Controllable Synthesis And Modification Of Cobalt-Aluminum-Based Layered Double Hydroxides For Enhanced Pollutant Adsorption Performance

Layered double hydroxides（LDHs）are a class of two-dimensional materials consisting of interlayer anions and positively charged laminates.LDHs are a very promising class of pollutant adsorbents,however,there is still room for further improvement in their pollutant adsorption performance.Since LDHs can be modified with different interlayer anions regulation and metal cation doping,the adsorption performance of LDHs can be optimized through the controllable synthesis and modification of LDHs materials.In this paper,the modification of LDHs and the increase of their adsorption activity sites were achieved through the modulation of the interlayer anions and the doping of the laminate cations in CoAl-LDHs.The modified LDHs exhibited a significant improvement in the adsorption performance of anionic pollutants such as methyl orange（MO）and Orange I（OI）.The experimental results of MO/OI adsorption on CoAl-LDHs demonstrated that:CoAl-LDH（Cl^–）had the fastest adsorption rate and the maximum adsorption capacity for MO reached 1372.1 mg/g;CoAl-LDH（NO₃^–）had the fastest adsorption rate and the maximum adsorption capacity for OI reached 757.6 mg/g.This is mainly attributed to their excellent ion exchange capacity(NO₃^–/Cl^–﹥CO₃^2–),large specific surface area and pore volume.Furthermore,the ion exchange driven by electrostatic interaction,hydrogen bonding,and surface complexation may be the main mechanisms for the adsorption of MO,OI on CoAl-LDH（Cl^–）and CoAl-LDH（NO₃^–）.However,the MO and OI adsorption on CoAl-LDH(CO₃^2–)was strongly p H-dependent and the optimal p H value was about 3,which indicated that the electrostatic interaction was the main mechanism for the adsorption of MO and OI on CoAl-LDH(CO₃^2–).The pseudo-second-order kinetic model and Langmuir/Sips isotherm model can fit the adsorption equilibrium data well,indicating that the adsorption process of MO and OI on CoAl-LDHs are multilayer adsorption at low MO/OI concentrations and monolayer surface adsorption at high MO/OI concentrations,and are controlled by chemisorption.Two ternary LDHs materials,Co Ni Al-LDH and Co Ni Fe-LDH,were designed and synthesized by hydrothermal method to remove MO from wastewater.The adsorption capacity of ternary LDHs is significantly better than that of binary CoAl-LDH(CO₃^2–).The maximum adsorption capacities of MO by Co Ni Al-LDH and Co Ni Fe-LDH reached 5146.8 and 2188.3 mg/g,respectively.This is mainly due to the fact that ternary Co Ni Al-LDH and Co Ni Fe-LDH have approximately 9 and 4 times higher specific surface areas and approximately 11 and 5 times higher total pore volumes,respectively,compared to binary CoAl-LDH(CO₃^2–).The large pore volume and specific surface area usually facilitate the effective exposure of active sites and can provide more efficient transport pathways,which are beneficial to improve adsorption performance.In addition,the initial p H of the solution has a strong influence on the MO adsorption performance of Co Ni Al-LDH and Co Ni Fe-LDH.Combined with the adsorption experiments and the characterization analysis,it can be concluded that the ion exchange driven by electrostatic interaction,hydrogen bonding and surface complexation may be the main mechanisms of MO adsorption on Co Ni Al-LDH and Co Ni Fe-LDH.The pseudo-second-order kinetic model and Langmuir/Sips isotherm model can fit the adsorption equilibrium data well,indicating that the adsorption process of MO on Co Ni Al-LDH and Co Ni Fe-LDH are multilayer adsorption at low MO concentrations and monolayer surface adsorption at high MO concentrations,and are controlled by chemisorption.