Efficiency And Mechanism Of Removing Phosphate From Water By Mineral-based Composites

With the rapid development of China's economy and the acceleration of urbanization,the discharge amount of phosphorus-containing wastewater has continued to increase,resulting in severe eutrophication of water bodies.The development of low-cost and efficient phosphate adsorbents is significant to protect the safety of the aquatic ecosystem and public health.Mineral materials are cheap,readily available,non-toxic and harmless,which are regarded as ideal water treatment adsorbent materials.However,the bottleneck problems of mineral materials are that they have few active adsorption sites and low adsorption capacity.In addition,the preparation conditions of mineral-based adsorbents targeting the efficient adsorption of phosphate pollutants in water are still unclear.The phosphate adsorption behavior of mineral-based adsorbents needs to be explored,and the mechanism of enhanced phosphate removal by materials should be analyzed.Aiming at solving the above problems,this project innovatively proposed new ideas for preparing mineral-based composites based on inexpensive palygorskite,periclase,hydrotalcite and lanthanum-based compounds.Lanthanum-loaded palygorskite-based adsorbent(PAL-La),periclase-based magnesium-lanthanum composite(PER-La),lanthanum-doped magnesiumaluminum hydrotalcite-like composite(LDH-La),and palygorskite and periclasebased hydrotalcite-like material(PP-LDH)were synthesized successively.Among them,PAL-La was based on the raw palygorskite with a unique nanorod crosslinked crystalline structure where lanthanum oxide was supported on the surface.After lanthanum loading,the palygorskite matrix was not blocked and the microporous volume was increased.PER-La was prepared based on cheap periclase and lanthanum-based compounds.Under the conditions of optimized urea dosing,PER-La obtained nano-needle-like morphology with the crystalline structure of Mg4(CO3)3(OH)2·3H2O and La CO3 OH.LDH-La was prepared by doping lanthanum-based compounds into magnesium-aluminum hydrotalcite,exhibiting a typical nano-grained microstructure.PP-LDH was prepared by acidification and urea hydrothermal method based on palygorskite and periclase under the optimized mixing ratios.PP-LDH demonstrated typical nano-sheet morphology of hydrotalcite-like material with crystal structure of Mg0.72Al0.28(CO3)0.15(OH)1.98·0.48H2 O.After being activated by calcination,its crystalline phase was transformed to magnesium-aluminum bimetal oxides.In order to determine the phosphate adsorption properties of mineral-based composites,this project examined phosphate adsorption isotherms,kinetics,reproducibility,and adsorption selectivity of the above 4 mineral-based materials.The results showed that PAL-La was able to quickly adsorb phosphate in water,and the adsorption rate constant calculated by the pseudo-secondary adsorption kinetic model was 0.19 g/mg·min.The surface-loaded lanthanum oxide exhibited excellent stability when no leakage was detected after suffering from 300 W highpower ultrasound challenge for 30 min.PER-La demonstrated excellent adsorption capacity for phosphate with a maximum adsorption capacity of 107.34 mg/g.In addition,PER-La exhibited good adsorption selectivity in a wide range of p H values.The coexistence of other anions at a concentration of 2 times as high as phosphate in solutions only reduced the adsorption capacity by about 20%.The maximum adsorption capacity of LDH-La for phosphate was 101.59 mg/g.The coexisting ions in the mixture had neglectable negative impact on the adsorption performance.LDH-La could be effectively regenerated after adsorption saturation.After four adsorption-desorption cycle experiments,the pollutant removal rate remained about 80%.PP-LDH demonstrated the most outstanding phosphate adsorption performance with a maximum adsorption capacity of 229.37 mg/g,which soared 448.58 mg/g after 400°C thermal activation.In order to analyze the enhanced phosphorus removal mechanism of the four mineral-based composites,this subject used a combination of Fourier Transform Infrared Spectroscopy,X-ray Diffraction,X-ray Photoelectron Spectroscopy and other detection techniques to systematically analyze the interaction between phosphate and various mineral-based composites.The results showed that the active lanthanum groups in PAL-La were widely distributed on the surface of the palygorskite support,which greatly increased the amount of effective adsorption sites for phosphate.Besides,the lanthanum groups on the surface of the material formed a stable lanthanum phosphate inner-sphere surface complex with phosphate by replacing the hydroxyl groups.PER-La possessed a unique magnesium-lanthanum bimetal oxide structure in which both magnesium and lanthanum metal cations could induce surface complex with phosphate by forming Mg3(PO4)2·x H2 O and La PO4·x H2 O complexes.exerted dual function of the characteristic layered structure of hydrotalcite and the high affinity of lanthanum active sites to phosphate.Thus,LDH-La achieved the efficient adsorption of phosphate through intercalation adsorption and strong complexation.PP-LDH had a hydrotalcite-like layered structure and phosphate ions were adsorbed into the inter-layered space.Thermal activation caused PP-LDH to undergo a dehydroxylation reaction,and the original interlayer anions fell off.In that case,phosphate ions were able to interact with metal ions directly by inner-sphere complexation.