Utilization of tungsten trioxide nanoparticles and nickel oxide pillared montmorillonite nanocomposites for the adsorption of the drug dexamethasone from aqueous solutions

Abstract

The present study was undertaken to expand an impressive adsorbent and to study the adsorption process captivated in the adsorption of the drug dexamethasone from aqueous solution using the tungsten trioxide (WO₃) nanoparticles and nickel oxide pillared montmorillonite (NiO/MMT) nanocomposites. Parameters such as pH, contact time, ionic strength and adsorbent concentration, and temperature were also studied. The adsorption data coordinated well with the Langmuir, Freundlich and Fakhri models. However, the Langmuir isotherm displayed a better fitting model than the other isotherms because of the higher correlation coefficient that the antecedent exhibited; thus, indicating the applicability of monolayer coverage of the drug dexamethasone on the surface of an adsorbent. The adsorption process was found to follow pseudo-second-order kinetics. The principal thermodynamic parameters such as Gibbs free energy (ΔG), enthalpy change (ΔH) and entropy change (ΔS) were calculated. The positive value of ΔH and negative value of ΔG show the endothermic and spontaneous nature of adsorption, respectively. The isosteric heats of adsorption (ΔH_X) were calculated and indicated that the mechanism of adsorption was a physical process.

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1. Introduction

In the past recent decades, the scientific community has become increasingly interested in the potential impact of new environmental pollutants originated from industrial, agricultural and human activities on public health. Many of the pharmaceuticals in the aquatic environment are chiral chemicals, which contain at least one element of incommensurability and leading to the substance of two or more stereoisomers and enantiomers. The introduction of these compounds into the environment through anthropogenic sources can constitute a potential venture for aquatic and geocentric organisms.

Dexamethasone (DEX) is a type of steroid drug recognized as a glucocorticoid. It is a man-made prescription for a hormone secreted by the adrenal glands. Glucocorticoids have a broad span of action on a multitude of parts of the body.¹ The chemical and physical characteristics of dexamethasone are summarized in Table 1.

Table 1 Physicochemical properties of DEX

	Molecular weight (g mol^−1)	Melting point (°C)	Chemical Structure
Dexamethasone	392.461	262–264°

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Over a remarkable period of time, nanomaterials, such as nanorods, nanoparticles, nanoribbons, and nanowires, have been of significant concern in materials science on the foundation of their vital properties and pragmatic applications in multiple areas. Among disparate nanomaterials, tungsten trioxide (WO₃), a major semiconductor material with a broad band gap confine from 2.4 to 2.8 eV,² has engrossed substantial attention because of its potential applications in agile windows,³ semiconductor gas sensors,⁴ solar energy devices,⁵ optical displays⁶ and photocatalysts.^7–9

Nickel oxide (NiO) has been the subject of deep research interest due to its unique electrical, optical and magnetic properties. Specifically, the electronic properties of NiO have been the focus of research on applications.^10–12 Nanoparticles of NiO and many other materials have shown unique behavior based on their size.

Clay minerals, such as montmorillonite, kaolinite and hydrotalcite, have recently received considerable attention because of their abundance, low cost, and environmental compatibility. Among clay substances, pillared clays (PILC) are interesting materials to be used as catalysts and adsorbents due to their multi-charged centers, large area, high interlayer space and thermal stability. PILC synthesized with oxide pillars and metal oxide precursors have been previously used in studies of adsorption of organic compounds.^13–16

In this study, tungsten trioxide (WO₃) nanoparticles and nickel oxide pillared montmorillonite (NiO/MMT) nanocomposites were investigated as potential adsorbents for the removal of dexamethasone (DEX) from aqueous solution. To attain this goal, the influence of experimental conditions such as pH, adsorbent dose, ionic strength, temperature and contact time on adsorption manner was investigated. The isotherm, kinetic, and thermodynamic parameters were also evaluated.

2. Material and methods

2.1. Materials

All the reagents, unless marked in brackets, were purchased from Merck. Dexamethasone (DEX) (C₂₂H₂₉FO₅) (maximum purity available) and montmorillonite K10 (MMT) (Cation exchange capacity (CEC) of 110 meq. per 100 g, and surface area of 220–270 m² g⁻¹) was obtained from Sigma-Aldrich Ltd. All the solutions were prepared with analytically pure grade (AR) solvent and deionized water (DI).

2.2. Procurement of WO₃ nanoparticles

For this object, 0.002 mol of H₄₂N₁₀O₄₂W₁₂·xH₂O was dissolved by magnetic stirring at 80 °C in 80 mL of a HNO₃ solution. Then, 0.07 mol of urea was added to the solution under continuous stirring and maintained with constant temperature until precipitation. This precipitate was used as a precursor for the WO₃ nanoparticles. The precursor obtained was disjointed by thermal therapy at 400 °C in order to obtain the monoclinic crystalline structure of the WO₃ NPs.

2.3. Procurement of nickel oxide pillared montmorillonite nanocomposites

A pillaring solution with a 1 M concentration of Ni(II) was prepared by dissolution of Ni(NO₃)₂·6H₂O salt at room temperature with the slow addition and vigorous stirring of a 2 M KOH solution. The pH of the solution was controlled to attain a value lower than 1.8 by adding drops of 6 M HCl when necessary. Afterward, the solution was placed at room temperature for 4 h with continuous stirring for aging.

Furthermore, a 300 mL deionized water dispersion of 3 g of MMT was mixed with the 100 mL pillaring solution. The mixture was stirred and allowed to react at room temperature for 24 h. Later on, the solid was obtained by centrifugation and washed by successive re-dispersions in deionized water followed by centrifugation. Subsequently, the product was air-dried, heated at 300 °C for 2 h and denoted as NiO–MMT nanocomposites.

2.4. Material characterization

A scanning electron microscope (SEM); JEOL JSM-5600 Digital Scanning Electron Microscope, transmission electron microscope (TEM, JEM-2100F HR, 200 kV), and X-ray diffractometer (XRD) Philips X'Pert were used to characterize the adsorbent for its morphological information. The Brunauer–Emmett–Teller (BET) surface area (S_BET) of the powder was obtained by nitrogen adsorption in an ASAP2020 surface area and porosity analyzer (Micromeritics, USA). Zeta-potential measurements were performed with a ZEN3600, Malvern. The catalyst compositions were obtained with an energy dispersive X-ray spectrometer (EDX-700HS, SHIMADZU).

2.5. Equilibrium studies

Batch experiments were performed in 25 mL Erlenmeyer flasks that included 10 mL of drug solution with different concentrations. pH values of the drug solution were adjusted by 0.1 N HCl and NaOH in the range between 1 to 12. The solution temperature was set between 20 and 80 °C to understand the temperature influence on adsorption process. To investigate the influence of salt concentration on the removal of adsorbate, different amounts of NaCl salt were dissolved in the mixture to get the Na⁺ concentration in the range of 0–100 mmol L⁻¹. Other parameters included the adsorbent concentration to determine its effects on the adsorption process by varying it between 0.10 to 1.00 g L⁻¹. Dexamethasone (C₂₂H₂₉FO₅) was used in the preparation of the stock solutions by dissolving it at a known concentration in distilled water. The amount of adsorbate removed was calculated using the following formula:¹⁷where C₀ and C_e are the adsorbate concentrations in mg L⁻¹ initially and at a given time, respectively, V is the volume of solution (L) and W is the mass of the adsorbent (g). The DEX concentration was distinguished with the aid of a two dimensional Gas Chromatography (GC × GC) (Kimia Shangarf Pars Research Co., Iran). The data analysis was carried out using correlation analysis employing the least-square method and the Marquardt's percent standard deviation is calculated using the following equation:¹⁷

This error function is similar in some respects to a geometric mean error distribution modified according to the number of degrees of freedom of the system.

2.6. Kinetic and equilibrium patterns

In this paper, batch kinetic test data for adsorbed DEX on WO₃ nanoparticles and NiO–MMT nanocomposites were analyzed using pseudo-first-order and pseudo-second-order models. These models are explained as follows:

(i) The pseudo-first-order kinetic model:^18,19

ln(q_e − q_t) = ln(q_e) − k₁t (3)

where q_e and q_t are the DEX adsorption capacity (mg g⁻¹) at equilibrium and at time t (min), respectively, and k₁ is the rate constant of the pseudo-first-order kinetic model (min⁻¹).

(ii) The pseudo-second-order kinetic model:^20,21

where k₂ is the rate constant of the pseudo-second-order kinetic model (g mg⁻¹ min⁻¹).

The equilibrium experimental data were analyzed applying the Langmuir, Freundlich and Fakhri isotherm models. These isotherms are listed as below:

(i) Langmuir isotherm model:²²

(ii) Freundlich isotherm model:^23,24

where q_m is the theoretical maximum adsorption capacity corresponding to monolayer coverage (mg g⁻¹), K_L is the Langmuir isotherm constant (L mg⁻¹) and K_F and 1/n are constants.

(iii) The Fakhri isotherm model:²⁵

Fakhri isotherm is a two parameter equation most widely applied in the pure equilibrium systems. The model has a linear dependence on the concentration in the numerator and an exponential function in the denominator to represent adsorption equilibrium over a wide concentration range. This isotherm model is defined as follows:

This equation is conveniently used in the following linear form:

ln(θ) = B_F ln(1 + K_FC_e) − ln(K_FC_e) (8)

where θ is the degree of surface coverage, B_F is Fakhri isotherm exponent, K_F is equilibrium constant of adsorption (L g⁻¹).

3. Results and discussion

3.1. Characterization of tungsten trioxide nanoparticles

The phase and structure of the product were determined by XRD, as shown in Fig. 1A. All the diffraction peaks can be attributed to the triclinic phase of WO₃ NPs. The XRD patterns of this specimen emphasize their crystallinity with apparently monoclinic structure. The crystallite sizes of WO₃ NPs specimens have been determined using the Scherrer equation: D = 0.9λ/B cos θ; where D is the crystallite size (nm), λ is the wavelength of the X-ray radiation, θ is the Bragg's angle and B is the full width at half maximum (FWHM) of the peak at 2θ. The crystallite sizes have been estimated to be 8.1 nm. Fig. 1B presents the SEM image of the WO₃ NPs powder. It can be seen that the nanoparticles were excellently synthesized, and have a round grain-shaped structure. Fig. 1C shows the change in uniform structure after the adsorption of DEX. Fig. 1D shows the typical nitrogen adsorption–desorption isotherms of WO₃ NPs. At a relative high pressure region (P/P₀ > 0.7), due to the capillary agglomeration phenomenon, the isotherms rapidly increase and form a lag loop. The specific surface area and total pore volume of WO₃ NPs calculated using the multipoint BET-equation is 25.61 m² g⁻¹ and 0.0775 cm³ g⁻¹, respectively. The morphology and particle size of the WO₃ nanoparticles were analyzed by TEM. Fig. 1E and F represent the TEM images and particles size plot for WO₃ nanoparticles, respectively. Fig. 1E shows the distinct spherical morphology of WO₃ nanoparticles. Fig. 1F shows that the mean size distribution of WO₃ NPs is 7.5–9.5 nm.

Fig. 1 X-ray diffraction analysis (A), SEM image before (B) and after reaction (C), nitrogen adsorption–desorption isotherm (D), TEM image (E) and particle size distribution (F) (obtained from TEM) of WO₃ nanoparticles.

3.2. Characterization of nickel oxide pillared montmorillonite nanocomposites

Fig. 2A shows the XRD of the raw montmorillonite (MMT) and NiO–MMT samples. A typical diffraction peak (001) for montmorillonite clays at 2θ = 6.6° corresponds to a basal spacing of 1.33 nm. This result indicates that NiO exists within the interlayer space of the modify clay and these species were introduced via an ion exchange process. Moreover, the XRD patterns of NiO–MMT show a characteristic diffraction peak for nickel oxide at 2θ = 43.5°, indicating the presence of NiO crystalline particles in the modified MMT. The Scherrer formula for the estimation of a crystallite size was used and a crystallite size of 7 nm was obtained.

Fig. 2 X-ray diffraction analysis (A), SEM image before (B) and after reaction (C), nitrogen adsorption–desorption isotherm (D), TEM image (E), EDX pattern (F) of NiO–MMT nanocomposites.

SEM images of NiO–MMT nanocomposites before and after the reaction are shown in Fig. 2B and C. The NiO–MMT nanocomposites before the reaction show less face to edge aggregations and consequently a more ordered morphology. The less smooth surface of MMT confirms the existence of another phase deposited on the flakes. Fig. 2C shows the change in the uniform structure after the adsorption of DEX. The adsorbed particles such as DEX as well as the pore particles are seen after adsorption.

The specific surface areas and total pore volume of NiO–MMT nanocomposites were 123 m² g⁻¹ and 0.130 cm³ g⁻¹, respectively. Fig. 2D shows the adsorption–desorption isotherms of nitrogen on the NiO–MMT nanocomposites isotherm.

A TEM image of the NiO–MMT sample is displayed in Fig. 1E. There are spectacular differences in the microstructure of the composites: the nanoparticles formed on montmorillonite are much larger, their surface coverage is higher, and they are dispersed quite homogeneously over the lamellae. EDX measurement reveal that the NiO–MMT nanocomposites contain O, Mg, Al, Si and Ni (Fig. 2F), which is also evidence that NiO nanoparticles were formed in the interlayers and on the surface of MMT. Cu was observed because the sample was placed on copper mesh.

3.3. Utilizable parameters

3.3.1. Effect of contact time and temperature. The impact of contact time between DEX with 0.4 g L⁻¹ of WO₃ nanoparticle and NiO–MMT nanocomposites was studied for solutions in which the concentration was 200 mg L⁻¹. The experiments were performed at 20, 50, and 80 °C. We observed the adsorption of a satisfactory amount of DEX onto the WO₃ nanoparticles and NiO–MMT nanocomposites after a very short contact time (Fig. 3). A weak association with the temperature was observed for the experiments carried out at 20 and 50 °C where both the samples demonstrated nearly the same adsorbed quantities at specific contact times. However, the test performed at 20 °C indicated lower adsorbed quantities. The diversity in the adsorption capability of the adsorbents and its dependence on the temperature could be due to the decrease in the removal equilibrium constant while decreasing the temperature. Better tests were performed with 11 min contact time in order to ensure the high adsorption capacity and a complete equilibrium situation.

Fig. 3 Effects of contact time (C = 200 mg L⁻¹; pH = 4; adsorbent dose = 0.4 g L⁻¹), initial pH solution (C = 300 mg L⁻¹; T = 25 °C; adsorbent dose = 0.4 g L⁻¹), ionic strength (C = 300 mg L⁻¹; pH = 4; T = 25 °C; adsorbent dose = 0.4 g L⁻¹) adsorbent dosage (pH = 4; T = 25 °C; C = 100 mg L⁻¹) on the adsorption of DEX.

3.3.2. Effect of initial pH. The most important parameter affecting the sorption tendency is the pH of the adsorption medium. In case of DEX, at a higher pH range, a reduced amount of removal was perceived and at a lower pH the amount of removal proliferated (Fig. 3). The acidic medium is desirable for the adsorption process of DEX. At higher pHs, the high negatively charged adsorbent surface sites did not favor the removal of deprotonated DEX due to electrostatic repulsion.

3.3.3. Effect of ionic strength. In this study, different amounts of NaCl were added to the mixture of DEX and WO₃ nanoparticles and NiO–MMT nanocomposites suspensions to investigate the ionic strength effect on the adsorption turnover (Fig. 3). It can be seen that the removal capacities slowly reduce with the addition of NaCl. The variation in the adsorption capacities in the NaCl concentration range is not significant. We supposed that the increase in ionic strength would resist the electrostatic interactions, and the cation–π bonding was weakened owing to electronic screening of the surface charge sites by the added Na⁺.

3.3.4. Effect of adsorbent dose. Fig. 3 shows that the quantity of the removal of the DEX proliferated with increase in the dose of the adsorbent. This may be due to the availability of surface active sites resulting from the increased dose and conglomeration of the adsorbent. The increase in the number of removal of DEX is found to be meager after a dose of 0.4 g L⁻¹ for WO₃ nanoparticle and NiO–MMT nanocomposites. Therefore, they are fixed as optimum dose of adsorbent for further studies.

3.4. Adsorption interaction

In the present study, q_m values increased with the reduction in pH, and thus with a reduction in the surface electronic density. These results may indicate that dispersion interactions between WO₃ nanoparticle and NiO–MMT nanocomposite surfaces, and the DEX species present in solution. In conclusion, electrostatic interactions play a major role in the mechanism of DEX adsorption, especially on adsorbents. The adsorption capacity of two adsorbents for DEX is enhanced because of an increase in (i) attractive electrostatic adsorbent–adsorbate interactions and (ii) the number of protons available for ion exchange with DEX.

3.5. Mechanism for DEX sorption on WO₃ nanoparticle and NiO–MMT nanocomposites

3.5.1. Assessment of adsorption isotherms. The Langmuir adsorption is based on the presumption of monolayer adsorption on a structurally homogeneous adsorbent. The total isotherm plot for the adsorption of DEX onto WO₃ nanoparticle and NiO–MMT nanocomposites is obtained in Fig. 4. The correlation coefficients of the isotherms are all higher than 0.999, which indicate that the adsorption of DEX onto WO₃ nanoparticle and NiO–MMT nanocomposites was best explained by the Langmuir isotherm model. The calculated values of q_m and K_L are 405.0, 470.9 mg g⁻¹ and 0.0384, 0.0278 L g⁻¹ for WO₃ nanoparticle and NiO–MMT nanocomposites, respectively. The calculated values of R_L (ref. 26) are all in the range of 0.5–0.6 for DEX, therewith confirming that the adsorption processes are all favorable. This quantitatively describes the formation of a monolayer adsorbate on the outer surface of the adsorbent, and after that no further adsorption takes place. Thereby, the Langmuir represents the equilibrium distribution of DEX between the solid and liquid phases. The Langmuir isotherm is valid for monolayer adsorption onto a surface containing a finite number of identical sites.

Fig. 4 Equilibrium adsorption isotherms for the removal of dexamethasone.

3.5.2. Assessment of adsorption kinetics. The parameters k₁ and q_e for the pseudo-first-order kinetic model (eqn (3)) could be determined from the slope and intercept of the plots of ln(q_e − q_t) versus t (figure not shown) and are given in Table 2. The values of the R² for the pseudo-first-order model changed in the lower confine. In addition, the experimental values of q_e,exp (mg g⁻¹) are far from the calculated q_e,cal (mg g⁻¹). This expresses that the adsorption of DEX did not follow the pseudo-first-order kinetic model, and it is not a diffusion-controlled phenomena.

Table 2 Kinetic parameters for the adsorption of dexamethasone

Kinetic parameters	WO3 nanoparticles			NiO–MMT nanocomposites
	20°	50°	80°	20°	50°	80°
qe,exp (mg g^−1)	168.22	177.00	185.20	172.10	181.00	190.20
Pseudo-first-order model
qe (mg g^−1)	24.223	20.721	23.847	35.442	34.222	36.752
k1 (min^−1)	0.0726	0.0805	0.0854	0.0759	0.0842	0.0895
R^2	0.9851	0.9840	0.9881	0.9837	0.9822	0.9875
MPSD	32.851	38.378	33.532	33.222	36.986	34.253
Pseudo-second-order model
qe (mg g^−1)	171.09	179.20	188.01	172.88	180.55	191.99
k2 (g mg^−1 min^−1)	0.0225	0.0212	0.0167	0.0201	0.0187	0.0144
R^2	0.9998	0.9991	0.9985	0.9999	0.9995	0.9990
MPSD	2.211	2.059	2.354	2.100	1.098	2.212

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For the pseudo-second-order kinetic model (eqn (4)), the q_e and k₂ values can be grabbed from the slopes and intercepts of plots of t/q_t versus t which are painted in Fig. 5. The R² of linear plots at different temperatures are higher values, which suggests that the adsorption of DEX onto WO₃ nanoparticle and NiO–MMT nanocomposites follows a pseudo-second-order kinetic model. In addition, it is consistent between the calculated data (q_e,cal) and the experimental data (q_e,exp). The parameters are listed in Table 2.

Fig. 5 Pseudo-second-order kinetic model for the removal of dexamethasone.

3.5.2.1. The intra-particle diffusion model. The kinetic results were further analyzed by the intra-particle diffusion model to elucidate the diffusion mechanism:²⁷

q_t = k_it^1/2 + C (9)

where C is the intercept and k_i is the intra-particle diffusion rate constant (mg g⁻¹ min^−0.5), which can be evaluated from the slope of the linear plot of q_t versus t^1/2 (Fig. 6).

Fig. 6 Intra-particle diffusion model for adsorption of dexamethasone onto WO₃ nanoparticle (A) and NiO–MMT nanocomposites (B).

The first sharper segment is due to the diffusion of adsorbate through the solution to the external surface of the adsorbent and the second segment represents the gradual adsorption procedure, i.e., the diffusion of adsorbate molecules inside the adsorbent. It is simple to find that k_i of first region was higher than k_i of second region. This demonstrates that the adsorption rate of DEX is higher in the beginning owing to the large surface area of the adsorbent accessible for the adsorption. The adsorbate formed a thick layer in the exterior inchmeal due to the intermolecular attraction and molecular association. This blocked the further adsorption was limited by the rate of the adsorbate was transported from the exterior to the interior sites of the adsorbent particles.

3.5.2.2. The liquid film diffusion model. However, for the transport of the solute molecules from the liquid phase up to the solid phase, the boundary plays the most significant role in adsorption; and the liquid film diffusion model may be applied as follows:

ln(1 − F) = −k_fdt (10)

where k_fd is the adsorption rate constant and F is the fractional attainment of equilibrium F = q_t/q_e. The liquid film diffusion plot is ln(1 − F) vs. t (figure not shown). The calculated values of k_fd for WO₃ nanoparticle and NiO–MMT nanocomposites are “0.2142, 0.2166, 0.2293” and “0.2241, 0.2279, 0.2310” at 20, 50, 80 °C, respectively. The value of R² indicated that the sorption process is controlled by diffusion through the liquid film surrounding the solid sorbent for the removal of dexamethasone using WO₃ nanoparticle and NiO–MMT nanocomposites.

3.5.3. Assessment of adsorption thermodynamics. Thermodynamic parameters can be distinguished using the adsorption constant K_L (L mol⁻¹) in the Langmuir isotherm, which depends on temperature. Changes in the free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) for the adsorption of dexamethasone onto WO₃ nanoparticle and NiO–MMT nanocomposites were calculated using the following equations:^28,29

ΔG° = −RT ln K_L (11)

ΔG° = ΔH° − TΔS° (12)

where R (8.314 J mol⁻¹ K⁻¹) is the universal gas constant and T is the temperature (K). The values of ΔH° and ΔS° of DEX can be calculated from the slope and intercept of the plot of ΔG° against T (Fig. 6). The results are listed in Table 3. The negative free energy changes (ΔG°) inferred that the adsorption of DEX onto two adsorbents were pragmatic and spontaneous thermodynamically. The positive value of ΔH° demonstrated that the adsorption is an endothermic process. The positive value of ΔS° infers DEX in bulk phase (aqueous solution) was in a considerably more disordered distribution in contrast to the relatively ordered state of solid phase (surface of adsorbent). Moreover, the positive value of ΔS° reflects the affinity of the adsorbents for DEX (Fig. 7).

Table 3 Change in thermodynamic parameters with temperature

Thermodynamic parameter	ΔH° (kJ mol^−1)	ΔS° (J mol^−1 K^−1)	ΔG° (kJ mol^−1)
			293 K	323 K	353 K
WO3 nanoparticles	4.5471	32.700	−5.029	−6.040	−6.992
NiO–MMT nanocomposites	3.1874	35.000	−7.021	−8.241	−9.123

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Fig. 7 Plot of Gibbs free energy change vs. temperature for adsorption of dexamethasone using WO₃ nanoparticle (A) and NiO–MMT nanocomposites (B).

3.5.3.1. Computation of activation parameters. From the pseudo-second-order rate constant k₂ (Table 2), the activation energy E_a for the adsorption of DEX using WO₃ nanoparticle and NiO–MMT nanocomposites was determined using the Arrhenius equation . By plotting ln k₂ versus 1/T (figure not shown), E_a was obtained from the slope of the linear plot. The values of E_a were 7.1228 and 11.4258 kJ mol⁻¹ for WO₃ nanoparticle and NiO–MMT nanocomposites, respectively. The importance of E_a may give an idea about the type of adsorption. The important information obtained from the activation energy turnover is whether the adsorption is mainly physical or chemical. Physisorption processes generally have energies in the range of 5–40 kJ mol⁻¹, while higher activation energies (40–800 kJ mol⁻¹) infer chemisorption.³⁰ The values of activation energies obtained confirm the physisorption nature of the process of DEX removal using WO₃ nanoparticle and NiO–MMT nanocomposites.
3.5.3.2. Computation of isosteric heat of adsorption. Isosteric heat of adsorption (ΔH_X, kJ mol⁻¹) specified as the heat of adsorption determined at constant amount of adsorbate adsorbed is one of the basic provisions for the characterization and optimization of an adsorption process. Science of the heats of sorption is very important for material and process design. The isosteric heat of adsorption at constant surface coverage was estimated using the Clausius–Clapeyron equation:³¹

For this object, the equilibrium concentration (C_e) at constant amount of DEX adsorbed was acquired from the isotherm data at different temperatures. The values of ΔH_X were acquired from the slope of a plot of ln C_e versus 1/T for different amounts of DEX onto WO₃ nanoparticle and NiO–MMT nanocomposites. The values of ΔH_X were determined from the slope of the plots. For physical adsorption ΔH_X must be below 80 kJ mol⁻¹ and for chemical adsorption it span between 80 and 400 kJ mol⁻¹.³¹ In the present system, the value of ΔH_X was 31.47 and 45.97 kJ mol⁻¹ denoting that the adsorption of DEX onto WO₃ nanoparticle and NiO–MMT nanocomposites were physical processes, respectively.

3.6. Regeneration of adsorbents

Preliminary regeneration experiments were carried out using spent WO₃ NPs and NiO–MMT nanocomposites adsorbent using 0.2 M and 0.5 M NaOH solution. 13.5%, 15%, 25% and 30% DEX was recovered from 200 mg L⁻¹ DEX solutions with 0.2 M and 0.5 M NaOH solutions, respectively. The lower percentage of desorption of DEX was due to the strong attraction between adsorbent active sites and DEX molecule. To check the efficiency of the regenerated adsorbents, WO₃ NPs and NiO–MMT nanocomposites were washed thoroughly in distilled water and dried properly under sunlight for 24 h. The regenerated adsorbents were used for the adsorption of 100 mg L⁻¹ of DEX at adsorbent dose of 0.4 g L⁻¹ for a contact time of 11 min. Results showed that only about a 5% and 6% decrease in adsorption efficiency for WO₃ NPs and NiO–MMT nanocomposites was observed, respectively. The maximum amount of adsorption by NiO–MMT nanocomposites is more than WO₃ nanoparticle (Table 4).

Table 4 Comparison of DEX adsorption with different adsorbents

Adsorbents	qm (mg g^−1)	Ref.
Carbon nanotubes	1.25	17
Activated carbon	3.38	17
WO3 nanoparticles	405.00	This study
NiO–MMT nanocomposites	470.90	This study

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4. Conclusions

Kinetic, equilibrium and thermodynamic studies were accomplished for the adsorption of dexamethasone onto WO₃ nanoparticle and NiO–MMT nanocomposites. Results of the adsorption studies showed that the two nano powders can be effectively used as adsorbents for the removal of DEX. Isotherm studies demonstrated that the Langmuir model fit the experimental data better than the Freundlich and Fakhri models. The adsorption equilibrium was explained well by the Langmuir isotherm model. The kinetic process was better explained by the pseudo-second-order kinetic model. The dependency of adsorption of DEX on temperature was surveyed and the thermodynamic parameters ΔG°, ΔH° and ΔS° were calculated. The results show a conceivable, spontaneous and endothermic adsorption process. Conforming to the activation energy calculated, adsorption is a physisorption process. The isosteric heats of adsorption were calculated by applying the Clausius–Clapeyron equation and indicated that the mechanism of adsorption was a physical process.

Acknowledgements

The author acknowledges Materials & Energy Research Center and Razi Metallurgical Research Center (RMRC) from Iran for performance support through the project.

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