Reduction of nitrate and nitrite in water by immobilized enzymes

Abstract

NITRATE, a common and serious contaminant of ground water, is removed at present either by physicochemical methods that do not degrade it, or via degradation by microorganisms, which is a slow process¹. We report here a rapid and efficient process for nitrate removal which involves catalytic reduction by immobilized enzymes. The reduction is driven by an electrical current, and results in complete conversion of nitrate to N₂ without residues. Our electro-bioreactor was constructed by co-immobilizing the enzymes (purified NADH: nitrate reductase from Zea mays² and crude nitrite reductase and N₂O reductase from Rhodop-seudomonas³) with electron-carrying dyes in a polymer matrix, which was then attached in thin layers to the surface of the cathode. Nitrate-laden water is pumped past the anode and through the active matrix on the cathode while a low voltage is applied, resulting in two-stage nitrate reduction to N₂ via nitrite. The enzyme activity is higher in the co-immobilized state than in free solution. In principle, such electro-bioreactors could be developed for removal of other water contaminants such as pesticides, if appropriate enzymes and cofactors can be identified.