Preparation And Transformation Mechanism Of Different Geopolymer Zeolite （NaA/NaX/SOD） Microsheres And Their Adsorption Properties For Pb²⁺

Geopolymer is a new type of sustainable inorganic gel material,which has the advantages of high strength,strong corrosion resistance and good adsorption performance.Under certain preparation conditions,it can be converted into zeolite molecular sieve for water pollution treatment.Geopolymer zeolite microspheres（GZMs）combine the advantages of both,with better porosity and specific surface area,which is beneficial to increase the effect of removing heavy metal ions in wastewater.It is a low-cost,low energy consumption,high efficiency and environmentally friendly adsorbent,which is expected to realize its industrial application in wastewater treatment of heavy metal ions.In this paper,metakaolin,nano-silica and sodium hydroxide were used as raw materials,and different kinds of GZMs were prepared by dispersion-suspension-solidification technology.The structure and properties of terrestrial zeolite microspheres were analyzed by a variety of characterization methods,the transformation mechanism between different zeolite was systematically studied,and the adsorption performance of Pb²⁺was studied.（1）The effects of n（Na₂O）/n（Al₂O₃）and n（H₂O）/n（Na₂O）on the sphericity of GZMs were studied using metakaolin and sodium hydroxide as raw materials.The optimal ratios of n（Na₂O）/n（Al₂O₃）=1.9 and n（H₂O）/n（Na₂O）=12 were determined.The influence of curing temperature and time on the crystal type of GZMs were explored,and the transformation law between NaA and SOD zeolite was obtained.The optimal curing conditions of three kinds of zeolite microspheres were obtained,which were NaA zeolite microspheres（NaA ZMs:curing temperature 65℃,curing time 24 h）,sodium zeolite microspheres（SOD ZMs:curing temperature 85℃,curing time 72 h）and two kinds of mixed crystal zeolite microspheres（NaA+SOD ZMs:curing temperature 85℃,curing time 12 h）.The BET surface areas of the three kinds of zeolite microspheres were 35.91 m²/g（NaA ZMs）,55.72 m²/g（NaA+SOD ZMs）and 58.85 m²/g（SOD ZMs）,respectively.The pore volumes were 0.15 cm³/g（NaA ZMs）,0.20cm³/g（NaA+SOD ZMs）and 0.21 cm³/g（SOD ZMs）,respectively.The effects of n（Si O₂）/n（Al₂O₃）,n（H₂O）/n（Na₂O）,curing temperature and time on sphericity and crystal type of GZMs were investigated by adding nano-silica,and the optimum basicity was determined as n（H₂O）/n（Na₂O）=11.The transformation law between NaX and NaA zeolite was obtained.The optimum preparation parameters of two kinds of zeolite microspheres were obtained,which were mixed crystal zeolite microspheres（NaX+NaA ZMs:n（Si O₂）/n（Al₂O₃）=3.4,curing temperature 85℃,curing time 24 h）and NaX zeolite microspheres（NaX ZMs:n（Si O₂）/n（Al₂O₃）=3.8,curing temperature 105℃,curing time 6 h）.The BET surface areas of the two kinds of zeolite microspheres were 147.03m²/g（NaX+NaA ZMs）and 633.02 m²/g（NaX ZMs）,and the pore volumes were0.29 cm³/g（NaX+NaA ZMs）and 0.41 cm³/g（NaX ZMs）,respectively.（2）The adsorption performance of NaA,NaA+SOD and SOD geopolymer zeolite microspheres on Pb²⁺showed that:The maximum adsorption capacity of NaA ZMs,NaA+SOD ZMs and SOD ZMs to Pb²⁺was 529.67 mg/g,345.00mg/g and 308.30 mg/g,respectively.The adsorption processes of Pb²⁺by three kinds of zeolite microspheres were in accordance with pseudo-second-order kinetic model and Langmuir isotherm model,and the adsorption mechanism were mainly ion exchange.The experimental results of NaA ZMs adsorption thermodynamics for Pb²⁺show that the adsorption process was spontaneous endothermic.The results of selective adsorption experiments show that NaA ZMs has a strong selective adsorption capacity for Pb²⁺,but the presence of Cu²⁺has a certain effect on the adsorption effect of Pb²⁺,and the stability experiments confirm that NaA ZMs has a certain acid,alkali and high temperature resistance.（3）The results showed that the maximum adsorption capacity of NaX+NaA ZMs and NaX ZMs for Pb²⁺was 460.86 mg/g and 460.03 mg/g,respectively.The adsorption data of Pb²⁺by the two adsorbents conforms to the pseudo-second-order kinetic model and Langmuir isotherm model.The results showed that the adsorption mechanism of NaX+NaA ZMs on Pb²⁺was mainly ion exchange,and the adsorption mechanism of NaX ZMs on Pb²⁺was mainly ion exchange and physical adsorption.The adsorption thermodynamics experiment of NaX ZMs showed that the adsorption process was spontaneous endothermic.The results of selective adsorption experiments show that NaX ZMs has strong selective adsorption capacity for Pb²⁺.The stability test results showed that NaX ZMs has acid resistance,alkali resistance and high temperature resistance.