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Properties and arsenic removal evaluation of polyvinyl alcohol nanofibers with embedded strontium hexaferrite nanoparticles
Materials Chemistry and Physics 234 (2019) 151–157
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Properties and arsenic removal evaluation of polyvinyl alcohol nanofibers with embedded strontium hexaferrite nanoparticles R. Murillo-Ortíz a, b, M. Mirabal-García a, J.J. Cruz-Rivera c, D. Valdez-P�erez d, J.R. Martínez e, F. P�erez-Moreno f, A. Lobo-Guerrero f, * a
� Instituto de Física, Universidad Aut� onoma de San Luis Potosí, Alvaro Obreg� on No. 64, 78000, San Luis Potosí, Mexico � Doctorado Institucional en Ingeniería y Ciencia de Materiales, Universidad Aut� onoma de San Luis Potosí, Alvaro Obreg� on No. 64, 78000, San Luis Potosí, Mexico c Instituto de Metalurgia, Universidad Aut� onoma de San Luis Potosí, Av. Sierra Leona 550, 78210, San Luis Potosí, Mexico d Instituto Polit�ecnico Nacional, UPALM, Edif. Z-4 3er Piso, 07738, M�exico D.F., Mexico e � Benem�erita Universidad Aut� onoma de San Luis Potosí, Alvaro Obreg� on No. 64, 78000, San Luis Potosí, Mexico f � Area Acad�emica de Ciencias de la Tierra y Materiales, Universidad Aut� onoma del Estado de Hidalgo. Carr. Pachuca-Tulancingo km. 4.5, 42039, Pachuca de Soto, b
Mexico
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Arsenic removal using PVA nanofiber magneto-composite. � Effect of strontium hexaferrite nano particles on the PVA nanofiber properties. � Improvement of arsenic removal effi ciency by effect of hexaferrite nanoparticles. � Thermoprotective effect of the hex aferrite nanoparticles on the PVA matrix.
A R T I C L E I N F O
A B S T R A C T
Keywords: Magnetic polymers Strontium hexaferrite nanoparticles Electrospinning Nanofibers Pechini method
This work deals with the study of physical properties exhibited the polyvinyl alcohol (PVA) fabricated as a nanofiber composite with embedded strontium hexaferrite nanoparticles (SrM-NPs). The nanofibers obtained from electrospinning have 165 nm average diameter, and the nanoparticles obtained from the sol-gel Pechini method have 78 nm diameter. A low-frequency sonication treatment performed to these nanoparticles allowed to deagglomerate and to reduce their size up to obtain 3.4 nm average diameter. Also, the magnetic nanoparticles exhibit an arrangement along the PVA nanofiber because of the nanoparticles interact with the electric field generated during the electrospinning. This arrangement improves the magnetic properties of the composite in comparison with the bulk nanoparticles. Also, the interaction of strontium hexaferrite nanoparticles with the PVA matrix induces changes on its physical properties, as well as on the ability of nanofibers to uptake arsenic from water.
* Corresponding author. E-mail address: [email protected] (A. Lobo-Guerrero). https://doi.org/10.1016/j.matchemphys.2019.05.043 Received 22 November 2018; Received in revised form 14 May 2019; Accepted 17 May 2019 Available online 18 May 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
R. Murillo-Ortíz et al.
Materials Chemistry and Physics 234 (2019) 151–157
1. Introduction
embedded strontium hexaferrite nanoparticles stands out as a trust worthy ensemble to remove arsenic from water, wherein the use of hard-magnetic nanoparticles allows the nanofiber extraction by using a low-intensity external magnetic field. The higher magnetic performance exhibiting the hard-magnetic nanoparticles as part of the composite allows extracting the nanofiber from water easier, regarding the usually used soft-magnetic nanoparticles. In addition, the hard-magnetic nanoparticles can be magnetically-arranged in a polymeric matrix to originate nanoparticles arrangement-dependent anisotropies which could modify some of the physicochemical properties in the composite. This work shows the feasibility of incorporating hard-magnetic nanoparticles in a nanostructured polymeric composite to be used in arsenic removal technology. The use of hard-magnetic nanoparticles, such as the hexaferrite ones, offers an unexplored research field among which the polymer-nanoparticle interaction plays an important role in determining functional properties of nanostructured composites. In this case, hexaferrite nanoparticles with diameters between 3 and 5 nm were for the first time used to develop a composite with potential application in the arsenic removal technology. Also, pure polyvinyl alcohol nano fiber and pure hexaferrite nanoparticles were used as reference, results showed an improvement on the arsenic adsorption uptake as the hex aferrite nanoparticles increase in the polyvinyl alcohol nanofiber, which makes of this system an interesting and low-cost candidate for envi ronmental remediation of water sources. In addition, a straightforward methodology to fabricate polyvinyl alcohol nanofiber with extremely small strontium hexaferrite nanoparticles is presented.
Arsenic in water is one of the most worrying problems due to its high toxicity for the human body [1,2]. At this moment, millions of people in all countries suffer from chronic arsenic poisoning due to the con sumption of contaminated water [3–5]. Arsenic compounds rank at the 20th position of major abundance in the earth’s crust, and they are in constant circulation in the environment [6]. Rocks weathering generates dust with arsenic compounds which propagate naturally in the envi ronment, and the human activity contributes to propagate this pollution. The adverse effect of arsenic on human health range from skin lesions to different types of cancer [7,8]. Arsenic contaminated drinking water has been reported worldwide which producing a significant environmental and public health problem [9–16]. Different methods have been proposed to separate arsenic from water, some of them, been adsorbent materials [17], oxidation and filtration [18], phytoremediation [19], ion exchange [20], electro chemical remediation [21], coagulation and flocculation [22], and so on [23–25]. Among these methods, adsorption stands out due to its high removal efficiency, easy operation, low cost and absences of toxic sludge [23,26]. Then, adsorbent materials attract lots of attention because it does not require the use of chemical compounds and toxic waste pro duction, the removal process is simple, and the adsorbent materials can be recycled without significant loss of its efficiency. Among the arsenic adsorbent materials, it has experimented with these based on iron oxides [27], alumina [28], and silica [29]. Also, polymeric materials as chito san and its derivatives have been tested to remove arsenic from water [30,31]. Recently, polyvinyl alcohol (PVA) has been recognized as a prom ising material for arsenic removal due to its ability to mobilize heavy metals in water media [32]. However, if any polymeric material remains in the water for a long time, it can create additional environmental is sues, so an efficient method is needed to extract the polymer with the adsorbed arsenic. In this sense, magnetic nanoparticles can act as a carrier allowing to extract the polyvinyl alcohol with the adsorbed arsenic. Besides, strontium hexaferrite with chemical formula SrFe12O19 is identified as a hard-magnetic compound widely used as permanent magnets. The properties of the strontium hexaferrite nanoparticles include high magnetization saturation, remanence, coercivity, Curie temperature, magnetocrystalline anisotropy, electrical resistivity, chemical stability, and they have low fabrication costs [33,34]. In this sense, by combining the polymer adsorption ability with the magnetic property of nanoparticles, a low-cost composite capable of removing arsenic could be developed. Therefore, polyvinyl alcohol nanofiber with
2. Experimental Strontium hexaferrite nanoparticles were fabricated using the Pechini method; this method is based on the formation of a polymeric resin containing strontium and iron ions in the stoichiometric ratio. For that, we used 9.0 g of ferric nitrate (Fe(NO3)3⋅9H2O, Sigma Aldrich) mixed with 0.38 g of strontium nitrate (Sr(NO3)2 Sigma Aldrich) stirred in 60 mL of deionized water at room temperature for 30 min. Then, in 20 mL of deionized water was added 2.0 g of citric acid and 0.5 mL of ethylene glycol. Subsequently, these two mixes were combined and heated at 70 � C with constant agitation until water evaporation pro motes a polyesterification reaction, obtaining a homogeneous resin where metal ions are uniformly distributed in the organic matrix [35]. The polymeric mix was heated 2 h in a Lindberg 51844 furnace at 200 � C to remove any solvent excess, and the obtained powder was heated to 5 � C/min up to 900 � C, maintaining the temperature for 2 h. After, 1.5 g of polyvinyl alcohol (MW ¼ 75,000) was dissolved in 50 mL of deionized water and stirred 40 min at 80 � C. Therefore, the mix was left to cool at
Fig. 1. Experimental set-up used to fabricate the nanofibers with the embedded SrM-NPs. 152
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Fig. 2. TEM micrograph of (a) SrM-NPs prepared from the Pechini method, (b) agglomerate conformed of various nanoparticles, (c) histogram of the strontium hexaferrite nanoparticles as a powder after sintered. TEM micrograph of (d) PVA nanofiber with SrM-NPs, (e) low-size SrM-NPs inside of the PVA nanofiber, (f) histogram of the strontium hexaferrite nanoparticles inside of the PVA nanofiber as part of the composite.
room temperature. Then, the nanofiber was prepared using 90 %wt of the previously obtained polyvinyl alcohol and incorporating 10 %wt of the hexaferrite nanoparticles. An additional sample was fabricated using 20 %wt of nanoparticles to evaluate the effect these nanoparticles on the arsenic (V) adsorption. After, strontium hexaferrite nanoparticles were put into ethanol (C2H6O) and sonicated for 2 h with a Branson 2510 ultrasonic bath operated at 40-kHz to disperse, deagglomerate, and fragment, the as-obtained nanoparticles [36,37]. Finally, a 5 mL syringe with a stainless-steel needle of 0.15 mm internal diameter was filled with the ultrasonicated mix and loaded in NE-300 New Era infusion pump programmed to deliver a flow rate of 0.3 mL/h. The syringe needle was connected to the positive electrode, and the negative elec trode connected to an aluminum foil which serves as a collector at a fixed distance of 5.3 cm from the syringe tip. The system was installed inside of an isolated chamber to minimize undesirable ambient effects, and the power supply was adjusted at 25 kV. Fig. 1 shows the compo nents and the experimental set-up of the electrospinning system. The collected fiber was left to rest at room temperature for 12 h; then, to avoid the polyvinyl alcohol dissolves in water, nanofibers were sub jected to a chemical crosslinking in methanol during 24 h. The samples were characterized with X-ray diffraction using a Siemens D5000 diffractometer which uses a cobalt source
(λ ¼ 1.7890 Å), the structural parameters were refined using Rietveld analysis incorporated in the MAUD program [38]. The morphological characteristics of nanofibers and nanoparticles were analyzed using a Hitachi S-570 transmission electron microscope (TEM) operated at 100 kV. Uranyl acetate was used to enhance the TEM contrast and make the strontium hexaferrite nanoparticles observables. Thermogravimetric analysis was performed to evaluate the effect of the embedded nano particles in the polymeric matrix by using a DSCQ200 differential scanning calorimetry with a heating rate of 10 � C/min in an N2 atmo sphere. The magnetic properties were obtained from the hysteresis loops measured at room temperature using LDJ-9600 vibrating sample magnetometer (VSM). The arsenic (V) removal efficiency of pure polyvinyl alcohol nano fiber was evaluated after 1, 3, 5, 7 and 10 min, and results compared with those obtained from the nanofibers incorporating 10 and 20 %wt of hexaferrite nanoparticles. Also, the arsenic removal efficiency of bare strontium hexaferrite nanoparticles has been verified, under the same initial conditions than the nanofibers. In each case, 0.2 g of nanofibers, or nanoparticles were immersed in a stock solution with initial arsenic (V) concentration of 3.26 μg/L. The solution with the nanofiber remained in constant stirring during all the experiment. The arsenic removal efficiency of the bare nanoparticles was evaluated at intervals 153
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Fig. 3. Experimental and calculated X-ray diffractograms of (a) SrM-NPs after sintering, and (b) PVA-SrM as a nanofiber composite.
of 2.5 min, where 1 min corresponds to the mechanical stirring, and 1.5 min is the time spent in clean the water from nanoparticles using a magnet. Measurements were done in triplicate with a PerkinElmer Op tima 8300 ICP-OES spectrometer, and the average absorbance was used to calculate the removal efficiency of arsenic (V) in the water using equation (1). Removal efficiency ¼ [(C0
Ce) / C0] (100)
Fig. 4. (a) TEM Micrograph of the polyvinyl alcohol composite as a nanofiber. (b) Histogram showing the nanofibers diameter distribution.
(1)
where C0 is the initial concentration, and Ce is the equilibrium concen tration given in μg/L.
refinement is based on the structure of the strontium hexaferrite: COD ID 100600 (hexagonal, P63/mmc). Results show the presence of strontium hexaferrite without secondary phases. The calculated unit cell parame ters were: a ¼ 5.8300 � 0.0004 Å and c ¼ 22.8716 � 0.0007 Å. The crystallite size calculated from the Rietveld method was 76 � 2 nm matches with the particle size observed in the TEM micrographs (78 nm). The final refinement output gives an Rwp ¼ 8.60, Rexp ¼ 4.59, and σ ¼ 1.87, which together with the good adjusted observed graphi cally in Fig. 3 (a), ensures the reliability of the Rietveld fit. Fig. 3 (b) shows the experimental X-ray and calculated X-ray dif fractograms of the PVA nanofiber with embedded strontium hexaferrite nanoparticles. The PVA exhibits an amorphous-like peak at 22.7� (Cokα) not showed in Fig. 3. The Rietveld refinement of the PVA with SrMNPs indicates only the presence of strontium hexaferrite as a crystalline phase. Although, in this case, the hexaferrite peaks are wider than the observed in Fig. 3 (a). This behavior is closely related with a size reduction of the hexaferrite nanoparticles from the initial 78 nm measured as a powder to SrM-NPs of 3.4 nm obtained as part of the PVASrM nanofibers. In this sense, X-ray results match with the observations made using TEM for the particle size of the hexaferrite. Fig. 4 (a) shows a TEM micrograph of the polyvinyl alcohol nanofiber with embedded strontium hexaferrite nanoparticles obtained from electrospinning. The PVA nanofibers have a circular section, the embedded nanoparticles are homogeneously distributed inside of the fiber, and they do not exhibit agglomeration. The nanofibers diameters go from 97 to 269 nm with an average diameter of 165 nm and a stan dard deviation of 39 nm. The histogram of the nanofibers diameters is presented in Fig. 4 (b). The embedded nanoparticles show an arrange ment along the nanofiber length as a result of the interaction forces between nanoparticles, and with the electric field generated inside of the electrospinning chamber. Fig. 5 shows the magnetization curves of the polyvinyl alcohol nanofiber with the embedded strontium hexaferrite nanoparticles, and
3. Results and discussion Fig. 2 (a) shows the TEM micrograph of the strontium hexaferrite nanoparticles with plate-like morphology obtained from the Pechini method after they were sintered at 900 � C. The TEM micrograph of Fig. 2 (b) shows that the particles which integrate an agglomerate have lengths of several tens of nanometers. Accordingly, Fig. 2 (c) presents the par ticles sizes histogram obtained from the measurement of 100 particles from various TEM micrographs, an average particle diameter of 78 nm with a standard deviation of 1.8 nm was obtained, and diameters dis tribution ranging from 37 nm to 179 nm. On the other hand, Fig. 2 (d) shows the PVA with SrFe12O19 nanoparticles after the sonication process and once nanofibers were fabricated. The TEM micrograph of Fig. 2 (e) shows a PVA nanofiber where inside there are dark spots dispersed along the nanofiber. In this case, these spots would correspond to the stron tium hexaferrite phase but with a significative particle size reduction. As before the nanofiber fabrication, only PVA and SrFe12O19 were present in the system, the polymeric phase forms the nanofiber, and the SrFe12O19 remains inside of the fiber as the magnetic phase, where the original particle size has been reduced by one order of magnitude. This affirmation is confirmed by the Rietveld analysis of the X-ray patterns, and from the measurement of the magnetic properties of the nanofibers. The strontium hexaferrite nanoparticles dispersed in the PVA matrix have an average diameter size of 3.4 nm with a standard deviation of 0.79 nm, accordingly with the histogram shown in Fig. 2 (f). The smaller and the bigger measured diameters were 2.3 nm, and 7.0 nm, respectively. Fig. 3 (a) shows the experimental X-ray diffractogram of the hex aferrite nanoparticles measured as a powder after sintered and its cor responding Rietveld refinement curve. The initial model for the 154
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compared with the one reported for strontium hexaferrite nanofibers sintered at 900 � C, but in this case, the nanoparticles coalescence gave the form of nanowires [39]. All hysteresis loops were normalized to its magnetization of saturation (Ms) to allow comparison. The squareness ratio (Mr/Ms) of the strontium hexaferrite nanoparticles is 0.63 and its coercivity (Hc) 6.22 kOe, while the hexaferrite nanowires reported in Ref. [39] has Mr/Ms ¼ 0.57 and Hc ¼ 521 kA/m (6.54 kOe). The hex aferrite nanoparticles and nanowires exhibit very similar magnetic properties. In contrast, when nanoparticles are diluted along the nano fiber, the remanent squareness increases 15% and the coercivity 1.2% (with respect of the nanoparticles). Table 1 shows the magnetic prop erties of the nanofiber with the embedded nanoparticles compared with those reported for similar systems. The magnetization squareness obtained for the polyvinyl alcohol nanofibers with the embedded strontium hexaferrite nanoparticles is one of the highest reported in literature for hexaferrite based materials, and only thick films wherein magnetization prefers the normal direction have a magnetization squareness of 0.96 [43], whereas for the strontium hexaferrite particles oriented randomly is about of 0.50 [44]. The high magnetization squareness obtained for the nanofibers is a consequence of the nanoparticle ordering in the polymeric matrix [45]. The effect of the embedded strontium hexaferrite nanoparticles on the thermal stability of the polyvinyl alcohol nanofiber was studied using thermogravimetric analysis (TGA) and differential thermogravi metric analysis (DTG). The continuous line of Fig. 6 (a) shown the TGA curves measured from RT to 1000 � C, and the dashed lines are the de rivative curves (DTG). The polyvinyl alcohol nanofiber degrades in a two-step process in the temperature range from 240 � C to 315 � C and from 315 � C to 480 � C. According to Peng and Kong [46], the first degradation process of the PVA involves mainly water elimination and decomposition of acetate groups, and then, its decomposition is controlled by chain-scission reactions. Fig. 6 (b) shows a maximum rate of mass decrement around 268 � C followed of a second minimum at 430 � C showed in detail in Fig. 6 (c). However, when strontium hex aferrite nanoparticles are embedded inside of the nanofibers, some dif ferences concerning the pure PVA nanofibers are observed. These differences are related to the influence of the strontium hexaferrite nanoparticles on the thermal behavior of the PVA. The strontium hex aferrite nanoparticles seem to have a thermo-protective function of the polyvinyl alcohol. This thermo-protective effect occurs in the PVA which surrounds the nanoparticles, delaying the chain-scission reactions occurred between 440� C and 500 � C up to 650� C-750 � C. Then, the PVA surrounding to the nanoparticles surface is decomposed at a higher temperature between 640 � C and 740 � C, as Fig. 6 (d) and 6 (e) shows. The protective function of the strontium hexaferrite nanoparticles on the polyvinyl alcohol delays its thermo-degradation for more than 150 � C concerning the PVA without nanoparticles. Finally, at 1000 � C the PVA
Fig. 5. Solid lines: Hysteresis loops of the Sr-M nanoparticles as powder, and in the nanofibers PVA composite. Dots: hysteresis loop of the Sr-M as nanowires reported in Ref. [39]. Table 1 Magnetic parameters obtained for de analyzed samples and those reported in the literature. Sample
Hc (kOe)
Mr/Ms
Reference
PVA/SrM (10:1) SrM-NPs SrM-Nanowires SrFe12–3Al3O19 SrM-thin film SrFe10.5Al1.5O19/Fe3O4
6.30 6.22 6.54 8.40 6.63 8.10
0.72 0.63 0.57 0.66 0.62 0.60
This work This work [39] [40] [41] [42]
Fig. 6. Thermogravimetric curves (TGA) and differential thermic analysis curve (DTG) of the polyvinyl alcohol nanofiber with and without strontium hexaferrite nanoparticles. DTG curves presented in detail at various tempera ture ranges. Fig. 7. (Ο) The arsenic removal efficiency of the PVA nanofiber. (Δ) Effect of the embedded strontium hexaferrite nanoparticles on PVA nanofiber at two different concentrations, and (□) arsenic removal efficiency of the bare stron tium hexaferrite nanoparticles.
as a reference, it was measured the hysteresis loop of the strontium hexaferrite nanoparticles obtained directly from the Pechini method before the sonication process. Both magnetization curves were 155
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Fig. 8. Extraction of nanofibers from water by applying an external magnetic field.
has been decomposed completely and only persists the hexaferrite phase. According to Fig. 6, water elimination from PVA results at a temperature below of 240 � C because of the incorporation of the hex aferrite nanoparticles, but at a higher temperature, the chain-scission reactions are avoided in polyvinyl alcohol which is in contact with the hexaferrite nanoparticles. The thermoprotective effect of the strontium hexaferrite nanoparticles on the polyvinyl alcohol has a trend to delay its thermal degradation. This result is consistent with the reported by Guo et al., for a PVA-Fe2O3 composite, where improvement on the polyvinyl alcohol thermal stability was attributed to a better cross-linkage as a result of the Fe2O3 nanoparticles incorporation [47]. Fig. 7 shows the arsenic removal efficiency of polyvinyl alcohol nanofiber with strontium hexaferrite nanoparticles at two different nanoparticle concentrations, with 10 and 20 % wt. of nanoparticles. Also, pure PVA nanofiber and strontium hexaferrite nanoparticles were evaluated as reference samples. Pure PVA nanofiber shows a poor arsenic adsorption efficiency contrasting with the one observed for the magnetic nanoparticles. However, when nanoparticles are embedded in the PVA nanofiber, the composite changes its arsenic adsorption effi ciency, showing an enhanced efficiency when the number of nano particles increases in the nanofiber. As the nanoparticles are located inside of the PVA nanofiber, they do not interact directly with the arsenic; and the improved adsorption ability arises from the strong interaction between nanoparticles and the PVA matrix as a consequence the higher surface area inherent with the smaller nanoparticles. The interaction of the PVA with small iron oxide nanoparticles can modify some physicochemical polymer properties. In this sense, incorporation of the hexaferrite nanoparticles in the PVA nanofiber could to promote a cross-linking of the polymer chains and induce changes in the van der Waals energy [47,48]. In sum, these ef fects result in the arsenic adsorption improvement. Although, more studies on the effect of the hexaferrite nanoparticles on the physico chemical properties of PVA nanofibers are required to fully understand the interactions between nanoparticles and the PVA matrix. On the other hand, different behavior in the hexaferrite nano particles has been observed in Fig. 7. In this case, arsenic adsorption majorly occurs below 3 min, which agrees with a previous study made by Patel et al. using barium hexaferrite nanoparticles [49]. Although the adsorption mechanism of the hexaferrite nanoparticles is not well un derstood, it was pointed out that the adsorption kinetics could be similar to the chemisorption mechanism observed in other iron oxides nano particles [50]. However, nowadays the use of bulk nanoparticles in environmental applications generates a certain amount of controversy. Then, the development of functional composites containing nano particles is a promising alternative. Among them, the PVA magnetic nanofibers stand out as an inexpensive potential adsorbent of arsenic in the water. Besides the improved arsenic removal efficiency observed by the polyvinyl alcohol nanofibers with embedded nanoparticles, they also have a remarkable ability to be extracted from water through its inter action with an external magnetic field. In contrast, the bare magnetic nanoparticles are dispersed uncontrollably in a water flow making difficult their localization, and consequently, their extraction from water. Fig. 8 shows a graphical sequence of the nanofiber extraction
using a magnet. In this sense, the hard-magnetic properties exhibited from the strontium hexaferrite nanoparticles allow to recover the nanofibers together with the arsenic adsorbed, and it is possible to extract more weight from water and to use fewer nanoparticles than if soft-magnetic nanoparticles were used. Moreover, these magnetic properties keep unchanged because nanoparticles remain confined in side the PVA matrix. 4. Conclusion A significant improvement in the arsenic adsorption ability was ob tained in polyvinyl alcohol nanofibers when strontium hexaferrite nanoparticles were embedded therein. Although this improvement is dependent on the number of nanoparticles in the fiber, using only 20 % wt of nanoparticles, it was achieved a similar efficiency than the bare strontium hexaferrite nanoparticles, with the advantage that the PVA composite is of easy handling. As the strontium hexaferrite nano particles with 3.39 nm average size are distributed along the nanofibers; the large surface area of these nanoparticles promoted a strong inter action with the polymeric matrix which modified some of its physico chemical properties, showing an increase in its magnetic properties regarding the bare nanoparticles, as well as an improvement of the thermal stability. Acknowledgments Authors thank L. P�erez-Tepetate (AACTyM-UAEH), H.G. Silva (IPI CYT), C.G. Elías (IM-UASLP), M.L. Gonz� alez and G.G. L� opez-Rocha (IFUASLP) for their assistance in the laboratories. R.M.O. thanks to Con acyt-Mexico for her 240837 scholarship. A.L.G. thanks to UAEH/PRO DEP-M� exico for partial support through the contract 511-6/17-8021. Thank are also due to Carolina Medina (UC-Davis) for their helpful suggestions to improve the manuscript. References [1] B. Daus, R. Wennrich, H. Weiss, Sorption materials for arsenic removal from water: a comparative study, Water Res. 38 (2004) 2948, https://doi.org/10.1016/j. watres.2004.04.003. [2] R. Bondu, V. Cloutier, E. Rosa, M. Benzaazoua, A review and evaluation of the impacts of climate change on geogenic arsenic in groundwater from fractured bedrock aquifers, Water, Air, Soil Pollut. 227 (2016) 296, https://doi.org/ 10.1007/s11270-016-2936-6. [3] H.J. Sun, B. Rathinasabapathi, B. Wu, J. Luo, L.P. Pu, L.Q. Ma, Arsenic and selenium toxicity and their interactive effects in humans, Environ. Int. 69 (2014) 148, https://doi.org/10.1016/j.envint.2014.04.019. [4] L. Rodríguez-Lado, G. Sun, M. Berg, Q. Zhang, H. Xue, Q. Zheng, C.A. Johnson, Groundwater arsenic contamination throughout China, Science 341 (2013) 866, https://doi.org/10.1126/science.1237484. [5] M. Banerjee, N. Banerjee, P. Bhattacharjee, D. Mondal, P.R. Lythgoe, M. Martínez, J. Pan, D.A. Polya, A.K. Giri, High arsenic in rice is associated with elevated genotoxic effects in humans, Sci. Rep. 3 (2013) 2195, https://doi.org/10.1038/ srep02195. [6] B.K. Mandal, K.T. Suzuki, Arsenic round the world: a review, Talanta 58 (2002) 201, https://doi.org/10.1016/S0039-9140(02)00268-0. [7] A.H. Smith, C. Hopenhayn-Rich, M.N. Bates, H.M. Goeden, I. Hertz-Picciotto, H. M. Duggan, R. Wood, M.J. Kosnett, M.T. Smith, Cancer risks from arsenic in drinking water, Environ. Health Perspect. 97 (1992) 259, https://doi.org/10.1289/ ehp.9297259.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Properties and arsenic removal evaluation of polyvinyl alcohol nanofibers with embedded strontium hexaferrite nanoparticles R. Murillo-Ortíz a, b, M. Mirabal-García a, J.J. Cruz-Rivera c, D. Valdez-P�erez d, J.R. Martínez e, F. P�erez-Moreno f, A. Lobo-Guerrero f, * a
� Instituto de Física, Universidad Aut� onoma de San Luis Potosí, Alvaro Obreg� on No. 64, 78000, San Luis Potosí, Mexico � Doctorado Institucional en Ingeniería y Ciencia de Materiales, Universidad Aut� onoma de San Luis Potosí, Alvaro Obreg� on No. 64, 78000, San Luis Potosí, Mexico c Instituto de Metalurgia, Universidad Aut� onoma de San Luis Potosí, Av. Sierra Leona 550, 78210, San Luis Potosí, Mexico d Instituto Polit�ecnico Nacional, UPALM, Edif. Z-4 3er Piso, 07738, M�exico D.F., Mexico e � Benem�erita Universidad Aut� onoma de San Luis Potosí, Alvaro Obreg� on No. 64, 78000, San Luis Potosí, Mexico f � Area Acad�emica de Ciencias de la Tierra y Materiales, Universidad Aut� onoma del Estado de Hidalgo. Carr. Pachuca-Tulancingo km. 4.5, 42039, Pachuca de Soto, b
Mexico
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Arsenic removal using PVA nanofiber magneto-composite. � Effect of strontium hexaferrite nano particles on the PVA nanofiber properties. � Improvement of arsenic removal effi ciency by effect of hexaferrite nanoparticles. � Thermoprotective effect of the hex aferrite nanoparticles on the PVA matrix.
A R T I C L E I N F O
A B S T R A C T
Keywords: Magnetic polymers Strontium hexaferrite nanoparticles Electrospinning Nanofibers Pechini method
This work deals with the study of physical properties exhibited the polyvinyl alcohol (PVA) fabricated as a nanofiber composite with embedded strontium hexaferrite nanoparticles (SrM-NPs). The nanofibers obtained from electrospinning have 165 nm average diameter, and the nanoparticles obtained from the sol-gel Pechini method have 78 nm diameter. A low-frequency sonication treatment performed to these nanoparticles allowed to deagglomerate and to reduce their size up to obtain 3.4 nm average diameter. Also, the magnetic nanoparticles exhibit an arrangement along the PVA nanofiber because of the nanoparticles interact with the electric field generated during the electrospinning. This arrangement improves the magnetic properties of the composite in comparison with the bulk nanoparticles. Also, the interaction of strontium hexaferrite nanoparticles with the PVA matrix induces changes on its physical properties, as well as on the ability of nanofibers to uptake arsenic from water.
* Corresponding author. E-mail address: [email protected] (A. Lobo-Guerrero). https://doi.org/10.1016/j.matchemphys.2019.05.043 Received 22 November 2018; Received in revised form 14 May 2019; Accepted 17 May 2019 Available online 18 May 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Materials Chemistry and Physics 234 (2019) 151–157
1. Introduction
embedded strontium hexaferrite nanoparticles stands out as a trust worthy ensemble to remove arsenic from water, wherein the use of hard-magnetic nanoparticles allows the nanofiber extraction by using a low-intensity external magnetic field. The higher magnetic performance exhibiting the hard-magnetic nanoparticles as part of the composite allows extracting the nanofiber from water easier, regarding the usually used soft-magnetic nanoparticles. In addition, the hard-magnetic nanoparticles can be magnetically-arranged in a polymeric matrix to originate nanoparticles arrangement-dependent anisotropies which could modify some of the physicochemical properties in the composite. This work shows the feasibility of incorporating hard-magnetic nanoparticles in a nanostructured polymeric composite to be used in arsenic removal technology. The use of hard-magnetic nanoparticles, such as the hexaferrite ones, offers an unexplored research field among which the polymer-nanoparticle interaction plays an important role in determining functional properties of nanostructured composites. In this case, hexaferrite nanoparticles with diameters between 3 and 5 nm were for the first time used to develop a composite with potential application in the arsenic removal technology. Also, pure polyvinyl alcohol nano fiber and pure hexaferrite nanoparticles were used as reference, results showed an improvement on the arsenic adsorption uptake as the hex aferrite nanoparticles increase in the polyvinyl alcohol nanofiber, which makes of this system an interesting and low-cost candidate for envi ronmental remediation of water sources. In addition, a straightforward methodology to fabricate polyvinyl alcohol nanofiber with extremely small strontium hexaferrite nanoparticles is presented.
Arsenic in water is one of the most worrying problems due to its high toxicity for the human body [1,2]. At this moment, millions of people in all countries suffer from chronic arsenic poisoning due to the con sumption of contaminated water [3–5]. Arsenic compounds rank at the 20th position of major abundance in the earth’s crust, and they are in constant circulation in the environment [6]. Rocks weathering generates dust with arsenic compounds which propagate naturally in the envi ronment, and the human activity contributes to propagate this pollution. The adverse effect of arsenic on human health range from skin lesions to different types of cancer [7,8]. Arsenic contaminated drinking water has been reported worldwide which producing a significant environmental and public health problem [9–16]. Different methods have been proposed to separate arsenic from water, some of them, been adsorbent materials [17], oxidation and filtration [18], phytoremediation [19], ion exchange [20], electro chemical remediation [21], coagulation and flocculation [22], and so on [23–25]. Among these methods, adsorption stands out due to its high removal efficiency, easy operation, low cost and absences of toxic sludge [23,26]. Then, adsorbent materials attract lots of attention because it does not require the use of chemical compounds and toxic waste pro duction, the removal process is simple, and the adsorbent materials can be recycled without significant loss of its efficiency. Among the arsenic adsorbent materials, it has experimented with these based on iron oxides [27], alumina [28], and silica [29]. Also, polymeric materials as chito san and its derivatives have been tested to remove arsenic from water [30,31]. Recently, polyvinyl alcohol (PVA) has been recognized as a prom ising material for arsenic removal due to its ability to mobilize heavy metals in water media [32]. However, if any polymeric material remains in the water for a long time, it can create additional environmental is sues, so an efficient method is needed to extract the polymer with the adsorbed arsenic. In this sense, magnetic nanoparticles can act as a carrier allowing to extract the polyvinyl alcohol with the adsorbed arsenic. Besides, strontium hexaferrite with chemical formula SrFe12O19 is identified as a hard-magnetic compound widely used as permanent magnets. The properties of the strontium hexaferrite nanoparticles include high magnetization saturation, remanence, coercivity, Curie temperature, magnetocrystalline anisotropy, electrical resistivity, chemical stability, and they have low fabrication costs [33,34]. In this sense, by combining the polymer adsorption ability with the magnetic property of nanoparticles, a low-cost composite capable of removing arsenic could be developed. Therefore, polyvinyl alcohol nanofiber with
2. Experimental Strontium hexaferrite nanoparticles were fabricated using the Pechini method; this method is based on the formation of a polymeric resin containing strontium and iron ions in the stoichiometric ratio. For that, we used 9.0 g of ferric nitrate (Fe(NO3)3⋅9H2O, Sigma Aldrich) mixed with 0.38 g of strontium nitrate (Sr(NO3)2 Sigma Aldrich) stirred in 60 mL of deionized water at room temperature for 30 min. Then, in 20 mL of deionized water was added 2.0 g of citric acid and 0.5 mL of ethylene glycol. Subsequently, these two mixes were combined and heated at 70 � C with constant agitation until water evaporation pro motes a polyesterification reaction, obtaining a homogeneous resin where metal ions are uniformly distributed in the organic matrix [35]. The polymeric mix was heated 2 h in a Lindberg 51844 furnace at 200 � C to remove any solvent excess, and the obtained powder was heated to 5 � C/min up to 900 � C, maintaining the temperature for 2 h. After, 1.5 g of polyvinyl alcohol (MW ¼ 75,000) was dissolved in 50 mL of deionized water and stirred 40 min at 80 � C. Therefore, the mix was left to cool at
Fig. 1. Experimental set-up used to fabricate the nanofibers with the embedded SrM-NPs. 152
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Fig. 2. TEM micrograph of (a) SrM-NPs prepared from the Pechini method, (b) agglomerate conformed of various nanoparticles, (c) histogram of the strontium hexaferrite nanoparticles as a powder after sintered. TEM micrograph of (d) PVA nanofiber with SrM-NPs, (e) low-size SrM-NPs inside of the PVA nanofiber, (f) histogram of the strontium hexaferrite nanoparticles inside of the PVA nanofiber as part of the composite.
room temperature. Then, the nanofiber was prepared using 90 %wt of the previously obtained polyvinyl alcohol and incorporating 10 %wt of the hexaferrite nanoparticles. An additional sample was fabricated using 20 %wt of nanoparticles to evaluate the effect these nanoparticles on the arsenic (V) adsorption. After, strontium hexaferrite nanoparticles were put into ethanol (C2H6O) and sonicated for 2 h with a Branson 2510 ultrasonic bath operated at 40-kHz to disperse, deagglomerate, and fragment, the as-obtained nanoparticles [36,37]. Finally, a 5 mL syringe with a stainless-steel needle of 0.15 mm internal diameter was filled with the ultrasonicated mix and loaded in NE-300 New Era infusion pump programmed to deliver a flow rate of 0.3 mL/h. The syringe needle was connected to the positive electrode, and the negative elec trode connected to an aluminum foil which serves as a collector at a fixed distance of 5.3 cm from the syringe tip. The system was installed inside of an isolated chamber to minimize undesirable ambient effects, and the power supply was adjusted at 25 kV. Fig. 1 shows the compo nents and the experimental set-up of the electrospinning system. The collected fiber was left to rest at room temperature for 12 h; then, to avoid the polyvinyl alcohol dissolves in water, nanofibers were sub jected to a chemical crosslinking in methanol during 24 h. The samples were characterized with X-ray diffraction using a Siemens D5000 diffractometer which uses a cobalt source
(λ ¼ 1.7890 Å), the structural parameters were refined using Rietveld analysis incorporated in the MAUD program [38]. The morphological characteristics of nanofibers and nanoparticles were analyzed using a Hitachi S-570 transmission electron microscope (TEM) operated at 100 kV. Uranyl acetate was used to enhance the TEM contrast and make the strontium hexaferrite nanoparticles observables. Thermogravimetric analysis was performed to evaluate the effect of the embedded nano particles in the polymeric matrix by using a DSCQ200 differential scanning calorimetry with a heating rate of 10 � C/min in an N2 atmo sphere. The magnetic properties were obtained from the hysteresis loops measured at room temperature using LDJ-9600 vibrating sample magnetometer (VSM). The arsenic (V) removal efficiency of pure polyvinyl alcohol nano fiber was evaluated after 1, 3, 5, 7 and 10 min, and results compared with those obtained from the nanofibers incorporating 10 and 20 %wt of hexaferrite nanoparticles. Also, the arsenic removal efficiency of bare strontium hexaferrite nanoparticles has been verified, under the same initial conditions than the nanofibers. In each case, 0.2 g of nanofibers, or nanoparticles were immersed in a stock solution with initial arsenic (V) concentration of 3.26 μg/L. The solution with the nanofiber remained in constant stirring during all the experiment. The arsenic removal efficiency of the bare nanoparticles was evaluated at intervals 153
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Fig. 3. Experimental and calculated X-ray diffractograms of (a) SrM-NPs after sintering, and (b) PVA-SrM as a nanofiber composite.
of 2.5 min, where 1 min corresponds to the mechanical stirring, and 1.5 min is the time spent in clean the water from nanoparticles using a magnet. Measurements were done in triplicate with a PerkinElmer Op tima 8300 ICP-OES spectrometer, and the average absorbance was used to calculate the removal efficiency of arsenic (V) in the water using equation (1). Removal efficiency ¼ [(C0
Ce) / C0] (100)
Fig. 4. (a) TEM Micrograph of the polyvinyl alcohol composite as a nanofiber. (b) Histogram showing the nanofibers diameter distribution.
(1)
where C0 is the initial concentration, and Ce is the equilibrium concen tration given in μg/L.
refinement is based on the structure of the strontium hexaferrite: COD ID 100600 (hexagonal, P63/mmc). Results show the presence of strontium hexaferrite without secondary phases. The calculated unit cell parame ters were: a ¼ 5.8300 � 0.0004 Å and c ¼ 22.8716 � 0.0007 Å. The crystallite size calculated from the Rietveld method was 76 � 2 nm matches with the particle size observed in the TEM micrographs (78 nm). The final refinement output gives an Rwp ¼ 8.60, Rexp ¼ 4.59, and σ ¼ 1.87, which together with the good adjusted observed graphi cally in Fig. 3 (a), ensures the reliability of the Rietveld fit. Fig. 3 (b) shows the experimental X-ray and calculated X-ray dif fractograms of the PVA nanofiber with embedded strontium hexaferrite nanoparticles. The PVA exhibits an amorphous-like peak at 22.7� (Cokα) not showed in Fig. 3. The Rietveld refinement of the PVA with SrMNPs indicates only the presence of strontium hexaferrite as a crystalline phase. Although, in this case, the hexaferrite peaks are wider than the observed in Fig. 3 (a). This behavior is closely related with a size reduction of the hexaferrite nanoparticles from the initial 78 nm measured as a powder to SrM-NPs of 3.4 nm obtained as part of the PVASrM nanofibers. In this sense, X-ray results match with the observations made using TEM for the particle size of the hexaferrite. Fig. 4 (a) shows a TEM micrograph of the polyvinyl alcohol nanofiber with embedded strontium hexaferrite nanoparticles obtained from electrospinning. The PVA nanofibers have a circular section, the embedded nanoparticles are homogeneously distributed inside of the fiber, and they do not exhibit agglomeration. The nanofibers diameters go from 97 to 269 nm with an average diameter of 165 nm and a stan dard deviation of 39 nm. The histogram of the nanofibers diameters is presented in Fig. 4 (b). The embedded nanoparticles show an arrange ment along the nanofiber length as a result of the interaction forces between nanoparticles, and with the electric field generated inside of the electrospinning chamber. Fig. 5 shows the magnetization curves of the polyvinyl alcohol nanofiber with the embedded strontium hexaferrite nanoparticles, and
3. Results and discussion Fig. 2 (a) shows the TEM micrograph of the strontium hexaferrite nanoparticles with plate-like morphology obtained from the Pechini method after they were sintered at 900 � C. The TEM micrograph of Fig. 2 (b) shows that the particles which integrate an agglomerate have lengths of several tens of nanometers. Accordingly, Fig. 2 (c) presents the par ticles sizes histogram obtained from the measurement of 100 particles from various TEM micrographs, an average particle diameter of 78 nm with a standard deviation of 1.8 nm was obtained, and diameters dis tribution ranging from 37 nm to 179 nm. On the other hand, Fig. 2 (d) shows the PVA with SrFe12O19 nanoparticles after the sonication process and once nanofibers were fabricated. The TEM micrograph of Fig. 2 (e) shows a PVA nanofiber where inside there are dark spots dispersed along the nanofiber. In this case, these spots would correspond to the stron tium hexaferrite phase but with a significative particle size reduction. As before the nanofiber fabrication, only PVA and SrFe12O19 were present in the system, the polymeric phase forms the nanofiber, and the SrFe12O19 remains inside of the fiber as the magnetic phase, where the original particle size has been reduced by one order of magnitude. This affirmation is confirmed by the Rietveld analysis of the X-ray patterns, and from the measurement of the magnetic properties of the nanofibers. The strontium hexaferrite nanoparticles dispersed in the PVA matrix have an average diameter size of 3.4 nm with a standard deviation of 0.79 nm, accordingly with the histogram shown in Fig. 2 (f). The smaller and the bigger measured diameters were 2.3 nm, and 7.0 nm, respectively. Fig. 3 (a) shows the experimental X-ray diffractogram of the hex aferrite nanoparticles measured as a powder after sintered and its cor responding Rietveld refinement curve. The initial model for the 154
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compared with the one reported for strontium hexaferrite nanofibers sintered at 900 � C, but in this case, the nanoparticles coalescence gave the form of nanowires [39]. All hysteresis loops were normalized to its magnetization of saturation (Ms) to allow comparison. The squareness ratio (Mr/Ms) of the strontium hexaferrite nanoparticles is 0.63 and its coercivity (Hc) 6.22 kOe, while the hexaferrite nanowires reported in Ref. [39] has Mr/Ms ¼ 0.57 and Hc ¼ 521 kA/m (6.54 kOe). The hex aferrite nanoparticles and nanowires exhibit very similar magnetic properties. In contrast, when nanoparticles are diluted along the nano fiber, the remanent squareness increases 15% and the coercivity 1.2% (with respect of the nanoparticles). Table 1 shows the magnetic prop erties of the nanofiber with the embedded nanoparticles compared with those reported for similar systems. The magnetization squareness obtained for the polyvinyl alcohol nanofibers with the embedded strontium hexaferrite nanoparticles is one of the highest reported in literature for hexaferrite based materials, and only thick films wherein magnetization prefers the normal direction have a magnetization squareness of 0.96 [43], whereas for the strontium hexaferrite particles oriented randomly is about of 0.50 [44]. The high magnetization squareness obtained for the nanofibers is a consequence of the nanoparticle ordering in the polymeric matrix [45]. The effect of the embedded strontium hexaferrite nanoparticles on the thermal stability of the polyvinyl alcohol nanofiber was studied using thermogravimetric analysis (TGA) and differential thermogravi metric analysis (DTG). The continuous line of Fig. 6 (a) shown the TGA curves measured from RT to 1000 � C, and the dashed lines are the de rivative curves (DTG). The polyvinyl alcohol nanofiber degrades in a two-step process in the temperature range from 240 � C to 315 � C and from 315 � C to 480 � C. According to Peng and Kong [46], the first degradation process of the PVA involves mainly water elimination and decomposition of acetate groups, and then, its decomposition is controlled by chain-scission reactions. Fig. 6 (b) shows a maximum rate of mass decrement around 268 � C followed of a second minimum at 430 � C showed in detail in Fig. 6 (c). However, when strontium hex aferrite nanoparticles are embedded inside of the nanofibers, some dif ferences concerning the pure PVA nanofibers are observed. These differences are related to the influence of the strontium hexaferrite nanoparticles on the thermal behavior of the PVA. The strontium hex aferrite nanoparticles seem to have a thermo-protective function of the polyvinyl alcohol. This thermo-protective effect occurs in the PVA which surrounds the nanoparticles, delaying the chain-scission reactions occurred between 440� C and 500 � C up to 650� C-750 � C. Then, the PVA surrounding to the nanoparticles surface is decomposed at a higher temperature between 640 � C and 740 � C, as Fig. 6 (d) and 6 (e) shows. The protective function of the strontium hexaferrite nanoparticles on the polyvinyl alcohol delays its thermo-degradation for more than 150 � C concerning the PVA without nanoparticles. Finally, at 1000 � C the PVA
Fig. 5. Solid lines: Hysteresis loops of the Sr-M nanoparticles as powder, and in the nanofibers PVA composite. Dots: hysteresis loop of the Sr-M as nanowires reported in Ref. [39]. Table 1 Magnetic parameters obtained for de analyzed samples and those reported in the literature. Sample
Hc (kOe)
Mr/Ms
Reference
PVA/SrM (10:1) SrM-NPs SrM-Nanowires SrFe12–3Al3O19 SrM-thin film SrFe10.5Al1.5O19/Fe3O4
6.30 6.22 6.54 8.40 6.63 8.10
0.72 0.63 0.57 0.66 0.62 0.60
This work This work [39] [40] [41] [42]
Fig. 6. Thermogravimetric curves (TGA) and differential thermic analysis curve (DTG) of the polyvinyl alcohol nanofiber with and without strontium hexaferrite nanoparticles. DTG curves presented in detail at various tempera ture ranges. Fig. 7. (Ο) The arsenic removal efficiency of the PVA nanofiber. (Δ) Effect of the embedded strontium hexaferrite nanoparticles on PVA nanofiber at two different concentrations, and (□) arsenic removal efficiency of the bare stron tium hexaferrite nanoparticles.
as a reference, it was measured the hysteresis loop of the strontium hexaferrite nanoparticles obtained directly from the Pechini method before the sonication process. Both magnetization curves were 155
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Fig. 8. Extraction of nanofibers from water by applying an external magnetic field.
has been decomposed completely and only persists the hexaferrite phase. According to Fig. 6, water elimination from PVA results at a temperature below of 240 � C because of the incorporation of the hex aferrite nanoparticles, but at a higher temperature, the chain-scission reactions are avoided in polyvinyl alcohol which is in contact with the hexaferrite nanoparticles. The thermoprotective effect of the strontium hexaferrite nanoparticles on the polyvinyl alcohol has a trend to delay its thermal degradation. This result is consistent with the reported by Guo et al., for a PVA-Fe2O3 composite, where improvement on the polyvinyl alcohol thermal stability was attributed to a better cross-linkage as a result of the Fe2O3 nanoparticles incorporation [47]. Fig. 7 shows the arsenic removal efficiency of polyvinyl alcohol nanofiber with strontium hexaferrite nanoparticles at two different nanoparticle concentrations, with 10 and 20 % wt. of nanoparticles. Also, pure PVA nanofiber and strontium hexaferrite nanoparticles were evaluated as reference samples. Pure PVA nanofiber shows a poor arsenic adsorption efficiency contrasting with the one observed for the magnetic nanoparticles. However, when nanoparticles are embedded in the PVA nanofiber, the composite changes its arsenic adsorption effi ciency, showing an enhanced efficiency when the number of nano particles increases in the nanofiber. As the nanoparticles are located inside of the PVA nanofiber, they do not interact directly with the arsenic; and the improved adsorption ability arises from the strong interaction between nanoparticles and the PVA matrix as a consequence the higher surface area inherent with the smaller nanoparticles. The interaction of the PVA with small iron oxide nanoparticles can modify some physicochemical polymer properties. In this sense, incorporation of the hexaferrite nanoparticles in the PVA nanofiber could to promote a cross-linking of the polymer chains and induce changes in the van der Waals energy [47,48]. In sum, these ef fects result in the arsenic adsorption improvement. Although, more studies on the effect of the hexaferrite nanoparticles on the physico chemical properties of PVA nanofibers are required to fully understand the interactions between nanoparticles and the PVA matrix. On the other hand, different behavior in the hexaferrite nano particles has been observed in Fig. 7. In this case, arsenic adsorption majorly occurs below 3 min, which agrees with a previous study made by Patel et al. using barium hexaferrite nanoparticles [49]. Although the adsorption mechanism of the hexaferrite nanoparticles is not well un derstood, it was pointed out that the adsorption kinetics could be similar to the chemisorption mechanism observed in other iron oxides nano particles [50]. However, nowadays the use of bulk nanoparticles in environmental applications generates a certain amount of controversy. Then, the development of functional composites containing nano particles is a promising alternative. Among them, the PVA magnetic nanofibers stand out as an inexpensive potential adsorbent of arsenic in the water. Besides the improved arsenic removal efficiency observed by the polyvinyl alcohol nanofibers with embedded nanoparticles, they also have a remarkable ability to be extracted from water through its inter action with an external magnetic field. In contrast, the bare magnetic nanoparticles are dispersed uncontrollably in a water flow making difficult their localization, and consequently, their extraction from water. Fig. 8 shows a graphical sequence of the nanofiber extraction
using a magnet. In this sense, the hard-magnetic properties exhibited from the strontium hexaferrite nanoparticles allow to recover the nanofibers together with the arsenic adsorbed, and it is possible to extract more weight from water and to use fewer nanoparticles than if soft-magnetic nanoparticles were used. Moreover, these magnetic properties keep unchanged because nanoparticles remain confined in side the PVA matrix. 4. Conclusion A significant improvement in the arsenic adsorption ability was ob tained in polyvinyl alcohol nanofibers when strontium hexaferrite nanoparticles were embedded therein. Although this improvement is dependent on the number of nanoparticles in the fiber, using only 20 % wt of nanoparticles, it was achieved a similar efficiency than the bare strontium hexaferrite nanoparticles, with the advantage that the PVA composite is of easy handling. As the strontium hexaferrite nano particles with 3.39 nm average size are distributed along the nanofibers; the large surface area of these nanoparticles promoted a strong inter action with the polymeric matrix which modified some of its physico chemical properties, showing an increase in its magnetic properties regarding the bare nanoparticles, as well as an improvement of the thermal stability. Acknowledgments Authors thank L. P�erez-Tepetate (AACTyM-UAEH), H.G. Silva (IPI CYT), C.G. Elías (IM-UASLP), M.L. Gonz� alez and G.G. L� opez-Rocha (IFUASLP) for their assistance in the laboratories. R.M.O. thanks to Con acyt-Mexico for her 240837 scholarship. A.L.G. thanks to UAEH/PRO DEP-M� exico for partial support through the contract 511-6/17-8021. Thank are also due to Carolina Medina (UC-Davis) for their helpful suggestions to improve the manuscript. References [1] B. Daus, R. Wennrich, H. Weiss, Sorption materials for arsenic removal from water: a comparative study, Water Res. 38 (2004) 2948, https://doi.org/10.1016/j. watres.2004.04.003. [2] R. Bondu, V. Cloutier, E. Rosa, M. Benzaazoua, A review and evaluation of the impacts of climate change on geogenic arsenic in groundwater from fractured bedrock aquifers, Water, Air, Soil Pollut. 227 (2016) 296, https://doi.org/ 10.1007/s11270-016-2936-6. [3] H.J. Sun, B. Rathinasabapathi, B. Wu, J. Luo, L.P. Pu, L.Q. Ma, Arsenic and selenium toxicity and their interactive effects in humans, Environ. Int. 69 (2014) 148, https://doi.org/10.1016/j.envint.2014.04.019. [4] L. Rodríguez-Lado, G. Sun, M. Berg, Q. Zhang, H. Xue, Q. Zheng, C.A. Johnson, Groundwater arsenic contamination throughout China, Science 341 (2013) 866, https://doi.org/10.1126/science.1237484. [5] M. Banerjee, N. Banerjee, P. Bhattacharjee, D. Mondal, P.R. Lythgoe, M. Martínez, J. Pan, D.A. Polya, A.K. Giri, High arsenic in rice is associated with elevated genotoxic effects in humans, Sci. Rep. 3 (2013) 2195, https://doi.org/10.1038/ srep02195. [6] B.K. Mandal, K.T. Suzuki, Arsenic round the world: a review, Talanta 58 (2002) 201, https://doi.org/10.1016/S0039-9140(02)00268-0. [7] A.H. Smith, C. Hopenhayn-Rich, M.N. Bates, H.M. Goeden, I. Hertz-Picciotto, H. M. Duggan, R. Wood, M.J. Kosnett, M.T. Smith, Cancer risks from arsenic in drinking water, Environ. Health Perspect. 97 (1992) 259, https://doi.org/10.1289/ ehp.9297259.
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