Comprehensive analysis of research trends and prospects in electrochemical advanced oxidation processes (EAOPs) for wastewater treatment

Bibliometric analysis and systematic review on the electrokinetic remediation of contaminated soil and sediment

Electrokinetic remediation (EKR) is a proficient, environmentally friendly separation technology for in-situ removal of contaminants in soil/sediment, distinguished for its ease of implementation and minimal prerequisites compared to other remediation technologies. To comprehensively understand the research focus and progress related to EKR of contaminated soil/sediment, a bibliometric analysis was conducted on 1593 publications retrieved from the Web of Science Core Collection (WOSCC) database. This analysis utilized data mining and knowledge discovery techniques through Bibliometrix, VOSviewer, and CiteSpace software. The results revealed a rising trend in annual publication numbers, with China leading in the number of publications. The primary journals in this field included the Journal of Hazardous Materials, Chemosphere, and Separation and Purification Technology. The primary disciplines contributed to this field included "Environmental Sciences", "Engineering, Environmental", "Engineering, Chemical", and "Electrochemistry". Keyword co-occurrence and burst analysis indicated that current EKR-related research mainly focuses on the remediation of soil/sediments contaminated by heavy metals (HMs) and organic pollutants (OPs). Furthermore, the EKR remediation improvement method emerged as the prevailing and future research hotspots and development directions. Future research could integrate numerical simulations and various methodologies to predict and assess the migration of pollutants and the efficiency of remediation efforts. Additionally, these studies could explore the effects of EKR on the physicochemical properties and microbial diversity of soil/sediment to provide a theoretical foundation for applying EKR in soil/sediment remediation.

Real-time in situ monitoring as a tool for comparison of electrochemical advanced oxidation processes for the decolourisation of azo and indigoid dyes

The widespread use of synthetic dyes has led to the release of substantial amounts of dye-contaminated wastewater, posing significant environmental and health concerns. This study focuses on the use of anodic and electrochemically activated persulfate oxidation for the degradation of organic contaminants. Specifically, the structural variations of nine dyes in the indigoid and azo families, and their impact on the efficiency of electrochemical oxidation were analysed. An in situ continuous monitoring apparatus with a UV-visible detector was employed to collect data in real-time. The electrochemically activated persulfate system demonstrated higher efficiency compared to the anodic oxidation approach. In both systems the efficiency of decolourisation was highly dependent on the structure of the pollutant. Electron-withdrawing substituents in direct conjugation with the chromophore, bulky auxochromes, and extended aromatic systems significantly decreased the decolourisation efficiency. Conversely, changing the location of electron-withdrawing groups and adding electron-donating substituents increased the decolourisation efficiency, even overcoming the detrimental effects of bulky groups and extended conjugation. This type of systematic structural comparison study is essential for highlighting the interconnected nature of pollutant structure and degradation speed so that efficient electrochemical oxidation systems can be designed for the treatment of genuine wastewater effluent containing more than one pollutant.

Internal cycling of Fe3+/Fe2+ within Fe3O4 on cathode promotes green degradation of reactive brilliant red X-3B on anode

Reactive brilliant red X-3B (RBRX-3B) wastewater is difficult to decolourise, not readily biodegradable, and large in quantity. Therefore, the efficient removal of RBRX-3B is crucial. In this paper, a green and efficient electrochemical-electro-Fenton system with Fe3O4-modified carbon felt bag cathode (ECEF-Fe3O4) was set up to degrade RBRX-3B wastewater. Experiments confirmed that the removal of RBRX-3B by ·OH or H2O2 is quite low, and RBRX-3B can be completely oxidised and degraded directly on the anode. Long-cycle experimental data further shows that the degradation efficiency of RBRX-3B on the anode is 100% at 70 min at the reaction rate constants (k) of 0.071 min-1 in ECEF-Fe3O4 while that of RBRX-3B on the cathode is only 16.8 ± 0.9%. The generation of ·OH is mainly catalysed through the internal cycling of Fe3+/Fe2+ within Fe3O4 on the cathode, and the generation and annihilation of ·OH on the cathode enhance the oxidation efficiency of the anode, achieving the green and effective removal of RBRX-3B by the anode in ECEF-Fe3O4.

Effect of the anode material, applied current and reactor configuration on the atenolol toxicity during an electrooxidation process

Atenolol (ATL) is a beta-blocker pharmaceutical product which is excreted mainly unchanged and may represent a long-term risk for organisms present in the sea and in fresh water. Due to its low biodegradation rate, electrochemical advanced oxidation processes (EAOPs) can be used to remove this compound. In this work, ATL ecotoxicity was analyzed in the presence of sodium sulfate (Na2SO4), which is widely used as supporting electrolyte in EAOPs. Ecotoxicity values were expressed as the pollutant concentration that leads to a 50% inhibition of the root elongation of Lactuca sativa seeds in relation to the control (EC50(5 days)). The obtained values for ATL showed an EC50(5 days) of 1377 mg L-1 towards Lactuca sativa. When Na2SO4 was added, the toxicity of the sample increased but no synergy was detected between both compounds. With 2 g L-1 Na2SO4, ATL showed an EC50(5 days) of 972 mg L-1; and with 4 g L-1 Na2SO4 and higher concentrations, EC50 value for ATL was 0 mg L-1. Statistical tools were used to obtain the zones of the [ATL]-[Na2SO4] plane which are toxic towards Lactuca sativa. Solutions containing ATL and Na2SO4 were treated by electrooxidation. Two anode materials (a boron-doped diamond electrode and a microporous Sb-doped SnO2 ceramic one); three operation currents (0.4, 0.6 and 1 A); and two reactor configurations (one-compartment reactor and two-compartment reactor separated by a cation exchange membrane) were used. Lactuca sativa seeds and Vibrio fischeri bacterium tests were employed to evaluate the toxicity of the solutions before and after applying the electrooxidation process. In all the tests, the ecotoxicity of the treated sample increased. This fact is owing to the persulfate presence in the solution due to the sulfate electrochemical oxidation. Nevertheless, none of the final samples were toxic towards Vibrio fischeri because ecotoxicity values were lower than 10 TU; and, in the case of the one-compartment reactor, practically all of them were also non-toxic towards Lactuca sativa. The toxicity of the treated samples increased when using the two-compartment reactor in the presence of the BDD anode, and when the operation current was increased. This is attributed to the highest formation of persulfates. Amongst all the tests performed in this work, the lowest toxicity value (i.e., 3 TU) together with the complete mineralization and degradation degrees was achieved with the two-compartment reactor using the BDD anode and operating at 0.6 A.