Adsorptive removal of acid yellow 17 (an anionic dye) from water by novel ionene chloride modified electrospun silica nanofibres

Accepted Manuscript Title: Adsorptive removal of Acid Yellow 17 (an anionic dye) from water by novel ionene chloride modified Electrospun Silica Nanofibres Authors: M.D. Teli, Gayatri T. Nadathur PII: DOI: Reference:

S2213-3437(18)30613-4 https://doi.org/10.1016/j.jece.2018.10.005 JECE 2690

To appear in: Received date: Revised date: Accepted date:

6-7-2018 25-9-2018 2-10-2018

Please cite this article as: Teli MD, Nadathur GT, Adsorptive removal of Acid Yellow 17 (an anionic dye) from water by novel ionene chloride modified Electrospun Silica Nanofibres, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.10.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Adsorptive removal of Acid Yellow 17 (an anionic dye) from water by novel ionene chloride modified Electrospun Silica Nanofibres

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M.D. Teli1* and Gayatri T. Nadathur1

Department of Fibres and Textile Processing Technology, Institute of Chemical Technology, N. P. Marg, Matunga (E), Mumbai, Maharashtra, 400019, India

Email: [email protected]

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*Corresponding

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Graphical Abstract

Highlights

Enhanced adsorption of Acid Yellow 17 in water on modified Electrospun Silica nanofibres (NF) Modified by etching Electrospun Silica NF, surface protected by ionene chloride polyelectrolyte Dye adsorption follows non-linear pseudo-second-order kinetics, and Freundlich isotherm Exothermic adsorption on modified silica NF but endothermic process on silica NF Modified Silica NF remains mostly efficient in dye removal even after 5 regeneration cycles

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    

ABSTRACT

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Sorbents for environmental remediation, based on Advanced Materials, have attracted research

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interest because of their better selectivity, reusability and resolution of disposal problems at end-

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of-life, relative to conventional activated carbon based sorbents. The adsorption behavior of non-

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calcinated, electrospun Silica nanofibres, with no additional co-electrospun fibre-forming polymer present, has not been much reported. This work describes the preparation and performance of a

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recyclable nanofibrous silica adsorbent, to effectively capture a highly water soluble anionic dye,

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Acid yellow 17(AY17), from water. The physical structure and chemical composition of the

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electrospun silica nanofibres and its modified form were elucidated by their infrared and X-ray Diffraction spectra, Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM),

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and nitrogen adsorption-desorption analysis. The adsorptive removal efficiency, of the modified and as-is silica nanofibres, against AY17 was 100% and 54.6%, respectively. Improved adsorption

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is achieved through micropore formation, and introduction of positively charged moieties on alkali etching of the silica nanofibres, after surface protection with the prepared ionene chloride polyelectrolyte. The nature of equilibrium adsorption data was more favorably described by nonlinear form of the Freundlich isotherm, and the non-linear pseudo-second-order kinetic model. The

adsorption of AY17 onto the modified silica nanofibres was an exothermic process, with larger decrease in free energy compared to the endothermic process in silica nanofibre. Keywords: silica nanofibre; acid yellow 17 anionic dye; ionene chloride; surface protected

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etching; dye adsorption; recycling 1 Introduction

The conservation and maintenance of available water resources, has gained urgency, in the face of significant threats to water security from climate change, pollution, and expanding population. According to World Bank estimates, textile dyeing and treatment is responsible for 17-20% of

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industrial water pollution [1]. The textile industry is culpable for being a voracious consumer of

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clean water for its wet processing, dyeing, finishing processes and simultaneously releasing highly

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polluted effluent contaminating ground water and running streams with dyes, organic chemicals,

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and salts. In the absence of 100% exhaustion of a dye onto fibre, dye effluents are generated from the dye bath. Dyes are designed to be sufficiently stable compounds that are not easily degraded.

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These dyes are not easily removed when subjected to municipal biological wastewater treatment

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procedures, and cause foaming, and colour persistence in sewage treatment plants [2]. Dye

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concentrations as low as 1.56 mg L-1 were detected in polluted water, while even 0.005 mg L-1 was visually observed in water runoffs [3]. Effluent dye molecules, absorbing sunlight, shield aquatic

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plant life from light essential for photosynthesis. This results in depletion of dissolved oxygen, increasing the organic load in water, making conditions unfavorable for the survival of piscine

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species and almost all aquatic life forms. Water treatment methods have become unavoidable, in light of increasingly stringent legislation and taxation on water pollution. They involve adsorption, coagulation-flocculation, filtration, and numerous catalyst promoted oxidation and reduction pathways, including electrochemical means [4]. Degradative methods of colour removal create

issues of removal of toxic by-products, and incur greater expenditure of energy and material. Adsorption methods which allow reuse and recycle of the dyestuff and adsorbent would be ideal. Being highly water soluble due to the sulfonate functional groups, anionic dye molecules have

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long residence times in water. Their azo bonds are not readily cleaved by water, and are resistant to biological treatment, with low adsorptivity on biomass, [5]. Acid Yellow 17 (AY17), a bisulfonated dichloro anionic dye, has a water solubility of 70 g L-1, and a pKa of 5.5 [6]. Sequential aerobic-anaerobic treatment of dye effluent does not remove AY17 [7, 8]. Aerobically non-degradable dyes are more toxic than aerobically biodegradable dyes [9]. AY17, having an

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LC50 of 180 mg L-1, describing respiratory toxicity in fish over 48 hours exposure, lies in the

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minimal toxicity range [10], though presence of chlorine substituents increases inhibition to algae

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and aquatic microbes. Data for low-level chronic exposure to commercial azo dyes are insufficient,

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but occupational and recurrent exposure, cause dermatitis and cancer of liver, blood and kidneys in humans [11, 12]. Reductive cleavage of azo dyes gives aromatic amines, which if they remain

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sulfonated have lower genotoxicity than non-sulfonated aromatic amines. Further metabolic

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activation of products from dye biodegradation, generates harmful superoxide free radical anions,

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besides their own genotoxicity [13, 14]. The increase in volume of the global trading market for acid dyes, necessitates the development of

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newer adsorbents for removal of anionic acid dyes. Unlike cationic dyes, anionic dyes, have relatively low adsorption capacity on commonly used adsorbents. Commercial activated carbon,

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though popularly used, is ineffective against disperse and vat dyes, is costlier with increased performance quality, and its source materials are also cost inefficient. The regeneration and reuse of carbon adsorbents is poor and cost intensive, and disposal of adsorbed carbon mass, usually by incineration, generates further pollution [15]. Inorganic sorbents based on abundant, cheap natural

sources of silica, have gained attention for decontamination capability, based on their porosity, high surface area, mechanical strength and low chemical cross-reactivity, as the acidic silanol functionality, is alone responsible for any reactivity. Silica, however, is susceptible to dissolution

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at higher pH. Nanofibrous materials suit sorbent applications in biotechnology and environmental remediation due to their characteristic high surface area to volume ratio. This feature drastically reduces the adsorbent dosage required for performance equivalent to conventional macro-structured sorbents, at similar initial dye or pollutant concentrations. Nanofibres have better ease of handling and

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stability in water, compared to other nanostructures such as nanoparticles. Nanosorbents score over

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conventional adsorbents, on considering material economy of fabrication, dosage used and

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regeneration ability [16]. Acid dye sorption studies on polyamide 6 nanofibrous membranes

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(NFM) reported sorption capacities of 20 – 44 mg g-1, for different acid dyes, decreasing with increasing molecular weight of dye molecule and structure complexity [17]. Micro-nano structured

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poly (ether sulfones)/poly (ethyleneimine) (PES/PEI) NFM, with crosslinking treatments post

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electrospinning, are reported to adsorb anionic dyes at very high adsorption capacities, though

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regeneration and reuse was not favored [18]. PVA/PEI NFM with Polydopamine/PEI functional coating showed selective adsorption at high capacity for anionic over cationic dyes [19]. There

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exist numerous reports of silylated silica as gel and as nanoparticle, when blended with polyvinyl alcohol, polyacrylonitrile etc., functioning as adsorbents [20, 21, 22].

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Water soluble polyelectrolytes, which have good flocculating and anti-fouling properties, have exhibited high efficiency in dye removal systems [23, 24]. In the present research, a non-aromatic ionene chloride polyelectrolyte is employed, to modify electrospun silica nanofibres (Fig. 1), by an aqueous reaction without any organic solvents, favoring the green chemistry requirement. To

the broadest extent of our knowledge, this is an original report of adsorption of anionic dye in aqueous solution on non-calcined electrospun silica nanofibre, and its non-silylated modification. 2 Materials and Methods

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2.1 Materials C.I.Acid Yellow 17, C.I.18965, CAS 6359-98-4, molecular weight 551.29 g/mol, C16H10Cl2N4Na2O7S2, sold under the commercial product name Sanolin Yellow E-2GL from Archroma (formerly Clariant), India, a specialty chemicals company, (λmax 400 nm), was used without further purification. TEOS (Tetraethylorthosilicate) 98% (Alfa aesar), Epichlorohydrin,

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for synthesis (Pallav chemicals, India), N, N, N’, N’-tetramethylethylenediamine, synthetic grade

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(Loba-chemie, India), Triethanolamine, synthetic grade (S.D. finechem ltd, India), conc. HCl

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35.4% AR, NaOH, NaCl, NaNO3, Na2SO4, (A. R. grade pellets and powders) absolute Ethanol,

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Methylene dichloride AR, Methanol AR, (S.D. finechem ltd, India) solvents were used. Dye solutions were made using deionized water.

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2.2 Specifications of the Electrospinning apparatus used

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ESPIN-NANO from Physics equipments co., Chennai. Model v1, was used, which has 0-40 kV

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power supply voltage applicable to electrospinning, with allowance for Multiple syringes, and a Plate collector 200 x 150 mm x mm, with 1 mA power supply current and input Power voltage

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220 V AC 50 Hz, 200 W.

2.2.1 Electrospinning of silica nanofibres

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Firstly, the silica-sol was prepared. The silica-sol was obtained from TEOS: ethanol: water: HCl blended in a molar ratio of 1:2:2:0.01[25]. After mixing of TEOS and ethanol, the water/HCl solution was added dropwise with vigorous stirring, at room temperature. After refluxing the solution over a period of 30 min at 80 °C the sol was allowed to cool to room temperature, with

continued stirring for 8 hours, and finally weighed. The ethanol solvent was evaporated by gently heating at temperature below 50°C, to obtain a controlled weight reduction of around 58 % [26]. This viscous solution was filled into a 5-ml plastic syringe fitted with a 24-gauge stainless steel

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needle, whose pointed tip was earlier cut. The syringe was positioned on ESPIN-NANO, the electrospinning apparatus, flow and voltage were set and then the viscous fluid was electrospun. The nanofibres were collected on a plate collector covered with a detachable aluminum foil; the applied voltage was 30 kV, flow rate of the syringe was set at 0.5 ml/h and the distance from the needle tip to collector surface was set at 15 cm. The collected silica nanofibres on aluminum foil

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were placed in a vacuum oven for 2 hours at 50 °C, to remove any remaining ethanol solvent and

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moisture. The membrane of nanofibres could be peeled off the aluminum foil and stored in

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resealable packets under dry atmosphere.

Preparation

of

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2.3 Synthesis of 2,3 ionene chloride polyelectrolyte

N’-tetramethyl-N,

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(3-chloro-2-

hydroxypropyl)

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ethylenediammonium dichloride (Precursor A) was carried out as per prior reported procedure

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[27]. This was reacted at reflux temperature with a tertiary amine, triethanolamine at a mole ratio

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of 2: 1 of tertiary amine (0.3 M) to Precursor A (0.15M), for 12 hours. The product was cleaned

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Fig. 1. (a) Scheme for chemical modification of electrospun silica nanofibre (b) surface protected etching of silica nanofibre

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by chromatography on neutral alumina with Methanol: methylene dichloride (1:3). It was

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characterized by its FTIR-ATR spectrum.

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2.4 Surface protected etching of Silica nanofibres

0.4 g silica NF was taken in an Erlenmeyer flask, to which a reagent solution was added (Fig. 1)

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[28]. The reagent solution was formulated as follows: 0.1 g of the polymeric ionene chloride in 20

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ml water, pH adjusted from 7.64 to pH 9 with drops of 1 % NaOH. The reagent solution with silica NF was heated at 90 ° C for 2 hours with continuous stirring on a magnetic stirrer. The solution

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was decanted, modified silica nanofibres were filtered and dried in the oven at 80 ° C for 4 hours.

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2.5 Characterization

2.5.1 Scanning electron microscopy / energy-dispersive X-ray spectroscopy

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Scanning electron microscope (SEM) and energy-dispersive spectroscopic (EDS) analysis was used to analyse the surface morphology of both the electrospun and further modified silica nanofibre. The identity of elements present and a quantitative estimate of their weight percentage were obtained from EDS analysis by a field emission scanning electron microscope JEOL, JSM-

7600F. The conductive agent was applied by means of sputter coating platinum over 600 s. The beam voltage was set at 10 kV and the samples were examined at a working distance of 8 mm. 2.5.2 ATR–FTIR analysis

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Attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectra of electrospun silica nanofibres and the modified silica nanofibres were obtained on a FTIR Shimadzu FTIR-8400S spectrometer attached with a PIKE MIRacleTM ATR module with diamond/ZnSe crystal. The spectra were scanned at a resolution of 1 cm-1 in the region of wavelength range 800–4000 cm-1. 2.5.3 X-ray Diffraction (XRD) Analysis

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XRD analysis of the electrospun silica nanofibre and the modified silica nanofibre samples was

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performed on the X-ray Diffractometer, Shimadzu 6100 model employing Cu Kα radiation (λ =

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1.54 Å). Data on reflected X-rays in terms of intensity in counts per second is collected at 2θ angle

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from 10° to 55°. The generator voltage was set at 40 kV, and generator current was set at 30 mA,

film and placed on the stub.

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2.5.4 Surface area Analysis

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with a step size of 0.02° and step time of 0.6 sec. The sample was prepared in the form of nanofibre

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The porosity characteristics of the mesoporous silica nanofibre and modified silica nanofibres were analysed by N2 adsorption-desorption experiments carried out at a temperature of 77 K on BET

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sorptometer, BET-201A, of Porous Materials, Inc. Sample degasing at 50°C, for 12 hours, was done prior to volumetric measurements. The linear part of the Brunauer-Emmett-Teller (BET)

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equation allowed determination of the specific surface area (SBET). The adsorbed volumes at a relative pressure (P/P0) of 0.994 for the silica NF sample and 0.996 for the modified silica NF, were used to estimate the total pore volumes (Vtot). Pore volume distribution was estimated from

the adsorption isotherm by Pierce’s simplified method of the Barrette, Joyner, and Halenda (BJH) procedure. The micropore volume (Vmicro) was obtained from method of t-plot analysis. 2.5.6 Thermogravimetric analysis

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Thermogravimetric analysis was performed on both the modified and as-is electrospun silica nanofibres on a differential thermogravimetric analyzer DTG–60H, Shimadzu, a simultaneous DTA–TG apparatus. The heating rate employed was 10°C/min, under nitrogen flow at a rate of 30 mL/min, starting from a room temperature of 30° C, and rising up to 600°C. 2.6 Preparation of dye solutions and concentration Measurements

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Stock solution (50 mg L−1) of the dye was made by dissolution in distilled water. Dilution was

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done to achieve desired concentrations. Visible Spectrometry measured the dye concentrations in

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solutions of the adsorption experiments. Calibration curves of the dye Acid Yellow 17 were made

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in the range 1 - 10 mgL-1 and 10 - 50 mgL-1 by measuring their absorbance at λmax 400 nm. Spectrometric measurements were made by scanning from 500 to 360 nm wavelength by a UV-

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1800 spectrophotometer, Shimadzu, using a sample cell of 1.0 cm path-length. Changes in the

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nature of the absorbance band with pH, or presence of salt were noted.

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2.7 Adsorption Studies

Adsorption experiments were performed using 100 ml Erlenmeyer flasks, containing the dye

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solution (30 mL) with initial dye concentration in the range 10-50 mg L-1. Both silica nanofibres and modified silica nanofibres were used as adsorbent. The stoppered flasks with dye solution and

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adsorbent were placed in a temperature controlled orbital shaker (TDR-72 / TC-09, A. R. V. Enterprises, Mumbai, India), and shaken at a constant speed of 120 rpm.

The adsorption

experiments were done at 3 different temperature points. Adsorption studies were also done by varying the pH of dye solutions. The pH levels were adjusted to a set point by adding a few drops

of HCl or NaOH (0.1 N HCl or 0.1 N NaOH), as required, to the diluent water. The adsorbent, 10 mg, weighed on a Wensar electronic weighing balance, DAB 220, (least count 0.1 mg), was added to the dye solutions. At the completion of equilibrium period on the temperature controlled shaker,

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7 ml of the aqueous sample was taken from the solution, and centrifuged on a Remi centrifuge at 3000 rpm, for 5 min, so that the supernatant liquid was clearly separated from the sedimented adsorbent nanofibres. The dye concentration in the supernatent was analysed with a UV-Vis spectrophotometer at 400 nm. The amount of dye removed at equilibrium qe (mg g-1) was calculated by the formula qe = (Ce – Co) * V / W

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equation (1)

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W: wt. of adsorbent nanofibre, 0.01 g; V: 30 mL

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Concentrations, Ce (at equilibrium), and Co (initial) were derived from the corresponding

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absorbances measured and the calibration curve constructed on the basis of Beer-Lambert’s Law. The level of dye adsorption with increasing contact time was determined by taking out samples at

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different time points from the start point, for measuring the residual concentration of dye. The

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adsorption isotherms were determined for measurements made after equilibration time of 4 hours.

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The effect of presence of electrolyte was studied by the addition of salt NaCl, NaNO3, Na2SO4 solutions, to make up the stock dye solution to 10 mg L-1 dye solution. A high concentration salt

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solution at pH 3.2 was used to dilute to final 1M concentration of salt in in 30 mL volume. The mean of measurements, with error bars representing the 95% CI for the mean, are used to plot the

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graphs. The data to graph were further analysed using the Solver module in Microsoft Excel™ 2013 software. 2.8 Regeneration of nanofibrous adsorbent for reuse

Desorption of AY17 anionic dye from the nanofibre was carried out with ethanol as desorbing agent. Dye-nanofibre aqueous solutions of volume 25 mL were centrifuged at 3000 rpm for 12 minutes, and the sediment of nanofibres collected. The nanofibre sediment was repeatedly

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centrifuged with fresh 15 mL portions of absolute ethanol per 0.5 g of nanofibre, until the supernatant was clear and colourless (did not require more than 2-3 rinses); prior centrifuge rinses composed of the yellow dye – containing ethanol were decanted off. The desorbed nanofibre was further rinsed with water, pH 7, and then dried for 6 hours in an oven at 78 °C. 3 Results and Discussion:

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3.1 Characterization of Adsorbent

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3.1.1 Scanning electron microscopy / Energy-dispersive X-ray spectroscopy analysis

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Fig. 2. SEM Images of (a, ai) silica nanofibre (SNF) and (b, bi) modified silica nanofibre (mSNF);

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Histogram of diameter distribution, with normal fit curve, of (c) SNF and (d) mSNF

Fig. 3. Elemental mapping of C, N and Si in mSNF SEM images (Fig. 2) show the nanofibrous nature of the electrospun Silica being retained even after undergoing surface protected etching. The image analysis for cross-sectional diameter of the

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nanofibre indicates that the electrospun SNF had a diameter of around 590 nm, with the greatest frequency in the population analysed, while after surface protected etching, the diameter decreased to 420 nm. Elemental analysis by EDX (Table 1) indicated that the starting material composition of tetraethyl ortho silicate of C, O and Si elements were present in the electrospun nanofibre; after alkaline etching with surface protection by a polymeric quaternary amine, there was a 0.1 %

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increase by atomic weight in C, and ~0.5% increase by atomic weight in O.

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Since light elements such as B, C, N, O and F have low photon yields from X-rays, the signal

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response from the comparatively low concentration of N atoms present in the modified SNF

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sample are overshadowed by the high response of the Si atom present at high concentrations, and not detected in EDX spectra, but are detected in the elemental mapping for C, N and Si distribution

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3.1.2 ATR–FTIR analysis

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in the sample as shown in Fig. 3.

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The FTIR-ATR spectra (Fig. 4.(a)) of the prepared ionene chloride film, of structure as in Fig 1(a) was similar to the spectra of the 2-hydroxy-3-ionene chloride type of compound, from literature

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[29].The broad band in the region 3600-3100 cm-1 is due to the free, inter- and intra-molecular bound –OH groups. Bound water present in ionenes contributes to the band at 1643 cm-1. The band

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at 1479 cm-1 is attributed to angular deformation of -CH2-, while the band at 1045 cm-1 is assigned to C-N str. This supports the chemical structure of the [2, 3] ionene chloride. After surface protection with this polymeric quaternary amine, and alkaline etching of the silica nanofibre, the band at ~938 cm-1 due to stretching of Si-O, in Si-OH groups decreases in intensity (Fig. 4.b, d).

This indicates the decrease in content of silica hydroxyl groups, which along with the slight increase in intensity of band at 800 cm-1, corresponding to Si-O-Si bending vibration, indicates changes in structure of Si-O-Si network. During the sol preparation for electrospinning, the

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polycondensation of Si-OH groups proceeds through linear -Si-O-Si structures which experience (transverse optical) mode of the Si-O-Si asymmetric stretching vibration at 1040 cm-1 and LO (the

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longitudinal optical) mode, at 1140 cm-1 [30].

Fig. 4. (a) FTIR-ATR spectra of prepared 2-hydroxy-3-ionene chloride, the [2, 3]–ionene polymer (b) Overlap spectra of Silica Nanofibre (SNF) and mSNF (c) Close-up features of the overlap spectra (d) Overlap spectra in the region 650-1200 cm-1

Some 4-membered and greater number of 6-membered siloxane rings, formed during the condensation to silicate polymer, also contribute to the TO, LO components of this stretching vibration. After surface modification, the peak in this region reduces in intensity, but with broader

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width; this could indicate the formation of greater porous structure, with etching, and incorporation of organic polyelectrolyte molecule, along with the relaxation of the remaining 4-membered rings to the 6-membered form [31]. The mSNF show appearance of new bands at 1440 and 2900 cm-1, attributed to -CH2- str from the incorporated polymeric quaternary amine, and , at 1600-1640 cm1

, due to N-H deformation (Fig. 4.c). The FTIR spectra thus indicate the bonding and incorporation

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of the polymeric quaternary amine in the mSNF.

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3.1.3 X-ray Diffraction (XRD) Analysis

Fig. 5. WAXRD Spectra of SNF and mSNF

The silica peak, (Fig. 5) is broad, indicating amorphous nature, and the 2θ of around 23° in the silica nanofibre and modified silica nanofibre is similar to that occurring in the cristobalite structure of silica. It arises from the (101) lattice plane having a characteristic d-spacing 0.4055

nm at around 22.0°. After surface modification of the silica NF, there is a 0.2° shift towards the lower degree, while the d-spacing increases slightly, along with a slight increase in crystallinity, (Table 2), due to introduction of the polyelectrolyte in the silica nanofibre.

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3.1.4 Surface area Analysis

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Fig. 6. N2 adsorption-desorption isotherm for (a) SNF (b) mSNF The silica NF displays a Type II adsorption isotherm characteristic of adsorption on macroporous

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adsorbent, with strong adsorbate-adsorbent interaction and unbounded monolayer-multilayer adsorption (Fig. 6). After modification, it exhibits Type IV adsorption isotherm with hysteresis of

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gradual desorption compared to adsorption [32]. From the graphs of pore size distribution, (Fig. 7) and Table 3, it is seen that total pore volume increases after surface protected etching on the

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silica nanofibre, with the formation of micropores, but there is a reduction in surface area and the

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Fig. 7. Pore size distribution of (a) SNF (b) mSNF

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average pore diameter. The BET surface area is lower though porosity increases after modification;

3.1.5 Thermogravimetric analysis

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the pore diameters are in the range for calcined silica fibres in literature [33].

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The thermal decomposition patterns of silica NF and modified silica NF could be inferred from

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their TGA and DTA curves (Fig. 8). Silica NF undergoes weight loss, occurring upto 120° C, of 13.02 % and subsequent weight, loss upto 600 °C, of 13.77%, in sum 26.78%. The mSNF

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experiences a lower weight loss of 8.07%, upto 120°C, and subsequent weight loss of 6.57%, upto

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600°C, in sum 14.64%. The modified silica NF is more thermally stable, as it experiences lower

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total weight loss.

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Fig. 8. (a) TGA curves (b) DTA curves of silica NF and mSNF

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DTA curves exhibit features accompanying weight loss, of which the first is an endothermic peak

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around 100°C due to desorption of water, which was previously physically adsorbed. The enthalpy

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change, accompanying loss of water, is greater in magnitude for the modified silica than the silica

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NF. Also it is centered closer to 40 °C in modified silica NF, than in silica NF where it is closer to 55°C. According to prior literature, thermal analysis of 2-hydroxy-3-ionene chloride showed an

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endothermic transition at 32-39°C, likely a glass transition Tg, which could arise due to the plasticizing intermolecular bonding exhibited by the hydroxyl group [34]. Above 200°C, vicinal

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hydroxyl groups start to condense, forming strained siloxane groups. By 500–600°C, this

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exothermic process finishes. This occurs to a greater degree in the as-is silica NF than the mSNF. The thermal stability of modified silica NF is attributed to it experiencing surface protected etching with sodium hydroxide, with the formation of micropores, and having lower surface area than silica NF [35]. 3.2 Adsorption Studies

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3.2.1 Effect of pH on adsorption behavior

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Fig. 9. (a) Influence of pH of dye solution on adsorption of AY17 on silica nanofibre and the

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polyelectrolyte (PEL) modified SNF; experimental conditions dye concentration 10 mg L-1,

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adsorbent dose 10 mg in 30 ml dye solution for a contact time of 3 h, 28 °C (b) chemical structure of AY17

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Dye adsorption of anionic dyes (which may be reactive, direct or acid) is closely related to the

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surface basicity of the adsorbents, with adsorption mechanism linked to the interaction between the electrophilic sites on the adsorbent and the electron dense regions of the dye molecule [36]. It

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is seen that AY17 which has a pKa of 5.5 [37], adsorbs with greater efficiency at pH 3.2 than at

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neutral pH (Fig. 9) ; at alkaline pH, the visible absorbance bands of the dye become broader and undergo a hypsochromic shift, with the delocalization of negative charges [38], and adsorb

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minimally on the negatively charged silica surface (hence the zero to negative removal efficiency at pH > 7, as adsorption is monitored through the change in intensity at absorbance wavelength of 400 nm). The two sulfonate groups on the AY17 molecule are the anionic sites, which interact with the multiple cationic quaternary N atoms on the [2, 3] ionene chloride polyelectrolyte modified silica, through the electrostatic mode. The [2,3] ionene chloride polyelectrolyte is an

internal cationic polymer possessing regular repeating cationic nitrogen atoms integral to the polymer backbone, and it modifies the silica nanofibre by coating the surface and micropore walls, even at relatively low concentrations.

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Sulfonic acid groups exhibit negative charge at highly acidic pH, as the pKa value of this functional group is negative, as in the case of p-toluenesulfonic acid having pKa of -2.8. Silica surface has a point of zero charge at 2.3, the pKa relevant to the ionization SiOH2+ ↔ SiOH + H+, above which it exhibits a negative charge, with increasing pH [39]. This causes electrostatic repulsion towards anionic dye, but this is screened and reversed to electrostatic attraction in the case of ionene

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chloride polyelectrolyte modified silica nanofibre, at pH 3.2. This almost doubles the removal

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efficiency of anionic dye, after modification. At higher pH, the attractive forces decrease as the

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negative charge on the silica increases and there is competition for the cationic adsorption sites

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between the anionic dye molecules and the hydroxyl groups in the aqueous solvent.

Fig. 10. (a) 3-d visualization of AY17 molecule with van der Waal volume (b) edge view of the

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AY17 molecular plane, courtesy open source project ‘visualizer’ at www.cheminfo.org Adsorption on modified silica nanofibre, may also be promoted by the cohesive hydrophobic interaction between the –CH2-CH2- carbon chain of the incorporated polyelectrolyte and the hydrocarbon aromatic rings in the dye molecule. The Acid Yellow 17 molecule has a hydrogen bond acceptor (electronegative atoms) count of 10, and a topological polar surface area of 189Å 2

[Pubchem Open Chemistry Database], which is defined as the summation of surface area ascribed to oxygen and nitrogen atoms; and hydrogen atoms bound to these electronegative atoms. This favors electrostatic attraction to electron deficient sites on the modified silica adsorbent. The

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computed van der Waal volume of this anionic dye molecule was found to be 590 Å3 [40]. It has been shown previously that van der Waals' (dispersion) forces account for adsorption of sulphonated dyes (Fig. 10), and their affinity increases linearly with the area of the adsorbed molecule interacting with the inorganic substrate, in an aqueous medium [41]. On this basis, AY17 has greater adsorption on the modified nanofibre which has greater porosity, and pore volume,

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though a slightly reduced surface area, relative to the electrospun silica nanofibre. Also, the most

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probable orientation of the dye adsorbate to the adsorbent surface is an edge–on orientation, with

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the two sulfonic acid end groups on opposite ends of the planar dye molecule.

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3.2.2 Effect of contact time

It is seen that more than 90% of the adsorption equilibrium is achieved in the first 100 minutes

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(Fig. 11), which is at least half the reported time required of 240 minutes, for adsorption on silica

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gel form [21]. Adsorption reaches equilibrium faster on nanofibrous surface area, and for AY17,

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complete removal is achieved on modified silica compared to 53.2% removal efficiency on the plain electrospun Silica NF. There are two steps visible in the adsorption, with a rapid initial rate

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of adsorption when the dye concentration is high and there are free adsorptive sites available for binding in the first 10 minute; after the initial burst, there is a slower rate of adsorption, with

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decreasing dye concentration and adsorption sites, until equilibrium is reached, with saturation of the available adsorption sites. The data points were further graphically analysed, on the basis of the pseudo-first order (PFO), pseudo-second order (PSO) and intra-particle diffusion models of

adsorption kinetics. The pseudo-first-order kinetic model considers the approximation that the

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adsorption rate relates to the number of the unoccupied, adsorptive sites.

Fig. 11. Effect of contact time on amount of AY17 dye adsorbed from dye solution on SNF and

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mSNF (dye concentration 10 mg L-1, adsorbent dose 10 mg in 30 ml dye solution, 28 °C)

qt= qe(1-e -k1t)

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The model is described by the equation in its non-linear form (2), and linear form (3) [42]

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ln (qe – qt) =ln qe - k1t

equation (2) equation (3)

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where k1 (min-1) is the pseudo-first-order rate constant for adsorption, and qe is the maximum adsorption capacity at equilibrium. Values of k1 and qe, can be calculated from the linear plot of

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ln (qe - qt) versus t, and the non-linear plot of qe against t. The pseudo-second order kinetic model is derived considering the relation of the adsorption rate to the square of the difference between the number of equilibrium adsorptive sites available on the adsorbent and the sites occupied. The non-linear form, equation (4), and linear form, equation (5), [43], is given as

qt= (k2qe2t) / (1+ k2qet)

equation (4)

t/qt = 1/ k2qe2 + t/qe

equation (5)

The values of second-order rate constant k2 and adsorbing capacity at equilibrium time (qe) can be

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determined from the linear plot of t/qt versus t, and the non-linear fitting of qe against t. The intraparticle diffusion model considers that the diffusion may extend beyond the external surface of the adsorbent and this step of intra-particle diffusion may be the slowest, rate-controlling step [44]. The equation describing the model is given as qt = kdif t0.5+ c

equation (6)

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where kdif is the intra-particle diffusion rate constant measured in mg g-1min1/2, and c, a constant

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relates to the boundary layer effects. The values of rate constant kdif and c, can be determined from

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the linear plot of qt versus t0.5.

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Fig. 12. Adsorption of AY17 dye: Linear fit of (a) Pseudo-First order and (b) Pseudo-second order kinetic model on SNF and mSNF

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Fig. 13. Adsorption of AY17 dye: Non-linear fit of (a) Pseudo-First order and (b) Pseudo-second order kinetic model relative to experimentally observed (exp) values on SNF.

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Fig. 14. Adsorption of AY17 dye: Non-linear fit of (a) Pseudo-First order and (b) Pseudo-second order kinetic model relative to experimentally observed (exp) values on mSNF.

Fig. 15. Adsorption of AY17 dye: linear fit of intra-particle diffusion model on SNF and mSNF

The comparisons of linear forms, Fig. 12(a, b), and non-linear forms, Fig. 13(a, b) and Fig. 14(a, b), of the models to the observed values of qt are depicted graphically. The coefficient of determination (R2) and chi-square test, standard error (SE) (in case of non-linear form) along with

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the extracted parameters of rate constants and adsorption capacity at equilibrium are shown in Table 4. It is seen that the linear form of pseudo-second order kinetic model has the highest coefficient of determination, for both types of adsorbents, while the non-linear form of the same has the least χ2 and SE, among the two non-linear models.

The non-linear form of the pseudo-second order kinetic model obtains the qe, equilibrium

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concentration of dye adsorbed per gram of adsorbent closest to the experimental values of the

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concentration of dye removed at equilibrium [45]. The equilibrium concentration of dye in solution

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becomes nil at contact time of 2 hours and 40 minutes, from the starting concentration of 10.52

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mg L-1, for the modified silica nanofibrous adsorbent.

The pseudo-first order model and the intraparticle diffusion model (Fig. 15) were less satisfactory,

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in modelling the experimental data, as seen in the graphs and their R2. However, the adsorption

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of the dye, especially on silica NF, occurring at a fast initial rate, followed by a slower rate until

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equilibrium is reached, indicates that intra-particle diffusion occurs in the slow stage, along with other adsorption mechanisms. Intra-particle diffusion does not determine the only rate-determining

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or slowest step since the regression does not pass through the origin, in case of both adsorbents; the magnitude of the intercept in this model points to the boundary layer being almost doubly

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thicker in the case of modified silica nanofibre, than the plain electrospun Silica nanofibre. 3.2.3 Effect of adsorbent dosage An increase in adsorbent dosage increases the % removal efficiency of the dye from the solution (Fig. 16). This increase can be due to the increase in available adsorption sites with increased adsorbent dosage. In case of silica NF adsorbent, the removal efficiency for an initial dye

concentration of 13.5 mg L-1 increases from 25.5 % to close to 40 %, for a change in adsorbent dosage from 10 to 20 mg per 30 ml dye solution; whereas the removal efficiency for modified

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silica NF adsorbent increases from 84.4 to 92% for similar variations in adsorbent dosage.

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Fig. 16. Variation in removal efficiency of AY17 from dye solution by varying adsorbent dosage of SNF and mSNF (experimental conditions dye concentration 13.5 mg L-1, adsorbent dose 10, 15 and 20 mg in 30 ml dye solution, 33 °C).

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The modified adsorbent has a much better adsorption capacity than the electrospun silica NF. 3.2.4 Adsorption Isotherms

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Equilibrium sorption data were obtained by varying the dye concentration in the range 10-50 mg L-1, for a constant adsorbent dose of 10 mg in 30 ml dye solution, for both the silica NF and ionene

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chloride modified silica NF. The equilibrium sorption data (amount of adsorbate per unit mass of adsorbent (qe mg g-1) with respect to concentration of the adsorbate solution at equilibrium (Ce mg L-1)), at constant temperature, Fig 17 (a)) were fitted on the non-linear and linear forms of Langmuir and Freundlich adsorption isotherm models, to elucidate the nature of adsorption of anionic dye from water on the silica nanofibrous adsorbent. The Freundlich isotherm describes a

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Fig. 17. Equilibrium Adsorption of AY17: (a) plot of experimental qe vs. Ce, on varying the initial dye concentration from 10-50 mg L-1 and (b) Linear form of Freundlich isotherm (on SNF and mSNF, for contact time of 4 hours, solution volume 30 mL, sorbent dosage 10 mg).

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Fig. 18. Equilibrium Adsorption of AY17: Linear form of Langmuir isotherm on experimental qe, Ce for dye concentration from 10-50 mg L-1 , on (a) silica NF and (b) mSNF (contact time of 4 hours, solution volume 30 mL, sorbent dosage 10 mg).

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process of multisite adsorption, while the Langmuir isotherm assumes a single binding site with adsorption restricted to a monolayer on a surface exhibiting heterogeneity, in which zones of siloxane-coated surfaces display hydrophobicity, whereas regions of high silanol densities show increased hydrophilicity [46]. The Langmuir model is represented by the non-linear form of equation

qe = (KL.qm.Ce)/ (1+ KL.Ce)

equation (7)

and its linear form Ce/qe = 1/KL.qm + Ce/qm

equation (8)

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where qm is the maximum theoretical adsorption capacity (mg g-1), qe, the amount of dye adsorbate adsorbed per unit mass of adsorbent (mg g-1), Ce, the equilibrium concentration of the dye molecule AY17 in solution (mg L-1), and KL, the Langmuir adsorption constant (L mg-1). The linear Langmuir plots of Ce versus Ce/qe, in Fig. 18 (a, b), and non-linear Langmuir curves on the experimental observations in Fig. 19(a, b) show the adsorption of the anionic dye on the silica NF

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and modified silica NF.

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A dimensionless equilibrium separation factor, RL, can be calculated from the KL, adsorption

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constant from the Langmuir equation, by the equation

equation (9)

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RL = 1/ (1+ KL.C0)

where C0 is the initial dye concentration (mg L-1). The values of RL indicate the nature of the

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adsorption being irreversible ((RL=0), favorable (0
Fig. 19. Equilibrium Adsorption of AY17: Non-Linear fit of Langmuir isotherm on experimental qe vs Ce, varying the initial dye concentration from 10-50 mg L-1 on (a) SNF and (b) mSNF (contact time of 4 hours, solution volume 30 mL, sorbent dosage 10 mg).

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Fig. 20. Equilibrium Adsorption of AY17: Non-linear fit of Freundlich adsorption isotherm on experimental qe vs Ce, varying the initial dye concentration from 10-50 mg L-1 on (a) SNF (b) mSNF (contact time of 4 hours, solution volume 30 mL, sorbent dosage 10 mg).

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Fig. 21. Variation of RL versus C0, for silica nanofibre and mSNF, on basis of KL derived from non-linear Langmuir adsorption of AY17 on adsorbent

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(RL>1).The calculated values of RL, (based on the KL from non-linear fit) for AY17 adsorption on both types of silica nanofibre adsorbent were found to lie between 0 and 1, (with values for the

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modified silica NF being almost 1/10th the value of RL for the silica NF), hence a favorable adsorption isotherm. The variations of the separation factor with the initial dye concentration are shown in Fig. 21. The Freundlich adsorption isotherm model is described by non-linear equation (1/n)

qe = KF. Ce

equation (10)

And in the linear form as ln qe = ln KF + (1/n) ln Ce

equation (11)

where KF is the Freundlich constant (L mg-1)) and (1/n), the heterogeneity factor, wherein value

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of n, 1
The Langmuir and Freundlich isotherm parameters and calculated values of error SE (standard error), RMSE (root mean square error), coefficient of determination (R2) and chi square of linear

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and non-linear fit, for the adsorption of the anionic dye on silica nanoadsorbents at varying initial

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dye concentrations from 10-50 mg L-1 and a constant adsorbent dosage of 10 mg in 30 mL dye

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aqueous solution, are shown in Table 5.

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Overall, the non-linear form of the Freundlich isotherm is found to best describe the adsorption in terms of observed values of qe and Ce in both nanofibrous adsorbents, (comparing Fig 19, 20, Table

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5). The values of the heterogeneity factor n, from the Freundlich isotherm are greater than 1 for

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both types of adsorbent; hence that mode of adsorption is considered favorable. The high value of

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KF, in the case of modified silica nanofibres indicates the comparatively high affinity of dye adsorbate for the adsorbent. According to the linear Langmuir model, the monolayer coverage of

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the adsorbent surface, gave maximum adsorption capacities of 40.32 and 84.75 mg g-1 for the silica nanofibre and modified silica nanofibre adsorbent respectively. These values of adsorption

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capacities are comparable to those for different adsorbent for acid dyes, as shown in Table 6. The increased adsorption capacity after modification of the silica nanofibre by protected surface etching, indicates that the dye adsorption is affected by the physico-chemical properties of the adsorbent, with respect to the dye molecular structure. The reduction in BET surface area, the

increase in total pore volume, formation of micropores and reduction in average pore diameter, after modification of the silica nanofibre, are correlated to dye uptake [47]. The decrease in dye uptake due to surface area reduction, are offset by the increase in dye uptake due to reduction in

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the average pore diameter, and increased pore volume. The equilibrium established between the adsorbent surface and aqueous phase, are controlled by the interaction between the dye adsorbate and silica surface functionality, as determined by the silica/water, water/dye and silica/dye modes of interaction.

Post modification, the silica/water and silica/dye modes of interaction are also changed. The ionene

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polyelectrolyte modifying the silica surface, exhibit a complex combination of hydrophobicity,

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charge interaction and specific ion effects, that modify the hydration structure of water, near the

A

silica interface, as they have rod-like or extended conformations, depending on the solvent polarity

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[48,49]. The nature of this interaction can be probed by the presence of salt in the aqueous phase, and temperature, which may favorably promote dye adsorption or instead retard the same.

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3.2.5 Effect of presence of salt

It is observed from Fig. 22, that the presence of salts at 1M concentration, changes the magnitude

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of q mg g-1, the amount of dye removed from the aqueous solution and adsorbed on the nanofibre. The presence of sodium nitrate drastically reduces the removal efficiency of the modified

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nanofibre by 41 %, in comparison to the absence of salt. For the silica nanofibre, there is a slight reduction by 3.4% in removal efficiency from the salt absent mode, but the modified nanofibres

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still perform slightly better, 10% more removal efficiency, than the silica nanofibre in presence of sodium nitrate.

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Fig. 22. Effect of presence of salt (1M) on adsorption of AY17 dye on silica and modified silica nanoadsorbent (at temperature 32°C, solution volume 30 mL, sorbent dosage 10 mg).

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The hydrated ionic radius and the hydration energy for the nitrate ion is the least in comparison to

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the chloride and sulfate anion (Table 7). This factor grants the greatest mobility for the nitrate anions, so that it competes with the anionic acid dye for the adsorption sites, and also screens the

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attractive electrostatic forces favoring the dye adsorption on the modified nanofibre.The

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magnitude of suppression of dye removal in presence of sodium chloride and sodium sulfate, is inverse the order of their ionic radius, greater for chloride than the sulfate salt. Sodium sulfate

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gives the least suppression of removal efficiency on the modified nanofibre. While both the chloride and sulfate salts increase the removal efficiency of dye on silica nanofibre, by the ‘saltingout’ effect, the chloride salt effects the greater magnitude of ‘salting-out’. The higher adsorption capacity of anionic dye on silica NF, is promoted by the salt ions corralling the dye molecules to cluster, and then deposit on the silica surface [36].This additionally shows the mechanism of

adsorption of anionic dye on the silica nanofibre to be different from that of the adsorptive process on the polyelectrolyte modified Silica nanofibre. 3.2.6 Effect of Temperature

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Temperature is highly significant to adsorption, since it affects the transport and movement of the dye molecules on to the adsorbent. In case of silica nanofibres, temperature elevation, increases the adsorptive removal of dye, which describes an endothermic adsorption; while in the ionene chloride modified silica NF, decreasing the temperature, increases the adsorption capacity, which is of an exothermic nature. Thermodynamic parameters including enthalpy change (ΔH°), entropy

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change (ΔS°) and standard Gibbs free energy change (ΔG°) at different temperatures (301, 305,

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311 and 316 K) were obtained, using the following equations:

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ln kd = (ΔS°/R) – (ΔH°/RT)

M

ΔG° = ΔH° - TΔS° kd = qe/Ce

equation (12) equation (13) equation (14)

D

where R is the universal gas constant (8.314 J mol-1 K-1), T is the reaction temperature (K) and kd

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is the distribution adsorption coefficient. The value of ΔH° was obtained from the slope of van’t

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Hoff linear plot of ln kd against 1/T, while the intercept, allowed ΔS° to be known (Fig. 23). These values enabled calculation of the free energy change, at the respective temperature using equation

A

CC

(13) (Table 8).

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Fig. 23. Effect of Temperature on adsorption of AY17: (a) qe vs T (b) van’t Hoff plot for SNF, and mSNF (solution volume 30 mL, sorbent dosage 10 mg, dye concentration ~10.5 mg L-1).

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The exothermic enthalpy, for the modified silica NF, could be due to the dye solute molecules

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having greater solubility in the aqueous phase, with increasing temperature, weakening the

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physical interaction between adsorbent surface and anionic dye molecule, retarding the adsorption.

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The value of enthalpy reflects the bonding strength between adsorbate and adsorbent or its degree

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of interaction. The absolute values of enthalpy lying between 40 to 120 kJ/mol, or close to it, in

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the case of both types of silica nanofibres indicate chemisorption, with modified silica nanofibre having a greater degree of chemisorption.

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The positive value of entropy ΔSo in case of silica nanofibre points to increased disorder at the adsorbent/solution interface during adsorption of dye. This could indicate the release of more than

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one water molecule (gaining more translational entropy) from the polymeric nanofibre by the

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adsorbate anionic dye, during adsorption. Adsorbent surface structural change during adsorption, could also contribute to the positive entropy change and it could be greater than the loss of freedom of the adsorbate dye molecule, leading to endothermic chemisorption [57]. The negative value of entropy for the modified silica nanofibre suggests that the process is enthalpy driven, with increased order at the solid/liquid boundary during adsorption. This causes the adsorbate dye ion

to release, with greater ease, from the solid to the liquid phase, i.e. desorb, with increase in temperature [58]. The Gibbs free energy ΔGo describes the spontaneity of the adsorption process. Negative value for

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the free energy favors adsorption (in modified silica NF and silica NF), which becomes more favored as the temperature increases to 43 °C, for silica NF. The adsorption process is spontaneous in the case of the ionene modified silica, and becomes less so as the temperature increases. Also the values of the change in free energy lying between -20 and 0 kJ mol-1 for ionene modified silica and silica NF at all the temperatures studied, point to physisorption as a major mode of adsorption

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in the process.

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When considering both the free energy and enthalpy values, the adsorption process is a mix of

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both physisorption and chemisorption, with an enthalpy driven chemisorption in the case of

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modified silica nanofibre.

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3.2.7 Reusability studies of silica nanofibres for adsorptive removal

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Fig. 24. Removal efficiency for Recycled nanofibres relative to number of cycles of regeneration ( temperature 33°C, solution volume 30 mL, sorbent dosage 10 mg, contact time 2 h 30 min,

D

mean initial dye concentration 10.5 mg L-1)

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Sorbent regeneration is a prerequisite for the economic use of the adsorbent, as there is minimal waste of sorbent material. For purposes of later potential usage, the recovery of both the adsorbent

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and the adsorbate anionic dye, is important [59]. Textile effluent should be considered a valuable

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resource, from the perspective of green economy. In this study, sorbent being a silica nanofibre, and the adsorbate being an anionic dye, selection of a reagent for recovery considered absolute

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ethanol, a common organic solvent readily available as distillery by-product. Alkaline treatment is not favorable, though desorption of the anionic dye occurs under those conditions, because silica surface may be etched by the sodium hydroxide solutions of pH ≥ 8. Dye may be recovered post distillation of the desorbing solvent ethanol, without further decomposition. After first

regeneration cycle, a drop in adsorption capacity by 0.3 mg g-1, for silica NF, and 7.0 mg g-1 for modified silica NF, occurs, leading to decreased removal efficiency (Fig. 24). Removal efficiency after 5 regeneration cycles remains almost constant for consecutive cycles,

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with a decrease of around 15% for modified silica NF, and a slight increase of 4 % for silica NF, relative to the first-use capacity, with the modified Silica nanofibre (NF) continuing to perform better. Hence, adsorbent could be used repeatedly while maintaining its favorable adsorption capacity. 4 Conclusion

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The modification of silica nanofibre was carried out by a flocculating and potentially anti-bacterial

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polymeric quaternary amine, at a low loading, that was not easily desorbed under regeneration

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conditions; the modification process was a facile, aqueous reaction. The adsorption of AY 17

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anionic acid dye onto silica nanofibres and modified silica nanofibres, from water, was then examined. The modified silica nanofibre possesses drastically improved dye adsorption capacity

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at acidic pH. Complete removal of AY17, anionic dye, is achieved on modified silica compared to

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53.2% removal efficiency on the plain electrospun Silica NF. The kinetic data were best-fitted

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using the non-linear form of the pseudo-second order kinetic model, while the nature of dye adsorption was suitably described by the non-linear form of the Freundlich isotherm. The

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adsorptive forces were more defined by electrostatic interaction and favorable hydrogen bonding, in the modified silica nanofibre. The adsorption on silica nanofibre was endothermic and

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spontaneous, but exothermic, and slightly less spontaneous on the modified silica nanofibres. This exemplifies the use of efficient, mainly inorganic, lithosphere compatible, nanofibrous adsorbents for water remediation.

Acknowledgement

One of the authors Ms. Gayatri Therani Nadathur is indebted to University Grants Commission-

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Basic Scientific Research (UGC-BSR), Govt. of India, for Ph. D. (Science) scholarship support through award letter number F.25-1/2014-15 (BSR)/No. F.5-65/2007(BSR) during the period of this study.

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Tables

Table 1 Elemental composition by atomic weight % of SNF and mSNF, by EDX Atomic Weight % C

O

Silica NF

6.38

49.33

Modified

6.49

49.8

SC RI PT

Sample

Si

44.29 43.71

U

Silica NF

Sample

2 θ (°)

Relative Intensity

Silica NF

23.02

100

0.3860

22.48

98

0.3952

22.86

100

0.3887

22.28

99

0.3987

CC

d nm

M

D

TE

EP

Modified Silica NF

A

A

N

Table 2 Comparison of the shift in 2θ and d, inter layer spacing, of major features of SNF and mSNF Crystallinity %

Icr (kcps *deg)

Ia (kcps *deg)

19.149

8.7665 37.012

21.5018

9.9803 36.436

Table 3 Physical properties related to surface area, pore volume, pore diameter and porosity S

V

V micro

Porosity based on skeletal density of 1.0000 g/cc, per gm of sample

(cm3/g)

(cm3/g)

Silica Nanofibre

58.132

0.0742

-

5.104

0.0691

Ionene modified Silica NF

44.083

0.3252

0.03198

3.894

0.2454

Sample

U

tot

SC RI PT

(m2/g)

Avg. Pore diameter (nm)

BET

Parameters qe

Linear Pseudo-first Order

qe

TE k

EP

CC A

Modified Silica NF -1

31.573 mg g

-1

15.231 mg g

-1

0.0201 min

9.489 mg g

0.0193 min

-1

0.971

0.977

χ

0.0257

0.0395

qe

15.252 mg g

k

0.1041 min

R

Non-linear Pseudo-first Order

2

-1

16.947 mg g

D

Observed

Silica Nanofibre

M

Model

A

N

Table 4 Kinetic parameters describing the adsorption of AY17 dye on nanofibre, based on the Linear and Non-linear forms of Pseudo-first order and Pseudo-second order Models, and Linear form of Intra-particle diffusion model.

2

RMSE

-1

-1

-1

29.178 mg g -1

0.1111 min

1.555

1.053

χ

0.970

1.281

SE

0.1979

0.0774

2

-1

k

0.00378 g mg min

0.00265 g mg min

0.995

0.998

χ

0.150

0.0487

qe

16.681 mg g

k

0.00749 g mg min

RMSE 2

-1

0.0593

0.0712

-1

-1

2

0.0177

A M D TE

17.878 mg g

-0.5

-1

-0.5

1.0376 mg g min

U

0.9924

-1

0.559

0.6671 mg g min

2

-1

0.00477 g mg min 1.709

8.3544 mg g

kdif

EP CC

-1

-1

c

-1

-1

0.973

SE

-1

31.459 mg g

-1

0.433

χ

A

-1

-1

χ

R

33.223 mg g

SC RI PT

2

2

Linear Intraparticle Diffusion

-1

18.018 mg g

R

Non-linear Pseudosecond Order

-1

qe

N

Linear Pseudosecond Order

0.9273 0.262

Table 5 Langmuir and Freundlich isotherm parameters for AY17 adsorption on nanofibre Parameters

-1

40.32 mg g

KL

0.0464 L mg 0.987

2

4.82*10

qm

39.87 mg g

KL

0.0473 L mg 0.4731 0.0758 0.0169

A

CC

TE

EP

Non-linear Freundlich form

KF

n RMSE 2

χ SE

-1

79.32 mg g

-1

N

A

D

2

χ

3.358 1.731 0.999

M

KF 2

3.8*10

-1

2

R

-1

1.494 mg 0.990

-1

χ SE

n

-1

-3

χ

RMSE

Linear Freundlich form

84.75 mg g

-3

2

Non-linear Langmuir form

-1

qm R

Modified Silica NF

SC RI PT

Linear Langmuir form

Silica Nanofibre

U

Model

-5

2.088 L mg 13.094 4.285 12.602 50.119 5.203 0.958 -3

1.87*10

4.67*10

3.3678 1.734 0.164

50.576 5.346 12.134 1.152 11.419

-3

7.41*10 0.00179

Table 6 Comparison of maximum adsorption capacities of various adsorbents for acid dyes. Adsorbent

Adsorption Capacity

Anionic Dye

Reference

Alumina/multi-walled carbon nanotubes

Reactive Blue 19

Cellulose

Reactive Red RB Acid Orange 8

Graphene oxide

Direct Red 23 Reactive Yellow 84 Acid Yellow 219

5.97

[54]

29.0

[52]

15.3

50.25

[51]

338.4

[53] [20]

1000

Acid Blue 113

769.23

Acid Yellow 17

40.32

This work

Acid Yellow 17

84.75

This work

Acid Red 114

M

pentaethylene hexamine functionalized nanoporous

A

CC

EP

TE

D

SBA-3

Ionene chloride modified Silica NF

[50]

A

N

Nanosized silica–titanium oxide (Si 80:Ti 20)

Silica Nanofibre

3.67

U

Hydroxyapatite

SC RI PT

(Langmuir Isotherm mg g-1)

Table 7 Characteristics of Anion species [55, 56] Ion

Ionic Radius (nm)

Hydrated Ionic radius (nm)

Hydration Energy

Cl-

0.181

0.347

376

NO3-

0.189

0.340

270

SO42-

0.230

0.380

1138

kJ mol-1

Table 8 Thermodynamic parameters for the adsorption of AY17, anionic dye, on nanofibre, at different temperatures Adsorbent

o

o

ΔS kJ mol-1.K-1

ΔG kJ mol-1

SC RI PT

o

ΔH kJ mol-1

TK

Silica NF

75.32

0.251

305

311

316

-1.24

-2.74

-3.996

TK

-130.66

-0.396

301

311

316

-11.46

-7.50

-5.52

A

CC

EP

TE

D

M

A

N

U

Ionene modified Silica NF

Figure Captions

Fig. 1 (a) Scheme for chemical modification of electrospun silica nanofibre (b) surface protected

SC RI PT

etching of silica nanofibre Fig. 2 SEM Images of (a,ai) silica nanofibre (SNF) and (b,bi) modified silica nanofibre (mSNF); Histogram of diameter distribution ,with normal fit curve, of (c) SNF and (d) mSNF Fig. 3 Elemental mapping of C, N and Si in mSNF

Fig. 4 (a) FTIR-ATR spectra of prepared 2-hydroxy-3-ionene chloride, the [2, 3]–ionene polymer

U

(b) Overlap spectra of Silica Nanofibre (SNF) and Modified SNF (c) Close-up features of the

N

overlap spectra (d) Overlap spectra in the region 650-1200 cm-1

A

Fig. 5 WAXRD Spectra of SNF and mSNF

M

Fig. 6 N2 adsorption-desorption isotherm for (a) SNF (b) mSNF Fig. 7 Pore size distribution of (a) SNF (b) mSNF

D

Fig. 8 (a) TGA curves (b) DTA curves of silica NF and mSNF

TE

Fig. 9 (a) Influence of pH of dye solution on adsorption of AY17 on silica nanofibre and the

EP

polyelectrolyte (PEL) modified SNF; experimental conditions dye concentration 10 mg L-1, adsorbent dose 10 mg in 30 ml dye solution for a contact time of 3 h, 28 °C (b) chemical structure

CC

of AY17

Fig. 10 (a) 3-d visualization of AY17 molecule with van der Waal volume (b) edge view of the

A

AY17 molecular plane, courtesy open source project ‘visualizer’ at www.cheminfo.org Fig. 11 Effect of contact time on amount of AY17 dye adsorbed from dye solution on SNF and mSNF (dye concentration 10 mg L-1, adsorbent dose 10 mg in 30 ml dye solution, 28 °C)

Fig. 12. Adsorption of AY17 dye: Linear fit of (a) Pseudo-First order and (b) Pseudo-second order kinetic model on SNF and mSNF Fig. 13. Adsorption of AY17 dye: Non-linear fit of (a) Pseudo-First order and (b) Pseudo-second

SC RI PT

order kinetic model relative to experimentally observed (exp) values on SNF Fig. 14. Adsorption of AY17 dye: Non-linear fit of (a) Pseudo-First order and (b) Pseudo-second order kinetic model relative to experimentally observed (exp) values on mSNF.

Fig. 15. Adsorption of AY17 dye: linear fit of intra-particle diffusion model on SNF and mSNF Fig. 16 Variation in removal efficiency of AY17 from dye solution by varying adsorbent dosage

U

of SNF and mSNF (experimental conditions dye concentration 13.5 mg L-1, adsorbent dose 10,

N

15 and 20 mg in 30 ml dye solution, 33 °C).

A

Fig. 17 Equilibrium Adsorption of AY17: (a) plot of mean experimental qe vs. Ce, on varying the

M

initial dye concentration from 10-50 mg L-1 and (b) Linear form of Freundlich isotherm (on SNF and mSNF, for contact time of 4 hours, solution volume 30 mL, sorbent dosage 10 mg).

D

Fig. 18 Equilibrium Adsorption of AY17: Linear form of Langmuir isotherm on experimental

TE

qe ,Ce for dye concentration from 10-50 mg L-1 , on (a) SNF and (b) mSNF (contact time of 4 hours,

EP

solution volume 30 mL, sorbent dosage 10 mg). Fig. 19 Equilibrium Adsorption of AY17: Non-Linear fit of Langmuir isotherm on experimental

CC

qe vs Ce, varying the initial dye concentration from 10-50 mg L-1 on (a) SNF and (b) mSNF (contact time of 4 hours, solution volume 30 mL, sorbent dosage 10 mg).

A

Fig. 20 Equilibrium Adsorption of AY17: Non-linear fit of Freundlich adsorption isotherm on experimental qe vs Ce, varying the initial dye concentration from 10-50 mg L-1 on (a) SNF (b) mSNF (contact time of 4 hours, solution volume 30 mL, sorbent dosage 10 mg).

Fig. 21 Variation of RL versus C0, for silica nanofibre and mSNF, on basis of KL derived from nonlinear Langmuir adsorption of AY17 on adsorbent Fig. 22 Effect of presence of salt (1M) on adsorption of AY17 dye on silica and modified silica

SC RI PT

nanoadsorbent (at temperature 32°C, solution volume 30 mL, sorbent dosage 10 mg). Fig. 23 Effect of Temperature on adsorption of AY17: (a) qe vs T (b) van’t Hoff plot for SNF

, and mSNF (solution volume 30 mL, sorbent dosage 10 mg, dye concentration ~10.5 mg L-1).

Fig. 24 Removal efficiency for Recycled nanofibres relative to number of cycles of regeneration ( temperature 33°C, solution volume 30 mL, sorbent dosage 10 mg, contact time 2 h 30 min,

A

CC

EP

TE

D

M

A

N

U

mean initial dye concentration 10.5 mg L-1)