ABIOTIC NITRATE REDUCTION TO AMMONIUM - KEY ROLE OF GREEN RUST

Leaching of nitrate from soils and sediments can be reduced in anoxic environments due to denitrification to N2O/N-2 Or reduction Of nitrate to ammonium. While microbial dissimilatory reduction of nitrate to am monia is well known, it is shown here that this conversion can also pr oceed at appreciable rates in abiotic systems in the presence of green rust compounds [(Fe4Fe2III)-Fe-II(OH)(12)SO4 . yH(2)O]. In the reacti on nitrate is stoichiometrically reduced to ammonium, and magnetite (F e3O4) is the sole Fe-containing product. At a constant pH of approxima tely 8.25 and 25 degrees C, the rate expression is given as: d[NH4+]/d t = k[Fe(II)](GR)[NO3-], where k = 4.93 x 10(-5) +/- 0.39 x 10(-5) L m ol(-1) s(-1). In anoxic soils and sediments, this reaction may also le ad to a nitrate to ammonium reduction, at rates of similar magnitude o r even higher than microbial reduction rates. Hence green rust should be considered a possible important reductant for nitrate reduction to ammonium in subsoils, sediments, or aquifers where microbially mediate d reduction rates are small.