Applied Geochemistry最新文献

Although light-independent production of reactive oxygen species (ROS) in subsurface environments has been increasingly documented, the processes controlling ROS transformations at redox interfaces remain poorly understood. The Fe(III)-reducing facultative anaerobe Shewanella oneidensis promotes light-independent ROS production in the presence of Fe(III) when alternately exposed to oxic and anoxic conditions and represents a good model of microbial processes at redox interfaces. In this study, S. oneidensis was incubated under three different orthophosphate concentrations from low natural conditions to high growth-promoting conditions to identify the processes regulating the light-independent production of ROS during redox oscillations and determine the impact of this important nutrient on ROS generation. Ferryl complexes were not significant intermediates in the redox cycling of Fe in this system. Instead, hydrogen peroxide (H₂O₂) and hydroxyl radicals (•OH) represented the main ROS produced in a process that is periodic during redox oscillations. Hydroxyl radicals were mainly produced during the aerated phases in concentrations inversely proportional to orthophosphate concentrations as a result of the complexation of Fe(III) and precipitation of oxidation-resistant vivianite. In relatively low orthophosphate concentration compared to the total reactive sites available on the Fe(III) oxides, •OH was produced at the mineral surface. Increased cell metabolic activities in the highest orthophosphate medium may have increased the •OH production efficiency during the aerated phases by enhancing O₂^•− formation and/or Fe(III) reduction. Unexpectedly, H₂O₂ was mainly detected under Fe(III) reduction conditions with net production rates that were proportional to orthophosphate concentrations, except when the solubility of vivianite was exceeded. Hydrogen peroxide is proposed to originate from the reduction of dissolved O₂ by Fe(II) produced during dissimilatory Fe(III) reduction in microaerobic conditions. Precipitation of vivianite likely affects the availability of Fe(II) for ROS reactions during redox cycles, thus decreasing overall H₂O₂ production rates. The rise in metabolic activity, outcompetition of Fe(III) hydrolysis by orthophosphate complexation, and potential dimerization of surface generated •OH are proposed to result in the accumulation of H₂O₂ under suboxic conditions.

Degradability of organic matter in river sediments differs in relation to origin and age. In order to explain previously observed spatial patterns of organic matter degradability and stabilization, this study investigated sediment organic matter (SOM) properties along a tidal Elbe river transect using dissolved organic matter (DOM) fractions, density fractions, carbon stable isotopes and thermometric pyrolysis (Rock-Eval 6©). These properties were linked to SOM decay rates and biological indicators such as chlorophyll a and silicic acid in the water phase, and sediment-bound extracellular polymeric substances (EPS), microbial biomass and oxygen consumption. Sediment source gradients were established using the concentration of Zn in the fraction < 20 μm as proxy.

The specific Zn concentration showed that the most upstream location was nourished primarily by upstream fluviatile sediments while the other locations carried a downstream signature. The upstream location was also characterised by the highest concentrations of chlorophyll a, microbial biomass, silicic acid, EPS, humic acids and hydrophilic DOM, the most negative δ¹³C signature and by the highest oxygen consumption rate, with decreasing trends towards downstream locations. This trend was also evident in the decreasing SOM lability from upstream to downstream, an increasing share of total SOM found in the acid-base-extractable fractions and a decreasing share of carbon in the light density fractions. Thermometric pyrolysis revealed the highest H-index (easily degradable SOM) for the most upstream location and the ratio of the I-index (immature SOM) to the R-index (refractory SOM) to correlate positively with measured SOM decay rates.

This study suggests that spatial patterns of SOM degradability can be explained by a source gradient, with young organic matter entering the system from upstream from predominantly biogenic sources, while downstream sources (North Sea sediment) deliver more refractory SOM that is stabilized in organo-mineral associations to a higher extent. In the investigated sediments, dissolved organic matter represented 0.23–1.20% of the total organic carbon (TOC) from anaerobically degradable SOM, while 4.10–11.46% TOC was liberated as CO₂ and CH₄ after long-term incubation (250 days). Thermometric pyrolysis is shown to serve as a useful proxy for SOM degradability in river sediments, with the Hydrogen-Index (HI) correlating well with degradability and the relationship between the I-index and R-index changing consistently towards lower I-indices and higher R-indices with an increasing degree of SOM stabilization.

Large rivers originating from the Tibetan Plateau play a crucial role in sediment production and transportation and thus influence the ocean composition and global climate. This study focuses on the geochemical characteristics, including major, trace and rare earth elements of overbank sediments from the Lancang River (Upper Mekong), which drains southwest China and is one of the largest international rivers in Asia. We characterize sediment maturity and recycling, weathering and provenance in a typical, tectonically active cold-humid climate region. Lancang River (LR) sediments with an average chemical index of alteration (CIA) of 73 ± 6 and an average index of compositional variability (ICV) of 1.1 ± 0.6 suggest an intermediate mineralogy and chemical maturity corresponding to moderate weathering in a slightly warmer climate. The provenance of LR sediments is characterized by felsic source rocks, similar to the upper continental crust (UCC), followed by recycled sedimentary rocks, which were possibly derived from the Yidun, North Qiangtang (NQ) and Songpan-Ganzi (SGZ) terranes at upstream locations when the Lingcang basalt (LCB) provided mafic rocks for LR sediments. However, recycled materials have limited impacts on sediment maturity, as reflected by the degree of weathering. Highly physical weathering (erosion) has created a massive source for LR sediments due to active tectonics, while the impacts of climate on weathering processes have increased. Meanwhile, the overbank sediment in large rivers could be a good sampling medium for evaluating terrestrial weathering, provenance and sediment characteristics using geochemical evidence.

The high concentrations of arsenic in the groundwater of Zimapán, Hidalgo, Mexico, have led to the search for options that contribute to resolving this problem. In this study, we use percolation columns to remove arsenic in aqueous solutions in coexistence with ions such as fluorides, chlorides, sulfates, or bicarbonates. This filtration technique is effective and low cost since the filter material is a native limestone belonging to the Soyatal Formation of the Upper Cretaceous. This article characterizes the mechanisms of arsenic removal by calcite, the limestone's main constituent, through X-ray absorption spectroscopy (XAS). The main processes of arsenic retention in calcite are adsorption (forming corner-sharing inner-sphere surface complexes) and coprecipitation (replacing AsO₄ in the carbonate site).

A comprehensive understanding of the geochemical behavior of ⁹⁹Tc is of great importance for safe disposal of radioactive waste and remediation of contaminated environmental sites. Illite is one of the most common constituents of clay rocks, and thus used in this work as a model system for studying the retention and transport of ⁹⁹Tc in clay-rich systems. In this study, a through-diffusion technique was applied to investigate the diffusion behavior of Tc in compacted illite clay under oxic and anoxic conditions. Particular focus of this investigation was on the role of Fe(II) on the redox state and mobility of Tc in clay. As the diffusion cells contained stainless steel filters for confining the clay plug, ⁹⁹Tc-diffusion in the filters was assessed first, followed by ⁹⁹Tc diffusion in the illite column with or without Fe-loading. Two types of Fe(II) loadings in illite were considered, surface complexed Fe(II) on illite edge sites and Fe(II) added as pyrite grains. COMSOL Multiphysics was used to analyze the diffusion data. The measured filter porosity was about 0.2 and the effective diffusion coefficient, D_e, for Tc (as TcO₄⁻) in the filter was 0.59 × 10⁻¹⁰ m²/s. Tc diffusion in illite under ambient conditions showed a typical anion diffusion behavior with D_e in the range 0.38-0.44 × 10⁻¹⁰ m²/s and the anion accessible porosity ε of approximately 0.2. In the presence of Fe(II), D_e for Tc migration in illite was one order of magnitude lower, showing that Fe(II) has a significant effect on the migration of ⁹⁹Tc. Analysis of the Tc distribution in the system suggests that most Tc was retarded at the filters, especially the ones connected to the high concentration reservoir. The remaining Tc quantities were immobilized at sample boundaries next to the filters with higher concentrations observed in the domains close to the reservoirs. Almost no Tc was immobilized in the middle part of the sample, where the Fe(II) was preloaded. This observation contradicts the anticipation, because Tc was expected to be immobilized in the middle of the sample preloaded with Fe(II). A possible explanation is that a delocalized redox reaction may occur, which means that electrons from the oxidation of the preloaded Fe(II) in the central part of the cell are transferred to the filter via the walls of the steel diffusion cell. Tc reacts with the electrons in the filter at a relative slow rate, which results in most of the Tc retarded in the filter, while some small amounts of it may diffuse through the clay.

Increased exposure to nitrate has been reported in many arid and semi-arid areas where groundwater is used as the drinking water source, presenting a major public health threat. Groundwater nitrate pollution is becoming serious due to intensive agricultural activities in the Nansi Lake Basin, Shandong Province, Northern China, through which the trunk line of the South-to-North Water Diversion Project passes. This study characterized potential nitrogen pollution sources by combined analysis of hydrochemical components and stable nitrate isotopes (δ¹⁵N–NO₃^-, δ¹⁸O–NO₃^-); the contribution of different sources was estimated using the SIAR model. The results show groundwater nitrate contamination in the Nansi Lake Basin is widespread both west (NSW) and east (NSE) of the lake. Manure and sewage (M&S), soil nitrogen (SN), and chemical fertilizer (CF) are potential NO₃⁻-N sources in the study area, with the order of contribution being M&S (78.38%) > SN (15.02%) > CF (6.60%) in areas west of the lake and M&S (59.40%) > SN (30.38%) > CF (10.22%) in areas east of the lake, indicating the significant impact of human activities. The relationship between d-excess and δ¹⁵N–NO₃^- suggests irrigation practices increase soil moisture, resulting in more N from ¹⁵N-depleted soils leaching into the groundwater, increasing the contribution of SN. The predominant processes of the nitrogen cycle in the basin are different in the NSW and NSE areas. In the upper-middle reaches of the NSW area, nitrification dominates the nitrogen cycle due to aerobic conditions and elevated salinity; however, denitrification occurs in local downstream aquifers at depths ranging from 10 to 50 m. In contrast, SN plays a bigger role in the NSE, where the mineralization of soil organic nitrogen is the predominant process, followed by nitrification of M&S and CF nitrogen sources due to rapid groundwater flow in the carbonate and metamorphic rock aquifers.

Speleothem δ¹⁸O, δ¹³C_DIC, and Mg/Ca are widely used to reconstruct past climatic and environmental changes. However, due to the rainfall regime and hydrological structure of caves, their application in reconstructing past hydroclimate in the East Asian monsoon region is poorly understood. In this study, we monitored two types of drip water in a subtropical cave to investigate the different responses of δ¹⁸O, δ¹³C_DIC, and Mg/Ca to hydroclimate changes at monthly to yearly scales. Our results showed that drip rate at all drip sites responded quickly to precipitation, whereas δ¹⁸O, δ¹³C_DIC, and Mg/Ca responded with some lag. The lag time for the parameters were in the order of δ¹³C_DIC (lag time ranging from 1 to 2 months) < Mg/Ca ratio (lag time of 2 months) < δ¹⁸O (lag time up to 5 months), and for fissure-fed drip sites (M-5 and M-6) < seepage flow drips (M-1 and M-2), as well as single-reservoir supply (M-5) < multi-reservoirs supply (M-6). A comprehension of both fast and slow flows is crucial in order to comprehend karst reservoirs and their paths, which in turn enables the exploration of the hydrological properties of unsaturated zones under the context of Asian monsoon. We also found that the hydrological structure was crucial in generating the different response times of δ¹⁸O, δ¹³C_DIC, and Mg/Ca in drip water. Additionally, the relationship between δ¹⁸O and δ¹³C_DIC may be due to the different residence times in different drip sites on seasonal- to annual timescales.

The injection and storage of anthropogenic CO₂ in the subsurface is being deployed as a climate change mitigation tool; however, diagenetic-paragenetic heterogeneity in sandstone reservoirs often contributes to interval specific chemomechanical changes that affect injection and can increase leakage risk. Here, we address reservoir heterogeneities’ impact on chemomechanical changes in a macroporous-dominated lithofacies of Morrow B sandstone, a formation containing several diagenetically-distinct hydraulic facies while undergoing enhanced oil recovery (EOR) and carbon dioxide (CO₂) sequestration. We performed three flow-through experiments using a CO₂-charged or uncharged formation water combined with four indirect tensile strength tests per post-test sample. We then used the microstructure and paragenetic sequence to understand chemomechanical weakening with key observations as follows: dissolution of carbonates and feldspars changed porosity; increased permeability led to reclassifying each sample in a different hydraulic flow unit; decreased ultrasonic velocity; and did not lead to a loss of tensile strength. Tensile strength maintenance occurred due to the low abundance and minor dissolution of siderite, the stability of quartz, and the relative position of diagenetic ankerite within feldspar. This macroporous-dominated lithofacies is the primary reservoir for the Morrow B Sandstone, and is analogous to other porous sandstone reservoirs. It represents an end-member of a chemomechanically low-risk siliceous CO₂ sequestration and CO₂-EOR reservoir.

Bentonite clay (MX-80, Wyoming, USA) is a proposed sealing material for the safe and long-term management of Canada's used nuclear fuel in a deep geological repository (DGR). Due to the physiochemical properties of bentonite, this buffer layer can limit radionuclide mobilization, and minimize air and water circulation, and hence suppress microbial activity near the used fuel container. However, natural organic matter (NOM) in mined Wyoming-type bentonite (MX-80) may serve as microbial substrates and cause further corrosion to the used fuel container. In the proposed DGR, bentonite will be exposed to high salinity due to saline groundwater and heat released from radioactive decay of radionuclide inventory of the used nuclear fuel. There is limited knowledge about how NOM quantity and quality in MX-80 may change after being exposed to the anticipated levels of salinity and heat in a DGR, especially at the molecular-level. To explore this, organic carbon contents and NOM composition (both soluble- and solid-phase) in MX-80 after exposure to different saline (NaCl) solutions (1 M, 2 M, 3 M, 4 M and 5 M) with and without heat exposure (90 °C) were analyzed. Dissolved organic carbon (DOC) solubility decreased with increasing salinity but these differences were not statistically significant when compared to the control (P < 0.05). DOC solubilization was not significantly altered with heat (P < 0.05). Dissolved organic matter (DOM) composition was analyzed with solution-state ¹H nuclear magnetic resonance (NMR) and ultraviolet–visible (UV–Vis) spectroscopy. The NMR results indicated that DOM samples with varied salinities and heat exposure had similar chemical signatures. The results from UV–Vis analysis revealed a small decrease in less unsaturated components in DOM with NaCl solution compared to deionized water. The chemistry of solid-phase NOM that remained in MX-80 after DOM isolation was similar and suggests an insensitivity to either heat and/or salinity. The solubilized DOC contributes less than 1% to the total organic carbon in MX-80, thus, the anticipated level of salinity and heat in a DGR is unlikely to enhance NOM solubilization and change NOM chemistry in the bentonite markedly. As the bentonite in this study was only exposed to the heat and salinity in a short-term experiment, further studies examining NOM composition and reactivity under long-term saline and heat conditions are recommended.

Many coal mining subsidence lakes have formed in eastern China due to underground coal mining. Lacustrine groundwater discharge (LGD) is of great importance to the hydrological cycle and the eco-environment of lakes. However, LGD in coal mining area is rarely reported. In the study, we quantified groundwater discharge into a Huainan coal mining subsidence lake (China) using water stable (D and ¹⁸O) and radioactive (²²²Rn) isotopes. ²²²Rn and ¹⁸O mass balance models were used to independently estimate LGD. The lake water was sampled based on the horizontal and vertical profile sampling. The isotope depth profiles were weighted according to the lake bathymetry to obtain a representative isotope inventory by geographic information system (GIS) analysis. The LGD rates estimated from two models were comparable, and the values were 18 mm d⁻¹ for the ¹⁸O and 14 ± 8 mm d⁻¹ for the ²²²Rn. Compared with the LGD of Ammelshainer See lake (Germany), a subsidence lake formed by coal mining, the LGD in the study was larger, which may be related to the different hydrological conditions in different areas. By integrating the LGD rates from these two models, this study quantitatively analyzed the role of groundwater in maintaining the water balance in subsidence lakes, which is of great significance to water resources assessment and sustainable utilization of coal mining subsidence lakes in eastern China.