3.1.3 Selenium
There are many natural pathways in which selenium is released into the environment. The processes range from volcanic activity; wildfires; volatilization from soils,
plants, and water bodies; and weathering of rocks (Sandy and DiSante 2010).
Selenium occurs naturally in soil, but at low concentrations, ranging between 0.1
and 2 μg/g soil, whereas in seleniferous soil the concentration can go up to
320–324 μg/g soil (Girling 1984). Of the selenium released into the environment,
37.5–40.6% can be ascribed to anthropogenic activities. It is released from the
earth’s crust by the mining of coal, by oil production, through the use of agricultural
products, as well as during the melting of nonferrous metals (Lenz and Lens 2009).
Landfill ash disposal generating toxic leachate poses a risk of groundwater contamination (Lemly 2004). According to different studies, acid mine drainage waters
contain selenium at concentrations ranging between 2.0 Â 10
À4 and 6.2 Â 10
À3 mM
(reported as total selenium) (Lenz et al. 2008). Wastewater from oil refineries in the
San Francisco Bay (USA) contains relatively low concentrations of selenium of
about 50–300 μg/L (Lawson and Macy 1995). The wastewater from a selenium
refinery plant in Japan contained an average of 30 mg/L selenium, with the most of it
present as selenite (Satoshi et al. 2012).
3.1.4 Mercury
Mercury is released to the environment from activities such as alkali and metal
processing, incineration of coal, and medical waste and other wastes. In the Southern
hemisphere, a lot of mercury is released into the environment as effluents from the
mineral purification processes. Mercury is used extensively in the separation of gold
from other metallic impurities. The process involves extraction of gold in liquid
mercury and boiling away the mercury to leave behind the pure gold. Fumes
produced result in atmospheric mercury pollution. Once in the atmosphere, mercury
is widely disseminated and can circulate for years. Other sources of atmospheric
mercury include natural sources such as volcanoes, geologic deposits, and volatilization from the ocean. Although all rocks, sediments, water, and soils naturally
contain small but varying amounts of mercury, scientists have found some local
mineral occurrences and thermal springs that are naturally high in mercury. In many
relatively pristine areas, however, mercury concentrations have actually increased
because atmospheric deposition has increased. For instance, concentrations of mercury in feathers of fish-eating seabirds from the northeastern Atlantic Ocean have
steadily increased for more than a century. In North American sediment cores,
sediments deposited since industrialization have mercury concentrations about 3–5
times those found in older sediments. Some sites may have become methylmercury
hot spots inadvertently through human activities (Gilmour and Henry 1991). Lake
acidification, addition of substances like sulfur that stimulate methylation, and
mobilization of mercury in soils in newly flooded reservoirs or constructed wetlands
2 Advances in Bioremediation of Toxic Heavy Metals and Radionuclides in. . .
25
There are many natural pathways in which selenium is released into the environment. The processes range from volcanic activity; wildfires; volatilization from soils,
plants, and water bodies; and weathering of rocks (Sandy and DiSante 2010).
Selenium occurs naturally in soil, but at low concentrations, ranging between 0.1
and 2 μg/g soil, whereas in seleniferous soil the concentration can go up to
320–324 μg/g soil (Girling 1984). Of the selenium released into the environment,
37.5–40.6% can be ascribed to anthropogenic activities. It is released from the
earth’s crust by the mining of coal, by oil production, through the use of agricultural
products, as well as during the melting of nonferrous metals (Lenz and Lens 2009).
Landfill ash disposal generating toxic leachate poses a risk of groundwater contamination (Lemly 2004). According to different studies, acid mine drainage waters
contain selenium at concentrations ranging between 2.0 Â 10
À4 and 6.2 Â 10
À3 mM
(reported as total selenium) (Lenz et al. 2008). Wastewater from oil refineries in the
San Francisco Bay (USA) contains relatively low concentrations of selenium of
about 50–300 μg/L (Lawson and Macy 1995). The wastewater from a selenium
refinery plant in Japan contained an average of 30 mg/L selenium, with the most of it
present as selenite (Satoshi et al. 2012).
3.1.4 Mercury
Mercury is released to the environment from activities such as alkali and metal
processing, incineration of coal, and medical waste and other wastes. In the Southern
hemisphere, a lot of mercury is released into the environment as effluents from the
mineral purification processes. Mercury is used extensively in the separation of gold
from other metallic impurities. The process involves extraction of gold in liquid
mercury and boiling away the mercury to leave behind the pure gold. Fumes
produced result in atmospheric mercury pollution. Once in the atmosphere, mercury
is widely disseminated and can circulate for years. Other sources of atmospheric
mercury include natural sources such as volcanoes, geologic deposits, and volatilization from the ocean. Although all rocks, sediments, water, and soils naturally
contain small but varying amounts of mercury, scientists have found some local
mineral occurrences and thermal springs that are naturally high in mercury. In many
relatively pristine areas, however, mercury concentrations have actually increased
because atmospheric deposition has increased. For instance, concentrations of mercury in feathers of fish-eating seabirds from the northeastern Atlantic Ocean have
steadily increased for more than a century. In North American sediment cores,
sediments deposited since industrialization have mercury concentrations about 3–5
times those found in older sediments. Some sites may have become methylmercury
hot spots inadvertently through human activities (Gilmour and Henry 1991). Lake
acidification, addition of substances like sulfur that stimulate methylation, and
mobilization of mercury in soils in newly flooded reservoirs or constructed wetlands
2 Advances in Bioremediation of Toxic Heavy Metals and Radionuclides in. . .
25