Particle Surfaces Surface Functional Groups Adsorption Surface Charge Points of Zero Charge

Surface Functional Groups Organic Fairly wide range of specific types exist, e.g., carboxyl, carbonyl, hydroxyl, phenol and so forth K a of benzoic acid = 6.3 x m-NO 2 benzoic acid = 32 x m-Cl = 15 x m-NH 2 = 1.9 x p-NO 2 = 670 x p-Cl = 120 x p-NH 2 = 1.6 x What is the K a of the soil organic matter carboxyl?

Inorganic Hydroxyl is common and functions as a Lewis base, undergoing protonation Occur in many soil minerals, e.g., oxides, oxyhydroxides, hyroxides and aluminosilicates (layer and amorphous)

As with surface functional groups of soil organic matter, the local electronic environment affects reactivity. Clearly, -OHs differ in electronic environment, e.g., O in position A is enriched with e - s compared to O in position B, so that it is relatively easily protonated

Generally, the lower the extent of coordination of the O in a –OH, the more reactive, with respect to both protonation and dissociation Also, the type of –OH, silanol or aluminol, affects reactivity with silanol dissociating but not tending to protonate, whereas aluminol undergoes both

e - charge density of siloxane groups varies depending on extent and proximity of isomorphic substitution In the absence of isomorphic substitution, the surface has weak affinity for + charge With isomorphic substitution, especially in the tetrahedral sheet, siloxane exhibits high affinity for + charge due to increased density of – charge near the surface Thus, electronic environment of any functional group is affected by its neighbors giving rise to ranges of reactive behavior

Complexes formed with surface functional groups called surface complexes. Among these distinguish inner- and outer-sphere surface complexes Outer-sphere have at least one water interposed between the surface functional group and the bound ion or molecule so that electrostatic forces alone bind the two whereas

Inner-sphere complexes involve immediate contact with covalent or ionic bonding Clearly, inner-sphere complexes are the more stable The displacement of 2 protonated –OHs on goethite by phosphate leads to a binuclear bridging complex (inner-sphere)

Adsorption Accumulation of matter at the interface between solid and solution phases Differs from precipitation (also a surface phenomenon) in that adsorption does not perpetuate upon itself Adsorbent, adsorbate and adsorptive

Particularly, note diffuse ions are electrostatically attracted to negative surface but without either outer- or inner-sphere complexation –in the diffuse ion swarm Outer-sphere and diffuse swarm adsorption involve only electrostatic attraction and do not depend on specific electronic structures of either the adsorbent or adsorbate and such adsorption is referred to as non-specific Inner-sphere complexation is specific Exchangeable ions = only non-specifically adsorbed ions?

Surface Charge Develops from isomorphic substitution and reaction of surface functional groups with solution ions Expressed in units of mol c kg -1 Net total particle charge, σ P = σ O + σ H + σ IS + σ OS Permanent structural charge, σ O, from isomorphic substitution

Net proton charge, σ H = q H + q OH, or excess / deficit of surface ionizable H + Al-OH + H + → Al-OH 2 +, increase in q H Al-OH → Al-O - + H +, decrease in surface ionizable H + = adsorption of OH - Similarly, surface hydrolysis of adsorbed cation, SM + + H 2 O = SMOH + H + is equivalent to adsorption of OH - (σ O + σ H ) = intrinsic charge since these components depend on the structure of the adsorbent σ O dominates the intrinsic charge for relatively un-weathered soils, whereas σ H dominates for highly weathered soils Former called permanent-charge soils, the latter, variable-charge soils

Other components are Inner-sphere complex charge, σ IS, net charge of ions other than H + and OH - Outer-sphere complex charge, σ OS, net charge of ions other than H + and OH -  σ P = σ O + σ H + σ IS + σ OS Diffuse-ion charge, σ D balances σ P, σ P + σ D = 0 σ Di = Z i / m s ∫ V [C i (x) – C 0i ] dV and Σ σ Di = σ D

σ0σ0 q H + q OH = σ H q +OS + q -OS = σ OS q +IS + q -IS = σ IS σDσD

Points of Zero Charge pH values at which one or more of the components of surface charge vanish Zero point of charge, ZPC pH at which σ P = 0 (  σ D = 0) Adjust pH to value at which soil particles do not move in an electric field

Point of zero net proton charge, PZNPC pH at which σ H = 0, i.e., pH at which q H - q OH = 0 Propose an experimental method for determining PZNPC If σ O = 0, σ H = -(σ IS + σ OS + σ D )  if pH where σ IS + σ OS + σ D = 0, PZNPC is also determined

Note that choice of electrolyte affects PZNPC For example, inner-sphere surface complex formation with a cation leads to decreased q H as by xFe-OH + M x+ = (Fe-O) x -M + xH + So that the PZNPC occurs at lower pH The opposite occurs for a specifically adsorbed anion  Indifferent electrolyte used, e.g., NaCl

3. σ H = pH / ( pH ) pH / ( pH ) – 8 What is PZNPC? pHTerm1Term2σ H ______________________________________________________ 4.What is q pH = 7? mol C kg -1 Also, 0.02 x 5.98 mol c kg -1 x 100 cmol c mol c -1 ~ 12 cmol c kg -1

Point of zero net charge, PZNC pH at which σ IS + σ OS + σ D = 0, i.e., q + - q - = 0 where q +/- is total over all components Propose an experimental method for determining PZNC One approach is simply to measure the CEC and AEC, the PZNC determined when these are equal

6. Find PZNC from given data. q K+ = f(pH)and q NO3- = g(pH) Intersect in pH = 3.7, 3.9 q K+ = a x pH + b q K+ = [(4.0 – 2.6) / (3.9 – 3.7)] x pH + b 4.0 = 7 x pH + b q K+ = 7pH Similarly, q NO3- = -2.5pH q K+ = q pH = 3.85

Point of zero salt effect, PZSE PZNPC determined at different ionic strengths generates family of curves For σ O = 0, intersection at PZNPC

Do problems 5 and 8.