Biochar can be widely produced by heating the rice straw under an oxygen-deficient condition at 500 ℃, leading to loose porosity, high specific surface area, and strong adsorption properties. Fortunately, compacted biochar with amended clay has been proposed as a sustainable alternative material for the final cover of landfills in recent years. The pore structure of soil can also be changed (such as the pore size and porosity), particularly for the variation in the permeability of the overlying soil on the landfill site. The permeability coefficient of soil can be dominated by both the biochar content and the dry density. It is a high demand to accurately determine the permeability coefficient of biochar-clay mixed soil, in order to fully meet the functional requirements of the overlying layer of the landfill. This study aims to explore the influence of biochar content and dry density on the permeability of biochar clay mixed soil. Five types of biochar-clay mixed soil were selected with the mass percentage of biochar of 0%, 5%, 10%, 15%, and 20%. The dry densities were taken as 1.42, 1.56, and 1.65 g/cm³, which were 85%, 95%, and 100% of the maximum dry density of clay. According to the Technical code for municipal solid waste sanitary landfill closure (GB 51220 2017), the degree of compaction of the landfill cover soil was not less than 90% of the standard proctor maximum dry density (SPMDD) of clay. The compactness of the slope was not less than 85%, according to the national standard (GB 51220 2017). The self-developed device was also used for the flexible wall water-air joint penetration to measure the saturated permeability coefficient and air permeability coefficient of biochar-clay mixed soil with different biochar content and dry density. The relationship was obtained between biochar content, dry density, saturated permeability coefficient and gas permeability coefficient. The applicability and accuracy were verified by validation group tests. A biochar content-dry density-dependent air-water permeability function was then established using the optimal relationships. The results show that the water permeability values of soil samples that were treated with 5%, 10%, 15% and 20% were 8.25×10⁻¹⁷, 8.89×10⁻¹⁷, 10.40×10⁻¹⁷, and 18.25×10⁻¹⁷ m², respectively, under the small dry density, compared with the pure clay. Among them, the permeability of soil samples mixed with biochar 20% increased by nearly an order of magnitude. The biochar content was obtained under the same initial dry density from the test, according to the relationship curve between the air permeability of biochar-clay mixed soil. The functional relationship between the dry density of the sample and the biochar content was termed as the water-gas permeability function of the soil sample mixed with biochar. Anyway, this function can be expected to rapidly and accurately determine the permeability coefficient of soil samples, particularly with considering the dry density and biochar content. The easily measured permeability coefficient was also taken as an independent variable. The validation group test showed that there was better consistence between the calculated and measured water permeability coefficient, indicating the better applicability of the improved function.

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.