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Abstract: The nature of soil iron oxides is of great interest from the viewpoints of both pedogenesis and environmental geochemistry as they are sensitive diagnostic indicators of soil hydromorphism on the one hand, and exhibit high sorption capacity for many toxic metal pollutants on the other. In this study, the association of 17 trace elements (Al, Ba, Ca, Cd, Co, Cr, Cu, K, Mg, Mn, Ni, Pb, Si, Sr, Ti, V and Zn) with Fe-rich nodules from a neutral-alkaline floodplain soil using various chemical extractions and dis-solution analyses was investigated.
The Fe-rich nodules consist of amorphous phases, probably of ferrihydrite. Other phases in the nodules are corresponding to those forming the soil matrix. Additionally, disordered goethite was found in the pseudomorphs after amphiboles. These amphiboles may have been the source of iron for nodule forma-tion in this soil. Cobalt and Mn showed the highest enrichment in the nodules when compared to their to-tal soil concentrations. Other metals, such as Cd, Ba, Ni, V, Zn, Cu and Pb can be characterized much smaller accumulation similarly to Fe. Additionally, Al, Mg, Si, Ti, K and Cr (e.g. elements bound to sili-cates and oxides), as well as Ca and Sr (e.g. elements bound to carbonates) also showed similar behavior during the extractions.
At the neutral-slightly alkaline conditions of the studied soil, the effect of pH may overwrite that of hy-dromorphism on trace metal accumulation in the Fe-rich nodules. Due to the intense sorption on and co-precipitation with oxide phases at these pH conditions, as well as the re-crystallization of oxides during the redox cycles, most of the cationic metals prefer Fe oxides over Mn oxides in their accumulation.

Keyword: ferrihydrite, goethite, concretion, aggregate, pseudomorph, oxalate extraction, dithionite ex-traction, hydromorphism