Role of biochar as a cover material in landfill waste disposal system: Perspective on unsaturated hydraulic properties

Biochar amended soil (BAS) has emerged as a sustainable and alternate material as a final cover system for landfills. This final cover is often subjected to change in matric suction due to seasonal change in evapotranspiration rates. The water retention characteristics, gas permeability and water infiltration vary for unsaturated soil depending on the suction in the soil. Main objective of this study was to investigate these unsaturated soil properties of BAS which are of major concern in landfill cover functioning. The biochar was produced from water hyacinth inside an in-house pyrolysis unit and mixed with a silty sand at 5% application rate. Water retention capacity increased with biochar amendment to silty sand. The air entry value increased by almost 100%, while the desorption rate marked by n parameter had no significant change, which was attributed to entrapped clay particles within the biochar and consequent alteration of soil porosity. Beyond suction range of 1 MPa, gas permeability of BAS decreased by almost 50% as compared to bare soil. This may be due to suppression of desiccation cracks in biochar amended soil. Infiltration rate was found to be relatively higher in case of biochar amended soil in comparison to bare soil. However, infiltration rate at different suction range were in same order, indicating marginal change in soil infiltration rate for actual field application.