Dr. Tratnyek's research interests concern a wide range of oxidation-reduction reactions that occur in the environment, and the contribution of these reactions to the fate of organic pollutants. In most of his work, the focus is on pathways, kinetics, mechanisms, and other fundamental, molecular aspects of environmental chemistry. The goal is to understand these redox processes at a process level, and to use this knowledge to develop deterministic models of environmental systems and remediation technologies.
In most cases, the research involves characterizing the reactivity of an organic pollutant or class of pollutants, and application of these results to environmental protection. However, environmental processes per se are also of interest. In these cases, organic "pollutants" may be employed, not so much as pollutants, but as probe substances with which to explore the geochemistry and microbiology of complex natural systems. Much of the areas of research outlined below involves organic pollutants in this dual role as probe and substrate.
Reductive transformation is the dominant reaction pathway for many organic pollutants in anoxic environments. Most interest in reductive transformations of environmental chemicals involves dehalogenation of chlorinated aliphatic or aromatic contaminants and the reduction of nitroaromatic compounds. Other reductive transformations that may occur abiotically in the environment include reduction of azo compounds, diamines, hydroquinones, disulfides, and sulfoxides. Additional background on these reactions can be found in various reviews. [1-3]
Some organic compounds undergo reduction reactions in natural anaerobic sediments, soils, and groundwaters. Examples of this reaction type include nitro reduction of the pesticide parathion, and dehalogenation of DDT or chlorinated hydrocarbon solvents like trichloroethylene (TCE). These reactions can be important contributors to the environmental fate of pollutants (e.g., in "natural attenuation"), but little is known about the specific reducing agents that cause them or the factors that control their rates. An example of our work on better characterizing natural reductants is Smolen et al. [4]
One specific issue that is central to understanding reduction of organic pollutants is the role of substances that can mediate electron-transfer (i.e., "electron shuttles"), like certain enzymes, porphyrins, and quinonoid compounds. Our most notable contribution in this area grew out of our early efforts to develop redox buffered ("poised") media for studying contaminant reduction reactions [5]. In that work, we made the first use of anthraquinone-2,6-disulfonate (AQDS) in environmental research. Subsequent work by Derek Lovley--and now many others--has made AQDS the most widely used electron shuttle compound in studies of biogeochemistry [6].
Placeholder caption..., ...or oxidation by heat activated persulfate. Figure from Waldemer et al. [12]. (Click for larger view.)
Despite gaps in our understanding of the fundamentals of environmental reduction reactions, research in this area has already led to some important practical applications, particularly with respect to the remediation of contaminated aquifers. Recently, the various approaches to groundwater remediation by reduction of contaminants have been grouped into into a category called "In situ Chemical Reduction" (ISCR). Our work applies to many aspect of ISCR.
The most important ISCR technology is also the area where our research has been most influential: the use of granular zero-valent iron metals (ZVMs) to degrade and/or sequester both organic and inorganic contaminants. We were among the first to investigate this chemistry in the context of groundwater remediation, which lead to the most highly cited paper in the field [7]. Some of the results that made that paper, and several of our subsquent papers [esp., 8,9], influential are summarized at http://cgr.ieh.ohsu.edu/iron/. As a by product of our continued research in this field, we maintain a searchable database of relevent biliographic data at http://cgr.ieh.ohsu.edu/ironrefs/.
Our most recent work on remediation with zero-valent metals has focused mainly on nano-size zero-valent iron (nZVI). The presumed advantages of nZVI over conventional (micron-sized) zero-valent iron are that the smaller particles are more reactive and more mobile. In actual practice, however, the effects of particle size on the behavior of ZVI are more complex, as we have emphasized in a recent review [10]. Our on-going work on nZVI concerns the effect of aging (i.e., diagensis) on the core-shell structure of nZVI and the effects of coatings of organic matter on the reactivity and mobility of nZVI.
Throughout our work in this area, we have strived to be quantitative and mechanistic, down to the molecular scale, while at the same time being rigorous about the holistic behavior of ZVMs in real world applications. This requires using multiple, complementary methods in our studies: ranging from molecular modeling, microscopy, spectroscopy, and mechanistic probe reactions to reactive-transport modeling, column experiments, and field tests. Electrochemical methods have become a signature method in our work, in part because they are applicable to phenomena across the whole range of relevant dimensions (nanometers to meters).
Placeholder caption..., ...or oxidation by heat activated persulfate. Figure from Waldemer et al. [12]. (Click for larger view.)
When surface waters absorb sunlight, a variety of transient oxidants are formed, including hydroxyl radical, superoxide anion, and singlet oxygen. These transient species (also known as reactive oxygen species, or ROS) are highly reactive and can have both beneficial and harmful effects. For example, singlet oxygen can oxidize organic substances such as phenols, polyaromatic hydrocarbons (PAHs), and pesticides, resulting in pollutant degradation. However, singlet oxygen can also attack cell constituents, resulting in phototoxicity. An example of this multiplicity of photoeffects can be found in a study we did on the fate/effects of textile dyes in sunlight stream waters [11].
Reactive oxygen species are used for contaminant degradation in a variety of processes classified as Advanced Oxidation Technologies (AOTs), or—when applied to groundwater—In Situ Chemical Oxidation (ISCO). The major approaches to ISCO employ permanganate, hydrogen peroxide, and persulfate. The latter two oxidants require "activation", which catalyzes their breakdown to hydroxyl and sulfate radicals. These radicals are responsible for most of the observed degradation of contaminants (although other ROS may play a role).
Our work on chemical aspects of ISCO has focused on two aspects. In general, we are developing a more comprehensive kinetic perspective on the kinetics of ISCO for groundwater contaminants. Part of this work involves summarizing previously published (second order) rate constants, and we have made the results of this available in a freely-accessible online database called IscoKin.
In addition, we are particularly interested in activated persulfate, because relatively little prior work has been done on its application to treatment of contaminants in environmental media. Persulfate is becoming popular for ISCO because it provides rapid degradation of a wide range of contaminants (like Fenton-type treatments) but is sufficiently stable for effective delivery to contaminated zones (like permanganate). One aspect that we have focused on is the role of temperature in activation of persulfate, which is unique relative to the other ISCO oxidants [12].
As noted above, much of our research on fate and remediation or organic contaminants involves the contaminant as probe (e.g., of environmental redox reactions) and as target substrate. The selection of target substrates involves similar criteria to those used to prioritize contaminants for regulation. Of particular interest are compounds that are regarded as emerging contaminants. Emerging contaminants are rarely new, but new developments (with respect to their use, distribution, toxicity, etc.) have given them new significance.
The emerging contaminant among chlorinated solvents is 1,2,3-trichloropropane (TCP). The combination of its high toxicity, mobility, and persistance has placed TCP among the the contaminants that are being considered by a number of organizations as a possible priority for future research and regulation (e.g., the 3rd Contaminant Candidate List prepared by the U.S. EPA under the Safe Drinking Water Act). To support on-going assessments of the risk from TCP-contaminated groundwater, and especially to help identify improved methods for its remediation, we have been studying TCP degradation under a variety of treatment conditions, including the whole range treatments used for in situ chemical reduction (ISCR) and in situ chemical oxidation (ISCO). An overview of prospects for remediation of TCP by ISCR or ISCO can be found in [13].
Most recently, nanoparticles (NPs) have come to be regarded as emerging contaminants. In some respects, assessing the fate and prospects for remediation of NPs is no different than for any other substance, but in other respects NPs are profoundly different. Our interest is primarily with NPs of iron and iron oxides, which probably are environmentally benign overall. However, there are aspects of this that merit further investigation, because the quantity of nZVI used in remediation applications is quite large and use of this technology in the field is spreading rapidly.
Comparison of perchloroethylene degradation by hydrolysis, oxidation by permanganate, or oxidation by heat activated persulfate. Figure from Waldemer et al. [12]. (Click for larger view.)
The analysis of correlations among chemical data serves a variety of purposes: ranging from validation of consistancy, to diagnosing mechansisms, to identificaiton of predictive models [14]. The latter leads to quantitative structure-activity relationships (QSARs), which are correlations between an important property for a series of related compounds (e.g., oxidation or reduction rate constants) and one or several more convenient independent variables.
For chemical properties that determine the fate and effects of environmental contaminants, the field is mature enough to have already engendered several generations of compilations of predictive models. However, the application of QSARs to environmental redox reactions is less well developed, and the only reviews on this topic are [15-16].
Placeholder caption..., ...or oxidation by heat activated persulfate. Figure from Waldemer et al. [12]. (Click for larger view.)