Synthesis and characterization of mixed Co-Zn-ZIF for arsenic(V) adsorption

One-pot synthesis of ZIF-8, ZIF-67, and a series of their mixtures was carried out in ambient conditions. Among the two reagents used for the deprotonation of 2-methylimidazole (2-mim), only ZIF-67 could be formed in the presence of both sodium hydroxide and trimethylamine. In contrary, no trace of ZIF-8 was observed when trimethylamine was used. About the solvents, because ZIF-67 could not be prepared in aqueous condition, for the given metal to ligand ratio, methanol was used for all. Once the most favorable condition was set, ZIF-8 (Zn(2-mim)(2)) and ZIF-67 (Co(2-mim)(2)), and the mixtures Zn0.9-0.1Co0.1-0.9(2-mim)(2) were successfully synthesized. The composition of the mixture ZIFs was found to be nearly in proportion with the mixing ratios of the Zn and Co salts. The XRD patterns of ZIF-8, ZIF-67, and the mixtures were found to be finely matching. However, the crystallite sizes of the mixtures were relatively larger than both the pure materials. FT-IR shows up to 2% red-shift of some major peaks with the change from ZIF-8 to ZIF-67. TG-DTA showed a sharp drop in the decomposition temperature with the introduction of Co into the framework. About the arsenate ion adsorption, all the materials could uptake AsO4 anion from KH2AsO4 solution, basically following the same mechanism, ligand exchange. Yet, for the basic pH solution, ZIF-67 and the Co-rich mixtures show surprisingly higher adsorption capacity.