Sedimentation Of Aggregates Research Articles

Insight into different adsorption behaviors of two fluoroquinolone antibiotics by sediment aggregation fractions.

Sediment, consisting of different aggregation fractions, is a hotspot site for transport and transformation of various pollutants including antibiotics. However, the fate of different antibiotics in aquatic sediments mediated by sediment aggregation fraction adsorption and the mechanism behinds are still unclear. In this study, we investigated the adsorption behavior of two fluoroquinolone antibiotics (ciprofloxacin and ofloxacin) on four aggregation fractions separated from the sediment of Taihu Lake, a typical lake contaminated by antibiotics in China. The results showed that the adsorption of ciprofloxacin and ofloxacin fitted the Freundlich model, irrespective of sediment aggregation size. The adsorption of ciprofloxacin and ofloxacin was depended on the size of sediment aggregation fractions, and the macroaggregation (> 200μm) exhibited the strongest capacity, followed by large microaggregation (63-200μm), medium microaggregation (20-63μm), and small and primary microaggregation (< 20μm). This fraction size-dependent effects of sediment aggregations on antibiotic adsorption might be closely related to the differences in their specific surface areas, organic matter contents, and surface functional groups. The adsorption of ciprofloxacin and ofloxacin by sediment aggregation fractions was characterized by a combination of chemical and physical adsorptions, with the former being the dominant process. Compared with ofloxacin, ciprofloxacin could be more rapidly and easily absorbed by four sediment aggregation fractions, and more readily complexed with carboxyl groups on macroaggregation surface. The adsorption of two antibiotics by extracellular polymeric substance showed that tryptophan and tyrosine protein-like, humic-like substance on the surface of sediment could bind to both antibiotics through a complexation reaction. The π-π electron donor-acceptor interaction and hydrogen bonds were responsible for the antibiotic adsorption by sediment aggregation.

Biomineral Flocculation of Kaolinite and Microalgae: Laboratory Experiments and Stochastic Modeling

AbstractIn estuaries and coastal waters, the flocculation of cohesive sediments is significantly affected by extracellular polymeric substances secreted by microalgae. In this study, laboratory experiments are carried out to explore the role of two typical microalgae (Skeletonema costatum and Cyclotella meneghiniana) on suspended fine‐grained sediments under various turbulent shear and environmental conditions. The results show that the shear rate is a dominant factor in controlling the biomineral flocculation processes, and the existence of microalgae generally enhances the aggregation of mineral sediments with microalgae. It is also found that floc size distributions (FSDs) and mean sizes are clearly influenced by different algal species and concentrations, especially under intensive turbulent environments. Moreover, a quasi‐Monte Carlo (QMC) based bivariate population balance model is developed to simulate the FSDs of sediment‐algae flocs. This model is reasonably validated by two analytical solutions and laboratory observations, and indicates a potential extension for tracking the properties of microalgae‐associated fine sediment flocs due to the discrete nature of the QMC method.

Review of the action of organic matter on mineral sediment flocculation

Sediment is found throughout the world’s alluvial plain rivers, estuarine coasts and adjacent seas and is thereby a key factor in major ecosystems. Suspended mineral sediment can affect the biological activity of microorganisms and plants, by reducing light penetration in the water column or by binding to organic matter. Biological processes can, in turn, affect the physical and chemical properties of the sediment particles and influence the adhesion between particles. They can facilitate the sediment aggregation (flocculation) through bridging, patching and sweep, while biological decay will mainly help to disintegrate organic matter rich flocs. Biological activity also affects the properties of flocs (structure, density, sedimentation rate and composition). This activity is itself influenced by environmental conditions (like temperature, light and nutrient fluxes). Sediment flocculation thus involves complex relationships between several physical, chemical and biological factors. The role of biology in particular needs to be better integrated in sediment transport models, through the interaction between mineral clay particles, microorganisms and their excreted polymers (Extra Polymeric Substances, i.e., EPS). In this article, a summary of the state-of-the-art research regarding sediment flocculation is given. In particular, the action of organic matter on fine-grained sediment flocculation is discussed. The aim of the article is to provide a more comprehensive understanding of bio-sediment dynamics and give an outlook on remaining research questions.

Sediment organic carbon dominates the heteroaggregation of suspended sediment and nanoplastics in natural and surfactant-polluted aquatic environments

The aggregation of nanoplastics (NPs) and suspended sediment (SPS) is the key to the transport and environmental fate of NPs. However, the influence of SPS composition and environmental conditions on this process and its mechanisms are still unclear. In this study, the heteroaggregation of NPs and SPS of different compositions is systematically explored under natural and surfactant-polluted aquatic environments (NaCl, humic acid, cetyltrimethylammonium bromide (CTAB)). The results showed that sediment organic carbon (SOC) dominates the aggregation and that different kinds of SOC (comprised of both amorphous organic carbon (AOC) and black carbon (BC)) contribute vary under distinct conditions. In natural freshwater, AOC represents a larger contribution to aggregation because of its weaker electrostatic repulsion compared to that of BC. However, BC represents a larger contribution in natural seawater resulting from decreased electrostatic repulsion and more hydrogen bonding. Conversely, in surfactant-polluted aquatic environments, both AOC and BC have a high contribution owing to the bridge effect plus hydrogen bonding. Notably, minerals’ contribution in aggregates remains low under all conditions. Furthermore, CTAB typically inhibits aggregation except under special conditions. The findings of this study contribute notably to a better understanding of the migration of nanoplastics in complex aquatic environments.

Strength and Stiffness Degradation Mechanisms of Stabilized/Solidified Sediments by Freeze–Thaw Cycles

Strength and stiffness of stabilized/solidified (S/S) sediments are supposed to be degraded by freeze–thaw cycles (FTs) when applied in constructions in cold regions. In order to reveal the degradation mechanisms, FTs were first simulated on S/S sediments in the laboratory. Then, macrotests including unconfined compression strength (UCS) tests and direct shear tests were conducted on S/S sediments at different FTs to investigate the strength and stiffness changes and microanalyses including scanning electron microscope analysis (SEM), mercury intrusion porosimetry (MIP), and nitrogen adsorption porosimetry (NAP) to investigate the microstructure changes. Macrotests show that strength indexes decrease gradually, stiffness parameters and brittleness increase first and then decrease, the compaction stage of stress–strain curves diminishes first and then increases, the decrease rate of friction angle decelerates first and then accelerates, and the decrease rate of cohesion decelerates first and then keeps stable, for S/S sediments in FTs. Microanalyses have precisely captured the damage chain of S/S sediments: meso and macropores inside sediment aggregates expand gradually during FTs; as a result, sediment aggregates are fractured and stripped off from the solidification skeleton gradually; losing support of sediment aggregates, solidification skeletons are adjusted first and then collapsed when sediment aggregate fracture reaches a limit value. Based on macro and microfindings, a damage model to explain strength and stiffness degradations by microstructure damage is successfully established. Based on the established damage model, a reasonable idea is proposed for promoting the resistance of S/S sediments to FTs.

Impact of oil−sediment interaction on transport of underwater spilled oil in the Bohai Sea

Based on a validated underwater oil spill model and validated ocean hydrodynamic background, the influence of oil−sediment interaction on the transport of underwater spilled oil is investigated through numerical experiments. Numerical result shows that interaction between oil and suspended sediment can form oil−sediment aggregates (OSAs), which can significantly decrease the amount of surface oil and increase the amount of underwater oil and its sweeping volume under certain conditions. Sensitivity of the model output (in terms of oil in OSAs, underwater oil amount, surface oil amount, and sweeping volume) to input factors (including oil spill volume, sediment diameter, and sediment concentration) is further investigated quantitatively through numerical experiments, which shows that increasing oil spill volume and sediment concentration and reducing sediment diameter can promote oil−sediment interaction. Among the three input factors, oil spill volume is the most sensitive factor, followed by sediment concentration and sediment diameter. This study is expected to provide useful information for spilled oil monitoring, contingency planning, and environmental damage assessment.

Significance of organic matter in the process of aggregation of suspended sediments in retention reservoirs

It is crucial to have a comprehensive understanding of the process by which suspended sediment (SS) in water is aggregated and deposited to ensure the proper use and management of storage reservoirs. The present study was an investigation into the varied granulometric composition of accumulated sediment, as well as an examination of the amounts of organic matter present and its origins. This study aimed to determine what underpins the process of aggregation of sediment suspended in reservoir water. The results of the study, as also analysed using multivariate statistics, reveal a process of sediment aggregation dependent not only on the amount of organic matter but also on its origin. Greater production of autochthonous organic matter was shown to be associated with an intensified process of suspended sediment flocculation, confirming that the metabolism of a reservoirs' aquatic organisms influences the granulometric composition of suspended sediment.

Analysis of Hydrological Characteristics: A Case Review of the Niger Delta

This paper reviews the significant contribution of hydrologic continuity in the development, evolution and fate of the Niger Delta. Formed from the aggregation of sediments from a drainage catchment area of over 2 million km2, the Niger Delta has evolved into a prolific sedimentary basin with a dense network of rivers and creeks. The ecological system is in a fragile state with fresh and saline water ecosystems maintaining a dynamic equilibrium. Rainfall typically varies from 2500 - 4000 mm per year compared to average annual evaporation of about 1500 mm, resulting in net antecedent moisture. Over 70% of the rainfall occurs in 4 months between May and September, creating a potential for flooding. Water level varies from less than 1.5 m in the estuaries to about 8 m at the apex of the delta, with Nun River having a slightly higher elevation compared to Forcados River, implying that canals connecting both rivers at the same reach would experience water movements from Nun to Forcados River. Similarly, water level gradients vary across the stretch of the rivers from 8.6 to 9.5 × 10-5 (cm/km) between Onitsha and Asamabiri, 7.5 to 7.7 × 10-5 (cm/km) on the Forcados River downstream of Asamabiri, and from 7.4 to 7.6 × 10-5 (cm/km) on the Nun River. At peak flood, about 23 × 103 m3/sec of runoff enters the Niger Delta. At the same time, about 16 × 103 m3/sec discharges from the estuaries into the ocean in a semi-diurnal tidal flow leaving a surplus that accumulates to cause flooding. The delta thus acts like a sponge, absorbing the shear amount of flow and releasing slowly stored water through at least twenty-one outlet estuaries into the Atlantic Ocean. The shear amount of discharge overwhelms the capacity of the network of distributaries and predisposes the delta to flooding. Assessment of the capacity of the rivers to evacuate pollutants showed that rivers to the west of Brass which are fed by Forcados River exhibited relatively stronger flux efficiencies prior to 1998 but are presently weaker compared to rivers fed by Nun. The rivers to the east between New Calabar and Imo Rivers exhibited very weak ebb tide asymmetry or net upstream flow largely because these rivers receive little or no freshwater influx from upstream sources. Flood and erosion are the major hydrological hazards in the region.

Soil and organic carbon redistribution in a recently burned Mediterranean hillslope affected by water erosion processes

Forest fires cause many changes in the physical, chemical and biological soil properties such as aggregation and soil organic carbon contents (SOC) as well as on soil hydrology and erosion processes. Most studies on post-fire soil erosion in Mediterranean environments have been plot-based and research at hillslope or broader scale is scarce. Understanding SOC nature, distribution and modifications, as produced by forest fires and erosion, has become crucial to model and define the role of soil erosion as source or sink of C, and to sustainably manage ecosystem services related to the soil resource. This research provides data about the loss and redistribution of soil and SOC in a Mediterranean forest hillslope burned with high severity, at the Natural Park of Sierra de Espadán, Spain. Soil was sampled in coupled hillslopes (ca. 0.25 ha) (BU: burned, CO: control) at bottom (depositional), middle (transport) and top positions (eroding) at two depths (0–2 cm, 2–5 cm), and under two environments (UC: under canopy soil, BS: bare soil). Sediments were collected after each erosive event along one year, and yields were calculated. Samples were analysed to assess aggregate stability (AS), size and density fractionations, SOC contents and stocks. The main hypothesis is that fire affects soil characteristics related to aggregation and SOC stabilization and, together with erosion processes, may modify SOC distribution within aggregates and the burned hillslope.Soils were in general very stable, but some differences in the results of the methods used were observed. Significant differences were found for the environment (under canopy vs bare) and soil depth but not for slope position. SOC content was high both at BU and CO with no significant differences. In the BU hillslope, a homogenization of SOC contents was observed along the hillslope, while in the CO, a higher SOC content was measured in the depositional and transport sites than in the eroding one. Similar trends were observed for SOC stocks. Only four erosive rain events were registered in this study, which generated no sediment yields in CO hillslope. In the BU one, sediment yields were measured (0.05–0.58 Mg ha−1, total 0.925 Mg ha y−1), which mobilised OC amounts ranging between 0.005 and 0.04 MgC ha−1. When samples were fractioned, changes were observed in the mass distribution of soil and sediment aggregates by size and density, and in the OC content between density fractions of BU soils with regard to sediment and CO soils.According to the results, effective post-fire management should be oriented to control and reduce the erosion of aggregates < 2 mm, which present the highest SOC content and are very prone to be transported off-site. This fraction should include all the partially burned biomass (free light material), which acts as a first mulching and contains high amounts of OC that should be kept within the burned hillslope to increase soil fertility, promote vegetation recovery and act as a C sink. In the BU hillslope, eroded free light material might be buried at the depositional site and if the conditions are favourable for its conservation, SOC accumulation would be promoted, which may have implications for its stabilization, and the role of soil erosion as a C sink.

Linking recent changes in sediment yields and aggregate-associated organic matter sources from a typical catchment of the Loess Plateau, China

The historically environmental information of soil erosion and human activity is thoroughly “recorded” in deposited organic matter (OM) in aggregates within the catchment. Although the changes in sediment-associated organic carbon (OC) concentration and source have great impact on the stability and fate of eroded OM on Earth, the response of OM sources of sediment aggregate fluxes in eroding landscapes to land uses still remain unclear. In this study, we selected three sediment deposit profiles in a typically dam-controlled catchment of the Chinese Loess Plateau to trace the 60-year changes in sediment yields and then explored the historical distributions of aggregate-associated OM sources through using the fallout radioisotope dating (137Cs and 210Pbex) and the isotope mixing model based on aggregate size-specific 13C and 15N isotope signatures. The results showed that the deposition rates of sediment trapped the check dam displayed a decreasing trend with year except for an abrupt increase in 2013 over the past 60 year, ranging from 4.47 × 103 to 89.16 × 103 t km−2 yr−1. The average SOC and TN concentrations in deposit profile showed the following order of macroaggregates (> 0.25 mm, 9.55 g kg−1 for SOC and 0.81 g kg−1 for TN) > microaggregates (0.053–0.25 mm, 4.16 g kg−1 for SOC and 0.42 g kg−1 for TN) > silt-clay (< 0.053 mm, 2.41 g kg−1 for SOC and 0.33 g kg−1 for TN), despite there was a opposite trend of mass fraction in aggregate sizes. The data of 13C and C/N among aggregate sizes indicated that the OM in silt-clay fraction had higher decomposition rate than that of other aggregates. The modeling outputs further showed that cropland was the primary contributing sources to transported OM of aggregate flux, accounting for > 20% of the total. Our results demonstrated the influence of erosion processes and changes in land use on the sediment yield and aggregate-associated OM dynamics in this Loess region.

Extent of impact of deep-sea nodule mining midwater plumes is influenced by sediment loading, turbulence and thresholds

Deep-sea polymetallic nodule mining research activity has substantially increased in recent years, but the expected level of environmental impact is still being established. One environmental concern is the discharge of a sediment plume into the midwater column. We performed a dedicated field study using sediment from the Clarion Clipperton Fracture Zone. The plume was monitored and tracked using both established and novel instrumentation, including acoustic and turbulence measurements. Our field studies reveal that modeling can reliably predict the properties of a midwater plume in the vicinity of the discharge and that sediment aggregation effects are not significant. The plume model is used to drive a numerical simulation of a commercial-scale operation in the Clarion Clipperton Fracture Zone. Key takeaways are that the scale of impact of the plume is notably influenced by the values of environmentally acceptable threshold levels, the quantity of discharged sediment, and the turbulent diffusivity in the Clarion Clipperton Fracture Zone.

Subglacial sediment deformation in the Ross Sea, Antarctica

The Ross Sea embayment is the outlet for one-third of all Antarctic ice flow, receiving input from both East and West Antarctica. Marine-based ice streams, whose stability is susceptible to global sea-level changes, expand and contract across the Ross Sea continental shelf, eroding and deforming the substrate and transporting sediment subglacially. Deposits from the Last Glacial Maximum in the western Ross Sea provide an opportunity to investigate paleo-ice stream processes using sediment cores associated with glacial landforms. This study combines micro-to macro-sedimentological analyses to characterize lithofacies that represent a glacial-interglacial succession. We observe subtle variations in sediment properties within the subglacial unit that suggest till heterogeneity. Physical characteristics that vary both laterally and vertically indicates different sediment response to glacial stress, resulting in non-pervasive deformation within the till. These observations are consistent with the interpretation that a dynamic ice sheet with variable flow regimes existed across the Ross Sea during the Last Glacial Maximum. Further microscopic evidence of dynamic subglacial conditions includes sediment aggregates preserved as discrete grains in the form of soft sediment clasts or till pellets. Till pellet-rich sediment is commonly associated with the transition from a subglacial to proximal glaciomarine environment, although here we demonstrate they are widespread in both facies. Their widespread occurrence downcore and distribution across the continental shelf provides sedimentological evidence of a deformable bed beneath an ice stream during the Last Glacial Maximum. Ultimately, better tools for discriminating between glacial deposits will allow for more detailed reconstructions of ice-sheet history, and thus determine the forcing mechanisms to which the ice is responding.

Formation of Oil-Particle-Aggregates: Numerical Model Formulation and Calibration

When oil spills occur in turbid waters, the oil droplets and mineral grains can combine to form oil-particle aggregates (OPAs). The formation of OPAs impacts the vertical transport of both the oil and the mineral grains; especially increasing deposition of oil to the seabed. Though the coastal oceans can be very turbid, to date, few numerical ocean models have accounted for aggregation processes that form OPAs. However, interactions between oil and mineral aggregates may be represented using techniques developed to account for sediment aggregation. As part of Consortium for Simulation of Oil Microbial Interactions in the Ocean (CSOMIO), we modified an existing, population dynamics-based sediment flocculation model to develop OPAMOD, a module that accounts for the formation of OPAs. A zero-dimensional model using OPAMOD is shown to be capable of reproducing the size distribution of aggregates from existing laboratory experimental results. Also using the zero-dimensional model, sensitivity tests were performed on two model parameters, the fractal dimension and collision efficiency. Results showed that fractal dimension played a role in the OPA size distribution by influencing the effective particle density, which modified the number concentration of flocs for a given mass concentration. However, the modeled particle characteristics and oil sequestration were relatively insensitive to collision efficiency. To explore OPA formation for an outer continental shelf site, two simulations were conducted using a one-dimensional (vertical) implementation of the model. One scenario had high sediment concentration near the seabed to mimic storm-induced resuspension. The other scenario represented river plume sediment delivery by having high sediment concentration in surface waters. Results showed that OPA formation was sensitive to the vertical distribution of suspended sediment, with the river plume scenario creating more OPA, and sequestering more oil within OPA than the storm resuspension scenario. OPAMOD was developed within the Coupled Ocean-Atmosphere-Wave-and-Sediment Transport (COAWST) modeling system, therefore the methods and parameterizations from this study are transferrable to a three-dimensional coupled oil-sediment-microbial model developed by CSOMIO within the COAWST framework.

Evaluating the Trend of Changes in Bed Sediments Gradation over the Sistan River

Sediment movement in rivers is one of the most important indicators to evaluate the health of an ecosystem. Bed load different specification such as shape, dimensions and other statistical parameters of this sediment reflect the hydraulic and hydrological conditions governing them, directly. To investigate the changes of particle aggregation of Sistan river bed, four sediment samples were collected from the Sistan river. After aggregation of sediments in the laboratory, sediment diameter (D50) and uniformity coefficient (Cu) was calculated, and using inverse distance weighting (IDW) method, the mentioned parameters for the Sistan river from Jarikeh up to the AfzalAbad plugs place were interpolated and parameters using ARC-GIS software was zoning along the river. Results showed that Sistan river bed material is sand and silt, 3 km to the beginning and at end of the study interval based on the English standard classification it is silty texture and the rest of bed is sandy texture. 11 kilometers beginning and the end of the river also from 20 +721 kilometer up to about 12 kilometers has also a good aggregation. The entire river except for the middle section of river from the 13 +244 km to extent of 4 km river bed has non-uniform soil.

Formation and weathering assessment of oil-suspended sediment aggregates through a laboratory investigation.

Formation of oil-suspended sediment aggregates (OSAs) is believed to be one of the natural cleaning processes in the marine environment. In this study, we have investigated the formation processes of OSAs under different mixing periods (continuous mixing and with the addition of sediments in between), oil-sediment ratios (1:1, 1:2 and 2:1) and crude oils (Arabian Light (AL), Kuwait (KW) and Murban (MB)). The results revealed that size of OSAs significantly increased (up to ≈ 1.41mm) with the addition of sediments. Aggregates (total 36) were extracted for n-alkanes and polycyclic aromatic hydrocarbons to quantify and assess their weathering and toxic levels. The maximum n-alkane depletion was 84% (111-02), 94% (212-02) and 84% (321-02) and PAH depletion was ≈ 72% (111-02), 79% (212-02) and 81% (311-03) for the OSAs of AL, KW and MB crude oils, respectively, for the different samples considered, indicating that n-alkanes were depleted relatively higher than the PAHs. The highest depletion of both n-alkane and PAHs has occurred in OSAs of 10-h continuous mixing. The depletion of both n-alkane and PAHs reduced after the addition of sediments, however, escalated the growth of OSAs, resulting in bigger size OSAs. The concentration of PAHs of all 36 OSAs is greater than 5000ng/g, indicating very high PAH pollution. Though the formation of OSAs helps in cleaning the spill sites, the carcinogenic threat to the marine ecosystem caused by these OSAs cannot be ignored.

Sugarcane bagasse biochar: Suitable amendment for inland aquaculture soils

The salt-affected soils cover vast area in more than 100 countries and increasingly decrease the arable land. It may lead to the global food insecurity that is needed to be solved urgently. Concurrently, these degraded areas are suitable for inland saline aquaculture on the conditions of improvement in sediment characteristics. In this connection, an experiment was conducted for 60 days to study changes in physicochemical properties of inland saline aquaculture pond sediments through biochar application. The biochar prepared from dried sugarcane bagasse at 500°C with 33% biomass recovery was characterized for its physicochemical properties and applied over surface and by mixing with the sediment. There was a significant (p < .05) increase in organic carbon (3.82%), available-P (2.13%), available-K (18%), Ca (5.62%), Mg (14%) and water-holding capacity (1.8 times), and decrease in pH (0.41 unit), EC (17%) and bulk density (7%) when biochar (18 t/ha) was mixed with sediment (T2 treatment), whereas increase and decrease in CEC (59%) and available-N (1.01 times), respectively, when biochar (18 t/ha) were applied over sediment surface (T4 treatment). Thus, mixing of sugarcane bagasse biochar with sediment at 18 t/ha is recommended for the improvement of soil characteristics in saline soils for aquaculture through this study. Furthermore, the SEM and FT-IR analysis of treatments showed that sediment aggregation and functional group characteristics improved over a short period of incubation along with microbial biomass.

Effect of aggregate breakdown on the unevenly enriched organic carbon process in sediments under a rain-induced overland flow

Soil organic carbon (SOC) could be enriched in sediments for rain-induced overland flow. However, the features of the enriched SOC distribution in each size of sediment particles and the effect of aggregate breakdown on SOC enrichment features have not been completely investigated. Thus, the SOC concentration of different sizes of sediment aggregates was tested for all samples obtained for 27 simulated rainfall experiments conducted with 5°, 10°, and 15° gradients at rainfall intensities of 45, 90, and 120 mm h−1, respectively. The changes of SOC composition in aggregates from the original soil to sediments were estimated through the ratios of the variation percentage of the effective and dispersed each size of class sediment particle to that of the original soil particles (Ap). The contribution percentage of aggregate breakdown to the SOC enrichment in sediments (Ca) was also quantified. Results showed that Ca was above 50 % even when under an extremely high rainfall intensity of 120 mm h−1 on a 15° slope, but Ca decreased with the increase in overall rainfall intensity and slope. When the soil erosion rate is not high enough, Ca decreases with time because of the increase in the soil erosion rate. The absolute values of Aps of all size class particles were increased with the rainfall intensity and slope. However, the absolute values of Aps of clay- or fine silt-sized class particles changing between 0.11 and 0.68 were generally lower than those of coarse silt, fine sand, and coarse sand changing from −3.11 to 1.38, −0.35 to 0.06, and −1.3 to 0, respectively. This result indicates that 0.05–0.25-mm aggregates were more easily broken than other size particles, and the different sizes of particulate organic carbon chippings were more preferred transport than clay/silt-bonded aggregate fragments. The preferred transport of large amounts of 0.25–2-mm-size particulate organic carbon resulted in higher SOC enrichment ratios (ERocs) of 2.58 in 0.25–2-mm particle size class than others. The preferred transport of light particles, for example, mineral-associated and particulate organic carbon chippings exposed by aggregate stripping, caused the uneven enrichment of SOC in sediments. Last, a significantly negative correlation is observed among the mean weight diameter, the enrichment ratio (Er) of each size class particles in sediments, and Ca and ERocs of sediments (P < 0.05). If the rainfall intensity and slope are small, then many aggregates will be broken, and the uneven enrichment of SOC will be enhanced. Overall, our research provides an important reference to further understand the SOC loss mechanisms of soil aggregate induced by water erosion.

Coupling between clay swelling/collapse and cationic partition

The semipermeability of clay aggregates in mudstones, shales, and marine sediments gives rise to geological ultrafiltration of ions and water. This behavior has been numerically analyzed based on the theory of the electric double layer (EDL). The model of EDL is suited for describing ionic partitions in clay pore with a width of a few nanometers. However, the interlayer space of smectite might be hardly described by the EDL model. An explicit atomic-scale description is more appropriate for the interlayer space. Besides, clay swelling or collapse corresponding to a variation of environmental conditions, and its impact on partitions of water and ions between interlayers and environment, have not been taken into account numerically. With atomic-scale molecular dynamics simulations and thermodynamic integrations based on them, the coupling between montmorillonite swelling/collapse and partitions of water and cations under different aqueous activity conditions is systematically disclosed here. With the combination of the Horinek’s force field to describe cations and ClayFF force field for atoms in solid layers, consistent swelling/collapse of Na- and K-montmorillonite with experimental observations as a function of water activity is shown. The swelling/collapse is connected to the transition between monolayer and bilayer hydration states. Decomposition of the swelling/collapse free energy shows both energy and entropy contributions are important in determining the thermodynamically stable hydration state. Taking environmental water activity and Na+/K+ ionic activity ratio as variables, a picture of cationic partition in response to those variables is established. It shows, under all circumstances, K+ ions are enriched in montmorillonite, which might explain K+ depletion in pore fluids of sediments. A low water activity improves K+ enrichment in montmorillonite. In addition, cationic partition accompanies variations of hydration states and alters the ionic concentration of the aqueous environment. These results can be applied to explain water and ionic partitions during subsurface water flow and the diagenesis processes of sediments.

Development of novel 2D and 3D correlative microscopy to characterise the composition and multiscale structure of suspended sediment aggregates

Suspended cohesive sediments form aggregates or ‘flocs’ and are often closely associated with carbon, nutrients, pathogens and pollutants, which makes understanding their composition, transport and fate highly desirable. Accurate prediction of floc behaviour requires the quantification of 3-dimensional (3D) properties (size, shape and internal structure) that span several scales (i.e. nanometre [nm] to millimetre [mm]-scale). Traditional techniques (optical cameras and electron microscopy [EM]), however, can only provide 2-dimensional (2D) simplifications of 3D floc geometries. Additionally, the existence of a resolution gap between conventional optical microscopy (COM) and transmission EM (TEM) prevents an understanding of how floc nm-scale constituents and internal structure influence mm-scale floc properties. Here, we develop a novel correlative imaging workflow combining 3D X-ray micro-computed tomography (μCT), 3D focused ion beam nanotomography (FIB-nt) and 2D scanning EM (SEM) and TEM (STEM) which allows us to stabilise, visualise and quantify the composition and multi-scale structure of sediment flocs for the first time. This new technique allowed the quantification of 3D floc geometries, the identification of individual floc components (e.g., clays, non-clay minerals and bacteria), and characterisation of particle-particle and structural associations across scales. This novel dataset demonstrates the truly complex structure of natural flocs at multiple scales. The integration of multi scale, state-of-the-art instrumentation/techniques offers the potential to generate fundamental new understanding of floc composition, structure and behaviour.

Methane hydrate formation in the stacking of kaolinite particles with different surface contacts as nanoreactors: A molecular dynamics simulation study

The heterogeneous nucleation behaviors of methane (CH4) hydrate in the presence of kaolinite (Kaol) were studied, and nucleation was found to be strongly affected by the surface contacts of Kaol particles. Molecular dynamics simulations were performed to investigate CH4 hydrate formation in Kaol particles with different surface contacts, such as silica-alumina face, alumina-alumina face, and silica-silica face. The results revealed two nucleation events, one in the bulk-like solution and the other near the siloxane surface of the Kaol. The crystal growth of CH4 hydrate tended to occur away from the Kaol surface. The silicon‑oxygen rings of the siloxane surface served as a plane of the cage, thereby forming a semi-cage arrangement in which the CH4 molecules appeared to have the ability to stabilize the arrangement of water structures. However, strong hydrogen bond interactions made it difficult for CH4 molecules to form clathrate-like structures on the hydroxyl surface. These results indicate that Kaol particles with a siloxane surface promoted CH4 hydrate nucleation and growth but that Kaol particles with a hydroxyl surface were unable to do so. Thus, the surface properties and surface contacts of Kaol particles were found to involve in the formation of aggregates in natural sediments, which shows that they are crucial for nucleation, distribution, and crystallinity of CH4 hydrate.