Johns Hopkins Craems Project

Objectives

The objectives of this CRAEMS initiative are threefold: (1) to elucidate the mechanisms through which environmental organohalide (R-X) contaminants undergo redox-mediated dehalogenation, either in "engineered" or in natural aquatic settings; (2) to apply the results to develop innovative means for treating or monitoring R-X compounds in environmental matrices; and (3) to provide an example to other universities for educating the next generation of chemists, who would benefit from an appreciation of how past practices have contributed to current environmental issues, how to redress such problems in a practical manner, and how to avoid them in the future.

We are addressing these objectives by establishing a research and educational program focused on R-X dehalogenation that engage a group of academic researchers, postdoctoral associates, graduate and undergraduate students, and industrial and National Lab partners around a sharply focused but multifaceted topic of broad, wide-ranging impact. This CRAEMS initiative involves five PIs from JHU (Meyer, Karlin, Goldberg, Fairbrother, and Roberts) - four from Chemistry and one (Roberts) from the Department of Geography and Environmental Engineering (DoGEE), who bring their collective expertise in synthesis, homogeneous and heterogeneous catalysis, biomimetic chemistry, electrochemistry, surface science, and environmental remediation to bear on the problem of delineating the fundamental and practical issues regarding R-X bond cleavage. This initiative affords diverse internship and enrichment opportunities for students in National Labs and industry. Through well-defined areas of collaboration between the PIs and outside partners, we expect to develop a synergy that will enable significant progress on a pressing environmental problem. The project also has the critical mass required for a first-rate educational program with long-term stability to train students who can ultimately contribute to the development of beneficial chemical technologies and processes.

Motivation

R-X as Environmental Contaminants. Underground aquifers supply drinking water to 130 million United States residents. Unfortunately, the quality of our nation's groundwater resources has been severely degraded by past activities. Among common groundwater pollutants, organohalides figure prominently. For example, of the 25 volatile organic contaminants most frequently detected in untreated groundwater in urban areas, 17 are alkyl or vinyl halides (Table 1). R-X compounds also comprise more than half of the 126 Priority Pollutants regulated by the USEPA (Table 2), as well as 22 of the 50 chemicals on the Chemical Contaminant List of compounds currently being considered for regulation in the future by EPA (Table 3). Many of the synthetic organic contaminants believed to serve as environmental endocrine disrupters contain chlorine. Volatile organohalides have also been shown to produce the stratospheric ozone depletion that occurs over Antarctica and is now beginning to appear over the Arctic. Such compounds are also potent greenhouse gases, contributing to global environmental change.

Table 1. Volatile organohalide contaminants in untreated groundwater in the US (1985-1995)

Compound/Structure	Freq.a(%)	Compound/Structure	Freq.a(%)
CHCl₃	26	Bromochloromethane CH₂BrCl	3.0
Tetrachloroethene	17	Chlorodibromomethane CHClBr₂	2.8
Trichloroethene	17	Dichlorodifluoromethane CF₂Cl₂	2.8
1,1,1-Trichloroethane	9.7	Tribromomethane CHBr₃	2.8
1,1-Dichloroethane	6.4	Dichloromethane CH₂Cl₂	2.4
trans-1,2-Dichloroethene	4.7	Chloroethane ClCH₂CH₃	2.1
cis-1,2-Dichloroethene	4.6	Chloroethene (vinyl chloride)	2.0
Trichlorofluoromethane CFCl₃	3.1	1,2-Dichloroethane	2.0
1,1-Dichloroethene	3.0

Table 2. Examples of organohalide contaminants designated as Priority Pollutants.

Class	Example	Class	Example
Dioxins		Alkyl (poly)halides	CCl₄
Aryl halides		Vinyl (poly)halides
Chlorinated ethers		Chlorophenols
Insecticides(DDT)		Polychlorinated biphenyls (PCBs)

Table 3. Examples of organohalides included on Contaminant Candidate List.

Class	Use	Structure
Alkyl/vinyl halide	Fumigant
Chloroacet-anilide	Herbicide
Substituted urea	Herbicide
Substituted terephthalic acid	Herbicide metabolite

Attributes of Halogens. The prevalence of R-X contaminants reflects their prominence in the chemical industry, and must be balanced by the invaluable roles they and often the halogenated compounds serve. Approximately 15,000 commercial chemicals (21% of the total) contain chlorine, and synthesis of many others involves chlorinated feedstocks. 85% of pharmaceuticals are manufactured using chlorinated compounds. In developing nations where drinking water is not chlorinated, waterborne diseases kill 25,000 children daily. The utility of organohalides stems from several attributes. Organic Cl and Br are facile leaving groups useful in synthetic intermediates and in biologically active compounds. Halogens modify local electronic environments, providing useful stability on exposure of R-X to O₂ or to oxygenase enzymes. Finally, halogens influence hydrophobicity, furnishing a means for designing pharmaceuticals and agrochemicals with optimal uptake and translocation properties. Despite problems associated with persistent, toxic, bioaccumulative pollutants, the benefits of R-X are compelling. Organohalides are destined to remain a cornerstone of the modern chemical industry. They are not just problems of the past; new R-X products (including chloroacetanilides, Table 3, and freon substitutes) are introduced regularly.

Each of the positive attributes of organohalogens has a negative aspect: the alkylating ability of R-X confers toxicity; excessive stability (as exemplified by freons) is an environmental liability, and excessive hydrophobicity leads to biomagnification. Halogenated byproducts of drinking water chlorination pose a risk to human health, and potential ecotoxic effects of pharmaceuticals in water are of concern. It is imperative that chemists recognize the link between chemical attributes and the attendant environmental consequences. A program providing classroom training in "green" chemistry and exposure to "real world" issues, as provided by this CRAEMS initiative, should represent a major step forward in educating the next generation of chemists.