Matthew Liu PhD Thesis Defense

Matthew Liu
PhD Candidate
Chemical Engineering
Academic advisor: Professor William Tarpeh

Abstract: Electrifying Chemical Transformations and Separations to Valorize Wastewater Nitrogen

Managing the nitrogen cycle has been identified as one of 14 Grand Challenges for Engineering in the 21st century, as defined by the U.S. National Academy of Engineering. Indeed, the U.S. Environmental Protection Agency considers nitrogen pollution “one of the costliest, most difficult environmental problems we face in the 21st century." Humanity has profoundly skewed the natural throughput of the nitrogen cycle through Haber-Bosch ammonia synthesis, which outpaces nitrogen removal rates from wastewater. As a result, nitrogen pollution continues to accumulate in the environment, threatening global water security and human health. However, as global populations continue to grow, society will require more ammonia than it ever has before. Electrochemical nitrogen recovery from wastewaters offers an avenue for bringing balance back to the nitrogen cycle by directly recovering ammonia from wastewater nitrogen. Two forms of reactive nitrogen in particular compose the majority of nitrogen pollution in wastewaters: ammonia and nitrate. Targeting these two pollutants for ammonia recovery is a key focus of this dissertation. Whereas ammonia requires a selective separation from other wastewater constituents to be recovered as a pure product, nitrate requires selective reduction to ammonia prior to separation. 

In my talk, I will discuss electrochemical stripping (ECS), a process that combines electrodialysis and membrane stripping into a single process unit to selectively remove ammonia from real wastewaters. With ECS as a validated platform for ammonia recovery, I will then investigate the use of heterogenous and homogeneous catalysts for selective electrochemical nitrate reduction to ammonia. A vignette on heterogenous catalysis will cover the use of synchrotron X-ray characterization to examine how the near-surface structure of titanium (Ti) electrodes evolve due to various nitrate reduction conditions. It is found that the near-surface is enriched in titanium hydride (TiH2) under more negative applied potentials or longer reaction durations. The electrochemical performance of unamended Ti electrodes is compared to preformed TiH2/Ti electrodes. Lastly, a vignette on homogeneous catalysis will examine the reaction mechanisms and kinetics of nitrate reduction on a Co(DIM), a cobalt-centered tetra-aza macrocycle. Electroanalytical studies demonstrate that prior to nitrate conversion, Co(DIM) must free its axial sites through bromide dissociation coupled with electron transfer. The kinetics of nitrate conversion are then quantified with foot-of-the-wave analysis to benchmark the performance of Co(DIM)-mediated nitrate reduction. Altogether, the insights from this dissertation advance resource recovery efforts and incorporate elements from electrochemical engineering, electrocatalysis, materials science, and molecular electrochemistry.