Colloidal Stability

Stability refers to the ability of particles to remain as separate entities or in a dispersion state for long periods rather than form aggregates (Gregory and Duan,

2001). Most colloidal particles in wastewater have a negative charge in nature (Koohestanian et al., 2008). Their colloidal dispersions are stabilized because of the

electrostatic repulsion between adjacent particles, which usually overwhelm the attractive van der Waals force and prevent particle aggregation.

Since colloids have a high surface area to weight ratio, they are hard to be removed by gravity or sedimentation.This phenomenon implies that the effective removal of colloidal dispersions is highly influenced by its electrokinetic properties.

Colloid electrokinetic properties are commonly illustrated by the electrical double-layer model and the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO) in Sections 2.8.2.1 and 2.8.2.2.

2.8.2.1 Electrical Double Layer Model

The combined Gouy–Chapman–Stern model is the most commonly used double-layer model, which plays a fundamental role in the mechanism of the electrostatic stabilization of colloids. The double-layer model explains the ionic environment and the setup of repulsive forces that surround a charged colloid.

A single, negatively charged colloid will initially attract some of the positive ions (or counterions) in the solution to form a firmly attached layer, known as the Stern layer, around the surface of the colloid. Additional positive ions are still attracted to the negative colloid, but they are also repelled by the Stern layer and by the other positively charged ions attempting to get close to the negatively charged colloid.

This constant attraction and repulsion results in the formation of a diffuse layer of charged ions that surrounds the colloid and the Stern layer. The diffuse layer can be visualized as a charged atmosphere that surrounds the colloid. The attached positively charged ions in the Stern layer and the charged atmosphere in the diffuse layer is together referred as the double layer. The charge is a maximum at the particle surface and decreases with distance from the surface. The thickness of this layer depends on the type and concentration of ions in the solution.

The Stern layer is considered to be rigidly attached to the colloid while the diffused layer is a dynamic layer of charged particles. Nerst Potential is the voltage measurement (in millivolts) in the diffuse layer. The potential is at its maximum at the Stern layer and drops exponentially through the diffuse layer. Zeta potential is the electrical potential that represents the difference in voltage between the surface of the diffused layer and the dispersant (Figure 2.11). Knowing the magnitude of the zeta potential is essential because it represents the strength of the repulsion between colloidal particles and the distance that must be overcome to bring the particles together.

Figure 2.11: Schematic representation of Zeta Potential (Source: Zetasizer Nano User Manual, 2008)

Since double layer electrokinetic quantification by Stern potential measurement is difficult and laborious, zeta potential would be an adequate substitute (Cosgrove, 2010; Bratby, 2006; Norde, 2003). Zeta potential is very dependent on pH and the pH for which zeta potential is zero, is defined as isoelectric point (IEP) or point of zero charge (PZC) (Kaszuba et al., 2010; Ravina, 2006). Both Sympatec GmbH (2009) and Shammas (2005) had illustrated colloid stability level (Tabble 2.6) and coagulation degree (Table 2.7) respectively based on zeta potential.

Table 2.6: Degree of colloid stability as a function of zeta potential

Zeta potential (mV) Colloid stability

from 0 to ±5 Rapid coagulation or flocculation

from ±10 to ±30 Incipient stability

from ±30 to ±40 Moderate stability

from ±40 to ±60 Good stability

more than ±61 Excellent stability

(Source: Sympatec GmbH, 2009)

Table 2.7: Degree of coagulation as a function of zeta potential

Zeta potential (mV) Degree of coagulation

from +3 to 0 Maximum

from -1 to -4 Excellent

from -5 to -10 Fair

from -11 to -20 Poor

from -21 to -30 Virtually none

(Source: Shammas, 2005)

According to Table 2.6 and 2.7, the magnitude of the zeta potential indicates the potential stability of a colloidal system. If all the particles in a suspension have a large negative or positive zeta potential, then they will tend to repel each other and no tendency for the particles to come together will occur. However, if the particles have low zeta potential values, then no force will prevent the particles from coming together and flocculating. As this electric potential approaches zero, particles tend to aggregate. Therefore, a lower zeta potential means a more effective coagulation.

Nevertheless, there are other coagulation mechanisms than charge destabilization which will be discussed in Section 2.14. Hence, indication of coagulation performance by zeta potential is only appropriate if charge destabilization is the predominant coagulation mechanism (Bratby, 2006). Deviation of zeta potential from zero to optimum coagulation conditions indicates other coagulation mechanisms surmount the coagulation process, not charge neutralization (Eikebrokk, Juhna and Østerhus, 2006).

2.8.2.2 DLVO Theory

Developed in the 1940s, DLVO theory deals with the stability of colloidal systems. According to DLVO theory, colloidal stability can be predicted by considering van der Waals and electrical double layer interactions. It calculates the balance of two opposing forces between charged colloidal particles, namely van der Waals attraction (attraction forces) and electrostatic repulsion (repulsive forces) (Binks and Horozov, 2006; Pashley and Karaman, 2004; Gregory and Duan, 2001).

Prediction of this model is in line with the Schulze-Hardy rule, which emphasizes the dependence of colloid stability or coagulations on the added electrolyte‟s concentration and valency (Tadros, 2007; Shammas 2005).

In water, colloidal particles collide with water molecules constantly and randomly, termed as Brownian movement. As particles with similar charges approach one another, the repulsive electrostatic forces increase to keep them separated. However, if they can neutralize the electrokinetic potential, then the attractive van der Waals force will dominate and the particles will remain close

together, forming larger flocs (in agreement with aforementioned zeta potential concept). In this case, coagulant can be introduced into the wastewater to do the trick.