Coupled fate and cascading risks of PFAS and heavy metals/metalloids along the soil-plant-human continuum: A critical review

Per- and polyfluoroalkyl substances (PFAS) and heavy metals/metalloids frequently co-occur in soils at industrial, e-waste and fire-training sites and in biosolid- or wastewater-affected agricultural systems. Yet their coupled fate and cascading risks along the soil–plant–human continuum remain uncertain. This critical narrative review separates established direct co-exposure evidence, emerging evidence and hypotheses derived from adjacent systems. Metal cations can increase retention of some anionic PFAS through acidification, charge neutralization and cation bridging, whereas oxyanions such as hexavalent chromium [Cr(VI)] and pentavalent arsenic [As(V)] can compete for positively charged sites and increase porewater mobility. In plants, short-chain PFAS are generally more mobile to shoots and long-chain PFAS more strongly retained in roots; under specific co-exposure conditions, however, metal-induced reactive oxygen species (ROS), membrane damage and barrier remodeling alter this baseline pattern. Rhizosphere exudates, arbuscular mycorrhizal fungi and soil fauna may either buffer or amplify transfer, but direct intact-soil mixture evidence remains sparse. Food-web and human studies indicate compound-, species- and endpoint-specific departures from additivity rather than universal synergy. We therefore propose an evidence-tiered framework in which soil sorption, rhizosphere ecology and root barriers constitute the first line of defense controlling the mobile fraction that enters food webs. Future studies should prioritize replicated, long-term field mixtures; simultaneous porewater, edible-tissue and soil-function endpoints; and mixture assessment using concentration addition, independent action and adverse outcome pathways.