Photocatalytic degradation of perfluorooctanoic acid under ambient conditions validated by duckweed as a sensitive ecotoxicity assay

Photocatalytic degradation of perfluorooctanoic acid (PFOA) has drawn great attention in the past. These studies have focused on developing high-efficacy photocatalysts and understanding the reaction mechanisms and structure-performance relationships. However, the photocatalytic degradation efficacy and kinetics under environmentally relevant conditions and the reduced toxicological impact of degraded PFAS products are lacking. To fill in this gap, we developed a photocatalytic system to deconstruct perfluorooctanoic acid (PFOA) under ambient conditions using sunlight as a renewable energy source. We evaluated the degradation pathway and reaction mechanisms using high-resolution mass spectrometry (LC/MS-MS) and product portfolio. The photocatalytic degradation of PFOA in environmental water matrices was set up over a two-week period using natural sunlight outdoors. The toxicity of degraded products was assessed in parallel using duckweed, or Lemna minor as a representative ecotoxicological assay during the same time course . Degraded products containing shorter carbon chain PFAS such as perfluoroheptanoic acid (PFHpA, C7) and perfluorohexanoic acid (PFHxA, C6) showed significantly reduced toxicity to L. minor , compared to untreated PFOA water. In the day and night cycles of PFOA photocatalytic treatment using natural sunlight, the measured toxicity of the photocatalytic reaction system continued to decline during the two-week period. However, the concentrations of measured PFAS, including degraded products of PFHpA and PFHxA, remained stable after six days for a period of 2 weeks. Our study suggested another potential detoxification mechanism exists with prolonged treatment, which leads to continuously reduced toxicity. Our results highlighted the need for a systematic approach combining analytical chemistry for degradation mechanisms and ecotoxicological models to perform time-dependent toxicological assessments of PFAS destruction.