13
2 Aim
The overall aim of this thesis was to obtain knowledge of the behaviour of
vanadium in soils in terms of sorption, toxicity and speciation, in order to
enable improved environmental risk assessments.
Specific objectives were to:
Assess the sorption pattern of vanadium to 2-line ferrihydrite in single
sorbate systems and in competition with phosphate; examine the vanadium
complex formed on the ferrihydrite surface by means of EXAFS
spectroscopy; and optimise surface complexation constants using the CD-
MUSIC model (Paper I).
Determine critical vanadium concentrations for microorganisms and higher
plants in soils with different vanadium amendments; freshly spiked, aged
and blast furnace slag (Papers II, III and IV).
Identify soil properties that explain vanadium bioavailability in soils
(Papers II, III, IV).
Assess the long-term solubility and speciation of vanadium in a forest soil
treated in the past with converter lime rich in vanadium (Paper V).
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3 Background
3.1 Vanadium in soils and waters
3.1.1 Sources
The vanadium content in soils and waters is primarily determined by the
geological parent material (Hope, 1997). However, anthropogenic emissions,
mainly from combustion of fossil fuels (Pacyna & Pacyna, 2001), may enhance
soil vanadium concentrations locally. Vanadium inputs to soils related to
human activities derive e.g. from phosphate fertilisers (Molina et al., 2009),
soil amendments and roadfill materials derived from steel slag (Shen &
Forssberg, 2003).
Many of the slags generated in the steel production process have properties
suitable for various applications within society, but slag reuse may be
restrained by elevated concentrations of trace elements (Motz & Geiseler,
2001). Swedish blast furnace slags are naturally high in vanadium, which can
reach concentrations above 500 mg kg
-1
(Nehrenheim & Gustafsson, 2008).
This is 10-fold higher than reported for e.g. blast furnace slags in the USA
(Proctor et al., 2000). Data on vanadium leaching from blast furnace slags are
scarce (Cornelis et al., 2008). Laboratory-based availability tests indicate that
the potential leaching capacity of vanadium from Swedish blast furnace slags is
approximately 10% of the total vanadium content (Fällman & Hartlén, 1994).
Two important factors that control the leaching from different slags are the pH
and redox conditions (De Windt et al., 2011; Fällman & Hartlén, 1994).
However, it should be noted that leaching tests performed in the laboratory
may not adequately represent the leaching conditions in the field (Chaurand et
al., 2007a; Fällman & Hartlén, 1994).
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3.1.2 Redox chemistry
Vanadium can exist in a range of oxidation states (from -2 to +5). The
prevailing valence states in nature are vanadium(III), vanadium(IV) and
vanadium(V) of which the latter two are the most soluble (Wanty &
Goldhaber, 1992). Vanadium(III) is stable in extremely reducing environments,
such as lake sediments, and is readily oxidised in the unsaturated zone of the
soil (Crans et al., 1998). In soils, the oxocation vanadyl(IV), VO
2+
, occurs at
lower pH values and in moderately reducing conditions (Figure 1).
Vanadyl(IV) forms strong complexes with organic matter, which may stabilise
the ion at higher redox potentials (Wehrli & Stumm, 1989). The oxyanion
vanadate(V), H
2
VO
4
-
, is usually the predominant form in soils. It prevails
above pH 3.6 and has three protonation states. However, protonation to
vanadic acid, H
3
VO
4
, is insignificant due to the formation of the oxocation
vanadyl(V), VO
2
+
(Crans et al., 1998). Vanadate resembles phosphate and is
the most soluble and toxic form of vanadium due to its ability to inhibit
phosphate-metabolising enzymes (Perlin & Spanswick, 1981; Seargeant &
Stinson, 1979). In waters, the relative concentrations of different vanadium(V)
species are affected by the total vanadium concentration, the ionic strength and
the pH. At concentrations above 100 μM, polymers of vanadium, in particular
decavanadates, start to form (Baes & Mesmer, 1976).
Figure 1. Eh-pH diagram of vanadium species formed in water at a vanadium concentration of
0.01 mM, in a background electrolyte of 0.01 M NaCl at 25ºC. Red lines indicate the transition
between oxidation states and the blue line is the stability limit for water. Source: Gustafsson &
Johnsson, (2004).