Microalgal synthesis of metal nanoparticles - towards a circular economy

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Heavy metals, such as cadmium, essential components of various dyes or other industrial processes, are listed among the ten chemicals of major public concern by the WHO for their potential to be carcinogenic and inflict acute organ damage. Algal systems for bioremediation (phycoremediation) are highly attractive as they are autotrophic - relying on sunlight and carbon dioxide as inputs. This project aims to address sustainable development goals by creating scalable methodologies for wastewater treatment using microalgae to extract and upgrade heavy metals into valuable metal nanoparticles. Using a model system, this project seeks to understand how photobioreactor technologies suitable for in-situ remediation might be developed. Under heavy metal exposure, plants and algae produce phytochelatins (PC) - glutathione-derived compounds. These peptides form stable complexes with metal, protecting the cells. The diatom alga Phaeodactylum tricornutum has been selected as a model organism for this project as it is known to bioaccumulate cadmium (Cd) and can tolerate a wide range of salinities. The aim of this project is to characterise the interactions between upstream cultivation parameters and cadmium nanoparticle formation and how these might relate to process scale up. Through the design of novel experimental systems for studying heavy metal uptake and engineering characterisation of the microenvironment, this project will seek to understand how the form of metal nanoparticles can be manipulated. The objectives identified: OBJECTIVESO1: To quantify the impact of cultivation conditions on cell growth and Cd uptake through growth experiments, cell viability testing and analysis. The nanoparticles formed will undergo morphological characterisation of size, shape and distribution. O2: To examine the impact of reactor design and operation on retention times, to identify strategies for maximising throughput by understanding geometries and fluid flow conditions for improved mass transfer characteristics.O3: To investigate the primary mode in which the bioreactor may operate, and how this may influence subsequent processing of cadmium by characterising the benefits/drawbacks of two mechanisms: bind and elute for recovery of metal bound to the cell surface vs recovery of cells and separation of cadmium from biomass. Green chemistry techniques will be evaluated for nanoparticle recovery. IMPACT: Our goal is to develop a phycoremediation system for application in the field. Following on from a Knowledge Exchange workshop supported by SfAM we propose to work with NGO partners to identify case study sites in India in order to apply a user-centered design approach to a future water treatment process and evaluate technoeconomic feasibility. Findings from this project will inform future design efforts and will provide an evidence base for a pilot remediation system.