Fractionation of marine algae to its constituents towards valuable chemicals and energy products

G5 Doctoral dissertation (article)


Internal Authors/Editors


Publication Details

List of Authors: Ricardo Miguel Pezoa Conte
Publisher: Åbo Akademi University
Place: Turku
Publication year: 2017
ISBN: 978-952-12-3599-3
eISBN: 978-952-12-3600-6


Abstract

There is an urgency for finding renewable resources for the replacement of fossil resources aiming at more environmentally friendly processing. The old concept of processing forestry biomass for pulp and paper that dominated the industry for centuries has evolved to the development of biorefineries, which convert most of the biomass into products and energy. One of the most valorized fractions of terrestrial plants is cellulose, which is utilized to produce pulp, paper and recently specialty cellulose. Regarding algae, while the cellulose content is rather low depending on the species, the presence of sulfate and uronic-acid-rich polysaccharides provides a new set of molecules with industrial potential. Consequently, the aim of this work is to demonstrate the efficient processing of seaweeds in line with a biorefinery concept.

In this thesis, brown and green algae were processed for the extraction of their main constituents using water and ionic liquids, following the principle of green chemistry. Subsequently, the work focused on the depolymerization of marine polysaccharides utilizing biotechnological and chemical catalysis. Additionally, the kinetics of these reactions was thoroughly studied using mathematical modeling. Several different analytical techniques were used for characterizing the products, such as high-performance liquid and size-exclusion chromatography, gas chromatography as well as ion chromatography, Fourier transform infrared spectrometry, thermogravimetric analysis, scanning electron microscopy and transmission electron microscopy, among others.

The results showed that it is feasible to extract molecules possessing antioxidant activities, such as phlorotannins, as well as polysaccharides such as the sulfated rhamnoglucuronan ulvan. By means of water processing of the brown alga Macrocystis pyrifera (giant kelp), two phlorotannins were identified: phloroeckol and a tetramer of phloroglucinol isomer. The treatment of the green alga Ulva rigida (sea lettuce) with the distillable ionic liquid 1,1,3,3-tetramethylguanidine propionate at 120°C allowed for the dissolving of 69 wt.% of the total carbohydrate content, while the extraction of ulvan with water at temperatures below 130°C resulted in yields of up to 99 wt.%. Further depolymerization of the polysaccharides contained in brown and green algae via enzymatic and chemical catalytic processing allowed for the obtaining of monomer yields up to 86 wt.% of d-mannuronic and l-guluronic acids and 82 wt.% l-rhamnose, respectively. The activation energy for the kinetics of the extraction of ulvan, to be estimated through mathematical modeling of the experimental data, was calculated to be 54 kJ mol-1. Regarding the hydrolysis of ulvan, an activation energy in the range 134–175 kJ mol-1 was estimated. These results suggested that the absence of lignin in U. rigida facilitated the extraction of sulfated polysaccharides. Admittedly, the presence of uronic acids represented challenges for the efficient processing of the biomass, since the conditions for depolymerization and stability of the molecules are different from those of neutral sugars.

The present work provides evidence that it is possible to obtain a high-yield of products from algal biomass using processing based on the concept of green chemistry. Still, many challenges remain open for research to optimize not only the processing parameters of the biomass, but also to optimize cultivation of the seaweeds and to explore fine chemical or nutraceutical applications for their constituents.


Keywords

Alginate, Biomass, Efficient extraction, fractionation, Kinetics, l-rhamnose, phlorotannins

Last updated on 2019-15-11 at 03:42