lab colloid chemistry & turbidity

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  • 1/29/2015

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    EVEN 3321

    Fall 2011EVEN 3321

    Objectives

    2

    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 chargeand pHpzc.

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

    What are colloids?

    4

    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

    11

    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

    13

    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

    15

    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 colloidselectrophoretic mobility is directly related to itszeta potential.

    (see next slide)

    A colloids surface charge (coulombs/m2) can be estimatedfrom its 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

    27

    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 pointoccurs 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 moreeffectiv

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