A Numerical Study of Elastic Wave Arrival Behavior in a Naturally Fractured Rock Based on a Combined Displacement Discontinuity-Discrete Fracture Network Model.

The arrival behavior of elastic waves in a naturally fractured rock is studied based on numerical simulations. We use the discrete fracture network method to represent the distribution of a natural fracture system and employ the displacement discontinuity method to compute the propagation of elastic waves across individual fractures. We analyze macroscopic wavefield arrival properties collectively arising from the interaction between elastic waves and numerous fractures in the system. We show that the dimensionless angular frequency ῶ = ωZ/κ exerts a fundamental control on the arrival behavior of a plane wave traveling through the fractured rock, where ω, Z, and κ are the angular frequency, seismic impedance, and fracture stiffness, respectively. An asynchronous arrival phenomenon of the wave energy occurs and becomes more significant with an increased ῶ. Two regimes are identified according to the two-branch dependency of the fractal dimension D of the FFAW on ῶ, where the wave arrival behavior is within a non-fractal regime for ῶ smaller than the critical frequency ῶ c ≈ 1.0, and enters the fractal regime for ῶ ≥ ῶ c. The self-affine properties of the FFAW, i.e., the roughness exponent α and the correlation length l c, both linearly decrease as a function of the exponent ξ (with ῶ = 10 ξ ) in the fractal regime. Early breakthrough of wave transport occurs in regions with relatively low fracture density, while late-time arrival happens in regions of high fracture density.

Investigation of Volumetric Block Proportion (VBP) Effect on Excavation-Induced Ground Response of Talus-like Rock Mass Based on DEM Simulations.

Due to the complexity of the talus-like rock mass with different values of volumetric block proportion (VPB), it is thus crucial to explore the VBP effect on the excavation-induced ground responses. We conduct a series of 2D DEM (discrete element method) simulations on a common circular tunnel excavation in the talus-like rock mass with different VBPs (0%, 15%, 50%, 85% and 100%). For each VBP, two support scenarios, i.e., unsupported and supported by a rigid lining, are considered. The micro characteristics of the excavation-induced ground responses, including the contact force, force chain, coordination number and shear-slip contact, and the stress distribution and ground settlement are elaborated in detail. Accordingly, three types of talus-like rock masses are identified as soil-, hybrid- and rock-types, corresponding to VBP = 0-15%, 50%, and 85-100%, respectively. It is found that the lining support is essential for maintaining the ground stability of a tunnel excavation in the soil- and hybrid-type talus-like rock masses while the backbones formed by rock blocks in the rock-type talus-like rock mass can provide a certain support for the surrounding ground. Our findings have important implications for optimizing the construction scheme of tunnel excavation in different types of talus-like rock masses.

Role of Particle Shape on Ground Responses to a Circular Tunnel Excavation in Sandy Soil: Consequences from DEM Simulations.

Due to the sensitivity of sandy soil's mechanical behavior to the particle shape, it is thus of importance for interpreting the effect of particle shape on the ground response induced by tunnel excavation in sandy formations. We conducted a series of 2D DEM (discrete element method) simulations on a common circular tunnel excavation in sandy soil with variable-shaped particles, which are characterized as two descriptors, i.e., aspect ratio (AR) and convexity (C). The macroscopic responses and the microscopic characteristics of the sandy ground are elaborated in detail. The simulation results show obvious asymmetrical features of the excavated ground, which results from the ground heterogeneity caused by the irregular particle shape. In addition, we investigate the roles of AR and C on the ground response and find that reducing AR or increasing C will enlarge the ground settlement, i.e., the sandy ground deformation is more sensitive to the particles with more irregular shapes. However, elongated particles are beneficial for the generation of soil arching with stronger bearing capacity and thus reduce the soil pressure on the tunnel lining. Our findings have important implications for the safety assessment of the tunnel excavation, as well as other underground structure construction in sandy formations.

Experimental and DEM-Based Numerical Studies on the Shearing Characteristics of Talus-like Rock Mass.

The talus-like rock mass is a special kind of geomaterial widely distributed in southwestern China, which has induced serious engineering disasters for tunneling engineering. However, the mechanical behavior of the talus-like rock mass remains unclear as the previous studies mainly focused on similar geomaterials such as the soil-rock mixtures. In this paper, we have carried out both experimental and discrete element method (DEM)-based numerical analyses to investigate the shearing characteristics of the talus-like rock mass collected from a real project site. Large-scale direct shear tests reveal that the strength parameters increase with the block content, which is different from the traditional soil-rock mixture. A dependence has been discovered in that the specimen dilation becomes more obvious under lower normal stress and larger block content. It is also observed that higher normal stress is beneficial for crushing blocks. The force chains obtained in the DEM simulations show that distinct internal structures are generated in the rock samples with different block contents. The distribution of coordination number establishes the dependence of fabric stability on block content during shearing. Bond-break evolution reveals the tendencies of crushed particles were consistent with those of experimental tests. The findings provide a more in-depth understanding about the mechanical behavior of the talus-like rock mass, which helps to uncover the cause of the collapse of the real tunnel project.

Total Synthesis of (-)-Rhodomollanol A.

An asymmetric approach for the first total synthesis of (-)-rhodomollanol A, a highly oxidized diterpenoid, is described. The efficient synthetic strategy features three key transformations: (1) an oxidative dearomatization-induced (5 + 2) cycloaddition/pinacol-type 1,2-acyl migration cascade to build up the bicyclo[3.2.1]octane skeleton; (2) a retro-Dieckmann fragmentation/vinylogous Dieckmann cyclization cascade to assemble the bicyclo[3.3.0]octane subunit; and (3) a photo-Nazarov cyclization/intramolecular cycloetherification cascade to forge the 7-oxabicyclo[4.2.1]nonane core structure of the natural product.

Facile Fabrication of NiCoAl-Layered Metal Oxide/Graphene Nanosheets for Efficient Capacitive Deionization Defluorination.

Capacitive deionization (CDI) has aroused extensive attention as a prospective technology for different ionic species removal from aqueous solutions. Traditional studies on the adsorption and desorption of fluoride from wastewater are energy-intensive and may have harmful effects on the environment. Herein, the feasibility of fluoride removal from wastewater by CDI has been investigated. NiCoAl-layered metal oxide (NiCoAl-LMO) nanosheets and reduced graphene oxide (rGO) composites (NiCoAl-LMO/rGO) were synthesized and used as CDI electrode materials for fluoride ion removal. The as-obtained NiCoAl-LMO/rGO with unique structure and high conductivity is beneficial to the adsorption of fluoride ions. In addition, the introduction of Co element in the laminate enhances the pseudocapacitive behavior of the electrode material. As expected, the CDI system with NiCoAl-LMO/rGO composites as anode and activated carbon treated by nitric acid (H-AC) as cathode exhibits outstanding defluorination performance. The maximum adsorption capacity of NiCoAl-LMO/rGO, 24.5 mg g-1, can be reached when the initial NaF concentration is 500 mg L-1 at 1.4 V applied voltage. The composites also show good cycle stability over 40 consecutive cycles of the CDI defluorination process. The excellent defluorination performance of NiCoAl-LMO/rGO makes it possible for its practical application in wastewater treatment.

Membrane-Free Hybrid Capacitive Deionization System Based on Redox Reaction for High-Efficiency NaCl Removal.

Capacitive deionization (CDI) is a promising technology for desalination due to its advantages of low driven energy and environmental friendliness. However, the ion removal capacity (IRC) of CDI is insufficient for practical application because such a capacity is limited by the available surface area of the carbon electrode for ion absorption. Thus, the development of a novel desalination technology with high IRC and low cost is vital. Here, a membrane-free hybrid capacitive deionization system (HCDI) with hollow carbon@MnO2 (HC@MnO2) to capture sodium via redox reaction and hollow carbon sphere with net positive surface charges (PHC) for chloride adsorption is introduced. The as-obtained HC@MnO2 with unique structure and high conductivity can improve the utilization of MnO2 pseudocapacitive electrodes. Meanwhile, the PHC can selectively adsorb Cl- and prevent the adsorption of Na+ due to electrostatic repulsion. As expected, the membrane-free HCDI system demonstrates excellent desalination performance. The system's IRC and maximum removal rate are 30.7 mg g-1 and 7.8 mg g-1 min-1, respectively. Moreover, the proposed system has a low cost because of the absence of expensive ion exchange membranes (IEM), which is suitable for practical application. The excellent performance of this HCDI makes it a promising desalination technology for future use.

Palladium-Catalyzed anti-Selective Fluoroalkylboration of Internal and Terminal Alkynes.

A Pd-catalyzed anti-stereospecific alkyne fluoroalkylboration, including mono-, di-, and perfluoroalkylboration, has been developed with fluoroalkyl halides and diboron reagents. The reaction is effective for both internal and terminal alkynes. It provides straightforward and streamlined access to functionalized 1,2-fluoroalkylboronated alkenes in a highly regio- and stereocontrolled manner. Preliminary studies suggest that this reaction is enabled by the combination of radical alkylation and metal-catalyzed borylation, thus leading to the realization of three-component trans-carboboration of alkynes for the first time.

Removal of antimonate from wastewater by dissimilatory bacterial reduction: Role of the coexisting sulfate.

The priority pollutant antimony (Sb) exists primarily as Sb(V) and Sb(III) in natural waters, and Sb(III) is generally with greater mobility and toxicity than Sb(V). The bio-reduction of Sb(V) would not become a meaningful Sb-removal process unless the accumulation of produced dissolved Sb(III) could be controlled. Here, we examined the dissimilatory antimonate bio-reduction with or without the coexistence of sulfate using Sb-acclimated biomass. Results demonstrated that 0.8mM Sb(V) was almost completely bio-reduced within 20h along with 48.6% Sb(III) recovery. Kinetic parameters qmax and Ks calculated were 0.54mg-Sb mg-DW-1h-1 and 41.96mgL-1, respectively. When the concentrations of coexisting sulfate were 0.8mM, 1.6mM, and 4mM, the reduction of 0.8mM Sb(V) was accomplished within 17, 9, and 5h, respectively, along with no final Sb(III) recovery. Also, the bio-reduction of sulfate occurred synchronously. The precipitated Sb2O3 and Sb2S3 were characterized by scanning electron microscopy coupled with energy dispersive spectrometer, X-ray diffraction, and X-ray photoelectron spectroscopy. Compared with bacterial compositions of the seed sludge obtained from anaerobic digestion tank in a wastewater treatment plant, new genera of Pseudomonas and Geobacter emerged with large proportions in both Sb-fed and Sb-sulfate-fed sludge, and a small portion of sulfate reduction bacteria emerged only in Sb-sulfate-fed culture.

Palladium-Catalyzed Site-Selective sp3 C-H Bond Thiocyanation of 2-Aminofurans.

Alkyl thiocyanates are prevalent in natural products, drugs, and biologically active compounds. We report here a novel, mild, and efficient Pd-catalyzed site-selective sp3 C-H bond thiocyanation of 2-aminofurans. Using Na2S2O8 as the oxidant and readily available NaSCN as the thiocyanation reagent, the kinetically favorable 2-amino-4-thiocyanatomethylfurans are selectively synthesized in promising yields with a broad substrate scope. This reaction represents the first example of transition-metal-catalyzed site-selective sp3 C-H bond thiocyanation, thus offering a novel strategy for the step- and atom-economic synthesis of alkyl thiocyanates.

Preparation of 2-Amino-5-homoallylfurans via Palladium-Catalyzed Tandem Cycloisomerization/Heck-Type Coupling of Homoallenyl Amides with Allyltrialkylsilanes.

The direct access to 2-amino-5-homoallylfurans has been realized by a palladium-catalyzed tandem cycloisomerization/Heck-type coupling between homoallenyl amides and allyltrialkylsilanes, using a novel DDQ/MnO2 combination as the efficient oxidant. The reaction exclusively affords γ-allylation products in good to excellent yields with broad substrate scope under exceptionally mild reaction conditions. It represents one of the rare examples of the Pd-catalyzed intermolecular Heck-type coupling of allytrialkylsilanes terminated by ß-silyl elimination, thus complementing traditional allylation methods because of the excellent γ-selectivity.

Impacts of nitrate and electron donor on perchlorate reduction and microbial community composition in a biologically activated carbon reactor.

The sensitivity of perchlorate reduction and microbial composition to varied nitrate and acetate loadings was studied in a biologically activated carbon reactor with perchlorate loading and empty bed contact time fixed at 5 mg/L and 226 min, respectively. In stage 1, the sole electron acceptor ClO4- realized complete removal with ≥21.95 mg C/L of acetate supply. As nitrate loading gradually increased to 5 mg/L (stage 2), perchlorate reduction was slightly promoted and both ClO4- and NO3- were completely removed at an acetate loading of 29.7 mg C/L. When nitrate loading continued increasing to 10-60 mg/L (stage 3), perchlorate reduction converted to be inhibited, along with nondetectable NO3- and approximately exhausted DOC in effluent. When acetate loading increased to 43.9 mg C/L in stage 4, both ClO4- and NO3- were again removed, though lags still existed in perchlorate reduction. ß-Proteobacteria accounted for about 60%, 55%, 58%, 61% and 12% in samples from the base and top of the filter in stage 1 and those from the base, middle and top in stage 4, respectively. These findings implied that ratio of NO3- to ClO4- loadings and acetate loading were two key factors impacting ClO4- reduction and microbial structure along the filter.

Bacterial reduction of highly concentrated perchlorate: Kinetics and influence of co-existing electron acceptors, temperature, pH and electron donors.

Perchlorate reduction kinetics and effects of various environmental conditions on removal of perchlorate from synthetic water were investigated to seek high-strength perchlorate removal using mixed perchlorate reducing bacteria. Results demonstrated that perchlorate (50-1500 mg L(-1)) could be degraded rapidly within 28 h under the optimal conditions. The maximum specific perchlorate reduction rate (qmax) and half saturation constant (Ks) were 0.92 mg-perchlorate (mg-dry weight)(-1) h(-1) and 157.7 mg L(-1), respectively. In the ClO4(-)-NO3(-) systems obvious but recoverable lags were caused in perchlorate reduction and the lag time increased with the ratio of nitrate to perchlorate concentration increasing from 0.5 to 3. While in the ClO4(-)-SO4(2-) systems inhibitions didn't occur until the ratio of sulfate to perchlorate concentration exceeded 10. The optimum temperature and pH value were 35 °C and 6.85, respectively. The optimal acetate-to-perchlorate ratio that could consume all perchlorate and acetate simultaneously was about 2. Dechloromonas, one of the most prominent perchlorate reducing bacteria, was identified as the dominant bacterium in the acclimated culture (69.33% of the whole clones). The study demonstrated that the perchlorate-acclimated mixed microorganisms can readily and efficiently realize reduction of highly concentrated perchlorate in wastewater.