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AN EXACT METHOD FOR RELATING ZWITTERIONIC MICROSCOPIC TO MACROSCOPIC ACIDITY CONSTANTS
Citation:
Loux, N T. AN EXACT METHOD FOR RELATING ZWITTERIONIC MICROSCOPIC TO MACROSCOPIC ACIDITY CONSTANTS. CHEMICAL SPECIATION AND BIOAVAILABILITY 15(2):47-51, (2003).
Impact/Purpose:
There are four objectives of this work:
A: Updating/Assessing EPA's MINTEQA2 Geochemical Speciation Model
EPA has distributed the MINTEQA2 geochemical speciation model to the professional research community for several decades. Although the model has undergone a number of improvements during this period, this effort will involve: 1) expanding the thermodynamic data base in MINTEQA2 to include components not currently in the model, and 2) assessing the error associated with applying the low ionic strength activity coefficient algorithms in MINTEQA2 to marine and hypersaline aquatic systems.
B: Advancing the State-of-the-Science in Ionic Toxicant Adsorption to Natural Surfaces Modeling
There does not currently exist an accurate mechanistic model applicable to all environments for predicting the partitioning behavior of ionic contaminants to natural surfaces. The absence of accurate mechanistic models of ionic contaminant partitioning impairs EPA's efforts to apply the NRC Risk Assessment Paradigm to assess aqueous ionizable contaminant exposures. This work is designed to support current efforts to develop rigorous and defensible mechanistic adsorption models.
The following sub objectives will be addressed: 1) developing improved surface complexation adsorption models to incorporate variable charging energies, 2) developing an improved model of the protonation behavior of zwitterionic species, and 3) exploring current adsorption model "phase additivity" and "surface coating" paradigms to account for trace ionic contaminant adsorptive behavior in heterogeneous systems.
C: Advancing the State-of-the-Science of Air/Water Toxicant Vapor Exchange
Models
Many toxicants of local, regional, continental and global significance display significant vapor phase transport and exchange between the atmosphere and underlying waters. It has been believed for several decades that temperature disequilibria between the atmosphere and underlying waters, and among atmospheric compartments around the globe, can have a significant effect on vapor phase contaminant migration. This work will extend the recently published temperature disequilibrium air/water exchange model for gaseous, elemental mercury to high windspeed conditions and to toxicants other than gaseous mercury.
Depending on the availability of resources, the following sub objectives will be addressed: 1) extending the current diel temperature disequilibrium gaseous mercury model to high wind speed conditions, 2) developing a rigorous method for assessing the affects of salinity and temperature on rates of elemental mercury air/water exchange, and 3) extending the model to contaminants other than mercury.
D: Assessing the Effects of Electrostatic Phenomena on Contaminant Fate
and Transport in Porous Media
Recent findings (Loux and Anderson, 2001. Colloids and Surfaces, A., 177:123-131) have indicated that the net charge and surface potential on environmental surfaces can significantly perturb the pH and oxidation reduction potentials in the solid/water interfacial regions (when compared to the bulk solution). There exists, however, a nearly total dearth of information in the technical literature concerning the electrostatic properties of natural surfaces. It can be inferred from first principles that the electrostatic properties of natural surfaces can potentially modify the transport behavior of ionic contaminants in sedimentary porewaters. Again, either very little or no data exists in the technical research literature to address this issue. This work will involve enhancing EPA's capabilities to account for these phenomena in MINTEQA2.
The following areas will be addressed: 1) characterizing the electrostatic properties of natural environmental surfaces, and 2) assessing the role of electrostatic phenomena on charged particle transport in porous media.
Description:
Zwitterions are aqueous solvated molecules simultaneously possessing one negatively and one positively charged site. Although electroneutral, the environmental interaction of zwitterions with other ionic species is likely to differ significantly from the behavior of comparable electroneutral species without charged sites. Amino acids, the zwitterionic species that have received the most historical scrutiny, are believed to possess at least four microscopic acidity constants: ka = [H+] [+H3NRCOO-]/[+H3NRCOOH], kb = [H+] [H2NRCOOH]/[+H3NRCOOH], kc = [H+] [H2NRCOO-]/[+H3NRCOO-], and kd = [H+] [H2NRCOO-]/[H2NRCOOH]. Unfortunately, due to their comparable energetics, these microscopic acidity constants cannot be discerned using standard potentiometric titration procedures. In response, experimentally observable macroscopic constants (K1 and K2) have historically been related to the microscopic constants with the following relationships: K1 = ka + kb and 1/K2 = 1/kc + kd. It will be demonstrated that these equations are approximations suitable for restricted pH ranges and that more exact expressions can be derived: K1 = ka + kb + kcka/[H+] + kdkb/[H+] and 1/K2 = 1/kd + 1/kc + [H+]/kakc + [H+]/kbkd.