Enhanced Arsenic Adsorption From Aqueous Solution Of Metal Organic Framework By Metallic Control Strategy And Its Mechanism Study

Arsenic contamination has caused serious risk to ecological environment and human’s health.Adsorption is an efficient technology to deal with arsenic contamination,the structural properties of adsorbent significantly influence its arsenic adsorption performance.Metal-organic frameworks（MOFs）are new kinds of porous materials with tunable porosity,as well as various of central metal and organic ligands.In order to better apply MOFs in the field of arsenic decontamination,the investigation about the relationship between the structural properties of MOFs and arsenic adsorption performance is urgently needed.In this present study,several novel MOFs with enhanced physicochemical property has been explored by metallic control strategies including defect engineering,synthesis of mix-metal MOFs and metallization of organic ligands,their adsorption behaviors and mechanism for different arsenic species has been systematically investigated.The main research works are shown as follow:（1）The chemical structure of arsenic species greatly influences their migration and transformation in the environment.The adsorption behaviors of arsenate（As（V））and its organic forms such as roxarsone（ROX）,p-arsanilic acid（p-ASA）and dimethyl arsenate（DMA）by MIL-101（Fe）,a type of highly porosity iron-based MOFs in aqueous environment were detailed investigated.The results show that MIL-101（Fe）exhibited excellent adsorption performance for arsenicals.The adsorption affinity of different arsenic species follows the order of As（V）>ROX>p-ASA>DMA,but the order of adsorption capacity is ROX(507.97 mg g^-1)>p-ASA(379.65 mg g^-1)>As（V）(232.98 mg g^-1)>DMA(158.94 mg g^-1).The formed Fe-O-As inner-sphere coordination between arsenic species and the incomplete-coordinated cationic Fe in the MIL-101（Fe）cluster is the primary adsorption mechanism,which is dominate adsorption mechanism for As（V）.Substituent aromatic units in ROX and p-ASA strengthen the adsorption on MIL-101（Fe）through hydrogen bonds andπ-πstacking interaction,resulting in higher adsorption capacities far beyond that of As（V）and DMA.Due to the steric hindrance caused by organic substituent inhibited the adsorption affinity of organoarsenicals,especially for DMA.（2）Based on defect engineering strategy,acetate modulated Yttrium based metal-organic frameworks MOF-76（Y）-Ac was explored for As（V）adsorption.MOF-76（Y）-Ac presented smaller particle size,higher porosity and larger number of coordinatively unsaturated sites,and consequently exhibited significantly enhanced adsorption affinity and capacity toward arsenate compared with that of pristine MOF-76（Y）.In the optimized operating condition（p H 9-11）,the maximum Langmuir adsorption capacity of arsenate by MOF-76（Y）-Ac can be 201.46 mg g^-1within 30 min.The arsenate adsorption capacity of MOF-76（Y）-Ac remained stable with the presence of coexisting anions and natural organic matters,even in practical simulated gold smelting alkaline wastewater,which can be attributed to the formation of strong Y-O-As coordination connected with Y nodes in MOF-76（Y）-Ac originating from ligand exchange between arsenate and the BTC linkers,besides the ligand change mechanism with Y-OH sites and chemical precipitation mechanism by forming YAs O₄.（3）Based on mix-metal MOFs strategy,a series of Zr/Ce-UIO-66 with different Zr/Ce ratio were synthesized,the relationship between As（V）adsorption performance and stability was investigated.Results show that the increase of Ce ratio of Zr/Ce-UIO-66 resulted in higher adsorption affinity for As（V）,but higher Ce percentage would lead to decreasing stability which resulted in the releasing organic ligands.The enhancement of As（V）adsorption can be attributed to the mechanism of ligand exchange between As（V）and Ce-O-C bond in Zr/Ce-UIO-66 structure as well as coprecipitation.To maintain the stability,ligand exchange between As（V）and Zr/Ce-OH should be the only one adsorption mechanism,such as Zr-UIO-66and Zr_0.7Ce_0.3-UIO-66.Therefore,the optimal Zr/Ce ratio is 7:3 to realize the increase As（V）adsorption performance and maintained stability.（4）Based on metallization of organic ligands strategy.Redox-active Zr-Fc MOFs,which was constructed using Zr cations and 1,1’-ferrocene dicarboxylic acid（Fc DA）as precursors,was driven by peroxymonosulfate（PMS）（Zr-Fc/PMS）to simultaneously realize the enhanced oxidation and adsorption of arsenite（As（III））.Results show that PMS enhanced the adsorption capacity of arsenic on Zr-Fc MOFs and significantly improved the adsorption affinity of As（III）,meanwhile,Zr-Fc MOFs promoted As（III）oxidation by PMS.This enhancement can be attributed to the heterogeneous catalytic oxidation and multi-sites adsorption mechanism,in which the bi-functional Fc ligands in Zr-Fc MOFs not only can activate PMS to produce reactive species(·OH,SO₄^·-,O₂^·-and ¹O₂)for As（III）oxidation,but also can be oxidized to form Fc⁺sites for extra arsenic adsorption.This part of work proved the efficiency and durability of this novel redox-active Fc-based PMS activating system for As（III）removal from aquatic environment.