Enhanced Adsorption Characteristics Of Steroid Estrogen By The Hydrophobically Modified Mesoporous Silica In Aqueous Solution

The most concerned steroid estrogens:estrone（E1）,17β-estradiol（E2）,and17α-ethinyl estradiol（EE2）,are considered as the high-risk endocrine disrupting chemicals to be removed from water and wastewater,due to their high estrogenic activity,which cause the adverse effects on the aquatic wildlife at the ng/L level.Adsorption as one of the reliable techniques of water pollution treatment shows great application potential for estrogen removal,due to the techno-economic feasibility,easy operation,and high removal efficiency.However,the reported adsorbents for estrogen removal had the problems like high adsorbent dosage,limited sorption capacity,slow pollutant uptake rate and the interference of the coexisted macromolecule.The development of the special adsorbents based on the physio-chemical characteristics of estrogens would be an effective approach to improve estrogen removal.Mesoporous silica materials have been widely studied as efficient adsorbents for trace pollutants adsorption due to their unique ordered pore structure,but their high hydrophilicity limits their application for estrogen removal.Therefore,the organic functionalized mesoporous silica materials were prepared based on physio-chemical properties of E1,E2,and EE2 in order to have the special adsorption capacities towards estrogens and utilize the pore structure of mesoporous silica materials.Based on microscopic characterization of materials and scientific adsorption experiments,effects of influence factors,adsorption mechanism,and application for estrogens removal in real water were explored.As E1,E2,and EE2 are neutral pollutants and moderate hydrophobicity(Log K_ow:3.43-4.12)with precursor structure of cyclopentanoperhydrophenanthrene,the phenyl functionalization and propyl functionalization were applied to mesoporous silica materials by one-pot co-condensation method.For the phenyl modified mesoporous silica materials,the different hydrolysable groups of trimethoxyphenylsilane and triethoxyphenylsilane led to the discrepancies in hydrolysis rates in the material synthesis,and resulted in the more hydrophobicity of the triethoxyphenylsilane-modified meso-silica than that of the trimethoxyphenylsilane-modified one.Nevertheless,the less available adsorption sites of the triethoxyphenylsilane-modified meso-silica led to the lower removal rates of estrogens than that of the trimethoxyphenylsilane-modified one.For the propyl modified meso-silica materials,the n-propyl modified meso-silica had higher estrogen removals than the thiopropyl modified one due to the more hydrophobic adsorption sites.Although the phenyl or propyl modification enhanced mesoporous silica to have higher estrogen adsorption,the enhancement was related to the usage of organosilanes.Based on the evaluated removal efficiencies of E1,E2,and EE2 by the hydrophobic functionalized meso-silica materials,effects of the factors were investigated.Acidic and neutral solutions（pH=3.0-7.0）benefited estrogen removal.The removal rates of E1,E2,and EE2 by the phenyl modified meso-silica were about 50%,54%,and 58%respectively,while the removal rates of E1,E2,and EE2 by the n-propyl modified one were about 75%,69%and 80%respectively.Alkaline solution,which induced estrogen deprotonation and negative surface charge of the functionalized meso-silica materials,inhibited estrogen adsorption by electrostatic repulsion and the decreased hydrophobic interaction.Ionic strength（0-0.1 M NaCl）,monovalent cation（0.1 M）,humic acid,and glucose（0-10 mg/L）did not affect E1,E2,and EE2 adsorption significantly,but the presence of Ca²⁺and Mg²⁺（0.1 M）inhibited estrogen removal.To reveal the adsorption mechanism,the adsorption characters were studied scientifically.The adsorption processes of E1,E2,and EE2 by the functionalized meso-silica materials were fitted the pseudo-second order model well.Although equilibria was achieved in 30 min for E1,E2,and EE2 by the phenyl modified meso-silica materials at adsorbent dosage of 0.025 g/L,the triethoxyphenylsilane-modified meso-silica exhibited higher estrogen adsorption rates than the trimethoxyphenylsilane-modified one due to the more hydrophobicity.The equilibrium time of E1,E2,and EE2 were 15,30,and 15 min respectively by the n-propyl modified meso-silica at the same adsorbent dosage,and the adsorption rates had a positive correlation with the adsorbents’hydrophobicity.Additionally,the intra-particle diffusion was not the only rate-limiting step.According to the Langmuir model,the maximum adsorption capacities of E1,E2,and EE2 by the phenyl modified meso-silica were 53.60,83.47,and 99.02 mg/g respectively at 25℃,while the maximum adsorption capacities of E1,E2,and EE2 by the n-propyl modified one were88.38,86.91,and 119.87 mg/g respectively.The thermodynamics analysis revealed that estrogen adsorption on the functionalized meso-silica materials was physical one with spontaneous,exothermic,entropy-driven,and the adsorption amount decreased with the increased temperature.The adsorption forces between estrogens and the phenyl modified meso-silica were mainlyπ-πeffect,hydrophobic effect,hydrogen-bond and dipole-dipole interactions,while the hydrophobic effect,hydrogen-bond and dipole-dipole interaction were considered as the predominant adsorption mechanism between estrogens and the n-propyl modified meso-silica.The estrogen removal was influenced by the coexisted multiple background constituents in actual water samples,and the inhibition rates of the maximum adsorption capacities were 30-50%and 40-60%respectively for the phenyl modified meso-silica and the n-propyl modified one.When E1,E2,and EE2 were coexisted in actual water samples,the competitive interactions were occurred on the hydrophobic functionalized meso-silica materials and led to the decreased adsorption amounts.The removal rates of estrogens with initial concentration of 100 ng/L in water samples by the phenyl modified meso-silica and the n-propyl modified one were above 60%and 80%respectively with the equilibrium time of 15 min.Considering the adsorbent dosage and estrogen removal,the phenyl modified meso-silica were suitable for trace E1,E2,and EE2 removal in tap water and river,while the n-propyl modified one was efficient in removal of trace E1,E2,and EE2 in the tap water,river,and secondary effluent of sewage treatment works.Based on the properties of estrogens,the two hydrophobic functionalized meso-silica materials were synthesized,and showed the selected removal towards estrogens in a short equilibrium time with high adsorption capacities at low adsorbent dosage,and successfully applied to the real water for estrogen removal.The results of this study extended the application of meso-silica to trace pollutants control,and also provided a new technique choice for estrogen removal in aquatic matrices.