17
3.1.3 Retention in soils
Vanadium is considered to be well retained in soils (Cappuyns & Swennen,
2014; Martin & Kaplan, 1998; Mikkonen & Tummavuori, 1994b). In one
study, less than 3% of the vanadium (0.56 mg m
-2
) applied to a coastal plain
soil migrated below the amended soil depth during a 30-month period (Martin
& Kaplan, 1998). In another study, between 70 and 80 % of the added
vanadium (510 mg kg
-1
soil) was found to be retained at pH 4 in three Finnish
mineral soils (Mikkonen & Tummavuori, 1994b). Application of different
extraction and leaching methods to field-contaminated soils and sediments has
demonstrated that a very small fraction (generally <1%) of the vanadium is
readily dissolved (Cappuyns & Swennen, 2014; Teng et al., 2011). Extremely
low pH values enhance the solubility (Cappuyns & Swennen, 2014; Mikkonen
& Tummavuori, 1994b). A sorption study conducted on 30 different mineral
soils with a range of vanadate(V) concentrations (from 25 to 125 mg V kg
-1
soil) found different sorption characteristics depending on the soil type (Gäbler
et al., 2009). The content of iron, aluminium and manganese (hydr)oxides was
the main property that controlled vanadium sorption, but it was also affected by
the clay and organic matter contents (Figure 2). Competition with other anionic
species such as phosphate and arsenate may also reduce vanadium sorption in
soils (Mikkonen & Tummavuori, 1994a). Over a long-term perspective, the
behaviour of vanadium in soils is less well known, but its solubility has been
shown to decrease with time (Martin & Kaplan, 1998).
Figure 2. Processes affecting vanadium cycling in soils. Background photo: Ann Kristin
Eriksson.
18
Metal (hydr)oxides
The concentration of vanadium in natural waters is positively correlated to the
iron concentration (Wällstedt et al., 2010; Auger et al., 1999). This has been
attributed to strong adsorption of vanadate(V) to colloidal iron (hydr)oxides
(Wällstedt et al., 2010). In natural soils, the metal (hydr)oxide content is
considered one of the most important properties for vanadium sorption (Gäbler
et al., 2009). The sorption depends on the type of (hydr)oxide, as well as on
solution pH and the ratio between solid and solute. Similar to other oxyanions
such as phosphate, molybdate and arsenate (Antelo et al., 2010; Su et al., 2008;
Gustafsson, 2003), vanadate(V) is strongly retained at lower pH, when
(hydr)oxide surfaces are positively charged (Naeem et al., 2007).
Spectroscopic measurements have shown the formation of a vanadate(V)
corner-sharing bidentate complex on the surface of goethite (Peacock &
Sherman, 2004). Although vanadate is somewhat more strongly adsorbed than
phosphate, high concentrations of phosphate in relation to vanadate can reduce
vanadate sorption (Blackmore et al., 1996). Sorption of vanadyl(IV) on metal
(hydr)oxides may also occur (Wehrli & Stumm, 1989).
Organic matter
Soil organic matter influences the speciation and mobility of vanadium in soils
(Gäbler et al., 2009; Lu et al., 1998). Firstly, dissolved organic matter may
occupy the binding sites on metal (hydr)oxides and thereby reduce the sorption
of oxyanions (Geelhoed et al., 1998), although the significance of this process
has been questioned (Guppy et al., 2005). Another aspect is the capacity of
organic matter to bind vanadium, in particular as vanadyl(IV) (Poledniok &
Buhl, 2003; Lu et al., 1998). The vanadyl ion coordinates to oxygen donor
atoms, most likely on carboxylate ligands (Lu et al., 1998). Humic substances
may also reduce vanadium(V) to vanadyl(IV).
Tyler (2004), who studied the vertical distribution of trace elements in the
soil profile of a Haplic Podzol, found that the surface (mor) layer, high in
organic matter, retained relatively large concentrations of vanadium. The
possibility to remove vanadium from waste waters by the use of a biosorbent
has been tested and the maximum adsorption is reported to occur around pH 4
(Urdaneta et al., 2008). In that study, vanadium sorption was found to be
around 50% which was low in comparison with the maximum of 95% removal
observed for other cationic metals such as lead(II), nickel(II) and
chromium(II).
19
3.1.4 Toxicity and bioavailability
Soil microorganisms are known to be sensitive to metals (Giller et al., 1998),
but knowledge of vanadium toxicity is scarce. Nitrification and nitrogen
mineralisation can be inhibited by vanadium addition (Liang & Tabatabai,
1978; Liang & Tabatabai, 1977). However, over a longer period of time (9
years) vanadium addition is reported to have no effects on nitrification (Wilke,
1989). Furthermore, the enzyme phosphatase, which is mainly released by soil
microorganisms to mineralise organic phosphorus, has shown reduced activity
as a result of vanadium addition to spruce needle mor (Tyler, 1976). In general,
metal toxicity to soil microorganisms differs between soils due to variations in
bioavailability, but also due to the natural diversity of microbial communities
(Giller et al., 1998).
Concerning vanadium toxicity to plants, vanadium is mainly accumulated in
the roots (Yang et al., 2011; Gil et al., 1995; Kaplan et al., 1990a). This is
probably due to the reduction of vanadate(V) to vanadyl(IV) during root
uptake, which decreases further translocation within the plant (Morrell et al.,
1986). For solution cultures of plants, the range of observed toxic vanadium
levels is relatively wide and varies between plant species. Inhibition of radicle
elongation, tested by solution cultures, has been reported for turnip, cabbage
and collard greens at solution concentrations between 2.5 and 3.0 mg V L
-1
(Carlson et al., 1991; Kaplan et al., 1990b). However, no decrease in radicle
elongation of wheat was observed at vanadium concentrations up to 40 mg V
L
-1
, at which concentration millet even showed a stimulatory effect (Carlson et
al., 1991). In a recent study, five different plant species grown in an artificial
soil with different nutrient additions showed a 50% reduction in plant biomass
at soil vanadium concentrations ranging from 21 to 180 mg V kg
-1
soil (Smith
et al., 2013).
As for other metals, soil type is important for vanadium bioavailability. For
example, no toxic effects were observed in a study on collard greens grown on
a loamy sandy soil at vanadium concentrations of 100 mg kg
-1
, whereas collard
green biomass was reduced at 80 mg V kg
-1
when grown on a sandy soil
(Kaplan et al., 1990b). In a pot experiment with soybean seedlings, vanadium
concentrations of 30 mg kg
-1
soil were toxic in a Fluvaquent but concentrations
of 75 mg V kg
-1
soil showed no negative effects on the seedlings in an Oxisol
(Wang & Liu, 1999). The differences in toxicity were attributed to the differing
vanadium sorption capacity of the soils.
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