Catalytic Reduction Of Nitrates From Groundwater

Nitrate is a wide spread contaminant of ground and surface waters worldwide, and it is a potential human health threat especially to infants, causing the condition known as methemoglobinemia (blue baby syndrome). With these in regards, it sitimulates people great concern on nitrate contamination in groundwater. WHO and USEPA have regulated the maximum nitrate concentration in drinking water not greater than 10 mg NO3-N/l. There are ways to prevent high concentrations, but since it will take many years to see the results of improved fertilizer management, nitrate removal may be the only option for many communities. The removal options that are discussed in this paper include physiccal denitrification (reverse osmosis, ion exchange), biological denitrification, and catalytic reduction. The physico-chemical approaches may impose high capital and operational costs, and nitrate is only shift to another form, it request post treatment of the concentrated nitrate solutions. On the other hand, the denitrification approach needs polishing of the effluent such as purification and disinfection in order to secure the water quality for potable purpose. In addition, the biological approach is also sensitive to great variations. However, catalytic nitrate reduction shows a promising technique for groundwater.In this paper, Batch experiments for catalytic reduction of nitrite or nitrate from groundwater with Pd or and/ Cu catalysts were conducted.Catalytic reduction of nitrite method from groundwater is introduced, the reaction is fast and completely compared to the previous methods, the products of the reaction are gaseous nitrogen and dissolved ammonia. On the basis of this, we found: the influence on catalytic activity and selectivity reduction by catalyst and carriers are discussed, and y-Al2O3 supported Pd showed a high nitrite reduction activity and a low formation of ammonia. But Pd catalyzed nitrate reduction proceeded slowly, even at the end of the experiment (time=240min). There was still around 40% of nitrate unreduced. Comparatively, ammonium concentration was increased almost linearly over the entire course of the experiment. Meanwhile 46.5% of nitrate was reducedIVinto nitrogen gas, showing the actual nitrate removal efficiency.A series of experiments of catalytic nitrate reduction over suspended catalysts Pd-Cu/y-Al2O3 were carried out and discussed. It has been found that Pd-Cu combined catalysts at a ratio of 4 can maximize the nitrate reduction into nitrogen; above 80% total nitrogen removal efficiency was realized in this study. With nitrate disappeared, nitrite as the primary intermediate, and N2 and ammonium ions as the final products were found. But no nitrite was detected as the intermediate in the hydrogenation of nitrate during the batch experiment. This illustrate that the step of nitrate reduced to nitrite under Pd monometallic catalyst was rate-controlled step. It was also found that the catalytic activity is affected by the amounts of catalysts, hydrogen flow rate and pressure, the initial concentration of nitrate. With an increase in the amount of the catalysts, both nitrite and ammonia intermediates can be kept at a low level. A high flow rate and pressure of hydrogen is in favor of the catalytic reduction of nitrate, but not benefit for the selectivity. The catalytic reduction follows a first order kinetics in term of the initial nitrate concentration. And reduction activity and the formation of ammonium are mainly controlled by diffusion limitations in the reaction and the mass transfer of the reactants.Moreover, the effect of the normal ions in groundwater on both the nitrate consumption rate and reaction selectivity to N2 was quantitatively discussed. It was found that chlorides and sulfates in the reaction medium have no effect on either the extent of nitrate reduction or reaction selectivity, while HCO3~ in drinking water decreases the nitrate disappearance rate and increases the ammonia formation. On the other hand, the initial nitrate activity was found to increase slight...