Brine-Free Regeneration of Ruthenium-Loaded Anion-Exchange Resin for Nitrate Removal

Abstract

Conventional regeneration of anion-exchange (AX) resin for nitrate (NO3–) removal relies on brine, generating nitrate-rich waste brine that requires costly post-treatment. In this study, we developed a brine-free regeneration method by introducing ruthenium nanoparticles into AX resin (Ru@AER), enabling catalytic decomposition of NO3– within the resin. Uniformly dispersed Ru nanoparticles (∼2 nm) were successfully formed throughout the resin, allowing regeneration of more than 99% of the ion-exchange sites. The regeneration efficiency strongly depended on the water content of the resin, and a water content of 48% achieved a regeneration rate exceeding 99%, owing to enhanced diffusion of NO3– and OH– within the resin matrix. Product analysis showed that NO3– was converted mainly to N2 with 86% selectivity, while NH3 was formed as a minor product (14% selectivity). A reaction mechanism is proposed in which NO3– diffuses through the aqueous phase in the resin matrix, is catalytically reduced on Ru nanoparticles, and is continuously replaced by OH– at ion-exchange sites, resulting in complete regeneration of the resin. Ru@AER catalyst maintained high regeneration performance over at least six consecutive cycles, demonstrating the feasibility of a sustainable brine-free regeneration process for nitrate-removal resins.