Catalyst-free persulfate activation by UV/visible radiation for secondary urban wastewater disinfection.

This study focuses on the treatment of secondary urban wastewater (W) to improve the effluent quality aiming at the reduction of pathogenic microorganisms for the safe reuse of the treated wastewater (TW). Catalyst-free persulfate activation by radiation-based oxidation was applied as a treatment technology. A parametric study was carried out to select the best operating conditions. Total enterobacteria inactivation (quantified by the log reduction (CFU/100 mL)) was achieved when using [S2O82-] = 1 mM, pH = 8.5 (natural pH of W), T = 25 °C, and I = 500 W/m2. However, storing TW for 3 days promoted the regrowth of bacteria, risking its reutilization. Therefore, in this study, and for the first time, the potential beneficial role of inoculation of wastewater treated by the radiation-activated persulfate process with a diverse bacterial community was evaluated in order to control the regrowth of potentially harmful microorganisms through bacterial competition. For this, TW was diluted with river water (R) in the volume percentages of 5, 25, and 50 (percentages refer to R content), and enterobacteria and total heterotrophs were enumerated before and after storage for 72 h. The results showed total heterotrophs and enterobacteria regrowth for TW and R + TW diluted 5 and 25% after storage. However, for R + TW diluted 50%, only the total heterotrophs regrew. Hence, the treated wastewater generated by the oxidative process diluted with 50% river water complies with the legislated limits for reuse in urban uses or irrigation.

Use of Pd-Ag Membrane Reactors for Low-Temperature Dry Reforming of Biogas-A Simulation Study.

Biogas is a valuable renewable energy source that can help mitigate greenhouse emissions. The dry reforming of methane (DRM) offers an alternative hydrogen production route with the advantage of using two main greenhouse gases, CO2 and CH4. However, its real application is limited mainly due to catalyst deactivation by coke formation and the reverse water gas shift (RWGS) reaction that can occur in parallel. Additionally, the typical dry reforming temperature range is 700-950 °C, often leading to catalyst sintering. A low-temperature DRM process could be in principle achieved using a membrane reactor (MR) to shift the dry reforming equilibrium forward and inhibit the RWGS reaction. In this work, biogas reforming was investigated through the simulation of MRs with thin (3.4 µm) and thick (50 µm) Pd-Ag membranes. The effects of the feed temperature (from 450 to 550 °C), pressure (in the range of 2-20 bar), and biogas composition (CH4/CO2 molar ratios from 1/1 to 7/3) were studied for the thin membrane through the calculation and comparison of several process indicators, namely CH4 and CO2 conversions, H2 yield, H2/CO ratio and H2 recovery. Estimation of the CO-inhibiting effect on the H2 molar flux through the membrane was assessed for a thick membrane. Simulations for a thin Pd-Ag MR show that (i) CO2 and CH4 conversions and H2 yield increase with the feed temperature; (ii) H2 yield and average rate of coke formation increase for higher pressures; and (iii) increasing CH4/CO2 feed molar ratio leads to higher H2/CO ratios, but lower H2 yields. Moreover, simulations for a thick Pd-Ag MR showed that the average H2 molar flux decreases due to the CO inhibiting effect (ca. 15%) in the temperature range considered. In conclusion, this work showed that for the considered simulation conditions, the use of an MR leads to the inhibition of the RWGS reaction and improves H2 yield, but coke formation and CO inhibition on H2 permeation may pose limitations on its practical feasibility, for which proper strategies must be explored.

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 (SBET), 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 (Smeso) 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.

Disinfection of treated urban effluents for reuse by combination of coagulation/flocculation and Fenton processes.

In this study, a combination of coagulation/flocculation and Fenton processes was studied as tertiary treatment in order to generate treated water susceptible to reuse. The combination of both processes has never been applied in disinfection of real urban wastewater. The best removals of turbidity and enterobacteria were achieved when applying a coagulant (FeCl3) dosage of 120 mg/L and the natural pH of the effluent (7.14). The following Fenton reaction presented the maximal enterobacteria inactivation after 120 min at 25 °C, when using hydrogen peroxide and added iron concentrations of 100 mg/L and 7 mg/L, respectively. The abundance of antibiotic resistant (amoxicillin and sulfamethoxazole) enterobacteria and total enterobacteria, enterococci, and heterotrophs, and antibiotic resistance genes - ARG - (sul1, blaTEM and qnrS) was evaluated before and after each step of the treatment. Values below 10 CFU/100 mL were achieved for total and resistant cultivable enterobacteria immediately after treatment and after storage for 72 h, therefore meeting the strictest limit imposed for E. coli. Physico-chemical parameters also met the established limits for water reuse. Despite harbouring a rich and diverse bacterial community, the final stored disinfected wastewater contained high relative abundance of potentially hazardous bacteria. Such results point out the need of a deep microbiological characterization of treated wastewater to evaluate the risk of its reuse in irrigation.

Degradation of Toluene from Gas Streams by Heterogeneous Fenton Oxidation in a Slurry Bubble Reactor with Activated Carbon-Based Catalysts.

A novel approach for the treatment of volatile organic compounds from gaseous streams was developed. In order to accomplish this, a semi-batch bubble reactor was used, aiming to assess the toluene (selected as model compound) degradation from gaseous streams via heterogeneous Fenton oxidation. Activated carbon-based catalysts-metal-free or iron-impregnated-with different textural and chemical surface properties were used for the first time as catalysts, in order to degrade gaseous toluene using such technology. Complementary characterization techniques, such as nitrogen adsorption at -196 °C, elemental analysis, pH at the point of zero charge (pHPZC), inductively coupled plasma optical emission spectrometry (ICP-OES) and transmission electron microscopy (TEM), were used. The materials' chemical surface properties, particularly the presence of N-surface groups, were herein found to play an important role in toluene adsorption and catalytic performance. The maximum amount of toluene transferred, 6.39 × 10-3 mol, was achieved using melamine-doped activated carbon (N-doped material) that was impregnated with iron (sample herein called ACM-Fe). This iron-based catalyst was found to be quite stable during three reutilization cycles.

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.

Intensification strategies for cytostatics degradation by ozone-based processes in aqueous phase.

Over the past decade there has been an increasing concern on the presence of cytostatics (also known as anticancer drugs) in natural waterbodies. The conventional wastewater treatments seem not to be effective enough to remove them, and therefore new processes must be considered. This work investigates the performance of ozonation (O3), catalytic ozonation (O3/Fe2+) and peroxone (O3/H2O2) processes, under dark or UV radiation conditions, for the degradation of cytostatics of worldwide concern. The degradation of bicalutamide (a representative of recalcitrant cytostatics) was firstly assessed in batch and then in a tubular column reactor (continuous flow mode runs) using a wastewater treatment plant (WWTP) secondary effluent. Bicalutamide removal ranged between 66 % (O3) and 98 % (O3/H2O2/UV) in continuous flow mode runs, the peroxone process being the most effective. The performance of these processes was then assessed against a mixture of twelve cytostatics of worldwide concern spiked in the WWTP effluent (25-350 ng/L). After treatment, seven cytostatics were completely removed, whereas the five most recalcitrant ones were eliminated to an extent of 8-92 % in O3/H2O2, and 44-95 % in O3/H2O2/UV. Phytotoxicity tests revealed a noticeable reduction in the effluent toxicity, demonstrating the feasibility of these processes in realistic conditions as tertiary treatment.

Assessment of the Potential of Using Nanofiltration Polymeric and Ceramic Membranes to Treat Refinery Spent Caustic Effluents.

Spent caustic effluents are very challenging due to their very hazardous nature in terms of toxicity as well as their extreme pH (approximately 12-14). Spent caustic has presented a challenge for wastewater treatment in refineries, due to its composition rich in mercaptans, sulfides and other aromatic compounds. To address such problems, membrane filtration was studied using real effluents from Sines Refinery, in Portugal. The present study attempts to assess the potential for spent caustic treatment with nanofiltration (NF) polymeric and ceramic membranes, assessing membrane life expectancy. For that, membrane aging studies in static mode were performed with the polymeric membrane before attempting NF treatment (dynamic studies). A ceramic membrane was also tested for the first time with this type of effluents, though only in dynamic mode. Although the polymeric membrane performance was very good and in accordance with previous studies, its lifespan was very reduced after 6 weeks of contact with spent caustic, compromising its use in an industrial unit. Contrarily to expectations, the ceramic membrane tested was not chemically more resistant than the polymeric one upon direct contact with spent caustic (loss of retention capacity in less than 1 h in contact with the spent caustic). The results obtained suggest that a pH of 13.9 is very aggressive, even for ceramic membranes.

Specific adsorbents for the treatment of OMW phenolic compounds by activation of bio-residues from the olive oil industry.

A series of adsorbents was developed by physical (CO2) and chemical (KOH) activation of two bio-residues: olive stones (OS) and wood from olive tree pruning (OTP). The physicochemical properties of such materials were determined and correlated with their adsorptive performance in the removal of phenolic compounds of olive mill wastewater (OMW). Adsorption isotherms and kinetics of single phenolic acids, as well as the kinetics for competitive multi-compound adsorption, were fitted by applying different models, though Langmuir and pseudo-second order models fitted better the experimental results, respectively. The intraparticle diffusion model pointed out that mesoporosity reduces the influence of phenolic compounds' restrictions in the external film diffusion of the adsorbent particle-solution interphase, but adsorption capacity linearly increases with the micropore volume accessible to N2 at -196 °C (and also with BET surface area), while diffusion into ultramicropores (<0.7 nm, determined by CO2-adsorption) is slow and presents minor influence on the total adsorption capacity. After saturation, thermal regeneration of spent adsorbents allows the removal of adsorbed products, enabling the reuse of samples whilst maintaining a significant performance.

Novel Adsorption-Reaction Process for Biomethane Purification/Production and Renewable Energy Storage.

This work proposes an innovative method for the simultaneous upgrading of biogas streams and valorization of the separated CO2, through its conversion to renewable methane. To this end, two sorptive reactors were filled with a layered bed containing a CO2 sorbent (K-promoted hydrotalcite) and a methanation catalyst (Ru/Al2O3). The continuous cyclic operation of the parallel sorptive reactors was carried out by alternately feeding a biogas stream (CO2/CH4 mixture) or H2. The CO2/CH4 mixture is fed to the sorptive reactor during the sorption stage, with CO2 being captured by the sorbent and CH4 exiting as a purified stream (i.e., as biomethane). During the reactive regeneration stage, the inlet stream is switched to pure H2, which reacts with the previously captured CO2 at the methanation catalyst active sites thus producing additional methane. For continuous operation, the two sorptive reactors were operated 180° out of phase and cyclic steady-state could be reached after ca. five cycles. The performance of the cyclic sorptive-reactive unit was assessed through a parametric study to evaluate the influence of different operating conditions, namely, the inlet flow rate and CO2 content during the sorption stage, the hydrogen inlet flow rate during the reactive regeneration stage, the stage duration, and temperature. The inclusion of an inert purge after the reactive regeneration stage was also tested. The performance of the unit was compared to the case of direct hydrogenation of biogas, and conclusions were drawn regarding future optimization, with special attention being given to CH4 productivity and purity. During the parametric study, a compromise between these process indicators, i.e., a productivity of 1.63 molCH4 kgcat -1 h-1 with 70.3% of CH4 purity, was obtained at 350 °C. However, biomethane purities above 80% were easily achieved, though at the expense of methane productivities.

Application of iron-activated persulfate for municipal wastewater disinfection.

To address the increasing contamination of aquatic environments and incidence of waterborne diseases, advanced oxidation processes with activated persulfate have emerged as tools to inactivate wastewater microorganisms and contaminants. In this work, the disinfection of a secondary effluent from a wastewater treatment plant by iron-based persulfate activation was studied. Experiments in a batch stirred tank reactor were carried out to evaluate the performance along reaction time and the effect of operational parameters in the oxidative process efficiency (oxidant and iron concentration, pH and temperature). After 60 min of reaction, persulfate and iron concentrations of 3 mM and 0.75 mM, respectively, combined with a neutral initial pH (7.5) and a temperature of 40 °C, allowed to reach values below the detection limit (<10 CFU/100 mL) of enterococci and enterobacteria with and without ciprofloxacin resistance, as well as a 91% inactivation of total heterotrophic organisms and a 70% removal of total organic carbon. Regrowth of microorganisms was evaluated 72 h after treatment and it was only noticed a slight increase in total heterotrophs. Evaluation of physico-chemical characteristics of the treated water showed that it meets the requirements imposed by European and Portuguese legislation for its reuse in irrigation and most urban utilities.

Ozonation of cytostatic drugs in aqueous phase.

As the number of cancer patients increases, so does the consumption of cytostatic drugs, which are commonly used in chemotherapy. These compounds are already ubiquitous in wastewater treatment plant (WWTP) effluents and natural water streams, revealing the urgent need for efficient technologies for their removal from the aqueous phase. This work presents the elimination of five cytostatics of concern, found in Portuguese WWTP effluents: bicalutamide (BICA), capecitabine (CAP), cyclophosphamide (CYC), ifosfamide (IFO) and mycophenolic acid (MPA), using non-catalytic ozonation. Experiments were performed starting from trace-level concentrations (1 µM) for all cytostatics at neutral pH (pH: 7.3 ± 0.1) and room temperature (23 ± 1 °C), employing different ozone dosages. Under the studied conditions, CAP and MPA were quickly eliminated by direct ozonation, whereas BICA, CYC and IFO were more slowly degraded, as they undergo a breakdown via hydroxyl radicals generation (HO) exclusively. Increasing the O3 dosage from 1 to 3 mgO3/mgDOC, CAP, MPA and IFO were completely removed, and BICA and CYC were converted more than 90% after 180 min. The presence of both inorganic ions and organic matter in real water matrices (river water, WWTP secondary effluent) did not affect the removal of CAP and MPA. Nonetheless, there was an inefficient and very fast O3 consumption that resulted in only around 30% elimination of BICA, CYC and IFO, even if the reaction time is extended.

Industrial wastewater treatment using a bubble photo-Fenton reactor with continuous gas supply.

The present study assesses the treatability of a real industrial wastewater (WW) with a high organic load (chemical oxygen demand (COD) above 5800 mgO2 L-1) by photo-Fenton's oxidation with the goal of improving the organic matter degradation reached previously, in another work, where the Fenton process was applied in a bubbling reactor. Thus, the process was carried out in a bubble photo reactor (BPR) wherein continuous air supply ensures an efficient mixing of the liquid phase. The effect of the main operatory parameters that influence the WW treatment (i.e., H2O2 and Fe2+ concentrations, initial pH, and UV-Vis radiation intensity) were evaluated, being found that in the best conditions tested (pH0 = 4.6, [Fe2+] = 0.1 g L-1, [H2O2] = 18 g L-1, Qair = 1.0 L min-1-measured at room temperature and atmospheric pressure-and irradiance of 500 W m-2), removals of 95% and 97% for total organic carbon (TOC) and COD, respectively, were achieved. Still, a high reduction of the concentration of the main constituents of this WW was reached, being total for aniline and 86% for sulfanilic acid. The continuous air supply reactor configuration was compared with magnetic stirring; similar mineralization was achieved. However, the air bubbling promotes a good heat transfer within the reactor, minimizing temperature gradients, which is quite advantageous due to the strong exothermicity of the oxidation process during the treatment of such highly loaded real effluents.

Simultaneous treatment of toluene-containing gas waste and industrial wastewater by the Fenton process.

This study reports a new perspective for the simultaneous oxidation of a volatile organic compound (VOC) - a toluene gas stream - and a real industrial liquid effluent by the Fenton's process; for that, a lab-scale bubbling reactor, operating in semi-continuous mode, was used. A parametric study was carried out to evaluate the effect of the aqueous matrix (water vs. real effluent), catalyst species nature (Fe2+ vs. Fe3+), concentration of organic matter in the liquid, and inlet toluene concentration in the gas phase. Their effects in the simultaneous gas-liquid treatment were assessed in terms of the toluene removal (from the gas stream) and wastewater mineralization (removal of dissolved organic carbon - DOC). The presence of organic matter in the liquid phase decreased toluene absorption. However, the simultaneous oxidation in the liquid phase extended the period of absorption until its saturation (and inherently the amount of toluene transferred) while still oxidizing 25% of the organic matter present in the industrial effluent. The application of the Fenton-like (H2O2 + Fe3+) process yielded a slightly reduced toluene transfer as compared to the Fenton one (H2O2 + Fe2+) - ca. 10%, although the overall mineralization has been similar. As expected, increasing the inlet toluene concentration reduces the process duration until liquid saturation, at the same time that a higher accumulation of by-products in the liquid due to oxidation was observed. Finally, a sequential treatment approach was performed, wherein liquid oxidation follows the previous simultaneous gas-liquid treatment, representing a strategy for long term operation, providing an opportunity for further VOC abatement in subsequent cycles. The main compounds resulting from oxidation remaining in the liquid phase after each stage were identified, allowing to close the carbon balance by ca. 80%.

Fitting Biochars and Activated Carbons from Residues of the Olive Oil Industry as Supports of Fe- Catalysts for the Heterogeneous Fenton-Like Treatment of Simulated Olive Mill Wastewater.

A series of biochars and activated carbons (ACs) was prepared combining carbonization and physical or chemical activation of cheap and abundant residues of the olive oil industry. These materials were used as Fe-support to develop low-cost catalysts for the heterogeneous Fenton-like oxidation of simulated olive mill wastewater (OMW), the highly pollutant effluent generated by this agroindustry. Commercial ACs were also used as reference. All catalysts prepared were extensively characterized and results related with their performances in the catalytic wet peroxide oxidation (CWPO). Results showed a linear relationship of the textural properties of the catalysts with the adsorptive and catalytic performance, as well as the preferential adsorption and degradation of some phenolic compounds (caffeic and gallic acids) by specific interactions with the catalysts' surface. Despite the best performance of catalysts developed using commercial supports, those prepared from agro-industrial residues present some advantages, including a smaller catalyst deactivation by iron leaching. CWPO results show that catalysts from physically activated olive stones are the most promising materials, reaching total organic carbon and toxicity reductions of 35% and 60%, respectively, as well an efficient use of H2O2, comparable with those obtained using commercial supports. This approach showed that the optimized treatment of this type of residues will allow their integration in the circular economic process of the olive oil production.

Insights into real industrial wastewater treatment by Fenton's oxidation in gas bubbling reactors.

In the present study, bubbling reactors (BRs) were chosen to design a new procedure for real industrial wastewater (WW) treatment by Fenton's oxidation. The process was carried out in BRs under batch mode for the treatment of a WW with a high organic load (chemical oxygen demand (COD) above 7000 mgO2/L), being the efficient mixing of the liquid phase ensured by the gas bubbling. The parameters that influenced the WW treatment (i.e., H2O2 and Fe2+ concentration, and initial pH) were optimized in a smaller BR (0.5 L volumetric capacity); the maximum oxidation efficiency (dissolved organic carbon (DOC) removal = 52% and COD removal = 83% after 60 min) was reached under the following conditions: Qair = 1.0 L/min (measured at room temperature and atmospheric pressure), [H2O2] = 22.5 g/L, [Fe2+] = 0.75 g/L, and pH = 4.6 - original WW pH. It was not detected any significant effect in the process efficiency of the air flow rate and gas phase composition (i.e., N2, and air), but when the process was performed with continuous O2 bubbling an increase in the DOC removal (from 43% to 53%) was observed after 5 min of oxidation. Even so, the high costs discourage the use of pure oxygen streams in real WWTPs. To understand the dynamics of the process, the continuous air bubbling was compared to another mixing mode (mechanical stirring), and similar mineralization was achieved, proving the feasibility of Fenton's process in a BR. In addition, the gas bubbling proved to be more efficient in terms of heat dissipation during the treatment, decreasing temperature profiles along the oxidation of heavily charged real effluents. An effective scale-up with a bubble column reactor with a higher volumetric capacity by a factor of almost one order of magnitude was also proved, providing similar mineralization. The final effluent was non-toxic and more biodegradable.

Treatment of biodigested coffee processing wastewater using Fenton's oxidation and coagulation/flocculation.

Biodigested coffee processing wastewater (CPW) presents a high organic load and does not meet the limits imposed by legislation (namely in Brazil) for discharge into water bodies. Anaerobic digestion generally cannot provide a satisfactory organic matter reduction in CPW as a significant fraction of recalcitrant compounds still persists in the treated effluent. So, this study aims to find alternative ways to remove refractory organic compounds from this wastewater in order to improve the biodegradability and reduce the toxicity, which will allow its recirculation back into the anaerobic digester. Three treatment approaches (Fenton's oxidation - Approach 1, Coagulation/flocculation (C/F) - Approach 2, and the combination of C/F with Fenton's process - Approach 3) were selected to be applied to the biodigested CPW in order to achieve that objective. The application of the Fenton process under the optimal operating conditions (initial pH = 5.0; T = 55 °C, [Fe3+] = 1.8 g L-1 and [H2O2] = 9.0 g L-1) increased the biodegradability (the BOD5:COD ratio raised from 0.34 ± 0.02 in biodigested CPW to 0.44 ± 0.01 after treatment) and eliminated the toxicity (0.0% of Vibrio fischeri inhibition) along with moderate removals of organic matter (51.3%, 55.7% and 39.7% for total organic carbon - TOC, chemical oxygen demand - COD and biochemical oxygen demand - BOD5, respectively). The implementation of a coagulation/flocculation process upstream from Fenton's oxidation, under the best operating conditions (pH 10-11 and [Fe3+] = 250 mg L-1), also allowed to slightly increase the biodegradability (from 0.34 to 0.47) and reduce the toxicity, whereas providing a higher removal of organic matter (TOC = 76.2%, COD = 76.5 and BOD5 = 66.3% for both processes together). Approach 1 and Approach 3 showed to be the best ones, implying similar operating costs (∼74 R$ m-3/∼17 € m-3) and constitute an attractive option for managing biodigested CPW.

Optimization of the NO photooxidation and the role of relative humidity.

Photocatalysis was recognised as a suitable process for the photoabatement of atmospheric pollutants. The photooxidation mechanism on TiO2 has been widely studied. However, recent studies demonstrated that the very often-assumed photooxidation intermediated by the hydroxyl radical cannot explain all the experimental observations. Indeed, this study contributes for a new understanding of NO photooxidation. First, the adsorption equilibrium isotherms of NO, NO2 and H2O on the photocatalyst, Aeroxide® P25 from Evonik Industries, were obtained. Also, the concentration of hydroxyl radicals was determined by photoluminescence. A comprehensive design of experiments was then followed; NO conversion and selectivity were obtained as a function of the relative humidity, irradiance, NO inlet concentration and residence time, following a response surface methodology (RSM). These results were then used to discuss the photooxidation mechanism of NO.

Application of the Fenton's process in a bubble column reactor for hydroquinone degradation.

The aim of this study was to assess the degradation and mineralization of hydroquinone (HQ) by the Fenton's process in a bubble column reactor (BCR). The effect of the main operating variables, namely, air flow rate, effluent volume, hydrogen peroxide (H2O2) concentration, catalyst (Fe2+) dose, initial pH, and temperature, were assessed. For all air flow rates tested, no concentration gradients along the column were noticed, evidencing that a good mixing was reached in the BCR. For the best conditions tested ([H2O2] = 500 mg/L, [Fe2+] = 45 mg/L, T = 24 °C, Q air = 2.5 mL/min, pH = 3.0, and V = 5 L), complete HQ degradation was reached, with ~ 39% of total organic carbon (TOC) removal, and an efficiency of the oxidant use-η H2O2-of 0.39 (ratio between TOC removed per H2O2 consumed normalized by the theoretical stoichiometric value); moreover, a non-toxic effluent was generated. Under these conditions, the intermediates and final oxidation compounds identified and quantified were a few carboxylic acids, namely, maleic, pyruvic, and oxalic. As a strategy to improve the TOC removal, a gradual dosage of the optimal H2O2 concentration was implemented, being obtained ~ 55% of mineralization (with complete HQ degradation). Finally, the matrix effect was evaluated, for which a real wastewater was spiked with 100 mg/L of HQ; no reduction in terms of HQ degradation and mineralization was observed compared to the solution in distilled water.

p-Nitrophenol degradation by Fenton's oxidation in a bubble column reactor.

This paper reports on a study of the oxidation of p-nitrophenol (PNP) in a bubble column reactor (BCR). The use of the air stream aimed to provide perfect mixing in the liquid phase, which was successfully achieved and checked experimentally; there were no concentration gradients along the column, even at the lowest air flow rate used (Q = 1 mL/min at room temperature and atmospheric pressure). The effect of the operating variables was assessed, and a total reduction of PNP was reached, as well as mineralization of 49.2%, oxidant consumption of 90.3%, and with an efficiency of use - ηH2O2 - of 0.09 mg C/mg H2O2, under the best operating conditions found - Q = 1 mL/min, [H2O2] = 1.6 g/L, [Fe2+] = 80 mg/L, pH = 3.0 and T = 22-24 °C - (after 120 min of reaction). Following this, various strategies were developed for improving the mineralization rate; it was found that the addition of H2O2 every 5 min and readjusting the pH after 30 min of reaction allow the attainment of a much higher TOC removal (75.1%) and efficiency of oxidant use (ηH2O2 = 0.17 mg C/mg H2O2) with less oxidant. A reaction mechanism was proposed, based on intermediates identified that include p-nitrocatechol - PNC, p-benzoquinone - PB, hydroquinone - HQ - and carboxylic acids (oxalic, maleic and fumaric). Since the performance achieved in the BCR was good, and very similar to that obtained in a conventional batch reactor, it was possible to verify the efficacy of carrying out the Fenton process in this reactor configuration, which in our future work will focus on the treatability of industrial effluents.