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. 2020 Feb 11;5(7):3346–3357. doi: 10.1021/acsomega.9b03537

Reactive Adsorption of Parabens on Synthesized Micro- and Mesoporous Silica from Coal Fly Ash: pH Effect on the Modification Process

Francisca F de Oliveira †, Karine O Moura ‡, Luelc S Costa §, Carla B Vidal †,*, Adonay R Loiola ∥, Ronaldo F do Nascimento †
PMCID: PMC7045554  PMID: 32118149

Abstract

graphic file with name ao9b03537_0007.jpg

Parabens are widely used as preservatives in food, pharmaceutical, and cosmetic products. These compounds are known for their estrogen agonist activity. This research investigates the synthesis of micro- and mesoporous silica from coal fly ash at different pH values (13, 11, 9, and 7) as well as its use as an adsorbent for the removal of parabens. The materials were characterized, and X-ray fluorescence (XRF) analysis revealed that the fly ash acid treatment reduced the presence of aluminum, iron, and calcium oxides and also that silica synthesized at lower pH values (7 and 9) showed a higher SiO2 content. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses revealed microporous silica formation for silica synthesized at pH 13 and mesoporous silica at pH 7, 9, and 11. Adsorption tests were performed with materials, and FA-AT7 showed a higher adsorption capacity. The effect of factors (A) adsorbent mass, (B) initial paraben concentration, and (C) agitation rate on the adsorption process was studied for the FA-AT7 adsorbent using a factorial experimental design. Standardized Pareto charts revealed a negative effect of factor A, positive effect of factor B, and negative interaction effects of factors A–B for all studied parabens. Isotherms and multicomponent kinetic studies were performed. A linear type-III isotherm was obtained, and adsorption equilibrium was reached at approximately 10 min.

1. Introduction

Parabens, esters of 4-hydroxybenzoic acid, which differ only in the ester group and may be an alkyl or benzyl group, are the most popular and widely used preservatives in food, pharmaceutical, and cosmetic products due to their broad spectrum of antimicrobial activity.1,2 However, recent studies have shown that parabens possess estrogen agonist activity, being is even detected in human breast tumor tissue with an average concentration of 20 ng g–1 of tissue.1−3 The widespread use of these compounds has led to environmental contamination.2 Parabens were detected in surface waters, soils and sediments, and indoor biota.1,4 These compounds can be released into the aqueous environment mainly through wastewater treatment discharges and also as runoff from nonpoint sources and deposition of particles from the atmosphere.4 Most parabens are frequently found in river water at concentrations varying from nanograms per liter to micrograms per liter, and their levels depend mainly on the extent of water dilution resulting from rainfall.5

Paraben adsorption using adsorbent materials has been studied for the removal of parabens from environmental samples.6,7 In this context, mesoporous silica from coal ash, as low-cost adsorbents, can be an effective alternative for the removal of parabens from aqueous media. In addition, this strategy can contribute to the reduction of the disposal of solid industrial waste from coal-based thermoelectric plants, such as coal ash, by transforming it into value-added products. It is estimated that around 750 million tons of coal fly ash are generated each year globally; however, the utilization rate of it is only up to 25%, especially in the production of cement and concrete, manufacture of ceramic materials, and for use as catalysts and adsorbents.8,9 Thus, it is important to study alternative uses of these wastes, as they are also a source of environmental pollution, generating large amounts of volatile organic pollutants.8−11

In this context, the mesoporous silicates and aluminosilicates known as M41S,12,13 in particular their main components MCM-41 (hexagonal phase), MCM-48 (cubic phase), and MCM-50 (lamellar phase), are highlighted.13 These materials have regular cylindrical pores with a controllable pore diameter (2–30 nm), high surface area, high chemical and thermal stability, and easy silica functionalization. These properties show that these materials present a high potential for applications in the areas of adsorption, catalysis, chemical separations, and biotechnology devices.14

The main principle of mesoporous silica synthesis is the use of surfactants or other species as templates through a sol–gel process.15 MCM-41 synthesis is generally carried out in a basic or neutral medium using a silicon source and a quaternary ammonium surfactant. A gel is formed and subjected to a hydrothermal treatment, followed by the removal of the template via either calcination or solvent extraction.13,15,16

Materials with different properties can be obtained by altering the synthesis conditions, such as the template/silica ratio; concentration and hydrophobicity of the template; and temperature, pH, and the nature of silica.16−18

This work investigates the synthesis of mesoporous silica from coal fly ash at different pH values (13, 11, 9, and 7) and its use as an adsorbent for the removal of parabens from aqueous solution.

2. Materials and Methods

2.1. Reagents and Materials

Fly ash was supplied by Eneva-Energia Pecém, located at São Gonçalo do Amarante-Ceara, Brazil. The reagents HCl and NaOH and the surfactant cetyltrimethylammonium bromide (CTAB) were obtained from Vetec, Brazil. For adsorption tests, individual high-purity standards (≥99%) of methylparaben (MP), ethylparaben (EP), propylparaben (PP), and butylparaben (BP) were purchased from Sigma-Aldrich. Their physical–chemical structures and properties are summarized in Table 1. For chromatographic analysis, methanol (PanReac AppliChem ITW Reagents, high-performance liquid chromatography (HPLC) grade) was used. Acetic acid used in the mobile phase was obtained from Vetec, and NaCl, used in the determination of the point of zero charge, was obtained from Sigma-Aldrich.

Table 1. Structures and Physical–Chemical Properties of the Parabens.

2.1.

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a

Ref (19).

b

Ref (20).

c

Ref (21).

2.2. Fly Ash Acid Treatment

Prior to the mesoporous material syntheses, fly ash was subjected to acid treatment so that the amount of metallic impurities present in the raw material could be reduced.22 This procedure consisted of mixing 8.00 g of fly ash (200 mesh) with 200 mL of HCl (2.0 mol L–1) in a reflux system at 90 °C for 2 h. Then, the treated material was centrifuged and washed with deionized water until the pH was between 5.0 and 6.0. Thereafter, the solid was dried at 80 °C for 18 h. The samples were named FA and FA-AT for materials before and after acid treatment, respectively.

2.3. Micro- and Mesoporous Material Syntheses

Prior to the material synthesis, silica was extracted from fly ash. For this, in a typical procedure, 1.00 g of the acid-treated fly ash was added to 6.5 mL of NaOH (4.0 mol L–1), in a reflux system, at 90 °C for 1 h. The mixture was then centrifuged at 6000 rpm for 5 min and filtered, and the supernatant was used as the silica source.23,24 The synthesis procedures were based on the works of Okada et al.25 and Santos et al.26 Herein, a reaction mixture of silicon oxide/NaOH/CTABr/H2O with molar ratio 1.0:0.25:0.08:103.5 was used. First, 0.8917 g of CTABr was dissolved in 20 mL of deionized water, under magnetic stirring at 350 rpm, Then, an aliquot of 18.5 mL of silica source was added. Finally, 2 mol L–1 HCl was added slowly until a gel (at pH 7, 9, or 11) or clear (pH 13) solution was observed. The total volume of the mixture was approximately 57.0 mL. By adding the silica source and making up the volume to 57 mL, the pH of the solution was already around 13, i.e., only pH adjustment to 11, 9, and 7 was required.

The pH effect over the syntheses was evaluated at pH values of 13, 11, 9, and 7, covering the basic to neutral pH range.15 In all of the procedures, the reaction mixtures were aged under magnetic stirring (350 rpm) for 24 h and then without stirring for 24 h more. After that, the reaction mixture was subjected to stirring for 30 min, and then transferred to a Teflon-lined stainless steel autoclave, and heated under autogenous pressure and static conditions at 110 °C for 24 h.

The resulting materials were washed three times with deionized water, dried at 110 °C for 12 h, and calcined at 560 °C for 6 h, with a heating rate of 1 °C min–1. After cooling, the samples were further ground and sieved through 200 mesh sieves. The materials obtained from this process were named FA-AT13, FA-AT11, FA-AT9, and FA-AT7, for the samples synthesized at pH values 13, 11, 9, and 7, respectively.

2.4. Material Characterization

The chemical composition of the materials before and after the acid treatment, as well as the synthesized solids at different pH values, was determined by X-ray fluorescence (XRF) using a Rigaku spectrometer, model ZSX Mini II, operating with a tube of Pd (40 kV, 1.2 mA). Powder X-ray diffraction (PXRD) measurements were performed in Panalytical model X-Pert PRO equipment with Cu Kα (λ = 0.154056 nm) radiation for the crystalline phase with a routine power of 1600 W (40 kV, 40 mA). The high-angle diffractograms were obtained in the 2θ range from 5 to 50°, while at a low angle, the range was from 1.5 to 10° with a counting time of 762 s. The identification of crystalline phases in the samples was performed using X-Pert HighScore software (Panalytical).

Fourier transform infrared (FTIR) measurements were performed using a PerkinElmer FTIR spectrum spectrometer, with a nominal resolution of 2 cm–1, in the region of 4000–400 cm–1. Samples were prepared in KBr pellets (3% mass).

Scanning electron microscopy (SEM) was carried out in Quanta 450 FEG. The samples were prepared on double-sided carbon tape on an aluminum support and coated with a thin layer of gold.

Transmission electron microscopy (TEM) images were acquired with a TEM-FEG (JEM 2100F) field-emission gun transmission electron microscope (acceleration voltage 200 kV, spot size 1, alpha selector 3). The samples were prepared by drying a drop of the isopropyl alcohol-dispersed nanoparticles for 24 h at room temperature on a carbon-coated copper grid (ultrathin carbon/holey carbon, 400 mesh copper grid, Ted Pella, Inc.). The TEM images were acquired with the sample on a single-tilt sample holder using a Gatan 831.J45M0 camera, Gatan Digital Micrograph, and EMMENU programs at different resolutions.

The textural characterization of the samples was performed by gas adsorption. Nitrogen adsorption–desorption isotherms were obtained at −196 °C over a wide range of relative pressure from 0.01 to 0.995 atm with a volumetric adsorption analyzer Autosorb iQ3 (Quantachrome Instruments). Prior to each measurement, the samples were outgassed at 300 °C to a vacuum of 4 mmHg for 12 h. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method. The pore size distribution (PSD) was determined using the density nonlocal density functional theory (NLDFT). The total pore volume was calculated from the adsorption isotherm at P/P0 = 0.992 for sample FA-AT13 and P/P0 = 0.985 for materials FA-AT11, FA-AT9, and FA-AT7.

2.5. Paraben Adsorption

2.5.1. Adsorbent and pH Selection

To identify the most efficient synthesized material for further adsorption studies, initial batch adsorption tests were performed.

A total of 150 mg of adsorbents was added to 40 mL of multicomponent paraben aqueous solution at 10 mg L–1 concentration, at room temperature (28 ± 2 °C), and at pH 5.0, adjusted using HCl, 1.0 mol L–1. The system was kept under 300 rpm agitation for 18 h, and after this, the solution was filtered using a 13 mm syringe filter with 0.22 μm hydrophilic membrane Durapore (poly(vinylidene difluoride), PVDF) (Millisul, Brazil). The adsorption capacity was determined according to eq 1

2.5.1. 1

where q is the adsorption capacity in mg g–1; Ci and Ce are the initial and equilibrium concentrations of parabens in solution, respectively; v is the solution volume in liters; and m is the adsorbent mass in grams.

The material that showed better performance in the initial adsorption tests was selected for further adsorption experiments, but first, the point of zero charge (pHPZC) was determined to select the most appropriate pH for the subsequent tests. pHPZC was determined for the FA-AT7 material by the pH drift method.27−29 A mass of 0.05 g was placed in contact with 20 mL of NaCl solutions (0.1 mol L–1), previously adjusted to the pH range from 2 to 12 for which was used 1 mol L–1 HCl or 1 mol L–1 NaOH solution was used. The solutions were stirred for 24 h at 200 rpm. After that, the pH of the final solutions was monitored, and the results were plotted as ΔpH versus pHi. The pHpzc was determined by taking into account the point at which the ΔpH was equal to zero.

Considering the pHpzc of the material as well as pKa of parabens, pH values ranging from 3 to 7 were studied in the adsorption test in batch, which was performed by adding 10 mg of adsorbent in 40 mL of multicomponent paraben solution at 10 mg L–1 concentration under 300 rpm agitation for 24 h. According to the results, the pH value was set to the experimental design tests.

2.5.2. Experimental Design and Adsorption Tests

The experimental screening design (STATGRAPHICS Centurion, StatPoint technologies, Inc., The Plains, VA) consisted of 16 + 2 (central points) experimental trials was developed. To determine the factors that influence the removal of parabens by the selected material and to investigate the interaction effects of various parameters, three factors were varied at two levels (23) (Table 2). The variables studied were initial concentration, agitation, and adsorbent mass. The adsorption capacity was used as a response variable. For this, adsorption experiments were performed in batch using the FA-AT7 adsorbent at pH 3 for 24 h.

Table 2. Levels of the Studied Factors in the Experimental Design.
    levels
factors symbols –1 0 +1
adsorbent mass (mg) A 2 6 10
initial concentration (mg L–1) B 5 10 15
agitation rate (rpm) C 150 225 300
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Based on this study and considering the concentration at which the parabens are found in the aquatic environment, the values of the parameters adsorbent mass, initial concentration of the analytes, and agitation were fixed for the kinetics and equilibrium studies.

2.5.3. Kinetics and Equilibrium Studies

Adsorption kinetics studies were carried out in a batch system using a 40 mL multicomponent paraben solution (5 mg L–1), 2 mg adsorbent mass, pH 3.0, and agitation at 300 rpm for 30 min at room temperature (28 ± 2 °C). Supernatants of the solutions were collected from the liquid phase at predetermined time intervals, and the analyte concentrations over time were verified.

The same conditions were used in equilibrium tests, where concentrations of multicomponent parabens varied from 1 to 20 mg L–1, and samples were kept in contact with the adsorbent for 30 min.

Equilibrium adsorption isotherm curves, which show the relationship between the solid-phase concentration “qe” of the adsorbed solute (mg g–1) and its concentration (Ce) in the liquid phase (mg L–1), were built according to the Langmuir (eq 2) and Freundlich (eq 3) equations.

2.5.3. 2
2.5.3. 3

where Ce and qe were previously defined; qmax is the monolayer capacity of the adsorbent (mg g–1); KL is the Langmuir adsorption constant, which is related to the energy of adsorption (L mg–1); and Kf (L mg–1) and n are the Freundlich adsorption isotherm constants related to the saturation capacity and intensity of adsorption, respectively.

2.6. HPLC/DAD Analyses

Quantitative analyses were performed using a liquid chromatograph (HPLC) (Shimadzu, 20A prominence) with a UV-DAD detector (SPD-M20A) and column-C18 (Hichrom5), with 250 × 4.6 mm2 i.d, and 5 μm particle size. MeOH/acetic acid 0.1% as mobile phase was carried out in an isocratic gradient (70:30),30 respectively, for 15 min and 1.0 mL flow. The column temperature was kept at 35°C. The injection volume was 20 μL and detection, UV, at 256 nm.

3. Results and Discussion

3.1. Sample Characterization

3.1.1. XRF Analysis

The chemical compositions of fly ashes without treatment (FA) and after acid treatment (FA-AT) are listed in Table 3. Fly ashes present diversified physical, chemical, and mineralogical characteristics. This variability is mainly related to the industrial process by which they were obtained.9

Table 3. Content of the Oxides Based on XRF Analysis (m/m%).
sample SiO2 Fe2O3 Al2O3 K2O CaO TiO2 Na2O other
FA 33.07 33.61 19.81 4.99 4.46 1.75   2.31
FA-AT 57.40 21.72 9.07 4.17 2.94 2.96   1.74
FA-AT13 51.23 8.99 27.08 3.37     7.96 1.36
FA-AT11 75.87 6.76 11.33 4.16       1.88
FA-AT9 94.27 0.90 3.04 1.13       0.66
FA-AT7 91.76 1.10 5.67 1.11       0.36
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It can be observed in Table 3 that the FA chemical compositions are mainly due to the presence of oxides of silicon, iron, aluminum, potassium, calcium, and titanium. Different oxides, such as SrO and Y2O3, are also detected; however, they are found at low concentrations (below 1%). The acid treatment reduced the presence of aluminum, iron, and calcium oxides, which results in an increase in silica by 24.33%. Yilmaz and Mermer10 studied the synthesis of MCM-41 from fly ash, and according to them, a higher Si/Al ratio is related to the best ability to form mesoporous silica. In the present study, the Si/Al ratios were 1.47 and 5.59 for FA and FA-AT, respectively, suggesting that the FA-AT material presents a higher mesoporous silica formation potential.

Table 1 shows the XRF analysis results for synthesized materials. According to the Table, the FA-AT9 and FA-AT7 samples showed higher SiO2 content (above 90%) compared to others (below 80%). Considerable fractions of Fe2O3, Al2O3, and K2O composed the FA-AT11 material (about 22%). In the FA-AT13 material, a significant presence of Fe2O3, Al2O3, K2O (about 40%), and Na2O (7.96%) was noted. Other components are around or below 1% by mass. Chao23 studied the pH effect on the MCM-41 synthesis process from fly ash, and they found greater incorporation of metallic ions into the structure of the synthesized material at high pH, which was observed in the present study for FA-AT13 and FA-AT11 materials.

3.1.2. XRD Analysis

Figure 1 shows high-angle X-ray diffractograms of FA and synthesized samples. According to FA XRD patterns (Figure 1a), the identified crystalline phases were mainly quartz, hematite, and magnetite.

Figure 1.

Figure 1

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High-angle X-ray diffractograms of (a) FA, (b) FA-AT13, (c) FA-AT11, (d) FA-AT9, and (e) FA-AT7.

It is observed that in the samples FA-AT11, FA-AT9, and FA-AT7 (Figure 1c–e), the XRD pattern presented amorphous phase silica characterized by a low-intensity signal at around 26°.24 However, crystalline phases were identified in the FA-AT13 sample (Figure 1b), indicating the formation of a mixture of zeolites X and Y.

The disappearance of the identified crystalline phases observed in FA or even the formation of new phases in the case of zeolite synthesis suggests that the silica in its natural crystalline form in FA has reacted with NaOH during the silicon extraction step, giving rise to soluble sodium silicate and subsequent formation of new materials.31 According to Chang et al.,32 an eventual absence of formation of the MCM mesoporous material can be explained to be due to the high concentration of sodium ions. In spite of the high content of sodium ions present in the fly ash supernatant, the formation of Al-MCM-41 and MCM-41 type materials can be worked out by means of controlled pH conditions.31,33−35 Thus, mesoporous silica synthesis is highly dependent on the pH of the reaction mixture.35 In the present work, the FA-AT13 sample showed the formation of a crystalline mixture of zeolites X and Y instead of the formation of mesoporous silica. This phenomenon is possibly due to reaction conditions, such as high amounts of sodium ions, which above a key concentration may act as a structure-directing agent,34 and the high pH value of the reaction mixture.

To investigate the existence of an ordered arrangement compatible to mesoporous structures, low-angle X-ray analyses were performed for the samples of FA-AT11, FA-AT9, and FA-AT7 materials (Figure 2). An intense peak (100) was observed for all three samples, which is indicative of the presence of MCM-41 type mesoporous silica.24,35 The highest intensity peak can be observed for samples FA-AT9 and FA-AT7 and might be interpreted as an indication of the higher ordering of the mesoporous channels.34,35 However, the diffraction peaks (110) and (200), which are characteristic of the formation of the ordered hexagonal pore structure, were not observed.10,35

Figure 2.

Figure 2

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Low-angle X-ray diffractograms of FA-AT11, FA-AT9, and FA-AT7.

3.1.3. IR Analysis

The FTIR spectroscopy technique has been used extensively for the identification of the functional group of nanomaterials. The broad band at around 3500 cm–1 may be attributed to surface silanol groups and physically adsorbed water molecules,23,24,35 while deformational vibrations of adsorbed water molecules caused the absorption bands at 1636 cm–1.23,35,36 The bands between 460 and 1250 cm–1 are assigned to framework vibrations of mesoporous silica. The asymmetric and symmetric stretching vibration bands of framework Si-O-Si appearing at 1082 and 800 cm–1, respectively, are assigned to the porous silica material.23,36

In the spectra of the FA-AT13 sample (Figure 3a), it is possible to see two signals consistent with the faujasite zeolite group, a band at 670 cm–1, which is related to symmetric stretching due to internal vibrations and a band at 560 cm–1 associated with the double-six-ring (D6R).37,38

Figure 3.

Figure 3

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FTIR spectra of (a) FA-AT13, (b) FA-AT11, (c) FA-AT9, and (d) FA-AT7 materials.

3.1.4. Scanning Electron Microscopy Analysis (SEM)

SEM images of FA and the synthesized materials are shown in Figure 4. In the micrographs of the fly ash sample (Figure 4a,b), it is possible to observe that the ashes are formed in the majority by hollow spheres, namely, cenospheres. A remarkable structural change in the materials is noticed after modification (Figure 4c–f). Other researchers who studied coal fly ash found similar morphologies.9,31,34,39 In the FA-AT13 micrographs (Figure 4c), the formation of crystalline structures has been identified, which were previously identified by XRD analysis as a mixture of X and Y zeolite phases. In the inset (Figure 4c), the high-resolution image shows that there are structures with needle-like morphologies and others with faceted crystals that may possibly be the two phases of the zeolites identified in XRD analysis. On the other hand, FA-AT11, FA-AT9, and FA-AT7 present particles of smaller size and agglomerates typical of mesoporous silica (Figure 4d–f).10

Figure 4.

Figure 4

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SEM images of (a, b) FA, (c) FA-AT13, (d) FA-AT11, (e) FA-AT9, and (f) FA-AT7. The yellow inset in (4c) corresponds to the high-resolution region of the image, and the scale bar value is 4 μm.

3.1.5. Transmission Electron Microscopy Analysis (TEM)

By means of the TEM technique, it was possible to carry out larger increases for the materials FA-AT11, FA-AT9, and FA-AT7 in more specific regions to elucidate the morphology of the materials. Figure 5 shows TEM images for FA-AT11, FA-AT9, and FA-AT7 materials.

Figure 5.

Figure 5

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TEM images for (a, d) FA-AT11, (b, e) FA-AT9, and (c, f) FA-AT7 materials.

The TEM images show that all materials have numerous pores of different sizes on the surface, corroborating with the data presented in the surface area technique. Although the XRD data at a low angle indicated that all samples have some arrangement, indicated by the presence of the main diffraction peak assigned to the plane (100) of the MCM-41 structure, the TEM images indicated ordering only in samples FA-AT9 and FA-AT7, Figure 5e,f, respectively. The cause may be the treatment at alkaline pH for this class of materials, which is found to be less aggressive when compared to the one performed for samples FA-AT11 and FA-AT13 because the latter presented the formation of zeolitic structures. It is known that SiO2 structures in the presence of high basic media can undergo structural collapse and can interfere in the hydrolysis of the TEOS reaction, in addition to the fact that the surfactant micelles in strongly alkaline media can lose shape/morphology and thus lead to the formation of disordered structures.40

3.1.6. N2 Adsorption–Desorption Isotherm Analysis

N2 adsorption–desorption isotherms and pore size distribution (PSD) curves of the synthesized samples are shown in Figure 6. The isotherms for FA-AT13 (Figure 6a) are classified as I type, related to microporous materials with a relatively small surface area and pores with narrow openings (pore width <1 nm) such as zeolite materials.41 In the PSD curve (Figure 6b), the peak below 10 Å confirms the existence of micropores.

Figure 6.

Figure 6

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N2 adsorption–desorption isotherms at 77 K and pore size distribution curves for (a, b) FA-AT13, (c, d) FA-AT11, (e, f) FA-AT9, and (g, h) FA-AT7 materials.

The isotherms for FA-AT11, FA-AT9, and FA-AT7 materials exhibit hysteresis loops demonstrating the presence of mesopores (Figure 6c,e,g), which can be classified as IV type.41 The pore size distribution (Figure 6d,f,h) shows, for the three samples, peaks centered in the range of 30–52 Å, showing the presence of mesopores in both materials. In the FA-AT7 sample, two peaks are identified, suggesting the presence of bimodal mesopores with two types of pore sizes.24 Samples FA-AT11 and FA-AT9 presented narrower pore size distributions.

The BET surface area and the pore volume of the samples ranged from 106 to 396 m2 g–1 and from 0.150 to 0.620 cm3 g–1, respectively, as listed in Table 4. The FA-AT7 material showed the largest BET surface area (SBET) (396 m2 g–1) and pore volume (0.62 cm3 g–1). Misran et al.34 studied the nonhydrothermal synthesis of mesoporous materials at pHs 7, 10, and 13, and they found pH 7 as the optimal value for the occurrence of the condensation–polymerization process of the sodium silicate precursor.

Table 4. Parameters of the Porous Structure Calculated from Nitrogen Adsorption Isotherms.
material SBET (m2 g–1) total pore volume (cm3 g–1) average pore diameter (Å)
FA-AT13 186 0.15 4.2
FA-AT11 106 0.29 52.9
FA-AT9 109 0.31 50.9
FA-AT7 396 0.62 50.9
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Surface areas with higher values have been reported in the literature,23,31,33−35 which may be related to the presence of the silica. High-purity silica sources are more reactive and have higher amounts of silicate anions present in the process, which may imply condensation and hydrolysis of these anions, resulting in a larger surface area.34

3.2. Paraben Adsorption

3.2.1. Adsorption Tests

Adsorption capacities of the synthesized materials for the homologous parabens (e.g., methylparaben, ethylparaben, propylparaben, and butylparaben) are shown in Figure 7. It can be seen that the adsorption efficiency of the materials follows the following order: butylparaben FA-AT7 > FA-AT9 > FA-AT11 > FA-AT13; propylparaben FA-AT7 > FA-AT9 ≥ FA-AT11 > FA-AT13; ethylparaben FA-AT13 > FA-AT7; methylparaben FA-AT13 > FA-AT7.

Figure 7.

Figure 7

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Initial adsorption tests for synthesized materials. Conditions: volume (40 mL), concentration (10 mg L–1), adsorbent mass (150 mg), temperature (28 ± 2 °C), pH (5.0), and agitation (300 rpm).

Thus, it is noted that FA-AT13 and FA-AT7 are able to remove all of the parabens studied, which differ from FA-AT11 and FA-AT9 materials that do not have the adsorptive capacity for methylparaben and ethylparaben compounds. However, this was only an initial test without adjusting the ideal conditions, so a better understanding of the process was achieved in the following stages of the experimental design.

Although FA-AT13 has a greater ability of adsorption for methylparaben and ethylparaben, however, its overall performance is surpassed by FA-AT7, which has adsorption capacities of 0.01, 0.07, 0.54, and 1.31 mg g–1 for methylparaben, ethylparaben, propylparaben, and butylparaben, respectively. It is likely that the higher adsorptive capacity of this material compared to the others can be due to its characteristics (Table 4), such as the specific surface area, pore specific volume, pore size distribution, and the adsorbate nature.42,43 This material was therefore chosen for subsequent adsorption studies.

The treatment carried out with FA-AT7 provided the largest BET surface area (SBET) (396 m2 g–1) and pore volume (0.52 cm3 g–1) and probably increased its hydrophobicity; as a result, its adsorption capacity increases as the paraben hydrophobicity increases. According to Cooney et al,42 the lower the solute solubility, the better the adsorption. Also, an increase in the molecular size of the substituent group usually enhances adsorption, especially for a homologous series of parabens. There is, in fact, a generalization, known as Traube’s rule, which states that “the adsorption of organic substances from aqueous solutions increases strongly and regularly as we ascend the homologous series”.42 Traube’s rule in fact had originally been applied to the adsorption of solutes from a polar solvent on a relatively nonpolar solid. A similar reversal of Traube’s rule was observed in the case of adsorption from a relatively nonpolar solvent by a polar solid. Examples are the short-chain fatty acids (heptanoic acid) that adsorb on the calcite surface to a lesser extent.44

3.2.2. Point of Zero Charge (pHPZC)

The solution pH may affect the degree of ionization, functional group dissociation, adsorbate structure, and adsorbent surface functionality.45 The pHpcz determination of an adsorbent allows us to identify trends in the variations of surface charge as a function of pH.46,47 pHpcz around 7.0 was found for FA-AT7 (Figure 8a), indicating the presence of a positive surface charge in solutions whose pH is below this value and a negative surface charge at a higher pH.48 Nairi et al.49 studied pHpcz for mesoporous silicas MCM-41 and SBA-15 types and found pHpcz to be around 6.0.

Figure 8.

Figure 8

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(a) FA-AT7 pHpcz results and (b) adsorption tests at varying initial pH.

The adsorption performance of FA-AT7, according to the pH, is shown in Figure 8b. At pH values 3, 5, and 7, an increase in the adsorption capacity with the increase of the paraben molecular size (or hydrophobicity) can be observed. On the other hand, there are no significant variations in the paraben adsorption at pH values 3, 5, and 7, and this is due to the pKa values of these compounds (Table 1). Since the parabens are majorly in the neutral form in the studied pH range, which means that there are no opposing charges, the electrostatic forces are not predominant in the adsorption mechanism. Hydrophobic forces and/or hydrogen bonds might be the principal adsorption mechanisms,50,51 taking into consideration the paraben proprieties studied (Table 1), mainly low water solubility and log Kow, since there is an increase in the adsorption capacity with the increase of the paraben carbon chain. At pH 3, the adsorption capacity was higher; thus, this pH was chosen for the later studies. At pH values 5 and 7 (values generally found in surface water),52−56 the material presented the adsorptive capacity approximate to the results identified at pH 3, indicating no relevant interference of this parameter in the studied range, which would facilitate the application of the adsorbent for resolving practical environmental problems.

3.2.3. Factorial Experimental Design

The factorial design methodology, unlike experiments that vary one factor at a time, is an excellent tool for the individual study, as well as for the interaction effects of all parameters simultaneously, since the variables can influence each other and the ideal value for one of these variables may depend on the values of other variables. This interaction between variables is a recurrent phenomenon.48

The studied factors were as follows: (A) adsorbent mass, (B) initial paraben concentration, and (C) agitation rate. The FA-AT7 adsorption capacity was used as a response. Standardized Pareto charts are shown in Figure 9.

Figure 9.

Figure 9

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Standardized Pareto charts showing the main effects of experimental parameters on responses at the confidence limit of 95% for (a) methylparaben, (b) ethylparaben, (c) propylparaben, and (d) butylparaben.

The factor A showed a significant negative effect. At higher mass dosage, the probability of collisions between the adsorbent particles is higher, generating aggregates, which might reduce the total surface area and cause difficulty in the diffusion of the adsorbates into the material surface.57 On the other hand, the factor B showed a positive effect for all studied parabens, which might be related to the increase in the driving force in the aqueous phase, increasing the rate of diffusion.58 It is worth noting that A–B interaction factors showed a negative effect, which means that although the initial concentration parameter has an individual positive effect, the increase of the two factors from the lower to the higher level reduced the adsorption capacity of the material.

The factor C showed a positive effect for methyl and ethylparabens, indicating that the increase of the agitation speed favored the interactions between the adsorbate and adsorbent, possibly due to its contribution in the process of mass transfer.43 While the interaction effect of agitation with the adsorbent mass (AC) is negative, the interaction of the same variable with the initial concentration of the analytes (BC) is positive, only for methylparaben. These results can be justified by the strong effect of adsorbent mass and initial concentration factors, indicating a considerable influence on their interaction.

3.2.4. Adsorption Kinetics

Based on the factorial experimental design results, for the kinetics and equilibrium tests, the adsorbent mass and agitation conditions were set at 2 mg and 300 rpm, respectively. Regarding the parabens’ initial concentration, the value used was 5 mg L–1. This concentration was selected considering the levels of parabens detected in the aquatic environment1,4 and the HPLC system used in quantification.

The equilibrium time for multicomponent adsorption of parabens onto the FA-AT7 adsorbent is shown in Figure 10. The paraben adsorption process in the mesoporous silica investigated is fast, with an equilibrium time reached in approximately 5 to 10 min.

Figure 10.

Figure 10

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Time of equilibrium for multicomponent adsorption of parabens onto the FA-AT7 adsorbent. Conditions: 5 mg L–1, pH 3.0, stirring rate: 300 rpm, mass adsorbent: 2.0 mg, and temperature (28 ± 2 °C).

Barczak et al.59 found fast adsorption; only a few minutes were needed to approach 90% of equilibrium values for diclofenac onto mesoporous silica. These authors associated their results with the medium pore size of the materials and, consequently, facilitated the diffusion of the molecules inside them.

3.2.5. Adsorption Equilibrium

It can be seen in Figure 11 that the experimental adsorption isotherm is linear (type-III), indicating that the mass of adsorbate retained per unit adsorbent mass is proportional to the adsorbate equilibrium concentration in the fluid phase.

Figure 11.

Figure 11

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Experimental and theoretical isotherms of (a) methylparaben, (b) ethylparaben, (c) propylparaben, and (d) butylparaben onto the FA-AT7 adsorbent in the multicomponent system. Conditions: 5 mg L–1, pH 3.0, stirring rate: 300 rpm, mass adsorbent: 2.0 mg, and temperature (28 ± 2 °C).

The adjustment using the Langmuir model is not adequate to estimate the Qmáx, which is evidenced by the high error values obtained (Table 5). The Freundlich model was better adjusted to the experimental data. KF and 1/n values indicated the following order: methylparaben < ethylparaben < propylparaben < butylparaben, which indicates a higher material surface heterogeneity effect for methylparaben (higher “n” value), leading to a strong interaction between the adsorbate and adsorbent. 1/n values above 1 indicate cooperative adsorption, which means that interactions between species of adsorbates are present in the system.60,61 Other studies also found a better fit using the Freundlich model in mesoporous materials.45,62,63

Table 5. Parameters of the Models for Adsorption Isotherms.
    methylparaben ethylparaben propylparaben butylparaben
Langmuir Qmax (mg g–1)        
KL (L mg–1) 0.0034 0.0017 0.0024 0.0017
R2 0.9926 0.9985 0.9966 0.9876
SQE 1.1243 1.0197 12.6321 225.4440
HYBRID 0.3726 0.1576 0.4509 6.2133
Freundlich KF (mg g–1 (mg L–1)−1/n) 0.8071 1.6329 4.0535 10.2246
1/n 0.9653 1.0100 1.0430 1.1297
R2 0.9934 0.9990 0.9986 0.9962
SQE 1.0064 0.6613 5.2004 68.7664
HYBRID 0.3356 0.1763 0.3001 1.3138
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4. Conclusions

The pH effect played a role in the mesoporous silica synthesis process; at pH 7, a more ordered structure and a higher surface area and pore volume were found, leading to an adsorbent with good efficiency to adsorb paraben compounds, and at pH 13, the formation of microporous crystalline structures such as X- and Y-zeolites is favored. Fly ash acid treatment prior to the syntheses was important to reduce metallic impurities in fly ash. The adsorption capacity of mesoporous silica increased with the molecular mass of the parabens; thus, propyl and butyl parabens were adsorbed more than others, probably due to their higher hydrophobicity.

Acknowledgments

The authors would like to thank the Laboratório de Pesquisa em Adsorção e Captura de CO2 (LPA) and professors Diana Azevedo and Moises Bastos Neto for nitrogen adsorption analysis. The authors are also grateful to Laboratório de Raios-X and Professor José Marcos Sasaki for XRD and XRF analyses and Central Analítica at Federal University of Ceará for the energy-dispersive X-ray spectroscopy (EDX) and MeV analysis (UFC/CT-INFRA/MCTI-SISNANO/Pró-equipamentos-CAPES). C.B.V. would like to thank CAPES (Coordination of Higher Level Personal Improvement) for the PNPD scholarship.

The authors declare no competing financial interest.

References

  1. Haman C.; Dauchy X.; Rosin C.; Munoz J.-F. Occurrence, fate and behavior of parabens in aquatic environments: A review. Water Res. 2015, 68, 1–11. 10.1016/j.watres.2014.09.030. [DOI] [PubMed] [Google Scholar]
  2. Nowak K.; Ratajczak–Wrona W.; Górska M.; Jabłońska E. Parabens and their effects on the endocrine system. Mol. Cell Endocrinol. 2018, 474, 238–251. 10.1016/j.mce.2018.03.014. [DOI] [PubMed] [Google Scholar]
  3. Shen X.; Liang J.; Zheng L.; Wang H.; Wang Z.; Ji Q.; Chen Q.; Lv Q. Ultrasound-assisted dispersive liquid-liquid microextraction followed by gas chromatography–mass spectrometry for determination of parabens in human breast tumor and peripheral adipose tissue. J. Chromatogr. B 2018, 1096, 48–55. 10.1016/j.jchromb.2018.08.004. [DOI] [PubMed] [Google Scholar]
  4. Błędzka D.; Gromadzińska J.; Wąsowicz W. Review: Parabens. From environmental studies to human health. Environ. Int. 2014, 67, 27–42. 10.1016/j.envint.2014.02.007. [DOI] [PubMed] [Google Scholar]
  5. Galinaro C. A.; Pereira F. M.; Vieira E. M. Determination of Parabens in Surface Water from Mogi Guaçu River (São Paulo, Brazil) Using Dispersive Liquid-Liquid Microextraction Based on Low Density Solvent and LC-DAD. J. Braz. Chem. Soc. 2015, 26, 2205–2213. 10.5935/0103-5053.20150206. [DOI] [Google Scholar]
  6. Chen H.; Chiou C.; Chang S. Comparison of methylparaben, ethylparaben and propylparaben adsorption onto magnetic nanoparticles with phenyl group. Powder Technol. 2017, 311, 426–431. 10.1016/j.powtec.2017.01.060. [DOI] [Google Scholar]
  7. Sophia A. C.; Lima E. C. Removal of emerging contaminants from the environment by adsorption. Ecotoxicol. Environ. Saf. 2018, 150, 1–17. 10.1016/j.ecoenv.2017.12.026. [DOI] [PubMed] [Google Scholar]
  8. Ahmaruzzaman M. A review on the utilization of fly ash. Prog. Energy Combust. Sci. 2010, 36, 327–363. 10.1016/j.pecs.2009.11.003. [DOI] [Google Scholar]
  9. Blissett R. S.; Rowson N. A. A review of the multi-component utilisation of coal fly ash. Fuel 2012, 97, 1–23. 10.1016/j.fuel.2012.03.024. [DOI] [Google Scholar]
  10. Yilmaz M. S.; Mermer N. K. Conversion of fly ashes from different regions to mesoporous silica: effect of the mineralogical composition. J. Sol-Gel Sci. Technol. 2016, 78, 239–247. 10.1007/s10971-016-3963-x. [DOI] [Google Scholar]
  11. Silva L. S.; Raulino G. S. C.; Vidal C. B.; Pires M. J. R.; Nascimento R. F. Peculiar properties of LTA/FAU synthetic composite zeolite and its effect on Cu2+ adsorption: factorial experimental design. Desalin. Water Treat. 2018, 107, 223–231. 10.5004/dwt.2018.22165. [DOI] [Google Scholar]
  12. Kresge C. T.; Leonowicz M. E.; Roth W. J.; Vartuli J. C.; Beck J. S. Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 1992, 359, 710–712. 10.1038/359710a0. [DOI] [Google Scholar]
  13. Beck J. S.; Vartuli J. C.; Roth W. J.; Leonowicz M. E.; Kresge C. T.; Schmitt K. D.; Chu C. T.; Olson D. H.; Sheppard E. W.; Mccullen S. B.; Higgins J. B.; Schlenker J. L. A new family of mesoporous molecular sieves prepared with liquid crystal templates. J. Am. Chem. Soc. 1992, 114, 10834–10843. 10.1021/ja00053a020. [DOI] [Google Scholar]
  14. Kumar S.; Malik M. M.; Purohit R. Synthesis Methods of Mesoporous Silica Materials. 5th International Conference of Materials Processing and Characterization (ICMPC 2016). Mater Today: Proc. 2017, 4, 350–357. 10.1016/j.matpr.2017.01.032. [DOI] [Google Scholar]
  15. Xu R.; Pang W.; Yu J.; Huo Q.; Chen J.. Chemistry of Zeolites and Related Porous Materials: Synthesis and Structure; John Wiley & Sons Pte Ltd: Asia, 2007. [Google Scholar]
  16. Braga R. M.; Teodoro N. M.; Aquino F. M.; Barros J. M. F.; Melo D. M. A.; Freitas J. C. O. Síntese da peneira molecular MCM-41 derivada da cinza da casca do arroz. Holos 2013, 5, 40–49. 10.15628/holos.2013.1364. [DOI] [Google Scholar]
  17. Schüth F. Surface Properties and Catalytic Performance of Novel Mesostructured Oxides. Bunsen-Ges. Phys. Chem. 1995, 99, 1306–1315. 10.1002/bbpc.199500076. [DOI] [Google Scholar]
  18. Chao Z.; Ruckenstein E. Effect of the Nature of the Templating Surfactant on the Synthesis and Structure of Mesoporous V-Mg-O. Langmuir 2002, 18, 734–743. 10.1021/la011391d. [DOI] [Google Scholar]
  19. Jewell C.; Prusakiewicz J. J.; Ackermann C.; Payne N. A.; Fate G.; Voorman R.; Williams F. M. Hydrolysis of a series of parabens by skin microsomes and cytosol from human and minipigs and in whole skin in short-term culture. Toxicol. Appl. Pharm. 2007, 225, 221–228. 10.1016/j.taap.2007.08.002. [DOI] [PubMed] [Google Scholar]
  20. Terasaki M.; Makino M.; Tatarazako N. Acute toxicity of parabens and their chlorinated by-products with Daphnia magna and Vibrio fischeri bioassays. J. Appl. Toxicol. 2009, 29, 242–247. 10.1002/jat.1402. [DOI] [PubMed] [Google Scholar]
  21. Soni M. G.; Garabin I. G.; Burdock G. A. Review: Safety assessment of esters of p-hydroxybenzoic acid (parabens). Food Chem. Toxicol. 2005, 43, 985–1015. 10.1016/j.fct.2005.01.020. [DOI] [PubMed] [Google Scholar]
  22. Nakanishi E. Y.; Villar-cociña E.; Santos S. F.; Rodrigues M. S.; Pinto P. S.; Savastano Junior H. Tratamentos térmico e químico para remoção de óxidos alcalinos de cinzas de capim elefante. Quim. Nova 2014, 37, 766–769. 10.5935/0100-4042.20140123. [DOI] [Google Scholar]
  23. Hui K. S.; Chao C. Y. H. Synthesis of MCM-41 from coal fly ash by a green approach: Influence of synthesis pH. J. Hazard. Mater. 2006, B137, 1135–1148. 10.1016/j.jhazmat.2006.03.050. [DOI] [PubMed] [Google Scholar]
  24. Zhou C.; Gao Q.; Luo W.; Zhou Q.; Wang H.; Yan C.; Duan P. Preparation, characterization and adsorption evaluation of spherical mesoporous Al-MCM-41 from coal fly ash. J. Taiwan Inst. Chem. E 2015, 52, 147–157. 10.1016/j.jtice.2015.02.014. [DOI] [Google Scholar]
  25. Okada K.; Yoshizaki H.; Kameshima Y.; Nakajima A. Effect of the crystallinity of kaolinite precursors on the properties of mesoporous silicas. Appl. Clay Sci. 2008, 41, 10–16. 10.1016/j.clay.2007.09.009. [DOI] [Google Scholar]
  26. Santos E. C.; Costa L. S.; Oliveira E. S.; Bessa R. A.; Freitas A. D. L.; Oliveira C. P.; Nascimento R. F.; Loiola A. R. Al-MCM-41 Synthesized from Kaolin via Hydrothermal Route: Structural Characterization and Use as an Efficient Adsorbent of Methylene Blue. J. Braz. Chem. Soc. 2018, 29, 2378–2386. 10.21577/0103-5053.20180115. [DOI] [Google Scholar]
  27. Hameed B. H.; Tan I. A. W.; Ahmad A. L. Adsorption isotherm, kinetic modeling and mechanism of 2,4,6-trichlorophenol on coconut husk-based activated carbon. Chem. Eng. J. 2008, 144, 235–244. 10.1016/j.cej.2008.01.028. [DOI] [Google Scholar]
  28. Postai D. L.; Demarchi C. A.; Zanatta F.; Melo D. C. C.; Rodrigues C. A. Adsorption of rhodamine B and methylene blue dyes using waste of seeds of Aleurites moluccana, a low cost adsorbent. Alexandria Eng. J. 2016, 55, 1713–1723. 10.1016/j.aej.2016.03.017. [DOI] [Google Scholar]
  29. Marković B. M.; Jankovic D. L.; Vukadinovic A. A.; Ranđelovic D. V.; Maksin D. D.; Spasojevic V. V.; Nastasovic A. B. A novel macroporous polymer–inorganic nanocomposite as a sorbent for pertechnetate ions. RSC Adv. 2017, 7, 21412–21421. 10.1039/C7RA02783D. [DOI] [Google Scholar]
  30. Gomes F. E. R.; Souza N. E.; Galinaro C. A.; Arriveti L. O. R.; Assis J. B.; Tremiliosi-Filho G. Electrochemical degradation of butyl paraben on platinum and glassy carbon electrodes. J. Electroanal. Chem. 2016, 769, 124–130. 10.1016/j.jelechem.2016.03.016. [DOI] [Google Scholar]
  31. Misran H.; Singh R.; Begum S.; Yarmo M. A. Processing of mesoporous silica materials (MCM-41) from coal fly ash. J. Mater. Process. Technol. 2007, 186, 8–13. 10.1016/j.jmatprotec.2006.10.032. [DOI] [Google Scholar]
  32. Chang H.; Chun C.; Aksay I. A.; Shih W. Conversion of Fly Ash into Mesoporous Aluminosilicate. Ind. Eng. Chem. Res. 1999, 38, 973–977. 10.1021/ie980275b. [DOI] [Google Scholar]
  33. Kumar P.; Mal N.; Oumi Y.; Yamana K.; Sano T. Mesoporous materials prepared using coal fly ash as the silicon and aluminium source. J. Mater. Chem. 2001, 11, 3285–3290. 10.1039/b104810b. [DOI] [Google Scholar]
  34. Misran H.; Singh R.; Yarmo A.; Kamarudin R. A. Non-hydrothermal synthesis of mesoporous materials using sodium silicate from coal fly ash. Mater. Chem. Phys. 2007, 101, 344–351. 10.1016/j.matchemphys.2006.06.007. [DOI] [Google Scholar]
  35. Majchrzak-Kucęba I.; Nowak W. Characterization of MCM-41 mesoporous materials derived from polish fly ashes. Int. J. Miner. Process. 2011, 101, 100–111. 10.1016/j.minpro.2011.09.002. [DOI] [Google Scholar]
  36. Romero A. A.; Alba M. D.; Zhou W.; Klinowski J. Synthesis and Characterization of the Mesoporous Silicate Molecular Sieve MCM-48. J. Phys. Chem. B 1997, 101, 5294–5300. 10.1021/jp970077i. [DOI] [Google Scholar]
  37. Visa M. Synthesis and characterization of newzeolite materials obtained fromfly ash for heavy metals removal in advanced wastewater treatment. Powder Technol. 2016, 294, 338–347. 10.1016/j.powtec.2016.02.019. [DOI] [Google Scholar]
  38. Sivalingam S.; Sen S. Optimization of synthesis parameters and characterization of coal fly ash derived microporous zeolite X. Appl. Surf. Sci. 2018, 455, 903–910. 10.1016/j.apsusc.2018.05.222. [DOI] [Google Scholar]
  39. Panek R.; Wdowin M.; Franus W.; Czarna D.; Stevens L. A.; Deng H.; Liu J.; Sun C.; Liu H.; Snape C. E. Fly ash-derived MCM-41 as a low-cost silica support for polyethyleneimine in post-combustion CO2 capture. J. CO2 Util. 2017, 22, 81–90. 10.1016/j.jcou.2017.09.015. [DOI] [Google Scholar]
  40. Mourhly A.; Khachani M.; Hamidi A. E.; Kacimi M.; Halim M.; Arsalane S. The Synthesis and Characterization of Low-cost Mesoporous Silica SiO2 from Local Pumice Rock. Nanomater. Nanotechnol. 2015, 5, 35 10.5772/62033. [DOI] [Google Scholar]
  41. Thommes M.; Kaneko K.; Neimark A. V.; Olivier J. P.; Rodriguez-Reinoso F.; Rouquerol J.; Sing K. S. W. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl. Chem. 2015, 87, 1051–1069. 10.1515/pac-2014-1117. [DOI] [Google Scholar]
  42. Cooney D. O.Adsorption Design for Wastewater Treatment; CRC Press: Boca Raton, Florida, 1999. [Google Scholar]
  43. Nascimento R. F.; Lima A. C. A.; Vidal C. B.; Melo D. Q.; Raulino G. S. C.. Adsorção: Aspectos Teóricos e Aplicações Ambientais; Editora UFC: Fortaleza, 2014. [Google Scholar]
  44. Gomari K. A. R.; Hamouda A. A. Effect of fatty acids, water composition and pH on the wettability alteration of calcite surface. J. Pet. Sci. Eng. 2006, 50, 140–150. 10.1016/j.petrol.2005.10.007. [DOI] [Google Scholar]
  45. Akpotu S. O.; Moodley B. Application of as-synthesised MCM-41 and MCM-41 wrapped with reduced graphene oxide/graphene oxide in the remediation of acetaminophen and aspirin from aqueous system. J. Environ. Manage. 2018, 209, 205–215. 10.1016/j.jenvman.2017.12.037. [DOI] [PubMed] [Google Scholar]
  46. Santos S. M. L.; Cecilia J. A.; Vilarrasa-García E.; Silva Junior I. J.; Rodríguez-Castellón E.; Azevedo D. C. S. The effect of structure modifying agents in the SBA-15 for its application in the biomolecules adsorption. Microporous Mesoporous Mater. 2016, 232, 53–64. 10.1016/j.micromeso.2016.06.004. [DOI] [Google Scholar]
  47. Santos S. M. L.; Cecilia J. A.; Vilarrasa-García E.; García-Sancho C.; Silva Junior I. J.; Rodríguez-Castellón E.; Azevedo D. C. S. Adsorption of biomolecules in porous silicas modified with zirconium. Effect of the textural properties and acidity. Microporous Mesoporous Mater. 2018, 260, 146–154. 10.1016/j.micromeso.2017.10.044. [DOI] [Google Scholar]
  48. Raulino G. S. C.; Silva L. S.; Vidal C. B.; Almeida E. S.; Melo D. Q.; Nascimento R. F. Role of surface chemistry and morphology in the reactive adsorption of metal ions on acid modified dry bean pods (Phaseolus vulgaris L.) organic polymers. J. Appl. Polym. Sci. 2018, 45879, 1–11. 10.1002/app.45879. [DOI] [Google Scholar]
  49. Nairi V.; Medda L.; Monduzzi M.; Salis A. Adsorption and release of ampicillin antibiotic from ordered mesoporous silica. J. Colloid Interface Sci. 2017, 497, 217–225. 10.1016/j.jcis.2017.03.021. [DOI] [PubMed] [Google Scholar]
  50. dos Santos S. M. L.; Nogueira K. A. B.; Gama M. S.; Lima J. D. F.; Silva Junior I. J.; Azevedo D. C. S. Synthesis and characterization of ordered mesoporous silica (SBA-15 and SBA-16) for adsorption of biomolecules. Microporous Mesoporous Mater. 2013, 180, 284–292. 10.1016/j.micromeso.2013.06.043. [DOI] [Google Scholar]
  51. Kim Y.; Bae J.; Park J.; Suh J.; Lee S.; Park H.; Choi H. Removal of 12 selected pharmaceuticals by granular mesoporous silica SBA-15 in aqueous phase. Chem. Eng. J. 2014, 256, 475–485. 10.1016/j.cej.2014.06.100. [DOI] [Google Scholar]
  52. Buarque P. M. C.; Firmino I. M.; Santos A. B.; Vidal C. B.; Buarque H. L.; Firmino P. M. Enhanced removal of emerging micropollutants by applying microaeration to an anaerobic reactor. Eng. Sanit. Ambiental 2019, 24, 667–673. 10.1590/s1413-4152201920190030. [DOI] [Google Scholar]
  53. Vidal C. B.; Santos A. B.; Nascimento R. F.; Bandosz T. J. Reactive adsorption of pharmaceuticals on tin oxide pillared montmorillonite: Effect of visible light exposure. Chem. Eng. J. 2015, 259, 865–875. 10.1016/j.cej.2014.07.079. [DOI] [Google Scholar]
  54. Vidal C. B.; Seredych M.; Rodríguez-Castellón E.; Nascimento R. F.; Bandosz T. J. Effect of nanoporous carbon surface chemistry on the removal of endocrine disruptors from water phase. J. Colloid Interface Sci. 2015, 449, 180–191. 10.1016/j.jcis.2014.11.034. [DOI] [PubMed] [Google Scholar]
  55. Pessoa G. P.; Souza N. C.; Vidal C. B.; Alves J. A. C.; Firmino I. M.; Nascimento R. F.; Santos A. B. Occurrence and removal of estrogens in Brazilian wastewater treatment plants. Sci. Total Environ. 2014, 490, 288–295. 10.1016/j.scitotenv.2014.05.008. [DOI] [PubMed] [Google Scholar]
  56. Vidal C. B.; Feitosa A. V.; Pessoa G. P.; Raulino G. S. C.; Oliveira A. G.; Santos A. B.; Nascimento R. F. Polymeric and silica sorbents on endocrine disruptors determination. Desalin. Water Treat. 2015, 54, 156–165. 10.1080/19443994.2014.880377. [DOI] [Google Scholar]
  57. Shao Y.; Wang X.; Kang Y.; Shu Y.; Sun Q.; Li L. Application of Mn/MCM-41 as an adsorbent to remove methyl blue from aqueous solution. J. Colloid Interface Sci. 2014, 429, 25–33. 10.1016/j.jcis.2014.05.004. [DOI] [PubMed] [Google Scholar]
  58. Melo D. Q.; Vidal C. B.; Medeiros T. C.; Raulino G. S. C.; Dervanoski A.; Pinheiro M. C.; Nascimento R. F. Biosorption of metal ions using a low cost modified adsorbent (Mauritia flexuosa): experimental design and mathematical modeling. Environ. Technol. 2016, 37, 2157–2171. 10.1080/09593330.2016.1144796. [DOI] [PubMed] [Google Scholar]
  59. Barczak M.; Wierzbicka M.; Borowski P. Sorption of diclofenac onto functionalized mesoporous silicas: Experimental and theoretical investigations. Microporous Mesoporous Mater. 2018, 264, 254–264. 10.1016/j.micromeso.2018.01.013. [DOI] [Google Scholar]
  60. Haghseresht F.; Lu G. Q. Adsorption Characteristics of Phenolic Compounds onto Coal-Reject-Derived Adsorbents. Energy Fuels 1998, 12, 1100–1107. 10.1021/ef9801165. [DOI] [Google Scholar]
  61. Foo K. Y.; Hameed B. H. Insights into the modeling of adsorption isotherm systems. Chem. Eng. J. 2010, 156, 2–10. 10.1016/j.cej.2009.09.013. [DOI] [Google Scholar]
  62. Ganiyu S. O.; Bispo C.; Bion N.; Ferreira P.; Batonneau-Gener I. Periodic Mesoporous Organosilicas as adsorbents for the organic pollutants removal in aqueous phase. Microporous Mesoporous Mater. 2014, 200, 117–123. 10.1016/j.micromeso.2014.07.047. [DOI] [Google Scholar]
  63. Wang X.; Wang H.; Lu M.; Teng R.; Du X. Facile synthesis of phenyl-modified magnetic graphene/mesoporous silica with hierarchical bridge-pore structure for efficient adsorption of pesticides. Mater. Chem. Phys. 2017, 198, 393–400. 10.1016/j.matchemphys.2016.12.017. [DOI] [Google Scholar]

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