Heterogeneous Ti/PbO2-electro-Fenton degradation of aromatic methane dyes using industrial pyrite waste slag as catalyst

A review of electro-Fenton and ultrasound processes: towards a novel integrated technology for wastewater treatment

Nowadays, the presence of persistent dissolved pollutants in water has received increasing attention due to their toxic effects on living organisms. Considering the limitations of conventional wastewater treatment processes for the degradation of these compounds, advanced oxidation processes such as electro-Fenton and sono-chemical process, as well as their combination, appear as potentially effective options for the treatment of wastewater contaminated with bio-recalcitrant pollutants. In view of the importance of the development of processes using real effluents, this review aims to provide a comprehensive perspective of sono-electro-Fenton-related processes applied for real wastewater treatment. In the first section, the fundamentals and effectiveness of both homogeneous and heterogeneous electro-Fenton approaches for the treatment of real wastewater are presented. While the second part of this work describes the fundamentals of ultrasound-based processes, the last section focuses on the coupling of the two methods for real wastewater treatment and on the effect of the main operational parameters of the process. On the basis of the information presented, it is suggested that sono-electro-Fenton processes substantially increase the efficiency of the treatment as well as the biodegradability of the treated wastewater. The combined effect results from mass transfer improvement, electrode cleaning and activation, water electrolysis, and the electro-Fenton-induced production of hydroxyl radicals. The information presented in this work is expected to be useful for closing the gap between laboratory-scale assays and the development of novel wastewater technologies.

Decomposition of metal-organic complexes and metal recovery in wastewater: A systematic review and meta-synthesis

Metals are rarely found as free ions in natural and anthropogenic environments, but they are often associated with organic matter and minerals. Under the context of circular economy, metals should be recycled, yet they are difficult to extract for their complex forms in real situations. Based on the protocols of review methodology and the analysis of VOS viewer, there are few reviews on the properties of metal-organic complexes, decomplexation methods, the effect of coexisting ions, the pH influence, and metal recovery methods for the increasingly complicated metal-organic complexes wastewater. Conventional treatment methods such as flocculation, adsorption, biological degradation, and ion exchange fail to decompose metal-organic complexes completely without causing secondary pollution in wastewater. To enhance comprehension of the behavior and morphology exhibited by metal-organic complexes within aqueous solutions, we presented the molecular structure and properties of metal-organic complexes, the decomplexation mechanisms that encompassed both radical and non-radical oxidizing species, including hydroxyl radical (OH), sulfate radical (SO˙4-), superoxide radical (O˙2-), hydrogen peroxide (H2O2), ozone (O3), and singlet oxygen (1O2). More importantly, we reviewed novel aspects that have not been covered by previous reviews considering the impact of operational parameters and coexisting ions. Finally, the potential avenues and challenges were proposed for future research.

Toward decentralized wastewater treatment: A flow-through module using microtubular gas diffusion electrodes for micropollutants removal

Electro-Fenton (EF) represents an eco-friendly and cost-effective advanced oxidation process that can remove highly persistent and hazardous pharmaceuticals, e.g., contrast media agents, from water bodies. However, up to date, EF modules incorporate a planar carbonaceous gas diffusion electrode (GDE) cathode containing fluorinated compounds as polymeric binders. Here, we introduce a novel flow-through module that deploys freestanding carbon microtubes (CMT) as microtubular GDEs, omitting any risks of secondary pollution by highly-persistent fluorinated compounds (e.g., Nafion). The flow-through module was characterized for electrochemical hydrogen peroxide (H₂O₂) generation and micropollutant removal via EF. H₂O₂ electro-generation experiments illustrated high production rates (1.1 ± 0.1-2.7 ± 0.1 mg cm^-2 h^-1) at an applied cathodic potential of - 0.6 V vs. SHE, depending on the porosity of CMTs. Diatrizoate (DTZ), as the model pollutant, with a high initial concentration of 100 mg L^-1 was successfully oxidized (95-100 %), reaching mineralization (TOC-total organic carbon removal) efficiencies up to 69 %. Additionally, Electro-adsorption experiments demonstrated the capability of positively charged CMTs to remove negatively charged DTZ with a capacity of 11 mg g^-1 from a 10 mg L^-1 DTZ solution. These results reveal the potential of the as-designed module to serve as an oxidation unit coupled with other separation techniques, e.g., electro-adsorption or membrane processes.

Efficient upcycling of iron scrap and waste polyethylene terephthalate plastic into Fe3O4@C incorporated MIL-53(Fe) as a novel electro-Fenton catalyst for the degradation of salicylic acid

The current research demonstrates the efficiency of a low-cost MIL-53(Fe)-metal-organic framework (MOF) derived Fe₃O₄@C (MIL-53(Fe)@Fe₃O₄@C) electrocatalyst in a batch-scale electro-Fenton (EF) process for the degradation of salicylic acid (SA) from wastewater. The electrocatalyst was prepared from the combination of polyethylene terephthalate (PET) and iron scrap wastes. The result showed 91.68 ± 3.61% degradation of 50 mg L^-1 of SA under optimum current density of 5.2 mA cm^-2, and pH of 7.0 during 180 min of electrolysis time. The degradation of SA from waste catalyst was similar to the chemical-based MIL-53(Fe)-derived Fe₃O₄@C (cFe) cathode catalyst. The presence of chloride ions (Cl^-) in the water matrix has shown a strong inhibitory effect on the elimination of SA, followed by nitrate (NO₃^-), and bicarbonate (HCO₃^-) ions. The multiple cyclic voltammetry (CV) analysis and reusability test of waste cathode catalyst showed only 8.03% drop of current density at the end of the 20th cycle and 5% drop of degradation efficiency after 6th cycle with low leaching of iron. The radical scavenging experiment revealed that the HO^• generated via electrochemical generation of H₂O₂ had a prominent contribution in the removal of SA compared to HO₂^•/O₂^•-. Besides, possible catalysis mechanism and degradation pathways were deduced. Furthermore, a satisfactory performance in the treatment of SA spiked in real water matrices was also observed by waste-derived Fe₃O₄@C cathode catalyst (wFe). Additionally, the total operating cost and toxicity analysis showed that the as-synthesized wFe cathode catalyst could be appropriate for removing organic pollutants from wastewater in the large-scale application.

State-of-the-art review and bibliometric analysis on electro-Fenton process

The electro-Fenton (EF) process was first proposed in 1996 and, since then, considerable development has been achieved for its application in wastewater treatment, especially at lab and pilot scale. After more than 25 years, the high efficiency, versatility and environmental compatibility of EF process has been demonstrated. In this review, bibliometrics has been adopted as a tool that allows quantifying the development of EF as well as introducing some useful correlations. As a result, information is summarized in a more visual manner that can be easily analyzed and interpreted as compared to conventional reviewing. During the recent decades under review, 83 countries have contributed to the dramatic growth of EF publications, with China, Spain and France leading the publication output. The top 12 most cited articles, along with the top 32 most productive authors in the EF field, have been screened. Four stages have been identified as main descriptors of the development of EF throughout these years, being each stage characterized by relevant breakthroughs. To conclude, a general cognitive model for the EF process is proposed, including atomic, microscopic and macroscopic views, and future perspectives are discussed.