Developing a Small-Scale De-Fluoridation Filter for Use in Rural Northern Ghana with Activated Alumina As the Sorbent

In northern Ghana, groundwater is the main source of household water and is generally considered safe to drink. However in some areas it contains fluoride (F^-) concentrations above the 1.5 ppm limit recommended by the World Health Organization, putting the users at risk of fluorosis. The study area in the Upper East Region of Ghana has pockets of groundwater F^- up to 4.6 ppm and, as a result, also has a high percentage of residents with dental fluorosis. They have no alternative water source and, because of poverty and limited access to technology, lack the capacity to set up advanced treatment systems. One proposed solution is to attach F^- adsorption filters to the wells, since adsorption is considered a simple and cost effective approach for treating high F^-drinking water.

This study evaluates activated alumina as a sorbent for use in de-fluoridation filters in the study area. We evaluated the long-term adsorption capacity of activated alumina, and changes in F^- adsorption rate and capacity with grain size. We measured differences in positive surface charge (C m^-2) via slow acid titration, as well as F^- loading with varied prior hydration time. Results from this research show no notable change in F^- adsorption or positive surface charge when the activated alumina surface was pre-equilibrated in distilled water from 24 hours to 30 weeks. The results of F^- loading show a maximum of ~3.4 mg F^- sorbed per gm activated alumina (initial pH ~6.9, initial F^- 1 to 60 ppm, 20 hr reaction time). The pH dependent surface charge is ~0.14 C m^-2 at pH of ~4.4 and is zero at pH ~8.6.

F^- loading experiments were conducted with grain size 0.125 to 0.250 mm and 0.5 to 1.0 mm to evaluate changes in F^- adsorption rate (initial pH ~6.9, initial F^- 10 ppm) and F^- loading (initial pH ~6.9, initial F^- 1 to 60 ppm, 20 hr reaction time). The F^- loading did not change with grain size. However time to equilibrium increased dramatically with a decrease in grain size - after one hour of reaction time, the larger grain size adsorbed only 59% of F^-, while at the finer grain size 90% was adsorbed. Future work will determine the volume of high F^- water that can be treated before activated alumina needs to be regenerated or changed. These data will aid in the design of a small-scale F^- adsorption filter in the study area, and will predict the longevity of activated alumina as the sorbent.