A Review on Advanced Oxidation Processes for Effective Water Treatment

Mechanisms in the photocatalytic breakdown of persistent pharmaceutical and pesticide molecules over TiO2-based photocatalysts: A review

Titanium dioxide (TiO2) based photocatalysts have been widely used as a photocatalyst for the degradation of various persistent organic compounds in water and air. The degradation mechanism involves the generation of highly reactive oxygen species, such as hydroxyl radicals, which react with organic compounds to break down their chemical bonds and ultimately mineralize them into harmless products. In the case of pharmaceutical and pesticide molecules, TiO2and modified TiO2photocatalysis effectively degrade a wide range of compounds, including antibiotics, pesticides, and herbicides. The main downside is the production of dangerous intermediate products, which are not frequently addressed in the literature that is currently available. The degradation rate of these compounds by TiO2photocatalysis depends on factors such as the chemical structure of the compounds, the concentration of the TiO2catalyst, the intensity, the light source, and the presence of other organic or inorganic species in the solution. The comprehension of the degradation mechanism is explored to gain insights into the intermediates. Additionally, the utilization of response surface methodology is addressed, offering a potential avenue for enhancing the scalability of the reactors. Overall, TiO2photocatalysis is a promising technology for the treatment of pharmaceutical and agrochemical wastewater, but further research is needed to optimize the process conditions and to understand the fate and toxicity of the degradation products.

Safeguarding drinking water: A brief insight on characteristics, treatments and risk assessment of contamination

Water pollution stands as a critical worldwide concern, bearing extensive repercussions that extend to human health and the natural ecosystem. The sources of water pollution can be diverse, arising from natural processes and human activities and the pollutants may range from chemical and biological agents to physical and radiological contaminants. The contamination of water disrupts the natural functioning of the system, leading to both immediate and prolonged health problems. Various technologies and procedures, ranging from conventional to advanced, have been developed to eliminate water impurities, with the choice depending on the type and level of contamination. Assessing risks is a crucial element in guaranteeing the safety of drinking water. Till now, research is continuing the removal of contaminates for the sake of supplying safe drinking water. The study examined physical, inorganic, organic, biological and radiological contaminants in drinking water. It looked at where these contaminants come from, their characteristics, the impact they have and successful methods used in real-world situations to clean the contaminated water. Risk assessment methodologies associated with the use of unsafe drinking water as future directives are also taken into consideration in the present study for the benefit of public concern. The manuscript introduces a comprehensive study on water pollution, focusing on assessing and mitigating risks associated with physical, inorganic, organic, biological and radiological contaminants in drinking water, with a novel emphasis on future directives and sustainable solutions for public safety.

Advanced oxidation/reduction processes (AO/RPs) for wastewater treatment, current challenges, and future perspectives: a review

Advanced oxidation/reduction processes (AO/RPs) are considered as effective water treatment technologies and thus could be used to solve the problem of water pollution. These technologies of wastewater treatment involve the production of highly reactive species such as •OH, H•, e-aq, SO4•-, and SO3•-. These radicals can attack the targeted contaminants present in aqueous media and result in their destruction. The efficiency of AO/RPs is highly affected by various operational parameters such as initial concentration of contaminant, solution pH, catalyst amount, intensity of light source, nature of oxidant and reductant used, and the presence of various ionic species in aquatic media. Among AO/RPs, the solar light-based AO/RPs are most widely used nowadays for contaminant removal from aqueous media because of their high environmental friendliness and cost effectiveness. By using these techniques, almost all types of pollutants can be easily removed from aquatic media within short intervals of time, and hence, the problem of water pollution can be solved effectively. This review focuses on various AO/RPs used for wastewater treatment. The effects of different operational parameters that affect the efficiency of these processes toward contaminant removal have been discussed. Besides, challenges and future recommendations are also briefly provided for the researchers in order to improve the efficiency of these processes.

Pharmaceutically active compounds in aqueous environment: recent developments in their fate, occurrence and elimination for efficient water purification

The existence of pharmaceutically active compounds (PhACs) in the water is a major concern for environmentalists due to their deleterious effects on living organisms even at minuscule concentrations. This review focuses on PhACs such as analgesics and anti-inflammatory compounds, which are massively excreted in urine and account for the majority of pharmaceutical pollution. Furthermore, other PhACs such as anti-epileptics, beta-blockers and antibiotics are discussed because they also contribute significantly to pharmaceutical pollution in the aquatic environment. This review is divided into two parts. In the first part, different classes of PhACs and their fate in the wastewater environment are presented. In the second part, recent advances in the removal of PhACs by conventional wastewater treatment plants, including membrane bioreactors (MBRs), activated carbon adsorption and bench-scale studies concerning a broad range of advanced oxidation processes (AOPs) that render practical and appropriate strategies for the complete mineralization and degradation of pharmaceutical drugs, are reviewed. This review indicates that drugs like diclofenac, naproxen, paracetamol and aspirin are removed efficiently by conventional systems. Activated carbon adsorption is suitable for the removal of diclofenac and carbamazepine, whereas AOPs are leading water treatment strategies for the effective removal of reviewed PhACs.

Construction and demolition waste as a high-efficiency advanced process for organic pollutant degradation in Fenton-like reaction to approach circular economy

The Fenton-like reaction is a promising organic wastewater treatment reaction among advanced oxidation processes (AOP), which has emerged to replace the conventional Fenton reaction. Recycled construction and demolition waste (CDW), which is porous and rich in iron, manganese, and magnesium, can be reused as a Fenton-like catalyst. This study proposes an AOP wastewater treatment strategy using recycled porous CDW mixed with hydrogen peroxide (H2O2) to decompose methylene blue (MB) wastewater. According to the apparent first-order rate (Kapp) of 10 ppm MB adsorption, CDW-3, having the highest specific surface area, also has the highest Kapp of 0.23 min-1 g-1. The optimized conditions recommended by the Taguchi method include a 0.3 g mL-1 CDW-3 concentration, a 0.254 g mL-1 H2O2 concentration, and 10 ppm MB, resulting in an about 2.01 min-1Kapp value. In addition, MB concentration is observed as the most influential factor for Kapp, which decreases with increasing MB concentration and is about 0.62 min-1 at 1000 ppm MB. Repeating the Fenton-like reaction five times at 100 p.m. MB using the same CDW-3, the Kapp is about 0.64 min-1, which is 86% of the initial run. The synergistic effect index (ξ) is defined to quantify the level of interaction between CDW and H2O2, which produces free radicals during the Fenton-like process. The ξ of CDW-3 is about 2.16. Overall, it is demonstrated that CDW is a promising catalyst for Fenton-like reactions, and the synergistic effect index (ξ) can be used as a reference index to evaluate the catalytic generation of free radicals between the catalyst and H2O2.

Fenton and photo-assisted advanced oxidative degradation of ionic liquids: a review

Ionic liquids (ILs) are the class of materials which are purely ionic in nature and liquid at room temperature. Their remarkable properties like very low vapour pressure, non-inflammable and high heat resistance are responsible for their use as a very appealing solvent in a variety of industrial applications in place of regular organic solvents. Because ILs are water soluble to a certain extent, the industrial wastewater effluents are found to contaminate with their traces. The non-biodegradability of ILs attracts the attention of the researchers for their removal or degradation from wastewater. Numbers of methods are available for the treatment of wastewater. However, it is very crucial to use the most efficient method for the degradation of ILs. Advanced oxidation process (AOP) is one of the most important techniques for the treatment of ILs in wastewater which have been investigated during last decades. This review paper covers the cost-effective Fenton, photochemical and photocatalytic AOPs and their combination that could be applied for the degradation of ILs from the wastewater. Theoretical explanations of these AOPs along with experimental conditions and kinetics of degradation or removal of ILs from water and wastewater have been reported and compared. Finally, future perspectives of IL degradation are presented.

A novel Corchorus olitorius-derived biochar/Bi12O17Cl2 photocatalyst for decontamination of antibiotic wastewater containing tetracycline under natural visible light

Herein, a novel composite of Corchorus olitorius-derived biochar and Bi12O17Cl2 was fabricated and utilized for the degradation of tetracycline (TC) in a solar photo-oxidation reactor. The morphology, chemical composition, and interaction between the composite components were studied using various analyses. The biochar showed a TC removal of 52.7% and COD mineralization of 59.6% using 150 mg/L of the biochar at a pH of 4.7 ± 0.5, initial TC concentration of 163 mg/L, and initial COD of 1244 mg/L. The degradation efficiency of TC increased to 63% and the mineralization ratio to 64.7% using 150 mg/L of bare Bi12O17Cl2 at a pH of 4.7 ± 0.5, initial TC concentration of 178 mg/L, and COD of 1034 mg/L. In the case of biochar/Bi12O17Cl2 composite, the degradation efficiency of TC and COD mineralization ratio improved to 85.8% and 77.7% due to the potential of biochar to accept electrons which retarded the recombination of electrons and holes. The synthesized composite exhibited high stability over four succeeding cycles. According to the generated intermediates, TC could be degraded to caprylic acid and pentanedioic acid via the frequent attack by the reactive species. The prepared composite is a promising photocatalyst and can be applied in large-scale systems due to its high degradation and mineralization performance in a short time besides its low cost and stability.

Photocatalytic degradation of naphthol blue from Batik waste using functionalized TiO2-based composites

This work provides a first-time comparative study examining the photocatalytic activity of functionalized TiO₂-based composites to eliminate naphthol blue in Batik wastewater. Reduced graphene oxide (RGO) was synthesized by oxidizing solid graphite using the Hummers' method followed by sonication and reduction. N-doped TiO₂ (N-TiO₂) was synthesized from titanium tetrachloride (TiCl₄) and urea (CH₄N₂O) precursors by the sol-gel method. N-TiO₂ modified RGO (RGO/NT) was synthesized using a hydrothermal method from N-TiO₂ and RGO. Prepared TiO₂-based composites and commercial TiO₂, for comparison were characterized using Fourier transform infrared spectrometer (FTIR), X-Ray diffractometer (XRD), scanning electron microscope-energy dispersive X-ray (SEM-EDX), and UV-Vis diffuse reflectance spectrometer (UV-Vis DRS). FTIR characterization indicated Ti-N bonding in N-TiO₂ and RGO/NT. XRD patterns showed that commercial TiO₂ had a rutile phase, while N-TiO₂ and RGO/NT had an anatase phase with crystal sizes of 30.09, 16.28, and 12.02 nm, respectively. SEM results displayed the presence of small and glossy white N-TiO₂ dispersed on the surface of RGO. Characterization using UV-Vis DRS showed that the band gap energy values for TiO₂, N-TiO₂, and RGO/NT were 3.25, 3.12, and 3.08 eV with absorption regions at the wavelengths of 382, 398, and 403 nm, respectively. The highest photocatalytic activity for RGO/NT for degrading naphthol blue was obtained at pH 5, with a photocatalyst mass of 60 mg, and an irradation of 15 min. Photocatalytic degradation by RGO/NT on Batik wastewater under visible light showed higher effectivity than under UV light.

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.

Insight into enhanced degradation of tetracycline over peroxymonosulfate activated via biochar-based nanocomposite: performance and mechanism

Rice husk biochars (BCs) doped with ferric chloride were prepared by one-pot method, characterized by SEM, EDS, BET, XRD, and FTIR, and utilized to catalyze peroxymonosulfate (PMS) for tetracycline (TC) degradation. Various influencing factors in the BC/PMS/TC system were investigated, as well as the recycling performance of the optimal BC. The mechanism of BC activation of PMS and degradation of TC were analyzed based on the free radicals quenching experiment and the pathways of TC degradation. The results demonstrated that bBC3 was an excellent catalyst with large specific surface area; the amounts of oxidant and catalyst were important factors affecting the catalytic performance of PMS, while pH had less effect on TC degradation; 10 mM of chloride ions inhibited the TC degradation, while 20 mM promoted the TC degradation; other ions and humic acid inhibited the TC degradation at the set concentrations; activation of PMS by bBC3 yielded species with strong oxidative activity, which were primarily responsible for TC degradation. The bBC3 obtained stable performance for removing TC. This study provided a pathway for the deep utilization of waste rice husks besides an effective method for degrading TC.

Pharmaceutical Transformation Products Formed by Ozonation-Does Degradation Occur?

The efficiency of an advanced oxidation process (AOP) using direct and indirect ozonation for the removal of pharmaceutical residues from deliberately spiked deionized water was examined. Both direct and indirect ozonation demonstrated 34% to 100% removal of the parent compounds. However, based on the products' chemical structure and toxicity, we suggest that despite using accepted and affordable ozone and radical concentrations, the six parent compounds were not fully degraded, but merely transformed into 25 new intermediate products. The transformation products (TPs) differed slightly in structure but were mostly similar to their parent compounds in their persistence, stability and toxicity; a few of the TPs were found to be even more toxic than their parent compounds. Therefore, an additional treatment is required to improve and upgrade the traditional AOP toward degradation and removal of both parent compounds and their TPs for safer release into the environment.

Advanced oxidation processes: Performance, advantages, and scale-up of emerging technologies

Advanced oxidation processes (AOPs) are promising technologies for partial or complete mineralization of contaminants of emerging concern by highly reactive hydroxyl, hydroperoxyl, superoxide, and sulphate radicals. Detailed investigations and reviews have been reported for conventional AOP systems that have been installed in full-scale wastewater treatment plants. However, recent efforts have focused on the peroxymonosulphate, persulphate, catalytic ozonation, ultrasonication and hydrodynamic cavitation, gamma radiation, electrochemical oxidation, modified Fenton, and plasma-assisted AOPs. This critical review presents the detailed mechanisms of emerging AOP technologies, their performance for treatment of contaminants of emerging concern, the relative advantages and disadvantages of each technology, and the remaining challenges to scale-up and implementation. Among the evaluated technologies, the modified electrochemical oxidation, gamma radiation, and plasma-assisted systems demonstrated the greatest potential for successful and sustainable implementation in wastewater treatment due to their environmental safety, compatibility, and efficient transformation of contaminants of emerging concern by a variety of reactive species. The other emerging AOP systems were also promising, but additional scale-up trials and a deeper understanding of their reaction kinetics in complex wastewater matrices are necessary to determine the technical and economic feasibility of full-scale processes.

Current Trends in the Utilization of Photolysis and Photocatalysis Treatment Processes for the Remediation of Dye Wastewater: A Short Review

Development in the textile industry leads to an increased demand for the use of various dyes. Moreover, there is the use of some dyes in the food industry as well as medical diagnostics. Thereby, increased demand for dyes in various fields has resulted in dye-containing wastewater. Only a small portion of the generated wastewater is adequately treated. The rest is usually dumped or otherwise directly discharged into the sewage system, which ultimately enters rivers, lakes, and streams. The handling and disposal of such concentrated wastewater, especially the dye-containing wastewater, is considered to be a major environmental issue from the moment of its generation to its ultimate disposal. Conventional water treatment methods such as flotation, filtration, adsorption, etc., are non-destructive physical separation processes. They only transfer the pollutants to other phases, thereby generating concentrated deposits. The advanced oxidation process (AOP) is one of the most effective emerging methods for the treatment of wastewater containing chemical pollutants. The method involves the formation and interaction of highly reactive hydroxyl radicals under suitable activation conditions. These radicals are non-selective and efficient for the destruction and eventual mineralization of recalcitrant organic pollutants. This review aims at the pros and cons of using photocatalysis as an efficient AOP to degrade dye-containing wastewater.

Addressing the water-energy nexus: A focus on the barriers and potentials of harnessing wastewater treatment processes for biogas production in Sub Saharan Africa

Several anthropogenic activities reduce the supply of freshwater to living organisms in all ecological systems, particularly the human population. Organic matter in derived wastewater can be converted into potential energy, such as biogas (methane), through microbial transformation during anaerobic digestion (AD). To address the current lack of data and values for wastewater generation in Sub-Saharan Africa, this review analyzes and estimates (at 50% and 90% conversion rates) the potential amount of wastewater-related sludge that can be generated from domestic freshwater withdrawals using the most recent update in 2017 from the World Bank repository and database on freshwater status in Sub-Saharan Africa. The Democratic Republic of the Congo (DRC) could potentially produce the highest estimate of biogas in Sub-Saharan Africa from domestic wastewater sludge of approximately 90 billion m3, which could be converted to 178 million MWh of electricity annually, based on this extrapolation at 50% conversion rates. Using same conversion rates estimates, at least nine other countries, including Guinea, Liberia, Nigeria, Sierra Leone, Angola, Cameroon, Central African Republic, Gabon, and Congo Republic, could potentially produce biogas in the range of 1-20 billion m3. These estimates show how much energy could be extracted from wastewater treatment plants in Sub-Saharan Africa. AD process to produce biogas and energy harvesting are essential supplementary operations for Sub-Saharan African wastewater treatment plants. This approach could potentially solve the problem of data scarcity because these values for Freshwater withdrawals are readily available in the database could be used for estimation and projections towards infrastructure development and energy production planning. The review also highlights the possibilities for energy generation from wastewater treatment facilities towards wastewater management, clean energy, water, and sanitation sustainability, demonstrating the interconnections and actualization of the various related UN Sustainable Development Goals.

Review on the Use of Heavy Metal Deposits from Water Treatment Waste towards Catalytic Chemical Syntheses

The toxicity and persistence of heavy metals has become a serious problem for humans. These heavy metals accumulate mainly in wastewater from various industries' discharged effluents. The recent trends in research are now focused not only on the removal efficiency of toxic metal particles, but also on their effective reuse as catalysts. This review discusses the types of heavy metals obtained from wastewater and their recovery through commonly practiced physico-chemical pathways. In addition, it covers the advantages of the new system for capturing heavy metals from wastewater, as compared to older conventional technologies. The discussion also includes the various structural aspects of trapping systems and their hypothesized mechanistic approaches to immobilization and further rejuvenation of catalysts. Finally, it concludes with the challenges and future prospects of this research to help protect the ecosystem.

Sustainable treatment of paint industry wastewater: Current techniques and challenges

Paint manufacturing industries produce wastewater containing high chemical oxygen demand and turbidity, besides organic matter, suspended solids, and heavy metals that cause enormous environmental damages. Safely treating this wastewater before being disposed to the natural water sources is essential for attaining the UN SDGs, particularly Goal 14: Life under water. Besides being efficient, wastewater treatment techniques must be sustainable - environmentally, economically, and ethically. While a few papers have reviewed specific treatment methods for certain pollutants, such as heavy metals, oils, and azo dyes from industrial wastewater, a comprehensive review of various treatment methods for all the pollutants of a particular industrial wastewater - paint industry - is lacking. This paper reviews the current treatment methods used for treating paint industry wastewater including the physicochemical, biological, and chemical treatment techniques. The physicochemical techniques produce large amount of sludge making it difficult for disposal while biological treatment techniques are difficult to maintain because of the uncertainties in the chemical compositions of the paint wastewater. Advanced oxidation processes are emerging as preferred methods among the chemical methods for reducing the toxicity of the various components of the paint wastewater with reduced sludge quantity. The review of various emerging techniques of paint industry wastewater treatments in this paper points to the need for paying greater attention to combining the oxidation and biological processes as they are emerging as sustainable methods for effective reduction of toxicity in paint wastewater while also reducing the sludge management challenges.

A Review on the Treatment of Petroleum Refinery Wastewater Using Advanced Oxidation Processes

The petroleum industry is one of the most rapidly developing industries and is projected to grow faster in the coming years. The recent environmental activities and global requirements for cleaner methods are pushing the petroleum refining industries for the use of green techniques and industrial wastewater treatment. Petroleum industry wastewater contains a broad diversity of contaminants such as petroleum hydrocarbons, oil and grease, phenol, ammonia, sulfides, and other organic composites, etc. All of these compounds within discharged water from the petroleum industry exist in an extremely complicated form, which is unsafe for the environment. Conventional treatment systems treating refinery wastewater have shown major drawbacks including low efficiency, high capital and operating cost, and sensitivity to low biodegradability and toxicity. The advanced oxidation process (AOP) method is one of the methods applied for petroleum refinery wastewater treatment. The objective of this work is to review the current application of AOP technologies in the treatment of petroleum industry wastewater. The petroleum wastewater treatment using AOP methods includes Fenton and photo-Fenton, H2O2/UV, photocatalysis, ozonation, and biological processes. This review reports that the treatment efficiencies strongly depend on the chosen AOP type, the physical and chemical properties of target contaminants, and the operating conditions. It is reported that other mechanisms, as well as hydroxyl radical oxidation, might occur throughout the AOP treatment and donate to the decrease in target contaminants. Mainly, the recent advances in the AOP treatment of petroleum wastewater are discussed. Moreover, the review identifies scientific literature on knowledge gaps, and future research ways are provided to assess the effects of these technologies in the treatment of petroleum wastewater.

Orbitrap mass spectrometry for the molecular characterization of water resource recovery from polluted surface water using membrane bioreactor

The increasing organic contamination of surface water hinders the conventional tap water treatment process. Membrane bioreactors (MBRs) are a promising alternative technology for recovering water from polluted surface water. In this study, the composition changes of dissolved organic matters (DOMs) in MBR and ultraviolet/ozone (UV/O₃)-MBR systems for polluted surface water treatment were investigated using Orbitrap mass spectrometry analysis with unknown screening. The intense DOM ions within a mass-to-charge ratio range of 100-500 was detected, and 2340 molecular formulae from 5743 peaks were assigned to the two systems. The most abundant components were formulae with C, H, O, N, and CHO only classes. The highest formulae decrease including CHO, CHON, CHOS, and CHONS were attributed to the bio-carrier used in both systems. Results showed that bioprocess was the main contributor in the DOM reduction, and the integration of UV/O₃ into the MBR improved the DOM composition changes. Biodegradable components with low O/C ratio in the CHO and CHON classes remarkably increased in the UV/O₃-MBR system. The integration of UV/O₃ as a polishing step in the recirculation stream of MBR system was effective in improve the DOM removal.

TiO2/Schwertmannite nanocomposites as superior co-catalysts in heterogeneous photo-Fenton process

The heterogeneous photo-Fenton reaction is an effective technique in combating organic contaminants for both soil and water remediation, and extensive studies have focused on enhancing its efficiency and reducing its costs. In this work, we developed novel photo-Fenton catalysts by simply milling commercially available TiO₂ (P25) with Schwertmannite (Sh), a natural iron-oxyhydroxysulfate nanomineral. We expect that the photo-generated electrons from TiO₂ could continuously migrate to Sh, which then could enhance the separation of electron-hole pairs on TiO₂ and accelerate the reduction of Fe(III) to Fe(II) on Sh, leading to high degradation efficiency of the target organic contaminants. SEM and TEM results showed the distribution of TiO₂ on Sh surface for the nanocomposites (TiO₂/Sh). Under simulated sunlight irradiation, the much higher content of Fe(II) was determined on TiO₂/Sh than on Sh via a common method in the iron ore, and the consumption of H₂O₂ and the production of •OH were more significant in the TiO₂/Sh system than those in the TiO₂ and Sh systems. These results well support our hypothesis that the photo-generated electrons could migrate from TiO₂ to Sh on the composites, and can also explain the much higher degradation efficiency of Rhodamine B (RhB) in the TiO₂/Sh system. Besides, TiO₂/Sh had lower Fe dissolution as compared with Sh, and retained high catalytic stability after four repeated cycles. Above merits of the TiO₂/Sh composites, in combining with their simple synthesis method and low-cost property, indicated that they should have promising applications as heterogeneous photo-Fenton catalysts.