Nitrate contamination in surface and ground waters is one of this century’s major engineering challenges due to negative environmental impacts and the risk to human health in drinking water. Electrochemical reduction is a promising water treatment technology to manage nitrate in drinking water. This critical review describes the fundamental principles necessary to understand electrochemical reduction technologies and how to apply them. The focus is on electrochemical nitrate reduction mechanisms and pathways that form undesirable products (nitrite, ammonium) or the more desirable product (dinitrogen). Factors influencing the conversion rates and selectivity of electrochemical nitrate reduction, such as electrode material and operating parameters, are also described. Finally, the applicability for treating drinking water matrices using electrochemical processes is analyzed, including existing implementation of commercial treatment systems. Overall, this critical review contributes to the understanding of the potential applications and constraints of electrochemical reduction to manage nitrate in drinking waters and highlights directions for future research required for implementation.

Electrocatalytic reduction of nitrate: Fundamentals to full-scale water treatment applications

Nitrate pollution of groundwater, which is mainly caused by the application of nitrogen-based fertilizers in intensive agriculture, is a widespread problem all over the world and a potential risk for public health. Reverse osmosis, ion exchange and electrodialysis are currently used for water denitrification, yielding a highly concentrated reject water that requires economic and environmental costs for disposal. Nitrate reduction technologies that are able to convert nitrate into inert nitrogen gas have appeared that are promising, cost effective and environmentally friendly. Among these technologies, attention has been focused on i) the chemical reduction over mono- and bimetallic catalysts with hydrogen as the reducing agent and ii) electrocatalytic reduction processes over metallic anodes. Although selectivity values towards N2 of greater than 90% are achieved with both technologies, the undesired formation of ammonium as a reaction by-product is still the main drawback preventing their implementation at larger scales. For this reason, the development of new catalytic and electrodic materials as well as novel reactor configurations to avoid ammonium formation have been extensively investigated in the last few years to increase the effectiveness and competitiveness of both technologies. In this paper, an overview of the current state-of-the-art of both catalytic reduction and electroreduction of nitrates is presented, highlighting their potential and their main drawbacks along with guidelines for future research.

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State-of-the-art and perspectives of the catalytic and electrocatalytic reduction of aqueous nitrates

The presence of high nitrate (NO3−) concentration in natural water constitutes a serious issue to the environment and human health. Therefore, the development of low-cost, stable non-precious metal catalysts is imminent for efficient NO3- reduction. In this study, we prepared a Co3O4-TiO2/Ti cathode via combining sol-gel and calcination methods and evaluated its performance for electrocatalytic NO3- reduction. The dispersion of the Co3O4 catalyst particles was improved by the addition of PVP to the coating liquid. The presence of anatase could effectively stabilize Co3O4 and prevent the releasing of toxic Co ions into the solution. The Co3O4-TiO2/Ti cathode with the optimized performance for NO3- reduction could be prepared by four times coating at calcination temperature of 500 °C. The electrocatalytic reduction of NO3- was negligibly impacted by solution pH in the range of 3.0–9.0, while it could be facilitated by elevating the current density from 2.5 to 25 mA cm2. Ammonium ions were the main final NO3- reduction product, and the presence of Cl- was capable to oxidize ammonium ions to N2 due to the electrochemical production of reactive chlorine species. The electrochemical analyses, scavenging experiments and density functional theory calculations collectively confirm that NO3- reduction was mainly induced by the Co2+–Co3+–Co2+ redox process instead of being directly resulted from the electrons generated at the cathode. Unlike noble metal (e.g., Pd and Ag) based catalytic reaction systems, in the present Co3O4 mediated electrocatalytic reaction process, atomic H* would more favorably turn to H2 by Heyrovsky and Tafel routes and therefore contributed marginally to the NO3- reduction. Generally, this study provided a new paradigm for designing the stable and cost-effective cathode for NO3- reduction.

Non-precious Co3O4-TiO2/Ti cathode based electrocatalytic nitrate reduction: Preparation, performance and mechanism

Due to their electrochemical and economical characteristics, pencil graphite electrodes (PGEs) gained in recent years a large applicability to the analysis of various types of inorganic and organic compounds from very different matrices. The electrode material of this type of working electrodes is constituted by the well-known and easy commercially available graphite pencil leads. Thus, PGEs are cheap and user-friendly and can be employed as disposable electrodes avoiding the time-consuming step of solid electrodes surface cleaning between measurements. When compared to other working electrodes PGEs present lower background currents, higher sensitivity, good reproducibility, and an adjustable electroactive surface area, permitting the analysis of low concentrations and small sample volumes without any deposition/preconcentration step. Therefore, this paper presents a detailed overview of the PGEs characteristics, designs and applications of bare, and electrochemically pretreated and chemically modified PGEs along with the corresponding performance characteristics like linear range and detection limit. Techniques used for bare or modified PGEs surface characterization are also reviewed.

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Pencil Graphite Electrodes: A Versatile Tool in Electroanalysis.

Magnetically separable ZnO/ZnFe2O4 and ZnO/CoFe2O4 photocatalysts supported onto nitrogen doped graphene for photocatalytic degradation of toxic dyes

The electrocatalytic reduction of NO3− and its intermediate NO2− in neutral medium was performed at a Cu-immobilized Pt surface. The voltammetric investigations showed that the bare Cu electrode has little effect on nitrate reduction reactions (NRR) whereas an enhanced catalytic effect (i.e. a positive shift of the peak potential and an increased reduction current) was observed when Cu particles were immobilized onto Pt surface. At the Cu–Pt electrode surface, the NRR process was observed to occur via a two-step reduction mechanism with a transfer of 2 and 6 electrons in the first and second steps, respectively. Similar results were obtained by chronoamperometric (CA) studies. Closer NRR mechanistic studies at the as prepared Cu–Pt electrode revealed concentration-dependent kinetics with a “critical” nitrate ion concentration of ca. 0.02 M. Moreover, NRR proceeded via a simple adsorption–desorption mechanism following a Langmuir isotherm with an adsorption Gibbs free-energy of ca. −10.16 kJ mol−1 (1st step) and ca. −10.05 kJ mol−1 (2nd step). By means of a Pt|Nafion|Cu–Pt type reactor without any supporting electrolyte, bulk electrolysis was performed to identify nitrate reduction products. It was found that after 180 min of electrolysis, 51% of NO3− was converted into NO2− intermediate. This percentage decreased to 30% in CO2 buffered conditions. However, when a tri-metallic Pt–Pd–Cu electrode was employed as a cathode, all of the NO2− produced could be successfully converted into NH3 and N2. The electrocatalysis of nitrate ion on Cu–Pt electrode surface showed no apparent surface poisoning as confirmed by its stability after excessive CV runs. This was further supported by surface analysis and morphology of the as-prepared catalyst with scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis.

Copper-immobilized platinum electrocatalyst for the effective reduction of nitrate in a low conductive medium: Mechanism, adsorption thermodynamics and stability

Electroreduction of nitrate ions at a platinum-copper electrode in an alkaline medium: Influence of sodium inositol phytate

Cathodic reduction of nitrate ions has been carried out using a sandwich type membrane reactor having a configuration of Pt|Nafion|Pt–Cu in absence of any supporting electrolyte. Both Pt and Cu metals are in polycrystalline form on the cathodic surface immobilized on Nafion membrane. The globular Cu particles have a wide range of sizes (70–120 nm). During the course of electrolysis in the reactor, the bimetallic Pt–Cu surface reduces NO 3 − into NH 3 and N 2 by means of electrochemical and catalytic hydrogenation reactions, respectively. The electrochemical contribution of nitrate reduction has been investigated in details by using voltammetric and electrolysis techniques. The NO 3 − ions are electrochemically reduced using a consecutive reaction. On the consecutive way of reduction, the intermediate NO 2 − and the final product NH 3 are generated at −0.74 V and −1.1 V versus Ag/AgCl (std. KCl), respectively. The molecular N 2 is generated by means of catalytic reactions followed by electrochemical hydrogen evolution.

Dissimilar catalytic trails of nitrate reduction on Cu-modified Pt surface immobilized on H + conducting solid polymer

Electrocatalytic nitrate reduction and sensing activities of Cu deposits from 0.01 M CuSO4 · 5H2O solution by fast scan (1000 mV/s) cyclic voltammetry on pencil graphite (PG) was investigated. The content of Cu particles on PG surface was controlled by fixing the deposition cycles between 0 and −300 mV. The performance of the different Cu/PG electrodes has been explained in terms of reduction current, kinetic order, exchange current density, specific electrical capacitance and nitrate sensing abilities. All these activities were modestly dependent on the content of Cu particles on the PG surface. The minimum catalytic sites on PG surface were generated even by the single Cu deposition cycle. Depending on the Cu content, the electrodes exhibited lowest nitrate detection limits in the range between 1.0 × 10−4 to 1.1 × 10−3 M. The nitrate detection performance of the Cu/PG electrode was justified with the ion chromatographic method.

S. Amertharaj

Papers

Copper-immobilized platinum electrocatalyst for the effective reduction of nitrate in a low conductive medium: Mechanism, adsorption thermodynamics and stability

Electroreduction of nitrate ions at a platinum-copper electrode in an alkaline medium: Influence of sodium inositol phytate

Dissimilar catalytic trails of nitrate reduction on Cu-modified Pt surface immobilized on H + conducting solid polymer

Nitrate detection activity of Cu particles deposited on pencil graphite by fast scan cyclic voltammetry