Evaluating gas breakthrough pressure and gas permeability in a landfill cover layer for mitigation of hazardous gas emissions

Variation and correlation between water retention capacity and gas permeability of compacted loess overburden during wetting-drying cycles

Landfill gases can have numerous detrimental effects on the global climate and urban ecological environment. The protective efficacy of the final cover layer against landfill gases, following exposure to periodic natural meteorological changes during long-term service, remains unclear. This study conducted centrifuge tests and gas permeability tests on compacted loess. The experiments examined the impact and relationship of wetting-drying cycles and dry density on the soil water characteristic curve (SWCC) and gas permeability of compacted loess. Research findings reveal that during the dehumidification process of compacted loess, the gas permeability increases non-linearly, varying the gas permeability of soil with different densities to different extents under wetting-drying cycles. Two models were introduced to describe the impact of wetting-drying cycles on gas permeability of loess with various dry densities, where fitting parameters increased with the number of wetting-drying cycles. Sensitivity analysis of the parameters in the Parker-Van Genuchten-Mualem (P-VG-M) model suggests that parameter γ's accuracy should be ensured in practical applications. Finally, from a microstructural perspective, wetting-drying cycles cause dispersed clay and other binding materials coalesce to fill minuscule pores, leading to an increase in the effective pores responsible for the gas permeability of the soil. These research results offer valuable guidance for designing water retention and gas permeability in compacted loess cover layers under wetting-drying cycles.

Applying the first microcapsule-based self-healing microbial-induced calcium carbonate materials to prevent the migration of Pb ions

Lead (Pb) accumulation can lead to serious threats to surrounding environments and damage to the liver and kidneys. In the past few years, microbial-induced carbonate precipitation (MICP) technology has been widely applied to achieve Pb immobilization due to its environmentally friendly nature. However, harsh pH conditions can cause the instability of the carbonate precipitation to degrade or dissolve, increasing the potential of Pb2+ migration into nearby environments. In this study, microcapsule-based self-healing microbial-induced calcium carbonate (MICC) materials were applied to prevent Pb migration. The highest sporulation rate of 95.8% was attained at 7 g/L yeast extract, 10 g/L NH4Cl, and 3.6 g/L Mn2+. In the germination phase, the microcapsule not only prevented the bacterial spores from being threatened by the acid treatment but secured their growth and reproduction. Micro analysis also revealed that cerussite, calcite, and aragonite minerals were present, while extracellular polymeric substances (EPSs) were identified via Fourier transform infrared spectroscopy (FTIR). These results confirm their involvement in combining Pb2+ and Ca2+. The immobilization efficiency of above 90% applied to MICC materials was attained, while it of below 5% applied to no MICC use was attained. The findings explore the potential of applying microcapsule-based self-healing MICC materials to prevent Pb ion migration when the calcium carbonate degrades under harsh pH conditions.

Biopolymer-assisted enzyme-induced carbonate precipitation for immobilizing Cu ions in aqueous solution and loess

Wastewater, discharged in copper (Cu) mining and smelting, usually contains a large amount of Cu2+. Immobilizing Cu2+ in aqueous solution and soils is deemed crucial in preventing its migration into surrounding environments. In recent years, the enzyme-induced carbonate precipitation (EICP) has been widely applied to Cu immobilization. However, the effect of Cu2+ toxicity denatures and even inactivates the urease. In the present work, the biopolymer-assisted EICP technology was proposed. The inherent mechanism affecting Cu immobilization was explored through a series of test tube experiments and soil column tests. Results indicated that 4 g/L chitosan may not correspond to a higher immobilization efficiency because it depends as well on surrounding pH conditions. The use of Ca2+ not only played a role in further protecting urease and regulating the environmental pH but also reduced the potential for Cu2+ to migrate into nearby environments when malachite and azurite minerals are wrapped by calcite minerals. The species of carbonate precipitation that are recognized in the numerical simulation and microscopic analysis supported the above claim. On the other hand, UC1 (urease and chitosan colloid) and UC2 (urea and calcium source) grouting reduced the effect of Cu2+ toxicity by transforming the exchangeable state-Cu into the carbonate combination state-Cu. The side effect, induced by 4 g/L chitosan, promoted the copper-ammonia complex formation in the shallow ground, while the acidic environments in the deep ground prevented Cu2+ from coordinating with soil minerals. These badly degraded the immobilization efficiency. The Raman spectroscopy and XRD test results tallied with the above results. The findings shed light on the potential of applying the biopolymer-assisted EICP technology to immobilizing Cu ions in water bodies and sites.

Immobilizing lead and copper in aqueous solution using microbial- and enzyme-induced carbonate precipitation

Inappropriate irrigation could trigger migration of heavy metals into surrounding environments, causing their accumulation and a serious threat to human central nervous system. Traditional site remediation technologies are criticized because they are time-consuming and featured with high risk of secondary pollution. In the past few years, the microbial-induced carbonate precipitation (MICP) is considered as an alternative to traditional technologies due to its easy maneuverability. The enzyme-induced carbonate precipitate (EICP) has attracted attention because bacterial cultivation is not required prior to catalyzing urea hydrolysis. This study compared the performance of lead (Pb) and copper (Cu) remediation using MICP and EICP respectively. The effect of the degree of urea hydrolysis, mass and species of carbonate precipitation, and chemical and thermodynamic properties of carbonates on the remediation efficiency was investigated. Results indicated that ammonium ion (NH4+) concentration reduced with the increase in lead ion (Pb2+) or copper ion (Cu2+) concentration, and for a given Pb2+ or Cu2+ concentration, it was much higher under MICP than EICP. Further, the remediation efficiency against Cu2+ is approximately zero, which is way below that against Pb2+ (approximately 100%). The Cu2+ toxicity denatured and even inactivated the urease, reducing the degree of urea hydrolysis and the remediation efficiency. Moreover, the reduction in the remediation efficiency against Pb2+ and Cu2+ appeared to be due to the precipitations of cotunnite and atacamite respectively. Their chemical and thermodynamic properties were not as good as calcite, cerussite, phosgenite, and malachite. The findings shed light on the underlying mechanism affecting the remediation efficiency against Pb2+ and Cu2+.