Abstract

In Gulu area, geothermal waters show a high concentration of alkali metals (Li, Rb and Cs) and some other typical pollution elements, e.g. As, Sb and Tl. They are also enriched in the geothermal deposits during the phase separation of silica sinter and residual geothermal water. A comprehensive sequential extraction was carried out on the Gulu silica sinter. The results indicated that significant Li and K amount is bound to the Fe-Mn oxide and the residual fractions of sinter while, the elements (B, Cs, Rb, As, Sb and Tl) are predominantly concentrated in the sinter crystal lattices. In addition, the elemental distribution law during phase separation of the residual geothermal water and associated evapoconcentrated deposits is also uncovered in this study. With the respect to the system of the natural silica sinters precipitated from the mother geothermal water, complex variations occur during the trace elements partition between the mother water and associated deposits under evapoconcentration. Thus, the miscellaneous minerals (Li, Rb, K, Cs and B) are also distributed in the solid phases when special minerals such as opal, borax or sylvite precipitate from the residual fluids. This confirms that the valuable minerals of K, B, Cs, Rb or Li can precipitate when the geothermal water is evapoconcentrated to a certain degree and within a very concentrative stage of density. However, these elements are remarkably depleted in the residual geothermal fluid at the stages corresponding to the precipitation of these minerals while they are highly enriched in the mother fluid during the late stages of the evapoconcentration. Thus, it can be stated for the first time that the high concentration of these elements in the residual waters at the final stages will be certainly used in the future. In contrast, the concentration of pollution elements (As, Sb and Tl) of the geothermal solids is higher in the early stage of the opal precipitation, while it is lower in the later stages. The concentration of these elements (As, Sb and Tl) in residual geothermal fluids distinctly increases and is very high at the final stages of the evapoconcentration. This scenario indicates that large quantities of toxic elements such as As, Tl and Sb may degrade the surface and ground waters resources located at or near the geothermal water by mixing process making them unsuitable for drinking or irrigation. Thus, in addition to the thermal energy provided, the geothermal area are very important prospect for exploration and extraction of the mineral resources of the elements such as K, Li, B, Rb or Cs that have very significant economic value. However, percentages of trace elements such as Li, Tl, Rb and K in the exchangeable and carbonates fractions of this sinter suggested that they could become rapidly bioavailable in the geothermal environment.