Influence of biochar addition on gas permeability in unsaturated soil

Biochar (BC) has been proposed as an alternative soil amendment in landfill cover. Its effects on soil hydraulic properties (e.g. soil water retention) have been explored to some extent, while its influence on landfill gas flow characteristics is not clear yet. The objective of this study is to investigate the water retention and gas permeability (k_g) of BC-amended soil and its relationship with soil water content in unsaturated state. In total, three BC contents (0, 5 and 10%) were considered in clay–sand mixtures. Soil suction, water content and gas pressure were measured continuously using a new in-house developed apparatus. Test results show that the presence of BC increased soil water content and the increased amount was almost the same over a wide range of suction values (up to 3 MPa). k_g increased linearly with soil suction in semi-log scale, regardless of the BC content. The addition of 5 and 10% of BC could decrease k_g by up to 50 and 65%, respectively, compared with that of the bare soil. k_g decreased with water content for both soil and BC -amended soils. Moreover, the decrease rate of k_g with water content became smaller when the BC percentage was higher.