 Abstract | 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. |
