Adsorptive removal of hazards from an aqueous solution using ZIF-8 decorated vermiculite

Abstract

Natural clay materials are common adsorbents for toxic pollutants in water. In spite of their high abundance, aluminosilicate-based clays with a negative surface charge are seldom utilized as adsorbents for anionic dyes due to their low adsorption capacity. To overcome this drawback, the surface of exfoliated vermiculite (eVMT) clay was chemically modified by zeolitic imidazolate framework-8 (ZIF-8) nanoparticles. The hybrid eVMT@ZIF-8 adsorbent exhibited that microporous ZIF-8 nanocrystals with an average size of 40 nm were strongly attached and densely distributed on the surface of planar eVMT, thus providing a great surface area and a reversal in surface charge polarity. This hybrid nanomaterial was investigated as an adsorbent for the removal of such as anionic congo red (CR) dye and Cu(II) ions from aqueous solution.

In the CR adsorption, experimental parameters including solution pH, initial CR concentration and contact time had been evaluated and optimized. Remarkably, the significant CR uptake of 126.6 mg/g was observed for eVMT@ZIF-8 via surface ZIF-8 modification. The adsorption data was well interpreted with the Langmuir isotherm model and a pseudo-second order kinetic, suggesting the chemisorption of CR on eVMT@ZIF-8 in monolayer. The adsorption mechanism was proposed to the electrostatic interactions between the positive nitrogen and metal cation of surface ZIF-8 and negatively charged groups (SO3- and –N=N-) of anionic CR dye.

Furthermore, the same eVMT@ZIF-8 material was tested to adsorb Cu(II) cations from an aqueous solution. The Cu adsorption also follows the pseudo-second order kinetics and Langmuir adsorption model. The adsorption equilibrium was reached only after 5 min, with 90% removal of Cu(II). The Cu(II) uptake was 143 mg/g, being 4.8 times higher than that of eVMT. The rapid and enhanced adsorption uptake was interpreted with the ion exchange reaction between Cu(II) and Zn(II) within ZIF-8. These results reveal that the novel eVMT@ZIF-8 adsorbent could be a cost-effective adsorbent for harmful pollutants in aqueous solution.