Mineralization of organic pollutants by anodic oxidation using reactive electrochemical membrane synthesized from carbothermal reduction of TiO2.

Reactive Electrochemical Membrane (REM) prepared from carbothermal reduction of TiO2 is used for the mineralization of biorefractory pollutants during filtration operation. The mixture of Ti4O7 and Ti5O9 Magnéli phases ensures the high reactivity of the membrane for organic compound oxidation through •OH mediated oxidation and direct electron transfer. In cross-flow filtration mode, convection-enhanced mass transport of pollutants can be achieved from the high membrane permeability (3300 LMH bar-1). Mineralization efficiency of oxalic acid, paracetamol and phenol was assessed as regards to current density, transmembrane pressure and feed concentration. Unprecedented high removal rates of total organic carbon and mineralization current efficiency were achieved after a single passage through the REM, e.g. 47 g m-2 h-1 - 72% and 6.7 g m-2 h-1 - 47% for oxalic acid and paracetamol, respectively, at 15 mA cm-2. However, two mechanisms have to be considered for optimization of the process. When the TOC flux is too high with respect to the current density, aromatic compounds polymerize in the REM layer where only direct electron transfer occurs. This phenomenon decreases the oxidation efficiency and/or increases REM fouling. Besides, O2 bubbles sweeping at high permeate flux promotes O2 gas generation, with adverse effect on oxidation efficiency.

Effects of sodium hypochlorite exposure mode on PES/PVP ultrafiltration membrane degradation.

Drinking water production plants using membrane filtration processes report membrane failure issues. According to the literature, membrane degradation is often induced by exposure to sodium hypochlorite, an oxidant widely used during in-place cleanings. The present study focused on quantifying the effect of membrane exposure mode to hypochlorite on properties modifications of a PES/PVP ultrafiltration membrane widely used for drinking water production. For this purpose effects of sodium hypochlorite concentration, contact duration and exposure mode (static or dynamic) were investigated. The pH of the hypochlorite solution was set to 8 as it was demonstrated in numerous previous works that the pH range 7-8 leads to the most severe modification in the membrane characteristics. Membrane degradation was monitored at molecular scale by attenuated total reflectance infrared spectroscopy and at macroscopic scale by pure water permeability and elongation at break measurements. The results obtained in static (soaking) and dynamic (filtration and filtration/backwashing cycles) hypochlorite exposure modes indicated that PES/PVP membrane degradation progress was predominantly governed by hypochlorite oxidation rate. In the tested conditions, mechanical stress (pressure differentials) did not significantly contribute to membrane ageing. The correlation between molecular and macroscopic characterizations demonstrated that PVP degradation is responsible for the membrane integrity loss. A linear relationship between the loss of ductility of the membrane and the progress of the PVP degradation was obtained whatever the exposure mode. Thanks to experiments conducted at various hypochlorite concentrations and exposure durations, the hypochlorite dose parameter (hypochlorite concentration times contact time), widely used in the literature, was demonstrated to be inappropriate to describe the degradation rate: the hypochlorite concentration impact was shown to be dominating the exposure time's one on the degradation rate.