Specific chemical adsorption of selected divalent heavy metal ions onto hydrous γ-Fe2O3-biochar from dilute aqueous solutions with pH as a master variable

A γ-Fe2O3-biochar was synthesized using ferrate as precursor. Results showed that ferrate treatment of biochar increased surface acidity and magnetism strength, which enhanced metal ion adsorption and enabled easy magnetic separation of spent biochar after use. The adsorption characteristics of selected metal ions including Cu(II), Pb(II), Zn(II), Ni(II), Cd(II) and Co(II) onto hydrous γ-Fe2O3-biochar were studied. The adsorption isotherms were well fitted by the Langmuir adsorption model. The γ-Fe2O3-biochar exhibited significantly high monolayer metal coverage capacity. For example, the maximum Cu(II) adsorption density was 55.45 × 10-5 mol/g, which was 4.0 times that of plain biochar (13.75 × 10-5 mol/g) at pH 6. Metal ion adsorption reactions could be described by surface complex formation, involving all M(II) hydroxy species and surface hydroxy species, i.e., ≡FeOH and ≡COH. Both experimental and calculation results suggested the formation of inner- and outer-sphere complexes. Coulombic and specific chemical forces contributed to the total adsorption energy, with specific chemical energy being the dominating component for the adsorption of hydrolyzable metal ions onto γ-Fe2O3-biochar surface. Under the context of surface complex formation, it is visualized that metal ion hydrolysis reactions occurred on γ-Fe2O3-biochar surface, in parallel to that in the bulk phase.