Article pubs.acs.org/IECR
Workplace Nitrous Oxide Sampling: Alternative Adsorbents Marianne Guillemot* and Blandine Castel INRS, Département Métrologie des Polluants, 1 rue du Morvan, CS60027, 54519 Vandoeuvre Cedex, France ABSTRACT: Nitrous oxide (N2O) is used in medical and veterinary procedures and in other sectors. Personnel exposure to this compound can have serious health effects. The current method used to sample N2O is based on diffusive sampling on a 5 Å molecular sieve, but this is not ideal because a back-diffusion phenomenon is observed. In this study, cationic ZSM5 zeolites were tested as possible alternative sampling supports. Zeolites were prepared, characterized, and tested for nitrous oxide adsorption. The influence of water vapor in the stream was also investigated. The performances of the various adsorbents tested were found to depend on the number and the nature of the exchange cations. The uptake rate of each zeolitic adsorbent was then determined to investigate whether back-diffusion occurred. The highest adsorption capacities, in the presence or absence of water vapor, were obtained with Ba-exchanged ZSM5 zeolites with an Si/Al ratio of 11; no back diffusion was observed on this adsorbent. BaZSM5(11) thus appears to be a suitable solid to replace 5 Å molecular sieve for diffusive N2O sampling.
1. INTRODUCTION
Numerous studies have investigated the adsorption of N2O over inorganic and organic media.8−17 Among the solids tested, zeolites have convenient characteristics for nitrous oxide sampling and analysis thanks to their stability at high temperatures, their good adsorption properties, and their easy regeneration. In addition, it has been demonstrated that the most efficient adsorbents at low pressures are those with pore dimensions close to the dimensions of the adsorbate molecule.18 Zeolites are natural or synthesized aluminosilicates based on a three-dimensional crystalline network of SiO4 and AlO4− tetrahedrons. Adsorbent−adsorbate interactions can be modified by dealumination or cationic exchange in channels and cavities. These solids have pore apertures ranging between 2 and 10 Å, which makes them efficient for adsorption of small molecules. For this study, ZSM5 zeolite was chosen due to its 10 atoms of oxygen apertures leading into cavities of 5.4 Å diameter. This correlates well with the kinetic diameter of nitrous oxide, which is calculated to be between 3.3 and 4.2 Å based on the LennardJones and Pauling models.10,19 ZSM5-type zeolites were exchanged with various cations and then investigated under dynamic N2O sampling conditions with a view to improving the current sampling method. Back-diffusion phenomena on the prepared zeolites were also examined during passive sampling experiments.
Nitrous oxide (N2O) is used as an anesthetic in medical, dental, and veterinary procedures. This gas is also used in food processes as a foaming agent for whipped cream, an oxidant for organic compounds, a nitrating agent for alkaline metals and as a component of some rocket fuels. Several toxicological studies have shown that occupational exposure to N2O can cause adverse effects such as reduced fertility, spontaneous abortions, neurological disorders, and renal and liver disease.1 The American Conference of Governmental Industrial Hygienists (ACGIH) has established a threshold limit value (TLV) for N2O of 50 ppmv for an 8 h time-weighted average.2 In France, decree DGS/3A/667 set the TLV for N2O to 25 ppmv during the maintenance phase of anesthesia. The method currently used at INRS to assess worker exposure to N2O is based on passive sampling on a thermal desorption sampler filled with 750 mg of 5 Å molecular sieve.3 Passive diffusion samplers are widely used in occupational monitoring to assess exposure to organic vapors or gas. Several investigations have highlighted the advantages of passive sampling for occupational exposure assessment.4−6 Compared to active sampling which requires the use of individual pumps, diffusive samplers are light, easy to use, and cost-effective devices. Analytes can be extracted from adsorbents used in passive samplers either by thermal desorption or using solvents. However, thermal desorption offers higher sensitivity and avoids the use of toxic substances. Pollution of the workplace atmosphere can be assessed using diffusive sampling on solid adsorbents provided these adsorbents allow complete retention (i.e., no breakthrough and no back diffusion, and complete recovery during analysis).7 The current method used to sample N2O has an important limitation due to the relatively low affinity between the adsorbent, 5 Å molecular sieve, and the adsorbate. This results in back-diffusion phenomena as the concentration of N2O decreases during sampling.3 © XXXX American Chemical Society
2. MATERIALS AND METHODS 2.1. Zeolite Preparation. Pure zeolites, without binder, were supplied by Zeolyst Corporation. ZSM5 type zeolites consist of 10 atoms of oxygen apertures and cavities of 5.4 Å diameter. The Cu-, Ag-, and Ba-exchanged zeolites were prepared from a NH4ZSM5 zeolite by ammonia removal and ion exchange. Received: March 3, 2015 Revised: July 27, 2015 Accepted: July 29, 2015
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DOI: 10.1021/acs.iecr.5b00832 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 1. Diagram of the dynamic adsorption apparatus.
NH4ZSM5 zeolites were first calcined under dry airflow (150 mL/min/g) with a 1 °C/min ramp from room temperature to 450 °C, a 1 h step at 110 °C, and a 4 h step at 450 °C. This produced HZSM5. Three consecutive exchanges were performed by mixing HZSM5 with the corresponding nitrate salt solution (at 0.5 mol/L) for 3 h under slow agitation, at 50 °C. The zeolite was isolated by centrifugation before repeating the exchange. Once the three exchanges had been performed, the zeolite was washed with deionized water and centrifuged three times prior to overnight drying at 100 °C. Zeolites were then pretreated under dry nitrogen at 350 °C for 4 h. The zeolites were pelletized, crushed, and then sieved to produce grains with a diameter of 0.355−0.8 mm. Each zeolite is named as follows: MxZSM5(y) where Mx represents the compensating cation and y represents the Si/Al ratio for the framework. 2.2. Zeolite Characterization. The hydration level of zeolites was determined by thermogravimetric analysis (TGA) from ambient temperature up to 350 °C with a ramp of 5 °C/ min. The exchange rate for each cationic zeolite was determined by atomic absorption spectrometry (ICP/OES). The samples were dissolved in nitric acid after treatment with sodium peroxide. Nitrogen adsorption−desorption measurements were performed at −196 °C in a gas adsorption system (Belsorp Max, Bel Japan). Prior to this, samples were pretreated under vacuum at 400 °C for 3 h. The total pore volume is measured at P/P0 = 0.97. The microporous volume was determined using the t-plot method. 2.3. Dynamic Adsorption of Nitrous Oxide. Adsorption experiments were performed at constant temperature (21 °C) under atmospheric pressure in a fixed-bed reactor consisting of a stainless steel tube (5 mm internal diameter, 9 cm length). A known amount of adsorbent was deposited between two
metallic grids. Before each test, the adsorbent was pretreated under dry nitrogen (50 mL/min) at 350 °C for 2 h. The inlet gas was obtained by diluting nitrous oxide from a gas bottle at 100 ppmv in air, through a mixing and a homogenization cells. For experiments performed with 50% relative humidity, a saturator containing water was added to the setup (Figure 1). The mixture containing N2O at 0 or 50% relative humidity was then passed through the adsorbent bed. Inlet and outlet N2O levels were measured online by a previously calibrated PhotoAcoustic System (PAS; Innova 1310). Breakthrough was defined as when N2O concentration in the outlet stream reaches 5% of the inlet concentration, i.e. 4 ppmv. Saturation was defined as when the inlet and outlet N2O concentrations merged. 2.4. Modeling Breakthrough Curves. The breakthrough curves show the evolution of Cout/Cin ratio as a function of time on stream. Cout is the concentration at the outlet of the adsorbent bed, and Cin is the concentration at the inlet. The adsorbent bed can be viewed as a column through which the analyte passes. The model used to fit experimental points is the integral of a Gaussian curve:20 y(t ) =
∫0
t
1 [ e 2π σ
(t − t1/2) 2σ
]
dt
where y(t) corresponds to the Cout/Cin ratio, t1/2 represents the time at which the Cout/Cin ratio equal 0.5, also called “time of half retention”, and σ represents the standard deviation of the distribution. These two parameters are different for each adsorbent. σ2 is the variance which characterizes the width of the distribution, such that a low σ2 value indicates rapid B
DOI: 10.1021/acs.iecr.5b00832 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research breakthrough. The value of σ2 depends on the diffusion characteristics of the mass transfer zone. Slow breakthrough on zeolite-type adsorbents suggests a large mass transfer resistance.21 2.5. Generation of Controlled Atmospheres To Determine N2O Uptake Rate. The gas test device used in this study was fully described in a previous paper.22 Nitrous oxide atmospheres were dynamically generated by dilution of bottled N2O at 50,000 ppmv in a constant air flow and mixed with a constant stream of humidified air. Air flows were controlled by gas mass flow controllers. The relative humidity was regulated using a motorized electric drive valve. The temperature of the atmosphere generated was regulated using externally autoregulated refrigeration/heating circulators. The exact N2O concentrations were determined online by the PAS analyzer. The uptake rate for each zeolite was determined by diffusive sampling in a controlled atmosphere of nitrous oxide and water vapor. For passive sampling, previously prepared stainless steel tubes of zeolites were fitted with a cap containing a Teflon filter and a grid which defines the sampling surface. The other end of the tube was kept sealed. According to Fick’s Law, the mass of analyte sampled depends on its concentration in the atmosphere and on the exposure time:23,5
Table 1. Thermal Desorption Conditions temperature (°C)
time (min)
transfer line valve trap low trap high trap rate (°C/s) options
210 200 −30 250 40
inlet split outlet split internal standard injections split mode
tube desorp. trap hold cycle purge trap desorp. pneumatics (mL/min)
on on NA 1 flow
tube desorp. column col/trap desorp. column mode
6.0 1.0 10.0 3.0 1.0 5.0 1.5 1.5 flow
hydrophilic character of the zeolites decreases as the number of compensating cations decreases. 3.2. Statistical Calculation. The uncertainty on the online concentration was calculated as follows, according to the European standard EN 482:1994: Um =
|C̅ − Cref | + 2 × Sp Cref
= 0.06
Um: Uncertainty of online measurement Sp: Standard deviation of the concentration measured by the PAS C̅ : Mean of the concentration measured by the PAS (ppmv) Cref: Theoretical concentration (ppmv) The overall uncertainty was calculated as follows:
A m i = K i (C i − C0)t l
mi: mass of analyte i sampled (pg) Ki: Coefficient of diffusion for analyte i in air (cm2/min) A: Cross sectional area of the tube (cm2) l: Length of the tube (cm) Ci: Concentration of analyte i in air (μg/m3) C0: Concentration of analyte i surrounding the adsorbent surface (μg/m3) t: Sampling time (min) The term Ki(A/l) corresponds to the uptake rate, Ui, for analyte i. Thermodesorption Analyses. The amount of N2O captured on diffusive samplers was determined by thermal desorption/ GC/MS (PerkinElmer TurboMatrix ATD linked to PerkinElmer GC Autosystem XL equipped with a PerkinElmer MS TurboMass Gold). The analytical column measured 60 m long × 0.32 mm i.d. × 1.4 μm film thickness (GASPRO, Agilent). The column temperature was 27 °C. Sampling tubes were heated to 250 °C to desorb analyte. The inlet and outlet split were set to 30 mL/min and 8 mL/ min, respectively. All other desorption conditions are listed in Table 1.
U=
[Um 2 + Ub 2] = 0.061
Ub: uncertainty of the concentration of the gas bottle used for calibration The repeatability of the analysis was evaluated by reproducing three time the same experiment on BaZSM5(40) in the absence of water, at an N2O concentration of 80 ppmv and a flow rate of 110 mL/min. Results are reported in Table 3. 3.3. N2O Adsorption: Influence of Cation Nature and of Si/Al Ratio. To determine the influence of the zeolites’ characteristics on N2O adsorption, experiments were first performed in the absence of water, at an N2O concentration of 80 ppmv and a flow rate of 110 mL/min. The breakthrough curves obtained with the six adsorbents prepared are presented in Figure 2. The adsorption capacities for each adsorbent before breakthrough and at saturation are reported in Table 4 in mg of N2O per gram of zeolite. Breakthrough corresponds to the amount of N2O adsorbed while the outlet concentration remains below 4 ppmv (i.e., 5% of the inlet concentration). Saturation of the zeolite is obtained when Cout/Cin ratio equals 1. The value of σ2 was also calculated by modeling the experimental data. These results indicate that both the nature of the cation and the Si/Al ratio influence N2O adsorption on ZSM5 zeolites. The adsorption capacity decreases as the Si/Al ratio increases. Thus, on cationic exchanged zeolites, N2O adsorption does not only depend on the available microporous volume.14,24 Two factors lead the adsorption process: the microporous volume (steric factor) and physicochemical interactions between adsorbate molecules and the zeolite surface. At low pressures, the amount of N2O adsorbed is determined by the second one, whereas at high pressures, the microporous volume has a
3. RESULTS AND DISCUSSION 3.1. Adsorbent Characteristics. The main characteristics of the cationic ZSM5 samples prepared and assessed in this work are shown in Table 2. Dealumination decreases the number of aluminum atoms and, consequently, the number of compensating cations. Consequently, a high Si/Al ratio corresponds to low cations content and a higher microporous volume than at low Si/Al ratio. The exchange ratio appears to mostly depend on the nature of the cation rather than the microporous volume. H corresponds to the hydration level for the zeolites. As expected, H decreased as the Si/Al ratio increased. The C
DOI: 10.1021/acs.iecr.5b00832 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Table 2. Adsorbent Characterization CuZSM5(11)
AgZSM5(11)
BaZSM5(11)
CuZSM5(40)
AgZSM5(40)
BaZSM5(40)
36 0.14 294 0.90 10.6
61 / / / 17.0
38 0.14 287 0.82 9.2
29 0.20 394 1.07 5.6
61 0.20 387 1.06 6.4
32 0.19 372 1.07 6.8
exchange ratioa (%) microporous volumeb (cm3STP/g) specific surface areac (m2/g) average pore diameterc (nm) Hd (%) a
Determined by ICP/OES. bDetermined by the t-plot method. cDetermined by the BET method. dDetermined by TGA.
Centi et al. and Zhang et al.15,26 They showed that a strong electric field could highly polarize the nitrous oxide molecules and thereby promote their adsorption. In parallel, Zhang et al. have observed that ZSM5 zeolite exchanged with cations of large radius exhibits the best strength of adsorption. Accordingly, as Ba2+, Ag+, and Cu2+ have a radius of respectively 135, 115, and 73 pm, nitrous oxide adsorption is favored on BaZSM5 zeolites. In addition, interactions of higher energy involve more diffusion resistance of the adsorbate molecules in the zeolite porous system.27 Therefore, N2O breakthrough occurs slower on BaZSM5 than on Ag- and Cu-ZSM5. The lower value of σ2 obtained with ZSM5(40) indicates that the breakthrough front is steeper with ZSM5(40) than with ZSM5(11). In general, adsorbate−adsorbent interactions are of higher energy than adsorbate−adsorbate ones. As the aluminum content of the zeolite framework decreases, the amount of cations decreases as well, so that the adsorbate− adsorbent interactions are reduced. On the other hand, dealumination frequently increases the macroporous volume of the zeolite which improves the diffusion and the transport of adsorbate molecules.27 Consequently, there is less diffusion resistance on ZSM5(40) than on ZSM5(11), which leads to a faster breakthrough. Comparison of the adsorption capacities before breakthrough and at saturation shows that adsorption of nitrous oxide is favored. Indeed, if the adsorption equilibrium was unfavorable, we should observe a non-null outlet concentration of N2O from the start of the experiment. Among the six different cationic zeolites assessed, BaZSM5(11) exhibits the best results for N2O adsorption. 3.4. N2O Adsorption: Influence of Water Vapor. To match real life conditions, adsorption was also tested in the presence of water vapor. Experiments were carried out with an N2O concentration of 80 ppmv and a relative humidity (RH) of 50%. The total flow rate was 110 mL/min. The breakthrough curves for N2O under humid conditions are shown in Figure 3, and the adsorption capacities for the different supports in these conditions are presented in Table 5. The adsorption capacity of all the zeolites studied is decreased in the presence of water vapor. N2O is still preferentially adsorbed on weakly dealuminated zeolites, as the adsorption capacity of ZSM5(11) is higher than that of ZSM5(40), even in the presence of water. However, it appears that on Ba-exchanged zeolites, N2O breakthrough is followed by a desorption peak as the Cout/Cin ratio exceeds 1. This means that some of the adsorbed N2O molecules are displaced from the adsorbent by water molecules.26 Similar behavior has previously been reported with cationic Faujasite.24 Thus, the final loading is lower than, or very similar to, the amount of N2O adsorbed before breakthrough.
Table 3. Adsorption Capacities in mg/g of N2O - Evaluation of the Repeatability before breakthrough
at saturation
0.825 0.844 0.905 0.858 0.042 4.88%
1.423 1.261 1.423 1.369 0.094 6.84%
exp. 1 exp. 2 exp. 3 average capacity standard deviation relative standard deviation
Figure 2. Breakthrough curves for N2O (80 ppmv) at 21 °C, 110 mL/ min, on: (purple △) CuZMS5(11), (yellow *) AgZSM5(11), (blue +) BaZSM5(11), (red ○) CuZSM5(40), (green ◊) AgZSM5(40), (turquoise ×) BaZSM5(40).
Table 4. Adsorption Capacities in mg/g of N2O CuZSM5(11) AgZSM5(11) BaZSM5(11) CuZSM5(40) AgZSM5(40) BaZSM5(40)
before breakthrough
at saturation
σ2
0.375 0.585 2.880 0.281 0.383 0.825
0.734 1.043 4.150 0.707 0.889 1.423
36 81 484 16 25 49
predominant role. The higher the number of cations in the framework, the higher the local electric field in the zeolite involving higher adsorbate−adsorbent interactions.25 The dealumination increases the available volume for nitrous oxide adsorption but also decreases the number of adsorption sites and consequently the amount of N2O molecules adsorbed. For both Si/Al ratios studied, the amount of adsorbed N2O follows the order Cu2+ < Ag+ < Ba2+. These results cannot be correlated to the microporous volume of the zeolite, as for the same Si/Al ratio the ZSM5 zeolites have closed microporous volume. The σ2 value follows the same ranking as the breakthrough occurs faster on Cu-exchanged zeolites than on Ag- and Ba-exchanged zeolites. Similar results were reported by D
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U: uptake rate in (mL/min) m: amount of N2O recovered (pg) t: duration of exposure (min) CN2O: concentration of N2O in the controlled gas test device (mg/m3) Kr: desorption coefficient N2O The uptake rate for the different zeolites roughly follows the same order as previously in dynamic conditions, thus: BaZSM5(11) > AgZSM5(11) = CuZSM5(11) BaZSM5(40) > AgZSM5(40) > CuZSM5(40)
On the basis of these values, back diffusion was assessed for each type of adsorbent by diffusive sampling in an atmosphere containing 80 ppmv N2O and 50% RH over 2 h followed by 2 h of diffusive sampling in an atmosphere without N2O, but still at 50% RH. The amount of N2O remaining on the adsorbent was then determined by TD/GC/MS. The N2O concentration calculated based on the amount of nitrous oxide retained on the sampler and its uptake rate, determined in the previous tests, was compared to the values returned by the PAS online concentration monitoring, as follows:
Figure 3. Breakthrough curves for N2O (80 ppmv) at 21 °C, RH = 50%, 110 mL/min, on: (△) CuZMS5(11), (yellow *) AgZSM5(11), (blue +) BaZSM5(11), (red ○ ) CuZSM5(40), (green ◊ ) AgZSM5(40), (turquoise ×) BaZSM5(40), (red +) 5 Å molecular sieve.
Table 5. Adsorption Capacities in wt % of N2O, RH = 50% CuZSM5(11) AgZSM5(11) BaZSM5(11) CuZSM5(40) AgZSM5(40) BaZSM5(40) 5 Å molecular sieve
before breakthrough
at saturation
σ2
0.290 0.394 1.137 0.192 0.308 0.428 0.621
0.465 0.533 0.672 0.532 0.719 0.502 0.576
16 30 12.25 6.25 9 2.25 36
Recovery rate (%) =
CuZSM5(11) AgZSM5(11) BaZSM5(11) CuZSM5(40) AgZSM5(40) BaZSM5(40) 5 Å molecular sieve
exp. 3
average
0.43 n.d. 0.67 0.31 0.33 0.52 0.71
0.35 0.42 0.62 0.29 0.34 0.46 0.58
0.42 0.36 n.d. 0.34 0.39 0.59 0.83
0.40 0.39 0.65 0.31 0.35 0.52 0.71
× 100
Table 7. Recovery Rate (%) of Diffusive Samplers CuZSM5(11) AgZSM5(11) BaZSM5(11) CuZSM5(40) AgZSM5(40) BaZSM5(40) 5 Å molecular sieve
exp. 1
exp. 2
exp. 3
average
83 n.d. 100 88 94 100 77
74 79 96 71 78 80 73
82 97 91 71 74 85 76
80 88 96 76 82 88 75
As with the adsorption capacity or uptake rate, the capacity of zeolites to retain adsorbed nitrous oxide as the ambient concentration drops to zero during passive sampling appears to be linked to the nature of the exchange cation. Thus, BaZSM5(11) has the highest uptake rate and displays no back-diffusion phenomenon.
Table 6. Uptake Rate in mL/min for Diffusive Samplers (80 ppmv N2O, 50% RH) exp. 2
C N2O
Table 7 reports the recovery rate for each diffusive sampler.
However, as the zeolites studied are intended to be used in passive sampling, this phenomenon is negligible compared to the effect it would have with active sampling use. 3.5. Determining the Uptake Rate and Evaluating Back Diffusion. In total, 80 ppmv of N2O was generated in an atmosphere with 50% RH, using the controlled gas test device, to mimic moderately polluted areas. Diffusive sampling was carried out over 2 h simultaneously on the six different zeolites and on a 5 Å molecular sieve. The same experiment was repeated three times to compare the results obtained. Following diffusive sampling, adsorbents were analyzed by TD/GC/MS (Table 6). A simplified method was used to
exp. 1
m T × t × Kr
4. CONCLUSION The present work was undertaken to investigate the capacity of Ba-, Ag-, and Cu-exchanged ZSM5 to adsorb and retain nitrous oxide. Dynamic adsorption experiments with N2O in the presence and absence of water vapor revealed that the adsorption capacity of ZSM5 samples depends on the nature of the cation and on the Si/Al ratio. Interestingly, a higher available microporous volume on ZSM5(40) does not lead to higher adsorption capacities. Rather, the most important factor in determining adsorption capacity is the number of cations in the framework. Thus, lower dealuminated zeolites ZSM5(11) provided a better adsorption efficiency. To a lesser extent, the nature of the cation influences both the amount of N2O adsorbed and the strength of adsorption, which is indicated by the value of σ2. Adsorption capacities can be ranked in the following order: BaZSM5 > AgZSM5 > CuZSM5; however, σ2 follows the same order.
determine the uptake rate, taking the amount of nitrous oxide on each sampler, the exposure time, the assigned N2O concentration, and the temperature and pressure readings for the day of the experiment, as follows: m U= t × C N2O × Kr E
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(15) Zhang, B.; Lu, Y.; He, H.; Wang, J.; Zhang, C.; Yu, Y.; Xue, L. Experimental and density functional theory study of the adsorption of N2O on ion-exchanged ZSM-5: Part II. The adsorption of N2O on main-group ion-exchanged ZSM-5. J. Environ. Sci. 2011, 23, 681. (16) Rakic, V.; Dondur, V.; Gajinov, S.; Auroux, A. Calorimetric study of room temperature adsorption of N2O and CO on Cu(II)exchanged ZSM5 zeolites. Thermochim. Acta 2004, 420, 51. (17) Rakic, V.; Rac, V.; Dondur, V.; Auroux, A. Competitive adsorption of N2O and CO on CuZSM-5, FeZSM-5, CoZSM-5 and bimetallic forms of ZSM-5 zeolite. Catal. Today 2005, 110, 272. (18) Pinto, M. L.; Pires, J.; Carvalho, A. P.; de Carvalho, M. B. On the Difficulties of Predicting the Adsorption of Volatile Organic Compounds at Low Pressures in Microporous Solid: The Example of Ethyl Benzene. J. Phys. Chem. B 2006, 110, 250. (19) Breck, B. W. Zeolites Molecular Sieves. Structure, Chemistry, and Use; John Wiley and Son: Hoboken, NJ, 1974. (20) Guillemot, M.; Mijoin, J.; Mignard, S.; Magnoux, P. Adsorption of tetrachloroethylene (PCE) in gas phase on zeolites of faujasite type: Influence of water vapour and of Si/Al ratio. Microporous Mesoporous Mater. 2008, 111, 334. (21) Huang, Z. H.; Kang, F.; Liang, K. M.; Hao, J. Breaktrough of methylethylketone and benzene vapors in activated carbon fiber beds. J. Hazard. Mater. 2003, 98, 107. (22) Delcourt, J.; Guenier, J. P.; Muller, J. Echantillonnage des polluants gazeux. Le badge INRS. 2 - Dispositif expérimental de validation. Cahier de notes documentaires 1990, 38, 23. (23) Palmes, E. D.; Lindenboom, R. H. Ohm’s law, Fick’s law, and diffusion samplers for gases. Anal. Chem. 1979, 51, 2400. (24) Guillemot, M.; Mijoin, J.; Mignard, S.; Magnoux, P. Adsorption of Tetrachloroethylene on Cationic X and Y Zeolites: Influence of Cation Nature and of Water Vapor. Ind. Eng. Chem. Res. 2007, 46, 4614. (25) Hernandez-Huesca, R.; Diaz, L.; Aguilar-Armenta, G. Adsorption equilibria and kinetics of CO2, CH4 and N2 in natural zeolites. Sep. Purif. Technol. 1999, 15, 163. (26) Centi, G.; Generali, P.; dall’Olio, L.; Perathoner, S.; Rak, Z. Removal of N2O from Industrial Gaseous Streams by Selective Adsorption over Metal-Exchanged Zeolites. Ind. Eng. Chem. Res. 2000, 39, 131. (27) Yu, W.; Yuan, P.; Liu, D.; Deng, L.; Yuan, W.; Tao, B.; Cheng, H.; Chen, F. Facile preparation of hierarchically porous diatomite/ MFI-type zeolite composites and their performance of benzene adsorption: The effects of NaOH etching pretreatment. J. Hazard. Mater. 2015, 285, 173.
Experiments performed in an atmosphere with 50% RH indicate that both water vapor and nitrous oxide are adsorbed on the zeolites tested, as all adsorption capacities are decreased in the presence of humidity. In the case of BaZSM5, water molecules were preferentially adsorbed, which results in desorption of nitrous oxide molecules. However, this is of minor importance as the adsorbents are intended for use in diffusive sampling. The uptake rate for each ZSM5 zeolite was calculated to determine whether back diffusion occurs in diffusive conditions. It appears that during passive sampling, the behavior of ZSM5 is similar to that in dynamic conditions. Thus, BaZSM5(11) exhibits the best properties for diffusive sampling of N2O. Further investigation will be required to determine the influence of concentration, temperature, and humidity level on the uptake rate for diffusive samplers containing this adsorbent.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.iecr.5b00832 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
