Effects of anionic hydrocarbon surfactant on the transport of perfluorooctanoic acid (PFOA) in natural soils

Transport and release behaviors of PFOA in saturated and unsaturated porous media with biochar amendment

The widespread usage of perfluorooctanoic acid (PFOA) in daily consumer products and its high mobility in porous media have resulted in ubiquitous contamination of PFOA in the natural environment. Developing techniques to immobilize and inhibit the transport of PFOA thus is critical to reduce its potential risks. In present study, biochar, one type of environmental-friendly material produced from cellulose, was utilized in porous media to test its addition on inhibiting the transport and release of PFOA before and after aging process. We found that although PFOA had high mobility in saturated/unsaturated porous media, biochar addition could significantly inhibit PFOA transport in porous media with different saturations due to its high adsorption capacity towards PFOA. The inhibited transport of PFOA by biochar also held true in solution with copresence of natural organic matter and in actual river water. Moreover, we found that negligible PFOA was released from porous media with biochar amendment even after exposure to freeze-thaw/dry-wet treatment. PFOA adsorbed onto biochar could be completely desorbed and the biochar could be reused for subsequent cycles after desorption. Clearly, amendment of porous media with biochar would be a feasible and cost-effective method to immobilize PFOA in natural environment and reduce its environmental risks.

Repeated Aqueous Film-Forming Foams Applications: Impacts on Polyfluoroalkyl Substances Retention in Saturated Soil

Historical practices at firefighter-training areas involved repeated aqueous film-forming foams (AFFFs) applications, resulting in source zones characterized by high concentrations of perfluoroalkyl and polyfluoroalkyl substances (PFAS). Repeated applications of AFFF composed of 14 anionic and 23 zwitterionic perfluoroalkyl substances (PFAS) were conducted on a single one-dimensional saturated soil column to quantify PFAS retention. An electrofluorination-based (3M) Milspec AFFF, which was above the mixture's critical micelle concentration (CMC), was at application strength (3%, v/v). Retention and retardation of PFAS mass increased with each successive AFFF addition, although the PFAS concentration profiles for subsequent applications differed from the initial. Greater degree of mass retention and retardation correlated with longer PFAS carbon-fluorine chain length and charged-headgroup type and as a function of AFFF application number. Anionic PFAS were increasingly retained with each subsequent AFFF application, while zwitterionic PFAS exhibited an alternating pattern of sorption and desorption. Surfactant-surfactant adsorption and competition during repeat AFFF applications that are at concentrations above the CMC resulted in adsorbed PFAS from the first application, changing the nature of the soil surface with preferential sorption of anionic PFAS and release of zwitterionic PFAS due to competitive elution. Applying a polyparameter quantitative structure-property relationship developed to describe sorption of AFFF-derived PFAS to uncontaminated, saturated soil was attempted for our experimental conditions. The model had been derived for data where AFFF is below the apparent CMC and our experimental conditions that included the presence of mixed micelles (aggregates consisting of different kinds of surfactants that exhibit characteristics properties different from micelles composed of a single surfactant) resulted in overall PFAS mass retained by an average of 27.3% ± 2.7% (standard error) above the predicted values. The correlation was significantly improved by adding a "micelle parameter" to account for cases where the applied AFFF was above the apparent CMC. Our results highlight the importance of interactions between the AFFF components that can only be investigated by employing complex PFAS mixtures at concentrations present in actual AFFF at application strength, which are above their apparent CMC. In firefighter-training areas (AFFF source zones), competitive desorption of PFAS may result in downgradient PFAS retention when desorbed PFAS become resorbed to uncontaminated soil.

Factors Affecting the Adsorption of Per- and Polyfluoroalkyl Substances (PFAS) by Colloidal Activated Carbon

The immobilization of per- and polyfluoroalkyl substances (PFAS) by colloidal activated carbon (CAC) barriers has been proposed as a potential in-situ method to mitigate the transport of plumes of PFAS in the subsurface. However, if PFAS are continuously released from a source zone, the adsorptive sites on CAC will eventually become saturated, upon which point the breakthrough of PFAS in the barrier will occur. To predict the long-term effectiveness of CAC barriers, it is important to evaluate the factors that may affect the adsorption of PFAS on CAC. In this study, the adsorption of 7 PFAS on a polymer-stabilized CAC (i.e., PlumeStop®) and on a polymer-free CAC was investigated using batch experiments. The adsorption affinity of PFAS to CAC was in the following order: PFOS > 6:2 FTS > PFHxS > PFOA > PFBS > PFPeA > PFBA. This result indicates that hydrophobic interaction was the predominant adsorption mechanism, and that hydrophilic compounds such as PFBA and PFPeA will break through CAC barriers first. The partition coefficient Kd for the adsorption of PFAS on the polymer-stabilized CAC was 1.3 - 3.5 times smaller than the Kd for the adsorption of PFAS on the polymer-free CAC, suggesting that the polymers decreased the adsorption, presumably due to competitive sorption. Thus, the PFAS adsorption capacity of PlumeStop CAC barriers is expected to increase once the polymers are biodegraded and/or washed away. The affinity of PFOS and PFOA to CAC increased when the ionic strength of the solution increased from 1 to 100 mM, or when the concentration of Ca2+ increased from 0 to 2 mM. In contrast, less PFOS and PFOA were adsorbed in the presence of 1 - 20 mgC/L Suwannee River Fulvic Acid, which represented dissolved organic carbon, or in the presence of 10 - 100 mg/L diethylene glycol butyl ether (DGBE), which is an important component in some aqueous film-forming foam (AFFF) formulations. The presence of 0.5 - 4.8 mg/L benzene or 0.5 - 8 mg/L trichloroethylene, the co-contaminants that may comingle with PFAS at AFFF-impacted sites, diminished PFOS adsorption but had no effect or even slightly enhanced PFOA adsorption. When the initial concentration of TCE was 8 mg/L, the Kd (514 ± 240 L/g) for the adsorption of PFOS was approximately 20 times lower than that in the TCE-free system (Kd = 9,579 ± 829 L/g). The results of this study provided insights into some key factors that may affect the adsorption of PFAS in in-situ CAC barriers.

Cotransport of different electrically charged microplastics with PFOA in saturated porous media

The fate and transport behavior of microplastics (MPs), emerging colloidal contaminant ubiquitous in natural environments, would be greatly affected by other copresent pollutants. PFOA (emerging surfactant pollutant) would interact with MPs after encounter with them in natural environments, which could alter the transport behavior of both pollutants. Relevant knowledge is still lacking, affecting accurate prediction the fate and distribution of these two emerging contaminants in natural porous media. The cotransport behavior of different surface charged MPs (negatively/positively charged, CMPs/AMPs) with PFOA (three concentrations ranged from 0.1 to 10 mg/L) in porous media in both 10 and 50 mM NaCl solutions thus was investigated in the present study. We found PFOA inhibited CMPs transport in porous media, while enhanced AMPs transport. The mechanisms leading to the altered transport of CMPs/AMPs caused by PFOA were found to be different. The decreased electrostatic repulsion between CMPs-sand induced by the decreased CMPs negative zeta potentials via the adsorption of PFOA led to the inhibited transport of CMPs in CMPs-PFOA suspension. The enhanced electrostatic repulsion between AMPs-sand due to the decreased positive charge of AMPs via the adsorption of PFOA together with steric repulsion induced by suspended PFOA resulted in the increased transport of AMPs in AMPs-PFOA suspension. Meanwhile, we found that the adsorption onto MPs surfaces also impacted the transport of PFOA. Due to the lower mobility of MPs than PFOA, the presence of MPs despite their surface charge decreased the transport of PFOA of all examined concentrations in quartz sand columns. This study demonstrates that when MPs and PFOA are co-existing in environments, their interaction with each other will alter the fate and transport behavior of both pollutants in porous media and the alteration is highly correlated with the amount of PFOA adsorbed onto MPs and original surface properties of MPs.

Sunrise of PFAS Replacements: A Perspective on Fluorine-Free Foams

One type of firefighting foam, referred to as aqueous filmforming foams (AFFF), is known to contain per- and polyfluoroalkyl substances (PFAS). The concerns raised with PFAS, and their potential environmental and health impacts, have led to a surge in research on fluorine-free alternatives both in the United States and globally. Particularly, in January 2023, a new military specification (MIL-PRF-32725) for fluorine-free foam was released in accordance with Congressional requirements for the U.S. Department of Defense. This paper provides a critical analysis of the present state of the various fluorine-free options that have been developed to date. A nuanced perspective of the challenges and opportunities of more sustainable replacements is explored by examining the performance, cost, and regulatory considerations associated with these fluorine-free alternatives. Ultimately, this evaluation shows that the transition to fluorine-free replacements is likely to be complex and multifaceted, requiring careful consideration of the trade-offs involved. Yet, the ongoing work will provide valuable insights for future research on alternatives to AFFF and enhancing the safety and sustainability of fire suppression systems.

A systematic review on distribution, sources and sorption of perfluoroalkyl acids (PFAAs) in soil and their plant uptake

Perfluoroalkyl acids (PFAAs) are ubiquitous in environment, which have attracted increasing concerns in recent years. This study collected the data on PFAAs concentrations in 1042 soil samples from 15 countries and comprehensively reviewed the spatial distribution, sources, sorption mechanisms of PFAAs in soil and their plant uptake. PFAAs are widely detected in soils from many countries worldwide and their distribution is related to the emission of the fluorine-containing organic industry. Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) are found to be the predominant PFAAs in soil. Industrial emission is the main source of PFAAs contributing 49.9% of the total concentrations of PFAAs (Ʃ PFAAs) in soil, followed by activated sludge treated by wastewater treatment plants (WWTPs) (19.9%) and irrigation of effluents from WWTPs, usage of aqueous film-forming foam (AFFFs) and leaching of leachate from landfill (30.2%). The adsorption of PFAAs by soil is mainly influenced by soil pH, ionic strength, soil organic matter and minerals. The concentrations of perfluoroalkyl carboxylic acids (PFCAs) in soil are negatively correlated with the length of carbon chain, log K_ow, and log K_oc. The carbon chain lengths of PFAAs are negatively correlated with the root-soil concentration factors (RCFs) and shoot-soil concentration factors (SCFs). The uptake of PFAAs by plant is influenced by physicochemical properties of PFAAs, plant physiology and soil environment. Further studies should be conducted to make up the inadequacy of existing knowledge on the behavior and fate of PFAAs in soil-plant system.

Effects of biofilms on the retention and transport of PFOA in saturated porous media

Understanding the fate and transport of perfluorooctanoic acid (PFOA) in soil and groundwater is essential to reliable assessments of its risks. This study investigated the impacts of Gram-positive Bacillus subtilis (BS), Gram-negative Pseudomonas aeruginosa (PA) and wild microbiota (WM) biofilm on the transport of PFOA in saturated sand columns at two ionic strengths (i.e., 1.0 and 20.0 mM NaCl). The retention of PFOA in biofilm-coated sand columns was higher than that in uncoated sand columns, due to biofilm-induced reinforced hydrophobic interactions and surface roughness, and decreased zeta potential. However, the retention effects varied among biofilm bacterial species with PFOA retardation factors in PA, WM and BS columns of 1.29-1.38, 1.21-1.29 and 1.11-1.15, respectively. Notably, PA biofilm had the most pronounced effect on PFOA retention. While increasing ionic strength promoted the retention of PFOA in BS biofilm-coated sand, it had no significant impact on PFOA transport in PA and WM biofilm-coated sand. This could be attributed to the differences in biofilm composition, deviating the ionic strengths effects on electrostatic double layer compression. The advection dispersion equation coupled with two-site kinetic retention model well described the transport of PFOA in all saturated columns. Our findings reveal that biofilm plays important roles in PFOA transport in porous media, instructive for risk assessment and remediation of PFOA contamination.