Cascading Biorefinery Approaches for the Sustainable Fractionation of Macroalgae

More Info
expand_more

Abstract

The growing population is accompanied by an increase in the global energy and resource demand. Biorefining can be considered a means to help meet these growing demands through a range of bio-based materials. A biorefinery approach for developing an upstream process for the production of valuable products from macroalgae would be essential to realize a feasible valorisation chain downstream, wherein these products can be tailored to various applications. On a global scale, the biorefinery concept incorporates mainly, terrestrial first and second-generation biomass which compete with food and other energy applications, as well as involving sustainability issues such as land use change. Third generation macroalgae or seaweed thus has tremendous scope within the biorefinery concept. Seaweed has rapid reproduction rates and can be cultivated with minimal resource inputs thereby offering the attractiveness of high biomass yields. This study deals with the sustainable fractionation of two seaweeds, namely, Ulva lactuca (green seaweed) and Palmaria palmata (red seaweed) for the extraction of rhamnose and xylose, respectively. These sugars are of interest as they can be used to produce 5-methylfurfural and furfural serving as platform chemicals for various applications, including fuel production. Focus is laid on alternative hydrolysis methods of low severity for the extraction of the targeted sugars including enzymatic hydrolysis, hot water treatment and hydrolysis with organic acids and chelating salts. The characterization of the seaweeds and determination of their biochemical compositions are performed by acid hydrolysis, after the application of pre-treatment steps like washing with water and soxhlet extractions in order to maximize the sugars recovered. These compositions then form a basis for the study of low severity and more sustainable hydrolysis methods.A series of low temperature enzymatic hydrolysis experiments are conducted in order to screen a number of potential enzymes for the two seaweed types, which led to enzyme selection for further investigation. It is followed by a study of the effect of operating conditions and parameters namely, the reaction temperature, the pH and enzyme dosage. Hydrolysis treatments with hot water, organic acids (acetic, formic, oxalic, citric) and chelating salts (sodium acetate, sodium citrate) are also conducted at different temperatures and for a selected range of reaction times. These are carried out in a multiclave, enabling the running of a large number of screening tests to determine the best combination of the hydrolysing agent, temperature and reaction time. The results from these low severity treatments and enzymatic hydrolysis tests are applied onto a larger reaction scale in order to implement a cascade biorefinery approach for the extraction of rhamnose and xylose. Hot water treatments at 120℃ and 140℃for the Ulva lactuca and Palmaria palmata, respectively are chosen as the feasible routes. As for the enzymatic hydrolysis studies, hydrolysis with ‘X’ enzyme cocktail and ‘Y’ at 50℃ are selected. Enzyme dosages of ‘a’ % and ‘b’%, respectively are deemed suitable for the release of the targeted sugars after 72 hours of hydrolysis.Ultimately, the cascading approach is applied to a larger amount of seaweed (1.5 kg) where the biomass is washed followed by subsequent centrifugation in a decanter-centrifuge to separate the washed seaweed. The washing liquid fraction is further fractionated through membrane filtration to recover MW fractions of >100kDa, 100-1kDa and <1kDa. The separated solids are thereafter subjected to enzymatic hydrolysis in the bioreactor as well as autoclave treatments at a 2-3L scale, in accordance with the conclusions from the screening treatments in order to further evaluate the scalability of the process. In this cascading approach, a complete mass balance of the seaweed biomass components of interest (sugars, inorganics, protein) is carried out in order to evaluate the efficiency of the cascade approach and maximize the same by realizing the potential of the by-product and residual streams.