Agricultural activity depends heavily on the use of pesticides with the consequent risks for humans and the environment. Many of them are endocrine disruptors (EDs), compounds that alter the function(s) of the endocrine system and consequently cause adverse health effects in an intact organism, or its progeny, or sub-populations [1
]. Owing to the required use of pesticides in intensive agriculture, their residues have important effects on the quality of aquatic ecosystems and drinking water resources. Recent studies have pointed to the presence of different pesticides in environmental (surface-, ground-, and seawater), waste- and drinking waters [2
]. For this reason, the European Water Framework Directive proposes a strategy to fight against water pollution. Therefore, to protect its citizens from dangerous effects, the European Union (EU) has set quality standards (0.1 mg L−1
for individual pesticides and 0.5 mg L−1
for the sum of all pesticides). However, many of them are biorecalcitrants and are not amendable to microbial degradation. Consequently, other more effective technologies such as advanced oxidation processes (AOPs) have been proposed in the past years.
The development of AOPs constitute a significant alternative to traditional technologies such as adsorption, chemical coagulation/flocculation, precipitation, membrane filtration, ion exchange, or biological degradation because they get the removal of organic pollutants by mineralization instead of transferring them from water to other medium. Therefore, the research of photochemical techniques based on the use of sunlight in order to catalyze the degradation of these toxic compounds toward innocuous derivatives is becoming an important field of research nowadays [3
]. These technologies are considered an advantageous option for the treatment of pesticide-polluted water because their effectiveness has been verified for different types of pesticides during the last decade [6
]. In this context, binary semiconductors (SCs) like TiO2
and ZnO have been extensively used as photocatalysts because of their chemical and optical properties and their use to remove pollutants is being increasingly explored by scientific community [7
]. ZnO is an excellent n-type semiconductor oxide that possesses excellent electrical, mechanical, and optical properties, similar to TiO2
although the influence of the synthesis parameters on the properties of the ZnO nanopowders is very important to improve their photocatalytic activity [13
and ZnO have been used in slurry form making their removal from the treated water cumbersome. To avoid this problem, different immobilized materials have been tested. In order to minimize the inherent risks of the systemic use of these materials and their prevalence in the environment, many attempts have been carried out to incorporate them into three-dimensional supports. In this sense, ZnO nanoparticles have been combined with cellulose supports [14
] or matrices of fibroin [15
], among others. On the other hand, TiO2
nanoparticles are also used for the removal of organic contaminants because of their activity and optimal electronic and optical behavior [17
] and its insertion into 3D-matrices is being studied [10
]. Some recent works expose the potential uses of materials combining SF and TiO2
in their composition. For example, Cai et al. [21
] stated a new methodology to produce silk fibers with enhanced mechanical properties by means of the feeding of silkworms with nanoparticles of TiO2
. Other authors have produced SF films [22
] or porous scaffolds [23
] containing them as biomaterials for tissue engineering, improving their mechanical and thermal properties. Fibrous mats of SF/TiO2
has also been manufactured as potential wound dressings [24
] or in the field of self-cleaning textiles [9
Electrospinning is a technique used to make fibrous scaffolds with different applications such as catalysts, filtration, electronic set ups or tissue engineering [25
]. Silk fibroin (SF) is a protein produced by the silkworm (Bombyx mori
), and it has been widely employed to produce electrospun materials [30
], because of their excellent properties in terms of biocompatibility and mechanical behavior [33
]. The electrospinning of this versatile protein allows the functionalization of the materials produced with different molecules of great interest. For instance, different growing factors [25
], polymeric blends [26
], conductive polymers [27
], or even nanoparticles have been successfully included in the composition of electrospun SF-mats [9
] with variety of applications. However, as far as we know, electrospun SF meshes have never been studied or produced as photocatalyst for the elimination of pesticides in polluted water.
With this aim, we propose a new methodology for manufacturing electrospun SF materials incorporating ZnO into their fibrillary network (SF/ZnO), analyzing their ability to degrade etoxazole (diphenyl oxazoline) and difenoconazole, myclobutanil and penconazole (triazole compounds) in water, pesticides commonly used in agriculture, by means of sunlight. In addition to this, their mechanical properties, infrared spectra, and appearance, using scanning electron microscopy, are also described. Finally, the photocatalytic efficiency of electrospun SF materials prepared with TiO2 (SF/TiO2) was also evaluated and compared with SF/ZnO for the reclamation of pesticide-polluted water.
3. Materials and Methods
3.1. Pesticides and Reagents
Pure standards (purity > 99%) were supplied by Ehrenstorfer GmBh (Augsburg, Germany). The physicochemical properties of the pesticides are listed in Table 3
]. The commercial formulations (Score 25% w
—difenoconazole, Borneo 11% w
—etoxazole, and Systhane Forte 24% w
—myclobutanil and Furabel 10% w
—penconazole) were supplied by Syngenta Agro, Kenogard, Dow Agrosciences Iberica, S.A. and Probelte, respectively. Catalyzers, zinc oxide (99.99%, 200 nm, BET 10 m2
) and titanium dioxide P25 (99.5%, <21 nm, 50 m2
) were purchased from Alfa-Aesar (Karlsruhe, Germany) and Nippon Aerosil Co Ltd. (Osaka, Japan), respectively. Deionized water was obtained from a Millipore Water Purification System (Bedford, MA, USA).
3.2. Silk Fibroin Processing
Cocoons of B. mori were chopped into four pieces and subsequently boiled in 0.02 M Na2CO3 during 30 min, in order to accomplish the elimination of the sericin. Later, the SF was washed with water and dried during 3 days at 22 ± 1 °C. It was dissolved using 9.3 M LiBr (Acros Organics, Belgium) for 3 h (60 °C), obtaining a 20 wt.% dissolution that was dialyzed with water for 3 days using Snakeskin Dialysis Tubing 3.5 KDa MWCO (Thermo Scientific, Waltham, MA, USA) carrying out eight changes of water (4 °C). The resultant 7 wt.% SF dissolution was retrieved and concentrated by means of dialysis in 30 wt.% polyethylene glycol (11,000 Da) for 24 h, in order to reach a concentration of 20–21 wt.% of SF, which is optimal for the electrospinning experiments.
3.3. Electrospinning and Post-Treatment of SF/TiO2 and SF/ZnO Mats
The electrospinning setup used in order to produce the materials was the same described in previous works of our research group [28
]. The working conditions were adapted so that the Taylor cone was stable, being the voltage implemented to the capillary needle 19 kV and −1.5 kV to the metallic collector (~375 cm2
). The distance between both elements of the setup was 42 cm and the selected injection rate of the SF dissolution was 1 mL h−1
. Either ZnO or TiO2
nanoparticles (contained in methanol) were sprayed, using an atomizer, throughout the electrospinning experiments over the surface of the mats, as it is clarified below. Each mat was produced from 6 mL of 20% wt SF dissolution. Total of 30 mL of methanol containing 1.5 g of either TiO2
or ZnO nanoparticles were sprayed during the electrospinning experiments. With this purpose, seven rounds of atomization (periodically distributed) were carried out (4.3 mL per round), covering homogeneously the collector.
A total of 1.5 mL of SF dissolution was electrospun before the first atomization and after the last one, in order to guarantee the fabrication of a compact net, avoiding the loss of the nanoparticles included. Pure SF electrospun mats were produced as negative controls and the materials including TiO2 or ZnO in their composition were named as SF/TiO2 or SF/ZnO, respectively.
The annealing of all the materials produced was performed by an immersion step in absolute methanol during 30 min, subsequently the mats were dried locating them between two portions of filter paper.
3.4. XRD, FESEM, XPS, XDS and SBET
XRD, FESEM, XPS, and XDS were used to characterize the materials as previously published by Garrido et al. [20
]. A Quadrasorb SI-MP (Quantachrome Instruments, Boynton Beach, FL, USA) was used to obtain the gas sorption (nitrogen, 77 K) surface area according to Brunauer–Emmett–Teller (BET) method. Outgassing was performed with a Masterprep Degasser (Quantachrome Instruments) at 120 °C for 12 h for SF/ZnO.
ATR-FTIR was the technique used to confirm the structural changes of SF from a water soluble conformation to a non-soluble state, with a characteristic increase in β-sheet content. IR spectra were also used to visualize the presence of the nanoparticles in the materials, by means of the analysis of some characteristic bands of ZnO or TiO2. For these purposes, a Nicolet iS5 spectrometer equipped with an iD5 ATR accessory (Thermo Scientific, USA) was used. The instrument worked in absorbance mode (resolution of 4 cm−1, a spectral range of 4000–550 cm−1, and 64 scans).
3.6. Analysis of Mechanical Properties and Fibre Diameter
Tensile tests were conducted using a universal test frame machine (Qtest; MTS Systems, Eden Prairie, MN, USA). Fragments of mats (10 mm × 30 mm) were proven at a crosshead speed of 0.1 mm·s−1 with a load cell of 10 N. A digital micrometer (Mitutoyo, IL, USA) with 0–25 mm, resolution of 0.001 mm, and precision of ±2 μm was used to measure the width of each sample. The stress–strain curves obtained were used in order to calculate the tensile strength (MPa), the strain at break (%) and the elastic modulus (MPa) in the linear elastic part of the curves. The tests were replicated three times per condition. Micrographs obtained by SEM were acquired and used to measure the diameter of the fibers of the materials produced.
3.7. Photoreaction Setup
Photocatalytic and photolytic trials were performed during August 2018 following the procedure described by Fenoll et al. with some modification [38
]. For both experiments we mixed Milli-RX water (pH 7.1, ORP 230 mV, conductivity 5.5 µS cm−1
, and TOC < 30 µg L−1
) with commercial formulations of the pesticides studied. Experiments were performed in vessels containing 100 mL of water and introducing a square fragment of SF/semiconductor oxide mats. The vessels were exposed to sunlight for 180 min (from 11 AM to 2 PM). Before starting experiments, the solution was stirred for 30 min in darkness and later a sample was collected to measure the adsorption of the pesticides onto each square fragment of SF/ZnO mats. Figure 7
illustrates a schematic picture of the experimental setup. 948.6 ± 52.4, 21.5 ± 2.3, and 1.7 ± 0.4 (all in W m−2
) of VIS plus NIR, UVA, and UVB, respectively, were recorded at noon.
3.8. Analytical Determinations
Pesticide residues in water samples were analyzed by an HPLC system (Agilent Series 1200 Agilent Technologies, Santa Clara, CA, USA) and a G6410A triple quadrupole mass spectrometer provided with an ESI+
interface following the methodology previously published by Fenoll et al. [55
]. Table 4
shows the analytical conditions of the studied pesticides. The presence of Ti and Zn in solution was determined using an Agilent 7900 ICP/MS system. The measure of dissolved organic carbon (DOC) content in water samples was performed by a Multi N/C 3100 TOC Analyzer (Analytic Jena AG, Jena, Germany) equipped with an NDIR detector (950 °C), following the procedure outlined by Fenoll et al. [38
3.9. Statistical Analysis
Statistical analyses and curve fitting were performed by SigmaPlot version 13.0 statistical software (Systat, Software Inc., San Jose, CA, USA). IBM SPSS 25 statistics software was used to analyze statistics of mechanical properties and fiber diameter. ANOVA (p < 0.05) or Mann–Whitney (p < 0.05) tests were determined by the data accomplished normality and homogeneity of variance requirements or not, respectively.