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SPECIATION OF COMPLEX ORGANIC CONTAMINANTS IN WATER WITH RAMAN SPECTROSCOPY
Citation:
Collette, T W. AND T. L. Williams. SPECIATION OF COMPLEX ORGANIC CONTAMINANTS IN WATER WITH RAMAN SPECTROSCOPY. Presented at 30th Annual International Symposium on Environmental Analytical Chemistry, Espoo, Finland, June 11-18, 2000.
Impact/Purpose:
Elucidate and model the underlying processes (physical, chemical, enzymatic, biological, and geochemical) that describe the species-specific transformation and transport of organic contaminants and nutrients in environmental and biological systems. Develop and integrate chemical behavior parameterization models (e.g., SPARC), chemical-process models, and ecosystem-characterization models into reactive-transport models.
Description:
Pesticides and industrial chemicals are typically complex organic molecules with multiple heteroatoms that can ionize, tautomerize, and form various types of hydrates in water. However, conceptual models for predicting the fate of these chemicals in the environment ignore these complex forms and, instead, assume that each chemical exists exclusively as a simple isolated neutral specie. This oversight is primarily due to the lack of experimental data on speciation of complex organics in water. It is clear that this erroneous assumption contributes large uncertainty in chemical exposure assessments when one considers the degree to which chemical behavior (e.g., sorption to soil) differs for different ionization species of the same chemical (e.g., a cation versus a neutral). Unfortunately, it has heretofore been prohibitively difficult to study organic chemical speciation in water; particularly when multiple species exist with the same net ionization charge (such as with zwitterions and tautomers).
We have recently developed an experimental and mathematical method, based on Raman spectroscopy, by which simultaneously occurring site-specific "micro-equilibrium" constants (such as with zwitterions and tautomers) can be determined with confidence. The method involves nonlinear regression modeling of temperature-variant spectral data according to the Gibbs-Helmholtz equation. Success of the method depends on capturing small changes in the observed series of spectra that are due to a systematic change (as a function of temperature) in relative concentration of the species that compose the equilibrium "mixture".
In order to glean the requisite information from our chemical systems, it is critical that the measurement method not perturb the equilibrium under study. Therefore, invasive techniques can not be considered. For example, methods that involve measurements at surfaces (such as infrared spectroscopy via attenuated total reflectance) can not be employed due to the possibility of surface affinity for one species over the other. Also, chromatographic separation of species is not feasible because equilibration times are too fast. We hope to illustrate that a modern dispersive Raman spectrometer is the ideal tool for this type of environmental investigation because measurements of bulk solutions can be made without contacting or perturbing the sample in any significant way. With this technique we can obtain unparalleled information regarding molecular speciation of organics in water. This information will significantly advance our understanding of the behavior complex organic chemicals in the aquatic environment.