Role In Geochemical Processes Research Articles

Soil organic matter dynamics and stability: Climate vs. time

Climate and time are among the most important factors driving soil organic carbon (SOC) stability and accrual in mineral soils; however, their relative importance on SOC dynamics is still unclear. Therefore, understanding how these factors covary over a range of soil developmental stages is crucial to improve our knowledge of climate change impact on SOC accumulation and persistence. Two chronosequences located along a climate gradient were investigated to determine the main interactions among time (age) and climate (precipitation and temperature) on SOC stability and stock with depth. Considering a common depth (0–15 or 0–30 cm), in the drier chronosequence, the older soil showed the highest SOC stock, while the younger exhibited the lowest carbon accumulation. Considering the whole profile, the SOC stock increased with age. In the wetter chronosequence, the younger soil showed the highest SOC stock considering a common depth, whereas, when the entire profile is taken into account, the older one accumulated 2–3 times more SOC than the others. In both chronosequences, significant stocks of SOC (∼42 %) were accumulated below 30 cm. Soil organic matter stability, assessed by thermal analysis and heterotrophic respiration, increases with depth and age only in the drier chronosequence. Soils from the wetter chronosequence were instead characterized by a greater quantity of labile and/or not-stabilized SOC; here, the amorphous Fe/Al-rich secondary mineral weathering products showed an essential predictor function of SOC storage, although they do not seem to be involved in SOC stabilization mechanisms. Otherwise, the interaction of SOC with fine particles, short-range order minerals, and organo-metal complexes represent the significant stabilization mechanisms in soils from drier climate. The results highlighted how the age factor plays an unassuming role in geochemical processes influencing SOC dynamics; however, climate determines different trajectories of soil development and SOC dynamics for a given soil age. Thus, soil age shows a key role in SOC stabilization especially in drier climatic conditions, while wetter conditions determine an accumulation of a higher yet more labile amount of SOC.

Acidophilic microorganisms enhancing geochemical dynamics in an acidic drainage system, Amarillo river in La Rioja, Argentina

The Amarillo River in La Rioja, Argentina, is a natural acidic environment that is influenced by an abandoned mine. The river is characterized by extremely low pH and high concentrations of metals and metalloids. Fe(III)-bearing neoformed precipitated minerals are widespread along the hydrological basin. This work reports the presence of different species of iron-oxidizing bacteria and demonstrates that their action has a significant role in geochemical processes of the Amarillo River, mainly by catalyzing Fe2+ oxidation and intensifying the Fe(III)-bearing mineral precipitation. Various iron oxidizers (i.e. Acidithiobacillus ferrivorans, Leptospirillum ferrooxidans, Ferrimicrobium acidophilum, Alicyclobacillus cycloheptanicus) were detected in enrichment cultures at different temperatures. Moreover, this is the first report confirming that Acidithiobacillus ferrivorans is able to grow at 4 °C. Other acidophilic bacteria (i.e., Acidiphilium iwatensii) and fungi (e.g., Fodinomyces uranophilus, Coniochaeta fodinicola, Acidea extrema, Penicillium sp. and Cladosporium pseudocladosporioides) were also detected. In vitro laboratory studies recreating natural Fe(III)-bearing mineral formation showed that mineral precipitation rate was higher than 350 mg L−1 day−1 in the presence of microorganisms whereas it was about 15 mg L−1 day−1 under abiotic conditions. Jarosite was the only mineral detected in the precipitates generated by microbial action and it was also identified in the Amarillo River bed sediments. Biological Fe2+ oxidation rates depend on temperature which range from 8 to 32 mM day−1 at 4 and 30 °C, respectively. Finally, a conceptual model recognizing the significant microbial role is proposed to gain a better understanding of the biogeochemistry dynamics of the Amarillo River.

Hydrogeodynamic processes in carbonate rocks. Part II. Karst and its influence on geological environment

Formulation of the problem. Chemical activity of groundwater in the natural geological environment is caused by chemical reactions in a complex physical-chemical system "rock-water-gas". Leaching and dissolution play the leading role in carbonate rocks. These processes can best be described by the laws of chemical thermodynamics, which can justify the existence of the relationship between the reactions themselves, and determine the amount of energy consumed by them. This enables to establish not only direction of physical and chemical transformations but also their results in certain geological, hydrogeological and geochemical conditions. The consequences of chemical activity of groundwater can be the following: 1) physical and chemical destruction and formation of minerals and rocks; 2) migration and concentration of chemical elements and their compounds in solutions; 3) formation of groundwater chemical composition.Presentation of main material. Sorption and ion exchange processes play an important role in carbonate water systems. Sorption is the process of selective absorption of liquid and gaseous substances by solid phase and contributes to penetration of various chemical composition in water solutions into rocks.Depending on the depth of karst cavities, karst is divided into young and ancient according to the time of formation. According to the development of covering sediments on the rocks, karst is distinguished by open (without soil-vegetation cover) and closed (blocked by younger formations). Formation and development of closed karst is almost exclusively due to the action of groundwater located in the aquifer, in the rocks where there is karst formation. Instead, open karst is formed with the participation of both underground and surface infiltration waters.Anthropogenic (man-made) factors play an important role in geochemical processes in the "rock-water" system in urban and industrial conditions. They can manifest both through physical and chemical destruction of rocks, on which buildings and structures were constructed, and due to artificial groundwater contamination up to the change of their geochemical type.Conclusions. Karst is a geodynamic consequence of geological activity of groundwater, manifested in dissolution and removal of rocks with the formation of a liquid runoff. For the karst to develop, the following basic conditions are required: a) physical - significant thickness of carbonate rocks and low groundwater level; b) chemical - alkalinity of underground waters of bicarbonate-sodium composition and presence of soluble and free carbon dioxide in them.Hydrogeological peculiarities of karst development on the territory of research are based on the role of cracks and cavities in the filtration of groundwater of different formations and ascending unloading of aggressive to carbonates water from deep horizons and carbon dioxide, as well as unfavorable physical-chemical conditions of marl and chalk rocks dissolution.

Chemical balance of the Yellow River source region, the northeastern Qinghai-Tibetan Plateau: Insights about critical zone reactivity

Rivers play a vital role in geochemical processes of the critical zone. As the Third Pole, the geochemical processes of the Qinghai-Tibetan Plateau are vital to global element cycles. The Yellow River source region is located in the northeastern Qinghai-Tibetan Plateau, yet there has been little research concerning water chemistry in this region. In this study, river water and rain samples were collected to analyze the stable isotopes and major ion chemistry in the Yellow River source region. The δ18O and δD values ranged from −12.9‰ to −4.5‰ and from −95.7‰ to −36.7‰, respectively. The stable isotopic values of tributaries were lower than those in the main stream, and the construction of the dam could result in the enrichment of δ18O and δD. The stable isotopic values showed that precipitation was the main source of river water. The total dissolved solids (TDS) in river water ranged from 214.9 to 563.2 mg/L, and calcium and bicarbonate were the dominant ions. The average contributions of atmospheric input, evaporites, silicates, and carbonates to the dissolved load of river water were 1.8%–12.5%, 12.4%–15.9%, 18.1%–18.4%, and 56.7%–64.3%. The silicate and carbonate weathering rates were 2.3–2.5 t/km2/yr and 17.4–19.6 t/km2/yr, respectively. The chemical denudation rate in the study area was about 23.0–26.8 t/km2/yr, comparable to the world average value. The CO2 consumption rates by silicate and carbonate weathering were 91–94 × 103 moL/km2/yr and 185–208 × 103 moL/km2/yr, respectively. The TDS flux in the study area was 6.1 × 109 kg/yr, which accounted for 38–47% of that of the Yellow River into the sea. This study showed that although as an alpine arid area, the Yellow River source region had a comparable chemical denudation rate to the world average value, and even higher CO2 consumption rates by silicate and carbonate weathering than some of the large rivers in the world.

The Role of Bacterial Spores in Metal Cycling and Their Potential Application in Metal Contaminant Bioremediation

Bacteria are one of the premier biological forces that, in combination with chemical and physical forces, drive metal availability in the environment. Bacterial spores, when found in the environment, are often considered to be dormant and metabolically inactive, in a resting state waiting for favorable conditions for them to germinate. However, this is a highly oversimplified view of spores in the environment. The surface of bacterial spores represents a potential site for chemical reactions to occur. Additionally, proteins in the outer layers (spore coats or exosporium) may also have more specific catalytic activity. As a consequence, bacterial spores can play a role in geochemical processes and may indeed find uses in various biotechnological applications. The aim of this review is to introduce the role of bacteria and bacterial spores in biogeochemical cycles and their potential use as toxic metal bioremediation agents.

Influence of anions and cations on the phase behavior of ternary salt solutions studied by high pressure differential scanning calorimetry

The knowledge of the phase behavior of salt solutions at elevated temperatures and pressures is important for processes using hot compressed water as the process medium, such as supercritical water gasification (SCWG) and supercritical water oxidation (SCWO). Furthermore hydrothermal salt solutions play an important role in geochemical processes in the Earth's crust. We studied ternary solutions related to biomass for obtaining relevant data for the conversion of wet biomass to synthetic natural gas (SNG) in a SCWG process. The experiments were conducted using isochoric high-pressure differential scanning calorimetry (HP-DSC) at a fixed average fluid density of 300kg/m3, a fixed concentration of 0.1mol/kg(H2O) and molar ratios between the two salts of 100:1–1:100. We investigated salts with Na+, K+ and Mg2+ as cations and SO42−, HPO42− and CO32− as anions.

Similarities in 2- and 6-Line Ferrihydrite Based on Pair Distribution Function Analysis of X-ray Total Scattering

Iron(III) hydroxides are abundant in near-surface natural environments and play an important role in geochemical processes and the fate of contaminants. The issue of the structure of the common nanophase material ferrihydrite (ferric hydroxide) is controversial and has been debated in the literature for many years without definitive resolution. At least two types of ferrihydrite, the 2-line and 6-line forms, are conventionally recognized. It has been suggested that these forms possess different structures built up by different mixtures of distinct nanophase components. However, traditional crystallographic methods provide depictions of structure that are most sensitive either to short-range order (X-ray absorption) or long-range periodicity (X-ray diffraction or electron diffraction). We used high-energy X-ray total scattering for pair distribution function analysis to observe both the short- and intermediate-range ordering (exceeding ∼15 Å) of synthetic ferrihydrite with three distinct average domain sizes of 2, 3, and 6 nm. We show that there are no significant differences in the underlying structures of these materials and that the differences in the diffraction patterns can be entirely interpreted by variations in the average size of the coherent scattering domains. The average crystallite sizes inferred from the PDF analysis are in good agreement with direct observation by high-resolution transmission electron microscopy.

Radiocarbon content and stable carbon isotopic ratios of individual fatty acids in subsurface soil: Implication for selective microbial degradation and modification of soil organic matter

A subsurface soil sample was studied for molecular composition and stable carbon isotopic ratios (δ13C) of fatty acids (FAs) using a capillary gas chromatography (GC) and GC/combustion/isotope ratio mass spectrometer, respectively. Compound specific radiocarbon analyses of FAs were also conducted using an off-line preparative capillary GC/accelerator mass spectrometer. Molecular distribution of FAs (C14-C32) is characterized by even carbon numbered predominance with two maxima at n-C16 and n-C28, being similar to that of plant leaf. However, branched chain C15 and C17 acids were abundant in soil, indicating bacterial degradation and modification of soil organic carbon. The δ13C of FAs (-35.2 to -23.0‰) are within the range of plant leaf δ13C (-36.4 to -31.2‰), except for heavy values (-23.0 to -28.1‰) of C14 to C18. The heavier δ13C values are most likely interpreted by microbial re-synthesis of shorter chain FAs in soil. We also report, for the first time, a significant diversity of Δ14C values (+17 to +127‰) in soil FAs, with higher values (+96 to +127‰) for saturated and unsaturated C18 and lower values (+16 to +19‰) for longer chain C28 and C30. The higher radiocarbon values can be explained by a combination of selective microbial decomposition of shorter chain FAs in subsurface soil, and the subsequent deposition and mixing of modern carbon with old carbon in soil. The modern carbon may be transported downward in the form of shorter chain FAs and other water-soluble organic compounds by meteoric water and/or tree roots, followed by microbial re-synthesis of lipids in soil. This study implies that microbial degradation and modification of soil organic matter play an important role in geochemical processes which control the carbon cycle on the Earth.

A novel protein-mineral interface.

Transferrins transport Fe3+ and other metal ions in mononuclear-binding sites. We present the first evidence that a member of the transferrin superfamily is able to recognize multi-nuclear oxo-metal clusters, small mineral fragments that are the most abundant forms of many metals in the environment. We show that the ferric ion-binding protein from Neisseria gonorrhoeae (nFbp) readily binds clusters of Fe3+, Ti4+, Zr4+ or Hf4+ in solution. The 1.7 A resolution crystal structure of Hf-nFbp reveals three distinct types of clusters in an open, positively charged cleft between two hinged protein domains. A di-tyrosyl cluster nucleation motif (Tyr195-Tyr196) is situated at the bottom of this cleft and binds either a trinuclear oxo-Hf cluster, which is capped by phosphate, or a pentanuclear cluster, which in turn can be capped with phosphate. This first high-resolution structure of a protein-mineral interface suggests a novel metal-uptake mechanism and provides a model for protein-mediated mineralization/dissimilation, which plays a critical role in geochemical processes.