The International Chemical Congress of Pacific Basin Societies 2021
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The acid mine drainage (AMD), a wastewater continuously discharged from abandoned mines, is highly acidic and contains hazardous heavy metal ions, causing serious environmental issues. To develop the treatment of AMD using natural zeolites, the equilibrium adsorption of model AMD using synthetic zeolite was studied to clarify the adsorption of metal ions by the zeolite in this study. Then, mordenite was employed as a representative zeolite contained in the natural zeolites and sodium type mordenite (NaMOR) was used.
The cation exchange capacity (CEC) of NaMOR was measured as 18.5×104 keq kg−zeo−1, comparable to that of ideal mordenite. The major exchangeable cation originally contained in NaMOR was Na+, sharing 78% of the CEC of the NaMOR. Then, the adsorption equilibria of Mn2+ and Zn2+, popular heavy metal ions in the AMD, using NaMOR were measured. The NaMOR could increase the pH in the model AMD due to adsorption of H+ and the zeolite could mitigate the high acidity. The pH at equilibrium increased as the initial concentration of heavy metal ions in the model AMD decreased. The NaMOR could adsorb and remove both of Mn2+ and Zn2+. The fractional removal of heavy metals increased as the initial metal concentration decreased and the initial pH increased. The adsorbed amount of heavy metal ions increased as the metal concentration and pH at equilibrium increased because of the competitive adsorption among H+ and metals in the model AMD. The extended Langmuir model for the multicomponent system approximately predicted the adsorption isotherms involving not only heavy metal ions, Mn2+ and Zn2+, but also Na+, exchangeable cation contained in NaMOR. While the saturated adsorbed amounts of Mn2+ and Zn2+ were the same as each other, that of Na+was larger than that of Mn2+ or Zn2+. The hydraulic diameters of Mn2+ and Zn2+ are similar to each other, and that of Na+ is smaller than those of the other cations, making the saturated adsorbed amounts of Na+ larger.
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