Burning questions: Current practices and critical gaps in evaluating removal of per- and polyfluoroalkyl substances (PFAS) during pyrolysis treatments of biosolids

Abstract

Concerns surrounding potential health and environmental impacts of per- and polyfluoroalkyl substances (PFAS) are growing at tremendous rates because adverse health impacts are expected with trace-level exposures. Extreme measures are required to mitigate potential PFAS contamination and minimize exposures. Extensive PFAS use results in the release of diverse PFAS species from domestic, industrial, and municipal effluents to wastewater, which partition to biosolids throughout secondary treatment. Biosolids generated during municipal wastewater treatment are a major environmental source of PFAS due to prevailing disposal practices as fertilizers. Pyrolysis is emerging as a viable, scalable technology for PFAS removal from biosolids while retaining nutrients and generating renewable, raw materials for energy generation. Despite early successes of pyrolysis in PFAS removal, significant unknowns remain about PFAS and transformation product fates in pyrolysis products and emissions. Applicable PFAS sampling methods, analytical workflows, and removal assessments are currently limited to a subset of high-interest analytes and matrices. Further, analysis of exhaust gases, particulate matter, fly ashes, and other pyrolysis end-products remain largely unreported or limited due to cost and sampling limitations. This paper identifies critical knowledge gaps on the pyrolysis of biosolids that must be addressed to assess the effectiveness of PFAS removal during pyrolysis treatment.

Keywords: Mass balance; Non-targeted analysis; Organofluorine emissions; Sampling; Thermal treatment.

Fig. 1.

Pyrolysis system processes, compartments, and critical sampling locations (red boxes). Shaded boxes identify processes amenable to aggregation for controlled, lab-scale evaluation.

Fig. 2.

Pyrolytic degradation pathways of PFAS (e.g. PFOS) to perfluoroalkanes, 1H-perfluoroalkanes, perfluoroalkenes, short-chain PFAS, and others, including acyl fluorides, carboxylates, and sulfonamide derivatives.

Fig. 3.

Schematic of critical, interconnected needs for PFAS analysis during pyrolysis treatment of biosolids, including atmospheric sampling (A-C), and important, characterization techniques (D–E).