Irreversible inactivation of multidrug-resistant Gram-positive bacteria using S-functionalized graphene sponge anode

Graphene sponges functionalized with sulfur were employed as anodes and coupled with N-doped graphene sponge cathodes for electrochemical inactivation of a Gram-positive multidrug-resistant bacterium Enterococcus gallinarum in drinking water. The application of 43.5 A m^-2 resulted in 2.3 log removal of E. gallinarum in one-pass, flow-through mode, at 2.7 kWh m^-3 of energy demand. In the case of non-functionalized graphene sponge electrode, 1.8 log removal of E. gallinarum required 3.8 kWh m^-3. Moreover, no bacterial regrowth was measured in any of the experiments conducted during storage of the treated samples for 16 h. Indeed, the storage of samples led to an additional 1 log removal for the S-functionalized graphene sponge anode, somewhat higher compared with the 0.7 log removal observed for the non-functionalized electrode. To further decrease the energy consumption and exploit the capacitance of graphene, the flow-through system was operated with intermittent current. Application of 43.5 A m^-2 in an intermittent mode, led to a similar, 2.4 log removal of E. gallinarum but with a significantly reduced energy consumption, from 2.7 with continuous current to 1.8 kWh m^-3. Scanning electron microscopy analyses of the inactivated bacteria confirmed the irreversible damage to the cell walls due to low-voltage electroporation that co-occurred with the presence of abundant cellular debris resulting from the leakage of intracellular material. Using two sequential reactors equipped with the S-doped graphene sponge anode and N-doped graphene sponge cathode operated at 43.5 A m^-2 of anodic current density resulted in an overall 5.8 log removal of E. gallinarum (including storage) from drinking water, and at the energy consumption of 5.4 kWh m^-3 (i.e., electric energy per order of 0.94 kWh m^-3). Overall, this study demonstrated the feasibility of using an S-functionalized graphene sponge anode for chlorine-free electrochemical inactivation of a multidrug resistant Gram-positive bacterium from low conductivity drinking water.