FRACTIONAL PARTITIONING FOR ASSESSING SOLID-PHASE SPECIATION AND GEOCHEMICAL TRANSFORMATIONS OF SOIL SELENIUM

Fractionation techniques have been applied widely to assess the behavi or of various geological trace elements. However, relatively few studi es have been conducted on solid-phase speciation and geochemical asses sment of selenium (Se) through fractional partitioning. Soils collecte d from contrasting range and mine environments of Wyoming were partiti oned fractionally to investigate the significance of different Se frac tions in assessing solid-phase speciation and geochemical transformati ons. The Se fractions extracted by the partitioning technique were aqu eous (0.25 M KCl), exchangeable (1 M KH2PO4), acid-extractable (4 M HC l), sulfide and humic bound (KClO3 + 12 M HCl), and siliceous (15.8 M HNO3 + 11.7 M HCLO(4) + 28.9 M HF); each fraction was also speciated f or selenite (Se+4) and selenate (Se+6). In addition, the soil samples were analyzed for total soil Se after a strong mixed-acid digestion (1 5.8 M HNO3 + 11.7 M HClO4 + 28.9 M HF). High percent recovery C85-104% ) was observed between total Se and sum of the partitioned fractions, which were also significantly correlated (r = 0.99). With increasing e xchangeable Se, there was a corresponding increase in the aqueous frac tion (r = 0.72). A similar increasing trend was found between the exch angeable and acid-extractable fractions (r = 0.51>. For the aqueous fr action, Se+6 was found to be the potential species, whereas in the exc hangeable fraction, Se+4 predominated. Both Se+4 and Se+6 were distrib uted almost evenly in the acid-extractable fraction. Speciation data f or sulfide and humic bound, and siliceous fractions, however, were not conclusive. Selenium in the available and conditionally available fra ctions increased as the weatherable fraction decreased (for both r = - 0.83). Total soil Se increased with increasing clay content (r = 0.81) , and a reverse trend was observed with sand content (r = -0.69), sugg esting Se is associated with the clay-sized soil fraction. Siliceous S e was Linearly related to total soil Se (r = 0.91), suggesting the pre sence of Se in soil minerals. This fraction was again found to increas e with increasing clay content (r = 0.74). Average distributions of th e five Se fractions indicated the soils were dominated by the siliceou s Se, with aqueous fraction comprising the least abundant Se level. St atistical analysis indicated all of the above mentioned correlations w ere significant at the P < 0.01 or 0.001 level.