GEOCHEMICAL RESEARCH ISSUES ASSOCIATED WITH ACID MINE DRAINAGE

1. GEOCHEMICAL RESEARCH ISSUES ASSOCIATED WITH ACID MINE DRAINAGE Nicholas T. Loux, Ph.D. U.S. EPA/ORD/NERL/ERD 960 College Station Road Athens, Georgia 30605 loux.nick@epa.gov

2. Geochemistry of Acid Mine Drainage (AMD) • L Oxidation of sulfide minerals generates acidity and may liberate toxic metals... • Permanent solution: • * Isolate sulfide minerals from oxygenated H2O. • Interim solutions: • * Neutralize acidity (e.g., wetlands, limestone basins) and remove toxics. • * Assess bioavailability of previously released toxicants in soils and sediments; remediate where necessary.

3. Some Individual Objectives in AMD Geochemical Research: * Develop accurate models of kinetically limited processes (e.g., precipitation reactions, nucleation reactions, phase transitions, oxidation-reduction reactions, diffusive processes, etc.). * Develop more accurate mechanistic models of significant equilibrium processes (e.g., adsorption reactions on heterogeneous sediments, estimating ion activity coefficients at high ionic strengths, estimating the effects of charged surfaces in low ionic strength media on: contaminant mobility, pH and oxidation-reduction potential sensitive reactions, colloidal particle migration etc.). * Develop models of toxicant mobility and bioavailability in historically contaminated soils and sediments. * Develop multimedia models describing environmental mercury alkylation, fate and transport.

4. Numerous federal, state and local government and private entities have ongoing research in many of these areas. Therefore, the remainder of this presentation will focus on relevant research being conducted at the Athens laboratory.

5. Developing Methodologies for Calculating Ion Activity Coefficients at High Ionic Strengths • Virtually all models describing the biogeochemical reactivity • of ionic species require an estimate of the chemical activity (a) of • each significant species in solution: • aMe(z+) = [Mez+]gMe(z+) • Where aMe(z+) is the chemical activity of the ion, [Mez+] is • the analytical concentration of the ion, and gMe(z+) is the single ion • activity coefficient of the ion. • Current generic methods for estimating gMe(z+) include the Davies and Extended Debye Huckel expressions. Unfortunately, these expressions are rated for use up to a maximum ionic strength of 0.5 M. What about saline, hyper saline, and high ionic strength AMD systems?

6. Ion Activity Coefficient Research Needs: * Quantitatively determine the ionic strength limitations of current broadly applicable activity coefficient estimation algorithms. * Develop interim activity coefficient algorithms for use in high ionic strength media common in the environment. * Ultimately, develop Pitzer relationships applicable to all trace toxicants.

7. Assessing the Role of Low Ionic Strength Electrostatic Phenomena on Toxicant Partitioning, Transformations, and Transport • Virtually all environmental surfaces possess a net surface charge. Associated with this charge is a net surface potential (Y). In low ionic strength systems, this potential can significantly modify solution pH and oxidation-reduction potentials (Davies and Rideal, 1968; Loux and Azarraga, 1987; Loux and Anderson, 2000): • pHsurface = pHsolution + eY/2.3kT • Esurface = Esolution - Y • Given that most environmental surfaces are negatively charged at ambient pH conditions, most interfacial regions are more acidic and oxidizing than is observed in bulk solution. • The surface potential also can play a key role in migration of colloidal particles (i.e., facilitated transport) and the partitioning behavior of ionic species.

8. Electrostatic corrections to diffuse layer adsorption models: • For a reaction of the form: • >SOH + Mez+ <==> >SOMe(z-1)+ + H+ • where >SOH designates a bound adsorptive site that reacts with an ion of valence z. The traditional diffuse layer model mass action expression describing this reaction is given by: • [>SOMe(z-1)+]aH(+)e-(1-z)eY/kT • K = ------------------------------- • [>SOH]aMe(z+) • where brackets designate concentrations, subscripted “a” terms indicate chemical activities, e represents the base of the natural logarithm, the superscripted e is the charge of the electron, Y is the surface potential, k is the Boltzmann constant and T is the absolute temperature.

9. Electrostatic corrections to diffuse layer adsorption models (continued): • Recent research (Loux, 2000) suggests that the following mass action expression may be more theoretically rigorous: • [>SOMe(z-1)+]aH(+)e-(2-t)(1-z)eY/kT • K = ------------------------------------- • [>SOH]aMe(z+) • where t represents the fraction of net surface charge that is neutralized by electrolyte counterions; t can deviate significantly from a value of one at low charge densities and ionic strengths (i.e., under conditions likely to be found in low ionic strength environments).

10. Some Adsorption Research Needs: * Quantitatively assess the significance of bound site charging energies (Loux, 2000) to environmental adsorption reactions. * Characterize the surface potential of natural solids in low ionic strength media. * Develop methods for calculating t for planar surfaces. * Recompute intrinsic reaction constants within the context of a charging energy paradigm.

11. Mercury at Mining Sites • Mercury contamination can be a problem at sites where the ore mercury content is high and at sites where mercury has been (or is) used to sequester precious metals. • Given that mercury may initially exist in the elemental form or may be converted from ionic species to the elemental form by environmental abiotic and biotic processes, evasion of elemental mercury to the atmosphere can be one means of mercury migration from regions with high concentrations of the contaminant. • Unfortunately, mercury contamination of fish is the leading cause of fish consumption advisories found in the majority of U.S. states. Hence, near background ambient mercury concentrations can lead to significant adverse ecological effects.

12. Mercury Research Needs: • Develop accurate, validated mechanistic models describing mercury air surface exchange (e.g., Loux, 2000). • Develop accurate, validated mechanistic models describing monomethylmercury formation from the more common nonalkyl species. • Develop large datasets relating to the environmental behavior of mercury in atmospheric, aquatic and soil/sediment media. • Assess the potential for a regional and/or global distillation effect with mercury.