Reusable Fe₃O₄/SBA15 nanocomposite as an efficient photo-Fenton catalyst for the removal of sulfamethoxazole and Orange II

nanomaterials Article Reusable Fe3O4/SBA15 Nanocomposite as an Efficient Photo-Fenton Catalyst for the Removal of Sulfamethoxazole and Orange II Jorge González-Rodríguez 1,* , Lucía Fernández 1, Zulema Vargas-Osorio 2,3 , Carlos Vázquez-Vázquez 2, Yolanda Piñeiro 2, JoséRivas 2, Gumersindo Feijoo 1and Maria Teresa Moreira 1   Citation: González-Rodríguez, J.; Fernández, L.; Vargas-Osorio, Z.; Vázquez-Vázquez, C.; Piñeiro, Y.; Rivas, J.; Feijoo, G.; Moreira, M.T. Reusable Fe3O4/SBA15 Nanocomposite as an Efficient Photo-Fenton Catalyst for the Removal of Sulfamethoxazole and Orange II. Nanomaterials 2021,11, 533. https:// doi.org/10.3390/nano11020533 Academic Editor: Lidija ´ Curkovi´c Received: 1 February 2021 Accepted: 12 February 2021 Published: 19 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 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/). 1CRETUS Institute, Department of Chemical Engineering, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain; [email protected] (L.F.); [email protected] (G.F.); [email protected] (M.T.M.) 2Laboratory of Magnetism and Nanotechnology, Departments of Physical Chemistry, Faculty of Chemistry and Applied Physics, Faculty of Physics, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain; [email protected] (Z.V.-O.); [email protected] (C.V.-V.); y.pineiro.r[email protected] (Y.P.); [email protected] (J.R.) 3Centre for Functional and Surface Functionalized Glass, Alexander Dubˇcek University of Trenˇcín, Študentská2, 911 50 Trenˇcín, Slovakia *Correspondence: jor[email protected]; Tel.: +34-8818-16771 Abstract: Today, the presence of recalcitrant pollutants in wastewater, such as pharmaceuticals or other organic compounds, is one of the main obstacles to the widespread implementation of water reuse. In this context, the development of innovative processes for their removal becomes necessary to guarantee effluent quality. This work presents the potentiality of magnetic nanoparticles immobilized on SBA-15 mesoporous silica as Fenton and photo-Fenton catalysts under visible light irradiation. The influence of the characteristics of the compounds and nanoparticles on the removal yield was investigated. Once the key aspects of the reaction mechanism were analyzed, to evaluate the feasibility of this process, an azo dye (Orange II) and an antibiotic (sulfamethoxazole) were selected as main target compounds. The concentration of Orange II decreased below the detection limit after two hours of reaction, with mineralization values of 60%. In addition, repeated sequential experiments revealed the recoverability and stability of the nanoparticles in a small-scale reactor. The benchmarking of the obtained results showed a significant improvement of the process using visible light in terms of kinetic performance, comparing the results to the Fenton process conducted at dark. Reusability, yield and easy separation of the catalyst are its main advantages for the industrial application of this process. Keywords: magnetic nanoparticles; visible light; photo-Fenton; sulfamethoxazole; Orange II; reuse; reactor 1. Introduction Freshwater is one of the most valuable resources of a territory, due to its essential role in life. Its consumption is closely influenced by climatic and anthropogenic factors [ 1 ]. Population growth in large cities and the unequal distribution of water, both within territories and over time, put pressure on freshwater resources [ 2 – 4 ]. In addition, changes in consumption patterns involve an increased presence of pharmaceuticals, personal care products (PPCPs) and other emerging pollutants in domestic and industrial effluents [ 5 ]. Moreover, agriculture causes the release of pesticides that may be present in surface and groundwater by leaching and transport through the soil [6,7]. WWTPs are the main alternative to prevent contamination of freshwater and groundwater. Beyond the removal of organic matter, nutrients and suspended solids, the most recalcitrant compounds present in the water, known as emerging contaminants, can bypass Nanomaterials 2021,11, 533. https://doi.org/10.3390/nano11020533 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021,11, 533 2 of 18 these treatments and reach the effluents. For this reason, some types of tertiary treatments are being studied in recent years in the field of WWTPs to remove these compounds, in order to limit or prevent their release into the environment [ 8 ]. Depending on the polarity and persistence of the products, they can leave the WWTPs adsorbed on the biosolids or in the aqueous stream. The diverted fraction can be treated by advanced oxidation processes (AOP), based on the generation of reactive oxygen species (ROS), which are capable of degrading a wide range of compounds through non-specific oxidations [9,10]. In these processes, highly oxidative compounds such as ozone or hydrogen peroxide are used to generate radicals such as · OH to achieve satisfactory removal efficiencies. In the case of ozone-based processes, removal rates above 90% can be achieved for PPCPs [ 11 ], the main drawbacks being the incomplete mineralization of the compounds, high energy consumption or limited oxidation of hazardous compounds due to their recalcitrant nature [ 12 ]. This removal rates can be improved by combining this technique with other strong oxidants such as H 2 O 2 , UV light or catalysts such as transition metals (Cu(II), Fe(II), ZnO(II), . . . ) or metal oxides (TiO 2 , Fe 2 O 3 , . . . ) [ 13 , 14 ]. Although ozonation is the process applied on a large scale for disinfection, color and odor removal and oxidation of contaminants in drinking water treatment plants, its use is not widespread in WWTPs [ 13 ]. In this context, other processes, such as semiconductor-based reactions or the Fenton process, are being investigated for the removal of emerging contaminants. On the one hand, semiconductor photocatalysis is based on the use of certain metal oxides (commonly TiO 2 and ZnO) that generate an electro-hole pair when irradiated by a suitable light source, causing the promotion of electrons from the valence band to the conduction band [ 15 ]. Oxidation and reduction reactions occur in these pairs involving the adsorption of oxygen on the catalyst surface and hydroxyl anions to produce ROS. To overcome mass transfer limitations, nanostructured materials are commonly used in both semiconductor and Fenton catalysis, since increasing the surface-to-volume ratio increases the reaction rate [ 16 ]. Moreover, surface modifications, supporting the nanoparticles onto membranes [ 17 , 18 ], doping with other elements [ 19 – 21 ], or targeted design of nanocomposites with magnetic properties have been evaluated to improve performance and enhance the separation stage [ 22 , 23 ]. Moreover, Fenton-based reactions have also generated interest in the field of micropollutant removal because these processes have shown good results even when the target compounds are present in low concentrations [ 11 , 24 ]. Furthermore, the improvement of Fenton processes using light sources up to 600 nm, such as UV or visible light, allows improved reaction rates [ 16 , 25 ]. These types of reactions are based on the use of Fe 2+ as catalyst to enhance the oxidation potential of H 2 O 2 under acidic conditions (around pH 3) [ 26 ] (Equation (1)), to produce mainly · OH (Equation (2)) and · OOH (Equation (4)). However, other reactive oxygen species (ROS) such as O 2− and singlet oxygen (1O2) can be produced during the reactions [27]. Fe2++H2O2→FeOOH++H+, (1) FeOOH++H+→Fe3++OH−+·OH, (2) Fe3++H2O2Fe(O2H)2++H+, (3) Fe(O2H)2+→Fe2++·OOH, (4) With the use of light sources, the equilibrium reaction for the regeneration of catalyst from Fe 3+ to Fe 2+ is favoured (Equation (3)), positively affecting the kinetics. Further research based on this reaction and applied to the field of wastewater treatment focused on the study and development of innovative catalysts to improve separation, removal rates or shift of reaction conditions towards circumneutral pH [ 28 ]. The main drawback of working under these conditions is the precipitation of Fe in the form of insoluble Fe(OH) 2 . Some authors have studied the stabilization of these species using iron complexing agents [29,30] or chemical compounds generated by microorganisms [ 31 ], but in these cases, the catalyst is released to the environment. However, the main advantages of Fenton reactions Nanomaterials 2021,11, 533 3 of 18 are the operation at standard pressure and temperature conditions, the use of low-risk chemicals and, in the case of homogeneous catalysis, no mass transfer limitations are observed [ 28 ]. Nevertheless, it requires the use of iron complexing agents to overcome the precipitation of the species, and separation of the catalyst is not possible, resulting in its continuous loss [ 32 ]. Substitution of dissolved iron species by nanostructured catalysts improves separation and reuse in successive removal cycles. Nonetheless, one of the main drawbacks of using heterogeneous catalysts is the decrease in reaction rates due to mass transfer limitations and the presence of active iron only on the surface [ 28 ]. At this stage, nanostructured magnetite, a combined oxide containing Fe(II) and Fe(III) species, and its superparamagnetic properties make it ideal for separation by the application of an external magnetic field, as demonstrated in other works [ 22 , 33 ]. However, one of the problems with this material is the formation of aggregates, which reduces it specific surface area negatively affecting the kinetics. Therefore, the enhancement of these NPs by coating with polymers such as polyethyleneimine (PEI) or polyacrylic acid (PAA) generates electrostatic repulsions to avoid agglomeration [33,34]. However, the changes in the surface charge affect the interaction between nanoparticles and target compounds, and for this reason, supporting the nanoparticles on mesoporous silica (SBA-15) increases the specific surface improving the kinetics. Nevertheless, the bottleneck of the Fenton reaction is the regeneration of the catalysts. The incorporation of a light source allows the formation of new reaction routes to regenerate Fe(III), while direct photolysis of hydrogen peroxide occurs. Both changes in the reaction pathways lead to increased reaction rates [ 28 ]. Different types of heterogeneous catalysts combining different types of iron oxides or other nanomaterials such as carbon- or cooper-based ones have been used in photo-Fenton reactions. Other authors investigated the use of hybrid materials such as iron oxide/graphene [ 35 ], Fe-sand [ 36 ] or a combination of different iron oxides or other metal oxides [ 37 , 38 ] to improve the catalytic performance. The main factors studied to evaluate the materials was the reusability of catalysts, their kinetic parameters. However, one of the most important parameters is the feasibility for industrial application, considering the catalyst separation mechanism. In this research work, the photo-Fenton process was studied, comparing the results obtained with the conventional Fenton process under the same conditions. Different target compounds and types of nanoparticles were analysed, selecting those that showed the best results. In addition, the optimization of the selected variables (catalyst loading, pH and hydrogen peroxide) was investigated using response surface methodology. Finally, the performance of the selected nanoparticles was evaluated under the best conditions in a 2 L reactor for the removal of Orange II (OII) and sulfamethoxazole (SMX), analysing the reuse of the nanoparticles, kinetic performance, degree of mineralisation and effluent toxicity. The use of visible light together with magnetic separation brings this process closer to an industrial application. The replacement of ultraviolet lights with more efficient ones allows reducing operating costs. Moreover, the use of magnetic fields to separate the catalyst improves the process by avoiding the use of a catalyst recovery unit. 2. Materials and Methods 2.1. Chemicals Iron (III) chloride hexahydrate (FeCl 3· 6H 2 O, 97% w/w), tetraethyl orthosilicate (TEOS, 98% w/w), polyethyleneimine (PEI, Mw 25000), polyacrylic acid (PAA, Mw 2000), triblock copolymer Pluronic P123 (PEO20-PPO70-PEO20), hydrochloric acid (HCl, 37% w/w), isopropyl alcohol (IPA, 99.7%) and hydrogen peroxide (H 2 O 2 , 30% w/w) were purchased from Sigma Aldrich (St. Louis, MI, USA). Iron (II) sulphate heptahydrate (FeSO 4· 7H 2 O, 99% w/w), ammonium hydroxide (NH 4 OH, 28% w/w), tetramethylammonium hydroxide (TMAOH, ≈ 10% w/w), orthophosphoric acid (H 3 PO 4 , 85% w/w) and acetonitrile (ACN, HPLC grade) were purchased from Fluka (Buchs, ZU, CH, Switzerland). Sodium hydroxide (NaOH, 95% w/w) was purchased from Panreac Química SLU (Barcelona, CT, Spain). Moreover, the reagents used as target compounds and their details are shown in Figure S1. Nanomaterials 2021,11, 533 4 of 18 Milli-Q deionized water was used in all synthesis experiments and was purchased from Merck-Millipore (Darmstadt, HE, Germany). Distilled water or wastewater was used for the preparation of the stock solutions, depending on the type of experiment. In the case of sulfamethoxazole, acetonitrile was used to prepare the initial stock due to the low solubility of this compound in water. 2.2. Synthesis and Characterization of Nanoparticles The synthesis of sterically stabilized black magnetite was conducted following the Massart’s method [ 39 ]. For this procedure, a mixture of FeCl 3· 6H 2 O and FeSO 4· 7H 2 O salts with a molar ratio of approximately [Fe 3+ ]/[Fe 2+ ] = 1.5 was dissolved into 100 mL of HCl 0.01 M. To promote the formation of black magnetite nanoparticles, 30 mL of NH 4 OH ( 28% w/w ) was added to the mixture and the solution was incubated for 1 h at 60 ◦ C. Once the reaction is over, deionized water was used to wash the synthesized nanoparticles three times and withdrawn by applying an external magnetic field. Finally, a TMAOH solution was used to reach pH 10 and stabilize the nanoparticles in solution [ 40 ]. Polymer coating with PEI or PAA was performed in a subsequent step by adding a HCl solution to acidify the medium [41]. On the other hand, the synthesis of magnetite supported by SBA-15 mesoporous silica was based on the Colilla method [ 42 ], using a Pluronic P123 (PEO20-PPO70-PEO20) triblock copolymer, HCl/H3PO4and TEOS. The obtained product was dried, and several washing cycles were performed with organic solvents to remove the remaining chemicals in order to reduce the adsorption of compounds that could negatively affect the catalytic performance. Furthermore, the synthesis of the immobilized magnetite on mesoporous silica was done by incorporating the SBA-15 matrix into the magnetite synthesis procedure when the iron salts are added [41]. The morphological characterization of the nanoparticles was performed using field emission scanning electron microscopy (Zeiss FE-SEM UltraPlus microscope) with the angle-selective backscatter electron detector (AsB detector) (Zeiss, Oberkochen, BW, Germany). For TEM analysis, a JEOL JEM-1011 transmission electron microscope (Tokyo, 13, Japan) was used. The analysis of the crystalline phases was carried out by X-ray diffraction (XRD) on powder samples with a Philips PW1710 diffractometer (Cu K α radiation source, λ = 1.54186 Å) (Eindhoven, NB, The Netherlands). Measurements were taken between 15 ◦ < 2 θ < 80 ◦ , increasing by 0.040 ◦ and a time per step of 20 s. Pore size distribution and specific surface area were estimated from N 2 adsorption-desorption isotherms obtained using a Quantachrome Autosorb IQ2 instrument. The ζ− potential was measured by laser-Doppler anemometry in diluted samples (Zetasizer ® , NanoZS, ZEN 3600, Malvern Instruments, Worcestershire, UK). The nanoparticle concentration of stocks was obtained by thermogravimetric analysis (TGA), and the thermogravimetric curves were recorded using a Perkin Elmer TGA 7 thermobalance (Waltham, MA, USA), operating under nitrogen atmosphere, from room temperature to 850 ◦C, at a scanning rate of 10 ◦C min−1. 2.3. Removal of the Target Compounds Both the chemical structure and the absorption wavelength of the target compounds are presented in Figure S1. The colored bonds and elements in the semi-structural formulas represent the chromophore molecules (in the case of dyes). The colorant concentrations were measured by visible spectrophotometry in a Powerwave XS2 spectrophotometer purchased from BioTek Instruments (Winooski, VT, USA) using a calibration curve up to 10 mg L −1 . On the other hand, sulfamethoxazole concentration was quantified by a high-performance liquid chromatography with diode array detector instrument (HPLCDAD), using a Jasco X-LCTM from Jasco Analítica Spain S.L. (Madrid, MD, Spain) with a Gemini ®C18 chromatographic column (3 µ m, 110 Å, 150 × 4.6 mm) from Phenomenex Inc. (Torrance, CA, USA). Nanomaterials 2021,11, 533 5 of 18 2.4. Photodegradation Experiments The photodegradation experiments were made in samples of 10 mL and were irradiated using three 11 W PL-G23 fluorescent tubes (4000 K) purchased from Philips (Amsterdam, The Netherlands), with an irradiance of 1.08 W m −2 . Moreover, the experiments made under UV-A (365 nm) Pen-Ray ® Lamp 11SC-1L (Analytic Jena, TH, Germany) were irradiated with an intensity of 15.68 W m −2 . On the other hand, the reactor experiments were made in a methacrylate cylindric 5 L small-scale reactor irradiated by three PL-GL23 fluorescent tubes located around the reactor at the same distance used for 10 mL experiments. After degradation experiments, the nanoparticles were separated from the medium by applying an external magnetic field. The paramagnetic properties of the nanocatalysts allow their complete separation from the effluent enabling their reuse in subsequent cycles. Both for visible and ultraviolet light, the values of irradiance were measured using a PCE-UV34 dosimeter (PCE Group Ibérica S.L., Tobarra, CM-ES, Spain), in the range of 290–390 nm. Control experiments of adsorption, photodegradation, stability against pH and stability against hydrogen peroxide were performed to determine the influence of each parameter on the final degradation of the compounds. Thus, the influence of pH medium, incidence of light and hydrogen peroxide concentration was evaluated. In addition, combinations between two factors such as H 2 O 2 and light were studied. The photodegradation yield (Y) was determined using the following equation (Equation 5): Y(t) = (CX,i−CX,t)/CX,i, (5) where C is the concentration of the target compound (X) at the starting point (i) or at a given time (t). In addition, for the optimization tests, the experimental data were adjusted according to a pseudo-first-order kinetic model. The linearization of Equation (6) and the expression used to calculate the half-life (Equation (7)) are shown below: ln(CX,i/CX,t) = kt, (6) t1/2,X = ln(2)/k, (7) where t 1/2 is the half-life of the compound, and k is the kinetic degradation constant for the given conditions. 2.5. Wastewater Characterization To evaluate the effect of the matrix, all the reuse experiments were conducted in a real secondary effluent obtained from a WWTP plant located on Calo-Milladoiro (A Coruña, Spain). This treatment plant has been designed for a capacity of 9000 equivalent inhabitants and consists of a primary treatment for the removal of solids and fats, a biological secondary treatment, and a UV-C treatment for the removal of pathogenic microorganisms. Moreover, the anionic and cationic characterization was performed by ion chromatography, using a Metrohm 861 Advanced Compact IC (Herisau, AR, CH, Switzerland), and the total organic carbon (TOC) was analyzed in Shimadzy TOC 5000 (Kyoto, Japan), both equipped with an autosampler. The pH values were measured using a pH-meter GLP 21 of Crison (Barcelona, CT, ES, Spain). Toxicity assessment was performed by analyzing Aliivibrio fischeri bioluminescence in a Microtox ® M500 purchased from Modern Water (Guildford, SRY, GB, UK). The chemical characterization of the wastewater is shown in Table S1. 3. Results and Discussion 3.1. Nanoparticle Characterization In this work, different types of magnetite-based nanoparticles were tested for the degradation of different target compounds using Fenton-based methods. Bare and coated magnetite nanoparticles covered with polymers such as polyacrylic acid (PAA) and polyethyleneimine (PEI) were evaluated. These coatings modify the interaction of nanopar- Nanomaterials 2021,11, 533 6 of 18 ticles with the target compounds [ 40 ] and improve the kinetic performance [ 33 ]. Furthermore, the use of mesoporous silica (SBA-15) as a carrier for bare magnetic nanoparticles was considered to improve magnetic separation, kinetic performance and reusability. The characterization of free nanoparticles was performed by X-ray diffractometry (XRD), to analyze the crystallographic structure of magnetite. The main peaks found coincide with the characteristic peaks of polycrystalline magnetite (Fe 3 O 4 , JCPDS PDF-2 card #19-0629), indicating that the inverse spinel structure is present in the sample. Furthermore, the absence of other peaks reveals that there are no impurities in the samples. No changes in the peaks were observed upon coating with polyacrylic acid (PAA) or polyethyleneimine (PEI), demonstrating that the coating step does not modify the structure. The complete XRD characterization of this type of nanoparticles was described by Fernández et al. [ 25 ]. Figure S2 shows the Fe 3 O 4 @PAA/SBA15 nanoparticles as prepared and after 1, 5 and 10 reuse cycles. As observed in these nanoparticles, the characteristic peaks of magnetite appear in the XRD pattern, but a higher signal/noise ratio was obtained in the measurements due to the presence of SBA15 mesoporous silica matrix. The size distribution of unsupported nanoparticles was measured by TEM images, calculating the mean and standard deviation of Feret diameters for N > 100 measurements. These diameters range from 7.6 ± 2.7 nm for PEI-coated nanoparticles to 10.9 ± 3.0 nm for PAA-coated ones. The bare magnetite nanoparticles have a diameter of 8.2 ± 3.8 nm. In addition, the zeta potential of the NPs was determined at a near-to-optimum pH for Fenton reactions (pH 3), resulting in − 11.452 mV for the Fe 3 O 4 nanoparticles and − 4.531 and 29.942 mV respectively for the PAA and PEI-coated magnetite. The zeta potential values of Fe 3 O 4 @PAA/SBA-15 as a function of pH are shown in Figure 1. Considering the results obtained, the point of zero charge (PZC) of these nanoparticles was reached around pH 2, reaching a value of −4.27 ±0.21 mV for the working point (pH 3). Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 19 3. Results and discussion 3.1. Nanoparticle Characterization In this work, different types of magnetite-based nanoparticles were tested for the degradation of different target compounds using Fenton-based methods. Bare and coated magnetite nanoparticles covered with polymers such as polyacrylic acid (PAA) and polyethyleneimine (PEI) were evaluated. These coatings modify the interaction of nanoparticles with the target compounds [40] and improve the kinetic performance [33]. Furthermore, the use of mesoporous silica (SBA-15) as a carrier for bare magnetic nanoparticles was considered to improve magnetic separation, kinetic performance and reusability. The characterization of free nanoparticles was performed by X-ray diffractometry (XRD), to analyze the crystallographic structure of magnetite. The main peaks found coincide with the characteristic peaks of polycrystalline magnetite (Fe₃O₄, JCPDS PDF-2 card #19-0629), indicating that the inverse spinel structure is present in the sample. Furthermore, the absence of other peaks reveals that there are no impurities in the samples. No changes in the peaks were observed upon coating with polyacrylic acid (PAA) or polyethyleneimine (PEI), demonstrating that the coating step does not modify the structure. The complete XRD characterization of this type of nanoparticles was described by Fernández et al. [25]. Figure S2 shows the Fe₃O₄@PAA/SBA15 nanoparticles as prepared and after 1, 5 and 10 reuse cycles. As observed in these nanoparticles, the characteristic peaks of magnetite appear in the XRD pattern, but a higher signal/noise ratio was obtained in the measurements due to the presence of SBA15 mesoporous silica matrix. The size distribution of unsupported nanoparticles was measured by TEM images, calculating the mean and standard deviation of Feret diameters for N > 100 measurements. These diameters range from 7.6 ± 2.7 nm for PEI-coated nanoparticles to 10.9 ± 3.0 nm for PAA-coated ones. The bare magnetite nanoparticles have a diameter of 8.2 ± 3.8 nm. In addition, the zeta potential of the NPs was determined at a near-to-optimum pH for Fenton reactions (pH 3), resulting in −11.452 mV for the Fe₃O₄ nanoparticles and −4.531 and 29.942 mV respectively for the PAA and PEI-coated magnetite. The zeta potential values of Fe₃O₄@PAA/SBA-15 as a function of pH are shown in Figure 1. Considering the results obtained, the point of zero charge (PZC) of these nanoparticles was reached around pH 2, reaching a value of −4.27 ± 0.21 mV for the working point (pH 3). Figure 1. Zeta potential of SBA15/Fe₃O₄@PAA nanoparticles for a pH range between 2 and 8. The blue dotted line is a guide to the eye. On the other hand, the morphological characterization of the nanoparticles immobilised on SBA15 mesoporous silica (SBA15/Fe3O4@PAA and SBA15/Fe3O4@PEI) was performed using SEM and TEM images. Figure 2 shows the TEM and SEM images for Fe₃O₄@PAA/SBA15 nanoparticles, being the complete characterization of the rest of the nanoparticles in Fernández et al. [25]. -40 -30 -20 -10 0 10 2 4 6 8 Zeta potential, ζ(mV) pH Figure 1. Zeta potential of SBA15/Fe 3 O 4 @PAA nanoparticles for a pH range between 2 and 8. The blue dotted line is a guide to the eye. On the other hand, the morphological characterization of the nanoparticles immobilised on SBA15 mesoporous silica (SBA15/Fe 3 O 4 @PAA and SBA15/Fe 3 O 4 @PEI) was performed using SEM and TEM images. Figure 2shows the TEM and SEM images for Fe 3 O 4 @PAA/SBA15 nanoparticles, being the complete characterization of the rest of the nanoparticles in Fernández et al. [25]. Nanomaterials 2021,11, 533 7 of 18 Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 19 Figure 2. (A) TEM and (B) SEM images of Fe₃O₄@PAA/SBA15 nanoparticles as prepared and (C) TEM and (D) SEM images after ten reuse cycles. In this case, magnetite nanoparticles are isolated or forming aggregates on the surface of the 2D hexagonal structure of SBA-15. Although the deposition of magnetite inside the pores cannot be clearly observed by microscopy, the presence of magnetite nanoparticles and aggregates is expected also inside the pores of SBA15 considering the synthesis procedure. Adsorption/desorption isotherms with N2 were performed to obtain the surface and volumetric parameters. The adsorption isotherms and pore-size distribution of the SBA15/Fe₃O₄@PAA nanoparticles are depicted in Figure 3. C A B D Figure 2. ( A ) TEM and ( B ) SEM images of Fe 3 O 4 @PAA/SBA15 nanoparticles as prepared and ( C ) TEM and ( D ) SEM images after ten reuse cycles. In this case, magnetite nanoparticles are isolated or forming aggregates on the surface of the 2D hexagonal structure of SBA-15. Although the deposition of magnetite inside the pores cannot be clearly observed by microscopy, the presence of magnetite nanoparticles and aggregates is expected also inside the pores of SBA15 considering the synthesis procedure. Adsorption/desorption isotherms with N 2 were performed to obtain the surface and volumetric parameters. The adsorption isotherms and pore-size distribution of the SBA15/Fe3O4@PAA nanoparticles are depicted in Figure 3. For SBA15/Fe 3 O 4 @PAA, the specific surface was 220.3 m 2 g −1 , and the pore volume was 0.810 cm 3 g −1 , with an average size of 7.588 nm. On the other hand, for SBA15/Fe 3 O 4 @PEI, the synthesis was conducted using a PEI with Mw of 25,000. In this case, the specific surface was 157.3 m2g−1, and the pore volume was 0.44 cm3g−1. 3.2. Potential of Nanocomposites for Dye Removal The non-specific oxidation mechanisms of the Fenton and photo-Fenton processes can oxidize a wide range of chemicals, regardless of the structure of the molecule. In the first step of the evaluation of the oxidative capacity of nanocatalysts, anionic and cationic dyes were used as model compounds: copper phthalocyanine (Cu(II)Pc), methyl green (MG), eriochrome black T (EBT), reactive blue 19 (RB19) and Orange II (OII), whose characteristics and structures are shown in Figure S1. Their use allows a rapid assessment of the concentration by spectrophotometric measurements in order to determine the kinetic data. In this section, experiments have been performed using SBA15/Fe 3 O 4 @PAA nanoparticles, and considering similar conditions to those used by other authors (pH 3, 300 mg L −1 of H 2 O 2 ) [ 28 , 33 ] for a dye concentration of 10 mg L −1 with white light irradiation. The results obtained are shown in Figure 4. Nanomaterials 2021,11, 533 8 of 18 Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 19 Figure 3. Adsorption isotherms and pore-size distribution of SBA15/Fe3O4@PAA nanoparticles. For SBA15/Fe3O4@PAA, the specific surface was 220.3 m2 g-1, and the pore volume was 0.810 cm3 g-1, with an average size of 7.588 nm. On the other hand, for SBA15/Fe3O4@PEI, the synthesis was conducted using a PEI with Mw of 25,000. In this case, the specific surface was 157.3 m2 g-1, and the pore volume was 0.44 cm3 g-1. 3.2. Potential of Nanocomposites for Dye Removal The non-specific oxidation mechanisms of the Fenton and photo-Fenton processes can oxidize a wide range of chemicals, regardless of the structure of the molecule. In the first step of the evaluation of the oxidative capacity of nanocatalysts, anionic and cationic dyes were used as model compounds: copper phthalocyanine (Cu(II)Pc), methyl green (MG), eriochrome black T (EBT), reactive blue 19 (RB19) and Orange II (OII), whose characteristics and structures are shown in Figure S1. Their use allows a rapid assessment of the concentration by spectrophotometric measurements in order to determine the kinetic data. In this section, experiments have been performed using SBA15/Fe3O4@PAA nanoparticles, and considering similar conditions to those used by other authors (pH 3, 300 mg L−1 of H2O2) [28,33] for a dye concentration of 10 mg L−1 with white light irradiation. The results obtained are shown in Figure 4. Figure 3. Adsorption isotherms and pore-size distribution of SBA15/Fe3O4@PAA nanoparticles. Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 19 (a) (b) Figure 4. Comparison of Fenton (a) and photo-Fenton (b) potentials for dye removal. [Cat] = 200 mg L−1; [H2O2] = 300 mg L−1; [MC]0 = 10 mg L−1. Adsorption control (green), light/dark + H2O2 control (blue) and degradation (yellow). The adsorption control experiments were performed using the same concentration of nanoparticles without the use of light or hydrogen peroxide, and the final concentration was measured after 2 h. For the case of photo-Fenton process, a combined control of hydrogen peroxide and light was conducted, and for the Fenton reaction, this control was performed under dark conditions. In terms of dye removal, OII is the most recalcitrant compound, since it presents the lowest percentage of degradation. To study the contributions of the independent factors in the degradation due to each factor, stability tests were performed. The stability control for pH indicates that no degradation occurred in the tests performed at acidic conditions. Adsorption values were fairly different between the target compounds according to their anionic or cationic nature. The planar structure of Cu(II)Pc and its low solubility of phthalocyanines in water lead to complete adsorption on the catalyst with the molecules stacked on top of each other. On the other hand, the zeta potential (ζ) of SBA15/Fe3O4@PAA at pH 3 is −1.468 mV, which explains that the negative surface charge causes electrostatic repulsions with the anionic dyes (OII and RB). The enhanced adsorption of EBT could occur due to the protonation of the azo groups, reducing the partial negative charge of the molecule and aiding adsorption onto the catalyst. The cationic nature of MG was responsible for the partial adsorption of the dye: 59%. The catalyst performance was strongly affected by the interaction between nanoparticles and dyes as a function of the electrostatic charges of the molecules. Compounds that reached high adsorption values such as MG or EBT were similarly removed in the Fenton and photo-Fen- ton processes, rendering low light irradiation efficiency. This phenomenon was possibly due to the fact that their adsorption on the catalyst surface caused a shielding of the light irradiation and consequently reduced the removal yield. Degradation due to hydrogen peroxide in an acidic medium (pH 3) occurred to a greater extent for EBT and RB, indicating that they are less recalcitrant compounds than MG and OII, whose degradation was not complete at the end of the experiment. Moreover, as observed in Figure 4, the dyes do not act as photosensitizers because the degradation percentages in the absence of nanoparticles are not significant (especially in the cases of Cu(II)Ph, MG and OII). The kinetic constants obtained in the degradation of MG and OII were 0.889–1.322 h−1 and 0.543–1.141 h−1 for the Fenton and photo-Fenton processes, respectively. In all cases, the correlation coefficients were greater than 0.99, demonstrating the good fit of the pseudo-first model. The differences between Fenton and photo-Fenton kinetic constants were 49% for photo-Fenton and 15% for Fenton, showing significantly lower OII removal rates in the latter case. Considering that the OII dye is the most recalcitrant dye, as long as this compound is removed by the nanocomposite, the rest of the dyes should be removed. 3.3. Screening of Different Types of Nanoparticles 0% 20% 40% 60% 80% 100% Cu(II)Pc MG EBT RB19 OII 0% 20% 40% 60% 80% 100% Cu(II)Pc MG EBT RB19 OII Figure 4. Comparison of Fenton ( a ) and photo-Fenton ( b ) potentials for dye removal. [Cat] = 200 mg L −1 ; [H2O2] = 300 mg L−1 ; [MC] 0 = 10 mg L −1 . Adsorption control (green), light/dark + H 2 O 2 control (blue) and degradation (yellow). The adsorption control experiments were performed using the same concentration of nanoparticles without the use of light or hydrogen peroxide, and the final concentration was measured after 2 h. For the case of photo-Fenton process, a combined control of hydrogen peroxide and light was conducted, and for the Fenton reaction, this control was performed under dark conditions. In terms of dye removal, OII is the most recalcitrant compound, since it presents the lowest percentage of degradation. To study the contributions of the independent factors in the degradation due to each factor, stability tests were performed. The stability control for pH indicates that no degradation occurred in the tests performed Nanomaterials 2021,11, 533 9 of 18 at acidic conditions. Adsorption values were fairly different between the target compounds according to their anionic or cationic nature. The planar structure of Cu(II)Pc and its low solubility of phthalocyanines in water lead to complete adsorption on the catalyst with the molecules stacked on top of each other. On the other hand, the zeta potential ( ζ ) of SBA15/Fe 3 O 4 @PAA at pH 3 is − 1.468 mV, which explains that the negative surface charge causes electrostatic repulsions with the anionic dyes (OII and RB). The enhanced adsorption of EBT could occur due to the protonation of the azo groups, reducing the partial negative charge of the molecule and aiding adsorption onto the catalyst. The cationic nature of MG was responsible for the partial adsorption of the dye: 59%. The catalyst performance was strongly affected by the interaction between nanoparticles and dyes as a function of the electrostatic charges of the molecules. Compounds that reached high adsorption values such as MG or EBT were similarly removed in the Fenton and photo-Fenton processes, rendering low light irradiation efficiency. This phenomenon was possibly due to the fact that their adsorption on the catalyst surface caused a shielding of the light irradiation and consequently reduced the removal yield. Degradation due to hydrogen peroxide in an acidic medium (pH 3) occurred to a greater extent for EBT and RB, indicating that they are less recalcitrant compounds than MG and OII, whose degradation was not complete at the end of the experiment. Moreover, as observed in Figure 4, the dyes do not act as photosensitizers because the degradation percentages in the absence of nanoparticles are not significant (especially in the cases of Cu(II)Ph, MG and OII). The kinetic constants obtained in the degradation of MG and OII were 0.889–1.322 h −1 and 0.543–1.141 h −1 for the Fenton and photo-Fenton processes, respectively. In all cases, the correlation coefficients were greater than 0.99, demonstrating the good fit of the pseudo-first model. The differences between Fenton and photo-Fenton kinetic constants were 49% for photo-Fenton and 15% for Fenton, showing significantly lower OII removal rates in the latter case. Considering that the OII dye is the most recalcitrant dye, as long as this compound is removed by the nanocomposite, the rest of the dyes should be removed. 3.3. Screening of Different Types of Nanoparticles Different types of magnetite-based catalysts were tested for their performance in Orange II dye removal. The influence of magnetite coatings using TMAOH-stabilized Fe 3 O 4 , Fe 3 O 4 @PAA and Fe 3 O 4 @PEI and the immobilization on mesoporous silica on Fe 3 O 4 @PAA/SBA15 and Fe 3 O 4 @PEI/SBA15 nanocomposites were evaluated on the basis of the use of different types of catalyst with equivalent magnetite loading. A concentration of 200 mg L −1 of Fe 3 O 4 and H 2 O 2 was used for the degradation of 10 mg L −1 of Orange II at pH 3. The degradation results are shown in Figure 5. Under these conditions, Fe 3 O 4 @PAA/SBA15 provides the best results for OII degradation, achieving complete removal of the dye for Fenton and photo-Fenton processes. However, the estimation of the kinetic constant showed that the reaction rate under irradiation was approximately 5 times higher, with a half-life of 15 min in the case of photo-Fenton, showing a R 2 value greater than 0.98 in all cases. The high positive value of the zeta potential for Fe 3 O 4 @PEI nanoparticles at pH 3 causes a remarkable adsorption of the OII dye. In order to quantify the adsorption on the NPs catalyst, the pH of the medium was changed to 7 at the end of the degradation experiments to desorb the remaining dye. The difference between the initial and final concentration was less than 10%, denoting the predominant contribution of adsorption. This phenomenon could be due to the fact that the high interaction between the compound and catalysts caused surface saturation, preventing radical formation and decreasing reaction rates. On the other hand, the uncoated nanoparticles showed better results than those coated with PAA. However, their stability in aqueous media is lower, and they precipitate out of water to form agglomerates, which could hinder their industrial application. Although bare magnetite NPs showed a high zeta potential value in absolute terms compared to those coated with PAA or PEI, electrostatic repulsions were not sufficient to prevent agglomeration. The low size of the NPs causes Nanomaterials 2021,11, 533 16 of 18 ica is still observed. In addition, the stability of similar type of nanoparticles has been demonstrated by other works [33]. On the other hand, XRD measurements were made initially and after the first, fifth and tenth cycles (Figure S2) to observe changes in morphology and structure. When comparing the results, no differences were observed in the morphology of the catalyst or in the wideangle XRD. However, the diffraction peak at 0.7 ◦ 2 θ disappears in the low-angle XRD analysis for the reused cycles. Both the TEM/SEM images and the XRD characterization of catalyst before and after the treatment showed that there is no evidence of inactivation of catalyst or structure changes in the nanoparticles. Moreover, the kinetic rates remain stable as early as the second cycle, suffering only slight changes compared to the initial values. 4. Conclusions Advanced oxidation processes based on Fenton and photo-Fenton reactions are promising technologies in the field of removing emerging contaminants in wastewater. In this work, the use of coated magnetic nanoparticles or their combination in nanocomposites supported on mesoporous silica for the removal of different dyes and sulfamethoxazole was investigated. According to the results, the best performance was obtained for magnetite nanoparticles supported on SBA15 and coated with polyacrylic acid. The response surface methodology was applied to identify the influence of pH as the most influential variable on the degradation of the target compounds, followed by H 2 O 2 . Surprisingly, for the range studied, the influence of catalyst loading was not significant. These optimum conditions were applied in the stability tests. Reusability of the catalyst was demonstrated after 10 cycles of reuse, showing low variability in the kinetics. Moreover, in all these experiments, an external magnetic field was used to separate the magnetite nanoparticles from the treated effluent, which points to this technology as a suitable process to apply in wastewater treatment. Supplementary Materials: The following are available online at https://www.mdpi.com/2079-499 1/11/2/533/s1. Figure S1: Target compounds used in degradation experiments. Figure S2: XRD characterization of fresh nanoparticles and after cycles 1, 5 and 10. Figure S3: Kinetic constants (h −1 ) (blue), TOC (green) and OII removal (orange) for lab-scale Fenton experiments. Figure S4: Kinetic constants (h −1 ) (blue), TOC (green) and SMX removal (orange) for lab-scale Fenton experiments. Table S1: Chemical characterization of the wastewater (N = 7), Table S2: Statistical parameters obtained from Box-Behnken optimization. Table S3: MicroTox ® results of different treated effluents after OII removal at lab-scale experiments. Table S4: Optimization results for Fenton removal of OII using SBA15/Fe3O4@PAA. Author Contributions: Investigation, J.G.-R.; writing—original draft preparation, J.G.-R.; writing— review and editing, J.G.-R., L.F., C.V.-V. and M.T.M.; resources, Z.V.-O. and Y.P.; conceptualization, C.V.-V. and M.T.M.; supervision, L.F., C.V.-V., J.R., G.F. and M.T.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by two projects granted by Spanish Ministry of Science, Innovation and Universities: HP-NANOBIO Project, grant number PID2019-111163RB-I00; CLUSTERCAT Project, grant number MAT2015-67458-P; and Fundación Ramón Areces (Spain), grant number CIVP18A3940. Data Availability Statement: The study did not report any data. Acknowledgments: J.G.-R. thanks the Xunta de Galicia Counseling of Education, Universities and Vocational Training (ED481A-2019/172) and Spanish Ministries of Science and Innovation and Universities (FPU19/00461) for his predoctoral fellowships. The authors also thank Xunta de Galicia for the CRETUS (AGRUP2015/02) and AEMAT (ED431E-2018/08) Strategic Partnerships, and the use of RIAIDT-USC analytical facilities. The authors belong to the Galician Competitive Research Groups ED431C-2017/22 and ED431C-2017/29, co-funded by FEGermanyR. Conflicts of Interest: The authors declare no conflict of interest. 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