Negatively charged particles repel each other due to electricity.
Objective
Reading Assignment
Along with the online lecture, read chapter 8 in Basic Chemistry for Water and Wastewater Operators.
Lecture
Colloids
A colloid is defined as very small, finely divided solids (particles that do not dissolve) that remain dispersed in a liquid for a long time due to their small size and electrical charge. These particles have negligible settling velocity because their small mass has a low gravitational force compared to surface frictional forces.
One property of colloid systems that distinguishes them from true solutions is that colloidal particles scatter light. If a beam of light passes through a colloid, the light is refelcted (scattered) by the colloidal particles and the path of the light can therefore be observed. When a beam of light passes through a true solution (ie salt in water) there is so little scattering of the light that the path of the light cannot be seen and the small amount of scattered light cannot be detected except by very sensitive instruments. The scattering of light by colloids is known as the Tyndall effect. Absorption is another characteristic of colloids, since the finely divided colloidal particles have a large surface area exposed. The presence of colloidal particles has little effect on the boiling and freezing points of a solution.
The particles of a colloid selectively absorb ions and acquire an electric charge. All of the particles of a given colloid take on the same charge (either positive or negative) and thus are repelled by one another.
Negatively charged particles repel each other due to electricity.
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Most particles dissolved in water have a negative charge, so they tend to repel each other. As a result, they stay dispersed and dissolved or colloidal in the water, as shown above.
The purpose of most coagulant chemicals is to neutralize the negative charges on the turbidity particles to prevent those particles from repelling each other. The amount of coagulant which should be added to the water will depend on the zeta potential , a measurement of the magnitude of electrical charge surrounding the colloidal particles. You can think of the zeta potential as the amount of repulsive force which keeps the particles in the water. If the zeta potential is large, then more coagulants will be needed.
Coagulants tend to be positively charged. Due to their positive charge, they are attracted to the negative particles in the water, as shown below.
The combination of positive and negative charge results in a neutral , or lack, of charge. As a result, the particles no longer repel each other.
The next force which will affect the particles is known as van der Waal's forces. Van der Waal's forces refer to the tendency of particles in nature to attract each other weakly if they have no charge.
Neutrally charged particles attract due to van der Waal's forces .
Once the particles in water are not repelling each other, van der Waal's forces make the particles drift toward each other and join together into a group. When enough particles have joined together, they become floc and will settle out of the water.
Particles and coagulants join
together into floc .
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Types of Colloids
A colloid is one of the three primary types of mixtures, with the other two being a solution and suspension. A colloid is a mixture that has particles ranging between 1 and 1000 nanometers in diameter, yet are still able to remain evenly distributed throughout the solution. These are also known as colloidal dispersions because the substances remain dispersed an do not settle to the bottom of the container. In colloids, one substance is evenly dispersed in another. The substance being dispersed is referred to as being inthe dispersed phase, while the substance in which it is dispersed is in the continuous phase.
To be classified as a colloid, the substance in the dispersed phase must be larger than the size of a molecule but smaller than what can be seen with the naked eye, or more precisely, between 1 and 1000 nanometers. If the dimensions are smaller than this the substance is considered a solution and if they are larger than the substance is, a suspension.
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Classifying Colloids
To be classified colloidal, a material must have one or more of its dimensions (length, width, or thickness) in the approximate range of 1-1000 nm. Because a colloidal solution or substance, like fog, is made up of scattered partiles (like dust and water in air), light cannot travel straight through. Rather, it collides with these microparticles and scatters, causing the effect of a visible light beam. This effect was observed and described by John Tyndall as the Tyndal Effect.
The Tyndall effect is an easy way of determining whether a mixture is colloidal or not. When light is shine through a true solution, the light passes cleanly through the solution, however, when light is passed through a colloidal solution, the substance in the dispersed phases scatters the light in all directions, making it readily seen.
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For example, light being shined through water and milk. The light is not reflected when passing through the water because it is not a colloid. It is, however, reflected in all directions when it passes through the milk, which is colloidal. A second example is shining a flashlight into fog; the beam of light can be easily seen because the fog is a colloid.
A common method of classifying colloids is based on the phase of the dispersed substance and what phase it is dispersed in. The types of colloids includes sol, emulsion, foam, and aerosol.
| Colloidal Type | Description |
| Sol | A colloidal suspension with solid particles in a liquid. |
| Emulsion | Occurs between two liquids. |
| Foam | Formed when many gas particles are trapped in a liquid or solid. |
| Aerosol | Contains small particles of liquid or solid dispersed in a gas. |
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Colloids are common materials with one material that is evenly distributed within another material at a very tiny scale. Some common examples of colloids include milk, fog, jelly, styrofoam, and whipped cream. Within colloids, a substance ("Substance A") is evenly distributed within another substance ("Substance B"). Depending on whether these are solid, liquid, or gas, the resulting colloidal materials are classified as:
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The most commonly encountered colloidal dispersions are solid-liquid (suspensions), liquid-liquid (emulsions), gas-liquid (foams), and solid-gas (aerosols).
Colloids can also be classified by being hydrophilic (having the tendency to mix with water) or hydrophobic (having the tendency to not mix with water). A hydrophilic colloid, or hydrocolloid, can be either reversible or irreversible (single-state). For example, agar is a reversible hydrocolloid of seaweed extract; it can exist in a gel or liquid state and can alternate between states with either heating or cooling.
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Many hydrocolloids are derived from natural sources. For example, gelatin is produced by hydrolysis of proteins from cows and fish, and pectin is extracted from citrus peel and apple pomace. A hydrophobic collid, occurring in emulsion, does not interact with the aqueous solvent. They are inherently unstable and generally do not form spontaneously. Energy input, through shaking, stirring, or homogenizing, is needed to form teh emulsion. Over time, an emulsion tends to separate, because separation puts it in a more stable state. An example of this is seen inthe separation of the oil and vinegar components of vinaigrette, an unstable emulsion that will quickly separate unless shaken almost continuously. Let's look at the different hydrophobic colloidal stages:
There are two immiscible liquids that are not yet emulsified. An emulsion has been formed because Phase II dispersed into Phase I. The unstable emulsion progressively separates. An emulsifier (purple outline around particles) positions itself on the interface between Phase II and Phase I to stabilize the emulsion. |
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There are three types of instability in emulsions: flocculation, creaming, and coalescence. In flocculation, the dispersed phase comes out of suspension in the form of flakes. In creaming, one of the substances migrates to the top or bottom, depending on the relative densities involved. In coalescence, small droplets of colloid bump into each other and combine to form progressively larger droplets. In order for the emulsion to stay stable, additional substances are needed to stabilize the colloid.
An emulsifier is a substance that stabilizes the colloid so that it does not change significantly with time. One important class of emulsifiers is known as "surface active substances" or surfactants. Soy and egg yolk lecithin are examples of surfactants. Surfactants are often used in food, such as mayo and salad dressing, to keep the emulsions mixed over time. Detergents are another class of surfactants, and they will physically interact with both oil and water, thus stabilizing the interface between the oil and water droplets in suspension. This principle is also exploited in soap, to remove grease for the purpose of cleaning.
Another classification of colloids is lyophilic (medium-loving) and lyophobic (medium-hating). An example of a lyophilic colloid is if a small amount of powdered gelatin is mixed in water and dispersion is allowed to stand, it sets to form a gel or jelly. The dispersed particles of gelatin have great affinity for the dispersing medium and become thoroughly hydrated. This traps the water in such a way that viscosity of dispersion increases, which is known as emulsoid. Other examples are soap, soluble starch, soluble protein, blood serum, and gum arabic.
Lyophobic colloids characterizes dispersed particles that have a poor afinity for the despersing medium, and thus there is no significant change in the viscosity of the medium. They are as fluid as the medium and are highly susceptible to coagulation, which is the separation of colloidal particles from the medium by adding electrolytes, e.g., coagulants like alum and ferric sulfate in water treatment. These colloids are known as suspensoids. The terms hydrophilic and hydrophobic are used when the dispersion medium is water.
Colloids can also be classified as positive and negative. All colloids are electrically charged, but the charge varies considerably in magnitude with the nature of the colloidal material. A colloidal charge may change with a change in external conditions. The charge is gained by the selective adsorption of ions from the surrounding medium or due to ionization of surface components of the colloid particles. Most colloids in water that cause turbidity and color are negative. Colloidal dispersion is an inherently thermodynamically unstable system because it tends to minimize surface energy. Hence, the stability of a colloidal system is inevitably linked to a notion of time, defined by process, use and/or application involved.
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Colloidal Stability
Two stability categories colloids can be affected by include colloidal stability and gravitational stability.
Colloidal stability relates to particle size change (e.g. aggregation or agglomeration). If particles are not subject to size variation, the dispersion is considered colloidally stable. Hence, colloidal stability depends on several types of interactions such as:
Therefore, it is tremendously difficult to theoretically predict the colloidal stability of a dispersion.
Gravitational stability refers to the ability of particles to resist particle migration (e.g. sedimentaiton or creaming) and mainly depends on the rheological properties (which is concerned with the flow and change of shape of matter) of the colloidal dispersion, such as viscosity and density of the continuous phase, size, and density of the particles. For diluted colloidal particles in a Newtonian fluid, this migration phenomenon can be described by Stokes law. Stokes law derived an expression for the frictional force, also called drag force, exerted on certain spherical objects. Knowing the terminal velocity, the size and density of the sphere, and the density of the liquid, Stokes' law can be used to calculate the viscosity of the fluid. This law is important for understanding the swimming of microorganisms as well as the sedimentation of small particles and organisms in water, under the force of gravity.
A Newtonian fluid is a fluid in which its viscosity remains constant, no matter the amount of shear applied for a constant temperature. These fluids have a linear relationship between viscosity and shear stress. Viscosity is defined as the state of being thick, sticky, and semifluid in consistency, due to internal friction. Examples of newtonian fluids are water, mineral oil, gasoline, and alcohol.
A non-Newtonian fluids are the opposite of Newtonian fluids. When shear is applied to non-newtonian fluids, the viscosity of the fluid changes. The behavior of the fluid can be described one of four ways:
Why do you need to know the difference? It's important to fully understand the properties of the fluids you're transferring, mixing, or pumping because viscosity plays a major role in sizing and selecting equipment. Understanding how it reacts to shear will help you properly size and select all the equipment it touches.
Stable vs Unstable Colloidal Dispersion
Sedimentation is sometimes confusingly considered as colloidal instability. For example, a particle dispersion in a solvent can be colloidally stable (there is no change in particle size) while it is gravitationally unstable (particles settle due to unmatched density with the solvent). It is worth noting that destabilizing colloidal dispersion can lead to gravitational instability (larger particles start to settle quickly).
The zeta potential is considered to be a reliable indicator of dispersion stability, but several parameter suhc as steric effects, sedimentation, or hydrophobic effects, will also have a strong influence. Zeta potential is basically the amount of repulsive force a particle has. The amount of coagulant needed will depend on the zeta potential. If the potential is large, then more coagulants will be needed.
Each colloid carries a "like" electrical charge which produces a force of mutual electrostatic repulsion between adjacent particles. Particle charge can be controlled by modifying the suspending liquid. Modifications include changing the liquid's pH or changing the ionic species in solution. Another more direct technique is to use surface active agents which directly adsorb to the surface of the colloid and change its characteristics.
Zeta Potential is a convenient way to optimize coagulant dosage in water and wastewater treatment. The most difficult suspended solids to remove are the colloids. Due to their small size, they easily escape both sedimentation and filtration. The key to effective colloid removal is reduction of their zeta potential with coagulants, such as alum, ferric chloride and/or cationic polymers. Once the charge is reduced or eliminated, then no repulsive forces exist and gentle agitation in a flocculation basin causes numerous successful colloid collisions. Microflocs form and grow into visible floc particles that settle rapidly and filter easily.
Remember, opposites attract, and like repels like. So the higher the zeta potential of a particle, the higher the charge it has, which gives it a higher ability to push other particles away. The goal is to neutralize this, so that the particles can begin to come together and form larger clumps, settling out.
The zeta potential, or repulsion between colloidal particles is counteracted by van der Waals forces, which states that particles in nature will attract to each if they have no charge. This phenomenon makes the colloids drift toward each other and form groups naturally. The objective of chemical coagulation is to reduce the zeta potential to almost zero for effective turbidity removal.
Coagulation of Colloidal Solutions
Let's start by understanding what coagulation is first. According to the general definition, coagulation is one of the various properties exhibited by colloidal solutions. A colloid is a heterogeneous mixture of one substance of very fine particles (dispersed phase) dispersed into another substance (dispersion medium). Substances like metals cannot be simply mixed with the dispersion medium to form a colloidal solution. Some special methods are used to make their colloidal solutions. We have already discussed how the particles all have electrical charges (positive and negative). If we can remove the charge present on the colloid, the particles get closer to each other and they accumulate to form aggregates and precipitate under the action of gravity. This process of accumulation and settling down of particles is known as coagulation or precipitation.
Coagulation/Flocculation in the Treatment Plant
In water treatment operations, the processes of coagulation and flocculation are employed to separate suspended solids from water. Finely dispersed solids (colloids) suspended in water are stabilized by negative electric charges on their surface, causing them to repel each other. Since this prevents these charged particles from colliding to form larger masses, called flocs, they do not settle. To assist in the removal of these particles from suspension, chemical coagulation and flocculation is required. Chemicals are first mixed with the water to promote the aggregation of the suspended solids into particles large enough to settle or be removed.
Once suspended particles are flocculated into larger partiles, they can usually be removed from the liquid by sedimentation, provided that a sufficient density difference exists between the suspended matter and the liquid. The speed in the flocculation chamber slows down as it nears the end so the floc that is formed does not break back apart. They need to stay clumped together so they will stay heavy enough to settle to the bottom.
See how different dosages of coagulant (usually alum) causes the floc to be different sizes. You don't want most of it to settle too fast, you have to give time for all the colloids to attach, but you don't want it to form too slowly. In the plant you may see a jar test procedure that will help you determine the proper dosage of coagulant to obtain the settling rate you desire.
The flocculation reaction not only increases the size of the floc particles to they will settle faster, but it also affects the physical nature of the floc, making these particles less gelatinous (viscous) and thereby easier to dewater.
Review
A colloid is defined as very small, finely divided solids (particles that do not dissolve) that remain dispersed in a liquid for a long time due to their small size and electrical charge. One property of colloid systems that distinguishes them from true solutions is that colloidal particles scatter light. The particles of a colloid selectively absorb ions and acquire an electric charge. All of the particles of a given colloid take on the same charge (either positive or negative) and thus are repelled by one another.
Zeta Potential refers to the electrostatic potential generated by the accumulation of ions at the surface of the colloidal particle. Particle charge can be controlled by modifying the suspending liquid. Modifications include changing the liquid's pH or changing the ionic species in solution.
In the treatment plant, to assist in the removal of colloidal particles from suspension, chemical coagulation and flocculation are required. Chemicals are mixed with wastewater to promote the aggregation of the suspended solids into particles large enough to settle or be
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Assignment
Complete the worksheet for this lesson. You must be logged into Canvas to submit this assignment. Make sure you choose the appropriate semester.
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Lab
Read the Turbidity and Jar Test labs.
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Quiz
Answer the questions in the lesson quiz. You must be logged into Canvas to take this quiz. You may take the quiz up to three times; an average will be taken for final grade calculation. Make sure you choose the appropriate semester.
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