Comment on simultaneous and efficient removal of Cr(VI) and methyl orange on LDHs decorated porous carbons

This article aims to concern on the X-ray photoelectron spectroscopy (XPS) of chromium-laden adsorbent (Ni/ Al@PAB) in the previously published work. The coexistence of Cr(III) and Cr(IV) on the laden adsorbent was confirmed by the Cr 2p spectra, suggesting that Cr(VI) anions were reduced to Cr(III) cations during the Cr(VI) adsorption process. The reduced Cr(III) ions in solution were adsorbed by Ni/Al@PAB. The XPS data of the original paper demonstrated that the percentage of Cr(III) species (similar to 80%) in the laden Ni/Al@PAB adsorbent was overwhelmingly higher than that of Cr(VI) ones. The mis-conclusion resulted from incorrectly identifying the binding energies of Cr(III) and Cr(VI) in the Cr 2p spectra of the laden adsorbent. It can be accurately concluded that hexavalent chromium is always characterized by higher binding energies than trivalent chromium. The binding energies of some standard Cr(VI) and Cr(III) chemicals from the literature were also reviewed.