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Amino acid functionalized chitosan magnetic nano-based particles for uranyl sorption Ahmed Galhoum, Mohammad Mahfouz , Asem A. Atia, Sayed Abdel-Rehem, Nabawia Gomaa, Thierry Vincent, and Eric GUIBAL Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03331 • Publication Date (Web): 19 Nov 2015 Downloaded from http://pubs.acs.org on November 23, 2015

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Industrial & Engineering Chemistry Research

1

Amino acid functionalized chitosan magnetic nano-based particles for uranyl sorption

2

Ahmed A. Galhouma,b, Mohammad G. Mahfouz a, Asem A. Atiac,

3

Sayed T. Abdel-Rehemd, Nabawia A. Gomaaa, Thierry Vincentb, Eric Guibalb,*

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a

Nuclear Materials Authority, P.O. Box 530, El-Maadi, Cairo, Egypt.

b

Ecole des Mines Alès, Centre des Matériaux des Mines d’Alès, 6 avenue de Clavières, Alès cedex, France. b

Chemistry Department, Faculty of Science, Menoufia University, Shebin El-Kom, Egypt.

c

Chemistry Department, Faculty of Science, Ain Shams University, Egypt.

9 10

Abstract

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Nano-based magnetic particles are synthesized by a one-pot hydrothermal precipitation of

12

chitosan in the presence of iron(II) and iron(III) salts. The material is then chemically modified

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by grafting alanine and serine amino acids. These materials are characterized by elemental

14

analysis, FTIR spectrometry, XRD analysis, TEM observations and by VSM (vibrating-sample

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magnetometry). In a second step these materials are tested for uranium(VI) sorption. The effect

16

of pH on sorption performance is first investigated before evaluating uptake kinetics (which are

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fitted by the pseudo-second order rate equation) and sorption isotherms (which are modelled by

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the Langmuir equation). Metal desorption and sorbent recycling are finally carried out for five

19

successive cycles of sorption/desorption. Thermodynamic parameters are determined by

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investigating sorption properties at different temperatures. Maximum sorption capacities reach 85

21

mg U g-1 and 116 mg U g-1 for alanine- and serine-based sorbents, respectively. The magnetic

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properties of the particles allow their efficient separation from the solution by external magnetic

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field.

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Keywords: alanine; serine; uranium(VI); amino acid functionalized chitosan; hybrid magnetic nano-based

25

particles; sorption isotherms; uptake kinetics.

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____________________________________________

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*Corresponding author:

28

[email protected] (Eric Guibal)

1

ACS Paragon Plus Environment


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1. Introduction

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Nuclear industry generates huge amounts of contaminated aqueous streams along the production

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chain: from mining activities to enrichment stage, including the reconditioning of spent material.

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The release of uranium from industrial units is strictly controlled because of the potential

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hazardous impact of this metal on human and animal health. The uptake of uranium by human

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beings may cause serious health problems, such as severe liver and kidney damages due to its

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extremely chemical and radioactive toxicity.1

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The strong demand for uranium worldwide has driven the attention of the research community

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for developing new processes for uranium recovery from dilute solutions.2 Separation and

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recovery of uranium are thus of great importance for both the reutilization of uranium resources,

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the sustainable development of nuclear energy, and for environmental protection.3, 4

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A number of processes have been developed for metal recovery including precipitation, solvent

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extraction, membrane processes, and ion-exchange and chelating resins. These different

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processes may face limitations depending on the type of metal, concentration, complexity of the

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solution: (a) technical limitations (difficulty to reach authorized discharge levels), (b)

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environmental constraints (production of huge amounts of contaminated sludge), and (c)

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economic limitations (expensive extractants). For effluents containing low levels of strategic or

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toxic-metal ions, ion-exchange and chelating resins are generally preferred.4 Biosorption, which

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consists in using waste biomass from agriculture, fisheries or industry, has received a great deal

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of attention for the last decades. These renewable materials bear functional groups similar to

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those found in synthetic resins and can be efficiently used for metal recovery from dilute

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solutions.5-7

2


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Chitosan is a naturally abundant and biodegradable polymer. This polysaccharide is obtained by

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partial alkaline deacetylation of chitin (a copolymer of glucosamine and N-acetyl-D-glucosamine

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linked by β(1→4) glycosidic bonds).8,

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chemically modified to elaborate new resins that bear functional chelating and ion-exchange

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groups.9,

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compared to conventional synthetic resins (like polystyrene-divinylbenzene, polyethylene, and

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polyurethane). This hydrophilic behavior may contribute to improve swelling and hydration

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properties, which, in turn, enhance uptake kinetics.

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Chelating or coordinating, ion-exchange resins are polymers with covalently bound functional

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groups containing one or more donor atoms that are capable of forming complexes with metal

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ions.11 These polymers can be used for specific separation of target metal ions from complex

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multi-component solutions.12-14 In chelating resins, the functional groupss the most frequently

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used are based on nitrogen (i.e., N as amine, azo, amide, nitrile groups), oxygen (i.e., O as

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carboxylic, hydroxyl, phenolic, ether, carbonyl, phosphoryl groups) and sulfur (i.e., S as thiol,

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thiocarbamate, thio-ether groups).15 Several chelating ligands such as catechol, iminodiacetic

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acid, iminodimetyl-phosphonic acid, phenylarsonic acid, or serine16 and amino acids moieties

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(glycine, valine, leucine, and serine) were used to functionalize cross-linked chitosan for sorption

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of uranium(VI) metal ions.17

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The major drawback of chitosan-based materials in sorption process is associated to the poor

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porosity of these raw materials.18-20 It is generally necessary improving the conditioning of the

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biopolymer (to prepare expanded non-crystalline structures, such as chitosan gel beads) or

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decreasing the size of sorbent particles (to reduce the impact of the resistance to intraparticle

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diffusion on the control of uptake kinetics). Nano-sized sorbents with high specific surface area

10

9

Chitosan is a very versatile material that can be

In addition, this biopolymer is characterized by its high hydrophilicity, at least

3



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and improved properties of intraparticle diffusion are promising alternatives to conventional

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materials.8, 9 However, the main drawback for these materials consists of the difficulty to separate

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sorbent particles at the end of the sorption by filtration or centrifugation. This problem can be

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overcome by incorporating a magnetic core in the nano-based particles: an external magnetic

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field allows recovering the particles at the end of sorption step.8,

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usually composed of a magnetic core (to ensure a strong magnetic response) and a polymeric

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shell (to provide selective functional groups).9, 25-27

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The present study focuses on the synthesis of hybrid materials associating a magnetic core and

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coating of chitosan that was functionalized by grafting amino-acids. Chitosan magnetic nano-

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based particles are synthesized in a one-pot procedure before being cross-linked to reinforce the

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stability of the sorbent. Amino acid grafting is operated through cross-linking with

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epichlorhydrin. Different techniques are used for the physicochemical characterization of the

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sorbent, including elemental analysis, FTIR spectrometry, TEM observation and VSM (vibrating

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sample magnetometry for magnetic properties). In a second step the sorption properties of the

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materials are tested for uranium(VI) recovery from dilute solutions. Main experimental

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parameters are investigated: pH effect, uptake kinetics and sorption isotherms. Thermodynamic

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constants are determined before investigating the possibility to desorb uranium and recycle the

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sorbent.

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2. Experimental

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2.1. Reagents and analysis

4


9, 21-24

Magnetic sorbents are

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Chitosan (90.5 % deacetylation degree) was supplied by Sigma-Aldrich (France). Alanine and

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serine amino acids were supplied by Sigma-Aldrich (France). Epichlorohydrin (> 98 %), 1,4-

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dioxane (99.9%) and ethanol were purchased from Fluka AG (Switzerland). Arsenazo III (A.R

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grade) was obtained from Fluka AG (Switzerland) and all other chemicals were Prolabo (France)

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products and were used as received. Uranium stock solution was prepared from

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UO2(OCOCH3)2·2H2O (supplied by Sigma-Aldrich, France) by dissolving in concentrated

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sulfuric acid under heating and finally diluted with demineralized water until final concentration

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of 1000 g L−1. The working solutions were prepared by appropriate dilution of the stock solution

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immediately prior to use. Uranium concentrations in both initial and withdrawn samples were

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determined by spectrophotometry using the Arsenazo III colorimetric method,28 and a

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“MetertechInc”(Taiwan) model SP-8001, UV–Visible spectrophotometer.

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2.3. Preparation of sorbent

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2.3.1. Preparation of cross-linked chitosan–magnetite nano-based particles

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Hybrid chitosan magnetic nano-based particles were prepared by a one-pot chemical co-

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precipitation of Fe(II) and Fe(III) ions by NaOH in the presence of chitosan, followed by

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hydrothermal treatment.29 Chitosan (4 g) was dissolved in 200 mL acetic acid solution (20 %,

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w/w) with FeSO4 and FeCl3 (1:2 molar ratio, corresponding to: 6.62 g FeSO4.7H2O and 8.68 g

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FeCl3, respectively). The solution was chemically precipitated at 40 oC by adding NaOH (2 M)

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dropwise with constant stirring, at controlled pH (i.e., 10–10.4). The suspension was heated at 90

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o

C for 1 h under continuous stirring, and the particles were recovered by magnetic separation. An

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alkaline solution of 0.01 M epichlorohydrin (0.01 M in 0.067 M NaOH, pH ≈ 10) was then added

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to the freshly prepared wet magnetic-chitosan particles (mass ratio 1:1) (referred (i)). The

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suspension (chitosan-magnetic nano-based particles and epichlorohydrin) was heated for 2 h at

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40-50 oC under continuous stirring.30 Finally, the cross-linked hybrid chitosan magnetic nano-

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based particles (referred (ii)) were removed by magnetic separation and washed intensively with

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demineralized water to remove any unreacted reagent.

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The amino acid moiety (alanine/serine) was grafted on cross-linked hybrid chitosan magnetic

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nano-based particles in two steps:17 first, the cross-linked chitosan magnetic nano-based particles

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(ii) were suspended in 150 mL ethanol/water mixture (1:1 v/v) before epichlorohydrin (15 mL)

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was added to the suspension; the mixture was refluxed for 4 h. After the reaction, the product (iii)

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was filtered and washed 3 times with ethanol and with ultrapure water (MilliQ) to remove any

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residual reagent. In a second step, the washed product (iii) and alanine (or serine 16 g) were

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suspended in dioxane (200 mL). The mixture was alkalinized to pH 9.5-10 using 1 M NaOH

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solution; the mixture was heated under reflux for 6 h. After the reaction, the final product was

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filtered and washed 3 times with ethanol and with ultrapure water. Finally, the sorbents were

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freeze-dried (Figure AM1, See Supporting Information).

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2.3.2. Characterization methods

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The amine content in the sorbent was estimated using a volumetric method:31 30 mL of 0.05 M

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HCl solution was added to 0.1 g of sorbent under agitation for 15 h. The residual concentration of

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HCl was estimated through titration against 0.05 M NaOH solution using phenolphthalein as the

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indicator. The number of moles of HCl having interacted with amino group and consequently the

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amino group concentration (mmol g-1) was calculated from Eq. (1):

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Concentration of amino group = (M1 −M2) × 30 / 0.1

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where M1 and M2 are the initial and final concentrations of HCl, respectively.

6


(1)

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The composition of the resin was characterized for C, H, and N contents using an automatic

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analyzer CHNS Vario EL III-elementar analyzer (Elementar, Germany). Powder X-ray

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diffraction (XRD) patterns were obtained, at room temperature, by a Philips X-ray diffractometer

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equipped with a PW 3710/31 controller, a X/Y PW generator and a PW Z/W goniometer for

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testing X-ray diffraction (using CuKα radiation in the range of 2θ = 10–90o). The dimension and

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morphology of sorbent were observed by high resolution transmission electron microscope

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HRTEM (JEOL-2100, Japan). The magnetic properties were measured on a vibrating sample

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magnetometer (VSM) (730T, Lakeshoper, America) at room temperature. Functional groups of

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sorbent were analyzed by Fourier Transform infrared spectroscopy using a FT-IR spectrometer

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Nicolet Nexus 870 (Nicolet, USA). The scanning range was set between 4000 and 400 cm-1 and

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the samples were prepared by inclusion in KBr pellets.

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2.4. Sorption and desorption experiments

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Batch experiments were carried out by contact of a mass of functionalized chitosan sorbent (m:

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0.05 g) with a fixed volume (V: 50 mL) of aqueous uranium solution (C0: 110 mg U L-1) in a

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conical flask. The flasks were maintained for 2 h in agitation (rotation speed, v: 200 rpm) at room

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temperature (i.e., T: 25 ±1 oC). Phase separation was performed by magnetic separation and the

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residual uranium concentration in the aqueous phase (Ceq, mg U L-1) was determined by the

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Arsenazo III method. The concentration of uranium in the sorbent (qeq) was calculated by the

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mass balance equation Eq. (2):

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159

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qeq = (C0 – Ceq) x V/M

(2)

The distribution coefficient Kd is obtained by the qeq/Ceq ratio and Eq. (3): Kd = [(C0 – Ceq)/ Ceq] x V/M

(3)

7



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Experimental conditions are systematically reported in the caption of the Figures: in most cases

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the equilibrium time was 40 min (experiments being performed at room temperature; i.e., 25 ± 1

163

o

C) and the pH was set at 3.6. Experiments that were duplicated or triplicated showed a standard

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deviation lower than 6 %.

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To evaluate the reusability of the sorbent, U(VI) sorption and desorption efficiencies were

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evaluated along five consecutive sorption–desorption cycles with the same sorbent. Experimental

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conditions for sorption correspond to: (a) mixing of 50 mg of sorbent with 50 mL of a U(VI)

168

solution (C0: 250 mg U L-1) for 45 min at room temperature in a conical flask, followed by (b) a

169

magnetic separation, and (c) metal analysis and mass balance calculation. After the sorption step,

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the sorbent was rinsed with demineralized water before processing to metal desorption. U(VI)

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desorption was operated at room temperature by contact of the collected sample with 50 mL of a

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urea solution (0.5 M, at slightly acidic pH; i.e., pH in the range 2-3). After magnetic separation,

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uranium concentration in the eluate was analyzed and the mass balance equation was used to

174

evaluate the desorption efficiency.

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3. Results and discussion

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3.1. Preparation of magnetic chitosan nanoparticles

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A simple one-pot in situ co-precipitation method was used to synthesize hybrid chitosan magnetic

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nano-based particles. In alkaline conditions, chitosan simultaneously precipitates to the synthesis

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of magnetic iron particles (reaction between Fe(II) and Fe(III) under hydrothermal alkaline

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conditions) resulting in the formation of composite magnetite-chitosan particles.29 The

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hydrothermal alkaline treatment produces magnetite particles as shown by X-ray diffraction

8


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analysis (see below). In addition, TEM analysis confirmed that nano-particles are formed (though

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they tend to aggregate to form micro-particles, see below). Chitosan-magnetite particles can be

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chemically modified to prevent their dissolution in acidic media; however, glutaraldehyde-

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crosslinking may result in the loss of sorption capacity because some amine groups are involved

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in the crosslinking reaction,32, 33 so epichlorohydrin (or chloromethyloxirane) had been used as

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the crosslinking agent. Indeed, the crosslinking mono-functional agent is used to form covalent

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bonds with the carbon atoms linked to the hydroxyl groups of chitosan, resulting in the rupturing

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of the epoxide ring and the release of a chlorine atom.34 Iron cations (Fe(II) and Fe(III)) in the

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solution (prior to reaction with epichlorohydrin) may interact with amino groups (and

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neighboring hydroxyl groups) and these interactions contribute to prevent epichlorhydrin to react

193

with these amino groups. This may also contribute in failing to orientate the cross-linking (and

194

spacer-arm) agent to react with other hydroxyl groups.35 Figure AM1 (See Supporting

195

Information) shows the synthesis route for amino acid functionalized magnetite-chitosan. In a

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second step, epichlorhydrin reacts with chitosan backbone: epoxide ring is opened and reacts

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with amino groups; chloride-ends remain available for reacting with amino group or hydroxyl

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group of the amino acid (with simultaneous release of chlorine). In the case of alanine, only

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amino groups are involved in amino acid grafting (due to the poor reactivity of –CH3 group),

200

while for serine two reactive groups may be involved in the grafting (i.e., amino groups and

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hydroxyl groups). Figure AM2 (See Supporting Information) reports the proposed chemical

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structure of the two sorbents. The effective chemical modification of chitosan was demonstrated

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by FT-IR spectrometry (see below).

204

3.2. Characterization of sorbents

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Table AM1 (See Supporting Information) reports the elemental analysis (C, H and N elements,

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mass

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chitosan/magnetic nano-based particles: the matrix) before and after grafting with alanine and

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serine, respectively. The increases of carbon and nitrogen contents show the efficient grafting of

209

the amino acids on the chitosan backbone: more specifically the mass percentage of nitrogen is

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doubled from 1.7 % to 3.3-3.4 %. It is noteworthy that the differences are not very marked

211

between the two derivatives. The concentrations of amino groups of alanine- and serine-type

212

sorbents (determined by volumetric titration) were found to be 3.94 and 3.60 mmol g-1,

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respectively. This means for the two derivatives about 1.38 and 1.26 times the amount of amino

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groups in the matrix. These values are not consistent with the elemental analysis; indeed, the

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mass percentage of N element was evaluated to 3.4 % (i.e., about 2.4 mmol N g-1). This is

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significantly less than the titration data: the volumetric titration is probably measuring other acid-

217

base groups than amino groups.

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Samples were analyzed by FTIR spectrometry, at the different steps of the synthesis, in order to

219

verify the grafting mechanisms (Figure 1). The band appearing at a wavelength close to 568 cm-1

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can be assigned to Fe-O stretching vibration in Fe3O4.29, 36 A characteristic strong and broad band

221

appears around 3399 cm−1 (in all spectra); this band corresponds to the combination of stretching

222

vibration of –OH group, the extension vibration of N–H group and the inter-hydrogen bonds of

223

polysaccharides. The characteristic peak of primary amine –NH2 appears at 1613 cm−1. The

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bands at 1463 and 1364 cm−1 can be attributed to the C–O–C stretching and –OH bending

225

vibrations, respectively. The absorption band at 893 cm−1 corresponds to β-D-glucose unit.37 This

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band (in conjunction with a band at 1250 cm-1) was also attributed of epoxy and oxirane rings.38

percentage)

of

the

hybrid

material

10

(composite


epichlorohydrin-cross-linked

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The absorption bands around 1320 and 1065 cm-1, correspond to the stretching vibration of

228

primary –OH group and the secondary –OH group, respectively.

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The intensities of these bands (–NH2 and –OH groups) decrease after biopolymer cross-linking

230

(reaction with epichlorohydrin): this result confirms the interaction of the cross-linking agent

231

(and spacer arm) with hydroxyl and amine groups.37 The epichlorohydrin cross-linking and the

232

introduction of spacer arms are confirmed by the appearance of a new band at 792 cm−1 that can

233

be attributed to C–Cl stretching vibration (in –CH2-Cl environment).38 The band at 1631-1637

234

cm–1 may be attributed to either (–COO–) carboxylate group vibration of the amino acid moiety,17

235

(new functional groups) or amide group (shift of initial bands appearing on the matrix

236

spectrum).38 The increasing intensity at 1422 and 1411 cm−1 in the spectra of alanine- and serine-

237

type sorbents (respectively), shows that the amount of amino groups increased with grafting the

238

amino acids (compared to matrix).33

239

The XRD patterns of amino acid functionalized chitosan magnetic nano-based particles are

240

shown in Figure AM3 (See Supporting Information), together with the assignments of the

241

different peaks representative of Fe3O4: (111), (220), (311), (400), (422), (511), (440), and (622).

242

These peaks are consistent with the database in JCPDS file (PDF No. 65-3107).36 This confirms

243

the presence of iron oxide particles (Fe3O4) with a spinel structure, which has magnetic properties

244

and can be used for magnetic separation.33 The full width at half maximum (FWHM) was used

245

for calculating the size of particles using the Debye-Scherrer equation:39

246

(3)

D = k λ / β cos θ

247

where D is the average diameter of nanoparticles, λ is the wavelength of X-ray radiation (1.5418

248

Å), θ is the diffraction angle, k=0.9 (shape parameter) and β is the full width at half maximum

11



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(FWHM) of selected X-ray diffraction peaks. The crystallite size were 13.0 nm and 11.6 nm

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(using the larger XRD peak; i.e., (311) index at 2θ = 35.4 degrees) for alanine-based and serine-

251

based sorbents, respectively. The size of magnetite nanoparticles was reported to depend on the

252

pH of precipitation and the temperature: high pHs and high temperatures usually contribute to

253

increase the size of the particles.40 In the case of hybrid chitosan-magnetite particles cross-linked

254

with glutaraldehyde, Morales et al. showed that the size of nanoparticles increased with

255

increasing the amount of cross-linking agent.41

256

The TEM images of the sorbents are shown in Figure AM4 (See Supporting Information): the

257

particles have a spherical and regular morphology and are homogeneously distributed in size. The

258

structure of the sorbents was monodisperse; however, probably due to dipole-dipole magnetic

259

attraction, the nanoparticles tended to aggregate and form particles of bigger size with an average

260

diameter close to 15–40 nm. Similar aggregation phenomena were observed on Fe3O4 magnetic

261

nanoparticles coated with chitosan that were developed for enterotoxin recognition and

262

enrichment.25

263

The magnetic performance of the sorbents was determined using VSM: Figure AM5 (See

264

Supporting Information) shows their respective magnetization loops. There was no remanence

265

and coercivity, contrary to certain supported-magnetite materials:42 these composites can be

266

characterized as superparamagnetic materials.43 This superparamagnetic behavior is frequently

267

associated to nano-sized magnetite particles: the critical size being less than 25 nm. This is

268

consistent with TEM observations. The saturation magnetization of alanine-based and serine-

269

based sorbents, were found to be about 14.0 and 10.6 emu g-1, respectively. These values are

270

much smaller than the levels reported for bulk phase magnetite (i.e., 92 emu g-1) and also smaller

271

than the values obtained for magnetite nanoparticles.40 The embedment of magnetite in non-

12


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magnetic supports has already been reported to significantly decrease saturation magnetization.44

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This decrease in saturation magnetization can be explained by several factors including size

274

effect and particle crystallization,40 and obviously by the fact that the magnetic core only

275

represents a 50 % fraction of the sorbent (w/w), as determined by the weight loss at 700 °C..45

276

Similar decrease in magnetization was observed for other chitosan-magnetite composites.46 High

277

temperatures and high pHs contribute to increase particle size which, in turn, increases saturation

278

magnetization.40 In the case of diethyleneatriamine-functionalized chitosan magnetic nano-based

279

particles the saturation magnetization was found close to 20 emu g-1.47 The magnetic sorbent

280

particles can be easily separated with the help of an external magnetic field. This may be very

281

helpful for solid/phase separation and for handling the material in hazardous environment.

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3.2. Sorption properties

283

3.2.1. pH effect on uranyl sorption

284

The pH of the solution plays a key role on the affinity of sorbents for target metals. This

285

parameter influences both the properties of the sorbent and metal speciation in solution. Indeed,

286

varying the pH contributes to the dissociation of functional groups (carboxylic, hydroxyl, amino

287

groups), the change in the surface charge of the sorbent, and then its affinity for metal ions. The

288

pH variation and more generally the composition of the solution (i.e., metal concentration,

289

presence of competitor ions, presence of ligands) control the speciation of the metal,48 which, in

290

turn, influences the affinity of metal species for target reactive groups, but also metal solubility.

291

The range of pH to be tested also depends on the stability of the sorbent; in the case of hybrid

292

chitosan/magnetite material, the stability of both the biopolymer and the magnetic core should be

293

considered. The cross-linking treatment contributes to improve the stability of the chemically

13



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Page 14 of 48

294

modified polymer; the main limitation in terms of pH stability is thus related to the stability of the

295

magnetic core. At pH below 1.5-2, Fe3O4 may partially and progressively dissolve (especially at

296

large contact time). On the other side, depending on metal concentration, the precipitation of

297

uranyl ions may begin at pH close to 6 (under the form of colloidal UO2(OH)2 species). The

298

experiments on pH effect were performed between pH 1.6 and pH 6.7 (with discussion of

299

stability issues).

300

The sorption of uranyl ions increases from 20 mg U g-1 to 72 and 92 mg U g-1 for alanine- and

301

serine-based sorbents respectively when the initial pH increases from 1.7 to 3.7 (Figure 2). Above

302

initial pH 4 the sorption capacity tends to stabilize. However, the sorption capacity tends

303

toincrease again above initial pH 6.5, due to metal precipitation (formation of UO2(OH)+ and

304

further UO2(OH)2, which may precipitate as colloidal species in the solution or on the sorbent).

305

The pH remains stable in the ranges 1.7-2.5 and 5-6.7, while it increases by 0.5-1.5 units in the

306

pHi range 2.5-4.7. The increase in sorption capacity can be explained by the progressive

307

deprotonation of reactive groups: protonated amino groups (which are converted to free amino

308

groups) and carboxylic acid groups (which are converted to carboxylate groups). Both free amino

309

groups and carboxylate groups are more favorable to the binding of metal cations than their

310

protonated forms. The affinity of uranyl species for sorbent may also change in function of the

311

pH due to change in metal speciation. Indeed, the hydrolysis of uranyl ions plays a significant

312

role on the predominance of metal species in solution at different pHs and metal concentrations:

313

formation of polynuclear and polyhydrolyzed species according to:49

314

UO22+ + 2 H2O ↔ UO2(OH)+ + H3O+

pK1 = 5.8

(4a)

315

2 UO22+ + 4 H2O ↔ (UO2)2(OH)22+ + 2 H3O+

pK2 = 5.62

(4b)

14


Page 15 of 48

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316


3 UO22+ + 10 H2O ↔ (UO2)3(OH)5+ + H3O+

pK3 = 15.63

(4c)

317

At low pH, divalent free UO22+ ions predominate in the solution; while with increasing the pH the

318

monovalent hydrolyzed species appear and predominate (i.e., UO2(OH)+, (UO2)2(OH)22+,

319

(UO2)3(OH)5+). At higher pH (i.e., pH> 5.5), dissolved solid schoepite (4UO3.9H2O) may exist in

320

the solution. Previous studies have shown the importance of the formation of polynuclear species

321

on uranium biosorption,50 and

322

sorbents.51, 52

323

In acidic conditions (pH below 2) the protonation of reactive groups induces the electrostatic

324

repulsion of free uranyl cations (UO22+); however, the sorption capacity is not negligible (about

325

20 mg U g-1). This means that another mechanism is probably involved in metal binding. The

326

protonation of amino groups makes possible the binding of anionic species by electrostatic

327

attraction or anion-exchange. Ritcey and Ashbrook reported the extraction of uranyl by tertiary

328

amine as anionic or neutral species in sulfuric acid solutions through the formation of anionic

329

complexes such as: UO2.(SO4)22- and UO2.(SO4)34-.53 Similar mechanisms have been identified in

330

the sorption of uranyl ions from sulfuric acid solutions using quaternary ammonium salt resins

331

(i.e., Amberlite IRA-910):54

332

First, in solution

on molybdate and vanadate recovery using chitosan-based

333

UO22+ + 2SO42- → UO2.(SO4)22-

(5)

334

UO2.(SO4)22+ SO42- → UO2.(SO4)34-

(6)

335

336

And sulfate binding on the resin: 2 R3N + H2SO4 ↔ (R3NH+)2SO42-

(7a)

15



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337

Page 16 of 48

Followed by anion exchange mechanism:53, 54

338

(2 R3NH+)2SO42- + UO2(SO4)34- ↔ (R3NH+)4UO2(SO4)34- + 2 SO42-

(7b)

339

(2 R3NH+)2SO42- + UO2(SO4)22- ↔ (R3NH+)2UO2(SO4)22- + SO42-

(7c)

340

For uranium from sulfate media, another extraction mechanism has been reported; namely the

341

extraction of a neutral uranium sulfate species in an adduct type mechanism:

342

(R3NH)2SO4 + UO2SO4 ↔ (R3NH)2UO2(SO4)2

(8)

343

The sorption of uranium(VI) is thus clearly pH dependent in terms of both sorption performance

344

and binding mechanism: in acidic solutions (below pH 3.6) uranyl binding occurs by ion

345

exchange of anionic uranyl sulfate species on protonated amine groups,49 while at near neutral

346

pH uranyl cations (free or polynuclear species) may be bound to free amino groups and/or

347

carboxylate groups by chelation. This is consistent with the respective behavior of alanine- and

348

serine-based sorbents that have similar pH-edge curves but with slightly higher sorption

349

capacities for serine-sorbent (92 vs 72 mg U g-1 or alanine- and serine-based sorbents,

350

respectively). Indeed in the case of this sorbent (serine-based sorbent), apart carboxylate groups,

351

the free amino groups (primary amine) are more reactive and accessible than the secondary amine

352

groups in the alanine-based sorbent.17 Oshita et al. tested several amino-acid derivatives of

353

chitosan (including serine, glycine, valine and leucine) for uranium sorption in the pH range 2-

354

7.17 They also observed that the serine-type sorbent was more efficient than the other derivatives

355

for uranium binding. They explained that uranyl ions were probably bound by the formation of a

356

chelate between U(VI) and the primary amino group and the carboxylic group of serine moiety.

357

On the opposite hand, in the case of the other amino-acid derivatives uranyl ions may have more

358

difficulty to form a chelate with secondary amino groups and carboxylic groups.

16


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359

More generally, carboxylate groups are able to bind metal cations (different cationic species of

360

uranyl; i.e. UO22+, UO2(OH)+, (UO2)2(OH)22+, (UO2)3(OH)5+) at mild pH (corresponding to

361

deprotonation of carboxylic groups; at pH above 2.3 and 2.2 for alanine and serine (pKa of

362

carboxylic acid moieties), respectively) through complexation and/or electrostatic attraction. The

363

isoelectric points for alanine and serine are 6.0 and 5.7, respectively (though their ionic properties

364

may be affected by the grafting on chitosan backbone). This means that between 3 and 5.5 the

365

ionization conditions of carboxylate groups are potentially favorable to the binding of uranyl

366

cations. In the case of uranium binding by alginate gel beads (which contain guluronic and

367

mannuronic carboxylic groups) the maximum sorption capacity did not exceed 20 mg U g-1 at

368

room temperature and at the optimum pH (which is close to pH 3): the optimum pH was

369

associated to the balance between the deprotonation of carboxylic acid groups and the speciation

370

of uranyl ions (mononuclear and polynuclear cationic species).

371

On the other hand the amino groups can bind metal cations in slightly acidic or near-neutral

372

solutions by complexation (though the sharing of free electron doublet on nitrogen): the pKa of

373

amine groups on free chitosan depends on the deacetylation degree (in most cases for commercial

374

samples with a deacetylation close to 90 % the pK of amine groups ranges between 6.4 and 6.7).

375

Inn slightly acidic solutions, a fraction of amino groups remain available for cation chelation.

376

This means that several co-existing reactive groups may be involved in the binding of uranyl ions

377

in the pH range 3.5-5.5, apart the mechanisms that could be involved (by analogy with synthetic

378

resins) in the binding of uranyl sulfate species.

379

3.2.2. Effect of contact time – Uptake kinetics

17



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Page 18 of 48

380

The time required for reaching the equilibrium has been evaluated by the plot of the sorption

381

capacity (obtained by mass balance) as a function of time, under selected experimental conditions

382

(reported in the caption of Figure 3). The sorption process can be described by three steps: (a) the

383

initial step lasting for a few minutes (less than 5 min) that counts for near 50 % of total sorption,

384

(b) a second step, standing between 5 and 50 min that corresponds to the progressive saturation of

385

the sorbents, and (c) the saturation plateau with a negligible residual sorption (which counts for

386

less than 2 % of total sorption). The sorption mechanism is fast compared to conventional

387

chitosan sorbents. Indeed, this biopolymer, in its raw form, is a poorly porous material whose

388

mass transfer properties are generally controlled by the resistance to intraparticle diffusion. In the

389

present case, the thin coating of magnetic nano-based particles reduces the possible impact of this

390

resistance to intraparticle diffusion. The initial stage is probably associated to film diffusion

391

resistance, which is favored by the large specific surface area (due to nano-sized particles), while

392

the second stage corresponds to diffusion through the thin polymer layer (supported on magnetic

393

core) with binding to reactive groups.53 The final stage corresponds to the binding of metal ions

394

on the internal reactive groups, which are becoming progressively accessible due to the slow

395

swelling of the hydrophilic polymer layer.

396

In order to investigate more deeply the steps that control uptake kinetics, the sorption kinetic

397

profile was fitted with the pseudo-first order rate equation (PFORE), the pseudo-second order

398

rate equation (PSORE) and the simplified resistance to intraparticle diffusion equation (RIDE)

399

(See Supporting Information). 55

400

The experimental data have been fitted by the aforementioned kinetic models, and the parameters

401

of these models are reported in Table 1. The linearized plots of the kinetic data are reported in

402

Supporting Information (Figure AM6: PSORE, and Figure AM7: RIDE). The analysis of their

18


Page 19 of 48

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403

correlation coefficients shows that the PSORE gives a best fit of experimental data for U(VI)

404

sorption on both alanine- and serine-based sorbents. The solid lines in Figure 3 show the

405

modeling of kinetic profiles with the PSORE: the model fits well kinetic data, though some

406

discrepancies can be observed in the curvature zone. The poor fit of experimental data with the

407

RIDE (Weber & Morris equation) confirms that the resistance to intraparticle diffusion is not the

408

rate controlling step for uptake kinetics. As expected, designing nano-based particles contributes

409

to significantly reduce the impact of resistance to intraparticle diffusion.

410

3.2.3. Effect of initial metal ion concentration – Sorption isotherms

411

Sorption isotherms (sorption capacity vs. equilibrium concentration; qeq vs. Ceq) are characterized

412

by a steep initial slope, followed by a progressive saturation (asymptotic trend) as shown in

413

Figure 4. The experimental maximum sorption capacities (qmax) reach 85.3 and 116.3 mg U g-1

414

(i.e., 0.36 and 0.49 mmol U g-1) for alanine and serine, respectively. The serine-based sorbent is

415

more efficient for recovering uranium from slightly acidic solutions; probably due to the higher

416

reactivity of primary amino groups compared to secondary amino groups (for alanine-based

417

sorbent). It is noteworthy that the maximum sorption capacities are much lower than the content

418

of nitrogen (and amino groups) in the sorbents. Plotting the curve qeq/qmax vs. Ceq (not shown)

419

demonstrates that the two sorbents have very similar affinity for uranyl ions: the curves are

420

superimposed and the progressive saturation of the material is obtained for similar uranium

421

concentrations.

422

Foo and Ahmed reviewed a series of equations for modeling sorption isotherms.55 However, the

423

most frequently used are the Langmuir, the Freundlich and the Dubinin-Radushkevich models.

424

Though the mathematical fit of experimental data by the model does not necessarily mean that

19



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Page 20 of 48

425

the relevant physical hypotheses are verified; this can be helpful for getting complementary

426

information on the sorption mechanisms.55, 56

427

Langmuir model is based on the assumption that sorption sites are identical and energetically

428

equivalent, and that sorption occurs through monolayer coverage.56, 57 It can be represented, in

429

the linearized form, by Eq. (9):

430

Ce/ qeq = (1/qm) Ce + (1/b.qm)

(9)

431

where qeq (mg U g−1) is the amount of metal ions sorbed at equilibrium, Ceq (mg U L-1) is the

432

equilibrium metal ion concentration in the aqueous phase, qm (mg U g−1) is the maximum

433

sorption capacity of the sorbent at saturation of the monolayer, and b (L mg−1) is the Langmuir

434

sorption constant, which represents the affinity of the sorbent (which is correlated to the initial

435

slope of the curve: qm x b), respectively.

436

Table 2 reports the parameters of the Langmuir equation. These parameters have been used for

437

plotting the simulated curves (bold lines) in Figure 4, while Figure AM8 (See Supporting

438

Information) shows the plots of linearized curves.

439

The dimensionless parameter, RL, also called separation factor, is obtained by (Eq. (10):56

440

RL = 1/ (1 + b Co)

(10)

441

where Co is the initial concentration of the U(VI) ions. The coefficient RL was calculated, at

442

different initial concentrations, for both alanine-based and serine-based sorbent. The separation

443

factor varies between 0.088 and 0.027 for alanine-based sorbent and between 0.067 and 0.02 for

444

serine-based sorbent. These values are systematically below 0.1: this means that the sorption is

445

very favorable, tending to irreversible behavior when RL tends to 0. The Freundlich model is an

20


Page 21 of 48

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446

empirical power-type equation that supposes an exponential profile for the sorption isotherm.

447

This is not consistent with the asymptotic trend of the experimental curve (for discussion of the

448

Freundlich equation, See Supporting Information).

449

D–R isotherm model is usually employed for determining the nature of the sorption process

450

(discriminating between physical and chemical mechanism). The D–R equation is given by Eq.

451

(11):57, 58

452

ln qeq = ln qD - KDR ε2

(11)

453

where qD is the theoretical saturation capacity, and ε is the Polanyi potential (which is equal to [ε

454

= RT ln(1 + 1/Ce )]). KDR is related to the mean free sorption energy per molecule of sorbate, R is

455

the universal gas constant (8.314 x 10-3 kJ mol-1K-1) and T is the absolute temperature, K.

456

The D–R constant (KDR) gives valuable information on the mean energy of sorption EDR

457

(kJ/mol), which is deduced from EDR = (-2K)-1/2. The plot of ln qeq versus ε2 gives a straight line

458

with the slope K and the intercept ln qD (Figure AM9, See Supporting Information); the

459

parameters of the model are reported in Table 2. The mean sorption energy (EDR) traduces the

460

free energy necessary for transferring one mole of solute from infinity (i.e., the bulk of the

461

solution) to the reactive groups at the surface of the sorbent. When the sorption process involves

462

only physical interaction, EDR falls in the range 1-8 kJ mol-1, while for chemical interactions

463

higher sorption energies are required (i.e., 8-16 kJ mol-1).57,

464

energy are 8.3 and 8.6 for alanine-based and serine-based sorbents, respectively. These values are

465

very close to the discriminating value: the sorption process is thus suspected to be a

466

chemisorption involving sorption of uranyl species on the chemical reactive sites present at the

467

surface of amino acid functionalized chitosan magnetic nano-based particles. This is consistent

21

58


The values for mean sorption


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Page 22 of 48

468

with the conclusions on the modeling of uptake kinetics by the PSORE (this model being usually

469

associated to chemical binding mechanism). It is noteworthy that the values of mean energies of

470

sorption are very close to the limit value: so the conclusions should be considered as indicative of

471

the nature of the interaction mechanism between the sorbent and metal ions. It is also possible

472

considering that depending on the different reactive groups and depending on the pH, metal

473

binding could proceed through electrostatic attraction/ion exchange (in more acidic solutions)

474

and/or chelation on amino groups (under mild acidic conditions).

475

The mean sorption energy is slightly lower than the value reached for U(VI) sorption using

476

diethylenetriamine-functionalized magnetic chitosan (i.e., 9.1 kJ mol-1),59 and magnetic Schiff

477

base sorbent (i.e., 9.0 kJ mol-1),36 but significantly higher than the mean sorption energy reported

478

for U(VI) binding on tetraeethylenepentamine-derivative of chitosan (supported on magnetic

479

core) (i.e., 1.8 kJ mol-1).60

480

The sorbents were only tested with synthetic and pure solutions. The presence of other metal

481

ions, other complexing agents is expected to influence metal sorption. A composite consisting of

482

the coating of magnetite nano-based particles with a diethylenetriamine derivative of chitosan

483

was previously developed for uranium sorption.47 The sorbent was tested for uranium recovery

484

from sulfuric-acid leachates of uranium ores (which contained also rare earths). The maximum

485

sorption capacity for uranium reached up to 157 mg U g-1 (unpublished personal data); this is

486

about 13 % less than the maximum sorption capacity obtained with synthetic pure solutions (i.e.,

487

180 mg U g-1). This result confirms the negative impact of complex solutions but shows that the

488

sorbent remains very active for uranyl ions in complex media. Though this result obviously

489

cannot be “transferred” to the present sorbents, this shows a trend.

22


Page 23 of 48

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490

3.2.4. Effect of temperature – Thermodynamic parameters

491

The thermodynamic parameters were determined by investigating uranyl sorption at different

492

temperatures under identical experimental conditions (i.e., SD: 1 g L-1; C0: 250 mg U L-1; pH:

493

3.6; contact time: 50 min; agitation speed: 200 rpm). It is noteworthy that selected experimental

494

conditions correspond to the plateau region of the sorption isotherms: the sorption capacities (and

495

the distribution coefficients) will be comparable. With increasing the temperature, the sorption

496

capacity decreases: the sorption process is exothermic. The distribution coefficient Kd (L g-1),

497

which was calculated by the ratio qeq/Ceq for each temperature, is correlated to enthalpy change

498

(∆Ho), and entropy change (∆So) by the van’t Hoff equation (Eq. 12), while the free energy (∆G°)

499

change can be deduced from Eq. 13.54

500

ln Kd = (-∆H°/R) 1/T + ∆S°/R

(12)

501

∆G° = ∆H° − T∆S°

(13)

502

Figure 5 shows the linear plots of ln Kd vs. 1/T: a good fit of experimental data is obtained and

503

allows determining the thermodynamic parameters, which are summarized in Table 3. The

504

negative values of ∆Ho confirm the exothermic nature of the sorption process: the reaction

505

becomes more favorable at low temperatures. The exothermic nature of sorption has also been

506

reported for U(VI) sorption on ethylenediamine-modified magnetic chitosan,33 magnetic Schiff

507

base,36 and ion-imprinted and non-imprinted magnetic chitosan resins.61 On the opposite hand,

508

the

509

functionalized hydrothermal carbon,3 or Amberlite IRA-910 resin were reported to be

510

endothermic.

511

chemical nature of the interaction between uranyl ions and the chelating groups. These values are

sorption

of

54

U(VI)

using

tetraethylenepentamine/GMA,62

and

salicylideneimine–

The values of enthalpy (in the range -9.5-10 kJ mol-1) are consistent with the

23



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Page 24 of 48

512

lower than the levels reached for Hg(II), Cu(II) and Ag(I) binding using magnetic nanoparticles

513

of cellulose grafted with tetraethylenepentamine (in the range -24/-31 kJ mol-1),31 or U(VI)

514

binding on tetraethylenepentamine-derivative of chitosan supported on magnetic nanoparticles

515

(i.e., -17.6 kJ mol-1).60

516

The negative values of ∆Go indicate that the sorption reaction is spontaneous; the decrease in ∆Go

517

with increasing temperature confirms that the spontaneity of the sorption process decreases with

518

increasing the temperature.4 In addition, the fact that, regardless of temperature,

519

│∆H°│

Amino Acid Functionalized Chitosan Magnetic ... - ACS Publications

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