Impact of redox cycles on accumulation of selected trace lithophile elements in iron-manganese nodules
Yana O. Timofeeva
Federal Scientific Center of the East Asia Terrestrial Biodiversity FEB RAS, Vladivostok, 690022, Russia
Iron- manganese nodules occur in soils of different origins and act as specific filters for soil self-purification. The nodules consist of different combinations of Fe and Mn oxides. The nodules are also characterized by a well-defined sorption activity for different elements. Elements differ in their selectivity for sorption by the Fe-Mn compounds in the nodules. Climate change and, respectively, soils redox cycles changes can affect the properties of the major Fe-Mn compounds in the nodules and of their associated elements. Despite extensive studies, little is known about how the accumulation of Sr, V, and Y in nodules varies according to variations in redox potential. Strontium and V are potentially dangerous for the natural environment. Yttrium is considered as "immobile elements" of soils. In this work, we collected nodules from Udepts on the west coast of the Pacific Ocean (south Russian Far-East) and investigated the dynamics of Sr, V, and Y accumulation in the nodules under controlled redox conditions in a column experiment, and studied Fe-Mn nodules using energy dispersive X-ray fluorescence spectroscopy, atomic absorption spectrometry, electron probe microanalysis, and field-emission scanning electron microscopy. The nodules consisted of a complex Mn-Fe-oxide matrix, soil mineral grains, and C-rich areas. In the nodules, Mn was the most important determinant for the accumulation of Sr and Y whereas V was predominantly associated with Fe under natural conditions of redox cycles that occur in surface soil horizons. The results show that the redox conditions above the value of 120 mV was sufficient to release Sr and Y from the nodules into the soil solution, while V was fixed into the nodules. These observations led us to suggest that V would not be released from the soil during nodule dissolution, whereas Sr and Y would be released.