An Investigation of Inorganic Background Soil Constituents With a Focus on Arsenic Species

A database was developed for the storage and convenient analysis of inorganic background soil constituent data within specific geological groups in Victoria, Australia. A statistical analysis of the data revealed the relative abundances of metals and, in particular, arsenic within soils of various geological units. These units included the Quaternary Aeolian (Qpw) (highest concentration of zinc, lowest concentration of chromium) the Quaternary Fluvial (Qrc) (highest chromium and nickel, equal highest copper, lowest lead and equal lowest arsenic); the Quaternary Newer Volcanics (Qvn) (equal lowest arsenic concentration); Silurian Anderson Creek Formation (Sla) (highest arsenic); Silurian Dargile Formation (Sud) (highest lead, equal highest copper); Tertiary Brighton Group (Tpb) (lowest nickel) and Older Volcanics (Tvo) (lowest copper and zinc). The identification of arsenic as a significant background constituent prompted a formal study of this element with respect to the nature of its sorption onto different kinds of soils, its bioavailability and speciation. Arsenic soil sorption analyses were conducted in the laboratory on clay loam, light clay, sand and silt loam soils. These experiments demonstrated that the sorption of arsenic was dependent on soil type and time of soil exposure to the arsenic solution. The bioavailability of arsenic from soil was also investigated using a relative bioavailability test method referred to as the 'geophagy gut simulation' extraction method. The adaptation of this method to these investigations showed it to be a viable, fast and simple technique. The experimental results indicated that the relative bioavailability of sorbed total arsenic was dependent on soil type. Given that the toxicity of arsenic is dependent on its speciation, techniques were also evaluated to assess arsenic speciation in soil extracts. To this end, the utility of electrospray mass spectrometry (ESI-MS) for the qualitative and quantitative assessment of arsenic and phosphorus speciation in solution was explored. Although this technique yielded interesting qualitative outcomes it was deemed not to be suitable for quantification. From the qualitative data, various postulates were formulated for the interaction between different species that were subsequently tested by quantum chemical calculations. A technique, based on extraction into chloroform, for quantifying the amount of AsIII in a sample was adapted to these investigations and was found to be highly accurate and discriminating, albeit time consuming. All phosphorus and arsenic species found to coexist in the ESI-MS experiments were modelled using high-level density functional theory (DFT). From these calculations, the relative energies of the species could be determined as well as reaction energies for their inter-conversion. This allowed hypotheses to be proposed for the distribution of such species in solution and how they might be taken up into clay structure. The DFT calculations also yielded geometrical information on a wide range of species as well as their electrostatic potential energy maps.