Investigation of different biogeochemical cover configurations for mitigation of landfill gas emissions: laboratory column experiments

Municipal solid waste (MSW) landfills are a major source of anthropogenic methane (CH4) and carbon dioxide (CO2) emissions, which are also major greenhouse gases. Apart from greenhouse gas emissions, MSW landfills are notorious for odor, and hydrogen sulfide (H2S) is a major contributor of odor in landfills. Recent studies have shown promise with biochar-amended soil covers to mitigate landfill CH4 emissions by enhancing microbial CH4 oxidation; however, mitigating only CH4 does not wholly resolve fugitive emissions as landfill gas (LFG) comprise of almost same proportion of CO2 as CH4. Also, H2S has very low odor threshold and numerous health risks. This study explores a novel biogeochemical MSW landfill cover integrating basic oxygen furnace (BOF) slag and biochar-amended soil to mitigate CH4, CO2 and H2S simultaneously from LFG. In this regard, column studies were carried out simulating four cover profiles: 1) soil control (column 1); 2) combination of BOF slag layer and 10% (by weight) biochar-amended soil layer (column 2); 3) combination of BOF slag layer and 5% (by weight) methanotrophically activated biochar-amended soil layer (column 3); and 4) combination of mixture of sand and BOF slag layer and 10% (by weight) methanotrophically activated biochar-amended soil layer (column 4). The cover profiles were exposed to simulated LFG (48.25% CH4, 50% CO2 and 1.75% H2S) at an average flux rate of 130 g CH4/m(2)-day. Terminal batch assays were conducted on the soil and biochar-amended soil samples obtained from various depths after exhumation from the columns to evaluate potential CH4 oxidation rates. Carbonate content tests and batch tests were conducted to evaluate carbonation potential of the BOF slag. The overall gas removal efficiencies of the cover profiles were in the order of column 3 > column 2 > column 4 > column 1. The CH4 oxidation rates were the highest in the 5% activated biochar-amended soil at 143 mu g CH4/g-day or 100 mu g CH4/g-day above soil control. Higher CH4 oxidation potential was associated with high moisture retention and biochar content. The BOF slag showed a maximum CO2 removal of 145 mg CO2/g BOF slag during column operation. Carbonation of BOF slag did not impede oxygen intrusion into the underlying biochar-amended soil layer and its CH4 oxidation efficiency. Overall, biogeochemical cover provides a holistic and sustainable solution to fugitive landfill emissions.