With the aim of removing binary metal ions from aqueous solutions, the adsorption characteristics of sawdust modified with citric acid are investigated using adsorption kinetics and isotherms in both an experiment and a numerical simulation. The adsorption of metal ions is characterized by the competitive adsorption in binary metal ion systems that results from the stability constants between metal ions and citrate in the modified sawdust. These findings suggest that competitive adsorption and the affinities of the metal ion adsorbents play important roles in the removal of metal ions and that the numerical simulation can predict the adsorption characteristics of the modified sawdust.