Table 2.
Chemical properties of soil clay minerals, Fe-, Al- (hydr)oxides, and organic matter.
| Species | Layer charge (eq/mol; half unit-cell) | Surface charge density (C/m2) | CEC (cmol/kg) | PZC | Surface functional groups | References |
|---|---|---|---|---|---|---|
| Clay minerals | ||||||
| Smectite | 0.5-1.2 [51] | 0.12 [66] | 70–150 [67] | ∼8.0 [52] | > X– (structural charged sites on basal and interlayer surfaces) > SiOH; > AlOH (silanol groups at edge) |
[52,59,68] |
| Vermiculite | 1.2-1.8 [51] | 0.248 [69] | 100-150 [67] | 8.5 [68] | ||
| Illite | 1.4-2.0 [51] | 0.49 [66] | 10–40 [67] | 2.5 [60] | > XNa+ (cation exchange on the planar sites) > SiOH; > AlOH (surface complexation reactions) |
[60] |
| Chlorite | 1.2–2.6 [51] | 0.56 [66] | 10–40 [67] | <3 [70] | > Si2O (cation exchange on the illite-like face) > Al2–OH (brucite-like or gibbsite-like sheet) > SiOH; > Al(Mg)–OH (silanol groups at edge) |
[70] |
| Kaolinite | <0.01 [51] | 0.064 [66] | 3–15 [67] | 5-6 [54] | > SiOH; > AlOH (edge surface) > Al2–OH (gibbsite-like sheet) |
[54] |
| (Hydr)oxides | Stern layer capacitance (F/m2) | |||||
| Goethite | 1.7 [71] | 8.5 [72] | > FeOH (surface groups) > Fe3OH (Fe at double octahedra chains or at junction of two double Fe octahedra chains) |
[72] | ||
| Hematite | 1.3 [73] | 8.1 [74] | > Fe–O (Lewis acid Fe surface sites) | [74,75] | ||
| Ferrihydrite | 1.17 [71] | 8.2 [76] | > FeOH (surface groups) > Fe3O (Fe octahedra at corner of two faces) |
[76] | ||
| Gibbsite | 1.4 [71] | 5.7 [77] | > AlOH (edge surface groups) | [77] | ||
| Organic matter | ||||||
| Acidic functional groups | R−COOH → R−COO− + H+ (carboxyl, R: aliphatic or aromatic carbon structure) Ar−OH → Ar−O− + H+ (phenolic-OH, Ar: aromatic structure) |
[65] | ||||
| Neutral functional groups and structures | Carbonyl (in esters, ketones, aldehydes, and anhydrides) Aliphatic-OH (alcoholic-OH) Aliphatic and aromatic ether Quinone (aromatic carbonyl) |
[65] | ||||
| Basic compounds by proton complexation | R–NH20 + H+ → R−NH3+ (R: amide, imine, aromatic ring nitrogen) | [65] |
Notes: >X– represents the permanent structural charged sites, and the ion exchange reaction is described by: >XH + Na+ >XNa + H+.