A Novel P@SiO2 Nano-Composite as Effective Adsorbent to Remove Methylene Blue Dye from Aqueous Media

This work aims to prepare a novel phosphate-embedded silica nanoparticles (P@SiO2) nanocomposite as an effective adsorbent through a hydrothermal route. Firstly, a mixed solution of sodium silicate and sodium phosphate was passed through a strong acidic resin to convert it into hydrogen form. After that, the resultant solution was hydrothermally treated to yield P@SiO2 nanocomposite. Using kinetic studies, methylene blue (MB) dye was selected to study the removal behavior of the P@SiO2 nanocomposite. The obtained composite was characterized using several advanced techniques. The experimental results showed rapid kinetic adsorption where the equilibrium was reached within 100 s, and the pseudo-second-order fitted well with experimental data. Moreover, according to Langmuir, one gram of P@SiO2 nanocomposite can remove 76.92 mg of the methylene blue dye. The thermodynamic studies showed that the adsorption process was spontaneous, exothermic, and ordered at the solid/solution interface. Finally, the results indicated that the presence of NaCl did not impact the adsorption behavior of MB dye. Due to the significant efficiency and promising properties of the prepared P@SiO2 nanocomposite, it could be used as an effective adsorbent material to remove various cationic forms of pollutants from aqueous solutions in future works.


Introduction
Many hazardous materials, such as heavy metals, dyes, drugs, pesticides, etc., have been discharged into the aquatic environment. This water pollution has become a severe universal subject and attracts attention worldwide from researchers, politicians, and simple people. Dyes are one of the more industrial effluents and are heavily used in several industries, such as food, wood, leather, paper, silk, etc. Discharging dyes in the aquatic sphere, even at low concentrations, will harm all living organisms that live in water, animals, and humans, where it is a toxic, carcinogenic, mutagenic, and non-degradable materials that can stay in the environment for a long time [1]. Methylene blue (MB) dye is the most famous water contaminant that badly impacts health through abdominal disorders, respiratory distress, skin sensitization, and blindness [2][3][4]. Also, methylene blue, with its in our present study, tri-sodium phosphate (Na 3 PO 4 ), an inexpensive and valuable material, was grafted into silica nanoparticles through a simple and green hydrothermal route. Trisodium phosphate and sodium silicate were dissolved in an aqueous solution, passed through highly acidic resin, and finally, hydrothermally treated to produce a white precipitate (P@SiO 2 nanocomposite) which will be used as an effective adsorbent to the methylene blue (MB). The characterization of the fabricated P@SiO 2 nanocomposite was obtained with various physicochemical techniques such as scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), elemental dispersive energy (EDX), and Fourier-transform infrared spectroscopy (FTIR). Also, the influences of the adsorption conditions, including P@SiO 2 nanocomposite dosage, initial MB dye concentrations, and solution pH, on the removal efficiencies were investigated. Also, the thermodynamic, kinetic and regeneration properties were studied.

Characterization and Analysis
The surface morphologies of the nanocomposite were detected using a scanning electron microscope (SEM, JEOL GSM-6610LV. Japan) operating at an acceleration voltage of 20 kV. The surfaces of the specimen were treated with a thin layer of gold before imaging. The dimension of the nanomaterial was measured using Image J software (Copyright 1998-2003 JEOL LDT) from the SEM captures in an original magnification of 30,000× and 50,000×. At least 25 isolated nanoparticles were randomly selected, and their diameters and diameter distributions were measured and averaged. The elemental analysis of the materials before and after the adsorption process was determined by the EDS unit connected with the SEM. The infrared spectrum of the nanoparticles was investigated by Fourier transform infrared spectrometer (FTIR, Shimadzu FTIR-8400 S, Japan) FTIR spectra. The infrared spectra were recorded in the transmission mode using nanomaterials mixed with KBr. The experiments were investigated in the range of 4000-400 cm −1 . The crystal structure of the composite was described by X-ray diffraction (XRD, Shimadzu, Japan XRD-7000) with a scanning speed of 12 • min −1 from 5 to 100 • .

Preparation of P@SiO 2 Nanocomposite
In a typical experiment, 9.0 g of di-sodium silicate and 2.0 g of tri-sodium phosphate were dissolved in 50 mL of double distilled water. After that, the previously prepared solution was loaded on a strong acid type of cation exchange resin column. The acidified solution was recovered from the column by elution. The eluted solution was charged into stainless steel autoclave reactor and placed in a muffle at 150 • C for 24 h. The resulting white powder centrifuge was washed several times with double distilled water and dried at 70 • C for 24 h. Figure S1 (in Supplementary Materials) shows the schematic diagram of the prepared powder.

Adsorption Studies
A methylene blue (MB) dye stock solution was investigated by stirring 1.0 g of the solid dye in 1.0 L of double distilled water, and the required concentrations were obtained by dilution. Batch removal experiments were prepared by stirring 10-25 mg of nanopowder (NPs) with 10 mL aqueous solution of methylene blue in 50 mL flasks at different concentrations (100-300 mg/L), pH (1.5-11), temperatures (25-80 • C), sodium chloride dose (0-2 g) and adsorbent tests take place at constant stirring speed. The nanopowder was isolated from the MB dye solution by centrifugation. The dye concentration residue was analyzed using UV-vis spectroscopy at λ = 664 nm. The dye removal percent, %R, can be measured by applying the Equation (1): where C o and C e are the initial and equilibrium concentrations of the liquid phase of the dye (mg/L), respectively.

Mathematical Modeling
Adsorption kinetic, isotherm, and thermodynamic models investigated in our study are explained in the Supplementary Material file (Sections S1.1-S1.3, respectively).

Characterizations
Scanning electron microscopy (SEM) was applied to study the surface morphologies of the prepared P@SiO 2 nanocomposite. The SEM images of typical phosphate-doped silica (Si-P) nanoparticles are shown in Figure S1a-d. The prepared nanoparticles are spherical and have a diameter range (85-173 nm). The dimensions of the P@SiO 2 nanoparticles were determined using image J software from the SEM captures in an original magnification of 30,000× and 50,000×. The average diameters and diameter distributions for 25 randomly selected isolated nanoparticles were measured and averaged, as represented in Figure 1. The data obtained showed that the morphologies of the synthesized P@SiO 2 nanocomposite are relatively homogenous.
was analyzed using UV-vis spectroscopy at λ = 664 nm. The dye removal percent can be measured by applying the Equation (1): where Cₒ and Ce are the initial and equilibrium concentrations of the liquid phase o dye (mg/L), respectively.

Mathematical Modeling
Adsorption kinetic, isotherm, and thermodynamic models investigated in our s are explained in the Supplementary Material file (Sections 2.4.1, 2.4.2 and 2.4.3, re tively).

Characterizations
Scanning electron microscopy (SEM) was applied to study the surface morphol of the prepared P@SiO2 nanocomposite. The SEM images of typical phosphate-dope ica (Si-P) nanoparticles are shown in Figure S1a-d. The prepared nanoparticles are s ical and have a diameter range (85-173 nm). The dimensions of the P@SiO2 nanopar were determined using image J software from the SEM captures in an original magn tion of 30,000× and 50,000×. The average diameters and diameter distributions for 25 domly selected isolated nanoparticles were measured and averaged, as represent Figure 1. The data obtained showed that the morphologies of the synthesized P@ nanocomposite are relatively homogenous.   FTIR spectra of silica and P@SiO 2 nanocomposite were investigated to obtain the basic information that illustrates the chemical structures of the prepared adsorbent material, as shown in Figure 2a.
FTIR spectra of silica and P@SiO2 nanocomposite were investigated to obtain the basic information that illustrates the chemical structures of the prepared adsorbent material, as shown in Figure 2a.  The common bands assigned to various vibrations of SiO 2 were observed. A broad band centered at around 3401 cm −1 corresponds to the stretching bands of the H-bonded H 2 O molecules in the interlayer [35]. The adsorbed water molecules show a bending vibrations band at 1597 cm −1 [35]. Two strong bands appear at 1016 cm −1 and 1136 cm −1 corresponding to the Si-O-Si asymmetric stretching vibrations [33]. Furthermore, the symmetric stretching and the bending mode vibrations of Si-O-Si appear at 738 cm −1 [39] and 405 cm −1 , respectively. Upon addition of phosphate to form Si-P, these peaks have a shift (405→467, 738→800, 1016→1066, 1136→1226, 1597→1646, and 3401→3465) [40]. A strong band at 1066 and 956 cm -1 , a characteristic of a PO 4 −3 group, was detected [41]. All of these mentioned spectral data prove that the P@SiO 2 nanocomposite was successfully prepared and has many active groups on the prepared P@SiO 2 nanocomposite surface that enhance the adsorption processes.
X-ray diffraction patterns were investigated to obtain information about the internal structures of SiO 2 and P@SiO 2 nanocomposites. The data obtained showed a broad peak at 2θ ≈ 20 • and broad peaks due to the amorphous nature of the synthesized P@SiO 2 nanocomposite, as represented in Figure 2b [42]. Also, other diffraction peaks were not observed at 2θ = 0.5-10 • , due to the exchanges of hydrated protons and cations between the interlayers [35].  Figure 2c. The diagram shows that the prepared P@SiO 2 powder consists of Si, O, P, and Na, as demonstrated in Figure 2c. After interaction with the MB dye, the introduction of C is detected in the analyzed powder (P@SiO 2 -MB) (as illustrated in Figure 2c). Moreover, to evaluate the influence of ionic strength on the adsorption of MB dye onto P@SiO 2 nanocomposite, the elements of Na and Cl were observed in the powder (P@SiO 2 -MB-NaCl). This result proves that NaCl does not has any influence on the adsorption process under the investigated conditions

Effect of Contact Time and P@SiO 2 Nanocomposite Dose
The effect of contact time on the adsorption percentage (%R) of MB dye was tested using various P@SiO 2 nanocomposite adsorbents mixed with a defined concentration of MB dye (10-25 mg/10 mL) at a period of 0.0-400 s. The removal percentages (%R) of the MB dye increased linearly with the time increase to 100 s ( Figure 3a). This is an attractive property of the promising P@SiO 2 nanocomposite and gives it added value to use as an effective and economical adsorbent material in wastewater treatment processes.
To study the effect of the P@SiO 2 nanocomposite dose on the uptake of the MB dye species, various weights of P@SiO 2 (10-25 mg/10 mL) were tested at constant other conditions ([MB] = 100 ppm, pH = 7, and T = 25 • C), as shown in Figure 3b. Also, it can be observed that the (%R) increased from 80.2 to 94.8%, with a further increase in the adsorbent dose from 10 to 25 mg, as illustrated in Figure 3b. This is attributed to increasing the number of active sites with an increase in the adsorbent dose, which enhances the removal percentage of MB dye molecules [43]. On the other hand, the increase in the adsorbent dose leads to a reduction in the amount of dye adsorbed to one gram of the adsorbent; this causes a decrease in q e value as the adsorbent dose increase [43], as shown in Figure 3b. Figure S2 represented the relation between q e vs. C e at different (a) P@SiO 2 nanocomposite doses and (b) initial MB concentrations.

Effect of Initial MB Dye Concentration
The effect of the initial MB dye concentrations on the removal percent (%R) and the equilibrium adsorption amount of the MB dye by P@SiO 2 nanocomposite from an aqueous solution was investigated (as in Figure 3c) using initial MB dye concentrations ranging from 100 to 300 mg L −1 at adsorbent dose = 25 mg/10 mL, pH = 7, and T = 25 • C. It was observed that the %R increased with the time of the initial concentration to 100 s. After that, the equilibrium state was reached, as shown in Figure 2c. Moreover, the adsorption capacity of MB dye (q e ) increases with a further increase in the initial dye concentration in the range (100-300 mg L −1 ), as presented in Figure 3d. The reason for this may refer to the increase in the MB dye concentrations causing improvement in the concentration gradient's driving force, which accelerates the MB dye species' diffusion velocity into the P@SiO 2 nanocomposite adsorbent particles [44]. Then, the concentration gradient reduces due to the adsorption of MB molecules on the active sites of the P@SiO 2 nanocomposite. using various P@SiO2 nanocomposite adsorbents mixed with a defined concentration of MB dye (10-25 mg/10 mL) at a period of 0.0-400 s. The removal percentages (%R) of the MB dye increased linearly with the time increase to 100 s ( Figure 3a). This is an attractive property of the promising P@SiO2 nanocomposite and gives it added value to use as an effective and economical adsorbent material in wastewater treatment processes. To study the effect of the P@SiO2 nanocomposite dose on the uptake of the MB dye species, various weights of P@SiO2 (10-25 mg/10 mL) were tested at constant other conditions ([MB] = 100 ppm, pH = 7, and T = 25 °C), as shown in Figure 3b. Also, it can be observed that the (%R) increased from 80.2 to 94.8%, with a further increase in the adsorbent dose from 10 to 25 mg, as illustrated in Figure 3b. This is attributed to increasing the number of active sites with an increase in the adsorbent dose, which enhances the removal percentage of MB dye molecules [43]. On the other hand, the increase in the adsorbent dose leads to a reduction in the amount of dye adsorbed to one gram of the adsorbent; this causes a decrease in qe value as the adsorbent dose increase [43], as shown in Figure  3b. Figure S2 represented the relation between qe vs. Ce at different (a) P@SiO2 nanocomposite doses and (b) initial MB concentrations.

Effect of Initial MB Dye Concentration
The effect of the initial MB dye concentrations on the removal percent (%R) and the equilibrium adsorption amount of the MB dye by P@SiO2 nanocomposite from an aqueous solution was investigated (as in Figure 3c) using initial MB dye concentrations ranging from 100 to 300 mg L −1 at adsorbent dose = 25 mg/10 mL, pH = 7, and T = 25 °C. It was In other words, at higher initial concentrations of MB, the binding sites of P@SiO 2 nanocomposite adsorbent were encompassed with many MB species in the solution. Hence, the adsorption capacity of the P@SiO 2 nanocomposite was enhanced by increasing the MB concentration, enhancing the adsorption capacity [45].

Effect of pH
The pH of the aqueous media is a very important factor in the adsorption process. The pH value affects the surface charge of the P@SiO 2 nanocomposite adsorbent and the degree of the ionization of the MB dye species. The effect of the pH was studied in the range 1.  Figure 4a. It was observed that the removal percentages were changed with further variation in the pH values as the following; at pH = 1.5, 3.0, 7.0, 9.0, and 11.0 the %R was 96.6, 67.1, 84.7, 99.7, and 95.3%, respectively. Consequently, the maximum adsorption capacities varied according to the pH values, as shown in Figure 4b.
degree of the ionization of the MB dye species. The effect of the pH was studied in the range 1.5-11 (by adjusting the pH with HCl (0.1 N) and NaOH (0.1 N) solutions) ([MB dye] = 150 ppm, [P@SiO2 nanocomposite] = 25 mg/10 mL, T = 25 °C) on the removal percent %R of the MB dye by P@SiO2 nanocomposite was performed in Figure 4a. It was observed that the removal percentages were changed with further variation in the pH values as the following; at pH = 1.5, 3.0, 7.0, 9.0, and 11.0 the %R was 96.6, 67.1, 84.7, 99.7, and 95.3%, respectively. Consequently, the maximum adsorption capacities varied according to the pH values, as shown in Figure 4b.  Therefore, with an increase in the pH value of 3-9, the active groups on the P@SiO 2 nanocomposite are deprotonated to carry negative charges, leading to electrostatic interactions with MB molecules increasing the removal percentages. At pH > 9, the removal percentage decreased, and this may be attributed to the -OH-anions of NaOH which can inhibit the electrostatic interactions between P@SiO2 nanocomposite and MB molecules by blocking the positive charges on MB molecules surfaces [46]. The color intensity of the MB dye sample varied according to pH value and is represented in Figure S3.

Effect of Temperature
The influence of the temperature, ranged from 25 to 80 • C, on the %R of the MB dye from aqueous media onto P@SiO 2 nanocomposite at pH = 7, [MB] = 150 ppm, [P@SiO 2 nanocomposite] = 25 mg/10 mL was investigated and is represented in Figure 4c. It was found that the %R of the MB dye increased with the time the temperature degrees were used overall. Consequently, the removal percentages decreased as the temperature increased, as illustrated in Figure 4c. Furthermore, as the dye solution temperature rose, the adsorbate's maximum adsorption capacity decreased [47], as plotted in Figure 4d. This can be attributed to the exothermic nature of the adsorption process.

Adsorption Kinetics
The kinetics of the MB dye adsorption on the P@SiO 2 nanocomposite for various P@SiO 2 nanocomposite doses were tested in contact times ranging from 0.0 to 7.0 min, and the results obtained are illustrated in Figure 5a,b. The calculated kinetic parameters are summarized in Table 1. As shown in Table 1, the correlation coefficient (R 2 ) related to the pseudo-second-order is higher than that obtained for the pseudo-first-order. Moreover, the calculated maximum adsorption capacity for the pseudo-second-order matches well with the experimental data. This indicated that the adsorption kinetics of various P@SiO 2 nanocomposite doses were described very well with the pseudo-second-order. For this model, it is suggested that the rate-limiting step might be chemical adsorption for the adsorption of MB dye onto P@SiO 2 nanocomposite [23]. The mechanism and kinetics of the removal of MB dye onto P@SiO 2 nanocomposite nanoparticles were evaluated by applying the data obtained from the dye concentration experiment using the pseudo-first-order (Equation (S3)) and pseudo-second-order models (Equation (S5)). Moreover, the kinetic parameters obtained from the linear plots (Figure 5c,d) of the two models were calculated and recorded in Table 2. Referring to the values of R 2 , the experimental data of the adsorption of MB dye on the P@SiO 2 nanocomposite showed a better fit with the pseudo-second-order models, which indicates that the dye species was chemically adsorbed and the adsorbent surface is the rate-limiting step [48]. the calculated maximum adsorption capacity for the pseudo-second-order matches well with the experimental data. This indicated that the adsorption kinetics of various P@SiO2 nanocomposite doses were described very well with the pseudo-second-order. For this model, it is suggested that the rate-limiting step might be chemical adsorption for the adsorption of MB dye onto P@SiO2 nanocomposite [23].  0.999 The mechanism and kinetics of the removal of MB dye onto P@SiO2 nanocomposite nanoparticles were evaluated by applying the data obtained from the dye concentration experiment using the pseudo-first-order (Equation (S3)) and pseudo-second-order models By plotting log (q e -q t ) versus t at different pH values of the MB dye solution (Figure 6a,b), the correlation coefficient (R 2 ), the first-order rate constant (k 1 ), and q e were calculated from the slopes and intercepts of the straight lines and listed in Table 3. Similarly, R 2 , k 2 , and q e related to the pseudo-second-order were calculated from the linear plot of the t/q t versus t at different pH values of the dye solution and recorded in Table 3. By comparing the values of R 2 related to the pseudo-first-order kinetic model with that in the case of the pseudosecond-order kinetic model, it can be observed that the R 2 in the pseudo-second-order kinetic model is higher than in the pseudo-first-order kinetic model. Moreover, the calculated q e value obtained from the pseudo-second-order kinetic model is closer to the experimental q e values. This indicates that the pseudo-second-order kinetic model best describes the adsorption kinetics rather than the pseudo-first-order model. To investigate the mechanism of the adsorption of MB dye at various temperatures, the Lagergren pseudo-first-order kinetic and pseudo-second-order kinetic models were applied (Figure 6c,d). The revealing parameters of the two models were evaluated and summarized in Table 4. According to the data obtained, it can be decided that the pseudo-second-order equation is the better-fitting model. This is due to it owning higher R 2 values [48]. In addition, the calculated maximum adsorption capacities, q e , from the pseudo-second-order model are close to the values of the experimental ones. This demonstrated that surface control mainly explains the adsorption processes rather than adsorbate diffusion. These results illustrated that chemical bonding or chemisorption between MB dye molecules and the active sites on the surface of P@SiO 2 nanocomposite might dominate the adsorption process, and this result agrees with what was reported in the literature [29].
pseudo-second-order kinetic model is higher than in the pseudo-first-order kinetic model. Moreover, the calculated qe value obtained from the pseudo-second-order kinetic model is closer to the experimental qe values. This indicates that the pseudo-second-order kinetic model best describes the adsorption kinetics rather than the pseudo-first-order model. To investigate the mechanism of the adsorption of MB dye at various temperatures, the Lagergren pseudo-first-order kinetic and pseudo-second-order kinetic models were applied (Figure 6c,d). The revealing parameters of the two models were evaluated and summarized in Table 4. According to the data obtained, it can be decided that the pseudosecond-order equation is the better-fitting model. This is due to it owning higher R 2 values [48]. In addition, the calculated maximum adsorption capacities, qe, from the pseudo-second-order model are close to the values of the experimental ones. This demonstrated that surface control mainly explains the adsorption processes rather than adsorbate diffusion. These results illustrated that chemical bonding or chemisorption between MB dye molecules and the active sites on the surface of P@SiO2 nanocomposite might dominate the adsorption process, and this result agrees with what was reported in the literature [29].

Adsorption Isotherm
The most famous isotherms used to describe the adsorption isotherm are Langmuir, Freundlich, and Tempkin isotherm expressions given by Equations (S6)-(S8), respectively.
Langmuir isotherm supposes a monolayer of the adsorbate adsorbed on homogenously active sites with the same adsorption energies. Moreover, once these sites are occupied, no more adsorption takes place. Langmuir constants Q • and b (Table 5) can be obtained from the slope and intercept of the linear plot of C e /q e versus C e as illustrated in Figure 7a. Dimensionless separation factor (R L ) for the MB dye adsorption onto the P@SiO 2 nanocomposite surface was determined from Equation (S6). If R L > 1, unfavorable; R L = 1, linear; 0 < R L < 1, favorable; R L = 0, irreversible (Figure 7b). It can be observed that the R L values between 0 and 1 indicate a favorable adsorption process.
The Freundlich isotherm suggested a heterogeneous surface with non-equivalent energetic binding sites. The Freundlich constants can be calculated by plotting ln q e versus ln C e (Figure 7c and Table 5). From the data in Table 5, 1/n < 1, which suggests a normal Langmuir isotherm.
The adsorption performance of MB dye onto P@SiO 2 nanocomposite was assessed by referring to the Tempkin isotherm model (Equation (S8)), and the linear relationship is plotted in Figure 7d. The correlation coefficient (R 2 = 0.940) shows the poorest fit to the experimental adsorption equilibrium data, as summarized in Table 5.  Dimensionless separation factor (RL) for the MB dye adsorption onto the P@SiO2 nanocomposite surface was determined from Equation (S6). If RL > 1, unfavorable; RL = 1, linear; 0 < RL < 1, favorable; RL = 0, irreversible (Figure 7b). It can be observed that the RL values between 0 and 1 indicate a favorable adsorption process.
The Freundlich isotherm suggested a heterogeneous surface with non-equivalent en- The obtained values of the correlation coefficient (R 2 ) (≈0.98) suggesting that the adsorption isotherm data of the adsorption of MB species onto P@SiO 2 nanocomposite fits better for both Langmuir and Freundlich isotherm models. Based on the closest values of the experimental and calculated Q max , the adsorption results of MB species onto P@SiO 2 nanocomposite were fitted well with the Langmuir model representing monolayer adsorption on homogeneous surfaces [17,26]. Hence, the MB species sequestration processes occurred at P@SiO 2 nanocomposite surfaces via the monolayer adsorption systems [26,47].

Adsorption Thermodynamics
Thermodynamic parameters were determined from the linear plot of Ln K c vis T −1 ( Figure S4) and according to Equations (S8)-(S10) and summarized in Table 6. The negative ∆G • values make it clear that the adsorption of MB dye on P@SiO 2 nanocomposite is a spontaneous adsorption process. Moreover, the decrease showed for the ∆G • values with the increasing temperature from 298 to 353 K, demonstrating the adsorption performance is favored at lower temperatures and the adsorption of MB species onto P@SiO 2 nanocomposite is a spontaneous process [26,35]. The ∆H • had a negative value confirming the exothermic nature of the adsorption process. The negative value of ∆S • suggests decreasing in the randomness of the solid/solution interfaces [26]. Activation energies, Ea, lower than 42 kJ/mol, suggest a diffusion-controlled mechanism, and higher than that value exhibits chemisorptions behavior. Here, the calculated value of E a is 45.3 kJ mol −1 ; this indicates that the adsorption of MB onto P@SiO 2 nanocomposite is a chemically controlled process. The data represented in Figure 4b confirms this result. Also, the P@SiO 2 nanocomposite illustrates excellent efficiency in adsorbing MB dye molecules from both acidic and alkaline media. Therefore, we can conclude that the P@SiO 2 nanocomposite will not be regenerated well. This could be considered an advantage where the MB dye molecules will be restricted from release again into the surrounding environment after adsorption due to the chemical bonding or chemisorption between MB dye molecules and the active groups on the surface of the P@SiO 2 nanocomposite. In practical application, studying the effect of NaCl dose is very important to evaluate the influence of ionic strength on the adsorption percent of MB dye onto P@SiO 2 nanocomposite. Here, we study the effect of NaCl dose in the range 0.00-2.00 g on the removal percent of MB dye, as shown in Figure S5. It was observed that the NaCl dose did not affect the adsorption percent of MB dye in the studied range, as shown in Figures S5 and S6.

Adsorption Mechanism
The P@SiO 2 nanocomposite was prepared by combining sodium silicate and sodium phosphate. Hence, the binding sites, which were responsible for the interaction with the MB species, were mainly composed of silanol and phosphate groups. Upon contacting with the positive MB species pollutants in the aqueous media, the negative binding sites P@SiO 2 nanocomposite interact with the MB species through ionic bond and oxygen lone pair sharing. The proposed adsorption mechanism is illustrated in Figure 8. The electrostatic attractions between the positive MB species and the negative active sites of P@SiO 2 nanocomposite mainly depends on the reaction pH [49,50]. In an acidic environment, the H-atom will convert Si=O into Si-OH which will enhance the activity of these groups towards interaction with the positive MB species. On the other hand, as the pH of the media increases, the ionization of P-O − Na + groups increase, which will increase the affinity of the P@SiO 2 nanocomposite towards the MB species. Also, the H-bonding becomes dominant in the adsorption mechanism and plays a vital role to improve the adsorption capacity [26,50]. P@SiO 2 nanocomposite and MB dye have many N-atoms; therefore, the adsorption capacities are enhanced due to the formation of N-H . . . N bonds between P@SiO 2 nanocomposite and MB species.

Comparison Study
A comparative evaluation of the maximum adsorption capacity of P@SiO2 nanocomposite to adsorb MB dye according to the Langmuir isotherm and other adsorbent materials in the literature is listed in Table 7  . Referring to the recent literature, equilibrium time and adsorption capacities are the main goals for scientists to investigate and develop many novel adsorbent materials. Comparatively, the prepared P@SiO2 nanocomposite illustrated considerable adsorption capacity compared to many novel materials and modified activated carbon materials. On the other hand, the obtained adsorption capacity of the prepared P@SiO2 nanocomposite was lower than some reported adsorbents; the major advantages of the prepared adsorbent over those reported materials were the rapid adsorption rate and easy recovery from aqueous solution after the adsorption process for reuse. Also, for the prepared P@SiO2 nanocomposite, the equilibrium time was reached rapidly within 100 s with an adsorption capacity of 76.9 mg/g. Therefore, the prepared P@SiO2 nanocomposite is suggested as an available, high-potential, and promising sorbent nanocomposite to remove MB dye from an aqueous media with considerable efficiency. Also, the P@SiO2 nanocomposite can meet commercial needs for water treatment applications.

Comparison Study
A comparative evaluation of the maximum adsorption capacity of P@SiO 2 nanocomposite to adsorb MB dye according to the Langmuir isotherm and other adsorbent materials in the literature is listed in Table 7  . Referring to the recent literature, equilibrium time and adsorption capacities are the main goals for scientists to investigate and develop many novel adsorbent materials. Comparatively, the prepared P@SiO 2 nanocomposite illustrated considerable adsorption capacity compared to many novel materials and modified activated carbon materials. On the other hand, the obtained adsorption capacity of the prepared P@SiO 2 nanocomposite was lower than some reported adsorbents; the major advantages of the prepared adsorbent over those reported materials were the rapid adsorption rate and easy recovery from aqueous solution after the adsorption process for reuse. Also, for the prepared P@SiO 2 nanocomposite, the equilibrium time was reached rapidly within 100 s with an adsorption capacity of 76.9 mg/g. Therefore, the prepared P@SiO 2 nanocomposite is suggested as an available, high-potential, and promising sorbent nanocomposite to remove MB dye from an aqueous media with considerable efficiency. Also, the P@SiO 2 nanocomposite can meet commercial needs for water treatment applications.

Conclusions and Future Perspectives
Here, we investigated a simple hydrothermal strategy to prepareP@SiO 2 nanocomposite to efficiently remove methylene blue dye from an aqueous solution. SEM, EDX, XRD, and FTIR techniques were employed to characterize the prepared nanomaterial. Various parameters that affected the adsorption process were investigated, such as preparing P@SiO 2 nanocomposite dose, MB dye concentration, pH, temperature, and NaCl dose in the kinetic study. An increase in the adsorbent dose leads to minimizing the amount of dye adsorbed to one gram of the adsorbent. While the adsorption capacity of MB dye (qe) increases with a further increase in the initial dye concentration. On the other hand, the maximum adsorption capacities of the MB dye have varied according to the pH values. Moreover, increasing the dye solution temperature will lead to a decrease in the maximum adsorption capacity of the adsorbate. Finally, NaCl at various doses does not affect MB adsorption. From the analysis of the experimental results, the pseudo-second-order was an excellent fit for the obtained data. Moreover, according to Langmuir isotherm, the P@SiO 2 nanocomposite shows excellent saturation capacity (76.92 mg g −1 ) which was suitable compared to other adsorbents in the literature. The thermodynamic studies showed that the adsorption process is preferred at low temperatures, exothermic, and ordered at the solid/solution interface. Also, the comparison study showed the promising properties and adsorption efficiency of P@SiO 2 nanocomposite compared with other adsorbent materials. Also, P@SiO 2 nanocomposite can be recommended as an eco-friendly absorbent material to purify wastewater from various cationic pollutants with significant efficiency in future works.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/ma16020514/s1. Figure S1: SEM images of as prepared Si-P nanoparticles; Figure S2: The relation between q e vs. C e at different (a) P@SiO 2 nano-composite doses and (b) initial MB concentrations; Figure S3: The color of MB-dye variation as a function of at time 5-420 s at different pH values; Figure S4: Thermodynamic plots for MB-dye removal from aqueous media by P@SiO 2 nano-composite; Figure