A review of ecotoxicity reduction in contaminated waters by heterogeneous photocatalytic ozonation

Solar-based photocatalytic ozonation employing novel S-scheme ZnO/Cu2O/CuO/carbon xerogel photocatalyst: effect of pH, salinity, turbidity, and temperature on salicylic acid degradation

This paper proposes the study of a solar-based photocatalytic ozonation process for the degradation of salicylic acid (SA) using a novel S-scheme ZnO/Cu2O/CuO/carbon xerogel photocatalyst. The incorporation of CuO and Cu2O aims to enhance charge mobility through the formation of p-n heterojunctions with ZnO, whereas the carbon xerogel (XC) was selected due to its eco-friendly nature, capacity to stabilize S-scheme heterojunctions as a solid-state electron mediator, and ability to function as a reducing agent under high temperatures. The characterization of the composites demonstrates that the presence of the XC during the calcination step led to the reduction of a fraction of the CuO into Cu2O, forming a ternary semiconductor heterojunction system. In terms of photocatalysis, the XC/ZnO-CuxO 5% composite achieved the best efficiency for salicylic acid degradation, mainly due to the stabilization of the S-scheme charge transfer pathway between the ZnO/CuO/Cu2O semiconductors by the XC. The total organic carbon (TOC) removal during heterogeneous photocatalysis was 80% for the solar-based process and 68% for the visible light process, after 300 min. The solar-based photocatalytic ozonation process was highly successful regarding the degradation of SA, achieving a 75% increase in the apparent reaction rate constant when compared to heterogeneous photocatalysis. Furthermore, a 78% TOC removal was achieved after 150 min, which is half the time required by the heterogeneous photocatalysis to obtain the same result. Temperature, salinity, and turbidity had major effects on the efficiency of the photocatalytic ozonation process; the system's pH did not cause any major performance variation, which holds relevance for industrial applications.

Application of BiOX Photocatalyst to Activate Peroxydisulfate Ion-Investigation of a Combined Process for the Removal of Organic Pollutants from Water

The persulfate-based advanced oxidation processes employing heterogeneous photocatalysts to generate sulfate radicals (SO4•−) from peroxydisulfate ion (PDS, S2O82−) have been extensively investigated to remove organic pollutants. In this work, BiOX (X = Cl, Br, and I) photocatalysts were investigated to activate PDS and enhance the transformation rate of various organic substances under UV (398 nm) and Vis (400–700 nm) radiation. For BiOCl and BiOBr, in addition to excitability, the light-induced oxygen vacancies are decisive in the activity. Although without organic substances, the BiOI efficiency highly exceeds that of BiOBr and BiOCl for PDS activation (for BiOI, 15–20%, while for BiOBr and BiOCl, only 3–4% of the PDS transformed); each BiOX catalyst showed enhanced activity for 1,4-hydroquinone (HQ) transformation due to the semiquinone radical-initiated PDS activation. For sulfamethoxypyridazine (SMP), the transformation is driven by direct charge transfer, and the effect of PDS was less manifested. BiOI proved efficient for transforming various organic substances even under Vis radiation. The efficiency was enhanced by PDS addition (HQ is wholly transformed within 20 min, and SMP conversion increased from 40% to 90%) without damaging the catalyst; its activity did change over three consecutive cycles. Results related to the well-adsorbed trimethoprim (TRIM) and application of biologically treated domestic wastewater as a matrix highlighted the limiting factors of the method and visible light active photocatalyst, BiOI.

Advanced oxidation process: a sustainable technology for treating refractory organic compounds present in industrial wastewater

The world faces tremendous challenges and environmental crises due to the rising strength of wastewater. The conventional technologies fail to achieve the quality water that can be reused after treatment means "zero effluent" discharge of the industrial effluent. Therefore, now the key challenge is to develop improved technologies which will have no contribution to secondary pollution and at the same time more efficient for the socio-economic growth of the environment. Sustainable technologies are needed for wastewater treatment, reducing footprint by recycling, reusing, and recovering resources. Advanced oxidation process (AOP) is one of the sustainable emerging technologies for treating refractory organic contaminants present in different industrial wastewaters like textile, paper and pulp, pharmaceuticals, petrochemicals, and refineries. This critical review emerges details of advanced oxidation processes (AOPs), mentioning all possible permutations and combinations of components like ozone, UV, the catalyst used in the process. Non-conventional AOP systems, microwave, ultrasound, and plasma pulse assisted are the future of the oxidation process. This review aims to enlighten the role of AOPs for the mineralization of refractory organic contaminants (ROC) to readily biodegradable organics that cannot be either possible by conventional treatment. The integrated AOPs can improve the biodegradability of recalcitrant organic compounds and reduce the toxicity of wastewater, making them suitable for further biological treatment.

Heterogeneous photocatalytic oxidation for the removal of organophosphorus pollutants from aqueous solutions: A review

Organophosphorus pollutants (OPs), which are compounds containing carbon‑phosphorus bonds or phosphate derivatives containing organic groups, have received much attention from researchers because of their persistence in the aqueous environment for long periods of time and the threat they pose to human health. Heterogeneous photocatalysis has been widely applied to the removal of OPs from aqueous solutions due to its better removal effect and environmental friendliness. In this review, the removal of OPs from aqueous matrices by heterogeneous photocatalysis was presented. Herein, the application and the heterogeneous photocatalysis mechanism of OPs were described in detail, and the effects of catalyst types on degradation effect are discussed categorically. In particular, the heterojunction type photocatalyst has the most excellent effect. After that, the photocatalytic degradation pathways of several OPs were summarized, focusing on the organophosphorus pesticides and organophosphorus flame retardants, such as methyl parathion, dichlorvos, dimethoate and chlorpyrifos. The toxicity changes during degradation were evaluated, indicating that the photocatalytic process could effectively reduce the toxicity of OPs. Additionally, the effects of common water matrices on heterogeneous photocatalytic degradation of OPs were also presented. Finally, the challenges and perspectives of heterogeneous photocatalysis removal of OPs are summarized and presented.

Challenges on solar oxidation as post-treatment of municipal wastewater from UASB systems: Treatment efficiency, disinfection and toxicity

The application of solar photo-Fenton as post-treatment of municipal secondary effluents (MSE) in developing tropical countries is the main topic of this review. Alternative technologies such as stabilization ponds and upflow anaerobic sludge blanket (UASB) are vastly applied in these countries. However, data related to the application of solar photo-Fenton to improve the quality of effluents from UASB systems are scarce. This review gathered main achievements and limitations associated to the application of solar photo-Fenton at neutral pH and at pilot scale to analyze possible challenges associated to its application as post-treatment of MSE generated by alternative treatments. To this end, the literature review considered studies published in the last decade focusing on CECs removal, toxicity reduction and disinfection via solar photo-Fenton. Physicochemical characteristics of effluents originated after UASB systems alone and followed by a biological post-treatment show significant difference when compared with effluents from conventional activated sludge (CAS) systems. Results obtained for solar photo-Fenton as post-treatment of MSE in developed countries indicate that remaining organic matter and alkalinity present in UASB effluents may pose challenges to the performance of solar advanced oxidation processes (AOPs). This drawback could result in a more toxic effluent. The use of chelating agents such as Fe³⁺-EDDS to perform solar photo-Fenton at neutral pH was compared to the application of intermittent additions of Fe²⁺ and both of these strategies were reported as effective to remove CECs from MSE. The latter strategy may be of greater interest in developing countries due to costs associated to complexing agents. In addition, more studies are needed to confirm the efficiency of solar photo-Fenton on the disinfection of effluent from UASB systems to verify reuse possibilities. Finally, future research urges to evaluate the efficiency of solar photo-Fenton at natural pH for the treatment of effluents from UASB systems.

Application of photocatalytic ozonation with a WO3/TiO2 catalyst for PFAS removal under UVA/visible light

This research evaluates photocatalytic ozonation for removing 5 PFAS (PFOA/PFHxS/PFBS/6:2 FTS/GenX) from water using a WO₃/TiO₂ catalyst under UVA-visible radiation. Four catalysts of varying WO₃ content (0/1/3/5 wt%) were synthesized by sol-gel and characterized by XRD, TEM, STEM-EDS, HAADF-STEM, adsorption/desorption N₂ isotherms, and DRS-UV-vis. 5 wt% WO₃/TiO₂ was the optimal composition based on physicochemical properties and photocatalytic activity tests with methylene blue. PFAS degradation showed that photocatalytic ozonation inefficiently degraded PFAS with WO₃/TiO₂ under UVA-visible light after 4 h (ΣPFAS removal 16 %, [range 4 %-26 %]). Photocatalysis had comparable removal to photocatalytic ozonation, photolysis and ozone photolysis showed lower removal, and ozonation had no effect. Microtox analysis showed the initial acute toxicity was no longer detectable after photocatalysis and photocatalytic ozonation treatment. Low PFAS removals under tested conditions require that future work evaluate different catalysts or treatment conditions, while disparities between tested PFAS removals demonstrate the need to evaluate multiple compounds. ENVIRONMENTAL IMPLICATION: The research presented in this manuscript involves the preparation and characterization of WO₃/TiO₂ catalysts used, for the first time, to remove multiple PFAS in water via photocatalytic ozonation. This manuscript supports the development of a catalytic process for the elimination of hard to degrade environmental pollutants, provides new knowledge on aspects of photocatalytic processes, and provides insights on environmental pollution abatement.

Semiconductors Application Forms and Doping Benefits to Wastewater Treatment: A Comparison of TiO2, WO3, and g-C3N4

Photocatalysis has been vastly applied for the removal of contaminants of emerging concern (CECs) and other micropollutants, with the aim of future water reclamation. As a process based upon photon irradiation, materials that may be activated through natural light sources are highly pursued, to facilitate their application and reduce costs. TiO2 is a reference material, and it has been greatly optimized. However, in its typical configuration, it is known to be mainly active under ultraviolet radiation. Thus, multiple alternative visible light driven (VLD) materials have been intensively studied recently. WO3 and g-C3N4 are currently attractive VLD catalysts, with WO3 possessing similarities with TiO2 as a metal oxide, allowing correlations between the knowledge regarding the reference catalyst, and g-C3N4 having an interesting and distinct non-metallic polymeric structure with the benefit of easy production. In this review, recent developments towards CECs degradation in TiO2 based photocatalysis are discussed, as reference catalyst, alongside the selected alternative materials, WO3 and g-C3N4. The aim here is to evaluate the different techniques more commonly explored to enhance catalyst photo-activity, specifically doping with multiple elements and the formation of composite materials. Moreover, the possible combination of photocatalysis and ozonation is also explored, as a promising route to potentialize their individual efficiencies and overcome typical drawbacks.

Ozone-enhanced TiO2 nanotube arrays for the removal of COVID-19 aided antibiotic ciprofloxacin from water: Process implications and toxicological evaluation

The purpose of this study was to evaluate the performance of synthesized TiO₂ nanotube arrays (NTAs) for the removal of the COVID-19 aided antibiotic ciprofloxacin (CIP) and the textile dye methylene blue (MB) from model wastewater. Synthesis of TiO₂ NTAs showed that anodization potential and calcination temperatures directly influence nanotube formation. The increased anodization potential from 10 to 40 V resulted in the development of larger porous nanotubes with a diameter of 36-170 nm, while the collapse of the tubular structure was registered at the highest applied potential. Furthermore, it was found that the 500 °C calcination temperature was the most prominent for the formation of the most photocatalytically active TiO₂ NTAs, due to the optimal anatase/rutile ratio of 4.60. The degradation of both model compounds was achieved with all synthesized TiO₂ NTAs; however, the most photocatalytically active NTA sample was produced at 30 V and 500 ^°C. Compared to photocatalysis, CIP degradation was greatly enhanced by 5-25 times when ozone was introduced to the photocatalytic cell (rates 0.4-4.2 × 10^-1 min^-1 versus 0.07-0.2 × 10^-1 min^-1). This resulted in the formation of CIP degradation by-products, with different mass-to-charge ratios from [M+H]⁺ 346 to 273 m/z. Even though the CIP degradation pathway is rather complex, three main mechanisms, decarboxylation, hydroxylation reaction, and piperazine ring cleavage, were proposed and explained. Furthermore, treated samples were placed in contact with the crustaceans Daphnia magna. It was found that 100% mortality was achieved when approximately 60% of the remaining TOC was present in the samples, indicating that toxic degradation by-products were formed.

Monitoring of a Broad Set of Pharmaceuticals in Wastewaters by High-Resolution Mass Spectrometry and Evaluation of Heterogenous Catalytic Ozonation for Their Removal in a Pre-Industrial Level Unit

The removal of contaminants of emerging concern (CECs) occurring in wastewater effluents, such as pharmaceutically active substances (PhACs) and personal care products, pose a big research challenge since they can be a major source of pollution for water bodies and a danger to public health. The objective of this work was to perform a comprehensive monitoring of a broad set of PhACs (>130) in a wastewater treatment plant (WWTP) close to Thessaloniki (Greece), as well as to evaluate the potential of heterogeneous catalytic ozonation for the removal of CECs from wastewater through a continuous flow system. The high-resolution mass spectrometry analysis revealed the highest average concentrations for irbesartan (1817 ng/L). Antihypertensives along with antibiotics, psychiatrics, and β-blockers were found to aggravate the effluents. Removal efficiency after conventional treatment was >30%. The results from catalytic ozonation unit operation indicate that the introduction of a proper solid material that acts as catalyst can enhance the removal of CECs. A preliminary risk assessment using the risk quotient (RQ) revealed that irbesartan and telmisartan entail high acute risk. The overall results underline the urgent need to incessantly monitor PhACs and expand the toxicological studies to establish the sublethal and chronic effects on aquatic organisms.

Adsorptive removal of herbicides with similar structures from water over nitrogen-enriched carbon, derived from melamine@metal-azolate framework-6

Based on the recent concern on the pollution of water bodies with herbicides, adsorptive removal of typical herbicides with similar chemical structures, e. g. clofibric acid (CLFA), methylchlorophenoxypropionic acid or mecoprop (MCPP) and 2,4-dichlorophenoxyacetic acid (2,4-D) from water was studied using a porous nitrogen-enriched carbon. To prepare the nitrogen-enriched carbon, pyrolysis of a melamine (MLM) incorporated metal-azolate framework-6 (MLM(x)@MAF6; x = 0-50 M % of the ligand 2-ethylimidazole for MAF6), that was prepared for the first time via an in situ method, was carried out. The MLM(x)@MAF6-derived carbons (MDC6M(x)s) were characterized and used in the removal of CLFA, MCPP and 2,4-D from water. We found that the MDC6M(25), obtained from MLM(25)@MAF6 with 25% MLM (as the optimum precursor composition), showed the highest maximum adsorption capacity (Q₀) of 1031 mg/g for CLFA, compared with any reported adsorbents, so far. The physicochemical properties of CLFA, as well as adsorbents and adsorptions under wide pH conditions, were employed to propose a plausible adsorption mechanism including hydrogen bonding. Remarkably, the porous carbon with enriched nitrogen, derived from MAF6 loaded MLM via in situ method, was very competitive in herbicides adsorption because of the contribution of well-dispersed nitrogen sties on the adsorbent. Finally, MDC6M(25) was suggested as a potential adsorbent for the removal of herbcides, including CLFA, MCPP and 2,4-D, from water, which is highly attractive to mitigate the environmental issue, especially, water pollution by various herbicides.