Solid speciation and fractionation of rare earth elements in a spodosol profile from northern Sweden as revealed by sequential extraction

A sequential extraction has been carried out on seven soil samples from a s podosol profile in till with granitic composition. The soil profile was sam pled in northern Sweden. Five fractions were selected for extraction: (A) C H3COONa-extractable (exchangeable/adsorbed/carbonate); (B) Na4P2O7-extracta ble (labile organics); (C) 0.25 M NH2OH . HCl-extractable (amorphous Fe-oxy hydroxides/Mn-oxides); (D) 1 M NH2OH . HCl-extractable (crystalline Fe-oxid es); and (E) KClO3/HCl-extractable (oganics and sulphides). Extracted rare earth elements (REE) were determined with High Resolution ICP-MS. In additi on to the soil, stream water suspended particulate matter was also analysed for REE. Total concentrations in the soil samples show that the REE have b een fractionated during weathering. In the acidic (pH 4.28) E-horizon all R EE are depleted relative to the unweathered till. The depletion decreases w ith increasing atomic number. Also in the B-horizon (pH 5.86) the REE are d epleted, although to a lesser extent compared to the E-horizon. Secondary p hases in the B-horizon fractionate the REE in different ways. Rare earth el ements extracted in extractions A and C were enriched in the intermediate R EE relative to heavy and light REE if normalized to local till. In extracti ons B and D, the heavy REE were enriched relative to the light REE, and in extraction E a large enrichment of light REE relative to heavy REE was foun d. Stream water suspended particulate matter is enriched in all REE relativ e to local till. The light REE are more enriched than the heavy REE. Normal ized La/Lu ratios in the suspended matter ranged from 1.5 to 2.4, and were negatively correlated with stream water discharge as well as with content o f detrital inorganic matter in the suspended load. The only extracted phase that potentially can explain the high normalized La/Lu ratio in the suspen ded matter is the phase extracted in extraction E, which has a ratio of 2.0 -4.2. The normalized La/Lu ratio for the rest of the extractions ranged fro m 0.5 to 1.2. It is concluded that organic matter seems to be important for the particulate transport of REE in northern coniferous areas. (C) 1999 El sevier Science B.V. All rights reserved.