Micronutrient Cation Complexes in Soil Solution: III. Characterization of Soil Solution Ligands and their Complexes with Zn²⁺ and Cu²⁺

Abstract

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.

Stability Constants of Metal–Humic Acid Complexes and Its Role in Environmental Detoxification

“…Also, H> and the metal ion compete for binding sites on the ligand and less metal is bound at the lower pH. Geering and Hodgson (1969) reported that at pH(4.5 and constant ionic strength, both Zn> and Cu> formed 1 : 1 molar complexes with what they referred to as the &&natural ligand'' of the soil solutions. O'Loughlin et al (1999) have studied the reductive dechlorination of trichloroethene mediated by humic metal complexes and reported that these complexes may represent mediators in natural environments.…”

Cu2+ – fulvic acid chelation equilibrium in 0.1 m KCl at 25.0 °C

“…Since the average polymer molecule has about three chelating sites, this result is to be expected if less than 113 of the chelating sites have reacted. In discussing the nature of the Cu2' -fulvic acid chelate, Geering and Hodgson (25) noted that the complexing reaction releases an otherwise non-titratable H + , and that the complex is more strongly formed than would be expected if only one functional group were bound to each metal ion. They postulated the existence of a chelate ring, but suggested that a phosphate acid group and a carboxyl group might constitute the bidentate chelating sites.…”

Aspects of the biogeochemistry of carbohydrates in aquatic environments

“…The restriction of the method to mononuclear complexes is not serious in acid solutions, since Geering and Hodgson (1969), Schnitzer andHansen (1970), andSchnitzer (1971) demonstrated that at pH values -( 4.5 and I = o _ 0.1, molar divalent metal-fulvic acid ratios are about unity. In the pH range of 4.5 to 10, the ratios gradually increase from 1 to 2.…”