By: Xanxin Wang, Ph.D.,
China University of Geosciences (Wuhan)
Visiting professor at the University of Waterloo (1998-1999)
The element fluorine, ranking thirteenth in abundance in the Earth's Crust, is used by higher life forms in the structure of bones and teeth. The importance of fluoride in forming human teeth and the role of fluoride intake from drinking water in controlling the characteristics of tooth structure was recognized during the 1930s. Fluoridation of drinking water since then has been a common practice in many countries.
But too much fluoride can be detrimental. Fluorosis, an endemic disease caused by the long-term ingestion of high fluoride drinking water, affects many millions of people, particularly children, in south Africa, Mongolia, India, Pakistan, Thailand and China. Severe forms of the disease typically develop only when the fluoride concentration of drinking water is greater than 5 to 10 mg/L. However, symptoms of the disease can develop with regular ingestion of water containing fluoride concentrations as low as 1 to 2 mg/L.
Fluoride can be enriched in natural waters by geological processes. Besides, there can also be formidable contributions from industries. High-fluoride (tens to thousands of mg/L) wastewaters are generated by coal power plants, rubber, fertilizer and semiconductor manufacturing, glass and ceramic production, and electroplating industry.
To find out cost-effective alternatives for removing too much fluoride from waters, many different geomaterials have been tested in recent years, including zeolite, heat-treated soils, fly ash, bauxite, volcanic ash, and limestone. Except limestone, all the other materials rely on sorption.
Recently, together with Eric J. Reardon, professor of Geochemistry at Waterloo, we experimented two different geomaterials to remove fluoride from waters: limestone and heat-treated soil.
Limestone was used in a two-column continuous flow system (we call it a limestone reactor) to reduce fluoride concentrations from wastewaters to below the MCL (Maximum Contaminant Level) of 4 mg/L. Calcite was forced to dissolve and fluorite to precipitate in the first column. The degassing condition in the second column caused the precipitation of the calcite dissolved in the first column, thus returning the treated water to its approximate initial composition.
In laboratory experiments, the fluoride concentration of the effluent from all tested feedwaters containing initial fluoride amounts from 10 to 100 mg/L were reduced to below 4 mg/L. And a steady state of the system performance was quickly achieved. For instance, in an experiment when the input fluoride concentration was 100 mg/L, effluent concentrations from both columns were below 4 mg/L after only 8 pore volumes had passed.
The major advantage of this technology over existing liming and ion exchange methods to treat wastewaters is that system monitoring is minimal, regular column regeneration is not required and the water is returned to its initial chemical composition. As confirmed by our preliminary experiments, the proposed reactor has potential application to reduce the concentrations from wastewaters of anionic elements similar in charge and size to carbonate ion, such as selenate and arsenate, and cations similar in size and charge to Ca2+, such as Cd2+.
In 1995, I found a Pleistocene soil in Xinzhou, China which is able to remove fluoride from the local groundwater. In this area of Shanxi Province, northwestern China, more than 300,000 residents suffer from fluorosis. A locally-available and cost-effective fluoride sorbent for this region is thus highly desirable. More detailed laboratory investigations on the activation and regeneration of the soil's fluoride-sorbing properties have been conducted at the University of Waterloo since September, 1998.
X-ray defraction analysis reveals that the soil is composed principally of quartz (50-60%), illite (30-40%), goethite (5-10%) and feldspar (5-10%).
A substantial improvement in both the permeability and the fluoride- removal capacity of the soil was achieved by heating it in a Muffle furnace. A granular material can then be obtained by crushing the heated product.
The experiment results show that heating at 400-500° C has the optimal effect on the enhancement of the material's fluoride removal capacity. A preliminary column experiment showed that 4.0 Kg of 400° C heat-treated soil can treat more than 300L of 5 mg/L F feedwater before the effluent fluoride concentration reaches 1.0mg/L.
Once the soil's fluoride-sorption capacity had been reached, the material could be regenerated in a cost-effective way: rinse the soil first with sodium carbonate solution; then with dilute hydrochloric acid; and finally with distilled water twice. After being air-dried, the material is ready for reuse.