Coral Limestone Modified by Magnetite and Maghemite Nanocomposites for Sequestration of Lead(II) and Chromium(VI) Ions from Aqueous Solution

Abstract

In present work, a one pot synthesis of magnetic nanocomposites was synthesized by using co-precipitation method in air atmosphere. The synthesis of magnetite and maghemite supported on biogenic coral limestone was done by varying the ratio of Fe(II)/Fe(III) in solution to obtain the two phases of iron oxide and capping with sucrose in air atmosphere. The nanocomposites were characterized by FTIR, where the results showed a distinct peak for Fe-O, while UV/vis showed an absorption in the visible region which is typical of iron oxide. Photoluminescence results showed that the nanocomposites were both red shifted for magnetite-PCLS (PCLS = pristine coral limestone) and magnetite-CCLS (CCLS = calcined coral limestone); while a blue shift and red shift was observed for the maghemite- PCLS and maghemite-CCLS. From the SEM a deviation of sphericity of the nanocomposites, with maghemite having an uneven distribution was observed. Equilibrium was reached within 60 min, of which maghemite showed higher metal uptake. The kinetic data fit PSOM better as compared to PFOM, this means that adsorption was due to the charge density on the surface of the nanocomposites. The good fit for intraparticle diffusion (IPD) also suggested that adsorption was also observed due to mass transfer, it was observed that the rate limiting step was due to surface adsorption. This was in good correlation with the better fit of PSOM. The mechanism of adsorption was found to be better explained by physisorption and the surface was heterogeneous whereby multilayer adsorption was possible. The data was also subjected to Dubinin-Radushkevich isotherm, which suggests that the uptake of the pollutants was due to physisorption. The adsorption process was spontaneous and favourable which is supported by the negative values of Gibb’s free energy for the system.