A permeable electrochemical reactive barrier for underground water remediation using TiO2/graphite composites as heterogeneous electrocatalysts without releasing of chemical substances

A sustainable activated carbon fiber/TiO2 cathode for the photoelectro-Fenton treatment of pharmaceutical pollutant enalapril

In this work, a TiO2-decorated electrode was fabricated by dip coating activated carbon fibers (ACF) with TiO2, which were then used as a cathode for the photoelectro-Fenton (PEF) treatment of the pharmaceutical enalapril, an angiotensin-converting enzyme inhibitor that has been detected in several waterways. The TiO2 coating was found to principally improve the electrocatalytic properties of ACF for H2O2 production via the 2-e- O2 reduction, in turn increasing enalapril degradation by PEF. The effect of the current density on the mineralization of enalapril was evaluated and the highest TOC removal yield (80.5% in 3 h) was obtained at 8.33 mA cm-2, in the presence of 0.5 mmol L-1 of Fe2+ catalyst. Under those conditions, enalapril was totally removed within the first 10 min of treatment with a rate constant k = 0.472 min-1. In contrast, uncoated ACF only achieved 60% of TOC removal in 3 h at 8.33 mA cm-2. A degradation pathway for enalapril mineralization is proposed, based on the degradation by-products identified during treatment. Overall, the results demonstrate the promises of TiO2 cathodes for PEF, a strategy that has often been overlooked in favor of photoelectrocatalysis (PEC) based on TiO2-modified photoanodes.

Hydroxyl radical induced Cr flocculation via redox reaction: The extending application of heterogeneous advanced oxidation processes on Cr removal

Flocculation is a traditional and effective method to remove Cr from wastewater, but the addition of flocculants inevitably leads to secondary pollution. In this study, Cr flocculation was induced using hydroxyl radical (•OH) (•OH flocculation) generated in an electro-Fenton-like system, achieving total Cr removal of 98.68% at initial pH = 8 within 40 min. The obtained Cr flocs showed significantly higher Cr content, lower sludge yield, and good settling properties compared to alkali precipitation and polyaluminum chloride flocculation. •OH flocculation behaved like a typical flocculant, introducing electrostatic neutralization and bridging. The mechanism proposed that •OH could overcome the steric hindrance of Cr(H₂O)₆³⁺ and combine with it as an additional ligand. Then Cr(III) was proved to undergo multi-step oxidation to form Cr(IV) and Cr(V). After these oxidation reactions, •OH flocculation took precedence over Cr(VI) generation. As a result, Cr(VI) didn't accumulate in solution until •OH flocculation was completed. This work provided a clean and eco-friendly strategy for Cr flocculation instead of flocculants and extended the application of advanced oxidation processes (AOPs), which is expected to enrich existing strategies of AOPs towards Cr removal.

Contaminant containment for sustainable remediation of persistent contaminants in soil and groundwater

Contaminant containment measures are often necessary to prevent or minimize offsite movement of contaminated materials for disposal or other purposes when they can be buried or left in place due to extensive subsurface contamination. These measures can include physical, chemical, and biological technologies such as impermeable and permeable barriers, stabilization and solidification, and phytostabilization. Contaminant containment is advantageous because it can stop contaminant plumes from migrating further and allow for pollutant reduction at sites where the source is inaccessible or cannot be removed. Moreover, unlike other options, contaminant containment measures do not require the excavation of contaminated substrates. However, contaminant containment measures require regular inspections to monitor for contaminant mobilization and migration. This review critically evaluates the sources of persistent contaminants, the different approaches to contaminant remediation, and the various physical-chemical-biological processes of contaminant containment. Additionally, the review provides case studies of contaminant containment operations under real or simulated field conditions. In summary, contaminant containment measures are essential for preventing further contamination and reducing risks to public health and the environment. While periodic monitoring is necessary, the benefits of contaminant containment make it a valuable remediation option when other methods are not feasible.

Iron-modified biochar-based bilayer permeable reactive barrier for Cr(VI) removal

Iron (Fe)-modified biochar (FeBC) has been developed to remove hexavalent chromium (Cr(VI)) from groundwater and is suitable for use in permeable reactive barriers (PRBs). However, Cr(VI) removal behavior and chemical processes in FeBC-based PRBs are not fully understood, and the potential for Fe release has not been addressed. In this study, three FeBC-based PRBs were assessed in column experiments for 563 days with respect to their ability to remove Cr(VI). Bilayer column filled with FeBC+limestone and BC+limestone in two separate layers (FeBC_Ca_BC) showed the best performance in terms of Cr(VI) removal with a low treatment cost. The corrosion of FeBC was mainly related to pH and Cr(VI) concentration rather than flow rate. Leached Fe was attenuated by BC and limestone and reutilized in FeBC_Ca_BC. Cr(VI) was reduced to Cr(III) and then adsorbed or precipitated on the biochars. Cr and Fe formed inner-sphere complexes and then transformed from double corner sharing to edge sharing. During the reaction, Cr penetrated from the surface to the interior of the biochars and became a more stable species. This study provides evidence of the effectiveness of a new combination of biochars for Cr(VI) removal and insights into the reaction mechanisms.