Investigation of biogeochemical landfill covers incorporating different biochars and alkaline industrial byproducts for landfill gas mitigation: A column experiment study

Fugitive emissions of methane (CH₄) and carbon dioxide (CO₂) from municipal solid waste (MSW) landfills pose significant environmental risks if not captured. To address this, researchers at the University of Illinois Chicago developed a biogeochemical cover (BGCC) comprising a biochar-amended soil (BAS) layer and a basic oxygen furnace (BOF) slag layer to simultaneously mitigate CH₄ and CO₂. Previous laboratory experiments with BGCC comprising pinewood-derived biochar (PW)-amended soil and BOF slag (BGCC-PWBOF) demonstrated substantial potential for CH₄ and CO₂ mitigation. However, reliance on PW biochar and BOF slag may face challenges due to limited availability near landfill site location, which motivated the investigation of BGCC, which consists of different biochar and alkaline materials. Therefore, this study compared two BGCC configurations-one with rice husk biochar and cement kiln dust (BGCC-RHCKD) and another with pinewood biochar and BOF slag (BGCC-PWBOF)-against a conventional soil cover (SC) system in a laboratory column experiment. All three covers were exposed to synthetic landfill gas across four phases with varying CH₄ influx rates. Surface emissions and gas concentrations were continuously monitored. After the experiment, the covers were dismantled, and samples from multiple depths were collected for physico-chemical characterization, microbial community analysis, and batch tests to determine CH₄ oxidation rates and residual carbonation capacity. Results showed that under moderate CH₄ influx rates (20.77-22.80 g CH₄/m²-day) in Phase 1, BGCC-RHCKD and SC achieved peak CH₄ removal efficiencies of 67.3 % and 74.2 %, respectively. The CKD layer in BGCC-RHCKD maintained 100 % CO₂ removal across all phases without breakthrough, whereas BOF slag in BGCC-PWBOF experienced breakthrough after 16-22 days due to desiccation. The highest CH₄ oxidation rates reached 2042.8 μg CH₄/g-day (BGCC-RHCKD), 455.3 μg CH₄/g-day (BGCC-PWBOF), and 372.1 μg CH₄/g-day (SC) and were strongly correlated with the relative abundance of methylotrophic bacteria, especially Methylobacter luteus, in 16S rRNA gene amplicon profiles. Overall, the BGCC-RHCKD system offers an effective solution for mitigating both CH₄ and CO₂ emissions, while the SC system is effective for CH₄ emissions.