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

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 −0.9V versus SCE, where the nitrate content was lowered from 200 down to 50mg/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.