Principal Mechanisms Of Colloidal Stabilization And Destabilization

Colloids of a given type (e.g., clays, fulvic acids, iron hydroxides) may aggregate together (homoaggregation) or with colloids of other types (heteroaggregation). General quantitative physicochemical theory exists only for the (irreversible) homoco-agulation of compact particles, based on the Smoluchowski equations and Derjaguin, Landau, Verwey and Overbeek (DLVO) theory.1,47,48 According to DLVO theory, the interaction energy between two compact, spherical colloids results essentially from (i) their surface charge, which defines the electrical field around each particle, and (ii) the attractive van der Waals forces between them. The electrical and van der Waals forces define an energy barrier which, if overcome by the kinetic energy of the particles moving in water, will lead to aggregation and an unstable suspension. For submicron colloids, this kinetic energy results mainly from Brownian motion and little from hydrodynamic or gravitational forces49,50 whereas for larger particles or aggregates, hydrodynamic processes such as differential sedimentation and fluid shear can provide the necessary energy (e.g., ref. [51,52]). The barrier to aggregation is decreased with increasing ionic strength or protonation of the colloidal surface. In fact, the critical coagulation concentration (CCC) of the suspension is defined as the ionic strength for which the energy barrier has been eliminated, resulting in a rapid, diffusion limited aggregation. Finally, for large particles (>micrometers), a secondary minimum, resulting in the formation of aggregates with relatively small cohesion energies, exists in solutions of moderate ionic strength (10-2 to 10-3 M).

Non-DLVO forces such as steric interactions, hydration pressure, hydrogen bonding, and hydrophobic effects, although rarely considered in quantitative attempts to model colloidal interactions, are more recently being recognized as important forces contributing to the coagulation of environmental colloids.53 For example, in environmental (aqueous) systems, it is now clear that non-DLVO hydration effects are possible due to the reorganization of surface bound water upon approach of the colloidal particles. Furthermore, hydrophobic surfaces have a greater tendency to aggregate than predicted by DLVO theory alone due to the migration of water from the solid-water interface to the bulk solution and the subsequent formation of hydrogen bonds. Indeed, under conditions for which the energy barrier for aggregation lies at subnanometer distances, there appear to be significant discrepancies between theory and experimentally measured coagulation rates (e.g., ref. [54-57]).

Little work has directly examined the role of biopolymers on the flocculation process. Based on the literature examining synthetic polymers, one would expect the biopolymers to influence the above processes either by being adsorbed to the colloidal surface58 or by being expulsed from the area between the particles (depletion layer).59,60 Indeed, the adsorption of small quantities of polymer is known to facilitate colloidal aggregation due to bridging flocculation or charge neutralization while the adsorption of larger quantities is thought to restabilize the colloidal suspensions due to charge inversion or steric restabilization. The presence of large quantities of non-adsorbing biopolymer can result in an excess pressure that pushes the colloids together, resulting in a destabilization of the colloidal suspension (i.e., depletion floc-culation). The adsorption of natural biopolymers to dispersed mineral particles can be facilitated by Coulombic interactions, surface complexation, hydrogen bonding, hydrophobic interactions, and even surface catalyzed polymerization reactions.61,62 In spite of an important literature on the physicochemical properties of the polymers and their interactions with compact colloids (e.g., ref. [15,16]), there is no general theory for aggregation involving biopolymers. One of the principal difficulties when evaluating the role of biopolymers on the aggregation process is that the conformation of the adsorbed biopolymer will greatly influence whether the colloidal system is stabilized or destabilized. Several parameters, including the affinity of the biopolymer for the particle surface, its chemical structure and molar mass, and the physico-chemical conditions existing at the particle-water interface will greatly influence the conformation of the macromolecule on the particle surface16 (Figure 7.4).

FIGURE 7.4 Schematic representation of some of the potential roles of organic matter on colloidal flocculation and stabilization processes in natural freshwaters.

Charge modification

Charge modification

FIGURE 7.4 Schematic representation of some of the potential roles of organic matter on colloidal flocculation and stabilization processes in natural freshwaters.

Increasing concentration of organic matter

Increasing concentration of organic matter

7.3.1 Charge Modification

A quantitative understanding of the role of biopolymer (polyelectrolyte) adsorption on colloidal stabilization and destabilization processes requires knowledge on: (i) the variation of the thickness of the adsorbed layer as a function of pH and ionic strength; (ii) the repulsive forces between noncovered particles; (iii) the competition between polymer segments and ions for surface sites; and (iv) the potential energy barrier between the adsorbed polymer and the solution. Destabilization by charge neutralization generally occurs at low concentrations of biopolymer for which the repulsive Coulombic forces between the particles, reflected by low electrophoretic mobilities or zeta potentials, approach zero.61,63 For a strong interaction between the biopolymer and the colloidal surface, adsorption and a subsequent charge inversion (and restabil-ization) is possible. Over time, the strongly adsorbing polymer will have a tendency to be flattened on the surface of the particle, greatly reducing the opportunity for interaction with other particles. In this case, the charge density of the polymer is more important than its molar mass.

Gregory demonstrated that in certain cases, the aggregation rate depended upon the molar mass of the polymer and the ionic strength of the solution.64 This mechanism is generally observed when particles having a low charge density are neutralized by polyelectrolytes with high charge densities. In such a case, a local charge heterogeneity can occur that results in the arrangement of the polyelectrolytes in patches on the particle surface.61 The most important consequence of the charge heterogeneity is that oppositely charged regions of different particles can react, even in the presence of an overall negligible particle charge. In that case, at low ionic strength, the maximal aggregation rates will be greater than those observed in the presence of a simple electrolyte. At high ionic strength, the additional attraction is reduced and the aggregation rate will approach that observed for a simple charge neutralization.

7.3.2 Bridging Flocculation

Bridging flocculation occurs when the loops and tails of a polymer adsorbed to one particle become attached to one or more other particles. The process is optimized for polymers which have several points of attachment on the colloidal surface and are large enough to have free segments (loops and tails) outside the zone of electrostatic repulsion and available to bind to other surfaces. The surface coverage of the adsorbed polymer appears to be a fundamental parameter controlling the probability of bridging65 with the half surface coverage postulated as the optimum condition for flocculation to occur.66 While flocculation can occur for polymers that are at equilibrium with the colloidal surface,67 nonequilibrium flocculation, occurring before the polymers are able to completely collapse on the colloidal surface, is thought to predominate.68 In that case, the dynamics of bridging flocculation are related to both the thermodynamics and kinetics of polymer adsorption, including: transport of the polymer to the colloidal surface, attachment of the polymers to the surface, and relaxation (reconformation) of the attached polymers. Furthermore, the time that the polymer remains in its nonequilibrium conformation will be an important parameter controlling flocculation efficiency. The time scale will be influenced by the particle : polymer ratio, the size of the particle, the surface area of the particle, the adsorption energy of the polymer segments, and the collision frequency among the particles.69,70 The polymer rigidity,71 charge and spacing of charged groups, thepoly-mer dosage, the particle surface charge,72 and the particle mixing regime52 are also important parameters controlling the flocculation efficiency.69 For neutral polymers, the optimal polymer dose will also depend upon the molar mass of the polymer and the ionic strength. In that case, larger polymers will flocculate particles more efficiently while a higher ionic strength will decrease the electrostatic repulsion among particles so that fewer polymer molecules are required to attain the same effect. In the presence of charged polymers, both polymer and colloid charge are influenced by the ionic strength. An increased ionic strength will generally decrease the rigidity of polyelectrolyte chains resulting in more limited possibilities for flocculation. On the other hand, as discussed earlier, the interparticle repulsive forces are screened at high ionic strength, allowing for an easier approach of the particles. For this reason, optimal bridging of particles by polyelectrolytes will generally occur at intermediate charge densities or salt concentrations.73

It is relatively difficult to distinguish between the bridging and charge neutralization mechanisms. In the presence of neutral polymers, there is little doubt that bridging flocculation is responsible for particle aggregation. On the other hand, polyelectro-lytes can both reduce electrostatic repulsion and act as bridging molecules. A charge neutralization mechanism is generally identified for cases in which the optimal polymer dosage (maximum aggregation rate) (i) coincides with a zero zeta potential;

(ii) decreases with increasing polymer charge density; and (iii) increases proportionally with particle concentration.74,75 In the presence of polyelectrolytes, the simple charge neutralization mechanism is not sufficient to explain the flocculation of colloidal particles for (i) failures of the above observations; (ii) effects due to the molar mass of the polymer (width of the flocculation zone; shifts in the optimal dosage), or

(iii) an observation of a higher aggregation rate than would be observed in the presence of a simple electrolyte.64,74 Discrepancies have been attributed to both an uneven distribution of charge on the particles (electrostatic patch model64) or to the likely possibility that both flocculation and charge neutralization mechanisms are likely to occur simultaneously.74

7.3.3 Depletion Flocculation

Depletion flocculation occurs for cases in which the polymer has a stronger affinity for the water than it does for the particle surface (or a polymer covered particle surface). In that case, the polymer remains predominantly as a nonadsorbing coil in the solution phase. When the particles in solution approach each other to a distance that is less than the effective polymer diameter (i.e., twice the gyration radius), then the polymer will be excluded from the space between the particles, reducing its concentration with respect to the bulk solution. The imbalance of osmotic pressure generates an attractive force among the particles, causing flocculation. The driving force for the existence of the depletion zone is due to conformational entropic restrictions of the polymer coils that are not compensated by an adsorption energy.59 In the presence of a sufficient quantity of nonadsorbing polymer, the interparticle interaction energy curve shows a secondary minimum at a distance corresponding to twice the radius of gyration of the polymer. Recent direct measurements of the interparticle interaction forces by atomic force microscopy have confirmed that the depth of the secondary minima increased with increasing free polymer concentration.76,77 The most important parameter governing depletion flocculation in aqueous solutions is the size of the polymer with respect to the particle.78 Although an increase in molar mass has opposing effects on the depletion energy,79 overall, an increase of the particle concentration or an increase of the molar mass of the polymer will favor depletion flocculation.77,79-81

7.3.4 Steric Stabilization

The adsorption of large concentrations of a neutral polymer or polyelectrolyte can lead to steric stabilization. In this case, the particle surface is saturated with polymer such that the polymer loops and trains form a relatively thick layer (several nanometers) of adsorbed polymer. Stabilization depends upon several factors including the thickness of the adsorbed polymer layer, the size of the particles, and the effective Hamaker constant of the covered particles. Indeed, the spatial extension of the adsorbed layer must be thick enough to prevent the particles from approaching to distances at which London-van der Waals attractive forces become significant. Although, the adsorbed polymers may overlap and become compressed, such overlap is generally unfavorable, due to a strong repulsion that increases very sharply with increasing penetration.82 Because van der Waals attractive forces increase with particle size, larger particles require thicker polymer layers to ensure a similar particle stability. Note that for concentrations of polymers that are greater than those required for steric stabilization, the excess polymer will remain unadsorbed in the bulk solution, potentially leading to depletion flocculation.

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