Study On Adsorption Of Simulated ¹³⁷Cs By Geopolymer Foams

Geopolymer is a cementitious material with a three-dimensional network structure,which is a highly promising absorbent due to its special zeolite-like pore structure.Particularly,alkali metal ions are the main charge balancing ions for geopolymer networks.Thus geopolymer is naturally advantageous to be utilized as an adsorbent for separating nuclide ¹³⁷Cs from the radioactive nuclear waste liquid.Nonetheless,poor permeability due to the macroscopic dense structure of geopolymer seriously affects the mass transfer rate of the adsorption process.In this study,we proposed to prepare geopolymer foams（GF）with hierarchical pore structure by physical foaming method,and investigated the static and dynamic adsorption performance and desorption-regeneration performance of the geopolymer foams for Cs⁺.Furthermore,we prepared a foam zeolite material derived from geopolymer foams（GFZ）by hydrothermal treatment,the static/dynamic adsorption performance and desorption-regeneration performance of the GFZ for Cs⁺were also investigated.All these above can provide theoretical support for the subsequent practical application of the geopolymer foam as an integrated Cs⁺filter/adsorption/immobilization device.（1）GF with each density level,hierarchical pore structure and proper mechanical performance were prepared by physical foaming method.The porosity of the 600 kg/m³GF was 68.4%and the compressive strength reached 3.72 MPa.As the densify level of GF decreased from 600 kg/m³to 300 kg/m³,the porosity increased from 68.4%to 83.8%.The higher adsorption capcity was reached with higher contents of geopolymer gel matrix in higher density level GF,in contrast,the lower adsorption capcity and higher permeability were obtained.（2）GF with a density level of 600 kg/m³was used to study the adsorption,desorption and regeneration performances of Cs⁺.It was found that the adsorption capacity of Cs⁺was up to 192.12 mg/g.The static adsorption behaviour was in accordance with the Pseudo-secondary kinetic model（R²=0.9627）and the Dubinin-Radushkevich model（R²=0.9949）,and the dynamic adsorption behaviour was in accordance with the Thomas model（R²=0.9848）.The adsorption mechanism was a coexistence of physical and chemical adsorption,and the chemical adsorption dominated the adsorption process.The second regeneration of Cs⁺for GF by desorption of 0.05 M EDTA-2Na solution reached 41.98%.（3）Effects of hydrothermal time,hydrothermal temperature and alkalinity of the hydrothermal medium on the crystallisation behaviour of the GF were investigated.GFZ with P-type zeolite structures as the main crystalline phase was obtained after hydrothermal treatment at 180°C for 12 h in 0.4 M Na OH solution.After the hydrothermal treatment,the pore structure of the GF was refined and the micro-mesoporous pores was mainly distributed in the pore size range of 1.766 nm-28.969 nm.GFZs were used to study the adsorption,desorption and regeneration performances of Cs⁺.It was found that the adsorption capacity of GFZ for Cs⁺was up to 88.27 mg/g.The static adsorption behaviour was in accordance with the Pseudo-secondary kinetic model（R²=0.9837）and the Langmuir model（R²=0.9808）,and the dynamic adsorption behaviour was fitted to the Bed Depth Service Time model（R²=0.9985）,and the adsorption mechanism was an ion-exchange based chemisorption process,with the fourth regeneration rate of Cs⁺was 62.13%by desorption of 0.05 M EDTA-2Na.