Titanate Fibroin Nanocomposites: A Novel Approach for the Removal

Dec 22, 2017 - Dipartimento di Ingegneria Industriale, Universitá di Padova, Via Marzolo, 9, 35131 Padova, Italy. #National Research Council of Italy...
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Titanate Fibroin Nanocomposites: A Novel Approach for the Removal of Heavy-Metal Ions from water Davide Magrì,†,∥ Gianvito Caputo,†,∇ Giovanni Perotto,†,∇ Alice Scarpellini,‡ Elena Colusso,⊥ Filippo Drago,§ Alessandro Martucci,⊥,# Athanassia Athanassiou,† and Despina Fragouli*,† †

Smart Materials, ‡Electron Microscopy Facility, and §Materials Characterization Facility, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy ∥ Universitá degli Studi di Genova, Via Balbi 5, 16126 Genova, Italy ⊥ Dipartimento di Ingegneria Industriale, Universitá di Padova, Via Marzolo, 9, 35131 Padova, Italy # National Research Council of Italy, Institute for Photonics and NanotechnologiesPadova, Via Trasea, 7, 35131 Padova, Italy S Supporting Information *

ABSTRACT: In this study, we report the fabrication of nanocomposites made of titanate nanosheets immobilized in a solid matrix of regenerated silk fibroin as novel heavy-metalion removal systems. The capacity of these nanocomposite films to remove lead, mercury, and copper cations from water was investigated, and as shown by the elemental quantitative analysis performed, their removal capacity is 73 mmol/g for all of the ions tested. We demonstrate that the nanocomposites can efficiently retain the adsorbed ions, with no release of titanate nanosheets occurring even after several exposure cycles to ionic solutions, eliminating the risk of release of potentially hazardous nanosubstances to the environment. We also prove that the introduction of sodium ions in the nanocomposite formulation makes the materials highly selective toward the lead ions. The developed biopolymer nanocomposites can be potentially used for the efficient removal of heavy-metal-ion pollutants from water and, thanks to their physical and optical characteristics, offer the possibility to be used in sensor applications. KEYWORDS: titanate nanosheets, silk fibroin, nanocomposites, cation exchange, water remediation

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INTRODUCTION The intense agricultural and industrial activity, related to the continuous increase of the world’s population, has caused a severe rise in the environmental pollutants. Among them, heavy-metal ions represent one of the most important classes of pollutants. Even at low concentration levels, in the range of few milligrams per liter (mg/L), heavy-metal ions can be a risk for human health, due to their tendency to accumulate in living organisms, affecting the food chain and provoking serious health implications.1,2 To name some, the presence of lead in a human organism, above the defined levels, can cause damage to kidneys and the nervous system, whereas organic complexes with mercury are associated with the destruction of mitochondria and with nervous system failures.1,3 Therefore, considerable attention is being devoted to the development of low-cost and environmentally safe materials for the removal of heavy-metal ions from polluted waters.4−6 To date, the removal of heavy-metal ions has been achieved by several methods, such as chemical precipitation, membrane filtration, reverse osmosis, biosorption, electrowinning, etc.7−9 One of the most widely used methods is the ion exchange, with a good compromise between removal rate and efficiency, © XXXX American Chemical Society

which also offers the possibility to regenerate and reuse the sorbent. It is based on the capability of some materials to exchange their own positive charges with heavy-metal cations.10 These systems are mainly based on synthetic resins, natural zeolites, and silicates,11 whereas recently, several kinds of nanostructured inorganic materials have been synthesized and studied for this purpose.12,13 In particular, metal oxides, such as iron oxides or aluminum oxides, and other nanomaterials have been already applied as pollutant sorbents due to their high surface/volume ratio, high reactivity, and ionexchange capacity, reaching in the case of lead ions removal capacities of 100 mg/g of sorbent.12−16 Titanates are titanium oxide-based compounds, which have been produced in various shapes, such as nanotubes, nanowires, and nanoflowers. Their performance as sorbents of heavy metal ions has been largely investigated with better performance compared to other metal oxides reaching a removal capacity of up to 550 mg/g in the case of lead Received: October 11, 2017 Accepted: December 14, 2017

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DOI: 10.1021/acsami.7b15440 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces ions.14,17−20 This is attributed to their surface-to-volume ratio, high concentration of hydroxyl groups, and their interlayer distance adaptability.14,17−20 Titanate nanosheets (TNSs), in particular, are novel two-dimensional (2D) nanomaterials with a layered nanostructure and a Ti(1−x)O24x− composition. Usually, depending on their preparation method, diverse TNS lamellar structures can be formed by single Ti−O sheets in a TiO6 octahedral configuration intercalated by organic cations.21−24 These 2D crystals of substoichiometric TiO2 have small sizes (one-unit-cell thickness and length of 2−6 nm) and very high surface-to-volume ratio, whereas they present high refractive index values.21,23,24 On top, it has been already demonstrated that the cations present in the TNS structures can be efficiently substituted by sodium or other metal ions, making them interesting materials for diverse types of applications, among which is also the environmental remediation by ion exchange.22,23,25 In particular, TNSs, due to their adaptable structure, which can rearrange during the ion exchange, have shown their appealing ability to irreversibly exchange and tightly immobilize among their layers hazardous metal ions, such as Pb2+.14,22 This stable immobilization is a concrete benefit for the safe removal and disposal of hazardous substances, such as heavy-metal ions. Despite the great potentiality of various nanomaterials for water remediation applications, there is a significant concern about their dispersion in the environment because of unknown long-term effects. It is, therefore, important to assure their safe manipulation especially if involved in environmental applications.26,27 Fixing the nanomaterials in a macroscopic solid matrix offers a possible approach to exploit their functional properties, as well as to limit the hazards related to their dispersion in the environment. A solid polymeric matrix can immobilize the nanoscale fillers and at the same time, if it has the appropriate wetting properties and porosity, may preserve their water remediation performance.28−30 In this way, solid nanocomposite materials can be achieved, capable of entrapping efficiently the pollutants due to the presence of the active fillers, improving the manageability, and offering the possibility to envision their large-scale applications in real-case situations. Titanates have been already successfully incorporated in diverse polymers, such as silk fibroin (SF). In particular, the natural and biocompatible SF biopolymer guarantees the enwrapping and the stable retention of the TNSs in a solid configuration.23,25 In this way, TNS−SF nanocomposites have been studied as potential systems for diverse optical applications, such as optical sensing of humidity, etc.23,25,31 Nevertheless, to the best of our knowledge, there is no report on the combination of titanates with a solid matrix for water remediation applications and specifically for the removal of heavy-metal ions, leaving unexploited the benefits that these nanomaterials can offer to the specific research field. In this study, we fabricate TNS−SF nanocomposites and investigate their ability to remove heavy-metal ions from aqueous solutions. The development of this material aims to find the most effective arrangement that combines the high efficiency and selectivity of the titanium oxide-based nanostructures to the suitable stabilizing effect given by the crystallized silk fibroin matrix, to obtain a safe and easy-to-use system able to remove hazardous metal ions from polluted water. The capacity of the developed material to uptake heavymetal ions has been analyzed, evaluating at the same time the specificity of the sorbents to remove lead ions from multi-ionic

solutions. We prove that the TNSs are stably retained in the SF matrix after the ion-exchange process and that the introduction of sodium ions in the nanocomposite formulation transforms the material in a highly selective system toward the lead ions. The effect of the ion exchange on the structure of the TNSs in the SF matrix has been investigated, and a preferential binding of lead cations to the TNS lamellar structure has been revealed. This study can open the possibility to create new bionanocomposites with good stability, selectivity, and performance suitable for environmental applications.

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EXPERIMENTAL SECTION

Fabrication of TNSs. TNSs were synthesized using a sol−gel process.21,32 Initially, 12 mmol of titanium tetraisopropoxide (Ti(OPri)4) were added to 107 mmol of warm (110 °C), dehydrated, and degassed ethylene glycol. Subsequently, 9 mmol of tetramethylammonium hydroxide (TMAH) (Sigma-Aldrich) dissolved in 54 mL of water were injected into the solution, which became optically clear after few seconds. The reaction runs for 4 h at 110 °C. After cooling the solution at room temperature (RT), an excess of acetone was added to induce flocculation. TNSs were then precipitated by centrifugation at 4000 rpm for 4 min. The TNS pellet was washed twice with acetone and twice with methanol and then dried under vacuum. A stock dispersion of TNSs was obtained by dispersing 200 mg of TNSs in 1 mL of 0.1 M TMAH aqueous solution. SF Extraction. SF aqueous solution was prepared according to a previously published protocol.33 Cocoons of Bombyx mori were cut into little pieces and boiled for 30 min in a solution of 0.02 M sodium carbonate (Sigma-Aldrich) to remove the sericin. The boiled silk was rinsed with Milli-Q water, and, after drying, the fibers were dissolved in 9.3 M LiBr (Sigma-Aldrich) solution at 60 °C for 4 h. The solution was dialyzed against distilled water using a dialysis membrane (molecular weight cutoff, 3500 Da; 6.74 mL/cm; Fisherbrand) for 2 days to remove the LiBr salts. The SF solution, obtained after dialysis, was transferred to a 50 mL tube, centrifuged twice at 9000 rpm at 4 °C for 20 min, and transferred to a clean tube to remove impurities. The concentration of fibroin in aqueous solution was in the range of 6−8% (w/v) for the different batches. The SF solution with a 5% concentration used in the present work was prepared by diluting the more concentrated solutions in Milli-Q water. TNS−SF Nanocomposites Formation. TNS−SF nanocomposites were formed by mixing the fibroin solution with the aqueous dispersion of TNSs. Aliquots of the TNS dispersion were added to aqueous solutions of SF (50 mg/mL) and mixed gently to avoid fibroin flocculation. The solutions were drop-casted and dried under the fume hood at RT to obtain films. The composite films were then dipped in methanol overnight at RT to induce fibroin crystallization and subsequently dried for 24 h under hood.23 The crystallized samples were stable in water permitting at the same time the efficient interaction of the polluted water with the TNS fillers.23,33−35 TNS− SF films with TNS-to-SF ratios of 50/50, 70/30, and 90/10 were synthesized. Ion-Exchange Process. To test the removal of heavy-metal ions from water, the nanocomposite films were incubated in aqueous solutions of lead nitrate (Pb(NO3)2, 99%), mercury chloride (HgCl2, 99.999%), copper chloride (CuCl2, 97%), and sodium chloride (NaCl, 95%) (Sigma-Aldrich). Different metal-ion concentrations were prepared (from 25 to 1000 mg/L) starting from stock solutions concentrated at 1000 mg/L. For each ion-exchange experiment, 20 mg of the TNS−SF films were enclosed in a dialysis membrane (molecular weight cutoff, 3500 Da; 1.15 mL/cm; Fisherbrand) to facilitate the sampling operations and dipped in 20 mL of ion solution. All of the tests were performed under stirring at 700 rpm. At various time points, aliquots of 250 μL of ion solution were collected for further characterization. At the end of the adsorption process, the solid films were collected and used for further analysis. Characterization. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was used to determine the ion concenB

DOI: 10.1021/acsami.7b15440 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces tration with an iCAP 6500 spectrometer (Thermo). All of the samples collected were previously treated with 2.5 mL of aqua regia (1HNO3/ 3HCl) overnight. Then, the digested samples were diluted up to 25 mL with Milli-Q water and filtered through Nylon syringe filters (diameter of 25 mm and pore size of 0.2 μm, Sartorius). From the ICP-OES data, the sorption capacity of the films qe (mg/g) was calculated as follows qe =

(C0 − Ce) × V W

where Ce (mol/L) is the concentration of the heavy-metal ions at equilibrium, C0 (mol/L) is the initial concentration, V (L) is the volume of the solution, and W (g) is the mass of the adsorbent. Energy-dispersive X-ray spectrometry (EDX) analysis was carried out using a scanning electron microscope (SEM) (JEOL JSM6490LA, Tokyo, Japan) equipped with a tungsten (W) thermionic electron source working in low vacuum. The composite samples were incubated in a 1000 mg/L Pb2+ solution for 24 h. After the incubation, the samples were washed in Milli-Q water and after drying were coated with a 10 nm thick AuPd film using a sputter coater (Cressington 208HR). Transmission electron microscopy (TEM) analysis was carried out using a JEM 1400-Plus JEOL microscope operating at an acceleration voltage of 120 kV. The composite samples before and after ion exchange were ground and dispersed in water. Few drops of the composite suspension were deposited onto copper grids coated with an amorphous layer of lacey carbon film. Then, the grids were dried under vacuum overnight to remove water. High-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) images were acquired using an image Cs-corrected JEOL JEM2200FS transmission electron microscope with a Schottky emitter equipped with a Quantax 400 STEM system, operated at 200 kV. EDX analyses were carried out in HAADF-STEM mode using the same microscope and an XFlash 5060 silicon drift detector (60 mm2 active area). Fourier transform infrared (FTIR) measurements were carried out on the films using a Vertex 80 (Bruker) spectrometer in attenuated total reflectance and in transmission mode. The spectra were acquired between 4000 and 400 cm−1 with a resolution of 2 cm−1. X-ray diffraction (XRD) measurements were performed on a PANalytical Empyrean X-ray diffractometer using a Cu Kα anode (λ = 1.5406 Å) operating at 45 kV and 40 mA. The diffraction patterns were collected in the 2θ range of 2−70° with a 0.04° step size. X-ray photoelectron spectroscopy (XPS) analysis was performed using a SpecsLab spectrometer with Al Kα source (hν = 1486.6 eV) operated at 15 kV with an emission current of 10 mA. A charge neutralizer consisting of low-energy (ca. 7 eV) electrons was applied, and energy-scale calibration was performed by setting the C−C/C−H component of C 1s spectrum at 285 eV.

Figure 1. (a) XRD patterns of bare TNSs (red trace) and TNS−SF nanocomposites before (orange trace) and after (blue trace) methanol treatment. TEM images of (b) as-prepared TNSs and (c) TNSs dispersed in the fibroin matrix. Data presented for the TNS−SF nanocomposite refer to a 50/50 composition.

TNSs are introduced in the SF matrix, they are exposed to a highly hydrated environment, which can cause the increase of the TNS interlamellar spacing. However, after the methanol annealing process necessary for the crystallization of the SF matrix, the TNS−SF composites dehydrate, and therefore with the material’s density increase while the interlamellar spacing of the TNSs decreases.23 Although the interlamellar spacing is modified, the nanostructure of TNSs is maintained also after their introduction into the SF matrix and the subsequent methanol annealing process. This is further confirmed by the TEM characterization of the as-synthesized TNSs and the TNS−SF nanocomposites. As shown in Figure 1b,c, the presence of the layered lamellar structures of length ∼10 nm is ascribable to the TNSs present in both cases. The dark layers indicate the presence of a stronger scattering material (i.e., TNSs) compared to the organic moieties, which lay invisible between each titanate lamella. The FTIR analysis shown in Figure S1 of the Supporting Information file confirms the influence of the hydration on the TNS structure. In fact, the vibrational modes of the TNS [Ti−O] lattice experience some modifications when the TNSs are incorporated in the silk matrix. Below 900 cm−1, it is possible to observe that the band of Ti−O bond related to stretching- and bending-type vibrations (i.e., infrared-active A2u and Eu modes, respectively) is modified, most possibly due to water absorption and thus due to the change in the coordination environment of surface atoms.36

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RESULTS AND DISCUSSION In Figure 1, the structural and morphological characterization of the as-synthesized TNS powder and the TNS−SF composite before and after methanol annealing is presented. All patterns present peaks ascribable to the TNSs consistent with the typical titanate layered structure.21,24 So far, the assynthesized TNS powder (Figure 1a, red trace) presents a strong reflection peak at low angle (7.8°), which corresponds to an interlamellar spacing of 1.1 nm, as derived from the Bragg’s law. The XRD patterns of the TNS−SF composites show a shift of the TNS characteristic reflection peak at lower angles: 2.3° for the sample before (Figure 1a, orange trace) and 5.1° for the sample after the methanol annealing process (Figure 1a, blue trace) corresponding to TNS interlamellar spacings of 3.8 and 1.8 nm, respectively. This variation can be attributed to the different degree of hydration of the films due to the nanocomposite fabrication procedure. In fact, when the C

DOI: 10.1021/acsami.7b15440 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a, b) SEM images of the TNS−SF (50/50) samples before and after ion exchange with Pb2+, respectively. The corresponding EDX maps are related to the Ti Kα and Pb Mα lines. (c) Representative EDX spectra of the TNS−SF samples before (black line) and after (red line) Pb2+ exchange. High-resolution N 1s XPS spectra of the TNS−SF (d) before and (e) after Pb2+ exchange.

To investigate any possible contribution of the SF matrix to the adsorption of the metal ions by the nanocomposites, pure SF films were dipped in 1000 mg/L Pb2+ aqueous solutions for 24 h. The subsequent ICP-OES analysis of the ion solution showed that the Pb2+ removal capacity of the polymer is negligible (0.1 mmol/g), mostly attributed to the presence of aminoacidic residues in its structure, such as glutamic acid, cysteine, and histidine, generally involved in the removal of metal ions (cf. histogram in Figure S2 in Supporting Information).37,38 On the other hand, under the same conditions, the removal capacity of the TNS−SF nanocomposite films (50/50, 70/30, and 90/10 TNS/SF) is directly dependent on the TNS concentration. Specifically, for the 50/50 TNS/SF, the removal capacity is 0.75 mmol/g, and it increases to 1.1 mmol/g for the 70/30 and to 1.3 mmol/g for the 90/10 TNS/SF (Figure S2, Supporting Information). Although the presence of higher concentration of TNSs increases the ion-uptake capacity, this is detrimental for the structural stability of the nanocomposite material itself. In fact, after 24 h in contact with the ion solution, the samples containing higher amount of TNSs with respect to SF (e.g., 70/30 and 90/10 TNS/SF) tend to swell and partially disintegrate, whereas the films of 50/50 composition retain their solid structure, not being affected by the aqueous environment. Nonetheless, in all cases, the ICPOES measurements carried out on the solutions after the adsorption process reveal a negligible presence of titanium. Therefore, it can be considered that no TNSs are released in the solution despite the samples swelling, indicating the stable retention of the nanostructures in the fibroin matrix. Due to the good compromise between the performance and the

structural integrity in the 50/50 TNS/SF samples, all of the tests that will be further presented below were performed with this type of samples. The scanning electron microscopy (SEM) analysis and the related titanium EDX mapping of the 50/50 composite show that the TNSs are well distributed in the fibroin matrix (Figure 2a), and this is preserved even after the dipping of the nanocomposite in the Pb2+ solution (1000 mg/L for 24 h) (Figure 2b). After dipping, the lead homogeneously distributes throughout the surface of the film, as revealed by the mapping of the Pb Mα line. The EDX analysis also gives information about the ion-exchange process that occurred upon the Pb2+ interaction with the nanocomposite. In particular, Figure 2c demonstrates that the Pb Mα signal arises with an intense band at 2.4 keV after the dipping process, with no modification of the amount of Ti and oxygen-containing species after the ion adsorption. This is proved by the almost stable intensity ratio of the signals of Ti Kα (4.5 keV) to those of O Kα (0.5 keV) before and after interaction with the Pb2+ ions (Ti/O ∼ 6.5). However, there is a significant decrease of the C Kα signal (0.3 keV) with respect to the unchanged O Kα (0.5 keV), with C/O intensity ratios of 5.2 and 2.3 before and after the interaction of the film with Pb2+, respectively. This may be attributed to the TMA+ ion exchange with Pb2+. The generally accepted mechanism of the ion-exchange process when TNSs are used as sorbent materials relies on the substitution of the ions intercalated in the titanate lamellar structure, in this case TMA+, with a mono- or a divalent cation, such as Pb2+23,39 (Figure S3, Supporting Information file). The TMA+ removal from the solid nanocomposite is further confirmed by highresolution XPS analysis of the N 1s band of the samples before D

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of the total ions removed when the TNS−SF films are incubated in solutions of concentration 25, 200, and 1000 mg/ L, respectively. To further investigate their ion removal performance, the TNS−SF films were dipped for 24 h in water solutions containing different concentrations of the three different ions, and the ion removal capacity in the equilibrium state (qe) is calculated in each case, as shown in Figure 4. In all cases, two

and after the interaction with the Pb2+ aqueous solution (1000 mg/L for 24 h) (Figure 2d). Specifically, in the TNS−SF films, the band analysis revealed the presence of two distinct N−C contributions: the first one centered at 399.8 eV is typical of the N−C bonds of the SF matrix40 and the second at 402.7 eV is consistent with the presence of the quaternary nitrogen present in the TMAH component.41 After the ion-exchange process, the latter peak disappears, whereas the N−C bonds of the SF component at 399.8 eV remain (Figure 2e). To determine the kinetics of the Pb2+ cation exchange and to explore the efficiency of the TNS−SF films to interact also with other metal ions, the 50/50 TNS−SF films were dipped in three different metal-ion (Pb2+, Hg2+, and Cu2+) aqueous solutions and the change of ion concentration during the time was monitored in each case. As shown in Figure 3, starting

Figure 4. Removal capacity qe in mmol/g after 24 h of incubation at different of ion concentrations (Pb2+, Hg2+, and Cu2+).

distinct sorption regimes can be distinguished. In the first one, between 25 and 200 mg/L for Pb2+ and Hg2+, and between 25 and 100 mg/L for copper, a sharp increase of the removal capacity was observed as the ion concentration increased. In the second one, for initial concentrations higher than 200 mg/ L for Pb2+ and Hg2+ and higher than 100 for Cu2+, the films reached their maximum sorption capacity (qmax), guiding us to the conclusion that at these respective concentrations the complete saturation of the active sites of the TNSs is reached, so no more ions can be uptaken. The qmax is similar for all of the ions studied, and a mean value of 0.73 ± 0.02 mmol/g was calculated at 1000 mg/L. Our results in terms of qmax are pretty similar to that previously described in the literature.14,17,18,22,43 Specifically, the so far reported qmax values of different-sized and -shaped titanates dispersed in Pb2+ aqueous solutions were between 0.50 and 2.64 mmol/g. However, in all of the previous studies, the titanates were used in the form of nanopowder, dispersed directly in the metal-ion-polluted water, whereas in our case the TNSs are embedded in the SF matrix, so the sorbent is a compact solid material. The obtained qmax of 0.73 mmol/g is calculated with respect to the weight of the nanocomposite, whereas when calculated with respect to the weight of the TNSs present in the nanocomposite, which is 50%, the qmax becomes 2 times higher, i.e., 1.46 mmol/g. Therefore, we can conclude that the incorporation of the TNSs in the polymer does not seem to alter the sorption capability of TNS ions, although the process is slower compared to the freely dispersed titanates in the polluted water,17,18,22,43 attributed to the presence of the solid SF matrix. Hence, the TNS−SF nanocomposites can efficiently remove heavy-metal ions from water, whereas the SF support offers the possibility to easily manipulate the samples and to prevent the release of the titanate fillers in the liquid environment. Even if the presented nanocomposite adsorbs the metal ions in a lower

Figure 3. Kinetics of removal of Pb2+, Hg2+, and Cu2+ ions after dipping the 50/50 TNS−SF film in the ion solutions for 24 h (1440 min). The analyzed samples were incubated in 20 mL solutions of 200 mg/L, which correspond to Pb2+ = 0.96 mM, Hg2+ = 0.99 mM, and Cu2+ = 3.15 mM. The standard deviation is below 0.6% for each point displayed in the plot.

from an initial concentration of 200 ppm in all cases, the metal ions’ concentration is reduced during the time. In the case of Pb2+ and Hg2+, there is a significant decrease of the concentration in the first 120 min of incubation with up to 25% of the total metal ions removed. Until 720 min, the films continue to entrap the ions, but slower compared to the initial phase, with removal efficiencies of 75% for Pb2+ and 60% for Hg2+ until the system reaches equilibrium. In the case of Cu2+ ions, the initial adsorption stage is concluded after 60 min, whereas at the second stage, the removal rate is lower compared to the other ions, reaching up to 20% of metal-ion removal efficiency in 720 min. This could be attributed to the higher initial concentration of the ions in the water solution, due to the smaller size of Cu2+, which might result in a faster saturation of the available active sites of the composite upon interaction with the ions.42 In fact, the initial concentration of the 200 mg/L ions in water corresponds to 3.15 mM of Cu2+, more than 3 times higher compared to the 0.96 and 0.99 mM for Pb2+ and Hg2+, respectively. This is further emphasized by the tests performed starting with the initial concentrations of 25 and 1000 mg/L (Figure S4a,b, Supporting Information). The Cu2+ adsorption kinetics is significantly affected by the increase of the initial ion concentration in contrast to the other two ions. In the first 120 min it is reached the 21, 30, and 78% E

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ACS Applied Materials & Interfaces rate than the titanates powder, compared to other solid systems used for the removal of metal ions from water, under similar conditions and metal-ion concentrations (in the order of mmol/L), it has similar or better removal kinetics. Specifically, zeolites and/or titanate nanofibers need up to 50−100 h to reach the sorption equilibrium,43,44 whereas in our case the equilibrium is reached after 24 h. To avoid concerns on the release of TMA+ to the environment upon its exchange with heavy-metal ions, it is possible to pretreat the TNS−SF films with Na+ solutions, substituting in this way the TMA+ with Na+ ions23 and resulting in a new type of films indicated as TNS−Na−SF. The FTIR analysis (Figure 5) of the TNS−SF films before and after their dipping in a 1000 mg/L Na+ solution for 24 h

Figure 6. ICP-OES result of a successive incubation of the TNS−SF nanocomposite in solutions of Na+ and Pb2+. After the initial substitution of TMA+ by Na+, the material was incubated alternatively in Pb2+ and Na+ of the same initial concentration of 2.1 mM for 24 h, each time.

exchange cycle. Therefore, although ion exchange of Pb2+ with Na+ is the dominant adsorption mechanism, further addition of Na+ cannot replace the adsorbed Pb2+, indicating that the adsorption of the ions is irreversible, facilitating thus the safe disposal after adsorption. To get insight into the ion exchange between Na+ and Pb2+, scanning transmission electron microscopy (STEM)−EDX analysis was carried out on the TNS−SF composites before and after ion sorption in the films. In Figures S5 and S6 of the Supporting Information file, the STEM images and related EDX mapping show that Na+ is homogeneously intercalated inside the TNSs matrix proving the efficacy of the TMA+ removal. Upon dipping the TNS−Na−SF in the Pb2+ solution, the STEM−EDX study reveals the presence of Pb2+ dispersed throughout the TNS structure without any signature of residual Na+. This is also proved by the XPS analysis of the TNS−Na−SF films. As the XPS survey spectra show (Figure 7), after the incubation in the Pb2+ solution, the signal related to Na 1s, as well as the strong Auger peak at 497 eV, disappears, whereas at the same time, the Pb signal emerges (cf. green line in Figure 7). Furthermore, high-resolution XPS spectra of the Na 1s signal (Figure S7 of the Supporting Information file) centered at the typical binding energy of 1072.2 eV45 becomes negligible after dipping in the Pb2+

Figure 5. FTIR spectra of the TNS−SF films before and after incubation in Na+ solution. The dotted line indicates the wavenumber at 949 cm−1 related to the identifiable peak of the TMAH.

confirms the complete TMA+ removal from the material: the absorption band at 949 cm−1 referred to the C−N stretching characteristic for the TMAH molecules disappears.23 The contribution of SF is still well recognizable from the relative amide I (1600−1700 cm−1) and II (1540 cm−1) bands.23 To confirm that the exchange of TMA+ with the Na+ ions does not alter the sorption capacity of the bionanocomposite, the uptake of Pb2+ by the Na+-modified system and the interactions taking place during such process were investigated. As shown in Figure 6, and in accordance with the data of Figure 5, after the incubation in a 1000 mg/L (43.5 mM) Na+ solution for 24 h, the TMA+ was exchanged with the Na+ and 0.65 mmol/g of such ions are entrapped in the TNS−SF system. Subsequently, the modified TNS−Na−SF nanocomposite was dipped in a Pb2+ ion solution of 435 mg/L (2.1 mM) for 24 h, and as shown, all of the Na+ ions were released and the Pb2+ ions took their place in the composite system (Pb2+ sorption capacity, 0.81 mmol/g). As shown in Figure 6, after the first uptake cycle, a second dipping of the Pb2+-modified TNS−SF system in the Na+ solution, with a concentration of 2.1 mM, does not cause any release of Pb2+ or uptake of Na+, whereas the subsequent dipping in Pb2+ ions causes a slight increase of the sorption capacity of the system because an additional small amount of Pb2+ was uptaken and an equivalent amount of Na+ was released (∼0.1 mmol/g), possibly due to the incomplete process of the first Pb2+

Figure 7. XPS survey spectra of TNS−Na−SF composites before (orange line) and after (green line) incubation with Pb2+. F

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ACS Applied Materials & Interfaces solution. On the other hand, the signal ascribed to the Pb 4f7/2 at 138.8 eV emerges after the dipping process. The difference of binding energies (Δ = 4.89 eV) between the Pb 4f5/2 and Pb 4f7/2 is consistent with the Pb in +2 oxidation state46 (Figure S7 of the Supporting Information file). The high-resolution O 1s XPS spectra of the TNS−SF nanocomposite after ion exchange with Na+ and Pb2+ (Figure S8a−c, Supporting Information file) show that the [Ti−O] component ascribed to the TNS lattice experiences very little changes (Tables S1 and S2, Supporting Information file). This behavior is consistent with the TNS structure retention and an ion-exchange process according to what was observed by Liu et al.14 From a theoretical point of view, a bivalent cation would exchange two monovalent cations, and this is in contrast to the observed behavior, as already described in Figure 6. In fact, after the Na+ exchange with the TMA+, the TNS−Na−SF sorbs ∼0.81 mmol/g of Pb2+ and releases a comparable amount of Na+ (∼0.73 mmol/g). This can be possibly explained by the analysis of the −OH groups population of each sample. In fact, the amount of the hydroxyl moieties increases after the Na+ exchange with the TMA+ component (2.4 ± 0.5% for the TNS−SF and 10.0 ± 0.5% for the TNS− Na−SF), indicating that the Na+ does not completely saturate the TNS interaction sites, leaving a higher content of available −OH groups, which belong to the TNS structure. On the other hand, after the interaction of the TNS−Na−SF sample with the Pb2+, the −OH component decreases to 3.8 ± 0.5%, indicating that the Pb2+ replaces not only all of the Na+ but also a great amount of the −OH groups (Table S2, Supporting Information file). Furthermore, the variation of the carboxylic moieties toward higher binding energies of the O 1s components (cf. Figure S8b,c and Table S1 of the Supporting Information file) upon the Na+ and Pb2+ ions sorption could be an indication of ion chelation also from the SF species.47 Finally, as shown in the XRD patterns presented and discussed in Figure S9 of the Supporting Information file, the interlamellar spacing of the TNSs embedded in the SF films increases after the TMA+ exchange with Na+ and even more after the exchange of the Na+ with Pb2+. The increase of the relative interlamellar spacing when the TNS−SF composites undergo ion exchange could be explained in terms of the corresponding ions’ salt hydration degree and ion size. As found in the TEM analysis, despite the variation in the lamellar structure of titanates, the integrity of the composite is not compromised by the ion exchange (Figure S10, Supporting Information file). Finally, the ion-exchange behavior of the nanocomposites and the effect of Na+ on the sorption process were studied when different ions were simultaneously present in the solutions. The histogram in Figure 8a shows the amount of ions exchanged (uptaken or released) by the TNS−Na−SF nanocomposite in a solution containing equal concentrations of Pb2+ and Cu2+ (2.1 mM of each ion, which corresponds to 435 mg/L for Pb2+ and 133 mg/L for Cu2+). It is evident that the Pb2+ sorption capacity is not affected considerably by the presence of Cu2+, reaching a final value of about 0.50 ± 0.01 mmol/g (Pb2+ sorption capacity was 0.81 mmol/g without the presence of Cu2+). However, the Cu2+ uptake is very low, showing a sorption capacity of 0.05 ± 0.01 mmol/g. At the same time, as previously observed, a similar amount of Na+ to that of the adsorbed ions was released in the solution demonstrating that the TNS−Na−SF samples selectively uptake Pb2+ in the presence of Cu2+.

Figure 8. (a) Histogram of ion exchange (uptaken = positive values, released = negative values) on a TNS−SF sample previously exchanged with Na+ (TNS−Na−SF) in a solution of Cu2+ and Pb2+ (2.1 mM each) for 24 h. (b) Histogram related to TNS−SF nanocomposite under competitive condition using a complex system constituted from three cations (Cu2+, Na+, and Pb2+ of 2.1 mM each) after 24 h. (c) Histogram of adsorption of a TNS−SF sample in a solution of Cu2+ and Pb2+ (2.1 mM each) for 24 h.

The fundamental role of Na+ in the selectivity of the nanocomposites toward Pb2+ is further demonstrated upon dipping a TNS−SF film in a solution containing equal concentrations of Na+, Cu2+, and Pb2+ (2.1 mM of each ion, which corresponds to 435 mg/L for Pb2+, 133 mg/L for Cu2+, and 48 mg/L for Na+) (Figure 8b). The TNS−SF sample demonstrated selectivity toward Pb2+, reaching a sorption capacity of 0.73 ± 0.01 mmol/g similar, and with similar kinetics (Figure S11, Supporting Information file), to that observed in the only-Pb2+ ions solution, reported in Figure 3. On the other hand, the final sorption capacity was only 0.11 mmol/g for the Cu2+, and no Na+ ions were sorbed. In the absence of Na+, as shown in Figure 8c, the TNS−SF sample is slightly more selective for Pb2+ compared to Cu2+, with sorption capacities of about 0.36 ± 0.01 and 0.26 ± 0.01 mmol/g, respectively, lower than the previous case for Pb2+ and also lower than the single-ion samples for both types of ions. Even if the absolute values of the sorption capacities are lower for each ion, the sum of both gives 0.62 mmol/g, close to the previously calculated maximum sorption capacity of the system. The stronger affinity of Pb2+ toward the TNSs compared to Cu2+ can be related to the hydration energy of the ions.14,48 Although ions with smaller radii, such as Cu2+ (0.73 Å, lower compared to Pb2+ having a radius of 1.19 Å),49 can be transported more easily in the structure, ions with a lower hydration energy, such as Pb2+ (the free energies of hydration of Pb2+ and Cu2+ are 1425 and 2085 kJ/mol, respectively),50 are more easily dissociated from water molecules, making their exchange with Na+ or TMA+ in the titanate layers more energy-favorable.14 This stronger affinity of the nanocomposite system toward the Pb2+ is significantly highlighted in the presence of Na+. During the sorption of the cations in the TNS structure, diverse mechanisms take place.20,36 Initially, the hydrated metal ions are dissociated into bare ions. This step is strictly dependent on the hydration energy of the ion because metal ions with lower hydration energy become more easily bare ions. Then, the cations are electrostatically attracted by negatively charged titanates, where metal cations with higher valence have larger electrostatic attraction force. Finally, the cations are exchanged with the existing interlayered ions, with G

DOI: 10.1021/acsami.7b15440 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces an efficiency related to the cation hardness or softness according to Pearson’s hard−soft acid−base principle,51 where hard acids bind strongly to hard bases and soft acids bind strongly to soft bases. Pb2+, which, as already discussed, has the lower hydration energy, is considered a softer acid than Na+ (absolute hardnesses of Pb2+ and Na+, 8.5 and 21.1, respectively)52 and therefore interacts with a higher affinity with a soft base, such as TNSs,39 compared to Na+. Although Na+ does not affect the sorption capacity of the highly favorable Pb2+, when also Cu2+ is present, it alters significantly the sorption of the less favorable interaction with Cu2+.14 Specifically, when Pb2+ and Na+ (either in the solution as free ion (Figure 8b) or as a component of the solid system (Figure 8a)) are present, the sorption capacity for Cu2+ reaches the minimum value of 0.1 mmol/g. These results further confirm the very high affinity of Pb2+ toward TNSs, as already observed for titanate nanotubes,14 paving the way to the application of TNS−SF-based composites under real condition, in complex aquatic environments.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Davide Magrì: 0000-0003-3790-5332 Gianvito Caputo: 0000-0002-8973-3109 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Author Contributions ∇

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G.C. and G.P. have contributed equally.

Notes

CONCLUSIONS In conclusion, we have reported the highly efficient ionexchange ability of a silk fibroin composite biomaterial. This study demonstrates that solid TNS−SF nanocomposites can be viable systems for the removal of heavy-metal ions from water. For all of the ions tested, the maximum removal capacity obtained is 0.73 ± 0.02 mmol/g. When transformed to the maximum removal capacity of the bare TNS fraction inside the composite, it reaches 1.46 mmol/g, which is in the upper limit of the values presented so far in the literature for freestanding titanates (between 0.50 and 2.64 mmol/g).14,17,18,22,43 This indicates that the presence of the solid SF matrix, which enwraps the TNSs, does not affect the ability of the titanates to efficiently interact with the metal ions, preventing at the same time any possible release of those nanostructures in the aqueous medium. The combination of good adsorption capacity, selectivity, and structural integrity make this material an ideal candidate for real-case scenarios. The presence of ubiquitous inorganic cations, such as Na+, increases the sorption selectivity of the nanocomposite toward Pb2+ without affecting the high affinity of TNSs for the specific ion, opening the way toward its application in selective Pb2+ ion collection in complex systems like seawater.

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components; XRD patterns of TNSs, TNS−SF, TNS− Na−SF, TNS−Pb−SF; TEM images of the TNS−SF after ion exchanges; absorption kinetics on a TNS−SF in a multi-ion solution (Na+, Cu2+, Pb2+) (PDF)

The authors declare no competing financial interest.

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ABBREVIATIONS TNSs, titanate nanosheets; SF, silk fibroin; TMAH, tetramethylammonium hydroxide REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15440. FTIR spectra of TNSs and TNS−SF samples, removal capacity properties of the composites at different composition percentages (TNS/SF ratios 50/50, 70/ 30, 90/10); scheme of ion exchange; kinetics of removal of Pb2+, Hg2+, and Cu2+ ions after the incubation of the TNS−SF nanocomposite in solutions with initial concentrations of 25 and 1000 mg/L; STEM−EDX of TNS−SF samples after ion exchange with Na+ and with Na+ and Pb2+; high-resolution Na 1s and Pb 4f XPS spectra of TNS−SF composites after incubation with Na+ and Pb2+; high-resolution O 1s XPS spectra of TNS−SF composites after incubation with Na+ and Pb2+; XPS analysis of the binding energies of the O 1s components; species percentage calculated on O 1s H

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DOI: 10.1021/acsami.7b15440 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX