Showing papers in "Elements in 2008"

CO2 Sequestration in Deep Sedimentary Formations

Abstract: Carbon dioxide capture and sequestration (CCS) in deep geological formations has recently emerged as an important option for reducing greenhouse emissions. If CCS is implemented on the scale needed to make noticeable reductions in atmospheric CO2, a billion metric tons or more must be sequestered annually—a 250 fold increase over the amount sequestered today. Securing such a large volume will require a solid scientific foundation defining the coupled hydrologic-geochemical-geomechanical processes that govern the long-term fate of CO2 in the subsurface. Also needed are methods to characterize and select sequestration sites, subsurface engineering to optimize performance and cost, approaches to ensure safe operation, monitoring technology, remediation methods, regulatory overview, and an institutional approach for managing long-term liability.

Mineral Carbonation of CO2

Abstract: A survey of the global carbon reservoirs suggests that the most stable, long-term storage mechanism for atmospheric CO2 is the formation of carbonate minerals such as calcite, dolomite and magnesite. The feasibility is demonstrated by the proportion of terrestrial carbon bound in these minerals: at least 40,000 times more carbon is present in carbonate rocks than in the atmosphere. Atmospheric carbon can be transformed into carbonate minerals either ex situ, as part of an industrial process, or in situ, by injection into geological formations where the elements required for carbonate-mineral formation are present. Many challenges in mineral carbonation remain to be resolved. They include overcoming the slow kinetics of mineral-fluid reactions, dealing with the large volume of source material required and reducing the energy needed to hasten the carbonation process. To address these challenges, several pilot studies have been launched, including the CarbFix program in Iceland. The aim of CarbFix is to inject CO2 into permeable basaltic rocks in an attempt to form carbonate minerals directly through a coupled dissolution-precipitation process.

The Global Phosphorus Cycle: Past, Present, and Future

Abstract: The cycling of phosphorus, a biocritical element in short supply in nature, is an important Earth system process. Variations in the phosphorus cycle have occurred in the past. For example, the rapid uplift of the Himalayan-Tibet Plateau increased chemical weathering, which led to enhanced input of phosphorus to the oceans. This drove the late Miocene “biogenic bloom.” Additionally, phosphorus is redistributed on glacial timescales, resulting from the loss of the substantial continental margin sink for reactive P during glacial sea-level lowstands. The modern terrestrial phosphorus cycle is dominated by agriculture and human activity. The natural riverine load of phosphorus has doubled due to increased use of fertilizers, deforestation and soil loss, and sewage sources. This has led to eutrophication of lakes and coastal areas, and will continue to have an impact for several thousand years based on forward modeling of human activities.

Nanoparticles in the Soil Environment

Abstract: Soils contain many kinds of inorganic and organic particles with at least one dimension in the nanoscale or colloidal range (<100 nm). Well-known examples are clay minerals, metal (hydr)oxides, and humic substances, while allophane and imogolite are abundant in volcanic soils. Apparently, only a small proportion of nanoparticles in soil occur as discrete entities. Organic colloids in soil, for example, are largely associated with their inorganic counterparts or form coatings over mineral surfaces. For this reason, individual nanoparticles are difficult to separate and collect from the bulk soil, and extraction yields are generally low. By the same token, the characterization of soil nanoparticles often requires advanced analytical and spectroscopic techniques. Because of their large surface area and the presence of surface defects and dislocations, nanoparticles in soil are very reactive towards external solute molecules. The focus of research in recent years has been on the interactions of nanoparticles with environmental pollutants and on their impact on the movement, fate, and bioavailability of contaminants.

Bone and Tooth Mineralization: Why Apatite?

Abstract: Through evolution, vertebrates have “chosen” the calcium phosphate mineral apatite to mineralize their teeth and bones. This article describes the key characteristics of apatite in biological mineralization and explores how the apatite structure allows biology to control mineral composition and functionality. Through the synthesis and testing of calcium phosphates for biomaterials applications, we have gained further understanding of how sensitive the chemical and physical properties of apatite are to its growth conditions.

The Magma Reservoirs That Feed Supereruptions

Abstract: The vigor and size of volcanic eruptions depend on what happens in magma reservoirs in the Earth’s crust. When magmatic activity occurs within continental areas, large reservoirs of viscous, gas-rich magma can be generated and cataclysmically discharged into the atmosphere during explosive supereruptions. As currently understood, large pools of explosive magma are produced by extracting interstitial liquid from long-lived “crystal mushes” (magmatic sponges containing >50 vol% of crystals) and collecting it in unstable liquid-dominated lenses.

Carbon Dioxide Sequestration A Solution to a Global Problem

Abstract: Human and industrial development over the past hundred years has led to a huge increase in fossil fuel consumption and CO 2 emissions, causing a dramatic increase in atmospheric CO 2 concentration. This increased CO 2 is believed to be responsible for a significant rise in global temperature over the past several decades. Global-scale climate modeling suggests that the temperature increase will continue, at least over the next few hundred years, leading to glacial melting and rising sea levels. Increased atmospheric CO 2 also leads to ocean acidification, which will have drastic consequences for marine ecosystems. In an attempt to solve these problems, many have proposed the large-scale sequestration of CO 2 from our atmosphere. This introductory article presents a summary of some of the evidence linking increasing atmospheric CO 2 concentration to global warming and ocean acidification and our efforts to stem this rise though CO 2 sequestration.

Iron Oxides as Geochemical Nanovectors for Metal Transport in Soil-River Systems

Abstract: Topsoils are often contaminated by trace metals, and it is important to understand how different processes govern the transport of such metals to fresh and marine waters This paper presents measurements of natural nanoparticles and colloidal organic matter in soil and river samples from Germany and Sweden In our analytical approach, a nanoparticle separation technique is combined with multielement detection and applied to soil and river samples to link the macroscale field observations with detailed molecular studies in the laboratory It was determined that lead is associated with iron oxide colloids, which are ubiquitous nanoparticles that can be efficiently transported Eventually both iron oxides and lead are removed by flocculation under conditions of estuarine mixing Iron-rich nanoparticles compete efficiently with natural organic matter (NOM) complexation for lead binding in both the soil and river systems studied

The Upper Mantle and Transition Zone

Abstract: The upper mantle is the source of almost all magmas. It contains major transitions in rheological and thermal behaviour that control the character of plate tectonics and the style of mantle dynamics. Essential parameters in any model to describe these phenomena are the mantle's compositional and thermal structure. Most samples of the mantle come from the lithosphere. Although the composition of the underlying asthenospheric mantle can be estimated, this is made difficult by the fact that this part of the mantle partially melts and differentiates before samples ever reach the surface. The composition and conditions in the mantle at depths significantly below the lithosphere must be interpreted from geophysical observations combined with experimental data on mineral and rock properties. Fortunately, the transition zone, which extends from approximately 410 to 660 km, has a number of characteristic globally observed seismic properties that should ultimately place essential constraints on the compositional and thermal state of the mantle.

Phosphorus Removal and Recovery from Municipal Wastewaters

Abstract: Phosphorus is a key pollutant in municipal wastewater. To minimise eutrophication, treatment facilities must often reduce phosphorus levels to less than 1 mg L -1 . Two main approaches to achieving this are chemical precipitation and enhanced biological uptake. Chemical precipitation is widely used and relatively simple; biological phosphorus removal is more complex but relies less on the addition of chemicals and also offers the opportunity to reuse the phosphorus. Phosphorus can be released from cells and converted to calcium phosphate or the mineral struvite. While the products have been shown to be excellent fertilisers, the economic drivers for recovery are still not clear.

Biogenic Uraninite Nanoparticles and Their Importance for Uranium Remediation

Abstract: Biogenic uraninite is of interest to geoscientists for its importance to bioremediation strategies, remarkably small particle size, and biological origin. Recent studies have begun to illuminate the chemical/structural complexities of this important natural nanomaterial. Intriguingly, in spite of its incredibly diminutive size, the molecular-scale structure, energetics, and surface-area-normalized dissolution rates of hydrated biogenic uraninite appear to be similar to those of coarser-particle, abiotic, stoichiometric UO 2 . These findings have important implications for the role of size as a moderator of nanoparticle aqueous reactivity and for the bioremediation of subsurface U(VI) contamination.

Supervolcanoes and their explosive supereruptions

Abstract: Earth's largest volcanic eruptions were an order of magnitude larger than any witnessed by humans since the advent of civilization. These “supereruptions” have played an important role in our species' past and they pose a serious future threat. In this issue of Elements , we consider key issues that reflect both the scientific and social importance of these awe-inspiring phenomena: the products and processes of the eruptions themselves, the nature and evolution of the shallow magma chambers that feed them, the monitoring of active supervolcano systems, and the potential consequences to humans of future supereruptions.

Nanoparticles in the Atmosphere

Abstract: The most continuous and intimate contact the average person has with nanoparticles is almost surely through the air, which is replete with them. Nanoparticles are being generated continuously and in large numbers by vehicles and industries in urban areas and by vegetation and sea spray in rural areas. Volcanoes are sporadic sources of huge numbers. Nanoparticles have large surface area to volume ratios and react rapidly in the atmosphere, commonly growing into particles large enough to interact with radiation and to have serious consequences for visibility and local, regional, and global climate. They also have potentially significant health effects.

Phosphate Mineral Reactivity and Global Sustainability

Abstract: Phosphorus is a unique element. It is the limiting nutrient controlling biological productivity in many terrestrial and marine environments. When in excess, however, dissolved phosphate leads to uncontrollable biological growth and water-quality problems through a process called eutrophication. The use of phosphate minerals and their products as fertilizers has increased tremendously global food production; it would not be possible to feed the current world population without phosphate fertilizers. Yet phosphate is a limited global resource; current estimates suggest economic phosphorus supply may be severely depleted over the next 100 years. Nevertheless, mineralogists and geochemists have invested little time investigating phosphate mineral stability, reactivity, and transformations. This issue attempts to bring phosphates to the forefront of our scientific endeavours.

Monitoring a Supervolcano in Repose : Heat and Volatile Flux at the Yellowstone Caldera

Abstract: Although giant calderas (“supervolcanoes”) may slumber for tens of thousands of years between eruptions, their abundant earthquakes and crustal deformation reveal the potential for future upheaval. Any eventual supereruption could devastate global human populations, so these systems must be carefully scrutinized. Insight into dormant but restless calderas can be gained by monitoring their output of heat and gas. At Yellowstone, the large thermal and CO2 fluxes require massive input of basaltic magma, which continues to invade the lower to mid-crust, sustains the overlying high-silica magma reservoir, and may result in volcanic hazard for millennia to come. The high flux of CO2 may contribute to the measured deformation of the caldera floor and can also modify the pressure, thermal, and chemical signals emitted from the magma. In order to recognize precursors to eruption, we must scrutinize the varied signals emerging from restless calderas with the goal of discriminating magmatic, hydrothermal, and hybrid phenomena.

Ore Deposits of the Platinum-Group Elements

Abstract: The formation of ore deposits of the platinum-group elements (PGE) requires that their concentrations be raised about four orders of magnitude above typical continental crustal abundances. Such extreme enrichment relies principally on the extraction capacity of sulfide liquid, which sequesters the PGE from silicate magmas. Specific aspects of PGE ore formation are still highly controversial, however, including the role of hydrothermal fluids. The majority of the world's PGE reserves are held in a handful of deposits, most of which occur within the unique Bushveld Complex of South Africa.

Phosphates and Nuclear Waste Storage

Abstract: A significant effort has been made by the scientific community to evaluate the potential of phosphate minerals and glasses as nuclear waste storage hosts. Radioactive waste-bearing phosphates, including monazites, apatites, and glasses, can be readily synthesized in the laboratory. Because of their low solubilities and slow dissolution rates, these phosphates are more resistant to corrosion by geological fluids than many other potential nuclear waste storage hosts, including borosilicate glass. Phosphates are, however, not currently being used for nuclear waste storage, in part because their synthesis at the industrial scale is relatively labor intensive, often requiring the separation of the waste into distinct fractions of elements. Such limitations may be overcome by adding phosphate amendments to backfill material, which could provoke the precipitation of stable radiactive waste-bearing phosphate minerals in situ.

Platinum-Group Elements: A New Set of Key Tracers for the Earth's Interior

Abstract: Due to their “iron-loving” properties, platinum-group elements (PGE) are expected to be stored in the Earth’s core. Although very low, at a few parts per billion, PGE concentrations measured in mantle-derived rocks are too high to be in chemical equilibrium with the core. The “late veneer” model offers the best explanation for this paradox—it postulates that a flux of primitive meteorites hit the early Earth after core formation had ceased. However, the inferred PGE composition of the hypothetical primitive mantle exhibits slight positive excesses of Ru, Rh, and Pd compared to the canonical chondritic signature. Such deviations have triggered considerable debate about the composition of the late veneer and the extent of reworking of PGE signatures by igneous processes within the Earth’s mantle.

Environmental Relevance of the Platinum Group Elements

Abstract: Platinum-group elements (PGE) are used in an increasing number of applications, and emissions are resulting in elevated environmental concentrations of these normally rare metals Automobile exhaust catalysts, which use Pd, Pt, and Rh as active components, are the main source of PGE emitted into urban and roadside environments, and they contribute to a global increase in PGE concentrations Emitted PGE are found in urban air and accumulate on the road surface and in roadside soil Transport of PGE via stormwater is resulting in contamination of aquatic environments There is now mounting evidence that a fraction of PGE in the environment is bioavailable, and potential uptake into the biosphere is raising concern over potential risks for humans and the environment

Nanogeoscience: From Origins to Cutting-Edge Applications

Abstract: At first glance, nano and Earth seem about as far apart as one can imagine. Nanogeoscience seems to be a word connecting opposites. But to a growing number of Earth scientists, this term makes sense. Although relatively difficult to detect and study, natural nanomaterials are ubiquitous in nature. Their properties are often different (sometimes dramatically different) from those of the same material at a larger size. In many cases, larger equivalents do not even exist. By understanding natural nanomaterials, we can acquire another perspective from which to view Earth's chemical and physical properties. Important insights into local, regional, and even global phenomena await our understanding of processes that are relevant at the smallest scales of Earth science research.

Ocean Storage of CO2

Abstract: One method for minimizing climate change is to capture CO2 from power plants and inject it into the deep ocean, thus reducing the magnitude and rate of change of CO2 concentration in the atmosphere and the surface ocean. Many discharge options are possible, with varied mixing and retention characteristics. The ocean's capacity is vast, and mathematical models suggest that injected CO2 could remain sequestered for several hundred years. While theoretical and laboratory studies support the viability of ocean storage, field experiments are necessary to realistically evaluate the environmental impact.

How Long Does It Take to Supersize an Eruption

Abstract: Along-recognized correlation between the volume of major eruptions and the time interval between them suggests that magma may accumulate for about a million years before a supereruption. However, radiometric ages and time-dependent phenomena like crystal growth and compositional homogenization show that the duration of supervolcano magma accumulation could be significantly shorter than this. Crystals in supervolcano magmas may have protracted growth histories and may grow from chemically different hosts as crystallization progresses. Semisolid crystal mushes rather than liquid-rich magma chambers may be the prevalent state of supervolcano feeder systems and should be the focus of geophysical studies aimed at predicting future supereruptions.

Consequences of Explosive Supereruptions

Abstract: Rare but extremely large explosive supereruptions lead to the catastrophic formation of huge calderas, devastation of substantial regions by pyroclastic flow deposits, and ash falls that cover continent-sized areas. The effects of future supereruptions will be felt globally or at least by a whole hemisphere. The most widespread effects are likely to derive from the volcanic gases released, particularly sulfur gases that are converted into sulfuric acid aerosols in the stratosphere. These will remain for several years, promoting changes in atmospheric circulation and causing surface temperatures to fall dramatically in many regions, bringing about temporary reductions in light levels and producing severe and unseasonable weather ('volcanic winter'). Major disruptions to global societal infrastructure can be expected for periods of months to years, and the cost to global financial markets will be high and sustained.

Structure, chemistry, and properties of mineral nanoparticles

Abstract: Nanoparticle properties can depart markedly from their bulk analog materials, including large differences in chemical reactivity, molecular and electronic structure, and mechanical behavior. The greatest changes are expected at the smallest sizes, e.g. 10 nm and below, where surface effects are expected to dominate bonding, shape and energy considerations. The precise chemistry at nanoparticle interfaces can have a profound effect on structure, phase transformations, strain, and reactivity. Certain phases may exist only as nanoparticles, requiring transformations in chemistry, stoichiometry and structure with evolution to larger sizes. In general, mineralogical nanoparticles have been little studied.

Phosphate Minerals, Environmental Pollution and Sustainable Agriculture

Abstract: The availability of phosphorus in soils is controlled by the ability of plants to dissolve phosphate-bearing minerals, including apatite and feldspars. To satisfy the requirement of plants for phosphate, mineral dissolution competes with precipitation such as, for example, reactions involving lead or other heavy metals. Plants exude organic acid anions that very effectively enhance mineral dissolution but that may also liberate harmful solutes, such as aluminium. To make readily soluble chemical fertilisers, apatite in igneous and sedimentary rocks is mined and processed; in organic farming, phosphate-rich rocks are crushed and applied directly to the soil, relying on compounds produced by plant roots (exudates) to extract the phosphorus that plants need.

“Fizzics” of Bubble Growth in Beer and Champagne

Abstract: Although beer and champagne are mostly enjoyed at leisure, the myriad physical and chemical processes in them are challenging. Furthermore, studying these processes sheds light on explosive volcanic and lake eruptions because bubble growth is a process common to all of them. We model the growth rate of rising bubbles in beer and champagne. Due to different initial gas concentrations, the eruption velocity of champagne is two orders of magnitude higher than that of CO2-based beer. In N2-based Guinness beer, bubble growth is slow, leading to smaller bubbles that can be entrained by downward flow; these are often seen as sinking bubbles.

CO2 Capture and Transport

Abstract: International interest in CO 2 capture and storage (CCS), as a method of reducing carbon dioxide emissions linked to global climate change, has been growing in recent years. CCS is particularly attractive for large industrial facilities, especially electric power plants, which contribute a large share of global CO 2 emissions from combustion of coal and other fossil fuels. This paper describes the current status of technologies to capture CO 2 and transport it to a storage site. The performance and cost of capture technologies are discussed, along with related environmental issues and the outlook for improved, lower-cost strategies. The key need now is financing of full-scale demonstrations of CCS at the various types of large coal-based power plants.

Platinum-Group Elements in Cosmochemistry

Abstract: In a cooling solar nebula, five of the six platinum-group elements (PGE) condense as refractory-metal alloys at temperatures above the condensation of Fe-Ni metal. Non-refractory Pd condenses in solid solution with Fe-Ni. Such refractory alloys are preserved in some meteorites, although they are often highly altered. The high resistance of PGE to oxidation leads to efficient extraction with metallic Fe-Ni during metal segregation and core formation. Experimentally determined PGE metal-silicate partition coefficients predict lower contents of PGE in planetary silicates than are found, supporting a late addition of PGE components. PGE are particularly useful as tracers of impacting extraplanetary materials in the strongly PGE-depleted crusts of the Earth and other planets.

Supereruptions and Supervolcanoes: Processes and Products

Abstract: Pyroclastic deposits and lava flows generated by supereruptions are similar to, but tens of times larger than, those observed in historic eruptions. Physical processes control eruption styles, which then dictate what products are available for sampling and how well the eruption sequence can be determined. These erupted products and their ordering in time permit reconstruction of the parental magma chamber. Supervolcanoes also have smaller eruptions that provide snapshots of magma chamber development in the lead-in to and aftermath of supereruptions. Many aspects of supereruption dynamics, although on a vast scale, can be understood from observations or inferences from smaller historic and prehistoric events. However, the great diversity in the timings of supereruptions and in the eruptive behaviour of supervolcanoes present continuing challenges for research.

The Platinum-Group Elements: “Admirably Adapted” for Science and Industry

Abstract: The platinum-group elements (PGE) tend to exist in the metallic state or bond with sulfur or other Group Va and VIa ligands, and often occur as trace accessory minerals in rocks. Combined with three isotopic systems that contain the PGE, these elements afford a unique view of early solar system evolution, planet formation and differentiation, and biogeochemical cycling. Initial purification of the PGE was accomplished in the late 1700s, at which time their unique properties, including high melting point, chemical inertness, and ability to catalyze chemical reactions, became apparent. This led to enormous industrial demand, most notably for fuel production and engine emission control, which combined with scarcity in crustal rocks, has made the PGE a highly valued commodity.