A comparative study on the synthesis of magnesium ferrite for the adsorption of metal ions: Insights into the essential role of crystallite size and surface hydroxyl groups

Metal ferrites with spinel structure are widely used as magnetic adsorbents and catalysts for solving myriads of environmental problems. This study was focused on the evaluation of factors influencing the adsorption properties of magnesium ferrite towards metal ions. Specifically, the effect of the synthesis method (glycine-nitrate, citrate–nitrate, and co-precipitation, abbreviated as gn, cn, cp) and the calcination temperature (range of 300–800 °C) on the crystal structure, texture, and adsorption characteristics of magnesium ferrite nanoparticles were studied. The adsorption affinity of heavy metal ions onto magnesium ferrites obeyed the following order gn-MgFe2O4 > cn– MgFe2O4 > cp-MgFe2O4. An increase in the calcination temperature of gn-MgFe2O4 and cn-MgFe2O4 from 300 to 800 °C leads to a decrease in the adsorption capacity and this may be attributed to a significant increase in the size of the crystallites, a decrease in the specific surface area and the removal of surface hydroxyl groups. The amorphous cp-MgFe2O4-80 and cp-MgFe2O4-800 samples demonstrated the equal adsorption capacity for metal ions and characterized by specific surface area of 179 and 13 m2/g, respectively. It was shown that the crystallites size and the concentration of surface hydroxyl groups are the main factors determining the adsorption performance of the synthesized novel adsorbents.