Page 289 - Plant-Based Remediation Processes

13 Phytoremediation Towards the Future: Focus on Bioavailable Contaminants 283

Table 13.1 Percentage of
Shoots (%) Roots (%)
bioavailable fraction removal
from different plant species in Brassica juncea CT 19.3 10.8
the first growing cycle TS 73.7 22.9
Helianthus annuus CT 13.4 30.8
TS 26.0 69.8
uptake larger amounts of the contaminant. The efficiency of plant removal was
determined by the ratio between total accumulation and total available mercury in
soil. For both species, efficiency was higher than 95 % Table 13.1.
For controls, the percentage was calculated with respect to the amount of
mercury extractable according to Milla ´n et al. (2006), while for treated soil the
percentage was determined with respect to the amount of Hg extractable by
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thiosulfate 0.27 mol L . As a matter of fact, there was a slight increase in biomass
production probably due to the fertilizing effect of ammonium thiosulphate. In the
growth period of 60 days in the untreated soils, the mean values of the aerial
biomass were 10.4 and 30.2 g DW for B. juncea and H. annuus, respectively.
After ammonium thiosulphate treatment, the obtained results were 16.1 and 34.5 g
in the case of B. juncea and H. annuus, respectively. Root biomass was not affected
by treatment with mean values of 1.5 g DW for B. juncea and 2.8 g DW for H.
annuus both in CT and TS soil. The results obtained for original (CT) and
thiosulphate treated (TS) soils are reported in Fig. 13.2.
The addition of the mobilizing agent promoted Hg uptake by plants of both
species. The data (Fig.13.2) clearly show that the uptake of the plants was depen-
dent on Hg concentration in the soil solution that determines the metal bioavail-
ability. The mean Hg concentrations in plants grown in the original soil were 2.20
and 32.9 mg kg 1 for aerial and root portions of B. juncea and 0.80 and 14.0 mg kg 1
in the case of H. annuus. The addition of TS solubilized greater quantities of Hg;
this in turn increased the Hg uptake by plants. For B. juncea the mean Hg
concentrations increased to 29.1 and 74.6 mg kg 1 in the aerial and root portions,
respectively, while for H. annuus results of Hg concentration were 10.5 and
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73.4 mg kg . The increase in Hg concentrations in plants after TS addition has
been reported also in different soils and plants (Moreno et al. 2004, 2005; Pedron
et al. 2011). The lower concentrations of Hg in shoots reflect the defense
mechanisms of plants that store the toxic metal in the root portion (Moreno et al.
2004, 2005). At the end of the experiments, the control analysis of leachates showed
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negligible Hg concentration, below the detection limit (2 μgL ), in any
mesocosms.
The time required to achieve reduction of bioavailable concentration required in
soil can be calculated by

W s ΔC
t ¼ ; (13.1)
C v B

where t is time (years), W s weight of soil (kg), and ΔC the decrease of metal
1
concentration necessary to achieve the remediation target (mg kg ), i.e.,