Association of dissolved aluminum with silica: Connecting molecular structure to surface reactivity using NMR

We studied uptake mechanisms for dissolved Al on amorphous silica by combining bulk-solution chemistry experiments with solid-state Nuclear Magnetic Resonance techniques (²⁷Al magic-angle spinning (MAS) NMR, ²⁷Al{¹H} cross-polarization (CP) MAS NMR and ²⁹Si{¹H} CP-MAS NMR). We find that reaction of Al (1 mM) with amorphous silica consists of at least three reaction pathways; (1) adsorption of Al to surface silanol sites, (2) surface-enhanced precipitation of an aluminum hydroxide, and (3) bulk precipitation of an aluminosilicate phase. From the NMR speciation and water chemistry data, we calculate that 0.20 (±0.04) tetrahedral Al atoms nm⁻² sorb to the silica surface. Once the surface has sorbed roughly half of the total dissolved Al (∼8% site coverage), aluminum hydroxides and aluminosilicates precipitate from solution. These precipitation reactions are dependent upon solution pH and total dissolved silica concentration. We find that the Si:Al stoichiometry of the aluminosilicate precipitate is roughly 1:1 and suggest a chemical formula of NaAlSiO₄ in which Na⁺ acts as the charge compensating cation. For the adsorption of Al, we propose a surface-controlled reaction mechanism where Al sorbs as an inner-sphere coordination complex at the silica surface. Analogous to the hydrolysis of , we suggest that rapid deprotonation by surface hydroxyls followed by dehydration of ligated waters results in four-coordinate (>SiOH)₂Al(OH)₂ sites at the surface of amorphous silica.     

Low-oxygen and chemical kinetic constraints on the geochemical niche of neutrophilic iron(II) oxidizing microorganisms

Neutrophilic iron oxidizing bacteria (FeOB) must actively compete with rapid abiotic processes governing Fe(II) oxidation and as a result have adapted to primarily inhabit low-O₂ environments where they can more successfully compete with abiotic Fe(II) oxidation. The spatial distribution of these microorganisms can be observed through the chemical gradients they affect, as measured using in situ voltammetric analysis for dissolved Fe(II), Fe(III), O₂, and FeS_(aq). Field and laboratory determination of the chemical environments inhabited by the FeOB were coupled with detailed kinetic competition studies for abiotic and biotic oxidation processes using a pure culture of FeOB to quantify the geochemical niche these organisms inhabit. In gradient culture tubes, the maximum oxygen levels, which were associated with growth bands of Sideroxydans lithotrophicus (ES-1, a novel FeOB), were 15-50 μM. Kinetic measurements made on S. lithotrophicus compared biotic/abiotic (killed control) Fe oxidation rates. The biotic rate can be a significant and measurable fraction of the total Fe oxidation rate below O₂ concentrations of approximately 50 μM, but biotic Fe(II) oxidation (via the biotic/abiotic rate comparison) becomes difficult to detect at higher O₂ levels. These results are further supported by observations of conditions supporting FeOB communities in field settings. Variablity in cell densities and cellular activity as well as variations in hydrous ferrous oxide mineral quantities significantly affect the laboratory kinetic rates. The microbial habitat (or geochemical niche) where FeOB are active is thus largely controlled by the competition between abiotic and biotic kinetics, which are dependent on Fe(II) concentration, P_O2, temperature and pH in addition to the surface area of hydrous ferric oxide minerals and the cell density/activity of FeOB. Additional field and lab culture observations suggest a potentially important role for the iron-sulfide aqueous molecular cluster, FeS_(aq), in the overall cycling of iron associated with the environments these microorganisms inhabit.     

Reaction mechanism of uranyl in the presence of zero-valent iron nanoparticles

The current study provides an investigation of abiotic reduction of an oversaturated uranyl solution driven by iron nanoparticle oxidation. The reactivity of nano-scale zero-valent iron (ZVI) under mildly oxic conditions (1.2% O₂ and 0.0017% CO₂) was studied in 1000 ppm uranyl solution in the pH range 3-7, at reaction times from 10 min to 4 h. Reductive precipitation of UO₂ was observed as the main process responsible for the removal of uranium from solution with the kinetics of reaction becoming increasingly favourable at higher pH. Despite working with an oversaturated uranium solution, the precipitation of UO₂ occurred in preference to precipitation of UO₃·2H₂O (metaschoepite) at reaction times between 1 and 4 h and for uranyl solutions initially set up at pH ?5. Characterisation of both solid and solution phases was performed using X-ray photoelectron spectroscopy (XPS), focused ion beam (FIB) imaging, X-ray diffraction (XRD) and inductively coupled plasma atomic emission spectroscopy (ICP-AES).     

Biotite weathering and nutrient uptake by ectomycorrhizal fungus, Suillus tomentosus, in liquid-culture experiments

Ectomycorrhiza-forming fungi (EMF) alter the nutrient-acquisition capabilities of vascular plants, and may play an important role in mineral weathering and the partitioning of products of weathering in soils under nutrient-limited conditions. In this study, we isolated the weathering function of Suillus tomentosus in liquid-cultures with biotite micas incubated at room temperature. We hypothesized that the fungus would accelerate weathering by hyphal attachment to biotite surfaces and transmission of nutrient cations via direct exchange into the fungal biomass. We combined a mass-balance approach with scanning electron microscopy (SEM) and atomic force microscopy (AFM) to estimate weathering rates and study dissolution features on biotite surfaces. Weathering of biotite flakes was about 2-3 orders of magnitude faster in shaken liquid-cultures with fungus compared to shaken controls without fungus, but with added inorganic acids. Adding fungus in nonshaken cultures caused a higher dissolution rate than in inorganic pH controls without fungus, but it was not significantly faster than organic pH controls without fungus. The K⁺, Mg²⁺ and Fe²⁺ from biotite were preferentially partitioned into fungal biomass in the shaken cultures, while in the nonshaken cultures, K⁺ and Mg²⁺ was lost from biomass and Fe²⁺ bioaccumulated much less. Fungal hyphae attached to biotite surfaces, but no significant surface changes were detected by SEM. When cultures were shaken, the AFM images of basal planes appeared to be rougher and had abundant dissolution channels, but such channel development was minor in nonshaken conditions. Even under shaken conditions the channels only accounted for only 1/100 of the total dissolution rate of 2.7 × 10⁻¹⁰ mol of biotite m⁻² s⁻¹. The results suggest that fungal weathering predominantly occurred not by attachment and direct transfer of nutrients via hyphae, but because of the acidification of the bulk liquid by organic acids, fungal respiration (CO₂), and complexation of cations which accelerated dissolution of biotite. Results further suggest that both carbohydrate source (abundant here) and a host with which nutrients are exchanged (missing here) may be required for EMF to exert an important weathering effect in soils. Unsaturated conditions and physical dispersal of nutrient-rich minerals in soils may also confer a benefit for hyphal growth and attachment, and promote the attachment-mediated weathering which has been observed elsewhere on soil mineral surfaces.     

Isotopic studies of leaf water. Part 1: A physically based two-dimensional model for pine needles

Understanding of isotopic variations in leaf water is important for reconstruction of paleoclimate and assessment of global biochemical processes. We report here a study of isotopic distributions within a single needle of two pine species, Pinus resinosa Ait and Pinus strobes L., with the objective of understanding how isotopic compositions of leaf water are controlled by environmental and physiological variables. A 2D model was developed to simulate along-leaf isotopic variations and bulk leaf water isotopic compositions. In addition to variables common to all leaf water isotopic models, this 2D model also takes into account the specific geometry and dimensions of pine needles and the isotopic transport in xylem and mesophyll. The model can successfully simulate oxygen isotopic variations along a single needle and averaged over a leaf (bulk leaf water). The simulations suggest that isotopic composition of the bulk leaf water does not always depend only upon the average transpiration rate, which in turn raises questions about using leaf water isotopic values to estimate transpiration rates. An unsuccessful attempt to simulate along-needle hydrogen isotopic variations suggests that certain unknown biological process(es) may not have been incorporated into our 2D model, and if so, it calls for a reevaluation of all other models for hydrogen isotopic simulations of leaf water since they too lack these processes.Existing leaf water isotopic models are reviewed in this work. In particular, we evaluate the most frequently used model, the stomatal boundary layer model (also referred to as the Craig-Gordon model). We point out that discrepancy between the boundary layer model and the measured bulk leaf water seems to depend upon relative humidity. Using our 2D model, we show that this humidity dependency is a result of an interplay between environmental and physiological conditions: if the transpiration rate of plant leaves decreases with increasing relative humidity, our 2D model can reproduce the pattern of isotopic discrepancy between boundary layer model predictions and observations, enabling us to understand better the reason behind this discrepancy.     

The effect of species diversity on metal adsorption onto bacteria

In this study, we measure proton, Pb, and Cd adsorption onto the bacteria Deinococcus radiodurans, Thermus thermophilus, Acidiphlium angustum, Flavobacterium aquatile, and Flavobacterium hibernum, and we calculate the thermodynamic stability constants for the important surface complexes. These bacterial species represent a wide genetic diversity of bacteria, and they occupy a wide range of habitats. All of the species, except for A. angustum, exhibit similar proton and metal uptake. The only species tested that exhibits significantly different protonation behavior is A. angustum, an acidophile that grows at significantly lower pH than the other species of this study. We demonstrate that a single, metal-specific, surface complexation model can be used to reasonably account for the acid/base and metal adsorption behaviors of each species. We use a four discrete site non-electrostatic model to describe the protonation of the bacterial functional groups, with averaged pK_a values of 3.1 ± 0.3, 4.8 ± 0.2, 6.7 ± 0.1, and 9.2 ± 0.3, and site concentrations of (1.0 ± 0.17) × 10⁻⁴, (9.0 ± 3.0) × 10⁻⁵, (4.6 ± 1.8) × 10⁻⁵, and (6.1 ± 2.3) × 10⁻⁵ mol of sites per gram wet mass of bacteria, respectively. Adsorption of Cd and Pb onto the bacteria can be accounted for by the formation of complexes with each of the bacterial surface sites. The average log stability constants for Cd complexes with Sites 1-4 are 2.4 ± 0.4, 3.2 ± 0.1, 4.4 ± 0.1, and 5.3 ± 0.1, respectively. The average log stability constants for Pb complexes with Sites 1-4 are 3.3 ± 0.2, 4.5 ± 0.3, 6.5 ± 0.1, and 7.9 ± 0.5, respectively. This study demonstrates that a wide range of bacteria exhibit similar proton and metal adsorption behaviors, and that a single set of averaged acidity constants, site concentrations, and stability constants for metal-bacterial surface complexes yields a reasonable model for the adsorption behavior of many of these species. The differences in adsorption behavior that we observed for A. angustum demonstrate that genetic differences do exist between the cell wall functional group chemistries of some bacterial species, and that significant exceptions to the typical bacterial adsorption behavior do exist.     

The Fe-C system at 5 GPa and implications for Earth’s core

Earth’s core may contain C, and it has been suggested that C in the core could stabilize the formation of a solid inner core composed of Fe₃C. We experimentally examined the Fe-C system at a pressure of 5 GPa and determined the Fe-C phase diagram at this pressure. In addition, we measured solid metal/liquid metal partition coefficients for 17 trace elements and examined the partitioning behavior between Fe₃C and liquid metal for 14 trace elements. Solid metal/liquid metal partition coefficients are similar to those found in one atmosphere studies, indicating that the effect of pressure to 5 GPa is negligible. All measured Fe₃C/liquid metal partition coefficients investigated are less than one, such that all trace elements prefer the C-rich liquid to Fe₃C. Fe₃C/liquid metal partition coefficients tend to decrease with decreasing atomic radii within a given period. Of particular interest, our 5 GPa Fe-C phase diagram does not show any evidence that the Fe-Fe₃C eutectic composition shifts to lower C contents with increasing pressure, which is central to the previous reasoning that the inner core may be composed of Fe₃C.     

Neoliberalizing environmental governance? Land trusts, private conservation and nature on the Oak Ridges Moraine

With recent changes in the ways that state agencies are implementing their environmental policies, the line between public and private is becoming increasingly blurred. This includes shifts from state-led implementation of environmental policies to conservation plans that are implemented and managed by multi-sectoral networks of governments, the private sector and environmental non-governmental organizations (ENGOs). This paper examines land trusts as private conservation initiatives that become part of neoliberal governance arrangements and partnerships that challenge our conceptions of environmental preservation and democratic participation. The paper starts with an examination of the concept of neoliberalized environmental governance. Next, it addresses the shifting social constructions of property and land in the context of protecting large scale ecosystems. Through a case study of the extension of new environmental governance arrangements on the Oak Ridges Moraine in Ontario, we examine the relationships that have formed between different levels of the state and environmental non-governmental organizations. Finally, we analyze the expansion of land trusts and private conservation initiatives that are predicated on private land ownership and the commodification of nature, the emerging discourses and practices of private conservation, and how these are implicated in the privatization and neoliberalization of nature.     

The persistence of lead from past gasoline emissions and mining drainage in a large riparian system: Evidence from lead isotopes in the Sacramento River, California

Lead concentrations and isotope ratios measured in river water colloids and streambed sediment samples along 426 km of the Sacramento River, California reveal that the influence of lead from the historical mining of massive sulfide deposits in the West Shasta Cu-mining district (at the headwaters of the Sacramento River) is confined to a 60 km stretch of river immediately downstream of that mining region, whereas inputs from past leaded gasoline emissions and historical hydraulic Au-mining in the Sierra Nevadan foothills are the dominant lead sources in the remaining 370 km of the river. Binary mixing calculations suggest that more than 50% of the lead in the Sacramento River outside of the region of influence of the West Shasta Cu-mining district is derived from past depositions of leaded gasoline emissions. This predominance is the first direct documentation of the geographic extent of gasoline lead persistence throughout a large riparian system (>160,000 km²) and corroborates previous observations based on samples taken at the mouth of the Sacramento River. In addition, new analyses of sediment samples from the hydraulic gold mines of the Sierra Nevada foothills confirm the present-day fluxes into the Sacramento River of contaminant metals derived from historical hydraulic Au-mining that occurred during the latter half of the 19th and early part of the 20th centuries. These fluxes occur predominantly during periods of elevated river discharge associated with heavy winter precipitation in northern California. In the broadest context, the study demonstrates the potential for altered precipitation patterns resulting from climate change to affect the mobility and transport of soil-bound contaminants in the surface environment.     

The composition and crystallinity of the near-surface regions of weathered alkali feldspars

Our ability to identify thin non-stoichiometric and amorphous layers beneath mineral surfaces has been tested by undertaking X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) work on alkali feldspars from pH 1 dissolution experiments. The outcomes of this work were used to help interpret XPS and TEM results from alkali feldspars weathered for <10,000 years in soils overlying the Shap Granite (north-west England). The chemistry of effluent solutions indicates that silica-rich layers a few nanometers in thickness formed during the pH 1 experiments. These layers can be successfully identified by XPS and have lower Al/Si, Na/Si, K/Si and Ca/Si values than the outermost ∼9 nm of unweathered controls. Development of Al-Si non-stoichiometry is coupled with loss of crystal structure to produce amorphous layers that are identifiable by TEM where >∼2.5 nm thick, whereas the crystallinity of albite is retained despite leaching of Na to depths of tens to hundreds on nanometers. Integration of XPS data over the outermost 6-9 nm of naturally weathered Shap feldspars shows that they have stoichiometric Al/Si and K/Si ratios, which is consistent with findings of previous TEM work on the same material that they lack amorphous layers. There is some XPS evidence for loss of K from the outermost couple of nanometers of Shap orthoclase, and the possibility of leaching of Na from albite to greater depths cannot be excluded using the XPS or TEM results. This study demonstrates that the leached layer model, as formulated from laboratory experiments, is inapplicable to the weathering of alkali feldspars within acidic soils, which is an essentially stoichiometric reaction.