Methane oxidation characteristics of biochar-methanotroph amended cover soil in landfill

The landfill cover is the simplest and most feasible method to control methane emissions. Besides its low gas conductivity, methane emissions can be further reduced due to the methane oxidation by soil microbes. However, ordinary soil covers have poor engineering properties, and the methane oxidation efficiency of soil and microbial methane-oxidizing bacteria is low. Biochar is a carbon-rich, black, refractory solid material obtained through pyrolysis of biomass (such as peanut shells, wheat straw, sawdust, and rice straw) under oxygen-limited conditions. Due to its porous structure, high surface area, and pH characteristics, biochar applied to soil can change the microenvironment of the soil, which in turn can affect the efficiency of methane biological oxidation. To investigate the methane oxidation efficiency of biochar-methane oxidizing bacteria modified soil, methane detectors were used to measure the changes in methane reduction rates of both sterile and non-sterile soil samples under different pH levels, methane concentrations, soil dry densities, and biochar dosages. The results showed that when the pH of the inorganic salt culture solution was 7, the methane-oxidizing bacteria had high oxidation efficiency. The methane oxidation curve at pH value 7 is located at the bottom, indicating that the methane oxidizing bacteria have the optimal methane oxidation efficiency in neutral solution (i.e., at pH value 7). When the solution reaches alkaline properties at pH value 9, the methane reduction rate of methane oxidizing bacteria is significantly lower than that at pH value 7. This suggests that the growth, reproduction, and metabolic activity of methane oxidizing bacteria are inhibited in an alkaline environment, leading to a decrease in the efficiency of biological methane oxidation. At a certain initial methane concentration, the oxidation efficiency of methane-oxidizing bacteria increases with the increase of initial methane concentration, but when the initial methane concentration exceeds a certain value, it inhibits the oxidation efficiency of methane-oxidizing bacteria. In the control group experiment without the addition of bacteria, the slope of the curve is close to zero, indicating that the methane emission reduction efficiency is significantly lower in the sterile state compared to the condition with bacteria added. Both the biochar content and dry density affect the methane reduction efficiency of methane-oxidizing bacteria. The methane reduction amplitude with the addition of biochar is greater than that without biochar incorporation. The reason for this is that biochar has a porous structure and low density. When added to the soil, it increases the porosity of the soil and reduces its density, indirectly increasing the physical adsorption capacity of methane. Therefore, the methane reduction rate is more significant. Under the same conditions, the greater the dry density of the sample, the lower the oxidation efficiency of methane. With the increase in biochar content, the methane oxidation efficiency of biochar-methane oxidizing bacteria modified soil gradually increases. When the dry density and biochar content are 1.20 g/cm³ and 15%, respectively, the methane reduction rates under sterile and inoculated conditions show more significant decreases, being 10.38% and 39.72% respectively. This demonstrates that the addition of biochar has altered the microenvironment of the landfill cover soil, enhancing the methane oxidation efficiency of methanotrophs. This research holds significant academic and practical value for reducing greenhouse gas emissions from landfills, preventing air pollution, and promoting soil carbon sequestration.