2.2. Pyrolysis of Rice Husk for Obtaining Bio-Oil and Bio-Silica in the Literature
3.2. Pyrolysis of Rice Husk for Obtaining Bio-Oil and Bio-Silica in the Literature
2.2.1. Bio-Silica Production
3.2.1. Bio-Silica Production
Pyrolysis of acid-leached rice husk in an N2 atmosphere was described by Liou et al. (2004) [24], obtaining carbon/silica powders with a surface area of 261 m atmosphere was described by Liou et al. (2004) [23], obtaining carbon/silica powders with a surface area of 261 m2
/g at 900 °C. No bio-oil is recovered from this process. In the same study, the acid-leached rice husks were calcined with air, and the surface area obtained was 235 m2
/g.
To our knowledge, the first pyrolysis performed, with the aim of obtaining pure silica nanoparticles, was reported in 2011 by Wang et al. [25]. However, although it is referred to as pyrolysis, it seems more like combustion since heating in the absence of oxygen does not achieve the obtention of pure silica but a composite of carbon and silica [25]. To our knowledge, the first pyrolysis performed, with the aim of obtaining pure silica nanoparticles, was reported in 2011 by Wang et al. [6]. However, although it is referred to as pyrolysis, it seems more like combustion since heating in the absence of oxygen does not achieve the obtention of pure silica but a composite of carbon and silica [6].
Liou et al. (2011) performed pyrolysis on acid-leached (hot HCl) RH at 700 °C for 1 h and extracted SiO2 by NaOH 1.5 M at 100 °C for 1 h to produce sodium silicate [26]. The addition of 1 M HCl to this sodium silicate solution to reach pH 7 at 50 °C allowed for the precipitation of SiO by NaOH 1.5 M at 100 °C for 1 h to produce sodium silicate [53]. The addition of 1 M HCl to this sodium silicate solution to reach pH 7 at 50 °C allowed for the precipitation of SiO2
NPs of 6–8 nm diameter (350 m2
/g, mesopore diameter 10.4 nm, V = 0.79 mL/g). These NPs are non-porous and agglomerated, and Na content (1.31 mg/g) is superior to the parent RH.
2.2.2. Combined Bio-Silica and Bio-Oil Production
3.2.2. Combined Bio-Silica and Bio-Oil Production
Research has been made on the production of bio-oil from pyrolyzed rice husks, studying its composition and its feasibility as a biofuel.
Lu Q et al. (2008) [10] analyzed the properties of bio-oil extracted from rice husk, which were similar to other bio-oils in terms of chemical composition. Acids, alcohols, ketones, sugars, etc., were compounds found in rice husk bio-oil similarly as for other bio-oils from different biomasses. However, they found high concentrations of nitrogen, inorganic elements and many low-boiling compounds, but significantly less heavy components, as well as high contents of water, solids and ash, and the presence of alkali metals, which is undesirable as well. The rice husks were not previously leached with acid, which explains the presence of these metals. Their main conclusion was that there is a need to upgrade the bio-oil produced from rice husk before it can be used as biofuel. In this case, the rice husk was not previously treated, and the bio-oil yield was 50 wt%. Lu Q et al. (2008) [16] analyzed the properties of bio-oil extracted from rice husk, which were similar to other bio-oils in terms of chemical composition. Acids, alcohols, ketones, sugars, etc., were compounds found in rice husk bio-oil similarly as for other bio-oils from different biomasses. However, they found high concentrations of nitrogen, inorganic elements and many low-boiling compounds, but significantly less heavy components, as well as high contents of water, solids and ash, and the presence of alkali metals, which is undesirable as well. The rice husks were not previously leached with acid, which explains the presence of these metals. Their main conclusion was that there is a need to upgrade the bio-oil produced from rice husk before it can be used as biofuel. In this case, the rice husk was not previously treated, and the bio-oil yield was 50 wt%.
Hsu et al. (2015) [27] studied the pyrolysis of RH in a fluidized bed pyrolizer with the aim of producing syngas and bio-oil, but it does not focus on the production of biochar or silica. The yields obtained with a N Hsu et al. (2015) [59] studied the pyrolysis of RH in a fluidized bed pyrolizer with the aim of producing syngas and bio-oil, but it does not focus on the production of biochar or silica. The yields obtained with a N2
gas flow of 40 L/min and feeding rate of 10 g/min were the following: syngas: 38.52%; bio-oil: 29.44%; and char: 32.04%. The composition of the bio-oil was analyzed, which showed that it was mainly composed of aromatic compounds.
3. Tailoring of SiO
4. Tailoring of SiO
2
from Rice Husk to Enhance Porosity
3.1. Mesoporous RH-SiO
4.1. Mesoporous RH-SiO
2
Nanoparticles
One of the main goals of the optimization of the extraction process of silica from rice husk is achieving a large surface area since the surface area is linked to the value of the produced material. Efforts to adapt acid-leaching, grinding and calcination parameters to enhance the porosity of silica from rice husk have been reported.
The effect of calcination temperature was studied by Manasa et al. (2021). Five silica materials were synthesized, with different calcination conditions in the oven after acid leaching. Temperatures of 550, 600 and 650 °C for 3 h combined with air or N2
flow were studied as parameters for the extraction of SiO2
. The highest BET surface area (SA) (376 m2
/g) was achieved with RH-SIO2
-600A calcined at 600 °C. RH-SiO2
-600A had a pore size of 4.5 nm and a pore volume of 0.38 mL/g. The same RH subjected to pyrolysis with only N2
at 600 °C was a composite C/SiO2
, named RH-SiO2
-600N, which presented a lower SA of 267 m2/g, a smaller pore diameter (2.7 nm) and a smaller pore volume (0.17 mL/g) [28]. With higher temperatures than 600 °C, the SA decreases, and the pore size increases, possibly due to nanoparticles sintering.
3.2. Ordered Mesoporous RH-SiO
/g, a smaller pore diameter (2.7 nm) and a smaller pore volume (0.17 mL/g) [62]. With higher temperatures than 600 °C, the SA decreases, and the pore size increases, possibly due to nanoparticles sintering.
4.2. Ordered Mesoporous RH-SiO
2
Nanoparticles
Ordered structures of silica, known as the MCM (Mobil Composition of Matter) family, have the potential for a wide range of processes. These materials combine amorphous characteristics with the ordered structure of homogeneous mesopores with adjustable diameters from 2 to 10 nm, developing a specific surface area as high as 1000 m
2/g due to very thin walls (1 nm) in between the pores. Rice husk has been explored as a raw material to produce biogenic MCM-41 by hydrothermal methods.
Most studies use similar methodologies for the production of biogenic MCM. Rice husk is pretreated with acid and then calcinated, then the silica in RHA is extracted by an alkali solution to form a sodium silicate solution. This solution is mixed with a surfactant solution such as CTAB, the pH is increased, and the solution is aged
[29][30][31][32][69,70,71,72]. The solid obtained needs to be treated, usually by calcination, in order to remove the surfactant.
3.3. Shaping RH-SiO
4.3. Shaping RH-SiO
2
NPs into Macroporous Structures
The assembly of mesoporous silica into macroporous structures such as monoliths presents great advantages for industrial applications, such as flow processes.
The use of rice husk as a silica precursor for this material has been explored by Bahrami et al. (2017)
[33][74] by freeze-casting a water-based bio-silica solution.
Both amorphous (ARHA) and crystalline (CRHA) (80/20 cristobalite/tridymite) rice husk ash were synthesized with acid-leaching pretreatment only for ARHA and calcination at 600 and 900 °C, respectively. Obtained ARHA had a specific surface area of 151 m
2/g and pore volume of 0.23 mL/g, while CRHA particles form agglomerates with a specific surface area of 1.5 m
2/g. Before freeze-casting, both RHA were ground with a high-energy planetary ball mill to produce a fine powder (d < 10 μm).
Freeze-cast monoliths with lamellar and cellular pore structures (macropores in the range of 10–50 μm) were obtained from ARHA and CRHA. It was found that the crystallinity of the starting material does not have a significant impact on the macropores’ structure compared to other parameters of the synthesis (freezing rate and solid content).
45. RH-SiO2 Applications
4.1. RH-SiO
5.1. RH-SiO
2
for Biomedical Applications
The main goals for biomedical applications such as drug delivery are to reduce side effects and treatment costs. The use of biogenic nanomaterials produced from waste resources accomplishes these objectives.
Biogenic silica nanoparticles (NPs) synthesized by acid leaching at 120 °C and calcination at temperatures of 500–700 °C were tested for cell viability for human stem cells revealing high biocompatibility
[34][75]. In the same way, SiO
2 NPs were produced and then milled after calcination to an average diameter of <150 nm in order to be used as a carrier of Penicillin-G. Once the drug was added to the silica, in vitro release was studied in simulated body fluid (SBF), concluding that the silica NPs can act as a carrier that delays the release of the drug in the body
[20][37].
4.2. RH-SiO
5.2. RH-SiO
2
for Energy Storage
Nanosized SiO
2 can also be used for photovoltaic and energy storage applications. SiO
2 NPs properties, such as high surface area and hydrophilic surface, enhance electrochemically active centers, which favors electrolyte transport through the material, which can be applied in the development of supercapacitors
[35][22]. SiO
2/C composites obtained by direct pyrolysis of rice husk were used for Li-ion sulfur batteries, showing high capacities as electrode material
[36][76]. Other researchers doped the silica with tin oxide in order to gain reactivity through the sol-gel method. These showed good performance as electrodes for making electrochemical supercapacitors
[37][77].
4.3. RH-SiO
5.3. RH-SiO
2
as Catalyst Support
Silica nanoparticles have been used as catalyst supports, taking advantage of their large surface area. Silica prepared through microwave-assisted acid leaching and posterior calcination was used as an iron-oxide-containing nanocatalyst for toluene alkylation and oxidation of benzyl alcohol
[38][66].
A SiO
2/C-nickel-based catalyst was obtained through rice husk pyrolysis for the selective hydrogenation of furfural derived from biomass
[39][55]. A similar nickel-based catalyst supported on RH silica showed high performance in yielding H
2 production during non-oxidative methane cracking
[28][62]. As can be seen, silica’s thermal stability is really attractive for use in catalytic reactions. Also, various studies have proved SiO
2 to be a better support for Ni than other traditional materials
[40][79].
4.4. RH-SiO
5.4. RH-SiO
2
as Adsorbent for Water Cleaning
45.4.1. Adsorption of Chemicals of Emerging Concern and Other Micropollutants
The current trends explored by researchers in adsorption are the utilization of agri-waste-based biomass due to its low cost, simple design and the added value of being a method of waste valorization
[41][80].
Activated carbon has been the most traditionally used adsorbent, and it has been demonstrated to be efficient with hydrophobic compounds but highly inefficient with hydrophilic and electrically charged molecules. Moreover, the efficiency of activated carbon is reduced in the presence of organic matter, and the regeneration of adsorbents is questionable
[42][81].
However, the ease of obtention of activated carbon from lignocellulosic residues via pyrolysis at 500–600 °C and posterior activation at 700 °C has always made this material extremely attractive. Moreover, it can be easily functionalized.
Therefore, the most recent advances that are being made in adsorption correspond to the need to develop efficient, reusable and cheaper adsorbents than activated carbon, as well as materials that can retain PMOCs (persistent mobile organic compounds) or PMTs (persistent mobile and toxic). The recent introduction of nanotechnology into wastewater treatment has shown to be a feasible solution to the issue of CECs
[41][43][80,82].
45.4.2. Adsorption of Metals in Water
The adsorptive capacities of silica nanoparticles can be exploited for water cleaning by the recovery of toxic metals present in wastewater. As mentioned before, for micropollutants, currently, there is a great interest in developing cost-effective, simple and environmentally friendly techniques to deal with this issue. SiO
2 has been applied to heavy metal removal from water
[44][45][46][47][48][98,99,100,101,102].
Silica derived from rice husk has been less studied as an adsorbent of metals. Silica NPs extracted from RHA via the sol-gel method with a particle size of around 50 nm and a surface area of 70 m
2/g were studied for the removal of Fe
2+ ions in water. Iron adsorption capacity was found to be 9 mg/g at its maximum at pHs higher than 5 in 20 min
[49][103].
SiO
2 NPs were produced from rice husk by incubation in fungal biomass for its biotransformation. These NPs were tested for their adsorption of Pb in water spiked with 88 ppb, the same concentration measured in samples from fish farms, showing maximum adsorption after 120 h of contact time. Then, the goal of this study was to test the adsorption ability of SiO
2 NPs in vivo in fishes supplemented with Pb and silica NPs, which showed effective adsorption of Pb at 1 ppm in the fish
O. niloticus, hence reducing its mortality rates
[50][104].
The removal of Cr
3+ and Cu
2+ by milled rice husk was tested. In this case, no extraction of SiO
2 was performed, and the husk was used directly in a batch test with 100 ppm of each metal. As previously seen in other studies, the highest adsorption capacity (22.5 mg Cr
3+/g and 30 mg Cu
2+/g) reaches its peak at pH > 5
[51][105]. The use of rice husk ash directly as an adsorbent has also been studied in the literature; however, rice husk ash has smaller surface areas compared with amorphous SiO
2 NPs
[52][106]. These adsorption abilities can be implemented for the treatment of real wastewater, such as tannery wastewater, which is rich in Cr
3+. Silica NPs were obtained from calcinated and leached rice husks using the sol-gel method. At pH 6, the highest adsorption capacity was determined to be 385 mg Cr
3+/g, and the chromium complexes were better adsorbed when the size of the pores was increased since their diffusion into the particles seems to be the limiting factor
[53][107].
To summarize, among the different procedures outlined previously, the following references focus on optimal techniques for generating high-surface silica nanoparticles from rice husk, cost-effective methods for shaping silica nanoparticles into monoliths suitable for flow applications, utilizing silica nanoparticles from rice husk for value-added materials like MCM-41, and the economical application of silica materials derived from rice husk for removing pollutants.
56. Conclusions
Silica nanoparticles have great, well-known properties such as a tunable specific surface area and particle shape and diameter. They can be obtained (i) directly after acid-leaching of rice husk and controlled pyrolysis and/or calcination, (ii) by rice husk calcination (leading to classical rice husk ash from industry) followed by silica dissolution by NaOH (or Na
2CO
3) and silica precipitation in acidic medium, and (iii) by pyrolysis of acid-leached rice husk followed by NaOH dissolution and silica precipitation in acidic medium. Pathways (i) and (iii) are recommended to obtain higher silica purity and high surface area. They can be easily functionalized and transformed into other mesoporous silica featuring very high surface areas, such as MCM-41 (1000 m
2/g), or into composites with other metals. These properties provide it with an excellent adsorption capacity, which can be applied to medicine as drug carriers or into catalysis as metal support, among others. When silica nanoparticles are extracted from biomass, they have been proven to have excellent biocompatibility and properties similar to silica produced by traditional methods but with a significant decrease in chemicals and energy consumption. SiO
2 derived from secondary agricultural waste such as rice husk is also advantageous since they are considered to be the major contributors in the future for sustainably sourced energy and materials since their growth does not compete with food crops. With the intent of harvesting most of the rice husk, applying circular economy to SiO
2 extraction through methods such as combined pyrolysis and calcination can lead to the obtention of valuable by-products such as bio-oil and pyrolysis gas. By varying processing parameters, SiO
2 nanoparticles can be tailored for specific applications.