[Acid buffer capacity of sewage sludge barrier for immobilization of heavy metals]

Employing the anaerobic activities of microorganisms, sewage sludge can be used as a barrier to immobilize the heavy metals leached from tailings. Due to the interactions between sewage sludge barrier and acid mine drainage (AMD), it is possible that the heavy metals that have been immobilized previously might be released out. The acid buffering capacity (ABC) of sewage sludge suspensions with various anaerobic incubation time and the effect of ABC on the mobility of heavy metals were investigated by acid titration tests. Test results showed that ABC of sewage sludge suspensions was increased with the solid-liquid ratio of the suspensions and the anaerobic incubation time, and that carbonate and organics play an important role in acid buffer of sewage sludge suspensions. During the acid titration test, Zn, Pb and Cu were released out obviously following the order of Zn > Cu > Pb as pH was decreased less than 6.2. A mathematical model was established to predict the ABC consumption of the sewage sludge barrier under AMD penetration condition. The simulation results showed that a sewage sludge barrier with 2.0 m thickness, even undergoing 666-years acidification by AMD under 10.0 m water head, can maintain a condition of pH > or = 6.2 and, therefore, keep immobilize the heavy metals of AMD in the barrier.