vvEPA
United States
Environmental Protection
Agency
Environmental
Research Laboratory
Athens, GA30613
Research and Development EPA/600/M-89/031 June 1990
ENVIRONMENTAL
RESEARCH BRIEF
Lanthanide Ion Probe Spectroscopy for Metal Ion Speciation
L.V. Azarragai and L.A. Carreiraa
Abstract
A unique process model that intimately involves the
experimental and mathematical aspects of metal-organic
interactions was developed. The technique, LIPSMIS (Lan-
thanide Ion Probe Spectroscopy for Metal Ion Speciation), in
its present form, provides for experimental verification of
predictions of the quantitative values derived from modeling
organic interactions. Although it is cast in its present form to
treat binding of metal ions by humic substances, LIPSMIS is
sufficiently general that it can be modified to model metal
ion binding with inorganic anions or with sites on organic
substances other than humics.
Introduction
Research in metal-organic interactions at the Environmental
Research Laboratory, Athens, Georgia has the prime
objective of providing the necessary scientific knowledge
with which to build decision-making tools for EPA's
regulatory offices. For example, implementation of both the
Hazardous and Solid Waste and Superfund Acts requires
tools for evaluation of alternative waste management and
treatment technologies, based on potential human health
and environmental impacts. These tools include exposure
assessment models for estimating the fate and transport of
toxic metals to either an environmental or human receptor.
Currently, the prime model for this purpose is MINTEQA2, a
thermodynamic equilibrium model for prediction of metal
Speciation, and thus of metal mobility.
Dissolved organic material (DOM), e.g., humic substances,
are an important component of most surface waters and soil
systems, and even occur to a significant extent in some
1 Environmental Research Laboratory, Athens, GA
2 Department of Chemistry, University of Georgia, Athens GA
aquifer solids. Toxic chemicals, such as metals, may bind
with DOM, leading to mobilization if the DOM is in a
dissolved <3r colloidal state. MINTEQA2 does not contain a
term representing the interactions of metals with naturally
occurring organic materials.
The long range goal of our research is to develop a
quantitative term (a mathematical process model) to
describe metal-natural organic binding. However, before this
is possible, it is necessary to develop techniques to
experimentally measure the strength and extent of metal-
organic interactions under the influence of environmental
variables such as pH, ionic strength, and metal ion
competition for the various binding sites of the DOM without
severely perturbing the state of the system as these
measurements are made.
Experimental
The LIPSMIS technique is based on the unique
fluorescence properties of the trivalent europium ion, Eu(lll).
The basis of this technique is the existence of a
hypersensitive transition associated with the fluorescence
emission spectrum of the Eu(lll) ion. The intensity of the
~616nm emission band of the Eu(lll) ion is extremely
sensitive to ligation, whereas the intensity of the ~592 nm
emission band is not. This results in an increase in the
intensity of the 616 band relative to the 592 band as the
Eu(lll) metal ion is bound to a ligand site, for example, a
carboxylic site on a humic material. This intensity ratio
(Ratio = L2/|616) can then be quantitatively related to the
concentrations of the free and bound metal species.
Therefore, the change in this ratio as a function of total
metal added is essentially a spectroscopic titration of the
metal binding sites of the humic (other) material present.
Two experimental problems had to be addressed, however,
before this type of measurement could be deemed viable.
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First, the cross section for spectroscopic absorption for the
Eu(lll) ion is vanishingly small (~10au as compared to
10000 for most organics). This was solved by exciting the
Eu(lll) emission with a high peak power, low average power
pulsed dye laser operating at the Eu(lll) absorption line at
~394nm. Second, at this wavelength the humic material
fluoresced with very high efficiency. The humic emission
could be up to 106 times as strong as the Eu(lll) signal at
low Eu(lll) concentrations. To overcome this large mismatch,
the spectral properties of Eu(lll) were used to separate the
signals. The fluorescence lifetimes of most organics are in
the nanosecond range, whereas the lifetime of the nearly
forbidden Eu(lll) transitions are in the millisecond range. By
using a combination of phototube pulsing and signal time
gating, the Eu(lll) fluorescence signal could be easily
extracted from the extremely intense humic emission.
Details of the experimental setup are discussed by Dobbs
and coworkers1.
A Model for Metal-Organic Binding
The present metal-organic interaction model is based on the
continuous distribution model used for proton binding1 with
humic substances. This distribution is characterized by
three descriptor variables: CL, the total concentration of
metal or proton binding sites, n, the mean Log K, value and
o, the breadth of the distribution of K, values. K, is the cation
- organic binding constant, a measure of binding strength.
The effect of competitive binding was incorporated into the
model by using a form of the competitive Langmuir equation
such that all cations (hydrogen ions and metal cations)
compete for the same distribution of ligand binding sites2.
The effect of ionic strength of the aqueous medium on
binding was found to be easily modeled by expressing the
activities of all species in equilibrium. The simple Davies
model was found to be satisfactory for calculating activity
coefficients.
Although the binding of Eu(lll) to humic materials is not of
particular environmental interest, it can be used as a
sensitive probe for measuring effects of pH and ionic
strength on metal binding. When a second metal, e.g., a
toxic environmental pollutant, is added, the competition for
sites between Eu(lll) and this metal can be monitored as a
displacement of Eu(lll) and concomitant increase in the
measured fluorescence intensity ratio
Effect of pH
Figure 1 shows the effect of pH on metal binding with
Suwannee River fulvic substances. The symbols are
experimentally observed values and the solid lines are
calculated from the metal organic interaction model
The effects of pH are most pronounced in pH ranges below
5. As the pH is decreased, the probe metal is released from
the humic substance, and the fluorescence ratio increases.
The complexation capacity of the humic material is a strong
function of the pH of the aqueous medium
Effect of Ionic Strength
Figures 2 and 3 show the effect of ionic strength on metal
binding with Suwannee River dissolved organic matter
(SRDOM) at pH 3.5 and pH 6 2
In both cases an increase in ionic strength results in a
release of metal ion and an increase in the fluorescence
ratio. The effect is most pronounced at lower pH.
Effect of metal competition
Figure 4 shows the effect of Pb(ll) ions competing with the
europium probe ion for humic binding sites.
The symbols represent observed values and the solid lines
are calculated from the model. The competitive binding
model developed here allows the LIPSMIS technique to
serve as a general tool for measuring the binding strength of
any metal that competes with the probe metal for binding
sites on the humic material.
Conclusions
Comparison of the experimental data with model predictions
shows that humic-bound metal ions can be released rapidly
as free metal ions as a result of changes in acidity and ionic
strength in the aqueous medium. LIPSMIS has yielded
Eu (III) Titrations
1.2
-2.5
Figure 1. Ratio is the fluorescence rate = |592/|616 • Cm is the
total metal added, x = pH of 2.5. * = pH of 3.0. +
= pH of 3.5. Humic concentration of 20 ppm.
20 E-1
NaCIOandO.lM pH=3.5
-6
-4
Log Cm
-2
Figure 2. Humic concentration 20 ppm. O = no NaCI x
0.1 M NaCI pH = 3.5.
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25 E-1 T
ra
o:
0 ;
NaCI 0 and 0.1M pH = 6.2
-6
-4
Log Cm
-2
Figure 3. Humic concentration 20 ppm. 0 = no NaCI x =
0.1M NaCI pH = 6.2.
reasonable values tor the humic complexation capacity, CL
(1.5 x 10-3 meq/g of SRDOM). Addition of environmentally
important metal ions such as Pb or Al results in competition
for binding sites with the probe metal and an observable
release of Eu. The ability to model and measure competitive
metal binding allows this technique to determine the binding
strengths of metals that are difficult if not impossible to
measure by other techniques, e.g. Al(lll). To date we have
measured the mean Log K, values of three metals: pEu = 6.4,
u,Pb = 4.8, and HAI = 55.
For more immediate application, portions of the model,
which have been verified to the extent shown in this paper,
will be incorporated into MINTEQA2 to allow an estimation
of metal-natural organic binding, thus providing a more
accurate prediction of metal speciation and fate.
20
Lead 0 and 2E-4M pH=3.5
o
a
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