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| by Alexis
S. Templeton, Thomas P. Trainor, and Gordon E. Brown, Jr.,
Stanford University
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Sorption reactions on
particle
surfaces can dramatically affect the speciation,
cycling and bioavailability
of essential micronutrients (i.e. PO43-, Cu, Zn etc.) and toxic
metals and metalloids (i.e. Pb, Hg, Se, As) in soils and aquatic environments.
Considerable attention has been focused on understanding metal sorption
reactions at a molecular/mechanistic level and the effects of metal
concentration, pH, ionic strength, and complexing ligands on the ways in
which metal ions bind to the surfaces of common mineral phases such as Fe-,
Mn- and Al-(hydr)oxides and clays. However, a significant fraction of mineral
surfaces in natural environments are extensively colonized by microbial
organisms, which can also be potent sorbents for metals due to the large
number of reactive functional groups that decorate the cell walls and outer
membranes of bacterial surfaces. |
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Bacteria are widespread in soil and aquatic environments and are
predominantly found in biofilm communities. Biofilms form when bacterial
consortia attach to mineral surfaces and produce films of hydrated
extracellular polymers. Once biofilms form, the bacterial-mineral
micro-aggregates create complex interfaces with the surrounding aqueous
solution. The biofilms may act as an "insulating layer" between the solution
and the mineral surface or form "microenvironments" in which the local
solution conditions are different than those in the bulk solution. Reactive
functional groups, such as carboxyl, hydroxyl, amino and phosphoryl moieties,
present on the bacterial surfaces and exopolysaccharide matrix, can provide a
large array of binding sites for metals, and they may simultaneously block
surface sites on the underlying substrate. In addition, bacterial activity
may catalyze the transformation of toxic metals into less (or more) toxic
species or enhance the dissolution of the underlying mineral substrate. All
of these combined interactions may strongly affect the mechanisms and
kinetics of metal sorption reactions in soils and aquatic environments.
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In Gordon Brown's group at Stanford University, new research has
focused on the application of synchrotron-based x-ray spectroscopic
techniques to the study of metal sorption reactions (e.g. Pb and Se) at
biofilm-mineral-solution interfaces, similar to previous studies of metal
sorption at biofilm-free mineral-solution interfaces. However, it is
particularly challenging to investigate metal-complexation reactions at these
more complex interfaces due to the variety of binding sites available for
metals, the small length-scale of variation in the metal distributions
(angstroms to nanometers) and the need to perform measurements under in
situ conditions. Recent work by Templeton et al. (PNAS
98,
11897 (2001)) has shown how the
long-period x-ray standing-wave (XSW) technique can be used to probe
metal-ion distributions within Burkholderia cepacia biofilms formed on
Al- and Fe-oxide single-crystal surfaces. XSW fluorescence-yield profiles were
collected for biofilms formed on  -Al2O3
and -Fe2O3 surfaces
that had been exposed to solutions with a range of Pb(II) concentrations. The
data show that Pb(II) ions preferentially bind to highly reactive sites on
the oxide surfaces. The order of Pb(II) reactivity with the
mineral substrates followed the order -Fe2O3
(0001)> -Al2O3(1
 02)
>
-Al2O3
(0001), as expected from previous work in "clean" model systems without
bacteria present. The combined data demonstrate that although the biofilm
does form a large sink for Pb(II), especially at high Pb concentrations,
the formation of the biofilm has not passivated the intrinsic reactivity of
the underlying mineral surfaces.
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Primary Citation
A. S. Templeton, T. P. Trainor, S. J. Traina, A. M. Spormann and G. E. Brown,
Jr., "Pb(II) Distributions at Biofilm-metal Oxide Interfaces ", Proc. Natl.
Acad. Sci. USA 98, 11897 (2001)
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