REDUCTION OF NITRATE TO AMMONIUM BY SULFATE GREEN RUST - ACTIVATION-ENERGY AND REACTION-MECHANISM

Iron(II)-containing minerals are potential inorganic nitrate reductant s in soils and sediments. Specifically, synthetic green rust (Fe4IIFe2 III(OH)(12)SO4 . yH(2)O, GR) reduces nitrate to ammonium. The reaction of GR with two different nitrate salts, NaNO3 and Ba(NO3)(2), has bee n compared. The reaction stoichiometry and the reaction order with res pect to Fe(II) in GR does not change in the examined temperature range (15-50 degrees C), irrespective of the nitrate salt used. Activation energies of 83.9 +/- 7.6 kJ mol(-1) and 90.5 +/- 6.9 kJ mol(-1) have b een determined for the reaction of GR with NaNO3 and Ba(NO3)2, respect ively. However, for the latter reaction the rate of reaction is increa sed 40 times. Based on X-ray and Mossbauer investigations, this accele ration of the reaction rates is attributed to the forced exchange of s ulphate with nitrate in GR interlayers caused by precipitation of BaSO 4 outside the GR particles, a reaction which does not occur in the pre sence of NaNO3. This difference in anion exchange behaviour is confirm ed by anion exchange experiments with the redox-inactive GR-analogue, pyroaurite. A reaction model initiated by nitrate electrostatically bo und at positively charged sites of GR is proposed.