Brian Barbaris

Initial snow chemistry survey of the Mogollon rim in Arizona

Abstract Fresh snowpack samples collected from high-elevation forests of north-central Arizona during late winter/early spring 1992–1994 were analyzed by ion chromatography and by instrumental neutron activation. Eleven major ions, including the organic species acetate and formate, and twenty-eight elements were determined. The results indicate a relatively pristine snowpack—most samples exhibited low ionic strengths (20 ± 15 μeq l−1) and moderate pH (range 4.9–5.7, mean 5.4). Typical snow-producing storms moved northeast under the influence of southwesterly flow of marine air. Northwesterly winds associated with a cutoff low pressure system centered over southern Utah on 31 January and 1 February 1993 brought snowfall with increased chemical loading. The snowpack was apparently enriched with Zn, As, Sb, and Cu when compared to regional soils. Regional source emissions (smelters, metropolitan Phoenix) may have influenced the snow chemistry but their impact appears to be minimal. These apparent trace metal enrichments are similar to those found in precipitation in remote and rural locations worldwide.

Thermocatalytic Destruction of Gas-Phase Perchloroethylene Using Propane as a Hydrogen Source

The use of propane in combination with oxygen to promote the destruction of perchloroethylene (PCE) over a platinum (Pt)/rhodium (Rh) catalyst on a cerium/zirconium oxide washcoat supported on an alumina monolith was explored. Conversions of PCE were measured in a continuous flow reactor with residence times less than 0.5s and temperatures ranging from 200 to 600 degrees C. The presence of propane was shown to increase significantly the conversion of PCE over oxygen-only conditions. Conversions close to 100% were observed at temperatures lower than 450 degrees C with 20% oxygen and 2% propane in the feed, which makes this process attractive from a practical standpoint. In the absence of oxygen, PCE conversion is even higher, but the catalyst suffers significant deactivation in less than an hour. Even though results show that oxygen competes with reactants for active sites on the catalyst, the long-term stability that oxygen confers to the catalyst makes the process an efficient alternative to PCE oxidation. A Langmuir-Hinshelwood competitive adsorption model is proposed to quantify PCE conversion.

Mixed redox catalytic destruction of chlorinated solvents in soils and groundwater.

A new thermocatalytic method to destroy chlorinated solvents has been developed in the laboratory and tested in a pilot field study. The method employs a conventional Pt/Rh catalyst on a ceramic honeycomb. Reactions proceed at moderate temperatures in the simultaneous presence of oxygen and a reductant (mixed redox conditions) to minimize catalyst deactivation. In the laboratory, stable operation with high conversions (above 90% at residence times shorter than 1 s) for perchloroethylene (PCE) is achieved using hydrogen as the reductant. A molar ratio of H2/O2= 2 yields maximum conversions; the temperature required to produce maximum conversions is sensitive to influent PCE concentration. When a homologous series of aliphatic alkanes is used to replace hydrogen as the reductant, the resultant mixed redox conditions also produce high PCE conversions. It appears that the dissociation energy of the C–H bond in the respective alkane molecule is a strong determinant of the activation energy, and therefore the reaction rate, for PCE conversion. This new method was employed in a pilot field study in Tucson, Arizona. The mixed redox system was operated semicontinuously for 240 days with no degradation of catalyst performance and complete destruction of PCE and trichloroethylene in a soil vapor extraction gas stream. Use of propane as the reductant significantly reduced operating costs. Mixed redox destruction of chlorinated solvents provides a potentially viable alternative to current soil and groundwater remediation technologies.