Lanthanum-Based Composites Synthesis For Phosphorus Adsorption From Water:Efficiency And Mechanism

Inorganic and organic phosphorus（P）are the main components of P in wastewater,while they enter the water environment and cause eutrophication.At present,the conventional wastewater treatment process is mainly for inorganic phosphorus removal.How to simultaneously remove inorganic and organic phosphorus economically and efficiently in wastewater is a difficult problem in-depth removal of P.Compared with other methods,adsorption can efficiently remove inorganic and organic phosphorus from wastewater,which has the advantages of simple operation,no secondary pollution and reusability.Among them,lanthanum-based adsorption materials have attracted extensive attention due to their selective adsorption capacity for inorganic phosphorus.However,the adsorption effect and mechanism for organic phosphorus remain to be studied in depth.Developing novel lanthanum-based materials and exploring the adsorption behavior and mechanism of simultaneous inorganic and organic phosphorus on their surface have important theoretical and practical significance for the removal and recovery of phosphorus in wastewater.However,there is no relevant research at present.This study aims to construct and optimize a new lanthanum-based material suitable for simultaneous removal of inorganic and organic phosphorus in wastewater,and reveal the adsorption mechanisms and provide references for designing novel La-based sorbents for phosphorus removal via combining adsorption experiments,materials characterization and DFT calculation.The main research results are as follows:（1）Lanthanum carbonate（LC）,lanthanum molybdate（LM）and lanthanum-based organic framework（La-MOF）were constructed and characterized.The adsorption experiment results showed that the equilibrium adsorption capacity of LC and LM was greatly affected by p H.The equilibrium adsorption capacity of La-MOF was less affected by p H.The co-existing anions in the water had little effect on the orthophosphate adsorption of the three materials.The equilibrium time of orthophosphate adsorption by LC,LM and La-MOF were 300 min,300 min and 60 min,respectively.For HEDP adsorption,the equilibrium time of the three La-based sorbents were 60 min,120 min and 240 min,respectively.The pseudo-second-order kinetic model could well describe the kinetics results of inorganic and organic phosphorus adsorption by the three materials.Langmuir model could well describe isotherms of phosphate adsorption by LC,LM and La-MOF,and the maximum sorption capacity were 63.25 mg-P/g,74.72 mg-P/g and 88.72 mg-P/g,respectively.Freundlich model could well describe isotherms of HEDP adsorption by the three materials.The study on the reuse of the three materials showed that LC and LM had a better reuse effects on inorganic phosphorus than La-MOF.For the removal of organic phosphorus,the three materials can maintain more than 90%of the initial after four times reuse.The mechanism of phosphate adsorption by LC was electrostatic attraction and ligand exchange,while ligan exchange and La PO₄ precipitation were the main mechanisms for LM and La-MOF.The mechanism of HEDP adsorption by LC and LM was electrostatic attraction and ligand exchange,while the ligand exchange was the main mechanism for La-MOF.Metal dissolution results showed that LC was more friendly to the environment.（2）In order to enhance the reusability of lanthanum-based materials,kaolinite/lanthanum carbonate（KT/LC）composite was prepared and optimized by using kaolinite as a carrier with a wide range of sources and low prices.The KT/LC was also characterized.The equilibrium time of orthophosphate and HEDP adsorption by KT/LC were both 300 min,and the pseudo-second-order kinetic model and intra-particle diffusion model could well describe the adsorption process.Freundlich model could well describe isotherms of inorganic and organic phosphorus adsorption by KT/LC,and KT/LC showed advantages in adsorption capacity compared with other adsorbents.The results of adsorption thermodynamics showed that the adsorption of inorganic and organic phosphorus by KT/LC was a spontaneous and endothermic process,and the degree of thermodynamic confusion increased with the temperature.The response surface analysis method was used to establish a polynomial model composed of three variables to successfully predict the phosphorus removal efficiency.The polynomial model showed that the adsorbent dosage and phosphorus concentration exhibited strong interactions and had a significant influence on phosphorus removal.The response surface analysis results provided a parameter basis for optimizing phosphorus advanced treatment process of wastewater.The reuse experiment showed that the adsorption capacity of inorganic phosphorus after five times reuse could be maintained at 74.5%of the initial adsorption capacity,and the adsorption capacity of organic phosphate was up to 99%of the initial adsorption capacity.（3）The efficiency and mechanism of KT/LC simultaneous removal of inorganic and organic phosphorus were studied.The adsorption kinetics results showed that the adsorption of phosphorus in the co-existence system of orthophosphate and HEDP fitted well with the pseudo-second-order kinetic model.Freundlich and Langmuir model could well describe isotherms of orthophosphate and HEDP adsorption by KT/LC,and there was a competitive adsorption relationship between the two phosphorus species,where HEDP plays the predominant role.The p H has a great influence on adsorption.Cl^-,SO₄^2-and NO₃^-had little effect on the removal of single orthophosphate and HEDP,however HCO₃^-inhibited inorganic and organic phosphorus adsorption.Ca²⁺and Mg²⁺had no effect on the removal of orthophosphate but inhibited HEDP.In order to simulate and predict the removal efficiency of orthophosphate and HEDP in the binary system and the importance of different influencing factors,the orthogonal experiment design and the neural network model were established by using the SPSS software.The selected seven input parameters can accurately predict the removal efficiencies of orthophosphate and HEDP in different water quality and reaction conditions,the significance of each parameters in the order:adsorbent dosage>HCO₃^-concentration>p H>Ca²⁺concentration>Mg²⁺concentration>SO₄^2-concentration>reaction time.Combined with a variety of characterization analyses and DFT calculations,it was shown that the adsorption mechanism of KT/LC on orthophosphate and HEDP was that ligand exchange of-OH and CO₃^2-on the material surface to form the La-O-P inner-sphere complex.Experiments of phosphorus removal from real wastewater showed that KT/LC had a good removal effect on the total phosphorus in the continuous flow of low-concentration phosphorus-containing wastewater,which was expected to be used in the advanced treatment of wastewater.