Selenate-selenium sorption on a Columbia river basalt (Umtanum basalt, Washington, U.S.A.)

Abstract

Sodium selenate and Na 2 75SeO 4 were used with a synthetic groundwater and hydrazine to determine selenate-selenium sorption characteristics of crushed Umtanum basalt between 40° and 60°C. Selenium sorption kinetics from both oxidizing and reducing solutions followed an equation of the type: C I − C= Kt b where C I is the initial selenium concentration in molarity; C is the selenium concentration in molarity at time t in hours; and K and b are constants. Relatively constant b-values of a wide C I range allowed K determination over the whole initial concentration range investigated ( C I = ∼10 −5–10 −12 M). Selenium desorption kinetics in 60°C oxidizing systems were described by: C= At d where A and d are constants and other quantities defined above. Equilibrium results conformed to a Freundlich sorption isotherm. The enthalpy change was independent of temperature within the range studied and yielded ΔH-values of from +27.23 kcal. mol −1 at 1.266 · 10 −14 mol Se/g basalt to +28.47 kcal. mol −1 at 2.061 · 10 −9 mol Se/g basalt. Chemisorption was indicated by the magnitude of the enthalpy change. Desorption kinetics at 60°C were only modestly less rapid than sorption kinetics, suggesting a relatively small sorption activation energy. Reduced (Eh = −150 to −200 mV) selenium did not desorb.