Extraction and physical characterization of amorphous silica made from corn cob ash at variable pH conditions via sol gel processing
Introduction
Corn cobs are a major waste from corn/maize production and contain 30–35% hemicellulose, 40–45% cellulose, and 10–20% lignin [1], [2]. As a low-cost lignocellulosic biomass, corn cobs are a reliable and renewable source for the production of various value-added products such as biofuel enzymes [3], proteins [4], fuels [5], absorbents [6], and cements [7]. With such a huge availability of raw materials, keeping corn cob waste as a resource is likely to become more economical for a greater number of commercial applications. Presently, having no inherent commercial value, corn cobs usually end up being burned in open spaces, creating environmental pollution and disposal problems. Due to the need to conserve energy and resources, efforts have been made to burn the cobs under controlled conditions and to utilize the resultant ash as building, semiconductor, composite, and abrasive materials [8], [9], [10], [11], [12]. Due to the high surface area present in ash, it can act as an active catalyst and has the potential to be a good catalytic support material. Corn cob ash (CCA) contains >60% silica by mass with minor amounts of metallic elements [7], [13], [14] and could be an economically-viable raw material for the production of silicates and silica nanoparticles. The chemical composition of CCA may vary as a factor of their environmental, geographical, and soil conditions were the corn was grown in addition to the methods used during sample preparation [13]. In a variety of fast growing research arenas, nano-silica (nS) occupies a prominent position because of its easy preparation and multi-industrial applications such as its use in catalysis, pigments, pharmaceuticals, electronics, ceramics, polymer material industries, thin film substrates, thermal insulators, and humidity sensors [15]. The size, shape, and distribution of these nanoparticles determine their quality.
In this study, nSs were prepared using sol–gel processing with an agro-industrial residue corn cob [16]. Generally, smelting quartz sand with sodium carbonate at 1300 °C is used to provide the precursor for silica [17]. Commercial silica is manufactured in a process involving multiple steps that involve high temperatures and pressure. This makes the process less cost effective and not very environment-friendly. Conversely, the extractable amorphous silica from CCA is cost effective, providing a low-energy method as an alternative to current high-energy methods. Additionally, natural silica from corn cobs is made up of organic materials and hydrated silicon [15], [16]. Earlier nSs have been synthesized using rice husks [17], wheat husks [18], [19], Fusarium oxysporum [20], and corn cobs [13], [14]. The emphasis of this paper is to optimize the conditions for the preparation of highly-purified amorphous nS from CCA at variable pH conditions via sol gel processing. Various characterizations and the stability analysis of these materials are also carried out.
Section snippets
Materials
The raw materials (corn cobs), which were obtained from a local agricultural field (Iksan-si, Jeollabuk-do, South Korea) during the process of removing kernels, were washed thoroughly with distilled water to remove adhering dust, soluble particles, and other contaminants. This process also removed heavy impurities such as sand. The material was then dried in an air oven at about 110 °C for 24 h. All of the reagents that were used were of analytical grade, and their solutions were made up twice
EDX spectra
Fig. 1(a) shows the EDS spectrum of the nS obtained at pH 7. A strong intensity of Si and O peaks between 0 and 5 keV, confirming the presence of silica, are predominant in this sample. Fig. 1(b) shows the EDS spectrum of the CCS obtained at pH 10; the spectrum clearly shows the impurities that are present in the silica prepared at pH 10. The silica surface consists of two types of functional groups, namely silanol (Si-OH) (SiOH) and siloxane (SiOSi), with silanol being more reactive and
Conclusions
In summary, we report a novel biomaterial (CCA) for the synthesis of nS. The synthesis of nS is eco-friendly, inexpensive, and rapid. At pH 7, nano-sized, highly purified silica 97.9 mass percentages) was produced with a high surface area, high reactivity, and that is 98.5% amorphous. The formed CCS had an average particle size of 60 nm and was amorphous in nature. Due to presence of more acidic sites at pH 7 (as compared to pH 10), CCS prepared at pH 7 yielded a good support material for
Acknowledgements
This study was supported by the research funds of USGS and the Nebraska Water Center, 2015. Funds were also received from the National Research Foundation of Korea (NRF) grant funded in association with the government (MEST) (No. 2011-0020202).
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2021, Biocatalysis and Agricultural BiotechnologyCitation Excerpt :Corncob ash is used as a raw material for the production of silicates, silica, and silica nanoparticles that includes more than 60% silica by mass and small quantities of metallic impurities (Velmurugan et al., 2015). Different studies have been reported that corn cob ash having a silica content of 27%–60% and the extracted silica mostly amorphous in nature (Adesanya and Raheem, 2009; Okoronkwo et al., 2013; Sapawe et al., 2018; Shim et al., 2015). It might be an economically feasible raw material for silicates, silica, and silica nanoparticles.