Based On Liquid Crystal Phase Conformation Construct Spatial Ordered Ligand And Its Selective Removal Of Heavy Metal Ions

The discharge of industrial wastewater is one of the main sources of heavy metals in the environment.According to the 2020 National Bureau of Statistics of China,the discharge into sewage of total lead was 26.68 tons,mercury was 1.129 tons,cadmium was 4.166 tons and arsenic was 10.241 tons.Heavy metals are biologically incompatible and highly toxic.Once inside an organism,heavy metals are very hard to biodegraded or excreted out of the system,as it easily accumulates in the food chain seriously affecting metabolism of the living organisms.Thus,an efficient,economy,and handy technology for removing and desorption heavy metals from wastewater is very necessary.Compared with other heavy metal treatments,adsorption is generally preferred for the removal of heavy metal ions due to its high efficiency,ease of handling,biological sludge and potential recovery and reuse of metals.The selective adsorbents（such as crown ethers and imprinting materials）utilize the formation of chemical bonds in the selective cavities,where the variability is extremely small and separation of metal ions is challenging.In this paper,a spatial ordered ligand constructed by conformation was designed.The specific method is as follows:mesogenic groups were used to anchor coordinating groups of the chelating resin through flexible chains and forming adsorbents with highly selective adsorb capacity.This ligand is different from the above-mentioned chemical method.Using physical method construct a selective cavity,which can selectively adsorb certain heavy metals.Based on the above design,A polyethyleneimine-based side-chain liquid-crystalline polymer（POMen）substituted with methoxybiphenyl as pendent mesogenic groups has been synthesized in which the flexible spacer is methylene（CH₂）_n.The structure of POMen was characterized by infrared spectroscopy and nuclear magnetic spectroscopy,and the degree of substitution was calculated.The liquid crystal behaviors of POMen were analyzed by DSC,POM and XRD.The results showed that the liquid crystal phase of the POMe4 and POMe6 are smectic B phase,POMe3 and POMe5 are smectic B and E phase.The structures of POMe3 and POMe5 are complex,which is not conducive to modeling.The structures of POMe4 and POMe6 are very similar.Thus,based on the above experiments,the spatially ordered structure of the coordination groups in the liquid crystal POMe6 was analyzed by using Materials Studio package implemented into the DMol³ program.The results show that the coordination groups on POMe6 formed a selective cavity,which is conducive to the adsorption of metal ions that match its space and forms a stable multidentate complex.The results obtained by the above calculation and simulation are consistent with our design and expected results.The synthesized polymer was heated to its isotropic state（>186°C）and crystalline state（126°C-178°C）,and then quenched in liquid nitrogen,respectively.POMe6 with three different states were obtained:POMe6 in isotropic state（I-POMe6）,POMe6 in liquid crystal state（I-POMe6）,and POMe6 in room temperature（H-POMe6）.Their maximum adsorption capacities for different metal ions were studied.The results showed that L-POMe6 is the largest,and I-POMe6 is the smallest.In the competition adsorption studies,L-POMe6 demonstrated excellent selective adsorption capacity relative to Pb²⁺.In the adsorption mechanism,the optimal conditions for the adsorption of Pb²⁺ions by L-POMe6 were discussed,and an isothermal adsorption model and a pseudokinetic model were established.It was found that the adsorption of Pb²⁺ions onto L-POMe6 fitted to the Langmuir adsorption and pseudosecond-order kinetic model.The regeneration capacities of L-POMe6 were studied,that the adsorption capability remains high after several cycles of adsorption-desorption process.Based on density functional theory,together with COSMO in DMol³were used to analyze adsorption mechanism of L-POMe6.The results show that L-POMe6 is not the best adsorb ability for Pb²⁺.In the meantime,it was found that the structure of L-POMe6 has changed too much after adsorption,which is not in accordance with the actual situation.All of these shows that our adsorption process is in solid-liquid system and is not suitable for COSMO solvent simulation.Based on previous experience,COSMO is no longer used.The adsorption mechanism of L-POMe6 was simulated again.It was found that Pb²⁺and one water molecule are coordinated by N atoms forming a tridentate complex,while Hg²⁺、Cu²⁺、Co²⁺、Cd²⁺and Zn²⁺ions and L-POMe6 all formed a unidentate complex.And the calculated binding energy of L-POMe6 and Pb²⁺is the lowest compared with other metal ions.These results show that L-POMe6 has excellent selective adsorption to Pb²⁺ions.The charge density difference was investigated,it is shown that N atoms are used as electron donors in the adsorption process.Moreover,the result of frontier orbital theory shows that the energy gap of the L-POMe6-Pb complex is the largest,indicating that its structure is the most stable.Compared to the other metal ions,Pb²⁺is the least easily desorbed from L-POMe6.Since the adsorption process is the interaction between L-POMe6 and hydrated metal ions,the data about hydrated metal ions in related literature reports are quite different.The Materials Studio package implemented into the Castep program was used to simulate the inner layer hydration structure of six metal ions.It was found that the maximum inner layer hydration numbers of Hg²⁺,Cu²⁺,Co²⁺,Cd²⁺and Zn²⁺ions were all 6,and their maximum hydration radii were all less than 2.5（?）.The maximum inner layer hydration number of Pb²⁺ions is 8,and the average bond length of Pb-O in Pb²⁺（H₂O）₈ is 2.69（?）,the hydration radius is the largest.Therefore,combined with the results of the previous POMe6 geometry optimization,L-POMe6 may be the most suitable for the adsorption of Pb²⁺（H₂O）₈.Based on the above calculation results,adsorption mechanism of L-POMe6 with hydrate metal ions were simulated under the DMol³ and Castep modules.It was found that Pb²⁺（H₂O）₈ is coordinated by N atoms forming a tridentate complex,while other hydrate metal ions and L-POMe6 all formed a unidentate complex.And the calculated binding energy of L-POMe6 and Pb²⁺（H₂O）₈ is the lowest compared with other metal ions.These results show that L-POMe6 has excellent selective adsorption to Pb²⁺（H₂O）₈.