Static and dynamic adsorptive removal of selenite and selenate by alkoxide-free sol-gel-generated Mg-Al-CO₃ layered double hydroxide: Effect of competing ions

Adsorption/ion exchange is a major separation approach capable of recovering the valuable Se component from various multicomponent solutions or to reduce its concentration. In this study, we report a method for selenite and selenate adsorptive removal based on the application of Mg-Al-CO₃ layered double hydroxide (LDH) generated via an alkoxide-free sol-gel synthesis method developed by the authors. The selenite and selenate removal capability of Mg-Al LDH was examined under static and dynamic adsorption conditions, focusing on the influence of the competing anions (phosphate, sulphate, carbonate, silicate, chloride). The adsorption capacities of Mg-Al LDH for selenite and selenate obtained from the equilibrium isotherms were not influenced by the presence of the competing sulphate, retaining the highest values of 168 and 103mg [Se]/g_dw for Se(IV) and Se(VI) at pH 5, respectively. This inorganic ion exchanger is capable of functioning across the broad range of pH values from 5 to 9. Mg-Al LDH could purify 16,200 and 4200 bed volumes (BVs) of the selenite/selenate-containing solutions (~50μg [Se]/L initial concentration), respectively, until reaching a selenium concentration of zero in the effluents. The presence of phosphate and a 74-times higher concentration of sulphate compared with selenate or selenite in the adsorbate showed nearly no influence on the dynamic adsorptive performance of Mg-Al LDH for selenite. An equivalent concentration of phosphate did not influence the dynamic adsorptive removal of selenate. Markedly higher concentrations of sulphate, however, decreased the time to breakthrough for selenate but did not affect the quality of its removal. Mg-Al LDH is a promising inorganic ion exchanger for the removal of both of the aqueous selenium species and will be tested on industrial scales.