Evaluation of Pd-In Supported Catalysts for Water Nitrate Abatement in a Fixed-Bed Continuous Reactor

Abstract

The increasing pollution of natural drinking-water sources brings about the development of new emerging technologies and processes for water remediation. In this work, the catalytic reduction of contaminated water containing nitrates (100 mg/L) was studied in a bubble column fixed-bed reactor, working at room temperature and atmospheric pressure and using hydrogen as a reducing agent. The activity and selectivity of these catalysts were evaluated under different reaction conditions (hydrogen flow, water flow, pH, acidifying agent) and verified in a stirred reactor, operating under batch conditions. On SiO2 supported catalysts, it was found that the nitrate conversion increased as theH2 flow was increased, while theN2 selectivity remained almost unaffected. On the other hand, in Al2O3 supported catalysts, an increase in H2 flow improved activity but worsened nitrogen selectivity. The best conversions and selectivity results were obtained when the feed solution was acidified with CO2. Very pronounced pH gradients or high amounts of OH- in the catalytic bed promoted nitrites and ammonium formation, provoking a notorious decrease of N2 selectivity. The highest conversion (100%) was obtained with the Al2O3 supported catalyst aged in the reaction. However, the selectivity to N2 under these conditions was 72%. On the other hand, the best selectivity to N2 was 97%, obtained with the aged SiO2 supported catalyst. In this case, the nitrate conversion was 30%. Characterization results showed that the metallic composition of the catalysts changed both after reduction and after contacting the liquid reaction media. These changes were observed by X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR). The catalysts stability was studied and discussed.