Functionalization of SBA-15 with EDTA and its application in removing Ca2+
and Mg2+ ions from hard water
Funcionalização da SBA-15 com EDTA e sua aplicação na remoção de íons Ca2+
e Mg2+ de água dura
Funcionalización de SBA-15 con
EDTA y su aplicación en la eliminación
de iones Ca2+ y Mg2+ del agua dura
Vanessa da Silva
Bezerra Marques1; Andarair Gomes dos
Santos2; Ricardo Henrique de Lima Leite3; Francisco Klebson Gomes dos Santos4
1Mestranda do Programa de Pós-Graduação em Ciência e Engenharia
de Materiais, da Universidade Federal Rural do Semi-Árido,
Mossoró, Rio Grande do Norte; vanessa.sirmarques@gmail.com; 2Professora da Universidade Federal Rural do Semi-Árido,
Mossoró, RN, +558433178200, andarair@ufersa.edu.br; 3Professor
da Universidade Federal Rural do Semi-Árido, Mossoró,
RN, ricardoleite@ufersa.edu.br;
4Professor da Universidade Federal Rural do Semi-Árido,
Mossoró, RN, klebson@ufersa.edu.br
Recebido: 20/01/2020; Aprovado: 26/06/2020
Abstract:
Poor management, changes in drinking parameters and drought subject many
parts of the world to survival in the face of shortages of drinking water and the
quality of drinking water needs to be assessed for adequate consumption. Hard
water can lead to the buildup of mineral deposits in pipes and appliances that
use water on a regular basis, affecting the performance and life cycle of these
items. In addition, in the presence of soap, fatty acids formed an insoluble
precipitate with calcium ions, making foam and cleaning difficult, as well as
other problems generated by the high content of Ca2+ and Mg2+
ions present in water. The goal of this paper was to study the removal of Ca2+
and Mg2+ ions using SBA-15, a mesoporous material, functionalized
with ethylenediaminetetraacetic acid and compared to its pure matrix. This
adsorbent was tested in Ca2+ and Mg2+ ions solution with
concentration of 250 mg.L-1 and pH = 9, varying the temperature in
25°C and 50°C. Mesoporous materials were synthesized by hydrothermal method and
characterized by TG/DTG (Thermogravimetry and Derivative Thermogravimetry), XRD
(X-Ray Diffraction) and BET (Brunauer, Emmett and
Teller method). Adsorption results showed removal of up to 50% of the Ca2+
and Mg2+ ions in a short time, approximately 5 min.
Key words: SBA-15 synthesis; EDTA
functionalization; Adsorption; Ca2+ and Mg2+.
Resumo: O mau gerenciamento, as mudanças nos parâmetros de consumo e a seca
sujeitam muitas partes do mundo à sobrevivência diante da escassez de água
potável e a qualidade da água potável precisa ser avaliada para um consumo
adequado. A água dura pode levar ao acúmulo de depósitos minerais em tubulações
e aparelhos que usam água regularmente, afetando o desempenho e o ciclo de vida
desses itens. Além disso, na presença de sabão, os ácidos graxos formam um
precipitado insolúvel com íons cálcio, dificultando a espuma e a limpeza, além
de outros problemas gerados pelo alto conteúdo dos íons Ca2+ e Mg2+
presentes na água. O objetivo deste trabalho foi estudar a remoção dos íons Ca2+
e Mg2+ usando a SBA-15, um material mesoporoso,
funcionalizado com ácido etilenodiaminotetracético e
comparado à sua matriz pura. Este adsorvente foi testado em solução de íons Ca2+
e Mg2+ com concentração de 250 mg.L-1
e pH 9, variando a temperatura em 25°C e 50°C. Os materiais mesoporosos
foram sintetizados pelo método hidrotérmico e caracterizados por TG-DTG (Termogravimetria e Termogravimetria
Derivada), DRX (Difração de Raios X) e BET (método Brunauer, Emmett, Teller). Os
resultados da adsorção mostraram uma remoção de até 50% dos íons Ca2+
e Mg2+ em um curto período de tempo,
aproximadamente 5 min.
Palavras-chave: Síntese de SBA-15;
Funcionalização de EDTA; Adsorção; Ca2+ e Mg2+.
Resumen:
La mala gestión, los cambios
en los parámetros
de consumo y la sequía someten a muchas partes del mundo a la supervivencia ante la escasez de agua potable y la calidad
del agua potable debe evaluarse para un consumo adecuado. El agua dura puede conducir a la acumulación
de depósitos minerales en tuberías y electrodomésticos que usan agua de manera regular, lo que afecta el
rendimiento y el ciclo de
vida de estos artículos. Además,
en presencia de jabón, los ácidos grasos formaron un precipitado insoluble con iones de calcio, dificultando la espuma y la limpieza, así
como otros problemas generados
por el alto contenido de
iones de Ca2+ y Mg2+ presentes en
el agua. El objetivo de este
trabajo fue estudiar la eliminación
de iones Ca2+ y Mg2+ utilizando SBA-15, un material mesoporoso,
funcionalizado con ácido etilendiaminotetraacético
y comparado con su matriz
pura. Este adsorbente se probó
en solución de iones Ca2+
y Mg2+ con una concentración
de 250 mg.L-1 y pH = 9, variando la
temperatura en 25°C y 50°C. Los materiales
mesoporosos se sintetizaron
por método hidrotérmico y se caracterizaron por TG /
DTG (termogravimetría y termogravimetría
derivada), XRD (difracción de rayos
X) y BET (método Brunauer, Emmett,
Teller). Los resultados de adsorción
mostraron la eliminación de hasta 50% de los
iones Ca2+ y Mg2+ en poco tiempo, aproximadamente 5 min.
Palabras
clave: Síntesis
de SBA-15; Funcionalización EDTA; Adsorción;
Ca2+ y Mg2+.
The quality and quantity of
available water resources, changes and management of consumption parameters, as
well as the drought itself make many parts of the world survive in the face of
scarcity of drinking water (MARINOSKI et al., 2018). Water quality is related
to various dissolved and suspended particles and impurities, as well as
physical, chemical and biological characteristics (MARCAL-SILVA et al., 2017).
Among the types of drinking water, it has those with hard characteristics (high
Ca2+ and Mg2+ ions content) that represent serious
environmental, industrial and domestic problems (OLIVEIRA JUNIOR et al., 2016). Furthermore, in the presence of
soap, fatty acids form an insoluble precipitate with leading to the formation
of soap foam.
For the treatment of
contaminated waters there are different techniques, such as solvent extraction,
micro/ultrafiltration, sedimentation and gravity separation, flotation,
precipitation, coagulation, oxidation, evaporation, distillation, reverse
osmosis, adsorption, ion exchange and electrodialysis (SANTHOSH et. al., 2016).
Among these techniques adsorption stands out for being
highly versatile and ecologically effective employed in wastewater treatment
due to its easy operation, low cost and availability of a wide range of
adsorbents. In addition, adsorption can also be applied for the removal of
soluble and insoluble organic, inorganic and biological pollutants (SANTHOSH et
al., 2016; FIORILLI et al., 2017).
New organic/inorganic
mesoporous ordered structures have been widely investigated as adsorbents for
waste or contaminated water treatment due to their high surface area, large and
uniform pore size. However, the surface area can be easily functionalized with
heteroatoms or organic groups to confer specific hydrophobicity/hydrophilicity
or to introduce specific binding sites in order to achieve greater interaction
(MAJDA et al., 2016; SOLTANI et al., 2017). Among the classes of mesoporous
materials stand out the SBA-15 (Santa Bárbara Amorphous) (ZHAO et al., 1998),
materials considered promising for application in adsorption because they have
a hexagonal array of mesoporous and uniform size with a highly ordered channel
matrix. Its surface is easily functionalized with numerous organic functions
without compromising the mesostructure (LUZ et al.,
2010; MORITZ and GESZKE-MORITZ, 2016; FONSECA-CORREA et al., 2016; THUNYARATCHATANON
et al., 2017).
In the adsorption
phenomenon, ethylenediaminetetraacetic acid (EDTA) has also attracted
considerable attention as adsorbent due to the formation of a strong
metal-binder complex, cost effectiveness and availability. Divalent ion capture
has been studied by immobilizing EDTA on some surfaces, but there is little
about the applications of EDTA-SBA-15 in the removal of ions such as Ca2+
and Mg2+ (IQBAL and YUN, 2017), as well as the capture of ions.
Calcium ions in EDTA functionalized mesoporous silica matrix have not been
found to date. Therefore, the goal of this paper is to synthesize SBA-15 and
its functionalization with EDTA to evaluate its potential as adsorbent material
in the removal of Ca2+ and Mg2+ ions in aqueous solutions
in finite bath. It is also made an analysis of the structural, textural and
zero charge point. Finally, kinetic and thermodynamic studies were also
evaluated.
MATERIAL AND METHODS
SBA-15 mesoporous support was synthesized by the hydrothermal method,
according to the methodology proposed by Zhao et al. (1998). The reagents used
in the syntheses are showed in Table 1.
|
Table 1. Reagents used in the synthesis of mesoporous
materials |
|||
|
Reagent |
Manufacturer |
Purity |
Chemical Formula |
|
Tetraethylorthosilicate (TEOS) |
Sigma-Aldrich |
98% |
Si(OC2H5)4) |
|
Pluronic P123 |
Sigma-Aldrich |
- |
HO[CH2CH2]20[CH2CH2(CH3)O]70(CH2CH2O)20H |
|
Hydrochloric acid |
Merck |
37% |
HCl |
Post graft modification was applied to modify the structure of the
synthesized mesoporous support and the EDTA was impregnated in the SBA-15
support by the wet spot impregnation method. In this technique a minimum volume
of solution is used to cover all the dust to the point of leaving it pasty (POPA
et al., 2011; DONG et al., 2013; SHENG et al., 2014). Initially, 2 mL of the
EDTA solution (0.2 M) is added to 1g of SBA-15 mesoporous support then dried at
30°C for 6h. Figure 1 shows a schematic of the synthesis and impregnation
process.
Table 2 summarizes a nomenclature to distinguish the
synthesized materials: the SBA-15 mesoporous matrix and functionalized by wet
spot impregnation.
|
Table 2. Nomenclature of synthesized mesoporous materials |
||
|
Mesoporous
Material |
Condition |
Nomenclature |
|
SBA-15 |
not calcined |
SBA-15 A |
|
SBA-15 |
Calcinated
(550°C/2h) |
SBA-15
B |
|
SBA-15 |
EDTA impregnated |
SBA-15 C |
Figure
1. A) Scheme of synthesis of SBA-15, B) impregnation
of SBA-15 with EDTA (DA’NA, 2017)
The thermogravimetric
curves (TG-DTG) of SBA-15 mesoporous support were obtained on a SHIMADZU DSC-60
and DTG-60H thermocouple with coupled DTA. For the analysis, approximately 5 mg
of sample were used in alumina crucibles that were heated to 750°C, with
heating rate of 10°C/min, in an inert nitrogen atmosphere. Through the
thermogravimetric curves generated from the samples of non-calcined materials,
it was possible to determine the amount of water and organic template present,
as well as the temperature ranges in which these molecules are removed. These
data are of fundamental importance in determining the lowest calcination
temperature required to remove the pore director from the mesoporous support.
For the X-ray analysis of SBA-15 mesoporous materials, the powder method
proposed by SETTLE (1997) was employed, which basically consists in
uniformizing the sample to make it a fine and homogeneous powder. Depending on
the type of material synthesized, analyzes were performed in two steps,
according to the low and high angle range. In the first stage, the low angle
diffractograms in the range of 0.5 to 3.0º were obtained by a Rigaku MineFlexTM II diffractometer, operating with Cukα radiation at 40kv, 30mA, velocity of 0.5ºg/min and
step of 0.01deg. In a second step, the high angle diffractograms in the range
of 10 to 60º were obtained by a Shimadzu model XRD - 6000 diffractometer,
operating with Cukα radiation at 40 kv, 30mA, velocity of 0.5deg/min and step of 0.02deg (GÓMEZ
et al., 2016). Structural analyzes as a function of diffractograms were
characterized based on reference standards calculated from the COD database for
standard chart (00-035-1779) and standard chart (00-035-1779) and using the
MAUD® program. Phase Identification from Powder Diffraction Version 2.064.
The zero charge point (ZCP) is defined as the
pH at which the surface of a material has a neutral charge. The methodology
used for its determination is called “11-point experiment” (BARBOSA et al.,
2014). 0.10 mol.L-1 NaCl solutions of
different pH values (ranging from 0 to 12) were prepared using distilled
water at room temperature, 25ºC. The pH of each solution was adjusted to the
required value with HCl or NaOH solution. SBA-15 ZCP and EDTA-SBA-15 were
estimated by direct pH measurements before and after contact with the solid in
standard solutions (HCl/NaCl and NaOH/NaCl). Approximately 0.1g of the matrix
was weighed and added in 50 mL pH adjusted solution starting from 0 to 12. The solutions
were constantly stirred for 24h, and then centrifuged and the pH of the final
solution was then measured. The obtained data were represented in a graph of
the pH variation (ΔpH = final pH - initial pH) as a
function of the initial pH of the solutions with the solid. The value of the zero charge point was determined, considering ΔpH equal to 0.
Nitrogen adsorption/desorption spectra were performed at 77K using the
BET method. Each analysis contained about 0.1g of sample previously calcined
which underwent a degassing process at 300°C for 10 hours. This treatment aims
to remove moisture from the surface of the solid. Nitrogen adsorption isotherms
for the samples were obtained in the P/P0 range (approximately from
0 to 0.95), providing important information about the materials, such as:
surface area, average pore diameter and pores volume. These parameters (SBET,
Dp and Vp)
were calculated using the autosorb Surface Area &
Pore Size Analyzer Program® version 1.55.
To verify the efficiency of removal of Ca2+ and Mg2+
ions from drinking water using the matrix SBA-15 B and SBA-15 C the adsorption
experiments were performed in a finite bath. To investigate the removal rate in
the pure (SBA-15 B) and impregnated (SBA-15 C) silica matrix, aqueous solutions
were used at a concentration of 250 mg.L-1 of CaCO3 and
pH=9. For each 1.0 g of adsorbent, 100 mL of the solution was used, varying the
temperature and constant stirring (200 rpm). At predetermined intervals
aliquots of solution were removed and filtered. The supernatant liquid was diluted and the metal concentration determined by titration.
The collected supernatants were analyzed by EDTA-Na titration method
according to standard method (19th edition, 1995). To study the removal of Mg2+
ions at initial concentrations of 250 mg.L-1,
a 0.01 M EDTA solution was used as titrator, a pH 10 NH4Cl/NH4OH
buffer solutions and the turning point Eriochrome Black T indicator from violet
to blue. Whereas for the study of Ca2+ ions removal in solutions
with initial concentrations 250 mg.L-1 a
0.01 M EDTA solution was used as titrant, pH 12 NaOH solution and the murexide
indicator with pink to purple turning point. For both Ca2+ and Mg2+
analyzes were performed in triplicate.
RESULTS AND DISCUSSION
Figure 2 shows TG-DTG curves for uncalcined
(SBA-15 A), calcined (SBA-15 B) and EDTA impregnated (SBA-15 C) mesoporous
materials, respectively.
Figure 2. TG-DTG (Thermogravimetry
and Derivative Thermogravimetry) curves: (A) SBA-15 mesoporous matrix, (B)
calcined SBA-15 B, (C) EDTA functionalized mesoporous silica (SBA-15 C)
As can be seen in Figure 2(A), the SBA-15 a TG curve (uncalcined) shows two mass losses. Such losses are related
to the physically adsorbed water output in the temperature range between 25°C
to 100°C and the second is directly related to the template output of the
mesoporous material structure relative to the peak in the range 150°C to 200°C
on the DTG curve, with 5% and 45% of mass loss, respectively. The stability of
the mesoporous silica structure begins from 550°C. Analyzing Figure 2 (B) the
material SBA-15 B shows a mass loss below 100°C which is attributed to water
desorption. The second mass loss occurs between 200°C and 600°C due to dehydroxylation of silicate networks. According to
EZZEDDINE et al., 2015, at high temperatures condensation of the silanol groups
of the silica matrix is observed. For EDTA functionalized mesoporous silica
(SBA-15 C), mass losses in the same temperature ranges are also observed, as
shown in Figure 2 (C). However, in this case EDTA has significant mass losses
in this same temperature range between 200°C and 600°C, which represents the breakdown
of CN (amine) bonds corresponding to the decomposition of EDTA (SENNA et al.,
2015). This can be verified by the decay in the DTG curves of Figure 2 (C),
proving the impregnation of the silica matrix with EDTA (EZZEDDINE et al.,
2015).
It is important to note that, as the SBA-15 support, the concept of
crystallinity cannot be used, as with zeolites, because its walls are made of
amorphous silica. The absence of peaks at greater angles indicates that the
material is not crystalline. However, it is known that there is an orderly
hexagonal network, where one pore is surrounded by six others, generating the
characteristic reflection of this material. Thus, phase identification occurs
when three to five peaks are observed corresponding to the planes (100), (110),
(200), (210) and (300) in the range between 0.5 and 3.0º, which are according
to the literature, characteristics of the hexagonal structure of the SBA-15
nanostructured mesoporous materials (GÓMEZ et al., 2016).
Figures 3 and 4 show the low and high angle diffractograms of mesoporous
silica without heat treatment, calcined and impregnated with EDTA, SBA-15 A,
SBA-15 B and SBA-15 C, respectively.
Figure 3. Low angle diffractograms for mesoporous silica (A) SBA-15 A;
(B) SBA-15 B and (C) SBA-15 C
The diffractograms in Figure 3 show three peaks, which confirm the
crystal structure of the synthesized materials. According to the powder X-ray
diffraction data of the adsorbent, as illustrated in Figure 3 presents three
peaks that appear in the range of 2θ between 0.5 to 2°. These peaks are indexed
to the reflections (100), (110) and (200), showing that the mesoporous
structure of the SBA-15 is maintained after both calcination and
functionalization with EDTA. However, the reduction in diffraction intensity
reveals some degree of disorder, especially for the less intense peak
attributed to organic monolayer installations within the pore structure when
functionalized with EDTA as can be seen in the diffractogram of Figure 4 (C)
(SBA-15-C) (WU et al., 2013).
Figure 4. High angle
diffractograms for mesoporous silica (A) SBA-15 A; (B) SBA-15 B and (C) SBA-15
C
The high angle X-ray diffraction pattern is typical of SBA-15 amorphous
walls with 2θ between 20 and 25° (IQBAL and YUN, 2017) and can be observed in
the three diffractograms presented. For the diffractogram for SBA-15 C (with
impregnation), was attributed to peaks concerning the molecular structure of EDTA are observed, indicated by * in Figure 4
(C). These peaks are characterized based on reference patterns calculated from
the COD database for the standard chart (00-035-1779) and the standard chart
(00-035-1779) and using the Phase Identification from Powder (MAUD®) program.
The result presented from the molecular crystalline characterization confirms
once again the presence of EDTA in the SBA-15 mesoporous silica matrix,
demonstrating that the adopted wet spot impregnation technique is efficient in
modifying and functionalizing the surface of the SBA-15.
The zero charge point (ZCP) is a parameter that
indicates the pH value at which a given solid has a zero charge on the surface.
This parameter is relevant because it allows predicting the adsorbent surface
charge as a function of pH. The results found for
calcined and functionalized SBS-15 are presented in Figure 5.
Figure 5. ZCP (Zero
Charge Point) measurement experiment
According to Figure 5, the ZCP of SBA-15 B and SBA-15 C occurs at pH 4.2
and 4.5, respectively. According to these values, it is possible to predict the
adsorbent behavior: for solution pH values lower than the ZCP value the
adsorbent surface is positive (adsorbs anions) and for solution pH values
greater than the ZCP, the surface charge adsorbent is negative (adsorb
cations). In this case, below these values, the hydroxyls are protonated and
therefore positively charged and for values above those obtained by ZCP
analysis, the hydroxyls are deprotonated and negatively charged (FERREIRA and
PARK, 2012).
It can be seen that SBA-15 C (functionalized) has a higher negative
surface charge than SBA-15 B, showing that the impregnation with EDTA was
successful, since the EDTA has coordinating oxygen and nitrogen atoms in its
structure, promoting a greater number of negative active sites in the
mesoporous silica matrix. In this context, the complexation reaction of calcium
and magnesium atoms by EDTA is facilitated at a more alkaline pH than in most
metals (eg Zn2+, Cd2+ and Pb2+),
since at more acidic pH their complexes are less stable, being protonated EDTA
instead of complexing calcium and/or magnesium (LEE et al., 2016). Thus, in
order to ensure that the adsorption process occurs without the phenomenon of
precipitation of Ca2+ and Mg2+ ions and the formation of
stable complexes, the study was carried out at pH 9, knowing that the
adsorption of cations occurs above the point. The zero-charge tests obtained
were performed at this pH based on studies already carried out in the capture
of calcium ions in zeolitic materials (HERRMANN and KLEIN, 1987; QIN et al.,
2010a, 2010b).
Figures 6 and 7 show the 77 K nitrogen adsorption/desorption isotherms
for the specific surface area obtained by the BET equation and the pore size
and pore volume distribution of synthesized mesoporous materials (SBA-15 A,
SBA- 15 B and SBA-15 C).
Figure 6.
SBA-15 A, SBA-15 B and SBA-15 C nitrogen adsorption/desorption isotherm
According to the IUPAC classification, type IV isotherms are observed,
as shown in Figure 6. The SBA-15 A, SBA-15 B and SBA-15 C also exhibit type H1
hysteresis, characteristic of mesoporous solids (BIRTH et al., 2014). In
addition, adsorption isotherms show a sharp inflection at high relative
pressure (P/P0 = 0.6 - 0.8), which according to Dong et al., (2013)
indicates typical capillary condensation within uniform pores.
Figure
7. SBA-15
A, SBA-15 B and SBA-15 C pore diameter distribution
Table 3 summarizes the surface area (SBET), pore diameter (Dp) and pore volume (Vp)
corresponding to pure SBA-15 with calcination and functionalized with EDTA
(SBA-15 A, SBA-15 B and SBA-15 C, respectively).
|
Table 3. Textural characteristics for SBA-15 A, SBA-15 B and SBA-15 C |
|||
|
Mesoporous material |
S BET (m 2/g) |
Dp (nm) |
Vp (cm3/g) |
|
SBA-15 A |
197.1 |
5.6 |
0.5 |
|
SBA-15 B |
744.9 |
6.6 |
1.9 |
|
SBA-15 C |
435.5 |
6.5 |
1.0 |
As shown in Table 3, the calcined and EDTA-impregnated mesoporous
material has a surface area of 197.1 m2g-1, 744.9 m2g-1
and 435.5 m2g-1, respectively. As expected, SBA-15
without calcination presents low surface area (197.12 m2g-1)
when compared to areas of calcined SBA-15 and EDTA-functionalized SBA-15. This
is due to the blockage of the pores by the organic director used in the
synthesis of the silica matrix. Meanwhile, SBA-15 B (calcined) exhibits the
largest surface area (744.9 m2g-1) even when compared to
EDTA functionalized SBA-15 C (435.5 m2g-1). This
reduction is a phenomenon also expected after mesoporous silica
functionalization, which occurs due to the implantation or suspension of
functional group portions inside and outside the walls of the silica structure.
Functional group loading tends to decrease specific surface area and pore
volume after functionalization. These functional group units block adsorption
of nitrogen molecules and suppress the dew point of functionalized mesoporous
silica toward lower relative pressure. Thus, it was observed a decrease in pore
size and surface area after functionalization (EZZEDDINE et al., 2015).
Chong et al. (2019)
synthesized SBA-15 with surface area, pore volume and pore diameter equal to
856 m2/g, 0.99 cm3/g and 7.45 nm, respectively. Although
it is the same synthesis method used, such variations can be attributed to the
shorter drying and/or calcination time of SBA-15, 12 hours and 3 hours,
respectively.
Pore and diameter volume
distributions also decreased with functionalization from 1.9 cm3 g-1
and 6.6 nm (SBA-15 B) to 1.0 cm3 g-1 and 6.5nm SBA-15 C.
This behavior is also related to implantation or suspension of functional group
portions within the pores of the silica thereby reducing pore volume. However,
no significant changes in the shape of the mesoporous matrix are observed after
functionalization and the structure remains intact, which is
in agreement with those presented by structural analysis (XRD).
Adsorption kinetics is one of the most relevant features as it
represents the efficiency of the system. The effect of different contact time
on the adsorption of Ca2+ and Mg2+ ions by SBA-15 B and
SBA-15 C is shown in Figures 8 to 11, respectively. The efficiency of SBA-15 as
a function of temperature (25°C and 50°C) and pH (without pH adjustment and
equal to pH 9) is presented.
Figure 8. Adsorption capacity of calcium ions (Ca2+) without
pH adjustment: (A) SBA-15 B (B) SBA-15 C at temperatures of 25° C and 50°C
|
|
|
As shown in Figure 8, initially there is a significant increase in Ca+2
adsorption at both 25°C and 50°C. However, SBA-15 B shows a 5% increase
in adsorption with increasing temperature over the same period
of time. After this time, at both temperatures, there is a small
reduction in adsorption, which remains constant over time, but more significant
at 25°C, ie, a reduction of 5% at 50°C and 8% at
25°C. It can be further observed that Ca+2 adsorption at 50°C is
almost 2 times higher than at 25°C.
When SBA-15 was functionalized with EDTA (SBA-15 C) (Figure 8b), a
similar adsorption profile was observed, that is, an increase in adsorption in
the first five minutes and then remained constant over time. However, with SBA-15
C the adsorption capacity is much higher at both temperatures (25°C and 50°C),
being more pronounced at 50°C, about 3 times higher when compared to SBA-15 B.
Finally, it can be concluded that in the system without pH adjustment,
the temperature increase favors the adsorption of calcium ions from the medium,
being a process that demands a higher energy to promote the reaction.
Figure 9. Adsorption capacity of calcium ions (Ca2+) at pH 9:
a) SBA-15 B b) SBA-15 C at temperatures of 25°C and 50°C
|
|
|
The profile of the Ca+2 ion removal curve is similar with and
without the pH adjustment of the medium at 9, as shown in Figure 10. However,
there is an inversion in temperature behavior with pH adjustment, ie adsorption is greater at 25°C than 50°C for SBA-15 B.
Analyzing Figures 8 (a) and 9 (a) calcined material (SBA-15 B) without
pH adjustment obtained a removal rate of about 12 and 20, while at pH 9 a
removal of 50 and 40% at the same temperatures, 25°C and 50°C, respectively. In
this context, the pH variation favors the adsorption process of calcium ions
(Ca2+) at room temperature, which proves to be a spontaneous
reaction without the need to increase the medium temperature, just adjusting
the pH to charge above the zero charge potential.
