Page 287 - Plant-Based Remediation Processes

13 Phytoremediation Towards the Future: Focus on Bioavailable Contaminants 281

13.4 A Case Study of Mercury-Contaminated Soil

Experiments were conducted to determine the possible utilization of EBCS in
mercury-contaminated soil. The soil derived from an industrial site located in
northern Italy. Soil samples were air dried and ground to pass through a 2-mm
sieve before laboratory analysis. All analytical determinations were carried out
according to the methods of soil analysis (Sparks 1998). Soil was characterized by:
1
pH 7.1, organic matter 1.1 %, and cation exchange capacity 17.3 cmol(+) kg . The
texture was sand 58.4 %, clay 22.5 %, and silt 19.1 %. Mercury was the only
1
contaminant. The total Hg concentration was 26.7 mg kg .
In this work, the scheme of the EBCS approach consisted in the following steps:
1. Evaluation of the original Hg bioavailable soil fraction by the use of the
sequential extraction with H 2 O and NH 4 Cl (Milla ´n et al. 2006).
2. Evaluation of the long-term potential Hg release from soil surfaces with time.
This step was performed by extraction with a specific solubilizing agent: ammo-
nium thiosulphate (NH 4 ) 2 S 2 O 3 (Moreno et al. 2004, 2005; Pedron et al. 2011).
3. Selection of plant species and evaluation of removable mercury. Experiments
were carried out at mesocosm scale, selecting the most efficient species.
4. Assessment of the existence of a residual bioavailable fraction in soil by both
chemical extractions and further plant growing cycles. When the metal concen-
tration in plants is negligible and no available fraction can be further extracted
from soil by (NH 4 ) 2 S 2 O 3 , the residual concentration of the metal in soil can be
considered not bioavailable and can be safely left in soil.

13.4.1 Experimental Procedure

13.4.1.1 Soil Sequential Extraction

The mercury available fractions were determined by a two-step sequential extrac-
tion procedure (Milla ´n et al. 2006) with H 2 O and NH 4 Cl. In the first step, 0.5 g of
soil was treated with 25 mL of deionized water at pH 7.0 for 1 h at room temperature
and the soil residue from water extraction was treated with 25 mL of ammonium
chloride 1 M at pH 7.0 for 1 h at room temperature. Mercury concentration was
determined in the surfactants from both extractions. The total available fraction was
calculated as the sum of the water-soluble fraction and the exchangeable fraction.
The long-term potential release from the soil solid phase was determined by 0.27 M
ammonium thiosulphate extraction at pH 5.0 with a ratio soil/extractant 1:20.

13.4.1.2 Mesocosm Experiments

The trials were carried out at mesocosm scale in a greenhouse where the tempera-
ture was kept between 18 C and 26 C. Mesocosms were polypropylene containers