Experimental Study And Application Of Multi-PRBs In Remediation Of Uranium-Manganese Contaminated Groundwater In Uranium Mines

With the rapid development of China’s nuclear energy industry,the demand for natural uranium resources will be further expanded.Under the premise of green development and ecological civilization construction in this new era,the exploitation and utilization of uranium resources and environmental protection are facing both opportunities and challenges.Under the natural conditions of weathering,acid rain and leaching,the residual radionuclides and associated heavy metals produced in the mining and smelting process of hard-rock type uranium deposits can be dissolved,then migrate into groundwater with the aqueous solution,thus not only form a complex and diversified groundwater contamination system,but also pollute the soil and water environment around the mine through convection,diffusion and other ways,which bring long-term health hazards to residents.Therefore,the study on the remediation of contaminated groundwater around uranium mines has become an urgent and important issue in the field of ecological environmental protection of nuclear industry in China.At present,in-situ remediation technology of permeable reactive barrier（PRB）has the advantages of non-powered operation,small environmental impact and low maintenance cost.It has become a reliable technology for remediation of groundwater pollution and has been applied in the site remediation of contaminants such as heavy metals.Especially,the multiple permeable reactive barriers（Multi-PRBs）,can target the removal of composite pollutants in groundwater,which has the advantages of breaking down the pollutants one by one,avoiding interference,as well as improving the remediation efficiency.However,the current research on Multi-PRBs technology are still in the stage of preparation and screening of superior reaction materials for single pollutants in the laboratory system,while the research on mechanism of synergetic removal of composite pollutants in the actual pollution system is far from sufficient.In addition,further exploration and research are needed on the migration and transformation mechanism of pollutants in Multi-PRBs,as well as the coupled remediation mechanism of multi-stage reaction units in Multi-PRBs.In this thesis,contaminated groundwater from downstream of a uranium tailings dam in Jiangxi province was taken as the research object.Through a serial research means like field investigation（geological drilling,pumping test,sample collection and test）,laboratory experiments（batch test,one-dimensional column experiment,three-dimensional tank experiment）,site experiments,microstructure characterization（SEM-EDS,XRD,BET,FT-IR）,numerical simulation（PHREEQC,MODFLOW）,and theoretical analysis,the efficiency of natural clinoptilolite,coconut shell activated carbon and type 717 anionic resin for remediation of uranium（U）and manganese（Mn）compound pollutants in groundwater,and the competitive adsorption mechanism between U and Mn in each reaction materials were elucidated,the migration and transformation of pollutants in the Multi-PRBs system under the condition of three-dimensional flow field were meticulous described and simulated,the multi-level coupling removal mechanism of composite pollutants over Multi-PRBs was revealed,and the regulation mechanism of Multi-PRBs in the remediation of contaminated groundwater at site scale was explored.The main research results are summarized as follows:（1）Hydrogeochemical characteristics and contamination evaluation of present groundwater in the study areaBased on the hydrogeological survey on the downstream area of uranium tailings dam,groundwater was found to be confined water in the study area.The average thickness of aquifer was 4.20 m,the permeability coefficient was 1.49 m/d,the p H value of groundwater was in the range of 6.52～9.67,and the hydrochemical type was HCO₃·SO₄-Ca.The concentration of U in groundwater ranged from 0 to 0.49 mg/L,with the highest contamination level in the samples collected during the mean-flow period.Over-standard rate of all boreholes reached 90%,while UO₂（CO₃）₂^2-,UO₂（CO₃）₃^4-and UO₂CO₃ were the main forms of U in the groundwater.The content of Mn in groundwater ranged from 0 to 37.22 mg/L.All samples of mean-flow period exceeding the groundwater-quality criteria,while Mn²⁺,Mn CO₃ and Mn SO₄ were the main forms.Pollution plumes with high concentration in the study area were distributed near the groundwater discharge area,which may pose a threat to the surrounding water environment.（2）Adsorption mechanism and interaction mechanism of U and Mn by PRB reaction materialsAccording to the stability,economy and producibility of reactive materials,natural clinoptilolite,coconut shell activated carbon and anion exchange resin were selected for the batch adsorption experiments under different contaminated systems.Results showed that the adsorption capacities of zeolite,activated carbon and anionic resin for U in single uranium contaminated system were 11.95 mg/kg,15.25 mg/kg and 44.15mg/kg,respectively,and the adsorption process were all in accordance with Langmuir isothermal adsorption model.The adsorption capacities of zeolite and activated carbon for Mn in single manganese contaminated system were 65.28 mg/kg and 355.88 mg/kg,respectively.The adsorption processes were both in accordance with Frendlich model.However,the adsorption performance of Mn by anionic resin was extremely poor.In the U-Mn dual system,distinct competitive effect was shown between U and Mn on the surface adsorption sites,resulting in the promotion of U adsorption and inhibition of Mn,which was because that the affinity of zeolite to U was higher than that of Mn.However,the removal of U by anionic resin was not affected significantly by the coexisting contaminant Mn.（3）Migration characteristics of U and Mn under one-dimensional column experimentsIn the one-dimensional flow simulation column experiments,the adsorption capacities of zeolite and activated carbon for U in single uranium contaminated system were 7.09 mg/kg and 44.48 mg/kg,respectively,while excellent adsorption performance was shown for Mn in single manganese system.The anionic resin had strong adsorption ability for U in single uranium contaminated system,but weak for Mn in single manganese system.Under the U-Mn combined contamination system,there were obvious differences in the adsorption capacity for three reactive materials,as well as the in the stages and degrees of competitive effects between U and Mn.To be specific,for zeolite,the competitive adsorptions of U and Mn on the surface of zeolite were carried out at the same time,and the competition became more intense with the increase of migration paths;the improvement of U adsorption mainly occurred in the middle and rear part of the column;the adsorption capacity of zeolite for U increased from 7.09 mg/kg to 15.61 mg/kg,while the adsorption efficiency for manganese decreased from 100%to 52.01%.For activated carbon,the competitive adsorptions of U and Mn were gradually obvious as the operation time increasing.Because the adsorption process of U on activated carbon was greatly affected by the number of reaction sites on surface,the promotion of U adsorption only occurred in the front of the column,while the inhibition of Mn adsorption occurred in the middle and rear part.For anion exchange resin,coexisting contaminant Mn had little effect on the removal of U.However,after 25 PV’s operation,Mn in the resin column was oxidized from dissolved Mn²⁺to precipitated Mn⁴⁺,resulting in the retention of Mn pollutants.（4）Migration and transformation of U and Mn and multi-stage coupling removal mechanism in three-dimensional fluid systemIn the three-dimensional seepage tank simulation experiments,U and Mn pollutants were removed step by step in Multi-PRBs under the combined effects of convection dispersion and adsorption reaction,while the removal efficiency showed a spatial heterogeneity affected by the dispersion in longitude.Numerical modeling of the ability of Multi-PRBs system to repair single pollutant of uranium was applied in MODFLOW.Results showed that the contaminant plume diffused from upstream to downstream evenly as the adsorption capacity of each reactive barrier reaching saturation gradually,predicting that the effluent concentration would not reach to the same as the influent concentration after continuous performance of 3650 days,which conformed a long-term effect.Compared to a single contamination system,the adsorption capacity for U was stronger in the coupled contamination system of U and Mn.The U removal performance of zeolite decreased under the influence of dispersion,while activated carbon showed minimum influence by hydrodynamic conditions and coexisting pollutants.For anion resin,extremely strong adsorption for U was presented as the increasing of hydraulic retention time,while no distinct difference was observed by the coexisting pollutant of Mn.In the coupled contamination system of U and Mn,because of a weak adsorption for Mn by anion resin,the overall removal efficiency of Multi-PRBs for Mn was lower than that of U,and reached to depletion after 46 PV’s operation,while adsorption capacity of zeolite for Mn increased from 68.71 mg/kg to182.00 mg/kg.The simulation results of PHREEQC showed that the existing forms and proportion of U and Mn had been transformed apparently with the migration of pollutants in Multi-PRBs.The anion resin mainly retained（UO₂）₂CO₃（OH）₃^-and Mn SO₄,but precipitated Mn O₂ was easily formed on the resin surface due to acidification and electrolysis of Mn SO₄,while the adsorption of Mn was mainly in the form of Mn CO₃ by activated carbon.（5）Research on regulation of pollution plume distribution in groundwater by Multi-PRBs remediation system at site scaleThe Multi-PRBs system was applied for the site remediation test in the downstream of a tailings dam of a uranium mine.After 150 days of operation,the groundwater samples collected from boreholes were analyzed.Results showed that the U concentration were76%-99%lower than that of before the running of the system,and Mn concentrations were reduced by 10%-60%.Those reduced U and Mn pollutants were fixed in the Multi-PRBs reaction zone with no diffusion ability.Comparing the dispersion characteristic of pollution plume before and after applying the Multi-PRBs system,high concentration areas of pollutants were adjusted and controlled from the groundwater discharge area to the tailings leakage area after applying the Multi-PRBs remediation system,indicating a source control effect for contaminants dispersion.The variation of groundwater table in the borehole was within2%after the remediation,suggesting no obvious interference happened to the original flowing system of groundwater due to the implementation of Multi-PRBs.Therefore,Multi-PRBs could be an effective method for collaborative removal of complex pollutants from groundwater in uranium mines.