An efficient nanofiltration system containing mixture of rice husk ash and Fe/CeO₂–SiO₂ nanocomposite for the removal of azo dye and pesticide

Chemicals and reagents

Chemical used were Hydrochloric acid HCl (Analytical grade), Sodium hydroxide (NaOH), Cerium (III) chloride heptahydrate CeCl₃·7H₂O (Sigma Aldrich), Iron (III) chloride hexahydrate (FeCl₃·6H₂O), 3-(N,N-Dimethyloctadecylammonio) propane sulfonate: SB3-18 (Sigma Aldrich), MB (Sigma Aldrich), CP (Sigma Aldrich). All chemicals were used without any further purification. Rice husk was taken from Al-Hameed Mills, Gujranwala, Pakistan.

Synthesis of Fe/CeO₂–SiO₂ nanocomposite

Fe/CeO₂–SiO₂ nanocomposite was synthesized in three steps by sol–gel method followed by hydrothermal method. In first step SiO₂ was synthesized, in second step CeO₂–SiO₂ nanocomposite was synthesized than in third step Fe/CeO₂–SiO₂ nanocomposite was synthesized.

Synthesis of SiO₂ nanoparticles

Seventeen grams of Rice husk was taken and washed it with tape water thrice and then with distilled water to remove dust particles, contaminations, and water-soluble impurities then dried under shade at room temperature after that it was placed in an oven at 80 °C for 1 h. The dried material was refluxed in 250 mL of 10 % HCl solution for 2 h. It was washed with distilled water until pH 7 was obtained. Then it was air-dried and placed in the oven at 80 °C for 1 h. The dried rice husk calcined at 700 °C for 2 h to remove all hydrocarbons. After calcination, white crystals were obtained. These white crystals are called as RHA.

Two gram white crystals (RHA) were refluxed in 30 mL 10 % HCl solution for 2 h then centrifuged and washed with distilled water until pH 7 and dried in the oven at 80 °C for 18 h. Silica white ash was obtained.

About 0.5 g of silica ash was taken and refluxed for 2 h with 15 mL of 1 M NaOH solution to dissolve silica ash. A clear solution of sodium silicate (Na₂SiO₃) was obtained which then centrifuged at 8000 rpm for 1 min to remove non-reactive impurities. A clear solution of sodium silicate was obtained which is then neutralized with 1 N HCl solution added drop-wise until the gel was formed at pH 7. The gel was aged for 24 h allowing the silica to precipitate then 25 mL of distilled water was added to make the slurry. The slurry was centrifuged at 8000 rpm for 2 min then washed several time with distilled water, followed by drying at 80 °C for 22 h to remove the solvent. The obtained pellet was crushed; white powder of SiO₂ xerogel was obtained with a 95 % yield.

Synthesis of CeO₂–SiO₂ nanocomposite

About 0.06 mM of SB3-18 surfactant was taken in 5 mL of distilled water and sonicated for 30 min. Twenty millimolar of SiO₂ nanoparticles and 20 mM of CeCl₃·7H₂O was taken in 5 mL distilled water and was placed on stirring on a hotplate. After 30 min the surfactant solution was charged into the salt solution and stirred for 30 min. Forty millimolar NaOH solution was made in distilled water and added into the salt solution at a feed rate of 0.5 mL/5 min until pH 9 was obtained. It was then centrifuged at 12 000 rpm for 2 min and washed with several times with distilled water and then dried at 50 °C for 16 h. The color of obtained pellet was yellowish. The pellet was crushed into powder and then calcined at 600 °C for 4 h. The obtained powder was CeO₂–SiO₂ nanocomposite.

Synthesis of Fe/CeO₂–SiO₂ nanocomposite

Twenty-four millimolar of CeO₂–SiO₂ nanocomposite was stirred for 30 min. After that solution of 4 mM of FeCl₃·6H₂O was added into the nanocomposite solution and stirred for 30 min. The solution was then transferred into the Teflon-lined autoclave and placed in an oven at 180 °C for 3 h. Then the product was centrifuged at 13 000 rpm for 4 min and washed several times with distilled water until pH 7, dried at 80 °C for 16 h. The obtained pellet was crushed and calcined at 700 °C for 3 h. The calcined product was Fe/CeO₂–SiO₂ nanocomposite.

Preparation of slurry

Fifteen grams of rice husk was washed several times with distilled water to remove dust particles and other impurities. Then it was calcined at 700 °C for 2 h, 2 g of brownish-white needles obtained. These needles were ground in mortar and pestle and then distilled water was added in it to make the slurry. The NF column was packed in three different ways (Fig. 1).

Fig. 1:

Packing of column (a) with RHA slurry (b) with nanocomposite with two layers of RHA slurry (c) with nanocomposite mixed with RHA slurry.

Column with RHA slurry

In column, Fig. 1(a) firstly set a small ball of cotton at the bottom and then poured the mass of 1 g of ground RHA slurry into the column with a height of 3 cm.

Column with nanocomposite with two layers of RHA slurry

In column (b), firstly set a small ball of cotton at the bottom of the glass column. The slurry of a mass of 1 g of ground RHA was prepared. A solution of 10 mg of Fe/CeO₂–SiO₂ nanocomposite was made in 10 mL distilled water. Poured 0.1 g of the RHA slurry then 10 mg of Fe/CeO₂–SiO₂ nanocomposite and then the remaining 0.9 g of RHA slurry as shown in Fig. 1(b) into the column to make the bands at a height of up to 3 cm. Fe/CeO₂–SiO₂ nanocomposite makes a thin sandwich layer between two layers of RHA slurry.

Column with nanocomposite mixed with RHA slurry

In column (c), firstly set a small ball of cotton at the bottom of the glass column. In this method, the column was packed by mixing 10 mg of Fe/CeO₂–SiO₂ nanocomposite with the RHA slurry of 1 g of ground RHA and then poured the mixture of slurry and nanocomposite into the column up to 3 cm as shown in Fig. 1(c).

All the columns were rinsed thoroughly with distilled water to remove the dissolved materials and colored impurities if any. The absorbance of the eluates was taken at λ_max 665 nm after a specific time of intervals by Agilent Cary-60 UV–visible spectrophotometer. The temperature of 25 °C was maintained by passing water in a double jacket cooling array attached to the spectrophotometer. The percentage of degradation of MB was examined by eq. 1 [27].

where A_t is the concentration of MB at specific time interval (t) and A_o is the initial concentration of MB.

Removal of chlorpyrifos with nanofiltration column

Removal of CP pesticide was studied by Fe/CeO₂–SiO₂ nanocomposite in the NF column. For degradation, a stock solution of 5 ppm of CP was prepared in methanol and water with a ratio of 1:3. The prepared solution was passed through the same column (Fig. 1). The absorbance of the eluates was taken at λ_max 290 nm after a specific time of intervals. The percentage of degradation of CP was examined by using eq. 1.

Characterization

The functional groups analysis of synthesized Fe/CeO₂–SiO₂ nanocomposite was done by fourier transform infrared spectroscopy (FTIR) (model V MIDAC 2000) with KBr powder. The crystalline determination was done by using powder X-ray diffractometer (Xpert Pro Panalytical) at 2θ range 20–80° with 4°/min scan speed by using CuKα (0.154 nm) radiation reflection mood. The morphology and composition of nanocomposite was determined by FEI quanta 200 F scanning electron microscope (SEM-EDX). Optical properties of nanocomposite were determined by UV–visible spectrophotometer (model Agilent Cary-60). Thermogravimetric analysis (TGA) was determined at a temperature range of 0–1200 °C by TA instrument (TA instrument, SDT Q600).

Powder X-ray diffraction (XRD)

X-ray diffraction (XRD) pattern of Fe/CeO₂–SiO₂ nanocomposite in Fig. 2 showed that CeO₂ is present in cubic flourite crystal structure. The diffraction peaks located at 2θ = 28.7°, 32.6°, 47.4°, 56.5°, 69.4° and 76.7° with hkl value (111), (200), (220), (331), (400) and (331) corresponds to CeO₂, all peaks are perfectly indexed with JCPDS card no. 03-065-5923 [27]. Peak at 2θ = 78.090 attributed to (110) corresponds to cubic structure of Fe [28] matched with JCPDS card no. 00-006-0696. No other potential phase of impurity such as FeCeO₃, Fe₂O₃ was observed which is confirmed by the absence of peak at 24.2° and 35.3° [29]. Diffraction peak at 2θ = 58.4° with hkl of (110) with quartz structure of SiO₂. The crystal system of SiO₂ is hexagonal which is perfectly matched with JCPDS card no. 01-081-0066 [28].

Fig. 2:

XRD pattern of Fe/CeO₂–SiO₂ nanocomposite.

Lattice parameters and d spacing

For cubic crystal, d spacing values between adjacent planes in Miller indices (hkl) was calculated by Bragg eq. 2.

After rearranging eq. 2 becomes

Wurtzite lattice parameters a = b = c of cubic crystal was calculated by the following eq. 4.

as the CeO₂ and Fe are present in cubic crystallite form so it’s all parameters have the same value. For hexagonal crystal system wurtzite lattice parameters a = b and c calculated by the following eqs. 5 and 6.

The d spacing value was calculated by the following eq. 7

Volume of the unit cell was calculated from lattice geometry eq. 8

Value of angel φ between two nearby planes of d pacing of Miller indices h₁k₁l₁ and h₂k₂l₂ was calculated by the following eq. 9 [30].

All calculations are displayed in (Table 1).

Determination of crystallite size and strain

Crystallite size of nanocomposite was calculated from XRD by using Debye-Scherrer and Williamson-Hall equations. The crystallite size of the synthesized and calcined nanocomposite was calculated from highly intense peak by Debye-Scherrer eq. 10.

where D is the average grain size, K is a shape factor with a value of 0.9, λ is the wavelength of X-rays with a value of 1.54 Å, β is the full width at half maximum, and θ is the diffraction angel (in degree). Crystallite size calculated as 5.33 nm from most intense peak (111) by Scherrer’s equation.

Average crystallite size and strain can be calculated by Williamson-Hall equation. The induced strain in nanocomposite due to crystal imperfections and lattice distortions in lattice can be calculated by the following eq. 11.

The calculated strain in nanocomposite was 1.34.

By combining eqs. 10 and 11 we get

Multiplying the above equation with cos θ, we get [26]

where ɛ is the induced strain in the crystal, D is the crystallite size which will be determined from the Williamson-Hall plot by comparing it with the straight line equation (Y = mx + c), where m is the slop and c is the intercept. The average crystallite size calculated by intercept of plot between β cos θ/λ on Y-axis and 4 sin θ/λ on X-axis so we obtained crystallite size from slop and strain from the intercept as shown in Fig. 3. The average size calculated from intercept is 5.26 nm and induced strain was 0.0091. λ is the wavelength of X-rays with a value of 0.154 nm, β is the full width at half maximum and θ is the diffraction angel (in degree).

Fig. 3:

Williamson-Hall plot of Fe/CeO₂–SiO₂.

Determination of dislocation density (δ)

Dislocation density is defined as the extent of defects present as per unit volume of the crystal which is in direct relation with the hardness of the crystal so, as the decrease in crystallite size, dislocation density increases results in strain increases. Dislocation density is calculated by eq. 14 [31].

where, D is the crystallite size while δ is the dislocation density. The dislocation density of Fe/CeO₂–SiO₂ nanocomposite was 3.528 × 10⁻² (nm)⁻².

Specific surface area (SSA)

Specific surface area (SSA) is the area per gram of sample. SSA is used for determination of the type and properties of the material. SSA is calculated by eq. 15 [32].

where D is the crystallite size (nm) and ρ is the density. The SSA depends on the size of the nanocomposite, as the size smaller the SSA is larger. The SSA of Fe/CeO₂–SiO₂ nanocomposite is 645.54 m² g⁻¹.

Determination of bond lengths

Bond length of Ce–O and Si–O bond is calculated by eq. 16 [31].

where μ is the positional parameter, a and c are the lattice constants which is calculated by eq. 17.

The calculated bond length of Ce–O is 0.30 nm while the reported bond length of CeO₂ in literature is 0.23 nm [33]. Bond length of Si–O bond is 0.46 nm.

Optical bandgap investigation through solid phase spectroscopy

Optical properties of Fe/CeO₂–SiO₂ nanocomposite were determined by UV–vis absorption measurements. The sample was prepared by mixing 8 mg of the Fe/CeO₂–SiO₂ nanocomposite powder with two drops of aceton on the glass slide to make a thin film and then immediately covered with a cover slip and then with another glass slide and subjected for assessment of optical properties. UV–vis absorption spectra was obtained by extrapolating a straight line between (αhv)² vs. hv showed in Fig. 4 using Wood and Tauc eq. 18 for allowed direct transition [34].

where α is the absorption coefficient, hv is photon energy, E_g is the band gap energy and n is ½ for direct band gap in semiconductors. Optical absorption coefficient α can be calculated by eq. 19.

Fig. 4:

Band gap of Fe/CeO₂–SiO₂ nanocomposite inside is band gap of CeO₂–SiO₂ nanocomposite.

The band gap values of CeO₂–SiO₂ nanocomposite was 4.07 eV as shown inside Fig. 4. The band gap value was found higher than bulk CeO₂ (3.15 eV) [35] while lower than bulk SiO₂ (9 eV) [36]. Doping of Fe on CeO₂–SiO₂ nanocomposite decreases the band gap value up to 3.66 eV as shown in Fig. 4. The results showed that doped nanocomposite showed red shift due to quantum confinement effect. The decrease in band gap value was due to the transfer of electron from 3d orbital of Fe to the absorption edge of CeO₂–SiO₂ nanocomposite thus led to red shift in optical band gap of doped nanocomposite [28].

Scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX)

SEM results in Fig. 5(a) showed that Fe/CeO₂–SiO₂ nanocomposite has irregular symmetry having agglomerated structure.

Fig. 5:

(a) SEM image and (b) EDX spectra of Fe/CeO₂–SiO₂ nanocomposite.

In Fig. 5(b) EDX spectra confirmed the presence of Fe/CeO₂–SiO₂ nanocomposite. The results showed that CeO₂, SiO₂, and Fe are present uniformly. Experimental values found for Ce, Si, O, and Fe were 67.41, 10.69, 21.90, and 2.27 %, respectively.

Thermogravimetric analysis (TGA)

Analysis of uncalcined Fe/CeO₂–SiO₂ nanocomposite was done by TGA instrument, SDT Q600 which showed the TGA-DSC curve of Fe/CeO₂–SiO₂ nanocomposite as shown in Fig. 6. The TGA-DSC curved showed the weight loss with temperature. The nanocomposite was scanned from 0 to 1200 °C. This analysis was done to describe the thermal stability of the sample.

Fig. 6:

TGA-DSC curve of Fe/CeO₂–SiO₂ nanocomposite.

First weight loss was 3.2 % at 90 °C was due to the removal of water adsorbed by hydrogen bonding on the surface. Second was 2 % at 250 °C due the removal of water in the microspores of the nanocomposite [37]. Third was 4 % at 600 °C due to removal of water molecules coordinately bonded at the surface of nanocomposite [28]. Fourth was 1 % at 850 °C due to removal of water due to dehydroxlation at high temperature [38]. No weight loss is observed for surfactant because the surfactant was already removed due to calcination of CeO₂–SiO₂ nanocomposite and doped iron by hydrothermal method was present in zero oxidation state due to these only water loss was observed.

Fourier transform infrared spectroscopy (FTIR)

Figure 7 showed the FTIR spectrum of Fe/CeO₂–SiO₂ nanocomposite in the range of 650–4000 cm⁻¹. Absence of peak at 677 cm⁻¹ confirmed the presence of Fe in zero oxidation state [38]. Flat region at 806 [39] and 973 cm⁻¹ [40] confirmed the absence of Si–OH bond. Peak at 812 cm⁻¹ is due to the absorption of CeO₂ [41]. Peak at 1639 cm⁻¹ is due to the stretching and bending frequency of H–O–H [42]. Peaks at 1067 and 695 cm⁻¹ are due to the asymmetric stretching vibrations of Si–O–Si and symmetric stretching vibrations of Si–O–Si [43]. Peak at 2320 cm⁻¹ is due to the adsorption of CO₂ from air at the highly reactive surface of the nanocomposite. Removal of surfactant by calcination and nanocomposite purity was assured by the absence of peak at 2850 cm⁻¹ [44]. A wide band at 3395 cm⁻¹ was appeared due to the stretching vibration of O–H of adsorb water (H₂O) molecules [35].

Fig. 7:

FTIR spectra of Fe/CeO₂–SiO₂ nanocomposite.

From FTIR spectra vibrational frequency and effective mass of Ce−O and Si−O bonds were determined by Eqs. 20–23 [45].

where, ν is the wavenumber, c is the velocity of light, k is force constant and μ is the effective mass which is determined by following equation

where, μ is the effective mass, M is the atomic masses of Ce, Si, and O. Effective mass (μ) of Ce−O and Si−O bonds are 14.92 and 10.98.

Force constant (k) determined by following equation

Force constant of the Ce–O bond is 3.88 × 10²⁵ kg while Si–O bond is 3.28 × 10²⁵ kg.

Removal of MB

Ten parts per million of MB solution was prepared and passed through all above mentioned columns as mentioned in Fig. 1 with a flow rate of 12mL/2 min. The absorbance of the eluates was measured at λ_max 665 nm by UV–vis spectrophotometer.

In NF column, two functions occurred at the same time, first was the adsorption and second was the degradation. In adsorption process, RHA contained silanol functional groups (–Si–OH). These silanol groups adsorbed MB by making hydrogen bond with the nitrogen atoms from the MB as shown in Fig. 8 because nitrogen atoms have high ability to attract electron due to small radius.

Fig. 8:

Hydrogen bonding between silanol groups of slurry and methylene blue (MB).

The results showed that 97 % MB was removed in all above mentioned columns by RHA while 3 % removed by nanocomposite.

Column packed with RHA slurry

As mentioned above only RHA as slurry was used in this column so silanol groups from the surface of the slurry adsorbed 97 % MB while no degradation was observed due to the absence of Fe/CeO₂–SiO₂ nanocomposite.

Column packed with nanocomposite with two layers of RHA slurry

The results showed that although Fe/CeO₂–SiO₂ nanocomposite was also in column but only 97 % removal of MB was observed because nanocomposite makes a very thin layer in column due to which it has a very small surface area and exposure in column due to which there was less removal showed by nanocomposite [23].

Column packed with nanocomposite mixed with RHA slurry

In this column completely removal of MB observed as shown in Fig. 9(b) because Fe/CeO₂–SiO₂ nanocomposite was mixed in the slurry as a result their surface area was increased so more number of active sites available for removal of MB. Hundred percent removal was achieved within 11 min and it remained 100 % until complete MB solution was passed. Best results were obtained by this column so it was chosen for reusability [46].

Fig. 9:

Change in absorption of MB due during its removal from column under visible light irradiation (A) in column (a, b) (B) in column (c).

Reusability of nanofiltration column

Column (c) was reused without washing for removal of MB. The flow rate of the solution was also slow because of the presence of already adsorbed MB [47].

MB shows two strong absorption bands at 291 and 665 nm [48]. These bands decreased up to almost zero only after 10 min in column (a) as shown in Fig. 9(a) and completely zero in column (c) as shown in Fig. 9(b) and its color changes from deep blue to transparent due to complete removal of MB.

Removal of chlorpyrifos

Five parts per million CP solution was prepared and passed through the column (c) (Fig. 1c) with flow rate of 3 mL/2 min. The absorbance of the eluates was taken at λ_max 290 nm by UV–vis spectrophotometer [25].

As mentioned above in NF column, two functions occurred at the same time, first was the adsorption and second was the degradation. In adsorption process, RHA contained silanol functional groups (–Si–OH). So these silanol groups adsorbed CP by making hydrogen bond with the nitrogen atom in ring structure of CP [49] as shown in Fig. 10. The affinity of nitrogen atom to make hydrogen bond is mentioned above.

Fig. 10:

Hydrogen bonding between silanol groups and nitrogen atom of chlorpyrifos (CP).

The results showed that 91 % removal was observed and with the passage of time degradation was decreased. This might be due to the decrease of the availability of adsorption sites.

Figure 11 showed the change in absorbance of CP. The strong absorption bands of CP did not shift depicting that the chemical structure of CP has been destroyed by removal under visible light irradiation and Fe/CeO₂–SiO₂ nanocomposite [50].

Fig. 11:

Change in absorption of CP due to adsorption and photodegradation under visible light irradiation.

Reaction mechanism in nanofiltration column

Photodegrdation process of Fe/CeO₂–SiO₂ nanocomposite can be evaluated by electron-hole pair mechanism. As Fe is doped on CeO₂–SiO₂ nanocomposite so it absorbs visible light and transfer electron from valence band to the conduction band by leaving hole (h⁺) at the valence band. 4f orbital of the CeO₂ has a very important rule in photo degradation process [51] because electrons can be easily transferred into the 4f orbital of CeO₂ due to exceptional electrontransfer mediator ability. Photogenerated electrons of the conduction band from the Fe transferred to the 4f orbital of the CeO₂. Oxygen of the CeO₂ captured these electrons and produce hydroperoxy (HO₂^•) radical, superoxide radical (O₂^•) that can then form hydrogen peroxide (H₂O₂) and hydroxyl radical (OH^•). Hydroxyl radical (OH^•) oxidized both MB and CP into their mineralized products Fig. 12. At the same time SiO₂ also plays a vital role to increase the catalytic activity of CeO₂ by not only adsorbing organic compounds but transfer these organic compounds on the active sites of CeO₂. It also helps in avoiding CeO₂ agglomeration [52]. The proposed mechanism of both MB and CP is as follow

Fig. 12:

Schematic representation of Fe/CeO₂–SiO₂ nanocomposite as catalyst under visible light for degradation of MB and CP.