Sedimentation Behavior Research Articles

Ink droplet drying analysis for understanding the ink-catalyst layer transition in proton exchange membrane fuel cells

There is still limited understanding of ink structure and the transition process from ink to the catalyst layer (CL) in proton exchange membrane fuel cells despite recent interest in the field. This study proposes a methodology for analyzing the CLs formed after drying single- and multi-ink droplets produced using an ultrasonic spray. Two model inks, Pt/Vulcan Carbon and Pt/Ketjen Black, are prepared using these catalysts, and their characteristics are analyzed through sedimentation behavior, ionomer adsorption quantification, and rheology. The structural properties of the spray-dried CLs are examined using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and scanning electron microscopy-Auger electron spectroscopy. These analyses reveal distinct differences in the CLs' structures, influenced by physical phenomena such as coffee ring formation and droplet coalescence, which are dependent on the ink properties. Furthermore, the structural characteristics of the CLs produced by the two model inks affect the electrochemical behavior of the 25 cm2 fuel cell. This study establishes a correlation between the ink structure, CL structure, and fuel cell performance, emphasizing the usefulness of the ultrasonic spray coating method in advancing such investigations.

Numerical investigation of local scour around twin piles under steady current using CFD-DEM coupling method

In recent years, the local scour has developed into a major threat to the safety of offshore group piles. The numerical simulation methods are useful tools in investigating the key mechanisms behind local scour. However, conventional simulation methods often neglect the microscopic sediment behavior, thus impeding a more profound understanding of the scour mechanism. In this research, the CFD-DEM method was applied to investigate the local scour around twin piles. And the coarse grain method and porous media model were incorporated to enhance simulation efficiency. The CFD-DEM model was constructed and validated using experimental data. Based on the validated model, a series of local scour simulations around twin piles were conducted under different arrangements. The time-development curves of scour depth were analyzed, and the variation of the equilibrium scour depth with pile spacing exhibits good agreement with the dates from previous studies. Based on the simulation results, the flow field, scour pit morphology, sediment flux, and microscopic behavior of sediment around twin piles were analyzed under different arrangements, and four key mechanisms of twin piles' local scour were observed and analyzed during this process.

Enhancing subsea asset performance: Investigating the biomimetic functionality of the Mushroom Reef design in hydrodynamics, stability, and sedimentation

In this study, laboratory and in situ experiments were performed to assess the stability of the Mushroom Forest Artificial Reef (MFAR) against the hydrodynamic forces of waves and currents and its functionality to avoid sedimentation. Physical modelling was performed in wave flume (waves only) and a long channel (currents only) with a smooth flat bed and a sediment bed. The results illustrated the positive and negative aspects, and then some modifications were made to the in situ experiments. A total of 130 prototype units were produced and deployed at an offshore site in the Arabian Gulf. Sediment traps were installed at the top and base of the mushroom units, and underwater visual assessments were performed to evaluate the stability and sediment behaviour around the studied reefs. The results demonstrated the stability of the units under hydrodynamic loading, with meagre resistance to current displacement (drag forces), with high stability (95% remained vertical), with horizontal vortices leading the sedimentation patterns, and with 46 times less sedimentation at the top than at the base. Thus encouraging coral growth at the mushroom's top with fewer sedimentation risks. In conclusion, MFAR was validated as a blue engineering technology despite some limitations.

Distinct rheological behavior of kaolinite dispersions containing gas bubbles at different initial pore water pressures

Discrete gas bubbles widely exist in the marine sediments, which are largely composed of kaolinite clay. They would evolve into kaolinite dispersions following submarine landslide, to impact and destroy seafloor infrastructures, such as oil/gas pipelines. The sliding distance and impact force are governed by the rheological behavior of the kaolinite dispersions, which are gas-dependent and remain poorly understood. In this study, effect of gas bubbles on the rheological behavior of a modeling clay sediment, a kaolinite dispersion, is investigated under both low and high initial pore water pressure. Results show that at the low initial pore water pressure, bubbles in the dispersion enhance the strength of the system, increasing both the viscosity and the yield stress. On the other hand, at the high initial pore water pressure, the presence of bubbles shows an opposite effect. Mechanisms on the bubble effect at both conditions are proposed, based on which a unified theoretical model predicting the yield stress of the gassy clay dispersion is developed.

Strength analysis of hydrate-bearing sandy sediments in excess gas and excess water based on drained triaxial compression tests

The safe and efficient production of methane hydrate (MH) has attracted much attention, and the engineering geological risks in MH production are highly valued. Safety evaluation of the MH reservoirs is necessary, and the strength characteristics are the key. In this study, the mechanical behaviors of MH-bearing sandy sediments in excess gas and excess water were studied by conducting drained triaxial compression tests, so as to provide reference for the mechanical evaluation of MH reservoirs in natural gas-rich and water-rich environments. The results show that: (1) the effects of MH saturation and effective confining stress on the mechanical properties of sediments are coupled. The hydrate cementation effect is restricted under high effective confining stress; (2) strain softening behavior is prone to occur under higher MH saturation and is hindered by the high effective confining stress; (3) the MH-bearing samples exhibit higher stiffness, failure strength and strain softening tendency in excess gas than in excess water; (4) in excess gas, the MH contribution to the failure strength of MH-bearing samples starts from the cementation component, while in excess water, it starts from the friction component; (5) theoretical analysis based on the Lade-Duncan criterion suggests that the sandy sediments containing MH in excess water can be considered cohesionless until the MH saturation exceeds 40%. This study provides a theoretical reference for geological safety evaluation during MH development.

A state-dependent constitutive model for gas hydrate-bearing sediment incorporating phase change

Extracting energy from hydrate-bearing sediment involves phase change, during which the strength of sediment is reduced and the risk of geohazards may subsequently increase. However, existing mechanical constitutive models cannot capture the responses of hydrate-bearing sediment during phase change since kinetics of phase change is ignored. Besides, most models are not suitable for hydrate-bearing sandy sediment since the dilatancy of sand is not considered well. In the current study, a state-dependent mechanical constitutive model for hydrate-bearing sediment considering phase change of hydrate is developed. The bounding surface plasticity is adopted. Fugacity is incorporated into the model to account for phase change of hydrate and the effects of thermodynamic states on mechanical behaviour of hydrate-bearing sediment. Additionally, a non-linear critical state line and a non-associated flow rule are adopted for hydrate-bearing sandy sediment, which is the most favourable for exploitation due to high hydrate content and permeability. These unique features can improve model performance regarding sediment responses during phase change of hydrate. The model is verified by comparing measured results of hydrate dissociation tests, drained triaxial tests and dissociation tests of hydrate-bearing sand. It is evident that stress-strain and volumetric responses of hydrate-bearing sandy sediments can be well captured by this model over a broad range of effective stresses from 0.1 MPa to 5 MPa. Additionally, the responses of hydrate-bearing sediment during dissociation under different stress states have been successfully reproduced. With an initial axial strain of 5%, dissociation of hydrate results in the development of over 20% axial strain and causes the specimen to collapse.

Research on benthic plume sedimentation characteristics and temperature effect in deep-sea nodule mining operation

The benthic plume generated by nodule mining operations has garnered significant attention due to its intricate environmental impact and the limitations of existing plume control techniques. Prior research has suggested that temperature effects can facilitate the aggregation of suspended sediments; however, a more detailed understanding is needed. In this study, a series of experiments were conducted to examine the sedimentation behavior of the benthic plume and the influence of temperature on its sedimentation process. Suspension concentrations of the plume were monitored over an hour, while flocs characteristics were investigated at 30 and 60-min intervals. A novel function describing the relationship between temperature effects and concentration changes was developed. Results indicate that higher temperatures can accelerate the deposition process by approximately 3–6 times, emphasizing the necessity to consider low water temperatures in laboratory studies of benthic plumes. Moreover, the integration of temperature effects and micro-eddies in plume aggregation research may offer new opportunities for developing advanced plume control and separation methods.

Numerical simulation of proppant migration and sedimentation behavior in complex fractures based on computational fluid dynamics

The migration and sedimentation behavior of proppant in complex fractures plays a vital role in the formation of fractures with high conductivity. Most of the existing studies on proppant sedimentation and migration are based on a single vertical fracture model that propagates in the same direction. However, due to in situ stress and reservoir heterogeneity, the propagation of fractures in reservoirs will produce multiple branch fractures that intersect with the main fractures and have a certain inclination angle. Therefore, it is necessary to study the sedimentation and migration behavior of proppants in inclined complex fractures. In this paper, the proppant particles sedimentation law in fractures is studied based on computational fluid dynamics. By changing the particle size, volume fraction, and inlet flow velocity of proppant, the migration and sedimentation law of proppant in a single fracture is simulated and verified. Then, through the establishment of a complex fracture model with multiple branches, and from the four dimensions of the vacant length of the front edge of the sand embankment, the length of the sand embankment, the highest point of the sand embankment, and the effective laying area of the proppant, the angle between the main and branch fractures in the complex fractures, the fracture angle and the inlet flow velocity, as well as the location relationship of the branch fracture setting and the migration and sedimentation of the proppant in the complex fractures are explored.

Study on creep behaviors and nonlinear creep constitutive model for sandy marine hydrate-bearing sediments

Marine natural gas hydrate (NGH) production is critically affected by reservoir damage, which is usually presented as creep deformation. Clarifying the creep behaviors of hydrate-bearing sediments (HBS) is essential for predicting potential geoengineering risks during long-term NGH mining. In this study, a series of multi-step loading creep tests on HBS samples are conducted and a modified creep model specialized for sandy HBS is proposed. The creep strain, creep rate, and long-term strength are experimentally evaluated under various hydrate saturation (Sh) and/or effective confining pressure (Pc). Three typical creep stages of deceleration creep, stable creep, and accelerated creep are observed in all experiments. The results show that the creep deformation of HBS is enhanced with the decrease in Sh under the same axial load. The long-term strength decays linearly with decreasing Sh and Pc. The creep model is established by modification of both the Kelvin element and the ideal viscoplastic element in the conventional Nishihara model. A nonlinear viscous coefficient and a time-damaged elastomer considering are introduced to describe the nonlinear creep and damage characteristics of HBS. The model is able to describe the whole failure process, especially the accelerated creep stage. The results might have some significance on the prediction of nonlinear failure behaviors of the reservoir during long-term NGH mining.

DEPTH-RESOLVED MODELLING OF SEDIMENT FLUXES UNDER BICHROMATIC WAVES IN THE SWASH ZONE

Traditionally, many numerical studies of sediment transport or morphodynamics in the swash zone use depth-averaged approach. This is relatively computationally cheap, however, the assumptions it makes on vertical structures of the flow velocity and suspended sediment concentration are not always fully met. Depth-resolving models, albeit more computationally expensive, offer the ability to investigate sediment transport without imposing assumptions on vertical structures. Therefore, these models are a great tool for investigating processes related to sediment transport in the swash zone. In this study, will look at the behavior of suspended sediment in the swash zone, and the role vertical structures therein.

A bounding surface plasticity model for methane hydrate-bearing sediments in deep seabed

This study proposes a new constitutive model to simulate the mechanical behavior of methane hydrate-bearing sediments, by modifying the SANISAND-C model originally designed for sands with low cement content. To capture the distinctive cementation effect of methane hydrate (MH), modifications are made to the state parameter and bonding degradation law of the SANISAND-C model. In addition, the initial bonding strength of MH is not only determined by MH saturation but also by a condition parameter that depends on temperature, pore pressure, and salinity. The densification effect caused by MH pore-filling and grain-coating is captured by replacing the original void ratio in the elastic and plastic moduli of the SANISAND-C model with a reduced void ratio. The model is verified through a comparison of its predictions not only with experimental data but also through discrete element method simulations that cover a wide range of loading paths including drained and undrained true triaxial tests. The model successfully captures the stiffness, strength and dilatancy enhancements induced by MH, as well as the temperature, pore pressure, and salinity-dependent behavior of sediment. Additionally, the model developed can account for different hydrate occurrence habits by adjusting a single parameter related to bonding strength.

Effect of food-to-microorganisms ratio on aerobic granular sludge settleability: Microbial community, potential roles and sequential responses of extracellular proteins and polysaccharides

The food-to-microorganism ratio (F/M) is an important parameter in wastewater biotreatment that significantly affects the granulation and settleability of aerobic granular sludge (AGS). Hence, understanding the long-term effects and internal mechanisms of F/M on AGS settling performance is essential. This study investigated the relationship between F/M and the sludge volume index (SVI) within a range of 0.23–2.50 kgCOD/(kgMLVSS·d). Thiothrix and Candidatus_Competibacter were identified as two dominant bacterial genera influencing AGS settling performance. With F/M increased from 0.27 kgCOD/(kgMLVSS·d) to 1.53 kgCOD/(kgMLVSS·d), the abundance of Thiothrix significantly increased from 0.20% to 27.02%, and the hydrophobicity of extracellular proteins (PN) decreased, which collectively reduced AGS settling performance. However, under high-F/M conditions, the gel-like polysaccharides (PS) effectively retained the granular biomass by binding to the highly abundant Thiothrix (53.65%). The progressive increment in biomass led to a concomitant reduction in F/M, resulting in the recovery of AGS settleability. In addition, two-dimensional correlation infrared spectroscopy analysis revealed the preferential responses of PN and PS to the increase and decrease of F/M, and the content and characteristics of PN and PS played important roles in granular settling. The study provides insight into the microbial composition and the potential role of extracellular polymer substances in the AGS sedimentation behavior, offering valuable theoretical support for stable AGS operation.

Insights into mineralogical distribution mechanism and environmental significance from geochemical behavior of sediments in the Yellow River Basin, China

Mineralogical investigations on fluvial sedimentary sequences could provide historical environmental information on the effects of human activities and natural events. This study aims to identify the mineralogical distribution mechanism and environmental significance of sediments of the Yellow River Basin based on topographic analysis, statistics, weathering and recycling indices. In total, 107 samples were collected from sedimentary sequences in the source area, and the upper, middle, and lower reaches and analyzed for grain size, major elements, and mineral composition. The results showed that the climate conditions were cold and arid, where weak hydrolysis under continental weathering and strong erosion accelerated physical weathering. Clay minerals in the upper reaches primarily originated from the Tibetan Plateau, whereas the middle and lower reaches received input of aeolian deposits from Northern China. Quartz and feldspar in the middle and lower reaches may derive from the source area and upper reaches. Meanwhile, calcite and dolomite formed through diagenesis, with loess input from the Chinese Loess Plateau. Regarding heavy minerals, the dominant determinative factors of pyrite were post-depositional diagenesis and leaching. Hematite and amphibole primarily formed through magnetite conversion and contribution from regional sources, respectively. Moreover, the mineral distribution mechanism significantly affected the mobility and distribution of geochemical elements through diagenesis and alteration. The findings are instrumental in reconstructing the environmental evolution of large-scale watersheds across multiple climatic zones.

Consolidation behaviors of gas-bearing sediments with modulus varying along depth under horizontal loads

Gas-bearing sediments are widely distributed in marine and lacustrine areas. The engineering properties of gas-bearing sediments, containing enclosed bubbles, are significantly different with that of saturated and unsaturated sediments. This paper investigates the consolidation behaviors of gas-bearing sediments with modulus varying along depth subjected to horizontal load using an extended precise integration method (XPIM). The compressibility of enclosed bubble is introduced to a new derived seepage equation and a varying modulus along depth is considered in governing equations. With the aid of integral transformation and Taylor expansion, such problems are solved by XPIM, which is proved to be markedly efficient and precise for boundary value problems of porous media than traditional numerical methods. Detailed comparisons against analytical solutions are performed to confirm the accurateness of XPIM. Extensive parametric investigations are conducted to examine the influence of saturation degree, varying modulus along depth and type of external load on the consolidation behaviors of gas-bearing sediments. The present work is conveniently utilized to evaluate the behavior of gas-bearing sediments, and subsequently the response of foundations built in gas-bearing soil areas.

Advances in Monitoring and Understanding the Dynamics of Suspended-Sediment Transport in the River Drava, Slovenia: An Analysis More than a Decade-Long

Managing sediment transport in streams is crucial to the surface water resource development strategy and has several implications for flood risk and water management, hydropower use, and balancing river morphology. This paper summarises the movement and behaviour of suspended sediment within the Slovenian portion of the River Drava, covering a span of thirteen years from 2005 to 2018. An analysis of relevant data collected during this period is also presented. Suspended-sediment dynamics strongly depend on flow velocity, seasonal variations in sediment sources, and human interventions in the riverbed. The transportation of material in the River Drava results in the accumulation of sediments in reservoirs and riverbeds, consequently impeding the natural hydrological cycle by reducing the outflow into aquifers. The 2018 high-water event is analysed in terms of the dependence of concentration of suspended sediments on discharge, where counterclockwise hysteresis was observed, providing an essential clue to the origin of sediment. Sediments from the River Drava in Slovenia are managed with some conventional processes and are mainly deposited or reintegrated into rivers and aquatic ecosystems. Some additional sediment management strategies with long-term solutions for efficient and comprehensive water management, hydropower, and ecological problems are proposed.

Instrumented column testing on long-term consolidation and desiccation behaviour of coal tailings under natural weather conditions

AbstractThe sedimentation, consolidation and desiccation behaviour of sub-aerially deposited tailings in tailings storage facilities, despite occurring simultaneously and being essential to maximise tailings dry density and dewatering efficiency, are usually analysed in laboratories using distinctive testing equipment, which is unable to reveal their interactions. A large instrumented column was constructed to test the three processes of tailings in a single apparatus, capable of monitoring the changes in the hydrological parameters of coal tailings with depth and surface settlement under natural weather conditions. The column was initially filled with coal tailings slurry and exposed to natural weather for two years, during which five wetting–drying periods were identified. The monitored results indicate that exposing the slurry tailings for one month would lead to the formation of an unsaturated tailings layer with a thickness of 150 mm, likely representing the optimised tailings deposition cycle and thickness, respectively, under the given tailings characteristics and semi-arid climates. Settlement mainly occurred after major rainfall events due to the collapse of cracks and cavities formed during the preceding desiccation. The maximum thickness of the unsaturated tailings increased over the first four periods as the predominant desiccation and settlement strengthened the evaporative capillary forces, despite it decreasing greatly in the last period due to insufficient desiccation. An empirical model was proposed to consecutively estimate the surface moisture content of deformable tailings and validated by achieving a good agreement in the water mass estimated by two methods during wetting–drying cycles.

Rheology Engineering for Dry-Spinning Robust N-Doped MXene Sediment Fibers toward Efficient Charge Storage.

MXene nanosheets are believed to be an ideal candidate for fabricating fiber supercapacitors (FSCs) due to their metallic conductivity and superior volumetric capacitance, while challenges remain in continuously collecting bare MXene fibers (MFs) via the commonly used wet-spinning technique due to the intercalation of water molecules and a weak interaction between Ti3 C2 TX nanosheets in aqueous coagulation bath that ultimately leads to a loosely packed structure. To address this issue, for the first time, a dry-spinning strategy is proposed by engineering the rheological behavior of Ti3 C2 TX sediment and extruding the highly viscose stock directly through a spinneret followed by a solvent evaperation induced solidification. The dry-spun Ti3 C2 TX fibers show an optimal conductivity of 2295 S cm-1 , a tensile strength of 64MPa and a specific capacitance of 948 F cm-3 . Nitrogen (N) doping further improves the capacitance of MFs to 1302 F cm-3 without compromising their mechanical and electrical properties. Moreover, the FSC based on N-doped MFs exhibits a high volumetric capacitance of 293 F cm-3 , good stability over 10000 cycles, excellent flexibility upon bending-unbending, superior energy/power densities and anti-self-discharging property. The excellent electrochemical and mechanical properties endow the dry-spun MFs great potential for future applications in wearable electronics.

Assessment of environmental parameters effect on potentially toxic elements mobility in foreshore sediments to support marine-coastal contamination prediction

Potentially toxic elements (PTEs) presence in marine sediments can significantly affect the environmental quality and negatively influence economy and recreational activities in related areas. Accordingly, contamination monitoring and control in the marine environment is a fundamental task. In this work, four PTEs behavior (i.e. As, Hg, Pb, and Zn) in sandy foreshore sediments (SFSs) was thoroughly investigated at different pH, redox potential and temperature conditions of the marine water. For all the tests, the released As was 2.7–6 times higher than its initial concentration in water. Nonetheless, final mass balances showed that preferential release in the liquid phase occurred for Pb and Hg (up to 10 % and 9.1 %, respectively). Moreover, final Zn and Hg content increase in SFSs labile fractions indicated their higher bioavailability after the tests. The obtained results outline an approach useful to predict the contaminants behavior in marine matrices and support environmental monitoring and preservation strategies.

Extreme rainstorm reshuffles the spatial distribution of heavy metals and pollution risk in sediments along the mangrove tidal flat

Mangroves as typical blue carbon ecosystems exhibit a high level of heavy metal accumulation capability. In this study, we investigated how extreme rainstorm effects the spatial variability and pollution risk of sediment heavy metals (i.e., Fe, Mn, Cr, Cu, Zn, Cd, Pb, As and Hg) at different compartments of a typical tidal flat, including the bare mudflat, mangrove zone, and tidal creek in Shenzhen Bay, China. The results showed that the extreme rainstorm can change the sediment particle size, which further regulated the spatial distribution, and source-sink pattern of heavy metals. Due to the strong rainstorm flushing, the concentrations of most heavy metals increased toward the sea and the comprehensive pollution level increased by 8.3 % after the extreme rainstorm. This study contributes to better understanding of how extreme rainstorm regulates heavy metal behavior in mangrove sediments to achieve sustainable development of mangroves under the pressures of extreme weather events.

Insights of mechanism into enhanced removal of Cr(VI) by Ginkgo biloba leaves synthesized bimetallic nano-zero-valent iron/copper

For purpose of green preparation of nano-zero-valent iron and copper (GB-nZVI/Cu), the use of Ginkgo biloba extract as a reducing agent is reported herein. Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction analysis (XRD) and X-ray photoelectron spectroscopy (XPS) analyzed the properties of nanoparticles. GB-nZVI/Cu exhibited a better Cr(VI) removal performance than nZVI/Cu materials synthesized by apricot leaves (Al-nZVI/Cu) and hawthorn leaves (Hl-nZVI/Cu). GB-nZVI/Cu could remove 97.2% Cr(VI) in 30 min. Low pH and 313 K promoted the reaction while humic acid (HA) had inhibitory effect. The promoting effects of coexisting anions were observed following the order: SO42->HCO3->Cl-. The rate constant K2 increased from 0.0303 to 4.8865 g·mg-1·min-1 as SO42− concentration increased. Na+ and low Mg2+ concentration promoted the removal of Cr(VI) while NO3−, K+, Ca2+ and high Mg2+ concentration had the negative effects. The removal reaction followed the pseudo-second-order kinetic model, with a chemical adsorption as a rate-controlling step. Visual MINTEQ models and mechanism studies showed that the removal of Cr(VI) by GB-nZVI/Cu was a combination of adsorption, reduction and precipitation. The Fe-Cu galvanic cell can accelerate the corrosion of Fe0, enhancing the reduction of Cr(VI). Cu0/Cu(I) could be oxidation into Cu(II) by receiving electrons from Fe3+, resulting in the generation of Fe2+. Cr(III) finally precipitated in the form of FeXCr1−X(OH)3 and Cr(OH)3. GB-nZVI/Cu had superior sedimentation behavior and more Fe2+ production to promote Cr(VI) reduction. GB-nZVI/Cu maintained a better removal performance of Cr(VI) than Al-nZVI/Cu and Hl-nZVI/Cu after five reuses.