Study On The Removal Of⁷⁹Se,⁹⁹Tc And¹²⁹? From Radioactive Wastewater Using Modified Bentonite

With the widespread use of nuclear energy,a large amount of radioactive waste was produced,of which mainly contains long-lived fission products.In radioactive wastewater,⁷⁹Se,⁹⁹Tc and ¹²⁹I with high fission yield and long half-life,often exist in solution in the form of anions such as ⁷⁹SeO₃^2-/⁷⁹SeO₄^2-,⁹⁹TcO₄^-and ¹²⁹I^-/¹²⁹IO₃^-,and migrate easily.Once leaked,the above radionuclides will enter biosphere with groundwater circulation,accumulate in the human body,and pose a long-term harm to human health.Accordingly,the efficient removal of radionuclides from wastewater is of great concern,which is also a research hotspot for researchers.Due to its large specific surface area,high cation exchange capacity and abundant reserves,bentonite is usually used as adsorbent in the recovery of metal cations in wastewater.But it is difficult to enrich anion radionuclides because of its permanent negative charge surface.In this work,two kinds of modified bentonite were prepared by hydrothermal method and applied for the removal of anionic radionuclides ⁷⁹Se,⁹⁹Tc and ¹²⁹I from aqueous.The specific research results are as follows:1.The adsorption of ⁷⁹Se onto Fe-OOH modified bentonite?Fe-OOH-bent?.The batch experiment results show that Fe-OOH-bent has high adsorption capacities and fast adsorption kinetics for both Se?IV?and Se?VI?,which is better than most reported inorganic materials.Combined with experimental data,the adsorption mechanism was deduced as surface coordination reaction through XPS,FT-IR and other characterization techniques.Specifically,Se?IV?forms inner-sphere complexes and Se?VI?forms both inner-sphere and outer-sphere complexes.The reutilization of Fe-OOH-bent was proved to be well by adsorption-desorption cycle experiments.Furthermore,the column experimental results indicate that Se?IV?and Se?VI?can be separated and recovered effectively from simulate wastewater using Fe-OOH-bent,which is a potential material for efficiently and selectively removal of ⁷⁹Se.2.The adsorption of ⁹⁹Tc onto hexadecylpyridinium chloride modified bentonite?HDPy-bent?.The mechanism of HDPy⁺loading on bentonite was investigated by XPS,XRD,FT-IR and other characterization techniques.It was found that HDPy⁺was arranged into bentonite layers as different patterns with the increase of loading HDPy⁺.Batch experiments show that HDPy-bent had high adsorption capacity for Re?VII?/Tc?VII?and an extremely fast adsorption kinetics?3min?.The effect of co-existing matters on the adsorption of Re?VII?/Tc?VII?was found to be insignificant even the molarity of co-existing anions(Cl^-,NO₃^-,HCO₃^-,CO₃^2-,SO₄^2-and PO₄^3-)and humic acid?HA?were over 1000 times.The reasonable adsorption mechanisms of ion exchange and surface precipitation were proposed.The column tests show that Re?VII?/Tc?VII?could be not only effectively enriched from wastewater,but also be recovered by 2.5M NaNO₃ and 8M HNO₃ solution.Furthermore,the labeled recovery experiment results show that the HDPy-bent is an adsorbent material for efficient removal of ⁹⁹Tc.3.The adsorption of ¹²⁹I onto HDPy-bent.The effect of different conditions on adsorption of I^-and IO₃^-was investigated by batch experiments.The results show that HDPy-bent had extremely fast adsorption kinetics and high adsorption capacities both for I^-and IO₃^-.The adsorption abilities of HDPy-bent for I^-and IO₃^-were better than that of most inorganic and organic adsorbent materials reported.The presence of excess co-existing anions(Cl^-,HCO₃^-,SO₄^2-and PO₄^3-)and humic acid?HA?have no evident effect on adsorption of I^-and IO₃^-.The adsorption mechanism of I^-onto HDPy-bent was proved to be ion exchange at low I^-concentration,and surface precipitation at high I^-concentration.For IO₃^-,an adsorption mechanism of ion exchange reaction was proposed.The column experiments show that HDPy-bent could not only effectively enrich I^-and IO₃^-from wastewater,but also recover them with 5M HNO₃ solution.The labeled recovery experiment results for water samples show that HDPy-bent could be used as an adsorbent material for efficient removal of¹²⁹I from radioactive wastewater.