Nitrate transformations in nitrate-contaminated soils and ground waters: thermodynamic aspects

In Saudi Arabia, contamination of soils and groundwaters with nitrate and its reduction products (i.e. nitrite and ammonium) at severe levels have been observed in many locations. Consequently, nitrate transformations are considered important parameters to estimate pollution impact on agricultural ecosystem in the Kingdom. Therefore, it was found necessary to test through thermodynamic relationships the spontaneity of some reactions describing the chemical transformations of nitrate to its reduction products (NO2-, N2O, N2 and NH4+) at pH values between 5 and 9, and at different sets of activities comparable to those really exist in air, groundwaters and soils.<br>The calculated results declared that at assigned pH range, nitrate is not stable and can easily transform to nitrite NO-2(aq), nitrous oxide N2O(g), dinitrogen N2 (g) and ammonium NH4+(aq). These transformations can proceed easier under acidic than under alkaline conditions. On the other hand, formation of nitrogen dioxide NO2 (g) can occur only at pH 7.52 and lower. However, at higher pH values, nitrate is stable and no formation of NO2 (g) takes place.<br>The study suggested that the formation of NO2 as an intermediate product of biological and chemical denitrification is very probable and should be considered in future practical research. <br> <br>Keywords: Greenhouse gases, Air pollution, Groundwater pollution, Denitrification, Ammonification, Nitrous oxide