Peng Zhang

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.

Damming plays a significant role in regulating global riverine phosphorus (P) cycling, with the general assumption that it led to the trapping of dissolved and particulate P in upstream sediments. However, in addition to riverine sediments, adjacent riparian zones may also serve as significant sinks for P species. This study investigated the distribution and transformation of groundwater P species in riparian zones along the bypass seepage pathway at two dam sites: the Three Gorges Dam (TGD) site on the Yangtze River and the Xinglong Dam (XD) site on the Han River. At the XD site, total P concentrations in groundwater were 1.5–13.2 times higher than in surface water, indicating significant P enrichment. In contrast, minimal P trapping was observed at the TGD site, where groundwater and surface water had similar total P concentrations and speciation. These differences were attributed to the variations in aquifer type, dam operation and aquifer geochemical conditions. In the fractured-karst aquifer at the TGD site, P species were readily transported, whereas transport was hindered in the Quaternary porous aquifer at the XD site. The water-bearing media in Quaternary porous aquifer had a high adsorption capacity for dissolved P species, resulting in the substantial trapping of P in riparian zones. Redox conditions were also crucial in regulating the distribution and transformation of groundwater P species in both aquifers. This study highlights the overlooked role of damming in regulating groundwater P distribution and transformation in riparian zones.

Riparian zones are natural hotspots for reactive oxygen species (ROS) generation, yet the spatiotemporal dynamics of ROS within these zones remain poorly understood. In this study, we combine results from a flume experiment and reactive transport modeling to show that H₂O₂ production is governed by a "chromatographic" separation process in which water flow modulates the interplay of O₂ and reductants. The riparian aquifer matrix acts as the stationary phase hosting various mobile and immobile reductants, while water flow serves as the mobile phase supplying oxidants, here dissolved oxygen (DO) and nitrate, during surface water inflow and flushing out of the mobile reductant species during flow reversal. The preferential consumption of DO by the reductants near the up-gradient boundary during surface water intrusion generates H₂O₂, whereas less reactive oxidants like nitrate are transported further into the riparian aquifer, where they consume the reductants that have not reacted with DO. Although the inflow of nitrate reduces the overall ROS production capacity, it enables deeper DO penetration, hence expanding the ROS production area. The resulting coupling between hydrodynamic solute transport and biogeochemical redox reactions regulates the spatial separation and temporal evolution of H₂O₂ across the simulated riparian aquifer. Overall, our study advances the mechanistic understanding of ROS dynamics in riparian zones with implications for redox-mediated contaminant attenuation and carbon cycling at the groundwater-river interface.