Ultraviolet A-driven photoelectrochemical system for efficient manganese removal and recovery from contaminated water: Continuous deposition and accelerated dissolution

Abstract

As the fourth most industrially applicable metal, the excessive manganese (Mn) mining and utilization generates an extensive quantity of Mn wastewater, leading to waste of valuable resource and numerous ecotoxicological effects. This study reported the novel photoelectrodeposition (PED) and photoassisted electrodissolution (PAED) processes driven by Ultraviolet A for Mn extraction and recycling from contaminated water. In the PED process with TiO2 nanorod arrays photoelectrode, efficient Mn removal above 83% was achieved over a wide pH range of 3–10, and selective removal of Mn2+ was achieved with Fe3+ coexistence. After deposition, Mn can be completely dissolved and recovered by the PAED process under acidic condition at pH 3. To the integration of the PED and PAED processes, 73.3 ± 0.9% of Mn can be recovered from the contaminated water with 20 mg/L Mn2+, and the energy consumption was 0.247 kWh/m3, which was only 63.5% of the conventional electrocatalysis process. The composite photocatalyst of TiO2-manganese oxides (TiO2-MnOx) film may form and develop on the electrode during the PED and PAED processes, accompanied by the Mn3+/Mn4+ redox cycle and the change in crystal morphology. As the predominant reactive species for Mn2+ oxidation, the continuous photogeneration of holes in the TiO2-MnOx film resulted in the continuous deposition of Mn. The mobile electrons photogenerated from the excited TiO2-MnOx film may also enhance the electron transfer and lead to the accelerated dissolution of Mn. The results of this study demonstrated that the PED-PAED process is an efficient and sustainable method for Mn removal and recovery.