capacity

3.3 Characterization

3.3.1 FTIR

3.3.2 SEM and energy dispersive X-ray studies

3.3.3 X-ray diffraction

3.3.4 Thermal studies

3.4 Dye removal studies

3.4.1 Effects of different parameters on dye removal

3.4.1.1 Effect of initial concentration of dye

3.4.1.2 Effect of contact time in dye removal

3.4.1.3 Effect of adsorbent dose

3.5 Isotherms for the dye adsorption

3.5.1 Langmuir isotherms

3.5.1.1 Separation factor

3.5.2 Freundlich isotherm

3.5.3 Temkin isotherm

3.6 Adsorption kinetics

3.6.1 Pseudo first-order equation

3.6.2 Pseudo second-order equation

3.6.3 Intra-particle diffusion study

4 Conclusions

Acknowledgements

References

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