Direct (utilize easily available and abundant precursors) and selective (both chemo- and regio-) aliphatic C-H functionalization is an attractive mean with which to streamline chemical synthesis. ...With many possible sites of reaction, traditional methods often need an adjacent polar directing group nearby to achieve high regio- and chemoselectivity and are often restricted to a single site of functionalization. Here we report a remote aliphatic C-H thiolation process with predictable and switchable regioselectivity through NiH-catalysed migratory hydrothiolation of two feedstock chemicals (alkenes/alkynes and thiols). This mild reaction avoids the preparation of electrophilic thiolation reagents and is highly selective to thiols over other nucleophilic groups, such as alcohols, acids, amines, and amides. Mechanistic studies show that the reaction occurs through the formation of an RS-Bpin intermediate, and THF as the solvent plays an important role in the regeneration of NiH species.
Accurately monitoring global vegetation dynamics with modern remote sensing is critical for understanding the functions and processes of the biosphere and its interactions with the planetary climate. ...The MODerate resolution Imaging Spectroradiometer (MODIS) vegetation index (VI) product has been a primary data source for this purpose. To date, the MODIS team had released several versions of VI products that have widely used in global change studies and practical applications. In this study, we re-examined the global vegetation activity by comparing the recent MODIS Collection 6 (C6) VIs with Collection 5 (C5) VIs including Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) from Terra (2001–2015) and Aqua Satellites (2003–2015). We found substantial differences in global vegetation trends betweenTerra-C5 and -C6 VIs, especially EVI. From 2001 to 2015, global vegetation showed a remarkable greening trend in annual EVI from the Terra-C6 (0.28% year−1; P<0.001), in contrast to the decreasing EVI trend from the Terra-C5 (−0.14% year−1, P<0.01). Spatially, large portions of the browning areas in tropical regions identified by Terra-C5 VIs were not evident in Terra-C6 VIs. In contrast, the widespread greening areas in Terra-C6 VIs were consistent with Aqua-C6 VIs and GIMMS3g NDVI. Our finding of a greening Earth supports the recent studies suggesting an enhanced land carbon sink. Our study suggests that most of the vegetation browning trends detected by MODIS Terra-C5 VIs were likely caused by sensor degradation, particularly for the period after 2007. Therefore, previous studies of temporal vegetation trends based solely on Terra-C5 VIs may need to be reevaluated. Our new analysis offers the most updated understanding of the global vegetation dynamics during the past 15years and contributes to accurately understanding the role of vegetation played in the Earth's biogeochemical and climatic systems.
•A significant greening Earth was detected by MODIS Terra-C6 VIs.•The widespread browning areas in Terra-C5 VIs were likely artifacts of sensor degradation.•A greening Earth is consistent with the enhanced land carbon sink suggested by recent studies.
Microgrippers driven by piezoelectric actuators have been widely applied in various fields demanding high accuracy. However, it is challenging to achieve a higher precision due to the presence of ...hysteresis and disturbances. This brief presents a new approach to achieve a precision motion control by overcoming the mentioned problems. The reported solution is derived by integrating three main components, i.e., the online estimation of unknown parameters for the ideal plant, the position prediction with Kalman filter, and an adaptive sliding mode control algorithm. The estimated parameters converge to their real values, which are guaranteed by Lyapunov criterion. Using the estimated parameters, the position is predicted with the Kalman filter. The adaptive law and the control law are designed based on sliding mode strategy to attenuate the influence of the unknown and unmodeled parts. The stability of the control system is proved by resorting to Lyapunov theorem. The effectiveness of the proposed control approach is verified through simulation and experimental investigations on a prototype gripper device.
This paper presents the design and control of a novel gripper device with a passive type of compliant constant-force mechanism. The function of constant-force output is achieved by using the ...combination of a positive-stiffness and a negative-stiffness mechanism. The negative stiffness is generated by a bistable buckled fixed-guided beam. Conventionally, when gripping a target-object using a high-stiffness gripper, both the displacement control and force control are required to prevent the damage of the object. The uniqueness of the developed constant-force gripper is that it can eliminate the dependence on force control while keeping the force constant via its mechanical structure. The gripper performance is evaluated by the established analytical model and nonlinear finite-element analysis, and validated through experimental study on a fabricated prototype. To achieve a precise position output of the gripper jaw, a discrete-time variable structure control strategy based on a nonswitching type of reaching law is realized. The effectiveness of the gripper system is verified by conducting experimental studies on grasp-hold-release operations of a micro copper wire.
In present study, the feasibility of applying a natural adsorbent with Fe3+ modification (Fe3+-modified argillaceous limestone, FAL) on the competitive adsorption of heavy metals (i.e., Cd2+, Pb2+ ...and Ni2+) was evaluated. The current results revealed an efficient adsorption on Cd2+, Pb2+ and Ni2+ in mono-metal system. Further experiments demonstrated a high selectivity of Pb2+ during the competitive adsorption of Cd2+, Pb2+ and Ni2+. The adsorption selectivity of the metal ions followed the order of Pb ≫ Cd > Ni. In addition, both pH and ionic strength are important factors affecting the metal adsorptions. It is interestingly that various NOMs (i.e., humic acid (HA) and glycine (Gly)) exerted different effects on the adsorption behaviors, probably due to the different affinities for Pb2+, Cd2+ and Ni2+ and the redistribution of newly-formed metal-DOM complexes. X-ray photoelectron spectroscopy (XPS) analysis together with X-ray diffraction (XRD) and energy dispersive spectrometer (EDS) analysis revealed that the metal adsorptions were mainly regulated via the synergistic mechanisms of ion exchange by Na+, Ca2+, and Al3+, precipitation to form CdCO3 and Pb2(OH)2(CO3)2, as well as complexes of FAL-OPb and FAL-ONi by hydroxyl groups on the surface of FAL. The application of FAL would be a promising option in leading to an efficient heavy metal removal.
Display omitted
•Fe3+-modified argillaceous limestone showed effective adsorption of Cd2+/Pb2+/Ni2+.•The adsorption selectivity of the metals followed the order of Pb ≫ Cd > Ni.•Both pH and ionic strength played key roles in metal adsorption.•Humic acid and glycine exerted distinct effects on adsorption behaviors of metals.•Mechanisms of metal adsorption were ion exchange, precipitation and complexation.
Land surface phenology (LSP) has been widely used as the “footprint” of urbanization and global climate change. Shifts of LSP have cascading effects on food production, carbon sequestration, water ...consumption, biodiversity, and public health. Previous studies mainly focused on investigating the effects of urbanization on the spatial patterns of LSP by comparing phenological metrics, e.g. start of season (SOS) and end of season (EOS), between urban center and the surrounding rural regions. However, it remains unclear how urbanization-induced land cover conversions and climate change jointly influence the temporal variations of SOS and EOS within the urban ecosystem. To fill this knowledge gap, we utilized daily two-band enhanced vegetation index, daily meteorological record, and annual land cover dataset to investigate the respective impacts of urbanization and climate change on temporal shifts of LSP between the post- and the pre-urbanization periods over 196 large cities in the northern mid-latitudes. We found 51% of the cities experienced an advanced SOS with an average of −6.39 ± 5.82 days after urbanization has occurred, while the remaining 49% of the cities had a delayed SOS with an average of 7.56 ± 5.63 days. We also found a later EOS at 53% of the cities and an earlier EOS at 47% of the cities with an average of 8.43 ± 7.59 and −5.57 ± 4.99 days between the post- and pre-urbanization periods, respectively. Multiple linear regression analysis indicates that climate variables (i.e. temperature, precipitation, and insolation) play dominant roles in regulating the temporal shifts of LSP. Furthermore, the earlier SOS and later EOS were significantly correlated with the amplitude of urbanization (i.e. increase of impervious surface area) in cities after controlling for effects of climate factors. These patterns were generally consistent across eight climate zones. Our findings provide critical information in modeling natural and anthropogenic effects on urban ecosystem, with important benefits for urban sustainability and biodiversity conservation.
•Post - and pre-urbanization periods were identified for 196 large cities.•Half of the cities experienced earlier SOS and delayed EOS after urbanization.•Climate change played a dominant role in regulating temporal shifts of phenology.•Urbanization can either amplified or off-set climate effects on phenology.•Influences of urbanization and climate change vary by climate background.
► We assess heavy metal pollution in the water level fluctuation zone of the Three Gorges Reservoir. ► There are higher concentrations of As, Cd, Pb, Cu, and Zn in the upper and low reaches. ► As and ...Cd are the primary pollutants originated from domestic sewage and industrial wastes before submergence. ► The major contaminants are Hg, Cd, and Pb with primary contribution from traffic exhaust and industrial effluent after submergence.
The water-level-fluctuation zone (WLFZ) between the elevations of 145–175
m in China's Three Gorges Reservoir has experienced a novel hydrological regime with half a year (May–September) exposed in summer and another half (October–April) submerged in winter. In September 2008 (before submergence) and June 2009 (after submergence), soil samples were collected in 12 sites in the WLFZ and heavy metals (Hg, As, Cr, Cd, Pb, Cu, Zn, Fe, and Mn) were determined. Enrichment factor (EF), factor analysis (FA), and factor analysis-multiple linear regression (FA-MLR) were employed for heavy metal pollution assessment, source identification, and source apportionment, respectively. Results demonstrate spatial variability in heavy metals before and after submergence and elements of As, Cd, Pb, Cu, and Zn are higher in the upper and low reaches. FA and FA-MLR reveal that As and Cd are the primary pollutants before submergence, and over 45% of As originates from domestic sewage and 59% of Cd from industrial wastes. After submergence, the major contaminants are Hg, Cd, and Pb, and traffic exhaust contributes approximately 81% to Hg and industrial effluent accounts about 36% and 73% for Cd and Pb, respectively. Our results suggest that increased shipping and industrial wastes have deposited large amounts of heavy metals which have been accumulated in the WLFZ during submergence period.
Strategies for reducing cadmium (Cd) content in polluted farmland soils are currently limited. A type of composite with nanoparticles incorporated into a hydrogel have been developed to efficiently ...remove heavy metals from sewage, but their application in soils faces challenges, such as organic hydrogel degradation due to oxygen exposure and slow Cd2+ release from soil constituents. To overcome these challenges, a composite with superior stability for long-term application in soil is required. In this study, ferrous sulfide (FeS) nanoparticle@lignin hydrogel composites were developed. The lignin-based hydrogels inherited lignin’s natural mechanical and environmental stability and the FeS nanoparticles efficiently adsorbed Cd2+ and enhanced Cd2+ desorption from soils by producing H+. The high sorption capacity (833.3 g kg−1) of the composite was attributed to four proposed mechanisms, including cadmium sulfide (CdS) precipitation via chemical reaction (84.06%), lignin complexation (13.19%), hydrogel swelling (0.61%), and nanoparticle sorption (2.15%). In addition, Fe2+ displaced from the composite was gradually oxidized to form solid iron oxide hydroxide, which increased Cd2+ sorption. The composite significantly reduced the total, surfactant-soluble, and fixed Cd in heavily and lightly polluted paddy soils by 22.4–49.6%, 13.5–68.6%, and 40.1–16.6%, respectively, in 7 days.
Display omitted
•The synthesized nFeS@lh reduced soil total and stable Cd by 16.6–49.6% in 7 days.•Lignin hydrogel realized the efficient dispersion and recovery of FeS nanoparticles.•The nFeS@lh had a high Cd sorption capacity (833.3 g kg−1) and mechanical strength.•The relative contributions of four sorption mechanisms were quantitively evaluated.
It is imperative to have an in-depth understanding of the change in methyl and methylene groups during low-temperature oxidation of coal not only for detecting and preventing coal spontaneous ...combustion in coal mining industry and for reducing emissions of hazardous gases. The Fourier transform infrared (FTIR) spectroscopy equipped with in-situ reactor cell was introduced to investigate the change in the methyl and methylene groups of different classes of coals (lignite, sub-bituminous and bituminous) during their low-temperature oxidation at temperatures below 230°C. The reaction kinetics of the methyl and methylene groups was obtained from the real-time measurements of the change in aliphatic hydrocarbon content in coal matrix. These results clearly show that the low-temperature oxidation of coal can be separated into three stages with respect to the activation energies for the change in methyl or methylene groups, which indicates that the reaction regime for the change in methyl or methylene occurrence switches during the spontaneous combustion of coal. At the first and second stages the reaction activation energies of the methyl or methylene groups showed significant difference for the three types of coal, and at the third stage the activation energies were very close for all of the coals. These results indicate that the spontaneous combustion propensity of coals can be evaluated on the basis of the kinetic parameters at the first and second stages. Furthermore, the relationship between the changes in methyl and methylene groups and the emission of carbon oxides (CO2 and CO) was also explored on the basis of the experimental findings.
•Real-time change in aliphatic hydrocarbon was investigated using in-situ FTIR.•Kinetics of change in aliphatic hydrocarbon from different coals was studied.•Reaction regime for change in aliphatic hydrocarbon varies during coal self-heating.•Relationship between aliphatic hydrocarbon and carbon oxides was explored.
Fracture initiation and propagation in fluid-saturated rocks are controlled by interaction between fluid flow and rock deformation. The description of hydromechanical coupling is essential for ...modeling the fracture process. In this paper, an improved hydromechanical model is proposed in the framework of the particle flow simulation method. This model provides a better description of hydraulic properties before and after breakage of bonds and can efficiently describe fluid flow through porous rock matrix and along fractures. The efficiency of the proposed model is first assessed by comparisons with analytical solutions and typical experimental evidences. A series of numerical simulations are then realized to investigate effects of some key parameters such as confining stress, fluid injection rate and viscosity on the initiation and propagation of fractures.
PDF
