BTX sorption on Na-P1 organo-zeolite as a process controlled by the amount of adsorbed HDTMA

Microporous and Mesoporous Materials 202 (2015) 115–123

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BTX sorption on Na-P1 organo-zeolite as a process controlled by the amount of adsorbed HDTMA Barbara Szala a,⇑, Tomasz Bajda a, Jakub Matusik a, Katarzyna Zie˛ba b, Beata Kijak b a b

AGH University of Science and Technology in Kraków, 30 Mickiewicza Av., 30-059 Kraków, Poland Jagiellonian University, Department of Environmental Chemistry, Faculty of Chemistry, Gronostajowa 3, 30-387 Kraków, Poland

a r t i c l e

i n f o

Article history: Received 15 May 2014 Received in revised form 2 September 2014 Accepted 9 September 2014 Available online 23 September 2014 Keywords: Zeolite Na-P1 Sorption of BTX Adsorption FTIR Organo-zeolite

a b s t r a c t The main objective of the study was to utilize fly ash by transforming it into Na-P1 zeolite. The obtained synthetic zeolite has been modified with a HDTMA surfactant in amounts of: 0.2, 0.4, 0.6, 0.8 and 1.0 of external cation exchange capacity (ECEC). The process reported hereunder was designed at room temperature (20 °C) and a low solid/solution ratio (1:2). Infrared spectroscopy (FTIR) was used to determine the quantity of the adsorbed surfactant on the crystallites’ surface. The normalized intensity of the selected bands was compared with CHN results. The results revealed a very strong correlation between spectroscopic and chemical analyses and enabled the preparation of calibration curves. The sorption properties of organo-zeolites towards benzene, toluene, p-xylene (BTX) were evaluated. The results showed that the modification of the Na-P1 zeolite by HDTMA improves the sorption properties in terms of benzene and toluene and that 1.0 ECEC modification proved to be the best sorbent of those compounds. Xylene was adsorbed in the greatest quantity, however, sorption efficiency from aqueous solutions does not depend on the amount of surfactant used. Experiments proved that it is possible to remove more than 95% of toluene and xylene and more than 85% of benzene contaminants from aqueous solutions. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Zeolites are well-known microporous crystalline solids with neutral SiO2 groups and negatively charged (AlO2) ions compensated by non-framework cations such as K+, Na+, Ca2+, Mg2+ or NH+4 [1]. Cation exchange properties of traditional aluminosilicate zeolites arise from the isomorphous positioning of aluminum in tetrahedral coordination within their Si/Al frameworks (Si+4 ? Al3+) [2]. Zeolites possess a few basic properties such as: high surface area, stability, ion exchange, adsorption and molecular sieving [3,4]. Those properties are exploited in a wide range of applications, e.g. in catalysis [5–7], ion-exchange (wastewater treatment) [8–11] and in the separation and removal of gases [12] and solvents [13,14]. However, natural zeolites contain admixtures which reduce the purity of their composition. Moreover, natural zeolites’ properties (CEC, charge, size of cavities etc.) make it difficult to use them in specific chemical processes – further perpetuating the demand for synthetic zeolites. Most of the world’s produced fly ashes are solid wastes of coal combustion in the generation of electricity. However, they are also very attractive for recycling [15]. In recent years, the possibility of ⇑ Corresponding author. Tel.: +48 126175223. E-mail address: [email protected] (B. Szala). http://dx.doi.org/10.1016/j.micromeso.2014.09.033 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.

using fly ash from the combustion of coal for the synthesis of zeolite materials has been examined [15–18]. The obtained results show that fly ash can be used for direct hydrothermal synthesis [19–21], hydrothermal synthesis from alkali-fused fly ash [22], and hydrothermal synthesis of zeolites using extracted silica [23] or mine waters [17]. Hydrothermal synthesis of fly ash with a solution of NaOH at 95 °C for 24 h brings a high purity zeolite and has been adapted to the industrial production of zeolites on a large scale [24]. The resultant zeolites possess both a high surface area and stability [16]. One of the many uses of zeolites is their application in removing harmful compounds from contaminated soils and waters [9,25,26]. Zeolites have been especially useful in removing cationic species such as ammonium [27,28] and heavy metals [29–32]. A significant number of studies have suggested that the area of application could be expanded by utilizing the zeolite’s surface [3]. Most zeolites are hydrophilic, therefore they are frequently modified to increase their surface hydrophobicity to adsorb low soluble compounds. The hexadecyltrimethylammonium bromide (HDTMA) is a long-chain cationic surfactant that possess a permanent positive charge and is one of the more popular surfactants [9]. In contact with the zeolite, HDTMA organic cations selectively exchange with the inorganic cations (K+, Na+, Ca2+ or Mg2+) on the surface of the zeolite crystals and form a surfactant layer with anion exchange

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properties [9,33]. Since the size of organic molecules is bigger than the pore diameter of zeolite, surfactants are attached on the external surface, leaving the internal pore accessible for adsorbate molecules [34]. Therefore, external cation exchange capacity (ECEC) is an important property in the synthesis of organo-zeolites. The surfactant layer on organo-zeolites is a solvent-like medium into which non-polar organic compounds tend to dissolve. It is now recognized that sorption of organic compounds on surfactant-modified zeolite (SMZ) is a feasible method for the removal of pollutants from wastewaters [35,36]. The use of zeolites to remove compounds such as phenol, acetone, trichloromethane, pentanol and cresol with different water solubilities onto SMZ are well described in the literature [35,37–39]. An important trend is to adapt the zeolites for removing volatile organic compounds such as BTEX (benzene, toluene, ethylbenzene, xylenes) [40–44]. The chemical and petrochemical industries are often the source of BTEX contaminants through the wastewater they generate. Rigorous regulations have been imposed in terms of acceptable concentrations of these compounds in water because of their toxicity to human health and for the environment [37,45]. So far, these compounds are removed through the use of methods such as thermal oxidation, catalytic oxidation, absorption, condensation, membrane separation and adsorption. It has been found that adsorption is one of the most efficient methods to remove organic pollutants in wastewaters [40–42]. Until now, in order to determine the effectiveness of modification, two methods were used: Analysis of HDTMA concentration in the solution before and after modification using chromatographic methods or comparing the content of carbon, hydrogen and nitrogen in the samples before and after modification [40–44]. The usefulness of the FTIR method to probe the structure, organization of surfactant molecules in the interlayer space of layered zeolites as well as the analysis of the influence of the carbon chain length on the appearance of the FTIR spectrum has been documented [38]. The fact that the intensity of the bands changes with the amount of surfactant attached has been previously recognized [38,45]. However, the adaptation of the FTIR method for rapid analysis of the effectiveness of modification has not been described before. The objective of our research was to optimize the modification process so that it is suitable for use on an industrial scale. Another challenge was to eliminate the problem of determining the effectiveness of the modification, as well as to examine whether the amount of surfactant affects the sorption efficiency.

2. Experimental 2.1. Zeolite Na-P1 As a substrate for the synthesis reaction of the zeolite Na-P1, the F-class fly ash (contains less than 20% lime – CaO) from the combustion of coal at the Rybnik power plant in Poland has been used. The chemical composition of fly ash was as follows (in wt%): SiO2 – 52.1, Al2O3 – 32.2, Fe2O3 – 5.2, MgO – 1.3, CaO – 1.2, Na2O – 0.5, K2O – 2.9. The 4.6 wt% was lost during calcination and through marginal components not relevant to the synthesis process [46,47]). In terms of mineralogy, quartz was the dominant phase, in addition, mullite, magnetite and hematite have been identified (Fig. 1A). Spherical forms of aluminosilicate glass dominated the mineralogical composition (Fig. 2A). For the synthesis of Na-P1 zeolite, technology developed by Franus (2011) has been used [24]. The following reaction conditions have been applied: 20 kg of fly ash; 12 kg NaOH, 90 dm3 of H2O; temperature of the process at 80 °C and for a duration of 36 h [24,47]. The resulting product was a high purity (81 wt%) Na-P1

zeolite. In addition to the Na-P1 phase, the presence of mullite and quartz phases have been identified (Fig. 1B). To calculate the quantitative content of Na-P1 the Rietveld method with the use of the computer program ‘‘FullProf’’ has been applied. Average ratios of individual cations are as follows: (Na + K + Ca + Mg)/ Si = 0.44; Si/Al = 1.42 [16,47]. Na-P1 zeolite has BET specific surface area of 88 m2/g, which is almost 6 times higher with respect to fly ash (15 m2/g) [47]. Sodium is the main-exchange cation in the zeolite’s structure, which means that Na+ mainly balances the charge of aluminosilicate framework. The surface morphology of the zeolite Na-P1 is shown in Fig. 2B. External cation exchange capacity (ECEC) was determined through the indication of the maximum amount of HDTMA-Br attached to the zeolite’s surface. For this purpose, 1 g of zeolite grinded in an agate mortar and 100 ml of double-distilled water was mixed with a magnetic stirrer for 3 h at 80 °C. Then, 1 g of dissolved HDTMA-Br was added to the suspension and stirred for a further 24 h at 80 °C. After this period, another portion of HDTMA-Br solution (1 g/100 ml) was added and mixed for 5 h. The zeolite was centrifuged and washed with hot (80 °C) water until the negative reaction occurred for the chlorides. The zeolite was washed with hot ethanol and dried. Using an automatic analyzer CHNS Elementar Vario EL III, the content of nitrogen and the carbon and hydrogen in the solid state were determined. The difference in the content of CHN between Na-P1 zeolite and that treated with HDTMA was used to calculate ECEC. To confirm that after the modification process the surfactant did not crystallize and was attached to the zeolite’s surface by ion exchange, XRD has been performed. On the diffraction pattern, peaks derived from HDTMA did not appear (Fig. 1C). Furthermore, as can be seen in the Fig. 2C, the surface morphology did not change. 2.2. Modification The objective was to design a process at low temperature which still effectively modified the zeolite. Another challenge was to reduce the amount of water used by lowering the ratio of the solid state (zeolite) in the solution (HDTMAaq). A series of experiments were planned. 50 g of zeolite was placed into a beaker and then saturated with a solution of HDTMA. Depending on the assumed temperature of the process, the mixture has been heated to 40 °C or 60 °C. Various volumes of the HDTMA solution were used, depending on the assumed ratio (Table 1). However, in a given volume, there was always 8.9 g of HDTMA. This amount of surfactant corresponds to one full external cation exchange capacity ECEC (ECEC = 24.4 meq/100 g ? 8.9 g of HDTMA/100 g of zeolite). In the case of an assumed room temperature (20 °C) the heating during HDTMA dissolution has been eliminated. The surfactant was dissolved using an ultrasonic disintegrator. It turned out that this method also allows for effective dissolution of HDTMA in a very small amount of water (50 ml). Following the selection of best conditions, five samples of organo-zeolites were prepared by mixing 50 g of zeolite Na-P1 with 100 g of the HDTMA-Br solution at concentrations of 0.2, 0.4, 0.6, 0.8, 1.0 of ECEC. After 6 h of stirring at room temperature, the samples were centrifuged and dried at 60 °C. 2.3. Sorption experiments All chemicals used in this study (benzene, toluene, p-xylene, and methanol) were of analytical grade. The solubility of the benzene, toluene and p-xylene in water at 20 °C is 0.188, 0.067, and 0.020%, respectively [48]. Therefore, to prevent confrontation with insolubility at higher concentrations, the adsorption of benzene, toluene and xylene were made in the mixture of distilled water

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Fig. 1. X-ray diffraction patterns of: (A) fly ash, (B) zeolite Na-P1, (C) zeolite Na-P1 modified with HDTMA in amount of 1.0 ECEC (organo-zeolite).

and methanol. The concentrations of working water solutions were approximately 50, 100, and 250 ppm. The maximum sorption capacity of zeolite Na-P1 in terms of benzene, toluene, p-xylene (BTX) was determined in dynamic conditions using a flow technique. In the syringe (volume 2.5 ml) a zeolite layer (0.6 g) has been placed and secured by glass wool on both sides. Then a triple volume of individual hydrocarbons (BTX) in relation to the volume of zeolite was poured. After sorption, hydrocarbons that were originally adsorbed by organo-zeolite were desorbed. For the extraction of hydrocarbons carbon disulfide was used. The maximum sorption capacity for organo-zeolite

was determined based on concentration of benzene, toluene and p-xylene in a solution of carbon disulfide after desorption. To conduct experiments of BTX sorption from aqueous solutions by various modifications of Na-P1 zeolite, 5 ml of each water solution was added to the 100 mg of zeolite (unmodified Na-P1 and modified with HDTMA in amount of 0.2, 0.4, 0.6, 0.8 and 1.0 of ECEC). The sorption of hydrocarbons was determined by a GC/MS instrument after 24 h of shaking and 10 min of centrifugation at 14,000 rpm of zeolite suspension in each hydrocarbon water solution. An analytical solution was prepared by 100-fold dilution of hydrocarbon water solution taken from the sample of zeolite after

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Fig. 2. SEM images of: (A) fly ash used for the synthesis. Magnification 10000, (B) zeolite Na-P1. Magnification 30000 (C) zeolite Na-P1 modified with HDTMA in amount of 1.0 ECEC (organo-zeolite). Magnification 30000.

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B. Szala et al. / Microporous and Mesoporous Materials 202 (2015) 115–123 Table 1 The effectiveness of modification by HDTMA in an amount of 1.0 ECEC in various process conditions. 20 °C Solid/Liquid Sorption of HDTMA [mmol 100 g Multiples of ECEC

1

zeolite]

1:2 24.7 1.01

40 °C 1:4 21.0 0.86

1:6 22.0 0.90

1:10 23.2 0.95

1:20 25.1 1.03

1:40 22.7 0.93

1:2 21.0 0.86

60 °C 1:4 22.5 0.92

1:6 23.1 0.95

1:10 22.7 0.93

1:20 23.7 0.97

1:40 23.9 0.98

1:20 24.6 1.01

Fig. 3. Calibration curves for benzene, toluene, p-xylene.

centrifugation. Fig. 3 presents the calibration curves for each tested hydrocarbon. The final stage of research was to examine the total amount of BTX removed from aqueous solutions. Zeolite Na-P1 and organozeolite modified by HDTMA in amount of 1.0 ECEC were chosen for the experiments. Selection was based on the fact that in the previously described studies (maximum sorption capacity and sorption of BTXaq) where zeolite Na-P1 modified with HDTMA in amount of 1.0 ECEC turned out to be the most effective sorbent. On the basis of the determined sorption capacity of the zeolite Na-P1 and organo-zeolite 1.0 ECEC, it was assumed that 1 g of zeolite had a large excess of sorbent with respect to 10 ml of a solution – even in the highest concentration. Therefore, 1 g of the sample was placed in an 12 ml centrifuge polypropylene tube and 10 ml of a solution containing BTX was added. The mixtures were shaken for 24 h and centrifuged at 14,000 rpm for 10 min until the washing solution was cleared and decanted. The concentration of BTX has subsequently been determined. By comparing the initial concentrations with equilibrium concentrations the sorption efficiency has been specified. 2.4. Analytical methods Air dried uncoated samples were analyzed by electron microscopy using a variable pressure field-emission scanning electron microscope (FEI Quanta 200) equipped with an energy dispersive spectrometer (EDS) for elemental microanalysis. The mineral composition of synthetic and modified zeolites was determined via powder XRD using a Philips X’pert APD diffractometer (PANalytical, Almelo, the Netherlands) with PW3020 goniometer, Cu lamp, and graphite monochromator from 5 to 65° 2h. Diffraction data was processed by Philips X’Pert and ClayLab ver. 1.0 software. For the determination of external cation exchange capacity (ECEC), as well as to determine the effectiveness of the modification the automatic analyzer, CHNS Elementar Vario EL III was used. The FTIR spectra were collected by Nicolet 6700 spectrometer using the DRIFT technique (3% wt. sample/KBr) with 64 scans at 4 cm 1 resolution in the 4000–400 cm 1 mid-region. Peak fitting was carried out with OMNIC v8.3 software (Thermo Fisher Scientific). Maximum sorption capacity of the zeolite as well as the percentage of hydrocarbons removed from solutions in terms of

benzene, toluene and p-xylene was measured using gas chromatography (GC Ultra by Thermo Scientific) fitted with a flame ionization detector, FID. Other samples’ analyses were performed with a Hewlett Packard GC/MS system (HP 6890/5973) coupled with a headspace sampler (HP 7694E). For the analysis of hydrocarbons, 10 mL of analytical solution was placed in 20-mL vials and sealed hermetically with silicone septum caps. These vials were in sequence introduced into the oven of the headspace sampler at a temperature of 70 °C for 30 min. Then, the volatiles filled the loop (120 °C) and were injected in the chromatographic system through a thermostatted transfer line heated to 160 °C. HP-5MS capillary column was used for the chromatographic separations (30 m  0.25 mm; film thickness 0.25 m, methyl-(5%)-phenylsiloxane phase), purchased from J&W Scientific. The column was initially maintained at 55 °C for 3 min, then the temperature increased to 120 °C at a rate of 15 °C/min, which was then held for an additional 1 min. Helium was used as the carrier gas at a flow rate of 1 mL/min. Data collection was performed using Enhanced ChemStation ver.A.03.00 software. The MS was operated in the SCAN mode with the m/z range of 24–160 amu. Operating parameters of MS were as follows: ionization voltage 70 eV, the temperature of ion source and quadrupol – 230 °C and 150 °C respectively. 3. Results and discussion 3.1. Effectiveness of modifications The calculated external cation exchange capacity (ECEC) was 24.4 meq/100 g. Based upon the amount of HDTMA adsorbed on the zeolites’ surface and the ECEC value, the efficiency of modification was determined. The obtained results indicate that the lowering of the temperature and ratio of solid to the solution slightly reduces the efficiency of the modification (Table 1). The elevated temperature as well as amount of solution does not always improve the efficiency of modification. However, the most of the accepted conditions lead to a product where the efficiency of the modification was more than 90%. It was assumed that the process of modification can be carried out at room temperature, assuming that for 1 g of the zeolite and 2 g of the solution is necessary. The use of such conditions is certainly a novelty in this type of research. The method in addition to being simple, fast and economical is also

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Table 2 Efficiency of zeolite Na-P1 modification to the organo-zeolite by adsorption of an organic surfactant HDTMA. Assumed amount of adsorbed HDTMA [multiples of ECEC] HDTMA adsorbed [mmol/100 g] The real amount of adsorbed HDTMA [multiples of ECEC]

0.20

0.40

0.60

0.80

1.00

4.88 0.20

10.98 0.45

13.42 0.55

19.52 0.80

22.94 0.94

straightforward to adapt on a larger scale. The first attempts have indicated that it is possible to produce 20 kg of zeolite at a time. The adopted process conditions (the ratio of solid to solution equal to 1:2, 20 °C, 6 h) proved to be an effective way of modification (Table 2). Experimental trials have shown that the efficiency of the modification process does not fall below 90%. Obtained for further experiments, organo-zeolites have been modified in amounts of: 20, 45, 55, 80 and 94% of calculated external cation exchange capacity (100% = 24.4 mmol HDTMA/100 g of zeolite). Each time, the determination of the carbon, hydrogen and nitrogen content in the samples was a long and expensive process. Therefore, the next step was to use Fourier transform infrared spectroscopy (FTIR) for the determination of modification efficiency. To the best of our knowledge, the influence of the amount of adsorbed surfactant on the intensity of the FTIR spectrum for HDTMA surfactant adsorbed on Na-P1 zeolite has yet to be investigated. Previous studies have revealed that FTIR is very useful to probe the structure, organization of surfactant molecules in the interlayer space of layered zeolites and how carbon chain length of the surfactant used can change the FTIR spectra of zeolites [38,49]. We investigated the dependence of the HDTMA amount attached to the zeolite’s surface on the intensity of FTIR spectrum.

Fig. 4. The intensity of the absorption spectrum determined based on the baseline. I1 = 2850 cm 1 and I2 = 2920 cm 1 are C–H stretching vibration modes of the methylene groups of hydrocarbons.

The spectra were normalized to the intensity of the Si-O stretching band. The normalized intensity of selected bands attributed to C–H stretching vibration in the 3000–2800 cm 1 region (Fig. 4) was compared with CHN results (Table 2). The bands at 2920 and 2850 cm 1 arise from the asymmetric and symmetric C–H stretching vibration modes of the methylene groups (CH2) of the hydrocarbon chains respectively [38]. Fig. 5 clearly shows the difference between the spectra of pure Na-P1 zeolite and organozeolites, where additional bands derived from HDTMA surfactant appear. The results reveal a very strong correlation between spectroscopic and chemical analyses and enabled the preparation of calibration curves (Fig 6). A linear relationship between the sum of the I1 and I2 band intensities and multiplicity of ECEC is clearly visible. The fit factor (R2 = 0.9984) confirms that it is possible to use the calibration curve to determine the amount of adsorbed HDTMA. 3.2. Maximum sorption capacity The maximum amount of hydrocarbons which were adsorbed on zeolite and organo-zeolites indicate an improvement in the sorption properties for a zeolite modified with an organic surfactant (Table 3). 1 g of zeolite Na-P1 modified with HDTMA in amount of 1.0 ECEC may adsorb 485 mg of benzene, 854 mg of toluene or 587 mg of p-xylene, which is 3.3, 1.2 and 1.4 times more respectively than the maximum adsorption capabilities of an unmodified zeolite. The maximum sorption capacity is the highest for toluene and the lowest for benzene. However, when looking at the results in terms of the quantity of attached molecules, it has been found that surface modification activates the surface, providing much more available positions

Fig. 5. The absorption spectra for zeolite Na-P1 and its modification: 0.2, 0.4, 0.6, 0.8, 1.0 of ECEC.

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Table 5 Amount of adsorbed benzene at equilibrium in terms of the mass of adsorbent (mg/g). Sample

Na-P1 0.2 ECEC 0.4 ECEC 0.6 ECEC 0.8 ECEC 1.0 ECEC

Adsorbed benzene (mg/g) Initial concentration (ppm) 50

100

250

1.38 1.36 1.38 1.50 1.59 1.66

2.53 3.06 2.89

6.11 5.98 7.09 8.17 9.99 10.28

2.70 3.07

Fig. 6. The dependence of the sum of I1 and I2 band intensities (respectively 2850 and 2920 cm 1) versus the amount of adsorbed HDTMA calculated based on CHN content (multiplicity of ECEC).

Table 3 The maximum amount of adsorbed hydrocarbons [mg/g of the zeolite].

Benzene Toluene p-Xylene

Zeolite Na-P1

1.0 HDTMA

145.6 748.8 413.9

485.5 854.4 587.4

Fig. 7. Adsorption of benzene by Na-P1 zeolite and HDTMA modified zeolite treated at different concentrations of initial surfactant.

Table 4 The amount of adsorbed hydrocarbons [mmol/g of the zeolite].

Benzene Toluene p-Xylene

Zeolite Na-P1

1.0 HDTMA

1.86 8.13 3.90

6.22 9.27 5.53

for hydrocarbons (Table 4). Maximum sorption capacity increased after modification and the surface of the organo-zeolite was able to adsorb: 4.4 mmol/g of benzene, 1.14 mmol/g of toluene and 1.6 mmol/g of p-xylene more in relation to the material where the original sodium cations present were not exchanged by organic cations from HDTMA salt. The obtained results allowed to design further experiments.

Fig. 8. Adsorption of toluene by Na-P1 zeolite and HDTMA modified zeolite treated at different concentrations of initial surfactant.

3.3. Sorption of BTXaq The equilibrium sorption isotherm is fundamentally crucial in design of sorption system. The adsorption isotherm is the relationship between the quantity sorbed per unit mass of sorbent and the concentrations of solute [35]. The adsorption isotherms of BTX onto zeolite Na-P1 and organo-zeolites: 0.2, 0.4, 0.6, 0.8 and 1.0 of ECEC are presented in Figs. 7–9. The benzene adsorption capacities for the zeolite Na-P1 and organo-zeolites modified with HDTMA in amounts of 0.2, 0.4, 0.6, 0.8 and 1.0 of ECEC with an initial concentration 51.3 mg/L were 1.38, 1.36, 1.38, 1.50, 1.59, 1.66 mg/g respectively. The sorption efficiency depended from the amount of surfactant which has been adsorbed. The least effective removal of benzene was for zeolite

Na-P1 and modifications 0.2 and 0.4 ECEC. With a rising amount of HDTMA attached to the zeolite surface, sorption efficiency increased. Similarly, the effect of the modifications on the efficiency of sorption in the case of higher concentrations 96.4 and 252 mg/L were also evident (Table 5). From the results it can be concluded that the modification over an amount of 0.4 ECEC brings a significant increase in adsorption capacities. The amount of HDTMA used clearly impacts the absorption efficiency of benzene from aqueous solutions (Fig. 7). The efficiency of sorption of toluene on zeolite Na-P1 and organo-zeolite is shown in Fig. 8. The toluene adsorption capacities for the modifications 0.2, 0.4, 0.6, 0.8 and 1.0 ECEC at initial concentration 246 mg/L were 8.2, 8.2, 8.7, 9.6 and 10 mg/g

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Table 6 Amount of adsorbed toluene at equilibrium in terms of the mass of adsorbent (mg/g). Sample

Na-P1 0.2 ECEC 0.4 ECEC 0.6 ECEC 0.8 ECEC 1.0 ECEC

Adsorbed toluene (mg/g) Initial concentration (ppm) 50

100

250

1.44 1.40 1.45 1.44 1.61 1.62

4.27 4.27 4.31 4.83

8.64 8.23 8.24 8.73 9.63 9.99

4.84

Fig. 10. Adsorption of benzene, toluene and p-xylene (BTX) by Na-P1 zeolite and HDTMA modified zeolite treated at different concentrations of initial surfactant. Modifications: the brightest color (Na-P1) to the darkest (1.0 ECEC).

Fig. 9. Adsorption of p-xylene by Na-P1 zeolite and HDTMA modified zeolite treated at different concentrations of initial surfactant.

Table 7 Amount of adsorbed p-xylene at equilibrium in terms of the mass of adsorbent (mg/g). Sample

Na-P1 0.2 ECEC 0.4 ECEC 0.6 ECEC 0.8 ECEC 1.0 ECEC

Adsorption of p-xylene (mg/g) Initial concentration (ppm) 50

100

250

1.81 1.79 1.79 1.81 1.88 1.83

4.06 4.05 4.06 4.12 4.07 4.09

11.41 11.41 11.35 11.41 11.47 11.42

respectively (Table 6). Zeolite Na-P1 adsorbed 6.64 mg toluene/g, which is more than the capability of 0.2 and 0.4 ECEC modifications. The improvement in adsorption capacity was observed for modifications 0.6, 0.8 and 1.0 ECEC. The most efficient sorption was observed at the initial concentration of 246 mg/L when 1.0 ECEC organo-zeolite adsorbed 10 mg toluene/g. The least effective sorption of toluene occurs at the initial concentration of 39.4 mg/L, when 0.2 ECEC organo-zeolite removed 1.4 mg toluene/g. The sorption effectiveness for removal toluene from its solution was almost 20% higher for modification 1.0 ECEC than for zeolite NaP1. With an increasing concentration, the removal of toluene by the organo-zeolites increases. The results indicate that only modifications in the amount over 60% of ECEC result in a good sorbent for toluene removal. p-Xylene was the best sorbed hydrocarbon from aqueous solution by zeolite Na-P1 and organo-zeolites. In Fig. 9, sorption efficacy according to the equilibrium concentration has been presented. The highest sorption of p-xylene equals

11.47 mg/g and has been recorded for the sample 0.8 ECEC in an initial concentration of 253 mg/L (Table 7). The differences in the efficiency of sorption for individual modifications are insignificant. The smallest amount of p-xylene, only 1.79 mg/g, was removed by 0.2 and 0.4 ECEC organo-zeolites at initial concentration 45.9 mg/L. There was no relationship between the amount of attached to the zeolite HDTMA-Br and the efficiency of sorption. This could be explained by p-xylene’s chemical properties. The presence of the methyl group makes it much more reactive than benzene. The only clear correlation is that with an increasing initial concentration of p-xylene the sorption efficiency increases. In Fig. 10 the correlation between adsorption of benzene, toluene and p-xylene (BTX) by Na-P1 zeolite and HDTMA modified zeolite treated at different concentrations of initial surfactant as well as different initial concentration of the solution are presented. The highest sorption efficiency for p-xylene and the lowest for benzene have been documented. The graph (Fig. 10) reveals a linear relationship between sorption efficiency and the amount of HDTMA attached to the zeolites’ surface. The difference between the efficiency of sorption of various volatile compounds are caused by their chemical and physical properties. Adsorption affinity of BTX occurs in the following order: p-xylene > toluene > benzene. This was due to the molecular size of the compounds in which p-xylene with the largest molecular size being the most adsorbed. Benzene with the molecular size 6.6 Å was less adsorbed by the organo-zeolites. Another reason for differences in sorption efficacy is the presence of a methyl group. The methyl group makes toluene around 25 times more reactive than benzene in such reactions. For p-xylene, were two methyl groups occurs, reactivity will be even higher. Dipole moment which is providing the charge and its distribution in the molecule may also be an important factor. Benzene, with the lowest sorption efficacy, has dipole moment 0. While toluene and p-xylene have dipole moment 0.36 and 0.07 D (Debye), respectively. 3.4. BTX removal efficiency The final step was to test the efficacy of BTX removal from aqueous solutions. For the experiments of benzene and toluene removal, the initial concentrations of 50, 250 and 500 ppm were selected (Fig. 11). Due to the very poor solubility of p-xylene in water, lower concentrations such as 50, 100, and 250 ppm were selected. Modification 1.0 ECEC removes more than 97% of p-xylene, more than 93% of toluene and above 84% of benzene from the aqueous solution. At the same time, zeolite Na-P1 has removed between 2% and 10% less hydrocarbons. The higher the initial

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Fig. 11. The efficacy of BTX removal from aqueous solutions.

concentration of the solution, the greater the percentage of the hydrocarbon removed from the aqueous solution. The differences obtained in the experiments are not too large. Although the differences are substantial, they are very important in terms of water treatment from BTX contaminations. Due to strict concentration limits of volatile compounds in the water, every 1% of BTX constitutes great value. The experiment of removing BTX compounds from water backed up previous studies in which adsorption affinity occurs in the following order: p-xylene > toluene > benzene. 4. Conclusions The results revealed a very strong correlation between spectroscopic and chemical analyses and allowed for the creation of calibration curves. This meant that FTIR could be used for rapid determination of surfactants in organo-zeolites. Zeolite Na-P1 and organo-zeolites after modification with HDTMA show a considerable ability to remove BTX contamination from the aqueous solution. Based on experimental data the removal efficiencies for a single-solute system follows the order: p-xylene > toluene > benzene. Experiments have shown that the use of various multiplicities of ECEC has an influence on the amount of BTX removal from the water solution. Selected research materials efficiently adsorb p-xylene, but it cannot be concluded that the surface modification significantly affects the process. The differences between the sorption efficiency of various hydrocarbons are visible. The results of this research can be used in environmental protection and for further study into the properties of surfactantmodified synthetic zeolites and their potential industrial applications; for example, in petrochemistry. Acknowledgements We gratefully acknowledge the support of NCBiR having provided Grant No. PBS1/A2/7/2012. References [1] Y. Yang, N. Burke, J. Zhang, S. Huang, S. Lim, Y. Zhu, RSC Adv. 4 (2014) 7279– 7287. [2] R.M. Barrer, Pure Appl. Chem. 51 (1979) 1091–1100. [3] R.E. Apreutesei, C. Catrinescu, C. Teodosiu, Environ. Eng. Manag. J. 7 (2008) 149–161. [4] H.P. Chao, S.H. Chen, Chem. Eng. J. 193–194 (2012) 283–289. [5] J. Weitkamp, Solid State Ionics 131 (2000) 175–188. [6] R. Gounder, E. Iglesia, Chem. Commun. 49 (2013) 3491–3509. [7] C. Paris, M. Moliner, A. Corma, Green Chem. 15 (2013) 2101–2109. [8] G.M. Heggerty, R.S. Bowman, Environ. Sci. Technol. 28 (1994) 425–458.

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