Advanced Removal Of Low-Concentration Heavy Metals In Water By Novel Three-Dimensional Framework Materials

A large number of heavy metal wastewater is produced during the process of mining,metal smelting and other industrial activities.After conventional treatment,the residual concentration of heavy metals still reaches the magnitude of mg L^-1,which substantially damages the ecological environment and human health.Most of heavy metals have high toxicity(such as Pb²⁺,Cu²⁺,Hg²⁺,Cd²⁺),while some of them have resource properties(such as La³⁺,Ce³⁺,Nd³⁺,UO₂²⁺).According to the purpose of treatment,it is urgent need for broad removal or specific capture of heavy metals from water.Adsorption is an effective method to remove low-concentration heavy metals.However,the heavy metals in water usually have the characteristics of low-concentration and various and complex types of coexisting ions,which requires capture materials with high adsorption affinity.However,the materials reported to remove heavy metals is still concentrated on improving the adsorption capacity while ignoring the enhancement of the adsorption affinity,so it is difficult to enrich heavy metals on the surface of the material.In view of the above problems,this work has carried out the following parts:（1）Uranium,a resource heavy metal,as the target pollutant of specific capture.A novel covalent organic framework（COF-Tp Dd-AO₂）with high density amidoxime groups was designed and synthesized,as the trap material.XRD,FT-IR,¹³C NMR,BET and other characterization methods were carried out to confirm the successful synthesis of COF-Tp Dd-AO₂and the introduction of amidoxime group sites in its structure.COF-Tp Dd-AO₂ exhibited different adsorption behaviors towards uranium with different concentrations,with the maximum affinity of 8.27×10⁵m L g^-1,which was 6.82 times and 9.81 times higher than that of COF-Tp Db-AO with a single amidoxime group and the pristine COF-Tp Dd.The removal efficiency exceeded 99.2%with the initial uranium concentration of 0.5 mg L^-1.Mechanism showed that the chelation interaction between multi-site amidoxime groups and uranium acted as the dominated adsorption force.Moreover,with the increasing density of the amidoxime group in COF-Tp Dd-AO₂,the chelation effect changed from one single site to multiple sites,thus further enhancing the uranium affinity.（2）For broad-spectrum removal of heavy metals,the wastewater containing La³⁺,Ce³⁺,Nd³⁺,UO₂²⁺,Pb²⁺and other heavy metals was selected as the target pollutants.A novel metal organic framework（doted as TMU-4-COOH）with high density active sites was designed,containing different types of soft and hard functional groups in each structure unit.XRD,FT-IR and XPS and other characterization methods were conducted to confirm the successful synthesis of TMU-4-COOH and the introduction of carboxyl and azine groups.To confirm the advantages of soft and hard functional groups for the broad removal of heavy metals,the control samples of 2D-N nanosheet containing only azine groups and 2D-COOH nanosheet containing only carboxyl groups were also carried out the adsorption experiments.The TMU-4-COOH achieved broad removal of La³⁺,Ce³⁺,Nd³⁺,UO₂²⁺and Pb²⁺,with the maximum removal efficiency of 97.7%.The adsorption affinity of TMU-4-COOH reached 1.38×10⁴,1.40×10⁴,1.35×10⁴,2.01×10⁴ and 1.04×10⁵ m L g^-1 for La³⁺,Ce³⁺,Nd³⁺,UO₂²⁺and Pb²⁺,which was 4.35and 3.35 times higher than that of 2D-N nanosheet and 2D-COOH nanosheet.Mechanism revealed that the cooperation interaction between carboxyl and azine groups and targeted heavy metals was the main adsorption force,which was assigned to the increasing density of functional groups in TMU-4-COOH.Moreover,the adsorption affinity of high-density functional groups with different properties towards the heavy metals followed the HSAB（Hard-Soft-Acid-Base）theory of"functional group-heavy metal"compatibility,thus extremely enhancing the affinity of TMU-4-COOH for low-concentration La³⁺,Ce³⁺,Nd³⁺,UO₂²⁺and Pb²⁺.