Synthesis Of Novel Inorganic Cationic Framework Materials For Oxyanionic Pollutants Removal

The cationic framework is positively charged and contains anions inside the framework compensating the positive charge of the host.Since the anions are exchangeable,the cationic frameworks are considered to be one of promising materials for the oxyanionic pollutants removal.Among the cationic framework materials,crystalline inorganic cationic frameworks usually show high thermal stability,high hydrolysis stability,and low cost,which make them good prospect in the field of adsorption separation,anion exchange,catalysis,etc..Compared to the large amount of neutral or anionic inorganic framework materials?such as zeolites?,the number and structure types of purely inorganic cationic framework materials are very limited.Layered Double hydroxides?LDHs?are the most common inorganic cationic framework materials.However,an energy-intensive activation by high-temperature calcination prior to performing anion exchange is required.In addition,they also suffer from slow sorption kinetics and poor selectivity mainly due to the strong affinity for CO₃^2-.Other main non-LDHs inorganic cationic framework materials include layered rare earth hydroxide?LRHs?,layered metal fluoride,layered oxyhydroxide,and layered hydroxide salts,etc..Similar to LDHs,most of them,especially LRHs possessing higher charge density than LDHs,have the ability to remove oxyantionic pollutants from aqueous solution.However,those materials contains rare earth elements,heavy metal elements,and radioactive elements,which seriously limit their application in reality.Thus,it is of great significance to develop novel inorganic cationic framework materials with low cost,environmentally benign nature,and superior performance.Aiming this challenging topic,this thesis developed a series of novel inorganic cationic framework materials which are practically applicable for oxyanionic pollutants removal.The main results are summarized as follows:1.Comprehensively investigated the physical-chemical properties of cationic aluminoborate BAC?10?and the performance in removal of Cr?VI?from aqueous solution.The thermal stability and hydrolytic stability of BAC?10?were systematically investigated and an unusual“structural memory effect”for 300°C calcined BAC?10?was observed.Its adsorption kinetic follows the pseudo second-order model and is faster than those of commercial anion exchange resin and layered double hydroxides?LDHs?.BAC?10?has a certain pH dependence on Cr?VI?adsorption,and the optimal pH is 3.Its adsorption isotherm follows the Langmuir isotherm model and the maximum sorption capacity is 139.1 mg/g,significantly surpassing that of commercial anion exchange resin?62.77 mg/g?and LDHs?81.43 mg/g?.More importantly,BAC?10?exhibits a remarkable selectivity for Cr?VI?even in the presence of 20-fold excess competing anions of NO₃^-,Br^-,Cl^-,and HCO₃^-.Moreover,BAC?10?also possess an excellent Cr?VI?removal performance for the solution with low concentration??10 mg/L?,and the residual concentration can be reduced to below0.05mg/L,meeting the WHO drinking water criterion.In addition to Cr?VI?,BAC?10?also has superior removal efficiency on other harmful oxyanions.Investigation on the adsorption mechanism by a series of characterizations unravel that the capture of Cr?VI?in the form of HCrO₄^-follow an anion exchange process with Cl^-in the framework of BAC?10?.These superior features indicate that BAC?10?is an emerging and promising candidate for Cr?VI?remediation as well as other harmful oxyanions from wastewater.2.Firstly synthesized a novel layered cationic aluminum oxyhydroxide JU-111with an empirical formula of Al₃O_2.5?OH?_3.5Cl_0.5·0.9H₂O.Its structure has been successfully determined by combining the 3D EDT and powder X-ray diffraction data.The structure of JU-111 consists of cationic aluminium oxyhydroxide layers stacked along the crystallographic b axis with Cl^-anions as interlamellar charge-balancing anions.JU-111 is stable up to 350 ^oC under air atmosphere and shows a better thermal stability than LDHs.JU-111 is stable in the water with a pH of 1-11,showing excellent hydrolysis stability.The batch sorption results indicate that JU-111 is a promising candidate for capturing Cr?VI?from polluted waters with a high sorption capacity and ultrafast kinetics,which is significantly superior to the extensively studied anion exchange resin and LDHs,and comparable with most of high performance cationic MOFs.Furthermore,due to the unique acidic/basic buffer ability,JU-111 shows excellent selectivity for Cr?VI?even in the presence of a large excess of CO₃^2-and is able to work steadily in a broad pH range of 3-10.JU-111 also has superior removal efficiency on other harmful oxyanions.The investigation on the sorption mechanism confirms that the removal of Cr?VI?from water is achieved via an anion exchange of HCrO₄^-with freely mobile interlayer Cl^-of JU-111.Such exceptional features of JU-111 along with their ultra-low cost,environmentally benign nature,and no any needed pre-treatment before usage make JU-111 a very promising candidate for practical application in toxic oxyanions elimination.3.By using calcium hydroxide,sodium aluminate,and sodium carbonate as raw materials,CO₃^2-intercalated hydrocalumite?Ca-Al-LDH?was synthesized via traditional co-precipitation method.The hydrocalumite was calcined at the temperature between 500 and 900 ^oC and a derivative of mayenite(Ca₁₂Al₁₄O₃₃)is obtained when the calcination is performed over 700 ^oC,which had excellent removal efficiency for low concentration ReO₄^-and high adsorption capacity.Compared to traditional Mg-Al-LDH and other four typical inorganic cationic framework materials?NDTB-1,Y₂?OH?₅Cl·1.5H₂O,BAC?10?,and JU-111?,the derivative of mayenite has a much better TcO₄^-/ReO₄^-adsorption performance.Comprehensive characterizations on the solid samples before and after the adsorption of ReO₄^-revealed that the adsorption of ReO₄^-was on the basis of the dissolution-reconstruction process of mayenite.After contacting water,mayenite was dissolved and Ca-Al-LDH was reformed,in which ReO₄^-would be intercalated into the newly generated Ca-Al-LDH,so as to achieve the purpose for ReO₄^-removal.