Constructed Wetland: An Ecotechnology for Wastewater Treatment and Conservation of Ganga Water Quality - Environmental Sustainability: Role of Green Technologies - page 246

life cycle of microorganisms implying the need
for additional removal pathways (Kosolapov
et al. 2004 ).

to methylate Hg, implying the need for control
when the process of sulphate reduction is aimed
to treat wastewaters that are also contaminated by
Hg. However, the precipitation of HgS has been
described to reduce Hg toxicity (Kosolapov et al.
2004 ).

4.2.1.2 Reduction of Heavy Metals
Anaerobic metal-reducing bacteria use metals as
terminal electron acceptors in their anaerobic res-
piration. Depending on the metal species
involved, this can lead to a mobilisation or immo-
bilisation of metals. The reduction of the soluble
and toxic Cr (VI) to Cr (III) results in its precipi-
tation with hydroxides and efficient removal
from wastewater. Reduction of Cr (VI) is per-
formed by a variety of microorganisms, includ-
ing some sulphate-reducing bacteria (SRB), and
can occur under oxic and anoxic conditions.
Whereas the reduction of Cr (VI) leads to its
effective immobilisation and retention in the sub-
strates of CWs, reduction of other metals includ-
ing Hg, Fe and Mn enhances their mobility and
releases from the wetland system. The reduction
of Hg (II) results in the release of elemental Hg to
the atmosphere. This strategy has been success-
fully applied to remediate Minamata Bay sedi-
ments. However, there are public concerns about
atmospheric pollution. Iron- and Mn-reducing
bacteria can dissolve insoluble Fe (III) and Mn
(IV) (oxy)hydroxides, resulting in the release of
soluble Fe (II) and Mn (II), as well as the trace
metals co-precipitated with these (oxy) hydrox-
ides. Intermediate redox conditions are detrimen-
tal to CW efficiency as they transfer metals to
their most soluble state (Kosolapov et al. 2004 ).

4.3 Uptake of Heavy Metals

by Aquatic Macrophytes

Uptake of heavy metals by aquatic macrophytes
is dependent on the life form of the macrophyte:
rooted emergent, floating or submerged
(Guilizzoni 1991 ). For emergent macrophytes,
root uptake is the primary source of metals.
Floating and submerged species, however, can
also accumulate metals directly from the water
by their shoots. Elucidation of a single metal
uptake pathway is difficult without the use of
tracers, and a comparison of several uptake pat-
terns simultaneously is even more complex
(Crowder 1991 ).
Aquatic macrophytes depend on the micronu-
trients available in the water and/or substrate, in
order to meet their nutritional demands. Among
the micronutrients that are required for the growth
and development of all higher plants, the follow-
ing heavy metals are encountered: Fe, Mn, Zn,
Cu, Mo and Ni (Welch 1995 ). Certain plants can
also accumulate metals with no known biological
function, including Cd, Cr, Pb, Co, Ag, Se and
Hg (Salt et al. 1995 ).

4.3.1 Uptake of Heavy Metals
by the Roots
Most metal ions enter plant cells by specific
metal ion carriers or channels. After entering the
roots, metals are either stored or transported to
the shoot. Transport to the shoot probably occurs
in the xylem, and metals can be redistributed in
the shoot via the phloem. Plants take up elements
in their ionic form. Aquatic macrophytes which
root in the substrate rely on the soluble metal
fraction which is affected by parameters includ-
ing pH, redox potential and organic matter con-
tent of the substrate (Jackson 1998 ). Plant roots
can mobilise metals by various strategies,

4.2.1.3 Methylation of Heavy Metals
Some metals, including Hg, As and Se, can be
biomethylated by aerobic and anaerobic bacteria
resulting in the production of volatile derivatives
such as dimethylmercury, dimethylselenide or
trimethylarsine (Salt et al. 1995 ; Kosolapov et al.
2004 ). These volatile derivatives are generally
more toxic than the inorganic metal species,
because of their lipophilic character. Another
toxic product of the methylation of Hg is methyl-
mercury (CH3Hg), a lipophilic product that is
easily accumulated by aquatic organisms and
transferred into the food chain. Some SRB
(sulphate-reducing bacteria) have been identified