Phytoextraction of Trace Metals: Principles and Applications - Environmental Sustainability: Role of Green Technologies - page 213

To be economically viable, phytomining
should be able to produce about $500 ha −1 , and
most phytomining operation can gain additional
revenue from incineration of biomass to generate
electricity. The vicinity of the phytomining farm
to the power plant is important to reduce
transportation costs and to facilitate the second-
ary use of biomass for bioenergy production in
addition to the recovery of the target metal.
Furthermore, as noted by many authors, the lack
of continuity of biomass supply from phytomin-
ing operation where harvest occurs once or twice
a year is an impediment to the use of biomass for
bioenergy unless other sources of biomass are
available locally to provide material for the con-
tinuity of combustion operation throughout the
year. One way to solve this problem is the co-
fi ring with other sources of biomass as explained
in Sect. 3 .
A critique related to the environmental sus-
tainability of phytomining addresses the effi -
ciency of phytomining relative to conventional
mining. Robinson et al. ( 2009 ) pointed out that
phytomining requires a large area and more time
to produce a ton of Ni than conventional mining
(2.5 ha year −1 vs. 22 m 3 in a few hours). In addi-
tion, Robinson et al. ( 2009 ) note that phytomin-
ing of surface serpentine soil will take from 3 to
18 crop cycles before the surface Ni is depleted,
and the authors argue that the surface soil needs
to be removed to continue phytomining of the
deeper soil layers. The authors are also concerned
with the introduction of a monoculture of possi-
bly exotic species and disruption of serpentine
ecosystem and native endemic fl ora.
Other authors (Anderson et al. 1999 ; Chaney
et al. 2010 ; Harris et al. 2009 ; Sheoran et al.
2009 ) are supportive of phytomining and con-
sider its environmental impact similar to that of
commercial farming. Phytomining is considered
to have a positive effect on soil erosion by the
effect of plant roots relative to the open pit and
the 'desert-like' landscape left after conven-
tional mining operation has ceased and substan-
tial site remediation is required at the end of the
mine. Phytomining, instead, can improve the
quality of the soil for post-mining operation

over the duration of the phytomine. It is neces-
sary, however, to study the potential environ-
mental impacts of the phytomine during the
planning phase and before the phytomining
operation is set in place.
Hence, this technology requires a team of
expert agronomists, ecologists, and soil scientists
to carefully choose the proper species for phyto-
extraction of a particular metal. Various steps can
be taken to reduce the ecological risks associated
with monoculture and introduction of potential
exotic species, for example, (1) analysis of phy-
toextraction potential of native vegetation and
selection of the best performing native species
for metal extraction effi ciency, (2) adoption of
principles of agro-ecology such as cultivation of
combination of various native species, and (3)
use of alternative fertilisers (sewage sludge bio-
solids) and bioherbicides.
Finally, conventional mining is not economi-
cally viable on low-grade ore, and hence a
comparison of the effi ciency between phytomin-
ing and conventional mining is not appropriate.
If phytomining proceeds beyond the theoreti-
cal and trail stage, the most likely scenario that
can be envisaged is that phytomining can be
farmed out to small-scale landholders and farm-
ers throughout the region where low-grade metal-
liferous soils are present. Small-scale operation
can be environmentally sustainable because usu-
ally small farms are farmed directly by the land-
holder, which is therefore interested to maintain
or increase the fertility of his/her land. Higher
revenues can be obtained if farmers unite in co-op
to reduce the costs of mechanisation and trans-
portation by economy of scale.
Fertility of land is likely to be increased after
phytomining operation because of increased root
and microbial activity in the soil profi le, Ni
removal, and pH and fertility management.
Therefore, landholders may want to use land for
food production after phytomining has reached
Ni depletion of the top layers and therefore
reduced Ni toxicity to crops occurs. The possi-
bility of restoring serpentine soils post-
phytomining and convert those soils to food
production is an important aspect of environ-