Transport Behavior of 6:2 Fluorotelomer Sulfonic Acid in Soil

BRIEF REPORTSignificance: Per- and polyfluoroalkyl substance (PFAS) alternatives like 6:2 fluorotelomer sulfonic acid (6:2 FTS) are increasingly used but their environmental transport risks remain poorly characterized, hindering accurate risk assessment. Under unsaturated conditions, air-water interface (AWI) adsorption dominates 6:2 FTS retention, accounting for 61%–98% of total adsorption and resulting in retardation up to 210% higher than in saturated soils. Crucially, reduced soil organic matter amplifies retardation by 118%, while finer particle size increases it by 65%, revealing synergistic soil property controls. This first quantification of multiphase adsorption mechanisms provides essential data for predicting 6:2 FTS mobility in natural environments.Introduction: PFAS have been widely applied in various fields including surfactants, firefighting foams, agrochemicals, and textiles due to their excellent surface activity, thermal stability, and amphiphobicity[1-2]. However, most PFAS exhibit bioaccumulation potential owing to their low biodegradability and may induce various toxic effects including immunotoxicity, genotoxicity, and carcinogenicity[2-3]. Perfluoroalkyl acids (PFAA), as the most extensively studied subclass of PFAS, have been widely detected in environmental media and biological organisms.　　With the gradual phase-out of traditional PFAA, certain PFAA precursors have been mass-produced and used as alternatives. Nevertheless, environmental behavior studies of these substitutes remain insufficient. Current research on PFAA precursors primarily focuses on species identification[15] and biotransformation[6-7], while investigations into their transport characteristics in soils are still limited. Previous studies have demonstrated that factors such as organic matter content[16], particle size[17], and water saturation[17-18] in aqueous media significantly influence the adsorption and transport behavior of conventional PFAS.　　To resolve this, our study systematically measured 6:2 FTS AWI adsorption coefficients (Kia), conducted saturated/unsaturated column experiments across soil types (QD1: high organic matter; QD2: low organic matter) and particle sizes (40–50/50–60 mesh), and decoupled retention contributions from solid-water versus AWI.Methods: 6:2 FTS (purity 98%, CAS#27619-97-2, Macklin) and 2,3,4,5,6-pentafluorobenzoic acid (PFBA, purity 98%, CAS#602-64-8, Macklin) were used as the target contaminant and non-reactive tracer (NRT), respectively. The 6:2 FTS was diluted to 100 g/L using 0.01 mol/L NaCl solution. Two types of aqueous media were selected: QD1 (surface soil from 20–40 cm depth) and QD2 (deep soil from 50–100 cm depth), both collected from farmland in the Laoshan district, Qingdao City. After natural air-drying, the samples were sieved to 40–50 mesh and 50–60 mesh sizes.　　The interfacial tension of 6:2 FTS at different concentrations (0, 0.001, 0.01, 0.1, 0.5, 1, 5, 10, 20, 50, 80, and 100 mg/L) was measured using a BZY-4B surface tensiometer. The sample temperature was maintained at 23 ± 0.3℃, with multiple parallel experiments conducted for each concentration until at least three consistent results (deviation <0.7%) were obtained.　　Miscible displacement column experiments were performed using acrylic resin columns (15 cm length × 2.5 cm inner diameter) packed with soil to simulate homogeneous conditions. CO2 and 0.01 mol/L NaCl solution were sequentially introduced from the bottom of the column to achieve saturation. The hydrodynamic characteristics within the column were characterized using NRT. Subsequently, 100 g/L 6:2 FTS solution was introduced under different experimental conditions, followed by elution with 0.01 mol/L NaCl solution. After saturated experiments, the top 1–2 cm of soil was removed and replaced with two layers of glass beads to ensure uniform solution distribution. The water saturation was then controlled by adjusting the pump flow rates at both ends. Unsaturated experiments followed similar procedures, with NRT characterization preceding 6:2 FTS introduction. The retardation factor (R) for each column experiment was determined through temporal moment analysis of breakthrough curves (BTCs).　　PFBA was analyzed at 262 nm using a UV-2800A spectrophotometer. 6:2 FTS quantification was performed by LC-MS equipped with a Waters C18 column (2.1 mm × 50 mm, 1.7 m) maintained at 30℃. The mobile phase consisted of ultrapure water and acetonitrile (3:7, V/V) at a flow rate of 0.2 mL/min.　　Column effluents were filtered through 0.45 m membranes prior to direct injection (2 L). System blank tests showed no detectable 6:2 FTS (detection limit <1 g/L) or other interferents. Strict quality control measures were implemented: calibration curves (n = 3) were run every 20 samples to monitor instrument stability, with external standard quantification. Both PFBA and 6:2 FTS calibration curves showed excellent linearity (R2 ≥ 0.999). Method validation through consecutive standard solution analyses demonstrated good precision, with intra-day relative standard deviation (RSD, n = 6) < 3% and inter-day RSD (3 consecutive days, n = 18) < 5%, confirming the method's stability and reproducibility for trace PFAS detection.Data and Results: （1） The surface tension characteristics of 6:2 FTS indicate pronounced surfactant properties and concentration-dependent interfacial behavior. As illustrated in Fig. 1, the surface tension of 6:2 FTS in 0.01 mol/L NaCl solution varied nonlinearly with concentration, exhibiting a critical reference concentration (CRC) of approximately 5 mg/L. When compared to perfluoroalkyl substances with equivalent carbon chain lengths, this value falls between PFOS (~1 mg/L) and PFOA (~10 mg/L), while GenX demonstrated a significantly higher CRC of about 30 mg/L. Additionally, the air-water interfacial adsorption coefficient (Kia) of 6:2 FTS was calculated from these measurements. The results demonstrated that the Kia values decreased significantly with increasing 6:2 FTS concentration, exhibiting distinct concentration dependence. At low concentrations (0.01 mg/L), the Kia value of 6:2 FTS reached 0.075 cm, which was significantly higher than those of PFOA (0.0037 cm) and GenX (0.001 cm) at the same concentration[23]. This finding further confirmed that 6:2 FTS exhibited stronger adsorption capacity at the AWI under low concentration conditions, revealing its excellent surface-active properties.　　（2） Under saturated conditions, 6:2 FTS exhibits minimal retardation and high mobility in porous media across varying particle sizes and organic matter contents. BTCs of 6:2 FTS in both 40–50 mesh and 50–60 mesh QD1 were steep and symmetric, closely matching those of the NRT (Fig. 2). R were low, measuring 1.2 and 1.3, respectively. Similar overlap with the NRT BTCs was observed in two additional soil types with differing organic matter contents, with no noticeable tailing (Fig. 3). These results suggest limited solid-phase adsorption and emphasize the high mobility of 6:2 FTS under water-saturated flow.　　（3） Under unsaturated conditions, the transport retardation of 6:2 FTS increased significantly, mainly due to adsorption at the AWI, which had considerable influence on soil properties. Retardation increased markedly under unsaturated flow, with R values up to 210% higher than under saturated conditions. AWI adsorption accounted for 61%–98% of total retention. Soil physicochemical properties substantially influenced transport behavior of 6:2 FTS, finer particle sizes (50–60 mesh) led to R values increasing from 1.7 to 2.8, while lower organic matter content caused R values to rise from 1.7 to 3.7, as shown in Table 3. Furthermore, the study revealed that saturated and unsaturated conditions differentially influenced the transport behavior of 6:2 FTS across soil types. In finer-textured 50–60 mesh QD1, the increased air-water interfacial area caused by reduced water saturation led to a substantially greater R-value increment (ΔR = 1.5) compared to 40–50 mesh soil (ΔR = 0.5). Under identical particle size conditions, the low-organic-matter QD2 exhibited dramatically higher R-value enhancement (ΔR = 2.5) than high-organic-matter QD1 (ΔR = 0.5).