Advanced Removal And Mechanism Of Low-concentration Heavy Metals From Water By Porous Cyclodextrin-based Functional Materials

While industrialization has brought the well-being of humans,it also inevitably contaminated the environment.Wastewater containing a variety of highly toxic heavy metal ions produced by metallurgy,chemical industry.Even after conventional precipitation,flocculation or other treatment,the residual concentration of heavy metals still reaches the magnitude of mg/L-μg/L.These toxic heavy metals will cause great damage to human health and ecological environment.So it is urgent to find efficient methods for advanced removal of heavy metal pollutants in water.Adsorption method is a simple and effective way to remove heavy metals.However,most of the materials reported to remove heavy metal still focus on the improvement of adsorption capacity,while ignore the requirement of high adsorption affinity for low-concentration of heavy metals and complex types of coexisting ions.So it is difficult to enrich heavy metals on the material surface.In view of the above problems,this work strengthened the coordination between heavy metals and materials through hydrogen bond network,and selectively improved the adsorption affinity of heavy metals.So as to achieve the advanced removal of heavy metals in complex water.In the environment where multiple heavy metal pollutants coexist,the adsorption affinity was enhanced by the synergistic enhancement of soft and hard functional groups to achieve broad-spectrum and advanced removal of heavy metal ions.In this work,a series of porous cyclodextrin materials were designed and modified by functional groups to improve the affinity of target heavy metals.（1）In the aspect of selective advanced removal,a porous cyclodextrin-based zirconium oxide composite（CDP-Zr）was designed.Arsenic as the specific capture object to study the removal effect of inorganic and organic arsenic.XRD,FTIR,XPS and other characterization methods proved that zirconium was successfully loaded onto porous cyclodextrin.CDP-Zr was used to adsorb inorganic and organic arsenic.The adsorption thermodynamic results showed that when the initial concentration of arsenic was 2.0 mg/L,the adsorption affinity of CDP-Zr for As（III）,As（V）and Roxarsone（ROX）was 4.0×10⁴m L/g,1.4×10⁸m L/g and9.9×10⁶m L/g,respectively.The removal efficiency of As（III）,As（V）and ROX could reach97.06%,99.98%and 99.73%,respectively.The mechanism of CDP-Zr for efficient removal of arsenic at low-concentration was studied by FTIR,XPS and EXAFS.The results show that the adsorption force of inorganic and organic arsenic mainly comes from the As-O-Zr coordination between the Zr and arsenic.At the same time,a large number of hydroxyl sites on the porous cyclodextrin generated hydrogen bonds with the oxygen-containing groups of inorganic arsenic and organic arsenic,which strengthened the As-O-Zr coordination.Thus,the affinity of the material to low-concentration of arsenic was improved.（2）In addition to zirconium oxide,iron based materials with strong affinity for arsenic were further selected to be combined with porous cyclodextrin,and an iron-based multi-pore cyclodextrin composite（CDP-Fe）was designed and synthesized.XRD,SEM,XPS and other characterization results confirmed that Fe₃O₄was successfully synthesized and loaded with a loading of 0.56 g Fe/g CDP.The BET results showed that the specific surface area of CDP-Fe was 106.6 m²/g,which was significantly higher than that of Fe₃O₄（84.5 m²/g）.It is proved that the composite enhanced the dispersion of iron oxide.The advanced removal effect of CDP-Fe on low arsenic concentration was studied by comparing pure Fe₃O₄.It was found that when the initial concentration of arsenic was 0.5 mg/L,the removal affinity of CDP-Fe for As（III）,As（V）and ROX was 2.9×10⁵m L/g,6.2×10⁵m L/g and 1.4×10⁶m L/g,respectively,which were 5.1,3.6 and 15.2 times of that of pure Fe₃O₄.The removal rate can reach 97.22%,98.65%and 99.40%,respectively.The mechanism of this affinity improvement was due to the enhanced As-O-Fe coordination through hydrogen bonding by a large number of hydroxyl sites on the porous cyclodextrin.This work confirmed the universality of the mechanism of hydrogen bonding assisted coordination enhancement,and provided theoretical support for the advanced removal of arsenic from water with low concentration in practice.（3）For broad spectrum advanced removal of heavy metals,a porous cyclodextrin modified material CDP-NH containing both hard group-OH and soft group-NH₂was designed.The successful synthesis of the material and the introduction of hydroxyl and amino groups were confirmed by means of XRD,FTIR and XPS.Taking Eu³⁺,Pb²⁺and Cd²⁺as the representative of hard,medium and soft heavy metals,the broad-spectrum advanced removal effect of CDP-NH on low concentration heavy metals in water was studied.The results showed that the adsorption affinity of CDP-NH for Eu³⁺,Pb²⁺and Cd²⁺was 1.1×10⁹m L/g,7.3×10⁹m L/g and 1.6×10¹⁰m L/g at 2.0 mg/L,respectively.Up to 538,468 and 9811 times of the sulfhydryl contrast sample（CDP-SH）and the hydroxyl sample（CDP）.Further,the broad-spectrum high-affinity mechanism of CDP-NH was studied by XPS,FTIR,etc.The results showed that hard group-OH and partial soft group-NH₂follow HSAB coordination principle,which greatly improved the material’s resistance to different materials.At the same time,when a variety of heavy metal pollutants coexisted,the mixed removal rate of Cd²⁺,Nd³⁺,Hg²⁺,La³⁺,Mn²⁺,Ce³⁺,Sn²⁺,Eu³+,Pb²⁺,Zn²⁺,Cu²⁺,Ni²⁺can still reach more than 98%.The broad spectrum advanced removal of low concentration heavy metals under complex conditions is realized.