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Article J. Braz. Chem. Soc., Vol. 00, No. 00, 1-11, 2022
©2022 Sociedade Brasileira de Química
https://dx.doi.org/10.21577/0103-5053.20220049
*e-mail: tarley@uel.br
Editors handled this article: Teodoro S. Kaufman and Luiz Ramos (Guest)
On-line Micro-Packed Column Solid-Phase Extraction of Cadmium Using Metal-
Organic Framework (MOF) UiO-66 with Posterior Determination by TS-FF-AAS
Ana Carla R. Carneiro,a Joziane G. Meneguin,a Paula M. dos Santos, a
MarcelaZ.Corazza,a Maiyara Carolyne Prete,a Andrelson Wellington Rinaldib and
CésarRicardo T. Tarley *,a,c
aDepartamento de Química, Universidade Estadual de Londrina, Rodovia Celso Garcia Cid, PR445
Km380, Campus Universitário, 86051-990 Londrina-PR, Brazil
bDepartamento de Química, Universidade Estadual de Maringá, Av. Colombo,
CampusUniversitário, 87020-900 Maringá-PR, Brazil
cDepartamento de Química Analítica, Instituto de Química,
Instituto Nacional de Ciência e Tecnologia em Bioanalítica (INCT-BIO),
Universidade Estadual de Campinas, (UNICAMP), Cidade Universitária Zeferino Vaz,
13083-970 Campinas-SP, Brazil
Metal-organic framework (MOF) UiO-66 was synthesized and evaluated as solid-phase
extractor support for cadmium preconcentration in a micro-ow injection system coupled to
thermospray ame furnace atomic absorption spectrometry (TS-FF-AAS) detection. The adsorbent
was characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), scanning
electron microscopy (SEM), Raman spectroscopy, and textural data from N2 adsorption/desorption
isotherm data. The optimized conditions were achieved by loading 10.0 mL of a sample, containing
0.05 mol L-1 of phosphate buffer solution at pH 8.0, at a high ow rate of 10.0 mL-1 through
20.0mg of UiO-66 packed into a mini-column, followed by elution with 1.0 mol L-1 HCl. At these
conditions, the method presented a preconcentration factor of 35.7, limit of detection of 0.03 µg L-1
and a dynamic linear range from 0.1 to 8.0 µg L-1. The adsorption performance of UiO-66 towards
cadmium was not inuenced by Pb2+, Hg2+, Fe2+, Co2+, Cu2+, Ni2+, Zn2+, Mg2+ and Ca2+ ions. Analysis
of different waters samples (tap water, physiological solution, mineral water, and lake water) was
carried out without matrix interference, yielding recovery values ranging from 92.0 to 111.9%.
Keywords: FIA system, metal-organic framework, UiO-66, TS-FF-AAS, cadmium
Introduction
The presence of cadmium in the aquatic environment,
represents, in general, a significant risk to aquatic
biota and water quality due to its extremely long
biological half-life and high toxicity. In humans, the
ingestion of cadmium may cause renal abnormalities and
nervous system diseases, apart from being considered a
carcinogenic element.1 The maximum allowed level of
cadmium in drinking water has been established as 5.0 and
3.0 µg L-1 by the United States Environmental Protection
Agency (USEPA)2 and the World Health Organization
(WHO)3, respectively. In Brazil, the Ministry of Health
also establishes the maximum of 3.0µgL-1 of cadmium
in drinking water.4
Cadmium determination in the µg L-1 concentration
range is usually performed by graphite furnace atomic
absorption spectrometry (GF AAS) and inductively
coupled plasma mass spectrometry (ICP-MS). However,
the acquisition and maintenance costs of these techniques
are relatively expensive.5 In turn, ame atomic absorption
spectrometry (FAAS) stands out due to its easy operation
and low cost of acquisition and maintenance. Nevertheless,
the main drawback of this technique is its low sensitivity.
Therefore, preconcentration methods prior to FAAS
determination have been widely reported to improve
the sensitivity.6,7 Additionally, analytical strategies for
sensitivity enhancement in FAAS based on total sample
introduction8 and atom-trapping techniques9 to prolong the
On-line Micro-Packed Column Solid-Phase Extraction of Cadmium J. Braz. Chem. Soc.
2
residence time of the analytes and higher atom absorption
volume also has been investigated. The thermospray ame
furnace atomic absorption spectrometry (TS-FF-AAS)
technique, reported in 2000 by Gáspár and Berndt,10
combine both features due to the efciency of sample
introduction into the Ni metallic atomizer placed on the
burner head, and the connement of atoms inside the ame
furnace with prolonged residence time.
Within the framework of current green analytical
chemistry requirements, with a particular emphasis on
preconcentration methods, the dispersive micro-solid
phase extraction (D-µSPE),11 dispersive liquid-liquid
microextraction (DLLME),12 cloud point extraction (CPE),13
supramolecular (SUPRA) solvent-based extraction14 and
micro-packed column ow solid-phase extraction15 have
been widely used. Although the rst ones make use of
microvolumes of elution solvent, microvolumes of extractor
solvent, relatively non-toxic surfactants or carboxylic
acid and alkanols-based extractors, the micro-packed
column ow solid-phase extraction makes possible the
use of large preconcentration volumes, thereby yielding
high preconcentration factors, in addition of providing
high sample throughput. Nevertheless, the criteria of
green chemistry will only be attended by this solid-phase
extraction method in ow analysis when ensuring the
elimination or minimization of highly toxic chemical
reagents, particularly toxic organic solvents and chelating
agents in the ow system.16 In this sense, the nature of
adsorbents plays an important role in the performance
of solid-phase extraction, in which the most desirable
characteristics include large surface area, good chemical
stability, high reusability and satisfactory selectivity.
Carbonaceous nanomaterials, such as carbon nanotubes17
and graphene18 are great examples of adsorbents, however,
their adsorptive performance depends on chemical or
physical surface modication to provide selectivity and
higher binding capacities towards metallic ions.
Metal-organic frameworks (MOFs) belong to a
fascinating class of porous inorganic materials with
widespread application including, for instance, catalysis,
sensing, gas storage and separation science.19 MOFs are
dened as 3D coordination polymers formed by metal ions
(or clusters) and multi-dentate organic ligands through
coordination bonds. The metal substitution in this structure
can directly alter the coordination units, creating new
properties once it may cumulate different catalytically
active sites and/or tune adsorptive, optoelectronic, and
magnetic properties.20 MOFs are named with an acronym
representing the name where they were originally obtained
or according to their framework, and the most common
are MIL-n (MIL: Matériaux de I’Institut Lavoisier),
HKUST-n (HKUST: Hong-Kong University of Science and
Technology), UiO-n (UiO: Universitetet i Oslo) and ZIFs
(zeolitic imidazolate frameworks). Some relevant properties
of MOFs are ultrahigh surface area (1000-10400 m2 g-1),
ultra-low densities, crystalline open structure with tunable
pore sizes, tailorable polarity, designable organic ligands,
and high thermal stability (300-600 ºC).21-24
MOFs-based separation methods fulll the aspects
of green analytical chemistry due to the low-toxicity of
metal ions used in MOFs synthesis, such as alkaline earth
metals (Mg or Ca), or Mn, Fe, Al, Ti and Zr, attested by
cytotoxicity studies, the absence of toxic chelating agents
in the framework, the possibility of miniaturization in the
separation process, as well as the absence of toxic organic
solvent as eluent of metal ions.19
It has been observed an increasing trend in the
research on the use of MOFs on the removal of heavy
metals pollutants from water samples, as well as solid-
phase extraction-based methods.25,26 However, owing to
their nanometer-size, MOFs have been mostly applied
as adsorbents for metal ions in D-µSPE24 or magnetic
dispersive solid-phase extraction (MDSPE).27 There
are also applications of MOFs as adsorbents for on-line
solid-phase extraction coupled to high performance liquid
chromatography (HPLC)28,29 and as monolithic column.30 At
the best of our knowledge, MOFs packed into closed mini-
columns in ow systems for metal ions preconcentration
has not been reported yet. This type of investigation is of
paramount importance to assess the chemical stability,
selectivity, and adsorption capacity of MOFs, since
equilibrium and mass transfer kinetics is rather different
from dispersive solid-phase extraction. Particularly in the
case of MOFs, the selectivity towards a target metal ion
depends on the exibility of highly porous structure of the
adsorbent, shape and size of the pores and the diffusion of
analyte into the bulk structure, which is fully dependent on
the hydrodynamic conditions of the packed mini-columns,
such as the swelling effect and ow rate.
Therefore, in this work, UiO-66 was evaluated as
adsorbent for the development of a novel on-line micro-
packed column solid-phase extraction method coupled
to TS-FF-AAS for cadmium determination in the µgL-1
concentration range in water samples. It is worth to
emphasize that, due to the nanostructure of UiO-66, a very
low amount of adsorbent can be used into the mini-column,
which results in advantages in the on-line coupling with
analytical techniques that works with low ow rates, such
as TS-FF-AAS. UiO-66 was herein used since it is a well-
known Zr-based cubic framework comprised of cationic
Zr6O4(OH)4 nodes and 1,4-benzene dicarboxylate organic
linkers. Moreover, UiO-66 exhibits high chemical stability
Carneiro et al. 3Vol. 00, No. 00, 2022
in a variety of organic solvents, as well as acidic and basic
aqueous media, besides its high surface area.31-34
Experimental
Reagents and instrumentation
All solutions were prepared using chemical reagents
of analytical grade and ultrapure water from ELGA
PURELAB Maxim system (High Wycombe, Bucks, UK),
18.2 MΩ cm resistivity. To prevent metal contamination,
all glassware was kept overnight in a 10% (v/v) HNO3
solution. Cd2+ working solutions at 5.0 µg L-1 were
prepared from a 1000.0 mg L-1 Cd2+ standard stock solution
(SpecSol®, Belo Horizonte, Brazil) by making appropriate
dilutions. Phosphate buffer solutions were prepared by
dissolving appropriate amounts of NaH2PO4.H2O (J. T.
Baker®, Chicago, USA) in ultrapure water, without further
purication, followed by pH adjustment to the desired
value with sodium hydroxide and/or nitric acid (Vetec,
Brazil) solutions. Zirconium(IV) oxychloride octahydrate
and terephthalic acid used in the MOF synthesis were
acquired from Sigma-Aldrich (Darmstadt, Germany), while
N,N-dimethylformamide (DMF) was purchased from Synth
(Diadema, Brazil) and hydrochloric acid (37%,v/v) was
purchased from Sigma-Aldrich (Darmstadt, Germany).
Solutions of Pb2+, Fe2+, Co2+, Cu2+, Ni2+, Mg2+, and Ca2+
used in interference studies were prepared from their
respective nitrate salts, while the solution of Hg2+ was
prepared from appropriate dilution of a standard stock
solution at 1000.0mg L-1 concentration (Specsol®, Belo
Horizonte, Brazil).
The measurements were performed by using an AA-6601
ame atomic absorption spectrophotometer (Shimadzu,
Kyoto, Japan) equipped with a cadmium hollow cathode
lamp (Hamamatsu Photonics K.K., Hamamatsu City,
Japan), operating at 8.0 mA and 228.8 nm, and a deuterium
lamp for background correction. The ame composition
was operated at an acetylene ow rate of 1.8Lmin-1 and
air ow rate of 15.0 L min-1. For construction of the on-
line preconcentration system coupled to TS-FF-AAS, a
peristaltic pump (Gilson Minipuls Evolution, Middleton,
USA), a home-made injector commutator made of Teon®
(PTFE, polytetrafluoroethylene), a 0.5 mm internal
diameter (i.d.) ceramic capillary (Al2O3 99.7%) (Friatec,
Germany), a 10 cm long and 2.5 cm i.d. nickel tube (72%
of Ni, 14-17% of Cr, 6-10% of Fe, 0.15% of C, 1% of Mn
and 0.5% of Si, Camacam, Brazil), containing 6 holes
of 2.5 mm i.d. and Tygon® tubes were used. Figure S1
depicts the ow injection system coupled to TS-FF-AAS
(Supplementary Information (SI) section).
The pH of samples was measured by an 826 pH mobile
Metrohm pHmeter (Herisau, Switzerland). In order to
identify the functional groups of the UiO-66 MOF, an
infrared spectrometer with Fourier transform (FTIR,
Bomem-Michelson, MB-100) operating in the transmission
mode between 4000 and 400 cm-1 and using the KBr pellet
method, was used. X-ray diffraction (XRD) spectra were
acquired from a Shimadzu XRD-6000 X-ray diffractometer
(Netherlands) operating with incident X-rays (Cu Kα of
1.54060 Å) with the 2θ angle between 5 and 40º, current
of 40 mA and voltage of 40 kV. For scanning electron
microscopy (SEM), analysis, the UiO-66 was coated with
a thin layer of gold (30 nm) using Bal-Tec SCD Equipment
Sputter Coater (New York, USA). The SEM UiO-66 images
were obtained with magnication of 6000 and 50000 times
with a range of 20.0 and 2.0 μm, respectively. The Raman
spectra were obtained on a Raman Senterra microscope
(Bruker©, Billerica, Massachusetts, USA), equipped with
a 785 nm beam and a resolution of 3-5cm-1, operating in
the spectral range from 70 to 3500 cm-1. Nitrogen sorption/
desorption experiments were performed on a Micromeritics
(ASAP2020) coupled to an automatic nitrogen gas
adsorption instrument (Quantachrome, Boynton Beach,
FL, USA), and the surface area of the UiO-66 MOF was
obtained according to the multipoint BET (Brunauer,
Emmett, Teller) method, while the average pore size
and pore volume were performed by the BJH (Brunauer,
Emmett, Teller) method.
Synthesis of UiO-66
UiO-66 was synthesized according to literature with
minor modication.32 As the chemical structure of UiO-66
has been widely reported in the literature we encourage
the readers to read the references35,36 to obtain a more
detailed elucidation of its chemical structure. For the
solution 1, vemmol of ZrOCl2.8H2O was dissolved in
50.0 mL of DMF and 8.0 mL of concentrated HCl under
magnetic stirring for 20 min, while solution 2 consisted
of the dissolution of 7.0 mmol of terephthalic acid in
100.0 mL of DMF under magnetic stirring for 20 min,
both at room temperature. Afterwards, solutions 1 and 2
were mixed and kept in an oven at 80 ºC for 21 h. After
complete crystallization, the resulting material was ltered
and washed with DMF to remove all residual terephthalic
acid and dried at 80 ºC for 12 h. This material was named
UiO-66 (DMF).
In order to avoid UiO-66 pores/channels blocking by
DMF, the MOF was calcined in a porcelain capsule with
heating ramp of 1º min-1 at 250 ºC for DMF removal. This
material was then named UiO-66 (calcined).
On-line Micro-Packed Column Solid-Phase Extraction of Cadmium J. Braz. Chem. Soc.
4
On-line preconcentration of Cd2+ procedure coupled to
TS-FF-AAS by the micro-packed column flow solid-phase
extraction method
Preconcentration of Cd2+ using the ow injection system
coupled to TS-FF-AAS was performed by percolating
10.0 mL of sample (buffered to pH 8.0 using 0.05molL-1
phosphate buffer) through a cylindrical mini-column
(length 1.5 cm and an internal diameter of 5.0 mm) made
of polypropylene packed with 20.0 mg of UiO-66 MOF,
at a ow rate of 10.0 mL min-1. After this step, the elution
was performed by switching the injector to the elution
position, using 1.0 mol L-1 HCl as eluent at a ow rate
of 1.0mL min-1, where the desorbed Cd2+ ions were
transported towards TS-FF-AAS detector. Pipette tips were
inserted at each extremity of the cylindrical mini-column
and pieces of cotton tissue were used to avoid losses of
UiO-66 during preconcentration and elution steps.
Sample preparation
The tap and mineral water samples were obtained from
the Chemistry Department of State University of Londrina
and local supermarkets, respectively, while the physiological
sample (NaCl 0.9% m/v) was acquired at a local drugstore.
Lake sample was collected with amber glass bottle from
Igapó Lake, located in Londrina (Brazil), and acidied with
concentrated nitric acid until pH 2.0 to prevent the growth of
microorganisms. The Igapó Lake water sample was ltered
under vacuum using 0.45 µm cellulose acetate membrane and
stored in freezer until analysis. Before use, an appropriate
amount of a stock solution of phosphate buffer (1.0 mol L-1)
at pH 8.0 was diluted in the samples to obtain a nal buffer
concentration of 0.05 mol L-1. For all samples, the analyses
were carried out in triplicate.
Results and Discussion
Characterization of the UiO-66 adsorbent
X-ray diffraction patterns of UiO-66 (calcined) and
UiO-66 (DMF) are depicted in Figure 1. As one can see,
the diffractogram shows a typical crystalline structure of
UiO-66 formed by clusters of octacoordinated zirconium
(ZrO4(OH)4), in which the triangular faces of the Zr6
octahedron are alternatively constituted by μ3-O and
μ3-OH groups, and each cluster is connected to 12 other
clusters via terephthalate ligands, in a face-centered
cubic structure.37 The appearance of distinctive peaks at
2θ=7.33, 8.48, 12.03, and 25.67º indicates that UiO-66
was well synthesized without any sign of impurity in its
crystalline structure.38,39 Additionally, the calcined UiO-66
showed preserved crystalline structure, although with
higher peak intensity.
The FTIR spectra of UiO-66 are shown in Figure 2.
The bands at around 1398 and 1662 cm-1 can be attributed
to the symmetric stretching of carboxylate groups whereas
the band at 1574 cm-1 can be ascribed to its asymmetric
stretching.40-42 The band at 741 cm-1 is attributed to C-H
vibration of the aromatic ring42,43 and the bands at the
region of 666 and 484 cm-1 are originated from O-Zr-O
vibration.40 The presence of low intensity bands at 2932
and 2858 cm-1 observed in the UiO-66 (DMF) spectrum
can be ascribed to the symmetric/asymmetric stretching of
C-H bonds of the adsorbed DMF molecules, which it is not
observed in the UiO-66 (calcined) spectrum. This indicates
that the total removal of the solvent was efcient and
unblocked the channels of the UiO-66 without damaging
its structure, thus favoring its application as adsorbent in
solid-phase extraction.
Figure 3 shows the SEM images of UiO-66 (calcined),
where aggregates of small crystallites resulting from the
Figure 1. XRD patterns of the UiO-66 (a) synthesized in DMF and (b)
after calcination.
Carneiro et al. 5Vol. 00, No. 00, 2022
direct reaction of ZrOCl2 with terephthalic acid were
observed. In addition to the granular structure, a uniform
particle size distribution was also observed, which is in
agreement with studies reported in the literature.44-48
Raman absorption spectrum of the UiO-66 (calcined)
is shown in Figure 4. The bands at 634 and 865 cm-1 are
attributed to the ring C–H curvature vibrations out of plane,
while the bands at 1146 and 1616 cm-1 are attributed to C=C
modes of the benzene ring present in terephthalic acid.
In addition, doublets at 1437 and 1452 cm-1 are usually
assigned to units ѵsym (C–O2) and ѵasym (C–O2).45,49-51
In order to investigate the surface properties of the
adsorbent UiO-66 (calcined) and UiO-66 (DMF), the N2
adsorption-desorption isotherms were carried out and are
shown in Figure 5, while the textural data obtained are
arranged in Table 1.
As shown in Figure 5, both isotherms are of type 1,
where the adsorption occurs with relatively low pressure,
indicating the microporosity of the material. The BET
surface area of UiO-66 (DMF) and UiO-66 (calcined) were
found to be 1262 and 1562 m2 g-1 with total pore volumes of
0.810 and 0.772 cm3 g-1, respectively. This small increase in
the surface area of UiO-66 (calcined) in relation to UiO-66
(DMF) is attributed to the removal of the solvent from the
micropores by thermal activation, which might favor the
adsorption capacity of material. Also, the average pore
diameter conrms that the material is microporous, since
it is lower than 2 nm.43,52
In order to evaluate the chemical stability of the UiO-66
after calcination and its crystalline structure, XRD patterns
were assessed at different pH values. This study is of great
importance, since cadmium preconcentration is carried
out under basic medium and the elution is performed with
diluted mineral acid. For this task, 100 mg of UiO-66
calcined was kept in contact with solutions at pH 1.0, 2.9,
5.8 and 8.0 during 48 h and, afterwards, the samples were
ltrated under vacuum and dried at 90 ºC for 12 h.
As can be seen in Figure 6a, the XRD patterns show
no crystalline structure failure when the UiO-66 is kept
in solutions at pH 2.9, 5.8, and 8.0, thereby showing
resistance to alkaline medium and slight acid medium.
On the other hand, with the increase in the acidity of the
Figure 2. FTIR (KBr) spectra of UiO-66 (a) synthesized in DMF and
(b) after calcination.
Figure 3. SEM images of UiO-66 (calcined).
On-line Micro-Packed Column Solid-Phase Extraction of Cadmium J. Braz. Chem. Soc.
6
medium (pH 1.0), a change in crystallinity patterns was
observed, suggesting a partial degradation of the crystal
structure (Figure 6b), evaluated mainly by the change in the
peak area referring to the diffraction angle at 2θ = 7.5º.53,54
Despite this nding, one should note that this study was
carried out by keeping the UiO-66 in contact with highly
acid solution for 48 h, which is a quite different condition
from the one used in the ow preconcentration system,
where the contact time of UiO-66 with acid at the elution
step is much lower. For this reason, the chemical stability
of UiO-66 was herein considered highly satisfactory,
since only one mini-column was used through all method
development, including the application in real samples,
without decreasing the adsorption capacity.
Optimization of the on-line micro-packed column solid-
phase procedure for cadmium preconcentration
For the optimization of the preconcentration procedure,
only the UiO-66 (calcined) adsorbent was evaluated, thus
it is going to be referred only by UiO-66 from this point.
Preliminary studies regarding the amount (20 and 50 mg)
of UiO-66 packed into the mini-column were carried out
to evaluate possible leakages due to resulting pressure and
the analytical performance in terms of preconcentration
(data not shown). The use of 20 mg of adsorbent provided
better results regarding the mini-column performance
and stability, thus this value was chosen for further
experiments. In addition, HCl was herein chosen instead
of HNO3 as eluent once the last one has oxidant properties,
which would imply in a decrease of chemical stability of
UiO-66. Also, metallic chlorides are more volatiles and,
therefore, more suitable for the TS-FF-AAS technique,
since the temperature inside the metallic tube is around
800-900 ºC. Other factors that play an important role in
the preconcentration procedure, including preconcentration
flow rate, eluent concentration, buffer concentration
and sample pH, were evaluated from a full 24 factorial
design. The levels of factors and the analytical responses
(absorbance as peak height) are shown in Table 2. The
experiments were carried out in triplicate by loading
10.0mL of standard solution of cadmium at 5.0 µg L-1.
The signicance of factors on the preconcentration
system was assessed from the Pareto chart (Figure 7) using
a condence level of 95%.
According to Pareto chart, sample pH and eluent
concentration (EC) were the most signicant factors at
their higher levels. It is known that binding/interaction sites
of MOF can be stablished between metallic ions and the
clusters, as well as by the functional groups of chelating
agents. Lewis acid-base interactions are considered as the
most common adsorption mechanism between metallic
ions and MOF.55 The functional groups containing oxygen
in the structure of UiO-66 from the organic ligands act as
Lewis bases and strongly interact with cationic species
Table 1. Textural parameters for the UiO-66 with DMF and after calcination obtained from N2 adsorption/desorption isotherms
Material Surface area / (m2 g-1) Total pore volume / (cm3 g-1) Average pore diameter / nm
BET
UiO-66 (DMF) 1262 ± 23 0.810 0.372
UiO-66 (calcined) 1562 ± 10 0.772 0.378
DMF: N,N-dimethylformamide; BET: Brunauer, Emmett, Teller.
Figure 4. Raman spectrum of UiO-66 (calcined).
Figure 5. N2 adsorption-desorption isotherms of the UiO-66 (calcined)
and UiO-66.
Carneiro et al. 7Vol. 00, No. 00, 2022
acting as Lewis acids, such as Cd2+. Therefore, the positive
standardized effect estimated for pH (3.63) indicates that
higher analytical signal was observed at pH 8.0 due to the
deprotonation of the functional groups that act as chelating
binding sites for Cd2+ ions adsorption on the MOF.
The eluent concentration (EC) effect also presented
positive standardized effect estimated (3.89), thereby
indicating that, for lower concentrations, the desorption of
cadmium from the UiO-66 is incomplete, which leads to
memory effect during further preconcentration and elution
steps. It is important to stress out that the interaction effect
of pH and EC has a negative and signicant standardized
effect estimated of -3.99, which suggests that for a nal
simultaneous optimization, the influence of pH at its
higher level is more pronounced when using lower EC
and the other way around is also true. Such result can most
likely be explained because, at high EC, there is a residual
concentration of acid into the mini-column, in which the
buffer solution has not enough buffering capacity to kept
Figure 6. (A) XRD patterns and (B) crystallinity indicator of UiO-66 (calcined) adsorbent after being kept for 48 h in solutions at different pHs.
Figure 7. Pareto chart obtained from the full 24 factorial design. EC:
eluent concentration (mol L-1), BC: buffer concentration (mol L-1) and
PFR: preconcentration ow rate (mL min-1).
Table 2. Factors, levels, and analytical responses from the full 24 factorial
design
Factor Levels
Low (-)High (+)
Preconcentration ow rate (PFR) /
(mL min-1)5.00 10.00
Buffer concentrationa (BC) / (mol L-1) 0.05 0.10
pH 6.00 8.00
Eluent concentration (EC) / (mol L-1) 0.50 1.00
PFR BC pH EC Average ± SD
++++0.71 ± 0.06
+++-0.65 ± 0.09
-+++0.89 ± 0.02
-++-0.93 ± 0.04
+-++0.69 ± 0.07
+-+-0.66 ± 0.05
--++0.79 ± 0.00
--+-0.93 ± 0.03
++-+0.47 ± 0.09
++--0.84 ± 0.02
-+-+0.83 ± 0.06
-+--0.35 ± 0.03
+--+0.73 ± 0.12
+---0.47 ± 0.03
---+0.56 ± 0.06
----0.52 ± 0.10
aPhosphate buffer solution. SD: standard deviation from analysis carried
out in triplicate.
On-line Micro-Packed Column Solid-Phase Extraction of Cadmium J. Braz. Chem. Soc.
8
constant the pH in the rst milliliters of sample loading in
the mini-column. Therefore, more complex experimental
designs such as the Doehlert matrix could not be performed
due to the memory effect drawbacks. Also, the memory effect
could also be observed when building the analytical curve.
In this sense, the best conditions were herein established as
1.0 mol L-1 HCl as eluent and samples at pH 8.0 and
Regarding the preconcentration ow rate (PFR), even
using high levels (5.0 and 10.0 mL min-1), no inuence
on the analytical response was observed, which clearly
indicates a fast mass transfer kinetics of cadmium towards
the UiO-66. Thus, in order to obtain a high sample
throughput to the method, the preconcentration ow rate
of 10.0 mL min-1 was chosen as the best condition. Buffer
concentration (BC) within experimental domain did not
feature any inuence on analytical response and, thus,
the lower concentration of 0.05 mol L-1 was adopted,
aiming at lower consumption of reagent, as well as cost
of analysis.
Effect of potentially interfering ions in the on-line micro-
packed column solid-phase preconcentration of cadmium
Selectivity of the preconcentration method was
evaluated through cadmium preconcentration of in the
presence of potential interfering ions, such as Pb2+, Hg2+,
Fe2+, Co2+, Cu2+, Ni2+, Zn2+, Mg2+ or Ca2+, which might be
present in different types of water samples. For this task,
binary solutions containing different ratios of analyte to
interferent were subjected to the proposed method, where
the Cd2+ concentration was xed at 5.0 µg L-1. The analytical
signal of binary solution was then compared to a solution
containing only 5.0 µg L-1 of Cd2+, and the other ion was
considered a potential interferent at certain concentration
when a relative error of ± 10% in the analytical signal
was obtained. Therefore, the degree of tolerance (µg L-1),
which is the high concentration that a potentially interfering
ions can be present in the solution without interfering the
preconcentration of cadmium, is shown in Table 3.
Even in the presence of high concentration for some
metals, no interference in the cadmium preconcentration
was observed, which might be attributed to high surface
area of UiO-66. Therefore, the proposed method shows
potentiality for an interference-free water samples analysis.
Analytical features and application of the proposed method
for cadmium determination in water samples
As can be seen in Figure 8, the dynamic linear
range of the proposed method was obtained within the
concentration range of 0.10 to 8.00 µg L-1 with linear
equation Abs=0.1213[Cd2+] + 0.0716, and determination
coefficient of R2 = 0.9996. For the measurements
without preconcentration step, the linearity was within
the concentration range of 50.00 to 250.00µg L-1,
Abs= 0.0034[Cd2+] + 0.0112 (R2= 0.9985). The
preconcentration factor (PF) was determined as the
ratio of the slopes of the analytical curves built with
and without preconcentration step. Thus, PF obtained
was 35.7. For 10.0mL of sample, the limit of detection
(LOD) of 0.03µg L-1 and the limit of quantification
(LOQ) of 0.10µg L-1, were defined according to
International Union of Pure and Applied Chemistry
(IUPAC) recommendations,56 as 3std/m and 10std/m,
respectively, where std is the standard deviation of 10
determinations of the analytical blank and m is the slope
of the analytical curve. The concentration efficiency
(CE), dened as the preconcentration factor obtained by
operating the preconcentration procedure for 1 min, and
the consumption index (CI), dened as the sample volume
required to reach a unit of PF, were found to be 0.595min-
Figure 8. Analytical curves obtained with and without preconception step
in TS-FF-AAS system under optimized conditions.
Table 3. Tolerable concentration of potentially interfering ions on
preconcentration of cadmium
Potentially
interfering ions
Tolerable
concentration / (µg L-1)Recovery / %
Pb2+ 5.00 96.2
Hg2+ 5.00 96.0
Fe2+ 25.00 91.2
Co2+ 50.00 102.9
Cu2+ 50.00 102.9
Ni2+ 50.00 103.5
Zn2+ 50.00 98.9
Mg2+ 500.00 105.3
Ca2+ 500.00 102.3
Carneiro et al. 9Vol. 00, No. 00, 2022
1 and 0.280 mL, respectively. The sample throughput of
proposed method was 11 h-1, considering the sample loading
of 10.0 mL at a ow rate of 10.0 mL min-1. The intra-day
precision of the method, calculated for 10consecutive
measurements of standard solutions of Cd2+ at 1.0, 5.0 and
7.0 µg L-1 were found to be, respectively, 3.69; 0.64 and
0.13% (relative standard deviation), whereas the inter-day
precision (two different days) for the same concentrations
of Cd2+ was found to be 4.64; 2.86 and 0.54%, respectively.
Analytical methods involving on-line preconcentration
coupled to TS-FF-AAS were compared to the proposed
method (Table 4). The developed method proved to be
highly efcient due to satisfactory sample consumption,
high preconcentration factor and low limit of detection.
In addition, the method is more environmentally-friendly
and inexpensive to be carried out than those that makes
use of fullerene and polyurethane foam due to the absence
of toxic chelating agent during the pre-concentration ow
system. Compared to hybrid imprinted polymer and the
nanocomposite based on multiwalled carbon nanotubes
(MWCNTs) and polyvinyl pyridine, the synthesis of
UiO-66 is easier and inexpensive to be performed. Although
the method that explores the precipitation-dissolution in
a knotted reactor (KR) features satisfactory analytical
features, it makes use of two peristaltic pumps and a
standard rotary injection valve, in which naturally makes
the method more expensive.
The accuracy of the method was attested by determining
Cd2+ in different types of water samples followed by spiking
known amounts of cadmium. The results are shown in
Table 5.
The recoveries values varied from 92.0 to 111.9%,
conrming the reliability of the proposed method for
preconcentration and determination of Cd2+ ions in water
samples with different matrix compositions.
Conclusions
The performance of UiO-66 as adsorbent for cadmium
preconcentration in an on-line micro-packed column
procedure coupled to TS-FF-AAS was evaluated for the
rst time. The use of UiO-66 attends the requirements
of green analytical chemistry in the ow system since no
organic solvents and toxic chelating agents were used.
Apart from these features, the synthesis of UiO-66, when
compared to other adsorbents previously used for cadmium
Table 4. Comparison of the proposed method with recent studies reported in the literature to determine Cd2+ ions using on-line preconcentration coupled
to TS-FF-AAS
Preconcentration method or adsorbent Sample
volume / mL
Chelating agent or
precipitant agent
LOD /
(µg L-1)Eluent PF Reference
Hybrid imprinted polymer 10 -0.03 HCl/ethanol 14.0 57
Nanocomposite based on MWCNTs and polyvinyl
pyridine 8.8 -0.03 HCl 19.5 58
Precipitation-dissolution in a knotted reactor 4.0 NH30.04 HNO334.0 59
Fullerene 1.5 APDC 0.10 ethanol 11 60
Polyurethane foam 2.0 DDTP 0.12 ethanol 5.2 61
Avocado seed activated carbon 10 -0.12 HCl 10.7 62
UiO-66 10 -0.03 HCl 35.7 this work
LOD: limit of detection; PF: preconcentration factor; MWCNTs: multiwalled carbon nanotubes; APDC: ammonium pyrrolidine dithiocarbamate; DDTP:
diethyl dithiophosphate ammonium salt.
Table 5. Application of the developed method in water samples and
evaluation of recovery tests
Sample Amount added of
Cd2+ / (µg L-1)
Amount
determined of
Cd2+ / (µg L-1)
Recovery / %
Tap water
0.00 < LOD -
1.00 1.03 ± 0.03 103.3
5.00 5.47 ± 0.01 109.3
8.00 7.56 ± 0.01 94.4
Saline water
0.00 < LOD -
1.00 1.01 ± 0.02 100.6
5.00 4.81 ± 0.01 96.2
8.00 8.63 ± 0.01 107.8
Mineral water
0.00 0.30 ± 0.01 -
1.00 1.27 ± 0.01 97.4
5.00 5.10 ± 0.00 96.2
8.00 7.64 ± 0.02 92.0
Lake water
0.00 < LOD -
1.00 1.12 ± 0.03 111.9
5.00 5.47 ± 0.04 109.3
8.00 7.45 ± 0.01 93.1
LOD: limit of detection. Numbers are mean concentration values ± SD
(standard deviation) of (n = 3).
On-line Micro-Packed Column Solid-Phase Extraction of Cadmium J. Braz. Chem. Soc.
10
preconcentration coupled to TS-FF-AAS, is easier and
inexpensive to be performed. In terms of analytical features,
the proposed method presents very low limit of detection,
high preconcentration factor, low sample consumption, high
reusability of UiO-66, since only one mini-column was
used through all the method development, and absence of
matrix effect for different types of water samples.
Supplementary Information
Supplementary data are available free of charge at
http://jbcs.sbq.org.br as PDF le.
Acknowledgments
The authors acknowledge the financial support
and fellowships of CAPES/Araucária Foundation
No.13/2018 - Post doctoral Fellowship, Coordenação de
Aperfeiçoamento de Nível Superior (CAPES) nance code
001, Conselho Nacional de Desenvolvimento Cientíco
e Tecnológico (CNPq) (grant No. 307432/2017-3), and
Instituto Nacional de Ciência e Tecnologia de Bioanalítica
(INCT) (FAPESP grant No. 2014/50867-3 and CNPq grant
No. 465389/2014-7).
Author Contributions
Ana Carla R. Carneiro and Paula M. dos Santos developed the
analytical method; Joziane G. Meneguin and Andrelson W. Rinaldi
performed the synthesis and characterization of UiO-66; Marcela
Z. Corazza and Maiyara Carolyne Prete participated in writing-
original draft preparation and César Ricardo T. Tarley participated
in conceptualization, funding acquisition and writing-original draft
preparation.
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Submitted: October 31, 2021
Published online: March 17, 2022
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