Harnessing microbially mediated redox processes for sustainable water treatment

Abstract

Water and wastewater treatment are essential for ensuring environmental and human health and identifying sustainable low carbon technologies for water treatment is now critical. However, the incomplete removal of organic micropollutants during conventional wastewater treatment continues to pose an emerging threat to our water resources. Encouragingly, AOPs, which rely mainly on the production of reactive oxidizing species such as OH radicals (•OH), have successfully been applied to degrade micropollutants during water treatment although currently applied AOPs are energetically expensive with high carbon footprints. Recent studies have shown that •OH can be effectively generated and degrade micropollutants during the oxygenation of natural ferrous iron-bearing clay minerals. To contribute to this research field, this PhD research provides the proof-of-concept for a sustainable clay-based advanced oxidation process (AOP) for water treatment through generation of hydroxyl radicals. Using mesocosm column experiments, this study has demonstrated that clay mineral iron can be effectively and sustainably ‘activated’ by microbial communities from local pond water, through microbial iron reduction, in the first essential step of this novel AOP. The introduction of oxygenated water produced hydroxyl radicals (•OH) in-situ as reduced clay mineral iron was oxidised and subsequently led to the degradation of a surrogate micropollutant, benzoic acid. Treatment efficiency and •OH production were measured during two time-series experiments and three activation treatment cycles in microbially-activated columns. Chemical activation of the same clay mineral was tested in a separate experiment and yielded comparable results for treatment efficiency and •OH production. Analysis of 16S rRNA sequencing data indicated that although microbial community diversity narrowed and species numbers decreased over time with increasing treatment cycles, known iron-reducing microbes appeared to be resilient to •OH exposure over three treatment cycles. Implications of this research include promise for this novel clay-based AOP although the long-term impact of microbial iron reduction on clay mineral integrity requires further study.