Fluoride‐selective electrode as a tool to evaluate the degradation of PFAS in groundwater: A bench‐scale investigation
By Janice Stonebridge, Rachel Baldwin, Neil R. Thomson, and Carol Ptacek
Groundwater Mon. & Rem.
April 27, 2020
Fluoride (F−) concentration in groundwater can be used as an independent measure of the degradation of per‐ and polyfluoroalkyl substances (PFAS) at contaminated sites. This study assessed the impact of the groundwater matrix, oxidizing reagents, quenching agent, and sample handling procedures (filtration) on determinations of F−. F− was quantified using a fluoride‐selective electrode (FSE) using matrix spike recovery (acceptable range 75–125%) and electrode slope performance (ideal range − 54 – −60 mV). For the unaltered aqueous matrices considered (ultrapure water, ultrapure water with sodium chloride, and simulated groundwater), matrix spike recoveries were better than 93% and acceptable electrode slopes (−59.5 ± 0.2 mV) were measured. Matrix spike recoveries and electrode slopes when persulfate or permanganate was added were 87 and 93%, and − 58.4 ± 1.3 and − 47.3 ± 2.2 mV, respectively. Therefore, quantifying F− in matrices containing permanganate should involve use of either a matrix‐matched calibration curve or the method of standard additions due to the low electrode slope. The addition of ascorbic acid as a quenching agent resulted in a decrease in matrix spike recoveries to <74% and suggests alternate quenching procedures should be used if an FSE is employed to measure F−. Removal of sediments by filtration prior to the addition of the fluoride spike led to improved matrix spike recoveries (> 96%), which were otherwise underestimated using the FSE in the presence of sediments. While the FSE may be a quick and portable tool, its significant limitations need to be fully understood before it can be used to quantify the production of F− resulting from the potential degradation of PFAS at field sites.