Synthesis and characterization of starch-based cationic flocculants for harvesting microalgae.

Abstract

Harvest of tiny microalgae cells is a technically and economically challenging step in algal biomass production and utilization. Many techniques have been developed and tested, and some of them are used with limited success. However none of these techniques has broad commercial applications, especially in the area of algae based fuels production. The purpose of this thesis project was to develop a novel flocculation process which would be able to concentrate algal biomass, allowing efficient separation and collection of algal biomass from culture broth. The core of the research was to develop a procedure to synthesize starch-based cationic flocculants which would be renewable, biodegradable, and non-toxic with harvest performance comparable with or better than commercial flocculants. The procedure involves cationization reaction between starch and glycidyltrimethylammonium chloride (GTAC), where the hydroxyl groups on starch are substituted by quaternary ammonium cations with the help of alkaline catalyst. In this project, experiments were designed and carried out to study the effects of key reaction variables, namely temperature, time, GTAC dosage, water content, and catalyst dosage on the degree of substitution (DS), an indication of how well GTAC was utilized in the reaction, and reaction efficiency (RE). The DS and RE generally increased and then decreased with increasing temperature, time, water content, and catalyst dosage. An increase in GTAC dosage increased DS but decrease RE. Pretreatments of starch using acid, alkaline, and microwave did not significantly affect the DS and RE. The resultant cationic starch flocculants and cationic polyacrylamide (CPAM), a commercial flocculant, were tested in harvesting experiments involving live algae grown in fresh water or animal manure. The harvest efficiency was affected by DS of the cationic starch, flocculant dosage, pH, and flocculation time. The cationic starch based flocculant performed better than CPAM in all conditions. Pilot scale cationic starch production and harvest tests were conducted. The results agreed well with those obtained from the lab scale experiments. Based on these results, an optimized cationic starch synthesis procedure was proposed. The novel procedure has great potential for commercial production of renewable, biodegrable, and non-toxic starch based flocculants for cost effective and eco-friendly harvest of algal biomass.