Yunsong Zheng

Fe(II)-bearing clay minerals are widespread reductants of hexavalent chromium (Cr(VI)) in subsurface environments. However, the influences of coexisting Fe (hydr)oxides on Cr(VI) reduction by Fe(II)-bearing clay minerals remain elusive. Here, Cr(VI) reduction was investigated in premixed systems of reduced nontronite NAu-2 (rNAu-2) and various Fe (hydr)oxides. Results showed that coexisting ferrihydrite significantly facilitated Cr(VI) reduction during the quick stage (0–2.5 min) across pH 5.5–9.0. During the subsequent slow stage (2.5–205 min), ferrihydrite inhibited Cr(VI) reduction at pH 5.5–7.0 but enhanced it at pH 9.0. In contrast, coexisting goethite had negligible effects. Without Fe (hydr)oxides, direct reduction by rNAu-2 dominated for Cr(VI) removal. In the presence of Fe (hydr)oxides, competitive adsorption was found to inhibit direct Cr(VI) reduction, yet Fe (hydr)oxide-mediated electron transfer effectively offset this inhibitory effect. Both competitive adsorption and mediated electron transfer were influenced by the type and dosage of Fe (hydr)oxides, as well as solution pH. Kinetic modeling revealed that ferrihydrite-mediated electron transfer contributed 1.8–71.5% of the total Cr(VI) reduction, with greater contributions observed at higher solution pH and ferrihydrite dosage. Our findings provide new insights into mineral–mineral electron transfer and its role in regulating Cr(VI) fate in subsurface environments.

In situ chemical oxidation (ISCO) is widely used for groundwater remediation, whereas the influence of aquifer heterogeneity, that is, the difference in physical structure and chemical compositions, on oxidant retention and consumption remains poorly understood. In low-permeability lens-containing sand columns, we found that the retention and consumption of oxidant (KMnO4) were largely influenced by the chemical reactivity of the lens besides the well-known permeability. Inert lens (represented by kaolin) mainly altered the transport pathways of oxidant, whereas chemically reactive lenses (represented by real sediments with different electron-donating capacities) further enhanced the retention and consumption within the lenses. A higher electron-donating capacity led to more oxidant retention and consumption. Moreover, the reduced components released from the reactive lens could also increase the oxidant consumption. This study highlights the importance of chemical reactivity besides the physical structure of lenses in heterogeneous aquifers for oxidant transport, retention, and consumption and also implicates the consideration of chemical reactivity for designing the injection dosages and position of oxidants in ISCO remediation.

In situ chemical oxidation (ISCO) has been widely used in groundwater remediation, whereas the injected chemicals pose secondary risks to the remediated aquifers. Here, we propose a noncontact oxidation scheme in which contaminants are treated by physically separated oxidants through wired electrodes, thereby eliminating the injection of chemicals into the aquifer. Groundwater circulation is applied to transport contaminated groundwater from surrounding aquifers to the in-well electrode. We serendipitously found that graphite electrodes after simple preoxidation showed superior performance to other commercial electrodes, exhibiting high and sustainable performance in mediating the long-distance electron transfer from contaminants to oxidants. For the noncontact oxidation of 10 μM phenol by 10 mM peroxysulfate (PDS), the removal reached 85% within 2 h and was maintained stable over 10 consecutive treatment cycles. Persulfate led to a larger contaminant removal than other common oxidants. Assisted with hydraulic circulation, this system achieved 95% of phenol removal within 36 h in a simulated aquifer. Contaminants with high electron-donating abilities were prone to be oxidized. Anode/contaminant and cathode/oxidant interfacial interactions played a critical role in noncontact oxidation. This noncontact oxidation strategy presents a new horizon for green groundwater remediation.

^•OH-based oxidation plays a crucial role in dissolved organic matter (DOM) transformation and carbon flux, whereas quantifying the contribution of this pathway remains challenging. Here we combined the concentration with the carbon isotope analysis of DOM and its generated CO₂ to quantify the contribution of ^•OH-based oxidation. Results showed that the ¹³C enrichment factors (ε values) were -8.1‰ to -8.9‰ for benzene ring oxidation in aromatic compounds, -4.2‰ to -28.9‰ for lower-molecular-weight organic acids, and -13.0‰ for DOM from sediment. The fractionation of sediment DOM reflects the average ε value of humic substances and organic acids. These ε values were more negative than those of the photochemical and microbial processes, enabling the identification of DOM transformation mechanisms. Using an end-member mix model, we found that the proportion of ^•OH-based mineralization in total CO₂ emission ranged from 20.9% to 39.8% for 100 g/L sediment oxidation by 5-20 mM H₂O₂ under pH-neutral condition within 2 h and was only 2% for oxidation by air under the same conditions. We also found that inorganic carbon degassing contributed greatly to CO₂ emission during sediment oxidation. This study presents a new isotope-based tool to quantitatively assess the contribution of ^•OH-based oxidation to the emission of CO₂ from DOM.