Electrochemical removal of contaminants of emerging concern with manganese oxide-functionalized graphene sponge electrode

Graphene sponge electrodes functionalized with amino-doped manganese oxide (anode) and nitrogen (cathode) were used for the electrochemical degradation of contaminants of emerging concern in real drinking water. Application of 29 A m-2 of anodic current density in a one-pass, flow-through reactor resulted in 25-64 % removal of contaminants, obtained via direct electron transfer and electrogenerated oxidants, e.g., hydroxyl radicals. Coupling of two flow-through reactors yielded 76-98 % removal of contaminants, representing a 1.5 to 3-fold improvement in the removal efficiencies compared with the single reactor. The application of intermittent current in the double reactor in 105 s ON - 105 s OFF periods decreased the energy consumption from 3.8 to 2.2-2.4 kWh m- 3, without compromising the reactor's performance. To gain insight into the electrochemical degradation mechanisms, five major transformation products of carbamazepine were elucidated, which were formed by direct electron transfer and subsequent oxidation of the highly reactive olefinic double bond of CBZ. This study demonstrates the capacity of the amino-doped manganese oxide graphene sponges to degrade persistent organic contaminants in low conductivity drinking water, the feasibility of coupling flow-through, one-pass electrochemical reactors to reach higher degradation of contaminants and their transformation products, and the opportunity to lower the energy consumption by applying intermittent current, thus exploiting the capacitance of graphene sponge electrodes.