Abstract
Groundwater pollutions by nitrates and nitrites have received increasing attention in many locations in the world. The present paper demonstrates that catalytic decomposition of trace amounts of toxic nitrate and nitrite in aqueous phase on a level with drinking water by reduction to molecular nitrogen could be carried out under mild conditions of atmospheric pressure of hydrogen at room temperature. Supported bimetallic catalyst, Cu/Pd/C, revealed excellent activity for nitrates, while single metallic Pd/C was much more effective than bimetallic catalyst for nitrites. Intermediate nitrite accumulated in solution in the decomposition of nitrate on bimetallic Cu/Pd/C, but the nitrite could be removed in situ with great ease in the presence of Pd/C free of Cu. Suspended Pd/C catalysts were highly active for nitrite reduction under vigorous stirring conditions, where the rate was proportional to the reciprocal of particle diameter, signifying the active Pd sites around external surface of the catalyst particle dominate the observed activity. Thus, catalysts should be used in fixed-bed for effective renewal of aqueous substrate at around the particle surface, but fine powders employed in slurry reactor are not adequate due to the pressure-drop at higher liquid-flow rates. Attempts were made in the present work to prepare Pd/C catalysts capable of fixed-bed operations without pressure-drop, employing carbon monolith, activated carbon powders coated on poly- urethane sponge-skeleton and non-porous alumina spheres as support.
