Removal of Fluorides from Public Water Supplies R. C. GOODWIN
AND JAMES B. LITTON Texas Technological College, Lubbock, Texas
T
HE disease now known as chronic endemic dental fluorosis was first recorded by Eager in 1901 (8) and was
Check of Rated against Actual Capacity
reported again by Black and McKay (4). The work of Smith, Lantz, and Smith (12’) and of Churchill (7) indicated that fluorides in water supplies were a causative factor in the mottling of enamel. Their work was rapidly followed by investigations on the determination, the effect, and the removal of fluorides from water. Endemic areas are found in many parts of the world. In the United States there are about 360 known or reported areas, of which about 27 per cent are in Texas. The West Texas Panhandle region is the largest affected area in the United States, if not in the world. Lubbock has the doubtful honor of being one of the largest cities within this area. The present investigation was carried out primarily to secure factual data concerning the removal of fluorides from the public water supply of this community. The removal of fluorides has been investigated by several (1, S, 6, 9,10, is, 14). Promising results have been secured by the use of a specially prepared mixture of calcium phosphate monohydrate and hydroxy apatite, 3Ca3P20&a(OH)2. Fluoride removal is probably due to the formation of a fluorapatite by the replacement of the hydroxyl. Regeneration by caustic replaces the hydroxyl, after which the excess caustic is washed out and finally neutralized by carbon dioxide. A pilot plant suitable for such operations was made available to us, and all results reported a t this time were secured with the use of this pilot plant.
Raw water was passed through the purifier until analysis showed the fluoride content of the effluent water had risen to 1 p. p. m. This usually required about 17 hours, when the rate of flow was adjusted to 1.5 gallons per minute. Fifty such determinations were made under the above described conditions. Slight variations in the rate of flow were noted. They were due to slight fluctuations in the water pressure and to a tendency of the phosphate to pack toward the end of each run. An average run is shown in Figure 1. The average fluoride content and the total fluoride content may be determined by graphical methods from this curve. The average
Pilot-Plant Experiments The water was from a well on the campus of the college. A complete analysis of this water is given in Table I; the fluoride concentration is 5.2 p. p. m. The relatively hi41 ratio of ma nesium to calcium is also noteworthy. Three dyfferent metho& were used in determining the fluoride content. Where the fluoride concentration did not exceed 3 p. p. m. the procedure outlined by Sanchis ( 1 1 ) was used. For higher concentrations such as were encountered in the raw waters, it was necessary to double the concentration of the zirconium-alizarin indicator. This prevented the complete bleaching of the reddish color, and it is believed that this modification gives more accurate results than are obtained by dilution of the raw water with distilled water. A second method followed was developed by the Texas Department of Health. I n this procedure the sulfates are first precipitated by barium chloride in an acid medium. After standing 48 hours, the sample was decanted and fluorides were determined colorimetrically by a zirconium-alizarin indicator. It was found that much time could be saved and a cleaner decantation made if the precipitated barium sulfate was thrown down by a centrifuge. The third method was that of Boruff and Abbott (6) where the fluorides were distilled as fluosilicic acid which was subsequently titrated against a standard thorium nitrate solution. The use of the alizarin indicator as suggested by Armstrong ( 2 ) apparently gave more accurate results. Taking into consideration the sulfate content of the raw water, the capacity of the calcium phosphate for fluoride removal was calculated to be 275 grains per cubic foot; 1.3 cubic feet of the phosphate were placed in the pilot plant, which gave I t a calculated capacity of 358 grains. 1046
The purpose of this investigation was to determine the efficiency of a commercial pilot plant i n the removal of fluorides from natural waters containing approximately 5 p. p. m. The pilot plant was charged with 1.3 cubic feet of phosphate with a rated capacity of 358 grains of fluorine. Fifty runs were made with aflow rate of 1.5 gallons per minute. In each case the run was continued until the fluoride content reached 1 p. p. m. The average value of the fluoride removal was 346 grains. The average value of the fluoride content of the effluent water was 0.42 p. p. m. It was shown that 1.5 gallons per minute was the optimum rate of flow for this particular pilot plant, and 1.4 pounds of sodium hydroxide were required for each regeneration. Approximately 0.65 pound of phosphate was lost during these determinations where 83,700 gallons of water were passed through the pilot plant. A n analysis of the regeneration and wash waters accounted for the fluorides removed. Fluorine determinations were made according to the methods of Sanchis, of the Texas Department of Health, and of Boruff and Abbott.
August, 1941
INDUSTRIAL A N D ENGINEERING CHEMISTRY
-
TABLE I. ANALYSISOF WATER Determinations Made, P. P. M. Physical 1.0 $us ended matter 1.0 Turgidity None Color None Odor Chemical 43 Calcium 57 Mapesium 96 Sodium 162 .Alkalinity (as COa) 149 Sulf a4e 89 Chloride N'ot detd. Nitrate Trace Iron 60 Silica 5.2 Fluorine 18 Free COS
Hypothetical Combinations, P. P. M. Calcium carbonate 107 137 Magnesium carbonate 88 M a p e s i u m sulfate 116 Sodium sulfate 147 Sodium chloride 60 Silica Total dissolved solids 055
-
1047
per minute. The optimum rate appears to be approximately that calculated. A flow rate of less than 1.5 gallons per minute does not result in any appreciable gain in capacity, while rates higher than 1.6 gallons per minute do show a continuously lower capacity for fluoride removal. These results are summarized on Figure 2.
0 270 343 107 236 281 551 263
Total sodium salts
7.4
PH
value of fluorides removed per run was found to be 346 grains with a maximum variation of 15 grains. The difference of 12 grains between the actual and rated capgcity of the purifier may be due to slight errors in analyses or in the calculations. The rated capacity of the purifier was based on results obtained by investigations made on a synthetic water. The difference of 12 grains may be due to the difference in behavior of the synthetic and natural waters. In fact, previous observations had led us to believe that the fluorine in the water of this area did not occur solely as a simple fluoride ion. We have not been able to establish this contention definitely] but with the possibility in view we were especially interested in determining the extent of its removal. I
16 8
€FFLUZNTIN GALLONS
FIGURE1. RELATION BBTWEEN EFFLUENT IN GALLONS AND FLUORIDE CONTENT DURING PURIFICATION RUN
The average value of the fluorides remaining in the effluent water in these tests was found to be 0.42 p. p. m. Using a similar procedure with a synthetic water containing 10 p. p. m. fluorides, Behrman and Gustafson (3)found the average fluoride content of the effluent water to be 0.15 p. p. m. Here again the natural water reacts somewhat differently from synthetic waters. I n view of present ideas regarding the effect of fluorides, the slightly higher fluoride content may not be objectional. Further investigations are now being carried out in that connection.
Best Conditions for Fluoride Removal With these basic results to use for comparison, further studies were made to determine the effect of altering the rate of flow, the quantities of sodium hydroxide needed for regeneration] the loss of calcium phosphate, and the amount of fluorides in the wash waters. A rate of flow of 1 gallon per minute per square foot area per 10-inch depth of the calcium phosphate is recommended. On this basis the rate of flow for the pilot plant used should be 1.57 gallons per minute, and therefore the original flow was adjusted to 1.5 gallons per minute. Additional runs were made with the rates of flow varying from 1.2 to 2.55 gallons
FIGURE 2. RELATION BETWEEN CAPACITY OF FLUORIDE REMOVAL AND RATE OF FLOW, AND BETWEEN CAPACITY AND ALKALI USEDPER REGENERATION
The regeneration of the calcium phosphate required t h e use of 1 per cent sodium hydroxide. Seven per cent stock solutions were prepared, and sufficient water was added during the injection process to lower the concentration to t h e final value. T o determine the optimum amount of alkali required per cubic foot for regeneration, quantities varying from 0.8 to 1.6 pounds were used. The capacity after regeneration increased with the amount of caustic, up to about 1.4 pounds. Roughly about 1 pound of caustic is required for each cubic foot of phosphate. Figure 2 shows the results obtained (broken line). Further studies were made on the wash waters with two points in view: We wanted to strike a fluorine balance if we could, and we desired to determine the stages in the regeneration of the phosphate a t which the fluorine was removed. The amount of fluorides removed from the water in the run preceding any regeneration could be calculated. By measuring the effluent wash water and determining the fluoride content] the total fluoride removal could be secured. Theoretically these figures should balance. Actually our analyses showed that more fluorine, by the amount of 13 grains, was removed in an average run than we calculated had been absorbed. Without doubt this figure is within the limits of our experimental procedure. At least it indicates that the fluo-
450
300
150
0
15 30 45 60 REGEN€RAT.JON WASH WATE'R, GALL ON5
7
I
FIGURE 3. RELATION BETWEEN REGENERATION WASHWATER AND FLUORIDE CONTENT
1048
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
rides removed from the water can actually be accounted for in the washings from the purifier. As was to be expected, the highest concentration of fluorides in the wash water was reached during the removal of the regenerating sodium hydroxide by a wash of fresh water (Figure 3).
Loss of Phosphate Since we were interested in the adaptability of such an installation by a municipality, we desired to check on the possible loss of phosphate. The original weight of the phosphate was 70.3 pounds. After fifty regenerations, 69.3 pounds remained. Analyses of the effluent water and of the wash waters showed a slight gain in phosphate content. The raw water showed 0.7 p. p. m., the effluent water 0.85, and the wash waters 1.4. Considering that during these fifty regenerations 78,000 gallons of raw water and 5700 gallons of wash water were used, we can account for 0.65 pound of phosphate. These figures are only an approximation, but they do show that there is a loss of phosphate. It is estimated that a phosphate replacement of 5 per cent might be necessary for every three hundred cycles. On the basis of these results a municipal installation large enough to treat 7,000,000 gallons of water per day would require 6200 cubic feet of phosphate, 6600 pounds of 90 per cent sodium hydroxide, and 2500 cubic feet of carbon dioxide. The raw water of this area shows a total hardness of 343 p. p. m. The ratio of magnesium to calcium is 57 to 43. Hence, if equipment for the removal of fluorides were coupled
voi. 33, N ~ a.
with a lime-soda softening plant, the quantities of material needed for the removal of fluorides would be decreased considerably, as the lime-soda softening of this water likewise reduces the fluoride content to 2.9 p. p, m. It is believed, therefore, that the removal of fluoride would be a practical operation even from waters of such high fluoride concent,ratioii; particularly would this be true if a limesoda softener were used.
Acknowledgment The pilot plant and calcium phosphate used in this work were made available by the International Filter Company.
Literature Cited Adler, Klein, and Lindsay, IND. ENG.CHEM.,30, 163 (19351. Armstrong, IND. ENG.CHEM.,Anal. Ed., 5, 300 (1933). Behrman and Gustafson, IND. ENG.CHEM,, 30, 1011 (1938). Black and McKay, Dental Cosmos. 58, 132 (1916). (6) Boruff, C. S., IND. ENQ.CHEM.,26, 69 (1934). (6) Boruff and Abbott, IND.ENG.CHEM.,Anal. Ed., 5, 236 (1933). (7) Churchill, H. V., IXD. EXG.CHEM.,23, 996 (1931). (8) Eager, J. M.,U. 8. Pub. HeaZth Repts., 16, 2576 (1901). (9) Fink and Lindsay, IKD. 33x0. C H E Y . , 28,947 (1936). (10) McKee and Johnston, Ibid., 26, 69 (1934). (11) Sanohis, J. B., IND. ENG.CHEM.,Anal. Ed., 6, 134 (1934). (12) Smith, Lantr, and Smith, Arizona -4gr. Expt. Sta., T e c h . Bull. 32, 264-82 (1931). (13) Smith and Smith, Water W o r k s Eng., 90, 1600 (1937). ENG.CHEM.,29, 424 (1937). (14) Swope and Hess, IND. (1) (2) (3) (4)
PRESBNTBD before the Division of Water, Sewage, and Sanitation Chemistry at the lOlst JIeeting of the hmerican Chemical Society, St. Louis, 310.
TWO ALCHEMISTS Artist Unknown
OUR
illustration, KO. 128 in the Berolzheimer series of Alchemical and Historical Reproductions, is from one of the earliest printed alchemical writings. It is from that excessively rare book “Geberi Philosophi ac Alchimistae maximi, de hlchimia libri tres” published in 1531 by Johannes Grieninger in Strassburg. The book is attributed to Geber (Abu Musa Jabir ibn Hayyan) that somewhat mystical and possibly mythical alchemist whose very existence has been questioned. He is supposed to have lived from 702 to 765. I t is alleged that he wrote in Arabic and that his manuscript writings v e r e collected and edited by a Spaniard well versed in both the Arabian language and Arabian chemistry. Entirely regardless of the authorship of the “writings” they are quite extensively considered “the most important of the medieval chemical works”. Our reproduction is from the title page of the above-mentioned book, the original being a woodcut. I t beautifully illustrates a very early form of fractional distillation. D. D. BEROLZHEIMER 50 East 41st Street
New S‘ork, N. Y . The lists of reproductions and directions for obtaining copies appear a8 follows: 1 t o 96, January, 1939, is.ue, page 124; 97 to 120, January, 1941, page 114. An additional reproduction appears each month.
