Per- and polyfluoroalkyl substances in the environment

Abstract

Over the past several years, the term PFAS (per- and polyfluoroalkyl substances) has grown to be emblematic of environmental contamination, garnering public, scientific, and regulatory concern. PFAS are synthesized by two processes, direct fluorination (e.g., electrochemical fluorination) and oligomerization (e.g., fluorotelomerization). More than a megatonne of PFAS is produced yearly, and thousands of PFAS wind up in end-use products. Atmospheric and aqueous fugitive releases during manufacturing, use, and disposal have resulted in the global distribution of these compounds. Volatile PFAS facilitate long-range transport, commonly followed by complex transformation schemes to recalcitrant terminal PFAS, which do not degrade under environmental conditions and thus migrate through the environment and accumulate in biota through multiple pathways. Efforts to remediate PFAS-contaminated matrices still are in their infancy, with much current research targeting drinking water.

Living with forever chemicals

Per- and polyfluoroalkyl substances (PFAS) are products of the modern chemical industry that have been enthusiastically incorporated into both essential and convenience products. Such molecules, containing fully fluorine-substituted methyl or methylene groups, will persist on geologic time scales and can bioaccumulate to toxic levels. Evich et al . review the sources, transport, degradation, and toxicological implications of environmental PFAS. Despite their grouping together, these compounds are heterogeneous in chemical structure, properties, transformation pathways, and biological effects. Remediation is possible but expensive and is complicated by dispersion in soil, water, and air. It is important that we thoroughly investigate the properties of potential replacements, many of which are merely different kinds of PFAS, and work to mitigate the harms of the most toxic forms already released. —MAF

Abstract

A review explains that per- and polyfluoroalkyl substances in the environment are a persistent hazard that we must understand and mitigate.

Abstract

BACKGROUND

Dubbed “forever chemicals” because of their innate chemical stability, per- and polyfluoroalkyl substances (PFAS) have been found to be ubiquitous environmental contaminants, present from the far Arctic reaches of the planet to urban rainwater. Although public awareness of these compounds is still relatively new, PFAS have been manufactured for more than seven decades. Over that time, industrial uses of PFAS have extended to >200 diverse applications of >1400 individual PFAS, including fast-food containers, anti-staining fabrics, and fire-suppressing foams. These numerous applications are possible and continue to expand because the rapidly broadening development and manufacture of PFAS is creating a physiochemically diverse class of thousands of unique synthetic chemicals that are related by their use of highly stable perfluorinated carbon chains. As these products flow through their life cycle from production to disposal, PFAS can be released into the environment at each step and potentially be taken up by biota, but largely migrating to the oceans and marine sediments in the long term. Bioaccumulation in both aquatic and terrestrial species has been widely observed, and while large-scale monitoring studies have been implemented, the adverse outcomes to ecological and human health, particularly of replacement PFAS, remain largely unknown. Critically, because of the sheer number of PFAS, environmental discovery and characterization studies struggle to keep pace with the development and release of next-generation compounds. The rapid expansion of PFAS, combined with their complex environmental interactions, results in a patchwork of data. Whereas the oldest legacy compounds such as perfluoroalkylcarboxylic (PFCAs) and perfluoroalkanesulfonic (PFSAs) have known health impacts, more recently developed PFAS are poorly characterized, and many PFAS even lack defined chemical structures, much less known toxicological end points.

ADVANCES

Continued measurement of legacy and next-generation PFAS is critical to assessing their behavior in environmental matrices and improving our understanding of their fate and transport. Studies of well-characterized legacy compounds, such as PFCAs and PFSAs, aid in the elucidation of interactions between PFAS chemistries and realistic environmental heterogeneities (e.g., pH, temperature, mineral assemblages, and co-contaminants). However, the reliability of resulting predictions depends on the degree of similarity between the legacy and new compounds. Atmospheric transport has been shown to play an important role in global PFAS distribution and, after deposition, mobility within terrestrial settings decreases with increasing molecular weight, whereas bioaccumulation increases. PFAS degradation rates within anaerobic settings and within marine sediments sharply contrast those within aerobic soils, resulting in considerable variation in biotransformation potential and major terminal products in settings such as landfills, oceans, or soils. However, regardless of the degradation pathway, natural transformation of labile PFAS includes PFAS reaction products, resulting in deposition sites such as landfills serving as time-delayed sources. Thus, PFAS require more drastic, destructive remediation processes for contaminated matrices, including treatment of residuals such as granular activated carbon from drinking water remediation. Destructive thermal and nonthermal processes for PFAS are being piloted, but there is always a risk of forming yet more PFAS products by incomplete destruction.

OUTLOOK

Although great strides have been taken in recent decades in understanding the fate, mobility, toxicity, and remediation of PFAS, there are still considerable management concerns across the life cycle of these persistent chemicals. The study of emerging compounds is complicated by the confidential nature of many PFAS chemistries, manufacturing processes, industrial by-products, and applications. Furthermore, the diversity and complexity of affected media are difficult to capture in laboratory studies. Unquestionably, it remains a priority for environmental scientists to understand behavior trends of PFAS and to work collaboratively with global regulatory agencies and industry toward effective environmental exposure mitigation strategies.