Défluoruration des eaux d'exhaure de Youssoufia (Maroc) par percolation sur les cendres volantes de charbon Defluoridation of drainage waters in Youssoufia (Morocco) by percolation with coal fly ash

Résumé

Dans cette étude, nous nous sommes intéressés à la défluoruration des eaux souterraines de Youssoufia (eaux d’exhaure) par percolation en colonne sur les cendres volantes de la centrale thermique d’El Jadida et à la désorption des fluorures par la suite. Les résultats obtenus ont montré que :

- le rendement d'élimination du F- est d'environ 97 % dans nos conditions expérimentales avec un temps de séjour supérieur à 72 heures ;

- la régénération des cendres est possible à l’aide de NaOH;

- le nombre de cycles successifs tolérés par les cendres permettant d’atteindre des efficacités convenables est d’au moins huit.

Mots-clés:

• Défluoruration,
• cendres volantes,
• percolation,
• régénération,
• eaux d’exhaure,
• Maroc

Abstract

The layer of black phosphate in Youssoufia is characterized by the presence of underground water in the building sites. This drainage water must be removed to allow the exploitation of these layers. Observations of the tonnage/flow relationship during previous years allowed the prediction of more than 35000 m3 drainage water/day from the year 2000. This water has particularly high levels of fluorides, which represents a permanent risk for the rural population, which relies on groundwater (wells) for its daily consumption. To minimize adverse health affects and to build on earlier work, the current study was focussed on the sorption performances of coal fly ash in the dynamic mode and on regeneration tests of these ashes for possible revalorization. In this study, we were interested in both the defluoridation of subsoil waters in Youssoufia (drainage waters) by percolation through a column of fly-ash collected from the power station of El Jadida and in fluoride desorption from the ash.

The first results from the characterization of this water showed that fluoride concentrations were elevated, exceeding the water quality standard established by the World Health Organization (WHO), 0.7 mg/L for a semi-arid climate. This abnormally elevated content in fluoride comes from the raw phosphates, which have a high fluoride content. Chemical analysis of the fly-ash, carried out by x-ray fluorescence, demonstrated that the principal components were silica, alumina, oxide iron (Fe2O3), and calcium oxide.

With respect to the first objective of this study, the sorption experiments of water soluble F- ions from the drainage waters were carried out at 25 °C in a glass column (33 cm high, 2.2 cm in internal diameter) filled with a well-defined mass of fly-ash. The process involved: placing 250 mL of the solution in the higher tank (placed in top of the column) and the flows were adjusted to 4.6, 7.7 and 15.4 mL/h. The concentration of fluorides was measured in the effluent every 24 h, with a fluoride ion selective electrode, pH model Orion SA 520 according to a standardized method (AFNOR, T90-004). Each experiment was carried out twice. The drainage water used had a fluoride content of 2.50 mg/L.

With respect to the second objective, the study of the desorption of fluorides from the fly-ash was carried out in a static reactor. The choice was related to chemical desorption with soda, and tests were carried out to evaluate the parameters likely to support desorption (concentration of soda, volume to be used, kinetics of desorption). The method used for desorption consisted of brewing the fly-ash in a basic solution (1g of ashes /100mL of soda) during one hour, at a stirring speed of 300 rpm. The concentration of fluorides was then measured in this solution by a potentiometric method following filtration. The fly-ash was removed and rinsed with distilled water and then re-used for a new sorption. The desorption tests were carried out on fly-ash that had been saturated by contact with the drainage waters from Youssoufia ([F] = 2.5 mg/L, pH=7.86). The sorption stage consisted of putting 10 g of ash in contact with 1 L of the drainage water with a stirring rate of 300 rpm and the tests were carried out at ambient temperature.

The results of the defluoridation by percolation through the fly-ash showed that the concentration of fluorides in the effluent decreased to 0 mg/L « < 5 10-7 M », when the flow decreased from 15.4 mL/h (flow 1) to 7.7mL/h (flow 2) to 4.8 mL/h (flow 3), after 96 to 120 hours. Similar results were also obtained by Piekos et al (1998).

The first regeneration results were encouraging for several reasons. First of all, the regeneration of support was possible and desorption was very important. This demonstrated that basic media are probably favorable for desorption. In parallel, the kinetics of desorption with soda were very fast and they did not exceed one hour for the various soda solution concentrations tested. Indeed, it was noted that at the end of one hour at least 90% of fluorides were desorbed. The optimum conditions for desorption were 60 min of contact and a soda concentration 6 M.

Regenerated ashes were placed again in contact with drainage waters containing fluoride concentrations of 2.50 mg/L. A new sorption of fluorides was noted. This result led us to study the effect of cycle numbers on the adsorption-desorption of fluorides in relation to the effectiveness of regeneration. A histogram of the results demonstrated that the quantities adsorbed and desorbed for the same cycle were practically equal for the eight cycles carried out, thus the effectiveness of ash was practically the same.

Keywords:

• Defluoridation,
• fly-ash,
• percolation,
• regeneration,
• drainage waters,
• Morocco