Subcritical water extraction for recovering cellulose fibres from Posidonia oceanica waste

Abstract

Posidonia oceanica (PO) is an abundant aquatic plant in the Mediterranean Sea that produces a great accumulation of leaves on the coast when detaching off the plant. PO waste collected at landfills could be used as a source of cellulose due to the considerable content of this component (30–40%). In this study, cellulose extraction from PO waste was studied in order to reduce chemicals in the process, in line with the green chemistry principles. Thus, subcritical water extraction (SWE) was applied to promote the separation of non-cellulosic compounds, such as hemicellulose and lignin, followed by bleaching treatments using hydrogen peroxide, alternatively to the usual sodium chlorite. Two SWE temperatures (150 and 170 oC) were tested, while hydrogen peroxide was used at 4 and 8% (v/v) at pH 12 in four one-hour bleaching cycles. This treatment was also carried out with sodium chlorite for comparison purposes. SWE efficiently reduced hemicellulose and lignin content in the solid extraction fraction, mainly at 170 oC, which yielded 63 wt. % of solid fraction, with 51% of cellulose content. This highest temperature also promoted the efficiency of the subsequent bleaching step. Using H2O2 as the bleaching agent, alternatively to chlorine agents, was effective at purifying cellulose but partially altered the cellulose structure through oxidative mechanisms. A combination of SWE at 170 oC and bleaching with H2O2 at 4 or 8% (v/v) yielded 24 wt. % bleached material from PO waste, with a high cellulose richness (near 90%).