Chromium(III) adsorption removal from acidic solutions by isomeric and tunnel-structural iron oxyhydroxides

Iron oxyhydroxides for heavy metal treatment have attracted wide attention. In this work, iron oxyhydroxides of isomeric FeOOH (GpI) and tunnel-structural schwertmannite/akaganéite (GpII) were selected to study chromium (Cr(III)) adsorption removal from acidic aqueous solutions by batch experiments, under various reaction time, adsorbate/adsorbent level, pH and anions. Adsorption processes well fitted to pseudo-second-order kinetics (R² = 0.992-0.999, except for 0.829 for Lep). Isotherm data could be fitted by Langmuir (R² = 0.901-0.985), Freundlich (R² = 0.884-0.985) and Temkin (R² = 0.845-0.961) models at pH 3.7. Langmuir maximum adsorption capacities (mg/g) were 10.4-18.8 (FeOOH, except for 3.08 for Gth2) in GpI, and 20.60/43.40 (Sch-Chem/Sch-Bio) and 12.80/24.70 (Aka-Chem/Aka-Bio) in GpII. Adsorption capacities would gradually increase as Cr(III) concentrations increased within 0-40 mg/L, and could be markedly affected by the SO₄^2- and H₂PO₄^- anions. There were stable adsorption capacities at about pH 3.7, and then increased at pH 3.7-4.1. The Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) results showed that adsorption mechanisms were electrostatic interaction and surface complexation. In addition, three optimal bio-/chem-schwertmannite and lepidocrocite adsorbents had good reusable properties and treating abilities of Cr(III)-polluted waters at pH 4.0. These results could provide a theoretical basis for the application of iron oxyhydroxides in removing Cr(III) from acid wastewaters.