  Boron-doped, graphene sponge anode was synthesized and applied for the
electrochemical oxidation of C4-C8 per and polyfluoroalkyl substances (PFASs).
Removal efficiencies, obtained in low conductivity electrolyte (1 mS cm-1) and
one-pass flow-through mode, were in the range 16.7-67% at 230 A m-2 of anodic
current density, and with the energy consumption of 10.1+-0.7 kWh m-3. Their
removal was attributed to electrosorption (7.4-35%), and electrooxidation
(9.3-32 %). Defluorination efficiencies of C4-C8 perfluoroalkyl sulfonates and
acids were 8-24% due to a fraction of PFAS being electrosorbed only at the
anode surface. Yet, the recovery of fluoride was 74-87% relative to the
electrooxidized fraction, suggesting that once the degradation of the PFAS is
initiated, the C-F bond cleavage is very efficient. The nearly stoichiometric
sulfate recoveries obtained for perfluoroalkyl sulfonates (91-98%) relative to
the electrooxidized fraction demonstrated an efficient cleavage of the
sulfonate head-group. Adsorbable organic fluoride (AOF) analysis showed that
the remaining partially defluorinated byproducts are electrosorbed at the
graphene sponge anode during current application and are released into the
solution after the current is switched off. This proof-of-concept study
demonstrated that the developed graphene sponge anode is capable of C-F bond
cleavage and defluorination of PFAS. Given that the graphene sponge anode is
electrochemically inert towards chloride and does not form any chlorate and
perchlorate even in brackish solutions, the developed material may unlock the
electrochemical degradation of PFAS complex wastewaters and brines.
