Remediation Of Farmland Soil Polluted By Uranium Tailings By Citric Acid Mild Leaching Combined With Cyclic Leaching Process

In this paper,the uranium contaminated soil around a uranium tailings in Fuzhou City,Jiangxi Province was repaired by mild cyclic leaching with citric acid combined adsorption material.The experiment designed a unique cyclic leaching device,used citric acid as a mild leaching agent,and carried out chemical cyclic leaching on the tested soil under the mild environment of p H = 4 and p H =5.Methods on the premise that the soil environmental quality and structure function are not destroyed,the dosage of eluent is reduced and the efficiency of chemical leaching is improved.In the experiment,different ion exchange resins and uranyl ion imprinting materials were used as the intermediate fixative of the circulating leaching system to ensure that the circulating leaching solution was always at the level of low metal elements and avoid the secondary pollution of soil by the leaching solution.The experiment adopts the step-by-step extraction experiment of soil heavy metals to detect the morphological distribution of uranium in soil before and after leaching.The soil before and after remediation was characterized by SEM-EDS and XRD.The main conclusions of this paper are as follows:(1)The chemical leaching remediation of uranium contaminated soil was carried out by mild cyclic leaching remediation.In the experiment,1.8 L,10 g/L and 15 g/L citric acid were used as leaching agents,and the optimal leaching efficiency was close to 96%.Compared with the conventional citric acid chemical leaching technology,this method ensures that the soil environmental quality is not damaged,improves the leaching efficiency,and has the advantage of low cost.(2)By designing a new type of rinsing device,the rinsing column is connected in series with the adsorption tower,and the peristaltic pump provides the power source of the circulating rinsing device to build a closed circulating rinsing system.By putting a fixed amount of eluent into the system to make it circulate repeatedly in the system,and using the adsorption tower to recover the uranium element in the eluent,the eluent that enters the soil again can regain the leaching effect.From the perspective of device structure,compared with the conventional rinsing device,the circulating rinsing device has unique tightness,which can improve the flow speed of rinsing agent in the rinsing system,and then improve the rinsing efficiency of the device.From the perspective of secondary pollution,by connecting the leaching column and adsorption tower in series,the leaching liquid of the circulating leaching device is no longer a mixed waste liquid containing a large number of heavy metal elements,and can be put into use again as leaching liquid.From the perspective of the economic effect brought by the circulating rinsing device,compared with the conventional rinsing device,the circulating rinsing device can greatly reduce the input of rinsing agent and reduce the economic cost in the rinsing work.(3)In the circulating leaching system of citric acid combined with two kinds of ionic resins,the concentration of various metal elements in the circulating leaching solution can be effectively reduced.The results showed that when p H = 4,the leaching column connected with two ion exchange resins in series could achieve good fixation effect on uranium in soil,and the removal rate of uranium in soil was 95.47%;When p H = 5,the uranium removal rate of two ion exchange resins in series can reach73.93%.Through a series of characterization and analysis of the soil,the soil crystal structure is almost not damaged,and the form of radioactive metal uranium in the soil is more transformed to a stable residue state after leaching.The experiment of citric acid combined with two exchange resins for cyclic leaching of uranium contaminated soil shows that this method can effectively fix the uranium in the soil and reduce the precipitation of uranium in the repaired soil,but there will be a phenomenon that a large number of conventional metal elements will be washed out in the leaching process.(4)Uranyl ion imprinting material was prepared experimentally.The material has a unique spatial structure and can selectively recognize and adsorb uranium elements.The material has good desorption performance and improves the reusability of the material.At the same time,the adsorption efficiency of imprinted materials for uranium can reach more than 70 % in citric acid p H environment of 2 ～ 5.Therefore,uranyl ion imprinting material can be perfectly suitable for citric acid mild cyclic leaching system.It is found that when uranyl ion imprinting material is used as the intermediate fixative of cyclic leaching system,the chemical leaching efficiency of uranium in uranium contaminated soil is 89.92 %,and the leaching amount of conventional metal elements in soil is reduced in varying degrees,with an average leaching amount reduced by about 10 %.The experiment verified the feasibility of citric acid as a mild leaching agent to participate in chemical leaching to repair uranium contaminated soil.By designing a cyclic leaching device,the leaching efficiency of uranium in soil was ensured and the leaching cost was reduced.The experiment was carried out in weak acid environment to ensure the safety of soil environment under this method.The selective adsorption of metal elements in the leaching solution by ion imprinting materials reduces the elution of conventional metal elements in the soil and further improves the soil environmental quality after remediation.