Interchange of Solutes between Solid, Liquid and Gas Phases - The Biogeochemistry of Submerged Soils - page 77

ions are those that can be replaced easily by leaching with an electrolyte solu-
tion. This is an empirical definition, but only fully solvated ions can be readily
exchangeable and therefore must be either in the diffuse-ion swarm or in outer-
sphere complexes.
Adsorption interacts strongly with complexation in solution. Table 3.13 indi-
cates the range of complexes between metal ions and inorganic and organic
ligands in soil solutions. In a submerged soil the organic ligands present include
acetate, formate and propionate at concentrations of 10-40 mM in the early stages
following submergence though less than 1mM after 3-4 weeks. In addition con-
centrations of amino acids, phenolic acids and larger molecular weight humic
acids may reach a few hundred M, though transiently. Figure 3.12 shows the

calculated effects of realistic concentrations of acetate, formate, propionate, glu-
tamate, glycine, benzoate and phenylacetate on Fe(II), Mn(II) and Zn(II) species.
The figure shows that for Fe(II) and Mn(II) the free ion dominates at all pHs,
except for Fe above pH 9 where hydroxy complexes are important. Complexes
with acetate are also significant at pHs above about 5, and FeHCO 3 + above
pH 6 and MnGlu above pH 5. Complexes with formate, propionate or either
of the phenolic acids are unimportant at all pHs. The picture is more compli-
cated for Zn(II) with many more significant species. The free ion dominates at
pH ≤ 7 . 5 but complexes with acetate, HCO 3 − , glutamate and especially CO 3 2 −
are important at various pHs. Hydroxy complexes are only important at pH > 9.
Figure 3.13 shows the solubility of Zn 2 + in soil at four Zn levels and different
pHs. The figure shows that the soil solution is under-saturated with respect to
likely pure Zn precipitates up to high pHs, and there is a marked minimum
in solubility at near neutral pH. The explanation involves cation exchange and
specific adsorption reactions, trace amounts of Zn 2 + being sorbed preferentially
over the main exchanging cations, and complexation reactions between Zn 2 + and
organic ligands in solution. The negative charge on soil surfaces increases as the
pH increases, tending to increase sorption of Zn 2 + on variable-charge surfaces.
But at near neutral pH the concentration of dissolved organic matter in solution

µ

Table 3.13 The main species of trace metals in soil solutions
Metal Acid soils Alkaline soils

Mn 2 + , MnSO 4 0 ,Org a Mn 2 + , MnSO 4 0 , MnCO 3 0 , MnHCO 3 + Mn(II)

Fe 2 + , FeSO 4 0 , FeH 2 PO 4 + FeCO 3 0 , Fe 2 + , FeHCO 3 + , FeSO 4 0

Fe(II)

Ni 2 + , NiSO 4 0 , NiHCO 3 + , Org NiHCO 3 0 , NiHCO 3 + , Ni 2 +

Ni(II)

Org, Cu 2 + CuCO 3 0 ,Org Cu(II)

Zn 2 + , ZnSO 4 ZnHCO 3 + , ZnCO 3 0 , Zn 2 + , ZnSO 4

Zn(II)

Cd 2 + , CdSO 4 0 , CdCl + Cd 2 + , CdCl + , CdSO 4 0 , CdHCO 3 + Cd(II)

Pb 2 + ,Org,PbSO 4 0 , PbHCO 3 + PbCO 3 0 , PbHCO 3 + , Pb ( CO 3 ) 2 2 − , PbOH + Pb(II)

a Org, organic complexes, e.g. with fulvic acids.
Source : adapted from Sposito (1983). Reproduced by permission of Elsevier.