Nitrogen is essential for plant growth and, in the form of nitrates, is their primary source of nitrogen. Nitrates occur naturally in soil and water, but extensive farming can deplete the soil of its natural nitrogen, so nitrogen-containing fertilisers are often added. However, when more nitrogen is added to me soil than plants can use, excess nitrate can leach into groundwater supplies and could become a potential health risk.
Technologies for the removal of nitrate from drinking water and waste water are therefore of great importance. Palladium-based catalysts for the hydrogenation of nitrates to nitrogen represent one of the most promising approaches.
Now, an Italian research group has investigated palladium-copper and palladium-tin supported on zirconia and titania catalysts for the hydrogenation of nitrate in drinking water (G. Sttukul, R. Gavagnin, F. Pinna, E. Modaferri, S. Perathoner, G. Centi, M. Marella and M. Tomaselli, Catal. Today, 2000, 55, (1-2), 139-149).
Catalysts were prepared either as microspheres or in the form of membranes deposited on alumina tubes. The use of catalysts under diffusion control conditions reduced the amount of the byproduct ammonia formed while retaining a high catalytic activity.
Researchers in Germany, however, used palladium-tin and palladium-indium catalysts for the nitrate reduction (U. Prüsse, M. Hähnlein, J. Daum and K.-D. Vodop, Catal. Today, 2000, 55, (1-2), 79-90). In situ buffering with formic acid as a reductant instead of hydrogen and polyvinyl alcohol-encapsulated catalysts with superior diffusional properties have both been demonstrated. The above Pd-Sn and Pd-In catalysts are reported to be more efficient than prior palladium-copper regarding the nitrogen formation activity, the selectivity and the long-term stability.
As the removal of nitrates from drinking water and waste water is becoming more necessary, these concepts may lead to the establishment of a technical-scale nitrate reduction process.