2.3. Analytical methods
All soil samples were air-dried and sieved (<2 mm) before
analyses, which were performed using standardized methods
(MAPA, 1994). The particle size
characterization was determined by the Robinson’s pipette method
(USDA, 1972). The pH was measured
potentiometrically in a 1:2.5 soil:water suspension and the electrical
conductivity (EC) in saturated paste extract. The calcium carbonate
content (CaCO3) was determined by the volumetric gas
method (Barahona, 1984) and the organic
carbon by the acid oxidation method (Tyurin,
1951), both parameters after fine grinding (<0.05 mm) of the
sample.
The total concentration of contaminants (Pb, As, Zn and Cu) was analyzed
by X-ray fluorescence (XRF), using the finely ground sample, with a
NITON XL3t-980 GOLDD+ portable analyser (Niton, Billerica, USA). The
precision and accuracy of the contaminating elements was made from
measurement (6 replicates) of a certified reference material (CRM
052-050 RT-Corporation Limited, Salisbury, United Kingdom), obtaining
the following results (certified vs. measured, mean values in mg
kg-1 and standard error in brackets): Pb: 82.6
(4.0) vs 92.6 (4.8), As: 14.6 (2.9) vs 15.5 (3.9), Zn:89.0 (4.0) vs 94.4 (6.2), Cu: 44.2 (6.9) vs 48.5 (8.0). In
all cases, the measured values were within the confidence interval of
the certified value.
The concentrations of water soluble contaminants were determined in a
1:5 suspension (soil:water) according to
Sposito et al. (1982), and the bioavailable
forms were extracted with 0.05 M EDTA at pH 7.0 according to
Quevauviller et al. (1998); in both
cases, the extracted elements were measured by inductively coupled
plasma mass spectrometry (ICP-MS), with a Perkin Elmer SCIEX ELAN-5000A
spectrometer (Waltham, MA, USA). The precision of the method was
corroborated by analysis (6 replicates) of a standard reference material
(SRM 2711), obtaining the following results (certified vs.
measured, mean values in mg kg-1 and standard error in
parentheses): Pb: 1162.0 (31.0) vs 1138.1 (11.0), As: 105.0
(8.0) vs 102.4 (1.1), Zn: 350.4 (4.8) vs 350.3 (3.4), Cu:114.0 (2.0) vs 115.0 (1.6). For all target elements, the measured
values were within the confidence interval of the certified value.