Evaluation of soil extraction methods for uranium

Abstract

Normally, U is present in most soils at very low concentrations. According to the United Nations, the normal concentration of U in soils is within the range of 0.3 - 11.7 mg kg^-1. The U behavior in soils is very complicated, this because U can be associated with numerous compounds and phases and many factors affect its behavior in soil. Few investigations have been published assessing the U bioavailability and the factors affecting the U bioavailability in soils. Better understanding of different factors governing the U behavior in soils is very useful because it leads to the best possible extraction procedures and the best remediation treatments in U-affected soils. The main goals of the present work were to evaluate the extractability of U from soil samples assessed by common heavy metal extractants, and compare the differences in the extractability potential between them. Also to find out which of these extractants best predict the plant uptake and concentration of U. And finally to study some of the most important factors affecting the U bioavailability for plants in a wide range of U contaminated soils. This research work was carried out at the Institute of Plant Nutrition and Soil Science (PB), Federal Agricultural Research Centre (FAL), Braunschweig, Germany. Four soil sample groups were collected covering a wide range of U concentration in soils. The first group of these samples was derived from a previously conducted greenhouse experiment by Rivas (2005). The second soil samples group was derived from a previously conducted incubation experiment under anaerobic conditions carried out by Lamas et al. (2005b). The third soil samples group was collected from the mining area of Schneeberg at the state of Saxony, Germany. The last soil samples group was collected from long term P fertilization experiments located at two sites in Braunschweig and Freising, Germany. Bioavailable U was determined using four extraction methods, which varied with respect to their extraction potential. Extractants used were: acid ammonium acetate-EDTA (AAAc-EDTA) at pH 4.65, 1N ammonium acetate (NH₄Ac) at pH 7.0, diethylene triamine pentaacetic acid (DTPA) at pH 7.3 and soil solution obtained by centrifugation (7,000 rpm for 20 min). Special emphasis was paid to the extraction of soil solution via centrifugation technique. The most important findings of the research work presented here were:
1- The results of the present research work demonstrated that the U solubility was found to be a function of the extraction solution. The AAAc-EDTA, NHsub>4Ac, DTPA and soil solution extractants had different capabilities to extract U from soil. The AAAc-EDTA extractant was usually the most powerful extractant in most soil samples tested here with few exceptions, followed by NHsub>4Ac and then DTPA while the concentration of soluble U in the soil solution was very low. Generally, the AAAc-EDTA extractant was definitely superior to the other extractants; this may be attributed to the presence of chelating agent (EDTA) in this extractant.
2- The results revealed that the maize has a lower ability to release U into the soil in all U levels after the growth than sunflower and faba bean and this observation is well in accordance with the literature. This may be attributed to fact that dicot roots (like sunflower and faba bean) release more U in the soil from its compounds than monocot roots (like maize).
3- The results of the present study implied that the prediction of U bioavailability for plant via the extraction of U from soil is a plant dependent. The AAAc-EDTA was the best predictor for maize U-uptake followed by NHsub>4Ac. The DTPA and AAAc-EDTA extractants were the best indicators for the U-uptake by sunflower plants. None of the tested extractants was able to predict the U-uptake by faba bean plants. On the other hand, none of the four extractions were able to predict the plant U concentration of any of the three crops tested here.
4- The total U concentration in the soil is one of the most important factors affecting the extractability of U assessed by the four extractants used in this study. Highly significant and significant correlation coefficients r were found between the total U in soil and the extractable U with the four extractants from soil. This phenomenon was true in most of the soil samples tested here with few exceptions.
5- Soil pH has various effects on the U extractability, a diverse effect of soil pH on the extractable U in soil was found in greenhouse experiment soil samples. The intensity of the effect of soil pH is dependent on the plant cultivated in the soil. In both soil samples of the incubation and long-term experiments, the effect of soil pH on the extractability of U in the soil was negligible. Positive correlations were found between soil pH and the U extracted by any of the extractants in all Saxony samples.
6- Like the effect of soil pH, the results revealed that the presence of P compounds in soil had an inconsistent effect on mobility of U in soil. The total P concentration in soil had a diverse effect on the extractability of U in greenhouse soil samples. The soluble P in soil solution of maize soil samples also had the same trend. Contrary to these findings, the effect of soil P on the extractability of U in Saxony soil samples was positive. The extractable U wasn’t affected significantly by soil total P in most samples of long-term trails soil.
7- The results also gave important information about the effect of soil organic matter on the extractability of U in the soil. Total organic carbon (OC) in the soil of long-term trials correlated negatively, but none significantly with AAAc-EDTA and DTPA extractants. While it correlated positively, none correlated significantly with NHsub>4Ac extractant. This means that part of the U in the soil may complex with organic matter in soil, and NH4Ac could extract some of U found in organic pool, while both AAAc-EDTA and DTPA couldn’t extract this pool. The results also demonstrated that the addition of different types of organic materials (i.e., cereal straw and milled alfalfa plants) to the U contaminated soil affected vary on the availability of U.
8- The results reported here implied that the effect of storage time (time since the contamination with U) is one of the most important factors affecting the availability of U in soil. The former U extractant was the more available than the current U extractant in the greenhouse experiment samples. It could be concluded that the more recently applied U was more available than the older U applied in the greenhouse experiment samples. The results also revealed that the effect of storage time on the U extractability is plant type dependent. The investigation of the effect of storage time in the incubation experiment was the opposite of that of the greenhouse experiment, and changed depending on the soil treatments. Due to these results we concluded that the effect of storage time on the U extractability for plants in the soil is not clear and better understanding of this factor is needed on the U availability.
9- The other factors studied in this work had negligible effects on the extractable U. The N and S fertilization rates had weak correlations with the extractable U from soil with the four extractants in all greenhouse soil samples. Similar results were found in the long-term trial samples. The total concentration of S in soil didn’t correlate with the extractability of U in all samples of long-term soil trials.
10- The presence of P compounds in the soil had an effect on the other heavy metals in the soil. The results of the long-term trials indicated that the total soil P had highly significant correlations with the total U, Cd and Ni in soil. This may be due to the addition of P fertilizers, which may be the source of these heavy metals in the soil. A similar phenomenon was found in Saxony soils, the results revealed that the U, Cd and P elements are highly correlated with each other in the soil.
11- The total content of S in the soil was found to be one of the important factors affecting the heavy metals concentration rather than U in the soil, because it correlated well with the total U, Cd, Pb, Ni in the soil. This means that all of the five elements are intrinsically present in the soil matrix.
12- Soil organic matter (expressed as OC in long-term samples) also impacted on the other heavy metals in the soil rather than U like Pb and Ni. The Pb was found to be complexed (positively correlated with OC) with organic matter, while Ni was not complexed (negatively correlated with OC) with organic matter in soils. On the other hand, the OC didn’t correlate with total Cd in the soil. Based on the results of the present work, it is therefore proposed that future investigations are needed for better comprehension of the behavior of U in soils with a wide range of U contamination and different soil characteristics. These investigations are also needed for better understanding of the factors affecting the extractability of U, which reflects the U bioavailability for plant in soils. A careful examination in the future of the detailed results of all investigations in the field of U should be useful in devising new soil extractants that may be of greater universal use than the extractants used in these investigations. With the limited results obtained from this research, it wasn’t possible to discover the ideal extractant for all soils with many different characteristics.