KINETICS OF NITRATE REDUCTION BY DETRITAL FE(II)-SILICATES

The ability of Fe(II)-bearing minerals to reduce nitrate was investigated experimentally in order to asses their potential for nitrate removal in aquifers. Experiments were carried out with a fluidized bed reactor, using arfvedsonite as an example for amphiboles and augite for pyroxenes. Results show that both Fe(II)-bearing silicates are able to reduce nitrate at low rates in the pH range 2 to 7. For arfvedsonite a maximum reduction rate was found around pH 4, while at higher values a pH independent rate of 4 · 10-17 N mol/cm2· sec (25°C) is found. Nitrate reduction rates for augite are on the same order of magnitude. The mechanism appears to be complex; apparently, it is not a direct reaction between nitrate and the dissolving mineral surface, but rather nitrate seems to react with secondary products of silicate dissolution. The most plausible explanation is that freshly precipitated FeOOH catalyzes nitrate reduction by Fe2+, as has been reported from other studies. A rough estimate for sandy aquifers indicates that Fe(II)-bearing silicates should be able to reduce nitrate at a rate on the order of magnitude of 4 · 10-5 NO3 mol/1 · a, and they can be of importance in aquifers with long groundwater residence times or low nitrate loads. © 1990.