Papers by Zacharias Frontistis
Higher alcohol (HA) synthesis via the hydrogenation of CO 2 constitutes a relatively new and exci... more Higher alcohol (HA) synthesis via the hydrogenation of CO 2 constitutes a relatively new and exciting field of research that has the potential to help towards the de-carbonization of the energy sector. The process poses formidable challenges, as it demands the formation of at least one C-C bond, when CO 2 is thermodynamically stable, fully oxidized and kinetically inert. This work provides a comprehensive and critical literature review of the catalytic formulations that have been employed, in both fixed-bed and batch reactors, which include noble metal catalysts, transition metal-based systems, post-transition metal catalysts, bimetallic, multimetallic/multifunctional catalysts, Metal Organic Frameworks (MOFs), perovskite-, and zeolite-based catalysts. The critical role of promoters and supports and the effect that the reaction conditions have on performance are also discussed. Emphasis has been given to single atom catalysts (SACs), as the high specific activity of these systems seems to hold great promise for the reaction at hand. Breakthroughs made by employing the concept of tandem catalysis are also critically analyzed. This review paper also discusses the thermodynamic aspects of the reaction and the insights that have been gained regarding the reaction mechanism. Finally, it provides an overview of the direction that research may move to into the future.
The sonochemical degradation of acesulfame (ACE), a widely used artificial sweetener, was investi... more The sonochemical degradation of acesulfame (ACE), a widely used artificial sweetener, was investigated. The impact of power density (40 and 71 W/L), initial ACE concentration (1.5-30.6 μМ), and initial solution pH for the elimination of the target pollutant from water was studied. It was found that nearly complete degradation of ACE was achieved within 120 min of treatment under conditions suitable for high production of free radicals. Degradation follows a pseudo-first order kinetic model. Coupling sonochemical degradation at optimal conditions with sodium persulfate (0.2-1.0 mM) enhanced the rate of ACE oxidation by six times. The influence of water complexity was also examined; the natural occurrence of humic acid in the water matrix reduced the degradation rate, despite the fact that at least 42 % of ACE removal was achieved. Finally, LC-QToF-MS was used to evaluate the generated transformation products (TPs). Using HRMS suspect and non-target screening, seven ACE TPs were identified, and their time-trend formation profiles showed a rising trend throughout the experiment. According to the ECOSAR model, most of the identified transformation products exhibited lower toxicity than acesulfame, with one notable exception that was classified as harmful due to its acute toxicity on fish and green algae.
Despite their compact design and superior effluent quality, resulting from the smart integration ... more Despite their compact design and superior effluent quality, resulting from the smart integration of separation processes with biodegradation, the widespread adoption of membrane bioreactors is limited due to the persistent issue of membrane fouling. This mini review encapsulates research on using ultrasound-induced vibration to mitigate membrane fouling in domestic wastewater systems. It discusses the dual physical and chemical mechanisms behind the use of ultrasound and highlights its limitations. Moreover, it proposes several future research directions, including further examination of operating conditions such as ultrasound power and frequency, combining ultrasound with other cleaning strategies scaling up the process, and the need for long-term experiments, where additional emphasis must be given (i) to the influence of ultrasound irradiation on membrane integrity and efficiency and (ii) estimating the cost of sonochemical integration and operation Finally, it underscores the need for a holistic assessment that considers energy and environmental issues and discusses a proposed roadmap toward implementing ultrasound technology in membrane bioreactor systems.
This work examines the use of pristine Mo 2 C as an intriguing sodium persulfate (SPS) activator ... more This work examines the use of pristine Mo 2 C as an intriguing sodium persulfate (SPS) activator for the degradation of the drug losartan (LOS). Using 500 mg/L Mo 2 C and 250 mg/L SPS, 500 µg/L LOS was degraded in less than 45 min. LOS decomposition was enhanced in acidic pH, while the apparent kinetic constant decreased with higher LOS concentrations. According to experiments conducted in the presence of scavengers of reactive species, sulfate radicals, hydroxyl radicals, and singlet oxygen participated in LOS oxidation, with the latter being the predominant reactive species. The presence of competitors such as bicarbonate and organic matter reduced the observed efficiency in actual matrices, while, interestingly, the addition of chloride accelerated the degradation rate. The catalyst showed remarkable stability, with complete LOS removal being retained after five sequential experiments. The system was examined for simultaneous LOS decomposition and elimination of Escherichia coli. The presence of E. coli retarded LOS destruction, resulting in only 30% removal after 3 h, while the system was capable of reducing E. coli concentration by 1.23 log. However, in the presence of simulated solar irradiation, E. coli was reduced by almost 4 log and LOS was completely degraded in 45 min, revealing a significant synergistic effect of the solar/Mo 2 C/SPS system.
In the last few years, the in situ H 2 O 2 electrogeneration through the 2e -oxygen reduction rea... more In the last few years, the in situ H 2 O 2 electrogeneration through the 2e -oxygen reduction reaction (ORR) pathway has increasingly intrigued the scientific community. In particular, a wide experimental campaign has addressed the synthesis of different carbonaceous cathodes. This has resulted in a high number of 2D and 3D cathode materials, either pristine or modified, whose ability to electrogenerate H 2 O 2 in an efficient manner has been tested in different types of electrochemical reactors. Although there is some experimental work correlating the properties of cathode materials with the H 2 O 2 production efficiency, there is no systematic study on this subject. The purpose of this critical review, which is focused on the literature published within the period 2010-2022, is to elucidate the role of the interfacial properties of carbonaceous cathodes in this field. These cathodes can be classified in two categories according to their structure (i.e., crystalline and fibrous), and in five subcategories: reticulated vitreous carbon (RVC), graphite, carbon felt, graphite felt and gas-diffusion electrode (GDE). These categories are briefly introduced and the performance of materials is compared. Additionally their interfacial properties (hydrophilicity/hydrophobicity, O 1s/C 1s ratio, pore size and specific surface area) are analyzed, trying to show their influence on the cathode performance in terms of current efficiency. The durability of the cathodes is also examined, along with density functional theory (DFT) studies conducted for the 2e -ORR. Finally, general conclusions and recommendations for future research are presented.
Among different biological methods used for advanced wastewater treatment, membrane bioreactors h... more Among different biological methods used for advanced wastewater treatment, membrane bioreactors have demonstrated superior efficiency due to their hybrid nature, combining biological and physical processes. However, their efficient operation and control remain challenging due to their complexity. This comprehensive review summarizes the potential of artificial neural networks (ANNs) to monitor, simulate, optimize, and control these systems. ANNs show a unique ability to reveal and simulate complex relationships of dynamic systems such as MBRs, allowing for process optimization and fault detection. This early warning system leads to increased reliability and performance. Integrating ANNs with advanced algorithms and implementing Internet of Things (IoT) devices and new-generation sensors has the potential to transform the advanced wastewater treatment landscape towards the development of smart, self-adaptive systems. Nevertheless, several challenges must be addressed, including the need for high-quality and large-quantity data, human resource training, and integration into existing control system facilities. Since the demand for advanced water treatment and water reuse will continue to expand, proper implementation of ANNs, combined with other AI tools, is an exciting strategy toward the development of integrated and efficient advanced water treatment schemes.
The development of efficient heterogeneous persulfate activators is one of the main research topi... more The development of efficient heterogeneous persulfate activators is one of the main research topics in the wastewater treatment area. The present work deals with the heterogeneous activation of sodium persulfate (SPS) using nickel oxide/strontium carbonate (NiO/SrCO 3 ) for the degradation of sulfamethoxazole (SMX), a representative compound from the group of antibiotics. Results showed that NiO/SrCO 3 exhibited high performance towards the activation of SPS, leading to SMX elimination in brief time spans. The impact of SPS (25-100 mg/L), NiO/SrCO 3 (50-250 mg/L), and SMX (0.25-3.00 mg/L) concentration, and initial pH on the decomposition of SMX was further examined. Experiments were also conducted in real matrices such as secondary effluent and bottled water, revealing the existence of retarding phenomena compared to ultrapure water. This behavior was further investigated with the addition of bicarbonates, chlorides, or humic acid in ultrapure water. It was found that organic matter significantly hampered SMX removal. The role of the main radicals (hydroxyl and sulfate radicals) was determined using appropriate radical traps (methanol and tertbutanol). These quenching experiments combined with the conducted electrochemical measurements revealed that both a radical and a non-radical mechanism contribute to the decomposition of SMX.
This study offers a review of machine learning (ML) applications in membrane bioreactor (MBR) sys... more This study offers a review of machine learning (ML) applications in membrane bioreactor (MBR) systems, an emerging technology in advanced wastewater treatment. The review focuses on implementing ML algorithms to enhance the prediction of membrane fouling, control and optimize the system, and predict faults early, thereby enabling the development of novel cleaning strategies. Key ML algorithms such as artificial neural networks (ANNs), support vector machines (SVMs), random forest, and reinforcement learning (RL) are briefly introduced, with an emphasis on their potential and limitations in advanced wastewater applications. The main challenges obstructing the implementation, namely data quality, interpretability, and transferability of ML, are identified. Finally, future research trends are proposed, including ML integration with big data, the Internet of Things (IoT), and hybrid model development. The review also underscores the need for interdisciplinary collaboration and investment in data management, along with the implementation of new policies addressing data privacy and security. By addressing these challenges, the integration of ML into MBRs has the potential to significantly enhance performance and reduce the energy footprint, providing a sustainable solution for advanced wastewater treatment.
The solar-induced semiconductor photocatalytic process is one of the greenest and most promising ... more The solar-induced semiconductor photocatalytic process is one of the greenest and most promising technologies for the elimination of pharmaceuticals in aqueous media. In the context of this study, a bismuth oxychloride (BiOCl) photocatalyst was fabricated and characterized by its morphology, crystallographic structure, and optical properties. Its photocatalytic efficiency was tested towards the degradation of Losartan (LOS), a medication used to treat high blood pressure, in water using a solar simulator. The as-prepared BiOCl exhibited significant photocatalytic efficiency, achieving complete degradation of 0.3 mg/L LOS in short periods of irradiation (15-30 min). The examined system showed optimal efficiency using 500 mg/L of BiOCL (k app = 0.21 min -1 ) and pH 3 (k app = 0.32 min -1 ). However, LOS removal significantly decreased in environmentally relevant water matrices, including wastewater (k app = 0.006 min -1 ) and bottled water (k app = 0.023 min -1 ). Additional tests carried out in synthetic water matrices showed that the LOS degradation rate was reduced by more than 40% in the presence of humic acid (k app = 0.016 min -1 ) and bicarbonates (k app = 0.029 min -1 ), while chlorides did not affect the overall efficiency. Moreover, photogenerated holes and singlet oxygen were the dominant oxidative species. The efficiency of the BiOCl photocatalyst towards LOS degradation was further studied using a flat plate pilot-plant scale photoreactor. It was found that more than 75% of LOS was removed after 100 kJ/L of accumulated solar irradiation. The results obtained in the pilot-plant unit confirmed the suitability of BiOCl as a potential photocatalytic material.
As the research community works towards the development of new catalytic materials aiming to over... more As the research community works towards the development of new catalytic materials aiming to overcome the main problem of today's catalysts (e.g., low selectivity and stability), the recently discovered single atom catalysts (SACs) appear to be promising alternatives to the widely used heterogeneous catalysts composed of nanoparticles of the active metal. SACs have rapidly attracted interest as, for certain reactions such as oxidation, WGSR and selective hydrogenation, they possess numerous advantages (high activity, stability, low cost, etc.) in comparison to their cluster and nanoparticle counterparts. Although, initially, the fabrication of these types of catalysts was considered unattainable, many researchers nowadays have successfully synthesized and tested them in a variety of catalytic reactions (oxidation, hydrogenation, etc.). Among the different reactions, the hydrogenation of CO 2 is perhaps the most interesting one since the catalysts that are industrially used today suffer from low activity and stability. This work provides a comprehensive literature review of the single atom catalytic systems that have so far been employed, including both noble and transition metals, focusing exclusively on the CO 2 hydrogenation reaction, in an effort to highlight the most promising features of these systems and to aid future research efforts.
In this work, the influence of supporting electrolytes (sodium sulfate and sodium chloride) on th... more In this work, the influence of supporting electrolytes (sodium sulfate and sodium chloride) on the electrochemical oxidation of the antihypertensive drug losartan (LOS) was studied under different operating conditions such as current density (4.1-12.5 mA cm -2 ), electrolyte concentration (0.05-0.5 M), initial pollutant concentration (250-1000 μg L -1 ) and solution pH. The role of cathodes on the removal of LOS has been investigated using five different cathodes with carbonaceous cathodes showing better LOS removal. The effect of matrix composition has been studied using simulated water spiked with various constituents and real water matrices such as bottled water (BW) and wastewater (WW). The removal of LOS was pronounced while using a chloride electrolyte as compared to the sulfate electrolyte. The apparent rate constant increased on adding persulfate (PS) up to concentrations of 150 mg L -1 and decreased in the presence of bicarbonates and organic matter. The transformation products (TPs) formed during the electrochemical oxidation depended on the supporting electrolyte and two common TPs were identified in both electrolytes with a total of 4 TPs identified in the chloride medium and 7 TPs in the sulfate medium. Degradation pathways for LOS in both electrolytes have also been proposed.
Water electrolysis is among the recent alternatives for generating clean fuels (hydrogen). It is ... more Water electrolysis is among the recent alternatives for generating clean fuels (hydrogen). It is an efficient way to produce pure hydrogen at a rapid pace with no unwanted by-products. Effective and cheap water-splitting electrocatalysts with enhanced activity, specificity, and stability are currently widely studied. In this regard, noble metal-free transition metal-based catalysts are of high interest. Iron sulfide (FeS) is one of the essential electrocatalysts for water splitting because of its unique structural and electrochemical features. This article discusses the significance of FeS and its nanocomposites as efficient electrocatalysts for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and overall water splitting. FeS and its nanocomposites have been studied also for energy storage in the form of electrode materials in supercapacitors and lithium-(LIBs) and sodium-ion batteries (SIBs). The structural and electrochemical characteristics of FeS and its nanocomposites, as well as the synthesis processes, are discussed in this work. This discussion correlates these features with the requirements for electrocatalysts in overall water splitting and its associated reactions. As a result, this study provides a road map for researchers seeking economically viable, environmentally friendly, and efficient electrochemical materials in the fields of green energy production and storage.
Biochar derived from pomegranate peel at different pyrolysis temperatures (450, 600, and 850 • C)... more Biochar derived from pomegranate peel at different pyrolysis temperatures (450, 600, and 850 • C) was synthesized and characterized by BET, XRD, FTIR, and SEM-EDX. Its catalytic efficiency in the degradation of the antihypertensive losartan (LOS) in the presence of sodium persulfate was examined. The biochar pyrolyzed at 850 • C exhibited higher catalytic activity, which was correlated with the greater surface area and higher concentration of minerals on its surface. Interestingly, despite adsorption being favored at alkaline pH, pH 3 showed the highest LOS degradation. LOS decomposition followed pseudo-first-order kinetics. The addition of persulfate significantly increased LOS reduction, while the presence of inorganic and organic water matrix constituents such as sodium chloride, bicarbonate, and humic acid inhibited the oxidation. Experiments conducted with radical scavengers revealed that both hydroxyl and sulfate radicals, as well as singlet oxygen, participated in LOS decomposition, with the former being the dominant species. Using a continuous flow reactor, the system exhibited a satisfactory steady-state performance of 90% LOS removal for 114 h. Afterward, a moderate decrease in performance was observed, which can be attributed to the alteration of the catalyst's surface and mineral dissolution due to acidity.
In this study, a series of Pd/CeO 2 catalysts were synthesized, characterized, and evaluated for ... more In this study, a series of Pd/CeO 2 catalysts were synthesized, characterized, and evaluated for the activation of persulfate and the degradation of the micropollutant, bisphenol A (BPA). The efficiency followed a volcano-type behavior with respect to Pd loading, and the 0.25% wt. Pd/CeO 2 exhibited the highest catalytic activity. However, this activity strongly depended on the operating conditions. The system was able to degrade 500 μg/L BPA in less than 30 min, and the removal was favored at near neutral pH (6.2). Scavenging experiments highlighted the role of superoxide and singlet oxygen, followed by sulfate radicals. The efficiency was found to be stable across several cycles, despite a slight decrease in the first cycle. The removal of BPA decreased with the complexity of the water matrices, showing the need for system optimization under real conditions. Five transformation products were identified using UHPLC/TOF-MS and their ecotoxicity was estimated using ECOSAR. Intriguingly, the system was capable of inactivating 99.99% of 2.4 × 10 5 CFU/mL E. coli, in less than 210 min making it an appealing alternative technology for the simultaneous inactivation of pathogens and degradation of micropollutants in environmental systems. conduction band ⟶ photocatalysis SO .- 4 + SO 2- 4 (2) Biochar surface -OH + S 2 O 2- 8 →Biochar surface -O . + SO .- 4 + HSO - 4
Adsorption is an effective and attractive solution for (waste) water treatment, with the dedicate... more Adsorption is an effective and attractive solution for (waste) water treatment, with the dedicated articles to be around 50,000 in 2023. Development of biomass-derived sorbents is assumed as a modern approach within the frame of circular (bio)economy. This opinion article underscores diverse preparation methods and applications of biomass-derived materials, highlighting the need for standardized protocols to enable comparability and activity evaluation across studies. Additionally, future challenges related to real-life implementation are discussed, such as incomplete material characterization, regeneration, disposal, and the need to move from laboratoryscale to industrial applications. Finally, establishing an international database for adsorption performance under fixed conditions, incorporating environmental and economic factors, is proposed as a crucial step for optimizing and standardizing biomass-derived adsorbents for water and wastewater treatment.
than DCF. Both acute toxicity and thyroid effects were mitigated with a prolonged degradation tim... more than DCF. Both acute toxicity and thyroid effects were mitigated with a prolonged degradation time. This study highlights the importance of integrating in vivo bioassays in the environmental risk assessment of novel degradation processes.
In this work, various MoB/BiOCl (0.5-2.0% wt. MoB) photocatalysts were fabricated, characterized ... more In this work, various MoB/BiOCl (0.5-2.0% wt. MoB) photocatalysts were fabricated, characterized using X-ray diffraction, diffuse reflectance spectroscopy, scanning electron microscopy/energy dispersive spectrometer (SEM/EDS) and chronoamperometry, and their photocatalytic activity was assessed for the degradation of the persistent contaminant losartan (LOS) in aqueous solutions under simulated solar irradiation. The 0.5MoB/BiOCl catalyst exhibited the highest efficiency, achieving complete degradation of 500 μg/L LOS in less than 10 min. Interestingly, the 0.5MoB/BiOCl sample also showed remarkable efficiency under visible irradiation (>420 nm), achieving 80% removal in 30 min. The decomposition kinetics followed pseudo-first-order, while the efficiency significantly decreased under alkaline conditions. According to the scavenging experiments, both h + , O •- 2 and 1 O 2 participate in the decomposition of LOS, while the role of • OH is minor. A slight increase in degradation was observed in the presence of NaCl, and a small retardation occurred in the presence of humic acid. On the contrary, the presence of bicarbonates significantly delayed the reaction. Although significant inhibition was observed for secondary effluent, the system was capable of completely degrading LOS in less than 15 min when pH was adjusted to 3.8. The process demonstrated promising results for the simultaneous removal of E. coli and micropollutants, achieving complete LOS removal in 15 min and 99.999% inactivation of E. coli after 180 min.
In this work, the decomposition of bisphenol S (BPS) by biochar derived from banana peel (BPB) pr... more In this work, the decomposition of bisphenol S (BPS) by biochar derived from banana peel (BPB) promoted by copper phosphide (Cu 3 P) was examined. Different materials with Cu 3 P loadings from 0.25 to 4.00 wt.% on biochar were synthesized, characterized using the Brunauer-Emmett-Teller (BET) method, X-ray diffraction (XRD) and a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS), and evaluated. Nearly all of the synthesized materials exhibited low to moderate adsorption capacity, attributable to their limited surface area (<3.1 m 2 /g). However, in the presence of sodium persulfate (SPS), the 2%Cu 3 P/BPB/SPS system was capable of removing 90% of 500 µg/L BPS in less than 10 min. The system's performance was enhanced under inherent pH, and the reaction rate followed pseudo-first-order kinetics with respect to BPS and persulfate concentrations. Interestingly, the presence of 250 mg/L of sodium chloride had a negligible effect, while low to moderate inhibition was observed in the presence of bicarbonates and humic acid. In contrast, significant retardation was observed in experiments performed in real matrices, such as secondary effluent (WW) and bottled water (BW). According to scavenging experiments, both radical and non-radical mechanisms participated in the BPS degradation. Four transformation products were identified using the UHPLC/TOF-MS system in negative ionization mode, with two of them having higher molecular weights than BPS, while the other two TBPs involved the ring-opening reaction, and a BPS decomposition pathway was proposed.
Developing highly active and available, environmentally friendly, and low-cost photocatalytic mat... more Developing highly active and available, environmentally friendly, and low-cost photocatalytic materials is one of the most popular topics in photocatalytic degradation systems. In the present study, a series of BiOCl/Sepiolite composite photocatalysts were prepared (in the range of 5%BiOCl/Sepiolite-30%BiOCl/Sepiolite). Their characterization was conducted using X-ray diffraction, diffuse reflectance spectroscopy, scanning electron microscopy, nitrogen physical physisorption at the temperature of liquid nitrogen (77 K), and attenuated total reflectance-Fourier transform infrared spectroscopy. Results showed that composite photocatalysts possess superior efficiency than the parent materials for losartan, an antihypertensive agent, degradation in water, with the sample with only 10%wt. BiOCl shows the highest performance. The beneficial effect of the addition of sepiolite to BiOCl is derived from the increase in surface area, the prevention of particle aggregation, and the efficient separation of photogenerated species. Increasing catalyst concentration from 125 mg/L up to 500 mg/L was accompanied by an increase in the apparent kinetic constant from 0.077 min -1 to 0.197 min -1 while varying losartan concentration from 0.25 to 5.00 mg/L slowed down the removal efficiency. In addition, losartan degradation was only partially hampered in the case of bottled water, whereas it was practically stopped in a secondary wastewater effluent. Overall, this study serves as a useful guide for using geopolymers in photocatalytic applications.
BACKGROUND: The present study investigates the in situ electrogeneration of H 2 O 2 by the 2e -ox... more BACKGROUND: The present study investigates the in situ electrogeneration of H 2 O 2 by the 2e -oxygen reduction reaction in an electrochemical flow cell using three commercially available cathodic electrode materials with different interfacial properties; that is, activated carbon felt (ACF), non-activated carbon felt (NACF) and gas diffusion electrode (GDE). RESULTS: Experiments were first performed to optimize the operating parameters and to determine the yield of H 2 O 2 electrosynthesis. The estimated maximum yield of ∼40% was achieved with the NACF electrode after 30 min of operation. Furthermore, the performance of the electrodes was investigated in two scenarios, namely (i) under conditions with O 2 -saturated electrolyte and (ii) under continuous air supply, to study the differences in O 2 distribution within the electrode material as a function of its interfacial properties. The maximum H 2 O 2 concentration for the first scenario was 3.86 mg L -1 after 60 min of operation, which was obtained with the NACF electrode. In contrast, in the second scenario, H 2 O 2 electrogeneration was significantly increased for all three electrodes, with the GDE exhibiting the best performance, at 43.3 mg L -1 after 60 min of operation. CONCLUSIONS: The properties of the electrode material play a decisive role in H 2 O 2 electrogeneration. The hydrophilicity of the electrode is very important when O 2 is dissolved in the electrolyte. Hydrophobicity is preferable when gaseous O 2 is applied to the electrode as a three-phase interface is formed, which favors the transfer/penetration of O 2 . The pore size of the electrode is of crucial importance, as mesoporous materials facilitate the distribution of gaseous O 2 in the electrode body.
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Papers by Zacharias Frontistis