Modelling nitrate reduction in a flow-through catalytic membrane contactor: Role of pore confining effects on water viscosity

Abstract

Nitrate hydrogenation has been largely considered as an eco-friendly strategy for nitrate abatement in drinking water to meet more and more stringent emission targets. Nitrate conversion on conventional Pd–Cu catalysts can be dramatically promoted in catalytic membrane reactors operating in flow-through configuration. In a previous study (Wehbe et al. [14]), we have reported an unexpected result: the nitrate conversion increases with the transmembrane flowrate when operating with mesoporous top layers (pore size < 10 nm), opposing to the expected inverse trend from basic Reactor Engineering concepts. Moreover, at a given flowrate, the catalytic activity increased with a reduction of the pore size. To provide a rationale on such effects, we present here a coupled model of concentration polarization and ionic solute transport within membrane pores. Mass transfer within the membrane top layer has been modelled using the extended Nernst–Planck equations (convection–diffusion–migration) combined with Donnan steric partitioning and dielectric exclusion at both membrane/solution interfaces. In addition to concentration polarization effects, a potential increase of the solvent viscosity under confinement, involving unexpectedly low ionic and hydrogen diffusivities under nanoconfinement, is argued as the underlying reason for such effects.