Studies on the hydrophobic and hydrophilic properties of humic substances (1991)
AuthorsTown, Raewynshow all
This thesis presents a study on the solubility and aggregation properties of humic and fulvic acids, and their interaction with hydrophobic metal complexes and with aqueous metal ions. A parallel study involved quantitative analysis of solution equilibria between Al(III), Cu(II) and simple ligand systems representing possible complexing moieties in humic substances. The solubility and aggregation properties of humic acid were studied as a function of pH and ionic strength by gel permeation chromatography and equilibrium dialysis. Predominantly smaller molecules were dissolved below pH 4; the solubility of the larger humic molecules increased with increased pH and with decreased ionic strength. The potential of XAD resins for the isolation of humic acid from soil extracts was investigated. These macroporous adsorbents were found to be unsuitable for this purpose. Large molecules were excluded from the XAD resins; those components which were adsorbed could not be completely desorbed by strongly alkaline solutions. In conjunction with the solubility studies these results highlighted the operational nature of the fraction defined as 'humic acid' and raised questions about the reported differences between aquatic and soil derived humic substances. The interaction of humic substances with hydrophobic Cu(II) complexes (L = 1-(2-pyridylazo)-2-naphthol and 8-hydroxyquinoline) was studied by ASV (at a NCTMFE) and by visible absorption spectroscopy. Humic substances (HS) interacted hydrophilically with these species (if necessary displacing a low molecular weight ligand) to form a ternary complex, HS-Cu-L. Algal assays established that humic acid could thus ameliorate the extreme toxicity of hydrophobic Cu(II) complexes provided that the displaced ligand itself is not toxic. The apparent lability of Cu(II) and Pb(II) complexes with humic and fulvic acids was studied by anodic stripping voltammetry (ASV). At a hanging mercury drop electrode (HMDE) the Cu(II) complexes of humic and fulvic acids were of similar lability; the apparent lability decreased as the pH increased. Complexes of humic substances with Cu(II) were less labile than those with Pb(II). Results were dependent on the electrode system used for the measurements. The effects of adsorption of humic substances on several electrode surfaces were characterized (HMDE, thin mercury film (TMFE), Nafion-coated TMFE, and bare glassy carbon). These effects were exerted predominantly in the deposition step. A method for measuring the apparent lability of metal-humic complexes was developed which corrected for the contribution from adsorption. Ion selective electrode (ISE) potentiometry was used to probe the complexation capacity of humic and fulvic acids for Cu(II) (at pH 5.0, 6.3, and 7.0) and the relative stability of these complexes (pH 2.5 - 7.5). For both humic substances the complexation capacity increased with pH. Assuming bidentate coordination then, for fulvic acid at pH 5.0, 6.3, and 7.0 respectively, complexation capacity measurements indicated that 82- 85%, 67- 72%, and 50 - 60% of carboxyl groups were not involved in strong Cu(II) binding under the experimental conditions. For humic acid the proportions were 73 - 79%, 33 - 43% and 5 - 25% respectively. Cu(II) complexes with fulvic acid were significantly less stable than those with humic acid. At the same carboxyl group concentration, Cu(II) binding curves for humic acid were displaced markedly to lower pH, indicating stronger binding. For a 1:20 Cu(II):COOH ratio the pH displacement between the humic and fulvic acid curves was 0.65 at pH 3.5 and 1.0 at pH 5.0. The larger humic acid molecules (fractionated by 0.025 μm filtration in weakly acidic solution) were stronger complexors than were the smaller moieties. The apparent stability of Cu(II)-humic complexes decreased with increased metal-to-ligand ratio and with increased ionic strength. The competition of Mg(II) and Al(III) for humic Cu(II) complexation sites was studied. Simulation of the Cu(II) binding curves for humic substances identified the discrete ligands malonate and citrate as reasonable models for humic chelating sites; complexation by salicylate and phthalate moieties was considerably weaker than that by humic substances. Quantitative pH potentiometric studies were made on the complexation of Cu(II) and Al(III) by carboxylate ligands (25°C, 0.1 mol L-1 KCl). The models which best described the solution equilibria in the Al(III) systems were: Al(III)-malonic acid: AlL (log K = 6.71), AlL2 (4.94), AlL3 (2.61), AlL20H (-7.0) Al(III)-isocitric acid: AlHL (10.07), AlL (6.96) The Cu(II)-5-methoxy-N-(2-hydroxybenzyl)sarcosine system (CuL (7.05), CuLH.1 (-3.98)) demonstrated the contribution of 'cascade binding' to metal complex stability. Formation of insoluble species in the Cu(II)-butane-1,2,3,4-tetracarboxylic acid system prevented quantitative analysis.