Preparation And Adsorption Properties Of Carboxyl Functionalized Covalent Organic Framework Materials And Study Of Their Adsorption Properties For Heavy Metal Ions

Water is one of the most necessary preconditions for life generation and maintenance on earth,and favorable water environment is an important guarantee for healthy growth of living things.However,since the acceleration of social industrialization,heavy metal pollution in water has become a serious threat to ecological environment health and human fitness.To date,adsorption is regarded as the most common method to treat heavy metal pollution in water.Therefore,the design and synthesis of highly selective adsorption materials are of great significance for the adsorption,enrichment and removal of heavy metal ions in water.Covalent organic frameworks（COFs）are new kinds of potential adsorption materials,which have high porosity,large specific surface area and stable properties.In this paper,two new carboxyl functionalized COFs materials were designed and synthesized.The structure and morphology of the materials were characterized through physical and chemical methods,and the adsorption performance and action mechanism of the materials on several metal ions were systematically studied.The application of keto-enamines COF materials was expanded,and the theoretical basis for the development of efficient heavy metal ion adsorbent in water has been provided.The main contents of this paper are as follows:（1）Design synthesis and structure characterization of carboxylated COFs materials.Covalent organic framework contained hydroxy that named COF@OH was synthesized through vacuum solvothermal method using 1,3,5-triformylphloroglucinol and 3,3’-dihydroxybenzidine as basic structural units.Subsequently,two carboxyl functionalized covalent organic framework materials,COF@SA and COF@EDTAD,were synthesized through post synthetic modification with succinic anhydride and ethylenediaminetetraacetic dianhydride,respectively.These two kinds of COFs both have good crystallinity stability.They are micropore and mesoporous materials with relatively higher specific surface area than others which can be used as excellent adsorbents.（2）Study on adsorption properties and mechanism of COF@SA.The removal rates of COF@SA to the five metal ions(Cd²⁺,Cu²⁺,Ni²⁺,Mn²⁺,Cr³⁺)in mixed solution are all close to 100%,which exhibit good heavy metal affinity.The influences of initial p H,contact time,reaction temperature and initial ion concentration on the adsorption effect were investigated.In the test condition,the adsorption capacity of COF@SA for Cd²⁺,Cu²⁺and Cr³⁺rises with the increase of p H.Increasing initial concentration of metal ions can help to improve the adsorption capacity.The adsorption kinetics can be described by pseudo-second-order kinetic model,which is dominated by chemisorption and the isotherms follow the Freundlich isotherm model.Ion exchange can occur between metal ions and H⁺in carboxyl on COF@SA,and coordination between metal ions and oxygen atoms on COF@SA surface also plays a part on heavy metal ions adsorption.（3）Study on adsorption properties and mechanism of COF@EDTAD.In the mixed solution of five metal ions(Cd²⁺,Cu²⁺,Ni²⁺,Mn²⁺,Cr³⁺),COF@EDTAD has the best adsorption effect on Cr³⁺,and the removal rate is close to 100%.Taking Cd2+,Cu2+and Cr3+as examples,various environmental factors affecting the adsorption performance of COF@EDTAD were investigated in detail.The adsorption behavior of Cd²⁺,Cu²⁺and Cr³⁺on COF@EDTAD is more consistent with Langmuir model.In the meantime,the adsorption kinetics follow the pseudo-second-order kinetic model.The adsorption of COF@EDTAD to Cu²⁺is endothermic,and the adsorption of Cr³⁺and Cd²⁺is exothermic.EDTAs on the COF@EDTAD are adsorption active sites which binds to metal ions through chelation.