Critical Role of Mineral Fe(IV) Formation in Low Hydroxyl Radical Yields during Fe(II)-Bearing Clay Mineral Oxygenation.

In subsurface environments, Fe(II)-bearing clay minerals can serve as crucial electron sources for O2 activation, leading to the sequential production of O2•-, H2O2, and •OH. However, the observed •OH yields are notably low, and the underlying mechanism remains unclear. In this study, we investigated the production of oxidants from oxygenation of reduced Fe-rich nontronite NAu-2 and Fe-poor montmorillonite SWy-3. Our results indicated that the •OH yields are dependent on mineral Fe(II) species, with edge-surface Fe(II) exhibiting significantly lower •OH yields compared to those of interior Fe(II). Evidence from in situ Raman and Mössbauer spectra and chemical probe experiments substantiated the formation of structural Fe(IV). Modeling results elucidate that the pathways of Fe(IV) and •OH formation respectively consume 85.9-97.0 and 14.1-3.0% of electrons for H2O2 decomposition during oxygenation, with the Fe(II)edge/Fe(II)total ratio varying from 10 to 90%. Consequently, these findings provide novel insights into the low •OH yields of different Fe(II)-bearing clay minerals. Since Fe(IV) can selectively degrade contaminants (e.g., phenol), the generation of mineral Fe(IV) and •OH should be taken into consideration carefully when assessing the natural attenuation of contaminants in redox-fluctuating environments.

Unveiling the inhibitory mechanisms of chromium exposure on microbial reductive dechlorination: Kinetics and microbial responses.

Chromium and organochlorine solvents, particularly trichloroethene (TCE), are pervasive co-existing contaminants in subsurface aquifers due to their extensive industrial use and improper disposal practices. In this study, we investigated the microbial dechlorination kinetics under different TCE-Cr(Ⅲ/VI) composite pollution conditions and elucidated microbial response mechanisms based on community shift patterns and metagenomic analysis. Our results revealed that the reductive dechlorinating consortium had high resistance to Cr(III) but extreme sensitivity to Cr(VI) disturbance, resulting in a persistent inhibitory effect on subsequent dechlorination. Interestingly, the vinyl chloride-respiring organohalide-respiring bacteria (OHRB) was notably more susceptible to Cr(III/VI) exposure than the trichloroethene-respiring one, possibly due to inferior competition for growth substrates, such as electron donors. In terms of synergistic non-OHRB populations, Cr(III/VI) exposure had limited impacts on lactate fermentation but significantly interfered with H2-producing acetogenesis, leading to inhibited microbial dechlorination due to electron donor deficiencies. However, this inhibition can be effectively mitigated by the amendment of exogenous H2 supply. Furthermore, being the predominant OHRB, Dehalococcoides have inherent Cr(VI) resistance defects and collaborate with synergistic non-OHRB populations to achieve concurrent bio-detoxication of Cr(VI) and TCE. Our findings expand the understanding of the response patterns of different functional populations towards Cr(III/VI) stress, and provide valuable insights for the development of in situ bioremediation strategies for sites co-contaminated with chloroethene and chromium.

Electrokinetic bioremediation of trichloroethylene and Cr/As co-contaminated soils with elevated sulfate.

Co-contaminants and complex subsurface conditions pose great challenges to site remediation. This study demonstrates the potential of electrokinetic bioremediation (EK-BIO) in treating co-contaminants of chlorinated solvents and heavy metals in low-permeability soils with elevated sulfate. EK-BIO columns were filled with field soils, and were fed by the electrolyte containing 20 mg/L trichloroethylene (TCE), 250 µM Cr(VI), 25 µM As(III), 10 mM lactate, and 10 mM sulfate. A dechlorinating consortium containing Dehalococcoides (Dhc) was injected several times during a 199-d treatment at ∼1 V/cm. Sulfate reduction, Cr/As immobilization, and complete TCE biodechlorination were observed sequentially. EK-BIO facilitated the delivery of lactate, Cr(VI)/As(III), and sulfate to the soils, creating favorable reductive conditions for contaminant removal. Supplementary batch experiments and metagenomic/transcriptomic analysis suggested that sulfate promoted the reductive immobilization of Cr(VI) by generating sulfide species, which subsequently enhanced TCE biodechlorination by alleviating Cr(VI) toxicity. The dechlorinating community displayed a high As(III) tolerance. Metagenomic binning analysis revealed the dechlorinating activity of Dhc and the potential synergistic effects from other bacteria in mitigating heavy metal toxicity. This study justified the feasibility of EK-BIO for co-contaminant treatment and provided mechanistic insights into EK-BIO treatment.

Response of TCE biodegradation to elevated H2 and O2: Implication for electrokinetic-enhanced bioremediation.

The study investigated the influences of pure H2 and O2 introduction, simulating gases produced from the electrokinetic-enhanced bioremediation (EK-Bio), on TCE degradation, and the dynamic changes of the indigenous microbial communities. The dissolved hydrogen (DH) and oxygen (DO) concentrations ranged from 0.2 to 0.7 mg/L and 2.6 to 6.6 mg/L, respectively. The biological analysis was conducted by 16S rRNA sequencing and functional gene analyses. The results showed that the H2 introduction enhanced TCE degradation, causing a 90.4% TCE removal in the first 4 weeks, and 131.1 µM was reduced eventually. Accordingly, cis-dichloroethylene (cis-DCE) was produced as the only product. The following three ways should be responsible for this promoted TCE degradation. Firstly, the high DH rapidly reduced the oxidation-reduction potential (ORP) value to around -500 mV, beneficial to TCE microbial dechlorination. Secondly, the high DH significantly changed the community and promoted the enrichment of TCE anaerobic dechlorinators, such as Sulfuricurvum, Sulfurospirillum, Shewanella, Geobacter, and Desulfitobacterium, and increased the abundance of dechlorination gene pceA. Thirdly, the high DH promoted preferential TCE dechlorination and subsequent sulfate reduction. However, TCE bio-remediation did not occur in a high DO environment due to the reduced aerobic function or lack of functional bacteria or co-metabolic substrate. The competitive dissolved organic carbon (DOC) consumption and unfriendly microbe-microbe interactions also interpreted the non-degradation of TCE in the high DO environment. These results provided evidence for the mechanism of EK-Bio. Providing anaerobic obligate dechlorinators, and aerobic metabolic bacteria around the electrochemical cathodes and anodes, respectively, or co-metabolic substrates to the anode can be feasible methods to promote remediation of TCE-contaminated shallow aquifer under EK-Bio technology.

Contaminant transformation during sediment oxygenation: Temporal variation of oxidation mechanisms mediated by hydroxyl radicals and aerobic microbes.

Sediment oxidation by oxygen is ubiquitous, whereas the mechanisms of concurrent contaminant oxidation, particularly the temporal variation of chemical and biological oxidation, remain inadequately understood. This study investigated the oxidation of two contaminants (phenol and trichloroethylene) with different responses during the oxygenation of four natural sediments with different redox properties. Results showed that contaminant oxidation was initially dominated by hydroxyl radicals (•OH) (first stage), stabilized for different time for different sediments (second stage), and was re-started by microbial mechanism (third stage). In the first short stage, the contribution of chemical oxidation by •OH was mainly determined by the variation of sediment electron-donating capacity (EDC). In the second long stage, the stabilization time was dependent on sediment redox properties, that is, the abundance and growth of aerobic microbes capable of degrading the target contaminants. A more reduced sediment resulted in a higher extent of oxidation by •OH and a longer stabilization time. When the third stage of aerobic microbial oxidation was started, the contaminants like phenol that can be utilized by microbes can be oxidized quickly and completely, and those refractory contaminants like trichloroethylene remained unchanged. The study differentiates chemical and biological mechanisms for contaminant oxidation during sediment oxygenation.

Dependence of Biotic and Abiotic H2O2 and •OH Production on the Redox Conditions and Compositions of Sediment during Oxygenation.

Reactive oxygen species (ROS) production in O2-perturbed subsurface environments has been increasingly documented in recent years. However, the constraining conditions under which abiotic and/or biotic mechanisms predominate for ROS production remain ambiguous. Here, we demonstrate that the ROS production mechanism, biotic and abiotic, is determined by sediment redox properties and sediment compositions. Upon the oxygenation of 10 field sediments, the cumulative H2O2 concentrations reached up to 554 µmol/kg within 2 h. The autoclaving sterilization experiments showed that H2O2 could be produced by both biotic and abiotic processes depending on the redox conditions. However, only the abiotic process could produce significant levels of •OH, and the production yield was closely related to the sediment components, particularly sediment Fe(II) and organic matter. Fe(II) bound with organic matter led to high yields of H2O2 and •OH production. Sediment oxygenation contributed to the appearance of H2O2 in groundwater, with the abiotic mechanism producing higher instantaneous H2O2 concentrations than the biotic mechanism. These findings reveal that the redox conditions, compositions, and texture of sediments collectively control abiotic and biotic mechanisms for ROS production, which assists the identification of ROS production hotspots and the understanding of ROS distribution and utilization in the subsurface.

Contaminant degradation by •OH during sediment oxygenation: Effect of abundant solid matrix in aquifer.

Aquifer oxygenation for hydroxyl radical (•OH) production has been recently proposed as a promising strategy for in-situ remediation. However, the high performance of this process was justified at low solid-to-liquid ratios (SLRs) of suspension systems. It remains unclear whether and how the performance is affected by abundant solid matrixes. Here we assessed the influence of SLR on •OH production and contaminant degradation during sediment oxygenation. Cumulative •OH increased from 21.8 to 165.2 µM when the SLR increased from 200 to 1600 g/L, while phenol degradation increased with the increase in SRL at the values lower than 1200 g/L and decreased at higher SLRs. As the main sediment component, silica exhibited a negligible effect on •OH production and phenol degradation because of the weak adsorption towards aqueous Fe(II). Whereas, the other component, alumina, significantly inhibited •OH production and phenol degradation because it strongly adsorbed Fe(II). •OH scavenging by solid reactive matrixes was mainly responsible for the inhibition at high SLRs. The scavenging effect could be mitigated by mediating the main reactive Fe(II) species from solid-adsorbed to dissolved phase with ligand addition. Our findings are important for understanding the side reactions and optimizing the remediation performance during aquifer oxygenation.

Mechanistic Insight into Electron Transfer from Fe(II)-Bearing Clay Minerals to Fe (Hydr)oxides.

Electron transfer (ET) is the essence of most biogeochemical processes related to element cycling and contaminant attenuation, whereas ET between different minerals and the controlling mechanism remain elusive. Here, we used surface-associated Fe(II) as a proxy to explore ET between reduced nontronite NAu-2 (rNAu-2) and Fe (hydr)oxides in their coexisting systems. Results showed that ET could occur from rNAu-2 to ferrihydrite but not to goethite, and the ET amount was determined by the number of reactive sites and the reduction potential difference between rNAu-2 and ferrihydrite. ET proceeded mainly through the mineral-mineral interface, with a negligible contribution of dissolved Fe2+/Fe3+. Control experiments by adding K+ and increasing salinity together with characterizations by X-ray diffraction, scanning electron microscopy/energy-dispersive spectrometry, and atomic force microscopy suggested that ferrihydrite nanoparticles inserted the interlayer space in rNAu-2 where structural Fe(II) in rNAu-2 transferred electrons mainly through the basal plane to ferrihydrite. This study implicates the occurrence of ET between different redox-active minerals through the mineral-mineral interface. As minerals at different reduction potentials often coexist in soils/sediments, the mineral-mineral ET may play an important role in subsurface biogeochemical processes.

Kinetic models for hydroxyl radical production and contaminant removal during soil/sediment oxygenation.

Hydroxyl radical (•OH) oxidation has been identified as a significant pathway for element cycling and contaminant removal in redox fluctuating environments. Fe(II) has been found to be the main electron contributor for •OH production. Despite the recognition of the mechanisms of •OH production from the oxidation of Fe(II) in soils/sediments by O2, the kinetic model about Fe(II) oxidation, •OH production and contaminant removal is not yet clear. To address this knowledge gap, we conducted a series of experiments to explore the variation of different Fe(II) species, •OH and trichloroethylene (TCE, a representative contaminant) during sediment oxygenation, followed by the development of a kinetic model. In this model, Fe(II) species in sediments was divided into three categories based on the sequential chemical extraction method: ion exchangeable Fe(II), surface-adsorbed Fe(II) and mineral structural Fe(II),. Results showed that the kinetic model accurately fitted the concentration time trajectories of different Fe(II) species, •OH and TCE in this study as well as in previous studies. Model analysis indicated that the relative contribution of surface-adsorbed Fe(II) and reactive mineral structural Fe(II) in •OH production was 16.4%-33.9% and 66.1%-83.6%, respectively. However, ion-exchangeable Fe(II) not only fails to contribute to •OH production but also reduces the •OH yield relative to H2O2 decomposition. Poorly reactive mineral structural Fe(II) can serve as an electron pool to regenerate these reactive Fe(II) and facilitate •OH production. Regarding TCE degradation, Fe(II) species plays a dual role in contributing to •OH production while competing with TCE for •OH consumption, with the quenching efficiency being related to their content and reactivity toward •OH. This kinetic model offers a practical approach to describing and predicting •OH production and associated environmental impacts at the oxic-anoxic interface.

Groundwater Quality and Health: Making the Invisible Visible.

Linking groundwater quality to health will make the invisible groundwater visible, but there are knowledge gaps to understand the linkage which requires cross-disciplinary convergent research. The substances in groundwater that are critical to health can be classified into five types according to the sources and characteristics: geogenic substances, biogenic elements, anthropogenic contaminants, emerging contaminants, and pathogens. The most intriguing questions are related to quantitative assessment of human health and ecological risks of exposure to the critical substances via natural or induced artificial groundwater discharge: What is the list of critical substances released from discharging groundwater, and what are the pathways of the receptors' exposure to the critical substances? How to quantify the flux of critical substances during groundwater discharge? What procedures can we follow to assess human health and ecological risks of groundwater discharge? Answering these questions is fundamental for humans to deal with the challenges of water security and health risks related to groundwater quality. This perspective provides recent progresses, knowledge gaps, and future trends in understanding the linkage between groundwater quality and health.

Electrokinetic-Enhanced Bioremediation of Trichloroethylene-Contaminated Low-Permeability Soils: Mechanistic Insight from Spatio-Temporal Variations of Indigenous Microbial Community and Biodehalogenation Activity.

Electrokinetic-enhanced bioremediation (EK-Bio), particularly bioaugmentation with injection of biodehalogenation functional microbes such as Dehalococcoides, has been documented to be effective in treating a low-permeability subsurface matrix contaminated with chlorinated ethenes. However, the spatio-temporal variations of indigenous microbial community and biodehalogenation activity of the background matrix, a fundamental aspect for understanding EK-Bio, remain unclear. To fill this gap, we investigated the variation of trichloroethylene (TCE) biodehalogenation activity in response to indigenous microbial community succession in EK-Bio by both column and batch experiments. For a 195 day EK-Bio column (∼1 V/cm, electrolyte circulation, lactate addition), biodehalogenation activity occurred first near the cathode (<60 days) and then spread to the anode (>90 days), which was controlled by electron acceptor (i.e., Fe(III)) competition and microbe succession. Amplicon sequencing and metagenome analysis revealed that iron-reducing bacteria (Geobacter, Anaeromyxobacter, Geothrix) were enriched within initial 60 d and were gradually replaced by organohalide-respiring bacteria (versatile Geobacter and obligate Dehalobacter) afterward. Iron-reducing bacteria required an initial long time to consume the competitive electron acceptors so that an appropriate reductive condition could be developed for the enrichment of organohalide-respiring bacteria and the enhancement of TCE biodehalogenation activity.

Integrated anaerobic-aerobic biodegradation of mixed chlorinated solvents by electrolysis coupled with groundwater circulation in a simulated aquifer.

Chlorinated solvents are widespread subsurface contaminants that are often present as complex mixtures. Complete biodegradation of mixed chlorinated solvents remains challenging because the optimal redox conditions for biodegradation of different chlorinated solvents differ significantly. In this study, anaerobic and aerobic conditions were integrated by electrolysis coupled with groundwater circulation for biodegradation of a mixture of chloroform (CF, 8.25 mg/L), 1,2-dichloroethane (DCA, 7.01 mg/L), and trichloroethylene (TCE, 4.56 mg/L). A two-dimensional tank was filled with field sandy and silty-clayed sediments to simulate aquifer conditions, a pair of electrodes was installed between an injection well and abstraction well, and groundwater circulation transported cathodic H2 and anodic O2 to produce multiple redox conditions. Microbial community analysis demonstrated that the system constructed a habitat suitable for the co-existence of aerobic and anaerobic microbes. After 50 days of treatment, 93.1%, 100%, and 87.3% of CF, 1,2-DCA, and TCE were removed without observed intermediates, respectively. Combined with compound specific isotope analysis, the degradation of 1,2-DCA and CF was mainly attributed to aerobic oxidation and reductive dechlorination, respectively, and TCE was removed by both aerobic and anaerobic biodegradation. Our findings provide a new and efficient strategy for in situ bioremediation of groundwater contaminated by mixed chlorinated solvents.

Quantification of the Influence of Citrate/Fe(II) Molar Ratio on Hydroxyl Radical Production and Pollutant Degradation during Fe(II)-Catalyzed O2 and H2O2 Oxidation Processes.

Ligand-enhanced hydroxyl radical (•OH) production is an important strategy for Fe(II)-catalyzed O2 and H2O2 oxidation processes. However, the influence of the molar ratio of ligands to Fe(II) on •OH production remains elusive. This study employed citrate and inorganic dissolved Fe(II) (Fe(II)dis) as the representative ligand and Fe(II) species, respectively, to quantify this relationship. Results showed that •OH production was highly dependent on the citrate/Fe(II) molar ratio. For instance, for the oxygenation of Fe(II)dis, the •OH accumulations were 2.0-8.5, 3.4-28.5 and 8.1-42.3 µM at low (0.25-0.5), moderate (0.5-1), and high (1-2) citrate/Fe(II) molar ratios, respectively. At low citrate/Fe(II) molar ratio (<0.5), inorganic Fe(II)dis mainly contributed to •OH production, with the increase in the citrate/Fe(II) molar ratio to a high level (1-2), Fe(II)-citrate complex turned to the electron source for •OH production. The change in Fe(II) speciation with the increase of citrate/Fe(II) molar ratio elevated •OH production. For pollutant degradation, 1 mg/L phenol was degraded by 53.6% within 40 min during oxygenation of Fe(II)-citrate system (1:1) at pH 7. Our results suggest that a moderate molar ratio of ligand/Fe(II) (0.5-1) may be optimal for Fe(II)-catalyzed O2 and H2O2 oxidation processes.

Effect of C/Fe Molar Ratio on H2O2 and •OH Production during Oxygenation of Fe(II)-Humic Acid Coexisting Systems.

Hydrogen peroxide (H2O2) and hydroxyl radical (•OH) production during oxygenation of reduced iron (Fe(II)) and natural organic matter (NOM) in the subsurface has been increasingly discovered, whereas the effect of the C/Fe molar ratio in Fe(II) and NOM coexisting systems remains poorly understood. In this study, aqueous Fe(II) and reduced humic acid (HAred) mixture at different C/Fe molar ratios (0-20) were oxygenated. Results show that both H2O2 and •OH accumulation increased almost linearly with the increase in the C/Fe ratio, with a more prominent increase in •OH accumulation at high C/Fe molar ratios. At low C/Fe molar ratios (C/Fe ≤ 1.6), electrons mainly transferred from dissolved inorganic Fe(II), surface-adsorbed Fe(II), and a low proportion of HA-complexed Fe(II) to O2; with the increase in the C/Fe ratio to a high level (C/Fe ≥ 5), the main electron source turned to HA-complexed Fe(II) and free HAred. The changes in the electron source and electron transfer pathway with the increase in the C/Fe ratio increased the yield of •OH relative to H2O2. This study highlights the important role of the C/Fe ratio in controlling H2O2 and •OH production and therefore in accurately evaluating the associated environmental impacts.

Significant Contribution of Solid Organic Matter for Hydroxyl Radical Production during Oxygenation.

Dark formation of hydroxyl radicals (•OH) from soil/sediment oxygenation has been increasingly reported, and solid Fe(II) is considered as the main electron donor for O2 activation. However, the role of solid organic matter (SOM) in •OH production is not clear, although it represents an important electron pool in the subsurface. In this study, •OH production from oxygenation of reduced solid humic acid (HAred) was investigated at pH 7.0. •OH production is linearly correlated with the electrons released from HAred suspension. Solid HAred transferred electrons rapidly to O2 via the surface-reduced moieties (hydroquinone groups), which was fueled by the slow electron transfer from the reduced moieties inside solid HA. Cycling of dissolved HA between oxidized and reduced states could mediate the electron transfer from solid HAred to O2 for •OH production enhancement. Modeling results predicted that reduced SOM played an important or even dominant role in •OH production for the soils and sediments possessing high molar ratios of SOC/Fe(II) (e.g., >39). The significant contribution of SOM was further validated by the modeling results for oxygenation of 88 soils/sediments in the literature. Therefore, reduced SOM should be considered carefully to comprehensively understand •OH production in SOM-rich subsurface environments.

Fe(II) oxygenation inhibits bacterial Mn(II) oxidation by P. putida MnB1 in groundwater under O2-perturbed conditions.

Bacterial Mn(II) oxidation plays a crucial role in Mn cycling and the associated biogeochemistry in natural waters and is of practical concern in the clean-up of excess Mn from drinking water. Fe(II) usually occurring together with Mn(II) in groundwater is oxidized prior to Mn(II) when perturbed by O2, but the impact of Fe(II) oxygenation on the subsequent bacterial Mn(II) oxidation remains unknown. Here we demonstrated that Fe(II) oxygenation largely inhibited the Mn(II)-oxidizing ability of MnB1 belong to Pseudomonas putida which is ubiquitous in groundwater. The mechanisms of the inhibition varied under different Fe(II) concentrations. At high Fe(II) concentrations (≥ 1 mM), the inhibition of bacterial Mn(II) oxidation was mainly because of cell death caused by intracelluar reactive oxygen species. At low Fe(II) concentrations (≤ 0.05 mM), the inhibition of bacterial Mn(II) oxidation was attributed to Fe(III) oxyhydroxides generated from Fe(II) oxygenation. Fe(III) oxyhydroxides attached to cell surface and damaged the cell membrane, resulting in the influx of dissolved Fe into the cell. Transcriptomic analysis revealed that the intracellular Fe suppressed the transcription initiation process and the subsequent generation of multicopper oxidases which were responsible for Mn(II) oxidation. These findings implicate that the inhibition effect of Fe(II) oxygenation on bacterial Mn(II) oxidation should be considered in groundwater-surface water interaction zone and the biological treatment of Fe-Mn containing drinking water.

Hydroxylamine promoted Fe(III) reduction in H2O2/soil systems for phenol degradation.

Production of hydroxyl radicals (•OH) upon the oxidation of solid Fe(II) by O2 or H2O2 in soils and sediments has been confirmed, which benefits in situ remediation of contaminants. However, Fe(III) reduction by H2O2 is rate-limiting. Accelerating the Fe(III)/Fe(II) cycle could improve the efficiency of remediation. This study intended to use hydroxylamine to promote Fe(III)/Fe(II) cycle during 100 g/L soil oxidation by H2O2 for phenol degradation. The removal of phenol was 76% in 3 h during soil oxidation with 1 mM H2O2 in the presence of 1 mM hydroxylamine but was negligible in the absence of hydroxylamine. Fe(III) in the soil was reduced to 0.21 mM Fe(II) by 1 mM hydroxylamine in 30 min. The accelerated cycle of Fe(III)/Fe(II) in the soil by hydroxylamine could effectively decompose H2O2 to produced •OH, which was responsible for the effective enhancement of phenol degradation during soil oxidation. Under the conditions of 1 mM H2O2 and 100 g/L soil, the pseudo-first-order kinetic constant of phenol degradation increased proportionally from 0.0453 to 0.0844 min-1 with the increase of hydroxylamine concentrations from 0.5 to 1 mM. The kinetic constant also increased from 0.0041 to 0.0111 min-1 with H2O2 concentration increased from 0.5 to 2 mM, while it decreased from 0.0100 to 0.0051 min-1 with soil dosage increased from 20 to 200 g/L. In addition, column experiments showed that phenol (10 mg/L) degradation ratio kept at about 48.7% with feeding 2 mM hydroxylamine and 2 mM H2O2 at 0.025 PV/min. Column experiments suggested an optional application of hydroxylamine and H2O2 for in situ remediation. The output of this study provides guidance and optional strategies to enhance contaminant degradation during soil oxidation.

Integration of water collection and purification on cactus- and beetle-inspired eco-friendly superwettable materials.

Freshwater shortage has been a terrible threat for the sustainable progress and development of human society in 21st century. Inspired from natural creatures, harvesting water from atmosphere has been a feasible and effective method to alleviate water shortage crisis. However, the recent works related to water collection just focuses on how to optimize fog-harvesting manners and efficiencies, the safety and availability of collected water are always ignored. In this paper, we proposed a new strategy accessed to freshwater resources through combining water collection and purification together on eco-friendly superwettable material inspired by cactus spines and desert beetles. Six superhydrophilic wedge-shaped patterns prepared by P25 TiO2 nanoparticles (NPs) were constructed on candle soot@polydimethylsiloxane (CS@PDMS) superhydrophobic coating. The special superhydrophilic regions not only effectively captured water from foggy environment but generated Laplace pressure gradient to faster drive water away. The bioinspired material exhibited an efficient water collection rate (WCR) of 14.9 ± 0.2 mg min-1 cm-2, which was 5.3 and 2.5 times larger than that on uniformed superhydrophilic and superhydrophobic surfaces, respectively. Because of the existence of photocatalytic P25 NPs in wetting areas, the harvested wastewater containing nine kinds of pesticides (0.5 mg/L) could be purified in low concentrations (< 5%) under UV light (365 nm, 5.0 ± 0.6 mW cm-2). Ten zebrafishes were still alive in such purified water for 72 h, as a contrast, the same number of fishes would almost die in untreated harvested wastewater in just 7 h. This work indeed opens up a new sight to freshwater accessibility, aiming to a promising project for alleviating water shortage around the world.

Mechanistic Insight into Humic Acid-Enhanced Hydroxyl Radical Production from Fe(II)-Bearing Clay Mineral Oxygenation.

Hydroxyl radical (•OH) production by electron transfer from Fe(II)-bearing clay minerals to oxygen has been increasingly reported. However, the influence of ubiquitous coexisting humic acid (HA) on this process is poorly understood. Here, we investigated the effect of different HA on •OH production during the oxygenation of reduced nontronite NAu-2 (rNAu-2), montmorillonite, and sediment. Results showed that HA could enhance •OH production, and the enhancement was related to the content of reactive Fe(II) in rNAu-2 and the electron-accepting capacity of HA. Coexisting HA leads to a new electron-transfer pathway from Fe(II) in rNAu-2 to HA (instead of the HA-Fe complex) and then to O2, changing the first step of O2 reduction from one- to two-electron transfer process with H2O2 as the main intermediate. Reduced HA decomposes H2O2 to •OH at a higher yield (13.8%) than rNAu-2 (8.8%). Modeling results reveal that the HA-mediated electron-transfer pathway contributes to 12.6-70.2% of H2O2 generation and 13.2-62.1% of •OH formation from H2O2 decomposition, with larger contributions at higher HA concentrations (5-100 mg C/L). Our findings implicate that HA-mediated electron transfer can expand the area of •OH production from the mineral surface to the aqueous phase and increase the yield of •OH production.

Redox transformation of structural iron in nontronite induced by quinones under anoxic conditions.

In natural anoxic subsurface environments, the geochemical cycles of iron are largely associated with the migration and transformation of organic matter. Intensive attention has been paid to the redox interaction of organic matter with aqueous Fe and iron (hydr)oxides. Whereas, the abiotic redox cycling of structural Fe in clay minerals induced by quinones has not been well understood. In this study, we selected nontronite (NAu-2) as a model Fe-bearing phyllosilicate clay mineral and 1,4-hydroquinone (H2Q)/1,4-benquinone (BQ) as a model quinone couple. Our results show that the structural Fe(III) in NAu-2, with tetrahedral Fe(III) priority, can oxidize H2Q into BQ, and octahedral Fe(II) in NAu-2 can reduce BQ to H2Q, with semiquinone radicals (SQ-) as intermediate. The extent of the redox reactions depends on the reduction potential difference between NAu-2 and H2Q/BQ. However, a fraction of Fe(II)-Fe(III)-OH and Fe(II)-Fe(II)-OH groups in the octahedral sheet are difficult to be oxidized by BQ, because the reduction potential gradient decreases to a low level as the reaction proceeds. And the structure of NAu-2 can only partially restored upon re-oxidation with tetrahedral Fe(III) irreversibility. Output of this study replenishes the understanding regarding redox cycling of structural Fe in clay minerals induced by quinones.