Journal of Hazardous Materials 178 (2010) 218–225
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Journal of Hazardous Materials
journal homepage: www.elsevier.com/locate/jhazmat
Removal of sulfonamide antibiotics from water: Evidence of adsorption into an
organophilic zeolite Y by its structural modifications
Ilaria Braschi a,∗ , Sonia Blasioli a , Lara Gigli b , Carlo E. Gessa a , Alberto Alberti b , Annalisa Martucci b
a
b
Dipartimento di Scienze e Tecnologie Agroambientali, Università di Bologna, Viale Fanin, 40 – 40127 Bologna, Italy
Dipartimento di Scienze della Terra, Università di Ferrara, Via Saragat, 1 – 44100 Ferrara, Italy
a r t i c l e
i n f o
Article history:
Received 23 July 2009
Received in revised form 11 January 2010
Accepted 12 January 2010
Available online 18 January 2010
Keywords:
Sulfadiazine
Sulfamethazine
Sulfachloropyridazine
Adsorption
XR diffractometry
a b s t r a c t
Sulfonamide antibiotics are persistent pollutants of aquatic bodies, known to induce high levels of bacterial resistance. We investigated the adsorption of sulfadiazine, sulfamethazine, and sulfachloropyridazine
sulfonamides into a highly dealuminated faujasite zeolite (Y) with cage window sizes comparable to sulfonamide dimensions. At maximal solubility the antibiotics were almost completely (>90%) and quickly
(t < 1 min) removed from the water by zeolite. The maximal amount of sulfonamides adsorbed was 18–26%
DW of dry zeolite weight, as evidenced by thermogravimetric analyses and accounted for about one
antibiotic molecule per zeolitic cage. The presence of this organic inside the cage was revealed by unit
cell parameter variations and structural deformations obtained by X-ray structure analyses carried out
using the Rietveld method on exhausted zeolite. The most evident deformation effects were the lowering
of the Fd-3m real symmetry in the parent zeolite to Fd-3 and the remarkable deformations which occurred
in the 12-membered ring cage window after sulfadiazine or sulfachloropyridazine adsorption. After sulfamethazine adsorption, zeolite deformation caused a lowering in symmetry up to the monoclinic P2/m
space group. The effective and irreversible adsorption of sulfonamides into organophylic Y zeolite makes
this cheap and environmentally friendly material a suitable candidate for removing sulfonamides from
water.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
In recent years, the occurrence and fate of pharmaceutically
active compounds in the aquatic environment has been recognized
as one of the emerging issues in environmental chemistry [1]. The
occurrence of antibiotics in hospital effluents and waste or surface waters may present two kinds of risks. Firstly, after supply,
antibiotics select for resistant bacteria in the treated individuals
themselves. Secondly, the presence of antibiotics in streams, lakes
and water supplies encourages the growth of resistant bacteria in
humans and wildlife [2]. Growing resistance means that what were
once effective and cheap antibiotics may become unsuitable for
treating infections.
Sulfonamide antibiotics comprise a class of synthetic sulfanilamide derivatives, widely used for the treatment of bacterial,
protozoal and fungal infections in human therapy, livestock production and aquaculture [3]. They act as competitive inhibitors of
p-aminobenzoate in folate biosynthesis. Sulfonamides are known
∗ Corresponding author. Tel.: +39 051 2096208; fax: +39 051 2096203.
E-mail addresses: ilaria.braschi@unibo.it (I. Braschi), sonia.blasioli@unibo.it
(S. Blasioli), laretta9@alice.it (L. Gigli), carloemanuele.gessa@unibo.it (C.E. Gessa),
alberto.alberti@unife.it (A. Alberti), annalisa.martucci@unife.it (A. Martucci).
0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jhazmat.2010.01.066
to induce high levels of resistance using a bypass mechanism—the
metabolic step which is inhibited by the antibiotic may be replaced
(bypassed) by an alternative metabolic step [4]. They are sufficiently stable in manure to maintain significant residual activity
until field application [5]. In liquid manure, N-aniline acetyl sulfonamides conjugates can be cleaved back to parent compounds
[6]. They may reach the soil through the faeces of treated grazing
livestock and/or the spreading of manure as fertilizer on agricultural soils where they may persist in an unmetabolized form for
months [7]. In non-acidic soils, sulfonamides may exist mainly in
anionic form due to the pKa value of the sulfonamide group (pKa
5.0–7.5 [8,9]) and therefore be potentially highly mobile and thus,
pollute water bodies [10]. In fact, the presence of a net negative
charge on soil surfaces makes this environmental compartment
ineffective in the retention of negative compounds. The adsorption of sulfonamide antibiotics in soil organic matter is reported
to be a time-dependent process [11], while on clay components it
exhibits pronounced acidic pH dependence [12]. Sulfonamides may
also directly reach water bodies through hospital and fish farming wastewaters [1] bypassing soil filtering and depuration activity.
Furthermore, in municipal sewage treatment plants, sulfonamides
may not be effectively eliminated owing to their anionic character.
In fact, pollutant biodegradation is largely achieved by sorption on
activated sludge, which is partly mediated through hydrophobic
I. Braschi et al. / Journal of Hazardous Materials 178 (2010) 218–225
interactions [13]. Approximately 20 years after industrial aquaculture began, evidence has emerged on the transfer of antibiotic
resistance between aquatic bacteria and human pathogens [14]. A
number of important studies indicate that the bacterial flora in the
environment surrounding aquaculture sites contain an increased
number of antibiotic-resistant bacteria [15].
Despite the need to clean up wastewaters which have been
highly polluted with sulfonamide antibiotics, no sorbents with
specific adsorption potential and favourable kinetics have been
identified to date. The aim of this study is to verify the efficiency of
sorbents such as zeolitic materials in removing sulfonamide antibiotics from water bodies.
Zeolites are crystalline alumino-silicates, characterized by
three-dimensional networks containing channels and cavities
whose dimensions are comparable with small organic molecules.
Such networks of well-defined micropores may act as adsorption
and reaction sites whose selectivity and activity can be modulated by acting on their structure and chemical composition.
The three-dimensional framework, consisting of nanometre-sized
channels and cages, imparts high porosity and a large surface area
to these materials. One of their defining features is that the shape
of their internal pore structure can strongly affect their adsorption
selectivity with respect to host molecules. The most fundamental consideration regarding the adsorption of chemical species by
zeolites is molecular sieving. The pores, or rather the active sites
within the pores, exclusively process molecules that fit them, so
that species with a kinetic diameter which makes them too large
to pass are effectively “sieved”.
Zeolites are commonly used in areas as diverse as laundry detergents, gas separation, oil refining and the petrochemical industries,
agriculture, wastewater and sewage treatment. The ability of zeolites with a low Si/Al ratio to remove cations by ionic exchange has
been largely demonstrated and utilized in water treatment plants
which produce drinking water [16]. On the contrary, zeolites characterized by a high Si/Al ratio are hydrophobic and organophilic
materials widely used in adsorption-related applications. To date,
studies and applications on organic pollutant adsorption in these
microporous materials from aqueous media are scarce [17–19].
Moreover, their purpose is the remediation of non-polar hydrocarbons pollutants which are frequently found in refineries and
gasoline station groundwater [20] and not of negatively ionisable
and polar compounds, such as sulfonamide antibiotics.
In this study, a Y faujasite hydrophobic zeolite is presented due
to its ability to specifically adsorb high amounts of sulfonamide
antibiotics inside its cavities with very favourable kinetics. These
findings were supported by X-ray analyses which demonstrated
the accommodation of antibiotic molecules inside the zeolite
cage.
219
Fig. 1. Pore size distribution of zeolite Y determined by nitrogen adsorption
isotherm at −196 ◦ C using a cylindrical pore NLDFT method in the desorption branch.
were sonicated for 15 min, shaken at 50 ◦ C for 30 min and, after
cooling at room temperature, filtered through 0.45 m Durapore®
membrane filters to eliminate the undissolved solute from the
solutions. The solubility of the antibiotics, measured by means of
high performance liquid chromatography (HPLC), was 71.9 ± 4.8,
135.8 ± 3.9 and, 173.6 ± 7.7 M for SD, SM and SC, respectively.
“Y” type faujausite zeolite powder (code HSZ-390HUA) with a
200 SiO2 /Al2 O3 (mol/mol) ratio, a 7.0 Å × 7.1 Å dimension and a
12-membered ring window diameter was purchased in its protonated form from the Tosoh Corporation (Japan). The specific
surface area was measured by means of nitrogen adsorption at liquid nitrogen temperature (−196 ◦ C) in the pressure range 5 × 10−6
to 760 torr (1 Torr = 133.33 Pa) using an Autosorb-1-MP (Quantachrome Instruments) (Fig. S-1 in Supporting Information). Prior
to adsorption, the samples were outgassed for 1 h at 90 ◦ C, 1 h at
130 ◦ C, and finally 16 h at 300 ◦ C under high vacuum conditions
(final pressure 1 × 10−9 Torr). The specific zeolite Y area, determined by the Brunauer–Emmett–Teller approach and using 0.01 as
the value of maximum relative pressure, was 853 m2 g−1 (Table 2).
The pore size distribution was calculated by applying the cylindrical
pore NLDFT method in the desorption branch (Fig. 1). A total specific
surface area of ca. 850 m2 g−1 , most of which related to the presence
of structural micropores (616 m2 g−1 ), was determined (Table 2).
As expected, zeolite Y shows structural 12 Å micropores whose
related volume is 0.21 cm3 g−1 . The presence of 22 Å (related volume = 0.12 cm3 g−1 ) and 50 Å pores (related volume = 0.08 cm3 g−1 )
were also found.
2.2. Adsorption screening
2. Experimental
2.1. Materials
Sulfadiazine (4-amino-2-N-pyrimidinyl-benzene sulfonamide,
SD), sulfamethazine (4-amino-N-(4,6-dimethyl-2-pyrimidinyl)
benzene sulfonamide, SM), and sulfachloropyridazine (4-amino-N(6-Cl-3-pyridazinyl)benzene sulfonamide, SC) were purchased as
analytical standards by Dr. Ehrenstorfer GmbH (Germany) with a
purity of 98.7%, 99.5%, and 98.0%, respectively. These antimicrobial
agents have been chosen because of their widespread consumption
and predominant occurrence in water bodies and soils [21–23].
The chemical structures of sulfonamides are shown in Table 1.
Stock solutions of antibiotics at maximal solubility were prepared
by adding SD, SM and SC antibiotics to distilled water in amounts
exceeding those required to saturate the solution. The suspensions
A preliminary study of the adsorption properties of zeolite Y
from distilled water towards a mixture of sulfonamide antibiotics
was conducted. The adsorption screening was performed at room
temperature and 65 ◦ C, by adding the zeolite to a solution containing SD, SM, and SC (ca. 40 M each, prepared by diluting antibiotics
stock solutions) with a zeolite:antibiotic solution ratio of 1 mg:2 mL
in polyallomer centrifuge tubes (Nalgene, NY, USA). Suspensions
were shaken for 24 h and then centrifuged at 20,000 × g for 15 min.
Finally, an aliquot of the supernatant was withdrawn and analyzed
by HPLC. The amount of antibiotics adsorbed by zeolite was calculated by the difference between the initial and final concentrations.
A control was run in the absence of zeolites in order to check the
stability of the antibiotics and the adsorption properties of the
centrifuge tube. No decrease in sulfonamides concentration was
recorded after 24 h.
I. Braschi et al. / Journal of Hazardous Materials 178 (2010) 218–225
220
Table 1
Structures and chemical characteristics of sulfonamide antibiotics under investigation.
Structure
a
3D structurea
Chemical name
(abbreviation)
Molecular
weight
(g mol−1 )
pKa
Sulfadiazine (SD)
250.3
6.4 [8]
C2–O1 (5.12)
O1–N4 (7.38)
Sulfamethazine (SM)
278.3
7.5 [8]
C12–N2 (10.50)
C11–O2 (6.72)
Sulfachloropyridazine
(SC)
284.7
5.5 [9]
Cl1–O2 (8.11)
O1–N4 (6.50)
Distance (Å)
References for 3D structures of sulfadiazine, sulfamethazine and sulfacloropyridazine are [28–30], respectively.
The effect of dissolved organic matter (DOM) on the adsorption
screening of zeolite Y towards sulfonamides was also investigated
at room temperature. DOM was extracted from the first 10 cm of
a forest soil sample (Lythic Ustorthents, sand:silt:clay = 40:44:16,
pH 5.2) from the north-east of Italy (Imola, BO). The soil was chosen because of its high organic carbon content (total organic carbon
32 g kg−1 ). The suspension formed by a 1:4 soil:distilled water ratio
was shaken in polypropylene copolymer centrifuge tubes on a horizontal shaker for 24 h at RT. After centrifugation at 15,000 × g
to remove the heaviest particulates, the DOM solution (pH 5.8,
electrical conductivity = 0.16 mS) was used without any additional
purification in order to verify if the presence of inorganic and
organic molecules with different dimensions from those of sulfonamides could interfere with their adsorption into zeolite Y. The
freeze–drying of the DOM solution gave 240 mg L−1 as a residue.
The nature and dimensional characterization of the DOM contained
in the soil solution was not investigated since this went beyond the
scope of this work.
2.3. Adsorption kinetics
With the aim of evaluating the time needed for antibiotics to
reach the adsorption equilibrium in zeolites, adsorption kinetics
for SD, SM, or SC on Y were followed over time. Zeolite Y was added
to sulfonamide solutions (ca. 20 M each) with a zeolite:antibiotic
aqueous solution ratio of 1 mg:2 mL. Samples were shaken at room
temperature and at different times, the supernatants were separated from the solid phase by centrifugation and directly analyzed
by HPLC. Adsorption kinetics was conducted in triplicate.
2.4. Adsorption isotherms
SD, SM, and SC sulfonamides adsorption isotherms from distilled
water were performed on zeolite Y in batches at room temperature with a zeolite:antibiotic solution ratio of 1 mg:2 mL. Owing
to the low solubility of sulfonamides, zeolite Y was exposed to a
number of adsorption cycles in the presence of a sulfonamide solu-
Table 2
Main textural properties of zeolite Y.
SBET a (m2 g−1 )
Smicrop b (m2 g−1 )
VP c (cm3 g−1 )
Vmicrop b (12 Å) (cm3 g−1 )
Vmesop d (22 Å) (cm3 g−1 )
Vmesop d (50 Å) (cm3 g−1 )
852
616
0.62
0.21
0.12
0.08
a
b
c
d
Brunauer–Emmet–Teller specific surface area.
Micropore surface area and volume by t-plot method.
Total pore volume.
Volume of mesopores.
I. Braschi et al. / Journal of Hazardous Materials 178 (2010) 218–225
221
tion at maximal solubility. At each adsorption cycle, the suspension
was shaken for 30 min and then centrifuged. The supernatant was
removed and analyzed by HPLC, and replaced by fresh sulfonamide
solution. Subsequent adsorption cycles were performed until the
zeolite Y reached its maximal adsorption capacity (exhausted zeolite). Adsorption experiments were conducted in triplicate.
The antibiotic concentration in aqueous phase at equilibrium
was expressed as Ce (M) while the amount of antibiotics adsorbed
in zeolite was calculated by the difference between the initial and
final (Ce ) concentrations and expressed as Cs (mol g−1 of adsorbent).
The antibiotic concentration used in these experiments (maximal solubility for each sulfonamide) is ca. 1 order of magnitude
higher than those measured in effluents from fish feedings [24]
and ca. 3 orders higher than their occurrence in natural waters
[3]. This procedure was considered opportune for obtaining the
maximal loadings of zeolite in the shortest time in order to subsequently perform thermogravimetric and diffractometric analyses
upon air-drying.
In a separate adsorption experiment, the exhausted zeolite
suspension with the remaining antibiotic solution after the last
adsorption cycle was used to test sulfa drugs desorption as
described below.
detector, using Cu K␣1 ,␣2 radiation in the 3–110◦ 2 range and
a counting time of 12 s step−1 . Rietveld structure refinement was
then performed using the GSAS package [25] with EXPGUI interface [26]. X-ray diffraction (XRD) patterns were then collected on
our zeolite Y after sulfadiazine, sulfamethazine and sulfachloropyridazine adsorption respectively, using the same experimental
conditions as the parent Y zeolite.
In all Rietveld structure refinements, the Bragg peak profile was
modelled using a pseudo-Voigt function with 0.01% cut-off peak
intensity. The background curve was fitted using a Chebyschev
polynomial with 16 variable coefficients. The 2-zero shift was
accurately refined into the data set pattern. The scale factor and
unit-cell parameters were allowed to vary for all the histograms.
The refined structural parameters for each data histogram were
the following: fractional coordinates and isotropic displacement
factors for all atoms (one for each tetrahedral sites and framework
oxygen atoms), and occupancy factors for extraframework ions.
Occupancy factors and isotropic displacement factors were varied
in alternate cycles. Soft constraints were imposed on tetrahedral
cations and coordinated framework oxygen atom distances during
the first stages of the refinements, and left free in the last cycles. The
positions of extraframework sites were determined using Fourier
and Difference Fourier maps.
2.5. Desorption isotherms
3. Results and discussion
Desorption isotherms was carried out on exhausted zeolite by
following this dilution technique. One-half volume of supernatant
containing the antibiotic was removed and substituted with distilled water. The system was then shaken for 24 h and centrifuged.
Then, a second half-volume of the diluted supernatant was replaced
by an equal volume of distilled water and analyzed. The dilution step was repeated several times, until no analytes could be
detected in the solution. The chemical concentration after each
desorption step (Ce ) was determined using HPLC, and the amount
which remained adsorbed in the zeolite (Cs ) was calculated by the
difference. Desorption experiments were conducted in triplicate.
Faujasite Y (200 SiO2 /Al2 O3 ratio) was the zeolite tested in
this adsorption study, on the basis of its pore dimension and high
hydrophobicity. Faujasite crystallizes in the cubic space group Fd3m, with a lattice constant ranging from about 24.2 to 25.1 Å,
depending on the framework aluminium concentration, cations,
and state of hydration [27]. The pore structure is characterized
by approximately 12 Å diameter cages, which are linked through
access windows which are 7.0 Å × 7.1 Å in diameter and are composed of rings of twelve linked tetrahedra (12-membered rings).
These cages and windows permit quite large molecules to enter,
making this structure potentially useful in the adsorption of the
sulfonamide antibiotics (sulfa drugs) under study.
Crystal structures for the three sulfa drugs, sulfadiazine [28],
sulfamethazine [29], and sulfachloropyridazine [30], were crystallographically characterized using the single crystal XRD method.
The molecular structure of these antibiotics is not linear, but
the benzene and heteroaromatic ring form a distorted “V”
configuration, with a dihedral angle of 76.0◦ , 75.5◦ , 83.0◦ in sulfadiazine, sulfamethazine, and sulfachloropyridazine, respectively.
The molecular dimension of sulfonamides is given in Table 1. For
all three molecules, at least one dimension is lower than the zeolite window diameter, thus making their diffusion through the cage
window possible.
2.6. Chromatographic analyses
The concentration of the three sulfonamides was determined by
HPLC-UV. The system was assembled with a Jasco 880-PU Intelligent pump, a Jasco AS-2055 plus Intelligent Sampler, a Jasco 875-UV
Intelligent UV–vis detector at 224 nm, Borwin v 1.2160 chromatography software, a Jones Chromatography model 7971 column
heater, and a 4.60 mm × 150 mm Synergi 4 m Hydro-RP 80A analytical column (Phenomenex, USA). The analytical column was kept
at 35 ◦ C and eluted with acetonitrile:water (23:77 by volume, pH
2.7 for H3 PO4 ) eluant at 1 mL min−1 flow. Under these chromatographic conditions, the retention times for SD, SM, and SC were 3.0,
4.0, and 6.0 min, respectively. All solvents were HPLC grade.
2.7. Thermogravimetric analyses
Thermogravimetric analyses were carried out using a TGDTA92
instrument (SETARAM, France). About 20 mg of pure antibiotic, Y
zeolite, or exhausted zeolite were weighed into an aluminium crucible and heated continuously from 30 to 700 ◦ C at a heating rate
of 10 ◦ C min−1 under airflow of 8 L h−1 . Calcinated kaolinite was
used as a reference material. The furnace was calibrated using an
indium transition temperature. The weight losses were referred to
the weight of the air-dried sample.
2.8. Diffractometric analyses
A powder pattern of the as-synthesized Y sample was measured
on a Bruker D8 Advance Diffractometer equipped with a Sol-X
3.1. Adsorption screening
The adsorption screening of a mixture of the three antibiotics
in zeolite Y gave positive results as shown in Fig. 2. All three sulfonamides were removed from the water to the same extent by
zeolite Y (>90% of the ca. 40 M initial concentration) revealing
no temperature effect in the range between room temperature
and 65 ◦ C. These results indicate a non-competitive effect among
the antibiotics for the zeolite adsorption sites. The dimensional
and physical–chemical differences among the three sulfonamides
structures (Table 1) are too small to differently influence their
adsorption to zeolite sites. The absence of a temperature effect
could be ascribed to covalent bonding. In our case, the formation
of covalent bonds between the host and guest is excluded by stoichiometric antibiotic recovery when the zeolite is treated with
an aqueous acetonitrile solution (50:50 by volume). Therefore, the
222
I. Braschi et al. / Journal of Hazardous Materials 178 (2010) 218–225
formance of our sorbent under real conditions. The adsorption
screening from water containing DOM from a forest soil, which is
highly rich in organic matter (TOC 32 g kg−1 ), showed an unmodified adsorption performance in comparison with those in distilled
water (Fig. 2). This finding seems promising for zeolite Y field application.
3.2. Adsorption kinetics
Fig. 2. Adsorption screening of a solution containing a mixture of sulfadiazine (SD),
sulfamethazine (SM), and sulfachloropyridazine (SC) (40 M each) in zeolite Y at
different temperatures and in the presence of dissolved organic matter from forest
soil (zeolite:antibiotic solution ratio of 1 mg:2 mL, mean of three replicates, standard
deviation <4%).
absence of temperature effects on adsorption can be explained by
assuming increased diffusivity of antibiotics molecules inside zeolite pores and cages which counterbalance the decreased extent of
adsorption at the highest temperatures.
As far as the occurrence of ionic binding between zeolite and
sulfonamides is concerned, both neutral and ionic forms of sulfonamides should be considered. Sulfa drugs are present in aqueous
solution as a mixture of cationic, neutral and anionic molecules
with the most abundant species as a function of their acidity along
with the medium pH. In our experiment, before adsorption, antibiotic solutions were left in equilibrium with atmospheric CO2 , thus
reaching a slightly lower pH than 6 (approximately 5.8). At this pH
value, along with the neutral species, all three sulfonamides present
their anionic form, which is the most abundant in sulfachloropyridazine (pKa 5.5, Table 1) and which is less abundant in sulfadiazine
(pKa 6.4, Table 1) and sulfamethazine (pKa 7.5, Table 1). Despite the
presence of the anionic form for all the sulfa drugs when adsorption
occurs, the adsorption of these anions has been ruled out as the zeolite which is used is not an anion exchanger. In fact, the high silica
zeolite used does not have the positive charges that are involved in
the stabilization of negative species.
Concerning the possibility for sulfonamides to be adsorbed in
their protonated form, it should be taken into account that aromatic
heterocyclic nitrogen is more basic than anilinic nitrogen. In fact,
despite the more basic character of the amino group nitrogen (N
in sp3 hybridization) compared with that of aromatic heterocyclic
nitrogen (N in sp2 hybridization), the conjugation of lone pair electrons of aniline nitrogen with benzene ring carbons makes these
electrons less available for protonation. The heterocyclic nitrogen
kb of sulfa drugs is too low (pKb > 12; [31]) since protonation occurs.
Nevertheless, sulfonamides could be protonated by the exchangeable protons (Brønsted sites) that are present on the zeolite surfaces
and subsequently interact as cations. In our case, this mechanism
of protonation followed by the binding of sulfa drugs under their
cationic form cannot be explained by the low isomorphic substitutions of Al for Si in the zeolite being studied (200 SiO2 /Al2 O3 ratio).
In fact, the adsorption of one drug molecule per zeolite cage on
average (vide infra) cannot be supported by the presence of approximately one exchangeable proton per eight cages. Therefore, the
adsorption mechanism proposed is of a hydrophobic type because
of its reversibility in organic solvents. In fact, if ionic interactions
did occur, they ought not to have been weakened by interactions
with an organic solvent which is less polar than water.
An adsorption screening of sulfonamides into zeolite Y from
water containing organic matter which could interfere with
antibiotic adsorption was performed in order to evaluate the per-
The adsorption kinetics of the isolated sulfonamides in the
zeolite Y was very favourable, and their removal from water
was complete in less the 1 min (see Fig. S-2 in supporting
information section). Therefore, adsorption in the subsequent
experiments was performed with a contact time of 30 min to permit
the process to reach an equilibrium state.
3.3. Adsorption/desorption isotherms
As the adsorption kinetics was favourable, an important question to be answered concerns the maximal zeolite adsorption
capacity. For this reason, adsorption cycles were performed on zeolites for each single antibiotic due to the low solubility of the sulfa
drugs. The maximal adsorption capacity of antibiotics in zeolite Y
measured using HPLC was in the order of SD < SM < SC and equalled
15%, 21%, and 26% of the air-dried zeolite weight, respectively (see
Fig. S-3 in supporting information section). The differences in the
maximal adsorbed amount can only to some extent be attributed
to the different antibiotic molar weight (see Table 1). In fact, the
expression of the maximal adsorption capacity for zeolite Y in terms
of antibiotics on a molar basis gives 583, 777, and 983 mol g−1
zeolite for SD, SM, and SC, respectively.
In order to define the adsorption isotherms, the amount of
antibiotic which remained in the solution at the equilibrium point,
after each adsorption cycle, was reported as a function of the antibiotic as adsorbed in zeolite Y (Fig. 3). The adsorption isotherm for all
sulfonamides clearly shows a two-stage trend, the first one is represented by the isotherm segment at low drug concentration (ranging
from 0 to ca. 500 mol g−1 zeolite along Cs axis) and the second one
is described by the isotherm segment at a higher concentration.
In the absence of degradation products, as in this case, these features could describe the different affinity of sulfonamides for zeolite
adsorption sites. This different affinity was hypothesized as occurring due to interaction with zeolite pores of different dimensions:
higher in micropores and lower in larger pores. It is likely that when
the sites at higher affinity (micropores) are completely filled with
antibiotics, adsorption continues on sites at lower affinity (pores
with dimensions higher than 20 Å). However the migration of sulfa
Fig. 3. Adsorption (black marks) and desorption (white marks) isotherms of sulfadiazine (SD), sulfamethazine (SM), or sulfachloropyridazine (SC) sulfonamide
antibiotics by zeolite Y.
I. Braschi et al. / Journal of Hazardous Materials 178 (2010) 218–225
drugs inside hydrophobic zeolite pores continued till pores were
completely filled. In fact, considering the number of cages in 1 g zeolite (4.2 × 1020 ) and the number of sulfonamide molecules adsorbed
in the same amount (4.0 × 1020 , 4.3 × 1020 and 5.3 × 1020 for SD, SM
and SC, respectively), the presence of about one molecule for each
zeolite cage can be calculated.
The reversibility of the adsorption process was evaluated by performing desorption experiments on exhausted zeolites by diluting
the antibiotic concentration at the equilibrium point. As reported in
Fig. 3, the desorption process did not have any significant effect on
the release of the antibiotics from the zeolite. In fact, in the case of
complete adsorption reversibility, the desorption isotherm derived
from data points obtained at each desorption step should overlap
the adsorption curve. On the contrary, according to our data, the
desorption curve is almost parallel to the x-axis, revealing the tendency of sulfa drugs to remain adsorbed in zeolite Y. These findings
are indicative of an irreversible adsorption process.
The amount of antibiotic adsorbed on exhausted zeolite as
evaluated by means of thermogravimetric analyses accounted for
15.9%, 20.2%, and 24.6% of SD, SM, and SC, respectively (Fig. 4,
left). These results are consistent with those measured using
HPLC and the difference between the two techniques can be
considered acceptable since the variation is ≤5%. Such a high
extent of adsorption for organics is usually displaced for organic
pollutants by other sorbents, for instance, activated carbon [32].
However, if compared to these, zeolites are usually preferable due
to their more favourable adsorption kinetics along with their lower
sensitivity to natural organic matter, which is always present in
natural environments [32].
For all antibiotics, the non-equivalence of the peak pattern in
the derivative thermogram for the adsorbed antibiotic (Fig. 4, right)
compared to its bare form is more indicative of different types of
interaction than a simple precipitation of antibiotics on a crystallite
surface. In fact, if this were the case, the thermogram of antibiotics
223
adsorbed in the zeolite should appear similar to that found in pure
antibiotics.
3.4. Diffractometric analyses
Diffractometric analyses were performed on pure and
exhausted zeolites in order to reveal whether structural modification occurred on the zeolitic framework as a consequence of
incoming sulfonamide antibiotics. Rietveld structure refinement
on zeolite Y (Table 3) was performed in the space group Fd-3m
starting from the crystallographic data reported for protonated
hydrophilic zeolite Y [33]. Difference Fourier maps revealed the
presence of water molecules (16 molecules u.f.) which occupy two
extraframework sites inside the supercage. Refinement parameters are reported in Table 3, while atomic coordinates, occupancies
and temperature factors are shown in the supporting information
section (Table S-1).
After adsorption, the incorporation of sulfa drugs inside the Y
cavities was assessed using the following.
3.4.1. Diffraction patterns analysis
After sulfadiazine or sulfachloropyridazine adsorption, Rietveld
structure refinement clearly indicated a lowering in real Fd-3m
symmetry to Fd-3 in the parent zeolite Y. After sulfamethazine
adsorption, the presence of reflections which are forbidden in the
cubic system indicated a lowering in symmetry up to monoclinic
P2/m space group, as shown by the diffraction pattern reported
in Fig. 5. Y-SM unit cell parameters (Table 3) were obtained using
the DICVOL program [34], and refined with LeBail fit, using the
multi-dataset capabilities of the GSAS software suite. As will be
discussed later, these lowerings in symmetry can be explained
as a consequence of distortions in the 12-ring channel caused
by the encapsulation of sulfonamides inside organophylic zeolite
Y cages.
Fig. 4. Thermogravimetry (left) and derivative thermogravimetry (right) in dry air atmosphere of sulfadiazine (SD), sulfamethazine (SM), or sulfachloropyridazine (SC) pure
and adsorbed into zeolite Y at maximal adsorption capacity.
I. Braschi et al. / Journal of Hazardous Materials 178 (2010) 218–225
224
Table 3
Lattice parameters and refinement details for Y zeolite before (Y) and after adsorption of sulfadiazine (Y-SD), sulfachloropyridazine (Y-SM), and sulfachloropyridazine (Y-SC).
Space group
Y
Fd-3m
24.259(4)
A = b = c (Å)
V (Å)3
˛=ˇ=
Y-SD
Fd-3
24.270(1)
14277.1(4)
( )
Refined pattern 2 range (◦ )
Rwp (%)
Rp (%)
RF2 (%)
No. of contributing reflections
Nobs
Nvar
14296.2(4)
90
◦
= 1.5417(1) Å. Rp = [Yio − Yic ]/Yio ; Rwp = [
90
3.3–110
12.8
12.5
9.95
982
5340
40
2
wi (Yio − Yic ) /
Y-SC
Fd-3
0.5
wi Yio 2 ]
; RF2 =
3.4.1.1. Variations in unit cell parameters. As reported in Table 3,
sulfadiazine or sulfachloropyridazine incorporation causes an
increase in all cell parameters, thus indicating noticeable structural
rearrangements. This result was also confirmed by a comparison
between powder patterns collected before and after sulfa drugs
adsorption on Y (Fig. S-4). In all the 2 range which was investigated, strong differences in both the position (which depend on cell
parameters values) and intensity (which depend on atomic parameters, such as positional coordinates x, y, z, isotropic temperature
factor, etc.) diffraction peaks were detected. After sulfamethazine
adsorption, unit cell parameters took on metrically monoclinic values, therefore the complexity of the system prevented Rietveld
structure refinement.
3.4.2. Fourier maps analysis
The observed and difference Fourier maps generated using
GSAS, revealed the presence of a number of extraframework ions
inside the supercages, which can be attributed to encapsulated
organic molecules. All localized atoms from antibiotics were at a
distance of more than 3.3 Å from the framework oxygen atoms,
indicating that they are only weakly bonded to the framework. The
largest peak in the difference Fourier map was attributed to the sulfur atom, which occupied a site with similar fractional coordinates
in both Y-SC and Y-SD. The atoms around sulfur are arranged in
a distorted tetrahedral configuration, similar in both the Y-SC and
Y-SD samples. Reasonable values were obtained for S–O1, S–O2,
S–N, and S–C bond distances, which were set free during all stages
a = 14.656(1)
b = 24.285(1)
c = 9.918(1)
24.266(1)
14288.1(4)
3384.4(4)
˛ = = 90
ˇ = 106.6(1)
90
3.3–120
12.7
9.84
7.17
1870
5844
59
|Fo2 − Fc2 |/
Y-SM
P2/m
3.3–110
10.0
7.76
7.27
1704
5344
56
3.3–110
11.2
8.10
–
–
5299
32
|Fo2 |. Estimated standard deviations in parentheses refer to the last digit.
Table 4
Free diameter (Å) of the twelve-membered ring viewed normal to [001], assuming
an oxygen ionic radius equal to 1.35 Å.
Sample
O1–O1
O4–O4
Y
7.00(1)
7.11(1)
Y-SD
Y-SC
7.15(1)
7.22(1)
7.12(1)
7.08(1)
of refinements. The positions occupied by atoms in pyridazine and
benzene rings appear strongly disordered. These findings could be
interpreted as being due to real static disorder or to dynamic disorder. For these reasons, the complete geometry of sulfonamide
molecules inside the zeolitic cage was not achieved.
3.4.3. Framework distortions
Rietveld structure refinements revealed that the presence of
encapsulated antibiotic molecules caused remarkable distortion
in the twelve-membered rings (see Table 4 for details). As can be
clearly seen, the ring shape became more circular (in the presence
of sulfadiazine) or more elliptical (in the presence of sulfachloropyridazine) when compared with that found in the parent zeolite Y.
The most remarkable change appears related to the O1–O1 oxygen
atoms distance, which increases as a consequence of adsorption
and as a function of sulfonamide dimensions.
In conclusion, very favourable adsorption kinetics along with
effective and highly irreversible adsorption for sulfonamide antibiotics in zeolite Y pores make this cheap and environmental friendly
material a tool with interesting applications for the removal of
sulfonamide antibiotics from water bodies.
Acknowledgements
Fig. 5. Observed powder diffraction patterns of zeolite Y after sulfamethazine
adsorption. The arrows reported in the patter indicate the reflections which are
forbidden in the cubic system.
The authors wish to acknowledge Dr. Francesco Di Renzo for
fruitful discussion and Dr. Giorgio Gatti and Dr. Daniela Montecchio
for sorbent characterization.
I. Braschi et al. / Journal of Hazardous Materials 178 (2010) 218–225
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jhazmat.2010.01.066.
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