Page 162 - Plant-Based Remediation Processes

8 A Multi-disciplinary Challenge for Phytoremediation of Metal-Polluted.. . 153

Generally the roots of crops were less restricted in their growth than shoots,
although roots had much higher metal concentrations, i.e., Cd 4-fold, Zn 7 ,Co
28 ,As 33 ,Pb51 and Cu 77 . They acted as a substantial accumulation sink for
most trace elements, particularly Cu, Pb and As—a result also found in coarse and
particularly fine roots of poplars and willow. Fine root biomass was quite modest,
2
estimates from soil cores in crops not exceeding 20 g m , and their contribution to
long-term metal stabilisation is probably negligible because of fast turnover (Goins
and Russelle 1996). For the coarse roots of trees and tap roots of annual species,
degradation is probably slower, but this is an issue to be further investigated in
phytostabilisation processes. From several aspects, root systems may hold the key to
understanding the possibilities and options for phytomanagement of pyrite waste,
although the maximum rooting depth (0.3 m) still remains to be greatly enhanced. In
this context, the very high number of spontaneous species with shallow fasciculate
roots is likely the result of severe selection of tap-rooted ones. Phytoextraction
enhancement through increased metal concentrations in biomasses (e.g., by soil
ploughing) turned out to reduce species differences, a strategy contrasting with
many other agricultural practices which are instead addressed to yield
improvements. The main information obtained from this trial was the need to reduce
soil contamination through soil amendment and to facilitate plant establishment—
for instance with the application of growth regulators.

8.5 Improving Pyrite Hospitality and Plant Metal Uptake

In order to improve the phytoextraction in pyrite, some pot trials with fodder radish
were set up in 2006 and 2007, the aim being to improve the environment for roots
and enhance above-ground productivity. In all experiments, plants were grown for 3
months in cylindrical 52-cm high pots (1.3 L volume), filled with a pyrite
cinder–sand mixture and regularly watered with 50 % diluted Hoagland solution.
Sand was added to attenuate contamination and improve water drainage, but
leachates were collected in order to check whether our treatments had environmen-
tal counter-indications. Treatments were compared with untreated controls with
five replicates.
We first thought of humic acid treatment for plants and pyrite directly. Humic
acids (HA) are characterised by acidic groups which play an important role in
enhancing the solubility, bioavailability, uptake and transport of metals (Evangelou
et al. 2004), and are known for their auxin-like effect (Delfine et al. 2005). HA came
from a commercial product (Humic super, Tiller—Italy) as liquid formulation
(10 % DW of HA) and were applied as follows: foliar spraying (0.1 g HA L 1
solution, once a week for 3 weeks), two doses mixed with the substrate before
1
sowing (0.1 and 1 g HA kg ) in combination or not with foliar treatment and a low
1
rate (0.1 g HA kg ) applied at sowing through irrigation. The highest amendant
dose of 1 g kg 1 positively increased shoot metal concentrations (overall elements:
+44 %) but, unfortunately, curbed shoot growth and worsened metal removals