Hardness and salt effects on catalytic hydrogenation of aqueous nitrate solutions

Liquid-phase hydrogenation using a solid Pd-Cu bimetallic catalyst provides a potential technique for the removal of nitrates from waters. In this study, hardness and salt effects on both the nitrate disappearance rate and the reaction selectivity were quantitatively evaluated. Reduction experiments were performed for a wide range of nitrate conversions in an isothermal semibatch slurry reactor. It was discovered in a series of runs using various nitrate salts as a source of nitrate ions that the apparent surface reaction rate constant increases in the order K+<Na+ <Ca2+ <Mg2+ <Al3+ and changes proportionally with the ionization potential of a cation present in the aqueous solution. Permanent hardness of drinking water exhibits no inhibitive impact either on the extent of nitrate removal or on reaction selectivity. On the other hand, the nitrate disappearance rate as well as nitrogen production yield decrease appreciably in the presence of hydrogencarbonates, which is attributed to identical structures of nitrate and hydrogencarbonate ions resulting in competitive adsorption of these species to Pd-Cu active sites. A detailed analysis of experimental data confirmed that efficiency of the Pd-Cu bimetallic catalyst in catalytic nitrate reduction is influenced by the migration of produced hydroxide ions from the Helmholtz layer. (C) 1998 Academic Press.