Treatment of nitrate loaded waters

, by Claude De Bellefon, Dominique Richard, Mohamad Ali Al Sawah

Chemical fertilizers containing nitrates, widely used by industrial agriculture as well as liquid manure resulting from intensive breeding have harmful effects upon the environment in many areas. High nitrate contents are one of the factors leading to a poor quality of water. Their ingestion by human beings rise health risks both acute and chronic.

Their abatement by catalytic hydrogenation was the object of many studies during the last years. The catalyst used were Pd-based and the most efficient incorporated a Pd-Cu catalytic system. The main drawback for a process based upon catalytic hydrogenation remains an incomplete selectivity towards nitrogen and thus the formation of ammonium ions during the reduction of the nitrate ions.

For this reason, the LGPC develops a new approach, taking into account this limitation through a process combining two steps:
- Catalytic hydrogenation allows for the abatement of nitrates ions with the formation of nitrogen and ammonium ions (étape 1).
- Photocatalytic oxidation of the ammonium ion leads to the formation of nitrate ion (étape 2).
The combination of both techniques in a recycle process should lead to a lower amount of nitrate at the outlet through a partial formation of gaseous nitrogen during each cycle.

However, photocatalytic oxidation of ammonium ion is only achievable at a reasonable rate in basic medium. For this reason, it is necessary to check that it is possible to use catalyst active in a basic medium such as Raney Nickel.

This has been done at the LGPC, where:
- The activity of Raney Nickel for catalytic hydrogenation of nitrates has been proved.
- An activity larger than 5000mg/(L.min.g_{cat}) for some Raney Ni has been measured, much larger than the one reported for Pd-Cu catalysts, with a selectivity towards nitrogen close to 70%.
- A slight deactivation has been observed during successive loads of concentrated nitrate solutions.
- The effect of promoting metal upon the activity and the selectivity of Raney Ni has been studied.