Influence of the synthesis parameters on the properties of the sepiolite-based magnetic adsorbents

The magnetic nanocomposites (MNCs), used as adsorbents for Cd2+ ions, were prepared by a co-precipitation method using both the untreated natural (SEP) and the partially acid-activated (ASEP) sepiolites as supports for magnetite. The synthesis conditions differed as follows: NaOH or NH3 was added in the SEP or the ASEP suspension before or after the addition of Fe3 and Fe2+. The SEP-based MNCs prepared using NaOH had a lower magnetization, but higher adsorption capacities than the MNCs synthesized using NH3. The order of the mixing of the reagents had a small influence on the properties of SEP-based MNCs, especially when NaOH was used for the synthesis. Due to acidity of the surface functional groups of the ASEP, the magnetic composite could be synthesized only when NH3 was added in the suspension of ASEP having Fe3+ and Fe2+ ions. Despite of the worse dispersibility of the magnetic particles, the adsorption capacityof the ASEP-based composite was higher than of the MNC with SEP prepared by the same procedure. All the MNCs showed a superparamagnetic behavior at room temperature and the adsorption capacities were higher than of pure compounds. The adsorption of Cd2+ on the composites was best fitted with the Sips model, which suggested the chemisorption on the heterogeneous surface at energetically non-uniform sites until a monolayer was formed