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Trace Elements in Abiotic and Biotic Environments
<0.2 to 0.5 mg/kg (Xiao et al. 2004). Soils of the remote region in Spitzbergen contain Tl within the range of 0.0006–0.0107 mg/kg (Gulińska et al. 2003).
The elevated Tl contents, at an average value of 59 mg/kg, in the top horizon of
forest soils in Sweden, are presumably affected by its strong sorption by the organic
matter (Tyler 2005). Soils derived from Tl sulfide-mineralized parent materials, and
soils of some industrial regions, may contain its highly elevated amounts, up to about
130 mg/kg (Xiao et al. 2004). In general, Tl contents in soils above 1 mg/kg indicate the pollution, and this content is proposed as maximum allowable concentration
value for agricultural soils. Thallium and its methylated species are toxic to microorganisms, and therefore, the nitrate formation is inhibited in Tl-contaminated soils,
which has an agronomic impact.
During weathering, Tl is readily mobilized, and then fixed by clay minerals,
SOM, and Mn and Fe hydroxides. In most soils, its highest proportion is in residual
soil fractions. It reveals a great affinity for micaceous minerals and exhibits an activity to replace K from these minerals. Under oxidation conditions, Tl may be oxidized
to Tl 3+ and become immobile. In acidic soils, especially under acid rain, Tl is highly
mobile, and thus easily phytoavailable.
Anthropogenic sources of Tl are from emission of cement factories (considered
to be the largest), coal-fired power plants, and sewage sludge from metal industries.
Slug dumps of Tl-rich pyrite mining, in China, contain this metal within the range of
30–70 mg/kg, which is of a great risk of Tl contamination of both soils and groundwaters (Xiao et al. 2004). Landfills from Zn and Pb mines, in Poland, contain Tl at
mean level of 39 mg/kg, with the maximum value of 76 mg/kg (Wierzbicka et al. vide
Kabata-Pendias 2011).
49.3 WATERS
The average concentration of thallium in seawaters is estimated within the range
of 10–15  ng/L. Its amounts in the North Pacific Ocean is reported to be 13 ng/L
(Nozaki 2005). Its amounts in river waters of different countries vary from 5 to
1350 ng/L, lowest in Poland and highest in Japan (Kabata-Pendias and Mukherjee
2007). Excluding its very elevated contents, its concentrations in polluted rivers
range from 5 to 56 ng/L.
The main Tl species is, most probably, TlCl 6
3
_
, in seawaters and Tl 3+ in river
waters. However, water pH has an impact on Tl speciation, and at acidic pH range
of 4–6, it predominates Tl(OH) 2+ , whereas at pH above 7 Tl(OH)
+
2 and Tl (OH) 3 are
present (Nozaki 2005). Also the redox potential of waters governs the Tl speciation.
The stability of Tl 3+ is limited, and apparently Tl + is dominant species in aquatic
environments.
The main source of elevated Tl concentrations in surface waters is its leaching
at ore processing operations. Its content in waters of some mining areas is reported
to reach 1620 μg/L. The mean Tl concentration in deep groundwaters of China is
62 μg/L, at the highest value of 1100 μg/L (Xiao et al 2004).
Compounds of Tl are readily soluble in waters, and this metal is easily fixed by bottom sediments. The content of Tl in bottom sediments of polluted rivers in Germany
is about 20 mg/kg (Gunther et al. vide Kabata-Pendias and Mukherjee 2007).