Comparison of Biogeochemical Cover Versus Conventional Cover: Long-Term Column Incubation Study

In this study, a novel biogeochemical cover system comprising of biochar amended soil and basic oxygen furnace (BOF) steel slag is being explored as a sustainable alternative cover system to mitigate methane (CH4), carbon dioxide (CO2) and hydrogen sulfide (H2S) simultaneously from landfill gas (LFG). Long-term column study was carried out simulating biogeochemical cover (BGCC) profile to investigate the CH4, CO2 and H2S removal potential of the cover system. The performance of the BGCC system was compared with a conventional soil cover (SC) profile. BGCC system showed higher overall gas removal potential than SC system. The CH4 oxidation rates of biochar amended soil in BGCC system were significantly higher ranging from 185 to 407 ug CH4/ g-day in comparison to barrier soil in SC system which ranged from 6 to 7.5 ug CH4/ g-day quantified based on the batch incubation on column exhumed samples. In addition, biochar amended soil showed higher relative abundance (maximum of 51% out of total sequences) in comparison to soil cover (maximum relative abundance of 27%). Both control soil in SC and biochar amended soil in BGCC cover showed complete attenuation of H2S at the bottommost part of the layers (75 cm bgs), preventing migration of H2S to the upper layers, and were inhabited with sulfur oxidizing bacteria such as Thiobacillus along with the methanotrophs. In addition, sulfur content was elevated at 70 cm depth (0.68%) in both control soil and biochar amended soil as a result of H2S removal. An inhibitory effect on CH4 oxidation rate was observed due to H2S whereas the effect was more prominent in control soil in SC than biochar amended soil in BGCC. The BOF slag in steel slag showed increase in carbonate content following column incubation (3.8% (initial) to 19% CaCO3) resulting in CO2 removal potential of 67 gCO2/ kg BOF slag. The hydraulic conductivity of carbonated BOF slag did not differ significantly from the virgin BOF slag and remained in the order of 10-3 cm/s before and after carbonation. Gas flow and CH4 oxidation efficiency of BGCC system was adversely affected following rainfall simulation event due to the capillary barrier effect at the topsoil and BOF slag interface. Hence, a thicker topsoil layer over slag surface is recommended to avoid water logging in the event of precipitation. Overall, the BOF slag layer did not affect the microbial community composition and CH4 oxidation potential of the underlying biochar amended soil suggesting BGCC system can be a sustainable alternative cover system to mitigate CH4, CO2 and H2S simultaneously.