COMPETITIVE INHIBITION OF CATALYTIC NITRATE REDUCTION OVER CU–PD-HEMATITE BY GROUNDWATER OXYANIONS

Abstract

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.