Electrochemical Gas Bubble Production Drives Mass Transport Enhanced Destruction of Short Chain PFAS

Short chain perfluoroalkyl substances (PFAS) (C4 to C6), exemplified herein by the C4 molecule perfluorobutanoic acid (PFBA), are considered harder to destroy than long chain PFAS. Herein, we report the use of an electrochemical flow cell containing two vertically orientated free-standing boron doped diamond (BDD) electrodes in a recirculating flow cell, treating solution volumes of 0.5 L. The cell is designed such 2 that recirculation flow zones (vortices) within the cell chamber are encouraged. This acts to increase the residency time for PFBA molecules and electrochemically produced intermediates in these flow regions. More importantly, with the cell orientated such that solution outlet is upwards, we show how bubbles produced at the electrode surface during water electrolysis further enhance the PFBA electro-oxidative destruction process. Increasing current density results in a concomitant increase in the density of gas bubbles leaving the electrode surfaces. This in turn locally induces microconvective enhancement of mass transport which is reflected in an increase in the firstorder rate constant for PFBA removal with increasing current density. At 700 mA cm-2 extremely high normalized volume/area first order rate constants for PFBA destruction are recorded (1 m hr-1 with a volume flow rate of 249 mL min-1). Coupled with the possibility of gas bubble-induced PFBA preconcentration and favorable hydroxyl radical production at the gas bubble-water-electrode interface, this leads to 100% defluorination efficiencies at current densities  175 mA cm-2 for ~ 20 mg L-1 PFBA in 0.5 M potassium sulfate.