Page 156 - Plant-Based Remediation Processes

8 A Multi-disciplinary Challenge for Phytoremediation of Metal-Polluted.. . 147

In the buffer zones, more than 80 different species were classified, mainly
herbaceous and only 10 woody. In zone I, close to the old factory, the latter were
mainly represented by Pyracantha coccinea M.J. Roemer (27.5 %, i.e., percentage
of the sum of all detected species), Salix spp. (21.6 %) and Populus alba L. (8 %).
The most widespread grasses were Solidago gigantea Aiton (8 %) and Dorycnium
pentaphyllum Scop. (8 %). Dominant species in zones II and III were Poa pratensis
L. (24 % and 11.4 %, respectively), Ambrosia artemisiifolia L. (18 % and 34 %),
Medicago lupolina L. (6 % and 9 %) and Bromus arvensis L. (11 %, only in zone
III). Lastly, in zone IV the prevailing species were Phragmites australis (Cav.)
Trin. ex Steud. (15 %), Solidago gigantea Aiton (10 %), Dactylis glomerata L.
(7 %) and Populus alba L. (10 %).
Zinc, Mn and Cu were the three most frequently accumulated elements in the
shoot tissues of all species, both herbaceous and woody (Fig. 8.3). The highest
values of Zn and Cu were found in Taraxacum officinale Weber (360 and
1 1
96 mg kg , respectively) and Mn in Carex hirta L. (393 mg kg ). The overall
metal concentrations (summation of various elements) were highest in Asteraceae
species, i.e., T. officinale, Eupatorium cannabium L. and A. artemisiifolia L., the
latter being the most widespread. Interesting accumulations were also found in the
hydrophyte C. hirta (family Cyperaceae), whereas trees and shrubs seemed to be
less efficient than herbs, except for the Salicaceae Salix alba L. and Populus
nigra L., which have been found to accumulate Zn efficiently in this and in other
contaminated sites (Rosselli et al. 2003; Pietrini et al. 2010). These preliminary
investigations confirmed the importance of species selection in phytoremediation.
Although a particular relationship between metal accumulation and kind of root
apparatus does not seem to exist (Fig. 8.3), the ability of our Asteraceae may
partially depend on their deeper tap roots. The application of spontaneous species
still raises the problem of seed supply and, with this in mind, screening of cultivated
species was considered necessary.

8.4 Experience in an On-Site Pilot Phytoremediation Plant

The sparse natural vegetation cover of the site meant that difficulties in plant
establishment and growth were predicted, but the presence of the capping unpol-
luted layer seemed useful for the vegetation. In a preliminary pot trial, we verified
whether some crops like sunflower, alfalfa and fodder radish (Raphanus sativus L.
var. oleiformis Pers.) could take advantage of a 7- or 15-cm top unpolluted soil layer
(Fig. 8.4). Indeed, mimicking site stratigraphy, regardless of the thickness of the
capping layer, all species showed regular growth both above- and below-ground
over a 60-day period of cultivation, comparable with that of the uncontaminated
reference soil of the University of Padova. However, roots tended to colonise
mainly the uncontaminated layer (length: 90 % vs. 80 % of pyrite alone and 50 %
of controls). The general marked reduction in plant growth with pyrite alone was
evident, i.e., 77 % in shoots and 63 % in roots (length) on average.