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FLUORIDE REMOVAL METHODS
The methods utilizing the fluoride exchange properties of the apatites, such as those involving the use of the constituents of bone, the ion exchange principle, and those depending on the sorptive properties of aluminum compounds, appear to show the most
promise for removing excess fluorides from water. In addition, where the fluoride concentration is not very high (less than 4.0 p.p.m.), processes involving the concurrent removal of magnesium are indicated, if hardness reduction is also contemplated.
The use of bone for removing fluorides is based on the long known affinity of bone for fluorides. Probably the most plausible theory of the chemistry of this affinity may be explained on the basis of the anion exchange properties of apatites. The carbonate radical in the apatite comprising bone, nCa3 (PO4) CaCO3 is replaced by the fluorides in the water, forming an insoluble fluorapatite.50 In the regeneration of the material with sodium hydroxide the fluorapatite probably becomes an hydroxyapatite and the fluorides are removed in the form of soluble sodium fluoride. The hydroxyapatite subsequently becomes available as an exchange material by the replacement of its hydroxy radical with fluoride.
ADSORPTION WITH ALUMINUM COMPOUNDS
In addition to the constituents of bone for fluoride removals, a large number of other materials have been tested. These materials include aluminum sulfate, sodium aluminate, zeolites, bauxite, silica gel, sodium silicate, ferric fluoride, lime,1 and various adsorbent clays and earths.4 With the exception of aluminum sulfate, all of these materials have been found to be impracticable probably because of the very low initial concentration of the fluoride ion, and in some instances because an undesirable proportion of the reagent remains in the water due to the excessive amounts of chemicals required. Aluminum sulfate and other aluminum salts have been used in combination with insoluble compounds in contact beds or as constituents of floc which is subsequently removed by settling and filtration. The fluorides might be removed by the formation of an aluminum fluoride complex or by adsorption on the floc. In addition to the fluoride removal characteristics of the aluminum compounds, they are relatively inexpensive, easy to use, and can be combined advantageously with aids to coagulation such as clays and activated silica.
ION EXCHANGE PROCESSES
While the compounds which are subjected to aluminum solution soaking probably involve a combination of ion exchange and adsorption principles in fluoride removal, several additional products are claimed to involve predominatelyion exchange processes. One of these is made from barium or ferric chloride and silicic acid with the formation of a complex metal chloride silicate. It is claimed53 that the fluorides are removed from water when in contact with this material by exchange with the chlorideions. Regeneration of the materials has not been advocated and no data are available on capacities or costs. …
The use of lime for fluoride reduction was known soon after the cause of fluorosis was discovered.11 In a study of the results of lime softening plants in Ohio,47 it was determined that fluoride reduction with lime was actually a function of the amount of magnesium removed. When waters were fluorinated with sodium fluoride and treated with magnesias, it was demonstrated that the magnesia first becomes partially hydrated, then magnesium fluoride and sodium hydroxide are formed by metathesis and finally the magnesium fluoride is attached to the magnesia, forming perhaps an oxyfluoride.61
… About one-half the fluorides originally present in water are removed using dosages of approximately 2,000 p.p.m. of either Wyoming bentonites, Fuller’s earths, celite, or silica gel. To obtain this removal, however, the pH of the raw water is reduced to 2.5. In no case is any fluoride removed when the pH of the water is 8.3.41 Because of the inefficiency of these materials and their pH requirements their use is impracticable for fluoride removals.
SUMMARY AND CONCLUSIONS
More than a million persons in over 500 communities in the United States are now using public water supplies containing in excess of 1.5 p.p.m. fluorides. The disfiguring dental condition which is caused by the use of such waters for drinking purposes can be prevented in future populations of these communities by removing the excess fluorides from the communal water supplies. Most of the removal processes now available appear to be either too expensive to operate or too complicated for routine application by the average small water treatment plant operator. An urgent need is the development of a process comparable to the lime-soda ash softening process in operating costs and simplicity.
The problem of choosing the most practical method of defluorination for a particular supply is difficult, because of the almost complete absence of operational data. The choice of method for the accomplishment of a similar purpose as in other treatment processes, depends, by and large, on the rate of water consumption and on the characteristics of the raw water. For large municipal plants which treat waters requiring a reduction in hardness, it is advisable, in general, to reduce the fluorides as much as possible by the lime softening process. In some cases it would be economical to balance the costs of pre-carbonation and sludge disposal against the addition of a limited quantity of magnesium compounds. Excess fluorides remaining after this process could be removed in gravity contact filters or by an aluminum-clayfloc, with subsequent separation of the floc from the water.
In small plants where softening is not desired, pressure contact filters appear to be more economical. Where the
water is hard, lime softening is indicated up to a raw water fluoride content of about 4.0 p.p.m. If the fluoride exceeds this figure in hard waters, the remaining fluorides can be removed by contact filters.
The tricalcium phosphates and the resinous ion exchangers, when used in contact filters, appear to have the highest exchange capacities for fluorides. Furthermore, the chemical cost for regeneration of these materials is relatively economical.
It is evident from this examination of fluoride removal methods that our current knowledge of the chemical processes involved is very limited and that additional fundamental research in this field is needed. Further, the practical usefulness of the available methods and those now undeveloped must be tested on a pilot plant scale, and their relative worth under varying conditions must be determined. Until this information is available, endemic fluorosis will continue to be a dental hazard to a large population in many communities in this country.
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