Patrick V. Brady

Direct effects of CO2 and temperature on silicate weathering: Possible implications for climate control

A critical uncertainty in models of the global carbon cycle and climate is the combined effect of organic activity, temperature, and atmospheric CO2 on silicate weathering. Here we present new dissolution rates of anorthite and augite which indicate that silicate weathering in organic-rich solutions is not directly affected by soil CO2 but is very sensitive to temperature. Apparently CO2 accelerates silicate weathering indirectly by fertilizing organic activity and the production of corrosive organic acids. The weathering dependencies highlight the ability of silicate weathering to act as a global thermostat and damp out climate change, when used as input in steady-state carbon cycle and climate models.

Seafloor weathering controls on atmospheric CO2 and global climate

Alteration of surficial marine basalts at low temperatures (<40°C) is a potentially important sink for atmospheric CO2 over geologic time. Petrologic analyses, thermodynamic calculations, and experimental weathering results point to extensive Ca leaching and consumption of marine CO2 during alteration. Basalt weathering in seawater-like solutions is sensitive to temperature. The activation energy for initial basalt weathering in seawater is 41–65 kJ mol−1. If seafloor weathering temperatures are set by deep ocean fluids under high fluid to rock ratios the feedback between weathering and atmospheric CO2 is indirect, but sizeable. If the bulk of seafloor weathering occurs in the presence of low-temperature hydrothermal fluids, the weathering feedback depends on the linkage between spreading rates and heat flow. In either case, the primary linkage between seafloor weathering and the global carbon cycle appears to be thermal as opposed to chemical.

Critical conditions for ferric chloride‐induced flocculation of freshwater algae

The effects of algae concentration, ferric chloride dose, and pH on the flocculation efficiency of the freshwater algae Chlorella zofingiensis can be understood by considering the nature of the electrostatic charges on the algae and precipitate surfaces. Two critical conditions are identified which, when met, result in flocculation efficiencies in excess of 90% for freshwater algae. First, a minimum concentration of ferric chloride is required to overcome the electrostatic stabilization of the algae and promote bridging of algae cells by hydroxide precipitates. At low algae concentrations, the minimum amount of ferric chloride required increases linearly with algae concentration, characteristic of flocculation primarily through electrostatic bridging by hydroxide precipitates. At higher algae concentrations, the minimum required concentration of ferric chloride for flocculation is independent of algae concentration, suggesting a change in the primary flocculation mechanism from bridging to sweep flocculation. Second, the algae must have a negative surface charge. Experiments and surface complexation modeling show that the surface charge of C. zofingiensis is negative above a pH of 4.0 ± 0.3 which agrees well with the minimum pH required for effective flocculation. These critical flocculation criteria can be extended to other freshwater algae to design effective flocculation systems. Biotechnol. Bioeng. 2012; 109:493–501.

Direct measurement of the combined effects of lichen, rainfall, and temperature onsilicate weathering ☆

Abstract A key uncertainty in models of the global carbonate–silicate cycle and long-term climate is the way that silicates weather under different climatologic conditions, and in the presence or absence of organic activity. Digital imaging of basalts in Hawaii resolves the coupling between temperature, rainfall, and weathering in the presence and absence of lichens. Activation energies for abiotic dissolution of plagioclase (23.1 ± 2.5 kcal/mol) and olivine (21.3 ± 2.7 kcal/mol) are similar to those measured in the laboratory, and are roughly double those measured from samples taken underneath lichen. Abiotic weathering rates appear to be proportional to rainfall. Dissolution of plagioclase and olivine underneath lichen is far more sensitive to rainfall.

Surface complexation clues to dolomite growth

Calcium and magnesium adsorb in near-stoichiometric proportions to dolomite over wide ranges in [Ca{sup 2+}]/[Mg{sup 2+}], ionic strength, and solution composition pointing to minimal mixing of metal cations between the CaCO{sub 3} and MgCO{sub 3} layer edges exposed at the dolomite surface. Near-neutral pH Mg and Ca adsorb as hydrated ions, or, in sulfate-rich solutions, as metal sulfate complexes. Near-stoichiometric adsorption of Ca and Mg points to dehydration and subsequent carbonation of adsorbed Mg as the likely rate-limiting step for dolomite growth at near-Earth surface conditions. We propose that one path for dolomite growth from low-temperature natural waters is through the initial adsorption of Mg-sulfate complexes onto either (1) growing dolomite crystals or (2) rate-limiting dolomite nucleii. Field relations, as well as homogeneous synthesis at low temperatures (25{degrees}C < T < 100{degrees}C) support this hypothesis and provide a mechanistic explanation for dolomite growth from sulfate-rich natural waters. 36 refs.

Surface chemistry of K-montmorillonite: ionic strength, temperature dependence and dissolution kinetics.

The surface chemistry of K-montmorillonite was investigated by potentiometric titrations conducted at 25, 50 and 70 degrees C and at ionic strengths of 0.001, 0.01 and 0.1 M KNO(3). Proton adsorption decreases with electrolyte concentration at all pHs. The pH of zero net proton charge (PZNPC) decreases from 8.1 to 7.6 when the ionic strength increases from 0.001 to 0.1 M. Temperature has a very small effect on surface charge. A constant capacitance model that accounts for protonation/deprotonation of aluminol and silanol edge sites and basal plane H(+)/K(+) exchange is used to fit the experimental data. H(+) and OH(-) adsorption to specific surface sites appear to account for the pH-dependence of the K-montmorillonite dissolution.

Effect of Al and organic acids on the surface chemistry of kaolinite

The cause of pH and ionic strength-dependent proton and hydroxyl adsorption onto kaolinite is specific binding at edge Al and Si sites, and it can be modeled as a function of temperature with a triple layer model (TLM) of the mineral-solution interface. Exchange of Al for protons and hydroxyls is observed at low pH, with a stoichiometry approaching 1:3 (Al:H+). Adsorption of organic acids from dilute solutions depends on: 1) solution pH; 2) the functionality of the acid; and, to a lesser extent, 3) temperature. Such adsorption may occur primarily at Al sites exposed on kaolinite edges, as indicated by sorption experiments on the constituent oxides, where negligible sorption was observed on SiO2 (quartz), but was significant on Al2O3 (corundum) surfaces. Under similar conditions, oxalate adsorbs more strongly than acetate or formate to aluminol sites.

Metal sorption to dolomite surfaces

Potential human intrusion into the Waste Isolation Pilot Plant (WIPP) might release actinides into the Culebra Dolomite where sorption reactions will affect of radiotoxicity from the repository. Using a limited residence time reactor the authors have measured Ca, Mg, Nd adsorption/exchange as a function of ionic strength, P{sub CO{sub 2}}, and pH at 25 C. By the same approach, but using as input radioactive tracers, adsorption/exchange of Am, Pu, U, and Np on dolomite were measured as a function of ionic strength, P{sub CO{sub 2}}, and pH at 25 C. Metal adsorption is typically favored at high pH. Calcium and Mg adsorb in near-stoichiometric proportions except at high pH. Adsorption of Ca and Mg is diminished at high ionic strengths (e.g., 0.5M NaCl) pointing to association of Na{sup +} with the dolomite surface, and the possibility that Ca and Mg sorb as hydrated, outer-sphere complexes. Sulfate amplifies sorption of Ca and Mg, and possibly Nd as well. Exchange of Nd for surface Ca is favored at high pH, and when Ca levels are low. Exchange for Ca appears to control attachment of actinides to dolomite as well, and high levels of Ca{sup 2+} in solution will decrease Kds. At the same time, to the extent that high P{sub CO{sub 2}} increase Ca{sup 2+} levels, JK{sub d}s will decrease with CO{sub 2} levels as well, but only if sorbing actinide-carbonate complexes are not observed to form (Am-carbonate complexes appear to sorb; Pu-complexes might sorb as well; U-carbonate complexation leads to desorption). This indirect CO{sub 2} effect is observed primarily at, and above, neutral pH. High NaCl levels do not appear to affect to actinide K{sub d}s.

Deep Borehole Disposal of High-Level Radioactive Waste.

Preliminary evaluation of deep borehole disposal of high-level radioactive waste and spent nuclear fuel indicates the potential for excellent long-term safety performance at costs competitive with mined repositories. Significant fluid flow through basement rock is prevented, in part, by low permeabilities, poorly connected transport pathways, and overburden self-sealing. Deep fluids also resist vertical movement because they are density stratified. Thermal hydrologic calculations estimate the thermal pulse from emplaced waste to be small (less than 20 C at 10 meters from the borehole, for less than a few hundred years), and to result in maximum total vertical fluid movement of ~100 m. Reducing conditions will sharply limit solubilities of most dose-critical radionuclides at depth, and high ionic strengths of deep fluids will prevent colloidal transport. For the bounding analysis of this report, waste is envisioned to be emplaced as fuel assemblies stacked inside drill casing that are lowered, and emplaced using off-theshelf oilfield and geothermal drilling techniques, into the lower 1-2 km portion of a vertical borehole ~ 45 cm in diameter and 3-5 km deep, followed by borehole sealing. Deep borehole disposal of radioactive waste in the United States would require modifications to the Nuclear Waste Policy Act and to applicable regulatory standards for long-term performance set by the US Environmental Protection Agency (40 CFR part 191) and US Nuclear Regulatory Commission (10 CFR part 60). The performance analysis described here is based on the assumption that long-term standards for deep borehole disposal would be identical in the key regards to those prescribed for existing repositories (40 CFR part 197 and 10 CFR part 63).

Rock-based measurement of temperature-dependent plagioclase weathering

Long-term ( > 105 years) weathering can be quantified by measuring microsopic dissolution of minerals in exposed rock surfaces. Digital backscattered (BSE) electron microscope images of plagioclase porosity in field exposures of known age resolves weathering at finer scales and over longer time spans than conventional solute budget and laboratory studies. Rock-based BSE imaging is therefore a potentially useful tool for quantifying steady-state weathering fluxes occurring over geologic time. Here, we compare results of the rock-based method against solute-budget and experimental data, using plagioclase weathering rates from 14C-dated basalt flows on Hualalai Volcano in Hawaii, USA. A rock-based field activation energy of 26.2 kcal mol−1 is somewhat higher than solute-budget and laboratory measurements of plagioclase weathering.