Page 163 - Plant-Based Remediation Processes

154 T. Vamerali et al.

probably as consequence of high metal availability in pyrite (Bandiera et al. 2009).
Improved translocation of all metals was the only positive effect of high HA
dosages. Actually, only at the small dose of 0.1 g HA kg 1 were there significant
increases in root length (+46 %) and—although with only slight above-ground yield
improvements—substantial enhancement of plant metal removals (+35 %). These
results confirm the auxin-like properties of humic substances, and their effective-
ness at low rates is a premise for low-cost large-scale applications.
More complicated was management of exogenous application of IBA, one of the
most powerful root-enhancing phytohormones. Starting from about 1 month after
sowing, we tentatively applied IBA five times (at 10-day intervals) to fodder radish
1
leaves at 10 mg L 1 or to the waste at 0.1 and 1 mg kg , in association or not with
foliar spraying. We obtained negative responses from this trial as—with the excep-
tion of foliar spraying alone—the hormone reduced shoot and root biomass ( 60 %
on average) when applied to the waste, probably due to unsuitable dosages and long
persistence caused by low microbial activity (Vamerali et al. 2011a). The expected
phytoextraction balance was thus greatly worsened, in spite of improvements in
concentrations due to the chelating effect of this phytohormone.
Lastly, verification of the applicability of chelant-assisted phytoextraction was
tested on pyrite, which is an uncommon substrate for this technique. We wished to
ascertain whether the recently available EDDS (ethylene diamine disuccinic acid),
characterised by higher degradability compared with EDTA (ethylene diamine
tetracetic acid), could improve metal uptake without causing substantial
phytotoxicity and leaching. The tested plants were fodder radish and Ethiopian
mustard treated with [S,S]-EDDS at various doses and application times: 2.5 and
5 mmol kg 1 substrate applied through irrigation 1 week before harvest (common
application time of chelators) and 1 mmol kg 1 soil repeated five times at 5- or 10-
day intervals, respectively starting 48 or 28 days after sowing. At these doses, the
chelator did successfully improve Cu, Co, Zn and Pb above-ground concentrations
(Table 8.3), together with Cu translocation, but reduced plant biomass, especially
with repeated applications and in radish (Bandiera et al. 2010). This may have a
direct effect on leaching, as the drop in transpiration caused by diminished leaf area
leads to significant losses of Cu, the metal with the greatest stability constant with
EDDS (Tandy et al. 2004). Better metal phytoextraction (+31 %) together with
1
minimal metal leaching was achieved with moderate (2.5 mmol kg ), traditional
close-to-harvest chelator applications, but in Ethiopian mustard only. Certainly,
these results on the use of EDDS and its management require on-site confirmation,
but the generally unfavourable phytoextraction balance, associated with the uncer-
tain fate of metal-EDDS compounds after plant harvest, gives rise to doubts about
its use.

8.6 Conclusions

Phytoremediation of pyrite waste is complicated to manage because of multiple
constraining factors which affect plant growth, beyond metal contamination.
Removal of the most labile fraction of metals with field crops seems to be a feasible