Electrochemical Oxidation of Perfluoro-octanoic Acid in Bi2O3-SnO2/Ti4O7 Membranes

The dissertation is focused on developing Bi2O3-SnO2 (BTO) deposited reactive electrochemical membranes (REMs) for electrochemical oxidation of perfluorooctanoic acid (PFOA). The developed BTO/REM was tested in different electrolytes and in a groundwater matrix to treat concentrated per- and poly-alkyl substances (PFAS) wastes that include ion exchange resin (IXR) still bottoms and foamates from foam fractionation of groundwater. In the initial part of the research different deposition methods, roles of Bi2O3 and SnO2 on defluorination, and oxidation in an inert electrolyte were investigated. Then, the effect of electrolytes, ions in the groundwater matrix, and methanol on PFOA oxidation and defluorination was studied. Based on the observation, defluorination pathways were proposed. Using BTO/REM, >98% PFOA removal with >85% fluoride yield was achieved at 4.2 V/SHE with hydraulic residence time (tr) of 60.4 s. The fluoride yield was enhanced from ~40.2% with ~99.4% PFOA removal with a fluoride yield of ~86.8% with ~99.9% PFOA removal at 4.7 V/SHE with tr of 15.1 s for 0.3% methanol in 100 mM (NH4)2SO4 using BTO/REM. The experiment conducted with IXR still bottoms surrogate in (NH4)2SO4 electrolyte in 0.3% methanol showed ~92% removal of perfluorocarboxylic acids (PFCAs), >82.6% removal of perfluorooctane sulfonic acid (PFOS), and ~67.5% defluorination at 4.7 V/SHE and tr of 15.1 s. Overall, results showed that surface blockage due to Ca2+ and Mg2+ precipitants led to higher PFCA product formation. Sulfate was proposed as the choice of electrolyte for groundwater and foamtes, and (NH4)2SO4 electrolyte was proposed as a regenerant for IXR to avoid formation of toxic chlorinated byproducts and a tolerable methanol amount was proposed. Precursors present in AFFFs formed PFCAs and PFSAs under anodic potentials. A few limitations include long-term stability of BTO/REM, formation of shorter chain PFCAs in groundwater matrix, and accumulation of removal short chain perfluorosulfonic acids (PFSAs). To overcome the limitations, future work will be focused on understanding the mechanism of PFAS oxidation on Bi2O3 and SnO2, synthesis of a stable BTO catalyst, defluorination of short chain PFSAs by utilizing reactors in series or by developing a new catalyst, and scale-up of BTO/REM with techno-economic assessment.