Evaluation of the catalytic reduction of nitrate for the determination of dissolved organic nitrogen in natural waters

The catalytic reduction of nitrate ions into nitrogen gas was tested to partly remove dissolved inorganic nitrogen (DIN) before the determination of dissolved organic nitrogen (DON). Experiments were conducted on nitrate solutions enriched with natural organic matter (NOM) isolates previously extracted from surface waters. Three catalysts Pd-In/Al2O3, Pd-Sn/Al2O3 and Pd/SnO2 were tested. Their noble metal (palladium) and promoter metal (indium or tin) contents are 5 and 1.75 wt%, respectively. Preliminary experiments performed on a solution containing 17 amino acids showed that most compounds were removed by less than 15%, probably due to sorption onto the catalysts. Reduction of nitrate in absence of NOM was complete after 20 min of reaction time and the removal of DIN was about 80% (about 19% formation of ammonium). In the presence of NOM (DOC 20 mgC/L, DON 0.67 mg N/L), the kinetic of nitrate reduction was slower and the reduction in DIN content was limited to 15% i.e. selectivity toward ammonium reached 85%. Adsorption tests showed a similar removal of both DOC and DON of about 70% and 30% onto Pd-Sn/Al2O3 and Pd/SnO2 catalysts, respectively, which confirmed that NOM probably compete with nitrate for active catalytic sites. In conclusion, catalytic reduction of nitrate before DON determination cannot be used because of DON sorption and low DIN removal.