# Addressing phosphate removal issues during peritoneal dialysis using colloidally stable iron oxide nanoclusters coated with tannic acid

Theo Lucante, Philippe Choquet, Joana Vaz-Ramos, Vincent Ball, Dominique Bégin, Cédric Leuvrey, Vasiliki Papaefthymiou, Spyridon Zafeiratos, Ariane Zaloszyc, Sylvie Bégin-Colin

► To cite this version:

Theo Lucante, Philippe Choquet, Joana Vaz-Ramos, Vincent Ball, Dominique Bégin, et al. Addressing phosphate removal issues during peritoneal dialysis using colloidally stable iron oxide nanoclusters coated with tannic acid. Applied Surface Science, 2026, 717, pp.164782. 10.1016/j.apsusc.2025.164782. hal-05333351

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# Addressing phosphate removal issues during peritoneal dialysis using colloidally stable iron oxide nanoclusters coated with tannic acid

Théo Lucante<sup>a,b</sup>, Philippe Choquet<sup>c</sup>, Joana Vaz-Ramos<sup>a,b D</sup>, Vincent Ball<sup>d D</sup>, Dominique Bégin<sup>b</sup>, Cedric Leuvrey<sup>a</sup>, Vasiliki Papaefthymiou<sup>b D</sup>, Spyridon Zafeiratos<sup>b</sup>, Ariane Zaloszyc<sup>b,e</sup>, Sylvie Bégin-Colin<sup>a,b,*</sup>

ARTICLE INFO

Keywords:

Phosphate adsorption
Iron oxide nanoclusters
Tannic acid
Colloidal stability
Ionic strength
Peritoneal dialysis

ABSTRACT

Removal of chronic excess phosphates in blood is critical for patients suffering from end-stage renal disease (ESRD). ESRD patients can be treated with peritoneal dialysis (PD), during which phosphates are transported from the blood to a specific solution, named dialysate, introduced into their peritoneal cavity. However, phosphate removal in PD is currently insufficient but could be improved by introducing in dialysate phosphate adsorbents. Iron oxide nanoparticles are biocompatible and well-known effective phosphate adsorbents, but their colloidal stability at physiological pH and adsorption capacity in high ionic strength aqueous media such as dialysate are significant challenges. Here, we have evaluated the potential of tannic acid-coated iron oxide raspberry-shaped nanoclusters (RSNs@TA) for improving phosphate adsorption in dialysate. We have investigated the influence of the TA coating on phosphate adsorption by performing adsorption experiments with uncoated and TA-coated RSNs. In parallel, we have studied the influence of electrolytes and compounds in dialysate on phosphate adsorption by conducting experiments in pH 7 water and dialysate. The TA coating was shown to provide a high colloidal stability to the nanoclusters and mitigates the inhibitory effect of electrolytes in dialysate on phosphate adsorption. Indeed, electrolytes in dialysate decreased the phosphate adsorption on both nanoclusters but especially on uncoated ones by disturbing phosphate outer-sphere complexes. Thus, RSNs@TA demonstrated enhanced adsorption capacity compared to uncoated RSNs in dialysate (20.7 ± 6.4 mg P.g⁻¹) and in pH 7 water (26.4 ± 8.1 mg P.g⁻¹). These results established RSNs@TA as promising adsorbents for phosphate removal in dialysate during a PD process.

1. Introduction

Phosphate accumulation in blood causes critical health issues for patients with End-Stage Renal Disease (ESRD). ESRD is related to the loss of the blood purification functions of the kidneys and leads to an accumulation of molecules and uremic toxins in the blood. Phosphates are among the most dangerous of these compounds, as their presence in chronically high concentration in blood conducts to life-threatening complications (vascular calcification, cardiac arrest, etc.). Waiting for

a kidney transplant, patients are dialyzed by either hemodialysis (HD) or peritoneal dialysis (PD) processes with HD being the most used treatment. However, PD has many advantages such as being the most suitable method for treating infants and children, allowing more autonomy in treatment, needing less drugs, being less expensive, etc. Furthermore, predictive analyses planned that the number of patients suffering from chronic kidney disease (CKD) will strongly increase by 2030, and current HD units will not be able to face this high increase in CKD patients [1]. Therefore, upgrading PD will allow facing such challenges. PD

<footnote><sup>a</sup> Université de Strasbourg, CNRS, Institut de Physique et Chimie des Matériaux (IPCMS), UMR 7504, 23 Rue du Loess, BP 43, Strasbourg 67034, France</footnote>

<footnote><sup>b</sup> Université de Strasbourg, CNRS, Institut de Chimie et Procédés pour l'Énergie, l'Environnement et la Santé (ICPEES), UMR-7515, 25 rue Becquerel, Strasbourg 67087, France</footnote>

<footnote><sup>c</sup> University of Strasbourg, CNRS and HUS, Laboratoire des Sciences de l'Ingénieur, de l'Informatique et de l'Imagerie (ICube) – CNRS/University of Strasbourg, UMR 7357 Preclinical Imaging Lab, Imaging Dpt, Hôpitaux Universitaires de Strasbourg, France</footnote>

<footnote><sup>d</sup> INSERM UMR_S 1121, EMR 7003 CNRS, "Biomaterials and Bioengineering", Université de Strasbourg, Faculté de Chirurgie Dentaire, F-67000 Strasbourg, France</footnote>

<footnote><sup>e</sup> Nephro-pediatry, CHU Hautepierre, Hopitaux Universitaire de Strasbourg, 1 avenue Molière, 67098 Strasbourg, France</footnote>

<footnote>* Corresponding author.
E-mail address: sylvie.begin@unistra.fr (S. Bégin-Colin).</footnote>
treatment consists in introducing a specific solution called dialysate in the patient's peritoneal cavity. In this cavity, the dialysate enters in contact with a biological membrane called the peritoneal membrane, and transport of solutes (comprising phosphate) occurs between the peritoneal blood capillaries and the dialysate through the peritoneal membrane. However, PD has a lower phosphate epuration efficiency compared to HD [2–4] and one solution to enhance the phosphate removal might be to add phosphate's adsorbents in the dialysate used in PD process.

Iron oxide nanoparticles are well-known phosphates absorbents and overcoming such phosphate accumulation by exploiting the affinity of phosphates for iron oxide surfaces [5] through adsorption is a very interesting strategy that has been already developed for depollution applications [5–7]. Indeed, among the various methods developed for eliminating phosphates (precipitation processes, biological processes, membrane separation processes, etc.), adsorption is notably known for its high adsorption capacity depending on the adsorbent used, the possibility of regenerating and reusing the adsorbent as well as not producing a precipitate [8,9]. Furthermore, adsorption is the most suitable solution considering the specification of the PD process. Therefore, iron oxide nanoparticles (IONPs) are very promising adsorbents to enhance phosphate removal in dialysate. They have been approved by the Food and Drug Administration (FDA) for clinical applications [10–12]. Adsorption of phosphates on IONPs in dialysate is expected to induce the transport of additional phosphates from blood during the PD session, improving the performance of this dialysis treatment [13].

For this application, IONPs should display a high phosphate adsorption capacity and colloidal stability in dialysate, which is an aqueous media at physiological pH (pH = 7.4). However, iron oxide isoelectric point (IEP) is located at approximatively pH = 7 [14], decreasing the IONPs colloidal stability in aqueous media at close pH values and weakening their efficiency for phosphate adsorption. A widely-used strategy to increase their colloidal stability consists in coating their surface with an appropriate surfactant that will promote electrostatic and/or steric interactions between colloids in suspension [15,16]. Though, such surfactants will occupy adsorption sites and may reduce the phosphate adsorption capacity of IONPs. In addition, dialysate contains different electrolytes and compounds (Table S1), increasing its ionic strength (here, $150 \pm 10 \text{ mmol.L}^{-1}$), which could affect the phosphate removal efficiency. Also, at pH = 7, the interaction between negatively charged phosphate anions (under the forms $H_2PO_4^-$/ $HPO_4^{2-}$) and the iron oxide surface are not favorable [5]. Indeed, the surface of iron oxide surface is quite neutral at the IEP, which does not favor phosphate adsorption through electrostatic interactions [5]. However, our previous works demonstrated that even at pH ≈ 7, phosphate adsorption occurs at the surface of IONPs of size 40 nm obtained by a hydrothermal treatment [5] and of nanoclusters obtained by a polyol solvothermal method [6].

In this work, we have evaluated the performances for phosphate adsorption in dialysate of tannic-acid coated iron oxide raspberry-shaped nanoclusters (RSNs@TA), with the objective to show that they could improve the phosphate removal performance in a PD process. RSNs@TA were synthesized, optimized, and extensively characterized in our previous work [17], where we demonstrated that the TA coating enabled a high colloidal stability in pH 7 water. Tannic acid (TA) was chosen among the various surfactants reported in the literature (humic acid [18,19], bovine serum albumin [20], polyallylamine hydrochloride [21], polyethylenimine [22], etc.) due to its good biocompatibility, its approval by the FDA as alimentary additive and its antimicrobial properties [17,23,24].

To assess the performances of RSNs@TA for this application, we started by selecting the most suitable method for quantifying phosphates under our experimental conditions. Then, we performed experiments of phosphate adsorption kinetics and isotherm in batch mode in dialysate. To study whether electrolytes in dialysate could alter phosphate adsorption, experiments were also conducted in pH 7 water (near-zero

ionic strength). In addition, to investigate the influence of the TA coating of RSNs@TA on phosphate adsorption, batch mode experiments were also performed with uncoated RSNs. We have studied the colloidal stability of both nanoclusters before and after batch mode experiments in each media and have characterized them to confirm the phosphates adsorption. Finally, the recyclability of RSNs@TA and the desorption of TA over time and after phosphate adsorption have been studied and discussed. All these results proved that the TA coating is particularly suitable for ensuring phosphate removal in high ionic strength media and that RSNs@TA are very promising adsorbents for phosphate removal in a PD process.

## 2. Materials and methods

### 2.1. Materials

Iron(III) chloride hexahydrate ($FeCl_3 \cdot 6H_2O$, ≥99 %, Sigma-Aldrich), succinic acid ($C_4H_6O_4$, ≥99.5 %, Sigma-Aldrich), urea ($CH_4N_2O$, 99.3+%, Alfa Aesar), ethylene glycol ($C_2H_6O_2$, 99%, Thermo Scientific) and tannic acid ($C_{76}H_{52}O_{46}$, Sigma-Aldrich) are used as received for the synthesis of RSNs and RSNs@TA.

Orthophosphoric acid ($H_3PO_4$, 85+%, Analytical Reagent, for analysis, Fisher Chemical) is used as adsorbate sources for adsorption experiments in batch mode.

Tin(II) chloride dihydrate ($Cl_2H_4O_2Sn$, 98%, Thermo Scientific), ammonium molybdate (para) tetrahydrate ($H_6MoNO_2$, 99%, Thermo Scientific), sulfuric acid ($H_2SO_4$, 96%, for analysis, CARLO ERBA) and glycerol ($C_3H_8O_3$, 99+%, Alfa Aesar) are used for phosphate quantification using the molybdenum blue method.

Ultrapure water produced by a purification unit PURELAB® flex, ELGA and dialysate BICAVERA (1.5 % GLUCOSE, 1.75 mmol.L⁻¹ calcium) are used as solvents in batch mode experiments. The composition of the dialysate used is detailed in Table S1.

### 2.2. Synthesis conditions of naked and TA-coated nanoclusters

RSNs and RSNs@TA are synthesized using a polyol solvothermal method, respectively according to a previously established protocol [6,25] and by a modified version of this protocol [17]. Briefly, iron(III) chloride hexahydrate (1.63 g), succinic acid (0.24 g) and urea (3.6 g) are added to ethylene glycol (60 mL) and placed under strong magnetic stirring. After dissolution of previous reagents (~5 min stirring), tannic acid (0.1 g) is optionally added to the reaction medium in order to obtain RSNs@TA. In both protocols, stirring is continued for 2 h. The reaction mixture is then placed in an ultrasonic bath for 3 cycles of 20 min sonication, cooling down the bath water with ice between each cycle. For solvothermal treatment, the reaction mixture is sealed in a stainless-steel autoclave lined with an inner Teflon reactor (75 mL capacity) and placed inside a programmable oven. The reaction mixture is heated from room temperature to 200 °C at 1.5 °C.min⁻¹, then maintained at 200 °C for 10.5 h before being left to cool outside the oven for 3 h. The obtained nanoclusters were magnetically separated from the supernatant and washed with a mixture of 50 % ethanol/50 % acetone nine times interspersed with 4 min sonication in an ultrasound bath. The washed nanoclusters are finally stored in ethanol until further characterization or use.

### 2.3. Characterization techniques

Micrographs of RSNs and RSNs@TA nanoclusters are obtained by Scanning Electron Microscopy (SEM) using a Zeiss GeminiSEM 500 electron microscope operating with an acceleration voltage of 3.00 kV. For sample preparation, suspension drops are deposited on silicon wafers and samples are observed after solvent evaporation.

Micrographs of RSNs and RSNs@TA nanograins are acquired by Transmission Electron Microscopy (TEM) using a Jeol 2100F
transmission electron microscope operating with an acceleration voltage of 200 kV. For observation, suspension drops are deposited on carbon-coated copper grids.

Hydrodynamic diameter and zeta potential of RSNs and RSNs@TA suspensions are measured by Diffusion Light Scattering (DLS) and Electrophoretic Light Scattering (ELS) using a Malvern Panalytical Zetasizer Nano ZS equipment. Samples are prepared by dilution of the working suspensions in ultrapure water or in dialysate to reach similar concentrations (attenuator = 8, count rate = 250 kcps). Each measurement is performed 3 times at 25 °C.

Weight change of nanostructures as a function of the temperature is determined by thermogravimetric analysis (TGA) with a TA Instrument SDT Q600 equipment. The measurements are performed on dried samples under an air flow from room temperature to 900 °C at a heating rate of 5 °C.min⁻¹.

X-ray Photoelectron Spectroscopy (XPS) experiments are conducted in a VSW-Scientific spectrometer equipped with a twin anode source. XPS spectra were obtained using Al Kα radiation and pass energies of 90 and 22 eV for survey and high-resolution spectra, respectively. All XPS spectra were referenced to the C 1 s peak, at 285 eV. Spectra analysis was done with the Casa-XPS software, with the application of Shirley background subtraction and a combination of Gaussian-Lorentzian symmetric and asymmetric line shapes for the peak fitting.

Iron concentration in nanoclusters is measured by NMR ¹H-relaxometry [26] using a Bruker Minispec 60 equipment working at a Larmor frequency of 60 MHz (1.41 T) at 37 °C. T₁ relaxation time measurements are performed on nanocluster aqueous solutions in the form of Fe³⁺ ions, previously dissolved in nitric acid HNO₃. The measured relaxation times are then compared with a calibration curve of relaxation rates (1/T₁) versus Fe³⁺ concentration, obtained with iron standards.

## 2.4. Adsorption experiments in batch mode

### 2.4.1. Phosphate adsorption kinetics experiments

Adsorption kinetics experiments are performed by stirring 20 mL suspensions of 1 g(Fe₃O₄).L⁻¹ nanocluster and 50 mg P.L⁻¹ phosphate in pH 7–7.5 water and dialysate during the following durations: 0.25, 0.5, 0.75, 1, 2, 3 and 4 h. All experiments are performed at room temperature. Nanoclusters adsorption capacity is then calculated using the following equation:

$$q_t = \frac{(C_0 - C_t) V}{m_{NS}} \qquad (1)$$

where q_t (in mg P.g⁻¹) is the nanocluster adsorption capacity at time t, C₀ (in mg P.L⁻¹) is the initial phosphate concentration, C_t (in mg P.L⁻¹) is the phosphate concentration at time t in the supernatant, V (in L) is the suspension volume and mₙₛ (in g) is the mass of nanoclusters.

In order to model and predict phosphate adsorption kinetics on the surface of nanoclusters, pseudo-first order (PFO) and pseudo-second order (PSO) kinetics models [27–29] are applied to the experimental data. The PFO kinetics model is given by the following equation:

$$q_t = q_E(1 - e^{-k_1 t}) \qquad (2)$$

where q_t and q_E (in mg P.g⁻¹) are the adsorption capacities at a given time t and at equilibrium respectively and k₁ (in h⁻¹) is the pseudo-first order kinetic model equilibrium rate constant.

While the PSO kinetic model is:

$$q_t = \frac{q_E^2 k_2 t}{1 + q_E k_2 t} \qquad (3)$$

where k₂ (in g.mg⁻¹.h⁻¹) is the pseudo-second order kinetic model equilibrium rate constant.

### 2.4.2. Phosphate adsorption isotherm experiments

Phosphate adsorption isotherm experiments are carried out by stirring 20 mL suspensions of 1 g(Fe₃O₄).L⁻¹ nanoclusters and phosphates of initial concentration in the range 10–300 mg P.L⁻¹, in pH 7–7.5 water and dialysate for a fixed duration of 4 h. All experiments are performed at room temperature. Nanocluster adsorption capacity at equilibrium is calculated using equation 3, with q_t and C_t becoming respectively q_E and C_E the adsorption capacity and the phosphate concentration at equilibrium.

To further investigate the adsorption behavior of phosphates on RSNs and RSNs@TA in different media, the data were fitted with two-parameter models – the Langmuir [30–32] and Freundlich [27,28,33] isotherm models, as well as the three-parameter Sips isotherm model [30,31,34,35].

Firstly, the Freundlich isotherm model is given by the following equation:

$$q_E = K_F C_E^{1/n} \qquad (4)$$

where q_E (in mg P.g⁻¹) is the adsorption capacity at equilibrium, C_E (in mg P.L⁻¹) is the phosphate concentration at equilibrium, K_F (in (mg.g⁻¹)(L.mg⁻¹)¹/n) is the Freundlich isotherm parameter and n is the adsorption intensity.

Then, the Langmuir isotherm model is expressed as:

$$q_E = \frac{q_{max} K_L C_E}{1 + K_L C_E} \qquad (5)$$

where q_max (in mg P.g⁻¹) is the maximum adsorption capacity and K_L (in L.mg⁻¹) is the Langmuir isotherm constant.

Finally, the Sips isotherm model is expressed as:

$$q_E = \frac{K_S C_E^{\beta_s}}{1 + \alpha_S C_E^{\beta_s}} \qquad (6)$$

where β_s is a model exponent, and α_S (in L.mg⁻¹) and K_S (in L.g⁻¹) are model constants.

# 3. Results and discussion

## 3.1. Effect of the TA coating on RSNs structural and colloidal properties

RSNs and RSNs@TA were synthesized by a one-pot polyol solvothermal synthesis detailed in references [6,25] and reference [17] respectively. A comparison of the characteristics of RSNs and RSNs@TA is given in Table 1. SEM images of RSNs and RSNs@TA in Figs. 1a,b show the morphology of the different nanoclusters. RSNs consist of oriented aggregates of nanograins and the addition of TA preserves their nanocluster structure but induces a decrease in their diameter and nanograin size, as well as the loss of the nanograin oriented aggregation [17]. Indeed, adding TA into the reaction medium during the synthesis process resulted in a change in the RSNs formation mechanism. The RSNs@TA surface specific area (SSA) increases as well as their colloidal stability at physiological pH. Indeed, the TA organic coating induces a shift in the iron oxide isoelectric point (IEP) towards acidic pH values (pHIEP = 5.6) in water. Their zeta potential (ζ) value in water is −29 ± 5 mV at pH 7 while it is almost zero for RSNs, as expected. The saturation magnetization of RSNs@TA (74 emu.g⁻¹) is slightly lower than that of RSNs (88 emu.g⁻¹) because of the loss of the oriented aggregation of nanograins, but stays quite high by comparison to that of individual nanograins thanks to the TA coating and aggregation state. RSNs@TA present thus a higher SSA and higher colloidal stability than RSNs. Both nanoclusters can be easily removed from suspensions by applying an external magnetic field.

In dialysate, RSNs display a very low colloidal stability as they sedimented very quickly (~minutes). By contrast, RSNs@TA display a higher colloidal stability in dialysate as shown by dynamic light
<caption>**Table 1**</caption>

Characteristics of nanoclusters synthesized by a polyol solvothermal method with and without TA [17] and phosphate adsorption results in batch mode.

<table><thead><tr><th></th><th>RSNs</th><th>RSNs@TA, molar ratio TA:Fe = 0.01</th></tr></thead><tbody><tr><td>SEM particle diameter (nm)</td><td>Population 1: 312 ± 126<br>Population 2: 391 ± 60</td><td>Population 1: 173 ± 30<br>Population 2: 292 ± 98<br>Population 3: 361 ± 50</td></tr><tr><td>TEM nanograin diameter (nm)</td><td>45 ± 22</td><td>6.4 ± 2.7</td></tr><tr><td>Lattice parameter (nm)</td><td>8.406</td><td>8.409</td></tr><tr><td>BET surface area (m²·g⁻¹)</td><td>27</td><td>82</td></tr><tr><td>Adsorption average pore size (nm)</td><td>2.7</td><td>1.6</td></tr><tr><td>Weight loss in TGA (%)</td><td>3.0</td><td>18</td></tr><tr><td>Mean hydrodynamic diameter in intensity in pH 7 water (nm)</td><td>~2000</td><td>378 ± 123</td></tr><tr><td>Zeta potential value in pH 7 water (mV)</td><td>0 ± 4</td><td>-29 ± 5</td></tr><tr><td>Mean hydrodynamic diameter in intensity in dialysate (nm)</td><td>—</td><td>426 ± 132</td></tr><tr><td>Zeta potential value in dialysate (mV)</td><td>—</td><td>-15 ± 1</td></tr><tr><td>Isoelectric point</td><td>6.9</td><td>5.6</td></tr><tr><td>Magnetization at saturation at 300 K (emu·g⁻¹)</td><td>88</td><td>74</td></tr><tr><td>Phosphate adsorption in pH 7–7.5 water (mg P·g⁻¹) after 4 h-isotherm experiment with 300 mg P·L⁻¹ phosphate</td><td>21.9 ± 6.8</td><td>26.4 ± 8.1</td></tr><tr><td>Phosphate adsorption in pH 7–7.5 dialysate (mg P·g⁻¹) after 4 h-isotherm experiment with 300 mg P·L⁻¹ phosphate</td><td>14.3 ± 4.4</td><td>20.7 ± 6.4</td></tr></tbody></table>

scattering (DLS) and electrophoretic light scattering (ELS) measurements. In each solvent, the hydrodynamic diameter ($\phi_{hydro}$) distribution of RSNs@TA is monomodal with a mean $\phi_{hydro}$ of 378 ± 123 nm and 426 ± 132 nm in pH 7 water and dialysate, respectively. Their $\zeta$ net value decreases from -29 ± 5 mV in water to -15 ± 1 mV in dialysate. This net decrease is attributed to electrolytes in dialysate, interacting with the TA coating and iron oxide surface, “shielding” thus the RSNs@TA surface charge [36]. Despite this net decrease, electrosteric interactions between RSNs@TA in suspension are high enough to ensure a high colloidal stability with only a slight increase in RSNs@TA mean $\phi_{hydro}$. This result is very promising as RSNs@TA are colloidally stable not only in pH 7 water but also in pH 7 aqueous media with higher ionic strength.

## 3.2. Phosphate adsorption studies

### 3.2.1. Choice of a phosphate quantification method

Two phosphate quantification methods have been tested and

compared: the Molybdenum Blue (MB) method and the use of an IDEXX VetTest 8008 blood biochemistry analyzer device. The protocols for quantifying phosphates using these two methods are detailed in the *Supplementary Information* section (Section SI.I). The MB method is an American Public Health Association (APHA) approved method for the detection and quantification of phosphate in water [5,6,37] and consists in forming a blue phosphate complex quantifiable by UV-visible spectroscopy. This method involves first the reaction between molybdate anions from ammonium heptamolybdate salt (NH₄)₆Mo₇O₂₄·4H₂O with phosphate under acidic condition to form phosphomolybdic acid H₃PMoVI₂O₄O (PMA), a heteropoly acid of the Keggin structural class where phosphate is located in its center [38]. Molybdenum in PMA are then partially reduced from MoVI to MoV by tin cations, leading to the formation of the blue complex [H₄PMoVI₂MoV₄O₄O]³⁻ [39].

However, as detailed in the *Supplementary Information* part (Section SI.I), the MB method leads to high standard deviations around measured concentrations and appeared to be very sensitive due to the low stability of the formed optical blue complex over time, making this method less reliable for phosphate quantification in our experimental conditions [6]. Indeed, the linear range of the calibration curve using this method is extremely narrow and in a concentration range largely below the concentrations studied here (Fig. S1a). It is thus necessary to make dilutions for quantification of our studied phosphate concentrations, which leads to broad measurement standard deviations. Therefore, a device used by veterinarians to perform blood analysis has been considered. Based on the experimental results detailed in Fig. S1c, the VetTest method has been selected for its lower standard deviation on measured concentrations, its higher accessible phosphate concentration range (i.e. no need of a dilution step, Fig. S1b), and its convenience for routine measurements and for comparing the different samples.

### 3.2.2. Phosphate adsorption kinetics experiments with RSNs and RSNs@TA in pH 7 water and dialysate

Phosphate adsorption on nanoclusters as a function of time in ultrapure water (pH = 7–7.5) and in dialysate is first performed to evaluate if RSNs and RSNs@TA are able to quickly adsorb phosphates even in the presence of electrolytes and glucose from dialysate. A fixed phosphate concentration of 50 mg P·L⁻¹ is selected for i) comparing our adsorption results with previous experiments performed in our laboratory with RSNs in pH 7 water [6], ii) because this value corresponds to roughly an average concentration investigated in the literature [6,40–42] and iii) because it corresponds to the hyperphosphatemia threshold (i.e. the plasma phosphate level above which the amount of phosphates is considered dangerous for health) in ESRD adults. The preparation of phosphate solutions and nanocluster suspensions for batch mode experiments is detailed in the *Supplementary Information* section (Section SI.II).

From results presented in Figs. 2a,b, the maximum adsorption

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** SEM images of a) RSN and b) RSN@TA nanoclusters.</figcaption></figure>
capacities of RSNs@TA after 4 h are very close in water (5.2 ± 1.6 mg P·g⁻¹) and in dialysate (5.5 ± 1.7 mg P·g⁻¹). For RSNs these values are globally lower, particularly in dialysate (1.9 ± 0.6 mg P·g⁻¹) compared to water (3.4 ± 1.1 mg P·g⁻¹). Moreover, one may underline that the maximum adsorption capacity for both nanoclusters is reached at durations lower than 2 h and even 1 h would be enough. These results are in quite good agreement with our previous studies on phosphate adsorption on similar RSNs [6] and evidence the fast adsorption of phosphates on these nanoclusters even in dialysate and in presence of the surfactant.

Kinetics data in Figs. 2a,b have been fitted using the widely-used pseudo-first order (PFO) and pseudo-second order (PSO) kinetic models [27–29] in non-linear form to obtain more information on phosphate adsorption mechanism on RSNs and RSNs@TA. Modeling attempts and associated parameters are reported in Figs. S2a–d and Table S2. In fact, adsorption is a multi-step phenomenon involving first the diffusion of adsorbates (here, phosphates) from the bulk solution until the adsorption sites of the adsorbent (here, the nanoclusters), where they could finally adsorb through chemical and/or physical interactions. In this context, compliance of experimental data with the PSO model is commonly interpreted as an indication that the rate-determining step of the adsorption process is linked to the diffusion step (diffusion-controlled adsorption). On the other hand, a better fit with the PSO model would suggest that chemisorption is the rate-determining step of the adsorption process.

However, in Figs. 2a,b, the maximum adsorption capacity values for each nanocluster is reached too fast, providing kinetics parameter values with a very high standard deviation. As a result, the coefficients of determination (R²) associated with each model in non-linear form are too close to discriminate the more suitable kinetic order. Nevertheless, one can notice that the fitting of kinetics data related to RSNs in dialysate is the worst for both models (lower R²), suggesting a different adsorption behavior of RSNs in dialysate compared to water. Theoretical equilibrium adsorption capacity qE = 3.1 ± 0.2 mg P·g⁻¹ (PFO model) / 3.3 ± 0.2 mg P·g⁻¹ (PSO model) are obtained for RSNs in water, in agreement with the experimental adsorption capacity (3.4 ± 1.1 mg P·g⁻¹). For RSNs@TA in water and dialysate, the qE values with both models are quite similar and in agreement with experimental results (Table S2).

To further analyze these data, they have been expressed in order to apply the linear form of the PFO and PSO models (Figs. S3a–h and Table S3). The linearized PSO model appears suitable for fitting all data, unlike the PFO model. Although great care must be taken with the limitations of interpreting these fits by linear models [43–45], it seems

that the PFO model can be excluded, meaning that adsorption would not be kinetically dependent on the chemisorption process. This interpretation is in agreement with our previous work in which the PSO model was also found to be the most suitable for modelling kinetic data related to naked and aluminum-doped RSNs at pH 7, in the same way as for many iron oxide-based materials reported in the literature [6]. Based on all these results, it appears more likely that phosphate adsorption on RSNs and RSNs@TA in pH 7 water and dialysate follows a PSO kinetics, interpreted as a quick phosphate adsorption on iron sites and then a slower phosphate diffusion to fill the remaining sites [6]. It is realistic since TA molecules in RSNs@TA are occupying adsorption sites and are negatively-charged, preventing phosphate anions to easily diffuse to the free sites. On the other hand, phosphates are known to have a high affinity for iron oxide surfaces, facilitating adsorption once they reach a free site [5].

### 3.2.3. Phosphate adsorption isotherm experiments with RSNs and RSNs@TA in pH 7 water and dialysate

Then, adsorption isotherm experiments have been carried out to study the nanoclusters adsorption capacity as a function of phosphate concentration. Experiment duration is kept to 4 h in line with the rapid adsorption observed in the previous kinetics experiments. Phosphate initial concentrations are chosen in the range 10–300 mg P·L⁻¹, i.e. conditions generally used for phosphate depollution experiments and comprising values encountered in PD application (blood critical phosphate concentration above 62 mg P·L⁻¹, i.e. 2 mmol P·L⁻¹).

Figs. 3a,b display the phosphate adsorption capacity of RSNs and RSNs@TA as a function of phosphate concentration. The phosphate adsorption capacity at equilibrium values increases with the phosphate concentration in ultrapure water and in dialysate for both RSNs and RSNs@TA. For RSNs, a phosphate adsorption of 21.9 ± 6.8 and 14.3 ± 4.4 mg P·g⁻¹ is reached in ultrapure water and dialysate respectively for the highest phosphate concentration (300 mg P·L⁻¹) while, for RSNs@TA, 26.4 ± 8.1 and 20.7 ± 6.4 mg P·g⁻¹ are noticed respectively (Table 1). Thus, the adsorption capacity of both nanoclusters is higher in ultrapure water than in dialysate in the whole range of concentrations. This may be explained by the higher ionic strength in dialysate calculated to be 150 ± 10 mmol·L⁻¹ (Table S1), which should hamper/per turbate the phosphate adsorption. Meanwhile, RSNs@TA adsorption capacity is higher than that of RSNs, which could be explained by their higher SSA. Nevertheless, the phosphate adsorption on RSNs@TA is not as high as expected considering their higher SSA. At first, it may be explained by the TA coating providing a “negative surface charge” to RSNs@TA (Table 1), which should not promote phosphate adsorption.

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Phosphate adsorption on RSNs and RSNs@TA as a function of time in (a) pH 7 water and (b) dialysate. Nanocluster concentration is 1 g·L⁻¹ and phosphate concentration is 50 mg P·L⁻¹.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 3. Phosphate adsorption on RSNs and RSNs@TA as a function of phosphate concentration at equilibrium in (a) pH 7 water and (b) dialysate. Nanocluster concentration is 1 g.L⁻¹ and experiment duration is 4 h. Data are here fitted with the Freundlich isotherm model except for data related to RSNs in dialysate fitted with the Sips isotherm model.</figcaption></figure>

Indeed, phosphate is also negatively charged and in the anionic forms H₂PO₄⁻/HPO₄²⁻ in pH 7–7.5 aqueous solvents, which should induce less favorable electrostatic interactions and affect the phosphate adsorption. There is also an effect of the TA coating, which occupy iron oxide surface adsorption sites and thus less phosphate adsorption sites are available.

Expected maximum phosphate adsorption capacities of RSNs and RSNs@TA may be calculated using their SSA and the theoretical area of a phosphate molecule (~0.24 nm²) [5]. Thus, ~4.2 phosphate molecules per nm² of iron oxide is calculated, leading to a theoretical maximum of ~6 mg P.g⁻¹ adsorbed on RSNs (SSA = 27 m².g⁻¹) and ~18 mg P.g⁻¹ for RSNs@TA (SSA = 82 m².g⁻¹). Experimental values for RSNs@TA and especially for RSNs are higher than the so-calculated adsorption capacity values. Such higher phosphate adsorption have been already observed in a previous study with similar uncoated nanoclusters and was suggested to be due to different surface complexes (i.e. monodentate and bidentate) or to the formation of a second phosphate layer by hydrogen interaction or a more favorable nanostructuration of RSNs [6]. The type of surface complex, as well as the nanostructuration, cannot explain such a high difference between expected and experimental adsorption values but these results rather suggest the presence of multiple adsorbed phosphate layers on the iron oxide surface. Indeed, phosphates adsorbed on iron oxide surfaces have been reported to possibly form two types of complexes: inner-sphere complexes and outer-sphere complexes by weaker interactions [5]. Therefore, one suggests that phosphates would also form outer-sphere complexes on RSNs and RSNs@TA surfaces [46,47]. As the adsorption capacity of RSNs and RSNs@TA in dialysate is lower than in water, they appear to be sensitive to the ionic strength of the solvent and one may thus suggest that electrolytes perturbate the formation of outer-sphere complexes.

Furthermore, isotherm data in Figs. 3a,b have been fitted with three models widely used in similar adsorption experiments in the literature: Langmuir [30–32], Freundlich [27,28,33] and Sips [30,31,34,35] isotherms and extracted parameters are given in Table S4. Except for RSNs in dialysate, all data are best fitted using the Freundlich isotherm model. This model assumes a multilayer adsorption on a heterogeneous surface possessing adsorption sites of different energies. This result is thus in line with the previous conclusions on the formation of phosphate outer-sphere complexes on our nanoclusters. For RSNs and RSNs@TA, the 1/n values between 0 and 1 (minimum is 0.64 ± 0.06 for RSNs@TA in dialysate and maximum is 0.90 ± 0.06 for RSNs in water) suggest a favorable chemisorption of phosphates. This value closer to 1 for RSNs in water than RSNs@TA tends to suggest a more homogeneous and strong

phosphate chemisorption on RSNs while RSNs@TA would have a more heterogeneous surface, which agrees with the presence of the TA coating. All these results support thus the presence of both phosphate inner- and outer-sphere complexes on RSNs and RSNs@TA. Concerning RSNs in dialysate, the data could only be fitted with the Sips isotherm model, which would indicate that electrolytes in dialysate strongly affect the phosphate adsorption on RSNs surface. Briefly, the Sips isotherm is a three-parameter model that have been used in the literature in the framework of water contaminant adsorption for fitting data exhibiting a sigmoid shape (type V isotherm according to the IUPAC classification) [48,49]. It is a combination of the Langmuir and Freundlich isotherms and used to predict adsorption on heterogeneous surface [30,31,34,35]. Thus, the dialysate electrolytic composition disturbed the phosphate adsorption and has a stronger influence on RSNs than on RSNs@TA. Following these interpretations, the TA coating seems to better preserve RSNs@TA from being strongly affected by the electrolytes in dialysate compared to naked RSNs.

### 3.2.4. Characterization of RSNs and RSNs@TA after adsorption experiments

To further confirm that phosphates interact chemically with the surface of each nanocluster, RSNs and RSNs@TA were characterized before and after phosphate adsorption experiments in ultrapure water and dialysate. This characterization study is detailed in the *Supplementary Information (Section SI.III)*. SEM, TEM and SAED images of nanoclusters before and after phosphate adsorption experiments (Figs. S4a-x) showed no structural changes neither in ultrapure water nor in dialysate. The analysis of FTIR spectra at each step has confirmed the adsorption of phosphate on the surface of both nanocluster whatever the liquid media with the presence of P-O-based bands (Figs. S5a,b) and the TA coating is always identified at the surface of RSNs@TA. TGA experiments allowed also confirming the phosphate adsorption in water for both RSNs and RSNs@TA but in dialysate, the presence of glucose has affected the results for RSNs@TA while a phosphate-indicating feature is observed with RSNs (Figs. S6a,b and Table S5). By photoelectron spectroscopy, the phosphorus 2p peaks are clearly identified in XPS spectra of both RSNs and RSNs@TA after adsorption (Fig. S7c). They are located at binding energies of respectively 133.3 and 133.8 eV, close to the value of 133.6 eV reported by Daou *et al.* for phosphated magnetite [5], as well as other groups [50,51]. All these characterizations confirm the presence of phosphates at the surface of both RSNs and RSNs@TA and confirmed thus that a chemisorption has occurred on both nanoclusters. These XPS results would confirm the presence of
inner- and outer-sphere phosphate complexes and the latter may
possibly result from hydrogen bonding between chemisorbed phos-
phates and phosphates for RSNs and RSNs@TA but also between phos-
phates and TA for RSNs@TA.

$\phi_{\text{hydro}}$ and $\zeta$ values of nanoclusters suspended in ultrapure water and dialysate were also measured before and after phosphate adsorption (Figs. S8a,b and Table S7). The RSNs in water or dialysate are aggregated due to the pH, which is very close to the IEP of iron oxide and the adsorption step has not changed this aggregation state overall. An increase in the mean $\phi_{\text{hydro}}$ of RSNs@TA is observed after adsorption but the size distribution stayed monomodal (Figs. S8a,b and Table S7). Adsorption on RSN@TA in water has not affected their colloidal stability (the mean $\phi_{\text{hydro}}$ and the $\zeta$ values are close). However, in dialysate, a screening effect due to electrolytes is observed as deduced from the net decrease of the $\zeta$ value from $-29 \pm 5$ mV in water to $-15 \pm 1$ mV in dialysate (without affecting the colloidal stability). One may suggest that the species (electrolytes and glucose) in dialysate should perturbate the surface of RSNs and thus affect the phosphate adsorption at their surface when for RSNs@TA, the species decreases the strength of unfavorable electrostatic interactions favoring thus the phosphate adsorption.

These results demonstrate a superior colloidal stability and phos-
phate adsorption capacity of RSNA@TA compared with RSNs and thus a
positive impact of the TA coating.

### 3.2.5. Discussion

Comparison of phosphate removal performance with results of the literature. We have tried to compare our adsorption results with those reported in the literature, first with surfactant-coated iron oxide-based adsorbents (Table S8) involved in phosphate removal from aqueous media at near-physiological pH. However, direct comparison is quite difficult because several characteristics of adsorbents and/or coatings such as SSA; $\phi_{\text{hydro}}$, IEP or $\zeta$ values are not systematically reported despite their crucial importance in confirming the performance of adsorbents in pollution control applications. Nevertheless, RSNs@TA adsorption capacity compares quite well with those of functionalized absorbents with similar negative $\zeta$ values, reporting values in the range 2 – 8.2 mg P.g⁻¹. Furthermore, when comparing our results with naked iron oxide-based adsorbents (Table S9) from experiments in pH 7.0 ± 0.4 water, with adsorbent concentration from 0.6 up to 2 g L⁻¹ and phosphate concentration in the range 150 – 200 mg P.L⁻¹, the reported adsorption maxima are ~ 9 mg P.g⁻¹ [52] and ~15 mg P.g⁻¹ [53]. These values are to be compared to 9.3 and 17.2 mg P.g⁻¹ for RSNs and RSNs@TA respectively (adsorption capacities at equilibrium, 150 mg P. L⁻¹ phosphates and 1 g.L⁻¹ nanoclusters), leading thus to slightly higher values. Also, Ajmal et al. [53] reported an adsorption capacity of 32 mg P.g⁻¹ from an experiment at higher phosphate concentrations (600 mg P.L⁻¹) after 2 h and an adsorbent concentration of 2 g.L⁻¹ with a SSA of 123.1 m².g⁻¹. This value compares well with that obtained with RSNs@TA at similar equilibrium concentration, considering that RSNs@TA displays a lower SSA (82 m².g⁻¹) and are in contact with phosphates in lower concentration (300 mg P.L⁻¹). Thus, the fast (~1 – 2 h) and quite high adsorption of phosphates to the surface of both nanoclusters at pH 7 – 7.5 represents a promising result for phosphate pollution control applications. In the context of PD, these results are promising not only for improving phosphate removal but also for reducing its duration.

Tannic acid desorption from RSNs@TA. In order to check a possible TA desorption from the surface of RSNs@TA over time and after phosphate adsorption, the supernatant of RSNs@TA suspensions has been analyzed at initial time in a fresh suspension at pH = 7, and after 24 h without and with 300 mg P.L⁻¹ phosphates. As references, two TA aqueous solutions at pH = 7 have been prepared with concentrations corresponding to the amount of TA at the surface of the RSNs@TA (0.2 mg(TA).mg (Fe₃O₄)⁻¹). One of the TA solutions contain phosphates at a concentration of 300 mg P.L⁻¹. At initial time and after 24 h mechanical

stirring, RSNs@TA are separated from the supernatant using a magnet. Then, free TA molecules in supernatant are analyzed using UV-visible spectroscopy and the resulting spectra are compared with that of TA solutions.

The UV-visible spectra of TA solutions without and with phosphates are shown in Fig. S9a, where two peaks are observed at 210 nm and 280 nm similarly as in our previous work [17]. These spectra are identical, suggesting that phosphates do not interact with the TA molecules. The UV-visible spectra at initial time and after 24 h of RSNs@TA supernatant without and with phosphates are shown in Figs. S9b,c. It can be observed that peak intensities increased after 24 h on each spectrum, indicating that a small fraction of the TA molecules in the coating has desorbed. This increase in intensity is higher in the presence of phosphates, suggesting that TA molecules are exchanged with phosphates during adsorption, as expected in the design of RSNs@TA. Then, by comparing the maximum absorbance of the most intense peak on the spectra of the TA solutions and the RSNs@TA supernatants, the amount of TA desorbed after 24 h without phosphate represents 2 ± 1 % of the total amount of TA in the coating (Figs. S9a,b). In the presence of phosphates, the quantity desorbed represents 4 ± 1 % of the total amount of TA in the coating at initial time, and 9 ± 2 % after adsorption (Figs. S9a,c). Therefore, these experiments confirm that a very low quantity of TA is desorbed from RSNs@TA over time without and with phosphates, and that a fraction of TA is exchanged with phosphates during their adsorption.

Recyclability of RSNs@TA. Our previous study demonstrated that phosphates could be desorbed from the surface of IONPs synthesized by a coprecipitation method under alkaline conditions (pH = 12), allowing the adsorbent to be reused in up to three adsorption-desorption cycles [5]. However, this process cannot be applied to RSNs@TA since the catechol groups of TA molecules anchored to the IO surface are known to undergo oxidation to a semi-quinone form, then quinone under mild alkaline conditions (pH ≥ ~8) [54–57]. As a quinone, the TA molecule no longer has an anchoring group and desorbs from the IO surface [55,58,59]. Thus, RSNs@TA cannot be recycled at alkaline pH without completely degrading the TA coating. However, RSNs@TA recyclability is not of utmost importance in the context of PD: such application requires RSNs@TA to have optimal adsorption capacity, which means they must be used fresh. This requirement was considered when designing our adsorbent material, with RSNs@TA being synthesized in high yield using a solvothermal polyol method in which the reagents used are inexpensive.

Potential uses of RSNs@TA for phosphate removal in high ionic strength aqueous media. In the context of phosphate removal during a PD process, and based on the results of batch mode experiments, phosphates adsorb in inner- and outer-sphere complexes on RSNs@TA. Compounds in dialysate (Ca²⁺, Na²⁺, Mg²⁺, Cl⁻, HCO₃⁻ and glucose, Table S1) have been found to disrupt the adsorption of outer-sphere complexes (sensitivity of the adsorption capacity of RSNs@TA to the dialysate composition for quantities of adsorbed phosphates higher than required for monolayer adsorption). In the literature, the sensitivity of phosphate adsorption to the ionic strength can also occur in wastewaters containing multiple ions (Cl⁻, SO₄²⁻, CO₃²⁻, etc.). To prevent hindrance of phosphate adsorption, highly effective adsorbents were developed. For example, Yang et al. [60–67] reported resilient hybrid adsorbents for which phosphate adsorption was not sensitive to the ionic strength of the medium and did not compete with the wastewater ions. Furthermore, Zelmanov et al. [68] reported the same observation for phosphate adsorption on iron oxide/hydroxide nanoparticles-based aggregates in water. We may notice that the ionic strength of dialysate (150 ± 10 mmol.L⁻¹) is very high compared to that of aqueous media used in the literature for mimicking wastewaters (~6 mmol.L⁻¹) [68]. In dialysate, phosphate adsorption on RSNs@TA in inner-sphere complexes is ensured due to the well-known high affinity phosphate-iron oxide surface, while adsorption in outer-sphere complexes is better preserved with the TA coating compared to uncoated RSNs. Therefore, RSNs@TA
could be suggested to be effective phosphate sorbents in other aqueous media at pH 7, with lower ionic strength than the dialysate used in this work.

## 4. Conclusion

In this work, we demonstrated that coating iron oxide nanoclusters (RSNs) with tannic acid (TA) enhanced their phosphate adsorption capacity in a dialysate used in a peritoneal dialysis (PD) process – a high ionic strength aqueous solutions at pH 7. Indeed, the TA-coated nano-clusters (RSNs@TA) exhibited a higher colloidal stability and a higher phosphate adsorption capacity at equilibrium compared to the uncoated ones (RSNs) in both ultrapure water (26.4 ± 8.1 mg P.g⁻¹) and dialysate (20.7 ± 6.4 mg P.g⁻¹). Both kinetics and isotherm adsorption experiments confirmed rapid and high-capacity phosphate uptake through the formation of inner- and outer-sphere complexes on the surface of RSNs@TA. Furthermore, we highlighted a protective effect of the TA-coating: the high electrolyte concentration in dialysate hindered phosphate adsorption in outer-sphere complexes for both RSNs and RSNs@TA but this effect was less pronounced on RSNs@TA. Chemical sorption of phosphates was confirmed though surface characterizations while colloidal stability of RSNs@TA was maintained in dialysate after phosphate adsorption experiments. It has been shown that a negligible amount of TA molecules desorbed from the surface of RSNs@TA over time. After phosphate adsorption on RSNs@TA, only a small amount of TA was exchanged with the phosphates for their adsorption. However, a more in-depth study will be conducted using an isothermal titration calorimetry (ITC) analysis method to better evaluate the possible influence of tannic acid coating on phosphate adsorption. All these results showed that RSNs@TA are biocompatible, colloidally stable and highly effective phosphate adsorbents for phosphate removal in dialysate. Therefore, they are very promising adsorbents for enhancing phosphate removal in a PD process, and next works will aim at testing them in dialysis conditions.

### CRediT authorship contribution statement

**Théo Lucante:** Writing – original draft, Methodology, Investigation, Formal analysis. **Philippe Choquet:** Validation, Methodology, Conceptualization. **Joana Vaz-Ramos:** Visualization, Methodology, Investigation. **Vincent Ball:** Validation, Methodology. **Dominique Bégin:** Validation, Methodology. **Cedric Leuvrey:** Validation, Investigation. **Vasiliki Papaefthymiou:** Validation, Investigation. **Spyridon Zafeiratos:** Visualization, Validation, Investigation. **Ariane Zaloszyc:** Validation, Methodology, Conceptualization. **Sylvie Bégin-Colin:** Writing – review & editing, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization.

### Declaration of competing interest

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

### Acknowledgments

This work has been conducted in the framework of the PHODIA project supported by ANR (ANR-21-CE09-0037), of the FIGHTVIRUS project supported by the region GRAND-EST and ANR (ANR-20-GES1-0013) and of the CAPTALL project funded by the MICA Carnot Institute.

The authors thank the technical and administrative departments and XRD, SEM and TEM platforms of IPCMS and ICPEES and the GDR NINO "Nanostructures Inorganiques par chimie en solution" for its scientific activities. The authors acknowledge the COST action COSY-STAR-CA21101 for its scientific structuration on CONFINED MOLECULAR

SYSTEMS: FROM A NEW GENERATION OF MATERIALS TO THE STARS and the Marie Curie project "Designing of multifunctional nano-materials for light-driven innovation technology" (DELIGHT) (Grant ref: 101131111) for collaboration and networking.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2025.164782.

### Data availability

The authors declare that the data supporting the findings of this study are provided in the article and its supplementary materials.

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