colloids and fine particles

8/12/2019 Colloids and Fine Particles

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Colloids and Fine Particles (Introduction)Example : Removal colloidal particle

from raw water

Solids are present in water in three main forms:

suspended particles, colloids and dissolved molecules.

Suspended particles, such as sand, vegetable matter

and silts, range in size from very large particles down to

particles with a typical dimension of 10 m.

Colloids are very fine particles, typically ranging from 10 nm to

10 m.

Dissolved molecules are present as individual

molecules or as ions.

There are two types of colloids: hydrophilic colloids and hydrophobic colloids.

Hydrophobic colloids, including clay and non-hydrated metal oxides,

are unstable. The colloids are easily destabilized.

Hydrophilic colloids like soap are stable. When these colloids are mixed with

water, they form colloidal solutions that are not easily destabilized. Most

suspended solids smaller than 0.1 mm found in water carry negative

electrostatic charges.


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Since the particles have similar negative electrical charges

and electrical forces to keep the individual particles separate,the colloids stay in suspension as small particles.

The magnitude of the zeta potential (Zp) is usually

used to indicate colloidal particle stability. The higher the

zeta potential, the greater are the repulsion forces between

the colloidal particles and, therefore, the more stable is the

colloidal suspension. A high Zp represents strong forces of

separation (via electrostatic repulsion) and a stable system,

i.e. particles tend to suspend. Low Zp indicates relativelyunstable systems, i.e. Particles tend to aggregate.


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To remove colloids, small particles have to be destabilized first and then

they will form larger and heavier flocks which can be removed by

conventional physical treatment. This process can be described by

clarification mechanisms, that includes: coagulation, flocculation andsedimentation.

Coagulation is the process of decreasing or neutralizing the negative

charge on suspended particles or zeta potential. This allows the van der

Waals force of attraction to encourage initial aggregation of colloidaland fine suspended materials to form microflock. Rapid, high energy

mixing is necessary to ensure the coagulant is fully mixed into the

process flow to maximize its effectiveness. The coagulation process

occurs very quickly, in a matter of fractions of a second.

Flocculation is the process of bringing together the particles to form

large agglomerations by physically mixing or through the bridging action

of coagulant aids, such as long chain polymer.


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Colloids and Fine Particles

-emergence of nanotechnology and

microfluids

Colloids- very fine particles between 1nm and 10 micron

Behaviour

dominated by surface forcerather than body force


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Surface force

Exhibited

cohesive nature of fine

particles

High viscosity ofconcentrated suspensions

Slow sedimentation of

dispersed colloidal

suspensions

Body force vs surface foce

Mass of fine particles and

colloids is so small-

magnitude of their bodyforce is less than the

magnitude of the forces

acting between their

surfaces.


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Surface Force

Other surface foces:

Van der waals

Electrical double layer

Bridging and steric force

* Control behaviour of

fine powders and

colloidal suspensions

Result from either

attraction or repulsion

between two particles

Depend on- material of the

particles

- type of fluid- distance between the

particles

B


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Brown an mot on- co o s sperse n a

liquid

Illustration of the random walk of a Brownian particle.

The distance the particle has moved over a period of time is L

Robert Brown (1827)

Thermal energy fromenvironment causes the

molecules of the liquid to

vibrate.

These vibrating

molecules collide

with each other

and with the

surface of the

particles.


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A kinetic model-determine the influence of key

parameters on the average velocity of

particles in suspension. Consider thermal

energy of the environment is transferred to

the particles as kinetic energy. Average

thermal energy is 3/2kT ( k is Boltmanns

constant =1.381 x 10-23J/K).Ignore

drag,collision..average velocity of the particle

can be estimated by equating the kineticenergy 1/2mv2


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x is the particle diameter

3kT

m

(Einstein, 1956)

f: friction coefficient = FD/U

k: Boltzmann constant = 1.381 x 10-23J/K

Extension to 3D case:

tL 6

2

2

1

2

3mvkT

Random thermal energyIgnoring drag, collision and other factors

Increase temperature or decrease mass, increase

Brownian motion

Based on statistical analysis of 1-D random walk to determine root mean square

distance traveled.


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Surface Force

Force between two particles(F) may be either attractiveor repulsive which dependson

-surface to surface separationdistance (D)

-between the particles and thepotential energy (V) at thatseparation distance

dVF

dD

Therodynamics dictates

that the pair of particles

move to separation

distance that results in

the lowest energy

configuration.


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Surface forces

dVF

dD


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Van der waals Forces-Wikipedia

van der Waals' force(or van der Waals' interaction),named after DutchscientistJohannes Diderik van derWaals, is the sum of the attractive or repulsive forcesbetween molecules(or between parts of the samemolecule) otherthan those due to covalent bonds,thehydrogen bonds, or the electrostatic interactionof ionswith one another or with neutral molecules or chargedmolecules.[1]The term includes:

force between two permanent dipoles(Keesom force)

force between apermanentdipoleand a correspondinginduced dipole (Debye force)

force between two instantaneously induced dipoles(London dispersion force).
http://en.wikipedia.org/wiki/Netherlandshttp://en.wikipedia.org/wiki/Scientisthttp://en.wikipedia.org/wiki/Scientisthttp://en.wikipedia.org/wiki/Scientisthttp://en.wikipedia.org/wiki/Johannes_Diderik_van_der_Waalshttp://en.wikipedia.org/wiki/Johannes_Diderik_van_der_Waalshttp://en.wikipedia.org/wiki/Johannes_Diderik_van_der_Waalshttp://en.wikipedia.org/wiki/Netherlandshttp://en.wikipedia.org/wiki/Scientisthttp://en.wikipedia.org/wiki/Johannes_Diderik_van_der_Waalshttp://en.wikipedia.org/wiki/Johannes_Diderik_van_der_Waalshttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Covalent_bondhttp://en.wikipedia.org/wiki/Hydrogen_bondshttp://en.wikipedia.org/wiki/Electrostatic_interactionhttp://en.wikipedia.org/wiki/Electrostatic_interactionhttp://en.wikipedia.org/wiki/Covalent_bondhttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Hydrogen_bondshttp://en.wikipedia.org/wiki/Electrostatic_interactionhttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Molecular_dipole_momenthttp://en.wikipedia.org/wiki/Keesom_forcehttp://en.wikipedia.org/wiki/Debye_forcehttp://en.wikipedia.org/wiki/Molecular_dipole_momenthttp://en.wikipedia.org/wiki/Debye_forcehttp://en.wikipedia.org/wiki/London_dispersion_forcehttp://en.wikipedia.org/wiki/Molecular_dipole_momenthttp://en.wikipedia.org/wiki/London_dispersion_forcehttp://en.wikipedia.org/wiki/London_dispersion_forcehttp://en.wikipedia.org/wiki/Molecular_dipole_momenthttp://en.wikipedia.org/wiki/Debye_forcehttp://en.wikipedia.org/wiki/Molecular_dipole_momenthttp://en.wikipedia.org/wiki/Keesom_forcehttp://en.wikipedia.org/wiki/Keesom_forcehttp://en.wikipedia.org/wiki/Keesom_forcehttp://en.wikipedia.org/wiki/Molecular_dipole_momenthttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Electrostatic_interactionhttp://en.wikipedia.org/wiki/Hydrogen_bondshttp://en.wikipedia.org/wiki/Covalent_bondhttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Johannes_Diderik_van_der_Waalshttp://en.wikipedia.org/wiki/Johannes_Diderik_van_der_Waalshttp://en.wikipedia.org/wiki/Johannes_Diderik_van_der_Waalshttp://en.wikipedia.org/wiki/Johannes_Diderik_van_der_Waalshttp://en.wikipedia.org/wiki/Johannes_Diderik_van_der_Waalshttp://en.wikipedia.org/wiki/Johannes_Diderik_van_der_Waalshttp://en.wikipedia.org/wiki/Johannes_Diderik_van_der_Waalshttp://en.wikipedia.org/wiki/Scientisthttp://en.wikipedia.org/wiki/Netherlands


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Van der waals Forces

Refer to group of

electrodynamics

interactions including

Keesom, Debye andLondon dispersion

Dominant contribution to

van der Waals

interaction betweentwo particles is

dispersion force

Dispersion Force

- Result of Columbic

interactions between

correlated fluctuatinginstantaneous dipole

moments within the

atom of the particles


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Van der waals Forces

Van der waals interaction

can be attractive or

repulsive depend on

dielectric properties ofthe two particles

Medium between the

particles

Hamaker constant : AA>0 interaction is attractive

A


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Van der Waals Forces

A group of

electrohydrodynamicinteractions that occur

between the atoms in

two different

particles.


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Electrical double layer forces when particles are immersed in an aqueos


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Electrical double layer forces-when particles are immersed in an aqueossolutions-oxide particles immersed in aqueous solution.


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Number density per unit area of neutral (M-OH), positive (M-OH2+)

and negative (M-O-)surface sites as a function of pH


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-13.29 [ ] (nm )c

2

EDL 0 0

D

V x e

A measure of the counterion cloud (thus the range of the repulsion)

is the Debye length , k-1

Approximate EDL potential energy


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Zeta potential of alumina particles as a function of pH and salt concentration


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Adsorbing polymers, bridging and steric forces

Schematic representation of (a) bridging flocculation and (b) steric repulsion

Net interaction force DLVO Theory:


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Net interaction force DLVO Theory:

l f f f b h i i i d


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Result of surface forces on behaviour in air and

water

I fl f i l i d f f


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Influences of particle size and surface forces on

solid/liquid separation by sedimentation

22 5216

P f

kTt

g x

m

223 18

P f x gkT

L t tx

m m


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Suspension rheology


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Relative viscosity ( ) of hard sphere silica particle

suspensions (black circles) and Einsteins relationship (line)

/s lm m

Einstein (1906), < 7% volume solids)

Batchelor (1977), 7-15%, volume solids


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The transition from Brownian dominated random structures to preferred flowstructures as shear rate is increased is the mechanism for the shear thinning behaviour

of concentrated suspensions of hard sphere colloids


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I fl f f f i fl


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Repulsive forces

Influences of surface forces on suspension flow

eff

volume of solid + excluded volume

total volume


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Attractive forces


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Force versus separation distance curves for alumina particles


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Force versus separation distance curves for oil droplets

colloids and fine particles

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