dc.description.abstract | The main goal of this Project has been the radiological
and alimentary assessment of the agriculture use
of phosphogypsum (PG), a by-product of the fertilizer
industry, as Ca-amendment in reclaimed salt-marsh
soils in SW Spain. The work-plan included: PG characterization
(concentrations of radionuclide and
heavy metals, 222Rn exhalation from PG stacks), experimental
field studies attending to the effect of PG
in agriculture soils and drainage waters, and studies
on soil-to-plant transfer of radionuclides and heavy
metals (through both, field and greenhouse studies).
PG characterization (PG stacks)
FERTIBERIA, a fertilizer factory located in Huelva
(SW Spain) owns a non-active PG stack lying in the
right bank of the Tinto River. From this PG stack, of
about 1 km2 and 8-10 m deep, PG could be extracted
for agriculture uses. PG has being traditionally
used since late 1970’s as a Ca-amendment for
the reclaimed soils from the salt-marsh area of the
Guadalquivir River.
222Rn exhalation in this non-active stack has been
measured following the USEPA method 115 (adapted
for using cylindrical charcoal canisters). Experimental
studies conducted in collaboration with the Institute of
Biophysics from the University of Salzburg, demonstrated
the repeatability and accuracy of the charcoal
canister method for the commented determinations.
The accuracy was demonstrated against the exhalation
chamber method. Finally, experiments with PG
columns of different heights demonstrated that the
major contribution to Rn exhalation comes from the
0-50 cm horizon. A total of 162 measurements of
222Rn exhalation have been carried out, corresponding
to 49 sampling points distributed through three
different regions in the stack (1, compacted and dry
top surface area; 2, loose –tilled- dry top surface and
3, sides). Three charcoal canisters were placed at
each sampling point, covering an area of ~1 m2
. 222Rn exhalation values ( 510 ± 470, 250 ± 270 y
360 ± 170 Bq h-1 m-2 for regions 1,2, y 3, respectively)
show high dispersion, but their mean values
were under the USEPA limit of 2664 Bq h-1 m-2.
Charcoal canister placed over big cracks and freshly
removed surfaces did not reveal any significant
change in 222Rn exhalation rates. The study of the
normalized exhalation rates at different spatial scales
(0.1, 0.75, 60 and 200 m) shows almost uniform
standard deviations of ~0.5. The major contribution
to this variability in the scale 0.1-1.0 m has to be attributable
to the variability, at the same spatial scale,
of the micro and meso-structure of pores and cracks.
For larger spatial scales, changes in 226Ra concentration
and percentage of humidity, contribute to the
variability in the 222Rn exhalation rates.
PG samples were taken at surface level (0-20 cm) in
20 sampling points, and at three different depths
(0-30 cm, 30-60 cm and 60-90 cm) in another 20
sampling points. 226Ra was measured by liquid scintillation,
providing activity concentrations of 720 ±
260 Bq kg-1 and 690 ± 180 Bq kg-1(dry weight), for
regions 1 and 2, respectively. These values are over
the threshold level of 370 Bq kg-1 established by the
USEPA to allow the agriculture use of PG. 210Po activity
concentrations (measured by alpha spectrometry)
of 660 ± 160 Bq kg-1 y 575 ± 95 Bq kg-1 were
determined for regions 1 and 2, respectively. U-isotopes
were measured by alpha spectrometry, obtaining
238U activity concentrations of 170 ± 110
Bq kg-1 for region 1, and 160 ± 80 Bq kg-1 for region
2. The isotopic ratio 238U / 234U was 0.97 ±
0.03, as expected from secular equilibrium. 210Pb
concentrations were measured by gamma spectrometry
in few samples, and their values were closed to
the corresponding values for 210Po activity concentrations.
The above activity concentration for 226Ra and
its daughters (and in less extend U-isotopes), are
more than one order of magnitude higher that those
found in agriculture soils from the area of Las
Marismas of Lebrija.
Multi-elemental analysis of the PG samples was carried
out by ICP-MS technique following USEPA 200.8
method. PG samples, after oven-dried, disaggregated
and sieved, were acid digested with supra-pure
nitric acid in a microwave-oven (pseudo-total recovery).
Cd concentrations were 2.0 g/g, one order of
magnitude higher than those found in the agriculture
soils from Lebrija. Thus, the Cd and the radionuclides
from the 238U series will be the elements of
major concern in the agriculture use of PG from
FERTIBERIA stack. For the rest of the elements considered
in the USEPA 200.8 method, concentrations
were not too much higher than the corresponding
ones for soils (Hg was not measured). Thus, taking
into account the important dilution of PG when is
applied as soil amendment (homogenised in the
0-30 cm soil horizon), the input of these elements
has to be considered as quantitatively negligible.
Nevertheless, all these elements has been considered
in the present study, since the scientific literature
reports some effects of the PG in the mobility
and bioavailability of some metals in soils. | es |