Layered Double Hydroxides and the Environment: An Overview - Advanced Materials for Agriculture, Food and Environmental Safety - page 5

clays and host-guest layered materials [1, 3, 32-35], which are quite rare in
nature. Most LDHs are synthetic phases and their structure resembles the
naturally occurring mineral hydrotalcite [Mg 6 Al 2 (OH) 16 ] CO 3 . 4H 2 O, hav-
ing the general formula of [M(II) 1-x M (III) x (OH) 2 ] (Y n- ) x/n . YH 2 O, where,
M(II), M(III) = divalent and trivalent metals respectively, 0.2 < x < 0.33,
and Y n- = the exchangeable anions between the layers [10, 36, 37].
h e basic layer structure of LDHs is based on brucite [Mg (OH) 2 ], typically
associated with small polarizing cations and polarizable anions. It consists of
magnesium ions surrounded approximately octahedrally by hydroxide ions.
h ese octahedral units form ini nite layers by edge-sharing with the hydrox-
ide ions sitting perpendicular to a plane of the layers. h e layers then stack
on top of one another to form a three-dimensional structure.
When Mg 2+ is replaced by a trivalent cation similar in radius, an over-
all positive charge results in the hydroxyl sheets and counter balance is
provided by carbonate ions which are positioned within the hydroxyl
interlayer. In addition to carbonate ions, water molecules are found in
the interlayer gallery. h e nature of the interlayer anion and the extent of
hydration ot en determine the layer spacing between each brucite-like sheet
[38]. h e brucite-like sheets may occur in two dif erent symmetries, namely
rhombohedral and hexagonal. In nature, the rhombohedral symmetry is
widespread. However, in mineral samples, the hexagonal symmetry is seen
to favor the interior of the crystallite samples, while the rhombohedral sym-
metry is found on the exterior. h is is a result of cooling during crystallite
transformation, in which the extrerior surface of the crystallite cools much
quicker than the interior and hence the interior hexagonal form cannot
transform due to a higher energy transformation barrier at lower tempera-
ture. From these observations, it has been deduced that the hexagonal sym-
metry is favored by high temperature [1, 4]. Naturally occurring minerals
that exhibit a LDH structure include manasseite, pyroaurite, sjogrenite, bar-
betonite, takovite, reevesite, desautelsite and stichtite. h ey dif er from one
another in the stacking arrangement of the octahedral layers [1, 39].
Conventionally synthesized LDHs are strongly hydrophilic materi-
als, either amorphous or microcrystalline with hexagonal habit, with the
dominant faces developed parallel to the metal hydroxide layers. Adjacent
layers are tightly bound to each other. Figure 1.1 shows the structure of
layered double hydroxides.
One of the advantages of LDHs among layered materials is the great
number of possible compositions and metal-anion combinations that can
be synthesized. Layered double hydroxides (LDHs) have high charge den-
sity. h e charge density is dependent on the metal ratio. Since it comprises
a divalent and trivalent metal cation, their ratio af ects charge density of the
layers. A lower divalent/trivalent ratio results in a higher charge density.