Preparation Of Chitosan Modified Bentonite And Their Adsorption Performance On Tetracycline

China is a big country in the production and use of tetracycline,and wastewater such as pharmaceutical wastewater contains a large amount of tetracycline,posing a huge threat to human health and the ecological environment.In recent years,adsorption method has been widely used in the treatment of tetracycline wastewater.Bentonite is a high-quality natural adsorbent with low price and good ion-exchange performance,which has the potential to be used in tetracycline wastewater treatment.However,due to its hydrophilicity and swelling properties,it is difficult to separate solid-liquid after adsorption,and it is difficult to avoid the loss of adsorbent during the recovery process,which limits its application in tetracycline wastewater treatment.Therefore,the development of a spherical bentonite adsorbent with simple solid-liquid separation after adsorption and no loss in the recovery process is of great significance for the treatment of tetracycline wastewater.In this study,natural calcium-based bentonite was modified by Na₂CO₃ and chitosan,and the modified bentonite was further prepared into spherical particles by"sol-gel method",and the preparation process was optimized.The structure of the prepared two adsorbents was characterized by means of scanning electron microscopy,X-ray diffraction,Fourier transform infrared absorption spectroscopy,thermogravimetric analysis and BET surface analysis.The influencing factors and mechanism of adsorption of tetracycline in simulated wastewater by chitosan modified bentonite and spherical chitosan modified bentonite were systematically studied.In this study,Na₂CO₃ and chitosan were used as modifiers,and chitosan modified bentonite was synthesized by co-precipitation method.When the content of Na₂CO₃ is 4%（weight of soil）and the mass ratio of chitosan to bentonite is 1:5,the modification effect is the best.Various characterization results showed that the chitosan successfully entered the bentonite layers,which increased the interlayer spacing,but did not change the layered mesoporous structure of the bentonite.The-CH₃ and-CH₂ groups of chitosan appeared on the bentonite modified by chitosan.The static adsorption experiment was carried out with tetracycline as the target pollutant.Under the optimal conditions,the adsorption capacity was 19.32 mg/g.The adsorption process was more in line with the pseudo-second-order kinetic model and the Frenudlich model,indicating that the adsorption process was a non-monolayer chemisorption process.The maximum monolayer adsorption capacity obtained by the Langmiur model was 119.46 mg/g.Thermodynamic parameters indicate that the adsorption process is a spontaneous,endothermic,entropy-increasing process.On the basis of chitosan-modified bentonite,this study further adopted"sol-gel method"to prepare spherical chitosan-modified bentonite.When the mass ratio of chitosan and bentonite was 1:50,the concentration of sodium alginate was 1%（w/v）,the concentration of CaCl₂ solution was 2%（w/v）,and the spherical adsorbent prepared by vacuum freeze drying for 24 hours,the adsorption effect of the spherical adsorbent is the best.Various characterization results show that the spherical adsorbent has fewer pores on the surface,and the specific surface area is 96.6%lower than that of the powder,but the layered mesoporous structure of the bentonite is not changed.The-COOH group of sodium alginate appeared in the spherical chitosan modified bentonite.The static adsorption experiment was carried out with tetracycline as the target pollutant.Under the optimal conditions,the adsorption capacity was 17.91 mg/g.The adsorption process also conforms to the pseudo-second-order kinetic model and the Frenudlich model,indicating that the adsorption process is a non-monolayer chemical adsorption process,and the influence of intra-particle diffusion on the adsorption rate is higher than that of powder.The maximum monolayer adsorption capacity obtained by the Langmiur model was 34.55mg/g.Thermodynamic parameters indicate that the adsorption process is also a spontaneous,endothermic,entropy-increasing process.Spherical adsorbent is simple for solid-liquid separation and will not be lost.It has a broad application prospect in wastewater treatment.There are some differences in the adsorption process of tetracycline between chitosan modified bentonite and spherical chitosan modified bentonite.