Page 290 - Plant-Based Remediation Processes

284 G. Petruzzelli et al.

Fig. 13.2 Hg concentrations a
in the aerial part (a) and roots 30
(b)of Brassica juncea and
25
Helianthus annuus grown in
mg kg-1 15
control (CT) and 20
thiosulphate-treated soil (TS)
10
5
0
CT TS CT TS
B. juncea H. annuus

b
80
70
60
mg kg-1 50
40
30
20
10
0
CT TS CT TS
B. juncea H. annuus

elimination of the bioavailable fractions. C v is the concentration of metal in plants
1
(mg kg ) and B is the annual biomass production per mesocosm (kg per year).
Both C v and B depend on the soil characteristics and bioavailable forms of metals.
These are the two essential parameters that determine the applicability of plant-
based remediation. Considering one growing cycle per year, and inserting the
specific values of this study in (13.1), the resultant time required is 1 year.
Step 4. To confirm the removal of all the bioavailable Hg, a second growing
cycle (without any further treatment) using the same plants was carried out. Results
showed a negligible uptake of the metal. Mesocosms were left to rest for 1 year and
then they were reseeded. The new trials were prepared with the following scheme:
some pots were sown with the same type of plants used in the past; in others, plant
species were reversed, sowing B. juncea in pots where H. annuus grew in previous
experiments and vice versa. At the end of the experiment, plants were collected and
analyzed. The results showed that also in this case Hg concentrations in plant
sample were below the detection limit. The soil extraction with TS did not extract
Hg amount over the detection limit. This confirms that the entire Hg bioavailable
portion has been removed and that new equilibria in the soil with subsequent release
of bioavailable Hg were not created. Clearly these results are site specific and it is
possible that in different contaminated soils more than one growth growing cycle
would be necessary to eliminate all the bioavailable metal fractions. Throughout the