Adsorption Behavior Of Montmorillonite And Sodium Alginate Based Adsorbents For Cs⁺ And Sr²⁺

The application of nuclear energy in military,energy,industry,aerospace and other fields has brought considerable social and economic benefits to mankind.Production of nuclear energy generates radioactive waste liquid.If the nuclear waste liquid was stored unreasonably or the nuclear power plant encounters natural disasters such as earthquakes and tsunamis,it will cause the leakage of nuclear waste liquid,which will bring great harm to the environment and human health.Therefore,how to properly settle the safety problems associated with nuclear energy has become a major concern in industry and society.In the nuclear waste liquid,radioactive strontium ⁹⁰Sr and radioactive cesium¹³⁴Cs and ¹³⁷Cs are high-risk radioactive pollutants.On the other hand,the stable isotopes of cesium and strontium and its derivatives are also important metal resources.Effective recovery of cesium and strontium from nuclear waste not only protects environment from radioactive contaminations but also profits industry from nuclear wastes.The dissertation focuses on the preparations and characterizations of new adsorbentsas for cesium ion and strontium ion,and investigations on their adsorption capacities,mechanism and regenerations.Details are presented asfollow:Based on magnetized montmorillonite（Mt）and embedded potassium copper ferricyanide,a low-cost,magnetically recyclable and superior composite adsorbent was prepared.The synthetic route includes:（ⅰ）Interlayer ion exchange between ferrous ions and montmorillonite followed by low temperature hydrothermal synthesis to form magnetic montmorillonite（Mt-Mag）;（ⅱ）Coating the magnetized Mt with polydopamine to complex it with copper ions,which was subsequently reacted with the hexacyanoferrite precursor to in situ grow potassium hexacyanoferrite-copper nanoparticles（HCF）to form the polydopamine-coated magnetic montmorillonite-copper hexacyanoferrate composite adsorbent（D-Mt-Mag-HCF）.The characterization by Fourier transform infrared,powder X-ray diffraction,and X-ray photoelectron spectroscopy confirmed the synthesis of the magnetic Mt-KCu HCF composite adsorbent and the presence of KCu HCF in the adsorbent.The cesium adsorption capacity of D-Mt-Mag-HCF was evaluated by adsorption experiments.The adsorption data were fitted by Langmuir adsorption model.The results showed that the adsorbent had excellent Cs⁺adsorption capacity（～159.2 mg/g）with an adsorption rate of 0.01 g mg^-1min^-1 determined by adsorption kinetics experiments.The adsorption process of cesium ions follows second-order kinetics,which is dominated by the chemical adsorption.The adsorbent has excellent selectivity for Cs⁺with Cs⁺selectivity greater than 8.2×10⁴ mL/g in concentrated brine,which can be attributed to the immobilized KCu HCF nanoparticles.The cubic lattice of KCu HCF acts as an ion sieve,which preferentially adsorbs Cs⁺and excludes ions with higher dynamic ionic radius including K⁺.Within a certain range,pH and temperature have little effect on the adsorption ability.What’s more,the magnetic composite adsorbent with magnetization and coercivity as high as 17.4 emu/g and 9.06 Oe,respectively,can be quickly recovered by an external magnetic field.After recovery,the composite material is recycled through redox reaction,and the regenerated adsorption capacity is 67.18 mg/g.The studies demonstrated that as a novel composite adsorbent D-Mt-Mag-HCF has the advantages of ease of use,environmental friendliness,good mechanical properties,potential radiation resistance and low cost.The composite adsorbent is promosing to be applied in the treatment of nuclear wastewater.Based on thiol group as the adsorption group and biomass material sodium alginate as the adsorbent matrix,a Sr²⁺adsorbent with low price,environmental friendliness and excellent performance was synthesized.The synthetic route includes:（ⅰ）Preparation of sodium alginate-strontium ion hydrogel by emulsion method;（ⅱ）Synthesis of thiol-rich carboxypropyl grafted pentaerythritol tetrakis（thioglycolic acid）ester（PA）by click chemistry and grafting of PA on sodium alginate-strontium ion hydrogel to synthesize SA-PA-Sr gel;（ⅲ）SA-PA-Sr gel was washed by acid to synthesize Sr²⁺-imprinted SA-PA-H adsorbent.The products were characterized by FTIR and NMR to confirm the synthesis of PA and SA-PA-H.The strontium adsorption capacity of SA-PA-H was evaluated by adsorption experiments.The data were fitted by Langmuir adsorption model.The adsorbent had excellent Sr²⁺adsorption capacity（～151.7 mg/g）.The adsorption process followed second-order kinetics,dominated by the chemical adsorption rate.The adsorption rate was 0.669 g mg^-1min^-1,and the adsorption equilibrium was almost reached in 30 min.The adsorbent has excellent selectivity for Sr²⁺,and the Sr²⁺selectivity in concentrated brine is greater than 1.97×10² mL/g,which is because of the exsistence of thiol group ion imprint by strontium.The strontium ion imprint form cavity in the adsorbent based on coordination geometry,charge and size of Sr²⁺,which selectively identify Sr²⁺.Meanwhile,the thiol preferentially adsorb strontium ions in the presence of competing‘hard’ions according to hard and soft Lewis acids and bases（HSAB）principle.Within a certain range of pH and temperature,the adsorbent is robust to adsorb Sr²⁺.Moreover,the adsorbent can be recycled and reused by centrifugation.After two consecutive adsorption-desorption cycles,the adsorption efficiency is only reduced by 3.33%,which demonstrate the high stability of the adsorbent.Studies have shown that SA-PA-H has the advantages of low cost,radiation resistance and good stability,and is expected to be applied to practical nuclear wastewater treatment.