33
fits were set to include a maximum of three standards and were ordered
according to the reported R-factor (Equation 4).
EXAFS spectroscopy
Vanadium K-edge EXAFS spectroscopy was performed on the ferrihydrite
samples to identify the vanadium complex(es) formed on the ferrihydrite
surface (Paper I). The method can be used to determine the distance between
the central atom (in this case vanadium) and other atoms in the first and second
coordination shells. Vanadium EXAFS spectra were collected for ferrihydrite
samples ranging from pH 3.6 to 9.4 and for two standards of solid and
dissolved vanadate (Na
3
VO
4
and H
2
VO
4
-
, respectively). A total of 3-6 scans
were collected for each sample, averaged and energy-calibrated by means of
the EXAFSPAK programme package (George & Pickering, 1993). The
programme was further used to draw and subtract the background (spline)
function and for modelling the spectra.
4.4.2 HPLC-ICP-MS with EDTA complexation
HPLC-ICP-MS measurements were performed on dissolved vanadium
samples. The method has been described by Aureli et al. (2008) and is
designed to prevent changes in vanadium speciation by adding EDTA prior to
analysis. The V-EDTA complexes are then run through a HPLC coupled to an
ICP-MS.
The method was applied to the Ringamåla soil samples, for which 10 g of
fresh soil were extracted using 20 mL 0.01 M CaCl
2
(Paper V). The aged soil
samples included in Paper III were extracted using 0.01 M CaCl
2
(20 g:20 mL
ratio) and water leachate of BF slag, together with sorption experiments
performed on the Pustnäs and Säby soils, were analysed following the
procedure described (Paper IV). A 50 mM aliquot of Na
2
EDTA was added to
the filtered samples immediately after extraction and stirred for 15 min. The
samples were then stored at 8 ºC until analysis which was performed within
three weeks.
35
5 Results and Discussion
5.1 Vanadium adsorption to ferrihydrite (Paper I)
Iron (hydr)oxides are considered important for the retention of vanadium in
soils and in the case of 2-line ferrihydrite, vanadium was strongly adsorbed
(Figure 7). The adsorption increased with decreasing pH similarly to the
oxyanions of e.g. molybdenum and phosphorus (Antelo et al., 2010;
Gustafsson, 2003), indicating adsorption of vanadate(V). The enhanced
adsorption at lower pH is due to the increase in positively charged surface
sites, which attract the negatively charged ion. Moreover, the adsorption edge
moved towards higher pH values with decreasing Fe:V ratio and hence the
adsorption strength increased with pH when the fraction of ferrihydrite
increased in relation to the vanadium. Adding phosphate to the system reduced
vanadium adsorption (Figure 7) but considering the large amount of phosphate
in comparison with vanadate (200 and 50 µM respectively), vanadate was a
strong competitor for the sorption sites.
Figure 7. Vanadium sorption to 2-line ferrihydrite (left) at different Fe and V concentrations and
(right) in competition with phosphate. Points are experimental observations and lines are
modelled fits.
36
The vanadium K-edge XANES spectra of the ferrihydrite samples
confirmed the adsorption of vanadate(V) (Figure 8). The pre-edge peak
features together with the E
1/2
corresponded well to the H
2
VO
4
-
(aq)
standard. In
addition, interpretations of the EXAFS region showed that the vanadate
adsorbed as an edge-sharing bidentate complex over the pH range studied (3.6-
9.4) (Figure 8). This was a different complex than that determined for
vanadate(V) adsorbed to goethite, which has been identified as a corner-
sharing bidentate complex (Peacock & Sherman, 2004). Differences in
complex formation between ferrihydrite and goethite have also been observed
for copper(II) (Peacock & Sherman, 2005; Scheinost et al., 2001) and for
arsenite (Ona-Nguema et al., 2005).
Adsorption was modelled with the CD-MUSIC model where three surface
complexes, representing different protonation states of the bound vanadate,
were used. The model was based on bidentate complexes as determined by
EXAFS spectroscopy. The sorption pattern could be explained fairly well when
applying the model to the system including vanadate and phosphate (Figure 7).
The model also fitted to a dataset on vanadium sorption to ferrihydrite reported
by Blackmore et al. (1996).
Figure 8. K-edge XANES spectra of vanadium adsorbed to 2-line ferrihydrite (V+Fh) together
with two vanadium standards (left) and the edge-sharing bidentate complex formed on the
ferrihydrite surface (right).