With this study, the removal mechanisms of arsenate by steel slag and its potential for treatment of contaminated water were elucidated. While original slag showed a poor fit to the Langmuir equation (R-2= 0.960), washed slag (the original slag is washed by low pH water solutions to remove readily soluble oxides) conformed better (R-2= 0.995). An initial pH of 2.0 give optimal adsorption, with a strong impact from the chemical speciation observed with highest efficiency for the fully protonated (OH)(3)AsO form. Adsorption capacity of the slag is 4.0 mg g(-1), while together with precipitation the retention capacity reaches 13.7 mg g-1. However, removal by precipitation is a non-steady process due to re-dissolution of Ca-3(AsO4)(2)(s). The washed slag shows a similar adsorption capacity to the original one but has not as strong alkaline properties. Batch experiment shows fast adsorption kinetics and column loading tests indicate an instant adsorption kinetics with 80 % As(V) removal for a 10mgL(-1) As(V) solution by 1.0 g of washed slag using a solution flowrate of 1 mL min(-1). Common ions like sulfate, carbonate, chloride, iron(III), humic acid and fulvic acid do not significantly interfere with the removal efficiency. In combination with limited hazardous metals leaching, the slag is thus appropriate for use as a filter material for treatment of contaminated water and it has been successfully applied as filter material for treatment of arsenate spiked natural water sample with average removal efficiency of 84 % (solid to liquid ratio of 200).