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Decontaminating Water via Zeta Potential
Stuart Wakefield, Regional Manager Middle East & India, Malvern Instruments Ltd Water destined for distribution into the public water system is extracted from a number of sources. Each of these sources needs to be treated in a specific way to remove suspended material, viruses and bacteria. This article discusses the role of zeta potential in helping understand the removal of such contaminants.

EEC drinking water regulations applied in 1989 and higher expectations of water quality worldwide mean that the traditional methods of treatment and control are inadequate to ensure that recommended contaminant levels are never exceeded. Water clarity is measured by monitoring the turbidity. This is useful to check the effectiveness of the treatment process, but is no use in controlling dosing or investigating the source of a problem if one arises.

The Answer
New control processes are needed which rely on the fundamental parameter controlling flocculation, the zeta potential. A typical water treatment plant is shown in Figure 1. The process involves altering the pH, adding salts or polyelectrolytes, or a combination of these. This flocculates the contaminants, which will then sediment or are filtered out.

These treatments work by altering the zeta potential of the suspended material. Maximising the efficiency of additive addition and control of flocculation can be done by routine measurement of the zeta potential.1 To destabilise a suspension or increase the rate of flocculation the zeta potential

must be reduced. The zeta potential is the parameter that determines the electrical interaction between particles, a high value prevents flocculation, reducing the value allows particles to approach each other and flocculate (Figure 2, on next page), the fastest rate of flocculation being the Point of Zero Zeta Potential (PZZP). The rate of flocculation can also be associated with the compactness of the floc, which affects the rate of filtration.2 The theoretical basis for the concept of zeta potential is described by the DLVO theory.3,4 This demonstrates how colloid stability is determined by the balance between the Van der Waals attractive forces and electrical repulsive forces (zeta potential) between particles. Plotting zeta potential over a range of electrolyte concentrations allows the Critical Coagulation Concentration (CCC) to be determined. Below this value the suspended material will not flocculate but may sediment slowly, forming a dense deposit. Above the CCC there will be flocculation; the material will sediment more quickly and form a lower density deposit.5

Mineral oxides form a significant proportion of the suspended material in ground water. The Point of Zero Zeta Potential (PZZP) for many mineral oxides is between 7 and 9 and H+, OH- are potential determining ions (Figure 3, on next page). This means that pH will be one of the main factors determining zeta potential.

Effect of Different Treatments pH: Source water will be typically pH 5 to 6, adding alkali will reduce the zeta potential of most contaminants and increase the rate of flocculation. pH adjustment is not usually used on its own, but in conjunction with other treatments to increase their effectiveness.

Electrolytes: The effect of adding salts to a dispersion, if they are not specifically adsorbed, is to reduce the zeta potential of the suspended particles. Aluminium sulphate is often used as a flocculate because aluminium is a trivalent cation. This is important because the suspended material in water is negatively charged and the flocculating effect is proportional to the sixth power of the valency of the positively charged ions added. Investigation of the correct pH for use of these ions is essential because the adsorption efficiency of the metal ions onto an oxide surface can vary from 0-100 per cent over a pH range of less than 1. Adsorption efficiency is directly related to the reduction in the zeta potential.6

Polyelectrolytes: Polyelectrolytes are charged polymers, which naturally adsorb onto particle surfaces. The polymer is of opposite charge to the particle so adsorption reduces the zeta potential. The quantity added and the rate of addition, both affect the rate of flocculation.7

Wastewater: Water used in the paper or mineral processing industries has to be treated before re-use or discharge into rivers etc. The same principles for domestic supply water treatment apply to waste water and ground water contaminated with toxic metals, insecticides and herbicides.8

Using an appropriate Zetasizer enables the measurement and control of the zeta potential of suspended material in water in a treatment plant. This will improve water quality and increase plant throughput.