doi:10.1016/j.gca.2004.12.029
Surface complexation model for multisite adsorption of copper(II) onto kaolinite
CAROLINE L. PEACOCK and DAVID M. SHERMAN*
Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, United Kingdom
(Received July 12, 2004; accepted in revised form December 15, 2004)
Abstract—We measured the adsorption of Cu(II) onto kaolinite from pH 3–7 at constant ionic strength.
EXAFS spectra show that Cu(II) adsorbs as (CuO
4
H
n
)
n-6
and binuclear (Cu
2
O
6
H
n
)
n-8
inner-sphere com-
plexes on variable-charge AlOH sites and as Cu
2+
on ion exchangeable X--H
+
sites. Sorption isotherms
and EXAFS spectra show that surface precipitates have not formed at least up to pH 6.5. Inner-sphere
complexes are bound to the kaolinite surface by corner-sharing with two or three edge-sharing Al(O,OH)
6
polyhedra. Our interpretation of the EXAFS data are supported by ab initio (density functional theory)
geometries of analog clusters simulating Cu complexes on the {110} and {010} crystal edges and at the
ditrigonal cavity sites on the {001}. Having identified the bidentate (AlOH)
2
Cu(OH)
2
0
, tridentate
(Al
3
O(OH)
2
)Cu
2
(OH)
3
0
and X--Cu
2+
surface complexes, the experimental copper(II) adsorption data can
be fit to the reactions
2 AlOH + Cu
2+
+ 2H
2
O = AlOH
2
CuOH
2
0
+ 2H
+
3AlOH + 2Cu
2+
+ 3H
2
O = Al
3
O(OH)
2Cu
2OH
3
0
+ 4H
+
and
X
-
--H
+
+ Cu
2+
= X
-
--Cu
2+
+ X
+
. (A1)
Copyright © 2005 Elsevier Ltd
1. INTRODUCTION
The aqueous geochemistry of copper can be strongly con-
trolled by sorption onto iron and manganese (hydr)oxides and
clay minerals. In soils, copper is concentrated into the clay
fraction (Le Riche and Weir, 1963) presumably by sorption
onto clay and colloidal FeOOH phases. Kaolinite, montmoril-
lonite and illite are the most common clay minerals in soil
systems (Du et al., 1997). Whether copper is associated with
the phyllosilicate minerals vs. the iron hydroxides is unclear.
The copper-kaolinite association and mechanism of retention in
altered copper-bearing rocks is also undetermined. In the Ra-
kha-Chapri mining block (Singhbhum copper belt, India) su-
pergene clay alteration products extend down to 60 m and are
comprised predominantly of kaolinite. Tenginkai et al. (1991)
reported copper as a component of the kaolinitic clay minerals
and suggested association via surface adsorption and/or lattice
binding. Mookherjee and Tenginkai (1987) also reported fixa-
tion of substantial Cu by supergene clay minerals. The exact
mechanism of retention is ambiguous.
This study presents a surface complexation model for the
adsorption of copper on kaolinite that is consistent with spec-
troscopic data and ab initio calculations. A surface complex-
ation model based on actual surface species will be more
reliable when applied to modeling reactive transport of Cu in
complex natural systems.
Kaolinite is a 1:1 aluminosilicate consisting of a tetrahedral
silica sheet bonded to an octahedral alumina sheet via the
sharing of oxygen atoms between silicon and aluminium atoms
in neighboring sheets. Consecutive layers stack by hydrogen
bonding and thus there are no interlayer spaces. Isomorphic
substitution of Al
3+
for Si
4+
in the tetrahedral sheet (e.g., van
Olphen, 1977) results in a small fixed negative charge within
the siloxane layer, which accounts for the small cation ex-
change capacity (CEC) of kaolinite (generally 0.02 mol
charge/kg in chemically pure kaolinites; Talibudeen, 1981). On
the face of the tetrahedral silica sheet, permanent negative
charge is located at the ditrigonal siloxane cavities (e.g., Davis
and Kent, 1990) and is commonly represented as X
-
surface
sites. Kaolinite CEC has also been suggested to occur due to the
presence of a small amount of aluminosilicate gel coating
(Ferris and Jepson, 1975) and/or contamination by small
amounts of 2:1 phyllosilicate minerals (e.g., Bolland et al.,
1976, 1980; Lim et al., 1980). In addition to permanent, neg-
atively charged X
-
sites, kaolinite has amphoteric, variable-
charge SOH sites at the crystal edges and on the octahedral
alumina sheet. SOH sites represent silanol SiO(H) sites at
the crystal edges (contributing only to the negative charge
through the formation of SiO
-
) and aluminol AlOH(H)
sites at the crystal edges and on the octahedral sheet (both
protonation and deprotonation can occur to form AlOH
2
+
,
AlOH, and AlO
-
). Al
2
OH(H) sites also exist at the
gibbsite basal plane and contribute to the positive surface
charge in the lowest pH regime through the formation of
Al
2
OH
2
+
(Huertas et al., 1998).
Previous studies examining the interaction between Cu(II)
and kaolinite have tended to focus on either the modeling of
adsorption behavior displayed in experimental adsorption
edges and isotherms, or direct spectroscopic investigation of
the metal-mineral association.
Two types of modeling approach have been followed: sur-
face complexation modeling (SCM) and a more empiric con-
sideration involving the use of Langmuir or similar equations to
* Author to whom correspondence should be addressed
(dave.sherman@bristol.ac.uk).
Geochimica et Cosmochimica Acta, Vol. 69, No. 15, pp. 3733–3745, 2005
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