Chemical transformations of Fe and Al from acid sulfate soils to coastal waters

Abstract

The aim of this thesis was to investigate the chemical processes involved in the production and transformation of the major acid sulfate soil (ASS) contaminants, iron (Fe) and aluminium (AI), and their associated acidity, as they are transported from the soil to coastal waters. The results of these investigations are summarised below. Within the body of the thesis, the results are discussed in relation to currently available remediation options aimed at reducing the export of these contaminants to neighbouring estuaries.

Laboratory studies established that high concentrations of dissolved silica (Si) and natural organic matter (NOM) present in coastal lowland acid sulfate soil (CLASS) groundwaters inhibit the Fe(II)-catalysed transformation of poorly crystalline Fe(Ill) minerals to thermodynamically stable Fe(III) minerals. This is consistent with CLASS environments being dominated by Fe(I]I) minerals that readily undergo reductive dissolution.

The release of soluble AI from the soil profile was found to be highly pH dependent. Solid phase and theoretical solubility calculations indicate the solubility of AI within CLASS groundwaters above approximately pH 4.5 is most likely controlled by the formation of a mixture of amorphous basaluminite (AI4(OH)J(..~D4.4H20) and AI(OH)3' Basaluminite is also predicted to be the major species controlling the solubility of Al at the interface of the CLASS flood drains and pH 6-8 buffered estuarine waters. Chemical Transformations of Fe and AI from Acid Sulfate Soils to Coastal Waters Abstract

Laboratory studies indicated that dissolved NOM-facilitated transport of Fe(III) or Al(III) is not a significant transport pathway in CLASS environments. For Fe(III), this is due to NOM concentrations limiting the concentration of soluble Fe(III)-NOM complexes which may form. In the case of Al(III), AI(III)-NOM complexes formed were found to readily dissociate, thereby limiting the ability for dissolved NOM to transport significant concentrations of Al(III) large distances off-site. The concentration of sulfate within ASS discharge waters was also found to severely limit the potential for the highly toxic inorganic polymeric Al13 species to form.

Field and laboratory studies demonstrate the photo-reduction potential of both dissolved and colloidal Fe(III) within the flood drains. This aids in maintaining higher concentrations of soluble Fe(II) than would otherwise be present in the absence of sunlight, increasing the transport of Fe to the main estuary. Laboratory studies also ascertained the degree to which SRFA catalyses the oxidation of Fe(II) at pH 3-5. A kinetic model encompassing the Fenton cycle, precipitation and catalysed Fe(II) oxidation satisfactorily modelled field results and identified the main chemical transformations of Fe occurring within the acidic flood drains.