Zeta Potential (ζ) for Predicting the Stability of Emulsions

Coagulants are commonly used in the drinking water treatment of surface water. The surface water can come from a variety of sources including rivers, lakes, and reservoirs. These source waters can have a high level of particulate that results in an increased level of turbidity. The EPA National Primary Drinking Water Regulations (NPDWR) sets a maximum level of turbidity based on the type of treatment. The turbidity is not exceed 1 nephelometric turbidity unit (NTU) for systems that use conventional or direct filtration with samples being less than 0.3 NTU 95% of the time. Systems other the conventional or direct filtration must meet state turbidity limits and at no time exceed 5 NTU. It is important that the particulate be removed from the source water to meet the National Primary Drinking Water Regulations for turbidity.

Drinking water treatment plants achieve the removal of particulate from surface water by the dosing of coagulants. There are a variety of coagulants available including ones based on aluminum and iron formulations. Aluminum based coagulants include aluminum sulfate, sodium aluminate, and a polymerized metal such as polyaluminum chloride (PACI). Iron based coagulants include ferric chloride and ferric sulfate. The main mechanism of action is to cause the particulate to agglomerate, which will then flocculate. The flocculent is then removed from the sedimentary basin.

Jar testing is a traditional method used to determine the amount of coagulant needed to treat the water. The goal of this method is to determine the minimum amount of coagulant required to reduce turbidity levels that will meet the NPDWR standard. With this method, a series of 1 L jars contain varying concentrations of coagulant. The jars are placed on a static mixer and allowed to stand to simulate a flocculator. The jars are observed for floc formation over time.

Although the jar testing method is useful, it is not a precise measurement

technique due to being a visual method. With this technique it is possible to over estimate the amount of coagulant needed resulting in higher chemical and waste disposal costs. An alternative to jar testing is the use of a zeta potential measuring instrument. This type of instrumentation is used to determine the surface charge of the particles in the water.  In nature, the charge is highly negative causing the particles to repel each other.

The role of the coagulant is to reduce the negative charge resulting in an unstable (destabilized) solution. Particles with a charge close to neutral (zero) will agglomerate into small groups, which then form larger aggregates, leading to a visible floc. The process can be optimized with some customers targeting a -8 mV zeta potential after the sedimentation basin. Any further reduction to a value closer to zero will require more chemical but with little benefit for the reduction in turbidity. The monitoring of zeta potential is a quantitative way to determine the feed rate required as variations in turbidity occur in surface water.

How is Zeta Potential / Surface Charge Measured?

There are two methods for measuring zeta potential from light scattering, both of which are based on electrophoretic light scattering (ELS). With these methods a sample is added to a cuvette and then blackened palladium electrodes are inserted. A laser light source is used to measure the light scattered by particles placed in an electric field.

The first method for measuring zeta potential is known as laser doppler electrophoresis, which uses a DC voltage to influence the movement of the charged particles. The electrophoretic velocity, or velocity of the particle in an electric field, is normalized to field strength, giving an electrophoretic mobility, which is then used to calculate a zeta potential from one of several standard models (Smoluchowski, Hückel, or Henry).¹  This method, typically referred to as ELS, works well for samples that are low in conductivity, including source water from lakes, streams, and rivers.

The second method, Phase-Analysis Light Scattering (PALS),² is based on similar principles, but instead utilizes an AC voltage. The use of AC field allows for a more sensitive measurement to reliably measure much lower magnitude zeta potentials. The PALS method is suitable for samples that have much higher conductivities including those with a high level of salt such as brackish and seawater.

Light Scattering Optics

Zeta Potential Analyzers typically use a photodetector for measuring light scattered at forward scattering angles (θ < 15˚).  This is done to minimize the effects of diffusion broadening. All of Brookhaven’s NanoBrook series Zeta Potential Analyzers utilize a dedicated reference beam path. In the case of PALS, the reference frequency, or demodulation frequency is set via a piezoelectric modulator.

The reference and sample scattering signals are then combined prior to being detected by the 15° fiber. The detector is a low dead-time avalanche photodiode detector (APD). The APD module used is an advanced single-photon counting device.

Regardless of the method, the electrophoretic mobility is determined by the movement of particles in an electric field. The zeta potential is then determined by using the  Smoluchowski, Hückel, or Henry equations. The most commonly used of the three models to calculate zeta potential is the Smoluchowski equation, where the zeta potential, ζ, can be calculated from the electrophoretic mobility (μ^elec), the viscosity (η), and the dielectric of the continuum liquid (ε_o) as follows:

ζ = μ^elec* η/ ε_o

The ZetaPlus offers ELS measurements suitable for freshwater while the ZetaPALS has both ELS and PALS measurement modes with the PALS being more suitable for saline water that have higher conductivity.