Per- and polyfluoroalkyl substances in sewage sludge: A global synthesis of persistence mechanisms, analytical complexities, and sustainable remediation strategies

Abstract

Per- and polyfluoroalkyl substances (PFASs), known as “forever chemicals” because of their persistent carbon–fluorine bonds, pose critical environmental risks. Sewage sludge is a major reservoir and secondary pollution source of PFASs. This review synthesizes the environmental behavior, analytical methods, global distribution, regulatory landscape, and mitigation strategies of PFASs in sludge. It also addresses complex interactions between PFAS and sludge components (organic matter, inorganic particles, extracellular polymeric substances, and microbial communities), which drive PFAS accumulation via hydrophobic interactions, ion bridging, and complexation. Analytical challenges arise from sludge heterogeneity, high moisture content, and diverse PFAS species. To address these challenges, integrated targeted/non-targeted approaches and total organic fluorine analysis are required. Global monitoring reveals stark regional disparities: U.S. sludge shows ∑PFAS up to 3,390 ng/g (dry weight) with dominant long-chain PFOS/PFOA, while Nordic countries exhibit lower levels (PFOS < 10 ng/g) owing to early regulations. Notably, the contents of short-chain PFASs and precursors (e.g., MeFOSAA and EtFOSAA) are increasing, reflecting industrial substitutions and incomplete wastewater treatment. Although regulatory frameworks are shifting from compound-specific limits (e.g., a PFOS of 5.2 ng/g in Maine, US) to class-based controls (e.g., ∑11 PFASs ≤ 3 ng/g in Sweden), many thresholds are insufficient to address contamination levels. Treatment technologies—from physical separation (adsorption and membrane treatments) and thermal destruction (incineration, pyrolysis and hydrothermal liquefaction) to advanced oxidation/reduction processes, plasma/supercritical water oxidation, and biological methods—show varying efficiencies. Thermal destruction approaches (≥650°C) achieve >99% mineralization but incur high energy costs, while biological routes achieve limited precursor transformation. Environmental fate assessments highlight three key behaviors of PFAS: migration in amended soils, plant uptake (particularly short-chain PFAS in leafy crops), and “delayed release” from precursor degradation. These behaviors threaten food chains and microbial ecosystems. Key future directions include developing high-sensitivity detection, elucidating binding mechanisms, establishing life-cycle risk models, innovating cost-effective green technologies, and advancing policy-driven management—providing a scientific basis for PFAS control and sustainable sludge utilization.