Page 49 - PR 2014 2016 07 Nuclear Science and Technology

Nuclear Science and Technology | Progress Report 191

Availability of Po present in the methodologies adopted, giving evidence
210
phosphogypsum used in agriculture that, although the radionuclides are present in
the PG, they are not available in the leachate.
The main phosphate industries in Brazil are re-
238
sponsible for the production of 5.5 × 10 metric Availability of U and Th in the utilization
232
6
tons of a TENORM residue, phosphogypsum of phosphogypsum in agriculture
(PG) annually, which is stored in stacks. The
presence of radionuclides puts restrictions Phosphogypsum (PG), a byproduct of the phos-
on the use of PG in building materials and in phate industry, is classified as Technologically
soil amendments. The Brazilian regulatory Enhanced Naturally Occurring Radioactive
body, CNEN, ruled that PG would only be per- Material (TENORM) or as a NORM waste. It is
mitted for use in agriculture or in the cement obtained during the attack of the phosphate
228
industry if the concentration of Ra and Ra rock with sulfuric acid for the production of
226
-1
does not exceed 1 Bq · g . In Brazil, PG has phosphoric acid. PG presents in its compo-
been widely used as soil amendment, to im- sition radionuclides of the natural U and Th
prove soil fertility. To assure a safe utilization decay series: mainly 226 Ra, 228 Ra, 232 Th, 210 Pb
in agriculture, it is important to estimate the and Po. The Brazilian producers stock the PG
210
lixiviation of the radionuclides in PG. The main in dry stacks, posing risks to the surrounding
objective is to evaluate the availability of Po environment. Therefore, several studies were
210
in the utilization of PG in agriculture as soil undertaken to evaluate viable ways of its re-
amendment. An experiment was carried out, use. One possible solution to this problem is
in which columns filled with sandy and clay to reuse PG in agriculture as a soil conditioner.
Brazilian typical soils and PG were percolated The Brazilian regulatory body, CNEN, estab-
with water, to achieve a mild extraction of lished exemption limits for the use of PG in
determination of Po. The technique used for agriculture or cement industry: the Ra and
210
226
the 210 Po in the samples of soil, soil + PG and 228 Ra activity concentration should be below
PG was alpha spectrometry which is suitable 1.0 Bq · g . However, for its safe application, it
-1
for the measurement of low activity of Po in is still necessary to ensure that the radionu-
210
environmental samples, since it presents high clides present in the PG will not be available
efficiency and sensibility. The concentration to the environment. The availability of 238 U
present in the leachate was compared with the and 232 Th in samples of soils amended with
total concentration of Po in soil, in the mix- PG through percolation with water, by calcu-
210
ture soil + PG and sole PG, in order to evaluate lating the available fraction, was determined.
the availability of the activity concentrations This fraction was obtained by the ratio of the
of 210 Po. The results obtained for 210 Po in the concentration of the radionuclides in the leach-
clay soil were approximately 2.5 times higher ate by a radiochemical procedure based on a
than the sandy soil. The results obtained for publication from International Atomic Energy
the radionuclides concentration in the PG var- Agency over the total concentration in the
ied from 155 ± 11 to 346 ± 7 Bq · kg for Po. samples, by Instrumental Neutron Activation
210
-1
However, the addition of PG to the soils stud- Analysis and gamma spectrometry. The results
ied did not represent any increase in the final demonstrated that the available fraction of the
activity concentration. The results obtained radionuclides were in all cases lower than 0.2%.
for the activity concentration of 210 Po in the This indicates that the use of phosphogypsum
leachate were close to the detection limits of as agricultural input does not contribute to