Mechanochemical Reactions at Solid Interfaces: The Destruction of Per- and Polyfluoroalkyl Substances (PFASs)

Abstract

The novel application of mechanochemistry for the destruction of hazardous organic substances presents a compelling solution for the treatment of environmentally recalcitrant pollutants in contaminated media (e.g., soil). This treatment technique, known as mechanochemical destruction (MCD), requires in-depth fundamental research to become a genuinely accepted technology with the potential to inform scale-up and implementation activities. However, the scope of work for the few existing MCD studies available in the literature was generally limited to establishing destruction efficiencies (DEs) as opposed to explicating the complex underlying mechanisms of the MCD process. This thesis systematically investigates the physical and structural transformation of solids subjected to mechanochemical conditions, as well as the subsequent reactivity of these mechanically activated solids to destroy persistent organic substances. Quartz sand was deemed the most suitable co-milling agent for organic pollutants due to its demonstrated ability to form reactive surfaces which facilitate MCD reactions. Therefore, before the commencement of pollutant degradation trials, quartz sand was mechanochemically milled and the samples interrogated by a range of analytical techniques to track matrix amorphisation and the generation of reactive surface species. This work revealed that surface area growth dominated the initial phases of mechanochemical milling and that reactive surface sites formed predominantly in the latter stages of milling when particles were more amenable to abrasive effects rather than particle size reduction. These observations offer a new perspective of the lag-phases encountered in both degradative and synthetic mechanochemical reactions. The kinetic evaluation and fitting of MCD reactions is not well-covered in the literature due, in part, to complicated overlapping reaction steps that often result in sigmoidal destruction curves. The Finke-Watzky (F-W) two-step kinetic model was employed in this work to expound the multi-step reaction progression of the MCD process; differentiating between the mechanical activation of solid matrices upon mechanochemical grinding and the subsequent effect these reactive surfaces have on organic pollutants. The F-W model was able to quantitatively fit the destruction kinetics for numerous organic pollutants subjected to MCD conditions, giving robust mathematical fits as well as physically meaningful justifications for the general MCD reaction process. The cornerstone of this PhD thesis focused on developing core foundational concepts associated with the molecular mineralisation of organic pollutants upon mechanochemical treatment. Per- and polyfluoroalkyl substances (PFASs) were selected as the target class of organic pollutants due to their recalcitrant properties and recent emergence as environmentally persistent contaminants that conventional treatment technologies have struggled to effectively destroy or remediate. In addition to establishing high DEs for PFASs (≥99.93%) under mild MCD conditions, this work describes a comprehensive and fundamental chemistry-based approach that elucidated key reaction mechanisms by extensive degradation product analysis and the determination of pollutant fate upon reaction completion. PFASs were eventually mineralised by an iterative chain-shortening mechanism that occurred in parallel with an array of other degradative reactions. The exhaustive set of analytical tools utilised in this work empirically validated the dynamic destruction profile of PFASs subjected to mechanochemical treatment. Finally, the MCD technique was applied to ‘real-world’ challenges, specifically for the destruction of organic pollutants in PFAS-containing obsolete products and PFASimpacted solid waste. Comprising substantial concentrations of PFASs, aqueous film-forming foams (AFFFs) present a major exposure pathway to the environment having been applied to land at fire-training sites globally for decades. This has led to significant contamination of environmental media, which has in-turn negatively impacted the health of communities within the vicinity of these facilities. Upon mechanochemical treatment, PFASs were indiscriminately and effectively destroyed in AFFF concentrates as well as highly contaminated soil, despite the incredible complexity of these substrates. The observed DEs ranged from 99.99% and 100% for a wide range of target PFASs, demonstrating the capability of the MCD technique to address industrially-relevant pollution challenges. The comprehensive and systematic approach employed throughout this PhD research provides a novel insight into the complex reactions that occur during mechanochemical treatment. Collectively, the distinct projects of this thesis form part of an overarching narrative to determine the fundamental mechanisms that initiate and propagate MCD reactions, which will inform ongoing research and scale-up development. As such, this work has created a platform for both fundamental mechanochemistry research and its application as a green treatment technique for hazardous organic pollutants.