Magnetic assisted separation of uranium(VI) from aqueous phase using diethylenetriamine modified high capacity iron oxide adsorbent

Journal Pre-proof MAGNETIC ASSISTED SEPARATION OF URANIUM(VI) FROM AQUEOUS PHASE USING DIETHYLENETRIAMINE MODIFIED HIGH CAPACITY IRON OXIDE ADSORBENT P. Amesh (Methodology) (Investigation), A.S. Suneesh (Investigation) (Validation), B. Robert Selvan (Investigation) (Resources), K.A. Venkatesan (Conceptualization) (Supervision) (Writing - review and editing) (Project administration), Manish Chandra (Investigation) (Resources)

PII:

S2213-3437(20)30009-9

DOI:

https://doi.org/10.1016/j.jece.2020.103661

Reference:

JECE 103661

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

30 September 2019

Revised Date:

31 December 2019

Accepted Date:

2 January 2020

Please cite this article as: Amesh P, Suneesh AS, Robert Selvan B, Venkatesan KA, Chandra M, MAGNETIC ASSISTED SEPARATION OF URANIUM(VI) FROM AQUEOUS PHASE USING DIETHYLENETRIAMINE MODIFIED HIGH CAPACITY IRON OXIDE ADSORBENT, Journal of Environmental Chemical Engineering (2020), doi: https://doi.org/10.1016/j.jece.2020.103661

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MAGNETIC ASSISTED SEPARATION OF URANIUM(VI) FROM AQUEOUS PHASE USING DIETHYLENETRIAMINE MODIFIED HIGH CAPACITY IRON OXIDE ADSORBENT 1,2

P. Amesh, 1A.S. Suneesh, 1B. Robert Selvan, 1,2K.A.Venkatesan, 1Manish Chandra

1Materials

Chemistry and Metal Fuel Cycle Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102. India 2Homi Bhabha National Institute, Anushaktinagar, Mumbai, Maharashtra 400094.

Corresponding Author: K. A. Venkatesan ([email protected])

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Graphical abstract

Research Highlights

Surface modification of magnetic adsorbent with diethylenetriamine.

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 

Characterization of the product by Raman, FT-IR, XRD, TG-DTA and SEM-EDX analysis.



Magnetic assisted solid phase extraction of uranium from aqueous phase.



High extraction capacity of uranium in the magnetic adsorbent (236 mg.g-1).



Recovery and recycling of the magnetic adsorbent.

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Abstract Processing of uranium ore for the production of reactor-grade uranium results in the generation of large amount of aqueous waste containing small quantities of uranium, but higher than the guideline value (15 μg.L-1) recommended by World Health Organization (WHO). The presence of uranium in aqueous waste poses several hazards and environmental issues due to the radioactive nature and migration behavior of uranium in the geosphere. In order to remove uranium (VI) from aqueous waste, the diethylenetriamine modified high capacity magnetic iron oxide adsorbent,

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abbreviated as Fe-DETA, was prepared and studied for the extraction of uranium from aqueous solutions. The Fe-DETA was characterized by X-ray diffraction, thermal analysis, infrared and Raman spectroscopy, and scanning electron microscopy. Significantly high amount (2 mmol.g-1) of

diethylenetriamine functional group (DETA) was anchored on magnetic iron oxide particles. The

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extraction of U(VI) from aqueous phase was studied as a function of pH of the aqueous phase,

duration of the contact between Fe-DETA and aqueous phase, concentration of U(VI) in aqueous

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solution etc. The data on the rate of extraction of U(VI) in Fe-DETA was fitted into the first order and second order rate equations. The apparent uranium extraction capacity on Fe-DETA was determined

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to be 236 mg.g-1, which correspond to the formation of 1:2 complex of U(VI) to diethylenetriamine ligand in Fe-DETA phase. The loaded U(VI) in Fe-DETA was quantitatively recovered using dilute Na2CO3 solution, and the recovered Fe-DETA was recycled for further extraction of U(VI), without any

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change in the apparent U(VI) extraction capacity.

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Keywords: Magnetic Separation; Iron oxide; Organo modification; Diethylene triamine anchoring; Uranium; Adsorption.

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1. Introduction

Among the different iron oxides, magnetite (Fe3O4), maghemite (γ -Fe2O3) and hematite (α-

Fe2O3) are popular candidates for the separation of heavy metal elements from the aquatic environment [1 - 3]. The magnetic properties of these iron oxides significantly differ from each other, primarily due to the structure of the iron oxide [2]. Generally, the hematite is regarded as a weak ferromagnetic material [2] at room temperature, with the saturation in magnetization occurs at 1 emu.g-1 (emu is electromagnetic units) [2]. However, the other iron oxides, namely magnetite and

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maghemite, are ferromagnetic [2] at room temperature, with the saturation in magnetization occurs up to 90 emu.g-1 [2]. In view of this, there is a great interest in the utilization of Fe 3O4 magnetic material for the adsorption of toxic and radiotoxic metal ions from aqueous medium followed by the magnetic assisted separation of Fe3O4 magnetic particles [1 - 12]. However, it should be noted that Fe3O4 could not exhibit the desirable selectivity for the separation of toxic elements, since the adsorption of metal ions on Fe3O4 involves only the interaction between the poorly selective surface hydroxyl groups (=Fe-OH) present on the surface of Fe3O4 with metal ion [2, 3]. Moreover, the adsorption capacity (defined as the maximum amount of the target metal ion extracted per gram of the adsorbent) was

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reported to be of the order of 1 mmol.g-1 for Fe3O4, which is quite low [2]. It is well recognized that the surface modification of iron oxide by organic ligands can

dramatically improve the selectivity of the magnetic Fe3O4 particle [2]. In this method, the surface of

iron oxide is initially modified to introduce the silanol (Si-OH) groups on the surface of Fe3O4 particles

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by silylation reaction, as discussed elsewhere [2 -13]. These surface Si-OH groups are amenable for further modification by organic ligands that can sequester the target metal ions [13 -16]. Moreover, the

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surface coating of silica over the magnetic particle protects the core of the adsorbent, namely Fe3O4, from acid and alkali attack during the extraction of metal ions from aqueous phase [2, 13 -16]. In this

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context, several organo modified/functionalized magnetic particles have been reported in the literature and studied for the extraction of a wide range of metal ions [15 -19].

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Natural uranium is employed as a fuel in nuclear reactors, world-wide, and the chemical and radiotoxicity associated with the fuel is essentially due to the specific chemical properties of uranium and to the radioactivity of its isotopes [20-25]. The abundance of natural uranium in the earth crust is

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quite small and that too limited to certain places across the globe. Mining and processing of uranium ore for the production of reactor-grade uranium results in the generation of large amounts of aqueous

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waste. The uranium bearing aqueous waste produced at processing site causes several hazards to the biosphere and environmental issues owing to the radioactive nature and migration behavior of uranium in the geosphere. The chemical toxicity of uranium has been studied in detail and reported in several papers published in literature [21-25]. The studies indicated that U(VI) can predominantly accumulate in bones, kidney, and liver [26]. In view of this, the World Health Organization (WHO) recommended a guideline value of 15 μg.L-1 for uranium as a tolerable daily intake (TDI) limit [26]. Since the TDI of uranium is very low and the uranium bearing aqueous waste has the concentration of

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uranium much higher than the guideline value, there is a growing interest on the removal of uranium from the aquatic streams [27]. Among the different methods reported for separation of uranium from the aqueous streams, the solid phase extraction technique has been regarded as an appropriate and technically viable method for the separation of trace amount of uranium present in the large volume of the aqueous wastes [1-12]. A number of solid-phase adsorbents composed of inorganic materials, organo functionalized inorganic materials, and organic polymeric materials have been employed for the separation of uranium [1-12, 28-34]. Excellent reports on the separation of uranium from aqueous

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streams by these materials are available in literature [28-34]. However, many solid-phase adsorbents reported in literature do not exhibit the desirable properties such as fast kinetics of extraction, high

selectivity and high capacity. Moreover, from the radioactive waste management point of view, it is

desirable to develop an adsorbent, in which the organics are completely incinerable and amenable for

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converting the spent adsorbent into solid/ceramic form for final disposal in deep geological

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repositories.

Among the different solid phase adsorbents, the method based on magnetic assisted

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separation offers a number of advantages over the other solid phase adsorbents. The first advantage is the ease of phase separation of magnetic particles by the application of external magnetic field [1,3]. Since the magnetite particles are separated by external magnetic field after adsorption, the size

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of the magnetic particles employed for the solid phase extraction can be as small as few micrometers to nanoparticle range, which is in contrast to the conventional solid phase adsorbents,

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wherein the average particle size of 300 to 500 m is required for convenient column operation [1-3]. Since the magnetic particles employed for extraction are in the range of few micrometer to

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nanometer, the kinetics of extraction observed using these magnetic particles are significantly faster as compared to those reported in conventional solid-phase adsorbents [1-12]. Moreover, the spent magnetic adsorbent material (when employed for the separation of radioactive elements) can be mixed with glass/ceramic forming additives, so that it can be converted to glass/ceramic form for disposal in deep geological repositories. In the recent past, Gdula et al. reported the amine-functionalized silica nanoparticles for the efficient extraction of uranium from aqueous solutions [30]. The U(VI) extraction capacity (maximum

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amount of uranium loaded per gram of adsorbent) of 118 mg.g-1 at pH 7 was reported by the authors. Aljarrah et al. reported the preparation of a new quaternary ammonium ion modified iron oxide nanoparticles for the removal of uranium from aqueous medium [31]. The composite material was characterized by advanced spectroscopic and microscopic methods, and the authors reported the uranium extraction capacity of 87 mg.g-1 [31]. Al-Harahshe et al. [32] reported the polyethyleneimine anchored carbon-coated iron oxide nanoparticles for the separation of uranium from aqueous phase. The authors reported the uranium extraction capacity of 127.5 g at pH 7. Lee et al. reported the arsenazo functionalized magnetic carbonaceous composite material for the extraction of uranium from

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aqueous solution [33]. Similarly, the synthesis of magnetite-porous carbonaceous materials was reported by Guo et al. and studied the extraction of uranium from aqueous medium [34]. The

adsorbent exhibited a capacity of 123 mg.g-1.The sequestration of U(VI) from aqueous solution (pH range) was studied by Cali et al., using phosphate-iron oxide nano particle, which is composed of

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((PO)x-Fe3O4) [35]. The adsorbent was prepared by the ligand–exchange reaction between the oleic acid capped-Fe3O4 with phosphate ions from the aqueous solution containing Na 2HPO4 and KH2PO4.

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It should be noted that the (PO)x is not chemically anchored on Fe 3O4 in (PO)x-Fe3O4. The uranium extraction capacity of 1690 mg per gram of the iron oxide present in the adsorbent ((PO)x-Fe3O4) was

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reported by the authors.

The present paper deals with the synthesis of diethylenetriamine anchored iron oxide,

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abbreviated as Fe-DETA, and studies on uranium extraction from the aqueous solution. The diethelenetriammine functional group was covalently linked on Fe3O4 magnetic particles in Fe-DETA. The product was characterized by X-ray diffraction, infrared and Raman spectroscopy,

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thermogravimetric analysis, and scanning electron microscopy. The extraction behavior of U(VI) in FeDETA was studied as a function of various parameters such as the effect of pH of aqueous phase,

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duration of equilibration, effect of uranium concentration in the aqueous phase, and recycling behavior of Fe-DETA. The results obtained from these studies reported in this paper.

2. Experimental 2.1. Materials

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High purity chemicals and solvents (HPLC or AR grade, >99% purity) were employed for the synthesis of the adsorbent, Fe-DETA, and for the extraction studies. The uranyl nitrate hexahydrate was obtained from Nuclear Fuel Complex, Hyderabad, India. A concentrated solution of uranyl nitrate (50 mg.mL-1) was prepared by dissolving the required quantity of uranyl nitrate in 0.1 M nitric acid, and the concentration of uranium in the stock was determined by Davies and Gray method, described elsewhere [36]. The chemicals used for the synthesis of Fe-DETA were ferric chloride hexahydrate (E-Merck, AR grade) and ferrous sulphate (E-Merck, AR grade), 3-aminotriethoxypropylsilane (APTS) (Aldrich, 99.5 % purity), and tetraethoxyorthosilane (TEOS) (Aldrich, 99.5 % purity), toluene (Aldrich,

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HPLC grade), and chloroform (Aldrich, HPLC grade). The AR grade nitric acid and Na2CO3 were obtained from SD fine chemicals, Chennai, India. The acetic acid (Ranbaxy, AR Grade) and sodium acetate (SD fine chemicals, AR Grade) were employed for the preparation of buffer solution. During equilibration studies, the adsorbent was separated from the aqueous phase by using a permanent

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hard ferrite magnet (strength 1.5 Tesla), which was purchased from Permanent Magnets Limited, Mumbai, India.

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2.2. Instrumentation

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The thermogravimetric analysis of Fe3O4, the silica-coated Fe3O4 (Fe-Si), and Fe-DETA was carried out using thermogravimetric analyzer (Metler Tolledo TGA/SDTA 851e, UK). For this purpose, a sample of ~10 mg was taken in an alumina crucible and heated from 313 K to 1273 K, at a constant

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heating rate of 10 K per minute under air atmosphere. Powder XRD pattern of Fe3O4 and Fe-DETA was recorded in a X-ray diffractometer (Philips 1011, Nederland) operating at 40 KV and 45 mA, with

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Cu K (1.5406 Å) radiation. Scanning electron microscope images of the Fe3O4, Fe-Si, and Fe-DETA, uranium adsorbed Fe-DETA, and Fe-DETA after uranium recovery were recorded using a Field

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Emission Gun-Scanning Electron Microscopy (Carl ZEISS, Germany). The elemental composition of the sample corresponding to the SEM image was obtained using Energy-dispersive X-ray spectroscopy analyzer (Oxford EDX analyzer, UK) equipped with the SEM unit. The FTIR spectrum of the sample was recorded by using the FT-IR spectrometer (Bruker Tensor II, Germany). The Raman spectra of samples were recorded using a Renishaw PLC Raman microscope system (Gloucestershire, UK) equipped with a Leica microscope and a 532 nm excitation laser.

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2.3. Preparation of Fe-DETA The Fe-DETA was prepared by the surface modification reactions on Fe3O4, shown in Fig. 1 Initially, the magnetite (Fe3O4) was prepared by the procedure described elsewhere [30-33]. Briefly, it involved the hydrolysis of a solution containing FeCl3 (0.033 moles) and FeSO4 (0.066 moles) present in Millipore water (100 mL). The hydrolysis was carried out by slow addition of 100 mL of aqueous ammonia (30 %) to the above solution at ambient temperature under argon atmosphere with vigorous stirring. A black precipitate was formed during the reaction. After the complete addition of ammonia (10 minutes), the mixture was stirred for about 2 h at 353 K under the argon atmosphere. Then the

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reaction mixture was allowed to cool, and the Fe3O4 particles were washed several times with millipore water to eliminate the traces of chloride and sulphate ions adhered on Fe3O4. The magnetite particles were separated from the aqueous phase by using a permanent magnet, followed by overnight drying in air.

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Later, the silylation reaction was carried out on Fe3O4 particles by the procedure described

elsewhere [37]. Initially, the surface of Fe3O4 was activated by dispersing Fe3O4 particles (1 gram) in

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0.5 M HCl (2 mL) for 10 minutes. The magnetite particles were separated and dispersed in a solution of ethanol (40 mL), aqueous ammonia solution (1 mL, 28 wt %), and deionized water (10 mL).

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Tetraethoxysilane (TEOS, 9.6 mmol) was added drop-wise to the above mixture under constant stirring at room temperature. After 10 h of stirring, the silylated iron oxide, abbreviated as Fe-Si, was

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separated from the aqueous phase using the permanent magnet, and the particles were washed with ethanol (50 mL x 6 times) and water (50 mL x 6 times). The particles were dried overnight in the air. The preparation of (4,7,10- trisaza-tridecyl) bis (trimethoxysilnae) (ATDTS) and (4,7,10-

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triazadecyl) trimethoxysilane (ADTS) is shown in Fig. 1 The diethylenetriamine (DETA, 48.4 mmol, 5 g) was taken in a round bottom flask equipped with a CaCl2 guard tube and mixed with triethylamine

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(48 mmol) present in chloroform (50 mL). 3-(Chloropropyl) trimethoxysilane (50 mmol, 8.7mL) was added drop-wise into DETA solution. After 6 hours of stirring, the solvent was removed by a method of rotary evaporation at 343 K under vacuum. The intermediate was obtained as a pale yellow colored viscous liquid containing both ATDTS and ADTS. The final product Fe-DETA was synthesized by the reaction between the silylated magnetite particle (Fe-Si) and viscous liquid (ATDTS and ADTS mixture). The procedure involved the reaction between Fe-Si (2 g) taken in toluene (125 mL) and yellow viscous liquid (10 g) in 25 mL toluene. The

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mixture was stirred for 8 hours at 363 K. After the reaction, the mixture was allowed to cool, and the magnetic particles were separated from the toluene by using an external magnet. The final product, Fe-DETA, was washed with fresh toluene, followed by isopropanol, acetone, water, and then finally by acetone, followed by drying in air overnight. The dried magnetic particles (Fe-DETA) were stored in a desiccator for further use.

2.4. Extraction of U(VI) The extraction behavior of U(VI) was studied by measuring the distribution coefficient of

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uranium (Kd, in mL.g-1) as a function of various parameters. The distribution coefficient of uranium on Fe-DETA is defined in equation 1, and it was measured by batch equilibration technique. The

extraction studies comprise of equilibration of 50 mg of the Fe-DETA with 10 mL of the aqueous solution containing uranyl nitrate (U = 200 mg.L-1) at 298 K. The pH of the aqueous phase was

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adjusted using dilute nitric acid for pH 1 and 2, using acetic acid – sodium acetate (0.2 M each)

buffers for pH 3 to 8, and using sodium hydroxide for pH 9 and 10. In some cases, sodium carbonate

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(CO32- = 500 mg.L-1) was added to the acetic acid – sodium acetate buffer. The amount of U(VI) in the aqueous phase was varied from 10 mg.L-1 to 2000 mg.L-1. The adsorbent and aqueous phases were

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taken in a stoppered glass test-tube (20 mL capacity), and the test tube was immersed in a constant temperature water bath (± 0.1 K). Mixing of the aqueous and adsorbent phases was facilitated with

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the help of a vortex mixer. After the specified duration, the equilibration was stopped, and the FeDETA was separated from the aqueous phase with the help of an external magnet. An aliquot was taken from the aqueous phase. The distribution coefficient of U(VI) in Fe-DETA was calculated by

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measuring the aqueous phase concentration of uranium before and at the end of equilibration and

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using the equation 1.

K d of U(VI) =

[U]ini −[U]fin [U]fin

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(m) mL. g −1

(1)

where [U]ini and [U]fin are the concentrations of U(VI) present in aqueous phase before and after equilibration respectively, and V (in mL) and m (in g) are the volume of the aqueous phase and weight of Fe-DETA taken for equilibration. The concentration of uranium present in the aqueous phase was measured by a spectrophotometric method using Arsenazo III as a colouring agent [38]. A Shimadzo spectrophotometer (UV-2100, Japan) was used for measuring the absorbance of the U(VI) samples at

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the wavelength of 650 nm. For kinetics experiment, the equilibration was stopped at different intervals of time and the amount of uranium extracted in Fe-DETA was determined using equation 2. V

q t = [U]ini − [U]fin (m)

mg. g −1

(2)

All the adsorption experiments were performed in triplicate, and the distribution coefficient of U(VI) in triplicate agreed well with less than a 5 % deviation. 2.6. Recycling studies The utilization of Fe-DETA depends upon the recycling ability of Fe-DETA. For this purpose,

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the extraction of U(VI) from aqueous solution was carried out in the recycled adsorbent, and apparent uranium extraction capacity in Fe-DETA was determined after each recycling. For the extraction

U(VI), about 50 mg of Fe-DETA was equilibrated with 10 mL of aqueous solution containing uranium

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(1500 mg.L-1) at pH 6, as discussed above. After equilibration, the amount of uranium extracted into Fe-DETA (known as apparent extraction capacity) was determined, as discussed above. After

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equilibration the aqueous phase was discarded, and the Fe-DETA phase was equilibrated with 10 mL of Na2CO3 solution (0.1 M) for the recovery of U(VI) from loaded Fe-DETA phase. The amount of

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uranium recoverd in the aqueous phase was determined, as discussed above. The recovered Fe-DETA phase was again subjected to equilibration with 10 mL of Na2CO3 solution (0.1 M). This procedure was repeated five times to remove the uranium quantitatively (more than 99 %) from the

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loaded Fe-DETA phase. The Fe-DETA obtained after recovery was once again subjected to uranium, extraction (1500 mg.g-1), as discussed above, and the apparent uranium extraction capacity was

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determined again. The procedure of loading of uranium in Fe-DETA upto the apparent extraction capacity, and recovery of uranium from the loaded Fe-DETA phase was repeated five times, and the

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apparent capacity in each loading was reported in this paper.

3. Results and discussion 3.1. Preparation and characterization of magnetic adsorbent The reaction scheme for the preparation of diethylenetriamine anchored iron oxide (Fe-DETA) is shown in Fig. 1 The synthesis involved the preparation of intermediates namely (4,7,10-tris-aza-

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tridecyl) bis(trimethoxysilane) (ATDTS) and (4,7,10-triaza decyl) trimethoxysilane (ADTS), followed by the reaction between the silylated iron oxide (Fe-Si) and ATDTS and ADTS. The intermediates ATDTS and ADTS together are abbreviated as “intermediate”. The silylated iron oxide was prepared by the reaction between Fe3O4 and tetraethoxysilane (TEOS) [37]. The weight ratio of the intermediate to Fe-Si was varied during the synthesis of Fe-DETA to optimise the degree of anchoring of diethylenetriamine (DETA) on iron oxide. The weight ratio of the intermediate to Fe-Si was varied from 5:1 to 15:1 and the product thus obtained after the reaction was subjected to the measurement of distribution coefficient of U(VI) at pH 6. The results are shown in Table 1. It can be seen that the

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distribution coefficient of U(VI) increases with increase in the weight ratio of the intermediate to Fe-Si. Beyond 10:1 weight ratio, the variation in the distribution coefficient of U(VI) is insignificant and

therefore, for the bulk preparation of Fe-DETA, the weight ratio of intermediate to Fe-Si was fixed at 10:1.

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The XRD patterns of the core Fe3O4 (as prepared) and the final product are shown in Fig. 1S (supplementary). The peaks observed in the XRD pattern of both the samples match well with the

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XRD pattern of the standard Fe3O4 (JCPDS 19 - 0629) [39]. This shows that the core of Fe-DETA is composed of Fe3O4 and it is not affected by functionalization of DETA on Fe3O4. Fig. 2S

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(supplementary) shows the comparison in the particle size distribution of Fe 3O4, Fe-Si and Fe-DETA. The particle size in all cases ranges from 0.1 μm to 100 μm. It appears that functionalization of both

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silanol (in Fe-Si) and DETA (in Fe-DETA) moieties on Fe3O4 is not affecting the particle size distribution to a significant extent. The average particle size in all cases is in the range of 1 to 10 μm. The thermogravimetric pattern of Fe-DETA is shown in Fig. 2 The TG pattern of Fe-DETA is

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compared with the TG of Fe3O4 and Fe-Si. The weight loss event happening in the temperature range from 303 K to 403 K on all solid samples is due to the loss of surface adsorbed water molecules. The

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loss of weight occurring in the temperature range 423 K to 1273 K in case of Fe3O4 and Fe-Si, could be assigned to the cross condensation of surface hydroxyl groups present on these samples leading to the loss of water molecules. The loss of weight occurred in this temperature range is quite insignificant for these samples. On the other hand, the Fe-DETA shows a significant weight loss event in the temperature range 523 K to 923 K, which can be attributed to the loss of organics present on the surface of Fe3O4. These organics undergo oxidative decomposition upon heating in air during thermomgravimetric measurements. About 30% weight loss occurs in the temperature range 523 K to

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923 K. Assuming that they are due to the organics present on the surface of Fe3O4, the amount of DETA anchored on the final product Fe-DETA is determined to be ~2 mmol.g-1. Above 973 K, the weight loss observed in case of Fe-DETA is negligible. The organics present on Fe-DETA was characterized by the spectroscopic techniques such as FT-IR and Raman spectroscopy. Fig. 3 compares the FT-IR spectrum of Fe-DETA with Fe3O4 and Fe-Si. The FT-IR spectrum of Fe3O4 shows a weak transmittance band at 593 cm -1, due to the Fe-O stretching frequency [40 - 42]. The FT-IR spectrum of silylated Fe3O4 (Fe-Si) shows a strong Si-O-Si transmittance band at 1098 cm -1, and a weak Si-OH band at 1640 cm-1, indicating the creation of the

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surface hydroxyl groups (Si-OH) on Fe3O4 by silylation reaction [42]. These surface hydroxyl groups are amenable for functionalization of Fe-Si by ATDTS and ADTS, as shown in Fig. 1 Anchoring of

ATDTS and ADTS on Fe-Si results in the covalent linking of diethylenetriamine (DETA) on Fe3O4. The FT-IR spectrum of Fe-DETA shows the presence of strong transmittance bands at 1098 cm-1, due to

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Si-O-Si stretching [41, 42], a eak transmittance band at 1636 cm -1, due to -N-H bending [41, 42], and a weak band at 2942 cm -1 due to C-H stretching. The spectrum also shows a broad transmittance

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adsorbed water molecules [41, 42].

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band at 3457 cm-1, attributed to both N-H stretching of DETA as well as the O-H stretching of

Previously, we investigated the structure of Fe3O4 [43] and succinic acid anchored on silica gel [44] by Raman spectroscopy. It was reported that the Raman spectrum of Fe3O4 phase, known as

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magnetite, showed a broad Raman band at 670 cm-1 and 306 cm-1 [45, 46]. Upon exposure of the sample to laser light from Raman spectrometer for excitation, the surface of Fe 3O4 was found to

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oxidise sequentially to maghemite (γ-Fe2O3) and hematite (α-Fe2O3) depending upon the power of laser light and exposure time [45, 46]. The hematite phase showed the Raman intensities at 596, 490,

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360, 281 and 219 cm-1 [47 - 49]. Based on our previous studies, the Raman spectrum of Fe3O4, Fe-Si and Fe-DETA was recorded and the traces are shown in Fig. 4. The trace A in Fig. 4 shows the Raman spectrum of Fe3O4 obtained after exposing the sample to the laser power of 11 W for 300 seconds. The Raman intensities obtained under this condition are located at 1295, 604, 484, 398, 281 and 219 cm-1. These lines correspond to the Raman pattern reported for hematite (α-Fe2O3) [46-48]. This indicates that the surface of Fe3O4 is oxidised to α-Fe2O3 under the condition of 11 W laser power with exposure time of 300 seconds. This observation is in good agreement with our previous

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studies on magnetite [43]. The same condition was maintained for recording the Raman spectra of other samples. The trace B in Fig. 4 shows the Raman spectrum of silylated Fe3O4. It should be noted that the spectrum was recorded under the same conditions, as discussed above. It can be seen that some peaks due to hematite are absent and new Raman lines at 596, 490 and 360cm -1 are also observed, perhaps due to the characteristic intensities of maghemite (γ -Fe2O3) [50, 51]. This shows that the silica coating on Fe3O4 is preventing the oxidation of the core Fe3O4 to some extent. It should be noted that silylation of Fe3O4 can result in the coating of few layers of silanol groups on the surface of

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Fe3O4 particles, as shown in Fig. 1 The silanol groups present on the surface of Fe3O4 seems to prevent the oxidation of the inner core made up of Fe3O4 [52, 53]. Based on this observation and

since the core of Fe3O4 in Fe-DETA is covered extensively with DETA organics (2 mmol.g-1), it can be expected that the core Fe3O4 in Fe-DETA would not undergo oxidation even to maghemite (γ -Fe2O3)

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or hematite (α-Fe2O3) upon exposing the laser light to the same duration and power.

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The trace C in Fig. 4 shows the Raman spectrum of Fe-DETA. As expected, the Raman peaks due to both hematite and maghemite are completely absent in this case. This shows that the

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organics present on the surface of Fe3O4 acts as shield, and prevents the oxidation of the core Fe3O4 to γ -Fe2O3 or α-Fe2O3. Moreover, a broad peak at 306 cm -1, characteristic to Fe3O4 is present in this case, but the other characteristic peak at 670 cm-1 is not observed [52, 53]. These observations

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confirm that the core of Fe3O4 is covered with the organics on Fe-DETA. To further confirm the presence of organics on Fe3O4, the Raman spectrum of Fe-DETA was recorded in the wavenumber

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range 3500 cm-1 to 900 cm-1, and the spectrum is displayed in Fig. 5. The trace A is for Fe-DETA. The spectrum shows the presence of few prominent Raman bands at 1615 cm -1, due to –N-H bending

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vibration, 2438 cm-1 due to –N-H stretching, and 2947 cm -1 due to –C-H stretching vibration [54]. All these observations discussed above confirm the presence of organics on the surface of Fe3O4. Since ATDTS and ADTS were employed for anchoring on Fe3O4, it is confirmed that DETA functional groups are present on the surface of Fe3O4 in Fe-DETA. Fig. 6 shows the scanning electron microscopic image of the sample and the corresponding average elemental composition obtained by EDX spectroscopic analysis. The SEM micrograph of Fe3O4, Fe-Si, and the adsorbent (Fe-DETA) reveals that, they consist of micro porous structure

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having irregular geometry. The EDX elemental composition of Fe3O4 shows the presence of Fe and O. Silylation of Fe3O4 results in the introduction of Si in Fe3O4, which is reflected in the EDX elemental composition of Fe-Si. The EDX analysis of Fe-DETA sample shows the presence of Fe, O, C, Si and N elements confirming the presence of these elements in Fe-DETA.

3.2. Effect of pH on the extraction of U(VI) Fig. 7 represents the variation in the distribution coefficient (Kd) of U(VI) in Fe-DETA as a

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function of the pH of aqueous phase. The distribution coefficient was measured in the presence and absence of sodium carbonate. It can be seen that the distribution coefficient of U(VI) in Fe-DETA

gradually increases with increase in the pH of the aqueous phase. It reaches a maximum Kd value at pH 6, followed by saturation. Previously, we studied the extraction behaviour of U(VI) on succinic acid

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anchored silica gel (abbreviated as Si-SUC) [44] as well as DETA anchored polystyrene-

divnylbenzene (abbreviated as PS-DETA) [55]. A maximum distribution coefficient of ~9300 mL.g-1

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and 3800 mL.g-1 was reported at pH 6 for Si-SUC [44] and PS-DETA [55] respectively. It was shown that the amidic carboxylic acid group present on Si-SUC and diethylenetriamine present on PS-DETA

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were responsible for the extraction of U(VI). The higher distribution coefficient of U(VI) obtained in the present study with Fe-DETA at all pH values (~ 43000 for mL.g-1 at pH 6 for instance) as compared to those obtained in Si-SUC and PS-DETA indicates that Fe-DETA forms a stronger complex with U(VI).

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Moreover, the average particle size of Fe-DETA is much lower (~ 10 μm) than the average particle size of PS-DETA (400 μm). As a result, the Kd of U(VI) observed in Fe-DETA is much higher than that

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mmol.g-1).

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observed in PS-DETA, even though the amount of DETA anchored on both adsorbents are similar (2

Fig. 7 also shows the distribution coefficient of U(VI) measured in the presence of sodium

carbonate at different pH values. Since, the leach liquors obtained from mining site is quite likely to contain small amount of sodium carbonate, the effect of carbonate ion on the distribution coefficient of uranium was studied [56]. It can be seen that the Kd of U(VI) observed in the presence of carbonate is lower than that observed in the absence of carbonate. This shows that carbonate is forming a stronger complex with U(VI) and retains the complex in aqueous phase. However, it should be noted

13

that the distribution coefficient of U(VI) in presence of carbonate is quite significant, and it is adequate for complete extraction of U(VI) even in the presence of carbonate ions up to 500 mg.L-1. 3.3. Kinetics of U(VI) extraction The contact time required for the establishment of equilibrium can be obtained by studying the rate of extraction of uranium from aqueous phase on Fe-DETA. The rate constants derived from these studies are useful for developing technologies involving large scale separations [57]. The rate of extraction of U(VI) on Fe-DETA was carried out at pH 3 and 6 and the results are shown in Fig. 8, in the form of loading of U(VI) in Fe-DETA (in mg.g-1) as a function of time. It can be seen that the

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amount of U(VI) loaded in Fe-DETA increases with increase in the duration of equilibration followed

by the establishment of equilibrium occurring around 200 minutes. The loading of U(VI) in Fe-DETA at a particular time increases with increase in the pH of the aqueous phase and also with the initial

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amount of U(VI) present in the solution. Usually, the adsorbents having larger particle size (for

instance 200 to 500 m) exhibits sluggish kinetic of adsorption [55], and require longer time for the

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establishment of equilibrium. Previously, we reported the extraction kinetics of U(VI) in PS-DETA [55]. It should be noted that the amount of DETA functional groups present on PS-DETA is similar to Fe-

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DETA (2 mmol.g-1). Even though, the amount of DETA functional groups are same, the equilibrium was found to be established only after 400 minutes in case of PS-DETA. This can be attributed to the

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bigger particle size of PS-DETA (400 μm) as compared to Fe-DETA (10 μm). The experimental data obtained for the extraction of uranium in Fe-DETA as a function of time can be modelled with the pseudo-first order or pseudo-second order rate equations shown in equation

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3 and 4 [58-60].

q t = q e (1 − e−k1t )

qt =

k 2 q2e t 1 + k2qet

(3)

(4)

where qt is the amount of U(VI) loaded in Fe-DETA at a time t, q e is the amount of U(VI) loaded in Fe-DETA at equilibrium and k1 (min-1), k 2 (mg.L-1min-1) are the first order and second order rate constants. Non-linear regression of the kinetic data with equations 3 (dotted line) and 4 (solid line) are

14

shown in Fig. 8 and the constants obtained from the regression analysis are shown in Table 1S (supplementary) along with the statistical parameter (2 - value) for the curve fitting. Lower 2 value indicates better fitting of the experimental data. It can be seen that the 2 value is lower for the second order fitting. The k 2 values (Table 1S, supplementary) determined from the curve fitting decreases with increase in the amount of U(VI) present in the feed solution and increases with increase in the pH of aqueous phase.

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3.4. Adsorption isotherms The adsorption isotherms provide information on the strength of interaction between the

adsorbent and adsorbate. It also provides the information on the adsorption capacities, monolayer or multilayer adsorption, energetics of adsorption and homogenous or the heterogeneous nature of the

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adsorbent [61 - 64]. The extraction behaviour of U(VI) in Fe-DETA was studied as a function of the

amount of U(VI) in aqueous phase at different pH values and the results are shown in Fig. 9 It can be

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seen that the loading of U(VI) in Fe-DETA (in mg.g-1) increases with increase in the equilibrium amount of U(VI) (in mg.L-1) in aqueous phase at both pH values. The experimental data for the

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adsorption isotherm was modelled to the versatile adsorption isotherms such as Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherms. The significance of these isotherms are

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described elsewhere [61 - 64]. . The non-linear expressions for the Langmuir, Freundlich, Temkin,

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and Dubinin-Radushkevich isotherms are provided in equations 5, 6, 7 and 8 respectively.

qe =

K L Qo Ce 1 + K L Ce 1⁄ n

q e = K F Ce qe =

(5)

(6)

RT ln AT Ce bT 2

q e = q m e−βЄ

(7)

(8)

15

where, K L and K F are the Langmuir and Freundlich constants represents adsorption energy (mg.L-1), Q o is the apparent U(VI) extraction capacity (in mg.g-1), Ce is the amount of U(VI) present in aqueous phase (in mg.L-1) at equilibrium,

1 n

is related to the heterogeneity parameter of the adsorbent, β is

Dubinin-Radushkevich constant related to the mean energy of extraction per mole of the adsorbent (mol2.J-1), and Є is the polanyi potential and AT and bT are the constants. Non-linear fitting of the extraction data with equations 5 to 8 are shown in Fig. 9 The fitting constants derived from the regression analysis and the statistical parameters (2 and R2) of the fitting are tabulated in Table 2. Lower the magnitude of 2, and the R2 value close to unity indicates better

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fitting of the model with the experimental data. It can also be seen from the 2 and R2 values shown in Table 2 that the adsorption isotherm shown in Fig. 9 is described well by the Langmuir adsorption

isotherm. The Langmuir constant K L obtained from the fitting increases with increase in the pH of the

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aqueous phase. Similarly, the extraction capacity of U(VI) in Fe-DETA increases from 71.5 mg.g-1 to the value of 236 mg.g-1 with increase of pH of the aqueous phase from 3 to 6. The U(VI) extraction

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capacity observed in this case 236 mg.g-1, corresponds to the extraction of about 1 mmol.g-1 of U(VI). It should be noted that the amount of DETA functional groups present on Fe-DETA corresponds to 2

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mmol.g-1, as discussed in section 3.1. This shows that two molecules of DETA in Fe-DETA are coordinated to U(VI) ion resulting in the uranium to DETA stoichiometry of 1:2 in Fe-DETA.

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In order to confirm the formation of 1:2 complex of U(VI) to DETA in Fe-DETA, the variation in the distribution coefficient of U(VI) was studied as a function of DETA concentration in Fe-DETA. To obtain different concentrations of DETA in the adsorbent phase, the Fe-DETA (DETA = 2 mmol.g-1)

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was mixed with Fe-Si at different weight ratios, so that the concentration of DETA in the adsorbent phase was varied from 0.4 mmol.g-1 to 2 mmol.g-1. The extraction of U(VI) in Fe-Si was negligible

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under this condition. It can be seen from Fig. 10 that the distribution coefficient of U(VI) increases with increase in the concentration of DETA in the adsorbent phase. Linear regression analysis of the extraction data results in a slope of 2.1, indicating two molecules of DETA are involved in the extraction of uranium from aqueous phase, which is in good agreement with the 1:2 stoichiometry of U(VI) to DETA obtained from Langmuir adsorption isotherm. However, more studies are needed to understand the mechanism of U(VI) extraction in Fe-DETA phase.

16

Table 3 compares the Langmuir adsorption capacities of U(VI) reported in literature for different adsorbents. It can be seen that the U(VI) adsorption capacity observed in the present case (Fe-DETA) compares well with the other adsorbents reported in literature. It is also noted that the adsorbents with nanoparticle size shows the adsorption capacities marginally above 300 mg.g-1. Since the particle size of the present adsorbent varies from 0.1 μm to 100 μm, which is higher than the nano-particles, the uranium adsorption capacity is determined to be 236 mg.g-1 in Fe-DETA. If the size of Fe-DETA is reduced to nano size, it is quite likely that the degree of functionalization and the extraction capacity of U(VI) in the resultant nano-particles can be improved to a large extent. Cali et

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al, [35] reported the uranium adsorption capacity of 1690 mg per gram of the iron oxide present in the adsorbent ((PO)x-Fe3O4). It should be noted that the adsorbent contains both iron oxide and

phosphates (POx) and since the ratio of iron oxide to phosphate (POx) is not known, it is difficult to

compare the adsorption capacity determined for Fe-DETA with (PO)x-Fe3O4. Moreover, the core of

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Fe-DETA being magnetic, the nano-particles can be easily separated from the aqueous phase with the aid of an external magnetic field, as shown in Fig. 3S In this context, the magnetic particles like

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3.5. Recycling of magnetic adsorbent

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Fe-DETA offer advantages over the other non magnetic adsorbents reported in Table 3.

The utilization of the adsorbent is dependent on the recycling ability of Fe-DETA. To understand the recycling ability of the adsorbent, the uranium was initially loaded to the maximum

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capacity of 236 mg.g-1 in Fe-DETA at pH 6. Then the loaded uranium was quantitatively recovered using 0.1 M sodium carbonate solution (3 times). The recovered adsorbent was again contacted with

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the U(VI) solution at pH 6 to load the maximum amount of uranium in the recovered Fe-DETA. The procedure of loading and recovery of U(VI) from Fe-DETA phase was repeated five times. If the

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functional groups present on Fe-DETA undergoes any chemical degradation during this procedure, then it is quite likely that the extraction capacity could decrease upon recycling. However, it was found that the differences in the extraction capacity of U(VI) was negligible upon recycling the same adsorbent five times. The extraction capacity was found to decrease marginally from 236 mg.g-1 for the first time loading, to 230 mg.g-1 after recycling the same adsorbent five times. This shows that the functional groups present on Fe-DETA were not affected upon recycling.

17

To confirm the presence of functional groups in the recycled Fe-DETA, the FT-IR spectrum of Fe-DETA was compared with U(VI) loaded Fe-DETA and recycled Fe-DETA, in Fig. 11 It can be seen that there is a transmittance band occurring at 1384 cm -1 is due to the COO- stretching of acetate ion coordinated to U(VI) [66 - 68]. It should be noted that U(VI) was loaded from acetic acid sodium acetate buffer solution. Under these conditions, the U(VI) forms a strong complex with acetate ion, and therefore the U(VI) loaded Fe-DETA contains acetate stretching frequencies of the uranyl acetate complex. A similar behaviour was also reported by other researchers [66 - 68]. After the recovery of U(VI) from Fe-DETA phase using dilute sodium carbonate solution, the FT-IR spectrum shows the disappearance of the acetate ion transmittance bands at 1384 cm-1, but the other bands due to Si-O-

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Si stretching, C-H stretching and –N-H stretching are retained in the FT-IR spectrum of the recycled Fe-DETA.

The Raman spectrum of Fe-DETA loaded with U(VI) and recycled Fe-DETA was also

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recorded and the results are shown in Fig. 5. The Raman intensity observed at 924 cm-1 is due to the U=O stretching frequency of the uranyl ion (trace B in Fig. 5) [67, 68]. This indicates that U(VI) has

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been loaded in Fe-DETA phase during extraction. After the recovery of uranium from the loaded FeDETA phase, the Raman intensity associated with U=O stretching at 924 cm-1 disappears, but the

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other Raman intensities are retained in the recycled Fe-DETA (trace C in Fig. 5). The scanning electron microscopic image and the corresponding EDX pattern of the uranium loaded Fe-DETA and

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recycled Fe-DETA are shown in Fig. 6. The EDX pattern of uranium loaded Fe-DETA shows the presence of elemental uranium, in addition to C, O, N and Si, which forms the base structure of FeDETA. The recycled Fe-DETA obtained after the recovery of uranium shows the absence of extracted

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uranium in the adsorbent, indicating that the recovery of uranium is nearly quantitative.

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The characterization of Fe-DETA, uranium loaded Fe-DETA and recycled Fe-DETA described above, and the results on the uranium adsorption capacities in recycled Fe-DETA indicates that the functional groups present on Fe-DETA are not affected upon recycling Fe-DETA five times. Therefore, Fe-DETA can be regarded as a promising candidate for the separation of U(VI) from aqueous solution. Fe-DETA can be also recycled efficiently for further extraction of uranium from aqueous phase. However, more studies are needed to understand the interference of other elements

18

present in the actual feed (mining) solutions and converting this technique into an industrially viable technology. Conclusions The diethylenetriamine functional group was anchored on Fe3O4 and the product obtained after anchoring was subjected for the extraction of uranium from aqueous solution. The concentration of DETA functional groups present on the surface of iron oxide was determined to be 2 mmol.g -1 by thermogravimetric analysis. The X-Ray diffraction pattern of sample revealed that the core structure of the magnetic particles was made up of Fe3O4 and it was not affected upon functionalization of DETA

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molecules on Fe3O4. The FT-IR and Raman spectra of the product revealed the presence of DETA functional groups on Fe3O4. The rate of extraction of uranium in Fe-DETA was quick in the initial stages of equilibrium followed by the establishment of equilibrium occurring in 200 minutes of

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equilibration. The small size of Fe-DETA (average 10 μm) facilitated rapid extraction of uranium from aqueous phase and enhanced the accessibility of uranium to different adsorption sites, and providing

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the Langmuir adsorption capacity of 236 mg.g-1 at pH 6. The U(VI) in Fe-DETA was found to form a 1:2 complex of U(VI) with DETA, which was also supported by the uranium extraction data and

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Langmuir adsorption capacity. The loaded uranium from Fe-DETA was quantitatively recovered using dilute Na2CO3 and the Fe-DETA was recycled for further extraction of uranium without any compromise in the apparent extraction capacity. In view of these promising results, the Fe-DETA can

ur

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be regarded as a potential candidate for the separation of uranium from aqueous solution.

Author Contribution Statement

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P. Amesh: Methodology, Investigation, Draft preparation and editing A.S.Suneesh: Document preparation, Investigation, Validation B. Robert Selvan: Investigation, Resources K.A. Venkatesan: Conceptualization, Supervision, Writing and reviewing, Project administration Manish Chandra: Investigation, Resources

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Conflicts of interest There are no conflicts to declare Acknowledgements The authors thank Dr. S. Balakrishnan for TG-DTA analysis, Dr. Afijit Nair for recording XRD, Dr. Sachin Srivastava for recording Raman spectrum. Thanks to Mr. Eldo George, CMS College,

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na

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Kottayam for assisting the authors in some experiments.

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Mater. Interfaces. 9 (2017) 12511-12517. https://doi.org/10.1021/acsami.7b01711

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Figure captions

Fig.1: Synthetic scheme for the preparation of Fe-DETA

29

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Fig.2: Thermogravimetric analysis of Fe3O4, Fe-Si, and Fe-DETA.

30

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Fig.3: Comparison in the FT-IR spectrum of Fe3O4, Fe-Si, and Fe-DETA.

31

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Fig.4: Comparison in the Raman spectrum of Fe3O4, Fe-Si, and Fe-DETA.

32

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Fig.5: Comparison in the Raman spectrum of Fe-DETA, Uranium loaded Fe-DETA (Fe-DETA-U), and Recycled Fe-DETA.

33

ro of -p re lP na ur Jo Fig.6: SEM image and the respective EDX spectrum of Fe3O4, Fe-Si, Fe-DETA, Fe-DETA loaded with uranium (230 mg.g-1) (Fe-DETA-U), and Fe-DETA recycled samples.

34

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Fig.7: Variation in the distribution coefficient of U(VI) as a function of pH.

35

ro of -p re lP na ur Jo Fig.8: U(VI) loading at various intervals of time. Dotted line: pseudo-first order rate equation. Solid line: pseudo-second order rate equation.

36

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Fig.9: Loading of U(VI) on Fe-DETA as a function of U(VI) concentration in the aqueous phase.

37

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Fig.10: Variation in the distribution coefficient of U(VI) as a function of the concentration DETA in Fe-DETA at 298 K.

38

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Fig.11: Comparison in the FT-IR spectrum of Fe-DETA, uranium loaded Fe-DETA (Fe-DETA-U), and recycled Fe-DETA.

39

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Table 1: Variation in the distribution coefficient of U(VI) obtained on Fe-DETA, which was prepared by varying the reaction amounts between the intermediate and Fe-Si at different weight ratios.

to Fe-Si 5

5:1

1

10

10:1

1

15

15:1

31,950 44,300 43,942

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na l

Pr

1

Kd of U(VI) at pH = 6/ mL.g-1

e-

intermediate/ g

pr

Ratio of intermediate Fe-Si/ g

40

pH 3

oo

f

Table 2: The Langmuir and Freundlich parameters derived by the non-linear fitting of experimental data. pH 6

Parameter Freundlich

Temkin

DR

Langmuir

R2

0.9831

0.9342

0.9735

0.9343

0.9741

2

15.8

61.9

24.9

61.7

Constant*

9.0x 10-2

20.5

33.4

69.4

Capacity/ mg.g-1

71.5

-

96.2

Temkin

DR

0.8306

0.8720

0.9631

219.8

1440.5

1088.4

313.2

4.2x 10-1

79.3

11.6

214.4

-

311.7

-

e-

Pr -

Freundlich

pr

Langmuir

231.3

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*Constants for Langmuir, Freundlich, Temkin Dubinin Radushkevich and models are KL, KF, ATand respectively

41

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Table 3: List of popular solid-phase adsorbents for comparing in the adsorption capacities reported for the uranium extraction from the aqueous medium with Fe-DETA. Langmuir apparent adsorption capacity / mg.g-1

Size of the particle

Reference

Fe-DETA (Present work)

236 at pH 6

1 -100 m

-

Quaternary ammonium on silica-coated magnetic nanoparticles

e-

[29]

118 at pH ≤ 4

40 – 80 nm

[30]

87 at 10 ≥ pH ≤ 4

-

[31]

123.45 at pH 6.3

7 – 12 nm

[32]

na l

Polyamine/amide functionalized magnetic nanoparticles

165 at pH 5.5

Pr

Acetylcysteine-functionalized microporous conjugated polymers Amine functionalized magnetite silica nanoparticles

pr

Solid-phase adsorbent

46.2 at pH 4

-

[33]

Phosphate functionalized iron oxide nanoparticles

1690 at pH 7

12 nm

[35]

Amidic succinic acid moiety anchored silica gel

61 at pH 6

100 m

[44]

Diethylenetriamine anchored polystyrenedivinylbenzene copolymer

85 at pH 6

400 m

[55]

Rhodamine-B modified silica

35.8 at pH 5

150 - 200 m

[69]

Functionalized porous aromatic framework

300 at pH 6

-

[70]

Jo ur

Arsenazo-functionalized magnetic carbon composite

42