title: Sorption and speciation of selenium in boreal forest soil.
abstract: Sorption and speciation of selenium in the initial chemical forms of selenite and selenate were investigated in batch experiments on humus and mineral soil samples taken from a 4-m deep boreal forest soil excavator pit on Olkiluoto Island, on the Baltic Sea coast in southwestern Finland. The HPLC-ICP-MS technique was used to monitor any possible transformations in the selenium liquid phase speciation and to determine the concentrations of selenite and selenate in the samples for calculation of the mass distribution coefficient, Kd, for both species. Both SeO32-and SeO42-proved to be resistant forms in the prevailing soil conditions and no changes in selenium liquid phase speciation were seen in the sorption experiments in spite of variations in the initial selenium species, incubation time or conditions, pH, temperature or microbial activity. Selenite sorption on the mineral soil increased with time in aerobic conditions whilst the opposite trend was seen for the anaerobic soil samples. Selenite retention correlated with the contents of organic matter and weakly crystalline oxides of aluminum and iron, solution pH and the specific surface area. Selenate exhibited poorer sorption on soil than selenite and on average the Kdvalues were 27-times lower. Mineral soil was more efficient in retaining selenite and selenate than humus, implicating the possible importance of weakly crystalline aluminum and iron oxides for the retention of oxyanions in Olkiluoto soil. Sterilization of the soil samples decreased the retention of selenite, thus implying some involvement of soil microbes in the sorption processes or a change in sample composition, but it produced no effect for selenate. There was no sorption of selenite by quartz, potassium feldspar, hornblende or muscovite. Biotite showed the best retentive properties for selenite in the model soil solution at about pH 8, followed by hematite, plagioclase and chlorite. The Kdvalues for these minerals were 18, 14, 8 and 7 L/kg, respectively. It is proposed that selenite sorption is affected by the structural Fe(II) in biotite, which is capable of inducing the reduction of SeO32-to Se(0). Selenite probably forms a surface complex with Fe(III) atoms on the surface of hematite, thus explaining its retention on this mineral. None of the minerals retained selenate to any extent.
authors: Laboratory of Radiochemistry, Department of Chemistry, P.O. BOX 55, FIN-00014 University of Helsinki, Finland. Electronic address: mervi.soderlund@helsinki.fi.
authors: Mervi
authors: M
authors: Söderlund
authors: Department of Geosciences and Geography, P.O. Box 64, FIN-00014 University of Helsinki, Finland. Electronic address: juhani.virkanen@helsinki.fi.
authors: Juhani
authors: J
authors: Virkanen
authors: Nuclear Chemistry, Department of Chemical Engineering, Chalmers University of Technology, Kemivägen 4, SE-41296 Gothenburg, Sweden. Electronic address: stehol@chalmers.se.
authors: Stellan
authors: S
authors: Holgersson
authors: Laboratory of Radiochemistry, Department of Chemistry, P.O. BOX 55, FIN-00014 University of Helsinki, Finland. Electronic address: jukka.lehto@helsinki.fi.
authors: Jukka
authors: J
authors: Lehto
mesh_terms: D000327:Adsorption; D005387:Finland; D064586:Selenic Acid; D020887:Selenious Acid; D012643:Selenium / Q000032:analysis* / Q000737:chemistry; D012987:Soil / Q000737:chemistry; D012989:Soil Pollutants / Q000032:analysis* / Q000737:chemistry; D066188:Taiga
chemical_list: D012987:Soil; D012989:Soil Pollutants; D020887:Selenious Acid; D012643:Selenium; D064586:Selenic Acid
keywords: HPLC–ICP-MS; Selenite and selenate; Soil; Sorption; Speciation