Page 170 - Plant-Based Remediation Processes

9 Phyto-transport and Assimilation of Selenium 161

Fate and transport of Se in water are strongly influenced by various environmen-
tal factors (Bowie and Grieb 1991). Selenate and selenite are two predominant
chemical species in water, where the former one is more stable under alkaline and
oxidizing conditions and the latter one is a dominant species in the mildly reducing
environment (Barceloux 1999; Belzile et al. 2000). Uptake of Se by various
organisms is able to immobilize Se temporarily (Simmons and Wallschla ¨ger
2005). Adsorption to clay, minerals, and dissolved organic carbon is also a process
that immobilizes/sequesters Se in aquatic environment (Belzile et al. 2000). Addi-
tionally, chemical reduction of oxidized forms of Se to elemental/colloidal Se can be
identified in water (Schlekat et al. 2000). It has been proposed that humic acids are
the main reservoir of Se in soils (Tokunaga et al. 1991). Indeed, majority of Se in
soils was detected in organic forms, namely salts of selenic acids and of selenious
acids (Barceloux 1999). Insoluble species of Se, e.g., elemental Se, selenide, and
selenium sulfides can also be identified in soils (Wang and Peng 1991). The
elemental Se in kerogen is more steadily mobilized and accumulated by vegetation,
whereas organically bound Se seems more resistant to chemical alteration and less
bioavailable (Wen et al. 2006). Abundant literatures show that selenate and selenite
rather than other Se species are easily taken up by plants (Terry et al. 2000; Zhang
and Moore 1997; Shardendu et al. 2003). Microorganisms are able to methylate/
convert elemental Se and selenite into volatile Se, DMSe, and dimethyl diselenide
(DMDSe) (Doran 1982).

9.2 Toxicity of Selenium

Marco Polo probably recorded the first observations of Se toxicity to horses in
western China in the thirteenth century (Dickerson and Smith 1994). Due to the
narrow margin of Se concentration among its essentiality, deficiency, and toxicity,
living organisms vary considerably in their physiological responses and tolerance
to Se (Barceloux 1999; Hamilton 2004). Gaseous Se of DMSe produced by plants
is 500–700 less toxic than selenate or selenite, with a lethal dose (LD 50 ) value of
1.6–2.2 g Se kg 1 for rat (Wilber 1980). An acceptable intake level has been
documented at 3.5 mg L 1 by the USEPA (Atkinson et al. 1990). Whole-body Se
threshold concentrations are suggested to be 6.0 and 9.0 mg kg 1 for cold water
fish and warm water fish, while the dietary threshold at 10 and 11 mg kg 1 has been
1
proposed, respectively (Hamilton 2003). The median LD 50 of 8.8 mg Se kg has
been determined for selenomethionine (SeMet) (Ammar and Couri 1981) and
selenite is fourfold more toxic than SeMet (Reid et al. 2004). The most common
symptoms of Se poisoning are loss of hair and nails or lesion of skin, but nervous
system and teeth may be affected in areas of higher incidence (Yang et al. 1983).
The intravenous administration of Se compounds in mice resulted predominantly
in cardiorespiratory effects, hind limb paralysis, and death (Ammar and Couri
1981). Growth inhibition, due to unacceptable high concentration of Se, has been
often observed in microorganisms, aquatic plants, and animals (Simmons and
Wallschla ¨ger 2005).