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Surface modification of a natural zeolite by treatment with cold oxygen plasma: Characterization and application in water treatment
Accepted Manuscript Title: Surface modification of a natural zeolite by treatment with cold oxygen plasma: Characterization and application in water treatment Authors: Paola S. De Velasco-Maldonado, Virginia Hern´andez-Montoya, Miguel A. Montes-Mor´an, Norma Aurea-Rangel V´azquez, Ma. Ana P´erez-Cruz PII: DOI: Reference:
S0169-4332(17)33246-4 https://doi.org/10.1016/j.apsusc.2017.11.023 APSUSC 37605
To appear in:
APSUSC
Received date: Revised date: Accepted date:
18-7-2017 28-10-2017 4-11-2017
Please cite this article as: De Velasco-Maldonado PS, Hern´andez-Montoya V, Montes-Mor´an MA, V´azquez NA-R, P´erez-Cruz MA, Surface modification of a natural zeolite by treatment with cold oxygen plasma: Characterization and application in water treatment, Applied Surface Science (2010), https://doi.org/10.1016/j.apsusc.2017.11.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Surface modification of a natural zeolite by treatment with cold oxygen plasma: Characterization and application in water treatment
Paola S. De Velasco-Maldonadoa, Virginia Hernández-Montoyaa,*, Miguel A. MontesMoránb, Norma Aurea-Rangel Vázqueza, Ma. Ana Pérez-Cruzc
a
Instituto Tecnológico de Aguascalientes, Av. Adolfo López Mateos No. 1801 Ote. C.P.
20256, Aguascalientes, Ags., México. b
Instituto Nacional del Carbón, INCAR-CSIC, Apartado 73 E-33080, Oviedo, Spain
Graphical Abstract
O Al S
*
Corresponding author. Tel.: +52 449 9105002 Ext. 137 E-mail address: [email protected] (Virginia Hernández-Montoya)
18
Highlights
Natural mesoporous clinoptilolite was treated with cold oxygen plasma
The crystallinity and chemical functionally was conserved after plasma treatment
The roughness of clinoptilolite was slightly affected by treatment
The mass loss of the ablated layer increases with the exposure time
Removal of dyes and lead was similar in clinoptilolite with and without treatment
Abstract In the present work the possible surface modification of natural zeolite using cold oxygen plasma was studied. The sample with and without treatment was characterized using nitrogen adsorption isotherms at -196 °C, FT-IR spectroscopy, SEM/EDX analysis and XRay Diffraction. Additionally, the two samples were used for the removal of lead and acid, basic, reactive and food dyes in batch systems. The natural zeolite was found to be a mesoporous material with a low specific surface area (23 m2/g). X-ray patterns confirmed that clinoptilolite was the main crystal structure present in the natural zeolite. The molecular properties of dyes and the zeolitic structure were studied using molecular simulation, with the purpose to understand the adsorption mechanism. The results pointed out that only the roughness of the clinoptilolite was affected by the plasma treatment, whereas the specific surface area, chemical functionality and crystal structure remained constant. Finally, adsorption results confirmed that the plasma treatment had no significant effects on the dyes and lead retention capacities of the natural zeolite.
Key words: Adsorption, Clinoptilolite, Cold Oxygen Plasma, Dyes, Surface modification 19
1. Introduction Zeolites are crystalline hydrated aluminosilicates of Na, K and Ca (±Ba, ± Sr y ± Mg), that consist of a three-dimensional framework, having a negatively charge lattice. This charge is balanced by cations that can be exchangeable with cations present in aqueous solutions [1]. Natural zeolites are used as adsorbents of organic and inorganic compounds due to the high ion-exchange capacity and shape-selective structure that acts as molecular sieve [2]. Specifically, natural zeolites are used in many types of industrial applications such as catalysis, construction, environmental remediation and restoration, gas separation, gas and steam adsorption/separation, removal of heavy metals and radioactive elements, water purification and anaerobic digestion processes, among others [3-6]. According with recent reports, there are more than 40 mineral species of zeolites. Particularly in Mexico, clinoptilolite, erionite and mordenite are the principal species found in the states of Oaxaca, Sonora, San Luis Potosi, and Michoacán [7]. Moreover, clinoptilolite species are the most abundant natural zeolites, with a typical unit cell formula Na6[(AlO2)6 (SiO2)30]. 24 H2O [8-9]. Clinoptilolite has a characteristic tubular morphology that shows an open reticular structure of easy access, formed by open channels of 8-10 membered rings. Specifically, clinoptilolites have been used for the removal of textile dyes as black reactive 5, red 239 and yellow 176 [8], methylene blue [10] and acid blue 25, basic blue 9, basic violet 3 [11]. Also, they have been used for the removal of heavy metals, i.e., manganese, copper, lead, zinc, cadmium, and nickel [1, 11-13]. In order to increase the adsorption capacity of clinoptilolite in the removal of dyes, different humid methods have been used extensively for the modification of natural clinoptilolite. For example, it has been modified with quaternary amine and 20
hexadecyltrimethylammonium bromide [8, 14]. Also, it was treated with HCl, NaOH, NaCl and NH4NO3 to improve the ion-exchange capacity for heavy metals [12, 15]. However, these humid methods have some disadvantages. For example, the use of acid and water could change the suspension properties and cause the hydrolysis of zeolite due to their siliceous nature. In this context, dry methods would be preferred. Cold oxygen plasma treatments have been used for the surface modification of some carbonaceous materials such as: activated carbons obtained from coconut shells, bamboo, pecan nut shells and peach stones; graphites, meso-carbon microbeads, glassy carbons, carbon blacks and carbon fibers [16-19]. Specifically, the modification of carbonaceous materials with cold oxygen plasma is intended to increase the amount of oxygen functional groups on the surface of the materials. In this context, the principal advantages of this technique are: shorter reaction times, clean process, fast solvent-free technique, simplicity and easy to control. Only two works have been found to report the modification of natural zeolites using cold oxygen plasma to enhance the chromium adsorption capacity [20] and to remove the organic template from nanosized beta zeolite nanocrystals [21]. Results of the effect of the plasma treatment are very limited and not conclusive. Considering the information described above, the purpose of this work is to modify a Mexican natural zeolite with cold oxygen plasma, to identify any relevant changes of its structure and surface chemistry and to test its influence in the adsorption of water pollutants. The zeolites with and without modification were thus characterized using different analytical techniques. The treated and untreated zeolites were finally used for the removal of textile and food dyes such as: acid blue 25, acid blue 41, acid orange 7, food red 17, basic blue 9, indigo carmine, reactive
21
black 5, reactive blue 4 and lead from water. Also, the possible adsorption mechanism was studied using molecular simulation.
2. Materials and Methods 2.1. Zeolite and surface modification The natural zeolite used in this study was supplied by San Francisco mine located in the state of San Luis Potosi, Mexico. The zeolite was milled and sieved for the retained fraction in 35 mesh US standard sieves and thoroughly washed with deionized water until constant pH is reached (~7.7). Subsequently, the zeolite was dried at 110 °C during 24 h and stored for later use. Surface modification of the natural zeolite was carried out by oxidation with cold oxygen plasma. The oxidation was performed in an Emitech K1050X plasma reactor where the oxygen was excited using radiofrequency (RF) energy (13.56 MHz). The plasma was maintained at 1 mbar by flowing oxygen into the reaction chamber. A defined amount of zeolite (0.5 g) was placed in a petri dish and after, it was oxidized during 9 min using a RF power of 75 W. Specifically, six treatments with plasma were performed in each sample in order to attain a homogeneous oxidation of the material surface. The oxidation conditions were selected considering results reported previously [22]. The natural zeolite was denoted as N-C and the sample oxidized with plasma as P-C.
2.2. Characterization of the natural zeolite The textural properties of the samples N-C and P-C were obtained from nitrogen adsorption isotherms at -196 °C using an automatic Micromeritics ASAP 2020 analyzer. Prior to the measurements, the samples were outgassed overnight at 200 °C under vacuum for 8 h. 22
Different models were used to calculate the principal textural parameters from the experimental data of isotherms. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) model using the 0.05 to 0.3 relative pressure (p/p0) interval. The total pore volume was determined from the amount of nitrogen adsorbed at a relative pressure of ~0.99; the micropore volume was obtained by applying the DubininRaduskevich method and finally, the pore size distribution was obtained with the BJH method applied to the N2 isotherm desorption branch. In order to identify the crystal phases present in the zeolites, X-ray diffraction patterns were recorded in a Bruker D8 Advance diffractometer equipped with CuKα X-ray source operated at 40 kV and 40 mA. The morphology and surface composition was also investigated using a FE-SEM system (Quanta FEG 650, FEI) equipped with an EDX detector. The solid particles were dispersed on a graphite adhesive tab placed on an aluminum stub and no further coating was required. Additionally, FT-IR spectra of N-C and P-C were obtained with a Nicolet IS10 spectrometer (Thermo Scientific) with an attenuated total reflectance (ATR) detector to collect the IR spectra in the 4000-500 cm-1 spectral range. Finally, the ablation of N-C surface layers was determined by gravimetry using a Denver Instrument TP-214 microbalance. The mass loss of the removed layer was determined from the measured change in weight of 2 samples of N-C before and after of each cycle of treatment of 9 min (a total of 6 cycles).
2.3. Adsorption of dyes and lead in aqueous solution Adsorption studies of dyes and lead in aqueous solutions were made on both the zeolite modified with plasma (P-C) and the sample without modification (N-C). Two types of 23
adsorption studies were performed in batch systems: preliminary tests using a constant initial concentration and adsorption studies at different initial concentrations. Preliminary tests were performed in polycarbonate cylindrical cells with lid. A defined amount of adsorbent (N-C or P-C) was suspended in 10 ml of a 500 mg/L dye or lead solution (Pb(NO3)2.6H2O). The suspension was then mixed at 160 rpm for 72 h, until equilibrium was reached. All the experiments were conducted by duplicated at 30 °C, pH 5 and using an adsorbent mass to volume ratio of 2 g/L. The dyes selected for these preliminary assays were the acid blue 25 (AB25), acid orange 7 (AO7), food red 17 (FR17), basic blue 9 (BB9), basic blue 41 (BB41), acid blue 74 (AB74), reactive black 5 (RB5), reactive blue 4 (RB4). All dyes were used without further purification. Selected properties of those dyes are shown in Table 1. Optimization of the molecular geometries of some selected dyes was performed using the AMBER molecular force field implemented in the HyperChem 8.0 software. Also, the Quantitative Structure Activity Relationship (QSAR) properties were calculated and the data are reported in Table 2. In this context, with the purpose to understand the adsorption mechanism, the optimization of the zeolite structure was made at 30 °C (temperature of the adsorption process), using Monte Carlo simulation. The molecular electrostatic potentials were thus determined (See Table 3). In the second type of adsorption test the effect of dye and lead initial concentration was analyzed. The experimental conditions were the following: batch systems with constant agitation (160 rpm), equilibrium time of 72 h, temperature 30 °C, pH 5 and mass to volume ratio of 2 g/L. The initial concentrations of dyes and lead were 100, 400, 500 and 750 mg/L. Dye concentration was determined by UV-Vis spectrophotometry at the maximum absorbance of each dye (see Table 1) using a UV-Vis HACH DR-5000 spectrometer. 24
Concentration of lead was analyzed by atomic absorption spectroscopy employing a Perkin Elmer Analyst 100 spectrometer. The dye and lead concentration at equilibrium was calculated from the respective calibration curve. Finally, the amount of adsorbed species, q (mg g-1), was calculated using a mass balance relationship given by Eq. (1). 𝐶0 −𝐶𝑒
𝑞=(
𝑊
)𝑉
(1)
where 𝐶0 and 𝐶𝑒 are the initial and equilibrium dye or lead concentration, respectively (mg L-1), 𝑉 is the volume of the solution (L) and 𝑊 is the weight of the zeolite used (g).
3. Results and discussion 3.1 Characterization of the natural zeolites The zeolite modified with cold oxygen plasma (P-C) and the original zeolite (N-C) were characterized with the purpose to identify the possible changes induced by the plasma treatment. The Fig. 1 shows the XRD patterns of the natural zeolite (N-C). Reflections are consistent with the 00-039-1383-Clinptilolite-Ca-KNa2Ca2(Si29Al7)O72.24H2O pattern (JCPDS cards). These results were also confirmed by EDX semi-quantitative analysis of the N-C sample (Table 4). The principal components of the natural zeolite (clinoptilolite) are Si (44.8 %), O (40.9 %) and Al (7.9 %), with the following exchange cations in the proportions: K≈Ca>Na, Fe≈Mg. Fig. 1 and Table 4 also show the results corresponding to the plasma-treated clinoptilolite (P-C), which are virtually the same that those of the untreated zeolite. This observation would agree with the data reported in the literature for a natural clinoptilolite from Iran, which was treated with a glow discharge plasma using 25
different O2 pressures and no changes were observed between the XRD patterns of the sample with and without treatment [23]. Fig. 2 shows the adsorption isotherms of N2 at 77 K of the two samples. According to the IUPAC classification the isotherms are type IV, which are characteristic of mesoporous materials. Table 5 summarizes selected textural parameters of the clinoptilolite samples and is clear that the percentage of mesopores (84 and 86 %) is greater than micropore amount (16 and 14 %) for N-C and P-C, respectively. According with the IUPAC pore size classification, the micropores have a pore diameter ≤ 2 nm and the mesopores between 2 and 50 nm. This size can be delimited the adsorption of dyes with different molecular properties and lead (see section 3.2). In this context, it seems clear that the modification with cold oxygen plasma did not cause any significant change in the specific surface area (SBET) (23 vs 24 m2/g for N-C and P-C, respectively). However, the micropore volume of N-C decreased from 0.015 to 0.013 cm3/g after the plasma treatment. Consequently, the micropore percentage also decreased (see Table 5). On the other hand, SEM/EDX results (see Fig. 3) suggest some changes in roughness and cleaning of the surface after the cold oxygen plasma treatment (see Fig. 3d). These results agree with the data reported in the literature for the modification of mordenite with Ar plasma treatment [24]. Fig. 2b shows the FT-IR spectra of the two zeolite samples, prior and after the plasma treatment. As for the rest of techniques, the FT-IR of P-C and N-C are essentially the same. The most intense peak is observed at 1015 cm-1, which is characteristic of Si-O stretching vibration [25]. The peak at ~1630 cm-1 is also identified in the FT-IR spectra and according with the literature it is due to H-O-H bending vibration of adsorbed water on the zeolitic structure [26]. The broad band between 3000 and 3600 cm-1 typical of isolated O-H stretching vibration of water is present in the FT-IR spectra of both samples [27]. 26
In short, the plasma treatment has little effect on most of the chemical or structural properties of the clinoptilolite. It is worth to mention, however, that a significant weight loss (approx. 5 wt%) was observed after the plasma treatment of sample N-C, most likely due to the dehydration of the zeolitic structure. This weight loss was found to be a consequence of both the vacuum conditions of the reaction chamber and the heating/annealing effect of the plasma treatment. In this context, the ablation of N-C surface layers was determined by gravimetry after each plasma treatment cycle (9 min by cycle). The mass loss of N-C induced by the plasma treatment was determined by weighing and Fig. 4 shows the results. A clear behavior was observed, and the mass loss of the ablated layer increases with the cycles or the exposure time until 45 and 54 min, where the loss mass was practically constant (0.01095 g). This ablation behavior was observed for other type of materials modified with plasma treatment as biaxially oriented polyethylene naphthalate foils, where the ablated layer increases with exposure time until 53 μg approximately [28].
3.2 Adsorption of dyes and lead from aqueous solution The plasma treated and untreated zeolites (P-C and N-C, respectively) were used for the removal of dyes such as acid blue 25 (AB25), acid orange 7 (AO7), food red 17 (FR17), basic blue 9 (BB9), basic blue 41 (BB41), acid blue 74 (AB74), reactive black 5 (RB5) and reactive blue 4 (RB4), from water. Fig. 5 shows the adsorption results of N-C and P-C, using an initial concentration of dye of 500 mg/L. In general, for all dyes, the adsorbed amount is very similar for the two samples (P-C and N-C), indicating that the plasma treatment is not affecting the removal of dyes. This behavior is congruent with the characterization results because the plasma treatment is not affecting the chemical 27
functionality and/or structure of the clinoptilolite. However, it is very interesting to note that P-C and N-C are more efficient in the removal of cationic dyes as basic blue 9 (BB9) and basic blue 41 (BB41), showing an adsorption amount of 23 and 24 mg/g, respectively. In this context, it is relevant to mention that the differences between the area, volume and hydrophobicity of BB41 and BB9 have not a determinant effect in the adsorption capacity of clinoptilolite (See Table 2). Moreover, the adsorption mechanism of basic dyes on these zeolites can be related with the electronic density distribution of the material as depicted in the molecular electrostatic potential (MEP) in Table 3. This particular natural zeolite shows a high electronic density: red color means regions with more negative EPs, blue are regions where EPs are more positive and green corresponds to regions with zero potential. This behavior is congruent with the permanent negative charge of the zeolite/water interface, which is balanced by the exchangeable metal cation and it is due to the isomorphs substitution of some Si4+ from Al3+ within the clinoptilolite lattice [29]. In this context, according with the data reported in literature, at acidic values of pH (3-5.3) the silanol groups are protonated in such an insignificant proportion that the competition between H3O+ ions and the cationic dyes (BB9 and BB41) lessens [30]. This observation was simulated and the results are shown in Table 3. It seems evident that the excess of hydronium ions is not affecting significantly the electronic density of clinoptilolite and that the adsorption of cationic dyes can be illustrated by the dissociated groups (≡SiO-), following the mechanism proposed by [30]:
≡SiO- + Dye+ ↔ ≡SiO- Dye +
28
Where Dye+ is BB9 or BB41, which are cationic dyes with pKa values of 3.8 [31] and 2.64 [32], respectively. The two dyes would have a positive charge at pH 5 (value used in this work for the adsorption tests). The effect of dye initial concentration was also studied and the adsorption results are congruent with the data reported in the preliminary tests, i.e., the plasma treatment of clinoptilolite has not a significant effect in the removal of dyes (See Fig. 6a and 6b). In this case, the removal of BB9 and BB41 was very similar for N-C and P-C. Additionally, is relevant to mention that the adsorbed amount was very similar when a dye solution of 400, 500 or 750 mg/L of BB9 or BB41 was employed in the adsorption tests. Particularly, the adsorbed amount of BB9 was 28.3 mg/g and 24.5 for BB41, for an initial concentration of 750 mg/L. These values are comparable with the data reported in the literature for the removal of BB41 (19.57 mg/g) using volcanic tuff, which is constituted by analcite, clinoptilolite and heulandite [30], but for the removal of BB9 the adsorbed amount reported in this work is lower than the value cited in [11], where the authors were testing a natural clinoptilolite obtained from Sonora state, Mexico (~80 mg/g) [11]. However, a direct comparison between both results is not feasible because the experimental adsorption conditions and the particle size of adsorbents were different. Finally, considering the results found in the literature showing that the removal of lead is increased by the oxidation of carbonaceous materials using cold oxygen plasma [18, 22], the adsorption of lead using clinoptilolite prior and after the plasma treatment was also studied. Fig. 6c shows the adsorption results of lead from water using N-C and P-C. As in the case of dye removal, there are not significant differences in the amount of lead removed by both clinoptilolites. Nevertheless, the lead adsorbed amounts reported in this work using an initial concentration of 750 mg/L (67 mg/g) is reasonably similar to other capacities 29
reported in the literature for the removal of lead using a natural clinoptilolite from Turkey (79 mg/g), Ukraine (26 mg/g) and Romania (73.9 mg/g) [33-35], bearing in mind the unfeasible fair comparison between experiments, due to crucial differences in experimental conditions such as pH, temperature and mass to volume ratio.
4. Conclusions In the present work a systemic analysis was made about the changes in the structure, texture and chemical functionality of natural clinoptilolite after cold oxygen plasma treatment. The obtained results are determinant and conclusive in comparison with the limited reports of the literature. Specifically, the crystallinity and chemical functionally of natural clinoptilolite was conserved after plasma treatment and the roughness was slightly affected by treatment. However, a weight loss of 0.01095 g was observed after the plasma treatment due to the vacuum conditions of the reaction chamber and the heating/annealing effect of the plasma treatment. Also, the mass loss of the ablated layer increases with the exposure time and this is due to dehydration of the zeolitic structure. Finally, the adsorbed amounts of acid, basic, reactive and food dyes and lead from water were very similar between the clinoptilolite prior and after the plasma treatment. ACKNOLEDGEMENTS This work was supported by CONACyT (project AGS-2012-C02-198207) and PCTIAsturias/FEDER (EU) (GRUPIN14-117) project. Paola acknowledges the grant (447346) received from CONACYT.
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L. Mihaly-Cozmuta, A. Mihaly-Cozmuta, A. Peter, C. Nicula, H. Tutu, Dan Silipas, E. Indrea, Adsorption of heavy metal cations by Na-clinoptilolite: Equilibrium and selectivity studies, Journal of Environmental Management 137 (2014) 69-80.
35
Figure Captions Fig. 1 XRD patterns of N-C and P-C samples. Fig. 2 N2 Adsorption isotherms at -196 ºC (a) and FT-IR spectra (b) of the plasma treated clinoptilolite (P-C) and natural clinoptilolite (N-C). Fig. 3 SEM images of the natural clinoptilolite (a, b) and the clinoptilolite treated with plasma (c, d). Fig. 4 Dependence of the mass loss of N-C during cold oxygen plasma treatment with exposure time (6 cycles of 9 min). Fig. 5 Adsorption results of acid, basic and reactive dyes using as adsorbents the natural clinoptilolite (N-C) and the clinoptilolite with plasma treatment (P-C). Experimental conditions: pH 5, temperature 30 °C, mass to volume ratio 2 g/L and initial concentration of dyes of 500 mg/L Fig. 6 Adsorption results of basic dyes (a, b) and lead (c) using as adsorbent the natural clinoptilolite (N-C) and the clinoptilolite with plasma treatment (P-C) and different initial concentrations. Experimental conditions: pH 5, temperature 30 °C, mass to volume ratio 2 g/L and initial concentration of dyes and lead of 100, 400, 500 and 750 mg/L.
36
Fig. 1
% Clinoptilolite
%
4000 %
3000
5000
%
2000
%
% %
1000
%
%
4000
% %
3000 0 2000
Intensity (counts)
Intensity (counts)
Plasma:P-C
Natural: N-C
5000
1000 0 0
20
40
60
2º
37
Fig. 2
80
3
Adsorbed volume (cm /g)
P-C N-C
(a)
60
40
20
0 0.0
0.2
0.4 0.6 0.8 Relative pressure, p/p0
1.0
110 (b)
100
Transmittance (%)
90
3400
1630
80 70 60
N-C P-C
50 40
1015
30 20 4000
3500
3000
2500
2000
1500
1000
-1
Wavenumber, cm
38
Fig. 3
(a)
(b)
(c)
(d)
39
Fig. 4
Mass Loss(g)
0.012
0.008
0.004
0.000 0
10
20
30
40
50
60
Exposure time(min)
40
Fig. 5
N-C P-C
30
Adsorbed amount, mg/g
25 20 15 10 5 0
25 B41 B B A
7 AO
1 FR
9 74 BB AB
5 RB
4 RB
Dyes
41
100 NC-BB41 PC-BB41
80
(a)
60 40 20 0 0
Adsorbed amount, mg/g
100
100
200
300
400
500
600
700
NC-BB9 PC-BB9
80
800 (b)
60 40 20 0 0
100
200
300
400
500
600
700
800
100 NC- Lead PC- Lead
80
(c)
60 40 20 0 0
100
200
300
400
500
600
700
800
Initial Concentration, mg/L
Fig. 6
42
Table 1. Selected properties of the studied dyes Dye and code assigned Molecular formula
Molecular weight (g
λmax (nm)
mol-1) Acid blue 25 (AB25)
C20H13N2NaO5S
416.38
600
Acid orange 7 (AO7)
C16H11N2NaO4S
350.32
484
Food red 17 (FR17)
C18H16N2Na2O8S2
496.42
503
Basic blue 9 (BB9)
C16H18ClN3S
319.85
664
Basic blue 41 (BB41)
C20H26N4O6S2
482.57
607
Acid blue 74 (AB74)
C16H8N2Na2O8S2
466.34
610
Reactive black 5 (RB5)
C26H21N5Na4O19S6
991.82
597
Reactive blue 4 (RB4)
C23H14Cl2N6O8S2
637.2
595
43
Table 2. QSAR properties of basic dyes calculated by molecular simulation Dye
Area (Å2)
Volume (Å3)
Log P
694.90
1236
-0.45
515.64
873.52
1.32
AB41
BB9
44
Table 3. Molecular electrostatic potential (MEP) maps in 3D of KNa2Ca2(Si29Al7)O72.24H2O with excess of hydronium ions by Monte Carlo simulation at 30 °C Geometry of optimization
Distribution of nucleophilic and electrophilic zones
45
Table 4. Elemental composition of the N-C and P-C samples obtained by EDX analysis Sample
Wt (%) ±0.1 O
Na
Mg
Al
Si
K
Ca
Fe
Si/O
N-C
40.9
0.6
0.5
7.9
44.8
2.7
2.1
0.6
1.095
P-C
42.8
0.7
0.9
9.0
40.2
2.7
3.0
0.7
0.939
Table 5. Textural parameters of N-C and P-C samples obtained from the N2 adsorption isotherms at -196 ºC Sample and code Parameter
Natural clinoptilolite: N-C
Plasma oxidized clinoptilolite: P-C
a
23
24
Vt (cm³/g)
0.094
0.095
VD-R (cm³/g)
0.015
0.013
% Micr
0.079 16
0.081 14
% Mes
84
86
SBET (m²/g)
b c
d
Vmes (cm³/g)
e f
a SBET: BET surface area Vt: Total pore volume, c VDR: Dubinin–Radushkevich micropore volume d VMes= Vt− VDR e Microporous (%) = (VDR/Vt)100 f Mesoporous (%)= (VMes/Vt)100 b
46
S0169-4332(17)33246-4 https://doi.org/10.1016/j.apsusc.2017.11.023 APSUSC 37605
To appear in:
APSUSC
Received date: Revised date: Accepted date:
18-7-2017 28-10-2017 4-11-2017
Please cite this article as: De Velasco-Maldonado PS, Hern´andez-Montoya V, Montes-Mor´an MA, V´azquez NA-R, P´erez-Cruz MA, Surface modification of a natural zeolite by treatment with cold oxygen plasma: Characterization and application in water treatment, Applied Surface Science (2010), https://doi.org/10.1016/j.apsusc.2017.11.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Surface modification of a natural zeolite by treatment with cold oxygen plasma: Characterization and application in water treatment
Paola S. De Velasco-Maldonadoa, Virginia Hernández-Montoyaa,*, Miguel A. MontesMoránb, Norma Aurea-Rangel Vázqueza, Ma. Ana Pérez-Cruzc
a
Instituto Tecnológico de Aguascalientes, Av. Adolfo López Mateos No. 1801 Ote. C.P.
20256, Aguascalientes, Ags., México. b
Instituto Nacional del Carbón, INCAR-CSIC, Apartado 73 E-33080, Oviedo, Spain
Graphical Abstract
O Al S
*
Corresponding author. Tel.: +52 449 9105002 Ext. 137 E-mail address: [email protected] (Virginia Hernández-Montoya)
18
Highlights
Natural mesoporous clinoptilolite was treated with cold oxygen plasma
The crystallinity and chemical functionally was conserved after plasma treatment
The roughness of clinoptilolite was slightly affected by treatment
The mass loss of the ablated layer increases with the exposure time
Removal of dyes and lead was similar in clinoptilolite with and without treatment
Abstract In the present work the possible surface modification of natural zeolite using cold oxygen plasma was studied. The sample with and without treatment was characterized using nitrogen adsorption isotherms at -196 °C, FT-IR spectroscopy, SEM/EDX analysis and XRay Diffraction. Additionally, the two samples were used for the removal of lead and acid, basic, reactive and food dyes in batch systems. The natural zeolite was found to be a mesoporous material with a low specific surface area (23 m2/g). X-ray patterns confirmed that clinoptilolite was the main crystal structure present in the natural zeolite. The molecular properties of dyes and the zeolitic structure were studied using molecular simulation, with the purpose to understand the adsorption mechanism. The results pointed out that only the roughness of the clinoptilolite was affected by the plasma treatment, whereas the specific surface area, chemical functionality and crystal structure remained constant. Finally, adsorption results confirmed that the plasma treatment had no significant effects on the dyes and lead retention capacities of the natural zeolite.
Key words: Adsorption, Clinoptilolite, Cold Oxygen Plasma, Dyes, Surface modification 19
1. Introduction Zeolites are crystalline hydrated aluminosilicates of Na, K and Ca (±Ba, ± Sr y ± Mg), that consist of a three-dimensional framework, having a negatively charge lattice. This charge is balanced by cations that can be exchangeable with cations present in aqueous solutions [1]. Natural zeolites are used as adsorbents of organic and inorganic compounds due to the high ion-exchange capacity and shape-selective structure that acts as molecular sieve [2]. Specifically, natural zeolites are used in many types of industrial applications such as catalysis, construction, environmental remediation and restoration, gas separation, gas and steam adsorption/separation, removal of heavy metals and radioactive elements, water purification and anaerobic digestion processes, among others [3-6]. According with recent reports, there are more than 40 mineral species of zeolites. Particularly in Mexico, clinoptilolite, erionite and mordenite are the principal species found in the states of Oaxaca, Sonora, San Luis Potosi, and Michoacán [7]. Moreover, clinoptilolite species are the most abundant natural zeolites, with a typical unit cell formula Na6[(AlO2)6 (SiO2)30]. 24 H2O [8-9]. Clinoptilolite has a characteristic tubular morphology that shows an open reticular structure of easy access, formed by open channels of 8-10 membered rings. Specifically, clinoptilolites have been used for the removal of textile dyes as black reactive 5, red 239 and yellow 176 [8], methylene blue [10] and acid blue 25, basic blue 9, basic violet 3 [11]. Also, they have been used for the removal of heavy metals, i.e., manganese, copper, lead, zinc, cadmium, and nickel [1, 11-13]. In order to increase the adsorption capacity of clinoptilolite in the removal of dyes, different humid methods have been used extensively for the modification of natural clinoptilolite. For example, it has been modified with quaternary amine and 20
hexadecyltrimethylammonium bromide [8, 14]. Also, it was treated with HCl, NaOH, NaCl and NH4NO3 to improve the ion-exchange capacity for heavy metals [12, 15]. However, these humid methods have some disadvantages. For example, the use of acid and water could change the suspension properties and cause the hydrolysis of zeolite due to their siliceous nature. In this context, dry methods would be preferred. Cold oxygen plasma treatments have been used for the surface modification of some carbonaceous materials such as: activated carbons obtained from coconut shells, bamboo, pecan nut shells and peach stones; graphites, meso-carbon microbeads, glassy carbons, carbon blacks and carbon fibers [16-19]. Specifically, the modification of carbonaceous materials with cold oxygen plasma is intended to increase the amount of oxygen functional groups on the surface of the materials. In this context, the principal advantages of this technique are: shorter reaction times, clean process, fast solvent-free technique, simplicity and easy to control. Only two works have been found to report the modification of natural zeolites using cold oxygen plasma to enhance the chromium adsorption capacity [20] and to remove the organic template from nanosized beta zeolite nanocrystals [21]. Results of the effect of the plasma treatment are very limited and not conclusive. Considering the information described above, the purpose of this work is to modify a Mexican natural zeolite with cold oxygen plasma, to identify any relevant changes of its structure and surface chemistry and to test its influence in the adsorption of water pollutants. The zeolites with and without modification were thus characterized using different analytical techniques. The treated and untreated zeolites were finally used for the removal of textile and food dyes such as: acid blue 25, acid blue 41, acid orange 7, food red 17, basic blue 9, indigo carmine, reactive
21
black 5, reactive blue 4 and lead from water. Also, the possible adsorption mechanism was studied using molecular simulation.
2. Materials and Methods 2.1. Zeolite and surface modification The natural zeolite used in this study was supplied by San Francisco mine located in the state of San Luis Potosi, Mexico. The zeolite was milled and sieved for the retained fraction in 35 mesh US standard sieves and thoroughly washed with deionized water until constant pH is reached (~7.7). Subsequently, the zeolite was dried at 110 °C during 24 h and stored for later use. Surface modification of the natural zeolite was carried out by oxidation with cold oxygen plasma. The oxidation was performed in an Emitech K1050X plasma reactor where the oxygen was excited using radiofrequency (RF) energy (13.56 MHz). The plasma was maintained at 1 mbar by flowing oxygen into the reaction chamber. A defined amount of zeolite (0.5 g) was placed in a petri dish and after, it was oxidized during 9 min using a RF power of 75 W. Specifically, six treatments with plasma were performed in each sample in order to attain a homogeneous oxidation of the material surface. The oxidation conditions were selected considering results reported previously [22]. The natural zeolite was denoted as N-C and the sample oxidized with plasma as P-C.
2.2. Characterization of the natural zeolite The textural properties of the samples N-C and P-C were obtained from nitrogen adsorption isotherms at -196 °C using an automatic Micromeritics ASAP 2020 analyzer. Prior to the measurements, the samples were outgassed overnight at 200 °C under vacuum for 8 h. 22
Different models were used to calculate the principal textural parameters from the experimental data of isotherms. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) model using the 0.05 to 0.3 relative pressure (p/p0) interval. The total pore volume was determined from the amount of nitrogen adsorbed at a relative pressure of ~0.99; the micropore volume was obtained by applying the DubininRaduskevich method and finally, the pore size distribution was obtained with the BJH method applied to the N2 isotherm desorption branch. In order to identify the crystal phases present in the zeolites, X-ray diffraction patterns were recorded in a Bruker D8 Advance diffractometer equipped with CuKα X-ray source operated at 40 kV and 40 mA. The morphology and surface composition was also investigated using a FE-SEM system (Quanta FEG 650, FEI) equipped with an EDX detector. The solid particles were dispersed on a graphite adhesive tab placed on an aluminum stub and no further coating was required. Additionally, FT-IR spectra of N-C and P-C were obtained with a Nicolet IS10 spectrometer (Thermo Scientific) with an attenuated total reflectance (ATR) detector to collect the IR spectra in the 4000-500 cm-1 spectral range. Finally, the ablation of N-C surface layers was determined by gravimetry using a Denver Instrument TP-214 microbalance. The mass loss of the removed layer was determined from the measured change in weight of 2 samples of N-C before and after of each cycle of treatment of 9 min (a total of 6 cycles).
2.3. Adsorption of dyes and lead in aqueous solution Adsorption studies of dyes and lead in aqueous solutions were made on both the zeolite modified with plasma (P-C) and the sample without modification (N-C). Two types of 23
adsorption studies were performed in batch systems: preliminary tests using a constant initial concentration and adsorption studies at different initial concentrations. Preliminary tests were performed in polycarbonate cylindrical cells with lid. A defined amount of adsorbent (N-C or P-C) was suspended in 10 ml of a 500 mg/L dye or lead solution (Pb(NO3)2.6H2O). The suspension was then mixed at 160 rpm for 72 h, until equilibrium was reached. All the experiments were conducted by duplicated at 30 °C, pH 5 and using an adsorbent mass to volume ratio of 2 g/L. The dyes selected for these preliminary assays were the acid blue 25 (AB25), acid orange 7 (AO7), food red 17 (FR17), basic blue 9 (BB9), basic blue 41 (BB41), acid blue 74 (AB74), reactive black 5 (RB5), reactive blue 4 (RB4). All dyes were used without further purification. Selected properties of those dyes are shown in Table 1. Optimization of the molecular geometries of some selected dyes was performed using the AMBER molecular force field implemented in the HyperChem 8.0 software. Also, the Quantitative Structure Activity Relationship (QSAR) properties were calculated and the data are reported in Table 2. In this context, with the purpose to understand the adsorption mechanism, the optimization of the zeolite structure was made at 30 °C (temperature of the adsorption process), using Monte Carlo simulation. The molecular electrostatic potentials were thus determined (See Table 3). In the second type of adsorption test the effect of dye and lead initial concentration was analyzed. The experimental conditions were the following: batch systems with constant agitation (160 rpm), equilibrium time of 72 h, temperature 30 °C, pH 5 and mass to volume ratio of 2 g/L. The initial concentrations of dyes and lead were 100, 400, 500 and 750 mg/L. Dye concentration was determined by UV-Vis spectrophotometry at the maximum absorbance of each dye (see Table 1) using a UV-Vis HACH DR-5000 spectrometer. 24
Concentration of lead was analyzed by atomic absorption spectroscopy employing a Perkin Elmer Analyst 100 spectrometer. The dye and lead concentration at equilibrium was calculated from the respective calibration curve. Finally, the amount of adsorbed species, q (mg g-1), was calculated using a mass balance relationship given by Eq. (1). 𝐶0 −𝐶𝑒
𝑞=(
𝑊
)𝑉
(1)
where 𝐶0 and 𝐶𝑒 are the initial and equilibrium dye or lead concentration, respectively (mg L-1), 𝑉 is the volume of the solution (L) and 𝑊 is the weight of the zeolite used (g).
3. Results and discussion 3.1 Characterization of the natural zeolites The zeolite modified with cold oxygen plasma (P-C) and the original zeolite (N-C) were characterized with the purpose to identify the possible changes induced by the plasma treatment. The Fig. 1 shows the XRD patterns of the natural zeolite (N-C). Reflections are consistent with the 00-039-1383-Clinptilolite-Ca-KNa2Ca2(Si29Al7)O72.24H2O pattern (JCPDS cards). These results were also confirmed by EDX semi-quantitative analysis of the N-C sample (Table 4). The principal components of the natural zeolite (clinoptilolite) are Si (44.8 %), O (40.9 %) and Al (7.9 %), with the following exchange cations in the proportions: K≈Ca>Na, Fe≈Mg. Fig. 1 and Table 4 also show the results corresponding to the plasma-treated clinoptilolite (P-C), which are virtually the same that those of the untreated zeolite. This observation would agree with the data reported in the literature for a natural clinoptilolite from Iran, which was treated with a glow discharge plasma using 25
different O2 pressures and no changes were observed between the XRD patterns of the sample with and without treatment [23]. Fig. 2 shows the adsorption isotherms of N2 at 77 K of the two samples. According to the IUPAC classification the isotherms are type IV, which are characteristic of mesoporous materials. Table 5 summarizes selected textural parameters of the clinoptilolite samples and is clear that the percentage of mesopores (84 and 86 %) is greater than micropore amount (16 and 14 %) for N-C and P-C, respectively. According with the IUPAC pore size classification, the micropores have a pore diameter ≤ 2 nm and the mesopores between 2 and 50 nm. This size can be delimited the adsorption of dyes with different molecular properties and lead (see section 3.2). In this context, it seems clear that the modification with cold oxygen plasma did not cause any significant change in the specific surface area (SBET) (23 vs 24 m2/g for N-C and P-C, respectively). However, the micropore volume of N-C decreased from 0.015 to 0.013 cm3/g after the plasma treatment. Consequently, the micropore percentage also decreased (see Table 5). On the other hand, SEM/EDX results (see Fig. 3) suggest some changes in roughness and cleaning of the surface after the cold oxygen plasma treatment (see Fig. 3d). These results agree with the data reported in the literature for the modification of mordenite with Ar plasma treatment [24]. Fig. 2b shows the FT-IR spectra of the two zeolite samples, prior and after the plasma treatment. As for the rest of techniques, the FT-IR of P-C and N-C are essentially the same. The most intense peak is observed at 1015 cm-1, which is characteristic of Si-O stretching vibration [25]. The peak at ~1630 cm-1 is also identified in the FT-IR spectra and according with the literature it is due to H-O-H bending vibration of adsorbed water on the zeolitic structure [26]. The broad band between 3000 and 3600 cm-1 typical of isolated O-H stretching vibration of water is present in the FT-IR spectra of both samples [27]. 26
In short, the plasma treatment has little effect on most of the chemical or structural properties of the clinoptilolite. It is worth to mention, however, that a significant weight loss (approx. 5 wt%) was observed after the plasma treatment of sample N-C, most likely due to the dehydration of the zeolitic structure. This weight loss was found to be a consequence of both the vacuum conditions of the reaction chamber and the heating/annealing effect of the plasma treatment. In this context, the ablation of N-C surface layers was determined by gravimetry after each plasma treatment cycle (9 min by cycle). The mass loss of N-C induced by the plasma treatment was determined by weighing and Fig. 4 shows the results. A clear behavior was observed, and the mass loss of the ablated layer increases with the cycles or the exposure time until 45 and 54 min, where the loss mass was practically constant (0.01095 g). This ablation behavior was observed for other type of materials modified with plasma treatment as biaxially oriented polyethylene naphthalate foils, where the ablated layer increases with exposure time until 53 μg approximately [28].
3.2 Adsorption of dyes and lead from aqueous solution The plasma treated and untreated zeolites (P-C and N-C, respectively) were used for the removal of dyes such as acid blue 25 (AB25), acid orange 7 (AO7), food red 17 (FR17), basic blue 9 (BB9), basic blue 41 (BB41), acid blue 74 (AB74), reactive black 5 (RB5) and reactive blue 4 (RB4), from water. Fig. 5 shows the adsorption results of N-C and P-C, using an initial concentration of dye of 500 mg/L. In general, for all dyes, the adsorbed amount is very similar for the two samples (P-C and N-C), indicating that the plasma treatment is not affecting the removal of dyes. This behavior is congruent with the characterization results because the plasma treatment is not affecting the chemical 27
functionality and/or structure of the clinoptilolite. However, it is very interesting to note that P-C and N-C are more efficient in the removal of cationic dyes as basic blue 9 (BB9) and basic blue 41 (BB41), showing an adsorption amount of 23 and 24 mg/g, respectively. In this context, it is relevant to mention that the differences between the area, volume and hydrophobicity of BB41 and BB9 have not a determinant effect in the adsorption capacity of clinoptilolite (See Table 2). Moreover, the adsorption mechanism of basic dyes on these zeolites can be related with the electronic density distribution of the material as depicted in the molecular electrostatic potential (MEP) in Table 3. This particular natural zeolite shows a high electronic density: red color means regions with more negative EPs, blue are regions where EPs are more positive and green corresponds to regions with zero potential. This behavior is congruent with the permanent negative charge of the zeolite/water interface, which is balanced by the exchangeable metal cation and it is due to the isomorphs substitution of some Si4+ from Al3+ within the clinoptilolite lattice [29]. In this context, according with the data reported in literature, at acidic values of pH (3-5.3) the silanol groups are protonated in such an insignificant proportion that the competition between H3O+ ions and the cationic dyes (BB9 and BB41) lessens [30]. This observation was simulated and the results are shown in Table 3. It seems evident that the excess of hydronium ions is not affecting significantly the electronic density of clinoptilolite and that the adsorption of cationic dyes can be illustrated by the dissociated groups (≡SiO-), following the mechanism proposed by [30]:
≡SiO- + Dye+ ↔ ≡SiO- Dye +
28
Where Dye+ is BB9 or BB41, which are cationic dyes with pKa values of 3.8 [31] and 2.64 [32], respectively. The two dyes would have a positive charge at pH 5 (value used in this work for the adsorption tests). The effect of dye initial concentration was also studied and the adsorption results are congruent with the data reported in the preliminary tests, i.e., the plasma treatment of clinoptilolite has not a significant effect in the removal of dyes (See Fig. 6a and 6b). In this case, the removal of BB9 and BB41 was very similar for N-C and P-C. Additionally, is relevant to mention that the adsorbed amount was very similar when a dye solution of 400, 500 or 750 mg/L of BB9 or BB41 was employed in the adsorption tests. Particularly, the adsorbed amount of BB9 was 28.3 mg/g and 24.5 for BB41, for an initial concentration of 750 mg/L. These values are comparable with the data reported in the literature for the removal of BB41 (19.57 mg/g) using volcanic tuff, which is constituted by analcite, clinoptilolite and heulandite [30], but for the removal of BB9 the adsorbed amount reported in this work is lower than the value cited in [11], where the authors were testing a natural clinoptilolite obtained from Sonora state, Mexico (~80 mg/g) [11]. However, a direct comparison between both results is not feasible because the experimental adsorption conditions and the particle size of adsorbents were different. Finally, considering the results found in the literature showing that the removal of lead is increased by the oxidation of carbonaceous materials using cold oxygen plasma [18, 22], the adsorption of lead using clinoptilolite prior and after the plasma treatment was also studied. Fig. 6c shows the adsorption results of lead from water using N-C and P-C. As in the case of dye removal, there are not significant differences in the amount of lead removed by both clinoptilolites. Nevertheless, the lead adsorbed amounts reported in this work using an initial concentration of 750 mg/L (67 mg/g) is reasonably similar to other capacities 29
reported in the literature for the removal of lead using a natural clinoptilolite from Turkey (79 mg/g), Ukraine (26 mg/g) and Romania (73.9 mg/g) [33-35], bearing in mind the unfeasible fair comparison between experiments, due to crucial differences in experimental conditions such as pH, temperature and mass to volume ratio.
4. Conclusions In the present work a systemic analysis was made about the changes in the structure, texture and chemical functionality of natural clinoptilolite after cold oxygen plasma treatment. The obtained results are determinant and conclusive in comparison with the limited reports of the literature. Specifically, the crystallinity and chemical functionally of natural clinoptilolite was conserved after plasma treatment and the roughness was slightly affected by treatment. However, a weight loss of 0.01095 g was observed after the plasma treatment due to the vacuum conditions of the reaction chamber and the heating/annealing effect of the plasma treatment. Also, the mass loss of the ablated layer increases with the exposure time and this is due to dehydration of the zeolitic structure. Finally, the adsorbed amounts of acid, basic, reactive and food dyes and lead from water were very similar between the clinoptilolite prior and after the plasma treatment. ACKNOLEDGEMENTS This work was supported by CONACyT (project AGS-2012-C02-198207) and PCTIAsturias/FEDER (EU) (GRUPIN14-117) project. Paola acknowledges the grant (447346) received from CONACYT.
30
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Figure Captions Fig. 1 XRD patterns of N-C and P-C samples. Fig. 2 N2 Adsorption isotherms at -196 ºC (a) and FT-IR spectra (b) of the plasma treated clinoptilolite (P-C) and natural clinoptilolite (N-C). Fig. 3 SEM images of the natural clinoptilolite (a, b) and the clinoptilolite treated with plasma (c, d). Fig. 4 Dependence of the mass loss of N-C during cold oxygen plasma treatment with exposure time (6 cycles of 9 min). Fig. 5 Adsorption results of acid, basic and reactive dyes using as adsorbents the natural clinoptilolite (N-C) and the clinoptilolite with plasma treatment (P-C). Experimental conditions: pH 5, temperature 30 °C, mass to volume ratio 2 g/L and initial concentration of dyes of 500 mg/L Fig. 6 Adsorption results of basic dyes (a, b) and lead (c) using as adsorbent the natural clinoptilolite (N-C) and the clinoptilolite with plasma treatment (P-C) and different initial concentrations. Experimental conditions: pH 5, temperature 30 °C, mass to volume ratio 2 g/L and initial concentration of dyes and lead of 100, 400, 500 and 750 mg/L.
36
Fig. 1
% Clinoptilolite
%
4000 %
3000
5000
%
2000
%
% %
1000
%
%
4000
% %
3000 0 2000
Intensity (counts)
Intensity (counts)
Plasma:P-C
Natural: N-C
5000
1000 0 0
20
40
60
2º
37
Fig. 2
80
3
Adsorbed volume (cm /g)
P-C N-C
(a)
60
40
20
0 0.0
0.2
0.4 0.6 0.8 Relative pressure, p/p0
1.0
110 (b)
100
Transmittance (%)
90
3400
1630
80 70 60
N-C P-C
50 40
1015
30 20 4000
3500
3000
2500
2000
1500
1000
-1
Wavenumber, cm
38
Fig. 3
(a)
(b)
(c)
(d)
39
Fig. 4
Mass Loss(g)
0.012
0.008
0.004
0.000 0
10
20
30
40
50
60
Exposure time(min)
40
Fig. 5
N-C P-C
30
Adsorbed amount, mg/g
25 20 15 10 5 0
25 B41 B B A
7 AO
1 FR
9 74 BB AB
5 RB
4 RB
Dyes
41
100 NC-BB41 PC-BB41
80
(a)
60 40 20 0 0
Adsorbed amount, mg/g
100
100
200
300
400
500
600
700
NC-BB9 PC-BB9
80
800 (b)
60 40 20 0 0
100
200
300
400
500
600
700
800
100 NC- Lead PC- Lead
80
(c)
60 40 20 0 0
100
200
300
400
500
600
700
800
Initial Concentration, mg/L
Fig. 6
42
Table 1. Selected properties of the studied dyes Dye and code assigned Molecular formula
Molecular weight (g
λmax (nm)
mol-1) Acid blue 25 (AB25)
C20H13N2NaO5S
416.38
600
Acid orange 7 (AO7)
C16H11N2NaO4S
350.32
484
Food red 17 (FR17)
C18H16N2Na2O8S2
496.42
503
Basic blue 9 (BB9)
C16H18ClN3S
319.85
664
Basic blue 41 (BB41)
C20H26N4O6S2
482.57
607
Acid blue 74 (AB74)
C16H8N2Na2O8S2
466.34
610
Reactive black 5 (RB5)
C26H21N5Na4O19S6
991.82
597
Reactive blue 4 (RB4)
C23H14Cl2N6O8S2
637.2
595
43
Table 2. QSAR properties of basic dyes calculated by molecular simulation Dye
Area (Å2)
Volume (Å3)
Log P
694.90
1236
-0.45
515.64
873.52
1.32
AB41
BB9
44
Table 3. Molecular electrostatic potential (MEP) maps in 3D of KNa2Ca2(Si29Al7)O72.24H2O with excess of hydronium ions by Monte Carlo simulation at 30 °C Geometry of optimization
Distribution of nucleophilic and electrophilic zones
45
Table 4. Elemental composition of the N-C and P-C samples obtained by EDX analysis Sample
Wt (%) ±0.1 O
Na
Mg
Al
Si
K
Ca
Fe
Si/O
N-C
40.9
0.6
0.5
7.9
44.8
2.7
2.1
0.6
1.095
P-C
42.8
0.7
0.9
9.0
40.2
2.7
3.0
0.7
0.939
Table 5. Textural parameters of N-C and P-C samples obtained from the N2 adsorption isotherms at -196 ºC Sample and code Parameter
Natural clinoptilolite: N-C
Plasma oxidized clinoptilolite: P-C
a
23
24
Vt (cm³/g)
0.094
0.095
VD-R (cm³/g)
0.015
0.013
% Micr
0.079 16
0.081 14
% Mes
84
86
SBET (m²/g)
b c
d
Vmes (cm³/g)
e f
a SBET: BET surface area Vt: Total pore volume, c VDR: Dubinin–Radushkevich micropore volume d VMes= Vt− VDR e Microporous (%) = (VDR/Vt)100 f Mesoporous (%)= (VMes/Vt)100 b
46