Study On The Performance And Mechanism For Adsorption Of Phosphate By LaFe (oxy)hydroxides

Phosphate is one of the main nutrients for the growth of aquatic organisms,however,excessive amounts of phosphate may cause eutrophication and subsequent deterioration of water quality.The discharge of low concentration PO₄^3-in the tail water of wastewater treatment plant is the main source of total phosphorus in water and one of the important causes of eutrophication.It has great theoretical significance and practice value for the application of basic research on deep PO₄^3-removal.In this work,the synthetic method of LaFe（oxy）hydroxides based on solvothermal reaction process were explored,a LaFe（oxy）hydroxide（LaFeOx Hy）and a LaFe（oxy）hydroxide nanoparticle-decorated rGO（LaFeO_xH_y-rGO）were synthesized for the removal of the low concentration PO₄^3-in the tail water of wastewater treatment plant.Futhermore,the removal efficiency and the influencing factors of phosphate adsorption were investigated,the interface reaction process and mechanism between complex material and PO₄^3-were also explored.The contents of this paper are:A LaFe（oxy）hydroxide（LaFeO_xH_y）was synthesized by solvothermal treatment method,taking La/Fe nitrate solution as the precursor and ethanol as the solvent,the maximum adsorption capacities of PO₄^3-onto Fe La2:1 prepared under 150℃and p H 10 were calculated to be 123.46 mg/g.LaFeO_xH_y had the structure characteristics of almost-spherical nanoagglomerates with the particle size about 10 nm.The surface complexation model can successfully simulate and predict the reaction process of LaFeO_xH_y with the PO₄^3-,the surface acidity constants p K_a1^int and p K_a2^int of LaFeO_xH_y were 4.85 and 7.51,which can further promote the understanding of the surface properties of LaFe（oxy）hydroxides,and the intrinsic stability constants p K₁^int and p K₂^int of the PO₄^3-on LaFeO_xH_y were-2.03 and-5.08,the interface reaction mechanism of PO₄^3-adsorption by LaFeOx Hy were initially clarified.The corresponding PO₄^3-adsorption mechanism based on Fe/La metal orbital hybridization activated lattice oxygen resulted in the shift of oxygen p character to unoccupied states to provide active sites was:Fe optimized the electron structure of La in LaFeOx Hy,Fe/La metal orbital hybridization resulted in the shift of oxygen p character to unoccupied states,the formation of bridging O^2-through the interaction of Fe 5d-O 2p orbitals with La5d-O 2p orbitals which futher affects O states in Fe/La-O,lattice oxygen orbital hybridization and split produced a greater number of unoccupied molecular orbitals that provide more active sites,meanwhile,the increase of pore size and volume provided a greater number of active sites for binding PO₄^3-,improving the adsorption and removal capacity of PO₄^3-.PO₄^3-was preferentially bonded with La to form the inner-sphere surface complexation La-PO₄.A LaFe（oxy）hydroxide nanoparticle-decorated rGO（LaFeO_xH_y-rGO）was synthesized by solvothermal-film pressing composite method,taking La/Fe nitrate-graphene solution as the precursor and ethanol as the solvent,the adsorption kinetics of PO₄^3-was significantly improved by the LaFeO_xH_y-rGO prepared under 150℃and p H 10,the equilibrium adsorption can be reached in 5 min,the maximum adsorption capacity of LaFeO_xH_y-rGO5:1 for phosphate removal at 1.2 V was 108.69 mg/g.A stable composite electrode material with electrical double-layer capacitor was successfully constructed after introducing LaFeO_xH_y,furthermore,the structural defects increased in composite electrode materials.The electro-promoted adsorption mechanism based on metal orbital hybridization activated lattice oxygen and LaFeO_xH_y-rGO electrode capacitance was:PO₄^3-bonded with La to form the inner-sphere surface complexation La-PO₄,at the same time,it should exact the charge-transfer between Fe site and PO₄^3-,lattice oxygen La-O and Fe-O of LaFeO_xH_y-rGO simultaneously provides adsorption sites for PO₄^3-,on the other hand,the LaFeO_xH_y nanocomposite doped on the surface of rGO increases the capacitance of the electrode material and improves its electrochemical activity,which promoting the PO₄^3-adsorption on LaFeO_xH_y-rGO.