Abstract
Due to their remarkable characteristics, cellulose nanocrystals are strategic materials that has various industrial applications, and are capable of being produced from vegetable fibers derived from the discards of agricultural practices. Peanut(Arachis hypogaea L.) peel is a residue considered of low commercial value and high polluting potential that needs new applications in order to mitigate these problems. Thus, in this study the feasibility of extracting cellulose nanocrystals was investigated. Two chemical routes were followed for this extraction. In the first, the fibers were bleached before acid hydrolysis whereas mercerization was used in the second. The second route was more efficient, as it enabled the elimination of proteins and phenolic compounds, which could be confirmed through solid-state 13C nuclear magnetic resonance (NMR) that revealed no signs of lignin residues. The cellulose nanocrystals composed of mainly type I cellulose presented a high degree of crystallinity index, 75 %, a thermal stability up to 200 °C, considerable stability in suspension (zeta potential of –48.1 ± 2.1 mV), and an aspect ratio of 125. They represent options that could add value to this residue, which would ease environmental problems.
Highlights
About 45 million tons of peanuts (Arachis hypogaea L.) were produced in 2019 in the world (National Peanut Board, 2019)
The nanocrystals obtained without the previous procedure of mercerization, route 1, maintained the coloration of peanut peel (Figure 1), possibly due to the presence of phenolic compounds, for the main part procyanidins (Constanza et al, 2012), since these compounds are strongly bound to the proteins (Xu et al, 2012) and may not have been totally eliminated from the material
For cellulose nanocrystals pre-mercerized (CNCPM) most of the lignin and hemicellulose contents were removed, which was evidenced by the FTIR and X-Ray diffractometry (XDR) results that will be shown below
Summary
About 45 million tons of peanuts (Arachis hypogaea L.) were produced in 2019 in the world (National Peanut Board, 2019). About 35 to 45 g of peanut peel (PP) are generated per kg of shelled peanut kernel, reaching over 1.5 million tons yearly (Hathorn and Sanders, 2012; Pandey et al, 2018). Lipids and proteins (Pandey et al, 2018; Zhao et al, 2012), PP can be used for production of active carbon (Saxena and Sarkar, 2012), antioxidants (Hathorn and Sanders, 2012), dyeing of textiles and as reinforcing filler in polymer composites, such as cassava starch-peanut skin based foams (Husen and Jawaid, 2020; Machado et al, 2019). Peanut skin contains approximately 18 % fiber and 41 % other carbohydrates (Machado et al, 2019)
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