Competitive sorption of As(V) and Cr(VI) on carbonaceous nanofibers

The competitive sorption of As(V) and Cr(VI) on the novel and uniform carbonaceous nanofibers (CNFs) was investigated in single/binary-metal systems. The presence of Cr(VI) showed a greater inhibitory effect on the sorption of As(V) in binary As-Cr system, especially at higher Cr(VI) concentrations. The maximum sorption capacities of Cr(VI) and As(V) on CNFs in single-metal systems at pH 5.0 and 303 K were calculated as 2.36 and 0.67 mmol/g, respectively, by Langmuir equation. XPS results demonstrated that only As(V) species was present at the CNFs surface, whereas both Cr(VI) and Cr(III) species existed on the CNFs surfaces, suggesting the partially reduction of Cr(VI) to Cr(III) by CNFs. According to the macroscopic and spectroscopic analysis, both inner-sphere and outer-sphere surface complexation contributed to the sorption process of Cr(VI), in contrast to the electrostatic outer-sphere sorption of As(V) on CNFs. The inner-sphere Cr complexes might arise from the covalent bonding between Cr(III) and oxygen-containing functional groups of CNFs. The findings herein highlight the higher chemical affinity of CNFs for Cr(VI) relative to As(V), as a consequence of the redox reaction of the strongly toxic Cr(VI) with CNFs.