Effect of Emerging Pollutant PFAS Exposure on Hydraulic Conductivity of Bentonite and Multi-Scale Mechanistic Investigation

Remediation and risk control of groundwater contaminated by emerging pollutants such as perfluoroalkyl and polyfluoroalkyl substances (PFAS) has received attention from global researchers. However, very limited studies have addressed whether exposure to PFAS can alter hydraulic conductivity (k) of bentonite and why this change is occurred. Accordingly, this research aims to fill the gap. A series of macroscopic tests were conducted to evaluate effects of exposure to PFAS on soil pH, free swell index, liquid limit, and k of sodium-activated calcium bentonite. The k of the bentonite filter cakes were evaluated via modified fluid loss test. Three types of PFAS were selected including perfluorobutane sulfonic acid (PFBS with short chain), perfluorohexanoic acid (PFHxA with short chain), and perfluorooctanoic acid (PFOA with long chain). Deionized water was tested as the control. In addition, a set of multi-scale, in terms of nanoscopic, microscopic, and mesoscopic scale tests were carried out to reveal the underneath mechanisms of interactions between PFAS and bentonite. These tests included dielectric constant measurement, Zeta potential measurement, ultraviolet absorption spectroscopy test, X-ray diffraction test, Fourier transform infrared spectroscopy test, and scanning electron microscopy tests. The results show that pH values of the PFBS, PFHxA and PFOA solutions, and deionized water were 3.6, 3.6, 3.8, and 6.8, respectively. Free swell index values of bentonite tested in the PFBS, PFHxA and PFOA solutions decreased by 3.1%, 1.6% and 5.5% respectively, as compared to that tested in the deionized water. Exposure to PFBS, PFHxA, and PFOA yielded a decrease in liquid limit by 2.9%, 2.3% and 3.0%, respectively, as compared to exposure to deionized water. Under the same void ratio, k of the bentonite filter cake to PFBS, PFHxA, and PFOA increased by 1.6-3.1 times, 1.5-3.0 times and 1.2-1.8 times, respectively, as compared to k to deionized water. The reasons why k of bentonite filter cake to three PFAS solutions were higher than that to deionized water were attributed to the synergistic effects of three factors as follows: (1) pH and relative dielectric constant of PFBS, PFHxA and PFOA solutions were higher than those of deionized water; (2) Total amount of hydrogen bonds formed in bentonite was lower in three PFAS testing solutions; and (3) Zeta potential of bentonite particles was less negative in three PFAS solutions as compared to that in deionized water.