Rahman, MS 2018, 'Sorption, kinetics and bio-availability of arsenic in contaminated soils', PhD thesis, Southern Cross University, Lismore, NSW.
Copyright MS Rahman 2018
Arsenic (As) is a highly toxic, class I, non-threshold and carcinogenic element, and therefore, As contamination of soils is a major environmental threat, where it may cause several adverse health effects, even at very low concentrations. The historical application of As-based pesticides has resulted in the contamination of soils with As in many parts of Australia. This thesis examines the sorption process and kinetics of As by laboratory batch sorption experiments, solid phase speciation, mobility, accessibility, and bioavailability of As using X-ray absorption spectroscopy (XAS), isotope exchange technique, chemical extraction methods and a biological test to drill down and elucidate some of the risk assessment and management issues around As-contaminated soils. Two different soil types are used, 1) cattle dip soils, such as, aged As-contaminated ferralitic and sandy soils from historical cattle dip sites and 2) control soils, which include pristine ferralitic and sandy soils collected from sites containing natural As background levels.
Arsenic sorption and the kinetics of this sorption strongly influenced the mobility of As in soil, which further influenced the bioavailability and toxicity of As. In As-contaminated soils, increasing pH caused a substantial decline in As(V) sorption. Similarly, the presence of P (PO43-) and S (SO42-) decreased the As(V) sorption amount by the aged contaminated soils with limited sorption sites, whereas P and S had little effect in pristine soils. However, Ca2+ amendments of cattle dip soils may contribute to decrease As accessibility. As(V) sorption kinetics suggest that the surface binding to exchangeable and specific sorption sites have an impact on the overall rate for the aged contaminated soils. Consequently, sorption to relatively accessible sites may contribute to increased mobilisation of newly adsorbed As. In contrast, As(V) sorption to pristine ferralitic soil seems mostly controlled by a complex combination of surface complexation, intra-particle diffusion, and/or surface precipitation; complexity that appears to contribute to greater As(V) retention to binding in soils.
Ageing in As-mobility for contaminated soils shows that historic soil As is strongly bound to the soils (> 90% irreversibly), and has a lower degree of As bio-accessibility (< 15%) and earthworm bioaccumulation (< 9%). This indicates that ageing time is an influential factor in defining the soil As labile fraction. Consequently, freshly added As (either added to historically loaded soils or pristine soils) has a significantly higher degree of As-accessibility. (XAS) data indicate that historic soil As is dominated as Ca-, Al- and Fe-mineral precipitates, which are key to lower bio-accessibility and bioaccumulation in these soil. Whereas newly added As is dominated by mineral sorption surfaces, particularly to iron oxy-hydroxides (goethite and hematite) and gibbsite and kaolin surfaces, and hence have substantially higher bio-accessibility and bioaccumulation effects.
The detailed knowledge generated in this thesis would be helpful for a more comprehensive risk characterisation and screening, reclamation, and management of the As-contaminated sites. Moreover, the combination of techniques applied in generating data provide enormous assistance for other environmental scientists in making judgement calls regarding the contamination state and environment/health risks associated with As-contamination in soils. Consequently, the findings of this thesis examine the validity of many current environmental quality guidelines, and may allow for future guideline assessments and adjustments.