Environmental Engineering Reference
In-Depth Information
resistance time of metals in plants and in contam-
inated area and the potential release of metals.
The biomass collection problems can be illus-
trated by the concerns raised by Khan et al.
(
2000
) and concerns regarding contaminated
plant biomass following accumulation.
Composting and compaction have been pro-
posed as postharvest biomass treatment (Garbisu
and Alkorta
2001
). Studies carried out by Hetland
et al. (
2001
) have showed that composting can
signifi cantly reduce the volume of harvested bio-
mass. However, metal-contaminated plant bio-
mass would still require treatment prior to
disposal. Total dry weight loss of contaminated
plant biomass by compaction is advantageous, as
it will lower the cost of transportation to a haz-
ardous waste disposal facility.
One of the conventional and promising routes
to utilize biomass produced by phytoremediation
in an integrated manner is through thermochemi-
cal conversion process. If phytoextraction could
be combined with biomass generation and its
commercial utilization as an energy source, then
it can be turned into a profi t-making operation,
and the remaining ash can be used as bio-ore
(Brooks et al.
1998
); this is also the basic princi-
ple of phytomining. Nicks and Chambers (
1994
)
also reported a second potential use for hyperac-
cumulator plants for economic gain in the mining
industry. This operation, termed phytomining,
includes the generation of revenue by extracting
saleable heavy metals produced by the plant bio-
mass ash, also known as bio-ore.
Phytomining is the recovery process of accu-
mulated trace metals, may be an additional ben-
efi t of phytoremediation. However, concerns
about plant matter getting into the food chain by
direct consumption or by decomposition path-
ways include considerable ecological and human
health problems (Khan et al
.
2000
). Also, the
recovery of metals is very costly. Hence, harvest-
ing and disposing of plant biomass are essential
to prevent recycling of accumulated metals when
plants are decomposed. Hetland et al
.
(
2001
) pre-
sented three approaches for processing of bioac-
cumulating plant tissues. Biomass recycling is a
foremost concern to address because the environ-
mental factors or the livestock may remove fresh
biomass prior to harvesting. The transfer of heavy
metals from the contaminated area into plant
matter may help to resolve biomass-recycling
concerns.
Combustion and gasifi cation are the most
important sub-routes for organized generation of
electrical and thermal energy. Recovery of this
energy from biomass by burning or gasifi cation
could help make phytoextraction more cost-
effective. Thermochemical energy conversion
best suits the phytoextraction biomass residue
because it cannot be utilized in any other way as
fodder and fertilizers. Combustion is a crude
method of burning the biomass, but it should be
under controlled conditions, whereby volume is
reduced to 2-5 % and the ash can be disposed
properly (Bridgwater et al.
1999
). It is not encour-
aged to burn the metal bearing hazardous waste
in the open, as the gases and particulates released
in the environment may be detrimental; only the
volume is reduced and the heat produced in the
process is wasted. Gasifi cation is the process
through which biomass material can be exposed
to series of chemical changes to yield clean and
combustive gas at high thermal effi ciencies. This
mixture of gases is called as pyro-gas that can be
combusted for generating thermal and electrical
energy.
Bridgewater et al. (
1999
) have reported that
pyrolysis is a novel method of municipal waste
treatment that might also be used for contami-
nated plant material. Pyrolysis decomposes mate-
rial under anaerobic conditions, and there is no
emission to the air. The fi nal products are pyro-
lytic fl uid oil and coke; heavy metals will remain
in the coke, which could be used in smelter.
Koppolu et al. (
2003
) reported that 99 % of the
metal recovered in the product stream was con-
centrated in the char formed by pyrolyzing the
synthetic hyperaccumulator biomass used in the
pilot scale reactor. Helson et al. (
1997
) conducted
low temperature pyrolysis experiments with Cr-,
Cu-, and As-treated wood, and it was concluded
that most of the metal was retained in the pyroly-
sis residue. Infl uence of metal ions on the pyroly-
sis of wood has been studied extensively by many
authors (Pan and Richards
1990
; Richards and
Zheng
1991
; Mohan et al.
2006
,
2014
).
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