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Keywords:

  • Adsorption kinetics;
  • Organic compound;
  • Activated carbon;
  • Carbon nanotubes

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES

Adsorption kinetics of two organic compounds on four types of carbonaceous adsorbents (a granular activated carbon [HD4000], an activated carbon fiber [ACF10], a single-walled carbon nanotube [SWNT], and a multiwalled carbon nanotube [MWNT]) was examined in aqueous solutions. The times needed for the adsorption to reach apparent equilibrium on the four carbons followed the order of ACF10 > HD4000 > SWNT > MWNT. Ultrasonication of the carbon nanotubes (CNTs) accelerated their adsorption kinetics but had no effect on their equilibrium adsorption capacities. The pseudo-second order model (PSOM) provided good fitting for the kinetic data. The fitting of kinetic data with the intraparticle diffusion model indicated that external mass transfer controls the sorption process in the organic compound–CNT systems, whereas intraparticle diffusion dominates in the sorption of organic compounds onto activated carbons. Environ. Toxicol. Chem. 2012;31:79–85. © 2011 SETAC


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES

Carbon nanotubes (CNTs) are carbonaceous adsorbents with hydrophobic surfaces that exhibit strong adsorption affinities to organic compounds 1–10. Adsorption is a heterogeneous process that consists of a combination of physical, chemical, and electrostatic interactions. The underlying mechanisms of the chemical and electrostatic interactions involved in the adsorption of organic compounds by CNTs and activated carbons are almost the same. The major difference between adsorption by activated carbons and CNTs exists in their physical interactions with organic compounds. Activated carbons have rigid and heterogeneous structures that consist of micro-, meso-, and macropores. Carbon nanotubes have uniform structural units but are prone to aggregate, forming bundles of randomly tangled agglomerates because of the strong van der Waals forces along the length axis. The outermost surface, inner cavities, interstitial channels, and peripheral grooves of CNTs constitute four possible sorption sites for organic compounds 10.

A wealth of experimental evidence and mechanistic theories is available in the literature regarding the adsorption kinetics of organic compounds on activated carbons 11. For a solid-liquid sorption system with porous activated carbons, essentially four steps make up the adsorption process: bulk diffusion, boundary layer diffusion (external mass transfer), intraparticle diffusion, and surface reaction. Bulk diffusion is the transport of solute from the bulk solution to the boundary layer surrounding the adsorbent particles, and boundary layer diffusion (external mass transfer) is the diffusion of solute through the boundary layer to the sorbent exterior surface. In certain cases, sorption occurs instantaneously on the external surface of the adsorbent. Intraparticle diffusion is the migration of solute within the pores of the adsorbent along the sorbent inner surface (surface diffusion) or within liquid contained in the pores of the particles (pore diffusion). Surface reaction is the interaction of solute with the available sites on the interior surface of the pores and capillary spaces of the sorbent. The overall rate of adsorption is controlled by the slowest step. Generally, the bulk diffusion and surface reaction steps are rapid and not rate-limiting. The external mass transfer controls adsorption for the systems with poor mixing, dilute concentration of sorbate, small particle sizes of sorbent, and high affinity of sorbent to sorbate, whereas the intraparticle diffusion controls the sorption process for a system with good mixing, large particle sizes of sorbent, high concentration of sorbate, and low affinity of sorbent to sorbate 12.

Although a wealth of equilibrium data are available in the literature 1–10 for the adsorption of organic compounds by CNTs, adsorption kinetics have been investigated only in recent years 13–19. These studies indicated that the pseudo-second order model (PSOM) 20 provided a good fit to the kinetic data. In a comparative study of the adsorption of 17α-ethinyl estradiol and bisphenol A on both single-walled CNT (SWNT) and multiwalled CNT (MWNT) 18, the PSOM showed better fitting performance than the pseudo-first order model (PFOM) 21, whereas both models failed to properly fit the kinetic data for their adsorption on an activated carbon. Because the equations expressed in PFOM and PSOM are empirical, the successful correlation of experimental data with either of these two models hardly can be considered as a proof of intraparticle diffusion or surface reaction being the rate-limiting step 22. The intraparticle diffusion model (IPDM) based on the theory proposed by Weber and Morris 23 has been widely used to elucidate the rate-limiting step of the adsorption process. The studies that applied IPDM on CNT adsorption reported that sorption on CNTs involved intraparticle diffusion; however, it was not the only rate-controlling step 13, 14, 17, and the sorption on CNTs was faster than that on activated carbons, because the contribution of intraparticle diffusion to the overall sorption on CNTs was not important, whereas it was the controlling factor for the sorption on activated carbons 15, 18.

Although equilibrium studies provide adsorption capacity information, sorption kinetics is important because it provides valuable insights into the sorption mechanisms. Considering the wide range of use and application of CNTs, and the relatively limited information on the adsorption kinetics of CNTs in the literature, our objective in the present study was to examine and compare sorption kinetics of CNTs and activated carbons. Our approach was to use the two small organic compound probe molecules, phenanthrene and biphenyl, with simple adsorption mechanisms to examine their sorption kinetics on well-characterized activated carbons and CNTs, including a granular activated carbon, an activated carbon fiber, an SWNT, and an MWNT. The results are helpful to further assess the use of CNTs in certain water treatment applications as well as their sorption behaviors in the environment if they are accidentally or intentionally released.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES

Materials

An SWNT (outer diameter: 1–2 nm, length: 5–30 µm, purity > 90%, from Chengdu Organic Chemicals, Chinese Academy of Sciences), an MWNT (inner diameter: 3–5 nm, outer diameter: 8–15 nm, length: 10–50 µm, purity > 95%, from Nanostructured & Amorphous Materials), a coal-based granular activated carbon (HD4000, 150–180 µm in diameter, from Norit), and a phenol formaldehyde-based activated carbon fiber (ACF10, in the form of cloth, from American Kynol) were used as received. As illustrated in Figure 1, the four adsorbents were different in pore structure: ACF10 was nearly 100% microporous; SWNT was hybrid in mesopores and micropores; MWNT was dominated by mesopores and macropores; and HD4000 had the highest heterogeneity in pore structure, with a wide distribution of pore sizes. The two probe molecules, phenanthrene and biphenyl (purchased from Sigma-Aldrich), are different in hydrophobicity and planarity. More detailed structural characteristics of the carbons and molecular properties of organic compounds have been reported earlier 10.

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Figure 1. Surface area distributions of the four carbons. ACF10 = phenol formaldehyde-based activated carbon fiber; HD4000 = coal-based granular activated carbon; SWNT = single-walled carbon nanotube; MWNT = multiwalled carbon nanotube.

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Adsorption kinetic experiments

Bottle point experiments were performed for the kinetic study. Concentrated stock solutions of each adsorbate were prepared in methanol. For adsorption experiments without sonication, phenanthrene and biphenyl solutions were first prepared in distilled and deionized water with the addition of sodium azide (200 mg/L) to control microbial growth. Glass bottles (255 ml) containing approximately 1 mg of adsorbent were completely filled with biphenyl or phenanthrene solutions with an initial concentration of approximately 0.2 to 0.9 mg/L and sealed with Teflon-lined screw caps. For CNT adsorption experiments with sonication, bottles containing approximately 1 mg of CNT were first filled with a 200 mg/L sodium azide solution to nearly full and were put in a bath sonicator (320W, 40 kHz, Branson 8510 Tabletop Ultrasonic Cleaner) for 1 h. After the sonicated solutions were cooled down to room temperature, a predetermined volume of stock solution was spiked into the bottles, and the sodium azide solution was added to completely fill the bottles. The volume percentage of the spiked methanol stock solution was kept below 0.1% (v/v) to minimize the co-solvent effect. The headspace-free bottles were placed on a tumbler at a speed of 0.6 g for predetermined times. After removal from the tumbler, the sample solutions were centrifuged at a speed of 1,575 g for 15 min (Fisher Scientific Centrific 225 Centrifuge). The supernatants were analyzed by high-performance liquid chromatography with a UV and a fluorescence detectors using 4.6 × 150 mm ZORBAX Extend-C18 Columns (Agilent). Detailed high-performance liquid chromatography analysis conditions have been reported in our previous work 10. Bottles with no adsorbent served as blanks to monitor the losses of adsorbates during the experiments, which were found to be negligible. All experiments were performed at room temperature (21 ± 2°C) without any buffer addition. The equilibrium pH of solutions ranged from 6.3 to 7.4.

Data analysis

Three widely used kinetic models, PFOM, PSOM, and IPDM, were employed to fit the kinetic data by using their linear forms.

The PFOM:

  • equation image(1)

where qe (mg/g) and q (mg/g) are the amounts of solute sorbed at equilibrium and at time t (h), and k1 is the rate constant of the pseudo-first order sorption (/h). The values of qe and k1 can be obtained from the intercept and the slope of the linear plot of ln (qe − q) versus t.

The PSOM:

  • equation image(2)

where qe (mg/g) and q (mg/g) are the same as defined in the PFOM, and k2 is the rate constant of the pseudo-second order sorption (g/mg/h). The slope and intercept of the linear plot of t/q versus t yield the values of qe and k2. As t [RIGHTWARDS ARROW] 0, the initial sorption rate, h (mg/g/h), is

  • equation image(3)

The IPDM:

  • equation image(4)

where θ is the intercept of the linear portion of the plot, and ki is the intraparticle diffusion rate constant (mg/g/h0.5), which can be evaluated from the slope of the linear plot of q versus t1/2.

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES

Sorption kinetics

As shown in Figure 2, different times were required for the adsorption of biphenyl and phenanthrene onto the four different types of sorbents to reach equilibrium. The kinetic data plotted in Figure 2 were modeled using Equations 1 and 2. The plots of ln (qe − q) versus t from the kinetic data were nonlinear (data not shown), indicating that the PFOM is not suitable for the simulation of these data. The PFOM has been widely used in the case of one-site-occupancy adsorption (that is, one adsorbed molecule occupies one adsorption site) 22. As explained by Rudzinski and Plazinski 22, in the case of one-site-occupancy adsorption, the Lagergren kinetic equation leads to Langmuir equation at equilibrium. In our previous work 10, we demonstrated that the adsorption of organic compounds on the adsorbents used in the present study did not follow the Langmuir model but was fitted well with the Freundlich model, because the adsorption sites on these carbons are not energetically homogeneous. The empirical Freundlich equation well described the general features of the adsorption systems with different geometric and energetic surface heterogeneity.

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Figure 2. Uptake versus time profiles of biphenyl (BP) and phenanthrene (PNT) on the four carbons. ACF10 = phenol formaldehyde-based activated carbon fiber; HD4000 = coal-based granular activated carbon; SWNT = single-walled carbon nanotube; MWNT = multiwalled carbon nanotube; S = sonication; L = low initial concentration; LS = low initial concentration coupled with sonication; H = high initial concentration; HS = high initial concentration coupled with sonication.

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All t/q versus t plots showed very good linearity (data not shown). As summarized in Table 1, the regression coefficients of t/q versus t plots were higher than 0.99 for the adsorption of both compounds on all carbons, indicating that the PSOM provided good fits to the experimental data.

Table 1. The pseudo-second order kinetic parameters for biphenyl (BP) and phenanthrene (PNT) adsorption on the four carbons
TermaBPPNT
SWNTSWNT-SMWNT-LMWNT-LSACF10HD4000SWNTSWNT-SMWNT-HMWNT-HSMWNT-LMWNT-LS
  • ACF10 = phenol formaldehyde-based activated carbon fiber; HD4000 = coal-based granular activated carbon; SWNT = single-walled carbon nanotube; MWNT = multiwalled carbon nanotube; S = sonication; L = low initial concentration; LS = low initial concentration coupled with sonication; H = high initial concentration; HS = high initial concentration coupled with sonication.

  • a

    C0 is the initial concentration of BP and PNT solutions in the unit of mg/L. Sl, Int, and r2 are the slope, intercept, and determination coefficient of the linearized pseudo-second order kinetic plot, respectively; qe_kin is the adsorbed amount of BP or PNT at equilibrium obtained from kinetic experiments; and qe_iso is calculated with the Freundlich isotherm parameters (KF and n, available in our previous work 10 at observed liquid phase equilibrium concentration Ce).

C00.8220.8480.2190.2240.7340.7340.8750.9000.8550.9130.2220.222
Sl0.01020.01080.09030.09180.00530.00670.00570.00620.01990.01860.03060.0300
Int0.00430.00120.01000.00300.01000.01310.00380.00080.00210.00070.00470.0030
r20.99640.99940.99990.99990.99640.99720.99860.99980.99970.99970.99970.9995
qe_kin989311.0710.8918914917516150.2553.7632.6833.33
k20.02420.09720.81542.80910.00280.00340.00860.04810.18860.49420.19920.3000
H2338331003331007626312504761429213333
k2*2.379.009.0330.600.530.511.507.759.4826.576.5110.00
qe_iso949711.1011.2512513217416452.0453.2430.0630.06

The better description of the adsorption kinetic data with the PSOM rather than with the PFOM might be because of the mathematical nature of the PSOM, and may not indicate a general pseudo-second order adsorption process 24. However, the successful description of the kinetic data with the PSOM provided some valuable insight into the adsorption process. The parameters in Table 1 demonstrate several factors.

First, ultrasonication of the CNT suspensions significantly accelerated their adsorption kinetics but had no effect on their equilibrium capacities.

Second, the k2 and h values showed different orders for the four carbons. Because k2 is dependent on solid phase concentration, the comparison of kinetic processes based on k2 values alone can be problematic 24. Therefore, a modified parameter, k2* (defined as k2qe, /h), was used to describe and compare adsorption kinetics 24. The modified pseudo-second order rate constants (k2* values) of the four carbons followed the order of MWNT > SWNT > ACF10 ≈ HD4000, indicating that the sorbents with a larger pore size and higher external surface areas possess faster sorption kinetics.

Third, the adsorption rate was faster at higher concentrations in comparison with low concentrations, as shown by all three parameters, k2, h, and k2*, of phenanthrene adsorption on MWNTs performed at two concentration levels. This phenomenon might be explained by the external mass transfer process—the diffusion of phenanthrene through the film surrounding the MWNT particles to the exterior surface of the MWNT is directly proportional to the linear concentration gradient across the film. A reverse trend was reported for the adsorption of 17α-ethinyl estradiol and bisphenol A on an MWNT and an SWNT; that is, a negative relationship existed between the k2* and the qe of 17α-ethinyl estradiol and bisphenol A on the CNT 18. The authors explained that porelike structures (inner pores and interstitial channels) may not contribute to the overall adsorption; therefore, diffusion was not a rate-controlling step in the adsorption of 17α-ethinyl estradiol and bisphenol A on CNT 18. The MWNT used in the above-mentioned work and in the present study have nearly identical structural characteristics. This indicates that both adsorbent and adsorbate properties should be considered in the investigation of adsorption kinetics. The two organic compounds selected in the present study do not have functional groups and have a small molecular size, making them ideal probe molecules to investigate adsorption kinetics. The role of diffusion in the adsorption of the organic compounds on the four types of carbons will be revisited in the next section.

Finally, the adsorption rates on CNTs were faster for biphenyl than phenanthrene. The three-dimensional molecular structures of the organic compounds, as discussed in the literature 18, could be attributed to the difference in their adsorption kinetics. Molecular structures determine the molecular diffusivity and hydrophobicity. Molecular diffusivity is inversely proportional to one-third the power of molar volume. The molar volume of biphenyl is smaller than that of phenanthrene (155.45 vs 167.67 cm3/mol), giving an advantage to biphenyl over phenanthrene in their adsorption on CNTs. Hydrophobic effect plays a significant role in the adsorption of organic compounds on CNTs. The qe values of biphenyl on CNTs were lower than those of phenanthrene at the same initial concentrations, because the hydrophobicity of biphenyl is lower than that of phenanthrene (the logarithms of octanol–water distribution coefficients of biphenyl and phenanthrene are 3.98 and 4.68, respectively). Molecular diffusivity impacts the rates in steps one to three of the adsorption process, whereas hydrophobicity is predominant during the final surface reaction step. Therefore, the faster adsorption of the nonplanar biphenyl than the planar phenanthrene indicated that the adsorption process of the two organic compounds on CNTs was diffusion-controlled and that their molecular configurations played a role in the adsorption kinetics.

Adsorption rate-limiting mechanism

The IPDM modeling of experimental data is shown in Figure 3. Piecewise linear regression method has been applied in the model fitting of these data. The linear segments were numbered as one to three to indicate the different adsorption stages. An obvious difference was seen between the Weber-Morris plots of biphenyl on CNTs and those on activated carbons. The plots of biphenyl on CNTs had positive intercepts, whereas the plots for biphenyl on activated carbons passed through the origin, suggesting that different mechanisms were involved in the adsorption. Various observations and interpretations in the application of IPDM to kinetic data 25 have been reported as follows: the regression of q versus t1/2 is linear and passes through the origin, in which intraparticle diffusion is the sole rate-limiting step; the regression of q versus t1/2 is linear, but it does not pass through the origin, suggesting that the adsorption involves intraparticle diffusion but it is not the only rate-limiting step; and the q versus t1/2 plot is multilinear, suggesting that two or more steps are involved in the process.

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Figure 3. Weber-Morris plots of biphenyl (BP) and phenanthrene (PNT) adsorption on the four carbons. The linear segments are numbered with 1, 2, and 3 to indicate the different stages. Dots = experimental data; lines = intraparticle diffusion modeling; ACF10 = phenol formaldehyde-based activated carbon fiber; HD4000 = coal-based granular activated carbon; SWNT = single-walled carbon nanotube; MWNT = multiwalled carbon nanotube; S = sonication; L = low initial concentration; LS = low initial concentration coupled with sonication; H = high initial concentration; HS = high initial concentration coupled with sonication.

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The intercept θ has been used widely as an index reflecting the thickness of the boundary diffusion layer 24, 26–28. However, interpretations have not been consistent in the literature. McKay et al. 26 stated that “extrapolation of the linear portions of the plots back to the axis provides intercepts which are proportional to the extent of the boundary layer thickness, i.e., the larger the intercept, the greater the boundary layer effect.” This is based on experiments performed at different mixing intensities, in which increased agitation led to decreased intercept on the time axis of the Weber-Morris plot. Because the effect of an increased mixing rate is to decrease the boundary layer and therefore the film resistance to external mass transfer surrounding the adsorbent particles, the results indicated that the boundary layer had a retardation effect on the intraparticle diffusion. However, the intercepts referred by the researchers were on the time axis, and the intercepts on the q axis were negative. McKay and co-workers' interpretation has been widely cited by other researchers. However, in some studies, the intercepts on q axis reported in the literature were positive 27, 28. In a recent report 28, the intercept values (1.935–27.296 mg/g) were found to increase with increasing temperature (273–333 K), and the increase in intercept was attributed to an enhanced boundary layer effect. In fact, this is probably because of an increase in the external mass transfer with temperature rather than the increase in the thickness of the boundary layer, because increasing temperature should promote external mass transfer rather than increasing the thickness of the boundary layer; that is, the larger the positive q intercept, the larger the contribution of external surface adsorption or instantaneous adsorption. Conversely, a negative q intercept indicates the boundary layer effect, as observed by McKay et al. 26.

The Weber-Morris plots of biphenyl adsorption on activated carbons, though not linear, passed through the origin, suggesting that intraparticle diffusion is a rate-limiting step in the adsorption of biphenyl to activated carbons. The positive intercepts of the Weber-Morris plots of biphenyl/phenanthrene adsorption on CNTs demonstrated that intermediate adsorption occurred in the adsorption of organic compounds onto CNTs. For adsorption without sonication, the kinetic data could be divided into three stages, whereas the adsorption on CNTs after sonication was composed of two (for MWNTs) or three (for SWNTs) stages. Moreover, the θ values of stage one in biphenyl and phenanthrene adsorption on sonicated CNTs were significantly higher than those on unsonicated CNTs. Before sonication, both the SWNT and the MWNT had agglomerated structures. Sonication dispersed the MWNT in the solution well, whereas no observable change was seen for the SWNT. The possible reason might be that the sonication treatment used in the present study was not powerful enough to overcome the van der Waals forces among the SWNT, which were higher than those among the MWNT. The change in the aggregation structure of the MWNT did not impact its adsorption capacities for the two organic compounds; however, it altered the relative contributions of the four different adsorption sites. More outermost surface and inner cavities were available, whereas peripheral grooves and interstitial channels were lost after sonication. Because the molecular sizes of the studied organic compounds were several times smaller than the inner diameter of the MWNT, the inner cavities of the MWNT with a cylindrical shape acted more as a curved surface rather than as pores for the adsorption of organic compounds. The interstitial channels were porelike structures, which may contribute to intraparticle diffusion in the adsorption process. Thus, the disappearance of stage 2 in the sonicated MWNT solutions suggested that stage 2 was attributable to intraparticle diffusion. Although the inner cavities of the SWNT were slightly larger than the organic compound in terms of size and theoretically could serve as porelike adsorption sites, the intraparticle diffusion in the adsorption on the SWNT occurred mainly in the interstices rather than in the inner cavities, because most of the individual SWNTs were close-ended, as characterized in our previous work 9. Thus, the difference between the adsorption sites of CNTs and activated carbons was that outer surface predominates in CNTs, whereas the space in the inner pores constitutes the main available adsorption sites in activated carbons. As a consequence, the adsorption of biphenyl on activated carbons was dominated by intraparticle diffusion, whereas intermediate adsorption played an important role in the adsorption of biphenyl on CNTs.

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES

The side-by-side kinetic study on the adsorption of organic compounds by activated carbons and CNTs demonstrated that the external mass transfer controls the sorption process for the organic compound–CNT systems. Therefore, the sorption rate of organic compounds on CNTs may be enhanced with improved mixing. The change in hydrodynamic conditions may have a negligible impact on the sorption rate of organic compounds on activated carbons because the sorption process in organic compound–activated carbon systems is controlled by internal mass transfer. For the system with good mixing and a high concentration of adsorbate, the use of CNTs may have an advantage over activated carbons in terms of sorption kinetics. The properties of adsorbate, as briefly discussed in the present study, also played a crucial role in the adsorption mechanism. The organic compounds used in the present study are hydrophobic, having strong adsorption affinities to the carbonaceous adsorbents. The kinetic behavior of additional adsorbates with different physical and chemical characteristics, and thus adsorption affinities, merits further research.

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES

The present study received support in part from a research grant from the National Science Foundation (CBET 0730694). However, the manuscript has not been subjected to the peer and policy review of the agency and therefore does not necessarily reflect its views.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES
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