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Langmuir 1990, 6, 596-602
Flocculation of Silica Colloids with Hydroxy Aluminum Polycations. Relation between Floc Structure and Aggregation Mechanisms J. Y. Bottero,*pt D. Tchoubar,$ M. A. V. Axelos,s P. Quienne,s and F. Fiessingerl CRVM ( E N S G ) , U A 235 CNRS, GS Traitement Chimique des Eaux, BP 40,54501 Vandoeuvre Cedex, France, Laboratoire de Cristallographie, U A 810 CNRS, GS Traitement Chimique des eaux, BP 6759, 45067 OrlBans Cedex 2, France, Laboratoire de physico-chimie des macromolhcules, I N R A BP 527, 44026 Nantes Cedex 03, France, and Lyonnaise des eaux, 44 rue de Lisbonne, 75008 Paris, France Received June 1, 1989. I n Final Form: September 26, 1989 The aggregation of silica colloids by various aluminum hydroxide polycations (Al13,octahedral polycations) has been studied. Both physicochemical variables, such as { potential and settling velocity, and floc structure on a semilocal scale (10-1200 A, by means of SAXS) have been studied as a function of chemical nature, concentration and size of the flocculant, and pH of the silica suspension. Between pH 4.5-5.0 and 7.0-7.5, the flocs have generally a fractal geometry with a fractal dimension varying from 1.7 to 2.2. Their effective density calculated from the distance distribution function obtained from SAXS was highest at low pH and with All, polycations as flocculants. When the pH increased, the density decreased but remained the highest for All, flocculants. The settling velocity and the effective density were conversely varied. The mechanisms of the flocculation depended on the reaction pH. A t low pH, the charge neutralization was the main driving force for the flocculation. At neutral pH, both charge neutralization and bridging were responsible for the formation of less dense flocs.
Introduction Coagulation and flocculation processes are important from industrial and environmental points of view; characterization and control of the flocs or sludges produced require control of the properties of the suspensions' and floc size distribution' and the development of on-line monitoring The efficiency of the liquid/solid separation by coagulation depends essentially on the chemical, physicochemical, and hydrodynamic conditions during mixing and transport of the flocs. These factors determine the structure of the aggregates, i.e., the density and the strength the flocs acquire. The first step of the flocculation is the adsorption of the flocculant at the liquid/ solid interface. In this step, ionization of the flocculant molecules is of primary importance. Generally, nonionic macromolecular flocculants have a weak affinity for solid surfaces, and the aggregation mechanism is essentially of bridging nature."' The consequence is that the structure of the flocs is loose and fragile. Conversely, the adsorption of cationic organic polyelectrolytes onto negatively charged solid surfaces is strong, and the compact CRVM (ENSG).
* Laboratoire de Cristallographie. Laboratoire de physico-chimie des macromol6cules. Lyonnaise des eaux. (1) Tadros, Th. F. Colloids S u r f . 1986,18,137. (2)Cahill, J.; Cummins, E. J.; Staples, E. J.; Thompson, L. Colloids S u r f . 1986,18, 189. (3)Gregory, J.; Nelson, D. W. Colloids Surf. 1986,18,175. (4) Eisenlauer, J.; Horn, D. Colloids Surf. 1985, 14, 121. Jennings, B. R.; Parslow, K. Colloids Surf. 1986, (5)Trimm, H.H.; 18,113. (6)Bottero, J. Y.;Bruant, M.; Cases, J. M.; Canet, D.; Fiessinger, F. J . Colloid Interface Sci. 1988,124,2,515. (7)Griot, 0.; Kitchener, J. A. Trans. Faraday. SOC.1965,61,1022. (8) Papenhuijzen, J.; Van der Schee, H. A.; Fleer, G. J. J . Colloid Interface Sci. 1985,104,540. (9)Gregory, J. J. Colloid. Interface. Sci. 1976,55, 35. (10)Mabire. F.;Audebert, R.; Quivoron, C. J . Colloid Interface Sci. 1984,97,120.
0743-7463/90/2406-0596$02.50/0
structure of the adsorbed layers leads to dense flocs. The mechanism is then one of charge neutralization.sJO The size of the flocs can change under the action of fluid stresses. From laboratory experiments with negative latex flocculated by NaC1, it was found that the size of flocs was proportional to the shear rate y raised to a power which is a function of the fractal dimension." In many industrial processes, aluminum or iron hydroxides are used as flocculants. It has been found that the breaking up of flocs obtained by precipitation of aluminum hydroxide in organic medium is sensitive to the shape of the mixing reactor." Although there are now many theories on the kinetics and thermodynamical aspect of coagulation-floc~ulation,~~~'~ few direct data exist on the structure of flocs obtained by heterocoagulation of negative mineral particles by positive mineral "particles". The coagulation-flocculation of negative colloids by aluminum hydroxide particles is of outstanding importance in soils and for water purification. The aim of this paper is to describe the structure of flocs, at the semilocal scale, obtained by mixing silica sols with various aluminum hydroxide species. To this end, we use small-angle X-ray scattering and try to relate the data to the macroscopic behavior during flocculation and settling.
Materials Ludox HS 40 (Du Pont de Nemours) silica colloids have been used throughout. The mean size is -14 nm in diameter as deduced from the manufacturer specification and from data obtained by small-angle X-ray scattering or small-angle neutron(11)Sonntag, R. C.;Russel, W. B. J. Colloid Interface Sci. 1986,113, 399. (12)Clark, M. Presented at the Annual A.W.W.A. Conference, Denver, 1986. (13)Ramsay, J. D.F.; Booth, B. 0. J. Chem. SOC.,Faraday. Trans. 1 1983,79,173.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. 3, 1990 597
Flocculation of Silica Colloids INITIAL SOLUTIONS 2.0
2.5
I
2.6
I I
Table I. Dosages of Aluminum Flocculants after Mixing with 0.8% Silica Suspension aluminum dosaee. ” . mol/L I
R
lo-’
2 X 1V8
2 2.5
+
+
2.6
WAC
Figure 1. Flocculants used in the present study ohtained by hydrolyzing CI,AI(H,O), with NaOH. For H = OH AI = 2.5 and 2.6, the flocculant is comprised of AI,, fractal aggregates with a fractal dimension D, = 1.43 and 1.72, respectively. scattering technique^?^." The sign of the surface charge determined by measuring the electrokinetic potential was negative over the whole range pH >2?5,16 All the experimentswere carried out at pH -4.5 or -7.5 so that the silica surface charge was always negative. The concentration of the silica sols was 0.8%. Two kinds of aluminum hydroxide flocculanta were used. The first were obtained by hydrolysis of aluminum chloride with sodium hydroxide following a procedure described previously.” The initial hydrolysis ratio of the aluminum hydroxides was R = [OH]/[Al] = 2, 2.5, and 2.6. The initial aluminum concentration was 0.1 M,and the pH was varied from -4.0 (R= 2) to 5.6 (R = 2.6). So the aluminum species present are mainly monomers and the polycation A1,,040H,,(H,0),~+. In R = 2.0 solution, there are only “AI,,” polycation and monomers (-10-20%).’T~18 The AI ions build fractal aggregates in R = 2.5 and R = 2.6 solution^?^ For R = 2.5, the fresh aggregates contained -69 AI,, and have a fractal dimension D, = 1.43 which corresponds to a very linear “polymer”. Fur R = 2.6, the aggregates are built with more than 512 AI,, and D, 1.86?0.21which correspondsto a branched ‘polymer” (Figure 1). The second aluminum flocdant used was an industrial hydrolyzed aluminum chlorosulfate named WAC (Atochem)with 3% of SO,2- ions. The pH of the mother solution was -2.2. The species present in the mother solution were essentially monomers and chains of octahedral aluminum?’
-
Experimental Procedure Focculation experiments were carried out by rapid mixing of a silica sol with a given amount of flocculants (R = 2, 2.5, 2.6) and WAC at a constant pH 4.5 or pH 7.5 following a jar-test procedure which consists of stirring the suspension in a 1-L vessel a t 250 rpm for 2 min and 40 rpm for 20 min; the experimental chemical conditions are given in Table I. The pH was adjusted by using concentrated NaOH. The flocs were allowed to settle in a 50-cm3 glass tube, vertically fixed in a carefully thermostated area and kept out of vibrations. The height of the sediment front was measured versus time. (14) Axelas, M. A. V.; Tchoubar, D.; Bottero, J. Y. Langrnuir 1989, 5,1186-1190. (15) Allen, L. H.; Matijevic, E. J. Colloid Interface Sci. 1969,31,287. (16) Capelle, P. Ph.D. Universith Cstholique de Louvain-La-Neuve, 1987, p 210. (171 Bottero. J. Y.:Cases. J. M.: Fiessineer. I . F.: . Poirier. J. E. J. Phvs. Chi& 1980.84,2933.
(18)Bottero. J. Y.; Marehal, J. P.; Cases, J. M.; Poirier, J. E.; Fiessinger, F. Bull. SOC.Chim. Fr. 1982,1-42,1-43. 1191 Bottero. J. Y.: Axelos. M. A. V.: Tehoubar. D.: Cases. J. M.: Fripi& J. J.; Fiessinger; F. J . Colloid Inierface. Sei: 1961, 117; 47. (20) Axelos. M. A. V.; Tchoubar, D.; Bottero, J. Y.; Fiessinger, F. J. Phys. (Les UlU. Fr.) 1985, 46, 1587. (21) Axelos. M. A. V.: Tehoubar.. D.:. Jullien. R. J. Phvs. . .(Lea Ufis. F~.)1986,54, k 7 0 . (22) Bottero. J. Y.; Tchoubar, D.; Cases, J. M.; Fripist, J. J.; Fie=inger. F. InlnterfocialPhenomena inBiotechnologyondMoterialsPlocossing; Attia, Y. A., Moudgil, B. M.. Chander, S., Eds.; Elsevier Seiences: Amsterdam, 1988: p 459.
+ + +
+ + +
4X
6X
+
8X
+ + + +
+ + +
10.’
+ + + +
t
+ + +
The electrokinetic potential of the flocs versus flocculant concentration was measured a t pH -4.5 and p H -7.5 by using a Laser Zee meter electrophoresis apparatus (Pen Kern Inc). The flocculation efficiency versus flocculant dosage was followed by measuring (i) the turbidity of the suspensions after 20 min of slow stirring and before settling and (ii) the turbidity of the supernataut after allowing the suspension to settle for l h. The optimum flocculation concentration (ofc) was determined as the lowest aluminum dosage (mol/L) corresponding to the lowest turbidity. Solid-state 27A1NMR Experiments. During the dilution of hydrolyzed aluminum in silica suspension at pH 7.0-7.5, further hydrolysis of previous species can proceed. NMR experiments were carried out on flocculants precipitated at pH 7.5 in order to detect possible restructuring on a local scale during hydrolysis. The spectra were obtained at 132 MHz a t magic angle spinning (-54’ and 3.8 KHz). The sequences, recording method, and the method for calculating the AI,, content in the precipitates was described earlier and will not he repeated.’9.23 Small-Angle X-ray scattering experiments were carried out on flocs obtained on both sides of the ofc and a t the ofc for each one of the systems investigated in Table I. The scattering from the suspensions is due t o silica colloids. The scattering by AI species (
