Experimental Observation And Mechanism Research On Phosphate Recovery From Wastewater By Superparamagnetic Nanomaterials Loaded With Transition Elements

Phosphorus,which plays an important role in life activities is an indispensable irreplaceable.A large amount of phosphorus resources are transformed in the form of pollutants after human production activities and finally transferred and concentrated into bio-sludge in wastewater treatment plant in.Therefore,the recovery of phosphorus resources from sewage and sludge has become an effective way to alleviate the shortage of resources.In this paper,superparamagnetic nanocomposites were used as adsorbents to study the adsorption and regeneration of phosphate,to explore the adsorption law and mechanism,and to investigate the potential of phosphate recovery from sludge digestion solution,so as to recycle phosphate resources.In this study,superparamagnetic nanocomposites with transition elements Hf,Zr,La and Ce were prepared with co-precipitation method,and their structures were characterized by scanning electron microscope,transmission electron microscope and X-ray diffraction.The results showed that these nanocomposites are 20-50 nm spherical particles,with regular shape and uniform distribution,and LDH structure.The crystallinity of LDHs was less significant after La-loading,therfore increasing the irregularity of the material surface and effectively promoting the phosphate adsorption.The influential factors on phosphate adsorption were also studied,including adsorption time,dosage,pH value as well as different transition elements during the adsorption process.The results showed that the composite material can adsorb phosphate rapidly,with adsorption capacity reaching more than 90% of the total adsorption capacity within 120 min.Besides,the adsorption capacity decreased with the increase of the dosage,and the adsorption efficiency increases with the increase of the dosage;These composites could maintain good phosphate adsorption performance in acid and neutral environment,and in strong acid environment(pH < 1),the adsorption performance of the composite decreases significantly;The adsorption capacity of MgAl-La composite for phosphate showed obviously superior adsorption capacity than the other three composites.In addition,the adsorption isotherm model for the phosphate adsorption process and the kinetic fitting results showed that the Freundlich model could accurately describe the P-adsorption process by all the four types of composite materials,indicating that the adsorption process should be probably multimolecular layer adsorption.The pseudo-second-order kinetic model can better describe the kinetic process,indicating that chemical adsorption mainly limits the adsorption rate;from the intra-particle diffusion model.It is known that the adsorption rate was controlled by both intra-particle diffusion and external diffusion.The strong alkali solution can realize the desorption and regeneration of the composite material,but the desorption effect of the composite material decreased with the cycles of adsorption-desorption;after 10 adsorption-desorption cycles,the desorption rate of MgAl-Zr,MgAl-La and MgAl-Ce composite decreased to 45.6%,50.4% and 45.6%,compared to the desorption rate of 12.5% by MgAl-Hf;The composite may exhibit different adsorption trends during the adsorption desorption cycle due to the loading of different transition elements.MgAlLa composite material was also employed to capture phosphorus from sludge digestion supernatant.It was found that the MgAl-La composite exhibited good phosphate adsorption performance,with the adsorption capacity of 71.88 mg/g;After one cycle of desorption,the phosphate adsorption capacity of MgAl-La composite decreased to 61.7% of original value and stayed stable after subsequent cylcles.The results of three-dimensional fluorescence spectroscopy showed that the biomacromolecule organics in the sludge digestion solution had little effect on the phosphate adsorption of MgAl-La,MgAl-Zr and MgAl-Ce,while the MgAlHf composite might react with the aromatic proteins in the water sample,thus producing competitive phosphate adsorption.In this study,the mechanism of phosphate adsorption and regeneration of superparamagnetic nanocomposites loaded with transition elements was inferred according to the results of energy spectrum quantitative,SEM mapping and X-ray fluorescence spectrum,it was presumablys considered that superparamagnetic nanocomposites mainly adsorbed phosphate through complexation reaction.Transition elements can form strong ligand complexation with phosphate,which enhanced the selective adsorption of phosphate by the composites.