# Functionalized maghemite nanoparticles for enhanced adsorption of uranium from simulated wastewater and magnetic harvesting

Yawen Xiao, Ahmed S. Helal, Eva Mazario, Alvaro Mayoral, Alexandre Chevillot-Biraud, Philippe Decorse, Rémi Losno, François Maurel, Souad Ammar, John S. Lomas, et al.

► To cite this version:

Yawen Xiao, Ahmed S. Helal, Eva Mazario, Alvaro Mayoral, Alexandre Chevillot-Biraud, et al. Functionalized maghemite nanoparticles for enhanced adsorption of uranium from simulated wastewater and magnetic harvesting. Environmental Research, 2023, 216, 10.1016/j.envres.2022.114569. insu-04582041

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Submitted on 23 Jul 2025

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1 Functionalized maghemite nanoparticles for enhanced adsorption of

2 uranium from simulated wastewater and magnetic harvesting

3 Yawen Xiao<sup>a#</sup>, Ahmed S. Helal<sup>a,b,c#</sup>, Eva Mazario<sup>a</sup>, Alvaro Mayoral<sup>d</sup>, Alexandre Chevillot-

4 Biraud<sup>a</sup>, Philippe Decorse<sup>a</sup>, Rémi Losno<sup>e</sup>, François Maurel<sup>a</sup>, Souad Ammar<sup>a</sup>, John S. Lomas<sup>a</sup>,

5 and Miryana Hémadi<sup>*a</sup>

6 <sup>a</sup> Université Paris Cité, CNRS, ITODYS, F-75013 Paris, France

7 <sup>b</sup> Department of Nuclear Science and Engineering, Massachusetts Institute of Technology,

8 USA.

9 <sup>c</sup> Nuclear Materials Authority, P.O. Box 540, El Maadi, Cairo, Egypt.

10 <sup>d</sup> Universidad de Zaragoza Instituto de Nanociencia de Aragón Zaragoza, Aragon, Spain.

11 <sup>e</sup> Université Paris Cité, Institut de physique du globe de Paris, CNRS, F-75005 Paris, France

12 #Contributed equally to this work.

13 * Corresponding author: Miryana Hémadi

14 Address: 15 rue Jean-Antoine de Baïf, 75013 Paris, France

15 E-mail address: [hemadi@univ-paris-diderot.fr](mailto:hemadi@univ-paris-diderot.fr) or [hemadi@u-paris.fr](mailto:hemadi@u-paris.fr)

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24 Abstract

Maghemite (γ-Fe₂O₃) nanoparticles (MNPs) were functionalized with 3-aminopropyltriethoxysilane (APTES) to give APTES@Fe₂O₃ (AMNP) which was then reacted with diethylenetriamine-pentaacetic acid (DTPA) to give a nanohybrid DTPA-APTES@Fe₂O₃ (DAMNP). Nano-isothermal titration calorimetry shows that DTPA complexation with uranyl ions in water is exothermic and has a stoichiometry of two DTPA to three uranyl ions. Density functional theory calculations indicate the possibility of several complexes between DTPA and UO₂²⁺ with different stoichiometries. Interactions between uranyl ions and DAMNP functional groups are revealed by X-photoelectron and Fourier transform infrared spectroscopies. Spherical aberration-corrected Scanning Transmission Electron Microscopy visualizes uranium on the particle surface. Adsorbent performance metrics were evaluated by batch adsorption studies under different conditions of pH, initial uranium concentration and contact time, and the results expressed in terms of equilibrium adsorption capacities (qₑ) and partition coefficients (PC). By either criterion, performance increases from MNP to AMNP to DAMNP, with the maximum uptake at pH 5.5 in all cases: MNP, qₑ = 63 mg g⁻¹, PC = 127 mg g⁻¹ mM⁻¹; AMNP, qₑ = 165 mg g⁻¹, PC = 584 mg g⁻¹ mM⁻¹; DAMNP, qₑ = 249 mg g⁻¹, PC = 2318 mg g⁻¹ mM⁻¹ (at 25 °C; initial U concentration 0.63 mM; 5 mg adsorbent in 10 ml of solution; contact time, 3 h). The pH maximum is related to the predominance of mono- and di-cationic uranium species. Uptake by DAMNPs follows a pseudo-first-order or pseudo-second-order kinetic model and fits a variety of adsorption models. The maximum adsorption capacity for DAMNPs is higher than for other functionalized magnetic nanohydrens. This adsorbent can be regenerated and recycled for at least 10 cycles with less than 10% loss in activity, and shows high selectivity. These findings suggest that DAMNP could be a promising adsorbent for the recovery of uranium from nuclear wastewaters.

46 **Keywords:** Magnetic nanohydrens; ligands; wastewater treatment; pH optimization; DFT calculations; actinide complexation; partition coefficient

48 Environmental Implication

The production of nuclear energy couples very high energy density with low greenhouse gas emissions. However, the release of hazardous radionuclides, notably uranium, into the environment by various associated processes (mining, refining, nuclear tests and nuclear disasters) represents a serious environmental problem and a danger to human health. Procedures are required for the safe handling and disposal of radioactive waste to avoid contamination and there is an important need to find a low-cost system for recovering uranium from wastewater. In this paper we propose a selective nanohybrid adsorbent for uranium, which can be recovered by magnetic harvesting, released from the uranium, and recycled.
## 1. Introduction

At the present stage in the development of the nuclear power industry and for the foreseeable future, uranium is and will be an essential primary resource for reliable and low carbon footprint electric energy baseload.¹ The fission of 1 kilogram of uranium releases a quantity of heat equivalent to about 2700 tons of standard coal,² which makes nuclear energy an extremely advantageous alternative to traditional fossil fuels, in terms of energy supply and effective mitigation of environmental issues such as climate change.³ However, the total amount of uranium-containing wastewater generated by nuclear activities continues to rise.⁴ The radioactive, biologically and chemically toxic nature of uranium, and its extremely long half-life,⁵ make the direct release of this contaminated wastewater a serious hazard to the ecosphere. Specifically, effluents coming from uranium mining and processing, normal discharges from nuclear power plants, and accidental leaks of contaminated water (Three Mile Island in Pennsylvania in 1979, Chernobyl in 1986, and Fukushima Daiichi in 2011), are a long-term environmental threat to human beings and bio-organisms.⁶,⁷ Hence, the enrichment and recovery of uranium from wastewaters will therefore not only reduce the damage caused by hazardous radioactive substances but also improve the utilization of uranium resources. The mobility of uranium in contaminated water depends on the behaviors of its different oxidation states and the surrounding environmental conditions, including water composition, pH, and temperature.

Over the past decade, considerable research has been carried out on remediation, using a great many techniques, such as ion exchange, filtration, solvent extraction, reductive precipitation, adsorption and so on.⁸⁻¹³ Amongst these, adsorption methods are the most widely utilized for the separation and recovery of uranium, due to their advantages of low cost, high capacity, high efficiency and eco-friendliness. Various advanced adsorption materials for solid-liquid separation of metal ions have been synthesized, such as membranes,¹⁴ modified fibers,¹⁵ carbon nanotubes,¹⁶ hydrogels,¹⁷ mesoporous silica¹⁸ and magnetic materials.¹⁹,²⁰ In particular, nanomaterials have attracted considerable interest,²¹ because they have larger surface areas, higher activities and more active sites than other materials. Moreover, their morphology and structure can be tuned to meet the needs of practical applications.²² When they are also superparamagnetic, they can be readily collected/removed from a complex multiphase system by an external magnetic field; they thus have obvious applications in separation science and technology.²³
The key to optimizing magnetic nanomaterials is functionalization, which modifies their surface properties, prevents them from aggregating, confers selectivity, and enhances adsorption capacity. In general, surface modification is accomplished by physical/chemical adsorption or coating with molecules appropriate to a specific application. Functionalized magnetic nanoparticles (MNPs) are excellent candidates for the selective extraction of trace metals from wastewater streams, industrial effluents or soil. After the adsorbed ions have been stripped, the MNPs can be recycled, making them a promising sustainable support for a green technology.²⁴ Many surface-modified particles have been proposed for the extraction of uranyl ions, such as: polyaniline-modified MNPs, prepared by self-assembly;²⁵ cysteine functionalized magnetic-chitosan NPs;²⁶ hydroxyapatite-modified NPs;²⁷ phosphoramid-functionalized MNPs.²⁸ Magnetite (Fe₃O₄), despite its excellent magnetic characteristics, is less suitable for such applications, since it is slowly oxidized to maghemite (γ-Fe₂O₃), with a consequent evolution in its properties.²⁹

An efficient uranium adsorbent has to have good chemical and mechanical stability, and low toxicity combined with high selectivity and high affinity toward uranyl ions even in the presence of interfering metal ions. In the present study, maghemite MNPs were surface-modified by 3-aminopropyl-triethoxysilane (APTES) to introduce reactive amine groups.³⁰ This makes it possible to graft on diethylenetriamine-pentaacetic acid (DTPA), a powerful ligand used in the treatment of wound and pulmonary contamination by radionuclides.³¹ Moreover, DTPA is a multifunctional ligand which shows high selectivity in forming thermodynamically stable complexes with uranyl ions.

The resulting nanohybrid adsorbent, DTPA-APTES@Fe₂O₃ (DAMNP), was fully characterized, and the uptake of U(VI) was investigated spectroscopically by X-photoelectron (XPS) and Fourier transform infrared (FTIR) spectroscopies. U(VI) uptake by free DTPA was also examined by nano-isothermal titration calorimetry. Uranium was visualized at the DAMNP surface by spherical aberration-corrected scanning transmission electron microscopy (STEM). Adsorption isotherms and uptake kinetics in batch systems were studied under various conditions of pH and uranium concentration. Magnetic properties, adsorbent selectivity, regeneration and recycling, critical parameters for the up-scaling of the process before investigating continuous systems, were tested. This work highlights DAMNP as a high-capacity, selective and efficient adsorbent for the removal of uranium from artificially contaminated water. It has excellent chemical stability, and can be recycled without leaching of the magnetic core or functional groups involved in the adsorption activity.
## 2. Materials and methods

### 2.1 Chemicals and reagents

All chemicals were of the purest grade, purchased from Merck, Sigma Aldrich, Fluka, Acros and VWR. A 1.00 g L⁻¹ stock solution of U(VI) was prepared by dissolving UO₂(Ac)₂·2H₂O (Ac = acetate) (1.782 g) in doubly distilled water (1 L) acidified with HNO₃ (5 mL, 2 M). Working standards were prepared by diluting different volumes of the stock solution to the desired concentrations.

### 2.2 Synthesis and functionalization of DTPA-ATPES@Fe₂O₃ NPs (DAMNPs)

Maghemite MNPs (diameter, 10 ± 2 nm) were synthesized in two steps. (i) Magnetite (Fe₃O₄) nanoparticles were first prepared by the polyol method, using an appropriate amount of iron(II) acetate as metal precursor and triethylene glycol (TEG) as solvent.³² The mixture was heated to boiling point (230 °C) at a rate of 6 °C min⁻¹ under reflux with mechanical stirring, and maintained at the final temperature for 3 h. The mixture was then cooled to room temperature, resulting in a black dispersion of magnetite. (ii) Magnetite was oxidized to maghemite (γ-Fe₂O₃) (brown) by several hot-water washes and centrifugations (15 min at 22500 rpm) before being dried at 50 °C for several hours.

The MNPs were then functionalized with 3-aminopropyltriethoxysilane (APTES).³³ MNPs (100 mg) were dispersed by ultrasonication in methanol (10 mL), and ethanol (250 mL) was added to the dispersion. APTES (1 mL) was added slowly to the above mixture which was then ultrasonicated for 1 h, boiled under argon for 2 h and finally cooled to room temperature. The APTES@Fe₂O₃ MNPs (AMNPs) were then washed at least 4 times with ethanol, collected by sedimentation on a laboratory magnet and dried overnight at 50 °C. The number of amino groups grafted at the MNP surface was determined by colorimetric titration after reaction with the Kaiser reagent (ninhydrin) with which they form a blue Ruhemann complex (absorption maximum at 575 nm).³⁰,³³,³⁴

N-hydroxysuccinimide (NHS, final concentration: 3.75 mM) and 1-ethyl-3-(3-dimethyl-aminopropyl)carbodiimide (EDC, final concentration: 1.5 mM) were added to DTPA in 2-(N-morpholino)ethanesulfonic acid (MES, 1 mL of a 100 mM solution) at pH 6. The final solution was stirred for 1 h at room temperature. AMNPs (5 mg) were dispersed in TRIS buffer (1 mL, 50 mM) at pH 9 and shaken vigorously for 1 h. The two solutions were mixed, and the suspension stirred overnight at 4 °C, and the pellet then washed several times with TRIS buffer at pH 7.4.
## 2.3 Characterization and instruments

Magnetic characteristics were evaluated from the first emanation curves at 10 K (maximum magnetic field, 5 T) using a Quantum Design SQUID magnetometer. Size and morphology were analyzed on a JEOL-100 CX transmission electron microscope (TEM) at 100 kV. XPS measurements were performed on a Thermo VG Scientific Escalab 250 equipped with a monochromatic Al Kα X-ray source at 1486.6 eV (spot size, 650 µm). High-resolution spectra of all elements are given in the supporting information section (Figure S1). FTIR spectra of dried samples in KBr pellets were recorded on a Bruker Equinox spectrometer at 400–4000 cm⁻¹. Zeta-potentials were measured at different pH values, with 0.01 M KNO₃ as the background electrolyte, using a Malvern Zetasizer Nano S instrument. Thermogravimetric analyses (TGA) were performed on a Setaram TGA92 apparatus where samples were heated in air from room temperature to 800 °C at 20 °C min⁻¹. Spherical aberration corrected (Cs-corrected) TEM analyses were performed on a cold-FEG JEOL Grand ARM 300 microscope operating at 300 keV in the scanning (STEM) mode and using a high-angle annular dark-field detector (HAADF). Samples were dispersed in ethanol and sonicated, and a few drops of the suspension were placed on carbon-coated copper grids for observation.

Adsorption/desorption isotherms (Figure S2) were recorded on a Belsorp-Max Instrument (BEL Japan Inc.) using nitrogen as probe gas. Sample was previously degassed at 150 °C for 3 h. The BET (Brunauer, Emmett and Teller) method was applied to obtain the specific surface area, and the pore size distribution was measured by the BJH method.

Uranium concentrations before and after the adsorption experiments were measured by inductively coupled plasma mass spectrometry (ICP-MS) on an Agilent 7900 Q-ICP-MSICP-MS. An aliquot of each U(VI) solution was diluted in 0.5 M HNO₃ until the ppb concentration range was reached.

## 2.4 Thermodynamics

Nano-Isothermal titration calorimetry (ITC) titrations were carried out at 25 °C in water on a TA Instruments (USA) calorimeter with an active cell volume of 940 µL and a 250 µL stirring syringe at 300 rpm. Measurements were run in the overfilled mode. Before each titration, all solutions were thoroughly degassed under vacuum. Titrations were performed by an automated sequence of 50 injections of 5 µL of U(VI) solution (0.25 mM), spaced at 300 s intervals to ensure complete equilibration, into the sample cell containing DTPA solution (0.02 mM). Results were fitted by Nano Analyze software.
## 2.5 Batch adsorption and desorption

Batch experiments were carried out by treating a mass (5 mg) of each adsorbent (MNPs, AMNPs and DAMNPs) with 10 mL of uranyl acetate solutions with initial metal concentrations ($C_0$) of 25, 50, 75, 100 and 150 mg L$^{-1}$. The samples were vortexed (at 2000 rpm) for 3 h at 25 ± 1 °C at several pHs. After equilibration, magnetic separation was performed to recover the adsorbent; the residual U(VI) concentration in the supernatant ($C_e$, mg L$^{-1}$) was measured by ICP-MS and the equilibrium adsorption capacity ($q_e$) obtained by Eq. 1. All experiments were run in triplicate, with a variation systematically less than 3%.

$$q_e = (C_0 - C_e) \times V/M \qquad (\text{Eq. 1})$$

where V is the volume of the solution (L) and M is the mass of adsorbent (g).

Recycling and reusability of the nanohybrids were ascertained by comparing the adsorption capacity at the end of each successive step in a series of 10 adsorption/desorption cycles: in the first cycle adsorbent (5 mg) was vortexed (3 h, 25 °C, pH 5.5) with U(VI) solution (10 mL, $C_0$: 150 mg L$^{-1}$) and analyzed as above to give a first value, $q_{e1}$. The recovered adsorbent was vortexed with $HNO_3$ (0.02 M, 100–250 µl) for 10 min at 25 °C to release the uranium and washed several times before reuse. In successive cycles, free adsorbent was incubated in a new solution of uranium under the same conditions to give $q_{e2}$ to $q_{e10}$, which are compared to $q_{e1}$, taken as 100%.

A selectivity test was performed by shaking DAMNPs (2 mg) in an aqueous solution (1.5 mL) containing 10 mg L$^{-1}$ of the following cations: $Cs^+$ (CsAc), $K^+$ (KCl), $Na^+$ (NaCl), $Ca^{2+}$ (CaCl$_2$), $Mg^{2+}$ (MgCl$_2$), $Al^{3+}$ (AlCl$_3$) and $U^{6+}$ (UO$_2$(Ac)$_2$) (3 h, 25 °C, pH 5.5). After magnetic separation, the supernatants were analyzed by ICP-MS, and the adsorption efficiency (A.E. %) calculated by the following equation:

$$A.E. = \frac{(C_0 - C_e)}{C_0} \cdot 100 \qquad (\text{Eq. 2})$$

## 2.6 Density functional theory (DFT) calculations on complexes of U(VI) with DTPA

DFT calculations of the geometries, vibrational frequencies, and electronic structures of the uranyl complexes with DTPA were performed with the Gaussian 16 software package.35 The M06 functional36 chosen for these calculations provides the most consistent results when multiple quantities (structure, properties and energetics) are evaluated simultaneously.37 The Stuttgart/Dresden small core (SC) relativistic effective core pseudo-potential (RECP), which
accounts for scalar relativistic effects by replacing 60 electrons with a relativistic pseudopotential,³⁸ was used for the U atom. For the C, H, O and N atoms, the total electron 6-31G(d) basis set is a good compromise between accuracy and computational cost in these systems. Solvation effects of water were taken into account by means of the integral equation formalism of the polarizable continuum model (IEFPCM)³⁹ with default convergence criteria. Molecular geometries were optimized without imposing symmetry constraints, and frequency calculations were performed at the same level to verify that the final geometries are local minima on the potential energy surfaces. Zero-point, thermal and entropic corrections were added to the electronic energies to calculate Gibbs free energies. Binding energy is defined as the difference between the electronic energy of the complex and the sum of those of its components (Eq. 3):

$$E_{\text{binding}} = E_{\text{complex}} - (m * E_{\text{DTPA}} + n * E(\text{UO}_2^{2+})) \quad (\text{Eq. 3})$$

where m and n are the numbers of DTPA anions and UO₂²⁺ cations, respectively, in a given complex. Gibbs free binding energies were calculated from the Gibbs free energies for each entity by the same equation. Basis set superposition errors (BSSE) were systematically corrected.⁴⁰

## 3. Results and discussion

### 3.1 Complexation of U(VI) by DTPA

The affinity of U(VI) for free DTPA was experimentally determined by ITC, a technique for measuring the thermodynamic properties of any chemical reaction initiated by the addition of a binding component, which either generates or absorbs heat. In a single experiment, the binding/affinity constant ($K_a$), reaction stoichiometry (n) and reaction enthalpy ($\Delta H$) are determined by means of appropriate models.⁴¹ The quantity of heat generated for each injection decreases with increase in the molar ratio of U(VI) to DTPA until the reaction is complete. The ITC data fit (Figure 1a) gives an apparent equilibrium constant of $K_a^* = 4.3 \times 10^4 \text{ M}^{-1}$ and n = 1.5 for the molar ratio of U(VI)/DTPA. This indicates that three U(VI) ions coordinate with two DTPA molecules in the presence of water.⁴² The reaction is exothermic, with a negative enthalpy, $\Delta H = -6.4 \text{ kJ mol}^{-1}$, which is consistent with the complexation of uranium by the ligands. The calculated values of the reaction entropy, $\Delta S$, and of the Gibbs free energy of reaction (at 25 °C), $\Delta G$, are 67.1 J mol$^{-1}$ K$^{-1}$ and -26.4 kJ mol$^{-1}$, respectively. Complex formation (spontaneous reaction, $\Delta G < 0$) is therefore mainly driven by the
magnitude of the entropy term, which can be attributed to changes in the conformation of the ligand and release of solvent water molecules. The affinity for uranium of DTPA immobilized on AMNPs (DAMNPs) was evaluated by adsorption experiments.

In order to understand the mechanism of metal-ligand complex formation and explain the ability of DTPA to coordinate with several uranyl entities, structures were modeled by means of density functional theory (DFT). The protonation states of the carboxylic acid groups depend on the pH. Given the pKₐ of DTPA, about 1.8,⁴³ they will be deprotonated above about pH 4. According to the speciation diagram (Figure 1c), two U(VI) species are predominant in the pH range studied, UO₂²⁺ and UO₂(OH)⁺. We therefore considered species where anionic forms of DTPA bind to UO₂²⁺, namely DTPA.UO₂, DTPA.(UO₂)₂, DTPA.(UO₂)₃ and (DTPA)₂.(UO₂)₃. Figure 1b shows the optimized geometries of these complexes, and Table S1 reports some geometrical parameters. The equatorial U...O distances for all complexes are in the 2.39–2.48 Å range, indicating strong coordination between the ligands and the uranium ion, and the axial U...O distances are in the 1.73–1.76 Å range. All these data are in agreement with previous studies.⁴³ Under these conditions, carboxyl groups are bonded in a bidentate fashion to UO₂²⁺ in the equatorial plane, and in the (DTPA)₂.(UO₂)₃ complex, each uranyl cation is complexed by two DTPA ligands.

The binding energies (M06/IEFPCM) and Gibbs free binding energies of formation, calculated as above (Table S2), show that U(VI) cations have a high affinity for carboxylate anions. For instance, the binding and free binding energies calculated for the 1:1 DTPA.UO₂ complex are –135.3 kJ mol⁻¹ and –84.7 kJ mol⁻¹, respectively. In the case of 1:2 and 1:3 complexes, the free binding energies are somewhat greater (–149.5 and –165.5 kJ mol⁻¹ per uranyl). This suggests that DTPA can coordinate several uranyl cations without loss of binding capacity. The high binding free energy calculated for the 3:2 U(VI)/DTPA complex clearly shows that the ratio found experimentally by ITC can be explained theoretically. Nevertheless, while these do not exhaust the possibilities for complexation of U(VI) cations by multiple DTPA configurations resulting from grafting onto a NP surface, DFT calculations on such configurations are beyond the scope of the present article.
<figure><img src="image_2.png" /><figcaption>**Figure 1.** (a) Nano-ITC measurement of exothermic binding of U(VI) to DTPA at pH 5.5 and 25 °C. (b) Optimized structures corresponding to the DTPA.UO₂, DTPA.(UO₂)₂, DTPA.(UO₂)₃ and (DTPA)₂.(UO₂)₃ complexes. (c) Diagram for speciation of U(VI).<sup>44,45</sup> **2-column fitting image.**</figcaption></figure>

**3.2 Elaboration and characterization of MNPs, AMNPs and DAMNPs**

Maghemite nanoparticles (MNP) were first synthesized (cf. 2. Materials and Methods) and then functionalized with APTES to obtain AMNP. DTPA was then grafted onto AMNP to give DAMNP (Figure 2).

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Different steps in the elaboration of DAMNPs. **2-column fitting image.**</figcaption></figure>
The particle size distribution for the inorganic core of MNPs, determined by TEM, corresponds to a diameter of 10 ± 2 nm (Figure 3a). BET measurements on the solid MNPs (Figure S2)⁴⁶ give a specific surface area of 129 m² g⁻¹. Calculation based on a particle diameter of 10 nm (see SI for details) gives a very similar, theoretical, value of 114 m² g⁻¹, which indicates that there is practically no porosity inside the MNPs in the solid phase.

The amount of organic material associated with MNPs was determined by TGA (Figure 3b). The relative weight loss curve of MNPs (black) shows a loss of 3.1%, attributable to organic residues remaining after their synthesis. For AMNPs (blue), the total loss is 5.8%, corresponding to the loss of APTES and residues from MNP synthesis. From this weight loss, the molecular weight of APTES and the mass of a 10 nm-diameter maghemite NP, the number of grafted APTES molecules per MNP is deduced to be 270, which is equivalent to 0.85 molecules per nm² of MNP surface. This calculation involves the assumption that one bond is formed between APTES and MNPs and that two ethoxy groups remain after this first functionalization. By colorimetric analysis of AMNPs, the number of amino functions was estimated to be about 300, which is a very good agreement with the value from TGA. The amount of DTPA lost from the final functionalized NPs, DAMNPs (Figure 3b, green curve), is 3.6%, given by the difference between the DAMNP and AMNP curves. In the same way, the number of DTPA molecules per DAMNP is estimated to be 175 (0.6 molecules per nm² of surface). This indicates that the yield of the second functionalization step is about 65%, and that the surface of the DAMNPs is decorated with 3-aminopropyloxydiethoxysilane and DTPA functions. These results were confirmed by the magnetic properties established by VSM analysis at room temperature (Figure 3c). As expected, the bare MNPs show the highest saturation magnetization, M<sub>sat</sub>, of 83.5 emu g<sup>-1</sup>, while those of AMNPs and DAMNPs are 80.1 emu g<sup>-1</sup> and 76.9 emu g<sup>-1</sup>, respectively. The progressive decrease in the saturation magnetization is due to the increased amount of non-magnetic organic material in the particles. However, this value is still sufficiently high to allow magnetic harvesting in environmental applications, even at room temperature.

The progressive functionalization of the MNPs by APTES and DTPA was monitored by FTIR spectroscopy (Figure 3d). The absorption bands in all spectra at 630 and 584 cm⁻¹ are typical of Fe–O in maghemite.<sup>42,47</sup> For MNPs (Figure 3d, black), the bands at 1640 and 3420 cm⁻¹ are those of δOH and vOH (water and residual polyol), respectively. A Si–O vibration band appears at about 928 cm⁻¹ after silane treatment (Figure 3d, blue). The bands at 1390 and
1642 cm⁻¹ are assigned to NH bending and C=O stretching, respectively, of amide bonds in DAMNPs. The broad band (Figure 3d, green) in the 3400–3700 cm⁻¹ region is typical of OH stretching.

Changes in the surfaces of MNPs, AMNPs and DAMNPs (Figure 3e) were evaluated by XPS determination of C/Fe, O/Fe, N/Fe and Si/Fe ratios (inset Figure 3e). The C/Fe ratio increases from 3.4 for MNPs (presumably due to residual polyol) to 7.6 for DAMNPs. The O/Fe ratio of 4.1 for MNPs is further increased by functionalization, while the N/Fe and Si/Fe ratios reflect the two functionalization steps. Notably, the latter, which is due to APTES, falls by only 30% when DTPA is grafted onto AMNPs, indicating that DTPA only partially covers the APTES layer, thus confirming the TGA results.

The zeta potential (ξ) is a key indicator of the surface charge of the NPs and the stability of their aqueous colloidal dispersions. Colloids with high zeta potentials (negative or positive) are electrically stabilized, while colloids with low, close to zero, zeta potentials tend to coagulate or flocculate.⁴⁸ Measurements at pH 2 to 10 show that the zeta potential varies from –40 mV to +40 mV (Figure 3f). The variation of the point of zero charge (PZC, pH value for ξ = 0 mV) indicates the presence of APTES and DTPA at the surface of the nanocomposite. PZC changes from 6.0 for MNPs, to 8.8 for AMNPs and 4.2 for DAMNPs. Under the conditions of the uranium adsorption tests (pH 5.5), the zeta potentials of AMNPs and DAMNPs are +40 mV and –21 mV, respectively, indicating good colloidal stability.

Overall, the results on the characterization of AMNPs and DAMNPs establish that the MNPs have been effectively functionalized, and the surface properties of both indicate that they have considerable potential as adsorbents.
<figure><img src="image_4.png" /><figcaption>**Figure 3.** (a) TEM image of MNPs; (b) TGA; (c) First magnetization curves measured at 10 K; (d) FTIR spectra of MNPs, AMNPs and DAMNPs; (e) XPS general spectra; (f) pH dependence of uranium adsorption by MNPs, AMNPs and DAMNPs (3 h; temperature, 25 °C)/Zeta potential and adsorption capacities (with C₀ at 150 mg L⁻¹) at equilibrium at pH 2 to 10. **2-column fitting image.**</figcaption></figure>

**3.3 Uranium adsorption by MNPs, AMNPs and DAMNPs**

Process optimization requires investigating different aspects of the coordination of uranium with the adsorbent: (a) pH dependence, (b) uptake kinetics, (c) adsorption isotherms, (d) selectivity and (e) regeneration/recycling of the nanocomposite. This information is fundamental for understanding not only the mechanisms of adsorption but also the technical and economic limitations of the process (pH range of application, durability of the adsorbent, etc.). The equilibrium adsorption capacity (qe), which is widely used to express adsorbent performance, is affected by variables such as the initial concentration of the adsorbate, the contact time, etc. and is not an objective metri.⁴⁹ For this reason, the partition coefficient (PC)
345 was also used in this work to express the performance of the adsorbent. In general, the higher
346 PC, the greater the adsorption affinity of the adsorben.⁵⁰ PC is defined as:

347 PC = equilibrium adsorption capacity ($q_e$)/final adsorbate concentration ($C_e$),
348 and can be expressed in $mg \ g^{-1} \cdot mM^{-1}$.

349 **3.3.1 Uranium adsorption effect factors**

350 Parameters studied as effect factors on uranium adsorption were pH, contact time and initial
351 uranium concentration. Of these, the most important is the pH which has a direct impact on
352 the performance of the adsorbent. The equilibrium adsorption capacity, $q_e$, of uranium on
353 MNPs, AMNPs and DAMNPs increases when the initial pH rises, going from about 16 $mg \ g^{-1}$ at pH 2 to maxima of 63, 165 and 249 $mg \ g^{-1}$ for MNPs, AMNPs and DAMNPs,
354 respectively, at pH 5.5 (at 25 °C; initial U concentration 0.63 mM; 5 mg adsorbent in 10 ml of
355 solution) (Figure 3f, Table 1). Likewise, PC, which is a better performance metric, has values
356 of 127, 584 and 2318 $mg \ g^{-1} \cdot mM^{-1}$ under the same conditions (Figure S3a). By both criteria
357 DAMNP is the best adsorbent but PC reveals a much greater disparity between the different
358 adsorbents than $q_e$. According to the PC criterion the performance of DAMNP at pH 5.5 is 20 times
359 better than that of MNP, whereas according to $q_e$ it is only 4 times better. With further increase to
360 pH 8, $q_e$ falls sharply to about $20 \ g^{-1}$ for MNPs and to $50 \ g^{-1}$ for AMNPs and
361 DAMNPs. The observed adsorption profiles are related to variations in (i) the distribution of
362 the U(VI) species and (ii) the surface charges on the NPs, due to protonation-deprotonation
363 reactions of the functional groups. The speciation of U(VI) in water depends markedly on the
364 pH (Figure 1c) and to a much lesser extent on the overall concentration. For a concentration
365 of 50 $µM^{42,44}$ from pH 2 to 5 the most predominant species is the di-cation $UO_2^{2+}$. At pH 5.5,
366 the optimal value for uranium adsorption, the most predominant species are the mono-cation
367 ($UO_2$(OH)⁺ and the di-cation. Beyond pH 7, the mono-cation is predominant. For DAMNP,
368 as the pH value increases the surface gradually becomes negatively charged (at pH 5.5, $\xi$ =
369 −21 mV) due to the presence of five carboxylate groups. Hence, the maxima for $q_e$ and PC
370 correspond to complexation between U(VI) mono- or di-cations and carboxylate or other
371 active sites of the adsorbent. Similar uranium extraction behavior with respect to pH variation
372 was reported in a previous study.⁴² In all cases the protonation state of the uranium determines
373 the adsorption-pH profile.
374

375 The contact time was also studied at fixed initial conditions of pH and uranium concentration.
376 Since a plateau in both $q_e$ and PC is observed after 120 minutes (Table 1, Figure S3), an
adsorption time of 120-180 minutes was chosen as standard in this work. The effect of initial uranium concentration is rather surprising, in that $q_e$ increases with increasing concentration while PC falls (Table 1). The increase in the adsorption capacity is clearly due to the increase in the amount of uranium available for complexation, whereas PC corrects $q_e$ for the amount of adsorbate remaining in solution.

<table><caption>**Table 1.** Detailed experimental data for the adsorption process with different effect factors.</caption><thead><tr><th>Effect Factor*</th><th>Adsorbent</th><th>Target Ion</th><th>Final Concentration (mM)</th><th>Adsorption Capacity (mg g<sup>-1</sup>)</th><th>Adsorption Capacity/Final Concentration (PC) (mg g<sup>-1</sup> mM<sup>-1</sup>)</th></tr></thead><tbody><tr><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>2</td><td></td><td></td><td>0.59</td><td>16.3</td><td>27.4</td></tr><tr><td>2.5</td><td></td><td></td><td>0.54</td><td>42.9</td><td>79.4</td></tr><tr><td>3</td><td></td><td></td><td>0.44</td><td>89.4</td><td>202</td></tr><tr><td>3.5</td><td></td><td></td><td>0.35</td><td>136</td><td>394</td></tr><tr><td>4</td><td></td><td></td><td>0.25</td><td>179</td><td>701</td></tr><tr><td>4.5</td><td></td><td></td><td>0.18</td><td>212</td><td>1146</td></tr><tr><td>5</td><td>DAMNP</td><td>UO<sub>2</sub><sup>2+</sup></td><td>0.14</td><td>231</td><td>1598</td></tr><tr><td>5.5</td><td></td><td></td><td>0.11</td><td>249</td><td>2318</td></tr><tr><td>6</td><td></td><td></td><td>0.18</td><td>214</td><td>1190</td></tr><tr><td>6.5</td><td></td><td></td><td>0.23</td><td>189</td><td>805</td></tr><tr><td>7</td><td></td><td></td><td>0.31</td><td>152</td><td>487</td></tr><tr><td>7.5</td><td></td><td></td><td>0.39</td><td>112</td><td>283</td></tr><tr><td>8</td><td></td><td></td><td>0.52</td><td>52.9</td><td>102</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>2</td><td></td><td></td><td>0.59</td><td>17.6</td><td>29.7</td></tr><tr><td>3</td><td></td><td></td><td>0.52</td><td>52.9</td><td>102</td></tr><tr><td>3.5</td><td></td><td></td><td>0.42</td><td>102</td><td>244</td></tr><tr><td>4.5</td><td></td><td></td><td>0.34</td><td>137</td><td>399</td></tr><tr><td>5.5</td><td>AMNP</td><td>UO<sub>2</sub><sup>2+</sup></td><td>0.28</td><td>165</td><td>584</td></tr><tr><td>6.5</td><td></td><td></td><td>0.36</td><td>129</td><td>358</td></tr><tr><td>7.5</td><td></td><td></td><td>0.45</td><td>86</td><td>191</td></tr><tr><td>8.5</td><td></td><td></td><td>0.51</td><td>58.9</td><td>116</td></tr><tr><td>9</td><td></td><td></td><td>0.56</td><td>33.2</td><td>59.2</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>2</td><td></td><td></td><td>0.60</td><td>14.8</td><td>24.6</td></tr><tr><td>3</td><td></td><td></td><td>0.57</td><td>27.5</td><td>48</td></tr><tr><td>3.5</td><td>MNP</td><td>UO<sub>2</sub><sup>2+</sup></td><td>0.54</td><td>43.6</td><td>80.9</td></tr><tr><td>4.5</td><td></td><td></td><td>0.52</td><td>51.6</td><td>98.9</td></tr><tr><td>5.5</td><td></td><td></td><td>0.49</td><td>63</td><td>127</td></tr><tr><td>6.5</td><td></td><td></td><td>0.52</td><td>51.6</td><td>98.9</td></tr></tbody></table>
<table><tbody><tr><td rowspan="12">Contact time (min)</td><td>7.5</td><td></td><td>0.54</td><td>44.9</td><td>83.8</td></tr><tr><td>8.5</td><td></td><td>0.59</td><td>20.1</td><td>34.2</td></tr><tr><td>9</td><td></td><td>0.61</td><td>7.37</td><td>12</td></tr><tr><td>30</td><td></td><td>0.24</td><td>188</td><td>793</td></tr><tr><td>60</td><td></td><td>0.16</td><td>222</td><td>1360</td></tr><tr><td>90</td><td>DAMNP</td><td>UO<sub>2</sub><sup>2+</sup></td><td>0.13</td><td>237</td><td>1772</td></tr><tr><td>120</td><td></td><td>0.11</td><td>249</td><td>2297</td></tr><tr><td>150</td><td></td><td>0.11</td><td>249</td><td>2297</td></tr><tr><td rowspan="10">Initial uranium concentration (mM)</td><td>0.11</td><td></td><td>0.01</td><td>56</td><td>5592</td></tr><tr><td>0.21</td><td></td><td>0.03</td><td>133</td><td>4165</td></tr><tr><td>0.32</td><td>DAMNP</td><td>UO<sub>2</sub><sup>2+</sup></td><td>0.05</td><td>181</td><td>3302</td></tr><tr><td>0.42</td><td></td><td>0.07</td><td>196</td><td>2772</td></tr><tr><td>0.63</td><td></td><td>0.13</td><td>269</td><td>2073</td></tr><tr><td>0.11</td><td></td><td>0.003</td><td>61</td><td>7585</td></tr><tr><td>0.21</td><td></td><td>0.05</td><td>98</td><td>3011</td></tr><tr><td>0.32</td><td>AMNP</td><td>UO<sub>2</sub><sup>2+</sup></td><td>0.09</td><td>134</td><td>2073</td></tr><tr><td>0.42</td><td></td><td>0.14</td><td>165</td><td>1171</td></tr><tr><td>0.63</td><td></td><td>0.32</td><td>186</td><td>597</td></tr><tr><td rowspan="5">0.11</td><td></td><td></td><td>0.06</td><td>29</td><td>517</td></tr><tr><td>0.21</td><td></td><td>0.13</td><td>46</td><td>342</td></tr><tr><td>0.32</td><td>MNP</td><td>UO<sub>2</sub><sup>2+</sup></td><td>0.22</td><td>55</td><td>250</td></tr><tr><td>0.42</td><td></td><td>0.31</td><td>65</td><td>208</td></tr><tr><td>0.63</td><td></td><td>0.51</td><td>73</td><td>145</td></tr></tbody></table>

* pH as effect factor: C₀ = [U] = 0.63 mM; contact time: 120 min; 5 mg absorbent in 10 mL solution.

Contact time as effect factor: C₀ = [U] = 0.63 mM, pH 5.5, 5 mg absorbent in 10 mL solution. C₀ as effect factor: pH 5.5; contact time, 120 min; 2 mg absorbent in 5 mL solution.

### 3.3.2 Stoichiometry of U(VI) complexation with MNPs, AMNPs and DAMNPs

The maximum adsorption performances (PC) of MNPs, AMNPs and DAMNPs are all located at about pH 5.5, increasing from about 130 mg g⁻¹ mM⁻¹ to 580 mg g⁻¹ mM⁻¹ to 2320 mg g⁻¹ mM⁻¹ in the order given (Figure 3f, Table 1). The first figure is surprising in that there is formally no complexing agent on the raw MNPs, but TGA shows that, in addition to maghemite, there is something like 3% w/w of organic material, revealed moreover by XPS and the FTIR spectra. Given the synthetic procedure, this is very likely residual polyol or oxygen-containing derivatives thereof, and these could act as complexation sites for U(VI) cations. The approximate stoichiometry of complexation for MNPs, AMNPs and DAMNPs can be determined from the number of MNPs in 1 g and the numbers of polyol, APTES and
DTPA residues. The former is obtained from the estimated volume of a 10 nm-diameter MNP, this being the mean value determined by TEM imaging (Figure 3a, inset), and the density of maghemite. For AMNPs and DAMNPs, information about the numbers of APTES and DTPA residues is provided by the TGA results above. Thus, we can estimate the number of U(VI) ions associated with one MNP or with one APTES or one DTPA residue, or with the sum of APTES and DTPA sites.

<table><caption>**Table 2.** Approximate stoichiometries for U(VI) complexation with APTES and DTPA sites in AMNP and DAMNP at pH 5.5 and 25 °C.</caption><thead><tr><th>NP</th><th>U/NP</th><th>APTES</th><th>DTPA</th><th>U/APTES</th><th>U/DTPA</th><th>U/DTPA+APTES</th></tr></thead><tbody><tr><td>MNP</td><td>420</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>AMNP</td><td>1110</td><td>270</td><td>-</td><td>4.1 (2.6)</td><td>-</td><td>-</td></tr><tr><td>DAMNP</td><td>1720</td><td>95*</td><td>175</td><td>-</td><td>9.8 (6.0)</td><td>6.4 (4.8)</td></tr></tbody></table>

* Based on the assumption that the difference between the number of APTES and DTPA sites in AMNPs and DAMNPs, respectively, represents unreacted APTES sites and that these remain active.

One MNP is able to adsorb over 400 U(VI) ions (Table 2). When MNPs are first functionalized by APTES the number of U(VI) ions adsorbed rises by almost 700 for the 270 APTES sites estimated by TGA (Table 2). This amounts to about 4 U(VI) cations per APTES residue, which in the usual representation has only one amino group likely to complex a metal cation. However, the estimation of the number of APTES groups grafted onto MNPs in AMNPs is based on the assumption that only one ethoxy group is involved in the sol-gel reaction with the iron oxide surface, and the result is in good agreement with that (300) from colorimetric assay of amine groups. The assumption that two of three ethoxy groups were involved leads to higher estimates, 360 or 550, respectively. The proximity of the lower value to the colorimetric value suggests that there may be some Si-O double-bonding of the APTES residues to the iron oxide surface, but in all cases there are at least one or, more often than not, two ethoxy (or hydroxyl) groups, available for complexation with a metal cation. We must also take into account the fact that the whole NP surface is not covered with APTES, and that free maghemite can play a role similar to that in the absence of APTES, or that there could be active sites involving combination of polyols and APTES residues. If we assume that the *difference* between MNP and AMNP adsorption (690 U) is entirely due to the 270 APTES residues, then the stoichiometry falls to about 2.6 (Table 2).

According to TGA there are only 175 DTPA sites per DAMNP (Table 2). Since we are unable to determine the fate of the remaining APTES residues, we shall assume that they are
unaffected, though they may now interact with the new DTPA residues as well. If we consider that the *difference* between AMNP and DAMNP adsorption (610 U) is due to the replacement of 175 APTES residues by DTPA and that each APTES residue continues to complex 2.6 U(VI) species, the stoichiometry of DTPA becomes 6.0 and the average is 4.8 U(VI) for one DTPA site or one APTES site in a DAMNP (Table 2). Even with these drastic assumptions and simplifications the stoichiometries of both APTES and DTPA residues remain very high. FTIR and XPS give information about bonds and atoms, respectively but, unfortunately we are no wiser as to the organization of the various residues on the NP surfaces and the possible interactions between them. Adsorption isotherms (below) shed some light on the real nature of the adsorbent.

### 3.3.3 Uranium adsorption kinetics

The optimal conditions for full-scale batch removal are determined by the kinetics of metal uptake and the adsorption efficiency of the adsorbent. Adsorption of U(VI) by DAMNPs is very rapid in the first hour, then slows and reaches a plateau after about 2 hours (Figure S3b). The time dependency of the distribution of U(VI) between the bulk solution and the adsorbent was analyzed by three kinetic models (pseudo-first-order (p1st), pseudo-second-order (p2nd) and intraparticle diffusion (ID), in order to investigate the adsorption mechanism and the rate-controlling step, an important factor in the process.⁵¹⁻⁵³

Lagergren's p1st equation in linear form is generally expressed as:⁵⁴

$$ \ln(q_e - q_t) = \ln q_e - k_1 t \qquad (\text{Eq. 4}) $$

where $q_e$ and $q_t$ are the amounts of U(VI) adsorbed at equilibrium and at time t, respectively (mg g⁻¹), and $k_1$ is the first-order adsorption rate constant (min⁻¹).

The linear form of the p2nd equation is given by:⁵⁵

$$ \frac{t}{q_t} = \frac{1}{k_2 q_e^2} + \frac{1}{q_e} t \qquad (\text{Eq. 5}) $$

where $k_2$ is the second-order adsorption rate constant (g mg⁻¹ min⁻¹).

The ID model is expressed as:⁵⁶

$$ q_t = C + k_p t^{0.5} \qquad (\text{Eq. 6}) $$
where $k_p$ is the intraparticle diffusion rate constant (mg g⁻¹ min⁻⁰.⁵) and C represents the boundary layer effect.

Values of the kinetic parameters derived from the rate plot are presented in Table 3 (Figure S4). The correlation coefficients, r², show that the p2nd model (r² = 0.999) describes the kinetics better than the p1st (r² = 0.931) and ID (r² = 0.895) models.

<table><caption>**Table 3.** Parameters of different kinetic models for U(VI) adsorption by DAMNP</caption><thead><tr><th>Model</th><th>Adsorption</th><th>Rate constant</th><th>Correlation coefficient</th></tr></thead><tbody><tr><td>p1st</td><td>q<sub>e</sub> = 249 mg g<sup>-1</sup></td><td>k<sub>1</sub> = 0.045 min<sup>-1</sup></td><td>0.931</td></tr><tr><td>p2nd</td><td>q<sub>e</sub> = 273 mg g<sup>-1</sup></td><td>k<sub>2</sub> = 2.7*10<sup>-4</sup> g mg<sup>-1</sup> min<sup>-1</sup></td><td>0.999</td></tr><tr><td>ID</td><td>C = 145 mg g<sup>-1</sup>*</td><td>k<sub>p</sub> = 9 mg g<sup>-1</sup> min<sup>-0.5</sup></td><td>0.895</td></tr></tbody></table>

* C is described as the boundary layer effect but has the same dimensions as adsorption.

Adsorption reactions can take place in several stages comprising: (i) external film diffusion, (ii) intraparticle diffusion and (iii) interaction between the adsorbate and the active site. In the present case, however, the first step is excluded by high-speed stirring, and the nanoparticles are not aggregated. Intraparticle diffusion is clearly not rate-limiting, as confirmed by the very low correlation coefficient for this model. Manifestly, the third step controls the adsorption rate,⁵⁷,⁵⁸ and it remains to determine which of the models is the more appropriate. The p2nd model looks to be the better but there is a systematic bias towards this mechanism,⁵⁴,⁵⁵ and in a similar situation it was rejected in favor of p1st because of the disagreement between the calculated q<sub>e</sub> (Eq. 5) and the experimental value⁵⁰ (Figure 3f). For the p1st mechanism the agreement is perfect. The p2nd model implies that there are two U(VI) ions involved in the transition state with the adsorbent, but it is very difficult to understand how adsorption could involve pairs of uranyl ions acting in concert with a given site. Moreover, neither mechanism is clearly compatible with the reaction of a complex surface, with several different types of adsorption site, some of which would be expected to react more rapidly than others. The easiest to accommodate is a p1st reaction, with single uranyl ions being adsorbed one by one, or a set of such reactions on different sites, but it is not feasible to extract rate constants and relative site concentrations from the data available, even if it could be assumed that sites were independent of each other.

### 3.3.4 Uranium adsorption isotherms

Adsorption isotherms indicate the efficiency of adsorption and how molecules are distributed between the liquid (or gas) and solid phases at equilibrium. The adsorption mechanism and
the adsorbent's affinity for the metal ion can be determined by analyzing the physicochemical parameters associated with some of the many isotherm equations available, notably those of Langmuir, Freundlich, Temkin, Elovich and Halsey.⁵⁹

The Langmuir isotherm model (Eq. 7) assumes that: (i) the solid surface presents a finite number of identical, energetically uniform sites; (ii) there is no interaction between adsorbed species, which means that the amount adsorbed has no influence on the extent of adsorption, and (iii) a monolayer is formed when the surface reaches saturation. This leads to the following equation:

$$ \frac{C_e}{q_e} = \frac{1}{q_{max}} C_e + \frac{1}{K_L q_{max}} \qquad (\text{Eq. 7}) $$

where $q_e$ is the equilibrium adsorption capacity (mg g⁻¹), the same as in Eqs. 1, 5 and 6; $q_{max}$ is the saturation adsorption capacity (mg g⁻¹), $C_e$ the equilibrium U(VI) concentration in solution (mg L⁻¹), and $K_L$ is directly related to the binding-site affinity (L mg⁻¹). This model fits well in all three cases, MNP, AMNP and DAMNP, with correlation coefficients of 0.996 to 0.998 (Figure 4, inset Table). It should be noted that the $q_{max}$ values based on this model (MNP, 90 ± 3 mg g⁻¹, AMNP, 200 ± 5 mg g⁻¹, DAMNP, 385 ± 9 mg g⁻¹) are higher than the maximum equilibrium adsorption capacities ($q_e$, Figure 3f, see Eq. 1) observed experimentally, and those of the kinetic models of the rate of adsorption by DAMNPs (Table 3).

The essential characteristic of the Langmuir isotherm is expressed in terms of a dimensionless constant, known as the separation factor ($R_L$),⁶⁰ and represented as:

$$ R_L = \frac{1}{1 + K_L C_0} \qquad (\text{Eq. 8}) $$

where $C_0$ is the initial concentration (mg L⁻¹). $R_L$ indicates that adsorption is either unfavorable ($R_L > 1$), linear ($R_L = 1$), favorable ($0 < R_L < 1$) or irreversible ($R_L = 0$). The $R_L$ values in the present study are 0.22, 0.04 and 0.095 for MNPs, AMNPs and DAMNPs, respectively, indicating in all cases that U(VI) adsorption is favored.

The Freundlich equation is an empirical expression for the isothermal variation of the quantity of metal ion adsorbed on a solid adsorbent.⁶¹ At equilibrium:

$$ \log q_e = \frac{1}{n} \log C_e + \log K_F \qquad (\text{Eq. 9}) $$

where $K_F$ is an indication of the adsorbent capacity (mg g⁻¹) and 1/n is an empirical parameter related to the surface heterogeneity, ranging between 0 and 1, the adsorbent being more
heterogeneous as 1/n approaches zero.⁶² Correlation coefficients for U(VI) adsorption on MNPs, AMNPS and DAMNPs are lower (0.969 to 0.981) than for the Langmuir equation, (Figure S5) with 1/n values of 0.43, 0.32 and 0.61, respectively. According to this criterion the surface is more heterogeneous in MNPs and AMNPs than in DAMNPs. While one can accept that the surface is more homogeneous in the presence of DTPA ligands, which largely cover the surface of the AMNPs, it is hard to understand why bare MNPs should have a more heterogeneous surface than the functionalized NPs, which clearly have more than one type of binding site.

<figure><img src="image_5.png" /></figure>

<table><caption>**Figure 4.** (a) Linear fits of Langmuir isotherms; (b) Parameters for U(VI) adsorption on MNPs, AMNPs and DAMNPs. **Single-column fitting image.**</caption><thead><tr><th></th><th>Adsorption</th><th>K<sub>L</sub></th><th>Correlation coefficient</th></tr></thead><tbody><tr><td>MNPs</td><td>q<sub>m</sub> = 90 mg g<sup>-1</sup></td><td>0.03 L mg<sup>-1</sup></td><td>0.996</td></tr><tr><td>AMNPs</td><td>q<sub>m</sub> = 200 mg g<sup>-1</sup></td><td>0.15 L mg<sup>-1</sup></td><td>0.997</td></tr><tr><td>DAMNPs</td><td>q<sub>m</sub> = 385 mg g<sup>-1</sup></td><td>0.07 L mg<sup>-1</sup></td><td>0.998</td></tr></tbody></table>

The Temkin isotherm takes into account indirect adsorbate-adsorbate interactions, and assumes that the heats of adsorption of all the molecules in a layer decrease linearly with surface coverage. It has been used for many years in a form which is dimensionally inconsistent and incomplete. A corrected version has been used in this work.⁶³ Unfortunately, in this treatment it is no longer possible to separate a constant related to the heat of adsorption from a maximum adsorption term (Eq. 10).

$$q = q_T \ln \left( \frac{1 + K_T C}{1 + K_I C} \right) \qquad (\text{Eq. 10})$$

$K_T$ and $K_I$ are equilibrium binding constants and $q_T = q_mRT/b_T$ where $q_m$ is the saturation capacity, $b_T$ is a heat of adsorption (in J mol⁻¹), R is the gas constant (J mol⁻¹ K⁻¹), and T is the temperature (K) (Figure S6, inset). Curve fitting of Eq. 10 leads to a good correlation coefficients for MNPs and DAMNPs, as good as for Langmuir. Only $q_T$ varies regularly from MNPs to DAMNPs, whereas $K_T$ and $K_I$ is much greater for AMNPs than for the other two materials.
The Elovich isotherm model assumes that the adsorption sites multiply exponentially with the extent of adsorption, which implies multilayer adsorption.⁶⁴

$$ \ln \frac{q_e}{C_e} = \ln(K_E \cdot q_m) - \frac{q_e}{q_m} \qquad (\text{Eq. 11}) $$

where $K_E$ is the equilibrium constant (L mg⁻¹), and $q_m$ is the maximum adsorption capacity (mg g⁻¹). The data fit this model (Figure S6, inset Table) slightly less well than the previous models. Values of $q_m$ are all smaller than the $q_{max}$ values associated with the Langmuir model.

The Halsey isotherm model describes a multilayer adsorption system for metal ion adsorption at a relatively large distance from the surface:⁶⁴

$$ \ln(q_e) = \left[ \left( \frac{1}{n_H} \right) \ln(K_H) \right] - \left( \frac{1}{n_H} \right) \ln\left(\frac{1}{C_e}\right) \qquad (\text{Eq. 12}) $$

where $K_H$ and $n_H$ are the Halsey constants. This model fits about as well as the others: r² = 0.967 – 0.981 (Figure S6, inset Table), except Langmuir.

The fact that several isotherm equations related to widely different adsorption models fit the data for all three NP materials makes it very difficult to conclude as to the nature of the surface and the mechanism of adsorption. All possibilities seem to be covered by these models, whether theory-based or empirical: homogeneous or heterogeneous; monolayer or multilayer; constant or variable adsorption energies and site numbers. The various parameters generated by the plots do not evolve from MNPs to DAMNPs in any regular fashion, with the data for AMNPs usually too small or too high compared to those for the other materials. Intuitively we would expect a regular progression following the increase in the adsorption capacity, and for any indicator of heterogeneity to increase with the complexity of the surface. This is not the case. Paradoxically, the present data fit best the Langmuir model, which is based on the simplest picture of the surface, with identical adsorption sites acting independently up to the completion of a monolayer. Though this model would fit MNPs, it is clearly in contradiction with the pictures of AMNPs and DAMNPs we have built up from the TGA study, unless the APTES and DTPA residues lose their identities and form adsorbent clusters including maghemite polyol residues.

In most adsorption studies, adsorbent performance is expressed by the saturation adsorption capacity ($q_{max}$), and unfortunately there are few studies in the literature where it is expressed by the partition coefficient (PC).⁴⁹ For this reason, in Table 4, we summarized the saturation
adsorption capacities ($q_{max}$) from previous studies and the present work, using the Langmuir isotherm equation in all cases. The value for DAMNPs makes them one of the most powerful adsorbents to date.

<table><caption>**Table 4.** Comparison of U(VI) adsorption capacities for various nanomaterials, as defined by the Langmuir isotherm model.</caption><thead><tr><th>Adsorbent</th><th>q<sub>max</sub><br>(mg g<sup>-1</sup>)</th><th>pH</th><th>Reference</th></tr></thead><tbody><tr><td>Humic acid-coated Fe<sub>3</sub>O<sub>4</sub> NPs</td><td>10.5</td><td>4.8</td><td>65</td></tr><tr><td>polyethylenimine -functionalized magnetic-chitosan NPs</td><td>134.6</td><td>5.0</td><td>66</td></tr><tr><td>Fe<sub>3</sub>O<sub>4</sub>@nSiO<sub>2</sub>@mSiO<sub>2</sub></td><td>104</td><td>5.5</td><td>67</td></tr><tr><td>Polyaniline combined CuFe<sub>2</sub>O<sub>4</sub> (CuFe<sub>2</sub>O<sub>4</sub>/PANI)</td><td>322.6</td><td>4.0</td><td>68</td></tr><tr><td>Ion-imprinted magnetic microspheres</td><td>122.7</td><td>6.0</td><td>69</td></tr><tr><td>CoFe<sub>2</sub>O<sub>4</sub>@SiO<sub>2</sub>@CS-PBTCA</td><td>105.3</td><td>4.0</td><td>70</td></tr><tr><td>Fe<sub>3</sub>O<sub>4</sub>/graphene composite</td><td>176.5</td><td>5.5</td><td>71</td></tr><tr><td>Amidoxime-functionalized flower-like Fe<sub>3</sub>O<sub>4</sub>@TiO<sub>2</sub></td><td>314</td><td>6.0</td><td>72</td></tr><tr><td>Succinyl-β-cyclodextrin-APTES@Fe<sub>2</sub>O<sub>3</sub> NPs</td><td>286</td><td>6.0</td><td>42</td></tr><tr><td>APTES-functionalized clay</td><td>45</td><td>7.0</td><td>73</td></tr><tr><td>DTPA cross-linked β-CD polymer</td><td>260</td><td>3.0</td><td>74</td></tr><tr><td>MNPs</td><td>90</td><td>5.5</td><td>this work</td></tr><tr><td>AMNPs</td><td>200</td><td>5.5</td><td>this work</td></tr><tr><td>DAMNPs</td><td>385</td><td>5.5</td><td>this work</td></tr></tbody></table>

### 3.3.5 Adsorption selectivity/reusability of DAMNPs

The selectivity of the magnetic nanohybrids was established by studying the matrix effect of coexisting ions on U(VI) adsorption in solutions containing the following cations: $Cs^+$, $K^+$, $Na^+$, $Ca^{2+}$, $Mg^{2+}$, and $Al^{3+}$. In Figure 5a the adsorption efficiencies (A.E. in %) are shown for all the metals tested. All foreign metal cations (i.e. not U(VI)), except $K^+$, are adsorbed by all NPs, slightly more by DAMNPs than by the others, but A.E. never exceeds 8%, whereas uranium is adsorbed almost as well in the presence of foreign cations as it is without. In other words, there is no significant matrix effect on uranium extraction by the nanohybrids studied.

After adsorption of U(VI) ions from the solution, elution of the immobilized material is required for further processing and regeneration of the sorbent. This was done by treating the loaded adsorbents with dilute nitric acid and shaking them for 10 min. Analysis of stripping solutions to investigate possible deterioration revealed no significant leaching of silicon and iron. It can be concluded that the material is stable during the time needed for complete
567 stripping of the loaded nanocomposite. To examine the reusability of DAMNPs,
568 adsorption/desorption experiments were repeated ten times. The results (Figure 5b) show that
569 they can be used five times without obvious loss in the initial binding affinity. Beyond five
570 cycles, the extraction efficiency of the adsorbent is slightly decreased. After ten cycles, the
571 extraction falls to 93% of the first cycle, but there is still no significant silicon or iron loss.
572 These results demonstrate that the DAMNP nanohybrid is reusable with almost the same
573 efficiency, which makes it a very promising sustainable material for uranium extraction.

<figure><img src="image_6.png" /><figcaption>**Figure 5.** (a) Selectivity test on MNPs, AMNPs and DAMNPs (2 mg of adsorbent) for U(VI) in 1.5 mL of a solution of U(VI) acetate alone and in a mixture containing U(VI) and six other metal cations (all concentrations, 10 mg L⁻¹). (b) Reusability test for DAMNPs. **Single-column fitting image.**</figcaption></figure>

574

**3.3.6 Mechanism of U(VI) adsorption on DAMNPs**

575

576 To understand the nature of uranium adsorption and identify possible binding sites, the XPS
577 and FTIR spectra of DAMNPs were recorded before and after adsorption (Figure 6a: survey
578 spectra). Important peaks at binding energies (BEs) of 382.1 and 392.9 eV after adsorption
579 are identified as the 4f<sub>7/2</sub> and 4f<sub>5/2</sub> peaks of uranium, respectively (Figure 6b). Both peaks and
580 the shake-down satellites are in the range reported previously.<sup>75</sup> The N1s binding energy
581 corresponding to NH-CHCOO- and H-N-C=O groups before adsorption is 401.0 eV, and the
582 corresponding peak appears at 400.3 eV after adsorption, confirming that the nitrogen atom is
583 involved in the uranium complex.<sup>76,77</sup> Small shifts of the C1s and O1s peaks were observed.
584 Changes in the BEs are consistent with the involvement of atoms (N, O, C) in complexation
585 with uranium, and establish that U(VI) is chemisorbed on DAMNPs.<sup>78</sup> Survey spectra of
586 MNPs and AMNPs before and after U(VI) adsorption are given in Figure S7.
After adsorption a new characteristic sharp peak at 909 cm⁻¹ appears in the FTIR spectrum, corresponding to covalently bound UO₂²⁺ (Figure 6c).⁷⁹ Shifts for the C=O group, from 1642 cm⁻¹ to 1622 cm⁻¹, and for OH stretching, from 3440 to 3380 cm⁻¹, as well as the very small shift for N–H bending, from 1390 to 1388 cm⁻¹, confirm that bonding involves the nitrogen and oxygen atoms of the ligand.⁸⁰,⁸¹

<figure><img src="image_7.png" /><figcaption>**Figure 6.** XPS spectra of DAMNPs: (a) Typical survey spectra before and after U(VI) adsorption. (b) High-resolution U4f<sub>7/2</sub> and U4f<sub>5/2</sub> core-level spectrum of U-loaded DAMNPs. (c) FTIR spectra of DAMNPs before and after U(VI) adsorption. (d) High-resolution C<sub>s</sub>-corrected STEM-HAADF image in grey, EELS images with Fe (green) and U (red) of DAMNPs after U(VI) adsorption. **2-column fitting image.**</figcaption></figure>

Finally, spherical-aberration-corrected STEM was performed on DAMNPs to visualize uranium adsorption (Figure 6d). When Cs-corrected STEM is combined with HAADF detection, the contrast is sensitive to the atomic number of the elements, and therefore uranium should be much brighter than the other elements in the NPs (Fe, C, N and O).⁸² The bright spots in the first images (grey) correspond to uranium atoms/cations. Electron energy loss spectroscopy (EELS) was performed to corroborate the NP composition. The compositional map of the extracted signals is presented for Fe (green) and for U (red). The HAADF image shows clearly the presence of uranium on the DAMNPs.

## 4. Conclusion

A major difficulty in summarizing the results of these various investigations of the properties of the different NPs and their ability to adsorb U(VI) is that there appears to be at least one,
two and three adsorption sites on MNPs, AMNPs and DAMNPs, respectively (Figure 2). All NP preparations adsorb U(VI) to different degrees, increasing with the complexity of the coverage. Nano-ITC measurements on free DTPA fit a simple, one-site model with 3:2 U/DTPA stoichiometry and a good affinity constant, and DFT calculations on the UO₂²⁺/DTPA system suggest a chemical process leading to a variety of coordination complexes with different stoichiometries, including 3:2, are possible. Calculations were not performed on DAMNP but numerical analysis of the TGA results for AMNP and DAMNP requires APTES and DTPA sites capable of complexing 2.6 and 6.0 U(VI) ions (at pH 5.5), respectively, somewhat exceeding what is expected on the basis of a simple complexation scheme. Adsorption isotherms favor chemical adsorption rather than physisorption, but a small contribution from physisorption cannot be totally ruled out. Clearly not everything can be explained by simple UO₂²⁺/ligand or UO₂(OH)⁺/ligand complexes. DAMNP remains a remarkably effective, selective and re-usable adsorbent for extracting uranium from aqueous solution (qₘₐₓ, 385 mg U g⁻¹ at pH 5.5, 25 °C by Langmuir isotherm and PC = 2318 mg·g⁻¹·mM⁻¹ at C₀ = 150 mg·L⁻¹) which can be recovered by simple magnetic harvesting (Figure S8). Much of the previous work in this field has used only the adsorption capacity as the performance criterion and, since both PC and the adsorption capacity depend on several effect factors, quantitative comparisons of different materials are very difficult. The relative performances of the materials studied in this paper are fully quantified: by all criteria, DAMNP is the best adsorbent, and the fact that some features about the adsorption mechanism are not completely understood does not detract from its potential applications. There are serious problems in the handling of wastes and the decommissioning of nuclear power plants, but nuclear energy has a carbon footprint comparable to that of sustainable energies based on water, sun and wind, and every effort to make it as clean and efficient as possible is worthwhile. Uranium remediation and recovery can make an important contribution to this effort.

## Credit Author Statement:

Y.X. and A.S.H. synthesized nanocomposites, performed adsorption tests and characterization; Y.X. and E.M. contributed to sample preparation, adsorption tests and characterization; J.S.L. contributed to analysis and interpretation of results; P.D. performed XPS analysis; A.C.B. performed thermogravimetric and FTIR analyses; A.M. visualized nanocomposites by STEM-HAADF; R.L. supervised the ICP experiments; F.M. performed DFT calculations; S.A.
performed magnetic and BET measurements; M.H. conceived the entire project, supervised
the experimental and theoretical campaign, and handled project administration and funding
acquisition; M.H., A.S.H. and J.S.L. drafted, reviewed and edited the manuscript. All authors
have read and agreed to the submitted version of the manuscript.

640 **Declaration of Competing Interest**

641 The authors declare that they have no known competing financial interests or personal
642 relationships that could have appeared to influence the work reported in this paper.

643 Acknowledgments

644 This work is supported by the ANR (Agence Nationale de la Recherche) program “DECRET”
645 (ANR-13-SECU-0001), by Campus France, and by the China Scholarship Council. ICP-MS
646 measurements were performed at the Chemistry department, Université Paris Cité with the
647 help of Gurvan Le Faucheur and under the supervision of Marie-Eveline Pinart. Financial
648 support from the ANR and the CGI (Commissariat Général à l’Investissement d’Avenir)
649 through Labex SEAM (Science and Engineering for Advanced Materials and devices): ANR
650 11 LABX 086, ANR 11 IDEX 05 02. The XPS-UPS facility is supported in part by the Ile-de-
651 France regional council, SESAME agreement n° 16016303. Michael Kirkpatrick (Centrale
652 Supélec) is warmly thanked for text editing.

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