Modulation of Soil Gas Permeability and Bioelectricity Generation in Chrysopogon zizanioides-Based Microbial Fuel Cells Using a Carbon Sink Material

Plant microbial fuel cells (PMFCs) are an emerging technology with potential applications in green infrastructure, including landfill covers and reactive barriers. Biochar amendments in landfill cover soils enhance hydromechanical properties and promote vegetation growth, thereby supporting ecological restoration. However, the effects of biochar on gas and water permeability and bioelectricity generation in PMFCs under unsaturated conditions remain insufficiently explored. This study examines the influence of biochar content (5% and 10% by mass) on gas permeability and PMFCs performance in sandy clay loam (CL) soil planted with vetiver grass (Chrysopogon zizanioides) and also estimates the corresponding hydraulic conductivity. Gas permeability, bioelectric current/potential, and soil properties (suction, saturation) were monitored. Results indicate that biochar significantly increases gas permeability—by 29.57% (5% biochar, SRS-5%) and 58.73% (10% biochar, SRS-10%)—and improves saturated hydraulic conductivity by 57.33% and 88%, respectively, compared to untreated soil (SP), due to enhanced porosity. Additionally, biochar improves PMFCs performance: 5% biochar increases bioelectric current and potential by 84.48% and 34.32%, respectively, with a 431.72% increase in power density; 10% biochar results in even higher enhancements—156.94% (current), 50.77% (potential), and 651.05% (power density)—attributed to its catalytic redox activity and reduced soil resistance. Gas permeability and hydraulic conductivity were negatively correlated (R² = 0.774–0.908) and positively correlated (R² = 0.714–0.843) with bioelectric output, respectively, indicating a trade-off between gas and water transport properties and bioenergy generation. These findings underscore the dual function of biochar in improving landfill soil aeration—thus potentially mitigating methane emissions—and enhancing bioelectricity generation, demonstrating its potential application in bioenergy recovery from agricultural residues.