Fate and behaviour of PFAS in natural resources: towards a safe circular economy

Resource scarcity has increased interest in the circular economy (CE) for environmental, economic, and social sustainability. The goal is to minimize waste generation and efficiently incorporate waste back into production processes without adversely impacting human health or the environment. By recognising the importance of assessing the potential accumulation of chemicals and associated  risks within the CE, the Horizon 2020 project PROMISCES focuses specifically on the so-called "forever chemicals" such as per- and polyfluoroalkyl substances (PFAS) in five CE routes, including semi-closed water cycles for drinking water (DW), wastewater reuse in agriculture, nutrient recovery from sewage sludge, material recovery from dredged sediments, and groundwater and land remediation for safe reuse. Based on the results from literature reviews, experiments and case studies, the project addresses the fate and transfer of PFAS across these CE routes. Despite the challenge of analysing PFAS in complex matrices such as sludge and wastewater, robust and sensitive methods have been developed and the following conclusions can be obtained: Wastewater treatment provides a limited removal efficiency, especially when wastewater treatment plants receive large contributions from industrial wastewater streams. Advanced wastewater treatment technologies implemented for micro-pollutant removal are not fully effective for all PFAS. Additionally, degradation of precursors can result in increased PFAS concentrations in the effluent. Consequently, until new treatment solutions are implemented, PFAS hotspots may not be able to implement wastewater reuse (e.g. for irrigation). Riverbank filtration, as a first DW treatment stage, demonstrates limited removal of PFAS. Accordingly, in the presence of an upstream emission source, DW providers may need to implement advanced water treatment technologies. During wastewater and landfill leachate treatment, particularly long-chain PFAS may accumulate in sludge. Although low level of targeted PFAS compounds were quantified, the presence of precursors in sludge is suspected and may present a barrier to its agronomic valorization. To date, PFAS content in sewage sludge is not regulated and depending on the country, sludge may be spread on agricultural land, incinerated, or disposed in landfills.  Valorisation of dredged sediment as secondary raw material has the advantage of limiting the cost of management and limiting the use of raw materials. Depending on the nature of the sediment (in particular organic matter content) and on the PFAS loads, different treatments result in different removal efficiencies. Moreover, treatments can result in the formation of new persistent PFAS from precursors. When initial loads are low, it seems possible to eliminate PFAS from the solid fraction. Nevertheless, the destruction of residual PFAS in the washing solution is necessary. In situ and on-site treatments of water and soil are confronted with environmental realities. Even if treatment trains can help overcome the complexity of PFAS treatment, the process efficiency is highly dependent on the alkyl chain length and the functional groups. As for sediment, although various treatment techniques exist, such as PFAS immobilisation, these do not result in complete degradation or removal of PFAS. This stands in the way of achieving a CE, as only after full removal of PFAS, safe reuse of resources can be guaranteed.