Défluoruration des eaux par dialyse de Donnan et électrodialyse Water defluoridation by Donnan dialysis and electrodialysis

Résumé

Les travaux réalisés dans cette étude avaient pour objectif l'application de techniques membranaires, dialyse de Donnan et électrodialyse, au traitement d'eaux dont la teneur en fluorure est supérieure à la valeur maximale admissible. Ces deux techniques qui mettent en jeu des membranes échangeuses d'ions, se distinguent par la nature des forces motrices de transfert. Trois eaux modèles relatives à trois pays différents (Maghreb, Sénégal et France) ont été préparées et traitées sur pilotes pré-industriels. Dans tous les cas étudiés, bien que divers anions (Cl^-, HCO₃^-, SO₄^2-) et cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) soient présents initialement dans les solutions à traiter, une concentration en fluorure conforme aux normes a pu être atteinte avec toutes les membranes testées.

L'électrodialyse qui abaisse la teneur de tous les ions présents dans l'eau, anions et cations, entraîne une déminéralisation partielle et par conséquent un adoucissement de la solution traitée. Par contre, la dialyse de Donnan, du fait de la diffusion du sel du compartiment receveur vers la solution traitée, augmente légèrement la minéralisation initiale. Cette technique qui, du point de vue énergétique, est un procédé plus économique que l'électrodialyse, semble donc plus adaptée au traitement d'eaux fluorurées à faible minéralisation.

Mots-clés:

• Membranes échangeuses d'ions,
• fluorure,
• défluoruration,
• dialyse de Donnan,
• électrodialyse,
• eau

Abstract

The purpose of this work was to apply Donnan dialysis (DD) and electrodialysis (ED) for removing fluoride ion from waters where the concentration exceeds acceptable values. The techniques both use ion-exchange membranes but involve different driving forces: the difference in the electrochemical potential on both sides of the membrane for DD and the difference in the electric potential in ED.

Both techniques were applied to treat model waters, the compositions of which were very close to those of natural waters contaminated by fluorides. Three standard waters related to three different countries (Maghreb, Senegal and France) were prepared and treated with pre-industrial pilots. The active membrane area was 1760 cm² for Donnan dialysis, 552 and 2000 cm² for electrodialysis. Two anion exchange membranes, DSV from Asahi Glass and AFX from Tokuyama Soda, were tested in Donnan dialysis. Three electrodialysis stacks equipped with different anion and cation exchange membranes, AMV-AM1/CMV from Asahi Glass, AMX/CMX and ACS/CMS from Tokuyama Soda, were used. Conductivity, pH and the concentrations of each ionic species were monitored during membrane treatment. The initial fluoride concentration were 9.5, 6.08 and 2.66 mg L^-1 in each standard water, respectively.

In all cases, despite the presence of different anions (Cl^-, HCO₃^-, SO₄^2-) and cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) generally present in ground waters, a fluoride concentration in agreement to the norms (< 1.5 mg L^-1) could be achieved regardless of the composition of the treated waters or the nature of the tested membranes.

Electrodialysis decreased the anion and cation concentrations and induced a partial demineralization (about 70%) and consequently a softening of the treated water. On the contrary in Donnan dialysis, due to the electrolyte diffusion from the receiving compartment to the treated solution, the mineralization of the treated water increased slightly (about 10%). In this latter process, the anion concentration declined while no changes were observed in the cation concentration, except for sodium because of the electrolyte leakage. The DSV membrane was the most effective anion exchange membrane to use in DD. In ED, the AMV-AM1/CMV stack was selected on the basis of the demineralization and softening ratio, and the energy consumption.

Donnan dialysis, which from an energy consumption point of view is more economical than ED, thus seems more adapted to the treatment of low mineralization waters.

Keywords:

• Ion exchange membranes,
• fluoride ion,
• defluoridation,
• Donnan dialysis,
• electrodialysis,
• water