Investigation of Short Chain PFAS Degradation Efficiency Using Free-Standing Boron Doped Diamond Electrodes at High Current Density in a Flow Cell

Short chain PFAS is known to be more challenging to destructively remove than its longer chain counterparts. Electrochemical oxidation at boron doped diamond (BDD) electrodes is one promising way forward. The majority of investigations are carried out using thin film, high grain density BDD electrodes (attached to the growth substrate) at low current densities of <50 mA cm^-2. In this work, the impact of high current density on short chain (C₄), perfluorobutanoic acid (PFBA), and perfluorobutanesulfonic acid (PFBS) removal rates and defluorination efficiency is investigated in a recirculating flow system. These studies are carried out using free-standing BDD electrodes, which are grown thick enough so that the BDD can be removed from the non-diamond growth substrate and thus contain a much lower grain density compared to thin-film BDD. The cell utilizes four BDD electrodes, where only the two outer electrodes are directly connected to a potential supply, the two inner electrodes are electrically unconnected, and driven in a bipolar arrangement. Solutions contain saturated potassium sulfate as the electrolyte. A current density of ≥390 mA cm^-2 (after correcting for surface roughness) is employed for times up to 9 h. PFBS/PFBA concentrations in the range range ∼1-60 mg L^-1 are investigated. Importantly when comparing rate constant data to literature for similar concentrations (after normalization of the rate constants to treatment volume/anode area), the values suggest removal rates approximately an order of magnitude higher than those at lower current density (in stirred solutions). Defluorination efficiency is also found to be higher with (close to) complete defluorination indicated at higher concentrations/longer times for short chain PFAS. Microscopy analysis of the free-standing electrodes after deployment for >90 h of advanced oxidation reveals no obvious signs of corrosion. This likely reflects both the reduced grain boundary density and lower sp² carbon content in thick free-standing BDD. The data highlight the potential for this electrode material in long term electrochemical treatment of short chain PFAS solutions in recirculating flow systems.