Cellular respiration via the alternative oxidase pathway (AOP) leads to a considerable loss in efficiency. Compared to the cytochrome pathway (COP), AOP produces 0-50% as much ATP per carbon (C) respired. Relative partitioning between the pathways can be measured in vivo based on their differing isotopic discriminations against O-18 in O-2. Starting from published methods, we have refined and tested a new protocol to improve measurement precision and efficiency. The refinements detect an effect of tissue water content (P < 0.0001), which we have removed, and yield precise discrimination endpoints in the presence of pathway-specific respiratory inhibitors [CN- and salicylhydroxamic acid (SHAM)], which improves estimates of AOP/COP partitioning. Fresh roots of Pinus sylvestris were sealed in vials with a CO2 trap. The air was replaced to ensure identical starting conditions. Headspace air was repeatedly sampled and isotopically analyzed using isotope-ratio mass spectrometry. The method allows high-precision measurement of the discrimination against O-18 in O-2 because of repeated measurements of the same incubation vial. COP and AOP respiration discriminated against O-18 by 15.1 +/- 0.3 parts per thousand and 23.8 +/- 0.4 parts per thousand, respectively. AOP contributed to root respiration by 23 +/- 0.2% of the total in an unfertilized stand. In a second, nitrogen-fertilized, stand AOP contribution was only 14 +/- 0.2% of the total. These results suggest the improved method can be used to assess the relative importance of COP and AOP activities in ecosystems, potentially yielding information on the role of each pathway for the carbon use efficiency of organisms.

Nanoconfinement of CO₂ in layered aluminosilicates contributes to the capture and release of this greenhouse gas in soils. In this work, we show that the residence times of CO₂ in montmorillonite are lowered by 15 min for each 1 degrees C increment in temperature during venting. Molecular simulations showed that activation energies of release are no more than half of the experimentally derived value of 34 kJ mol(-1). This raised the possibility of additional processes limiting CO₂ mobility in real materials, including (chemi)sorption at reactive sites or frayed edges or defects. The residence times (approximate to 1616 min at -50 degrees C to approximate to 6 min at 60 degrees C) for some of the driest (approximate to 1.4 mmol H₂O per g) montmorillonites that can be produced at ambient temperatures are readily lowered by inclusion of additional water. They are, in turn, prolonged again as the water content and interlayer spacing become smaller through venting. These efforts showed that soil-building clay minerals will lose their propensity to dynamically exchange CO₂ as temperatures continue to rise, yet they may retain CO₂ more efficiently in cold seasons as soils will become depleted in moisture content.

Atmospheric water vapor binding to soils is a key process driving water availability in unsaturated terrestrial environments. Using a representative hydrophilic iron oxyhydroxide, this study highlights key mechanisms through which water vapor (i) adsorbs and (ii) condenses at mineral surfaces coated with Leonardite humic acid (LHA). Microgravimetry and vibrational spectroscopy showed that liquid-like water forms in the three-dimensional array of mineral-bound LHA when present at total C/Fe ratios well exceeding similar to 73 mg C per g Fe (26 C atoms/nm(2)). Below these loadings, minerals become even less hydrophilic than in the absence of LHA. This lowering in hydrophilicity is caused by the complexation of LHA water-binding sites to mineral surfaces, and possibly by conformational changes in LHA structure removing available condensation environments for water. An empirical relationship predicting the dependence of water adsorption densities on LHA loadings was developed from these results. Together with the molecular-level description provided in this work, this relationship should guide efforts in predicting water availability, and thereby occurrences of water-driven geochemical processes in terrestrial environments.

This study investigated interfacial reactions between aqueous Ce(III) and two synthetic nanosized Mn (hydr-)oxides (manganite: γ-MnOOH, and vernadite: δ-MnO2) in the absence and presence of Nordic Lake fulvic acid (NLFA) at circumneutral pH by batch experiments and cryogenic X-ray photoelectron spectroscopy (XPS). The surfaces of manganite and vernadite were negatively charged (XPS-derived loadings of (Na + K)/Cl > 1) and loaded with 0.42–4.33 Ce ions nm−2. Manganite stabilized Ce-oxidation states almost identical to those for vernadite (approximately 75% Ce(IV) and 25% Ce(III)), providing the first experimental evidence that also a Mn(III) phase (manganite) can act as an important scavenger for Ce(IV) and thus, contribute to the decoupling of Ce from its neighboring rare earth elements and the development of Ce anomaly. In contrast, when exposed to Ce(III)-NLFA complexes, the oxidation of Ce by these two Mn (hydr-)oxides was strongly suppressed, suggesting that the formation of Ce(III) complexes with fulvic acid can stabilize Ce(III) even in the presence of oxidative Mn-oxide surfaces. The experiments also showed that Ce(III) complexed with excess NLFA was nearly completely removed, pointing to a strong preferential sorption of Ce(III)-complexed NLFA over free NLFA. This finding suggests that the Ce(III)-NLFA complexes were most likely sorbed by their cation side, i.e. Ce(III) bridging between oxide groups on the Mn (hydr-)oxides and negatively-charged functional groups in NLFA. Hence, Ce(III) was in direct contact with the oxidative manganite and vernadite but despite that not oxidized. An implication is that in organic-rich environments there may be an absence of Ce(IV) and Ce anomaly despite otherwise favorable conditions for Ce(III) oxidation.

Silicate sorption on ferrihydrite surfaces, as monomers, oligomers, and polymers, strongly affects ferrihydrite crystallinity, thermodynamic stability, and surface reactivity. How these silicate species bind on ferrihydrite surfaces is, however, not well understood. We have determined silicate binding geometries using a combination of X-ray absorption spectroscopy (XAS), differential atomic pair distribution function (d-PDF) analysis, and density functional theory (DFT) calculations. Silicon K-edge absorption pre edges and DFT-predicted energies indicate that silicate forms monomeric monodentate mononuclear (MM) complexes at low silicate sorption loadings. With increasing silicate loading, the pre-edge peak shifts to higher energies, suggesting changes in the silicate binding geometry toward multidentate complexation. The d-PDF analysis determines the Si Fe interatomic distance to be 3.25 A for the high-loading samples. The DFT calculations indicate that such distance corresponds to an oligomer in the bidentate binuclear (BB) binding geometry. The transition of the silicate sorption geometry accompanied by polymerization can affect stability of ferrihydrite and its adsorption and redox reactivity and increase the degree of Si isotopic fractionation upon silicate sorption on Fe oxides. MM monomeric complexes and BB oligomeric complexes should be used for surface complexation models predicting silicate sorption on Fe oxide surfaces.

The vibrational spectral profiles of Na- and Ca-montmorillonite (MMT) of controlled water layer populations (nW) was extracted by chemometric analysis of new Fourier transform infrared (FTIR) spectroscopy data and validated by mixed-layer modeling of previously published X-ray diffraction data. These efforts resolved FTIR spectral profiles of 0W, 1W, and 2W interlayers, which can now be used to explore the distinct hydration states of MMT. These spectral profiles reflect water populations organized around interlayer cations (Na⁺, Ca²⁺), interacting with siloxane groups of the basal face of the interlayer, and with other bound and “free” water molecules. This cohesive description of water-bearing clays provides the link needed to relate vibrational to structural attributes of these geochemically important materials.

Hydrated clay minerals that are common to Earth’s atmosphere and terrestrial and aquatic environments can form gels that host saline solutions. Using cryogenic electron microscopy and vibration spectroscopy, we show that saline gels of montmorillonite frozen at < −90 °C host elongated hexagonal ice (Ih) microcrystals embedded in a network of honeycomb micropores. Freezing segregates salts into walls of aggregated clay nanoparticles sharing face-to-face contacts. Above ∼ −50 °C, clay gels that are sufficiently dense (≫10 g/L) and flexible (Na-exchanged montmorillonite) also host the cryosalt mineral hydrohalite (NaCl·2H2O), either co-existing or entirely replacing Ih in the gels. Hydrohalite does not form in gels of low-density (<10 g/L) or rigid (Ca-exchange montmorillonite) clay particles. These results suggest that hydrohalite forms in expandable clay gels that are sufficiently dense and flexible to retain saline solutions within their walls, possibly through interparticle capillary and hydration forces. These forces effectively oppose water diffusion to growing ice microcrystals within micropores, thus prolonging the lifetime of hydrohalite within these hydrated clay gels. Our findings tie the fate of ice and cryosalt nucleation and growth to the water-retention capability of expandable clay gels.

To exploit the full potential of hematite (α-Fe2O3) as an efficient photoanode for water oxidation, the redox processes occurring at the Fe2O3/electrolyte interface need to be studied in greater detail. Ex situ doping is an excellent technique to introduce dopants onto the photoanode surface and to modify the photoanode/electrolyte interface. In this context, we selected antimony (Sb5+) as the ex situ dopant because it is an effective electron donor and reduces recombination effects and concurrently utilize the possibility to tuning the surface charge and wettability. In the presence of Sb5+ states in Sb-doped Fe2O3 photoanodes, as confirmed by X-ray photoelectron spectroscopy, we observed a 10-fold increase in carrier concentration (1.1 × 1020 vs 1.3 × 1019 cm–3) and decreased photoanode/electrolyte charge transfer resistance (∼990 vs ∼3700 Ω). Furthermore, a broad range of surface characterization techniques such as Fourier-transform infrared spectroscopy, ζ-potential, and contact angle measurements reveal that changes in the surface hydroxyl groups following the ex situ doping also have an effect on the water splitting capability. Theoretical calculations suggest that Sb5+ can activate multiple Fe3+ ions simultaneously, in addition to increasing the surface charge and enhancing the electron/hole transport properties. To a greater extent, the Sb5+- surface-doped determines the interfacial properties of electrochemical charge transfer, leading to an efficient water oxidation mechanism.

Silica-bearing waters in nature often alter the reactivity of mineral surfaces via deposition of Si complexes and solids. In this work, Fourier transform infrared (FTIR) spectroscopy was used to identify hydroxo groups at goethite (alpha-FeOOH) and lepidocrocite (gamma-FeOOH) surfaces that are targeted by ligand exchange reactions with monomeric silicate species. Measurements of samples first reacted in aqueous solutions then dried under N-2(g) enabled resolution of the signature O-H stretching bands of singly (-OH), doubly (mu-OH), and triply coordinated (mu(3)-OH) groups. Samples reacted with Si for 3 and 30 d at pH 4 and 7 revealed that -OH groups were preferentially exchanged by silicate and that mu-OH and mu(3)-OH groups were not exchanged. Based on knowledge of the disposition of -OH groups on the major crystallographic faces of goethite and lepidocrocite, and the response of these groups to ligand exchange prior oligomerization, our work points to the predominance of rows of mononuclear monodentate silicate species, each separated by at least one -OH group. These species are the attachment sites from which oligomerization and polymerization reactions occur, starting at loadings exceeding similar to 1 Si/nm(2) and corresponding to soluble Si concentrations that can be as low as similar to 0.7 mM after 30 d reaction time. Only above such loadings can reaction products grow away from rows of -OH groups and form hydrogen bonds with nonexchangeable mu-OH and mu(3)-OH groups. These findings have important repercussions for our understanding of the fate of waterborne silicate ions exposed to minerals.