LETTER TO THE EDITOR
Surface Precipitation Reactions on Oxide Surfaces
Retention of heavy metal ions on solid surfaces is an important process for many catalytic and electrochemical reactions and for maintaining environmental quality. Determining reaction mechanisms is
essential for understanding such processes. However, various mechanisms have been proposed for the
sorption of cationic heavy metals on oxide surfaces. In this study, we provide direct evidence using highresolution transmission electron microscopy (HRTEM) for the formation of a surface precipitate prior
to bulk solution precipitation. Furthermore, the type of surface present influenced the onset of surface
precipitation. At pH 5 and 400 ttM A1(III), a surface precipitate was observed on M nO2 (the birnessite
phase) but was not apparent on TiO2 (the rutile phase). Thus, surface precipitation reactions must be
considered in modeling the sorption mechanisms of hydrolyzable metal ions on oxide surfaces. ©1992
Academic Press, Inc.
The mechanisms of metal ion retention on colloidal
particles are of great interest to colloidal and environmental
scientists. The reactivity, high surface area, and coating
ability of oxide materials make them very influential in
the sorption of metal ions. Numerous thermodynamic approaches have been used in describing metal sorption processes on oxide surfaces (1-8). However, these macroscopic approaches do not give direct evidence for the
mechanisms of sorption reactions. Spectroscopic studies
have indicated various mechanisms for the sorption of
metal ions. Here, we provide direct experimental evidence,
using an advanced structure analysis technique, high-resolution transmission electron microscopy (HRTEM), that
surface precipitation can occur prior to bulk solution precipitation and is influenced by the type of surface. Consequently, such phenomena need to be considered in catalytic and environmental processes. Mechanistic models
proposed to describe cationic metal ion sorption reactions
should incorporate surface precipitation reactions rather
than modeling the total sorption process as just an isolatedsite binding mechanism.
In this study, samples were prepared for HRTEM examination by dispersing powdered oxide, MnO2 (the birnessite phase) and TiO2 (the rutile phase), on a holey
carbon film supported by a copper mesh grid: The surface
areas of the oxides were determined by an EGME method
(9) and were 223 m 2 g-i for MnO2 and 44 m 2 g-1 for
TiO2. To have similar surface areas of each oxide, 1 mg
TiO2 and 0.2 mg MnO2 were dispensed on the grids--at
10 ml solution volumes this resulted in 0.1 g liter -a TiO2
and 0.02 g liter -l MnO2. Reactions were carried out by
suspending the oxide-containing grid in a 10 ml solution
of 0.1 M NaNO3 and 400 #M _AI(III) at pH 5, 20°C, and
1 atm pressure. After a 24-h reaction period the grids were
removed from solution, rinsed in deionized water, and air
dried at 20°C. Results were consistent between replicates
and within each sample; thus, the results do not appear
295
Journal of Colloid and lnter/~tce Science. Vol. 148, No. 1, Januapy 1992
to be artifacts of individual preparations. Micrographs illustrated in this paper are representative of each treatment.
Further details on the HRTEM imaging technique are
given elsewhere (10).
High-resolution TEM images show that the unreacted
MnO2 was partially crystalline (Fig. la). The characteristic
balls of needles of synthetic birnessite are depicted ( 11 ).
Interspersed with amorphous material, there are layers of
parallel atomic planes exhibiting severe bends and twists
which in some cases form distinctive needle-shaped protrusions. In the unreacted MnO2; the needles are prominent and clearly delineated. After reaction with AI(III),
the basic structure of the MnO2 appears to be unchanged
(Fig. lb). However, under closer inspection a structural
alteration of the colloid can be discerned. The characteristic
needles are still present, but are no longer as sharply defined
as in the unreacted material. An amorphous layer can be
seen along the edges of the MnO2 particles, filling in between the needles as well as coating the edges. In addition,
an overall loss of detail in the image is apparent. This
persisted over a range of focus settings bracketing the optimum value, indicating that the electron beam has passed
through an incoherently scattering (noncrystalline) layer
in addition to the material that was previously being imaged. Therefore, we conclude that an amorphous layer
developed after reaction with AI(III) and completely enveloped the MnO2 surface. The amorphous material is
expected to be a precipitate of A1(OH)x, where x denotes
molecularity of hydroxyl species associated with each A1;
x may vary depending upon the bonding arrangement of
A1 with the OH groups and with the surface.
The rutile polymorph of TiO2 is depicted in Fig. 2a
prior to reaction with A1(III). The micrograph shows that
this material is polycrystalline, with distinctly faceted grains
of fairly uniform size. Atomic planes are visible within
many of the grains and allow orientation differences between grains to be observed directly. After reacting with
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296
LETTER TO THE EDITOR
FIG. 1. High-resolution TEM image of (a) unreacted MnO2 (the bimessite phase) and (b) MnO2 after
reaction with 400 # M AI(III) at pH 5. After MnO2 has reacted with AI(III), an amorphous layer has
enveloped the MnO2 surface, which is shown by the amorphous material deposited at the edges of the needle
structures and a resulting overall loss of detail in the image.
A1(III ) (Fig. 2b), the TiO2 appears to be unaltered. Even
very close inspection of the images does not reveal the
formation of a surface precipitate; the crystalline structure
remains unchanged, and no amorphous material is present.
The MnOz and TiO2 surfaces differ in their ability to
induce surface precipitation in systems of 400 I~M A1(III)
at pH 5. At pH 5, the MnO2 (zero point of charge, ZPC
Journal of Colloid and Interface Science, Vol. 148, No. 1, January 1992
= 2.8) (13) surface was strongly negatively charged, while
TiO2 (ZPC = 6.7) was positively charged. The absence of
a precipitate on the surface of the TiO2 discounts the possibility that a colloid acts only as a nucleation site for the
formation of a precipitate. The enhancement of precipitation in between the needles of the MnO2 may be due to
electrostatic effects, which are increased due to the close
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LETTER TO THE EDITOR
F1G. 2. High-resolution TEM image of (a) unreacted TiO2 and (b) TiO2 after reaction with 400 u M
AI(III) at pH 5. The TiO2 structure appears to be unaltered after reacting with AI(III); no amorphous
material can be seen on the surface structure.
proximity of the charged surfaces, rather than simply an
abundance of nucleation sites. Hence, the lack of precipitation in the TiO2-AI(III) system substantiates (i) that
surface precipitation can occur prior to bulk solution precipitation (as exemplified by the MnO2 system in this
study), and (ii) that the properties of the surface do affect
the formation of a surface precipitate.
Hypothesized mechanisms for cationic metal sorption
on oxide surfaces include adsorption of the free ion or
hydrolysis products (3-7), adsorption of polymerized
species ( 13, 14), surface cluster formation (15), and the
formation of a surface precipitate ( l, 8, 16-19 ). The multinuclear metal species observed at lower surface coverages
by spectroscopic techniques ( 14, 15) may be a precursor
Journal ~fColloid and lnter/bcz, Science,
Vol. 148,No. 1, January1992
298
LETTER TO THE EDITOR
to the formation of a surface precipitate. The mass action
balance proposed by Bleam and McBride (15) to explain
the progression from isolated-site binding to the formation
of multinuclear sorbed clusters with increases in solution
metal concentration may explain the formation of a surface
precipitate at the even greater solution metal ion concentrations used here. Direct evidence has also been obtained
using X-ray photoelectron spectroscopy (XPS) for the
formation of a Co(H) surface precipitate on oxide surfaces
( 17-19). Crowther et al. (19) observed XPS spectra characteristic of Co(OH)2 on the surface of a synthetic birnessite when critical pH and Co (II) concentrations were
exceeded, However, the results of the XPS studies have
been questioned (14). As demonstrated here, HRTEM
provides a tool for obtaining direct evidence for the formation of a surface precipitate. Clearly, the MnO2 surface
structure underwent an alteration, while the TiO2 surface
appeared unaltered.
The formation of a surface metal precipitate prior to
bulk solution precipitation has important implications for
environmental quality and catalytic processes. Surface
precipitation represents a means of metal removal from
solution which will be less subject to desorption reactions
releasing metals back to solution--only dissolution of the
precipitate will release the metal to solution. Moreover,
the surface precipitate will mask the properties of the original sorbent, and only the surface properties of the metal
hydroxide will be exhibited by the resulting conglomerated
colloid.
ACKNOWLEDGMENT
The HRTEM facilities used were at the National Center
for Electron Microscopy, Lawrence Berkeley Laboratory,
which are funded by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences
Division of the U:SI Department of Energy.
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SCOTT E. FENDORF 1
DONALD L. SPARKS
Department of Plant and Soil Science
University of Delaware
Newark, Delaware 19717-1303
MARK FENDORF
RONALD GRONSKY
Materials" Science Division
Lawrence Berkeley Laboratory and Department o f
Materials Science and Mineral Engineering
University of California
Berkeley, California 94720
Received July 15, 1991; accepted October 8, 1991
To whom correspondence should be addressed.