Copper retention by Danish Spodosols in relation to contents of organic matter and aluminum, iron, and manganese oxides

The importance of various soil components on copper (Cu) retention by Spodosols was investigated. Copper sorption and extraction were conducted on samples from the B horizon from six Danish Spodosols. The investigation was conducted on untreated samples, on hydrogen peroxide-treated samples (to remove organic matter), on oxalate-treated samples [to remove amorphous to poorly crystalline aluminum (Al) and iron (Fe) oxides], on hydroxylamine-treated samples [to remove manganese (Mn) oxides]. Subfractions treated with hydrogen peroxide (H2O2) were further treated with oxalate and c citrate-bicarbonate- dithionite (CBD). Sorption of Cu from an initial 10(-6) M solution after 48 hours was determined in the pH range 3 to 7 using 0.1M sodium nitrate (NaNO3) as the background electrolyte. The pH-dependent sorption curve (sorption edge) was shifted to a higher pH with decreasing Al oxide content en the soils, and for the treated sample after removal of organic matter and Al and Fe oxides. A negligible effect was seen after removal of the Mn oxides because of their low abundance. Extraction of sorbed Cu at pH 4 to 6 with 0.1M nitric acid (HNO3) for 24 hours confirmed the sorption results, fn inasmuch as removal of the Al (and Fe) oxides increased Cu extractability. Therefore, it was concluded that in the soils investigated, Cu retention is mainly determined by the oxalate-extractable Al fraction with a minor contribution due to crystalline Fe oxides.