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Please use this identifier to cite or link to this item: http://hdl.handle.net/123456789/267

Authors: Yoseph, Abebe
Advisors: Prof. B. S. Chandravanshi
Dr. Feleke Zewge
Copyright: 2007
Date Added: 16-Nov-2007
Abstract: Health problems occurring due to high fluoride concentrations in drinking water are a widespread problem in the East African Rift system including the Ethiopian Rift Valley. Excessive fluoride in groundwater is the most serious water quality problem and many people are being affected by both dental and skeletal fluorosis. The WHO and national standard (ES 261:2001) permit only 1.5 mg/l as a safe limit for human consumption whereas several residents of Rift Valley region of Ethiopia are consuming water with fluoride concentration much more than the permissible limit. Current methods of fluoride removal from water include adsorption onto activated alumina, bone char and clay, precipitation with lime, dolomite and aluminium sulfate, the Nalgonda technique, ion exchange and membrane processes such as reverse osmosis, electrodialysis and nano filtration. Most of the available materials for defluoridation are expensive and technically non-feasible in rural communities in Ethiopia. Hence the development of defluoridation method based on locally available materials is desirable. The technology must be technically simple, cost effective, easily transferable, use local resources and must be accessible to the rural community. The present work has been undertaken to explore the feasibility of aluminium oxide hydroxide for the removal of fluoride from water. Aluminium hydroxide was prepared by the reaction between aluminium sulfate (Alum) and sodium hydroxide at room temperature. The white preciptate of aluminium hydroxide was dried at 50 oC in air circulated oven and heated at 300 oC in oxidizing atmosphere to produce two forms of aluminium hydroxides respectively. The products were characterized by XRD and BET SA measurements by BET-nitrogen method. A surface area of 110 m2/g and pore volume of 0.29 cm3/g was obtained for the sample prepared at 300 oC (Boehmite) where as a surface area of 37 m2/g and pore volume of 0.19 cm3/g was obtained for the sample prepared at ordinary conditions (Pseudo-boehmite structure).XRD results show that heat treated aluminium hydroxide comprised AlOOH as the major component and traces of FeO(OH), Fe (OH) 2 and Al2O3.. The Langmuir sorption capacity, qm, and adsorption coefficient, b, are 101.63 mg/g and 4.0 × 10-2 L mg -1 for psedoboehmite and 72.72 mg/g and 2.11×10 –1 L mg -1 for boehmite, respectively. The D-R isotherm adsorption energy values of 6.04 and 13.35 kJ mol-1 are noted for F –adsorption onto pseudoboehmite and AlOOH, respectively which indicates the sorption process is predominantly physisorption. The Temkin constant, KT, of boehmite and pseudoboehmite for F - are 20,735.34 l mg -1 and 7,369.996 l mg -1, respectively indicating a lower adsorbent/fluoride ion potential (interaction) for pseudoboehmite. The Temkin adsorption potential, 1/bT, was 0.00742 and 0.00116 for AlOOH and pseudoboehmite, respectively. The relatively higher adsorption potential for AlOOH was probably due to the high surface coverage of F- ions onto its surface. The AlOOH media used has the capability of producing water with a residual fluoride concentration of less than 0.05 mg/l from an initial fluoride of 20.0 mg/l. In the previous fluoride adsorption studies using this material, the adsorption capacity of AlOOH for fluoride ions has been tested in batch and continuous mode and found out as 23.7 mg F - /g and 25.79 mg F -/g , respectively. A comparable breakthrough time, t, was observed between a laboratory scale column (down flow) experiment and mini column(up flow) of the previous adsorption experimental results with breakthrough time values 1150 min and 1350 minutes, respectively. The presence of certain cations may be effective in fluoride retention. Exchangeable cations such as Na, Ca and Mg may form solid precipitate with fluoride. A domestic defluoridator has been developed and tested at a pilot domestic unit. AlOOH particles with 1.0 – 2.0 mm size were used as filter media in this unit. Upward flow technique has been used in the defluoridation process. The performance of the pilot defluoridator has been monitored at regular intervals to evaluate its fluoride removal performance. The pilot household defluoridator has a capacity of treating about 20 L of water and this quantity of water could be used by a family of five each consuming four liters per day for cooking and drinking purposes. About 0.785 Kg of AlOOH granules has reached breakthrough in 1.4 month when the fluoride concentration in water was around 20 mg/l. The defluoridation price per liter of water was estimated as 0.55 birr/L. Hence it is concluded from the pilot study that the developed technology is simple, efficient, effective and feasible for defluoridation of water for fluoride affected regions both at the remote and rural settlements at large and urban households in Ethiopia.
Description: A Thesis Submitted to The School of Graduate Studies of Addis Ababa University in Partial Fulfillment of the Requirement for the Degree of Master’s of Science in Environmental Science
URI: http://hdl.handle.net/123456789/267
Appears in:Thesis - Environmental Sciences

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