Removal of NO3 from water by electrochemical reduction in different reactor configurations

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Abstract

A series of cathode materials were investigated to establish their performance towards the electrochemical reduction of nitrate ions in unbuffered aqueous solutions. In particular, Ti/PdO–Co3O4 (for simplicity Ti/Pd–Co), nanostructured SS/Pd–Cu (Cu content: 15%, w/w), Ti/Pd–Co loaded with small amounts of electrodeposited Cu (Ti/Pd–Co–Cu) electrodes were prepared and studied by using both cyclic voltammetry and exhaustive electrolysis experiments. Cyclic voltammetry was mainly employed to establish the potentials at which the reduction of NO3 and of one of the intermediate reduction products (NO2) occurred at the different materials, and the effect of copper, employed as a promoter, for the nitrate ion reduction. With this respect a bulk copper working electrode was also investigated. Exhaustive electrolysis were performed under stagnant and stirred conditions in either a single-cell batch or membrane reactor, operating at a constant potential. For the latter reactor, a thick Nafion 117 membrane was used to separate the cathodic from the anodic compartment. The experiments showed that good results, in terms of nitrate removal, were obtained by using the membrane reactor equipped with the Ti/Pd–Co–Cu cathode operating at −0.9 V versus SCE, where the nitrate content was lowered from 200 down to 50 mg/L, which represents the upper limit for drinking water. The performance of the various electrodes in the presence of NaClO4 as base electrolyte in both compartments of the reactor, or NaCl as base electrolyte in the anodic compartment only, was also investigated in order to simultaneously remove nitrate and ammonia. The latter, which is the main final product formed in the reduction process, diffused through the Nafion membrane into the anodic compartment where it was oxidized by electrogenerated active chlorine. For the anodic generation of chlorine the Ti/Pt–Ir material was chosen, due to its previously assessed excellent electrocatalytic properties towards this reaction. The current efficiency of nitrate reduction and energy consumption under different conditions were also evaluated.

Introduction

Nitrate is one of the inorganic pollutants whose concentration in the wastewater discharged into the surface water and in drinking water is regulated by law [1], the respective limits being 15 and 50 mg/L. Pollution of ground and surface waters by nitrate is a common environmental problem throughout intensive agricultural areas. In fact, a high concentration of the nitrate ion has detrimental effects on health and environment, and its removal has gained renewed attention [2]. Possible technologies for the treatment of nitrate include ion exchange, biological treatment, reverse osmosis, heterogeneous catalysis and electrochemical reduction [3]. While elimination of this ion from the wastewater containing also organic pollutants is a well established, low cost biological process, for the removal of nitrate from the wastewater of such industries as metal plating, electronic, nuclear or fertilizers and from drinking water, there is not a widely accepted, cost-effective, method of treatment. New technologies, in particular those based on the reduction of nitrates, are currently under study [4]. Among these processes the electrochemical reduction is a promising route. In fact, generally, electrochemical methods offer such advantages as the lack of the requirement for chemicals before or after the treatment, no sludge production, small area occupied by the plant and relatively low investment costs.

The main cathodic reactions which can be involved in the electrochemical reduction of nitrate are [5]:NO3 + H2O + 2e  NO2 + 2OH, E0 = 0.01 VNO3+3H2O+5e12N2+6OH,E0=0.26VNO3 + 6H2O + 8e  NH3 + 9OH, E0 = −0.12 VNO2+2H2O+3e12N2+4OH,E0=0.406VNO2 + 5H2O + 6e  NH3 + 7OH, E0 = −0.165 VNO2 + 4H2O + 4e  NH2OH + 5OH, E0 = −0.45 VThe standard potentials are versus SHE.

A cathodic side reaction can be the hydrogen evolution:2H2O + 2e  H2 + 2OHIn case no other ions are present, the main anodic reaction is oxygen evolution:4OH  O2 + 2H2O + 4eIf Cl ions are present, chlorine can also evolve at the anode, according to the overall reaction:2Cl  Cl2 + 2e

The reaction mechanism involved in the electro-reduction of nitrates, principally depends on the type of the electrocatalytic material, the cathode potential and the pH of the solution. Depending on the experimental conditions, the main products of the reduction are: NO2, NH3, N2 and other oxygen-containing nitrogen species, including hydroxylamine. When the nitrate concentration is high, ammonium can constitute the desired final product of the reduction, as it can be further recovered and utilized, e.g. for the production of a fertilizer. Conversion of nitrate into ammonia can be successfully accomplished on such electrocatalytic materials as gold [6], [7], Co Nafion coated electrodes [8] or Co (cyclam) analogues [9]. For a low NO3 concentration elimination of all nitrogen compounds lower than the allowable limits is desirable.

Several metal and bimetallic electrode systems, most of them from the group VIII (Ir, Pt [10], [11], [12]) have been employed in numerous laboratory studies on the electrocatalytic reduction of NO3. Among the various materials employed, Pd–Cu bimetallic systems have displayed an efficient catalytic effect and transformation of nitrate prevalently into N-NH3 [13]. Major problems still exist, for the selective reduction of NO3 into nitrogen, which would be the most convenient process when dilute nitrate solutions are treated.

The objective of the present study was to compare the electrocatalytic behavior of the material prepared by electrochemical deposition: Ti/Pd–Co, copper modified Ti/Pd–Co, and of SS/Pd–Cu nanostructured cathodes, obtained by combustion synthesis, during the reduction of NO3. These cathode materials contain metals which singly or in combination proved some catalytic activity for the nitrate or nitrite reduction reactions [14], [15], [16], [17]. Thus, it was interesting to investigate the influence of preparation and of the weight composition on the electrocatalytic properties.

The performance of the reactor equipped with these materials was followed with the objective of solving the problem of N-NH3 generation. To understand the role of each element in the nitrate and its intermediate, nitrite ion reduction, a bulk copper electrode was also investigated. A preliminary assessment of the electrochemical performance of the various catalytic materials was done by cyclic voltammetry. For bulk electrolysis two different processes were considered: electrocatalytic reduction and a hybrid process comprising electrocatalytic reduction and oxidation of the excess ammonia into N2 with the electro-generated chlorine [18]. A single cell reactor and its counterpart, comprising an ion exchange membrane separating cathode and anode chambers, were tested. The latter was investigated in order to propose an alternative option for the complete elimination of NO3 and other nitrogen compounds based on electrochemical reduction and chlorine mediated oxidation, of either oxidized or reduced nitrogen products.

Section snippets

Chemicals

NaNO3, NaNO2, NaCl, NaClO4, CuSO4, Cu(NO3)2·2.5H2O and Pd(NO3)2 were provided by J.T Baker. All chemicals were of analytical-reagent grade and used as received. The solution were prepared by using deionized water.

Nanostructured SS/Pd–Cu catalyst

A palladium-copper catalytic layer was deposited over a steel (AISI 316) electrode by means of a technique that combines solution combustion synthesis (SCS) with spray pyrolysis [19]. SCS takes advantage of highly exothermic and basically self-sustaining reactions that, with low energy

Cyclic voltammetric investigations

The performance of the different electrode materials was investigated in unbuffered aqueous solutions containing 0.1 mol L−1 NaClO4 as base electrolyte and different concentrations (1–100 mmol L−1) of either NaNO3 or NaNO2. The latter is one of the intermediate NO3 reduction products, which can be further and easily reduced at the electrode surface. In the following discussion, the first scan of the various cyclic voltammetric pictures, recorded with the different electrode materials, is

Conclusions

Information gained from voltammetric and exhaustive electrolysis measurements indicate that all the tested materials (Ti/Pd–Co, SS/Pd–Cu nanostructured, Ti/Pd–Co loaded with Cu) exhibit appreciable activity towards the reduction of nitrate. The best performance was displayed by the membrane reactor equipped with Ti/Pd–Co–Cu cathode and an applied cathode potential of about −0.9 V. These conditions allowed to lower the nitrate content from 200 to 50 mg/L, which is the upper limit for drinking

Acknowledgement

The authors thank Dr. Andrea Civera for preparation of the SS/Pd-Cu electrode.

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