Cotransport and deposition of biochar with different sized-plastic particles in saturated porous media

Cotransport of 6PPD-Q and pristine/aged microplastics in porous media: An insight based on transport forms and mechanisms

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.

Influence of macromolecules and electrolytes on heteroaggregation kinetics of polystyrene nanoplastics and goethite nanoparticles in aquatic environments

Fate and transport of nanoplastics in aquatic environments are affected by their heteroaggregation with minerals in the presence of macromolecules. This study investigated the heteroaggregation of polystyrene nanoplastics (PSNPs) with goethite nanoparticles (GNPs) under the influence of macromolecules [humic acid (HA), bovine serum albumin (BSA), and DNA] and electrolytes. Under 1 mg C/L macromolecule, raising electrolyte concentration promoted heteroaggregation via charge screening, except that calcium bridging with HA also enhanced heteroaggregation at CaCl2 concentration above 5 mM. At all NaCl concentrations and CaCl2 concentration below 5 mM, 1 mg C/L macromolecules strongly retarded heteroaggregation, ranking BSA > DNA > HA. Raising macromolecule concentration strengthened such stabilization effect of all macromolecules in NaCl solution and that of DNA and BSA in CaCl2 solution by enhancing steric hindrance. However, 0.1 mg C/L BSA slightly promoted heteroaggregation in CaCl2 solution due to stronger electrostatic attraction than steric hindrance. In CaCl2 solution, raising HA concentration strengthened its destabilization effect via calcium bridging. Macromolecules having more compact globular structure and higher molecular weight may exert greater steric hindrance to inhibit heteroaggregation more effectively. This study provides new insights on the effects of macromolecules and electrolytes on heteroaggregation between nanoplastics and iron minerals in aquatic environments.

Heteroaggregation kinetics of nanoplastics and soot nanoparticles in aquatic environments

Heteroaggregation between polystyrene nanoplastics (PSNPs) and soot nanoparticles (STNPs) in aquatic environments may affect their fate and transport. This study investigated the effects of particle concentration ratio, electrolytes, pH, and humic acid on their heteroaggregation kinetics. The critical coagulation concentration (CCC) ranked CCCPSNPs > CCCPSNPs-STNPs > CCCSTNPs, indicating that heteroaggregation rates fell between homoaggregation rates. In NaCl solution, as the PSNPs/STNPs ratio decreased from 9/1 to 3/7, heteroaggregation rate decreased and CCCPSNPs-STNPs increased from 200 to 220 mM due to enhanced electrostatic repulsion. Outlier was observed at PSNPs/STNPs= 1/9, where CCCPSNPs-STNPs= 170 mM and homoaggregation of STNPs dominated. However, in CaCl2 solution where calcium bridged with STNPs, heteroaggregation rate increased and CCCPSNPs-STNPs decreased from 26 to 5 mM as the PSNPs/STNPs ratio decreasing from 9/1 to 1/9. In composite water samples, heteroaggregation occurred only at estuarine and marine salinities. Acidic condition promoted heteroaggregation via charge screening. Humic acid retarded or promoted heteroaggregation in NaCl or CaCl2 solutions by steric hindrance or calcium bridging, respectively. Other than van der Waals attraction and electrostatic repulsion, heteroaggregation was affected by steric hindrance, hydrophobic interactions, π - π interactions, and calcium bridging. The results highlight the role of black carbon on colloidal stability of PSNPs in aquatic environments.

Microplastics/nanoplastics in porous media: Key factors controlling their transport and retention behaviors

Till now, microplastics/nano-plastics(M/NPs) have received a lot of attention as emerging contaminant. As a typical but complex porous medium, soil is not only a large reservoir of M/NPs but also a gateway for M/NPs to enter groundwater. Therefore, the review of the factors controlling the transport behavior of M/NPs in porous media can provide important guidance for the risk assessment of M/NPs in soil and groundwater. In this study, the key factors controlling the transport behavior of M/NPs in porous media are systematically divided into three groups: (1) nature of M/NPs affecting M/NPs transport in porous media, (2) nature of flow affecting M/NPs transport in porous media, (3) nature of porous media affecting M/NPs transport. In each group, the specific control factors for M/NPs transport in porous media are discussed in detail. In addition to the above factors, some substances (colloids or pollutants) present in natural porous media (such as soil or sediments) will co-transport with M/NPs and affect its mobility. According to the different properties of co-transported substances, the mechanism of promoting or inhibiting the migration behavior of M/NPs in porous media was discussed. Finally, the limitations and future research directions of M/NPs transport in porous media are pointed out. This review can provide a useful reference for predicting the transport of M/NPs in natural porous media.

Small microplastic particles promote tetracycline and aureomycin adsorption by biochar in an aqueous solution

Biochar (BC) has been used to remove antibiotics from wastewater. Microplastics are emerging contaminants of wastewater. The capacities of microplastics for adsorbing antibiotics and the effects of microplastics of different types and particle sizes on antibiotic adsorption by BC have not been studied. Here, adsorption isotherm and kinetics experiments were performed to investigate tetracycline and aureomycin adsorption to polyvinyl chloride particles with diameters of 10, 100, 500, and 2000 μm, polylactic acid particles with diameters of 30, 100, 500, and 2000 μm (PLA30, PLA100, PLA500, and PLA2000, respectively), and wheat straw BC. The highest tetracycline adsorption capacity (25.00 mg g-1) was found for a PLA30 + BC. The tetracycline adsorption capacities of the other microplastic particles were 20.44-24.57 mg g-1. The highest aureomycin adsorption capacity (39.50 mg g-1) was found for 10 μm polyvinyl chloride particles and BC. The aureomycin adsorption capacities of the other microplastic particles were 32.21-38.42 mg g-1. The tetracycline adsorption capacities were 13.69%, 6.28%, 5.49%, and 4.54% higher for PLA30 + BC, PLA100 + BC, PLA500 + BC, and PLA2000 + BC, respectively, than for only BC. This may have been because there were more sites available per unit mass of microplastic for adsorbing tetracycline and dissolved organic carbon on small microplastic particles than large microplastic particles. The results indicated that microplastics can adsorb antibiotics and increase the amounts of antibiotics adsorbed by BC. Therefore, it is essential to consider potential interactions between BC and microplastics when BC is used to remove antibiotics from wastewater.

Scavenging microplastics and heavy metals from water using jujube waste-derived biochar in fixed-bed column trials

Extensive production and utilization of plastic products have resulted in the generation of microplastics (MPs), subsequently polluting the environment. The efficiency of biochars (BCs) derived from jujube (Ziziphus jujube L.) biomass (300 °C and 700 °C) for nylon (NYL) and polyethylene (PE) removal from contaminated water was explored in fixed-bed column trials. The optimum pH for the removal of both MPs was found 7. Both of the produced biochars demonstrated >99% removal of the MPs, while the sand filter exhibited a maximum of 78% removal of MPs. BC produced at 700 °C (BC700) showed 33-fold higher MPs retention, while BC produced at 300 °C (BC300) exhibited 20-fold higher retention, as compared to sand filters, indicating the higher efficiency of BC produced at higher pyrolysis temperature. Entrapment into the pores, entanglement with flaky structures of the BCs, and electrostatics interactions were the major mechanism for MPs retention in BCs. The efficiency of MPs-amended BCs was further explored for the removal of Pb(II) and Cd(II) in fixed-bed column trials. BC700 amended with PE and NYL exhibited the highest 50% breakthrough time (2114.23 and 2024.61 min, respectively, for Pb(II) removal and 2107.92 and 1965.19 min, respectively, for Cd(II) removal), as compared to sand filters (38.07 and 60.49 min for Pb(II) and Cd(II) removal, respectively). Thomas model predicted highest adsorption capacity was exhibited by BC700 amended with PE (584.34 and 552.80 mg g-1, for Pb(II) and Cd(II) removal, respectively), followed by BC700 amended with NYL (557.65 and 210.59 mg g-1 for Pb(II) and Cd(II) removal, respectively). Therefore, jujube waste-derived BCs could be used as efficient adsorbents to remove PE and NYL from contaminated water, while MPs-loaded BCs can further be utilized for higher adsorption of Pb(II) and Cd(II) from contaminated aqueous media. These findings suggest that BC could be used as an efficient adsorbent to remove the co-existing MPs-metals ions from the environment on a sustainable basis.

Cotransport of different electrically charged microplastics with PFOA in saturated porous media

The fate and transport behavior of microplastics (MPs), emerging colloidal contaminant ubiquitous in natural environments, would be greatly affected by other copresent pollutants. PFOA (emerging surfactant pollutant) would interact with MPs after encounter with them in natural environments, which could alter the transport behavior of both pollutants. Relevant knowledge is still lacking, affecting accurate prediction the fate and distribution of these two emerging contaminants in natural porous media. The cotransport behavior of different surface charged MPs (negatively/positively charged, CMPs/AMPs) with PFOA (three concentrations ranged from 0.1 to 10 mg/L) in porous media in both 10 and 50 mM NaCl solutions thus was investigated in the present study. We found PFOA inhibited CMPs transport in porous media, while enhanced AMPs transport. The mechanisms leading to the altered transport of CMPs/AMPs caused by PFOA were found to be different. The decreased electrostatic repulsion between CMPs-sand induced by the decreased CMPs negative zeta potentials via the adsorption of PFOA led to the inhibited transport of CMPs in CMPs-PFOA suspension. The enhanced electrostatic repulsion between AMPs-sand due to the decreased positive charge of AMPs via the adsorption of PFOA together with steric repulsion induced by suspended PFOA resulted in the increased transport of AMPs in AMPs-PFOA suspension. Meanwhile, we found that the adsorption onto MPs surfaces also impacted the transport of PFOA. Due to the lower mobility of MPs than PFOA, the presence of MPs despite their surface charge decreased the transport of PFOA of all examined concentrations in quartz sand columns. This study demonstrates that when MPs and PFOA are co-existing in environments, their interaction with each other will alter the fate and transport behavior of both pollutants in porous media and the alteration is highly correlated with the amount of PFOA adsorbed onto MPs and original surface properties of MPs.

Influence of microplastics on the transport of antibiotics in sand filtration investigated by AFM force spectroscopy

Microplastics and antibiotics were frequently detected in the effluent of sand filtration, while the presence of microplastics may change the interactions between the antibiotics and the quartz sands. However, the influence of microplastics on the transport of antibiotics in sand filtration has not been revealed. In this study, ciprofloxacin (CIP) and sulfamethoxazole (SMX) were respectively grafted on AFM probes to determine the adhesion forces to the representative microplastics (PS and PE) and the quartz sand. CIP and SMX exhibited low and high mobilities in the quartz sands, respectively. Compositional analysis of the adhesion forces indicated that the lower mobility of CIP in sand filtration columns could be attributed to the electrostatic attraction between the quartz sand and CIP compared with repulsion for SMX. Moreover, the significant hydrophobic interaction between the microplastics and the antibiotics could be responsible for the competitive adsorption of the antibiotics to the microplastics from the quartz sands; meanwhile, the π-π interaction further enhanced the adsorption of PS to the antibiotics. As a result of the high mobility of microplastics in the quartz sands, the carrying effect of microplastics enhanced the transport of antibiotics in the sand filtration columns regardless of their original mobilities. This study provided insights into the mechanism of the microplastics on enhancing the transport of antibiotics in sand filtration systems from the perspective of the molecular interaction.

Microplastic migration and distribution in the terrestrial and aquatic environments: A threat to biotic safety

Plastics production has been increasing over years, while their recycling rate is lower, resulting in huge amounts of microplastics (MP) accumulating in the environment. Although the environmental behaviors of MPs have been focused on in recent years, the migration, distribution and adverse effects of MPs in terrestrial and aquatic environments are still not systematically understood. In this review, based on the newest publications from the core database of the Web of Science, both the migration and distribution of MPs were summarized, as well as MPs transfer in biota and their biological effects were also focused on. Generally, the complicated and numerous pathways of MPs migration lead to their distribution throughout or nearly all environments on a global scale. However, the migration mechanisms of MPs with various sizes, shapes, and colors by physicochemical and biological processes, and the prediction models of MP migration and distribution, are deficient, despite these properties being highly related to MPs migration and bio-safety. Although MPs have already invaded microorganisms, plants, animals, and even human beings, the biological effects still need more study, so far as their sizes and shapes and also their composition and adsorption are concerned. Moreover, based on the highlights and deficiencies of current studies, further studies have also been proposed. This review aims to help people re-evaluate the uncertain behaviors of MPs in various environments, and could be helpful to fully understand their biological effects in different environmental conditions.

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.

Impact of biochar coexistence with polar/nonpolar microplastics on phenanthrene sorption in soil

Microplastics and biochar normally coexist in soil. In this study, two microplastics of different polarities (nonpolar polyethylene (PE) and polar polybutylene adipate-co-terephthalate (PBAT)) and two wheat straw biochars produced at 400 (W4) and 700 °C (W7) were selected to investigate the sorption behaviors of phenanthrene in soil where microplastics and biochar coexisted. The results showed that the presence of PE more significantly weakened the adhesion of soil particles onto biochar than the presence of PBAT. Meanwhile, the presence of biochar enhanced the soil particle attachment on the microplastic surface. As a result, the sorption behavior of phenanthrene was significantly different in soil where biochar coexisted with microplastics of different polarities. The K_oc values of PE-biochar-soil mixtures at C_e= 0.005 C_s were up to 42 % lower than those of PBAT-biochar-soil mixtures, which is related to lower micropore area of particles isolated from the former. However, at C_e = 0.05 C_s and 0.5 C_s, the K_oc values of PE-biochar-soil mixtures were up to 1.4 times higher than those of PBAT-biochar-soil mixtures because of a more significant reduction in biochar surface polarity when it coexisted with nonpolar PE.

Biochar and flow interruption control spatio-temporal dynamics of fecal coliform retention under subsurface drip irrigation

Applying wastewater in subsurface drip irrigation helps address water shortage in arid and semi-arid environments. Microbial contamination may result, but soil amendments such as biochar could help protect soil and water resources. To improve understanding, this study investigated spatio-temporal dynamics of fecal coliform retention in a biochar-treated soil under subsurface drip irrigation. Two dripper discharges rates of 2 and 4 l hr^-1 containing fecal coliforms (3.99 ± 1.4 × 10⁵ CFU ml^-1), three maximum allowable depletion levels (MAD; 30, 50, and 70%), and three biochar levels (0, 0.5 and 1%, w/w) were used. The lysimeters were filled with biochar-treated soil and had drippers located at 20 cm depth. Three irrigations were performed for either Q, and soil samples were taken after each irrigation at different times at the lysimeter centre at 5, 15, 25, and 50 cm depths and at the edge at 20 cm depth (labeled 20 L). Soils containing biochar had much greater recovered coliforms from soil than the unamended controls. The greatest retention was at 15 and 25 cm depth (within 5 cm of the dripper) for 1% biochar, with the recovered cells about 70% for 2 l hr^-1 discharge and 60% for 4 l hr^-1 discharge. The greatest concentration occurred immediately after irrigation, but over 10 days the number of coliforms gradually decreased and inactivated. Therefore, the coliform residence time, soil depth, and biochar rate all influenced coliform retention if the discharge rates and MADs were considered. We recommend slower subsurface dripper rates with high MAD and biochar amendment to minimize fecal coliform contamination from subsurface field wastewater irrigation.

Biochar for the Removal of Emerging Pollutants from Aquatic Systems: A Review

Water contaminated with emerging pollutants has become a serious environmental issue globally. Biochar is a porous and carbon-rich material produced from biomass pyrolysis and has the potential to be used as an integrated adsorptive material. Many studies have shown that biochar is capable to adsorb emerging pollutants from aquatic systems and could be used to solve the water pollution problem. Here, we provided a dual perspective on removing emerging pollutants from aquatic systems using biochar and analyzed the emerging pollutant removal efficiency from the aspects of biochar types, pollutant types and coexistence with heavy metals, as well as the associated mechanisms. The potential risks and future research directions of biochar utilization are also presented. This review aims to assist researchers interested in using biochar for emerging pollutants remediation in aquatic systems and facilitate research on emerging pollutants removal, thereby reducing their environmental risk.

Transport of plastic particles in natural porous media under freeze-thaw treatment: Effects of porous media property

Freeze-thaw (FT) cycles would alter physical and chemical properties of soil and thus influence the transport of plastic particles (one type of emerging contaminant with great concerns). This study was designed to investigate the effects of FT treatment on the mobility of plastic particles (nanoplastics as representative) in columns packed with natural soils (i.e. loamy sand and sandy soil, quartz sand employed as comparison). We found that FT treatment of different types of porous media would induce different transport behaviors of plastic particles. Specifically, FT treatment of quartz sand did not affect plastic particles mobility. While FT treatment of loamy sand and sandy soil increased plastic particles transport. The increased pore sizes and disintegration of small soil particles from soils (the detached soil would serve as mobile vehicle for the transport of plastic particle) led to the facilitated mobility of plastic particles in two types of soils after FT treatment. The presence of preferential flow paths induced by FT treatment also drove to the enhanced mobility of plastic particles in sandy soil with FT treatment. This study clearly showed that the mobility of model plastic particles in two types of natural soils was greatly enhanced by FT treatment.

Adsorptive behavior of micro(nano)plastics through biochar: Co-existence, consequences, and challenges in contaminated ecosystems

The abundance of micro(nano)plastics in natural ecosystems is a crucial global challenge, as these small-sized plastic particles originate from land-based and marine-based activities and are widely present in marine, freshwater, and terrestrial ecosystems. Micro(nano)plastics can significantly be reduced through various methods, such as biological, chemical, and physical techniques. Biochar is a low-cost adsorbent and is considered an efficient material and its application is ecologically effective carbon-negative for remediation of organic and inorganic pollutants. Therefore, this review critically discusses the fate and transport of micro(nano)plastics and their interactions with different biochar in aqueous and column porous media. This review outlines the implications of biochar with the co-existence of micro(nano)plastics in efforts to understand their coupled effects on soil physicochemical properties, microbial communities, and plant growth, along with the removal of heavy metals and other toxic contaminants. In batch experiments, biochar synthesized from various biomasses such as corn straw, hardwood, pine and spruce bark, corncob, and Prosopis juliflora had shown high level of removal efficiency (>90 %) for microplastic adsorption under varying environmental conditions viz., pH, temperature, ionic strength, particle size, and dose due to chemical bonding and electrostatic attractions. Increased temperature of the aqueous solutions encouraged higher adsorption, while higher pH and dissolved organic matter and nutrients may show decreased adsorption capacities for micro(nano)plastics using biochar. Compared to other available physical, chemical, and biological methods, biochar-amended sand filters in column experiments have been very efficient in removing micro(nano)plastics. In saturated column porous media, various microplastics could be inhibited using biochar due to decreased electrostatic repulsion, steric hindrance, and competitive sorption due to humic acid, ionic strength, and cations. Finally, this review provides in-depth insights on further investigations and recommendations for overall micro(nano)plastics removal using biochar-based materials.

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.

Micro(nano)plastic pollution in terrestrial ecosystem: emphasis on impacts of polystyrene on soil biota, plants, animals, and humans

Pollution with emerging microscopic contaminants such as microplastics (MPs) and nanoplastics (NPs) including polystyrene (PS) in aquatic and terrestrial environments is increasingly recognized. PS is largely used in packaging materials and is dumped directly into the ecosystem. PS micro-nano-plastics (MNPs) can be potentially bioaccumulated in the food chain and can cause human health concerns through food consumption. Earlier MP research has focused on the aquatic environments, but recent researches show significant MP and NP contamination in the terrestrial environments especially agricultural fields. Though PS is the hotspot of MPs research, however, to our knowledge, this systematic review represents the first of its kind that specifically focused on PS contamination in agricultural soils, covering sources, effects, and ways of PS mitigation. The paper also provides updated information on the effects of PS on soil organisms, its uptake by plants, and effects on higher animals as well as human beings. Directions for future research are also proposed to increase our understanding of the environmental contamination of PS in terrestrial environments.

Transport characteristics of polystyrene microplastics in saturated porous media with biochar/Fe3O4-biochar under various chemical conditions

Magnetically modified biochar, with a rougher surface and more positive surface charge, may interact with microplastics (MPs) after being applied to soil, potentially altering the fate and transport of MPs in porous media. In this study, the transport and retention behavior of polystyrene microplastics (PSMPs) in a sandy porous media mixed with biochar/Fe₃O₄ modified biochar (Fe₃O₄-biochar) was investigated under various chemical conditions (humic acid (HA), ionic strength (IS) and cationic types (Na⁺/Ca²⁺)). The results showed that the addition of biochar and Fe₃O₄-biochar can hinder the transport of PSMPs in porous media without HA, and that Fe₃O₄-biochar was more effective in inhibiting the transport of PSMPs through electrostatic adsorption and complexation, with an optimum retention efficiency of 92.36 %. HA significantly attenuated the retention of PSMPs in both porous media through electrostatic repulsion, steric resistance and competitive adsorption under 1 mM Na⁺ solutions, and the mobility of PSMPs in Fe₃O₄-biochar/sand was enhanced more significantly than in biochar/sand with the increase of HA concentration. IS significantly inhibited the transport of PSMPs in both porous media in the absence of HA, but there was an antagonistic effect of HA and IS on the transport of PSMPs in the presence of HA, with the facilitative effect of HA being stronger than the inhibitory effect of IS. Ca²⁺ was consistently more effective in inhibiting the transport of PSMPs than Na⁺ under all test conditions, and HA promoted the transport of PSMPs in all Na⁺ solutions, while it inhibited the transport of PSMPs in high IS (10 mM) with Ca²⁺ solutions. In addition, HA, Fe₃O₄-biochar and PSMPs tend to form larger aggregates under the complex interactions of Ca²⁺, leading to increased retention of PSMPs in porous media. The two-site kinetic retention models suggested that the retention of PSMPs in porous media with biochar was predominantly reversible attachment effect, while retention in porous media with Fe₃O₄-biochar was predominantly an irreversible straining effect.

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.

Aging of colloidal contaminants and pathogens in the soil environment: Implications for nanoplastic and COVID-19 risk mitigation

Colloidal contaminants and pathogens are widely distributed in soil, whose tiny sizes and distinct surface properties render unique environmental behaviours. Because of aging, colloids can undergo dramatic changes in their physicochemical properties once in the soil environment, thus leading to diverse or even unpredictable environmental behaviour and fate. Herein, we provide a state-of-art review of colloid aging mechanisms and characteristics and implications for risk mitigation. First, we review aging-induced formation of colloidal contaminants and aging-associated changes. We place a special focus on emerging nanoplastic (NP) contaminants and associated physical, chemical, and biological aging processes in soil environments. Second, we assess aging and survival features of colloidal pathogens, especially viruses. Viruses in soils may survive from several days to months, or even several years in groundwater, depending on their rates of inactivation and the reversibility of attachment. Furthermore, we identify implications for risk mitigation based on aging mechanisms. Hotspots of (photo)chemical aging of NPs, including plastic gauzes at construction sites and randomly discarded plastic waste in rural areas, are identified as area requiring greater research attention. For COVID-19, we suggest taking greater care in regions where viruses are persist for long periods, such as cold climate regions. Soil amendment with quicklime (CaO) may act as an effective means for pathogen disinfection. Future risk mitigation of colloidal contaminants and pathogens relies on a better understanding of aging mechanisms and more sophisticated models accurately depicting processes in real soil environments.

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

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

Effects of clay minerals on the transport of polystyrene nanoplastic in groundwater

Microplastics are widely detected in the soil-groundwater environment, which has attracted more and more attention. Clay mineral is an important component of the porous media contained in aquifers. The transport experiments of polystyrene nanoparticles (PSNPs) in quartz sand (QS) mixed with three kinds of clay minerals are conducted to investigate the effects of kaolinite (KL), montmorillonite (MT) and illite (IL) on the mobility of PSNPs in groundwater. Two-dimensional (2D) distributions of DLVO interaction energy are calculated to quantify the interactions between PSNPs and three kinds of clay minerals. The critical ionic strengths (CIS) of PSNPs-KL, PSNPs-MT and PSNPs-IL are 17.0 mM, 19.3 mM and 21.0 mM, respectively. Experimental results suggest KL has the strongest inhibition effect on the mobility of PSNPs, followed by MT and IL. Simultaneously, the change of ionic strength can alter the surface charge of PSNPs and clay minerals, thus affecting the interaction energy. Experimental and model results indicate both the deposition rate coefficient (k) and maximum deposition (S_max) linearly decrease with the logarithm of the DLVO energy barrier, while the mass recovery rate of PSNPs (R_m) exponentially increases with the logarithm of the DLVO energy barrier. Therefore, the mobility and associated kinetic parameters of PSNPs in complex porous media containing clay minerals can be predicted by 2D distributions of DLVO interaction energy. These findings could help to gain insight into understanding the environmental behavior and transport mechanism of microplastics in the multicomponent porous media, and provide a scientific basis for the accurate simulation and prediction of microplastic contamination in the groundwater system.

Application of α-Fe2O3 nanoparticles in controlling antibiotic resistance gene transport and interception in porous media

Metal oxide nanoparticles (MONPs) with a large specific surface area are expected to bind with antibiotic resistance genes (ARGs), thereby controlling ARGs' contamination by reducing their concentration and mobilization. Here, adsorption experiments were carried out and it was found that α-Fe₂O₃ NPs could chemically bind with ARGs (tetM-carrying plasmids) in water with an adsorption rate of 0.04 min^-1 and an adsorption capacity of 7.88 g/kg. Mixing α-Fe₂O₃ NPs into quartz sand column markedly increased the interceptive removal of ARGs from inflow water. The interception rate of 1.0 μg/mL ARGs in ultrapure water (25 mL, 5 pore volumes) through the sand column (plexiglass, length 8 cm, internal diameter 1.4 cm) with 1 g/kg α-Fe₂O₃ NPs was 1.73 times of that through the pure sand column; the interception rate overall increased with increasing addition of α-Fe₂O₃ NPs, reaching 68.8% with 20 g/kg α-Fe₂O₃ NPs. Coexisting Na⁺ (20 mM), Ca²⁺ (20 mM), and acidic condition (pH 4.0) could further increase the interception rate of ARGs by 1 g/kg α-Fe₂O₃ NPs from 21.1% to 86.2%, 90.7%, and 96.2%, respectively. The presence of PO₄^3- and humic acid at environmentally relevant concentrations would not significantly affect the interception of ARGs. In the treatment groups with PO₄^3- and humic acid, the removal rate decreased by only 1.8% and 0.1%, respectively. In addition, the interceptive removal of ARGs by α-Fe₂O₃ NPs-incorporated sand column was even better in actual surface water samples (87.2%) than that in the ultrapure water (21.1%). The findings provide a promising approach to treat ARGs-polluted water.

Oil spills enhanced dispersion and transport of microplastics in sea water and sand at coastal beachheads

The coastal zone is being under the threat by accumulation of microplastics (MPs), with much of MPs ending up on the beachhead. Oil spills, which frequently happen in coastal zones due to oil pipe leakage or oil drilling, may affect the behavior of MPs in the beachheads. Herein, sea water and sea sand were collected from three different coastal beachheads including Bohai Sea (BS), East Sea (ES), and South Sea (SS), China, to investigate how the oil spills affect the dispersion and transport of MPs in sea water and sand. The oil spills greatly enhanced the dispersion of MPs in all three sea waters by forming MPs-oil-dispersant agglomerates, which increased the electrostatic repulsion and steric hindrance between MPs particles. Accordingly, the aggregation rates of MPs were reduced from 1.7-8.86 nm min^-1 to 0.39-1.29 nm min^-1. The lowest salinity and highest dissolved organic carbon content in SS sea water favored the highest dispersion of MPs, compared to BS and ES sea water. The improved dispersion of MPs with oil spills enhanced their transport in sea sand with an increase of effluent rates from 0-18.8 % to 5.78-42.2 % for BS and from 30.5-45.2 % to 35.0-60.0 % for SS one. However, the transport of MPs in ES sea sand was lower than 3.62 %, even with oil spills, which was attributed to the strong adsorption of MPs by the rich Fe/Al oxides in ES sea sand through electric attraction. Modeling also showed that oil spills increased the migration rate of 10 mg g^-1 MPs accumulated in the surface 0-1 cm sea sand from 6.50-13.8 cm year^-1 to 8.17-16.7 cm year^-1 after 1500 mm rainfall for 3 years, and the strongest transport of MPs was observed in SS sea sand, with the highest cumulative flux and the longest maximum migration depth as 0.089-0.120 mg/cm² and 50 cm, respectively. These results indicated that the dispersion and transport of MPs can be enhanced by oil spills, but regulated by sea water salinity for MPs dispersion and sea sand Fe/Al oxides for MPs transport, which advanced our understanding of the transport and transformation of MPs in coastal zones.

Aggregation kinetics of biochar nanoparticles in aqueous environment: Interplays of anion type and bovine serum albumin

The colloidal particles, especially those at the nanoscale, are the most active part of the pyrogenic carbon (biochar). Increasingly applied biochar has resulted in a large number of biochar nanoparticles (NPs) being released into the environment. The aggregation of biochar NPs affects their environmental behavior and fate. The complex effects of anion type (Cl^-, SO₄^2-) and protein (bovine serum albumin, BSA) on the aggregation of wheat straw biochar (WB) and pinewood biochar (PB) NPs in solutions were investigated by the time-resolved dynamic light scattering method. The critical coagulation concentration (CCC) of WB and PB NPs in Na₂SO₄ solution was higher than their CCCs in NaCl solution, which was consistent with the Hofmeister series that SO₄^2-, a kosmotrope anion, increased the interaction between water molecules, thus enhancing the hydrophobic interactions between biochar NPs in solution and promoting their aggregation, while Cl^-, a chaotropic agent, exhibited the opposite effect. When BSA was added into the solution, BSA was adsorbed on the surface of biochar NPs and BSA corona was formed, which inhibited the aggregation of biochar NPs by inducing steric force. The enhanced stability of biochar NPs by BSA was more significant in NaCl than in Na₂SO₄ solution because BSA corona had a more negatively charged surface and a more steric structure in NaCl solution, thus generating stronger electrical repulsion and steric hindrance. The classical DLVO theory and the XDLVO theory incorporating the steric repulsion (in the presence of BSA) were used to interpret the aggregation and dispersion of biochar NPs. Through this study, we found that anion type indirectly affected the aggregation of biochar NPs by influencing the interaction between water molecules, while the aggregation of BSA-biochar NPs conjugates is mainly influenced by the surface charge and structure of BSA corona.

Micro- and nanoplastics retention in porous media exhibits different dependence on grain surface roughness and clay coating with particle size

The presence and/or coating of natural colloids (e.g., clays and metal oxides or hydroxides) on collector surfaces has frequently been demonstrated to enhance the retention of engineered colloids that are negatively charged due to favorable electrostatic interactions. However, this work demonstrates that the presence of natural clay coating can lead to reduced or nonmonotonic retention of micro- and nanoplastics (MNPs). Column experiments were carried out to systematically investigate the transport of MNPs with different sizes in relatively smooth and rough sands that had various clay coating fractions. These coating fractions on the collector were found to significantly influence MNP retention in a complex manner that changed with the colloid size and the roughness properties of the sand. This observation was attributed to the impact of clay coatings on the roughness and morphology properties of collector surfaces that were dominant over surface charge. Scanning electron microscopy and interaction energy calculations on surfaces with pillars or valleys indicate that mechanisms that contributed to MNP retention changed with the colloid size. In particular, retention of nanosized plastics was mainly controlled by interactions on convex/concave locations that changed with the solution chemistry, whereas microsized plastics were also strongly influenced by the applied hydrodynamic torque and straining processes. Additionally, the significant sensitivity of MNP retention under a low-level ionic strength also reflects the importance of roughness and charge heterogeneities. These observations are important for investigating the mechanisms of colloid transport in natural systems that ubiquitously exhibit clay coating on their surfaces.

Transport and retention of microplastics in saturated porous media with peanut shell biochar (PSB) and MgO-PSB amendment: Co-effects of cations and humic acid

Biochar particles are extensively used in soil remediation and interact with microplastics (MPs), especially metal oxide-modified biochar may have stronger interactions with MPs. The mechanism of interactions between humic acid (HA) and different valence cations is different and the co-effect on the transport of MPs is not clear. In this study, the co-effects of HA and cations (Na⁺, Ca²⁺) on the transport and retention of MPs in saturated porous media with peanut shell biochar (PSB) and MgO-modified PSB (MgO-PSB) were systematically investigated. Breakthrough curves (BTCs) of MPs were fitted by the two-site kinetic retention model for analysis. In the absence of HA, the addition of PSB and MgO-PSB significantly hindered the transport of MPs in saturated porous media, and the retention of MPs increased from 34.2% to 59.1% and 75.5%, respectively. In Na⁺ solutions, the HA concentration played a dominant role in controlling MPs transport, compared to the minor role of Na⁺. The transport capacity of MPs always increased gradually with the increase of HA concentration. Whereas, in Ca²⁺ solutions, Ca²⁺ concentrations had a stronger effect than HA. The transport ability of MPs was instead greater than that in Na⁺ solutions as the HA concentration increased at low ionic strength (1 mM). However, the transport capacity of MPs was significantly reduced with increasing HA concentrations at higher ionic strength (10, 100 mM). The two-site kinetic retention model indicated that chemical attachment and physical straining are the main mechanisms of MPs retention in the saturated porous media.

The response of life history defense of cladocerans under predation risk varies with the size and concentration of microplastics

Microplastics are an emerging and increasingly serious pollutant in freshwater environment, which have become a threat to freshwater organisms. However, whether microplastics interfere with the responses of organisms to their predators is still unclear. In this study, we investigated the effects of microplastics with tiny different particle size (diameter: 0.7 and 1 µm) on the anti-predation (Rhodeus ocellatus as the predator) defense responses of different body-sized cladocerans, Daphnia pulex and Moina macrocopa. Results showed that microplastics had a size-based inhibitory effect on the induced defense of both D. pulex and M. macrocopa. Specifically, 0.7 µm microplastics had stronger effects on reduced survival time, delayed maturation time, and decreased offspring numbers. In addition, the effects of microplastics also varied with different body-sized cladocerans, i.e. medium-sized cladoceran (D. pulex) were more sensitive than the small-sized one (M. macrocopa) regarding the maturation time. This study illustrated for the first time that the effect of microplastics on induced defense was related to cladoceran species and microplastics size, and further revealed the extensive negative effects of microplastics from the perspective of interspecific relationship.

Effect of Agricultural Organic Inputs on Nanoplastics Transport in Saturated Goethite-Coated Porous Media: Particle Size Selectivity and Role of Dissolved Organic Matter

The transport of nanoplastics (NPs) through porous media is influenced by dissolved organic matter (DOM) released from agricultural organic inputs. Here, cotransport of NPs with three types of DOM (biocharDOM (BCDOM), wheat strawDOM (WSDOM), and swine manureDOM (SMDOM)) was investigated in saturated goethite (GT)-coated sand columns. The results showed that codeposition of 50 nm NPs (50NPs) with DOM occurred due to the formation of a GT-DOM-50NPs complex, while DOM loaded on GT-coated sand and 400 nm NPs (400NPs) aided 400NPs transport due to electrostatic repulsion. According to the quantum chemical calculation, humic acid and cellulose played a significant role in 50NPs retardation. Owing to its high concentration, moderate humification index (HIX), and cellulose content, SMDOM exhibited the highest retardation of 50NPs transport and promoting effect on 400NPs transport. Owing to a high HIX, the effect of BCDOM on the mobility of 400NPs was higher than that of WSDOM. However, high cellulose content in WSDOM caused it to exhibit a 50NPs retardation ability that was similar to that of BCDOM. Our results highlight the particle size selectivity and significant influence of DOM type on the transport of NPs and elucidate their quantum and colloidal chemical-interface mechanisms in a typical agricultural environment.

Transport and deposition behaviors of microplastics in porous media: Co-impacts of N fertilizers and humic acid

Due to the interaction of fertilizers with microplastics (MPs) and porous media, fertilization process would influence MPs transport and distributions in soil. The co-impacts of N fertilizers (both inorganic and organic N fertilizers) and humic substance on MPs transport/retention behaviors in porous media were examined in 10 mM KCl solutions at pH 6. NH₄Cl and CO(NH₂)₂ were employed as inorganic and organic N fertilizers, respectively, while humic acid (HA) was used as model humic substance. We found that for all three sized MPs (0.2, 1 and 2 µm) without HA, both types of N fertilizers decreased their transport/increased their retention in porous media (both quartz sand and soil). N fertilizers adsorbed onto surfaces of MPs and sand/soil, lowering the electrostatic repulsion between MPs and porous media, thus contributed to the enhanced MPs deposition. MPs with N fertilizers in solutions more tightly attached onto porous media and thus were more difficult to be re-mobilized by low ionic strength solution elution. Via steric repulsion and increasing electrostatic repulsion between MPs and porous media due to adsorption onto their surfaces, HA could increase MPs transport with N fertilizers in solutions.

Effects of selected functional groups on nanoplastics transport in saturated media under diethylhexyl phthalate co-contamination conditions

The production and degradation of plastic remains can result in nanoplastics (NPs) formation. However, insufficient information regarding the environmental behaviors of NPs impedes comprehensive assessment of their significant threats. In this study, the transport behavior of unmodified NPs (PSNPs), carboxyl-modified NPs (PSNPs-COOH), and amino-modified NPs (PSNPs-NH₂) was investigated using column experiments in the presence and absence of goethite (GT) and diethylhexyl phthalate (DEHP). Quantum chemical computation was performed to reveal the transport mechanisms. The results showed that GT decreased the transport of NPs and the presence of DEHP decreased it further. Van der Waals forces and small electrostatic interactions coexisted between the PSNPs and GT and caused deposition. Ligand exchange caused greater deposition of PSNPs-COOH on GT-coated sand than that of PSNPs. Although hydrogen bonding existed between the DEHP and NPs with functional groups, an increase in the positive charge and chemical heterogeneity of the collector was the main reason for DEHP promoting the deposition of NPs. Because of low absolute negative zeta potential values, PSNPs-NH₂ was sensitive to chemical heterogeneity, and thus fully deposited (over 96.9%) in GT and GT-DEHP-coated columns. Generally, the deposition of NPs due to chemical heterogeneity was more significant than that due to the formation of chemical bonds and van der Waals, electrostatic, and hydrogen interactions. Our results highlight that the surface charge and functional groups significantly influence the transport behaviors of NPs and elucidate the fate of NPs in the terrestrial environment.

New insights into the enhanced transport of uncoated and polyvinylpyrrolidone-coated silver nanoparticles in saturated porous media by dissolved black carbons

Silver nanoparticles (AgNPs) are among the most applied nanomaterials and have great potential to be present in the environment. Dissolved black carbon (DBC) is ubiquitous in soil as a result of large-scale application of biomass-derived black carbon as soil amendments, while its impacts on the transport of AgNPs remain unclear. In this study, two DBCs with different functional groups were prepared at 300 and 500 °C (DBC300 and DBC500), and their impacts on the transport of uncoated AgNPs (Bare-AgNP) and polyvinylpyrrolidone-coated AgNPs (PVP-AgNP) in saturated quartz sand were investigated. The transport of PVP-AgNP was much higher than Bare-AgNP under the same conditions because of the increased steric hindrance provided by PVP surface coating. The transport of two kinds of AgNPs was both enhanced by the DBCs under all the experimental conditions. DBC500 displayed a stronger enhancement effect than DBC300 on PVP-AgNP transport, but DBC300 facilitated the migration of Bare-AgNP more significantly than DBC500. The higher aromaticity and stronger hydrophobicity of DBC500 drove it to be adsorbed on the surface of PVP-AgNP, thus providing stronger steric hindrance and promotion effect on PVP-AgNP transport. However, DBC300 contained surface sulfhydryl groups, which bound with the Bare-AgNP tightly, therefore it greatly promoted Bare-AgNP transport via enhanced steric hindrance. (X)DLVO calculations indicated DBCs generally increased the energy barrier between the AgNPs and sand grains. The results shed light on the vital roles of both the properties of AgNPs and DBCs on the fate and environmental behaviors of silver nanomaterials in complex environments.

Transport and deposition of microplastic particles in saturated porous media: Co-effects of clay particles and natural organic matter

Natural colloids such as clays and natural organic matter (NOM) are universally present in environments, which could interact with microplastics (MPs) and thus alter the fate and transport of MPs in porous media. The co-effects of clays and NOM on MPs transport in saturated porous media were systematically explored at both low and high ionic strength (IS) conditions. Specifically, bentonite and humic acid (HA) were employed as representative clays and NOM. 5 mM NaCl and 1 mM CaCl₂ solutions were used as low IS conditions, while 25 mM NaCl and 5 mM CaCl₂ solutions were employed as high IS conditions. We found that formation of MPs-bentonite heteroaggregates had great effects on MPs transport under different conditions. Without HA, the small MPs-bentonite heteroaggregates formed under low IS increased MPs transport via serving as mobile carriers, while larger MPs-bentonite heteroaggregates formed at high IS led to the decreased MPs mobility. When both HA and bentonite were copresent in MPs suspension, we found that HA could inhibit the formation of larger sized MPs-bentonite heteroaggregates. Particularly, when the two types of natural colloids copresent in MPs suspensions, MPs transport behaviors were similar to those with only bentonite present in MPs suspensions at low IS, while MPs transport was greatly increased at high IS comparing with those only with bentonite in suspensions. Clearly, without HA in suspensions, bentonite played the dominant role on MPs transport under all examined conditions concerned in this study. Instead, when both HA and bentonite copresent in MPs suspensions, MPs transport was mainly controlled by bentonite at low IS, while both bentonite and HA had major contributions at high IS. The results showed that under solution conditions concerned in present study, MPs mobility in porous media would be greatly affected (either enhanced or inhibited) by the two types of natural colloids.

Microplastics in the soil-groundwater environment: Aging, migration, and co-transport of contaminants - A critical review

Microplastic contamination in soil has received increasing attention since excessive plastic debris has been emitted directly into the terrestrial environment. Once released into the terrestrial environment, microplastics can be aged via photo- and thermally-initiated oxidative degradation, hetero-aggregation, and bioturbation. Aging affects the physiochemical properties of microplastics with the increase of surface roughness and oxygen-containing groups, which could enhance the sorption and mobility of microplastics in the soil and groundwater environment. However, the interactions among aging, sorption, and transport of microplastics in the terrestrial system have not been unveiled. This review clarifies the key processes of microplastics transport pathways in soil and groundwater ecosystems influenced by aging and sorption under various scenarios. Co-transport of microplastics and sorbed contaminants are also addressed to help understand the risks associated with heavy metals, organic contaminants, and engineered nanoparticles in the soil environment. Overall, this review elaborates the most pressing research limitations on the present literature and highlights the future perspectives to investigate the possible broad transport pathways of microplastics in soil.

Bacteria have different effects on the transport behaviors of positively and negatively charged microplastics in porous media

Bacteria, biological colloids with wide presence in natural environments, would interact with plastic particles (emerging colloids with great concern recently) and thus would influence the fate and distribution of plastics in environment. In present research, the impacts of bacteria (both Gram (-) E. coli and Gram (+) B. subtilis) on the transport/deposition of model microplastics (MPs) in porous media were examined in NaCl salt solutions (5 and 25 mM, pH = 6). Both negative carboxylate-modified MPs (CMPs) and positive amine-modified MPs (AMPs) were concerned. We found that under both solution conditions, the presence of both types of bacteria decreased CMPs transport and enhanced retention of CMPs in sand columns. In contrast, the presence of bacteria (regardless of cell type) yet increased AMPs transport and decreased their deposition in sand columns under both ionic strength conditions. The mechanisms leading to the altered transport of CMPs and AMPs by bacteria were different. The formation of larger sized CMPs-bacteria clusters and the extra deposition sites resulted from bacteria adsorbed on quartz sand contributed to the decreased CMPs transport and enhanced their deposition in sand columns. Whereas, the formation of AMPs-bacteria clusters with overall negatively surface charge improved AMPs transport in quartz sand.

Transport characteristics of fragmental polyethylene glycol terephthalate (PET) microplastics in porous media under various chemical conditions

Transport characteristics of fragmental polyethylene glycol terephthalate (PET) microplastics in porous media were elucidated via column experiments under a series combination of electrolytes, pH, and humic acid (HA) conditions. Fragmental PET microplastics showed low mobility in porous media with a small mass recovery rate (<50.1%) even under unfavorable retention conditions. The electrolyte, pH, and HA showed combined impact on PET microplastic transport. PET microplastics mobility was enhanced with decreasing electrolyte concentration, increasing pH, and increasing HA concentration. Basic properties (e.g. destiny and shape) of PET microplastics showed stronger effect on their transport behaviors in porous media rather than the experimental chemical conditions. In general, both environmental factors and basic properties played important roles in controlling the retention and transport of PET microplastics in porous media. A numerical model considering the second order kinetic deposition sites was applied to depict the retention and transport of PET microplastics in porous media. Model simulations well matched the experimental breakthrough curves. Given the fragmental PET microplastics have more realistic and irregular shapes, results from this study can improve present knowledge of the environmental fate and risk of microplastics in underground soil and water systems.

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.

Environmental fate, toxicity and risk management strategies of nanoplastics in the environment: Current status and future perspectives

Tiny plastic particles considered as emerging contaminants have attracted considerable interest in the last few years. Mechanical abrasion, photochemical oxidation and biological degradation of larger plastic debris result in the formation of microplastics (MPs, 1 μm to 5 mm) and nanoplastics (NPs, 1 nm to 1000 nm). Compared with MPs, the environmental fate, ecosystem toxicity and potential risks associated with NPs have so far been less explored. This review provides a state-of-the-art overview of current research on NPs with focus on currently less-investigated fields, such as the environmental fate in agroecosystems, migration in porous media, weathering, and toxic effects on plants. The co-transport of NPs with organic contaminants and heavy metals threaten human health and ecosystems. Furthermore, NPs may serve as a novel habitat for microbial colonization, and may act as carriers for pathogens (i.e., bacteria and viruses). An integrated framework is proposed to better understand the interrelationships between NPs, ecosystems and the human society. In order to fully understand the sources and sinks of NPs, more studies should focus on the total environment, including freshwater, ocean, groundwater, soil and air, and more attempts should be made to explore the aging and aggregation of NPs in environmentally relevant conditions. Considering the fact that naturally-weathered plastic debris may have distinct physicochemical characteristics, future studies should explore the environmental behavior of naturally-aged NPs rather than synthetic polystyrene nanobeads.

Biochar Aging: Mechanisms, Physicochemical Changes, Assessment, And Implications for Field Applications

Biochar has triggered a black gold rush in environmental studies as a carbon-rich material with well-developed porous structure and tunable functionality. While much attention has been placed on its apparent ability to store carbon in the ground, immobilize soil pollutants, and improve soil fertility, its temporally evolving in situ performance in these roles must not be overlooked. After field application, various environmental factors, such as temperature variations, precipitation events and microbial activities, can lead to its fragmentation, dissolution, and oxidation, thus causing drastic changes to the physicochemical properties. Direct monitoring of biochar-amended soils can provide good evidence of its temporal evolution, but this requires long-term field trials. Various artificial aging methods, such as chemical oxidation, wet-dry cycling and mineral modification, have therefore been designed to mimic natural aging mechanisms. Here we evaluate the science of biochar aging, critically summarize aging-induced changes to biochar properties, and offer a state-of-the-art for artificial aging simulation approaches. In addition, the implications of biochar aging are also considered regarding its potential development and deployment as a soil amendment. We suggest that for improved simulation and prediction, artificial aging methods must shift from qualitative to quantitative approaches. Furthermore, artificial preaging may serve to synthesize engineered biochars for green and sustainable environmental applications.

Influence of biofilm on the transport and deposition behaviors of nano- and micro-plastic particles in quartz sand

Biofilm, community of bacteria ubiquitously present in natural environment, may interact with plastic particles and affect the transport of plastic particles in environment. The significance of biofilm (Escherichia coli) on the transport and deposition behaviors of three different sized plastic particles (0.02 μm NPs, 0.2 μm MP and 2 μm MP) were examined under both 10 mM and 50 mM NaCl solutions by comparing the breakthrough curves and retained profiles of plastic particles in bare sand versus those in biofilm-coated sand. Regardless of ionic strengths, the presence of biofilm increases the deposition of all three sized plastic particles in porous media. Via employing X-ray microtomography imaging (XMT) and Scanning electron microscope (SEM), we find that the presence of biofilm could narrow the flow path especially near to the inlet of the column and increase the surface roughness of porous media (by decreasing DLVO repulsive interaction), which contributes to the enhanced the deposition of plastic particles. Extracellular polymeric substances (EPS) present on the biofilm are found to contribute to the enhanced deposition of plastic particles. Packed column experiments, quartz crystal microbalance with dissipation (QCM-D) as well as parallel plate flow chamber experiments all show that three major components of EPS, proteins, polysaccharide, and humic substances all contribute to the enhanced deposition of plastic particles. O-H and N-H groups present on cell surfaces are highly likely to form hydrogen bond with plastic particles and increase the deposition plastic particles. Elution experiments show that decreasing solution ionic strength could release small portion of plastic particles from both bare and biofilm-coated sand columns especially from the segments near to the column inlet (with slighter lower percentage from biofilm-coated columns based on the total mass of retained plastics). In contrast, increasing flow rate does not obviously detach the plastic particles that already deposited onto porous media. The results of this study clearly show that the presence of biofilm in natural environment could enhance the deposition and decrease the transport of plastic particles.