Biogenic synthesis of NiO nanoparticles using <i>Ficus religiosa</i> and their application to sequestrate anionic dye

This study demonstrates the use of Ficus religiosa (FR) leaves as a natural, cost-effective, green stabilizing and capping agent for the synthesis of nickel oxide (NiO) nanoparticles (NPs) for the sequestration of anionic Congo Red (CR). Various techniques, including FTIR, XRD, field-emission scanning electron microscopy, dynamic light scattering, Brunauer–Emmett–Teller, HRTEM, and a UV–Vis spectrophotometer, were employed to demonstrate the efficiency of the synthesized NiO (FR) NPs as a biosorbent. The crystallite size of the NPs was found to be 5 nm for NiO NPs (control) and 2.8 nm for Ni (FR) NPs, calculated using XRD. The adsorption behavior of the biosorbent was examined by varying several parameters, such as pH, dye concentration, adsorbent amount, and equilibrium time. Adsorption isotherms and kinetic models were also fitted. The maximum adsorption capacity of CR on biogenically synthesized NiO NPs was found to be 17 mg g−1, achieved at 37 °C or 310.15 K. Isotherm and kinetic studies revealed that Langmuir and pseudo-first order were found to be the best fit. The recyclability of the adsorbent remains good after 5 cycles of regeneration, with the adsorbent achieving ∼83% removal using the real wastewater sample. Overall, NiO (FR) NPs, as a sorbent based on dried FR leaves, exhibit strong potential as an effective biosorbent for the adsorption of CR via contaminated water.