Interference effects of oxyanions commonly found in natural waters on the catalytic reduction of nitrate in water

In this work, the influence of oxyanions on the catalytic reduction of nitrates using formic acid as the reducing agent was studied as well as the influence of bicarbonate, sulfate, and phosphate co-anions on the catalytic nitrate reduction with Pd:In/Al₂O₃ (1:0.25 wt.%). A negative effect on nitrate conversion was observed in the following order: phosphate > sulfate > bicarbonate, showing a strong influence of electrostatic adsorption on the catalytic reduction of nitrate. However, no direct trend was observed relating the levels of interferents to the impact on the selectivity of the bimetallic catalyst using formic acid as a reducing agent. For both bicarbonate and phosphate, at lower levels, higher selectivity to nitrogen was obtained than for the reaction in the absence of interferents. On the other hand, increasing sulfate concentration led to a decrease in nitrate conversion. The mixtures of co-anions also showed a decrease in the catalytic activity. At 120 min, a N₂ selectivity higher than 95% was obtained, except for the C50-S20 (bicarbonate 50 ppm-sulfate 20 ppm) mixture which showed the lowest selectivity to N₂ value (87.3%). The loss of catalyst activity was found to be reversible and not permanent.