Activity and stability of the catalytic hydrogel membrane reactor for treating oxidized contaminants

The catalytic hydrogel membrane reactor (CHMR) is an interfacial membrane process that uses nanosized catalysts for the hydrogenation of oxidized contaminants in drinking water. In this study, the CHMR was operated as a continuous-flow reactor using nitrite (NO2-) as a model contaminant and palladium (Pd) as a model catalyst. Using the overall bulk reaction rate for NO2- reduction as a metric for catalytic activity, we evaluated the effect of the hydrogen gas (H-2) delivery method to the CHMR, the initial H-2 and NO(2)(- )concentrations, Pd density in the hydrogel, and the presence of Pd-deactivating species. The chemical stability of the catalytic hydrogel was evaluated in the presence of aqueous cations (H+, Na+, Ca2+) and a mixture of ions in a hard groundwater. Delivering H-2 to the CHMR lumens using a vented operation mode, where the reactor is sealed and the lumens are periodically flushed to the atmosphere, allowed for a combination of a high H-2 consumption efficiency and catalytic activity. The overall reaction rate of NO(2)(- )was dependent on relative concentrations of H-2 and NO(2)(- )at catalytic sites, which was governed by both the chemical reaction and mass transport rates. The intrinsic catalytic reaction rate was combined with a counter-diffusional mass transport component in a 1-D computational model to describe the CHMR. Common Pd-deactivating species [sulfite, bisulfide, natural organic matter] hindered the reaction rate, but the hydrogel afforded some protection from deactivation compared to a batch suspension. No chemical degradation of the hydrogel structure was observed for a model water (pH > 4, Na+, Ca2+) and a hard groundwater after 21 days of exposure, attesting to its stability under natural water conditions. (C) 2020 Elsevier Ltd. All rights reserved.