Agriculture Reference
In-Depth Information
and dissolution of the melt in dilute nitric acid. Analysis was conducted at SARM (CRPG,
Nancy). Ignition loss was determined by heating the samples at 980
o
C during 3 hours
.
Vegetation samples and Reference soil samples (IAEA-405) were accurately weighed to
approximately 0.25 g directly in microwave PTFE vessels and subjected to 1 mL H
2
O
2
(30%)
and 9 mL concentrated HNO
3
(65%). The digestion program itself consisted of a 10 min
gradual increase in temperature to 200 °C, a 15 min
step at 200
ⓦ
C (1,000 W; 106 Pa) and then
a stage of ventilation cooling. The digests were filtered through a 0.45 µm and analyzed using
an Atomic Absorption Spectroscopy (AAS) ―Analytical Gena Zeenit 700‖ to determine the
content of Pb, Zn, Cu, and Cd in roots and leaves plants.
The sequential extractions were carried out progressively on an initial weight of 5 g in
polypropylene centrifuge tubes of 50 mL. Five fractions were separated in 5 steps as follows:
(1) exchangeable with 40 mL of 1 M Mg(NO
3
)
2
; (2) acid soluble extracted by 40 mL of 1 M
CH
3
COONa adjusted to pH 5 with CH
3
COOH; (3) reducible extracted by 20 mL Na-citrate
(78,4 g L
-1
) and 20 mL NaHCO
3
(9.82 g L
-1
) then adding 1 g of Na
2
S
2
O
4
(80 °C for 4 h); (4)
oxidizable extracted by adding 8 mL HNO
3
(0.02M) and 20 mL of H
2
O
2
(35%) in small
aliquots then 12 mL of 3.2 M CH
3
COONH
4
in nitric acid (85 °C for 2 h); (5) residual
extracted by 40 mL HNO
3
(65%). Each extraction step was followed by centrifugation the
mixtures at 5,000 rpm for 30 minutes and the solutions were separated from the solid residue.
Liquors were first filtered on with a membrane filter of ester cellulose of 0.45 µm porosity.
All the solutions were stored in polyethylene vials at 4 °C until analysis and then
analyzed by Atomic Absorption Spectroscopy (AAS) to determine the content of Pb, Zn, Cu
and Cd in each extracted fraction. Reagent blanks were also analyzed in order to monitor
analytical accuracy and precision.
Mineralogical Analysis
Samples were crushed and homogenized before analysis by X-ray diffraction. About 500
mg of homogenized and ground samples were deposited as sub compacted powders within a
thin layer of 1mm thickness on a plexiglass disc of 2 cm diameter. Diffractograms were
collected and recorded by using a D8-Bruker diffractometer (cobalt radiation source, λ =
1.7889 Å) operating with reflection mode and a 3 s of acquisition time.
Electron microscopy observations were performed with an S-2500 Hitachi SEM (Scan
Electron Microscopy) equipped with a Kevex 4850-S EDX (energy dispersive X-ray
spectrometer). Trace elements carriers were identified from elemental analysis of individual
particles. Stoichiometric ratios were first calculated from atomic percentages given by EDX
spectra, and then compared with known mineralogical compositions. For SEM imaging and
microanalysis, the powder sample was re-suspended in ethanol under ultra-sonication, and a
drop of suspension was evaporated on a carbon-coated copper grid (EuroMEDEX, Mesh200).
Samples for SEM-EDX examination were sprinkled onto 2 cm
2
plates and carbon coated.
Backscattered Electron Imaging (BEI) was used to identify the particles of interest. In that
mode, brightness is related to the average atomic number of materials, and the mineral
particles appear as bright spots within the matrix of sediments. The relative abundance of a
given heavy metal carrier can then be assessed by conducting systematic microanalysis of
bright spots. It should nevertheless be noted that this procedure largely overlooks mineral
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