Metal-organic complexing in natural soil systems
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A series of metal-ligand equilibria have been studied which may be pertinent to the dissolution and transport of sesquioxides in soil systems. In particular the interactions of A1(III) and Fe(III) with polyphenols (1,2-dihydroxybenzenes) in aqueous solution have been investigated. The complexing reactions between A1(III) and catechol, protocatechuic acid and catechin, and between Fe(III) and protocatechuic acid have been studied quantitatively. The reactions of epicatechin, an epicatechin dimer (designated B2) and an epicatechin polymer (designated B13) with these metal ions were studied semi-quantitatively. It was observed that those ligands with a standard reduction potential < 0.9 V underwent both complexing and redox reactions with Fe(III) at pH ≤ 5.6. The polyphenols chosen for study are representative of species found in soil solutions. These polyphenols may be important in solubilizing, and hence mobilizing, Fe(III) and A1(III) in soil systems. This study reports the protonation constants for catechol, protocatechuic acid, catechin and epicatechin derived from both potentiometric and spectrophotometric measurements. By reference to model compounds the protonation sequence for the four phenolate oxygens in catechin and in epicatechin have been assigned. The epimerization of catechin to epicatechin and vice versa was investigated. The protonation and complexing studies used a glass/calomel electrode system as a probe for hydrogen ion concentrations in the pH range 3.0 - 11.5. A method of electrode calibration using a series of titration-generated o-phthalic acid buffers of known [H+] was further developed in this work. Spectrophotometric and visual tests for extractable Fe(II) and A1(III) in the soil have been developed and their possible use in the field investigated. A computer model was used to rank some of the above polyphenols and a representative series of other ligands in their ability to complex aluminium(III) and to dissolve gibbsite, A1(OH)₃.