23
4 Materials and Methods
This thesis is based on five studies (Papers I-V) dealing with various aspects of
vanadium behaviour in soils. These include the sorption pattern of vanadium to
2-line ferrihydrite (Paper I); vanadium sorption and speciation in soils and
toxicity and bioavailability to soil microorganisms and plants in different
mineral soils and with different vanadium treatments (Papers II, III, and IV);
and the long-term impact of vanadium solubility and speciation in a forest soil
(Paper V). The main experimental approach used throughout the work
consisted of batch experiments and toxicity assays (Table 1). The batch
experiments were applied to different soils and soil constituents to evaluate
vanadium sorption, solubility and speciation. Speciation analysis was
conducted on both solid samples and solutions by applying XANES
spectroscopy and HPLC-ICP-MS, respectively (Table 1). In addition, EXAFS
spectroscopy was used to determine the structure of the vanadium surface
complex(es) formed on ferrihydrite.
Table 1. Description of experimental and analytical methods applied in Papers I-V.
Paper I
Paper II
Paper III
Paper IV
Paper V
Batch experiments
√
√
√
√
√
Toxicity assays
√
√
√
EXAFS
√
XANES
√
√
√
HPLC-ICP-MS
√
√
√
24
4.1 Vanadium sorption to 2-line ferrihydrite (Paper I)
Ferrihydrite (Fh) is a poorly ordered naturally occurring iron (hydr)oxide with
a large surface area that can retain a number of different elements (Cornell &
Schwertmann, 2003). The ferrihydrite used in this study was synthesised in the
laboratory using an adapted version of the method described by Swedlund and
Webster (1999) and Schwertmann and Cornell (2000), resulting in 2-line
ferrihydrite. The sorption experiments were conducted by adding dissolved
vanadate(V), in a background electrolyte of 0.01 M NaNO
3
, to Fh in a series of
centrifuge tubes. In four single-sorbate series at different Fh:V ratios,
vanadium sorption was studied as a function of pH by addition of HNO
3
or
NaOH. In one ternary system, sorption competition was evaluated by adding
dissolved phosphate together with vanadate(V) at different pH values. In
addition, three series involved pH-dependent phosphate sorption in single
sorbate systems and at different Fh:P ratios. The samples were equilibrated
during 48 h in an end-over-end shaker and then centrifuged to separate the
dissolved phase from the sorbent. The pH value was measured in the
supernatant, which was then filtered (0.2 µm Acrodisc PF filter) and analysed
for vanadium and other relevant elements (e.g. Fe, Al & P). The amount of
sorbed vanadium was estimated by subtracting the measured vanadium
concentration in solution from the total added vanadium.
Vanadium speciation and coordination to ferrihydrite were evaluated by
XANES and EXAFS spectroscopy, respectively (see below). The results were
used to define surface complexation reactions and constants in the CD-MUSIC
model (Hiemstra & van Riemsdijk, 1996) within the Visual MINTEQ
equilibrium software (Gustafsson, 2013). In the model, the surface area of the
ferrihydrite was set at 650 m
2
g
-1
and the site density at 7.8
sites nm
-2
(Tiberg et
al., 2013). The inner and outer layer capacitances were set at 1.15 and 0.9 F m
-
2
, respectively. The model was calibrated using data from the single sorbate
systems of vanadate(V) and phosphate. The optimised constants were then
used to predict vanadate(V) and phosphate sorption in the binary systems. The
final model was also applied to previously published data on vanadate sorption
(Blackmore et al., 1996).
4.2 Vanadium toxicity and bioavailability (Paper II-IV)
Vanadium toxicity and bioavailability were evaluated in detail for six different
European mineral soils (Table 2). The soils were taken from the 20 cm surface
horizon (A-horizon) and were selected to cover ranges of soil textures, pH
values and metal (hydr)oxide contents. Three different vanadium amendments
were analysed; soils freshly spiked with vanadate(V) (Paper II), soils that had
Table 2. Name, origin and soil properties of different soils used for the toxicity assays.
Mineral fraction
b
Oxalate
extractable
Included
in
Land use
pH
Org. C CaCO
3
Sand Silt Clay
eCEC
V
c
P-AL
d
Al
Fe
Mn
paper
0.01 M CaCl
2
%
%
%
cmol
c
kg
-1
mg kg
-1
mg kg
-1
g kg
-1
Guadalajara (ES)
a
Olive orchard
7.8
0.5
23
23
57
24
14.1
17
0
58
0.4 0.2 <0.1
II
Zwijnaarde (BE)
Arable land
5.2
1.6
n.d.
85
10
6
0
3.0
15
225
1.2 0.9 <0.1
II
Ter Munck (BE)
Arable land
6.6
0.9
n.d.
19
64
17
0
7.3
38
141
0.6 2.2
<
0.4
II
Pustnäs (S)
Grassland
5.9
1.1
n.d.
86
3
11
0
4.3
27
0
93
0.8 1.4
<
0.1 II, III and IV
Säby (S)
Arable land
5.5
2.5
n.d.
34
37
29
10.2
58
0
41
1.3 4.4 <0.1 II, III and IV
Hygum (DK)
Grassland
5.2
2.1
n.d.
56
31
13
0
7.6
31
n.d.
1.8 3.4
<
0.7
III
n.d.= not determined
a
ES=Spain, BE= Belgium, S= Sweden and DK=Denmark
b
Reported as percentage of the mineral fraction
c
Vanadium soil concentration determined by aqua regia digestion
d
Soil phosphorus soil content determined by ammonium lactate extraction