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

Ka of benzoic acid = 6.3 x 10-5

m-NO2 benzoic acid = 32 x 10-5

m-Cl = 15 x 10-5

m-NH2 = 1.9 x 10-5

p-NO2 = 670 x 10-5

p-Cl = 120 x 10-5 p-NH2 = 1.6 x 10-5

What is the Ka 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 surfacebut 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 molc kg-1

Net total particle charge, σP = σO + σH + σIS + σOS

Permanent structural charge, σO, from isomorphic substitution

Net proton charge, σH = qH + qOH, or excess / deficit of surface ionizable H+

Al-OH + H+ → Al-OH2+, increase in qH

Al-OH → Al-O- + H+, decrease in surface ionizable H+ = adsorption of OH-

Similarly, surface hydrolysis of adsorbed cation,

SM+ + H2O = SMOH + H+ is equivalent to adsorption of OH-

(σO + σH) = intrinsic charge since these components depend on the structure of the adsorbent

σO dominates the intrinsic charge for relatively un-weathered soils, whereas σH dominates for highly weathered soils

Former called permanent-charge soils, the latter, variable-charge soils

Other components are

Inner-sphere complex charge, σIS, net charge of ions other than H+ and OH-

Outer-sphere complex charge, σOS, net charge of ions other than H+ and OH-

σP = σO + σH + σIS + σOS

Diffuse-ion charge, σD balances σP, σP + σD = 0

σDi = Zi / ms ∫V [Ci(x) – C0i] dV

and Σ σDi = σD

σ0

qH + qOH = σH

q+OS +q-OS =σOS

q+IS + q-IS = σIS

σD

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 σP = 0 (σD = 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 σH = 0, i.e., pH at which qH - qOH = 0

Propose an experimental method for determining PZNPC

If σO = 0, σH = -(σIS + σOS + σD)

if pH where σIS + σOS + σD = 0,

PZNPC is also determined

Note that choice of electrolyte affects PZNPC

For example, inner-sphere surface complex formation with a cationleads to decreased qH as by

xFe-OH + Mx+ = (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.

σH = 6 104.7-pH / (1 + 104.7-pH) + 2 109.2-pH / (1 + 109.2-pH) – 8

What is PZNPC?

pH Term1 Term2 σH

2 5.98805 2.00000 -0.011953 5.88263 2.00000 -0.11738

7 0.02992 1.98746 -5.98262

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4. What is q+ @ pH = 7? 5.98262 molC kg-1

Also, 0.02 x 5.98 molc kg-1 x 100 cmolc molc-1 ~ 12 cmolc kg-1

Point of zero net charge, PZNC

pH at which σIS + σOS + σD = 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.

qK+ = f(pH) and qNO3- = g(pH)

Intersect in pH = 3.7, 3.9

qK+ = a x pH + b

qK+ = [(4.0 – 2.6) / (3.9 – 3.7)] x pH + b

4.0 = 7 x pH + b

qK+ = 7pH - 23.3

Similarly, qNO3- = -2.5pH + 13.25

qK+ = qNO3- @ pH = 3.85

Point of zero salt effect, PZSE

PZNPC determined at different ionic strengths generates family of curves

For σO = 0, intersection at PZNPC

Do problems 5 and 8.