To minimize the effects of water pollution and alleviate water scarcity, urban water infrastructure will have to be re-invented through implementation of innovative low-cost, energy-efficient decentralized water and wastewater treatment systems, and use of alternative water resources. Electrochemical systems offer several advantages as they do not use chemical reagents, do not form a residual waste stream, operate at ambient temperature and pressure, are robust, versatile and have a small footprint. However, despite all the above-mentioned benefits, they have two major limitations: i) high energy consumption, in particular for the degradation of persistent contaminants such as , poly- and per-fluoroalkyl substances (PFAS), and ii) formation of toxic organic and inorganic chlorinated byproducts in the presence of chloride, a naturally occurring anion.
ELECTRON4WATER project developed a new electrode material, graphene sponge electrode, produced using a low-cost, bottom-up synthesis method with an estimated cost of the synthesis at lab scale being below 50€ per m2 of the material, which is orders of magnitude lower compared to the price of commercial electrode materials (3-6,000€ per m2). The bulk synthesis method developed allows easy functionalization of the graphene-based coating, thus making it possible to tailor the electrode properties and performance. Given its three-dimensionality and porous structure, graphene sponge electrode offers high surface area and can be operated in flow-through mode, thus reducing the mass transfer limitations. The main advantage of the developed material is that it demonstrates electrochemical inertness to chloride, and does not form any chlorine, chlorate, and perchlorate due to chloride oxidation, even at high anodic current densities. This characteristic, coupled with the capacity of graphene sponge anode to effectively oxidize and degrade even the most persistent contaminants such as PFAS, represents a major breakthrough in electrochemical water treatment, and is a step towards practical applications of electrochemical systems in water and wastewater treatment. The invention enables electrochemical treatment of even brackish, highly polluted wastewater (e.g. reverse osmosis brines, landfill leachate), often rich in PFAS and other contaminants, but without compromising the process performance due to the formation of toxic and persistent byproducts. The synthesis and application of graphene sponge electrodes for water treatment has been patented and is the focus of an ERC Proof of Concept Grant GRAPHEC.
Furthermore, ELECTRON4WATER project developed an electrode material based on nanostructured coating of manganese oxide, capable of selectively oxidizing sulfide to elemental sulfur, which could be easily separated and recovered from the solution. Given that manganese is an earth-abundant element, and that the electrode is produced using a controllable and low-cost electrochemical deposition process, the developed synthesis method is both low cost and easily scalable. This material enabled a selective and rapid removal of sulfide in the form of elemental sulfur particles. Given that the formed sulfur particles are not retained at the electrode surface but accumulated in the solution in the conditions of wastewater treatment, sulfur can be easily separated, which effectively excludes any possibility of sulfide reformation, as well as avoids the passivation of the anode surface. The anode developed in the ELECTRON4WATER project is the first reported anode material capable of such performance and represents a significant step towards in situ electrochemical sulfide control in sewage and other parts of wastewater treatment systems where sulfide formation is an issue (e.g. anaerobic digestion).
