An apparatus and method for removing NO.sub.x from the exhaust of internal combustion engines. The apparatus has a porous absorbent body saturated with a liquid alkaline electrolyte for absorption of NO.sub.x as nitrate and nitrite, in which body electrodes are distributed pairwise for electrochemical decomposition of the nitrate and nitrite to form nitrogen. The method uses the apparatus of the invention to decompose nitrates and nitrites into diatomic nitrogen.