Study On Recovery Of Phosphorus And Selective Adsorption Mechanism Of Superparamagnetic Nanocomposites

Phosphate resources are being depleting with the continuous exploitation of phosphate deposit while the water bodies are suffering from phosphorus pollution eutrophication..Therefore,it is urgent need to explore innovative approaches towards capturing phosphate from sewage,which is also an effective way to reduce phosphate contamination.Developing superparamagnetic nanocomposites is promising due to their large adsorption capacity,good selectivity,rapid separation and recyclability.In this study,the superparamagnetic nanocomposites were prepared by modified microwave hydrothermal method.The Fe₃O₄magnetic core was synthesized under alkaline conditions and coated with SiO₂.The layered double hydroxides（LDHs）with different metal combinations were prepared by coprecipitation method.The superparamagnetic nanocomposites are obtained by mixing the two materials and performing microwave treatment.The superparamagnetic nanocomposites were characterized by scanning electron microscopy,transmission electron microscopy,X-ray powder diffractometry and X-ray fluorescence analyzer.The results showed that the superparamagnetic nanocomposites had a rough surface and remarkable crystal structure with diameter of about 20-50 nm.LDHs had an amorphous structure.In this study,the phosphate adsorption capacity of the superparamagnetic nanocomposites was investigated and the adsorption kinetics model and thermodynamic model were established.The results showed that the nanocomposites with different metal LDHs needed different time intervals to reach saturation.The saturated adsorption capacities of MgAl-LDH,MgAlHf-LDH,MgFe-LDH,and MgFeHf-LDH nanocomposites were 13.11 mg P/g LDH,17.9 mg P/g LDH,11.35 mg P/g LDH,and 20.09 mg P/g LDH,respectively.The pseudo-second-order equction model and the Langmuir isotherm adsorption model can better describe the adsorption process,indicating the monolayer adsorption occurred.The factors that can influence the regeneration properties of nanocomposites were also investigated.The results showed that the saturated adsorption capacity of the four composites decreased with the increase of pH and ionic strength of the wastewater.The doped rare metal hafnium（Hf）enhanced the selectivity of the nanocomposite when capturing phosphate from the sewage.The influence other common anions in wastewater on the adsorption capacity of MgAl-LDH,MgAlHf-LDH and MgFe-LDH composites followed the order:CO₃^2->SO₄^2->Cl^->NO₃^-;Nevertheless,the influence on the adsorption capacity of MgFeHf-LDH composites followed the order:NO₃^->SO₄^2->CO₃^2->Cl^-.The nanocomposite can be regenerated by an alkaline solution.After 5 cycles of adsorption-desorption tests,the adsorption capacity of MgAl-LDH,MgAlHf-LDH,MgFe-LDH and MgFeHf-LDH composites still achieved 8.25mgP/g LDH,11.2mgP/g LDH,7.91mgP/g LDH.11.32mgP/g LDH,which were 65.4%,64%,71.82%,and 63.2%of the initial adsorption capacity,respectively.These composites were of high reusability.The superparamagnetic nanocomposites prepared by the subject can be used for phosphorus recovery of domestic sewage,therefore increasing the phosphorus reusability and realizing the optimal allocation of resources.