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Abstract Submarine volcanos are the most active areas in the deep sea, but the environmental consequences of frequent volcanic activity on the geophysical fields and biogeochemical processes near hydrothermal chimneys have not been fully understood yet. In particular, how continuous high‐flux thermal energy, the most typical form of energy in active submarine volcanic systems, affects electron transport and geoelectric field remains unknown. This study provides the first evidence that thermal energy can be efficiently converted to electrical energy at an extremely small spatial scale of the submarine black chimneys. The Seebeck coefficient of sulfide chimneys can reach more than 200 μV/K, with high electrical conductivity of 104 S/m and low thermal conductivity of 1.0 W/(m·K) within 300–700 K. A maximal potential gradient of 300 mV/cm under a temperature difference of 300–700 K can be generated by the thermoelectric conversion of sulfide chimneys, with a maximum energy converting efficiency up to 1%. The thermoelectric conversion effect of a global‐scale submarine volcanos could enable electroactive bacteria to fix appromaxiately 105–106 tons of carbon per year. In addition, the thermal‐electrochemical experiments indicated sulfides underwent rapid oxidation under thermoelectric effects, which may help explain the intense oxidative weathering of sulfides in some anoxic deep‐sea hydrothermal zones.

Li and coworkers quantitatively evaluate the tendency and direction of electron transfer in the deep Earth using a machine learning method to predict the electronegativity of atoms and work function of minerals under pressure.

Reductive immobilization has been a commonly used technique to detoxify Cr(VI) from soil; however, it's challenging to remove the reduced Cr from soil to prevent its re-oxidation. This work explored a natural magnetic composite for the remediation, mineralization, and magnetic removal of Cr(VI) from the soil. It consists of 77% magnetite and 23% pyrrhotite with strong magnetic properties. A series of characterization tests show that composites of magnetite and pyrrhotite are interlaced and closely bonded, and contain no other heavy metals. The Cr(VI) removal rate increases with the decrease in composite particle size. A kinetics study shows that removing Cr(VI) by the composite is likely through both adsorption and reduction. Acidic conditions are more favorable for the immobilization of Cr(VI), at 45.8 mg Cr(VI) removal per g of composite at pH 2. After 100 days of in-situ treatment by the composite, the leaching concentration (TCLP) of Cr(VI)-contaminated soil was 1.95 mg L−1, which was below the EPA limit (5 mg L−1) for hazardous waste. After reduction, the composite was separated from soil by magnetic characteristics, and 58.2% of Cr was found mineralized. The post-treatment Cr-containing composite was analyzed by SEM-EDS, Raman spectra, and XPS. It was found that Cr was mineralized on the surface of the composite in the form of Cr(OH)3, Cr2O3, and FeCr2O4. This indicates that reduction and mineralization of Cr(VI) in the soil can be accomplished through natural magnetic mineral composites and easily separated and removed from the soil, achieving a complete soil cleanup.

Dolomite exhibits a wide distribution in geological strata. The metabolic activities of microorganisms in marine sediments play a crucial role in the formation of dolomite. Semiconducting minerals, such as hematite, goethite, and rutile, generate photoelectrons when exposed to sunlight, which can impact the community structure and metabolic activities of microorganisms. In this study, a simulated photoelectron system was conducted to investigate the response of the microbial community, as well as the regulation of sulfate reduction, to photoelectrons using high-throughput sequencing of the 16S rRNA gene. The regulatory effect of semiconducting mineral photoelectrons on the induction of carbonate precipitation by sulfate-reducing bacteria was explored. X-ray diffraction and Raman spectroscopy were used to characterize carbonate precipitation. During cultivation, the pH values of the system increased from 8.0 to approximately 8.5 and the rate of sulfate reduction was significantly enhanced under the influence of simulated photoelectrons. The alpha diversity of the microbial community decreased, and the semiconducting mineral photoelectronic system had a promoting effect on the enrichment of sulfate-reducing bacteria, mainly Desulfovibrio. Under the regulation of photoelectrons, sulfate-reducing bacteria can effectively oxidize organic matter and reduce sulfate in the environment, and proto-dolomite can be formed at a low Mg/Ca ratio. This process has important implications for carbon and sulfur element cycling in estuarine and oceanic photic zones, and provides a new explanation for the formation of large amounts of dolomite in geological history.

In the estuary and nearshore environments, suspended particulate matter (SPM) plays a particularly important role. This article presents a study on the suspended particulate matter and microbial communities in the euphotic zone of China’s nearshore and estuarine regions. The study used various analytical techniques, including ICP-OES, SR-XRD, confocal Raman microscopy, ESEM, and EDX, to investigate the spatial distribution and elemental and mineral compositions of the suspended particulate matters. The study found that semiconducting minerals, such as iron oxide, sulfide minerals (hematite, goethite, and pyrrhotite), and titanium oxide minerals (rutile and anatase), were widely present in the suspended particulate matter. This discovery highlights the photochemical activity of suspended particulate matter in the euphotic zone. Furthermore, a correlation analysis of microbial communities revealed that the content of suspended particulate matter was positively correlated with denitrifying bacteria and metal-reducing bacteria in seawater. This study provides valuable insight into the ecosystem dynamics and biogeochemical cycling of estuaries and coastal seas, which are critical for sustaining the biodiversity and productivity of these ecosystems.

Marine euphotic zone is the pivotal region for interplay of light-mineral–microorganism and elements cycle, in which semiconducting minerals exist widely and iron-bearing goethite is a typical and widespread one. In this work, we have conducted in-depth researches on the effect of ferrous [Fe(II)] ions dissolved by photoreduction of goethite on microbial community structure and diversity. The mineral phase, structure and morphology of synthesized goethite were characterized by Raman, X-ray diffraction (XRD), energy disperse spectroscopy (EDS), environmental scanning electron microscope (ESEM), and atomic force microscope (AFM). Photoelectrochemical measurements tested photoelectric response and redox activity of goethite, having proved its significant property of photoelectric response with 44.11% increment of the average photocurrent density relative to the dark current density. The photoreduction experiments of goethite were conducted under light condition in simulated seawater. It has suggested the photoreduction of goethite could occur and Fe(III) was reduced to Fe(II). The dissolved Fe(II) from the photoreduction of goethite under light condition was nearly 11 times than that group without light after a 10-day reaction. Furthermore, results of microbial community sequencing analysis indicated that dissolved Fe(II) could affect the structure and regulate the decrease of microbial community diversity. The emergence of dominant bacteria associated with iron oxidation and transport protein has suggested their obvious selectivity and adaptability in the environment with adding dissolved Fe(II). This work revealed the photoreduction process of semiconducting goethite was remarkable, giving rise to a non-negligible dissolved Fe(II) and its selective effect on the structure, diversity, as well as the function of microbial community. This light-induced interaction between minerals and microorganisms may also further regulate correlative metabolic pathways of carbon cycle in the marine euphotic zone.

Rock varnish is a thin coating enriched with manganese (Mn) and iron (Fe) oxides. The mineral composition and formation of rock varnish elicit considerable attention from geologists and microbiologists. However, limited research has been devoted to the semiconducting properties of these Fe/Mn oxides in varnish and relatively little attention is paid to the mineral–microbe interaction under sunlight. In this study, the mineral composition and the bacterial communities on varnish from the Gobi Desert in Xinjiang, China were analyzed. Results of principal components analysis and t-test indicated that more electroactive genera such as Acinetobacter, Staphylococcus, Dietzia, and Pseudomonas gathered on varnish bacterial communities than on substrate rock and surrounding soils. We then explored the culture of varnish, substrate and soil samples in media and the extracellular electron transfer (EET) between bacterial communities and mineral electrodes under light/dark conditions for the first time. Orthogonal electrochemical experiments demonstrated that the most remarkable photocurrent density of 6.1 ± 0.4 μA/cm2 was observed between varnish electrode and varnish microflora. Finally, based on Raman and 16S rRNA gene–sequencing results, coculture system of birnessite and Pseudomonas (the major Mn oxide and a common electroactive bacterium in varnish) was established to study underlying mechanism. A steadily growing photocurrent (205 μA at 100 h) under light was observed with a stable birnessite after 110 h. However, only 47 μA was generated in the dark control and birnessite was reduced to Mn2+ in 13 h, suggesting that birnessite helped deliver electrons instead of serving as an electron acceptor under light. Our study demonstrated that electroactive bacterial communities were positively correlated with Fe/Mn semiconducting minerals in varnish, and diversified EET process occurred on varnish under sunlight. Overall, these phenomena may influence bacterial–community structure in natural environments over time.

The solar light response and photoelectrons produced by widespread semiconducting mineral play important roles in biogeochemical cycles on Earth’s surface. To explore the potential influence of photoelectrons generated by semiconducting mineral particles on nitrate-reducing microorganisms in the photic zone, a marine heterotrophic denitrifier Halomonas sp. strain 3727 was isolated from seawater in the photic zone of the Yellow Sea, China. This strain was classified as a Halomonadaceae. Whole-genome analysis indicated that this strain possessed genes encoding the nitrogen metabolism, i.e., narG, nasA, nirBD, norZ, nosB, and nxr, which sustained dissimilatory nitrate reduction, assimilatory nitrate reduction, and nitrite oxidation. This strain also possessed genes related to carbon, sulfur, and other metabolisms, hinting at its substantial metabolic flexibility. A series of microcosm experiments in a simulative photoelectron system was conducted, and the results suggested that this bacterial strain could use simulated photoelectrons with different energy for nitrate reduction. Nitrite, as an intermediate product, was accumulated during the nitrate reduction with limited ammonia residue. The nitrite and ammonia productions differed with or without different energy electron supplies. Nitrite was the main product accounting for 30.03% to 68.40% of the total nitrogen in photoelectron supplement systems, and ammonia accounted for 3.77% to 8.52%. However, in open-circuit systems, nitrite and ammonia proportions were 26.77% and 11.17%, respectively, and nitrogen loss in the liquid was not observed. This study reveals that photoelectrons can serve as electron donors for nitrogen transformation mediated by Halomonas sp. strain 3727, which reveals an underlying impact on the nitrogen biogeochemical cycle in the marine photic zone.

Developing simple and cheap electrocatalysts or photocatalysts for cathodes to increase the oxygen reduction process is a key factor for better utilization of microbial fuel cells (MFCs). Here, we report the investigation of natural wolframite employed as a low-cost cathode photocatalyst to improve the performance of MFCs. The semiconducting wolframite was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy. The band gap and photo respond activities were determined by UV-vis spectroscopy and linear sweep voltammetry (LSV), respectively. Compared with the normal graphite cathode, when MFCs were equipped with a wolframite-coated cathode, the maximum power density was increased from 41.47 mW·m−2 to 95.51 mW·m−2. Notably, the maximum power density further improved to 135.57 mW·m−2 under light irradiation, which was 2.4 times higher than with a graphite cathode. Our research demonstrated that natural wolframite, a low-cost and abundant natural semiconducting mineral, showed promise as an effective photocathode catalyst which has great potential applications related to utilizing natural minerals in MFCs and for environmental remediation by MFCs in the future.

The Danxia landform is representative of the Cretaceous continental red sediment. The careful identification and potential environmental effects of minerals in Danxia red beds have yet to be clearly reported. In this work, reddish sandstone samples were collected from Lang Mountain Danxia landform in Xinning, Hunan province, China, and their mineral phases, element distribution, microstructure, and the spatial relationship of different minerals were investigated using polarizing optical microscope, environmental scanning electron microscopy, energy-dispersive X-ray analysis, electron probe microanalysis, micro-Raman spectra, micro- X-ray diffraction, X-ray fluorescence spectroscopy, and high-resolution transmission electron microscopy. The results revealed that iron oxide (mainly hematite) and titanium oxide (mainly anatase) were the dominant minerals in Danxia red layers. Microcrystalline hematite was suggested as being the coloring mineral. Anatase, reported here for the first time in Danxia red beds, constituted the content of titanium in the red layer (0.17⁻0.57%) and was present in a significantly higher amount than the adjacent limestone formation (0.13%). Over 95% of Fe/Ti oxides served as a cementation agent along the framework of coarse-grain minerals (quartz and feldspar). The hematite and anatase were visible-light-responsive semiconductors, with a band gap of 2.01 eV and 3.05 eV, respectively. Photoelectrochemical experiments were performed on synthetic hematite, anatase, and their coupled material. The inactive hematite displayed an enhanced 23-fold photocurrent at 0.8 V (vs. Ag/AgCl) when coupled with anatase. Furthermore, in a photodegradation experiment using methyl orange dye under simulated sunlight, the coupled material showed decolorizing efficiency 2.4 times that of hematite. The anatase, therefore, prominently improved the photocatalytic activities of hematite. It is proposed that these semiconducting minerals in red beds produce oxygen reactive species and have significant environmental effects, which is of great importance.

The cost-effective exfoliation of layered materials such as transition metal dichalcogenides into mono- or few- layers is of significant interest for various applications. This paper reports the preparation of few-layered MoS2 from natural SiO2-containing molybdenite by exfoliation in isopropanol (IPA) under mild ultrasonic conditions. One- to six-layer MoS2 nanosheets with dimensions in the range of 50-200 nm are obtained. By contrast, MoS2 quantum dots along with nanosheets are produced using N-methyl-pyrrolidone (NMP) and an aqueous solution of poly (ethylene glycol)-block-poly (propylene glycol)-block-poly (ethylene glycol) (P123) as exfoliation solutions. Compared with molybdenite, commercial bulk MoS2 cannot be exfoliated to nanosheets under the same experimental conditions. In the exfoliation process of the mineral, SiO2 associated in molybdenite plays the role of similar superfine ball milling, which significantly enhances the exfoliation efficiency. This work demonstrates that isopropanol can be used to exfoliate natural molybdenite under mild conditions to produce nanosheets, which facilitates the preparation of highly concentrated MoS2 dispersions or MoS2 in powder form due to the volatility of the solvent. Such exfoliated MoS2 nanosheets exhibit excellent photoconductivity under visible light. Hence, the direct mild exfoliation method of unrefined natural molybdenite provides a solution for low-cost and convenient production of few-layered MoS2 which is appealing for industrial applications.

Here we reported the investigation of enhanced performance for the removal of hexavalent chromium (Cr (VI)) by a new microbial fuel cell (MFC) with natural pyrrhotite-coated cathode. By comparisons of the graphite-cathode, the MFCs equipped with a pyrrhotite-coated cathode generated the maximum power density of 45.4 mW·m−2 that was 1.3 times higher than that of with bare graphite cathode (35.5 mW·m−2). Moreover, the Cr (VI) removal efficiency of 97.5% achieved after 4.5 h compared with only 46.1% by graphite cathode MFC. In addition, Cr (VI) removal rate with different initial Cr (VI) concentrations for 10 mg/L and 30 mg/L was investigated and a decreased removal percentage with increasing Cr (VI) concentration was observed. Batches of experiments of different pH values from 3.0 to 9.0 in catholyte were carried out to optimize system performance. The complete Cr (VI) removal was achieved at pH 3.0 and 99.59% of Cr (VI) was removed after 10.5 h, which met the requirement of the Cr (VI) National Emission Standard. When the value of pH was decreasing, the removal rate was obviously increased and Cr (VI) could be removed successfully with a broad pH range indicating pyrrhotite-coated cathode MFC had more extensive usage scope. Furthermore, cathode treatment products were studied by X-ray photoelectron spectroscopy (XPS), Cr2O3, Cr (III)-acetate were detected on the cathode by the XPS Cr2p spectra and no Cr (VI) founded, indicating that the Cr on the surface of cathode was Cr (III) and Cr (VI) were reduced. On cathode, pyrrhotite not only played a significant role for catalyst of MFCs, but also acted as reactive sites for Cr (VI) reduction. Our research demonstrated that pyrrhotite, an earth-abundant and low-cost natural mineral was promised as an effective cathode material. Which had great potential applications in MFCs for reduction of wastewater containing heavy metals and other environmental contaminants in the future.

Exploring the interplay between sunlight, semiconducting minerals, and microorganisms in nature has attracted great attention in recent years. Here we report for the first time the investigation of the interaction between a hematite photoelectrode and Pseudomonas aeruginosa PAO1 under visible light irradiation. Hematite is the most abundant mineral on earth, with a band gap of 2.0 eV. A hematite electrode was electrochemically deposited on fluorine-doped tin oxide (FTO). It was thoroughly characterized by environmental scanning electron microscopy (ESEM), Raman, and UV–Vis spectroscopy, and its prompt response to visible light was determined by linear sweep voltammetry (LSV). Notably, under light illumination, the hematite electrode immersed in a live cell culture was able to produce 240% more photocurrent density than that in the abiotic control of the medium, suggesting a photoenhanced extracellular electron transfer process occurring between hematite and PAO1. Different temperatures of LSV measurements showed bioelectrochemical activity in the system. Furthermore, I–t curves under various conditions demonstrated that both a direct and an indirect electron transferring process occurred between the hematite photoanode and PAO1. Moreover, the indirect electron transferring route was more dominant, which may be mainly attributed to the pyocyanin biosynthesized by PAO1. Our results have expanded our understanding in that in addition to Geobacter and Shewanella it has been shown that more microorganisms are able to perform enhanced extracellular electron transfer with semiconducting minerals under sunlight in nature.

Developing cheap electrocatalysts for cathodic oxygen reduction in neutral medium is a key factor for practical applications of microbial fuel cells (MFCs). Natural hematite was investigated as a low-cost cathode to improve the performance of microbial fuel cells (MFCs). With hematite-coated cathode, the cell current density stabilized at 330.66 ± 3.1 mA·m−2 (with a 1000 Ω load) over 10 days under near-neutral conditions. The maximum power density of MFC with hematite cathode reached to 144.4 ± 7.5 mW·m−2, which was 2.2 times that of with graphite cathode (64.8 ± 5.2 mW·m−2). X-ray diffraction (XRD), Raman, electrode potential analysis, and cyclic voltammetry (CV) revealed that hematite maintained the electrode activities due to the stable existence of Fe(II)/Fe(III) in mineral structure. Electrochemical impedance spectroscopy (EIS) results indicated that the cathodic electron transfer dynamics was significantly improved by using hematite to lower the cathodic overpotential. Therefore, this low-cost and earth-abundant natural mineral is promised as an effective cathode material with potential large-field applications of MFCs in future.

Sunlight drives photosynthesis and associated biological processes, and also influences inorganic processes that shape Earth's climate and geochemistry. Bacterial solar-to-chemical energy conversion on this planet evolved to use an intricate intracellular process of phototrophy. However, a natural nonbiological counterpart to phototrophy has yet to be recognized. In this work, we reveal the inherent "phototrophic-like" behavior of vast expanses of natural rock/soil surfaces from deserts, red soils, and karst environments, all of which can drive photon-to-electron conversions. Using scanning electron microscopy, transmission electron microscopy, micro- Raman spectroscopy, and X-ray absorption spectroscopy, Fe and Mn (oxyhydr)oxide-rich coatings were found in rock varnishes, as were Fe (oxyhydr)oxides on red soil surfaces and minute amounts of Mn oxides on karst rock surfaces. By directly fabricating a photoelectric detection device on the thin section of a rock varnish sample, we have recorded an in situ photocurrent micromapping of the coatings, which behave as highly sensitive and stable photoelectric systems. Additional measurements of red soil and powder separated from the outermost surface of karst rocks yielded photocurrents that are also sensitive to irradiation. The prominent solarresponsive capability of the phototrophic-like rocks/soils is ascribed to the semiconducting Fe- and Mn (oxyhydr)oxide-mineral coatings. The native semiconducting Fe/Mn-rich coatings may play a role similar, in part, to photosynthetic systems and thus provide a distinctive driving force for redox (bio)geochemistry on Earth's surfaces. [ABSTRACT FROM AUTHOR]

Background: Mn oxides occur in a wide variety of geological settings and exert considerable influences on the components and chemical behaviors of sediments and soils. Microbial reduction of Mn oxides is an important process found in many different environments including marine and freshwater sediments, lakes, anoxic basins, as well as oxicanoxic transition zone of ocean. Although the pathway of Mn anaerobic reduction by two model bacteria, Geobacter and Shewanella, has been intensively studied, Mn bio-reduction is still the least well-explored process in nature. Particularly, reduction of Mn oxides by other bacteria and in the presence of O2 has been fewly reported in recent publishes. Results: A series of experiments were conducted to understand the capability of Dietzia DQ12-45-1b in bioreduction of birnessite. In anaerobic systems, Mn reduction rate reached as high as 93% within 4 weeks when inoculated with 1.0 × 1010 cells/mL Dietzia DQ12-45-1b strains. Addition of AQDS enhanced Mn reduction rate from 53 to 91%. The anaerobic reduction of Mn was not coupled by any increase in bacterial protein concentration, and the reduction rate in the stable stage of day 2–14 was found to be in good proportion to the protein concentration. The anaerobic reduction of birnessite released Mn(II) either into the medium or adsorbed on the mineral or bacteria surface and resulted in the dissolution of birnessite as indicated by XRD, SEM and XANES. Under aerobic condition, the reduction rate was only 37% with a cell concentration of 1.0 × 1010 cells/mL, much lower than that in parallel anaerobic treatment. Bacterial growth under aerobic condition was indicated by time-course increase of protein and pH. In contrast to anaerobic experiments, addition of AQDS decreased Mn reduction rate from 25 to 6%. The reduced Mn(II) combined with carbon dioxide produced by acetate metabolism, as well as an alkaline pH environment given by cell growth, finally resulted in the formation of Mn(II)-bearing carbonate (kutnohorite), which was verified by XRD and XANES results. The system with the highest cell concentration of 1.0 × 1010 cells/mL gave rise to the most amount of kutnohorite, while concentration of Mn(II) produced with cell concentration of 6.2 × 108 cells/mL was too low to thermodynamically favor the formation of kutnohorite but result in the formation of aragonite instead. Conclusion: Dietzia DQ12-45-1b was able to anaerobically and aerobically reduce birnessite. The rate and extent of Mn(IV) reduction depend on cell concentration, addition of AQDS or not, and presence of O2 or not. Meanwhile, Mn(IV) bioreduction extent and suspension conditions determined the insoluble mineral products. [ABSTRACT FROM AUTHOR]

Chemoautotrophic organisms have once been excluded from the development of universally applicable CO2 fixation technology due to its low production yields of biomass. In this study, we used Acidithiobacillus ferrooxidans ( A.f.) as a model chemoautotrophic microorganism to test the hypothesis that exogenetic photoelectrons from semiconducting mineral photocatalysis can enable the regeneration of Fe2+ that could be then used by A.f. and support its growth. In a simulated electrochemical system, where exogenetic electrons were provided by an electrochemical approach, an accelerated growth rate of A.f. was observed as compared with that in traditional batch cultivation. In a coupled system, where light-irradiated natural rutile provided the primary electron source to feed A.f., the bacterial growth rate as well as the subsequent CO2 fixation rate was demonstrated to be in a light-dependent manner. The sustaining flow of photogenerated electrons from semiconducting mineral to bacteria provided an inexhaustible electron source for chemoautotrophic bacteria growth and CO2 fixation. This finding might contribute to the development of novel effective CO2 fixation technology. [ABSTRACT FROM AUTHOR]

The article focuses on the study of the Photo reduction of inorganic carbon(+IV) by elemental sulfur, and implications for prebiotic synthesis in terrestrial hot springs. It mentions that the show that elemental sulfur (S0), as the dominant mineral in terrestrial hot springs, can reduce carbon dioxide (CO2) into formic acid (HCOOH) under ultraviolet (UV) light below 280 nm.

The utilization by Alcaligenes faecalis of electrodes as the electron donor for denitrification was investigated in this study. The denitrification rate of A. faecalis with a poised potential was greatly enhanced compared with that of the controls without poised potentials. For nitrate reduction, although A. faecalis could not reduce nitrate, at three poised potentials of +0.06, -0.06, and -0.15 V (versus normal hydrogen electrode [NHE]), the nitrate was partially reduced with -0.15- and -0.06-V potentials at rates of 17.3 and 28.5 mg/liter/day, respectively. The percentages of reduction for -0.15 and -0.06 V were 52.4 and 30.4%, respectively. Meanwhile, for nitrite reduction, the poised potentials greatly enhanced the nitrite reduction. The nitrite reduction rates for three poised potentials ( -0.06, -0.15, and -0.30 V) were 1.98, 4.37, and 3.91 mg/liter/h, respectively. When the potentials were cut off, the nitrite reduction rate was maintained for 1.5 h (from 2.3 to 2.25 mg/liter/h) and then greatly decreased, and the reduction rate (0.38 mg/liter/h) was about 1/6 compared with the rate (2.3 mg/liter/h) when potential was on. Then the potentials resumed, but the reduction rate did not resume and was only 2 times higher than the rate when the potential was off. [ABSTRACT FROM AUTHOR]