Properties of cellulose from four freshwater macroalgae cultivated under high nutrient conditions

Abstract The four freshwater algae genera, Rhizoclonium, Zygnema, Oedogonium and Stigeoclonium are often used in filamentous algae phycoremediation systems, a process of wastewater treatment using algal bioremediation. This study explores the potential to use the algal biomass that is produced by the phycoremediation systems as a valuable feedstock for cellulose production. Cellulose extraction involved bleaching, alkali treatment and acid hydrolysis, followed by freeze-drying. The characteristics of macroalgal cellulose were assessed in terms of yield, thermal properties, crystallinity index and microstructure. Oedogonium had the highest cellulose yield (8.4%), while Zygnema had the lowest (2.2%), with Stigeoclonium and Rhizoclonium yielding 4.8% and 6.9% respectively. Notably, Rhizoclonium cellulose had a 92% crystallinity index and was thermally stable up to 345 °C. X-ray diffraction analysis revealed a unique crystalline structure for Rhizoclonium cellulose differing from typical cellulose I patterns but resembling Cladophora cellulose. Morphologically, Rhizoclonium cellulose exhibited a web-like structure, characteristic of nanofibril formation. Rhizoclonium, in particular stands out as a promising sustainable source of cellulose, with high crystallinity, thermal stability and unique Iα-rich nanofibril morphology making it particularly suitable for application as a structural reinforcement in biodegradable composites. The algae were cultivated under high-nitrogen, low-carbon nutrient conditions typical of large-scale phycoremediation systems. Using a consistent extraction methodology, this study provides a comprehensive comparison of the physicochemical properties of cellulose from four freshwater algal genera, commonly used for phycoremediation. Given the limited research on cellulose from freshwater macroalgae, this work significantly contributes to the understanding of physicochemical properties of freshwater macroalgal cellulose.