Manganese oxide-based porous electrodes for rapid and selective (electro)catalytic removal and recovery of sulfide from wastewater

Abstract

Sulfide and its removal is a major concern in wastewater treatment as it represents a threat to human health and the structural integrity of the water distribution system. In this study, we demonstrated for the first time an exceptional performance of manganese oxide-coated graphite felt (GF-MnxOy) electrodes for selective sulfide oxidation to sulfur. Oxidation state, loading, morphology and crystallinity of MnxOy coating was tuned using electrodeposition synthesis method to enable an efficient and selective sulfide oxidation to sulfur. Excellent (electro)catalytic activity of GF-MnxOy yielded up to 25-fold increase in sulfide removal rates compared to pristine GF, both in the open circuit (OC, no applied potential) and under anodic polarization at 0.4 V vs Standard Hydrogen Electrode (SHE). Although anodic polarization did not further enhance sulfide oxidation rate compared to OC, it enabled a continuous re-oxidation of the reduced MnxOy coating after its reaction with sulfide. Thus, restoring of the catalytic properties of MnxOy coating enabled higher sulfide removal rates compared with the OC experiment. The formed elemental sulfur remained at the surface of GF-MnxOy leading to a gradual electrode passivation. The deposited sulfur was successfully dissolved by reversing the polarity of the GF-MnxOy electrode to -0.8 V vs SHE. However, full electrode recovery and restoring of the initial sulfide removal rates could not be achieved as cathodic polarization at -0.8 V vs SHE during longer time (>3 hours) required to remove S0 also caused a partial dissolution of the MnxOy coating. (Electro)catalytic sulfide removal was somewhat decreased in real sewage (2.42 ± 0.02 h-1 vs 0.93 ± 0.15 h-1 in NaNO3 supporting electrolyte and real sewage, respectively). The selectivity of the process towards deposition of elemental sulfur was decreased in real sewage due to partial production of colloidal sulfur, which was presumably caused by diffusion limitation imposed by presence of other ions. Due to lesser extend of GF-Mn2O3 electrode passivation in real sewage, sulfide removal rates remained stable over six subsequent application cycles. In summary, MnxOy-based electrodes demonstrated exceptional (electro)catalytic activity and selectivity for sulfide oxidation to sulfur and thus its complete separation from water. Upscaling of the proposed electrochemical system and its application for the treatment of complex wastewater streams requires further efforts to maintain the selectivity towards deposited sulfur as a final product and allow its complete recovery