Acidification of lead/zinc mine tailings and its effect on heavy metal mobility

The acid-forming potential of lead/zinc (Pb/Zn) mine tailings at Lechang Ci ty of Guangdong Province was studied using both net acid generation (NAG) a nd acid-base accounting (ABA) methods. The pyritic and total sulfur content s of the tailings were 12.6% and 18.7%, respectively. The mean acid neutral ization capacity (ANC) was 63.5 kg H2SO4/t while three oxidized tailings sa mples had an ANC less than zero. The NAG and net acid production potential (NAPP) values were 220 and 326 kg H2SO4/t, and both the NAG and NAPP result s indicated that the tailings had a high acid-forming potential. NAG was mo re accurate than NAPP in predicting acid-forming potential of the tailings due to uncompleted oxidization of pyritic sulfur. Analysis of samples from two profile tests indicated that acidification mainly occurred at the surfa ce (0-20 cm) and had little effects at deep layer of the tailings. Total co ncentrations of Pb, Zn, Cu, and Cd were increased greatly with depth at the acidified tailings profile, while heavy metal concentrations at different depths of nonacidified tailings profile were similar. The results indicated that depletion of heavy metals at the acidified surface was due to acidifi cation. The diethylenetetramine pentaacetic acid (DTPA)-extractable Pb, Zn, Cu, and Cd concentrations of acidified tailings surface (0-20 cm) were sig nificantly higher than those of nonacidified tailings, which further reveal ed that acidification enhanced the mobility of heavy metals in the tailings . (C) 2001 Elsevier Science Ltd. All rights reserved.