Fundamental investigation on removal and recovery of heavy metals from synthetic wastewater using magnetic nanoparticles

Nanoparticle adsorption coupled with magnetic separation was applied for the removal and recovery of heavy metals from industrial wastewater. Magnetic nanoparticles (e.g., magnetite, maghemite, metal ferrite) with a diameter of 10-20 nm were successfully synthesized in the laboratory. A comprehensive study was carried out in batch tests, involving the effects of adsorbate properties (e.g., metal concentration, coexisting ions and ligands), adsorbent properties (e.g., particle size, surface area and surface pretreatment), and operational parameters (e.g., pH, temperature and shaking speed). Maghemite (γ-Fe2O3) nanoparticles from simple oxidation of magnetite (Fe3O4) were effectively used for the selective removal of Cr(VI), Cu(II) and Ni(II). Modified MnFe2O4 nanoparticles were successfully applied for the fast removal and recovery of Cr(VI) within 5 minutes and the initial 100 mg/L of Cr(VI) was reduced to less than 50 μg/L. The magnetic nanoparticles could be completely recovered for reuse. Two methods involving metal-doping and surface-coating were developed to enhance the adsorption capacity of the original materials while preventing the nanoparticle dissolution under extreme conditions. The results from comparative studies showed that the Al-doping or δ-FeOOH-coating could promote the adsorption of Cr(VI) significantly without sacrificing other properties too much. The dissolution of modified nanoparticles was inhibited by 30-40%. Especially, the Al-doped γ-Fe2O3 nanoparticles were found able to treat very acidic (pH 1) industrial wastewater. Adsorption mechanism studies were conducted using XPS, FTIR and Raman spectroscopy. It is suggested that the uptake of Cr(VI) by Fe3O4 nanoparticles could be a combination of physical adsorption and chemical redox reaction, the adsorption of Cr(VI) and Cu(II) by γ-Fe2O3 or modified MnFe2O4 nanoparticles could be due to electrostatic attraction and ion exchange; and the adsorption of Ni(II) by γ-Fe2O3 nanoparticles could be as a result of electrostatic attraction only. Al-doping on γ-Fe2O3 changed the force between adsorbed CrO42- and nanoparticles from weak electrostatic attraction into specific complexation. Keywords: adsorption, Al-doping Cr(VI), desorption, ferrite, heavy metal, Langmuir isotherm, maghemite, magnetic nanoparticles, recover