Temperature-constrained sorption of inorganic and organic selenium by Fe-oxide minerals: Significance for selenium bioavailability in Earth's critical zone

Temperature influences hydrobiogeochemical processes and consequent fate of selenium (Se) in the Earth's critical zone. Quantitative understanding of the temperature-constrained sorption behaviors of both inorganic and organic Se species, however, is critically lacking. Using ferrihydrite and goethite as comparative sorbents in batch experiments under wide ranges of environmental temperature (5-40 °C) and pH (5-9), we mechanistically identify the effects of temperature on sorption kinetics and thermodynamics of four key Se species: selenate (Se(VI)) and selenite (Se(IV)) as inorganic Se oxyanions, as opposed to DL-selenocystine (SeCys) and DL-selenomethionine (SeMet) as organic Se zwitterions. Although sorption of the four Se species occurred more rapidly on ferrihydrite than on goethite, increasing temperature overall reduced Se retention by Fe-oxide minerals. Despite the overall rapid sorption observed for all Se species, model-derived rate constants demonstrate higher sorption rates for ferrihydrite than goethite, and this kinetic contrast is amplified as temperature increases. Temperature-dependent kinetic fitting indicates that sorption of Se(IV), Se(VI), and SeCys is more consistent with surface reaction-controlled (activation-controlled) complexation than SeMet, which shows weaker temperature sensitivity agreeing with less specific interaction. Thermodynamic parameters derived from the revised Langmuir isotherms indicate exothermic Se sorption on Fe oxides and affinity shift with temperature, pH, and Se speciation. A diffuse-layer surface complexation model captures the observed temperature and pH dependence by accounting for coupled changes in aqueous Se speciation, surface protonation (thus effective site availability), and surface complexation thermodynamic constants. Therefore, temperature as a key driver of Se sorption plays a nonnegligible role in controlling Se mobility and potential bioavailability in the critical zone.