Flocculation, polymer adsorption

Phase titrations have well confirmed that the precipitated phase contains polymer, then the turbidity cannot be due to a flocculation of Al(0H)3 particles without polymer adsorption. pH 7 with added NaCl... 

If only electrostatic effects are responsible for polymer adsorption and flocculation, our results can be explained according to the same scheme as that used by Furusawa et Al.(20) to interprete the destabilization of negatively charged latex by a cationic polymer. In a first step, the adsorption of the polymer leads to the neutralization of the particles which are no more stabilized by electrostatic repulsions and there is flocculation (we have not studied this step since in our experiments polymer was always in large excess with respect to Al(0H)3). In a second step the adsorption inverses the charge and (we have indeed measured by... 

As with experimental work on polymer adsorption, experiments in the area of dispersion stability in the presence of polymers require detailed characterisation of the systems under study and the various controlling parameters (discussed above) to be varied in a systematic way. One should seek the answer to several questions. Is the system (thermodynamically) stable If not, what is the nature of the equilibrium state and what are the kinetics of flocculation If it is stable, under what critical conditions ( s, T, x> p etc.) can flocculation be induced ... 

A major advantage of the simple model described in this paper lies in its potential applicability to the direct evaluation of experimental data. Unfortunately, it is clear from the form of the typical isotherms, especially those for high polymers (large n) that, even with a simple model, this presents considerable difficulty. The problems can be seen clearly by consideration of some typical polymer adsorption data. Experimental isotherms for the adsorption of commercial polymer flocculants on a kaolin clay are shown in Figure 4. These data were obtained, in the usual way, by determination of residual polymer concentrations after equilibration with the solid. In general, such methods are limited at both extremes of the concentration scale. Serious errors arise at low concentration due to loss in precision of the analytical technique and at high concentration because the amount adsorbed is determined by the difference between two large numbers. 

Polymer adsorption is important in the flocculation and stabilization of colloidal sols and has been reviewed by Vincent et al. (1) and Tadros (2). Polyvinyl alcohol (PVA) has been used in these studies because of its practical application in textiles, adhesives, and coatings. The adsorption of PVA has been studied on silver iodide by Fleer (3) and Koopal (4), and on polystyrene (PS) latex particles by Garvey (5). The adsorption isotherms reported by these workers extend up to 600 ppm PVA. The adsorption at... 

Understanding how polymer functions as a stabilizer and flocculant is obviously a problem of polymer adsorption and its conformation at the particle/liquid interface(l, 2). The process of polymer adsorption is fairly complicated the behavior depends on many factors, e.g., the nature of the adsorbent, the molecular weight of the polymer, the temperature, the effect of the solvent,... 

Flocculation is indeed dependent on polymer adsorption, and there are hypotheses correlating the two phenomena, but often these have been put forth without detailed measurement of the two phenomena simultaneously (10-13). In this paper, flocculation is investigated as a function of polymer and solution properties and hydrodynamic conditions by measuring different properties of the system, including adsorption, using well characterized kaolinite and polymer samples prepared specifically for this purpose. Also, the role of concentration and charge density of polyacrylamide and polyacrylamide-polyacrylic acid co-polymers in determining kaolinite flocculation is examined under controlled hydrodynamic conditions. 

These differences in the effect of polymers on various flocculation responses have important theoretical and practical implications and can be explained in terms of various characteristics of floes and floc-aggregates. Polymer adsorption or attachment of particles to polymer can occur in any number of configurations, and as a result the aggregation of particles also can take place in many ways, leading to different floe and suspension structures which will respond differently to different tests. 

In order to generate information on the mechanism of flocculation by polymers it is, however, necessary to correlate flocculation with various system properties, particularly adsorption. Thus, if particle/polymer-polymer/particle contact is the aggregation mechanism, the flocculation responses should be expected to continuously increase with surface coverage. On the other hand, if particle/polymer-particle contact is predominant and if the polymer adsorption is essentially irreversible, maximum flocculation might be expected under submonolayer conditions. In order to determine the nature of this relationship for the present systems, selected flocculation responses are plotted in Figures 8 and 9 as a function of surface coverage for the nonionic and the anionic polymer respectively. The assumptions involved in the computation of the surface coverage are to be noted at this point ... 

Polymer Adsorption and Particle Flocculation in Turbulent Flow... 

This approach is based on the assumption that polymer adsorption is fast ("instantaneous") compared with flocculation. In other words the surface coverage is taken to be constant during the flocculation process. Equation (1) states that the flocculation rate tends to zero when 0 tends to 0 or 1. The maximum rate occurs at 0 = 0.5, i.e., at 50% surface coverage. 

Studies on orthokinetic flocculation (shear flow dominating over Brownian motion) show a more ambiguous picture. Both rate increases (9,10) and decreases (11,12) compared with orthokinetic coagulation have been observed. Gregory (12) treated polymer adsorption as a collision process and used Smoluchowski theory to predict that the adsorption step may become rate limiting in orthokinetic flocculation. Qualitative evidence to this effect was found for flocculation of polystyrene latex, particle diameter 1.68 pm, in laminar tube flow. Furthermore, pretreatment of half of the latex with polymer resulted in collision efficiencies that were more than twice as high as for coagulation. 

In summary, polymeric flocculants generally increase peri-kinetic flocculation rates compared with perikinetic coagulation rates. This is not necessarily true for orthokinetic flocculation, and experimental results in the literature are seemingly in conflict. Collision rate theory predicts that the polymer adsorption step may become rate limiting in orthokinetic flocculation. The present study was designed to elucidate the relationship between polymer adsorption rates and particle flocculation rates under orthokinetic conditions. 

Flocculation rate limitation. The adsorption step was rate limiting for the overall flocculation process in this system. Polymer adsorption rate measurements for dispersed systems reported in the literature (2,26) do not lend themselves to direct comparisons with the present work due to lack of information on shear rates, flocculation rates, and particle and polymer sizes. Gregory (12) proposed that the adsorption and coagulation halftimes, tA and t, respectively, should be good indications of whether or not the adsorption step is expected to be rate limiting. The halftimes, tA and t, are defined as the times required to halve the initial concentrations of polymer and particles, respectively. Adsorption should not limit the flocculation rate if... 

Finally, as pointed out in the results of the present study, there is a possibility of determining a "pseudo" OFC in an orthokinetic experiment if the extent of flocculation is not taken to its equilibrium value. This may erroneously lead to the conclusion that the polymer adsorption step is not rate limiting for the overall flocculation process. 

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