Adsorption Characteristics And Mechanisms Of Phosphate And Nitrate From Aqueous Solution By Chitosan-based Modified Hydrogel Beads

The rapid development of agriculture and urbanization in our country,along with the massive use of chemical fertilizers and daily necessities,has led to excessive discharge of phosphate and nitrate into natural water body.A series of environmental problems,including algae blooms caused by eutrophication and blue baby syndrome caused by excessive nitrates,have attracted more and more attention.However,phosphate and nitrate are recyclable resources,in which phosphorus resources are non-renewable and the total amount is declining.Therefore,it is urgent to develop a technology which not only can efficiently remove phosphate and nitrate,but also recycle them.Adsorption is the first choice because of the advantages as follows:simple operation,high removal rate,low concentration of phosphate and nitrate,no sludge and other pollution,and the potential to recover phosphate and nitrate resources.In this study,chitosan was used as a matrix carrier,and zirconium or lanthanum was introduced into the chitosan network structure to form beads under alkaline conditions.Various characterization methods are used to explore the various physical and chemical properties of the beads and the influence of different factors on the adsorption performance of the beads.The main research results are as follows:（1）Zirconium metal was introduced into chitosan network structure to form chitosan-zirconium beads（CS-Zr）.Then polyamine-based substance polyvinyl polyamine（PEPA）was grafted on its surface to form a polyvinyl polyamine modified chitosan-zirconium beads（CS-Zr-PEPA）.SEM,FTIR,BET,XRD,and XPS were used to explore the changes in morphology,functional groups,thermogravimetry,crystal properties,specific surface area and pore size,and energy spectrum of each element before and after adsorption.The results showed that the CS-Zr-PEPA beads own well spherical,stable property,a larger specific surface area and big pores,and excellent adsorption potential.When exploring the performance of CS-Zr-PEPA beads in different pH solutions,co-existing anion interference,repeated regeneration desorption rate,isothermal,kinetics and column adsorption,it was found that the adsorption capacity reached the maximum at pH 3.The presence of SO₄^2-has a more significant interference effect on the removal of phosphate and nitrate than Cl^-and NO₃^-.After five cycles of adsorption,it still maintained 81%and 87%of the adsorption performance,respectively.The Freundlich model well described isothermal adsorption data and the pseudo-second-order kinetics well fitted kinetic data.It had excellent dynamic adsorption capacity on the adsorption of phosphate column.（2）In order to maintain a high and stable adsorption capacity of the beads in a wide pH solution,lanthanum was introduced into chitosan network structure to form CS-La beads in this study.Then the beads were soaked in the organic ligand solution,reacting in a reaction kettle at 120°C for 24 hours.La-MOF was grown in situ inside the CS-La beads to prepare CS-La-MOF beads.The results of characteristic showed that La-MOF had good dispersibility on well spherical,and had excellent thermal stability.Lanthanum had been successfully converted into La-MOF.The large specific surface area and the high internal porosity of the beads were measured.In the experiment of different factors affecting its adsorption of phosphate and nitrate,the results showed that CS-La-MOF beads still maintained a stable adsorption capacity from 49.43mg-P/g to 26.27 mg-NO₃^-/g in a solution with a pH of 3-10.The change of pH value had less influence on the adsorption capacity than CS-Zr-PEPA.The influence of the interfering ion SO₄^2-was greater than that of Cl^-and NO₃^-.The Freundlich model and the pseudo-second-order kinetics well fitted the isotherm adsorption data and the kinetic data,respectively.In the dynamic column adsorption experiment,the breakthrough and the depletion time point were higher than the CS-Zr-PEPA,which had the potential for practical engineering applications.