Mechanism of Co(II) adsorption by zero valent iron/graphene nanocomposite

Highlights: • Nanoscaled zero valent iron/graphene show a high performance for Co(II) removal. • The Co-substituted iron oxides transformed to CoFe₂O₄-like structure at pH 3.0 and 6.0. • The Co-substituted iron oxides changed to green rust-like phases at pH 9.0. • Co(II) removal involved in isomorphous substitution, electrostatic action and precipitation. - Abstract: Nanoscale zero valent iron (ZVI)/graphene (GF) composite was prepared and characterized by Brunauer–Emmett–Teller (BET) surface area measurement and zeta potential determination. The adsorption isotherm of Co(II) in aqueous solution, as well as the influence of pH values and ionic strengths was studied. The mechanism of Co(II) adsorption by GF was investigated through analyzing the sorption products at initial pH of 3.0, 6.0 and 9.0 using high-resolution transmission electron microscope with energy dispersive X-ray detector (HRTEM-EDX), X-ray diffraction (XRD), vibrating-sample magnetometer (VSM), Raman spectra, X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) measurement. The results indicated that Langmuir isotherm model fitted well and the adsorption capacity was 131.58 mg g⁻¹ at 30 °C. Adsorption capacity was not significantly influenced by ionic strength and kept high at pH 4.0 ∼ 9.0. The detail information of GF–Co interaction at different initial pH values was obtained using XAFS analysis combined with other characterization methods. Coordination numbers (CN) and interatomic distances (R) of both Fe and Co were given. At pH 3.0 and pH 6.0, the Co-substituted iron oxides transformed to CoFe₂O₄-like structure, while at pH 9.0 they changed to green rust-like phases. Co occupied preferentially in the octahedral sites in acid solution. The adsorption mechanism of Co(II) was attributed to inner-sphere complexation and dissolution/re-precipitation of the substituted metal oxides.