Magnetite, the main component of converter slag in the steel industry, can be used for the adsorption of Ni(II) from aqueous solutions, over a range of conditions: initial metal concentration (10–100 mg l−1), stirring times (2–240 min), adsorbent dosage (1 g for 0.5 l of metal solution) and temperatures (20, 30 and 38°C). The adsorption rate increased with initial concentration but decreased with increase in temperature, due to competition of the dissolution process. The adsorption process obeyed the Freundlich isotherm model.