Sand Surface Research Articles

Impact of mineralogy, salinity, and temperature on the adsorption characteristics of a novel natural surfactant for enhanced oil recovery

Surfactants are considered as beneficial chemical additives in enhanced oil recovery due to their capability of reducing the interfacial tension and altering the wettability. However, the loss of surfactant by adsorption on the surface of the porous rock is one of the significant drawbacks of surfactant flooding. The mechanism, equilibrium, and kinetics of adsorption of a natural surfactant derived from the weed, Eichhornia crassipes, on sandstone and sand surfaces have been investigated through batch experiments at different concentrations (i.e., 1000–5000 mg dm−3), temperatures (i.e., 298–333 K), and a fixed salinity [NaCl (76.2%), CaCl2 (3.7%), MgCl2 (1.5%), and Na2SO4 (18.6%)]. The mineralogy and morphology of the adsorbent samples were examined by the XRD and SEM analyses. The mechanism of surfactant adsorption and maximum adsorption were determined by various isotherms and kinetic models. for adsorption on sandstone and sand was found to be −21.48 and −20.80 kJ mol−1, respectively.

Synergistic effect of cyanobacteria and nano-sand-stabilizer on biocrust formation and sand fixation

The biocrust formation by cyanobacteria inoculation is considered to be a substantial and promising biotechnology method for combating desertification. However, due to the harsh ecological environment in desert, it is difficult to form stable biocrust quickly on sand surface. Herein, we fabricated a network-structured nanocomposite as nano-sand-stabilizer (named as AC) using attapulgite (ATP) and carboxymethyl cellulose (CMC). The AC with abundant ‒COOH and ‒OH had a high retention water capacity and could bind sand particles together through netting effect, providing a stable and suitable microenvironment for growth of mixed cyanobacteria (MC). A one-year field experiment confirmed that synergistic effect of AC and MC could improve the soil nutrition content and microbial structure, which was beneficial for biocrust formation and sand fixation. Therefore, this study provides a new method for fast sand fixation and promoting the biocrust formation, which could achieve the goal of sustainable desert restoration.

Surface Energy of the Titan Aerosol Analog “Tholin”

Abstract The photochemical haze produced in the upper atmosphere of Titan plays a key role in various atmospheric and surface processes on Titan. The surface energy, one important physical property of the haze, is crucial for understanding the growth of the haze particles and can be used to predict their wetting behavior with solid and liquid species on Titan. We produced Titan analog haze materials, so-called “tholins,” with different energy sources and measured their surface energies through contact angle and direct force measurements. From the contact angle measurement, we found that the tholins produced by cold plasma and UV irradiation have a total surface energy around 60–70 mJ m−2. The direct force measurement yields a total surface energy of ∼66 mJ m−2 for plasma tholin. The surface energy of tholin is relatively high compared to common polymers, indicating its high cohesiveness. Therefore, the Titan haze particles would likely coagulate easily to form bigger particles, while the haze-derived surface sand particles would need a higher wind speed to be mobilized because of the high interparticle cohesion. The high surface energy of tholins also makes them easily wettable by Titan’s atmospheric hydrocarbon condensates and surface liquids. Thus, the haze particles are likely good cloud condensation nuclei for hydrocarbon clouds (methane and ethane) to nucleate and grow. And if the haze particles are denser compared to the lake liquids, they will likely sink into the lakes instead of forming a floating film to dampen the lake surface waves.

Automated measurement of long-term bower behaviors in Lake Malawi cichlids using depth sensing and action recognition

In the wild, behaviors are often expressed over long time periods in complex and dynamic environments, and many behaviors include direct interaction with the environment itself. However, measuring behavior in naturalistic settings is difficult, and this has limited progress in understanding the mechanisms underlying many naturally evolved behaviors that are critical for survival and reproduction. Here we describe an automated system for measuring long-term bower construction behaviors in Lake Malawi cichlid fishes, in which males use their mouths to sculpt sand into large species-specific structures for courtship and mating. We integrate two orthogonal methods, depth sensing and action recognition, to simultaneously track the developing bower structure and the thousands of individual sand manipulation behaviors performed throughout construction. By registering these two data streams, we show that behaviors can be topographically mapped onto a dynamic 3D sand surface through time. The system runs reliably in multiple species, across many aquariums simultaneously, and for up to weeks at a time. Using this system, we show strong differences in construction behavior and bower form that reflect species differences in nature, and we gain new insights into spatial, temporal, social dimensions of bower construction, feeding, and quivering behaviors. Taken together, our work highlights how low-cost tools can automatically quantify behavior in naturalistic and social environments over long timescales in the lab.

Graphene oxide nanoparticles and hematite colloids behave oppositely in their co-transport in saturated porous media

Since iron oxide minerals are ubiquitous in natural environments, the release of graphene oxide (GO) into environmental ecosystems can potentially interact with iron oxide particles and thus alter their surface properties, resulting in the change of their transport behaviors in subsurface systems. Column experiments were performed in this study to investigate the co-transport of GO nanoparticles and hematite colloids (a model representative of iron oxides) in saturated sand. The results demonstrated that the presence of hematite inhibited GO transport in quartz sand columns due to the formation of less negatively charged GO-hematite heteroaggregates and additional deposition sites provided by the adsorbed hematite on sand surfaces. Contrarily, GO co-present in suspensions significantly enhanced the transport of hematite colloids through different mechanisms such as the increase of electrostatic repulsion, decreased physical straining, GO-facilitated transport of hematite (i.e., highly mobile GO nanoparticles served as a mobile carrier for hematite). We also found that the co-transport behaviors of GO and hematite depended on solution chemistry (e.g., pH, ionic strength, and divalent cation (i.e., Ca2+)), which affected the electrostatic interaction as well as heteroaggregation behaviors between GO nanoparticles and hematite colloids. The findings provide an insight into the potential fate of carbon nanomaterials affected by mineral colloids existing in natural waters and soils.

Comparative Transport of Legionella and E. coli through Saturated Porous Media in a Two-Dimensional Tank

This study investigated bacterial transport in a two-dimensional (2-D) tank to evaluate the bacterial behavior of Legionella pneumophila as compared to Escherichia coli under saturated flow to simulate aquifer conditions. The experiments were performed in a 2-D tank packed with 3700 in3 (60,632 cm3) of commercially available bagged play sand under saturated conditions. The tank was disinfected by backwashing with 10% chlorine solution and subsequently neutralized by backwashing with tap water containing sodium thiosulphate (Na2S2O3) to ensure no chlorine residual. Bacterial transport was measured using samples collected from ports located at vertical transport distances of 5, 15 and 25 inches (12.7, 38.1 and 63.5 cm, respectively) below the sand surface along two vertical sections in the tank. An influent concentration of 105 CFU/mL was used for bacterial cells and the vertical fluid transport rate was 10.3 in/day (26.2 cm/day), controlled using a peristaltic pump at the bottom outlet. Legionella breakthroughs were recorded at 8, 22 and 35 h for the ports on the right side and 9, 24 and 36 h for the ports on the left side, at 5, 15 and 25 inch depths, respectively. At the same depths, E. coli breakthroughs were recorded at 5, 17 and 30 h for the ports on the right side and 7, 19 and 31 h for the ports on the left sides. The delay in Legionella transport compared to E. coli is homologous to Legionella’s pleomorphic nature. This study provides evidence of the mobility of both E. coli and Legionella in saturated aquifer conditions at a scale more representative of actual aquifer conditions. This study also provides a substantive basis for the premise that cell characteristics affect transport characteristics under those conditions.

Influence of metal cation and surface iron oxide on the transport of sulfadiazine in saturated porous media

Sulfadiazine (SDZ) is an antibiotic frequently detected in soil and groundwater. The transport of SDZ in subsurface environment is a critical process affecting its retention in soil. To date, the effects of iron oxide and metal cation on the transport of SDZ remain largely unknown, so we investigated the transport properties of SDZ in cleaned and iron oxides coated quartz sand, as affected by the presence of conventional cations (Ca2+, Mg2+, K+, and Na+) and Cd2+ through column experiments and simulation. We found that iron oxide coating on sand surface inhibited the transport of SDZ, mainly due to hydrophobic effect, complexation, and electrostatic attraction. The inhibitory effect became more marked with increasing concentration of Cd2+. It favors the transport of Cd2+ due to the electrostatic repulsion between positively charged iron oxide and Cd2+. Ca2+ promoted the transport of SDZ in coated sand, while all the conventional cations had no effect on the transport of SDZ in cleaned sand. The increase in the concentration of Cd2+ favors the transport of SDZ in cleaned sand. However, in iron oxide coated sand, the influence of Cd2+ on the transport of SDZ was dependent on the concentration of Cd2+. At lower concentration of Cd2+ and by competition, the transport is favored. At high concentration, the transport is inhibited mainly due to the formation of ternary surface complexes. A convective–dispersive transport model was applied to simulate and interpret experimental data. Breakthrough curves fitted well with a one-site model (OSM), indicating that SDZ adsorption on the sand experiences reversible kinetic. A low level of KF values with nearly linear sorption isotherm shows high mobility of SDZ and a high potential risk of surface and groundwater contamination. However, such high mobility can be reduced by increasing the content of iron oxides in porous media.

Scours Evolution at Bridge Abutments Under Unsteady Flow Events

Extreme floods become more frequent and is the reason rising load on structures in rivers and the risk to be damaged. The methods of scour calculation at bridge abutments are based on that design flood up to 1% probability (Arneson et al. 2012) accepted by many authors is constant during the maintenance period of the bridge and the equilibrium depth of scour often used for foundation design. Inspite of that equilibrium depth of scour sometimes can be overestimated, because in nature scour is formed under unsteady flow. The bridge abutments usually are constructed on the floodplain, where is no sediment movement, here we considering scour at clear water conditions and estimating scour evolution when sediments are removing out of scour hole. As a basis, the equation of the bed sediment movement was used and a semi-analytical model for computing depth of scour evolution under unsteady flow at the abutments is elaborated. The tests were made in flume for free surface flow conditions, for rigid and sand bed of the bridge crossing model with different contraction rate, depth, discharge, Froude number, steady and unsteady flow, with uniform or stratified bed. To study different parameters impact on scour at the abutment, More than 50 tests were made. Tests results are in good agreement with calculated data by model presented. In the paper is presented the relative local and critical velocities evolution, impact the bed material, flow contraction and unsteady flow on scour depth under clear water conditions at vertical wall abutment.

Wet regeneration of waste artificial sand used in sand casting using chemical solutions

Natural sand and organic binders have been the main materials used in sand casting. However, inorganic binders are used in novel casting technologies, and research on artificial sand as a substitute of natural sand has been conducted. Herein, the wet regeneration of waste artificial sand was performed using a sodium silicate binder. Acidic, neutral, and basic solutions were used for wet regeneration. The effects of binder removal and reusability of the regenerated sand were investigated by comparing the characteristics and strengths of the artificial and regenerated sand. It was demonstrated that the basic solution effectively removed the binder from the surface of the waste artificial sand. The Si-O-Si bond cleavage in the binder occurred due to the high concentration of hydroxide ions in the basic solution. The strength of the regenerated sand treated with the basic solution and that of the artificial sand were similar. Moreover, the strengths of regenerated sands treated with the acidic or neutral solutions were lower than that of artificial sand due to the presence of residual binder. These results indicated that the basic solution was suitable for removing the sodium silicate binder. The quality of the regenerated sand was similar to that of artificial sand.

Mechanical behaviors of interaction between coral sand and structure surface

Based on the interface shear tests, the macro- and meso-mechanical behaviors of interaction between coral sand and different structure surfaces are studied, in which CCD camera is used to capture digital images to analyze the evolution of the interaction band and a particle analysis apparatus is applied to studying the distribution characteristics of particle morphology. This study proposes four-stage evolution process based on the shear stress-strain curve. During the shear process, coral sand particles slide and rotate within the interaction band, causing the changes in shear stress and vertical displacement. In addition, the effects of structure surface roughness on shear strength, volume change and particle breakage are illustrated that the greater the roughness of slabs is, the larger the shear stress is, the more obvious the contraction effect is and the more the particles break. Furthermore, the change in particle’s 3D morphology during the breakage will change not only their size but also other morphological characteristics with convergence and self-organization.

INVENTARIO DE CARBONO EN LA RECUPERACIÓN DE PLAYAS CUBANAS

The objective of this study is to construct an inventory of carbon for the nourishment of Cuban sand beaches. It uses data of a beach nourishment project executed in 2016 in the archipelago of Jardines del Rey in Ciego de Avila, Cuba, according to an algorithm of carbon inventory for the recovery of beaches. The carbon budget was 62531.04 Mg, of which 952.36 were contained in consumed fossil fuels, 2338.95 were retained in the protected or rehabilitated vegetation, and 59239.73 Mg were retained in carbonate sands. The mole ratio in carbon compounds was used to transform the inventory into a net capture of 5084.16 Mg of carbon dioxide and the net depositions of calcium carbonate at 493652.11 Mg. These amounts were valued at 127 104.06 and 30 413 906.67 Cuban pesos, respectively. The total amounts were compared with the cost of investment. A positive net present value was deducted, representing a profitable investment in terms of carbon capture and increased carbonate sand surfaces on the studied beaches. We recommend to continue the study of sand beaches, in other to complete the inventory by our further research on carbon emission due to changes produced in the vegetation of the submarine zone.

Effects of phosphate on the transport of graphene oxide nanoparticles in saturated clean and iron oxide-coated sand columns

In this study, transport behaviors of graphene oxide (GO) in saturated uncoated (i.e., clean sand) and goethite-coated sand porous media were examined as a function of the phosphate. We found that phosphate enhanced the transport of GO over a wide range of solution chemistry (i.e., pH 5.0–9.0 and the presence of 10 mmol/L Na+ or 0.5 mmol/L Ca2+). The results were mainly ascribed to the increase of electrostatic repulsion between nanoparticles and porous media. Meanwhile, deposition site competition induced by the retained phosphate was another important mechanism leading to promote GO transport. Interestingly, when the phosphate concentration increased from 0.1 to 1.0 mmol/L, the transport-enhancement effect of phosphate in goethite-coated sand was to a much larger extent than that in clean sand. The observations were primarily related to the difference in the total mass of retained phosphate between the iron oxide-coated sand and clean sand columns, which resulted in different degrees of the electrostatic repulsion and competitive effect of phosphate. When the background solution contained 0.5 mmol/L Ca2+, phosphate could be bind to sand/ goethite-coated sand surface by cation bridging; and consequently, promoted competition between phosphate and nanoparticles for deposition sites, which was an important mechanism for the enhanced effect of phosphate. Moreover, the DLVO theory was applicable to describe GO transport behaviors in porous media in the absence or presence of phosphate. Taken together, these findings highlight the important status and role of phosphate on the transport and fate of colloidal graphene oxide in the subsurface environment.

Groundwater Flow and Moisture Dynamics in the Swash Zone: Effects of Heterogeneous Hydraulic Conductivity and Capillarity

AbstractA density‐dependent, variably saturated groundwater flow and solute transport model was used to investigate the influence of swash motions on subsurface flow and moisture dynamics in beach aquifers with heterogeneous distributions of hydraulic conductivity (K) and capillarity. The numerical simulations were performed within a Monte Carlo framework using field measurements conducted in the swash zone of a sandy beach. Our results show that heterogeneous capillarity causes spatially variable capillary rise above the groundwater table. In response to swash motions, heterogeneity creates capillary barriers that result in pockets of elevated moisture content beneath the swash zone. These moisture hotspots persist within the unsaturated zone even at ebb tide when the swash motions recede seaward. Heterogeneous capillarity also results in highly tortuous preferential flow paths and alters the flow rates from the sand surface to the water table. Heterogeneous K greatly enhances the seawater infiltration into the swash zone and modulates its spatial distribution along the beach surface. Due to heterogeneous K and capillarity, complex mixing patterns emerge. Both strain‐dominated and vorticity‐dominated flow regions develop and dissipate as tides and waves move across the beach surface. Complex mixing patterns of seawater percolating from the swash zone surface to the water table, with localized areas of high and low mixing intensities, are further demonstrated by analysis of dilution index. Our findings reveal the influence of geologic heterogeneity on swash zone moisture and flow dynamics, which may have important implications for sediment transport and chemical processing in beach aquifers.

Application of Lost-foam casting Technology in Grate of Hot blast stove and Solution of Defects

Lost-foam casting Technology has been applied in Manufacturing the grate of hot blast stove. According to the requirements of structure and tolerance dimension, the lost-foam unit of grate is prepared with expandable polystyrene resin beads. RTCr2 is used as the material of grate, which is poured at 1360-1380°C by Top post system. And the negative pressure range is 0.04-0.05MPa. The results show that the surface of the casting is relatively complete, and the internal structure of the casting is dense. Through inspection, the quality performance, structure dimension and manufacturing precision of casting products can meet the requirements well. But there are also some disadvantages, such as inner hole sticking sand, surface sand hole, shrinkage cavity, air hole, sand inclusion, slag inclusion, mechanical processing edge breaking. Then some corresponding effective repair methods are used to solve the defects, which guarantee the quality of the products. Some lost-foam casting data and control measures have been accumulated, which are beneficial to continuously improve the lost-foam casting process and master the key casting technologies such as lost-foam casting and hot metal casting.

Influence of slow sand filter cleaning process type on filter media biomass: backwashing versus scraping

Biomass was assessed as a new approach for evaluating backwashed slow sand filters (BSF). Slow sand filtration (SSF) is a simple technology for water treatment, where biological mechanisms play a key role in filtration efficiency. Backwashed slow sand filters were previously recommended for small-scale filters (~1 m² of filtration area) as an alternative to conventional filters that are usually cleaned by scraping (ScSF). Biomass was never evaluated in BSF, which is a gap in the knowledge of this technology, considering the importance of its biological mechanisms. Therefore, for the first time, two filters operating under the same conditions were used to compare the influence of backwashing on biomass; one filter was cleaned by backwashing and the other by scraping. Biomass along the filter media depth (40 cm) was assessed by different techniques and compared in terms of cellular biomass (by chloroform fumigation), volatile solids, bacterial community (by 16S rRNA gene sequencing), and observations by scanning electron and fluorescence microscopy. Filters were also monitored and compared regarding filtered water quality and headloss; their differences were related to the different cleaning processes. Overall, filtered water quality was acceptable for slow sand filter standards (turbidity < 1 NTU and total coliform removal > 1 log). However, headloss developed faster on scraped filters, and biomass was different between the two filters. Backwashing did not significantly disturb biomass while scraping changed its surface sand layers. Cell biomass was more abundant and spread across the filtration depth, related to lower headloss, turbidity, and cyanobacterial breakthrough. These results agreed with the water quality and microscopy observations. The bacterial community was also less stratified in the backwashed filter media. These results expand the knowledge of backwashing use in slow sand filters, demonstrating that this process preserves more biomass than scraping. In addition, biomass preservation can lead to bacterial selectivity and faster filter ripening. Considering the importance of biomass preservation on slow sand filtration and its biological filtration mechanisms, the results presented in this paper are promising. The novel insight that BSF can preserve biomass after backwashing may contribute to increasing its application in small communities.

Dynamic Behaviour of High Performance of Sand Surfaces Used in the Sports Industry

The sand surface is considered a critical injury and performance contributing factor in different sports, from beach volleyball to greyhound racing. However, there is still a significant gap in understanding the dynamic behaviour of sport sand surfaces, particularly their vibration behaviour under impact loads. The purpose of this research was to introduce different measurement techniques to the study of sports sand surface dynamic behaviour. This study utilised an experimental drop test, accelerometry, in-situ moisture content and firmness data, to investigate the possible correlation between the sand surface and injuries. The analysis is underpinned by data gathered from greyhound racing and discussed where relevant.

Improve the mechanical properties of lightweight foamed concrete by using nanomodified sand

The current study aims to solve the agglomeration problem of carbon nanotubes (CNTs). Many researchers have been solved this task by using dispersion by ultrasonic along with adding a dispersant to prevent carbon precipitation for as long as possible. However, some researchers have confirmed that the dispersed materials affect the mechanical properties of the concrete when added with carbon nanotubes. This problem was solved in this work via the growth of the carbon nanotubes on a sand surface using the chemical vapor deposition method (CVD). The modified sand by (CNTs) was incorporated into the main components of the lightweight concrete. A catalyst solution for the carbon nanotubes growth on the sand surface was prepared by a chemical vapor deposition method. Through this method, the direct use of carbon nanotubes could be achieved without using dispersion methods. Also, this method ensured the distribution of the nanomodified sand grains in the concrete body. In addition, two types of solutions were applied for controlling the density of the carbon nanotubes growing on the sand surface. The first method was without water dilution, while the second was with water dilution. The percentages of the modified sand were 0.1%, 0.2%, 0.5% and 1.0% of the sand weight, and the particle sizes were 0.63 mm, 0.315 mm and 0.16 mm. The results showed an increase of the compressive strength of the sample containing modified sand without diluted 11.0%, 19.0%, 29.0%, and 40.0%, diluted sand 11.0%, 20.0%, 26.0%, and 35.2%, for additives 0.1%, 0.2%, 0.5%, and 1.0% from the sand weight. The highest increase of the flexural strength of the sample containing modified without diluted sand was 22.0%, 28.0%, 36.0% and 45.0%, and for diluted sand 15.8%, 19.7%, 18.1%, and 20.3% for additives 0.1%, 0.2%, 0.5% and 1.0% from the sand weight.

Fabrication of highly hydrophobic sand@soot with core–shell structure and large-scale production possibility for oil/water separation

One of the ever increasing environmental concerns for marine areas is oil spillage, and seeking a suitable method for the separation of oil spills from water is of vital importance. Here, highly hydrophobic sand as a very low-cost material has been synthesized for oil/water separation. By kerosene combustion using a burner, nano/micro-soot can be easily coated on the surface of sand. By this one-step method, highly hydrophobic sand with a water contact angle of 134 ± 5° has been obtained. The prepared hydrophobic sand covered with soot has been fully characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Hydrophobic core–shell sand@soot has been used as a filter that allows the passage of oil, but prevents the infiltration of water. The time consumption for oil/water separation is low and the separation system does not need any special force. A continuous separation system based on simple equipment has been designed for the removal of oil from the surface of water with high efficiency. Due to the abundance of the precursor materials, the simplicity of the synthesis process, and the feasibility of large-scale production, the prepared hydrophobic sand@soot can be regarded as a suitable material for oil/water separation in real systems.

Experimental measurements and numerical simulations of the transport and retention of nanocrystal CdSe/ZnS quantum dots in saturated porous media: effects of pH, organic ligand, and natural organic matter.

The risks of environmental exposures of quantum dot (QD) nanoparticles are increasing, but these risks are difficult to assess because fundamental questions remain about factors affecting the mobility of QDs. The objective of this study is to help address this shortcoming by evaluating the physico-chemical mechanisms controlling the transport and retention of CdSe/ZnS QDs under various environmental conditions. The approach was to run a series of laboratory-scale column experiments where QDs were transported through saturated porous media with different pH values and concentrations of citrate and Suwannee River natural organic matter (SRNOM). Numerical simulations were then conducted and compared with the laboratory data in order to evaluate parameters controlling transport. QD suspensions were injected into the column in an upward direction and ICP-MS used to analyze Cd2+ concentrations (C) in column effluent and sand porous media samples. The increase in the background solution pH values enhanced the QD transport and decreased the QD retention. QD transport recovery percentages obtained from the column effluent samples were 2.6%, 83.2%, 101.7%, 96.5%, and 98.9%, at pH levels of 1.5, 3.5, 5, 7, and 9, respectively. The effects of citrate and SRNOM on the transport and retention of QDs were pH dependent as reflected in the influence of the electrostatic and steric interactions between QDs and sand surfaces. QDs were mobile under unfavorable deposition conditions at environmentally relevant pHs (i.e., 5, 7, and 9). Under favorable pH conditions for deposition (i.e., 1.5), QDs were completely retained within the porous media. The retention profiles of QDs showed a non-exponential decay with distance to the inlet, attributed to multiple deposition rates caused by the QD particles and surface heterogeneities of the quartz silica sand. Results of the diameter ratios of QDs to the median sand grains, in suspensions of DI water at pH 1.5, of citrate at pH 1.5, and of citrate at pH 3.5 indicate straining as the dominating mechanism for QD retention in porous media. The blocking effect and straining were significant under favorable deposition conditions and the detachment effect was non-negligible under unfavorable deposition conditions. Physico-chemical attachment and straining are the governing mechanisms that control the retention of QDs. Overall, experimental results indicate that aggregation, deposition, straining, blocking, and DLVO-type interactions affect the advective transport and retention of QDs in saturated porous media. The simulations were conducted using models that include terms describing attachment, detachment, and straining terms-model 1: M1-attachment, model 2: M2-attachment and detachment, model 3: M3-straining, and model 4: M4-attachment, detachment, and straining. The results from simulations with M2-attachment and detachment and M4-attachment, detachment, and straining matched best the observed breakthrough curves, but all four models inadequately described the retention profiles. Our findings demonstrate that QDs are mobile in porous media under a wide range of physico-chemical conditions representative of the natural environment. The mobility behavior of QDs in porous media indicated the potential risk of soil and groundwater contamination.

Sterile White Basidiomycete Fungus Marasmius graminum: A New Pathogen Causing Seedling Blight in Rice.

In April 2018, damping-off of rice (Oryza sativa L.) seedlings at the 2-to-3-leaf stage was observed in three fields in the counties of Wharton and Matagorda of Texas and Jefferson-Davis Parish of Louisiana. All affected areas were 1 ha or greater, with 10 to 20% of the seedlings showing the symptoms. Infected seedlings showed dark-brown necrotic lesions on the roots and/or mesocotyls where white superficial mycelium was usually present. Symptomatic tissues excised from 10 diseased seedlings of each field were surface sterilized with 1% NaOCl, double rinsed in sterilized distilled water, and plated on potato dextrose agar (PDA). The plates were incubated at 25°C with a 12-h photoperiod in a growth chamber. After 48 h, hyphal tips of fungal colonies were transferred onto PDA and 12 isolates were obtained. Clamp connections and dolipore septa were observed in young hyphae, indicating that these isolates were a basidiomycete fungus. Young hyphal cells were binucleate based on safranin O stain (Bandoni 1979). No fruiting bodies or sclerotia produced on PDA after one month of incubation. Based on these morphological characteristics, these isolates were identified as belonging to sterile white basidiomycetes (SWB) (Howard et al. 1977). To further identify the isolates into the species level, the internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) of a representative isolate was sequenced with primer ITS1 and ITS4 (Vinnere et al. 2005). The ITS sequence (GenBank acc. no. MT524457) had more than 97% sequence similarity with known Marasmius graminum strains from Denmark (JN943595) (Schoch et al. 2012) and Sweden (MH857692) (Vu et al. 2019). Pathogenicity was tested with three representative isolates in a growth chamber using a modified method (Carling and Leiner 1990). Pots (6.5 cm in diameter x 7.5 cm height) were filled with 100 g of sterilized sand and watered to field capacity. Five PDA plugs (4 mm in diameter) from 5-day-old growing culture were placed on the sand surface of each pot. Pots inoculated with PDA plugs without fungus served as the controls. Five seeds of rice cv. Presidio were planted into each pot and covered with 10 g of sterilized sand. Pots were maintained at 25±2°C in a growth chamber with a 12-h photoperiod for 14 days. There were four replicated pots for each treatment and the experiment repeated twice. After 2 weeks, severe damping-off and associated symptoms similar to those observed in the field appeared in the inoculated pots. No symptoms developed in the control pots. The same fungus was consistently re-isolated from infected plants. Based on morphological characteristics and rDNA-ITS sequencing, these isolates were identified as M. graminum. The SWB fungus was first reported as a causal agent of stem rot of snap bean in Florida (Howard et al. 1977) and Nebraska (Harveson 2002), root or hypocotyl rot of corn, snap bean, squash and peanut in Georgia (Sumner et al. 1979; Bell and Sumner 1984), and crown rot of pigeon pea (Cajanus cajan) in Puerto Rico (Kaiser et al. 1987). Later, the SWB strain (ATCC 28344) causing stem rot of snap bean in Florida was further identified as M. graminum based on nuclear large subunit rRNA gene (Vinnere et al. 2005). Comparing the ITS region of this isolate (AY445120) with our isolate revealed a 99% similarity. To our knowledge, this is the first report that the SWB fungus M. graminum causes seedling blight in rice. Identification of this new disease will help to develop management strategies for control of stand loss in rice.