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Defect Engineering in Two-Dimensional Graphitic Carbon Nitride and Application to Photocatalytic Air Purification
No full textSince the pioneering work on polychlorinated biphenyl photodegradation by Carey in 1976, photocatalytic technology has emerged as a promising and sustainable strategy to overcome the significant challenges posed by energy crisis and environmental pollution. In photocatalysis, sunlight, which is an inexhaustible source of energy, is utilized to generate strongly active species on the surface of the photocatalyst for triggering photo-redox reactions toward the successful removal of environmental pollutants, or for water splitting. The photocatalytic performance is related to the photoabsorption, photoinduced carrier separation, and redox ability of the semiconductor employed as the photocatalyst. Apart from traditional and noble metal oxide semiconductors such as P25, bismuth-based compounds, and Pt-based compounds, 2D g-C3N4 is now identified to have enormous potential in photocatalysis owing to the special pi-pi conjugated bond in its structure. However, some inherent drawbacks of the conventional g-C3N4, including the insufficient visible-light absorption ability, fast recombination of photogenerated electron-hole pairs, and low quantum efficiency, decrease its photocatalytic activity and limit its application. To date, various strategies such as heterojunction fabrication, special morphology design, and element doping have been adopted to tune the physicochemical properties of g-C3N4. Recent studies have highlighted the potential of defect engineering for boosting the light harvesting, charge separation, and adsorption efficiency of g-C3N4 by tailoring the local surface microstructure, electronic structure, and carrier concentration. In this review, we summarize cutting-edge achievements related to g-C3N4 modified with classified non-external-caused defects (carbon vacancies, nitrogen vacancies, etc.) and external-caused defects (doping and functionalization) for optimizing the photocatalytic performance in water splitting, removal of contaminants in the gas phase and wastewater, nitrogen fixation, etc. The distinctive roles of various defects in the g-C3N4 skeleton in the photocatalytic process are also summarized. Moreover, the practical application of 2D g-C3N4 in air pollution control is highlighted. Finally, the ongoing challenges and perspectives of defective g-C(3)N(4 )are presented. The overarching aim of this article is to provide a useful scaffold for future research and application studies on defect-modulated g-C3N4
Constraining Paleoearthquakes by Combining Faulted Stratigraphy and Microgeomorphology: A Case Study on the Haiyuan Fault, Northwestern China
No full textSurface-rupturing strong earthquakes will leave evidence distributed along fault zones. The combination of paleoearthquake trench excavation and faulted microgeomorphic analysis at the same site provides more comprehensive knowledge of paleoearth-quakes than either method could accomplish alone. In this article, we report on our use of trench excavation and dating, together with a 5-cm resolution digital elevation model obtained from an unmanned aerial vehicle based on the structure from motion photogrammetry technology, to investigate the timing and size of strong paleoearth-quake events in the Dashagou site near the west end of the Haiyuan fault, which rup-tured in the 1920 Haiyuan earthquake. The result reveals that at least four strong paleoearthquake events with the same or even higher magnitude (including the 1920 Haiyuan earthquake) have occurred along the west end of the Haiyuan fault since the mid-Holocene. Event IV occurred shortly before 6.0 ka with a horizontal displace-ment of 4.27 +/- 1.50 m and a vertical displacement of 0.70 +/- 0.39 m. Event III occurred at approximately 4.65 +/- 0.45 ka with a horizontal displacement of 5.45 +/- 1.25 m and a vertical displacement of 0.38 +/- 0.23 m. Event II occurred at approximately 1.0 ka with a horizontal displacement of 3.86 +/- 0.90 m and a vertical displacement of 0.55 +/- 0.27 m. The most recent event was the 1920 Haiyuan earthquake, with a horizontal displace-ment of 2.15 +/- 0.82 m and a vertical displacement of 0.26 +/- 0.12 m. From the results of these four events, we can certainly conclude that the fault has mainly maintained the strike-slip kinematic pattern over the past 6 ka. These observations highlight the benefits of combining trench excavation and faulted microgeomorphology to gain a more complete understanding of paleoearthquakes
Holocene Environmental Archaeology of the Yangtze River Valley in China: A Review
No full textThe Yangtze River Valley is an important economic region and one of the cradles of human civilization. It is also the site of frequent floods, droughts, and other natural disasters. Conducting Holocene environmental archaeology research in this region is of great importance when studying the evolution of the relationship between humans and the environment and the interactive effects humans had on the environment from 10.0 to 3.0 ka BP, for which no written records exist. This review provides a comprehensive summary of materials that have been published over the past several decades concerning Holocene environmental archaeology in the Yangtze River Valley, to further understand large-scale regional Holocene environmental and cultural interaction within this area. The results show that: (1) in recent years, Holocene envi-ronmental archaeology research in the Yangtze River Valley has primarily taken paleoflood and sea-level change stratigraphical events to be the foundational threads for study. This began with research on the spatiotemporal distribution of archaeological sites, typical archaeological site stratigraphy, and research on background features concerning environmental evolution recorded by the regional natural sedimentary strata. (2) Significant progress has been made at the upper, middle, and lower reaches of the Yangtze River, indicating that Holocene environmental ar-chaeology research along the Yangtze River Valley is deepening and broadening. (3) Dramatic changes to Neolithic cultures that occurred approximately 4.0 ka BP were influenced by climate change and associated consequences, although the impacts differed on the various Neolithic cultures in the Yangtze River Valley. Local topography, regional climate, and varying survival strategies may have contributed to these differences. (4) Newly-published research pays particular attention to the sedimentary records of the past with resolutions as high as one year to several months, the degree to which humans altered the quality of their natural environment, and human adjustments to settlement and subsistence practices during periods of Holocene climate change. The application of technologies such as remote sensing, geographic information systems (GIS), and molecular biological analysis are also gradually being extended into the research field of Holocene environmental archaeology in the Yangtze River Valley
Determination of ultra-low level Am-241 in soil and sediment using chemical separation and triple quadrupole inductively coupled plasma mass spectrometry measurement with He-NH3 as collision-reaction gas
No full textInductively coupled plasma mass spectrometry (ICP-MS) has been becoming a competitive technique for the measurement of trace americium isotopes, but the isobaric and polyatomic ions interference (Pu-241, (PbCl)-Pb-206-Cl-35, (PbCl)-Pb-204-Cl-37, etc.) will deteriorate the analysis accuracy for soil and sediment samples with high concentration of interfering elements (e.g., Pb). This study developed a novel analysis method to determine Am-241 using ICP-MS with tandem quadrupoles and collision/reaction cell. The interference of isobaric and polyatomic ions was effectively removed by the mass filter of quadrupole and reactions with NH3, with the contribution efficiency of interfering elements at m/z 241 or 243 lower than 1 x 10(-8), and the measurement sensitivity in this mode relatively (1170 Mcps/(mg/L)) higher than in other modes. The detection limit of 0.091 fg/g for Am-241 was achieved, 3 times better than other types of ICP-MS (Q-ICP-MS, SF-ICP-MS, etc.). The collision focusing by He and the chemical reaction with NH3 played an important role in the improvement of Am sensitivity and elimination of polyatomic ions. This study suggested that the presence of Cl- could significantly increase the polyatomic ions interference ((PbCl)-Pb-206-Cl-35, (PbCl)-Pb-204-Cl-37, (PbCl)-Pb-208-Cl-35, (PbCl)-Pb-206-Cl-37, etc.) because of the high Pb concentration in the soil or sediment samples, and thus should be completely removed. The developed method had been validated with two certified reference materials of soil (IAEA-375 and IAEA-Soil-6) and successfully applied to measure Am-241 concentrations in seven soil samples collected in different regions of China and one sediment sample collected in Denmark
Climate Change in Rwanda: The Observed Changes in Daily Maximum and Minimum Surface Air Temperatures during 1961-2014
No full textRwanda has experienced high temperature rising phenomena over the last decades and hence, highly vulnerable to climate change. This paper examined the spatial and temporal variations of daily maximum and minimum surface air temperature (Tmin and Tmax) and diurnal temperature range (DTR). It studied variables at monthly, seasonal and annual time-scales from 1961 to 2014. The study applied various statistical methods such as ordinary least-square fitting, Mann-Kendall, Sen' slope and Sequential Mann-Kendall statistical test to the new reconstructed ENACTS dataset that cover the period from 1983 to 2014 while pre-1983s recorded data from 24 meteorological stations have been added to complete the lengthiness of ENACTS data. The January to February season did not show a significant trend at seasonal time-scales. The authors decided only to consider March-to-May, June-to-August and October-to-December seasons for further analyses. Topography impacts on temperature classified stations into three regions: region one (R1) (1,000-1,500 m), region two (R2) (1,500-2,000 m) and region three (R3) (>= 2,000 m). With high confidence, the results indicate a significant positive trend in both Tmin and Tmax in all three regions during the whole study period. However, the magnitude rate of temperatures change is different in three regions and it varies in seasonal and annual scale. The spatial distributions of Tmax and Tmin represent a siginificant warming trend over the whole country notably since the early 1980s. Surprisingly, Tmin increased at a faster rate than Tmax in R3 (0.27 vs. 0.07 degrees C/decade in March-to-May) and (0.29 vs. 0.04 degrees C/decade in October-to-December), resulting in a significant decrease in the DTR. This is another confirmation of warming in Rwanda. The mutation test application exhibited most of the abrupt changes in the seasonal and annual Tmax and Tmin trends between 1984 and 1990. The present work mainly focus on the spatial and temporal variability of Tmin, Tmax and DTR in Rwanda and their relationship with elevation change, leaving a gap in other potential cause factors explored in the future
A study of the gas-water characteristics and their implications for the coalbed methane accumulation modes in the Southern Junggar Basin, China
No full textGas and water samples were collected from coalbed methane (CBM) wells, rivers, and springs in the southern Junggar Basin (SJB). These samples were analyzed for gas composition, stable isotopes, 16s ribosomal ribonucleic acid sequence, chemical compositions, and radioisotopes. The objective of this study was to understand CBM genesis in the Junggar Basin, the reason for abnormal CO2 accumulation, the development of microbial communities, the source of coalbed water, and the timing of methanogenesis. The CBM genesis is complex in the SJB, but it is closely related to microbial activities. The stagnant zone, which experiences limited groundwater recharge, may represent a relatively closed system where CO2 is easily trapped and the residual CO2 becomes progressively enriched in C-13. Only two families of methanogens (i. e., Methanobacteriaceae and Methanospirillaceae) are present in the coalbed waters, indicating that CO2 reduction is the main pathway for generating microbial gas. The coalbed water samples from the Houxia and Manasi-Hutubi regions plot around the local meteoric water line (LMWL), indicating recharge by modern meteoric water and rivers. However, the samples from the Miquan and Fukang regions plot below the LMWL, reflecting older snowmelt water recharge. Isotopic dating indicates that the age of coalbed water in the Miquan and Fukang regions is 43.5-2000 ka. Early coalification and later hydrological events collectively determined the regional variations in CBM genesis and gas composition in the SJB
Pre-nuclear values for I-129/I-127 in Chinese sediments and their geochronological implications
No full textLarge variations in pre-nuclear I-129/I-127 ratios in terrestrial environments have been observed, but few investigations have been carried out on how the isotopic composition varies and how it is affected by the terrestrial environment. In this work, lake and river sediments were studied for the first time with the aim of exploring the possible connections between the natural iodine isotopic composition and its sources. The observed higher natural I-129/I-127 ratios compared to those in the marine system suggested that the isotopes did not reach a steady state. Decreasing I-129/I-127 ratios with increasing I-127 contents indicated perturbation of old iodine, and a narrower range of initial equilibrium ratios was suggested based on pre-nuclear ratios derived in the studied sediments
Improved estimation of PM2.5 brown carbon contributions to filter light attenuation
No full textMultiwavelength light attenuation measurements have been acquired as part of thermal/optical carbon analysis in the U.S. Chemical Speciation Network (CSN) and the Interagency Monitoring of PROtected Visual Environments (IMPROVE) network beginning in 2016. These are used to estimate PM2.5 brown carbon (BrC) contributions to light absorption at various wavelengths, a useful method for separating biomass burning contributions from other sources. Attenuation of light transmitted through the filter deviates from Beers Law as the mass of light absorbing materials increase. This study estimates the effects of these deviations with empirical adjustment factors applied to samples for CSN from 2016 to 2017 and for IMPROVE from 2016 to 2019. Accounting for the filter loading effect results in an annual average increase of similar to 6-7% BrC contribution to light attenuation: from 3.6% to 10.7% for the urban, more heavily loaded CSN samples; and from 23.7% to 29.5% for the non-urban IMPROVE samples. An alternative method is examined for BrC and black carbon (BC) adjustments by calculating the Absorption Angstrom Exponent (AAE) for BC (i.e., AAE(BC)) based on the ratios of 635 nm/780 nm light attenuation rather than assuming AAE(BC) of unity. These paired-wavelength calculations result in a median AAE(BC) of 0.76 for CSN and 0.8 for IMPROVE, with the majority of samples (i.e., 91% of CSN and 70% of IMPROVE) showing AAE(BC) < 1. By assuming negligible contributions from BrC to AAE at longer wavelengths, the amount of light attenuation at shorter wavelengths (e.g., 405 nm) where BrC is dominant can be calculated. The paired-wavelength method applied to the filter loading adjusted data has a greater effect on urban (fresh) than on non-urban (aged) aerosols, resulting in a factor of two increase in annual averaged BrC light attenuation (from 10.7% to 21.6%) for CSN and by a factor of 1.11 (from 29.5% to 32.7%) for IMPROVE samples. This result demonstrates the importance of particle loading and AAE correction on quantifying BrC light attenuation from multi-wavelength thermal/optical analysis. Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences
Cryogenic cave carbonates in the Dolomites (northern Italy): insights into Younger Dryas cooling and seasonal precipitation
No full textIn the European Alps, the Younger Dryas (YD) was characterised by the last major glacier advance, with equilibrium line altitudes being similar to 220 to 290 m lower than during the Little Ice Age, and also by the development of rock glaciers. Dating of these geomorphic features, however, is associated with substantial uncertainties, leading to considerable ambiguities regarding the internal structure of this stadial, which is the most intensively studied one of the last glacial period. Here, we provide robust physical evidence based on Th-230-dated cryogenic cave carbonates (CCCs) from a cave located at 2274 m a.s.l. in the Dolomites of northern Italy coupled with thermal modelling, indicating that early YD winters were only moderately cold in this part of the Alps. More precisely, we find that the mean annual air temperature dropped <= 3 degrees C at the Allerod-YD transition. Our data suggest that autumns and early winters in the early part of the YD were relatively snow-rich, resulting in stable winter snow cover. The latter insulated the shallow subsurface in winter and allowed the cave interior to remain close to the freezing point (0 degrees C) year-round, promoting CCC formation. The main phase of CCC precipitation at similar to 12.2 ka coincided with the mid-YD transition recorded in other archives across Europe. Based on thermal modelling we propose that CCC formation at similar to 12.2 ka was most likely associated with a slight warming of approximately +1 degrees C in conjunction with drier autumns and early winters in the second half of the YD. These changes triggered CCC formation in this Alpine cave as well as ice glacier retreat and rock glacier expansion across the Alps
High light absorption and radiative forcing contributions of primary brown carbon and black carbon to urban aerosol
No full textTo investigate the seasonal variation of black carbon (BC) and brown carbon (BrC) light absorption at an urban site in Xi'an, China, an annual measurement was conducted by using a 7-wavelengths Aethalometer (model AE33). The results showed the aerosol absorptions were observed approximately 5-8 times greater in winter (395 Mm(-1) at 370 nm and 99 Mm(-1) at 880 nm, respectively) than those in autumn (49 Mm(-1) at 370 nm and 18 Mm(-1) at 880 nm, respectively). BC was the dominant light absorbing carbonaceous component at all wavelengths (from 370 nm to 950 nm). BrC absorption (370 nm) maximized during winter with 41% of the total aerosol absorption, and was lower in autumn (16%). The light absorption contributions of primary BrC (BrCpri) were higher than that of secondary BrC (BrCsec) in four seasons, with more than 67% of total BrC light absorption. The seasonal absorption Angstrom exponents (AAEs) of BrC, BrCpri, and BrCsec were directly derived with the highest values (3.91, 3.69, and 4.94, respectively) in wintertime. The BrCpri contributed much higher solar absorptions percentage to BC than BrCsec. The results underlined the primary emissions of urban BrCpri and BC, implying that the strategies of energy efficiency enhancement and energy structure reformation will be very important in reducing primary emissions in China. Such investigations should be conducted in other developing countries with severe air pollution.
(c) 2020 Published by Elsevier B.V. on behalf of International Association for Gondwana Research