Effect of transition-metal cations on the adsorption of H2S in modified pillared clays

Article abstract of DOI:10.1021/jp0223509

Sodium montmorillonite was intercalated with the Keggin ion (hydroxyaluminum polycation) and calcined at 400°C. Then, doping with Fe³⁺, Cu²⁺, or Zn²⁺ was carried out. On the obtained samples, H₂S breakthrough capacity tests were carried out under wet conditions. Doping with metals significantly improved the capacity of adsorbents, despite a noticeable decrease in microporosity. The sample doped with copper showed the best performance as a hydrogen sulfide adsorbent. The reason for this is likely the affinity of the metal to bind hydrogen sulfide, the degree of metal dispersion, and the accessibility of small pores.

Full text of DOI:10.1021/jp0223509

5812
J. Phys. Chem. B 2003, 107, 5812-5817
Effect of Transition-Metal Cations on the Adsorption of H2S in Modified Pillared Clays
Danh Nguyen-Thanh† and Teresa J. Bandosz*,‡
Department of Chemistry, The City College of the City UniVersity of New York, and The Graduate School of
the City UniVersity of New York, New York 10031
ReceiVed: October 31, 2002; In Final Form: March 26, 2003
Sodium montmorillonite was intercalated with the Keggin ion (hydroxyaluminum polycation) and calcined
at 400 °C. Then, doping with Fe3+, Cu2+, or Zn2+ was carried out. On the obtained samples, H2S breakthrough
capacity tests were carried out under wet conditions. Doping with metals significantly improved the capacity
of adsorbents, despite a noticeable decrease in microporosity. The sample doped with copper showed the best
performance as a hydrogen sulfide adsorbent. The reason for this is likely the affinity of the metal to bind
hydrogen sulfide, the degree of metal dispersion, and the accessibility of small pores.
Introduction
oxidation-reduction properties (iron) or the affinity of metals
to form stable sulfides.
From their first syntheses during the late 1970s,1-3 PILCs
(pillared interlayered clays) have attracted a lot of attention and
have been the subject of numerous studies. They are generated
through the exchange of the small charge-compensating cations
(usually alkali) of the clays, situated between the negatively
charged clay layers, with metal polyoxycations.4,5 Calcination
at 400 °C or higher temperatures leads to the dehydration and
dehydroxylation of hydroxycations and their conversion to metal
oxide pillars separating the clay layers.4,6,7 Hence, for example,
pillaring a clay with Al implies first the intercalation of the
Keggin ion [AlO4Al12(OH)24(H2O)12]7+ 8,9 and then the forma-
tion of Al2O3 pillars as a result of heat treatment.9-12
Experimental Section
Materials. Sodium Montmorillonite (Na-M). The same
original (unmodified) clay was used for the preparation of all
of the samples. It is a Na-rich montmorillonite from Crook
County, Wyoming purchased from The Source Clay Minerals
Repository from the University of Missouri in Columbia. In all
experiments, it was used as received without any purification.
Al-Pillared Montmorillonite (Al-M). For Al pillaring, a
50 wt % solution of chlorhydrol from Reheis Inc. was used. It
is a solution of aluminum chlorohydrate, which has a molecular
formula of Al2Cl(OH)5‚(2.5H2O). The chlorhydrol was diluted
with distilled water to give a 0.2 M solution with respect to
aluminum content. The montmorillonite clay sample (20 g) was
dispersed to water and aged for 24 h, and then the intercalating
solution was added dropwise, giving a final Al/clay ratio of 20
mmol/g. The obtained suspension was stirred for 24 h at room
temperature. Excess pillaring agent was washed out with distilled
water until its conductivity reached 30 µS. After drying, the
intercalated montmorillonite was obtained. The samples were
calcined at 400 °C for 4 h. This process led to the formation of
pillared montmorillonite, which is referred to as Al-M.1,2,13,26,30,31
Me-Doped Al-Pillared Montmorillonite (MeAl-M) with
Me ) Fe3+, Zn2+, Cu2+. A 0.2 M NaCl solution (1 L) was
prepared. Al-M (10 g) was dispersed into that solution with a
Na/clay ratio of 20 mmol/g. The suspension was heated to 60
°C and stirred until all of the clay was well-dispersed. At that
point, the pH of the solution was around 4.6. A dilute solution
of 0.05 M NaOH was added dropwise to raise the pH of the
suspension to 9. Once that pH was reached, small amounts of
NaOH were added to maintain the pH at 9. After 24 h of stirring
under these conditions, the suspension was washed by dialysis
until the conductivity of the water was below 5 µS. Then the
clay suspension was filtered and dried. A 20-g sample of treated
Al-M was dispersed in 1 L of water, and 0.2 M solutions of
either Fe3+, Zn2+, or Cu2+ were added dropwise until the Me/
clay ratios were equal to 20 mmol/g. After the addition of the
Me (iron, zinc, or copper) solution, the suspensions were stirred
for 24 h. Then the clays were washed out using a dialysis
membrane, filtered, dried, and calcined at 400 °C for 4 h. After
Pillaring of clay with metal oxides results in the appearance
of new and interesting properties. First, the presence of these
large pillars instead of alkali cations in the clay interlayer space
causes the development of micropores (pores with a width
smaller than 20 Å).6,13,14-23 Another valuable feature of PILCs
is their acidity, whether it is of the Brønsted or of the Lewis
kind. Even though clays originally have Brønsted acidic
character,8 introducing metal oxide pillars, especially aluminum,
brings Lewis acid sites.12 These properties explain why PILCs
have been so extensively studied. They are potentially good
materials for many industrial applications, mostly in cataly-
sis4,6,24 and gas adsorption.7 But so far, most studies of PILC
applications have been on their use as either catalysts or as
catalyst supports. Meanwhile, studies using PILCs as gas
adsorbents have been rather scarce.25 Research on PILCs is
currently going in new directions, one of them being the doping
of PILCs with metallic cations.12,26 This is done to introduce
new acidic or oxido-reductive properties into the clay.
In this study, we synthesize transition-metal-doped Al PILCs
and test them as adsorbents of hydrogen sulfide. Earlier studies
showed that various adsorbents (carbonaceous and mineral)
modified with metal cations have improved capacities for H2S
adsorption.27-29 The metal cations used here for doping are Fe3+
,
Zn2+, and Cu2+. They were chosen by taking into account either
* To whom correspondence should be addressed. E-mail: tbandosz@
ccny.cuny.edu. Tel: (212) 650-6017. Fax: (212) 650-6107.
† Department of Chemistry.
‡ The Graduate School of the City University of New York.
10.1021/jp0223509 CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/23/2003

Effect of Cations on H2S Adsorption in Clays
J. Phys. Chem. B, Vol. 107, No. 24, 2003 5813
this process, the samples are referred to as AlFe-M, AlZn-
M, and AlCu-M.
Methods. X-ray Diffraction (XRD). XRD (40 kV, 30 mA)
was performed on a Rigaku X-ray diffractometer that uses Cu
KR (1.54184 Å) as a source of radiation. The analyzed clay
samples were spread as thin layers on a glass slide. The
diffraction patterns were obtained by varying the 2θ (θ being
the Bragg angle) from 4 to 8° in increments of 0.008°.
N2 Adsorption. N2 adsorption measurements were performed
at -193 °C on an ASAP 2010 (Micrometrics). In each case,
about 0.2 g of sample was used. Prior to the measurement, all
samples were degassed at 120 °C and until constant low pressure
was reached (0.004 µm of Hg). For a precise determination of
the microporosity, adsorption isotherms were measured at a very
low relative pressure (P/Po) of around 10-6. Successive doses
of N2 were then added until the relative pressure reached 0.1.
Then the preprogrammed pressure table was followed. The
surface area, SN , was calculated following the BET approach.32
2
The total pore volume, Vt, was determined by nitrogen uptake
at P/Po ≈ 0.99. The micropore volume, Vmic, and pore size
distributions (PSDs) were obtained using the Kelvin equation
with the Halsey thickness curve.33-34
Elemental Analysis. The Fe, Zn, and Cu contents of the
different samples were determined using the ICP method of
Huffman Laboratories (Golden, CO).
Figure 1. X-ray diffraction results.
Thermal Analysis. Thermal analyses were carried out on
an SDT 2960 from TA Instruments. The sample (about 20-30
mg) to be analyzed was heated from room temperature (∼30
°C) to 1000 °C at a rate of 10 °C/min with a flow of nitrogen
at 100 mL/min. TG (thermal gravimetry) and DTG (derivative
thermal gravimetry) curves were then recorded.
H2S Breakthrough Capacity. Dynamic tests were carried
out at room temperature to evaluate the capacity of the sorbents
for H2S removal under wet conditions. Adsorbent samples (2-3
mm granule size) were packed into a column (length 60 mm,
internal diameter 9 mm, bed volume 6 cm3) and prehumidified
with moist air (relative humidity 80% at 25 °C) for 1.5 h. The
amount of adsorbed water was estimated from the increase in
the sample weight. Moist air containing 0.3% (3000 ppm) H2S
was then passed through the column of adsorbent at 0.5 L/min.
The breakthrough of H2S was detected using an Interscan LD-
17 electrochemical sensor. The test was stopped at the break-
through concentration of 1000 ppm. The adsorption capacities
of each sorbent in terms of milligrams of H2S per gram of
adsorbent were calculated by integrating the area above the
breakthrough curves and from the H2S concentration in the inlet
gas, the flow rate, the breakthrough time, and the mass of
sorbent. For each sample, the H2S test was repeated at least
twice. The determined capacities agreed to within 5%.
To determine if sulfur dioxide was generated during H2S
adsorption/oxidation on modified clays, the SO2 concentration
in the effluent gas was also monitored with a MicroMax
electrochemical sensor.
Figure 2. N2 adsorption isotherms at -196 °C.
diffraction peak at 2θ ) 5.08°, the interlayer spacing is 7.8 Å.31
This is in agreement with the expected values described in the
literature.4 After doping the clay with zinc and copper, the
diffraction peaks are broader, but their maxima are at the same
positions as that for the aluminum pillared material. It is
interesting that AlFe-M lost its crystallinity as a result of the
incorporation of iron and the delamination of the mineral
structure.35,36
Although the N2 adsorption isotherms of all of the clay
samples are similar in shape (Figure 2), the amounts that were
adsorbed significantly differ. After intercalation, nitrogen uptake
significantly increased because of the formation of micropores
as a result of the opening of the interlayer space for the nitrogen
molecule. The steep rise at the beginning of the isotherms
indicates the presence of microporosity. Doping with metals
decreases the amount adsorbed and changes the width of the
hysteresis loop, which indicates changes in the micro- and
mesoporosity of the materials studied.7,18 In the case of doping
with iron, these changes are the most extended, suggesting the
blocking of small pores along with the aggregation of mineral
particles (changes in mesoporosity).36 Both zinc and copper have
similar effects on micropores; however, they affect larger pores
differently. The hysteresis loop in the case of zinc is broader
pH. The suspensions of clays were prepared by dispersing
0.4 g of the sample into 20 mL of water. They were stirred for
24 h and then filtered, and the pH of the filtrate was measured.
Results and Discussion
The X-ray diffraction patterns for our samples are presented
in Figure 1. The Na-M sample shows a diffraction peak at
2θ ) 6.88°, which, according to Bragg’s law, corresponds to a
basal spacing of 12.8 Å. Taking into account the thickness of
a montmorillonite clay layer equal to 9.6 Å,7,12 an interlayer
spacing for this sample is equal to 3.2 Å. For Al-M with a

5814 J. Phys. Chem. B, Vol. 107, No. 24, 2003
Nguyen-Thanh and Bandosz
area occurred. Similar changes are observed for the volume of
micropores; in fact, they are expected because micropores
mainly contribute to the surface of materials.40 When the total
pore volume is analyzed, the changes observed for the sample
after doping with Fe3+ are not as drastic as changes in the
surface area. This is the result of changes in the total pore
structure of materials and the formation of new mesopores. This
effect was observed previously for the samples modified with
iron.35,36,45
Besides difference in the nature of surface interactions,
changes in the pore volume can originate from differences in
the amount of metal deposited on the surface. Elemental analysis
showed that after modifications the content of metal was of 9.82,
4.03, and 7.72% for the iron-, zinc-, and copper-modified
samples, respectively. Although the content of iron, which is
twice that of zinc, can cause changes in the structure of the
clay with pore blocking for AlFe-M, this is not the case when
copper and iron are compared. This indicates that Cu2+ cations,
besides being present in a highly dispersed state in micropores
(small decreases in volume and pore size), are also present in
significant quantities in mesopores and macropores. In the case
of AlFe-M, the micropores practically do not exist. These
differences can be caused by the propensity of iron to
incorporate into aluminum pillars as a result of acid-base
interactions.26 This process leads to a decrease in the thermal
stability of pillars and the collapse of the sorbent structure during
calcination at 400 °C. It is likely that for AlZn-M, where the
content of metals is twice smaller than for the copper-modified
counterpart, Zn2+ cations are located mainly in the micropores.
Figure 3. Pore size distributions.
TABLE 1: Structural Parameters Calculated from Nitrogen
Adsorption Isothermsa
SBET
Vmic
Vt
sample
[m2/g]
[cm3/g]
[cm3/g]
Na-M
34
324
97
0.001
0.129
0.017
0.017
0.091
0.078
0.082
0.064
0.090
0.207
0.131
0.133
0.161
0.147
0.169
0.126
Al-M
AlFe-M
Al-Fe-M/E
AlZn-M
AlZn-M/E
AlCu-M
AlCu-M/E
98
228
197
220
155
a E refers to the exhausted samples after H2S adsorption.
The H2S adsorption experiments carried out on Al-M showed
negligible capacity; the sample was exhausted after 1.6 min,
resulting in a breakthrough capacity of 0.6 mg of H2S per g of
sample. This happened even though the pore volume of the
Al-M sample is quite significant compared to that of other
adsorbents.46 It has to be mentioned here that the physical
adsorption of hydrogen sulfide under our experimental condi-
tions is expected to be small because of the small size of the
molecule and the high temperature of this process. The only
factor that can enhance the capacity is chemisorption of
hydrogen sulfide or its catalytic oxidation. In both of these cases,
the pore volume is an important factor because the products of
surface reactions, either salts or sulfur, have to be stored within
the pore system of the adsorbents. The small capacity of Al-M
obviously indicates the lack of any chemical or catalytic effect
of the surface.
than that for copper-doped material. The hysteresis loops are
of type B for slit-shaped capillaries with parallel walls.37 Such
porosity is expected between the small particles of clays.35,36
Figure 3 collects pore-size distributions (PSDs). In the
calculation of PSDs, the classical method (Kelvin equation with
Halsey thickness curve) was used,33,34 assuming that the pores
have a cylindrical shape. So far, many approaches leading to
the PSDs for PILC have been discussed in the literature, and
none of them gives unambiguous results.38-42 Because all of
the materials have the same origin, in our view, the changes in
pore structure can be compared if the same approach is used,
even though it is not necessarily perfect. Figure 3 shows that
all of the studied samples, besides the initial material and that
modified with iron, have an intense peak at about 11 Å. The
intensity of this peak corresponds to the volume of the pores.
On the basis of XRD, the main pore size should be about 8 Å,
and the shift is caused by the imperfection of the model that
was used.40,41 From the analyses of the obtained plots, it is
clearly seen that after pillaring the volume of micropores
significantly increased and doping with zinc or copper decreased
it only slightly. This suggests that zinc and copper, as expected,
are dopped on the mineral layers rather than on the surface of
pillars. The PSD looks different for the sample modified with
iron. For this material, the significant volume of the micropores
is unavailable. As indicated above, it is likely due to the
adsorption of metals on the external surface, delamination, and
the aggregation of mineral particles.36,43 Moreover, as indicated
in the literature,26 iron destroys the structural integrity of
aluminum pillars.
On the contrary, running the H2S breakthrough experiment
on the three doped samples, AlFe-M, AlZn-M, and AlCu-
M, showed that doping with Me considerably improves the
capacity of Al-M for H2S adsorption. The breakthrough curves
of the doped samples are shown in Figure 4. An analysis of the
curve shapes suggests differences in the mechanisms of the
removal of hydrogen sulfide on metal-modified pillared clays.
For AlCu-M, the concentration rises linearly with a constant
slope, suggesting a constant rate of the adsorption/reaction.
However, the slope changes for the samples modified with zinc
or iron. For these samples, after steep rises at the beginning,
the slopes decrease. This indicates that after the initial step of
adsorption the nature of the surface process changes, which is
favorable for further retaining hydrogen sulfide.
Structural parameters calculated from the isotherms are
collected in Table 1. After pillaring, the surface area increased
10-fold. Doping with zinc and copper has similar effects on
the surface accessibility: the surface area decreases about 25%
compared to that of the sample pillared with aluminum species.
In the case of doping with iron, a 70% decrease in the surface
Table 2 collects the breakthrough capacities for the materials
studied. For AlCu-M, the capacity is almost 3 times higher
than that for iron-modified counterparts. The capacity of
AlZn-M is only slightly greater than that for AlFe-M. All of
these differences follow neither the changes in the content of

Effect of Cations on H2S Adsorption in Clays
J. Phys. Chem. B, Vol. 107, No. 24, 2003 5815
Figure 4. H2S breakthrough curves.
Figure 5. Comparison of pore size distributions for Zn- and Cu-doped
samples before and after H2S adsorption.
TABLE 2: H2S Breakthrough Capacity Results and pH of
Surface
weight
[g]
bth time
[min]
bth cap.
[mg/g]
sample
pHinit
pHE
Al-M
5.29
3.28
3.47
2.83
1.60
33.23
42.07
71.27
0.60
17.64
20.94
48.77
7.18
5.59
6.09
5.35
AlFe-M
AlZn-M
AlCu-M
5.59
6.06
5.05
metal nor the changes in the pore volumes. However, although
the small adsorption on iron- and zinc-containing samples can
be linked to the small pore volume or small metal content,
respectively, the high adsorption on Cu-modified samples does
not exactly follow the effect of metal loading. However, the
comparison of the results obtained for the Zn- and Cu-modified
samples does not stress the importance of the pore volume. The
reason for the differences in the observed performance of the
materials studied as H2S adsorbents is in the combination of
surface chemistry and porosity.
Figure 6. TG curves in nitrogen.
The analysis of the changes in the pore structure of the
samples after adsorption may shed some light on the under-
standing of the H2S removal mechanism of those materials.47
The structural parameters are collected in Table 1. Comparison
with the data obtained for exhausted samples to those for the
samples not exposed to hydrogen sulfide adsorption clearly
shows that in the case of zinc- and copper-modified samples
the products of the adsorption/reaction are deposited on the
surface, resulting in a decrease in the surface area and volume
of pores. This decrease is the most pronounced for the sample
modified with copper where the adsorption of hydrogen sulfide
was the greatest.
becomes acidic after pillaring because of the dehydration and
the dehydroxylation of the Keggin ion at 400 °C and the
formation of Al2O3 pillars. As a result, H+ cations are generated:
2[AlO4Al12(OH)24(H2O)12]7+ f 13Al2O3 +14H+ + 29H2O
(1)
An unchanged pH value for the iron-modified sample supports
our hypothesis about the conversion to weakly adsorbed SO2.
A decrease in the pH for AlCu-M/E is related to the formation
of new species. This hypothesis is discussed below.
Results of thermogravimetric analyses are collected in Figures
6 and 7. A comparison of TG curves (Figure 6) once again
indicates that in the case of the sample modified with iron only
adsorbed water or weakly retained SO2 can be detected. It is
seen as an increase in weight loss at about 100 °C. At higher
temperatures, the curves remain almost parallel, with a weight
loss at about 600 °C related to the dehydroxylation of hydroxy-
aluminum layers.30 For the Cu- and Zn-modified samples, an
increase in weight loss is noticed after H2S adsorption.
On DTG curves (Figure 7), the weight losses in certain
temperature ranges are represented as peaks whose surface areas
can be directly compared. An analysis of the curves obtained
for zinc- and copper-modified samples before and after hydrogen
sulfide adsorption clearly shows the thermal desorption of new
species present on the surface after the materials were exposed
to hydrogen sulfide. Because the first peak with a maximum at
about 100 °C represents mainly water, its intensity is not
discussed here. An interesting temperature range where changes
Because the structural parameters for the iron-modified
sample were unchanged after H2S adsorption, in Figure 5 we
collect the PSDs obtained for the samples modified with zinc
and copper. For both samples, it is clearly seen that pores smaller
than 200 Å are affected and that their volume is decreased.
Although in the case of AlZn-M after H2S adsorption the
changes occurred mainly in the volume of mesopores, for
AlCu-M/E the volume of the micropores is much more affected
than that of mesopores. Although in the case of AlFe-M the
amount of hydrogen sulfide adsorbed was similar to that on
the zinc-modified sample, changes in the pore volume did not
occur. It is likely that in the case of this sample Fe3+ was
reduced to Fe2+, thus oxidizing hydrogen sulfide to weakly
adsorbed SO2, which was desorbed from the surface immediately
or during outgassing when high vacuum was applied.
Differences in the products of surface reactions are also seen
in the values of the surface pH. Slightly basic Na-M (pH 7.18)

5816 J. Phys. Chem. B, Vol. 107, No. 24, 2003
Nguyen-Thanh and Bandosz
TABLE 3: Number of Moles of Metals Doped on the
Surface and Number of Moles of Hydrogen Sulfide
Adsorbed
Me
H2S
sample
[mmol/g]
[mmol/g]
AlFe-M
AlZn-M
AlCu-M
1.70
0.61
1.10
0.51
0.61
1.43
which suggests a direct chemical reaction between zinc and
hydrogen sulfide and optimum conditions of doping manifested
by the accessibility to all of the zinc cations. In the case of
AlCu-M, the number of moles of doped Cu is 30% smaller
than the number of moles of adsorbed H2S. This indicates that
on the surface of this material there are copper cations that can
act as catalysts for H2S oxidation. However, for AlFe-M, the
number of moles of doped Fe3+ cations far exceeds (by a factor
of more than 3) the number of moles of H2S, which suggests
that most Fe3+ in AlFe-M do not participate in H2S im-
mobilization. It is likely that the doping level was too high.
Excess Fe3+ resulted in the destruction of micropores and the
formation of large iron clusters. Another reason for this may
be the difference in the oxidation states of iron (+3) and copper
and zinc (+2).
Figure 7. Comparison of DTG curves in nitrogen for Zn- and Cu-
doped samples before and after H2S adsorption.
On the basis of the results described above addressing the
chemical structure and performance of materials such as
hydrogen sulfide adsorbents, the mechanism for H2S im-
mobilization on Me-doped Al-M seems to be based on acid-
base reactions. In the first step, H2S is adsorbed on the sample
surface through a metathesis reaction, where hydrated Men+ acts
as a Lewis acid toward the base S2- and H+ and H2O compose
the other acid-base pair:
Men+(H2O)m + H2S f MeS(n - 2)+(H2O)m - 2 + 2H3O+ (2)
Figure 8. Differences in DTG curves in exhausted and initial samples.
are observed is the one between 200 and 700 °C. To make this
more unambiguous, we plotted in Figure 8 the differences
between the exhausted and the initial samples. It is clearly seen
that whereas for the iron-modified sample the curve is almost
flat in the cases of zinc and especially of copper significant
features can be noticed. The well-defined peak between 220
and 280 °C likely represents the decomposition of copper sulfide
with a boiling point of 220 °C.48 The second broad peak,
between 400 and 700 °C, is the result of the decomposition of
copper sulfate (boiling point 650 °C)48 to sulfur oxide and copper
oxide. This latter copper sulfate was likely formed on the surface
from copper sulfide when enough oxygen from the air was
present for the further oxidation of sulfides. Indeed, for this
sample a noticeable decrease in the pH was found after H2S
adsorption.
It is also noteworthy to mention how the samples change their
appearance after H2S adsorption. Although AlFe-M (brick red)
and AlZn-M (pale gray) hardly change color after H2S
adsorption (they become slightly lighter), AlCu-M is the only
sample that really undergoes a color change, from dark gray to
dark green. This supports our hypothesis that first CuS is formed
and then it is oxidized to CuSO4 (dark green).
Indeed, Lewis acids and bases preferentially bind if they are
both hard or soft.49,50 Although H+ and H2O are both considered
to be hard species, S2- is a soft base.51 However, Fe3+ is a
hard acid, whereas Zn2+ and Cu2+ are borderline acids (which
means that they are halfway between hard and soft acids).50
Consequently, when the formation of metal sulfide with Zn2+
and Cu2+ is very favorable, only a weak bond with Fe3+ can
form. This would explain why chemisorption occurs more
favorably on AlZn-M and AlCu-M than on AlFe-M. Without
doping, the only available Lewis acids are Si4+ and Al3+, which
are also strong acids. But because they belong to the clay layers
or to the pillars, they are bound to O2-, a hard base,50 which
makes them even harder acids,49 especially harder than Fe3+
.
Binding S2- to these cations is then unlikely, which explains
the negligible capacity that was found for Al-M. The H3O+
formed during this first step would be responsible for the pH
decreases observed with AlZn-M and AlCu-M. These sulfides
can be oxidized to sulfates by oxygen from the air, and the
occurrence of this process was observed in the case of the
AlCu-M sample. Taking into account that water is present in
the system and adsorbed in pores, CuSO4 dissociates, leaving
copper ion available to bind S2- further. This can account for
the fact that the amount of H2S that is adsorbed is greater than
the number of copper ions introduced to the surface of the
mineral. Because the amount of copper sulfate dissolved is
limited by the amount of water adsorbed in pores, the
breakthrough occurs when no more H2S can react on the surface.
Moreover, another product of the surface reaction is sulfur
dioxide, whose low concentration (a few ppm) was detected in
the effluent gas.
To better understand the mechanisms of H2S adsorption on
the three doped samples, for each sample, the number of moles
of doped Me cations, calculated from the percentage of doped
Me cations (from elemental analysis), was compared to the
number of moles of H2S adsorbed on the sample, which was
evaluated from the breakthrough capacity results. The results
are collected in Table 3. It is interesting that for AlZn-M the
number of moles of metal and hydrogen sulfide are identical,

Effect of Cations on H2S Adsorption in Clays
J. Phys. Chem. B, Vol. 107, No. 24, 2003 5817
(4) Pinnavaia, T. J. Science 1983, 220, 356.
The smaller capacity for hydrogen sulfide immobilization
observed for the zinc-doped material is due to the smaller doping
effect than in the case of copper (twice as much copper as zinc).
To explain the “catalytic” effect of copper (more moles of H2S
adsorbed than Cu cations present), the kinetics of sulfide
oxidation should be considered. Because in the case of the
copper-doped material the number of binding sites for sulfur
was greater than in the case of the zinc-doped counterpart, the
breakthrough to 1000 ppm of H2S for the former sample took
longer than for the latter one. The consequence of this is the
longer contact of CuS with oxygen than that for ZnS. These
differences in time would be critical for the oxidation of sulfides
to sulfates. In fact, in the case of AlZn-M, changes in the slope
of the breakthrough curves could be noticed. This phenomenon
can be related to the initialization of ZnS oxidation on the
surface. Besides this, there are also differences in the solubility
(5) Cheng, L. S.; Yang, R. T. Microporous Mater. 1997, 8, 177.
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48
1.27 × 10-36 compared to 2.93 × 10-25
for ZnS, the
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Conclusions
Doping of montmorillonite pillared with aluminum oxides
with Fe3+, Zn2+, or Cu2+ results in a significant increase in
hydrogen sulfide adsorption on the surface. As a result of surface
reaction, hydrogen sulfide is immobilized as metal sulfides.
Because of the presence of oxygen in the system, further
oxidation occurs, and sulfates are formed. The best performing
materials are the ones doped with Cu, which is due to the high
propensity of copper to form sulfides. When iron was present,
the removal of hydrogen sulfide was not efficient because of
the clustering of iron and the oxidation of H2S to elemental
sulfur and then to SO2, which is very weakly adsorbed on the
surface.
Acknowledgment. We thank Professor Maria Tamargo and
Ms. Yanting Liao for XRD measurements. This work was
partially supported by PSC-CUNY (grant no. 64358-00-33).
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