Morphological and mineral features of nZVI induced precipitation on quartz particles

Nano Zero-Valent Iron (nZVI) is a versatile nanomaterial that can not only efficiently remove contaminants in soil, but also improve the soil’s geotechnical strength by changing their physicochemical properties. Inert solid mineral particles are the most common ingredients in soils, they present universal surface modification after the nZVI treatment. This study presents an investigation on the morphological and mineral features of nZVI induced iron mineral precipitations on quartz particles. Lead was employed as the artificial contaminant, while quartz was used to mimic the inert solid mineral particles in soil. Scanning Electron Microscope (SEM), Digital Image Analysis (DIA), Transmission Electron Microscopy (TEM), laser particle size analyzer, X-ray diffraction (XRD) and Raman spectrum were carried out for the characterization. The results indicate that iron minerals precipitated heterogeneously on the surface of quartz particles with plush-like and flake-like structure. They are made of deuterogenic plumbiferous minerals and ferriferous minerals. XRD analysis demonstrated that these minerals are amorphous. The curly flake-like mineral clusters were scatteredly distributed on the surface of quartz along with the of corroded nZVI aggregation. The thickness of the curly flake-like precipitation varied from 20 nm to 60 nm, and 20 nm to 35 nm for the plush-like precipitation. The generation of these iron mineral precipitations led to a slight increase in the average particle size and a decrease in the surface area of the soil. However, no clear difference in the shape and roughness of quartz was found after the nZVI treatment. This study is provided to improve the understanding of mass transfer from nZVI to inert solid particles in soil and its effect in soil improvement

Study of the slippage of particle / supercritical CO2 two-phase flow

In this paper, the slippage velocity and displacement between particles and supercritical CO2 (SC-CO2) were studied to reveal the particle-SC-CO2 two-phase flow behavior. Visualization experiments were performed to directly measure the slippage velocity and displacement. Eight groups of experiments involving various pressures (7.89–10.96 MPa), temperatures (38.6–47.5 °C), particle diameters (0.3–0.85 mm), particle densities (2630 and 3120 kg/m3) and SC-CO2 flow rates (0.920–1.284 m/s) were conducted. The measured particle slippage velocities in the flowing direction were approximately 10.3% of the SC-CO2 flow rate. The measured particle slippage displacements were all at the centimeter level, which indicated that SC-CO2 had a superior particle transporting capability that was similar to those of liquids even if it had a low viscosity that was similar to those of gases. A numerical model was built, and analytic slippage calculations were performed for SC-CO2 for additional analyses. The density of SC-CO2 was found to have a greater influence on the slippage than the viscosity. Moreover, a comparison of the slippage between SC-CO2 and water showed that the particle slippage in water was constant, while the particle slippage in SC-CO2 continually accumulated at an extremely slow rate

An integrated geophysical approach for investigating hydro-geological characteristics of a debris landslide in the Wenchuan Earthquake area

Debris landslides are one of the most widely distributed types of landslides in the Wenchuan earthquake area. The hydro-geological structure characteristics are the fundamental basis for stability evaluation, performing protection and administration of a landslide. The rock and soil mass of a debris landslide was highly non-uniform and preferential seepage paths were normally developed in it. Therefore, in situ identification of the underground water seepage system became particularly important. Recently, investigations on the seepage paths of underground water in debris landslides were restricted to indoor model testing and site observation, which were far from meeting the actual demand for landslide prevention and mitigation. To locate the seepage paths, we conducted survey work on a debris landslide seated in the Xishan Village, Li County, Sichuan Province, China, by combing four different geophysical methods. They were multichannel analysis of surface wave (MASW), electrical resistivity tomography (ERT), ground penetrating radar (GPR) and microtremor survey method (MSM). The geophysical interpretation was verified with field engineering surveys and monitoring data. The results suggested that a dendritic pipe-network seepage system usually developed in debris landslides. Varisized infiltration pipes showed the characteristics of inhomogeneity and concentration of the seepage. This work highlighted that geophysical parameters (shear wave velocity Vs, dielectric constant ε and resistivity value ρ) could provide reliable qualitative and quantitative information about the colluvial layer, bedrock interface, potential sliding surface and underground water seepage system of a landslide. The optimum combination of geophysical methods was suitable to survey the hydro-geological characteristics of debris landslides in the Wenchuan earthquake area

Swelling and embedment induced by sub- and super-critical-CO2 on the permeability of propped fractures in shale

Swelling and embedment exert significant influence on the evolution of permeability in propped fractures, potentially consuming significant proportions of the original gain in permeability. We measure the evolution of permeability in propped fractures of shale to both adsorbing CO2 and non-adsorbing He – accommodating the impacts of aperture change due to proppant pack compaction and both reversible and irreversible modes of embedment. A linear relation between pressure and log-permeability is obtained for He, representing the impact of effective stresses in proppant pack compaction, alone. Permeability change with pressure is always concave upwards and U-shaped for gaseous subcritical CO2 and W-shaped for supercritical CO2. One exception is for liquid CO2 at high injection pressure where effective stress effects and swelling contribute equally to the change in permeability and result in a linear curve with the lowest permeability. Approximately ~50–70% of the permeability recovers from the recovery of swelling after the desorption of CO2. The magnitude of swelling is recovered from measurements of permeability change and ranges from 0.005 to 0.06 mm, which contributes ~9–56% of the total swelling and induced embedment as evaluated from the adsorbed mass. Swelling also increases embedment by a factor of ~1.84–1.93 before and after the injection of CO2. A new calibration equation representing swelling and induced embedment is generated accommodating Langmuir isothermal sorption and verified against experiments on rocks both admitting and excluding swelling and embedment and for various sorbing and non-sorbing gases. Stability and accuracy of the predictions demonstrate the universality of the approach that may be applied to both enhanced gas recovery and CO2 sequestration

Consolidation considering clogging

In land reclamation projects, the vacuum preloading method has been widely used to strengthen dredged fills by removing water. However, during the improvement process, clogging inevitably occurs in the drains and soils, hindering water drainage and causing inhomogeneous consolidation results. Therefore, it is essential to evaluate the effect of clogging on the consolidation behavior of dredged slurry at different radii. In this study, analytical solutions are derived under an uneven strain assumption to calculate the consolidation in the clogging zone and the normal zone, with time-dependent discharge capacity and clogging in the soil considered. Results calculated by the proposed solutions indicated that the clogging effect slows down the development of consolidation, reduces the final consolidation degree, and increases the difference between consolidations at different radii. It is found that the influence of the clogging effect's varies with the speed of the discharge capacity decay, the value of the initial discharge capacity of the drain, the permeability, and the radius of the clogging zone. Finally, a practical application of the proposed solution is discussed, and the proposed solution is suggested for the calculation of consolidation when treating high-water-content slurry

Microstructure and morphological characterization of lead-contaminated clay with nanoscale zero-valent iron (nZVI) treatment

The increasing use of nanoscale zero-valent iron (nZVI) for soil and groundwater remediation has raised concerns about its potential effect on soil properties. Numerous laboratory and field studies have demonstrated its excellent capability to immobilize contaminants and enhance contaminated soil. However, a few studies have shed light on the changes in the microstructure and morphology of the soil due to nZVI treatment. This study explores the variation in particle morphology and microstructure in nZVI-treated soil. A series of microscale experiments, including field emission scanning electron microscopy (FESEM), particle size analysis, mercury injection porosimetry (MIP), optical microscopic analysis, and particle shape tests, were conducted on nZVI-treated samples. The dosages of nZVI used were 0%, 0.2%, 1%, 5%, and 10% of the contaminated soil. Morphological characterization suggested that the addition of nZVI resulted in the occurrence of larger-sized particles, on-particle branched structures, finer pore size distribution, aggregation, and a flocculent network in the soil structure. The aggregated and bonded soil particles via nZVI could be one of the mechanisms for its variation in geotechnical characteristics. The findings of this study may improve our understanding of soil improvement using nZVI treatment

Simultaneous stabilization of Pb and improvement of soil strength using nZVI

This study demonstrates the feasibility of nanoscale Zero-Valent Iron (nZVI) for simultaneous stabilization of Pb and improvement of soil strength via batch experiments. The soil samples were prepared using slurry and pre-consolidation method at nZVI doses of 0.2%, 1%, 5%, and 10% (by dry weight). The physicochemical and geotechnical properties of Pb-contaminated soil treated by nZVI were analyzed. The results indicate that the contamination of Pb(II) resulted in a notable reduction in the undrained shear strength of soil from 16.85 kPa to 7.25 kPa. As expected, the Pb in exchangeable and carbonate-bound fractions decreased significantly with the increasing doses of nZVI. Meanwhile, the undrained shear strength of Pb-contaminated soil enhanced substantially as the increase of nZVI, from 25.83 kPa (0.2% nZVI treatment) to 69.33 kPa (10% nZVI treatment). An abundance of bubbles, generated from the oxidation of nZVI, was recorded. The mechanisms for simultaneous stabilization of Pb and soil improvement primarily include: 1) the precipitation and transformation of Pb-/Fe-hydrated oxides on the soil particles and their induced bounding effects; 2) the increased drainage capability of soil as the occupation of nZVI aggregates and bubbles in the macropores space and 3) the lower soil density derived from the increase in microbubbles retained in the soil. This study is provided to facilitate the application of nZVI in the redevelopment of contaminated soil

Fall cone test on biopolymer-treated clay

Fall cone tests were conducted to evaluate the consistency variations of clay soils treated with six types of biopolymers, e.g. carrageenan kappa gum (KG), locust bean gum (BG), xanthan gum (XG), agar gum (AG), guar gum (GG) and sodium alginate (SA) at various concentrations (e.g. between 0.1% to 5% biopolymer to soil mass ratio). The dependences of shear viscosity on water content, and undrained shear strength on water content were established. The results indicated that KG and SA increased the liquid limit (LL) of treated soils after the biopolymer content exceeded a certain limit (e.g. 0.5%), BG and GG contributed to a peak point in LL at biopolymer concentration of 1% to 2%, while XG and AG almost did not change the LL at all. The plastic limit (PL) was about 25% to 50% of the LL, leading to a trend of plasticity index (PI) similar to liquid limit. In order to further simplify the testing procedure and get the Atterberg limits for biopolymer-treated soil, one-point method was adopted

The role of foam in improving the workability of sand : insights from DEM

Foam as a soil conditioner can transform the mechanical properties of the excavated natural muck and lubricate the interface between the cutting tools and muck, thus reducing the tools’ wear and promoting the efficiency of earth pressure balance (EPB) shield tunneling. This paper aims to explore the meso-mechanism of foam in improving the workability of sand by combining discrete element modeling (DEM) with experimental investigations of slump tests. A “sand-foam” mixture DEM model was generated by simplifying the sand grains and foam as individual particles with different properties. The particle-scale simulated parameters were calibrated based on a series of experimental observations. The effects of foam on the inter-particle contact distribution and the evolution of contact forces during the slumping process were investigated in detail through numerical modeling. It was found that injecting foam into sand specimens could increase the coordination number and the contact number around sand grains. Although the force transmission pattern changes from “sand-sand” into the coexistence of “sand-foam”, “sand-sand” and “foam-foam” contacts, the magnitude of contact forces transferred by foam particles is significantly lower than that by sand particles. The presence of foam reduces contact-scale frictional strength and thus reduces the stability of the microstructures of sand. In addition, the normal direction of inter-particle contact force deflects from the vertical to the horizontal and the magnitude of contact force decreases significantly with the influence of foam