Study on Mechanical Properties of Basalt Fiber Shotcrete in High Geothermal Environment

With the development of infrastructure, there are growing numbers of high geothermal environments, which, therefore, form a serious threat to tunnel structures. However, research on the changes in mechanical properties of shotcrete under high temperatures and humid environments are insufficient. In this paper, the combination of various temperatures (20 °C/40 °C/60 °C) and 55% relative humidity is used to simulate the effect of environment on the strength and stress–strain curve of basalt fiber reinforced shotcrete. Moreover, a constitutive model of shotcrete considering the effect of fiber content and temperature is established. The results show that the early mechanical properties of BFRS are improved with the increase in curing temperature, while the compressive strength at a later age decreases slightly. The 1-day and 7-day compressive strength of shotcrete at 40 °C and 60 °C increased by 10.5%, 41.1% and 24.1%, 66.8%, respectively. The addition of basalt fiber can reduce the loss of later strength, especially for flexural strength, with a increase rate of 11.9% to 39.5%. In addition, the brittleness of shotcrete increases during high temperature curing, so more transverse cracks are observed in the failure mode, and the peak stress and peak strain decrease. The addition of basalt fiber can improve the ductility and plasticity of shotcrete and increase the peak strain of shotcrete. The constitutive model is in good agreement with the experimental results.

Cement / rock interaction in geothermal wells

<p>One of the main issues associated with the exploitation of geothermal energy is the durability of the cement that is used downhole to cement the steel casing to the formation. Cement durability can have a major impact on the lifetime of geothermal wells, which do not usually last as long as desirable. The cement formulations used in the construction of geothermal wells are designed to provide mechanical support to the metallic well casings and protect them against the downhole harsh environment, which often leads to corrosion. This research is focused on the way that these formulations interact with the surrounding rock formation in geothermal environments, and aims to understand whether these are likely to affect the cement durability and, consequently, the geothermal well lifetime. The experimental work in this thesis consists of examining the changes in the interfacial transition zone (ITZ) that forms between geothermal cements and the volcanic rocks, after hydrothermal treatment. Holes were drilled in blocks of volcanic rocks and cement slurries with distinct formulations were poured into the cavities. The assemblages were autoclaved under typical geothermal conditions. The main variables under study were the cement formulation, the temperature of curing (150°C and 290°C), the presence of drilling mud, CO₂ exposure and the type of rock. The results show that with all the Portland cement based systems a series of chemical reactions occur at the interface between the cement and the rock, the ITZ, where migration of Ca²⁺ and OH⁻ ions occurs from the cement into the rock pores. These reactions are ongoing, which occur faster during the first days/few weeks of curing, mostly driven by physical process of cement movement into the rock, followed by a slower second stage, controlled mostly by chemical driving forces. This work highlights the interdependence between the chemical and physical interactions between geothermal cements and volcanic rocks which are complex. Variables such as temperature and time of curing and silica addition affect the cement phases that form, while the amount of amorphous silica and rock permeability dictate the extent of rock interaction. The presence of carbon dioxide influences the extent of rock/cement interaction and this can be controlled by the rock permeability and cement formulation. Consequently, most of the above mentioned variables were found to have an impact on the geothermal cement durability, which depends on the way these factors are combined.</p>

Cement / rock interaction in geothermal wells

<p>One of the main issues associated with the exploitation of geothermal energy is the durability of the cement that is used downhole to cement the steel casing to the formation. Cement durability can have a major impact on the lifetime of geothermal wells, which do not usually last as long as desirable. The cement formulations used in the construction of geothermal wells are designed to provide mechanical support to the metallic well casings and protect them against the downhole harsh environment, which often leads to corrosion. This research is focused on the way that these formulations interact with the surrounding rock formation in geothermal environments, and aims to understand whether these are likely to affect the cement durability and, consequently, the geothermal well lifetime. The experimental work in this thesis consists of examining the changes in the interfacial transition zone (ITZ) that forms between geothermal cements and the volcanic rocks, after hydrothermal treatment. Holes were drilled in blocks of volcanic rocks and cement slurries with distinct formulations were poured into the cavities. The assemblages were autoclaved under typical geothermal conditions. The main variables under study were the cement formulation, the temperature of curing (150°C and 290°C), the presence of drilling mud, CO₂ exposure and the type of rock. The results show that with all the Portland cement based systems a series of chemical reactions occur at the interface between the cement and the rock, the ITZ, where migration of Ca²⁺ and OH⁻ ions occurs from the cement into the rock pores. These reactions are ongoing, which occur faster during the first days/few weeks of curing, mostly driven by physical process of cement movement into the rock, followed by a slower second stage, controlled mostly by chemical driving forces. This work highlights the interdependence between the chemical and physical interactions between geothermal cements and volcanic rocks which are complex. Variables such as temperature and time of curing and silica addition affect the cement phases that form, while the amount of amorphous silica and rock permeability dictate the extent of rock interaction. The presence of carbon dioxide influences the extent of rock/cement interaction and this can be controlled by the rock permeability and cement formulation. Consequently, most of the above mentioned variables were found to have an impact on the geothermal cement durability, which depends on the way these factors are combined.</p>

Complete Genome Sequence of a Novel Sulfolobales Archaeon Strain, HS-7, Isolated from the Unzen Hot Spring in Japan

The order Sulfolobales includes thermoacidophilic archaea that thrive in acidic geothermal environments. A novel Sulfolobales archaeon strain, HS-7, which may represent a novel genus, was isolated from an acidic hot spring in Japan. We report the 2.15-Mb complete genome sequence of strain HS-7.

Expanded Diversity and Phylogeny of mer Genes Broadens Mercury Resistance Paradigms and Reveals an Origin for MerA Among Thermophilic Archaea

Mercury (Hg) is a highly toxic element due to its high affinity for protein sulfhydryl groups, which upon binding, can destabilize protein structure and decrease enzyme activity. Prokaryotes have evolved enzymatic mechanisms to detoxify inorganic Hg and organic Hg (e.g., MeHg) through the activities of mercuric reductase (MerA) and organomercury lyase (MerB), respectively. Here, the taxonomic distribution and evolution of MerAB was examined in 84,032 archaeal and bacterial genomes, metagenome assembled genomes, and single-cell genomes. Homologs of MerA and MerB were identified in 7.8 and 2.1% percent of genomes, respectively. MerA was identified in the genomes of 10 archaeal and 28 bacterial phyla previously unknown to code for this functionality. Likewise, MerB was identified in 2 archaeal and 11 bacterial phyla previously unknown to encode this functionality. Surprisingly, homologs of MerB were identified in a number of genomes (∼50% of all MerB-encoding genomes) that did not encode MerA, suggesting alternative mechanisms to detoxify Hg(II) once it is generated in the cytoplasm. Phylogenetic reconstruction of MerA place its origin in thermophilic Thermoprotei (Crenarchaeota), consistent with high levels of Hg(II) in geothermal environments, the natural habitat of this archaeal class. MerB appears to have been recruited to the mer operon relatively recently and likely among a mesophilic ancestor of Euryarchaeota and Thaumarchaeota. This is consistent with the functional dependence of MerB on MerA and the widespread distribution of mesophilic microorganisms that methylate Hg(II) at lower temperature. Collectively, these results expand the taxonomic and ecological distribution of mer-encoded functionalities, and suggest that selection for Hg(II) and MeHg detoxification is dependent not only on the availability and type of mercury compounds in the environment but also the physiological potential of the microbes who inhabit these environments. The expanded diversity and environmental distribution of MerAB identify new targets to prioritize for future research.

Verrucomicrobial methanotrophs grow on diverse C3 compounds and use a homolog of particulate methane monooxygenase to oxidize acetone

Short-chain alkanes (SCA; C2-C4) emitted from geological sources contribute to photochemical pollution and ozone production in the atmosphere. Microorganisms that oxidize SCA and thereby mitigate their release from geothermal environments have rarely been studied. In this study, propane-oxidizing cultures could not be grown from acidic geothermal samples by enrichment on propane alone, but instead required methane addition, indicating that propane was co-oxidized by methanotrophs. “Methylacidiphilum” isolates from these enrichments did not grow on propane as a sole energy source but unexpectedly did grow on C3 compounds such as 2-propanol, acetone, and acetol. A gene cluster encoding the pathway of 2-propanol oxidation to pyruvate via acetol was upregulated during growth on 2-propanol. Surprisingly, this cluster included one of three genomic operons (pmoCAB3) encoding particulate methane monooxygenase (PMO), and several physiological tests indicated that the encoded PMO3 enzyme mediates the oxidation of acetone to acetol. Acetone-grown resting cells oxidized acetone and butanone but not methane or propane, implicating a strict substrate specificity of PMO3 to ketones instead of alkanes. Another PMO-encoding operon, pmoCAB2, was induced only in methane-grown cells, and the encoded PMO2 could be responsible for co-metabolic oxidation of propane to 2-propanol. In nature, propane probably serves primarily as a supplemental growth substrate for these bacteria when growing on methane.

Genomic adaptations enabling Acidithiobacillus distribution across wide-ranging hot spring temperatures and pHs

Background Terrestrial hot spring settings span a broad spectrum of physicochemistries. Physicochemical parameters, such as pH and temperature, are key factors influencing differences in microbial composition across diverse geothermal areas. Nonetheless, analysis of hot spring pools from the Taupo Volcanic Zone (TVZ), New Zealand, revealed that some members of the bacterial genus, Acidithiobacillus, are prevalent across wide ranges of hot spring pHs and temperatures. To determine the genomic attributes of Acidithiobacillus that inhabit such diverse conditions, we assembled the genomes of 19 uncultivated hot spring Acidithiobacillus strains from six geothermal areas and compared these to 37 publicly available Acidithiobacillus genomes from various habitats. Results Analysis of 16S rRNA gene amplicons from 138 samples revealed that Acidithiobacillus comprised on average 11.4 ± 16.8% of hot spring prokaryotic communities, with three Acidithiobacillus amplicon sequence variants (ASVs) (TVZ_G1, TVZ_G2, TVZ_G3) accounting for > 90% of Acidithiobacillus in terms of relative abundance, and occurring in 126 out of 138 samples across wide ranges of temperature (17.5–92.9 °C) and pH (1.0–7.5). We recovered 19 environmental genomes belonging to each of these three ASVs, as well as a fourth related group (TVZ_G4). Based on genome average nucleotide identities, the four groups (TVZ_G1-TVZ_G4) constitute distinct species (ANI < 96.5%) of which three are novel Acidithiobacillus species (TVZ_G2-TVZ_G4) and one belongs to Acidithiobacillus caldus (TVZ_G1). All four TVZ Acidithiobacillus groups were found in hot springs with temperatures above the previously known limit for the genus (up to 40 °C higher), likely due to significantly higher proline and GC contents than other Acidithiobacillus species, which are known to increase thermostability. Results also indicate hot spring-associated Acidithiobacillus have undergone genome streamlining, likely due to thermal adaptation. Moreover, our data suggest that Acidithiobacillus prevalence across varied hot spring pHs is supported by distinct strategies, whereby TVZ_G2-TVZ_G4 regulate pH homeostasis mostly through Na+/H+ antiporters and proton-efflux ATPases, whereas TVZ_G1 mainly relies on amino acid decarboxylases. Conclusions This study provides insights into the distribution of Acidithiobacillus species across diverse hot spring physichochemistries and determines genomic features and adaptations that potentially enable Acidithiobacillus species to colonize a broad range of temperatures and pHs in geothermal environments.

Bacterial biosilicification: A new insight into the global silicon cycle

Biosilicification is the process by which organisms incorporate soluble, monomeric silicic acid, Si(OH)4, in the form of polymerized insoluble silica, SiO2. Biosilicifying eukaryotes, including diatoms, siliceous sponges, and higher plants, have been the targets of intense research to study the molecular mechanisms underlying biosilicification. By contrast, prokaryotic biosilicification has been less well studied, partly because the biosilicifying capability of well-known bacteria was not recognized until recently. This review summarizes recent findings on bacterial extracellular and intracellular biosilicification, the latter of which has been demonstrated only recently in bacteria. The topics discussed herein include bacterial (and archaeal) extracellular biosilicification in geothermal environments, encapsulation of Bacillus spores within a silica layer, and silicon accumulation in marine cyanobacteria. The possible contribution of bacterial biosilicification to the global silicon cycle is also discussed.