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• Sn-Pd-kaolinite showed faster NO 3 − reduction kinetics. • Sn-Pd-red mud showed higher N 2 selectivity. • A low Sn:Pd allowed their optimal proximity on the red mud support. • Sn-Pd-red mud showed stable kinetics and selectivity over a wide range of CO 2 :H 2. • The reactivity of Sn-Pd-kaolinite was strongly affected by groundwater impurities. In this study, two different materials i.e., naturally occurring kaolinite (Al-Si containing structured clay) and industrially produced red mud (mixture of metal oxides) were used as support surfaces for Sn-Pd bimetallic catalysts to investigate the effects of support on catalytic nitrate (NO 3 −) reduction. Under the same experimental conditions, the Sn-Pd-kaolinite reduced NO 3 − at a higher rate (k = 15.46 × 10−2 min−1) and lower N 2 (70 %) selectivity compared to that of Sn-Pd-red mud, which exhibited slower reduction kinetics (k = 9.16 × 10−2 min−1) but higher N 2 selectivity (85 %). We found that support material with a relatively low surface area (i.e., red mud) provided optimal Sn-Pd proximity at relatively low Sn:Pd ratios, but suffered from metallic overlap at high Sn:Pd ratios. Temperature program reduction (TPR) analysis showed that the kaolinite interaction with the Sn-Pd ensembles maintained a higher H 2 activation capacity, which resulted in fast reduction kinetics but low N 2 selectivity. However, the red mud support exhibited a strong alloying effect toward the Sn-Pd ensembles and regulated the H 2 capacity of the Pd to suppress the reduction kinetics and NH 4 + selectivity. The NO 3 − removal and byproduct selectivity was sustainable over a wide range of CO 2 :H 2 ratios. Sn-Pd-red mud showed a 100 % NO 3 − removal and ∼85 % N 2 selectivity at CO 2 :H 2 ≥ 0.6, while complete NO 3 − removal was obtained by Sn-Pd-kaolinite at CO 2 :H 2 ≥ 1. A groundwater NO 3 − removal experiment showed that the reactivity of the Sn-Pd-red mud was stable toward fouling by impurities. [ABSTRACT FROM AUTHOR]

In this study, a novel and highly reactive Sn–Pd catalyst supported by environmentally benign kaolinite (Sn–Pd-kaolinite) was developed and evaluated for stability for effective nitrate (NO 3 −) reduction in batch and continuous mode. Complete NO 3 − removal with fast reduction kinetics (k = 18.16 × 10−2 min−1) and 71% selectivity toward N 2 were achieved by the Sn–Pd-kaolinite catalyst during batch reactions. During continuous tests, 100% NO 3 − removal and 80% N 2 was achieved for 60 h. However, NO 3 − removal efficiency gradually decreased to 80% in170 h. The catalyst was then successfully regenerated in the system by increasing H 2 flow which achieved a complete NO 3 − removal again. The metal leaching from catalyst surface was negligible (Sn 0.01% and Pd 0.006%) and the structure was stable during the continuous test, confirming that the Sn–Pd-Kaolinite catalyst had a superior reaction kinetics and operational durability. Image 1 • A novel Sn–Pd-kaolinite catalyst was synthesized for an accelerated NO 3 − reduction. • Complete NO 3 − removal with high N 2 selectivity was achieved by the bimetallic catalyst. • The catalyst showed a remarkable durability and stability in a continuous reactor system. • Catalyst regeneration was successfully implemented with an increased H 2 flow. [ABSTRACT FROM AUTHOR]

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 NO 3 − reduction due to the inhibition of its catalytic reactivity and alteration of product selectivity. The batch experiments showed that the reduction kinetics of NO 3 − was strongly suppressed by ClO 4 −, PO 4 3−, BrO 3 − and SO 3 2− at low concentrations (>5 mg/L) and HCO 3 −, CO 3 2−, SO 4 2− and Cl− at high concentrations (20–500 mg/L). The presence of anions significantly changing the end-product selectivities influenced high N 2 selectivity. The selectivity toward N 2 increased from 55% to 60%, 60%, and 70% as the concentrations of PO 4 3−, SO 3 2−, and SO 4 2− increased, respectively. It decreased from 55% to 35% in the presence of HCO 3 − and CO 3 2− in their concentration range of 0–500 mg/L. The production of NO 2 − was generally not detected, while the formation of NH 4 + was observed as the second by-product. It was found that the presence of oxyanions in the NO 3 − reduction influenced the reactivity and selectivity of bimetallic catalysts by i) competing for active sites (PO 4 3−, SO 3 2−, and BrO 3 − cases) due to their similar structure, ii) blockage of the promoter and/or noble metal (HCO 3 −, CO 3 2−, SO 4 2−, Cl− and ClO 4 − cases), and iii) interaction with the support surface (PO 4 3− case). The results can provide a new insight for the successful application of catalytic NO 3 − reduction technology with high N 2 selectivity to the contaminated groundwater system. [Display omitted] • Evaluation of oxyanion influence on the catalytic NO 3 − reduction kinetics. • Cl−, PO 4 3−, BrO 3 − and SO 3 2− significantly inhibited the catalytic NO 3 − reduction. • Inhibition effect of PO 4 3−, SO 3 2−, and BrO 3 − by competing for active catalytic sites. • N 2 production remained unchanged with the addition of Cl−, ClO 4 − and BrO 3 − • N 2 selectivity increased with higher concentrations of SO 3 2−, SO 4 2− and PO 4 3-. [ABSTRACT FROM AUTHOR]

• Removal of NO 3 − by metallic catalysts supported by diverse supports were reviewed. • Fe materials themselves cannot achieve high N 2 selectivity at optimized conditions. • Passive supports showed better selective NO 3 − removal to N 2 than active supports. • Metals, their contents, support materials, and pH influence selective NO 3 − removal. • Iron and zeolite can be used as proper support materials for the long-term use. Contamination of surface and groundwater by nitrate (NO 3 −) is one of the most serious environmental issues worldwide because of the increase in discharge of N-containing wastewater due to agricultural and industrial activities. Among various NO 3 − removal techniques, catalytic denitrification by metallic catalyst has been considered as one of innovative and promising techniques to convert NO 3 − to an ideal gas (N 2) in natural and engineered environments. Extensive studies have developed diverse metallic catalysts for the enhanced catalytic removal of aqueous NO 3 −. Generally, the removal kinetics of NO 3 − and selectivity towards N 2 are varied with experimental conditions such as type of the catalyst (mono- or bimetallic), support material, promoter, and noble metals and their contents, concentrations of catalyst and NO 3 −, suspension pH, and additional reductant (H 2) flow. Therefore, selection of a proper combination of metals and support material and optimization of experimental factors are main challenges to achieve an efficient catalytic NO 3 − reduction, leading to the successful application of the technology to real water and wastewater treatment problems. This review presents an analysis of recent research works on NO 3 − reduction by the metallic catalysts, and discusses: (i) an assessment of various combinations of promoter and noble metals for mono- and bimetallic catalysts, based on their reactivity and selectivity to N 2 ; (ii) investigation of the reaction mechanism; (iii) evaluation of effect of diverse environmental factors on the performance of the catalyst; and (iv) stability and durability of the catalyst. In addition, environmental implication and future prospect of the denitrification by metallic catalyst are provided. [ABSTRACT FROM AUTHOR]