No abstract

Zinc deficiencies are generally more widespread on calcareous soils than are Cu deficiencies. The concentration and the degree of complexing of Zn and Cu were measured in the soil solution of 20 calcareous soils from Colorado. The Zn levels in solution were generally less than 2 ppb with rarely more than 75% of the Zn being complexed. Copper levels generally ranged from 5 to 16 ppb in the soil solution with 98 to 99% of the Cu present as organic complexes. Thus, complexing increased the total Cu concentration in solution by a factor of about 100, whereas it increased the total Zn in solution by a factor of 4 or less. Compared to acid soils studied previously, soluble Zn levels were much lower and complexing of Cu was somewhat higher in calcareous soils.

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.

Specific reactions between montmorillonite and Co in low concentrations were studied in the presence of excess CaCl2. Two forms of bound Co were distinguished. One form, characterized as being slowly dissociable, appears to be bound in a monolayer by chemisorption and will exchange with Cu++, Zn++, or other Co++ and to a lesser extent with ions of other transition metals but not with Ca++, Mg++, or NH4+. A second form of Co did not dissociate and could not be extracted from the mineral with 2.5% acetic acid. This fraction, which was not characterized, is thought to either enter the crystal lattice or be occluded in the precipitation of another phase.

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.

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.

No abstract

The proposition that hydrolysis may account for the selective adsorption of heavy metals by clay surfaces is compared with an alternate explanation of specific exchange of the metal with weakly dissociable H+. It is shown that past evidence offered in support of the two hypotheses is somewhat ambiguous. Theoretical treatment is presented for the competition of ions in adsorption reactions which provides a means of distinguishing between the two alternatives through the effect of H+ on the metal‐clay reaction in the presence of a competing ion. Experimentally, the effect of H+ on the reaction of Co2+ with montmorillonite is studied in the presence and absence of Mg2+, a weakly competing ion. Ionic strength is held constant, and nonspecific reactions are minimized with an excess of CaCl2. The results are interpreted as favoring the hydrolysis hypothesis. Inherent assumptions are considered.