Page 172 - Plant-Based Remediation Processes

9 Phyto-transport and Assimilation of Selenium 163

leading to nonspecific replacement of S by Se in proteins and other sulfur
compounds (Pilon-Smits and LeDuc 2009). Additionally, a nonspecific integration
of the selenoamino acids, selenocysteine (SeCys) and selenomethionine (SeMet),
into proteins has been proposed to be the major contributor of Se toxicity in plants
(Brown and Shrift 1982).

9.3 Remediation of Selenium

Various processes, ranging from intensive engineering techniques to bio-
treatments, have been developed to remediate Se-contaminated soils and waters
(Zayed et al. 1998). The majority of processes used for cleaning Se-contaminated
wastewater are through physiochemical methods, such as chemical precipitation,
electrochemical treatment, and catalytic reductions (Zayed et al. 1998). The use of
ion exchange resins has primarily been used for the removal of soluble Se (Kashiwa
et al. 2000). Aluminum oxide, manganese nodules, titanium oxide, hematite, and
magnetite are adsorbents used for removing Se (EI-Shafey 2007). Bioremediation is
a biological response to environmental abuse when the concentrations of the
pollutants are below the threshold of toxicity to the exposed organisms. A broad
range of bacteria, fungi, and yeasts have been identified or isolated to be capable of
converting soluble Se into elemental Se and DMSe (Milne 1998). Because of the
insolubility of elemental Se in aquatic systems, reduction of soluble Se to elemental
Se is considered to be a useful technique for removing Se from Se-contaminated
water (Zhang and Frankenberger 2005). In spite of the capabilities of
microorganisms to reduce both Se chemical species, one conclusive result is that
reduction of selenate by microorganisms is a more difficult process than selenite
(Maiers et al. 1988). Indeed, selenate is often considered a spectator in living
organisms (Milne 1998). Additionally, an algal–bacterial removal system has
been designed, in which microalgae produced by the system provide sources of
carbon and energy for the specific bacterial reduction of the soluble Se from
wastewater (Lundquist et al. 1994).

9.4 Uptake and Transport of Selenium by Plants

The first interaction between Se and plants is during its uptake process. In spite of
the capabilities of plants to take up different species of Se readily from soil solution,
namely selenate, selenite, and organic Se compounds, great differences in the
uptake and transport pathways between these Se species have been observed
(Brown and Shrift 1982; Arvy 1993; White et al. 2004; Sors et al. 2005). Indeed,
selenate is mainly absorbed by roots through cell membranes by sulfate transporters
owing to the chemical similarity between selenate and sulfate (Brown and Shrift
1982; Smith et al. 1995). But very little is known about the mechanism of selenite