From forever to fluoride: A critical review of per- and polyfluoroalkyl substance (PFAS) occurrence, fate, and the transition from sequestration to destruction-based remediation in groundwater.

Per- and polyfluoroalkyl substances (PFAS) comprise thousands of anthropogenic fluorinated compounds whose exceptional stability conferred by the carbon-fluorine bond, the strongest single bond in organic chemistry has earned them the epithet "forever chemicals." Decades of use in aqueous film-forming foam (AFFF), industrial processes, and consumer products have created persistent, highly mobile groundwater plumes that are now detected on every inhabited continent. The promulgation of the first legally binding U.S. drinking water limits (4 ng/L for PFOA and PFOS) in 2024, and the regulatory controversy that followed in 2025-2026, have transformed PFAS from an emerging concern into an urgent remediation imperative. This review critically synthesizes contemporary knowledge of PFAS occurrence, geochemical behaviour, and remediation in groundwater, and is organized around a central argument: the field is undergoing a paradigm shift from sequestration to destruction. We first examine sources and subsurface transport, highlighting the physicochemical controls air-water interfacial adsorption, precursor transformation, and matrix interactions that govern plume evolution and complicate site characterization. We then show that the prevailing sequestration technologies (granular activated carbon, ion exchange, and high-pressure membranes) concentrate rather than destroy PFAS, generating problematic residual streams. Against this backdrop, the rapidly developing suite of destruction technologies, electrochemical oxidation, UV/sulfite reductive defluorination by hydrated electrons, supercritical water oxidation, plasma-based treatment, sonolysis, and base-mediated low-temperature mineralization is critically appraised using a common set of criteria: defluorination efficiency, by-product formation, energy demand, and field readiness. A multi-criteria comparison shows that no single technology is superior across all criteria, and that separation and destruction are most rationally deployed as complementary stages of an integrated "concentrate-then-destroy" treatment train. The principal contribution of this review is to place that paradigm on a quantitative footing. We develop a first-order energy model, E_total = E_sep + E_d/CF, which expresses the energy demand of a complete train as a function of the concentration factor CF and thereby reinterprets technology selection in energetic terms. The model yields an explicit design criterion-a technology-specific break-even concentration factor, CF* = E_d/E_sep that identifies how much concentration each destruction route requires before it ceases to dominate the energy budget, and shows that beyond this point the separation step governs whole-train energy. Energy is not the sole determinant of technology choice, and the framework is therefore applied alongside defluorination extent, by-product formation, technology readiness, cost, and matrix compatibility. A simple energy model formalizes this logic, showing that concentration is what renders destruction energetically viable and that, beyond a technology-specific break-even concentration factor, it is the separation step that governs the energy budget of the whole train. We close by identifying priority research needs, including validated destruction metrics, management of short-chain compounds and precursors, and treatment-train optimization. The field's central challenge is no longer removing PFAS from water, but transforming them-cost-effectively and verifiably into harmless fluoride.