Single Atom Catalysts on Biochar for the Removal of Organic Contaminants in Water

Abstract

Extensive uses of pharmaceuticals in hospitals, livestock farms, and human activities have resulted in high concentrations of these chemicals in discharged water. However, conventional wastewater treatment processes are ineffective in removing these contaminants, leading to a significant presence of pharmaceuticals in surface water, and posing threat to both wildlife and humans. Metal nanoparticles have been widely explored for the removal of organic contaminants in water. However, their propensity for aggregation accompanied by possible metal leaching hinders their broad applications. Single-atom catalysts, as an ultimate version of metal nanoparticles, offer a promising solution to overcome these limitations and provide an excellent method for environmental contaminant removal. This dissertation aims to develop two cost-effective single-atom catalysts on biochar support to effectively remove pharmaceuticals from water without additional oxidants or energy input.

Chapter I provides an introduction to this dissertation, offering background information on pharmaceuticals and highlighting the potential use of biochar as a support material for single-atom catalysts to achieve direct degradation of pharmaceuticals without the need for additional oxidants and energy.

Chapter II presents a comprehensive review of biochar, including its synthesis and the influence of different parameters on its properties. Special emphasis is placed on novel findings regarding the electron exchange capacity of biochar.

Chapter III focuses on the review of single-atom catalysts (SACs), covering common characterization methods and emphasizing the importance of support materials for the functionality of catalysts. The chapter primarily explores the application of various SACs for the removal of organic contaminants in water for environmental purposes.

Chapter IV details the synthesis of single-atom Zn catalyst on biochar support and reports the first instance of direct oxidation of TMP by single-atom Zn catalyst on biochar. The study also identifies superoxide and hydroperoxyl radicals as the main reactive species for TMP oxidation. Overall, this study demonstrates a combined radical and nonradical way for TMP oxidation by single-atom Zn catalyst alone, without other oxidant and energy input.

Chapter V investigates the influence of light irradiation on the performance of single-atom Zn on biochar for the degradation of IBP, TMP, SMX, and CBZ. The study reveals enhanced generation of reactive species and the production of hydroxyl radicals under light irradiation. Most importantly, the study demonstrates that the redox potential of contaminants is a crucial factor impacting their degradation by the single-atom Zn catalyst. For instance, a high redox potential of IBP will withdraw electrons from the catalyst and be reduced, while low redox potential chemicals such as TMP and SMX are oxidized.

Chapter VI explores the synthesis of single-atom Fe catalyst on biochar support, achieving more efficient degradation of TMP. This is attributed to the presence of high-valent Fe-oxo species in addition to the reactive oxygen species in the system. Direct electron transfer between the catalyst and contaminant was also observed. The chapter also highlights the significant impact of pH on the catalyst performance, with enhanced degradation under acidic conditions due to increased reactive species generation. However, direct electron transfer is less affected by pH variations.

Chapter VII provides a comprehensive summary of the dissertation and offers recommendations to further strengthen current studies on single-atom catalysts. Several research questions are recommended for further exploration.