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

Tailings from mining industry can generate acid mine drainage (AMD) containing high-concentration heavy metals. When the sulfide minerals are exposed in an oxidized condition, and result in the contamination of soils and groundwater around the tailings. Benefit from the microbial activities especially the anaerobic sulfate reduction processes, sewage sludge could be used as a barrier to immobilize the heavy metals leached from tailings. Sewage sludge barrier is an enrichment and improvement of the available barriers.AMD solution, generated from weathering and leaching process, shows a stronger oxidizing capability than the sludge, because of the dissolved of free oxygen in it. Redox reactions between AMD solution and sludge barrier inevitably lead the sludge barrier to an oxidation state. Heavy metals originally immobilized by sulfate reduction in sludge barrier may be released out secondarily because of the oxidation of sludge. The objectives of this research are to analyse the source of the oxidation buffering capacity of sludge, to discuss the relationship between the consumption of oxidation buffer capacity and dissolution of heavy metals, and to calculate the immobilization time of heavy metals in sewage sludge barriers in a simulated thicknesses conditions. This research try to establish a theoretical basis for the practical application of sludge barrier.At first, specially designed titrations experiments were conducted in this research. Sludge suspensions were prepared by mixing sludge with distilled water, detailed treatment including addition of nutrients, strengthening of the target heavy metals Zn, Cu, Pb, pH adjusting. Followed by an anaerobic cultivation of the sludge suspensions by placing them in water bath with a constant temperature. After anaerobic cultivation for a definite time, a certain quantity of sludge suspension was used for oxidation titration experiment. During the oxidation titration tests, some suspension was taken for chemical analysis at the moment that the Eh of suspension reached a designated Eh value, to determine the concentrations of target heavy metals and inorganic ions in suspension. The source of oxidation buffering capacity (OBC) of sludge and the release characteristics of heavy metals caused by oxidation were discussed based on the test results.Test results showed that OBC of sludge suspensions was decreased slightly with the solid-liquid ratio of the suspensions, but increased with the anaerobic incubation time. More than 50% of OBC was attributed by the sludge existing in strongly-reduction conditions (Eh≤-150mV). During the oxidation titration tests, it was found that when the pH of sludge was decrease around 6.0±0.5 and Eh≥-150mV, Zn was released obviously, while Cu and Pb released obviously when Eh≥150mV. When the pH of sludge was decrease to 5.0±0.5, the release of target heavy metals were significantly greater than those of pH=6.0±0.5. Especially when the pH of sludge was 7.5±0.5, almost no of heavy metals were found to be released. According to the test results, a mathematical model was established to predict the OBC consumption of the sludge barrier under AMD penetrating conditions. The simulation results showed that a sludge barrier with 2m thickness, even undergone 38787-years oxidation by AMD under 10m water head, keep in a strongly-reduced condition and, therefore, promote an immobilization of heavy metals from AMD in the barrier. As a final conclusions, the sludge barrier can be applied to mine tailings, which has a strong oxidation buffer capacity and ensure a long-term immobilization of heavy metals from AMD solution.