MXenes as sustainable functional nanomaterials for photocatalytic degradation of dye pollutants: Performance, effect of process parameters, stability and re-useability evaluation – A critical review

MXene, an emerging cutting-edge functional material with unique properties like high electron conductivity, improved surface functionality, efficient charge separation, good optical properties, and lower Schottky barrier for photocatalytic applications, has gained a high interest of the scientific community in the recent decade. This work aimed to critically review the advances in the application of MXENE-based materials (MXBM) for photocatalytic degradation of various hazardous dye contaminants for water and wastewater treatment applications. Specifically, the effect of various technical operating parameters that are of high importance for the evaluation of MXBM's real applicational value was analyzed. Mechanisms of photocatalytic degradation in MXBM were rationally discussed. Usefulness of MXenes in photocatalytic materials include enhancement of photo-generated charge carrier separation, serving as stable support, limitation of photocatalyst size, improving reactant adsorption. Ti 3 C 2 (Ti 3 C 2 T x ) is the most popular MXene for this application, while few attempts have been made also for Nb 2 CT x . Effective nanocomposites were synthesized with TiO 2 , MOFs, g-C 3 N 4 , chitosan, cellulose, BiOBr, CdS. Other attempts included hybrid materials containing Ag 3 PO 4 , CuO, MoS 2 , NiCo 2 S 4 , Nd-NiCo 2 O 4 , ZnO. Coating the MXenes with Ag and Pd nanoparticles (NPs) was another effective method. Several effective applications were developed for degradation of Congo Red (CR), Crystal violet (CV), Direct Red 31 (DR31), Malachite Green (MG), Methylene Blue (MB), Methyl Orange (MO), Orange G, Rhodamine B (RhB). Based on the performed review, it was found that for many applications, the photocatalytic degradation efficiency exceeds 90 % for 60–80 min treatment. Typically, the optimal dose of MXBM photocatalyst is in the range of 10–100 mg/L, and the optimal pH is in the range 2–7.5. The major reactive species in the photocatalytic reaction catalysed by MXBM were hydroxyl radicals ( • OH) and superoxide radical anions ( • O 2 - ), which are accountable for the degradation of the dye contaminants via Advanced Oxidation Processes (AOPs). The reusability of the materials is an important advantage of MXBM, as it can be treated mostly employing pure water or with addition of ethanol and can be reused up to 3–8 cycles while still maintaining degradation efficiency of >75 %. In the end, the knowledge gap and problematic issues that can help to improve the MXBM, as well as provide future research directions for investigators, were highlighted. Future studies should focus, beside the degradation effectiveness, on intermediate products (by-products) formation, cost analysis, sustainable scale-up strategies under real-case scenario conditions. Importantly, a long-term toxicity assessments of MXBMs should be studied along with research on particle size-dependent toxicity of different types of photocatalytic MXene-based materials. Due to utilisation of hydrated electrons in specific conditions MXBMs could find applicability also in Advanced Reduction Processes (ARPs). Photocatalyzed reactions could utilize sulphite, dithionate or gaseous hydrogen as reductans and sources of reductive radicals. This application of MXBMs seems to be overlooked by scientific community. • Good MXENE stability and possibility of its regeneration for long-term applications. • Importance of scavenging tests and proper identification of radical species in the process. • Degradation studies must be evaluated, including the final mineralization rate of the pollutant. • Optimization of pollutant concentration must include the aspect of photocatalyst capacity. • Controversial, not confirmed, conclusions about CO2 formation during treatment.