Adsorption Study Of Heavy Metal Ions On Zeolite-Imidazolate Frameworks

With the rapid development of industrialization, people pay much attention to industrial water pollution. In fact, heavy metal ions wastewater is the most dangerous to human and the environment, so removing heavy metal ions from wastewater is a very important issue. Zeolite-imidazolate frameworks (ZIFs) possess many excellent properties such as large specific surface area and high porosity, high thermal stability, chemical stability. They have been widely studied in some areas including gas storage and separation, catalysis and luminescent probe etc. In addition, it has important research value in wastewater treatment especially the adsorption of heavy metal ions from wastewater.The main work in the experiment included:(1) ZIF-8and ZIF-67were synthesized by solvothermal method. Their morphologies, specific surface area and pore volume as well as pore diameter were observed or tested by scanning electron microscope and surface area analyzer. ZIF-8is a flaky hexahedron while ZIF-67is composed of wafer-like shape. They have bumpy grooves, large specific surface area and high porosity, which are beneficial to the adsorption of metal ions.(2) Through studying the adsorption effect of adsorption time, adsorbent dose and initial concentration, it indicated that ZIF-8and ZIF-67both had excellent adsorption effect for Pb2+、Cu2+and Cr3+. In fact, ZIF-67showed better performance in respects of adsorption rate, adsorption capacity and removal efficiency than ZIF-8, which may have something to do with its larger specific surface area. The maximum adsorption capacities of Pb2+、 Cu2+、Cr3+are1119.80mg·g-1.454.71mg·g-1、137.39mg·g-1for ZIF-8and1348.41mg·g-1、617.50mg·g-1、216.73mg·g-1for ZIF-67. As for the effect of adsorption time, adsorption process included initial rapid adsorption, the following adsorption at slow speed and final equilibrium stage. For the effect of adsorbent dose, the adsorption capacity and removal efficiency of Cr3+on ZIF-8and ZIF-67could achieve the maximum value simultaneously, indicating that the adsorbent doses were optimum. Moreover, when initial concentration was200mg·L-1and adsorbent dose was2.0g·L-1, the removal efficiency of Pb2+and Cr3+by both adsorbents could reach almost100%. When initial concentrations of Cu2+were within certain limits, the adsorption capacity qe of Cu2+on both adsorbents and initial concentration ci showed linear relationships and both correlation coefficients were more than0.999.(3) Pseudo-first-order kinetics model, pseudo-second-order kinetics model and intra-particle diffusion model were employeed to fit the adsorption data of Pb2+、Cu2+、Cr3+on ZIF-8and ZIF-67. The results showed that all the adsorption processes fitted best with pseudo-second-order kinetics model and most correlation coefficients were higher than0.999, indicating the adsorption may include chemical adsorption. The fit of intra-particle diffusion model showed that adsorption rate was controlled by external diffusion and intra-particle diffusion simultaneously instead of only intra-particle diffusion. Langmuir and Freundlich isothermal adsorption models were used to fit the adsorption of Cu2+and Cr3+on ZIF-8and ZIF-67. Compared to Freundlich model, Langmuir model fitted better with experimental results, indicating that the adsorptions of Cu2+and Cr3+on both adsorbents were mainly a monolayer adsorption. XPS studies indicated that the adsorption mechanism was probably the coordination bond between Pb2+、Cu2+or Cr3+and nitrogen element of2-methyl imidazole in ZIF-8and ZIF-67.