# Diclofenac Removal by Alkylammonium Clay Minerals Prepared over Microwave Heating

Denise França, Alice Silva, Josy Osajima, Edson Silva-Filho, Santiago

Medina-Carrasco, Maria del Mar Orta, Maguy Jaber, Maria Fonseca

► To cite this version:

Denise França, Alice Silva, Josy Osajima, Edson Silva-Filho, Santiago Medina-Carrasco, et al. Diclofenac Removal by Alkylammonium Clay Minerals Prepared over Microwave Heating. ACS Omega, 2024, 9 (49), pp.48256-48272. 10.1021/acsomega.4c05763. hal-04852942

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Submitted on 21 Dec 2024

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Diclofenac removal by alkylammonium clay minerals prepared over microwave heating

Denise B. França<sup>a,1</sup>, Alice P. N. Silva<sup>a</sup>, Josy A. Osajima<sup>b</sup>, Edson C. Silva-Filho<sup>b</sup>, Santiago Medina-Carrasco<sup>c</sup>, Maria del Mar Orta<sup>d</sup>, Maguy Jaber<sup>e</sup>, Maria G. Fonseca<sup>a*</sup>

<sup>a</sup>Universidade Federal da Paraíba, Núcleo de Pesquisa e Extensão - Laboratório de Combustíveis e Materiais (NPE - LACOM), Cidade Universitária s/n – Campus I, 58051-900, João Pessoa-PB, Brazil.

<sup>b</sup>Universidade Federal do Piauí, Laboratório Interdisciplinar de Materiais Avançados (LIMAV), Avenida Universitária s/n, 64049-550, Teresina-PI, Brazil.

<sup>c</sup>Universidad de Sevilla, SGI Laboratorio de Rayos X - Centro de Investigación, Tecnología e Innovación de la Universidad de Sevilla (CITIUS), Avenida Reina Mercedes, 4B, 41012, Sevilla, Spain.

<sup>d</sup>Universidad de Sevilla, Departamento de Química Analítica da Facultad de Farmacia, Calle Profesor García González 2, 41012, Sevilla, Spain

<sup>e</sup>Sorbonne Université, CNRS UMR 8220, Laboratoire d'Archéologie Moléculaire et Structurale (LAMS), Case courrier 225, 4 pl. Jussieu, 75252, Paris Cedex 05, France.

*mgardennia@quimica.ufpb.br

<footnote n="1">¹ Universidade Federal do Piauí, Laboratório Interdisciplinar de Materiais Avançados (LIMAV), Avenida Universitária s/n, 64049-550, Teresina-PI, Brazil.</footnote>
Diclofenac is an emerging contaminant widely detected in water and has caused adverse effects on the biota. In this study, the adsorbents were prepared by reacting tetradecyl-(C₁₄), hexadecyl-(C₁₆), and octadecyltrimethylammonium (C₁₈) bromides with sodium vermiculite (Ver) and used for the removal of the first time for diclofenac sodium from aqueous solution. Synthesis was carried out in a microwave-assisted reactor operating at 50 °C for 5 min, using proportions of organic salts in 100% and 200% of the phyllosilicate cation exchange capacity. The stability of loaded alkylammonium solids was evaluated under drug adsorption conditions. Adsorption was mainly influenced by the amount of surfactant incorporated into the clay mineral, according to the thermogravimetric and CHN elemental analysis data. Samples prepared with 200% CEC presented lower stability at pH 6.0 and 8.0. Drug adsorption was more effective for C₁₄-Ver-200%, C₁₆-Ver-200% and C₁₈-Ver-200% samples, with a maximum retention of 97.8, 110.1, and 108.0 mg g⁻¹, respectively. The adsorptive capacities of C₁₄-Ver-200%, C₁₆-Ver-200%, C₁₈-Ver-200%, C₁₄-Ver-100%, C₁₆-Ver-100%, and C₁₈-Ver-100% were reduced to 29.0%, 36.8%, 41.0%, 61.0%, 50.4%, and 58.0% compared with their initial value after three adsorption cycles. X-ray diffractometry (XRD) patterns revealed that diclofenac was adsorbed into the interlayer region of organovermiculites. Fourier transform infrared spectroscopy (FTIR), Zeta potential results, and the pH study on adsorption indicated that Van der Waals interactions are dominant in the adsorption mechanism.

**KEYWORDS:** environmental remediation, anti-inflammatory drug, diclofenac sodium, organoclay, adsorption
INTRODUCTION

Nonsteroidal anti-inflammatory drugs (NSAIDs) are among the groups of pharmaceuticals that threaten the ecosystem and human health due to their presence in water.<sup>1,2</sup> Sodium diclofenac (sodium 2-[2-(2-dichloroanilino) phenyl]acetate) is a NSAIDs highly consumed by hundreds of tons annually around the world for both human and veterinary medical care.<sup>3,4</sup> The drug is among the most frequently detected in aquatic environments and has been involved in the European Union's top 10 priority list for detection.<sup>1,3,4</sup> The average concentrations of the drug in aquatic environments were higher than 0.1 µg L<sup>-1</sup> in surface waters in Europe,<sup>5</sup> while the concentration in Brazilian waters was 759.06 µg L<sup>-1</sup>.<sup>6</sup>

Diclofenac can lead to adverse effects on aquatic organisms,<sup>7,8</sup> and the byproducts formed through biotic and abiotic transformations can pose even greater toxicity than the original molecule.<sup>9,10</sup> Therefore, it is imperative to remove diclofenac from aquatic ecosystems. Adsorption is an interesting water treatment method due to its simplicity, cost-effectiveness, and high removal efficiency of pollutant, in addition to absence of by-product generation.<sup>1</sup>

Clay minerals are versatile, low-cost, and highly available materials that can be used as adsorbents for drugs, among which montmorillonite (Mt) has been widely used for this proposal.<sup>11,12</sup> More recently described in the literature, vermiculite is a clay mineral that also acts as an adsorbent for drugs.<sup>13-19</sup> Vermiculite is a 2:1 phyllosilicate that exhibits an idealized negative layer charge per formula unit (~0.6-0.9) and exchange cations in the interlayer region, normally Mg<sup>2+</sup>.<sup>20</sup> The clay mineral is characterized by having tetrahedral silicate sheets that can be substituted by aluminum or other elements of lower valency, while the octahedral sites are generally occupied by
Al³⁺, Mg²⁺, and Fe³⁺.²⁰ Furthermore, vermiculite is a two-dimensional (2D) material²¹ which can be modified through the intercalation of organic compounds,¹³,¹⁴ acid activation,²² silylation,²³ among others, to obtain materials with desired properties.

The adsorption of anionic drugs is significantly restricted in untreated clay mineral.¹⁵,¹⁶ However, drug adsorption performance of clay minerals can be further significantly improved by reacting with surfactants.¹¹,¹³⁻¹⁵ Organovermiculites prepared with 1,3–2(hexadecamide propyl dimethylammonium chloride) *n*-butane, 1,3–2(hexadecamide propyl dimethylammonium chloride)-2-hydroxypropane dichloride and 1,3–2(hexadecamide propyl dimethylammonium chloride)-*p*-xylene exhibited ibuprofen adsorption capacity of 322.6, 404.7, and 489.9 mg g⁻¹, respectively, while drug adsorption by sodium vermiculite was negligible.¹⁴

Organoclays based on bentonite²⁴⁻²⁷ or montmorillonite,²⁸⁻³¹ kaolinite,²⁷,³² halloysite,³³ illite³¹, and sepiolite³⁴ were investigated as adsorbents for diclofenac sodium. In addition to clay minerals, other materials such as hydroxyapatite@chitosan hybrids,³⁵ carbons,³⁶,³⁷ zeolites,³⁸ carbon sphere@polyaniline@layered double hydroxides composites,³⁹ and metal–organic frameworks⁴⁰,⁴¹ were also studied.

For organoclays performance, the effects of experimental parameters such as pH adsorbent dosage, time, temperature, drug concentration, and ionic strength on the diclofenac adsorption were evaluated.²⁴,²⁶,²⁸,⁴² Although the drug has been detected in the environment at concentrations in the µg L⁻¹ range, studies have been carried out at higher concentrations (10-2000 mg L⁻¹) to understand the mechanisms of adsorption and factors that alter the performance of adsorbents.²⁷,²⁸,³² The effect of the type and amount of surfactant loading in the clay mineral matrix on the diclofenac adsorption performance was also evaluated.²⁴,²⁸⁻³¹ In summary, the increase in surfactant loading in the clay mineral improved the diclofenac adsorption.²⁴,²⁵,²⁹⁻³¹,⁴³
Despite extensive research in organoclays, the stability of the matrixes under drug adsorption conditions has been neglected and very few studies investigated the regeneration of these adsorbents.²⁵,³³ Since surfactant leaching is one of the factors that control organoclay-induced ecotoxicity,⁴⁴ its stability must be known. The influence of pH on the stability of organovermiculites prepared with hexadecyltrimethylammonium and hexadecylpyridinium at 100% CEC for the adsorption of naphthalene was reported⁴⁵,⁴⁶ Results showed that the both samples were stable in the pH range of 4-10.⁴⁵ However, the effect of the chain size and the amount of loaded surfactant on the stability of the organophilic clay mineral was not evaluated.

In the current investigation, a Brazilian vermiculite sample was modified with alkyl trimethylammonium salts with different chain lengths ($C_{14}$, $C_{16}$, and $C_{18}$) by microwave heating (MW) and used for the first time as adsorbents for sodium diclofenac, a predominantly anionic drug at pH 6.0 (pKa = 4.15). Reactions over MW heating have been shown to be an effective route for the modification of clay minerals.⁴⁷,⁴⁸ Brazil has abundant reserves of vermiculite⁴⁹, as well as a demand for sodium diclofenac, which has been detected in Brazilian surface waters,⁶,⁵⁰,⁵¹ due to the inefficiency of conventional water treatment methods used.⁶ No studies regarding the use of organophilic vermiculites for the adsorption of diclofenac have been verified until this point. The evaluation of diclofenac adsorption by the resulting materials was carried out under varying experimental conditions that included pH levels, adsorbent dosage, contact time, and diclofenac concentrations. The stability of organophilic vermiculites under adsorption conditions was also verified. Therefore, the impacts of the composition and size of the alkyl chain of surfactants on the stability of organovermiculites and diclofenac adsorption performance of diclofenac were evaluated, and the potential for reusing organoclay was also explored.
# RESULTS AND DISCUSSION

## Characterizations

### X-ray diffractometry

The X-ray diffractometry (XRD) patterns of Ca,Mg-Ver, Na-Ver and organovermiculites are presented in Figure 1. The results suggested that the Ver sample (Figure 1i-a) is composed of vermiculite (ICDD 00-034-0166), with impurities of hydrobiotite (ICDD 00-049-1057) and quartz (ICDD 00-046-1045). Hydrobiotite (Hb) is a regular interstratified biotite/vermiculite phase in a 1:1 proportion as a result of the weathering of micas,²⁰ and its presence is frequently reported in vermiculite samples.⁵²,⁵³ The principal reflection of vermiculite occurred at 2θ = 6.13° (002 plane) and resulted in a basal spacing of 1.46 nm.⁵⁴,⁵⁵ The reflection at 2θ = 60.13° (d = 0.154 nm, 060 plane) was assigned to the trioctahedral clay mineral⁵⁶. For the Hb phase, reflections occurred at 2θ = 3.45° (d~2.56 nm, 001 plane) and 7.00° (d = 1.26 nm, 002 plane).⁵⁷

After the Na⁺ exchange reaction, basal space changed to 1.21 nm (Figure 1i-b), as a result of the substitution of the interlayer cations in the raw sample (normally Mg²⁺) and the reduction of the water molecules in a monolayer arrangement in the interlayer region.⁵⁸ In the Hb phase, the changes in d₀₀₁ and d₀₀₂, measuring 2.38 nm and 1.14 nm, respectively, align with the saturation of the samples with sodium following the exchange process.⁵⁹
<figure><img src="image_2.png" /><figcaption>**Figure 1.** XRD patterns of (i): (a) Ca, Mg-Ver, and (b) Na-Ver (Ver = vermiculite, Hb = hydrobiotite, Q = quartz, \*unidentified phase); (ii): (a) Na-Ver, (b) C₁₄-Ver-100%, and (b) C₁₄-Ver-200%; (iii): (a) Na-Ver, (b) C₁₆-Ver-100%, and (b) C₁₆-Ver-200%; and (iv): (a) Na-Ver, (b) C₁₈-Ver-100%, and (b) C₁₈-Ver-200%.</figcaption></figure>

In organophilic samples, two reflections were observed at 2θ < 4.0° (Figures 1ii-iv) corresponding to basal distances ranging between 2.70-3.40 nm, and exceeding 4.00
nm which could be due to intercalation of alkylammonium cations in vermiculite and
hydrobiotite, respectively.⁶⁰,⁶¹ The basal distances increased with the chain size of
surfactants C₁₄ (2.27 nm), C₁₆ (2.53 nm) and C₁₈ (2.79 nm).⁶² Considering that the 2:1
layer thickness is about 0.96 nm²⁰ and based on the basal spacings and the surfactant
size, the intercalation of organic cations in paraffin-like monolayer arrangements is
proposed for all organophilic samples, for both Ver and Hb phases.⁶⁰ An illustration of
this conformation is presented in Figure S1. For the Ver phase, second-order reflections
were also observed at 1.33-1.36 nm, 1.39-1.45 nm and 1.45-1.64 nm for C₁₄-Ver, C₁₆-
Ver, and C₁₈-Ver, respectively.⁶³

Thermogravimetry (TG/DTG)

TG/DTG was used for the quantification of organic content in the organophilic vermiculites. Results are shown in Figure 2 and summarized in Table 1. For the Na-Ver sample, the curve exhibited two thermal decomposition events, resulting in 9.4% total mass losses in the 30-800 °C range, while the organophilic samples presented 23.6-38.6% total mass losses in the same temperature range. The initial mass loss event for Na-Ver (30-125 °C) corresponded to the loss of the physically adsorbed on the clay mineral surface. The subsequent mass loss (366-800 °C) is associated with the condensation of silanol groups of the Ver and Hb phases.⁶⁴,⁶⁵
<figure><img src="image_3.png" /><figcaption>**Figure 2.** (i) TG and (ii) DTG curves for (a) Na-Ver, (b) C₁₄-Ver-100%, (c) C₁₆-Ver-100%, (d) C₁₈-Ver-100%, (e) C₁₄-Ver-200%, (f) C₁₆-Ver-200% and (g) C₁₈-Ver-200%.</figcaption></figure>

For organophilic samples, the decrease in mass loss during the initial event suggests an enhancement in hydrophobicity.⁶³,⁶⁶ The mass loss events in the range of about 120~537 °C were assigned to the decomposition of organic cations incorporated in the clay mineral, and were used to estimate the percentage of organic content in the organophilic samples (see Table 1). Higher percentages of organic content (24.8-32.8%) were observed for organovermiculites prepared with surfactant amounts of 200% CEC. The final thermal decomposition event was related to structural hydroxyl condensation, and organophilic samples did not show significant differences compared to Na-Ver.⁶³,⁶⁷
<table><caption>Table 1. Summary of mass losses and temperature intervals based on DTG curves for Na⁺-Ver and organo-vermiculites</caption><thead><tr><th>Sample</th><th>Event</th><th>T (°C)</th><th>Mass loss (%)</th><th>Total mass loss (%)</th><th>Total organic content<sup>a</sup> (%)</th></tr></thead><tbody><tr><td rowspan="2">Na-Ver</td><td>I</td><td>30-125</td><td>7.2</td><td rowspan="2">9.4</td><td rowspan="2">-</td></tr><tr><td>II</td><td>366-800</td><td>2.2</td></tr><tr><td rowspan="6">C<sub>14</sub>-Ver-100%</td><td>I</td><td>30-126</td><td>5.0</td><td rowspan="6">23.6</td><td rowspan="6">16.2</td></tr><tr><td>II</td><td>126-232</td><td>3.9</td></tr><tr><td>III</td><td>235-335</td><td>7.4</td></tr><tr><td>IV</td><td>335-433</td><td>3.7</td></tr><tr><td>V</td><td>433-522</td><td>1.3</td></tr><tr><td>VI</td><td>522-800</td><td>2.3</td></tr><tr><td rowspan="7">C<sub>14</sub>-Ver-200%</td><td>I</td><td>30-120</td><td>5.5</td><td rowspan="7">32.5</td><td rowspan="7">24.8</td></tr><tr><td>II</td><td>120-221</td><td>6.9</td></tr><tr><td>III</td><td>221-262</td><td>5.6</td></tr><tr><td>IV</td><td>262-337</td><td>6.5</td></tr><tr><td>V</td><td>337-436</td><td>4.0</td></tr><tr><td>VI</td><td>436-532</td><td>1.8</td></tr><tr><td>VII</td><td>532-800</td><td>2.1</td></tr><tr><td rowspan="6">C<sub>16</sub>-Ver-100%</td><td>I</td><td>30-130</td><td>5.6</td><td rowspan="6">24.5</td><td rowspan="6">16.8</td></tr><tr><td>II</td><td>126-223</td><td>3.2</td></tr><tr><td>III</td><td>223-340</td><td>8.9</td></tr><tr><td>IV</td><td>340-435</td><td>3.5</td></tr><tr><td>V</td><td>435-504</td><td>1.1</td></tr><tr><td>V</td><td>570-800</td><td>2.1</td></tr></tbody></table>
<table><tbody><tr><td rowspan="7">C<sub>16</sub>-Ver-200%</td><td>I</td><td>30-128</td><td>5.2</td><td rowspan="7">36.0</td><td rowspan="7">28.8</td></tr><tr><td>II</td><td>128-210</td><td>4.6</td></tr><tr><td>III</td><td>210-267</td><td>9.8</td></tr><tr><td>IV</td><td>267-343</td><td>8.2</td></tr><tr><td>V</td><td>343-440</td><td>4.1</td></tr><tr><td>VI</td><td>440-519</td><td>2.1</td></tr><tr><td>VII</td><td>573-800</td><td>2.0</td></tr><tr><td rowspan="6">C<sub>18</sub>-Ver-100%</td><td>I</td><td>30-126</td><td>4.4</td><td rowspan="6">24.5</td><td rowspan="6">17.0</td></tr><tr><td>II</td><td>126-229</td><td>4.5</td></tr><tr><td>III</td><td>229-363</td><td>10.4</td></tr><tr><td>IV</td><td>363-459</td><td>1.4</td></tr><tr><td>V</td><td>459-517</td><td>0.7</td></tr><tr><td>VI</td><td>517-779</td><td>3.0</td></tr><tr><td rowspan="6">C<sub>18</sub>-Ver-200%</td><td>I</td><td>30-138</td><td>4.0</td><td rowspan="6">38.6</td><td rowspan="6">32.8</td></tr><tr><td>II</td><td>138-275</td><td>19.4</td></tr><tr><td>III</td><td>275-348</td><td>7.6</td></tr><tr><td>IV</td><td>348-442</td><td>4.1</td></tr><tr><td>V</td><td>442-537</td><td>1.6</td></tr><tr><td>VI</td><td>507-800</td><td>1.9</td></tr></tbody></table>

## CHN elemental analysis

The quantification of surfactants incorporated in organophilic samples was also performed using CHN elemental analysis (Table 2). The total percentage of the values

<footnote>ªValues were obtained considering the sum of mass losses in the events, excluding dehydration and dehydroxylation.</footnote>
of organic content were close to those obtained by the TG / DTG analysis. The amounts of surfactants in the C₁₄-Ver-100%, C₁₆-Ver-100% and C₁₈-Ver-100% samples were close to the initial values used in their preparation (0.67 mmol/g), and high organic incorporations were obtained for the surfactant proportions at 200% CEC.

<table><caption>Table 2. Results of the CHN elemental analysis of organophilic vermiculites</caption><thead><tr><th rowspan="2">Sample</th><th colspan="2">C</th><th colspan="2">H</th><th colspan="2">N</th><th colspan="2">α<sup>a</sup></th><th colspan="2">Q<sup>b</sup></th></tr><tr><th>(%)</th><th>(mmol/g)</th><th>(%)</th><th>(%)</th><th>(%)</th><th>(mmol/g)</th><th>(%)</th><th>(mmol/g)</th><th>(%)</th><th>(mmol/g)</th></tr></thead><tbody><tr><td>C<sub>14</sub>-Ver-100%</td><td>11.6</td><td>9.7</td><td>3.2</td><td>0.9</td><td>0.6</td><td></td><td>15.7</td><td></td><td>0.6</td><td></td></tr><tr><td>C<sub>16</sub>-Ver-100%</td><td>13.0</td><td>10.8</td><td>3.3</td><td>0.9</td><td>0.7</td><td></td><td>17.2</td><td></td><td>0.7</td><td></td></tr><tr><td>C<sub>18</sub>-Ver-100%</td><td>14.1</td><td>11.8</td><td>3.6</td><td>0.8</td><td>0.6</td><td></td><td>18.5</td><td></td><td>0.6</td><td></td></tr><tr><td>C<sub>14</sub>-Ver-200%</td><td>18.2</td><td>15.2</td><td>4.3</td><td>1.4</td><td>1.0</td><td></td><td>23.9</td><td></td><td>1.0</td><td></td></tr><tr><td>C<sub>16</sub>-Ver-200%</td><td>21.3</td><td>17.8</td><td>4.8</td><td>1.4</td><td>1.0</td><td></td><td>27.5</td><td></td><td>1.0</td><td></td></tr><tr><td>C<sub>18</sub>-Ver-200%</td><td>23.8</td><td>19.8</td><td>5.2</td><td>1.4</td><td>1.0</td><td></td><td>30.3</td><td></td><td>1.0</td><td></td></tr></tbody></table>

## FTIR spectroscopy

Infrared spectroscopy is widely used to obtain qualitative information about the organophilization of clay minerals with surfactants, as well as the conformation of ammonium cations in the interlayer region.⁶⁸ The FT-IR spectra of Na-Ver and organophilic vermiculites are shown in Figure 3. The spectrum of the Na-Ver sample (Figure 3a) shows a shoulder at 3669 cm⁻¹, attributed to the OH stretching of the structural groups of clay minerals, and a broad band at 3393 cm⁻¹, assigned to the OH

<footnote><sup>a</sup>Total organic content determined from CHN elemental analysis; <sup>b</sup>Amount of surfactant in the samples</footnote>
stretching vibrations of water molecules.⁵⁴ The band at 1645 cm⁻¹, associated with the deformation vibrations of water molecules.⁵⁷ Bands at 973 cm⁻¹ and 815 cm⁻¹ were assigned to Si-O stretchings, while bands at 731 cm⁻¹ and 683 cm⁻¹ are linked to the in-plane deformation vibration of Al-O-Si bonds.⁵⁴

<figure><img src="image_4.png" /><figcaption>**Figure 3.** FTIR spectra of (a) Na-Ver, (b) C₁₄-Ver-100%, (c) C₁₄-Ver-200%, (d) C₁₆-Ver-100%, (e) C₁₆-Ver-200%, (f) C₁₈-Ver-100%, and (g) C₁₈-Ver-200%.</figcaption></figure>

The presence of new bands in the infrared spectra assigned to the surfactants was observed in all organophilic vermiculites (Figure 3b-c). Bands at 2917 cm⁻¹ and 2850~2851 cm⁻¹ were attributed to antisymmetric and symmetric stretchings in the CH₂ groups, respectively.¹⁴ These bands are very close to the free surfactant frequency, observed at 2916 and 2849 cm⁻¹, indicating that the organic chains adopt an ordered
conformation (all-trans conformation) in organovermiculites.⁶⁸ The band at 1488 cm⁻¹ was related to CH₃ deformation, while the bands in 1471~1469 cm⁻¹ and 719 cm⁻¹ were assigned to CH₂ deformation.⁶⁸

## Electron microscopy

Morphology of the Na-Ver and organophilic vermiculites were followed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. SEM images of the Na-Ver and organophilic vermiculites are shown in Figure 4. Na-Ver exhibited a plate-like morphology characteristic of the clay mineral.⁶⁹ which was maintained after modification with surfactants.
<figure><img src="image_5.png" /><figcaption>**Figure 4.** SEM images of (a) Na-Ver, (b) C₁₄-Ver-100%, (c) C₁₄-Ver-200%, (d) C₁₆-Ver-100%, (e) C₁₆-Ver-200%, (f) C₁₈-Ver-100%, and (g) C₁₈-Ver-200%.</figcaption></figure>

TEM images are present in Figures 5 and S2. The interplanar distances for Na-Ver were measured at 1.00 and 1.11 nm, lower than the XRD basal space, potentially attributed to sample dehydration under vacuum.⁷⁰ In organophilic samples, the basal
spaces were larger than those observed for Na-Ver, indicating the intercalation of
surfactants (Table S1). The values were close to those obtained by XRD; nevertheless,
the d values ≥ 4.00 nm of Hb phase were only evident in the TEM images for the C₁₆-
Ver-100% and C₁₈-Ver-100% samples. In certain regions, organophilic samples
displayed basal distances ranging from 1.02 to 1.17 nm, closely resembling those of the
Na-Ver and hydrobiotite phase. This suggests that not all interlayers - clay mineral are
intercalated by organic cations.⁷¹

<figure><img src="image_6.png" /><figcaption>**Figure 5.** TEM images of (a) Na-Ver, (b) C₁₄-Ver-100%, (c) C₁₄-Ver-200%, (d) C₁₆-Ver-100%, (e) C₁₆-Ver-200%, (f) C₁₈-Ver-100%, and (g) C₁₈-Ver-200%.</figcaption></figure>
## Textural properties

The N₂ adsorption-desorption isotherms and textural parameters (specific surface area, pore volume, and pore diameter) of Na-Ver and organovermiculites (C₁₄ and C₁₆) prepared at 100%CEC are shown in Figure S3(a-c) and Table 3. For C₁₈-Ver-100%, Kr adsorption was performed Figure S3(d) and the N₂ isotherm was not obtained possibly due to the nature of the sample. The samples presented a type H3 loop in the IUPAC classification with no plateau at high P/Po and this type indicates that the adsorption branch resembles a Type II isotherm and that the lower limit of the desorption branch is normally located at the cavitation-induced P/Po.⁷² H3 loops are given by nonrigid aggregates of plate-like particles like clay minerals.⁷² The specific surface area of Na-Ver is within the range reported for Santa Luzia vermiculite samples reported in the literature (16 to 34 m² g⁻¹),⁵⁷,⁶⁹ whose values depend on the size of the particle.⁷³ The presence of surfactant in the samples decreased the specific surface area and the volume of the pore of the clay mineral, while the diameter of the pores increased. This behavior has also been reported for other organophilic vermiculites prepared with C₁₄,¹⁶ C₁₆¹⁶,⁷⁴ and other surfactants,⁶⁶,⁷⁵,⁷⁶ and occurs due to blockage of the structural pores of the clay mineral by incorporation of organic cations.¹⁶,⁶⁶

<table><caption>**Table 3. Textural parameters of Na-Ver and organovermiculites prepared with 100% CEC**</caption><thead><tr><th>Sample</th><th>S<sub>BET</sub><sup>a</sup> (m<sup>2</sup> g<sup>-1</sup>)</th><th>Pore volume (cm<sup>3</sup> g<sup>-1</sup>)</th><th>Pore diameter (nm)</th></tr></thead><tbody><tr><td>Na-Ver</td><td>29</td><td>0.064</td><td>13</td></tr><tr><td>C<sub>14</sub>-Ver-100%</td><td>4</td><td>0.031</td><td>35</td></tr><tr><td>C<sub>16</sub>-Ver-100%</td><td>3</td><td>0.027</td><td>34</td></tr><tr><td>C<sub>18</sub>-Ver-100%</td><td>3</td><td>-</td><td>-</td></tr></tbody></table>
Adsorption

pH effect

The test results illustrating the influence of pH on sodium diclofenac adsorption by organophilic vermiculite are shown in Figure 6a. No adsorption experiments were carried out at pH < 6 to avoid precipitation of diclofenac due to the presence of the neutral form under this condition, as shown in the speciation diagram as a function of pH (Figure 6b), which is even less soluble than salt.³⁷ The results demonstrate that adsorption was slightly higher at pH 8 for C₁₄-Ver-100%, C₁₄-Ver-200%, C₁₆-Ver-100%, and C₁₈-Ver-100%, producing values of 6.9 mg g⁻¹, 9.0 mg g⁻¹, 3.9 mg g⁻¹, and 2.9 mg g⁻¹, respectively. In the case of C₁₆-Ver-200% and C₁₈-Ver-200%, adsorption was independent of the pH range (6 to 10). Na-Ver did not exhibit diclofenac adsorption within the pH range, therefore, alkyltrimethylammonium salts provided sites for diclofenac adsorption on organophilic vermiculites.
<figure><img src="image_7.png" /><figcaption>**Figure 6.** (a) Influence of pH on drug adsorption by organophilic vermiculites (conditions: 24 h, 25 °C, 25 mg mass adsorbent and $C_i$ = 10 mg L⁻¹), (b) diclofenac speciation as a function of pH and (c) zeta potential (ζ) measurements of Na-Ver and organophilic vermiculites.</figcaption></figure>

The results of the Zeta potential measurements of Na-Ver and organophilic vermiculites are shown in Figure 6c. Na-Ver exhibits negative charge throughout the entire pH range due to isomorphic substitutions in the lattice, which generates a permanent negative charge on the surface,⁷⁷⁻⁷⁹ while the decrease in charge with increasing pH results from dissociation of the hydroxyl groups of the edge surfaces.⁷⁷ On the other hand, diclofenac (pKa 4.15) is predominately anionic at pH 6.0 (98.61%),
8.0 (99.98 %) and 10.0 (99.99%), Figure 6b, which complicates adsorption on clay mineral due to repulsion between charges.

Organophilic vermiculites showed Zeta potential values higher than those of Na-Ver at the pH of adsorption, which reduces repulsion by anionic species. The increase in surface charge occurs due to the adsorption of cationic surfactants on the negatively charged surface of the clay mineral.⁸⁰,⁸¹ The results show that there was no relationship between the amount of diclofenac adsorbed and the surface charge of organophilic vermiculites (Figure S4), which suggests that Van der Waals interactions between diclofenac and the alkyl chain of the surfactant played a role in the adsorption mechanism.²⁴

## Adsorbent dosage

The influence of the dosage of the adsorbent on the removal of diclofenac by organophilic vermiculites (Figure 7) was studied under the optimal pH conditions obtained for each adsorbent. The findings revealed that the highest percentage of drug removal (96-99%) was observed for C₁₄-Ver-200%, C₁₆-Ver-200%, and C₁₈-Ver-200% samples, achieved with 25 mg of each adsorbent. However, for C₁₄-Ver-100%, C₁₆-Ver-100%, and C₁₈-Ver-100%, higher doses of 125 mg, 50 mg, and 75 mg were required, resulting in removal percentages of 89%, 85% and 88%, respectively. This indicates that larger amounts of adsorbents were necessary for samples with lower surfactant contents, directly influencing the availability of adsorption sites.⁸²,⁸³
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Effect of the dosage of the adsorbent on the adsorption of the drug by organovermiculites (a) C₁₄-Ver, (b) C₁₆-Ver and (c) C₁₈-Ver (24 h, pH 6.0 or 8.0, 25 °C and Cᵢ = 10 mg L⁻¹).</figcaption></figure>

Adsorption kinetics

The results obtained in the kinetic study (Figure 8) demonstrated that the drug was rapidly adsorbed on organophilic vermiculites at equilibrium times of only 5 min for all hybrids. The result obtained was very close to that observed for diclofenac adsorption by modified C₁₆Br kaolinites, ~ 6 min,⁴³ and was shorter than those obtained with other organophilic clay minerals, such as modified C₁₆Br Mt (60 min),²⁵
commercial organoclay Spectrogel Type C (500 min)⁸⁴ and alkypyridinium bentonites
(10 and 60 min).²⁴

<figure><img src="image_9.png" /><figcaption>**Figure 8.** Kinetic adsorption isotherms for adsorption of diclofenac sodium by the organo-vermiculites (a) C₁₄-Ver, (b) C₁₆-Ver and (c) C₁₈-Ver (25 °C, pH 6.0 or 8.0 and Cᵢ = 10 mg L⁻¹).</figcaption></figure>

The fitting of experimental data to the adsorption kinetic models was not
performed because of rapid adsorption. The use of data at or very close to, equilibrium,
is likely to lead to erroneous conclusions regarding adsorption kinetics.⁸⁵,⁸⁶
Adsorption isotherms

The adsorption isotherms are present in Figure 9 and the data were evaluated for
adjustment to the Langmuir, Freundlich, and Temkin models, and the parameters
obtained are shown in Table S2. Taking into account R² and SD, the experimental data
were fitted to the Langmuir model for all investigated solids.

The adsorption isotherms illustrated a greater adsorption of diclofenac by
organophilic vermiculites with an increasing initial concentration of the pollutant. The
maximum adsorption capacities observed for the hybrids C₁₄-Ver-200%, C₁₆-Ver-200%,
and C₁₈-Ver-200% were 97.75 mg g⁻¹, 110.06 mg g⁻¹ and 107.97 mg g⁻¹, respectively,
while the adsorption values were 36.30 mg g⁻¹, 52.90 mg g⁻¹, and 17.88 mg g⁻¹ for C₁₄-
Ver-100%, C₁₆-Ver-100%, and C₁₈-Ver-100%.
<figure><img src="image_10.png" /><figcaption>**Figure 9.1** Equilibrium isotherms and their fit to the Langmuir, Freundlich and Temkin models for the adsorption of diclofenac sodium by organo-vermiculites (a) C₁₄-Ver, (b) C₁₆-Ver, and (c) C₁₈-Ver at 25 °C (pH 6.0 or 8.0, 25 °C and Cᵢ = 1 - 500 mg L⁻¹).</figcaption></figure>

Drug adsorption increased with the amount of surfactant in the C₁₄-Ver, C₁₆-Ver and C₁₈-Ver samples (Figure S5). These results are in agreement with data found for diclofenac adsorption in benzyldimethyltetradecylammonium-modified Mt,²⁹ Mt and kaolinite modified by C₁₆Br,²⁵,³¹,⁴³ and dodecy and hexadecylpyridinium-bentonites.²⁴ Samples with a higher amount of surfactant incorporated have more available active sites for diclofenac interaction, improved the adsorption of pollutant.²⁷

The number of carbons in the alkyl chain of the surfactants influenced the adsorption of the drug only for samples prepared with 100% CEC and the performance
followed the order C₁₆-Ver-100% > C₁₄-Ver-100% > C₁₈-Ver-100%. The difference in the adsorption of the C₁₄-Ver-100%, C₁₆-Ver-100% and C₁₈-Ver-100% samples can occur due to the contribution of a series of interrelated factors, such as the length of the alkyl chain, the packing density of the surfactants and the organic content of the samples in concordance with other organophilic clay minerals.²⁴,⁸⁷

The adsorption performance of the C₁₄-Ver-200%, C₁₆-Ver-200%, and C₁₈-Ver-200% samples was better than that obtained by other organoclays prepared with higher amounts of surfactants (≥ 200% CEC)²⁴,²⁸, including pristine clay mineral with CEC close to sample used in present study (Table S3). The CEC influence the amount of loaded surfactant into the clay mineral,⁸⁸,⁸⁹ and consequently the availability of drug adsorption sites.

## Organovermiculites stability

The stability of organophilic vermiculites after treatment at pH 6.0 and pH 8.0 was monitored by TG/DTG (Figure S6 and Table S4) and CHN elemental analysis (Table S5). Results indicated a small reduction in the total organic content of the samples after the stability test (Figure 10). The lower leaching of organic cations and the highest stability were observed for samples prepared with 100% CEC. Previous studies also showed that vermiculites modified with C₁₆ and hexadecylpyridinium cations at 100% CEC were stable in the pH range of 4–10, and a lower leaching of organic cations was observed.⁴⁵
<figure><img src="image_11.png" /><figcaption>**Figure 10.** Total organic content of (a) C₁₄-Ver-100%, (b) C₁₆-Ver-100%, (c) C₁₈-Ver-100%, (d) C₁₄-Ver-200%, (e) C₁₆-Ver-200% and (f) C₁₈-Ver-200% before and after stability test at pH 6.0 and 8.0 determined by (i) CHN, and (ii) TG/DTG.</figcaption></figure>

## Characterizations of the diclofenac-loaded samples

XRD and FTIR analysis after adsorption were useful to understand the mechanism of drug/clay minerals interactions. The XRD patterns are shown in Figure 11i. After interacting with diclofenac, all samples showed an increased basal spacing, suggesting that the drug can be intercalated to access the active adsorption sites. XRD data were also compared with the molecular dimensions of diclofenac, which are 1.0 nm in length, 0.5 nm in width and 0.4 nm in height,³¹ and are in line with the possible intercalation of the drug or an interlayer rearrangement of surfactant chains after the entrance of diclofenac in both Ver and Hb phases. Similar behavior was observed in the adsorption of naphthalene by organovermiculites.⁴⁶
<figure><img src="image_12.png" /><figcaption>**Figure 11.2** (i) XRD patterns and (ii) FTIR spectra of (a) C₁₄-Ver-100%, (b) C₁₄-Ver-200%, (c) C₁₆-Ver-100%, (d) C₁₆-Ver-200%, (e) C₁₈-Ver-100%, and (f) C₁₈-Ver-200% after diclofenac adsorption, (25 °C, pH 6.0 or 8.0 and Cᵢ = 500 mg L⁻¹) and (g) free diclofenac sodium.</figcaption></figure>

The FTIR data also provided information about the groups of organophilic vermiculites and diclofenac involved in the adsorption. In the infrared spectra for diclofenac-loaded samples (Figure 11ii), the shift in the v<sub>as</sub>(CH<sub>2</sub>) band of organophilic samples (initially at 2917 cm<sup>-1</sup>) to higher frequencies (2922 cm<sup>-1</sup>) indicated that the interaction with the drug caused a disorganization or rearrangement of the alkyl chains of the intercalated surfactants,<sup>14,68</sup> supporting the XRD results. The changes in the CH<sub>2</sub> stretching frequencies indicate the interaction between long hydrophobic tails of surfactants and the nonpolar moiety of diclofenac.<sup>90,91</sup> In addition, several bands characteristic of the organic structure of sodium diclofenac were observed (Table S6). Variation in the position of the v<sub>s</sub>(COO<sup>-</sup>) band, initially at 1398 cm<sup>-1</sup> for the free drug, to
frequencies around 1376~1380 cm⁻¹ in loaded diclofenac samples also suggested electrostatic interactions with the carboxylate group during adsorption.²⁴,²⁶

## Mechanism of interaction

Drawing from the results, a comprehensive schematic of the mechanisms governing diclofenac adsorption on organophilic vermiculites was devised (Figure 12), highlighting the primary involvement of hydrophobic interactions, according to FTIR, Zeta potential results and study of the effect of pH on adsorption. FTIR data also show that electrostatic interactions can also contribute to the adsorption mechanism. However, considering the negative surface charge of most adsorbents at the adsorption pH, electrostatic interactions should play a less important role in the adsorption mechanism. Furthermore, XRD results showed that drug intercalation occurred in the interlayer space of all organophilic vermiculites. The presence of surfactant on the surface was considered on PCZ results. In this scheme, diclofenac molecules undergo intercalation and interact with (1) alkyl tails of the organic chain through Van der Waals interactions and (2) $-N^+-(CH_3)_3$ surfactants groups through electrostatic interactions. However, mechanism (1) predominates in the adsorption of diclofenac.
<figure><img src="image_13.png" /><figcaption>**Figure 12.** Proposed mechanism for diclofenac adsorption by the organophilic vermiculites through (1) Van der Waals forces and (2) electrostatic interactions.</figcaption></figure>

## Reuse tests

Several factors influence the selection of an adsorbent, including its production, affinity for the adsorbate and reusability, among other parameters.<sup>44,92</sup> Therefore, the regeneration capacity of drug-loaded organophilic vermiculites using ethanol as a desorption agent was evaluated over three adsorption/desorption cycles (Figure 13).
<figure><img src="image_14.png" /><figcaption>**Figure 13.** Results of reuse tests performed for (a) C₁₄-Ver-100%, (b) C₁₄-Ver-200%, (c) C₁₆-Ver-100%, (d) C₁₆-Ver-200%, (e) C₁₈-Ver-100%, and (f) C₁₈-Ver-200%.</figcaption></figure>

A reduction in adsorption capacity was observed with an increasing number of cycles, which could be attributed to adsorbent losses during the adsorption/desorption and washing processes, as well as the potential reduction or blocking of adsorption sites
during regeneration steps.²⁵,³³ It is worth noting that ethanol molecules may also be adsorbed on organophilic vermiculites during the regeneration process.³³ In the last cycle, maximum adsorption capacities were maintained at 61.0%, 50.4%, 58.0% for C₁₄-Ver-100%, C₁₆-Ver-100%, C₁₈-Ver-100% and 29.0%, 36.8% and 41.0% for the C₁₄-200%-Ver, C₁₆-Ver-200% and C₁₈-Ver-200%, respectively. The reduction in diclofenac adsorption capacity was higher for organophilic vermiculites prepared with 200% CEC probably due to the lower stability of these samples, in agreement with the stability test results. Additionally, the ethanol used in the washing can also leach surfactants.⁹³

## CONCLUSIONS

The adsorption of diclofenac on organophilic vermiculites was predominantly influenced by the level of organofunctionalization of the adsorbents. Optimal performance was observed in hybrids prepared with 200% CEC that exhibited higher organic content compared to those with 100% CEC. However, during reuse tests, these samples did not show enhanced performance. The stability of the samples prepared at 200% CEC was lower compared to organovermiculites prepared with 100% CEC. The leaching of surfactants during the adsorption of diclofenac and regeneration with ethanol decreased the performance of the adsorbents in subsequent cycles. Diclofenac was effectively adsorbed into the interlayer region of organophilic vermiculites, and hydrophobic interactions between the tail group of surfactants intercalated into the clay mineral and the nonpolar moiety of the drug play an important role in the adsorption process. However, FT-IR results showed that electrostatic interaction can also occur
between the carboxylate group (COO⁻) of diclofenac and the -N⁺-(CH₃)₃ surfactants groups.

# EXPERIMENTAL

## Materials and Chemicals

A Brazilian vermiculite sample (Ca,Mg-Ver) originating from Santa Luzia (Paraiba state, Brazil) was used as the starting material. The chemical composition of Ca,Mg-Ver in mass percentage was previously evaluated by chemical analysis: SiO₂ (40.08), Al₂O₃ (12.35), Fe₂O₃ (6.83), TiO₂ (1.43), CaO (2.32), MgO (18.74), Na₂O (3.37), K₂O (2.86) and a mass loss of 11.85% after heating at 950 °C.⁹⁴ Its cation exchange capacity (CEC) was 67 cmol(+) kg⁻¹, measured by the ammonium exchange method.⁹⁵,⁹⁶ All chemicals were used without prior treatment. Sodium chloride (99% purity), ammonium salts tetradecyltrimethylammonium, (C₁₄Br), hexadecyltrimethylammonium (C₁₆Br) and octadecyltrimethylammonium (C₁₈Br) bromides) (99% purity) were supplied from Sigma-Aldrich. Sodium hydroxide (99% Loba Chemie), nitric acid (75% Vetec), and ethanol (95% - Anidrol) were used. The sodium diclofenac (CAS number 15307-79-6, MM = 318.13 g mol⁻¹) was purchased from Sigma-Aldrich.

## Preparation of Na-Vermiculite (Na-Ver)

The Na-Ver sample was prepared from the Ca, Mg-Ver sample by repeated reactions with a 1.0 mol L⁻¹ NaCl solution stirred at 25 °C for 72 h, following a previous procedure⁶⁴ and was carried out in triplicate to ensure complete saturation. The resulting
Na-Ver was washed with distilled water until the AgNO₃ test for chloride anions showed negative in the supernatant solution and dried at 70 °C for 48 h. Na-Ver was ground and classified by sieving in Tyler sieves (Granutest, Brazil) to obtain a particle size of less than 0.074 mm.

## Preparation of organovermiculites

Organovermiculites were prepared by reacting between Na-Ver and each alkylammonium salt (C₁₄Br, C₁₆Br and C₁₈Br) based on a previous procedure.²⁴,⁸³ The reaction was carried out as follows: In a Teflon vessel reactor, 4.0 g of Na-Ver was dispersed in 100.0 ml of solution of ammonium salts at 100 and 200% CEC of clay mineral and heated in a microwave reactor (IS-TEC MW reactor model RMW-1, Brazil, with a power of 1100 Watt 2.45 GHz) for 5 min at 50 °C. Samples were repeatedly washed with distilled water until the AgNO₃ test for bromide anions showed negative results. The washed organovermiculites were dried at 50 °C at 24 h on a stove under air atmosphere.

## Adsorption studies

Adsorption tests were performed according to a previous method,²⁴,⁴⁸,⁸³ whose methodology consisted of evaluating the influence of experimental parameters such as pH, adsorbent dosage, contact time, and diclofenac concentration in adsorption.

In a typical procedure, organovermiculite samples were dispersed in 20 mL of diclofenac solution, stirring at 25 °C. The evaluation conditions (pH, adsorbent dosage, drug concentration, and reaction time) were systematically varied according to each test,
as listed in Table S7. Adsorption at different pH values was also performed for Na-Ver under the same conditions as used for organophilic vermiculites as a control.

After each test, the adsorbents were separated by centrifugation and the final drug concentration was determined by UV-vis absorption spectroscopy at 276 nm. The amount of drug adsorbed (q) and the drug removal efficiency (R%) by organo-vermiculites was determined by Equation (1) and (2), respectively:

$$q = \frac{(C_i - C_e)V}{m} \quad (1)$$

$$R(\%) = \frac{(C_i - C_e)}{C_i} * 100 \quad (2)$$

Where Cᵢ and Cₑ are the initial and equilibrium drug concentrations (mg L⁻¹), respectively, m refers to the mass of the adsorbent (g) and V (mL) is the volume of solution.

## Adsorption models

Adsorption models of Langmuir,⁹⁷ Freundlich,⁹⁸ and Temkin⁹⁹ were applied to adjust and analyze the experimental adsorption data employing the nonlinear method (see Table S8).

The models were also evaluated by standard deviation (SD root mean square error),¹⁰⁰ described in the Equation (3),

$$SD = \sqrt{\frac{1}{n_p-p} \sum_i^n (q_{i,exp} - q_{i,mode})^2} \quad (3)$$
where $q_{i,exp}$, and $q_{i,mode1}$ are the amount of experimental adsorbed drug predicted by the fitted model, $n_p$ is the number of experiments performed, and $p$ is the number of parameters of the fitted model.

## Reuse studies of adsorbents

Regeneration of the adsorbents was carried out according to a previous procedure²⁵ by dispersing the loaded organophilic diclofenac vermiculites in 50 mL of ethanol stirred for 6 h at 30 °C. After each desorption cycle, the solids were recovered by centrifugation at 7500 rpm, washed with distilled water and dried at 50 °C to be used for the next adsorption cycle. The readsorption tests were under the same conditions as adsorption experiments.

Drug adsorption/desorption from organovermiculites was calculated as percentage (%), where the initial adsorption amount was taken as 100%. For instance, the % desorption was calculated by Equation (4)

$$ \text{desorption (\%)} = \left( \frac{q_{e_a}}{q_{e_d}} \right) 100 \quad (4) $$

where $q_{e_a}$ and $q_{e_d}$ are the quantities of adsorbed and desorbed drugs per unit mass of the adsorbent (mg g⁻¹).

## Evaluation of the organovermiculites stability

The stability of organovermiculites was carried out under the same conditions used in the adsorption isotherms, without the presence of diclofenac, to follow changes by the adsorption process, according to the method previously reported.⁴⁵ For this, organovermiculites were suspended in 20 mL of water, using the optimal adsorbent
dosage. The pH values were adjusted 6.0 and 8.0 with HCl (0.1 mol L⁻¹) and NaOH (0.1 mol L⁻¹). Finally, the solids were recovered by centrifugation at 7500 rpm for 10 min and dried at 50 °C by 24 h.

## Characterizations

X-ray diffraction (XRD) data were recorded using an X-ray diffractometer (D8 Advance Bruker-AXS), with the 2θ ranging from 1° to 10° at the scanning, using CuKα radiation (λ = 1.5406 nm) at 30 kV and 30 mA. Elemental analysis of C and N was performed using a Perkin-Elmer PE-2400 microelemental analyzer. Thermogravimetric analyzes of organovermiculites were performed using a Discovery TGA instrument under argon atmosphere with a 100 mL min⁻¹ flux from 30 to 800 °C with a heating rate of 10 °C min⁻¹. The samples obtained after the stability test were analyzed in a TGA Q500 equipment under N₂ atmosphere with a 100 mL min⁻¹ flux from 30 to 800 °C with a heating rate of 10 °C min⁻¹. Fourier transform-infrared (FT-IR) spectra were obtained using an IR Prestige-21 spectrometer (Shimadzu) equipped with an ATR accessory, from 4000-600 cm⁻¹ with resolution of 4.0 cm⁻¹ and 32 scans. The Zeta potentials (ζ) were measured at different pH levels using a Zetasizer Nano ZS90 (Malvern Instrument). TEM was performed using a Talos S200 FEI instrument. A voltage acceleration of 200 kV and a current of 4 mA in STEM were used to obtain HAADF images. SEM was performed using a FEI Quanta FEG 250 microscope, operating at an accelerating voltage of 15 kV. The nitrogen adsorption isotherms were measured in an ASAP 2420 Micromeritics analyzer. Before measurement, the samples were degassed at 100 °C, and the N₂ isotherms of adsorption were measured at -196 °C in a P/P₀ range of 0.0-1.0. SBET value for C₁₈-Ver-100% sample was obtained from the Kr adsorption
isotherm in a P/P₀ range of 0.0-0.27. The specific surface area (S<sub>BET</sub>) of the samples was calculated by Brunauer, Emmet, and Teller (BET) method, while the pore volume and pore diameter were estimated by Barrett-Joyner-Halenda (BJH) method.

## ASSOCIATED CONTENT

### Supporting Information

Tables contain basal distance values obtained in TEM, adsorption parameters for equilibrium data calculated according Langmuir, Freundlich, and Temkin models; comparison of the diclofenac adsorption capacity of organovermiculites with others organoclays, summary of mass losses and temperature intervals for events in the DTG curves and CHN results the samples after stability tests at pHs 6 and 8, assignments of the bands in FT-IR spectra of diclofenac-loaded samples, additional experimental details for adsorption evaluation and equations of equilibrium adsorption models. Figures describe the interlayer arrangements of organic chains in the organophilic vermiculites, TEM images for sodium and organophilic vermiculites, N₂ adsorption-desorption isotherm Na-Ver, C₁₄-Ver-100%, and C₁₆-Ver-100%, and Kr adsorption isotherm for C₁₈-Ver-100%, and organovermiculites, graph of zeta potential versus amount of drug adsorbed by the organophilic vermiculites, relation between adsorption capacity and nitrogen content in the adsorbents, and TG/DTG data of the samples after stability tests at pHs 6 and 8 (DOC).

## ACKNOWLEDGMENT

This research was supported and funded by CAPES and CNPq in the form of a research fellowship awarded to M.G. Fonseca (Grant 310921-2017-1) and D.B. França (Grant
140661/2017-4), Paraíba State Research Foundation (FAPESQ) (Grant number 0012/2019 - FAPESQ/CNPq), Paraíba State Research Foundation grant 2021/3094, the University of Seville through the VII Plan Propio de Investigación by Proyect 2022/00000444, for the creation of the International Thematic Network on the use of Clays as a Drug Support and Environmental Remediation and by Proyect Stay Program for Researchers from Other National and Foreign Centers in US Departments and Research Institutes-2023, for the mobility of MG. Fonseca in the US was granted to M.M. Orta. We would thank to the Centro de Investigación, Tecnología e Innovación de la Universidad de Sevilla (CITIUS). We also thank to Dr Alessandra de C. Ramalho, Dr Wilton José da Rocha Lima, and Dr Michele Rocha (IQ-USP) for kindly help in TG and CHN elemental analysis and Luis Humberto Oliveira (UFPI) for SEM measurements.

**Author's contributions** All authors contributed to the conception and design of the present study as follows: Denise Brito França: Methodology, Investigation, Roles/Writing – original draft, Alice Priscila Nunes Silva: Methodology, Investigation, Roles/Writing – original draft, Edson Cavalcanti da Silva Filho: Methodology, Investigation, Writing- Reviewing and Editing, Josy Anteveli Osajima: Methodology, Investigation, Santiago Medina-Carrasco: Methodology, Investigation, Writing- Reviewing and Editing, Maria Del Mar Orta: Methodology, Investigation, Funding acquisition, Writing-Reviewing, and Editing, Maguy Jaber: Methodology, Investigation, Writing-Reviewing and Editing; Maria Gardennia Fonseca: Supervision, Conceptualization, Funding acquisition, Project administration, Roles / Writing - Original draft; Writing- Reviewing and Editing. All authors read and approved the final manuscript.
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