N. Lee Wolfe

Green plants: A terrestrial sink for atmospheric CH3Br

Methyl bromide is reactively removed from air by the foliage of all 9 herbaceous, 18 deciduous, and 12 coniferous plants we have tested, in a process that appears enzymatic. Excised plant leaves yielded removal rates directly proportional to leaf surface area and first-order in CH3Br concentration from 10 ppmv to 500 pptv, the current limit of our experimental technique. Observed rate constants for different plants vary within a factor of about 100 with a lower value of 1×10−3 hr−1cm−2 in systems where the rate is not diffusion limited. This sink for atmospheric CH3Br could be significant in calculations of the global methyl bromide budget.

New perspectives in aquatic redox chemistry: Abiotic transformations of pollutants in groundwater and sediments☆

Abstract Presented is a review of recent advances in the chemistry of abiotic redox transformations of organic pollutants in anaerobic ecosystems. The goal is to provide an indication of the state of knowledge and the remaining difficulties, rather than an exhaustive review of the existing literature. Particular attention is given to the types of functional groups that undergo reaction and the findings concerning physical and chemical parameters of ecosystems that govern the rates and products of redox transformations. Classes of compounds and structural features within these classes of compounds provide information about the intrinsic nature of the natural reductants. Further information is provided by studies which consider system variables such as sediment concentrations, organic carbon levels, pH, Eh and temperature. While the identity of reducing agents that transform organic pollutants in anaerobic systems remains elusive, the reactivities of these agents are being characterized and compared with surrogate (model) reductants. It is apparent that chemical and biological reduction processes are strongly coupled, and there is increasing evidence for widespread mediation of reductive reactions by bio-organic molecules.

Organophosphate and organophosphorothionate esters: Application of linear free energy relationships to estimate hydrolysis rate constants for use in environmental fate assessment

Abstract Hydrolysis rate constants required for assessing the environmental fate of certain organophosphate and organophosphorothionate esters may be estimated by use of linear free energy relationships (LFERs). LFERs for the second-order alkaline hydrolysis rate constants and the pKa of the conjugate acid of the leaving groups were established for 0,0-dimethyl- and 0,0-diethyl-0-substituted phosphates and phosphorothionates. Also, the second-order alkaline hydrolysis rate constants of selected triaryl phosphates were correlated with the Hammett-sigma constants. Existing LFERs for diaryl phosphate ester anions along with monoaryl phosphate ester mono- and dianions were used to predict hydrolytic half-lives under reaction conditions that are characteristic of aquatic environments.

N-nitrosamine formation from atrazine.

ConclusionsPrevious studies have focused on N-nitroso derivatives of relatively non-persistent pesticides or pesticides that form unstable N-nitroso derivatives. We feel that more persistent pesticides or their degradative products capable of forming N-nitroso derivatives should be investigated. Also pathways of degradation, both chemical and biological, should be elucidated.

Use of structure-reactivity relationships to estimate hydrolytic persistence of carbamate pesticides

Abstract Linear free energy relationships are given for use in estimating alkaline hydrolysis of carbamate pesticides in water as applied to environmental conditions. Plots of the second-order alkaline hydrolysis rate constants versus p K a of the resulting alcohol are given for N , N -dimethyl-, N -methyl- N -phenyl-, N -methyl-, and N -phenyl-carbamates. Use of these relationships allows one to estimate rate constants for specific compounds and make general statements about hydrolytic stability of classes of compounds.

On the degradation of methyl bromide in sea water

Methyl bromide degradation in sea water can be described by a summation of the hydrolysis and chloride ion exchange reactions. Laboratory experiments covered chloride concentrations of 0.1 to 1.0 mol/1, and temperatures from 20 to 60°C. The first-order hydrolysis rate constant is and the second-order chloride ion exchange rate constant deduced from the experiments is At a sea water surface temperature of 21.9 °C and a chloride concentration of 0.56 mol/1, the calculated degradation half-life of methyl bromide in sea water is 4 days. At 35 °C, τ1/2 = 22 hr.

Analysis of volatile N-nitroso compounds in drinking water at the part per trillion level

ConclusionVolatile gc-amenable N-nitroso compounds have not been found to be present in drinking water, even at ultra trace levels. Since sensitive analytical procedures do not currently exist for detecting the non-volatile N-nitroso compounds, such as the N-nitrosotriazines it should be clearly understood that these results refer only to the gas-chromatograph amenable N-nitroso compounds. Nothing is to be inferred as to the presence or absence of the more thermally labile N-nitroso compounds.

QSARS for predicting biotic and abiotic reductive transformation rate constants of halogenated hydrocarbons in anoxic sediment systems

Abstract Quantitative structure-activity relationships (QSARs) are developed relating biotic and abiotic pseudo-first-order disappearance rate constants of halogenated hydrocarbons in anoxic sediments to a number of readily available molecular descriptors. Based upon knowledge of the underlying reaction mechanisms the following descriptors were selected: • - Carbon-halogen bond strength. • - The summation of the Hammett (aromatics) and Taft (aliphatics) sigma constants and the inductive constants (aromatics) of the additional substituents. • - Carboncarbon bond dissociation energy (aliphatics). • - Steric factors of the additional substituents (aromatics). Comparison of the abiotic and biotic QSARs clearly showed the close similarities between both processes. By correlating the rate constants for reduction of a number of halocarbons obtained in a number of distinct sediment samples to the organic carbon content of the samples, the QSARs were made operative for predicting rates of reduction of given halocarbons in given sediment-water systems. The correlations were enhanced by taking into account the fraction of the compounds sorbed to the solid phase.

Methylmercuric complexes in aquatic systems.

Abstract Equilibrium calculations indicate that methylmercuric ion is almost completely complexed under conditions that exist in fresh waters. Sulfur-bonded complexes, CH3HgS−, (CH3Hg)2S, and CH3HgSR, are likely to be the predominant forms of methylmercury in fresh natural waters. With no reduced sulfur species or sulfhydryl-containing organics present, methyl-mercuric hydroxide or methylmercuric chloride would predominate. Common chemical species such as orthophosphate, ammonia, phenolic groups, and protein amino groups have little effect upon complexation of methylmercury.

Abiotic Transformations of Toxic Organic Chemicals in the Liquid Phase and Sediments

Increasing concern over the disposal of hazardous chemicals in commercial waste sites and municipal landfills in the United States has served to focus additional emphasis on the transport and transformation of organic compounds in ground waters, sediments and soils. In particular, mechanisms of transformations of the parent compounds as well as daughter products are receiving increased attention. Because of the large number of disposal sites (> 20,000) and an expanding list of hazardous organic chemicals to be assessed, models are being increasingly used to predict the impact of these chemicals on the environment as well as their potential for human exposure (Brown et al. 1986).