Microbial Methane Oxidation and Gas Adsorption Capacities of Biochar-Modified Soils

Biochar has been recently considered as a potential soil additive to mitigate the landfill gas emissions in the soil cover. Some physical and chemical properties of biochar, like high phosphorus and organic matter contents, porous structures, and high specific surface area, are prone it a good soil amendment material to enhance the microbial methane oxidation and gas adsorption capacities of the soils. Three different types of biochar: wood chip, two herbaceous biomasses (corn straw and rice straw), were used to modify a silty soil in this study. Microbial CH4 oxidation and CH4 and CO2 adsorption capacities of the modified soils were investigated by the batch tests. The test results showed that the maximum methane oxidation rate (MOmax) of the soil modified by 30% biochar content (Bc) was 3–4 times that of the host soil. An optimum Bc corresponding to the peak value of MOmax was identified. For the three biochars tested, the optimum Bc ranged between 20 and 30% and soil modified by wood-derived biochar exhibits the highest MOmax, because woody biochar has the highest specific surface area and lower pH. Moreover, MOmax also increased with the preincubation time. It is suggested that preincubated sample has the advantage over the fresh sample for building up the content of methanotrophs in the soil before constructing the cover system. The adsorption kinetics and isotherms of CH4 and CO2 in the modified soils followed the pseudo-second-order equation and Langmuir model, respectively. By adding 20% Bc, the maximum adsorption capacity of CH4 and CO2 in the modified soil was about 54 times and 80 times that of the host soil, respectively.