Synthesis of ZIF-8 in high yield and simultaneous removal of Mn(II), Cu(II), and Cd(II): performance and mechanism

The mining and smelting of manganese results in a composite pollution that is primarily manganese with coexisting copper and cadmium. This pollution is extremely dangerous for both human health and the environment. Scholars have found that ZIF-8 has a saturated adsorption capacity of up to 276.75 mg/g for the single Mn(II) pollution solution. However, there are common defects in the preparation of ZIF-8, such as low yield, serious waste of 2-methylimidazole, and commonly used toxic methanol as a solvent. This study focused on the common defects in the preparation of ZIF-8 and the gaps in the treatment of Mn-Cu-Cd composite pollution. Water-TEA (water volume was 27.0 mL, TEA addition amount was 4.5 mL), which was both economical and environmentally friendly, was chosen as the solvent. Under the conditions of a 2:1 molar ratio of 2-methylimidazole to zinc nitrate hexahydrate, ZIF-8, with a yield of 92.93% (the yield was 3.79 times more than methanol as a solvent, significantly reducing costs), was quickly synthesized and was excellent for treating the single Mn(II) pollution solution and the Mn-Cu-Cd composite pollution. According to the research findings, the competitive adsorption capacity of ZIF-8 for Mn(II), Cu(II), and Cd(II) was in descending order: Cu(II) >Mn(II) >Cd(II). In the single Mn pollution solution and the Mn-Cu-Cd composite pollution solution, the adsorption of Mn(II) by ZIF-8 was more corresponding to the quasi-second-order kinetic model and Langmuir equation, with chemical adsorption and single-layer adsorption as the main modes. The saturated unit adsorption capacities were 185.19 mg/g and 199.49 mg/g, respectively. ZIF-8's adsorption capacity increased, but its adsorption rate for Mn(II) decreased when a small amount of Cu(II) and Cd(II) were added. ZIF-8's primary mechanisms of adsorption on Mn(II), Cu(II), and Cd(II) were hydroxyl coordination, amino coordination, and ion exchange. In addition to lowering costs dramatically, the water-TEA system's high-yield synthesis of ZIF-8 offers many potential applications in the management of both single and composite pollution control.