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.