Efficient Removal of Dyes from Aqueous Solution by Adsorption on L-Arginine-Modified Mesoporous Silica Nanoparticles

Abstract

The use of mesoporous silica modified with L-arginine (Ar-MSNPs) for the removal of ionic dyes from aqueous solutions has been investigated. Several analytical techniques have been used to determine the characteristics of nanoadsorbents. The removal of crystal violet and fluorescein was performed using the batch method to investigate the effects of cultivation pH, initial concentrations of dyes, and exposure time on adsorption efficiency. The optimum adsorption of fluorescein was achieved at pH 2, whereas the optimum adsorption of crystal violet was achieved at pH 13. The equilibrium was established in both systems at 20 min at low concentrations, and approximately 30 min at high concentrations. The equilibrium adsorption data was analyzed using Langmuir and Freundlich isotherm models. The correlation coefficient (R²) values of the isotherms presented the best fit with the Langmuir isotherm. The adsorption kinetic data was fitted with the pseudo second-order model for both systems.

1. Introduction

In recent years, the excessive use of dyes, due to their entry into the environment, has raised concerns globally. Approximately 7 million tons of dyes are consumed in the world per year [1,2]. Dyes are utilized widely in the cosmetic, textile, pharmaceutical, paper, and food industries. During the dyeing process, approximately 10% (up to 15%) of dyes are discharged into aquatic environments, without any restrictions [3,4]. Although in some cases, the entry of such compounds into the aquatic ecosystem may be low, the cumulative effect is considered to have potential risk to the environments and microorganisms [5].

Fluorescein is an organic dye that has been widely used as a fluorescent tracer for labelling membranes and proteins [6,7]. Fluorescein shows different structural forms (cation, neutral molecule, monoanion, and dianion) depending on pH values [8,9,10,11]. Crystal violet is an organic cationic dye that has a triphenylmethane group in its structure [12]. Crystal violet is widely used in the manufacturing of various products, such as veterinary medicine, textiles, pens, and printers [13,14]. It is carcinogenic and toxic to mammalian cells, and it can be harmful when encountered by inhalation, ingestion, or through skin contact [15,16]. Therefore, there is a need to develop an efficient system to remove these dyes from the wastewater. So far, various methods have been used for water remediation, including membrane separation, chemical oxidation, and biodegradation [17,18,19]. The main disadvantage of these techniques is the secondary pollutants produced from the process. Adsorption is one of the most efficient, easiest, and cheapest methods for removing organic dyes [20,21]. The adsorption performance is directly dependent on the adsorbent properties. Different adsorbents have been studied in recent decades, including activated carbon [22], zeolite [23], clay [24], and mesoporous silica nanoparticles (MSNPs) [25,26].

Mesoporous silica nanoparticles (MSNPs) have been studied as a nanoadsorbent for removing dyes from wastewater [27,28,29]. For instance, methylene blue has been removed from contaminated water using surface-modified MSNPs [28,30]. Naseem et al. synthesized mesoporous silica via a green route for the removal of crystal violet from wastewater [31]. The equilibrium adsorption data was found to follow the Langmuir model, with adsorption capacity ca. 26 mg/g. Hasan et al. reported the synthesis of MSNPs coated by a copolymer chain of L-ascorbic acid (AS) and polyaniline (PAni) as an effective adsorbent for the removal of crystal violet [32]. The experimental data was fitted by the Langmuir model, with a maximum adsorption capacity of 88 mg g⁻¹ at 298 K.

Most of ionic dyes are water soluble, which could have environmental and health risks [33,34,35]. Thus, the removal of ionic dyes from wastewater has attracted a large number of research activities. In the present study, mesoporous silica modified with L-arginine (Ar-MSNPs) was prepared and characterized. The adsorption behaviour of crystal violet and fluorescein on the modified silica was studied. The kinetics of the adsorption process have been investigated. The adsorption isotherms were studied to determine the probable mechanism of adsorption.

2. Materials and Methods

2.1. Materials

L-Arginine (Ar, ≥98%), 3-isocyanatopropyl)triethoxysilane (ICPTES, >95%), ammonium hydroxide (NH₄OH, 32 wt%), N-cetyltrimethylammonium bromide (CTAB, 98%), tetraethyl orthosilicate (TEOS, 98%), ethanol (HPLC grade), n-hexane (HPLC grade), fluorescein (95%), and crystal violet (ACS reagent, ≥90.0%) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl, 36.5% v/v), dimethylformamide (DMF, HPLC grade), pyridine (analytical grade), and methanol (HPLC grade) were obtained from Fisher Scientific. All chemicals were used as received, without any further purification.

2.2. Preparation of the Adsorbent

2.2.1. Mesoporous Silica Nanoparticles (MSNPs) Preparation

CTAB (1 g) was dissolved in 150 cm³ of distilled water and sonicated for 5 min. To the aqueous solution, 4 cm³ of NH₄OH was added and stirred at 40 °C. TEOS (20 cm³), and 5 cm³ of expanding agent (n-hexane) was mixed and added slowly to the aqueous solution. After 12 h, MSNPs with CTAB were filtered and washed with distilled water and methanol. To extract the templet, the nanoparticles were dispersed in a mixture containing 12.5 cm³ of acetic acid and 12.5 cm³ of hydrogen peroxide, under stirring and at 120 °C. CTAB free MSNPs were separated and washed with distilled water (DW) and methanol [26,27].

2.2.2. MSNPs Modified with L-Arginine (Ar-MSNPs)

Ar-MSNPs were achieved in two steps. First step: CTAB free MSNPs were suspended in dried toluene (30 cm³) and sonicated for 5 min. To the mixture, 0.2 cm³ of ICPTES was added and heated at 120 °C for 12 h. MSNPs-ICP were centrifuged and washed with dry toluene and dry DMF [36].

Second step: The separated nanoparticles were added to dry DMF (10 cm³) and sonicated for 10 min. Then, 10 mg of L-arginine in 10 cm³ of dry DMF was added to the suspension. The mixture was sonicated for 10 min and left under stirring at room temperature for 12 h. Ar-MSNPs were separated and washed with DW and ethanol.

2.3. Characterization

The morphology, structure, and performance of the nanomaterials were characterized by several physiochemical techniques. FTIR spectra were obtained using a Thermo Scientific (Waltham, MA, USA) Nicolet iS10 FT-IR spectrometer instrument, in KBr pellets with a spectral range 4000–400 cm⁻¹, at 25 °C. The chemical composition of the organic film (elemental analysis) was obtained from a Perkin Elmer Series II-2400 analyzer. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained using JEOL instruments, model JSM-6380 LA and model JEM-1230, respectively. The surface charge of the nanoparticles was measured at different pH values by a Malvern instrument (Zetasizer Nano ZS). Dye concentration was evaluated using a UV–vis spectrophotometer—Shimadzu instrument model UV-2600.

2.4. Adsorption Studies

2.4.1. Batch Adsorption

To evaluate the adsorption performance, the adsorption experiments were conducted using fluorescein and crystal violet as probes. The adsorption behavior of both dyes on Ar-MSNPs was investigated at different initial concentrations, pH values, and contact times. Contaminated water was first prepared by dissolving known amounts of fluorescein and crystal violet in distilled water. Second, 10 mg of the adsorbent was placed in a sample tube containing 10 cm³ of the contaminated water at 200 r/min for a certain amount of time. Different concentrations of fluorescein and crystal violet were used. Finally, the adsorbent was separated by centrifuge, and the concentration of unabsorbed dyes was determined by UV–vis spectrophotometer. The amount of dye adsorbed onto the nanoparticles surface and the removal efficiency were determined by:

q_t = (C₀ − C_t)V/m

(1)

Removal efficiency (%) = C₀ − C_e/C₀ × 100%

(2)

where q_t is the amount of dye adsorbed per unit mass of the nanoparticles at time t in mg.g⁻¹. C₀ and C_e (in mg.dm⁻³) are the dye initial and equilibrium concentrations, respectively. C_t is the dye concentration at time t (min) in mg.dm⁻³. V is the volume of the solution in dm⁻³; m is the mass of the nanoparticles in g.

2.4.2. Adsorption Isotherms

The adsorption behavior of the selected dyes on the Ar-MSNPs surface was determined using two isotherms: Langmuir and Freundlich equation models. The isotherm equations are expressed by:

Langmuir equation:

$$\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{bq}}_{\mathrm{m}}}+\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{m}}}$$

(3)

Freundlich equation:

$${\mathrm{lnq}}_{\mathrm{e}}={ \mathrm{lnK}}_{\mathrm{F}}+\frac{1}{\mathrm{n}}{\mathrm{lnC}}_{\mathrm{e}}$$

(4)

where ${\mathrm{q}}_{\mathrm{m}}$ in mg.g⁻¹) is the maximum adsorption capacity, and b (in dm⁻³.mg⁻¹) is a constant value, related to the strength of the adsorbent/adsorbate interaction (Langmuir constant). ${\mathrm{K}}_{\mathrm{F}}$ is a constant value, related to the adsorption intensity; 1/n is a value related to the surface heterogeneity and homogeneity. If the value is small, the surface is a more heterogeneous surface. If the value is equal or closer to one, then the adsorbent has relatively more homogeneous binding sites.

2.4.3. Adsorption Kinetics

The kinetic adsorption of the dyes onto the Ar-MSNPs surface was determined using three adsorption models. The kinetic adsorption equations are expressed by:

• Pseudo first-order model:

$$\begin{gathered} \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2}+\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}} \\ \lg \left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)={ \mathrm{lgq}}_{\mathrm{e}}-{\mathrm{k}}_1\mathrm{t} \end{gathered}$$

(5)

• Pseudo second-order model:

$$\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2}+\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}$$

(6)

• Intraparticle diffusion model:

$${\mathrm{q}}_{\mathrm{t}}={ \mathrm{k}}_{\mathrm{dif}}{\mathrm{t}}^{1/2}+\mathrm{C}$$

(7)

where ${\mathrm{q}}_{\mathrm{e}}$ and q_t is the amount of adsorbed dyes at equilibrium and at time t, respectively, in mg.g⁻¹; ${\mathrm{k}}_1$ and ${\mathrm{k}}_2$ are the pseudo first-order and pseudo second-order rate constants, respectively, in mg.min.g⁻¹; ${\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2$ is the initial sorption rate in mg.g⁻¹.min⁻¹ (shows dye molecules movement rate); ${\mathrm{k}}_{\mathrm{dif}}$ is the intraparticle diffusion rate constant in mg.g⁻¹.min^−1/2; C is the thickness of the boundary layer.

3. Results and Discussion

3.1. General Characterization

Ar-MSNPs nanoadsorbents were synthesized in four steps. Scheme 1. First, MSNPs with templates were obtained after the condensation reaction of TEOS in basic media in the presence of CTAB (template) and n-hexane (expanding agent). A mixture of acetic acid and hydrogen peroxide was used to extract the template to allow for the reaction with the inner surface. ICPTES was attached to CTAB free MSNPs via condensation reactions between the silanol groups and alkoxysilane groups to obtain MSNPs modified with the isocyanato groups. Primary amine groups in L-arginine can react with isocyanato groups present in MSNPs-ICP to obtain Ar-MSNPs.

The FTIR spectrum of the fabricated nanoparticles shows transmittance peaks at various frequencies, indicating the presence of different functional groups. FT-IR spectra of MSNPs (with CTAB), CTAB free MSNPs, MSNPs-ICP, and Ar-MSNPs are presented in Figure 1. For all samples, it was observed that wave numbers at 450 cm⁻¹ and 800 cm⁻¹ were assigned to the tetrahedron vibration of SiO₄ and the symmetric vibration of Si–O–Si, respectively. Strong absorbance was observed at ~1100 cm⁻¹, which was attributed to the Si–O–Si stretch of silica. The absorbance at ~1640 and 3455 cm⁻¹ was observed, as assigned to the surface hydroxyl groups of mesoporous silica. MSNPs (with CTAB) spectra show peaks at wave numbers ~1450 and 2990 cm⁻¹, assigned to –CH vibrations of the template molecules, which disappeared after the extraction process. MSNPs-ICP presented an additional peak at 2280 cm⁻¹, supporting that the –NCO groups were introduced to the MSNPs surface successfully. After MSNPs-ICP was reacted with L-arginine, a new peak at 1700 (C = O group) was apparent, while that at 2280 cm⁻¹ disappeared, supporting that the reaction between MSNPs-ICP and L-arginine was successfully initiated.

The chemical compositions of the organic layer formed on the silica was also quantified by elemental analysis. Figure 2 shows the results of elemental analysis of the CTAB free MSNPs and modified MSNPs. The low carbon content on CTAB free MSNPs could come from the template and unreacted TEOS and ethoxy groups that were not fully extracted and oxidized by the extraction process. The carbon, hydrogen, and nitrogen contents of modified MSNPs samples are higher due to the grafting of the organic layer on the surface of the particles. After L-arginine reaction with MSNPs-ICP, the carbon and hydrogen contents increased because of the successful attachment of L-arginine onto the MSNPs surfaces.

To determine the surface charge of the selected samples, the zeta potential of CTAB free MSNPs, MSNPs-ICP, and Ar-MSNPs were measured. The zeta potential measurements as a function of pH values were presented in Figure 3. CTAB free MSNPs had a negative value of zeta potential, as the silanol groups were negatively charged at a pH value above 5. Zeta potential measurements of CTAB free MSNPs reached −31 mV at pH 9. However, after functionalization with ICPTES, the values were between +11 mV and –13 mV, under a wide range of pH conditions (acidic and basic media). The value increased to +25 mV, as the L-arginine molecules attached to the MSNPs surface were positively charged at a pH below 3 (pKa of carboxylic acid). The zeta potential was more negative at a pH above 11, due to the deprotonations of the highly ionic functional groups. Therefore, the changes observed in the zeta potential indicated that the functional groups attached successfully on the MSNPs surface.

The textural properties of CTAB free MSNPs and Ar-MSNPs were measured by N₂ adsorption/desorption using an Autosorb-iQ instrument (Quantachrome). The selected samples were heated at 120 °C and degassed for 6 h under flow with N₂ prior to measurement. The BET surface areas, pore volume, and pore size of both samples were quantified, and the results are presented in

Table 1. Physical properties of CTAB free MSNPs and Ar-MSNPs obtained from N₂ adsorption/desorption studies.

	BET Surface Area (m2/g)	Pore Volume (cm3/g)	Pore Diameter (nm)
CTAB free MSNPs	931	1.29	5.6
Ar-MSNPs	627	0.95	4.8

. The pore diameters and surface areas were slightly decreased 1.3- and 1.14-fold, respectively, due to the formation of a thin organic layer on the surface of the mesoporous structures. Figure 4A shows the BJH pore size distribution (uniformed). The pore volume was also decreased after surface modification due to pore filling with the propyl terminated L-arginine. The nitrogen adsorption/desorption isotherms of the samples indicated a IUPAC type IV isotherm, corresponding to the mesoporous structure, as shown in Figure 4B. A different capillary condensation between CTAB free MSNPs and Ar-MSNPs was observed at higher relative pressures. The hysteresis loop was broader for non-modified MSNPs, compared to Ar-MSNPs, indicating the successful attachment of the propyl terminated L-arginine.

The scanning electron microscope (SEM) images of CTAB free MSNPs and Ar-MSNPs are shown in Figure 5A,B. Similar to what was reported by Alswieleh (2021), the diameters of the particles were in nanometres, with an average particle size of 260 nm. No difference was observed in morphology when the MSNPs surface was modified in the organic layer, in agreement with previously reported work. To further explore detailed information on morphology, transmission electron microscopy (TEM) was applied to estimate the particle size and observe the pore structure of the silica nanoparticles. The TEM images of CTAB free MSNPs and Ar-MSNPs are shown in Figure 5C,D. It can be clearly seen that the sphere surface of the samples was smooth and they contained pores in their structures. Both samples have a similar average particle size of 280 nm. The size of the majority of the pores was estimated to be between 5 and 6 nm.

3.2. Adsorption Studies

3.2.1. Effect of pH

The absorption properties of the selected dyes (crystal violet and fluorescein) are strongly dependent upon pH. Crystal violet (CV) has a relatively high stability in various pH values. CV has a positive value at a pH value above the pKa value (1) [37]. Fluorescein (Fl) has different forms (cation, neutral molecule, and dianion), depending on the pH environments [38]. The cationic form is the predominate species in a strongly acidic media. Fl is found in neutral species at a pH between 2 and 4, and the anion form is present at a pH above these levels.

A series of batch experiments were carried out to investigate the effect of pH on the dyes adsorption onto the Ar-MSNPs surface at two different concentrations: 40 ppm and 100 ppm for Fl and CV, respectively. These concentrations have been chosen based on the dye solubility. Based on our results, the adsorption of Fl and CV on Ar-MSNPs shows maximum values at pH 2 and 11, respectively (shown in Figure 6). The adsorption of the dyes could be explained by electrostatic attraction between the negatively charged deprotonated amino and carboxylic acid groups on the Ar-MSNPs surface at a pH above 9 and the positive charge of the CV. The maximum adsorption capacity of CV at pH 13 was ca. 138 mg.g⁻¹ at 298 K. Similar results have been reported for L-ascorbic acid-g-polyaniline mesoporous silica nanocomposite (113 mg.g⁻¹ at 333 K) [32]. Moreover, the surface of Ar-MSNPs may have been zwitterionically charged at pH levels higher than 4, which would decrease the number of Fl attracted to the adsorbent surface. The maximum adsorption capacity of Fl was estimated to be 32 mg.g⁻¹ at pH 2 and 298 K.

3.2.2. Adsorption Isotherms

The behavior adsorption was described using two models: the Langmuir and Freundlich models (Figure 7). The difference between these models is that Langmuir makes assumptions that the analytes are adsorbed on the surface of the adsorbent as a monolayer, and the maximum adsorption occurs when the surface is completely covered. Freundlich assumes that the surface is heterogeneous, and that different affinities and analytes are adsorbed as a multilayer adsorption. The correlation coefficients (R²) value was used to determine the suitable isotherm model. For both dyes, the Langmuir adsorption model was found to be the best fitting model. The values of Langmuir constants q_m and b were calculated from the slopes and intercepts of the lines, as shown in

Table 2. Langmuir parameters for the adsorption process at 298 K.

Dye	qe (Experimental) (mg.g−1)	qm (mg.g−1)	b (dm3.mg−1)	RL	R2
Fl	37	58	1.01	0.098	0.945
CV	128	201	0.41	0.087	0.951

. The separation factor (or equilibrium parameter) (R_L) is an essential feature of the Langmuir adsorption isotherm, describing the nature of the adsorption isotherm. R_L can be expressed by [39]:

$${\mathrm{R}}_{\mathrm{L}}=\frac{1}{1+\mathrm{b}{\mathrm{C}}_0}$$

If the value of R_L is equal to zero, then the adsorption is irreversible. The adsorption process is favourable if the R_L value is between 0 and 1, and unfavourable when R_L = 1. As presented in Table 2, the R_L values were found between 0 and 1, indicating that the adsorption of both dyes on the Ar-MSNPs is favourable under the conditions used in this study.

Moreover, the comparison of the non-linear form of the Langmuir and the Freundlich isotherm models, with experimental data for the sorption of dyes onto Ar-MSNPs at 25 °C, has been shown in Figure 8, and the nonlinear Langmuir isotherm parameters are summarized in

Table 3. Parameters of nonlinear Langmuir isotherm model for the adsorption process at 298 K.

Dye	qe (Experimental) (mg.g−1)	qm (mg.g−1)	b (dm3.mg−1)	R2
Fl	37	61	0.83	0.883
CV	128	183	0.85	0.978

. For both dyes, it was found that the Langmuir adsorption model showed a better fit to the experimental data than did the Freundlich model. Fitting the experimental data with the non-linear Langmuir isotherm had a lower coefficient of determination in the case of adsorption isotherms of Fl, compared to the linear method. The R² values differed, and the parameter values between the linear and nonlinear parameters were different. The Langmuir adsorption isotherm of CV showed a remarkable similarity between nonlinear and linear parameters.

3.3. Kinetics

3.3.1. Effect of Initial Concentration of Dye Solutions

To investigate the effects of the initial concentration of the dyes on the adsorptions, the concentration varied in the range of 10–40 ppm of Fl and 25–200 of CV at a constant adsorbent dosage of 10 mg, 23 °C and a shaking speed of 200 rpm. Figure 9A,B illustrates the change in the removal dyes percentage over the initial concentration. The amount of the adsorbent adsorbed on the Ar-MSNPs surface increased with an increasing initial concentration of the dyes. This may be explained by the increasing driving force between the solid phase and aqueous phase, and increasing the collisions between nanoadsorbent and dyes molecules. Generally, the amount of dyes removed by the Ar-MSNPs increased sharply and reached equilibrium in 20 min. At high concentrations, however, the amount of dye adsorbed on the surface increased gradually within 40 min and reached equilibrium after 50 min exposure time. This phenomenon could be interpreted by the fact that the adsorption sites on the nanoadsorbent were sufficient in the early adsorption stage. As time passed, sufficient interactions occurred between the adsorbent and the dyes, until all adsorption sites were occupied by dyes molecules [40].

3.3.2. Adsorption Models

The adsorption data can be fitted using kinetics models to determine the dynamics of dye adsorption on the nanoparticles surface with respect to the order of the rate constant. Such parameters can provide important information for designing the adsorption process [41]. Pseudo first-order, pseudo second-order, and intraparticle diffusion kinetics were applied to analyze the adsorption kinetic data, as described by Equations (5)–(7). The pseudo first-order and pseudo second-order kinetic models are useful to determine whether a physical or chemical adsorption process occurred on the surface. If the pseudo first-order model fits better with the result, then the adsorption process can be described as a physical process. If the pseudo second-order model provides better fit, then it is described as a chemical adsorption. Figure 10A,B shows the fit result with the pseudo first-order kinetics of Fl and CV, respectively, and Figure 9A,B shows the experimental values with the pseudo second-order kinetics of Fl and CV, respectively.

Table 4. Pseudo first-order and pseudo second-order parameters for the adsorption process at 298 K.

	Pseudo First Order
	k1 (min−1)	R2
Fl removed by Ar-MSNPs	−0.008	0.14
CV removed by Ar-MSNPs	−0.015	0.58
	Pseudo Second Order
	k2 (g.mg−1.min−1)	R2
Fl removed by Ar-MSNPs	0.130	0.98
CV removed by Ar-MSNPs	0.065	0.97

shows the pseudo first-order and pseudo second-order parameters for the adsorption process. According to the correlation coefficients (R²), the adsorption data for both dyes was better represented with the pseudo second-order model than with the pseudo first-order model.

Figure 10E,F shows the adsorption process presented by the intraparticle diffusion model. The first sharp line in the plot displays the adsorption on the external surface (the instantaneous adsorption stage). The larger slope of the first sharp line suggests that the dye adsorption rate was high at the initial time, due to the presence of plenty of active adsorption sites. The second line shows the slow adsorption process, indicating the intraparticle diffusion, or micro-sore diffusion. The lower slopes of the second sharp line suggested that the dye molecules take a long time to diffuse on the mesopores of the Ar-MSNPs, due to a decreased concentration profile [42].

To reduce costs and protect the environment, the regeneration of the adsorbents is essential. For this purpose, regeneration experiments have been performed in successive cycles. It was observed that the adsorption efficiency of the nanoadsorbent gradually decreases with an increasing recycle number of the adsorbent, as shown in Figure 11. The reduction in removal efficiency could be related to the reduction of adsorbent sites and the blockage of pores [43]. After four cycles, the removal efficiencies of Fl and CV were found to be 76 and 72%, respectively.

4. Conclusions

In the present work, experiments were performed to evaluate the use of mesoporous silica modified with L-arginine as an adsorbent for ionic dyes. In the first stage of the study, the adsorbent was fully characterized. The average particle size of the adsorbent was found to be ca. 260 nm. BET analysis shows that the surface area was relatively high (ca. 931 m²/g), with a pore size of 5 nm. The elements of carbon, hydrogen, and nitrogen were found to be increased after each modification step. The effects of operating parameters such as pH, exposure time, and initial dye dose on the sorption capacity of Ar-MSNPs were studied for both selected dyes. The experimental results suggested that the adsorption was influenced by pH solution, contact time, and the concentration of the dyes. The optimum pH of fluorescein and crystal violet were 2 and 13, respectively. It was found that the adsorption of both dyes onto Ar-MSNPs is a fast process, with an equilibrium time of 20 min at low concentrations. The adsorption equilibrium of both systems was described by the Langmuir isotherm. The adsorption kinetics indicated that the adsorption processes fitted well with the pseudo second-order model.