Enlèvement intensif des nitrates et de l’azote ammoniacal par methode électrochimique

Abstract

The discharge of effluents with a high concentration of nitrogen pollutants (nitrates and ammoniacal nitrogen) presents a great threat to the balance of the receiving aquatic environment (eutrophication, toxicity for certain species, and asphyxiation of the environment). Several techniques for treating nitrates and ammoniacal nitrogen (biological, physicochemical and electrochemical) have been developed in order to alleviate environmental problems. However, the electrochemical technique is interesting from an energy, ecological and economic viewpoint. The general objective of this thesis is to establish an electrochemical treatment of two nitrogen pollutants (ammoniacal nitrogen and nitrates). The objective is divided into two main parts, the first consists in evaluating and comparing the effectiveness of the reduction of nitrates in a fixed and dynamic system with the addition of reagents promoting electroreduction and aluminosilicates (electrocatalyst). The second part consists in the electroconversion of ammoniacal nitrogen in a solution fixed on the surface of a solid (zeolite). The experimental methodology used consists in carrying out an electrochemical elimination of the two main nitrogen pollutants treated in this thesis (NOଷି and NHସା). The various tests were carried out using a set of electrodes connected to a current generator exerting a constant potential of 5 V, over a period varying between 120 and 180 minutes depending on the experimental condition, the regime, and the type of pollutant a treated. Concerning the electroreduction of nitrates, the first tests took place without supplementing additives with a potential of 5 V, and the best reduction resulting from these experiments was around 47% after a period of 180 minutes. Second, accelerators (salts and acids) were added in order to improve the ionic mobility of the solution and at the same time the nitrate removal yield. The results obtained after adding salt and acid accelerators such as: NaCl, KCl, MgCl2, CaCl2 and HCl, CH3COOH, H3PO4, allow to conclude that an effective nitrate electroreduction can be obtained in the presence of alkali metal salts, resulting in nitrate removal yields of up to 95% after 30-60 min. The electroreduction of nitrates in the presence of aluminosilicates such as: clinoptilolite, kaolinite, bentonite, illite and the 1: 1 mixture [illite + montmorillonite] revealed that the average values of the cation exchange capacity play a key role in providing Na + cations and retaining a sufficient amount of NHସା to inhibit the formation of ammonia. The use of aluminosilicates of different types has led to an improvement in the electroreduction process. This use has also revealed the detrimental role of their cation exchange capacity in the capture of ammonium cations, favoring the shift of chemical equilibrium towards the formation of ammonia. The previous saturation in NHସା of aluminosilicates has become an essential requirement for a high catalytic activity with a possible improvement of the selectivity with respect to nitrogen. In a closed dynamic system, the electroreduction of nitrates by the addition of salt (monovalent and bivalent), acid, and aluminosilicate accelerators has proved conclusive. However, this efficiency varies according to the recirculation speed of the circulation of the solution to be treated. The yields obtained during the addition of aluminosilicates (clays and zeolites) are less significant than those obtained with the salts; the efficiency gain obtained fluctuates between 1 and 16% compared to control experiments. The various experiments carried out within the framework of this work made it possible, on the one hand, to study the influence of the recirculation speed on the electrochemical reduction of nitrates and, on the other hand, to optimize this yield with the addition of the various reagents. The aluminosilicate (clinoptilolite) catalyzed electroconversion of the contaminant NHସା in water was investigated using copper-nickel electrodes. The electroconversion was carried out in the presence of different salt and acid accelerators (NaCl, KCl, MgCl2, CaCl2 et HCl, CH3COOH, H3PO4). The results showed the beneficial role of Cl- chloride and moderately acidic media. The NHସା adsorbed on the zeolite (clinoptilolite) was converted to nitrogen gas with a selectivity of 98 %. The NHସା electroconversion process obeyed zero order kinetics in the presence of clinoptilolite and first order kinetics when NaCl is added. The buffering effects of zeolite and acids beneficial to the electrochemical reaction have been recorded. During the electroconversion of ammonium, clinoptilolite acts as both a catalyst and a reservoir for NHସା.

Date	1 Oct 2020
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Frédéric Monette (Supervisor) & Abdelkrim Azzouz (Co-supervisor)