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

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

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

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

Figure 7.2

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);  = 4q/D;  = thickness of diffuse layer; D = dielectric constant + - *  x cm 

Figure 3.1

Table 10.6

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)

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

Figure 10.7

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

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 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: 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+}

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)

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

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

Example 4-27

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

Figure 11.1

Figure

Figure 11.14

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

Figure 11.25

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

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

Aggregation of Particles – cont. Assuming Fluid shear Predominates: dn/dt = -4nG/ n = colloids/L @ 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

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)