The binding between heavy metals and corresponding ligands affects their chemical behavior and toxicity in soil environments. The mechanisms of competitive complexation and/or chelation between Cd2+ free cations and preferential concentrations of Cl-, SO42-, and fulvate anions were investigated in simulated soil solutions at pH 4.00, 5.00 and 6.00. The Cd2+ concentrations were calculated by a proposed equation, simulated by MINTEQ software, and directly determined by ion chromatography (IC). When Cl-/Cd or Cl-/Cd with SO42-/Cd molar ratios of 3.18 and 4.05, the differences among Cd2+ concentrations calculated by equation, simulated by MINTEQ software, and directly determined by IC were not significant. but their differences were pH independent for considering Cl-/Cd molar ratio and pH dependent for Cl-/Cd and SO42-/Cd molar ratios. When Cl-/Cd. SO42-/Cd, and additional FA/Cd molar ratios of 3.18 and 4.05, the Cd2+ concentrations calculated by equation were significantly larger than those simulated by MINTEQ and determined by IC because in simulation and determination of Cd2+ concentrations by IC, the complexation of Cd2+ with ligands to form CdCl+, CdSO4, FACd(+) and FA(2)Cd had been considered, whereas in calculation this complexation aspect was ignored. Though IC can be used to determine Cd2+ concentration in rhizosphere soil solutions ion chromatographic peak of Cd2+ in 0.1 M HCl saturation extract of slightly acidic soil and in deionized distilled water saturation extract of acidic soils still may be shielded by the vicinal chromatographic peaks of Mg2+ and Mn2+, respectively. The Cd2+ concentrations in rhizosphere soil solutions of acidic or slightly acidic soils calculated by equation and/or simulated by Model may thus be used as potential alternatives for those determined by IC. (C) 2009 Elsevier B.V. All rights reserved.