Facile synthesis of ZIF-8@GO composites for enhanced adsorption of cationic and anionic dyes from their aqueous solutions

Abstract

In this study, zeolitic imidazolate frameworks (ZIFs) and ZIF-8-graphene oxide (ZIF-8@<i>x</i>GO) composites were prepared at room temperature to be used as adsorbents for cationic (methylene blue (MB)) and anionic dyes (methyl orange (MO)) from their aqueous solutions. The structural characteristics confirmed the successful preparation of amorphous ZIF-8 and its ZIF-8@GO composites at room temperature. The BET surface area of the ZIF-8@0.5GO composite was estimated to be 286.22 m² g^-1, with a mean pore diameter of 3.34 nm. The adsorption study confirmed that dye removal efficiency of ZIF-8 was significantly enhanced when blended with GO. The maximum removal efficiency of the ZIF-8@0.5GO composite was achieved within 60 min, and the removal percentages of MB and MO dyes were 95.2% and 94.6%, respectively. These values were close to those achieved by GO at 60 min (96.2% for MB and 96.3% for MO). The kinetic study confirmed that the adsorption data of MB onto GO, ZIF-8, and the ZIF-8@<i>x</i>GO composites fitted the non-linear pseudo-first-order kinetic model, while the adsorption of MO dye obeyed the non-linear pseudo-second-order kinetic model. Moreover, the adsorption isotherm study confirmed that the adsorption of both MB and MO dyes onto the ZIF-8 and its ZIF-8@<i>x</i>GO composites were fitted to the Langmuir model, which indicates a chemical adsorption process. The estimated maximum adsorption capacity of the ZIF-8@0.5GO composite towards MB and MO were 87.39 and 82.78 mg g^-1, which are much higher than that achieved by pure ZIF-8 and very close to that obtained by pure GO. This indicates that our prepared ZIF-8@GO composites are comparable to pure GO. The thermodynamic study confirmed that adsorption of both the dyes onto the prepared materials is endothermic, spontaneous, and thermodynamically favorable.