Study On Self-sealing/Self-healing Capacity Of Modified Landfill Clay Liner

Landfill is one of the main means of MSW(municipal solid waste) treatment. Clay (natural clay, compacted clay, etc.) is widely used in the landfill liner system due to its strong pollutants interception ability, low permeability coefficiency, and low cost. However, owing to containing hydrophilic minerals, clay tends to expansion and contraction when gains or loses water. The cycle of evaporation and infiltration or freezing and thawing can result in clay liner's cracking. What's more, landfill leachate can dissolve some components in the clay, which will also increase liner's permeability coefficient. One common problem with these barriers is that they cannot be repaired without laborious excavation and reconstruction if they are damaged. So it not only causes lots of trouble, but also increases the landfill operation and maintenance costs, and causes the surrounding groundwater and soil pollution.In order to improve these problems, enhance the clay liner's shear strength, reduce swelling and achieve self-healing/self-sealing ability, this paper selected economical adding materials and changed the structure of the liner. The modification could greatly reduce the operational and maintenance costs of the landfill. At the same time, the impact factors of the self-sealing process and the mechanisms of the self-healing are explored by column experiments. The results showed that the self-healing capacity of the clay liner not only fulfilled by pozzolanic reaction, but also accomplished by precipitation. And the realse of the pollutants influenced by both the adding materials and the structure of the liner. Specific experimental contents and results are as follows:(1) lime, fly ash was selected based on the principle of self-healing/ self-sealing and the theory of pozzolanic. after a series of physical, chemical and engineering experiments (plasticity index, shear strength), it was found out that the composite soil made of clay and adding materials weakened the tendency of cracking caused by clay's swelling and shrinkage, enhanced the clay liner's poor shear strength and increased the pollutants removal capacity from the landfill leachate. Since fly ash is widely available, and economical industrial waste, so it became the optimal experimental materials added into clay liner.(2) column experiments, structured by two-layer, taking clay as a medium, added lime, fly ash and activator (Na2SO4 or CaCl2) respectively to the top and the lower layer, filled with water or landfill leachate, explored the self-healing/ self-sealing reaction mechanism and influence factors. Experimental findings were as follows:1) The permeability coefficient of the liner influenced by adding materials, activators, and the leachate.SH/SS could be achieved by adding lime and fly ash to the clay liner. As the penetration of leachate, activators (Na2SO4 and CaCl2) significantly reduced the permeability of the liners. However, they functioned with different mechanism; Na2SO4 mainly strengthened pozzolanic reaction, while CaCl2 played an important role in promoting precipitation reaction to plug the pores of the liner. Na2SO4 can speed up the pozzolanic reaction between fly ash and lime, thus it effectively reduced clay liner's initial permeability coefficient to 10-8 cm·s-1.CaCl2 weakened volcanic reaction, but in the presence of leachate, it will precipitate with a large amount of CO32-, SO42-, S2-.and the precipitation plugged pores, thereby greatly reduced liner's permeability coefficient from 10-7cm·s-1 with water to 10-11cm·s-1 with leachate. At the same time when the SO42- in Na2SO4 is reduced to S2-, the majority of metal ions could be precipitated. Therefore precipitation reactions can be thought as another kind of self-sealing/self-healing reaction mechanism.2) SS ability of the liner will gradually diminish with the extension of time, and the addition of lime should be controlled within a certain range.under the experimental condition (hydraulic pressure was 20 cm, lime content was 20%, no activator, liner thickness was 5 cm,leachate is water) the liner had kept self-sealing ability within 13 days, but after 30 days the ability was significantly crippled. Under the same experimental conditions when the content of lime controlled within 10% to 20%, the self-sealing exerted better effect. Too high or too low lime content both weakened self-sealing effect.(3) Comparing different liner material and structures'capacity for pollutants releasing and blocking by using different leachate (water and landfill leachate), to provide a theoretical basis for application. The results were as follows, 1) The value of pH, COD, NH4 + in the discharged water had significantly risen after the clay liner added lime. While the double-layer structure liner not only used less lime, but also have higher pollutants retardant capacity. So it had both economic and environmental benefits.2) The addition of Na2SO4 markedly elevated the conductance value of the discharged water, due to the dissolution of itself and the function of accelerating the hydration process of the ash. The dissolution of CaCl2 released a large number of Cl- also causing serious environmental pollution.In short, when it comes to the physical and chemical properties and engineering characters, fly ash, lime were favorable adding materials to clay liner; As far as the SH/SS is considered, activators of Na2SO4 and CaCl2 both reduced the permeability, by promoting pozzolanic reaction and precipitation reactions in the liner; from preventing the release of pollutants point of view, clay liner with double-layer structure and without activators could meet the landfill liner's demands. Therefore the modified clay liner for SH/SS with double-layer structure, 10% -20%lime proportion was proposed.As China is a developing country, clay liner is still playing an important role in landfill. Hence the need to develop a kind of economical clay liner with SH/SS ability is urgent. However the researches in this area have not been carried out yet in China. Although in some foreign countries, some relevant researches have been done on the SH/SS landfill liner, but there are also many aspects need to be made clear. For example, the parent materials added, the impact factors of the SH/SS, and so on.This paper identified lime and fly ash can be used as adding materials of the SH/SS clay liner; Na2SO4 and CaCl2 can also realize SH/SS by precipitation. At the same time, the impact factors were investigated under the experimental conditions, which provided as a reliable theoretical basis for engineering applications.