particle surfaces surface functional groups adsorption surface charge points of zero charge
DESCRIPTION
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 10 -5 - PowerPoint PPT PresentationTRANSCRIPT
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Particle Surfaces
Surface Functional Groups
Adsorption
Surface Charge
Points of Zero Charge
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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?
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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)
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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
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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
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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
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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
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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)
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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
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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?
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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
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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
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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
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σ0
qH + qOH = σH
q+OS +q-OS =σOS
q+IS + q-IS = σIS
σD
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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
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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
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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
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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
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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
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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
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Point of zero salt effect, PZSE
PZNPC determined at different ionic strengths generates family of curves
For σO = 0, intersection at PZNPC
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Do problems 5 and 8.