ABSTRACT
Undoped and Fe-doped NiO nanoparticles were successfully synthesized using a lyophilization method and systematically characterized through magnetization techniques over a wide temperature range, with varying intensity and frequency of the applied magnetic fields. The Ni1-xFexO nanoparticles can be described by a core-shell model, which reveals that Fe doping enhances exchange interactions in correlation with nanoparticle size reduction. The nanoparticles exhibit a superparamagnetic blocking transition, primarily attributed to their cores, at temperatures ranging from above room temperature to low temperatures, depending on the Fe-doping level and sample synthesis temperature. The nanoparticle shells also exhibit a transition at low temperatures, in this case to a cluster-glass-like state, caused by the dipolar magnetic interactions between the net magnetic moments of the clusters. Their freezing temperature shifts to higher temperatures as the Fe-doping level increases. The existence of an exchange bias interaction was observed, thus validating the core-shell model proposed.
ABSTRACT
Novel magnetic zeolite type 5A nanocomposites were synthesized by the co-precipitation method and applied to lead removal from aqueous ambient. Maghemite nanoparticles were mixed with zeolite and, by controlling its content, transmission electron microscopy results gave sizes of 5 to 15 nm and selected area electron diffraction patterns confirmed the presence of zeolite. The nanocomposites have high specific surface area with values up to 194 m2/g. Magnetization measurements proved superparamagnetic behavior with saturation values of ~35 emu/gFe. Kinetic adsorption experiments showed removal efficiencies of 99.9% and an enhanced equilibrium time of 5 min. The lead concentrations after adsorption experiments lay under the permissible levels of 10 µg L-1, according to the World Health Organization. The maximum adsorption capacity, estimated by Sips model, was 265 mg L-1 at 300 K. The removal efficiency was significantly improved in the range of pH > 6, as well as in the presence of cation interferents such as Ca(II), Cu(II), and Cd(II). The adsorption mechanism was explained with cation exchange between Pb(II), the zeolite framework, and the protonated maghemite surface. Besides, our system revealed recyclability even after seven regeneration cycles. Thus, our synthesized materials have remarkable adsorption properties for lead water remediation processes.
