Synthesis of nickel-iron layered double hydroxide via topochemical approach: Enhanced surface charge density for rapid hexavalent chromium removal.

Hexavalent chromium (Cr(VI)) is considered to be a potential metal contaminant because of its toxicity and carcinogenicity. In this work, the surface charge density of nickel-iron layered double hydroxide (NiFe LDH) is tuned through iron valence change to improve the performance in adsorption of Cr(VI). The addition of iron divalent in the precursor enhances the surface positivity and reducibility of Fe2+-NiFe LDH, resulting in a nearly 150% Cr(VI) maximum adsorption capacity improvement. The increase of hydroxyl groups and charge density on the surface of NiFe LDH is due to the topological chemical transition from Ni2+-Fe2+ LDH to Ni2+-Fe3+ LDH. The adsorption of Cr(VI) onto Fe2+-NiFe LDH prepared via topochemical approach is highly pH-dependent. The adsorption dynamics and isotherms results may be clearly elucidated by the pseudo-second-order model and Langmuir isotherm model. Electrostatic attraction, interlayer anion exchange and adsorption-coupled reduction are proven to be the main Cr(VI) removal mechanisms for Fe2+-NiFe LDH. This finding demonstrates that Fe2+-NiFe LDH adsorbents have potential application for efficient removal of Cr(VI) pollutants.