Adsorption of selenate onto ferrihydrite, goethite, and lepidocrocite under neutral pH conditions

This study evaluates the adsorption of dissolved Se(VI) onto three synthetic Fe oxy-hydroxides (i.e., 2-line ferrihydrite, goethite and lepidocrocite). Selenate adsorption was measured as a function of sorbent concentration (0.01–50 g L−1) at near neutral pH (∼7) and room temperature (25 °C) via batch experiments. Inductively coupled plasma-mass spectrometry (ICP-MS) analyses on the aqueous samples showed that 2-line ferrihydrite is by far the strongest adsorbent for Se(VI) when compared to goethite and lepidocrocite at all sorbate:sorbent ratios tested. Goethite has a slightly greater affinity for Se(VI) than lepidocrocite, irrespective of a lower specific surface area under similar experimental conditions (pH, solids concentration). Selenium(VI) surface complexes on the solids were not observed in Raman spectra or attenuated total reflectance-infrared (ATR-IR) spectra of selenate adsorbed phases, likely due to low surface coverage. However, X-ray absorption spectroscopy (XAS) showed that all three Fe oxy-hydroxides adsorbed Se(VI) via inner-sphere surface complexation irrespective of their dissimilar affinities for selenate at pH ∼ 7. Similarly, sorption data for the three synthetic solids was modeled using the Langmuir isotherm. Overall, 2-line ferrihydrite appears to efficiently and effectively adsorb Se(VI) under near neutral conditions and, as such, should be considered a potential sink for Se(VI) in a transient system.