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# Intercalation mechanism of hexavalent chromium with Mg2Al-NO3 layered double hydroxide nanoparticles

Abdelhak Fezraoui, Damien Cornu, Marc Hébrant

► To cite this version:

Abdelhak Fezraoui, Damien Cornu, Marc Hébrant. Intercalation mechanism of hexavalent chromium with Mg2Al-NO3 layered double hydroxide nanoparticles. ISICXXI: 21st International Symposium on Intercalation Compounds, Jun 2023, Nancy, France. hal-05057604

HAL Id: hal-05057604

https://hal.science/hal-05057604v1

Submitted on 6 May 2025

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## Introduction

Layered double hydroxides (LDH) nanomaterials are well-known for their bipolar adsorption properties, attributed to their positively charged external surfaces and abundant interlayer anions [1]. Numerous research efforts have been made in recent years to gain insights into the adsorption mechanisms of hexavalent chromium (Cr(VI)). The proposed mechanisms include anionic exchange [2], adsorption-coupled reduction [3][4], and electrostatic attraction [5] onto LDH. This study aims to provide a comprehensive analytical understanding of the kinetics of these mechanisms within very short timeframes using the stopped-flow technique. Furthermore, the analysis will be linked to the adsorption parameters to differentiate between the potential mechanisms and establish a clear understanding of the interaction between LDH and Cr(VI).

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Possible adsorption mechanisms of Cr(VI) onto Mg₂Al-NO₃-LDH.

## Physico-chemical characteristics

<figure><img src="image_6.png" /><figcaption>X-ray diffraction (XRD)</figcaption></figure>

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A single band is observed at 1056 cm⁻¹ and 717 cm⁻¹, corresponding to the NO₃⁻ anion, confirming the presence of nitrate within the LDH matrix supporting the notion introduced by Di Bitetto, A [6] of a single conformation for the nitrate anions in the LDH structure.

<table><thead><tr><td>a(Å)</td><td>c(Å)</td><td>Mg/Al ratio</td></tr></thead><tbody><tr><td>3.04</td><td>8.87</td><td>2.01</td></tr></tbody></table>

Crystallographic parameters of Mg₂Al-NO₃-LDH Mg/Al ratio was measured by ICP-MS

<figure><img src="image_8.png" /><figcaption>Scanning electron microscopy (SEM)</figcaption></figure>

<table><thead><tr><td>pH</td><td>Size (nm)</td><td>Polydispersity index</td></tr></thead><tbody><tr><td>≈ 9 (Natural)</td><td>183.4</td><td>0.13</td></tr></tbody></table>

The average size of the nanoparticles obtained from SEM and DLS is close to 180 nm.

<figure><img src="image_9.png" /><figcaption>Zeta potential</figcaption></figure>

Zeta potential of Mg₂Al-NO₃-LDH nanoparticles. [LDH] = 0.25 g/L, T = 25°C.

The nanoparticles of Mg₂Al-NO₃-LDH exhibit a homogeneous distribution.

## Conclusion & Perspectives

* Improving and adjusting the synthesis process to diminish the Mg leaching during from LDH nanoparticles.

* Investigating potential phenomena using the stopped-flow technique to gain insight into the kinetic aspects of the interaction.

* Validating the complete anionic exchange of Cr(VI) by Mg₂Al-NO₃-LDH nanoparticles by comparing the experimental adsorption quantity and by Raman spectroscopy.

* Thorough characterization of the Mg₂Al-LDH nanoparticles that are fully loaded with Cr(VI) will be conducted to provide an affirmation of the proposed mechanisms.

## Experimental procedure

### Optimized LDH synthesis

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### Ultrafiltration system

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### Kinetics studies

### Stopped-flow

$$OD(t) = A \cdot \exp(-k_{\text{obs}} \cdot t^{0.5}) + C$$

Abs (t) : optical density function which is a time-dependent absorbance.

k<sub>obs</sub> : observed rate constant of the reaction.

Kinetics model of ions uptake by colloidal particles of synthetic resins in non-stationary conditions [7].

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The disappearance of the band at 1056 cm⁻¹ confirms the complete removal of the NO₃⁻ anions from the LDH matrix. Additionally, the appearance of new peaks at 216, 360, and 904 cm⁻¹ under both pH conditions indicates the presence of Cr(VI) within the LDH matrix. These observations confirm the successful intercalation of Cr(VI).

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### Observations :

1 step - Electrostatic Attraction : jump of the absorbance in the very first seconds which may be attributed to the high electrostatic attraction between the positively charged LDH nanoparticles (ζ ≈ +50 mV) and Cr(VI) anions.

2 step - Slower process presumably consisting in diffusion of species in the aqueous layer surrounding the nanoparticle or an intraparticle diffusion toward a shrinking core corresponding to the ionic exchange.

<figure><img src="image_14.png" /></figure>

Raman spectra of LDH loaded with Cr(VI), [Cr(VI)]₀ = 500 mg/L, [LDH]₀ = 3,33 g/L, t = 24 hours, v = 500 rpm,

### Ultrafiltration tests

Langmuir model:

$$\frac{C_e}{q_e} = \frac{1}{K_L \cdot q_m} + \frac{C_e}{q_m}$$

* q<sub>e</sub> (mg/g): the adsorption capacity of Mg₂Al-NO₃-LDH in adsorption equilibrium.

* q<sub>m</sub> (mg/g): the maximum adsorption capacity.

* k<sub>L</sub> (L/mg): the adsorption constant of Langmuir model.

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### Langmuir Isotherm model

<table><thead><tr><td></td><td>q<sub>m</sub>(mg/g)</td><td>K<sub>L</sub>(L/mg)</td><td>R<sup>2</sup></td></tr></thead><tbody><tr><td>pH = 4</td><td>115</td><td>0.371</td><td>0.989</td></tr><tr><td>pH = 9</td><td>127</td><td>0.341</td><td>0.977</td></tr></tbody></table>

The adsorption isotherm parameters of the adsorption of Cr(VI) by Mg₂Al-NO₃-LDH.

The theoretical adsorption capacity of Cr(VI) by Mg₂Al-NO₃-LDH, calculated based on the stoichiometric chemical formula (Mg₂Al(OH)₆·NO₃·H₂O), is 101 mg/g.

<footnote>[1] Tran, H. N et al., (2019). *Journal of hazardous materials*, 373, 258-270.</footnote>

<footnote>[2] N.K. Lazaridis et al., (2004). *Ind. Eng. Chem. Res.*, 43(9), 2209-2215.</footnote>

<footnote>[3] D. Park et al., (2005). *Chemosphere*, 60(10), 1356-1364.</footnote>

<footnote>[4] H.-P. Chao et al., (2018). *Environ. pollut.*, 243 620-629.</footnote>

<footnote>[5] M. Khitous et al., (2016). *Chin. J. Chem. Eng.*, 24 433-445.</footnote>

<footnote>[6] Di Bitetto, A. (2017). (Doctoral dissertation, Université de Lorraine).</footnote>

<footnote>[7] Hébrant, M. (2007). *Journal of colloid and interface science*, 311(2), 368-374.</footnote>

<footnote>[8] Szabó, M et al., (2018). *Inorganica Chimica Acta*, 472, 295-301.</footnote>