1. Surface Chemistry: Complexation 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), Calcite
Bio-particles, Bio-Colloids, Detritus (e.g., cell fragments)
Significance: Turbidity, Contaminant Carriers, Pathogens
Stationary Surfaces:
Lake and Aquifer Sediments

2. Reactions at Surfaces Complexation: Ligand + Surface (Metal) Site or
Metal + Surface (Ligand) Site
Complexation vs. Adsorption
vs. Partitioning (hydrophobic effects)

3. Characteristics 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

4. Surface 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

5. Figure 7.2

6. 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);  = oC
ZP vs. Charge, q (coulombs or C/m2);  = 4q/D;  = thickness of diffuse layer; D = dielectric constant + - *
 x cm 

7. Figure 3.1

8. Table 10.6

9. Adsorption 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 m  *
S (ug/g) C0
 Ceq
C (ug/L)

10. Origin of (Surface) Charge
Ionizable Function Groups on Surface
Let “>” or “” or “{}” represent a surface site
>SiOH2+  >SiOH  >SiO-
+ H H+
Ka1s Ka2s
>SiOH or SiOH or {SiOH}
Ka1s = [H+]{SiOH}/{SiOH2+}
Amphoteric Acid/Base Behavior
Figure 10.7

11. Figure 10.7

12. 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. 2s (multidendate, polynuclear)
For Clays, Isomorphous Substitution
Clay: Aluminum Silicate
Al(III)  Clay  Si(IV); “-’ charge

13. 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 -

14. Metal 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

15. Figure 8.3

16. Reactions @ Metal Oxide Surface: Modeling 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+}

17. Reactions @ Metal Oxide Surface: 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)

18. Reactions @ Metal Oxide Surface: 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, pKa2s = 10.0, CT,s = 1.3/nm2
System:
Five surface species
Four aqueous species
Ki expressions + Kw
CT,M, CT,L, CT,s 
(Aqueous) Charge Balance

19. Colloidal 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
H+ or OH- or M or L 
Al2O3
 pH or Mfree or Lfree

20. Example 4-27

23. Surface Coatings and Common Mineral Surfaces
Figure 11.1
Bacterium: Protein/Amino Acids; Amphoteric Behavior
Common Mineral Surfaces
Silica, alumina, Geothite, Hematite
Figure, Figure 11.14

24. Figure 11.1

25. Figure

26. Figure 11.14

27. Surface 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

28. Figure 11.25

29. Movement 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

30. Aggregation 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

31. Aggregation of Particles – cont.
Assuming Fluid shear Predominates:
dn/dt = -4nG/
n = time t
G = velocity gradient (T-1; (L/T)/L)
 = volume fraction of colloids
per unit volume of suspension
ln (n/n0) = -4nGt/
Stability Ratio, W
W = 1/
Discrete Particles vs. Aggregates
slope: 
ln n/n0 t

32. Environmental 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)