Handouts for a presentation about colloid chemistry and turbidity in relation to water quality.

1. 1/29/2015
1
EVEN 3321
Fall 2011EVEN 3321
Objectives
2
1. To understand what colloids are & why theyare
important in environmental engineering.
2. To understand theelectric double layer theory of
colloidal 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).

2. 1/29/2015
2
Objectives (cont.)
3
5. Tounderstand how colloidscan be destabilized &
coagulated (e.g., by increasing ionic strength or
adjusting pH).
6. Tounderstand the meaning of “pointof zerocharge”
and pHpzc.
7. Tounderstand thecauses & significance of turbidity
inwatersupplies.
What are “colloids”?
4
Colloids = particlesof 1-1000 nm (1nm = 10-9m).
Canexistas 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

3. 1/29/2015
3
General properties of colloids
5
Stability = resistanceof colloid to removal by settling
orfiltration
Stabilityof colloids in solutionaffected by:
Particle size
Particle surface charge
pH, ionic strength, & organic content of water
Surfacearea to mass ratio isvery high then surface
phenomenapredominate.
Electrical properties of colloids
Colloidal particlesgenerally have surfacecharge.
Can be positiveor negative.
Likecharges 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 higher
electrophoretic mobility (higher velocity in electric field).
This allows colloid surface charge to be quantified.
6

4. 1/29/2015
4
Brownian Movement & Tyndall
Effect
7
Brownian movement:
Colloidal particles are constantly in motion due to
collisions with molecules in solution.
Tyndall effect:
A beam of light passing throughacolloidal dispersion will
beref lected
Ref lected light can be observed at a right angle to beam of
light.
True solutions & coarse suspensionsdo not produce this
phenomenon.
Thus Tyndall effect used to prove presence of colloids.
Adsorption by colloids
8
Colloids havegreatadsorptioncapacitydue tovery
largesurface area.
Colloidswill preferentiallyadsorb positiveor negative
charged ionsgiving the colloid a net surface charge.
Surfacecharge providesstability (preventing
agglomerationand coagulation) forcolloids in
solution.
Transportof environmentallysignificant contaminants
(e.g., metals) is facilitated by adsorption tocolloids.

5. 1/29/2015
5
Colloidal solids in liquids
9
Colloidal dispersionsof solids in liquidsare “sols”.
Hydrophobiccolloidsall havea surfacecharge.
easierto remove than hydrophiliccolloids.
The surface charge (or “primarycharge”) dependson:
Characterof thecolloid.
ionic characteristics of solution, including pH.
Surfacecharges tend to be negative.
However, low pHs tend to result in morepositive
surfacecharge.
Colloids in solutiondo not settledue togravity.
Electric “double layer” theory
10
Sol as a whole must be neutral
Primarycharge – charged groupswithin particlesurface
+ adsorptionof layerof ionsat surface.
(see next slide)
Primarycharge must be balanced by counter ions near
thesurface & in solution.
(see next slide)
Result is an electricdouble layer:
Fixed or Stern layerof counter ions
Diffuselayerof a mixtureof charged ions.
(see next slide)

6. 1/29/2015
6
Electrical double layer of negatively
charged colloid
Surface charge
(or primary charge)
BACK
11
Electric double layer theory (cont.)
12
Fixed & diffuse layersareseparated by a shearsurface.
The fixed layerwill movewith colloid if it is subjected toan
electric field.
(see next slide)
Counter-ions in fixed layerareattracted electrostatically.
However, counter ionscan diffuse away from fixed layer
due to Brownian motion.
(see next slide)

7. 1/29/2015
7
Electrical double layer of negatively
charged colloid
Surface charge
(or primary charge)
BACK
13
Electric double layer theory (cont.)
14
Competing forces of electrical attraction & diffusion
(due to concentration difference) spread charge over
theelectrical double layer.
Conc. of counter ions is greatest at surface & decreases
withdistance from surface.
(see next slide)
The primarycharge producesan electric potential
between the surface & the solution.
Theelectric potential isgreatest at thesurface &
decreases with distance from the surface.
(see next slide)

8. 1/29/2015
8
Electrical double layer of negatively
charged colloid
Surface charge
(or primary charge)
BACK
15
Electric double layer theory (cont.)
As two negatively-charged colloidscome closer (r
smaller), the electrostaticrepulsive force between the
twoprimarycharges (samesign) increases (Frepel
(see next slide)
1/r2).
Theelectrostaticrepulsive forcesarecounteracted by an
intermolecularattractive force.
Theattractive “van der Waals’ force” decreases rapidly
withdistance from surface (-Fattract
(See next slide)
16
1/r6).

9. 1/29/2015
9
Effect separating distance between colloids
on forces of interaction between them
BACK
17
Electric double layer theory (cont.)
WeakvanderWaals’ intermolecularattractiveforces arisewhen
moleculesare in verycloseproximity (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)
18

10. 1/29/2015
10
Effect separating distance between colloids
on forces of interaction between them
19
Electric double layer theory (cont.)
20
Todestabilize & coagulatecolloidal particles:
Need to provide kinetic energy (by stirring) toovercome the
energy barrier, or
Reduce theenergy barrier by some means.
(see next slide)

11. 1/29/2015
11
Effect separating distance between colloids
on forces of interaction between them
21
Electric double layer theory (cont.)
22
Oneway todecreaseenergy barrier is to increase ion
concentration in solution (high ionic strength).
Thisdecreases thicknessof theelectric double layer.
(see next slide)

12. 1/29/2015
12
Effect of ionic strength on energy barrier that
prevents coagulation of colloids
23
Colloid electrokinetic properties
Topredictconditions that will destabilizecolloids, it is
useful toestimate theirsurfacecharge.
The surfacecharge of colloidscan beestimated by
experimentallymeasuring theirelectrophoretic mobility
(essentially theirvelocity in an applied electric field).
+ E -
V
v
colloid with negative
surface charge
24

13. 1/29/2015
13
Instrument for measuring electrophoretic mobility &
zeta potential
25
Colloid electrokinetic properties
(cont.)
26
Acolloid’selectrophoreticmobility is directly related to its
zeta potential.
(see next slide)
Acolloid’ssurface charge (coulombs/m2) can be estimated
from it’s zeta potential.
Zetapotential measurementsare used tocharacterize
effectivenessof lowering energy barrier between colloids:
byadding electrolyte
byadjusting pH

14. 1/29/2015
14
Electrical double layer of negatively
charged colloid
Surface charge
(or primary charge)
BACK
27
Effect of ionic strength on energy barrier that
prevents coagulation of colloids
28

15. 1/29/2015
15
“Point of zero charge” & pHpzc
29
The pzc “pointof zerocharge” or “isoelectric point”
occurswhen the colloid surface charge is zero.
Surfacechargechanges with pH:
ThepH at pointof zero charge is called pHpzc.
Colloidsaregenerally least stable (i.e., tend to
coagulatereadily) at pHpzc.
(see next slide)
Effect of pH on surface charge of clay, iron, &
aluminum colloids
pHpzc
30

16. 1/29/2015
16
Colloid destabilization &
coagulation
31
Destabilizing colloidsallows them tocoagulate into
largerparticles thatcan be removed by settling.
Fourbasic mechanisms forcoagulating colloids:
Electrical double layercompression.
Charge neutralization.
Entrapment in precipitate.
Interparticle bridging.
(1) Electrical double layer
compression
High electrolyteconcentration:
increasesconcentration of ions in double layer
decreasesdouble layerthickness
decreasesenergy barrier
increasescolloidal coagulation
Ionswith highercharge (e.g., Al3+) are moreeffective
than ions with lowercharge (e.g., Na+).
(see next slide)
32

17. 1/29/2015
17
Effect of ionic strength on energy barrier that
prevents coagulation of colloids
33
“alum”
34

18. 1/29/2015
18
(2) Charge neutralization
5+
Additionof hydrophobicmoleculesof oppositechargethatcan
adsorbontothecolloids:
neutralizescolloidal surfacecharge
reduceselectrostaticrepulsive forces increasingcolloidal
coagulation
Dodecylammonium(C12H25NH3
+) is anexample.
Additionof Al(III) & Fe(III) saltsproducesproducevarious
hydroxidecomplexes [e.g., Al13(OH)34 ]:
Positively-chargedcomplexesadsorbtocolloids.
Results in chargeneutralization.
Overdosingcan result in chargereversal & formationof stable
positively-chargedparticles.
35
(3) Entrapment in precipitate
Additionof high dosesof Al(III) & Fe(III) salts rapidly
forms hydroxideprecipitates[e.g., Al(OH)3(s) &
Fe(OH)3(s)]:
Colloids becomeenmeshed in settling sweep floc.
Lowestsolubility for Al(OH)3(s) occurs near neutral
pH.
Thus coagulation bestcarried outat neutral pH.
(see next slide)
36

19. 1/29/2015
19
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)4
5+
37
(4) Interparticle bridging
Additionof long-chain polymers (polyelectrolytes)can
form bridges betweencolloidal particles:
Numerouscommercial polyelectrolytesavailable.
Interparticle bridging between colloids
using long-chain charged polymers
38

20. 1/29/2015
20
39
Turbidity
40
Turbidity is caused by suspended matterthat interferes
with passageof light.
Suspended mattercan range fromcolloidal tocoarse
dispersions & can be eitherorganicor inorganic.
Colloidal rock particles
Topsoil, clays, and silt
Domestic & industrial wastewater
Street runoff
Bacteria, algae, & other microorganisms

21. 1/29/2015
21
Environmental significance of turbidity
41
Aesthetics
Filterability
Operationof rapid sand filters requires prior removal of
turbidity by chemical coagulation.
Increasing turbidity increases difficulty & cost of filtering
water supplies.
Disinfection
Usuallyuseschlorine, 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
42
Turbidity is measured using nephelometry:
Lightsource illuminates water sample
Photoelectric detectors measure intensity of light
scattered at rightangles.
Turbidity measurementsare reported in
nephelometricturbidity units (NTU).
EPA set morestringent turbidity standards for
drinking waters in 2002.
Turbidity must never exceed 1 NTU.
Turbidity must not exceed 0.3 NTU in 95% of daily
samples in any one month.

22. 1/29/2015
22
Application of turbidity data
43
Turbidity measurementscan helpdetermine the
following forwatersupplytreatmentplants:
Whethera raw water supplyrequires chemical
coagulation prior to sand filtration.
Optimal coagulant [e.g., Al(III)or Fe(III) salts].
Coagulantdose required.
Sand filtereffectiveness.
Conformitywith regulatory standards.