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Title: REMOVAL OF FLUORIDE FROM WATER USING GRANULAR ALUMINIUM HYDROXIDE: ADSORPTION IN A FIXED-BED COLUMN
Authors: Eyobel, Mulugeta
Keywords: Fluoride
Adsorption
Breakthrough
Fixed-bed column
Aluminium hydroxide
Bed
Date Added: 7-Sep-2007
Abstract: Excessive and undesirable level of fluoride in drinking water supplies is a major problem in the Rift Valley of Ethiopia. It has been reported that people in the Rift Valley of Ethiopia are consuming water up to 33 mg L-1 of fluoride. The WHO has set 1.5 mg L-1 as the maximum permissible limit for fluoride in potable water. Sustained intake of fluoride beyond this limit can cause dental or skeletal fluorosis, which is a chronic disease manifested by mottling of teeth in mild cases, softening of bones and neurological damage in severe cases, therefore adequate measures for the reduction of the level are important. It is also expected that the fluoride concentration may increase mainly because of the excessive utilization of ground water as these area are characterized by water scarcity. In areas where alternative water sources are not available, physical or chemical treatment of water is the best option to control fluorosis. The methods of fluoride removal used by industrialized countries require more technical support for operation and maintenance than is possible in the rural areas of developing countries. In this study, the removal of fluoride using aluminium hydroxide was studied in a fixed bed column system. The Bed Depth Service Time design model, Empty Bed Residence Time and Thomas model were used to analyze the performance of the column and the effect of the different operating variables such as bed depth, flow rate and initial concentration were tested on these simplified fixed bed design models. Desorption experiments were conducted to evaluate the possibilities of regeneration and reuse of the media. The effects of co-existing ions on the adsorption capacity of aluminium hydroxide were also investigated in batch mode. The breakthrough curves for the adsorption of fluoride on to aluminium hydroxide confirmed that the breakthrough volume and breakthrough time were decreased with increasing flow rate and initial fluoride concentration or decreasing bed depth. The data estimated from bed depth service time model showed that the adsorption capacity (No) of the adsorbent were found to be 24.07, 25.79 and 12.7 mg g-1 for 12, 23 and 40 mL min-1 flow rate, respectively. The operating line seems flatten and no significant reduction in adsorbent exhaustion rate is gained with contact time greater than about 3, 6 and 7 min for 40, 23 and 12 mL min-1 flow rates, respectively, with the corresponding usage rate of 2.2, 0.9 and 1.3 g L-1. The optimum dose for batch system was 1.6 g L-1 and it is close to the adsorbent exhaustion rate of 12 mL min-1. The application of Thomas model has showed that the adsorption capacity is strongly dependent on the flow rate, initial fluoride concentration, and bed depth and is greater under conditions of a lower concentration of fluoride, lower flow rate and higher bed depth. And the Thomas rate constant decreases with increasing bed depth, decreasing initial concentration, and flow rate. Results concerning the effects of anions on the adsorption of fluoride on to the aluminium hydroxide showed that Cl- and SO4 2- have very little effect on the fluoride removal capacity of adsorbent but HCO3 - and PO4 3- had a profound effect on the removal capacity of the adsorbent. Hence it is concluded that using granular aluminium hydroxide as an adsorbent for fluoride removal in a fixed-bed adsorption process is feasible.
Description: A Thesis Submitted to The School of Graduate Studies of Addis Ababa University in Partial Fulfillment of the Requirement for the Degree of Masters of Science in Environmental Science
URI: http://hdl.handle.net/123456789/108
Appears in:Thesis - Environmental Sciences

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