Direct and Indirect Electron Transfer Routes of Chromium(VI) Reduction with Different Crystalline Ferric Oxyhydroxides in the Presence of Pyrogenic Carbon

Effect of straw decomposition on hexavalent chromium removal by straw: Significant roles of surface potential and dissolved organic matter

Mobility and bioavailability of hexavalent chromium (Cr(VI)) in agricultural soils are affected by interactions between Cr(VI) and returned crop straws. However, the effect of straw decomposition on Cr(VI) removal and underlying mechanisms remain unclear. In this study, Cr(VI) removal by pristine and decomposed rice/rape straws was investigated by batch experiments and a series of spectroscopies. The results showed that straw decomposition inhibited Cr(VI) removal, regardless of straw types. However, the potential mechanisms of the inhibition were distinct for the two straws. For the rice straw, a lower zeta potential after decomposition suppressed Cr(VI) sorption and subsequent reduction. In addition, less Cr(VI) was reduced by the decomposed rice straw-derived dissolved organic matter (DOM) than the pristine one. In contrast, for the rape straw, due to the increased zeta potential after decomposition, the decreased Cr(VI) removal was mainly ascribed to less Cr(VI) reduction by the rape straw-derived DOM. These results emphasized the significant roles of straw surface potential and DOM in Cr(VI) removal, depending on straw types and decomposition, which facilitate the fundamental understanding of Cr(VI) removal by straws and are helpful for predicting the environmental risk of Cr and rational straw return in Cr(VI)-contaminated fields.

Immobilization or mobilization of heavy metal(loid)s in lake sediment-water interface: Roles of coupled transformation between iron (oxyhydr)oxides and natural organic matter

Iron (Fe) (oxyhydr)oxides and natural organic matter (NOM) are active substances ubiquitously found in sediments. Their coupled transformation plays a crucial role in the fate and release risk of heavy metal(loid)s (HMs) in lake sediments. Therefore, it is essential to systematically obtain relevant knowledge to elucidate their potential mechanism, and whether HMs provide immobilization or mobilization effect in this ternary system. In this review, we summarized (1) the bidirectional effect between Fe (oxyhydr)oxides and NOM, including preservation, decomposition, electron transfer, adsorption, reactive oxygen species production, and crystal transformation; (2) the potential roles of coupled transformation between Fe and NOM in the environmental behavior of HMs from kinetic and thermodynamic processes; (3) the primary factors affecting the remediation of sediments HMs; (4) the challenges and future development of sediment HM control based on the coupled effect between Fe and NOM from theoretical and practical perspectives. Overall, this review focused on the biogeochemical coupling cycle of Fe, NOM, and HMs, with the goal of providing guidance for HMs contamination and risk control in lake sediment.

Three-dimensionally structured MoS2@biochar breaks through the bottleneck in antibiotic wastewater treatment: Greater efficiency and self-motivated oxidation pathway

Two-dimensional (2D) MoS2 has been widely used to remove antibiotics. However, low selectivity for antibiotic pollutants, dependence on applied energy and oxidant, and secondary contamination are still the bottlenecks of this system for treating antibiotic wastewater. In this study, we proposed a three-dimensional (3D) material (3MoS2/BMBC@MF) based on MoS2 and biochar with melamine sponge as the backbone. Compared with the 2D material (MoS2/BMBC), 3MoS2/BMBC@MF performed significantly better in enrofloxacin (ENR) removal, with an increase in the removal degree from 60.8 % to 88.1 %, and acted mainly through the degradation pathway rather than relying solely on the adsorption effect. It was shown that the direct oxidation process (DOP) behind the 3D materials is the key to the self-activated oxidation pathway. The three-dimensional structure enhances the generation and transfer pathways of persistent free radicals (PFRs) and electrons, realizing a multi-dimensional activation mechanism through its unique three-dimensional network, which greatly improves the redox capacity of the material. Upon exposure to pollutants, 3MoS2/BMBC@MF generates carbon-centered radicals of PFRs, which degrade ENR through mediated electron transfer. Coupled with the three-dimensional structure that contributes to the homogeneous dispersion of the active substances, dense steric active centers are formed in the grid skeleton by redox cycling of Mo ions to degrade antibiotics via DOP. Meanwhile, 3MoS2/BMBC@MF possesses good recyclability and maintains high efficiency in recycling. The structural design of this material not only enhances the removal efficiency and reduces the environmental impact, but also provides new potentials and solutions for practical water treatment of antibiotic contaminants.

Cysteine-Facilitated Cr(VI) reduction by Fe(II/III)-bearing phyllosilicates: Enhancement from In-Situ Fe(II) generation

Structural Fe in phyllosilicates represents a crucial and potentially renewable reservoir of electron equivalents for contaminants reduction in aquatic and soil systems. However, it remains unclear how in-situ modification of Fe redox states within Fe-bearing phyllosilicates, induced by electron shuttles such as naturally occurring organics, influences the fate of contaminants. Herein, this study investigated the processes and mechanism of Cr(VI) reduction on two typical Fe(II/III)-bearing phyllosilicates, biotite and chlorite, in the presence of cysteine (Cys) at circumneutral pH. The experimental results demonstrated that Cys markedly enhanced the rate and extent of Cr(VI) reduction by biotite/chlorite, likely because of the formation of Cr(V)-organic complexes and consequent electron transfer from Cys to Cr(V). The concomitant production of non-structural Fe(II) (including aqueous Fe(II), surface bound Fe(II), and Cys-Fe(II) complex) cascaded transferring electrons from Cys to surface Fe(III), which further contributed to Cr(VI) reduction. Notably, structural Fe(II) in phyllosilicates also facilitated Cr(VI) reduction by mediating electron transfer from Cys to structural Fe(III) and then to edge-sorbed Cr(VI). 57Fe Mössbauer analysis revealed that cis-coordinated Fe(II) in biotite and chlorite exhibits higher reductivity compared to trans-coordinated Fe(II). The Cr end-products were insoluble Cr(III)-organic complex and sub-nanometer Cr2O3/Cr(OH)3, associated with residual minerals as micro-aggregates. These findings highlight the significance of in-situ produced Fe(II) from Fe(II/III)-bearing phyllosilicates in the cycling of redox-sensitive contaminants in the environment.

Critical role of soil-applied molybdenum dioxide composite biochar material in enhancing Cr(VI) remediation process: The driver of Fe(III)/Fe(II) redox cycle

Heavy metal contamination of agricultural land due to sewage irrigation, over-application of fertilizers and pesticides, and industrial activities. Biochar, due to its rich functional groups and excellent electrochemical performance, is used for the remediation of heavy metal-contaminated farmland. However, the remediation mechanism remains uncertain due to the influence of minerals and multi-element composite pollution on soil. Therefore, introducing transition metal oxide MoO2 to prepare biochar composite remediation materials enhances the adsorption and reduction of soil Cr (Ⅵ). This study compared the differences in Cr (Ⅵ) improvement under different pollution systems and pH conditions and explored the potential mechanism of Fe (Ⅲ)/Fe (Ⅱ) redox cycling in Cr (Ⅵ) remediation. The results showed that both biochar MoO2 ball-milling composite (BC + M) and biochar-loaded MoO2 (BC/M) retained the original biochar (BC) remediation method for Cr (Ⅵ). Among them, the remediation of BC/M was the most stable, with the maximum remediation value ranging from approximately 6.52 to 58.58 mg/kg. In different pollution systems, Cd and Pb exhibited competitive adsorption toward Cr (Ⅵ), but they enhanced Cr (Ⅵ) remediation by promoting adsorption and self-complexation. In acidic conditions (pH = 4), BC/M showed the best remediation effect, with a reduction kinetic constant of 34.61 × 10-3 S-1 and a maximum adsorption capacity of 61.64 mg/g. Fe (Ⅲ)/Fe (Ⅱ) redox cycling accelerated the reduction of Cr (Ⅵ) (R2 = 0.81), and MoO2 promoted the Fe (Ⅲ)/Fe (Ⅱ) redox cycle. BC/M enhanced the Fe (Ⅱ) formation efficiency by 66.39% and 71.81% compared to BC + M and BC at pH = 4. The introduction of MoO2 and biochar composite materials enhanced the reduction process of Cr (Ⅵ), with BC/M achieving the optimal remediation level. This study reveals the potential mechanisms of MoO2 and biochar composite materials in soil Cr (Ⅵ) remediation, providing a reference and insight for the preparation of Cr (Ⅵ) remediation materials and the treatment of contaminated farmland.

Time-varying contributions of FeII and FeIII to AsV immobilization under anoxic/oxic conditions: The impacts of biochar and dissolved organic carbon

Engineering black carbon (e.g. biochar) has been widely found in natural environments due to natural processes and extensive applications in engineering systems, and could influence the geochemical processes of coexisting arsenic (AsV) and FeII, especially when they are exposed to oxic conditions. Here, we studied time-varying kinetics and efficiencies of AsV immobilization by solid-phase FeII (FeIIsolid) and FeIII (FeIIIsolid) in FeII-AsV-biochar systems under both anoxic and oxic conditions at pH 7.0, with focuses on the effects of biochar surface and biochar-derived dissolved organic carbon (DOC). Under anoxic conditions, FeII could rapidly immobilize AsV via co-adsorption onto biochar surfaces, which also serves as the dominant pathway of AsV immobilization at the initial stage of reaction (0-5 min) under oxic conditions at high biochar concentrations. Subsequently, with increasing biochar concentrations, FeIIIsolid precipitation from aqueous FeII (FeIIaq) oxidation (5-60 min) starts to play an important role in AsV immobilization but in decreased efficiencies of AsV immobilization per unit iron. In the following stage (60-300 min), FeIIsolid oxidation is suppressed and leads to AsV release into solutions at >1.0 g·L-1 biochar. The decreasing efficiency of AsV immobilization over time is attributed to the gradual release of DOC into solution from biochar particles, which significantly inhibit AsV immobilization when FeIIIsolid is generated from FeIIsolid oxidation in the vicinity of biochar surfaces. Specifically, 4.06 mg·L of biochar-derived DOC can completely inhibit the immobilization of AsV in the 100 μM FeII system under oxic conditions. The findings are crucial to comprehensively understand and predict the behavior of FeII and AsV with coexisting engineering black carbon in natural environments.

Chromium(VI) Adsorption and Reduction in Soils under Anoxic Conditions: The Relative Roles of Iron (oxyhr)oxides, Iron(II), Organic Matters, and Microbes

Chromium (Cr) transformation in soils mediated by iron (Fe) (oxyhr)oxides, Fe(II), organic matter (OM), and microbes is largely unexplored. Here, their coupling processes and mechanisms were investigated during anoxic incubation experiments of four Cr(VI) spiked soil samples with distinct physicochemical properties from the tropical and subtropical regions of China. It demonstrates that easily oxidizable organic carbon (EOC, 55-84%) and microbes (16-48%) drive Cr(VI) reduction in soils enriched with goethite and/or hematite, among which in dryland soils microbial sulfate reduction may also be involved. In contrast, EOC (38 ± 1%), microbes (33 ± 1%), and exchangeable and poorly crystalline Fe (oxyhr)oxide-associated Fe(II) (29 ± 3%) contribute to Cr(VI) reduction in paddy soils enriched with ferrihydrite. Additionally, exogenous Fe(II) and microbes significantly enhance Cr(VI) reduction in ferrihydrite- and goethite-rich soils, and Fe(II) greatly promotes but microbes slightly inhibit Cr passivation. Both Fe(II) and microbes, especially the latter, promote OM mineralization and result in the most substantial OM loss in ferrihydrite-rich paddy soils. During the incubation, part of the ferrihydrite converts to goethite but microbes may hinder the transformation. These results provide deep insights into the geochemical fates of redox-sensitive heavy metals mediated by the complicated effects of Fe, OM, and microbes in natural and engineered environments.

Effect of biochar-derived DOM on contrasting redistribution of chromate during Schwertmannite dissolution and recrystallization

Biochar-derived dissolved organic matter (BDOM), is extensively involved in the recrystallization of minerals and the speciation alteration of associated toxic metals. This study investigates how BDOM extracted from tobacco petiole (TP) or tobacco stalk (TS) biochar influences the speciation repartitioning of Cr(VI) in environments impacted by acid mine drainage (AMD), focusing on interactions with secondary minerals during Schwertmannite (Sch) dissolution and recrystallization. TP-BDOM, rich in lignin-like substances, slowed down the Cr-Sch dissolution and Cr release under acidic conditions compared to TS-BDOM. TP-BDOM's higher O/C component exerts a delayed impact on Cr-Sch stability and Cr(VI) reduction. In-situ ATR-FTIR and 2D-COS analysis showed that carboxylic and aromatic N-OH groups in BDOM could interact with Cr-Sch surfaces, affecting sulfate and Cr(VI) release. It was also observed that slight recrystallization occurred from Cr-Sch to goethite, along with increased Cr incorporation into secondary minerals within TS-BDOM. This enhances our understanding of BDOM's role in Cr(VI) speciation changes in AMD-contaminated sites.

Fe Crystalline Phases in Fe/C Composites Modulated the Selective Adsorption of Pb(II) from Industrial Wastewater with Cd(II): An Electronic-Scale Perspective

Fe oxide or Fe0-based materials display weak removal capacity for Pb(II), especially in the presence of Cd(II), and the electronic-scale mechanisms are not reported. In this study, Fe3C(220) modified black carbon (BC) [Fe3C(220)@BC] with high adsorption and selectivity for Pb(II) from industrial wastewater with Cd(II) was developed. The quantitative experiment suggested that Fe species accounted for 80.5-100 and 18.4-33.8% of Pb(II) and Cd(II) removal, respectively. Based on X-ray absorption near-edge structure analysis, 57.3% of adsorbed Pb2+ was reduced to Pb0; however, 61.6% of Cd2+ existed on Fe3C@BC. Density functional theory simulation unraveled that Cd(II) adsorption was attributed to the cation-π interaction with BC, whereas that of Pb(II) was ascribed to the stronger interactions with different Fe phases following the order: Fe3C(220) > Fe0(110) > Fe3O4(311). Crystal orbital bond index and Hamilton population analyses were innovatively applied in the adsorption system and displayed a unique discovery: the stronger Pb(II) adsorption on Fe phases was mediated by a combination of covalent and ionic bonding, whereas ionic bonding was mainly accounted for Cd(II) adsorption. These findings open a new chapter in understanding the functions of different Fe phases in mediating the fate and transport of heavy metals in both natural and engineered systems.

Marine microplastics enhance release of arsenic in coastal aquifer during seawater intrusion process

Microplastics (MPs), omnipresent contaminants in the ocean, could be carried by seawater intrusion into coastal aquifers, which might affect the fate of heavy metals existing in aquifers. Herein, we investigated the release behavior of arsenic (As) in coastal aquifers during MPs-containing seawater intrusion by applying laboratory experiment and numerical simulation. We found that seawater with marine MPs enhanced the release of As in aquifers, especially for dissolved As(V) and colloidal As. Negatively charged MPs competed with As(V) for the adsorption sites on iron (hydr)oxides in aquifers, resulting in the desorption of As(V). In addition, MPs could promote the release of Fe-rich colloids by imparting negative charge to its surface and providing it with sufficient repulsive force to detach from the matrix, thereby leading to the release of As associated with Fe-rich colloid. We also developed a modeling approach that well described the transport of As in coastal aquifer under the impact of MPs, which coupled variable density flow and kinetically controlled colloids transport with multicomponent reactive transport model. Our findings elucidated the enhancement of MPs on the release of As in aquifers during seawater intrusion, which provides new insights into the risk assessment of MPs in coastal zones.

Insights into the Crystallinity-Dependent Photochemical Productions of Reactive Oxygen Species from Iron Minerals

Iron minerals are widespread in earth's surface water and soil. Recent studies have revealed that under sunlight irradiation, iron minerals are photoactive on producing reactive oxygen species (ROS), a group of key species in regulating elemental cycling, microbe inactivation, and pollutant degradation. In nature, iron minerals exhibit varying crystallinity under different hydrogeological conditions. While crystallinity is a known key parameter determining the overall activity of iron minerals, the impact of iron mineral crystallinity on photochemical ROS production remains unknown. Here, we assessed the photochemical ROS production from ferrihydrites with different degrees of crystallinity. All examined ferrihydrites demonstrated photoactivity under irradiation, resulting in the generation of hydrogen peroxide (H2O2) and hydroxyl radical (•OH). The photochemical ROS production from ferrihydrites increased with decreasing ferrihydrite crystallinity. The crystallinity-dependent photochemical •OH production was primarily attributed to conduction band reduction reactions, with the reduction of O2 by conduction band electrons being the rate-limiting key process. Conversely, the crystallinity of iron minerals had a negligible influence on photon-to-electron conversion efficiency or surface Fenton-like activity. The difference in ROS productions led to a discrepant degradation efficiency of organic pollutants on iron mineral surfaces. Our study provides valuable insights into the crystallinity-dependent ROS productions from iron minerals in natural systems, emphasizing the significance of iron mineral photochemistry in natural sites with abundant lower-crystallinity iron minerals such as wetland water and surface soils.

Crucial roles of soil inherent Fe-bearing minerals in enhanced Cr(VI) reduction by biochar: The electronegativity neutralization and electron transfer mediation

Biochar has been used for soil Cr(VI) remediation in the last decade due to its enriched redox functional groups and good electrochemical properties. However, the role of soil inherent Fe-bearing minerals during the reduction of Cr(VI) has been largely overlooked. In this study, biochar with different electron-donating capacities (EDCs) was produced at 400 °C (BC400) and 700 °C (BC700), and their performance for Cr(VI) reduction in soils with varied properties (e.g., Fe content) was investigated. The addition of BC400 caused around 14.2-36.0 mg g-1 Cr(VI) reduction after two weeks of incubation in red soil, paddy soil, loess soil, and fluvo-aquic soil, while a less Cr(VI) was reduced by BC700 (2.57-16.7 mg g-1) with smaller EDCs. The Cr(VI) reduction by both biochars in different soils was closely related to Fe content (R2 = 0.93-0.98), so red soil with the richest Fe (14.8% > 1.79-3.49%) showed the best reduction capability, and the removal of soil free Fe oxides (e.g., hematite) resulted in 71.9% decrease of Cr(VI) reduction by BC400. On one hand, Fe-bearing minerals could increase the soil acidity, neutralize the surface negative charge of biochar, enhance the contact between Cr(VI) and biochar, and thus facilitate the direct Cr(VI) reduction by biochar in soils. On the other hand, Fe-bearing minerals could also facilitate the indirect Cr(VI) reduction by mediating the electron from biochar to Cr(VI) with the cyclic transformation of Fe(II)/Fe(III). This study demonstrates the key role of soil Fe-bearing minerals in Cr(VI) reduction by biochar, which advances our understanding on the biochar-based remediation mechanism of Cr(VI)-contaminated soils.

Efficacy and mechanism of enhanced Sb(V) removal from textile wastewater using ferric flocs in aerobic biological treatment

Antimony contamination from textile industries has been a global environmental concern and the existing treatment technologies could not reduce Sb(V) to meet the discharge standards. To overcome this shortcoming, ferric flocs were introduced to expedite the biological process for enhanced Sb(V) removal in wastewater treatment plant (WWTP). For this purpose, a series of laboratorial-scale sequential batch reactor activated sludge processes (SBRs) were applied for Sb(V) removal with varied reactor conditions and the transformation of Fe and Sb in SBR system was investigated. Results showed a significant improvement in Sb(V) removal and the 20 mg L-1 d-1 iron ions dosage and iron loss rate was found to be only 15.2%. The influent Sb(V) concentration ranging 153-612 μg L-1 was reduced to below 50 μg L-1, and the maximum Sb(V) removal rate of the enhanced system reached about 94.3%. Furthermore, it exhibited high stability of Sb(V) removal in the face of antimonate load, Fe strike and matrix change of wastewater. Sludge total Sb determination and capacity calculation revealed decreasing in Sb adsorption capacity and desorption without fresh Fe dosage. While sludge morphology analysis demonstrated the aging and crystallization of iron hydroxides. These results verify the distinct effects of fresh iron addition and iron aging on Sb(V) removal. High-throughput gene pyrosequencing results showed that the iron addition changed microbial mechanisms and effect Fe oxidized bacterial quantity, indicating Sb(V) immobilization achieved by microbial synergistic iron oxidation. The present study successfully established a simple and efficient method for Sb(V) removal during biological treatment, and the modification of biological process by iron supplement could provide insights for real textile wastewater treatment.

Dual role of pyrogenic carbon in mediating electron transfer from clay minerals to chromium in aqueous and solid media

Clay minerals (CMs) and pyrogenic carbons (PCs) often co-exist in the environment and participate in the redox cycling of pollutants. This study unveiled the dual role of PCs in CM-dominated chromium transformation in both aqueous and agar solidification media. The findings showed that CMs and PCs adsorbed minimal Cr(VI), while reduced CMs and PCs displayed a substantial difference by directly reducing Cr(VI) to solid/dissolved Cr(III) through reactive structural Fe(II) and functional groups, respectively. Moreover, dissolved PCs were found to mediate electron transfer from reduced CMs to Cr(VI) in aqueous and solid media. Interestingly, the effect of solid PCs on Cr(VI) reduction by reduced CMs was concentration-dependent. At lower concentrations, solid PCs dispersed reduced CMs, acting as electron mediators and facilitating both direct and indirect Cr(VI) reduction, resulting in solid Cr(III) rather than dissolved Cr(III). Conversely, at higher concentrations, solid PCs served as redox buffers, storing electrons transferred from reduced CMs to Cr(VI). In either case, the transformed chromium was primarily immobilized on the surface of CMs rather than PCs. These findings offer valuable insights into pollutant transformations associated with CMs and PCs, deepening our understanding of their geochemical processes.

Systematic evaluation of methods for iron-impregnation of biochar and effects on arsenic in flooded soils

There is a need for innovative strategies to decrease the mobility of metal(loids) including arsenic (As) and cadmium (Cd) in agricultural soils, including rice paddies, so as to minimize dietary exposure to these toxic elements. Iron (Fe)-modified biochars (FBCs) are used to immobilize As and Cd in soil-water systems, but there is a lack of clarity on optimal methods for preparing FBCs because there are only limited studies that directly compare BCs impregnated with Fe under different conditions. There is also a lack of information on the long-term performance of FBCs in flooded soil environments, where reductive dissolution of Fe (oxy)hydroxide phases loaded onto biochar surfaces may decrease the effectiveness of FBCs. This study uses material characterization methods including FTIR, SEM-EDX, BET, and adsorption isotherm experiments to investigate the effects of Fe-impregnation methods (pH, pyrolysis sequence, and sonication) on the morphology and mineralogy of Fe loaded onto the biochar surface, and to FBC adsorbent properties for arsenate (As(V)), arsenite (As(III)), and Cd. Acidic impregnation conditions favored the adsorption of As(III) onto amorphous Fe phases that were evenly distributed on the biochar surface, including within the biochar pore structure. The combination of sonication with acidic Fe-impregnation conditions led to the best adsorption capacities for As(V) and As(III) (4830 and 11,166 μg As g-1 biochar, respectively). Alkaline Fe-impregnation conditions led to the highest Cd adsorption capacity of 3054 μg Cd g-1 biochar, but had poor effectiveness as an As adsorbent. Amending soil with 5% (w/w) of an acid-impregnated and sonicated FBC was more effective than an alkaline-impregnated FBC or ferrihydrite in decreasing porewater As concentrations. The acid-impregnated FBC also had greater longevity, decreasing As by 54% and 56% in two flooded phases, probably due to the greater stability of Fe(III) within the biochar pore structure that may have a direct chemical bond to the biochar surface. This study demonstrates that FBCs can be designed with selectivity towards different As species or Cd and that they can maintain their effectiveness under anaerobic soil conditions. This is the first study to systematically test how impregnation conditions affect the stability of FBCs in soils under multiple drying-rewetting cycles.

Effect of coupled physical and chemical heterogeneity on the transport of pristine and aged pyrogenic carbon colloids in unsaturated porous media

Due to extensive application and recurrent wildfires, an increasing number of pyrogenic carbon (PyC) colloids are present in the environment, experiencing processes of environmental aging. Subsurface environments are typically heterogeneous in unsaturated conditions, which may affect the transport of PyC colloids. This study focused on the transport of both pristine and aged PyC colloids in physically (clean coarse and fine sand) and physicochemically (iron oxides-coated coarse and clean fine sand) heterogeneous porous media at three different water saturations (100 %, 70 %, and 40 %). In physically heterogeneous porous media, the decrease in water saturation from 100 % to 40 % led to a shift in the main water flow from the clean coarse sand to the clean fine sand domain, resulting in a continuous decrease in the transport of PyC colloids. In physicochemically heterogeneous porous media, the primary water flow shifted from the iron oxides-coated coarse sand to the clean fine sand domain, resulting in an initial increase and subsequent decrease in PyC colloid transport. Aging enhanced the transport of PyC colloids, attributed to the increasingly negative and hydrophilic surface. Retention profiles revealed substantial PyC colloid retention at the interface between coarse and fine sand domains. The release of retained PyC colloids exhibited two peaks at 100 % and 70 % water saturations, along with a single peak at 40 % water saturation. Additionally, the increased irreversible retention was observed at lower water saturation. This study underscores the significance of water content, environmental aging, and heterogeneity in PyC colloid transport. It provides essential insights into the environmental fate of PyC colloids in natural field conditions.

Characteristics of iron (hydr)oxides and Cr(VI) retention mechanisms in soils from tropical and subtropical areas of China

Though both iron (hydr)oxides and soil organic matter (SOM) significantly influence heavy metal behaviors in soils, studies on the characteristics of natural minerals and the synergic effects of the two on Cr(VI) transformation are limited. This study investigated Cr(VI) retention mechanisms in four soils from tropical and subtropical regions of China based on a comprehensive characterization of Fe (hydr)oxides. These soils exhibited varying quantities of hematite, ferrihydrite and goethite, with distinct Al substitution levels and varied exposed crystallographic facets. Adsorption experiments revealed a positive correlation between Fe (hydr)oxide content and Cr(VI) fixation amount on colloid, which was influenced by the mineral types, Al substitution levels and facet exposures. Further, Cr(VI) was sequestered on soil by adsorption and reduction. In soils enriched with crystalline Fe (hydr)oxides, Cr(VI) reduction was primarily governed by SOM, while in soils enriched with poorly crystalline Fe (hydr)oxides, mineral-associated Fe(II) also contributed to Cr(VI) reduction. Aging experiments demonstrated that SOM and mineral-associated Fe(II) expedited Cr (VI) passivation and diminished the Cr leaching. These results improve our understanding of natural Fe (hydr)oxide structures and their impact on Cr(VI) behavior in soils, and shed light on complex soil-contaminant interactions and remediation of Cr(VI) polluted soils.

Mineral-mediated stability of organic carbon in soil and relevant interaction mechanisms

Soil, the largest terrestrial carbon reservoir, is central to climate change and relevant feedback to environmental health. Minerals are the essential components that contribute to over 60% of soil carbon storage. However, how the interactions between minerals and organic carbon shape the carbon transformation and stability remains poorly understood. Herein, we critically review the primary interactions between organic carbon and soil minerals and the relevant mechanisms, including sorption, redox reaction, co-precipitation, dissolution, polymerization, and catalytic reaction. These interactions, highly complex with the combination of multiple processes, greatly affect the stability of organic carbon through the following processes: (1) formation or deconstruction of the mineral-organic carbon association; (2) oxidative transformation of the organic carbon with minerals; (3) catalytic polymerization of organic carbon with minerals; and (4) varying association stability of organic carbon according to the mineral transformation. Several pieces of evidence related to the carbon turnover and stability during the interaction with soil minerals in the real eco-environment are then demonstrated. We also highlight the current research gaps and outline research priorities, which may map future directions for a deeper mechanisms-based understanding of the soil carbon storage capacity considering its interactions with minerals.

Coactivation of Hydrogen Peroxide Using Pyrogenic Carbon and Magnetite for Sustainable Oxidation of Organic Pollutants

Pyrogenic carbon and magnetite (Fe3O4) were mixed together for the activation of hydrogen peroxide (H2O2), aiming to enhance the oxidation of refractory pollutants in a sustainable way. The experimental results indicated that the straw-derived carbon obtained by pyrolysis at 500-800 °C was efficient on coactivation of H2O2, and the most efficient one was that prepared at 700 °C (C700) featured with abundant defects. Specifically, the reaction rate constant (kobs) for removal of an antibiotic ciprofloxacin in the coactivation system (C700/Fe3O4/H2O2) is 12.5 times that in the magnetite-catalyzed system (Fe3O4/H2O2). The faster pollutant oxidation is attributed to the sustainable production of •OH in the coactivation process, in which the carbon facilitated decomposition of H2O2 and regeneration of Fe(II). Besides the enhanced H2O2 utilization in the coactivation process, the leaching of iron was controlled within the concentration limit in drinking water (0.3 mg·L-1) set by the World Health Organization.

Tracking of the conversion and transformation pathways of dissolved organic matter in sludge hydrothermal liquids during Cr(VI) reduction using FT-ICR MS

In this study, the remediation effects of two types of sludge (ferric-based flocculant and non-ferric-based flocculant) on Cr(VI)-polluted wastewater were evaluated to clarify the key components in sludge hydrothermal solutions responsible for reducing Cr(VI) and understand the underlying molecular-level transformation mechanisms. The results revealed that the primary reactions during the hydrothermal processes were deamination and decarboxylation reactions. Correlation analysis highlighted proteins, reducing sugars, amino groups, and phenolic hydroxyl groups as the major contributors. In-depth analysis of the transformation process of functional groups within dissolved organic matter (DOM) and synergistic redox process between Cr(VI) and DOM in hydrothermal solutions demonstrated that phenolic hydroxyl and amino groups gradually underwent oxidation during reduction of Cr(VI) by DOM, forming aldehyde and carboxyl groups, among the others. Time-dependent density functional theory calculations revealed notable shift of reducing functional groups from ground state to excited state following iron complexation, ultimately facilitating reduction reaction. Subsequent investigations, including soil column leaching and seed germination rate tests, indicated that synergistic redox interaction between Cr(VI) and DOM significantly reduced waterborne heavy metal and toxic organic pollution. These findings carry substantial implications for sludge treatment and remediation of heavy metal pollution in wastewater, offering valuable insights into effective environmental remediation strategies.

Effect of different production methods on physicochemical properties and adsorption capacities of biochar from sewage sludge and kitchen waste: Mechanism and correlation analysis

Different pyrolysis methods, parameters and feedstocks result in biochars with different properties, structures and removal capacities for heavy metals. However, the role of each property on adsorption capacity and corresponding causal relationships remain unclear. Here, we investigated various physicochemical properties of biochar produced via three different methods and two different feedstocks to clarify influences of biomass sources and pyrolysis processes on biochar properties and its heavy metal adsorption performance. Experimental results showed biochars were more aromatic and contained more functional groups after hydrothermal carbonization, while they had developed pores and higher surface areas produced by anaerobic pyrolysis. The inclusion of oxygen resulted in more complete carbonization and higher CEC biochar. Different biochar properties resulted in different adsorption capacities. Biochar produced by aerobic calcination showed higher adsorption efficiency for Cu and Pb. Correlation analysis proved that pH, cation exchange capacity and degree of carbonization positively affected adsorption, while organic matter content and aromaticity were unfavorable for adsorption. Microstructure and components determined biochar macroscopic properties and ultimate adsorption efficiency for metal ions. This study identifies the degree of correlation and pathways of each property on adsorption, which provides guidance for targeted modification of biochar to enhance its performance in heavy metal removal.

Metal toxin threat in wildland fires determined by geology and fire severity

Accentuated by climate change, catastrophic wildfires are a growing, distributed global public health risk from inhalation of smoke and dust. Underrecognized, however, are the health threats arising from fire-altered toxic metals natural to soils and plants. Here, we demonstrate that high temperatures during California wildfires catalyzed widespread transformation of chromium to its carcinogenic form in soil and ash, as hexavalent chromium, particularly in areas with metal-rich geologies (e.g., serpentinite). In wildfire ash, we observed dangerous levels (327-13,100 µg kg-1) of reactive hexavalent chromium in wind-dispersible particulates. Relatively dry post-fire weather contributed to the persistence of elevated hexavalent chromium in surficial soil layers for up to ten months post-fire. The geographic distribution of metal-rich soils and fire incidents illustrate the broad global threat of wildfire smoke- and dust-born metals to populations. Our findings provide new insights into why wildfire smoke exposure appears to be more hazardous to humans than pollution from other sources.

Novel heterogeneous Fenton catalysts for promoting carbon iron electron transfer by one-step hydrothermal synthesization

Carbon materials play a crucial role in promoting the Fe(III)/Fe(II) redox cycle in heterogeneous Fenton reactions. However, the electron transfer efficiency between carbon and iron is typically low. In this study, we prepared a novel heterogeneous Fenton catalyst, humboldtine/hydrothermal carbon (Hum/HTC), using a one-step hydrothermal method and achieved about 100 % reduction in Fe(III) during synthesis. Moreover, the HTC continuously provided electrons to promote Fe(II) regeneration during the Fenton reaction. Electron paramagnetic resonance (EPR) and quenching experiments showed that Hum/HTC completely oxidized As(III) to As(V) via free radical and non-free radical pathways. Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) and two-dimensional correlation spectroscopy (2D-COS) analyses revealed that monodentate mononuclear (MM) and bidentate binuclear (BB) structures were the dominant bonding methods for As(V) immobilization. 40 %Hum/HTC exhibited a maximum As(III) adsorption capacity of 167 mg/g, which was higher than that of most reported adsorbents. This study provides a novel strategy for the efficient reduction of Fe(III) during catalyst synthesis and demonstrates that HTC can continuously accelerate Fe(II) regeneration in heterogeneous Fenton reactions.

Vitamin B12 and iron-rich sludge-derived biochar enhanced PFOA biodegradation: Importance of direct inter-species electron transfer and functional microbes

Owing to the strong C-F bond in nature and the rigidity of the poly-fluoroalkyl chain, perfluorooctanoic acid (PFOA) is difficult to be eliminated by reactive species and microbes in environments, thus posing a serious threat to ecosystems. Vitamin B12 as a cofactor for enzymes, and biochar as the electron providers and conductors, were integrated to enhance PFOA biodegradation. The raw material of biochar was the sludge after dewatering by adding 50 mg/g DS of Fe(III). After pyrolysis under high temperature (800 °C), biochar (SC800) detected high content of Fe(II) (197.64 mg/g) and abundant oxygen-containing functional groups, thus boosting PFOA biodegradation via donating electrons. 99.9% of PFOA could be removed within 60 d as 0.1 g/L SC800 was presented in the microbial systems containing vitamin B12. Moreover, vitamin B12 facilitated the evolution of Sporomusa which behaved the deflorination. Via providing reactive sites and mediating direct inter-species electron transfer (DIET), SC800 boosted PFOA biodegradation. Corresponding novel results in the present study could guide the development of bioremediation technologies for PFOA-polluted sites.

Zero-valent iron-peroxydisulfate as synergistic co-milling agents for enhanced mechanochemical destruction of 2,4-dichlorophenol: Coupling reduction with oxidation

Mechanochemical (MC) remediation with zero-valent iron (ZVI) as co-milling agent enables the non-combustion and solvent-free disposal of solid halogenated organic pollutants (HOPs) via solid-phase reaction, but suffers from incomplete dechlorination (especially for less chlorinated chemicals). Herein, a reduction-oxidation coupling strategy using ZVI and peroxydisulfate as synergistic (ZVI-PDS) co-milling agents was investigated, with 2,4-dichlorophenol (2,4-DCP) as probe contaminant. By revisiting the MC destruction process of 2,4-DCP by ZVI, the contribution of both reductive and oxidative routes is confirmed, and the inefficient •OH generation is addressed. With ball-to-material and reagent-to-pollutant mass ratios of 30:1 and 13:1, respectively, ZVI-PDS achieves higher dechlorination ratio (86.8%) for 2,4-DCP within 5 h, outcompeting sole ZVI (40.3%) or PDS (33.9%), due to the accumulation of numerous SO4•-. As suggested by a two-compartment kinetic model, the optimal ZVI/PDS molar ratio of 4:1 is determined, which balances the relative contribution of reductive/oxidative routes and leads to a maximum mineralization efficiency of 77.4%. The analysis on product distribution verifies the generation of dechlorinated, ring-opening and minor coupling products (with low acute toxicity). This work validates the necessity to couple reduction with oxidation in MC destruction for solid HOPs, and may provide information on reagent formulation.

Sulfurization alters phenol-formaldehyde resin microplastics redox property and their efficiency in mediating arsenite oxidation

Microplastics weathering by various types of oxidants in the oxic environment and their interaction with environmental contaminants have drawn numerous scientific attention. However, the environmental fate of microplastics under a reducing environment has been largely unresolved. Herein, the change of physicochemical and redox properties of microplastics during the weathering under a sulfate-reducing environment and the interaction with arsenite were addressed. The sulfurization of phenol-formaldehyde resin microplastics under a sulfate-reducing environment generated smooth and porous particles with the induction of organic S species. Multiple spectroscopic results demonstrated thioether and thiophene groups formed by the substitute removal of O-containing functional groups. Moreover, the sulfurization process induced the reduction of carbonyl groups and oxidation of phenolic hydroxyl groups and resulted in the formation of semiquinone radicals. The O-containing functional groups contributed to microplastics redox property and As(III) oxidation while S-containing functional groups showed no obvious effect. The sulfurized microplastics had lower efficiency in mediating arsenite oxidation than the unsulfurized counterparts due to the decreased electron donating capacity. Producing hydrogen peroxides by electron-donating phenol groups and semiquinone radicals and the direct semiquinone radicals oxidation could mediate arsenite oxidation. The findings of this study help us understand the fate of microplastics in redox fluctuation interfaces.

Quantification of the redox properties of microplastics and their effect on arsenite oxidation

Microplastics have attracted global concern. The environmental-weathering processes control their fate, transport, transformation, and toxicity to wildlife and human health, but their impacts on biogeochemical redox processes remain largely unknown. Herein, multiple spectroscopic and electrochemical approaches in concert with wet-chemistry analyses were employed to characterize the redox properties of weathered microplastics. The spectroscopic results indicated that weathering of phenol-formaldehyde resins (PFs) by hydrogen peroxide (H2O2) led to a slight decrease in the content of phenol functional groups, accompanied by an increase in semiquinone radicals, quinone, and carboxylic groups. Electrochemical and wet-chemistry quantifications, coupled with microbial-chemical characterizations, demonstrated that the PFs exhibited appreciable electron-donating capacity (0.264-1.15 mmol e- g-1) and electron-accepting capacity (0.120-0.300 mmol e- g-1). Specifically, the phenol groups and semiquinone radicals were responsible for the electron-donating capacity, whereas the quinone groups dominated the electron-accepting capacity. The reversible redox peaks in the cyclic voltammograms and the enhanced electron-donating capacity after accepting electrons from microbial reduction demonstrated the reversibility of the electron-donating and -accepting reactions. More importantly, the electron-donating phenol groups and weathering-induced semiquinone radicals were found to mediate the production of H2O2 from oxygen for arsenite oxidation. In addition to the H2O2-weathered PFs, the ozone-aged PF and polystyrene were also found to have electron-donating and arsenite-oxidation capacity. This study reports important redox properties of microplastics and their effect in mediating contaminant transformation. These findings will help to better understand the fate, transformation, and biogeochemical roles of microplastics on element cycling and contaminant fate.

Distinct chromium removal mechanisms by iron-modified biochar under varying pH: Role of iron and chromium speciation

Iron-modified biochar (Fe-biochar) has been widely developed to attenuate Cr(VI) pollution in both acid and alkaline environments. However, there are few comprehensive studies on how the iron speciation in Fe-biochar and chromium speciation in solution influencing the removal of Cr(VI) and Cr(III) under varying pH. Here, multiple Fe-biochar containing Fe3O4 or Fe(0) were prepared and applied to remove aqueous Cr(VI). Kinetics and isotherms suggested that all Fe-biochar could efficiently remove Cr(VI) and Cr(III) via adsorption-reduction-adsorption. The Fe3O4-biochar immobilized Cr(III) by forming FeCr2O4, while amorphous Fe-Cr coprecipitate and Cr(OH)3 was formed when using Fe(0)-biochar. Density functional theory (DFT) analysis further indicated that pH increase caused more negative adsorption energies between Fe(0)-biochar and the pH-dependent Cr(VI)/Cr(III) species. Consequently, the adsorption and immobilization of Cr(VI) and Cr(III) species by Fe(0)-biochar was more favored at higher pH. In comparison, Fe3O4-biochar exhibited weaker adsorption abilities for Cr(VI) and Cr(III), which were in consistent with their less negative adsorption energies. Nonetheless, Fe(0)-biochar merely reduced ∼70% of adsorbed Cr(VI), while ∼90% of adsorbed Cr(VI) was reduced by Fe3O4-biochar. These results unveiled the importance of iron and chromium speciation for chromium removal under varying pH, and might guide the application-oriented design of multifunctional Fe-biochar for broad environmental remediation.

Remediation of Cu, Cr(VI) and Pb polluted soil with industrial/agricultural by-products in seasonally frozen areas

Soils contaminated with potentially toxic elements (PTEs) may face serious environmental problems and pose health risks. In this study, the potential feasibility of industrial and agricultural by-products as low-cost green stabilization materials for copper (Cu), chromium (Cr(VI)) and lead (Pb) polluted soil was investigated. The new green compound material SS ∼ BM ∼ PRP was prepared by ball milling with steel slag (SS), bone meal (BM), and phosphate rock powder (PRP) which had an excellent stabilization effect on contaminated soil. Under 20% SS ∼ BM ∼ PRP addition into the soil, the toxicity characteristic leaching concentrations of Cu, Cr(VI) and Pb were reduced by 87.5%, 80.9% and 99.8%, respectively, and the phytoavailability and bioaccessibility of PTEs were reduced by more than 55% and 23%. The freezing-thawing cycle significantly increased the activity of heavy metals, and the particle size became smaller due to the fragmentation of the soil aggregates while SS ∼ BM ∼ PRP could form calcium silicate hydrate by hydrolysis to cement the soil particles, which inhibited the release of PTEs. Different characterizations indicated that the stabilization mechanisms mainly involved ion exchange, precipitation, adsorption and redox reaction. Overall, the results obtained suggest that the SS ∼ BM ∼ PRP is a green, efficient and durable material for remediation of various heavy metal polluted soils in cold regions and a potential method for co-processing and reusing industrial and agricultural wastes.

Reductive Sequestration of Cr(VI) and Immobilization of C during the Microbially Mediated Transformation of Ferrihydrite-Cr(VI)-Fulvic Acid Coprecipitates

Cr(VI) detoxification and organic matter (OM) stabilization are usually influenced by the biological transformation of iron (Fe) minerals; however, the underlying mechanisms of metal-reducing bacteria on the coupled kinetics of Fe minerals, Cr, and OM remain unclear. Here, the reductive sequestration of Cr(VI) and immobilization of fulvic acid (FA) during the microbially mediated phase transformation of ferrihydrite with varying Cr/Fe ratios were investigated. No phase transformation occurred until Cr(VI) was completely reduced, and the ferrihydrite transformation rate decreased as the Cr/Fe ratio increased. Microscopic analysis was uncovered, which revealed that the resulting Cr(III) was incorporated into the lattice structure of magnetite and goethite, whereas OM was mainly adsorbed on goethite and magnetite surfaces and located within pore spaces. Fine line scan profiles showed that OM adsorbed on the Fe mineral surface had a lower oxidation state than that within nanopores, and C adsorbed on the magnetite surface had the highest oxidation state. During reductive transformation, the immobilization of FA by Fe minerals was predominantly via surface complexation, and OM with highly aromatic and unsaturated structures and low H/C ratios was easily adsorbed by Fe minerals or decomposed by bacteria, whereas Cr/Fe ratios had little effect on the binding of Fe minerals and OM and the variations in OM components. Owing to the inhibition of crystalline Fe minerals and nanopore formation in the presence of Cr, Cr sequestration and C immobilization can be synchronously favored at low Cr/Fe ratios. These findings provide a profound theoretical basis for Cr detoxification and synchronous sequestration of Cr and C in anoxic soils and sediments.

Electron donation of Fe-Mn biochar for chromium(VI) immobilization: Key roles of embedded zero-valent iron clusters within iron-manganese oxide

The dense surface passivation layer on zero-valent iron (ZVI) restricts its efficiency for water decontamination, causing a poor economy and waste of resources. Herein, we found that the ZVI on Fe-Mn biochar could afford a high electron-donating efficiency for the Cr(VI) reduction and immobilization. Over 78.0% of Fe in the Fe-Mn biochar was used for the Cr(VI) reduction and immobilization, i.e., 56.2 - 161.7 times higher than the commercial ZVI (0.5%) and modified ZVI (0.9 -1.3%), indicating that the unique ZVI species in Fe-Mn biochar offered an outstanding Fe utilization efficiency. We proposed that oxygen atoms in the FeO in the FeMnO₂ precursor were removed during pyrolysis with biochar while the MnO skeleton was preserved, forming the embedded ZVI clusters within Fe-Mn oxide. The unique structure inhibited the formation of the Fe-Cr complex on Fe(0), which would facilitate the electron transfer between core Fe(0) and Cr(VI). Moreover, the surface FeMnO₂ inhibited the diffusion of Fe and facilitated its affinity with pollutants, thus supporting higher efficiency for pollutant immobilization. The preserved performance of Fe-Mn biochar was proved in industrial wastewater and after long-term oxidation process, and the economic benefit was evaluated. This work provides a new approach for developing active ZVI-based materials with high Fe utilization efficiency and economics for water pollution control.

Biochar-derived dissolved black carbon accelerates ferrihydrite microbial transformation and subsequent imidacloprid degradation

The effects of an electron shuttle (dissolved black carbon (DBC) derived from biochar) on the microbial reduction of ferrihydrite and subsequent imidacloprid (IMI) degradation were studied. The results showed that DBC addition enhanced the microbial reduction of Fe(III) in ferrihydrite and increased the quantity of Fe(II) released into the liquid phase. The electron transfer capacity of DBC was significantly influenced by the content of redox-active oxygen-containing functional groups (e.g., quinone, hydroquinone, and polyphenol groups), which was dependent on the pyrolysis temperature. The electrochemical characteristics of DBC resulted in enhanced electron transfer, which promoted Fe(III) reduction and mediated the microbial transformation of ferrihydrite. The microbial transformation of ferrihydrite resulted in the formation of secondary minerals such as siderite and vivianite. The IMI degradation efficiency was related to the Fe(III) reduction rate and the pyrolysis temperature used in DBC production, and the degradation pathways were nitrate reduction and imino hydrolysis induced by the Fe(II) generated from the reduction of Fe(III) in ferrihydrite. The results obtained in this study provide new data for understanding the multifunctional roles of biochar-derived DBC in the redox and transformation processes of iron minerals induced by iron-reducing bacteria, the related biogeochemical cycles of iron and the fate of pollutants.

Interactions between Cr(VI) and the hydrochar: The electron transfer routes, adsorption mechanisms, and the accelerating effects of wood vinegar

Conversion of the low-valued invasive plant biomass into high-grade carbonaceous materials may provide a novel strategy to tackle the global issues of climate changes and exotic plant invasion. In this study, the hydrochar was fabricated from the biomass of Eupatorium adenophorum spreng. via hydrothermal carbonization (HTC) process to remove Cr(VI). The adsorption thermodynamics and kinetics were investigated via batch experiments, and the electron transfer routes and adsorption mechanisms were further revealed based on systematic characterization. The adsorption isotherms were well fitted by the Langmuir model with a maximum adsorption amount of 7.76 mg/g. The adsorption was spontaneous, and the surface adsorption and intraparticle diffusion may be the speed-limiting steps. Both -OH group and furan structures may donate the electrons to reduce Cr(VI), and the adsorption was governed by the surface complexation with the oxygen-containing functional groups including hydroxyl and carboxyl. Furthermore, the wood vinegar, as the by-product, can significantly accelerate the reduction rate of Cr(VI). Thus, this study provided a new strategy to fabricate carbonaceous materials which may facilitate to boost the carbon neutrality and control of invasive plants.

Effect of manganese oxides on arsenic speciation and mobilization in different arsenic-adsorbed iron-minerals under microbially-reducing conditions

The oxidation and immobilization of arsenic (As) by manganese oxides have been shown to reduce As toxicity and bioavailability under abiotic conditions. In this study, we investigate the impact of manganese oxide (δ-MnO₂) on the fate of different Fe-minerals-adsorbed As in the presence of As(V)-reducing bacteria Bacillus sp. JQ. Results showed that in the absence of δ-MnO₂, As release in goethite was much higher than in ferrihydrite and hematite during microbial reduction. Adding 3.1 mM Mn reduced As release by 0.3%, 46.3%, and 6.7% in the ferrihydrite, goethite, and hematite groups, respectively. However, aqueous As was dominated by As(III) in the end, because the oxidation effect of δ-MnO₂ was limited and short-lived. Additionally, the fraction of solid-phase As(V) increased by 9.8% in ferrihydrite, 39.4% in goethite, and 7.4% in hematite in the high-Mn treatments, indicating that δ-MnO₂ had the most significant oxidation and immobilization effect on goethite-adsorbed As. This was achieved because goethite particles were evenly distributed on δ-MnO₂ surface, which supported As(III) oxidation by δ-MnO₂; while ferrihydrite strongly aggregated, which hindered the oxidation of As(III). Our study shows that As-oxidation and immobilization by manganese oxides cannot easily be assessed without considering the mineral composition and microbial conditions of soils.

Roles of soil active constituents in the degradation of sulfamethoxazole by biochar/persulfate: Contrasting effects of iron minerals and organic matter

The biochar/persulfate (BC/PS) has been extensively applied in the degradation of organic contaminants in the aqueous solutions. However, much less work has been done on the degradation of organic contaminants in soil by BC/PS, especially on the unclear roles of soil active constituents in the degradation. This study was conducted to investigate the degradation of sulfamethoxazole (SMX) in two soils through PS oxidation activated by biochar. Biochar was produced via the pyrolysis of peanut shell at 400 °C and 700 °C, which was denoted as BC400 and BC700, respectively. Two soils used were red soil and paddy soil, mainly differing in iron minerals and organic matter. Both biochar promoted SMX degradation (42.6 %-90.7 %) in two soils, compared to PS alone (20.9 %-41.7 %). In BC400/PS system, the free radicals were the dominant reactive species for SMX degradation, while the electron transfer pathway played a vital role in the SMX degradation by BC700/PS. Higher SMX degradation was observed in red soil (41.7 %-97.8 %) than that in paddy soil (20.3 %-94.8 %), which was ascribed to the promotion of iron minerals in red soil yet the inhibition of organic matter in paddy soil. Specifically, the reaction between ≡Fe(III)/≡Fe(II) and PS on the surface of iron minerals in red soil generated more SO₄^•- and ^•OH, resulting in the enhanced SMX degradation. However, the consumption of free radicals and suppression of electron transfer pathway by organic matter in paddy soil inhibited SMX degradation. As the comparative carbonaceous materials to biochar, graphite exerted no obvious degradation effect, whereas activated carbon exhibited the comparable promoting efficacy to BC700. Both biochar, especially BC700, significantly (p < 0.05) alleviated the adverse effects of PS treatment on wheat (Triticum aestivum L.) growth. Overall, this study demonstrates that biochar/persulfate was effective in SMX degradation in soil and the degradation was affected by soil iron minerals and organic matter, which should be paid more attention in the persulfate remediation of organic contaminated soils at a specific site.

Biochar colloids facilitate transport and transformation of Cr(VI) in soil: Active site competition coupling with reduction reaction

Biochar has been demonstrated as an efficient amendment for immobilizing contaminants. However, a certain number of micro/nano-scale particles are inevitably present in the fresh or aged biochar, which may facilitate the downward transport of contaminants along the soil profile, posing a detrimental impact on the groundwater. Herein, the effects of biochar colloids derived from wood chip and wheat straw at two temperatures (350 °C and 500 °C) on the transport and transformation of Cr(VI) in soil were investigated. All biochar colloids facilitated the transport of Cr(VI) in a loam clay Ultisol, which was attributed to the competition between biochar colloids and Cr(VI) for the available sorption sites on the soil surface. Wheat straw biochar colloids caused more transport of Cr(VI) than wood chip ones due to the more negative charge and higher polarity, which resulted in stronger electrostatic repulsion and competition with Cr(VI). It is soluble Cr(VI) that dominated the transport of Cr in the effluent solution, however, the particulate Cr(VI) could be reduced into Cr(III) before being carried by biochar colloids for co-transport. The 350 °C biochar colloids had higher electron donating capacities than 500 °C ones, resulting in more reduction of Cr(VI) and more co-transport as biochar colloids-associated Cr(III) in the effluent. Moreover, the more negatively charged 350 °C biochar colloids could also attach more soil Fe oxides, further facilitating the cotransport of Cr via the formation of a binary or ternary complex. Modeling showed the experimental-consistently results that biochar colloids caused 0.5-7.0 times faster transport of Cr(VI) than no biochar colloids in the long-term period. Our findings demonstrate that biochar colloids can enhance transport and transformation of Cr(VI) in soils, which arouse migration risk concern about in-situ remediation of Cr(VI)-contaminated soils by biochar.

Two-step ball milling-assisted synthesis of N-doped biochar loaded with ferrous sulfide for enhanced adsorptive removal of Cr(Ⅵ) and tetracycline from water

Nitrogen-doped biochar loaded with FeS (FeS@NBC_BM) was synthesized by two-step ball milling processes. Characterization results revealed that N-doping process successfully introduced pyridinic, pyrrolic, and graphitic N structures, and FeS was subsequently embedded in N-doped biochar (NBC_BM). The resultant FeS@NBC_BM presented predominant adsorption capacity for Cr(VI) (194.69 mg/g) and tetracycline (TC, 371.29 mg/g) compared with BC (27.28 and 37.89 mg/g) and NBC_BM (71.26 and 81.26 mg/g). In addition, the Cr(VI)/TC elimination process by FeS@NBC_BM was basically stable with multiple co-existing ions with slight decrease on adsorption performance after three desorption-regeneration cycles. Most importantly, FeS@NBC_BM was found to achieve Cr(VI) elimination not only by electrostatic attraction, ion exchange and complexation, but also by electrons-triggered reduction provided by different species of N, Fe²⁺ as well as S(Ⅱ). Meantime, pore filling, hydrogen bonding, and π-π stacking interactions were demonstrated to contribute to TC adsorption. These results suggested the co-modification of N-doping and FeS loading by ball milling as an innovative decorating method for biochar to adsorptive purification of Cr(VI) and TC-contaminated water.

Electroactive Fe-biochar for redox-related remediation of arsenic and chromium: Distinct redox nature with varying iron/carbon speciation

Electroactive Fe-biochar has attracted significant attention for As(III)/Cr(VI) immobilization through redox reactions, and its performance essentially lies in the regulation of various Fe/C moieties for desired redox performance. Here, a series of Fe-biochar with distinct Fe/C speciation were rationally produced via two-step pyrolysis of iron minerals and biomass waste at 400-850 °C (BCX-Fe-Y, X and Y represented the first- and second-step pyrolysis temperature, respectively). The redox transformation of Cr(VI) and As(III) by Fe-biochar was evaluated in simulated wastewater under oxic or anoxic conditions. Results showed that more effective Cr(VI) reduction could be achieved by BCX-Fe-400, while a higher amount of As (III) was oxidized by BCX-Fe-850 under the anoxic environment. Besides, BCX-Fe-400 could generate more reactive oxygen species (e.g.,^•OH) by reducing the O₂, which enhanced the redox-related transformation of pollutants under the oxic situation. The evolving redox performance of Fe-biochar was governed by the transition of the redox state from reductive to oxidative related to the Fe/C speciation. The small-sized amorphous/low-crystalline ferrous minerals contributed to a higher electron-donating capacity (0.43-1.28 mmol g^-1) of BCX-Fe-400. In contrast, the oxidative surface oxygen-functionalities (i.e., carboxyl and quinoid) on BCX-Fe-850 endowed a stronger electron-accepting capacity (0.71-1.39 mmol g^-1). Moreover, the graphitic crystallites with edge-type defects and porous structure facilitated the electron transfer, leading to a higher electron efficiency of BCX-Fe-850. Overall, we unveiled the roles of both Fe and C speciation in maneuvering the redox reactivity of Fe-biochar, which can advance our rational design of electroactive Fe-biochar for redox-related environmental remediation.