The main objectives of the GRAPHEC project were: i) to determine the fate of fluoride released from PFAS molecules and its impact on the graphene sponge electrode lifetime, ii) to verify the occurrence of fouling/scaling when dealing with real waste streams, iii) to optimize the reactor design and determine the range of PFAS influent concentrations it can deal with, and iv) to evaluate key performance indicators (KPI) such as electric energy per order of the developed reactor.
The results achieved so far demonstrated that electrochemical cleavage of the C-F bond at the anode surface leads to the release of highly reactive hydrofluoric acid (HF), which reacts rapidly with the reduced graphene oxide (RGO) coating of the anode, resulting in the fluorination of RGO. Nevertheless, no impact of RGO fluorination could be observed during several months of operation of the graphene-enabled electrochemical system treating highly concentrated PFAS solution. Even though the experiments are still ongoing and will be further continued in the FOREVER-WATER project, data obtained so far indicate that PFAS degradation does not shorten the electrode lifetime.
Long-term continuous operation of graphene-enabled electrochemical system showed a decrease in PFAS removal over time due to the accumulation of PFAS and other anions at the positively charged anode surface, which over time start blocking the access of PFAS molecules and prevent their anodic oxidation. This was resolved by periodic polarity reversals, where switching the anode to negative potentials (e.g. -1.2 V vs Standard Hydrogen Electrode, SHE) sheds off the accumulated anions and restored the anode performance in terms of PFAS removal and degradation.
No fouling of the graphene sponge anode could be observed during >1 year, including storage of the material. This can be explained by the antimicrobial properties of RGO that prevent the development of a biofilm. Cathode scaling due to the formation of calcium and magnesium carbonates is only observed when employing stainless steel cathode and can be avoided by using graphene sponge as a cathode, too.
Lab-scale work performed within the ERC StG ELECTRON4WATER allowed the use of stainless steel anodic current feeder, yet this material undergoes rapid corrosion when employed with real waste streams. This was resolved by using an iridium oxide-based metallic current feeder, which not only eliminates the corrosion but also suppresses chloride oxidation reaction. The performance of graphene-enabled electrochemical system with the new current feeder has been extensively studied and will be published in a scientific paper, currently in preparation.
Scalable reactor developed for housing the graphene sponge electrodes was demonstrated to be able to deal with PFAS influent concentrations up to 500 ng/L of short chain PFBS. The obtained electric energy per order (Eeo) of the new reactor design is in the range of 20-50 kWh/m3 and degrades 90-99% of C4-C8 PFAS in one-pass, flow-through mode in synthetic electrolytes and real wastewater. This compares favorably to the destruction methods based on supercritical water oxidation (SCWO), hydrothermal alkaline treatment (HALT) and plasma, where their energy consumption is ~1,000 kWh/m3 and ~100 kWh/m3, respectively. Moreover, these processes cannot be operated in continuous mode (SCWO, HALT) or treat larger volumes of waste (plasma). SCWO and HALT operate at high temperatures (≥370°C) and pressures (>20 MPa), whereas plasma-based water treatment is difficult to upscale, e.g. due to the limited depth of plasma penetration.
