One of the main components of rice husk ash (RHA) is silica, which can be used as raw material for fertilizer. The study examined influences of various volume and concentration on silica extraction. The silica was extracted using a technical alkaline grade solvent (NaOH) with a variation in volume/the ratio of RHA to the amount of solvents (1: 4, 1: 5, and 1: 6) and the solvent concentration (0.50, 0.75 and 1 N). The highest average yield was found in the concentration of 1 N solvent with the ratio of RHA to the number of solvents 1: 6 is 62.83%. Based on the physical characteristics of silica from RHA which include whiteness (93.2496.66%), moisture content (0.49-2.81%), and density (0.56-0.95 g / mL) indicates that the use of technical alkaline solvents tends to decrease the purity of the proven silica also with the presence of major contaminants such as Na and Cl and other elements through SEM-EDX, XRF, and XRD test. In saline soils, the presence of contaminants in silica with an amorphous phase (2θ = 22.29°) especially Na and Cl may have a negative effect if used as raw material for fertilizers.

Rice husk is an abundant and underutilized agricultural by-product in Indonesia. The silica contained in the waste could be used as a substitute natural silica for various purposes. The extraction of silica from rice husk (RH) has been done using the sol-gel method without involving high energy. Extraction was carried out with KOH at concentrations of 7, 10, and 13% for 90, 120, and 150 minutes. Experimental results showed that the highest extraction yield was obtained at the extraction process using KOH 7% (average 9.03%). The minimum concentration of KOH could produce the maximum yield of silica. Characterization by XRD showed that the silica had the same pattern as that of commercial products and it was categorized as amorphous silica indicated with the broad peaks at 2θ = 22°. Amorphous silica with high purity was produced from rice husk by KOH 7 % at 120 min (95.34 %). The process condition resulted in high reactivity and a high BET surface area. The washing process needs to be intensified to improve the purity of silica.

Rice husk is a rich source of biogenic silica nanoparticles (SNPs) having excellent physico-chemical properties and numerous potential applications. Extraction yield of the biogenic SNPs may be influenced by many factors. In this study, morphology and the influence of solvent type (sodium hydroxide/NaOH, potassium hydroxide/KOH) and aging time (0, 6, 12, 18, 24h) on extraction yield and properties of selected biogenic SNPs from Indonesian rice husk were investigated. The biogenic SNPs were produced by a simple sol-gel method. The presence of biogenic SNP aggregates in the rice husk tissues was successfully identified by SEM-EDX micrographs. NaOH produced better SNPs extraction results than KOH. Aging time for 6h was the optimum condition for production of SNPs. The TEM image showed that SNPs from the best treatment were approximately spheroidal with particle sizes in the range of 20–50nm. The XRD patterns revealed the amorphous nature of the SNPs. Nitrogen adsorption–desorption analysis showed that the SNPs had BET surface area of 235,78m2/g, total pore volume of 0.58cc/g, and average pore radius of 4.91nm. The SNPs exhibited a type IV isotherm and H1 hysteresis loop indicating a characteristic of mesoporous materials. The results suggest that the sol-gel method developed could provide a simple and relatively fast extraction technique producing biogenic amorphous mesoporous SNPs that could extend their potential applications and increase the economic benefits of utilizing rice husk.

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