Chromium [Cr, 24]
79
However, under natural soil conditions, the Cr oxidation–reduction potential seems to
be directly associated with Fe and Mn-oxides contents (Chung and Sa 2001).
Increased levels of mobile Cr species may be toxic to both microorganisms and
crop plants. Activities of microbial enzymes decrease under elevated Cr contents, especially sensitive are nitrification processes. More toxic is Cr 6+ than Cr 3+ . However, toxic
effects of Cr at various oxidation states may differ depending on species of the microorganisms (Wyszkowska and Kucharski 2004). Some microorganisms are resistant to
Cr 6+ due to chromate reduction processes (Ramirez-Diaz et al. 2008).
Cr contents of surface soils have increased due to pollution from various sources,
of which the main are chromite-ore processing residue, municipal, and tannery and
leather-manufacturing wastes. A number of studies have been recently carried out to
understand Cr behavior in contaminated soils, and to apply effective remedial treatments (Bini et al. 2008; Geelhoed et al. 2003).
Municipal and industrial wastes may contain Cr up to 10,200 mg/kg. Its contents
of sewage sludge applied to agricultural soils usually vary between 100 and 200 mg/kg
(Maján et al. 2001). Its deposition to agricultural soils is estimated at 0.5–46 g/ha/yr,
being the lowest in Finland and the highest in Italy (Nicholson et al. 2003). According
to these authors, the total annual input of Cr to agricultural soils of the United
Kingdom in 2000 was 327 t, of which 126 t was from inorganic fertilizers (mainly
phosphate), 83 t from atmospheric deposition, and 78 t from sewage sludge (Maján
et al. 2001).
Reported Cr contaminations of soils from industrial sources in various countries
are as follows:
r Italy, leather tannery: range 50–10,000 and mean 210 mg/kg (Bini et al. 2008)
r Portugal, electroplating plant: Up to 27,132 and mean 1,000 mg/kg (Morgado
et al. 2001)
r Albania, metal smelter: Up to 20,300 and mean 3117 mg/kg (Shtiza et al. 2005)
The criteria for contaminated land (Dutch List 2013), following Cr concentrations in
soils and groundwater, are established (in mg/kg, and μg/L) as follows: uncontaminated, 100 and 20; medium contaminated, 250 and 50; and heavily contaminated,
800 and 200. Soil quality Cr 3+ levels are estimated, based on various criteria, within
the broad range of 25–1,000,000 mg/kg (Siebielec et al. 2012).
The main purpose of remediation treatments of Cr-contaminated soils is the
reduction of easily mobile Cr 6+ to slightly mobile Cr 3+ , or Cr adsorption by mineral or organic compounds. The phytoremediaton methods are rather limited due
to a relatively low soil-to-plant transfer factor. Recently, the bioremediation of
Cr-contaminated soils by actinomycetes, partly due to the bioreduction of Cr 6+ , has
been proposed (Amoroso and Abate 2012; Farmer et al. 1999).
12.3 WATERS
The worldwide median concentration of Cr in ocean water is given as 0.3  μg/L
(Reimann and de Caritat 1998). In the North Pacific Ocean it is a bit lower. Its concentration in the Baltic Sea (Swedish coast) is extremely high, up to about 185,000 μg/L