283
Selenium [Se, 34]
The Se contents of soils have received much attention, mainly in countries where
its role in human and animal health has been recognized. The program for increasing
Se levels in soils of Finland began in 1984 (Eurola et al. vide Kabata-Pendias 2011)
and was based on the addition of Se (as sodium selenate) to inorganic fertilizers, at
levels 6–10 mg/kg. The mean hot-water extractable Se in soil before this program was
0.006 mg/L, and after the program (in 1998) was 0.01 mg/L. However, Se that was
easily available to plants was present only in the first summer after Se fertilizing, and
quickly turned into unavailable species to plants species. The fixation of Se in soils
was correlated positively with clay content, Fe content, and negatively with sulfuric
acid-extractable P. However, nonsoluble Se compounds were, later on, changed into
mobile species. Secondary effects, after this program, carried out during the period
1983–1992, resulted in increased Se levels in river water (up to 180 mg/L), and especially in bottom sediments (up to 4 mg/kg). Results of this experiment showed that Se
phytoavailability varies significantly, depending on soil parameters (Hécho et al. 2012).
Elevated Se amounts in soils (called seleniferous soils) are in some ferrasols,
organic soils, and other soils derived from Se-rich parent materials. They occur
mainly in some central regions of the United States, and in Asia. Also, salt-affected
soils may contain higher Se levels.
Selenium behavior in soils is controlled by pH–Eh soil system. Usually it is easily
mobile and also oxidized in soils. Selenite ions resulting from oxidation processes
are stable and readily adsorbed on minerals and organic particles. Clay minerals
(particularly montmorillonite) and Fe oxides (especially goethite) reveal a great
adsorption capacity. In acid soils, Se is likely to occur as Se 4+ , which is adsorbed by
Fe oxide and form ferric selenite [Fe 2 (OH) 4 SeO 3 ] and iron selenite (FeSe). Its maximum adsorption occurs at the pH range of 3–5, and decreased as pH increases. In
alkaline Se-rich soils, predominated species is Se 6+ , which is very weakly adsorbed.
Thus, Se is very mobile in soils of arid and semiarid regions. The biological methylation of Se compounds affects its easy volatilization, which plays a significant role in
its geochemical cycling.
Due to variable oxidation states of Se, its behavior in soils is very variable. The
impact of pH–Eh is as follows:
r pH 7, Eh >400, oxidation Se 6+ , major species: SeO 4
2–
r pH >7, Eh 200–400, oxidation Se 6+ , major species: SeO
2
3
–
r pH <7, Eh 200, oxidation state Se 4+ , major species: HSeO 4
–
r pH <3.2, Eh <200, oxidation state Se 2− , major species: HSe − , H 2 Se°
Inorganic species of Se reveal variable properties, depending on its oxidation state,
which are as follows:
r Selenates (Se 6+ ) are mobile as inorganic forms, especially in neutral and alkaline soils, and are not adsorbed on hydrous sesquioxides (mainly Fe 2 O 3 ·H 2 O)
r Selenites (Se 4+ ) are slightly mobile in neutral and acid soils, and are easily
absorbed hydrous sesquioxides and organic matter
r Selenides (Se 2− ) are very slightly mobile in acid soils, due to the formation
of stable minerals and organic compounds
Selenium [Se, 34]
The Se contents of soils have received much attention, mainly in countries where
its role in human and animal health has been recognized. The program for increasing
Se levels in soils of Finland began in 1984 (Eurola et al. vide Kabata-Pendias 2011)
and was based on the addition of Se (as sodium selenate) to inorganic fertilizers, at
levels 6–10 mg/kg. The mean hot-water extractable Se in soil before this program was
0.006 mg/L, and after the program (in 1998) was 0.01 mg/L. However, Se that was
easily available to plants was present only in the first summer after Se fertilizing, and
quickly turned into unavailable species to plants species. The fixation of Se in soils
was correlated positively with clay content, Fe content, and negatively with sulfuric
acid-extractable P. However, nonsoluble Se compounds were, later on, changed into
mobile species. Secondary effects, after this program, carried out during the period
1983–1992, resulted in increased Se levels in river water (up to 180 mg/L), and especially in bottom sediments (up to 4 mg/kg). Results of this experiment showed that Se
phytoavailability varies significantly, depending on soil parameters (Hécho et al. 2012).
Elevated Se amounts in soils (called seleniferous soils) are in some ferrasols,
organic soils, and other soils derived from Se-rich parent materials. They occur
mainly in some central regions of the United States, and in Asia. Also, salt-affected
soils may contain higher Se levels.
Selenium behavior in soils is controlled by pH–Eh soil system. Usually it is easily
mobile and also oxidized in soils. Selenite ions resulting from oxidation processes
are stable and readily adsorbed on minerals and organic particles. Clay minerals
(particularly montmorillonite) and Fe oxides (especially goethite) reveal a great
adsorption capacity. In acid soils, Se is likely to occur as Se 4+ , which is adsorbed by
Fe oxide and form ferric selenite [Fe 2 (OH) 4 SeO 3 ] and iron selenite (FeSe). Its maximum adsorption occurs at the pH range of 3–5, and decreased as pH increases. In
alkaline Se-rich soils, predominated species is Se 6+ , which is very weakly adsorbed.
Thus, Se is very mobile in soils of arid and semiarid regions. The biological methylation of Se compounds affects its easy volatilization, which plays a significant role in
its geochemical cycling.
Due to variable oxidation states of Se, its behavior in soils is very variable. The
impact of pH–Eh is as follows:
r pH 7, Eh >400, oxidation Se 6+ , major species: SeO 4
2–
r pH >7, Eh 200–400, oxidation Se 6+ , major species: SeO
2
3
–
r pH <7, Eh 200, oxidation state Se 4+ , major species: HSeO 4
–
r pH <3.2, Eh <200, oxidation state Se 2− , major species: HSe − , H 2 Se°
Inorganic species of Se reveal variable properties, depending on its oxidation state,
which are as follows:
r Selenates (Se 6+ ) are mobile as inorganic forms, especially in neutral and alkaline soils, and are not adsorbed on hydrous sesquioxides (mainly Fe 2 O 3 ·H 2 O)
r Selenites (Se 4+ ) are slightly mobile in neutral and acid soils, and are easily
absorbed hydrous sesquioxides and organic matter
r Selenides (Se 2− ) are very slightly mobile in acid soils, due to the formation
of stable minerals and organic compounds