Treatment of acid rock drainage by clinoptilolite — Adsorptivity and structural stability for different pH environments

The adsorptive capacity for aluminum, copper, iron and zinc ions of clinoptilolite was studied based on batch equilibrium tests using synthetic and natural acid rock drainage (ARD). Clinoptilolite efficiently adsorbed Cu 2+, Fe 2+, Al 3+, and Zn 2+ within 20–30 min. Stability tests were carried out for pH from 1 to 5 to determine the leaching of ions from clinoptilolite. After acid treatment at pH < 2, the clinoptilolite released significant amounts of Al, Fe, Na and K cations. XRD patterns revealed that clinoptilolite treated at pH 1–5 for 144 h had similar diffraction patterns and minimal change in crystallinity. Elemental analysis by ED-XRF confirmed that the Al content for treatments at pH 1 and > 3 were similar to that of untreated clinoptilolite. TEM showed that clinoptilolite had more rounded corners after treatment at pH 1, whereas at higher pH the morphology was similar to that of the untreated clinoptilolite. At low pH clinoptilolite might dissolve only at the surface. Clinoptilolite appears to be a good candidate for ARD treatment.