29
BF slag-treated soils was determined by 0.01 M CaCl
2
extraction at a 20 g
soil:20 ml CaCl
2
ratio with an equilibrium time of 24 h. The two extraction
methods were compared for some samples of the freshly spiked soils and the
vanadium concentrations were found to be within the same range.
The vanadium concentration in the plant (plant vanadium) was determined
for the barley plants from the shoot assay by digesting 200 mg of plant material
with 3-4 mL of 67% nitric acid at 180 ºC. All vanadium concentrations
determined for soils and plants were measured by ICP-OES.
4.2.4 Statistical evaluation
The EC10 and EC50 values represent the concentration of added vanadium in
the soil at which a 10 and 50% reduction in the response occurs, respectively.
These values were determined for all toxicity assays performed on the freshly
spiked and aged soils by a log-logistic dose response model (Equations 1-2):
?????? =
??????
1 +
1
9
??????????????????(?????? × ????????????
??????
????????????10
)
????????????????????????????????????????????????1
?????? =
??????
1 + ??????????????????(?????? × ????????????
??????
????????????50
)
????????????????????????????????????????????????2
where Y represents the response (i.e. barley shoot biomass), c is the response
parameter in the control, b is the slope parameter and X is the added vanadium
concentration (total vanadium concentration minus the background vanadium
in the soils from aqua regia digestions).
Significant differences in threshold values between soils and assays were
pair-wise tested by single-sided t tests with 95% confidence limits.
4.2.5 Soil sorption properties
Vanadium sorption isotherms were determined for five of the mineral soils
(Paper II). The isotherms were determined by performing batch experiments in
which a range of dissolved vanadate(V) concentrations (0-15 mg V kg
-1
) was
added to the soils and equilibrated for six days. The vanadium sorption
isotherms were then determined according to the Freundlich equation
(Equation 3):
??????
????????????????????????
+ ??????
????????????????????????
= ??????
??????
× ??????
??????
????????????????????????????????????????????????3
where the total concentration of sorbed vanadium is the initially sorbed
vanadium (n
init
) plus the vanadium sorbed from additions (n
sorb
) and c is the
measured dissolved vanadium concentration in solution. K
F
(the Freundlich
30
coefficient) and m (non-ideality parameter) are adjustable parameters. The n
init
was fitted by trial and error, using the linear regression tool on log-transformed
values in Microsoft Excel. The best fit was selected based on the highest
obtained R
2
value and the optimised m and K
F
could be obtained from the
linear equation derived. Furthermore, the Freundlich sorption strength (FSS)
was determined from the sorption isotherms for each soil and represented the
amount of sorbed vanadium when the solution contained 2.6 mg V L
-1
.
In one experiment in paper III, the vanadium reaction kinetics were studied.
In addition to the three aged soils, a fourth soil (Hygum, Table 2) was included.
The four soils were treated with two different concentrations of vanadium (32
and 100 mg V kg
-1
) and incubated at 20 ºC. Between 3 and 100 days after soil
spiking, sub-samples were taken and extracted with 0.01 M CaCl
2
to evaluate
the change in soluble vanadium over time. Oxalate extractions was also
performed on the amended and non-amended Pustnäs, Säby and Ter Munck
soils to quantify the amount of vanadium retained by metal (hydr)oxides.
The Pustnäs, Säby and Ter Munck soils were also subjected to speciation
analysis by XANES spectroscopy and/or HPLC-ICP-MS, as described in the
“Analytical methods” section.
4.3 Long-term field study (Paper V)
The long-term field study was carried out in a pine forest stand at Ringamåla in
southern Sweden that had been amended with converter lime in 1984, 26 years
prior to sampling (Figure 6). The converter lime contained 14.6 g V kg
-1
and
had been added manually to the soil surface at concentrations of 0, 0.2, 0.7 and
1.0 kg V m
-2
in 10 m × 10 m plots, corresponding to vanadium additions of 2.9,
10.2 and 14.6 g m
-2
. Each lime addition was made in triplicate plots except for
the highest addition, for which only one replicate was available. Separate
Figure 6. (Left) The Ringamåla field site and (right) schematic diagram of the soil layers
sampled.