Efficient sonoelectrochemical decomposition of sulfamethoxazole adopting common Pt/graphite electrodes: The mechanism and favorable pathways

A review on reactive oxygen species generation, anode materials and operating parameters in sonoelectrochemical oxidation for wastewater remediation

The application of sonoelectrochemical (SEC) oxidation technique involving the incorporation of ultrasound irradiation into an electrochemical oxidation system has found enormous success for various purposes, especially for organic synthesis and water treatment. Although its industrial application towards the removal of organic contaminants in water is not popular, its success on the laboratory scale is often attributed to the physical and chemical effects. These effects arise from the influence of ultrasound irradiation, thus eliminating electrode passivation or fouling, improving mass transfer and enhancing reactive oxygen species (ROS) generation. The continuous activation of the electrode surface, improved reaction kinetics and other associated advantages are equally occasioned by acoustic streaming and cavitation. This review hereby outlines common ROS generated in SEC oxidation and pathways to their generation. Furthermore, classes of materials commonly employed as anodes and the influence of prominent operational parameters on the performance of the technique for the degradation of organic pollutants in water are extensively discussed. Hence, this study seeks to broaden the significant promises offered by SEC oxidation to environmentally sustainable technology advances in water treatment and pollution remediation.

Impact of Condition Variations on Bioelectrochemical System Performance: An Experimental Investigation of Sulfamethoxazole Degradation

Bioelectrochemical systems (BESs) are an innovative technology for the efficient degradation of antibiotics. Shewanella oneidensis (S. oneidensis) MR-1 plays a pivotal role in degrading sulfamethoxazole (SMX) in BESs. Our study investigated the effect of BES conditions on SMX degradation, focusing on microbial activity. The results revealed that BESs operating with a 0.05 M electrolyte concentration and 2 mA/cm2 current density outperformed electrolysis cells (ECs). Additionally, higher electrolyte concentrations and elevated current density reduced SMX degradation efficiency. The presence of nutrients had minimal effect on the growth of S. oneidensis MR-1 in BESs; it indicates that S. oneidensis MR-1 can degrade SMX without nutrients in a short period of time. We also highlighted the significance of mass transfer between the cathode and anode. Limiting mass transfer at a 10 cm electrode distance enhanced S. oneidensis MR-1 activity and BES performance. In summary, this study reveals the complex interaction of factors affecting the efficiency of BES degradation of antibiotics and provides support for environmental pollution control.

Ultrasound for the degradation of endocrine disrupting compounds in aqueous solution: A review on mechanisms, influence of operating parameters and cost estimation

Availability of drinking water is one of the basic humanitarian goals but remains as a grand challenge that the world is facing today. Currently, water bodies are contaminated not only with conventional pollutants but also with numerous recalcitrant pollutants, such as PPCPs, endocrine disrupting compounds, etc. These emerging pollutants require special attention because of their toxicity to living organisms, bio-resistant and can sustain even after primary and secondary treatments of wastewater. Among different treatment technologies, sonolysis is found to be an innovative and promising technique for the treatment of emerging pollutants present in aqueous solution. Sonolysis is the use of ultrasound to enhance or alter chemical reactions by the formation of free radicals and shock waves which ultimately helps in degradation of pollutants. This review summarizes several studies in the sonochemical literature, including mechanisms of sonochemical process, physical and chemical effects of ultrasound, and the influence of several process variables such as ultrasound frequency, power density, temperature and pH of the medium on degradation performance for endocrine disrupting compounds. In addition, this review highlighted techno-economic perspectives focusing on the total cost required for translating the ultrasound-based processes on a large scale. Overall, the objective of this study is to exhibit a critical review of information available in the literature to encourage and promote future research on sonolysis for the degradation of Endocrine Disrupting Compounds (EDCs).

An anode and cathode cooperative oxidation system constructed with Ee-GF as anode and CuFe2O4/Cu2O/Cu@EGF as cathode for the efficient removal of sulfamethoxazole

This study aimed to further improve the degradation efficiency of pollutants by electrochemical oxidation system and reduce the consumption of electric energy. A simple method of electrochemical exfoliation was used to modify graphite felt (GF) to prepare an anode material (Ee-GF) with high degradation performance. An anode and cathode cooperative oxidation system was constructed with Ee-GF as the anode and CuFe₂O₄/Cu₂O/Cu@EGF as the cathode to efficiently degrade sulfamethoxazole (SMX). Complete degradation of SMX was achieved within 30 min. Compared with anodic oxidation system alone, the degradation time of SMX was reduced by half and the energy consumption was reduced by 66.8 %. The system displayed excellent performance for the degradation of different concentrations (10-50 mg L^-1) of SMX, different pollutants, and under different water quality conditions. In addition, the system still maintained 91.7 % removal rate of SMX after ten consecutive runs. At least 12 degradation products and seven possible degradation routes of SMX were generated in the degradation process by the combined system. The eco-toxicity of degradation products of SMX was reduced after the proposed treatment. This study provided a theoretical basis for the safe, efficient, and low energy consumption removal of antibiotic wastewater.

Sonoelectrochemical oxidation of sulfamethoxazole in simulated and actual wastewater on a piezo-polarizable FTO/BaZr x Ti(1-x)O3 electrode: reaction kinetics, mechanism and reaction pathway studies

The sonoelectrochemical (SEC) oxidation of sulfamethoxazole (SMX) in simulated and actual wastewater on FTO/BaZr_(0.1)Ti_(0.9)O₃, FTO/BaZr_(0.05)Ti_(0.95)O₃ and FTO/BaTiO₃ electrodes is hereby presented. Electrodes from piezo-polarizable BaZr_(0.1)Ti_(0.9)O₃, BaZr_(0.05)Ti_(0.95)O₃, and BaTiO₃ materials were prepared by immobilizing these materials on fluorine-doped tin dioxide (FTO) glass. Electrochemical characterization performed on the electrodes using chronoamperometry and electrochemical impedance spectroscopy techniques revealed that the FTO/BaZr_(0.1)Ti_(0.9)O₃ anode displayed the highest sonocurrent density response of 2.33 mA cm^-2 and the lowest charge transfer resistance of 57 Ω. Compared to other electrodes, these responses signaled a superior mass transfer on the FTO/BaZr_(0.1)Ti_(0.9)O₃ anode occasioned by an acoustic streaming effect. Moreover, a degradation efficiency of 86.16% (in simulated wastewater), and total organic carbon (TOC) removal efficiency of 63.16% (in simulated wastewater) and 41.47% (in actual wastewater) were obtained upon applying the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode for SEC oxidation of SMX. The piezo-polarizable impact of the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode was further established by the higher rate constant obtained for the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode as compared to the other electrodes during SEC oxidation of SMX under optimum operational conditions. The piezo-potential effect displayed by the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode can be said to have impacted the generation of reactive species, with hydroxyl radicals playing a predominant role in the degradation of SMX in the SEC system. Additionally, a positive synergistic index obtained for the electrode revealed that the piezo-polarization effect of the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode activated during sonocatalysis combined with the electrochemical oxidation process during SEC oxidation can be advantageous for the decomposition of pharmaceuticals and other organic pollutants in water.

Electrified ceramic membrane actuates non-radical mediated peroxymonosulfate activation for highly efficient water decontamination

Electrified ceramic membranes (ECMs) achieve high water decontamination efficiency mainly through implementing in situ radical-mediated oxidation in membrane filtration, whereas ECMs leveraging non-radical pathways are rarely explored. Herein, we demonstrated a Janus ECM realizing ultra-efficient micropollutant (MP) removal via electro-activating peroxymonosulfate (PMS) in a fast, flow-through single-pass electro-filtration. The Janus ECM features two separate palladium (Pd) functionalized electrocatalytic reaction zones engineered on its two sides. We confirmed that the PMS/electro-filtration system induced non-radical pathways for MP degradation, including singlet oxygenation and mediating direct electron transfer (DET) from MP to PMS. Under the design of the ECM featuring dual electrocatalytic reaction zones in the ceramic membrane intrapores, the Janus ECM showed over one-fold increase in micropollutant removal rate as 94.5% and lower electric energy consumption as 1.78 Wh g^-1 MP in the PMS electro-activation process, as compared with the conventional ECM assembly implementing only half-cell reaction. This finding manifested the Janus ECM configuration advantage for maximizing the PMS electro-activation efficiency via singlet oxygenation intensification and direct usage of cathode for DET mediation. The Janus ECM boosted the PMS electro-activation and water decontamination efficiency by enhancing the convective mass transfer and the spatial confinement effect. Our work demonstrated a high-efficiency PMS electro-activation method based on electro-filtration and maximized the non-radical mediated PMS oxidation for MP removal, expanding the ECM filtration strategies for water decontamination.

Pharmaceuticals and personal care products in water streams: Occurrence, detection, and removal by electrochemical advanced oxidation processes

Pharmaceutical and personal care products (PPCPs) are part of the emerging contaminants (ECs) in the environment due to their known or suspected adverse effects in aquatic and terrestrial organisms, as well as in human health. Presence of PPCPs in aquatic and terrestrial ecosystems has been mainly attributed to the effluents of wastewater treatment plants (WWTPs). Although several PPCPs have been detected in wastewater, their removal from wastewater via biological processes is limited. Removal of PPCPs depends on their chemical structure, concentration, solubility, and technology used to treat the wastewater. Electrochemical Advanced Oxidation Processes (EAOPs) are some of the most sought-after methods for dealing with organic pollutants in water including PPCPs, due to generation of strong oxidants such as •OH, H₂O₂ and O₃^- by using directly or indirectly electrochemical technology. This review is focused on the removal of main PPCPs via EAOPs such as, anodic oxidation, electro-Fenton, photoelectron-Fenton, solar photoelectron-Fenton, photoelectrocatalysis and sonoelectrochemical processes. Although more than 40 PPCPs have been identified through different analytical approaches, antibiotics, anti-inflammatory and antifungal are the main categories of PPCPs detected in different water matrices. Application of EAOPs has been centered in the removal of antibiotics and analgesics of high consumption by using model media, e.g. Na₂SO_4. Photoelectrocatalysis and Electro-Fenton processes have been the most versatile EAOPs applied for PPCPs removal under a wide range of operating conditions and a variety of electrodes. Although EAOPs have gained significant scientific interest due to their effectiveness, low environmental impact, and simplicity, further research about the removal of PPCPs and their by-products under realistic concentrations and media is needed. Moreover, mid-, and long-term experiments that evaluate EAOPs performance will provide knowledge about key parameters that allow these technologies to be scaled and reduce the potential risk of PPCPs in aquatic and terrestrial ecosystem.

Recent progress on ultrasound-assisted electrochemical processes: A review on mechanism, reactor strategies, and applications for wastewater treatment

The electrochemical advanced oxidation processes (EAOPs) have received significant attention among the many other water and wastewater treatment technologies. However, achieving a desirable removal effect with a single technique is frequently difficult. Therefore, the integration of ultrasound technique with other processes such as electrocoagulation, electro-Fenton, and electrooxidation is a critical way to achieve effective organic pollutants decomposition from wastewater. This review paper is focused on ultrasound-assisted electrochemical (US/electrochemical) processes, so-called sonoelectrochemical processes of various organic pollutants. Emphasis was given to recently published articles for discussing the results and trends in this research area. The use of ultrasound and integration with electrochemical processes has a synergistic impact owing to the physical and chemical consequences of cavitation, resulting in enhancing the mineralization of organic pollutants. Various types of sonoelectrochemical reactors (batch and continuous) employed in the US/electrochemical processes were reviewed. In addition, the strategies to avoid passivation, enhanced generation of reactive oxygen species, and mixing effect are reviewed. Finally, concluding remarks and future perspectives on this research topic are also explored and recommended.

Complete decomposition of sulfamethoxazole during an advanced oxidation process in a simple water treatment system

A simple water treatment system consisting of a deep UV light (λ = 222 nm) source, a mesoporous TiO₂/boron-doped diamond (BDD) photocatalyst, and a BDD electrode was prepared and used to decompose sulfamethoxazole (SMX) in an advanced oxidation process. The mesoporous TiO₂/BDD photocatalyst used with the electrochemical treatment promoted SMX decomposition, but the mesoporous TiO₂/BDD photocatalyst alone had a similar ability to decompose SMX as photolysis. Fragments produced through photocatalytic treatment were decomposed during the electrochemical treatment and fragments produced during the electrochemical treatment were decomposed during the photocatalytic treatment, so performing the electrochemical and photocatalytic treatments together effectively decomposed SMX and decrease the total organic carbon concentration to a trace.

Preparation of CeO2-doped carbon nanotubes cathode and its mechanism for advanced treatment of pig farm wastewater

The effluent from conventional treatment process (including anaerobic digestion and anoxic-oxic treatment) for pig farm wastewater was difficult to treat due to its low ratio of biochemical oxygen demand to chemical oxygen demand (BOD₅/COD_Cr) (<0.1). In the present study, electro-Fenton (EF) was used to improve the biodegradability of the mentioned effluent and the properties of self-prepared CeO₂-doped multi-wall carbon nanotubes (MWCNTs) electrodes were also studied. An excellent H₂O₂ production (165 mg L^-1) was recorded, after an 80-min electrolysis, when the mass ratio of MWCNTs, CeO₂ and pore-forming agent (NH₄HCO₃) was 6:1:1. Results of scanning electron microscopy (SEM), transmission electron microscope (TEM) and x-ray photoelectron spectroscopy (XPS) showed that addition of NH₄HCO₃ and the doping of CeO₂ could increase the superficial area of the electrode as well as the oxygen reduction reaction (ORR) electro-catalytic performance. The BOD₅/COD_Cr of the wastewater from the first stage AO process increased from 0.08 to 0.45 and COD_Cr reduced 71.5% after an 80-min electrolysis, with 0.3 mM Fe²⁺ solution. The non-biodegradable chemical pollutants from the first stage AO process were degraded by EF. The non-biodegradable pollutants identified by LC-MS/MS in the effluent from AO process including aminopyrine, oxadixyl and 3-methyl-2-quinoxalinecarboxylic acid could be degraded by EF process, with the removal rates of 81.86%, 34.39% and 7.13% in 80 min, and oxytetracycline with the removal rate of 100% in 20 min. Therefore, electro-Fenton with the new CeO₂-doped MWCNTs cathode electrode will be a promising supplement for advanced treatment of pig farm wastewater.

Janus electrocatalytic flow-through membrane enables highly selective singlet oxygen production

The importance of singlet oxygen (1O2) in the environmental and biomedical fields has motivated research for effective 1O2 production. Electrocatalytic processes hold great potential for highly-automated and scalable 1O2 synthesis, but they are energy- and chemical-intensive. Herein, we present a Janus electrocatalytic membrane realizing ultra-efficient 1O2 production (6.9 mmol per m3 of permeate) and very low energy consumption (13.3 Wh per m3 of permeate) via a fast, flow-through electro-filtration process without the addition of chemical precursors. We confirm that a superoxide-mediated chain reaction, initiated by electrocatalytic oxygen reduction on the cathodic membrane side and subsequently terminated by H2O2 oxidation on the anodic membrane side, is crucial for 1O2 generation. We further demonstrate that the high 1O2 production efficiency is mainly attributable to the enhanced mass and charge transfer imparted by nano- and micro-confinement effects within the porous membrane structure. Our findings highlight a new electro-filtration strategy and an innovative reactive membrane design for synthesizing 1O2 for a broad range of potential applications including environmental remediation.

Enhanced electrochemical decontamination and water permeation of titanium suboxide reactive electrochemical membrane based on sonoelectrochemistry

Reactive electrochemical membrane (REM) allows electrochemical oxidation (EO) water purification under flow-through operation, which improves mass transfer on the anode surface significantly. However, O₂ evolution reaction (OER) may cause oxygen bubbles to be trapped in small-sized confined flow channels, and thus degrade long-term filterability and treatability of REM. In this study, ultrasound (ultrasonic vibrator, 28 kHz, 180 W) was applied to EO system (i. e. sonoelectrochemistry) containing titanium suboxide-REM (TiSO-REM) anode for enhanced oxidation of 4-chlorophenol (4-CP) target pollutant. Both experimental and modeling results demonstrated that ultrasound could mitigate the retention of O₂ bubbles in the porous structures by destructing large-size bubbles, thus not only increasing permeate flux but also promoting local mass transfer. Meanwhile, oxidation rate of 4-CP for EO with ultrasound (EO-US, 0.0932 min^-1) was 216% higher than that for EO without ultrasound (0.0258 min^-1), due to enhanced mass transfer and OH production under the cavitation effect of ultrasound. Density functional theory (DFT) calculations confirmed the most efficient pathway of 4-CP removal to be direct electron transfer of 4-CP to form [4-CP]⁺, followed by subsequent oxidation mediated by OH produced from anodic water oxidation on TiSO-REM anode. Last, the stability of TiSO-REM could be improved considerably by application of ultrasound, due to alleviation of electrode deactivation and fouling, indicated by cyclic test, scan electron microscopy (SEM) observation and Fourier transform infrared spectroscopy (FT-IR) characterization. This study provides a proof-of-concept demonstration of ultrasound for enhanced EO of recalcitrant organic pollutants by REM anode, making decentralized wastewater treatment more efficient and more reliable.

Mechanistic insight into ultrasound-induced enhancement of electrochemical oxidation of ofloxacin: Multi-response optimization and cost analysis

In this paper, a hybrid advanced oxidation process of sonoelectrochemical, in which ultrasound and electrochemical are applied simultaneously, has been used for the degradation of ofloxacin (bio-recalcitrant pharmaceutical pollutant). Response surface methodology based central composite design was applied to understand the parametric effects of ultrasonic power, current density, initial pH, and electrolyte dose. Enhanced ofloxacin degradation was obtained using sonoelectrochemical (≈95%) process in comparison to the electrochemical (≈60.6%) and sonolysis alone (≈7.2%) after 120 min treatment time. Multi-response optimization was used so as to maximize COD removal (70.12%) and minimize specific energy consumption (11.92 kWh (g COD removed)^-1)at the optimized parametric condition of pH = 6.3 (natural pH), ultrasonic power = 54 W, current density = 213 A m^-2, and Na₂SO₄ electrolyte dose = 2.0 g L^-1. It was revealed that ^•OH radicals contribute major to the ofloxacin degradation reaction among the other oxidizing agents. Degradation of the ofloxacin followed pseudo-first-order kinetics with a higher reaction rate, which confirmed the synergistic effect of 34% between ultrasound and electrochemical approaches. The degradation pathway of ofloxacin removal was elucidated at optimum condition by the temporal evolution of the intermediate compounds and final products using gas chromatography coupled with mass spectroscopy (GC-MS), liquid chromatography-mass spectroscopy (LC-MS), high-resolution mass spectroscopy (HR-MS), and Fourier transform infrared spectroscopy (FTIR). Atomic force microscopy (AFM) and field emission scanning electron microscope (FE-SEM) coupled with energy dispersed X-ray (EDX) were used to determine the morphology of electrodes. Operational cost analysis was done based on the reactor employed in the present study.

Ultrasound-enhanced electrokinetic remediation for removal of Zn, Pb, Cu and Cd in municipal solid waste incineration fly ashes

Low-frequency ultrasound generated by a transducer was investigated to activate the raw municipal solid waste incineration (MSWI) fly ashes in the electrokinetic process, aiming at enhancing heavy metal (HM) removal and achieving better remedial efficacy. The maximum removal efficiencies of 69.84%, 64.24%, 67.74% and 59.93% were obtained in the orthogonal tests of ultrasonication for Zn, Pb, Cu and Cd, respectively. The acoustic time of 30 min and controlling temperature of 45 °C in the operating parameters were quantitatively determined to optimize the ultrasonication of the MSWI fly ash matrices. The changes of acoustic time had a significant effect on the extraction efficiencies of all the four heavy metal elements in the sonication optimal experiments. The longer running time was preferred for the pretreatment of the fly ashes in according to the marginal mean removal results. The voltage gradient of 2 V/cm was most likely to improve the removals of four HMs during the electrokinetics in the range of 0.5-2 V/cm. The synergetic application of acidification and ultrasonication for the media treatment was demonstrated to be most effective in enhancing the remedial efficiencies in the further electrokinetic experiments compared with the other activation systems. Correspondingly, the leaching concentrations of Zn, Pb, Cu and Cd in the samples were reduced by 85.92%, 98.22%, 88.53% and 98.34%, respectively. The contaminants were continuously extracted from the solid grains of the fly ashes by the protonic attack and bubble implosion. The obtained risk-assessment-code values indicated the adoption of AUS-EKR system reduced the environmental toxicity for the fly ashes to the maximum extent.