Association of naturally occurring radionuclides with organic matter in soils in Mediterranean ecosystems

Organic matter in soil has many sources and is able to form complexes with anthropogenic and naturally occurring radionuclides that may have significant influence on their environmental behaviour, such as their bioavailability or mobility in soils. In this study, three different types of soil (sandy, silt loam and sandy loam) from a Mediterranean region were selected, and the association of ²³⁴,²³⁸U, ²²⁶Ra, ²¹⁰Po and ²²⁸,²³⁰,²³²Th to soil organic matter was estimated through three methodologies: (i) extraction of humic and fulvic acids (NaOH); (ii) oxidizable organic matter (NaClO); and (iii) organic-bound and sulphide-bound fraction from Tessier's method (H₂O₂+NH₄OAc). The radionuclides were mainly associated with humic and fulvic acids. Uranium and thorium were mainly associated with humic acids; while it was fulvic acids for radium, and it depended on soil type for ²¹⁰Po. NaClO and H₂O₂+NH₄OAc were not able to extract as much radionuclides as for humic and fulvic acids, which might imply that these reagents were not able to extract all organic matter in soil.