Simultaneous removal of cationic and anionic dyes by highly efficient and recyclable ZIF-67/expanded vermiculite (ZIF-67/EV) composites.

Abstract

This study focuses on the synthesis of composite materials using Zeolitic imidazolate frameworks (ZIF-67) nanoparticles as an effective adsorbent, along with different concentrations (2-10%) of thermally expanded vermiculite (EV) as a low-cost and natural adsorbent substrate. The pristine materials and their composites were fully characterized using XRD, FTIR, BET, SEM, zeta potential, and EDS techniques. The pseudo-second-order kinetic model described both organic dyes' adsorption on synthesized adsorbents. Accordingly, the calculated adsorption capacities of Congo Red (CR) and Malachite Green (MG) dyes over the synthesized adsorbents were found to be about 22.72 and 49.02 mg/g for pure EV, 100 and 100 mg/g for pure ZIF-67, 90.91 and 100 mg/g for ZIF-67/EV-2, 100 and 100 mg/g for ZIF-67/EV-5, 95.24 and 99.01 mg/g for ZIF-67/EV-7, and 92.59 and 97.09 mg/g for ZIF-67/EV-10, respectively. The Langmuir isotherm model fits experimental isotherm data best in the studied temperature range (298-313 K). Among the synthesized adsorbent materials, the ZIF-67/EV-5 composite (containing 5% EV flakes) showed the highest maximum adsorption capacities of 1428.6 and 1114.2 mg/g for MG and CR dyes at pH 7 and 298 K. Moreover, it showed the highest removal efficiency (up to 99.5%) toward both cationic MG and anionic CR dyes in the binary mixture of both dyes. Finally, the regeneration and recyclability of this composite showed a 12% decrease in dye removal after five adsorption cycles. The synthesized ZIF-67/EV composites may therefore be used as efficient and inexpensive adsorbent materials for the simultaneous removal of cationic and anionic dyes from contaminated water. PRACTITIONER POINTS: ZIF-67/expanded vermiculite composites were synthesized and used to simultaneously remove cationic and anionic dyes from wastewater. Kinetics, isotherms, and thermodynamics of adsorption were studied showing good removal of both dyes. The ZIF-67/EV-5 composite achieved maximum adsorption capacities of 1428.6 and 1114.2 mg/g for cationic Malachite Green and anionic Congo Red dyes, respectively. Various interactions like π-π stacking and coordination are proposed as mechanisms of adsorption. The composite showed good selectivity in separating dyes and maintained high removal efficiency even after 5 reuse cycles.