Metal organic complexing in soil systems (1987)
AuthorsGregor, Jan E.show all
The work described in this thesis is concerned with properties of soil derived fulvic acids. A method for isolating soil fulvic acids is described. The method makes use of the metal complexing agent pyrophosphate (pH 3) as the extractant. The hydrophobic XAD-7 resin was used for the selective separation of the non-ionic fulvic acids from the ionic extractant at low pH. Fulvic acids obtained after a single XAD-7 treatment had acceptably low ash contents (< 0.6%). Chemical and physical properties of these fulvic acids were compared with those of fulvic acids that had been subjected to high and low pH conditions (conditions normally encountered during traditional methods of extraction). The resultant changes in molecular weight (gel exclusion chromatrography), equivalent weight and protonation constants (potentiometric titrations) and in metal binding properties (ion selective electrode potentiometry, fluorescence spectroscopy and anodic stripping voltammetry) are discussed. The acid-base titration curves for fulvic acid have been interpreted in terms of fulvic acid having four structurally independent carboxyl groups, having mean pKn values pK₁ˈ .... pK₄ˈ. The results for six fulvic acids were in the range log K₁ˈ 7.2 - 6.5 (c 8% of total carboxyl acidity), log K₂ˈ 5.9- 5.3 (c 11%), log K₃ˈ 5.0- 4.1 (c 20%) and log K₄ˈ 3.2- 2.4 (c 60%). These residues indicate a low percentage (not more than 8%) of polyprotic acid residues (e.g. 1,2,3-tricarboxylic acid); these residues may be important complexing sites. A comparative study of fulvic acid complexing, both between fulvic acids and with model ligands, is reported. Ion selective electrode potentiometry, fluorescence spectroscopy and anodic stripping voltammetry were used to study copper(II) and lead(II) complexing. A variety of binding sites with different affinities for metal ions was indicated. The stronger binding sites are involved in binding at low metal concentration followed by weaker binding sites at higher concentrations of metal. Copper(II)-fulvic acid binding curves (i.e. % free metal vs. pH) are similar to those for citrate, malonate and aspartate, and were modelled numerically by mixtures of these competing ligands. The citrate ion was established as a useful model for describing the complexing reactions of fulvic acid. A detailed study has been made of the equilibrium reactions ofcitrate with protons and aluminium(III). The protonation constants for citrate are log K₁ 5.90, log K₂ 4.35 and log K₃ 2.91. The model best describing the A1(III)-citrate equilibria was found to be A1HL + (log β 11.02), AlL (8.35), A1 (HL)L²- (17.36), A1L₂ ᶟ- (13.40), A1L₂H₁₄⁴- (pK A1L₂ ᶟ-, 6.07) and A1 (LH₁)₂⁵- (pK A1L₂H₁⁴-, 7.09).