Sustainable and Green Synthesis of Nano Zero-Valent Iron for Hexavalent Chromium Adsorption

This research aimed to assess the potential of Ethiopian coffee extracts in synthesizing nano zerovalent iron (nZVI) from ferric chloride hexahydrate (FeCl₃·6H₂O) and to explore their ability to adsorb hexavalent chromium (Cr (VI)) from solution. The resulting nano zerovalent iron (nZVI) was analyzed using transmission electron microscopy (TEM) and Fourier-transform infrared (FTIR) spectroscopy. The particle sizes of nZVI derived from Wenbera (W-nZVI) and Sidama (S-nZVI) coffee extracts were below 10 nm. Cr (VI) adsorption to W-nZVI and S-nZVI were 91 and 94%, respectively, achieved at a pH of 3, an initial Cr (VI) concentration of 10 mg/L, and a temperature of 30 °C. The results confirmed that both W-nZVI and S-nZVI adsorbed significant amounts of Cr (VI). Adsorption isotherm models indicate that favorable removal of Cr (VI) by W-nZVI followed the Langmuir equation (R ² = 0.9913; 0 < R _L < 1) better than the S-nZVI adsorption data (R ² = 0.9795). However, adsorption behavior for S-nZVI fitted the Freundlich isotherm model better (R ² = 0.9999; n = 2.029) compared with the W-nZVI adsorption data (R ² = 0.999; n = 0.559). The maximum adsorption capacities for W-nZVI and S-nZVI were 35 and 38 mg/g, respectively. The adsorption of Cr (VI) from aqueous solutions was spontaneous, accompanied by a negative Gibbs energy and exothermic, accompanied by a negative enthalpy of adsorption. The adsorption kinetics were better described by the pseudo-second-order model for both adsorbents. Taken together, nZVI synthesized by using coffee extract may provide a sustainable approach to remove hexavalent chromium from solution.