The potential application of red mud and soil mixture as additive to the surface layer of a landfill cover system

Red mud, the by-product of aluminum production, has been regarded as a problematic residue all over the world. Its storage involves risks as evidenced by the Ajka red mud spill,an accident in Hungary where the slurry broke free, flooding the surrounding areas. As an immediate remediation measure more than 5 cm thick red mud layer was removed from the flooded soil surface. The removed red mud and soil mixture（RMSM） was transferred into the reservoirs for storage. In this paper the application of RMSM is evaluated in a field study aiming at re-utilizing waste, decreasing cost of waste disposal and providing a value-added product. The purpose was to investigate the applicability of RMSM as surface layer component of landfill cover systems. The field study was carried out in two steps： in lysimeters and in field plots. The RMSM was mixed at ratios ranging between 0 and 50% w/w with low quality subsoil（LQS） originally used as surface layer of an interim landfill cover. The characteristics of the LQS ＋ RMSM mixtures compared to the subsoil（LQS） and the RMSM were determined by physical–chemical, biological and ecotoxicological methods. The addition of RMSM to the subsoil（LQS） at up to 20% did not result any ecotoxic effect, but it increased the water holding capacity. In addition, the microbial substrate utilization became about triple of subsoil（LQS） after 10 months. According to our results the RMSM mixed into subsoil（LQS） at20% w/w dose may be applied as surface layer of landfill cover systems.

In-situ neutralize methane emission from landfills in loess regions using leachate

In loess regions, landfilling is the predominant solid waste disposal and loess is usually used as landfill cover soil. However, the methane(CH_4) bio-oxidation activity of virgin loess is usually below 0.01 μmol/(h g-soil). In this study, we proposed a method to improve CH_4 removal capacity of loess by amelioration with mature landfill leachate, which is in-situ, easily available, and appropriate. The organic matter content of the ameliorated loess increased by 180%, reaching 19.69–24.88 g/kg-soil, with more than 90% being non-leachable. The abundance of type I methane-oxidizing bacteria and methane monooxygenase gene pmoA increased by 5.0 and 79 times, respectively. Consequently, the maximum CH_4 removal rate of ameliorated loess reached 0.74–1.41 μmol/(h g-soil) at 25°C, which was 4-fold higher than that of water-irrigated loess. Besides, the CH_4 removal rate peaked at 10 vt% CH_4 concentration and remained at around 1.4 μmol/(h g-soil) at 15°C–35°C. The column test confirmed that the highest CH_4 removal efficiency was at 30–40 cm below the surface, reaching 26.1%±0.4%, and the 50-cm-thick loess layer irrigated with leachate achieved more than 85% CH_4 removal efficiency. These results could help to realize carbon neutrality in landfill sites of global loess regions.