From cells to flocs: a study of algal organic matter and bubble interactions in flotation-based harvesting of microalgal biomass

Abstract

Human activities have irreversibly disrupted the ecological balance, primarily by releasing vast amounts of CO2 into the atmosphere. This has resulted in increases in global temperature and ocean acidification, and therefore, thawing of the polar ice reserves, disturbance of the ocean’s global conveyor belt, an increase in the frequency of extreme weather events, and last but not least, loss of biodiversity. There is a pressing need for eco-friendly methods of producing food and materials that do not exacerbate climate change, considering the growth of the human population and rising consumerism. Microalgae cultivation can play a crucial role in these efforts. Environmentally, with the rise of algal blooms, it is vital to efficiently remove excess microalgae from water bodies before they decompose and create an anoxic environment or release toxins. In this regard, coagulation combined with flotation offers a promising solution to these challenges in both freshwater and saline environments. Furthermore, microalgae cultivation has the potential to replace many traditional biomass production processes, using minimal land and water resources to address various challenges faced by modern society. However, despite their ability to sequester large amounts CO2, the valorization of microalgae biomass demands high energy expenditures, which increases both the carbon footprint and production costs. Hence, optimizing the harvesting process, which accounts for a significant portion of the total costs, can result in commercially competitive microalgae products and commodities. A comprehensive investigation into the coagulation and flotation separation of the model microalgal Chlorella species aims to improve the efficiency of these processes. This study examines the biotechnological aspects of microalgae, focusing on the interactions between cells, coagulants and bubbles. Microalgae undergo changes in their intracellular and cell wall composition as they absorb inorganic minerals and release organic matter, known as AOM. These dynamic natural processes can affect the coagulation and flotation efficiency of the algal biomass. Therefore, interdisciplinary research integrating biology, chemistry, and material science is essential to uncover sustainable and efficient solutions for harvesting microalgal biomass. The methodology used in this study involves experimental testing of the separation performance of the eco-friendly chitosan coagulant in combination with flotation. Coagulation-flotation provides several advantages, mainly that it is a process that is fast, highly concentrates the biomass, and can be integrated in-line in scaled-up operations. The cells and the AOM were analyzed to better understand their characteristics through chemical and instrumental analyses relative to the separation response of the culture and gain insight into the interplay between the biological and chemical differences. The findings highlight a strong influence of environmental conditions and specific interactions between the chitosan, AOM, and the cell surface. These interactions increase during the stationary growth phase, leading to the formation of environments based on hydrogen bonding and electrostatic interactions. The latter occurs when the chitosan’s amine groups are protonated and interact with the acidic residues of polysaccharides present in the AOM. Additionally, the impact of the growth phase (in terms of cells, AOM, and medium) on floc formation and its characteristics was recorded. This study revealed the advantages and disadvantages of different dosing strategies of chitosan, depending on the growth phase. Findings also endorse a strong influence of the environmental conditions in the experimental set-up and the specific type of interaction of the functional groups between chitosan, AOM, and the cell surface. It is inferred that the interaction multiplies in the stationary phase of growth, creating environments as described before.