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Vol. 9. Issue 1.
Pages 307-313 (January - February 2020)
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Vol. 9. Issue 1.
Pages 307-313 (January - February 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.10.059
Open Access
Green technology extraction and characterisation of silica nanoparticles from palm kernel shell ash via sol–gel
Patrick E. Imoisili, Kingsley O. Ukoba, Tien-Chien Jen
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Corresponding author.
University of Johannesburg, Kingsway and University Road, 2092, P.O. Box 524, Auckland Park, 2006 Johannesburg, South Africa
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Silica nanoparticles have numerous applications including drug delivery, lightweight aggregates, and energy storage. It has been manufactured from different agricultural bioresources with limited research on palm kernel shell ash (PKSA). This study produced silica nanoparticles from palm kernel shell ash. Modified sol–gel extraction technique was used to produce the silica nanoparticles from PKSA. The extracted silica nanoparticles were characterized using X-ray diffraction (XRD), Scanning electron microscope (SEM) with Energy dispersive X-ray (EDX), Fourier transform infrared (FT-IR) techniques, Brunauer–Emmett–Teller (BET) method and Thermogravimetric analysis (TG). The microstructural analysis reveals that the unit size of the extracted silica nanoparticles is between 50–98nm, with a very high specific surface area (438m2g−1). EDX confirmed the presence of SiO2 in the sample. FT-IR analysis shows the existence of silanol and siloxane groups. This success means, decrease in environmental contamination caused by indiscriminate disposal of palm kernel shell (PKS) and silica nanoparticles for advanced material applications.

Palm kernel shell ash
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America, Asia, and Africa, especially in Nigeria are home to a large quantity of oil palm trees [1–3]. Elaeis guineensis commonly known as palm kernel consists of different parts [4]. Palm kernel shell PKS is the rigid endocarp of palm fruit which borders the seed and is alternatively known as Oil Palm Shell [5,6]. It is gotten as the remaining waste in the removal of the kernel in the nut once palm oil has been extracted from the mesocarp of oil palm pod [7]. PKS was utilized by scientists as lightweight aggregates (LWA) to substitute traditional normal weight aggregates (NWA) in essential elements and motorway construction [8–11]. PKS has also been utilized in various applications extending from energy storage, biomass, bio-fertilizer, to supercapacitor electrode [12–15]. Silica nanoparticles are also used for biomedical applications, including drug delivery, due to the large surface area [16]. It constituents environmental waste if not utilized [17].

Silica gel is a stiff three-dimensional linkage of colloidal silica. Silica gel is categorized based on the synthesis process as aqua gel, xerogel and aerogel. The aqua gel contains holes packed with water. The aqueous stage in which the holes are removed through vaporization is known as xerogel. However, the aerogel involves the removal of the solvent by supercritical extraction. Silica xerogels are primarily used in preparing compact ceramics. Furthermore, the high surface area and permeability of xerogels allow for uses in such as ultra-filters, catalytic substrates and column stuffing ingredients for chromatography [18]. Silica has been effectively extracted from several agrarian bioresources such as rice husk ash (RHA) [19–23], sugar-cane [24–27], coffee husk [28], wheat husk [29,30] and corn cob ash (CCA) [31–34].

The cumulative interest in the synthesis of SiO2 nanoparticles is owed to the wide usage as a basic raw material in the growth of new classes of innovative materials for high-tech applications [35–37]. Silica nanoparticles have been produced using Chemical Vapour Condensation (CVC) [38], Reverse Micro Emulsion (RME) [39], precipitation method [40]. Sol–gel is methods used to synthesis silica, porcelain materials and glass owing to its capability to produce pure consistent products at trifling situations [41–43]. This study produced silica nanoparticles from agricultural bioresources of PKSA using green technology via the modified sol–gel method. It has a combined advantage of reducing disposal as well as pollution problems and making valued silica particles at a lesser cost. The prepared silica nanoparticles from palm kernel shell ash (PKSA) were characterized using XRD, SEM with EDX, FTIR, BET and TG techniques.

2Materials and methods2.1Materials

The palm kernel shell PKS was obtained from southwestern Nigeria. PKS collection, washing and drying were performed in the open air. Combustion of PKS was done for 3h at 750°C with a heating speed of 10°C/min and allow to cool in the muffle furnace.

2.2Extraction of silica

Extraction of silica from palm kernel shell ash, (PKSA) was done using the sol–gel, template-free method as previously reported by Okoronkwo et al. [44]. 500ml rations of 3M NaOH was added to 50g PKSA samples and heated for 2h using a hot plate with continuous stirring to dissolve the silica present in the ash and producing a silicate solution. Ashless filter paper was used to filter the solutions and 100ml boiled distilled water was used to wash the residue. After cooling, the filtrate was titrated with 3M HCl to pH in the range of 7.5–8.5 with continual stirring and nurtured for 24h to allow gel development. The gel was softly broken after ageing and centrifuged at 4000rpm for 4min. The supernatant was discarded while the aqua gels were placed inside an oven to dry at 80°C for 24h to yield silica xerogels.

2.3Extraction of nano silica

The silica xerogels were refluxed at 70°C with 3M HCI for 4h and repeatedly washed with deionised water after which it was dissolved with 3M NaOH by uninterrupted stirring with a magnetic stirrer for 10h and the pH adjusted in the range of 7.5–8.5 by adding concentrated H2SO4. The silica precipitate was collected and repetitively washed using warm deionised water till the silica became wholly alkali-free and were dried for 48h at 80°C in a vacuum oven. A flow illustration of the extraction procedure is presented in Fig. 1.

Fig. 1.

A flow illustration of the process used to produce nano-silica from PKSA.

2.4Material characterization

XRD of palm kernel shell ash (PKSA) and extracted silica was scanned using GBC EMMA X-ray diffractometer having CuKα emission at 25kV acceleration voltage and 400μA current from 2θ 15° to 60° at a speed of 4.00°/min. Morphology and particle dimension of produced nano-silica were observed with an SEM (Zeiss Ultra Plus) and EDX at Secondary Electron Image (SEI) and high vacuum (HV) mode with 20kV accelerating voltage. FTIR spectra were recorded using Tianjin GangDong FT-IR 650 in the range of 4000–350cm−1. Specific surface area (SBET) was estimated using the Brunauer–Emmett–Teller (BET) method. Thermal gravimetric analysis (TG) was assessed using Perkin-Elmer Pyris 6 TGA analyzer. Sample weight of 10mg was heated from 40 to 800°C with a heating rate of 10°C/min in nitrogen atmosphere.

3Results and discussions3.1XRD

The silica yield was 54.35%. X-ray diffractogram of palm kernel shell Ash and extracted silica nanoparticles are presented in Fig. 2. The XRD pattern for the Ash shows the present quartz at theta=20, 26, 40, 47 and 55° (SiO2 PDF Card #331161) and Calcite at theta=22, 44, 47 and 55° (CaCO3 PDF Card #050586). The broad XRD array of extracted silica nanoparticles at theta=22.5°, which is distinctive of amorphous solid, confirms the formation of amorphous silica; similar results were obtained by other researchers [20–34].

Fig. 2.

XRD of palm kernel shell ash, extracted xerogel and nano-silica.


Fig. 3 shows the SEM micrograph of silica nanoparticles produced from palm kernel shell ash at ×200,000 magnification. The particles were observed to be spherical with reduced silica-silica agglomeration. The average particle sizes of the extracted silica particles sizes were found to be between 50 and 98nm.

Fig. 3.

SEM micrograph of nano-silica produced from palm kernel shell ash.


A strong intensity of Si and O as shown in the EDX spectra in Fig. 4, confirms silica (SiO2) as the predominant element in the sample. This represents about 96.59% of the total element. This is negligible impurities commonly associated with green technology method.

Fig. 4.

EDX micrograph of nano-silica produced from palm kernel shell ash.


FTIR spectral identified the key chemical compound existing in the nano-silica as revealed in Fig. 5. Band 463cm−1–475cm−1 is linked with vibration network of OSiO, while band 791cm−1–807cm−1 was allotted to symmetric stretching vibration network of SiOSi [34]. Band 1071cm−1–1090cm−1 was due to SiOSi irregular stretching vibration [14] and broadband at 1633cm−1–1645cm−1, is due to OH bond bending vibration from SiOH silanol groups, while 3338cm−1 to 34,750cm−1 is due to OH bond stretching vibration from SiOH silanol groups and are due to adsorbed H2O molecules on the silica surface [18,19].

Fig. 5.

FT-IR spectra of produced silica from palm kernel shell.


The surface area (SBET) of prepared nano-silica produced from palm kernel shell ash was assessed after calcination at 750°C for 2h. Nitrogen adsorption/desorption isotherms of produced nano-silica are shown in Fig. 6. The isotherms exhibit the typical Type IV isotherms, typical of mesoporous materials [47,48].

Fig. 6.

Nitrogen adsorption/desorption isotherms of produced nano-silica.


Pore size distribution is shown in Fig. 7, reveals narrow pore size distributions with average pore diameters ranging from 2.2nm to 6.3nm.

Fig. 7.

Pore size distributions of produced nano silica.


The nano-silica SBET surface area was found to be 438m2g−1. This specific surface area is greater than 327m2g−1 recorded for commercially available silica, and the values 120 to 288m2g−1 reported for the silica nanoparticles extracted from RHA [45]. It also seems comparable to the 413m2g−1 from rice straw and 422m2g−1 from mineral pumice rock powder [46]. However, this method produced a better yield.


The results of thermal gravimetric analysis (TG) are shown in Fig. 8. Two-step weight losses were observed. The loss in weight up to 130°C (step 1) is ascribed to dehydration caused by the loss of physically adsorbed H2O. However, chemically bound water from the sol–gel production method was ascribed to the loss in weight from 130 to 570°C (step 2) [49]. Above 600°C, no further weight loss was observed indicating thermal stability of extracted nano-silica.

Fig. 8.

TG of produced nano silica from palm kernel shell.


Silica nanoparticles were extracted from palm kernel shell ash (PKSA) using a modified sol–gel procedure. The results show that amorphous silica nanoparticles with 54.35% silica yield with negligible mineral contaminants can be produced from palm kernel shell ash using a modified sol–gel technique. XRD investigation revealed the presence of silica in the ash and amorphous nature of extracted silica. SEM micrograph shows silica nanoparticles in the range of 50–98nm. The surface area of extracted silica nanoparticles was 438m2g−1 and SiO2 presence in the samples was confirmed by EDX result. Key chemical group existing in the samples were indicated by FT-IR data. This will contribute to knowledge and help reduce environmental pollution caused by the disposal of PKS and valuable silica nanoparticles for advanced materials for high-tech applications can be produced.

Conflicts of interest

The authors declare no conflicts of interest.


Authors appreciate funding of URC of the University of Johannesburg. Prof T. C. Jen would like to acknowledge the financial support from NRF as well.

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