The solubility of Se in seven soils indicate that Se concentration in solution is governed primarily by a ferric oxide‐selenite‐adsorption complex (Se oxidation state +4). However, under certain conditions Se may also exist in the oxidation states +6, 0, and −2. The proportions of Se in the four oxidation states are treated theoretically as they are affected by the redox potential, soil pH, and ions with which Se combines.
Procedures were developed for the partition of heavy metals (Cu, Co, and Zn) between complexed and uncomplexed forms in soil solution. Competition for the above cations by complexing agents naturally present in soil solution and added complexing agents that form metal complexes soluble in organic solvents was determined by measuring the distribution of metal between aqueous and organic phases in a two-phase system. The degree to which the metal was complexed in the original soil solution could be deduced by comparing the amount of metal extracted from soil solution with that extracted from water at the same pH. Examination of soil solutions from several mineral soils indicated that in the case of Cu as much as 99% of the metal may exist in a complexed form.
Soil solutions from several areas of the US were analyzed by atomic absorption for total Mn and by resin exchange for percent Mn complexed. Values for the latter (84%–99%) in soil solution from the A horizon were intermediate to those previously reported for Zn and Cu. Soil solution from a New York forest soil at pH 7 contained 13 ppm Mn, of which 93% was complexed. Spectrophotometric and polarographic methods were developed to determine the oxidation state of Mn in this soil solution. Both methods indicated the Mn in solution was present in the +2 oxidation state. The presence of such large concentrations of Mn2+ in a soil solution of a neutral soil was still less than that predicted from measured values of the Eh of the soil solution and the Mn2+/MnO2 half‐cell potential. Soil microorganisms oxidized and precipitated Mn2+, presumably as MnO2, but the presence of the neutral soil solution that contained 13 ppm Mn partially inhibited this precipitation.
At least three groups of compounds were found to complex Zn2+ and Cu2+ in soil solution from the A horizon of a Williamson silt loam. A nondialyzable fraction of soil solution ligands had a poorly defined acid dissociation, pKa, ranging from 3 to 4.7. The dialyzable fraction had acid dissociation constants of 4.5 and 9.5, which were attributed to aliphatic and amino acids, respectively. The equivalent concentration of the nondialyzable acid fraction was only 1/40 that of the dialyzable fraction, but was more effective in complexing Zn and Cu in the soil solution. The average metal/ligand ratio was equal to unity in complexes of both the dialyzable and nondialyzable fractions with Zn2+ and Cu2+. Estimated, log10 (average formation constants) of the nondialyzable‐metal complexes were calculated on the basis of titratable acidity as 4.3 ± 0.1 and 5.5 ± 0.1 for Zn2+ and Cu2+, respectively.
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