Hexavalent Chromium Removal Using Ionic Liquid Coated Magnetic Nano Zero-Valent Iron Biosynthesized by Camellia sinensis Extract

Nano zero-valent iron (nZVI) coated with magnetite and an imidazolium-based ionic liquid (1-hexyl-3-methylimidazolium hexafluorophosphate (HMIMPF6)) core/shell nanoparticles (NPs) (nZVI@Fe3O4@HMIMPF6) were successfully green synthesized for the first time using industrial iron waste. Polyphenols were extracted from dried leaves of Camellia sinensis and applied as the reducing agent for nZVI synthesis. Synthesized NPs were characterized by FT-IR, XRD, VSM, BET, SEM, TEM, and mapping analysis. According to the results, NPs were in the size range of less than 30 nm and the values of $${{a}}_{{{s}}\text{, BET}}$$ = $${3.5975} {\text{ m}}^{2}{{\text{g}}}^{-{1}}$$ , $${{V}}_{{m}}$$ (BET monolayer capacity) =  $${0.8265 \,}{\text{cm}}^{3}\left({\text{STP}}\right) \, {\text{g}}^{-{1}}$$ , C = 47.378, total pore volume ( $$\frac{{p}}{{{p}}^{0}}=0.990) \, = \, {0.14998 \,}{\text{cm}}^{3}{{\text{g}}}^{-1}$$ and mean pore diameter = $$\text{16.68 nm}$$ were obtained. These NPs were used for the Cr (VI) removal from aqueous samples. Taguchi orthogonal array design was used to optimize the various affecting variables on the efficiency of Cr (VI) removal. Results showed that about 90.0% of Cr (VI) $$\mathrm{were}$$ removed at the optimum conditions. Kinetic and isotherm studies were conducted, and the pseudo-second order kinetic model was determined as the most appropriate kinetic model. The equilibrium data displayed proper linearity with the Freundlich isotherm model (R2 = 0.9697), and the maximum adsorption capacity was found to be $$\text{56.49 mg }{\text{g}}^{-{1}}$$ (in terms of CrO42− ions). Also, Redlich-Peterson, Sips, and Toth as three-parameter isotherm models were compared to two-parameter models using MATLAB’s fmincon function. Thermodynamic studies showed negative ΔG°, ΔH° = 50.255 kJ mol−1, and ΔS° =  + 0.1799 kJ mol−1 K−1 indicating spontaneous and endothermic adsorption process. According to the results, the $$\mathrm{nZVI}@{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}@\mathrm{IL}$$ NPs can be recovered by an external magnet and exhibit good applicability for Cr (VI) removal from real samples.