Subcritical water extraction for recovering cellulose fibres from Posidonia oceanica waste

In this study, cellulose extraction from Posidonia oceanica (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 °C) 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 °C, 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 °C 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 %).