Nano-structured Mg-doped Fe2O3-ferrihydrite powder--a new adsorbent for cation removal from aqueous solutions

The present studies were undertaken with a view to developing a low-cost, efficient adsorbent for removal of heavy metal ions from aqueous solutions. A new cost-effective nano-structured Mg-doped Fe2O3-ferrihydrite adsorbent exhibiting high uptake capacities for various cations was synthesized. After preliminary testing of a series of Mg-doped (0.38 to 0.98%) samples for Pb(II), Cd(II), Cu(II) or Co(II) adsorption, a typical sample containing 55.7% Fe and 0.38% Mg was chosen for detailed characterization studies (XRD, TG-DTA, FTIR and TEM techniques). The adsorption behaviour of cations on the prepared nano powder was studied under various experimental conditions. The generated data were fitted to kinetic and isotherm models. The Langmuir monolayer capacities were 99.1, 83.3, 111.1 and 151.5 mg/g for Pb(II), Cd(II), Cu(II) and Co(II), respectively. It is the first time that such a high loading capacity for Co(II) has been reported. The effect of the presence of chloride or sulphate was dependent on the nature of the cation. Lead(II) adsorption was endothermic in nature with increased randomness at the solid-liquid interface, while for the rest of the three cations the adsorption process was exothermic. The XRD and FTIR studies on the loaded samples revealed structural changes during the adsorption process. The high cation loading capacities along with the stability of the loaded adsorbent make the synthesized ferrihydrite-Fe2O3 nano powder a potential candidate for contaminated water purification.