http://wslar.epfl.ch/perso/plso03_fulltext.pdf
http://www.psat.wa.gov/Publications/LID_tech_manual05/14_app6.pdf
http://www.diva-portal.org/diva/getDocument?urn_nbn_se_su_diva-307-1__fulltext.pdf
Phytoremediation of Hg
Phytoextraction
In phytoextraction, metal-tolerant plants with high metal accumulation and high biomass
production are preferably used. Our results showed a large variation among the six clones
of willow in their sensitivity to Hg (paper II). The tolerant clone Björn was used to study
the phytoextraction of Hg both in pots with aged Hg-spiked soil or industrial Hgcontaminated
soil and in the field. Results showed that this willow clone could grow
successfully without significant measurable toxic effects except with 1mM KI addition
(Papers III and IV). The toxic effects found in the test with 1 mM KI addition was thought
to be mainly due to the toxicity of iodide to the plants (Paper IV). It suggests that selected
willow clones are able to tolerate Hg while being used for phytoextraction of such types of
aged Hg-contaminated soil.
A possible release of Hg into air by plants may contribute to air contamination when
using phytoextraction in practice. However, our study showed that plant leaves do not
release Hg into the air in any of the investigated plant species (Paper I). This suggests that
there is no consequent increase of Hg burden in the atmosphere by phytoextraction.
Willow roots accumulated Hg from aged industrial Hg-contaminated soil (Papers III,
IV), as shown earlier for other plant species (Lenka et al., 1992). The plants used for
phytoextraction must have an ability to efficiently accumulate metal via their roots. Our
studies showed that willow roots efficiently accumulated Hg in hydroponics, where they
could accumulate more than 300 µg Hg g-1DW from of 1 µM Hg(NO3)2 (200 µg Hg L-1)
within 4 hours (Fig. 8) and reduce the Hg concentration in Hg(NO3)2 solution from initial
1 µM to 0.05 µM after 3 days of cultivation. Moreover, willow could accumulate Hg by
more than 1000 µg g-1DW in its roots without significant toxic effects (Paper II).
However, Hg accumulation in willow grown in soil was much less efficient than that of
willow grown in hydroponics (Papers II, III, and IV). Other plant species, e.g., western
thistle with the highest Hg accumulation among plant species grown in Hg-contaminated
soil at the Bohus site, accumulated similar low levels of Hg as willow (Table 4). The low
accumulation of Hg in plants from soil was believed to be due to the low bioavailability of
Hg in the soil. Indeed, the results of the sequential extraction showed that Hg in soil was
mainly bound to residual organic matter (53%) and sulphides (43%), which remained
stable during the cultivation of willow.
The low bioavailability of Hg in contaminated soil is a restricting factor in
phytoextraction of Hg. Compared with chelating agents, e.g. EDTA, iodide is more
efficient in mobilizing Hg in soil, which mainly forms the soluble complex HgI4
2- with a
stability constant of 29.8 (Wasay et al., 1995). However, too high iodide concentrations
may be toxic to willow (Paper IV; Mackowiak and Grossl, 1999; Zhu et al., 2003).
Therefore, the iodide concentrations used to increase the bioavailability should be
tolerated by plants. Additions of up to 1 mM KI increased the Hg concentrations to about
5, 3 and 8 times, respectively, in the leaves, branches and roots (Paper IV).
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The plants used for phytoextraction should have high translocation of accumulated
metals to an easily harvestable part of the plant, i.e. the shoot in the case of willow.
However, both hydroponics and soil studies showed that willow had a low translocation of
Hg to the shoots (Papers I–IV), and similar results were found in other plant species (Paper
I; Beauford et al., 1977; Godbold and Hütterman, 1988). Moreover, although iodide
addition could increase the amount of Hg extracted by plants from soil, it could not
improve the low translocation of Hg from the roots to the shoots (Paper IV). The low
translocation of Hg to plant shoots detected leads to a low efficiency of Hg removal from
the contaminated soil if plant shoots alone are harvested. Hence, Hg-accumulating roots
should also be harvested together with shoots, which is apparently not feasible in practice.
Therefore, it might not be realistic to use this plant for phytoextraction of Hg in practice,
even though iodide could enhance the phytoextraction efficiency.
To estimate the time required to remove all Hg from a Hg-contaminated soil by using
phytoextraction, model calculations were made based on the data from field trials and pot
tests in Paper IV (Table 6). The calculations show that extremely long time is needed to
clean up the Hg-contaminated soils if stem alone is harvested. Moreover, industrial Hgcontaminated
soil needs longer time to be cleaned up than Hg-spiked soil. This is due to
the differences in bioavailability of Hg between the two kinds of soils. The soil used in the
pot test was 1-year-old Hg-spiked agricultural soil and well homogenised with relatively
higher Hg bioavailability than that of the aged-soil in the field trial. The soil for the field
trial was polluted with Hg more than 30 years ago and was extremely heterogeneous.
Furthermore, it probably contained large amount of sulphur, as sulphur was previously
used by the company to produce sulphuric acids. The long ageing effect and the high
concentrations of sulphur lead to the extremely low bioavailability of Hg in the soil,
because the bioavailable Hg decreased with time by leaching, bacterial volatilization and
formation of stable Hg complexes with the soil matrix, especially with sulphur.
Table 6 Estimation of the time required to remove all Hg from two kinds of Hg-contaminated
soils by phytoextraction, assuming that the metal taken up by plants is from the top 50 cm of soil
Soil harvest Biomass production
Kg(ha*yr)-1 §
Hg in plant
µg (g DW)-1 years
stem 23000 0.46 23600 Industrial Hg-contaminated soil with
50 mg Hg kg-1DW † root 16000 27.6 574
stem 23000 0.70 15500 One-year-old Hg-spiked soil with
50 mg Hg kg-1DW ‡ root 16000 274 57
† Calculation is based on the data from field trial at the site of a chlor-alkali plant in the vicinity of
Gothenburg (Sweden) with 0.5mM KI addition (Paper IV).
‡ Calculation is based on the data from pot tests with 0.2mM KI addition (Paper IV).
§ Biomass production of stem is based on the data from Labrecque and Teodorescu (2003). The root
biomass was based on the root/stem biomass ratio in the hydroponics cultivation.