The influence of sodium and sulfate ions on total solidification and encapsulation potential of iron-rich acid mine drainage in silica gel

Official URL: http://dx.doi.org/10.1016/j.jece.2018.05.037

Upon neutralization by acid mine drainage (AMD) a semi-solid silica gel forms from sodium silicate. During the solidification of silica gel, the surrounding positively charged toxic heavy metals can also be adjoined to the negatively charged silica network in a similar manner by forming a newly metal-silica bond. Simultaneously, sodium silicate addition increases the pH value of the AMD and restricts the solubility and leachability of the most metal ions. Thus, this experimental study investigated the adjoining potential of the metal ions (particularly iron) into the silica gel at different acidic-alkaline conditions (pH: 5.5, 7, and 9) in the presence of excessive Na+ and SO4-2 ions originated from sodium silicate solution and AMD respectively. As described via infrared spectroscopy (FTIR), X-ray diffractometry (XRD), scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), and thermogravimetric (TGA) analyses, the extended gelation time at pH 5.5 and 9 resulted in a higher precipitation of secondary minerals such as sodium and iron sulfates while the only formed encapsulant mineral, sodium iron silicon oxide was constantly formed at any pH value. The acid leaching of solid AMD specimens up to 96 hours resulted in 7 to 16% iron ions release that can be attributed to the dissolution of secondary formed iron-based minerals.