Theoretical Design Of Layered Double Hydroxides And The Performance For Arsenic Removal

Layered double hydroxides(LDHs)have tunable compositions and structures with strong arsenic removal potential.However,due to the lack of theoretical guidance,the relationship between the structure and arsenic removal performance of LDHs has not been clarified,which seriously hinders the development of high-performance arsenic removal LDHs.In this work,theoretical designs were carried out around the composition,structure and arsenic removal performance of LDHs.In addition,a new approach of theoretical calculations was provided for the design and development of new arsenic removal materials.The main results are as follows.(1)The relationship between the structure and arsenic removal performance of LDHs was established.Density functional theory(DFT)calculations of the large system were applied to investigate the structure and arsenic removal performance of conventional Mg Al-LDHs.The results indicated that the bonding,weak interactions between arsenic and laminates within the structure could influence the arsenic removal performance of Mg Al-LDHs.Moreover,the arsenic removal performance of Mg Al-LDHs was governed by their electronic structure.As light metals Mg and Al were unable to provide the electronic structure of active sites for LDHs due to their relatively simple electronic states,seriously limiting the arsenic removal performance of Mg Al-LDHs,predicted by calculation to be in the range of about 18.08 ～ 53.28 mg/g.(2)The feasibility of enhanced arsenic removal performance by laminate metal replacement was confirmed.The arsenic removal performance of LDHs after laminate metal substitution was investigated by taking transition metal Mn as an example.The results indicated that Mn substitution into the laminate could significantly improve the electronic structure of Mg Al-LDHs.The replacement of Mn into the laminate as the active center improved the chemical activity of the laminate components and strengthened the hydrogen bonding between arsenic and laminate,creating favorable conditions for the improvement of arsenic removal performance.However,the unreplaced light metals Mg and Al could not provide the prevailing active sites for arsenic removal in LDH.(3)The composition of metals constituting the laminate for efficient arsenic removal from LDHs was further determined.The preferred metals constituting the effective arsenic removal LDHs were selected,based on the charge density of metal ions.The results suggested that Ni(II),Co(II),and Mn(III)with high charge densities,could stably form M-O octahedra and stabilize into the LDHs laminate,thus being candidates for the metal composition of the LDHs.In addition,different metals played different roles in their respective centered single-doped laminates,yet all could effectively improve the electronic structure of LDHs.The main role of Co(II)contributed to the charge density,Mn(III)controlled the electronic structure activity of the laminate,and Ni(II)played the role of activating the laminate-OH.(4)High-performance arsenic-removal multivariate NiCoMn-LDHs were designed and prepared.The optimal metal ratio was determined.The results suggested that the NiCoMn-LDHs with Ni(II):Co(II):Mn(III)of1:2:1 possessed the strongest arsenic removal performance.In addition,the coexistence of Ni(II),Co(II),and Mn(III)in the laminate exhibited a synergistic effect,resulting in the enhancement of the dominant effect of each metal for the improvement of the arsenic removal performance.Finally,based on the guidance of theoretical calculations,highperformance NiCoMn-LDHs was successfully prepared,and its maximum arsenic removal capacity reached 407.23 mg/g according to the arsenic removal verification experiments,outperforming the existing similar materials and with potential application prospects.104 Figures,10 Tables,and 170 References.