11 Search Results

Both thioarsenites and thioarsenates have been demonstrated to exist in sulfidic waters, yet there is uncertainty regarding the geochemical conditions that govern the formation of these arsenic species. The purpose of this research was to use advanced spectroscopy techniques, speciation modeling, and chromatography to elucidate the chemical speciation of arsenic in sulfidic solutions initially containing arsenite and sulfide. Results of X-ray absorption spectroscopy (XAS) show that experimental solutions contained mixtures of arsenite and thioarsenites with increasing substitution of sulfur for oxygen on arsenic as the sulfide concentration increased. Experimental samples showed no evidence of polymeric arsenic species, or transformation of thioarsenites to thioarsenates. The arsenic speciation measured using XAS was similar to predictions obtained from a thermodynamic model for arsenic speciation, excluding thioarsenate species in sulfidic systems. Our data cast some doubt on the application of chromatographic methods for determining thioarsenates and thioarsenites (or mixtures) in natural waters in cases where the arsenic oxidation state cannot be independently verified. The same chromatographic peak positions proposed for thioarsenates can be explained by thioarsenite species. Furthermore, sample dilution was shown to change the species distribution and care should be taken to avoid sample dilution prior to chromatographic analysis.

The mechanism of sulfate adsorption on goethite was investigated in situ using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Sulfate adsorption was investigated at ionic strengths between 0.005 and 0.1 M, reactant concentrations between 5 and 500 {micro}M, and pH values between 3.5 and 9.0. It was determined that sulfate forms both outer-sphere and inner-sphere surface complexes on goethite at pH less than 6. At pH values greater than 6, sulfate adsorbs on the goethite only as an outer-sphere complex. The relative amount of outer-sphere sulfate surface complexation increased with decreasing ionic strength. The spectrum of sulfate adsorbed on goethite was also compared to the infrared spectrum of synthetic schwertmannite, an iron(III) oxy-hydroxy-sulfate. It was determined that in situ spectra of both schwertmannite and adsorbed sulfate are quite similar, suggesting that a continuum of outer- and inner-sphere sulfate occurs in both cases.

In a Functionally Graded Material (FGM), the composition and structure gradually change over volume, resulting in corresponding changes in the properties of the material. By applying the many possibilities inherent in the FGM concept, it is anticipated that materials will be improved and new functions for them created. A comprehensive description of design, modeling, processing, and evaluation of FGMs as well as their applications is covered in this book. The contents include: lessons from nature; graded microstructures; modeling and design; characterization of properties; processing and fabrication; applications; and summary and outlook.

The formation of transition-metal surface precipitates may occur during sorption to clay minerals under ambient soil conditions. This process may lead to significant long-term stabilization of the metal within the soil profile. However, the rates and mechanisms controlling surface precipitate formation are poorly understood. The authors monitored changes in the reversibility of Ni sorbed to a clay mineral, pyrophyllite, in model batch experiments maintained at pH 7.5 for up to 1 year. The macroscopic sorption and dissolution study was complemented by a time-resolved characterization of the sorbed phase via spectroscopic and thermal methods. They found that nickel became increasingly resistant, over time, to extraction with EDTA. Initially, the sorbed phase consisted of a Ni-Al layered double hydroxide (LDH). With time, the anionic species in the interlayer space of the LDH changed from nitrate to silica polymers transforming the LDH gradually into a precursor Ni-Al phyllosilicate. The authors believe that this phase transformation is responsible for a substantial part of the observed increase in dissolution resistance. Thus, clay mineral weathering and the time-dependent release of Al and Si ions controlled Ni precipitate nucleation and transformation. The results suggest a potential pathway for long-term Ni stabilization in soil.

Hydrous iron oxides can play a significant role in controlling the mobility of trace metals in the environment. Metals adsorbed to or coprecipitated with the poorly-crystalline ferrihydrite may form solid solutions with goethite and hematite as solid aging progresses. This will lead to a fraction of partitioned metal resistant to desorption. The magnitude of this fraction will depend on the extent of metal miscibility (ionic radius) and the stability of the metal-ferrihydrite complex (logK{sup surf}{sub int}). In Fe-Ni and Fe-Pb int binary systems, an apparent miscibility limit is exceeded at (Pb)t = 5x10{sup -4} M and (Fe)t = 0.01 M as measured by increasing (Pb)aq with aging, contrary to the behavior of Ni. Extraction of the solid phase with an acidic oxalate solution to remove ferrihydrite indicates that 70% of (Ni)t is nonreversibly associated with hematite and goethite. However, x-ray diffraction indicates that both Ni and Pb are substituted into these crystalline iron oxides.

Metals sorbed to or coprecipitated with ferrihydrite can significantly inhibit transformation to more crystalline endproducts. The authors hypothesized that metals with a higher stability constant for a metal-ferrihydrite surface complex would retard the transformation process to a greater extent. To test this hypothesis, they examined the influence of Ni or Pb sorption on the kinetics of ferrihydrite crystallization to goethite/hematite. Reported surface stability constants for Ni and Pb sorbed to ferrihydrite are logK{sub 1,int} = 0.37 and 4.0, respectively (Dzombak and Morel, 1990). The structural evolution of nickel- and lead-ferrihydrite coprecipitates was studied for various metal loading during aging at pH 6 or 11 and 70 C. Results of aging studies demonstrated that the influence on transformation kinetics was not related to the magnitude of the stability constant of the Ni- or Pb-ferrihydrite surface complex. At pH 11, crystallization was retarded more significantly in the presence of Ni and rates decreased with increasing Ni/Pb surface loading. At pH 6, crystallization rates were accelerated in the presence of Pb, and this was also true for system at the lowest Ni loading. However, crystallization rates in the presence of Ni were always slower relative to systems containing Pb. Characterization of crystalline iron (hydr)oxide endproducts by x-ray diffraction and high-resolution thermogravimetric analysis showed that hematite was formed to a greater extent than goethite in the presence of Ni. X-ray absorption fine structure spectroscopy suggested that the majority of sorbed Pb was present as an inner-sphere surface complex. The distribution of coprecipitated Ni or Pb on aged solids, as assessed via continuous dissolution with oxalic acid, suggested that a significant fraction of Ni was partitioned into the structure of a crystalline iron (hydr)oxide. In contrast, Pb desorption/dissolution behavior confirmed that this metal was primarily associated with surface sites or poorly ordered iron (hydr)oxide phases. The relative metal-specific influence on crystallization rate and endproduct, and the apparent Ni and Pb distribution in aged solids suggest that Pb forms a more kinetically labile sorption complex than Ni with iron (hydr)oxides.

The objective of this study was to model the risks to selected species of marine mammal and seabird populations in the Southern California Bight from oil spills during OCS oil and gas development and operations. Risk analysis is a procedure designed to investigate the possible negative effects of projects and activities. The conventional approach to analyzing oil and gas reserves is through the use of the MMS Oil Spill Risk Analysis Model (OSRAM). OSRAM was developed to aid in estimating the environmental hazards of developing oil resources in OCS lease areas. Two other computer models were used in these analyses. They are: (2) the short-term oil response model, STORM and (3) the oil-spill population response model, OSPREY. In the report, a methodology for describing the range of consequences which oil spills might have on Southern California Bight seabird and marine mammal populations and the likelihood of those effects were developed. Two general categories of spill consequences were examined: (1) the immediate mortality to a population caused by a spill from a given source, and (2) the long term marine mammal and seabird populations effects of the projected Southern California Bight OCS development.

The specification discloses a solar energy collector having a manifold with a manifold supply tube and a manifold return tube disposed therein. Fluid is supplied to the apparatus through the manifold supply tube and is received from the apparatus through the manifold return tube. A plurality of spaced apart riser U-tubes extend away and upwardly from the manifold with one end of the U-tube being in fluid communication with the manifold supply tube and the other end of the U-tube being in fluid communication with the manifold return tube. Dimples are formed in spaced apart positions along the supply tube and an orifice is formed in the center of the dimple. The U-tube is connected to the supply tube at the dimple, covering the orifice so that the orifice meters the fluid flow to the U-tube. A solar energy absorber tube encompasses each of the U-tubes.

The incorporation of a high reflectivity stationary concentrator directly into an evacuated solar collector tube provides dramatically increased high temperature performance and improved lifetime and reliability compared to more conventional evacuated tube designs. An experimental prototype has demonstrated that both of these features are readily achievable. The next phase in the development of this concept is the fabrication of a manufacturable field prototype and its testing in an operating system. Several alternative design configurations for the field prototype and the essential requirements for the system tests are reviewed. 10 refs., 4 figs., 1 tab.

Estimates of territory and home range size can yield widely varying results depending upon methods of data collection and analysis. The merits of different methods of space-use patterns of Pectoral Sandpipers (Calidris melanotos) and Red Phalaropes (Phalaropus fulicarius) were examined on their breeding ground. Empirical data from these species was used to generate a series of computer-simulated home ranges. The efficiency of a non-probabilistic estimator of territory size (minimum convex polygon method) vs two probabilistic techniques, one parametric (Jennich and Turner 1969) and one nonparametric (Ford and Krumme 1979) were examined, testing for their sensitivities to sample size and to temporal dependence between successive observations. All methods are sensitive to temporal dependence and sample size, but the probabilistic techniques provide better estimates from small samples. Both the minimum convex polygon method and the parametric Jennrich-Turner technique overestimate area utilized by the species studied here, both of which deviated from a bivariate normal distribution. The Ford-Krumme approach provided the most accurate estimate of utilized area.