Evaluación de las propiedades químicas de los filetes de trucha

areas. Electrostatic attachment of particles leads to

flocculation.

polymers flocculate latex particles at a higher rate than simple salts, which reduced the surface charge in a uniform manner. The increase in molecular weight of polymer caused an increase in the breadth of the optimum flocculation dose region, as well as an increased rate of flocculation. When flocculation is expressed in ’equivalent’ amounts of charge neutralisation, the effect of the polymer molecular weight disappears (Gregory, 1976).

Cationic polyelectrolytes are subject to changes in charge and molecular size by alteration of the pH and ionic strength; their adsorption onto oppositely charged surfaces may also change with these properties. Lindquist and Stratton (1976) noted that the mechanism by which polyethyleneimine (PEI) flocculated colloidal silica depended upon the solution conditions. It was noted that at pH 9 and above, flocculation is brought about by a bridging mechanism, due to the low cationic character of the PEI molecule. At pH values less than 9, where the cationic charge is higher, it was proposed that flocculation is brought about by electrostatic interactions (charge neutralisation). At high pH values, the optimum polymer dosage decreases with increasing molecular weight; this dependence is not present at lower pH values.

The effect of a high shear field on the aggregates is one method of distinguishing between the polymer bridging and charge neutralisation mechanisms of flocculation. In the case of bridging, the floes are not readily reformed, whilst those produced by charge neutralisation can recover after being disrupted.

1.2.3 BRIDGING FLOCCULATION.

Bridging flocculation occurs when segments of a polymer chain adsorb onto more than one particle, without necessarily removing or neutralising the charge barrier. For adsorption to occur there must be some favourable interaction between the polymer segments and the particle surface. This may arise from ionic interactions due to the opposite charges on the polymer and particle, giving rise to a strong interaction. For non-ionic polymers, the mechanisms of interaction include hydrophilic and hydrogen bonding. The above interactions are short range and could not operate if a strong

electrical repulsion exists between the particles and the polymer chains; also an electrolyte (salt) is required to promote adsorption (Atkinson & Jack, 1973). This is not simply an ionic strength effect, but thought to be due to anions promoting adsorption by binding groups on the polymer chain to oppositely charged sites on the particles surface.

Ruerhwein and Ward (1952) first suggested the bridging flocculation mechanism, since a typical polymeric flocculant molecule has similar dimensions to many colloid particles (0.1 - 1 pm) so that attachment of the polymer chain to more than one particle could occur. Bridging flocculation requires that the absorbed polymer extend far enough from the particle surfaces to attach to other particles and that there is sufficient surface available for adsorption. If excess polymer is adsorbed, then the suspension can become restabilised due to surface saturation or steric stabilisation. The DLVO theory (Gregory, 1978b) states that the repulsion between two particles is dependent upon the ionic strength; at low ionic strengths the electrical double layer around a particle can be large ( approx. 100 nm), and so inter-particle repulsion is the dominant factor. At high electrolyte concentrations, the degree of double layer repulsion will fall to about 1 nm in 1 M NaCl (Gregory, 1978b), hence bridging is more likely to occur. In the case of polyelectrolytes the situation is complicated, since the size of the polymer is a direct function of the ionic strength; high ionic strengths will give rise to smaller polymers due the screening of repulsion between the charged segments on the molecule.

Michaels (1954) considered the effects of charge density and molecular weight on the action of polymeric flocculants. The flocculation of clay suspensions by polyacrlyamide has been found to improve with increasing chain length. There was also found to be an optimum degree of hydrolysis (anionic character) of approximately 30%. This was explained in terms of an increasing expansion of the polymer chain by mutual repulsion between charged segments (as the degree of hydrolysis increased), and a decreasing tendency of the anionic polymer to adsorb onto negative particles. The expansion of the chain would give better flocculation by acting as a bridge, whilst reduced adsorption would give the opposite effect; the optimum degree of hydrolysis would be a compromise between the two effects. Hence for a given polymer there would be an optimum concentration; this is

consistent with bridging theory which requires that the particle’s surface should be partly covered with adsorbed polymer. Excess flocculant causes the surfaces to become saturated and hence the particles are restabilised. Floes broken by shear may not reform when the shear is removed, since the polymer chains might adopt a flatter configuration on the particle.

In La M er’s (1966) quantitative theory of bridging flocculation the most important param eter is the fractional coverage (f) of the particle surface by the adsorbed polymer segments. Since bridging requires the attachment of adsorbed polymer onto vacant sites of other particles, the effect is greatest when the term /(I-/) is maximal

{ie> f = 0.5), hence ’half surface coverage’. However the precise definition of coverage is difficult, since it depends on the configuration of the adsorbed polymer chains. If the polyelectrolyte is present in excess, then the particle becomes restabilised due to surface saturation or steric stabilisation. It is clear that / is dependent on polymer adsorption, this should not be too strong since a proportion of the segments must remain unattached for adsorption to other particles (Gregory, 1978b).

The above listed mechanisms are not mutually exclusive, and it is possible for more than one to occur in a given process. The stability of suspensions of hydrophobic colloids is largely due to the surface potential, hence the interaction between colloid and polyelectrolyte may lead to destabilisation as a result of charge neutralisation. However most hydrophilic colloids are stable due to the ionisation and hydration of certain functional groups at the surface of the material. Factors affecting interactions between colloidal surfaces and polymers include particle size and density, the concentration, configuration and molecular weight of the polymer, pH and ionic strength of the solution, and intensity and time of agitation. For the polymer bridging mechanism, the optimal polymer concentration for aggregation will be a function of the total surface area of the colloidal suspension. At low concentrations or after shearing, the absorbed polymer molecules have a greater chance of being adsorbed onto the same colloidal particle, thus reducing the total amount of polymer bridging in the system. The pH and ionic strength of a solution have a profound effect on flocculation. These effects include the surface (zeta) potential, the charge and nature of the double layer surrounding each particle, as well as the overall charge, charge