Critical coagulation concentration solutions

Here, Co(OH)2 represents the solid hydroxide. The solution data show that at pH values of 7.5 and 6.5 the dominant cobalt (II) species is the free (aquo) ion by factors of 100 and 1000 respectively. It is therefore highly unlikely that the coagulation at pH 6.5-7.5 and 10"4Af Co (II) and the reversal of charge can be caused by the free CoOH+ species. If it is caused by polynuclear charged species then the log-linear relationship (9) between the critical coagulation concentration and the valence of the coagulating ion would require a polynuclear species to have a charge of +5 or +6. Such a species has not been identified. (It is of interest to note that if this species did exist it would have to be a compact ion,... 

Fig. 6.8. Log-log plot of Wrap versus electrolyte concentration for hematite ( Fe203) colloids suspended in either CaCl2 or NaCI solution at pH 10.5. Arrows indicate critical coagulation concentrations [Eq. 6.76 data from L Liang, Effects of surface chemistry on kinetics of coagulation of submicron iron oxide particles (a-Fe2Oi) in water, Ph.D. dissertation, California Institute of Technology, Pasadena, CA, 1988. Environmental Quality Laboratory Report No. AC-5-88].

The critical coagulation concentration for a colloid suspended in an aqueous electrolyte solution is determined by the ions with a charge opposite in sign to that on the colloid and is proportional to an inverse power of the valence of the ions. 

In this chapter, mathematical procedures for the estimation of the electrical interactions between particles covered by an ion-penetrable membrane immersed in a general electrolyte solution is introduced. The treatment is similar to that for rigid particles, except that fixed charges are distributed over a finite volume in space, rather than over a rigid surface. This introduces some complexities. Several approximate methods for the resolution of the Poisson-Boltzmann equation are discussed. The basic thermodynamic properties of an electrical double layer, including Helmholtz free energy, amount of ion adsorption, and entropy are then estimated on the basis of the results obtained, followed by the evaluation of the critical coagulation concentration of counterions and the stability ratio of the system under consideration. 

Go, C uptake of counterions, and critical coagulant concentration (CCC), are usually compared at constant concentration of ions (salts). In very concentrated solutions, the activities of different ions in salt solutions of equal concentrations can be very different (see Section 4.3). 

A low CCC is desirable because low levels of salts in soils are then sufficient to prevent dispersion of aggregates and the consequent structural degradation. At the critical coagulation concentration, there are enough salts in solution to induce an attractive particle-particle interaction, and the clay-sized particles associate into aggregates composed of a sufficient number of particles that gravity supersedes Brownian motion. The aggregates then settle out of suspension. 

FIGURE 13.4 Idealized curves of the stability factor W in perikinetic aggregation for small particles stabilized by electrostatic repulsion, as a function of the salt concentration m. Numbers near the curves denote the valence of the ions in the solution. The dotted lines indicate the critical coagulation concentrations mCI. 

Aggregate size depends on solution chemistry, hydrodynamic conditions when aggregates are formed, and the presence of organics. To understand the aggregation formation, at first simple systems were looked at, that is 10 mgL hematite, pH 3 and ionic strength varied using KCl. The critical coagulation concentration for KCl in these systems was 65 mM as shown in Chapter 6. 

The critical concentration (critical coagulation concentration) is thus found to depend on the type of electrolyte used and on the valency of the counterion. It is seen that divalent ions are 60 times as effective as monovalent ions. Trivalent ions are several hundred times more effective than monovalent ions. However, ions that specifically adsorb (such as surfactants) will exhibit different behavior. Based on these observations, in the composition of washing powders, one has used multivalent phosphates (or similar kinds of poly-ions), for instance, to keep the charged dirt particles from attaching to the fabrics after having been removed off. Another example is the wastewater treatment, where for coagulation purposes one uses multivalent ions (Cheremisinoff, 2002 Kim and Platt, 2007). Colloidal solutions are characterized by the degree of stability or instability. 

Probably the most studied system is silver iodide, Agl, for which many reliable experimental data are available, such as the critical coagulation concentrations (ccc) in different salt solutions and the surface charges as a function of Ag+ concentration (see Fig. 10). 

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