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Highly Selective Photocatalytic CO2 Methanation with Water Vapor on Single-Atom Platinum-Decorated Defective Carbon Nitride
No full textSolar-driven CO2 methanation with water is an important route to simultaneously address carbon neutrality and produce fuels. It is challenging to achieve high selectivity in CO2 methanation due to competing reactions. Nonetheless, aspects of the catalyst design can be controlled with meaningful effects on the catalytic outcomes. We report highly selective CO2 methanation with water vapor using a photocatalyst that integrates polymeric carbon nitride (CN) with single Pt atoms. As revealed by experimental characterization and theoretical simulations, the widely explored Pt-CN catalyst is adapted for selective CO2 methanation with our rationally designed synthetic method. The synthesis creates defects in CN along with formation of hydroxyl groups proximal to the coordinated Pt atoms. The photocatalyst exhibits high activity and carbon selectivity (99 %) for CH4 production in photocatalytic CO2 reduction with pure water. This work provides atomic scale insight into the design of photocatalysts for selective CO2 methanation
Seasonal River Chemistry and Lithium Isotopes in the Min Jiang at Eastern Tibetan Plateau: Roles of Silicate Weathering and Hydrology
No full textRiverine lithium (Li) isotopes have been considered as a robust tracer for silicate weathering, but processes controlling riverine delta Li-7 ratios remain controversial. To address the impacts of weathering and hydrology on riverine delta Li-7, the seasonal variation of water chemistry in the Min Jiang at the eastern Tibetan Plateau was investigated over December of 2009 to the end of 2010. The results showed distinct seasonal variations in ionic chemistry and delta Li-7. Increased river discharge in the monsoon season diluted dissolved ions, and monsoonal hydrological changes caused frequent delta Li-7 fluctuations. High discharge caused by monsoonal rainfall reduced Li isotope fractionation by shortened rock-fluid interaction time, resulting in lower delta Li-7, whereas the input of high delta Li-7 groundwater and landslide seepage elevated riverine delta Li-7, together with lengthened rock-fluid interaction time in less rain intervals. Based on the high-resolution sampling strategy and dataset over one hydrological year, this study highlights that changes of hydrological conditions can have a significant impact on weathering processes and water sources, and therefore on riverine delta Li-7 variation
Nitrous acid emission from soil bacteria and related environmental effect over the North China Plain
No full textSoil bacteria could be one of the important sources for ambient HONO. However, the HONO emission from soil bacteria over North China Plain (NCP) with vast croplands has not yet been evaluated. In this study, high-resolution simulations are created to explore the HONO emission from soil bacteria over NCP and related in-fluences on atmospheric chemistry. Ground measurements of critical air pollutants including O-3, HONO, and PM2.5 compositions are incorporated to constrain the model simulations. Results show that abundant HONO is emitted from soil bacteria over NCP during summertime and the emission rate varies dramatically for different areas (about 0.2 kg km(-2) d(-1) - 2.0 kg km(-2) d(-1)). The HONO emission rate presents clear diurnal cycles with peaks of 1.5 kg km(-2) d(-1) in the afternoon and valleys of 0.4 kg km(-2) d(-1) during the early morning hours. The resulting HONO concentration ranges from 0.2 mu g m(-3) to 1.4 mu gm(-3), which predominates the total HONO concentration in ambient air, particularly in western NCP. The soil bacteria source can significantly alter the diurnal cycles of ambient HONO and OH concentrations over NCP, but only slightly change O-3 and PM2.5 concentrations via participating photochemistry and secondary aerosol formations. These results highlight the pressing need for the involvement of HONO emission from soil bacteria in modeling studies regarding atmo-spheric chemistry, particularly in rural areas
Controls on seasonal erosion behavior and potential increase in sediment evacuation in the warming Tibetan Plateau
No full textGlobal warming and intensified climate variability have greatly affected Earth's surface processes and continental sediment evacuation. River suspended sediment is a sensitive indicator for tracing seasonal surface erosion, but details of the rates of sediment generation and evacuation, and their connections with nowadays warming climate are not entirely clear, particularly in Tibet and other high-altitude areas where field observations remain scarce. Here, we investigate daily to seasonal river sediment transport dynamics between the cold, permafrost-dominated northeastern Tibetan Plateau and warm, non-permafrost Sichuan and Taiwan regions. Our results show that at a given river water discharge, greater river suspended sediment was evacuated during the pre-monsoon season (April-Mid June) relative to other seasons in the cold NE Tibetan catchments. In contrast, no such phenomenon was observed in the warm, non-permafrost regions. These comparisons likely indicate a center role of freeze-thaw processes on loose sediment generation, which enhanced sediment output. Hydmmeteorological records show up to similar to 2 degrees C warming in the NE Tibetan Plateau since the past 30 years, coupled with an 8-fold increase in sediment transport. We suggest that continuous warming climate may further accelerate sediment and soil carbon release in the Tibetan Plateau and other global permafrost-dominated areas, which in turn influences climate feedback
Efficacy of rice husk biochar and compost amendments on the translocation, bioavailability, and heavy metals speciation in contaminated soil: Role of free radical production in maize (Zea mays L.)
No full textThe present study investigated the effectiveness of rice husk biochar (RHB) and chicken manure compost (CMC) amendments (T1: Control, T2: 2% RHB, T3: 2% CMC, T4: 1% RHB + 1% CMC) on the potential mobility of heavy metals (HMs) and the growth of maize plants in contaminated soil. Results indicated that the potential mobile fraction of Cr (44.43%), Ni (29.93%), and Zn (44.68%) accounted for T2, whereas Cu (69.33%) and Pb (34.38%) for T4 of their total amount. Amending the soil resulted in a significant increase in maize growth and biomass production, whereas T4 showed an increment of 156% in fresh maize weight. While root biomass was highest with T2 treatment, which increased fresh and dry weight by 35% and 90%. The bioaccumulation factor (BF) and Translocation factor (TF) were less than one indicating that the low content of HMs accumulation in roots from soil and slight transferred to shoot (edible tissues). The results of electron paramagnetic resonance (EPR) revealed the generation of hydroxyl radical (center dot OH) as reactive oxygen species (ROS) in root and shoot of maize seedling. The X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectrum confirmed the complexation and the adsorption of HMs as prominent mechanisms owing to their complexation with hy-droxyl, carbonyl and carboxyl groups. The results indicate that the RHB and CMC, especially their combination can be used effectively for the remediation of HMs by reducing their toxicity and bioavailability in food matrices and soils
Carbonate weathering dominates magnesium isotopes in large rivers: Clues from the Yangtze River
No full textCarbonate weathering regulates the short-term carbon (C) cycle and global climate due to its fast response to hydrological processes. The carbonate weathering flux needs to be well constrained to better understand the climate change at short time scale. Riverine magnesium (Mg) isotopes are sensitive to primary mineral dissolution and so have great potential to trace carbonate weathering. Global large rivers draining continental crust dominate weathering flux to the oceans, but how riverine Mg isotopes respond to carbonate weathering remains unclear. The Yangtze River drainage basin (YRDB) was selected to test the robustness of riverine Mg isotopes (delta Mg-26) in tracing continental carbonate weathering because it spans a wide range in lithology, geomorphology and climate. The riverine delta Mg-26 values within the YRDB show a decreasing trend from the headwater to the mainstream ranging from -1.36 parts per thousand to -0.59 parts per thousand The dissolved delta Mg-26 have strong negative correlations with carbonate weathering rate and intensity within the YRDB, indicating a sensitive response of riverine delta Mg-26 to the carbonate weathering flux. In a compilation of Mg fluxes and delta Mg-26 in the world's largest rivers, there is similar dominance of carbonate weathering on riverine Mg fluxes and isotopes. Therefore, we propose that riverine delta Mg-26 in large rivers are a robust tracer of carbonate weathering intensity. Intensifying carbonate weathering under global warming tends to increase riverine Mg and C fluxes to the oceans and thus the atmospheric CO2 sink at the millennial time scale
A Route Map for Successful Applications of Geographically Weighted Regression
No full textGeographically Weighted Regression (GWR) is increasingly used in spatial analyses of social and environmental data. It allows spatial heterogeneities in processes and relationships to be investigated through a series of local regression models rather than a single global one. Standard GWR assumes that relationships between the response and predictor variables operate at the same spatial scale, which is frequently not the case. To address this, several GWR variants have been proposed. This paper describes a route map to decide whether to use a GWR model or not, and if so which of three core variants to apply: a standard GWR, a mixed GWR or a multiscale GWR (MS-GWR). The route map comprises 3 primary steps that should always be undertaken: (1) a basic linear regression, (2) a MS-GWR, and (3) investigations of the results of these in order to decide whether to use a GWR approach, and if so for determining the appropriate GWR variant. The paper also highlights the importance of investigating a number of secondary issues at global and local scales including collinearity, the influence of outliers, and dependent error terms. Code and data for the case study used to illustrate the route map are provided
Lithium isotopic composition of soil pore water: Responses to evapotranspiration
No full textLithium isotopes show great potential to trace Earth surface processes due to the large mass discrimination between Li-6 and Li-7 associated with clay uptake. However, factors controlling the Li isotopic composition (delta Li-7) of river water, especially those with a water-bedrock delta Li-7 difference higher than that of the equilibrium fractionation associated with clay formation (Delta(W-C)), have not yet been fully resolved. Traditional interpretation involves the Rayleigh distillation, but it unrealistically separates the stage of clay formation from that of silicate dissolution using fractionation factors that are much lower than laboratories can constrain. We propose an in situ mechanism that simulates high delta Li-7 by evapotranspiration. A model with coupled mineral dissolution and clay precipitation shows that evaporative enrichment of pore-water Li progressively increases the incorporation of Li into clays with light delta Li-7, resulting in higher delta Li-7 values in the residual water than Delta(W-C). We also provide evidence from the Chinese Loess Plateau, where an evaporative effect readily explains the observed delta Li-7. The influence of evapotranspiration on riverine delta Li-7 implies that changes in aridity may partly explain the variations of seawater delta Li-7. The same principle may also apply to other stable isotopic systems whereby incorporation into secondary precipitates controls the isotopic fractionation
Topographic and Climatic Control on Chemical Weathering of Mountainous Riverine Sediments of Hainan Island, South China Sea
No full textHainan Island, the largest island in the northwestern South China Sea (SCS), is characterized by many tropical mountainous rivers that flow into the SCS. The geochemistry of these riverine sediments and the factors controlling the weathering intensity are still not well understood. In this study, sedimentary geochemistry has been investigated by using 45 riverine sediments collected from 18 major rivers on Hainan Island. The Hainan riverine sediments are only first-cycle rather than polycyclic sediments, and they faithfully reflect the present weathering intensity. The high and steady values of the Chemical Index of Alteration (CIA) indicate that the riverine sediments at different parts of the Hainan Island have overall undergone intensive chemical weathering. The low values of Weathering Index of Parker (WIP) and high alpha(Ca) , alpha(K), alpha(Sr), and alpha(Ba) values of north Hainan indicate the highest weathering degree, which is mainly influenced by the flat topography. The values of alpha(Na) , alpha(K), alpha(Sr), and alpha(Ba) of southwest Hainan which are higher than those of east Hainan suggest that the leaching of elements such as Na, K, Sr, and Ba is more extreme (i.e., more intensive weathering). These are mainly caused by the different physical denudation due to different summer precipitation. Overall, the Hainan Island is featured by intensive chemical weathering and is classified as a typical transport-limited weathering regime. Therefore, the geochemistry of the riverine sediments of the Hainan Island is different from that of sediments in tectonically active regions (e.g., Taiwan Island)
Quantifying the spatio-temporal variability of total water content in seasonally frozen soil using actively heated fiber Bragg grating sensing
No full textActively heated fiber-optic (AHFO) method has become a research focus for soil water content measurement in recent years due to its advantages such as small size, distributed measurement and good durability. However, the AHFO method is presently coupled to distributed temperature sensing (AHFO-DTS) which is inevitably limited by low temperature measurement accuracy and spatial resolution. Furthermore, the existing studies about AHFO method do not consider the existence of frozen soil caused by temperature changes, which further limits its application in the field. Here a new method for total water content measurement in frozen soil using actively heated fiber Bragg grating (AHFO-FBG) sensing is proposed for the first time, which directly determines the calibration formulas of frozen soil under different soil temperatures from the calibration formula of unfrozen soil. Moreover, the feasibility and reliability of AHFO-FBG technology are proved through laboratory calibration tests and in-situ monitoring data of Chinese loess, and the temporal and spatial distribution of total water content in shallow seasonally frozen loess is also revealed. It is suggested that the AHFO-FBG technology can complement with the traditional techniques to achieve quasi-distributed and in-situ total water content monitoring in seasonally frozen soil