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DEFLUORIDATION OF ALGERIAN SOUTH WATER BY ADSORPTION ON NATURAL AND ACTIVATED BENTONITE

Congress: 2008
Author(s):
Samia ACHOUR and Leila YOUCEF Research Laboratory in Subterranean and Surface Hydraulics (LARHYSS), Biskra University, Po Box 145, RP, 07000, Biskra, Algeria. Tel. (+213) 33 74 50 90 E-mail : samia.achour@larhyss.net

Keyword(s): fluoride, defluoridation, adsorption, bentonite, acid activation, groundwater
AbstractFluoride is a trace element with a dual biological effect on humans, which depends on various factors, the most important being the dose. Fluoride in small quantity is known beneficial for bones and dental enamel .However, in several third world regions, its presence in excessive amounts has been detrimental to the people health due to the resulting endemic dental and skeletal fluorosis. In Algeria, the endemic fluorosis is prevalent in the north-eastern Sahara and is largely of hydrogeochemical origin. In this region, people almost exclusively depend on the local ground water for their drinking water and the available data indicate that fluoride levels often far exceed recommended limits, namely 0,6 to 0,8mg/l depending on the maximum day time temperature. Defluoridation of drinking water is the only practicable option to overcome the problem of excessive fluoride where alternate source is not available. Defluoridation techniques are predominately based on the addition of chemicals, the use of membranes or the use of a sorption media. The abundance of bentonite from deposits of Algerian west and its low cost are likely to make it the most appropriate sorbent for fluoride removal from south Algerian waters. The aim of this work is to study the adsorption of fluoride ions from aqueous solutions onto natural and acid activated bentonitic clay. Batch adsorption experiments were carried out by shaking natural and activated bentonite of Maghnia with fluoride solution. In the first step, the optimization of the process was achieved under various experimental parameters (contact time, adsorbent mass, pH, activation time, and acid / clay ratio) with a synthetic solution. In the second step, the process was used to treat drinking waters of Biskra’s region under optimal conditions previously determined. After equilibration, solutions were centrifuged and the fluoride residual concentration determining by a fluoride ion selective electrode. The results showed that the removal of fluoride increases with an increase in both contact time and mass of natural bentonite. A maximum removal of 46% was observed at 10 g/l of clay. Kinetic of reaction was fast enough and equilibrium was reached in 3 hours. Between 30 and 180 minutes of reaction time, these might be controlled by rates of interparticulate diffusion. Defluoridation efficiencies increases with the decrease in pH from 9 to 4. Chemical treatment of clay with sulphuric acid solution enhances adsorption capacity of Maghnia bentonite. Optimal results were obtained for an acid /clay ratio of 0,2 and 3 hours of activation time. The constants of Langmuir and Freundlich models confirm this. Results obtained support the notion that fluoride adsorption onto clay followed an ion exchange, a complexation or precipitation mechanisms. The defluoridation of drinking waters of Biskra was performed and showed high removal efficiencies of fluoride, especially using activated bentonite. The presence of the competitive ions such as sulphate or chloride may explain the performance variability process for the tested waters.
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