Efficient extraction of uranium from environmental samples using phosphoramide functionalized magnetic nanoparticles: Understanding adsorption and binding mechanisms

Journal Pre-proof Efficient extraction of uranium from environmental samples using phosphoramide functionalized magnetic nanoparticles: Understanding adsorption and binding mechanisms Pallavi Singhal, Bal Govind Vats, Ashok Yadav, Vandana Pulhani

PII:

S0304-3894(19)31307-X

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121353

Reference:

HAZMAT 121353

To appear in:

Journal of Hazardous Materials

Received Date:

26 April 2019

Revised Date:

28 August 2019

Accepted Date:

28 September 2019

Please cite this article as: { doi: https://doi.org/ This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier.

Efficient Extraction of Uranium from Environmental Samples using Phosphoramide Functionalized Magnetic Nanoparticles: Understanding Adsorption and Binding Mechanisms

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Pallavi Singhal±*, Bal Govind Vats#*, Ashok Yadav$ and Vandana Pulhani±

±

Environmental Monitoring and Assessment Division, Bhabha Atomic Research Centre,

Atomic & Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai 400085,

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$

Fuel chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India

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#

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Mumbai 400085, India

India

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*E-mail: [email protected], [email protected] Phone : +91-22-2559 2349, +91-22-2559 0643, Fax: +91-22-2550 5313, +91-22-2550 5151

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TOC

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  Highlights 

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 Phosphoramide functionalized Fe3O4 sorbent was synthesized for uranium extraction.   A maximum sorption capacity of 95.2 mg of U/g of sorbent has been achieved. 

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   >90% uranium extraction from tap water, drinking water and sea water was observed.   EXAFS studies suggests that uranium bind with oxygens of three P=O group.    Sorbent is low cost having high sorption capacity and requires less contact time.   

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Efficient Extraction of Uranium from Environmental Samples using Phosphoramide Functionalized Magnetic Nanoparticles: Understanding Adsorption and Binding Mechanisms

Abstract Phosphoramide functionalized Fe3O4 nanoparticles (NPs) were synthesized by a three step procedure and its application for uranium extraction from different enviornmental matrices has been demonstrated. A maximum adsorption capacity of 95.2 mg of U/g of the sorbent has been achieved which is higher as compared to many reported magnetic NPs. pH dependent adsorption studies were performed at 1 ppm uranium concentrations which suggests more than

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80% adsorption in pH range of 4-8 with maximum adsorption at pH 6. Interestingly this is the

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pH range of most naturally occurring water bodies suggesting the potential of this material to extract uranium from real environmental samples. Adsorption studies were carried out with tap

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water, drinking water and sea water and more than 90% uranium extraction was observed. Desorption studies were performed with different reagents suggesting that the material can be

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reused again. EXAFS studies have been carried out which suggests that the uranium binds with oxygens of three P=O group at the surface of phosphoramide functionalized NPs and based on

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this, binding mode of uranium with the synthesized sorbent is proposed.

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Keywords: Magnetic Nanoparticles, Uranium, EXAFS, Sea Water, Extraction.

1. Introduction Uranium is a well known nephrotoxic element with a maximum permissible

contamination level of 30 ppb in drinking water1. It called a need to find suitable materials for its separation if the contamination level is higher than the prescribed limit. In addition, the worldwide concern over the scarcity of fossil fuels and global warming has pushed a lot of

efforts towards the use of non-conventional sources of energy. Nuclear power is one of the most important sources among these2. Countries like France and Ukraine produce 76.3 and 56.5%, respectively, of their total energy requirement from nuclear energy only3. However, the sustainability of any nuclear energy programme depends upon the availability of nuclear fuel and uranium is one of the key elements for this purpose. The terrestrial ores of uranium account for about 6.3 million tons of uranium and will only last for less than 100 years at the current

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consumption rate considering that the fuel is not recycled4. Alternatively, seawater is the largest reservoir of uranium with a total global pool of ~4.3 billion tons, representing 99.9% of

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uranium inventory on earth5. Such a large amount of uranium can support nuclear power production at the current capacity for nearly 72,000 years6. However, a paramount challenge

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in this is extraction at extremely low concentrations (~3.3 ppb uranium in sea water) and the

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presence of high concentration of coexisting ions5,7. This causes a serious limitation on uranium extraction from seawater and a lot of work is going on to overcome these limitations1,6-25.

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Moreover, by different activities employing uranium, there are chances of contamination and considering its chemical toxicity26,27, it is important to remove it from different environmental

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matrices.

Over the past years, a lot of research is going on to extract uranium from different environmental matrices and various methods like solvent extraction28, ion exchange29,30, and adsorption1,6-10 have been explored for the same. Recently, much attention has been given to

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adsorption methods because of their simplicity, low cost and wide adaptability31. While considerable progress has been achieved, the current state-of-the-art adsorbents suffer from

severe deficiencies such as appropriate species selectivity8, large contact time9,10,12,13, high cost, low adsorption capacity, tedious material synthesis29, difficult separation from the matrix etc. For example, traditional sorbents such as clay minerals and oxides are low cost, and non-toxic but suffer from low adsorption capacities. Although, nanomaterials, such as carbon-based

composites, metal oxides and nanocarbon exhibit excellent adsorption capacity32-35, their high dispersibility in aqueous solutions makes separation from the matrix tedious which restricts the application in real water bodies32,33. Nowadays the most popular sorbents for uranium extraction is amidoxime-based materials7,16,36,37 and adsorption upto 3.3 mg U/g of sorbent has been achieved in marine test using these ligands36 but the method has several shortcomings such as high cost, large equilibration time (>24 hrs), and high adsorption of other seawater

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cations (e.g., vanadium)36. Such limitations prompt further research in this direction that can lead to feasible technologies for extracting uranium.

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In the present work, phosphoramide functionalized Fe3O4 nanoparticles (NPs) were synthesized and their application in extracting uranium from different environmental matrices

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has been demonstrated. Here the use of Fe3O4 NPs as sorbent serves three main purposes (1)

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its magnetic character enables the fast separation of the sorbent from the matrix which is important in a large volume sample, (2) the material is non-toxic considering its use in

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environmental samples8,37, and (3) is low cost hence large scale synthesis is economical. Few studies have been carried out earlier using Fe3O4 based NPs as a sorbent for uranium

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extraction20-22,39,40. Zhao et. al.16 have used amidoxime-functionalized Fe3O4@SiO2 magnetic microspheres for adsorption of uranium and obtained a maximum adsorption capacity of 104 mg of U/g of sorbent. However, the experiment was performed at pH 5 which does not represent the pH of seawater (7.8-8.2). Rezaei et. al.41 have used salicylaldehyde modified

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Fe3O4@SiO2 NPs for uranium extraction. The experiments were performed under ambient condition but the adsorption capacity was less (~49 mg of U/g of sorbent). Li et. al.6 have used organic ligand functionalized magnetic mesoporous silica NPs for uranium separation and

observed an adsorption capacity of 54 mg of U/g of sorbent. Our group has demonstrated the application of humic acid coated Fe3O4 NPs for extraction of uranium from seawater, though the sorbent was not selective8. These studies show that a remarkable progress has been made

in separation of uranium from environment; however, further improvements are needed. Moreover, in many cases, the binding of the sorbent with uranium is not known which is important to design the better sorbents. Phosphoramide functional group has a highly basic oxygen atom and recently it was proposed as a very high and selective group for uranyl ion extraction from water42. Ouadi et al.42 shown that phosphoramide containing ionic liquid enhance the extraction of uranyl ion significantly. Kong et. al.43 describe that phosphoramide

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functionalized mesoporous SBA-15 have shown very promising adsorption of uranium due to the presence of phosphoramide functionality. Recently, Zhuo et.al.44 have shown that

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phosphoramidate functionalized ionic liquid is superior extractant for uranyl due to the synergistic effect of coordination and hydrogen bonding. But there are no reports available in

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literature for functionalization of magnetic NPs with this group. In our present study, we work

2. Experimental 

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from real environmental samples.

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on these limitations by using phosphoramide functionalized Fe3O4 NPs for uranium extraction

All chemicals were of analytical reagent grade and purchased from Aldrich and used as

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such; Milli-Q water was used throughout the sample preparation. Phosphoramide functionalized Fe3O4 NPs were synthesized via a three step procedure as shown in scheme 1. First Fe3O4 NPs were synthesized by co-precipitation route as reported earlier8. These NPs were used to synthesize dopamine functionalized45 Fe3O4 NPs. For this,

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200 mg of Fe3O4 NPs were taken in 10 mL water and 500 mg of dopamine in 40 mL water was

added to it under Ar flow at 60 °C. The mixture was stirred for 4 hrs followed by cooling. The material was separated using a magnet, washed with water and acetone and dried. Phophoramide functionalized Fe3O4 NPs were synthesized from dopamine functionalized NPs. 200 mg suspension of dopamine functionalized Fe3O4 in 10 mL toluene was taken and 140 μL of triethylamine was added. The solution was stirred for 15 minutes followed by addition of

200 μL of diphenylphosphinic chloride dropwise and again stirred for 3 hours. After the reaction, the NPs were separated by a magnet. The obtained solid is washed with water and ethanol three times each, respectively. The final solid residue is dried to obtain the phosphoramide functionalized Fe3O4 NPs and were characterized by different characterization techniques such as X-ray diffraction (XRD), Transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric (TG) analysis, Brunauer–

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Emmett–Teller (BET), Dynamic light scattering (DLS) and, Zeta potential (Supporting Information; SI). The elemental analysis was performed by CHNS analyzer and inductively

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coupled plasma - optical emission spectrometry (ICP-OES) studies.

Scheme 1: Synthesis procedure for phosphoramide functionalized Fe3O4 NPs For ICP-OES analysis 1 mg of sample was dissolved in 600 μL of HCl and was further diluted with 1% HNO3. Adsorption studies were performed by adding NPs (10 mg) to uranium solutions of different concentrations (10 mL) followed by sonication (Bath sonication, 100 W, 40 kHZ,

25 °C and 75 min). Sorbent and solution were separated with the a magnet and the concentration of uranium in supernatant was analyzed using laser fluorimetry (LF 003 uranium analyser fabricated by Laser Applications and Electronics Division, RRCAT, Department of Atomic Energy, Indore, India). For pH dependent studies, initially solutions of different pH were prepared by addition of dil. HNO3 and dil. NaOH and then NPs were added to it followed by sonication. The sorbent and solution was separated and the concentration of uranium in

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supernatant was analyzed using LF.

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To understand the binding mode of uranium with the sorbent, X-ray absorption Spectroscopy (XAS) measurement, which comprises of both X-ray near edge structure

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(XANES) and extended X-ray absorption fine structure (EXAFS) techniques, have been carried out at Fe K and U L3 edges. For the EXAFS measurement, sample of NPs with adsorbed

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uranium was collected by centrifugation and dried. The XAS measurements have been carried out at the Energy-Scanning EXAFS beamline (BL-9) at the Indus-2 Synchrotron Source (2.5

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GeV, 100 mA) at Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India. The detail of the instrumental set up is given in the SI.

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3. Results and Discussion

3.1. Characterization of Phosphoramide functionalized Fe3O4 NPs XRD pattern of phosphoramide functionalized Fe3O4 NPs is shown in Figure 1A. The

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synthesized NPs have a cubic structure with calculated lattice parameter a = 8.3489 Å which matches well with the reported lattice parameter for Fe3O4 (8.397 Å JCPDS No.- 19-629). The

average crystallite size of NPs was found to be ~15 nm after correcting with silicon as a standard (Figure S1). FTIR studies have been carried out to confirm the coating of dopamine and phosphoramide on NPs. FTIR spectra of dopamine functionalized Fe3O4 and phosphoramide functionalized Fe3O4 are shown in Figure 1B. As it is clearly visible from the

FTIR spectra that a strong peak at 1130 cm-1 appears in phosphoramide functionalized Fe3O4

which is a P=O signature peak in phosphine oxide compounds. This indicates the functionalization of dopamine coated Fe3O4 with P=O functionality46. TG analysis has been carried out in synthesized NPs and is shown in Figure 1C. TG analysis reveals that in phosphoramide functionalized Fe3O4 NPs an appreciable weight loss occur at ~ 230 -650 °C. This indicates a successful coating of NPs with organic moiety. CHNS analysis suggests that the C, H, N and S content in the synthesized NPs is 8, 0.4, 0.5 and 0% respectively. From ICP-

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OES analysis the Fe and P content was found to be 62 and 1% respectively. This further confirms the presence of C, H, P in the synthesized NPs. BET analysis has been carried out to

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determine surface area. BET analysis shows that the surface area of phosphoramide functionalized Fe3O4 NPs was 29.11 m2/g. TEM images of NPs were taken to find out the size

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of NPs. Figure 1D shows the TEM image of Fe3O4 NPs where particles were found to be

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spherical and separated. The particle size was observed to be ~13 nm±5 nm and the size distribution is shown in Figure S2. High resolution TEM (HRTEM) image of NPs shows the

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crystalline nature of NPs (Figure 1E). Interestingly TEM image of NPs after phosphoramide functionalization is shown in Figure 1F and suggest that the particles are agglomerated which

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is due to the phophoramide functionality on NPs surface which allows particle-particle interaction. This was further validated by the DLS measurements. DLS study of uncoated and phosphoramide coated NPs suggests that the average hydrodynamic diameter of the particles are 116 nm and 420 nm with polydispersity index of 0.05 and 0.17 respectively (Figure S3).

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This further confirms that after phosphoramide functionalization particles come closer and agglomerate.

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Figure 1A: Powder XRD pattern of phosphoramide functionalized Fe3O4 NPs (Blue) and cubic

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Fe3O4 JCPDS No. 19-629 (Red). 1B: FTIR spectra of Dopamine (Red) and phosphoramide (Blue) functionalized Fe3O4 NPs. 1C: TG curve of phosphoramide functionalized Fe3O4 NPs.

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1D: TEM image of Fe3O4 NPs. 1E: HRTEM image of Fe3O4 NPs. 1F: TEM image of phosphoramide functionalized Fe3O4 NPs.

3.2.1.

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3.2. Adsorption Studies Effect of pH

Due to the existence of oxygen and nitrogen-containing functionalities (scheme 1),

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phosphoramide functionalized Fe3O4 NPs has a potential for capturing uranium from aqueous solutions. First pH dependent adsorption studies have been carried out to determine the

working pH range of the synthesized sorbent and are shown in Figure 2. pH dependent adsorption studies suggests that the material adsorbs uranium in pH range of 3 -11,

80 60 40 20 0 2

4

6

pH

8

10

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% Uranium Sorption

100

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Figure 2: Effect of pH on uranium adsorption efficiency at a contact time of 60 min. pH was

ratio of volume of solution to the sorbent amount).

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adjusted using dil. HNO3 and NaOH. [UO22+] = 1 ppm, V/m = 1 mL /mg (where V/m is the

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however, more than 80% adsorption was observed from pH 4 to pH 8. Interestingly, this is the pH range of most of the naturally occurring water bodies suggesting the appreciable potential

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of this material for uranium extraction from natural water bodies. The maximum adsorption was observed at pH 6. Such pH dependent adsorption behaviour has been observed earlier

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also15,16 and can be correlated to the different distribution of uranium species and charge on NPs surface at different pH. Uranium speciation at 1 ppm concentration has been calculated and is shown in Figure S4. It is clear from Figure S4 that uranium speciation is pH dependent and different species forms at different pH47. Zeta potential studies have been carried out to

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determine the surface charge and are shown in Figure S5. Zeta potential studies suggest that the surface charge of NPs is positive till pH 2 and then becomes negative. Hence at low pH value, pH 1 or 2, the positive charge surface does not interact with UO22+ ion which is the major

existing species in pH range 1-2 (Figure S4). At pH > 2, the other species of uranium such as [(UO2)(OH)]+, (UO2)(OH)2, [(UO2)2(OH)2]2+, [(UO2)3(OH)5]+, exist and higher adsorption can be correlated to the interaction of these species with the negatively charged surface. Adsorption

again decreases at pH > 8 since at higher pH, the negatively charged species of uranium; UO2(CO3)22-, UO2(CO3)34- forms and the interaction of these species with the negatively charged NPs surface is poor. Effect of contact time

3.2.2.

To attain the maximum adsorption capacity of the sorbent, it is important to determine the equilibration time between the sorbate and the sorbent. Uranium adsorption as a function

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of contact time has been studied and shown in Figure 3. The result shows that the uranium

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adsorption increases rapidly and the relative amount of uranium sorbed has reached more than 90% after 10 min of interaction and reach ~98% after 75 min (Figure 3). The results were

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analyzed by fitting the data into three kinetic models namely pseudo-first-order, pseudosecond-order and intra-particle models11 (Figure S6, Figure 3B) and the values of

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corresponding kinetic parameters calculated from these models are given in table 1. It is evident

is described by equation 111.  

 

 

 

 



    

 

 

 

 

(1)

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from table 1 that the best fit of data has been achieved by pseudo-second-order kinetics which

where k2 is the pseudo-second-order rate constant of adsorption (g mg−1 min−1), qe and qt are the amounts of metal ion adsorbed (mg/g) at equilibrium and at time t, respectively, and t is adsorption time (min). The plots of t/qt vs t of the kinetics data showed a perfect linear relation

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(Figure 3B) with qe =0.98 mg g-1 and K2 = 0.78 g mg-1 min-1. Interestingly, the theoretically calculated qe values match well with the experimental ones (qe =0.95 mg/g; described later in the results) further confirming the applicability of the pseudo-second-order model. Best fitting of the data into pseudo-second-order kinetics suggests that the adsorption mechanism of uranium onto NPs is chemisorption11, 14. To validate it further and to ensure that the interaction mechanism is chemisorption, a separate experiment was performed. In

t/qt (min gmg-1)

% Uranium Sorption

120

A

100

95

90

R2= 0.9936

B

80

40

0

0

20

40

60

80

100

0

120

20

60

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100

120

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Time (min)

40

Time (min)

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Figure 3A: Adsorption of uranium as a function of contact time. [UO22+] = 1 ppm, V/m = 1 mL/mg. 3B: Plot of t/qt vs t of the current kinetics data which is well fitted with the pseudo-

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second-order kinetic model.

Table 1: Fitting parameters of pseudo-first-order, pseudo-second-order and intra-particle

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diffusion kinetic models.

Fitting parameters at 25°C, pH 6

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Kinetic Models Pseudo-first-order model

R2 = 0.91251

K1 = 4.5×10-4 mg g-1 min-1

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Pseudo-second-order model

K2 = 0.78 g mg-1 min-1 qe= 0.98 mg/g R2 = 0.95661 K1 = 0.01197 mg g-1 min-1

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Intra-particle diffusion

R2 = 0.9936

this experiment, phosphoramide functionalized Fe3O4 NPs sorbed with uranium were taken and Mill-Q water was added to these NPs. The solution was sonicated for 1 hr followed by separation using a magnet. The concentration of uranium in water was determined and found to be ~100 ppb while the NPs were sorbed with 30 ppm of uranium (V/m = 1 mL/mg). This suggests that even though the samples were sonicated; no appreciable loss of uranium from

sorbent was observed, further confirming that the interaction between Fe3O4 NPs and uranium is strong and both are bound chemically. 3.2.3.

Adsorption isotherm study Adsorption experiment allows identifying the mechanism of interactions between the

sorbates and sorbents and determining the sorption capacity of the process. To determine the maximum adsorption capacity of NPs and mechanism of interaction, adsorption experiments

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were performed by adding a different concentration of uranium solutions to the NPs. The mass

experimental data using equation 2  

 

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of uranium sorbed onto the NPs surface (qe, mg U/g) was calculated based on batch

 

(2)

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where C0 (mg/L) is the initial uranium concentration, Ce (mg/L) is uranium concentration in the solution at equilibrium, V is the volume of the solution (L) and m is the mass of the sorbent

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(g). In the present experiment, V/m is kept as 1 mL /mg. The result of the sorption experiment is shown in Figure 4A. The results were fitted into different adsorption models namely

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Langmuir, Freundlich, and Temkin isotherm11, 14 and are shown in Figure 4B, 4C and 4D respectively. The fitting parameters of these isotherms are given in table 2. It is interesting to observe from Figure 4 and Table 2 that the results are best fitted in Langmuir isotherm model which is one of the most common isotherm models suggesting that all sites are equal and

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uniform and the uptake occurs on a homogeneous surface by monolayer adsorption without interacting with neighbouring adsorbed molecules11, 14. The maximum adsorption capacity was evaluated to be 95.2 mg of U/g of sorbent and is comparatively high as compared to many other reported sorbents. Adsorption studies were also carried out in sea water by spiking it with different concentration of uranium. The results are shown in Figure S7 and were fitted with different s

adsorption models. The best fit was obtained with Frendlich isotherm which suggests that the adsorption mechanism is different in both Mill-Q water and sea water and multilayer adsorption process is favourable in sea water. This might be due to the different species formation in sea water as compared to Milli-Q water. As reported the major species of uranium in Milli-Q water at pH-6-7 are8  (UO2)3(OH)5+, (UO2)4(OH)7+, (UO2)2CO3(OH)3-, UO2(OH)2, (UO2)2(OH)22+, (UO2)3(OH)82-. However the species in sea water9 is [Ca2 UO2(CO3)3](aq). Since different

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species of uranium exist in both the waters, the mechanism of adsorption is different.

A

Ce/qe

2.5 2.0

B

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qe

40

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3.0

60

20

1.5

50

100

150

200

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Ce

C

1.2

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log qe

1.6

0.8 0.4 0.0

0.0

0.5

1.0

log Ce

1.5

2.0

0

50

Ce 100

150

200

60

D

40

qe

0

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1.0

0

20 0 0.0

0.5

1.0

1.5

log Ce

2.0

2.5

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Figure 4A: Adsorption of uranium on phosphoramide functionalized Fe3O4 NPs surface. Fitting of adsorption data into 4B: Langmuir isotherm, 4C: Freundlich isotherm and 4D: Temkin isotherm. Experimental conditions were, pH 6, contact time 75 min and V/m = 1 mL /mg. Table 2: Fitting parameters of uranium adsorption on phophoramide functionalized Fe3O4 NPs

into various Isotherms. qmax is the maximum adsorption capacity (mg/g), KL is the Langmuir

equilibrium constant (L/mg), KF and n are the Freundlich constants related to the adsorption capacity and the adsorption intensity, respectively, KT denotes the Temkin isotherm equilibrium binding constant (dm3/g), bT is the Temkin isotherm constant related to the heat of adsorption (J/mol). Fitting parameters at 25°C, pH 6, V/m = 1 mg/ mL

Langmuir isotherm

R2 = 0.9944, qmax= 95.2 mg g-1, KL = 11.03 L g-1

Freundlich isotherm

R2 = 0.9838, KF = 1.34 L g-1, n = 1.25

Temkin isotherm

R2 = 0.8358, bT = 85 J/mol, KT = 0.377 L g-1

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Adsorption Models

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3.2.4. Comparison between the synthesized and reported sorbent

As noted from the adsorption isotherm that a maximum adsorption capacity of 95.2 mg

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of U/g of the sorbent has been achieved. A number of extractants have been reported earlier for uranium extraction from different environmental matrices8-22,48-51. Table 3 summarizes few

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of these sorbents with their maximum adsorption capacity and equilibration time8-26, 48-51. It is interesting to note that the adsorption capacity of the synthesized material is comparatively higher as compared to many of the sorbents mentioned in table 3. Also, for few sorbents based on magnetic NPs such as polymeric-magnetite cryobead, amidoxime modified Fe3O4@SiO216,

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magnetic oxime20, Fe3O4@C@Ni−Al LDH21, Fe3O4@SiO2 nanospheres functionalized with N/O-containing

group48,

amino/amine-functionalized

magnetic

mesoporous

silica

nanosphere49 and, Magnetic graphitic carbon nitride nanosheets50 a high adsorption capacity

was observed but involves a large equilibration time which makes the process slow and tedious. Carbon nanotube coated magnetic NPs show exceptional high adsorption capacity, ~250 mg/g of sorbent with an equilibration time of 40 min. However the synthesis requires a number of

organic solvents51. For other non magnetic sorbents such as silica gel, aminosilica gel, benzoimidazol-2yl-phenylphosphinic acid/aminosilica, amine-functionalized metal−organic framework,

layered

organic−inorganic

hybrid

thiostannate,

melanin-functionalized

agarose−chitosan, serine sorbent, imidazol-2-yl-phosphonic acid/silica a high adsorption capacity was observed but separation from large volume sample is not economical. Here we would like to mention that the less equilibration time has advantage of fast processing which

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is essential in large batch of samples and at industrial scale. In the present work, the synthesized sorbent; phosphoramide functionalized Fe3O4 NPs has the properties of high adsorption

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capacity, less equilibration time and easy separation from the matrix suggesting their effective

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use in large volume samples.

Table 3: Comparison of the uranium loading capacity (qmax) and the equilibration time of the

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studied adsorbent with the other adsorbents

qmax (mg/g)

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Sorbent

Equilibration Time

Ref

95.2

75 min

This work

Fe3O4/humic acid

39.4

1h

8

PDA-functionalized magnetic NPs, mesoporous carbon, glass fiber carpet

44.5

168 h

9

resin Tulsion CH-96

70

24 h

10

magnetic Schiff base

94.3

6h

11

polymeric-magnetite cryobead

97

24 h

12

silica gel

144

1h

14

aminosilica gel

166

1h

14

benzoimidazol-2-yl-phenylphosphinic acid/aminosilica

176

1h

14

amine-functionalized metal−organic framework

200

10 h

13

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Phosphoramide functionalized Fe3O4 NPs

338.43

24 h

10

melanin-functionalized agarose−chitosan

435

3h

17

polyacrylic acid hydrogels

445.11

2h

19

imidazol-2yl-phosphonic acid/silica

618

2h

18

Quercetin modified Fe3O4 NPs

12.3

<30 min

15

Amidoxime modified Fe3O4@SiO2

105

24 h

16

Magnetic oxine

125

4h

20

Fe3O4@C@Ni−Al LDH

227

3h

21

Alanine sorbent

88

Serine Sorbent

122

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layered organic−inorganic hybrid thiostannate

22

~0.8 h

22

139.1

3h

48

amino/amine-functionalized magnetic mesoporous silica nanosphere

153.68

3h

49

Magnetic graphitic carbon nitride nanosheets

572.78

3h

50

250

40 min

51

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Fe3O4@SiO2 nanospheres functionalized with N/O-containing group

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~0.8 h

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Carbon nano tube coated magnetic NPs

3.2.5. Analysis in real environmental samples and cyclability To check the feasibility of the material in real samples, adsorption experiments were

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performed with tap water, drinking water and sea water all spiked with 1 ppm uranium and the concentration of uranium in solution was determined after sorption. The water quality parameters of these water systems are given in Table S1. The experiments were performed as such without any pH adjustment to ensure the real environmental conditions. The samples were sonicated for 75 min with V/m = 1 mL/mg (10 mL of sample).The results are shown in Figure 5A. It is interesting to observe from Figure 5A that more than 90% of uranium was adsorbed

on sorbent suggesting the potential of this material to extract uranium from seawater and decontaminate drinking water. We have also carried out experiment with larger quantity of sea water; 20 mL and 50 mL by keeping V/m = 1 mL /mg. For this purpose 1 ppm of uranium was spiked in sea water samples. Result shows ~85% uranium adsorption. The experiment was also performed in simulated sea water spiked with 3.3 ppb of uranium to check whether the NPs can sorb uranium at such low concentrations. Result shows <1 ppb of uranium concentration

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in solution after adsorption confirming that the compound can adsorb uranium from sea water at such low concentration also.

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We have also checked adsorption capacity of the material at different cycles without

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uranium desorption to make it more simple and economical. The experiments were performed with 1 ppm uranium solution with V/m = 1 mL /mg (1 mL uranium solution). The result of the

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experiment is shown in Figure 5B. It is interesting to observe from Figure 5B that upto 6 cycles, the adsorption capacity was more than 90% after which it has declined to ~80%. It suggests

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that depending on the extraction efficiency needed, initial uranium concentration in solution

100

20

0

Sea Water

40

Drinking Water

60

Tap Water

80

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% U Sorption

100

% U Adsorption

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and economics of the process, the material can be used without any uranium desorption.

80 60 40 20 0

1

2

3

4

5

6

7

8

9 10

Number of Cycles

Figure 5A: Adsorption studies of uranium in drinking water, tap water and sea water on phosphoramide functionalized Fe3O4 NPs. 5B: Adsorption of uranium at different cycles on

phosphoramide functionalized Fe3O4 NPs. Experimental conditions were contact time 75 min and V/m = 1 mL /mg and initial uranium concentration = 1ppm. 3.2.6. Selectivity Analysis Selectivity of the sorbent for uranium was tested in presence of different ions such as Fe2+, Mn2+, Sr2+, Zn2+, Pb2+ and Na+. The experiment was performed by adding 10 ppm

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concentration of all ions and the concentration of these ions after adsorption in supernatant was analyzed by ICPMS. These results are shown in Figure 6. It is interesting to observe that the

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material is selective for most of the ions mentioned except for Pb2+. This suggests that the

Spiked Concentration Concentration in Solution

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10

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8 6 4 2

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Concentration (ppm)

12

0

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material is reasonably selective and can be used in a contaminated system.

2+ Fe

2+ Mn

2+ Sr

2+ Zn

+ Na

2+ Pb

2+ UO2

Figure 6: Concentration of different ions in solution before and after adsorption. Experimental

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conditions were: contact time 75 min and V/m = 1 mL /mg. 3.2.7. Regeneration studies Desorption study was also carried out to check the reusability of the material. For

desorption study, EDTA (Ethylene diamine tetraacetic acid), NaHCO3, Na2CO3 and HNO3 solution was selected. 10 mM; 1mL solution of each of the reagent was added to 1 mg of nanoparticles having 28 g of sorbed uranium. The mixture is sonicated for 1 hr and then the

NPs are separated from solution and uranium concentration in the solution was measured. It was observed that EDTA, NaHCO3, Na2CO3 and HNO3 desorb 89, 85, 86 and 83% of uranium respectively. This suggests that the material can be reused after adsorption and the extracted uranium can be used for various applications. 3.2.8. Binding mechanism: An EXAFS study As the material shows a high binding affinity for uranium; EXAFS study has been carried

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out to understand the binding of uranium with the sorbent. Results of the k3-weighted EXAFS

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fitting at Fe K-edge (Figure S8) shows that the phosphoramide functionalized Fe3O4 NPs retain its phase after sorption of uranium. EXAFS study clearly shows that iron oxide form inverse

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spinal structure with octahedral and tetrahedral coordination and bond lengths are in agreement to the literature values (Fe – O (1.94 Å) for tetrahedral coordination and Fe – O (1.77 Å) for

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octahedral coordination) (Table S2)52. The uranium L3-edge EXAFS studies in liquid (before

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sorption) and at phosphoramide functionalized Fe3O4 NPs (after adsorption) has been carried out and the fitting results are summarized in Table S3. The best fit was obtained with a structural model having three coordination shell of oxygen in both the samples. In liquid, the

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two axial oxygen (Oax) at a distance of 1.820.01 Å comprises of the first shell while two equatorial oxygen (Oeq) at a distance of 2.300.01 Å forms the second shell. In the third shell, there are three oxygens (Oeq) at a distance of 2.49±0.01 Å. After sorption at solid, the fitting

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parameter shows two axial oxygens (Oax) at 1.790.01 Å, two equatorial oxygens at (Oeq.) at 2.320.01 Å and three equatorial oxygens at (Oeq.) 2.42±0.02 Å. The fourth shell i.e. second

coordination sphere is also fitted. In the liquid it shows three more oxygens atom at a distance of 3.490.04 Å. These may be a result of hydrogen bonding interactions with coordinated water molecules in liquid. In solid, the fourth shell comprises of three phosphorus atom at a distance of 3.92±0.02 Å. This shows that at the solid surface the three oxygens which are bonded to

uranium (VI) are from phosphine oxide functional group present at the surface. The fitted spectra are shown in Figure 7. From the EXAFS studies, it can be inferred that the majority of uranium (VI) present in the solution at 1 ppm concentration exists as UO2(H2O)n Lm (where L can be OH or CO32-). This inference is in synchronization with the observation of speciation diagram of uranium(VI) at 1 ppm concentration (Figure S4), where the major mononuclear species are UO2(OH)+, UO2CO32- and UO2(CO3)22- at pH 6-8. The parameters obtained from

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the fitting of EXAFS spectra of solid U@Fe3O4 suggest that the P=O group on the surface of Fe3O4 binds to the uranyl center by replacing water molecule making the complex

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UO2Lm.3(P=O) (where P=O is surface functional groups and L can be OH or CO3). The

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EXAFS) 2.42 Å and P=O bond length (1.50 Å)53.

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obtained U-P distance of 3.92 Å agrees well with the sum of U-O bond distance (obtained from

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Figure 7: Fourier transformed EXAFS spectra of (A) liquid and (B) solid at U L3 edge. The experimental spectra are represented by scatter points and the theoretical fit is represented by a solid line. 4.

Conclusions

The study report synthesis of phosphoramide functionalized Fe3O4 NPs via a three step route which is economical and simple. The maximum adsorption capacity of the sorbent was evaluated and is found to be 95.2 mg U/g of sorbent. pH dependent adsorption experiments were performed which suggests that the working pH range of the sorbent is 4-8 with maximum adsorption at pH 6. The material was tested for real environmental samples namely; seawater, tap water, and drinking water and adsorption >90% was achieved. This suggests the huge

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potential of this material for uranium extraction from real environmental samples. EXAFS studies have been carried out which suggests that the uranium bind with the oxygens of three

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P=O group in phosphoramide functionalized NPs. A number of sorbents are reported in literature but here high adsorption capacity, less equilibration time, low cost, non toxicity of

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the material, and easy separation from matrix adds a lot of advantages when the process is

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visualized in an industrial scale where the above mentioned processes has always an advantage

ASSOCIATED CONTENT

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Supporting Information

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as compared to the conventional processes and are always favourable.

Details about the characterization, analytical techniques used as well as Figure S1, S2, S3 S4, S5, S6, S7, S8 and Table S1, S2, S3 are given in supporting information. AUTHOR INFORMATION

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Corresponding Author

[email protected] (Pallavi Singhal), Phone: +91-22-2559 2349; [email protected] (BalGovind Vats), Phone: +91-22-2559 0643. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS PS and VP would like to acknowledge Dr. A Vinodkumar, and Dr. K.S. Pradeepkumar, HS&EG, BARC for their support. BGV would like to acknowledge Dr. S. Kannan, FCD BARC for his support. We acknowledge Shri. T.V.V. Rao , FCD, BARC for recording the BET isotherms, Prof. H. N. Ghosh, Institute of Nanoscience and Technology for providing TEM

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measurements and Dr. P. Sawant for providing uranium analysis facility.

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