COMPARING INDIGENOUS LATERITE AND BAUXITE WITH IMPORTED ACTIVATED ALUMINA FOR USE IN SMALL-SCALE FLUORIDE ADSORPTION FILTERS IN RURAL NORTHERN GHANA
One proposed solution is to attach fluoride adsorption filters to the wells, since adsorption is one of the simplest and most cost effective methods for treating high F- drinking water. We tested three promising sorbents for use in small-scale filters in the affected areas: two indigenous rocks, laterite and bauxite, which have been recommended as low cost sorbents for F-removal, and activated alumina, which is a proven effective sorbent. However, activated alumina must be purchased and imported.
The results of batch adsorption experiments show that laterite and bauxite adsorb only ~8% F- at the pH of local groundwater (6.8-7.5), and an initial solution concentration of 10 ppm F-. In comparison activated alumina adsorbs ~ 97%. The large difference in adsorption capacities is primarily due to two properties. First, at I = 0.01 M the pH point of zero charge (pHPZC) of laterite and bauxite are 7.3 and 7.1 respectively (as determined by acid-base titration), so they are in the range of the pH of the local groundwater, whereas the pHPZC of activated alumina is 8.3 making it a better F- sorbent. Also the specific surface areas for bauxite (5.5 m2 gm-1) and laterite (21.9 m2 gm-1) are many times lower than that of activated alumina (288.9 m2 gm-1), thus limiting the number of fluoride adsorption sites. The indigenous sorbents, therefore, have been discounted as potential sorbents, leaving activated alumina as the most viable material for use in the adsorption filters. Because of the cost, maximizing the effectiveness of activated alumina needs to be investigated, including regeneration and re-use.