Flocculation : A New Way to Treat the Waste Water

Journal of Physical Sciences, Vol. 10, 2006, 93 – 127

Flocculation : A New Way to Treat the Waste Water

Tridib Tripathy1 and Bhudeb Ranjan De2

1 Narajole Raj College, Narajole, Paschim Medinipur 2 Department of Chemistry, Vidyasagar University, Paschim Medinipur

Received 7 November, 2006 ; accepted 8 December, 2006

ABSTRAC

Pure water is an essential requirement for the survival of human beings. To meet the requirements of potable, industrial and agricultural water, the immediate need is to treat waste water, particularly the sewage sludges and slimes from the municipal and industrial effluents respectively. These effluents are highly undesirable and unsafe. The removal of contaminants from waste water is a must before they can be reused. The removal of contaminants from these effluents involves the process of flocculation and coagulation. The purpose of the present article is to clarify and explain the processes in details along with the materials involved in the processes. This will be essential for the present day life.

1. Coagulation and Flocculation

Colloidal particles in nature normally carry charges on their surface, which lead to the stabilisation of the suspension. By addition of some chemicals, the surface property of such colloidal particles can be changed or dissolved material can be precipitated so as to facilitate the separation of solids by gravity or filtration. Conversion of stable state dispersion to the unstable state is termed destabilisation and the processes of destabilisation are coagulation and flocculation1,2. Often the terms coagulation and flocculation are used synonymously inspite of existing a subtle difference between the two1,3. If destabilisation is induced through charge neutralisation by addition of inorganic chemicals, the process is called coagulation. On the other hand, the process of forming larger agglomerates of particles in suspension or of small agglomerates already formed as a result of coagulation through high molecular weight polymeric materials is called flocculation. No substantial change of surface charge is accomplished in flocculation. The agglomerates formed by coagulation are compact and loosely bound, whereas the flocs are of larger size, strongly bound and porous in case of flocculation. In mineral processing industries, the scope of application of flocculants is much greater than the coagulants. Coagulants find use in the processing of coal, taconite, soda ash,

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sand and gravel and to some extent in the uranium industry. Flocculants are widely used in the processing of coal, bauxite, phosphate, potash sand, gravel, cement, soda ash, copper, silver, gold, beryllium, lead and zinc.

1.1. Stability of colloidal suspension

The attractive force between particles, known as Van der Waal force exit in case of colloidal particles in suspension. But the electrostatic repulsion of surface charges opposes the particles to come closer and form agglomerates. The principal mechanism controlling the stability of both hydrophobic and hydrophilic particles is the electrostatic repulsion4. Hydrophobic surfaces may acquire an excess of anions or cations at the interface producing an electrical barrier that can repulse particulates of similar surface potential. Hydrophillic particles acquire surface charge from dissociation of inorganic groups (carboxylic or other organic acid groups) located on the particle surface or interface. Besides electrical repulsion, a suspension may be stable due to the presence of adsorbed water molecules that provide a physical liquid barrier preventing particulates from making collisions and destabilisation. Particles may acquire surface charges due to unequal distribution of constituent ions on the particle surface, preferential adsorption of specific ions, ionisation of surface groups, crystal imperfection, or any combination of these.

1.2. Electrical double layer

Oppositely charged ions in an electrolytic solution are attracted to the surface of a charged particle and can either be closely associated with the surface or distributed some way into the solution5. Thus the two opposite forces, electrostatic attraction and ionic diffusion, produce a diffuse cloud of ions surrounding the particulate, which can extend up to 300 nm. This co-existence of original charged surface and the neutralizing excess of counter-ions over co-ions distributed in a diffused manner are known as the electrical double layer6. Fig.1 gives a schematic diagram114 showing the nature of electrical forces around a colloidal particle in bulk solution and the various electrical potentials thus developed in the double layer. The double layer consists of two major regions, an inner layer (called Stern layer) where the initial layer of adsorbed ions and molecules are located at the particle surface; and the outer layer (called Gouy-Chapman layer) of oppositely charged counter-ions. The stability of colloidal suspension is greatly influenced by the potential of the Stern layer. Though this potential cannot be measured directly, it is approximated to the zeta potential representing the electrical potential between the shear plane and the bulk solution7. According to Deryagin and Landau8, Verwey and Overbeek9, if the kinetic energy of the particle is larger enough to surmount the potential hump created between them by way of double layer formation, the particles would coalesce otherwise they would remain as a stable suspension. This theory is popularly known as DLVO theory.

1.3. Zeta potential

Flocculation : A new way to treat the waste water 95

A charged particle dispersed in an ionic medium tends to have a concentration of opposite ions attracted towards it. For example, a negatively charged particle collects a number of positive counter-ions. As one move further away from the particle, concentration of counter- ions decreases due to diffusion until ionic equilibrium is reached. A plot of the charge contributed by these ions versus distance from the particle surface (Fig.1) reveals the familiar exponential decay. Now, if the particles were imagined to be moving, it would tend to drag its counter- ions along with it while leaving behind the ions that are further away from its surface. This would set up a plane of shear – the potential difference at which is called the zeta potential (ζ).

1.3.1.Principle of measuring zeta potential

Zeta potential is measured10 using the technique of micro-electrophoresis, which was invented by Ware and Flygare and independently by Uzgiris in the early 1970s. However, as per Friend and Kitchener11, zeta potential was calculated using Smouluchowski equation as early as 1903. The sample to be measured is dispersed in suitable liquid phase and placed in the path of a beam of laser light. A pair of electrodes is introduced into the sample. Charged particles in the sample will move under the influence of an electric field applied across the electrodes. The direction of the motion indicates the sign of the charge on the particles: negatively charged particles will gravitate towards the positive electrode and vice versa. The velocity of particles, per unit electric field, can be measured and is called the electrophoretic mobility (u). Thus,

u=v/E (2.1) Where v is the particle velocity and E is applied field strength. The equation used for converting the observed mobilities into effective electrokinetic potential depends upon the value of dimensionless quantity ‘ka’ in which ‘a’ is the radius of the particle (assumed spherical) and ‘k’ is the quantity given by12.

4πe2∑nZ2 k= εKT (2.2)

where, e = Electronic charge, ε = Electrical permittivity of the solvent K = Boltzman constant T = Absolute temperature.

From the expression given, k = 1 × 106 cm at 25oC in water (ε/εo = 78) containing 1 mM of 1:1 electrolyte. Values of ‘k’ at other concentration follow by simple proportion. If ‘ka’ > 200 it will be usually sufficiently accurate to use the Smoluchowski formula which in the original unrationalised form is u = εζ/4πη, where ε is the permittivity of the suspending medium and η is the viscosity.

96 Tridib Tripathy et al. Typical units would be to have u in micron per second under one volt per cm, in

which case for water at 25oC, zeta potential in mV would be given by ζ = 12.83 u (2.3)

1.3.2. Significance of the measured zeta potential

The zeta potential is best seen as the potential at the surface of the ‘electrokinetic unit’ moving through the solution. This will not be the mean ‘wall potential’, often called Wo, but can be taken as the mean potential at the ‘outer Stern plate’ (OSP). The OSP is seen as removed from the surface by one hydrated radius of the principal counter-ions (0.2 to 0.5 nm). The electrokinetic entity may well include ions specifically adsorbed from the solutions and this will be reflected in the value of zeta potential. Certainly the double layer on the solution side of zeta potential will be purely diffused so that zeta potential is the relevant potential for all the effects that depend on diffuse layer effects (e.g., inter plate repulsion). Calculation of particle charge from zeta potential is also possible12. 1.3.3. Zeta potential and suspension stability

Knowledge of zeta potential can be used to predict and control the stability of colloidal suspensions or emulsions. Greater the zeta potential, more likely the suspension is to remain in stable form.

Zeta potential is very much dependent on the pH of the suspension. A plot of zeta potential vs pH is called an iso-electric curve. The pH for which zeta potential is zero, is called the ‘iso-electric point’ or ‘point of zero charge’ (PZC).

It

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