Acid Buffer Capacity Of Sewage Sludge Barrier For Immobilization Of Heavy Metals

Sulfide minerals in tailings can be oxidized and generate acid mine drainage (AMD) under natural weathering condition. The AMD is characterized by a low pH and elevated concentration of heavy metals, all of which have a detrimental effect on the ecosystems of mining districts. AMD from mine wastes is a serious environmental problem associated with mining industry around the world.Benefit from the reduced condition due to anaerobic microorganisms, sewage sludge can be used to make an innovative barrier, sewage sludge barrier, for controlling the heavy metals leached from tailings. Compared with traditional hydraulic barrier such as compected clay liner, sewage sludge barrier does not only have a low hydraulic conductivity as traditional hydraulic barrier, but also employ the anaerobic activities of microorganisms to immobilize the heavy metals.The interactions between sewage sludge barriers and acid mine drainage (AMD) will result in the acidification of the sewage sludge, as a result, 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 times and the effect of ABC on the mobility of heavy metals were investigated in the thesis. Acid titration tests were conducted to measure the ABC of sewage sludge suspensions, to analyze the sources of ABC, to discuss the relationship between the consumption of ABC and the release of heavy metals, and to estimate the immobilization time of heavy metals in sewage sludge barriers in simulated thicknesses conditions. This research tries to establish a theoretical basis for practical application of sewage sludge barriers.Acid titration 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 2.0m thickness, even undergoing 666-years acidification by AMD under 10.0m water head, can maintain a condition of pH>6.2 and, therefore, keep immobilize the heavy metals of AMD in the barrier.