GEOCHEMICAL RESEARCH ISSUES ASSOCIATED

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GEOCHEMICAL RESEARCH ISSUES ASSOCIATED WITH
ACID MINE DRAINAGE
Nicholas T. Loux, Ph.D. U.S. EPA/ORD/NERL/ERD 96
0 College Station Road Athens, Georgia
30605 loux.nick_at_epa.gov
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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.

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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.
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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.
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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) MezgMe(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?

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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.
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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.

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Electrostatic corrections to diffuse layer
adsorption models
For a reaction of the form gtSOH Mez ltgt
gtSOMe(z-1) H where gtSOH 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 gtSOMe(z-1)aH()e-(1-z)eY/kT
K ------------------------------- gtS
OHaMe(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.
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Electrostatic corrections to diffuse layer
adsorption models (continued)

• Recent research (Loux, 2000) suggests that the
  following mass action expression may be more
  theoretically rigorous
• gtSOMe(z-1)aH()e-(2-t)(1-z)eY/kT
• K -------------------------------------
• gtSOHaMe(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).

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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.
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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.

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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.