Ben D. Holt

Carbon and chlorine isotope fractionation of chlorinated aliphatic hydrocarbons by evaporation

Two pure solvents (trichloroethene and dichloromethane) were evaporated at room temperature (24±1°C). The carbon and chlorine isotopic compositions of the residual solvents were measured as a function of the fraction remaining. Isotopic fractionation factors (α) derived from these experimental results are 1.00031±0.00004 for C and 0.99818±0.00022 for Cl in trichloroethene, and 1.00065±0.00002 for C and 0.99952±0.00006 for Cl in dichloromethane. The negative correlation between δ13C and δ37Cl values caused by evaporation of chlorinated aliphatic hydrocarbons can be used to distinguish evaporation from biodegradation in contaminated systems.

Oxygen-18 study of nonaqueous-phase oxidation of sulfur dioxide☆

Abstract In a study of the mechanisms of atmospheric sulfate formation, oxygen isotope ratios were measured in sulfates and in the SO 2 and water vapors from which they were formed, in the absence of liquid water. In a 3-l glass chamber, SO 2 and water vapor of various 18 O contents were isotopically equilibrated, and then air oxidation of the SO 2 to sulfate was performed by four different methods: high-voltage discharges, NO 2 addition, gamma irradiation and adsorption on activated charcoal. Isotopic equilibration between SO 2 and water vapor proceeded rapidly, resulting in a strong dependence of the δ 18 O of the sulfate on that of the water vapor. Oxidation of SO 2 on dry charcoal occurred through the apparent formation of 9-oxygen, 2-sulfur, chemisorbed molecules which decomposed to sulfate in leach water. The δ 18 O SO 2− 4 vs δ 18 O H 2 O relationships observed for these four nonaqueous-phase oxidations of SO 2 to sulfate, together with those in three previously reported aqueous-phase oxidations ( Fe 3+ -catalyzed air oxidation, charcoal-catalyzed air oxidation and H 2 O 2 oxidation), were compared to sulfate in rain and snow collected at Argonne, IL. The δ 18 O of sulfate in precipitation water was significantly higher than could be accounted for by any of the several oxidation reactions that were investigated as possible pathways in the formation of secondary sulfates in the atmosphere, either singly or in combination.

Primary Sulfates in Atmospheric Sulfates: Estimation by Oxygen Isotope Ratio Measurements

The relative amounts of primary and secondary sulfates in atmospheric aerosols and precipitation can be estimated from measurements of the stable oxygen isotope ratios. The oxygen-18 content of sulfates formed in power plant stack gases before emission into the atmosphere is significantly higher than that of sulfates formed from sulfur dioxide after emission. Results show that 20 to 30 percent of the sulfates in rain and snow at Argonne, Illinois, are of primary origin.

Oxygen-18 study of high-temperature air oxidation of SO2☆

Abstract The oxygen-18 enrichment in sulfates formed at high temperatures (475–500°C) by platinum-catalyzed air oxidation of SO2 to SO3 in humidified air, was found to be several parts per thousand higher than in the air oxygen, SO2, or water vapor from which the sulfates were formed. Variation of the δ18O in the sulfates showed little dependence on variation of the δ18O in the water vapor. The mechanism of sulfate formation involved isotopic exchange between the air oxygen and water vapor, isotopic exchange between the water vapor and SO2, and formation of the hydrate, H2SO4· 3H2O. When Fe2O3 or V2O5 was heated in mixtures of air, water vapor, and SO2, some of the SO2 was analytically oxidized (via SO3 formation) to sulfate of relatively high δ18O and the remainder to chemisorbed sulfate of relatively low δ18O. Charcoal and fly ash (containing unburned carbon and basic oxides) reacted with the SO2 to yield chemisorbed sulfates of relatively low δ18O.

Ultrasonic vacuum extraction of gases from water for chemical and isotopic analysis

A method is described for ultrasonic vacuum extraction of gases from groundwater samples. Ultrasonic vacuum extraction provides near-quantitative gas yields within 30 min, and allows on-line preparation of carbon dioxide, nitrogen and oxygen for isotopic analyses. This method eliminates uncertainties involved in correcting headspace gas analyses for solubilities and liquid-vapor isotopic fractionation. In addition to developmental data, some field results are given to demonstrate the applicability of the method to geochemical studies.

Extraction of chlorinated aliphatic hydrocarbons from groundwater at micromolar concentrations for isotopic analysis of chlorine.

A method is described for near-quantitative extraction of micromolar concentrations of chlorinated aliphatic hydrocarbons (CAHs) from water for determination of chlorine (Cl) isotope ratios. A low pressure, carrier-gas procedure of extraction was proven to be applicable to CH2Cl2, CCl4, C2H2Cl2, and C2HCl3. The pH of the water was adjusted with NaOH to prevent extraction of CO2 from air and/or dissolved inorganic carbonate species. Recoveries of CAH samples (approximately 15 mumol), added to and extracted from approximately 340 ml of water, averaged approximately 96%. Average changes in the delta 37Cl values of the CAHs, attributable to the extraction process, were -0.01 +/- 0.06@1000. Significant isotopic fractionation of Cl was measured during partial extraction of C2CHCl3 from water, indicating that near-quantitative extraction is required for reliable stable Cl isotope analysis of CAHs. This method is also suitable for the extraction of dissolved CAH for gas chromatography-combustion-isotope ratio mass spectrometric measurements of hydrogen and carbon.

Seasonal Variations of Oxygen-18 in Atmospheric Sulfates

Abstract Oxygen-isotope analyses were made on samples of aerosol sulfates, SO2, water vapor, precipitation water, and precipitation sulfates collected over a two-year period near Chicago, Illinois, U.S.A. The purpose of this isotopic study was to help to elucidate the mechanisms of sulfate formation in the atmosphere. Oxygen-18 enrichments in precipitation sulfates varied seasonally and in phase with the corresponding enrichments in precipitation water. The ratio of the amplitudes of the enrichment-vs.-time curves indicated isotopic equilibration between SO, and atmospheric water prior to oxidation. Oxygen-18 enrichments in aerosol sulfates appeared to vary randomly with season, but averaged about the same as precipitation sulfates. If aerosol sulfates and precipitation sulfates were formed by the same hydrolysis-oxidation mechanism in clouds, relatively long residence times and transport distances of sulfate aerosols may have provided sufficient mixing to obscure seasonal effects such as were observed in...

Oxygen-18 Study of the Atmospheric-Aquatic Linkage in Adirondack Watersheds

AbstractTwelve monthly measurements were made of the δ18O of the water and of the dissolved sulfates in inlet streams and in outlet streams of lakes in three watersheds in the Adirondack Park region of New York. The average

Conversion of Chlorinated Volatile Organic Compounds to Carbon Dioxide and Methyl Chloride for Isotopic Analysis of Carbon and Chlorine

\delta ^{18} {\text{O}}_{{\text{H}}_{\text{2}} {\text{O}}}