Clinoptilolite zeolite use as adsorbent for removal of metal contaminants from natural acid mine drainage

Natural clinoptilolite zeolite was investigated for potential use as an adsorbent for the removal of Al, Fe and Zn from natural acid mine drainage (AMD). The AMD (pH of 3.01) was polluted water obtained from an abandoned coal mine in South Africa. A batch reactor experiment was set up for the treatment conducted at the mixing ratios of 67:1, 20:1, 14:1, 6.7:1 and 5:1 by weight of AMD to zeolite. The reactors were agitated for 15, 30, 60, 120, 180 and 300 min, then the AMD was analysed. Kinetics of metals removal and the adsorbent’s efficacy, were measured. Absorption ability was simulated using the Langmuir and Freundlich adsorption isotherms. The optimal ratios were 6.7:1 and 5:1 by weight of AMD to zeolite, which increased the pH levels of AMD to the ideal circumneutral values of 6–8 within 15 min of the treatment. Results showed that the natural zeolite effectively removed 82–99% of Al, Fe and Zn from AMD. Both isotherms showed a realistic depiction of the zeolite’s rate and capacity for adsorption of the contaminant metals; however, the Langmuir equation fitting was more suitable than that of the Freundlich isotherm.