Effect of acid buffering capacity on the remobilization of heavy metals in sewage sludge barrier for tailings

Remobilization of heavy metals from the bottom liner system due to the seepage of acid mine drainage (AMD) is an important concern in the long-term management of tailing impoundment. Titration tests were carried out to evaluate the acid buffering capacity (ABC) of sewage sludge and to investigate its effect on the remobilization of heavy metals. Test results demonstrate that the ABC increases with solid/liquid ratio and anaerobic incubation time and it is mainly attributed to the abundant organic matters contained and increasing carbonate loads. The added heavy metals (Zn, Pb, and Cu) were well immobilized during the anaerobic incubation stage but were released out dramatically during the acidifi cation especially when pH drops below 6.0 because of dissolution of carbonates and cation exchange of clay minerals. The calculated results, from a simplifi ed model, indicate that high levels of remobilization of heavy metals are not expected during the typical management time because of the high ABC of compacted sewage sludge barrier. These results support that sewage sludge is a suitable bottom liner material for the management of AMD from tailings. Effect of acid buffering capacity on the remobilization of heavy metals in sewage sludge barrier for tailings 63 2010, Zhang et al. 2016). Additionally, sewage sludge contains abundant organic substances and microorganisms. Not only does this benefi t the strong absorption of heavy metals presented in the leachate, but also it induces weak alkaline and reduced condition that mediates the process of sulfate reduction (Zhang et al. 2014, Tuttle et al. 1969, Kamon et al. 2002). Furthermore, sewage sludge is a cost-effective and easy-to-access material and its reuse might be of signifi cant advance in the consideration of sustainability. ABC of sewage sludge barrier and its impacts on the remobilization of heavy metals, however, has not been evaluated. Actually, there are some contradictory results regarding this question. For instance, laboratory test on the saturated hydraulic conductivity of compacted sewage sludge permeated with simulated AMD (pH=6.0) by Wang et al. (2010) showed that the pH of effl uent varied from 7.0 to 7.9 and the heavy metals (Zn and Cd) were well immobilized in the specimen. That is, the sewage sludge seems to have enough ABC to maintain the pH condition in the barrier system. Another similar test by Zhang et al. (2014) revealed that under the seepage of synthetic AMD with low pH (pH=2.1), the concentration of Zn and Cd in the leachate increased. It seems that ABC of sewage sludge could be consumed out and it loses its function under low pH conditions. As previously mentioned, any comprehensive evaluations about the feasibility of a barrier material for tailings and the impaction of heavy metal from tailing impoundment on the adjacent environment ask for a thorough understanding of the attenuation mechanisms of heavy metal pollutants within the barrier system. In this study, acid titration test was conducted on the sewage sludge suspensions to evaluate the ABC of sewage sludge barrier system and to reveal the relationship between remobilization of heavy metals and decreasing pH. A simplifi ed model was designed to describe the depletion of ABC within the sewage sludge barrier and therefore the prediction of its failure time. The ultimate goal of this study was to make additional estimation on the feasibility of sewage sludge as a probable barrier material for management of tailings. Materials and methods Material Sewage sludge sampled from the bottom of a settling tank in Yanerwan municipal sewage water treatment plant, located in Lanzhou, China, was used for the experiment as the barrier material in this study. The physical and chemical properties of the sewage sludge are shown in Table 1. Sampled sewage sludge was air-dried for at least 30 days before any testing procedures because it was over-saturated under natural conditions. Firstly, the water content of natural and air-dried sludge was determined. Then the air-dried sample was ground and passed through 0.5 mm sieve for other tests. In this study, the water content is defi ned as the mass quantity of water in a totally dry material, which can be expressed as a ratio of the weight of water to the dry weight of sludge (often presented in percentage). Dry weight of sludge was measured with oven-drying method at the temperature of 60°C instead of the standard temperature of 105oC to prevent any mass loss of contained organic substances due to charring. The difference value in water content between natural and air-dried sludge can be as large as 302.33%, which indicates good water holding capacity of the sampled sewage sludge. The liquid limit and plastic limit were measured following cone penetrometer method, which was often used in previous studies (Zhang et al. 2016). The specifi c gravity, whose value is equivalent to the Table 1. Properties of sewage sludge used