Phytoextraction of Trace Metals: Principles and Applications - Environmental Sustainability: Role of Green Technologies - page 212

et al. ( 2003b ) demonstrated the feasibility of Ni
recovery from the ash (bio-ore) of genetically
improved Alyssum spp. Bio-ore contains higher Ni
concentration (6-16 %) than normal Ni ores and
are free of Mn, Fe, and Si oxides as in conventional
ores (Robinson et al. 2009 ). Chaney et al. ( 2007 )
discuss the importance of fertilisation, pH optima,
plant density, and development of improved culti-
vars for increasing shoot Ni concentration and
yield of hyperaccumulator Alyssum murale . Weed
management is also necessary when serpentine
soils are fertilised, as other plants can compete
with the hyperaccumulator grown for phytomin-
ing. In addition, in wet climate when soils are
poorly drained, ridge tilling helped reduce adverse
effect of heavy spring rainfall and consequent
fl ooding on the survival and yield potential of
Alyssum grown on a fi eld trial in Canada (Chaney
et al. 2007 ).
Further studies (Anderson et al. 1999 ; Harris
et al. 2009 ; Keller 2004 ; Robinson et al. 1997b )
have shown that other species could be grown for
phytomining and that other metals could be
extracted, such as thallium (Tl) and gold (Au).
There are only a few Tl hyperaccumulators,
namely, Iberis intermedia and Biscutella laevi-
gata , from southern France (Leblanc et al. 1999 )
that can take up high level of Tl in dry matter,
0.4 mg kg −1 and 1.4 mg kg −1 , respectively. Tl is
quite rare in nature, representing only 0.7 mg kg −1
in the Earth's crust. It is very toxic and is used in
rat poison and for the control of ants. There is
clearly potential for Tl phytomining if large areas
are contaminated with Tl to obtain the advantage
of large-scale operations. I. intermedia can pro-
duce 10 t ha −1 , as determined from fi eld observa-
tion in France, and it should produce about
700 kg ha −1 of bio-ore containing 8 kg Tl
(Anderson at al. 1999 ). Plants do not normally
accumulate Au, and the metal must be made sol-
uble before uptake can occur. Au is extracted by
induced phytoextraction, which consists of using
a chelating agent, usually thiocyanate, to make
Au available to plant roots. It requires standard
ore mining and grinding of the ore and then plac-
ing the ground ore over a plastic surface to avoid
leaching of the cyanide and thiocyanide used to
induce phytoextraction.

4 Phytomining

Commercial mining is usually performed on ores
with high concentration of the target metal (for
Ni, at least 30 g kg −1 ) and are environmentally
costly and energy- and capital-intensive practice
of mining. Few ore bodies of this kind occur on
the Earth's surface and are present in small local-
ised areas, and some of these are becoming
exhausted due to expanding economies and
industrialisation.
Sub- or low-grade ores contain concentration
of target metal below the content required to be
economically extracted and smelted by conven-
tional methods. Most of these ore bodies are
associated with ultramafi c deposits that generate
serpentine soils after weathering of the ultramafi c
rocks. Serpentine soils are characterised by pH of
6-8; low Ca/Mg ratio and low levels of N, P, and
K; and potentially toxic concentrations of Ni (Li
et al. 2003a ; Sheoran et al. 2009 ); they are not
economic to mine and are unsuitable for agricul-
ture due to high trace metal content (especially
Ni). These deposits are scattered around the
world and usually support a characteristic fl ora of
endemic plants (Brooks 1987 ) that are able to tol-
erate and/or (hyper) accumulate the metals pres-
ent in the serpentine soil. The use of plants to
extract Ni and few other metals to produce a bio-
ore is called phytomining. Dried plant material of
hyperaccumulators grown on Ni-rich serpentine
soil is reduced to an ash, and the metal is recov-
ered using conventional metal refi ning methods
such as acid dissolution or electrowinning.
The fi rst fi eld trial on Ni phytomining used a
naturally occurring stand of Streptanthus polyga-
loides grown on serpentine soils in CA, and it
extracted up to 100 Ni kg ha −1 , worth $550 ha −1 at
the prices of Ni in 1994 (Nicks and Chambers
1998 ). In a second phytomining fi eld trial, Alyssum
bertolonii (Robinson et al. 1997a ) was used on a
serpentine soil in Italy, and plants were fertilised
with N, P, and K over a 2-year period. Fertilisation
induced a threefold increase in dry biomass, indi-
cating that agricultural practices similar to those
applied to crops are important to increase yield in
hyperaccumulator plants used for phytomining. Li