Synthesis of Tributyl Phosphate-Coated Hydroxyapatite for Selective

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Synthesis of tributyl phosphate-coated hydroxyapatite for selective uranium removal Hyunju Kim, wooyong um, Won-Seok Kim, and Seeun Chang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04491 • Publication Date (Web): 05 Mar 2017 Downloaded from http://pubs.acs.org on March 5, 2017

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Industrial & Engineering Chemistry Research

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Synthesis of tributyl phosphate-coated

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hydroxyapatite for selective uranium removal

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HyunJu Kima, Wooyong Uma,

4

Changa

b,*

, Won-Seok Kima, and Seeun

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a

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(POSTECH), 77 Chongam-ro, Nam-Gu, Pohang 790-784, Republic of Korea

Division of Advanced Nuclear Engineering, Pohang University of Science and Technology

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b

Energy and Environment Directorate, Pacific Northwest National Laboratory (PNNL), 902

Battelle Blvd., MSIN P7-54, Richland, WA 99354, USA

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*Corresponding Author. Tel.: +1-509-372-6227; Fax.: +1-509-376-1638

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E-mail addresses: [email protected]; [email protected]

14 15

16

1

ACS Paragon Plus Environment


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Abstract

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Efficient and rapid removal of radioactive contaminants is crucial when they are released

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into the environment following nuclear accidents. Here, different types of apatite were

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synthesized using tributyl phosphate (TBP) and tested for uranium removal from various

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solutions using different reaction times and uranium concentrations. The uranium adsorption

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results showed that uranium adsorption reached steady state within 24 h in tests open to

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atmosphere at a slightly alkaline pH in different background solutions. TBP-coated

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hydroxyapatite showed better U removal compared to hydroxyapatite, itself. The U removal

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mechanism was considered as multilayer adsorption, showing the best fit to the Freundlich

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isotherm. The maximum U adsorption capacity determined from the Langmuir isotherm is 38

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mg-U·g−1. Because of high U removal efficiency, even at high pH condition in this study,

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TBP-coated hydroxyapatite appears to be a promising adsorbent for U removal from various

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waste streams as well as for U recovery from seawater.

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1.

Introduction

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Owing to depleting natural resources and the need for eco-friendly energy due to

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climate change, the competition for energy resources has been receiving considerable

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attention worldwide.1 Many countries have highlighted the importance of nuclear energy in

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meeting energy needs. In countries that import petroleum as a source of energy, nuclear

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power has contributed significantly to economic growth. However, because the toxic effects

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of uranium (U) contamination pose a significant risk to the environment, many studies are

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still under way for developing technologies to safely manage the U waste generated by

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nuclear fuel production and spent nuclear fuel reprocessing.2-5 Reprocessing and/or selective

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separation technologies are being promoted to increase the safe disposal of spent nuclear fuel

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and the removal of radionuclides during radioactive waste decontamination. An innovative

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technology for the treatment and removal of radionuclides is also needed for radioactive

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waste remediation after nuclear accidents.

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Recent environmental issues6-8 related to fossil fuels and the rapid industrial

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development, which is driving the demand for energy, are increasing the consumption of U as

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an energy resource.9 However, currently available U resources are not commensurate with the

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demand, and a flexible and independent supply of U to fuel nuclear power is needed. One

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source of U is seawater, but the concentration of U in seawater is very low (~3.3 ppb).3

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Nevertheless, seawater represents an essentially inexhaustible and eco-friendly source of U; it

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contains an estimated total U content of 4.5 billion tons—1,000 times more than the

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remaining known terrestrial deposits of U. If half of the seawater U source could be

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recovered, it could provide for about 6,500 years of 3,000 GW of nuclear capacity (with 75%

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capacity factor) based on high temperature gas cooled reactors fuel consumption.10 Due to the 3



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oxidizing environment of the Earth’s oceans, U exists as hexavalent U(VI) oxidation state

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species. The predominant anionic species of U in seawater is [UO2(CO3)3]4− at pH levels of

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approximately 7–8.11 However, because the U concentration in seawater is very low

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compared to others major elements such as sodium and chlorine, the extraction or recovery of

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U from seawater is very difficult and expensive. For economic reasons, a highly selective

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adsorbent that can be used to recover U from the seawater would be very valuable.

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Recent studies12-14 indicate a trend of developing adsorbent materials that are

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increasingly efficient, selective, and economically inexpensive. Building upon this trend to

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address the problem of U extraction from seawater and secure independent technology for U

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remediation from various radioactive waste streams, this study aims to develop a new

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efficient adsorbent for the selective removal of U using tributyl phosphate (TBP)-coated

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hydroxyapatite. The hydroxyapatite crystal phase—Ca10(PO4)6(OH)2—is generally hexagonal

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and it is the most common form of calcium phosphate found in nature. The main

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characteristics of hydroxyapatite are its high thermal stability, low solubility (Ksp=6.3 ±

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2.1×10-59 in phosphoric acid solution at pH 5 to 7 at 25°C),15 and strong capacity to retain a

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large variety of elements, due to complexation reactions with the functional groups on its

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surface.11 Phosphate is one of the strong complexing ligands influencing U(VI) adsorption

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and mobility in the subsurface.16, 17 In addition, U removal is expected to result from the

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selective reaction between TBP and U. As an organophosphorus extractant, TBP is one of the

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most favored extractants in the nuclear industry and nuclear fuel separation (i.e., PUREX

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process),18-22 because of TBP’s high stability constants for actinide-organophosphorus

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complexes.23,

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hydroxyapatite was synthesized under different pH conditions, characterized, and tested for

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its capability to remove U from various solutions.

24

Since TBP exhibits high selective reaction with U, TBP-coated

4


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2.

Experimental section

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2.1

Synthesis of different apatite samples

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Hydroxyapatite was prepared using calcium nitrate tetrahydrate [Ca(NO3)2•4H2O]

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and ammonium phosphate dibasic [(NH4)2HPO4] as starting materials and ammonia solution

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as the agent for pH adjustment.25 A suspension of 0.24 M Ca(NO3)2•4H2O in 350 mL of

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deionized (DI) water was vigorously stirred and its temperature was maintained at 25°C. A

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second solution, 0.29 M (NH4)2HPO4 in 250 mL of DI water, was slowly added dropwise to

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the Ca(NO3)2•4H2O solution. In all experiments, the pH of the mixed solution, adjusted using

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6 M ammonia solution, was close to 11. After mixing, the precipitated hydroxyapatite was

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removed from the solution by centrifugation at a rotation speed of 3,000 rpm for 5 min. The

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resulting precipitate was placed in a drying oven for 24 h at 80°C; it was then calcined at

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600°C in a muffle furnace (SH-MF2A, Samheung, Korea) for 3 h before use. The

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hydroxyapatite synthesis process followed the reaction shown in Eq. (1):

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10Ca(NO3)2•4H2O + 6(NH4)2HPO4 + 8NH4OH → Ca10(PO4)6(OH)2 + 20NH4NO3 + 46H2O

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(1)

100 101

2.2 Surface functionalization of hydroxyapatite using TBP

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The TBP surface functionalization method was revised based on an optimization

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process for TBP-coated magnetic polystyrene divinylbenzene by Wang et al,.26 TBP-coated

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hydroxyapatite was prepared at different pH levels (pH = 4, 7, and 10). The synthesized

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hydroxyapatite was washed repeatedly with DI water and dried in an oven at 60°C prior to

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use. TBP (20 mL) was diluted in 100 mL ethanol, and this mixing solution was adjusted to 5



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pH 4, 7, or 10 with 1 M hydrochloric acid or 1 M sodium hydroxide as needed. After pH

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adjustment, 5 g of hydroxyapatite was added to a prepared TBP mixing solution at each pH

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condition. The slurry was placed in a heating mantle at 100°C and stirred constantly for 30

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min until the ethanol had evaporated. The mixture was then baked in an oven at 130°C for

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two days. The final product was washed using DI water, then ethanol several times, and dried

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at 60°C in an oven for 3hr.

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2.3 Characterization methods

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The synthesized hydroxyapatite and TBP-coated hydroxyapatite were investigated

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using X-ray diffraction (XRD, D/MAX-2500/PC, Rigaku). XRD was performed from 20 to

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70° (2Θ region) with a 0.02° step size using monochromatized Cu-Kα (λ = 1.541 Å) radiation;

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the instrument was operated at a potential of 40 kV and a current of 100 mA.

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Phosphorus-31 400 MHz solid-state nuclear magnetic resonance spectroscopy (31P

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NMR, AVANCE 400WB, Bruker) and Fourier transform infrared spectroscopy (FTIR,

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Varian 670-IR, Agilent Technologies) were used to identify the functional groups of the

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TBP-coated hydroxyapatite. The 31P MAS NMR measurements were performed at a spinning

123

rate of 7 kHz. In addition, the TBP solution was analyzed using 600 MHz solution 31P NMR.

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To calibrate

125

tetramethylsilane (TMS) and dimethyl sulfoxide-d6 (DMSO-d6) to create solid and solution

126

samples.27 FTIR measurements were obtained in the attenuated total reflection mode using a

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ZnSe crystal. IR spectra were measured between 650 and 4,000 cm−1. All spectra were

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recorded with a resolution of 4 cm−1 and 32 scans.

31

P NMR spectra as a standard, the synthesized samples were added to

6


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The total carbon (TC) of the synthesized materials was determined using a CM5015

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CO2 coulometric carbon analyzer (UIC Corp.) under pure oxygen after combustion at 950 °C.

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The hydroxyapatite and TBP-coated hydroxyapatite samples were transferred to an alumina

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boat. The alumina boat was placed into a small quartz tube in an oxygen atmosphere and the

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temperature of the oxygen atmosphere was heated to 900°C in 2 hours and maintained for 90

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minutes. Morphology information about the hydroxyapatite before and after TBP coating was

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obtained using a high-resolution field-emission scanning electron microscope (FE-SEM,

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JSM-7401F, JEOL)/energy dispersive spectrometer (EDS). The EDS was used to analyze the

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elemental compositions of the samples.

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Specific surface areas and pore structure information on the TBP-coated

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hydroxyapatites were measured by N2 sorption isotherms at 77 K using an ASAP-2010

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surface analyzer (Micromeritics Instrument Corporation, USA). Samples were heated at

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150°C to remove water and other physically adsorbed molecules. Nitrogen adsorption-

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desorption was performed at relative pressures from 0.990 to 0.01.

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Thermal decomposition was obtained for mass loss using a DTG-60 thermoanalyzer

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(DTG-60, Shimadzu, JAPAN). Approximately 50 mg of sample was prepared in an

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aluminum pan and heated from 35°C to 300°C under nitrogen atmosphere (10 mL/min) at the

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heating rate of 10 °C/min.

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The zeta potential of the particles was measured using the Nano-ZS instrument

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(Malvern Instr., UK). The pH of 0.1 M KCl solution was adjusted to 4, 5, or 6 by adding

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0.001M HCl or 0.001M NaOH solution. At each pH solution, 0.002 mg of TBP coated-

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hydroxyapatite (pH=7) was mixed with 2 mL of 0.1 M KCl solution in 15-mL polypropylene

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test tubes before measuring the zeta potential. 7



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2.4 Batch U adsorption experiment

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Uranium batch adsorption experiments were performed in duplicate under an open

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atmosphere conditions using 15-mL polypropylene test tubes. Various apatite solids (0.05 g)

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were used with 10 mL of different background solutions to prepare a constant solid-to-

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solution ratio of 5 g·L−1. An U(VI) standard solution (1,000 mg·L−1 uranyl nitrate

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hexahydrate in 2% HNO3), a PerkinElmer inductively coupled plasma mass spectrometry

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(ICP-MS) standard, was diluted with DI water, 0.001 M NaHCO3, or 0.1 M NaHCO3

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background solutions and used in the U batch adsorption experiments; 0.7 M NaCl solution

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was also prepared to simulate seawater. Using the various apatites, U adsorption experiments

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were conducted for different reaction times (1 min, 1 h, and 1 day) with various initial U

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concentrations (10 ppb, 100 ppb, and 1,000 ppb) in the background solutions. Because the

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total U concentration in seawater is very low (~3.3 ppb), 10 ppb of initial U concentration

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was prepared in 0.7 M NaCl solution for U adsorption experiments. The test tubes were

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placed on a platform shaker and individual effluent samples were collected after each

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reaction time. After reaction, the supernatant was separated using a 0.45 µm syringe filter and

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the filtrate U concentration was measured by ICP-MS. Uranium adsorption isotherm

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experiments were performed with varying solid-to-solution ratios and a constant initial U

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concentration at 298.15 K and pH = 8.0 ± 0.6. For isotherm experiments, a series of 15-mL

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polypropylene test tubes were used. Each test tube was filled with apatite-based adsorbent

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and 200 ppm of the initial U concentration in 0.01 M NaHCO3 solution at different solid-to-

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solution ratios. The samples were reacted in an open system for 24 h. Because U adsorption

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reached steady state within 24 h from independent U adsorption kinetic experiments, an

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isotherm contact time of 24 h was justified. After 24 h reaction, the samples were filtered

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using a 0.45 µm syringe filter and the U concentration in the supernatant was measured by 8


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ICP-MS. The equilibrium adsorption capacity of the adsorbent (mg·g−1) was calculated using

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Eq. (2):

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ܳ = (‫ܥ‬଴ − ‫ܥ‬௘ ) ×

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where C0 is the initial U concentration (mg·L−1), Ce is the equilibrium U concentration in

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solution (mg·L−1), V is the volume of U sample solution (mL), and M is the mass of the

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adsorbent (g).28

௏

(2)

ெ

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To evaluate the U adsorption mechanism, the experimental data were analyzed by

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Langmuir and Freundlich models.29-33 The Langmuir isotherm assumes monolayer adsorption

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on the surface of the adsorbent material. The Langmuir isotherm can be represented as Eq.

185

(3):

186

஼೐ ௤೐

ଵ

= ொ•௕ +

஼೐

(3)

ொ

187

where Ce is the concentration of ions in solution at equilibrium (mg·L−1), qe is the adsorbed

188

U amount per unit adsorbent mass at equilibrium (mg·g−1), Q is the maximum adsorption

189

capacity of the adsorbent for U at monolayer coverage (mg-U·g−1), and b is the Langmuir

190

constant (L·mg−1). The Freundlich isotherm allows for various adsorption sites on

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heterogeneous solid surfaces. The Freundlich isotherm can be represented as Eq. (4):

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log ‫ݍ‬௘ = log ‫ܭ‬௙ + ௡ log ‫ܥ‬௘

193

where Kf is the a adsorption constant of the Freundlich model and n is a parameter of the

194

adsorption tendency.

ଵ

(4)

195 9



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3. Results and discussion

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3.1 Characterization of the synthesized apatite minerals

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The XRD patterns of the synthesized hydroxyapatite and TBP-coated hydroxyapatite

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after calcination at 600°C for 3 h are shown in Fig. 1. The XRD patterns of the

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hydroxyapatite agreed well with the reference pattern for hydroxyapatite (Powder Diffraction

202

File number: 00-009-0432).34 Major peaks are attributed to (002), (211), (112), (300), (202),

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(222), and (213) lattice planes. Lattice dimensions determined from the fitted peaks are a =

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0.941 nm, b = 0.941 nm, and c = 0.684 nm, with angles α = β = 90° and γ = 120°, showing

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hexagonal structure and space group p63/m. These parameters were in good agreement with

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standard values (a = b = 0.94180 nm and c = 0.68840 nm). The XRD patterns of the

207

synthesized TBP-coated hydroxyapatite at different pHs are almost the same as those of

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hydroxyapatite, itself. As can be seen, the hydroxyapatite structure did not change or collapse

209

during the TBP functionalization and calcination processes.

210

Functional groups associated with hydroxyapatite were identified by FTIR

211

spectroscopy (Fig. 2). Because each chemical bond has a unique vibrational frequency,

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structural and bond information about complex materials can be determined.35 The FTIR

213

spectra of the synthesized materials showed the two peaks characteristic of hydroxyapatite at

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3,570 cm−1 and 1,034 cm−1.36 The band around 3,570 cm−1 confirms the presence of the

215

stretching hydroxyl group from hydroxyapatite, though these are relatively weak. The

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asymmetric stretching vibration for the phosphate functional group was also observed around

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1,030−1,040 cm−1. The wavenumber region for the C-H-containing functional group (i.e., the 10


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C-H group at 3,000–2,950 cm−1) from TBP was also found in the TBP-coated

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hydroxyapatites prepared at three different pHs (Fig. 2b). Among the three samples, the TBP-

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coated hydroxyapatite prepared at pH = 10 shows distinct multiple C-H-bonding peaks

221

compared to the two other TBP coated hydroxyapatites prepared at lower pHs.

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Analysis of phosphorus from TBP on the hydroxyapatite surface was performed 31

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using

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The solution

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Figure S1(b). Commercial pure TBP exhibits a 31P NMR signal at δ = 0.3 ppm.37 The solid-

226

state

227

samples was phosphate (PO43−) functional group, but the TBP-coated hydroxyapatite sample

228

prepared at pH = 10 showed an additional peak around δ = −2 ppm, due to the O = P(OR)3

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structure derived from the TBP coating on the hydroxyapatite surface.38

31

P NMR spectroscopy and the results are shown in the Supporting Information (SI). 31

P NMR spectrum measured for the commercial TBP solution is shown in

P MAS NMR spectra in Figure S1(a) showed that all P in the synthesized apatite

230

The TC data for hydroxyapatite and TBP-coated hydroxyapatite are provided in

231

Table 1. The TBP-coated hydroxyapatite sample prepared at pH = 10 has 8–13 times more

232

carbon present compared to other hydroxyapatite samples. This means that more TBP

233

coatings on hydroxyapatite occurred at pH = 10, consistent with FTIR and NMR results.

234

The thermal degradation of hydroxyapatite-adsorbents was observed in terms of mass

235

loss by TGA carried out in nitrogen atmosphere. The mass loss was almost negligible for the

236

hydroxyapatite, while the TBP-coated hydroxyapatite samples showed about 3~6 wt% loss

237

between 150 and 250°C (Figure S2). This mass loss is attributed to the vaporization of

238

physically adsorbed water and thermal decomposition of the surface TBP, because the boiling

239

point of TBP is about 289°C.39 Based on TGA results, about 3 wt%, 5 wt%, and 6 wt% of

11



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TBP was found on the surface of hydroxyapatites prepared at pH=7, 4, and 10, respectively.

241

This result also showed a same tendency as the TC results (Table 1).

242

Based on the combination of various characterization results from XRD, FTIR,

31

P

243

NMR, TGA, and TC analyses, more TBP coating occurred on the hydroxyapatite surfaces at

244

pH = 10 than other pH conditions. The surface of hydroxyapatite has two types of adsorbing

245

sites, positively charged ≡CaOH2+ and negatively charged ≡OPO3H−.37 Under neutral and

246

alkaline pH conditions, the hydroxyapatite surface is more negatively charged because of the

247

deprotonation.40 The bonding mechanism of TBP-coated hydroxyapatite is considered to be

248

dominated by hydrogen bonding between one of the oxygen atoms in TBP and the hydroxyl

249

group present in the hydroxyapatite surface. The more negatively charged surface sites are

250

present in hydroxyapatite at pH=10, when more TBP coating is occurring. In addition,

251

hydroxyapatite surfaces become more reactive by alkali treatment at high pH condition.41

252

Then, subsequent heating inside an oven can evaporate the ethanol and fix the TBP on the

253

hydroxyapatite surfaces.42

254

The hydroxyapatite morphology before and after the TBP coating process were

255

examined using SEM and representative images are shown in Fig. 3. The SEM images

256

indicated that the hydroxyapatite surface changed after TBP coating. A typical SEM image of

257

the synthesized hydroxyapatite showed mixed spherical and needle-shaped particles (Fig. 3a).

258

However, numerous agglomerations of smaller spherical particles are observed in TBP-

259

coated hydroxyapatites synthesized under the different pH conditions (Figs. 3b–d). The TBP-

260

coated hydroxyapatite at pH = 10 showed more irregular and relatively larger agglomerated

261

particles compared to other two TBP-coated hydroxyapatites. Atomic percentage of major

262

elements C, Ca, and P in the synthesized materials was determined by EDS analysis (Table

263

S1). In addition, relative atomic ratios of Ca/P and C/P (or C/Ca) were calculated for the 12


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synthesized materials. The Ca/P ratio of the synthesized hydroxyapatite materials was

265

approximately 1.5, similar to the Ca/P ratio found in “pure” hydroxyapatite.43 Relatively

266

lower C/P and C/Ca ratios were found in the TBP-coated hydroxyapatite at pH = 7 compared

267

to two other TBP-coated hydroxyapatites.

268

Surface area and pore properties of the synthesized materials were analyzed using

269

nitrogen adsorption-desorption isotherms. The surface area and pore size distributions of the

270

synthesized materials were determined using the Brunauer–Emmett–Teller (BET) and

271

Barrett–Joyner–Halenda (BJH) methods, respectively. Figure S3 shows the nitrogen

272

adsorption-desorption isotherms for hydroxyapatite and TBP-coated hydroxyapatites, which

273

are in accordance with the classical type-III isotherm in the International Union of Pure and

274

Applied Chemistry classifications.44 A type-III isotherm increases at higher relative pressures

275

because of weak adsorbate–adsorbent interactions. The surface area, pore volume, and pore

276

size of the synthesized sample materials based on the isotherms of the three materials are

277

summarized in Table S2. The pore diameter also decreases with the amount of TBP coated on

278

the hydroxyapatite surface under the different pH conditions. The BET surface area of the

279

hydroxyapatite is 31.7 m2·g-1, which decreases with the amount of TBP coating on the

280

hydroxyapatite surface prepared at pH = 10 (7.26 m2·g-1) and pH = 7 (5.27 m2·g-1).

281

The zeta potential of TBP-coated hydroxyapatite (pH=7) was performed in 0.1M

282

KCl solution under different pH conditions. The measured zeta potential values of TBP-

283

coated hydroxyapatite (pH=7) are shown in Table 1. Generally, the pHiep (pH of isoelectric

284

points) of hydroxyapatite in 0.01M KCl solution was determined to 7.7.

285

measured pHiep of TBP-coated hydroxyapatite (pH=7) was about 5.7 from the changing

286

charge of zeta potential from negative to positive between pHs 5.1 and 6.0. This shift of pHiep

45

However, the

13



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from 7.7 to 5.7 was attributed to the TBP coated on hydroxyapatite surfaces. Even though the

288

background ionic strength (KCl) is a little bit different, this effect can be ignored. According

289

to Brunelle 46, when the pH is below the pHiep, the surface charge of adsorbent becomes more

290

positively charged due to protonation process. If the pH is higher than the pHiep, the surface

291

charge becomes more negative. Likewise, surface charge of TBP-coated hydroxyapatite

292

(pH=7) is considered to be negative in most of the experimental solutions due to high pH

293

(~8.0) in this study. In addition, the dominant U aqueous species in 0.1 M NaHCO3 solution

294

(pH > 7) is negatively charged UO2(CO3)34− (Figure S4). Therefore, because both aqueous U

295

species and surface of TBP-coated hydroxyapatite have the same negative charge,

296

electrostatic adsorption is not a major binding mechanism between U and TBP-coated

297

hydroxyapatite (pH=7). However, more strong chemical bonding controls the U adsorption

298

on TBP-coated hydroxyapatite (pH=7).

299 300

3.2 Uranium adsorption results

301

The U batch adsorption experiments conducted using various background solutions

302

with different carbonate concentrations showed that the U removal by most of the

303

hydroxyapatite materials was almost 100% within 1 day of reaction when the low carbonate

304

background solution (0.001 M NaHCO3, blue column in Fig. 4) was used. As the bicarbonate

305

concentration increased from 0.001 M to 0.1 M, the U removal amount decreased by more

306

than half because of the formation of strong U-carbonate aqueous complexes. In the

307

Geochemist’s Workbench (GWB) model simulation shown in Figure S4, uranyl species exist

308

predominantly as UO2(CO3)34− in 0.1 M NaHCO3 solution (pH > 7). As the concentration of

309

the uranyl-carbonate complexes was increased, the uranyl adsorption tendency decreased 14


Page 15 of 37

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310

because the strong negatively charged aqueous species is repelled by the negative surface

311

charges on the hydroxyapatite solids.47

312

The TBP-coated hydroxyapatites showed better U removal than pure hydroxyapatite

313

due to the interaction of phosphate in TBP with U (Fig. 5). In particular, TBP-coated

314

hydroxyapatite prepared at pH = 7 exhibits the best U removal capacity for the TBP-coated

315

hydroxyapatites prepared under the three different pH conditions. As mentioned in the

316

discussion on the characterization of the synthesized materials, more TBP coating occurred

317

on the hydroxyapatite surface at pH = 10. However, the TBP-coated hydroxyapatite sample

318

prepared at pH = 7 exhibits the highest U adsorption capacity. After the adsorption

319

experiment, glass electrodes were used to measure the pH of the experimental solutions. The

320

final pH of the TBP-coated hydroxyapatite sample prepared at pH = 7 is slightly less than

321

that of the other samples. Because U adsorption varies significantly at different pH levels in

322

the presence of phosphate,48 the formation of U-phosphate complexes can greatly increase U

323

adsorption at low pH conditions.49 As the pH increases, U adsorption (%) tends to decrease in

324

open atmosphere systems. The highest U adsorption occurs when the pH is approximately ~4

325

in the presence of phosphate.48 The final pH values of all batch adsorption final solutions in

326

this study were not significantly different from the initial pH values of NaHCO3 background

327

solutions (pH = ~8.3). Among these, the final pH of the uncoated hydroxyapatite (pH = ~8.5)

328

is the lowest compared to the TBP-coated hydroxyapatite samples (pHs = 8.6~8.8); however,

329

the adsorption capacity of the uncoated hydroxyapatite is still the lowest among all the

330

adsorbents tested. This also confirms that phosphate in the TBP-coated hydroxyapatites can

331

enhance U removal, even from high carbonate and alkaline waste streams. Among the TBP-

332

coated hydroxyapatites, the highest U adsorption (~70 %) was found in the TBP-coated

333

hydroxyapatite prepared at pH = 7 due to slightly lower pH condition of the final batch test 15



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Page 16 of 37

334

leachates, compared to the other TBP-coated hydroxyapatites at pH = 4 (~27 %) and 10

335

(~57%) under 0.1 M NaHCO3 solution as shown in Fig. 5c.

336

The U removal from the simulated seawater was also tested, because the developed

337

adsorbent needs to have high selectivity and rapid separation capacity for separating low-

338

concentrated U (~ 3.0 ppb) from seawater. In Fig. 6, the results of U adsorption experiments

339

using the four hydroxyapatites in simulated seawater (0.7 M NaCl solution) showed that the

340

TBP-coated hydroxyapatite prepared at pH = 7 still had the best U removal capacity among

341

the four solids, suggesting that TBP-coated hydroxyapatite can be used an efficient adsorbent

342

to recover U from seawater.

343 344

3.3 Uranium adsorption isotherm

345

The U isotherm adsorption data were fit using both Langmuir and Freundlich

346

isotherm models.50, 51 Linear plots of U adsorption by the four hydroxyapatites are presented

347

in Figure S5. The adsorption constants and their correlation coefficients are also presented in

348

Table S3. According to the fits, the maximum adsorption capacity for U removal is close to

349

38 mg·g-1 for the TBP-coated hydroxyapatite prepared at pH = 7. The correlation coefficients

350

(R2) for the two isotherm models in Table S3 indicate that the Langmuir isotherm gives a

351

better fit to the U adsorption behavior of hydroxyapatite and TBP-coated hydroxyapatite

352

prepared at pH = 4, while the Freundlich isotherm gives a better fit to the U adsorption by

353

TBP-coated hydroxyapatites prepared at both pH = 7 and 10. The better fit of these two

354

materials (TBP-coated hydroxyapatite prepared at pH = 7 and 10) to the Freundlich isotherm

355

suggests multilayer U(VI) adsorption sites on their surfaces because of more TBP coatings at

356

high pH (Fig. 2). 16


Page 17 of 37

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357

The maximum U adsorption capacities on various previously studied adsorbents were

358

compared with the TBP-coated hydroxyapatite prepared at pH = 7 (Table S4). The U

359

adsorption capacity of TBP-coated hydroxyapatite prepared at pH = 7 was higher than those

360

of other adsorbents, except for graphene oxide nanosheets,20 cross-linked chitosan,55 titanium

361

dioxide,57 and iron oxyhydroxide.58 However, our study was performed at higher carbonate

362

concentrations and higher pH conditions (closer to that of seawater which ranges from 7.5 to

363

8.4, the U adsorption isotherm tests were run at pH = ~8.5) than the other studies, which used

364

(pH values that ranged from 4 to 6). Based on the results of the experiment conducted using

365

zirconium oxophosphate and goethite at pH (7-7.5), the Qmax value is lower at higher pH.

366

Compared to these two adsorbents, Qmax of TBP-coated hydroxyapatite (pH=7) adsorbent still

367

shows 10-30 times higher even in 0.01 M NaHCO3 solution similar to seawater. Therefore,

368

TBP-coated hydroxyapatite (pH=7) could potentially be used to adsorb U from seawater

369

more efficiently compared to other adsorbents in Table S4.

370

371

4. Conclusions

372

The hydroxyapatite and TBP-coated hydroxyapatite adsorbents were successfully

373

fabricated and tested as adsorbents for U removal from various solutions. As carbonate

374

concentrations in solutions increased, the U removal decreased because of the formation of

375

strong anionic aqueous U-carbonate complexes. However, under the same conditions, the

376

TBP-coated hydroxyapatite showed better U-removal capability than ordinary uncoated

377

hydroxyapatite. In particular, the TBP-coated hydroxyapatite synthesized at pH = 7 showed

378

the highest U adsorption capacity (38 mg·g-1). The results of this study indicate that TBP-

379

coated hydroxyapatite can be used as a new adsorbent to efficiently and selectively remove U 17



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Page 18 of 37

380

from seawater. In addition, the technology for the separation and removal of U developed in

381

this research should be applicable for U separation and remediation from radioactive waste

382

streams with varying pH and ionic strength conditions.

383

384

Supporting Information

385

The Supporting Information is available free of charge on the ACS Publications website at

386

DOI:

387

Five Figures: additional material analysis conducted upon the synthesized apatites

388

including NMR, TGA, BET data, GWB simulation of species of U under the different

389

background solutions, and Langmuir isotherm results; Four Tables presented (1) EDS results,

390

(2) value from the BET analysis, (3) isotherm constants, and (4) comparison of maximum U

391

adsorption capacities with other adsorbents (PDF)

392

393

Acknowledgements

394

This research was supported by the BK21+ program and Basic Science Research Programs

395

(NRF-2013R1A1A2063649,

396

through the National Research Foundation of Korea (NRF) funded by the Ministry of

397

Education, Science, and Technology.

NRF-2016R1C1B1014163,

and

2016R1D1A1B02013310)

398 399 18


Page 19 of 37

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400

AUTHOR INFORMATION

401

Corresponding Author

402

*To whom correspondence should be addressed (e-mail: [email protected];

403

[email protected]).

404

Present Addresses

405

† Present address: Division of Advanced Nuclear Engineering, Pohang University of Science

406

and Technology (POSTECH), 77 Chongam-ro, Nam-Gu, Pohang 790-784, Republic of Korea

407

‡ Energy and Environment Directorate, Pacific Northwest National Laboratory (PNNL), 902

408

Battelle Blvd., MSIN P7-54, Richland, WA 99354, USA

409 410

Author Contributions

411

The manuscript was written through contributions of all authors. The experimental design

412

was prepared by HyunJu Kim and Wooyong Um, and the experiments were conducted by

413

HyunJu Kim, Won-Seok Kim, and Seeun Chang. All authors have given approval to the final

414

version of the manuscript.

415

416

417

19



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418

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567

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571 572

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573

574 575 576 577 578 579 580 581 27



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Page 28 of 37

List of Figures

583 584

Fig. 1. XRD patterns of the synthesized apatites with hydroxyapatite reference. (Pink line

585

shows reference pattern for hydroxyapatite)

586

Fig. 2. FTIR spectra of the synthesized apatites: (a) analysis of the 4,000–650 cm−1 region;

587

(b) an expanded view of the 3,200–2,900 cm−1 range of the C-H functional group.

588

Fig. 3. SEM images of (a) hydroxyapatite, (b) TBP-coated hydroxyapatite (pH = 4), (c) TBP-

589

coated hydroxyapatite (pH = 7), and (d) TBP-coated hydroxyapatite (pH = 10).

590

Fig. 4. Uranium removal (%) by hydroxyapatite from different adsorption times and

591

background solutions at 293.15 K (blue column (□): 0.001 M NaHCO3; red column (▨): 0.1

592

M NaHCO3).

593

Fig. 5. Uranium removal (%) using different synthesized apatites at different initial U

594

concentrations (a) 10 ppb, (b) 100 ppb, and (c) 1,000 ppb in 0.1 M NaHCO3 background

595

solution at 293.15 K [blue (▨): hydroxyapatite and final pH = ~8.5; green (▤): TBP-coated

596

hydroxyapatite (pH = 4) and final pH = 8.6~8.7; red (▥): TBP-coated hydroxyapatite (pH =

597

7) and final pH = 8.6~8.8; yellow (▦): TBP-coated hydroxyapatite (pH = 10) and final pH =

598

~8.7].

599

Fig. 6. Uranium removal percentage of synthesized apatites using the simulated seawater [0.7

600

M NaCl] spiked with 10 ppb U(VI) in at 293.15 K [blue (▨): Hydroxyapatite and final pH =

601

~8.5; green (▤): TBP-coated hydroxyapatite (pH = 4) and final pH = 8.5~8.6; red (▥): TBP-

602

coated hydroxyapatite (pH = 7) and final pH = ~8.5; yellow (▦): TBP-coated hydroxyapatite

603

(pH = 10) and final pH = ~8.5].

604 605 606 28


Page 29 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

607


List of Table

608 609

Table 1. Total Carbon amounts for the Hydroxyapatite and TBP-coated hydroxyapatite

610

samples and Zeta potential of TBP-coated hydroxyapatite (pH=7) in different pHs.

611 612 613 614 615 616 617 618 619 620 621 622 623

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 37

624 625

Fig. 1. XRD patterns of the synthesized apatites with hydroxyapatite reference. (Pink line

626

shows reference pattern for hydroxyapatite)

627 628 629 630 631 632 633 634

30


Page 31 of 37

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(a)

(b)

635 636

Fig. 2. FTIR spectra of the synthesized apatites: (a) analysis of the 4,000–650 cm−1 region;

637

(b) an expanded view of the 3,200–2,900 cm−1 range of the C-H functional group.

638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

653

(a)

100nm

(c)

100nm

(b)

100nm

(d)

100nm

654

Fig. 3. SEM images of (a) hydroxyapatite, (b) TBP-coated hydroxyapatite (pH = 4), (c) TBP-

655

coated hydroxyapatite (pH = 7), and (d) TBP-coated hydroxyapatite (pH = 10).

656 657 658 659 660 661 662 32


Page 33 of 37

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663 664

Fig. 4. Uranium removal (%) by hydroxyapatite from different adsorption times and

665

background solutions at 293.15 K (blue column (□): 0.001 M NaHCO3; red column (▨): 0.1

666

M NaHCO3).

667 668 669 670 671 672 673 674 675 676 33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 34 of 37

(b)

(c)

677 678

Fig. 5. Uranium removal (%) using different synthesized apatites at different initial U

679

concentrations (a) 10 ppb, (b) 100 ppb, and (c) 1,000 ppb in 0.1 M NaHCO3 background

680

solution at 293.15 K [blue (▨): hydroxyapatite and final pH = ~8.5; green (▤): TBP-coated

681

hydroxyapatite (pH = 4) and final pH = 8.6~8.7; red (▥): TBP-coated hydroxyapatite (pH =

682

7) and final pH = 8.6~8.8; yellow (▦): TBP-coated hydroxyapatite (pH = 10) and final pH =

683

~8.7].

684 685 686 687 688 689 690 34


Page 35 of 37

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691

692 693

Fig. 6. Uranium removal percentage of synthesized apatites using the simulated seawater [0.7

694

M NaCl] spiked with 10 ppb U(VI) in at 293.15 K [blue (▨): Hydroxyapatite and final pH =

695

~8.5; green (▤): TBP-coated hydroxyapatite (pH = 4) and final pH = 8.5~8.6; red (▥): TBP-

696

coated hydroxyapatite (pH = 7) and final pH = ~8.5; yellow (▦): TBP-coated hydroxyapatite

697

(pH = 10) and final pH = ~8.5].

698 699 700 701 702 703 704 705 35



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Page 36 of 37

706

Table 1. Total Carbon amounts for the Hydroxyapatite and TBP-coated hydroxyapatite

707

samples and Zeta potential of TBP-coated hydroxyapatite (pH=7) in different pHs.

Sample

Total Carbon [ppm]

Hydroxyapatite

905

TBP-coated

TBP-coated

TBP-coated

hydroxyapatite

hydroxyapatite

hydroxyapatite

(pH = 4)

(pH = 7)

(pH = 10)

10,400

6,090

82,600

-6.84 (at pH=4.34) Zeta Potential

-2.59

(mV)

(at pH=5.10) 1.16 (at pH=6.00)

708 709 710 711 712 713 714 715 716 717 718 719 36


Page 37 of 37

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For Table of Contents Only

720 721 U U

722

U TBP

Hydroxyapatite [Ca10(PO4)6(OH)2]

U

U

37


Synthesis of Tributyl Phosphate-Coated Hydroxyapatite for Selective

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