Abstract
Our study was motivated by the large amount of crop residues produced in Brazil, which represent underutilized waste biomass and a serious threat to the environment because of the landfilling. For example, even though the corn crop is not popular in Brazil, it is produced in the area of around 18-20 million hectares with an average production of 5-6 t ha-1. That is the reason to seek for the crop residues value-added applications as proposed in the present study. Four Brazilian crop waste biomass: orange bagasse, corn husks, sugarcane straw and coffee residues were used for cellulose nanofibers (CNF) production. CNFs were prepared using alkali treatment, followed by bleaching with sodium chlorite and extraction with oxalic acid. All steps were applied under moderate conditions of temperature and pressure, such as temperature of 120 °C and below, water solutions with chemicals’ concentrations lower that 10% and short sonication pulses. CNFs with diameters in the range 50-70 nm were obtained and products from orange bagasse and corn husks presented high crystallinity indexes (CI), 0.72 and 0.75, respectively. The highest CNF yield was obtained from corn husks (38.5%) followed by sugarcane straw (24.0% with CI of 0.69). CNFs obtained from coffee residues showed a CI value of 0.65 after two bleaching steps. Different CNF morphologies were obtained according to the raw material. The four-crop waste biomass can be considered as excellent starting materials for CNF production in the four-steps process that adds new value to agro-industrial waste and might bring great economical valorization to Brazilian crops production.
Highlights
Brazil is a country with many advantages for agriculture production due to the intense solar radiation and fertile soil
Other study[35] explored coffee husks at reflux temperature for alkaline cooking, an exhausted bleaching at reflux temperature for about 24 h, an acid treatment with H2SO4 (64%) and additional step of ultrasonic shearing for 30 min. These processes resulted in cellulose nanocrystals (CNC) with mean diameters of 8-20 nm and crystallinity indexes (CI) of 0.72-0.92, somewhat higher when compared to the results reported (0.65) from coffee residues
Analogous heterogeneous network structure of cellulose nanofibers (CNF) with widths between 10-50 nm were reported by hydrolysis of cellulose from sugarcane bagasse with 64% sulfuric acid;[12,43,49] while Mariño et al.[42] obtained CNF from orange bagasse using 5% sulfuric acid. These results show that despite having applied hydrolysis with concentrated acid to cellulose from sugarcane bagasse, homogeneous CNF or CNC were not obtained from biomass with a high lignin content at moderate conditions
Summary
Brazil is a country with many advantages for agriculture production due to the intense solar radiation and fertile soil. A huge quantity of a biomass coming from different agricultural residues is generated per year, which by far are underutilized as resources for fine chemicals’ production. Among the most cultivated crops, stand out oranges, coffee, sugar cane and corn, which leave around 80 million tons of waste per year. The rests of corn and sugar produced in Brazil, which in 2018 corresponded to 8 and 20% of world production, respectively,[2] such as corn husks and sugarcane bagasse, are mainly used to produce biogas and/or bioethanol.[3] Besides, coffee produced in Brazil corresponded to about 32% of world production and its residues are less explored,[2] mostly burned, disposed or used as fertilizers.
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