Synthesis and characterization of silica xerogel from corn husk waste as cationic dyes adsorbent

Preparation of silica xerogel from corn husk

The dried corn husk, which was collected from July to December 2020, was burned to ash in a muffle furnace at 600°C for two hours at a heating rate of 10 °C/min to produce corn husk ash. Silica was extracted from 0.25 g of corn husk ash using 6 mL of 1 M sodium hydroxide. The mixture was stirred continuously at 80 °C for one hour to dissolve the silica. The mixture was cooled and centrifuged at 12,000 rpm for 10 minutes to remove the residue from the sodium silicate solution. The sodium silicate solution was neutralized with 3 M hydrochloric acid under constant stirring until it reached a pH of 7 to produce sol. The sol was aged at room temperature for 18 hours to promote hydrogel formation. After aging, the hydrogel was washed repeatedly using deionized water then centrifuged for five minutes at 4000 rpm. The hydrogel was dried at 60 °C for five hours to obtain silica xerogel. The yield of silica was calculated by the following equation:

Characterization of silica xerogel from corn husk

The identification of functional groups in the silica xerogel was conducted by Fourier transform infrared spectroscopy (FTIR, Thermo Scientific iS 5), scanned from 600 to 4000 cm^-1. The crystallinity of the xerogel was confirmed by X-ray diffraction (XRD, PANalytical) with Cu-Kα radiation and 2-theta ranging from 10–90°. The morphology, particle size, and elemental composition were examined using scanning electron microscope and energy dispersive X-ray spectroscopy (SEM-EDS, Desktop Phenom ProX). The Brunauer, Emmett, and Teller (BET) specific surface area as well as Barrett, Joyner, and Halenda (BJH) average pore diameters were measured by Micromeritics Tristar II 3020.

Adsorption studies

The adsorption experiments were performed in a batch setup by adding 10 mg of silica xerogel to 10 ml of cationic dyes (crystal violet and methylene blue) at the initial concentrations of 10, 20, 30, 40, and 50 mg/L at 25°C for 30, 60, 90, 120, and 150 minutes. The final concentration of the solution was analyzed using a UV-Vis spectrophotometer by measuring absorbance at λ = 664 nm for methylene blue and λ = 590 nm for crystal violet. The following equation was used to calculate the adsorption capacity:

The following equation was used to calculate the removal efficiency (%) of dyes:

where C_o and C_e (mg L^-1) represent the initial and final concentration of crystal violet and methylene blue, w (g) is the mass of silica xerogel, and V (L) is the volume of solution. Then, the data were fitted against the Langmuir and Freundlich isotherm model based on the following equations:

Preparation of silica xerogel from corn husk

The transformation of corn husk ash to hydrogel and xerogel by the sol-gel method is shown in Figure 1.

Figure 1.

Transformation of (a) sol (b) hydrogel and (c) xerogel via the sol-gel method.

In the preparation of silica xerogel, sol was formed at sodium silicate acidification when hydrochloric acid was added to sodium silicate until pH=7 was obtained, as shown in Figure 1(a). The acidification reaction was then followed by the creation of a siloxane linkage between silanol groups on surface sites¹⁴. This resulted in the formation of the hydrogel as displayed in Figure 1(b). The hydrogel was dried to form xerogel in solid form as shown in Figure 1(c) with a maximum yield of 22.457%.

Characterization of silica xerogel

The FTIR spectrum of silica xerogel from corn husk is shown in Figure 2¹⁵. The peaks at 1650 cm^-1 and 3500 cm^-1 are associated with symmetric bending vibration and asymmetric stretching vibration of Si-OH, respectively. Meanwhile, peaks at 1100 and 800 cm^-1 correspond to the Si-O stretching vibration in Si-O-Si bonds.^6,10. Thus, we can conclude that silica xerogel has been successfully synthesized from corn husk waste.

Figure 2. Fourier transform infrared spectroscopy (FTIR) spectrum of silica xerogel prepared from corn husk.

X-ray diffraction patterns of silica xerogel in the range of 2-theta 10–90° are shown in Figure 3¹⁶. A broad peak at 2-theta of 17–28° indicates the characteristic amorphous silica structure. No other peak was detected implying the sample has high purity.

Figure 3. X-ray diffractograms of silica xerogel from corn husk.

SEM micrographs of silica xerogel are displayed in Figure 4¹⁷. The morphology of the obtained particles gives some resemblance to flakes or shards with sizes ranging from 80–250 μm.

Figure 4. Scanning electron micrographs of silica xerogel from corn husk.

The EDS spectrum of silica xerogel in Figure 5 shows the high purity of the silica xerogel, which has silicon and oxygen as the main constituents without any other impurities¹⁷.

Figure 5. Energy dispersive X-ray spectra and composition of silica xerogel from corn husk.

The N₂ adsorption-desorption isotherms of silica xerogel from corn husk are displayed in Figure 6¹⁸. According to IUPAC classification, the adsorption isotherm of as-synthesized silica xerogel belongs to the type-IV, which is typical of mesoporous material with size ranging from 2 to 50 nm. The BET surface area and the BJH average pore diameters are 363.72 m²/g and 6.35 nm, respectively.

Figure 6. N₂ adsorption-desorption isotherms of silica xerogel from corn husk.

Adsorption studies

The relationship between contact time and the adsorption of both cationic dyes at different initial concentrations is shown in Figure 7^19,20. In general, the removal efficiency of silica xerogel for crystal violet and methylene blue increases with prolonged contact time. However, the adsorption rate becomes slower as it approaches equilibrium. This may be caused by the unavailability of active sites on the adsorbent surface after a certain period^21,22.

Figure 7.

The effect of contact time on the removal efficiency of (a) crystal violet and (b) methylene blue.

As shown in Figure 7(a), the optimum adsorption condition for crystal violet is obtained at a contact time of 90 minutes with a removal efficiency of 98.539%. Meanwhile, as displayed in Figure 7(b) the optimum adsorption condition for methylene blue is obtained at a contact time of 120 minutes with a removal efficiency of 96.184%.

From the calculated correlation coefficients (R²) in Table 1^19,20, the adsorption process of both dyes on silica xerogel is a good fit with the Langmuir isotherm model. This suggests that the adsorption process is inclined toward monolayer chemisorption due to electrostatic interactions between the positively charged cationic dyes molecule and negatively charged surface sites^23,24.

The maximum adsorption capacity (q_max) for crystal violet is as high as 69.44 mg/g and 59.17 mg/g for methylene blue. Compared to other crystal violet adsorbents from previous research, silica xerogel from corn husk has a higher maximum adsorption capacity^13,24–26. The as-synthesized silica xerogel also performed better in methylene blue adsorption compared to other adsorbents^11,27–29. The high surface area and suitable pore diameter of silica xerogel arguably also contributes to the favorable interactions between the adsorbate molecules and the adsorbent surface.

Underlying data

Figshare: Underlying data for ‘Synthesis and characterization of silica xerogel from corn husk waste as cationic dyes adsorbent’.

The following underlying data is provided:

Data are available under the terms of the Creative Commons Attribution 4.0 International license (CC-BY 4.0).