Enhanced removal of humic acid from aqueous solution by combined alternating current electrocoagulation and sulfate radical

Unveiling electron transfer and radical transformation pathways in coupled electrocatalysis and persulfate oxidation reactions for complex pollutant removal

The degradation of multiple organic pollutants in wastewater via advanced oxidation processes might involve different radicals, of which the types and concentrations vary upon interacting with different pollutants. In this study, electrochemical activation of peroxymonosulfate (E/PMS) using advanced activated carbon cloth (ACC) as electrode was applied for simultaneous degradation of mixed pollutants, e.g., metronidazole (MNZ) and p-chloroaniline (PCA). 92.5 % of MNZ and 91.4 % of PCA can be degraded at the cathode and anode at a low current density and PMS concentration, respectively. The rate constants for the simultaneous removal of MNZ and PCA in the E/PMS/MNZ(PCA) system were 118 times and 6 times higher than those in the sole PMS system, and 2.5 times and 1.6 times higher than those in the E/Na2SO4/MNZ(PCA) system, respectively. Different electrochemical characteristics, EPR spectra and radical quenching tests verified that the degradation of MNZ and PCA in the optimal system proceeded primarily through non-radical-dominated oxidation, involving electron transfer and 1O2 effect. The system also exhibited low energy consumption (0.215 kWh/m-3·order-1), broad operational pH range, excellent removal efficiency for water matrix, and low by-products toxicity, indicating its strong potential for practical applications. The ACC, with its super stable, low cost, and electrochemical activity, make it as a promising materials applicable in the E/PMS system for degradation of multiple pollutants. The study further elucidated the mechanism of pollutant interaction with electrode materials in terms of radical and non-radical transformation, providing fundamental insight into the application of this system for treatment of complex wastewater.

Unique effect of bromide ion on intensification of advanced oxidation processes for pollutants removal: A systematic review

Advanced oxidation processes (AOPs) have been developed to decompose toxic pollutants to protect the aquatic environment. AOP has been considered an alternative treatment method for wastewater treatment. Bromine is present in natural waters posing toxic effects on human health and hence, its removal from drinking water sources is necessary. Of the many techniques advanced oxidation is covered in this review. This review systematically examines literature published from 1997 to April 2024, sourced from Scopus, PubMed, Science Direct, and Web of Science databases, focusing on the efficacy of AOPs for pollutant removal from aqueous solutions containing bromide ions to investigate the impact of bromide ions on AOPs. Data and information extracted from each article eligible for inclusion in the review include the type of AOP, type of pollutants, and removal efficiency of AOP under the presence and absence of bromide ion. Of the 1784 documents screened, 90 studies met inclusion criteria, providing insights into various AOPs, including UV/chlorine, UV/PS, UV/H2O2, UV/catalyst, and visible light/catalyst processes. The observed impact of bromide ion presence on the efficacy of AOP processes, alongside the AOP method under scrutiny, is contingent upon various factors such as the nature of the target pollutant, catalyst type, and bromide ion concentration. These considerations are crucial in selecting the best method for removing specific pollutants under defined conditions. Challenges were encountered during result analysis included variations in experimental setups, disparities in pollutant types and concentrations, and inconsistencies in reporting AOP performance metrics. Addressing these parameters in research reports will enhance the coherence and utility of subsequent systematic reviews.

Ascorbic acid enhanced the circulation between Fe(II) and Fe(III) in peroxymonosulfate system for fluoranthene degradation

In this research, ascorbic acid (AA) was used to enhance Fe(II)/Fe(III)-activated permonosulfate (PMS) systems for the degradation of fluoranthene (FLT). AA enhanced the production of ROS in both PMS/Fe(II) and PMS/Fe(III) systems through chelation and reduction and thus improved the degradation performance of FLT. The optimal molar ratio in PMS/Fe(II)/AA/FLT and PMS/Fe(III)/AA/FLT processes were 2/2/4/1 and 5/10/5/1, respectively. In addition, the experimental results on the effect of FLT degradation under different groundwater matrixes indicated that PMS/Fe(III)/AA system was more adaptable to different water quality conditions than the PMS/Fe(II)/AA system. SO4·- was the major reactive oxygen species (ROS) responsible for FLT removal through the probe and scavenging tests in both systems. Furthermore, the degradation intermediates of FLT were analyzed using gas chromatograph-mass spectrometry (GC-MS), and the probable degradation pathways of FLT degradation were proposed. In addition, the removal of FLT was also tested in actual groundwater and the results showed that by increasing the dose and pre-adjusting the solution pH, 88.8 and 100% of the FLT was removed for PMS/Fe(II)/AA and PMS/Fe(III)/AA systems. The above experimental results demonstrated that PMS/Fe(II)/AA and PMS/Fe(III)/AA processes have a great perspective in practice for the rehabilitation of FLT-polluted groundwater.

Enhanced electrochemical treatment of humic acids and metal ions in leachate concentrate: Experimental and molecular mechanism investigations

Membrane technologies are effective for treating leachate, but they generate leachate concentrates (LCs), which contain elevated humic acids (HAs) and metals. LCs are very challenging and expensive to treat; but in-situ coagulation-electrochemical oxidation (CO-EO) treatment is promising. We previously hypothesized and proved that substituting the widely used graphite cathode with an Al cathode will generate Al(OH)3 floccules that would enhance HAs removal in CO-EO systems. However, the fundamental mechanisms are unclear. Here, we examined this hypothesis using laboratory experiments (using an Al cathode and a Ti/Ti4O7 anode CO-EO system) and performed molecular dynamics (MD) simulation to investigate the underlying mechanisms. Up to 84.2% HAs was removed by the Al-cathode system, which is ∼10% higher than a graphite cathode-based system. Based on MD simulation we found that enhanced HAs removal occurred via two steps: (1) degradation by oxidants produced at the anode, and (2) subsequent coagulation with the Al(OH)3 generated from the Al cathode. This finding challenges the current belief that whole HAs and Al(OH)3 directly flocculate. Meanwhile, metal removal efficiency by the graphite cathode system was only 0.8-13.9%, which increased up to 13-folds at most when in the Al cathode system. This work provides new molecular-level insights into an efficient electrochemical treatment of LC.

Mesoporous bimetallic S-doped nanoparticles prepared via hydrothermal method for enhanced photodegradation of 4-chlorophenol

Magnesium oxides (MgO) have gained shown significant promise for a variety of applications, which can be modified by ions doping. In this study, bimetallic Ag-doped S-MgO nanoparticles were prepared by hydrothermal method and used for photocatalytic degradation of 4-chlorophenl (4-CP). EDX suggested the presence of no impurities, which mainly contained Mg, Ag, and S elements, suggesting that S and Ag were incorporated into the lattice of MgO as a result of successful doping. Estimated bandgap of Ag-doped S-MgO nanoparticles was 3.7 eV, lower than MgO (7.8 eV), but useful to improve optical characteristics and photocatalytic efficiency to degrade 4-CP up to a maximum of 99.60 ± 0.50%. The synergetic parameter during photocatalysis of 4-CP was 6.91, confirming the degradation of 4-CP. Quenching experiments proved the presence of hydroxyl radicals (•OH) and singlet dioxygen (1O2) that were critical in 4-CP degradation. The kinetics rate constant was increased by 24.8% from 0.086 ± 0.004 to 0.108 ± 0.005 min-1 by the addition of sulfate in the reaction medium. The work proposes a new synthetic method for preparing catalysts that are capable of producing in-situ •OH radicals and 1O2 to decompose the organic contaminants.

Efficient removal of naphthenic acids from real petroleum wastewater by natural pyrite activated persulfate system

The petroleum wastewater (PWW) contains a diverse range of recalcitrant organic contaminants. Of particular concern is the removal of naphthenic acids (NAs) due to the high toxicity and persistence. Persulfate (PS) based oxidation processes have shown promising in treating refractory wastewater, while the high costs of prepared catalysts limited their widespread implementation. This study aims to develop a cost-effective natural pyrite activated PS system for PWW treatment. The removal of NAs by pyrite/PS system was initially investigated. More than 90% of cyclohexanoic acid (CHA), a model NA, was removed in pyrite/PS system (2.0 g/L pyrite, 4.0 mM PS) at initial pH of 3-11. Scavenging experiments revealed that Fe(II) on pyrite surface was the reactive site for PS activation to generate reactive species, including sulfate radical (SO4·-), Fe(IV) and hydroxyl radical (·OH) for CHA degradation. Reactions of Fe(III) with S helped restore Fe(II) and enhance PS activation, resulting in the sustained catalytic activity of pyrites over five cycles. Cl-, SO42- and NO3- below 10 mM had minimal impact on CHA degradation in pyrite/PS system. However, over 1 mM of HCO3- inhibited 80% of CHA removal due to the buffer effect to maintain the high solution pH. Removing HCO3- from real PWW restored the removal of CHA and of total organic carbon (TOC) to over 90% and 71.3% in pyrite/PS system, respectively. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) results indicated that O2‒6 species including NAs were primarily eliminated through mineralization and oxygen addition. Besides, O3-5S, NO3-5S and N3O2‒4 species were the most susceptible to oxidation in PWW, resulting in the increase of the oxidation level (i.e., O/Cwa) from 0.41 to 0.56 after treatment. This study provides valuable insights into the treatment of NAs in real PWW, and potential application of natural minerals in the treatment of industrial wastewater.

Hydrodynamic cavitation coupled with zero-valent iron produces radical sulfate radicals by sulfite activation to degrade direct red 83

In the present research, hydrodynamic cavitation (HC) and zero-valent iron (ZVI) were used to generate sulfate radicals through sulfite activation as a new source of sulfate for the efficient degradation of Direct Red 83 (DR83). A systematic analysis was carried out to examine the effects of operational parameters, including the pH of the solution, the doses of ZVI and sulfite salts, and the composition of the mixed media. Based on the results, the degradation efficiency of HC/ZVI/sulfite is highly dependent upon the pH of the solution and the dosage of both ZVI and sulfite. Degradation efficiency decreased significantly with increasing solution pH due to a lower corrosion rate for ZVI at high pH. The corrosion rate of ZVI can be accelerated by releasing Fe2+ ions in an acid medium, reducing the concentration of radicals generated even though ZVI is solid/originally non-soluble in water. The degradation efficiency of the HC/ZVI/sulfite process (95.54 % + 2.87%) was found to be significantly higher under optimal conditions than either of the individual processes (<6% for ZVI and sulfite and 68.21±3.41% for HC). Based on the first-order kinetic model, the HC/ZVI/sulfite process has the highest degradation constant of 0.035±0.002 min-1. The contribution of radicals to the degradation of DR83 by the HC/ZVI/sulfite process was 78.92%, while the contribution of SO4•- and •OH radicals was 51.57% and 48.43%, respectively. In the presence of HCO3- and CO32- ions, DR83 degradation is retarded, whereas SO42- and Cl- ions promote degradation. To summarise, the HC/ZVI/sulfite treatment can be viewed as an innovative and promising method of treating recalcitrant textile wastewater.

Effect of iron ion configurations on Ni2+ removal in electrocoagulation

Electrocoagulation (EC) has been widely used to treat the heavy metal wastewater in industry. A novel process of sinusoidal alternating current electrocoagulation (SACC) is adopted to remove Ni²⁺ in wastewater in this study. The morphology of precipitates and the distribution of the main functional iron configurations were investigated. Ferron timed complex spectroscopy can identify the monomeric iron configurations [Fe(a)], oligomeric iron configurations [Fe(b)] and polymeric iron configurations [Fe(c)]. The optimal operating conditions of SACC process were determined through single-factor experiments. The maximum Ni²⁺ removal efficiency [Re(Ni²⁺)] was achieved under the conditions of pH₀=7, current density (j) = 7 A/m², electrolysis time (t) = 25 min, c₀(Ni²⁺) = 100 mg/L. At pH=7, the proportion of Fe(b) and Fe(c) in the system was 50.4 at.% and 23.1 at.%, respectively. In the SACC process, Fe(b) and Fe(c) are the main iron configurations in solution, while Fe(c) are the vast majority of the iron configurations in the direct current electrocoagulation (DCC) process. Re(Ni²⁺) is 99.56% for SACC and 98.75% for DCC under the same optimum conditions, respectively. The precipitates produced by SACC have a high proportion of Fe(b) configurations with spherical α-FeOOH and γ-FeOOH structures which contain abundant hydroxyl groups. Moreover, it is demonstrated that Fe(b) has better adsorption capacity than Fe(c) through adsorption experiments of methyl orange (MO) dye. Fe(a) configurations in the homogeneous solution had no effect on the removal of nickel.

The role of the current waveform in mitigating passivation and enhancing electrocoagulation performance: A critical review

Electrocoagulation (EC) can be an efficient alternative to existing water and wastewater treatment methods due to its eco-friendly nature, low footprint, and facile operation. However, the electrodes applied in the EC process suffer from passivation or fouling, an issue resulting from the buildup of poorly conducting materials on the electrode surface. Indeed, such passivation gives rise to various operational problems and restricts the practical implementation of EC on a large scale. Therefore, it has been suggested that using pulsed direct current (PDC), alternating pulse current (APC), and sinusoidal alternating current (AC) waveforms in EC as alternatives to conventional direct current (DC) can help mitigate passivation and alleviate its associated detrimental effects. This paper presents a critical review of the impact of the current waveform on the EC process towards the capabilities of the PDC, APC, and AC waveforms in de-passivation and performance enhancement while comparing them to the conventional DC. Additionally, current waveform parameters influencing the surface passivation of electrodes and process efficiency are elaborately discussed. Meanwhile, the performance of the EC process is evaluated under different current waveforms based on pollutant removal efficiency, energy consumption, electrode usage, sludge production, and operating cost. The proper current waveforms for treating various water and wastewater matrices are also explained. Finally, concluding remarks and outlooks for future research are provided.

New combination of pulsed light and iron (II) for carbonate radical production to enhanced degradation of bisphenol A: Parameter optimization and degradation pathway

Bisphenol A(BPA) is a common industrial chemical with significant adverse impacts on Environment and human health. The present work evaluates the efficacy of pulsed light (PL) and Fe²⁺ ions in activation of sodium percarbonate (SPC) to produce hydroxyl (OH^•) and carbonate (CO₃^•-) radicals for efficient degradation of BPA. The effects of operational parameters such as solution pH, SPC and Fe²⁺ dose as well as the mixture composition were analyzed and the decomposition pathway of BPA proposed. The BPA was successfully degraded at the initial concentration of 15.0 mg/L and optimized conditions by the PL/Fe²⁺/SPC process (99.67 ± 0.29%). A rapid reduction in the degradation of BPA was observed with increasing pH due to OH^• radicals quenching and also the precipitation of Fe²⁺. Under the optimized conditions, degradation of BPA by PL/Fe²⁺/SPC process was five-times faster than the individual process. The quenching experiments revealed that radical and non-radical pathways on BPA degradation was accomplished with OH^•, CO₃^•-, O₂^•-, and ¹O_2, while OH^• and CO₃^•- radicals (as a dominant radicals) have the contributions of 80.23% and 8.30%, respectively. Based on the detected byproducts, ring cleavage can be considered as the main transformation mechanism of BPA by the PL/Fe²⁺/SPC process.

Rapid removal of PFOA and PFOS via modified industrial solid waste: Mechanisms and influences of water matrices

Emerging perfluoroalkyl and polyfluoroalkyl substances contaminate waters at trace concentrations, thus rapid and selective adsorbents are pivotal to mitigate the consequent energy-intensive and time-consuming issues in remediation. In this study, coal combustion residuals-fly ash was modified (FA-SCA) to overcome the universal trade-off between high adsorption capacity and fast kinetics. FA-SCA presented rapid adsorption (t_eq = 2 min) of PFOX (perfluorooctanoic acid and perfluorooctanesulfonic acid, collectively), where the dynamic adsorption capacity (q_dyn = q_m/t_eq) was 2-3 orders of magnitude higher than that of benchmark activated carbons and anion-exchange resins. Investigated by advanced characterization and kinetic models, the fast kinetics and superior q_dyn are attributed to (1) elevated external diffusion driven by the submicron particle size; (2) enhanced intraparticle diffusion caused by the developed mesoporous structure (V_meso/V_micro = 8.1); (3) numerous quaternary ammonium anion-exchange sites (840 μmol/g), and (4) appropriate adsorption affinity (0.031 L/μmol for PFOS, and 0.023 L/μmol for PFOA). Since the adsorption was proven to be a synergistic process of electrostatic and hydrophobic interactions, effective adsorption ([PFOX]_ini = 1.21 μM, concentration levels of highly-contaminant-sites) was obtained at conventional natural water chemistries. High selectivity (>85.4% removal) was also achieved with organic/inorganic competitors, especially compounds with partly similar molecular structures to PFOX. In addition, >90% PFOX was removed consistently during five cycles in mild regeneration conditions (pH 12 and 50 °C). Overall, FA-SCA showed no leaching issues of toxic metals and exhibits great potential in both single-adsorption processes and treatment train systems.

Enhanced removal of humic acid from piggery digestate by combined microalgae and electric field

Microalgae technology is a promising method for treating piggery digestate, while its removal ability of humic acids (HAs) is poor. Here, an electric field-microalgae system (EFMS) was used to improve the removal of HAs from the piggery digestate. Results indicated that the removal of HAs by EFMS relied on the initial concentration of HAs, electrical intensity, the initial inoculation concentration of microalgae and pH. Values of these parameters were optimized as electrical intensity of 1.2 V/cm, microalgae initial inoculation concentration of 0.1 g/L and pH 5.0. The HAs removal efficiency by EFMS (55.38%) was 13% and 38% higher than that by single electric field and microalgal technology. It was observed that oxidation, coagulation and assimilation contributed to the removal of HAs, suggesting that EFMS could serve as an attractive and cost-effective technique for the removal of HAs from the piggery digestate.

Magnetized Activated Carbon Synthesized from Pomegranate Husk for Persulfate Activation and Degradation of 4-Chlorophenol from Wastewater

The compound 4-chlorophenol (4-CP) is known to be a highly toxic compound having harmful effects on human health and the environment. To date, the removal of 4-CP by advanced oxidation processes (AOPs) has attracted tremendous attentions. The persulfate-based AOPs show higher oxidation, better selectivity, wider pH range, and no secondary pollution compared to the traditional Fenton-based AOPs. Carbon materials with low cost and chemical stability are useful for the activation of persulfate (PS) to produce reactive species. Herein, we magnetized activated carbon synthesized from pomegranate husk (MPHAC). By using 4-CP as a model organic pollutant, tests of the activation of PS via MPHAC for the removal of 4-CP were performed. Batch processes were carried out to study the influence of different parameters (initial solution pH, catalyst dose, PS dose, and initial 4-CP concentration) on the adsorption of 4-CP on PHAC with ferric oxide (Fe3O4-PHAC). The results show that under the obtained optimal conditions (MPHAC dose: 1250 mg/L, PS dose: 350 mg/L, solution pH 5, an initial 4-CP concentration of 100 mg/L, and a contact time of 60 min), a 4-CP removal factor of 99.5% was reached by the developed MPHAC/PS system. In addition, it was found that reusing MPHAC in five successive cycles is feasible because the catalyst in the last cycle kept exhibiting a high potential for 4-CP absorption, indicating the economically viable procedure. Therefore, this study provides a comprehensive understanding on the degradation of 4-CP by the magnetized activated carbon persulfate system.

Efficient removal of Cu-EDTA complexes from wastewater by combined electrooxidation and electrocoagulation process: Performance and mechanism study

In this study, combined electrooxidation and electrocoagulation (EO-EC) reactor using RuO₂-IrO₂/Ti and Al electrodes has been built for treatment of Cu-EDTA wastewater. Effects of current density, electrolyte, NaCl concentration, pH and initial concentration on EO-EC performance were investigated. In this study, Cu-EDTA removal efficiency increased with a higher current density. The electrolyte type exerted a significant role in EO-EC process, compared with Na₂SO₄ and NaNO₃, NaCl was a superior supporting electrolyte because the oxidation of Cl^- into Cl₂ provided additional highly reactive oxidant ClO^- for Cu-EDTA oxidation or mineralization. In neutral or alkaline solution, EO-EC reactor performed better than when it was acid. At the condition of current density 10.29 mA cm^-2, C₀(NaCl) 1 g L^-1, C₀(Cu) 50 mg L^-1 and pH 7, the Cu and COD removal efficiency reached 99.85% and 85.01%, respectively within 60 min. The possible mechanism of Cu-EDTA removal was proposed based on SEM, EDS, XRD, FTIR and XPS analysis of the products. Cu-EDTA chelates were degraded or mineralized by direct charge transfer, chemisorbed M(·OH) and active chorine species produced on anode surface, in which degradation intermediates and mineralization products of Cu-EDTA were generated. Meanwhile, residual degradation intermediates and mineralization products were removed by electrocoagulation. In this study, EO-EC process has been proved to be an effective way for the treatment of Cu-EDTA contaminated wastewater.

Cephalexin removal by persulfate activation using simulated sunlight and ferrous ions

This study presents the main results related to the use of activated persulfate (PS) in the elimination of the beta-lactam antibiotic cephalexin (CPX). Experiments were done using K₂S₂O₈ and simulated sunlight. A face-centered central composite experimental design was used to analyze the effects of the solution pH and the PS concentration on the reaction, and to determine the optimized conditions that favor the CPX elimination. The results indicated that the removal of CPX is promoted by an acidic pH and under the higher evaluated PS dose (7.5 mg L^-1). CPX total removal was achieved in 30 min. The analysis of the effect of the pollutant initial concentration indicated that a pseudo-first-order kinetics model can be used to describe the reaction. Likewise, the use of Fe²⁺ ions for PS activation (in the dark) was evaluated and established that a higher concentration of ions favors the pollutant removal. Control tests and under the presence of scavenger agents indicated that both HO• and SO₄^-• radicals would be present in the solution and promote the CPX elimination. The assessment of the solution dissolved organic carbon, nitrates and sulfates was also carried out, and indicated that a portion of the organic matter was mineralized.