Efficiency And Mechanism Of Immobilizing Uranium By Natural And Synthetic Pyrite Oxidation In Neutral Aerobic Environment

Pyrite is a typical reducing mineral,which has a good immobilization effect on the radioactive heavy metal such as uranium in a reducing environment.However,pyrite is prone to oxidation in an oxidizing environment and produce Fe²⁺and SO₄^2-,resulting in a decrease in the p H of the aqueous solution,thereby greatly reduce its ability to immobilize uranium.Therefore,in this paper,Tris was selected as the p H buffer.The pure phase pyrite synthesized by hydrothermal method and natural pyrite were used for static batch experiments and acid dissolution experiments.Adsorption kinetics simulation and various characterization analysis were carried out.This study aims to reveal the efficiency and mechanism of immobilizing uranium by natural and synthetic pyrite oxidation in a neutral aerobic environment,so as to provides a certain theoretical guidance for industrial production of neutral in-situ leaching of uranium.（1）Batch experiments on immobilizing uranium by natural and synthetic pyrite oxidation were carried out.The results showed that the oxidation of pyrite in an aerobic environment produced H⁺and SO₄^2-,resulting the acidity of the solution increased and the removal rate of U（VI）decreased to 21.70%.Adding Tris as a buffer to maintain a stable neutral environment was beneficial for the high-efficiency immobilization of uranium by pyrite.The immobilization efficiency of uranium by pyrite under aerobic conditions was significantly higher than that of anaerobic conditions.Under aerobic conditions,the removal rate of U（VI）by oxidation of synthetic pyrite（99.55%）was higher than the removal rate of（VI）by natural pyrite oxidation（95.41%）.（2）In the presence of CO₂and O₂,the effect of oxidation of pyrite on the concentration of U（VI）in the solution under different conditions was studied.The results showed that under neutral aerobic conditions,the injection of CO₂promoted the dissolution of calcite,inhibiting the adsorption of uranium by pyrite oxidation and facilitating the leaching of uranium on the surface of minerals.（3）Acid dissolution experiments showed that uranium was dissolved in preference to iron.When[H⁺]≥0.1 M,the dissolution reached equilibrium and about 84%and 63%of uranium in natural and synthetic pyrite precipitates were dissolved and released,respectively.In both cases,only about 2%of Fe was dissolved.The dissolution and release process of uranium in the precipitate mainly included two stages:1)before the dissolution of iron oxides,5%and 16%of uranium corresponding to natural/synthetic pyrite were released firstly;2)79%and 47%of uranium was gradually dissolved with～2%of iron.（4）Characterization analysis showed that under neutral aerobic conditions,uranium was successfully immobilized on the surface of the precipitate as hexavalent uranium,and part of U（VI）reduced to U（IV）by synthetic pyrite.Pyrite was oxidized to form a small amount of goethite,lepidocrocite and ferric hydroxide colloid,and its crystal structure would not be destroyed.The immobilization mechanism of pyrite to uranium was mainly the adsorption and co-precipitation of pyrite and iron（hydroxide）oxides formed by oxidation of uranium in solution.Synthesis pyrite had the reduction effect on uranium partly.The adsorption behavior of pyrite to U（VI）conformed to the quasi-secondary adsorption model,which indicated that the reaction was mainly chemical adsorption.In summary,pyrite has strong stability and efficiency,and can be used as a long-term adsorption material to treat low-concentration uranium-containing wastewater.The research results have certain theoretical guiding significance for the improvement of the uranium leaching concentration and the shortening of the long leaching cycle in the industrial production of neutral in-situ leaching of uranium.