The kinetics of cadmium sorption by polyelectrolyte-coated fly ash from aqueous solution

In this study, the sorption efficiency of different layers of polyelectrolyte-coated fly ash (PEFA) on cadmium (Cd) ions adsorption from aqueous solution was investigated. Using layer-by-layer technique, PEFA materials were synthesized in which fly ash treated with acid was coated with 5 and 20 layers of polyelectrolyte. The morphology of the modified fly ash was characterized using field-emission scanning electron microscope equipped with energy dispersive X-ray spectroscopy and Brunauer–Emmett–Teller surface area analysis. The synthesized PEFA materials were then tested for their efficiency to remove cadmium, Cd(II) ions from aqueous solution under different number of PEFA layers, pH, adsorbent dosage, shaking speed, and treatment time. The results revealed that 5-layer PEFA had better efficiency in removing Cd than the 20-layer PEFA. At pH 7, more than 96% removal of Cd(II) ions was achieved using 5-layer PEFA compared with 58% removal using the 20-layer PEFA within 15 min. Langmuir, Freundlich, Dubinin–Radushkevich, and Scatchard plot isotherm models were used to assess the Cd sorption data at equilibrium. The equilibrium adsorption isotherm models showed good fit to the experimental data for both 20 and 5 layers PEFA followed pseudo-second-order kinetics. The study also indicated a favorable sorption process between the adsorbents and Cd(II) ions in aqueous solution. On the other hand, the Scatchard plot analysis revealed the presence of multiple binding sites on PEFA surface, which confirmed the good fit in the Freundlich model.