Preparation And Performance Of Silicon-based Metal Ferrocyanide Cesium Adsorbents

Cesium is an extremely important rare alkali metal resource.In recent years,with the continuous deepening of scientific research and technological development,cesium and its compounds have been widely used in optical fiber communications,catalysts,medical treatment and energy.Cesium salt used as a co-catalyst in the sulfuric acid production industry can improve the conversion efficiency of SO₂,reduce the ignition temperature and the concentration of SO₂ in the tail gas.There is abundant cesium reserve in the Salt Lake brine in Tibet and Qinghai of China needs to be exploited.In addition,the radioactive waste liquid produced by nuclear fission from nuclear power plants contains a large amount of radioactive cesium elements such as ¹³⁷Cs,which poses a threat to the environment and biological safety.Therefore,it is of great importance to carry out research on cesium separation technology.In this thesis,non-toxic and inexpensive diatomite was used as raw material instead of the traditional silicon source,and mesoporous silica（Al-MCM-41 and SBA-15）prepared by hydrothermal method are used as carriers.KMe FC/Al-MCM-41 and KZn FC/SBA-15 adsorbents were prepared with metal ferrocyanide(KMe FC;Me=Cu²⁺,Ni²⁺,Zn²⁺,Co²⁺,Fe³⁺)as the effective adsorption component.The structure and performance of the adsorbents were systematically characterized.The effect of different adsorption conditions on the adsorption of cesium ions（Cs⁺）by adsorbents were investigated.The related adsorption kinetics,isotherms and thermodynamic properties were analyzed,and the main adsorption mechanism for Cs⁺was analyzed in combination with instrument characterization.The detailed research content and results are as follows:1.Preparation of KMe FC/Al-MCM-41 adsorbents and its adsorption for Cs⁺The technic conditions for preparing Al-MCM-41 with good mesoporous order,high specific surface area(1190.3 m²·g^-1)and pore volume(1.12 cm³·g^-1)using diatomite as raw materials were explored.The results of ²⁷Al-NMR showed that Al is mainly in the structure of Al-MCM-41.WAXS and FT-IR showed that KMe FC/Al-MCM-41 were successfully prepared.All adsorbents can reach almost adsorption equilibrium for Cs⁺in 15 mins,which conform to the quasi-second-order adsorption kinetic model.The adsorption capacity of adsorbents for Cs⁺is KZn FC/Al-MCM-41(1.1602 mmol·g^-1)>KNi FC/Al-MCM-41(1.0920 mmol·g^-1)>KCo FC/Al-MCM-41(0.8174 mmol·g^-1)>KCu FC/Al-MCM-41(0.6973 mmol·g^-1)>KFe FC/Al-MCM-41(0.4541 mmol·g^-1)>Al-MCM-41(0.2641 mmol·g^-1).All adsorbents conform to the Langmiur adsorption isotherm model.The Cs⁺adsorption capacity of Al-MCM-41prepared from diatomite is 2.34 times that of pure silicon MCM-41.It is due to the Al doping that makes the framework of Al-MCM-41 show negatively electricity and easier to trap Cs⁺.KMe FC/Al-MCM-41 can keep relatively stable adsorption performance for Cs⁺at p H=4～11 and in the presence of competitive cations.Moreover,the adsorption process are spontaneous and endothermic.The adsorption of Cs⁺by KZn FC/Al-MCM-41 is dominated by chemical adsorption,mainly due to the ion exchange between K⁺in the KZn FC structure and Cs⁺in the solution.2.Preparation of KZn FC/SBA-15 adsorbents and its adsorption for Cs⁺The mesoporous silica SBA-15 with good mesoporous order,high specific surface area(864.2 m²·g^-1)and pore volume(1.36 cm³·g^-1)was prepared using diatomite as raw material.KZn FC-Ⅰ/SBA-15 and KZn FC-Ⅱ/SBA-15 adsorbents were prepared by taking SBA-15 as a carrier to change the order of Zn SO₄ solution and K₄Fe（CN）₆solution addition sequence.Combining the results of adsorption experiments and chemical composition analysis,KZn FC-I/SBA-15 has better adsorption performance,mainly because when the content of KZn FC in the two adsorbents is the same,the content of K⁺that can exchange with Cs⁺in KZn FC-Ⅰ/SBA-15 is relatively higher.When the KZn FC content in KZn FC/SBA-15 is 45.6%,the Cs⁺adsorption capacity can reach 1.4938 mmol·L^-1.The adsorption rate and distribution coefficient of KZn FC/SBA-15 to Cs⁺is much larger than Li⁺,Na⁺,Mg²⁺and Ca²⁺.Under the same initial concentration conditions,the adsorption selectivity order of the adsorbent for each ion is Cs⁺>Na⁺>Ca²⁺>Mg²⁺>Li⁺,indicating that the adsorbent has good selectivity to Cs⁺.However,when there is a large amount of K⁺in the solution,it will have a greater impact on the adsorption of Cs⁺by the adsorbent.In the simulated salt lake brine with high salinity,the adsorbent still maintains an adsorption rate of 72.5%for Cs⁺.0.5 mol·L^-1 NH₄Cl solution was used to desorb KZn FC/SBA-15 after saturated adsorption of Cs⁺,the maximum single desorption rate is 67.4%.