Science Inventory

MODELING MONOMETHYLMERCURY AND TRIBUTYLTIN SPECIATION WITH EPA'S GEOCHEMICAL SPECIATION MODEL MINTEQA2

Citation:

Loux, N T. MODELING MONOMETHYLMERCURY AND TRIBUTYLTIN SPECIATION WITH EPA'S GEOCHEMICAL SPECIATION MODEL MINTEQA2. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-05/063 (NTIS PB2006-100890), 2005.

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:

Given the complexity of the various, simultaneous (and competing) equilibrium reactions governing the speciation of ionic species in aquatic systems, EPA has developed and distributed the geochemical speciation model MINTEQA2 (Brown and Allison, 1987, Allison et al., 1991; Hydrogeologic, 1999a, 1999b). The present work is designed to at least partially address a limitation found in earlier versions of MINTEQA2 by initiating a capability for modeling the aqueous speciation behavior of MMHg and TBT in environmental aquatic systems. Generally speaking, geochemical speciation models require both an innate database of reaction constants that enable one to model complex competitive geochemical speciation simulations and user input containing the total analytical concentrations of the reacting species of interest. It is the purpose of this document to develop a "first cut" modeling capability for both MMHg and TBT in EPA's geochemical speciation model MINTEQA2 by extending the existing reaction constant database. There is extensive ongoing research concerning the environmental speciation behavior of both MMHg and TBT and it is anticipated that future upgrades in this area will be necessary as more data becomes available.

URLs/Downloads:

MODELING MONOMETHYLMERCURY AND TRIBUTYLTIN SPECIATION WITH EPA'S GEOCHEMICAL SPECIATION MODEL MINTEQA2   (PDF,NA pp, 187 KB,  about PDF)

Record Details:

Record Type: DOCUMENT (PUBLISHED REPORT/REPORT)
Product Published Date: 05/31/2005
Record Last Revised: 09/03/2015
OMB Category: Other
Record ID: 96167

Organization:

U.S. ENVIRONMENTAL PROTECTION AGENCY

OFFICE OF RESEARCH AND DEVELOPMENT

NATIONAL EXPOSURE RESEARCH LABORATORY

ECOSYSTEMS RESEARCH DIVISION

ECOSYSTEMS ASSESSMENT BRANCH