Electrochimica Acta 71 (2012) 270–276 Contents lists available at SciVerse ScienceDirect Electrochimica Acta j ourna l ho me pag e: www.elsevier.com/locate/electacta Reaction pathways in the electrochemical reduction of nitrate on tin Ioannis Katsounaros a,1 , Maria Dortsiou a , Christos Polatides a , Simon Preston b , Theodore Kypraios b , Georgios Kyriacou a,∗,1 a Department of Chemical Engineering, Aristotle University of Thessaloniki, 54 124 Thessaloniki, Greece b School of Mathematical Sciences, University of Nottingham, NG7 2RD Nottingham, United Kingdom a r t i c l e i n f o Article history: Received 7 November 2011 Received in revised form 24 March 2012 Accepted 31 March 2012 Available online 5 April 2012 Keywords: Nitrate Reduction Tin Reaction pathways Parameter inference a b s t r a c t The reaction pathways that lead to the formation of intermediates and final products of the reduction of nitrate on tin at high overpotential were studied in this paper. Possible chemical or electrochemical reactions of the intermediates were investigated and discussed. A complex mechanistic scheme was proposed which describes the formation of all products apart from nitrogen, even though the latter is the main electrolysis product. Several simplified reaction schemes were investigated and each fitted to experimental data using a Bayesian approach. It was concluded that in order to describe the rate of nitrogen formation, another intermediate must be considered between nitrite and nitrogen. It is finally postulated that this intermediate is nitramide; however, further work in order to develop a method for determining the nitramide concentration is required to confirm that this is indeed the precursor of nitrogen. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction The electrochemical conversion of nitrate to the non-toxic nitro- gen gas has a great importance for the treatment of a wide range of nitrate-contaminated wastewater in which the biological denitri- fication cannot be applied. Such solutions are the low-level nuclear wastes [1–4] and the solution which results from the regeneration of ion exchangers [5–8]. In both cases the bacteria do not survive in the solution environment, due to the high concentration of nitrate or other salts, or because of the presence of radioactive materials. However, the selective conversion of nitrate to nitrogen is very difficult due to the complexity of the reaction mechanism [9–11] which includes chemical as well as electrochemical (series, parallel, dimerization and possibly autocatalytic) reactions. Therefore, the reduction of nitrate usually leads to the formation of side-products which are more toxic than the reduced nitrate, such as nitrite, ammonia or hydroxylamine [12–21], whereas many other inter- mediate (nitric oxide, hyponitrite) or final (nitrous oxide) products may be additionally formed. The reaction paths that lead to the final products of the reduction of nitrate (mainly N 2 and N 2 O) have not been clarified. Apart from the fact that the chemistry of nitrogen is too complex [11], the elucidation of the mechanism is also difficult because there are not reliable methods for the determination of all the intermediates of the reduction. ∗ Corresponding author. Tel.: +30 2310 99 62 38. E-mail address: kyriakou@eng.auth.gr (G. Kyriacou). 1 ISE member. Previous works from our group showed that nitrate can be reduced with both high rate and high selectivity (%S) to nitrogen (65–85%) on a tin [22] or a bismuth [23] cathode at very negative potentials (-2.0 V to -2.8 V vs. Ag/AgCl). The rate of the reduction and the distribution of the products depend on the overpotential [22,24] and the concentration of nitrate [25], whereas the con- centration and the nature of the supporting electrolyte influence mainly the rate of the reduction [24]. The aim of this paper is to study the mechanism of the electro- chemical reduction of nitrate by investigating the reaction paths that lead to the formation of intermediate and final products. 2. Experimental A Teflon electrochemical cell with a total volume of 24 mL was used in all experiments. The cell was equally separated into two compartments using a Nafion 117 (H + form) cation exchange mem- brane. A tin foil of 2 cm 2 purchased by Sigma–Aldrich (99.9%) was used as working electrode whereas the anode was a platinized Pt foil (Alpha Metal) of 6 cm 2 . The working electrode was polished in a Struers DP–U2 grinding equipment with a No. 1200 paper and rinsed with ultra-pure water before the measurement. The potential was controlled by a Wenking POS 73 (Bank Elektronik) potentiostat and the reference was a saturated Ag/AgCl electrode. All chemicals were reagent grade (Aldrich) and the solutions were freshly prepared using ultra pure water from a Sation 9000 appa- ratus. The determination of nitrate, nitrite and hyponitrite was per- formed by ion chromatography using an AS19-HC (DIONEX 4500i) 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.03.154