COMPARING INDIGENOUS LATERITE AND BAUXITE WITH IMPORTED ACTIVATED ALUMINA FOR USE IN SMALL-SCALE FLUORIDE ADSORPTION FILTERS IN RURAL NORTHERN GHANA

The Upper East Region in northern Ghana is poor and rural, with groundwater from communal wells as the main source of drinking water. While groundwater is generally considered a safe and economical source of drinking water, it often contains elevated concentrations of fluoride (F^-) (up to 4.6 ppm) from dissolution of F^- containing minerals in the local granite. One community within the study area has a high number of cases of dental fluorosis, due to drinking water with F^- concentrations above the World Health Organization recommended limit of 1.5 ppm. There is no alternative water source in the community and, because of poverty and limited access to technology, there is currently no water treatment system.

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, the pH point of zero charge (pH_PZC) of laterite and bauxite are 7.2 and 7.5 respectively (as determined by acid-base titration), so they are in the range of the pH of the local groundwater, whereas the pH_PZC of activated alumina is 8.3 making it a better F^- sorbent. Also the specific surface areas for bauxite (6 m² gm^-1) and laterite (21 m² gm^-1) are many times lower than that of activated alumina (298 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.