Nylon 6 electrospun nanofibers mat as effective sorbent for the removal of estrogens: kinetic and thermodynamic studies
© Qi et al.; licensee Springer. 2014
Received: 30 May 2014
Accepted: 28 June 2014
Published: 15 July 2014
Nylon 6 electrospun nanofibers mat was prepared via electrospinning for the removal of three estrogens, namely, diethylstilbestrol (DES), dienestrol (DS), and hexestrol (HEX) from aqueous solution. Static adsorption as well as the dynamic adsorption was evaluated by means of batch and dynamic disk flow mode, respectively. The kinetic study indicated that the adsorption of the target compounds could be well fitted by the pseudo-second-order equation, suggesting the intra-particle/membrane diffusion process as the rate-limiting step of the adsorption process. The adsorption equilibrium data were all fitted well to the Freundlich isotherm models, with a maximum adsorption capacity values in the range of 97.71 to 208.95 mg/g, which can be compared to or moderately higher than other sorbents published in the literatures. The dynamic disk mode studies indicated that the mean removal yields of three model estrogens were over 95% with a notable smaller amount of adsorbent (4 mg). Thermodynamic study revealed that the adsorption process was exothermic and spontaneous in nature. Desorption results showed that the adsorption capacity can remain up to 80% after seven times usage. It was suggested that Nylon 6 electrospun nanofibers mat has great potential as a novel effective sorbent material for estrogens removal.
Synthetic estrogens are some of the most potent endocrine-disrupting chemicals (EDCs) found in municipal wastewater, despite of low concentration (ng/L) . Given pervasive contamination and the highly toxic nature of synthetic estrogens, there is considerable interest in the development of techniques to remove these compounds from contaminated water. Since these compounds are hydrophobic compounds of low volatility, adsorption plays an important role in their removal [2–4].
In principle, the heart of the sorption technique is the sorbent material. Several kinds of materials have been used as adsorbent for estrogens, such as carbon nanomaterials , activated charcoal [6, 7], fullerene-containing membranes , multi-walled carbon nanotubes , granular activated carbon, chitin, chitosan, ion-exchange resin and a carbonaceous adsorbent prepared from industrial waste [10, 11], iron (hydr)oxide-modified activated carbon fibers , etc. These materials showed good performance for the removal of estrogens from wastewater. However, they are suffering a common problem that it needs a next separation process from the wastewater, which will increase the operation cost. Thus, further research is needed to find new adsorbents with optimized disposal process and high removal performance.
Recently, there is a growing interest on sorbents based on nanofibers for their characteristics . As reported by the literatures, polymer nanofibers obtained by electrospinning show excellent heavy-metal ions and organic pollutants removal ability from water [14–16]. However, to our knowledge, no reports using electrospun nanofibers as adsorbent for the removal of estrogens have appeared up to now.
Nylon 6 is a general chemical material, consisting of amide groups which are separated by methylene sequences, where nonpolar interactions are expected between hydrophobic compounds and the methylene chains of Nylon 6. Our previous research, using the Nylon 6 electrospun nanofibers mat as solid-phase extraction (SPE) sorbent, has demonstrated the highly effective extraction nature of the Nylon 6 nanofibers mat for nonpolar and medium polarity EDCs, such as natural and synthetic estrogens [17, 18], bisphenol A , and phthalate esters [20, 21] in environmental water. It is indicated from the results of our work that the extremely large surface-to-volume ratio and numerous micropores make nanofibers mat a promising high-performance adsorbent material that can achieve a larger specific surface and more active sites for adsorption, compared with microscale adsorbents. Accordingly, the adsorption of the target compounds is facilitated and a small amount nanofiber (2 ~ 3 mg) is sufficient [17–21]. Furthermore, some researchers have indicated that polymer fiber mat as the adsorbent could avoid the subsequent separation process . All the facts mentioned above revealed that the Nylon 6 electrospun nanofibers mat has a great potential as an efficient adsorbent.
The objective of this research is to go deeper on the basis of our previous work and investigate the detailed adsorption characteristics and thermodynamics of estrogen removal by Nylon 6 electrospun nanofibers. Diethylstilbestrol (DES), dienestrol (DS), and hexestrol (HEX) were chosen as the model target estrogens. The static adsorption as well as the dynamic adsorption was evaluated by means of batch and dynamic disk flow mode. Kinetic and thermodynamic studies of removal of estrogens were investigated based on the experimental data for the understanding of the adsorption characteristic. Results from this study were used to evaluate the feasibility of Nylon 6 electrospun nanofibers as sorbent for estrogen removal in real-wastewater treatment.
High-purity standards of three estrogens including DES, DE, and HEX were purchased from Sigma Company, St. Louis, MO, USA. Methanol, acetonitrile, and acetone of HPLC grade used for analysis were obtained from Tedia Inc, Fairfield, OH, USA. Cresol, formic acid, hydrochloric acid, and sodium hydroxide were analytical reagent grade, which were purchased from Chemical Reagent Factory, Shanghai, China. Nylon 6 material was purchased from DebioChem, Nanjing, China.
Preparation of Nylon 6 nanofibers mat
The Nylon 6 nanofibers mat was fabricated by electrospinning described previously [17–21]. The procedure was briefly as follows. An appropriate amount of Nylon6 was dissolved in a composite solvent of formic acid and m-cresol (6:4, v/v). This solution was loaded into a glass syringe (volume 5 mL). The glass syringe was fitted to a stainless needle (diameter 0.5 mm) with a flat tip connected to the anode. With an interval of 20 cm, a grounded aluminum foil was served as the collection screen, and a voltage of 15 kV (DW-P403-1 AC high-voltage generator, Dongwen Factory, Tianjing, China) was applied between the tip and the aluminum foil. The rate of movement of the syringe was controlled and fixed at 0.5 mL/h by a syringe pump (model TCI-I, SLGO, Beijing, China). A dense mat of Nylon 6 nanofibers with its thickness in the range of 70 to 200 μm was collected on the aluminum foil while the electronspun time was 2 to 8 h. A scanning electron microscope (SEM, Hitachi S-3000 N, Tokyo, Japan) was utilized to characterize the Nylon 6 nanofibers mat. The surface-to-volume ratio of Nylon 6 nanofibers was measured by the ASAP 2020 Accelerated Surface Area and Porosimetry system (Micromeritics Instrument Corporation, Norcross, USA).
Instrument and analytical conditions
The quantitative method of the three estrogens was established in our previous work . Briefly, a Thermo Finnigan TSQ Quantum Ultra tandem mass spectrometer equipped with an electrospray ionization (ESI) source (San Jose, CA, USA), a Finnigan surveyor LC pump, and an auto sampler were used for LC-MS/MS analysis. Data acquisition was performed with Xcalibur 1.1 software (Thermo-Finnigan, San Jose, CA, USA). Peak integration and calibration were carried out using LC Quan software (Thermo-Finnigan).
An ODS column (250 mm × 4.6 mm i.d., particle size 5 mm) was used for analysis at 35°C. A mixture of methanol and water (80:20, v/v) at a flow rate of 1.0 mL/min was used as the mobile phase, and the split ratio was 4:1. The ionization of each compound was tested in negative multiple reaction monitoring (MRM) mode. Nitrogen was used as the sheath gas (35 psi) and the auxiliary gas (5 psi). The capillary temperature was 350°C, and the spray voltage was 3.5 kV. The injection volume was 10 μL throughout the study.
where Co is the initial concentration of estrogens in solution (mg/L) and Ce is the equilibrium concentration (mg/L). m is the mass of adsorbent (g), and V is the volume of solution (L).
where q t is the adsorption capacity at time t, Co is the initial concentration of estrogens in solution (mg/L), Ct is the concentration at time t (mg/L), m is the mass of adsorbent (g), and V is the volume of solution (L).
For desorption studies, 1.5 mg Nylon 6 nanofibers mat was first contacted with 50 mL 2 mg/L estrogens for 6 h at 298 K. Then the adsorbent was eluted by 0.5 mL methanol/water (80:20, v/v, i.e., mobile phase for HPLC separation) for 20 min. Before the second adsorption, Nylon 6 nanofibers mat was washed with 0.5 mL water on a magnetic stirrer at 200 rpm. The above procedure was repeated for seven times to test the reusability of the adsorbent.
Results and discussion
Morphology of the nanofibers mat
The morphology of Nylon 6 nanofibers mat was studied by SEM; the results are shown in Figure 1. We can see that the surface of Nylon 6 nanofibers was smooth, the average diameter is about 200 nm, and the average specific surface of Nylon 6 fibers was 23.90 m2/g.
where K1 and K2 are the pseudo-first-order and second-order rate constants, respectively.
Adsorption kinetic model rate constants for three estrogens adsorption on Nylon 6 nanofibers mat at different temperatures
R 1 2
R 2 2
The values of the correlation coefficients (R2) clearly indicated that the adsorption kinetics closely followed the pseudo-second-order model rather than the pseudo-first-order model (the results draw the same conclusion under initial concentration 0.1 to 2.0 mg/L; to be concise, kinetic parameters obtained from initial concentration 2.0 mg/L are presented in Figure 3 and Table 1 only). The pseudo-second-order rate constant (K2) of DES, DE, and HEX decreased from 0.00239 to 0.000525 g/mg/min, 0.00123 to 0.000346 g/mg/min, and 0.00130 to 0.000533 g/mg/min, respectively, with an increase in initial concentration from 0.1 to 2.0 mg/L. Moreover, the qe, calculated values obtained from the pseudo-second-order kinetic model appeared to be very close to the experimentally observed values than the values from the pseudo-first-order kinetic model. The results accordingly indicated that the adsorption kinetics of three estrogens adsorbed onto the Nylon 6 nanofiber mat closely followed the pseudo-second-order kinetic model (Figure 3a) rather than the pseudo-first-order kinetic model (Figure 3b), suggesting that intra-particle/membrane diffusion process was the rate-controlling step of the adsorption process . So, it was necessary to analyze the intra-particle/membrane diffusion model in order to describe the adsorption process more clearly.
The Weber-Morris intra-particle/membrane diffusion model has often been used to determine if intra-particle/membrane diffusion is the rate-limiting step [24, 25]. According to this model, a plot of q t versus t1/2 should be linear if intra-particle/membrane diffusion is involved in the adsorption process, and it is essential for the plots to cross the origin if the intra-particle/membrane diffusion is the sole rate-controlling step . In this work, the plot did not pass through the origin; instead, three linear portions were obtained (Figure 3c); and this suggested that adsorption occurred in three phases, involving diffusion to the external surface, intra-particle/membrane diffusion or gradual adsorption being the rate-controlling stage, and the final equilibrium stage where the intra-particle/membrane diffusion slowed down due to the extremely low solute concentration in solution . As the plots did not pass through the origin, intra-particle/membrane diffusion was not the only rate-limiting step. It might be estimated that the surface diffusion assisted by agitation or oscillation could benefit to the adsorption process, considering the condition of static adsorption in this work was just standing to establish adsorption equilibrium.
where KF and n are Freundlich isotherm constants related to adsorption capacity and adsorption intensity, respectively and C e is the equilibrium concentration (mg/L).
Where KL is the Langmuir constant related to the energy of adsorption and qmax is the maximum adsorption capacity (mg/g).
Langmuir and Freundlich constants for the adsorption of three estrogens on Nylon 6 nanofibers mat
Comparison with other sorbent materials in literatures
Amount of sorbents
Maximum adsorption capacity or constant
17α-Ethinyl estradiol, bisphenol A
50 to 600 mg/g
Multi-walled carbon nanotubes
Estriol, 17α-ethinyl estradiol
0.52 μg/g, 5.59 μg/g
9290 mL/g, 12200 mL/g
Benzo(a)antracene, β-estradiol, bisphenol A
42.9 to 84 mg/g
Iron(hydr)oxide modified activated carbon fibers
Estrone, 17α-ethinyl estradiol
Nylon 6 nanofibers mat (this work)
Diethylstilbestrol, dienestrol, and hexestrol
208.95 mg/g, 135.21 mg/g, 97.71 mg/g
The possible reason might be the large surface area and high porosity of Nylon 6 nanofibers mat. Furthermore, as the primary chemical structure of nylon consists of amide groups separated by methylene sequences, nonpolar interactions are expected between hydrophobic estrogens and the methylene chains of nylon, and meanwhile, the hydrophilic amide groups are expected to enhance the water molecule movement into the sorbent, improving mass transfer and the chance for uptake. The higher adsorption capacity of the adsorbent used in this study may be coming from these properties of Nylon 6 nanofibers mat.
Negative values of ∆G0 of the three estrogens indicated spontaneous adsorption and the degree of spontaneity of the reaction decrease with increasing temperature. Because the physical sorption energies are in the range of 0 to −20 kJ/mol and the chemisorption energies in the range of −80 to −400 kJ/mol . The interaction between the three estrogens and Nylon 6 nanofibers mat can be considered as a physical adsorption rather than chemisorption. The negative values of ∆H0 indicated that the adsorption process of estrogens on Nylon 6 nanofiber mat was exothermic process. The negative values of ∆S0 indicated the decreased randomness at the solid/solution interface during the adsorption of three estrogens in aqueous solution on the nanofibrous membrane.
Dynamic disk mode studies
Continuous adsorption trials in dynamic flow mode were performed in a home-made disk filter device for the removal of three model estrogens in 100 mL solution. Since the adsorption performance of adsorbents usually depends on available sorbent amount for adsorption, the effect of the Nylon 6 nanofibers mat amount was examined in the range of 1.0 to 5.0 mg (the initial concentration was 5.0 mg/L and flow rate was 1.0 mL/min). The results indicated that the amount of adsorbent strongly influenced estrogens adsorption yield. The removal yields of DES, DS, and HEX increased from 70.15 ± 1.93% to 97.59 ± 2.26%, 62.47 ± 1.96% to 96.72 ± 1.81%, and 60.32 ± 2.23% to 96.26 ± 1.68%, respectively, with an increase in the adsorbent amount from 1.0 to 4.0 mg, and the variations of removal for target contaminants using 5.0 mg nanofibers were not remarkable. The higher adsorption yields for higher adsorbent amount are due to the increase of more available binding sites for the adsorption. And then, after a certain point (4.0 mg), the adsorption yield stayed nearly constant may be due to the saturation of binding sites on the adsorbent surface. Therefore, 4.0 mg of the Nylon 6 nanofibers mat was found to be optimum of the further dynamic flow mode adsorption.
The effect of the flow rate on the estrogen adsorption in continuous mode was also investigated. The flow rate of estrogens solution was varied from 0.5 to 4.0 mL/min while the initial concentration (5.0 mg/L) and adsorbent amount (4.0 mg) were kept constant. It was found that the flow rate strongly influenced estrogen uptake capacity, and lower flow rates favored estrogen adsorption. The maximum removal yields were obtained at flow rates of 0.5 and 1.0 mL/min (p > 0.05). The adsorption capacity significantly decreased with increased flow rate from 2.0 to 4.0 mL/min (p < 0.05). This was due to a decrease in the residence time of estrogens within the Nylon 6 nanofibers mat at higher flow rates. This caused a weak distribution of the liquid inside the mat, which leaded to a lower diffusivity of the adsorbates to the binding sites for the adsorption. Therefore, removal yields of DES, DS, and HEX decreased from 97.95 ± 1.28% to 75.13 ± 2.14%, 96.55 ± 1.46% to 79.37 ± 1.95%, and 96.85 ± 1.62% to 74.65 ± 2.74%, respectively, with an increase in the flow rate from 1.0 to 4.0 mL/min. The optimal flow rate for estrogens adsorption was chosen as 1.0 mL/min in this study, given an overall consideration of adsorption efficiencies and the cost of the increment of the treatment time.
If the amount of adsorbates was larger than breakthrough adsorption amount of adsorbent materials, target compounds could flow away with solution. In order to obtain high removal efficiency, breakthrough amount should be investigated. Under the optimum conditions, the breakthrough amount was investigated by pumping 100 mL solution with initial concentration of the three target estrogens in the range of 1.0 to 20.0 mg/L through the disk filter device. The results indicated that satisfactory removal yields (above 90%) were obtained during 1.0 to 15.0 mg/L. When the initial concentration was increased to 20.0 mg/L, a drop about 11.29% to 14.76% of removal yields of all the three target estrogens was occurred. The marked decline indicated the adsorption breakthrough of Nylon 6 nanofibers mat. According to the experimental results, the breakthrough initial concentration of all the three estrogens was 15.0 mg/L, while the removal yields of DES, DS, and HEX were 97.55 ± 1.36%, 95.13 ± 1.65%, and 93.37 ± 1.49%, respectively. Therefore, the maximum dynamic adsorption capacity of DES, DS, and HEX by Nylon 6 nanofibers mat was calculated as 365.81, 356.74, and 350.13 mg/g for DES, DS, and HEX, respectively. It was evident that highly dynamic estrogen adsorption performance could be obtained using Nylon 6 nanofibers mat as sorbent material.
Desorption performance and reusability of Nylon 6 nanofibers mat
Adsorption technology plays an important role in pollutant removal in environmental water. The key research is to find new adsorbents and clear the detailed adsorption characteristics. This study investigated the kinetics and thermodynamics characteristics of estrogen removal by Nylon 6 electrospun nanofibers for the first time, with an expectation of taking advancement in the feasibility of applications of nanofiber-based adsorption technique for contaminated water treatment. It is suggested by our experimental results that adsorption based on Nylon 6 electrospun nanofibers mat has great potential for estrogen removal in wastewater treatment, moderately higher than other sorbents published in the literatures. Furthermore, a small amount nanofiber is sufficient and regenerated readily and presents better reuse performance.
This work was supported by the National Natural Science Foundation of China (Project No. 81172721), Suzhou Social Development Projects (Project No. SS201124), and Suzhou Nanoresearch Special Plan (Project No. ZXG2013026).
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