<p>Iron-loaded Japanese cedar (JC) charcoal adsorbents were synthesized under various conditions to investigate their ability to adsorb 3d-block metal ions. The metal ions (Mⁿ⁺) examined in this study were Cr³⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺. Ordinary JC charcoal adsorbents used as references were also prepared under the same synthetic conditions. The aqueous Mⁿ⁺ solutions were prepared using metal chlorides and deionized water, and the concentration of Mⁿ⁺ was set to 2.5 × 10^− 2 mmol/L or 2.5 mmol/L. Fe(NO₃)₃ was used to synthesize iron-loaded charcoal. The Fe³⁺ content in JC wood, which was the crude material for iron-loaded charcoal, was set to 3 or 7% w/w by adding an appropriate amount of aqueous Fe(NO₃)₃ solution. JC wood powder impregnated with Fe³⁺ was heated at 600 or 800&#xa0;°C for 1.0&#xa0;h under a N₂ gas flow. The charcoal adsorbents were analyzed using Raman spectroscopy and X-ray diffractometry. The adsorption abilities of the JC charcoal adsorbents for Mⁿ⁺ ions were assessed based on adsorption isotherms, which consisted of double logarithmic plots of the adsorbed Mⁿ⁺ concentrations on charcoal versus the dissolved Mⁿ⁺ concentrations in aqueous solution.</p>