Abstract

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 Cd 2+ free cations and preferential concentrations of Cl −, SO 4 2−, and fulvate anions were investigated in simulated soil solutions at pH 4.00, 5.00 and 6.00. The Cd 2+ 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 SO 4 2−/Cd molar ratios of 3.18 and 4.05, the differences among Cd 2+ 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 SO 4 2−/Cd molar ratios. When Cl −/Cd, SO 4 2−/Cd, and additional FA/Cd molar ratios of 3.18 and 4.05, the Cd 2+ concentrations calculated by equation were significantly larger than those simulated by MINTEQ and determined by IC because in simulation and determination of Cd 2+ concentrations by IC, the complexation of Cd 2+ with ligands to form CdCl +, CdSO 4, FACd + and FA 2Cd had been considered, whereas in calculation this complexation aspect was ignored. Though IC can be used to determine Cd 2+ concentration in rhizosphere soil solutions ion chromatographic peak of Cd 2+ 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 Mg 2+ and Mn 2+, respectively. The Cd 2+ 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.

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