Reduction Of Nitrate In Water Research Articles

Highly selective chemical reduction of nitrate in water in carbon-based contactor catalytic membrane reactors

A carbon-based catalytic membrane reactor in a contactor (interfacial) configuration has been used as an approach to control selectivity in the reduction of NO3- in water. Water and hydrogen fed to the reactor circulate tangentially on opposite sides of the membrane, which acts as a separator and contactor, enabling hydrogen transfer to the water phase. Powdered carbons (carbon black, carbon nanofiber, and activated carbon) were used to prepare catalytic membranes that also acted as contactors to transfer H2 from the gas phase to water. The membranes based on the Pd-Cu/carbon black catalyst showed lower selectivity towards unwanted NH4+ than the Pd-Cu and Pd-Sn catalysts supported on carbon nanofibers and activated carbon, due to a good balance among conductivity, external surface area, and particle size. Sensitivity analysis for H2 partial pressure demonstrated that the contactor membrane reactor can be operated with control of H2 mass transfer, hence enabling enhanced control of the selectivity to NH4+. Additional control of selectivity was achieved by reducing the catalytic layer thickness of the membrane, which improved NO3- transport and decreased the H/N ratio at the active centers. With these approaches, NO3- conversion values above 40 % with negligible NH4+ production and NO3- conversions close to 60 % with NH4+ selectivity values around 3 % were achieved.

Electrochemical nitrate reduction over bimetallic Pd–Sn nanocatalysts with tunable selectivity toward benign nitrogen

Nitrate is recognized as a highly impactful water contaminant among various pollutants in water. To address the ever-growing demand for water purification, this work investigates the bimetallic palladium (Pd) and tin (Sn) catalysts, which are electrochemically deposited on stainless steel mesh support (Pd–Sn/SS) for the selective conversion of harmful nitrate (NO3−) into benign nitrogen (N2) gas. Results indicate that the bimetallic composition in Pd–Sn/SS electrodes substantially influenced the reaction route for nitrate reduction as well as the performance of nitrate transformation and nitrogen selectivity. It is found that the electrode prepared from Pd:Sn = 1:1 (mole ratio) demonstrates an outstanding nitrate conversion of 95%, nitrogen selectivity of 88%, and nitrogen yield of 82%, which outperform many reported values in the literature. The electrochemically synthesized bimetallic electrode proposed herein enables a new insight for promoting the reactivity and selectivity of nitrate reduction in water.

Electrosynthesis of hydroxylamine from nitrate reduction in water

Electrosynthesis of hydroxylamine from nitrate reduction in water

Electrocatalytic reduction of nitrate by copper/iron oxides supported on nitrogen doped carbon spheres

In recent years, electrocatalytic reduction has gained attention as a promising method for environmentally friendly nitrate treatment. The key to this approach is finding cathode materials with high activity, N2 selectivity, and stability. Researchers are therefore focused on creating efficient catalysts. In this study, nitrogen-doped carbon spheres (NMCS) loaded with Cu and Fe3O4 were synthesized from non-precious metals Fe and Cu and tested for nitrate reduction in water. The bimetal combination effectively reduces the formation of toxic NO2− intermediates and improves material impedance, accelerating electron transfer during the reaction. The mesoporous structure with high surface area also supports metal nanoparticle loading and reduces metal leaching during the process. The resulting Cu/Fe-NMCS-1–3 catalysts showed excellent NO3− reduction (97.04%) and 100% N2 selectivity using electrochlorination. After four cycles, no decline in electrocatalytic performance was observed, suggesting good stability and potential for electrocatalytic nitrate removal.

Synergistic effect of oxygen vacancies and built-in electric field in GdCrO3/BiVO4 composites for boosted photocatalytic reduction of nitrate in water

As a promising approach for nitrate nitrogen removal in water, photocatalysis required sufficient photogenerated charges to achieve a high photocatalytic efficiency. Herein, a novel GdCrO3/BiVO4 composite was prepared by the solid-state combustion method combined with hydrothermal treatment. The synergistic effect of oxygen vacancies introduced by chemical reduction method and built-in electric field formed in GdCrO3/BiVO4 heterostructures suppressed the charge recombination, which eventually improved the photocatalytic activity. Effects of photocatalyst composition, HCOOH dosage and oxygen vacancy concentration on photocatalytic activity were investigated. It was found that 0.30-GdBiOVs composites exhibited highest photocatalytic activity with nitrate removal ratio of 97.05% and N2 selectivity of 92.76% in 40 min. Moreover, end-products of nitrate nitrogen elimination and stability of photocatalysts were revealed. The existences of oxygen vacancies and built-in electric field were confirmed by EPR and HRTEM analyses. Ultimately, the possible mechanisms of nitrate nitrogen elimination were clarified.

Comparing methods to deposit Pd-In catalysts on hydrogen-permeable hollow-fiber membranes for nitrate reduction

Catalytic hydrogenation of nitrate in water has been studied primarily using nanoparticle slurries with constant hydrogen-gas (H2) bubbling. Such slurry reactors are impractical in full-scale water treatment applications because 1) unattached catalysts are difficult to be recycled/reused and 2) gas bubbling is inefficient for delivering H2. Membrane Catalyst-film Reactors (MCfR) resolve these limitations by depositing nanocatalysts on the exterior of gas-permeable hollow-fiber membranes that deliver H2 directly to the catalyst-film. The goal of this study was to compare the technical feasibility and benefits of various methods for attaching bimetallic palladium/indium (Pd/In) nanocatalysts for nitrate reduction in water, and subsequently select the most effective method. Four Pd/In deposition methods were evaluated for effectiveness in achieving durable nanocatalyst immobilization on the membranes and repeatable nitrate-reduction activity: (1) In-Situ MCfR-H2, (2) In-Situ Flask-Synthesis, (3) Ex-Situ Aerosol Impaction-Driven Assembly, and (4) Ex-Situ Electrostatic. Although all four deposition methods achieved catalyst-films that reduced nitrate in solution (≥ 1.1 min−1gPd−1), three deposition methods resulted in significant palladium loss (>29%) and an accompanying decline in nitrate reactivity over time. In contrast, the In-Situ MCfR-H2 deposition method had negligible Pd loss and remained active for nitrate reduction over multiple operational cycles. Therefore, In-Situ MCfR-H2 emerged as the superior deposition method and can be utilized to optimize catalyst attachment, nitrate-reduction, and N2 selectivity in future studies with more complex water matrices, longer treatment cycles, and larger reactors.

Activity and Selectivity of Bimetallic Catalysts Based on SBA-15 for Nitrate Reduction in Water

Nitrate from the application of nitrogen-based fertilizers in intensive agriculture is a notorious waste product, though it lacks cost-effective solutions for its removal from potential drinking water resources. Catalytic reduction appears to be a promising technique for converting nitrates to benign nitrogen gas. Mesoporous silica SBA-15 is a frequently used catalyst support that has large surface areas and highly ordered nanopores. In this work, mesoporous silica SBA-15 bimetallic catalysts for nitrate reduction were investigated. The catalyst was optimized for the selection of promoter metal (Sn and Cu), noble metal (Pd and Pt) and loading ratios of these metals at different temperatures and reduction conditions. The catalysts prepared were characterized by FT-IR, N2 physisorption, XRD, SEM, and ICP. All catalysts showed the presence of cylindrical mesoporous channels and uniform pore structures that remained even after metals loading. In the presence of a CO2 buffer, the catalysts 4Pd-1Cu/SBA-15 and 1Pt-1Cu/SBA-15 reduced at 100?C under H2 and 1Pd-1Cu/SBA-15 reduced at 200°C under H2 demonstrated very high nitrate conversion. Furthermore, the forementioned Pd catalysts had higher N2 selectivity (88% - 87%) compared to Pt catalyst (80%). Nitrate conversion by the 4Pd-1Cu/SBA-15 catalyst was significantly decreased to 81% in the absence of CO2.

Efficient nitrate reduction in water using an integrated photocatalyst adsorbent based on chitosan-titanium dioxide nanocomposite.

Globally, there exists a huge concern on the increased discharge of nitrates to the natural water resources out of various anthropogenic activities as it causes serious environmental pollution and associated harmful effects. In the present work, sol-gel-derived functional nanocomposites based on silver (Ag) and nitrogen (N)-doped titanium dioxide (TiO2)-coated chitosan nanocomposites were successfully synthesized in the form of beads, and their application for the reduction of nitrates in water was studied. The synthesized nanocomposite beads were characterized for their structural, textural, and morphological features using X-ray diffraction analysis, Fourier transform infrared spectroscopy, UV-visible spectroscopy, BET surface area analysis, Scanning electron microscopy, Transmission electron microscopy, and X-ray photoelectron spectroscopy. A uniform coating of doped titania species on the chitosan porous structure was achieved through electrostatic interaction. Adsorption/photocatalytic reduction of nitrates was further carried out using functional nanocomposite beads by monitoring the nitrate concentration of the model contaminated water, in an adsorption study under dark condition and photocatalytic study under UV/sunlight for a definite time period. Drying conditions of the nanocomposite beads were found to have a significant effect on the adsorption cum photocatalytic efficiencies of the nanocomposite. The freeze-dried chitosan-titania nanocomposite beads containing 0.5mol% Ag exhibited an adsorption efficiency of ~ 43.5% (under dark for 30min) and photocatalytic reduction capability of ~ 95% (under sunlight for 2h), whereas the oven dried beads of the same composition exhibits adsorption and photocatalytic efficiencies of 40% (under dark for 30min) and 70% (under UV light for 2h) respectively, towards the reduction of nitrate ions in an aqueous solution. Continuous flow adsorption cum photocatalytic study using the oven-dried nanocomposite beads was also carried out with the help of an experimental setup fabricated in-house and under varying experimental conditions such as flow rate, bed height, and concentration of feed solution. Nitrate reduction efficiency of 87.6% and an adsorption capacity of 7.9mgg-1 were obtained for the nanocomposite beads in the continuous flow adsorption cum photocatalysis experiment for up to 8h when using an inlet concentration of 100ppm, bed height 12cm, and flow rate 5.0mLmin-1. A representative fixed-bed column adsorption experiment performed with oven dried nanocomposite beads in a real groundwater sample collected from the Palakkad District of Kerala shows promising results for nitrate reduction (85.9% efficiency) along with a significant removal rate for the other anions as well. Thus, the adsorption cum photocatalytic nitrate reduction efficiency of the functional nanocomposite material makes them suitable for the reduction of nitrates from water/wastewater through an integrated nanocomposite approach.

Interference effects of oxyanions commonly found in natural waters on the catalytic reduction of nitrate in water.

In this work, the influence of oxyanions on the catalytic reduction of nitrates using formic acid as the reducing agent was studied as well as the influence of bicarbonate, sulfate, and phosphate co-anions on the catalytic nitrate reduction with Pd:In/Al2O3 (1:0.25 wt.%). A negative effect on nitrate conversion was observed in the following order: phosphate > sulfate > bicarbonate, showing a strong influence of electrostatic adsorption on the catalytic reduction of nitrate. However, no direct trend was observed relating the levels of interferents to the impact on the selectivity of the bimetallic catalyst using formic acid as a reducing agent. For both bicarbonate and phosphate, at lower levels, higher selectivity to nitrogen was obtained than for the reaction in the absence of interferents. On the other hand, increasing sulfate concentration led to a decrease in nitrate conversion. The mixtures of co-anions also showed a decrease in the catalytic activity. At 120min, a N2 selectivity higher than 95% was obtained, except for the C50-S20 (bicarbonate 50ppm-sulfate 20ppm) mixture which showed the lowest selectivity to N2 value (87.3%). The loss of catalyst activity was found to be reversible and not permanent.

Phosphomolybdic acid-modified polyaniline nanowires with abundant acidic sites for efficient photocatalytic reduction of nitrate to nitrogen in water

Although polymeric conductive polyaniline (PANI) is an effective material for catalytic nitrate reduction in water, its catalytic efficiency is limited. Here, we report that phosphomolybdic (PMoA) acid-modified PANI nanowires exhibit superior activity in the catalytic reduction of nitrate to harmless N2. Excellent catalytic efficiency is attributed to the construction of Z-scheme heterojunctions and the combined contribution of chemical-photocatalytic reduction. Pyridine infrared analysis confirms that the introduction of PMoA adds an abundance of acidic sites to PANI, which explains the extraordinary N2 selectivity. Additionally, DFT calculations are used to determine the band structures and surface electron transfer pathways of the two materials. Impressively, the effectiveness of catalytic nitrate reduction under various conditions is evaluated together with the practicality of PMoA-PANI by simulating saltwater. This work sheds new light on the mechanism of polymer for efficient reduction of nitrate and develops innovative technology for the remediation of practical wastewater.

Photocatalytic reduction of nitrate pollutants by novel Z-scheme ZnSe/BiVO4 heterostructures with high N2 selectivity

Eliminating nitrate pollutants in water efficiently and environmentally friendly has attracted widespread attention. As a promising technology to solve problems of environmental pollution and energy shortage, photocatalysis also displayed great potential in nitrate removal. In this work, a novel Z-scheme ZnSe/BiVO4 heterostructures were synthesized by in-situ growth of ZnSe nanospheres on BiVO4, and employed for photocatalytic reduction of nitrate in water. Experiments demonstrated that ZnSe/BiVO4 composites obtained improved photocatalytic activity compared with pristine ZnSe and BiVO4, and an optimal performance with nitrate removal rate as 89.84% and N2 selectivity as 91.03% in 50 min was achieved when initial nitrate was 100 mgN/L. The enhancements may be due to the boosted light harvesting capability and stronger redox ability of photogenerated carriers provided by Z-scheme ZnSe/BiVO4 heterostructures. Moreover, HCOOH played a better role as hole scavenger than CH3COOH and C2H5OH. The HCOOH dosage and initial solution pH greatly affected the photocatalytic removal of nitrate. Effects of impurity anions in water such as Cl−, SO42− and CO32− on nitrate removal were also investigated. The charge transfer pathway as well as photocatalytic mechanism for nitrate removal over ZnSe/BiVO4 heterostructures were proposed at the end.

Catalytic membrane reactor based on Pd-Sn supported on nanocarbons for the reduction of nitrate in water

This work studies the reduction of NO3- in water using a catalytic membrane reactor in flow-through configuration (FTCMR) for enhanced control of H2 availability and generation of NH4+. The catalytic membrane was prepared with metal catalysts supported on carbon materials with different structural and physicochemical properties (graphite, carbon nanofibers, reduced graphene oxide, activated carbon and carbon black). The catalysts were firstly tested in a batch reactor for screening and assessing influence of regime control on activity and selectivity. Pd-Sn catalysts showed higher production of NH4+ under chemical control than Pd-Cu ones, but equivalent performance was reached for Pd-Sn supported on carbon nanofibers and carbon black in conditions of H2 mass transfer control. Catalytic membranes were prepared with Pd-Sn catalyst according to higher impact of H2 availability in NH4+ generation. FTCMR was less selective to NH4+ compared to the batch reactor due to better control of H2 mass transfer. Reduction of NH4+ generation was achieved at the expense of activity due to lower availability of H2. However, membranes based on Pd-Sn supported on carbon nanofibers and carbon black were able to operate at higher H2 concentration with low selectivity to NH4+, making possible the use of membrane reactors at advantageous conditions.

Electrostatic self-assembled layered polymers form supramolecular heterojunction catalyst for photocatalytic reduction of high-stability nitrate in water

In this work, two polymers are connected by electrostatic self-assembly method to form a supramolecular heterojunction to remove pollutants. g-C3N4-Cl/PANI catalyst can be used for photocatalytic reduction of nitrate in water, and the nitrogen selectivity reaches 98.2%. Specially, charge density analysis and comparative experiments showed that the introduction of covalent chlorine increased in electron transfer conduction between layers. In addition, differential charge density and solid EPR tests reveal high electron density and electron transfer pathways for supramolecular heterostructures. The results of the work function give direct evidence for the high catalytic performance of the supramolecular heterojunction. The reasons and active species of photocatalytic reduction of nitrate by g-C3N4-Cl and g-C3N4-Cl/PANI are compared. The catalyst exhibits the performance of highly reducing nitrate to harmless nitrogen with the contribution of supramolecular heterojunction and covalent chlorine. In short, a new idea of constructing a supramolecular photocatalyst is proposed, which can be applied to efficiency reduce nitrate in water.

In-situ polymerization of polyaniline modified by phosphotungstic acid on the surface of hollow carbon for two-way efficient reduction of nitrate in water

• Chemical and photocatalytic reduction are combined to remove nitrate in water. • H 3 PW 12 O 40 -PANI-C shows superb photocatalytic reduction activity with less hole scavenger. • Interface electron transport station is built to realize the fast electron conduction. • The Lewis acidic site is the key active site for nitrate reduction. Catalytic reduction of nitrate to harmless nitrogen is a significant and promising green environmental protection technology. In this paper, the reduction of nitrate by two pathways is realized without light (Reaction I) and under light (Reaction II) conditions by wrapping polyaniline modified by phosphotungstic acid on the hollow carbon surface (H 3 PW 12 O 40 -PANI-C, HPC). Modified PANI chain and Z-scheme heterojunction make the hollow carbon surface form an interfacial electron transport network, which not only shortens electron migration distance but also promotes the carrier separation and transfer. HPC catalysts concentrate the advantages of adsorption, high conductivity and abundant catalytic sites to promote the reduction efficiency. Satisfied nitrogen generation and fewer hole scavengers using make HPC catalysts more practical. This work emphasizes the rapid conduction of interfacial electrons and the effect of catalytic sites on nitrate removal, which provides new guidance for the treatment of pollutants in water.

Facile fabrication of nano zerovalent iron – Reduced graphene oxide composites for nitrate reduction in water

Nano zerovalent iron is used to destruct a wide range of organic and inorganic contaminants in water. However, its performance is limited due to rapid aggregation and surface passivation. To minimise aggregation, we fabricated nano zerovalent iron on the reduced graphene oxide sheets using green tea derived polyphenols (hereafter rGO-nZVI-P) or borohydride ions (hereafter rGO-nZVI-B). Both rGO-nZVI-P and rGO-nZVI-B composites were characterised by electron microscopic, molecular spectroscopic and electrochemical methods. The spherical nZVI particulates (e.g. ~4–15 mm diameter) are well dispersed among rGO sheets. Polyphenols act as a capping agent for Fe (0) to prevent its aggregation. The X-ray diffraction and X-ray photon spectroscopic results show an admixture of Fe (0) with rGO and Fe oxides (e.g. FeOOH, Fe2O3, and Fe3O4 phases). The association of Fe (0) on the reduced graphene oxide matrix is believed to occur via π–π framework thus minimising surface passivation. The reduction efficiency of the nano zerovalent iron composites was determined using nitrate as index ion. When compared with rGO-nZVI-B, the rGO-nZVI-P reduces 70% of 0.8064 mM nitrate within an hour. Although traces of NO and NO2− are observed, ammonia is the dominant product that accounts for 95% nitrogen mass balance. The nitrate reduction by the rGO-nZVI composites follows pseudo-second-order kinetics. Fe (0) or its oxidation products are environmentally benign. The rGO-nZVI-P also has the potential to destruct excess nitrate in water remediation.

Efficient UV-light photocatalytic activity of TiO2@Phosphogypsum composite synthesized by sol–gel method

There is no doubt that novel and efficient photocatalyst with excellent properties based on TiO2 are highly desirable for practical applications to degrade high-concentration pollutants. Until now TiO2 nanoparticles have been successfully coupled with phosphogypsum prepared via a simple sol–gel method after calcination at different temperatures. The efficiency of the photocatalytic activity of composite can be explained by the difference of calcination temperatures in the structure of TiO2. Structures, morphologies, and surface areas were examined in all samples using specific characterization methods. In this process, the anatase and rutile structure of TiO2 nanoparticles were determined. The results obtained showed that the forms of the composite are spherical, although some nanoparticles are agglomerated. Surface area analyses showed that higher surface areas were obtained when the composite calcinated at 450 °C. We can attribute this to the small size of the particles. This study demonstrates that the synthesized composite display excellent photo-activity for degrading nitrate from aqueous solution up to 93.5% after 180 min of UV light irradiation time. Hence, we demonstrate a simple and efficient method to valorize phosphogypsum and to prepare a promising composite photocatalyst for reduction of nitrate in water and wastewater treatment process.

Electrocatalytic reduction of nitrate in water using Cu/Pd modified Ni foam cathode: High nitrate removal efficiency and N2-selectivity

Electrocatalytic reduction of nitrate is a promising technology for nitrate removal from groundwater or surface water to eliminate the negative influence on human. The activity of an electrode usually depends on many factors such as catalytic metal, accessible active centers and electrode geometry. In this work, we prepared Cu/Pd modified nickel foam electrode (NF-Cu/Pd) according to two-step electrodeposition method using nickel foam (NF) as substrate. Cu/Pd particles with a grape-cluster-like shape with pores around 100 nm between clusters were formed on the NF surface according to the field emission scanning electron microscope with energy dispersive spectroscopy (FESEM-EDS) results. The crystal structure and the valence state of the prepared NF-Cu/Pd electrode were analyzed by using powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) technology. Linear sweep voltammetry (LSV) tests indicated that the prepared NF-Cu/Pd electrode had high electrocatalytic activity for nitrate reduction either using Na2SO4 as electrolyte or using NaCl as electrolyte. According to the dynamic experiment, 97% of the initial 100 mg/L NO3–-N were removed with a N2-selectivity of 99% at the cathode potential of −1.2 V vs. Ag/AgCl (sat. KCl) when using 0.05 M Na2SO4 as electrolyte. Using NaCl as electrolyte could improve the N2-selectivity of the nitrate electrocatalytic reaction under general conditions. The cathode potential and the current density affects the reaction pathways of nitrate reduction. In the microenvironment around the active sites, the mole ratio of NO2– and Had dominated the main product of NO3– reduction. Long time cyclic voltammetry test indicated that the prepared NF-Cu/Pd electrode has a good long-term electroactive stability for nitrate reduction from water solution. And the prepared NF-Cu/Pd showed high nitrate removal efficiency (96.4%) and N2-selectivity (65.4%) from the municipal wastewater treatment plant (WWTP) effluent.

Catalytic reduction of nitrate in water over alumina-supported nickel catalyst toward purification of polluted groundwater

Pollution of groundwater with NO3– is a serious problem in the world. While catalytic reduction of NO3– over Pd-bimetallic catalysts including Cu-Pd and Sn-Pd is a promising method for purification of the groundwater, the use of precious metal is a major obstacle for practical applications. In the present study, we applied Ni/Al2O3 for the catalytic reduction of NO3– and compared the catalytic performance with that of unsupported Ni catalyst. The reaction rate over 5 wt.% Ni/Al2O3 was about 5 times higher than that of the unsupported Ni catalyst, based on unit weight of catalyst. While the unsupported Ni catalyst was completely deactivated in low partial pressure of H2 (=0.75 atm) and high concentration of NO3– (=800 ppm), Ni/Al2O3 was still active even under less reductive conditions ([NO3–]0 = 800 ppm and P(H2) =0.5 atm). The unsupported Ni catalyst had the Ni0 particles formed by the reduction of NiO with H2 at 310–420 °C. On the other hand, 5 wt.% Ni/Al2O3 possessed the Ni0 particles formed from NiAl2O4 on Al2O3 by the reduction with H2 above 450 °C. It is plausible that those Ni0 particles had different properties, giving different catalytic properties. The Ni loadings for Ni/Al2O3 had a significant impact on the catalytic properties. The reaction orders with respect to both NO3– and H2 were 0.8 for 5 wt.% Ni/Al2O3, while those were 0 and –0.2, respectively, for 10 wt.% Ni/Al2O3. On 10 wt.% Ni/Al2O3, there were two kinds of the Ni° particles, which were formed by low (310-420 °C) and high (above 450 °C) temperature H2 reductions. Unlike the Pd-bimetallic catalysts, the reduction of NO3– over Ni/Al2O3 did not proceed through NO2–.

Highly fast and selective removal of nitrate in groundwater by bimetallic catalysts supported by fly ash-derived zeolite Na-X

A novel and sustainable Pd–Sn bimetallic catalyst supported by fly ash-derived zeolite Na-X (Zeolite-XF) was developed for highly reactive and N2-selective reduction of nitrate in water.

Effect of other anions on reduction of nitrate in water over Pd–In catalyst loaded on various supports

The effect of the presence of other anions on nitrate reduction was investigated using catalysts supported by TiO2 and nitrate adsorbents (calcium-chloride-treated charcoal and an anion exchange resin). When the TiO2-supported catalyst was used, the nitrate decomposition rate was greatly reduced in the presence of Cl−, SO42−, and PO43−, and the decrease was large for Cl− and PO43−. The activity of the nitrate-adsorbent-supported catalysts in the presence of SO42− and PO43− showed a smaller decrease than that of the TiO2-supported catalyst. In the presence of SO42−, the amount of nitrate adsorbed on the supports was correlated with the decrease in activity, and calcium-chloride-treated charcoal, which has high nitrate adsorption selectivity, showed the smallest decrease in activity in the presence of SO42−. By contrast, in the presence of PO43−, no clear correlation was found between the decrease in the amount of nitrate adsorption on the supports and the decrease in activity.