Aluminium geochemistry in coastal lowland acid sulfate soils (CLASS): speciation, reactivity and mobility

The drainage of coastal floodplain sulfidic sediments for agricultural activities in Australia, and worldwide, has resulted in the production of coastal lowland acid sulfate soil (CLASS) environments in which the transport of acidity and Al is the main cause of adverse effects on organisms in adjacent aquatic systems. The controls on Al solubility, speciation and mobility in CLASS are not well researched and the aim of this thesis was to further understanding on the: 1) secondary mineral reactive pools of Al; 2) speciation of Al in soil pore-waters by combining Donnan dialysis and isotope exchange techniques; 3) partitioning of Al between the soil-solid and -solution phases and measurement of exchangeable Al through isotope exchange; 4) extraction salt solutions that best represent the exchangeable pool of Al determined by isotope exchange. From the combination of Donnan dialysis with isotope exchange between radioactive 26Al and stable 27Al, it was observed that Al was present almost solely (> 99%) as negatively charged complexes in CLASS porewaters, presumably with natural organic matter. Secondly, the Al in these complexes was also > 99% isotopically exchangeable and therefore in dynamic equilibrium with the solid and solution phases. Isotopically exchangeable concentrations of Al (E values) were high in a set of 28 CLASS samples. As expected, the sorption coefficient (Kd) of Al was highly pH dependent, but with an unexpected sorption maximum at ~ pH 4.5. Similarly, the E values of Al were also pH dependent and reasonably strong correlations were obtained between E-values and CEC. 1 M KCl extraction solutions always underestimated isotopically exchangeable Al concentrations in these soils. The sequential addition of a 0.2 M CuCl2 extraction step generally improved agreement between both methodologies. A sequential extraction procedure showed that substantial amounts of Al have already been dissolved from primary aluminosilicates initially present in the soils and remain in the soils as reactive secondary Al and Fe minerals with the former being quantitatively a much more important sink for Al. The outcomes of this thesis significantly further our understanding of Al geochemistry in CLASS environments and this will improve upon land management practices