Electrochemical degradation of estrone using a boron-doped diamond anode in a filter-press reactor

Sustainable application of electrocatalytic and photo-electrocatalytic oxidation systems for water and wastewater treatment: a review

Wastewater treatment and reuse have risen as a solution to the water crisis plaguing the world. Global warming-induced climate change, population explosion and fast depletion of groundwater resources are going to exacerbate the present global water problems for the forthcoming future. In this scenario, advanced electrochemical oxidation process (EAOP) utilising electrocatalytic (EC) and photoelectrocatalytic (PEC) technologies have caught hold of the interest of the scientific community. The interest stems from the global water management plans to scale down centralised water and wastewater treatment systems to decentralised and semicentralised treatment systems for better usage efficiency and less resource wastage. In an age of rising water pollution caused by contaminants of emerging concern (CECs), EC and PEC systems were found to be capable of optimal mineralisation of these pollutants rendering them environmentally benign. The present review treads into the conventional electrochemical treatment systems to identify their drawbacks and analyses the scope of the EC and PEC to mitigate them. Probable electrode materials, potential catalysts and optimal operational conditions for such applications were also examined. The review also discusses the possible retrospective application of EC and PEC as point-of-use and point-of-entry treatment systems during the transition from conventional centralised systems to decentralised and semi-centralised water and wastewater treatment systems.

Formation of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) in the electrochemical oxidation of polluted waters with pharmaceuticals used against COVID-19

The COVID-19 pandemic has produced a huge impact on our lives, increasing the consumption of certain pharmaceuticals, and with this, contributing to the intensification of their presence in wastewater and in the environment. This situation demands the implementation of efficient remediation technologies, among them, electrochemical oxidation (ELOX) is one the most applied. This work studies the application of ELOX with the aim of eliminate pharmaceuticals used in the fight against COVID-19, assessing its degradation rate, as well as the risk of formation of toxic trace by-products, such as unintentional POPs like polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). To this end, model solutions containing 10 mg L^-1 of dexamethasone (DEX), paracetamol (PAR), amoxicillin (AMX), and sertraline (STR) with two different electrolytes (NaCl and Na₂SO₄) have been evaluated. However, electrochemical systems that contain chloride ions in solution together with PCDD/Fs precursor molecules may lead to the formation of these highly toxic by-products. So, PCDD/Fs were quantified under conditions of complete degradation of the drugs. Furthermore, the presence of PCDD/Fs precursors such as chlorophenols was determined, as well as the role of Cl^-, Cl^• and SO 4 • - radicals in the formation of the by-products and PCDD/Fs. The maximum measured concentration of PCDD/Fs was around 2700 pg L^-1 for the amoxicillin case in NaCl medium. The obtained results emphasise the importance of not underestimating the potential formation of these highly toxic trace by-products, in addition to the correct selection of oxidation processes and operation variables, in order to avoid final higher toxicity in the medium.

Removal of selected pesticides, alkylphenols, hormones and bisphenol A from domestic wastewater by electrooxidation process

In this study, seven compounds of environmental and health concern were treated by electrooxidation to determine their removal efficiencies from domestic wastewater. A batch type lab-scale reactor was used for the treatment process, and the analytes studied included two obsolete pesticides, two alkylphenols, two hormones, and bisphenol A. Titanium oxide and graphite electrodes were used as anode and cathode, respectively. Parameters of the electrooxidation process including pH of wastewater, ionic strength, applied current and treatment period were optimized by the univariate approach to maximize the removal efficiency of the analytes from wastewater. The optimum conditions were determined as nonadjusted pH of wastewater, 1.5 A current, 15 min treatment period and 5.0 g/L sodium chloride. Dispersive liquid-liquid microextraction was used to preconcentrate analytes before and after treatment in order to calculate the removal efficiency of analytes. The removal efficiency obtained under the optimum conditions was satisfactory for all seven analytes at different influent concentrations.

Recent Trends in Pharmaceuticals Removal from Water Using Electrochemical Oxidation Processes

Nowadays, the research on the environmental applications of electrochemistry to remove recalcitrant and priority pollutants and, in particular, drugs from the aqueous phase has increased dramatically. This literature review summarizes the applications of electrochemical oxidation in recent years to decompose pharmaceuticals that are often detected in environmental samples such as carbamazapine, sulfamethoxazole, tetracycline, diclofenac, ibuprofen, ceftazidime, ciprofloxacin, etc. Similar to most physicochemical processes, efficiency depends on many operating parameters, while the combination with either biological or other physicochemical methods seems particularly attractive. In addition, various strategies such as using three-dimensional electrodes or the electrosynthesis of hydrogen peroxide have been proposed to overcome the disadvantages of electrochemical oxidation. Finally, some guidelines are proposed for future research into the applications of environmental electrochemistry for the degradation of xenobiotic compounds and micropollutants from environmental matrices. The main goal of the present review paper is to facilitate future researchers to design their experiments concerning the electrochemical oxidation processes for the degradation of micropollutants/emerging contaminants, especially, some specific drugs considering, also, the existing limitations of each process.

Simultaneous degradation of 30 pharmaceuticals by anodic oxidation: Main intermediaries and by-products

The anodic oxidation (AO) of 30 pharmaceuticals including antibiotics, hormones, antihistaminics, anti-inflammatories, antidepressants, antihypertensives, and antiulcer agents, in solutions containing different supporting electrolytes media (0.05 M Na₂SO₄, 0.05 M NaCl, and 0.05 M Na₂SO₄ + 0.05 M NaCl) at natural pH was studied. A boron-doped diamond (BDD) electrode and a stainless-steel electrode were used as anode and cathode, respectively, and three current densities of 6, 20, and 40 mA cm^-2 were applied. The results showed high mineralization rates, above 85%, in all the tested electrolytic media. 25 intermediaries produced during the electrooxidation were identified, depending on the supporting electrolyte together with the formation of carboxylic acids, NO₃^-, SO₄^2- and NH₄⁺ ions. The formation of intermediates in chloride medium produced an increase in absorbance. Finally, a real secondary effluent spiked with the 30 pharmaceuticals was treated by AO applying 6 mA cm^-2 at natural pH and without addition of supporting electrolyte, reaching c.a. 90% mineralization after 300 min, with an energy consumption of 18.95 kW h m^-3 equivalent to 2.90 USD m^-3. A degradation scheme for the mixture of emerging contaminants in both electrolytic media is proposed. Thus, the application of anodic oxidation generates a high concentration of hydroxyl radicals that favors the mineralization of the pharmaceuticals present in the spiked secondary effluent sample.

On the Degradation of 17-β Estradiol Using Boron Doped Diamond Electrodes

This work focuses on the evaluation of the degradation of 17β-estradiol in a mixture of synthetic urine and methanol, trying to determine in which conditions the hormone can be more easily degraded than the urine compounds. This is in the frame of an overall study in which the pre-concentration stage with adsorption/desorption technology is evaluated to improve electrolysis efficiency. Results show that this pollutant can be efficiently removed from mixtures of urine/methanol by electrolysis with diamond electrodes. This removal is simultaneous with the removal of uric acid (used as a model of natural pollutants of urine) and leads to the formation of other organic species that behave as intermediates. This opens the possibility of using a concentration strategy based on the adsorption of pollutants using granular activated carbon and their later desorption in methanol. Despite methanol being a hydroxyl radical scavenger, the electrolysis is found to be very efficient and, in the best case, current charges lower than 7 kAh·m−3 were enough to completely deplete the hormone from urine. Increases in the operation current density lead to faster but less efficient removal of the 17β-estradiol, while increases in the operation flowrate do not markedly affect the efficiency in the removal. Degradation of 17β-estradiol is favored with respect to that of uric acid at low current densities and at high flowrates. In those conditions, direct oxidation processes on the surface of the anode are encouraged. This means that these direct processes can have a higher influence on the degradability of the hazardous species and opens the possibility for the development of selective oxidation processes, with a great economic impact on the degradation of the hazardousness of hospitalary wastewater.

Recent Trends in Removal Pharmaceuticals and Personal Care Products by Electrochemical Oxidation and Combined Systems

Due to various potential toxicological threats to living organisms even at low concentrations, pharmaceuticals and personal care products in natural water are seen as an emerging environmental issue. The low efficiency of removal of pharmaceuticals and personal care products by conventional wastewater treatment plants calls for more efficient technology. Research on advanced oxidation processes has recently become a hot topic as it has been shown that these technologies can effectively oxidize most organic contaminants to inorganic carbon through mineralization. Among the advanced oxidation processes, the electrochemical advanced oxidation processes and, in general, electrochemical oxidation or anodic oxidation have shown good prospects at the lab-scale for the elimination of contamination caused by the presence of residual pharmaceuticals and personal care products in aqueous systems. This paper reviewed the effectiveness of electrochemical oxidation in removing pharmaceuticals and personal care products from liquid solutions, alone or in combination with other treatment processes, in the last 10 years. Reactor designs and configurations, electrode materials, operational factors (initial concentration, supporting electrolytes, current density, temperature, pH, stirring rate, electrode spacing, and fluid velocity) were also investigated.

Study of the atenolol degradation using a Nb/BDD electrode in a filter-press reactor

The present paper deals with the atenolol (ATL) degradation by advanced anodic oxidation using a boron-doped diamond anode supported on niobium (Nb/BDD). Cyclic voltammetry performed on this electrode revealed that it presents a high quality (diamond-sp³/sp²-carbon ratio), high potential for OER and that ATL can be oxidized directly and/or indirectly by the electrogenerated oxidants, such as hydroxyl radicals, persulfate ions and sulfate radicals. Electrolysis experiments demonstrated that ATL degradation and mineralization follow a mixed (first and zero) order kinetics depending on the applied current density. At high applied current densities, the amount of OH radicals is very high and the overall reaction is limited by the transport of ATL (pseudo first-order kinetics) whereas for low applied current densities, the rate of OH radicals generation at the anode is slower than the rate of arrival of ATL molecules (pseudo-zero order kinetics). Estimated values of k_zero and k_first based on the assumption of pseudo-zero or pseudo-first order kinetics were carried oud as a function of the supporting electrolyte concentration, indicating that both parameters increased with its concentration due the higher production of sulfate reactive species that play an important role in degradation. Finally, MCE increased with the decrease of current density, due to the lower amount of OH present in solution, since this species could be rapidly wasted in parasitic reactions; and the increase of sulfate concentration due to the more efficient production of persulfate.

Insights into Mechanisms of Electrochemical Drug Degradation in Their Mixtures in the Split-Flow Reactor

The recirculating split-flow batch reactor with a cell divided into anolyte and catholyte compartments for oxidation mixture of cytostatic drugs (CD) was tested. In this study, kinetics and mechanisms of electrochemical oxidization of two mixtures: 5-FU/CP and IF/CP were investigated. The order of the CD degradation rate in single drug solutions and in mixtures was found to be 5-FU < CP < IF. In the 5-FU/CP mixture, k_app of 5-FU increased, while k_app of CP decreased comparing to the single drug solutions. No effect on the degradation rate was found in the CP/IF mixture. The presence of a second drug in the 5-FU/CP mixture significantly altered mineralization and nitrogen removal efficiency, while these processes were inhibited in IF/CP. The experiments in the different electrolytes showed that •OH and sulphate active species can participate in the drug's degradation. The k_app of the drugs was accelerated by the presence of Cl^- ions in the solution. Chlorine active species played the main role in the production of gaseous nitrogen products and increased the mineralisation. Good results were obtained for the degradation and mineralisation processes in mixtures of drugs in municipal wastewater-treated effluent, which is beneficial from the technological and practical point of view.

Electrocatalytic degradation of the herbicide metamitron using lead dioxide anode: influencing parameters, intermediates, and reaction pathways

In the present study, the electrocatalytic degradation of triazine herbicide metamitron using Ti/PbO₂-CeO₂ composite anode was studied in detail. The effects of the current density, initial metamitron concentration, supporting electrolyte concentration, and initial pH value were investigated and optimized. The results revealed that an electrocatalytic approach possessed a high capability of metamitron removal in aqueous solution. After 120 min, the removal ratio of metamitron could reach 99.0% in 0.2 mol L^-1 Na₂SO₄ solution containing 45 mg L^-1 metamitron with the current density at 90 mA cm^-2 and pH value at 5.0. The reaction followed the pseudo-first-order kinetics model. HPLC and HPLC-MS were employed to analyze the degradation by-products in the metamitron oxidization process, and the degradation pathway was also proposed, which was divided into two sub-routes according to the different initial attacking positions on metamitron by hydroxyl radicals. Therefore, the electrocatalytic approach was considered as a very promising technology in practical application for herbicide wastewater treatment.

Pharmaceutically active compounds in aqueous environment: A status, toxicity and insights of remediation

Pharmaceutically active compounds (PhACs) have pernicious effects on all kinds of life forms because of their toxicological effects and are found profoundly in various wastewater treatment plant influents, hospital effluents, and surface waters. The concentrations of different pharmaceuticals were found in alarmingly high concentrations in various parts of the globe, and it was also observed that the concentration of PhACs present in the water could be eventually related to the socio-economic conditions and climate of the region. Drinking water equivalent limit for each PhAC has been calculated and compared with the occurrence data from various continents. Since these compounds are recalcitrant towards conventional treatment methods, while advanced oxidation processes (AOPs) have shown better efficiency in degrading these PhACs. The performance of the AOPs have been evaluated based on percentage removal, time, and electrical energy consumed to degrade different classes of PhACs. Ozone based AOPs were found to be favorable because of their low treatment time, low cost, and high efficiency. However, complete degradation cannot be achieved by these processes, and various transformation products are formed, which may be more toxic than the parent compounds. The various transformation products formed from various PhACs during treatment have been highlighted. Significant stress has been given on the role of various process parameters, water matrix, oxidizing radicals, and the mechanism of degradation. Presence of organic compounds, nitrate, and phosphate usually hinders the degradation process, while chlorine and sulfate showed a positive effect. The role of individual oxidizing radicals, interfering ions, and pH demonstrated dissimilar effects on different groups of PhACs.

Analysis of operational parameters, reactor kinetics, and floc characterization for the removal of estrogens via electrocoagulation

Estrogenic compounds can cause human and ecological health issues and have been detected in surface and drinking water. In this research a reactor analysis determined the impact of operational parameters, the best fit kinetic model for the removal of estrone (E1), 17β-estradiol (E2), estriol (E3), and 17α-ethynylestradiol (EE2) using a bench-top iron electrocoagulation reactor, and characterized the floc generated in-situ. The parameters investigated were current density, conductivity, stir rate, and polarity reversal. Estrogen removal correlated well with an increase in current density, while conductivity did not impact removal but did reduce potentials. High stir rates and frequent polarity reversal demonstrated greater removal. The operating parameters that achieved the greatest estrogen removal were a current density of 16.7 mA cm^-2, conductivity of 1000 μS cm^-1, stir rate of 500 rpm, and a polarity reversal time of 30 s. These parameters led to average removal efficiencies of 81%, 87%, 85%, and 97% for E1, E2, E3, and EE2, respectively. The removal data for all estrogenic compounds best fit a pseudo-first order relationship with kinetic rate constants of 0.015 min^-1 for E1 and E2, 0.016 min^-1 for E3 and 0.040 min^-1 for EE2. The floc formed in-situ were characterized by determining the crystalline phases with X-ray diffraction, the size and zeta potential, and the shape and major components using scanning electron microscope with energy-dispersive X-ray spectrometer. The iron coagulant generated during electrocoagulation was lepidocrocite with a point of zero charge of 5.67 and an average floc diameter of 2255 nm.

Optimization of the electrochemical degradation process of the antibiotic ciprofloxacin using a double-sided β-PbO2 anode in a flow reactor: kinetics, identification of oxidation intermediates and toxicity evaluation

The electrochemical degradation of ciprofloxacin-CIP (50 mg L^-1 in 0.10 mol L^-1 Na₂SO₄) was investigated using a double-sided Ti-Pt/β-PbO₂ anode in a filter-press flow reactor, with identification of oxidation intermediates and follow-up of antimicrobial activity against Escherichia coli. The effect of solution pH, flow rate, current density, and temperature on the CIP removal rate was evaluated. All of these parameters did affect the CIP removal performance; thus, optimized electrolysis conditions were further explored: pH = 10, q_V = 6.5 L min^-1, j = 30 mA cm^-2, and θ = 25 °C. Therefore, CIP was removed within 2 h, whereas ~75% of the total organic carbon concentration (TOC) was removed after 5 h and then, the solution no longer presented antimicrobial activity. When the electrochemical degradation of CIP was investigated using a single-sided boron-doped diamond (BDD) anode, its performance in TOC removal was similar to that of the Ti-Pt/β-PbO₂ anode; considering the higher oxidation power of BDD, the surprisingly good comparative performance of the Ti-Pt/β-PbO₂ anode was ascribed to significantly better hydrodynamic conditions attained in the filter-press reactor used with this electrode. Five initial oxidation intermediates were identified by LC-MS/MS and completely removed after 4 h of electrolysis; since they have also been determined in other degradation processes, there must be similarities in the involved oxidation mechanisms. Five terminal oxidation intermediates (acetic, formic, oxamic, propionic, and succinic acids) were identified by LC-UV and all of them (except acetic acid) were removed after 10 h of electrolysis.

Emerging pollutant treatments in wastewater: Cases of antibiotics and hormones

Because of the intensive use of pharmaceutical substances in human life, studies on the detection of these chemical compounds and their metabolites as pollutants in water bodies are continuously reported. Some pharmaceutical agents are associated with adverse effects to aquatic life, even at very low concentrations (ng L^-1 to μg L^-1). For instance, the presence of antibiotics and hormones has been associated with increasing proliferation of antibiotic resistant pathogens and feminization and masculinization of some aquatic organisms. Currently, new attempts are being made to minimize or fully remove these types of pollutants from aquatic systems to protect the environment and human health. In this regard, physicochemical and biological treatments are among the most promising technologies for the treatment of wastewater containing pharmaceutical pollutants. These treatments are green alternatives for the degradation of hazardous organic compounds into nontoxic by-products. Here, we review some of the physicochemical and biological treatment methods used for the removal of the most extensively used antibiotics and hormones. Enzymatic oxidation, photocatalysis and electrochemical oxidation are described in terms of the aforementioned pharmaceutically active compounds (PhACs). The use of membrane technologies to separate different groups of antibiotics and hormones prior to biologic or physicochemical treatment methods is also addressed.

Soft drink wastewater treatment by electrocoagulation-electrooxidation processes

The aim of this work was to implement a coupled system, a monopolar Electrocoagulation (EC)-Electrooxidation (EO) processes, for the treatment of soft drink wastewater. For the EC test, Cu-Cu, anode-cathode were used at current densities of 17, 51 and 68 mA cm^-2. Only 37.67% of chemical oxygen demand (COD) and 27% of total organic carbon (TOC) were removed at 20 min with an optimum pH of 8, this low efficiency can be associated with the high concentration of inorganic ions which inhibit the oxidation of organic matter due to their complexation with copper ions. Later EO treatment was performed with boron-doped diamond-Cu electrodes and a current density of 30 Am^-2. The coupled EC-EO system was efficient to reduce organic pollutants from initial values of 1875 mg L^-1 TOC and 4300 mg L^-1 COD, the removal efficiencies were 75% and 85%, respectively. Electric energy consumption to degrade a kilogram of a pollutant in the soft drink wastewater using EC was 3.19 kWh kg^-1 TOC and 6.66 kWh kg^-1 COD. It was concluded that the coupled system EC-EO was effective for the soft drink wastewater treatment, reducing operating costs and residence time, and allowing its reuse in indirect contact with humans, thus contributing to the sustainable reuse as an effluent of industrial wastewater.

Photoelectrochemical removal of 17β-estradiol using a RuO2-graphene electrode

A new electrode for the photoelectrochemical removal of 17β-estradiol from water using ruthenium oxide nanoparticles supported on reduced graphene oxide is proposed in this study. The morphology, microstructure and the electrochemical performance of the material were characterized using HRTEM, XRD and Raman spectroscopy. The characterization showed the formation of reduced graphene oxide from a micro-wave assisted hydrothermal method with a particle size of 5.0 nm + 2.4 nm. The electrochemical measurements point to a high performance of the electrode in the presence of a white light source. The hormone removal efficiency in water containing 50 μmol L(-1) of 17β-estradiol was evaluated using chronoamperometry at +1.0 V and the process was monitored using liquid chromatography. The reaction is pseudo first order with the removal of 92.2% of 17β-estradiol after 60 min of photoelectrocatalytic treatment.