Description
Disposing of leachate-containing per- and poly-fluoroalkyl substances (PFAS) is one of the most pressing issues surrounding the operation of landfills due to limited destructive technologies and high disposal costs. In this project, six leachate samples were collected from various point sources and characterized. Two novel treatment processes, namely (1) electrical discharge plasma (EDP) and (2) electrochemical oxidative filtration (EOF) were employed for PFAS removal. The effects of co-pollutants on PFAS removal efficiency were also explored.
Physico-chemical characterization of leachate samples indicated that the type and concentration of PFAS and other compounds present varied depending on the leachate’s source and age. For the six leachate samples examined, Cl- had the highest anion concentration (80 to 650 mg/L). Total organic carbon (TOC) concentrations varied between 13500 and 150 mg/L, indicating there were also high concentrations of organic carbon present. There were 15 PFAS compounds found (three precursors, seven short-chain, and five long-chain) with concentrations between 90 to 17500 ng/L.
Non-thermal plasma was evaluated as a method for PFAS destruction. Within 40 min of plasma treatment all five long-chain PFAS compounds, and within 60 min all precursor compounds (n=3), were removed to below detection limits (BDL) of approximately 35 ng/L. However, the seven short-chain PFAS compounds required more than 120 min of plasma treatment for their removal. PFBA (C4) and PFPeA (C5) were not completely removed because these short-chain PFAS were not effectively transported to the gas-liquid interface due to fewer –CF2- groups (low hydrophobicity) and thus not able to be removed by plasma-produced active species in the gas-liquid interface. In addition, these compounds can be produced as breakdown products of longer chain precursor compounds. The addition of a cationic surfactant like cetyltrimethylammonium bromide (CTAB) significantly enhanced the destruction of PFBA and other short-chain PFAS compounds. Pearson`s correlation indicated that the presence of organic compounds (TOC, COD) and inorganic ions did not negatively (statistically insignificant) influence PFAS destruction kinetics. However, precursor PFAS compound concentrations significantly negatively impacted the value of the k(PFOA+PFOS) removal rate constant. The electrical conductivity of the sample was not statistically significantly correlated with k(PFOA+PFOS) even though high conductivity of the solution decreases the plasma discharge area at the gas-liquid interface.
EOF was also evaluated as a destructive leachate treatment process by treating a raw leachate sample and plasma-treated leachate. It was found that long-chain PFAS were removed to BDL in the raw leachate treatment. EO also demonstrates reactivity toward the removal of short and ultrashort chain PFAS (i.e., trifluoracetate). However, more than 100 Wh/L energy input was needed to remove about 80% of short-chain PFAS. The required treatment time was inversely correlated with the chain length. The oxidative capability of the electrode used (e.g., oxygen evolution overpotential) is not the key factor determining the PFAS treatment in landfill leachate. Instead, the mass transfer efficiency for the PFAS to attach to the electrode surface controlled the removal rate. It was determined that incorporating EOF after plasma treatment could be an effective strategy to remove short-chain PFAS without the need for surfactant addition, although plasma alone with surfactant addition would be more energy efficient.
Electrical energy per order (EE/O) is widely used to calculate the energy efficiency of a process. The EE/O value of the Ar gas-bubbled enhanced contact plasma reactor ranged between 8 and 14.5 Wh/L, which is twofold lower than the reported value in the literature. To remove 90% of PFOA+PFOS in the E2 leachate sample, the 2.2 L plasma reactor consumed about 8 Wh/L, but the EOF reactor required about 20 Wh/L to treat 0.35 L sample, indicating that the plasma reactor is about 10 times more energy efficient than EOF reactor. This finding implies that the developed enhanced contact multi-pin-to-plate plasma reactor could be used for PFAS destruction from the leachate samples.
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