Adsorption of Co(II) Ions from Aqueous Solutions onto Hybrid Geopolymers

Geopolymers are aluminosilicates characterized by a structure tectosilicate like and are prepared from an alumino-silicate material, treated with an alkaline solution of sodium silicate. By adding a silicone component of 10% (w/w) polydimethylsiloxane1 to the geopolymer, a material with good mechanical properties as well as high fire resistance is obtained. This solid is used for the removal of toxic metals such as Cobalt found in the environment as the radioactive isotope (60Co) is mainly used for the sterilization of medical equipment as well as in medical radiotherapy. In the present work, the adsorption of Co(II) by using a hybrid geopolymer at 15 °C, 25 °C and 50 °C in 0.1 M NaClO4 as ionic medium has been studied (Fig. 1). By potentiometric measurements it has been deduced that the solid presents superficial acid−base properties, due to silanol and aluminolic groups. In addition, the solid shows a point of zero charge (PZC) of 6.9 ± 0.2. Adsorption is described using isotherms obtained as a function of contaminant concentration, pH and ionic strength, and the data interpreted according to the Langmuir or Freundlich model. Adsorption process reaches equilibrium after about 6 hours. Kinetic studies show a pseudo-second order process; from data collected at different temperatures it is possible to obtain thermodynamic2 (G°, H°, and S°) and kinetic (activation energy) parameters.