Page 288 - Plant-Based Remediation Processes

282 G. Petruzzelli et al.

(height 30 cm, volume 8.15 L) that are arranged to collect leachates by a hole in the
bottom connected to a plastic tank with a PVC tube. Two plant species were
selected: Brassica juncea var. scala and Helianthus annuus var. paola. Coarser
materials (>2 cm) were eliminated from soil before filling the mesocosms. The
amount of soil per pot was 5 kg. Plants were sowed using 0.5 g of seeds for
B. juncea and nine seeds for H. annuus. During the growing period, plants were
watered daily with deionized water. Treatment with mobilizing Hg additive started
45 days after sowing, with the same solution used for Hg extraction, 0.27 M
ammonium thiosulphate. The solution was added to mesocosms by splitting the
total dose, 250 mL in 5 consecutive days to avoid or at least to minimize possible
toxic effects on plant species. Three replicates of treated mesocosms (TS) were
prepared, with controls (CT) (untreated soil) run simultaneously. Experiments
lasted 60 days. Plants were harvested 15 days after additive treatment. Aerial
parts were separated from the roots and all samples were washed with deionized
water. The roots were subjected also to a washing in an ultrasound bath (Branson
Sonifier 250 ultrasonic processor) for 10 min to eliminate soil particles that could
have remained on root surfaces. Vegetal samples were left in a ventilated oven at a
temperature of 40 C until a constant weight was obtained. The dry mass of shoots

and roots was gravimetrically determined. Materials were grinded and
homogenized by the use of Knife Mill Grindomix GM 300 Retsch for analysis.

13.4.2 Results from EBCS

Step 1. Among the several reagents used for evaluating the bioavailable Hg
fractions, the sequential extraction with H 2 O and NH 4 Cl was selected (Milla ´n
et al. 2006) since it properly individuates the amounts in the Hg soil solution
(H 2 O) or easily solubilizable (NH 4 Cl) available for plant uptake. The analysis on
extracts from the sequential extraction procedure showed that Hg soluble and
exchangeable fractions represented a very low portion of the total concentration,
1
2.6 and 9.6 μgkg , respectively. The sum of these two fractions was considered
the “total available mercury” that can be immediately uptaken by plants.
Step 2. To assess the potential ability of the soil to replenish the available metal
pool over time, extraction with a highly specific Hg-mobilizing agent, 0.27 M
ammonium thiosulphate, was performed. The action of this extractant is much
stronger than any natural process, and the amount released in this step can be safely
considered as the maximum possible amount of metal available to plants. Ammo-
1
nium thiosulphate extracted, as a mean, 0.12 mg kg Hg before plant growth.
A further extraction on the residual soil showed negligible amount of Hg below the
detection limit. This extraction gives an estimate of the likely long-term bioavail-
able Hg.
Step 3. According to EBCS scheme, the extractant 0.27 M ammonium
thiosulphate used to evaluate the release of mercury in the long term has been
added to soil. In this way, a new bioavailable pool is created from which plants can