Biomedical Engineering Reference
In-Depth Information
H
He
Li Be
B
CNOFNe
Na
Mg
Al
Si
P
Se
Cl
Ar
KCa Sc
Ti V rMnFeCoNiCuZnGa
Ge As
Se
Br
Kr
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
Cs
Ba
La*
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
Fr
Ra
Ac**
Rf
Db
Sg
Bh
Hs
Mt
Uuu
Uuu
*
Ce
Pr Nd Pm Sm Eu
Gd
Tb Dy Ho Er Tm Yb Lu
**
Th
Pa
U
Np
Pu
Am
Cm
Bk
Cf
Es
Fm
Md
No
Lr
FIGURE 11.1
Metals (white), metalloids (grey), and non-metals (black) of the periodic table.
Trace metals generally do not occur as discrete minerals, but are rather “minor
substituents in silicates and aluminosilicates (olivines, pyroxenes, amphiboles, micas,
and feldspars); hydrous metal (iron, aluminum, and manganese) oxides; iron sul-
fides; calcium and magnesium carbonates; calcium, iron, and aluminum phosphates”
(Essington, 2004). Anthropogenic activities, as discussed in the next section, have
resulted in sites where soils are contaminated with metal concentrations well above
background levels. The metals, as a result of processing, are no longer incorporated
into mineral components, for example, silicates. As a result, they have increased bio-
availability and the potential to exert toxicity. Such metal contaminants can become
widely dispersed. For example, aerosols generated from metal processing or from
metal-containing waste sites can contain elevated metal concentrations resulting
in higher metal distribution via aerosol emission/deposition (Csavina et al., 2011).
These aerosols can be dispersed over large areas, and since deposition is often a
gradual process, elevated levels develop over time as metals accumulate.
Metal contamination cannot be mineralized to innocuous end products such as car-
bon dioxide and water like organic contaminants. Once in a system, metals can only
be modified through changes in redox state, and thus speciation. For example, toxic
species of selenium (Se
6+
, Se
4+
, and Se
0
) can be methylated by bacteria to the volatile
and less toxic species dimethylselenide [(CH
3
)
2
Se] or dimethyldiselenide [(CH
3
)
2
Se
2
]
(Maier et al., 2009). Thus, in order to treat a metal contamination event, the metal must
either be physically removed or chemically modified to reduce toxicity and mobility.
Due to the large area of most contamination events, the former solution is often infea-
sible or economically prohibitive; the latter solution is inevitably temporary as changes
in environmental conditions due to natural processes or lack of management may
result in the contamination returning to its initial or potentially worse condition. In the
earlier example, microbial methylation of selenium accomplishes both chemical treat-
ment via the generation of less toxic methylated species and physical removal via the
generation of a volatile compound that is released to the atmosphere.
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