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EVEN 3321

Fall 2011EVEN 3321

Objectives

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1. To understand what colloids are & why they are important in environmental engineering.

2. To understand the electric double layer theory ofcolloidal surface charge.

3. To understand thedifference between electrostatic repulsive forces & van der Waals’ attractive forces between colloidal particles.

4. To understand theelectrokinetic propertiesof colloids(e.g., zeta potential & electrophoretic mobility).

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Objectives (cont.)

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5. To understand how colloids can be destabilized & coagulated (e.g., by increasing ionic strength or adjusting pH).

6. To understand the meaning of “pointof zero charge”and pHpzc.

7. To understand the causes & significance of turbidityin water supplies.

What are “colloids”?

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� Colloids = particlesof 1-1000 nm (1nm = 10-9m).

� Can exist as dispersions in solids, liquid, orair.

� Sols = solid colloids in liquid (this lab)

� Emulsions = liquid colloids in liquid

� Foams = gas colloids in liquid

� Smokes = solid colloids in gas

� Fogs = liquid colloids in gas

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General properties of colloids

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� Stability = resistance of colloid to removal by settling or filtration

� Stabilityof colloids in solution affected by:

� Particle size

� Particle surface charge

� pH, ionic strength, & organic content of water

� Surface area to mass ratio is very high then surfacephenomena predominate.

Electrical properties of colloids

� Colloidal particlesgenerally have surface charge.

� Can be positiveor negative.

� Like charges repel, preventing colloids from agglomerating(coagulating) into larger particles.

� Thus, colloidal stability largely due to surface charge.

• When charged colloids are placed in electric field they migrate towards pole of opposite charge.

� Particles with a greater surface charge exhibit a higherelectrophoretic mobility (higher velocity in electric field).

� This allows colloid surface charge to be quantified.

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Brownian Movement & Tyndall

Effect

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� Brownian movement:

� Colloidal particles are constantly in motion due tocollisions with molecules in solution.

� Tyndall effect:

� A beam of light passing through a colloidal dispersion will be ref lected

� Ref lected light can be observed at a right angle to beam oflight.

� True solutions & coarse suspensionsdo not produce thisphenomenon.

� Thus Tyndall effect used to prove presence of colloids.

Adsorption by colloids

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� Colloids have great adsorption capacity due to very large surface area.

� Colloidswill preferentiallyadsorb positive or negativecharged ions giving the colloid a net surface charge.

� Surface charge provides stability (preventing agglomerationand coagulation) for colloids in solution.

� Transportof environmentallysignificant contaminants(e.g., metals) is facilitated by adsorption to colloids.

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Colloidal solids in liquids

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� Colloidal dispersions of solids in liquids are “sols”.

� Hydrophobiccolloids all have a surface charge.� easier to remove than hydrophiliccolloids.

� The surface charge (or “primary charge”) depends on:� Character of thecolloid.

� ionic characteristics of solution, including pH.

� Surface charges tend to be negative.

� However, low pHs tend to result in more positivesurface charge.

� Colloids in solution do not settle due to gravity.

Electric “double layer” theory

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� Sol as a whole must be neutral

� Primarycharge – charged groupswithin particle surface+ adsorptionof layerof ions at surface.

� (see next slide)

� Primarycharge must be balanced by counter ions nearthe surface & in solution.

� (see next slide)

� Result is an electric double layer:

� Fixed or Stern layer of counter ions

� Diffuse layer of a mixture of charged ions.

� (see next slide)

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Electrical double layer of negatively charged colloid

Surface charge

(or primary charge)

BACK

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Electric double layer theory (cont.)

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� Fixed & diffuse layers are separated by a shear surface.

� The fixed layerwill movewith colloid if it is subjected to anelectric field.

� (see next slide)

� Counter-ions in fixed layerare attracted electrostatically.

� However, counter ions can diffuse away from fixed layerdue to Brownian motion.

� (see next slide)

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Electrical double layer of negatively charged colloid

Surface charge

(or primary charge)

BACK

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Electric double layer theory (cont.)

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� Competing forces of electrical attraction & diffusion(due to concentration difference) spread charge overthe electrical double layer.� Conc. of counter ions is greatest at surface & decreases

with distance from surface.� (see next slide)

� The primarycharge produces an electric potentialbetween the surface & the solution.� Theelectric potential is greatest at the surface &

decreases with distance from the surface.� (see next slide)

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Electrical double layer of negatively charged colloid

Surface charge

(or primary charge)

BACK

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Electric double layer theory (cont.)

� As two negatively-charged colloids come closer (r smaller), the electrostatic repulsive force between thetwo primary charges (same sign) increases (Frepel

� (see next slide)

1/r2).

� The electrostatic repulsive forces are counteracted by anintermolecularattractive force.

� Theattractive “van der Waals’ force” decreases rapidlywith distance from surface (-Fattract

� (See next slide)

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1/r6).

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Effect separating distance between colloidson forces of interaction between them

BACK

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Electric double layer theory (cont.)� Weakvan derWaals’ intermolecularattractive forces arisewhen

moleculesare in veryclose proximity (a few angstroms – 10-10m).� “Synchronized” induced dipoles result in weak electrical attraction

between molecules:

• If two colloids can be brought sufficiently close so van der Waals’ forces are greater than electrostatic repulsive forces, the two colloids will coagulate together.

o (see next slide)

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Effect separating distance between colloidson forces of interaction between them

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Electric double layer theory (cont.)

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� Todestabilize & coagulate colloidal particles:

� Need to provide kinetic energy (by stirring) to overcome theenergy barrier, or

� Reduce the energy barrier by some means.� (see next slide)

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Effect separating distance between colloidson forces of interaction between them

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Electric double layer theory (cont.)

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� One way to decrease energy barrier is to increase ionconcentration in solution (high ionic strength).

� This decreases thickness of the electric double layer.� (see next slide)

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Effect of ionic strength on energy barrier thatprevents coagulation of colloids

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Colloid electrokinetic properties� To predict conditions that will destabilize colloids, it is

useful to estimate their surface charge.

� The surface charge of colloids can be estimated by experimentally measuring their electrophoretic mobility (essentially theirvelocity in an applied electric field).

+ E -

V

v

colloid with negative surface charge

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Instrument for measuring electrophoretic mobility &

zeta potential

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Colloid electrokinetic properties

(cont.)

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� A colloid’selectrophoretic mobility is directly related to itszeta potential.

� (see next slide)

� A colloid’s surface charge (coulombs/m2) can be estimatedfrom it’s zeta potential.

� Zeta potential measurementsare used to characterize effectiveness of lowering energy barrier between colloids:

� by adding electrolyte

� by adjusting pH

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Electrical double layer of negatively charged colloid

Surface charge

(or primary charge)

BACK

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Effect of ionic strength on energy barrier thatprevents coagulation of colloids

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“Point of zero charge” & pHpzc

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� The pzc “point of zero charge” or “isoelectric point”occurs when the colloid surface charge is zero.

� Surface charge changes with pH:

� The pH at point of zero charge is called pHpzc.

� Colloidsare generally least stable (i.e., tend tocoagulate readily) at pHpzc.

� (see next slide)

Effect of pH on surface charge of clay, iron, &

aluminum colloids

pHpzc30

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Colloid destabilization &

coagulation

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� Destabilizing colloids allows them to coagulate into largerparticles that can be removed by settling.

� Four basic mechanisms for coagulating colloids:

� Electrical double layer compression.

� Charge neutralization.

� Entrapment in precipitate.

� Interparticle bridging.

(1) Electrical double layer

compression� High electrolyteconcentration:

increases concentration of ions in double layer

decreases double layer thickness

decreases energy barrier

increases colloidal coagulation

� Ions with higher charge (e.g., Al3+) are moreeffectivethan ions with lowercharge (e.g., Na+).

� (see next slide)

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Effect of ionic strength on energy barrier thatprevents coagulation of colloids

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“alum”

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(2) Charge neutralization

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� Addition of hydrophobic moleculesof oppositechargethatcanadsorbonto the colloids:

neutralizescolloidal surfacecharge

reduceselectrostaticrepulsive forces increasing colloidalcoagulation

� Dodecylammonium (C12H25NH3+) is an example.

� Addition of Al(III) & Fe(III) salts producesproducevarioushydroxidecomplexes [e.g., Al13(OH)34 ]:

Positively-charged complexesadsorbto colloids.

Results in charge neutralization.

� Overdosing can result in chargereversal & formationof stablepositively-charged particles.

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(3) Entrapment in precipitate� Addition of high doses of Al(III) & Fe(III) salts rapidly

forms hydroxide precipitates [e.g., Al(OH)3(s) & Fe(OH)3(s)]:

Colloids become enmeshed in settling sweep f loc.

Lowest solubility for Al(OH)3(s) occurs near neutralpH.

Thus coagulation best carried out at neutral pH.

(see next slide)

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Solubility of Al(OH)3(s) vs. pH

Lines are for five hydroxide complexes: AlOH2+,Al(OH)2+, Al(OH)3(aq), Al(OH)4-, & Al(OH)45+

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(4) Interparticle bridging� Addition of long-chain polymers (polyelectrolytes) can

form bridges between colloidal particles:

Numerouscommercial polyelectrolytesavailable.

Interparticle bridging between colloids using long-chain charged polymers

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Turbidity

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� Turbidity is caused by suspended matter that interfereswith passage of light.

� Suspended mattercan range from colloidal to coarsedispersions & can be either organic or inorganic.

� Colloidal rock particles

� Topsoil, clays, and silt

� Domestic & industrial wastewater

� Street runoff

� Bacteria, algae, & other microorganisms

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Environmental significance of turbidity

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� Aesthetics

� Filterability

� Operation of rapid sand filters requires prior removal ofturbidity by chemical coagulation.

� Increasing turbidity increases difficulty & cost of filtering water supplies.

� Disinfection

� Usually uses chlorine, ozone, ClO2, or UV radiation.

� Disinfecting agents must be in contact with theorganism.

� In turbid water supplies, microorganismscan be encased in particles & thus protected from disinfection.

Turbidity measurement, units, & standards

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� Turbidity is measured using nephelometry:� Light source illuminates water sample

� Photoelectric detectors measure intensity of lightscattered at right angles.

� Turbidity measurementsare reported innephelometric turbidity units (NTU).

� EPA set more stringent turbidity standards fordrinking waters in 2002.� Turbidity must never exceed 1 NTU.

� Turbidity must not exceed 0.3 NTU in 95% of dailysamples in any one month.

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Application of turbidity data

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� Turbidity measurements can help determine the following for water supply treatment plants:

� Whether a raw water supply requires chemical coagulation prior to sand filtration.

� Optimal coagulant [e.g., Al(III) or Fe(III) salts].

� Coagulantdose required.

� Sand filter effectiveness.

� Conformity with regulatory standards.