No abstract

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

Pollution of surface and groundwater by nitrates is one of the major worldwide environmental problems. Pd/In-based catalysts were prepared by the wet impregnation method using Al2O3, SiO2, and TiO2 as supports. The influence of the synthesis procedure in the search for catalysts with high selectivity to N2 was studied. The materials were characterized by X-ray fluorescence and scanning electron microscopy-EDS analysis and evaluated in the catalytic nitrate removal from water employing a batch reactor. The catalytic performance was influenced by the support used in the synthesis, obtaining the following order of better nitrate removal: Al2O3 > TiO2 > SiO2. Moreover, the selectivity to N2 is affected by the support used, being Al2O3-based catalyst being the most selective to N2 and TiO2-based catalysts the less one due to the strong metal–support interaction. On the other hand, the order of the synthesis stages affects the nitrate removal efficiency and the selectivity, being the catalysts in which In was added after Pd with intermediate calcination and reduction stages and a posterior reduction, the most selective to N2 (94.4% at 90% of nitrate conversion).