Separation Of Chromate Anions By Selective Crystallization Based On The Guanidinium Cation Ligands

Chromium is considered as a heavy metal with strong biological toxicity and easy migration in the environment,and it mainly exists in the oxidation states of trivalent Cr(?)and hexavalent Cr(VI).Chromium is widely used in electroplating,metal finishing,leather tanning,and the nuclear spent fuel reprocessing process due to its redox properties and transition metal properties.The hexavalent chromium species of chromate(CrO42-)hydrogen chromate(HCrO4-)and dichromate(Cr2O72-)may cause severe problems towards environments and human beings due to their toxicity,carcinogenicity,and fast mobility in aquatic environments.Thus,the sequestration of Cr(VI)from industrial wastewater is highly desirable.The traditional materials for treating chromium-containing industrial wastewater are very limited,and several disadvantages remain,such as low removal efficiency,poor selectivity and subsequent environmental pollution.Therefore,this work provides a new paradigm to separate chromium from industrial wastewater.The cationic guanidium-based ligand could be facilely prepared through the imine condensation of an aldehyde and aminoguanidine hydrochloride.This type of cationic ligands showed a high removal efficiency for chromate anions(>99%)in the solution.Rapid decontamination of an industrial waste water containing toxic metal ions was achieved by precipitating the chromate anions in the form of crystals.The recovery of the guanidium-based ligand made it possible to the utility of this method for real-world applications.The charge-assisted hydrogen bondings between cationic ligand and chromate-water cluster can account to the superior performance of this materials in the removal of Cr(?),which was revealed by the single-crystal X-ray diffraction analysis and density functional theory(DFT)calculations.A combination of powder X-ray diffraction(PXRD),Fourier-transform infrared(FT-IR)spectroscopy and proton nuclear magnetic resonance spectroscopy(1H NMR)was used to characterize this kind of material.The concentrations of CrO42-in solutions were determined by an inductively coupled plasma-atomic emission spectrometry(ICP-AES).The major results are summarized as follows:(1)First,condensation of aminoguanidine hydrochloride with bidentate ligands in ethanol yields the cationic 1,4-benzene-bis(iminoguanidinium)(BBIG-Cl).Suitable crystals of BBIG-CrO4 were obtained by slow evaporation of solvent from the mother liquid.Single-crystal X-ray diffraction analysis combining with DFT calculations reveal that these BBIG2+cations are stacked in an antiparallel fashion with an ABAB pattern in the crystal.Each(CrO42-)2(H2O)4 cluster is immobilized with 20 hydrogen bonds provided by the surrounding 12 guanidine groups,including 14 hydrogen bonds between NH…O(CrO42-)and 6 hydrogen bonds between NH…O(H2O).For each(CrO42-)2(H2O)4 cluster,such a super-dense HBs network yields a binding energy(EbL-C)of-153.93 kcal/mol.DFT calculation indicated the BBIG molecule with positive electrostatic potential(ESP)in the electron density map.The bidentate guanidinium ligand showed a high removal efficiency for chromate anions(99%)when the molar ratio of the cationic ligands and CrO42-anions is 5,and the removal process reached the equilibrium within 30s.Finally,the reusability of the ligand was confirmed by cycling experiments(2)On the basis of above-mentioned results,the tridentate cationic imine-linked guanidium ligand was synthesized and exhibited a more superior removal performance.This ligand showed a high efficiency for chromate anions(-97%)in the 1:1 mixture of ligand and CrO42-anions.Excellent selectivity towards CrO42-(74.6%)was observed even in the presence of high concentrations of NO3-anions(a 100-fold excess with respect to the amount of CrO42-).Single-crystal X-ray diffraction analysis shows that the TBIG ligands crystallize with chromate anions in the form of layered structures which are held together by multiple anion-guanidium hydrogen bonding.The cationic TBIG ligands are perfectly planar and stacked in an antiparallel fashion with an ABBA pattern in the crystal.The two parallel TBIG cations are stacked by?-? stacking interaction in the form of cofacial stacking modes.There are six chromate anions and seven hydrated water molecules in the asymmetric unit,involving 47 hydrogen bonds,among which 45 NH…O bonds from guanidinium groups,and 2 OH…O hydrogen bonds from the water molecules.