Surface Chemistry:Surface Chemistry:Complexation at the Solid/Water InterfaceComplexation at the Solid/Water Interface

Types of Particles and Colloids in Water:Supra-micron (Particles) and Sub-micron (Colloids)Clays (e.g., kaolinite, montmorillonite)

Metal Oxides (e.g., alumina (Al2O3), geothite (Fe2O3), hematite (FeOOH))

Silica (SiO2), CalciteBio-particles, Bio-Colloids, Detritus (e.g., cell fragments)Significance: Turbidity, Contaminant Carriers, Pathogens

Stationary Surfaces:Lake and Aquifer Sediments

Reactions at SurfacesReactions at Surfaces

Complexation:

Ligand + Surface (Metal) Site

or

Metal + Surface (Ligand) Site Complexation vs. Adsorption

vs. Partitioning (hydrophobic effects)

Characteristics of Particles/ColloidsCharacteristics of Particles/Colloids

Size (um); Optical Particle Counters (OPC); Scanning Electron Microscopy (SEM)

Specific Surface Area (m2/g) Porous vs. Non-porous; Diffusional

Considerations Charge; Electrophoretic

Mobility; Zeta Potential Site Density (eq/m2, #/nm2); Acid/Base,

Metal, and Ligand Sites; Colloidal Titrations Colloidal/Particle Stability

Surface ChargeSurface Charge

Electric Double Layer Charged Surface + Counter-ions in Counter-layer Net Charge, Dictated by Shear Plane Colloidal Sol: No Net Charge Colloidal Stability: Interaction of Double Layers Electrostatic vs. Van Der Waals Forces Figure 7.2

Figure 7.2

Surface Charge – cont.Surface Charge – cont.

Measurement by Electrophoresis:

Electrophoretic Mobility (EPM) {(um/s)/(V/cm); EPM = f(pH); Amphoteric; pHZPC; Acidic (e.g. silica) vs. Basic (e.g., alumina) Surfaces

Figure 3.1, Table 10.6 EPM vs. Zeta Potential, (mV); = 12.9 EPM @ 25 oC ZP vs. Charge, q (coulombs or C/m2); = 4q/D; =

thickness of diffuse layer; D = dielectric constant

*+ -

x cm

Figure 3.1

Table 10.6

Adsorption IsothermAdsorption Isotherm

Isotherm Plot:

S = (C0 – Ceq)/M; S = solid-phase conc. (ug/g); C = aqueous-phase conc. (ug/L); m = sorbent conc. (g/L)

Linear (Partitioning) vs. Curvilinear (Adsorption) Linear; S = KPC, KP = Partition Coefficient; Freundlich: S = KCn;

Langmuir: S = abC/(1 + bC), a = monolayer saturation

S (ug/g)

C (ug/L)

* * **

C0

m

Ceq

Origin of (Surface) ChargeOrigin of (Surface) Charge

Ionizable Function Groups on SurfaceLet “>” or “” or “{}” represent a surface site

>SiOH2+ >SiOH >SiO-

+ H+ + H+

Ka1s Ka2

s

>SiOH or SiOH or {SiOH}

Ka1s = [H+]{SiOH}/{SiOH2

+}

Amphoteric Acid/Base BehaviorFigure 10.7

Figure 10.7

Origin of (Surface) Charge – cont.Origin of (Surface) Charge – cont.

Surface Complexes

>O- + Mg2+ >O-Mg+; >O- = surface ligand site

>M+ + SO42- >MSO4

-; >M+ = surface metal site

Figure 10.7

K1s vs. 2

s (multidendate, polynuclear)

For Clays, Isomorphous Substitution

Clay: Aluminum Silicate

Al(III) Clay Si(IV); “-’ charge

Origin of (Surface) Charge – cont.Origin of (Surface) Charge – cont.

Surface Adsorption of NOM (e.g. Fulvic Acid); Complexation, Hydrophobic Effects

Surface Complexation vs. Ion Exchange (e.g., alumina) vs. Hydrophobic Effects (e.g., HA) vs. Electrostatic Barriers (e.g., silica)

Ca2+ Binding to Humic Coating: Reduction in Charge

pH

* * * *

+

-* = w/o FA; = w/FA

Metal Binding and Ligand Exchange at a SurfaceMetal Binding and Ligand Exchange at a Surface

Surface Ligands and Surface Metals; Ligand Sites and Metal Sites

Figure 8.3 Metal Binding:

OH + M2+ OM+ + H+ (proton competition) Ligand Exchange;

OH + L- L + OH- (exchangeable ligand) Multidendate and Polynuclear Behavior Possible

Figure 8.3

Reactions @ Metal Oxide Surface: Reactions @ Metal Oxide Surface: Modeling FrameworkModeling Framework

Consider:Acid/Base (Protonation/Deprotonation)Metal Complexation (M2+)Ligand Exchange (L2-)Use Silica (Si) as Example

Let {Bi} = conc. of surface species i (mol/cm2)

[Bi] = conc. of surface species i (mol/L) Acid/Base Reactions

{SiOH2+} {SiOH} + [H+]

Ka1s = {SiOH}[H+]/{SiOH2

+}

{SiOH2+} {SiO-} + [H+]

Ka2s = {SiO-}[H+]/{SiOH2

+}

Reactions @ Metal Oxide Surface: Reactions @ Metal Oxide Surface: Modeling Framework – cont.Modeling Framework – cont.

Metal Complexation{SiOH} + [M2+] {SiOM+} + [H+]

KMs = {SiOM+}[H+]/{SiOH}[M2+]

Ligand Exchange{SiOH} + [L2-] {SiL-} + [OH-]

KLs = {SiL-}[OH-]/{SiOH}[L2-]

Surface Charge

= F({SiOH2+} - {SiO-} + {SiOM+} - {SiL-} )

F = Faraday constant (90,490 C/mole)

Reactions @ Metal Oxide Surface: Reactions @ Metal Oxide Surface: Modeling Framework – cont.Modeling Framework – cont.

Mass Balance, CT,s

CT,s = {SiOH2+} + {SiOH}+ {SiO-} + {SiOM+} + {SiL-}

CT,s = Total sites (mol/cm2 or #/cm2)

e.g., Al2O3: pHZPC = 8.7, pKa1s = 7.4, pKa2

s = 10.0, CT,s = 1.3/nm2

System:Five surface speciesFour aqueous species

Ki expressions + Kw

CT,M, CT,L, CT,s

(Aqueous) Charge Balance

Colloidal TitrationsColloidal Titrations

e.g., Suspension of Al2O3, Titrated with Acid or Base or Metal (M) or Ligand (L):

Infer Conditional Bind Constant(s) from Shape of Titration Curves (Alkametric, Compleximetric, etc.)

Note: {SiOH} can protonate, deprotonate, complex metals, exchange ligands; competitive effects

Example 4-27

Al2O3

H+ or OH- or M or L

pH or Mfree or Lfree

Example 4-27

Surface Coatings and Surface Coatings and Common Mineral SurfacesCommon Mineral Surfaces

Surface Coatings

Figure 11.1

Bacterium: Protein/Amino Acids; Amphoteric Behavior

Common Mineral Surfaces

Silica, alumina, Geothite, Hematite

Figure, Figure 11.14

Figure 11.1

Figure

Figure 11.14

Surface and Aqueous Complexation of MetalsSurface and Aqueous Complexation of Metals

Figure 11.25 Cu-NOM Cu2+ Cu-Mineral

Cu-Mineral-NOM

Binding and Sorption Constants Metal Partitioning vs. Metal Transport Stationary vs. Mobile Phases

Figure 11.25

Movement of Particles/ColloidsMovement of Particles/Colloids

By Settling

Water Column in Lake; Sedimentation Basin; Differential Settling

By Fluid Shear

Velocity Gradient; Mixing By Brownian Motion

<0.1 Colloids By Advection

Advective Flow

Aggregation of ParticlesAggregation of Particles

Particle-Particle Interactions Zeta Potential vs. Van Der Waals Forces Particle Collisions; Attachment

Sticking Factor: Lake: = 0.01 to 0.10

Treatment Plant (Coagulation): = 0.10 to 1.0

Aggregation of Particles – cont.Aggregation of Particles – cont.

Assuming Fluid shear Predominates:dn/dt = -4nG/n = colloids/L @ time tG = velocity gradient (T-1; (L/T)/L) = volume fraction of colloidsper unit volume of suspension

ln (n/n0) = -4nGt/

Stability Ratio, WW = 1/

Discrete Particles vs. Aggregates

ln n/n0

t

slope:

Environmental PartitioningEnvironmental Partitioning

Lake

Unique: Phases: Water (Column), Sediments, Colloids/NOM, Biomass, Atmosphere

Associated Binding, Partitioning, Sorption Constants

Cu2+ vs. Benzene Groundwater (Aquifer)

Unique Phases: Water, Aquifer Media, NOM/Colloids

Saturated Zone vs. Unsaturated Zone (Pore Gas)