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![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](https://image.slidesharecdn.com/lab-colloidchemistryturbidity-150129135813-conversion-gate02/75/Lab-colloid-chemistry-turbidity-18-2048.jpg)
![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.](https://image.slidesharecdn.com/lab-colloidchemistryturbidity-150129135813-conversion-gate02/75/Lab-colloid-chemistry-turbidity-22-2048.jpg)
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Lab colloid chemistry & turbidity
This document outlines objectives and concepts related to colloids and their importance in environmental engineering. It discusses the electrical double layer theory of colloidal surface charge and how colloids can be destabilized through processes like increasing ionic strength or adjusting pH. It also addresses turbidity in water supplies and its environmental significance.
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Lab colloid chemistry & turbidity
- 1. 1/29/2015 1 EVEN 3321 Fall 2011EVEN3321 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. Tounderstandhow 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 ofcolloids 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 inliquids 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 layerof 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 layerof 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 layerof 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 distancebetween 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 distancebetween 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 distancebetween 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 ionicstrength 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 measuringelectrophoretic 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 layerof 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 zerocharge” & 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 Destabilizingcolloidsallows 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 ionicstrength on energy barrier that prevents coagulation of colloids 33 “alum” 34
- 18. 1/29/2015 18 (2) Charge neutralization 5+ Additionofhydrophobicmoleculesof 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 causedby 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 ofturbidity 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 turbiditydata 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.