Nitrate Reduction of Brines from Water Desalination Plants Employing a Low Metallic Charge Pd, In Catalyst and Formic Acid as Reducing Agent

Abstract

Brines from desalination plants lead to adverse environmental impact; therefore, their treatment by catalytic reduction has become a topic of increasing interest. This work aims to apply catalytic reduction to the treatment of water with high nitrate concentration, employing a low noble metal loading in the catalyst (Pd 1%, In 0.25%). Catalytic nitrate removal was evaluated under various conditions, e.g. at different pH values, by assessing the N₂ stripping effect, and by modifying the method of pH correction. A change on the final step of the synthesis was also evaluated as well as the reduction of the active phase by means of H₂ flux or by the reduction in the aqueous phase with hydrazine. The synthesis method through hydrazine reduction showed catalytic activity and selectivity comparable to those shown by the hydrogen-reduction method. It was found that activity and selectivity can be manipulated. The ratio between nitrate eliminated and Pd mass employed proved to be very high, and among the best ratios reported so far. The best conditions to obtain total nitrate conversion were obtained when the pH was controlled between 4 and 5. The stripping with N₂ to the reaction media resulted in lower selectivity to ammonium. The hydrazine-reduced catalyst characterization showed that the nanoparticles were well dispersed on the surface and had an indium: palladium surface ratio higher than that in the bulk, while the H₂-reduced catalyst had more Pd on the surface with lower dispersion.

Graphical Abstract