39
5.2.2 Bioavailability
The dissolved vanadium speciation was estimated by HPLC-ICP-MS on
CaCl
2
-extracts of the aged Pustnäs, Säby and Ter Munck soils (Paper III), and
on the solutions obtained from sorption experiments performed with the
Pustnäs and Säby soils (Paper IV). The results were consistent with the
hypothesis that the prevailing oxidation state is vanadium(V). In most soils
vanadium(V) comprised more than 90% of total dissolved vanadium
concentration (Table 6). Considering the conditions in the soils used in the
toxicity experiments, such as pH and organic matter content, the measured
bioavailable vanadium was expected to mainly consist of vanadium(V) in all
soils.
It was evident from the large range of toxic threshold values that different
soils and different vanadium treatments affected the concentration of
bioavailable vanadium. The FSS value, which was determined for the soils
used in the freshly spiked treatment, was positively correlated to the measured
EC50 values (Figure 10). In a correlation analysis performed on the EC50
values and different soil properties, none of the soil properties included could
statistically be identified as significantly affecting the threshold values.
However, there was a correlation between EC50 values and oxalate-extractable
iron for four of the soils, excluding the Guadalajara soil. Metal (hydr)oxides
are important for vanadium sorption in soil, but they are not the only
controlling factor (Gäbler et al., 2009).
Ageing of the soils decreased the vanadium bioavailability. This was shown
not only by the EC50 values obtained, but also by the kinetic experiments in
which the concentration of soluble vanadium decreased by approximately a
half between 14 and 100 days after vanadium spiking. The importance of metal
Table 6. Vanadium speciation in the dissolved phase of the soil, determined by HPLC-ICP-MS on
CaCl
2
-extracts of aged soils and on the solution of soils subjected to sorption experiments.
Soil
V addition
Dissolved V
V(IV)
V(V)
(mg kg
-1
)
(mg L
-1
)
(%)
Aged soils (Paper III)
Pustnäs
H
2
VO
4
-
(aq)
150
3.03
4
96
Säby
H
2
VO
4
-
(aq)
290
0.65
9
91
Ter Munck
H
2
VO
4
-
(aq)
270
3.16
4
96
Sorption experiments (Paper IV)
Pustnäs
VO
2+
(aq)
115
2.00
3
97
Pustnäs
H
2
VO
4
-
(aq)
115
4.67
4
96
Säby
VO
2+
(aq)
115
0.23
32
68
Säby
H
2
VO
4
-
(aq)
115
0.42
25
75
40
Figure 10. (Left) Vanadium sorption strength and (right) oxalate-extractable iron in relation to
estimated EC50 values in the plant growth assays.
(hydr)oxides for vanadium sorption was confirmed by oxalate extractions of
the aged soils, according to which extractable vanadium increased with ageing
time. This is probably because vanadium was incorporated into the metal
(hydr)oxides during ageing, as suggested by Martin and Kaplan (1998).
The plant vanadium content increased linearly with increasing aqua regia-
extractable vanadium concentration in the soil when plotted separately by soil
and vanadium treatment (Figure 11). Thus the vanadium concentration in the
plant could be used as an estimate of bioavailable vanadium content in the soil.
Even though vanadium is mainly accumulated in the roots, this relationship has
also been observed in a pot experiment performed with alfalfa (Yang et al.,
2011). The largest increase in vanadium uptake (slope) was observed for the
freshly spiked soils and the lowest for the aged and BF slag-treated soils
(Figure 11). As discussed above, a fraction of the vanadium in the aged soils
was incorporated into metal (hydr)oxides, but for the BF slags the slow
dissolution of vanadium from the slag matrix was an additional process that
controlled the bioavailability.
When all soils and vanadium treatments were plotted together, the
correlation between vanadium bioavailability (i.e. plant vanadium) and the
aqua regia-extractable vanadium concentration in the soil was weak, having
large variation (Figure 12). This variation was however reduced significantly
by comparing the plant vanadium against the dissolved vanadium
concentration in the soil. In that case, the variation in plant vanadium was
much smaller, regardless of vanadium treatment and soils. In other words, the
dissolved vanadium concentration in the soil was a much better estimate of
bioavailable vanadium in different soils and vanadium treatments.