Estudo do potencial das macroalgas Sargassum sp. e Padina sp. para extração de nanocelulose

Abstract

In recent years, there has been an increase in the search for new materials that are more sustainable and of renewable origin. Among these materials, cellulose nanocrystals (CNC) have gained prominence due to their origin from cellulose-rich raw materials. CNCs can be used in various industrial sectors to improve optical and mechanical properties, as well as provide stability and environmental benefits to a variety of bioproducts. They can be obtained from various sources, such as plant fibers, agroindustrial residues, and seaweed, the latter having the advantage of a lower lignin content. However, to ensure quality and reduce costs in the extraction of CNCs, it is essential to understand the parameters that influence the process. In this work, the potential of brown macroalgae Sargassum sp. and Padina sp. as a source of cellulose nanocrystals (CNC) was explored, investigating the impact of alkaline pretreatment (performed with 2% NaOH), bleaching (with 30% H2O2), and combined treatment (alkaline followed by bleaching), as well as the effects of operational parameters time (45 minutes and 90 minutes) and temperature (45°C and 60°C) on hydrolysis with sulfuric acid. The pretreatment of biomass reduced its impurities, increasing the cellulose content from 29.57% to 82.23% in Sargassum sp. and from 30.27% to 79.23% in Padina sp., thus favoring the extraction of CNC by acid hydrolysis. The CNCs produced under the mildest operational conditions evaluated, specifically at 45°C and 45 minutes, resulted in the best mass yields, ranging from 25.81% to 87.41% for Sargassum sp. and from 39.87% to 62.89% for Padina sp. These CNCs were characterized by Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), zeta potential, X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The main results showed that the samples subjected to alkaline treatment and combined treatment showed FTIR spectra with a decrease in the peaks characteristic of lignin and hemicellulose, as well as an increase in the intensity of the peaks characteristic of cellulose. A reduction in the intensity of the peak associated with water molecule absorption was also observed, due to the decrease in the number of free hydroxyls, suggesting the dissolution of amorphous regions. The zeta potential results indicated that all suspensions showed values between -42.5 to -66.4 mV, demonstrating good suspension stability. The XRD analysis revealed that the pretreatment did not change the native cellulose configuration to cellulose type II. However, the crystallinity index (CI) was significantly higher in the Sargassum sp. samples subjected to alkaline treatment (CI=70.95%) and combined treatment (CI=88.13%) compared to the merely bleached biomass (CI=36.31%). A similar behavior was observed for Padina sp., with CI=78.76% for alkaline treatment and CI=87.46% for combined treatment, while the bleached biomass showed CI=39.81%. These results indicate that bleaching alone is not sufficient to improve the extraction of CNC from macroalgae. The thermogravimetric analysis showed that the combined treatment resulted in a decrease in thermal stability for both macroalgae, due to the substitution of hydroxyl groups by sulfate groups during the hydrolysis process. The DLS results indicated that the hydrolysate obtained from Padina sp. subjected to the combined treatment showed a size distribution of over 70% in dimensions below 250 nm, while the hydrolysate from Sargassum sp. showed a size distribution of over 90% in dimensions below 100 nm, indicating the presence of CNC. The mercerized Padina hydrolysate showed a size distribution of over 25% in dimensions below 250 nm, and Sargassum subjected to the same pretreatment showed a size distribution of over 15% in dimensions below 250 nm, suggesting that alkaline treatment has potential for the production of nanocellulose under other operational conditions. The results suggest that it is possible to obtain CNCs with a high crystallinity index from the acid hydrolysis of macroalgae from the genera Sargassum sp. and Padina sp. subjected to alkaline and combined pretreatment, with a probable decrease in environmental costs associated with nanocellulose extraction stages.