Interactions between soil organic matter and aluminium
Al equilibria with soil organic matter (SOM) was studied in terms of two basic reactions involving exchangeable Al and chemically bound (complexed) Al. Ca ↔ Al exchange on eight soils of variable SOM content resulted in cation exchange capacities (CEC) which varied with the degree of Al saturation. This effect was attributed to the release of organically bound Ca by 1M KC1 as shown by the relative homogeneity of Al adsorption strength and heterogeneity of Ca adsorption revealed by Langmuir analysis. Acetylacetone extraction gave virtually no 'non-exchangeable' Ca but the exchangeable Ca following Ca saturation, was invariably greater than the corresponding Al fraction even on a wholly organic soil. A persistant pH drop, despite repeated equilibration was definitely attributed to continuing Al adsorption. The weak acid nature of the SOM polycarboxylates was analysed by slow auto-titration. Analysis of the apparent acidity constant variation with polymeric charge showed trends consistent with 'electrostatic principles' where weakening of carboxyl-acid strength, occurs as a result of polymeric charge build-up. The extent of this depended on the COOH content/density. A high COOH content also resulted in a low 'intrinsic' pKa (at zero charge) and this was assumed to be a product of chemical dipole effects. However, when the SOM fractions were compared with poly-maleic acid fractions of various molecular weights, low m.w. pure polycarboxylates and pure dicarboxylic acids some other influence on pKa values was in evidence. The exceptionally low intrinsic pKa values for fulvic acids and low m.w. water soluble soil acids were considered to be due not only to COOH clustering but also to carbonyl or hydroxyl substitution around COOH groups. A semi-empirical study of pH buffer capacity and coagulation of soluble SOM by Al was conducted on peat, and the relative complexing and coagulating powers of six metals were assessed. Complexing power depended on the individual metal, whereas coagulation was largely a function of the metal ion valence. This meant that for any degree of complex formation, the solubility of different metal complexes was variable. Transition metals apparently produced the most soluble complex species. An attempt was made to assess 'differential scanning calorimetry' as an analytical tool with respect to metal-SOM complexes. The degree of metal saturation of a SOM fraction gave regular changes in the main DeltaT peaks and different metals produced different patterns. However, it was clear from the study that the major influence on the temperatures of thermal decomposition was of a physical rather than a chemical nature. Accordingly, it is possible that the micro-configuration of coagulated SOM-metal species dictate the DSC pattern rather than the strength of metal-SOM bond as suggested by other workers. The reproducibility of the technique as well as its qualitative interpretation limit its use. In order to test the electrostatic treatment of SOM with respect to Al bonding, Cu chelation was studied so that free ionic Cu could be determined by the use of an ion selective electrode (I.S.E.) and so provide a check on the more theoretical 'pH method' with respect to species prediction. The method used required computer analysis using successive approximations to resolve a 'proton competition' constant which proved to be approximately constant over a wide range of metal:SOM ratios. Furthermore, the true co-ordination number was virtually invarient at ≈2, indicating only a single bidentate chelate with respect to Cu bonding. The pH and ISE derived stability constants showed good agreement. From the stability constants for Cu-SOM chelation, a model of the soil solution was constructed where the proportion of Cu present as organically complexed metal was expressed exactly as a function of mg SOM l-1 and p(L-) and approximately as a function of mg SOM l-1 and pH. The techniques used to study Cu-polycarboxylate equilibria were applied to Al-SOM and complex formation with low m.w. aromatic and aliphatic carboxylic acids was studied also. The bidentate chelation constant of Al-SOM at a polymer charge of zero closely resembled Al-succinic/glutaric/adipic acid equilibria and a 1:2 Al-acetate complex. As the polycarboxylate charge increased, the Al bond strength increased. The lower m.w. soil acids bonded with Al more weakly because of the same inductive effects controlling proton association. A model of the soil solution with respect of Al chelation was constructed including allowance for hydrolysis species. The relevance of the model with respect to solubility of Al-SOM and soluble SOM in soil solution was confirmed by a coagulation study and by measuring soluble SOM concentration in agricultural topsoil solutions covering a pH range of 4.5--7.5.