Advancing macro and microalgal valorisation via green technologies

This research explores the valorisation of renewable algal biomass feedstocks into materials and chemicals via green technologies in the context of a zero-waste biorefinery. The work reported in this thesis is divided into two areas corresponding to the two algal feedstocks explored: namely i) the production and characterisation of naturally-inherent sulfur-containing porous carbons derived from unrefined carrageenan extracts obtained from Irish Moss macroalgae; and their novel application to copper(II) remediation, and ii) the fractionation of commercial microalgae, ALG01, into antioxidant-rich lipids via supercritical CO2 extraction, hydrogel forming defibrillated cellulose via hydrothermal microwave processing, and proteins via tandem cell disruption and ultrafiltration methodologies.
The pyrolysis of carrageenan aerogels afforded porous chars rich in sulfur (3.9-7.6 %). Low pyrolysis temperatures (250 °C) developed acidic, sulfate surface chemistry facilitating copper(II) adsorption (54 mg g-1) under acidic conditions (pH 1) described well by Langmuir adsorption isotherm and pseudo-second order kinetic models. In contrast, high temperatures (800 °C) incorporated sulfur as organic thiol/thiophene functionalities and carbothermic reduction of deposited sulfate salts developed alkaline magnesium oxide composites which promoted basification and deposition of copper(II) (86 mg g-1, pH 4). These results prompt future development of carrageenan-derived sulfur-containing carbons for metal adsorption.
Lipid extracts rich in PUFAs, pigments and phenolics with significant antioxidant activity were obtained from ALG01 via supercritical CO2 extraction (300 bar, 35-50 °C). Supercritical CO2 pretreatment perturbed hemicellulose hydrolysis, however, hydrogel forming defibrillated cellulose was still afforded following acid-free 220 °C hydrothermal microwave treatment of the deoiled ALG01. Cell disruption methods, ultrasonication and microwave treatment, were successfully implemented and evolved larger water-soluble proteins (100-150 kDa). A maximum yield of 3% and 12% purity was achieved following tandem ultrasonication and ultrafiltration. This research has contributed significant new knowledge to the concept of zero-waste algal biorefineries.