Platinum Metals Rev., 1993, 37, (4), 211
Nitrate and Nitrite Removal from Drinking Water
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.