Formatted Title
Applicability of PFAS-Contaminated Groundwater Treatment by Powdered Activated Carbon Attached Pleats Filter
Background/Objectives
In recent years, environmental pollution by per- and polyfluoroalkyl substances (PFAS) has become a major social problem worldwide. In Japan, perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) were added to the monitored substances of Environmental Quality Standards (EQS) for water pollution in public and EQS for groundwater pollution and guideline values (provisional) “0.00005 mg/L (=50 ng/L) or less” was set in May 2020. In addition, perfluorohexane sulfonic acid (PFHxS) was added to the required survey items in March 2021, and the Ministry of the Environment (MOE) is striving to accumulate new knowledge on toxicity information and its existence in the water environment. As a technology for efficiently treating contaminated groundwater by PFAS, the authors considered that the technology for collecting PFAS by passing contaminated groundwater pumped into a pleated filter attached powdered activated carbon (PAC) was effective. And to confirm the applicability of water treatment equipment using this technology, we grasped the PFOA, PFOS and PFHxS adsorption characteristics of PAC used.
Approach/Activities
Adsorption property tests of two types of PAC (PAC-A: coal-based, PAC-B: coconut shell based) and water flow test of PAC-B attached filter were carried out. In adsorption properties tests, adsorption isotherms of PFOA and PFOS in single solutions of PFOA or PFOS with a concentration of 0.1 mg/L were obtained at pH 5, 7 and 9 for PAC-A and PAC-B, and adsorption isotherms of PFOA, PFOS and PFHxS in mixed solutions of these three substances with a concentration of 0.1 mg/L each were obtained at pH 5, 7 and 9 for PAC-B. Regarding the mixed solution at pH 7, the relationship between the amount of potassium chloride (KCl) used as a buffer and the adsorption characteristics of these substances were determined by adjusting pH with hydrochloric acid (HCl). In the water flow test, a 0.1 mg/L PFOA solution at pH 7 was continuously passed at 200 LMH (=L/m2/h) through a PAC-B attached filter (PAC attaching amount: 500 g/m2) with a diameter of 70 mm, and changes of PFOA remove rate and breakthrough time of adsorption by PAC-B were observed.
Results/Lessons Learned
Results of adsorption property tests in single solutions of PFOA or PFOS showed that PAC-B adsorbed more PFOA and PFOS than PAC-A at any pH. The fact that coconut shell-based PAC, which has a higher proportion of micropores, adsorbs PFAS more strongly than coal-based PAC was the opposite of other previous studies. Based on these test results in single solutions, PAC-B was selected as the PAC to be attached to the pleated filter. From adsorption isotherms of PFOA and PFOS at each pH when PAC-A or PAC-B was used in the single solution, the relationship between pH of the solution and equilibrium adsorption amounts (q) of PFOA and PFOS, and the correlation of q between PFOS and PFOA were understood. In the adsorption property tests in the mixed solution at pH 7, adsorption isotherms of PFOA and PFHxS were almost the same even if the amount of KCl was different. However, adsorption isotherms of PFOS were unstable, and stabilized as the amount of KCl increased. From adsorption isotherms of PFOA, PFOS and PFHxS at each pH when PAC-B was used in the mixed solution, the relationship between pH of the solution and q of these three substances, and the correlation of q between these three substances were understood. As the result of the water flow test using PAC-B, the PFOA concentration in the treated water was less than 10 ng/L (removal rate: >99.99 %) after 49.9 hours, less than 50 ng/L after 122.7 hours, and 500 ng/L after 154.5 hours and spiked to 5,900 ng/L over 170.7 hours. PFOA cumulative adsorption amount of PAC-B at 154.5 hours before breakthrough was 6,600 µg/g. Considering the actual water treatment system (approximately 3 m x 2 m x height 2 m) of three adsorption towers (filter area: 150 m2), 75 kg of PAC-B is attached at 500 g/m3, and 30 m3/h of contaminated water is treated at 200 LMH. Even if PFOA-contaminated water with a high concentration of 0.1 mg/L is treated with this system and specification with a target PFOA concentration of 50 ng/L, PAC-B will be replaced every 122.7 hours or less.