Significantly Accelerated Photochemical Perfluorooctanoic Acid Decomposition at the Air-Water Interface of Microdroplets

Accurate detection and high throughput profiling of unknown PFAS transformation products for elucidating degradation pathways

The accurate detection of unknown per- and polyfluoroalkyl substances (PFAS) transformation products (TPs) is essential for elucidating degradation pathways and advancing remediation strategies. Herein, we developed a workflow that combined Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with a paired mass distance (PMD) network. This study achieved high throughput profiling of PFAS TPs with mDa resolving power and sub-ppm mass error. UV treatment revealed chain-shortening pathways, while plasma treatment uncovered competing mechanisms of chain shortening and lengthening, generating oxygen-rich TPs with increased hydrophilicity. Specifically, UV treatment of a 15-PFAS mixture and contaminated natural water showed disappearance of 7 unknown PFAS homologues and the emergence of 12 unknown PFAS homologues. Despite PFAS persistence under UV exposure, previously undetected low-abundance PFAS species were identified, indicating non-negligible photochemical transformation. Under plasma treatment of isolated PFOS, 39 unknown PFAS homologues including 142 suspect and 34 unknown PFAS TPs were identified, highlighting the extensive transformation of emerging and persistent PFAS. Overall, our approach enabled accurate and high-throughput profiling of unknown PFAS TPs and their degradation pathways, providing new insights into persistent unknown PFAS.

Accelerated Pollutant Degradation by UV/H2O2 at the Air-Water Interface of Microdroplets

Ultraviolet light-induced homolysis of hydrogen peroxide (UV/H2O2) can generate powerful hydroxyl radicals (•OH) for sustainable water purification. However, the efficiency of the conventional bulk-phase UV/H2O2 system is limited by the low yield and utilization of •OH, in turn necessitating high UV energy input and long purification period. In this study, we present an innovative UV/H2O2 microdroplet system for enhanced pollutant degradation. The degradation of pollutants in sprayed microdroplets was accelerated by 8.5-63.3-fold compared to those in bulk water, demonstrating universal effectiveness across a range of pollutant types and diverse aqueous matrices. This enhancement stems from elevated •OH production at the air-water interface due to the enhanced UV absorbance of H2O2. The production of •OH in the microdroplet system was 45-fold higher than that in bulk water, facilitating rapid •OH-mediated pollutant degradation. Moreover, pollutants accumulate at the air-water interface, where •OH is concentrated, leading to higher utilization of •OH for mediating pollutant degradation before quenching. Our findings provide a solution to overcome the bottlenecks in •OH production and utilization, offering insights for improving the efficiency of UV/H2O2 water treatment systems.

Programmable C-N Bond Formation through Radical-Mediated Chemistry in Plasma-Microdroplet Fusion

Non-thermal plasma discharge produced in the wake of charged microdroplets is found to facilitate catalyst-free radical mediated hydrazine cross-coupling reactions without the use of external light source, heat, precious metal complex, or trapping agents. A plasma-microdroplet fusion platform is utilized for introduction of hydrazine reagent that undergoes homolytic cleavage forming radical intermediate species. The non-thermal plasma discharge that causes the cleavage originates from a chemically etched silica capillary. The coupling of the radical intermediates gives various products. Plasma-microdroplet fusion occurs online in a programmable reaction platform allowing direct process optimization and product validation via mass spectrometry. The platform is applied herein with a variety of hydrazine substrates, enabling i) self-coupling to form secondary amines with identical N-substitutions, ii) cross-coupling to afford secondary amine with different N-substituents, iii) cross-coupling followed by in situ dehydrogenation to give the corresponding aryl-aldimines with two unique N-substitutions, and iv) cascade heterocyclic carbazole derivatives formation. These unique reactions were made possible in the charged microdroplet environment through our ability to program conditions such as reagent concentration (i. e., flow rate), microdroplet reactivity (i. e., presence or absence of plasma), and reaction timescale (i. e., operational mode of the source). The selected program is implemented in a co-axial spray format, which is found to be advantageous over the conventional one-pot single emitter electrospray-based microdroplet reactions.

Accelerated Indirect Photodegradation of Organic Pollutants at the Soil-Water Interface

Indirect photolysis driven by photochemically produced reactive intermediates (PPRIs) is pivotal for the transformations and fates of pollutants in nature. While well-studied in bulk water, indirect photolysis processes at environmental interfaces remain largely unexplored. This study reveals a significant acceleration of indirect photodegradation of organic pollutants at the soil-water interface of wetlands. Organic pollutants experienced ubiquitously enhanced indirect photodegradation at the soil-water interfaces, with rates 1.41 ± 0.01 to 4.27 ± 0.03-fold higher than those in bulk water. This enhancement was observed across various natural and artificial wetlands, including coastal wetlands and rice paddies. In situ mapping indicated that soil-water interfaces act as hotspots, concentrating both organic pollutants and PPRIs by 9.30- and 4.27-folds, respectively. This synchronized colocation is the primary cause of the accelerated pollutant photolysis. Additionally, the contribution of each PPRI species to pollutant photolysis and a coupled transformation pathway at the soil-water interface significantly differed from those in bulk water. For instance, the contribution of singlet oxygen to metoxuron photolysis increased from 10.1% in bulk water to 44.4% at the soil-water interface. Our study highlights the rapid indirect photolysis of organic pollutants at the soil-water interfaces, offering new insights into the natural purification processes in wetlands as "Earth's kidneys."

Spontaneous Degradation of the "Forever Chemicals" Perfluoroalkyl and Polyfluoroalkyl Substances (PFASs) on Water Droplet Surfaces

Because of their innate chemical stability, the ubiquitous perfluoroalkyl and polyfluoroalkyl substances (PFASs) have been dubbed "forever chemicals" and have attracted considerable attention. However, their stability under environmental conditions has not been widely verified. Herein, perfluorooctanoic acid (PFOA), a widely used and detected PFAS, was found to be spontaneously degraded in aqueous microdroplets under room temperature and atmospheric pressure conditions. This unexpected fast degradation occurred via a unique multicycle redox reaction of PFOA with interfacial reactive species on the droplet surface. Similar degradation was observed for other PFASs. This study extends the current understanding of the environmental fate and chemistry of PFASs and provides insight into aid in the development of effective methods for removing PFASs.