current research Division of Colloid and Surface Chemistry, joint with the Divisions of Petroleum Chemistry and Water, Air, and Waste Chemistry, this symposium was presented at 156th Meeting. ACS,Atlantic City, N.J., September 1968. Those presiding were: Hendrik van Olphen, Chairman Office of Critical Tables National Academy of Sciences 21 01 Constitution Ave. Washington, D.C.
S Y M P O S I U M O N COLLOID A N D
C. R. O’Melia, Cochairman School of Public Health University of North Carolina Chapel Hill, N. C.
SURFACE C H E M I S T R Y IN A I R A N D WATER POLLUTION
G. M. Hidy, Cochairman Science Center N. A. Rockwell Corp. Thousand Oaks, Calif.
The first two articles in this issue, pages 825-35and the first three articles in the June issue, pages 551-67, were given a t this symposium. Other papers from this symposium will appear in subsequent issues.
Flocculation of Negatively Charged Colloids by Tnorganic Cations and Anionic Polyelectrolytes Arthur S. Teot and Stacy L. Daniels Chemicals Laboratory, The Dow Chemical Co., Midland, Mich. 48640
The rates and degrees of flocculation of a monodisperse synthetic latex and a polydisperse raw sewage were determined optically after each system had been treated sequentially with inorganic cations and anionic polyelectrolytes. These anionic colloids are not flocculated significantly by anionic polyelectrolytes in the absence of multivalent cations. A lower concentration of multivalent cation was required to flocculate the monodisperse latex in the presence of an anionic polyelectrolyte. The cation must be in an unhydrolyzed form, however, before this synergism occurs. Both rates and degrees of flocculation of raw sewage were logarithmic functions of polyelectrolyte and multivalent cation concentrations. 1
I
t is not immediately apparent why anionic polyelectro-
lytes should flocculate negatively charged colloids. Flocculation does not occur in the absence of appropriate cations. Flocculating AgBr/Br- with gelatin, Nemeth and Matijevic (1968) found that the concentration of simple electrolytes required to cause flocculation followed the Schulze-Hardy rule regarding the valence of the electrolyte. The) concluded that the polyelectrolyte functioned by adsorbing onto the surface of the AgBr/Br- and reduced the surface potential. The simple electrolytes were effective at lower concentrations, assuming a classical double-layer compression mechanism. Sommerauer, Sussman, et a/. (1968) also observed that a lower electrolyte concentration was required to coagulate AgBr/Br- in the presence of sodium polyacrylate or sodium polystyrene sulfonate. They found the concentration of Ca2+
and Cuzt required for coagulation to be below that necessary to produce complex formation in the bulk of the solution. They proposed that complex formation occurs in the electrical double layer adjacent to the colloidal surface where the counterion concentration is greater than in the bulk. Complex formation is then followed by attachment of the polymer to the colloid surface and bridging to cause flocculation. To define further the factors involved in the flocculation of anionic colloids by anionic polyelectrolytes, a well-characterized monodisperse latex was used as a model colloid. Prior work on coagulation of latexes with metal ions has been reported by Ottewill and Shaw (1966) and Force and Matijevic (1968). The effects of various metal cations, metal ion valence, and pH upon flocculation of this latex were studied. A second aspect of this work was to correlate the flocculation of monodisperse latex particles with the flocculation of raw sewage colloids. This extension is of importance from a practical viewpoint. It is also virtually impossible to do mechanistic work on the heterogeneous colloids contained in sewage. Sewage colloids have modest negative charges, but can be flocculated by anionic polymers. Production of small flocs in sewage by a multivalent metal counterion, such as calcium or aluminum, is necessary prior to the addition of the anionic polyelectrolyte. It has also been suggested that the flocculation of kaolinite by hydrolyzed polyacrylamide labeled with is dependent upon Ca2+ to reduce particle-particle, polyelectrolyte-particle, and adsorbed polyelectrolyte-polyelectrolyte repulsive forces (Black, Birkner, et a/., 1965).
Materials and Methods The monodisperse polystyrene latex was prepared using a sulfonate emulsifier. Its diameter was determined by electron microscopy to be 1090 27 A. (Bradford, Vanderhoff, et al.,
*
Volume 3, Number 9, September 1969 825
1956). It was purified by dialysis, and titration with a cationic surfactant yielded a charge of 2.4 x 104 e.s.u. per sq. cm. of geometrical latex surface or one charge per 200 sq. A. of surface area. The raw sewage studied had the following analysis: suspended solids, 162 mg. per liter; hardness, 28 mg. per liter as Ca2+;alkalinity, 128 mg. per liter as HC03-; and pH 6.9. This sewage was diluted with water of comparable hardness and alkalinity, to 16.2 mg. per liter of suspended solids. The polyelectrolyte used in the studies of latex flocculation was dialyzed sodium polystyrene sulfonate (SPSS), having a molecular weight of approximately 8,000,000 as determined by viscosity. Commercially available SPSS (Purifloc A-21 flocculant, Dow Chemical Co.) was used on the sewage. Flocculation of the latex was monitored by measuring transmittance of monochromatic light of 5460-A. wavelength in vacuo using a Brice-Phoenix photometer. Flocculation of sewage was monitored by measuring transmittance of white light through a suspension of sewage contained in a 3-liter glass resin kettle. Microelectrophoretic measurements were made with the Zetameter (Zetameter, Inc., New York, N. Y . ) . The critical coagulation concentration and critical restabilization concentration (ccc and csc, respectively) for the latex flocculations were determined by the reciprocal time extrapolation method (Teot, 1969), previously shown to correlate with the turbidity extrapolation method (Tezak, Matijevic, et ul., 1951). Dilute solutions of the latex are fairly transparent optically because of the small particle size. Addition of flocculating agent initially decreases transmittance as the average particle size increases. This decrease is followed by an increase due to sedimentation of the flocculated latex out of the light beam. Two parameters, derived from time ( t ) - transmittance ( T ) data, were used to describe the flocculation of the nonspherical, polydisperse sewage colloids. The initial rate of flocculation, during the experiment, r F , is defined as the slope, (fl/dt)t=a, of the transmittance us. time curve, measured at the time of polyelectrolyte addition. The degree of flocculation, ep, is defined as a dimensionless absorbance ratio (1 - A/Ao)l=lsmin, where the absorbance, A = -log& The degree is zero in the case of no flocculation and approaches unity for a theoretically ideal flocculation. It is measured after the polyelectrolyte has had sufficient time (15 minutes) to exert essentially all of its flocculation ability. The flocculated sewage particles remain in suspension during the experiment, although the particle number decreases and the average particle size increases. Experimental Results Flocculation of Monodisperse Latex. The flocculation abilities of various metal ions were first studied as functions of pH. The critical coagulation (ccc) and critical restabilization concentration (csc) of monodisperse latex with Al(N0~)afor the pH range 2 to 6 are shown in Figure 1. The pH of the latex and metal solutions was preadjusted to the required levels before mixing. The appearance of the diagram is practically identical with a similar plot for AgBr/Br- (Matijevic, Mathai, et ul., 1961). Both AgBr and polystyrene latex are, therefore, presumed to be classical lyophobic colloids. Higher absolute concentrations of aluminum salt are required in the case of the latex because of its greater inherent stability. Since the surfaces of AgBr and polystyrene latex are markedly different, the flocculation differences at various acidities must depend upon the properties of the solution. The Ala+ion species in solution coordinates with hydroxyl ion 826 Environmental Science & Technology
PH
Figure 1. Critical coagulation (ccc) and critical restabilization (csc) concentration of 1 X l o w 4gram per cc., 10904. diameter, monodisperse latex by AI(RTO& at various acidities
- - -
in a manner approximate by the following equation : Ala+
A10H2+
Al(OH)+
Als(OH)zs4T, etc.
+Al(0H)a
Acidic t
Al(OH)d-
*alkaline
In strongly acidic solutions, the species causing flocculation is Al(H20)03+. In solutions of pH approximately 4 to 7, cationic polynuclear hydroxo complexes are the accepted structure. The aluminum hydroxo complexes are about 30 times more effective as flocculants than Ala+. Matijevic, Janauer, et nl. (1964) believe that this is due to Als(OH)204+, which acts as a quadrivalent ion at low total metal concentrations. Excess concentrations of aluminum result in electrostatic charge reversal and restabilization of the colloids in the pH 4 to 5 region but not at pH 5 3. Charge reversal is caused by adsorption of the aluminum hydroxo complexes at higher concentrations. A plot of the counterion valence cs. the logarithm of the concentrations of Na+, Ca2+,and Ala' required to flocculate the standard 1090-A. latex is shown in Figure 2. The solutions were acidic (pH 2) to ensure coordination of the metal ions only with water. The coagulation ratios for mono-, di-, and trivalent ions should be 1 to (1/2)6 to (1/3)6, respectively, according to a theoretically derived relationship (Verwey and Overbeek, 1948). A straight line is a better fit of the experimental data. This feature has been observed with other colloids (Matijevic, Broadhurst, et al., 1959). The curved relationship of Verwey and Overbeek is converted to a straight line on a semilogarithmic plot if the discrete ion cffect is taken into account (Levine and Bell, 1962). Charge reversal readily occurs with ferric and aluminum salts but not with La3+ in the pH region investigated, because of hydroxo complex formation with the two former cations. These metal cations are all trivalent in highly acidic solution, but differ in coordination ability for hydroxyl ions in aqueous solution. Their ccc values are compared in Figure 3. The order of hydroxo coordination of these metals is Fe>Al >> La. This is also the general order of their flocculation activities as
c- I
I
I
I
4
5
6
I
I U
I
1
1
2
3
VAL E NC E
Figure 2. Relationship between Falence of coagulating cation and experimentally determined optimum coagulation concentration ( 0 ) for 1 X 10 gram per cc.. 1090-4. diameter monodisperse latex
- - - From Verwe, -0ierbeek theorj -Discrete ion effect judged by the relative acidities where a lower concentration of metal ion causes flocculation. Essentially the same molar concentration of all ions is required for coagulation if the pH is low. This concentration is in agreement with that predicted by the Schulze-Hardy rule for trivalent species. The ccc values of aluminum or ferric ions, at concentrations where appreciable amounts of hydroxo complexes form, are much lower. This demonstrates the superiority of the complex ions over the simple ions. hfetal Ions plus bionic Polyelectrolytes. Aluminum and iron salts have been used for many years as coagulants in water and waste treatment to supplement the naturally occurring divalent cations. Aluminum was the metal ion chosen for evaluation in systems containing both SPSS and monodisperse latex. According to certain authors (Cohen, Rourke, et d., 1958), in water treatment, “prior production of floc with alum is a requisite for the benefits (faster rates and less residual turbidity) obtainable from Anionic A (hydrolyzed polyacrylamide) treatment.” These authors were dealing with “sweep” coagulation (Stumm and O’Melia, 1968) in which a
LIj
I 3
PH
Figure 3. Critical coagulation concentration (ccc) of Fe(NO&, Al(NO3)3, and La(NO& at various acidities for 1 X gram per cc., 1090-A. diameter, monodisperse latex
voluminous hydrated alumina precipitate mechanically entrapped the particles, causing turbidity. The function of the polyelectrolyte was to collect this precipitate into larger and faster settling flocs. In the present work, only soluble aluminum species exist in the latex suspensions. The total concentration is belo& the ccc, so that negligible flocculation occurs when the metal salt (or polyelectrolyte) is added singly. In the case of raw sewage, the aluminurn salt solution was acidic but the sewage was basic. The aluminum species are initially soluble. A series of flocculations of latex was conducted by selecting an AI(NOJd concentration below the ccc and treating successively in separate experiments with increasing concentrations of SPSS. Flocculation was studied for the pH range 2 to 6. A different AI(NO& concentration was employed at each pH to remain below the ccc. Results obtained for Al(NO& SPSS are shown on the second line in Table I. The ccc for
+
Table 1. Flocculation of Latexu by SPSS and Various Metal Salts at Different pH and Molarities (M) pH 2
hZ 63 25 45 33 50 38 1:
1, c
PH 5
Qua1.l
G G G G G G
M .__
2 2 3 2 31 25
5 4 5 4
Qual.
G N G N G G
M
-
2 2 3 2 23 19
-
4 3 0 9
M
Qual. ~
~
G N G N G G
2.9
1.5 -
22 19
Qual. __
G N ._ ._
G G
MonodispeIsepol)si!rene latex. 1090 A,, 1 X g./dc. g./cc. Quality offlocculation (G good, S some, N none). Critical coagulation concentration (csc) for metal ion alone ( X 106)). Selected metal ion concentration etnployed with SPSS ( X 106).
Volume 3, Number 9, September 1969 827
Al(N03)3 coagulation at various acidities is included in the first line for comparison. The polyelectrolyte does not cause flocculation of the latex when the aluminum ion concentration is below the ccc, except at pH
