Agriculture Reference
In-Depth Information
radionuclides, decreasing the 241 Am and 244 Cm content in plants; while for 239+240 Pu, 232 Th,
and 238 U presented no effect (Whicker et al., 1999). The NH 4 + content in the soil solution had
also influence on the bioavailability of 137 Cs. Its increase led to an increase of the 137 Cs
content in the soil solution by a factor of 3-4 (Nisbet et al., 1993). It can also displace K + from
exchange sites in soils, increasing its concentration in the soil solution (Nisbet et al., 1993).
The application of NH 4 + and manure can also reduce the uptake of 137 Cs, due probably to the
release of potassium and other ions from the manure when NH 4 + is applied (Fuhrmann et al.,
2003). The addition of phosphate stimulates root growth and may also increase the caesium
uptake (Shaw et al., 1993)
The reduction of the uptake and transfer of radiostrontium to plants is carried out by
means of calcium addition to soil. This addition is usually in the form of lime, which is a
general term for inorganic compounds containing calcium. The success of this
countermeasure is more limited than that of radiocaesium, about 20% of 90 Sr (Lembrechts,
1993). The highest reductions were obtained in soils with low calcium content (Shaw et al.,
1993). Although in some occasions, this limited success is because the rates used, 1.6-15.6
ton Ca·ha -1 , were not able to modify significantly the calcium content in the soil solution
(Vidal et al., 2001; Camps et al., 2004). It has also been observed that it depended on the type
of soil. The addition of lime can be beneficial for mineral soils and possibly deleterious for
organic ones (Nisbet et al., 1993). Liming in excess can also fail to further reduce the
radiostrontium uptake (Lembrechts, 1993). The addition of lime to acidic soil can increase the
pH and modify also charges in clay (Massas et al., 2010). Therefore, the radiocaesium
bioavailability can vary because it is mainly controlled by its attachement to clay minerals in
the soil (Ohnuki, 1994), becoming to fixed to them (Zibold et al., 2009; Massas et al., 2010).
The addition of potassium fertilizers increased both the calcium and strontium content in the
soil solution in the same way. So the ratio Ca: 90 Sr remained approximately the same (Nisbet
et al., 1993b). The phosphate amendments might also be effective to suppress the
radiostrontium uptake due to the probable formation of insoluble strontium phosphate (Shaw
et al., 1993).
C ONCLUSION
The industrial production of fertilizers, based mainly of phosphate ores, is considered a
NORM activity, due to the high activity levels of naturally occurring radionuclides present in
the raw material. Their occurrence depended on the origin of the phosphate rock. Sedimentary
phosphate ores presented higher 238 U, 226 Ra, and 232 Th contents than others. During the
industrial procedure, the naturally occurring radionuclide in the ore is fractioned into different
by-products and wastes. Phosphoric acid and fertilizers are enriched in uranium and thorium;
while wastes, mainly phosphogypsum, are enriched in radium, 210 Pb, and 210 Po. As a
consequence of these enhanced radionuclide contents, they can pose a radiological hazard.
Regarding phosphogypsum, different hazards can be considered according to the waste
management. Their release into rivers increased the radium content in water and sediments
downstream of the discharging point. After the banning of these discharges, a self-cleaning
process of the affected aquatic ecosystems has been reported due mainly to the redistribution
of the contamination by tides and currents. The accumulation of phosphogypsum in stacks
Search WWH ::




Custom Search