Study On Phosphorus Mineralization Recovery From Water By Mg/Ca-based Nano-Magnetic Composites

Phosphorus is an essential element for all lives,which is also a nonrenewable resource.The phosphate rock resource crisis will become increasingly severe.Besides,the excessive release of phosphorus is a main cause of eutrophication.Phosphorus recovery could not only reduce the pollution,but also reach the aim of resource recycle.Phosphorus removal methods mainly include crystallization method,chemical precipitation method,biological method,etc.Crystallization is regarded as a promising method and widely used for removing phosphorus from wastewater,but the high costs of calcium-and magnesium-containing reagents limited its practical applications.Developing cheap,available,efficient materials is required.In this study,magnesium oxide,calcite and steel slag were used as raw materials to prepare magnetic Mg/Ca nanocomposites.The effects of solution pH,dosage concentration,composition proportion,contact time,and initial phosphorus concentration on the phosphorus recovery efficiency were extensively examined by the batch experiments.Moreover,the interaction mechanisms between phosphate ions and magnetic nanocomposites was elucidated.The important results of this dissertation were summarized as follows:(1)MgO-Fe3O4 nanocomposite were prepared by microwaveassisted method.The phosphate could be highly efficient recovered by the MgO-Fe3O4 nanocomposites with the maximum removal capacity of 318.6 mg P/g and 138.2 mg N/g.The saturation magnetisation of 13.72 emu/g allowed for effective separation from water.Quantitative and qualitative analyses confirmed that the struvite was harvested in high phosphate concentration effluent,and the interaction mechanisms between the phosphate and the nanocomposites was the synergistic of sorption and struvite crystallization.Moreover,ultralow level phosphate(below 0.01 mg P/L)could be achieved in low-level phosphate solution ranging 0.1 to 6.2 mg P/L,due to the coupling sorption effects of Fe3O4 and MgO in MgO-Fe3O4 nanocomposites.At low concentrations,the phosphorus was mainly removed by adsorption and electrostatic interaction;at higher concentrations,the phosphorus was mainly removed by adsorption-MgNH4PO4·6H2O crystalline coupling mechanism.Considering the easy magnetic separation,efficient phosphorus recovery efficiency and high removal capacity,the designed magnetic MgO-Fe3O4 nanocomposites thus could be potentially used for sustainable and efficient phosphorus recovery from water and wastewater.(2)Ca-Fe nano-magnetic composite with large specific surface area and good magnetism was prepared by microwave assisted method,calcination and ultrasonication coupling techniques.The magnetic nanocomposite was effective in recovering phosphate from water in the pH range of 3.0-6.0,with a maximum removal rate of 189.21 mg P/g.The saturation magnetisation of 18.50 emu/g allowed for effective separation from water.At low concentrations,the phosphorus was mainly removed by adsorption and electrostatic interaction;at higher concentrations,CaHPO4·2H2O was formed,and the phosphorus was mainly removed by adsorption-CaHPO4·2H2O crystalline coupling mechanism.The prepared CaCO3-Fe3O4 magnetic composites have the advantages of easy magnetic separation,strong phosphorus removal ability and environmental friendliness,which have potential applications in phosphorus resource recovery.(3)Steel slag,a bulk solid waste rich in metal elements such as Mg,Fe and Ca,was selected as the basic raw material and the composites were prepared by ball milling and calcination modification of magnesium hydroxide after ball milling.The saturation magnetisation of the modified steel slag before and after calcination were 3.12 emu/g and 1.77 emu/g respectively,both of which achieved effective separation from water.The magnesium hydroxide ball mill modified as well as calcined steel slag showed good phosphorus removal efficiency,with an increased phosphorus removal capacity of 131.76 and 151.37 mg P/g respectively.The phosphorus removal mechanism of the modified steel slag was the synergistic effect of adsorption,electrostatic effect and mineralization reaction.At initial phosphate concentrations ranging 3.131 mg/L,the residual phosphorus concentration of the modified slag was less than 0.5 mg/L.At higher concentrations,phosphate was effectively removed by mineralization of CaHPO4·2H2O.In addition,the modified steel slag has good magnetic separation ability and high efficiency in phosphorus recovery,which has good application prospects for phosphorus recovery in aquatic environment.