Defluoridation using clay and a chemical technique were tested with the objective of choosing a suitable method for purifying the F- polluted Ethiopian Rift Valley waters. Fluoride sorption experiments with clay were conducted in columns packed with clay by pouring F- containing water on top of a column and consecutively collecting and determining the concentration of the residual F- in the leachate. whereas, chemical flocculation was performed by mixing Ca(OH)2 with the F- containing water and gradually titrating with a solution of Al2(SO4)3.18H2O while stirring with a magnetic stirrer. The residual F- was then determined in the filtrate, after filtering on a filter paper.
Clay was also heated at different temperatures to determine the most favorable temperature at which it yields optimal F- sorption. The results show that clays heated at 400 °C had the maximal F- loading capacity (350 mg F- /kg DW clay) while When the clay was heated at 700 °C F- loading capacity decreased to 95 mg F- /kg DW clay. The study showed that 115 mg F-/kg DW clay could sorb to clays heated at 400 °C from 15 mg F-/l containing synthetically prepared water to reach a residual F- concentration of 1.5 mg/l. At this rate, about 172 kg of clay can reduce the F- concentration from 15 to 1.5 mg F-/l for the annual drinking water requirement of one person. In addition, the oxalic acid extractable fractions of amorphous Al2O3 and Fe2O3 contents of the heat-treated clays were also examined. The experiment showed that, clays heated at 400 °C have had the highest concentration of amorphous Al2O3 (6488 mg/kg DW) and Fe2O3 (3732 mg/kg DW) while, those clays heated at 105 °C had the lowest concentrations of amorphous Al2O3 (3390 mg/kg DW) and Fe2O3 (2116 mg/kg DW).
With the chemical flocculation experiment, optimum residual F- concentration (1.5 mg F-/l) was reached at a dose of 15 mg Ca2+/l and 930 mg Al2(SO4)3.18 H2O/l.
Comparing chemical defluoridation with F- sorption onto clay showed that both techniques successfully removed F-. However, the chemical technique is less expensive but needs skilled personnel, while, clay is locally available in Ethiopia but needs to be processed and made ready for use by digging, transporting, heating and grinding. Therefore, defluoridation with clay is more expensive than the chemical technique when the labor requirement to make the clay ready for use was taken into consideration. As a final point, Ethiopian clay with high Al and Fe contents may be used for the defluoridation of drinking waters having high F-concentrations in areas where alternative water sources are not available and where chemical or other technology dependent techniques are very remote for the time being.


Auteur: Kulule Mekonnen Duffera
Herkomst: GGS ENVIRONMENTAL SANITATION- CMS UGent
Referentie: Referentie