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

• Published in: Water research (Oxford) Vol. 174; p. 115593
• Main Authors: Marks, Randal, Seaman, Joseph, Kim, Junyeol, Doudrick, Kyle
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.05.2020
• Subjects: Catalysis; Catalytic; Hydrogel; Hydrogels; Hydrogenation; Nano; Nitrites; Oxidation-Reduction; Oxidized contaminant; Palladium; Oxidized contaminant; Nano; Catalytic; Palladium; Hydrogel; Hydrogenation
• ISSN: 0043-1354; 1879-2448
• DOI: 10.1016/j.watres.2020.115593

The catalytic hydrogel membrane reactor (CHMR) is an interfacial membrane process that uses nano-sized 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 (H2) delivery method to the CHMR, the initial H2 and NO2− 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 H2 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 H2 consumption efficiency and catalytic activity. The overall reaction rate of NO2− was dependent on relative concentrations of H2 and NO2− 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. [Display omitted] •Lumen cyclic venting provided optimal reactivity and H2 consumption efficiency.•The overall reaction rate depended on influent reactive species concentrations at catalyst sites.•A 1-D AQUASIM model described the species reaction profiles in the hydrogel.•The CHMR provided some protection for Pd from deactivating species in groundwater.•The CHMR reactivity was stable in groundwater over long-term operation.