Paul R. Bloom

An evaluation of rate equations for calcite precipitation kinetics at pCO2 less than 0.01 atm and pH greater than 8

We used a reproducible seeded growth technique with a pH-stat to study the kinetics of calcite precipitation at 25°C. We performed different experiments at initial Ca2+ and HCO3− concentrations ranging from 0.7–2 and 4–7 mmol L−1, pH values ranging from 8.25 to 8.70, pCO2 values ranging from 0.0006 to 0.01 atm, and ionic strengths ranging from 0.015 to 0.10 mol L−1. With this experimental data set, we used initial rate measurements and integral methods to test several precipitation rate equations. Rate equations that possess a disequilibrium functional dependence, such as the BURTON et al. (1951) dislocation model, forms of the Davies and Jones (1955) model, and the model used by Reddy and Nancollas (1973), did not adequately describe the kinetics of calcite precipitation at pH greater than 8 and pCO2 less than 0.01 atm. Rate equations that describe independent dissolution and precipitation mechanisms with elementary reactions, such as the equation presented by Plummeret al. (1978), and nancollas and Reddy (1971) were more successful. However, Plummers model did not adequately describe the rate of all experiments due to the presence of an OH− surface term in the precipitation rate equation. The elementary reaction of the Nancollas and Reddy model is written in terms of bulk Ca2+ and CO3− concentrations, and appears to be the most successful model which describes calcite precipitation at pH > 8 and pCO2 < 0.01 atm. The Nancollas and Reddy model, altered to account for varying ionic strengths, adequately described the rate of all experiments and yielded a precipitation rate constant of 118.2 ± 13.9 dm6 mol−1 m−2 s−1, with an apparent Arrhenius activation energy of 48.1 kJ mol−1.

Bonding of methyl mercury to reduced sulfur groups in soil and stream organic matter as determined by x-ray absorption spectroscopy and binding affinity studies

Abstract We combined synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy and binding affinity studies to determine the coordination, geometry, and strength of methyl mercury, CH3Hg (II), bonding in soil and stream organic matter. Samples of organic soil (OS), potentially soluble organic substances (PSOS) from the soil, and organic substances from a stream (SOS) draining the soil were taken along a short “hydrological transect.” We determined the sum of concentrations of highly reduced organic S groups (designated Org-SRED), such as thiol (RSH), disulfane (RSSH), sulfide (RSR), and disulfide (RSSR), using sulfur K-edge XANES. Org-SRED varied between 27% and 64% of total S in our samples. Hg LIII-edge EXAFS analysis were determined on samples added CH3Hg (II) to yield CH3Hg (II)/Org-SRED ratios in the range 0.01–1.62. At low ratios, Hg was associated to one C atom (the methyl group) at an average distance of 2.03 ± 0.02 A and to one S atom at an average distance of 2.34 ± 0.03 A, in the first coordination shell. At calculated CH3Hg(II)/Org-SRED ratios above 0.37 in OS, 0.32 in PSOS, and 0.24 in SOS, the organic S sites were saturated by CH3Hg+, and O (and/or N) atoms were found in the first coordination shell of Hg at an average distance of 2.09 ± 0.01 A. Based on the assumption that RSH (and possibly RSSH) groups take part in the complexation of CH3Hg+, whereas RSSR and RSR groups do not, approximately 17% of total organic S consisted of RSH (+ RSSH) functionalities in the organic soil. Corresponding figures for samples PSOS and SOS were 14% and 9%, respectively. Competitive complexation of CH3Hg+ by halide ions was used to determine the average binding strength of native concentrations of CH3Hg (II) in the OS sample. Using data for Org-SRED, calculated surface complexation constants were in the range from 1016.3 to 1016.7 for a model RSH site having an acidity constant of mercaptoacetic acid. These values compare favorably with identically defined stability constants (log K1) for the binding of methyl mercury to thiol groups in well-defined organic compounds.

Aluminum toxicity in forests exposed to acidic deposition: The ALBIOS results

The ALBIOS project was conducted to examine the influence of acidic deposition on aluminum transport and toxicity in forested ecosystems of eastern North America and northern Europe. Patterns of aluminum chemistry were evaluated in 14 representative watersheds exposed to different levels of sulfur deposition. Controlled studies with solution and soil culture methods were used to test interspecific differences in aluminum sensitivity for one indicator species (honeylocust - Gleditsia triacanthos L. ) and six commercial tree species (red spruce - Picea rubens Sarg., red oak - Quercus rubra L., sugar maple - Acer saccharum Marsh., American beech - Fagus grandifolia Ehrh., European beech - Fagus sylvatica, and loblolly pine - Pinus Taeda L. ). Overall, red spruce was the tree species whose growth was most sensitive to soluble aluminum, with significant biomass reductions occurring at Al concentrations of approximately 200–250 umol/L. Analyses of soil solutions from the field sites indicated that the conditions for aluminum toxicity for some species exist at some of the study areas. At these watersheds, aluminum toxicity could act as a contributing stress factor affecting forest growth.

Characterization of laboratory weathered labradorite surfaces using X-ray photoelectron spectroscopy and transmission electron microscopy

Altered surfaces of labradorite resulting from laboratory weathering at pH 4 and 25°C were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). SEM micrographs showed nonuniform surface alteration of labradorite weathered at pH 3.7 for 415 days. TEM micrographs showed exsolution lamellae of a more calcic-rich and more sodic-rich phase, each averaging approximately 700 a thick. The more calcic phase was preferentially weathered to average depths of 1350 A in excess of the more sodic phase, producing a corrugated surface. The surface roughness caused by preferential weathering of the more calcic phase affects the relative exposure of calcic and sodic phases to the XPS detector. A three-dimensional analysis of possible surface exposures was used to predict the influence of a corrugated surface on XPS measurements. Actual XPS data showed significant Ca depletion, slight Al depletion, slight Si enrichment, and slight Na enrichment relative to unweathered labradorite. Sputter depth profiling with an Ar ion gun showed that surface alteration was significant up to depths of 500 A, similar to the depth of preferential weathering of the more calcic lamellae observed with TEM. Predicted XPS data accounting for the topographic effects of a corrugated surface showed similar trends of Ca depletion, slight Al depletion, slight Si enrichment, and moderate Na enrichment. Furthermore, predicted XPS sputter depth profiles of Ca, Al, Si, and Na were similar to actual sputter depth profiles, indicating that a significant amount of the surface alteration on labradorite can be explained by preferential weathering of the more calcic lamellae, and the subsequent surface roughness effects this has on XPS spectra. Other surface processes such as H+ or H3O+ exchange for Ca, Na, or Al and preferential weathering at sites of excess surface energy (dislocations, twin boundaries, etc.) not accounted for in the predicted XPS data may also contribute to the surface composition of weathered labradorite. Results showing preferential weathering of more calcic-rich lamellae and its effect on XPS spectra indicate the importance of understanding the micro-structure of feldspars used for laboratory weathering studies.

Prehistoric lowland Maya environment and subsistence economy

A collection of essays presenting original data that have allowed the author to reconstruct prehistoric Maya environment and subsistence.

Effects of Manure and Cultivation on Carbon Dioxide and Nitrous Oxide Emissions from a Corn Field under Mediterranean Conditions

The use of organic residues as soil additives is increasing, but, depending on their composition and application methods, these organic amendments can stimulate the emissions of CO(2) and N(2)O. The objective of this study was to quantify the effects of management practices in irrigated sweet corn (Zea mays L.) on CO(2) and N(2)O emissions and to relate emissions to environmental factors. In a 3-yr study, corn residues (CR) and pasteurized chicken manure (PCM) were used as soil amendments compared with no residue (NR) under three management practices: shallow tillage (ST) and no tillage (NT) under consecutive corn crops and ST without crop. Tillage significantly increased (P < 0.05) CO(2) and N(2)O fluxes in residue-amended plots and in NR plots. Carbon dioxide and N(2)O fluxes were correlated with soil NH(4) concentrations and with days since tillage and days since seeding. Fluxes of CO(2) were correlated with soil water content, whereas N(2)O fluxes had higher correlation with air temperature. Annual CO(2) emissions were higher with PCM than with CR and NR (9.7, 2.9, and 2.3 Mg C ha(-1), respectively). Fluxes of N(2)O were 34.4, 0.94, and 0.77 kg N ha(-1) yr(-1) with PCM, CR, and NR, respectively. Annual amounts of CO(2)-C and N(2)O-N emissions from the PCM treatments were 64 and 3% of the applied C and N, respectively. Regardless of cultivation practices, elevated N(2)O emissions were recorded in the PCM treatment. These emissions could negate some of the beneficial effects of PCM on soil properties.

Factors affecting bicarbonate chemistry and iron chlorosis in soils

Abstract The bicarbonate ion has been implicated as a causative agent in iron chlorosis induced by high moisture in calcareous soils. Our studies have demonstrated a correlation between soil solution bicarbonate and chlorosis in soybeans in fields in western Minnesota and under growth chamber conditions. In the growth chamber studies, we found that the severity of chlorosis increases with increasing soil moisture to near the saturated water content. At high moisture, soil pores were filled with water and the partial pressure of CO2 in the soil air increased. Since the soil pH is strongly buffered by the high cation exchange capacity of the soils, soil pH remained constant and [HCO‐ 3] increased. In a system in which CaC03 is in equilibrium with the solution phase, the [HCO‐ 3] should be controlled by CaCO3 solubility. In soils, however, the soil solutions were as much as 20‐fold over‐saturated with respect to CaCO3 solubility and [HCO‐ 3] was much greater than predicted by solubility calculations.

Change in surface area and dissolution rates during hornblende dissolution at pH 4.0

Abstract Four particle-size fractions (0.045–0.075, 0.075–0.11, 0.11–0.25, and 0.50–1.00 mm sieve sizes) were used to study the relationships of bulk dissolution rates (mol g−1s−1) to particle size and measured specific surface area. All reactions were performed on sonically cleaned samples in pH 4.0, 0.01 M HOAc-LiOAc buffer at 298 K. To minimize the effects of grinding, dissolution rates were not determined until 30 days of weathering had occurred; subsequent bulk dissolution rates were still nonlinear with time and the reaction was incongruent. Bulk reaction rates were higher for the smaller particle size fractions. The rates of release of Al, Fe, and Mg were first order with respect to surface area, but the order of release for Si was 0.39. These results suggest that the direct relationship between the quantity of exposed crystalline defects and dissolution rates suggested by previous investigators does not hold true for hornblende. The surface area of hornblende grains increased 99% on average during the first 30 days of weathering, and an additional 12% during the next 29 days of weathering. Scanning electron micrographs and N2 adsorption-desorption isotherms (77 K) indicated that the formation of etch pits and hollow dissolution cores contributed to the increased surface area. High resolution TEM observations of surface materials removed by ultrasonic treatment and N2 adsorption hysteresis data suggest weathering along cleavage planes with the formation of 0.5–13 nm pores. Most of the increased adsorption of N2 in the surface area determinations, however, was due to the formation of larger etch features.

A comparative study of soil solution chemistry associated with chlorotic and nonchlorotic soybeans in western Minnesota

Abstract We planted three varieties of soybeans (Glycine max (L.) Merr.) with varying Fe‐ efficiency ratings (Swift, Hodgson, Anoka) across a high‐lime chlorotic spot in western Minnesota. Our objective was to determine the chemical components in soil solutions associated with chlorotic and nonchlorotic areas along transects through the chlorotic spot. We also measured soil temperature, soil moisture and bulk density along the transects. Chlorosis, determined by visual ratings and chlorophyll content, was associated with high CaCO3(s) and mobile ion (i.e. Mg2+, Na+, Cl‐) accumulations at the edge of the depressional area. High soil solution Mg2+ (≈10 mM) and high plant Mg (1.5%) were consistently associated with low chlorophyll content, suggesting a possible causative relationship in reducing Fe2+ uptake. High Mg/Ca ratios (?1.8) in chlorotic areas were related to the over‐saturation of the soil solutions with respect to calcite. High HCO‐ 3 activities (8 ?M) correlated with chlorosis for one transect, bu...

Mobilization of non-exchangeable K by ryegrass in five Moroccan soils with and without mica

Intensive cropping of Italian ryegrass (Lolium multiforum L.) in pots was used to assess the contribution of non-exchangeable K to plant uptake. The soils used were: two soils high in mica (illite) developed on recent alluvium plus two smectitic (beidellitic) soils and a soil of mixed mineralogy rich in mica. Four K treatments were used (0, 28.6, 143, and 286 mg kg-1 soil) with 8 successive monthly cuttings. A response of plant K uptake to added K was observed in all soils. Both 1.0 M NH40Ac and 0.2 M CaCl2 extractable K were depleted to a minimum level specific for each soil. The minima were lower in the old upland soils compared to the young alluvial soils. Uptake of K by Italian ryegrass induced K release from the non-exchangeable K to replenish the plant available pool of K ions. The release of mica interlayer K in the alluvial and in the high K smectitic soil supplied sufficient K to plants even under intensive cropping. The rate of mobilization of interlayer K was low in the smectitic soil with lower K. The lowest release rate was in the old high mica soil. Iron coatings may have inhibited mobilization of interlayer K. The rates of mobilization cannot be predicted from mineralogical and K-extraction data only. The rates of K uptake and the rates of K release by ryegrass under intensive cropping are potential values which can be used for modelling K availability to plants in the soils studied.