| Acid mine drainage, formed when sulfide-rich mine waste oxidize in the presence of water and oxygen, is characterized by a low pH and elevated concentration of heavy metal ions and sulfate. Acid mine drainage is a worldwide concern facing the mining industry. Solid contaminant barriers might be an effective means in the source control of acid mine drainage by limiting or precluding the contact of mine waste with water or oxygen.In this paper, sewage sludge was chosen as barrier material based on principle of Reducing Barrier. Permeability, heavy metal stabilization and geotechnical properties of sewage sludge were evaluated by a series of laboratory tests. According to the tests results, a cooper mine was chosen as an case study and applications of sewage sludge barrier in the containment of mine waste were proposed to investigate the feasibility of pollution control of acid mine drainage. The main research contents and results are as follows:(1) Properties of sewage sludgeNatural water content, air-dried water content, plastic limit, liquid limit, organic matter content, specific gravity, total heavy metal content, optimum water content and maximum dry density were determined in the laboratory. The test results indicate that organic matter with a high hydrophilicity and low specific gravity poses a significant effect on properties of sewage sludge.(2) Permeability properties of sewage sludge barrierHydraulic conductivities of compacted sewage sludge permeated with distilled water, dilute sulphuric acid and simulated acid mine drainage were determined using a flexible-wall permeater. The results show that hydraulic conductivities of all sewage sludge specimens with permeation time. Hydraulic conductivities of sewage sludge specimens were in the order of 10-11 cm·s-1 after the 75th day, which is far below the recommended hydraulic conductivity (1×10-7 cm·s-1) for barrier materials. Compacted sewage sludge shows excellent compatibility with simulated acid mine drainage and might be a suitable barrier material when permeability considered alone.(3) Heavy metal stabilization capacity of sewage sludge barrierHeavy metals speciation within sewage sludge specimen before or after permeation were tested as per Tessier and potential mobility index (PMI) was introduced to assess stability of stability and potential mobility of heavy metal in all specimens. Heavy metal speciation analyses indicate that permeation time and permeation liquid have no significant effect of speciation distribution of heavy metal within specimens. Within all specimens, Fe was mainly distributed in residual fraction with high stability; Zn, Cu and Ni were mainly distributed in organic matter fraction with moderate stability; Cd had no dominant fraction but showed relative high fraction of exchangeable and carbonate with poor stability. No obvious increase of potential mobility was detected when sewage sludge permeated with simulated acid mine drainage. Compacted sewage sludge generally shows high stabilization capacities of all heavy metal exception Cd.(4) Geotechnical properties of sewage sludge barrierPlastic limit, liquid limit of sewage sludge, coefficient of compressibility and shear strength of compacted sewage sludge were determined with distilled water and simulated acid mine drainage as pore liquid. Plastic limit and liquid limit of sewage sludge was 207%,57% respectively and decrease to 195%,53% when pore liquid was switched form distilled water to simulated acid mine drainage. Compacted sewage sludge had a high compressibility and the value of a1-2 was 0.85 MPa-1. The a1-2 was 0.90 MPa-1 when the pore liquid was simulated acid mine drainage. Cohesion and friction angle of compacted sewage sludge was 9.1kPa,1.85° and changed to 9.75kPa,1.64° respectively when the pore liquid was simulated acid mine drainage. Sewage sludge was high plastic and shows high compressibility and low shear strength after compacted with both pore liquid.(5) Application of sewage sludge barrierA copper mine was chosen as case study to evaluate feasibility of sewage sludge barrier of three models. Based on its low permeability, high compressibility and fluidity, a barrier with low permeability and high heavy metal stability capacity could be formed under the gravity of mine waste if lined or co-disposal with sewage sludge. Compacted sewage sludge would be a suitable barrier for pollution control of acid mine drainage in mine sites.In summary, accurate determination of hydraulic conductivity of compacted sewage sludge and reasonable evaluation of heavy metal stability within sewage sludge were realized and principle of Reducing Barrier was verified in this research. In addition, feasibility of compacted sewage sludge as barrier for pollution control of acid mine drainage was assessed by a case study when permeability, heavy metal stabilization and geotechnical propertied of sewage sludge were considered together. |
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