| The percentage of 235U in the natural uranium is 0.714 percent. Depleted uranium (DU) is the by-product of the process of extracting 235U from the natural uranium. After extraction , 235U account for only 0.2~0.3 percent. The radioactive intensity of DU is sixty percent of natural uranium. DU emitsα,βparticals, sometimes accompanied by ? rays, having radiotoxicity and heavy metal toxicity. Because of its high density, high toughness and low cost, DU has been widely used in the manufacture of military armor piercing projectiles, incendiary bombs and tank armors. In 1991 Gulf war, DU weapons were used by US army for the first time, up to 94,000 sets. Later, in the 1994~1995 war in Bosnia-Herzegovina and Kosovo War in 1999, nearly 50,000 depleted uranium weapons were dropped. Soldiers who had participated in combat missions suffered chronic fatigue, muscle pain, insomnia, dizziness, hair loss, trance and other strange diseases, known as the "Gulf War Syndrome" and the "Balkan Syndrome", which was with relation to the using of DU weapons according to many reports.DU is long-lived nuclide, the half-life of which is 4.49×109 years. Although external exposure generated by depleted uranium weapons on the human body is weak, once the aerosol produced in burning is inhaled, it will accumulate in the body, resulting in internal exposure and heavy metal toxicity, which will bring long-term harm. After a long period of settlement, aerosols comed into the surface of soil or water, and about 70% to 80% of the depleted uranium drilled into the ground. The DU in the soil may migrate into the groundwater , transfer to the plants and animals, which is harm to human health through the food chain. So it is necessary to study the migration rule of DU in the soil in order to adopt appropriate repair measures.This study has further improved the laboratory model of DU migrating in the soils. The method of detecting the concentration of uranium and the ratio of 235U/238U in soils by ICP-MS has been established .And the influence on the migration of DU in soil by humic acid (HA) and acid rain has been studied with the laboratory model.Firstly, the improved laboratory model of DU migrating in the soils.The research of elements migration mainly include the laboratory simulation and the field on-site experiments. The laboratory simulation plays an important role, for that it can control different experimental conditions to study the influence on the migration of elements by various environmental factors, which can't be studied in field experiment .In this study, the laboratory model that had been built was further improved. Circular columns used in the original model was made of ordinary glass,φ30mm×400mm.Peristaltic pump provide power to make the liquid in the storage bottle delivering from the bottom of the column to the top, and the eluent was recycled during the experiment .The depth of DU migrating in soils for 20 years in stimulation by the device was no more than 8cm, while DU migrated at least 25 cm in the surrounding soil of a DU simulation experiment module in China. The disparity between the experimental results and the field measurements was large, so the simulative circulation device has been improved.After Improved, the simulation device is made of plexiglass ,strong and durable; In order to simulate the natural rainfall leaching the soil more accuratly, cross-sectional area leached should be maximized, However, it will increase the quantity of the needed soil and the workload of filling column, so in comprehensive consideration, the diameter of the plexiglass column has been expanded to 50mm . The length of the plexiglass column was 350mm, which has been divided into seven sections and each section is 50mm long, which is benefit to layered sampling and cleaning. The liquid flow rate was regulated by adjusting the peristaltic pump speed first, and then adjusted by flowmeter for more accurate regulation; Leachate was no more used in recycle in experiment.There are two reasons, one was that the leachate pH value and element content was different from the drizzle liquid ,which will influence the experimental results, the other was that it facilitated the monitoring of the leachate pH during the experiment, the uranium content and the change of 235U/238U in order to determine when the DU migrates from soil into the leachate. Secondly, to establish the method of detecting the concentration of uranium and the ratio of 235U/238U in soils by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) .For the complex matrix of soil samples and trace amount category of uranium in soil, it is not easy to determine the concentration of uranium and the ratio of 235U/238U accurately. Traditional analysis methods, includingα,γspectrum measurements, neutron activation analysis, liquid scintillation and solid track method, which have complicated procedures and take a long counting time, have no advantage if the number of samples is large, and are vulnerable to interference from natural radioactive elements in the environment, so the measurement sensitivity is not high. In contrast, ICP-MS techniques are able to determine the concentration and the ratio of isotope simultaneously, having the advantages of measurement speed, high sensitivity, low detection limit, less interference. Therefore, we measured the concentration of uranium and the ratio of 235U/238U in soils by ICP-MS.Ash-Acidolysis Method for pre-treatment of soil. 100mg soil was placed in the box resistance furnace ashing at 450℃for 2h. After it cooled to the room temperature, the soil was taken out and transferred to the teflon tank , adding 4ml HNO3, 2mlHF. Acidolysis was performed on electric board at 120℃, the teflon tank was removed until the remanent acid was about 0.5ml. After cooling, digestion liquid was diluted with Milli-Q ultrapure water to 50.0g, then filtrated and measured by the ICP-MS. Using online addition of internal standard method for sample collection, 1μg/ml of Pt internal standard solution was introduced by the internal standard tube, so that the ratio of the amount of collected sample and internal standard solution remained relatively constant, in order to reduce system errors caused by the instrument volatility and eliminate matrix effect. The acquisition time of 235U, 238U and 195Pt was 1.5s, 0.3s and 0.1s, and the acquisition repeated 5 times.The linear correlation coefficient of the standard curve drawn in the 0 ~ 320μg/L range is 0.9997(r=0.9997), in the 0~80μg / L range is 1.0000(r = 1.0000), so standard solution has good detection accuracy within this range. For this method, the detection limit was 0.0063μg / L, and the precision was 0.71 percent. For the measurement of three national standard soil, the measured values are within the scope of the standard reference values, indicating that the accuracy of the method is high.Thirdly, impact of the humic acid (HA) and the acid rain on the migration of DU in the soil. The migration of depleted uranium in the soil will be influenced by the chemical nature of the DU, rainfall,soil structure and other factors.HA is a class of widely distributed in soil, water and sediments of the polymer. It has complex structures and large specific surface area, with phenolic groups, carboxyl, hydroxyl, methoxy, carbonyl and other active groups which has strong complexation and adsorption capacity, so it could adsorb and complex uranium in the environment. The migration of the complexes formed depends on the interactions between complexes and the soils. Therefore, the environmental behavior of HA plays an important role in the migration of DU in soils. Acid rain has now become a global environmental problem, which has been taken seriously. Researches show that acid rain will bring down the pH value of soils and the base saturation level. decreased and aggravate the leaching intensity of metal cations, which is closely related with soil properties. Acid rain may enhance the leaching of heavy metal ions in contaminated soils by heavy metals, therefore, it is also bound to influence the migration of DU in soils.The influence of various amount of HA and pH values of acid rain on the migration of DU has been studied with the laboratory model. The results showed that: The migration depth of DU of the controled group (simulated rainfall, the pH value of which is 5.6, leached the soil of Beijing area that no HA was added in) has reached to 9~11cm. DU has migrated deeper in the experimental groups that 2% HA, 5% HA, 10% HA was added in, corresponding to the migration depth of 21~23cm , 15~17cm ,11~13cm respectively; Leached by the simulated acid rain whose pH was 4.0 and 3.0, DU migrated to 29~31cm and more than 35cm respectively.However,the combined action of humic acid and acid rain caused the migration of DU to 5~7cm only.After leached by acid rain,the pH value of soils changed unsignificantly.In this experiment, adding HA has promoted the migration of depleted uranium. And with the increasing amount of HA, the promoting effect has been weakened;Acid rain leaching has significantly promoted the migration of DU, the greater the acidity, the stronger the promoting effect has been; with acid rain of pH = 4.0 leaching the soil column containing five percent of HA, the migration of DU has been inhibited; In a short period of time, acid rain has not changed the pH value of the alkaline soil significantly; The results of the laboratory model simulating the migration of DU in the soil is basically consistent with results from the field tests, indicating that the model has been successfully designed in the main. |
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