Nitrate contamination of ground water in intensively farmed arable areas may result from the over application of natural or artificial ferilisers. As nitrate can constitute a health risk to humans such contamination must be minimised and the European Community has stipulated that the nitrate concentration in drinking water should not exceed 50 mg/1, and in fact it recommends significantly lower levels. Although a number of different processes are available for the removal of nitrate from water they have disadvantages; now researchers in Germany are working to develop a catalytic process to reduce both nitrate and nitrite with hydrogen (S. Hörold, K.-D. Vorlop T. Tacke and M. Sell, “Development of Catalysts for a Selective Nitrate and Nitrite Removal from Drinking Water”, Catal. Today, 1993, 17, (1–2), 21–30).

Nitrite, which is more toxic than nitrate, can be reduced to nitrogen by the use of a number of supported platinum metals hydrogenation catalysts. The removal of nitrate can be achieved by first reducing it to nitrite, which is then reduced to gaseous nitrogen and/or dissolved ammonia, the latter being undesirable in drinking water. The type of hydrogenation catalyst, activated by the addition of a second metal, determines the reduction products. Of the hydrogenation catalysts tested for the reduction of nitrite, 5 per cent platinum on carbon displayed the highest activity but resulted in substantial ammonia formation; with 5 per cent palladium on alumina the ammonia was significandy less. Palladium-copper bimetallic catalysts were studied for nitrate reduction, and optimum activity and selectivity were achieved with a palladium:copper ratio of 4:1.

A combination of suitable platinum metals nitrate and nitrite reducing catalysts, and strict hydrogen limitation, has enabled the complete removal of 100 mg/1 nitrate to be achieved while the formation of ammonia is kept below 0.5 mg/1, which is the level permitted in drinking water. Thus it is suggested that, when optimised, bimetallic catalysts could possibly be applied to the purification of drinking water.