ZIF-Based Novel Green Nanomaterial for Cr(VI) Removal

Cobalt (Co)-based zeolitic imidazolate frameworks (ZIF)-L is a porous material having a high surface area, and high thermal, chemical, and hydrothermal stability. ZIF-L (Co) finds various applications in catalytic reactions, drug delivery, gas separation, adsorption of heavy metals, and wastewater treatment. Hexavalent chromium [Cr(VI)] is a heavy metal that is extremely carcinogenic in nature. According to WHO, the permissible limit for chromium in drinking water is 50 µg/L^–1, confirming its removal to be paramount. The objective of this work includes optimization of ZIF –L (Co) nanoparticles (NPs) synthesis by varying the addition rate of 2-methyl imidazole solution to the metal precursor solution (Cobalt salt) and studying the removal efficiency of chromium by carrying out adsorption studies using the prepared NPs. The synthesized NPs were characterized for evaluation of morphology, crystal structure, and functional groups using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR), respectively. The leaf-like structure revealed by SEM images confirmed the formation of ZIF-L (Co) NPs, and the average size of NPs was found to be 250 ± 88 nm. The XRD diffractogram confirmed all the peaks pertaining to ZIF-L, whereas the FTIR spectra revealed the functional groups on ZIF-L (Co) NPs bearing important peaks at 427, 754, 1143 and 1386 cm^-1 corresponding to vibrations of Co-N, imidazole ring, C-H, and C=N, respectively. Batch adsorption studies were conducted to evaluate the Cr(VI) removal efficiency by varying the contact time, adsorbent dosage, and concentration of Cr(VI) ions. The results depicted that equilibrium was reached within 20 min for an initial concentration of 20 mg/L of Cr(VI) when the adsorbent dosage was 0.6 g/L. This study investigates the use of ZIF-L (Co) NPs for the adsorption of Cr(VI), and further studies are required to use this material for the remediation of the Cr(VI) contaminated water.