Carbonized Apples and Quinces Stillage for Electromagnetic Shielding

9.24.3 Carbonized Apples and Quinces Stillage for Electromagnetic Shielding Mila Milenkovic, Warda Saeed, Muhammad Yasir, Dusan Milivojevic, Ali Azmy, Kamal E. S. Nassar, Zois Syrgiannis, Ioannis Spanopoulos, Danica Bajuk-Bogdanovic, Snežana Maletić et al. Article https://doi.org/10.3390/nano14231882 Citation: Milenkovic, M.; Saeed, W.; Yasir, M.; Milivojevic, D.; Azmy, A.; Nassar, K.E.S.; Syrgiannis, Z.; Spanopoulos, I.; Bajuk-Bogdanovic, D.; Maleti´c, S.; et al. Carbonized Apples and Quinces Stillage for Electromagnetic Shielding. Nanomaterials 2024,14, 1882. https:// doi.org/10.3390/nano14231882 Academic Editor: Jose L. Arias Received: 3 October 2024 Revised: 29 October 2024 Accepted: 22 November 2024 Published: 23 November 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Carbonized Apples and Quinces Stillage for Electromagnetic Shielding Mila Milenkovic 1, Warda Saeed 2, Muhammad Yasir 2,* , Dusan Milivojevic 1, Ali Azmy 3, Kamal E. S. Nassar 3, Zois Syrgiannis 3, Ioannis Spanopoulos 3, Danica Bajuk-Bogdanovic 4, Snežana Maleti´c 5, Djurdja Kerkez 5, Tanja Barudžija 1and Svetlana Jovanovi´c 1,* 1Vinˇca Institute of Nuclear Sciences—National Institute of the Republic of Serbia, University of Belgrade, P.O. Box 522, 11000 Belgrade, Serbia; [email protected] (M.M.) 2Department of Computing Science, Microrobotics and Control Engineering, Carl von Ossietzky Universität Oldenburg, 26129 Oldenburg, Germany; [email protected] 3Department of Chemistry, University of South Florida, Tampa, FL 33620, USA; [email protected] (I.S.) 4Faculty of Physical Chemistry, University of Belgrade, Studentski Trg 12–16, 11158 Belgrade, Serbia 5 Department of Chemistry, Biochemistry and Environmental Protection, Faculty of Science, University of Novi Sad, Trg Dositeja Obradovi´ca 3, 21000 Novi Sad, Serbia *Correspondence: [email protected] (M.Y.); [email protected] (S.J.) Abstract: Electromagnetic waves (EMWs) have become an integral part of our daily lives, but they are causing a new form of environmental pollution, manifesting as electromagnetic interference (EMI) and radio frequency signal leakage. As a result, the demand for innovative, eco-friendly materials capable of blocking EMWs has escalated in the past decade, underscoring the significance of our research. In the realm of modern science, the creation of new materials must consider the starting materials, production costs, energy usage, and the potential for air, water, and soil pollution. Herein, we utilized biowaste materials generated during the distillation of fruit schnapps. The biowaste from apple and quince schnapps distillation was used as starting material, mixed with KOH, and carbonized at 850 ◦ C, in a nitrogen atmosphere. The structure of samples was investigated using various techniques (infrared, Raman, energy-dispersive X-ray, X-ray photoelectron spectroscopies, thermogravimetric analysis, BET surface area analyzer). Encouragingly, these materials demonstrated the ability to block EMWs within a frequency range of 8 to 12 GHz. Shielding efficiency was measured using waveguide adapters connected to ports (1 and 2) of the vector network analyzer using radiofrequency coaxial cables. At a frequency of 10 GHz, carbonized biowaste blocks 78.5% of the incident electromagnetic wave. Keywords: graphene; graphene oxide; biowaste; carbonization; electromagnetic interference shielding 1. Introduction Aside from the traditional forms of pollution caused by chemicals in gaseous, liquid, or solid states, such as microand nano-plastics, modern society is now grappling with a new type of pollution, i.e., electromagnetic waves (EWs). The proliferation of devices in our daily lives, the expansion of telecommunication infrastructure, and the evolution of wireless technologies has underscored the critical need for innovative materials capable of blocking the propagation of EWs. Electromagnetic interference (EMI) is particularly detrimental in the case of sensitive measurement devices, potentially leading to a reduction in the instrument’s lifespan, unwanted interference signals, and prolonged measuring times. Thus, new materials, in the form of powder, foil, and fabric, which can efficiently block electromagnetic wave propagation, have become more critical in recent years [ 1 – 3 ]. Studies have suggested that metals, metal composites, and conductive polymers have excellent EMI shielding properties [ 4 – 6 ]. New nanomaterials, such as silk-flower-like NiO with a hierarchical structure, showed a reflection loss (R L ) of –65.1 dB at 13.9 GHz due to its Nanomaterials 2024,14, 1882. https://doi.org/10.3390/nano14231882 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2024,14, 1882 2 of 16 hierarchical and porous structures, multiple scattering effects from nanosheets, folds and voids, different impedance matching characteristics, and clustered defects, where oxygen vacancies could disrupt the charge distribution balance and induce dipole polarization and related relaxations [ 7 ]. A flexible, semiconductive device was fabricated using carbon nanotubes with a tunable electromagnetic wave absorption frequency [ 8 ]. However, these materials are often sensitive to environmental conditions, such as humidity [ 9 ], or have limited processability, e.g., conductive polymer [ 10 ]. Furthermore, the question of the sustainability of the production, carbon footprint, and recyclability of these new materials is open. Graphene and its derivates show promising EMI shielding properties [ 11 – 13 ]. Defectfree single-layer graphene shows an EMI shielding efficiency (SE) of 2.27 dB [ 14 ], while few-layered graphene, graphene doped with more electronegative atoms, and multilayer film made of small and large graphene flakes show the ability to block 99% of the incident electromagnetic wave [ 11 , 15 – 18 ]. The main shielding mechanism of graphene is absorption [ 14 ], which is a desirable shielding mechanism compared to materials that reflect EMWs, such as metals, which cause secondary pollution [ 19 ]. Apart from electrical conductivity, both high dielectric loss and low density make graphene a very favorable material as an EMW absorber, while a derivate of graphene, graphene oxide (GO), possesses oxygen functional groups that induce strong dipolar polarization and interfacial polarization, improving microwave attenuation at GHz frequencies [ 20 ]. Composites based on graphene and its derivates block EMWs by absorption, reflection, and multiple reflections [21,22]. Combining graphene or graphene oxide with a metal-based nanostructure such as silver nanowires (AgNWs) enhances the conductivity and improves EMI SE, from 17 dB as measured for reduced graphene oxide to 38 dB as obtained for a composite of graphene with AgNWs [ 23 ]. By adding AgNWs as a middle layer between two reduced graphene oxide layers, Kumar et al. created a sandwich structure, with an electrical conductivity of 6.5 × 10 4 S m −1 , able to block EMWs dominantly by absorption of waves [ 23 ]. Considering that metals are efficient shielding materials based mainly on EMW reflection, conducive graphene-based composites also show desirable EMI SE, with the advantage that the shielding mechanism often involves both reflection and absorption [22]. Herein, we investigated the possibility of fruit biowaste usage as a starting material in producing new, environment-friendly EMI shielding materials. Therefore, we selected a side product from the fruit schnapps manufacturing process. These byproducts are called distillery stillage. Stillage is discharged in large quantities by distilleries: for 1 L of produced alcohol, between 8 and 15 L of stillage is generated [ 24 ]. Due to high levels of compounds with nitrogen and various organic molecules, these waste materials must be properly managed, which demands additional costs in the disposal of or treatment of stillage [ 25 , 26 ]. One solution for stillage management is to produce fertilizer or livestock feed, both low-value products [ 27 ]. Recently developed technology for recovering phenolic acids has been reposted, contributing to more efficient stillage valorization [28]. In this study, we used stillage collected after distillation of apple and quince schnapps and pyrolyzed at 850 ◦ C. We analyzed the structural and morphological properties, as well as the ability of the materials to block EMWs in the X-band frequency region. To investigate the structure of carbonized materials at both qualitative and quantitative levels, several spectroscopic techniques were used: infrared, Raman, energy-dispersive X-ray, X-ray photoelectron spectroscopies, thermogravimetric analysis, and BET surface area analyzer. It was previously reported that biochar produced from lignin increased the SE of cement [ 29 ], while sewage sludge biochar showed an SE value >10 dB [ 29 , 30 ]. By exploring the possibility of converting stillage into value-added, new, and highly demanded material, this study aims to answer the need for these new sustainable products and give a new perspective on reducing the environmental burden of stillage discharge. Nanomaterials 2024,14, 1882 3 of 16 2. Materials and Methods Samples of carbonized biomass were produced starting from residual waste material after fruit schnapps distillation, i.e., distillery stillage. Two different fruit schnapps residual distillery stillages were used. Namely, residuals from apples and quinces were collected. The distillery “Skrbic”, Nestin, Republic of Serbia, donated the stillages. The experimental procedure was conducted in the following stages: 1. Stillages were first diluted, using demineralized water produced by the Millipore Milli-Q ® water system, Burlington, MA, USA. The volume ratio of stillage to water was 1:2. 2. Diluted stillage was mechanically homogenized: the mixtures were homogenized mechanically using a kitchen chopper with a power of 2000 W in 4 cycles for 15 min. Large particles and fragments of the starting materials that could lead to inhomogeneity in the resulting material were removed by filtration through filter paper. 3. Homogenized, diluted stillage was filtrated: the resulting solution was filtered through a Whatman Filter Paper, Grade 4 (20–24 µ m particle retention, Whatman plc, Maidstone, UK). The collected filtrate was used for further production, while the material left on the surface of the filter paper was discarded. 4. Drying of the filtrates: after collecting filtrates, they were dried at atmospheric pressure at 85 ◦C. Consequently, black powders were obtained. 5. Pre-carbonization step: to achieve carbonization, dried biomass was mixed with KOH (reagent grade, 90%, flakes, Sigma Aldrich, St. Louis, MO, USA) in a mass ratio of 1:1. 6. Carbonization step: samples were heated for 1 h at 850 ◦C in the stream of N2. 7. Cleaning step: after carbonization, the samples were washed with demineralized water to remove KOH. Specifically, black powders obtained after carbonization were immersed in water. The powders were separated from water by simple decantation after clean water was added to the powder. The procedure was repeated until the pH was 7. 8. After KOH was removed, black powders were collected and dried at 70 ◦ C at a reduced pressure. The sample produced from residuals that remained after ethanol distillation in the process of apple schnapps production was named BA, and the other sample produced from quince schnapps residuals was named BQ. These powders were then used for further analysis. Raman spectra were recorded on a DXR Raman microscope (Thermo Fisher Scientific, Waltham, MA, USA). Each spectrum was obtained at room temperature using a 532 nm excitation line with a power of 2 mW. The data were analyzed through a spectrometer equipped with 800 lines/mm aperture and a 50 µ m pinhole. The acquisition time was 10 ×10 s. For Fourier-transform infrared spectroscopy (FTIR) analysis, powdered biomass samples were mixed with KBr and compressed into pellets. FTIR spectra were recorded using an FTIR Spectrometer (Thermo Scientific Nicolet iS20 FTIR Spectrometer, Thermo Scientific™, Waltham, MA, USA), in the range of 4000–400 cm −1 at 32 scans per spectrum and a resolution of 4 cm−1. The thermal stability of the BA and BQ samples was investigated using a Mettler Toledo TGA/DSC 1 instrument (Mettler Toledo, Columbus, OH, USA). Per each analysis, around 3 mg of powder samples were used. Samples were heated from 40 ◦ C to 750 ◦ C at a heating rate of 5 K min −1 at a nitrogen flow rate of 20 mL min −1 . Each measurement was repeated two times. The structural analysis of the synthesized material was conducted using the X-ray diffraction (XRD) method by employing a SmartLab ® X-ray diffractometer (Rigaku Co., Tokyo, Japan), with Cu K α radiation at a continuous scanning mode (40 kV, 30 mA, and λ= 1.542 Å ). Measurements were made in the various diffraction angles ranging from 6 to 35◦with a step size of 0.02◦and a measurement speed of 2◦min−1. Nanomaterials 2024,14, 1882 4 of 16 Scanning electron microscopy with an energy-dispersive X-ray spectroscopy (SEM– EDS) was performed on an INCAx-act LN2-free analytical silicon drift detector of characteristic X-rays with the PentaFET ® Precision and Aztec 4.3 software package (Oxford Instruments, Oxford, UK) connected to a TESCAN Mira3 XMU (20 kV, SE detector, Brno, Czech Republic). Samples of pyrolyzed biomass were deposited on double-sided conductive and adhesive type and were analyzed. To investigate the structure of BA and BQ in detail, X-ray Photoelectron Spectroscopy (XPS) was used. These measurements were performed using a Physical Electronics Industries PHI 5400 LS XPS (Physical Electronics, Chanhassen, MN, USA) equipped with a 10-360 Spherical Capacitor Analyzer (SCA). The SCA was set at aperture 2 small, yielding an analysis area of 600 microns. Aluminum K-alpha X-rays were used as a source. The power was set to 350 watts. SCA pass energy was set to 178.95 eV. The eV/step was 0.250 eV. Powdered BA and BQ samples were pressed onto Indium foil, which served as support. The specific surface area was obtained by performing nitrogen (N 2 ) adsorption. The mesopore and micropore volumes were determined using the BJH and t-test methods, and utilizinga Quantachrome autosorb TMiQ (Quantachrome Instruments, Boynton Beach, FL, USA) surface area analyzer. To investigate the shielding efficiency, BA and BQ powders were weighed and mixed with a sodium silicate resin to produce a homogenous paste. The concentration of prepared BA and BQ pastes was 0.4 g mL −1 (0.004 g/10 µ L). BA and BQ pastes were deposited on a 0.2 mm thick plexiglass sheet covered with a paper mold and formed into a 22.86 mm ×10.16 mm thin film. The thickness of the films was also 0.2 mm, corresponding to the thickness of the plexiglass sheet. This size corresponds to the inner dimensions of WR-90 waveguide adapters which were used for the electromagnetic shielding measurement. The prepared films of BA and BQ samples are shown in Figure 1. The transmission coefficients in the X-band (8–12 GHz frequency range) were measured using a Rohde & Schwarz ZVA 24 Vector Network Analyzer (VNA, Munich, Germany). − − ff ff α λ ff − tt ffi − (a) (b) Figure 1. Prepared films of BA (a) and BQ (b) samples. The measurement setup is shown in Figure S1. It consisted of two WR90 waveguide adapters connected to ports 1 and 2 of the VNA using RF coaxial cables. The thin film formed was placed between the two waveguide adapters and the S 21 scattering parameter was measured for both films, as previously reported [31]. The shielding effectiveness due to transmission (SE T ), Shielding Effectiveness due to dissipation (SE A ) and Shielding Effectiveness due to reflection (SE R ) are plotted using Equations (1)–(3): SET=−S21 dB (1) SER=−10log(1 −|S11|2) (2) SEA=−10log(|S21|2/(1 −|S11|2)) (3) Nanomaterials 2024,14, 1882 5 of 16 where S 11 and S 21 represent the reflection and transmission coefficients, respectively, which have been obtained from the Vector Network Analyzer after the measurement of the biomass samples. 3. Results and Discussion To investigate the chemical composition, SEM–EDS analysis was initially performed. Figure 2shows SEM images of the pyrolyzed biomass samples. It can be observed that both BA and BQ are homogeneous and have granular morphology. tt ff ff ff tt − − − − − ffi Figure 2. SEM images and associated EDS spectra of BA (a,b) and BQ (c,d), respectively. According to EDS analysis (Figure 2b,d and Table 1), the main chemical elements in both samples are C (77.82 to 75.66 wt%) and O (13.90 and 16.49 wt%), while other elements, such as Mg, Ca, Si, S, K, and Al, were also detected but in very small amounts. The presence of toxic metals in biochar provides important information for estimating the environmental impact of these newly produced materials [ 32 ]. EDS analysis shows that Fe is present in 1.3 wt% in BA, while other heavy metals were not detected. Table 1. Elemental composition of BA and BQ samples in wt% and atomic%. BA BQ Element wt% Atomic% Element wt% Atomic% C 77.82 85.18 C 75.66 83.05 O 13.90 11.43 O 16.49 13.59 Mg 0.56 0.30 Mg 1.90 1.03 Al 0.48 0.24 Al 0.25 0.12 Si 4.21 1.97 Si 2.33 1.09 S 0.11 0.04 P 0.26 0.11 K 0.62 0.21 S 0.21 0.09 Ca 0.99 0.32 K 0.28 0.09 Fe 1.30 0.31 Ca 2.22 0.73 Nanomaterials 2024,14, 1882 6 of 16 EDS maps were obtained to investigate homogeneity in terms of chemical composition (Figure 3). These results indicated an equal distribution of all detected elements on the surface of both samples. − tt − Figure 3. SEM–EDS maps of BA (a,b) and BQ (c,d) for C, O, Si, Ca, and Mg. FTIR spectroscopy was used to investigate the structure of carbonized biomass samples, and collected spectra are presented in Figure 4. All spectra show the band at 3412 cm −1 . This band is assigned to OH groups, attached to hydroxyl groups of the carbon skeleton of the material, or could be associated with physically absorbed water. Two low-intensity bands at 2922 and 2851 cm −1 are also observed. These bands are commonly observed in FTIR spectra of various carbon-based nanomaterials, in graphene oxide, graphene quantum dots, and carbon nanotubes [ 33 , 34 ], and stem from the asymmetric and symmetric stretching vibrations of alkyl and aliphatic CH 2 bonds [ 35 ]. These bands are prominent in the FTIR spectrum of BQ but could hardly be observed in the BA spectrum. This change is associated with a lower content of methyl groups in the sample of BA [ 36 ]. All spectra show a prominent band at 1570 cm −1 resulting from vibrations of C=C bonds and proves the presence of sp 2 domains in all samples. A low-intensity band at 1384 cm −1 is detected and assigned to C-O bonds in COOH groups. Additionally, two bands at 1240 and 1110 cm −1 are associated with stretching vibrations of C-O bonds in epoxy and alkoxy groups, respectively [33,34]. Nanomaterials 2024,14, 1882 7 of 16 − − − − − tt − tt − ff − Figure 4. FTIR (a) and Raman (b) spectra of BA (black curve) and BQ (red curve). Raman spectroscopy was used to investigate the structure of BA and BQ samples, and spectra are presented in Figure 4b. The two most prominent bands are observed in all spectra, one at 1349 cm −1 and the second at 1585 cm −1 . The first band stems from defects in graphitic, sp 2 structure due to grain edges, various vacancies, amorphous carbon, and functional groups that create sp 3 sites [ 37 – 39 ]. This band is called the D-band and its intensity is associated with the level of structural disorder. The second band is called the graphitic or G-band. This band stems from the first-order scattering of the E 2g phonon of the bonds between sp 2 carbon atoms [ 40 ]. The band around 2700 cm −1 was also detected. This band is called G’ band or 2D which involves a two-phonon double resonance Raman scattering process [ 41 ]. An additional low-intensity band could be noticed at 2875 cm −1 . This band is named the D + G band and is observed in other graphene samples produced from biomass [ 42 ]. Applying the Knight and White equation [ 43 ], crystalline size (La) was calculated and its values are presented in Table 2. These calculations show that an adequate size of the basal plane is similar for BA and BQ samples obtained from different starting materials and comparable to GO produced using the modified Hummers method. Table 2. Position of G bands, ID/IGratios, and crystalline size (La) values for BA and BQ. Sample G Position ID/IGLa = 4.4 (ID/IG)−1 BA 1583 0.96 4.58 BQ 1588 0.98 4.48 GO 1580 0.88–0.95 [34] 5.00 Graphite 1575 0.04 [44] 110 Figure 4a,b indicates that BA and BQ possess both sp 2 and sp 3 C in their structure, as well as various oxygen-containing functional groups, such as hydroxyl, carbonyl, and carboxyl. Figure 5a shows the TGA curves of carbonized biomass samples. Both TGA curves are similar to previously observed thermograms obtained for carbonized rice husks [ 45 ]. The pyrolysis process is divided into three main phases: dehydration, devolatilization, and carbonization [ 46 , 47 ]. In the first stage, up to 150 ◦ C, a major weight loss of 21.08% for BQ and 32.01% for BA was measured. This weight loss is associated with the evaporation of physically absorbed water, including inbound water [ 48 ]. Additionally, degradation of volatile compounds with a small molecular weight could also occur at this stage [ 49 , 50 ]. In the second step, in the temperature range of 150–400 ◦ C, the minor mass weights were measured: 3.80% for BQ and 6.27% for BA. In graphene-based materials, weight loss in this temperature range is often associated with the decomposition of oxygen-containing functional groups, such as OH and epoxy groups [ 51 ]. These weight losses could be Nanomaterials 2024,14, 1882 8 of 16 associated with the decomposition of oxygen-containing functional groups, such as OH, and epoxy groups from the graphitic regions of carbonized biomass [ 52 – 54 ]. As a third step (in the temperature range of 400–600 ◦ C), weight losses of 2.64% for BQ, and 2.73% for BA were measured. FTIR analysis showed the presence of different oxygen-containing functional groups, such as carboxyl, carbonyl, and hydroxyl. It is assumed that these groups were decomposed during stages 2 and 3. Thus, the BQ sample showed less, while BA showed significantly larger amounts of the total % of oxygen-containing groups. At the final stage, at 740 ◦ C, the residual sample weight was 69.73% for BQ and 56.12% for BA. These results indicate that the BQ sample is terminally the most stable with the lowest content of functional groups. ff tt tt ff θ ff tt ff tt tt Figure 5. TGA curves (a) and XRD patterns (b) of BA (black) and BQ (red). XRD patterns of BA and BQ show two diffraction peaks in the 2 θ range of 25.5 ◦ and 43.5 ◦ (Figure 5b). These bands correspond to different types of crystalline graphite: (002) and (100) planes, respectively [ 55 , 56 ]. Notably, the (002) lattice diffraction peak patterns are more pronounced. These peaks suggest the formation of the graphitic crystalline structure during the carbonization process [ 57 , 58 ]. The higher intensity of the peaks at 25.5 ◦ for BA than in the BQ sample is related to a higher-degree graphitic crystalline structure. Both samples show a peak at 29.46 ◦ assigned to the (104) plane of CaCO 3 [ 59 ]. The experimental data were compared with the standard ICDD database (#00-005-0586). Element Ca was observed in EDS spectra of BQ and BA samples, but higher at and mass% was detected in BQ (Table 1), analogous to the intensity of the peak at 29.46 ◦ in the XRD pattern. The structure of carbonized biomasses was investigated using XPS and the results of these measurements are displayed in Figures 6and 7. Figure 6shows survey spectra and reveals that both samples contain the following elements: C, O, N, and In. The indium (In) presence in both spectra is due to the In foil being used as a sample support. The regions indicated by C1s, O1s, and N1s were further analyzed (see Figure 7). In both spectra, in the C 1s region, three main contributions were identified ( Figure 7a,d ): carbon in C-C/C=C bonds at 284.8 eV and 284.6 eV, the C − O carbon attribute to both epoxy and hydroxy groups at 286.2 and 285.4 eV, and the C=O carbon at 291.1 and 288.2 eV, in BA and BQ spectra, respectively. Bonds identified in the C 1s region were confirmed by inspection of the O 1s region, where oxygen in single and double bonds were identified in the BA spectrum at 533.0 for C − O, and 531.9 at C=O eV, C-O, ∼ = 535.3 eV, and C=O, ∼ =532.8 eV in BQ. 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