Comparative studies on the solubility of uranium and phosphorus in phosphate-fertilisers and their uranium transfer to

Abstract

Phosphate fertilisers are commonly used in agricultural lands to supply phosphorus nutrients for enhancing quantity and quality of crop products around the world. According to the literature, phosphorus-fertilisers may contain considerable amounts of uranium varying from less than 10 to more than 360 mg kg^-1, and uranium can be loaded onto soils from 7 to 23 g ha^-1 yr^-1 with applying of 22 kg P ha^-1 yr^-1 (50 kg P₂O₅ ha^-1) from various phosphorus-containing fertilisers. With increasing uranium content and concentration in soil, uranium concentration may increase in plant tissues, animal bodies, and the human body in the long run; it can cause damage to health of human and animals mainly based on its chemical toxicities as a heavy element. Total amount of uranium, uranium-to-phosphorus ratio, and uranium solubility and transferability vary in different P-containing fertilisers. The objectives of the present study were three points: comparison of uranium and phosphorus solubility in chemical extractants commonly used for P-fertilisers, investigation of the effect of U-containing P-fertilisers on the uranium concentration and uptake of different crops (a monocotyledon: maize, a dicotyledon: sunflower, and a root crop: carrot), and investigation of relationships between chemical extractability and plant uptake of uranium coming from different P-containing fertilisers. To this end, the following experiments were carried out:
a) Determination of U and P solubility of 37 P-containing fertilisers in 5 standard chemical extractants for P solubility defined in the EU Fertiliser Ordinances (W, NAC, AAC, 2% FA, and 2% CA) and in aqua regia for socalled total amounts.
b) Neubauer pot experiments with maize and sunflower.
c) Kick-Brauckmann pot experiments with carrot. The most important findings of the research work presented here on chemical extractions were:
• Relative uranium solubility (i.e. referred to AR) significantly varied in the different extractants as well as in the various types of P-fertilisers, the same results were found for phosphorus. The highest and lowest mean relative uranium solubility were equal to 60.6 % and 10.2 % of so-called total uranium content (digested by aqua regia) and were found in 2 % citric acid and water, respectively.
• The lowest and highest mean relative uranium solubility over all extractants were found in phosphate rocks and compound fertilisers which were equal to 14.7 % and 67.8 %, respectively. In contrast, the highest mean relative solubility of phosphorus was found in straight phosphorus-fertilisers with a value of about 100 %.
• The results of the present research work showed significant differences between uranium and phosphorus solubility in various extractants when looking separately at all four types of phosphorus-containing fertilisers. Uranium showed somewhat lower relative solubility than phosphorus in all extractants except in AAC. In the first year, two pilot experiments were carried out to test the suitability of the Neubauer method for the research objective, suitability of maize and sunflower crops as test plants in Neubauer method, suitability of different carrot varieties, and behaviour of different substrates. The pilot experiments revealed that:
- Maize and sunflower seedlings could grow under special condition of the method in Neubauer pots in a high plant density for about 3 weeks. The results of the pilot Neubauer pot experiments also showed significant effects of fertiliser-derived uranium on uranium concentration and uptake while the dry matter yield was independent from phosphorus availability in substrate in a short term growing period of 22 days.
- The results of pilot Kick-Braukmann pots with carrot showed significant effects of P-fertilisers and substrate on dry matter yield, and U uptake by carrot plants, while the effect of variety on roots’ dry matter yield and total U uptake was not significant. The effect of fertiliser-derived U on carrot uranium concentration and uptake in soil substrate was not significant. In the second year, a number of phosphorus sources varying in composition (RP, straight P-fertiliser, mineral compound P, organo-mineral compound P), P-solubility, uranium content, U/P-ratio, and uranium solubility were tested in a Neubauer and Kick-Braukmann pot experiments, yielding the following results:
- The effect of different phosphorus sources (fertiliser-derived uranium) on uranium concentration in roots of maize and sunflower seedlings on sand and mixed soil/sand substrate was strongly significant (P<0.001). In contrast to uranium concentration in roots, uranium concentration in shoots of maize seedlings was not affected significantly by different phosphorus sources. In sunflower seedlings on sand substrate, only shoots uranium concentration in the organo-mineral fertiliser, NPK+Mg (MD19), was significantly higher than that of other treatments.
- The effects of different phosphorus sources on uranium concentration of carrot shoots were significant in both substrates, i.e U concentration of carrot shoots in the organo-mineral fertiliser, NPK+Mg (the MD19) and the NP-compound fertiliser (the MD28) were significantly higher than in other treatments. The effects of different phosphorus sources on uranium concentration in roots of carrot were only significant in sand substrate varying from 12 to 32 ng g-1 in the Control and the MD35 treatment (a PARP fertiliser), respectively.
- Uranium concentration in roots of maize and sunflower seedlings was about 10-100 times (varying in different treatments) higher than uranium concentration in their shoots, but in carrot plants these concentrations were completely similar and even on the contrary, uranium concentration in roots of carrot plants was lower than that in their shoots in the most of treatments. These results can show lower risk of fertiliser-derived uranium for entering to the food chain via carrot roots.
- Uranium uptake by roots and total uranium uptake were significantly influenced by fertilizer-derived uranium in maize seedlings at the 0.05 level and in sunflower seedlings at the 0.001 level.
- In maize and sunflower plants total uranium uptake from all P-fertilisers (except PK-fertiliser, MD38, in that uranium uptake on both substrates was equal) on sand substrate was considerably higher than on mixed soil/sand. With regard to decreasing the concentration of uranyl ions and complexes in soil solution due to precipitation, adsorption, and cation exchange reactions in presence of soil colloids, the attenuating effect of soil substrate on uranium transferring to plants was expected. In contrast to P-fertiliser treatments, in the KH₂PO₄-Control and Blank total and root uranium uptake in mixed soil/sand was higher than in sand substrate, caused by a low initial amount of uranium in the sand substrate.
-The highest total uranium uptake by maize and sunflower seedlings (2.33 and 4.12 μg pot^-1 for maize and 2.04 and 5.64 μg pot^-1 for sunflower in mixed soil/sand and sand substrate, respectively) were observed in straight phosphorusfertilizers (superphosphates).
-Uranium uptake by carrot roots and shoots was affected significantly by phosphorus sources. The highest mean uptake by carrot plants (shoots, roots, and total amount) was found in the OMF, NPK+Mg (MD19), and significantly different from those of other treatments, this could imply that some organic component in the MD19 fertilisers can be easily taken up by hair roots and transferred to carrot roots.
-Total uranium uptake by carrot plants on sand substrate was lower than that on mixed soil/sand substrate in all phosphorus-fertilisers except the organo-mineral fertiliser which contained highest total and water soluble amounts of uranium. The results of investigation of relationships between chemical extractability and plant uptake of uranium coming from different P-containing fertilisers were:
-The best prediction of uranium and phosphorus bioavailability for carrot plants from various P-fertilisers tested in this research was found by water extractant. The relations between water extractable form of uranium and phosphorus and their uptake by carrot plants were significant on both substrates with correlation coefficient values of 0.79 and 0.85 for U, and 0.83 and 0.90 for P in mixed soil/sand and sand substrate, respectively.
-As for carrot plants, for sunflower seedlings also the best extraction for assessing bioavailability of uranium and phosphorus was water extraction. Correlation coefficients between sunflower uranium uptake and water soluble proportion of uranium in both substrates and between sunflower phosphorus uptake and its water extractable form in sand substrate were the highest among all extractants, and statistically significant. The relationship between phosphorus uptake by sunflower and its water solubility proportion on mixed soil/sand was strongly significant (P<0.01, r=0.96).
-The results revealed that a common extractant can not be introduced for assessing phosphorus and uranium bioavailability in various type of P-Fertiliser for maize seedlings. Anyhow, the best assessing extractants for uranium and phosphorus bioavailability for maize were aqua regia and neutral ammonium citrate (NAC), respectively. Based on the results of this study it was concluded that uranium and phosphorus behave differently in various chemical extractants, and solubility of uranium and phosphorus of P-containing fertilisers depend on both the extractant and the type of P-containing fertilisers. Also, it was concluded that fertiliser-derived uranium can significantly affect plant uranium uptake in maize and sunflower seedlings with regard to total uranium concentration and its solubility. On the other hand, total uranium uptake by all tested plants was considerably less than water soluble portion of uranium in straight P-fertilisers and the organo-mineral fertiliser (the MD19) implying a potential risk of fertiliser-derived uranium transferring to the ground water. Transferability of fertiliser-derived uranium to carrot roots and shoots was not significant except in the case of an organo-mineral fertiliser (NPK+Mg, the MD19). These results suggest that the risk of uranium transferring from mineral phosphorusfertiliser to the food chain via root crops is rather low; however, further studies should be carried out to confirm this.