Insights into the adsorption mechanism of Al30 polyoxocations-modified graphene oxide nanosheets for efficient removal of phosphate, chromate and selenate oxyanions: A comparative study

Abstract

Keggin-type aluminum polyoxocation species, Al30, with interesting features resulting from a variety of surface oxygen functional groups, has high adsorption ability of different species so it can be served either as nanohybrids or singly in water purification. Insight into surface complex structures of various oxyanions with different activities on Al30 is fundamental for both mechanism and application in wastewater treatment. Reported here is an adsorption behavior investigation of three environmentally problematic oxyanions (phosphate, chromate, and selenate) with different activities on Al30. For this purpose, firstly, Al30 species were consolidated on graphene oxide to use as the sorbent material. Performance of the proposed sorbent evaluated by kinetics and isotherm studies was representative of good agreement of adsorption data with Langmuir isotherm model and pseudo-second-order kinetics for all oxyanions. The underlying molecular binding mechanisms of these oxyanions onto Al30 were revealed by FT-IR spectroscopy, desorption studies, TGA, as well as studying adsorption trends in different pH values. The obtained results boded predominant formation of inner-sphere surface complexes of phosphate and chromate and common contribution of inner- and outer-sphere complexes for selenate, increasing the mobility of selenate. Competitive adsorption studies exhibited a more adsorption affinity and selectivity of Al30 towards phosphate than chromate and selenate, while this was contrary to the result of the single-ion adsorption case, where less adsorption capacity was obtained for phosphate than the other ones.