Reduction of nitrate and nitrite in water by immobilized enzymes

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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. © 1992 Nature Publishing Group.

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