{"paper_id":"1928d35f-e2d9-4e1e-ac65-dda50ad8f7b8","body_text":"License and Terms: This document is copyright 2024 the Author(s); licensee Beilstein-Institut.\nThis is an open access work under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0). Please note that the reuse,\nredistribution and reproduction in particular requires that the author(s) and source are credited and that individual graphics may be subject to special legal provisions.\nThe license is subject to the Beilstein Archives terms and conditions: https://www.beilstein-archives.org/xiv/terms.\nThe definitive version of this work can be found at https://doi.org/10.3762/bxiv.2024.59.v1\nThis open access document is posted as a preprint in the Beilstein Archives at https://doi.org/10.3762/bxiv.2024.59.v1 and is\nconsidered to be an early communication for feedback before peer review. Before citing this document, please check if a final,\npeer-reviewed version has been published.\nThis document is not formatted, has not undergone copyediting or typesetting, and may contain errors, unsubstantiated scientific\nclaims or preliminary data.\nPreprint Title Clays enhanced with niobium: potential in wastewater treatment and\nreuse as pigment with antibacterial activity\nAuthors Silvia Jaerger, Patricia Appelt, Mário A. A. da Cunha, Fabián C.\nAyma, Ricardo Schneider, Carla Bittencourt and Fauze J. Anaissi\nPublication Date 11 Sept. 2024\nArticle Type Full Research Paper\nSupporting Information File 1 BJ-ANANO-Suplemmentary-3nd vers.docx; 241.6 KB\nORCID® iDs Patricia Appelt - https://orcid.org/0000-0003-4302-534X; Carla\nBittencourt - https://orcid.org/0000-0002-3330-6693; Fauze J.\nAnaissi - https://orcid.org/0000-0002-5454-472X\n\n \n1 \nClays enhanced with niobium: potential in \nwastewater treatment and reuse as pigment with \nantibacterial activity \nSilvia Jaerger*1,2, Patricia Appelt ‡2, Mario Antônio Alves da Cunha ‡3, Fabián \nCcahuana Ayma ‡2, Ricardo Schneider ‡1, Carla Bittencourt ‡4, Fauze Jacó Anaissi *‡2 \n \n1Federal University of Technology - Paraná - UTFPR, Campus Toledo, Rua Cristo \nRei, 19. 85902-490, Toledo, Brazil. \n2Chemistry Department, Universidade Estadual do Centro-Oeste, Guarapuava \n85040-167, PR, Brazil. \n3 Department of Chemistry, Universidade Tecnológica Federal do Paraná, Pato \nBranco 85503-390, Brazil; \n4 Chimie des Interactions Plasma-Surface (ChIPS), Research Institute for Materials \nScience and Engineering, University of Mons, 7000 Mons, Belgium \n \nEmail: Silvia Jaerger sjaerger@gmail.com and Fauze J. Anaissi \nanaissi@unicentro.br \n* Corresponding author \n‡ Equal contributors \n \n  \n\n \n2 \nAbstract \nRaw smectite clay  (SM) sourced from the Guarapuava region, Brazil, underwent \nmodification with niobium oxide (SMO x) and niobium phosphate ( SMPh) to act as \nadsorbent and photocatalyst in the remediation of wastewater containing methylene \nblue (MB) dye. Additionally , these materials were evaluated for their potential  as \nantibacterial hybrid pigments. The characterization of the SM, SMO x, and SMPh \nsamples was conducted using various analytical techniques to assess the \nmodifications induced by the incorporation of niobium compounds into the clay matrix \nand to evaluate the colorimetric properties and dye removal efficiency. Notably, X-ray \ndiffractometry (XRD), X -ray photoelectron spectroscopy (XPS) , and laser -induced \nbreakdown spectroscopy (LIBS) were used to detail  characterization. The  results \nindicate successful modification of SM through the intercalation of niobium oxide and \nniobium phosphate within the interlayer space s of the clay  structure. Following \ncharacterization, the SMO x and SMP h samples were used for the treatment of \nsolutions containing methylene blue at 25 oC. The initial concentration was 400 mg L-\n1. Subsequently, the efficacy of the dye removal was assessed using the minimum \ninhibitory concentration (MIC) assay against two bacteria strains : Bacillus cereus \n(ATCC 10876) and Proteus mirabilis (ATCC 35649). The analysis revealed remarkable \nantibacterial activity against Proteus mirabilis, suggesting a preferential selectivity for \nGram-negative bacteria. \n \nKeywords \nSmectite; niobium; adsorption; photocatalysis; hybrid pigment  \n  \n\n \n3 \n1.0 Introduction  \nThe most found dye pollutants in wastewater on a global scale originate from \ntextile, plastic, paper, food, cosmetics, mineral, and pharmaceutical industries, among \nothers, resulting in significant environmental impacts [1]. Dyes , as  chemical \ncompounds used to impart color to different materials, play a crucial role in industries \nrequiring coloring, such as textile, food, cosmetics, rubber, printing, paper, and plastic. \nGlobally, an estimated 7 x 10 5 tons of dyes are produced, with 10 -15% typically \ndisposed of as wastewater waste  [2]. Among the most used dyes, methylene blue \n(MB) is an intense blue cationic dye important in medical sciences, chemistry , and \nbiology, as well as widely used in the textile industry  [2] . Prolonged exposure to MB \ncan result in adverse health effects , including abdominal disorders, respiratory \ndistress, skin sensitization, and blindness [3] . The dark blue color of MB in wastewater \nreduces light penetration into aquatic organisms, disturbing the balance of the \necosystem and harming various forms of life [3]. Effluents and water bodies containing \nMB require prioritized treatment due to its adverse effect on water quality. Therefore, \nit is crucial to explore remedial strategies for MB, especially considering the water \nscarcity challenges that many countries face [3]. \nTo satisfy environmental regulations, a range of wastewater treatment \ntechnologies with inherent advantages and limitations are available, encompassing \nprocesses such as advanced oxidation, extraction , and biodegradation [4]. \nUnfortunately, these methods exhibit inefficiencies due to the generation of secondary \npollution and high operational costs. Biological and anaerobic degradation of dyes may \nyield carcinogenic by-products [4,5], highlighting the significant challenge in purifying \nwater contaminat ed with dyes  necessitating the development of cost -effective \ntechnologies for their removal from industrial effluents. \nAdsorption emerges as a  widely used  method for pollutant  removal from \nwastewater due to its design simplicity, operational ease, and relatively straightforward \nregeneration of adsorbent. Various a dsorbents such as chitosan, cellulose, \norganophilic clays, kaolinite and montmorillonite clays, and activated carbon, among \nothers,have been use for removing toxic compounds from polluted water [6]. Among \nthese adsorbents, smectite clay exhibits advantageous properties as an adsorbent, \ncharacterized by its low cost, abundant availability, non-toxic nature, and large surface \narea  [2,7]. Additionally, its negatively charged surface renders it favorable for the \n\n \n4 \nadsorption of cationic [7]. In the region of Guarapuava, Paraná, Brazil, smectite clay is \nabundantly found. This natural clay has predominantly the smectite phase (at least \n50%), known as montmorillonite. Isomorphic substitution of cations betwe en the \ninterlayer space of montmorillonites by exchanging Na+, Ca2+, Mg2+, and Cu2+ cations \nadd other functionalities to the resulting material [7]. \nHeterogeneous photocatalysis emerges as a cost -effective alternative to \nbiological treatment methods for purifying polluted water  [8]. Using semiconductors as \nheterogeneous catalysts proves to be more efficient compared to traditional methods, \nas the photocatalytic process gradually decomposes contaminating molecules without \ngenerating residue s from the ori ginal organic matter, thus avoiding the disposal of \nsludge [8]. This approach allows the removal of various organic pollutants , including \ntextile dyes, using solid semiconductor s (for example, NbOPO 4 and Nb 2O5) and \nphotons (with energy greater than the ban d-gap energy of the semiconductor) to \ngenerate OH  radicals (strong oxidants), leading to the mineralization of organic \npollutants, including textile dyes [8]. \nIn this study, smectite clay samples modified with niobium oxide and niobium \nphosphate were characterized by X-ray diffractometry (DRX), vibrational spectroscopy \n(FTIR), spectroscopy in the ultraviolet -visible region (UV -Vis), X -ray photoelectron \nspectroscopy (XPS), static laser scattering (SLS), laser -induced breakdown \nspectroscopy (LIBS) and co lorimetry (CIE L*a*b*).  Subsequently, the as-prepared \nsamples were evaluated for their color properties , and the niobium-modified samples \nwere applied as adsorbents and MB dye photocatalysts. The novel-developed \npigments, specifically the  smectite clay modified with niobium-containing adsorbed \ndye, were investigated as antibacterial pigments. Additionally, colorimetric analysis of \nthe synthesized pigments dispersed in paint was evaluated. \n2.0 Experimental \n2.1 Materials \nThe smectite clay from the Guarapuava region in the Parana State, Brazil, was \npurchased from a local supplier. Niobium phosphate (NbOPO4) and niobium pentoxide \n(Nb2O5) were provided as donations by Companhia Brasileira de Metalurgia e \nMineração (CBMM). Methylene blue, with molecular mass 319. 8513 g mol -1, was \nobtained from Nuclear (Brazil). \n\n \n5 \n \n2.2 Clay modified with niobium \nFirst, the clays were swollen; for this purpose, 2 g of smectite clay was \ndispersed in 100 mL of water, and the resulting suspension was kept under stirring for \n24 h. Then, 3 .14 g of niobium phosphate (NbOPO 4) and niobium pentoxide (Nb 2O5) \nwere added. The clay/Nb suspension was continuously stirred for 72 hours at 65 oC. \nFinally, the suspensions, after being cooled to room temperature, were subjected to \nthermal treatment at 50 0 °C, with a heating rate of 5 °C/min. These samples were \nnamed SMPh and SMOx for modification with NbOPO4 and Nb2O5, respectively. \n \n2.3 Adsorption and photocatalysis tests \nBefore the adsorption and photocatalysis assessments, a stock solution of the \nMB dye was prepared at a concentration of 1 g L-1. The calibration analytical curve was \nestablished using a UV-Vis spectrophotometer at a wavelength of 664 nm. \nAdsorption experiments were conducted in batches containing 250 mg of the \nSMPh and SMO x samples un der agitation at 25°C. 100 mL of MB solutions at a \nconcentration of 400 mg L -1 were used for 3 hours.  The adsorption experiment was \ncarried out considering light ambient  laboratory conditions. Following the adsorption \nprocess, the clay/Nb samples were centrifuged at 3500 rpm for 10 minutes, and the \nfinal concentration of the solutions was determined using a UV-vis spectrophotometer. \nThese samples were named A-SMPh and A-SMOx for modification with NbOPO4 and \nNb2O5, respectively. \nPhotocatalytic tests were performed using 100 mL of an MB solution at a \nconcentration of 400 mg L -1. In this experiment, 250 mg of the SMP h and SMO x \nsamples were used as catalysts. The experimental setup system included a  \nthermostatic Pyrex glass reactor at 25°C (open), a magnetic stirrer, and a UV lamp \n(253.7 nm, 15 W, 220 V) within a dark chamber. After 3 hours of exposure, the \nsolutions were centrifuged at 3500 rpm for 10 minutes, and their final concentrations \nwere determined using a UV-vis spectrophotometer. These samples were labeled as \nA-SMPhP and A-SMOxP for modification with NbOPO4 and Nb2O5, respectively. \nThe adsorption efficiency of MB by the clays was calculated using Equation 1: \n%𝑅𝑒𝑚𝑜𝑡𝑖𝑜𝑛 = 100.\n(𝐶0−𝐶𝑓)\n𝐶0\n          Equation 1 \n\n \n6 \nwhere C 0 (mg L −1) is the initial concentration of the solution, C f (mg L −1) is the final \nconcentration of the solution. \nThe efficiency of MB photodegradation (X%) was determined by Equation 2: \n𝑋(%) =  \n(𝑀0−𝑀𝑓)\n𝑀0\n . 100    (Equation 2) \nwhere M0 and Mf are the concentrations of MB at the beginning and at the end of the \nphotocatalytic test, respectively. \n \n2.4 Dispersion of the pigments clay/Nb and clay/Nb/MB in colorless \ncommercial paint \nThe samples A -SMPh, A -SMOx, A -SMPhP, and A -SMOxP were separated \nthrough centrifugation and dried in an oven at 70°C. The clay powders, clay/Nb, and \nclay/Nb/MB powders were tested as pigments in colorless commercial paint. For this \npurpose, a 10% (w/w) proportion of the pigments in colorless commercial p aint was \nused. A sodium hydroxide solution (NaOH, 1 mol L -1) was dripped onto the clay \npowders until reaching a pH between 8 and 10. Subsequently, this suspension was \nblended with the transparent paint. Plaster molds were painted with both colorless and \npigmented paint. After the paint dried, the color was characterized through colorimetric \nanalysis (CIEL*a*b*) and UV-Vis spectroscopy. \n \n2.5 Antimicrobial activity test \nThe antimicrobial properties of the SMP h, SMO x, A -SMPh, A -SMOx, A -\nSMPhP, and A-SMOxP samples were investigated against the bacteria Bacillus cereus \n(ATCC 10876) (Gram-positive) and Proteus mirabilis (ATCC 35649) (Gram-negative). \nThe samples of smectite clay modified with niobium were dispersed in water. The \nassay followed the protocols described by the Clinical and Laboratory Standards \nInstitute (CLSI). The samples were evaluated using the minimum inhibitory \nconcentration (MIC) method [26,27] at concentrations ranging from 1.25 to 0.09 \nmg/mL. \nThe bacterial stock cultures were activated by culturing in brain heart infusion \n(BHI) broth at 37°C for 24 hours. Then, the cellular concentrations were standardized \naccording to the McFarland 0.5 scale (≅ 1.5 × 108 CFU/mL) using a spectrophotometer \nat a wavelength of 625 nm in saline water tubes. S ubsequently, 100 µL of Mueller -\n\n \n7 \nHinton broth was added to all wells in 96-well plates, followed by duplicate addition of \nthe samples using serial microdilution, and finally, 10 µL of the inoculum. The plates \nwere then incubated for 24 hours at 37°C. After i ncubation, 20 µL of the TTC dye \n(0.125% w/v - 2,3,5-triphenyltetrazolium chloride 0.125%) (NEON®) was added to all \nwells, and the plate was kept in an oven for an additional two hours. The antibacterial \nactivity was determined by MIC, observing the presenc e/absence of viable bacteria \ndue to the reaction of the TTC dye with the enzyme succinate dehydrogenase (present \nin the mitochondria), leading to the formation of a salt called Formazan with a pink -\nreddish color. \n \n2.6 Characterization \nX-ray diffraction (XRD) measurements of the powder were conducted using a \nRigaku SmartLab SE 3kW diffractometer equipped with Cu Kα radiation (λ=1.5410 Å) \noperating at 40 kV and 30 mA. Data were collected in a scanning mode in steps \nbetween 4° and 75° (2θ) with a step size of 0.05°/s. The basal distance was obtained \nusing Bragg's Law. \nFourier-transform infrared spectroscopy (FTIR) spectra were collected on a \nPerkin Elmer Frontier FTIR spectrometer using KBr pellets containing 1% by weight of \nthe samples. Analyses of all samples were performed in the range of 4000 to 400 cm-\n1 with a resolution of 4 cm-1, accumulating 10 scans.  \nAbsorbance measurements of the supernatant solutions were analyzed using \na UV -Vis spectrophotometer (UV -1800 SHIMADZU) with a 1 cm path length glass \ncuvette at λ max nm (maximum absorbance).  \nSamples collected after the adsorption process had the electronic spectra \nanalyzed using an Ocean Optics USB -2000 instrument for solid samples with a \ntungsten lamp in the range of 200–800 nm in diffuse reflectance mode.  \nPowder and paint -applied samples were analyzed by colorimetry, based on \nthe CieLab system, using a portable colorimeter (NR60CP – 3NH). \nThe oxidation state and elemental composition of the samples were evaluated \nusing X-ray photoelectron spectroscopy (XPS) with a PHI Genesis instrument from \nPhysical Electronics (Chanhassen, MN, USA), equipped with a monochromatic Al Kα \nX-ray source. The binding energy was calibrated based on the C 1s peak at 284.6 eV. \n\n \n8 \nLaser-induced breakdown spectroscopy (LIBS) analyses were carried out by \nApplied Spectra J200 equipment. The clay samples were pelletized in circular discs of \n1.2 cm and approximately 0.2-0.3 cm of height. The discs were prepared using  5 mg \nof each sample, and then it was pressed at 10 tons. The spectra were collected under \nair atmosphere between 186 and 1050 nm using a laser line at 50 % as source \nperforming 10 shots per spot with 50 μm of diameter and gate delay of 0.5 μs. \nThe powder particle size distribution measurements were assessed by a Static \nLaser Scattering (SLS) Horiba LA -960 equipment using a 15 mL cuvette accessory \nand water as dispersion medium. The refractive index was set to 1.640 for red and \nblue lines. \n3.0 Results and Discussions \nFigure 1 shows the particle distribution for the SM, SMPh, and SMOx samples. \nThe average particle size for the sample SM was 11.71 μm. The SMPh and SMOx \nsamples presented different particle size distributions, so the average particle size D50 \nfor them was 0.18 μm and 62.20 μm, respectively. These values indicate that the clay \naggregates are dissociated and well dispersed, as shown in the study by Yang et al. \n(2023) [9]. \n \nFigure 1. Particle size distribution for the samples SM (a), SMPh (b) and SMOx (c). \n \na) \n\n \n9 \nFigure 2 shows the X -ray photoelectron spectroscopy (XPS) analysis of \nniobium in SMOx and SMPh samples (Figures  2 a and b, respectively ). The Nb 3d \nspectra exhibit two distinct peaks centered at 207.5 eV and 210.2 eV, corresponding \nto Nb 3d5/2 and Nb 3d3/2, respectively, indicative of niobium +5. The O 1s XPS spectra \nare shown in Figure 2c for SMOx and Figure 2d for SMPh. The spectra of samples \nSMOx and SMPh are reproduced with two components centered at 530.9 and 533.5 \neV. The component centered at 531.0 e V can be attributed to photoelectrons emitted \nfrom oxygen atoms in  Si –O, Al –O, or Nb –O bonds, whereas the low -intensity \ncomponent at higher binding energy can be associated with the hydroxyl OH− group \nof Nb –OH located in the interlayer region of the clay.  Figure 2e shows the O1s \nspectrum recorded on sample SM. The high-intensity component centered at 532.0 eV \nis associated with oxygen bonds in Si-O-Si bonds  [10]. \n \n216 212 208 204\n \n \nIntensity (a. u.)\nBinding energy (eV)\nNd 3d\nSMOx\na)\n216 212 208 204\n \n \nIntensity (a. u.)\nBinding energy (eV)\nNb 3d\nSMPh b)\n \n540 535 530 525\n \n \nIntensity (a. u.)\nBinding energy (eV)\nO 1s\nSMOx\nc)\n540 535 530 525\n \n \nIntensity (a. u.)\nBinding energy (eV)\nO 1s\nSMPh\nd)\n \n\n \n10 \n540 535 530 525\n \n \nIntensity (a. u.)\nBinding energy (eV)\nO 1s\nSM\ne)\n \nFigure 2: Photoelectron spectroscopy (XPS) spectra for Nb3d (a and b for the samples \nSMOx and SMPh, respectively), O  1s (c, d and e for the samples SMOx , SMPh and \nSM respectively). \n \nTable 1 presents the chemical compositional analysis of the samples SM, \nSMOx, and SMPh determined by XPS. The results indicate that the smectite clay \nmodified with niobium phosphate (SMP h) exhibit a phosphorous content of 2.0 % (wt \n%), thereby confirming the successful modific ation of the clay with this niobium \ncompound. Furthermore, the samples, SMO x and SMPh, displayed niobium relative \nconcentrations of 6.4 wt% and 4.0 wt%, respectively. These results suggest the \nincorporation of niobium into the clay matrix. \n \nTable 1: Composition of the samples SM, SMox, and SMPh determined by XPS. \nSamples \nwt (%) \nC N O F Na Mg Al Si P Ca Nb Fe \nSMOx 6.4 - 62.5 1.1 2.6 2.1 4.8 13.1 - 1.0 6.4 - \nSMPh 7.9 1.5 62.7 1.0 1.7 1.3 4.4 12.5 2.0 1.0 4.0 - \nSM 9.3 - 55.0 - 6.0 3.8 6.5 17.5 - 1.4  0.5 \n \nIn recent years, laser -induced breakdown spectroscopy (LIBS), an optical \nemission spectroscopic technique, has emerged as a rapid qualitative and quantitative \nanalysis  [11]. This spectroscopic technique can be explained by the short -duration, \nhigh-intensity pulsed laser being focused on a material, producing a plasma called \nlaser-induced plasma (LIP) [11]. Qualitative and quantitative information about a \nsample is obtained by measuring the spectral delivery of the laser-induced plasma [11]. \n\n \n11 \nFigure 3 shows LIBS spectra for the samples SM, SMPh and SMOx. It is observed that \nthe samples containing niobium show a higher density of spectral lines. \nFor the sample SM, the principal emissions lines for Mg, Al, Na, at around 279 \nnm, 309 nm and 589 nm, respectively, are consistent for the montmorillonite structure, \nand these results are coherent with XPS composition in Table 1. The modification of \nthe clay samples with niobium was characterized by the presence of many other lines \nthroughout the spectrum region, which highlighted the lines at 666 nm. XPS analysis \n(Table 1) as well as the XRD results (Figure 4) verified that the niobium compounds \n(NbOPO4 and Nb2O5) were intercalated in the interlayer space of the clay. LIBS results \nshowed the presence of lines of Mg and Na, at around 280 nm and 819 nm, \nrespectively. As demonstrated in Table 1, the compositions of Mg and Na were \nreduced when the SM sample was modified with niobium compounds. \n \n\n \n12 \n \nFigure 3: Laser-induced breakdown spectroscopy (LIBS) analysis for the samples SM \n(a), SMPh(b) and SMOx (c). \n \nThe X-ray diffraction (XRD) profile for smectite clay and its modifications with \nniobium phosphate and niobium oxide (SMPh and SMOx, respectively), as well as the \nsamples obtained after adsorption/photocatalysis of MB (A-SMPh, A -SMOx, A -\nSMPhP, A -SMOxP), are shown in Figure 4. The XRD analysis for smectite before \nmodification with niobium indicates dioctahedral montmorillonite (M -COD 9 002779 \nAl2Na0.5O12Si4) with an amount of kaolinite (K -COD 1011045 Al 2H4O9Si2) and quartz \n(Q-COD 9012600 SiO2) at 13.8%, 41.6%, and 44.6%, respectively. \n \n \n \n\n \n13 \n \nFigure 4: X -ray diffraction patterns of the smectite samples and those modified with  \nNiobium Oxide (A) and  Niobium Phosphate (B) \n \nA similar diffraction profile for iron-rich bentonite was observed by Fontaine et \nal., 2020 [13]. The characteristic reflections of montmorillonite for the basal spacing \ncorrespond to approximately 15 Å (d001 = 14.88 Å), re lated to the interlayer distance \nof 2:1 clays, resulting in a spacing between 14.0 Å - 15.0 Å [14]. The quartz phase was \nidentified by the presence of reflections at 2 θ = 20.96°, 26.64°, and 50.02°. The \ncharacteristic peaks that allowed the identification of the kaolinite phase were at 2θ = \n12.48°, 20.14°, 25.63°, and 36.48°. \nFigure 4.a,b presents diffraction patterns with amorphous characteristics of the \nniobium compounds NbOPO 4 and Nb 2O5. Two broad peaks were identified, one at \napproximately 25.54° (2θ) and the other at 49.73° (2θ) [14,15]. The amorphous pattern \ncharacteristic of niobium compounds was maintained upon modification of the smectite \n\n\n \n14 \nclay with these compounds . For these compounds, was observed that the \ncharacteristic reflection of the (001) basal stacking plane of montmorillonite appeared \nto shift towards lower 2θ values (below 5 (2θ) values), indicating intercalation of Nb2O5 \nand NbOPO4 structures, for the samples SMOx and SMPh, respectively. \nFurthermore, characteristic peaks corresponding t o the quartz and kaolinite \nphases were observed at reflections at 2 θ = 20.14° and 68.5° (phase K), 26.64° and \n50.02° (phase Q). It can be observed that for the A -SMPh, A-SMOx, A-SMPhP, and \nA-SMOxP samples, those obtained after adsorption/photocatalysis of MB, the basal \nstacking (001) reflection shifted towards higher 2 θ values compared to the clay \nmodified with niobium but without MB. Beyond the displacement of the basal stacking \n(001) reflection of the sample SM, the results suggest that the Nb 2O5 and NbOPO4 \nstructures, also the MB were intercalated between the clay layers, due to the reduced \nNa and Si elements, as observed in LIBS (Figure 1) results and XPS composition \n(Table 1 and Figure 2).  \nThe Fourier-transform infrared spectroscopy (FTIR) spectra of t he smectites \nbefore and after modification with niobium are shown in Figure 5. It can be observed \nthat for the SM samples and all those modified with NbOPO4 (Figure 5 A), the spectra \nshow a narrow band in the region of 3638 cm -1, associated with the (Al-OH-Al) Al2OH \nvibrational stretching, indicative of smectite with a high aluminum content in octahedra \n[7,16]. The spectra for the NbOPO 4 samples and the smectites modified with niobium \nphosphate (SMPh) before and after adsorption/photocatalysis (Figure 5A) show a band \nin the region of 1043 cm-1, due to the vibrational mode (ν) of asymmetric stretching of \nthe phosphate ion. \nThe broadband in the region of 3385 cm -1 and the narrow band in the region \nof 1600 or 1637 cm -1 present in the spectrum for SM and the clay samples modified \nwith NbOPO4 and Nb2O5 are attributed to the stretching of hydroxyl (OH) and angular \ndeformation of water molecule (H 2O), respectively [7,17] . The band in the region of \napproximately 1115 cm -1 is related to the stretching vibrations of Si -O. Bands in the \nregion of 520 and 463 cm -1, which appear in all spectra of smectites and smectites \nmodified with niobium, are attributed to the stretching and bending of Si -O present in \nthe clay layers [7]. \n\n \n15 \n \nFigure 5: FTIR spectra for the smectite samples modified with Niobium phosphate (A) \nand Niobium oxide (B). \n \nThe spectra for Nb 2O5 and NbOPO4 and the smectites modified with niobium \nbefore and after adsorption/photocatalysis present an intense band in the region of 630 \ncm-1 related to the stretching of the Nb-O bond. The A-SMPh, A-SMPhP, A-SMOx, and \nA-SMOxP samples exhibit a set of bands in the region from 1477 to 1277 cm-1 typical \nfor the identification of the MB dye, indicating the intercalation of the dye between the \nlayer of clay [18]. \nThe absorbance spectra profiles in the visible region for the smectite samples \nand smectites modified with niobium are presented in Figure 6. It can be observed that \nthe NbOPO4 and Nb2O5 powder samples do not show an absorption band. However, \nthe smectite modified with these compounds exhibited a band with a maximum of 493 \nnm. The samples SMOx e SMPh obtained a fter adsorption/photocatalysis present \n\n\n \n16 \nprofiles like SM clay, with intense absorption in the UV region with a sharp drop of \naround 550 nm [8]. This fact indicates the feasibility of activating the A -SMPh, A -\nSMPhP, A-SMOx, and A-SMOXPh samples under visible light (above 400 nm)  [8]. \nThe indirect band-gap energy values for the SM, SMPh, SMOx, A-SMPhP, and \nA-SMOxP samples were estimated by the Tauc method [19]. The band gap values are \npresented in Table 2. The band -gap value of the SM sample (2.23 eV) was sli ghtly \nhigher than for the NbOPO 4 and Nb2O5 compounds. The SMPh and SMOx samples \nmodified with niobium remained with band-gap values close to those of the clay, 2.22 \nand 2.23 eV, respectively. \nHowever, the values of the smectite samples modified with niobium recorded \n(A-SMPhP and A -SMOxP) after the photocatalysis experiment showed much lower \nvalues, namely 1.53 eV for the A -SMPhP and A -SMOxP samples (Table 2). The \ndecrease in values occurred due to the smectite clay with small band -gap values, \nwhich generated impurity energy levels above the valence band edge. This results in \nlower energy values required to excite charge carriers, reducing the optical band [20]. \n \nTable 2: Band-gap values. \nSample Band-gap values Sample Band-gap values \nSM 2.23 --- --- \nNbO(PO4) 2.12 Nb2O5 2.21 \nSMPh 2.23 SMOx 2.22 \nA-SMPhP 1.53 A-SMOxP 1.53 \n \n \n\n \n17 \n \nFigure 6: The absorbance spectrum in the visible region for the samples of smectite \nand their modifications with Niobium Oxide (A)  and Niobium Phosphate(B). \nFigures 7 show the comparison of the results obtained regarding the \npercentage of removal from adsorption and heterogeneous photocatalysis tests. The \nphotocatalysis mechanism can be explained as follows: a semiconductor such as the \nSMPh and SMOx samples absorbs a photon, promoting an electron from the valence \nband (VB) to the conduction band (CB), creating a hole in the valence band (hBV+)  [8]. \nThese holes induce the oxidative decomposition of organic molecules adsorbed on the \ncatalytic surface. They also react with water molecules, producing the hydroxyl radical \n(OH•). This radical rapidly attacks the dye molecules in the solution, leading to \nmineralization into CO2 and H2O [8]. \nAccording to the observed results, the SMO x and SMPh samples exhibited \nnotably higher efficiency in the adsorption process (99.01% and 99.99%, respectively) \ncompared to photocatalysis. This phenomenon can be attributed to the negative \n\n\n \n18 \nsurface charge of the modified clays SMO x and SMPh, which exhibit a strong affinity \nwith the positively charged structure of the MB dye. On the other hand, the SMOx and \nSMPh samples demonstrated significant efficacy in MB removal, with removal rates of \n94.5% and 99.81%, respectively.  The preferential electron -hole (hBV+) favoring of the \nSMOx catalyst hindered its photocatalytic activity [8]. \nSMOx SMPh\n0\n94\n96\n98\n100\n102MB dye treatment efficiency (%)\nAdsorbents and Photocatalytic materials\n99,01 %\n94,50 %\n99,99 % 99,81 %\n Adsorption Efficiency  (%)\n Photocatalysis Efficiency (%)\nA\n \n \nFigure 7: Adsorption and photocatalysis efficiency (A) and UV-Vis spectra following the  \ndigital images of the MB solutions after adsorption and photocatalysis assays (B) using \nthe starting materials SMOx and SMPh. \n \nIn a study by Asencios et al. (2019)  [8], niobium-modified clay was explored \nfor the photocatalysis of Rhodamine B dye, yielding removal rates close to 95% \nremoval. Additionally, Lacerda et al. (2020)  [21] achieved up to 90% efficiency in the \nremoval of reactive blue 19 dye using niobium -modified bentonite. These reports \n\n\n \n19 \ndemonstrate that the material obtained in this study presents high values of MB \nremoval under UV light, standing out as a novel material. \nThe absorbance profiles of the niobium -modified samples dispersed in clear \npaint are shown in Figure S1. It is possible to observe that the SMOx and SMPh \nsamples dispersed in clear paint, despite presenting a yellow coloration, did not show \nabsorption bands. The samples collected after the adsorption/photocatalysis assays \nexhibited bands in the maximum region at 664 nm, corresponding to the π → π* \nelectronic transitions of the adsorbed MB dye [22]. \nTables 3 -4 and Figures S2-S3 present the values of the CIE L*a*b* colorimetric \nparameters for the samples SMOx and SMPh (powder sample, cycle) even as they \ndisperse in colorless paint (paint sample, square), respectively. The tables also include \ncolor difference values between the samples in powder form and those  dispersed in \ncolorless paint. Colorless paint does not contain white pigment in its matrix, enhancing \nthe dispersed pigments' color. The ΔE parameter quantified the color difference \nbetween two samples, SMOx and SMPh in powder form and those dispersed in \ncolorless paint. As observed, the results of ΔE shows that the samples A -SMPhP \n(12.17) and SMOx (9.82) before the adsorption/ photocatalysis process when \ndispersed in colorless paint demonstrate strong color parameter difference (ΔE = 6 -\n12). The ΔE values  obtained for the samples SMPh (13.62) before the adsorption/ \nphotocatalysis process, A-SMPh (23.38), A-SMOx (15.97) and A-SMOxP (13.80) were \nabove 12, indicating a very strong color parameter difference [23]. \nThe colorimetric parameters presented in Table s 3-4 and in Figures S2 -S3 \ndemonstrate that the test specimens painted with niobium -modified clay before the \nadsorption/photocatalysis of MB show higher luminosity (L). When the clay samples \nwere colored MB, after the adsorption/photocatalysis process, the  luminosity \ndecreased. The A-SMPhP sample show a slight tendency towards green coloration (-\na*). On the other hand, when evaluating the b* parameter, it is observed that all \nsamples obtained after adsorption/photocatalysis of MB (A-SMPh, A-SMPhP, A-SMOx \nand A -SMOxP), whether in powder form or dispersed in paint, show a tendency \ntowards blue coloration, as observed in the negative b* values. \n\n \n20 \nTable 3: Colorimetric parameters obtained by the CieLab system for the SMPh \nsamples before and after the adsorption/photocatalysis assays in powder form (circle) \nand dispersed in paint colorless (square). \nSample Colorimetric Parameters Images of \nthe sample L* a* b* C*  h ΔE \nSMPh 77.02 2.47 10.38 10.67 76.67 - \n \nSMPh 90.54 2.19 12.03 12.23 68.01 13.62 \n \n        \nA-SMPh 13.45 4.91 -36.34 36.67 277.69 - \n \nA-SMPh 32.05 3.15 -22.29 22.51 278.05 23.38 \n \n        \nA-SMPhP 31.91 -5.08 -21.03 21.63 256.42 - \n \nA-SMPhP 32.98 5.97 -26.01 26.68 282.92 12.17 \n \n \nTable 4: Colorimetric parameters obtained by the CieLab system for the SMPh \nsamples before and after the adsorption/photocatalysis assays in powder form (circle) \nand dispersed in paint colorless (square). \nSample Colorimetric Parameters Images of \nthe sample  L  a* b* C*  h ΔE \nSMOx 80.96 3.07 13.17 11.05 73.85 - \n \nSMOx 90.42 0.71 12.03 9.63 85.78 9.82 \n \n\n\n \n21 \n        \nA-SMOx 22.91 13.94 -36.80 39.55 290.74 - \n \nA-SMOx 30.93 6.79 -24.99 25.90 285.20 15.97 \n \n        \nA-SMOxP 18.25 7.90 -30.06 31.00 284.72 - \n \nA-SMOxP 30.65 7.15 -24.06 25.10 286.54 13.80 \n \n \nThe SMPh, SMOx, A -SMPh, A -SMOx, A -SMPhP, A -SMOxP samples were \nevaluated for their in vitro antibacterial capacity using the minimum inhibitory \nconcentration (MIC) method against the pathogenic bacteria Proteus mirabilis  and \nBacillus cereus. The results obtained are presented in Table 5 \n \nTable 5: MIC (mg/mL) for the in vitro antibacterial activity in water. \nSample SMPh SMOx A-SMPh A-SMOx A-SMPhP A-SMOxP \nMIC  \nP. mirabilis * * 0.31 0.31 0.15 0.62 \n* MIC values were not determined \nThe samples A -SMPh, A -SMPhP, A-SMOx, and A -SMOxP demonstrated \nantibacterial activity against the Proteus mirabilis  bacterial strains, with minimum \ninhibitory concentration (MIC) values recorded at 0.31, 0.15, 0.31, and 0.62 mg/mL, \nrespectively. In contrast, the SMPh and SMOx samples exhibited no inhibitory effects \nat the tested concentrations for this microorganism. \nThe A -SMPhP sample exhibited the best MIC value at 0.15 mg/mL. In \ncomparison, the A-SMPh sample, which did not undergo the photocatalysis process, \ndemonstrated an MIC of 0. 31 mg/mL. This indicates that the antibacterial activity for \nA-SMPh was approximately twice as high, suggesting that the photocatalytic process \nmay enhance inhibitory efficiency. In contrast, the A-SMOxP sample yielded an MIC of \n0.62 mg/mL, while its precu rsor, the A -SMOx which lacked photocatalytic treatment, \n\n\n \n22 \nhad an MIC of 0.31 mg/mL, indicating a more effective antibacterial action against the \ntested strains. \nIt is noteworthy that antibacterial activity was only observed in materials that \nhad the addition of methylene blue, highlighting the influence of the dye on the assay \noutcomes. According to Thesnaar (2021) [24] , many studies have demonstrated that \nmethylene blue, either alone or in combination with other compounds, possesses \nantibacterial activity; however, the precise mechanism of action remains unclear. The \nSMPh, SMOx, A -SMPh, A-SMOx, A-SMPhP, and A -SMOxP samples did not exhibit \nminimum inhibitory activity against the other studied bacterial strain, Bacillus cereus. \nThese results highlight the sele ctivity of the studied samples (A -SMPh, A -\nSMPhP, A-SMOx, and A-SMOxP) in inhibiting Gram-negative bacteria, which typically \nexhibit increased resistance due to the presence of an outer membrane that protects \nthem from certain antimicrobial agents [25] . Therefore, the preliminary results indicate \nthat smectite clay modification with niobium oxide (SMOx) and niobium phosphate \n(SMPh), when adsorbed and/or photocatalyzed for the treatment of wastewater \ncontaining MB, demonstrates promising selective antibacter ial activity against Gram -\nnegative bacteria. \n4.0 Conclusions \nIn conclusion, the presented study demonstrates  that the modification of \nsmectite clay with niobium phosphate and niobium oxide can be achieved through a  \nsimple and low -cost procedure. The XRD results revealed that the raw smectite \ncontains additional phases such as kaolinite and quartz. The modified smectite clay \nwith niobium ( SMPh and SMO x) retained the amorphous characteristic typical of \nniobium compounds, as indicated by the XRD re sults. Furthermore, analyses using  \nXPS and LIBS confirmed that the niobium compounds were intercalated in the clay \nstructure. \nThe smectite sample, along with the SMPh and SMO x samples, exhibited \nadsorption and photocatalytic efficiencies exceeding 99 % for  the removal of MB in \nwater. This process resulted in a  final product blue color ed as evidenced by  the \ncolorimetric parameters obtained through the CieL*a*b* color space . The samples \nderived from the  adsorption/photocatalysis tests (A-SMPh, A-SMPhP, A-SMOx, and \n\n \n23 \nA-SMOxP) demonstrate significant antibacterial activity against the  Gram-negative \nbacteria Proteus mirabilis, highlighting the influence of the dye in the assay outcomes. \nThe implications of this work extend to the development of a novel hybrid \npigment. This pigment, synthesized from abundant natural clays of the Guarapuava \nregion in conjunction with niobium, an abundant metal in Brazil, is easily synthesized, \ncost-effective and has potential as a pigment in commercial paints . Moreover, it \nexhibits antibacterial properties against pathogens responsible for various diseases, \nincluding ocular and auditory infections. \n \nAcknowledgments \nThe authors would like to thank the following agencies for their support: Capes, CNPq, \nFinep, and Fundacão Araucária. S.J . is thankful for a CNPq PDJ post-doctorate grant \n(152230/2022-0). 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