Assessing the Mobilization of Cadmium, Lead, and Nickel Using a Seven-Step Sequential Extraction Technique in Contaminated Floodplain Soil Profiles Along the Central Elbe River, Germany

The mobilization of toxic metals in soils strongly depends on their bounding in different geochemical fractions. However, the relations between soil properties and the vertical horizon-specific distribution of different geochemical fractions of cadmium (Cd), lead (Pb), and nickel (Ni) in various floodplain soil types are limited and have not been studied up to date. Therefore, seven soil profiles in three areas along the Elbe River, Germany, which represent the two soil groups Mollic Fluvisols and Eutric Gleysols, were selected to determine geochemical fractions of Cd, Pb, and Ni. A sequential extraction procedure which fractionate metals into the seven fractions:F1:soluble+exchangeable, F2: easily mobilizable, F3:bound to Mn oxides, F4:bound to soil organic matter, F5:occluded into amorphous Fe oxides, F6:occluded into crystalline Fe oxides, and F7: residual fraction was used. Concentrations of pseudo-total Cd, Ni, and Pb were exceeded the international trigger action values and governed mainly by soil organic carbon (SOC), cation exchange capacity, and Fe-Mn sesquioxides. The mobile fraction (Sigma F1-F2) was dominant for Cd, whereas Pb was mainly bounded in F4/F5, and Ni in F7/F6. Cadmium and Pb reveal a higher potential mobility (Sigma F1-F6) than Ni. The potential mobile fraction ranged from 90 % to 97 %, 44 % to 61 %, and 83% to 92% of the (pseudo) total Cd, Ni, and Pb, respectively. The Gleysols showed a higher mobile fraction and potential mobile fraction than Fluvisols. The mobile fraction of the metals correlated positively with clay, SOC, and total sulfur (S-t), and negatively with pH and Fe-Mn oxides. Our results indicate that the studied soils exhibit elevated concentrations of Cd, Ni, and Pb, as well as a high potential mobilization of these metals. Our findings suggest that a release of these toxic metals in floodplain soils should be considered due to an increased mobilization and the potential environmental risks such as uptake by plants, and thus, the transfer of these metals into the grassland and food chain, as well as transport via waters during periods of flooding.