Mercury [Hg, 80]
203
The greatest amounts of Hg came from sewage sludge, which may contain it up to
6 mg/kg. Total amount of Hg in waste of 11 EU countries, in 1999–2000, has been
estimated at 752 t; however, the authors have pointed out that several countries have
not calculated this carefully (Mukherjee et al. 2004).
Besides industrial sources of Hg, it has been added to agricultural soils with fungicides and seed dressing (now prohibited, but used for over 20 years). Also, fly ash
used for the land reclamation, and to limit the mobility of Cd, Cu, Pb, and Zn, may
be a serious source of Hg in soils (Kumpiene et al. 2005).
Mercury, as a contaminant is soils, occurs mainly as Hg 0 , but small amounts of
1+
Hg 2 and Hg
2+ are also present (Melo 2012). Methylated Hg is present as a result
of natural microbial transformation. Its complexes in soils are as follows: Hg–Fe,
Mn > Hg residual > Hg–OM (Melo 2012). Soil Hg contamination is often an effect
of its anthropogenic deposition over the last 100 years (Fitzgerald vide KabataPendias and Mukherjee 2007).
27.3 WATERS
The worldwide mean Hg concentration in seawater is estimated at 30 ng/L (Reimann
and de Caritat 1998). However, it widely varies depending on sampling sites; in
water from open seas, it is within the range of 0.5–3.0  ng/L, and in coastal and
bay water, it varies from 2 to 65 ng/L. Uncontaminated rivers and streams contain Hg within the range of 1–7 ng/L. It occurs in water in several forms such as
0
+
2+
0
2
− , and in various methylated species (Me–Hg). Up to
60% of Hg 2+ and Me–Hg are particle-bound, and the rest of Hg is dissolved or dissolved organic carbon-bound phases (Nriagu vide Kabata-Pendias and Mukherjee
2007). Unfiltered water from Hg mine in Spain contain Hg up to 13,000 ng/L and
methyl Hg up to 30 ng/L, and mine water in California contain up to 19,000 and
Hg , Hg , Hg , HgCl , HgS 2
4.5 ng/L, respectively (Gray et al. 2004). Several techniques are proposed for cleaning wastewater from Hg, by adding Na sulfide or some coagulants (e.g., FeCl 3 ).
These methods may clean up water with Hg concentration <10,000 ng/L (Chen
2013).
Mercury methylation and bioaccumulation in water depend on several parameters, such as Hg loading, microbial activity, pH and redox potential, temperature, and
several other factors. Me–Hg species are relatively easily accumulated by water biota
(Perelomov and Chulin 2013). A great part (>95%) of total Hg in both, fresh and saltwater fish is in the form of Me–Hg. It is taken up by fish straight from the water, as
well as from their fodder. In general, the highest Hg content is in fish muscles and the
lowest is in gonads. In muscles of fish from the Baltic Sea, its contents vary between
0.2 and 0.33 mg/kg, whereas in female gonads its range is 0.02–0.05 mg/kg (Voigt
vide Kabata-Pendias and Mukherjee 2007). The highest means Hg levels in commercial fish in the United States are reported to be 0.73 and 1.45 mg/kg FW, in mackerel
king and tilefish, respectively; and the lowest contents are 0.01 and 0.11 mg/kg FW,
in salmon and cod, respectively.
Mercury contamination of fish from gold-mining areas in America, Asia, and
Africa has been investigated. Hg levels in fish from Brazilian Amazon, near mining
areas, varied from 2.21 to 6.11 mg/kg in Caras and Trairas fish, respectively.