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^-, PO₄^3-, BrO₃^- and SO₃^2- at low concentrations (>5 mg/L) and HCO₃^-, CO₃^2-, SO₄^2- 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 PO₄^3-, SO₃^2-, and SO₄^2- increased, respectively. It decreased from 55% to 35% in the presence of HCO₃^- and CO₃^2- 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 (PO₄^3-, SO₃^2-, and BrO₃^- cases) due to their similar structure, ii) blockage of the promoter and/or noble metal (HCO₃^-, CO₃^2-, SO₄^2-, Cl^- and ClO₄^- cases), and iii) interaction with the support surface (PO₄^3- 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.