Removal of chromium(VI) from aqueous solution using manganese oxide nanofibers

Manganese oxide nanofibers (MONFs) were synthesized, characterized, and successfully used for the removal of the toxic Cr(VI) ions from a model and the real solution. The results showed that the MONFs were fibers, whose shape consisted of an average diameter of 10 nm to 16 nm and a specific surface area equal to 94.1 m2 g−1, that was mainly composed of crystalline α-MnO2 (i.e., as was confirmed with the XRD measurement). The effect of different parameters that affected the adsorption process was studied and optimized. The results showed an efficient removal at pH 2.0, within 60 min, and by using 150 mg MONFs at the ambient temperature. The adsorption of Cr(VI) ions from an aqueous solution was studied kinetically at different temperatures. The results showed that the removal process followed the pseudo-second-order model, with an adsorption capacity of 14.6 mg g−1 at ambient temperature. The thermodynamic parameters were calculated and the results showed that the adsorption of Cr(VI) ions from aqueous solution by MONFs is physical, spontaneous, and exothermic (ΔH = −39.3 kJ mol−1) in nature, with negative entropy. Finally, MONFs were used for the removal of Cr(VI) ions from a real sample, and the result indicated that the MONFs are highly efficient at removing the toxic Cr(VI) ions.