Vanadium in Soils Chemistry and Ecotoxicity
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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.
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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).
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
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. Yüklə 456,72 Kb. Dostları ilə paylaş:
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