A magnetite supported monometallic Pd catalyst was synthesized by a co-precipitation process followed with the reduction in pure hydrogen at 453 K. The catalyst was composed of ultrafine Pd nanoparticles (similar to 2 nm) highly dispersed on the surface of superparamagnetic Fe3O4 nanoparticles. Aside from its roles as the catalyst support and the magnetic separation medium, Fe3O4 was found to be a good promoter for the nitrate reduction, where nitrate was firstly reduced to nitrite by the Fe(II)/Fe(III) redox couple, and subsequently reduced to nitrogen and ammonium. Further mechanistic studies demonstrated that besides the Pd sites, active sites for the nitrite reduction also exist on the surface of Fe3O4. Part of the nitrite reduction occurred on the surface of Fe3O4, which may also be attributed to the Fe(II)/Fe(III)redox couple. In the present study, ammonium was the main product because of the different denitrification mechanisms compared with bimetallic catalysts. (C) 2012 Elsevier B.V. All rights reserved.