Variable Redox Conditions Research Articles

Paleodepositional conditions and organic matter accumulation mechanisms in the Upper Ordovician-lower Silurian Wufeng-Longmaxi shales, Middle Yangtze region, South China

The deposition of black shales has been widely attributed to primary productivity, preservation controlled by redox conditions, and detrital dilution controlled by sedimentation rates. It is difficult to distinguish the relative importance of productivity and redox conditions in organic matter accumulation. On the Upper Ordovician-Lower Silurian Wufeng-Longmaxi black shales, we combined petrographic features, major and trace elemental proxies, and organic carbon isotopic analysis to determine the relative importance of productivity versus anoxia on organic matter accumulation and the evolution of factors controlling organic matter accumulation across the Ordovician-Silurian boundary in the Middle Yangtze region. Our research also sheds light on the temporal variations in redox conditions that caused the Late Ordovician mass extinction.Productivity, redox conditions, detrital dilution, and basin restriction changes over time as the Wufeng-Longmaxi Formations were deposited (from upper Katian to Rhuddanian stage). Based on the stratigraphic profiles of productivity (biogenic Si and P/Al), redox conditions (Mo/Al, U/Th ratios, and DOPT values), detrital fraction flux (Al2O3), and the cross-plots of biogenic Si versus Co*Mn values, our findings indicate that organic matter accumulation was a combined effect of productivity and anoxia; however, the elevated productivity caused by upwelling that introduced plenty of nutrients to the oceans was the direct factor driving organic matter accumulation. Previous research found a simultaneous variation in stratigraphic profiles of productivity and redox proxies; however, in our datasets, we found an offset between productivity geochemical proxies (biogenic Si content) and redox conditions (Mo/Al). Biogenic Si profile peaks occur stratigraphically beneath Mo/Al profile peaks. Anoxia does not coexist with increased paleoproductivity across the Ordovician-Silurian boundary in the Middle Yangtze region, and redox conditions may be disrupted by local geological events.

Oxygen-Deficient (Ln,Sr)2NiO4- δ Nickelates for Oxygen Electrodes of Solid Oxide Fuel and Electrolysis Cells: Anisotropic Thermochemical Expansion and Thermomechanical Constraints

Perovskite-like and perovskite-related oxides based on variable-valence transition metals may exhibit substantial changes in oxygen content in the course of temperature cycling or under variation of oxygen chemical potential at elevated temperatures. This gives rise to a chemical contribution to the thermal expansion (chemical expansion/contraction or chemical strain) originating mainly from the accompanying changes in the average oxidation state of transition metal cations and, consequently, their ionic radii. Excessive dimensional changes caused by the chemical expansion impose constraints on the applicability of functional oxides at elevated temperatures and under variable redox conditions (due to thermomechanical instability or incompatibility with other materials) and complicate the processing of oxide ceramics at high temperatures. Another unfavorable phenomenon is microcracking caused by a strongly anisotropic expansion of crystal lattice, with effects on the microstructural and mechanical integrity and electrical and electrochemical properties. The present work is a representative study case that focuses on layered Ln2NiO4-based ceramic materials for oxygen electrodes of solid oxide fuel/electrolysis cells (SOFC/SOEC).Ln2NiO4+δ–based oxides (Ln = La, Pr, Nd) with perovskite-related K2NiF4-type structure demonstrate high oxygen diffusivity and surface exchange kinetics in combination with comparatively high electronic conductivity and are considered, therefore, as attractive mixed ionic-electronic conductors for oxygen electrodes of high-temperature SOFC/SOEC [1]. Acceptor-type substitution of Ln3+ by Sr2+ in Ln2NiO4+ δ is compensated by comparatively high oxygen deficiency in Sr-rich Ln2-xSrxNiO4-δ phases at elevated temperatures [2,3]. The transition from oxygen excess to oxygen deficiency is expected to be accompanied by a change in the ionic transport mechanism from prevailing interstitial oxygen diffusion in rock-salt layers to oxygen vacancy diffusion in perovskite-type layers.Ceramic powders of Ln2-xSrxNiO4-δ (Ln = La, Nd, Pr, x = 1.0-1.6) were prepared by the Pechini method with final calcination steps at 1150-1200°C in oxygen. Thermogravimetric studies confirmed that acceptor-type substitution by strontium is compensated by the formation of oxygen vacancies and electron-holes and progressively increases high-temperature oxygen nonstoichiometry (Fig.1A), which reaches as high as δ = 0.36-0.40 for x = 1.6 at 900°C in air. High-temperature XRD studies demonstrated that Ln2-xSrxNiO4-δ nickelates exhibit strongly anisotropic lattice expansion interrelated with oxygen deficiency changes on heating (Fig.1B). Highly anisotropic expansion induces microcracking phenomenon, which, in turn, results in a strong hysteresis in dimensional changes (Fig.1C) and also in variations of electrical conductivity (Fig.1D) on cycling in air for the ceramic samples sintered at 1200-1300°C.An appropriate approach to minimize or neglect the undesirable effects of anisotropic expansion and microcracking is to preserve the grain size in Ln2-xSrxNiO4-δ ceramics or porous electrodes at ~1 micron or submicron level. In particular, spark plasma sintering with subsequent careful oxidation treatment was adopted to avoid grain growth and to produce mechanically stable ceramics with smooth reversible thermochemical expansion and enhanced electrical properties. Acknowledgments: This work was supported by the FCT/MCTES/FEDER (projects CARBOSTEAM (POCI-01-0145-FEDER-032295) and CICECO-Aveiro Institute of Materials (UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020), and PhD grant SFRH/BD/138773/2018).

Multiple geochemical and microbial processes regulated by redox and organic matter control the vertical heterogeneity of As and Cd in paddy soil

The heterogeneity of arsenic (As) and cadmium (Cd) in paddy soils seriously hinders the assessment of contamination status and prediction of rice uptake. Their vertical patterns across different environmental conditions and the underlying mechanisms remain largely unexplored. In this study, maximum vertical differences of bioavailable As and Cd within 0–30 cm depth in paddy soils were 4.1-fold and four orders of magnitude, respectively. The vertical patterns of As and Cd followed the vertical redox gradient in long-term reduced paddies, but were shaped by the vertical pH gradient derived from acidic wastewater irrigation in partly oxidized soils. Iron(III)- and sulfate-reducing bacteria played key roles in the formation of vertical pH gradient and the immobilization of As and Cd by iron (hydr)oxides and sulfides under varied redox conditions. Soil redox and organic matter determined the transition between these two mechanisms via regulating microbial iron(III) and sulfate reduction processes. The work proposes that soil vertical As and Cd patterns directly affect the accumulation of As and Cd in different rice cultivars with different vertical root patterns. This is the first study elucidating the controlling mechanisms governing the vertical As and Cd patterns in paddy fields, providing important references to identify, manage and remediate contaminated paddy fields.

Palaeoenvironments and elemental geochemistry across the Permian–Triassic boundary at Ursula Creek, British Columbia, Canada, and a comparison with some other deep‐water Permian–Triassic boundary shelf/slope sections in western North America

The Permian–Triassic boundary sediments at Ursula Creek accumulated in a continental shelf basin, or on the continental slope of the western Canadian passive margin, at 30°N palaeolatitude along the eastern Panthalassic Ocean margin. The area lay within the cold northerly ocean currents at the junction of westerly and north-easterly trade wind belts, the latter causing summer coastal upwelling. The shift from uppermost Permian grey radiolarian cherts and grey shales to lowermost Triassic grey and black shales and fine-grained dolomites is typical of deep-water Panthalassic sediments. The palaeogeographical situation and palaeoenvironments are comparable to those of the present Canadian north-western Pacific margin. The Ursula Creek section reveals the progressive decline of seafloor oxygen values in the Changhsingian Stage), followed by the persistent development of euxinic conditions in the latest Changhsingian and throughout the Early Triassic; a transition that coincides with the disappearance of a siliceous sponge fauna and the loss of diverse radiolarian populations. Much of the detrital sediment was supplied by summer north-east Trade winds from the deserts of western North America, although variable amounts may have come across the Panthalassic Ocean as dust from contemporary volcanic eruptions. Relative palaeoproductivity changes show no consistent change in productivity across the Permian–Triassic boundary producing results that are comparable with those from the similar Opal Creek section to the south-east. The Ni/Co, Cu/Zn, U/Al and Th/U ratios indicate variable redox conditions in all sections, but with a tendency for oxic conditions to change to dysoxic across the Permian–Triassic boundary. The lack of consistent element geochemical changes across the boundary accompanied by significant isotopic changes, here and elsewhere, suggests that atmospheric and oceanic chemistry rather than physical changes, like provenance and sea-level changes, drove Permian–Triassic environmental changes and extinctions.

Metagenomic Insights Into Competition Between Denitrification and Dissimilatory Nitrate Reduction to Ammonia Within One-Stage and Two-Stage Partial-Nitritation Anammox Bioreactor Configurations.

Anaerobic ammonia oxidizing bacteria (Anammox) are implemented in high-efficiency wastewater treatment systems operated in two general configurations; one-stage systems combine aerobic ammonia oxidizing bacteria (AOB) and Anammox within a single aerated reactor, whereas two-stage configurations separate these processes into discrete tanks. Within both configurations heterotrophic populations that perform denitrification or dissimilatory nitrate reduction to ammonia (DNRA) compete for carbon and nitrate or nitrite and can impact reactor performance because DNRA retains nitrogen in the system. Therefore, it is important to understand how selective pressures imposed by one-stage and two-stage reactor configurations impact the microbial community structure and associated nitrogen transforming functions. We performed 16S rRNA gene and metagenomic sequencing on different biomass fractions (granules, flocs, and suspended biomass) sampled from two facilities treating sludge dewatering centrate: a one-stage treatment facility (Chambers Creek, Tacoma, WA) and a two-stage system (Rotterdam, Netherlands). Similar microbial populations were identified across the different samples, but relative abundances differed between reactor configurations and biomass sources. Analysis of metagenome assembled genomes (MAGs) indicated different lifestyles for abundant heterotrophic populations. Acidobacteria, Bacteroidetes, and Chloroflexi MAGs had varying capacity for DNRA and denitrification. Acidobacteria MAGs possessed high numbers of glycosyl hydrolases and glycosyl transferases indicating a role in biomass degradation. Ignavibacteria and Phycosphaerae MAGs contributed to the greater relative abundance of DNRA associated nrf genes in the two-stage granules and contained genomic features suggesting a preference for an anoxic or microoxic niche. In the one-stage granules a MAG assigned to Burkholderiales accounted for much of the abundant denitrification genes and had genomic features, including the potential for autotrophic denitrification using reduced sulfur, that indicate an ability to adapt its physiology to varying redox conditions. Overall, the competition for carbon substrates between denitrifying and DNRA performing heterotrophs may be impacted by configuration specific selective pressures. In one-stage systems oxygen availability in the bulk liquid and the oxygen gradient within granules would provide a greater niche space for heterotrophic populations capable of utilizing both oxygen and nitrate or nitrite as terminal electron acceptors, compared to two-stage systems where a homogeneous anoxic environment would favor heterotrophic populations primarily adapted to anaerobic metabolism.

Retention of immobile Se(0) in flow-through aquifer column systems during bioreduction and oxic-remobilization

Selenium (Se) is a toxic contaminant with multiple anthropogenic sources, including 79Se from nuclear fission. Se mobility in the geosphere is generally governed by its oxidation state, therefore understanding Se speciation under variable redox conditions is important for the safe management of Se contaminated sites. Here, we investigate Se behavior in sediment groundwater column systems. Experiments were conducted with environmentally relevant Se concentrations, using a range of groundwater compositions, and the impact of electron-donor (i.e., biostimulation) and groundwater sulfate addition was examined over a period of 170 days. X-Ray Absorption Spectroscopy and standard geochemical techniques were used to track changes in sediment associated Se concentration and speciation. Electron-donor amended systems with and without added sulfate retained up to 90% of added Se(VI)(aq), with sediment associated Se speciation dominated by trigonal Se(0) and possibly trace Se(-II); no Se colloid formation was observed. The remobilization potential of the sediment associated Se species was then tested in reoxidation and seawater intrusion perturbation experiments. In all treatments, sediment associated Se (i.e., trigonal Se(0)) was largely resistant to remobilization over the timescale of the experiments (170 days). However, in the perturbation experiments, less Se was remobilized from sulfidic sediments, suggesting that previous sulfate-reducing conditions may buffer Se against remobilization and migration.

Alteration of birnessite reactivity in dynamic anoxic/oxic environments

Although the oxidative capacity of manganese oxides has been widely investigated, potential changes of the surface reactivity in dynamic anoxic/oxic environments have been often overlooked. In this study, we showed that the reactivity of layer structured manganese oxide (birnessite) was highly sensitive to variable redox conditions within environmentally relevant ranges of pH (4.0 – 8.0), ionic strength (0–100 mM NaCl) and Mn(II)/MnO2 molar ratio (0–0.58) using ofloxacine (OFL), a typical antibiotic, as a target contaminant. In oxic conditions, OFL removal was enhanced relative to anoxic environments under alkaline conditions. Surface-catalyzed oxidation of Mn(II) enabled the formation of more reactive Mn(III) sites for OFL oxidation. However, an increase in Mn(II)/MnO2 molar ratio suppressed MnO2 reactivity, probably because of competitive binding between Mn(II) and OFL and/or modification in MnO2 surface charge. Monovalent cations (e.g., Na+) may compensate the charge deficiency caused by the presence of Mn(III), and affect the aggregation of MnO2 particles, particularly under oxic conditions. An enhancement in the removal efficiency of OFL was then confirmed in the dynamic two-step anoxic/oxic process, which emulates oscillating redox conditions in environmental settings. These findings call for a thorough examination of the reactivity changes at environmental mineral surfaces (e.g., MnO2) in natural systems that may be subjected to alternation between anaerobic and oxygenated conditions.

Geochemical focusing and burial of sedimentary iron, manganese, and phosphorus during lake eutrophication

AbstractThe redistributive transport of sedimentary, redox‐sensitive metals such as iron (Fe) and manganese (Mn) along depth gradients in lakes or marine basins, and in particular the concentration of these elements at local depressions, has been described as “geochemical focusing.” However, the impact of variable redox conditions at the sediment–water interface on this process is not entirely understood. In this study, we report that the enrichment and burial of Fe, Mn, and associated phosphorus (P) in dimictic Lake Arendsee, Germany, has been affected by a transition from oligo‐mesotrophic to eutrophic conditions in the first half of the 20th century. Eleven sediment cores (<50 cm long) were obtained from water depths ranging from 38.5 to 49.5 m, and analyzed for their elemental composition. The older, non‐varved section, which was deposited prior ca. 1940 during oligo‐mesotrophic conditions, contains up to 10 times higher Mn and 2.5 times higher Fe contents at the deep sites compared to corresponding intervals at shallower sites. Due to their high Fe content, sediments in this interval also contain up to eight times more P, at least partly owing to vivianite authigenesis. Burial of Mn occurred via Mn‐carbonate authigenesis, possibly as rhodochrosite. Since ca. 1940, calcite and diatom varves have formed under eutrophic conditions, whereas Fe enrichment decreased and finally stopped due to fixation as pyrite in the shallow sites. Our results suggest that redox‐controlled geochemical focusing determines the distribution of Mn and Fe, as well as the occurrence of authigenic minerals such as vivianite and Mn‐carbonate.

Mediation of arsenic mobility by organic matter in mining-impacted sediment from sub\u2010Arctic lakes: implications for environmental monitoring in a warming climate

Arsenic (As) is commonly sequestered at the sediment–water interface (SWI) in mining-impacted lakes through adsorption and/or co-precipitation with authigenic iron (Fe)-(oxy)hydroxides or sulfides. The results of this study demonstrate that the accumulation of organic matter (OM) in near-surface sediments also influences the mobility and fate of As in sub-Arctic lakes. Sediment gravity cores, sediment grab samples, and porewaters were collected from three lakes downstream of the former Tundra gold mine, Northwest Territories, Canada. Analysis of sediment using combined micro-X-ray fluorescence/diffraction, K-edge X-ray Absorption Near-Edge Structure (XANES), and organic petrography shows that As is associated with both aquatic (benthic and planktonic alginate) and terrestrially derived OM (e.g., cutinite, funginite). Most As is hosted by fine-grained Fe-(oxy)hydroxides or sulfide minerals (e.g., goethite, orpiment, lepidocrocite, and mackinawite); however, grain-scale synchrotron-based analysis shows that As is also associated with amorphous OM. Mixed As oxidation states in porewater (median = 62% As (V), 18% As (III); n = 20) and sediment (median = 80% As (-I) and (III), 20% As (V); n = 9) indicate the presence of variable redox conditions in the near-surface sediment and suggest that post-depositional remobilization of As has occurred. Detailed characterization of As-bearing OM at and below the SWI suggests that OM plays an important role in stabilizing redox-sensitive authigenic minerals and associated As. Based on these findings, it is expected that increased concentrations of labile OM will drive post-depositional surface enrichment of As in mining-impacted lakes and may increase or decrease As flux from sediments to overlying surface waters.

Operando electrochemical SERS monitors nanoparticle reactions by capping agent fingerprints

Nanomaterials are frequently employed in daily life goods, including health, textile, and food industry. A comprehensive picture is lacking on the role of the capping agents, added ligand molecules, in case of nanoparticle reactions and degradation in aqueous solutions, like surface waters or biofluids. Here, we aim to elucidate the capping agent influence on nanoparticle reactivity probing two commonly employed capping agents citrate and polyvinylpyrrolidone (PVP). Their influence on silver nanoparticle (AgNP) transformation is studied, which is particularly important due to its application as an antimicrobial agent. We induce oxidation and reduction processes of AgNPs in halide solutions and we monitor the associated transformations of particles and capping agents by spectro-electrochemical surface-enhanced Raman spectroscopy (SERS). Raman bands of the capping agents are used here to track chemical changes of the nanoparticles under operando conditions. The sparingly soluble and non-plasmon active silver salts (AgBr and AgCl) are formed under potential bias. In addition, we spectroscopically observe plasmon-mediated structural changes of citrate to cis- or trans-aconitate, while PVP is unaltered. The different behavior of the capping agents implies a change in the physical properties on the surface of AgNPs, in particular with respect to the surface accessibility. Moreover, we showcase that reactions of the capping agents induced by different external stimuli, such as applied bias or laser irradiation, can be assessed. Our results demonstrate how SERS of capping agents can be exploited to operando track nanoparticle conversions in liquid media. This approach is envisaged to provide a more comprehensive understanding of nanoparticle fates in complex liquid environments and varied redox conditions.

Chemical composition of garnet from the Xintianling skarn W deposit in southern Hunan and its geological significance

新田岭矿床是南岭钨锡成矿带中的一个大型矽卡岩型钨矿床，产于骑田岭岩体东北部与石炭系碳酸盐地层的接触带位置。本文运用LA-ICP-MS技术对该矿床矽卡岩中的石榴子石进行了系统的成分分析，获得了其主量、微量和稀土元素含量。结果显示，新田岭矿床中的石榴子石属于钙铁榴石-钙铝榴石固溶体系列（And₂₄Gro₆₆-And₇₁Gro₂₂），石榴子石的端元成分在富钙铝榴石和富钙铁榴石之间变化。稀土元素的配分模式也同时出现了左倾、Eu负异常和右倾、Eu正异常两种类型，暗示新田岭矿床石榴子石结晶过程中热液流体存在不同的氧化还原环境和水/岩比条件，这也与其晶体中是否出现振荡环带相对应。将不同矽卡岩型矿床中石榴子石的W、Sn含量进行对比显示，含W矿化的矽卡岩型矿床中石榴子石的W、Sn含量整体上显著高于不含W矿化的矿床，指示石榴子石中的W、Sn含量在一定程度上具有预测矽卡岩型矿床成W矿潜力的作用。此外，石榴子石中Fe、Eu、U等元素的含量还可以进一步区分矽卡岩W矿床中的伴生金属元素类型（包括W-Mo、W-Sn、W-Cu-Fe和W-Mo-Cu-Fe等）。本文研究表明，石榴子石的成分特征不仅可以指示矽卡岩的成矿环境，还可用于评估矽卡岩中金属（尤其是W）的成矿潜力，具有一定的理论意义和应用价值。;Xintianling is a large skarn W deposit in the Nanling W-Sn metallogenic belt, and it is located in the contact zone between the northeastern Qitianling pluton and the Carboniferous carbonate rocks. In this study, the typical skarn mineral, garnet, has been analyzed by LA-ICP-MS to quantify the concentrations of major elements, trace elements, as well as rare earth elements (REEs). The results reveal that the garnet from the Xintianling deposit mainly belongs to the andradite-grossular solid solution (And₂₄Gro₆₆ to And₇₁Gro₂₂), which shows a transitional trend dominated by either andradite or grossular. The REEs of the garnet show two distinct patterns, one is characterized by HREE enrichment and LREE depletion with negative Eu anomaly, and the other is featured with LREE enrichment and HREE depletion with positive Eu anomaly. These differences suggest that the garnet crystals grow under variable redox conditions and water/rock ratios in the hydrothermal system, which has also been reflected in the oscillatory zones. A compilation of available data from other skarn deposits combined with the results from this study indicates that W and Sn concentrations of garnet from skarn deposits with economic W mineralization are significantly higher than those without W mineralization, implying that the W and Sn concentrations could probably predict the potential economic metals (especially W) in a skarn system. Furthermore, the results also indicate that the Fe, Eu and U concentrations of garnet can provide useful information for distinguishing the associated metal(s) of a W skarn system, including W-Mo, W-Sn, W-Cu-(Fe) and W-Mo-Cu-(Fe). In conclusion, the current study shows that the compositional characteristics of garnet have certain guiding significance in understanding the ore-formation conditions and evaluating mineralization potentials of metals in a skarn system, especially for these W skarns like Xintianling.

Local and Global Sensitivity Analysis of a Reactive Transport Model Simulating Floodplain Redox Cycling

AbstractReactive transport models (RTMs) are essential tools that simulate the coupling of advective, diffusive, and reactive processes in the subsurface, but their complexity makes them difficult to understand, develop and improve without accompanying statistical analyses. Although global sensitivity analysis (SA) can address these issues, the computational cost associated with most global SA techniques limits their use with RTMs. In this study, we apply distance‐based generalized sensitivity analysis (DGSA), a novel and computationally efficient method of global SA, to a floodplain‐scale RTM and compare DGSA results to those from local SA. Our test case focuses on the impact of 17 uncertain environmental parameters on spatially and temporally variable redox conditions within a floodplain aquifer. The input parameters considered include flow and diffusion rates, geochemical reaction rates, and the spatial distribution of sediment facies. Sensitivity was evaluated for three distinct components of the model response, encompassing both multidimensional and categorical output. Parameter rankings differ between local SA and DGSA, due to nonlinear effects of individual parameters and interaction effects between parameters. DGSA results show that fluid residence time, which is controlled by aquifer permeability, generally exerts a stronger control on redox conditions than do geochemical reaction rates. Sensitivity indices also demonstrate that sulfate reduction is key for establishing and maintaining reducing conditions throughout the aquifer. These results provide insights into the key drivers of heterogeneous redox processes within floodplain aquifers, as well as the main sources of uncertainty when modeling complex subsurface systems.

Saturated biomarkers adsorbed and occluded by the asphaltenes of some Venezuelan crude oils: Limitations in geochemical assessment and interpretations

This paper presents the study of saturated biomarkers in maltenes and the compounds adsorbed and occluded by the asphaltenes of six Venezuelan crude oils. The study is intended to know to what extent geochemical information stored in the adsorbed and occluded compounds matches the information in maltenes when it comes to evaluating source input, depositional environment, and thermal maturity. Our findings suggest that geochemical information obtained from the adsorbed and occluded compounds agrees with the information provided by maltenes of unaltered crude oils such as Alturitas, Boscan, Guafita, and Silvestre. We confirmed a clear correlation of Alturitas and Boscan crude oils with a carbonate source rock sedimented under anoxic conditions and containing marine organic matter, while Guafita and Silvestre crude oils resulted from a marl-type source rock characterized by mixtures of marine and terrigenous organic matter inputs preserved under varying redox conditions (anoxic to sub-oxic). Regarding maturity, Alturitas and Silvestre resulted more mature than Boscan and Guafita. Despite the good correspondence between the geochemical information provided by the maltenes and the adsorbed and occluded compounds, we observed compound enrichments that caused limitations in the use of some geochemical indicators. The adsorbed compounds of the six studied crude oils were enriched in linear alkanes higher than n-C24, regular steranes (C27–C28), and pentacyclic terpanes, while in the occluded compounds pregnanes, tricyclic terpanes, and n-alkanes higher than n-C24 were more abundant. Thus, maturity relationships defined by the relative abundances of low and high molecular mass steranes and terpanes yielded values that confused when the adsorbed and occluded compounds were compared with the respective maltenes. However, when these relationships were used to compare the occluded compounds of two or more different crude oils, they yielded a reliable appraisal of the maturity stage of the respective source rock. Our results strongly suggest that the adsorbed compounds still preserve their geochemical information as they probably undergo a very low exchange with the surrounding maltenes. So, these compounds together with the occluded ones become a valuable source of geochemical information to assess altered crude oils such as Ayacucho and Junín that have lost most of their maltene biomarkers due to biodegradation. In these cases, the information provided by the adsorbed and occluded compounds confirmed that Ayacucho crude oil is more mature than Junín and that the former derives from a carbonate, marine-organic-matter source rock sedimented under anoxic conditions while the latter correlates with a marl-type source rock sedimented in anoxic conditions and containing mixtures of terrigenous and marine organic matter. Compounds such as oleanane, C27 25,28,30-trisnorhopane (TNH), C28 17α,21β,28,30-dinorhopane (DNH) and C29 17α(H),21β(H)-25-norhopane (NH) detected in the occluded material of the six studied oils, indicate that the major contributions to the occluded compounds probably came from the original material initially adsorbed and occluded in the parent kerogen.

Quinones contained in wastewater as redox mediators for the synergistic removal of azo dye in microbial fuel cells

The present paper aimed to investigate the roles of quinones contained in wastewater and the enhanced effects on microbial fuel cells (MFCs) under different redox conditions. The feasibility of using wastewater rich in quinones to act as co-substrate and redox mediators (RMs) library to strengthen the synergistic removal of azo dye in MFCs was evaluated. The results demonstrated that quinones achieved enhanced effects on electricity generation and COD removal of MFC better at higher current intensity. The addition of pure quinone decreased electron transfer resistance (Rct) of MFCs from 4.76 Ω to 2.13 Ω under 1000 Ω resistance and 1.16 Ω–0.75 Ω under 50 Ω resistance. Meanwhile, higher coulombic efficiency was achieved. Compared with sodium acetate, using quinone-rich traditional Chinese medicine (TCM) wastewater as the co-substrate enhanced the synergistic removal of reactive red 2 (RR2) in MFCs from 79.58% to 92.45% during 24 h. RR2 was also degraded more thoroughly due to the accelerated electron transfer process mediated by RMs. Microbial community analysis demonstrated that the presence of quinone in TCM wastewater can enrich different exoelectrogens under varied redox conditions and thus influenced the enhanced effects on MFC. Metagenomic functional prediction results further indicated that the abundance of functional genes involved in carbohydrate metabolism, membrane transport metabolism, biofilm formation, and stress tolerance increased significantly in presence of RMs. Redundancy analyses revealed that RMs addition was the more important factor driving the variation of the microorganism community. This study revealed the potential effect of quinones as redox mediators on the bioelectrochemical system for pollutants removal.

Impact of organic matter on arsenic mobilization induced by irrigation in the unsaturated and saturated zones

Organic matter (OM) plays an important role in the biogeochemical transformation of arsenic (As) in groundwater and sediments. High-As groundwater used for irrigation introduces dissolved As into the ground. Moreover, water table fluctuations and variations in redox conditions affect As mobilization in both the unsaturated and saturated zones. A field experiment was conducted at the DY site, Datong Basin, China to explore the effect of OM on As and iron (Fe) mobilization and transformation in the unsaturated and saturated zones during irrigation. Soil water/groundwater and sediments sampled at different depths during the experiment were analyzed to investigate the hydrochemical processes and correlations between As, Fe, and OM concentrations. Irrigation activities led to the accumulation of aqueous As in the saturated zone and increasing As content in the topsoil. Analyses of Fourier-transform infrared (FTIR) spectra suggest the existence of As-Fe-OM ternary complexation. Parallel factor analysis (PARAFAC) of the three-dimensional fluorescence excitation–emission matrix (EEM) reveals that water-extractable organic matter (WEOM) of sediments comprises three major components, two terrestrial humic-like components (C1 and C2) and one protein-like component (C3). And terrestrial humic-like components with high molecular weight promotes As-Fe-OM complexation reactions. The results of this study demonstrate As mobilization under irrigation conditions and contribute to a better understanding of the role of OM under high-As groundwater irrigation. This study has implications for controlling and managing OM or As-contaminated water for irrigation in high-As groundwater regions.

Gold enrichment and the Au–Te association in the Huangtan Au–Cu–Zn volcanogenic massive sulfide deposit, Kalatag, East Tianshan, China: An in-situ LA–ICP–MS analysis

Huangtan Au–Cu–Zn is an auriferous VMS deposit hosted in tuff breccia, and tuff of Silurian age in the Kalatag area that located in the northern part of East Tianshan. Mineralization can be divided into two principal ore facies: massive, semimassive ore and stockwork ore. Pyrite, chalcopyrite, and sphalerite, as the predominant sulfide minerals and Au–bearing minerals, were investigated using a combination of ore microscopy, EMPA, SEM–EDS, and in situ LA–ICP–MS. There is no clear systematic variation in the trace element distribution with different types of pyrite, chalcopyrite, and sphalerite in the massive ore, stockwork ore, and hydrothermal alteration zone. Combined with mineralogical characteristics, this deposit is considered to be the product of the same ore-forming fluid and belongs to the VMS metallogenic system. Gold in the Huangtan deposit is present in three main mineralogical forms: auriferous tellurides, “invisible” gold, and native gold. Tellurides identified in the Huangtan deposit include: calaverite, sylvanite, petzite, native tellurium, hessite, altaite, and tellurobismuthite. LA–ICP–MS depth profiles, and mapping imaging for Au, Ag, and Te suggest that the invisible gold in pyrite, chalcopyrite, and sphalerite are micro- to nanoscale inclusions of Au–Ag–Te assemblages. Native gold contains smaller amounts of Ag, Bi, Te, and Se. The concentrations of trace elements Au, Te, Ag, Bi, and Se in pyrite, chalcopyrite, and sphalerite are strongly correlated, indicating that the ore-forming fluid was likely of magmatic origin. In addition, the sulfur isotope compositions (–3.3‰ to 2.3‰) of different types of major sulfides have a narrow range, indicating a homogeneous sulfur source, and show that sulfur sources were mainly derived from magmatic sulfur. The mineral assemblages reveal that Au–Te precipitated under variable redox conditions with log ƒTe2 being less than −8 in the massive ore, and log ƒTe2 being greater than −12.5 in the stockwork ore. The formation of tellurides is sensitive to the temperature of the mineralization system. With the decreasing temperature, the auriferous tellurides gradually precipitated and formed the Huangtan Au–Cu–Zn deposit with an auriferous telluride ore body.

Evidence of a new geothermal prospect in the Northern-Central trans-Mexican volcanic belt: Rancho Nuevo, Guanajuato, Mexico

Thermal manifestations are commonly found in central Mexico as result of the volcanic activity originating from the formation of the Trans-Mexican Volcanic Belt during the Quaternary. The Rancho Nuevo hot spring is one of them that has not been described before with a discharge temperature near 92 °C. The goal of the present study is to provide geothermal characteristics of thermal manifestations at Rancho Nuevo location based on geochemical and mineralogical results to explain deep-subsurface processes that occurred in the geothermal system. The presence of kaolinite, montmorillonite, opal, zeolite, barite, pyrite, and stibnite in altered soil sediments or around the hot springs identified by the techniques used in the present study, confirms the presence of hydrothermal activity. In addition, based on the X-ray diffraction, calcite precipitates at the surface of the thermal springs. This mineral association reflects deep geothermal processes and is eventually deposited in shallow zones. Fluid mixing processes and variations in redox conditions are suggested by mineral association and isotopic sulfur data. Finally, based on the physicochemical data provided by the water samples and the discharge conditions of the springs, stability diagrams were constructed for pyrite, barite, and zeolites using the Geochemist’s Work Bench program to corroborate these data with the mineralogical results. The mineralogical results and distribution, as well as the N-S trend of mineral associations suggest interaction processes between geothermal fluid and rocks of the stratigraphic sequence, and active major faults, enabling the upward flow of deep geothermal fluids. The approach to the conceptual model of the Rancho Nuevo geothermal prospect reveals an attractive potential for the exploration of a viable geothermal resource in central Mexico.

Elucidating the redox-driven dynamic interactions between arsenic and iron-impregnated biochar in a paddy soil using geochemical and spectroscopic techniques

Iron (Fe)-modified biochar, a renewable amendment that synthetizes the functions of biochar and Fe materials, demonstrates a potential to remediate arsenic (As)-contaminated soils. However, the effectiveness of Fe-based biochar to immobilize As in paddy soils under varying redox conditions (Eh) has not been quantified. We tested the capability of the raw (RBC) and Fe-impregnated (FeBC) biochars to immobilize As in a paddy soil under various Eh conditions (from -400 to +300 mV) using a biogeochemical microcosm system. In the control, As was mobilized (686.2–1535.8 μg L-1) under reducing conditions and immobilized (61.6–71.1 μg L-1) under oxidizing conditions. Application of FeBC immobilized As at Eh < 0 mV by 32.6%–81.1%, compared to the control, because of the transformation of As-bound Fe (hydro)oxides (e.g., ferrihydrite) and the formation of complexes (e.g., ternary As-Fe-DOC). Application of RBC immobilized As at Eh < -100 mV by 16.0%–41.3%, compared to the control, due to its porous structure and oxygen-containing functional groups. Mobilized As at Eh > +200 mV was caused by the increase of pH after RBC application. Amendment of the Fe-modified biochar can be a suitable approach for alleviating the environmental risk of As under reducing conditions in paddy soils.

Sedimentological and geochemical characterisation of the Lower Oligocene Menilite shales from the Magura, Dukla, and Silesian nappes, Polish Outer Carpathians - A new concept

The Oligocene Menilite Beds (Formation) represent the most important hydrocarbon source rock in the Carpathian Belt. The formation is laterally uniform across long distances but it shows strong internal heterogeneity that reflects changes in depositional environments, which controls the source rock potential. The age of the Menilite Formation in the study area is Lower Oligocene – Rupellian - Lower Chattian (nannofossil zone NP22-NP24). Generally, Menilite Beds are dominated by fine-grained (claystone, mudstone, and siltstone) facies, but in some areas they are replaced by thick sandy facies or both types are present. In the study area, both types of facies are present; however, only fine-grained facies have been sedimentologically analysed here because these types can also provide good analytical material for organic geochemistry. The conducted research was aimed at verifying the concepts discussed in the literature that the Menilite Beds were formed in one or more different sedimentary basins, and thus also different palaeogeographic conditions in a small area of the Polish Carpathians, which is located on the border of three tectonic units, precisely in the research area. Therefore, a comparison of these areas was carried out both in terms of facies interpretation and geochemical characteristics. The sedimentological features of 11 different sections of the Menilite Beds from seven localities have been examined: two connected to the Magura Nappe, six to the Dukla Nappe (including tectonic windows) and three to the Silesian Nappe, and total of eight facies (F1 to F8) have been distinguished. Facies F1 from Magura Nappe shows aerobic deposition conditions for sediments with a decisive majority of terrestrial material originating from both angio- and gymnosperms, but from Dukla Nappe facies F1 biomarkers indicate a mixed type of marine-terrestrial matter, with a majority of the terrestrial substance and low-oxygen conditions of the sedimentation environment, with an indication of a lacustrine or marine-lacustrine environment, while in Silesian Nappe facies F1 shows marine and typically anoxic features of sedimentation environment. Facies F2, which is presence in all units, characterise a high content of terrestrial-type substance or there is no clear domination of a terrestrial, marine, or lacustrine component. Facies F2 in Magura Nappe has many features in common with facies F1 but differs higher share of angiosperms (high content of terrestrial-type substance), compared to in the Dukla and Silesian units. It does not show clear features or biomarkers that can be unambiguously interpreted to indicate one specific type of sedimentation condition. Facies F3, which was found in the Silesian unit, stands out due to its high deposition environment anoxicity index and a lack of markers proving a share of angiosperms with higher content of algae-type material. The structures found within these facies shows clear lamination, which frequently accompanies algae mats, hence its sedimentation environment may be similar to lacustrine conditions. Facies F4 is present only in the Dukla unit with all its geochemical features and indicates the deltaic and marine shallow-water aerobic conditions of its deposition environment, which has a share of organic matter of marine and terrestrial types. Facies F5, which is only present in the Silesian Nappe, originated in an anaerobic environment. A high input of organic matter from freshwater and terrestrial origins, indicated by biomarkers, is consistent with shallow-water deposition. Facies F6, present in the Magura Nappe, similar like in Dukla Nappe in most cases stands out due to its low generation potential and high deposition environment aerobicity indices. The origin of organic matter is typically terrestrial originating from angiosperms matter that was deposited probably in lacustrine environment, contrary to the domination of gymnosperms observed in F1 and F2 in the Dukla unit. Stratification of this facies with light and dark colours of fine-grained deposits can indicate cyclically oxidation changes in the sedimentary basin. Generally, biomarker data suggest varied types of organic matter from marine in the Silesian Nappe through mixed in the Dukla Nappe to typically terrestrial in the Magura Nappe which may indicate a deposition in separate basins with their periodical joining. Most of the geochemical parameters indicate also varying redox conditions during deposition in shallow water. Additionally, a variety of the sedimentary structures within accompanying thin-bedded sandstone beds were also recognised. These sedimentary structures provide a possibility to interpret the sedimentary environments of those facies. Most of them are connected to wide-type shallow-water sedimentary environments and are found in the zones between the storm-wave base and the fair-weather water wave base (facies F2, F4, F5 and F6). The analysis confirmed the variability of the sedimentation conditions of the Menilite Beds in terms of the deposition conditions, which can also be used to infer the paleogeographic variability of the original deposition areas.

Source, sink and preservation of organic matter from a machine learning approach of polar lipid tracers in sediments and soils from the Yellow River and Bohai Sea, eastern China

Transport and transformation of organic matter (OM) from the river to the marginal sea is a significant part of the global carbon cycle. Biomarkers are of indispensable advantage in precisely identifying the origin of OM that is crucial to understand the organic carbon cycle. Application of more biomarker molecules with mutually confirmable information commonly implies stricter constraint of the source but also brings challenges to the data analysis and interpretation due to a large amount of molecular information. Here we used random forest (RF) classification models to analyze 123 polar lipid biomarkers of six categories, including fatty alcohols, fatty acids, alkan-2-ones, steroids, triterpenoids, and 1-O-monoalkylglycerol ethers (MAGEs) from the sediments and soils in the Yellow River and the Bohai Sea of eastern China. The environmental specificity of biomarkers was assessed based on the effective distinguishment of samples from different habitats by RF models. The sources of polar lipid biomarkers were constrained according to their environmental specificity, and four genetic classifications, i.e., bacteria, algae and zooplankton, terrestrial higher plants, and anthropogenic input were identified. The spatial distribution of OM sources provides a reasonable scheme of the sink for biospheric OM in this typical “land-river-ocean” system. A type of MAGEs as the most important variables for the RF models was effectively used to be a potential bottom-water oxygen proxy to assess the preservation of OM, and ca. 37% of marine in-situ fresh OM was estimated to decompose under varying redox conditions in the surface sediments of Bohai Sea.