Application of Subcritical Water for Conversion of Macroalgae to Value-Added Materials

Subcritical water technology was used for treatment of Codium fragile (C. fragile) as a model for green macroalgae. Subcritical water reactions were carried out for 10 min at temperatures ranging from 100 to 240 degrees C. After reaction, three main phases were isolated: aqueous, residual solid, and hexane soluble phases. The aqueous phase showed a very high total organic carbon (TOC) content due to decomposition and solubilization of the cellulosic portion and other organic molecules of the algae. The main products identified in the aqueous phase were soluble sugars, derived from hydrolysis reactions. The production of other biobased compounds was also examined and several value-added compounds were identified. Investigation of the residual solid phase after treatment showed that with increasing subcritical water temperature, the amount of total residual solid declined and finally reached a constant amount at higher temperatures, attributed to the inorganic composition of the algae sample. The maximum inorganic composition of the sample was identified as approximately 36% based on dry matter. We also evaluated and compared the higher heating values (HHVs) of the residual solid residue and initial sample base on ultimate analysis. Finally, the hexane soluble content of this type of algae was found to be very low.