Effect of temperature on methane oxidation and community composition in landfill cover soil.

Municipal solid waste (MSW) landfills are the third largest anthropogenic source of methane (CH4) emissions in the United States. The majority of CH4 generated in landfills is converted to carbon dioxide (CO2) by CH4-oxidizing bacteria (MOB) present in the landfill cover soil, whose activity is controlled by various environmental factors including temperature. As landfill temperature can fluctuate substantially seasonally, rates of CH4 oxidation can also vary, and this could lead to incomplete oxidation. This study aims at analyzing the effect of temperature on CH4 oxidation potential and microbial community structure of methanotrophs in laboratory-based studies of landfill cover soil and cultivated consortia. Soil and enrichment cultures were incubated at temperatures ranging from 6 to 70 °C, and rates of CH4 oxidation were measured, and the microbial community structure was analyzed using 16S rRNA gene amplicon sequencing and shotgun metagenome sequencing. CH4 oxidation occurred at temperatures from 6 to 50 °C in soil microcosm tests, and 6-40 °C in enrichment culture batch tests; maximum rates of oxidation were obtained at 30 °C. A corresponding shift in the soil microbiota was observed, with a transition from putative psychrophilic to thermophilic methanotrophs with increasing incubation temperature. A strong shift in methanotrophic community structure was observed above 30 °C. At temperatures up to 30 °C, methanotrophs from the genus Methylobacter were dominant in soils and enrichment cultures; at a temperature of 40 °C, putative thermophilic methanotrophs from the genus Methylocaldum become dominant. Maximum rate measurements of nearly 195 µg CH4 g-1 day-1 were observed in soil incubations, while observed maximum rates in enrichments were significantly lower, likely as a result of diffusion limitations. This study demonstrates that temperature is a critical factor affecting rates of landfill soil CH4 oxidation in vitro and that changing rates of CH4 oxidation are in part driven by changes in methylotroph community structure.

Effect of basic oxygen furnace slag particle size on sequestration of carbon dioxide from landfill gas.

The mineral carbon sequestration capacity of basic oxygen furnace (BOF) slag offers great potential to absorb carbon dioxide (CO2) from landfill emissions. The BOF slag is highly alkaline and rich in calcium (Ca) containing minerals that can react with the CO2 to form stable carbonates. This property of BOF slag makes it appealing for use in CO2 sequestration from landfill gas. In a previous study, CO2 and CH4 removal from the landfill gas was investigated by performing batch and column experiments with BOF slag under different moisture and synthetic landfill gas exposure conditions. The study showed two stage CO2 removal mechanism: (1) initial rapid CO2 removal, which was attributed to the carbonation of free lime (CaO) and portlandite [(Ca(OH)2)], and (2) long-term relatively slower CO2 removal, which was attributed to be the gradual leaching of Ca2+ from minerals (calcium-silicates) present in the BOF slag. Realising that the particle size could be an important factor affecting total CO2 sequestration capacity, this study investigates the effect of gradation on the CO2 sequestration capacity of the BOF slag under simulated landfill gas conditions. Batch and column experiments were performed with BOF slag using three gradations: (1) coarse (D50 = 3.05 mm), (2) original (D50 = 0.47 mm), and (3) fine (D50 = 0.094 mm). The respective CO2 sequestration potentials attained were 255 mg g-1, 155 mg g-1, and 66 mg g-1. The highest CO2 sequestration capacity of fine BOF slag was attributed to the availability of calcium containing minerals on the slag particle surface owing to the highest surface area and shortest leaching path for the Ca2+ from the inner core of the slag particles.