Improvement of an integrated ion-exchange/catalytic process for nitrate removal by introducing a two-stage denitrification step

A novel scheme of integrated ion-exchange/catalytic denitrification process for the removal of excessive quantities of nitrate ions from groundwater is proposed, which employs two separate reactor units to considerably minimize the production of ammonium ions as a side product in subsequent regeneration cycles. In the first reactor unit, selective reduction of nitrates to nitrites is carried out at high pH value (above 11.0) in the presence of Pd-Cu/γ-Al2O3 bimetallic catalyst. At T = 298 K and hydrogen partial pressure of 1.0 bar, up to 97% transformation of nitrates to nitrites was obtained, even at complete conversion of nitrate ions; the rest was found in the form of ammonium ions. In the second reactor unit, selective reduction of nitrites to nitrogen takes place at low pH value (4.5) over a Pd/γ-Al2O3 monometallic catalyst. A catalyst with higher dispersion (39%) of Pd clusters on the alumina carrier enables lower production of ammonium ions, the concentration of which was found below 0.5 mg/L for initial nitrite content up to 70 mg/L. Both Pd-Cu bimetallic and Pd monometallic catalysts were found chemically resistant in the investigated range of pH values (4.5–12.4). In comparison to the previous scheme of integrated ion-exchange/catalytic process, in which the denitrification step was conducted in a single, “liquid-full” reactor unit packed with a Pd-Cu/γ-Al2O3 bimetallic catalyst, the production of ammonium ions was reduced by an order of magnitude.