Facet-Dependent Competitive Adsorption Mechanisms of Chromate and Oxalic Acid on γ-FeO(OH) Nanocrystals

Abstract

Facet-dependent toxic metal adsorption of iron oxides widely occurred in natural environments. It is known that organic acids can alter the adsorption behaviors of trace elements by cooperative or competitive effects. However, the coadsorption mechanisms of the specific facets are still not fully understood. In the current investigation, Cr(VI) adsorption onto the lepidocrocite (γ-FeO(OH))-exposed facets in the presence of oxalic acid (OA) was studied using macroexperiments, in situ attenuated total reflectance Fourier transform infrared spectroscopy, X-ray adsorption fine structure, and density functional theory calculations. Rod-like lepidocrocite (R-LEP) with a high ratio of {001}/{010} facet showed excellent Cr(VI) adsorption capacity than that of plate-like lepidocrocite (P-LEP, the dominant facet is {010}) in the absence/presence of OA. Interestingly, OA reacted with R-LEP would be easier to diminish Cr(VI) adsorption than with P-LEP. The competitive adsorption occurred on the {001} facet due to the formation of inner-sphere OA configurations (monodentate mononuclear and bidentate mononuclear structures) and a bidentate binuclear Cr(VI) complex. However, OA coordinated with {010} facets via the outer-sphere complexes, while Cr(VI) could form a protonated monodentate binuclear configuration. These observations suggest that the competitive adsorption processes between OA and Cr(VI) exhibit facet dependence. Furthermore, lepidocrocite-exposed facets determine the interfacial interactions and geochemical behaviors of Cr(VI) in polluted environments.

Cationic Metal–Organic Framework with Amino-Functionalized Isonicotinic Acid for Chromium(VI) Removal

“…Cr­(VI) species in water exist in the forms of hydrogen chromate (HCrO 4 – ), chromate (CrO 4 2– ), and dichromate (Cr 2 O 7 2– ) depending on the Cr­(VI) concentration and the pH of the solution (Figure S5). On the other hand, the surface charge of the adsorbent can be altered according to the pH in the adsorption system, resulting in different adsorption performances. , Thus, the effect of the pH on the adsorption capacity needs to be evaluated. Before Cr­(VI) adsorption experiments were conducted, the structural stability of the two MOFs in Cr­(VI) solution with various pH was investigated by immersing 10 mg of the two adsorbents into corresponding solutions.…”

Effects of Ionic Interferents on Electrocatalytic Nitrate Reduction: Mechanistic Insight

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