AbstractThe study aims at determining the importance of various soil components on cobalt retention by Spodosols. Therefore, Co sorptions and extractions were carried out on samples from the B horizon in six Danish Spodosols. The investigation was carried out on untreated samples, on hydrogen peroxide-treated samples (to remove organic matter), on oxalate-treated samples (to remove amorphous to poorly crystalline Al and Fe oxides), on hydroxylamine-treated samples (to remove Mn oxides). Subfractions treated with hydrogen peroxide were further treated with oxalate and CBD (citrate-bicarbonate-dithionite). Sorption of Co from an initial 10⁻⁶ M solution after 48 h was determined in the pH range 3 to 8 using 0.1 M NaNO₃ as background electrolyte. The pH-dependent sorption curve (sorption edge) was shifted to higher pH with decreasing Al oxide content in the soils and, for the treated samples, after removal of Al and Fe oxides. Negligible effects were seen after removal of the Mn oxides, because of their low abundance, and of organic matter. Extraction of at pH 5-7.5 with 0.1 M HNO₃ for 24 h confirmed the sorption results, inasmuch as removal of the Al (and Fe) oxides increased Co extractability. Therefore, it is concluded that in the soils investigated Co retention is mainly determined by the oxalate-extractable Al fraction with minor contribution from crystalline Fe oxides.