Fabrication And Functionalization Of Geopolymer Microspheres For Dye Adsorption

The continuous developments of modern industry and agriculture have greatly promoted social and economic progresses,but there are lots of pollutants released into the environment,causing water pollution.The discharge of synthetic dyes into water could affect the growth of aquatic organisms,with mutagenic and carcinogenic risks.Dyes have seriously threats to the survival environment of human beings and aquatic organisms.Adsorption is one of the common methods for treating dye wastewater,but it is limited by the high cost of adsorbents,difficult to recover and regenerate.So it is necessary to develop green,environmental,low-cost,renewable and continuous processing adsorbents.In this paper,slag-based geopolymer microspheres(SM)and metakaolin-based geopolymer microspheres(MM)were prepared,and their adsorption behavior and mechanism to CV and MB were studied.Moreover,in order to solve the problem of poor removal of anionic azo dyes by geopolymers,co-immobilization of laccase and ABTS onto MM were synthesized.The optimum loading conditions,immobilized enzyme properties,and removal of the antiazo dye CR were studied.The main conclusions are as follows:(1)The slag-based geopolymer microspheres(SM)was prepared using dispersion suspension solidification method with sodium hydroxide as alkali activator and slag as the raw material,that the particle size ranged from 100 to900μm,the specific surface area was 13.38 m~2/g,and the average pore diameter was 20.95 nm.The adsorption process of CV and MB onto SM could be described by the pseudo-first-order model and Langmuir adsorption isotherm model.The maximum theoretical adsorption capacity of CV and MB onto GM was 34.94 and 45.31 mg/g,respectively.The adsorption processes of CV and MB by GM were spontaneous and endothermic processes.The increase of temperature has positive influence on the reactions.After 6 times of cycles of SM,the removal rate of CV(MB)by SM can still reach 86.45(81.56)%.Due to electrostatic interaction,SM had selective adsorption ability to cationic dyes.(2)The metakaolin-based geopolymer microspheres(MM)was prepared using dispersion suspension solidification method with modified water glass as alkali activator and metakaolin as the raw material,that the particle size ranged from 100-180μm and 280-450μm,the specific surface area was 64.87 m~2/g,the average pore diameter was 8.77 nm,and the isoelectric point was 3.5.The adsorption processes of CV and MB onto MM more fit to pseudo-first-order model.Found by the fitting of intra-particle diffusion model,the process of adsorption has intra-particle diffusion.However,the rate of diffusion is affected by other factors.The absorption processes of MM to CV and MB are Langmuir models with maximum adsorption capacity onto CV and MB were 88.34 and100.1 mg/g,better than the main geopolymer adsorbents reported so far.The adsorption processes of CV and MB by MM were spontaneous and endothermic processes.The increase of temperature has positive influence on the reactions.After 6 times of cycles of MM,the removal rate of CV(MB)by MM can still reach 75.45(81.56)%.Since the adsorption of MM on CV and MB includes electrostatic interaction,hydrogen bond,and n-πinteraction,MM had better selective adsorption ability to cationic dyes.In the processing of dynamic adsorption,the experimental results show that low flow rate,high bed height,small particle size and low initial concentration are beneficial to extend adsorption penetration time,which could be described by the Thomas and Yoon-Nelson models.(3)The immobilized Lac-ABTS@MM was obtained using physical adsorption with co-immobilizing laccase and ABTS on MM.The optimum conditions were:0.5 mmol/L ABTS concentration,12 h ABTS adsorption time,4.0 p H buffer,1 g/L laccase concentration,and 6 h enzyme loading time.Lac-ABTS@MM was optimized with p H=4.5 and temperature 40℃,and the p H stability,thermal stability and operational stability of laccase were significantly improved.The removal rates of Lac-ABTS@MM were 86.54%,72.70%and 49.91%higher than MM、Laccase and Lac@MM,and realize synchronous double loop of Lac and ABTS.