High-capacity strong cation exchanger prepared from an inactivated immobilized enzyme and its application to the removal of methylene blue from water

Abstract

Environmental protection has become a very important issue. The transformation of solid waste into profitable products to remove hazardous chemicals from environmental samples has attracted much attention. In this study, inactivated immobilized enzymes were reutilized by converting into an adsorbent for the first time, and poly(sodium 4-styrene sulfonate) (poly(NASS)) was grafted from the surface of the particles using surface-initiated atom transfer radical polymerization (SI-ATRP) to produce a strong cation exchanger. X-ray photoelectron spectroscopy (XPS) confirmed the successful grafting of poly(NASS). The cation exchanger exhibited very good performance for the adsorption of methylene blue (MB) with a high capacity (409.8 mg g⁻¹) and fast speed (equilibrium achieved within 1 min), the removal ratio was more than 96.4–99.2% when the MB concentration ranged from 10 to 800 mg L⁻¹, and the removal efficiency of the cation exchanger for MB was 397.1 mg (g⁻¹ min⁻¹), these properties are much better than those previously reported materials. The cation exchanger can be used for five times with removal ratio higher than 80%. Additionally, the adsorption exhibited no significant pH and temperature dependence, and more than 80% capacity was maintained at high concentrations of sodium chloride up to 0.4 mol L⁻¹, therefore the exchanger is very favorable for removal of MB from wastewater with different pH values and salt concentrations, and can be used in a wide temperature range.

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1. Introduction

With the development of industrial and agricultural areas, the amount of generated solid waste has greatly increased. These waste products are often hazardous and need to be treated prior to disposal. To reduce waste treatment expenses, certain waste products can be recycled or reused. Adsorbents are functional materials that have been broadly used in various applications. The conversion of solid waste to specific adsorbents has become an attractive area of study in recent years.^1–5 Hadi et al.⁶ investigated the nonmetallic fraction of recycled printed circuit boards as adsorbents for copper and lead removal, and the metal ion removal capacity was 3 and 3.4 mmol g⁻¹ for Cu and Pb ions, respectively. Zhu et al.⁷ prepared a novel magnetic adsorbent from iron sludge and studied its adsorption properties for methylene blue. Its maximum adsorption capacity was 99.4 mg g⁻¹, which was higher than that of granular active carbon. Rovani et al.¹ developed a new adsorbent from agro-industrial waste and successfully employed it in the removal of an endocrine disruptor compound from aqueous solutions.

With the rapid expansion of biocatalysis, immobilized enzymes have been extensively employed in many manufacturing industries, such as pharmaceuticals, food, bioscience and biomedical engineering.⁸ As a type of biocatalyst, immobilized enzymes are employed to catalyze certain chemical reactions with high selectivity and high efficiency under mild conditions.⁹ However, the activity of the immobilized enzymes largely decreases after several repeated uses, producing a large amount of inactivated immobilized enzyme. Therefore, the reuse of inactivated enzymes is desirable for environmental protection and sustainable development. Currently, very few studies have focused on the recovery and reuse of inactivated immobilized enzymes. To date, the reported strategies for the recovery and reuse of inactivated immobilized enzymes include reactivation^10,11 and enzyme re-immobilization.¹² To the best of our knowledge, no studies of the reuse of inactivated immobilized enzymes by converting them to adsorbents have been reported.

Currently, adsorption is one of the most broadly employed methods to remove dyes from wastewater.¹³ The adsorption capacity is a very important index for evaluating adsorbents. An improvement in the adsorption capacity could reduce the amount of adsorbents used and improve the adsorption efficiency. Tethering polymer chains onto solid substrates is an effective method for preparing high capacity adsorbents. “Grafting to” and “grafting from” are the two frequently used methods for surface modification using polymer chains. In comparison to the “grafting to” method, the “grafting from” approach has the ability to graft polymer chains onto solid supports with high grafting densities.¹⁴ Therefore, higher polymer grafting densities can typically be obtained via the “grafting from” method. Surface-initiated atom-transfer radical polymerization (SI-ATRP) is one of the most broadly used “grafting from” methods in recent years. In general, SI-ATRP is advantageous due to a high polymer grafting density and controllable polymer chains, (i.e., low polydispersities, homogeneous structures and desirable architectures).^15–17 In recent years, SI-ATRP has been increasingly used to prepare high capacity adsorbents.^18–23

Synthetic dyes represent a relatively large group of organic chemicals that are encountered in all spheres of daily life. The broadened application of dyes in various industries, such as the leather, paper, cosmetics and foods industries, has resulted in the production of large quantities of dye wastewater, which has received extensive attention due to environmental protection.^24,25 Most of the dyes are chemically stable and have a complex structure. In addition, these dyes have toxic effects on the environment. Industrial dye effluents pose threats not only to humans but also to aquatic life, plants and animals.²⁶ Therefore, synthetic dye polluted water must be treated prior to disposal in nature.

In this study, a most frequently used solid biowaste, inactivated immobilized enzymes were reutilized by converting it into an adsorbent for the first time, via grafting poly(sodium 4-styrene sulfonate) (poly(NASS)) from the surface of the particles using SI-ATRP to produce a strong cation exchanger. The adsorption properties of methylene blue (MB) on the cation exchanger were investigated, and various factors that affected the adsorption were optimized, and its capacity to remove MB from wastewater was evaluated. The cation exchanger exhibited much better performance for the adsorption of methylene blue (MB) than those previously reported materials in terms of adsorption capacity, adsorption and desorption speed, removal efficiency.

2. Experimental

2.1. Materials

Immobilized penicillin G amidase E.C.3.5.1.11 (Im-PGA, particle size 100–300 μm) was purchased from Zhejiang Haider Biochemical Corp. (Hangzhou, China). 2-Bromoisobutyrylbromide (2-BIBB, ≥98.0%) was obtained from Aladdin Inc. (Shanghai, China). Copper(I) bromide (CuBr, ≥98%), 2,2′-bipyridyl (Bpy, ≥99.5%) and methylene blue (MB) were purchased from Sinopharm, (Shanghai, China). Sodium 4-styrenesulfonate (NASS) was supplied by TCI. The other chemicals were of analytical grade.

2.2. Instruments

The surface composition of the adsorbent was determined by X-ray photoelectron spectroscopy (XPS, PHI-5400, Perkin Elmer). The concentrations of MB were measured by UV-Vis spectrophotometer (UV-2250, Shimadzu).

2.3. Immobilization of ATRP initiator onto inactivated Im-PGA

2 g of inactivated Im-PGA were dispersed in 20 mL of H₂SO₄ (0.1 mol L⁻¹) and maintained at 60 °C for 8 h with stirring followed by sequential washing with water and methanol and drying under vacuum at 40 °C. Next, 2 g of the dried particles were immersed in 50 mL of anhydrous THF for 20 min with gentle stirring in an ice bath. Then, triethylamine (TEA, 0.5 mL) and 2-BIBB (0.5 mL) were added to the suspension, and the reaction proceeded in an ice bath for 3 h and continued for an additional 12 h at 35 °C. The obtained initiator-functionalized particles were extensively and sequentially washed with methanol and distilled water followed by drying under vacuum at 40 °C.

2.4. Grafting PNASS onto the inactivated Im-PGA by SI-ATRP

2 g of NASS, 2 g of the initiator-functionalized particles and 0.44 g of 2,2′-bipyridyl were added to a reaction vessel containing 24 mL of methanol/water (1 : 1, v/v), and the mixture was de-oxygenated using two freeze–pump–thaw cycles. Then, 0.2 g of copper(I) bromide were introduced into the tube under a stream of high-purity nitrogen followed by two freeze–pump–thaw cycles. The reaction was performed at 30 °C for 3 h under a high-purity nitrogen atmosphere with gentle stirring. Next, the modified particles were collected by filtration and sequentially washed with methanol and water. Then, the copper ion was removed by extensive washing of the particles with a 0.1 mol L⁻¹ HCl solution. The particles were rinsed with distilled water and methanol followed by drying under vacuum to afford the cation exchanger. The synthetic process is shown in Fig. 1.

Fig. 1 Preparation of a strong cation exchanger from inactivated immobilized enzymes.

2.5. Adsorption experiments

The adsorption experiments were carried out by adding 20 mg of the adsorbents to a 50 mL centrifugal tube containing 10 mL of a methylene blue (MB) solution at various concentrations in 20 mmol L⁻¹ phosphate buffered solutions (pH 7), and the adsorption was allowed to proceed for 1 min at room temperature with gentle shaking. After adsorption, the suspension was centrifuged at 10 000 rpm for 5 min to separate the particles and the supernatant. The dye concentration was determined using a UV-Vis spectrophotometer at 665 nm. The equilibrium adsorption capacity (Q_e), removal ratio (R) and desorption ratio (D) of the adsorbent were calculated using the following equations, respectively:


where Q_e is the equilibrium adsorption capacity (mg g⁻¹), C₀ is the initial concentration of MB (mg L⁻¹), C_e is the concentration of MB at equilibrium after adsorption (mg L⁻¹), V is the volume of the solution (L), m is the mass of the cation exchanger (g), W_a is the amount of MB adsorbed on the cation exchanger (mg) and W_d is the amount of MB desorbed to the elution medium (mg). In this study, “removal” means “removal of MB from bulk solution through adsorption”.

3. Results and discussion

3.1. Preparation of cation exchanger from inactivated immobilized enzyme

In the current study, one of the most widely used immobilized enzyme (i.e., Im-PGA) was taken as an example to establish a simple route for converting inactivated immobilized enzymes to a cation exchanger via SI-ATRP using 2-BIBB as the initiator and NASS as the monomer. The matrix of Im-PGA consists of poly(glycidyl methacrylate) (PGMA) microspheres with unreacted epoxy groups on their surface. To introduce more initiator molecules onto the inactivated immobilized enzymes, the inactivated immobilized enzymes were hydrolyzed in diluted sulfuric acid to convert the unreacted epoxy groups to hydroxyl groups that could readily react with 2-BIBB. Hydroxyl, amine and imine groups in the amino acid residues linked to the microspheres could also react with 2-BIBB. In addition, some microdomains that are sensitive to hydrolysis could be removed after hydrolysis, which is beneficial to the stability of the final cation exchanger. It is important to note that NASS is a very active monomer and one of the extensively employed monomers in ATRP, which guarantees facile implementation of this approach.

3.2. Chemical characterization of the cation exchanger

To confirm the successful grafting of NASS, the chemical compositions of the inactivated immobilized enzymes, initiator-functionalized and PNASS-modified inactivated immobilized enzymes were characterized using XPS (Fig. 2). As shown in Fig. 2, the nitrogen content significantly decreased from 5.49% to 2.53% after initiator immobilization and further decreased to 1.31% after grafting poly(NASS) due to the initiator and NASS molecules not containing nitrogen atoms. In comparison to the original inactivated immobilized enzyme, the characteristic electronic energy of the bromine atom at 71.09 eV (Br 3d) was observed in the microspheres after initiator functionalization, and the bromine content increased from zero to 0.82% after initiator functionalization. This result indicated that the initiator was successfully coupled to the surface of the inactivated immobilized enzyme (Fig. 2b). After grafting poly(NASS) via SI-ATRP, the characteristic binding energies of S 2s and S 2p were observed at 231.13 and 167.90 eV, respectively, and the sulfur content increased from zero to 1.72%. The XPS results confirm that poly(NASS) was immobilized on the surface of the inactivated immobilized enzyme.

Fig. 2 XPS spectra of the original (a), initiator-functionalized (b) and poly(NASS)-modified (c) inactivated immobilized enzymes.

3.3. Adsorption isotherm of MB

The experimental adsorption results for various concentration of MB with the cation exchanger are shown in Fig. 3. The adsorption capacity increased substantially as the initial dye concentration increased up to 800 mg L⁻¹ and then gradually reached a plateau. A similar trend was observed for MB adsorption on pine-fruit shells.²⁷ Two commonly used isotherm models (i.e., Langmuir and Freundlich isotherms) were employed to simulate the adsorption data. The Langmuir adsorption model was based on the assumption that the maximum adsorption capacity corresponds to a saturated monolayer of molecules on the adsorbent. The Freundlich isotherm was derived assuming a heterogeneous surface with a non-uniform distribution of heat of adsorption over the surface.²⁸

Fig. 3 Adsorption isotherm for MB. Adsorbent dosage: 2 g L⁻¹; temperature: 298 ± 2 K; equilibrium time: 30 min.

The linear form of the Langmuir isotherm model can be expressed as follows:

where Q_e (mg g⁻¹) and C_e (mg L⁻¹) are the quantities of adsorbed dye per unit mass of adsorbent and unadsorbed dye concentration, respectively, in solution at equilibrium. Q_m is the maximum amount of dye per unit mass of adsorbent, and b is a constant related to the affinity of the binding sites (L mg⁻¹). Q_m and b can be determined from the intercept and the slope, respectively, of the plot of C_e/Q_e as a function of C_e.

The linear form of the Freundlich isotherm model can be expressed as follows:

where K_f [(mg g⁻¹) (L mg⁻¹)^1/n] and n are the Freundlich constants, which are indicators of the adsorption capacity and adsorption intensity, respectively. A value of n > 1 represents favorable adsorption conditions.

The adsorption data for MB on the cation exchanger was simulated using the Langmuir and Freundlich models, and the resulting parameters are listed in Table 1. The Langmuir model provided a better fit to the adsorption data than the Freundlich model with a correlation coefficient R² value of 0.9995. Therefore, the adsorption of MB on the cation exchanger proceeded according to a monolayer adsorption process, and the cation exchanger possessed homogeneous active sites for MB adsorption on its surface.²⁴ The maximum adsorption capacity of MB was 409.8 mg g⁻¹, which is much higher than that on previously reported materials (Table 2).

Table 1 Adsorption isotherm parameters for MB adsorption simulated using the Langmuir and Freundlich equations

Isotherms	Parameters
Langmuir	Qm (mg g^−1)	409.8
	b (L mg^−1)	0.14
	R^2	0.9995
Freundlich	Kf	93.7
	n	57.5
	R^2	0.6565

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Table 2 Comparison of the results for the prepared cation exchanger to those of other reported adsorbents for the removal of MB^a

Adsorbents	Adsorption capacity (mg g^−1)	CMB (mg L^−1)	Contact time	Removal ratio (%)	Removal efficiency [mg (g^−1 min^−1)]	Reference
a MCC: acid activated morindacoreiabuch-ham bark carbon, MWCNTs: multi-walled carbon nanotubes, PLP: natural-based pineapple leaf powder, MIIE: modified inactive immobilized enzyme.
Rejected tea	147.0	50	120 min	90.4	1.51	36
Activated rice husk	0.2	10	80 min	89	0.0028	37
MCC	62.5	60	40 min	64.4	0.97	38
Alginic acid fiber	187.4	200	20 min	93.7	9.31	39
Tea waste	85.2	50	300 min	49.5	0.083	40
MWCNT	152.3		30 min			25
Bamboo-based activated carbon	454.2	500	24 h	84	0.29	41
PLP	332.0	150	20 min	95.0	2.38	42
Tyre char	129.6	1200	21 d			43
Cellulose nanofibrils	122.2	100	1 min			44
Silkworm exuviae	25.5	100	6 h			35
MIIE	409.8	800	1 min	96.4	397.1	This study

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The characteristics of the adsorption isotherm process were also investigated using the dimensionless separation factor (R_L) and the values defined by the following equation:²⁹

where C₀ (mg L⁻¹) is the initial dye concentration and b (L mg⁻¹) is the Langmuir constant related to the affinity of the binding sites. The R_L value indicates whether the adsorption process is favorable or unfavorable. The adsorption is considered to be irreversible when R_L = 0, favorable when 0 < R_L < 1, linear when R_L = 1, and unfavorable when R_L > 1. In the current study, the R_L values were in the ranges of 0.432–0.004 when the initial concentration increased from 10 to 2000 mg L⁻¹, indicating that the adsorption process of MB on the cation exchanger is favorable.

3.4. Effect of pH on MB adsorption

The solution pH may play a significant role in the adsorption of adsorbates onto ion exchangers. To investigate the effect of pH on MB adsorption on the prepared cation exchanger, batch adsorption experiments were carried out, and the results are shown in Fig. S1.† The adsorption amount of MB was not significantly influenced by the pH, which is most likely due to MB being a cationic dye that exists as positively charged ions (MB⁺) at the studied pH. In addition, the surface of the cation exchanger maintained a fixed charge density due to all of the sulfonic groups that are fully ionized in this pH range.³⁰ Therefore, the electrostatic interactions between MB⁺ and the cation exchanger did not vary substantially. This feature is very favorable for removal of MB from wastewater with different pH values.

3.5. Effect of the salt concentration on MB adsorption

Fig. 4 shows the effect of the ionic strength on the adsorption performance of the cation exchanger. As shown in Fig. 4, the adsorption amount decreased as the sodium chloride concentration increased. This result is consistent with typical ion exchange processes. However, the decrease was gentle, and the adsorption amount of MB only decreased 16.9% from 50.4 mg g⁻¹ in the absence of sodium chloride to 41.9 mg g⁻¹ in the presence of 0.4 mol L⁻¹ sodium chloride. The results indicated that the adsorption of MB on the cation exchanger is tolerant to salt, which is beneficial for MB removal from wastewater containing salt.

Fig. 4 Effect of the NaCl concentration on the MB adsorption. Initial MB concentration, 100 mg L⁻¹; adsorbent dose, 2 g L⁻¹; temperature, 298 K ± 2; adsorption time, 30 min.

3.6. Adsorption kinetics

The adsorption kinetics are an important characteristic that define the adsorption efficiency. The adsorption kinetics were investigated using a dye solution with a constant concentration and varying contact times at a temperature of 25 °C. The spectra and photometry was determined after diluted the solutions 625-fold on a UV-Vis spectrometer. The effect of the contact time on the adsorption of MB was shown in Fig. 5 and the spectral changes before and after adsorption with time was shown in Fig. S2.† It can be seen that the adsorption could reach dynamic equilibrium within 1 min, which is much faster than that previously reported (Table 2). This result indicates very fast mass transfer. Therefore, the cation exchanger is desirable for fast dye removal in practice.

Fig. 5 Effect of contact time on the adsorption of MB. Initial MB concentration, 2 g L⁻¹; adsorbent dose, 2 g L⁻¹; temperature, 298 K ± 2; pH 7.0.

Two kinetic models (i.e., pseudo-first order^31,32 and pseudo-second order³³) were used to fit the experimental data obtained from the MB adsorption experiments. The linear form of the pseudo-first order equation can be expressed as follows:

where k₁ is the rate constant of the pseudo-first-order equation and Q_e is the equilibrium adsorption capacity (mg g⁻¹). k₁ can be determined from the slope of the plot of log(Q_e − Q_t) as a function of t.

The pseudo-second order equilibrium adsorption model can be expressed as follows:

where k₂ is the rate constant of the pseudo-second-order equation. A plot of t/Q_t as a function of t yields a straight line where k₂ can be calculated from the intercept. When fitting the adsorption data with the pseudo-second-order kinetic model, a very good linear regression was obtained (Fig. S3†) with a R² value of 0.9997. The pseudo-first-order kinetic model did not provide a good fit to the data (Fig. S4†). Similar phenomena have been observed for the sorption of methylene blue onto Mansonia altissima wood sawdust³⁴ and Silkworm exuviae.³⁵ The pseudo-second-order rate constants are k₂ = 0.0155 g (mg⁻¹ min⁻¹) and Q_e = 421.9 mg g⁻¹. It is important to note that the saturated adsorption capacity that was obtained from the kinetics data fitted to the pseudo-second-order kinetic model (421.9 mg g⁻¹) was very similar to that acquired from the adsorption isotherm data fitted by Langmuir model (409.8 mg g⁻¹). This result indirectly indicates that the data simulation by the Langmuir isotherm and pseudo-second order equilibrium adsorption model is reasonable.

3.7. Effect of temperature

The temperature primarily affects the diffusion rate of adsorbate molecules and internal pores of the adsorbent.³⁶ The effect of temperature on the adsorption of MB was investigated at seven different temperatures in a range from 20 to 50 °C. As shown in Fig. S5,† the equilibrium adsorption capacity increased slightly when the temperature increased from 20 °C to 25 °C and then remained stable, which indicates that the cation exchanger can be used in a wide temperature range.

3.8. Removal ratio of MB from water

The removal ratio of MB was assessed in a concentrations range from 10 to 800 mg L⁻¹. The experimental results are shown in Fig. 6. The removal ratio of MB was 96.4–99.2% in the studied concentration range. The removal efficiency of the cation exchanger for MB was calculated to be 397.1 mg (g⁻¹ min⁻¹). A comparison of our results with other recently reported results for the removal of MB is shown in Table 2. The removal ratio and removal efficiency of our cation exchanger are superior to those of the other reported adsorbents.

Fig. 6 Removal ratio of MB with various initial concentrations. Adsorbent dose, 2 g L⁻¹; pH, 7.0; adsorption time, 1 min.

3.9. Desorption and reusability

The economic feasibility of an adsorbent to remove dyes from wastewater is significantly affected by its reusability in multiple adsorption–desorption cycles. Prior to evaluating its reusability, the regeneration of the cation exchanger after adsorption of MB was investigated using several regeneration solutions (Fig. S6†). The results indicated that 5 mol L⁻¹ HCl containing 30% ethanol is a better reagent for the regeneration. Therefore, the adsorption of MB was not only controlled by electrostatic interactions but also by some other interactions, such as hydrophobic interactions, that may also participate in the adsorption process. A similar phenomenon has also been previously reported.⁴⁵ The regenerated cation exchanger was reused for the adsorption of MB, and the results from the adsorption–desorption cycles are shown in Fig. 7. The removal ratio of MB decreased very slightly as the number of adsorption–desorption cycles increased, and the removal ratio remained as high as 92.8% after five cycles. The removal ratios decreased significantly with a further increase in the number of adsorption–desorption cycles, and the removal ratios were 74.8% and 68.5% after the sixth and seventh cycles, respectively. Therefore, the cation exchanger can be used five times with the acceptable removal ratio is higher than 80%. The results indicated that the cation exchanger exhibits good reusability. It is important to note that the desorption ratio of MB was maintained at 74.9% to 90.7% during the seven adsorption–desorption cycles, this indicated that the desorption condition employed could strip the vast majority of MB from the cation exchanger and allow it show good performance in repeating dye removal cycles. When comes to the reason why the removal ratios decreased considerably after five adsorption–desoprtion cycles, while desorption ratios were almost unchanged during seven cycles, two factors may contribute to these results. On one hand, when a fresh batch of cation exchanger particles were taken to adsorb MB, adsorption sites on the cation exchanger were not fully covered, the residual MB on the cation exchanger due to incomplete desorption would not affect the adsorption; However, with the adsorption cycles increased, the adsorption sites on the cation exchanger would be fully occupied, leading to significant decline of removal ratio. On the other hand, multiple cycles of desorption in strong acid solution might lead to chemical stability problem, which could produce decreasing adsorption capacity.

Fig. 7 Reusability of the cation exchanger for removal of methylene blue. Adsorbent dose, 2 g L⁻¹; pH, 7.0; adsorption time, 1 min.

4. Conclusions

A most frequently used solid biowaste, inactivated immobilized enzymes were reutilized by converting it into an adsorbent for the first time, via grafting poly(sodium 4-styrene sulfonate) (poly(NASS)) from the surface of the particles using SI-ATRP to produce a strong cation exchanger. The maximum adsorption capacity for MB was 409.8 mg g⁻¹, and the adsorption reached equilibrium within 1 min. These values are much higher and faster than those of the most recently reported adsorbents. The Langmuir adsorption isotherm and the pseudo-second order kinetic model provided a good fit of the experimental data for various concentrations and contact times. The pH and temperature did not substantially affect the adsorption of MB. Additionally, the adsorption of MB was not substantially affected by a high concentration of sodium chloride, favoring practical application of the cation exchanger for the removal of MB from wastewater. The removal ratio was more than 96% when the MB concentration ranged from 10 to 800 mg L⁻¹. In addition, the cation exchanger exhibited good reusability. The results indicated that the inactivated immobilized enzymes can be reutilized via conversion to a cation exchanger. The inactivated immobilized enzymes could also be converted to other types of adsorbents, such as metal-chelating and reversed-phase adsorbents, to remove heavy metal ions and organic pollutants from wastewater.

Acknowledgements

This work was supported by the National Natural Science Foundation in China (No. 21575114, 21475104) and the Research Project on Social Development by the Science and Technology Department of Shaanxi Province (No. 2013K13-02-10).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10243c

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