NiFe Layered Double Hydroxides with Controlled Composition and Morphology for the Efficient Removal of Cr(VI) from Water

A layered double hydroxide (LDH) composed of Ni²⁺ and Fe³⁺ with a Fe³⁺/(Ni²⁺ + Fe³⁺) ratio of 0.05, which is not commonly available, was successfully prepared by coprecipitation from an aqueous solution of glycerol containing nickel nitrate and iron nitrate. Precipitation using NaOH as a precipitating agent at room temperature or 120 °C under hydrothermal conditions gave products with micrometer-sized aggregates of nanometer-sized unshaped particles, while that using urea yielded LDHs with a foam-like porous architecture composed of platy particles with a size of 100-300 nm. The products were examined to remove Cr(VI) from an acidic (pH = 3) aqueous solution of K₂Cr₂O₇ by adsorption and photocatalytic reduction. The foam-like porous NiFe-LDH exhibited the highest adsorbed amount (122 mg g^-1) and rate (0.017 g mg^-1 min^-1) in the dark and the highest rate (0.012 min^-1) of photocatalytic Cr(VI) reduction among the NiFe-LDHs prepared in the present study, which can be explained as a positive effect of the foam-like porous architecture. These performances were superior to those of other reported LDHs, showing the importance of the composition and the particle morphology to boost the removal of Cr(VI).