Variações macro e microestruturais na permeabilidade ao lixiviado em camada de base de aterro sanitário pela interação solo-contaminante

Abstract

Bentonite-soil mixtures in landfills have made it possible to obtain compacted layers with low water permeability and reduced cost. However, this bentonite soil is highly sensitive to contact with leachate. Therefore, variations in the permeability of soil layers exposed to this contaminant liquid can reduce the efficiency of the lining and allow the migration of contaminants to the subsoil and water bodies. To this end, this study aims to determine the effect of soil-contaminant interaction on the behavior of permeability to water and leachate in the base layer of a landfill at macro and microscopic scales. The experimental field of the research was the Sanitary Landfill located in the municipality of Campina Grande - PB - Brazil (ASCG). Local Soil (SL), Bentonite and leachate from this Landfill were collected and characterized. Bentonite was added to the SL in proportions of 5 (SB5), 10 (SB10), 15 (SB15) and 20% (SB20), in dry weight. These soil mixtures were also characterized and then samples were molded, according to the parameters obtained in the compaction test at the Normal Proctor energy. Macro and microstructural tests were performed with percolation of distilled water and leachate from the ASCG. The water and leachate retention curves of the mixtures were obtained by the filter paper method. The samples of the soil mixtures were subjected to permeability tests in a Triflex-2 flexible wall permeameter. The free swell levels of the soil layers flooded with the fluid of interest were also verified. After percolation of the liquid in the soil layers, samples were extracted and taken for optical microscopy and scanning electron microscopy tests. From a 10% increase in bentonite, the results showed that the presence of contaminant liquid in the samples promoted a reduction in matric suction and an increase in osmotic suction. This increase in osmotic suction increases as the percentage of bentonite in the mixture increases. The increase in bentonite promoted considerable reductions in the permeability coefficient to both fluids. With a 20% increase, it was possible to observe a 33-fold decrease in the permeability coefficient to water and a 65-fold decrease in the leachate permeability coefficient, enabling the use of the mixture as a base coating. The permeability to the contaminant liquid was greater than to water in all samples studied. This difference can be explained by volumetric reductions of the clay minerals that could be verified in the free swell of the soil samples in contact with the contaminant fluid. Microscopy tests show that there was a retraction in the expansion of clay minerals in contact with the percolate, reducing the thickness of the diffuse double layer and altering the repulsive forces in contact between the clay particles. This may have caused structural rearrangement and flocculation of the particles, increasing the pore space and allowing greater flow in contaminated soil layers. However, the difference in the permeability coefficient for the two fluids, for these soil layers, was not considered relevant, demonstrating the viability of using bentonite in composition with the local soil. The normative criteria were met, and the sustainability of the solution enables the promotion of geoenvironmental safety and cost reduction.