Factors affecting the immobilization of metals in geopolymerized flyash

Geopolymerization, a fairly new technology based on a very old principle, has emerged during the last few years as a possible solution to some waste stabilization and solidification problems. Some commercial successes have been achieved, although the technique remains fairly unknown and seemingly unpopular. It has been shown that most waste materials containing sources of silica and alumina should be capable of taking part in a geopolymerization reaction. In this article, flyash was used as a reactant in creating a geopolymeric matrix for the immobilization of process water containing 25 000 ppm of Cu or Pb cations. By means of x-ray diffraction, scanning electron microscopy (SEM), infrared spectroscopy, Brunauer-Emmett-Teller (BET), compressive strength, and kinetic leaching analyses, the main factors influencing matrix stability, immobilization efficiency, and therefore leaching behavior were investigated and discussed qualitatively. It was found that relatively high strengths could be obtained using low Ca flyash. The environment and coordination number of source aluminum and silica seemed to play a major role in the eventual matrix stability. Other factors influencing matrix stability include the alkali metal cation used and the type of metal being immobilized. The kinetics of leaching of immobilized metals from the geopolymerized flyash were qualitatively found to proceed along a combination of pore diffusion and boundary diffusion control mechanisms. It is finally concluded that immobilization of metals in geopolymerized flyash proceeds by a combination of physical encapsulation and chemical bonding, with adsorption also thought to play a role.