Per- and polyfluoroalkyl substances, PFAS, are a group of synthetic chemicals used globally since the 1940s. Their resistance to water, grease, and stains led to widespread application in products like non-stick cookware, water-repellent clothing, and firefighting foams. PFAS are a concern because their strong carbon-fluorine bonds make them highly persistent in the environment and resistant to breakdown, earning them the nickname “forever chemicals”. Their persistence allows accumulation in soil, water, air, and living organisms, causing long-term environmental contamination and potential health impacts. Remediation processes aim to clean up or mitigate this contamination.
Technologies for Removing PFAS
Technologies for removing PFAS physically separate these compounds from contaminated water and soil. These methods concentrate PFAS into a smaller waste volume, requiring further treatment or disposal.
Granular Activated Carbon (GAC) is a common method for PFAS removal, especially in drinking water. GAC works through adsorption, as PFAS molecules adhere to its porous surface. The effectiveness of GAC depends on factors such as the type of PFAS, the specific carbon used, and the presence of other organic matter in the water. It effectively removes long-chain PFAS like perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), and can treat large water volumes.
Ion exchange resins effectively remove PFAS from water, particularly short-chain PFAS less effectively captured by carbon-based adsorption processes. These resins are typically polymeric beads, approximately 0.5 to 1 millimeter in diameter, with cationic adsorption sites. Negatively charged PFAS molecules exchange with ions bound to these sites via electrostatic and hydrophobic interactions. Exhausted resins, containing captured PFAS, are then disposed of, often by incineration, to prevent re-release.
Reverse Osmosis (RO) and Nanofiltration (NF) are membrane-based separation processes highly effective in removing a wide range of PFAS, including shorter-chain types. They apply high pressure to force water through semi-permeable membranes, blocking PFAS passage. The primary mechanisms for PFAS rejection by these membranes include size exclusion, where the membrane pores are smaller than the PFAS molecules, and electrostatic interactions, which repel charged PFAS molecules. Though efficient, these processes generate a concentrated PFAS waste stream (about 20% of initial water volume) requiring subsequent management.
Technologies for Destroying PFAS
Technologies for destroying PFAS break down the robust carbon-fluorine bonds within PFAS molecules, transforming them into less harmful or inert substances. The goal is mineralization, converting PFAS into simple, stable compounds like carbon dioxide, water, and fluoride ions.
High-temperature incineration is a thermal oxidation method that destroys PFAS by exposing them to extreme heat in an oxygen-rich environment. Hazardous waste incinerators typically operate at temperatures ranging from 980°C to 1200°C, with some studies suggesting that complete decomposition of certain PFAS compounds may require temperatures exceeding 1400°C. While effective for destroying PFAS in various waste streams, concerns remain about whether typical incinerator temperatures are sufficient for complete breakdown and the potential for incomplete combustion products.
Electrochemical oxidation (EO) uses electricity to generate reactive species that degrade PFAS. An electrical current across electrodes in contaminated water leads to direct electron transfer and indirect oxidation via hydroxyl radicals. EO breaks down longer-chain PFAS into shorter compounds, aiming for complete mineralization into carbon dioxide, hydrogen gas, and fluoride ions. Effectiveness is influenced by electrode material, solution pH, temperature, and initial PFAS concentration.
Supercritical Water Oxidation (SCWO) uses water’s unique properties above its critical point (374°C and 22.1 MPa). In this state, water behaves as both liquid and gas, enhancing organic compound solubility and accelerating oxidation. SCWO generates reactive free radicals that rapidly cleave PFAS structures, leading to their destruction. It has shown over 99% reduction of total PFAS in various contaminated matrices, including dilute aqueous film-forming foam, and is considered a promising alternative to incineration for permanent PFAS destruction.
Advanced Oxidation Processes (AOPs) generate highly reactive radicals, like hydroxyl radicals, to degrade organic pollutants, including PFAS. They often combine ultraviolet (UV) light with oxidants like hydrogen peroxide or persulfate. For example, UV light can activate reducing agents to produce hydrated electrons and other reducing radicals that initiate reductive defluorination, breaking down PFAS. While promising for PFAS destruction, their efficiency varies based on oxidant type and concentration, solution pH, and co-contaminants.
In-Situ and Developing Remediation Methods
In-situ remediation methods are applied directly at contamination sites, offering potential cost-effectiveness for large, diffuse contamination. Several developing technologies also show promise for future PFAS cleanup.
In-situ solidification/stabilization (ISS) reduces PFAS leaching from contaminated soils by encapsulating or binding the contaminants within a solid matrix. It treats contaminated soils with amendments like activated carbon or cementitious materials to immobilize PFAS and reduce water infiltration. While ISS does not destroy PFAS, it significantly reduces their mobility, preventing spread into groundwater. For instance, adding 2% activated carbon with cements has been shown to reduce leaching of perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) by 98% in laboratory studies.
In-situ chemical oxidation/reduction involves injecting chemical oxidants or reductants directly into contaminated ground to transform or degrade PFAS. Research in this area is ongoing, with heat-activated persulfate showing promise for degrading certain PFAS compounds, such as PFOA, by transforming them into shorter-chain perfluorocarboxylic acids at low pH. This approach can reduce remediation costs and enhance subsequent bioremediation, especially for co-occurring chemicals. However, effectiveness can be limited by factors like chloride and aquifer sediments.
Bioremediation uses microorganisms to break down PFAS, a challenging area due to the strong carbon-fluorine bonds making PFAS highly resistant to biological degradation. While some fungal and bacterial strains have been identified with the ability to degrade PFAS, information on the exact mechanisms of degradation is limited. Bioremediation offers a cost-effective, large-scale in-situ strategy for PFAS removal from soils, but it proceeds slowly and may not lead to harmless end-products. Ongoing research uses advanced microbial ecology technologies, like metagenomics, to understand and identify PFAS biodegradation pathways.
Phytoremediation uses plants to absorb, degrade, or stabilize contaminants, offering a nature-based solution for PFAS removal from soil and groundwater. Plants can bioaccumulate PFAS into their tissues; shorter-chain PFAS accumulate in shoots, while longer-chain PFAS often remain bound to roots. While phytoremediation primarily extracts and stores PFAS within plant biomass rather than extensively degrading it, this process removes PFAS from contaminated environments. Research indicates that plant species like willow and sunflower show high PFAS removal efficiency, with up to 34% removal of short-chain PFAS (C3-C6) after 90 days of exposure in pot experiments.
Sonolysis uses high-frequency sound waves to create cavitation bubbles in contaminated water. The implosive collapse of these bubbles generates high temperatures and pressures (averaging 5000 K), leading to pyrolysis and radical reactions that break down PFAS. This method can mineralize aqueous PFAS into inorganic substances like fluoride ions, carbon monoxide, and carbon dioxide without harmful byproducts. Sonolysis has shown promise for degrading PFAS in various matrices, including groundwater and landfill leachate, and is not chain-length selective, effectively degrading both long and short-chain PFAS.