Environmental Engineering Reference
In-Depth Information
4.1.1 Sedimentation and Filtration
Sedimentation and filtration are important physical
processes allowing the removal of heavy metals
associated with particulate matter. Sedimentation
has long been recognised as a principle process in
the removal of heavy metals from wastewaters
(Sheoran and Sheoran
2006
).
Iron and Mn precipitate sequentially because the
oxidation of Mn is inhibited by the presence of
Fe (II) (Kosolapov et al.
2004
). Co-precipitation
of metals with Fe and/or Mn (oxy) hydroxides is
an additional important removal pathway in oxi-
dised substrates. Co-precipitation with Fe and/or
Mn (oxy) hydroxides is however not considered
as a long-term removal mechanism as it is redox
sensitive (Sheoran and Sheoran
2006
).
In anaerobic conditions and the presence of an
organic carbon source, sulphate is reduced by
sulphate-reducing bacteria (SRB), and sulphides
are formed. Sulphides then react with divalent
metals to form insoluble metal sulphide precipi-
tates (Gambrell
1994
; Kosolapov et al.
2004
):
4.1.2 Sorption
Sorption of heavy metals can occur by processes
of adsorption and precipitation. Heavy metals
may be adsorbed either electrostatically, result-
ing in the formation of relatively weak complexes
(physical adsorption), or chemically, resulting in
the formation of strong complexes (chemisorp-
tion). Metals are more strongly bound by chemi-
sorption than by physical adsorption (Evangelou
1998
).
2−
+ − −
+ →++
SO
422
CH OHHS
2
HCO
3
+→
()
HHSHSg
+
2
4.1.3 Precipitation
and Co-precipitation
Metals can also precipitate with (oxy)hydrox-
ides, sulphides, carbonates, etc. when solubility
products are exceeded (Kadlec and Knight
1996
).
The stability of metals precipitated as inorganic
compounds is primarily controlled by pH. At
near-neutral or alkaline pH, metals are effectively
immobilised (Gambrell
1994
). Bacterial produc-
tion of bicarbonate by bacterial sulphate reduc-
tion, or the presence of limestone in the substrate,
can lead to sufficiently high bicarbonate levels to
form precipitates with metals. Hydrolysis and/or
oxidation of metals leads to the formation of
(oxy)hydroxides. The solubility of these oxides is
very low in the range of pH normally encoun-
tered in most substrates (Evangelou
1998
).
Dissolved reduced forms of Fe (II) are oxidised
by abiotic reactions and bacteria and then pre-
cipitated mainly as hydroxides, a reaction which
increases the acidity of the wastewater (Kosolapov
et al.
2004
):
HS M SH
−
+→↓+
2
+
+
With CH2O and M
2+
representing, respectively, a
simple organic compound and divalent metal ion.
The process of sulphate reduction buffers the pH
of the solution.
4.2
Biological Removal
Mechanisms of Heavy Metals
4.2.1 Microbial Removal Processes
Microorganisms affect the behaviour of heavy
metals in constructed wetlands by (i) their role in
the biogeochemical cycles which affect metal
speciation, (ii) biosorption, (iii) reduction and
(iv) methylation of heavy metals (Kosolapov
et al.
2004
).
4.2.1.1 Biosorption of Heavy Metals
The microbial biomass can also sequester metals
by the processes of active (energy-dependent)
and passive (energy-independent) metal uptake,
respectively, called bioaccumulation and bio-
sorption (Kosolapov et al.
2004
). Some bacteria
are also able to accumulate metals inside their
cells, forming amorphous mineral inclusions.
However, the storage of metals by the microbial
biomass is relatively short-term due to the short
2
+
+ +
++ →+
3
Fe HOFe
41 2
212
HO
+ →
( )
↓+
After the oxidation of Fe (II), Mn (II) is oxi-
dised to Mn (IV) which is mainly precipitated as
MnO2 but can also be precipitated as Mn(OH)2.
Fe HO Fe OH
3
+
32
33
H
+
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