Antagonistic effects of humic acid and iron oxyhydroxide grain-coating on biochar nanoparticle transport in saturated sand

From Bulk to Nano: Formation, Features, and Functions of Nano-Black Carbon in Biogeochemical Processes

Globally increasing wildfires and widespread applications of biochar have led to a growing amount of black carbon (BC) entering terrestrial ecosystems. The significance of BC in carbon sequestration, environmental remediation, and the agricultural industry has long been recognized. However, the formation, features, and environmental functions of nanosized BC, which is one of the most active fractions in the BC continuum during global climate change, are poorly understood. This review highlights the formation, surface reactivity (sorption, redox, and heteroaggregation), biotic, and abiotic transformations of nano-BC, and its major differences compared to other fractions of BC and engineered carbon nanomaterials. Potential applications of nano-BC including suspending agent, soil amendment, and nanofertilizer are elucidated based on its unique properties and functions. Future studies are suggested to develop more reliable detection techniques to provide multidimensional information on nano-BC in environmental samples, explore the critical role of nano-BC in promoting soil and planetary health from a one health perspective, and extend the multifield applications of nano-BC with a lower environmental footprint but higher efficiency.

Co-transport of citrate-modified biochar nanoparticles and released plant-available silicon in saturated porous media: Effect of LMWOAs and solution chemistry

Citrate-modified biochar nanoparticles (CBCNPs) represent a promising amendment with plant-available silicon (PASi) releasing capacity. However, the co-transport behavior with released PASi remain poorly understood. This study investigated their co-transport in saturated porous media under various solution chemistry and low molecular weight organic acids (LMWOAs). Experimental and two-site kinetic model results revealed that higher ionic strength caused favorable aggregation and size-selective, hindering CBCNPs transport. Divalent Ca2+ ions retained CBCNPs more effectively than K+ due to stronger charge screening and cation bridging. The pH buffering capacity of CBCNPs resulted in consistent transport behavior across a broad pH range (4-8). XDLVO calculation clarified the impact mechanisms of IS, ion types and pH on CBCNPs transport. Furthermore, LMWOAs exhibited a time-dependent blocking effect on CBCNPs transport. Oxalic acid (OA) and citric acid (CA) facilitated CBCNPs transport though mechanisms beyond XDLVO, including steric hindrance, competitive adsorption, and surface hydrophilicity. The presence of LMWOAs significantly hindered PASi co-transport, with the inhibitory effect ranked as acetic acid (AA) ≈ CA > OA > absence of organic acids. The inhibition is attributed to the blocking effect and formation of Si-organic acid complexes, as evidenced by breakthrough curves and density functional theory calculations. This study provides novel insights into the co-transport of CBCNPs with released PASi through mutual mechanisms, indicating both potential environmental benefits and risks.

A comprehensive review of the "black gold catalysts" in wastewater treatment: Properties, applications and bibliometric analysis

A significant amount of effort has been devoted to the utilization of biochar-based catalysts in the treatment of wastewater. By virtue of its abundant functional groups and high specific surface area, biochar holds significant promise as a catalyst. This article presents a comprehensive systematic review and bibliometric analysis covering the period from 2009 to 2024, focusing on the restoration of wastewater through biochar catalysis. The production, activation, and functionalization techniques employed for biochar are thoroughly examined. In addition, the application of advanced technologies such as advanced oxidation processes (AOPs), catalytic reduction reactions, and biochemically driven processes based on biochar are discussed, with a focus on elucidating the underlying mechanisms and how surface functionalities influence the catalytic performance of biochar. Furthermore, the potential drawbacks of utilizing biochar are also brought to light. To emphasize the progress being made in this research field and provide valuable insights for future researchers, a scientometric analysis was conducted using CiteSpace and VOSviewer software on 595 articles. Hopefully, this review will enhance understanding of the catalytic performance and mechanisms pertaining to biochar-based catalysts in pollutant treatment while providing a perspective and guidelines for future research and development efforts in this area.

Humic acid-mediated transport of a typical soil passivation remediation product (chloropyromorphite) in saturated porous media

Conversion of labile Pb species into chloropyromorphite (CPY) using phosphorus-bearing amendments was considered to be an ideal strategy in soil passivation remediation. However, the fate and transport of CPY in the soil are poorly understood. This study aims to fill the knowledge gap by evaluating the fate and transport of CPY under environmentally relevant conditions of humic acid (HA), pH, electrolyte concentration, and species through the saturated sandy medium. Results showed that bare CPY colloids are basically immobile in sandy porous media while the co-existence of HA made the transport of CPY improved by 30%-93.5%. Facilitated transport of CPY was attributed to the increased stability of CPY and the repulsive interaction between CPY particles and sands due to HA adsorption. The mobility of CPY was also increased with increasing pH from 5.0 to 9.0. When the pH was 9 with a 10 mmol/L NaCl background solution, the stronger energy barrier between CPY and sand led to enhanced transport behavior. The divalent Ca2+ had a more dramatic effect than monovalent Na+ on the aggregation and sedimentation of CPY colloids due to its effectivescreening of the surface charge of CPY and bridging interaction with CPY particles. Derjaguin-Landau-Verwey-Overbeek theory and attachment efficiency calculation indicated that high energy barriers were responsible for the high mobility of CPY colloids, while the retention of CPY in sands was mainly caused by secondary energy minimum and physically straining. The findings of this work can help to evaluate the fate of soil passivation remediation products in natural water and soil.

Dual roles of goethite coating on the transport of plastic nanoparticles in heterogeneous porous media: The significance of collector surface roughness

This study systematically examines the roles of positive goethite on the retention and release of negative plastic nanoparticles (PSNPs) with different surface functional groups (Blank, -COOH, and -NH2). It provides the first evidence for the dual roles of goethite coatings on colloid transport; e.g., increased transport caused by surface morphology modification or decreased transport due to increased surface roughness and charge heterogeneity. Although previous work has shown that goethite-coated sand increases the retention of negative colloids, this work demonstrates that collector surface roughness can also reduce the retention of PSNPs due to increased interaction energy profiles. Nonmonotonic retention of all the different functionalized PSNPs was observed in goethite-coated rough sand, and the magnitude of variations was contingent on the PSNP functionalization, the solution ionic strength (IS), and the goethite coating. The release of PSNPs with IS decrease (phase I) and pH increase (phase II) varied significantly due to differences in energy barriers to detachment, e.g., release in phase I was inhibited in both goethite-coated sands, whereas release in phase II was enhanced in coated smooth sand but completely inhibited in rough sand. The findings of this study provide innovative insight into transport mechanisms for colloidal and colloid-associated contaminants.

Properties of biochar colloids and behaviors in the soil environment: Influencing the migration of heavy metals

Biochar pyrolyzed by biomass shows excellent application prospects for heavy metal (HM) remediation, but a part of biochar can be inevitably broken into micro- and nano-sized biochar colloids (BCs) under biological and physicochemical actions in soil. BCs derived in the process of remediation have rough surface, rich elemental species and contents, and multiple functional groups, which are similar to biochar. However, BCs have some unique colloidal properties because of their micro and nano scale size. Due to these properties, BCs exhibit strong mobilities in the soil environment, and the mobilities may be influenced by a combination of colloidal properties of BCs and environmental factors including soil colloids and other soil environmental conditions. In addition, BCs may have affinity effects on HMs through electrostatic adsorption, ion exchange, surface complexation, precipitation/co-precipitation, and redox because of the properties such as large specific surface area, and rich oxygen-containing functional groups and minerals on the surface. This review summarizes the physicochemical and migratory properties of BCs, and the internal and external factors affecting the migration of BCs in the soil environment, and the possible effects of BCs on HMs are high-lighted. This review provides a theoretical basis for the optimization of soil contaminated with HMs after remediation using biochar. Notably, the innovative idea that BCs may influence the presence of HMs in soil needs to be further confirmed by more targeted detection and analysis methods in future studies to prevent the possible environmental toxicities of the lateral and vertical diffusion of HM caused by BCs in soil.

Organic matters adsorbed on goethite inhibited the heterogeneous aggregation and adsorption of CdSe quantum dots: Experiments and extended DLVO theory

The widespread use of Cd-based quantum dots (Cd-QDs) has led to their inevitable release into the environment, and the prevalent iron oxides and natural organic matter (NOM) are the key factors affecting the environmental behavior and fate of Cd-QDs. However, the impact of NOM adsorbed on iron oxides on the behavior of Cd-QDs with iron oxides and the mechanism of its interaction are not clear. In this study, two kinds of water-soluble QDs (CdSe QDs and core-shell CdSe/ZnS QDs) were selected to study the aggregation and adsorption behavior on goethite (Goe) and goethite-humic acid/fulvic acid composites (Goe-HA/FA). Aggregation kinetics and adsorption experiments between QDs and Goe(-HA/FA), characterization, and extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory calculations indicated that electrostatic interaction was the dominant force for QDs adsorption on Goe(-HA/FA). HA/FA changed the surface charge of Goe and increased the electrostatic repulsion and steric hindrance between the particles, which in turn inhibited the adsorption of QDs on Goe. Besides, unsubstituted aromatic carbons, carboxy carbons, and carbonyl carbons played an important role in the adsorption process, and chemisorption occurred between QDs and Goe(-HA/FA). Our findings are important for better assessing the transport, fate, and potential environmental impacts and risks of Cd-QDs in iron-rich environments.

Transport and retention of functionalized graphene oxide nanoparticles in saturated/unsaturated porous media: Effects of flow velocity, ionic strength and initial particle concentration

The widespread use of nanomaterials has raised the threat of nanoparticles (NPs) infection of soils and groundwater resources. This research aims to investigate three parameters including flow velocity, ionic strength (IS), and initial particle concentration effects on transport behavior and retention mechanism of functionalization form of graphene oxide with polyvinylpyrrolidone (GO-PVP). The transport of GO-PVP was investigated in a laboratory-scale study through saturated/unsaturated (Saturation Degree = 0.91) sand columns. Experiments were conducted on flow velocity from 1.20 to 2.04 cm min-1, initial particle concentration from 10 to 50 mg L-1, and IS of 5-20 mM. The retention of GO-PVP was best described using the one-site kinetic attachment model in HYDRUS-1D, which accounted for the time and depth-dependent retention. According to breakthrough curves (BTCs), the lower transport related to the rate of mass recovery of GO-PVP was obtained by decreasing flow velocity and initial particle concentration and increasing IS through the sand columns. Increasing IS could improve the GO-PVP retention (based on katt and Smax) in saturated/unsaturated media; katt increases from 2.81 × 10-3 to 3.54 × 10-3 s-1 and Smax increases from 0.37 to 0.42 mg g-1 in saturated/unsaturated conditions, respectively. Our findings showed that the increasing retention of GO-PVP through the sand column under unsaturated condition could be recommended for the reduction of nanoparticles danger of ecosystem exposure.

Definitive Review of Nanobiochar

Nanobiochar is an advanced nanosized biochar with enhanced properties and wide applicability for a variety of modern-day applications. Nanobiochar can be developed easily from bulk biochar through top-down approaches including ball-milling, centrifugation, sonication, and hydrothermal synthesis. Nanobiochar can also be modified or engineered to obtain "engineered nanobiochar" or biochar nanocomposites with enhanced properties and applications. Nanobiochar provides many fold enhancements in surface area (0.4-97-times), pore size (0.1-5.3-times), total pore volume (0.5-48.5-times), and surface functionalities over bulk biochars. These enhancements have given increased contaminant sorption in both aqueous and soil media. Further, nanobiochar has also shown catalytic properties and applications in sensors, additive/fillers, targeted drug delivery, enzyme immobilization, polymer production, etc. The advantages and disadvantages of nanobiochar over bulk biochar are summarized herein, in detail. The processes and mechanisms involved in nanobiochar synthesis and contaminants sorption over nanobiochar are summarized. Finally, future directions and recommendations are suggested.

Pore size distribution and Al oxide content significantly regulated the effects of humic acid on perfluorooctanoic acid transport in natural soils

The ubiquity of dissolved organic matter (DOM) makes it encounter the released perfluorooctanoic acid (PFOA) in subsurface environment. However, the effect of DOM (e.g., humic acid, HA) on PFOA transport in soils and the critical influencing factors and mechanisms remain obscure. Column experiments were conducted to explore PFOA transport with the presence of different concentrations of HA in three types of soils and two types of Al oxide coated sand. Results revealed soil properties significantly regulate the effects of HA on PFOA transport, for which pore size distribution, minerals content (e.g., Al oxide) and pH were critical influencing soil-properties. For soil with large mesopore volume, pore blockage caused by HA controlled the effect of HA on PFOA transport. Large mesopore volume significantly alleviated pore blockage of HA, and led to insignificant effects of HA on PFOA transport. For soil exhibited minimum mesopore volume, Al oxide content and pH dominated the effect of HA on PFOA transport. Results from Al oxide coated sand (low mesopore volume) columns further proved that higher Al oxide content and lower pH caused more significant facilitating effect of HA on PFOA transport via site competition. Results highlighted the importance of considering pore size distribution and Al oxide content when assessing PFOA mobility capacity with co-transport with DOM in soils.

Overlooked encounter process that affects physical behaviors of stabilized nanoscale zero-valent iron during in situ groundwater remediation

Dynamic encountering between groundwater matrices and nanoscale zero-valent iron (NZVI) injected for in situ subsurface remediation affects NZVI's mobility and has not been well recognized. Polyacrylic acid (PAA)-stabilized NZVI (NZVI-PAA) and Mg(OH)2-coated NZVI (NZVI@Mg(OH)2) were investigated as representative NZVIs stabilized by enhanced electrostatic repulsion and reduced magnetic attraction, respectively. Encounters with divalent cations and humic acid (HA) induced the drastic aggregation and sedimentation (presedimentation) of NZVI-PAA owing to Lewis acid-base interactions and heteroaggregation. In addition, encountered groundwater electrolytes could not effectively provide electrostatic repulsion for NZVI-PAA, resulting in breakthrough ripening dynamics. The presedimentation and ripening behaviors of NZVI-PAA were eliminated and unheeded after mixing the NZVI slurry with groundwater by sonication. In comparison, the encountering process barely impacted NZVI@Mg(OH)2, for which settling was hindered. Although the particle-collector attraction promoted NZVI@Mg(OH)2 adsorption on pristine and hybrid-coated sands, the Langmuirian blocking dynamics of the NZVI@Mg(OH)2 breakthrough demonstrated its high mobility after adsorption sites of sand surface were exhausted. Extended Derjaguin-Landau-Verwey-Overbeek analysis and transport modeling provided insights into overlooked effects of encountering on physical behaviors of different stabilized NZVIs, which should be considered during practical applications under diverse subsurface conditions.

Effect of reduced inherent organic matter on stability and transport behaviors of black soil colloids

Soil organic matter plays an important role in the stability, transport, and fate of soil colloids. At present, studies have mostly focused on the effects of adding exogenous organic matter on soil colloidal properties, while there is very limited research on the effect of reduced inherent soil organic matter on the environmental behavior of soil colloids. This study investigated the stability and transport behavior of black soil colloids (BSC) and black soil colloids with reduced inherent organic matter (BSC-ROM) under different ionic strength (5, 50 mM) and background solution pH (4.0, 7.0, and 9.0) conditions. Meanwhile, the release behavior of two soil colloids in the saturated sand column under transient ionic strength conditions was also studied. The results showed that both ionic strength reduction and pH increase increased the negative charges of BSC and BSC-ROM, and improved the electrostatic repulsion between soil colloids and grain surface, thereby promoting the stability and mobility of soil colloids. The decrease in inherent organic matter had little effect on the surface charge of soil colloids, suggesting that the electrostatic repulsive force was not the main force affecting the stability and mobility of BSC and BSC-ROM, and reducing inherent organic matter might significantly reduce the stability and mobility of soil colloids by weakening the steric hindrance interaction. The decrease of transient ionic strength reduced the depth of the energy minimum and activated the soil colloids retained on the surface of the grain at three pH conditions. This study is helpful to predict the potential impact of soil organic matter degradation on the fate of black soil colloids in natural environment system.

High-efficient removal of arsenic(III) from wastewater using combined copper ferrite@biochar and persulfate

Arsenic (As) is a potentially toxic element with variable valence states. Due to high toxicity and bioaccumulation, As can pose a severe threat to the quality of the ecology as well as human health. In this work, As(III) in water was effectively removed by biochar-supported copper ferrite magnetic composite with persulfate. The copper ferrite@biochar composite exhibited higher catalytic activity than copper ferrite and biochar. The removal of As(III) could reach 99.8% within 1 h under the conditions of initial As(III) concentration at 10 mg/L, initial pH at 2-6, and equilibrium pH at 10. The maximum adsorption capacity of As(III) by copper ferrite@biochar-persulfate was 88.9 mg/g, achieving superior performance than mostly reported the metal oxide adsorbents. By means of a variety of characterization techniques, it was found that ∙OH acted as the main free radical for removing As(III) in the copper ferrite@biochar-persulfate system and the major mechanisms were oxidation and complexation. As a natural fibre biomass waste-derived adsorbent, ferrite@biochar presented a high catalytic efficiency and easy magnetic separation for As(III) removal. This study highlights the great potential of copper ferrite@biochar-persulfate application in As(III) wastewater treatment.

Insight into the effect of pyrolysis temperature on photoreactivity of biochar-derived dissolved organic matter: Impacts of aromaticity and carbonyl groups

Practical application of biochar may result in more biochar-derived dissolved organic matter (denoted as BDOM) inevitably release into surface waters by infiltration and surface runoff. The photochemical reaction of BDOM has gained intense attention, which played a key role in the fate of organic contaminants. However, the relationships between specific characteristics of BDOM and its photoreactivity are still uncertain. In this study, the characteristics of BDOM pyrolyzed from rice husk derived biochar at different temperature (from 400 °C to 700 °C) and their effect on the photodegradation of oxytetracycline (OTC) were carefully investigated. The ¹³C NMR and EEM results indicated the dominated component of BDOM was gradually turned from humic acid like substances with low aromaticity to high aromaticity with abundant oxygen-containing functional groups as pyrolytic temperature increases. Experimental results showed that the apparent rate constants (k_obs) of BDOM700 (4.53 × 10^-2 min^-1) for OTC photodegradation was approximately one order of magnitude higher than BDOM400 (4.52 × 10^-3 min^-1), which was closely correlated with their aromaticity (R² = 0.944). It was found that ³BDOM* rather than ¹O₂ played the major role in BDOM mediated photodegradation of OTC (80.13 % vs 14.34 %), and the carbonyl-containing group was identified as the main ³BDOM* precursor by NaBH₄ reduction experiment. This work highlighted both aromaticity and carbonyl group contents were critical indicators for assessing the potential to generate ³BDOM* and corresponding photoreactivity.

Co-transport of polystyrene microplastics and kaolinite colloids in goethite-coated quartz sand: Joint effects of heteropolymerization and surface charge modification

This study investigated the transport behavior of polystyrene microplastics (MPs) in saturated quartz sand and goethite-coated sand in the presence of coexisting kaolinite colloids. Column experiments were conducted under a wide range of solution chemistry conditions, including pH levels of 6.0, 7.0, and 9.0, as well as background Na⁺ concentrations of 5 mM and 25 mM. We found that: (1) The individual transport of MPs in porous media diminished both with increasing background ion strength and decreasing pH, and its transport ability was significantly dominated by the interactions between MPs and porous media rather than the interplay between MPs, which has been further corroborated by the aggregation stability experiments of MPs particles. (2) MPs had a much lower ability to move through goethite-coated sand columns than quartz sand columns. This is because goethite coating reduces the repulsion energy barriers between porous media and MPs. The increased specific surface area and surface complexity of sand columns after goethite coating should also account for this difference. (3) MPs transport would be subjected to the differentiated impact of co-transported kaolinite colloids in the two types of porous media. The promotion effect of kaolinite colloid on MPs' transport capacity is not significantly affected by background ionic strength changes when quartz sand is served as the porous medium; however, the promotion effect is highly correlated with the background ionic strength when goethite-coated sand is served as the porous medium. In comparison with low background ionic strength conditions, kaolinite colloids under high background ionic strength conditions significantly facilitated MPs transport. This is mainly because under high background ionic conditions, kaolinite colloids are more likely to be deposited on the surface of goethite-covered sand, competing with MPs for the limited deposition sites. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory is applicable to describe the transport behavior of MPs.

Potential disintegration and transport of biochar in the soil-water environment: A case study towards purple soil

Biochar has been widely applied in soil and water. However, the fate and transport of biochar are not yet fully understood. Here, biochar's disintegration, transport, and the effect of temperature on biochar transport in soil (purple soil)-water systems were investigated. The results showed that the potentially transportable components (PTC) of biochar for corn straw, wheat straw, rice straw, rice husk and wood biochar reached 6.22-7.60%, 5.96-12.29%, 11.77-12.45%, 5.34-6.26% and 5.08-6.14% by mass, respectively. An external force (ultrasound exposure) intensified the physical disintegration, including colloidal and nanoparticles from larger particles, thereby increasing the transport potential. The mass recovery rates of PTC for rice straw biochar after penetrating through soil at 5, 20 and 35 °C reached 44.25%, 32.97% and 10.98%, respectively, which was supported by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory results. Elevated temperatures increased the hydrodynamic average diameter of PTC, and the Zeta potential of PTC and soil at 35 °C were less negative than those at 5 and 20 °C. As a result, biochar's transportability decreases with increasing temperature in the soil-water system, during which the enhanced PTC aggregation and the decreased electrostatic repulsion between biochar and soil particles played a crucial role. The increase in electrical conductivity in the soil-water system may be the main reason for the decrease in electrostatic repulsion at higher temperatures. The findings are helpful for an in-depth understanding of the environmental fate and managing the transport risk of biochar.

Influence of Humic Acids on the Removal of Arsenic and Antimony by Potassium Ferrate

Although the removal ability of potassium ferrate (K₂FeO₄) on aqueous heavy metals has been confirmed by many researchers, little information focuses on the difference between the individual and simultaneous treatment of elements from the same family of the periodic table. In this project, two heavy metals, arsenic (As) and antimony (Sb) were chosen as the target pollutants to investigate the removal ability of K₂FeO₄ and the influence of humic acid (HA) in simulated water and spiked lake water samples. The results showed that the removal efficiencies of both pollutants gradually increased along the Fe/As or Sb mass ratios. The maximum removal rate of As(III) reached 99.5% at a pH of 5.6 and a Fe/As mass ratio of 4.6 when the initial As(III) concentration was 0.5 mg/L; while the maximum was 99.61% for Sb(III) at a pH of 4.5 and Fe/Sb of 22.6 when the initial Sb(III) concentration was 0.5 mg/L. It was found that HA inhibited the removal of individual As or Sb slightly and the removal efficiency of Sb was significantly higher than that of As with or without the addition of K₂FeO₄. For the co-existence system of As and Sb, the removal of As was improved sharply after the addition of K₂FeO₄, higher than Sb; while the latter was slightly better than that of As without K₂FeO₄, probably due to the stronger complexing ability of HA and Sb. X-ray energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD), and X-ray photoelectron spectroscopy (XPS) were used to characterize the precipitated products to reveal the potential removal mechanisms based on the experimental results.

Embedding of biochar in soil mineral fractions: Evidence from benzene polycarboxylic acids molecular biomarkers

Investigators are debating on the positive and negative priming effects of biochar on native soil organic carbon (SOC), which is largely attributed to the technical barrier of identifying biochar contribution to the apparently measured SOC or mineralized CO₂. We combined benzene polycarboxylic acids (BPCAs) molecular biomarkers and soil particle density fractionation to identify biochar contributions to the carbon content in three representative allitic soils in Yunnan. The soil-biochar mixture was incubated for one-month to avoid significant biodegradation of biochar. The results showed that BPCAs were mainly distributed in free light fractions (fLF) up to 87 % of the total BPCAs contents after one month incubation. Recognition of BPCAs in occluded light fractions (oLF) and heavy fractions (HF) suggested a significant interaction between biochar and soil mineral particles. In addition, the percentage of B6CA is comparable or even higher in HF than in fLF or oLF. Thus, biochar-mineral interactions may be an additional stabilization mechanism besides the condensed aromatic structures in biochar. The apparently measured carbon contents increased after biochar application, and both positive and negative priming effects to native SOC were observed after deducting biochar contents based an accurate calculation from BPCAs. The most native SOC depletion (positive priming effects) was noted for the soil with the most favored biochar embedding in soil mineral compositions. This study emphasized that combining BPCAs molecular biomarkers and soil particle density fractionation could accurately quantify different carbon pools, and thus facilitate a comprehensive understanding on the stabilization and turnover of biochar in soils.

Recent progresses, challenges, and opportunities of carbon-based materials applied in heavy metal polluted soil remediation

The application of carbon-based materials (CBMs) for heavy metal polluted soil remediation has gained growing interest due to their versatile properties and excellent remediation performance. Although the progresses on applications of CBMs in removing heavy metal from aqueous solution and their corresponding mechanisms were well known, comprehensive review on applications of CBMs in heavy metal polluted soil remediation were less identified. Therefore, this review provided insights into advanced progresses on utilization of typical CBMs including biochar, activated carbon, graphene, graphene oxide, carbon nanotubes, and carbon black for heavy metal polluted soil remediation. The mechanisms of CBM remediation of heavy metals in soil were summarized, mainly including physical adsorption, precipitation, complexation, electrostatic interaction, and cationic-π coordination. The key factors affecting the remediation effect include soil pH, organic matter, minerals, microorganisms, coexisting ions, moisture, and material size. Disadvantages of CBMs were also included, such as: potential health risks, high cost, and difficulty in achieving co-passivation of anions and cations. This work will contribute to our understanding of current research advances, challenges, and opportunities for CBMs remediation of heavy metal-contaminated soils.

The roles of Fe oxyhydroxide coating and chemical aging in pyrogenic carbon nanoparticle transport in unsaturated porous media

Pyrogenic carbon (PyC) nanoparticles are widespread in the environment, which is important to global carbon cycle. PyC can exist for millions of years and undergo various environmental aging processes. To better understand the roles of Fe oxyhydroxides and water content on the pristine and aged PyC transport, adsorption and column experiments were conducted under three saturations (100%, 70%, and 40%) and three pH (5, 7, and 9) in both clean and Fe oxyhydroxide-coated sand. At high water saturations (100% and 70%), the mobility of both the pristine and aged PyC was enhanced at high pH due to strong electrostatic repulsion, and the aged PyC showed higher mobility than the pristine PyC because of its more negative charge and hydrophilic surface. The coating of Fe oxyhydroxides on sand decreased the mobility of both the pristine and aged PyC due to weak electrostatic repulsion, large specific surface area, and high roughness. At low saturation (40%), solution pH showed little effect on both the pristine and aged PyC mobility, and water saturation became the main factor affecting PyC mobility. Almost no pristine or aged PyC transported out from the Fe oxyhydroxide-coated sand column because Fe oxide increased the roughness of the sand surface, which led to a sharp increase in the air-water-solid interface and retention sites. This study demonstrates that water content, environmental aging, and Fe oxyhydroxides are significant in the fate and transport of PyC nanoparticles in environments, which provides a good fundamental understanding for the assessment of pyrogenic carbon application in environmental protection and carbon sequestration.

Transport behaviors of biochar particles in saturated porous media under DC electric field

The mobility of biochar in saturated quartz sand under a direct current (DC) electric field was investigated by column transport test. The effects of biochar preparation temperature (350 and 550 °C), solution chemistry (pH of 4, 7, and 10, and ion strength of 1, 10, 100 mM) and voltage gradient (0, 0.5 and 1.0 V cm^-1) on the mobility of biochar were explored. It was found that DC electric field could significantly promote the migration of biochar, and the recovery rate of particles could be improved by 0.5-6.1 folds under 0.5 V cm^-1. Higher voltage potential, solution pH and ionic strength were more favorable for biochar migration. The transport of biochar could be well interpreted by deterministic nonequilibrium convection-dispersion equation model. The enhanced mobility caused by DC electric field was attributed to the following reasons: enhanced electromigration following electrostatic attraction from the anode; increasing surface negative charges and functional groups on biochar surface as a result of electrochemical oxidization; reducing size blocking of biochar particles by decreasing particle size. Moreover, the interaction between biochar particles and electrode could alter solution chemistry, in particular, increasing solution pH, which in turn facilitated the transport of biochar. This study provided a perspective to modulate the transport behavior of biochar particle in the soil for the remediation of polluted sites.

Transport of oxytetracycline through saturated porous media: role of surface chemical heterogeneity

The current state of knowledge on the transport behaviors of oxytetracycline (OTC, a typical tetracycline antibiotic) in porous media with heterogeneous chemical surfaces is inadequate. In this work, the mobility properties of OTC through saturated porous media with different chemical heterogeneities (i.e., quartz sand, montmorillonite (MMT)-, humic acid (HA)-, and goethite (Goe)-coated sands) were investigated. In comparison with the mobility of OTC in the quartz sand, HA and goethite coatings inhibited the mobility of OTC, whereas montmorillonite coating enhanced OTC mobility. HA coating inhibited the transport of OTC that stemmed from the strong interactions between HA and OTC via complexation, π-π stacking, hydrogen bonding, and hydrophobic interaction. The positively charged iron oxide coating on Goe-coated sand provided favorable sites for OTC deposition through complexation and electrostatic attraction. The enhanced transport of OTC through MMT-coated sand was mainly due to the strong electrostatic repulsion between the anionic OTC species (i.e., OTC^-) and negatively charged porous media. Solution pH (5.0-9.0) posed a negligible effect on the trend of OTC mobility in different porous media. Furthermore, Ca²⁺ inhibited the transport of OTC mobility through various porous media via cation-bridging. The findings of this work contribute significantly to our understanding of the influence of aquifer surface chemical heterogeneities on OTC mobility behaviors in the subsurface environment.

Influence of dissolved organic matter, kaolinite, and iron oxides on aggregation and transport of biochar colloids in aqueous and soil environments

The aggregation and transport of biochar colloids (BCs) in the soil and groundwater are critical for applying biochar in the field and assessing long-term environmental risk. This research aimed to study the influence of dissolved organic matter (DOM) with different molecular weights (including humic acid, HA; bovine serum albumin, BSA; deoxyribonucleic acid, DNA) and three minerals (including kaolinite, goethite, and hematite) on the aggregation and transport behaviors of BCs. The adsorption of DOM on the surface of BCs increased the stability, inhibited aggregation, and promoted the transport of BCs. As the molecular weight of DOM increased, the thicknesses of the adsorption layer of HA, BSA, and DNA on BCs surface were 2.2 nm, 5.3 nm, and 5.6 nm, respectively, resulting in increasing steric hindrance and improving the stability and mobility of BCs. Kaolinite also significantly enhanced the stability and mobility of BCs by increasing the electrostatic repulsion. Goethite and hematite quickly combined with BCs through electrostatic attraction, resulting in stronger aggregation and retention of BCs. Compared to hematite, goethite provided more adsorption sites for BCs due to its needle-like shape, so goethite caused a larger heteroaggregation rate. Overall, the presence of DOM with different molecular weights and the minerals with varying surface charges in the soil environment had a significant and distinct impact on the stability, aggregation, and transport of BCs, which advances the knowledge of colloidal biochar fate in the soil and groundwater.

Enhancing effects of dissolved and media surface-bound organic matter on titanium dioxide nanoparticles transport in iron oxide-coated porous media under acidic conditions

Natural organic matter (NOM) and iron oxides have been proved to be crucial factors controlling the behaviors of nanoparticles in heterogenous environment. Here, we conducted experimental and modeling study on the transport of titanium dioxide nanoparticles (TiO₂ NPs) in iron oxide-coated quartz in the presence of NOM under acidic conditions. Results showed the antagonistic effects of iron oxides and NOM on TiO₂ NPs mobility. The inhibition of iron oxides coated on quartz was crystal form-dependent other than quantity-dependent. Amorphous ferric oxyhydroxide with higher specific surface area brought more positive charge and favorable deposition sites onto quartz, and induced more retention of nanoparticles than two crystalline iron oxides, goethite and hematite. Dissolved organic matter (DOM) facilitated TiO₂ NPs transport in iron oxide-coated quartz. In comparation with the limited enhancing effects of DOM, the NOM coatings on media surface partially or largely offset the inhibition of goethite on nanoparticles mobility through direct occupation of attachment sites and sites screening due to the steric repulsion of the macromolecules. Owing to the higher steric hindrance, humic acid, both in dissolved and media surface-bound states, exerted stronger facilitating effects on TiO₂ NPs mobility relative to fulvic acid.

Nano-biochar: A novel solution for sustainable agriculture and environmental remediation

Currently, the applications of biochar (BC) in agricultural practices and for environmental remediation purposes have demonstrated multifaceted advantages despite a few limitations. Nano-BC offers considerable opportunities especially for the remediation of hazardous contaminants as well as the improvement of crop productivity. Positive outcomes of nano-BC on soil physico-chemical and biological characteristics have indicated its suitability for agricultural applications. Nano-BC may effectively regulate the mobilization and sorption of important micro- and macro-nutrients, along with the hazardous contaminants including potentially toxic metals, pesticides, etc. Additionally, the sorption characteristics of nano-BC depends substantially on feedstock materials and pyrolysis temperatures. Nevertheless, the conducted investigations regarding nano-BC are in infant stages, requiring extensive field investigations. The nano-enhanced properties of BC on one hand dramatically improve its effectiveness and sustainability, on the other hand, there may be associated with toxicity development in diverse aquatic and/or terrestrial environments. Therefore, risk assessment on soil organisms and its indirect impact on human health is another area of concern linked with the field application of nano-BC. The present review delineates the potentiality of nano-BC as an emerging sorbent for sustainable agriculture and environmental applications.

Aging significantly increases the interaction between polystyrene nanoplastic and minerals

With the massive use and discarding of plastic products, plastic particles, including nanoplastics (NPs), which are continuously released under the action of environmental factors, are posing greater risk to the ecosystem and human health. NPs exposed to the environment experience aging, which can significantly change their physical and chemical properties and affect their environmental behavior. Here, we examined the adsorption behavior of polystyrene nanoplastic (PSNP) aging by ultraviolet (UV) exposure on different minerals (goethite, magnetite, kaolinite and montmorillonite). Aging not only changes the surface morphology of PSNP, but also increases the surface negative charge and produces a large number of oxygen-containing functional groups (OFGs). Incubation of aged PSNP with minerals indicated that iron oxides (goethite and magnetite) showed stronger interactions with aged PSNP than pristine PSNP, and there was an interaction between clay minerals and aged PSNP. The adsorption experiments and scanning electron microscopy (SEM) suggested that the higher adsorption capacity of a mineral surface to aged PSNP may be related to electrostatic attraction and ligand exchange. The Fourier transform infrared (FTIR) spectra after adsorption showed that the adsorption affinity between the functional groups was different, and two-dimensional correlation spectroscopy (2D-COS) analysis further indicated that the mineral preferentially adsorbed the aged PSNP in accordance with the order of OFGs. The findings provide a theoretical basis for scientific evaluation of ecological risks of NPs in the environment.

Transport of silver nanoparticles coated with polyvinylpyrrolidone of various molecular sizes in porous media: Interplay of polymeric coatings and chemically heterogeneous surfaces

Silver nanoparticles (AgNPs) are usually capped with stabilizing agents to protect their activities and improve stability. Polyvinylpyrrolidone (PVP) is one of the most used capping agents of AgNPs, and may affect the transport of AgNPs in porous media. The transport and retention of AgNPs capped with PVPs of different molecular weights (PVP10-AgNP, PVP40-AgNP and PVP360-AgNP) in uncoated, and humic acid (HA)-, kaolinite (KL)- and ferrihydrite (FH)-coated sand porous media were investigated. Among the three AgNPs, PVP360-AgNP exhibited the highest mobility and eluted from all types of porous media. This is because PVPs of higher molecular weight provided stronger steric effect and electrostatic repulsive forces among PVP-AgNPs, inducing stronger blocking and shadow effects. The transport of the PVP-AgNPs increased in the HA-Sand columns, while decreased in the KL- and FH-Sand columns, especially for PVP10-AgNP and PVP40-AgNP. The simulation results using one-site kinetic model indicated that HA-Sand reduced the maximum retention capacity (Smax), while KL- and FH-Sand increased the Smax as well as the first-order attachment rate coefficients (katt), particularly at high ionic strength. The results shed light on the interplay of the capping agents of AgNPs and the surface heterogeneity on the transport of AgNPs in porous media.

Co-transport of U(VI) and colloidal biochar in quartz sand heterogeneous media

Biochar has attracted much attention for remediating the sites contaminated with heavy metals and radionuclides due to its low cost and high adsorption affinity. However, little is known about how colloidal biochar influences U(VI) transport in the environment. In this study, column experiments were conducted to investigate the individual and co-transport of U(VI) and biochar colloids (BC) in quartz sand heterogeneous media. Results showed that the transport of U(VI) in the individual transport system was pH-dependent and insensitive to ionic strength, whereas the individual BC transport was more sensitive to the changes in ionic strength compared to those in pH, indicating that electrostatic interaction plays a major role during BC transport but chemical interaction dominates U(VI) transport. In the presence of BC, the transport of U(VI) was significantly facilitated because of U(VI) adsorption on BC. The existence of low concentration of U(VI) (2.5 × 10^-6 M), however, did not affect the breakthrough curves (BTCs) of BC, except for the co-transport at relatively high ionic strength (100 mM) where BC transport was impeded due to the decrease of colloid suspension stability. Colloid size exclusion effect was evidenced by the evolution of particle size and zeta potential of the effluents. The transport of BC in both the individual and co-transport systems could be described by a two-site kinetic attachment/detachment model. This work implies that a risk assessment of BC facilitated heavy metal transport should be carefully considered when biochar is applied to the remediation of heavy metal contaminated sites.

Strengthened erosion resistance of compacted bentonite by layered double hydroxide: A new electrostatic interaction-based approach

For the geological repository of high-level radioactive waste (HLW) built in granitic host rock，the control of buffer material (compacted bentonite) erosion and subsequent loss caused by groundwater in granite fissures is an unresolved problem of major concern. We propose here new insight into enhancing the erosion resistance of compacted bentonite by means of its electrostatic interaction with oppositely-charged layered double hydroxide (LDH). The interaction between bentonite and LDH was studied by dropwise addition of colloidal LDH into colloidal bentonite suspension, during which the variation in electrical conductivity, zeta potential and particle size proved a strong interaction between these two materials. Interestingly, in addition to their aggregation, intercalated structures of LDH and montmorillonite were found in the composite (BEN@LDH) by a combined characterization of X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM), and were confirmed by density functional theory (DFT) calculation. Colloid generation of compacted BEN@LDH under ultrasonic conditions is negligible comparing with that of compacted bentonite, indicating a significantly higher erosion resistance. Besides, a small amount of LDH by mechanically mixing with bentonite (mass ratio 1:99) can also effectively improve the erosion resistance of compacted bentonite. Moreover, BEN@LDH displayed stronger retention performance towards U(VI) and Se(IV) than bentonite under near-neutral/weakly alkaline conditions. Our results indicate that LDH is a promising additive in compacted bentonite, and this approach may be extended to common geotechnical structures built with clays and soils.

Systematic Research on the Transport of Ball-Milled Biochar in Saturated Porous Media: Effect of Humic Acid, Ionic Strength, and Cation Types

Ball-milled biochar (BMBC) is a typical engineering material that has promising application prospects in remediating contaminated soil and water. It is fundamental to rate the transport behaviors of BMBC in the underground environment before extensive use. In this study, the effects of the ubiquitous cations (Na+, Mg2+, and Al3+) and model organic matter (humic acid) on the transport of BMBC were investigated using laboratory column experiments. The results demonstrated the facilitated effect of HA on the transport of BMBC due to the negatively charged surface and steric effect under neutral conditions. HA and ionic strength manifested an antagonistic effect on the transport of BMBC, where the presence of one could weaken the effect from the other. We also found the charge reversal of the BMBC surface in the presence of Mg2+, thus enhancing the deposition of BMBC onto the medium surface. On the other hand, the charge reversal from Al3+-coupled acid conditions led to the restabilization and transport of BMBC in porous media. Therefore, the rational usage of BMBC is indispensable and more attention should be paid to the composition and change in underground water that might facilitate the transport of BMBC and thus lead to negative environmental implications.

Enhanced sequestration of molybdenum(VI) using composite constructed wetlands and responses of microbial communities

The molybdenum (Mo) non-point source pollution in the mining area has an irreversible impact on the surrounding water and soil ecosystems. Herein, three integrated vertical subsurface flow constructed wetlands (CWs) were constructed to assess the effects of combination substrates and plant on the removal of Mo(VI). Results showed that CW1 with combination substrates and cattail exhibited a favorable removal performance for Mo(VI) at 80.90%. Moreover, most Mo(VI) retained in the CWs was retained in the substrate (58.13-88.04%), and the largest fraction of Mo(VI) retained was the water-soluble fraction on the surface of the combination substrates. Mo(VI) removal was also influenced by the microbial community composition in substrate, especially their co-occurrence networks. The species that showed significant positive correlation with Mo(VI) removal were Planctomycetes, Latescibacteria, Armatimonadetes, and Gemmatimonadetes. Moreover, CWs added plants showed that more co-occurrences interaction between taxa occurs, which means that the wetlands efficiently select recruitment of potential microbial consortia and change the co-occurrences to remove pollution in the substrate. These results could be useful in providing an ecology-based solution for the treatment of Mo(VI) in wastewater, especially in adjusting the microbial communities for Mo(VI) removal at the genetic level.

Biochar nanoparticles with different pyrolysis temperatures mediate cadmium transport in water-saturated soils: Effects of ionic strength and humic acid

Biochar is advocated as an environment-friendly and cost-effective material for removing both heavy metals and organic contaminants in soil remediation. However, our understandings on the cotransport potential of contaminants with the nanoscale biochar downward along soil profiles (e.g., potential environmental risks towards groundwater) remain largely unknown. This study investigated the effects of wheat straw-derived biochar nanoparticles pyrolyzed at 350 °C and 500 °C (BNP350 and BNP500) on the transport of cadmium (Cd(II)) in water-saturated soil packed columns. Different ionic strengths (ISs) without/with humic acid (HA) were tested to mimic the scenarios during soil remediation. BNPs could act as a vehicle mediating Cd(II) transport in soils. At a low IS (1.0 mM KCl), compared to the limited transport of individual Cd(II), BNP500 enhanced (69 times) Cd(II) transport (Cd(II) mass recovery (M) = 7.59%) in soils, which was greater than that by BNP350 (54 times, M = 5.92%), likely due to the higher adsorption of Cd(II) onto BNP500. HA further increased the Cd(II) transport by BNPs (M = 8.40% for BNP350 and M = 11.95% for BNP500), which was mainly due to the increased mobility of BNPs carrying more absorbed Cd(II). In contrast, at a high IS (10 mM KCl), BNP500 dramatically inhibited the transport of Cd(II) (M = 12.9%), decreasing by about 61.6%, compared to the BNPs absence (M = 33.6%). This is because a large amount of BNP500-Cd(II) was retained in soils at a high IS. This inhibition effect of Cd(II) transport by BNPs was reinforced with the presence of HA. Our findings suggest that the pyrolysis temperature of biochar should be carefully considered when applying biochar for in-situ remediation of soils contaminated by heavy metals such as Cd(II) under various organic matter and IS conditions.

Investigating the effects of biochar colloids and nanoparticles on cucumber early seedlings

Understanding about the influence of biochar colloidal and nanoscale particles on plant is limited. We therefore extracted the colloids and nanoparticles from hot pepper stalk biochar (CB600 and NB600), and examined physiological responses of cucumber early seedlings through hydroponic culture and pot experiment. CB600 had no significant effect on shoot at 500 mg/L, while it decreased root biomass and inhibited lateral root development. The biomass and root length, area, and tip number dramatically reduced after 500 mg/L NB600 treatment. Water content of NB600-exposed shoot was lower, suggesting water uptake and transfer might be hindered. For resisting exposure stress, root hair number and length increased. Even, the study observed swelling and hyperplasia of root hairs after direct exposure of CB600 and NB600. These adverse effects might be associated with the contact and adhesion of CB600 and NB600 with sharp edges to root surface. For a low concentration of 50 mg/L, NB600 did not influence cucumber early seedlings. In soil, CB600 and NB600 did not cause inhibitory effect at relatively high contents of 500 mg/kg and 2000 mg/kg. This study provides useful information for understanding phytotoxicity and environmental risk of biochar colloids and nanoparticles, which has significant implications with regard to biochar application safety.

Potential hazards of biochar: The negative environmental impacts of biochar applications

Biochar has been widely used as an environmentally friendly material for soil improvement and remediation, water pollution control, greenhouse gas emission reduction, and other purposes because of its characteristics such as a large surface area, porous structure, and abundant surface O-containing functional groups. However, some surface properties (i.e., (i) some surface properties (i.e., organic functional groups and inorganic components), (ii) changes in pH), and (iii) chemical reactions (e.g., aromatic C ring oxidation) that occur between biochar and the application environment may result in the release of harmful components. In this study, biochars with a potential risk to the environment were classified according to their harmful components, surface properties, structure, and particle size, and the potential negative environmental effects of these biochars and the mechanisms inducing these negative effects were reviewed. This article presents a comprehensive overview of the negative environmental impacts of biochar on soil, water, and atmospheric environments. It also summarizes various technical methods of environment-related risk detection and evaluation of biochar application, thereby providing a baseline reference and guiding significance for future biochar selection and toxicity detection, evaluation, and avoidance.

Synthesis of montmorillonite-supported nano-zero-valent iron via green tea extract: Enhanced transport and application for hexavalent chromium removal from water and soil

The nano-zero-valent iron composite (nZVI@TP-Mont) was successfully prepared using a low-cost and environmental-friendly green synthesis via tea leaves extract (tea polyphenols, TPs) and the montmorillonite (Mont). The batch and column experiments and characterization were conducted to investigate the transport behavior and Cr(VI) remediation by nZVI@TP-Mont in water/soil. Due to its particular surface characteristics and morphology (i.e., the Fe⁰ core wrapped by TPs, the doped sulfur, and interlayer structure), the nZVI@TP-Mont composite showed a great removal capacity of Cr(VI) and sufficient mobility under different soil conditions. We opine the increase in the Cr(VI) reduction of nZVI@TP-Mont was attributed to the tethering of Fe₂O₃ on the surface of Fe⁰ core by the support of Mont interlayer, especially the TP-coverage around nZVI@TP-Mont surface unwrapped, thereby increasing the regenerated reactive Fe²⁺ and the exposed reaction sites of Fe⁰ cores to Cr(VI). The increased transportability of nZVI@TP-Mont slightly depends on the heterogeneous soil properties (i.e., ionic strength, sand/soil ratio, and pH). The two-site kinetic attachment model fitting results suggest Cr(VI)/Cr(III) speciation associated with the agglomerated nZVI@TP-Mont were efficiently immobilized in soil. Therefore, this study would benefit the efficient application of the green-synthesized nZVI@TP-Mont in in-situ remediation of soils contaminated by Cr(VI).

Comparison of the effects of large-grained and nano-sized biochar, ferrihydrite, and complexes thereof on Cd and As in a contaminated soil-plant system

Cd and As are difficult to co-remediate in co-contaminated soils. In this study, remediation materials comprising large-grained and nano-sized biochar (BC), ferrihydrite (FH), and complexes thereof were added to Cd- and As-contaminated soil. The uptake of Cd and As by pak choi (Brassica chinensis L.) was then evaluated using a pot experiment and the Cd and As concentrations of the soil pore water and leaching water were measured. The Cd and As concentrations of the pore and leaching water were slightly increased with the addition of BC, and decreased with addition of FH and the biochar-ferrihydrite complex (BC-FH). However, nano-sized BC (BC_N), FH (FH_N), and BC-FH (BC-FH_N) had little influence on the decreases in Cd and As of the two monitored water types. Large-grained remediation materials, rather than nanomaterials, decreased the Cd and As concentrations of the two monitored water types. Nonetheless, nanomaterial treatments more effectively decreased the Cd and As concentrations in plants by an average of >10% relative to the large-grained treatments. The DLVO theory analysis suggested that BC_N, FH_N, and BC-FH_N, immobilized in the topsoil, adsorbed heavy metals in the rhizosphere soil. The remainder of the nano-sized materials was dispersed in the rhizosphere soil pores, shielding the uptake of Cd and As by the roots. Although the doses of nanomaterials used in this study were less than one-fortieth of those of the large-grained materials, changes in the plant rhizosphere microenvironment caused by the nanomaterials decreased the risk of toxicity transfer from the soil to the plants.

Co-transport of ball-milled biochar and Cd2+ in saturated porous media

The combination of ball milling technology and biochar materials provides new prospects for environmentally friendly and sustainable environmental pollution control technologies, but comes with opportunities and risks. In this study, column experiments were used to evaluate the environmental behavior of ball‑milled biochar (BMBC). The results of the column experiments showed that BMBC transport increased with a high flow velocity, large medium size, high pH, and low ionic strength. Owing to the strong adsorption of Cd²⁺ by BMBC, the presence of BMBC in the medium led to a decrease in effluent Cd²⁺. The presence of Cd²⁺ in the solution slightly inhibited the transport of BMBC. The transport of Cd²⁺ was facilitated by BMBC due to the high affinity. Therefore, attention should be paid to favorable conditions for BMBC transport. This study provides a perspective to assess the behavior of BMBC in the environment and whether its interaction with Cd²⁺ will introduce new environmental hazards.

Evolution of redox activity of biochar during interaction with soil minerals: Effect on the electron donating and mediating capacities for Cr(VI) reduction

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. β-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 × 10^-2 h^-1 for original biochars to 0.71-1.95 × 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.

Different electrically charged proteins result in diverse transport behaviors of plastic particles with different surface charge in quartz sand

The influence of proteins on the transport and deposition behaviors of microplastics (MPs) in quartz sand was examined at both low (5 mM) and high ionic strength (25 mM) in NaCl solutions at pH 6. Carboxylate- and amine-modified polystyrene latex microspheres with size of 200 nm were employed as negatively (CMPs) and positively surface charged MPs (AMPs), respectively, while bovine serum albumin (BSA) and bovine trypsin were utilized as representative negatively and positively charged proteins, respectively. The results showed that for two examined protein concentrations (both 1 and 10 mg/L TOC) under both ionic strength conditions, the presence of BSA increased the transport of both CMPs and AMPs, while the presence of trypsin decreased the transport of CMPs yet increased the transport of AMPs in porous media. The mechanisms driving to the changed transport of MPs induced by two types of proteins were found to be different. Particularly, steric interaction induced by BSA corona adsorbed onto CMPs surface as well as the repel effects resulted from BSA suspending in solutions were found to contribute to the enhanced CMPs transport with BSA copresent in suspensions. The increased sizes and the decreased electrostatic repulsion of CMPs due to the adsorption of trypsin onto CMPs, together with the addition of extra deposition sites due to the adsorption of trypsin onto quartz sand drove to the decreased CMPs transport with trypsin copresent in suspensions. The increased electrostatic repulsion due to the adsorption of BSA onto AMPs surfaces caused the enhanced AMPs transport with BSA in solutions. While, the decreased electrostatic attraction of AMPs due to the adsorption of trypsin onto AMPs, as well as the competition of deposition sites due to the adsorption of trypsin onto quartz sand contributed to the increased AMPs transport with trypsin copresent in suspensions. The results showed that the presence of different types of proteins would induce different transport behaviors of microplastics with different surface charge in porous media. Since proteins are widely present in aquatic systems, to more accurately predict the fate and transport of MPs in natural environments, the effects and mechanisms of proteins on the transport of MPs should be considered.

Enhanced Cr(VI) removal from water using a green synthesized nanocrystalline chlorapatite: Physicochemical interpretations and fixed-bed column mathematical model study

Apatite-based minerals have attracted much attention in the remediation of heavy metal-contaminated environment. However, exploring apatite with efficient adsorption performance for inorganic oxyanions such as Cr(VI) remains a challenge. Herein, a novel nanocrystalline chlorapatite (ClAP) was promptly prepared by a green method using eggshell wastes as calcium source with the purpose to enhance Cr(VI) adsorption capability. The generated ClAP was characterized by XRD, SEM-EPMA, TEM, FTIR, and BET analyses. Batch and column experiments were subsequently carried out to explore the influencing factors, adsorption capacity and removal mechanism. Results showed that ClAP exhibited excellent stability and adsorption performance for Cr(VI) (63.47 mg g^-1), which was much greater than that of hydroxyapatite and most reported materials. The adsorption process was fitted well by the pseudo-second-order model and the Langmuir model. In fixed bed column experiments, a novel time-fractional derivative model exhibited much better suitability in interpreting the observed breakthrough curves of Cr(VI) than traditional models. Furthermore, the reusability of ClAP in column was evaluated. Results showed that the adsorption capacity maintained well after consecutively reused for five cycles. Studies of the effect of pH, as well as FTIR and XPS investigations indicated that Cr(VI) adsorption was mainly ascribed to electrostatic interactions and surface complexation, while the reduction of Cr(VI) to the low-toxicity Cr(III) also existed in the adsorption process. The ClAP adsorbent was also successfully used for Cr(VI) remediation from real wastewater. Hence, nanocrystalline ClAP can be a promising material for enhancing the elimination of oxyanion contaminants such as Cr(VI) from water.

Co-transport of biochar colloids with organic contaminants in soil column

Co-transport of biochar (BC) colloids with coexisting organic contaminants (OCs) in soil involves complex interactions among BC colloids, OCs, and soil particles, which is significant for the environmental application and risk assessment of BC and yet has not been well addressed. This study explored co-transports of three typical OCs (i.e., phenanthrene (PHN), atrazine (ATZ), and oxytetracycline (OTC)) and BC colloids obtained from bulk BCs with different charring temperatures (200-700 °C) and particle sizes (250 nm, 500 nm, and 1 μm) in a soil column of 9 cm in height. Considerable transport of BC colloids alone was observed and the maximum breakthrough concentration (C/C_o) increased from 0.08 to 0.77 as the charring temperature decreased from 700 to 200 °C. The mobilities of PHN, OTC, and ATZ alone were very low but were greatly increased by co-transports with BC colloids, and their maximum C/C_o values were within 0.05-0.33, 0.03-0.44, and 0.05-0.62, respectively, in the absence and presence of various BC colloids. The enhancement effect of BC colloids on the OC transport decreased with increasing charring temperature or particle size of BC colloids. BC colloids mainly acted as a vehicle to facilitate the transport of OCs, and dissolved organic carbon from BC colloids also contributed to the increased mobility of OCs in dissolved form. These findings provide new insights into co-transport of BC colloids and contaminants in soil.

Sulfide-induced reduction of nitrobenzene mediated by different size fractions of rice straw-derived black carbon: A key role played by reactive polysulfide species

Here we investigated the mediation efficiency of different size fractions of rice straw-derived black carbon (BC) using sulfide-induced nitrobenzene reduction as a model system. The bulk BC was divided into three size fractions: dissolved BC (size <0.45 μm), colloidal BC (0.45 μm < size < 1 μm), and particulate BC (size > 1 μm). With the presence of BC fractions (250 mg/L) nitrobenzene reduction by Na₂S was significantly facilitated, wherein the mediation efficiency was positively correlated with the BC fraction's oxygen group content in an order of particulate BC < colloidal BC ≪ dissolved BC. Consistently, the oxidation treatment of particulate BC with O₃ or HNO₃ improved the mediation efficiency, whereas the reduction treatment with NaBH₄ reduced the mediation efficiency. The supernatant collected with dialysis or filtration of suspension of BC materials pre-reacted with Na₂S could effectively reduce nitrobenzene, suggesting that reactive reducing sulfur species were produced in aqueous solutions by reacting sulfide only with BC materials. This was evidenced by the fact that polysulfides and polysulfide radicals were both detected in the supernatant. As demonstrated by electron paramagnetic resonance analysis, the quinone moieties at the surface of BC materials accepted electrons from sulfide and turned into semiquinone free radicals, consequently leading to formation of reactive reducing sulfur species and thus enhanced nitrobenzene reduction. The strong mediation efficiency on redox reactions observed for colloidal BC and dissolved BC combined with their significant mobility in subsurface environments indicate that these carbonaceous materials may play an important role in the fate process of organic contaminants as both carriers and catalysts. CAPSULE: The surface quinone moieties of BC induce the formation of reactive reducing sulfur species by acting as one-electron acceptors and facilitate nitrobenzene reduction by sulfide.

The effect of particle size of bamboo biochar on the phytoremediation of Salix psammophila C. to multi-metal polluted soil

Biochar shows great potential in soil remediation. The benefits of biochar on soil depend onits intrinsic properties and soil characteristics. However, the influence of particle sizes of biochar on soil remediation is not clear. In a pot experiment, we evaluated the effects of bamboo biochar (BBC) particle sizes (P1 < 0.15 mm, 0.15 mm < P2 < 0.25 mm, 0.25 mm < P3 < 0.50 mm) on phytoremediation efficiency of Salix psammophila C. cultivated in multi-metal polluted soil. We added the BBC at 3% (w/w) in tested soil. Next, the BBC was thoroughly mixed with soil and weighting to the pot, and S. psammophila cuttings were planted and grown for six months in the amended soil under model growth condition.Results revealed the addition of different sizes of BBC particles affected soil quality, plant growth, and HMs accumulation in plants. All sizes of BBC treatments improved Cd and Zn accumulation, whereas plants in P2 treatment showed the greatest accumulation, increased by 52.41 and 25.55% compared with the control (1,503 and 19,928 μg·plant^-1). Overall, the results indicated BBC enhanced the phytoremediation efficiency of S. psammophila. Plants cultivated in P2 treatment showed the most significant effect on remediating contaminated soil.

Surface properties and suspension stability of low-temperature pyrolyzed biochar nanoparticles: Effects of solution chemistry and feedstock sources

Intensive application of biochar requires better understanding of their environmental behaviors such as stability, fate, and mobility. The release of bulk biochar into biochar nanoparticles (NPs) may bring risks because of their potential flowing into downstream water bodies with nutrients/containments attached. Low-temperature pyrolyzed biochars, namely fruit tree branch biochar of 350/450/550 °C (FB350, FB450 and FB550), corn straw biochar of 350 °C (CB350) and peanut straw biochar of 350 °C (PB350), were produced, and their NPs were extracted. The yield, elemental composition, mineral composition, surface functional groups and zeta potential of biochar NPs were characterized. Subsequently their suspension stability was evaluated in NaCl and CaCl₂ solutions by dynamic light scattering technique. The Hamaker constants and particle interaction energy of the biochar NPs were calculated by adopting Derjaguin-Landau-Verwey-Overbeek theory. For biochar NPs of same feedstock, the stability of FB350/450/550-NPs could be predicted well by their zeta potential values. The types of their surface functional groups were the same while their adsorption intensity differed. The scenarios for biochar NPs of different feedstock sources were different, that is, inconsistent variation was observed between their zeta potential and suspension stability, which were rooted in the variable type and quantity of surface functional groups. In conclusion, feedstock was the most significant factor that influenced the suspension stability of biochar NPs, followed by the pyrolysis temperature and solution chemistry, which were highly dependent on surface potential. The findings provide references for the environmental risk evaluation of biochar NPs and reasonable application of biochar in field.

Surface heterogeneity mediated transport of hydrochar nanoparticles in heterogeneous porous media

The effects of clay particles (montmorillonite, M) and phosphate (P) on the transport of hydrochar nanoparticles (NPs) in water-saturated porous media (uncoated and aluminum (Al) oxide-coated sands) were explored in NaCl (1-50 mM) solutions. Our results showed that the deposition behaviors of hydrochar NPs affected by M and phosphate were significantly different between pH 6.0 and pH 9.0, especially in Al oxide-coated sand. This can be attributed to their distinct surface characteristics: hydrochar agglomerates with a larger pore size distribution, more carboxylate groups, and less negative charges on the surface at pH 9.0 than those at pH 6.0. In Al oxide-coated sand, block adsorption of hydrochar was alleviated appreciably with the presence of M due to the preferential preoccupies of M on these favorable retention sites. On the contrary, M substantially increased the hydrochar retention on uncoated sand due to the formation of nanoaggregates between hydrochar and M. Differently, phosphate substantially enhanced the transport of hydrochar, even in coated sand, due to the strong phosphate adsorption onto Al oxide on the surface of sand and hydrochar. Our findings will provide useful insights into designing effective strategies for land application of hydrochar while minimizing potential environmental risks. Graphical abstract.

Sulfide reduction can significantly enhance transport of biochar fine particles in saturated porous medium

The release of fine particles from biochar materials applied in the environment may have important environmental implications, such as mobilization of environmental contaminants. In natural environments biochar fine particles can undergo various transformation processes, which may change their surface chemistry and consequently, the mobility of the particles. Here, we show that sulfide reduction can significantly alter the transport of wheat-straw- and pine-wood-derived biochar fine particles in saturated porous media. Counterintuitively, the sulfide-reduced biochar particles exhibited greater mobility in artificial groundwater than their non-reduced counterparts, even though reduction led to decrease of surface charge negativity and increase of hydrophobicity (from the removal of surface O-functional groups), both should favor particle deposition, as predicted based on extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. Using transport experiments conducted in single-cation background solutions containing K⁺, Mg²⁺ or Ca²⁺ under different pH conditions, we show that the surprisingly greater mobility of sulfide-reduced biochar particles was attributable to the removal of surface carboxyl groups during reduction, as this markedly alleviated particle deposition through cation bridging, wherein Ca²⁺ acted as the bridging agent in linking the surface O-functional groups of biochar particles and quartz sand. These findings show the critical roles of surface properties in dictating the mobility of biochar fine particles and call for further understanding of their transport properties, which apparently cannot be simply extrapolated based on the findings of other (engineered) carbonaceous nanomaterials.