Performance of electrochemical oxidation and photocatalysis in terms of kinetics and energy consumption. New insights into the p-cresol degradation

Techno-economic evaluation of UV light technologies in water remediation

Disinfection commonly follows conventional treatments in wastewater treatment and remediation plants aiming at reducing the presence of pathogens. However, the presence of the so called "micropollutants" has emerged as a serious concern, therefore developing tertiary treatments that are not only able to remove pathogens but also to degrade micropollutants is worth investigating. Nowadays, UV-C photo-degradation processes are widely used for disinfection due to their simplicity and easy operation; additionally, they have shown potential for the removal of contaminants of emerging concern. Conventional mercury lamps are being replaced by light-emitting diodes (LEDs) that avoid the use of toxic mercury and can be switched on and off with no effect on the lamp lifetime. This work aims to comparatively evaluate the performance of several photo-degradation technologies for the removal of two targeted micropollutants, the pharmaceutical dexamethasone (DXMT) and the herbicide S-metolachlor (MTLC), using UV irradiation doses typical of disinfection processes. To this end, the technical performance of UV-A/UV-C photolysis, UV-A/UV-C photocatalysis, UV-C/H₂O₂ and UV-C/NaOCl has been compared. The influence of operating conditions such as the initial concentration of the pollutants (3 mg L^-1 - 30 mg L^-1, concentrations found in membrane or adsorption remediation steps), pH (3-10), and water matrix (WWTP secondary effluent, and ultrapure water) on the degradation efficiency has been studied. The economic evaluation in terms of electricity and chemicals consumption and the carbon footprint has been evaluated. UV-C photolysis and UV-C photocatalysis appear as the most suitable technologies for the degradation of DXMT and MTLC, respectively, in terms of kinetics (1.53·10^-1 min^-1 for DXMT and 1.96·10^-2 min^-1 for MTLC), economic evaluation (1 € m^-3 for DXMT and 32 € m^-3 for MTLC) and environmental indicators (0.5 g-CO₂ for DXMT and 223.1 g-CO₂ for MTLC).

Degradation of o-, m-, p-cresol isomers using ozone in the presence of V2O5-supported Mn, Fe, and Ni catalysts

Oxidative degradation of o-, m- and p-cresols using ozone in the presence of V2O5-supported metal (Mn, Fe, Ni) catalysts was studied under ambient reaction conditions. Metal (Mn, Fe, Ni) loaded V2O5 catalysts were prepared using a wet-impregnation method, thereafter, characterized, and analyzed by use of the XRD, FT-IR, SEM-EDX, TEM, and ICP-OES. Results show the effect of the amount of a metal that was loaded on the support, particularly, how it affects the resultant catalysts’ (i) crystallite size, (ii) dispersion of an active metal over the surface of a support, and (iii) catalytic activity. Mn-loaded catalysts were found to be relatively more active for the conversion of individual cresol isomers and the activity of this catalyst was significantly enhanced at a lower Mn to V2O5 ratio (2.5 wt%). Mn(2.5 %)/V2O5 catalyst led to conversions of 66.78, 71.01 and 73.68 % with o-, m-, and p-cresols respectively within 24 h of oxidation. Oxidation products were derivatized by ethanol and a few were positively detected using GC-MS. o-Tolyl acetate and 2,5-dihydroxy toluene were detected from o-cresol, m-tolyl acetate, and 2,3-dihydroxy toluene from m-cresol and p-tolyl acetate and 3,4-dihydroxy toluene from p-cresol oxidation. Dimethyl maleate and dimethyl oxalate were detected as common products in all three isomers’ oxidation.

Heterogeneous Advanced Oxidation Processes: Current Approaches for Wastewater Treatment

Nowadays, water pollution is one of the most dangerous environmental problems in the world. The presence of the so-called emerging pollutants in the different water bodies, impossible to eliminate through conventional biological and physical treatments used in wastewater treatment plants due to their persistent and recalcitrant nature, means that pollution continues growing throughout the world. The presence of these emerging pollutants involves serious risks to human and animal health for aquatic and terrestrial organisms. Therefore, in recent years, advanced oxidation processes (AOPs) have been postulated as a viable, innovative and efficient technology for the elimination of these types of compounds from water bodies. The oxidation/reduction reactions triggered in most of these processes require a suitable catalyst. The most recent research focuses on the use and development of different types of heterogeneous catalysts, which are capable of overcoming some of the operational limitations of homogeneous processes such as the generation of metallic sludge, difficult separation of treated water and narrow working pH. This review details the current advances in the field of heterogeneous AOPs, Fenton processes and photocatalysts for the removal of different types of emerging pollutants.

Performance of TiO2/UV-LED-Based Processes for Degradation of Pharmaceuticals: Effect of Matrix Composition and Process Variables

Ultra-violet light-emitting diode (UV-LED)-based processes for water treatment have shown the potential to surpass the hurdles that prevent the adoption of photocatalysis at a large scale due to UV-LEDs' unique features and design flexibility. In this work, the degradation of five EU Watch List 2020/1161 pharmaceutical compounds was comprehensively investigated. Initially, the UV-A and UV-C photolytic and photocatalytic degradation of individual compounds and their mixtures were explored. A design of experiments (DoE) approach was used to quantify the effects of numerous variables on the compounds' degradation rate constant, total organic carbon abatement, and toxicity. The reaction mechanisms of UV-A photocatalysis were investigated by adding different radical scavengers to the mix. The influence of the initial pH was tested and a second DoE helped evaluate the impact of matrix constituents on degradation rates during UV-A photocatalysis. The results showed that each compound had widely different responses to each treatment/scenario, meaning that the optimized design will depend on matrix composition, target pollutant reactivity, and required effluent standards. Each situation should be analyzed individually with care. The levels of the electrical energy per order are still unfeasible for practical applications, but LEDs of lower wavelengths (UV-C) are now approaching UV-A performance levels.

Influence of dimethylphenol isomers on electrochemical degradation: Kinetics, intermediates, and DFT calculation

Dimethylphenol isomers (DMP) pose a great threat to the environment, and the electrooxidation (EO) process proves to be an extraordinarily effective method to degrade DMP. However, the EO performance is affected by the molecular structure of DMP and the adopted experimental parameters. In this study, the effects of 2,4-DMP and 2,6-DMP on the working potential, limiting current density (J_lim), and pH were systematically analysed, with Ti-mesh plates used as the cathode and Ti/PbO₂ as the anode. The peak potentials of 2,4-DMP and 2,6-DMP were determined to be 0.83 V and 0.77 V by cyclic voltammetry, with J_lim were 2.5 mA·cm^-2 and 2.0 mA·cm^-2, respectively. The whole process exhibited pseudo-first-order kinetics, and the kinetic constants (K) for the degradation of 2,4-DMP and 2,6-DMP were determined to be 0.0041 min^-1 and 0.0150 min^-1, respectively. Additionally, the optimal initial pH value for 2,4-DMP and 2,6-DMP was 5.0, where the highest hydroxyl (OH) radical density, as determined by the electron spin technique (ESR), was achieved at a higher current density. Comparatively, the OH radical density in the 2,6-DMP solution was lower than that in 2,4-DMP. In situ Fourier infrared (FT-IR) spectroscopy, GC-MS, and density functional theory (DFT) were employed to explore three possible degradation pathways. The main intermediates for 2,4-DMP degradation were determined to be quinone and ether, while that for 2,6-DMP degradation was quinone. According to the results of this study, the molecular structure (different methyl group positions on the benzene ring) has a great influence on the EO process.

Degradation synergism between sonolysis and photocatalysis for organic pollutants with different hydrophobicity: A perspective of mechanism and application for high mineralization efficiency

Despite extensive studies, the fundamental understanding of synergistic mechanisms between sonolysis and photocatalysis for the abatement of persistent organic pollutants (POPs) remains uncertain. As different phases formed under ultrasound irradiation, hydrophilic POPs, sulfamethoxazole (SMX, K_ow: 0.89), predominantly resides in bulk liquid and is ineffectively degraded by sonolysis (k_US = 3.33 × 10^-3 min^-1) since <10% of hydroxyl radicals (·OH) formed at the gas-liquid interface of cavitation is diffused into the bulk, whereas the other fraction rapidly recombines into hydrogen peroxide (H₂O₂). This study provides a proof-of-concept for the mechanism by presenting various analytical results, endorsing the synergistic role of photoexcited electrons in splitting sonolysis-induced H₂O₂ into ·OH, particularly in the bulk phase. In a sonophotocatalytic system, the hydrophobic POPs such as bisphenol A (BPA) and atrazine (ATZ) were mainly degraded in gas-liquid interface indicated by the low synergistic values correlation compared to SMX [i.e., SMX has a higher synergistic factor, f_syn (3.26) than BPA (1.30) and ATZ (1.35)]. Also, f_syn was found linearly correlated with the contribution factor of photocatalysis to split H₂O₂. Three times of consecutive kinetics using an effluent of municipal (MP) wastewater spiked by POPs presented >98% POPs and >96% total organic carbon (TOC) removal.

Enhanced visible-light driven multi-photoredox Cr(VI) and p-cresol by Si and Zr interplay in fibrous silica-zirconia

Multiple contaminants including heavy metals and phenolic compounds are normally co-exist in wastewater, which caused the treatment process is rather complicated. Herein, the synergistic photoredox of Cr(VI) and p-cresol (pC) by innovative fibrous silica zirconia (FSZr) photocatalyst was reported. The high surface area of FSZr comprised of microspheres with a bicontinuous concentric lamella structure morphology consisted of silica, while its core consisted of ZrO₂ structure. The rearrangement of FSZr framework increased the crystallinity, formed Si-O-Zr bonds and narrowed the band gap of ZrO₂ for enhanced of photoredox of Cr(VI) and pC. Compared to the reaction, the photoredox efficiency of FSZr for removing Cr(VI) and pC in simultaneous system was found to be 96 % and 59 %, respectively which are higher than that in its single system owing to the efficient electron-hole charge separation. Phenolic compound with high degree of electron donating group gave beneficial effect to photoreduction of Cr(VI). Consequently, a proposed mechanism involving multi-photoredox pathway were proposed based on photoredox reaction and scavengers studies. FSZr sustained the simultaneous photoredox activities after five runs demonstrating its possibility to be use in the wastewater treatment of various pollutants.

Comprehensive Kinetics of the Photocatalytic Degradation of Emerging Pollutants in a LED-Assisted Photoreactor. S-Metolachlor as Case Study

Although the potential and beneficial characteristics of photocatalysis in the degradation of a good number of emerging pollutants have been widely studied and demonstrated, process design and scale-up are restrained by the lack of comprehensive models that correctly describe the performance of photocatalytic reactors. Together with the kinetics of degradation reactions, the distribution of the radiation field in heterogeneous photocatalytic systems is essential to the optimum design of the technology. Both the Local Volumetric Rate of Photon Absorption (LVRPA) and the Overall Volumetric Rate of Photon Absorption (OVRPA) help to understand this purpose. This work develops a Six-Flux radiation absorption–scattering model coupled to the Henyey–Greenstein scattering phase function to evaluate the LVRPA profile in a LED-assisted photocatalytic reactor. Moreover, the OVRPA has been calculated and integrated into the kinetic equation, accounting for the influence of the radiation distribution on the reaction rate. The model has been validated with experimental data for the degradation of S-Metolachlor (MTLC), and the set of operating variables that maximize the reactor performance, 0.5 g/L of TiO2 P25 and pH 3, has been determined.

Degradation of hydroxychloroquine by electrochemical advanced oxidation processes

In this work, the degradation of hydroxychloroquine (HCQ) drug in aqueous solution by electrochemical advanced oxidation processes including electrochemical oxidation (EO) using boron doped diamond (BDD) and its combination with UV irradiation (photo-assisted electrochemical oxidation, PEO) and sonication (sono-assisted electrochemical oxidation, SEO) was investigated. EO using BDD anode achieved the complete depletion of HCQ from aqueous solutions in regardless of HCQ concentration, current density, and initial pH value. The decay of HCQ was more rapid than total organic carbon (TOC) indicating that the degradation of HCQ by EO using BDD anode involves successive steps leading to the formation of organic intermediates that end to mineralize. Furthermore, the results demonstrated the release chloride (Cl-) ions at the first stages of HCQ degradation. In addition, the organic nitrogen was converted mainly into NO3 - and NH4 + and small amounts of volatile nitrogen species (NH3 and NOx). Chromatography analysis confirmed the formation of 7-chloro-4-quinolinamine (CQLA), oxamic and oxalic acids as intermediates of HCQ degradation by EO using BDD anode. The combination of EO with UV irradiation or sonication enhances the kinetics and the efficacy of HCQ oxidation. PEO requires the lowest energy consumption (EC) of 63 kWh/m3 showing its cost-effectiveness. PEO has the potential to be an excellent alternative method for the treatment of wastewaters contaminated with HCQ drug and its derivatives.

Reuse and recycle solutions in refineries by ozone-based advanced oxidation processes: A statistical approach

Fresh water sources are under pressure globally by the increasing population and consequently increasing production, which increases the water demand day by day. Thus, decreasing the industrial fresh water demand and wastewater production became crucial both for the water availability in the future and for its impact to the environment. This study examined the ozone-based treatments as the possible solution to a refinery to treat the effluent already treated by the traditional techniques to reach the final requirements for reuse and recycle purposes. The screening tests performed by fractional factorial design revealed that the significant parameters for the treatment were ozone feed ratio, H₂O₂ amount and processing time while pH was found insignificant for this case. Based on the box-Behnken response surface methodology for effluent collected after biological treatment, the significant parameters were optimized as the ozone ratio of 0.9 g/h, H₂O₂ amount of 47 mg/L and 60 min duration. However, in case of increasing the H₂O₂ amount to 80 mg/L the duration can be minimized to 37.5 min decreasing the energy and reagent consumption costs by a 37%, reaching a final total organic carbon (TOC) under 4 mg/L, that is the target for reuse possibilities.

Pharmaceutically active compounds in aqueous environment: A status, toxicity and insights of remediation

Pharmaceutically active compounds (PhACs) have pernicious effects on all kinds of life forms because of their toxicological effects and are found profoundly in various wastewater treatment plant influents, hospital effluents, and surface waters. The concentrations of different pharmaceuticals were found in alarmingly high concentrations in various parts of the globe, and it was also observed that the concentration of PhACs present in the water could be eventually related to the socio-economic conditions and climate of the region. Drinking water equivalent limit for each PhAC has been calculated and compared with the occurrence data from various continents. Since these compounds are recalcitrant towards conventional treatment methods, while advanced oxidation processes (AOPs) have shown better efficiency in degrading these PhACs. The performance of the AOPs have been evaluated based on percentage removal, time, and electrical energy consumed to degrade different classes of PhACs. Ozone based AOPs were found to be favorable because of their low treatment time, low cost, and high efficiency. However, complete degradation cannot be achieved by these processes, and various transformation products are formed, which may be more toxic than the parent compounds. The various transformation products formed from various PhACs during treatment have been highlighted. Significant stress has been given on the role of various process parameters, water matrix, oxidizing radicals, and the mechanism of degradation. Presence of organic compounds, nitrate, and phosphate usually hinders the degradation process, while chlorine and sulfate showed a positive effect. The role of individual oxidizing radicals, interfering ions, and pH demonstrated dissimilar effects on different groups of PhACs.

Comparative Analysis of Photocatalytic and Electrochemical Degradation of 4-Ethylphenol in Saline Conditions

We evaluated electrochemical degradation (ECD) and photocatalytic degradation (PCD) technologies for saline water purification, with a focus on rate comparison and formation and degradation of chlorinated aromatic intermediates using the same non-chlorinated parent compound, 4-ethylphenol (4EP). At 15 mA·cm^-2, and in the absence of chloride (0.6 mol·L^-1 NaNO₃ was used as supporting electrolyte), ECD resulted in an apparent zero-order rate of 30 μmol L^-1·h^-1, whereas rates of ∼300 μmol L^-1·h^-1 and ∼3750 μmol L^-1·h^-1 were computed for low (0.03 mol·L^-1) and high (0.6 mol·L^-1) NaCl concentration, respectively. For PCD, initial rates of ∼330 μmol L^-1·h^-1 and 205 μmol L^-1·h^-1 were found for low and high NaCl concentrations, at a photocatalyst (TiO₂) concentration of 0.5 g·L^-1, and illumination at λ_max ≈ 375 nm, with an intensity ∼0.32 mW·cm^-2. In the chlorine mediated ECD approach, significant quantities of free chlorine (hypochlorite, Cl₂) and chlorinated hydrocarbons were formed in solution, while photocatalytic degradation did not show the formation of free chlorine, nor chlorine-containing intermediates, and resulted in better removal of non-purgeable hydrocarbons than ECD. The origin of the minimal formation of free chlorine and chlorinated compounds in photocatalytic degradation is discussed based on photoelectrochemical results and existing literature, and explained by a chloride-mediated surface-charge recombination mechanism.

Degradation of anti-inflammatory drugs in municipal wastewater by heterogeneous photocatalysis and electro-Fenton process

Non-steroidal anti-inflammatory drugs (NSAID) are compounds frequently found in municipal wastewater and their degradation by conventional wastewater treatment plants (WWTP) is generally incomplete. This study compared the efficiency of two advanced oxidation processes (AOP), namely heterogeneous photocatalysis (HP) and electro-Fenton (EF), in the degradation of a mixture of common NSAID (diclofenac, ibuprofen and naproxen) dissolved in either deionized water or effluent from a WWTP. Both processes were effective in degrading the NSAID mixture and the trend of degradation was as follows, diclofenac > naproxen > ibuprofen. EF with a current density of 40 mA cm^-2 and 0.3 mmol Fe²⁺ L^-1 was the most efficient process to mineralize the organic compounds, achieving up to 92% TOC removal in deionized water and 90% in the WWTP effluent after 3 h of reaction. HP with 1.4 g TiO₂ L^-1 at pH 7 under sunlight, produced 85% TOC removal in deionized water and 39% in WWTP effluent also after 3 h treatment. The lower TOC removal efficiency shown by HP with the WWTP effluent was attributed mainly to the scavenging of reactive species by background organic matter in the wastewater. On the contrary, inorganic ions in the wastewater may produce oxidazing species during the EF process, which contributes to a higher degradation efficiency. EF is a promising option for the treatment of anti-inflammatory pharmaceuticals in municipal WWTP at competitive electrical energy efficiencies.

Photocatalytic degradation of trace carbamazepine in river water under solar irradiation

An interesting ZnIn₂S₄/TiO₂ composite catalyst was prepared by a hydrothermal method and thoroughly characterized. The photocatalytic degradation of trace carbamazepine (CBZ) in two river waters was primarily investigated through a batch experiment under solar irradiation, and the effects of dissolved organic matter (DOM), inorganic salt (IS), suspended solids (SS) and ultraviolet (UV) on CBZ degradation were researched. The influential degree was DOM ≈ IS » SS and CBZ with an initial concentration of 100 μg/L in the Bahe River water was completely degraded under a catalyst dosage of 75 mg/L and solar irradiation of 240 min. Compared with direct photolysis, the reaction rate constant enhanced 45 times and the half-life reduced to 1/82 in photocatalysis after the removal of all SS, IS and DOM. A certain adsorption capacity of composite catalyst with a specific surface area of 91.9 m²/g and a strong interaction between TiO₂ and ZnIn₂S₄ effectively improved the photocatalytic degradation of CBZ. The increase of light intensity was confirmed to be of benefit to CBZ photocatalysis. Most of CBZ was degraded by visible light and UV effect was negligible. Although photo-etching and acidic corrosion by course products had negative effect on ZnIn₂S₄/TiO₂, the removal of CBZ was mainly kept at 86% after five times usage of the catalyst.

Performance of Bi2O3/TiO2 prepared by sol-gel on p-Cresol degradation under solar and visible light

Photocatalytic degradation of p-Cresol was evaluated using the mixed oxide Bi₂O₃/TiO₂ (containing 2 and 20% wt. Bi₂O₃ referred as TB2 and TB20) and was compared with bare TiO₂ under simulated solar radiation. Materials were prepared by the classic sol-gel method. All solids exhibited the anatase phase by X-ray diffraction (XRD) and Raman spectroscopy. The synthesized materials presented lower crystallite size and Eg value, and also higher surface area as Bi₂O₃ amount was increased. Bi content was quantified showing near to 70% of theoretical values in TB2 and TB20. Bi₂O₃ incorporation also was demonstrated by X-ray photoelectron spectroscopy (XPS). Characterization of mixed oxides suggests a homogeneous distribution of Bi₂O₃ on TiO₂ surface. Photocatalytic tests were carried out using a catalyst loading of 1 g L^-1 under simulated solar light and visible light. The incorporation of Bi₂O₃ in TiO₂ improved the photocatalytic properties of the synthesized materials obtaining better results with TB20 than the unmodified TiO₂ under both radiation sources.

TiO2 structures doped with noble metals and/or graphene oxide to improve the photocatalytic degradation of dichloroacetic acid

Noble metals have been used to improve the photocatalytic activity of TiO₂. Noble metal nanoparticles prevent charge recombination, facilitating electron transport due to the equilibration of the Fermi levels. Furthermore, noble metal nanoparticles show an absorption band in the visible region due to a high localized surface plasmon resonance (LSPR) effect, which contributes to additional electron movements. Moreover, systems based on graphene, titanium dioxide, and noble metals have been used, considering that graphene sheets can carry charges, thereby reducing electron-hole recombination, and can be used as substrates of atomic thickness. In this work, TiO₂-based nanocomposites were prepared by blending TiO₂ with noble metals (Pt and Ag) and/or graphene oxide (GO). The nanocomposites were mainly characterized via transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transformed infrared (FTIR), Raman spectroscopy, and photocurrent analysis. Here, the photocatalytic performance of the composites was analyzed via oxidizing dichloroacetic acid (DCA) model solutions. The influence of the noble metal load on the composite and the ability of the graphene sheets to improve the photocatalytic activity were studied, and the composites doped with different noble metals were compared. The results indicated that the platinum structures show the best photocatalytic degradation, and, although the presence of graphene oxide in the composites is supposed to enhance their photocatalytic performance, graphene oxide does not always improve the photocatalytic process. Graphical abstract It is a schematic diagram. Where NM is Noble Metal and LSPR means Localized Surface Plasmon Resonance.