Simultaneous adsorption of fluoride and hexavalent chromium by synthetic mesoporous alumina: performance and interaction mechanism

Combined pollution by fluoride (F−) and hexavalent chromium (Cr(VI)) in water bodies causes much environmental concern and its treatment is of the utmost importance. This study first evaluates the efficiency of the simultaneous adsorption of F− and Cr(VI) from aqueous solutions using a sol–gel synthesized mesoporous alumina (MA) adsorbent. Adsorption properties show that the sorption of F− is spontaneous and endothermic, while that of Cr(VI) is non-spontaneous and exothermic in nature. Both adsorption processes are well described by a pseudo-second-order kinetic model and Langmuir isotherms. The maximum adsorption capacities are 14.39 and 9.19 mg g−1 for F− and Cr(VI), respectively. F− and Cr(VI) present a similar adsorption mechanism on the MA adsorbent which primarily involves electrostatic attraction and chemisorption. Interaction adsorption experiments and FT-IR and XPS spectra demonstrate that the co-existence of F− and Cr(VI) in solution impedes the sorption of Cr(VI) and accelerates the sorption of F−. The former is ascribed to competition for surface adsorption active sites. The formation of a surface chromium hydroxyl group (CrOH or CrOH2+) could become a new adsorption site for F− adsorption which mainly contributes to the latter. Furthermore, these findings indicate that MA is a promising adsorption material for application in the treatment of combined F− and Cr(VI) polluted wastewater.