ABSTRACT
Migration of microplastics (MPs) in soil-groundwater systems plays a pivotal role in determining its concentration in aquifers and future threats to the terrestrial environment, including human health. However, existing models employing an advection-dispersion equation are insufficient to incorporate the holistic mechanism of MP migration. Therefore, to bridge the gap associated with MP migration in soil-groundwater systems, a dispersion-drag force coupled model incorporating a drag force on MPs along with dispersion is developed and validated through existing laboratory and field-scale experiments. The inclusion of the MP dispersion notably increased the global maximum particle velocity (vmaxp) of MPs, resulting in a higher concentration of MPs in the aquifer, which is also established by sensitivity analysis of MP dispersion. Additionally, increasing irrigation flux and irrigation areas significantly accelerates MP migration downward from soil to deep saturated aquifers. Intriguingly, vmaxp of MPs exhibited a nonlinear relationship with MPs' sizes smaller than 20 µm reaching the highest value (=1.64 × 10-5 m/s) at a particle size of 8 µm, while a decreasing trend was identified for particle sizes ranging from 20 to 100 µm because of the hindered effect by porous media and the weaker effect of the drag force. Moreover, distinct behaviors were observed among different plastic types, with poly(vinyl chloride), characterized by the highest density, displaying the lowest vmaxp and minimal flux entering groundwater. Furthermore, the presence of a heterogeneous structure with lower hydraulic conductivity facilitated MP dispersion and promoted their migration in saturated aquifers. The findings shed light on effective strategies to mitigate the impact of MPs in aquifers, contributing valuable insights to the broader scientific fraternity.
ABSTRACT
Perfluorooctane sulfonamide (PFOSA) is an immediate perfluorooctanesulfonate (PFOS) precursor (PreFOS). Previous studies have shown PFOSA to induce stronger toxic responses compared to other perfluorinated compounds (PFCs). However, the specific nature of PFOSA-induced toxicity, whether autonomous or mediated by its metabolite PFOS, has not been fully elucidated. This study systematically investigates the immunomodulatory effects of PFOSA and PFOS in zebrafish (Danio rerio). Exposure to PFOSA compromised the zebrafish's ability to defend against pathogenic infections, as evidenced by increased bacterial adhesion to their skin and reduced levels of the biocidal protein lysozyme (LYSO). Moreover, PFOSA exposure was associated with disruptions in inflammatory markers and immune indicators, along with a decrease in immune cell counts. The findings from this study suggest that the immunotoxicity effects of PFOSA are primarily due to its own toxicity rather than its metabolite PFOS. This conclusion was supported by dose-dependent responses, the severity of observed effects, and multivariate analysis. In addition, our experiments using NF-κB-morpholino knock-down techniques further confirmed the role of the Nuclear factor-κappa B pathway in mediating PFOSA-induced immunotoxicity. In conclusion, this study reveals that PFOSA impairs the immune system in zebrafish through an autotoxic mechanism, providing valuable insights for assessing the ecological risks of PFOSA.
ABSTRACT
Natural attenuation of organic contaminants can occur under anoxic or oxic conditions. However, the effect of the coupling anoxic-oxic process, which often happens in subsurface soil, on contaminant transformation remains poorly understood. Here, we investigated 2,4-dichlorophenol (2,4-DCP) transformation in Fe-rich soil under anoxic-oxic alternation. The anoxic and oxic periods in the alternating system showed faster 2,4-DCP transformation than the corresponding control single anoxic and oxic systems; therefore, a higher transformation rate (63.4%) was obtained in the alternating system relative to control systems (27.9-42.4%). Compared to stable pH in the alternating system, the control systems presented clear OH- accumulation, caused by more Fe(II) regeneration in the control anoxic system and longer oxygenation in the control oxic system. Since 2,4-DCP was transformed by ion exchangeable Fe(II) in soil via direct reduction in the anoxic process and induced ·OH oxidation in the oxic process, OH- accumulation was unbeneficial because it competed for proton with direct reduction and inhibited â¢OH generation via complexing with Fe(II). However, the alternating system exhibited OH--buffering capacity via anoxic-oxic coupling processes because the subsequent oxic periods intercepted Fe(II) regeneration in anoxic periods, while shorter exposure to O2 in oxic periods avoided excessive OH- generation. These findings highlight the significant role of anoxic-oxic alternation in contaminant attenuation persistently.
ABSTRACT
With the increasing production and application, more molybdenum disulfide (MoS2) nanosheets could be released into environment. The aggregation and dispersion of MoS2 nanosheets profoundly impact their transport and transformation in the aquatic environment. However, the colloidal stability of MoS2 remains largely unknown in natural surface waters. This study investigated the colloidal stability of MoS2 nanosheets in six natural surface waters affected by both light irradiation and water chemistry. Compared to that of the pristine MoS2 nanosheets, the colloidal stability of MoS2 photoaged in ultrapure water declined. Light irradiation induced the formation of Mo-O bonds, the release of SO42- species, and the decrease in 1T/2H ratio, which reduced negative charge and enhanced hydrophobicity. However, the colloidal stability of MoS2 photoaged in natural surface waters was increased relative to that in ultrapure water not only for the smaller extent of photochemical transformation but more importantly the surface modification by water chemistry. Furthermore, the colloidal stability of MoS2 photoaged in natural surface waters followed the order of sea water > lake water > river water. The abundant cations (e.g., Ca2+ and Mg2+) in sea water facilitated the covalent grafting (S-C bonds) of more dissolved organic matter (DOM) on MoS2 via charge screening and cation bridging, thus inducing stronger electrostatic repulsion and steric effect to stabilize nanosheets. The crucial role of the covalent grafting of DOM was further confirmed by the positive correlation between the critical coagulation concentration values and S-C ratios (R2 = 0.82, p < 0.05). Our results highlighted the dominant role of water chemistry than light irradiation in dictating the colloidal stability of MoS2 photoaged in natural surface waters, which provided new insight into the environmental behavior of MoS2 in aquatic environment.
Subject(s)
Colloids , Disulfides , Light , Molybdenum , Molybdenum/chemistry , Colloids/chemistry , Disulfides/chemistry , Nanostructures/chemistry , Water/chemistryABSTRACT
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.
ABSTRACT
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.
Subject(s)
Charcoal , Chromium , Environmental Restoration and Remediation , Iron , Minerals , Oxidation-Reduction , Soil Pollutants , Soil , Charcoal/chemistry , Chromium/chemistry , Soil Pollutants/chemistry , Soil/chemistry , Minerals/chemistry , Iron/chemistry , Environmental Restoration and Remediation/methods , ElectronsABSTRACT
Soil arsenic (As) contamination associated with the demolition of smelting plants has received increasing attention. Soil As can source from different industrial processes, and also participate in soil weathering, making its speciation rather complex. This study combined the usage of chemical sequential extraction and advanced spectroscopic techniques, e.g., time of flight secondary ion mass spectrometry (ToF-SIMS), to investigate the mineralogical transformation of soil As at different processing sites from a typical copper smelting plant in China. Results showed that the stability of arsenic species decreased following the processes of storage, smelting, and flue gas treatment. Arsenic in the warehouse area was incorporated into pyrite (FeS2) as well as its secondary minerals such as jarosite (KFe3(SO4)2(OH)6). At the smelting area, a large proportion of As was adsorbed by iron oxides from smelting slags, while some As existed in stable forms like orpiment (As2S3). At the acid-making area, more than half of As was adsorbed on amorphous iron oxides, and some were adsorbed on the flue gas desulfurization gypsum. More importantly, over 86% of the As belonged to non-specifically and specifically adsorbed fractions was found to be bioaccessible, highlighting the gypsum-adsorbed As one of the most hazardous species in smelting plant soils. Our findings indicated the importance of iron oxides in As retention and suggested the potential health risk of gypsum-adsorbed As. Such detailed knowledge of As speciation and bioaccessibility is vital for the management and remediation of As-contaminated soils in smelting plants.
Subject(s)
Arsenic , Ferric Compounds , Soil Pollutants , Arsenic/analysis , Copper/analysis , Calcium Sulfate , Iron/chemistry , Soil/chemistry , Oxides/analysis , Soil Pollutants/analysisABSTRACT
The groundwater table fluctuation (GTF) zone is an important medium for the hydrologic cycle between unsaturated soil and saturated aquifers, which accelerates the migration, transformation, and redistribution of contaminants and further poses a potential environmental risk to humans. In this review, we clarify the key processes in the generation of the GTF zone and examine its links with the variation of the hydrodynamic and hydrochemistry field, colloid mobilization, and contaminant migration and transformation. Driven by groundwater recharge and discharge, GTF regulates water flow and the movement of the capillary fringe, which further control the advection and dispersion of contaminants in soil and groundwater. In addition, the formation and variation of the reactive oxygen species (ROS) waterfall are impacted by GTF. The changing ROS components partially determine the characteristic transformation of solutes and the dynamic redistribution of the microbial population. GTF facilitates the migration and transformation of contaminants (such as nitrogen, heavy metals, non-aqueous phase liquids, and volatile organic compounds) through colloid mobilization, the co-migration effect, and variation of the hydrodynamic and hydrochemistry fields. In conclusion, this review illustrates the limitations of the current literature on GTF, and the significance of GTF zones in the underground environment is underscored by expounding on the future directions and prospects.
Subject(s)
Groundwater , Water Pollutants, Chemical , Humans , Soil , Reactive Oxygen Species , Water Movements , Groundwater/chemistry , Colloids , Water Pollutants, Chemical/analysisABSTRACT
Naturally occurring iron (Fe) minerals have been proved to activate persulfate (PS) to generate reactive species, but the role of soil-inherent Fe minerals in activating PS as well as the underlying mechanisms remains poorly understood. Here, we investigated sulfamethoxazole (SMX) degradation by PS in two Fe-rich soils and one Fe-poor soil. Unlike with the radical-dominant oxidation processes in Fe-poor soil, PS was effectively activated through nonradical pathways (i.e., surface electron-transfer) in Fe-rich soils, accounting for 68.4%-85.5% of SMX degradation. The nonradical mechanism was evidenced by multiple methods, including electrochemical, in situ Raman, and competition kinetics tests. Inherent Fe-based minerals, especially those containing Fe(II) were the crucial activators of PS in Fe-rich soils. Compared to Fe(III) minerals, Fe(II) minerals (e.g., ilmenite) were more liable to form Fe(II) mineral-PS* complexes to initiate the nonradical pathways, oxidizing adjacent SMX via electron transfer. Furthermore, mineral structural Fe(II) was the dominant component to coordinate such a direct oxidation process. After PS oxidation, low-crystalline Fe minerals in soils were transformed into high-crystalline Fe phases. Collectively, our study shows that soil-inherent Fe minerals can effectively activate PS in Fe-rich soils, so the addition of exogenous iron might not be required for PS-based in situ chemical oxidation. Outcomes also provide new insights into the activation mechanisms when persulfate is used for the remediation of contaminated soils.
Subject(s)
Soil , Sulfamethoxazole , Sulfamethoxazole/analysis , Sulfamethoxazole/chemistry , Ferric Compounds , Iron/chemistry , Minerals/chemistry , Oxidation-Reduction , Ferrous Compounds/chemistryABSTRACT
The simulated in-vitro gastrointestinal method provides a simple way to evaluate the health risk of human body exposed to soil contaminants. Several in-vitro methods have been successfully established for soil As, Pb, and Cd. However, the method development for soil Cr failed up to now, which could be resulted from alteration in the species of Cr (e.g., Cr(VI)/Cr(III)) caused by the gastrointestinal digestion components, ultimately affecting the accessibility of Cr. This study explored the transformation and bioaccessibility of Cr in two Cr-contaminated soils during the physiologically based extraction test. The water-soluble and exchangeable Cr in soil was dissolved in gastrointestinal tract, accompanied with reduction of Cr(VI) into Cr(III), and the reduction occurred after the chemical extraction in two soils rather than during the extraction. Pepsin and organic acids in gastric phase could reduce Cr(VI) into Cr(III) and reduction efficiency were 20.4%- 53.0%, while in intestinal phase, pancreatin and bile salt had little effect on the Cr(VI) reduction, instead, more Cr(VI) was released from soil. In the gastric solution, Cr(VI) was mainly present as HCrO4- and Cr(III) as free Cr3+ ion. In the intestinal phase, Cr(VI) mainly occurred as CrO42- and Cr(III) as Cr(OH)3 (aq). Cr in the soil solid phase was dominated as the precipitates of Cr-Fe oxide, which was hardly extracted. Bioaccessibility of Cr in gastric phase increased as extraction duration increased and decreased in the intestinal phase, the contrary trend was observed for the hazard quotient of Cr in two phases due to Cr(VI)/Cr(III) transformation. This study indicates that the gastrointestinal components could influence the Cr transformation and subsequently affect the Cr bioaccessibility, which would help for a successful establishment of in vitro determination method for soil Cr bioaccessibility.
Subject(s)
Soil Pollutants , Soil , Humans , Chromium/toxicity , Chromium/analysis , Stomach/chemistry , Digestion , Soil Pollutants/analysisABSTRACT
Nanoplastics (NPs), as a type of newly emerging pollutant, are ubiquitous in various environmental systems, one of which is coexistence with organic pollutants in wastewater, potentially influencing the pollutants' biodegradation. A knowledge gap exists regarding the influence of microbial consortium and NPs interactions on biodegradation efficiency. In this work, a 2,4-dichlorophenol (DCP) biodegradation experiment with presence of polystyrene nanoplastics (PS-NPs) with particle sizes of 100 nm (PS100) or 20 nm (PS20) was conducted to verify that PS-NPs had noticeable inhibitory effect on DCP biodegradation in a size-dependent manner. PS100 at 10 mg/L and 100 mg/L both prolonged the microbial stagnation compared to the control without PS-NPs; PS20 exacerbated greater, with PS20 at 100 mg/L causing a noticeable 6-day lag before the start-up of rapid DCP reduction. The ROS level increased to 1.4-fold and 1.8-fold under PS100 and PS20 exposure, respectively, while the elevated LDH under PS20 exposure indicated the mechanical damage to cell membrane by smaller NPs. PS-NPs exposure also resulted in a decrease in microbial diversity and altered the niches of microbial species, e.g., they decreased the abundance of some functional bacteria such as Brevundimonas and Comamonas, while facilitated some minor members to obtain more proliferation. A microbial network with higher complexity and less competition was induced to mediate PS-NPs stress. Functional metabolism responded differentially to PS100 and PS20 exposure. Specifically, PS100 downregulated amino acid metabolism, while PS20 stimulated certain pathways in response to more severe oxidative stress. Our findings give insights into PS-NPs environmental effects concerning microflora and biological degradation.
Subject(s)
Environmental Pollutants , Nanoparticles , Water Pollutants, Chemical , Microbial Consortia , Microplastics , Biodegradation, Environmental , Climate , Phenols , PolystyrenesABSTRACT
Two phosphorus (P)-rich biowastes, sewage sludge (SS) and bone dreg (BD), were selected to clarify P footprints among biowaste, biochar, soil, and plants by introducing a novel "3R" concept model. Results showed that pyrolysis resulted in P transformation from an unstable-organic amorphous phase to a stable-inorganic crystalline phase with a P retention rate of 70-90% in biochar (P reservation). In soil, SSBC released more P in acid red soil and alkaline yellow soil than BDBC, while the opposite result appeared in neutral paddy soil. The P released from SSBC formed AlPO4 by combining with Al in soil, whereas P from BDBC transformed into Ca5(PO4)3F(or Cl) in conjunction with Ca in the soil (P replenishment). Various plants exhibited an uptake of approximately 2-6 times more P from biochar-amended soil than from the original soil (P reception). This study can guide the application of biochar in various soil-plant systems for effective nutrient reclamation.
Subject(s)
Soil Pollutants , Soil , Soil/chemistry , Phosphorus/chemistry , Charcoal/chemistry , Sewage/chemistry , Soil Pollutants/analysisABSTRACT
Photoexcitation of natural semiconductor Fe(III) minerals has been proven to generate Fe(II), but the photogeneration of Fe(II) in Fe-rich surface soil as well as its role in the redox biogeochemistry of Cr(VI) remains poorly understood. In this work, we confirmed the generation of Fe(II) in soil by solar irradiation and proposed a new mechanism for the natural reductive detoxification of Cr(VI) to Cr(III) in surface soil. The kinetic results showed that solar irradiation promoted the reduction of Cr(VI) in Fe-rich soils, while a negligible Cr(VI) reduction was observed in the dark. Fe(II), mainly in the form of silicate-bound Fe(II), was generated under solar irradiation and responsible for the reduction of Cr(VI) in soils, which was evidenced by sequential extraction, transmission electron microscopy with electron energy loss spectroscopy, and electron transfer calculation. Photogenerated silicate-bound Fe(II) resulted from the massive clay-iron (hydr)oxide associations, consisting of iron (hydr)oxides (e.g., hematite and goethite) and kaolinite. These associations could generate Fe(II) under solar irradiation either via intrinsic excitation to produce photoelectrons or via the ligand-to-metal charge transfer process after the formation of clay-iron (hydr)oxide-organic matter complexes, which was proven by photoluminescence spectroscopy and X-ray photoelectron spectroscopy. These findings highlight the important role of photogenerated Fe(II) in Cr(VI) reduction in surface soil, which advances a fundamental understanding of the natural detoxification of Cr(VI) as well as the redox biogeochemistry of Cr(VI) in soil.
Subject(s)
Ferric Compounds , Soil , Ferric Compounds/chemistry , Soil/chemistry , Clay , Chromium/chemistry , Iron/chemistry , Silicates , Oxidation-Reduction , Oxides , Ferrous CompoundsABSTRACT
Bioelectrochemical system is a prospective strategy in organic-contaminated groundwater treatment, while few studies clearly distinguish the mechanisms of adsorption or biodegradation in this process, especially when dense biofilm is formed. This study employed a single chamber microbial electrolysis cell (MEC) with two three-dimensional electrodes for removing a typical organic contaminant, 2,4-dichlorophenol (DCP) from groundwater, which inoculated with anaerobic bacteria derived from sewage treatment plant. Compared with the single biodegradation system without electrodes, the three-dimensional electrodes with a high surface enabled an increase of alpha diversity of the microbial community (increased by 52.6% in Shannon index), and provided adaptive ecological niche for more bacteria. The application of weak voltage (0.6 V) furtherly optimized the microbial community structure, and promoted the aggregation of microorganisms with the formation of dense biofilm. Desorption experiment proved that the contaminants were removed from the groundwater mainly via adsorption by the biofilm rather than biodegradation, and compared with the reactor without electricity, the bioelectrochemical system increased the adsorption capacity from 50.0% to 74.5%. The aggregated bacteria on the surface of electrodes were mainly dominated by Delftia tsuruhatensis (85.0%), which could secrete extracellular polymers and has a high adsorption capacity (0.30 mg/g electrode material) for the contaminants. We found that a bioelectrochemical system with a three-dimensional electrode could stimulate the formation of dense biofilm and remove the organic contaminants as well as their possible more toxic degradation intermediates via adsorption. This study provides important guidance for applying bioelectrochemical system in groundwater or wastewater treatment.
Subject(s)
Bioelectric Energy Sources , Delftia , Electricity , Phenols , Biofilms , Electrodes , Bioelectric Energy Sources/microbiologyABSTRACT
Large amounts of coastal silt produced annually is urgent to be treated with a feasible strategy. This study converted it into subgrade soil by cement solidification for resource utilization. Biochar was used as exogenous additive for enhancing compressive strength of the product, simultaneously achieving carbon sequestration. Three biochars derived from peanut shells (PSBC), cow dung (CDBC) and sewage sludge (SSBC) at 300 °C, 500 °C and 700 °C pyrolysis, were added into raw materials with 1%, 2% and 5%, respectively. All biochars significantly improved the compressive strength of the subgrade soil by 20-110%. Biochar catalyzed cement hydration reactions to produce more Ca(OH)2, CaCO3 and calcium silicate hydrates (C-S-H gel). The catalytic capacity of different biochars followed the order of SSBC > PSBC > CDBC. Addition of 2% SSBC500 induced the greatest increase in 28 d-strength from only 1.0 MPa-2.1 MPa, which was due to that 500 °C biochar had a suitable specific surface area and porosity. Biochar facilitated CO2 capture (absorption) during the hydration reactions at the initial 48 h with 55-70 mg g-1. The high alkalinity and water holding capacity of biochar contributed to the absorption of CO2; the high content of minerals in SSBC compared to CDBC and PSBC promoted chemical conversion of CO2 to carbonate. Besides, the biochar itself as carbon rich material was encapsulated in the subgrade soil, which can be regarded as a long-term carbon sequestration strategy. Carbon budget analysis demonstrated that converting one ton dry silt into subgrade soil with addition of 2% biochar could increase CO2 sequestration from 11 kg to 36-94 kg. This study proposes a novel strategy of using biochar to strengthen the subgrade soil simultaneously achieve long-term carbon sequestration.
Subject(s)
Carbon , Soil , Carbon/chemistry , Soil/chemistry , Carbon Dioxide , Charcoal/chemistry , Sewage/chemistry , Carbon SequestrationABSTRACT
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.
ABSTRACT
Biochar has been proven to participate in the biotic reduction of hexavalent chromium (Cr(VI)) in environment since its involvement may accelerate the extracellular electron transfer (EET). However, roles of the redox-active moieties and the conjugated carbon structure of biochar in this EET process remain unclear. In this study, 350 °C and 700 °C were selected to produce biochar with more O-containing moieties (BC350) or more developed conjugated structures (BC700), and their performances in the microbial reduction of soil Cr(VI) were investigated. Our results showed that BC350 presented a 241% increase of Cr(VI) microbial reduction after 7-day incubation, much higher than that of BC700 (39%), suggesting that O-containing moieties might play more important roles in accelerating the EET process. Biochar, especially BC350 could serve as an electron donor for microbial anaerobic respiration, but its contribution (73.2%) as an electron shuttle for EET was dominant to the enhanced Cr(VI) reduction. The positive correlation between electron exchange capacities (EECs) of pristine and modified biochars and the corresponding maximum reduction rates of Cr(VI) evidenced the crucial role of redox-active moieties in electron shuttling. Moreover, EPR analysis suggested the nonnegligible contribution of semiquinone radicals in biochars to the accelerated EET process. This study demonstrates the crucial role of redox-active moieties, i.e., O-containing moieties in mediating the EET process during the microbial reduction of Cr(VI) in soil. Findings obtained will advance the current understanding of biochar as an electron shuttle participating in the biogeochemical processes of Cr(VI).
Subject(s)
Electrons , Soil Pollutants , Soil/chemistry , Soil Pollutants/analysis , Charcoal/chemistry , Oxidation-Reduction , Chromium/chemistry , AdsorptionABSTRACT
Transformation of phosphorus (P) species during pyrolytic production of biochar from P-rich biowastes with a subsequent soil amendment is important to P reclamation. Aiming at increasing the content of plant-available P and restraining the formation of easily mobile P in pyrolysis product, this study used exogenous calcium ions (20 wt% CaCl2) addition prior to pyrolysis to regulate the pyrolytic transformation of P chemical fractions from sewage sludge and bone dreg. Results showed that active Ca catalyzed the decomposition of organic P to transform into inorganic orthophosphate. Based on Hedley's sequential extraction method, this study found that addition of Ca ions remarkably reduced the content of soluble P, exchange P, Fe/Al bound P, and occluded P in biochar, while increased Ca bound P from 78 to 85% to 85-96%. Liquid 31P NMR indicated that exogenous Ca induced the crack of the P-O-P bond in pyrophosphate to become orthophosphates. It also explained why new orthophosphates including chlorapatite (Ca5(PO4)3Cl) and calcium hydroxyapatite (Ca10(PO4)6(OH)2) appeared in the Ca-composite biochar compared to pristine biochar. Combined with rapid P-release test in paddy soil (pH 6.27) and 30-days rice seedling growth test under flooded condition (10 wt% biochar addition ratio), it was confirmed that compared to pristine biochar, Ca-composite biochar released more P in paddy soil, but also promoted more P to be taken in by rice root and stalk. These results suggested that pretreating biowaste with Ca ion was a friendly approach to enhance P reclamation during biochar formation, making it a promising P fertilizer.
Subject(s)
Oryza , Soil Pollutants , Phosphorus , Calcium , Fertilizers , Soil/chemistry , Soil Pollutants/analysis , Phosphates , Charcoal/chemistry , SewageABSTRACT
Although zero-valent iron-embedded biochar (ZVI-BC) has been proposed as an effective amendment for arsenic (As)-contaminated soils, the impacts of soil characteristics and treatment conditions on the remediation process remained poorly understood. Herein, the immobilization of As in four As-contaminated soils (i.e., smelting soil, storage soil, agricultural soil, and mining soil) by ZVI-BC under different amendment dosages, cultivation temperatures, and soil moisture contents were investigated. ZVI-BC showed high As immobilization capacity in all four soils via forming the AsFe co-precipitation, and the liable As was reduced by 82.4-97.0 % with a 2 % (w/w) amendment. The higher temperature could raise the concentration of liable As in all four soils, especially for the storage soil, in which liable As at 35 °C was almost 3 times of that at 25 °C after 50-days treatment, because the elevated temperature enhanced the destruction of the generated AsFe coprecipitation as well as the desorption of As in soils. Too much soil moisture was unfavorable for the As immobilization after 50-days treatment. Flooding tended to inhibit the community diversity of As-detoxicated bacteria, e.g., Halomonas, Bryobacter, and Anaerolinea, thus resulting in the release of liable As. According to the correlation analysis, the crucial influencing factor for As immobilization was different in four soils, which was determined by the soil properties and proportion of liable As. Our study indicates that ZVI-BC is an effective amendment for As immobilization under various conditions, and the biogeochemical processes of As-associated Fe minerals determine the As immobilization during amendment.
Subject(s)
Arsenic , Soil Pollutants , Iron/chemistry , Arsenic/analysis , Soil/chemistry , Soil Pollutants/analysis , Charcoal/chemistryABSTRACT
In this study, the zero valent iron-biochar composite (ZVI-BC) was applied to construct an active surface soil layer for the simultaneous remediation of As-Zn contaminated soil and groundwater, focusing on the influence of the infiltration mode of pumped-up groundwater into soil. The active surface soil treated more contaminated groundwater for As (4.45-5.46 L kg-1 soil) than that for Zn (2.52-3.13 L kg-1 soil) under both continuous and intermittent infiltration modes, with about 98% As and 95% Zn removed from groundwater and retained in the soil. As(V) precipitated with Fe(III) due to ZVI oxidation, which was responsible for the As immobilization. The soil under the intermittent infiltration mode was enriched by the Sphingomonas with arsenate reductase gene, which promoted more reduction of As(V) into As(III) and facilitated coprecipitation of As(III) with Fe(III). The Mn oxide determined the sorption of Zn in the active soil layer, where the Hyphomicrobium, one type of manganese oxidizing bacteria, was much higher under the continuous infiltration mode, which accounted for the more Zn immobilization. After the remediation, both As and Zn immobilized in the active surface soil showed high stability, with the average downward migration rate of only 0.207-0.368 cm year-1 within 20-year rainfall exposure. Our findings indicate that this active surface soil layer is applicable for simultaneous immobilization of As and Zn in both contaminated soil and groundwater, and the groundwater intermittent infiltration could be a better option considering the remediation effectiveness, the immobilization mechanism, the long-term stability, and the energetic efficiency.