A systematic investigation of single solute, binary and ternary PFAS transport in water-saturated soil using batch and 1-dimensional column studies: Focus on mixture effects

Elucidating per- and polyfluoroalkyl substances (PFASs) soil-water partitioning behavior through explainable machine learning models

In this study, an optimized random forest (RF) model was employed to better understand the soil-water partitioning behavior of per- and polyfluoroalkyl substances (PFASs). The model demonstrated strong predictive performance, achieving an R2 of 0.93 and an RMSE of 0.86. Moreover, it required only 11 easily obtainable features, with molecular weight and soil pH being the predominant factors. Using three-dimensional interaction analyses identified specific conditions associated with varying soil-water partitioning coefficients (Kd). Results showed that soils with high organic carbon (OC) content, cation exchange capacity (CEC), and lower soil pH, especially when combined with PFASs of higher molecular weight, were linked to higher Kd values, indicating stronger adsorption. Conversely, low Kd values (< 2.8 L/kg) typically observed in soils with higher pH (8.0), but lower CEC (8 cmol+/kg), lesser OC content (1 %), and lighter molecular weight (380 g/mol), suggested weaker adsorption capacities and a heightened potential for environmental migration. Furthermore, the model was used to predict Kd values for 142 novel PFASs in diverse soil conditions. Our research provides essential insights into the factors governing PFASs partitioning in soil and highlights the significant role of machine learning models in enhancing the understanding of environmental distribution and migration of PFASs.

Spatial distribution, source, and fate of per- and polyfluoroalkyl substances in the surrounding environment of closed and converted fluorochemical factories in Fujian, China

Following the closure of perfluorooctanesulfonic acid (PFOS) production to comply with the Stockholm Convention regulations or restrictions, manufacturers have shifted to developing short-chain alternatives like perfluorobutane sulfonic acid (PFBS). However, limited research has been conducted to evaluate the impact of this transition on the surrounding environment. This study focused on the spatial distribution, source, and fate of 18 per- and polyfluoroalkyl substances (PFAS) in the surrounding environment of the closure and transformation of two PFAS manufacturing plants in Fujian, China. The total concentrations of PFAS in surface water, sediment, and fish were within the range of 48.9-72,400 ng/L, 0.930-57.6 ng/g dw, and 3.33-1245 ng/g dw, respectively. The predominant compounds were PFBS, PFOS, and perfluorooctanoic acid (PFOA) among the three matrices. Principal component analysis highlighted significant differences in PFAS profiles across different regions of the Futun River, suggesting diverse sources of PFAS. Source apportionment indicated that despite being closed or converted for almost three years, the two factories still significantly impacted the surrounding environment. The shutdown factory mainly released PFAS characterized by perfluoroalkyl sulfonic acids. In contrast, the PFAS were released from conversion plant with the fingerprint being PFBS and perfluoroalkyl carboxylic acids. The conversion of the factories has resulted in the coexistence of long-chain and short-chain PFAS, which has complicated the composition of PFAS in the environment. As sewage treatment plant could not effectively remove PFBS and perfluorobutanoic acid (PFBA) in wastewater, and due to their strong migration ability, these chemicals had a wider impact range, increasing the difficulty of environmental restoration and management. Risk assessment showed that PFAS downstream of the two factories posed high or moderate ecological risks. Specifically, PFBS, PFOS, and PFOA displayed the highest risk quotients and should be paid further attention.

Modeling 1-D aqueous film forming foam transport through the vadose zone under realistic site and release conditions

Aqueous film forming foams (AFFFs) have been used to extinguish fires since the 1960s, leading to widespread subsurface contamination by per- and polyfluoroalkyl substances (PFAS), an essential component of AFFF. This study presents 1-D simulations of PFAS migration in the vadose zone resulting from AFFF releases. Simulation scenarios used soil profiles from three US Air Force (USAF) installations, encompassing a range of climatic conditions and hydrogeologic environments. A three-component mixture, representative of major constituents of AFFF, facilitated the exploration of competitive and synergistic effects of co-constituents on PFAS migration. To accurately capture unsaturated transport of PFAS in porous media, the model considers (1) surfactant-induced flow, (2) non-linear sorption to the solid phase, (3) competitive accumulation at the air-water interface, and (4) the moisture-dependence of the air-water interfacial area. Defined PFAS releases were consistent with fire training exercises, emergency responses, and accidental spills of record. Simulation results illustrate the importance of hydrogeologic, climatic, geochemical, and AFFF release conditions on PFAS transport and retention. Comparison of field observations and model simulations for Ellsworth AFB indicate that much of the PFOA and PFOS mass is associated with the air-water interface and the solid phase, which limits their migration potential in the vadose zone. Results also show that rates of migration in the aqueous phase are largely controlled by hydrogeologic properties, including recharge rates and hydraulic conductivity. AFFF spill scenarios varying in volume, concentration, and frequency reveal the importance of release characteristics in determining rates of PFAS migration and concentration peaks. Variability is attributed to non-linear sorption processes, where, contrary to simple linear partitioning formulations, transport is strongly affected by the concentration of PFAS species. Simulations also demonstrate the importance of modeling the AFFF as a mixture since competitive interfacial accumulation effects are shown to enhance the mobility of less surface-active PFAS compounds.