| Under the leaching and infiltration of water,the harmful metallicions in the uranium tailings deposit could be released to the surface water or shallow underground water,which will bring serious harm to human health and the ecologicar environment.Therefore,preventing the migration of harmful metal ions from uranium tailings becomes an important topic in the environment field both at home and abroad.Bentonite and zeolite minerals have superior performance in adsorbing and immobilizing the radionuclides and heavy metals.They are potentially excellent solidification matrixes.Based on the study of chemical and mineral composition,the content,occurrence,and migration activity of harmful metal ions were investigated.Besides,the effects of natural particle size of tailings and pH of leachate on the release and migration of harmful metal ions in uranium tailings deposit were discussed.Due to the excellent cation exchange property,impermeability and colloidal dispersion,the bentonite was utilized to adsorb the harmful metals.Furthermore,the bentonite was prepared as grouting material to inject into the uranium tailings deposit to form the bentonite/uranium tailings complex.The in-situ adsorption of bentonite on the injurious metals and its characteristics of pore sealing and watertight were verified.Zeolite minerals were synthesized from bentonite in the alkaline water system,and the cementation and impermeable characteristics of the montmorillonite or zeolite minerals/uranium tailings complex as well as the immobilization effect on the harmful metal ions were revealed.Combining with theoretical analysis and experimental characterization,the contents include the chemical and mineral composition,the physical and chemical properties of the deposits in the typical uranium tailing impoundments,and the injection properties of bentonite colloidal materials for the deposit of uranium tailings,as well as the selective adsorption of bentonite for the harmful metal ions were identified.In addition,the migration law and migration mechanism of harmful metal ions from uranium tailings,adsorption law and adsorption mechanism of harmful metals in uranium tailings with bentonite,and the in-situ closed and solidifying effects,as well as the blocking mechanism of bentonite and zeolite minerals for reducing the release of the harmful metal ions from uranium tailings deposit were revealed.After studied the properties of uranium tailings deposit,the principal mineral composition,chemical composition,distribution characteristics and occurrence of harmful metal ions in uranium tailings deposit were found out.The primary composition of uranium tailings deposite was silicate minerals with mineral phases of albite,quartz,gypsum,and chlorite.The chemical composition of uranium tailings were SiO2,Al2O3,CaO and SO3 with a small amount of harmful metal ions,such as Mn,Sr,Pb and U.It was found that Sr and Pb were much more likely to adsorb in the silicate minerals and gypsum,respectively,and the U and Mn were uniformly distributed in the doposit.In addition,most of U and Pb were in the residue fraction with a minor in migration fractions.The Sr and Mn were of intense migration activity,due to most of them were in the water-soluble and exchangeable fractions.After calculated,the accumulation degree of Mn,Sr,Pb and U in the uranium tailings was in the order of U>Pb>Sr>Mn,the migration degree was in the order of Mn>Sr>U>Pb,and the relationship of risk factor was Sr>Mn>Pb>U.Moreover,the comprehensive hazard effect of environment was U>Sr>Mn>Pb.The migration behavior of harmful metals in uranium tailings was studied by static leaching and dynamic leaching methods.Furthermore,the migration and migration mechanism of harmful metal elements in uranium tailings under simulated natural conditions were revealed.The static leaching showed that the Mn,Sr,Pb and U in uranium tailings could be released and migrated to the aqueous solution,and the release was deeply affected by environmental factors such as soaking time,solid-liquid ratio,pH and natural particle size of tailings.The dynamic leaching of uranium tailings with different natural particle sizes showed that the migration of harmful metals in uranium tailings was different with the different fractions of the tailings.The release of Mn decreased with the decrease of particle size of tailings,while the release amount of Sr,Pb and U did not change with the decrease of particle size of tailings.In the dynamic leaching of uranium tailings with different pH leachate,the release of Mn,Sr,Pb and U in uranium tailings displayed two phases:fast release and slow release.The release of Mn and U decreased with the increase of leachate pH,while the release of Sr and Pb changed little with the change of leachate pH.It was also found that the release of Mn,Sr,Pb and U in uranium tailings was related to the chemical reactivity,migration activity and occurrence state of Mn,Sr,Pb and U.In the uranium tailings,Mn,Sr,Pb and U in the migration fraction were released first.The higher the content,the more the release rate.However,some ions were also controlled by the mechanism of dissolution-precipitation-dissolution.Under water chemical conditions,release of Mn from uranium tailings was controlled by wash off.The release of Sr and Pb was affected by dissolutions.And the wash off and difussion were the primary mechanism for the release of U from uranium tailings.Based on the release of Mn,Sr,Pb and U in uranium tailings,the bentonite was used to adsorb the Mn,Sr,Pb and U.The removal behavior of Mn,Sr,Pb and U on bentonite was studied with different initial concentrations,pH,contact times,and temperatures.It was found that the bentonite was an effective adsorbent for Mn,Sr,Pb and U.The adsorption process was strongly influenced by initial concentrations and pH.Kinetic calculations prove that the removal of Mn,Sr,Pb and U on bentonite was consistent with pseudo-second-order dynamic model.Besides,the adsorption process was in accordance with Langmuir isotherm.The adsorption mechanism of bentonite on Mn,Sr,Pb and U varied slightly with the pH value.When pH was lower than 4,the adsorption mechanism was cation exchange.The exchange cations in interlayer of montmorillonite exchanged with Mn,Sr,Pb and U in the solution,and then entered the interlayer of montmorillonite to be stabilized.When pH was located between 4 and 5,the adsorption was the specific adsorption mechanism of hydrolysis complexation.When pH was higher than 5,the adsorption of Mn,Sr,Pb and U on bentonite was mainly controlled by precipitation.Bentonite was used as feedstock materials for the synthesis of zeolite minerals with a novel one-step strategy in a water bath under atmospheric pressure.The effect of Si/Al molar ratio,Na/Si molar ratio,reaction temperature and reaction time on the structure of synthetic zeolite minerals were revealed.Besides,the optimum conditions for synthesizing zeolite A were as follows:the Na/Si molar ratio was 4,the Si/Al molar ratio was 1.0,the reaction temperature was 90?,the reaction time was 12 h,and the addition of water was 35 mL.Optimum synthesis conditions of sodalite were Na/Si=16,Si/Al?1,reaction temperature=90?,reaction time=12 h,and the addition of water=35 mL.Zeolite A synthesized under optimization of process conditions was excellent cube shape with an average particle size of 1 ?m.And sodalite synthesized under optimization of process conditions was spherical aggregates formed by petal shaped particles with an average particle size of 1?2 ?m.Bentonite/uranium tailings complex was successfully prepared by injecting bentonite gel into uranium tailings deposit,and the zeolite mineral/uranium tailings complex was synthesized in situ from the bentonite/uranium tailings complex.The closure of tailings pore and the inhibition of harmful metal ions by bentonite and zeolite minerals were studied.It was found that the bentonite/uranium tailings complex could be transformed into zeolite A/uranium tailings complex and sodalite/uranium tailings complex under the control of alkalinity.The synthetic zeolite minerals/urnaium tailings complex had good compactness and could effectively reduce the permeability of water.Besides,the zeolite minerals/uranium tailings complex can stabilize uranium tailings well and decrease the leaching and migration activity of harmful metal ions from uranium tailings deposit.The blocking mechanism of zeolite minerals/uranium tailings complex for reducing the release and leaching of pollutants from uranium tailings deposits was as follows:The injection of bentonite gel and in situ transformation into zeolite minerals/uranium tailings complex could block and seal of pores in uranium tailings deposit,which formed a isolation barrier with decrease of the seepage migration channels.Therefore,the migration of harmful metal ions with the penetration of pore water could be effectively reduced.In addition,zeolite minerals could be regarded as a crystalline geopolymer with good stability and could bind uranium tailings particles.Its three dimensional network structure had a trapping effect on harmful metal ions.It was demonstrated by SEM the uranium tailing particles were tightly wrapped by the zeolite minerals in the zeolite minerals/uranium tailings complex,reducing the direct contact between uranium tailings particles and water.Thereby,the migration of particle pollution from tailings with the water penetration and diffusion was further reduced. |
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