Increasing Color in Some Fluorosilicic Acid Products
In the past several months, the US Centers for Disease Control (CDC) have been asked to comment on the observed increase in color in some fluorosilicic acid (FSA) products. This question about exceeding the American Water Works Association (AWWA) color standard for FSA is one that is currently being addressed by the AWWA Fluoride Standards Committee. FSA can range from "water-white" to straw colored. The characteristic straw color is related to the iodine and phosphoric acid content. As iodine and phosphoric acid increase, the color of FSA will increase. The current color standard of 100 APHA units was established during a time when there was an excess of fluoride products, and the 100 APHA units criterion was based on aesthetic considerations, not health concerns. The thought was that if you can choose between something with minimal color versus something with minor color, choose the minimal color. FSA with color in the 65-75 range can typically contain Total Iodine in the 80-125 mg/L range (with scattering due to an imperfect data correlation). There is no operational, health, or scientific basis for the 100 APHA unit limit. After being diluted 250,000 times when added to drinking water, the color and iodine are not detectable.
The level of iodine in apatite ore varies; ore from some mines in central Florida has more iodine than the levels historically experienced. The same process that extracts fluoride from the ore also extracts iodine in much the same manner. In the past, the "off spec" FSA would be sold for other uses such as aluminum or glass flux, but those demands have disappeared. Recently, the total supplies of FSA have decreased for various reasons.
Since the US EPA does not have a Maximum Contaminant Level (MCL) for iodine, the National Science Foundation (NSF) has prepared an analysis on the levels of iodine that may have a health effect, either as a direct result of consumption or with respect to an ozonation derivative. The permissible levels from the NSF criteria are significantly greater than the maximum iodine observed in the colored FSA product. The preliminary data indicate that presence of regulated contaminants in the colored FSA product is the same as for lower-color product. At the June 2007 AWWA committee meeting in Toronto, the AWWA Fluoride Standards Committee began the process of considering the data relevant to this issue. One confounding issue is that phosphoric acid also contributes to color; the Committee is attempting to quantify the relative contributions to color by iodine and phosphoric acid. It should be noted that the European standard for drinking water FSA does not have a color standard. AWWA may continue to stipulate a color standard or instead may decide to implement a numerical value for iodine and phosphoric acid. Because of AWWA procedures, all this may take 1 to 2 years before the standard is actually changed.