Insight into the TiO2 Nanostructures Stabilized with Cashew Tree Gum to Remove Emerging Contaminants as a Green Technology

Insight into the TiO 2 nanostructures stabilized with cashew tree gum to remove emerging contaminants as a green technology Marcelo Xisto Ribeiro a , Francisca P. Araujo a , Edson Cavalcanti da Silva-Filho a , Luciano C. Almeida b , Durcilene Alves da Silva a , Ram´ on R. Pe˜ na Garcia a,c , Juan Antonio Cecilia d , Marcelo Barbosa Furtini a , Josy A. Osajima a,* , Maria Del Mar Orta Cuevas e,* a Federal University of Piauí, Interdisciplinary Laboratory Advanced Materials, Teresina, PI, Brazil b Federal University of Pernambuco, Chemical Engineering Department, Recife, PE, Brazil c Federal University of Pernambuco, Cabo de Santo Agostinho Academic Unit, Cape St. Augustine, PE, Brazil d Department of Inorganic Chemistry, Crystallography, and Mineralogy, Faculty of Sciences, University of M´ alaga, M´ alaga 29071, Spain e Departamento de Química Analítica, Facultad de Farmacia, Universidad de Sevilla, C/Profesor García, Gonz´ alez 2, Sevilla 41012, Spain ARTICLE INFO Keywords: Plant polysaccharide Stabilizer Nanostructures Dye Drugs ABSTRACT Searching for technologies to combat emerging pollutants such as dyes and pharmaceuticals is arduous. Plant polysaccharides have been used to synthesize photocatalysts as strategies for green technology. The present study aimed to synthesize TiO 2 nanostructure photocatalytic using cashew tree gum. Structural, morphological, and optical characterization was carried out to verify the gum’s effect on the growth of nanostructures. Photocatalytic tests were performed with Methylene dye (MB) and Ibuprofen (IBP) pollutants under UV irradiation. Structural characterization demonstrated the formation of anatase-type TiO 2 , with a band gap at 3.15 eV. Photocatalytic assays using CGT showed that the material was more efficient in removing MB (43.17 %) when compared to IBP (29.86 %). In addition, scavenger studies indicated that electrons are the species involved in dye degradation. Therefore, the results are promising for using TiO 2 nanoparticles obtained with gum to remove pollutants. 1. Introduction The excessive and inadequate release of materials or energy has caused high pollution levels in all the planet’s ecosystems. Some predictions have estimated that around 9 millon people die due to pollution yearly [1]. Regarding water resources, pollution levels have worsened due to the high percentage of untreated wastewater discharged into surface water bodies and the high rates of chemical contamination in drainage water in some regions of the planet [2]. Among the primary pollutants are dyes, pesticides, medicines, beauty products, fragrances, and hormones [3], now known as emerging pollutants. Their presence and persistence in water, even in low concentrations, pose severe risks to human health and the general well-being of other species in the environment [4]. In Brazil, initial research dates to the 1990s, when pesticides, drugs, and hormones were identified in different aquatic matrices [5]. However, studies have been reported more intensively since 2012 [6], justified by the range of toxic substances that were found in the various regions of the country, from more developed areas where a high consumption of pesticides has always been used intensively in extensively cultivated areas, to less developed areas plagued by precarious basic sanitation. Although Brazil does not have official methodologies for grouping these substances, regulatory bodies and sanitation companies such as the Environmental Company of the S˜ ao Paulo State and the Brazilian Association of Sanitary and Environmental Engineering have based their decisions on academic research [5]. CONAMA Resolutions, No. 357/2005 [7] and No. 396/2008 [8], as well as Ordinance MS 2914/2011 of the Ministry of Health [9], are the principal regulations for dealing with emerging pollutants and at the same time reinforce the weakness of Brazilian legislation because, among the thousands of emerging pollutants identified, only 7 % of all pesticides authorized for use in the country are covered by these * Corresponding authors. E-mail addresses: [email protected] (J.A. Osajima), [email protected] (M.D.M. Orta Cuevas). Contents lists available at ScienceDirect Nano-Structures & Nano-Objects journal homepage: www.elsevier.com/locate/nanoso https://doi.org/10.1016/j.nanoso.2024.101362 Received 24 June 2024; Received in revised form 12 September 2024; Accepted 22 September 2024 Nano-Structures & Nano-Objects 40 (2024) 101362 Available online 25 September 2024 2352-507X/Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). documents. It is, therefore, imperative to learn about the sources, transformation, and fate of these contaminants using cleaner and more economically sound technologies that make it possible to protect the environment [5]. Developing low-cost remediation techniques prioritizing the environmental component is essential and has mobilized several research groups. Photocatalysis is a popular and highly efficient method for removing recalcitrant compounds from water bodies. It consists of using suitable catalysts to accelerate the degradation of organic contaminants and convert them from toxic to harmless forms [10]. This technology is based on activating a semiconductor using electromagnetic radiation (UV or visible), which is preceded by the formation of charge carriers. From this, different oxygen radical species can be formed, which destroy the structure of the target molecule through oxidation reactions. As a result, mineralization of the pollutant is expected [11–14]. Among some materials with photocatalytic properties, TiO 2 is a reference semiconductor in water decontamination due to its low cost, good stability, and high efficiency. However, difficulties such as the rapid recombination of charge carriers and the wide intrinsic band gap (E g =3.2 eV) have been reported as some of the obstacles to a superior photocatalytic performance [15]. Therefore, synthesis strategies have been suggested over the years as alternatives to increasing the photocatalytic potential of TiO 2 , such as lattice doping using metallic and non-metallic elements, immobilization in materials with high surface area, and obtaining heterojunctions with different catalysts and materials [16,17]. However, the process can become expensive due to the high cost of some metallic reagents and the metallic lixiviation process that can aggravate the contamination scenario [16]. In recent years, the scientific community’s interest in developing more ecological synthesis methods has grown, given the need for rational use of natural resources and as an alternative to minimize risks to human health [18–20]. These methods are inserted into a set of principles and practices that encourage the design of products and processes that reduce harmful effects on man and the environment. In this context, using plant-based substrates to replace chemical solvents or stabilizers is highly relevant, as they significantly reduce the toxicity of the process [21]. When synthesizing nanoparticles that generally have a solid tendency to agglomerate, plant substrates stand out for the protective and stabilizing action of nanoparticles [22]. Gum polysaccharides are natural polymers exploited to obtain semiconductor nanostructures due to their unique structure and functionality. It is possible to use various types of gums with different functional and structural properties to synthesize a variety of nanostructures from different sources with structural and functional characterization suitable for specific applications, as well as providing access to various metal oxides [23,24]. Studies have shown that using gums to obtain oxide nanostructures directly impacts properties such as morphology, surface area, and optics. All these parameters can be decisive in the photocatalytic performance of semiconductor materials. The diversity of Brazilian flora allows regional species to be explored as potential substitutes for these commercial gums. The cashew tree (Anacardium occidentale) is a large tree common in northeastern Brazil, and the gum obtained from the exudate of the plant trunk – Cashew Gum (CG) – has been mentioned as a possible alternative to replace Arabic gum [25]. CG polysaccharide is formed from galactose in its main chain (72 %) plus side chains with glucose (14 %), arabinose (4.6 %), rhamnose (3.2 %), and uronic acid (4.7 %) [26] In order to expand the use of Anacardium occidentale exudate and cooperate with the development of water remediation technologies using ecological methods, this work proposes the synthesis of TiO 2 using CG polysaccharide to remove Methylene Blue (MB) dye. 2. Experimental part 2.1. Reagents In the synthesis process were used the following reagents: Ethyl alcohol 99.8 % (Aldrich), Titanium isopropoxide 97 % (Aldrich), CG gum was collected in Parnaiba city (Piauí State), and ultrapure water. All the reagents were analytical grade used without prior purification. The cashew tree is registered in SisGen number ABD61DA. 2.2. Sample preparation CG-stabilized TiO 2 nanoparticles were obtained by the sol-gel method, according to a previous study [27,28]. An illustrative scheme of the synthesis is shown in Fig. 1. CG polysaccharide (2 % w/v relative to the titanium precursor - TTIP) was initially added to 100.0 mL of the ethyl alcohol. The mixture was kept under magnetic stirring for 30 min for complete homogenization of the gum. Then, a volume (6.0 mL) of metallic precursor was dropped into the system. After 30 min, an equal volume of ultrapure water was slowly added to the reaction. The system was stirred for another 30 minutes, and the gel obtained was aged for 24 h. After that, the material was dried overnight in an oven at 75 ◦C and calcined at 400 ◦C in an oven for 2 h. The sample was named at CGT. 2.3. Characterization The material powder obtained was structurally characterized by X- ray diffraction (XRD) using a Bruker diffractometer (D8 Advanced) with Cu-K α radiation and a scan rate 2◦min −1 . From the XRD data, the values of crystallite size (D) and lattice strain ( ε ) were calculated using the Williamson and Hall method, expressed by Eq. 1: βhklcos(θ) = Kλ D+4 ε sinθ(1) where β corresponds to line broadening at half maximum intensity (FWHM), θ is Bragg’s angle, K is Scherrer’s constant that admits a value of 0.9 for the spheroid format, λ is the wavelength for Cu K- α of the X- rays used (1.540 Å), D is crystallite size, and ε represent the lattice strain. Using a linear adjustment of the data, the crystalline size was estimated from the intercept, and the lattice strain was estimated from Fig. 1. Schematic diagram of the synthesis of TiO 2 by sol-gel method. M.X. Ribeiro et al. Nano-Structures & Nano-Objects 40 (2024) 101362 2 the slope of the linear adjustment. Diffuse reflectance analysis was performed using a Shimadzu UV-Vis spectrophotometer, model UV-3600 with diffuse reflectance accessory, monitoring the region between 200 and 800 nm. The optical band gap (E g ) was determined using Kubelka - Munk function F(R)expressed in Eq. 2: F(R) = (1−R))2 2R=K S(2) Where the function F(R) corresponds to the absorbance, R is the absolute reflectance, K is the absorption coefficient, and S is the scattering coefficient The function F(R) is directly proportional to the absorption coefficient α , as demonstrated in Eq. 3: F(R)h υ ≈ (h ν −Eg)n(3) where: h is the Planck constant, ʋ is the frequency, and n corresponds to the different transitions (direct or indirect). The band gap energies (E g ) can be estimated from a plotting ( α h ν )2 versus the photon energy (h ν ), considering the intersection of the extrapolated linear part. Surface morphology was obtained through Scanning Electron Microscopy using TESCAN MIRA3 FIB-SEM equipment, and a semiquantitative analysis of the surface of the sample was performed using an Energy Dispersive Spectroscopy (EDS) analyzer coupled to SEM equipment. 2.4. Photocatalytic experiment The photocatalytic activity of TiO 2 was investigated through the degradation of MB (1.0×10 −5 mol.L −1 ) under UV irradiation. Thus, 0.5 g.L −1 of the photocatalyst was added to 200 mL of pollutant solution. The adsorption equilibrium was reached in 30 min in the dark. The tests were conducted in a borosilicate reactor under magnetic stirring and strict temperature control (25◦C ±1◦C). The light source was a commercial lamp (160 W) whose potency was monitored using a radiometer (HANNA – HI 97500 – Luxmeter). During the photocatalytic test, aliquots were removed at different times 0, 5, 10, 15, 30, 45, 60, 90 and 120 min). After irradiation, the samples were immediately centrifuged, and the absorbance measurements were carried out in a CARY 300 model spectrophotometer. The degradation rate was determined by the Eq. 4: Degradation =(C0−C) C0x100 (4) C 0 and C represent the initial concentration of the pollutant after the determined irradiation time, respectively. The recyclability of CGT in consecutive cycles of photocatalysis was also investigated, and removal capacity was determined in each case. The role of reactive species produced by CGT in the photocatalysis of contaminants was studied in an inhibitor test. Thus, the reagents ethylenediamine acid – EDTA (2.4×10 −6 mol L −1 ), Isopropyl alcohol - IPA (1.6×10 −6 mol L −1 ), or silver nitrate (5.0×10 −4 mol L −1 ), inhibitors of the hole (h + ), hydroxyl radical ( • OH) and electrons (e - ), respectively, were added to the reaction medium and the degradation rate determined in each case. 3. Results and discussion 3.1. Characterization The structural characterization of the CGT samples using XRD analyses and diffractograms is shown in Fig. 2. Peaks were identified at 2θ = 25.7◦, 37.9◦, 47.7◦, 54.6◦, 62.8◦, 69.9◦, 75.6◦and 82.5◦that corresponding to planes (101), (004), (200), (211), (204), (116), (215) and (224), respectively, considering JCPDS (21–1272) card for TiO 2 in anatase phase [29,30]. Additional peaks relative to other phase were not identified in the diffractogram. This result indicates that the synthesis method allowed crystalline structures of anatase TiO 2 to be obtained. Based on the XRD result, the crystallite size was estimated using the Williamson-Hall method [31], obtaining the values at 0.48309 nm for the crystallite size and 0.12443 of lattice strain. Herein, gum polysaccharide plays a hole protective during the growth of nanoparticles, stabilizing them and hindering the disorderly growth of these structures [32–34]. The possible mechanism involved in the green synthesis of TiO 2 nanoparticles involves the reduction of the titanium precursor by bioactive reducing groups, forming nuclei and growth of TiO 2 , and stabilizing the nanoparticles by the green agent used [35]. In the first case, the Ti 4+ present in the metal precursor undergoes hydrolysis, forming titanium hydroxide species (Ti(OH) 4 ). The donation of electrons to Ti(IV) by the reducing agents allows the reduction of Ti(IV) to lower oxidation states, such as Ti(III). Then, nuclei are formed, which grow and originate TiO 2 nanoparticles. During this process, polysaccharides can be adsorbed on the surface of the nanoparticles, stabilizing them and inhibiting the disordered growth of the structures [36]. FTIR technique was used to investigate the functional groups in the CGT structure, as shown by the spectrum in Fig. 3. The band at 3336 cm −1 is attributed to stretching vibration in the O-H bond [37]. The band observed at Fig. 2. XRD diffractogram of the CGT synthesized by sol-gel method and calcinated at 400 ◦C. Fig. 3. FTIR analysis CGT sample. M.X. Ribeiro et al. Nano-Structures & Nano-Objects 40 (2024) 101362 3 1632 cm −1 may be related to vibration in the C – – O bond, present in the uronic groups found in the gum [34]. Other bands identified in 1390 and 1145 cm −1 are due to O-H vibration in the acid groups and C–O stretching vibrations, respectively [38,39]. In general, bands associated with vibrations of Ti-O and Ti-O-Ti bands are found in the region below 800 cm −1 [40]. Thus, it is possible to affirm that the band found at 665 cm −1 in the CGT spectrum is typical of vibration in the metal-oxygen bond. The morphology of the CGT material was investigated using the SEM technique, and the image is shown in Fig. 4a. As seen, an agglomeration of nanoparticles was observed. Additionally, semi-quantitative analysis was performed using the EDS technique (Fig. 4b), where the presence of peaks associated with Titanium (Ti) and Oxygen (O), which are related to TiO 2 was noted. Other peaks identified, such as Carbon (C) and Gold (Au), are due to the sample preparation process and carbon tape fixation in the sample holder. The UV–Vis reflectance analysis was performed to determine the optical band gap energy (Eg), an essential measurement for materials with photocatalytic properties. These results are demonstrated in Fig. 5. An Eg value at 3.15 eV was found from the data, and successive mathematical transformations showed in 2.3 section. The slight decrease in the band gap energy value has been reported in other studies involving Fig. 4. (a) CGT morphology investigated by SEM demonstrating the presence of TiO 2 nanoparticles and (b) semi-quantitative analysis of the material performed by EDS. Fig. 5. (a) Band gap energy (Eg) calculated from the Kubelka-Munk method and (b) size distribution. Fig. 6. (a) Nitrogen adsorption and (b) pore volume for the CGT sample. M.X. Ribeiro et al. Nano-Structures & Nano-Objects 40 (2024) 101362 4 the production of TiO 2 nanoparticles using materials of natural origin [27,35,41,42]. For example, using Karaya Gum, [26] observed an Eg value greater than 3.20 eV for TiO 2 nanoparticles synthesized with Karaya gum. In another study reported in the literature, the synthesis of TiO 2 using plant extract resulted in a material with a band energy of 3.13 eV [35]. This behavior is due to the formation of surface defects in TiO2 that act as electron traps and hinder the electron-hole pair’s recombination. For photocatalytic applications, this is advantageous because the charge carriers formed become more available to trigger redox reactions that will cause the degradation of the pollutant. N 2 adsorption-desorption isotherm of the prepared CGT sample is shown in Fig. 6a. The isotherm pattern demonstrated to be type IV, considering the International Union of Pure and Applied Chemistry (IUPAC) classification, with an H3 hysteresis loop [43,44]. It indicates that CGT has a mesoporous structure [28,40]. Based on the isotherms, the specific surface area (S BET ) was 110 m 2 .g −1 . This result is superior to that demonstrated in a study in which TiO 2 was synthesized using Karaya Gum [27]. A high surface area indicates greater availability of reactive sites. Thus, reactions that occur in the surface region, such as photocatalysis, can benefit materials with an increase in this parameter. The insert graph of Fig. 6b shows the size distribution. The nanoparticles showed a concentrated size distribution between 5 and 20 nm, confirming the existence of mesopores [45]. 3.2. Photocatalytic test The photocatalytic response of the CGT sample under UV irradiation was tested against MB dye and IBP drug, considered a model pollutant in several photocatalysis studies [28,46–48]. Variations in concentration at MB e IBP due to the photocatalytic activity of CGT were monitored by UV–vis analysis, and the results are shown in Fig. 7a-b. As seen, the typical band of each pollutant decreased during the photocatalysis test. Furthermore, an increase in absorption in the region between 240 and 300 nm of the IBP spectrum was evidenced. This behavior is expected in IBP degradation and has been reported in previous study [49]. Under a quantitative approach, the C/C 0 ratio was obtained, shown in Fig. 7c, and MB and IBP removal capacity by CGT sample was determined during the experiment. For MB, in dark adsorption, dye removal was 37.08 %. After 120 min of UV irradiation, the percentage of MB removed was 43.17 %. For the test performed in the IBP solution, CGT’s adsorption by this pollutant is irrelevant, suggesting that CGT has a greater affinity for MB. After 120 min of irradiation, the capacity for IBP removal was 29.86 %. The degradation kinetics were investigated using the pseudo-first-order model. Plotting ln C/C 0 versus time irradiation and obtaining the linear fit of the data, as seen in Fig. 7d, where the slope represents the reaction rate constants (k) of the contaminant solution irradiated [50]. The values found were 4.71×10 −3 and 3.75×10 −3 min −1 to MB and IBP degradation, respectively. When TiO 2 is irradiated with adequate wavelength, holes (h + ) are formed in the valence band (VB) due to the electronic jump from them to Fig. 7. Photocatalytic activity in an experiment using CGT sample: spectral variation for (a) MB and (b) IBP, (c) C/C 0 ratio, and (d) kinetic rate. M.X. Ribeiro et al. Nano-Structures & Nano-Objects 40 (2024) 101362 5 the conduction band (CB). Thus, photogenerated charge carriers react with molecules adored on the surface of the semiconductor. For example, in VB, the holes promote the oxidation of H 2 O molecules, producing hydroxyl radicals ( • OH). Simultaneously, electrons in CB cause the reduction of O 2 molecules, forming superoxide anions. After successive reactions, • OH radicals are also generated. The power of • OH radicals in photocatalytic systems has been reported in the degradation of different pollutants [51,52]. The degradation of MB on the TiO 2 surface has been reported in the literature through the classical mechanism as shown in the Eqs. (5–8) below [50,53,54]: TiO 2 +h ν → h +(VB) +e - (CB) (5) h +(VB) +H 2 O (adsorbed) →Н + +ΟН • (adsorbed) (6) e - (CB) +O 2 → O 2 •  (7) MB +OH • → Intermediate photoproducts (8) Scavenger studies were performed to identify reactive species involved in the photocatalytic activity of CGT photocatalysts. Thus, AgNO 3 , EDTA, or IPA reagents were added to the system as inhibitors of electrons, holes, and hydroxyl radicals. As seen in Fig. 8, after 120 minutes of irradiation, the MB removal capacity was 51.4, 24.6, and 18.2 % in the tests performed with EDTA, IPA, and AgNO 3 , respectively. Comparing these results with each other, the activity of the species involved in the CGT photocatalytic response is in the following order: e - > • OH >h + . The abrupt decrease in the degradation rate of MB in the presence of AgNO 3 indicates that electrons are the main species involved in the photocatalytic activity of CGT. It is known that the Ag + ions act by sequestering the photogenerated electrons during semiconductor activation [55]. Thus, the inhibition of electrons caused a dramatic reduction in the rate of MB degradation. Similar behavior was observed for modified TiO 2 nanoparticles [48]. Based on these results, a proposed mechanism involved in the photocatalytic activity of MB is shown in Fig. 9. Photocatalyst reuse experiments were performed to verify the stability of the material in consecutive photocatalytic tests. These results are presented in Fig. 10. The ability of CGT to remove the pollutant during reuse tests was 32.3, 27.4, 23,.7, and 19.5 % after the first, second, third, and fourth cycles, respectively. The decrease in photocatalytic efficiency during reuse can be explained by the partial deactivation of the material surface due to accumulated intermediates, as reported in others studies [56,57]. Fig. 8. Inhibitor test in photocatalysis mediated by CGT material. Fig. 9. Mechanism involved in the MB removal by CGT photocatalysis. Fig. 10. Percentage of photo discoloration of MB solution after consecutive cycles. M.X. Ribeiro et al. Nano-Structures & Nano-Objects 40 (2024) 101362 6 4. Conclusion TiO 2 nanostructures anatase type can be synthesized by using stabilized cashew gum. The photocatalyst demonstrated an excellent ability to remove MB dye pollutant removal. The surface area must have contributed to the pollutant removal because these processes occur at the solid-liquid interface. In the photocatalytic removal of MB by CGT, electrons are an important species, as observed in inhibitor tests. Intermediate photoproducts from MB degradation can accumulate on the CGT surface, decreasing the material’s degradation efficiency. Finally, TiO 2 synthesized with natural polysaccharides may be a candidate for pollutant removal. CRediT authorship contribution statement Marcelo Xisto Ribeiro Xisto Ribeiro: Writing – original draft, Investigation, Data curation. María del Mar Orta Cuevas: Resources, Funding acquisition. Josy A. Osajima: Supervision, Project administration. Marcelo Barbosa Furtini: Writing – review & editing. Juan Antonio Cecilia: Writing – review & editing. Ram´ on R. Pe˜ na Garcia: Formal analysis. Durcilene Alves da Silva: Writing – review & editing. Luciano C. Almeida: Methodology. Edson Cavalcanti da Silva-Filho: Visualization. Francisca P. Araújo: Formal analysis. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability No data was used for the research described in the article. Acknowledgments The authors thank the CAPES, CNPq, FAPEPI and their institutes: UFPI, UFRPE, UFPB, Universidad de Malaga, Universidad de Sevilla, and Interdisciplinary Laboratory for Advanced Materials - LIMAV for providing work research conditions and the Fundacíon Carolina for fellowship. INCT/ Polysaccharides (National Technology-Science Institute for Polysaccharides. 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