XPS determined mechanism of selenite (HSeO3−) sorption in absence/presence of sulfate (SO42−) on Mg-Al-CO3 Layered double hydroxides (LDHs): Solid phase speciation focus

Abstract

Success in adsorptive removal depends on solid phase speciation, which predetermines the extraction mechanism. This work investigates the mechanism of selenite (HSeO3−) sorption on Mg-Al Layered double hydroxides (LDHs), prepared by the original alkoxide-free sol-gel method, which already demonstrated outstanding removal of aqueous selenium species. Using X-ray Photoelectron Spectroscopy (XPS) as most relevant technique allowed to reveal the phase speciation in solid state as a function of two variables, competing sulfate and pH (8.5; 7.0; 4.5), and to detect the phases responsible for selenite uptake. Despite the leading initializing role of Mg-containing phases, selenite ended up to be chemically bound to one aluminium-based phase. The involvement of the second sorption mechanism via anion exchange of the interlayer carbonate with aqueous selenite depended on absence/presence of competing sulfate and pH. Comparison with the analogues results on selenate (SeO42−) sorption on the same material (published) resulted in discovery of the differences in XPS feature changes upon either physisorption or chemisorption. This is the first report, which discloses that physisorption of aqueous anion to a particular solid phase shifts the respective envelopment binding energies, whist chemisorptive binding causes considerable alterations (decreases/shifts) of XPS features of the specific binding solid species, such as Al(OH)3, however, it preserves nearly the same XPS characteristics of the envelopment peaks. Based on these discoveries, here we suggest a methodological idea on using the results of XPS analysis for interpretation of molecular mechanism of ion sorption on inorganic ion exchangers and to reliably distinguish the chemisorption from physisorption.