The environmental fate and risks of microplastics (MPs) and their associated contaminants have attracted increasing concern in recent years. In this study, the cotransport of six kinds of pristine and aged MPs and the antiager ozonation product N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q) were investigated via a series of batch and transport experiments, and characteristic analysis (e.g., SEM, FTIR and XPS). Generally, pristine MPs exhibit higher adsorption ability than aged MPs due to the hydrophobic interaction. The 6PPD-Q usually exhibited both free moving and bond-MPs moving during transport process in presence of MPs, but none free 6PPD-Q was detected in presence of pristine PP MPs. The mobility of 6PPD-Q was generally facilitated in presence of MPs by bond-MPs moving due to the hydrogen bonding, halogen bonding, π-π interaction (the maximum total mass recovery of 84.11%), which efficiency was influenced with the combined effect of adsorption ability and mobility of MPs. The pristine PVC MPs showed highest facilitation on 6PPD-Q transport. The retained 6PPD-Q in porous media also was released by various MPs with different mass recovery ranged from 15.72% to 56.26% via surface moving of MPs around porous media. Both the dissolved and retained 6PPD-Q decreased the MPs mobility with the minimum mass recovery of 34.02%. Findings from this study contribute to the prediction and assessment of the combined risks of MPs and 6PPD-Q.

For advancing current knowledge on the transport of microplastics (MPs) in the environment, this study used a real-time pore-scale visualization and quantitative system to examine the motions and mobility of fibrous and fragmental MPs under various surfactant (AEO, CTAC, and AES) and electrolyte conditions. The videos showed that fibrous MPs formed tangles through entanglement, which moved in an axial direction aligned with the flow streamline. Both fibrous and fragmental MPs showed suspended movement as well as surface movement (e.g., sliding, rolling, and saltating) in the porous media. Some deposited fibrous MPs showed flexible deformation due to shear flow. Compared to fragmental MPs, fibrous MPs showed lower mobility due to the tendency to deposit and clog the porous media. The mobility of fragmental MPs was enhanced in the presence of AEO but remained relatively unchanged with AES. In the presence of CTAC, the mobility of fragmental MPs was slightly inhibited under low ionic strength (IS) conditions but remarkably enhanced under high IS conditions. However, the mobility of fibrous MPs was largely unaffected by the surfactants. Both the numerical model and FDLVO calculations effectively described the transport and deposition of MPs in porous media.

Understanding the transport behaviors of microplastics (MPs) in porous media is crucial in controlling MPs pollution. Given nitrogen is one of the most important nutrients in soil and groundwater systems, unclearness of the transport behaviors of microplastics (MPs) under various nitrogen conditions may inhibit the acknowledgment of MPs fate. For this reason, this study innovatively investigates the transport characteristics of four kinds of typical MPs (PVC MPs, PMMA MPs, PET MPs, and PP MPs) under various NaNO3, NH4Cl and urea conditions via column experiments numerical models. The FTIR and XPS analysis were conducted to excavate the transform of MPs. The MPs mobility was generally reduced with the increasing nitrogen concentrations. The polarity and density properties of different MPs played combined roles in transport under similar conditions. Compared to NO3−, NH4+ may neutralize the negative charge of MPs and then restrain their transport in porous media. Urea may coat the surface of MPs and promote the mobility, however, increasing concentrations of urea may result in the interattraction between MPs and porous media via hydrogen-bond and π-π interaction. PET MPs and PP MPs showed barely transform during transport under the tested conditions. Particularly, the chlorines on PVC MPs could react with the amide on urea and produce amidogen, which may improve PVC MPs transport. The N–H and C–N bond also generated on PMMA MPs in presence of urea also may enhance the mobility.

Microplastics (MPs) inevitably undergo multiple aging processes during their life cycle in the environment. However, the information regarding the mechanical fragmentation behavior of MPs remained unclear, including the changes in the intrinsic properties of aged MPs, the measurement of aging degree, the underlying mechanism, and the interaction with heavy metals. Here, MPs (PS, PP, PET) were aged by crushing (-CR) and ball-milling (-BM) to simulate mild and severe mechanical fragmentation, respectively. Our results indicated that mechanical fragmentation significantly affected the morphology of MPs. The aging degree of MP-BM was deeper compared to MP-CR owing to smaller particle size, larger specific surface area, poorer heat resistance, better hydrophilicity, and richer oxygen-containing functional groups. The carbonyl index (CI) and O/C ratio were used to measure the aging degree of the two mechanical aging treatments. Besides, the mechanism was proposed and the discrepancy between the two treatments was elaborated from three aspects including the excitation energy source, reaction interface, and reaction dynamics. Furthermore, the extrinsic properties of MPs altered with the increase of aging degree; specifically, the adsorption capacities of heavy metals were enhanced. Meanwhile, it was unveiled that the CI value and O/C ratio played a vital role in estimating the adsorption ability of heavy metals. The findings not only reveal the mechanical fragmentation behavior of MPs but also provide new insights into the assessment of the potential risks of the aged MPs via chemical indexes.

In this work, column experiments were applied to investigate the transport of four kinds of microplastics (MPs) under a series of ionic strength (IS) conditions. Under 0.1 mM IS, PMMA MPs showed the highest mobility, as well as the PET MPs showed the lowest mobility. With the IS increased, the transport of all kinds of MPs in porous media was generally reduced to the minimum. The transport reducing efficiency of PMMA MPs and PET MPs was lower than that of the PVC MPs and PP MPs. It was found that both the hydro-chemical conditions and basic properties showed combined effect on MPs transport in porous media. The DLVO results were well used to describe the deposition of MPs onto sand surface and excavate the transport behaviors of MPs. The one-site kinetic deposition model was successfully conducted to fit the observed breakthrough curves. Findings from this study elucidated the key factors controlling the MPs transport in porous media, contributing to the prediction and assessment of the environmental risks of MPs.

Electron transfer mediated by iron minerals is considered as a critical redox step for the dynamics of pollutants in soil. Herein, we explored the reduction process of Cr(VI) with different crystalline ferric oxyhydroxides in the presence of pyrogenic carbon (biochar). Both low- and high-crystallinity ferric oxyhydroxides induced Cr(VI) immobilization mainly via the sorption process, with a limited reduction process. However, the Cr(VI) reduction immobilization was inspired by the copresence of biochar. Low-crystallinity ferric oxyhydroxide had an intense chemical combination with biochar and strong sorption for Cr(VI) via inner-sphere complexation, leading to the indirect electron transfer route for Cr(VI) reduction, that is, the electron first transferred from biochar to iron mineral through C-O-Fe binding and then to Cr(VI) with Fe(III)/Fe(II) transformation on ferric oxyhydroxides. With increasing crystallinity of ferric oxyhydroxides, the direct electron transfer between biochar and Cr(VI) became the main electron transfer avenue for Cr(VI) reduction. The indirect electron transfer was suppressed in the high-crystallinity ferric oxyhydroxides due to less sorption of Cr(VI), limited combination with biochar, and higher iron stability. This study demonstrates that electron transfer mechanisms involving iron minerals change with the mineral crystallization process, which would affect the geochemical process of contaminants with pyrogenic carbon.

Zero-valent iron-embedded biochar (ZVI/BC) is considered as an effective material for arsenic (As) immobilization in soil, but the stability of As after remediation against aging remains unknown. Herein, the effects of dry wet and freeze-thaw aging on the immobilization of As in two As-contaminated soils amended by ZVI/BC were evaluated. ZVI/BC showed high immobilization capacity for As-contaminated soils with an over 82% decrease of bioavailable As, mainly due to the As-Fe co-precipitation accompanied with ZVI oxidation. The aging of dry-wet and freeze-thaw had an opposite effect on the bioavailability of As. After 35 rounds of dry-wet aging, bioavailable As concentration increased from 1.25-9.50 to 1.83-21.75 mg/kg, because of the oxidation dissolution of ZVI and the formation of mobile reduced As(III). By contrast, the crystallization of amorphous iron with the structural incorporation of sorbed As and the oxidation of As(III) into stable As(V) occurred during the 35 rounds of freeze thaw aging, leading to the decrease of bioavailable As concentration from 9.50-1.25 to 5.42-0.45 mg/kg. Our results revealed that the stability of soil As after remediation by ZVI/BC varied with the different aging process, which needs more consideration for the long-term soil As immobilization in the different whether areas.

Biochar in soil is susceptible to natural aging along with soil minerals, which might alter its electrochemical properties and redox reactions with contaminants. In this study, soluble mineral salts (FeCl3, MnCl2, AlCl3, CaCl2) and clay mineral (kaolinite) were selected to investigate the impact of co-aging with soil minerals on the redox activity of peanut-shell biochar for Cr(VI) reduction. Natural aging for 3-month induced oxidation of biochar with the decrease of reducing moieties, i.e., -C-OH from 26.8-43.7% to 18.4-24.1%. Co-aging with minerals except for Mn(II) further decreased the proportion of -C-OH to 6.94-22.2% because of the interaction between mineral ions and biochar, resulting in the formation of mineral-biochar complex and new minerals, e.g. beta-FeOOH. Due to its reductivity, Mn(II) presented the least decrease or even slight increase of -C-OH while itself was oxidized to Mn(III) and Mn(IV). The decline of -C-OH caused the decrease of Cr(VI) reduction rate constant from 2.18 to 2.47 x 10-2 h-1 for original biochars to 0.71-1.95 x 10-2 h-1 for aged ones, of which co-aging with Fe(III) showed the lowest reduction rate constant among all minerals. The electron mediating capacity of biochar also decreased after aging alone or co-aging with Al, Ca, and kaolinite, while co-aging with Fe(III) and Mn(II) facilitated the electron transfer process, increasing the rate constant by 219.3-1237% due to electron mediation through valence transformation of Fe(III)-Fe(II) and Mn(II)-Mn(III). Given the abundance of soil minerals, it was essential to consider this crucial factor for redox reactions when applying biochar for soil remediation.

Sludge-derived biochar has been considered as a promising sorbent for As(V) due to its high content of Fe. However, role of different endogenous iron minerals and the bulk carbon in this process remained unclear. Here, biochar with different Fe content and species were prepared from two types of sludge (SSA and SSB) at 350 degrees C or 700 degrees C for As(V) immobilization. Although SSB biochar had considerable Fe (3.6%-8.6%), they showed limited sorption capacity for As(V) due to the electrostatic repulsion related to the negatively charged surface at the experimental pH. By contrast, SSA biochar demonstrated superior sorption abilities for As(V) with the maximum capacities of 60.2-90.2 mg g(-1). Amorphous iron determined the removal of As(V) by SSA350 due to the adsorption and the co-precipitation of released Fe2+ with H2AsO4-, which accounted for 77.6% and 22.4%, respectively. For SSA700, iron minerals were converted to more crystalline species, including gamma-FeOOH, Fe-4(O, OH,SO4)(9), and Fe-2(SO3)(3)center dot xH(2)O, and the co-precipitation with As(V) caused 74.6% of As(V) removal by SSA700. The bulk carbon of biochar was proved to serve as heterogeneous nucleation sites for the co-precipitation, showing its synergistic effect with iron on As(V) removal, which raised the removal efficiency from 0.9-2.9% by Fe2+ alone to 13.9-25.5% by Fe2+ co-existing with bulk carbon. Findings of this study not only demonstrate the importance of iron species in biochar to As(V) removal, but also provide new insights on the synergistic effect of iron interaction with bulk carbon.

Iron-modified biochar has been demonstrated as an effective sorbent for the removal of aqueous organic contaminants. However, the effects of different iron species in the iron-biochar composites on the sorption and degradation of organic contaminants remain unclear. In this study, the iron-biochar composites containing different iron species were produced by one-pot pyrolysis of peanut shell with Fe2O3 at 400 degrees C and 700 degrees C, and then were used to remove two model dyes of different properties, methylene blue (MB) and acid orange 7 (AO7). The main Fe species in the biochar composites generated at 400 degrees C were Fe2O3, Fe3O4, FeO, and Fe-0, while Fe3O4, FeO, and Fe-0 were dominant in the biochar composites produced at 700 degrees C. Compared to the pristine biochar, the iron-biochar composites possessed higher ability to remove both MB and AO7 with the removal efficiency increasing from 33%-72% to 48%-92% and from 49%-70% to 72%-85%, respectively. The removal efficiency increased as Fe contents increased and biochar produced at 400 degrees C was more effective in removing MB and AO7 than biochar produced at 700 degrees C. Fe2O3, Fe-0, and Fe3+ facilitated the degradation of MB, while FeO, Fe-0, and Fe3+ had significant positive impact on AO7 degradation. The degradation was more obvious than sorption in MB removal by the biochar composites, while the opposite trend was observed for the removal of AO7. Degradation of MB by iron-biochar composites underwent the aromatic ring cleavage and oxidation, whereas hydroxylation, oxidization and decarboxylation were found in AO7 degradation. This study indicated that the iron-biochar composites were effective in removing the dyes and iron species played important roles in the dye removal.

Biochar could be involved in environmentally relevant redox reactions, and their redox-active moieties may change with pyrolysis temperature. In this study, pyrolysis-temperature depended electron donating and mediating ability of biochar for Cr(VI) reduction were evaluated. All biochar derived from peanut shell at 400-800 degrees C effectively reduced Cr(VI) into Cr(III), and the reduction capability decreased as the pyrolysis temperature increased (400-600 degrees C), and then increased (600-800 degrees C). The electron donating moieties transformed from the-OH functional groups at lower pyrolysis temperature (< 600 degrees C) to the functional groups associated with conjugated structure at higher temperature (> 600 degrees C). Biochar could mediate the reduction of Cr(VI) by lactate, with the reduction rates of Cr(VI) increased up to 16.3 and 345 times that by either biochar or lactate alone, respectively. The redox ability of biochar was the premise, but its conductivity was the dominant factor, for the mediating reduction of Cr(VI). The mediation capability of biochar increased with elevated pyrolysis-temperature due to its increased conductivity related to the growth of conjugated clusters and carbon defects. Our results indicated that pyrolysis-temperature affects the formation of functional groups and conjugated carbon structure of biochars which have a distinguishable influence on the electron donating and mediating ability for Cr(VI) reduction.