DEFLUORIDATION OF DRINKING WATER WITH HYDROTALCITE-LIKE COMPOUNDS

In semi-arid regions, the ground water in many areas contain high levels of fluorine. People depending on water with high concentrations of fluorine for their daily drinking water may develop fluorosis. It is estimated that in the order of 80 million people worldwide suffer from fluorosis. Fluorosis is particularly widespread in rural communities in northern China, where people almost exclusively depend on the local groundwater for their drinking water.

The use of hydrotalcite-like compounds (HT) and their calcined product (HT500) to remove fluorine ion from drinking waters has been studied. This research assessed the effectiveness of crystallinity of HT and HT500 in absorbing fluorine ions in water. [Mg₃Al(OH)₈]₂CO₃·nH₂O was prepared by the co-precipitation method and subjected to hydrothermal treatment, with samples heated to air temperatures of 80 centigrade and 130 centigrade for 24 hours. Characterisation of fluoride sorption behaviour by HTCO₃ and HTCO₃500 was achieved through absorption and equilibrium experiments. The absorption of fluorine was followed by absorption isotherms, quantitatively described by the application of the Henry equation. Factors found to influence fluoride sorption include the solution pH, crystallinity of HTCO₃ and the presence of exchangeable anions.

These exchange reactions were not significantly modified by the crystallinity, especially in the case of carbonate-samples. The specific affinity of carbonate anion for HT layer strongly influences its displacement. On the calculated anion exchange capacity (AEC) basis, the HTCO₃ in the untreated sample absorbed 32% of the fluorine, whereas the hydrothermally treated samples heated to 80 centigrade and 130 centigrade adsorbed 37% and 43% of fluorine, respectively. The calcined HTCO₃500 adsorbed fluorine more effectively than the HTCO₃ samples, the HTCO₃500 hydrothermally treated at 130 centigrade absorbed 93% of the fluorine. The HTCO₃500 sample hydrothermally treated at 130 centigrade absorbed significant amounts of fluorine, bringing the F-levels from 10 ppm down to around 1 ppm or even less. The absorption of fluorine with HTCO₃ and HTCO₃500 by anionic exchange is significantly affected by the pH values. From the above results, the method of defluoridation using HTCO₃500 and hydrothermal treatment is considered effective.