A controlled hydroponic experiment was undertaken to investigate Cd uptake in relation to the activity of Cd species in solution other than the free ion(Cd2+)by maintaining a constant Cd2+activity under variable SO2-4 and Cl-concentrations exposed to maize(Zea mays var.Cameron)plants.The objectives of these experiments were:(1)to distinguish and quantify the different uptake rates of free and inorganic-complexed Cd from nutrient solution,and(2)to model the uptake of Cd by maize with a Biotic Ligand Model(BLM)in a system which facilitates the close examination of root characteristics.Results of the current experiments suggest that,in addition in the free ion,CdSO04 complexes are important factors in determining Cd uptake in nutrient solution by maize plants.Higher nominal SO2-4 concentrations in solution generally resulted in a greater Cd accumulation by maize plants than predicted by the Cd2+activity.A better integration of the complete dataset for the 3 harvest times(6,9 and 11 days after treatment)was achieved by including consideration of both the duration of Cd exposure and especially the root surface area to express Cd uptake.Similarly,the fit of the BLM was also improved when taking into account exposure time and expressing uptake in terms of root morphological parameters.