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6 Concluding discussion
The aim of this thesis was to improve existing knowledge regarding vanadium
sorption, toxicity and speciation in soils, with the ultimate aim of improving
environmental risk assessments.
Vanadium adsorbed strongly to ferrihydrite forming a vanadate(V) edge-
sharing bidentate complex. This complex could be used to describe the
vanadium adsorption pattern in competition with phosphate by the CD-MUSIC
model. The importance of iron (hydr)oxides for vanadium retention in soil was
confirmed by adsorption experiments and the long-term field study. Hence,
since ferrihydrite is an important sorbent in many Swedish soils, this model
could be utilised in a more generalised model explaining vanadium sorption in
soils. However, it was also shown that iron (hydr)oxides may not be the only
determining factor for the sorption. Other soil constituents, such as organic
matter and aluminum (hydr)oxides, may play a significant role in some soils
and more detailed information regarding their role for vanadium sorption is
needed. This would gain a more profound understanding of vanadium sorption
as well as vanadium bioavailability in different soils.
The soil properties did not only affect vanadium sorption but also vanadium
speciation in the soil. By combining two vanadium speciation methods, it was
shown that vanadium speciation in soils was mainly determined by the
conditions in the soil, and not by the vanadium species added to the soil. The
two speciation methods used proved promising in terms of estimating the
vanadium speciation with almost no pretreatment. Vanadium K-edge XANES
spectroscopy had a distinct advantage with the strong correlation between pre-
edge peak intensity and the position of the main edge with the oxidation state.
However, evaluation of vanadium K-edge XANES spectra for soil samples
needs to be improved. A larger library of vanadium standards would help
determine the variation in pre-edge peak and main edge with oxidation state. It
46
could also provide a possibility to extend the linear combination fitting
approach.
The relationship demonstrated in this thesis between bioavailability and the
vanadium concentration in the soil solution represents a great step forward in
assessing the toxicity risks arising from vanadium in soils. Toxicity risk
assessment could be even more accurate if more were known about factors in
the soil solution that affect vanadium uptake. One aspect is the effect of
concentration of other oxyanions in terms of competition for uptake by
organisms as well as competition for sorption sites. Phosphate and dissolved
organic acids are probably the most important compounds to consider since
they occur in much higher concentrations in the soil compared to vanadium.
Increased knowledge about the competition with other constituents could also
be beneficial in terms of formulating remediation measures.
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7 Vanadium and risk assessments
In principle, all blast furnace slags generated in Sweden today are re-used in
e.g. road materials. One of the main concerns about their use in the
environment is the elevated concentrations of vanadium they contain. The
Swedish generic guideline values for vanadium in soil are 100 and 200 mg V
kg
-1
soil for sensitive and less sensitive land use, respectively, and are based on
the pseudo-total vanadium concentration in the soil. In the first phase of an
environmental risk assessment, the measured vanadium concentration in the
soil is normally compared with generic guideline values. However, as shown in
this thesis, vanadium bioavailability varies considerably between different soils
and vanadium treatments. Hence the generic guideline values may be
misleading by either over- or under-estimating the ecotoxicological risk in a
specific soil.
One way to improve site specific risk assessments would be to relate the
ecotoxicological risk to the soil solution concentration, which would narrow
the range of uncertainty considerably. Soil solution chemistry not only gives a
better estimate of vanadium bioavailability, but is also directly linked to the
risk of vanadium leaching to groundwater and surface waters. From a practical
perspective, a simple leaching test based on CaCl
2
extraction would probably
be the most appropriate in this respect.
The risk of vanadium contamination of soils through addition of
metallurgical slags seems very limited based on the experiments performed in
this thesis. However, different slags differ in vanadium concentrations and
solubility. Depending on the slag and the soil, the application may pose a risk
of vanadium contamination. One aspect that needs to be considered is the
leaching of vanadium from the material under field conditions, which should
be based on in situ measurements over longer time periods. Another aspect is
to consider the soil properties in areas where the material is applied, in order to
assess the risk of vanadium contamination of waters and organisms.