Effect of Cl−, SO42−, and fulvate anions on Cd2+ free ion concentrations in simulated rhizosphere soil solutions

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 FA2Cd 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.