Increased amounts of available biomass residues from agricultural food production are present widely around the globe. These biomass residues can find essential applications as bioenergy feedstock and precursors to produce value-added materials. This study assessed the production of biogenic silica (SiO2) from different biomass residues in Africa, including cornhusk, corncob, yam peelings, cassava peelings and coconut husks. Two processes were performed to synthesize the biogenic silica. First, the biomass fuels were chemically pre-treated with 1 and 5% w/v citric acid solutions. In the second stage, combustion at 600 °C for 2 h in a muffle oven was applied. The characterization of the untreated biomasses was conducted using Inductively coupled plasma—optical emission spectrometry (ICP-OES), thermal analysis (TG-DTA) and Fourier-transform infrared spectroscopy (FTIR). The resulting ashes from the combustion step were subjected to ICP, nitrogen physisorption, Energy dispersive X-ray spectroscopy (EDX) as well as X-ray diffraction (XRD). ICP results revealed that the SiO2 content in the ashes varies between 42.2 to 81.5 wt.% db and 53.4 to 90.8 wt.% db after acidic pre-treatment with 1 and 5 w/v% acid, respectively. The relative reductions of K2O by the citric acid in yam peel was the lowest (79 wt.% db) in comparison to 92, 97, 98 and 97 wt.% db calculated for corncob, cassava peel, coconut husk and cornhusk, respectively. XRD analysis revealed dominant crystalline phases of arcanite (K2SO4), sylvite (KCl) and calcite (CaCO3) in ashes of the biomass fuels pre-treated with 1 w/v% citric acid due to potassium and calcium ions present. In comparison, the 5 w/v% citric acid pre-treatment produced amorphous, biogenic silica with specific surface areas of up to 91 m2/g and pore volumes up to 0.21 cm3/g. The examined biomass residues are common wastes from food production in Africa without competition in usage with focus application. Our studies have highlighted a significant end-value to these wastes by the extraction of high quality, amorphous silica, which can be considered in applications such as catalyst support, construction material, concrete and backing material.

Porous silica was synthesized from cornhusk using the sol–gel polymeric route and compared with ash obtained from the direct combustion process under laboratory conditions. The unmodified ash from the direct combustion process was dissolved in NaOH for 1 h to form sodium silicate, which was subsequently hydrolyzed with citric acid to yield a silica xerogel. The obtained xerogel was characterized using inductively coupled plasma–optical emission spectrometry (ICP-OES), Fourier transforms infrared (FTIR) spectroscopy, X-ray diffraction (XRD), simultaneous thermal analysis (STA), gas sorption techniques to determine their elemental constituents, functional groups, crystalline phases, thermal stability, and porosity, respectively. The results showed that the synthesized silica xerogel exhibited porous network structures with a high-specific surface area and mesopore volume of 384 m2/g and 0.35 cm3/g, respectively. The pore size distribution revealed a complete transformation of the pore network structures of the unmodified ash from a monomodal to a bimodal pore system, with micro- and mesopore peaks centered around 1.5 and 3.8 nm, respectively. The ICP-OES results showed that the silica content significantly increased from 52.93 to 91.96 wt.% db after the sol–gel treatment. XRD diffraction confirmed the amorphicity of the silica particles obtained from the sol–gel extraction method. In addition, the STA data showed that the silica xerogel has high thermal stability compared to the unmodified ash, as the latter exhibited poor thermal stability and low textural properties. The high surface area and narrow pore cavity size distribution of the porous silica xerogel make it an ideal substrate for catalysts and an excellent template for growing other nanoparticles within the pores.

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