20
3.2 Determining vanadium speciation in soils
Vanadium speciation in soils is important not only from a purely chemical
perspective but also when evaluating toxic risks. Several analytical methods
have been developed to determine vanadium speciation in environmental
samples (Pyrzynska & Wierzbicki, 2004a). The analytical procedure is
complex due to the low vanadium concentrations that commonly occur in
natural samples. Moreover, interference by other metals is a common problem.
Some methods require changes of e.g. pH and redox conditions in the samples
that may change the initial vanadium speciation. Consequently, methods that
involve minimal pre-treatments are preferable.
3.2.1 Extraction and separation techniques
There are few published methods that cover vanadium speciation in soil
samples, but extraction with e.g. phosphate to quantify leachable vanadium(V)
has been suggested (Mandiwana et al., 2005). Determination of vanadium
speciation in natural waters is more common, and those methods may also be
applicable to soil water. The use of chelating resins to separate the vanadium
species has been proposed (Wang & Sanudo-Wilhelmy, 2008; Pyrzynska &
Wierzbicki, 2004b; Soldi et al., 1996). The resins tested in those studies
(Chelex 100 and Cellex P) achieved a maximum sorption of vanadium(IV) and
vanadium(V) at about pH 4.5. It was therefore necessary to adjust the sample
pH before the samples could be run through the column. In principle, the
vanadium species could then be eluted separately at different pH, or by
addition
of
ethylenediaminetetraacetic
acid
(EDTA)
or
trans-1,2-
diaminocyclohexane-N,N,N',N'-tetraacetic acid (CDTA). The complex-
forming ligand EDTA shows good vanadium selectivity (Chen & Naidu,
2002). It forms complexes with both the cationic vanadyl(IV) and the anionic
vanadate(V) by forming the anionic complex [VO(EDTA)]
2-
and
[VO
2
(EDTA)]
3-
, respectively (Komarova et al., 1991). This principle has been
used to determine the vanadium speciation in bottled mineral waters (Aureli et
al., 2008). In that process, the vanadium-EDTA complexes are separated by
high-performance liquid chromatography (HPLC) with an anion-exchange
column with different retention times for the two complexes. The low
vanadium concentrations can then be measured by inductively coupled plasma
mass spectrometry (ICP-MS) (Aureli et al., 2008).
3.2.2 X-ray absorption spectroscopy
X-ray absorption spectroscopy (XAS) is a technique that utilises X-ray
radiation generated by cyclic particle accelerators, synchrotrons (Kelly et al.,
2008). In principle, the specific binding energy of core electrons in atoms can
21
be used to determine the oxidation state, coordination and binding geometries
of different elements. The core electrons are tightly bound closest to the
nucleus in an atom and their binding energy differs between elements and
oxidation states. In XAS, samples are exposed to an X-ray energy range that
covers the core electron binding energy for the element of interest. The
electron absorbs the X-ray photons and is subsequently excited to higher
orbitals or out into the continuum. The core hole thus formed is filled by
another electron in an outer shell, which emits energy that gives rise to the
absorption spectrum.
The XAS technique comprises two different methods; X-ray absorption
near edge structure (XANES) spectroscopy and extended X-ray absorption fine
structure (EXAFS) spectroscopy. XANES spectroscopy is applied to determine
the oxidation state and coordination geometry of single elements. The binding
environment of the element under study can be evaluated by EXAFS
spectroscopy due to scattering of the photoelectron when it interacts with other
atoms that surround the absorber atom. The advantages with this technique
include the ability to detect low concentrations of a single element using
minimum pre-treatment of the sample. In addition, its applicability to both
solid and liquid samples makes the XAS method suitable for soil samples.
In the case of vanadium, the K absorption edge is at 5465 eV and the
oxidation state may be determined from the main edge and features of the pre-
edge peak (Figure 3). Wong et al. (1984) studied the XANES spectra of a large
set of different vanadium minerals and laboratory standards with different
Figure 3. Vanadium K-edge X-ray absorption spectrum including the XANES and EXAFS
regions. Inserted: Enlargement of the XANES region, showing its main features.
22
oxidation states and coordination geometries. The intensity and area of the pre-
edge peak, and the position of the main edge, generally increased with
increasing oxidation state, but they were also affected by the symmetry of the
compound. Hence there may be overlaps in the main edge position and pre-
edge peak intensity between oxidation states (Chaurand et al., 2007b). Despite
some limitations, these absorption features are still commonly evaluated and
compared with vanadium standards when determining the vanadium oxidation
state in unknown samples (Burke et al., 2012; Sutton et al., 2005; Mansour et
al., 2002; Rossignol & Ouvrard, 2001). There are also methods available that
involve analysis of the pre-edge peak position plotted against the pre-edge peak
intensity or area, which can provide further insights into vanadium symmetry
(Chaurand et al., 2007b; Giuli et al., 2004). So far, vanadium K-edge XANES
spectroscopy has commonly been applied to more heterogeneous samples, such
as those originating from metallurgical processes, rather than to soils.
However, it has been applied to soils for other elements, such as phosphorus
(Prietzel et al., 2010; Eveborn et al., 2009). In that case, the shape of the main
edge is of interest as it changes depending on the soil constituents with which
the phosphorus is associated. This can be evaluated by linear combination
fitting (LCF), where the sample spectrum is fitted to a set of standards
representing the possible phosphorus forms in the soil. In the case of
vanadium, the shape of the main edge also changes with binding mode (Wong
et al., 1984). LCF analysis is not commonly applied, but it has been tested for
assessing vanadium binding to iron pipe corrosion by-products (Gerke et al.,
2010).