Effect of interlayer anions on chromium removal using Mg–Al layered double hydroxides: Kinetic, equilibrium and thermodynamic studies

The influence of interlayer anions such as NO3−, SO42 − and Cl− on Mg–Al hydrotalcites for Cr(VI) removal from aqueous solution was studied. The structure of the prepared LDHs was characterized by XRD, SEM, FTIR, TGA, BET surface area and pHzpc. The sorbent ability and sorption mechanisms were also investigated. The LDHs exhibit high removal for Cr(VI), and the sorbed amount depends on the nature of interlayer anion, which decreased in the following order: NO3− > Cl− > SO42 −. Nitrate-containing LDH reached a Cr(VI) sorption equilibrium within only 30 min. The effects of operating conditions, including initial concentration, solution pH, agitation time and sorbent amount have been studied in batch mode. The optimum conditions were observed at an initial concentration of 100 mg·L− 1, pH = 6, agitation time of 60 min and a sorbent dose of 2 g·L− 1. The equilibrium data were fitted to the Langmuir, Freundlich and Dubinin–Radushkevich isotherm models. The Langmuir model was found to sufficiently describe the sorption process, offering a maximum sorption capacity of 71.91 mg·g− 1. The sorption kinetic follows pseudo-second-order reaction with high accuracy. Thermodynamic parameters suggested that the sorption process is spontaneous and endothermic in nature.

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