Effect of sulphate and nitrate anions on heavy metal immobilisation in ferronickel slag geopolymers

Industrial wastes need to be valorised and new techniques need to be developed for the production of secondary materials or immobilisation of hazardous elements in order to improve sustainability of the respective industrial sector. In the present experimental study the effect of monovalent and divalent anions, such as sulphate and nitrate, on heavy metal immobilisation in ferronickel slag geopolymers was investigated. Low calcium electric arc ferronickel slag was geopolymerised using KOH and Na ₂SiO ₃ as activators, in the presence of limited quantities of sulphate and nitrate salts of Pb, Cu, Cr or Ni. Gel properties, final strength of the specimens produced and immobilisation of heavy metals were determined, in order to explore the potential of geopolymerisation in developing a hazardous waste encapsulation matrix. In addition, the Toxicity Characteristics Leaching Procedure was used to assess the potential toxicity of the produced geopolymers. The experimental results indicate that even limited quantities of sulphates and nitrates in the starting mixture affect adversely geopolymerisation of ferronickel slag. XRD (X-ray diffraction), SEM (scanning electron microscopy) and FTIR (Fourier transform infrared spectroscopy) studies were carried out to identify new phases, reactions taking place and thus elucidate the main mechanisms involved.