Browsing by Author "Ribeiro, Rui"
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- Activated carbon xerogels for the removal of azo dyes by adsorption and catalytic wet peroxide oxidationPublication . Ribeiro, Rui; Fathy, Nady; Silva, Adrián; Faria, Joaquim; Gomes, Helder
- Activated carbon xerogels for the removal of the anionic azo dyes Orange II and Chromotrope 2R by adsorption and catalytic wet peroxide oxidationPublication . Ribeiro, Rui; Fathy, Nady; Attia, Amina; Silva, Adrián; Faria, Joaquim; Gomes, HelderActivated carbon xerogels (ACXs) were tested for the removal of azo dyes in aqueous solutions, either by adsorption or by catalytic wet peroxide oxidation (CWPO). Two azo dyes, Orange II (OII) and Chromotrope 2R (C2R), were chosen as model pollutants. The ACXs were produced by activation of an organic resorcinol–formaldehyde xerogel (RFX). Three different activation procedures were carried out producing five distinct ACXs: steam at 1073 K (ACX-S), chemical impregnation with H3PO4 at 773 K (ACX-P) and alkali activation with dry KOH at 973 K (ACX-K), using three different mass ratios of KOH/RFX, namely 1:1 (ACX-K1), 2:1 (ACX-K2) and 4:1 (ACX-K4). The results obtained in the adsorption experiments carried out at pH = 3, T = 303 K, adsorbent load of 0.1 g L 1 and azo dye concentration of 100 mg L 1 show that the interaction between the carbon materials and the anionic dyes is enhanced with the basicity of the carbon surfaces. ACX-K materials, the carbon materials with higher basicity amongst those prepared, exhibit high adsorption performances for the removal of both dyes, namely from over 215 mg g 1 (for adsorption of C2R on ACX-K2 after 150 min) up to 499 mg g 1 (for adsorption of OII on ACX-K4 at the same period of time). Furthermore, with ACX-K materials in CWPO (i.e., using H2O2) increments in the removal of C2R as high as 33%, 24% and 20%, in comparison to the removals obtained by adsorption, where obtained when ACX-K1, ACX-K2 and ACX-K4 were respectively tested at 303 K. Increasing the operating temperature (T = 323 K), the removal increments achieved by CWPO, compared to the removals obtained by adsorption at the same temperature, increase 67%, 59% and 49%, when ACX-K1, ACX-K2 and ACX-K4 were respectively tested. Recycling studies with ACX-K1 puts in evidence the high stability of this catalyst in CWPO, since it was observed, after a first reaction run, that the catalytic activity of this material is not affected by its successive reuse. Increasing the operating temperature (T = 323 K) and the adsorbent load (0.5 g L 1), ACX-K4 is able to completely remove the C2R content by adsorption. In the case of ACX-K1 and ACX-K2, adsorption removals over 97% of the C2R content are attainable.
- Activation of sodium persulfate by magnetic carbon xerogels (CX/CoFe) for the oxidation of bisphenol A: Process variables effects, matrix effects and reaction pathwaysPublication . Outsiou, Alexandra; Frontistis, Zacharias; Ribeiro, Rui; Antonopoulou, Maria; Konstantinou, Ioannis; Silva, Adrián; Faria, Joaquim; Gomes, Helder; Mantzavinos, DionissiosAn advanced oxidation process comprising sodium persulfate (SPS) and a novel magnetic carbon xerogel was tested for the degradation of bisphenol A (BPA), a model endocrine-disrupting compound. The catalyst, consisting of interconnected carbon microspheres with embedded iron and cobalt microparticles, was capable of activating persulfate to form sulfate and hydroxyl radicals at ambient conditions. The pseudo-first order degradation rate of BPA in ultrapure water (UPW) was found to increase with (i) increasing catalyst (25–75 mg/L) and SPS (31–250 mg/L) concentrations, (ii) decreasing BPA concentration (285–14,200 μg/L), and (iii) changing pH from alkaline to acidic values (9–3). Besides UPW, tests were conducted in drinking water, treated wastewater, groundwater and surface water; interestingly, the rate in UPW was always lower than in any other matrix containing several organic and inorganic constituents. The effect of natural organic matter (in the form of humic acids) and alcohols was detrimental to BPA degradation owing to the scavenging of radicals. Conversely, chlorides at concentrations greater than 50 mg/L had a positive effect due to the formation and subsequent participation of chlorine-containing radicals. Liquid chromatography time-of-flight mass spectrometry was employed to identify major transformation by-products (TBPs) of BPA degradation in the absence and presence of chlorides; in the latter case, several chlorinated TBPs were detected confirming the role of Cl-related radicals. Based on TBPs, main reaction pathways are proposed.
- Beyond batch experiments: unveiling the potential of bimetallic carbon xerogels for catalytic wet peroxide oxidation of hospital wastewater in continuous modePublication . Silva, Adriano S.; Roman, Fernanda; Ribeiro, Rui; Garcia, Juan; Gomes, HelderSingle- and bimetallic carbon xerogels were prepared by incorporating iron and iron-cobalt precursors during their synthesis, respectively, and tested in the catalytic wet peroxide oxidation (CWPO) of ibuprofen spiked into a simulated matrix in batch mode. The bimetallic catalyst outperformed single and non-metallic catalyst by 25 and 85% after 360 min of reaction, at mild temperature (30 °C). The best-performing catalyst was further used to treat hospital wastewater in a CWPO system operating in full continuous mode. Process optimization was carried out considering different catalyst loads, temperatures, and pH. The results obtained showed that the best conditions are initial pH 3, T = 80 °C, and a catalyst load of 35.4 mg cm− 3. Having maintained values of chemical oxygen demand (COD) removals as high as 80% after 24 h of continuous operation, the results herein reported revealed the high potential of the bimetallic carbon xerogel for CWPO of hospital wastewater beyond conventional applications in batch mode. Despite some catalytic deactivation, the bimetallic carbon xerogel still delivered a mineralization degree as high as 55% of the initial total organic carbon (TOC) content of the hospital wastewater in the third 24-h cycle of CWPO in continuous mode of operation with successive catalyst reuse, as opposed to a 73% TOC removal in the first cycle. Therefore, our results open prospects for the implementation of CWPO for hospital wastewater treatment in continuous mode of operation.
- Carbon based materials for photocatalytic solar applications in water treatmentPublication . Faria, Joaquim; Silva, Adrián; Gomes, Helder; Ribeiro, Rui; Pinho, Maria; Morales-Torres, Sergio; Figueiredo, José; Pastrana-Martínez, Luisa; Silva, CláudiaWhen dealing with chemical wastewater treatments one depends on the addition of auxiliary oxidants, which may include molecular oxygen, ozone, and hydrogen peroxide, working on their own, or activated by means of a catalyst or photocatalyst. Typical solutions are the thermal processes at elevated temperatures and pressures. Alternatively, it is possible to use heterogeneous photocatalysis at room temperature and atmospheric pressure. With the development of new carbon allotropes a whole range of advanced oxidation processes, traditionally based on the action of the HO● radical, can be explored using carbon materials as common denominator.
- Carbon materials with different properties for the removal of 2-nitrophenol by catalytic wet peroxide oxidationPublication . Ribeiro, Rui; Silva, Adrián; Faria, Joaquim; Gomes, Helder
- Carbon nanotubes as base materials for water treatment processesPublication . Faria, Joaquim; Ribeiro, Rui; Silva, Adrián; Silva, Cláudia; Figueiredo, José; Gomes, HelderChemical wastewater treatments are dependent on the addition of auxiliary oxidants, which may include molecular oxygen, ozone, and hydrogen peroxide, working on their own, or activated by means of a specialized catalyst or photocatalyst. Chemical treatments are by nature definitive processes, since they can lead to complete mineralization of the existing pollutants. However, this is seldom the case, when looking for a rational solution from the socio-economical point of view. In the case of industrial effluents, special treatments are often required, even when only a partial oxidative degradation is targeted, due to the complex nature of the pollutants (e.g. dyes, pharmaceuticals, oils, organics, inorganics and bio-compounds). Some compounds, like nitrophenols, are particularly refractory to aerobic biodegradation and in addition to that toxic, requiring strong oxidative solutions. Typical solutions are the thermal processes at elevated temperatures and pressures, or using metal supported catalysts. Alternatively, it is possible to use heterogeneous photocatalysis based on the efficient production of hydroxyl radicals. Somewhere between these two limits lies the catalytic wet peroxide oxidation (CWPO), an advanced oxidation process (AOP) involving the use of hydrogen peroxide (H2O2) as oxidation source and a suitable catalyst (typically iron based catalysts). The main role of the catalyst is to promote H2O2 decomposition through the formation of hydroxyl radicals (HO●) with high oxidizing potential, effective in the destruction of a huge range of pollutants [1,2]. This type of technology is especially attractive due to the use of mild conditions, simple equipment and the environmental safe oxidant H2O2.
- Carbon nanotubes as catalysts for catalytic wet peroxide oxidation of highly concentrated phenol solutions: towards process intensificationPublication . Pinho, Maria; Gomes, Helder; Ribeiro, Rui; Faria, Joaquim; Silva, AdriánCommercial multi-walled carbon nanotubes with different properties (two samples from Sigma-Aldrich,SA1 and SA2; one sample from Nanocyl, NC; and two samples from Shenzhen Nanotech, SZ and LSZ),and SA2 modified by hydrothermal treatment with concentrated sulfuric acid (SA2-H), were tested ascatalysts in wet peroxide oxidation. Phenol was selected as model compound since it represents a classof noxious compounds for human health and for the environment and, due to this, phenol is typically considered in wastewater treatment studies. The experiments were carried out under the following inten-sified conditions: phenol concentration = 4.5 g L−1, hydrogen peroxide concentration = 25 g L−1, catalystload = 2.5 g L−1, pH 3.5, T = 353 K and 24 h.The results demonstrated that phenol is poorly adsorbed in this type of carbon materials (11% as max-imum when using the NC sample). However, in the catalytic experiments, complete removal of phenol isachieved when using some of the carbon nanotubes (SA1, NC and SA2), together with a remarkable total organic carbon removal (77, 69 and 67%, respectively). These materials have the less pronounced acidiccharacter, which is often considered favorable for oxidation reactions in advanced oxidation processesand may explain the higher performance of SA1, NC and SA2 regarding the other materials. Leaching of Fespecies into the solution was also observed in all cases (that can also have some influence on the degra-dation of phenol), SA1 leading to the highest concentration of Fe species leached (26 mg L−1), followedby SA2 (2 mg L−1) and NC (1 mg L−1).Considering the lower Fe leaching levels observed for SA2 and NC, these catalysts were then testedin consecutive reusability cycles. SA2 showed a superior performance than NC, but temperature-programmed desorption as well as thermogravimetric analysis suggested that the carbon material isoxidized by hydrogen peroxide at the employed conditions and/or that carboxylic acids are adsorbed onthe catalyst surface after consecutive runs (mainly after the first use). However, only a slight decrease inthe catalyst activity was observed.
- Carbon nanotubes as catalysts for wet peroxide oxidation: structure-reactivity relationshipsPublication . Ribeiro, Rui; Martin-Martinez, Maria; Machado, Bruno; Serp, Philippe; Morales-Torres, Sergio; Silva, Adrián; Figueiredo, José; Faria, Joaquim; Gomes, HelderMagnetic neat and N-doped carbon nanotubes with different properties have been synthesized by chemical vapour deposiüon and tested in the catalytic wet peroxide oxidation of 4-nitrophenol solutions (5 g L') at relatively mild operating conditions (atmospheric pressure, T = 50 °C, pH = 3)~using a catalyst load = 2.5 g L-' and [H202]o = 17.8 g L-1. The results demonstrate that the catalyst hydrophobicity/ hydrophilicity is a detenninant property in the CWPO reaction, since it affects the rate ofH202 decomposition. The controlled formation ofreactive radicais (HO* and HOO*) at hydrophobic surfaces avoids the formation of non-reactive species (02 and H20), increasing.
- Carbon nanotubes: a suitable material for catalytic wet peroxide oxidation of organic pollutants?Publication . Ribeiro, Rui; Silva, Adrián; Faria, Joaquim; Gomes, HelderCarbon materials, such as activated carbons (AC), graphite and activated carbon xerogels, have been explored as metal-free catalysts for the catalytic wet peroxide oxidation (CWPO) of bio-refractory organic compounds, such as azo dyes and phenolic compounds [1-3]. At the same time, the application of carbon nanomaterials in catalysis, such as carbon nanotubes (CNT), has grown exponentially [4]. In the present work, commercial multiwalled carbon nanotubes (MWNT) were used in the CWPO of 2-nitrophenol and its activity compared to a commercial AC, used as received and after chemical modifications.