A critical review on latest innovations and future challenges of electrochemical technology for the abatement of organics in water

Updated water directives and ambitious targets like the United Nations’ Sustainable Development Goals (SDGs) have emerged in the last decade to tackle water scarcity and contamination. Although numerous strategies have been developed to remove water pollutants, it is still necessary to enhance their effectiveness against toxic and biorefractory organic molecules. Comprehensive reviews have highlighted the appealing features of the electrochemical technologies, but much progress has been made in recent years. In this timely review, a critical discussion on latest innovations and perspectives of the most promising electrochemical tools for wastewater treatment is presented. The work describes the performance of electrocatalytic anodes for direct electrochemical oxidation, the oxidation mediated by electrogenerated active chlorine, the electrocatalytic reduction as well as coupled approaches for synchronous anodic and cathodic processes combined with homogeneous and heterogeneous catalysis. The last section is devoted to the assessment of scale-up issues and the increase in the technology readiness level

Performance of Electro-Fenton Process for Phenol Degradation Using Nickel Foam as a Cathode

Toxic substances have been released into water supplies in recent decades because of fast industrialization and population growth. Fenton electrochemical process has been addressed to treat wastewater which is very popular because of its high efficiency and straightforward design. One of the advanced oxidation processes (AOPs) is electro-Fenton (EF) process, and electrode material significantly affects its performance. Nickel foam was chosen as the source of electro-generated hydrogen peroxide (H2O2) due to its good characteristics. In the present study, the main goals were to explore the effects of operation parameters (FeSO4 concentration, current density, and electrolysis time) on the catalytic performance that was optimized by response surface methodology (RSM). According to the results, nickel foam made an excellent choice as cathode material. The pH value was adjusted at 3 and the airflow at 10 L/h for all experiments. It was found that the optimal conditions were current density of 4.23 mA/cm2, Fe2+ dosage of 0.1 mM, and time of 5 h to obtain the removal rates of phenol and chemical oxygen demand (COD) of 81.335% and 79.1%, respectively. The results indicated that time had the highest effect on the phenol and COD removal efficiencies, while the impact of current density was the lowest. The high R2 value of the model equation (98.03%) confirmed its suitability

PHENOL REMOVAL WITH ELECTROCHEMICAL CARBON NANOTUBES FILTER COUPLED WITH IN SITU GENERATED H202

Master'sMASTER OF SCIENC

Water treatment with boron-doped diamond electrodes – A review on by-products formation

The formation and identification of by-products is crucial for understanding the possible environmental and public health risks of applying electrochemical advanced oxidation processes (EAOPs) in water treatment, involving boron-doped diamond (BDD) anodes. Despite the substantial research work concerning the use of BDD technology in water treatment review articles with a specific focus on by-products formation and their hazardous effects in the major environmental fields of application in water treatment are not available. This comprehensive review provides the current knowledge on the reactive species and by-products formed during electrochemical oxidation with BDD in municipal landfill leachate treatment, industrial wastewater treatment, disinfection of drinking water, municipal wastewater treatment, and removal of micropollutants. The reactive species electrogenerated in EAOPs with BDD, including the reactive oxygen species (ROS), reactive sulfate species (RSS), reactive chlorine species (RCS) and other oxidant species, are systematically discussed. In general, halogenated and perhalogenated compounds are the common by-products identified in the different fields of application. The concentration and type of by-products were influenced by process conditions and the BDD anode material itself. Recommended conditions to minimize the formation of halogenated and perhalogenated compounds are acidic pH, low current densities, high chloride concentrations, NaNO3 as a supporting electrolyte and avoiding active cooling temperatures (e.g., 5ºC). The few available toxicological studies reported that BDD lowered the toxicity. However, in terms of toxic effects in mammalian’s (i.e., rats and humans) health, there is a considerable number of by products classified as possible carcinogenic, endocrine disruptive and mutagenic. This review paper provides better understanding of the by-products formed during electrochemical oxidation with BDD anodes and raises awareness of the improvements that still need to be made when applying this technology in water treatment plants.A formação e identificação de sub-produtos resultantes da utilização de processos eletroquímicos avançados de oxidação de diamante dopado com boro (BDD), para o tratamento de água, é essencial para a compreensão dos respetivos riscos para a saúde pública e o ambiente. Contudo, apesar da considerável bibliografia relativa à utilização desta tecnologia em tratamento de água, um estado da arte sobre a formação de sub-produtos e respetivos efeitos nocivos nas principais áreas de tratamento de água, ainda não foi elaborado. Esta dissertação tem como objetivo apresentar o conhecimento atual sobre as espécies reativas e sub-produtos formados durante a oxidação eletroquímica com BDD no tratamento de lixiviados de aterro municipal, água residual industrial, água potável, água residual municipal e remoção de micropoluentes. Foram abordadas as principais espécies reativas, nomeadamente, de oxigénio (ROS), de sulfato (RSS), de cloro (RCS) e, outras espécies oxidantes. Relativamente aos sub- produtos identificados, a presença de compostos halogenados e per-halogenados é recorrente nas diferentes áreas de tratamento de água. A concentração dos mesmos foi influenciada por condições experimentais e pelo material do BDD. Consequentemente, para minimizar a formação destes compostos, é recomendável a utilização de um pH ácido, baixa densidade de corrente, concentrações elevadas de cloretos, NaNO3 como eletrólito e evitar o uso de temperaturas baixas (e.g., 5oC). A diminuição da toxicidade com esta tecnologia foi verificada em alguns dos estudos. No entanto, em termos de efeitos tóxicos em mamíferos (i.e., roedores e humanos), existe um número considerável de sub-produtos classificados como possíveis carcinogénicos, disruptor endócrinos e mutagénicos. Esta dissertação contribui para uma melhor compreensão dos sub-produtos formados durante a oxidação eletroquímica com BDD, e proporciona um aumento da consciencialização sobre os progressos a realizar na implementação desta tecnologia em estações de tratamento de água

The application of in situ generated hydrogen peroxide for corrosion inhibition, disinfection and pollutant degradation

This thesis describes the utilisation and activation of in situ generation of hydrogen peroxide (H2O2) from dioxygen (O2) for corrosion inhibition, disinfection and pollutant degradation. MnCl2· 4H2O and Tiron (disodium 4,5-dihydroxy-1,3-benzenedisulfonate) rapidly remove O2 from aqueous solution at a rate of ~20 mg∙ L^(-1) min^(-1) using hydroxylamine (NH2OH) as reducing substrate. A mechanism is proposed that involves two 1-electron transfers from bound NH2OH to bound O2 to produce H2O2 concomitant with two proton transfers from catecholate oxygen atoms. This system can act as an anti-corrosion formulation as the catalytic reduction of O2 results in the removal of O2 from open aqueous solutions and the in situ generated H2O2 can be used as a biocide e.g. to kill L. pneumophila. The same system, which involves manganese(II) ions (Mn(II)) and Tiron as the co-catalyst for the in situ generation of H2O2, was also utilised for the oxidative degradation of Calmagite (CAL, 2-hydroxy-1-(2-hydroxy-5methylphenylazo)-4-naphthalenesulfonic acid) at room temperature. Percarbonate (HCO4-) was found to be the main reactive species for CAL degradation in the added H2O2 system buffered by carbonate at pH 9.0 in the absence of Mn(II). Manganese(IV)=O (Mn(IV)=O) and manganese(V)=O (Mn(V)=O) are the main reactive species in the added H2O2/Mn(II) system buffered by carbonate and non-carbonate buffers respectively. This system was enhanced by activation using ultrasound and copper(II) ions (Cu2+) as catalyst, forming the Cu2+/O2/ultrasound/NH2OH (COUN) system for the degradation of bisphenol AF (BPAF). Using a two-stage kinetic model, quantitative analysis of the catalytic efficiency showed that Cu2+ was relatively stable in the COUN system in contrast to the Cu2+/H2O2/ultrasound (CHU) system. This work contributes to a better understanding of the use of Cu2+, NH2OH and O2 for the in situ generation of H2O2, as well as the role of Cu2+ and NH2OH in Fenton-like systems

From Fenton and ORR 2e−-Type Catalysts to Bifunctional Electrodes for Environmental Remediation Using the Electro-Fenton Process

Currently, the presence of emerging contaminants in water sources has raised concerns worldwide due to low rates of mineralization, and in some cases, zero levels of degradation through conventional treatment methods. For these reasons, researchers in the field are focused on the use of advanced oxidation processes (AOPs) as a powerful tool for the degradation of persistent pollutants. These AOPs are based mainly on the in-situ production of hydroxyl radicals (OH center dot) generated from an oxidizing agent (H2O2 or O-2) in the presence of a catalyst. Among the most studied AOPs, the Fenton reaction stands out due to its operational simplicity and good levels of degradation for a wide range of emerging contaminants. However, it has some limitations such as the storage and handling of H2O2. Therefore, the use of the electro-Fenton (EF) process has been proposed in which H2O2 is generated in situ by the action of the oxygen reduction reaction (ORR). However, it is important to mention that the ORR is given by two routes, by two or four electrons, which results in the products of H2O2 and H2O, respectively. For this reason, current efforts seek to increase the selectivity of ORR catalysts toward the 2e(-) route and thus improve the performance of the EF process. This work reviews catalysts for the Fenton reaction, ORR 2e(-) catalysts, and presents a short review of some proposed catalysts with bifunctional activity for ORR 2e(-) and Fenton processes. Finally, the most important factors for electro-Fenton dual catalysts to obtain high catalytic activity in both Fenton and ORR 2e(-) processes are summarized.Ministry of Science and Innovation, Spain (MICINN) Spanish Government PID2021-127803OB-I00Junta de Andalucia B.RNM.566.UGR2

Solving challenges in electrochemical water treatment for a circular economy

Erweiterte elektrochemische Oxidationsverfahren (engl.: Electrochemical Advanced Oxidation Processes, EAOP) sind vielversprechende Technologien für die dezentrale Wasseraufbereitung und haben das Potential wichtige Elemente bei der Realisierung einer Kreislaufwirtschaft zu werden. Bislang wurde die Technologie trotz ihrer nahezu konkurrenzlosen Reinigungsleistung, aufgrund der Bildung von Hydroxylradikalen, noch kaum im technischen Maßstab angewendet. Daher wurden innerhalb der vorliegenden Arbeit vier Anwendungsfälle in verschiedenen Bereichen betrachtet und fünf Herausforderungen für EAOPs auf Basis von bordotierten Diamantelektroden (BDD) identifiziert, die eine technische Realisierung bislang erschweren: 1. Nur die Anode wird zur Erzeugung eines direkten Oxidationsmittels verwendet 2. Die kathodische Wasserstoffentwicklung trägt zu einem höheren Energieverbrauch und zur Schaumbildung bei 3. Hohe Betriebskosten aufgrund preisintensiver BDDs mit begrenzter Lebensdauer 4. Die Kathodenoberfläche wird während der Elektrolyse alkalisch. Dies führt beim Vorliegen von Härtebildnern in der Wassermatrix zu einer Verkalkung der Kathode, bis hin zur Isolierung der aktiven Oberfläche 5. Chlorierung von organischen Verbindungen wenn Chloridionen im zu behandelnden Wasser vorliegen. Diese Herausforderungen wurden durch zwei neuartige Reaktorsysteme und Prozessführungen gelöst. Durch die Verwendung eines Reaktors auf Grundlage einer BDD der nächsten Generation (auf Basis von Tantal anstatt Niob) in Kombination mit einer Wasserstoffperoxid-bildenden Gasdiffusionselektrode (BDD-GDE-System) wurden die Herausforderungen eins bis drei gelöst. Es konnte gezeigt werden, dass dieser Reaktor bei der Behandlung von künstlichen pharmazeutischen Abwässern eine wesentlich höhere Abbaueffizienz (135 %) bei einem wesentlich geringeren Energieverbrauch (75 %) gewährleistet als dem Stand der Technik entsprechende Zellkonzepte und die konkurrierenden Technologien Ozonierung und Perozonierung. Daneben ermöglichen BDD-GDE-Systeme einen nahezu 100 %-igen Abbau. Die neue Ta-basierte BDD zeigt eine deutlich reduzierte Degradation während des Betriebes auf und es wird eine erhöhte Lebensdauer von 18 Jahren anstelle von 2 Jahren (Nb-basierte BDD-Anoden) erwartet. Dies führt zu einer Reduzierung der Betriebskosten um bis zu 80 %. Dieses System stößt an seine Grenzen, wenn Härtebildner in hohen Konzentrationen im Wasser vorhanden sind (Ca2+, Mg2+). Diese Herausforderung wurde durch die Entwicklung eines neuartigen Zellendesigns auf der Grundlage einer in-situ bewegten Graphit-Polymer-Compound Kathode (BDD-GPC-System) gelöst. Ein Test (120 Stunden) mit künstlichem Zugtoilettenabwasser zeigte die Langzeitstabilität des BDD-GPC-Systems auf und demonstrierte, dass die periodisch magnetisch induzierte Bewegung der GPC-Kathode in-situ Ablagerungen auf ihrer Oberfläche entfernt und den wartungsarmen Betrieb des Systems ermöglicht. Neben der Abwasserreinigung ermöglicht die BDD-GPC-Kombination die elektrochemische Fällung von anorganischen Stoffen und eröffnet neue Anwendungsbereiche wie die elektrochemische Wasserenthärtung (Enthärtungsgrade bis > 90 %) und Metallfällung (Fällungsgrade bis > 98 %). Die fünfte Herausforderung wurde durch die Identifizierung eines neuartigen Betriebspunktes gelöst, der den Abbau organischer Verbindungen ohne die Bildung von Chloremissionen an BDD-Anoden und damit ohne die Bildung chlorierter Kohlenwasserstoffe und Produkte ermöglicht. Um diese chlorfreie Abwasserbehandlung an der BDD zu nutzen, muss der pH-Wert des Anolyts während der Behandlung bei über 14,2 gehalten werden. Mit diesen Entwicklungen kann die Kreislaufwirtschaft in der pharmazeutischen Industrie und in der metallverarbeitenden Industrie, insbesondere zur Wiederverwendung von Wasser, erreicht werden. Die chlorfreie Reinigung von organisch belasteten NaCl-haltigen Prozesswässern aus der Kunststoffproduktion ermöglicht die Wiederverwendung beispielsweise in der Chlor-Alkali-Industrie. Durch den Einsatz in Zügen kann das durchschnittliche Frischwassertankintervall deutlich verlängert werden, da das Abwasser im Zug aufbereitet und als Spülwasser wiederverwendet werden kann. Darüber hinaus ermöglicht ein Wasserenthärtungsreaktor weitere erhebliche Einsparpotentiale - lokale Frischwasserversorgungen der Züge werden von der Wasserhärte entkoppelt.Electrochemical advanced oxidation processes (EAOP) are promising technologies for decentralized water treatment and have the potential to be important components in achieving a circular economy. To date, EAOPs are rarely applied at technical scale despite their nearly unrivaled treatment performance owing to hydroxyl radical formation. Therefore, four use cases in diverse fields were considered and five challenges of boron-doped diamond electrode (BDD) based EAOPs were identified, which impede the application in technical scale: 1. Solely the anode is used for oxidant generation and only one oxidant is directly formed 2. Cathodic hydrogen evolution contributes to a higher energy consumption and foam formation 3. High operation costs due to expensive BDDs with limited lifetimes 4. The local pH at the cathodes surface becomes highly alkaline, which results in a calcification of the cathode in the presence of hardness minerals 5. Chlorination of organic compounds, in case of chloride ions in the wastewater matrix, generating toxic byproducts. These challenges were successively solved by two novel reactor systems and unique process controls. By using a reactor based on a "next-generation" BDD (tantalum-based instead of niobium) combined with a hydrogen peroxide-forming gas diffusion electrode (BDD–GDE system), challenges one to three were solved. Compared to the state-of-the-art cell design and also to the competitive technologies ozonation and peroxone it was shown that this novel reactor ensures much higher degradation efficiency (135 %) with much lower energy consumption (75 %) when treating artificial pharmaceutical wastewater. Next to the low energy demand of BDD–GDE systems, the investigations revealed a treatment efficiency rate of nearly 100 % with the lowest specific energy consumption per mass organic compared to the mentioned technologies and electrochemical processes reported in the literature. The new Ta-based BDDs show drastically reduced degradation during operation and an increased lifetime of 18 years is predicted instead of 2 years for Nb-based BDD anodes. This results in a reduction of up to 80 % of the operational costs. The system reaches its limits in presence of a high concentration of hardness minerals in the water (Ca2+, Mg2+). This challenge was solved by developing a novel cell design based on an in situ moving graphite-polymer-composite (GPC) cathode (BDD–GPC system). A treatment test (120 h) of artificial vacuum toilet wastewater indicated the long-term stability of the BDD–GPC system and demonstrated that the in situ periodically magnetic-induced movement of the GPC cathode removed deposits from its surface and ultimately resulted in a low-maintenance operation of the system. Besides the wastewater treatment, the BDD–GPC combination can be used for electrochemical precipitation of inorganics and opens new application areas, such as electrochemical water softening (softening levels up to > 90 %) and metal precipitation (efficiencies up to > 98 %). Challenge five was solved by experimentally determining an operation point for the purification of organic substances while avoiding the formation of chlorine species at BDD anodes. Therefore, the treated water remained free of chlorinated hydrocarbons. To reach the chlorine-free purification at the BDD, the pH value of the anolyte must be maintained above 14.2 during treatment. With the presented developments, a circular economy for water can be achieved in the pharmaceutical industry and in the metal processing industry. The chlorine-free purification of organic-polluted sodium chloride-containing water from plastics production can led to the reuse of water and brine in the chlor-alkali industry. The application in trains extend the average fresh water tank interval significantly due to an on-train wastewater treatment and reuse of the water as flushing water. For areas with high water hardness levels significant savings for water utilization can be achieved by the use of the above-mentioned water softening reactor

Advanced Oxidation Processes

Advanced Oxidation Processes – Applications, Trends, and Prospects constitutes a comprehensive resource for civil, chemical, and environmental engineers researching in the field of water and wastewater treatment. The book covers the fundamentals, applications, and future work in Advanced Oxidation Processes (AOPs) as an attractive alternative and a complementary treatment option to conventional methods. This book also presents state-of-the-art research on AOPs and heterogeneous catalysis while covering recent progress and trends, including the application of AOPs at the laboratory, pilot, or industrial scale, the combination of AOPs with other technologies, hybrid processes, process intensification, reactor design, scale-up, and optimization. The book is divided into four sections: Introduction to Advanced Oxidation Processes, General Concepts of Heterogeneous Catalysis, Fenton and Ferrate in Wastewater Treatment, and Industrial Applications, Trends, and Prospects

Treatment of azo dyes in industrial wastewater using microbial fuel cells

Due to the extensive use of xenobiotic azo dyes in the colour industry and their proven mutagenic and cytotoxic nature, their treatment prior to discharge is essential and is legally enforced. However, currently used wastewater treatment technologies such as activated sludge systems, anaerobic digestion, electrochemical destruction, adsorption and membrane filtration are ineffective in removing azo dyes due to reasons such as inefficient dye degradation, slow degradation kinetics, toxic metabolite formation, inhibitory costs and generation of secondary waste streams. Therefore, in this study, microbial fuel cells (MFCs) were studied as possible systems that could effectively degrade azo dyes with an additional benefit of concomitant biogenic electricity generation. The co-metabolic degradation of the model azo dye Acid Orange-7 (AO-7) using Shewanella oneidensis and mixed anaerobic cultures in MFC was carried out with particular emphasis on AO-7 degradation kinetics in the initial study. The effect of using various carbon sources including cheaper complex ones such as molasses and corn steep liquor as electron donors for azo dye degradation in MFCs was also investigated. The outcomes of this study demonstrated that fast AO-7 reductive degradation kinetics using cheap, sustainable co-substrate types can be achieved with concomitant bioelectricity generation in two-chamber MFCs. Power densities up-to 37 mWm-2 were observed in the two-chamber MFC system during AO-7 decolourisation. Co-metabolic reductive degradation of azo dye mixtures using dye acclimated mixed microbial populations under industrially relevant conditions (high temperatures and salinities) and changes in microbial community structure in the MFCs in presence of complex azo dye mixtures in two-chamber MFCs was investigated. The outcomes of this work demonstrated that efficient colour and organic content removal can be achieved under high temperatures and moderate salinities using azo dye adapted mixed microbial populations in two-chamber MFCs. Microbial community analysis of the original anaerobic consortium and the azo dye adapted microbial culture following MFC operation indicated that both cultures were dominated by bacteria belonging to the phylum Firmicutes. However, bacteria belonging to phyla Proteobacteria and Bacteroidetes also became selected following MFC operation. Peak power densities up-to 27 mWm-2 were observed in this study during decolourisation of complex azo dye mixtures. The complete degradation of the azo dye AO-7 using a sequential reductive – oxidative bioprocess in a combined MFC-aerobic bioreactor system operating at ambient temperature in continuous mode was studied. The outcomes of this study demonstrated that the azo dye AO-7 can be fully decolourised and degraded into non-toxic and simpler metabolites. Maximum power densities up-to 52 mWm-2 were observed during azo dye degradation. A modular scale-up version (with a volumetric scale-up factor of 6) of the two stage integrated bioreactor system demonstrated the capability to efficiently treat two types of real wastewater originating from colour industry without any apparent deterioration of reactor performance in terms of dye decolourisation and COD removal. The use of applied external resistance (Rext) and redox mediators as tools for enhancing azo dye degradation kinetics in dual chamber MFCs was studied. The outcomes of this work suggest that azo dye reductive degradation kinetics in MFC anodes can be influenced by varying Rext. Furthermore, AO-7 reductive degradation kinetics was improved in a concentration-dependent manner by exogenous addition of two electron shuttling compounds anthraquinone-2,6-disulfonic acid and anthraquinone-2-sulfonic acid in MFC anodes. The overall outcomes of this study implies that MFCs could be successfully applied for achieving enhanced azo dye reductive biodegradation kinetics in MFC anodes coupled with concomitant bioelectricity generation. It further demonstrated that MFC systems can be successfully integrated with existing wastewater treatment technologies such as activated sludge systems for complete degradation and toxicity removal of azo dyes and their biotransformation metabolites

Treatment of Antibiotics in Wastewater Using Advanced Oxidation Processes (AOPs)

Antibiotics are nonbiodegradable, can survive at aquatic environments for long periods and they have a big potential bio-accumulation in the environment. They are extensively metabolized by humans, animals and plants. After metabolization, antibiotics or their metabolites are excreted into the aquatic environment. Removal of these compounds from the aquatic environment is feasible by different processes. But antibiotics are not treated in conventional wastewater treatment plants efficiently. During the last years studies with advanced oxidation processes (AOPs) for removal of these pharmaceuticals from waters has shown that they can be useful for removing them fully. Advanced oxidation processes (AOPs) can work as alternatives or complementary method in traditional wastewater treatment, and highly reactive free radicals, especially hydroxyl radicals (OH) generated via chemical (O3/H2O2, O3/OH-), photochemical (UV/O3, O3/H2O2) reactions, serve as the main oxidant. This study presents an overview of the literature on antibiotics and their removal from water by advanced oxidation processes. It includes almost all types of antibiotics which are consumed by human and veterinary processes. It was found that most of the investigated advanced oxidation treatment processes for the oxidation of antibiotics in water are direct and indirect photolysis with the combinations of H2O2, TiO2, ozone and Fenton?s reagent