Suspension-sedimentation

Objective:

1. To obtain a good formulation in controlling the rate of flocculation and sedimentation

2. To study the formation of flocculation

3. To study the relationship between flocculation and sedimentation

Introduction:

In colloid chemistry, a heterogeneous mixture contains the solid particles with a diameter more than 1μm which are large enough to settle out from the liquid through sedimentation is known as suspension. Suspension is a coarse dispersion in which the insoluble particles dispersed in a liquid medium and it will settle out of the liquid after left for some time. Suspension shows difference with a solution in which the solute does not exist in the solid form and the solvent and solute are homogeneously mixed. Suspension is thermodynamically unstable due to the properties of suspension is always changing with time as they will undergo sedimentation. Suspension may be flocculated or deflocculated.

Sedimentation is a process of the particles settle down to the bottom of the solution. This is due to the instability of the particles in the system. The factors that affect the sedimentation are particles size, densities of dispersed phase and dispersed medium, temperature of liquids and the viscosity of the liquid. In a colloidal system, the particles are apart from each other due to their double layer charge. In suspension, the particles also possess the particular surface charges, but the attractive force in between the suspending particles are greater (after flocculating agent was added) than the repulsive force causes the particles to bind together and form larger particles. Usually, the forces acting on the particles that cause sedimentation occurs due to the gravity. The larger particles will sink to the bottom through sedimentation process which will accumulate in the bottom and the particles stack on the sediment. The sedimentation rate can be calculated by using the formula as shown in the following:

The second factor is the densities between the dispersed phase and dispersed medium. Generally, the density of dispersed phase (particles) is greater than the dispersed medium, however in certain cases particle density is less than dispersed phase, so suspended particle floats is difficult to distribute uniformly in the system. If density of the dispersed phase and dispersion medium are equal, the rate of settling becomes zero hence sedimentation does not occur. Temperature and viscosity of liquid are highly in related in which the temperature might affect the viscosity of the liquid. If the liquid is highly viscous, the sedimentation rate of the particles will be slower compared to the less viscous liquid. This is because when the terminal velocity decreases, the dispersed phase will settle at lower rate which may remain dispersed for longer time. The viscosity is known as the resistance of a substance to motion under an applied force.

Flocculation is a process of destabilizing the suspending particles in suspension by adding in certain chemicals called flocculating agent. In industry, this process is purposely used to cause the particles suspended in solution to aggregate into clumps or masses that then sink or else it can be removed easily by filtering. The formation of floc is due to the decrease in Zeta potential (in slipping plane) between the particles until Van der Waals forces predominate. Usually, the larger particles are known as floccules. The floc may float on the surface of liquid instead of sediment at the bottom if the density of floc is less dense than liquid’s density. This is known as creaming. The flocculating agent are includes neutral electrolyte (eg: KCl, NaCl), surfactant, and polymers. When the neutral electrolyte added into the suspension, it will affect the electrical barrier between the particles and hence the Zeta potential could be altered. For surfactant, the cationic and anionic surfactant could bring about the flocculation of suspended particles. However, optimum concentration is necessary because these compounds acting as the wetting agents to achieve dispersion. Surfactant decreases the surface energy by reducing the interfacial tension between the particles which attracted by van der Waals force and the solvent-particles’ interfacial tension. Besides, the addition of polymeric flocculating agents can induce the floc formation. The polymers possess long chain in their structure. The long chain will adsorb on the surface of the particles and the remaining part projecting out into the dispersed medium. The formations of bridging between these later portions tend to cause the formation of floc.

The flocculation process is reversible if the interaction forces between the floc are not strong. Floccules form when weak van der Waals forces are holding the particles together. However, the floccules are easily to re-suspend by shaking. If the floccules trap solvent during sedimentation, they are easily to be broken apart. Otherwise, the flocculation is considered as non reversible if the particles settle into a tighter aggregate by the strong interaction force without trapped solvent. Most of the flocculants are multivalent cation and anion which are able to neutralize the different charges (positive charge or negative charge) of the particles suspended in liquid. After overcome the surface charges of the suspending particles, the particles can easily to form floccules since their barrier have been minimized. Other than that, the deflocculation to form may be occurred once the magnitude of Zeta potential of the particles achieve sufficiently positive or negative (usually more than +30mV or less than -30mV). Thus, the phenomenon of flocculation and deflocculation depends on zeta potential carried by particles.

Apparatus: boiling tube, stopwatch, test tube rack

Materials: potassium dihydrogen phosphate, purified water, bismuth subnitrate, methylcellulose, parafilm

Procedure:

1. 40ml of flocculating reagent (potassium dihydrogen phosphate) was prepared which will be used in four of five suspensions.

2. Five 25ml graduated cylinder were assembled and were numbered consecutively. 2g of bismuth subnitrate was added to each boiling tube.

3. Sufficient water was added to make the 25ml of suspension in the 1^st boiling tube.

4. 10ml of anionic flocculating reagent solution was added into the 2^nd boiling tube and enough water was added to make 25ml.

5. 10ml of anionic flocculating reagent solution was added into the 3^rd boiling tube and 0.5% methylcellulose was added to make 25ml.

6. 10ml of anionic flocculating reagent solution was added into the 4^th boiling tube and 1.0% methylcellulose was added to make 25ml.

7. 10ml of purified water was added into the 5^th boiling tube and sufficient 0.5% methylcellulose was added to make 25ml.

8. All the boiling tubes were covered with parafilm.

9. Each of them was inverted for several time to mix tehm well. The height of suspension were measured and recorded after 15minutes, 60minutes, and 120minutes.

Results:

Table 1 The height of sedimentation in each boiling tube over the time

Number of boiling tubes	Mixture in the boiling tubes	0min	15mins	60mins	120mins
First	2g bismuth nitrate + water	6.5cm	0.3cm	0.3cm	0.5cm
Second	2g of bismuth subnitrate+10ml of flocculating agent+water	6.5cm	1.2cm	1.5cm	1.6cm
Third	2g of bismuth subnitrate+10ml of flocculating agent+0.5% methylcellulose solution	6.5cm	2.0cm	1.2cm	1.6cm
Fourth	2g of bismuth subnitrate+10ml of flocculating agent+1% methylcellulose solution	6.5cm	1.4cm	1.5cm	1.3cm
Fifth	2g of bismuth subnitrate+10ml of purified water+0.5% methylcellulose solution	6.5cm	0.0cm	0.0cm	0.0cm

Table 2 The height of suspension in each boiling tube over the time

Number of boiling tubes	Mixture in the boiling tubes	0min	15mins	60mins	120mins
First	2g bismuth nitrate + water	0.0cm	6.2cm	6.2cm	6.0cm
Second	2g of bismuth subnitrate+10ml of flocculating agent+water	0.0cm	5.3cm	5.0cm	4.9cm
Third	2g of bismuth subnitrate+10ml of flocculating agent+0.5% methylcellulose solution	0.0cm	4.5cm	5.3cm	4.9cm
Fourth	2g of bismuth subnitrate+10ml of flocculating agent+1% methylcellulose solution	0.0cm	5.1cm	5.0cm	5.2cm
Fifth	2g of bismuth subnitrate+10ml of purified water+0.5% methylcellulose solution	0.0cm	6.5cm	6.5cm	6.5cm

Precaution steps:

1. Do not shake or move the boiling tubes consist of suspension.

2. Make sure the content in each boiling tube is mixed well.

3. Make sure all the boiling tube are properly covered by parafilm