Adsorptive removal of Cr3+, Cu2+, and Ni2+ ions by magnetic Fe3O4@alkali-treated coal fly ash

With the rapid development of industrial economy, water pollution by heavy metals is becoming more and more serious and efficient treatment of the polluted water is still a challenge. To solve this problem, we synthesized magnetic nanoparticles of Fe3O4@alkali-treated coal fly ash (MNFFA) by the coprecipitation method at 298.15 K and 101.3 kPa. The structures of the MNFFA were characterized by the scanning electron microscope, X-ray diffraction, and surface area analyses. The effects of adsorbent composition and dosage, equilibration time, temperature, pH, and co-existing ions on the adsorption were investigated to optimize the operating condition for the removal of Cr3+, Cu2+, and Ni2+ ions by the MNFFA. The adsorption equilibrium and kinetics follow the Langmuir isotherm and pseudo-second-order kinetic model, respectively. The adsorption capacity was calculated to be 212.3, 229.9, and 26.2 mg/g for Cr3+, Cu2+, and Ni2+, respectively, by fitting equilibrium data to the Langmuir model. The selectivity of Cu2+ over Cr3+ and Ni2+ is, respectively, 1.06 and 8.68 in the binary cationic solutions and 1.21 and 4.67 in the ternary cationic solution. The high adsorption capacity was kept after five cycles, indicating that the adsorbent is stable enough to be regenerated and re-used efficiently. The results demonstrated that the MNFFA is an effective, low-cost, and environment-friendly adsorbent and therefore is very promising to be applied in the adsorptive removal of the heavy metal ions from polluted water.