We successfully developed mesoporous γ-Fe₂O₃ structures with a specific surface area of 35.7 m² g⁻¹ through atmospheric calcination of composite Fe₃O₄/phenol-formaldehyde resin structures, which were prepared by the addition of phenol during a hydrothermal process. For comparison, aggregated γ-Fe₂O₃ nanoparticles with a specific surface area of 29.6 m² g⁻¹ were also prepared. The mesoporous γ-Fe₂O₃ structures were used to adsorb arsenic ions, and the maximum uptake of arsenic ions was calculated to be 73.2 mg g⁻¹, which is higher than that of the aggregated γ-Fe₂O₃ nanoparticles (32.3 mg g⁻¹) because of the larger specific surface area, pore volume and pore sizes of the mesoporous γ-Fe₂O₃ structures. In this study, we developed a feasible approach to wastewater treatment, including the removal of arsenic ions.