Synthesis and applicatión of manganese dióxide coated magnetite for removal of metal ións from aqueous solutións

Abstract

A microsorbent with magnetic properties has been successfully synthesized by means of the deposition of manganese dioxide onto magnetite particles.For the physicochemical characterization of the synthesized magnetic microsorbent different measurements were carried out. Magnetic properties were determined by means of an altemating gradient magnetometer (AGM). The specific surface area was determined based on BET theory. Further information about composition and morphology was gathered by theuse of an environmental scanning electron microscopy with energy-dispersive X-rayspectroscopy analysis (ESEM/EDAX). By means of microelectrophoresis measurements of the zeta potential of the particle surface were carried out.The results showed, that the resulting microsorbents have a particle size between 0.5 and 18 ?m. Magnetization ofthe material is between 41 and 75 Am2/kg, in the arder ofmagnitude ofthat of magnetite. Therefore efficient solid-liquid separation by means of the application of a modera te magnetic field is possible. Due to the properties of Mn02 the point of zero charge of the surface was found to be clase to 3. Therefore, the adsorbent is almost exclusively a cation exchanger although sorption of oxyanions is also possible.Sorption experiments were carried out and evaluated by means of the Langmuir andFreundlich relationships. The influence of pH, competitive sorption, and complexing ligands was studied. Results of the equi librium experiments canied out with Cd2+, Ni2+, and Pb2+ ions,showed that the sorption process was highly dependent on the pH of the solution and the uptake of heavy metal ions is preferred against the sorption of other cations. Sorption of alkaline earth ions such as Ca2+ affects the uptake only at high concentrations. Other ions suchas chloride ions affect also the uptake dueto changcs in the metal ion speciations.Kinetic experiments were carried out in batch systems. The sorption kinetics was evaluated by means of four models; film diffusion, Lagergren or pseudo-first-order, pseudo-second-order, and the Elovich model. It was found, that the film diffusion model can successfully describe the sorption kinetics and in conjunction with the equilibrium parameters, the sorption at determined conditions can be modeled well.Beca use of the small size of the particles and the respective large specific surface, sorption is fast. It was observed that within the first 10 minutes around 90% of the final uptake takes place and sorption equilibrium was reached within one hour.Besides the experiments with heavy metal cations, kinetic and equilibrium sorption experiments with the oxyanion MoO42- were carried out. It was found that the sorption of oxyanions onto the magnetic microsorbent was possible too. As well as for the metal ions the sorption process was fast and highly dependent on the pH of the solution. On the basis of the results the order of sorption capacity was found to be: Pb2+ > Ni2+ > Cd2+ >> MoO42. Finally, results of consecutive sorption-desorption cycles demonstrated that the magnetic microsorbent can be recycled, although the sorption capacity drops considerably within the first few cycles. Nevertheless, the high selectivity for bivalent heavy metal ions makes the synthesized microsorbents an interesting alternative to conventional ion exchangers. Therefore, in the outlook a possible set-up to apply the magnetic microsorbents in industrial scale is suggested.