Temperature and nitrogen-induced modification of activated carbons for efficient catalytic ozonation of salicylic acid as a model emerging pollutant

The occurrence of emerging pollutants on effluents of wastewater treatment plants makes unfeasible their reutilization and consequently to comply with the sixth goal of 2030 Agenda for sustainable development. Thus, it is extremely important to find ways to remove these pollutants without compromising the quality of reclaimed water. Ozonation has been successfully explored for this purpose, but it still presents limitations towards some oxidant-resistant pollutants. To surpass this, the conversion of ozone (O3) into more reactive species is required, which can be accomplished by using catalysts. Carbon catalysts, such as activated carbons (ACs), represent a more environmentally attractive option than traditional metal-based catalysts, with the advantage of being easily modified to tune their textural and surface properties to the reaction chemistry. In this study, two different sources of ACs were tested in the catalytic ozonation of a frequently detected emerging pollutant: salicylic acid (SalAc). These ACs were submitted to thermal treatment under H2 and functionalization with N precursors, such as melamine and poly(ethyleneimine), to induce changes in the surface properties, especially in the nitrogen content. Although no correlation was found between the N-content and catalytic activity, the thermal treatment under H2 increased the mesopores surface area (Smeso), which reflected in greater catalytic activity. As that, the best-performing AC was the one with the highest Smeso, which revealed also to be resistant to O3 and able to convert O3 into more reactive species, evidenced by the capacity of oxalic acid, a well-known ozone-resistant by-product. The same AC was then submitted to three consecutive reutilization cycles and a more significant activity loss was observed in terms of SalAc degradation rate (⁓ 40%) then total organic carbon removal (⁓ 25%), from the first to the third cycle. This decline in efficiency was ascribed to the presence of by-products adhered to the catalyst surface, which impede its ability to react effectively with O3.

Combination of CNTs with Classical Drugs for Treatment in Human Colorectal Adenocarcinoma (HT-29) Cell Line

Due to the increase in new types of cancer cells and resistance to drugs, conventional cancer treatments are sometimes insufficient. Therefore, an alternative is to apply nanotechnology to biomedical areas, minimizing side effects and drug resistance and improving treatment efficacy. This work aims to find a promising cancer treatment in the human colorectal adenocarcinoma cell line (HT-29) to minimize the viability of cells (IC₅₀) by using carbon nanotubes (CNTs) combined with different drugs (5-fluorouracil (5-FU) and two repurposing drugs-tacrine (TAC) and ethionamide (ETA). Several CNT samples with different functional groups (-O, -N, -S) and textural properties were prepared and characterized by elemental and thermogravimetry analysis, size distribution, and textural and temperature programmed desorption. The samples that interacted most with the drugs and contributed to improving HT-29 cell treatment were samples doped with nitrogen and sulfur groups (CNT-BM-N and CNT-H₂SO₄-BM) with IC₅₀ 1.98 and 2.50 µmol∙dm^-3 from 5-FU and 15.32 and 15.81 µmol∙dm^-3 from TAC. On the other hand, ETA had no activity, even combined with the CNTs. These results allow us to conclude that the activity was improved for both 5-FU and TAC when combined with CNTs.

Degradation of p-Nitrophenol by activated persulfate with carbon-based materials

The removal of p-nitrophenol (PNP) from wastewater was evaluated by the activated persulfate process using different materials - carbon xerogels (XG), carbon nanotubes (CNT), and activated carbon (AC) -, and also using such materials doped with nitrogen (XGM, CNTM and ACM). These carbon materials were impregnated with 2 wt.% of iron and tested in the oxidative process to assess the influence of their textural and surface chemical properties. The carbon-based materials' properties influence the efficiencies of the adsorption and oxidative processes; in adsorption, the materials with higher specific surface areas (S_BET), i.e. AC (824 m²/g) and Fe/AC (807 m²/g), have shown to be the most promising (having achieved a PNP removal of about 20%); on the other hand, in the activated persulfate process the carbon or iron-containing carbon materials with the highest mesoporous areas (S_meso) were the preferential ones - XG and Fe/XG, respectively - reaching removals of 47.3% and 75.7% for PNP and 44.9 and 63.3% for TOC, respectively. Moreover, the presence of nitrogen groups on the samples' surface benefits both processes, being found that PNP degradation and mineralization increase with the nitrogen content. The stability of the best materials (XGM and Fe/XGM) was evaluated during four cycles, being noticed that while XGM lost catalytic activity, the Fe/XGM sample remained stable without leaching of iron. The quantification of intermediate compounds formed during persulfate oxidation was performed, and only oxalic acid was detected, in addition to PNP, being that their contribution to the TOC measured was higher than 99%. Experiments carried out in the presence of radical scavengers proved that only the sulfate radical is present under the acidic conditions used. Complete PNP oxidation and TOC removal of ∼96% were reached for the activated persulfate process, proving to be more attractive than the Fenton one.

Gaseous toluene abatement by the heterogeneous Fenton-like process using iron/carbon-coated monolith as catalyst: Proof of concept

The degradation of toluene from a gas stream by the heterogeneous Fenton process was evaluated over a carbon-coated monolith impregnated or not with iron as catalyst in a bubble column reactor (BCR). The carbon-coated monolith support (CM) was prepared by chemical vapor deposition and the catalyst (CM impregnated with iron - herein called CM-Fe) by adsorption. In the screening of processes (absorption, adsorption and reaction), it was shown that the heterogeneous Fenton process catalyzed by CM-Fe presents the best efficiency (toluene transfer (η) = 10 × 10^-3 mol, for 300 mL of liquid solution and 0.69 g of catalyst). Finally, the stability of CM and CM-Fe was evaluated, wherein ten consecutive runs were carried out, the results showing a considerable deactivation of CM during the first five cycles. In contrast, the CM-Fe sample only slightly decreases its activity from the 1st to 2nd cycle (due to a small amount of iron leached from the monolith, 0.7%), remaining stable after that, which is important for applying this technology at the industrial level. This work showed for the first time that the treatment of gaseous effluents containing organic compounds by the Fenton process (which takes place in the liquid phase) using a carbon-coated monolith impregnated with iron is plausible, so the proof of concept was successfully accomplished.

Solar Light-Induced Methylene Blue Removal over TiO2/AC Composites and Photocatalytic Regeneration

TiO₂-containing photocatalysts, which combine TiO₂ with carbon-based materials, are promising materials for wastewater treatment due to synergistic photodegradation and adsorption phenomena. In this work, TiO₂/AC composites were produced by the in situ immobilization of TiO₂ nanoparticles over activated carbon (AC) derived from spent coffee grains, using different TiO₂/AC proportions. The TiO₂/AC composites were tested as adsorbents (dark) and as photocatalysts in a combined adsorption+photocatalytic process (solar irradiation) for methylene blue (MB) removal from ultrapure water, and from a secondary effluent (SecEf) of an urban wastewater treatment plant. All the materials were characterized by XRD (X-ray powder diffraction), N₂ adsorption–desorption isotherms at −196 °C, SEM (scanning electron microscopy), UV-Vis diffuse reflectance, FTIR (Fourier-transform infrared spectroscopy), TPD (temperature programmed desorption), XPS (X-ray photoelectron spectroscopy) and TGA (thermogravimetric analysis). The TiAC60 (60% C) composite presented the lowest band gap (1.84 eV), while, for TiAC29 (29% C), the value was close to that of bare TiO₂ (3.18 vs. 3.17 eV). Regardless of the material, the solar irradiation improved the percentage of MB discolouration when compared to adsorption in dark conditions. In the case of simultaneous adsorption+photocatalytic assays performed in ultrapure water, TiAC29 presented the fastest MB removal. Nevertheless, both TiAC29 and TiAC60 led to excellent MB removal percentages (96.1–98.1%). UV-induced photoregeneration was a promising strategy to recover the adsorption capacity of the materials, especially for TiAC60 and AC (>95%). When the assays were performed in SecEf, all the materials promoted discolouration percentages close to those obtained in ultrapure water. The bulk water parameters revealed that TiAC60 allowed the removal of a higher amount of MB, associated with the overall improvement of the SecEf quality.

Highly N2-Selective Activated Carbon-Supported Pt-In Catalysts for the Reduction of Nitrites in Water

The catalytic reduction of nitrites over Pt-In catalysts supported on activated carbon has been studied in a semi-batch reactor, at room temperature and atmospheric pressure, and using hydrogen as the reducing agent. The influence of the indium content on the activity and selectivity was evaluated. Monometallic Pt catalysts are very active for nitrite reduction, but the addition of up to 1 wt% of indium significantly increases the nitrogen selectivity from 0 to 96%. The decrease in the accessible noble metal surface area reduces the amount of hydrogen available at the catalyst surface, this favoring the combination of nitrogen-containing intermediate molecules to promote the formation of N₂ instead of being deeply hydrogenated into NH⁴⁺. Several activated carbon-supported Pt-In catalysts, activated under different calcination and reduction temperatures, have been also evaluated in nitrite reduction. The catalyst calcined and reduced at 400°C showed the best performance considering both the activity and the selectivity to nitrogen. This enhanced selectivity is ascribed to the formation of Pt-In alloy. The electronic properties of Pt change upon alloy formation, as it is demonstrated by XPS.

Zero-valent iron supported on nitrogen-doped carbon xerogel as catalysts for the oxidation of phenol by fenton-like system

Nitrogen-free and nitrogen-doped carbon xerogel materials, from urea and melamine precursors, were prepared at different pH and evaluated as adsorbents/catalysts in the removal of phenol. Then, zero-valent iron (ZVI) was supported on these carbon xerogel materials and its activity was again evaluated for phenol removal by adsorption and catalytic wet peroxide oxidation (CWPO). The prepared samples were characterized by N₂ adsorption at -196°C, pH at the point of zero charge (pH_PZC) and elemental analysis. The textural properties of the N-free and N-doped carbon xerogels are strongly influenced by pH of the preparation solution and precursor used. The presence of ZVI on all carbon xerogel supports improved the phenol removal efficiency. ZVI supported on urea- and melamine-doped carbon xerogels show a good performance, reaching above 87% phenol conversion after 60 min of CWPO. On the contrary, pure adsorption and CWPO using the same materials without the presence of ZVI gives low phenol removal efficiency. A correlation was found between the activity of ZVI catalysts in CWPO and the N-content of the supports.

Effect of cobalt loading on the solid state properties and ethyl acetate oxidation performance of cobalt-cerium mixed oxides

Cobalt-cerium mixed oxides were prepared by the wet impregnation method and evaluated for volatile organic compounds (VOCs) abatement, using ethyl acetate (EtAc) as model molecule. The impact of Co content on the physicochemical characteristics of catalysts and EtAc conversion was investigated. The materials were characterized by various techniques, including N₂ adsorption at -196°C, scanning electron microscopy (SEM), X-ray diffraction (XRD), H₂-temperature programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS) to reveal the structure-activity relationship. The obtained results showed the superiority of mixed oxides compared to bare CeO₂ and Co₃O₄, demonstrating a synergistic effect. The optimum oxidation performance was achieved with the sample containing 20wt.% Co (Co/Ce atomic ratio of ca. 0.75), in which complete conversion of EtAc was attained at 260°C. In contrast, temperatures above 300°C were required to achieve 100% conversion over the single oxides. Notably, a strong relationship between both the: (i) relative population, and (ii) facile reduction of lattice oxygen with the ethyl acetate oxidation activity was found, highlighting the key role of loosely bound oxygen species on VOCs oxidation. A synergistic Co-Ce interaction can be accounted for the enhanced reducibility of mixed oxides, linked with the increased mobility of lattice oxygen.

Naphthopyran-Based Silica Nanoparticles as New High-Performance Photoresponsive Materials

Hybrid nanomaterials based on the covalent grafting of silylated naphthopyrans (NPTs) onto silica nanoparticles (SiO2 NPs) were successfully prepared and studied as new photochromic materials. They were prepared by a two-step protocol consisting of (i) NPTs (derivatives from 2H-naphtho[1,2-b]pyran (2H-NPT) and 3H-naphtho[2,1-b]pyran (3H-NPT)) silylation by a microwave-assisted reaction between hydroxyl-substituted NPTs and 3-(triethoxysilyl)propyl isocyanate, followed by (ii) covalent post-grafting onto SiO2 NPs. In order to study the role of the silylation step, the analogous non-silylated nanomaterials were also prepared by direct adsorption of NPTs. The characterization techniques confirmed the successful NPTs silylation and subsequent grafting to SiO2 NPs. All SiO2-based nanomaterials revealed photoswitching behavior, following a biexponential decay. The SiO2 NPs functionalized with silylated 3H-NPTs (SiO2@S3 and SiO2@S4) presented the most promising photochromic properties, showing fast coloration/decoloration kinetics (coloring in 1 min under UV irradiation and fading in only 2 min) and high values of total color difference (ΔE*ab = 30-50). Also, the 2H-NPTs-based SiO2 NPs (SiO2@S1 and SiO2@S2) presented fast coloration and good color contrasts (ΔE*ab = 54), but slower fading kinetic rates, taking more than 2 h to return to their initial color. In contrast, the SiO2 NPs functionalized with non-silylated NPTs (SiO2@1 and SiO2@3) showed weaker color contrasts (ΔE*ab = 6-10) and slower fading kinetics, proving that the NPT silylation step was crucial to enhance the photochromic behavior of SiO2 NPs based on NPTs. Furthermore, the silylated-based nanomaterials showed good photostability upon prolonged UV light exposure, keeping their photochromic performance unchanged for at least 12 successive UV/dark cycles, anticipating interesting technological applications in several areas.

Ozonation of textile effluents and dye solutions under continuous operation: Influence of operating parameters

Ozonation experiments were carried out under continuous operation in a bubble column. The effect of several parameters (inlet dye concentration, applied ozone dose, pH and conductivity) in colour and TOC removal of an acid dye solution was investigated with the aim to optimize the operation conditions. The ozone consumption was measured in each experiment. Ozonation was found to be effective for decolourisation of an acid dye; however, it only has a slight effect on TOC removal. Increasing the inlet dye concentration leads to a decrease in the decolourisation efficiency and an increase in the ozone consumption. The decolourisation increases with the applied ozone dose. Colour removal efficiencies for different ozone doses were between 76 and 100%. In the pH range 5-9, the decolourisation efficiency decreases with pH only when buffered solutions were used. The presence of salt decreases the decolourisation efficiency. Several dyes of different classes were also studied and ozonation was found to be effective for decolourisation but considerably less efficient for TOC removal. Under the conditions tested, only the disperse and sulphur dyes presented a colour removal lower than 86%. Practical application of this process was validated by treating two industrial textile effluents collected after two different biological treatments.