Glycerol-Modified Binary Layered Double Hydroxide Nanocomposites

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Glycerol-Modified Binary Layered Double Hydroxide Nanocomposites for Uranium Immobilization via EXAFS Technique and DFT Theoretical Calculation Yidong Zou, Yang Liu, Xiangxue Wang, Guodong Sheng, Suhua Wang, Yuejie Ai, Yongfei Ji, Yunhai Liu, Tasawar Hayat, and Xiangke Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b00439 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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ACS Sustainable Chemistry & Engineering

Authors Information

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The full mailing address of all authors:

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Yidong Zou: No.2 Beinong Road, Huilongguan Town, Changping District, School of

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Environment and Chemical Engineering, North China Electric Power University, Beijing

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102206, P. R. China

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Yang Liu: No.2 Beinong Road, Huilongguan Town, Changping District, School of

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102206, P. R. China

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Xiangxue Wang: No.2 Beinong Road, Huilongguan Town, Changping District, School

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of Environment and Chemical Engineering, North China Electric Power University,

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Beijing 102206, P. R. China

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Guodong Sheng: No.2 Beinong Road, Huilongguan Town, Changping District, School

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of Environment and Chemical Engineering, North China Electric Power University,

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Beijing 102206, P. R. China

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Suhua Wang: No.2 Beinong Road, Huilongguan Town, Changping District, School of

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102206, P. R. China

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Yuejie Ai: No.2 Beinong Road, Huilongguan Town, Changping District, School of

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102206, P. R. China

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Yongfei Ji: Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute

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of Technology, Roslagstullsbacken 15, 10691 Stockholm, Sweden

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Yunhai Liu: No.418 Guanglan Avenue, Changbei Economic and Technological

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ACS Paragon Plus Environment


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Development Zone, School of Chemistry, Biological and Materials Sciences, East China

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Institute of Technology, Nanchang, 330013, P. R. China

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Tasawar Hayat: NAAM Research Group, Faculty of Science, King Abdulaziz University,

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Jeddah 21589, Saudi Arabia

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Xiangke Wang: No.2 Beinong Road, Huilongguan Town, Changping District, School of

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102206, P. R. China

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*: Corresponding authors. [email protected] or [email protected] (X. Wang);

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[email protected] (Y. Ai), [email protected] (Y. Liu).

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Glycerol-Modified Binary Layered Double Hydroxide Nanocomposites

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for Uranium Immobilization via EXAFS Technique and DFT

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Theoretical Calculation

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Yidong Zoua,b, Yang Liua, Xiangxue Wanga, Guodong Shenga, Suhua Wanga, Yuejie Aia*,

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Yongfei Jic, Yunhai Liub*, Tasawar Hayatd, Xiangke Wanga,d*

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a

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University, Beijing 102206, P. R. China

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b

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Technology, Nanchang, 330013, P. R. China

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c

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Technology, Roslagstullsbacken 15, 10691 Stockholm, Sweden

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d

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Saudi Arabia

School of Environment and Chemical Engineering, North China Electric Power

School of Chemistry, Biological and Materials Sciences, East China Institute of

Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of

NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589,

47

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ABSTRACT: Novel efficient glycerol-modified nanoscale layered double hydroxides

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(rods Ca/Al LDH-Gl and flocculent Ni/Al LDH-Gl) were successfully synthesized by

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simple one-step hydrothermal synthesis route and showed excellent adsorption capacities

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for U(VI) from aqueous solutions under various environmental conditions. The advanced

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spectroscopy analysis confirmed the existence of abundant oxygen-containing functional

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groups (e.g., C-O, O-C=O and C=O) on the surfaces of Ca/Al LDH-Gl and Ni/Al LDH-

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Gl, which could provide enough free active sites for the binding of U(VI). The maximum

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adsorption capacities of U(VI) calculated from Sips model were 266.5 mg·g-1 for Ca/Al

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LDH-Gl and 142.3 mg·g-1 for Ni/Al LDH-Gl at 298.15 K, and the higher adsorption

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capacity of Ca/Al LDH-Gl might be due to more functional groups and abundant high-

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activity “Ca-O” groups. Macroscopic experiments proved that the interaction of U(VI) on

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Ca/Al LDH-Gl and Ni/Al LDH-Gl was due to surface complexation and electrostatic

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interactions. The EXAFS analysis confirmed the non-transformation of U(VI) to U(IV)

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on solid particles, and stable inner-sphere complexes were not formed by reduction

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interaction but by chemical adsorption. The DFT calculations further evidenced that the

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higher adsorption energies (i.e., Ead = 4.00 eV for Ca/Al LDH-Gl-UO22+ and Ead = 2.43

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eV for Ca/Al LDH-Gl-UO2CO3) were mainly attributed to stronger hydrogen bonds and

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electrostatic interactions. The superior immobilization performance of Ca/Al LDH-Gl

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supports a potential strategy for decontamination of UO22+ from wastewater, and it may

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provide new insights for the efficient removal of radionuclides in environmental pollution

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cleanup.

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KEYWORDS: Nanocomposites, Layered double hydroxides, Immobilization, U(VI),

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EXAFS

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INTRODUCTION

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Recently, erosion-environmental effect and global environmental deterioration issue

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caused by the application of fossil fuels have attracted intense interests due to their severe

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hazard to human beings and environmental toxicity.1,2 It leads more and more countries

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to search for new energy to satisfy their basic energy demand and reduce environmental

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pressure. Nuclear power has become the potential energy and essential method for

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solving this energy crisis.3,4 With the development of nuclear technology and extensive

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utilization of nuclear energy, exhaustion of nuclear resources has been the current urgent

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problem for maintaining sustainable development of nuclear energy.5,6 In addition,

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radioactive pollution, which was caused by unreasonable utilization and exploitation of

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nuclear sources, has already become one of the forefront environmental and energy issues

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because of its potential life-threatening and environmental effects.7-9 Nevertheless,

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uranium, the main material of nuclear power, a toxic radionuclide, has been excessively

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disposed into the natural environment in hexavalent form (U(VI)).2,10,11 It can cause

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serious comprehensive environmental radiation and potential toxicological effects, which

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results in different levels of water pollution or soil pollution,12,13 so it is essential and

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urgent to recycle and aggregate uranium from natural environment.

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In order to remove and recycle radioactive U(VI) from water system, various

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treatment technologies (e.g., ion-exchange,14 chemical (co)precipitation,15 solvent

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extraction,14,16 coagulation,17 filtration,18 permeation19 and membrane separation20) have

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been applied to separate or aggregate radionuclides from wastewater or groundwater

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system. However, a series of defects, such as secondary pollution, high investment and

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complex operation, significantly limited these methods’ application in environmental

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remediation.21 Compared with these methods, adsorption, an efficient and environmental-

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friendly technology, is widely applied with great potential application in the removal of

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U(VI) from aqueous solutions.3,5,22,23 Various adsorbents, such as graphene-based

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materials,1,5,23,24 carbon-based materials,3,16 clay minerals-based materials,6,12,25 polymer-

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based materials26,27 or metal-based materials,10 have been applied for the removal and

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enrichment of U(VI) from wastewater or groundwater. Nonetheless, the effectiveness and

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practicability of these materials depend on the adsorption efficiency, production cost and

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complexity.21 Thus, novel and multi-functional adsorbents should be developed and

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applied for the efficient removal of U(VI) from natural environment.

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Layered double hydroxides (LDHs), a typical 2D-structured anionic clay of mineral

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brucite, have aroused increasing attention in adsorption, photocatalytic, coprecipitation or

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other fields based on their novel structures and easily-exchanged intercalated layer

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anions.28,29 In general, LDHs are described by the chemical formula [M1-

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x

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metal cations, respectively (e.g., Mg2+, Zn2+, Co2+, Mn2+, Ni2+, Ca2+, Fe3+, Cr3+ and Al3+),

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and An- represents the interlayer anions with high activity (e.g., Cl-, ClO4-, NO3-, CO32-

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and SO42-), x is regarded as the molar ratio of M2+/(M2+ + M3+).30-35 LDHs and their

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composites have been considered as superior adsorbents for the removal and enrichment

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of U(VI) from aqueous solutions, which may be attributed to the special structure and

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physicochemical properties of LDHs, such as high stability, excellent anion exchange

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capacities, high specific surface area and effective active sites.28,36,37 In order to obtain

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novel LDHs or their derivatives with higher performance, lamellar, flakes, spheres or

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porous LDHs have been investigated in environmental pollution cleanup and displayed

2+

Mx3+(OH)2]x+(An-)x/n·yH2O, where M2+ and M3+ represent the divalent or trivalent

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excellent removal efficiency for various pollutants.28,30,33 For example, Zou et al.28

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applied lamellar LDH-CO3 and LDH-Cl to coagulate graphene oxides (GO) and

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exhibited high coagulation capacities for (GO) in aqueous solutions. To further evaluate

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the adsorption behavior and interaction mechanism of LDHs with U(VI), herein, rods

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glycerol-modified nanocrystallined Ca/Al LDHs (Ca/Al LDH-Gl) and flocculent

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glycerol-modified nanocrystallined Ni/Al layered double hydroxide (Ni/Al LDH-Gl)

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were synthesized and applied as superior adsorbents for the efficient removal of U(VI)

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from aqueous solutions.

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The objectives of this paper are: (1) to fabricate rods Ca/Al LDH-Gl and flocculent

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Ni/Al LDH-Gl with a facile-simple hydrothermal process; (2) to investigate the

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adsorption behavior of U(VI) on the as-prepared Ca/Al LDH-Gl or Ni/Al LDH-Gl under

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various conditions (e.g., solution pH, ionic strength, solid contents, contact time,

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temperature and concentration of CO32-); (3) to evaluate the interaction mechanism

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between U(VI) and Ca/Al LDH-Gl or Ni/Al LDH-Gl by SEM, TEM, XRD, FT-IR, XPS,

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EXAFS and theoretical calculations. This paper provides new insights and scientific

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understanding to adsorption or aggregation of U(VI) on LDHs by advanced EXAFS and

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density functional theory (DFT) calculations, which is beneficial for reasonable

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application of LDHs in radionuclide pollution cleanup during nuclear industry processes.

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MATERIAL AND METHODS

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The synthesis and characterization of Ca/Al LDH-Gl and Ni/Al LDH-Gl composites,

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and batch adsorption experiments are described in Supporting Information. Ca/Al LDH-

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Gl and Ni/Al LDH-Gl were fabricated by a typical facile-cheap hydrothermal method

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(Figure 1).

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RESULTS AND DISCUSSION

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Morphology Analysis and Structure Characterization. The microstructures and

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morphologies of as-prepared Ca/Al LDH-Gl and Ni/Al LDH-Gl were characterized by

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various spectroscopy techniques. According to SEM (Figure 2) and TEM images (Figure

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S1), significant differences were existed in SEM and TEM images between Ca/Al LDH-

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Gl and Ni/Al LDH-Gl. It showed that Ca/Al LDH-Gl possessed a typical rod-like 3D

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structure, and the average diameter could be adjusted in the range of 0.247 µm ~ 0.524

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µm. The rod-like structure is beneficial for the surface adsorption and particle

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diffusion.35,38 Interestingly, Ni/Al LDH-Gl samples exhibited flocculent-like structures,

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and the flocculent composites were aggregated with smooth layer-by-layer nano-plates.

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Specifically, the TEM image of Ni/Al LDH-Gl (Figure S1(b)) showed that flocculent

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composites with a ternary network were formed, and the pileup-pellets were beneficial

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for the removal of U(VI) on the surface of flocculent, which was attributed to its high

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specific surface area.

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The bonding types of Ca/Al LDH-Gl and Ni/Al LDH-Gl were evaluated by FT-IR

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technique. As shown in Figure 3(a), the FT-IR spectra exhibited no significant changes of

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Ca/Al LDH-Gl and Ni/Al LDH-Gl in the main peaks. The peak at 3443 cm-1 is assigned

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to the stretching mode of hydroxyl (ν(OH)),28 and the wide bonds between ~3000 cm-1

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and ~3180 cm-1 are defined as the water molecules, which are formed with the

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hybridization of hydrogen-bonded and CO32- in the interlayer (H2O-CO32-).35 Compared

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with Ni/Al LDH-Gl, a special peak at 2515 cm-1 in the FT-IR spectrum of Ca/Al LDH-Gl

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exhibits the typical C-H stretching vibration,39 which is due to the dehydration of glycerol

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molecule.40 Similarly, the special peak at 2185 cm-1 in the FT-IR of Ni/Al LDH-Gl

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demonstrates the ionic structures associated with C=N conjugation formed by the cross-

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linking among glycerol, CO32- and NO3- in aqueous solution.41 The FT-IR spectra

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indicated that glycerol was successfully introduced into the LDHs, and the grafted of

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glycerol molecule could beneficial for the improvement of stability, dispersity and

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decreasing of chemical-resistance. The platform from ~1375 cm-1 to ~1560 cm-1 may be

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attributed to the ν3 vibration of CO32-,42 and the strong peaks at ~1102 cm-1 and ~859 cm-1

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represented the ν1(CO32-) and ν2(CO32-) of various LDHs, respectively.37 Typically, a

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series of characteristic peaks at 980 cm-1, 765 cm-1, 635 cm-1 and 485 cm-1, are exhibited

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the M-O lattice vibrations and M-O-H bending (M: Ca or Al or Ni).29,43

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The phase distribution and purity were characterized by XRD (Figure 3(b)), the

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strong diffraction peak at 2θ = 11.55° was assigned to the (002) planes of Ni/Al LDH-

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Gl,29,31 with the basal spacing d(002) of 7.61 nm. The diffraction peaks at 2θ = 15.11°,

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26.80°, 30.58°, 34.77°, 39.36°, 44.30° and 52.67° corresponded to the (003), (006), (009),

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(012), (015), (018) and (1010) planes of Ca/Al LDH-Gl and Ni/Al LDH-Gl, which

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indicated that the LDHs had a special hydrotalcite structure with relatively well-formed

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crystalline.28,44 In addition, the weak peak at 2θ = 29.31° was attributed to the (100) plane

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of Ca/Al LDH-Gl (d(100) = 3.03 nm), and the similar weak peak at 2θ = 62.43° may

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correspond to the (110) plane of Ni/Al LDH-Gl, which was due to the synergistic effects

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between various divalent cations and Al3+ in the composites.33,29 Furthermore, according

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to Bragg’s law,45 it can be seen that the d(003) and d(006) space are independent of the

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types of divalent cations (d(003) = 5.83 nm, d(006) = 3.32 nm for Ca/Al LDH-Gl and d(003) =

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5.81 nm, d(006) = 3.31 nm for Ni/Al LDH-Gl).

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The XPS spectra of Ca/Al LDH-Gl and Ni/Al LDH-Gl were displayed in Figure 3(c),

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various peaks (e.g., Al 2s, Al 2p, Ca 2p, Ni 2p, C 1s, O 1s and O 2s) indicated that Al, O,

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C and Ni were the predominant elements in Ni/Al LDH-Gl sample, and Al, O, C and Ca

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were the main elements in Ca/Al LDH-Gl sample.46 In order to further explain the

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distribution of functional groups, the high XPS O 1s spectra resolution of Ni/Al LDH-Gl

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and Ca/Al LDH-Gl were shown in Figure 3(d), and the relative contents of different

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groups were listed in Tables S1 and S2. The O 1s spectrum of Ca/Al LDH-Gl can be

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deconvoluted into two components at 531.0 eV (bridging –OH, 13.81%) and 531.9 eV

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(C=O 86.19%), and the O 1s spectrum of Ni/Al LDH-Gl can be also deconvoluted into

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two components at 530.8 eV (bridging –OH, 18.74%) and 531.8 eV (C=O, 81.26%).26

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Similarly, as shown in Figure 3(e) and 3(f), the C 1s peak of Ca/Al LDH-Gl can be

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deconvoluted into four components at 284.6 eV (C=C, 51.75%), 285.5 eV (C-C, 15.41%),

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286.8 eV (C-O, 8.47%) and 289.3 eV (O-C=O, 24.37%), and the C 1s peak of Ni/Al

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LDH-Gl can also be deconvoluted into four components at 284.6 eV (C=C, 56.80%),

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285.9 eV (C-C, 16.27%), 287.3 eV (C-O, 9.23%) and 288.7 eV (O-C=O, 17.70%),

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respectively.32,28,47 The content of total oxygen-containing functional groups calculated

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from C 1s spectrum was 32.84% (C-O and O-C=O) for Ca/Al LDH-Gl, which was higher

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than that for Ni/Al LDH-Gl (26.93%). More oxygen-containing functional groups can

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provide more active sites for the binding of U(VI) in adsorption process.

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Effect of Solid Content. In natural application of environmental remediation, solid

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content can affect the adsorption efficiency of U(VI) and economic benefit.48,49 In order

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to achieve excellent removal efficiency and obtain considerable economic value, the

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effect of solid content on the adsorption process was investigated with the range of 0.0-

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0.6 g/L. As shown in Figure 4(a) and 4(b), the removal percentage of U(VI) from

10


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aqueous solutions on Ca/Al LDH-Gl increased from ~8% to ~57% with the solid content

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increasing from 0.01 g/L to 0.10 g/L, while the removal percentage of U(VI) on Ni/Al

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LDH-Gl increased from ~11% to ~43% at same conditions. The higher adsorption

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efficiency of Ca/Al LDH-Gl might be attributed to more oxygen-containing functional

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groups and active sites on the surface of Ca/Al LDH-Gl,9,

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functional groups (e.g., -OH, C-O, C=O and O-C=O) could improve adsorption energy

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and specific binding between guest molecules and targets, moreover, these highly active

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groups could provide electronic for the adsorption process through participating in

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protonation/deprotonation reaction, which was consistent with the results of XPS spectra

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analysis. Moreover, it can be clearly seen that the Kd values were independent of solid

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contents, which was accordance with the properties of Kd.47 In general, the distribution

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coefficient (Kd) value was independent on solid contents when the competition adsorption

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among various solid particles was negligible and in-apparent at low solid contents.39

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Once increasing the solid content from 0.10 g/L to 0.60 g/L, the adsorption efficiency

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increased slowly, thus 0.10 g/L was the satisfied content for the adsorption process.

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Effect of pH and Ionic Strength. During the adsorption process, solution pH plays an

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important role because it can change the surface charge, the protonation/deprotonation

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process of solid particles and U(VI) ions.2,47 Figure 4(c) and 4(d) exhibited the effect of

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solution pH on U(VI) adsorption on Ca/Al LDH-Gl and Ni/Al LDH-Gl (I = 0.01 M

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NaNO3). Dependent on the high-activity of Ca-O and charge-effect of Ca/Al LDH-Gl, the

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adsorption process of UO22+ could be divided into three various stages, which was the

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result of synergistic effect including surface complexation, electrostatic interaction and

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chemical precipitation. In addition, according to the high adsorption capacities of U(VI)

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36

and oxygen-containing


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on both adsorbents under low pH, it indicated that Ca/Al LDH-Gl and Ni/Al LDH-Gl

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were stability and highly-activity. One can see that the removal percentage of U(VI) from

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aqueous solutions on Ca/Al LDH-Gl increased from ~1% to ~56% with the increase of

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pH from 2.3 to 5.0 (process I). In process I, the positive charge of U(VI) species (e.g.,

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UO22+, UO2OH+, (UO2)2(OH)22+ and (UO2)3(OH)5+) are observed from Figure 4(e) and

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4(f), and the positive charged adsorbents can cause electrostatic repulsion with U(VI).28,50

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With the increase of C[H+], the protonation process was promoted and produced more

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complexes. In addition, changing the concentration of NaNO3 from 0.001 M to 0.1 M, the

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removal percentage was also changed obviously, which indicated that the adsorption

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process was dependent on ionic strength, and demonstrated that the adsorption behavior

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in process I was mainly dominated by outer-sphere surface complexation and

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electrostatic repulsion.50,51 While continues to increase the pH from 5.0 to 7.2, the

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removal percentage decreased from 56% to 37%, which was caused by the deprotonation

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process and the strong electrostatic repulsion interaction in process II.12,13 At alkaline

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condition (7.2 < pH < 10.9) (process III), the adsorption efficiency was improved again

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and achieved to 85%, which may be due to the formation of precipitates (schoepite)

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(Figure 4(f)), and the process III was dominated by surface precipitation and outer-sphere

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surface complexation. Furthermore, the negative derivatives of U(VI) (e.g., UO2(OH)3-

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and UO2(OH)42-) can improve the electrostatic attraction to positively charged Ca/Al

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LDH-Gl.9,10 Compared to Ca/Al LDH-Gl, the adsorption behavior of U(VI) on Ni/Al

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LDH-Gl was more simple. At low pH values (2.3 < pH < 6.0), the removal percentage

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increased from ~2% to ~70%, and the process was little independent of ionic strength,

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which demonstrated that the adsorption reaction was controlled by inner-sphere surface

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complexation and electrostatic attraction.50 In addition, based on the stabilizer effect of

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glycerol molecule, both adsorbents decreased the proteolytic phenomenon and exhibit

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highly-activity and reactivity even in acidic solution. At high pH values (6.0 < pH < 10.9),

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the process was strong dependent on ionic strength, indicating that the main interaction

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was also dominated by surface precipitation and outer-sphere surface complexation.52

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Interestingly, with the increasing of background electrolyte, the adsorption capacity of

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U(VI) on Ca/Al LDH-Gl was decreased while that on Ni/Al LDH-Gl was increased, and

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it was attributed to the following reasons: (1) stronger competitive adsorption among

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NaNO3, UO22+ and Ca/Al LDH-Gl based on the higher active of M-O (e.g., Ca-O) on the

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surface of Ca/Al LDH-Gl; (2) higher electrostatic repulsion between UO22+ and Ca/Al

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LDH-Gl with the increasing of Na+.39 The different adsorption behavior between Ca/Al

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LDH-Gl and Ni/Al LDH-Gl may be attributed to the easily changeable charge of Ca2+

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and its derivatives.

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Adsorption Isotherms and Thermodynamic Behavior. In order to explore the

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adsorption mechanism and thermodynamic behaviors, adsorption isotherms of U(VI) on

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Ca/Al LDH-Gl and Ni/Al LDH-Gl were investigated at three temperatures (298.15 K,

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313.15 K and 333.15 K). As shown in Figure 5(a) and 5(b), the adsorption efficiencies of

273

U(VI) on Ca/Al LDH-Gl and Ni/Al LDH-Gl were promoted obviously at higher

274

temperature, especially in high U(VI) concentration. In addition, three traditional

275

isotherms, i.e., Langmuir, Freundlich, and Sips models, were applied to simulate the

276

experimental data. Typically, Langmuir isotherm represents monolayer adsorption, and

277

the adsorption reaction is carried on the surfaces and bulk phase of homogeneous

278

adsorbents, which is described as47:

13



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Ce Ce 1 = + C s C s, max K L C s, max

279

Page 14 of 43

(1)

280

Freundlich isotherm stands for multilayer adsorption, and the adsorption process is

281

considered to produce on the surface of heterogeneous adsorbents, which can be

282

described as3,26: log C s = log K F +

283 284 285

1 log Ce n

(2)

Sips isotherm is regarded as a combined form of Langmuir and Freundlich equations, which can predict the heterogeneous adsorption process, and it is expressed as51: n

Cs =

286

K s Ce s n

1 + a s Ce s

(3)

287

where Ce (mg·L-1) and Cs (mg·g-1) are the equilibrium concentration and the amount of

288

U(VI) adsorbed on adsorbents, Cs,max (mg·g-1) is the maximum amount under per unit

289

weight of adsorbents, KL (L·mg-1) is a constant related to Langmuir isotherm, and with

290

the increase of KL, the affinity and bonding of adsorbent can be promoted. KF (mg1-

291

n

292

corresponds to the adsorption intensity. ns is the Sips’ heterogeneity parameter, and Ks =

293

Cs,max·ns (L·g-1) is a Sips constant.

·Ln·g-1) is a constant related to the adsorption capacity of Freundlich model, and 1/n

294

The corresponding parameters calculated from the three models were tabulated in

295

Tables S3. One can see that the adsorption isotherms of U(VI) on Ca/Al LDH-Gl were

296

well fitted by Sips mode (R2 > 0.95), and the maximum adsorption capacities of U(VI)

297

calculated from Sips model was 266.5 mg·g-1 at 298.15 K (Tables S3). It demonstrated

298

that the adsorption process was multilayer adsorption at low concentration of U(VI) and

299

monolayer adsorption at high concentration.51 However, the adsorption curves of U(VI)

14


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300

on Ni/Al LDH-Gl were well fitted by Langmuir model (R2 > 0.91) and Sips model (R2 >

301

0.90), and the maximum adsorption capacity calculated from Sips model was 142.3 mg·g-

302

1

303

higher adsorption capacity of U(VI) on Ca/Al LDH-Gl might be attributed to more

304

oxygen-containing functional groups (e.g., C-O, O-C=O and C=O), which provided more

305

active sites, promoted the deprotonation reactions and thereby enhanced the binding of

306

U(VI) ions.10,12,52

at 298.15 K, indicating that the adsorption reaction was monolayer coverage.3 The

307

To further study the feasibility and stability of the adsorption reaction, various

308

thermodynamic parameters (e.g., standard entropy change (∆S0, J·K-1·mol-1), standard

309

free energy change (∆G0, kJ·mol-1) and standard enthalpy change (∆H0, kJ·mol-1)) were

310

calculated according to the following equations12:

311

ln K 0 =

∆S 0 ∆ H 0 − R RT

(4)

312

∆ G 0 = − RT ⋅ ln K 0

(5)

313

where K0 is the adsorption equilibrium constant, and the values of ∆H0 and ∆S0 are

314

obtained from the slope and intercept of linear regression of Ln K0 versus T-1 (Figure. S2).

315

The values of ∆G0 at different temperatures were calculated by Eq. (5), and T is the

316

absolute temperature in Kelvin and R is the gas constant (8.314 J·mol-1·K-1). The relative

317

values of thermodynamic parameters for the adsorption of U(VI) at different

318

temperatures (i.e., 298.15 K, 313.15 K, and 333.15 K) were shown in Table 1.

319

As shown in Table 1, the positive values of △H0 suggested that the adsorption

320

process of U(VI) on Ca/Al LDH-Gl and Ni/Al LDH-Gl was an endothermic reaction

321

process.3 In addition, the positive △S0 values demonstrated that the reaction was a

15



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322

spontaneous adsorption process with high affinity, which indicated that more ligands

323

incorporated with UO22+ and more H+ or OH- participated the adsorption process.56 The

324

negative △G0 values further proved the spontaneous adsorption process,47,56 and the

325

decrease of △G0 with the increase of temperature showed that better adsorption efficiency

326

could be obtained under higher temperature, which was due to the easy dehydration of

327

U(VI) adsorbed on Ca/Al LDH-Gl or Ni/Al LDH-Gl.56 The adsorption capacities of

328

U(VI) on different materials were summarized in Table 2, and it was clear that Ca/Al

329

LDH-Gl and Ni/Al LDH-Gl could be the promising potential adsorbents for the efficient

330

removal of U(VI) from aqueous solutions in environmental pollution remediation and

331

cleanup.

332

Adsorption kinetics. In the practical application, adsorption rate is also an

333

significant factor for the evaluating various adsorbents. The effect of contact time on

334

U(VI) adsorption on Ca/Al LDH-Gl and Ni/Al LDH-Gl were compared in Figure 5(c), it

335

demonstrated that the adsorption of U(VI) on Ni/Al LDH-Gl increased rapidly and

336

reached ~40% in the first 4 h of contact time and then increased slowly. While the

337

adsorption on Ca/Al LDH-Gl increased to ~60% in the first 7 h of contact time and

338

maintained the high level with further increase of contact time. At the first stage (t < 4 h

339

for Ni/Al LDH-Gl or t < 7 h for Ca/Al LDH-Gl), abundant free binding sites and active

340

sites were available for the removal of U(VI) from aqueous solutions, and the adsorption

341

was quickly at the initial contact time. Especially, in the initial 4 h, the adsorption rate of

342

U(VI) on Ni/Al LDH-Gl was higher than that on Ca/Al LDH-Gl, which was controlled

343

by the higher stability of Ni/Al LDH-Gl in weak-acid solution and lower diffusion

344

resistance of solid particles. However, at the later stage (t > 4 h for Ni/Al LDH-Gl or t > 7

16


Page 16 of 43

Page 17 of 43

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345

h for Ca/Al LDH-Gl), most of the active sites were occupied by U(VI) ions and the

346

adsorption of U(VI) would proceed in the inside of adsorbents, which needed longer

347

diffusion ranges to bonding with inner-free binding sites, and the adsorption increased

348

slowly.9,47 As shown in Figure 5(d), the highest peak position of U(VI) UV-vis spectra in

349

both adsorbent suspensions was found at the wavelength of 650 nm, and it did not change

350

with the increase of contact time, suggesting that the basic characteristic of U(VI) (i.e.,

351

species, microstructures etc) was maintained and did not transfer to other new

352

components.47

353

To evaluate the mass transfer process in adsorption system, two typical adsorption

354

kinetic models (pseudo-first-order and pseudo-second-order kinetic) were applied to

355

simulate the kinetic adsorption process, which can be described as follows3,47:

356

ln(q e − qt ) = ln q e − k1t

(6)

357

t t 1 = + qt k 2 q e2 q e

(7)

358

where qe (mg·g-1) and qt (mg·g-1) are the adsorption capacity at equilibrium and time t

359

(h), respectively. k1 (min-1) and k2 (g·mg-1·min-1) represent the constant of pseudo-first

360

oeder and pseudo-second order rate, respectively. The values of k1, k2 and qe can be

361

calculated from the linear plot of ln (qe - qt) versus (t) or t/qt versus (t), and the basic

362

parameters are listed in Table S4. As inserted in Figure 5(c) and Figure S3, the higher R2

363

values (R12 = 0.996 and R22 = 0.999) demonstrated that the adsorption kinetics of U(VI)

364

on Ca/Al LDH-Gl and Ni/Al LDH-Gl followed the pseudo-second order model, and the

365

adsorption process was mainly attributed to chemical reaction.3,47

366

Effect of Carbonate Ions. In the natural environment, various anions and cations

17



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367

are widespread existed in aqueous solution, and many researchers have studied the effect

368

of these ions in the removal of environmental pollutants.7,11 However, the results

369

demonstrated that CO32- ions could produce significant influence on the removal of U(VI),

370

which was due to the abundant carbon dioxide in the natural condition and its high water

371

solubility,56 thus the effect of CO32- ions should be considered and discussed.

372

As shown in Figure 6(a) and 6(b), the observed increase in the enrichment of U(VI)

373

at low concentration of CO32- (0.001 < C[CO32-] < 0.02 for Ca/Al LDH-Gl and 0.001

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