PdIn Catalysts in a Continuous Fixed Bed Reactor for the Nitrate Removal from Groundwater

Abstract

The ground water (GW) is a real sample of water which contains nitrates (81 mg/L), carbonates and sulphates. 3 This sample was treated by catalytic reduction in a bubble column fixed-bed reactor. The reaction conditions 4 were room temperature and atmospheric pressure, and hydrogen was used as reducing agent. A comparison 5 between the catalytic activity of Pd,In/SiO2 and Pd,In/Al2O3 catalysts was performed, giving the latter the best 6 results regarding the nitrate conversion and the former the selectivity to nitrogen. Synthetic water containing 7 nitrates (SW) and the groundwater (GW) with high salinity (81 mg/L nitrates) and humic acid content were 8 treated in these conditions using HCl and CO2 as acidifiers. When the SW was acidified with CO2 at an H2 flow 9 (2.17 mL/min), more than 50% of conversion was obtained with an ammonia and nitrite concentrations of 0.44 10 and 0.92 ppm respectively. These results reached the level established by the WHO and the USEPA. However, 11 both Pd,In/SiO2 and Pd,In/Al2O3 catalysts showed evidence of some deactivation process. This deactivation was 12 higher in the GW sample, possibly due to the presence of Ca+2 and Mg+2 cations that precipitate on the active 13 sites, and/or the presence of humic acids. The characterization studies indicated that, after the reaction, the 14 composition of the metal phases was modified, specifically on their surface. As the catalyst activity is related to 15 surface ratio Pd/In, a relative increase in the concentration of Pd decreases the activity and causes changes in the 16 selectivity to N2. Some sintering of the metal particles was also observed to contribute to catalyst deactivation. 17 Nevertheless, promising results were obtained since a high water volume could be treated with a relatively low 18 amount of low metal loading catalysts.