Chemical Reaction

Complex networks describe a wide range of systems in nature and society. Frequently cited examples include the cell, a network of chemicals linked by chemical reactions, and the Internet, a network of routers and computers connected by physical links. While traditionally these systems have been modeled as random graphs, it is increasingly recognized that the topology and evolution of real networks are governed by robust organizing principles. This article reviews the recent advances in the field of complex networks, focusing on the statistical mechanics of network topology and dynamics. After reviewing the empirical data that motivated the recent interest in networks, the authors discuss the main models and analytical tools, covering random graphs, small-world and scale-free networks, the emerging theory of evolving networks, and the interplay between topology and the network's robustness against failures and attacks.

A potential function is presented that can be used to model both chemical reactions and intermolecular interactions in condensed-phase hydrocarbon systems such as liquids, graphite, and polymers. This potential is derived from a well-known dissociable hydrocarbon force field, the reactive empirical bond-order potential. The extensions include an adaptive treatment of the nonbonded and dihedral-angle interactions, which still allows for covalent bonding interactions. Torsional potentials are introduced via a novel interaction potential that does not require a fixed hybridization state. The resulting model is intended as a first step towards a transferable, empirical potential capable of simulating chemical reactions in a variety of environments. The current implementation has been validated against structural and energetic properties of both gaseous and liquid hydrocarbons, and is expected to prove useful in simulations of hydrocarbon liquids, thin films, and other saturated hydrocarbon systems.

Cyclodextrins are a family of cyclic oligosaccharides composed of ␣-(1,4) linked glucopyranose subunits. Cyclodextrins are useful molecular chelating agents. They possess a cage-like supramolecular structure, which is the same as the structures formed from cryptands, calixarenes, cyclophanes, spherands and crown ethers. These compounds having supramolecular structures carry out chemical reactions that involve intramolecular interactions where covalent bonds are not formed between interacting molecules, ions or radicals. The majority of all these reactions are of 'host-guest' type. Compared to all the supramolecular hosts mentioned above, cyclodextrins are most important. Because of their inclusion complex forming capability, the properties of the materials with which they complex can be modified significantly. As a result of molecular complexation phenomena CDs are widely used in many industrial products, technologies and analytical methods. The negligible cytotoxic effects of CDs are an important attribute in applications such as rug carrier, food and flavours, cosmetics, packing, textiles, separation processes, environment protection, fermentation and catalysis.

Platinum-based heterogeneous catalysts are critical to many important commercial chemical processes, but their efficiency is extremely low on a per metal atom basis, because only the surface active-site atoms are used. Catalysts with single-atom dispersions are thus highly desirable to maximize atom efficiency, but making them is challenging. Here we report the synthesis of a single-atom catalyst that consists of only isolated single Pt atoms anchored to the surfaces of iron oxide nanocrystallites. This single-atom catalyst has extremely high atom efficiency and shows excellent stability and high activity for both CO oxidation and preferential oxidation of CO in H2. Density functional theory calculations show that the high catalytic activity correlates with the partially vacant 5d orbitals of the positively charged, high-valent Pt atoms, which help to reduce both the CO adsorption energy and the activation barriers for CO oxidation.

Novel gold catalysts were prepared by coprecipitation from an aqueous solution of HAuCl 4 and the nitrates of various transition metals. Calcination of the coprecipitates in air at 400 °C produced ultrafine gold particles smaller than 10 nm which were uniformly dispersed on the ...

This study shows that closest packed, oriented monolayers of n-alkanoic acids can be formed on oxidized aluminum substrates by adsorption from dilute solution. The formation of these organic surface phases is characterized by complicated kinetics in which surface and/or monolayer defects, as well as impurities, seemingly play important roles. The structures so obtained are dynamic in nature in that they undergo rapid exchange with ligands in solution. The data further suggest an important limitation to self-assembly in this system which is directly related to the length of the n-alkanoic acid t a i l . The influences of experimental variables on the formation of oriented monolayers is discussed.

Graphite oxide is one of the main precursors of graphene-based materials, which are highly promising for various technological applications because of their unusual electronic properties. Although epoxy and hydroxyl groups are widely accepted as its main functionalities, the complete structure of graphite oxide has remained elusive. By interpreting spectroscopic data in the context of the major functional groups believed to be present in graphite oxide, we now show evidence for the presence of five-and six-membered-ring lactols. On the basis of this chemical composition, we devised a complete reduction process through chemical conversion by sodium borohydride and sulfuric acid treatment, followed by thermal annealing. Only small amounts of impurities are present in the final product (less than 0.5 wt% of sulfur and nitrogen, compared with about 3 wt% with other chemical reductions). This method is particularly effective in the restoration of the p-conjugated structure, and leads to highly soluble and conductive graphene materials.

A bstract. The methodology, characteristics and application of the sensitive C,H.,-C2H4 assay for N2 fixation by nitrogenase preparations and bacterial cultures in the laboratory and by legumes and free-living bacteria in situ is presented in this comprehensive report. This assay is based on the N9ase-catalyzed reduction of C2Ho to C2H4, gas chromatographic isolation of C2H2 and C9H4, and quantitative measurement with a H,-fiame analyzer. As little as 1 ,u,lmole C..H4 can be detected, providing a sensitivity 103-fold greater than is possible with '-5N analysis.

We present an overview of the current status of transition-state theory and its generalizations. We emphasize (i) recent improvements in available methodology for calculations on complex systems, including the interface with electronic structure theory, (ii) progress in the theory and application of transition-state theory to condensedphase reactions, and (iii) insight into the relation of transition-state theory to accurate quantum dynamics and tests of its accuracy via comparisons with both experimental and other theoretical dynamical approximations.

An updated genome-scale reconstruction of the metabolic network in Escherichia coli K-12 MG1655 is presented. This updated metabolic reconstruction includes: (1) an alignment with the latest genome annotation and the metabolic content of EcoCyc leading to the inclusion of the activities of 1260 ORFs, (2) characterization and quantification of the biomass components and maintenance requirements associated with growth of E. coli and (3) thermodynamic information for the included chemical reactions. The conversion of this metabolic network reconstruction into an in silico model is detailed. A new step in the metabolic reconstruction process, termed thermodynamic consistency analysis, is introduced, in which reactions were checked for consistency with thermodynamic reversibility estimates. Applications demonstrating the capabilities of the genome-scale metabolic model to predict high-throughput experimental growth and gene deletion phenotypic screens are presented. The increased scope and computational capability using this new reconstruction is expected to broaden the spectrum of both basic biology and applied systems biology studies of E. coli metabolism.

The approach we will take is motivated by the work of Becke 1 on hybrid Hartree−Fock density functional theory, as justified by the adiabatic connection formula. 2 This theory involves mixing various amounts of the Hartree−Fock (HF) nonlocal exchange operator 3 with local DFT ...

Arsenite [As(III)] and arsenate [As(V)] are highly toxic inorganic arsenic species that represent a potential threat to the environment and human health. Iron oxides including poorly crystalline oxides, e.g., ferrihydrite, play a significant role in controlling dissolved As concentration and limit the mobility and bioavailability of As(III) and As-(V). Adsorption occurs by ligand exchange of the As species for OH 2 and OHin the coordination spheres of surface structural Fe atoms. The objective of this study was to evaluate H + /OHrelease stoichiometry and changes in surface charge properties of the adsorbent during the adsorption of arsenite and arsenate on ferrihydrite in the pH range of 4-10. This information, which is not directly accessible through spectroscopic studies, provides important clues to bonding mechanism. While arsenate adsorption resulted in the net release of OHat pH 4.6 and 9.2, arsenite adsorption resulted in net OHrelease at pH 9.2 and net H + release at pH 4.6. The amount of H + or OHrelease per mole of adsorbed As varied with the As surface coverage, indicating that different mechanisms of arsenic adsorption predominate at low versus high coverage. The experimentally observed surface charge reduction and net OHrelease stoichiometry were compared with the theoretical stoichiometry of the surface adsorption reactions that might occur. The results provide evidence that during arsenite adsorption at low pH, i.e., pH 4.6, the oxygen of the Fe-O-As bond remained partially protonated as Fe-O(H)-As. There is evidence that the monodentate bonding mechanism might play an increasing role during arsenate adsorption on ferrihydrite with increasing pH (at pH > 8). The results of this study have provided ancillary evidence to support the experimentally observed reduced adsorption of arsenite at low pH and of arsenate at high pH.

In 1991 Morris proposed an effective screening sensitivity measure to identify the few important factors in models with many factors. The method is based on computing for each input a number of incremental ratios, namely elementary effects, which are then averaged to assess the overall importance of the input. Despite its value, the method is still rarely used and instead local analyses varying one factor at a time around a baseline point are usually employed.

is the primary database resource of the Japanese GenomeNet service (http://www.genome.ad.jp/) for understanding higher order functional meanings and utilities of the cell or the organism from its genome information. KEGG consists of the PATHWAY database for the computerized knowledge on molecular interaction networks such as pathways and complexes, the GENES database for the information about genes and proteins generated by genome sequencing projects, and the LIGAND database for the information about chemical compounds and chemical reactions that are relevant to cellular processes. In addition to these three main databases, limited amounts of experimental data for microarray gene expression profiles and yeast two-hybrid systems are stored in the EXPRESSION and BRITE databases, respectively. Furthermore, a new database, named SSDB, is available for exploring the universe of all protein coding genes in the complete genomes and for identifying functional links and ortholog groups. The data objects in the KEGG databases are all represented as graphs and various computational methods are developed to detect graph features that can be related to biological functions. For example, the correlated clusters are graph similarities which can be used to predict a set of genes coding for a pathway or a complex, as summarized in the ortholog group tables, and the cliques in the SSDB graph are used to annotate genes. The KEGG databases are updated daily and made freely available (http://www.genome.ad.jp/kegg/).

The purity of wafer surfaces is an essential requisite for the successful fabrication of VLSI and ULSI silicon circuits. Wafer cleaning chemistry has remained essentially unchanged in the past 25 years and is based on hot alkaline and acidic hydrogen peroxide solutions, a process known as "RCA Standard Clean." This is still the primary method used in the industry. What has changed is its implementation with optimized equipment: from simple immersion to centrifugal spraying, megasonic techniques, and enclosed system processing that allow simultaneous removal of both contaminant films and particles. Improvements in wafer drying by use of isopropanol vapor or by "slow-pull" out of hot deionized water are being investigated. Several alternative cleaning methods are also being tested, including choline solutions, chemical vapor etching, and UV/ozone treatments. The evolution of silicon wafer cleaning processes and technology is traced and reviewed from the 1950s to August 1989.

Solar fuel production often starts with the energy from light being absorbed by an assembly of molecules; this electronic excitation is subsequently transferred to a suitable acceptor. For example, in photosynthesis, antenna complexes capture sunlight and direct the energy to reaction centres that then carry out the associated chemistry. In this Review, we describe the principles learned from studies of various natural antenna complexes and suggest how to elucidate strategies for designing light-harvesting systems. We envisage that such systems will be used for solar fuel production, to direct and regulate excitation energy flow using molecular organizations that facilitate feedback and control, or to transfer excitons over long distances. Also described are the notable properties of light-harvesting chromophores, spatial-energetic landscapes, the roles of excitonic states and quantum coherence, as well as how antennas are regulated and photoprotected.

Azo dyes are the most important group of synthetic colorants. They are generally considered as xenobiotic compounds that are very recalcitrant against biodegradative processes. Nevertheless, during the last few years it has been demonstrated that several microorganisms are able, under certain environmental conditions, to transform azo dyes to non-colored products or even to completely mineralize them. Thus, various lignolytic fungi were shown to decolorize azo dyes using ligninases, manganese peroxidases or laccases. For some model dyes, the degradative pathways have been investigated and a true mineralization to carbon dioxide has been shown. The bacterial metabolism of azo dyes is initiated in most cases by a reductive cleavage of the azo bond, which results in the formation of (usually colorless) amines. These reductive processes have been described for some aerobic bacteria, which can grow with (rather simple) azo compounds. These specifically adapted microorganisms synthesize true azoreductases, which reductively cleave the azo group in the presence of molecular oxygen. Much more common is the reductive cleavage of azo dyes under anaerobic conditions. These reactions usually occur with rather low specific activities but are extremely unspecific with regard to the organisms involved and the dyes converted. In these unspecific anaerobic processes, low-molecular weight redox mediators (e.g. flavins or quinones) which are enzymatically reduced by the cells (or chemically by bulk reductants in the environment) are very often involved. These reduced mediator compounds reduce the azo group in a purely chemical reaction. The (sulfonated) amines that are formed in the course of these reactions may be degraded aerobically. Therefore, several (laboratory-scale) continuous anaerobic/aerobic processes for the treatment of wastewaters containing azo dyes have recently been described.

Literature data (through mid-1996) concerning the gas-phase reactions of alkanes and alkenes (including isoprene and monoterpenes) leading to their first generation products are reviewed and evaluated for tropospheric conditions. The recommendations of the most recent IUPAC evaluation [J. Phys. Chern. Ref. Data, 26, No.3 (1997)] are used for the :::;C 3 organic compounds, unless more recent data necessitates reevaluation. The most recent review and evaluation of Atkinson [J. Phys. Chern. Ref. Data, Monograph 2, 1 (1994)] concerning the kinetics of the reactions of OH radicals, N0 3 radicals, and 0 3 is also updated for these two classes of volatile organic compounds.

The structural properties of oxygen-deficient YBazCu30& z have been determined by neutron powder diffraction for 0.07 & 5 &0.91. The samples were produced by quenching into liquid nitrogen from controlled oxygen partial pressures at 520'C, and they exhibit a clearly defined "plateau" behavior of T, versus 6. Superconductivity disappears at the orthorhombic-to-tetragonal transition that occurs near 6=0.65. Structural parameters, including the copper-oxygen bond lengths, vary smoothly with 6 within each phase but exhibit different behavior in the superconducting and nonsuperconducting phases. These observations are consistent with a model in which superconducting behavior is controlled by charge transfer between the conducting two-dimensional CuOz planes and the CuO"chains, which act as reservoirs of charge.

The chemicali mechanism of Field, Karas, and Noyes for the osclllatory Belousov reaction has been generalized by a model composed of five steps involving three independent chemical intermediates, The behavior of the resulting differential equations has been examined numerically, and it has been shown that the system traces a stable closed trajectory in three dimensional phase space, The same trajectory is attained from other phase points and even from the point corresponding to steady state solution of the differential equations. The model appears to exhibit limit cycle behavior. By stimy coupling the concentrations of two of the intermediates, the limit cycle model can be simplified to a system described by two independent variables; this coupled system is amenable to analysis by theoretical techniques already developed for such systems.

Allicin, one of the active principles of freshly crushed garlic homogenates, has a variety of antimicrobial activities. Allicin in its pure form was found to exhibit i) antibacterial activity against a wide range of Gram-negative and Gram-positive bacteria, including multidrug-resistant enterotoxicogenic strains of Escherichia coli; ii) antifungal activity, particularly against Candida albicans; iii) antiparasitic activity, including some major human intestinal protozoan parasites such as Entamoeba histolytica and Giardia lamblia; and iv) antiviral activity. The main antimicrobial effect of allicin is due to its chemical reaction with thiol groups of various enzymes, e.g. alcohol dehydrogenase, thioredoxin reductase, and RNA polymerase, which can affect essential metabolism of cysteine proteinase activity involved in the virulence of E. histolytica. © Elsevier, Paris alliinase / allicin / thiol enzymes / antibiotics / garlic / antimicrobials * Correspondence and reprints Figure 1. Generation of allicin in a garlic clove.

The recent literature on the kinetics of water-rock interactions is reviewed. The data are then extended to provide a quantitative framework for the description of weathering and alteration. The available experimental data on dissolution of silicates verifies quantitatively the usual mineral stability series in sedimentary petrology. The rate of hydration of carbonic acid is shown to be a possible limiting factor in water-rock interactions. The framework is developed to enable use of laboratory dissolution experimental results and thermodynamics to arrive at a rate law applicable up to equilibrium and therefore applicable to natural systems. The kinetic justification for the significance of a water-rock ratio is discussed. With a proper treatment of fluid flow, the equations are applied to the weathering profile leading to the development of bauxites from nepheline syenites. Enstatite? <2.4 Diopside? <1.6 Tremolite? <1.0 Anorthite$ < 10 *Petrovich et al. [1976]. ?Schott et al. [1981]. $Fleer [1982]. This last equation is the classic "parabolic" concentration versus time curve. An obvious consequence of the leached layer hypothesis is that given a certain mineral surface area and a certain amount of dissolution, the thickness of the leached layer can be estimated. The results for several minerals gave numbers in the range 10-100 nm. However, the search for this leached layer was not undertaken until recently. A variety of tools are available for surface chemical analysis. Some of the more widely known are low-energy electron diffraction (LEED), scanning electron microscopy (SEM), Auger spectroscopy, secondary ion mass spectrometry (SIMS), and X ray photoelectron spectroscopy (XPS or also called ESCA, electron spectroscopy for chemical analysis). An important point is that in techniques such as Auger, SIMS, or XPS the depth from the surface that is "seen" by the analysis is only 1-5 nm. Therefore these techniques are ideal for following the process of mineral dissolution. XPS (or ESCA) was recently used to obtain a maximum depth for the leached layer of several minerals. XPS uses X ray to bombard a sample and measures the kinetic energy of core electrons given off from the induced photoionization of surface atoms. By comparing the intensities of fresh and altered mineral surfaces, any deviations due to a leached layer of sufficient depth can be detected. (See Defosse and Rouxhet [1980] for a review.) Table 1 gives the results. The obvious conclusion is that there is no leached layer in these minerals, at least in the extensive sense mentioned previously. In light of the new evidence the explanation for the nonlinear concentration versus time curves stems from the sample preparation. Careful analysis by SEM revealed the presence of submicron particles coating the surface of minerals. Recently, Petrovich [1981a, b] has analyzed the effects of grinding the samples on the structure of the surface and the size and shape of the various small grains. If these small particles are removed (e.g., by cleaning with 5% HF + 0.1 N H2SO4 for 20 min), then linear kinetics are obtained from the beginning of dissolution. A clear example was shown by Holdren and Berner [1979] as recast in Figure 1. A similar result was found by Grandstaff [1980], who obtained linear dissolution kinetics for forsterite after it had been cleaned. P.

For fluid-solid reactions, a random pore model is developed which allows for arbitrary pore size distributions in the reacting solid. The model can represent the behavior of a system that shows a maximum in reaction rate as well as one that does not, and it identifies an optimal pore structure for either of such systems. It is demonstrated that the new model subsumes several earlier treatments as special cases. By comparison with the grain model, a relationship is derived between the effective grain shape factor and a pore structure parameter defined here. When the variance of the pore size distribution is effectively zero, the new results approach those predicted by the Petersen (1957) model over a large range of conversions. The char gasification data of Hashimoto et al. (1979) are shown to produce correlations consistent with the expectations of the new model.

We present a two-dimensional (2-D) fitting algorithm (Galfit, Version 3) with new capabilities to study the structural components of galaxies and other astronomical objects in digital images. Our technique improves on previous 2-D fitting algorithms by allowing for irregular, curved, logarithmic and power-law spirals, ring and truncated shapes in otherwise traditional parametric functions like the Sérsic, Moffat, King, Ferrer, etc., profiles. One can mix and match these new shape features freely, with or without constraints, apply them to an arbitrary number of model components and of numerous profile types, so as to produce realistic-looking galaxy model images. Yet, despite the potential for extreme complexity, the meaning of the key parameters like the Sérsic index, effective radius or luminosity remain intuitive and essentially unchanged. The new features have an interesting potential for use to quantify the degree of asymmetry of galaxies, to quantify low surface brightness tidal features beneath and beyond luminous galaxies, to allow more realistic decompositions of galaxy subcomponents in the presence of strong rings and spiral arms, and to enable ways to gauge the uncertainties when decomposing galaxy subcomponents. We illustrate these new features by way of several case studies that display various levels of complexity.

Louisiana 70803-75 1 1 H The influence of redox potential and pH on arsenic speciation and solubility was studied in a contaminated soil. Alterations in the oxidation state of arsenic, as influenced by redox potential and pH, greatly affected its solubility in soil. At higher soil redox levels (500-200 mV), arsenic solubility was low and the major part (65-98% ) of the arsenic in solution was present as As(V). An alkaline pH, or the reduction of As(V) to As(III), released substantial proportions of arsenic into solution. Under moderately reduced soil conditions (0-100 mV), arsenic solubility was controlled by the dissolution of iron oxyhydroxides. Arsenic was coprecipitated [as As(V)] with iron oxyhydroxides and released upon their solubilization. Upon reduction to -200 mV, the soluble arsenic content increased 13-fold as compared to 500 mV. The observed slow kinetics of the As(V)-As(II1) transformation and the high concentrations of Mn present indicate that, under reduced soil conditions, arsenic solubility could be controlled by a Mn3(AsO& phase.

Ethylene and ethane (Cp's) have been synthesized by catalytic oxidative coupling of methane at atmospheric pressure and temperatures of 500-1OOCtC. A great number of metal oxides, supported on an o-alumina support, have been screened for activity and selectivity in the normal, concurrent feeding mode of reactants, as well as, in the sequential or feed programming mode. The concurrent feeding experiments gave low selectivities of O-20%, while feed programming experiments gave selectivities of -50%. In the latter case, however, a stainless-steel reactor became catalytically active for burning of methane to carbon oxides. Although a stainless-steel reactor was used in most of the work reported here, a quartz reactor was found to be inert. The most active catalysts for CZ formation were the oxides of Sn, Pb, Sb, Bi, Tl, Cd, and Mn, while Li, Mg, Zn, Ti, Zr, MO, Fe, Cr, W, Cu, Ag, Pt, Ce, V, B, and Al showed little or no activity. The low C,-forming activity of Pt and Ce, however, may be due to the secondary burning of C,'s on the stainless-steel reactor walls. This also makes it diflicult to quantify the &-forming selectivity of the active metal oxides. The active metals seem to exhibit a common characteristic: they can cycle between at least two oxidation states. Although there are differences itrselectivities in CI formation and carbon oxides formation, no correlation seems to exist with the free-energy changes in the oxidation states. A possible mechanism for Cx formation from methane is proposed. 9 0@21-9517/82/010009-11$02.00/O

Cellular rhythms are generated by complex interactions among genes, proteins and metabolites. They are used to control every aspect of cell physiology from signaling, motility and development to growth, division and death. By considering specific examples of oscillatory processes, we pick out three general requirements for biochemical oscillations: delayed negative feedback, sufficient 'nonlinearity' of the reaction kinetics, and proper balancing of the timescales of opposing chemical reactions. Positive feedback is one mechanism to delay the negative feedback signal. Biological oscillators can be classified according to the topology of the positive and negative feedback loops in the underlying regulatory mechanism. Biochemical oscillations occur in many contexts (metabolism, signaling, development, etc.) where they control important aspects of cell physiology, such as circadian rhythms, DNA synthesis and mitosis, and the development of somites in vertebrate embryos (see Table 1). In the 1950s and 60s, the first clear examples of biochemical oscillations (in metabolic systems) were recognized in glycolysis1, 2, in cyclic AMP production3, and the horseradish peroxidase reaction4, 5. Soon after these discoveries, theoreticians were thinking about the general requirements for chemical oscillations and the specific mechanisms of these examples6, 7. After the molecular biology revolution of the 1980s, many new examples of oscillations in protein interaction networks and in gene regulatory networks came to light, such as the PERIOD proteins in animal circadian control8, the CYCLIN proteins in eukaryotic cell cycle control9, 10, and the Repressilator11 in genetically engineered bacteria. Understanding the molecular basis of cellular oscillations is more than an exercise in experimental genetics and biochemistry. Oscillators have systems-level characteristics (periodicity, robustness, entrainment) that transcend the properties of individual molecules or reaction partners and involve the full topology of the reaction network. These properties can only be fully understood by viewing experimental data from a theoretical perspective, by quantitative mathematical modeling of chemical oscillatory processes. These models address general concepts of dynamical systems, such as feedback, time delays, bistability and hysteresis.

Gordon E. Brown, Jr., received his BS in chemistry and geology in 1965 from Millsaps College, Jackson, MS, and his MS and Ph.D. in mineralogy and crystallography from Virginia Polytechnic Institute & State University in 1968 and 1970, respectively. He is currently the ...

We have developed a three-dimensional Eulerian regional acid deposition model to calculate episodic chemical concentrations and dry and wet deposition of acids in North America. This transport, transformation, and deposition modeling system subdivides the troposphere over the eastern United States, southeastern Canada, and the western Atlantic Ocean into a six-level, 30 by 30 horizontal grid with a horizontal grid size of 80 x 80 km 2. Transport and vertical diffusion of 24 trace gases and particles are calculated using temporally and spatially varying meteorology, provided by a mesoscale meteorological model. A gas phase chemical reaction mechanism is used to simulate concentrations and chemical conversion rates for 36 species, including 14 stable organics and 11 short-lived radicals. Altitude-, latitude-, and season-dependent photolysis rates for nine reactions in the chemical mechanism are specified using a delta-Eddington radiative transfer model which includes 0 2 and 0 3 absorption, scattering and absorption by clouds and aerosols, Rayleigh scattering, and ground reflections. Subgrid scale vertical transport, aqueous chemical conversions, and trace gas and particle scavenging by clouds are parameterized using a one-dimensional dynamical and microphysical cloud model and a box aqueous chemistry and scavenging submodel. The aqueous phase chemistry model includes sulfur oxidation by H202, 03, trace metals, and two organic peroxides, with numerous equilibria between all soluble trace species. Dry deposition rates for 13 compounds are computed using species-specific deposition velocities that depend on the local meteorology, season, land type, insolation, and surface wetness conditions. Emissions of SO 2, SO4 =, NO, NO2, CO, NH3, and 10 classes of volatile organic compounds are included in the model. Trace gases are emitted into different vertical levels of the model according to a plume rise submodel. Hourly emissions are adjusted according to season and weekday or weekend activities. This model provides a framework to examine the relative importance and sensitivity of numerous physical and chemical processes responsible for the formation and deposition of tropospheric acidity. INTRODUCTION Wet and dry depositions of acidic species on material surfaces of aquatic and forest ecosystems have become the subject of widespread scientific attention in recent years. The primary precursors to acidity in the troposphere are sulfur and nitrogen oxides and organic compounds which are emitted and subsequently oxidized into more acidic forms through a complex series of interactions involving numerous chemical, meteorological, physical, and biological processes. Since many of these precursors are anthropogenically produced, it may be possible to mitigate the undesirable effects of acidic deposition by reducing pollutant emissions. Unfortunately, it is difficult to directly determine the benefits of implementing a potentially costly pollutant emissions reduction program. Direct experimental study of the complex sequence of physical and chemical processes involved with acid deposition is extremely costly and highly perturbing to segments of society. Therefore it is useful to develop an integrated theoretical modeling system which includes the relevant coupling and feedback among several physical and chemical phenomena. With this system it is possible to identify and analyze the processes responsible for the formation, transport, and deposition of acidity in the atmosphere and estimate the relative merits of alternative control strategies in a manner which cannot be obtained by other means. Lagrangian puff models [e.g.

The chemical activation of anthracites with hydroxides has been shown to be of interest for the production of activated carbons with a highly microporous structure. In a previous paper, attention was placed on the reactions occurring during the chemical activation of an anthracite by NaOH and KOH. In the present work, the process of chemical activation by hydroxides has been extended to different coal precursors to confirm that such a chemical activation process starts through a solid-solid reaction and continues as a solid-liquid reaction. In such a solid reaction, the reactivity of the solid (precursor) should be a key parameter. The importance of the carbon reactivity on its reaction with hydroxides has been confirmed: the lowest rank coal reacts much easily and has a much lower temperature for the beginning of reaction than the highest rank coal.

AI&met-We used a combination of porewater and solid phase analysis, as well as a series of sediment incubations, to quantify organic carbon oxidation by dissimilatory Fe reduction, Mn reduction, and sulfate reduction, in sediments from the Skagerrak (located off the northeast coast of Jutland, Denmark). In the deep portion of the basin, surface Mn enrichments reached 3.5 wt%, and Mn reduction was the only important anaerobic carbon oxidation process in the upper 10 cm of the sediment. In the less Mnrich sediments from intermediate depths in the basin, Fe reduction ranged from somewhat less, to far more important than sulfate reduction. Most of the Mn reduction in these sediments may have been coupled to the oxidation of acid volatile sulfides (AVS), rather than to dissimilatory reduction. High rates of metal oxide reduction at all sites were driven by active recycling of both Fe and Mn, encouraged by bioturbation. Recycling was so rapid that the residence time of Fe and Mn oxides, with respect to reduction, ranged from 70-250 days. These results require that, on average, an atom of Fe or Mn is oxidized and reduced between 100-300 times before ultimate burial into the sediment. We observed that dissolved Mn2+ was completely removed onto fully oxidized Mn oxides until the oxidation level of the oxides was reduced to about 3.8, presumably reflecting the saturation by Mn2+ of highly reactive surface adsorption sites. Fully oxidized Mn oxides in sediments, then, may act as a cap preventing Mn2+ escape. We speculate that in shallow sediments of the Skagerrak, surface Mn oxides are present in a somewhat reduced oxidation level (~3.8) allowing Mn2+ to escape, and perhaps providing the Mn2+ which enriches sediments of the deep basin.-INTRODUCIION THE COMBINED PROCESSES of oxic respiration and sulfate reduction have been argued to account for most of the organic carbon oxidation in marine sediments (JORGENSEN, 1983; HENRICHS and REEBURGH, 1987; CANFIELD, 1989a). Evidence seems to suggest that while oxic respiration dominates in deep-sea sediments of relatively low deposition rate (J&~R-GENSEN, 1982; BENDER and HEGGIE, 1984; CANFIELD, 1989a), in near-shore sediments, sulfate reduction and oxic respiration may be of a similar magnitude (JORGENSEN, 1982; CANRELD, 1989a). Denitrification is considered to be a relatively minor process (BENDER and HEGGIE, 1984, CHRIS-TENSEN and ROWE; 1984) except for low bottom water oxygen environments, where in some cases, it may dominate carbon oxidation (JAHHKE et al., 1990; CANFIELD, 1993). Rates of dissimilatory Fe and Mn reduction have not been well quantified, but, based on the steepness of dissolved porewater Fe'+ and Mn2+ gradients, metal oxide (Fe and Mn) reduction has been thought to be of minor importance (BENDER and HEGGIE, 1984; CHRISTENSEN and ROWE, 1984; BENDER et al., 1989). There has, however, been the rather recent discovery of bacteria which can gain energy from the reduction of Fe and Mn oxides, using a wide range of organic substrates, including some aromatic compounds (LOVLEY and PHILLIPS, 1988a; LOVLEY and LONERGAN, 1990, MYERS and NEALSON, 1988). Also, CANFIELD (1993) has demonstrated that rates of Fe and Mn reduction based on chemical gradients, at least in some near shore sediments, may underestimate the actual rates of reduction. Hence, metal oxide reduction may be more important than previously thought.

Quantitative predictions of the effects of acid deposition onterrestrial and aquatic systems require physically based, process-oriented models of catchment soil water and streamwater chemistry. A desirable characteristic of such models is that they include terms to describe ...

Photosensitizers in photodynamic therapy allow for the transfer and translation of light energy into a type II chemical reaction. In clinical practice, photosensitizers arise from three families—porphyrins, chlorophylls, and dyes. All clinically successful photosensitizers have the ability to a greater or lesser degree, to target specific tissues or their vasculature to achieve ablation. Each photosensitizer needs to reliably activate at a high enough light wavelength useful for therapy. Their ability to fluoresce and visualize the lesion is a bonus. Photosensitizers developed from each family have unique properties that have so far been minimally clinically exploited. This review looks at the potential benefits and consequences of each major photosensitizer that has been tried in a clinical setting.