Assessing the Effectiveness of Limestone from Oterkpolu Area in the Eastern region of Ghana as a Suitable Adsorbent for Water Defluoridation

ABSTRACT

Fluoride-contamination of groundwater [above the World Health Organization (WHO) recommended limit of 1.5 mg/L] in the Upper East and Northern regions of Ghana is a well-known problem. Fluoride is, however, beneficial to humans if present in drinking water at levels between 0.7 – 1.5 mg/L. Although, there are some efficient methods for defluoridation of drinking water using various adsorbents, the magnitude of the problem has made it imperative to develop economically viable water defluoridation techniques using readily available natural resource as adsorbent. This will complement the existing defluoridation techniques in order to alleviate the difficulty faced by inhabitants of the affected communities. In addition, a method which is cost effective, easy to use by a layman, does not add other contaminants to water, and efficient in the long term is highly desirable. In this study, the effectiveness of readily available limestone from Oterkpolu (Yilo-Krobo district, Eastern region of Ghana) as fluoride adsorbent was assessed. A drinking water defluoridation technique was subsequently developed using the limestone with various grain sizes (i.e., 500 – 1000 μm, 1000 – 2000 μm and 2000 – 6350 μm) through Batch Adsorption Experiment (using NaF solution concentrations of 1, 5 and 10 mgF-/L), followed by Column Adsorption Experiment using fluoride contaminated groundwater water samples from Bongo district. This was achieved through the geochemical and mineralogical characterization of Oterkpolu limestone using X-ray Powder Diffraction (XRD) and Petrographic Thin Section (PTS). In addition, the radiological risk associated with the use of the limestone for water defluoridation was assessed through the determination of the activity concentration of the Naturally-Occurring Radioactive Materials (NORMs) using a High-Purity Germanium (HPGe) γ-ray detector [γ-ray spectrometry], and computed Annual Effective Dose (AED). The

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study also evaluated the fluoride adsorption efficiency (Sorption capacity and % Adsorption) of different limestone types from Oterkpolu with respect to varying: (i) adsorbent dose (ii) particle sizes of the adsorbent (limestone) (iii) residence time (iv) fluoride concentration. The developed technique was applied to fluoride contaminated water samples collected from affected the communities (Bongo district) through a Column Adsorption Experiment. From the Batch Adsorption Experiment, the maximum percentage adsorption of fluoride was 57.27%, 62.96% and 50.96% (for 1, 5 and 10 mg/L respectively) for sample EKL-R102 at the 60th minute. These results were recorded for 1000 – 2000 μm limestone grain size. The mean activity concentrations for 238U, 232Th and 40K were found to be 2.0 ±1.5, 1.7 ±1 and 21.9 ±13.4 Bq/kg respectively for the limestone samples from Oterkpolu. The calculated Annual Effective Dose of the adsorbent (0.027 mSv/yr) was lower than the recommended 0.40 mSv/yr proposed by United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). Mineralogically, (PTS and XRD analysis), the limestone sample is made up of 96% calcite and 4% quartz (PTS) and 83% calcite, 11% serandite and 6% silicon oxide (XRD). This indicates high calcite content in the sample which suggests that, it is an effective material for fluoride adsorption. The application of the developed methodology (Column Adsorption Analysis) yielded 80% and 67% fluoride removal from two groundwater water samples (BNN8 and BNB6) respectively from the Bongo district of the Upper East region of Ghana. Thus, the fluoride levels in the 330 mL samples were reduced from 7.5 and 6.2 mg/L to 1.5 and 2.0 respectively. The variation was as a result of co-existing anions present in the water sample. The results suggest that Oterkpolu limestone can be used effectively for the removal of fluoride ions in fluoride-contaminated groundwater in general and from the northern regions of Ghana in particular Bongo district.