Recycling Of Organic Modified Montmorillonite And Its Adsorption/Desorption Behavior Of PPCPs

Aspirin,diclofenac and sulfa antibiotics are common used substances in pharmaceuticals and personal care products（PPCPs）.These substances are difficult to be removed in sewage treatment plants due to their unbiodegraded and untransformed characteristics.The high efficient removal technology of PPCPs thus becomes the hot issue in environmental field.The clay adsorption is a low-cost,high operable and feasible PPCPs treatment method.Among the clay minerals,montmorillonite possesses the property of rich,low price,high specific surface area,and good adsorption removal efficiency.Because of the surface negative charge,it is favorable in adsorption of cationic PPCPs,and unfavorable in adsorption of acidic diclofenac（DFC）and amphoteric sulfamethoxazole（SMX）.To solve the mentioned problems,the cationic and amphoteric surfactants were selected to be organically modified on the structure of montmorillonite.The organically modified montmorillonite is then applied for increasing the adsorption capacity of DFC and SMX.Addtionally,the methanol,ethanol,and water were chosen as the desorption reagents for exploring the regeneration of modified montmorillonite.The results obtained in this study are as follows:（1）The cation exchange capacity（CEC）of montmorillonite measured by ammonium chloride-ethanol method was 0.96 mmol/g,and montmorillonite was modified with cetyl trimethyl ammonium bromide and dodecyl dimethyl betaine.After organic modification,it was characterized by various characterization equipments such as XRD,FTIR,BET and Zeta potential instruments.The results showed that the specific surface area reduced,the characteristic diffraction peaks of montmorillonite gradually became smaller or disappeared after modification.The surfactant molecules partially entered the montmorillonite layer,and the arrangement was regular and highly ordered.With the increase of the ratio,the intensity of the diffraction peaks increased,but at2θ=36.1°and 61.9°,the characteristic peaks did not change significantly,maintaining the original layered structure of montmorillonite.After modification,the zeta potential value showed a positive value,which confirmed that the surface of the adsorbent had a positive charge due to the modification,which also showed that the adsorption of DFC and SMX was mainly by electrostatic force and Van der Waals force.The adsorption capacity increased significantly when comparison to the montmorillonite without modification.（2）The DFC and SMX saturated adsorption capacity of montmorillonite modified by cationic surfactant can be increased from 22.3 mg/kg,28.3 mg/kg to 16904.19 mg/kg,431.59 mg/kg,respectively while the DFC and SMX saturated adsorption capacity of montmorillonite modified by active agent can be increased to 217.99 mg/kg and 170.69mg/kg,respectively.The adsorption kinetics and isotherms results indicated that pseudo first order model can describe the kinetics well while.Langmuir equation was consistent with the adsorption of DFC;Langmuir and Freundlich equations were both consistent with the adsorption of SMX.Different environmental factors（pH,ionic strength）also affected the adsorption efficiency of DFC and SMX.Under acidic and neutral conditions,the cationic surfactant modified montmorillonite had the better adsorption efficiency on DFC and SMX.When K⁺and Ca²⁺were added,adsorption would be promoted.The amphoteric surfactant-modified montmorillonite would be less affected with coexisting cation in adsorbing DFC,but cation would promote its adsorption to SMX.（3）The regenerative experiment results of organically modified montmorillonite showed that after modification,montmorillonite possessed good regeneration characteristics in adsorbing DFC and SMX.After 4 cycles of adsorption/desorption,the removal efficiency of DFC and SMX adsorbed by active agent-modified montmorillonite was maintained above 40%.Our results also found that methanol and ethanol could be the ideal desorption reagent,with the desorption efficiency≥65%.The results obtained here were significant for guiding the modification of clay minerals,understanding adsorption mechanism of PPCPs,and regenerability of adsorbents.Importantly,this study can provide theoretical basis and technical support for efficient and rapid removal of PPCPs in water bodies.