Fluoride Removal from Groundwater by Adsorption Technology

High fluoride concentration in groundwater has made many drilled boreholes unusable for drinking in parts of Northern region of Ghana. Treatment of the groundwater by adsorption is also hampered by the lack of suitable locally available adsorbents. This thesis highlights through principal component analysis and saturation indices calculations that predominant mechanisms controlling the fluoride enrichment probably include calcite precipitation and Na/Ca exchange processes, which deplete Ca from the groundwater, and promote the dissolution of fluorite. Additionally, aluminium oxide coated media is capable of reducing fluoride in water, suggesting it is a promising defluoridation adsorbent.

In the Eastern corridor of Northern region of Ghana, presence of high fluoride concentration in the groundwater has made many drilled boreholes unusable for drinking. Little is, however, known about the factors contributing to the occurrence of high fluoride in this part of Ghana and it’s spatial distribution. Treatment of the fluoride-contaminated groundwater by adsorption is also hampered by the lack of suitable adsorbents that are locally available.

Based on principal component analysis, and saturation indices calculations, this thesis highlights that, the predominant mechanisms controlling the fluoride enrichment probably include calcite precipitation and Na/Ca exchange processes, both of which deplete Ca from the groundwater, and promote the dissolution of fluorite. The mechanisms also include F^-/OH^- anion exchange processes, as well as evapotranspiration processes which concentrate the fluoride ions, hence increasing its concentration in the groundwater. Spatial mapping showed that the high fluoride groundwaters occur predominantly in the Saboba, Cheriponi and Yendi districts.

The thesis further highlights that, modifying the surface of indigenous materials by an aluminium coating process, is a very promising approach to develop a suitable fluoride adsorbent. Aluminum oxide coated media reduced fluoride in water from 5. 0 ± 0.2 mg/L to = 1.5 mg/L (which is the WHO health based guideline for fluoride), in both batch and continuous flow column experiments in the laboratory. Kinetic and isotherm studies, thermodynamic calculations, as well as analytical results from Fourier Transform Infrared Spectroscopy and Raman spectroscopy, suggest the mechanism of fluoride adsorption onto aluminium oxide coated media involved both physisorption and chemisorption processes.

Field testing in a fluoritic community in Northern Ghana showed that the adsorbent is also capable of treating fluoride-contaminated groundwater in field conditions, suggesting it is a promising defluoridation adsorbent. The adsorbent also showed good regenerability potential that would allow re-use, which could make it practically and economically viable. Additional research is, however, required to further increase the fluoride adsorption capacity of developed adsorbent.