MCM-41 Synthesized By Diatomite And Study On Adsorption Performance For Phosphate After Modification

The excessive presence of phosphate in water bodies often triggers eutrophication,and it is imperative to control phosphate emissions.The adsorption process has attracted much attention due to its simple operation,high removal efficiency and fast adsorption rate,and the research on environmentally-friendly and efficient adsorbents for phosphate removal has become a hot topic.In this study,MCM-41 mesoporous molecular sieves were prepared from natural diatomite,then two kinds of adsorbent were synthesized by loading lanthanum oxide and magnesium oxide on the surface of MCM-41,the phosphate adsorption performance and mechanism on two adsorbents was studied systematically.The main results are listed as follows:Taking natural diatomite as the raw material,hexadecyltrimethylammonium bromide（CTAB）as the template,the hexagonally ordered mesoporous material was prepared by the hydrothermal method,characterization of adsorbents by X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）,Transmission electron microscopy（TEM）and N₂adsorption-desorption measurements.The results showed that the material had a highly ordered mesoporous structure when CTAB/SiO₂ molar ratio was 0.4,its specific surface area from BET measurements was 815.42 m²/g,the corresponding pore sizes was 4.32 nm,and the total pore volume amounted to 0.82 cm³/g.La₂O₃-modified MCM-41 was synthesized with the optimum La/SiO₂ molar ratio of 0.2.La_0.2M41wascharacterizedbyX-raydiffraction（XRD）,Transmissionelectron microscopy（TEM）and N₂ adsorption-desorption measurements.Characterization results indicated that mesopores with long range order was preserved after La₂O₃ was introduced into MCM-41.However,hexagonal arrays of mesopores was decreased with La₂O₃ in the mesoporous structure.The influences of contact time,initial pH,amount of adsorbent and coexisting anions on the adsorption effect were explored.The results of adsorption experiments showed that adsorption process of phosphate by La_0.2M41 was quite fast,the removal efficiency was high,and the removal rate can reach 97.56%when the adsorbent dosage was 0.08 g.High phosphate adsorption capacities were achieved within the pH range of 3.0 and 6.0.In addition,phosphate removal at different initial pH values and pH change after phosphate adsorption indicated that ligand exchange and electrostatic attraction are the main mechanisms for phosphate removal by adsorbent.The influence of anion on phosphate adsorption followed the order:CO₃^2->F^-?>NO₃^->?Cl^->SO₄^2-.Adsorption kinetics and thermodynamic are studied.It is concluded that the adsorption kinetics followed the pseudo-second-order kinetic model,the Langmuir isothermal model well described the adsorption isotherm data,showing a maximum adsorption capacity for phosphate up to263.16 mg/g.The regeneration experiment showed that the phosphate resolution of the adsorbent was as high as 92.68%,indicating that La_0.2M41 possessed excellent regeneration ability.MgO-modified MCM-41 was prepared,and the optimum modification concentration of MgCl was determined to be 1.5 mol/L.Mg_1.5M41 was characterized by X-ray diffraction（XRD）,Transmission electron microscopy（TEM）and N₂ adsorption-desorption measurements,characterization results indicated that mesopores with long range order was decreased significantly after MgO was introduced into MCM-41.The influences of contact time,initial pH,amount of adsorbent and coexisting anions on the adsorption effect were investigated.The results of adsorption experiments showed that Mg_1.5M41 still had an excellent performance in adsorbing phosphate,and the phosphate removal rate can reach98.25%when the adsorbent dosage was 0.12 g.The adsorption capacity was higher in the range of pH 5.0-8.0,physical adsorption and electrostatic forces are the main mechanisms for phosphate remove by adsorbents.Cl^-,NO₃^-and F^-had little effect on phosphate removal,but CO₃^2-and SO₄^2-had greater influence.Adsorption kinetics and thermodynamic are studied.It is concluded that adsorption of phosphate was also well described by both the Langmuir isotherm and pseudo second-order models,showing a maximum adsorption capacity for phosphate up to 250 mg/g.The regeneration experiment showed that the resolution rate was as high as 90.08%,and the adsorbent regenerability was strong.