Competitive inhibition of catalytic nitrate reduction over Cu-Pd-hematite by groundwater oxyanions.

The presence of various oxyanions in the groundwater could be the main challenge for the successive application of Cu-Pd-hematite bimetallic catalyst to aqueous NO3- reduction due to the inhibition of its catalytic reactivity and alteration of product selectivity. The batch experiments showed that the reduction kinetics of NO3- was strongly suppressed by ClO4-, PO43-, BrO3- and SO32- at low concentrations (>5 mg/L) and HCO3-, CO32-, SO42- and Cl- at high concentrations (20-500 mg/L). The presence of anions significantly changing the end-product selectivities influenced high N2 selectivity. The selectivity toward N2 increased from 55% to 60%, 60%, and 70% as the concentrations of PO43-, SO32-, and SO42- increased, respectively. It decreased from 55% to 35% in the presence of HCO3- and CO32- in their concentration range of 0-500 mg/L. The production of NO2- was generally not detected, while the formation of NH4+ was observed as the second by-product. It was found that the presence of oxyanions in the NO3- reduction influenced the reactivity and selectivity of bimetallic catalysts by i) competing for active sites (PO43-, SO32-, and BrO3- cases) due to their similar structure, ii) blockage of the promoter and/or noble metal (HCO3-, CO32-, SO42-, Cl- and ClO4- cases), and iii) interaction with the support surface (PO43- case). The results can provide a new insight for the successful application of catalytic NO3- reduction technology with high N2 selectivity to the contaminated groundwater system.