Nano-silica from white silica sand functionalized with PANI-SDS (SiO2/PANI-SDS) as an adsorbent for the elimination of methylene blue from aqueous media

Natural resources including sand are one of the best approaches for treating dye-polluted wastewater. The SiO2/PANI-SDS nanocomposite was synthesized by self-assembly and intermolecular interaction. The physicochemical features of the SiO2/PANI-SDS nanocomposite were explored by FT-IR, XRD, SEM, TEM, EDX, and N2 adsorption–desorption techniques to be evaluated as an adsorbent for the MB. The surface area of the SiO2/PANI-SDS is 23.317 m2/g, the pore size is 0.036 cm3/g, and the pore radius is 1.91 nm. Batch kinetic studies at different initial adsorbate, adsorbent and NaCl concentrations, and temperatures showed excellent pseudo-second-order. Several isotherm models were applied to evaluate the MB adsorption on the SiO2/PANI-SDS nanocomposite. According to R2 values the isotherm models were fitted in the following order: Langmuir > Dubinin–Radushkevich (D–R) > Freundlich. The adsorption/desorption process showed good reusability of the SiO2/PANI-SDS nanocomposite.


Experimental Chemicals
The natural raw silica sand material was obtained from the Abu Zenima area, Sinai Peninsula, Egypt.The methylene blue (MB) (C 16 H 18 ClN 3 S) dye was obtained from Sigma Aldrich and used as received.Aniline (C 6 H 5 NH 2 , 99.5%) from Sigma Aldrich was distilled twice before being used.Ammonium peroxydisulfate (NH 4 ) 2 S 2 O 8 98%) and sodium dodecyl sulfate, SDS, (CH 3 (CH 2 ) 11 SO 4 Na 99%) are from Sigma Aldrich and used without further purification.C 2 H 5 OH 99%, HCl 37%, H 2 SO 4 99%, NaOH 98%, and NaCl 99.5% were of analytical grade reagents from Sigma Aldrich.The structures of MB and SDS are shown in Fig. 1.The distilled water was the main solvent over the entire work.

Instrumental measurements
All materials were characterized by these instruments.FTIR spectra were recorded by JASCO FT-IR-4100 (Japan) spectrometer in the range of 4000-400 cm −1 .The XRD was obtained by Brukeraxs D8, Germany.A copper Ka radiation target with a wavelength of 1.54 Å was employed.The morphology of the composite was examined using a scanning electron microscope (SEM: JEOL and JSM-6510LV) and a transmission electron microscope (TEM: JEM-2100 JEOL (Japan).The energy dispersive X-ray spectroscopy (EDX) was analyzed by an IT100LA operating at an accelerating voltage of 20.00 keV that was attached to the SEM device.The XPS analysis was made by XPS (PHI Quantera SXM; ULVAC-PHI, Inc., Osaka, Japan).
The specific surface area was measured using the BET (Belsorb III equipment, Japan) with nitrogen as the adsorbate.The pH data were obtained with a Mettler Delta 320 pH meter.A UV/Vis double beam spectrophotometer (PG T80+, U.K.) was employed to monitor the progress of MB adsorption with time.The aniline (3 mL) was added to 10 mL of HCl (1 M) and stirred.Sodium dodecyl sulfate (SDS, 2 g) was dissolved in 50 mL of water.The SDS solution was added to the aniline solution and sonicated for 10 min.Ammonium peroxydisulfate solution (50 mL, 0.5 M) was added and magnetically stirred for 24 h to ensure complete polymerization of aniline.The obtained dark-green precipitate was filtered and rinsed several times with HCl (1 M), followed by water and alcohol before being dried in an oven at 60 °C for 12 h.These steps were repeated without using SDS to synthesize the pristine polyaniline (PANI), Fig. 1 13 .

Synthesis of silica nanoparticles (SN)
The production of silica nanoparticles proceeds through two steps.The white sand is first purified, and then the pure silica is converted into nanoparticles 10 .
Natural white sand purification Natural white sand (5 g) was washed several times with water to remove soluble contaminants.The sample was dispersed in HCl (5 M) under vigorous magnetic stirring at room temperature overnight.The sand was meticulously rinsed with sufficient water to eliminate the excess of HCl and the remaining free ions until pH 7. It was subsequently filtered and dried overnight in an oven at 100 °C.

Synthesis of silica nanoparticles (SN)
3 g of the pure silica and 5 g of NaOH pellets were crushed in a dry fusion procedure.The crushed material was fused for 4 h at 450 °C in a muffle furnace to form sodium silicate (Na 2 SiO 3 ), which was leached with water according to Eq. (1).The solution was filtered to eliminate any unreacted silicate residue.This procedure was conducted twice to obtain a clear and colorless solution.H 2 SO 4 (10 N) was then added to the Na 2 SiO 3 solution under constant agitation to bring the solution to pH 9 and form SiO 2 gel.Equation (2) indicates that the Na 2 SiO 3 was hydrolyzed and condensed to form silica gel.The silica gel was rinsed with cold and hot water multiple times.It was dried at 80 °C for 32 h followed by 3 h at 120 °C.The solid material was crushed into powder using a plenary ball mill at 400 rpm for 3 h to hinder particle accumulation.It was then calcined in air at 400 °C for 1 h.

Synthesis of SiO 2 /PANI-SDS
The SiO 2 /PANI-SDS nanocomposite was synthesized following the same procedure described in "Synthesis of PANI-SDS" section for synthesizing the PANI-SDS.Briefly, SiO 2 nanoparticles (1.6 g) were added to the aniline-HCl solution, sonicated for 15 min, and magnetically stirred for 24 h.The black precipitate was rinsed several times with water and alcohol and dried overnight at 70 °C, Fig. 2.

Procedure of MB adsorption
The MB adsorption run was conducted as follows: 1.06 mL of MB was withdrawn from its stock solution (300 mg/L) and transferred into an Erlenmeyer conical flask (100 mL), 48.8 mL of H 2 O, and 0.02 g of the SiO 2 / PANI-SDS nanocomposite were added, the total volume of the reaction mixture was 50 mL at pH 6.The flask was replaced into a shaking water thermostat at an agitating speed of 140 rpm and 30 °C.The time was noted at the moment when the nanocomposite was added to the reaction mixture.The kinetics measurements began immediately once the nanocomposite was added.Subsequently, 3 mL of the reaction mixture were withdrawn by a 0.45-μm Millipore filter to remove suspended particles, and the supernatant was transferred to a 1-cm quartz cell.The absorbance of the remained MB was then taken at λ max = 665 nm.The tests were repeated until no additional change in the absorbance was detected.Finally, the removal efficiency was evaluated using Eq. ( 3), the amount of MB adsorbed at equilibrium was calculated by Eq. ( 5).
(1) where C o is the initial concentration of MB, C e is the equilibrium concentration, and C t indicates its concentration at time t.The q t (mg/g) and q e (mg/g) refer to the adsorption capacities of the SiO 2 /PANI-SDS nanocomposite at time t (min) and equilibrium.

Reusability test
The previously used SiO 2 /PANI-SDS/MB nanocomposite was collected and separated from the medium by filtration.To explore the reusability of the SiO 2 /PANI-SDS nanocomposite toward the MB, the nanocomposite was suspended in 100 ml of ethanol-water mixture (30:70, mL) and stirred in a closed Erlenmeyer flask at 140 rpm for 12 h at 25 °C.Subsequently, it was centrifuged at 5000 rpm for 20 min, followed by repeated washing with water until a clear supernatant solution was obtained.Finally, the recycled SiO 2 /PANI-SDS nanocomposite was used in fresh adsorption runs under identical conditions.This procedure was repeated for six cycles.

Characterization of adsorbent
FT-IR Figure 3a,b depict the FT-IR spectra of PANI, PANI-SDS, SiO 2 , and SiO 2 /PANI-SDS nanocomposite, and the main unique bands of the four materials were assigned.The materials were explored in the range of 4000-400 cm −1 .For the pure PANI, the peaks at 3400, and 1290 cm −1 are attributed to the secondary aromatic amine's N-H and C-N stretching 14 .Meanwhile, the bands at 1470 and 1570 cm −1 are ascribed to the benzenoid and quinoid units 15 .The peak at 1108 cm −1 is attributed to the benzenoid ring's C-N stretching mode, and the peak at 795 cm −1 is designated to the plane bending vibration of C-H, which is formed during the protonation 16 .The bands at 612 and 504 cm −1 can be assigned to the aromatic C-H out of the plane bending vibrations 17 .The PANI-SDS composite was subjected to FTIR analysis to confirm the successful functionalization of PANI with the SDS.In the FTIR spectrum of the PANI-SDS, the outstanding absorption bands at 3375 and 2922 cm −1 are ascribed to the C-H stretching vibrations of the SDS and N-H of the PANI 18 .The bands at 2925 and 2024 cm −1 confirmed the presence of the C-H stretching mode on the long alkyl tail of SDS and S=O extending of the SO 3 group in the PANI-SDS.The peak at 1121 cm −1 is assigned to the vibration mode of the NH + structure,  showing the high degree of electron delocalization in the PANI-SDS and the strong interchain NH + -N hydrogen bonding 19 .The results indicate the successful formation of the PANI-SDS.The inclusion of SiO 2 nanoparticles and SDS into the PANI chains induced the shifting of some PANI-SDS nanocomposites.The SiO 2 /PANI-SDS nanocomposite spectrum was compared to those of the PANI, PANI-SDS, and SiO 2 to demonstrate its formation.The distinctive absorption of SiO 2 /PANI-SDS expresses a peak at 1585 cm −1 which is characteristic of the C=N in quinoid units and a peak at 3474 cm −1 for the N-H stretching.The peak at 3261 cm −1 belongs to the N-H stretching frequency of amine.The peaks at 495, 735, and 696 cm −1 are assigned to the Si-O bond, out of plane C-H bending, and out of plane C-C deformation vibrations in monosubstituted aromatic rings, respectively 20,21 .
The strong peak at 1095 cm −1 describes the skeletal vibrations of the Si-O-Si obtained from silica 22 .The small peak at 2371 cm −1 indicates that carbon dioxide is adsorbed in the SiO 2 /PANI-SDS nanocomposite 23 .In summary, the discrepancies in the FTIR spectra of the PANI, SiO 2 , PANI-SDS, and SiO 2 /PANI-SDS nanocomposite guarantee the successful formation of the novel SiO 2 /PANI-SDS nanocomposite.
The FT-IR spectra of the SiO 2 /PANI-SDS nanocomposite before and after MB adsorption are compared in Fig. 3c to obtain additional information on the interaction between the SiO 2 /PANI-SDS nanocomposite and MB molecules.Although some of the FT-IR data are almost identical, others are relatively strengthened and displaced.The identical peaks at 1581, 1092, 1093, and 1296 cm −1 reveal the in-degradability of the SiO 2 /PANI-SDS nanocomposite after MB adsorption 24 .The peak at 3474 cm −1 is shifted to 3445 cm −1 .It may be attributed to the stretching vibrations of the OH group.The novel peaks at 2922, 2856, and 2030 cm −1 correspond to the C-H asymmetric and symmetric stretching vibrations of the methylene blue molecules on the PANI/SDS/SiO 2 nanocomposite surface, including the C-H of the aromatic ring 25 .XRD X-ray diffraction is a non-destructive method that assesses the phase purity and crystallinity of inorganic materials.Figure 3d depicts the diffraction peaks in the 2θ range 10°-70° and shows the amorphous structure of the PANI/SDS/SiO 2 nanocomposite.The two peaks at 2θ = 19.38°and 25.47° correspond to the growth directions of (010) and (200) planes based on the JCPDS card no.53-1891.They are linked to similar periodicity of the PANI polymer chain 26 .The peaks at 2θ = 17°, 23.11°, and 28.13° refer to the PANI-emeraldine salt state 27 .The XRD pattern of the nanocomposite indicates that the peaks are highly intense and count for the high crystallinity in the backbone of the PANI polymer 28 .

BET
The surface area of the adsorbent material is critical to the extent of the adsorbent.It is therefore a quality characteristic.Following the IUPAC classification, the SiO 2 /PANI-SDS nanocomposite exhibits a type-IV isotherm with the usual H3 hysteresis loop of mesoporous materials, (Fig. 4a).The pore size distribution was also measured using the Barret, Joyner, and Halenda (BJH) method and is shown in Fig. 4b.The SiO 2 /PANI-SDS nanocomposite has a particular surface area of 23.317 m 2 /g, a pore volume of 0.035 cm 3 /g, and a pore radius of 1.91 nm 29 .SEM, TEM, and EDX SEM, TEM, and EDX analyses were performed to get more precise information on the particle morphology of the SiO 2 /PANI-SDS nanocomposite.The SEM image of the nanocomposite shows an asymmetrical microscopic scale with granular floccules, Fig. 5a.The TEM micrographs of the nanocomposite reveal a globular cluster of PANI matrix with SiO 2 particles that have particle size in the range from 8 to 15 nm as given in Fig. 5b.The EDX analysis indicates the existence of silicon (6.28%) and oxygen (26.28%) from SiO 2 , carbon (54.31%), sulfur (3.29%), and nitrogen (9.21%) from PANI/SDS in the nanocomposite, Fig. 5c.The results show that the SiO 2 / PANI-SDS nanocomposite is effectively constructed from the five main components, namely S, Si, C, O, and N 30 .

XPS
The chemical nature and the composition of the SiO 2 /PANI-SDS nanocomposite were evaluated by X-ray photoemission spectroscopy (XPS).The XPS survey scan given in Fig. 6 shows the existence of five major peaks for C 1s peak at 285. 38  The Si 2p spectrum could be deconvoluted into two subpeaks at 102.95 eV and 103 eV which could be attributed to the presence of different Si environments such as Si-O, and Si-Si from silica nanoparticles, respectively.After the adsorption of MB into SiO 2 /PANI-SDS nanocomposite, the XPS survey shows that the composition has a little changed ratio of carbon, oxygen, nitrogen, sulfur, and silicon into 63.39:20.76:7.21:2.06:6.58confirming the adsorption of MB on the surface of the SiO 2 /PANI-SDS nanocomposite.The peak deconvolution of each element is still the same indicating no changes in the functional groups of the nanocomposite.

Effects of adsorbent type
The present adsorption study determines which is the most active adsorbent among the SiO 2 , PANI-SDS, and SiO 2 /PANI-SDS nanocomposite for the removal of MB from aqueous solution.The removal efficiencies of these adsorbents were investigated under the same conditions, which were [MB] o = 6.36 mg/L, dose = 0.02 g, pH 6, T = 30 °C, and stirring speed 140 rpm, Fig. 7a.The results revealed that the removal efficiency of PANI-SDS, SiO 2 , and SiO 2 /PANI-SDS adsorbents are respectively 53.68, 84.18, 92.56% and their adsorption capacities (q e ) are 8.5, 13.39, and 14.97 mg/g, respectively.According to these results, it is clear that the SiO 2 /PANI-SDS nanocomposite is the most active adsorbent in the MB removal process, Fig. 7b.Hence, it was selected for the rest of the adsorption investigations.

Effect of dose
The quantity of nanocomposite in the system significantly impacts the adsorption rate and is critical for preventing the waste material from desorbing after reaching equilibrium.The effect of nanocomposite dose was evaluated in the range from 0.005 to 0.04 g at a constant concentration of MB (6.36 mg/L), pH 6, and 30 °C with a constant stirring speed of 140 rpm.The results revealed that the SiO 2 /PANI-SDS nanocomposite can remove MB at a low dosage implying that it has adequate active sites for MB adsorption even if present in a small amount.Furthermore, the results showed that on increasing the nanocomposite dose from 0.005 to 0.04 g, the removal efficiency of MB raised from 28.39 to 93.52% within 14 min, Fig. 8a.This can be attributed to the increasing surface area of the nanocomposite and consequently the increasing number of adsorption sites that are available for MB adsorption.Such an increase in the removal efficiency of MB with the increasing dose continues rapidly www.nature.com/scientificreports/until all the MB molecules get adsorbed on the nanocomposite surface.At this stage, the removal efficiency reaches an almost constant value at 0.04 g, indicating no more adsorption takes place 31 .

Effect of initial dye concentration
The effect of initial MB concentration was investigated at a constant dose of SiO  Therefore, the uptake of the molecules is fast and then the removal efficiency is high.Further, with the gradual increase in the MB concentration, the adsorption sites get occupied more and more with the MB molecules until the surface reaches the saturation state.In this case, the adsorption process of MB on the SiO 2 / PANI-SDS nanocomposite declines hence the removal efficiency is dropped.

Effect of pH
The pH of the solution substantially impacts the adsorption process because it regulates the adsorbent surface charge, the magnitude of dye ionization, and the dissociation of functional groups on the adsorbent.The pH was varied from 3 to 12 to assess the effect of pH on MB adsorption.As seen in Fig. 9a, the increase in solution pH enhances the MB removal efficiency.The removal efficiency was maximum at pH 12 and minimum at pH 3.
The law efficiency at pH 3 may be elucidated in two aspects.The first may be due to the protonation of the SiO 2 / PANI-SDS surface by the H + ions present in solution, which enhance the electrostatic repulsion between the surface and the cationic MB molecules.The second may be linked to the competition between the H + present in the medium and the positively charged MB molecules for the active adsorption sites of the nanocomposite.On increasing the pH, the removal efficiency increases owing to the increase in the negative charge density created on the surface, which in turn results in an electrostatic attraction between the SiO 2 /PANI-SDS surface and the  www.nature.com/scientificreports/positively charged MB molecules.Given these results, it seems that the electrostatic interaction taking place between the SiO 2 /PANI-SDS surface and the MB molecules over the entire investigated range of pH (3-12) is playing a predominant role in the adsorption process 32 .

Effect of salt
The effectiveness of the adsorption sites, the solubility, and the hydrophobicity of the dye are all impacted by co-existing salt, making it a vital component to investigate in the adsorption tests.The effect of NaCl on MB adsorption was evaluated at various NaCl concentrations ranging from 0.175 to 1.75 mg/L.When the NaCl concentration was increased from 0.175 to 1.75 mg/L, the MB removal efficiency dropped from 81 to 66% within 14 min, Fig. 9b.The reduction in the electrostatic contact between the MB molecules and the SiO 2 /PANI-SDS surface may be the cause behind the drop in removal efficiency.Furthermore, the Na + ions and the cationic MB molecules compete for the active sites of SiO 2 /PANI-SDS, and the Na + might comfortably occupy the active sites of the adsorbent faster than the MB molecules.This may be due to the smaller ionic radius of Na + than that of the MB.Therefore, some of the MB molecules cannot reach the SiO 2 /PANI-SDS surface and then the removal efficiency is decreased 33 .

Effect of temperature
The influence of temperature on MB adsorption was investigated since it is one of the most significant elements affecting the adsorption rate.The effect of temperature was investigated at variable temperatures ranging from 291 to 323 K and fixed [MB] o at 6.36 mg/L, pH at 6 rpm at 140, and SiO 2 /PANI-SDS dosage at 0.02 g.The results showed that as the temperature was elevated from 291 to 308 K, the MB adsorption increased from 67 to 89%, then declined slightly, Fig. 10a.The viscosity of the MB solution was lowered as the temperature was raised allowing the MB molecules to flow readily and migrate faster toward the nanocomposite surface.Therefore, the removal efficiency is increased.Furthermore, the slight decrease in the MB removal efficiency beyond 313 K may be due to the reduction of the attraction forces between the MB molecules and the active sites on the nanocomposite surface, resulting in a loss in the adsorption capacity 34 .As the temperature was raised, the volume of dissolved gas in the reaction mixture expanded, creating gaps between the MB molecules and the active sites of the SiO 2 /PANI-SDS.These gaps obstructed the MB movement to the surface of the SiO 2 /PANI-SDS nanocomposite.However, some of the MB molecules could not reach the nanocomposite surface and consequently, the removal efficiency is decreased 35 .

Activation parameters
The rate of most chemical reactions increases as the temperature increases.Therefore, the Arrhenius equation determines the activation energy (E) and the frequency factor (A) of the adsorption process.The Arrhenius equation is where k is the pseudo-second-order rate constant (g mg −1 min −1 ), A is the Arrhenius factor, E is the activation energy, R is the ideal gas constant (8.314J mol −1 K −1 ), and T (K) is the absolute temperature of the adsorption medium.The E and A values were obtained from the slope of ln k vs 1/T, Fig. 10b.The value of activation energy (E) shows whether the adsorption process of MB on the SiO  where, K (mg/g) is the standard thermodynamic equilibrium constant, T (K) is the absolute temperature, and R (J/mol K) is the gas constant.By graphing the linear plot between ln K vs. 1/T, the ∆S° and ∆H° values can be estimated, Fig. 10c.The thermodynamic parameters are calculated and presented in Table 2.The negative value of Gibbs free energy ∆G° (− 23.08 kJ/mol) suggests that MB adsorption occurs spontaneously and feasibly.The positive value of ∆S° (192.35J/mol K) shows the improved randomness at the interface of the SiO 2 /PANI-SDS nanocomposite and the MB solution during the adsorption process.In contrast, the positive value of ∆H° (49.75 kJ/mol) suggests that the MB adsorption process was endothermic 37 .

Adsorption kinetics
Important details about the adsorption mechanism can be revealed from the adsorption kinetics.Non-linear kinetic models like pseudo-first-order and pseudo-second-order were applied to analyze the MB adsorption kinetics on the SiO 2 /PANI-SDS nanocomposite.Based on an error study that include calculating the non-linear chi-square (χ 2 ) and the Root Mean Square Error (RMSE), the best kinetic model was selected.The best model has an R 2 value that is close to 1, a χ 2 value that is close to 0, and a lower RMSE.The estimated coefficients are displayed in Table 3 39 . (

Pseudo-first-order model
The pseudo-first-order non-linear form of the model is expressed as where q t (mg/g) and q e (mg/g) are the adsorption capacities of the SiO 2 /PANI-SDS nanocomposite at time t (min) and equilibrium.k 1 (min −1 ) is the pseudo-first-order rate constant.To calculate values of k 1 and q e, q t was plotted against t and illustrated in Fig. 11a.Based on the results, the value of q e. cal has a significant disparity from the relevant value of q e.exp .Additionally, most adsorption data have low correlation coefficient (R 2 ) values as given in Table 3.Therefore, it was concluded that the MB adsorption process onto SiO 2 /PANI-SDS did not fit well with the pseudo-first-order kinetics 38 .
(9) q t = q e 1 − e −k 1 t ,  The non-linear form of this model is represented by, k 2 (g/mg) is the pseudo-second-order rate constant.k 2 and q e can be obtained by graphing q t against t as depicted in Fig. 11a.The results obtained from all concentrations of MB show high values of the R 2 , and the q e.cal values are very near to the q e.exp values, Table 3.According to these findings, the pseudo-second-order kinetic model is the best model that describes well the MB adsorption on the SiO 2 /PANI-SDS nanocomposite 39 .

Intra-particle diffusion model
The intra-particle diffusion model developed by Weber and Morris 40 was applied to investigate the kinetics of MB adsorption on the SiO 2 /PANI-SDS surface.This model is expressed in the following equation.
where, c (mol/g) is a constant associated with the thickness of the boundary layer, and k p (mol/g min 0.5 ) refers to the intra-particle diffusion rate constant.Values of these constants determined from the slope and intercept of the q t vs. t 0.5 plot shown in Fig. 11b are provided in Table 4. Based on this model, if a straight line is produced by plotting q t vs. t 0.5 , the intra-particle diffusion is involved in the adsorption process.However, when this line intersects the origin, the intra-particle diffusion is the only rate-limiting step in the adsorption process 41 .Indicating that intra-particle diffusion was not the only rate-limiting step and that the MB adsorption process involved three steps rather than one, the straight segments in the plot do not pass through the origin.The first segment is attributable to the diffusion of MB molecules through the film to the SiO 2 /PANI-SDS exterior surface (film diffusion), and it is characterized by k p1 .The second segment is due to the diffusion of MB molecules within the SiO 2 /PANI-SDS pores (intra-particle diffusion), measured as k p2 .The final segment depicts the slow adsorption of MB molecules into the interior particles of the SiO 2 /PANI-SDS.Consequently, the adsorption of MB decreases until it reaches an equilibrium state at the nanocomposite/liquid interface.Furthermore, values of adsorption rates of the three stages showed that the MB adsorption process was originally faster and subsequently slowed down with time 42 .

The Elovich model
Elovich's empirical model applies to a broad range of adsorption systems with heterogeneous adsorbent surfaces and is frequently valid for chemisorption kinetics, such as electron exchange in the liquid phase.This model relies on the adsorption site energy heterogeneity in the rectangular distribution form 43 .The Elovich nonlinear equation is given as, where α is the initial adsorption rate, (mg g −1 min −1 ), and β is the desorption constant and denotes the surface coverage extent and energy of activation for chemisorption (g mg −1 ). Figure 11c depicts the nonlinear plots of q t vs. t.The constants α and β were determined and are presented in Table 4.The model fit well the experimental data, indicating that the MB adsorption was chemical on the SiO 2 /PANI-SDS surface.Since the values of α are greater than those of β, then the adsorption process is feasible and has irreversible feature 44 .

Adsorption isotherm
Adsorption isotherms explain how the adsorbent interacts with the adsorbate during the equilibrium state of the adsorption process.It also depicts the distribution of adsorbate molecules between liquid and solid.It is critical to fit the experimental data into several isotherm models to characterize the adsorption phenomenon.Thus, the adsorption of MB onto SiO 2 /PANI-SDS was studied using the most commonly used isotherm models, Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) 45 .
(11) q t = k p t 0.5 + c, (12)     The model illustrates the existence of a homogenous adsorption process and assumes that the adsorbate molecules will adsorb in a monolayer on the adsorbent surface.According to the Langmuir theory, there are a set number of equally energetic sites on the adsorbent surface, the adsorbent molecules do not interact with the other molecules, and each adsorbent site can only store one adsorbate molecule.Hence the adsorption is a monolayer adsorption 46 .The Langmuir equation is, C e (mg/L) represents the concentration of MB in solution at equilibrium.q e (mg/g) is the adsorption capacity of MB at equilibrium, q m is the maximum adsorption capacity, and K L is the Langmuir constant.Figure 12a shows the plot of C e /q e vs. C e from which the values of K L and q m are determined.These values are summarized in Table 5.In addition, the dimensionless factor R L was also calculated to evaluate the feasibility of the MB adsorption on the SiO 2 /PANI-SDS surface.The factor is defined by Eq. ( 12). ( 13)  From the R L value, the features of MB adsorption can be determined.If 0 < R L < 1, the adsorption is favorable, R L > 1 it is unfavorable, linear at R L = 1, and irreversible when R L = 0. Since R L = 0.16 in the present study it then verifies the favorability of MB adsorption on the SiO 2 /PANI-SDS surface.

The Freundlich isotherm model
Unlike the Langmuir isotherm model, the Freundlich model describes the adsorption characteristics of the multilayer and heterogeneous surfaces.It is assumed that the strongest active sites are first occupied.Subsequently, the binding strength of the active sites decreases with increasing occupation 47 .Its mathematical expression is given in Eq. ( 13).K F (mg g −1 ) is the Freundlich constant which refers to the intensity of adsorption while 1/n denotes the favorability of adsorption.Both constants are determined from the slope and intercept of the plot of log C e vs. log q e shown in Fig. 12b.In the case of 0 < 1/n > 1, the adsorption is favorable.When 1/n is zero, the isotherm is irreversible.If 1/n > 1, the isotherm is unfavorable 48 .In this system, the value of 1/n is 0.313, thus indicating that MB has favored adsorption on the surface of the SiO 2 /PANI-SDS (Table 5).

The Dubinin-Radushkevich (D-R) isotherm model
The D-R model was developed to deal with the vapor adsorption on the solids with micropores via the porefilling mechanism.Subsequently, it was developed based on the Polanyi theory and the assumption that the pore distribution in the adsorbent follows the Gaussian energy distribution to describe the adsorption on the heterogeneous surface.Unlike layer-by-layer adsorption on pore walls, the adsorption occurs by filling the micropore volume.Because it distinguishes between physical and chemical adsorption and does not need a homogeneous surface or a constant adsorption potential, the D-R isotherm model works better than the Langmuir and Freundlich models 49 .The following equations provide the linear form of the isotherm.
where, q e (mg/g) is the amount of MB adsorbed at equilibrium, ε (kJ/mol) is the Polanyi sorption potential, and q s (mg/g) is the theoretical saturation capacity of the SiO 2 /PANI-SDS nanocomposite.The constant β is related to the adsorption energy (E) as given in Eq. (18).The slope and intercept of the ln q e vs. ε plot were used to determine the q m value (Fig. 12c).In contrast the mean free energy (E) of adsorption is given as, E (kJ/mol) is the energy required to remove the molecules from the SiO 2 /PANI-SDS surface.Therefore, if E < 8, the adsorption is physical and it is chemical adsorption when 8 < E < 16 kJ/mol 50 .Since the estimated value of E = 7.63 kJ/mol (Table 5), demonstrates that MB was physically adsorbed on the surface of SiO 2 /PANI-SDS.

Reusability of the nanocomposite
The regeneration process is crucial in developing economical and cost-effective adsorbents.The results revealed that the SiO 2 /PANI-SDS nanocomposite has low adsorption capacity at low pH.Therefore, acid treatment might be an appropriate approach for the nanocomposite regeneration.Figure 13 depicts the adsorption efficiency of the SiO 2 /PANI-SDS nanocomposite throughout six successive cycles of the MB adsorption-desorption process.( 15) [MB] o = 6.36 mg/L, dose = 0.02 g, pH 6, at 30 °C.
The results reveal a depletion in the adsorption efficiency of the nanocomposite across the cycles.Its activity decreased from ~ 92 to 71% along the first three cycles and from ~ 66 to 47% over the next three cycles.Taking into account this behavior of the SiO 2 /PANI-SDS/MB system it can be concluded that such nanocomposite still has a potential for large-scale application in the depollution of wastewater with low costs 51 .

Adsorption mechanism
To predict how the MB molecules, get adsorbed on the surface of the SiO 2 /PANI-SDS nanocomposite, it is important to identify the active functional groups on the nanocomposite surface that can interact with the MB molecules.These active functional groups were identified by the FT-IR and XPS analysis.From the FT-IR results, the nanocomposite has S=O, Si-O -, and -NH + -, demonstrating a high degree of electron delocalization on the SiO 2 /PANI-SDS and a strong interchain NH + -N-hydrogen bonding.The XPS data of the deconvoluted peaks of Si, S and O atoms also confirm the existence of Si-O, S-O, H-O, and Si-NO functional groups that are associated with the nanocomposite surface.Therefore, the adsorption of MB by the nanocomposite may proceed via hydrogen bonding and electrostatic interaction.The electrostatic interaction is formed between the Si-O, Si-NO, and S-O groups of the nanocomposite, and the -N + -of the dimethylamino group in the MB 52 .The intermolecular hydrogen bond in the form of -N…H-O-forms between the -N-of dimethylamino group of the MB and the Si-O, Si-NO, and S-O on the nanocomposite.At pH 3, the nanocomposite exhibits very low removal efficiency relative to that observed at higher pH, (Fig. 8a).At low pH, the ionized H + ions in the reaction medium tend to bind via an electrostatic interaction with the Si-O, Si-NO, and S-O moieties of the nanocomposites surface faster than the cationic MB + leading to a decrease in the removal efficiency.Above pH 3, the removal efficiency increases with increasing the pH, suggesting that the Si-OH silanol groups becomes more deprotonated to generate the Si-O -.The Si-O -groups increase more and more in the reaction medium in addition to the Si-NO and S-O groups of the nanocomposite 53 .These groups possess strong electrostatic interactions with the -N + -centres of MB molecules.Hence the removal efficiency is enhanced.A proposed mechanism for the MB adsorption onto the SiO 2 /PANI-SDS nanocomposite is illustrated in Fig. 14.

Conclusion
The current study demonstrated the successful synthesis of a novel SiO 2 /PANI-SDS nanocomposite and assessed its efficiency for MB removal by batch mode experiment.At pH 6, 6.36 mg/L of MB, and 2.5 g/L of SiO 2 /PANI-SDS dose, the SiO 2 /PANI-SDS nanocomposite can remove ~ 92.5% of the MB within 60 min.The initial MB concentration, nanocomposite dose, pH of the solution, presence of NaCl, and temperature had a great influence on the adsorption process.The removal percentage increased with increasing the dose, pH, and temperature while decreasing with increasing the dye and NaCl concentrations.The kinetic study indicated that the MB adsorption follows the pseudo-second-order and Elovich models.The adsorption process was spontaneous, favorable, and endothermic.It followed Langmuir isotherm with a good fit and maximum adsorption capacity of 24.9 mg/g.According to the Dubinin-Radushkevich isotherm, the micro pores of the SiO 2 /PANI-SDS were first filled with the MB molecules instead of the layer-by-layer adsorption.After four adsorption/desorption cycles, the efficiency of the generated SiO 2 /PANI-SDS nanocomposite only decreased by about 26%.Because of these outstanding properties of the SiO 2 /PANI-SDS nanocomposite, it can thus be considered a cost-effective adsorbent for wastewater treatment.

Consent to participate
All the authors are agreed to participate in this work.

Figure 4 .
Figure 4. (a) N 2 adsorption-desorption isotherms at 77.35 K. (b) BJH Pore size distribution and the insert is the multi-point plot of SiO 2 /PANI-SDS nanocomposite.

Figure 8 .
Figure 8.(a) Dependence of removal efficiency on the SiO 2 /PANI-SDS dose for the removal of MB at [MB] o = 6.36 mg/L, (b) removal efficiency as a function of initial concentration of MB with adsorbent 0.02 g.Conditions: t = 14 min, pH 6, T = 30 °C.