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Obtaining a comprehensive understanding of solute transport in fractured rocks is crucial for various geoengineering applications, including waste disposal and construction of geo-energy infrastructure. It was realized that solute transport in fractured rocks is controlled by stochastic discrete fracture-matrix systems. However, the impacts and specific uncertainty caused by fracture network structures on solute transport in discrete fracture-matrix systems have yet not been fully understood. In this article, we aim to investigate the influence of fracture network structure on solute transport in stochastic discrete fracture-matrix systems. The fluid flow and solute transport are simulated using a three-dimensional discrete fracture matrix model with considering various values of fracture density and size (i.e., radius). The obtained results reveal that as the fracture density or minimum fracture radius increases, the corresponding fluid flow and solute transport channels increase, and the solute concentration distribution range expands in the matrix. This phenomenon, attributed to the enhanced connectivity of the fracture network, leads to a rise in the effluent solute concentration mean value from 0.422 to 0.704, or from 0.496 to 0.689. Furthermore, when solute transport reached a steady state, the coefficient of variation of effluent concentration decreases with the increasing fracture density or minimum fracture radius in different scenarios, indicating an improvement in the homogeneity of solute transport results. The presented analysis results of solute transport in stochastic discrete fracture-matrix systems can be helpful for uncertainty management in the geological disposal of high-level radioactive waste.

The classical deviatoric hardening models are capable of characterizing the mechanical response of granular materials for a broad range of degrees of compaction. This work finds that it has limitations in accurately predicting the volumetric deformation characteristics under a wide range of confining/consolidation pressures. The issue stems from the pressure independent hardening law in the classical deviatoric hardening model. To overcome this problem, we propose a refined deviatoric hardening model in which a pressure-dependent hardening law is developed based on experimental observations. Comparisons between numerical results and laboratory triaxial tests indicate that the improved model succeeds in capturing the volumetric deformation behavior under various confining/consolidation pressure conditions for both dense and loose sands. Furthermore, to examine the importance of the improved deviatoric hardening model, it is combined with the bounding surface plasticity theory to investigate the mechanical response of loose sand under complex cyclic loadings and different initial consolidation pressures. It is proved that the proposed pressure-dependent deviatoric hardening law is capable of predicting the volumetric deformation characteristics to a satisfactory degree and plays an important role in the simulation of complex deformations for granular geomaterials.

This work reports an upgrading reuse of construction-generated sludge waste (CGSM) to manufacture value-added building materials using hydrothermal mineralization (HM). The optimization of mixture proportions and the influential factors of curing temperature and duration were explored. Results showed that high-strength HM-CGSW blocks over 50 MPa with a strength-to-calcium ratio of 2.95 MPa/percentage can be produced under an optimal Ca/Si ratio of 0.5 and HM curing of 200 °C for 12 h. The production of HM-CGSW block would generate 305.81 kg e-CO2/m3 and cost of 222.44 CNY/m3, lower than those of concrete block and autoclaved fly-ash block. An industrial application of HM-CGSW was proposed for upgrading use of CGSW to produce building materials for new constructions. The findings in this study would not only deepen the mechanistic understandings of HM treatment of sludge waste, but also shed much light on developing an industrial path that enables sustainable building material production.

The presence of inactive components in siliceous and calcareous wastes tends to cause a misunderstanding in their mix design for hydrothermal solidification. The contents of typical reactive elements (i.e. Caact, Siact and Alact) in raw materials are quickly determined by simulating hydrothermal conditions. A generic mix design framework is proposed, which utilizes the molar ratio of Caact/Siact as the controlling parameter. The framework was demonstrated through a series of studies, which involved the production of granular materials using a mixture of clayey soil with slaked lime, as well as the production of compacted cylinders using a mixture of clayey soil with either slaked lime or calcium carbide slag. Experimental results showed that the hydrothermal samples achieved their maximum strength when the molar ratio of Caact/Siact in raw materials approached the theoretical Ca/Si molar ratio (i.e. 0.83) in tobermorite, provided that the molar ratio of Alact/(Alact + Siact) remained below 21%.

Heavy metal contamination has long been a tough challenge. Recently, enzyme-induced carbonate precipitation (EICP) has been proposed to handle this problem. This paper aims to explore the efficacy, process, and mechanisms of EICP using crude sword bean urease extracts to remediate Zn2+, Ni2+, and Cr(VI) contamination. A series of liquid batch tests and geochemical simulations, as well as microscopic analyses, were conducted. The liquid batch test results show that Zn2+, Ni2+, and Cr(VI) can be effectively immobilized by the EICP method, and the highest immobilization percentage was observed for Zn2+, reaching up to 99%. Ni2+ and Cr(VI) were immobilized at 62.4% and 24.4%, respectively. Additionally, the immobilization percentage of heavy metals increased with the concentration of added Ca2+. The simulation results and XRD results reveal that the organic molecules in crude sword bean urease can promote ZnCO3, Zn(OH)2, Zn5(CO3)2(OH)6, and NiCO3 precipitation. The FTIR and SEM-EDS results provide evidence for heavy metal adsorption by the functional groups in crude urease and calcium carbonate. The liquid batch test results, as well as the simulation results and the microscopic analysis results, indicate that the mechanism of EICP in heavy metal remediation can be summarized as biomineralization to form heavy metal carbonate precipitates and metal hydroxide precipitates, adsorption by calcium carbonate, and adsorption or complexation or promoting nucleation by organic molecules.

As a geological barrier for high-level radioactive waste (HLW) disposal in China, granite is crucial for blocking nuclide migration into the biosphere. However, the high uncertainty associated with the 3D geological system, such as the stochastic discrete fracture networks in granite, significantly impedes practical safety assessments of HLW disposal. This study proposes a Monte Carlo simulation (MCS)-based simulation framework for evaluating the long-term barrier performance of nuclide migration in fractured rocks. Statistical data on fracture geometric parameters, on-site hydrogeological conditions, and relevant migration parameters are obtained from a research site in Northwestern China. The simulation models consider the migration of three key nuclides, Cs-135, Se-79, and Zr-93, in fractured granite, with mechanisms including adsorption, advection, diffusion, dispersion, and decay considered as factors. Subsequently, sixty MCS realizations are performed to conduct a sensitivity analysis using the open-source software OpenGeoSys-5 (OGS-5). The results reveal the maximum and minimum values of the nuclide breakthrough time Tt (12,000 and 3600 years, respectively) and the maximum and minimum values of the nuclide breakthrough concentration Cmax (4.26 × 10−4 mSv/a and 2.64 × 10−5 mSv/a, respectively). These significant differences underscore the significant effect of the uncertainty in the discrete fracture network model on long-term barrier performance. After the failure of the waste tank (1000 years), nuclides are estimated to reach the outlet boundary 6480 years later. The individual effective dose in the biosphere initially increases and then decreases, reaching a peak value of Cmax = 4.26 × 10−4 mSv/a around 350,000 years, which is below the critical dose of 0.01 mSv/a. These sensitivity analysis results concerning nuclide migration in discrete fractured granite can enhance the simulation and prediction accuracy for risk evaluation of HLW disposal.

Biochemical decomposition of high kitchen wastes in landfills in China results in (i) a high content of vapor in landfill gas (LFG) and (ii) a noticeable temperature gradient across the earthen final cover (EFC). Yet, it is unclear how vapor and temperature gradients affect moisture and gas transport through EFC and microbial aerobic methane oxidation (MAMO). A theoretical model of moisture-gas-heat reactive transport considering MAMO and effects of vapor flow and temperature gradients was developed, and a series of parametric studies were included. The model was verified by published test results. The parametric simulation results show that vapor from landfill wastes can replenish water to EFCs in dry seasons, which results in (i) the relief of water shortage of vegetation and MAMO and (ii) LFG emission reduction. Effects of vapor inflow become more significant under larger temperature gradient. In cold seasons, temperature gradients can promote MAMO to reduce methane emissions, and enhance evaporation to reduce percolation. When evaporation rate at the surface exceeds 30 times vapor influx, vapor inflow has negligible effects on moisture and gas transport in EFCs. Neglecting effects of vapor and temperature gradient can lead to misjudgments on anti-seepage performance and methane emission reduction performance of EFCs.

Soil contamination is a serious global hazard as contaminants can migrate to the human body through the soil, water, air, and food, threatening human health. Human Health Risk Assessment (HHRA) is a commonly used method for estimating the magnitude and probability of adverse health effects in humans that may be exposed to contaminants in contaminated environmental media in the present or future. Such estimations have improved for decades with various risk assessment frameworks and well-established models. However, the existing literature does not provide a comprehensive overview of the methods and models of HHRA that are needed to grasp the current status of HHRA and future research directions. Thus, this paper aims to systematically review the HHRA approaches and models, particularly those related to contaminated sites from peer-reviewed literature and guidelines. The approaches and models focus on methods used in hazard identification, toxicity databases in dose–response assessment, approaches and fate and transport models in exposure assessment, risk characterization, and uncertainty characterization. The features and applicability of the most commonly used HHRA tools are also described. The future research trend for HHRA for contaminated sites is also forecasted. The transition from animal experiments to new methods in risk identification, the integration and update and sharing of existing toxicity databases, the integration of human biomonitoring into the risk assessment process, and the integration of migration and transformation models and risk assessment are the way forward for risk assessment in the future. This review provides readers with an overall understanding of HHRA and a grasp of its developmental direction.