Colloidal Chemistry in Water Treatment J
EDWARD S . I-IOPKINS Montebello Filters, Baltimore, Md.
Water purification has made practical use of the concepts of colloidal chemistry. Coagulation is now governed by pH factors, and the operation and design of mixing basins are in accordance with the principles reviewed in this paper. Utilization of sulfate adsorption has increased plant efficiency b y giving a precise value to optimum flocculation, while adsorptive beds are no longer considered as a phase of the rule-of-thumb operation. Since proper coagulation of water is the controlling factor in plant efficiency, it is obvious that better sedimentation and filtration procedures follow in relationship. Therefore, the assumption that efficient purification processes are in accordance with the advancement of colloidal chemical knowledge is correct.
T
HE experiments by Fuller in 1896 a t Louisville, Ky. (191, established the principle of coagulation by electrolytes to remove turbidity from water. This study disclosed that the trivalent iron and aluminum salts, particularly the sulfate, gave best results. By the utilization of chemical precipitation as pretreatment to filtration, modern filtration rates were made possible, and the present type of filter plant was established upon a sound operating basis. For approximately twenty years water purification was operated by the “rule of thumb” in relation to the fundamental factors concerning the coagulation of alum floc. Velocity of flow and time of coagulation in mixing basins were empirically adopted, and it is commendable that more serious errors in plant design were not made. During this period the chemical background was not sufficiently developed to enable the designing civil engineer to install proper types of purification equipment.
Clark and Lubs in 1917 (3) published their classic papers in which were developed new and practical methods for the accurate determination of hydrogen ion concentrations. By using Sorensen’s pH scale, they provided a simple measurement for the control of water purification processes and opened the door to a series of investigations of fundamental principles. Upon carefully reviewing the then existing theories concerning the formation of the hydrated aluminum oxide floc. Wolman and Hanan (32) suggested that the hydrogen ion concentration was of controlling importance in coagulation. They expressed the belief that the isoelectric point for floc precipitation would be a t a definite pH value, and that many of the then existing criticisms of residual alum in filtered water were due to a failure to understand the reactions involved. They also indicated that maximum sedimentation and complete coagulation of the sol were controlled by varying factors not necessarily related. This article dispelled the belief that complex definite aluminum compounds were formed in waters on the alkaline or acid side of neutrality. Hatfield’s work in 1922 (14) demonstrated that the quantity of alum to produce floc is not entirely dependent on the turbidity, and that with a moderately hard water (210 p. p. m. alkalinity) flocculation occurred between pH values of 6.6 and 7.6. He operated the Highland Park, Mich., plant under pH control from August 1, 1921, to April 16, 1922, with increased efficiency of coagulation. In so far as can be learned, this was the initial plant operation with pH procedure. He also indicated that TThen excessive amounts of alum are required to neutralize high alkalinities the pH value is decreased to the flocculating zone by the coagulant. This neutralization utilizes the alum and it is not necessarily required as a coagulant. Upon surveying various plants, Buswell and Edwards (6) indicated that a maximum precipitation of alum occurred a t a pH value of approximately 6.0. They concluded that the exact hydrogen ion concentration at which maximum precipitation occurred was not a t a definite point for all waters. Because of the wide range of insolubility of alum floc, that there was no reason for residual alum to pass filter beds in objectionable quantities was also proved by them. Smith in 1920 (25) determined the optimum concentration for the precipitation of hydrated aluminum oxide for the re-
Value of Hydrogen Ion Concentration to Coagulation In 1916 Blum ( 4 ) ,studying the reaction of sodium hydroxide and aluminum chloride, noted that precipitation of the hydrated oxide took place between pH values of 3.0 and 7.0. He stated that “it therefore becomes desirable to select the degree of alkalinity which will insure most nearly complete precipitation and a t the same time avoid resolution of the precipitate”. The fact that alum floc would redissolve as a function of pH value was not recognized by water plant operators until some years later. About two decades ago Morison (24) reported upon a series of studies governing the clarification of the water supply of Ponna, India. He pointed out that “by increasing the dose of alum up to a certain point, the clarification improves up to a n optimum where the settlement is excellent; any alum in excess of this optimum point again results in incomplete settlement”. This optimum was found by him to be a t a concentration of the alkalinity necessary to react completely with half the weight of the alum. He concluded that probable precipitation of the colloids were not so much due t o the neutralization of the charge of one sol by an electrolyte, but that the acid and base ion balance in the solvent were a controlling factor. 263
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moval of silicic acid to be a t pH 8.0. He also discovered that silicic acid retards the coagulation of clay suspensoids. The classic study of Theriault and Clark (29) established the value of a precise hydrogen ion control for floc formation. This work indicated that a pH value of 5.5 was the optimum condition for floc formation for dilute solutions and that the phenomenon occurred in a narrow zone of not more than about 0.3 pH. They considered that a minimum time of flocculation would occur only at the isoelectric point. It was also noted that the floc forming in a minimum of time settled most rapidly.
n ‘
L I l l
~ “ 1 1 0
FIGURE1. ASSEMBLYFOR COAGUL~4TIONCONTROL CORDING POTENTIOMETER
WITH
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a definite formula of A1,03.S03to the floc containing sulfates. In a later article (21) he advanced the proposition that this floc compound was not due to adsorption but was a solid solution phenomenon. since it behaved as a single solid phase in its displacement relation. In this report he pointed out significantly: “In water purification by alum, it may be that the composition of the raw water, especially with respect to the negative ions it contains, and the character of the negative ions in the reagents added, will have a profound influence on the character and efficiency of the floc formed.” The excessive alum concentrations of 100 to 4000 p. p. m. used by Miller in his study of this floc caused Hopkins (18) to repeat this work in 1924 with quantities from 50 to 200 p. p. m. These solutions were mechanically agitated a t 100 p, p. m., and the peripheral relocity of water in the test beakers duplicated actual plant operation. The study with these more dilute solutions demonstrated that sulfate could be washed from the floc, but that if the concentration were raised above 10 p. p. m. it was present as a function of the pH value a t which the floc was precipitated. In this publication Miller’s formula of a definite compound r a s supported, although it is now believed that this phenomenon is one of adsorption rather than of a solid solution. This belief is supported by the work of Charriou (7) which showed that an increase in particle size of the sol was a direct function of pH value, and that these particles decreased under alkaline conditions. He also reported that divalent ions found in the precipitate could be displaced by ions of a higher valence. Therefore, this would indicate that the larger particles having a greater surface area adsorb more sulfate, and that floc precipitated a t low hydrogen ion concentration utilizes this sulfate as a solution link, since it was impossible to wash it free of this compound without dispersing the floc itself.
RE-
A , inlet line from mixing basin; B . “Standard” Calomel flow cell C, Chapman suction pump
Baylis (9) in the same year recognized the value of pH control and demonstrated that a maximum precipitation of alum floc occurred a t a p H value of 5.5; this confirmed the study made by Theriault and Clark. His work was based upon actual demonstration of residual alum in solution, and the concentrations used were of magnitudes comparable t o that in water purification processes. This confirmatory work was accepted by the water works profession as establishing the value of pH control for coagulation. Continuing the initial work of Clark and Lubs, Miller (20) a t the Hygienic Laboratory demonstrated that floc precipitated a t definite pH values was not completely hydrated aluminum oxide but contained sulfate. Miller’s work is characterized by two factors that were not comparable to water plant operation. He did not mechanically agitate the solutions but shook them by hand in a bottle; and the concentrations of alum were in excess of that normally used in the coagulation of water. He found that a t pH values between 4.0 and 5.5 the ratio of sulfate to aluminum is constant, but that above this value sulfate could be washed from the hydrated floc, and that the amount removed was a function of the increasing pH value. At about pH 9.0, when the stoichiometric quantity of alkali was added, none remained. Miller also noted that the maximum insolubility was between pH 6.7 and 7.0, confirming Wolman and Hanan’s original postulate. I n addition, he reported that this zone of insolubility extended between pH 5.4 and 8.5 with “no general agreement as to the particular p H a t which aluminum is most insoluble under these varying conditions”. The greatest optimum concentration of sulfates was also found in this zone. At points below the stoichiometric relationship, Miller assigned
FIGURE2. COMPACT FLOC Miller (22) later indicated the importance of anion content of the water for floc coagulation. He showed that “the hydrogen ion zone of coagulation may be controlled a t will by varying the negative ions present in solution. Coagulation must therefore be partially dependent upon the coagulating effect of the anions and partially upon the hydrogen ion concentration. Although it appears that polyvalent ions are in general more efficient than monovalent anions in producing
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confirmed the laboratory data previously obtained and proved that the sulfate ion displaced the optimum coagulating pH value in direct proportion to its concentration. In every instance this was toward the acid side.
Mixing Basin Design
FIGURE3. A PROPERLY PROPORTIONED “ROUND-THEEND”MIXINGBASIN
Much has been written on the value of efficient mixing to the formation of alum floc. Velocities of mixing have been recorded which vary from 0.5 to 2.0 feet per second. Langelier (19) in 1921 concluded that the initial velocity in a mixing basin should be high-namely, 2.0 feet per second-with a gradual reduction a t each succeeding step until the last portion, which should just keep the floc in suspension. He also concluded that an extended mixing period from 20 to 40 minutes is necessary, depending upon the type of basin or mechanical agitator employed. This principle of design was confirmed by later experiments utilizing coagulation with pH control and is standard practice today. Utilizing the mixing basin a t Baltimore, Hopkins in 1923 (16) disclosed that after addition of the coagulant, the hydrogen ion content of the water did not vary throughout the basin. This fact proved rapid diffusion of the alum solution with immediate dissociation and stabilization of the hydrogen ion concentration. This study also supported Langelier’s early theory that an initial rapid mixing, followed by a definite period of agitation, produced the best floc. By obtaining this data with coagulation a t the optimum pH value, mixing basin design was placed upon the fundamental principle of colloid aggregation into clumps by agitation, in accordance with the von Weimarn theory. Later studies by Cox (10) a t Reading, Penna., demonstrated that water which coagulated a t the optimum pH value settled rapidly when mixed a t a definite velocity of flow, for a given period of time. These tests initially established the relation between floc formation and sedimentation period. Ellms (11) at Cleveland, Ohio, accepted the principle of a rapid initial mix, followed by decreased agitation, for floc formation in the design of the hydraulic jump flume (Figure 4). This device has proved to be a satisfactory mixing installation. Later, Smith (d7) recognizing that floc insufficiently compacted by inadequate mixing will not be satisfactorily deposited in sedimentation basins, developed the flocculator (Figure 5). These devices are installed a t the inlet of sedimentation basins t o prolong the agitating period and thereby increase the von Weimarn clumping action (6).
coagulation, the effect of each anion seems to be specific in determining the particular regions of hydrogen ion concentration over which flocculation will occur. The sulfate anion is unique among the anions studied, in that it produces (under the conditions described) a comparatively good floc over a broad range of hydrogen ion concentration. The existence of a colloidal, opalescent suspension has never been observed with pure alum solution. It occurred with all other anions studied”. It became apparent that Theriault and Clark’s study, based upon a rapid flocculation of alum, and those of Hatfield and of Baylis, based upon the maximum flocculation, were in close agreement, although these hydrogen ion concentration relations may not necessarily coincide because of the adsorption of sulfate by the floc. This fact, that waters of varying anion concentrations will flocculate a t different pH values, reconciled the unexplained discrepancies obtained by various investigators. This study indicated that three factors are necessary for successful clarification of water: There must be a minimum quantity of aluminum ion; a strong anion such as sulfate must be present; and the hydrogen ion concentration must be a t a definite value. I n 1928 Peterson and Bartow (25) determined the effect of anions as dissolved buffer salts on the initial formation of alum floc. I n this study the alkali was added to the alum solution and the mixture was mechanically agitated a t precise speeds for definite periods. Floc formation was determined by the initial appearance of coagulation in a Tyndall beam. Concentrations approximating 34 p. p. m. of alum were used. Color Flocculation Varying concentrations of different anions proved conclusively The removal of turbidity is but one of the problems enthat the sulfate ion moved the initial flocculation point to the countered in water purification. Upon the establishment of acid side. This shifting below neutrality was in direct proportion to the salt concentration present. A concentration of 25 p. p. m. had little affect, but 500 p. p. m. decreased the pH about 2 units. Chlorides and carbonates were of little value in decreasing the optimum pH point, even in concentrations of 500 p. p. m. Additional confirmatory work as to the value of the sulfate ion in regulating the optimum pH value of alum precipitation was reported by Black, Rice, and Bartow ( 3 ) . They employed mechanical stirring a t 52 r. p. m. with rotating paddles and 34 p. p. m. of alum. The time for the initial formation of floc by this controlled method was in accordance with Peterson and Bartow’s previous Courtesy, K. J . W.Ellms experiment. These investigators extended FIGURE 4. HYDRACLIC JUMP F L U M~ their work to the operation of the experimental A . Inlet pipe E. Turbulent section of jump B. Rising well F. Effluent end of Bume water plant with a capacity of 310 gallons per C. Throat t o flume G. Settling basin hour a t the University of Florida. This study D. Sloping and expanding seation of flume
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Courtesu, M . C. Smith
FIGURE 5. FLOCCULATOR
p H control for alum floc, the procedure was naturally extended to other fields of coagulation. The removal of organic color fromywaters is probably the most classic precipitation of a trueccolloid encountered in water purification, since Saville demonstrated many years ago that these particles carry a negative charge and obey the laws of cataphoresis. Previous work by Hale (13)had indicated that color reactions were a stoichiometric function. It remained for Miller to support Saville’s original conclusions (26) and definitely demonstrate that colored waters from organic materials carry negative colloidal charges. He also proved that the best clarification can be obtained around the pH value of 5.4. His work demonstrated that this coagulation is generally produced by the positive aluminum ion. Coagulation a t this low pH value necessitates a final increase of the pH to prevent pipe corrosion. It is noted that such procedure causes a secondary flocculation of the hydrated aluminum oxide. With the development of ferric coagulants, Hedgepeth and Olsen (16) demonstrated that chlorinated ferrous sulfate is an effective coagulant at a pH of 4.8, provided a secondary flocculation is obtained with sodium aluminate a t a value of 6.6. This procedure reduced the normal color of Cape Fear River water from 500 to 7 p. p. m. and established the general use of these coagulants for the purpose. Coagulation with Iron Salts For many years it has been recognized that coagulation with ferrous sulfate and lime a t high p H values effectively removes turbidity in the hard waters of the West. This procedure has been successfully practiced by the aid of phenolphthalein indicator and was originally advocated by C. Arthur Brown. A study by Miller (23) of the hydrated oxide floc obtained from ferric salts indicated that this material is not amphoteric and therefore does not redissolve a t high pH values. He also demonstrated that floc formed from these salts was quantitatively precipitated a t pH values above 3.0. This floc adsorbed sulfate as a function of pH value, similarly to the alum floc. Continuing these investigations with floc produced by fer-
ric salts, Bartow, Black, and Sansbury ( f ) obtained aminimum time for floc formation, with 27 p. p. m. of coagulant, in a zone between pH 4.8 and 7.0. Upon addition of 25 p. p. m. of sulfate ion, this zone extended to 3.5 on the acid side. They also found a zone between pH 7.0 and 8.0 in which floc did not form within a 45-minute stirring period, followed by flocculation again a t pH value above 8.1 to 9.6. This is in sharp contrast to previous coagulation studies with ferrous sulfate and lime and also with chlorinated copperas made by Hopkins (17 ) . I n this older work the maximum precipitation of floc from ferrous sulfate and lime, as indicated by the analysis of residual iron, was a t a pH value of about 9.4. Actual plant operating results over many years at Baltimore support this laboratory data in that the removal of iron and manganese is accomplished a t a pH of 9.0 with only 0.03 p. p. m. of residual iron in solution. The maximum turbidity removal is a t the same pH value. Additional experiments with coagulation from chlorinated copperas gave a compact floc a t any pH value above 3.5. Analysis of the various hydrated oxides for sulfate disclosed that these anions were not adsorbed when less than 85 p. p. m. of ferric salts were used. When present a t higher concentrations of coagulant, sulfate could not be washed from the floc precipitated a t pH values on the acid side but could be removed as a function of decreasing hydrogen ion, t o that of maximum precipitation a t pH 9.4. This factor indicates that the sulfate is adsorbed by the hydrated oxide and may be considered as a “solution link”; it thus stabilizes the sol a t low pH values in accordance with the theory advanced by Thomas and Frieden (30). This fact is further supported in that the stability of a sol is increased with increase in hydrogen ion concentration; therefore, adsorption of precipitating ion would be influenced by the pH of the sol. “Adsorption of sulfate falls off quite rapidly with increasing concentration of the hydroxyl ion, the carrying down of sulfate being completely nullified a t a pH value of about 9.2” ( S I ) . Therefore, as the hydrogen ion concentration decreases, sulfate may be washed from the precipitate. This also indicates that the isoelectric point occurs a t a pH of 9.0, since this is the point of minimum solubility and also the point showing ab-
INDUSTRIAL AND ENGINEERING CHEMISTRY
FEBRUARY, 1940
sen= of adsorbed sulfate. This theory is a t variancewith the opinion that these compounds have a definite formula as previously quoted.
Surface Adsorption Soluble manganese and iron may be removed from water by aeration followed by adsorption in a contact bed of coke, pyrolusite, or manganese or iron-coated gravel or stone. It is believed that tho initial precipitation of the hydrated oxide in the bed is due to stoichiometric reactions following the conversion of the bicarbonate to the hydrated oxide. However, when the initial film of oxide has been precipitated in the bed, i t catalytically TemoYes these soluble salts as a phenomenon of surface adsorption followed by oxidation (9). Utilization of these phenomena is the hasis for the purification of many %-ellsupplies.
Literature Cited (11 . . Baitow. Erlwmrd, . ...Black, .. A. P., and Ssnshury. W. E., IND.Exo.
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CIIEX., 25, W 8 (1WdJ. (2) Ba~lis.J. R.. J . Am. Wctm A'orks Asuoc.. 10,366 (1923).
(3) Black, A.
P.,Rioe. Owen, and Bartow, Edward, IND.
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Came., 25. 811 (1933). (4) Blum. William. .I. Am. Chem. Soc., 38,7 (1916). (5) Buswell, A. M., "Chemistry of Water and Sewage", p. 47, New
York. Chemic81 Catulog Co., 1928.
(6) . . Buswell. A. M.. and Ed\rmdu. E. P., Chem. & Met. Ew.. 28.
826 (1922). (7) Clumiou. A,, Cornpt. Tend., 176, 679 (1933) (8) Clark. W.M.. and Lubr, f l . A,, J . Aact.. 2, 1, 109,191 (1917).
267
Corson. H. P., Univ. Illinois. Bull. 13, 145 (1915), (1915). Cor, C C.. R., Eng. News-Record, 93, 101 (1994). Ellms, J. W., "Water Purification", 2nd ed.. p. 74, New York. McGiaw-Hill Book Co., 1928. Fuller, G.W., "W-ater Purifioetion at Louisville". p. 271, New York. D. Van Nostrnnd Co., 1898. Hale. F. E., J. IND.ENG.Cnx;~., 6, 632 (1014).
Hatfield. N.D.. Zbid., 14,1038 (1922). Hedgepeth, L. L., and Olsen. W. C., J . Am. Water Ii'orks Assoo., 20, 467 (1928). (16) Wopkins, E. S., Ew. News-Record. 90,204 (1923). (17) flopkine, E. S.. IND.ENU. CHEM.,21, 581 (1929). (18) Eopkins, E. S..J . Am. Water Wodm A ~ M c . 12, , 405 (1924). (19) Lsngelier, N. F., Eng. News-Record, 86, 924 (1921). (20) Miller. L. B,, Pub. Health Rep&.. 38, 1995 (1023). (21) r&d., 39, 1502 (1924). (22) Zbid.. 40,351 (1925). (23) Ibtd., 40, 1413 (1925). (24) Moriaon. J.,Indian .I. Medical Research, 3. 4 (April, 1916). (25) Peterson. B. H., and Bartow, Edward. IND. ENG.CHEM..20, 51
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(26) Saville, Thorndike, J . New E n d . Water Works Assoc., 31. 78 (10171. (97) Sn&h. M. C., Wder Wmks & Sewerwe, 79, 103 (1932). (28) Smith. 0.XI., J . Am. C h a . Soe., 42,460 (1920). (99) Therimlt, E. J.. and Clark, W. M., Pub. Health Repts., 38, 181 (1923). (30) Thomad. A. N., and Frieden, A.. J. Am. Chem. Soc., 45, 2522
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(31) Weiser, H. B.. "Hydrous Oxides", p. 142,Now York, MoGrawIiill Book Go.. 1926. (32) Wolman, Abel, and Hanan, Frank,Chem. & Met. Ew.. 24, 728 (1921). PBEBENTBD before the Diviaion of Water, Sewage. and Sanitation Chemistry at the 97th Meeting of the Arnericnn Chemical Society. Baltimore, Md.
L'ALCHEMIST By Tnoh4As WIJCK (1617-1677)
T h r o u g h rhr kind cooperation of Mr. Arthur Linr n e are enabled m bring as No. 110 in the Beiolzheimrr series of Alchemical and Historical Reprodrrctions anorher painting by char master of somber arc, Thomas Wijck. Our half-tom CUT war made from an cngraving by L. LeGrand, purchased by Mr. Linr in Paris last summer. Thc engraving berrs the following inscription: "TiiC du Cabiner de Mr. IC Bran d'apds IC Tablcru Original de Thomas Wyck dc la Grandeur de 24 p a c e sur 22 de large. "A Paris c h h Cheican, rue de Mathuiins au coin de cclle dc Sorbonnc." Atrcntion is called to the strong rescmblvlce of this work of rhe Dutch master to No. 5 in rhc series, also by Wijck, but engisved by Tcnier. Thc mom is undoubtedly rhc same one aod many of rhe "props" are idenrical in the two pictures.
D.D. B ~ n o i z n s i ~ s n Strcct New York. N. Y. 50 East 41st
