May 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
RESULTS. The pressure-temperature curves for sulfur hexafluoride with various amoullts of nitrogen (from 0 to o.s4% nitrogen) are shown in Figure 4. The pressure readings at 27" and 50' C. are as given in Table IV. From these pressure readings the quantities of nitrogen in the svstem were calculated, and are tabulated in Table V along with the Bunsen absorption coefficients. The absorption coefficients 'Onstant in agreement with Henry's law, and a rather are high solubility of nitrogen in sulfur hexafluoride is noted.,*
1129
LITERATURE CITED
(1) Lowry, H. H., and Erickson, W. R., J. Am. Chem. Soc., 49, 272934 (1927). (2) Schumb, W. C., IND.ENO.CHEM.,39, 421-3 (1947). (3) Schumb, W. C., Trump, J. G., and Priest, G. L., Ibid., 41,1348-51 (1949). RECEIVED October 12, 1950. Presented before the Fluorine Ohemistry Subdivision, Division of Industrial and Engineering Chemistry, Symposium on Fluorine Chemistry, a t the 118th Meeting of the AUERICAN CHEXICAL SOCIETY, Chicago, Ill.
Gelation Times of Various Silica Sols
4
EFFECT OF LOW TEMPERATURES R. W. SPENCER, A. B. MIDDLETON, AND R. C. MERRILL Philadelphia Quarts Co., Philadelphia, Pa. Activated silica sols are becoming increasingly important as coagulant aids in the various phases of raw water and waste water treatment. A more thorough understanding of the fundamental reactions and the times involved in the formation of these sols is therefore important. Gelation times for mixtures of dilute sulfuric acid or ammonium sulfate with a dilute 3.3 ratio sodium silicate containing 1 to 69'0 silica at 25" and 8" C. are given. The influence of the concentrations of gelling agent and the sodium silicate solutions is investigated, The longer gel
times that are evident with the ammonium sulfate as compared to sulfuric acid are illustrated and discussed. Although the curves follow for the most part the expected relationship of increased gel times at lower temperatures, there are several conditions of pH and silica concentration in which this relationship is reversed. The new data should enable those concerned with the chemical treatment of water to utilize activated silica sols more fully and more efficiently. Improved flocculation and coagulation can be obtained when silica sols are prepared in accordance with the results here presented.
T
Hurd and his associates have done considerable work on gelation times for various concentrations of sodium silicates with hydrochloric, acetic, and sulfuric acids, but these have not been complete within the range in which the authors are interested (4, 6). Hay, in his patent on sodium silicate-ammonium sulfate mixtures, has also given some gel times (3). Because actual appljcation of coagulant aids frequently involves operation under near freezing conditions, the authors have determined the times required for gelation of sols made with sulfuric acid and ammoniumsulfate a t 8" C. These supplement previous data at 25" C. (IO).
HE increasing use of activated silica sols as coagulants and
h
coagulant aids in raw and waste water conditioning has made imperative a more thorough understanding of the time factors involved in the formations of the different sols. The work described in this paper has established the limits of time, temperature, and concentration within which satisfactory silica sols can be made without the attendant fears of gelation taking place. In 1936 Baylis a t Chicago developed a process for making silica sols based on partial neutralization of a sodium silicate (Naz0.3.3Si02)with an acid (1, ,$). The authors' present interest in this application of sodium silicates is an extension of these first successful efforts of Baylis in the use of silica sols as an aid in water coagulation. The term "activated silica" designates a negatively charged colloidal particle formed by the reaction of a dilute sodium silicate solution with a dilute solution of an acidic material. This process generally involves the dilution of a concentrated commercial sodium silicate solution containing about 8.9% sodium oxide and 28.7% silica with a specific gravity of 41.0" Baum6. This silic a b is diluted with water until the silica concentration reaches about 2%. Then the addition of a solution of an acidic material such as sulfuric acid, ammonium sulfate, chlorine, sodium bicarbonate, etc., neutralizes a part of the alkali and further reduces the silica content. Aging for 1.5 to 2 hours a t this concentration permits the growth of the silica micelle. By addition of water to bring the silica concentrations down to about 1%, further growth is then essentially arrested and gelation is prevented with resultant increased stability of the sol. Factors other than gelation which influence the commercial appliqations of activated silica sols have been discussed (8,9).
EXPERIMENTAL PROCEDURE
All gel times were determined by adding a dilute solution of the gel-forming reagent to diluted sodium silicate solutions, mixing well, and allowing the mixture to stand in a &ounce bottle kept a t either 8" or 25" C. until the mixture gelled. The mixing time was a matter of seconds and in all cases was considered to be a negligible part of the gel time. Gelation was determined by 109s of uniform fluid flow, the appearance of breakage planes when the mixture was tilted, and the adherence of solid gel t o the glass wall. The gel times obtained in this manner are reproducible within 2y0 and should correspond fairly closely to the "time of set" determined by the "tilted rod" method used by Hurd (6) and his associates. All acids and the ammonium sulfate used were reagent grade chemicals and all water used in diluting the silicate and acids was distilled. The silicate used in all cases was the N sodium silicate manufactured by the Philadelphia Quartz Go., containing S.9y0 sodium oxide, 28.7% silica, and 62.4% water.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1130
Vol. 43, No. 5
10,OOl 5,001
2,00( I,OOC
500
20c
g IOC 3
z
i
.. E
50
w
w"a
20 10
5
2
I MOLE
RATIO-^;^^ 100
Figure 2
Figure 1
G E L TIMES
Figure 1 shows the gel times of sodium silicate (Na2O.3.3SiO~)-ammonium sulfate solutions at constant silica concentrations from 1.0 to 6.0% at 8" and 25' C. as a function of molecular ratio of sulfate t o sodium oxide. In Figure 2 the gel time8 are given as a function of the silica concentration at constant ratios of sulfate (SOa--) to sodium oxide. The percentage figures moles of SO4-on the curves in Figure 2 are moles of XazO X 100. No attempt was made to keep these solutions a t a constant pH. The p H values of these solutions a t 8" C. as deter11.20 mined with a Beckman Model G p H meter are shown in Figure 3. For each silica con11.25 centration the gel times of the solutions with the higher ratios of ammonium sulfate 1 I 00 to sodium silicate are longer at 8" C. than a t 25" C., whereas the more alkaline solu10 75 tions of lower sulfate to silicate ratio require I longer time to gel at 25" C. than at 8" C. This apparently anomalous situation is I O 50 similar t o that found previously by Hurd and his associates, who observed that 10 25 partially neutralized sodium silicate solutions having initial pH's higher than 10.5 10.00 required longer times to gel a t higher temperatures (7). This is in contrast to the 9.75 more acid silicate solutions which gel faster a t higher temperatures. The longer gel 0 20 times a t higher temperatures for the more alkaline mixtures probably involve the greater solution or dispersion of silica
by alkali, although the effect may be modified by the presence of other ions. The gel times of sodium silicate-sulfuric acid solutions at 8 O and 25" C. are shown in Figure 4 as a function of the molecular ratio of acid to silicate. Figure 5 gives the gel times as functions of the silica concentrations at constant acid-silicate ratios, and Figure 6 shows the p H a t 8" 6. as a function of per cent neutralization for these solutions. The same reversal of the effect of
MOLE RATIO Vs pH FOR N - S O L A SOLS CONSTANT Si02 CONCENTRATIONS TEMPERATURE 8 8%
40
60 MOLE
80
100
120
140
RATIO=-^ io0 Figure 3
160
180
200
220
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1951
1131
10,OOl
2,ooc I,OOC
. 500
20c
Y
F
E i
20
IO S
OASHED LINES : 25'0.
% SiOg
Figure 4
Figure 5
temperature on gel times observed with the sodium silicateammonium sulfate solutions is also obtained in this case. Comparison of Figures 3 and 6 shows that silica sols a t equivalent silica contents have much higher pH's when prepared with ammonium sulfate than when made with sulfuric acid The data of Figures 1 and 5 demonstrate that the ratio of ammonium sulfate to silicate does not affect the gel times at both 8" and 25" C. as much as does the ratio of sulfuric acid t o silicate. Thus when operating under conditions close to gelation, a small change in the
amount of sulfuric acid added to the silicate may produce gelation, whereas addition of an equivalent amount of ammonium sulfate would not. Figure 4 shows that a 2% silica sol will gel in about 5 minutes a t 25" C. or approximately 15 minutes at 8" C. if made with 90% neutralization of the silicate alkali with sulfuric acid. However, if the 2% silica sol is made with the same equivalent of ammonium sulfate per equivalent of silicate, it will not gel for about 1800 minutes a t either temperature. When this ratio of ammonium sulfate to silicate is used in making the sol, the silica concentration can be increased to more than 3% before the sol will gel in 20 minutes a t either temperature. This increased stability of silica sols made with ammonium sulfate as compared with those produced with sulfuric acid makes them easier to handle in actual plant practice because of the lessened tendency to form an actual gel. The higher p H of the ammonium sulfate sols is in some measure responsible for their increased stability. For economic reasons it is usually advisable to prepare sols with as high a silica concentration as can be obtained with the desired maximum gel times under the conditions of storage. The data in this paper show that a t both 8" and 25" C. these requirements can be met with ammonium sulfate-silicate sols. If the silica sols are to be used as coagulant aids, the extent to which the gel time can be increased by decreasing the sulfatesilicate I ratio is limited because, according to present indications, silica sols with too low a ratio lose their effectiveness as coagulant aids.
b
11
I,
""I 7.0
0 NEUTRALIZATION VapH FOR HaSO4-N" SILICATE SOLS 5.0
,
CONSTANT S i 0 2 CONCENTRATIONS TEMPERATURE 8-c.
Figure 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
1132
It is apparent that the gelling power of ammonium salts a t both 8' and 25' C. is not entirely due to acid formed by hydrolysis, as gels are formed a t a p H only slightly less alkaline than the p H of the sodium silicates themselves. This ability of ammonium salts to form sols and gels is probably due to the formation of slightly ionized, heavily hydrated, insoluble gelatinous salts. The data are in agreement with the theory most widely accepted a t the present time conccrning the structure of silica gels. According to this theory, a silica gel consists of heavily hydrated interlaced fibrillar or brush heap structure of very large polysilicic acid molecules with the spaces filled with water or dilute silicate solutions. These chains are formed by the condensation of molecules of simpler silicic acids or hydroxides forming water and high Dolvmeric Dolvsilicic acids. If we assume that eelation is due to a reaction of this type, fastestgelation should and probably does occur near the p H of least ionization where there -
I
-
I
Y
Anaerobic
Vol. 43, No. 5
is a minimum of repulsive forces between similarly charged silicic acids or hydroxides. LITERATURE CITED (1) Baylis, J. R., J . Am. W a t e r W o r k s Assoc., 34, 1397 (1942). ( 2 ) Baylis, J. R., w o r k s & Sewerage, 83, 469 (1936). (3) Hay, H. R., U. S. P a t e n t 2,444,774 (July 6, 1948). (4) H u d C. B.9 C'hem. Rezs.1 221 403 (1938). J . Phys. Chem., 44, 847 (1940). (5) Hurd, C. B., and Barclay, R. W., Hurd, C, B,, and hIlller, p. s,, 36, 2194 (1932). (7) Hurd, C. B., Pomattl, R. C., Spittle. J. H., and iiloes, F. J., J . Am. Chem. Soc., 66, 388 (1944). (8) Merrill, R. C., IND.ENG.CHEM.,40, 1355 (1948). (9) hIerrill, R. and Bolton, z,, Chem. Progress, No.1, 27 (1947). (10) PIerrill, R. C., and Spencer, R. W., J . Phys. $. Colloid Chem., 54, 806 (1950). RECEIVED July 7, 1950. Presented before the Division of Water, Sewage, and Sanitation Chemistry a t the 117th Meeting of the AVERICAVCHEMICAL sOCIETY, Detroit, Mioh,
water
c.,
sistence of o-Cresol
enol
111. B. ETTIKGER, W. ALLAN MOORE, AND R. J. LISHKA Federal Security Agency, Public Health Service, Environmental Health Center, Cincinnati, Ohio Factors bearing on the resistance of phenolic materials to destruction by natural agencies are of importance because of the difficulty which the presence of trace amounts of such materials causes in the production of a safe palatable water from surface water supplies. Phenol and o-cresol are observed to undergo complete destruction under anaerobic conditions similar to those that might be encountered in polluted streams or waste lagoons. The end products of such decomposition are presumed to be carbon dioxide and methane. The over-all
persistence of the phenolic materials studied under anaerobic conditions was found to be significantly greater than the persistence noted for such materials in polluted aerobic waters. Microorganisms are responsible for the chemical changes observed. This work indicates that controlled storage may be satisfactory as a means of destroying the phenol content of wastes under certain circumstances. Phenolic material will be transported farther with a minimum loss in a deoxygenated stream under ice cover.
I
was used. A 5-gallon carboy was preferred because the smaller size made possible fairly effective mixing by manual shaking. To prevent photosynthesis, the carboys used were given several coats of a heavy black paint and placed in the laboratory where they received no direct sunlight. Incubations were carried out a t both room temperature and a t 4" C. I n the latter case, the entire apparatus was placed in a 4' C. incubator. In making a run, the carboy was Bled with a mixture of sewage and a selected diluent. Diluents used have consisted of up to 95% of tap water, formula C dilution water (S), or various surface waters. The mixture was allowed to stand until oxygen-free on the basis of the Winkler test ( 2 ) . The phenolic material was then introduced through the thistle tube. The holding bottle was shaken thoroughly. Samples for examination were withdrawn through the siphon tube. When required, air was pumped through the deoxygenating train into the bottle to maintain nitrogen pressure in the system slightly greater than atmospheric pressure.
N AN earlier series of investigations, the factors governing the persistence of phenolic materials in surface waters were studied in Borne detail (5). This study is a natural sequel. It represents an attempt to determine some of the principal factors which govern the persistence of phenol and 0-cresol under anaerobic conditions such as might be encountered in badly polluted surface waters or waste lagoons. The scope of the work has been limited to: (1) a qualitative evaluation of some of the most important factors involved, and (2) making sufficient observations to permit qualitative comparison of the anaerobic persistence of phenol and o-cresol with the aerobic persistence of these materials in polluted waters previously reported. Tarvin and Buswell ( 7 ) have shown that phenol and a number of other aromatic compounds will undergo anaerobic decomposition, with the carbon involved appearing in the end products as carbon dioxide and methane. The extended series of investigations reported by Buswell and his coworkers ( 1 ) make it evident that the anaerobic diesimilation of phenol reflects the metabolic activity of microorganisms. METHOD O F INVESTIGATION
The apparatus illustrated in Figure 1 was used t o make this series of investigations. According to the need of a particular experiment, either a 5gallon or 10-gallon carboy of accurately known total capacity
*
In setting up the deoxygenation train, it was found desirable to stagger the levels of the alkaline pyrogallol as indicated by Figure 1. In practice it was found that while the apparatus was standing the first several bottles of the washing train would remove some oxygen from the quiescent gas above them. This set up vacuum conditions which resulted in sucking back of the pyrogallol solution toward the inlet end of the train. In order to avoid this difficulty, particularly because some experiments were run for several months, the system of filling the washing bottles was used as indicated in Figure 1. Determinations of phenol and 0-cresol were made using the
