Desilication of calcined pulverised fly ash and use of silicate solution to prepare a mesoporous silica adsorbent for heavy metals in acid mine drainage

Mesoporous silica (MCM41) adsorbent was prepared from silicate solution produced from silica leaching of calcined pulverised fly ash. MCM41 was used for the removal of heavy metals and neutralisation of acid mine drainage. Silica leaching was optimised using two level full factorial and 13 run central composite rotable uniform design with five centre points. A pH of 7 produced the most pure MCM41 with an external surface area of 98,000 m2m-3. A 4% m/v solid loading of MCM41 could remove 95.5%, 97.3% and 99.6% of Ni, Cu and Fe respectively. The adsorption process was found to fit the Langmuir isotherm and pseudo second order kinetics. The Gibbs free energy of adsorption for all metals was below-20 kJ/mol for all metals. The enthalpy for Fe, Cu and Ni removal were 19.97 kJ/mol, 21.40 kJ/mol and 16.44 kJ/mol respectively and thus endothermic. The significance of the research was that silica could be leached from calcined pulverised fly ash and MCM41 can be used as an adsorbent for heavy metals in acid mine drainage treatment.