Abstract:
Apparatus and method are provided for the treatment of uranium-contaminated soil by using comprehensive joint technology. The apparatus include the pumping system, the electrokinetic remediation system, elution system, remediation-separation system and recharge system. The remediation technologies (i.e. chemical, photolysis and electrokinetic) are used to remedy the uranium-contaminated soil. First, extract uranium from the contaminated areas and make the ionized uranium extract from the soil phase to the solution phase. Then, use the electrokinetic remediation technology to drive uranium enrichment electromigrate to near the anode. Finally, return the repaired-soil and groundwater back to anode area and recharge well, respectively. This comprehensive joint apparatus can reduce the uranium volume in the contaminated soil or water, and recycle the obtained uranium, which are cleaning processes and have no secondary pollution.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to Chinese Patent Application No. CN 201410168041.1 filed on Apr. 24, 2014, the disclosure of which is hereby incorporated by reference in its entirety. 
       BACKGROUND 
       [0002]    The present disclosure relates to uranium contamination treatment, and specifically it is a comprehensive joint technology-based device and method for the treatment of uranium-contaminated soil. 
         [0003]    With China&#39;s increasingly rapid exploration and mining of uranium, there emerge numerable uranium tailings and waste ores. Currently, the dedicated storage facility of uranium tailings is the uranium tailings impoundment. According to incomplete statistics, the total amount of uranium tailings worldwide has reached 20 billion tons. Since 1940s, uranium and other radionuclides from a large quantity of in-service or retired uranium tailings have went into the ground water, and flowed and spread with the groundwater, causing serious harm to the ground water and the ecological environment. Uranium is primarily a a-ray source (uranium-235 is 4.679 MeV, uranium-238 is 4.270 MeV), and its half-life period is up to a million years (the half-life of uranium-235 is 704 million years, and the half-life of uranium-238 is 4.46 billion years), with an extremely slow convection velocity. Uranium in the soil will be accumulated in the human body due to the bioaccumulation action, while its chemical behavior and physiological toxicity are similar with lead, which means it will cause chemical toxicity to human and animal kidney. When the volume of uranium in human body is over 3 mg/kg, it will do harm to human body or even cause cancer. 
         [0004]    The soil contamination caused by uranium is mainly of three kinds The first kind of contamination is caused by the test and use of depleted uranium bombs. For example, the UK and the US once conducted the depleted uranium bomb tests, respectively leading to 45t and 70t depleted uranium substances left in the environment. The second kind of contamination is caused by the production and test of nuclear weapons. The contaminated soil caused by the nuclear weapon production of US Department of Energy is estimated to be 3.0×10 7 -8.0×10 7  m 3 , and the contaminated water is of 1.8×10 9 -4.7×10 9  m 3 . The third kind of contamination is caused by waste water, waste gas, waste residue, etc. discharged by nuclear power plants. On Mar. 11, 2011, an explosion occurred in Fukushima Daiichi, which used uranium-235 as the fuel for nuclear reactor, and the leaked radioactive material—uranium received much concern all over the world. The fourth kind of contamination is caused by waste residue of uranium tailings. The volume of uranium tailings waste is estimated to reach 9.38×10 8  m 3  due to the uranium mining. China&#39;s uranium mines are located in more than 30 counties of 15 provinces and cities nationwide, ⅔ or more of uranium mines are located in mountainous, humid and rainy regions, while nearly ⅓ are located in hills and arid regions. Over the years, the total amount of waste ores mined from uranium mines are about 28 million tons, covering an area of 250 hm 3 . And uranium tailings charged by waste water treatment plants are about 30 million tons, which is covering an area of 375 hm 3  if calculated as its average stacking height of 4 m. 
         [0005]    Uranium exists in the soil in an adsorbed state (in soil particles and pore water), the compound state, the sediment state and the exchangeable state, while these sates will greatly affect the uranium migration. Due to the huge nuclear pollution hazards, the depollution of nuclear contamination in a large scale is a worldwide problem. All walks of life are looking for ways to clear nuclear contamination, and they have achieved some results. The depollution methods adopted by most countries include: physical methods, chemical methods, electrochemical methods, physical-chemical methods, microbial clearance and soil nuclear contamination removal method, but these methods are usually of a high cost and easy to cause a secondary pollution to the environment, failing to solve the problem of radionuclides removal fundamentally. Although the forest restoration method can remedy the contaminated soil in a large scale, and can be used for the development of industrial raw material forest and firewood forest, but its required radionuclide concentration can only be at a not high level, greatly restricting its application. Foreign scholars mainly adopt chemical method for the treatment of uranium-contaminated soil, for example, Caroline et. al (1998) uses sodium bicarbonate for the extraction of uranium in soil solution, and then adopts the ion exchange technology to remove the uranium in the soil solution. Kantar et al. (2006) points out that the citric acid elution technique can be used to improve the clearance efficiency of uranium in soil under the weak acid condition. And Crean et. al (2013) uses multi-level chemical extraction techniques for the remediation of depleted uranium particles in contaminated soil. Currently, in China, researches on the uranium-contaminated soil are mainly focused on bioremediation, for example, the patent CN200810030860.4 discloses a method of using American phyotalacca acinosa seedling for the treatment of the soil contaminated by uranium tailings; patent CN201210056468.3 discloses a method of using water spinach for the treatment of soil contaminated by high-concentration uranium, and the patent CN201210155561.X discloses a method of using fertilizer to promote the treatment of soil contaminated by uranium or cadmium with broad beans. 
       SUMMARY 
       [0006]    Considering the current technology deficiency, in this paper a comprehensive joint technology-based device for the treatment of uranium-contaminated soil is provided. 
         [0007]    In order to achieve the goal above, this invention is adopting the following technical program: 
         [0008]    The comprehensive joint technology-based device for the treatment of uranium-contaminated soil is of the following structure: Set the cathode zone  5  and anode zone  6  at both sides of uranium-contaminated soil, and set the cathode and anode respectively in those two zones, which are respectively connected to the power transformer  9  with wires; set several sprinklers  10  between cathode zone  5  and anode zone  6  in the form of an array; set pumping well  1  in the vicinity of the cathode zone  5 , with pumping pipeline  3  in it, and pumping pipeline  3 , pump  11  and pumping controller  12  should be connected to the water collecting sump  13  sequentially; set the soil storage tank  14  near the water collecting sump  13 , and connect one end of the remediation-separation system  15  to the soil storage tank  14  and water collecting sump  13 , while connect the other end to the soil recovery tank  16 , ground water recovery tank  17  and uranium recovery tank  18 ; set recharge well  2  in the vicinity of the anode zone  6 , with the recharge pipeline  4  in it, and the recharge pipeline  4 , recharge pump  19  should be connected to the groundwater storage tank  17  sequentially. 
         [0009]    Said comprehensive joint technology-based device for the treatment of uranium-contaminated soil further comprises the catholyte processing system  7  and anolyte processing system  8 . The catholyte processing system  7  comprises a catholyte storage compartment, a catholyte pH controller and a peristaltic pump; the catholyte pH controller is connected to the catholyte storage compartment, while the catholyte storage compartment is connected to the cathode zone  5  through the corresponding pipeline and the peristaltic pump. The anolyte processing system  8  comprises an anolyte storage compartment, an anolyte pH controller and a peristaltic pump; the anolyte pH controller is connected to the anolyte storage compartment, while the anolyte storage compartment is connected to the anode zone  6  through the corresponding pipeline and the peristaltic pump. 
         [0010]    The remediation-separation system  15  mentioned above is composed by the photoelectric remediation device  22 , the screen  23 , the solid phase extraction device  24 , the aqueous phase processing device  25 , and the solid phase processing device  26 . One end of the photoelectric remediation device  22  is connected to the water collecting sump  13  and the soil storage tank  14 , and the other end is connected to the screen  23 ; the screen  23  is respectively connected to the aqueous phase processing device  25  and the solid phase extraction device  24 ; the aqueous phase processing device  25  is connected to the ground water recovery tank  17 ; the solid phase extraction device  24  is connected to the solid phase processing device  26  and the uranium recovery tank  18 ; the solid phase processing device  26  is connected to the soil recovery tank  16 . 
         [0011]    The pumping pipeline  3  and the recharge pipeline  4  mentioned above are both non-sand concrete tube wells. Their inner diameter is of 250-500 mm and depth of 35-50 m, and their periphery is filled with backfill and gravels, with the filling layer of 60-100 mm in thickness. 
         [0012]    The cathode and anode mentioned above are both plate electrodes. 
         [0013]    A uranium-contaminated soil treatment method with the use of the comprehensive joint technology-based device for the treatment of uranium-contaminated soil mentioned above is of the following steps: 
         [0014]    Step 1: Install the comprehensive joint technology-based device for the treatment of uranium-contaminated soil at the site contaminated by uranium, with the cathode, anode, pumping pipeline  3  and recharge pipeline  4  inserted into the soil, and the sprinklers  10  above the contaminated soil; 
         [0015]    Step 2: Spray the eluting solution via multiple sprinklers  10  to the soil until the concentration of eluting solution in the soil is 10-30 ml/g; 
         [0016]    Step 3: Add the catholyte and anolyte respectively in the cathode zone  5  and the anode zone  6 , and maintain the voltage between the cathode and anode of 150-600V via the power transformer  9 ; keep the electrokinetic remediation for 2-8 days, and then uranium substances will migrate to the vicinity of anode, leading to the gathering of uranium pollutants; 
         [0017]    Step 4: During the electrokinetic remediation, start up the pump  11  to transport the contaminated ground water in the phreatic layer to the water collecting sump  13  through the pumping pipeline  3 , with the pumping volume of 5-10 m 3 /h and lasting for 1.5-6 days; 
         [0018]    Step 5: When the electrokinetic remediation is finished, excavating the soil in the area within the diameter of 1-2.5 m and the depth of 10-20 m centered on the anode zone  6 , and then transport it to the soil storage tank  14 ; 
         [0019]    Step 6: Transport the contaminated soil in the soil storage tank  14  and the contaminated water in the water collecting sump  13  to the remediation-separation system  15 , and mix the soil and ground water to form a suspension; adjust the pH of the suspension to 2.8-3.5, and irradiate it with the fluorescent light for 24-36 h for the photolysis remediation, then the uranium compound in the suspension will become the uranyl hydroxide and deposit; after the remediation, deposit and separate the soil, water and uranyl hydroxide, and respectively transport them to the soil recovery tank  16 , the ground water recovery tank  17  and the uranium recovery tank  18 ; 
         [0020]    Step 7: Transport the soil remedied through the photolysis in the soil recovery tank  16  to the vicinity of the anode zone  6 , and start up the recharge pump  19  to transport the remedied ground water in the ground water recovery tank  17  to the recharge well  2  via the recharge pipeline  4 ; 
         [0021]    Step 8: Test the soil remediation effect, and repeat Step 1 to Step 7 if needed until the uranium in the soil meets the requirement of the safety standards or is totally removed. 
         [0022]    Specifically: 
         [0023]    Before Step 1, conduct the ground investigation of the soil contaminated by the radioactive uranium that remains to be treated, so as to get information of the contaminated soil area, hydrogeological conditions, types of pollutants, and data of the physical and chemical nature, thus determining the contamination status. 
         [0024]    After Step 1, the pumping pipeline  3  and the recharge pipeline  4  should be 0.5-2 m in depth across the phreatic layer. 
         [0025]    After Step 1, the cathode and anode should be inserted 10-20 m in depth into the contaminated soil surface, tangent with and the phreatic layer. 
         [0026]    The used eluting solution in Step 2 is the 0.2-0.6M citric acid solution or sodium bicarbonate solution. 
         [0027]    The spray duration in Step 2 is 2-4 h. 
         [0028]    The weight/volume of soil and ground water in the suspension used in Step 6 is 1 g: (10-20) ml. 
         [0029]    The separation of soil, water and uranyl hydroxide in Step 6 can be achieved as follows: when the photolysis remediation is finished, solid substances and the solution in the suspension will be separated through the screen  23 , and the screen whose bore diameter is 0.22 micron is preferred; transport the obtained ground water through the separation to the aqueous phase processing device  25  to adjust a neural pH, and then transport it to the ground water recovery tank  17 ; transport the obtained soil to the solid phase extraction device  24  to recover the uranyl hydroxide precipitate in the soil with the use of the TBP-sulfonated kerosene extraction technique, and then transport the recovered uranium substances to the uranium recovery tank  18 ; transport the rest soil to the solid phase processing device  26  to adjust a neural pH, and then transport it to the soil recovery tank  16 . 
         [0030]    In Step 8, adopt the inductively coupled plasma atomic emission spectrometry (ICP-AES) to measure the residual volume of uranium in soil and in the uranium recovered solution, so as to get the soil remediation effect. 
         [0031]    Beneficial effects of the invention include: 
         [0032]    (1) This comprehensive joint technology can not only reduce the uranium volume in the contaminated soil or water, but also recycle the obtained uranium; (2) the electrokinetic technique can gather the uranium to the vicinity of the anode, which is convenient for the centralized processing; (3) add the extracting agent, citric acid or sodium bicarbonate solution to the soil can effectively extract the uranium from the soil phase to the aqueous phase; (4) the photolysis can deposit the uranium, which is convenient for further recycling; (5) the treatment process is clean and will not cause secondary pollution with a shorter processing cycle and guaranteed security. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  is the diagram of the comprehensive joint technology-based device and method for the treatment of uranium-contaminated soil; 
           [0034]      FIG. 2  is the diagram of the remediation-separation system  15 ; and 
           [0035]      FIG. 3  is the treatment effect of the uranium-contaminated soil with different voltages and electrokinetic remediation duration. 
       
    
    
       [0036]    Numerals in the drawing represent:  1 —pumping well;  2 —recharge well;  3 –pumping pipeline;  4 —recharge pipeline;  5 —cathode zone;  6 —anode zone;  7 —catholyte processing system;  8 —anolyte processing system;  9 —the power transformer;  10 —sprinklers;  11 —pump;  12 —pumping controller;  13 —water collecting sump;  14 —soil storage tank;  15 —remediation-separation system;  16 —soil recovery tank;  17 —ground water recovery tank;  18 —uranium recovery tank;  19 —recharge pump;  20 —soil excavating zone;  21 —soil surface;  22 —photoelectric remediation device;  23 —screen;  24 —solid phase extraction device;  25 —aqueous phase processing device;  26 —solid phase processing device. 
       DETAILED DESCRIPTION 
       [0037]    Here, a further introduction of this invention will be made combined with the attached figures and specific examples: 
         [0038]      FIG. 1  is the diagram of the invented comprehensive joint technology-based device for the treatment of uranium-contaminated soil, which is composed by the pumping system, the electrokinetic remediation system, elution system, remediation-separation system and recharge system. The specific structure is as follows: set the cathode zone  5  and anode zone  6  at each side of the uranium-contaminated soil, and respectively set the cathode and anode inside the cathode zone  5  and anode zone  6 , which are respectively connected to the power transformer  9 . Control the volume, pH and other parameters of catholyte and anolyte in the cathode zone  5  and anode zone  6  through the catholyte processing system  7  and anolyte processing system  8 . The catholyte processing system  7  is composed by a catholyte storage compartment, a catholyte pH controller and a peristaltic pump; the catholyte pH controller is connected to the catholyte storage compartment, while the catholyte storage compartment is connected to the cathode zone  5  through the peristaltic pump. The anolyte processing system  8  is composed by an anolyte storage compartment, an anolyte pH controller and a peristaltic pump; the anolyte pH controller is connected to the anolyte storage compartment, while the anolyte storage compartment is connected to the anode zone  6  through the peristaltic pump. Multiple sprinklers  10  will be set between the cathode zone  5  and anode zone  6  in the form of an array, and when the device is installed at the site of the contaminated soil, these sprinklers can spray on the contaminated soil between the cathode zone  5  and anode zone  6 . Set the pumping well  1  in the vicinity of the cathode zone  5 , and set the pumping pipeline  3  in it. The pumping pipeline  3  is connected to the pump  11 , and the pump  11  and the pumping controller  12  are sequentially connected to the water collecting sump  13 , so as to transport the contaminated ground water in the in shallow water layer to the water collecting sump  13 ; set the soil storage tank  14  in the vicinity of the water collecting sump  13 , which is used for the storage of soil after the electrokinetic remediation. One side of the remediation-separation system  15  is connected to the water collecting sump  13  and the soil storage tank  14 , while the other side is respectively connected to the soil recovery tank  16 , the ground water recovery tank  17  and the uranium recovery tank  18 . As is shown in  FIG. 2 , adopt the remediation-separation system  15  for the photolysis remediation of contaminated ground water and the obtained soil after the electrokinetic remediation, and specifically, the remediation-separation system  15  is composed by the photoelectric remediation device  22 , the screen  23 , the solid phase extraction device  24 , the aqueous phase processing device  25  and the solid phase processing device  26 . And one side of the photoelectric remediation device  22  is connected to the water collecting sump  13  and the soil storage tank  14 , and the other side is connected to the screen  23 , while the screen  23  is respectively connected to the aqueous phase processing device  25  and the solid phase extraction device  24 ; the aqueous phase processing device  25  is connected to the ground water recovery tank  17 ; the solid phase extraction device  24  is connected to the solid phase processing device  26  and the uranium recovery tank  18 ; and the solid phase processing device  26  is connected to the soil recovery tank  16 . Set the pumping recharge well  2  in the vicinity of the anode zone  6 , and set the recharge pipeline  4  inside it. The recharge pipeline  4  is connected to the recharge pump  19 , and the recharge pump  19  is connected to the ground water recovery tank  17 . 
         [0039]    The pumping pipeline  3  and the recharge pipeline  4  adopted by this research are both non-sand concrete tube wells. Their inner diameter is of 250-500 mm and depth of 35-50 m, and their periphery is filled with backfill and gravels, with the filling layer of 60-100 mm in thickness. And the cathode and anode mentioned above are both normal plate electrodes. 
       Embodiment 1 
       [0040]    Remedy the uranium-contaminated soil with the use of said device, and the specific steps are as follows: 
         [0041]    Step 1: Conduct the ground investigation of the soil contaminated by the radioactive uranium that remains to be treated, so as to get information of the contaminated soil area, hydrogeological conditions, types of pollutants, and data of the physical and chemical nature, thus determining the contamination status. 
         [0042]    Step 2: Install the comprehensive joint technology-based device for the treatment of uranium-contaminated soil at the site contaminated by uranium, with the cathode, anode, pumping pipeline  3  and recharge pipeline  4  inserted under the surface soil  21 , and the sprinklers  10  uranium above the contaminated soil; ensure that the pumping pipeline  3  and the recharge pipeline  4  is 0.95 m in depth across the phreatic layer, and the cathode and anode is inserted 12 m in depth into the contaminated soil surface, tangent with and the phreatic layer. 
         [0043]    Step 3: Spray the citric acid solution via multiple sprinklers  10  to the contaminated soil, and then measure the volume of eluting solution in different soils (10, 20, 30 ml/g) and the uranium removal efficiency of different eluting duration (2, 3, 4 h). 
         [0044]    Step 4: Use the catholyte pH controller and the anolyte pH controller to control the pH of catholyte and anolyte, and transport the catholyte and anolyte respectively to the cathode zone  5  and the anode zone  6  with the corresponding pumps, and among them the catholyte and anolyte are preferably graphite electrode solution, purchased through the business approach. Maintain the voltage between the cathode and anode of 150V via the power transformer  9 , and keep the electrokinetic remediation for two days so as to make uranium substances migrate to the vicinity of anode, leading to the gathering of uranium pollutants; 
         [0045]    Step 5: During the electrokinetic remediation, start up the pump  11  to transport the contaminated ground water in the phreatic layer to the water collecting sump  13  through the pumping pipeline  3 , with the pumping volume of 5 m 3 /h and lasting for 1.5 days; 
         [0046]    Step 6: When the electrokinetic remediation is finished, excavating the soil in the area within the diameter of 1-2.5 m and the depth of 10 m centered on the anode zone  6 , just as the soil excavating zone  20  shown in  FIG. 1 , and then transport it to the soil storage tank  14 ; 
         [0047]    Step 7: Transport the contaminated soil in the soil storage tank  14  and the contaminated water in the water collecting sump  13  to the remediation-separation system  15 , and mix the soil and ground water to form a suspension, while weight/volume of soil and ground water is 10 g:120 ml. Adjust the pH of the suspension to 3.5, and irradiate it with the fluorescent light for 24 h for the photolysis remediation, then the uranium compound in the suspension will become the uranyl hydroxide and deposit. After the remediation, deposit and separate the suspension with the 0.22 micron screen  23 . Transport the obtained ground water to the aqueous phase processing device  25  to adjust a neural pH, and then transport it to the ground water recovery tank  17 . Transport the obtained soil to the solid phase extraction device  24  to recover the uranyl hydroxide precipitate in the soil with the use of the TBP-sulfonated kerosene extraction technique, and then transport the recovered uranium substances to the uranium recovery tank  18 . Finally, transport the rest soil to the solid phase processing device  26  to adjust a neural pH, and then transport it to the soil recovery tank  16 . 
         [0048]    Step 8: Transport the soil remedied through the photolysis in the soil recovery tank  16  to the vicinity of the anode zone  6 , and start up the recharge pump  19  to transport the remedied ground water in the ground water recovery tank  17  to the recharge well  2  via the recharge pipeline  4 ; 
         [0049]    Step 9: Adopt the inductively coupled plasma atomic emission spectrometry (ICP-AES) to measure the residual volume of uranium in soil and in the uranium recovered solution, so as to get the soil remediation effect. Repeat Step 2 to Step 8 if needed until the uranium in the soil meets the requirement of the safety standards or is totally removed. 
         [0050]    The test result is shown in the following table: 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 The uranium removal efficiency of different 
               
               
                 eluting duration and volumes 
               
             
          
           
               
                   
                 Eluting solution volume in soil (ml/g) 
               
             
          
           
               
                   
                 10 
                 20 
                 30 
               
             
          
           
               
                   
                 Eluting duration (h) 
               
             
          
           
               
                   
                 2 
                 3 
                 4 
                 2 
                 3 
                 4 
                 2 
                 3 
                 4 
               
               
                   
                   
               
             
          
           
               
                 Uranium 
                 51.3 
                 62 
                 60.9 
                 63.4 
                 75.1 
                 83.7 
                 69.7 
                 80.2 
                 79.8 
               
               
                 removal 
               
               
                 efficiency 
               
               
                 (%) 
               
               
                   
               
             
          
         
       
     
       Embodiment 2 
       [0051]    Specific approaches are the same with the implementation example 1, but change the voltage and remediation duration of the electrokinetic remediation, and then analyze the uranium removal efficiency changes of ICP-AES under the different voltages (200V, 300V, 400V, 500V, 600V) and electrokinetic remediation duration (2, 4, 6, 8 day). The result is shown in  FIG. 3 . It can be seen that increasing the voltage or prolonging the electrokinetic remediation duration time will help to improve the removal efficiency of uranium in soil, and when the voltage is 500V with the electrokinetic remediation time of 8 d, the repair efficiency will be up to 82.9%.