Kinetics of Iron and Manganese Release from Contaminated Calcareous Soils

Knowledge of the release of heavy metals (HM) and their chemical speciation is necessary for characterizing HM behavior in soils. The kinetics and characteristics of iron (Fe) and manganese (Mn) release were studied in 10 contaminated calcareous soils using 0.01 M calcium chloride (CaCl₂), 0.01 M ethylenediamine tetraacetic acid (EDTA), and 0.01 M malic acid (malic acid) extractions. Iron and Mn in soil samples were fractionated before and after 2084 h kinetic release using a sequential extraction procedure. The proportion of Fe and Mn released by EDTA was greater than that with CaCl₂ and malic acid. A power model satisfactorily described Fe and Mn release from soils. In general, the mean release rate of Fe was greater than that of Mn, indicating a greater rate of Fe release from contaminated soils. It was shown that Fe and Mn distributions were similar in native soils and they were mainly found in Fe-Mn oxides and organic-matter (OM) fractions. There were changes in the proportional distribution of Fe and Mn in all soils during the 2084 h kinetic study with different extraction solutions. In general, the proportions of Fe and Mn associated with carbonate (CARB) and OM fractions tended to decrease, with corresponding increases in the Fe-Mn oxides for Mn and residual (RES) fractions for Fe during the kinetic study with all extraction solutions. The Fe and Mn solubility at the initial and final stages of release was controlled by siderite (FeCO₃), vivianite (Fe)₃(PO₄)₂·8H₂O], MnCO₃(am), MnHPO₄, and rhodochrosite (MnCO₃) minerals in all extraction solutions. Based on a risk assessment and percentage of release of metals, there is a high potential for Mn release into the food chain from contaminated soils.