Display omitted •A low-cost adsorbent was successfully prepared and had high capacity for removing Cd(II).•Adsorbent characterizations were investigated using modern instrument analytical technique.•pH was the most critical factor affecting the removal of Cd(II).•Adsorption kinetic, equilibrium and thermodynamicaspects were examined.•Cd(II) removal mechanism was investigated. In this work, the original and modified steel-making slags were tested to remove of Cd(II) ions from acidic aqueous solution. The physicochemical characteristics of the materials were investigated to understand the effect of surface properties on the Cd(II) adsorption behavior. The effects of contact time, pH, adsorbent dosage and temperature on adsorption process were studied in batch experiments. The results showed that the adsorption capacity of modified steel-making slag was found to sharply increase as a result of modification, and the adsorbent has excellent acid neutralization capacity. The optimum condition for removal was found to be 10g/L adsorbent on the treatment of 100mg/L Cd(II) at pH 4.0, and the Cd(II) removal rate could reach 99.1%. Kinetic data were best described by pseudo-second-order model. The adsorption isotherms were good fit to the Langmuir model with a maximum adsorption capacity of 10.16mg/g at 25°C. The thermodynamic studies indicated that the adsorption was a spontaneous and endothermic process. It was concluded that the mechanism of Cd(II) removal by modified steel-making slag is mainly based on chemisorptions including chemical precipitation and coordination reactions.